WO2000041014A1

WO2000041014A1 - Distributed bragg reflectors with preferred properties in optical fibers

Info

Publication number: WO2000041014A1
Application number: PCT/US1999/030373
Authority: WO
Inventors: Ron Straayer
Original assignee: Alpha And Omega Imaging, Llc
Priority date: 1998-12-30
Filing date: 1999-12-20
Publication date: 2000-07-13

Abstract

A Bragg grating (16) is imaged in optical media as a series of longitudinally spaced fringes/zones, each located at a fixed longitudinal position in the core and having a measurably different index of refraction than the nominal index, wherein each fringe/zone is circularly symmetric about the longitudinal axis. Bragg gratings (16) formed in optical waveguide media can exhibit properties of a main wavelength transmission region and a secondary transmission region.

Description

"DISTRIBUTED BRAGG REFLECTORS WITH PREFERRED PROPERTIES IIM OPTICAL FIBERS"

Field of the Invention

The present invention pertains to passive optical filters formed with Bragg gratings and an innovative and preferred imaging technique for the fabrication of such devices. These devices when applied to other waveguide propagation articles such as optical fibers are used broadly in the fields such as telecommunication and remote sensing.

Background This invention concerns devices known as DBR's (distributed

Bragg reflectors) and the techniques and equipment used to manufacture such devices. DBR's are optical fibers or other media that have been modified by modulating the longitudinal index of refraction of the fiber core, cladding or both to form a pattern. This pattern is generally known as a Bragg grating or image. A fiber equipped Bragg grating functions to modify the optical passband of the fiber (transmission characteristic) in such a way as to only transmit a narrow and controlled wavelength band.

Such DBR is typically a "lossless" device. That is, the sum of the optical transmission and reflection is unity for all wavelengths of interest. A telecommunication system equipped with DBR's can divide a single fiber into multiple channels (40, 80, or more) by dividing the full passband of the fiber into discrete channels which are assigned to specific wavelengths. An optical fiber laser having a DBR terminated optical cavity is described by G.A. Ball and W.W. Morey in

"Continuously Tunable Single-Mode Erbium Fiber Laser", Optics Letters, 1 7 ( 1 992) pp.420-422.

These DBR's are very useful in fiber-based telecommunications because of their high selectivity to wavelength (channel selection), their stability, and their packaging factor that allows them to directly couple to the telecommunications fiber and its pump amplifiers. Such a system which use DBR's for pump radiation reflection in a telecommunication application with optical pumping via doped fiber amplifiers is described in U.S. Patent No. 5,21 8,655 (Mizrahi, et al) . Remote sensing systems with DBR's use the passive and very sensitive nature of the DBR passband characteristics to sense various physical phenomenons (temperature, pressure, vibration, chemical content, etc.).

DBR devices are fabricated by exposure to "actinic" radiation in an appropriate spectral range (typically UV) much like photoresists and the like. This actinic exposure causes the index of refraction to permanently change in the exposed portion of the medium (i.e., the core or cladding of the optical fiber. A period pattern can be formed on the surface of the fiber by superimposing two optical beams and forming an optical interference pattern. Two intersecting beams form a pattern whose grating period (distance between maxima) is given by

Λ =Λ/((2*sin (φ/2)) where λ is the optical wavelength and φ is the angle between the beams. Sufficient exposure to UV radiation in such an interference pattern will produce a Bragg grating within a fiber and thereby, a DBR. A technique for fabricating such DBR's is described in U.S. Patent No. 4,807,950 (Morey, et al).

DBR's fabricated with this technique are limited by several factors. First, the grating spacing is periodic. It is well known that quasiperiodic gratings (specifically "chirped" gratings) have preferred properties in certain applications. A chirped grating is a grating where the grating period changes (increases or decreases) monotonically down the fiber. Chirped gratings are useful in making broadband optical reflectors. Additionally, chirped gratings can be useful in removing undesirable wavelengths (and signals) as described in U.S. Patent No. 5,625,472 (Mizrahi, et al) . U.S. Patent No. 5,309,260 teaches that the normal exposure of DBR's made with the technique of U.S. Patent No. 4,807,950 will suffer from exhibiting one or more subsidiary peaks or a regularly spaced series of peaks which may adversely affect the operation of telecommunication systems or remote sensing systems with active wavelength stabilization.

DBR fabrication techniques according to U.S. Patent Nos. 4,807,950; 5,309,560; 5,388, 1 73; 4,807,950; 5,625,472, and 5,694,248 all teach lateral imaging of the fiber. This lateral exposure has the effect of inducing a polarization birefringence in the fiber that is undesirable. Furthermore, these are inherently incompatible with specialty fibers such as tapered index fibers which may require further non-linearities in the grating period and index change profile as a function of the longitudinal position down the fiber. These DBR fabrication techniques are also highly labor intensive, subject to high levels of "trial and error", and therefore unsuited for high production environments. DBR's produced with these methods are, therefore, of low yield and must be subject to stringent quality control testing.

Known DBR fabrication techniques do not afford the DBR designer automatic and arbitrary control over the transmission and phase response characteristics of the imaging system for rapid production of custom DBR's.

Summary of the Invention

The present invention includes a number of improvements that overcome these and other deficiencies with an imaging system that in the preferred embodiment:

1 . Allows the user wide control of the fringe spacing along the grating;

2. Allows the user wide control of the fringe amplitude along the grating; 3. Includes an interactive design station which defines the fringe spacing and amplitude characteristics in response to a user selected transmission target; 4. Is capable of producing DBR's in a "reel-to-reel" production environment;

5. Is capable of producing DBR's of different characteristics automatically;

6. Produces DBR's without tooling; and 7. Produces DBR's without birefringence.

In a broad system aspect of the invention, optical media is imaged by a combination of components including means for splitting a coherent beam into two co-propagating optical beams; means for converging the optical beams to produce a pattern of interference fringes having a pitch; means for varying the pitch of the interference pattern fringes; means for axially rotating the fringes around the media; and means for axially rotating the co-propagating optical beams so as to maintain rotary synchronism with the axially rotating fringes as the permanent fringes are formed in the media. Preferably, the system includes means for linearly translating the means for splitting, the means for converging, the means for rotating the fringes, and the means for rotating the optical beams, parallel to the grating direction. In this manner, relatively narrow interfering beams produce a relatively short length of fringes which can be "marched along" the media to produce a complete, relatively long fringe group, with varying properties.

In a preferred system and associated method for modulating the longitudinal index of refraction along an optical fiber that is positioned as a work piece along a workpiece axis, a source of actinic light is modulated to generate a beam having a known amplitude. The beam of modulated light is passed through a phase mask or similar component having the nominal pitch of the desired modulations to be formed in the fiber. From the beam passing through the phase mask or the like, at least two diffracted beams of nominally equal intensity are generated. All additional diffracted orders and the "zero" or undiffracted beams are blocked, (except the plus one and minus one order) . From the blocked diffracted beams, two collimated beams having a predetermined cross sectional shape, and a nominal separation distance are folded with a tiltable mirror or the like, to travel in a direction substantially parallel to the workpiece. The folded collimated beams are passed through a prism assembly or the like that is nominally centered on and is rotatable about the workpiece axis such that at any rotational angle about the workpiece axis the collimated beams emerge from the prism assembly with a nominal separation distance or gap between them. The optical fiber is fed through the gap and into a recording head assembly which surrounds the fiber, folds the collimated beams to project perpendicularly to the fiber, and focuses the beams within the fiber. The fiber is clamped on the workpiece axis and the phase mask, the prism assembly, and the recording head assembly are co-translated parallel to the workpiece axis. The prism assembly and the recording head assembly are co-rotated at different angular velocities (in ratio 1 :2), thereby maintaining rotary synchronism of the beams in the recording head assembly with the beams in the prism assembly. Preferably, the rate and distance of translation of the phase mask assembly, the prism assembly and recording head assembly; the angular velocities of rotation of the prism and the recording head; the amplitude of the modulated beam; and the degree of tilt of the tiltable mirror, are all independently variable during the modulation of fringe grid pattern in the optical fiber. The independently variable parameters identified immediately above, can, according to the preferred control system of the present invention, be specified in advance with a high degree of flexibility. For example, a plurality of identical, axially spaced apart gratings can be produced, followed by a plurality of gratings which have a very different design, all without human intervention or stoppage of the fabrication process. Moreover, the flexibility includes fabrication of a plurality of gratings, each of which is different from the other. As a final level of flexibility, within a grating pattern, the characteristics of individual fringe/zones can differ, according to a predetermined plan or design. Another advantage of the control system is that the settings for a particular grating pattern can be stored and readily retrieved for duplicating gratings which may have been fabricated at a considerably earlier time.

In a broad method embodiment for imaging light transmitting media oriented along a workpiece axis by directing two co-propagating coherent light beams having known beam amplitudes and beam pitch at the media transversely to the workpiece axis so that the beams interfere to produce a fringe pattern in the media having a commensurate fringe amplitude and fringe pitch, the improvement according to the present invention comprises as a first step, directing the co propagating coherent beams parallel to the workpiece axis. The beams are then redirected from a direction parallel to the workpiece axis to a direction perpendicular to the workpiece axis while maintaining the beam pitch. The beams are further redirected from a direction perpendicular to the workpiece axis to respective converging directions whereby the beams interfere to produce the fringe pattern in the media. The coherent beams and the converging beams are synchronously rotated about the workpiece axis while the rotating beams are translated relative to the media in parallel with the workpiece axis, thereby producing permanent fringes in the media, preferably having circular symmetry. This method permits amplitude control and, independently, pitch control of the fringes.

The foregoing novel method also permits the fabrication of novel optical fibers, in that the modulations of the longitudinal index of refraction of the fiber, are substantially circularly symmetric about the longitudinal axis of the fiber.

In particular, a passive optical filter can be fabricated, comprising a core of material having a first nominal index of refraction, and a cladding of material having a second nominal index of refraction, which surrounds the core and extends concentrically therewith along a longitudinal axis of the optical filter. A Bragg grating is imaged in the core, and defined by a series of longitudinally spaced fringes/zones, each fringe/zone located at a fixed longitudinal position in the core and having a measurably different index of refraction than the nominal index, wherein each fringe/zone is circularly symmetric about the longitudinal axis.

The invention further permits the fabrication of Bragg gratings formed in optical wave guide media that exhibit properties of a main wavelength transmission region and a secondary transmission region with a small additional sideband at the low wavelength and high wavelength side respectively.

Practitioners in this field of technology can readily appreciate that the present invention affords greater flexibility in design, and more efficient fabrication, than is currently available. Moreover, the fringes or zones constituting the grating, can be fabricated with circular symmetry, with great ease, thereby producing DBRs which do not exhibit birefringences.

Brief Description of the Drawings

The preferred embodiments and other objects and advantages of the invention will be described below with reference to the accompanying drawings wherein:

FIG. 1 is a schematic representation of a DBR device installed between two optical fibers; FIG. 2 is a section view of the DBR device, taken through a fringe maximum along line 2-2 of Figure 1 ;

FIG. 3 is an enlarged view of the portion of the core and surrounding cladding, showing in diagrammatic form, the fringe maxima and minima forming part of the fringe pattern in the grating of Figure 1 ; FIG. 4 is a cross-section view of the core portion of the DBR device of Figure 2, having a radius R_{1 ;}-

FIG. 5 is a graphic representation of monotonic variations in the index of refraction of each zone in the core, as a function of the radial distance from the longitudinal axis to the radius R^- FIG. 6 is a graphic representation of more complex variations of the index of refraction as a function of radial distance from the longitudinal axis;

FIG. 7 is a schematic representation of a system according to the invention, for imaging circularly symmetric DBR images on an optical fiber that is fed from its supply reel to a take up reel;

FIGs. 8 and 8A are detailed views of the tiltable mirror portion of the system shown in Figure 7;

FIG. 9 is a graphic representation of the use of the system shown in Figure 7 to produce a monotonically decreasing (chirped) fringe pitch along the grating direction;

FIG. 10 is a graphic representation of the cumulative fringes added to the nominal pitch phase mask to create the chirped pattern represented in Figure 9;

FIG. 1 1 is a top view of a dove prism assembly component of the system as depicted in Figure 7; FIG. 12 is a side elevation view of the dove prism of Figure 1 1 ;

FIG. 13 is a side elevation view schematic of the recording head assembly component of the system shown in Figure 7;

FIG. 14 is a schematic representation of the fabrication of custom DBR grating devices, with each device receiving a marker while it is still part of the continuous fiber played from the supply reel to the take-up reel;

FIG. 15 is a schematic representation of the components associated with a design and control sub-system for the imaging system shown in Figure 7;

FIG. 16 is a schematic of an alternative recording head assembly for use with imaging multiple fibers;

FIG. 17 is a graphic representation of one transmission characteristic that can be achieved with the imaging according to the present invention; and

FIG. 18 is a graphic representation of a second transmission characteristic usable in conjunction with the transmission pattern shown in Figure 1 7, for together achieving a particularly advantageous pattern for use in communications systems.

Description of the Preferred Embodiments

Figures 1 -4 show a DBR device 1 0 which in a typical end use, consists of a core 1 2 having a diameter D 1 of approximately 5 microns, and a first nominal optical index of refraction (η^ = 1 .46), surrounded by a cladding 1 4 having an outer diameter D₂ of approximately 1 25 microns, and a second nominal index of refraction (r/₂ = 1 .45) . The device may be on the order of several centimeters in length, but the Bragg grating 1 6 typically extends along the longitudinal axis of the device, for a distance L of 6-1 Omm. Conventionally, the pitch Λ , i.e., distance between fringe maxima 1 8a, 1 8b, is on the order of 0.5 micron. Thus, a DBR grid according to the present invention, can have from 1 2,000 to 20,000 fringes formed along length L, preferably with varying amplitude and pitch as will be described in greater detail below. As is known conventionally, actinic beams directed at an angle σ to the perpendicular passing through a fiber, interfere constructively and destructively to form respective fringe maxima 1 8a, 1 8b and fringe minima 20a, 20b at the surface of (and penetrating into) the fiber core. If the raw or nominal index r/₁ of the core is 1 .460000, the fringe maxima will have indexes of, for example, 1 .460080 to 1 .460090, whereas the fringe minima will have indexes that are much closer to the raw or nominal index of 1 .46000. As depicted in Figure 3, the maxima 1 8 and minima 20 are spaced axially by one half of the fringe pitch. The index variation as one moves in the axial direction between the maxima and minima, is not necessarily sinusoidal, but zones of high index are measurably different from the nominal so that individual fringe regions or zones 1 8a, 1 8b are identifiable.

Whereas conventionally, these fringe maxima are located within, for example, about one quadrant of the cross-section of a fiber core, according to the present invention as shown in Figure 2, these fringes 1 8a can be formed circularly symmetric, i.e., the maxima lie in co-planar circles or annuli which are perpendicular to the axis of the fiber. Figures 4, 5, and 6 include graphic representations showing alternatives available for producing indexes of refraction that have variations in magnitude which depend (e.g., functionally) on the radius r from the axis 22 to the outer radius R^ of the core 1 2. Variation A is constant with radius. Variation B is simply a monotonic, e.g., substantially linear, increase from the axis to radius R Variation C monotonically decreases in value. Variation D shows an increase from the core axis to a peak at the radial position r' substantially equal to Vi R_v with a return at radius R₁₇ to the value at the axis. Variation E is the mirror image of variation D, with the lowest value of the index of the fringe, rather than the highest, appearing at position r' . It should be appreciated that even the lowest index values represented in Figures 5 and 6, are nevertheless higher than the nominal index of the core. Each of the variations shown in Figures 5 and 6, depends on the distance in the direction r shown in Figure 4, keeping in mind that the variable r shown in Figure 4 projects along a substantially disc-like, planar, preferably circular surface of a zone which is perpendicular to the axis of the fiber or device. In Figure 3, each of the zones 1 8a, 1 8b defined by the fringe maxima is shown as a vertical solid line. If each zone in Figure 3 had a variation of the type shown as curve D in Figure 6, then the maximum index in each zone, would be located at substantially the same radial distance r' from the axis.

It should be understood that in the present description, a sequence of fringe maxima such as the four spanned by the interval 24 shown in Figure 3, can be considered a group of zones. The optical fiber or device 10 can therefore be considered as having modulations in the longitudinal index of refraction, wherein the modulations are defined by a group of longitudinally spaced zones, each one of the zones having a measurably different index of refraction than the

"spaces" 26 on either longitudinal side of each zone. As shown in Figure 3, the spaces between the zones can be uniform, but as will be described with greater detail below, these spaces can with the present invention be made non-uniform. These zones are contiguous and seamless with respect to each other. This invention uniquely offers control over both fringe pitch and amplitude on a group-to-group basis.

The following description of implementing hardware is made with reference to Figures 7-1 2 and pertains to direct imaging of an optical fiber via a traveling interferometer imaging system 1 00. By logical extension, this same system and technique could be used to image other optical media either by direct exposure or by exposure of a photoresist coating followed by conventional lithographic processing. In the preferred embodiment as represented in Figure 7 an actinic laser light source 102 is used. This is a source of semi-coherent laser light in the appropriate UV portion of the spectrum. By further illustration, this might be a diode pumped, frequency quadrupled, coherent light output of a Vandate or Yttrium Aluminum Garnet (YAG) laser crystal followed by a quadrupling crystal (or equivalently two serial doubling crystals) which emits 250 milliWatts of power at 266 nanometers in continuous wave (CW) output mode. Alternatively, frequency doubled dye lasers pumped by an Excimer laser can be used effectively to expose these fibers with actinic radiation. This preferred laser light source could be replaced with alternative sources, including pulsed lasers, which have sufficient coherence, brightness, and spectral distribution for this application.

A modulator 104 follows the laser, to control the amplitude of the light 1 06 under the synchronized direction of the system controller (Figure 1 5) . The preferred modulation device 1 04 is an acousto-optic modulator fabricated from fused quartz. This device can be operated in either the "0" order or loss modulation mode or in the + 1 order or gain modulation mode. This device would be characterized by typical optical efficiencies exceeding 85% and modulation bandwidths exceeding 1 00 KHz.

The light beam output 108 of the modulator 1 04 is folded 1 10 by mirror M 1 and directed at spherical lens L1 and cylindrical lens C1 .

These lenses form an anamorphic spot on the surface of phase mask 1 1 2. In the preferred embodiment, the focal length of these lenses is selected to form an optical spot of Gaussian intensity distribution (in both axes) at a size of about 1 5 microns in the longitudinal (or grating) direction 1 1 4 followed by about 50 microns in lateral (or cross-grating) direction. Those skilled in the art will recognize that there are many combinations of lenses and optical beam diameters which will accomplish such a result and that those preferred results can be further modified and adjusted to accomplish desired variations of implementation. This mask 1 1 2 lies in an orientation parallel to the grating direction and is an optical grating at the nominal pitch of the resultant DBR to be produced in fiber 1 1 6. In the preferred embodiment the mask consists of sinusoidally alternating thickness variations (i.e. mask fringes) at twice the nominal pitch Λ_oof the DBR. This mask generates two equal intensity diffracted optical beams 1 1 8a, 1 1 8b.

Alternatively, those familiar with the art will recognize that this mask 1 1 2 could be constructed of alternating indexes of refraction, a triangularly or square varying thickness pattern, or constructed as an amplitude mask. An aperture stop 1 20 follows the mask to block all diffracted orders other than the desired + 1 and - 1 order.

In the preferred embodiment, all of the optical elements with exception of the phase mask 1 1 2 are fixed in position. The phase mask (together with the rotating dove prism assembly and rotating record head assembly to be described subsequently) are mounted on a translating slide assembly (TSA) 1 22 which provides motion in a single axis in the direction 1 14 of the gratings.

With reference also to Figure 8, in the preferred embodiment, a second spherical lens L2 follows the phase mask 1 1 2. This lens (nominal focal length of 1 5 mm) is selected to form two collimated beams 1 26a, 1 26b of nominally 0.34 mm in the grating direction and nominally 0.1 mm in the cross grating direction. A constant nominal distance of 8.3 mm separates the centerlines of these two beams. This lens is followed by another fold mirror, M2, which folds the optical path by 90°to be nominally parallel to the direction 1 1 4 of the translating slide assembly. In this preferred embodiment all of the optical elements L2 and M2 are nominally stationary in position and do not translate on the TSA.

Mirror M2 is equipped with a controllable tilt feature such that the angle of reflection in the grating direction can be controlled. This control can be implemented in extremely small increments using available technology such as the Newport U200-A mirror mount equipped with a Newport ESA micromanipulator under the control of a Newport ESA-C driver which is interfaced to the system controller (Figure 1 5) . During the advance of the phase mask through the illumination spot 1 28, changing the tilt M2 in the phase mask motion direction 1 30 via the micromanipulator has the effect of retarding the apparent motion of the phase mask fringes 1 34. Conversely, changing the tilt of M2 opposite 1 32 to the motion direction has the effect of advancing the apparent motion of fringes. For purposes of illustration in this preferred embodiment, let the pitch of the phase mask, 2Λ_o, be nominally constant at 1 .0 microns, the length L of the DBR be 5 mm, and let L2 have a 1 5 mm focal length. In this illustration the goal of imaging the desired DBR is to produce a grating with a linear 2% (or 300 Angstrom) chirp centered about 0.5 microns. Therefore, this DBR will have a pitch linearly varying from 2020 cycles/mm to 1 980 cycles/mm over the length of 5 mm, as illustrated in Figure 9. In order to fabricate this chirped DBR, DBR fringes will need to be added at the leading end (2020 c/mm) at a progressively slower rate until no fringes are added at the center crossing followed by fringes being retarded starting at the center and increasing until the 1 980 c/mm end is reached. The number of cumulative fringes added to the constant 0.5 mm pitch phase mask to create the chirped DBR is shown in Figure 1 0. Twenty five DBR fringes is the maximum number of fringes that were required to be added in this illustration. The M2 angular change that is required to add a DBR fringe is given by:

φ = atan (2Λ/F₂) where φ is the angular increment for a single DBR fringe change 2Λ is the pitch of the phase mask and

F₂ is the focal length of the lens

In the preferred embodiment this angular increment is approximately 33 microradians for a one fringe change. This preferred configuration is able to provide angular adjustment at a much finer increment (e.g., 0.1 microradian) . Then, during the travel of the TSA, the mirror M2 will be adjusted in these small increments creating a smooth variation of the adjusted pitch of the fabricated DBR. This pitch is adjusted during fabrication under the control of a system controller that is using pre-calculated values for the fringe amplitude and spacing and for the motion of the mirror M2 that controls that fringe spacing.

This small incremental motion of M2 will support the fabrication of a 1 angstrom chirped DBR with a linearity error of less than 1 % .

It is significant to note that motion of mirror M2 is required to modify the fringe spacing. If the mirror M2 is left stationary in any position, there will be no adjustment to the recorded fringe spacing over the "raw" fringe spacing of the phase mask. Those familiar with the art will recognize that this fringe controlling feature of mirror M2 can be implemented in a variety of alternative ways such as but not limited to rotating plane parallel plates located between the focused spot 1 28 on the phase mask 1 1 2 and L2, rotating prism assemblies, translatable lens

L2, and controlled micromovement of the phase mask with respect to the TSA. In the preferred embodiment, there is a common linear motion of the phase mask 1 1 2, the dove prism assembly 1 36, and the rotating head assembly 138. Alternatively, motion of the phase mask can be replaced with the motion of mirror M 1 , lens L2, cylinder lens C 1 , lens L2, and controllable mirror M2, all in combination.

With further reference to Figures 1 1 and 1 2, the two diffracted beams 1 26a, 1 26b then enter a dove prism assembly (DPA) 1 36. This

DPA is nominally centered on the optical axis 1 39 of the fiber 1 1 6 as positioned 1 1 6a for creation of the DBR fringes. The DPA 1 36 is mounted to and within a motor 140 (indicated in phantom lines) that provides rotation of the DPA about the nominal optical axis. (Such rotation is also referred to herein as "axial rotation") . This motion is supported by a motor driver and is under the synchronized control of the system microcontroller. This motor and DPA are further attached to the translating slide assembly 1 22 that provides motion 1 1 4 in the z-axis (i.e., parallel to workpiece axis) . At any rotational angle of the DPA, the collimated beams 126a, 126b emerge from the DPA with a minimum separation or gap 142 therebetween.

Figure 1 3 further illustrates the preferred embodiment of the recording head assembly (RHA) 1 38. In this embodiment, the RHA comprises another spherical lens, L3, and a series of right angle prisms P1 , P2, P3 that fold the optical path as shown. The lens, L3, is selected to have a nominally matching focal length to the previous spherical lens, L2, which is 1 5 mm in the preferred embodiment. The spacings, thicknesses, and positions of the lens and prisms are selected so that the beams 148a, 1 48b converge at angles a and interfere to form an optical spot 1 46 of nominal size 1 5 microns in the grating direction and 50 microns in the cross grating direction within the center of the nominal 100 micron fiber 1 1 6a. The relatively large depth of optical focus provides a high degree of immunity to varying fiber diameters and mis-position of the fiber or beam. The preferred RHA configuration illustrated in Figure 1 3 could be modified by those skilled in the art with a variety of combinations which replace prisms with mirrors, move the location of the lens along the optical path including between prisms or mirrors, and which replace the function of the lens with a holographic optical element, curved mirror, gradient index optic, or other element which causes the light to come to an optical focus.

Figure 7 for the preferred embodiment illustrates the relative positions and mounting of the phase mask 1 1 2, the rotating dove prism assembly 1 36, and the rotating recording head assembly. These are co- mounted on the translating slide assembly 1 22 and thereby move commonly and together as a unit. The translating slide assembly in the preferred embodiment includes a compressed air supported carriage traveling along a linear guide rail together with a forcing or driving linear motor. Those familiar with this art will recognize that such linear TSA's, together with the linear motors, drive electronics, and motion controllers are commercially available from manufacturers such as Dover

Precision, Anorad, and NEAT, among others. The linear position of the carriage is continuously measured by a linear interferometer such as the Zygo Axiom or equivalent that provides position feedback to a precision of 2.5 x 1 0^"3 microns. This TSA motion system is under the control of the system microcontroller (Figure 1 5) .

Those familiar with the art of optical interferometry will recognize that the co-location of these elements as illustrated in the preferred embodiment is particularly useful. When the rotating dove prism assembly 1 36 is driven at an angular velocity equal to one half the angular velocity of the rotating record head assembly 1 38 and is phase locked appropriately, the fringes formed at 1 46 by the imaging process are fixed in position both during the rotation of the RHA around the fiber and during the motion of the TSA. Moreover, the fixity of the fringes is unaffected by the velocity or position of the TSA carriage along the rail. A recognized limitation of this approach of a traveling interferometer is the size of the traveling spot 146 as opposed to the extent of change of the fringe pitch constant. If the rate of change of the fringes generated by the interference of these actinic beams is excessive, the incoherent addition of the fringes will cause smearing or erasure of the newly written fringes as the "tail" of the overlap region passes over it. As a rough guideline:

(dΛ/Λ)ω spot < Λ/4 where c Λ/Λ is the pitch rate of change ω spot is the spot diameter (e ²) and

Λ is the pitch

In the case of the preferred embodiment, this guideline places an upper limit of 3% on the pitch rate change, which is substantially more than is required for typical quasiperiodic gratings. This system is configured in the preferred embodiment to produce quasiperiod (or chirped) gratings with chirp rates which vary between 1 Angstrom and 200 Angstroms along the length of the DBR (nominally 1 0 mm in length) with an error in chirp linearity of substantially less than 5 %. The previously described angularly actuated adjustable mirror M2 provides this control of pitch change over the range of at least - 1 .3% to + 1 .3% . Since this upper rate of change can be applied nominally every 1 5 microns (the size of the optical spot 1 46 in the grating direction) in this preferred embodiment, this limitation is inconsequential to the application of imaging DBR's in practice. In fact, this preferred embodiment exhibits the control and freedom to fabricate DBR's having specific desired transmission response characteristics without practical limitation.

In Figure 1 3 of the preferred embodiment, the RHA 1 38 is shown to have a clear access aperture or bore 1 50 which extends along the optical (z) axis 1 39. This clear aperture continues through both the drive motor and the rotary feedback encoders so as to provide effective mechanical clearance about the optical axis. This clearance, in the preferred embodiment, is assigned to provide effective "threading" of the optical fiber portion 1 1 6b along the z-axis. The gap in the collimated beams upstream of the RHA also accommodates the fiber. In this configuration, the fiber portion 1 1 6a is clamped in position during the writing operation, and advanced upon completion. As is typical of DBR's, the exposed fiber is approximately 6 to 1 0 mm in length which sets the required travel range of the TSA. In this preferred embodiment, the modulator would be "blanked" during the period when either one of the rotating pair of beams encounters the optical fiber portion 1 1 6c laterally crossing its path (see Figure 7) . Figures 7 and 14 show the details of a device for marking the fiber in the "reel-to-reel" automated fabrication system of Figure 7, with a code which indicates the demarcation between imaged DBR's along the fiber 1 1 6, the type of DBR which is adjacent to the mark, and additional markings as appropriate. These additional marks may be used to indicate the type of fiber, date of manufacture, plant where fabricated, channel assignment, customer, or other manufacturing data as required by the DBR fabricator. The preferred embodiment utilizes an ink jet marker 1 56, by which control of color and dot placement are readily adaptable to such marking requirements. The general sequence of imaging in series along a continuous fiber 1 1 6 advanced in direction

1 59 from supply reel 1 52 to take-up reel 1 54 is that ( 1 ) the fiber portion 1 1 6a is undamped and advanced by some set distance sufficiently large for the fabrication of a DBR, marking, and subsequent connector terminations (e.g., 1 00 mm); (2) the fiber portion 1 1 6a is clamped in position; and (3) the fiber portion is imaged and marked (a DBR is fabricated) .

It is also understood by those familiar with the art that additional alternatives are equally feasible. These include the less desirable option of replacing the motion of the DPA and the RHA with motion of the actual fiber.

As previously described, the position and registration of the fringe is independent of the speed and position of the TSA 1 22. In the preferred embodiment, the RHA 1 38 and the DPA 1 36 will be in constant rotation during the record process, thereby ensuring a uniform and smooth exposure around the fiber (i.e., it is a helical recording process) . The advance (or pitch) of this helix is a variable which can be chosen at will. Those familiar with the art will recognize that the system as described will record DBR's effectively across a wide range of helix pitches: from 0.1 micron of RHA 1 38 advance per revolution to infinity (no RHA rotation during recording).

For purposes of illustration with reference to Figure 7, the helix pitch will be assumed as set to 1 micron. Further, the 250 mW UV laser 1 02 will support an exposure rate of approximately 0.1 mm/sec. Therefore, in this embodiment, the RHA rotational rate will be 1 00 revolutions/sec and the DPA will be 50 revolutions/sec. The typical recording time for a 6 mm DBR will be 1 minute. Further, the preferred embodiment will assign values to the pitch and amplitude matrix of the specific DBR design to be fabricated every 1 micron of longitudinal travel. The data that controls the fringe pitch and the amplitude of the recorded fringes longitudinally along the DBR is provided by the design system.

This design system 200, illustrated in Figure 1 5, is implemented with computer programs residing on a computer workstation 202. For purposes of illustration, this workstation has a Pentium or Pentium II class of processor running under Microsoft Windows NT or similar operating system. The purpose of the design system is to provide the designer a flexible environment by which a DBR with desired properties of transmission response can be effectively designed for the application of interest in telecommunications or remote sensing or some other field of interest. A further objective is to maintain a set of imaging performance data for the imaging system herein described when operated at a specific system setting which represents a combination of fiber type, exposure settings, record linear velocity, pitch helix, and other relevant parameters. This set of imaging performance data is used both to quickly return the system to a known working setting and also to predict the response of the imaging system 1 00 to the forthcoming DBR design. This predicted system response (imaging system modulation transfer function or MTF to those familiar with this art) is developed with calibration operations during the setup of the imaging system for specific optical fibers. This predicted imaging system MTF is of great utility since the design system software application can use it effectively to produce DBR's which must more closely meet the performance objectives of the designer.

It may be appreciated that the control system 200 would typically include specially adapted driver programs, in particular, driver 21 0 for the modulator 1 04; driver 21 2 for operating the actuator 1 60 on tilt mirror M2; driver 21 4 for operating the DBA motor 1 40; driver 21 6 for operating the RHA motor 1 58; and driver 21 8 for operating the TSA 1 22. It is well within the skill of the ordinary practitioner in this field to acquire and if necessary program the drivers identified above, and to acquire and connect the equipment and components operated by the drivers. Similarly, any computer programming that may be required to define the user interface and the linkages among the various drivers and components, would be within the ordinary skill of such persons. By way of further illustration in the preferred embodiment the sequence of production is as follows:

1 -The system is configured through the design application menu and the variables of fiber type, exposure settings, record linear velocity, pitch helix, and other relevant parameters are selected. A set of all these selected variables are kept in a record in "setup" file stored at

204. Multiple setups can be configured and characterized at will.

2-A test recording of a DBR 206 is made on a section of the same optical fiber as intended for production, at a specific setup

(combination of fiber type, exposure settings, record linear velocity, pitch helix, and other relevant parameters as selected) using an available set of DBR pitch and intensity data from a calibration design DBR 208.

3-The calibration design DBR 208 at this setup is measured and characterized using an instrument such as a HP external cavity tunable laser source Model 81 68A/F and power detector. 4-The actual performance data of DBR 206 is compared with the calibration DBR 208 design and the system MTF is calculated for this setup.

5-This calculated MTF is then used to predict the performance of the subsequent production DBR when fabricated with this setup. 6-The MTF can be further used to adjust the production DBR pitch and amplitude data using Fourier transform processing calculations as known in the art. This will force the DBR as produced at this setup to more closely match the DBR design target.

This technique as outlined herein can be effective at designing and quickly producing custom DBR's with quasiperiod gratings. This technique is particularly effective at reducing the "trial and error" of the earlier manually intensive techniques.

A first alternative travelling interferometer system is similar in all regards to the system described with respect to Figures 7 and 1 3, except the RHA 1 38 is modified. This modification allows the focus of the spot 146 to be formed not on the optical axis 1 38, but on the surface of an imaginary cylinder. This RHA 1 60 is depicted in Figure 1 6. The imaging geometry allows for the imaging of multiple fibers 1 62, 1 64, 1 66, 1 68 arranged side-by-side around the surface of a cylinder 1 70. The traveling interferometer is configured to have the focus of the spot formed by the two beams nominally aligned with the center 1 72 of the fibers distributed around the circumference of the cylinder. During the imaging operation all of these optical fibers are clamped into fixed position by a clamping mechanism (not shown) . The alternative system is also distinguished in the routing of the optical fibers into the imaging station. The system of Figure 7 requires a coaxial routing of a single fiber. The alternative system does not use coaxial routing and additionally images multiple fibers. Therefore, the RHA does not require mechanical clearance about the optical axis. The RHA configuration illustrated in Figure 1 6 could be modified by those skilled in the art with a variety of combinations which replace the prism with a mirror, move the location of the lens along the optical path including before the prism or mirror, or replace the function of the lens with a holographic optical element, curved mirror, gradient index optic, or other element which causes the light to come to an optical focus.

Those familiar with the art will also recognize that because substantially more area is optically scanned by the alternative configuration, for the same longitudinal motion of the TSA, there is a higher requirement for optical power. However, this does not present a problem in view of recent advances in available coherent laser power in this UV spectrum and in the construction and sensitivity of optical fibers.

A second alternative system is similar in all regards to the first alternative system, except the scan arc is limited in extent. Instead of the full circumference of a circle, the scan arc can be in the range of, e.g., 90° - 270° to accommodate a more limited quantity of optical fibers to be imaged as DBR's. This system can include a "back and forth" sweeping motion, where the scanner stops and reverses direction at the end of each scan. During the time of stopping and reversing of direction, the TSA can be incrementally advanced. Those familiar with the art will recognize that this imaging process can be implemented by a wide variety of techniques, e.g., monodirectionally, bidirectionally, sinusoidally, step and settle, etc.

Further advances in telecommunications, remote sensing and other applications of Wavelength Division Multiplexing (WDM) technology are anticipated which will require complex and intricate control of the desired passband transmission characteristic. An example of this is given in Figures 1 7 and 1 8 which show a center passband with a small additional sideband at the low wavelength and high wavelength side, respectively. A pair of DBR's of this type could be highly effective in sensing drift in the center wavelength and then enabling a closed-loop correction of the center wavelength as understood by those who are familiar and skilled in this art. The transmission in the main region is greater than 90%, has a wavelength range of transmission greater than 5nm, and an optical transmission attenuation outside the main region greater than 20db, followed by a secondary transmission region separated from the first region by a distance approximately equal to the width of the main transmission region. The secondary transmission region is characterized by a secondary transmitted wavelength that is either longer or shorter than the wavelength transmitted in main transmission region, a wavelength range of transmission that is no greater than that of the main region, and a secondary transmission attenuation in the secondary region which is less than 20db from the main transmission region.

In another embodiment gratings are formed in optical wave guide media and exhibit the properties of a main wave length transmission region and a secondary "band stop" non-transmission region. Transmission in the main region is greater than 90% over a wavelength range greater than 5nm, with an optical transmission attenuation outside the region greater than 20db. A secondary "band stop" non- transmission region within the wave length range of the main region is provided wherein the band stop non-transmission wave length range is less than 1 nm and the peak attenuation is greater than 1 5 db.

Those familiar with this technology would recognize additional benefits of the present invention. A DBR made according to the invention could be readily designed to provide compensation for the non-linear gain of an Erbium fiber pump laser as is commonly used in long-haul telecommunication systems. A DBR according to the invention could be readily designed to provide compensation for dispersion of signals in the present long-haul telecommunication systems. For potential additional advantages that may arise in the use of DBRs fabricated in polarization preserving but none-symmetrical fibers, the exposure amplitude according to the invention could be readily controlled as a function of the radial position around the fiber (as well as the longitudinal position along the fiber) thereby producing, e.g., an elliptical or other shape.

Claims

I CLAIM:

1 . An optical fiber having a longitudinal axis and a modulated longitudinal index of refraction, wherein the improvement comprises that the modulations are substantially circularly symmetric about the longitudinal axis.

2. The optical fiber of claim 1 , wherein the index of refraction of each modulation varies with radial distance from the axis.

3. The optical fiber of claim 2, wherein each modulation has a maximum index of refraction and the maximum for each modulation is located at substantially the same radial distance from the axis.

4. The optical fiber of claim 1 , wherein the modulations are defined by a group of longitudinally spaced zones, each one of the zones having a measurably different index of refraction than the spaces on either longitudinal side of said one zone.

5. The optical fiber of claim 4, wherein each measurably different index of refraction of a respective zone is greater than the index of the respective spaces.

6. The optical fiber of claim 4, wherein the spacing between the zones of a group is uniform.

7. The optical fiber of claim 4, wherein the spacing between the zones of a group is nonuniform.

8. The optical fiber of claim 4, wherein each zone has a maximum index of refraction and the maximum of each zone is located at substantially the same radial distance from the longitudinal axis.

9. The optical fiber of claim 4, wherein each zone of a group is substantially disc-shaped, and the index of refraction varies with radial distance from the axis.

1 0. The optical fiber of claim 9, wherein each zone comprises a plurality of substantially coplanar regions, each region having a measurably different uniform index of refraction.

1 1 . The optical fiber of claim 4, wherein each zone of a group is substantially annular, and the index of refraction varies with radial distance from the axis.

1 2. An optical waveguide having Bragg gratings, comprising: a main wavelength transmission region in which the transmission is greater than 90%, the wavelength range of transmission is greater than 5nm, and the optical transmission attenuation outside the main transmission region is greater than 20db; and a secondary transmission region separated from the first transmission region by a distance approximately equal to the width of the main transmission region, having a secondary transmitted wavelength that is measurably longer or shorter than the wavelength transmitted in the main transition region, a secondary wavelength range of transmission that is no greater than the wavelength range of transmission of the first transmission region, and a secondary transmission attenuation which is less than 20db from the main transmission region.

1 3. The optical waveguide of claim 1 2, wherein the Bragg gratings are defined by fringes which are substantially circularly symmetric.

14. An optical waveguide having Bragg gratings, comprising: a main wavelength transmission region having a transmission greater than 90%, a wavelength range of transmission greater than 5nm, and an optical transmission attenuation outside the main region greater than 20 db; and a secondary non-transmission region within the wavelength range of the main region, having a wavelength range less than 1 nm and a peak attenuation greater than 1 5db.

1 5. The optical waveguide of claim 14, wherein the Bragg gratings are defined by fringes which are substantially circularly symmetric.

1 6. A passive optical filter, comprising: a core of material having a core nominal index of refraction; a cladding of material having a cladding nominal index of refraction, surrounding the core and extending concentrically therewith along a longitudinal axis; a Bragg grating in said core, defined by a group of longitudinally spaced zones, each zone located at a fixed longitudinal position in the core and having a measurably different index of refraction than said core nominal index, said group of zones extending along a length L; wherein each zone is circularly symmetric about said longitudinal axis.

1 7. The optical filter of claim 1 6, wherein each zone has a radial dimension and the index of refraction of each zone varies as a function of radial position within the zone.

1 8. The optical filter of claim 1 7, wherein the index of refraction is a non-linear function of radial position.