WO2001069136A1

WO2001069136A1 - Laser ignition

Info

Publication number: WO2001069136A1
Application number: PCT/US2000/006255
Authority: WO
Inventors: James W. Early; Charles S. Lester
Original assignee: The Regents Of The University Of California
Priority date: 2000-03-10
Filing date: 2000-03-10
Publication date: 2001-09-20
Also published as: AU2000235236A1

Abstract

An excitation light source (10) and an ignitor laser (50) in tandem provide a compact, durable, engine deployable fuel ignition system. A single remote excitation light source is used for one or more small lasers located proximate to one or more combustion zones. In two embodiments the excitation light source beam is split. A first portion goes to the ignitor laser. A second portion combines either with the first portion before injection into the ignitor laser or with the output of the ignitor laser. In another embodiment short and long pulses from the excitation light source are directed into the ignitor laser. In another embodiment the excitation light source is a laser with more than one resonating cavity. In another embodiment the excitation light source is capable of producing alternating beams of light having different wavelengths for pumping. Each of the embodiments can be multiplexed.

Description

LASER IGNITION

This application is a continuation in part of Patent Application No. 08/861,214 filed

May 21, 1997, which had benefit of Provisional Patent Application No. 60/044,483 filed April

21, 1997.

TECHNICAL FIELD

This invention relates to a method and apparatus for laser ignition.

This invention was made with government support under Contract No. -7405-ENG-36

awarded by the U.S. Department of Energy. The government has certain rights in the

invention.

BACKGROUND ART

Laser light has been used to initiate the ignition of fuel/oxidizer mixtures by use of

laser-spark, air-breakdown ignition methods in which a single, high peak power laser light

pulse from a Q-switched laser is used to initiate fuel ignition by generating high temperatures

and an ionization plasma. These laser ignition methods and apparatuses are generally

unreliable except within narrow ranges of fuel parameters such as fuel/oxidizer ratios, fuel

droplet size, number density and velocity within a fuel aerosol, and initial fuel and air

temperatures.

After initial ignition, sustaining ignition of fuel/oxidizer mixtures is typically

accomplished by use of a laser light pulse from a Q-switched laser with a pulse width and pulse energy which will provide the peak power density required to initiate plasma formation

and to satisfy concurrently the need for time-averaged power for sustaining the ignition. This

requires fragile, bulky laser excitation sources such as flashlamps or laser diodes which are

often difficult to fit proximate to fuel combustion zones, particularly in fuel combustion zones

in places such as aircraft engines. Fuel combustion zones are usually harsh, mechanically

adverse environments that necessitate sturdy design or frequent replacement of ignition

devices subjected to those environments.

Thus there is still a need for a laser ignition process which can reliably ignite gaseous

or aerosol fuel mixtures within a broad range of parameters such as fuel/oxidizer ratios, fuel

droplet size, number density and velocity within a fuel aerosol, and initial fuel temperatures as

well as a need for means for ignition within small spaces under mechanically adverse

conditions.

Therefore, it is an object of this invention to provide a reliable ignition method and

apparatus.

It is another object of this invention to provide a method and apparatus for laser

ignition of gaseous or aerosol fuel mixtures wi in a broad range of parameters such as

fuel/oxidizer ratios, fuel droplet size, number density and velocity within a fuel aerosol, and

initial fuel temperatures.

It is yet another object of this invention to provide an economical method and

apparatus for laser ignition of gaseous or aerosol fuel mixtures. It is a further object of this invention to provide a method and apparatus for laser

ignition of gaseous or aerosol fuel mixtures within small spaces under mechanically adverse

conditions.

It is yet a further object of this invention to provide a method and apparatus for

elimination of fragile and bulky laser excitation sources such as flashlamps or laser diodes

from fuel igniting lasers located proximately to fuel combustion zones.

Additional objects, advantages and novel features of the invention will be set forth in

part in the description which follows, and in part will become apparent to those skilled in the

art upon examination of the following or may be learned by practice of the invention. The

objects and advantages of the invention may be realized and attained by means of the

instrumentalities and combinations particularly pointed out in the appended claims which are

intended to cover all changes and modifications within the spirit and scope thereof.

DISCLOSURE OF INVENTION

To achieve the foregoing and other objects, and in accordance with the purposes of the

present invention, as embodied and broadly described herein, there has been invented a fuel

ignition apparatus and method in which an excitation light source is used to activate one or

more small ignitor lasers located remotely from the excitation light source and more

proximately to one or more fuel combustion zones. Using a separate excitation light source to

pump the lasers which ignite the fuel eliminates the need for large, heavy, fragile or complex

excitation sources in the harsh operating environments of fuel combustion chambers. Durable, reliable, economical ignition of gaseous or aerosol fuel mixtures can be

accomplished with the invention apparatus and method.

In a first embodiment of the invention, a long duration low peak power pulse from an

excitation light source is split and injected into at least two optical fibers. The first optical

fiber transports the light into an ignitor laser. The second optical fiber is longer than the first

optical fiber and transports a long duration low peak power pulse of the light into a beam

combiner where, after the delay caused by the greater length of the second optical fiber, the

long duration low peak power pulse is combined with the output of the ignitor laser. The

output of the ignitor laser is a short duration high peak power pulse because the ignitor laser

functions as a beam compressor. The combined beam of short duration high peak power

pulses from the ignitor laser and long duration low peak power pulses from the fiber optic

delay line is then focused into a focal point in an aerosol spray or cloud of combustible fuel.

In a second embodiment of the invention, a long duration low peak power pulse from a

remote excitation light source is split and injected into at least two optical fibers, one of which

is longer than the other. The first optical fiber and the second optical fiber both transport light

into a beam combiner, with the portion of the light carried by the longer optical fiber being

delayed in reaching the beam combiner by the greater distance it travels. A third optical fiber

transports the combined beam into an ignitor laser which is activated by the combined beam. Because the ignitor laser functions as a laser pulse length compressor, the first output of the ignitor laser is a short duration high peak power pulse. Before the Q-switch on the ignitor

laser has time to reset, a long duration low peak power pulse from the fiber optic delay line

comes through the ignitor laser. The sequenced operation of the ignitor laser provides ignitor

laser output of an alternating sequence of short duration high peak power pulses and long

duration low peak power pulses. Output of the ignitor laser is focused into a focal point in an

aerosol spray or cloud of combustible fuel.

In a third embodiment of the invention, light beams from a remote excitation light

source are transported by an optical fiber to at least one ignitor laser which is positioned to

direct a beam into a focal point in an aerosol spray or cloud of a combustible fuel. The

excitation light source is used to provide a sequence of two low peak power pulses separated

by a short time interval. After the ignitor laser is pumped by the first of the pulses from the

excitation light source, the ignitor laser outputs a short duration high peak power pulse which

breaks down and ignites the fuel. Before the ignitor laser Q-switch is reset, the second of the

long duration low peak power pulses from the excitation light source re-energizes the ignitor

laser to produce a long duration low peak power pulse in the output of the ignitor laser. The

long duration low peak power pulse is also focused into the focal point in the aerosol spray or

cloud of the combustible fuel to sustain the plasma formed by contact of the short duration

high peak power pulse with the fuel.

In a fourth embodiment of the invention, excitation light is provided by a laser with at

least two resonator cavities with a common high reflecting end mirror and separate output couplers. The sequenced pulses of light from the single excitation laser are transported by

two optical fibers, one directly from the excitation laser to at least one ignitor laser and

another directly from the excitation light source to a beam combiner where it is combined

with the output from the ignitor laser. The combined beam of the sequenced pulses is focused

into a focal point in an aerosol spray or cloud of a combustible fuel where the first short

duration high peak power pulse from the ignitor laser breaks down and ignites the fuel and the

second long duration low peak power pulse directly from the excitation laser sustains the

plasma formed by contact of the short duration high peak power pulse with the fuel.

In a fifth embodiment of the invention, a light source capable of producing more than

one wavelength of light is used as the excitation light source for activating a remote ignitor

laser and sustaining a breakdown plasma in fuel. Two different wavelengths of light, the

second of which is not within the absorption band of either the laser rod or Q-switch of the

ignitor laser, are sequentially injected into a single optical fiber and transported to the remote

ignitor laser. The pulse of light with the wavelength which is absorbable by the laser rod is

compressed by the Q-switched ignitor laser to produce an ignitor laser output of a short

duration high peak power pulse that is focused into a focal point in an aerosol spray or cloud

of combustible fuel, thereby forming a breakdown plasma. The subsequent pulse of light

having the wavelength which is not absorbable by the laser rod or Q-switch passes unimpeded

through the ignitor laser as a long duration low peak power beam is also focused into the focal point in the aerosol spray or cloud of the combustible fuel to sustain the plasma formed by

contact of the short duration high peak power pulse with the fuel.

Any of the invention apparatuses can have an optical switching feature positioned to receive output of a single excitation light source and to direct beams in an ordered sequence or

in a random sequence through beam splitters into pairs of first and second optical fibers, each

pair of which is associated with an ignitor laser. When it is desired not to use a fiber optic

delay line or a second optical fiber direct to the fuel combustion zone, as is the case of the

third and fourth embodiments, the multiplexing feature is positioned to receive output of a

single excitation light source and to direct beams in an ordered or random sequence into single

optical fibers which are each connected to an ignitor laser.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of the

specification, illustrate five preferred embodiments of the present invention and, together with

the description, serve to explain the principles of the invention. In the drawings: Figure 1 is a schematic diagram of a first embodiment of the invention employing

a remote excitation laser to pump a single ignitor laser and to provide light which is delayed

and then combined with the output of the ignitor laser for application of dual pulses to a focal point in a combustion zone. Figure 2 is a schematic diagram of a second embodiment of the invention employing a single remote excitation laser to pump an ignitor laser with light pulses which are split into a

portion that goes directly to a beam combiner and a portion that is delayed before going into

the beam combiner, with the output of the beam combiner being directed into the ignitor laser.

Figure 3 is a schematic diagram of a third embodiment of the invention which operates

without use of an optical delay line or splitting of the beam from the remote excitation light

source.

Figure 4 is a schematic diagram of a fourth embodiment of the invention employing a

remote excitation laser to provide at least two separate beams of light which are separately

transported away from the excitation laser, one to an ignitor laser and another separate beam

to a beam combiner where it is combined with the output from the ignitor laser.

Figure 5 is a schematic diagram of a fifth embodiment of the invention which

employs a plurality of wavelengths of excitation light and operates without use of an optical

delay line or splitting of the beam from the remote excitation light source.

Figure 6 is a schematic diagram of a multiplexed laser ignition system in accordance

with the invention.

Figures 7a and 7b are schematic diagrams of two steps of the sequential operation of

an optical switching system such as one which can be used in the embodiment of the

invention shown in Figure 6. BEST MODES FOR CARRYING OUT THE INVENTION

The invention ignition method utilizes a combination of short and long duration light

pulse lengths from a single excitation light source and one or more lasers, or two or more

lasers in series, to provide fuel ignition performance which is superior to conventional laser-

based methods with respect to reliability, laser energy efficiency and insensitivity to fuel/air

composition and fuel temperatures.

Dual pulse ignition such as that described in U. S. Patent 5,756,924 can be

accomplished using the apparatus and methods of this invention. Dual pulse ignition requires

application of a short duration high peak power laser pulse to an aerosol spray or cloud of fuel

to generate an air-breakdown plasma within the fuel, followed by application of a second, lower peak power, longer duration laser pulse to sustain the plasma and achieve efficient fuel

ignition. Unique laser pulse temporal formats and sequencing are necessary.

In each embodiment of the present invention, a single excitation light source is used to

provide low peak power long duration pulses of excitation light to one or more small ignitor

lasers located more proximately than the excitation light source to one or more fuel

combustion zones. By serving as a pulse compressor, the small ignitor laser or lasers provide

high peak power short duration laser light pulses for the air-breakdown plasma formation in

the fuel aerosol spray or cloud. Long duration low peak power pulses from the excitation

light source sustain the air-breakdown plasmas for efficient and stable ignition. Thus, the light from the excitation light source serves a dual role: (a) providing light

for excitation of the ignitor laser; and (b) providing light for energizing and sustaining the

plasma produced in the fuel by light from the ignitor laser.

The excitation light source generally is a laser, but may also be a light emitting diode

or a flashlamp. Any of a variety of laser systems may be used as an excitation laser light

source. For example, excitation laser light may be generated by a Q-switched, cavity dumped

or free running laser. For the fifth embodiment of the invention, an excitation light source

which can produce more than one wavelength of light is needed.

Q-switched pulses can be obtained from the excitation laser by either active or passive

Q-switching of the laser. Excitation lasers which are active or passive Q-switched can be used

for all four embodiments of the invention. Generally presently preferred, particularly for the

third, fourth and fifth embodiments of the invention, are active Q-switched solid-state laser

systems to reduce timing jitter.

A wavelength tunable, Q-switched Cr:LiSAF laser that can be tuned over a

wavelength range from about 800 to about 1000 nanometers is presently preferred. The

output of such a laser can be tuned to a wavelength of 808 nanometers, which is a wavelength

that can efficiently pump a Nd: YAG ignitor laser rod and many other Nd-doped host laser

materials that would be used in a Nd: YAG ignitor laser.

Other good choices for the excitation laser are: an alexandrite laser operating at 750

nanometers, which can also be used to pump a Nd: YAG ignitor laser; or a Ti:sapphire laser.

Other tunable lasers or light emitting diodes or flashlamps can be employed as excitation light sources in the practice of the invention, depending upon specific ignition conditions and the type of ignitor laser employed.

The excitation laser may be energized by flashlamps or diodes. The excitation laser

may be cooled by any suitable means such as water cooling, air cooling, or thermo-electric

cooling.

For all embodiments of the invention, the excitation laser can be operated in

continuous or pulsed mode. Pulsed mode is generally preferred for economics of energy.

A summary of the operating parameters for the excitation laser in each embodiment of the

invention are shown in Tables 1, 2, 3, 4 and 5.

Any wavelength in the range from the ultraviolet to the infrared portion of the light

spectrum could be useful for the longer duration lower peak power pulse from the excitation

light source, depending upon the choice of laser material for the ignitor laser, the type of fuel, and the combustion conditons. The excitation light source must be operated at a wavelength

within the range which is absorbed by the laser material utilized as the active lasing medium

of the ignitor laser. Absorption of the excitation light by the ignitor laser material is required

for energizing the laser material of the ignitor laser. Generally, for the portion of the light

from the excitation light source used to sustain the plasma, longer wavelengths up to about 12

microns of light are preferred.

The light pulse from the excitation light source must be of sufficient peak power to

efficiently pump the plasma generated by the ignitor laser, but need be of no greater peak power. A peak power of greater than 70 kW is generally sufficient, depending upon the type

of fuel used and combustion environment conditions.

Generally presently preferred excitation light pulse energy is in the range from about

50 to about 300 mJ, depending upon fuel conditions and the type of ignition to be performed.

The duration or temporal length (pulse width) of the ignitor laser light pulse

appropriate for the plasma generation function generally may be anywhere from 10 or fewer

nanoseconds to about 200 nanoseconds in duration, depending upon the light pulse energy

available. The temporal length of the long duration lower peak power laser pulse from the

excitation laser light source is preferably at least as large or larger than that of the short

duration higher peak power laser pulse from the ignitor laser. Presently preferred are

excitation laser light pulse lengths in the range between about 50 and about 1 microsecond,

depending upon the laser pulse energy.

In the first two embodiments of the invention, the two portions into which the beam

from the excitation light source is split are generally of the same intensity although portions of

the beam with differing intensities can be used.

In the fifth embodiment, the two different wavelengths of light from the excitation light

source can be of the same or different intensities. In the third and fifth embodiments of the

invention, the beam or beams from the excitation light source are not split, but are delivered

through a single optical fiber to the ignitor laser in the selected sequenced pulse format.

Presently preferred optical fibers are multiple-mode optical fibers, but multiple fiber

bundles can also be used for transporting the laser light pulses. Single mode optical fibers are generally not as useful because the smaller fibers cannot carry the high peak power needed. A

multiple-mode fiber with a core diameter in the range from about 50 microns to about 1

millimeter generally can accommodate transport of the typical peak power without damage to

the optical fiber. Multiple-mode fibers with core diameters of about 400 microns are

presently preferred because these are large enough to carry the high peak power needed

without damage to the optical fiber.

A taper at the input end of the optical fiber can be used to enhance efficiency of light

injection and inhibit fiber damage.

Any suitable device can be used for combining the beams, including, but not limited

to, beam combiners, reflective devices and optical fiber couplers.

The ignitor laser or lasers for any of the embodiments of this invention can be and

generally is a modified laser having a laser rod, optical resonator cavity, and a Q-switch, but

little else.

The ignitor laser needs no pumping diodes or flashlamps because it is pumped by light

coming through at least one fiber optic line from the excitation light source. Thus the ignitor

laser can be a small, durable unit which can withstand a harsh fuel ignition environment.

Passive Q-switched ignitor lasers are presently preferred because of the high voltage

needed for active Q-switching. High voltage in the proximity of a fuel combustion zone is

undesirable because of maintenance and safety problems associated with high voltage in the

fuel combustion zone. The ignitor laser rod (lasing medium) can be made of any Q-switchable, solid-state

laser material which will provide light output within the desired wavelength and peak power

range. Although many types of solid-state lasing material can be employed for the active

lasing medium, neodymium doped yttrium aluminum garnet (Nd: YAG) is presently preferred

because of its economy and good thermal properties. Neodymium doped yttrium aluminum

garnet lases at a wavelength of 1064 nanometers. Other useful ignitor laser rod materials

include, but are not limited to, Cr:Nd:GSGG, Nd:glass or neodymium doped yttrium lithium fluoride (Nd:YLF).

The resonant cavity of the ignitor laser can be of either stable or unstable configuration

and is bounded by light reflective coatings placed upon optical surfaces. The coated optical

surfaces may be curved or flat, depending upon cavity configuration (stable or unstable). A

first coating can be upon the surface of the collimating lens facing the input end of the ignitor

lasing rod or upon the input end of the ignitor lasing rod. The first coating must be of a

dichroic nature, that is, highly reflective for lasing wavelengths and highly transmissive for

the excitation wavelengths. A second coating is generally placed upon the output end of the

Q-switch to form an output coupling reflector for the selected wavelength of light.

The size of the ignitor laser rod can vary greatly, depending upon the application.

The diameter of the Q-switch is preferably matched to the rod diameter with thickness

of the Q-switch depending upon the dopant level needed for the selected optical density. All components of the ignitor laser can be mounted in a cylindrical tube to become a

virtually monolithic unit. Metal or ceramic material is generally considered most suitable for

the igmtor laser encasement, although other materials can be used with good results.

For aircraft applications ignitor laser rods with a length in the range from 0.5 to no

more than 20 centimeters and with a rod diameter from about 1 mm to 10 or more millimeters are generally preferred, depending upon the absorptive properties of the rod to the excitation

light and the maximum pulse energy desired. Presently preferred dimensions of an ignitor

laser to be used for aircraft engine ignition are in the range from 1 to 12 centimeters in length

and from 1 to 6 millimeters in diameter. These smaller ignitor lasers are preferred because

larger sizes would defeat the purposes of having a small, economical, easily positioned ignitor laser which directly replaces the currently used capacitive discharge spark ignitor.

The ignitor laser can be cooled by any suitable means such as air cooling, circulated water cooling, thermal-electric cooling, or use of phase transition material. Air cooling is

presently preferred because of the simplicity, economy and ignitor dimensions.

For each of the embodiments of the invention it is generally presently preferred that

the ignitor laser be operated in pulsed mode.

The ignitor laser is used to produce laser light having temporal lengths or pulse widths

in the range from about 1 to about 60 nanoseconds, with pulse energies in the range from

about 10 mJ to about 250 mJ being presently preferred, depending upon type of fuel used and

combustion temperatures. Which wavelengths from the igmtor laser are most effective depends upon the fuel

breakdown processes. Resonant excitation and ionization of the fuel/oxidizer components are

typically obtained by utilizing short light wavelengths which are preferred due to the greater

efficiency in the ionization yield. For non-resonant ionization of fuel/oxidizer components,

wavelengths from as short as 200 nanometers to as long as 12 microns can be effectively used

depending upon choice of fuel. Generally presently preferred for most common fuels are

wavelengths in the infrared range from about 700 nanometers to about 3 microns.

Operating parameters for the ignitor laser or lasers in each embodiment of the

invention are shown in Tables 1, 2, 3, 4 and 5.

A short focal length lens is positioned to focus the output of the ignitor laser into the

fuel region to be ignited. Short focal lengths are generally preferred because the longer the

focal length, the larger the spot size, which, for efficient operation of the invention, is limited

by the minimum power density required to break down the fuel at the focal point. The focal

length of this lens can be anywhere in the range from about 1 cm to about 100 cm, although

for aircraft engine ignition applications, a focal length in the range from about 5 to about 30

cm is presently preferred.

If desired, a laser window can be used to protect the ignitor laser focal lens from

combustion chamber products.

In the first embodiment of the invention, a single excitation light source is used to

pump at least one ignitor laser located more proximately to a fuel combustion zone than the

excitation light source and to sustain plasmas formed in the fuel combustion zone by application of output from the igmtor laser. An example of this embodiment of the invention

is shown in Figure 1 and detailed in Example I.

In this first embodiment of the invention, a long duration low peak power pulse of

light from the excitation light source is split into two portions, with the first portion being

injected into an optical fiber for transportion to the ignitor laser. The two portions into which

the light beam from the excitation light source is split can be equal or unequal. When the first

portion of the long duration low peak power pulse of light from the excitation light source

reaches the ignitor laser, the lasing material is pumped, the Q-switch is activated, and the

ignitor laser outputs a short duration high peak power pulse of light, typically from about 10

to about 20 microseconds. The short duration high peak power pulse from the ignitor laser is

directed into a beam combiner.

The second portion of the beam from the excitation light source laser is directed

through a fiber optic delay line long enough to delay the beam, generally from about 50 to 100

nanoseconds, then through a short focal length lens into the beam combiner where it is

combined with the short duration high peak power pulse from the ignitor laser.

Other methods for introducing a temporal delay between these two pulses, including

the use of reflective or diffractive multiple pass delay lines, can effectively serve the same

role.

Any suitable device can be used for combining the beams including, but not limited to,

beam combiners, reflective devices and optical fiber couplers. The combined beams are then directed through a common focusing lens into a focal

point in an aerosol spray or cloud of the fuel to be ignited. A common lens having a short

focal length is generally preferred because of the high laser light power density achieved,

although multiple focusing elements can be used effectively. For example, a 10 cm focal

length lens is found to be useful for generation of an air breakdown spark by efficient

contacting of the breakdown plasma with the short duration high peak power pulse from the

ignitor laser as well as for sustaining of the plasma by reliable contact of the plasma with the

second pulse light from the second portion of the beam from the excitation light source.

Pulse widths or temporal durations, wavelengths, peak powers and pulse energies

which are generally most useful in operating the excitation light source and ignitor laser or

lasers of the first embodiment of the invention are set forth in Table 1.

TABLE 1

FIRST EMBODIMENT

Excitation Laser

Pulse Sequence: single output pulse split into two beams

Ignitor Laser

Pulse Sequence: single output pulse

In the second embodiment of the invention, a single excitation light source providing a

long duration low peak power pulse is used to pump at least one ignitor laser with split

portions of the pulse arriving at the ignitor laser at sequential intervals. An example of this

embodiment of the invention is shown in Figure 2 and detailed in Example II.

Getting the light to arrive at the ignitor laser in sequential intervals is accomplished by

having a beam of light from the excitation light source split into two portions, with the first

portion of the beam being transported tlirough an optical fiber into a beam combiner. The two portions into which the light beam from the excitation light source is split can be equal or

unequal.

The second portion of the beam from the excitation light source is transported to the

beam combiner by an optical fiber which is longer than the optical fiber carrying the first

portion of the beam and which serves as a fiber optic delay. Other means of delaying the second portion of the beam from the excitation light source such as reflective or diffractive

multiple pass delay lines can be used.

After transportation through the fiber optic delay line, the second portion of the beam

from the excitation light source is combined with the first portion of the beam from the

excitation light source before the combined beams are directed into the ignitor laser.

to, beam combiners, reflective devices and optical fiber couplers. The combined light beam from the beam combiner having a pulse of long duration low

peak power light followed by a second pulse (the second portion of the split beam) of long

duration low peak power light is focused into an ignitor laser located more proximately to a

fuel combustion zone than the excitation light source.

In this second embodiment of the invention, the time interval between the pulses of

light from the excitation light source is created by splitting the beam, delaying a portion of the beam, and then recombining the beam so that pulses of it reach the ignitor laser at intervals of

time that are controlled by choice of length of the optic delay line.

As the first portion of the beam from the excitation light source reaches the ignitor

laser, the ignitor laser is activated and outputs a short duration high peak power laser light

pulse. The short duration high peak power laser light pulse from the ignitor laser is focused

into a focal point in an aerosol spray or cloud of combustible fuel, thereby causing breakdown

and ionization of the combustible fuel to form a plasma.

Then, before the bleached Q-switch on the ignitor laser has had time to reset (typically

about 1 microsecond), the second portion of the long duration low peak power beam from the

excitation laser by way of the beam combiner reaches the ignitor laser and re-energizes the

ignitor laser. Because the ignitor laser Q-switch is bleached and has not had time to recover,

the ignitor laser simply outputs the same low power long duration pulse received from the

excitation light source. This long duration low peak power pulse output from the ignitor laser

is also focused into the same focal point in the aerosol spray or cloud of combustible fuel where it sustains and pumps the breakdown plasma which was created by the short duration

high peak power pulse from the ignitor laser.

In other words, in the second embodiment of the invention, the two portions of the

excitation laser pulses are used to sequentially Q-switch and gain switch the ignitor laser.

Sequentially Q-switching and gain switching the ignitor laser produces the sequential high

and low peak power pulses in the output of the ignitor laser. This sequence of alternating

pulses is reiterated continuously during the entire time fuel combustion is desired.

Generally useful choices of time intervals between the pulses, pulse widths or

temporal durations, pulse energies, wavelengths, and peak powers for this second embodiment

of the invention are set out in Table 2.

TABLE 2

SECOND EMBODIMENT Excitation Laser

Pulse Sequence: single output pulse split into two beams

Ignitor Laser

Pulse Sequence: alternating first and second pulses

In the third embodiment of the invention, a single excitation light source is used to

pump at least one ignitor laser without use of a beam splitter, an optical delay line, or beam

combiner. The excitation laser produces two laser pulses in its output. An optical fiber

transports pulses of light from the excitation light source to a lens which focuses the light into

an ignitor laser located more proximately to a fuel combustion zone than the excitation light

source. Output from the ignitor laser is focused into a focal point in the aerosol spray or cloud

of fuel. An example of this third embodiment of the invention is shown in Figure 3 and

detailed in Example III.

In this third embodiment of the invention, active modulation of the excitation light

source is used to provide a time interval between the pulses of light from the excitation light

source. A first long duration low peak power pulse from the excitation light source is injected

into the optical fiber which transports the beam to the ignitor laser.

The first pulse of the excitation light source beam directed into the laser rod of the

ignitor laser provides excitation of the ignitor laser. The ignitor laser subsequently outputs a

short duration high peak power laser light pulse which breaks down and ionizes the

combustible fuel in the focal point to form a plasma.

Then, before the bleached Q-switch on the ignitor laser is reset, a second long duration

low peak power pulse from the excitation light source comes into the ignitor laser, causing a

re-excitation of the ignitor laser rod that results in output of a long duration low peak power

laser light pulse from the ignitor laser. Because the ignitor laser Q-switch is bleached and has

not had time to recover, the ignitor laser simply outputs the same low peak power long duration pulse received from the excitation laser. This long duration low peak power pulse is

used to directly pump and sustain the breakdown plasma produced by the high peak power,

short duration output of the ignitor laser.

In other words, in the third embodiment of the invention, the two excitation laser

pulses are used to sequentially Q-switch and gain switch the ignitor laser. Sequentially Q-

switching and gain switching the ignitor laser produces sequential high and low peak power pulses in the output of the ignitor laser.

This sequence of alternating pulses is reiterated continuously during the entire time fuel combustion is desired.

In the third embodiment of the invention, the temporal duration or pulse width of the

light from the excitation light source can be longer than the pulse widths usually used in the

first and second embodiments of the invention because the time delay between pulses is

adjusted by active modulation of the excitation laser rather than being set by delay line length.

The presently preferred ranges of time intervals between pulses, pulse widths,

wavelengths, pulse energies and peak powers for the third embodiment of the invention are set

forth in Table 3. Long duration low peak power pulses from the excitation light source are

separated by a time interval in the range from about less than 1 nanosecond to about 1

microsecond. Generally presently preferred are time intervals in the range from about 50 to about 200 nanoseconds. The long duration low peak power pulses from the excitation light

source can be of equal duration and power or they can be different. One particular fuel ignition process for which the third embodiment of the present invention is particularly suitable is achieved by application of two identical, low peak power

laser pulses from an excitation laser to the lasing material of an ignitor laser. The first of the

two excitation laser pulses pumps the lasing material of the ignitor laser to an inversion level

which induces the formation of a short duration (typically 10 to 15 nanoseconds), high peak

power pulse at the output of the ignitor laser, through a passive Q-switching process. Before

the passive Q-switch can recover to a high loss state (typically 1 microsecond), the application

of the second excitation pulse re-energizes the lasing medium of the ignitor laser, thereby causing the ignitor laser to lase again. This time gain-switched pulse produced by the ignitor

laser has a pulse width approximating that of the excitation pulse (typically about 150 nanoseconds). In this manner the required dual laser pulse format for efficient fuel ignition is

obtained. Energy provided by each ignitor laser pulse is typically about 50 to 100 mJ.

Alternatively, the excitation laser in the third embodiment of the invention can provide

two very different pulse widths. The first pulse which pumps the ignitor laser may be of very

long duration (up to about 1 microsecond). After the ignitor laswer has been sufficiently

energized to cause the ignitor laser to Q-switch, the second excitation laser pulse with a pulse width of only about 10 to 200 nanoseconds is produced to sustain the fuel plasma generated

by the output of the ignitor laser. The interval between the Q-switched pulse of the ignitor

laser and the second pulse from the excitation laser is typically from about 25 to about 200

nanoseconds in this alternative. TABLE 3

THIRD EMBODIMENT

Excitation Laser

Pulse Sequence: multiple output pulses

Igmtor Laser

Pulse Sequence: altemating short and long pulses

In the fourth embodiment of the invention, two sequential laser pulses are produced by

the excitation laser and transported to the ignitor laser using two optical fibers. A single

excitation light source, more specifically, a laser with a plurality of resonator cavities and

separate output couplers, is used to pump at least one ignitor laser located more proximately

to a fuel combustion zone than the excitation laser and to sustain plasmas formed in the fuel

combustion zone by application of output from the ignitor laser. An example of this

embodiment of the invention is shown in Figure 4 and detailed in Example IV.

In this fourth embodiment of the invention, light in the excitation laser produces an

output of a plurality of pulses by developing light beams from each of the resonator cavities.

Switching of the light path from cavity to cavity is achieved by selective reorientation of the

polarization of the light in a Pockels cell or other light modulator and direction or redirection of the light with a polarization analyzer.

A common high reflecting end mirror can be used for two resonating cavities, but

separate output couplers are associated with each of the resonating cavities.

When no voltage is applied to the Pockels cell, all intracavity light is directed to a first

output coupler and the excitation laser output is directed into a first optical fiber for

transportation to an ignitor laser. When voltage equal to the halfwave voltage (about 3500 V

for LiNO₃) is applied to the Pockels cell, the intracavity lasing light is directed to a second

output coupler and the excitation laser output is directed into a second optical fiber for

transportation to a beam combiner where it is combined with the output of the ignitor laser. If the excited-state population inversion of the excitation laser rod is sufficiently high by proper

choice of the first output coupler, the excitation laser will Q-switch when a voltage is quickly

applied to the Pockels cell. The Q-switched pulse which results is directed to and injected

into the second optical fiber.

The pulses of laser light injected into each of the separate output couplers and optical

fibers can be of equal or unequal peak power, pulse energy, or pulse width.

Light injected into the first optical fiber is transported to an ignitor laser of the sort

described in the first embodiments of the invention. The ignitor laser is pumped by the long

duration low peak power pulse of light from the first optical fiber and outputs a short duration

high peak power pulse of light which is directed into a beam combiner.

Long duration low peak power light injected into the second optical fiber is

transported to the beam combiner using focusing lenses and reflecting mirrors as needed to direct the beam into the beam combiner.

Light from the ignitor laser and light from the second output coupler of the excitation

laser reaches the beam combiner in sequential pulses because of the sequencing of the

production of the light in the excitation laser. The pulses of light from the two sources

reaching the beam combiner are spatially overlapped and co-axially propagated and directed

through a short focal length lens which focuses the alternating sequence of short and long

pulses of light into a focal point in an aerosol spray or cloud of fuel. The first short duration

high peak power pulse from the ignitor laser breaks down and ignites the fuel. The long duration low peak power pulse from the excitation laser sustains the breakdown plasma

formed by the first pulse.

The presently preferred ranges of time intervals between pulses, pulse widths,

forth in Table 4.

TABLE 4

FOURTH EMBODIMENT

Excitation Laser

Pulse Sequence: alternating first and second pulses

Ignitor Lager

Pulse Sequence: single output pulse from one or more ignitor lasers

Operating Broad Intermediate Narrow Feature Range Range Range

In the fifth embodiment of the invention, a single excitation light source that can

produce two laser pulses, each with a different wavelength of light, is used for: (a) excitation

of at least one ignitor laser; and (b) to sustain the breakdown plasma in a focal point in an

aerosol spray or cloud of combustible fuel. An example of this fifth embodiment of the

invention is shown in Figure 5, and as detailed in Example V.

In this fifth embodiment, two beams (pulses) of long duration low peak power light

from the excitation light source are produced by any suitable means. A laser with an electro-

optic Q-switch is presently preferred for producing the two beams by sequentially operating the laser in free-running and Q-switched modes. Active double Q-switched lasers are

presently preferred as excitation light sources for the fifth embodiment of the invention.

Alternatively, any other light source capable of producing alternately sequenced beams of two

different wavelengths of light can be used. Also, two separate excitation light sources can be

used.

A birefringement filter, Brewster plate, prism, or other wavelength selecting device

can be used between the polarization analyzer and reflecting mirror as needed to fine tune the

lasing wavelength.

The first pulse or beam from the excitation light source must be of a wavelength that is

capable of being absorbed by the ignitor laser rod so as to pump the ignitor laser. This first

pulse of long duration low peak power light from the excitation light source is injected into an

optical fiber which transports the beam to the ignitor laser. As with other embodiments of the invention, the multimode optical fiber may be a

tapered fiber to facilitate beam alignment or a fiber bundle may be used. A multimode fiber

with a core diameter of 200 microns or greater is presently preferred.

At the ignitor laser the first beam is directed into the laser rod of the ignitor laser to

energize the ignitor laser. The igmtor laser then outputs a short duration high peak power Q-

switched laser light pulse. Output of the short duration high peak power laser light pulse from

the ignitor laser is directed through a focusing lens such as that described for the first

embodiment into a focal point in an aerosol spray or cloud of fuel. At the focal point the short

duration high peak power laser light pulse from the ignitor laser breaks down and ionizes the

combustible fuel, forming a plasma.

The second pulse of excitation light must be of a wavelength that can be transported

through the ignitor laser without absorption by the laser rod and Q-switch of the ignitor laser.

Thus, the second pulse of excitation light passes unimpeded through the ignitor laser and is

focused through the same lens used to focus the short duration high peak power ignitor laser

output into the focal point in the aerosol spray or cloud of fuel. The long duration low peak

power second pulse sustains the breakdown plasma in the combustion zone.

Pulse widths or temporal durations, wavelengths, peak powers and pulse energies

lasers of the fifth embodiment of the invention are set forth in Table 5. TABLE 5 FIFTH EMBODIMENT Excitation Laser

Pulse Sequence: alternating first and second pulses

Ignitor Laser

Pulse Sequence: single output pulse from

In any of the embodiments described, more than one ignitor laser can be used with a

single excitation light source.

Multiple ignitor laser configurations are desirable for applications in which multiple

ignitors are required for each engine combustion chamber or where multiple combustion

chambers or engines are used. The smaller, stripped down ignitor lasers can be placed

proximate to the fuel jets for each of the cylinders of an aircraft engine and a single remote excitation light source used to power the smaller ignition lasers. The smaller, low-cost

ignition lasers with no pumping elements such as flashlamps or diode lasers, and with no

electro-optic devices, can be built to tolerate the extreme temperature variations and vibrations

in the engine operating environment while the more expensive, more fragile excitation light source laser is safely positioned in the fuselage of the aircraft.

The capability of multiplexing an excitation light from the excitation light source with

several different ignitor lasers reduces the cost that would be associated with having to have

two excitation light sources at each of the combustion chambers of an aircraft and the cost that

would be associated with maintenance of fragile, temperature sensitive and expensive excitation light sources, such as flashlamps or diode lasers in the extreme environments of

combustion chambers.

To achieve this multiplexing function, an optical switching system is used to

sequentially direct excitation light from the single excitation light source into multiple pairs of

optical fibers, with each pair of optical fibers connected to an individual ignitor laser. Any suitable means for optical switching can be used. Electro-optically controlled

means such as that described in patent application Serial Number 60/076,301 can be

successfully employed for optical switching in the invention. Although electro-optical

switching is presently preferred, various mechanical switching systems can be employed in any of the embodiments of the invention. For example, multiplexing systems based upon a

rotatable prism can be utilized.

EXAMPLE I

In an example of the first embodiment of the invention, the lasers can be arranged as shown in the schematic diagram of Figure 1. An excitation light source laser K) having a

CπLiSAF (chromium-doped, lithium-strontium-aluminum fluoride) rod is operated at a

wavelength of 80δ nanometers to produce laser light pulses with a pulse energy of about 250

mJ. The excitation light source laser 10 is pumped by either flashlamps or light emitting

diodes. The excitation light source laser 10 is operated in a Q-switched mode to produce a

long duration (for example, about 100 nanosecond) light pulse at the output of the laser.

The Q-switched light output from the excitation light source laser JO is directed into a

beam splitter 20. No fiber optic lines are needed between the excitation source light laser 10

and the beam splitter 20.

The output of the excitation light source laser J_0 is split into at least two beams by the beam splitter 20. A first portion of the output from the excitation light source laser 10 which

is split is directed through a focusing lens 24 and injected into a first optical fiber 38. In this example, 400-micron diameter multiple-mode optical fiber is used for the first optical fiber

38. The peak power density of the laser light within the multiple-mode optical fiber 38 is more than a factor of 3 below the threshold for optical damage to the fiber.

The excitation light source laser 10 is operated at sufficiently long pulse times to

provide excitation energy for an ignitor laser 50 having an Nd: YAG rod 54. In this first

embodiment of the invention, the two long duration pulses from the excitation light source

laser 10 generally have a pulse energy of about 125 mJ and a temporal pulse length from

about 50 to about 200 nanoseconds.

The first portion of the laser light from the excitation light source laser 10 coming

through the first optical fiber 38 is collected and focused by a lens 52 into the Nd:YAG laser rod 54 of the ignitor laser 50. Laser light at the wavelength (808 nanometers) used in this

example of the invention is strongly absorbed within the neodymium-doped YAG lasing

material of the ignitor laser 50 and provides the excitation energy required for the ignitor laser

50 to operate.

A lasing condition is quickly established for the ignitor laser cavity within the 50 to

200 nanosecond duration of the light pulse that pumps the ignitor laser 50.

In this example, the mirrors for the optical resonator of the ignitor laser 50 consist of

high reflectivity dielectric coatings deposited directly upon one end of the laser rod 54 (end

facing the excitation light focusing lens 52) and one surface of the Q-switch 56 (surface facing

the beam combiner 64). The optical coating placed upon the rod end is highly transmitting of

808 nanometer excitation light although it is highly reflecting to the 1064 nanometer lasing light produced by the ignitor laser 50. The coated end of the laser rod 54 is curved to provide

a spherically reflecting cavity end mirror. The coated surface of the Q-switch 56 is optically

flat so that, in combination with the spherical mirror at the laser rod end, it forms the resonant

cavity within the ignitor laser 50.

The short duration, Q-switched laser pulse (generally having a duration of about 10 to

15 nanoseconds) generated by the ignitor laser 50 is provided by a passive, solid-state,

saturable absorber which is contained within the resonator of the ignitor laser 50.

The pulse energy of the laser light at the output of the ignitor laser 50 is calculated to

be about 50 mJ.

A second portion of the laser light from the excitation light source laser 10 which goes

into the beam splitter 20 is reflected by a reflecting or turning mirror 26 which directs the

beam through a lens 30 which focuses the beam into a fiber optic delay line 32. In this

example, 400-micron diameter multiple-mode optical fiber is also used for the fiber optic

delay line 32. The fiber optic delay line 32 is sufficiently longer than the first multiple-mode

optical fiber 38 to provide a temporal delay of a number of nanoseconds in the arrival of the

second portion of the output of the excitation light source laser 10 at the beam combiner 64

beyond the ignitor laser 50. For example, a delay of approximately 50 nanoseconds can

usually be accomplished by having the fiber optic delay line 32 about 35 feet longer than the

first multiple-mode optical fiber 38.

Laser light from the fiber optic delay line 32 is collimated using a short focal length

lens 60 and then directed by way of a reflecting mirror 62 to the beam combiner 64 where it is spatially overlapped and co-axially propagated with the light output of the ignitor laser 50,

although it is delayed by a number of nanoseconds relative to the output of the ignitor laser

50. Generally a delay of about 25 to about 150 nanoseconds is most useful, depending upon the properties of the fuel to be ignited.

Laser light from the beam combiner 64 having both long and short duration laser

pulses from the excitation light source laser 10 and the ignitor laser 50, respectively, is then

directed to a common focusing lens 66 in which both laser pulses are focused through a laser

window 68 to a preselected position or focal point 70 within the fuel spray 72 from a fuel nozzle 74 in the combustion zone.

A spark breakdown plasma in the fuel spray 72 is formed by the output of the ignitor

laser 50, followed by sustaining of the plasma with the long duration pulse from the second

portion of the beam from the excitation light source laser 10. Thus, an effective dual pulse of

laser light within the temporal format needed to achieve optimal fuel ignition performance is

provided.

EXAMPLE II

In an example of the second embodiment of the invention, the manner in which the

ignitor laser is pumped is altered to produce two sequential pulses which conform to the selected dual pulse fuel ignition format, thus eliminating the need for a beam combiner between the small ignitor laser and the combustion zone. The portions of the beam from the

excitation light source are combined before transportation to the ignitor laser.

In this second embodiment, shown in the schematic of Figure 2, the excitation light 12

from the excitation laser light source 10 is split into two beams by a beam splitter 20. A first

portion of the laser light is focused by a lens 24 and injected into a first multiple-mode optical

fiber 38. A second portion of the excitation light from the beam splitter 20 is reflected by a

mirror 26 and lens 30 into a fiber optic delay line 32. As described in Example I of the first

embodiment of the invention, the fiber optic delay line 32 is longer than the first multiple-

mode optical fiber 38 in order to provide a temporal delay in the delivery of the second

portion of the excitation light.

In this second embodiment shown in Figure 2, the light output of the two optical fibers

38 and 32 is combined using an optical fiber coupler 44 which combines both beams. The

combined beam is injected into a single fiber 46 which carries the two excitation light pulses

through a focusing lens 52 to the ignitor laser rod 54. The ignitor laser 50 is in the same

configuration as that described in Example I.

The first excitation pulse quickly establishes a lasing condition in the laser rod 54

which results in the formation of a short duration pulse (generally from about 10 to about 20

nanoseconds) in the output of the ignitor laser 50 by the action of the saturable absorber Q-

switch 56. Before the bleached Q-switch can recover or reset, the arrival of the second

excitation pulse re-establishes a lasing condition in the laser rod 54 which results in the

formation of a gain-switched light pulse in the output of the ignitor laser 50. The pulse width of the gain-switched pulse is approximately equal to that of the excitation pulse (generally

about 100 nanoseconds). In this manner, two sequential laser pulses with high and low peak

powers are generated and separated in time by the temporal delay between the excitation

pulses. Both laser pulses from the ignitor laser 50 are focused within the fuel by a common

lens 66.

EXAMPLE III

An example of the third embodiment of the invention is shown in Figure 3.

In this example of the third embodiment of the invention, the excitation laser K) is

operated to produce two sequential, low peak power pulses. A Cr:LiSAF laser as described in

Example I is used as the excitation light source 10.

The two pulses are produced by Q-switching the excitation laser 10 twice within a

time frame from about 50 nanoseconds to about 1 microsecond. The provision of the two

sequenced pulses separated by a time interval from the excitation laser 10 eliminates the need

for an optical delay line or beam splitters or beam combiners.

The temporal length of the two pulses is typically from about 50 to about 200

nanoseconds. The length of the two pulses can be the same or different. The time interval

between the two pulses is typically from about 25 to about 2000 nanoseconds.

An excitation laser peak power of less than about 200 MW/cm² was used.

The light pulses 12 from the excitation light source laser 10 are focused through a

short focal length lens 14 into a single multiple-mode optical fiber 38 with a core diameter of

about 400 microns and with a taper at the input end. The multiple-mode optical fiber

transports both excitation laser light pulses sequentially to the ignitor laser 50.

The configuration of the ignitor laser 50 is the same as that described in Example I.

The transported laser light output of the optical fiber 38 is focused through another

lens 52 into the laser rod 54 of the ignitor laser 50. Any suitable lens capable of uniformly illuminating nearly the full diameter of the input end of the laser rod 54 of the ignitor laser 50

can be used for this second lens 52. The second lens 52 may be a conventional short focal

length lens or may be a graded refractive index type lens.

The first excitation light pulse arriving in the ignitor laser 50 causes ignitor laser

output of a short duration high peak power pulse, as described in operation of the ignitor laser

in Examples I and II. The second excitation light pulse arriving in the ignitor laser causes output of a long duration low peak power pulse from the ignitor laser 50.

The alternating sequence of laser light output of the ignitor laser 50 is then focused

through a third lens 66 into the selected location in the fuel spray 72.

EXAMPLE IV

In an example of the fourth embodiment of the invention, the lasers can be arranged as

shown in the schematic diagram of Figure 4. An excitation laser 110 having a CπLiSAF rod

112, high reflection end mirror 114 which is highly reflective of 808 nm wavelength light, a

Pockels cell 116, a polarization analyzer 1T8, a reflecting mirror 120, two output couplers 122

and 124, and two output focusing lenses 126 and 128, was used to produce a dual pulse

format.

The excitation laser 110 is pumped by either flashlamps or light emitting diodes. The

excitation laser 110 is operated at a wavelength of 808 nanometers to produce laser light

pulses with a pulse energy of about 250 mJ. Typical pulse widths for the first pulse from the

excitation laser 110 is in the range from about 200 ns to about 300 microseconds.

A first excitation light pulse is produced by having all intracavity light directed to the

first output coupler 122 by not activating the Pockels cell 116 and allowing the light to pass

unimpeded through the polarization analyzer H8. The first pulse of light passing through the

polarization analyzer 118 and first output coupler 122 is focused by a first output focusing

lens 126 into a first optical fiber 130.

In this example, 400-micron diameter multiple-mode optical fiber is used for all

optical fibers.

A second excitation light pulse is produced by applying voltage equal to the halfwave

voltage (about 3500 V) to the Pockels cell 116, thereby changing the polarization of the light. The re-polarized light is rejected by the polarization analyzer 18 and redirected by a

reflecting mirror 120 through the second output coupler 124 into the second focusing lens

128. The second focusing lens 128 focuses the second light pulse into the second optical fiber

132, which transports the second light pulse to a beam combiner 138 where it is combined

with output of the ignitor laser Jj>0. In this example an additional short focal length lens 134

and reflecting mirror 136 are used to collimate and direct the second light pulse to the beam

combiner.

The pulse width of the second excitation laser pulse is typically from about 50 to about

200 ns.

A Nd: YAG laser 150 , having an input focusing lens 140, neodymium-doped YAG

laser rod 142 and passive Q-switch 144 as described in the first embodiment of the invention

in Example I is used as the ignitor laser.

The first pulse of light from the excitation laser U0 is transported through the second

fiber optic 130 into the ignitor laser 150, causing output of a short duration high peak power

pulse. Generally the ignitor laser output had a duration about 10 to 30 nanoseconds and a

calculated pulse energy of about 50 mJ.

The combined output of the ignitor laser 150 and the second pulse of light from the

excitation laser 110 are directed into a short focal length lens 146 and focused through a laser

window 148 into a focal point 152 within the fuel spray 154 from a fuel nozzle 156 in the

combustion zone. A spark breakdown plasma in the fuel spray 154 is formed by the output of the ignitor

laser 110 followed by sustaining of the plasma with the long duration second pulse from the

excitation laser 110. Thus, an effective dual pulse of laser light within the temporal format

needed to achieve optimal fuel ignition performance is provided.

EXAMPLE V

An example of the fifth embodiment of the invention is shown in the schematic

diagram in Figure 5. In this example of the invention, the excitation laser is operated to produce two sequential low peak power pulses having two different wavelengths.

Wavelengths of 808 nanometers and 850 nanometers were selected because the

Nd:YAG rod of the ignitor laser will absorb the 808 nm wavelength and will not absorb the

850 nm wavelength light.

In the excitation laser 210 for this embodiment, an output coupler 212 is positioned

between the output focusing lens 214 and the laser rod 216. A Pockels cell 218 is positioned

to receive light from the laser rod 216 and direct the light from the laser rod 216 into a

polarization analyzer 220.

The excitation laser 210 contains two end mirrors 222, 224 that are dielectric coated so

as to select the particular wavelength at which the excitation laser will operate. End mirror 222 is coated for high reflectivity at 808 nanometers; end mirror 224 is coated for high

reflectivity at 850 nanometers. The mirror reflecting light employed at any given time is determined by the voltage

applied to the Pockels cell 218. When no voltage is applied to the Pockels cell 218, the

polarization of the light is unchanged and all the laser light is directed to end mirror 222

through the polarization analyzer 220 to laser rod 216. Output of laser rod 116 goes to an

output coupler 212, thusly producing laser output with a wavelength of 808 nm. The output

coupler 212 is a broad-band reflector with a reflectivity in the range from about 30% to about

70%.

A brewster plate 226, birefringement filter, or other wavelength tuning element can be

positioned between the polarization analyzer 220 and the reflecting end mirror 222 to fine

tune the lasing wavelength.

Typically, the excitation laser 210 outputs a first pulse with a duration of about 50

nanoseconds to about 300 microseconds at the selected 808 nm wavelength. Typical peak

power for the first excitation laser pulse is from about 1 kW to about 1 MW.

Once the ignitor laser 230 has been fully energized by the first light pulse and the

ignitor laser 230 has Q-switched, voltage is quickly applied to the Pockels cell 218. The

magnitude of this voltage generally is equal to the voltage of the Pockels cell 218, i.e., a halfwave voltage of about 3,500 V. When the halfwave voltage is applied to the Pockels cell

218, the polarization of the light is changed by 90°. The polarization analyzer 220 will direct

all the light of the second pulse to end mirror 224 and a Q-switched pulse with a wavelength of 850 nm and a duration in the range from about 50 to about 200 ns is produced by the excitation laser. Typical peak power for the second excitation laser pulse is from about 100

kW to about lO MW.

The light pulses from the excitation light source laser 210 are focused through a short

focal length lens 214 into a single multiple-mode optical fiber 228 a core diameter of about

400 microns and with a taper at the input end. The multiple-mode optical fiber 228 transports both excitation laser light pulses sequentially to the ignitor laser 230.

The configuration of the ignitor laser 230 is the same as that described in Example I.

Light from the optical fiber 228 is focused into the ignitor laser 230 through a lens 232 into

the ignitor laser rod 234 and thence to the passive Q-switch 236 of the ignitor laser 230. The first 808 nm wavelength pulse from the excitation light laser 210 is absorbed by

the ignitor laser rod 234 and energizes the ignitor laser 230, thereby producing a short

duration high peak power pulse of laser light. The short duration high peak power pulse from the ignitor laser 230 is focused through a focusing lens 238, then through a laser window 240

to a preselected position or focal point 242 within the fuel spray 244 from a fuel nozzle 246 in

the combustion zone. A breakdown plasma is produced at the focal point 242 by the first 808

nm wavelength pulse of light.

The interval between the end of the first pulse and the start of the second pulse is

typically from about 50 to about 200 ns.

After the selected interval between the two pulses, the second 850 nm wavelength pulse from the excitation light laser 210 is similarly transported to the ignitor laser 230.

However, since the 850 nm wavelength of the second pulse of light is not within the absorption band of either the laser rod 234 or Q-switch 236, the collimated light of the second

pulse passes unimpeded tlirough each of the ignitor laser components. The lens 238 focuses

the 850 nm wavelength light which has passed through the ignitor laser 230 and through the laser window 240 to the focal point 242 within the fuel spray 244 from the fuel nozzle 246 in

the combustion zone. This second long duration low peak power pulse sustains the ignition of

the fuel/air plasma induced earlier by the short duration high peak power pulse from the

ignitor laser 230.

EXAMPLE VI

A schematic of a mechanically multiplexed arrangement of the ignition lasers in a

sixth embodiment of the invention is shown in Figure 6. As depicted in Figure 6, a single

excitation light source laser 10 is used to provide low peak power long duration light pulse

energy for several ignitor lasers 48a, 48b, . . .. The excitation laser light 12 is directed into a

beam multiplier 16 or other means for optical switching of the excitation laser light beam 12

in random or ordered sequence from one pair of optical fibers 34a and 40a to another 34b and

40b, and to as many other pairs of optical fibers as are used.

The laser light is transported by each of the pairs of optical fibers to an ignitor laser

48a, 48b, . . . and a beam combiner 64a, 64b, . . . and thence through an additional lens 66a,

66b, . . . into the focal point 70a, 70b, . . . in a fuel spray 72a, 72b, . . . as would be used with

the first embodiment of the invention and shown in Figure 6. Alternatively, the laser light

transported by each pair of optical fibers is directed into a beam combiner before injection

into the ignitor laser associated with each of the pairs of optical fibers, as would be used with

the second embodiment of the invention shown in Figure 2. '

EXAMPLE VII

A presently preferred simple and economical optical switch which can be used in the

multiplexed embodiment of the invention described in Example VI is based upon a rotatable

90-degree prism 18 as shown in Figures 7a and 7b.

With reference to Figure 7a, excitation light 12 is directed into the lower (bottom) face

of the prism 18 where the excitation light is reflected through a 90° angle. Excitation light

exiting the prism 18 is then directed to a beam splitter 20a where the excitation laser light is

split into two equal intensity beams. The two beams are then focused through lenses 24a and

30a, and injected into two optic fibers 36a and 42a, one of which transports laser light directly

to the ignitor laser and the second of which is an optical delay line. The optical fibers 36a and

42a transport the excitation light to a first single ignitor laser in a manner substantially

identical to that described for either the first or second embodiment of the invention, each of

which has a first multiple-mode op'tical fiber and a fiber optic delay line transporting

excitation light in sequential pulses to an ignitor laser.

With reference to Figure 7b, to excite a second ignitor laser, the same prism 18 is

rotated to another angular position so that the excitation light is directed to a second beam

splitter 20b which then directs excitation light to a second pair of optical fibers 36b and 42b

that transport excitation light to the second ignitor laser.

Likewise, third, fourth, and more ignitor lasers can be powered by the single excitation

laser light source 10, by rotating the prism 18 through more angles to direct laser light through other beam splitters into other pairs of optical fibers that take sequential pulses of laser light to each of the other ignitor lasers in sequential turns. In this manner, numerous remotely located

ignitor lasers can be energized sequentially by a single excitation light source.

EXAMPLE VIII

In another alternative device and method analagous to that shown in Figure 3, similar switching action can be performed in an apparatus using only a single excitation light source,

a single multiple-mode optical fiber and no fiber optic delay line. Only one optical fiber is

used to link the excitation source with each of the ignitor lasers. A multiplexing device is

used to direct light from the excitation light source sequentially into the optical fiber

associated with each of the ignitor lasers.

In each of the embodiments of the invention, the small physical size and simplicity of design of the ignitor laser enables an effective, compact, robust and cost effective laser ignitor

package which is about the same size as the spark plug which it replaces in various

combustion applications.

The multiplexed embodiments of the invention are particularly useful for aerospace

fuel ignition applications. The laser ignition hardware of this invention is suitable for use in harsh aerospace operating environments because it is compact, insensitive to engine

vibrations, and is able to withstand extreme temperature variations. There is no need to provide bulky and fragile excitation sources such as flash lamps or light

emitting diodes at the ignitor laser proximate to the ignition site. Instead, bulky and fragile

excitation sources are located at the excitation light source, which can be located within the

aircraft cabin or other remote location away from the propulsion engine. The reduced size of

the ignition equipment at the engine reduces cooling requirements and reduces sensitivity to both vibrations and temperature.

No electrical connections of any kind are required to operate the invention

apparatuses. Therefore, problems with high voltage containment at the engine, which are

experienced with the failure of conventional capacitive discharge type ignitors (spark plugs)

are eliminated.

Because the ignitor lasers are basically monolithic units with no need for excitation

sources such as flash lamps or light emitting diodes, virtually all maintenance is confined to

the single excitation light source which can be placed in an easily accessable location. Since

the excitation light source is located remotely from the ignition site, its environment can be

more stringently controlled and the size of the excitation light source is less likely to be

problematic.

While the apparatuses and methods of this invention have been described in detail for the purpose of illustration, the inventive apparatuses and methods are not to be construed as

limited thereby. This patent is intended to cover all changes and modifications within the

spirit and scope thereof. INDUSTRIAL APPLICABILITY

The apparatus and method of the invention can be used as an ignition source for

turbojet engines, internal combustion engines, diesel engines and gas turbines for electrical

power generation.

Claims

WHAT IS CLAIMED IS:

1. An ignition apparatus comprising:

(a) an excitation light source; and

(b) at least one optical fiber positioned to collect light from said excitation light

source and transport said light into an ignitor laser.

2. An ignition apparatus comprising:

(a) an excitation light source;

(b) a beam splitter positioned to receive output from said excitation light source;

(c) a first end of a first optical fiber positioned to collect a first portion of a beam

from said beam splitter;

(d) a second end of said first optical fiber connected to an ignitor laser so as to

permit transport of a first portion of a beam from said excitation light source into said ignitor

laser;

(e) a first end of a second, longer optical fiber positioned to collect a second

portion of a beam from said beam splitter;

(f) a second end of said second optical fiber positioned to deliver a second portion

of a beam from said beam splitter into a beam combiner positioned such that a beam from said

ignitor laser can also be directed into said beam combiner; and

(g) a lens to direct a combined beam from said beam combiner into a combustible

fuel.

3. An apparatus as recited in Claim 2 wherein said excitation light source is a laser.

4. An apparatus as recited in Claim 3 wherein said excitation light source is a solid

state Q-switched laser selected from the group of Cr:LiSAF, Ti:sapphire and alexandrite lasers.

5. An apparatus as recited in Claim 2 wherein said ignitor laser is a Q-switched

laser selected from the group of Nd:YAG lasers, Nd:glass lasers and Nd: YLF lasers.

6. An apparatus as recited in Claim 5 wherein said ignitor laser is a Nd: YAG laser.

7. An apparatus as recited in Claim 2 wherein said ignitor laser has no pumping

diodes or flashlamps.

8. An apparatus as recited in Claim 2 wherein said excitation light source is

remote from said ignitor laser.

9. An apparatus as recited in Claim 2 wherein said optic delay line is long enough

to delay the beam from about 50 to about 100 nanoseconds.

10. An apparatus as recited in Claim 2 wherein said beam combiner is an optical

fiber coupler.

11. An apparatus as recited in Claim 2 wherein said ignitor laser is proximate to

fuel in an engine combustion chamber.

12. An apparatus as recited in Claim 2 wherein said ignitor laser is proximate to

fuel in the cylinder of an aircraft engine.

13. An apparatus as recited in Claim 2 wherein said ignitor laser is proximate to

fuel in a turbine engine.

14. An ignition apparatus comprising:

(a) an excitation light source;

from said beam splitter;

(d) a second end of said first optical fiber positioned to deliver a first portion of a

beam into a beam combiner; (e) a first end of a second, longer optical fiber positioned to collect a second

portion of a beam from said beam splitter;

of a beam from said beam splitter into said beam combiner;

(g) an optical fiber connecting said beam combiner to an ignitor laser so as to

transport combined beam output from said beam combiner to said ignitor laser; and

(h) a lens to direct output from said ignitor laser into a combustible fuel.

15. An apparatus as recited in Claim 14 wherein said excitation light source is a

laser.

16. An apparatus as recited in Claim 15 wherein said excitation light source is a solid state Q-switched laser selected from the group of Cr:LiSAF, Ti:sapphire and alexandrite

lasers.

17. An apparatus as recited in Claim 14 wherein said ignitor laser is a Q-switched laser selected from the group of Nd: YAG lasers, Nd:glass lasers and Nd:YLF lasers.

18. An apparatus as recited in Claim 17 wherein said ignitor laser is a Nd: YAG

laser.

19. An apparatus as recited in Claim 14 wherein said ignitor laser has no pumping

diodes or flashlamps.

20. An apparatus as recited in Claim 14 wherein said excitation light source is

remote from said igmtion laser.

21. An apparatus as recited in Claim 14 wherein said optic delay line is long

enough to delay the beam from about 50 to about 100 nanoseconds.

22. An apparatus as recited in Claim 14 wherein said beam combiner is an optical

fiber coupler.

23. An apparatus as recited in Claim 14 wherein said ignitor laser is proximate to

fuel in an engine combustion chamber.

24. An apparatus as recited in Claim 14 wherein said ignitor laser is proximate to

fuel in the cylinder of an aircraft engine.

25. An apparatus as recited in Claim 14 wherein said ignitor laser is proximate to

fuel in a turbine engine.

26. An ignition apparatus comprising:

(a) an excitation light source;

(b) a lens positioned to focus output from said excitation light source into an optical

fiber; (c) a lens positioned to focus output from said optical fiber into an ignitor laser; and

(d) a lens positioned to focus output from said ignitor laser into a combustible fuel.

27. An apparatus as recited in Claim 26 wherein said excitation light source is a

laser.

28. An apparatus as recited in Claim 27 wherein said excitation light source is a

solid state Q-switched laser selected from the group of Cr:LiSAF, Ti:sapphire and alexandrite

lasers.

29. An apparatus as recited in Claim 26 wherein said ignitor laser is a Q-switched

laser selected from the group of Nd:YAG lasers, Nd:glass lasers and Nd:YLF lasers.

30. An apparatus as recited in Claim 29 wherein said ignitor laser is a Nd: YAG

laser.

31. An apparatus as recited in Claim 26 wherein said ignitor laser has no pumping diodes or flashlamps.

32. An apparatus as recited in Claim 26 wherein said excitation light source is

remote from said ignition laser.

33. An apparatus as recited in Claim 26 wherein said ignitor laser is proximate to

fuel in an engine combustion chamber.

34. An apparatus as recited in Claim 26 wherein said ignitor laser is proximate to fuel in the cylinder of an aircraft engine.

35. An_. ignition apparatus comprising :

(a) an excitation light source having at least two resonator cavities;

(b) a first optical fiber positioned to receive light pulses from one of said resonator

cavities and transport said light pulses to an ignitor laser;

(c) a second optical fiber positioned to receive light pulses from another of said

resonator cavities and transport said light pulses to a beam combiner;

(d) a third optical fiber positioned to receive output from said ignitor laser and

transport said output to said beam combiner; (e) wherein said beam combiner is positioned to direct combined beams through a focusing lens into a combustible fuel.

36. An ignition apparatus comprising:

(a) an excitation laser having a laser rod, high reflection end mirror, Q-switch, light

modulator, polarization analyzer, and a plurality of output couplers;

(b) a first optical fiber positioned to receive light pulses from one of said output

couplers and transport said light pulses to an ignitor laser;

output couplers and transport said light pulses to a beam combiner;

transport said output to said beam combiner;

(e) a lens to focus a combined beam from said beam combiner into a combustible fuel.

37. An apparatus as recited in Claim 36 wherein said excitation laser is a solid state

Q-switched laser selected from the group of Cr:LiSAF, Ti:sapphire and alexandrite lasers.

38. An apparatus as recited in Claim 36 wherein said excitation laser is a CπLiSAF

laser.

39. An apparatus as recited in Claim 36 wherein said light modulator is a Pockels

cell.

40. An apparatus as recited in Claim 36 wherein said high reflection end mirror is

highly reflective of light having a wavelength in the range from about 750 nanometers to

about 850 nanometers.

41. An apparatus as recited in Claim 36 wherein said polarization analyzer is

positioned to direct light into a selected one of said first or second output couplers depending

upon polarization of said light.

42. An apparatus as recited in Claim 36 wherein said ignitor laser is a Q-switched

laser selected from the group of Nd: YAG lasers, Nd:glass lasers and Nd: YLF lasers.

43. An apparatus as recited in Claim 36 wherein said ignitor laser is a Nd:YAG

laser.

44. An apparatus as recited in Claim 36 wherein said ignitor laser has no pumping

diodes or flashlamps.

45. An apparatus as recited in Claim 36 wherein said excitation laser is remote from said ignitor laser.

46. An apparatus as recited in Claim 36 wherein said beam combiner is an optical

fiber coupler.

47. An apparatus as recited in Claim 36 wherein said ignitor laser is proximate to

fuel in an engine combustion chamber.

48. An apparatus as recited in Claim 36 wherein said ignitor laser is proximate to

fuel in the cylinder of an aircraft engine.

49. An apparatus as recited in Claim 36 wherein said ignitor laser is proximate to

fuel in a turbine engine.

50. An ignition apparatus comprising :

(a) an excitation light source having the capability of producing two

different wavelengths of light; (b) a first end of a first optical fiber positioned to collect output from said

excitation light source;

(c) a second end of said first optical fiber connected to an ignitor laser so as to permit transport of pulses of light from said excitation light source into said ignitor laser; and

(d) a lens positioned to focus output of said ignitor laser into a combustible fuel.

51. An apparatus as recited in Claim 50 wherein said excitation light source is a

double Q-switched laser.

52. An apparatus as recited in Claim 51 wherein said excitation light source further

comprises a plurality of reflection end mirrors, a light modulator, polarization analyzer, and a

plurality of output couplers.

53. An apparatus as recited in Claim 52 wherein one of said reflection end mirrors

reflects wavelengths of light different from the wavelengths of light reflected by at least one

other reflection end mirror.

54. An apparatus as recited in Claim 52 wherein said light modulator is a Pockels

cell.

55. An apparatus as recited in Claim 52 wherein each of said output couplers is positioned to receive light reflected by a separate one of said reflection end mirrors.

56. An apparatus as recited in Claim 52 wherein further comprising at least one

wavelength selecting device positioned between said polarization analyzer and at least one of said plurality of reflecting end mirrors.

57. An apparatus as recited in Claim 51 wherein said excitation light source is a

solid state double Q-switched laser selected from the group of Cr:LiSAF, Ti:sapphire and

alexandrite lasers.

58. An apparatus as recited in Claim 50 wherein said ignitor laser is a Q-switched

59. An apparatus as recited in Claim 57 wherein said ignitor laser is aNd:YAG

laser.

60. An apparatus as recited in Claim 50 wherein said ignitor laser has no pumping

diodes or flashlamps.

61. An apparatus as recited in Claim 50 wherein said excitation light source is remote from said ignition laser.

62. An apparatus as recited in Claim 50 wherein said ignitor laser is proximate to fuel in an engine combustion chamber.

63. An apparatus as recited in Claim 50 wherein said ignitor laser is proximate to fuel in the cylinder of an aircraft engine.

64. An apparatus as recited in Claim 50 wherein said ignitor laser is proximate to

fuel in a turbine engine.

65. An apparatus as recited in Claim 2 further comprising a multiplexing device

positioned to receive output from said excitation light source and deliver output into pairs of

said first and second optical fibers.

66. An ignition apparatus comprising:

(a) an excitation light source;

(b) an optical switch positioned to receive output from said excitation light source

and direct said output from said excitation light source into a plurality of beam splitters; (c) a plurality of pairs of a first and a second optical fiber having first ends

positioned to receive, respectively, a first and second portion of a beam from each of said

beam splitters;

(d) wherein said second optical fiber of each pair is longer than said first optical

fiber of each pair;

(e) a second end of each of said first optical fibers connected to an ignitor laser so

as to permit transport of a first portion of a beam from said excitation light source into each

said ignitor laser;

(f) a second end of each of said second optical fibers positioned to deliver a

second portion of a beam from each said beam splitter into a beam combiner positioned such

that a beam from each said ignitor laser can also be directed into said beam combiner; and

(g) a lens to direct a combined beam from each said beam combiner into a

combustible fuel.

67. An apparatus as recited in Claim 66 wherein said excitation light source is a

laser.

68. An apparatus as recited in Claim 66 wherein said excitation light source is a

lasers.

69. An apparatus as recited in Claim 66 wherein said ignitor laser is a Q-switched

70. An apparatus as recited in Claim 66 wherein said ignitor laser is a Nd: YAG

laser.

71. An apparatus as recited in Claim 66 wherein said ignitor laser has no pumping

diodes or flashlamps.

72. An apparatus as recited in Claim 66 wherein said excitation light source is

remote from said ignition laser.

73. An apparatus as recited in Claim 66 wherein said optic delay line is long

enough to delay the beam from about 50 to about 100 nanoseconds.

74. An apparatus as recited in Claim 66 wherein said beam combiner is an optical

fiber coupler.

75. An apparatus as recited in Claim 66 wherein said optical switch is based upon a

rotatable prism.

76. An apparatus as recited in Claim 66 wherein said optical switch is an electro¬

mechanical switch.

77. An apparatus as recited in Claim 66 wherein said ignitor laser is proximate to

fuel in an engine combustion chamber.

78. An apparatus as recited in Claim 66 wherein said ignitor laser is proximate to

fuel in the cylinder of an aircraft engine.

79. An apparatus as recited in Claim 66 wherein said ignitor laser is proximate to

fuel in a turbine engine.

80. An apparatus as recited in Claim 14 further comprising a multiplexing device

positioned to receive output from said beam combiner and to deliver output to a plurality of

ingitor lasers.

81. An ignition apparatus comprising :

(a) an excitation light source;

and direct said output from said excitation light source laser into a plurality of beam splitters; (c) a plurality of pairs of a first and a second optical fiber having first ends

beam splitters;

(d) wherein said second optical fiber of each pair is longer than said first optical fiber of each pair;

(e) a second end of each of said first optical fibers positioned to deliver a first

portion of a beam from each said beam splitter into a beam combiner;

(f) a second end of each of said second optical fibers positioned to deliver a

second portion of a beam from each said beam splitter into each said beam combiner; (g)

wherein each said beam combiner is positioned such that a beam from each said beam combiner can be directed into an ignitor laser; and

(h) a lens to direct a combined beam from each said beam combiner into a

combustible fuel.

82. An apparatus as recited in Claim 81 wherein said excitation light source is a

laser.

83. An apparatus as recited in Claim 81 wherein said excitation light source is a

solid state Q-switched laser selected from the group of Cr:LiSAF and alexandrite lasers.

84. An apparatus as recited in Claim 81 wherein said ignitor laser is a Q-switched

laser selected from the group of Nd:YAG lasers, Nd:glass lasers andNd:YLF lasers.

85. An apparatus as recited in Claim 81 wherein said ignitor laser has no pumping

diodes or flashlamps.

86. An apparatus as recited in Claim 81 wherein said excitation light source is

remote from said ignition laser.

87. An apparatus as recited in Claim 81 wherein said optic delay line is long

enough to delay the beam from about 50 to about 100 nanoseconds.

88. An apparatus as recited in Claim 81 wherein said beam combiner is an optical

fiber coupler.

89. An apparatus as recited in Claim 81 wherein said optical switch is based upon a

rotatable prism.

90. An apparatus as recited in Claim 81 wherein said optical switch is an electro¬

mechanical switch.

91. An apparatus as recited in Claim 81 wherein said ignitor laser is proximate to

fuel in an engine combustion chamber.

92. An apparatus as recited in Claim 81 wherein said ignitor laser is proximate to

fuel in the cylinder of an aircraft engine.

93. An apparatus as recited in Claim 81 wherein each said ignitor laser is proximate

to fuel in a turbine engine.

94. An ignition apparatus comprising:

(a) an excitation light source;

and direct said output from said excitation light source laser into a plurality of optical fibers;

(c) wherein each of said plurality of optical fibers is positioned to transport laser

light from said laser through a focusing lens into an ignitor laser; and

95. An apparatus as recited in Claim 94 wherein said excitation light source is a

laser.

96. An apparatus as recited in Claim 94 wherein said excitation light source is a

lasers.

97. An apparatus as recited in Claim 96 wherein said excitation light source is a

Cr:LiSAF laser.

98. An apparatus as recited in Claim 94 wherein said ignitor laser is a Q-switched

99. An apparatus as recited in Claim 98 wherein said ignitor laser is a Nd: YAG

laser.

100. An apparatus as recited in Claim 94 wherein said ignitor laser has no pumping

diodes or flashlamps.

101. An apparatus as recited in Claim 94 wherein said excitation light source is

remote from said ignition laser.

102. An apparatus as recited in Claim 94 wherein said beam combiner is an optical

fiber coupler.

103. An apparatus as recited in Claim 94 wherein said optical switch is based upon a

rotatable prism.

104. An apparatus as recited in Claim 94 wherein said optical switch is an electro¬

mechanical switch.

105. An apparatus as recited in Claim 94 wherein said ignitor laser is proximate to

fuel in an engine combustion chamber.

106. An apparatus as recited in Claim 94 wherein said ignitor laser is proximate to

fuel in the cylinder of an aircraft engine.

107. An apparatus as recited in Claim 94 wherein each of said plurality of ignitor

lasers is proximate to fuel in a turbine engine.

108. An apparatus as recited in Claim 35 further comprising a multiplexing device

positioned to receive output from said beam combiner and direct output to a plurality of

ignition lasers.

109. An ignition apparatus comprising: (a) an excitation light source having at least two resonator cavities;

(b) a plurality of optical switches each of which is positioned to receive light pulses

from each of said resonator cavities;

(c) a plurality of optical fibers each of which is positioned to receive light pulses

from each of said plurality of optical switches;

(d) wherein at least one of said plurality of optical fibers is positioned to transport

light pulses from at least one of said output couplers to one of a plurality of ignitor lasers;

(e) wherein at least one of said plurality of optical fibers is positioned to transport

light pulses from at least one of said output couplers to a beam combiner positioned to receive

output from said one of a plurality of ignitor lasers; and

(e) a lens to focus a combined beam from said beam combiner into a combustible

fuel.

110. An apparatus as recited in Claim 109 wherein said optical switch is based upon

a rotatable prism.

111. An apparatus as recited in Claim 109 wherein said optical switch is an electro¬

mechanical switch.

112. An apparatus as recited in Claim 109 wherein said ignitor laser is proximate to

fuel in an engine combustion chamber.

113. An apparatus as recited in Claim 109 wherein said ignitor laser is proximate to fuel in the cylinder of an aircraft engine.

114. An apparatus as recited in Claim 109 wherein said ignitor laser is proximate to fuel in a turbine engine.

115. An ignition apparatus comprising:

modulator, polarization analyzer, and a plurality of output couplers;

from each of said plurality of output couplers;

from each of said plurality of optical switches;

output from said one of a plurality of ignitor lasers; and

(e) a lens to focus a combined beam from said beam combiner into a combustible

fuel.

116. An apparatus as recited in Claim 115 wherein said optical switch is based upon

a rotatable prism.

117. An apparatus as recited in Claim 115 wherein said optical switch is an electro¬

mechanical switch.

118. An apparatus as recited in Claim 115 wherein said ignitor laser is proximate to

fuel in an engine combustion chamber.

119. An apparatus as recited in Claim 115 wherein said ignitor laser is proximate to

fuel in the cylinder of an aircraft engine.

120. An apparatus as recited in Claim 115 wherein said ignitor laser is proximate to

fuel in a turbine engine.

121. An apparatus as recited in Claim 50 further comprising a multiplexing device

positioned to receive output from said excitation laser and to direct output sequentially into a

plurality of said optical fibers.

122. An ignition apparatus comprising: (a) an excitation light source having the capability of producing two

different wavelengths of light;

(b) an optical switch positioned to receive output from said excitation light

source and direct said output from said excitation light source sequentially into a plurality of

ignitor lasers.

(c) a plurality of optical fibers each of which is positioned to receive light

from said optical switch and transport said light to each of said plurality of ignitor lasers; and

(d) a plurality of lenses positioned to focus output of each of said ignitor

lasers into a combustible fuel.

123. An apparatus as recited in Claim 122 wherein said optical switch is based upon

a rotatable prism.

124. An apparatus as recited in Claim 122 wherein said optical switch is an electro¬

mechanical switch.

125. An apparatus as recited in Claim 122 wherein said ignitor laser is proximate to

fuel in an engine combustion chamber.

126. An apparatus as recited in Claim 122 wherein said ignitor laser is proximate to

fuel in the cylinder of an aircraft engine.

127. An apparatus as recited in Claim 122 wherein said igmtor laser is proximate to

fuel in a turbine engine.

128. An ignition method comprising:

(a) splitting a beam from an excitation light source into at least two portions;

(b) directing a first portion of said excitation light beam through a first optical fiber into an ignitor laser, causing output of an ignitor laser beam;

(c) directing a second portion of said excitation light beam through a second optical

fiber into a beam combiner;

(d) combining said second portion of said excitation light beam with said ignitor

laser beam to form a combined laser beam; and

(e) directing said combined laser beam into a focal point in a combustible fuel.

129. A method as recited in Claim 128 wherein:

(a) said first portion of said excitation light beam is transported through a first optical

fiber into said ignitor laser;

(b) said second portion of said excitation light beam is transported through a second

optical fiber into said beam combiner; and

(c) said second optical fiber is longer than said first optical fiber, thereby delaying

arrival of said second portion of said excitation light beam at said beam combiner.

130. A method as recited in Claim 129 wherein said second optical fiber delays said

second portion of said excitation light beam so that said second portion of said excitation light

beam arrives at said beam combiner at a later time than said first portion of said first light

beam from said excitation light source.

131. A method as recited in Claim 129 wherein said second portion of said excitation

light beam is delayed in arriving at said beam combiner by an amount of time in the range

from about 20 nanoseconds to about 1 microsecond.

132. A method as recited in Claim 128 wherein said beam from said excitation light

source is a pulsed beam, having long duration low peak power pulses.

133. A method as recited in Claim 128 wherein said combined beam has both long

duration low peak power pulses from said excitation light source and short duration high peak

power pulses from said ignitor laser.

134. A method as recited in Claim 128 wherein said first portion and said second

portion of said excitation light beam are of approximately the same intensity.

135. A method as recited in Claim 128 wherein said excitation light source outputs

light with a wavelength in the range from about 200 nanometers to about 12 microns.

136. A method as recited in Claim 128 wherein said excitation light source outputs

light with a peak power in the range from about lOOkW to about 40MW.

137. A method as recited in Claim 128 wherein said excitation light source outputs

light with a pulse energy in the range from about 20mJ to about 400mJ.

138. A method as recited in Claim 128 wherein said excitation light source outputs

light with a pulse width in the range from about 20 nanoseconds to about 200 nanoseconds.

139. A method as recited in Claim 128 wherein said ignitor laser beam has a

wavelength in the range from about 200 nanometers to about 12 microns.

140. A method as recited in claim 128 wherein said ignitor laser beam has a peak

power from about 200kW to about 250MW.

141. A method as recited in Claim 128 wherein said ignitor laser outputs light with a

pulse energy in the range from about lOmJ to about 250mJ.

142. A method as recited in Claim 128 wherein said ignitor laser outputs light with a

pulse width in the range from about 1 nanosecond to about 60 nanoseconds.

143. A method as recited in Claim 128 wherein prior to direction into said beam

splitter said long duration low peak power light beam from said excitation light source is

injected into a plurality of optical fibers using an optical switch which sequences the

injections into said optical fibers.

144. An ignition method comprising :

(a) splitting a an excitation light beam from an excitation light source into at least

two portions;

(b) directing a first portion of said excitation light beam into a beam combiner;

(c) delaying and then directing a second portion of said excitation light beam into

said beam combiner;

(d) combining said first portion of said excitation light beam with said second

portion of said excitation light beam to form a combined light beam having pulses from both said first portion and said second portion of said first light beam;

(e) directing said combined light beam into an ignitor laser; and

(f) directing output of said ignitor laser into a focal point in a combustible fuel.

145. A method as recited in Claim 144 wherein: (a) said first portion of said excitation light beam is transported through a first optical

fiber into said beam combiner;

optical fiber into said beam combiner; and

146. A method as recited in Claim 144 wherein said second portion of said excitation

from about 20 nanoseconds to about 1 microsecond.

147. A method as recited in Claim 144 wherein said beam from said excitation light

source is a pulsed beam, having long duration low peak power pulses.

148. A method as recited in Claim 144 wherein said combined beam has both long

power pulses from said ignitor laser.

149. A method as recited in Claim 144 wherein said first portion and said second portion of said excitation light beam are of approximately the same intensity.

150. A method as recited in Claim 144 wherein said beam from said excitation light source outputs light with a wavelength in the range from about 200 nanometers to about 12

microns.

151. A method as recited in Claim 144 wherein said output of said excitation light

source has a wavelength in the range from about 200 nanometers to about 12 microns.

152. A method as recited in Claim 144 wherein said excitation light source outputs

light with a peak power in the range from about lOOkW to about 40MW.

153. A method as recited in Claim 144 wherein said excitation light source outputs

light with a pulse energy in the range from about 20mJ to about400mJ.

154. A method as recited in Claim 144 wherein said excitation light source outputs

light with a pulse width in the range from about 10 nanoseconds to about 200 nanoseconds.

155. A method as recited in Claim 144 wherein said igmtor laser outputs short pulses

of light with a wavelength in the range from about 200 nanometers to about 12 microns and

long pulses of light with a wavelength from about 200 nanometers to about 15 microns.

156. A method as recited in Claim 144 wherein said ignitor laser outputs light with a pulse energy in the range from about lOmJ to about 250mJ.

157. A method as recited in Claim 144 wherein said ignitor laser outputs short pulses

of light with a peak power in the range from about 200 kW to about 250 MW and long pulses

of light with a peak power in the range from about 20 kW to about 25 MW.

158. A method as recited in Claim 144 wherein said ignitor laser outputs short pulses

of light with a pulse energy in the range from about 10 mJ to about 250 mJ and long pulses of

light with a pulse energy in the range from about 10 mJ to about 250 mJ.

159. A method as recited in Claim 144 wherein said ignitor laser outputs short pulses of light with a pulse width in the range from about 1 nanoseconds to about 50 nanoseconds

and outputs long pulses of light with a pulse width in the range from about 10 nanoseconds to

about 500 nanoseconds.

160. A method as recited in Claim 144 wherein prior to direction into said beam

injections into said optical fibers.

161. An ignition method comprising :

(a) injecting a pulsed long duration low peak power light beam from an excitation

light source into at least one optical fiber;

(b) directing said pulsed long duration low peak power light beam from said optical

fiber into an ignitor laser, thereby pumping said ignitor laser; and

(c) directing output from said ignitor laser into a focal point in a combustible fuel.

162. A method as recited in Claim 161 wherein said beam from said excitation light

source outputs light with a wavelength in the range from about 200 nm to about 12 microns.

163. A method as recited in Claim 161 wherein said excitation light source outputs

light with a peak power in the range from about 20 kW to about 40 MW.

164. A method as recited in Claim 161 wherein said ignitor laser outputs light with a

peak power in the range from about 20 mJ to about 400 mJ.

165. A method as recited in Claim 161 wherein said excitation light source outputs

166. A method as recited in Claim 161 wherein said ignitor laser outputs short pulses

of light with a wavelength in the range from about 200 nanometers to about 12 microns and outputs long pulses of light with a wavelength in the range from about 200 nanometers to

about microns.

167. A method as recited in Claim 161 wherein said igmtor laser outputs short pulses

of light with a peak power in the range from about 100 kW to about 200 MW and outputs

long pulses of light with a peak power in the range from about 10 kW to about 25 MW.

168. . A method as recited in Claim 161 wherein said ignitor laser outputs short pulses

of light with a pulse energy in the range from about 10 mJ to about 200 mJ and outputs long

pulses of light with a pulse energy in the range from about 10 mJ to about 250 mJ.

169. A method as recited in Claim 161 wherein said ignitor laser outputs short pulses

of light with a pulse width in the range from about 1 nanosecond to about 1 microsecond and

outputs long pulses of light with a pulse width in the range from about 10 nanoseconds to

about 1 microsecond.

170. A method as recited in Claim 161 wherein said beam from said excitation light

source is a pulsed beam, having long duration low peak power pulses.

171. A method as recited in Claim 161 wherein said combined beam has both long

duration low peak power pulses from said excitation light source and short duration high peak power pulses from said ignitor laser.

172. A method as recited in Claim 161 wherein said long duration low peak power

light beam from said excitation light source is injected into a plurality of optical fibers using

an optical switch which sequences the injections into said optical fibers.

173. An ignition method comprising :

(a) generating a first beam of light with an excitation light source;

(b) directing said first beam of excitation light through a first optical fiber into an

ignitor laser, causing output of an ignitor laser beam;

(c) generating a second beam of light with said excitation light source;

(d) combining said second beam of excitation light with said ignitor laser beam to

form a combined beam; and

(e) directing said combined beam into a focal point in a combustible fuel.

174. An ignition method as recited in Claim 172 wherein said first beam of light

from said excitation light source is a pulsed beam, having long duration low peak power

pulses.

175. An ignition method as recited in Claim 172 wherein said second beam of light

from said excitaiton light source is a pulsed beam, having long duration low peak power

pulses.

176. An ignition method as recited in Claim 172 wherein said combined beam has

both long duration low peak power pulses from said excitation light source and short duration

high peak power pulses from said ignitor laser.

177. An ignition method as recited in Claim 172 wherein said said first beam of light

from said excitation light source and said second beam of light from said excitation light

source are of approximately the same intensity.

178. An ignition method as recited in Claim 172 wherein said first beam of light

from said excitation light source has a wavelength in the range from about 200 nanometers to

about 15 microns.

179. An ignition method as recited in Claim 172 wherein said first beam of light

from said excitation light source has a peak power in the range from about 200 nanometers to

about 15microns.

180. An ignition method as recited in Claim 172 wherein said first beam of light

from said excitation light source has a pulse energy in the range from about lOmJ to about

500mJ.

181. An ignition method as recited in Claim 172 wherein said first beam of light

from said excitation light source has a pulse width in the range from about to about 10

nanoseconds to about 1 microsecond.

1 δ2. An ignition method as recited in Claim 172 wherein said second beam of light

from said excitation light source has a wavelength in the range from about 200 nanometers to aboutl5 microns.

183. An ignition method as recited in Claim 172 wherein said second beam of light

from said excitation light source has a peak power in the range from about 20kW to about

40MW.

1 δ4. An ignition method as recited in Claim 172 wherein said second beam of light

400mJ.

185. An ignition method as recited in Claim 172 wherein said second beam of light

from said excitation light source has a pulse width in the range from about 10 nanometers to about 1 microsecond.

186. An ignition method as recited in Claim 172 wherein there is a delay from about

25 nanoseconds to about 300 nanoseconds between said first beam of light and said second

beam of light from said excitation light source.

187. An ignition method comprising:

(a) generating a first beam of light with an excitation light source;

(b) injecting said first beam of light into a first plurality of optical fibers using an

optical switch which sequences the injections into said optical fibers;

(c) sequentially transporting said first beam of light in said first plurality of optical fibers to a plurality of ignitor lasers thereby causing output of a ignitor laser beam from each of said plurality of ignitor lasers;

(d) generating a second beam of light with said excitation light source;

(e) injecting said second beam of light into a second plurality of optical fibers using

an optical switch which sequences the injections into said second plurality of optical fibers;

(f) combining output light from each of said second plurality of optical fibers with

output from one of said plurality of ignitor lasers to form a combined beam;

(g) focusing each said combined beam into a focal point in a combustible fuel.

188. A method as recited in Claim 173 wherein said combustible fuel is in one or

more engine combustion chambers.

189. A method as recited in Claim 173 wherein said combustible fuel is in one or

more cylinders of an aircraft engine.

190. A method as recited in Claim 173 wherein said combustible fuel is in one or

more turbine engines.

191. A method as recited in Claim 187 wherein said combustible fuel is in one or more engine combustion chambers.

192. A method as recited in Claim 187 wherein said combustible fuel is in one or

more cylinders of an aircraft engine.

193. A method as recited in Claim 187 wherein said combustible fuel is in one or

more turbine engines.

194. An ignition method comprising :

(a) generating a first beam of light with an excitation light source; (b) directing said first beam of excitation light through a first optical fiber into an

ignitor laser, causing a first output of an ignitor laser beam;

(c) generating with said excitation light source a second beam of light having a

wavelength different from the wavelength of said first beam of light ;

(d) directing said second beam of excitation light through said optical fiber into

said ignitor laser, causing a second output of an ignitor laser beam; and

(e) directing output of said ignitor laser into a focal point in a combustible fuel.

195. A method as recited in Claim 194 wherein said first beam of light has a

wavelength which is substantially absorbable by lasing material in said ignitor laser and said

second beam of light has a wavelength which is substantially not absorbable by lasing

material in said ignitor laser.

196. A method as recited in Claim 194 wherein said first beam from said excitation

light source is a long duration low peak power pulsed beam.

197. A method as recited in Claim 194 wherein said second beam from said

excitation light source is a low peak power long duration pulsed beam.

198. A method as recited in Claim 194 wherein said first beam from said excitation

light source and said second beam from said excitation light source are of approximately the same intensity.

199. A method as recited in Claim 194 wherein said first beam from said excitation

light source has a wavelength in the range from about 200 nanometers to about 12 microns

and said second beam from said excitation light source has a wavelength in the range from

about 200 nanometers to about 12 microns.

200. A method as recited in Claim 194 wherein said ignitor laser beam has a

wavelength in the range from about 200 nanometers to about 12 microns.

201. A method as recited in Claim 194 wherein said first beam of excitation light has

a peak power in the range from about 10W to about 50MW and said second beam of

excitation light has a peak power in the range from about 20kW to about 40MW.

202. A method as recited in Claim 194 wherein said first beam of excitation light has

a pulse energy in the range from about lOmJ to about 500mJ and said second beam of

excitation light has a pulse energy in the range from about lOmJ to about 400mJ.

203. A method as recited in Claim 194 wherein said first beam of excitation light has

a pulse width in the range from about 10 nanoseconds to about 1 microsecond and said second

beam of excitation light has a pulse width in the range from about 10 nanoseconds to about lmicronsecond.

204. A method as recited in Claim 194 wherein said first output of said ignitor laser

has a wavelength in the range from about 200 nanometers to about 12 microns and said

second output of said ignitor laser has a wavelength in the range from about 200 nanometers

to about 12 microns.

205. A method as recited in Claim 194 wherein said first output of said ignitor laser

has a peak power in the range from about 25 kW to about 250 MW and said second output of

said ignitor laser has a peak power in the range from about 25kW to about 250MW .

206. A method as recited in Claim 194 wherein said first output of said ignitor laser

has a pulse energy in the range from about 5mJ to about 250mJ.

207. A method as recited in Claim 194 wherein said first output of said ignitor laser has a pulse width in the range from about 1 nanosecond to about 200 nanoseconds.

208. A method as recited in Claim 194 further comprising allowing a time interval in

the range from about 25 nanoseconds to about 300 nanoseconds to occur between the end of

said first beam and the beginning of said second beam.

209. A method as recited in Claim 194 wherein said first and said second beams

from said excitation light source are injected into a plurality of optical fibers using an optical

switch which sequences the injections into said optical fibers.