WO2001011736A1

WO2001011736A1 - Waveguide laser

Info

Publication number: WO2001011736A1
Application number: PCT/US2000/020959
Authority: WO
Inventors: Stuart Mac Cormack; Robert G. Waarts
Original assignee: Sdl, Inc.
Priority date: 1999-08-09
Filing date: 2000-08-01
Publication date: 2001-02-15
Also published as: CA2381170A1

Abstract

Embodiments of 980 nm ytterbium-doped fiber lasers illustrate features of novel high-efficiency optical lasers. In a cladding-pumped fiber laser comprising a core region (31) in optical proximity to a cladding region (32), a rare-earth dopant (35) is disposed within the cladding region in such a manner that higher levels of population inversion can be achieved and the cladding region itself is arranged to confine the pump energy such that at least one-fourth of the pump energy is absorbed while achieving optical transparency. Various dopants may be disposed within the core region and/or the cladding region to provide a desired refractive index profile, to provide a photosensitive core region in which reflective gratings (37, 38) may be written, and to dissipate unwanted emissions at longer wavelengths. A tapered fiber (21) may be used to improve the coupling efficiency between a pump energy source (10) and the optical laser.

Description

DESCJ IPTION

WAVEGUIDE LASER

TECHNICAL FIELD

The present invention is related generally to optical lasers and is related more particularly to improving the efficiency of laser operation.

BACKGROUND ART There is a growing interest to provide a high-efficiency three-level operation lasers such as, for example, ytterbium-doped optical fiber lasers that emit light having a wavelength of about 980 nm. Such lasers would be very useful as a pump-laser for fiber amplifiers or other fiber lasers. Unfortunately, such lasers that are implemented according to present designs are very inefficient. Throughout this disclosure, more particular mention is made of 980 nm ytterbium-doped fiber lasers for ease of discussion; however, the principles discussed herein are applicable to a broad range of laser structures, dopants and wavelengths. In addition, references to specific wavelengths should generally be understood to be nominal wavelengths. For example, references to 980 nm and 1060 nm should be understood to represent ranges of wavelengths such as 970 to 980 nm and 1020 to 1120 nm, respectively.

The inventors have determined that three requirements must be satisfied to achieve high efficiency for a ytterbium (Yb) laser operating at 980 nm: (1) pump energy intensity must be high enough to pump a majority of the Yb-dopant electrons into an excited state for three-level operation, which is known as population inversion, (2) an undesirable optical gain competition between 980 nm and longer wavelengths must be reduced, and (3) a significant proportion of the pumping energy must be absorbed.

Only one known technique is capable of satisfying all three requirements. According to this technique, which is disclosed in Zenteno et al., "0.65 W single- mode Yb-fiber laser at 980 nm pumped by 1.1 W Nd:YAG", Advanced Solid State Letters '99, post deadline PD-10, 1999, which is incorporated herein by reference, high-efficiency 980 nm emissions are achieved by confining the 946 nm pumping energy of a Nd:YAG pump laser to the single-mode core of a Yb-doped fiber laser. This solution is not totally satisfactory because the output power of the Yb fiber laser is limited by the available single- mode power of the Nd YAG pump laser and by the fraction of the pumping energy that can be launched into the core

According to another technique disclosed in Nilsson et al , "Ring-doped cladding-pumped single-mode three-level fiber laser", Optics Letters, vol 23, no 5, March 1, 1998, pp 355-357, which is incorporated herein by reference, high pumping energy intensity is achieved by using a high-power pumping source to pump the inner cladding of a short length of double-clad optical fiber The undesirable gain competition at longer wavelengths is reduced by disposing the Yb dopant in a ring within the inner cladding that surrounds the core of the double-clad fiber The ring- doping structure reduces the optical gain at all wavelengths, however, the reduction in gain for the longer wavelengths allows a higher population inversion to be achieved, which recovers at least part of the gain that is otherwise lost for 980 nm The net effect of ring doping is a relative increase in optical gain for wavelengths at 980 nm Unfortunately, this approach imposes limits on the length of the optical fiber, which limits the absorption of pumping energy and limits the overall efficiency of the laser The laser is very inefficient because a considerable amount of the pumping energy is wasted as it passes unabsorbed through the optical fiber

DISCLOSURE OF INVENTION

It is an object of the present invention to improve the efficiency of lasers such as, for example, Yb-doped 980 nm fiber lasers The teachings of the present invention may be applied to a broad range of laser structures, dopants and wavelengths

According to one aspect of the present invention, a method for operating a laser that comprises a core region and a first region that is in optical proximity to the core region and contains atoms of a rare-earth dopant comprises receiving pumping energy at a first wavelength into the first region, confining the pumping energy such that at least one-fourth of the pumping energy is absorbed and such that the pumping energy that overlaps the rare-earth dopant atoms has an intensity throughout the first region that exceeds a threshold for achieving gain transparency at a second wavelength that is longer than the first wavelength, and emitting energy having the second wavelength According to another aspect of the present invention, a laser powered by one or more pumping energy sources that emit pumping energy at a first wavelength comprises a core region and a first region that is in optical proximity to the core region and receives the pumping energy, wherein the first region contains atoms of a rare-earth dopant that have electrons capable of transitions that emit energy at a second wavelength that is longer than the first wavelength, and wherein the first region confines the pumping energy such that at least one-fourth of the pumping energy is absorbed and such that the pumping energy that overlaps the rare-earth dopant atoms has an intensity throughout the first region that exceeds a threshold for achieving gain transparency at a second wavelength that is longer than the first wavelength.

According to yet another aspect of the present invention, a laser comprises a core region that contains atoms of a photosensitive dopant and a first region that is in optical proximity to the core region that contains atoms of a rare-earth dopant, wherein refractive indices of the core region and the first region are such that the core region and the first region confine an optical mode that overlaps the core region and the rare-earth dopant atoms.

According to a further aspect of the present invention, a laser comprises a core region; a first region that is in optical proximity to the core region and contains atoms of a rare-earth dopant, and that contains atoms of a first dopant that at least partially compensate for changes in the first region refractive index caused by the rare-earth dopant atoms; and a second region that is in optical proximity to the first region.

The various features of the present invention and its preferred embodiments may be better understood by referring to the following discussion and the accompanying drawings in which like reference numerals refer to like elements in the several figures. The contents of the following discussion and the drawings are set forth as examples only and should not be understood to represent limitations upon the scope of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a block diagram of a laser optically coupled to an optical pump. Fig 2 is a schematic illustration of a double-clad optical fiber laser with a non- uniform distribution of a rare-earth dopant within the inner cladding that overlaps the evanescent tails of the fiber's optical mode

Figs 3 A and 3B are schematic cross-section illustrations of a double-clad optical fiber with various refractive index profiles

Fig 4 is a graphical representation of the absorption and emission characteristics of silica with a ytterbium dopant

Figs 5 A and 5Bare graphical representations of fiber laser output power and efficiency as a function of fiber length Fig 6 is a schematic perspective illustration of a double-clad optical fiber laser

Fig 7 is a graphical representation of the efficiency of a ring-doped fiber laser as a function of the ring doping radius

Fig 8 is a graphical representation of the output power of a ring-doped fiber laser as a function of fiber length

Figs 9 A and 9B are graphical representations of the output power and overall efficiency of a particular embodiment of a double-clad fiber laser with and without a pumping energy reflector

Fig 10 is a schematic diagram of fiber laser that receives pumping energy from two pump sources

Fig 11 is a schematic diagram of a pump laser module that is coupled through a tapered optical fiber to a fiber laser

MODES FOR CARRYING OUT THE INVENTION A. Overview

Fig 1 is a block diagram showing laser 30 optically coupled to pump 10 through coupler 20 Throughout the following disclosure, more particular mention is made of Yb-doped embodiments of laser 30 that emit light at a nominal wavelength of 980 nm Specific numerical examples are based on simulations B. Laser

1. Embodiments a) Various Features

An embodiment of laser 30 according to the present invention may be implemented by a wide range of structures such as double- and single-clad optical fibers or planar waveguides. The essential features are a core region that is optically proximate to a first region in which atoms of a rare-earth dopant are disposed. The first region receives pumping energy from pump 10 at a first wavelength and the rare- earth dopant atoms have electrons capable of transitions that emit energy at a second wavelength that is longer than the first wavelength.

Preferably, the atoms of the rare-earth dopant are disposed in the first region in such a way that the overlap between the rare-earth dopant atoms and the emitted energy at the second wavelength is reduced. This reduction in overlap reduces the optical gain of laser 30 at all wavelengths. The reduction in gain at the second wavelength is offset at least partially by a decrease in absorption at the second wavelength due to a higher population inversion made possible by the reduced gain for longer wavelengths.

Fig. 2 is a schematic illustration of a double-clad optical fiber 39 with a non- uniform distribution of a rare-earth dopant 35 within the inner cladding or first region 32 that reduces the overlap with the emitted energy at the second wavelength by overlapping only the evanescent tails of the fiber's optical mode 34. The shape of the optical mode shown in the figure is Gaussian; however, no particular shape is critical to the present invention. Rare-earth dopant 35 is disposed in such a way that the overlap between the dopant and optical mode 34 provides enough optical gain for lasing at the second wavelength but not so much optical gain that amplified spontaneous emission (ASE) at longer wavelengths cuts off lasing by depleting the population inversion of dopant atoms with electrons in an excited state. Essentially any disposition of rare-earth dopant 35 that provides a suitable optical gain may be employed. A disposition in the form of rings is discussed in more detail below. The core region may contain atoms of a photosensitive dopant such as germanium, which allows gratings to be written directly into the core region. These gratings may be used to form a lasing cavity or to dissipate longer wavelengths such as those within a range from about 1020 nm to about 1120 nm. The core region and/or the first region may also contain atoms of a dopant that absorb these longer wavelengths. b) Refractive Index Profile The profile of the refractive indices of the core region and the first region is arranged such that the core region and the first region confine an optical mode that overlaps the core region and the first region. Atoms of various dopants may be added to some or all of the regions to achieve a desired refractive index profile. Figs. 3A and 3B are schematic cross-section illustrations of double-clad optical fiber 39 with various refractive index profiles. The profiles illustrated in these figures are also applicable to other embodiments including single-mode optical fibers and planar waveguides. The following examples assume core region 31 and first region 32 are made of a glass such as fused silica.

Referring to Fig. 3 A, the refractive index 41 of core region 31 is higher than the refractive index 42 of first region 32, which, in turn, is higher than the refractive index 43 of second region 33. This profile may be obtained in a variety of ways. One way adds a dopant such as germanium, aluminum or phosphorous to the core region to increase the core refractive index, adds a dopant to first region 32 to compensate for changes in the refractive index brought about by the inclusion of rare-earth dopant 35, and adds an index-depressing dopant such as fluorine to a silica second region 33 to change its refractive index.

The changes in refractive index that are caused by rare-earth dopant 35 can be compensated in a variety of ways. One way disposes the rare-earth dopant and the compensating dopant together in one or more portions of first region 32 as necessary. Another way disposes the dopants separately in distinct portions such as in alternating layers; however, the size of these distinct portions should be small as compared to the wavelength of energy emitted by laser 30 so that this emitted energy will react to the average refractive index. Yet another way modulates the disposition of the dopants in one or more portions of first region 32. If a rare-earth dopant such as Yb is disposed in a silica first region 32, this rare-earth dopant will increase the refractive index 42 of first region 32; therefore, a dopant such as fluorine may be used to compensate for this increase in the refractive index. The compensation may be less than, equal to or greater than the increase caused by the rare-earth dopant, as desired. Referring to Fig 3B, the refractive indices 51, 52 of core region 31 and the portion of first region 32 immediately adjacent to core region 31 are substantially equal, and are higher than the refractive index 53 of portions of first region 32 that are not adjacent to core region 31 This profile may be obtained in a variety of ways One way disposes an index-increasing rare-earth dopant within a portion of first region 32 that is immediately adjacent to core region 31 and adds a dopant to core region 31 to increase the core refractive index by substantially the same amount as the rare-earth dopant increases the refractive index of first region 32 The lower refractive index 53 at the outer edges of first region 32 is inherent because no rare-earth dopant is disposed there The refractive index 53 may be reduced further by adding an index- depressing dopant such as fluorine

A low refractive index 54 in a silica second region 33 may be obtained by adding a dopant such as fluorine to the region Alternatively, a material with a lower refractive index such as a polymer or air may be used for second region 33 2. Operation a) Three-Level Operation

In a preferred embodiment, the dominant lasing action of laser 30 is due to three-level transitions of a rare-earth dopant such as ytterbium The difficulties encountered in achieving three-level operation of a Yb-doped laser may be better understood by referring to Fig 4, which is a graphical representation of the absorption and emission characteristics of ytterbium in a silica host material

The solid line 61 in Fig 4 represents a typical emission spectrum for Yb glass as a function of wavelength An emission peak 63 exists at approximately 980 nm A region of lower emission exists for a range of longer wavelengths from about 1020 nm to about 1120 nm The dotted line 62 in Fig 4 represents a typical absorption spectrum for Yb glass An absorption peak 64 exists at approximately 980 nm A region of lower absorption exists for a range of shorter wavelengths from about 910 nm to about 930 nm The 980 nm peaks 63, 64 for emission and absorption correspond to electron transitions between the ²F₅/ and ²F₇/₂ levels Three-level operation of ytterbium for 980 nm emission requires a very high population inversion to achieve a net optical gain, which is the overall difference between the level of optical emission and absorption at 980 nm Unfortunately, ytterbium electrons are capable of multiple transitions from the same excited state which emit energy at longer wavelengths These longer-wavelength transitions reduce the population inversion by depleting the population of excited electrons and make efficient three-level operation difficult to achieve The levels of emission at these longer wavelengths is lower than the levels of emission at 980 nm, however, the much lower absorption at these longer wavelengths generally results in an optical gain at these longer wavelengths that is much higher than the optical gain achieved at 980 nm

The deleterious effect of the undesirable gain at these longer wavelengths can be appreciated by referring to the graphs illustrated in Figs 5A and 5B The graph in Fig 5 A represents the output power at 980 nm for one particular embodiment of a Yb-doped double-clad fiber laser as a function of fiber length In this particular embodiment, the fiber comprises a uniformly doped single-mode core region surrounded by an inner cladding that has a circular cross section with a diameter of about 200 μm The lower curve 71 represents output power in response to 50 watts of pump power launched into the inner cladding of the fiber laser The upper curve 72 represents output power in response to 100 watts of launched pump power The two curves show that pump power increases with increasing fiber length up to only about 35 cm, which is very short for conventional fiber lasers For longer lengths, the output power falls off rapidly The graph in Fig 5B represents the efficiency of the same double-clad fiber laser with respect to the level of absorbed pump power The lower curve 75 represents the efficiency for a launched pump power of 50 watts, which is about 50% for fiber lengths up to about 30 cm The upper curve 76 represents the efficiency for a launched pump power of 100 watts, which is about 60% for fiber lengths up to about 35 cm The drastic reduction in output power and efficiency for fiber lengths above about 30 cm is attributable to ASE at longer wavelengths that depletes the population inversion As explained above, this condition is due to undesirable gain competition between 980 nm and the longer wavelengths b) Reducing Gain at Longer Wavelengths This undesirable gain competition between 980 nm and the longer wavelengths can be reduced by either or both of two approaches The first approach increases losses at the longer wavelengths The second approach reduces losses at the 980 nm emission wavelength The losses at the longer wavelengths may be increased by a number of techniques including (1) using long-period gratings or other structures to scatter the longer wavelengths, (2) bending the core and first regions to increase losses at the longer wavelengths by scattering the longer wavelengths out of the waveguide, (3) disposing a dopant such as samarium (Sm) in the core region and/or the first region that absorbs energy at the longer wavelengths but is reasonably transparent at 980 nm, and (4) designing the refractive index profile and the geometry of the core and first regions to provide propagation properties that attenuate energy above a cutoff wavelength of, for example, 1020 nm The losses at the 980 nm emission wavelength can be reduced by obtaining a high population inversion If all or a significant proportion of the Yb-dopant electrons can be pumped into the excited state, then the first region in which the Yb-dopant is disposed will be essentially transparent at 980 nm A high population inversion reduces absorption losses at all wavelengths, however, because the net optical gain at 980 nm is affected by background absorption much more strongly than at other wavelengths, this technique achieves a significant improvement in net optical gain at 980 nm relative to the gain at longer wavelengths

The population inversion in a laser will grow with increasing pump power as the laser approaches the lasing threshold and then is clamped at this threshold value even as the pump power increases further Achieving a lasing condition with a high population inversion requires that the laser threshold occur at a relatively high pump power level A high laser threshold can be achieved by designing the first region to have a low optical gain per unit of absorbed pump power

A low optical gain may be obtained by confining the optical mode of the waveguide mainly to the core region that is substantially devoid of a rare-earth dopant In a fiber laser, for example, atoms of the rare-earth dopant may be disposed within one or more rings within the first region that surround the core region of the fiber Preferably, the optical mode should contain long evanescent tails that extend through the Yb-doped portion of the first region that is optically proximate to the core region A suitable optical gain can also be achieved by a graded density of rare-earth dopant that diminishes as a function of distance from the core region, following the intensity profile of the optical mode for example A step in the refractive index at the edge of the doped region may be employed to confine the optical mode and prevent the evanescent tail from extending outside the doped region The volume of the doped region can be made relatively large, thereby increasing the amount of pumping energy absorption per unit length along the first region 32

A suitable reduction in optical gain may be achieved by disposing the rare- earth dopant in a wide variety of ways as discussed above in connection with Fig 2 A ring-shaped disposition such as that shown in Fig 6 is one example

Fig 6 is a schematic perspective illustration of a double-clad optical fiber implementation of laser 30 that comprises core region 31, which is adjacent to first region 32, which in turn is adjacent to second region 33 In this particular embodiment, first region 32 is the inner cladding layer of double-clad fiber 39, which includes a portion containing atoms of a rare-earth dopant 35 such as ytterbium that are disposed nonuniformly in a ring-like structure around core region 31 Second region 33 is the outer cladding layer of double-clad fiber 39 Reflectors 37 and 38 in the form of Bragg gratings are written in core region 31 to form a lasing cavity for laser 30

Fig 7 illustrates the effects of ring doping on the efficiency of a two meter double-clad fiber laser that is similar to the embodiment mentioned above in connection with Fig 5 A The efficiency is illustrated as a function of the ring doping radius with a constant dopant cross section of approximately 50 μm² The launched pump power is 50 watts As shown, the efficiency is much higher for ring doping radii between about 6 μm and 8 μm than it is for smaller radii

The graph in Fig 8 illustrates output power as a function of fiber length for the same 980 nm fiber laser represented by Fig 7 with a ring-doping radius of 7 5 μm Peak power is obtained for a fiber length of about 15 meters, which is a significant increase over the lengths that are possible without ring doping c) Increasing Overall Efficiency Laser 30 cannot be operated efficiently unless a significant portion of the pumping energy is absorbed by the rare-earth dopant that is disposed in first region 32 In embodiments of laser 30 that receive pumping energy through a first end, the pumping energy that passes through laser 30 and emerges at a second end represents wasted energy that is not absorbed Prior art attempts to operate a 980 nm laser efficiently have been unable to reduce the amount of this wasted energy because a minimum level of pumping energy intensity is required throughout the doped region of laser 30 to achieve optical transparency, i e no optical loss, at 980 nm The minimum pumping intensity required to achieve optical transparency can be estimated from the information discussed above and illustrated in the figures Referring again to Fig 8, it can be seen that the optimum length for this particular embodiment of a Yb ring-doped silica fiber is about 15 m for a launched pump power of 50 watts into an inner cladding that has a diameter of 200 μm The optimum fiber length is established by a requirement that the pumping intensity throughout this length be sufficient to achieve optical transparency, i e a balance between optical gain and absorption, at 980 nm If the fiber is too short, less than an optimum amount of the pumping energy is absorbed If the fiber is too long, the pumping intensity along part of the fiber drops below the level required for optical transparency, which causes unwanted absorption of the laser output at 980 nm

The magnitude of the unabsorbed pump power that passes through the fiber at the optimum fiber length, together with the cross-sectional area of the inner cladding, establishes the pumping intensity that is required to reach optical transparency The magnitude of the unabsorbed pump power at the transparency point for the 200 μm inner cladding fiber is 18 watts This corresponds to a pumping intensity of 60 kW/cm² If the fiber length exceeds this optimum value, then optical transparency is lost because the additional fiber beyond this optimum length absorbs the 980 nm signal and the output power diminishes accordingly

A similar evaluation was performed for a double-clad fiber laser with an inner cladding diameter of 130 μm and a lower launched pump power of 20 watts to compensate for the reduced cross-sectional area of the cladding In this case, the magnitude of the unabsorbed pump power at the transparency point for the 130 μm inner cladding fiber is 7 5 watts This also corresponds to a pumping intensity of 60 kW/cm²

This pumping intensity is the minimum pumping intensity for a Yb-doped glass fiber that is required at the end of the fiber away from the pumping source to achieve optical transparency at 980 nm Table I below lists the minimum unabsorbed power through this end of the fiber laser that is required for optical transparency as a function of the inner-cladding diameter Inner cladding Minimum unabsorbed diameter (μm) pump power (W)

200 18

130 7.5

100 4.7

50 1.2

20 0.19

8 0.030

Table I Since the pumping energy is uniformly distributed across the inner-cladding cross-sectional area, the level of pumping energy that is required to achieve optical transparency may be calculated from the product of the inner-cladding cross-sectional area and the pumping intensity required for optical transparency. As the inner- cladding diameter is reduced, the amount of pumping energy that is required to achieve optical transparency is reduced.

As the inner-cladding diameter is reduced, the amount of unabsorbed pumping energy that passes through the fiber is also reduced, allowing pumping energy absorption to increase well above 25%, for example 50%, yet still allowing optical transparency to be attained. In preferred embodiments, the inner-cladding diameter is approximately 50 μm or less. A reduction in diameter may also reduce the amount of pumping energy that can be launched into the fiber laser. This is discussed more fully below.

It should be noted that the pump power required to achieve the lasing threshold is not simply equal to the pumping intensity required for optical transparency times the inner cladding area. Instead, it is the launched pump power that is required to realize the necessary pumping intensity for transparency throughout the length of the fiber laser.

The graph in Fig. 9A illustrates the output power and overall efficiency of a 4 meter, 130 μm cladding-diameter Yb ring-doped fiber laser. Solid line 81 shows the output power as a function of launched pump power and dotted line 82 shows the overall efficiency of the laser as a function of launched pump power. This graph shows a lasing threshold pump power of about 11.5 W, which results in 5.5 W of pump power passing unabsorbed through the fiber. This threshold is slightly lower than the calculated threshold because lasing can be obtained even with a net optical loss along a portion of the fiber length.

The overall efficiency of the laser may be improved by reflecting the unabsorbed pumping energy that passes through the laser back toward the end through which the pumping energy is launched. The graph in Fig. 9B is similar to the graph illustrated in Fig. 9A but illustrates the output power and overall efficiency obtained using a pumping energy reflector. One embodiment of a fiber laser with a pumping energy reflector is illustrated in Fig. 11.

The foregoing discussion assumes the laser is pumped from only one end; however, the laser may be pumped from each end as illustrated in Fig. 10. In the illustrated embodiment, laser 30 receives pumping energy from pump 10 through coupling fiber 20 and also receives pumping energy from pump 90 through coupling fiber 92 and optical coupler 91. The 980 nm output of laser 30 is launched into fiber 93. As shown in this embodiment, laser 30 is bent into a loop 36 to increase transmission losses at longer wavelengths. Optical coupler 91 may be implemented in a wide variety of ways as known in the art. No particular implementation is critical in principle to the practice of the present invention. d) Cladding Structure Conventional double-clad fibers comprise a core and inner cladding made of silica surrounded by an outer cladding made of a polymer. The use of a polymer for the outer cladding is attractive because it provides a low index of refraction, increasing the typical numerical aperture (NA) of the fiber to about 0.45, so that it can receive and guide light from a broad range of angles with respect to the principal optical axis of the fiber. Unfortunately, the mechanical reliability of conventional silica-polymer fiber designs imposes a restriction on the extent to which the inner-cladding diameter can be reduced. It is anticipated that advances in optical fiber manufacturing and design may permit the inner-cladding diameter of a silica-polymer fiber to be reduced to 50 μm or so without compromising mechanical reliability. Nevertheless, unless and until such advances are realized, inner-cladding diameters of 50 μm or less are not practical for conventional silica-polymer fibers. A double-clad fiber with an arbitrarily small inner cladding may be realized without compromising mechanical reliability by using silica to make the outer cladding as well as the core and inner cladding The use of silica to make the outer cladding generally will reduce the NA of the fiber, however, the reduction in NA can minimized by controlling the refractive index profile of the fiber as discussed above in connection with Figs 3 A and 3B An NA of about 0 30 or so is possible e) Reflectors Thus far, little has been said about the reflectors that may be employed to form the lasing cavity of laser 30 While no particular implementation is essential for carrying out the present invention, it is anticipated that lasing cavity reflectors in the form of gratings may be written directly into the core of preferred embodiments of fiber lasers This may be facilitated by disposing in the core a dopant such as germanium that is photosensitive In one particular embodiment illustrated in Fig 11, reflectors 37 and 38 in the form of gratings are written in the core of an optical fiber to form the lasing cavity of laser 30

For three-level operation at 980 nm, these reflectors may be advantageously designed to optimize reflection at 980 nm and to scatter longer wavelengths from about 1020 to about 1120 nm The reflectors should also be designed to transmit pumping energy at either end through which laser 30 is pumped The pumping energy may be within a range of wavelengths from about 910 nm to about 930 nm, for example

Preferably, the reflector at one end of the lasing cavity should have a reflectivity for 980 nm that is as close as practically possible to 100%, for example 98%o or more At the other end of the lasing cavity, the reflector preferably should have a reflectivity for 980 nm that is greater than or equal to about 70% and less than or equal to 95% This preferred level of reflectivity for the second reflector is higher than that normally used in fiber lasers because the fiber in preferred embodiments of the present invention for 980 nm laser operation is shorter

As discussed above in connection with Fig 9B, the overall efficiency of laser 30 may be improved by using a reflector to reflect back into laser 30 the unabsorbed pumping energy that passes through laser 30 Referring again to the embodiment illustrated in Fig 11, reflector 94, which is located outside the lasing cavity, is employed as a pumping energy reflector If the output of laser 30 that passes though reflector 94 is intended to be used in some application such as for pumping an optical gain medium, reflector 94 is preferably designed to pass this output with as little loss as is reasonably possible. Reflector 94 could be implemented by a dichroic mirror that is interposed between two segments of optical fiber. Coupling losses through the mirror between the fiber segments should be kept as low as possible.

C. Pumping Energy Source Laser 30 implemented according to the present invention may be pumped by essentially any type of pump 10 including one or more laser diodes. Important considerations in selecting the implementation of pump 10 include output power and wavelength, and size and shape of pump 10 output.

Preferably, pump output power is as high as possible while meeting any requirements for reliability that may be imposed by the application. In addition, preferably the pump emits pumping energy with as small an optical divergence and as much optical brightness as is practical at the required output power. Suitable pumps include reduced-divergence broad-area lasers and flared semiconductor lasers.

Preferably, for pumping the 980 nm fiber laser mentioned above, pump 10 provides pumping energy within a range of wavelengths from about 910 nm to about 930 nm. In one embodiment, pump 10 is a 915 nm broad-area laser diode such as that embodied in an SDLO 4000 laser package available from SDL Optics, Inc. of British Columbia, Canada.

Size and shape of the pump output should be selected to optimize coupling efficiency. Ideally, the size and shape of the pump output should match the size and shape of the region into which pumping energy is to be launched. Unfortunately, a match in shape is usually difficult to achieve. For example, it is very difficult to match the shape of the inner cladding of a double-clad fiber laser with practical pump sources such as the SDLO 4000 laser package mentioned above.

Embodiments of practical broad-area lasers that are suitable for use as pump 10 emit light through a narrow stripe that is only few microns high and perhaps 60 to 120 μm wide. Generally, the intensity of the pump output is constant along the stripe width; therefore, the amount of pump power that can be launched into the cladding increases linearly with increasing stripe width. The cross-sectional area of the inner cladding, however, increases quadratically with increasing diameter. If the width of the pump output is exactly matched to the diameter of the inner cladding, the intensity of the pumping energy that is launched into the cladding decreases linearly with increasing stripe width

For example, a 60 μm wide broad-area laser that is rated at 17 mW/μm of stripe width will deliver half of the pump power as a 120 μm wide broad-area laser that is rated at 17 mW/μm, however, if the two pump lasers are coupled to a fiber cladding having a diameter exactly matched to the laser stripe width, the intensity of the pumping energy launched by the 60 μm wide laser will be twice as great

D. Coupler Optical pumping energy is conveyed from pump 10 to laser 30 by coupler 20, which may be implemented by essentially any form of optical coupling such as optical fiber or other type of optical waveguide, bulk or micro optics, graded-index (GRIN) lenses, or a direct butt-coupling between pump 10 and laser 30

Differences in size and shape between pump 10 and laser 30 can be accommodated to at least some extent by coupler 20 In one embodiment illustrated in Fig 11, coupler 20 is implemented by tapered fiber 21 that receives pumping energy from broad-area laser module 11 into its round cladding layer having a diameter that is commensurate with the width of the output of pump 10 The tapered fiber increases the intensity of the pumping energy by guiding it into a length of the inner cladding that has a smaller diameter that matches the diameter of the region in laser 30 that is to receive the pumping energy

Preferably, the tapered fiber increases the intensity of the pumping energy with no more than an insignificant loss of pump power This characteristic may be attained if proper attention is paid to the NA of the fiber The NA of a conventional silica-polymer fiber would usually not be a concern, however, if an all-silica fiber is used, more care may be required to achieve a sufficiently high NA

As explained above, an all-silica fiber may be desirable to obtain a fiber that has both a very small inner-cladding diameter, for example 50 μm or less, and sufficient mechanical reliability for routine handling Because silica normally has a higher refractive index than polymers, the all-silica design of the fiber may impose severe restrictions on the NA of the fiber As explained above, however, the refractive index of the core and cladding layers can be controlled by various dopants to obtain a NA on the order of 0 3 or so This NA may be sufficient to implement a coupler with a 2: 1 taper in cladding diameter, as shown in the specific embodiment discussed below.

In one embodiment, pump 10 is implemented by the SDLO 4000 laser package mentioned above. The output of this broad-area laser is coupled into a depressed-index cladding of a multimode round fiber having a NA of 0.30 and a cladding diameter substantially equal to the width of the broad-area laser output. In this example, the diameter of the cladding is 100 μm. Because the output of pump 10 fills only 0.14 of the available NA of the fiber, a fiber that is tapered to a 50μm cladding diameter will increase the NA requirements of the pump output to 0.28, which is still within the NA of the fiber. As a result, a 2: 1 taper can intensify the pumping energy with little if any loss of pump power during transmission. In this example, the cladding and NA of the tapered coupling fiber are designed to match the cladding of laser 30, allowing coupler 20 and laser 30 to be spliced with minimal loss. If desired, the cross section of coupler 20 may have a rectangular shape or some other shape that departs significantly from a circle. For example, coupler 20 with a generally rectangular or elliptical cross section may be used advantageously with an implementation of pump 10 that emits pumping energy in a rectangular or elliptical shaped beam.

Preferably, coupler 20 is designed to dissipate any energy received from laser 30 having a wavelength within a range from about 1020 nm to about 1120 nm.

Claims

CLAEVIS

1. A method for operation of a laser comprising a core region and a first region that is in optical proximity to the core region, wherein the first region contains atoms of a rare-earth dopant and the method comprises the steps of: receiving pumping energy at a first wavelength into the first region; confining the pumping energy such that at least one-fourth of the pumping energy is absorbed and such that the pumping energy that overlaps the rare-earth dopant atoms has an intensity throughout the first region that exceeds a threshold for achieving gain transparency at a second wavelength that is longer than the first wavelength; and emitting energy having the second wavelength.

2. A method according to claim 1 that receives the pumping energy into the first region at a first position, and reflects unabsorbed pumping energy received at a second position back toward the first position.

3. A method according to claim 1 or 2 that receives the pumping energy into the first region at a first end of the laser and at a second end of the laser.

4. A method according to any one of claims 1 through 3 that comprises dissipating energy at a third wavelength that is longer than the second wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength.

5. A method according to any one of claims 1 through 4 where the rare-earth dopant atoms are disposed nonuniformly in the first region and the method comprises limiting an overlap of the emitted energy having the second wavelength with the rare- earth dopant atoms to reduce net optical gain at a third wavelength that is longer than the second wavelength relative to net optical gain at the second wavelength.

6. A method according to any one of claims 1 through 5 where at least some of the atoms of rare-earth dopant are disposed immediately adjacent to the core region and the core region contains a first dopant that causes the core region to have an index of refraction equal to or greater than an index of refraction in the first region immediately adjacent to the core region

7 A method according to claim 6 where the first dopant increases photosensitivity of the core region and one or more gratings are formed in the core region

8 A method according to any one of claims 1 through 7 where the first region contains a second dopant that at least partially compensates for changes in the first region index of refraction caused by the rare-earth dopant

9 A method according to any one of claims 1 through 8 where the lasei comprises a glass second region that that is in optical proximity to the first region

10 A method according to claim 9 where the second region contains a third dopant that reduces the second region index of refraction

11 A method according to any one of claims 1 through 10 that comprises emitting the energy having the second wavelength from three-level operation of the laser, wherein the first wavelength is within a range from 910 to 930 nm, and the second wavelength is within a range from 970 to 980 nm

12 A method according to any one of claims 1 through 11 that comprises emitting the energy having the second wavelength from three-level operation of the laser and confining all or substantially all of the pumping energy to the first region that is made of glass and has a cross-section diameter that is 50 μm or less

13 A method according to any one of claims 1 through 12 that receives the pumping energy into the first region at two ends of the laser

14 A method according to any one of claims 1 through 13 that comprises reflecting the light emitted at the second wavelength between a first reflector having a level of reflectivity greater than or equal to 98% and a second reflector having a level of reflectivity greater than or equal to 70% and less than or equal to 95%

15 A method according to any one of claims 1 through 14 that comprises receiving the pumping energy from one or more sources, increasing intensity of the received pumping energy, and delivering the pumping energy with increased intensity to the first region

16 A laser powered by one or more pumping energy sources that emit pumping energy at a first wavelength, wherein the laser comprises a core region, and a first region that is in optical proximity to the core region and receives the pumping energy, wherein the first region contains atoms of a rare-earth dopant that have electrons capable of transitions that emit energy at a second wavelength that is longer than the first wavelength, and wherein the first region confines the pumping energy such that at least one-fourth of the pumping energy is absorbed and such that the pumping energy that overlaps the rare-earth dopant atoms has an intensity throughout the first region that exceeds a threshold for achieving gain transparency at a second wavelength that is longer than the first wavelength

17 A laser according to claim 16 that receives the pumping energy into the first region at a first position and comprises a reflector at a second position that reflects unabsorbed pumping energy back toward the first position

18 A laser according to claim 16 or 17 that comprises atoms of a dopant that dissipate energy at a third wavelength that is longer that the second wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength

19 A laser according to any one of claims 16 through 18 that is bent to dissipate energy at a third wavelength that is longer that the second wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength

20 A laser according to any one of claims 16 through 19 that comprises gratings that dissipate energy at a third wavelength that is longer that the second wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength

21 A laser according to any one of claims 16 through 20 wherein the rare- earth dopant atoms are disposed nonuniformly in the first region and the indices of refraction of the core region and the first region are arranged to limit an overlap of the emitted energy having the second wavelength with the rare-earth dopant atoms to reduce net optical gain at a third wavelength that is longer than the second wavelength relative to net optical gain at the second wavelength

22 A laser according to any one of claims 16 through 21 wherein at least some of the atoms of rare-earth dopant are disposed immediately adjacent to the core region and the core region contains atoms of another dopant that causes the core region to have an index of refraction equal to or greater than an index of refraction in the first region immediately adjacent to the core region

23 A laser according to any one of claims 16 through 22 that comprises one or more gratings formed in the core region, wherein the core region is photosensitive

24 A laser according to any one of claims 16 through 23 where the first region contains atoms of a dopant that at least partially compensate for changes in the first region index of refraction caused by the rare-earth dopant

25 A laser according to any one of claims 16 through 24 that comprises a glass second region that that is in optical proximity to the first region

26 A laser according to claim 25 where the second region contains atoms of a dopant that reduce the second region index of refraction

27 A laser according to any one of claims 16 through 26 that emits the energy having the second wavelength from three-level operation of the laser, wherein the first wavelength is within a range from 910 to 930 nm, and the second wavelength is within a range from 970 to 980 nm

28 A laser according to any one of claims 16 through 27 that emits the energy having the second wavelength from three-level operation of the laser, wherein indices of refraction of the core region and the first region are such that all or substantially all of the pumping energy is confined to the first region, which has a cross-section diameter that is 50 μm or less

29 A laser according to any one of claims 16 through 28 that comprises a lasing cavity formed between a first reflector having a level of reflectivity at the second wavelength that is greater than or equal to 98% and a second reflector having a level of reflectivity at the second wavelength that is greater than or equal to 70% and less than or equal to 95%

30 A laser according to any one of claims 16 through 29 that comprises a coupling waveguide that receives pumping energy from at least one of the one or more pumping energy sources, increases intensity of the received pumping energy, and delivers the pumping energy with increased intensity to the first region

31 A laser according to claim 30 wherein the coupling waveguide receives pumping energy at a first end and delivers the pumping energy at a second end, and wherein the coupling waveguide has a cross section at the first end that is substantially rectangular or elliptical

32 A laser according to claim 30 or 31 wherein the coupling waveguide receives pumping energy from a plurality of laser diodes

33. A laser that comprises a core region that contains atoms of a photosensitive dopant, and a first region that is in optical proximity to the core region that contains atoms of a rare-earth dopant, wherein refractive indices of the core region and the first region are such that the core region and the first region confine an optical mode that overlaps the core region and the rare-earth dopant atoms

34 A laser according to claim 33 wherein the photosensitive dopant is germanium

35 A laser according to claim 33 or 34 wherein the rare-earth dopant is ytterbium

36 A laser according to any one of claims 33 through 35 wherein the rare- earth dopant atoms are disposed nonuniformly within the first region

37 A laser according to any one of claims 33 through 36 wherein the rare- earth atoms are disposed within one or more cylindrical portions of the first region that surround the core region

38 A laser according to any one of claims 33 through 37 that comprises a second region that is in optical proximity to the first region and contains atoms of a dopant that reduces the second region refractive index

39 A laser according to claim 38 wherein the second region is made of glass

40 A laser according to any one of claims 33 through 39 wherein the first region has a first refractive index in one or more portions of the first region immediately adjacent to the core region that is increased by the rare-earth dopant atoms and the core region has a refractive index that is greater than or equal to the first refractive index

41 A laser according to claim 40 wherein the refractive index of the core region is increased by a disposition within the core region of atoms of germanium, aluminum or phosphorous

42 A laser according to any one of claims 33 through 41 wherein the first region has a first refractive index in one or more first portions of the first region that is higher than a second refractive index in one or more second portions of the first region that are farther from the core region than the one or more first portions

43 A laser according to any one of claims 33 through 42 wherein the first region contains atoms of a dopant that at least partially compensate for changes in the first region refractive index caused by the rare-earth dopant atoms

44 A laser according to any one of claims 33 through 43 wherein the first region has a cross-section diameter that is 50 microns or less

45 A laser according to claim 44 that comprises a second region that is in optical proximity to the first region, wherein the first region and the second region are made of glass

46 A laser according to any one of claims 33 through 45 that is powered by one or more pumping energy sources that emit pumping energy at a first wavelength, wherein the rare-earth dopant atoms have electron energy levels capable of emitting energy at a second wavelength that is longer than the first wavelength and at a third wavelength that is longer than the second wavelength

47 A laser according to claim 46 that comprises atoms of a dopant that dissipate energy at the third wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength

48 A laser according to claim 46 or 47 that is bent to dissipate energy at the third wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength

49. A laser according to any one of claims 46 through 48 that comprises gratings that dissipate energy at the third wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength.

50. A laser according to any one of claims 33 through 49 that comprises a lasing cavity formed between a first reflector having a level of reflectivity at the second wavelength that is greater than or equal to 98% and a second reflector having a level of reflectivity at the second wavelength that is greater than or equal to 70% and less than or equal to 95%.

51. A laser according to any one of claims 33 through 50 that comprises a coupling waveguide that receives pumping energy from a pumping energy source, increases intensity of the received pumping energy, and delivers the pumping energy with increased intensity to the first region.

52. A laser according to claim 51 wherein the coupling waveguide receives pumping energy at a first end and delivers the pumping energy at a second end, and wherein the coupling waveguide has a cross section at the first end that is substantially rectangular or elliptical.

53. A laser according to claim 51 or 52 wherein the coupling waveguide receives pumping energy from a plurality of laser diodes.

54. A laser that comprises: a core region; a first region that is in optical proximity to the core region and contains atoms of a rare-earth dopant, and that contains atoms of a first dopant that at least partially compensate for changes in the first region refractive index caused by the rare-earth dopant atoms; and a second region that is in optical proximity to the first region.

55. A laser according to claim 54 where the core region contains atoms of a dopant that are photosensitive.

56. A laser according to claim 54 or 55 wherein the rare-earth dopant atoms and the first dopant atoms are disposed together within one or more portions of the first region.

57. A laser according to any one of claims 54 through 56 wherein the rare- earth dopant atoms and the first dopant atoms are disposed in one or more distinct portions of the first region.

58. A laser according to any one of claims 54 through 57 wherein the rare- earth dopant is ytterbium.

59. A laser according to any one of claims 54 through 58 wherein the rare- earth dopant atoms are disposed nonuniformly within the first region.

60. A laser according to any one of claims 54 through 59 wherein the rare- earth atoms are disposed within one or more cylindrical portions of the first region that surround the core region.

61. A laser according to any one of claims 54 through 60 that comprises a second region that is in optical proximity to the first region and contains atoms of a dopant that reduces the second region refractive index.

62. A laser according to claim 61 wherein the second region is made of glass.

63. A laser according to any one of claims 54 through 62 wherein the first region has a first refractive index in one or more portions of the first region immediately adjacent to the core region that is increased by the rare-earth dopant atoms and the core region has a refractive index that is greater than or equal to the first refractive index.

64 A laser according to claim 63 wherein the refractive index of the core region is increased by a disposition within the core region of atoms of germanium, aluminum or phosphorous

65 A laser according to any one of claims 54 through 64 wherein the first region has a first refractive index in one or more first portions of the first region that is higher than a second refractive index in one or more second portions of the first region that are farther from the core region than the one or more first portions

66 A laser according to any one of claims 54 through 65 wherein the first region has a cross-section diameter that is 50 microns or less

67 A laser according to claim 66 that comprises a second region that is in optical proximity to the first region, wherein the first region and the second region are made of glass

68 A laser according to any one of claims 54 through 67 that is powered by one or more pumping energy sources that emit pumping energy at a first wavelength, wherein the rare-earth dopant atoms have electron energy levels capable of emitting energy at a second wavelength that is longer than the first wavelength and at a third wavelength that is longer than the second wavelength

69 A laser according to any one of claims 54 through 68 that comprises atoms of a dopant that dissipate energy at the third wavelength to reduce net optical gain at the third wavelength relative to net optical gam at the second wavelength

70 A laser according to any one of claim 54 through 69 that is bent to dissipate energy at the third wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength

71 A laser according to any one of claims 54 through 70 that comprises gratings that dissipate energy at the third wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength

72 A laser according to any one of claims 54 through 71 that comprises a lasing cavity formed between a first reflector having a level of reflectivity at the second wavelength that is greater than or equal to 98% and a second reflector having a level of reflectivity at the second wavelength that is greater than or equal to 70% and less than or equal to 95%

73 A laser according to any one of claims 54 through 72 that comprises a coupling waveguide that receives pumping energy from a pumping energy source, increases intensity of the received pumping energy, and delivers the pumping energy with increased intensity to the first region

74 A laser according to claim 73 wherein the coupling waveguide receives pumping energy at a first end and delivers the pumping energy at a second end, and wherein the coupling waveguide has a cross section at the first end that is substantially rectangular or elliptical

75 A laser according to claim 73 or 74 wherein the coupling waveguide receives pumping energy from a plurality of laser diodes

76 A laser powered by one or more pumping energy sources that emit pumping energy at a first wavelength, wherein the laser comprises a core region, first and second reflectors forming a lasing cavity at a second wavelength, and a first region that is in optical proximity to the core region and receives the pumping energy, wherein the first region contains atoms of a rare-earth dopant that have electrons capable of transitions that emit energy at the second wavelength that is longer than the first wavelength and at a third wavelength that is longer than the second wavelength, and wherein the atoms of the rare-earth dopant are disposed within the first region such that they provide an optical gain that is sufficient for the laser to achieve a lasing threshold for the lasing cavity at the second wavelength and limit amplified spontaneous emission at the third wavelength to a level that is no more than one-tenth the pumping energy.

77. A laser according to claim 76 that receives the pumping energy into the first region at a first position and comprises a reflector at a second position that reflects unabsorbed pumping energy back toward the first position.

78. A laser according to claim 76 or 77 that comprises atoms of a dopant that dissipate energy at the third wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength.

79. A laser according to any one of claims 76 through 78 that is bent to dissipate energy at the third wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength.

80. A laser according to any one of claims 76 through 79 that comprises gratings that dissipate energy at the third wavelength to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength.

81. A laser according to any one of claims 76 through 80 wherein the rare- earth dopant atoms are disposed nonuniformly in the first region and the indices of refraction of the core region and the first region are arranged to limit an overlap of the emitted energy having the second wavelength with the rare-earth dopant atoms to reduce net optical gain at the third wavelength relative to net optical gain at the second wavelength.

82. A laser according to any one of claims 76 through 81 wherein at least some of the atoms of rare-earth dopant are disposed immediately adjacent to the core region and the core region contains atoms of another dopant that causes the core region to have an index of refraction equal to or greater than an index of refraction in the first region immediately adjacent to the core region.

83 A laser according to any one of claims 76 through 82 that comprises one or more gratings formed in the core region, wherein the core region is photosensitive

84 A laser according to any one of claims 76 through 83 where the first region contains atoms of a dopant that at least partially compensate for changes in the first region index of refraction caused by the rare-earth dopant

85 A laser according to any one of claims 76 through 84 that comprises a glass second region that that is in optical proximity to the first region

86 A laser according to claim 85 where the second region contains atoms of a dopant that reduce the second region index of refraction

87 A laser according to any one of claims 76 through 86 that emits the energy having the second wavelength from three-level operation of the laser, wherein the first wavelength is within a range from 910 to 930 nm, the second wavelength is within a range from 970 to 980 nm, and the third wavelength is within a range from 1020 to 1120 nm

88 A laser according to any one of claims 76 through 87 that emits the energy having the second wavelength from three-level operation of the laser, wherein indices of refraction of the core region and the first region are such that all or substantially all of the pumping energy is confined to the first region, which has a cross-section diameter that is 50 μm or less

89 A laser according to any one of claims 76 through 88 wherein the first reflector has a level of reflectivity at the second wavelength that is greater than or equal to 98% and the second reflector has a level of reflectivity at the second wavelength that is greater than or equal to 70% and less than or equal to 95%

90 A laser according to any one of claims 76 through 89 that comprises a coupling waveguide that receives pumping energy from at least one of the one or more pumping energy sources, increases intensity of the received pumping energy, and delivers the pumping energy with increased intensity to the first region.

91. A laser according to claim 90 wherein the coupling waveguide receives pumping energy at a first end and delivers the pumping energy at a second end, and wherein the coupling waveguide has a cross section at the first end that is substantially rectangular or elliptical.

92. A laser according to claim 90 or 91 wherein the coupling waveguide receives pumping energy from a plurality of laser diodes.