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`
`US 20050207454/X1
`
`(19) United States
`(12) Patent Application Publication
`Starodoumov et al.
`
`(10) Pub. No.: US 2005/0207454 A1
`(43) Pub. Date:
`Sep. 22, 2005
`
`(54) WAVELENGTH STABILIZED DIODE-LASER
`ARRAY
`
`(76)
`
`Inventors: Andrei Starodoumov, Cupertino, CA
`(US); Murray Keith Reed, Menlo
`Park, CA (US)
`
`Correspondence Address:
`STALLMAN & POLLOCK LLP
`SUITE 2200
`.
`353 SACRAMENTO STREET
`SAN FRANCISCO, CA 94111 (US)
`
`(21) Appl. No.:
`
`10/802,616
`
`(22)
`
`Filed:
`
`Mar. 16, 2004
`
`Publication Classification
`
`(51)
`
`Int. Cl? ..................................................... .. H01S 3/30
`
`(52) U.S. Cl.
`
`................................................... .. 372/4; 372/5
`_
`ABSTRACT
`(57)
`A fiber laser including doped-core fiber having inner and
`outer cladding is optically pumped by plurality of diode-
`lasers. Light emitted by the diode—1asers is coupled into a
`single multimode optical fiber. Light from the multimode
`optical fiber is directed to a wavelength selective reflecting
`device that is partially reflective in a narrow reflection band
`about a peak refiection wavelength. A portion of the light
`having the peak reflection wavelength is reflected from the
`wavelength selective reflecting device back along the mul-
`timode optical fiber and back into the plurality of diode—
`lasers. This locks the emitting wavelength of the light
`emitted from each of the diode—lasers to the peak reflection
`wavelength. Light at the emitting wavelength that is not
`reflected from the wavelength selective reflective device is
`coupled into the inner cladding of the doped—core fiber for
`optically pumping the fiber laser.
`
`
`
`ASML 1313
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`

`
`Patent Application Publication Sep. 22, 2005 Sheet 1 of 4
`
`US 2005/0207454 A1
`
`

`
`Patent Application Publication Sep. 22, 2005 Sheet 2 of 4
`
`US 2005/0207454 A1
`
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`_ Patent Application Publication Sep. 22, 2005 Sheet 3 of 4
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`US 2005/0207454 A].
`
`Sep. 22, 2005
`
`WAVELENGTH STABILIZED DIODE-LASER
`ARRAY
`
`TECHNICAL FIELD OF THE INVENTION
`
`[0001] The present invention relates generally to cladding-
`pumped fiber—lasers. The invention relates in particular to a
`fiber-laser, cladding pumped by multiple diode-laser emit-
`ters that are wavelength locked by a common wavelength-
`locking device.
`
`DISCUSSION OF BACKGROUND ART
`
`[0002] Fiber-lasers are commonly pumped by light from a
`diode-laser. One preferred fiber—laser type that is suitable for
`diode-laser pumping is referred to by practitioners of the art
`as a double—clad fiber-laser or a cladding pumped fiber-laser.
`The double clad fiber-laser has a doped core that provides
`optical gain when energized by the pump light from the
`diode-laser. Surrounding the doped core is an inner cladding
`and surrounding the inner cladding is an outer cladding. The
`diode-laser light (pump light) is directed into the inner
`cladding of the fiber-laser and propagates through the inner
`cladding while being progressively absorbed in the doped
`core, thereby energizing (pumping) the core.
`
`If a fiber—laser is required to provide a high power
`[0003]
`output, for example, greater than about 3.0 Watts (VV), a
`single diode-laser emitter may not be capable of providing
`sufiicient pump light power. It this case, it will be necessary
`to provide pump-light from a plurality of emitters. It is
`usually found convenient
`to provide an integrated linear
`array of such emitters or diode-lasers in what is termed a
`“diode-laser bar” by practitioners of the art. The emitters in
`the bar are preferably multimode emitters.
`I
`[0004] A multimode emitter usually has a higher power
`output than single mode emitter of the same length and
`heterostructure. The output power and the number of emitted
`lateral (spatial) modes of such an emitter usually increases
`as the width of the emitter increases. By way of example, a
`multimode emitter having an emitter width of 100 microme-
`ters (,um) can emit as much as twenty or more times the
`power of a corresponding single mode emitter having a
`width of between 1 ,um and 5 pain. In a common pumping
`arrangement, multimode radiation from a laser emitter is
`coupled into a multimode optical
`fiber. Light from the
`multimode optical fiber is, in turn, coupled to the fiber—laser.
`
`[0005] Optimum absorption of pump light in a doped fiber
`core usually occurs in a relatively narrow band of wave-
`lengths. By way of example, in a ytterbium (Yb) doped core,
`there is a strong absorption peak at a wavelength of about
`977 nanometers (nm). The absorption peak has a full width
`at 90% maximum absorption (FWNM) of only about 1.0
`nm. A diode-laser having a peak gain at 977 nm has a gain
`bandwidth of between about 4 and 6 nm. Accordingly, it is
`desirable that pump light have a wavelength equal to the
`peak absorption wavelength and have a bandwidth about
`equal to the peak absorption bandwidth.
`
`lasing wavelengths of indi-
`In a diode-laser bar,
`[0006]
`vidual diode-lasers or emitters may be spread over a range
`of a few nanometers. Further, the individual emitters in the
`bar will exhibit a strong, temperature—induced wavelength
`shift. Byway of example, for emitters nominally lasing at a
`wavelength of 977 nm, the wavelength variation with tern-
`
`perature change is about 0.3 nm per degree Kelvin (0.3 nm/°
`K). This relatively high temperature sensitivity, combined
`with the range of emitting wavelengths, makes a multimode
`diode-laser bar unsuitable for pumping a fiber—laser lasers
`wherein pump light must be absorbed in a narrow band of
`wavelengths. To provide an efficient absorption of pump
`light in a doped fiber core having a narrow absorption peak,
`wavelength locking or wavelength stabilization of diode-
`laser bars and narrowing of bandwidth is required.
`
`[0007] Wavelength stabilization and relative insensitivity
`of the emitting wavelength to temperature change has been
`achieved, in a single—mode laser diode, by locking the lasing
`wavelength to the refleeting wavelength of a wavelength
`selective reflector arranged to form an external cavity or
`resonator for the diode-laser. The wavelength selective
`reflector is provided by a fiber Bragg grating (FBG) formed
`on a length of a single-mode fiber. Single mode radiation
`from the diode-laser is launched into the core of the single
`mode fiber and is partially reflected and partially transmitted
`by the FBG. The FBG typically has a reflection coeflicient
`between about 0.5% and 8% at a wavelength near the peak
`gain wavelength of the diode-laser and has a reflection
`bandwidth of about 1 nm or less. The reflected radiation
`wavelength is defined by the optical period (hereinafter
`simply “period”) of the FBG. The emitting wavelength of
`the laser diode is locked to the peak reflection wavelength
`(resonance wavelength) of the FBG, and the emission band-
`width less than 1 nm. The resonance wavelength of an FBG
`is less sensitive to temperature change than the emitting
`wavelength of a (unstabilized) diode-laser. By way of com-
`parison, the temperature sensitivity of the resonance wave-
`length for a FBG is about 0.01 nm/° K, while temperature
`sensitivity of lasing wavelength is about 0.3 nm/° ‘K, as
`discussed above.
`
`[0008] The FBG wavelength locking scheme is efiective
`because the FBG is written in a single-mode fiber. In a single
`mode fiber, radiation is incident on the FBG at only one
`angle of incidence such that the wavelength of radiation
`reflected is determined only by the period of the FBG.
`Radiation from a multimode diode-laser must be coupled
`into a multimode fiber for efficient coupling. However, in a
`multimode fiber different modes propagate at different
`angles to the fiber axis. Were a FBG with fixed period
`written into such a multimode fiber, different lasing modes
`coupled into the fiber would be incident on the FBG at
`different angles, and, accordingly, would be reflected at
`differentwave1engths.A result of this is thatthe output of the
`multimode diode-laser could not be locked to a single lasing
`wavelength. There is a need for a wavelength locking and
`stabilization scheme that
`is effective for a plurality of
`multimode diode—lasers the output of which is coupled into
`a plurality of multimode fibers.
`
`SUMMARY OF THE INVENTION
`
`[0009] The present invention is directed to a method and
`apparatus for stabilizing the lasing wavelength of a plurality
`of multimode diode~lasers.
`In one aspect,
`the inventive
`method comprises providing a wavelength selective reflect-
`ing device having a peak reflection wavelength within the
`emitting bandwidth of the diode—lasers. Light emitted by the
`plurality of diode—lasers is coupled into a single multimode
`optical fiber. Light from the multimode optical fiber is
`directed to the wavelength selective reflecting device. A
`
`

`
`US 2005/0207454 A1
`
`Sep. 22, 2005
`
`portion of the light having the peak reflection wavelength is
`reflected from the wavelength selective refiecting device
`back along the multimode optical fiber and back into the
`plurality of diode-lasers, thereby locking the wavelength of
`the light emitted from each of the diode.-lasers to the peak
`reflection Vvavelength.
`
`[0010] The wavelength selective reflective devices suit-
`able for use with the inventive method include a fiber Bragg
`grating and a volume Bragg grating (VBG). Preferably, the
`light from the multimode fiber is collimated prior to reflect-
`ing the light from the wavelength selective reflecting device.
`In a preferred embodiment of the inventive method, the light
`emitted from the plurality of diode-lasers is coupled into the
`multimode fiber via a corresponding plurality of other
`multimode fibers bundled and fused together, with the fused
`I bundle being tapered to the diameter of the single optical
`fiber.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0011] The accompanying drawings, which are incorpo-
`rated in and constitute a part of the specification, schemati-
`cally illustrate a preferred embodiment of the present inven-
`tion, and together with the general description given above
`and the detailed description of the preferred embodiment
`given below, serve to explain the principles of the present
`invention.
`
`[0012] FIG. 1 schematically illustrates one preferred
`embodiment of apparatus in accordance with the present
`invention including a plurality of multimode diode-lasers, an
`equal plurality of multimode optical fibers bundled, fused,
`and tapered into a common multimode optical fiber, an
`optical fiber collimator including a fiber Bragg grating, and
`a length of doped-core optical fiber coupled to the optical
`fiber collimator by a second common multimode fiber and
`arranged to function as an optical fiber-laser.
`
`the
`[0013] FIG. 2 schematically illustrates detail of
`bundled and fused optical fibers in the apparatus of FIG. 1.
`
`[0014] FIG. 3 is a cross-section View seen generally in a
`direction 3-3 of FIG. 1, schematically illustrating the details
`of a junction between the second common multimode opti-
`cal fiber and the doped-core optical fiber of FIG. 1.
`
`[0015] FIG. 4 schematically illustrates another preferred
`embodiment of apparatus in accordance with the present
`invention including four pluralities of multimode diode-
`lasers, and, for each of the pluralities of diode-lasers, an
`equal plurality of multimode optical fibers, each thereof
`bundled fused and tapered into a corresponding one of four
`first common multimode optical fibers, each of the first
`common multimode optical°fibers coupled into a corre-
`sponding one of four optical fiber collimators each thereof
`including a fiber Bragg grating, the four collimators being
`coupled to a single second common multimode fiber, and a
`length of doped-core optical fiber coupled to the optical fiber
`collimators by the second common multimode fiber and
`arranged to provide an optical fiber-laser.
`
`[0016] FIG. 5 schematically illustrates yet another pre-
`ferred embodiment of apparatus in accordance with the
`present invention including four pluralities of multimode
`diode-lasers, and, for each of the pluralities of diode-lasers,
`an equal plurality of multimode optical fibers each thereof
`bundled fused and tapered into a corresponding one of four
`
`first common multimode optical fibers, each of the first
`common multimode optical fibers coupled into a corre-
`sponding one of four optical fiber collimators each thereof
`including a fiber Bragg grating, and a length of doped-core
`optical fiber coupled to the four optical fiber collimators by
`a corresponding four second common multimode optical
`fibers.
`
`[0017] FIG. 6 schematically illustrates still another pre-
`ferred embodiment of apparatus in accordance with the
`present invention including a plurality of multimode diode-
`lasers, an equal plurality of multimode optical fibers bundled
`fused and tapered into a common multimode optical fiber, a
`collimating lens for collimating light from the common
`optical fiber, and a volume Bragg grating positioned to
`receive light from the collimating lens.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`[0018] Referring now to the drawings, wherein like fea-
`tures are designated by like reference numerals, FIG. 1,
`FIG. 2, and FIG. 3 schematically illustrate one embodiment
`of a cladding pumped fiber-laser 20, optically pumped by a
`diode-laser array 22. Diode—laser array 22 is wavelength
`stabilized by a preferred embodiment of the wavelength
`stabilizing method of the present invention. Diode-laser
`array 22 includes a plurality of individual diode-lasers or I
`emitters 24. These may be emitters in a diode-laser bar,
`indicated in phantom in FIG. 1 by dotted rectangle 26, or
`could be individual diode-lasers on separate substrates. A
`multimode optical fiber 28 is provided for each emitter 24.
`Light (not shown) emitted from each of theemitters 24 is
`coupled into a corresponding multimode fiber 28 as illus-
`trated in FIG. 1. As methods for coupling light from a
`diode-laser array into a corresponding array of optical fibers
`are well known in the art to which the present invention
`pertains, a detailed description of such coupling is not
`presented herein. One suitable method is described in U.S
`Pat. No. 5,949,932 the complete disclosure of which is
`hereby incorporated by reference.
`
`[0019] Optical fibers 28 are collected into an optical fiber
`multiplexing arrangement (multiplexer) 30. Here, individual
`fibers 28 are grouped and fused together, in a fused region
`32 (see FIG. 2) of the mutiplexer, such that there is no
`interstitial space between the optical fibers. Fused tapered
`region 32 is tapered to a smallest diameter about equal to the
`diameter of one of the individual fibers 28, providing a
`straight multimode fiber region 34 (see again FIG. 2) into
`which light from all of the fibers 28 is coupled by multi-
`plexer 30. The multimode fiber region of multiplexer 30 is,
`here, extended by splicing a length of multimode fiber 38
`onto the multiplexer as indicated by dashed line 36 in FIG.
`1 and FIG. 2. Light exiting the multiplexer is coupled into
`multimode fiber 38. It should be noted, here, that a detailed
`description of the construction and operation of a multi-
`plexer, such as multiplexer 30, is not necessary for under-
`standing principles of the present invention. A detailed
`description of one such multiplexer is included in US. Pat.
`No. 5,864,644, the complete disclosure of which is hereby
`incorporated by reference.
`
`[0020] Continuing with reference to FIG. 1, light coupled
`into multimode optical fiber 38 propagates therealong and is
`coupled into a fiber optic collimator 40. At one end of
`
`

`
`US 2005/0207454 A1
`
`sep. 22, 2005
`
`collimator 40 is an adiabatically up-tapered tapered region
`42. Tapered region 42 has a smallest diameter thereof
`spliced to multimode optical fiber 28 as indicated in FIG. 1
`by dashed line 48. Preferably, this narrowest diameter of
`tapered region 42 is about equal to the diameter of multi-
`mode fiber 38. Tapered region 42 tapers up in diameter from
`the smallest diameter
`to a largest diameter preferably
`between about two and five times this smallest diameter.
`Tapered region 42 then transitions to a straight region 44
`having this largest diameter.
`
`[0021] An optimum diameter of collimator 40 can be
`calculated using the Bragg reflection condition, the required
`' bandwidth of light A)», fiber parameters, and brightness
`theorem. By way of example, for a required bandwidth of
`A?»=1 nm at a central emitted wavelength of 7»:-977 nm, and
`for a multimode fiber 38 having core and cladding diameters
`of 400 and 440 microns, respectively, with a numerical
`aperture (NA) of 0.22, the core diameter D2 of a straight
`region 44 can be calculated using the following equation:
`
`D2 =Dl ‘N/‘ii
`
`A
`2vAA-I17
`
`(1)
`
`[0022] where D] is the core diameter of fiber 38, NA] is
`the numerical aperture of the light in fiber 38, n is the
`refractive index of collimator 40. In one preferred example
`of fiber optic collimator 40, tapered region 42 has a length
`L between about 7.0 and 30.0 milllimeters (mm) providing
`smooth adiabatic up-taper from a fiber of 440 micron
`diameter to a fiber with 1480 micron diameter.
`
`[0023] A FBG 46 is written into straight region 44 of
`collimator 40. FBG 46 has period selected such that the V
`grating has a peak reflectivity for collimated light at a
`wavelength within the gain-bandwidth (emitting bandwidth)
`of emitters 24. Usually, such a grating would have a reflec-
`tion bandwidth at half maximum reflection (FWHM) of less
`than 1.0 nm. As light propagates along tapered region 42 of
`collimator 40, difierent propagating angles of dilferent
`modes with respect to the longitudinal axis of the collimator
`progressively decrease until, at the largest diameter of the
`collimator, in straight region 44 thereof, the angles of all of
`the modes are sufficiently close to parallel to the longitudinal
`fiber axis that light propagating in straight region 44 can be
`considered to be collimated. Accordingly, FBG 46 has about
`the same peak reflectivity wavelength for all modes. Pref-
`erably, this peak reflectivity is between about 0.5% and 50%.
`[0024] Light reflected from FBG 46 is directed back
`through tapered region 42 of collimator 40. On propagating
`back through tapered -region 40, the propagating angles of
`the different modes increase and become characteristically
`diiferent, however, all of these back reflected modes have
`the same wavelength. The back reflected light propagates
`along multimode fiber 38 back to multiplexer 30. In multi-
`plexer 30, the back reflected light is distributed back into
`individual multimode fibers 28 and is fed back into each of
`emitters 24. This locks the emitting wavelength of all of the
`emitters at the peak reflection wavelength of FBG 46 and
`constrains the emitting bandwidth to about the bandwidth of
`FBG.
`
`~
`
`It should be noted, here, that while FBG 46 essen-
`[0025]
`tially forms an external (feedback) resonator for each emitter
`
`24, it is not necessary that the optical path lengths from the
`grating to the emitters be equal, provided that this path is
`longer than about 0.5 meters (m). In this case, there will be
`sufiicient individual
`lasing modes in the each individual
`external resonator within the reflection bandwidth of FBG
`46 that the emitting wavelength of all of the emitters 24 will
`be locked to about the same wavelength.
`
`[0026] Continuing now with reference to FIG. 1 and
`additionally to FIG. 3, optical fiber collimator 40 includes a
`second down-tapered region 50 having a diameter tapering
`from the diameter of straight region 44 of the collimator to
`a lesser diameter about equal to the diameter of a multimode
`optical fiber 52. Fiber 52 has a multimode core 54 sur-
`roundedby cladding 56 (see FIG. 3). Tapered region 50 of
`the collimator is coupled to optical fiber 52 by a splice joint
`indicated in FIG. 1 by dashed line 58. Light propagates
`through straight region 44 of optical fiber collimator 40 and
`is concentrated by tapered region 50 before being coupled
`into multimode optical fiber 52. Light propagates in core 54
`of multimode fiber 52.
`
`[0027] A fiber-laser 60 includes a single mode fiber 62
`having a doped core 64 surrounded by inner cladding 66,
`which is, in tu rn, surrounded by outer cladding 68 (see FIG.
`3). It should be noted, here, that in FIG. 3, a longitudinal
`cross section View of fiber 62, traditional cross-hatching of
`material is omitted for clarity. Multirnode optical fiber 52
`preferably has a diameter about equal to single-mode optical
`fiber 62 and is coupled thereto via a splice joint indicated in
`FIGS. 1 and 3 by a dashed line 58. Light propagating in
`multimode optical fiber 52 is coupled into single-mode
`optical fiber 62 and propagates primarily in inner cladding
`66 thereof, progressively being absorbed in doped, single-
`mode core 64. As a result of this,
`the propagating light
`provides pump-light for fiber-laser 60. FBGs 70 and 72
`written into ends regions 60A and 60B respectively of
`optical fiber 62, each being selectively reflective at the lasing
`wavelength of the optical fiber define a resonator for fiber-
`laser 60, Laser output is delivered from free end 62B of
`optical fiber 62.
`
`[0028] Although FIG. 3 illustrates the more conventional
`arrangement where the doped core 64 is located in the center
`of the fiber, other arrangements are possible. For example,
`the doped core can be in the form of an annular with the
`central region of the fiber being undoped as described in
`U.S. Pat. No. 6,288,835, the disclosure of which is incor-
`porated herein by reference.
`
`[0029] FIG. 4 schematically illustrates another embodi-
`ment 80 of a cladding pumped fiber-laser, optically pumped
`by a plurality of diode-laser arrays 22, each of the arrays
`being wavelength stabilized by the wavelength stabilizing
`method of the present invention. In laser 80 there are four
`diode-laser arrays 22 each thereof including a plurality of
`emitters 24. There are four fiber optic collimators 40, one for
`each of the diode-laser arrays. Light from the plurality of
`emitters 24 in each array is coupled via a plurality of
`multimode fibers 28 and one of four multiplexers 30 into one
`of four multimode optical fibers 38. Light from each of the
`multimode fibers 38 is coupled into a corresponding one of
`the fiber optic collimators 40. Each of the fiber optic
`collimators 40 has a FBG 46 written into straight region 44
`thereof. Light having the peak reflection wavelength of the
`FBG is reflected back into emitters 24 of the diode-laser
`
`

`
`US 2005/0207454 A1
`
`Sep. 22, 2005
`
`array thereby locking the emitting wavelength of those
`emitters to the peak reflection wavelength of the FBG.
`
`In each fiber optic collimator 40, light not reflected
`[0030]
`back from the FBG propagates through straight region 44 of
`the collimator, is concentrated in down-tapered region 50 of
`the collimator then coupled into one of four multimode
`optical fibers 52. Light from the four multimode fibers 52 is
`coupled via a multiplexer 31 into a single multimode optical
`fiber 53. Light from multimode fiber 53 is coupled into
`single mode fiber 62 of a fiber-laser 60 for pumping the
`fiber—laser, as discussed above.
`
`[0031] FIG. 5 schematically illustrates one embodiment
`of a cladding pumped fiber amplifier 80 optically pumped by
`a plurality of diode—laser arrays 22, each one the arrays being
`wavelength stabilized by the wavelength stabilizing method
`of the present invention. In fiber amplifier 82 there are four
`diode-laser arrays 22 each thereof including a plurality of
`emitters 24 as described above with reference to laser 80.
`Light from the plurality of emitters 24 in each array is
`coupled via a plurality of multimode fibers 28 and one of
`four multiplexers 30 into one of four multimode optical
`fibers 38. Light from each of the multimode fibers 38 is
`coupled into a corresponding one of four fiber optic colli-
`mators 40. Each of the fiber optic collimators 40 has a FBG
`46 written into straight region 44 thereof. Light having the
`peak reflection wavelength of the FBG is reflected back into
`emitters 24 of the diode—laser array,
`thereby locking the
`emitting wavelength of those emitters to the peak reflection
`wavelength of the FBG.
`
`In each fiber optic collimator 40, light that is not
`[0032]
`reflected back from the FBG propagates through straight
`region 44 of the collimator, is concentrated in down-tapered
`region 50 of the collimator, then Coupled into one of four
`multimode optical fibers 52. Light from the four multimode
`fibers 52 is coupled into inner the cladding of a single
`multimode optical fiber 62 for energizing the doped core of
`the fiber-laser. Light to be amplified, for example, from a
`laser or from another amplifier, is coupled into end 62A of
`optical fiber 62. Amplified light is delivered from end 62B
`of optical fiber 62. The coupling, here,
`is effected by
`grouping multimode fibers 52 around single—mode fiber in
`an arrangement 33. Each of the multimode fibers 52 is
`tapered and fused into the cladding of the single—rnode fiber.
`This mode of coupling light from multimode fibers 52 into
`single—mode fiber 62, however, should not be construed as
`limiting. Any other coupling arrangement known in the art
`may be used without departing from the spirit and scope of
`the present invention.
`
`another
`schematically illustrates yet
`6
`[0033] FIG.
`embodiment 84 of a cladding pumped fiber-laser optically
`pumped by a diode—laser arrays 22 including a plurality of
`emitters 24. Laser 84 is similar to above—described laser 20
`of FIG. 1, with an exception that a bulk optics arrangement
`86 is used for collimation and back reflection of light, in
`place of a fiber optic collimator and FBG. Light from the
`plurality of emitters 24 is coupled via a corresponding
`plurality of multimode optical fibers 28 into a single mul-
`timode optical fiber 38. Light is delivered from end 38E of
`optical fiber 38 as a diverging bundle of rays designated in
`FIG. 6 by rays 88D. Ray 88D are received by a positive lens
`90 and collimated as indicated by parallel rays 88P. Parallel
`rays 88 traverse a volume Bragg grating (VBG) 92. VBG 92
`
`the grating has a peak
`has a period selected such that
`reflectivity for collimated light at a wavelength within the
`gain—bandwidth (emitting bandwidth) of emitters 24. The
`peak reflectivity is preferably between about 0.5% and 50%
`as discussed above for FBG 46.
`
`[0034] Light reflected from VBG 92 is coupled back into
`multimode optical fiber 38 by lens 90 and propagates back
`along multimode fiber 38 to multiplexer 30. In multiplexer
`30 the back reflected light is distributed back into individual
`multimode fibers 28 and is fed back into each of emitters 24.
`This locks the emitting wavelength of all of the emitters at
`the peak reflection wavelength of FBG 46. Light that is not
`reflected by VBG 92 is collected by a lens 90 and coupled
`into multimode optical fiber 52 as indicated by converging
`rays 86C. Light is then coupled from optical fiber 52 into
`fiber-laser 60 as described above with reference to laser 20
`of FIG. 1.
`
`It should be noted here that, in theory at least, VBG
`[0035]
`92 could be replaced by a vacuum-deposited multilayer
`reflector having a comparable reflection bandwidth, [for
`example, a bandwidth of about 0.1% of the nominal wave-
`length of light to be reflected. Fabricating such a reflector,
`however, would require deposition of hundreds of layers
`even for the relatively low reflectivity required. Further,
`thickness and refractive index of the layers would need to be
`extremely precisely controlled, for example, to within about
`0.1%, to limit the occurrence of unwanted sideband reflec-
`tions. It is believed that depositing such a reflector is not
`practical within the scope of present vacuum technology.
`
`[0036] The present invention is discussed above in terms
`of a preferred and other embodiments. The invention is not
`limited, however,
`to the embodiments described and
`depicted. Rather the invention is limited only by the claims
`appended hereto.
`
`What is claimed is:
`1. Amethod of stabilizing the wavelength of light emitted
`by each one of a plurality of multimode diode-lasers, each
`of said diode—lasers having an emitting bandwidth,
`the
`method comprising the steps of:
`
`providing a wavelength selective reflecting device having
`a peak refiection wavelength within the emitting band-
`width of the diode-lasers;
`
`- coupling the light emitted from the plurality of diode-
`lasers into a first multimode optical fiber;
`
`directing said light from said first multimode optical fiber
`onto said wavelength selective reflecting device; and
`
`reflecting a portion said light having said peak reflection
`wavelength from said wavelength selective reflecting
`device back along said first multimode optical fiber and
`back into said diode—lasers thereby locking the wave-
`length of the light emitted from said diode—lasers to said
`peak reflection wavelength.
`2. The method of claim 1, wherein said wavelength
`selective reflective device is aFBG.
`3. The method of claim 1, wherein said wavelength
`selective reflecting device is a volume Bragg grating.
`4. The method of claim 1, wherein said wavelength
`selective reflecting device has a bandwidth less than about 1
`nanometer and a reflectivity between about 0.5% and 50%.
`
`

`
`US 2005/0207454 A1
`
`Sep. 22, 2005
`
`5. The method of claim 1 further including the step of
`collimating said light from said first multimode optical fiber
`prior to said reflecting step.
`6. The method of claim 1, wherein the light emitted from
`each of the plurality of diode-lasers is coupled into said first
`multimode optical fiber via a corresponding plurality of
`second multimode optical fibers.
`7. A method of stabilizing the wavelength of light emitted
`by each one of a plurality of multimode diode-lasers, each
`of said diode-lasers having. an emitting bandwidth,
`the
`method comprising the steps of:
`
`coupling the light emitted from the plurality of diode-
`lasers into a first multimode optical fiber;
`
`coupling said light from said multimode optical fiber into
`an optical fiber collimator, said optical fiber collimator
`including a fiber Bragg grating having a peak reflection
`wavelength within said emitting bandwidth; and
`
`reflecting a portion of said light having said peak wave-
`length from said fiber Bragg grating back into said
`diode-lasers thereby locking the wavelength of the light
`emitted from said diode-lasers to said peak refiection
`wavelength.
`8. The method of claim 7, wherein said first multimode
`optical fiber has a first diameter and said optical fiber
`collimator has a diameter tapered at one end thereof from
`said first diameter to a second diameter greater than said first
`diameter, said first diameter of said tapered region being
`coupled to said first multimode optical fiber.
`9. The method of claim 8 wherein said optical fiber
`collimator has a straight region following said tapered
`region, said straight region of said optical fiber collimator
`having said second diameter and including said fiber Bragg
`grating.
`10. The method of claim 7, wherein the light emitted from
`each of the plurality of diode~lasers is coupled into said first
`multimode optical fiber via a corresponding plurality of
`second multimode optical fibers.
`11. The method of claim 7, further including the steps of
`providing a single mode optical fiber including a single
`mode doped-core surrounded by a cladding, and coupling
`said light from said optical fiber collimator into said clad-
`ding of said single-mode optical fiber.
`12. A method of stabilizing the wavelength of light
`emitted from each of a plurality of multimode diode—lasers,
`each of said diode-lasers having an emitting bandwidth, the
`method comprising the steps of:
`
`providing a plurality of first multimode optical fibers
`equal in number to the plurality of diode-lasers;
`coupling the light emitted from each of the plurality of
`diode-lasers into a corresponding one of the multimode
`optical fibers;
`
`Coupling the light from the plurality of multimode fibers
`into a single second multimode optical fiber;
`
`coupling the light from the second multimode optical fiber
`into an optica