WO2020264126A1 - Amplificateur à fibre à pompage inverse avec suppresseur de lumière de gaine entre segments de fibre active - Google Patents

Amplificateur à fibre à pompage inverse avec suppresseur de lumière de gaine entre segments de fibre active Download PDF

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Publication number
WO2020264126A1
WO2020264126A1 PCT/US2020/039570 US2020039570W WO2020264126A1 WO 2020264126 A1 WO2020264126 A1 WO 2020264126A1 US 2020039570 W US2020039570 W US 2020039570W WO 2020264126 A1 WO2020264126 A1 WO 2020264126A1
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WIPO (PCT)
Prior art keywords
fiber
pump
combiner
segment
active
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PCT/US2020/039570
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English (en)
Inventor
Christopher LUETJEN
C. Geoffrey Fanning
Roger Farrow
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Nlight, Inc.
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Publication of WO2020264126A1 publication Critical patent/WO2020264126A1/fr

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/262—Optical details of coupling light into, or out of, or between fibre ends, e.g. special fibre end shapes or associated optical elements
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094003—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094003—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre
    • H01S3/094007—Cladding pumping, i.e. pump light propagating in a clad surrounding the active core
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02066—Gratings having a surface relief structure, e.g. repetitive variation in diameter of core or cladding
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/268—Optical coupling means for modal dispersion control, e.g. concatenation of light guides having different modal dispersion properties
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/0675—Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06754—Fibre amplifiers

Definitions

  • the fiber laser industry continues to increase laser performance metrics, such as average power, pulse energy, and peak power.
  • One technique to increase fiber laser system power is with reverse pumping where pump light is launched into an active fiber in propagation direction counter to the direction signal light is propagated out of the system (e.g., though an output coupler).
  • the reverse pump light may excite gain medium within the active fiber, and while in a well designed system most of the pump light will be absorbed by the gain medium, not all of pump light can be absorbed by an active fiber of practical length. This may be particularly true where thermal effects within the fiber system cause pump light wavelengths to drift off of narrow absorption peaks of an active fiber.
  • Reverse pumping fiber lasers can therefore subject some fiber system components to greater heat loads.
  • Unabsorbed pump light exiting an end of the active fiber may enter one or more elements of the system that are sensitive to optical power and/or have a limited ability to dissipate such back-propagating optical energy.
  • a fiber pump combiner may be designed to combine light from multiple fibers into a single fiber.
  • the pump combiner may not also be equally well suited to couple unabsorbed pump light back-propagating from the single fiber side into the multiple fibers.
  • a number of combined pump fibers and/or total pump power applied in a reverse pumped fiber laser system may therefore be limited so as to avoid damaging a reversed fiber pump combiner.
  • FIG. 1 illustrates a schematic of a fiber laser system including reversed fiber combiners separated by segments of active fiber and an intervening cladding light stripper, in accordance with some embodiments;
  • FIG. 2 is block diagram illustrating methods of fabricating the fiber laser system illustrated in FIG. 1, in accordance with some embodiments;
  • FIG. 3 is block diagram illustrating methods of operating the fiber laser system illustrated in FIG. 1, in accordance with some embodiments
  • FIG. 4 illustrates a schematic of a reverse pumped fiber laser system, in accordance with some embodiments
  • FIG. 5 illustrates a schematic of a reverse pumped fiber laser system, in accordance with some other embodiments
  • FIG. 6A and 6B depict orthogonal cross-sectional views of a single-clad fiber, in accordance with some embodiments
  • FIG. 7A and 7B depict orthogonal cross-sectional views of a double-clad fiber, in accordance with some embodiments.
  • FIG. 8A is an isometric view of a fiber pump-signal combiner, in accordance with some embodiments.
  • FIG. 8B and 8C depict cross-sectional views of a portion of the fiber pump-signal combiner illustrated in FIG. 8A, in accordance with some embodiments; and FIG. 9 is a cross-sectional view of a fiber segment including a cladding light stripper (CLS), in accordance with some embodiments;
  • CLS cladding light stripper
  • first embodiment may be combined with a second embodiment anywhere the particular features, structures, functions, or characteristics associated with the two embodiments are not mutually exclusive.
  • singular forms“a”, “an” and“the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term“and/or” as used herein refers to and
  • Coupled may be used to indicate that two or more elements are in direct physical, optical, or electrical contact with each other.
  • Connected may be used to indicate that two or more elements are in direct physical, optical, or electrical contact with each other.
  • Coupled may be used to indicated that two or more elements are in either direct or indirect (with other intervening elements between them) physical or electrical contact with each other, and/or that the two or more elements co-operate or interact with each other (e.g., as in a cause an effect relationship).
  • the terms“substantially equal,”“about equal” and“approximately equal” mean that there is no more than incidental variation between among things so described. In the art, such variation is typically no more than +/- 10% of a predetermined target value.
  • a list of items joined by the term“at least one of’ or“one or more of’ can mean any combination of the listed terms.
  • the phrase“at least one of A, B or C” can mean A; B; C; A and B; A and C; B and C; or A, B and C.
  • FIG. 1 illustrates a schematic of a fiber laser system 100 including opposing fiber cladding light combiners 105 and 106 separated by cladding light dissipation elements 101 and 102.
  • Each of cladding light dissipation elements 101 and 102 dissipate cladding light energy received from cladding light combiners 105 and 106, respectively.
  • Elements 101 and 102 may dissipate cladding light energy through any mechanism, with some amount of residual cladding light energy that is not dissipated within element 101 and 102 expected to propagate through the dissipation elements and then (re)enter an output port of combiners 105, 106.
  • each of cladding light combiners 105 and 106 may be subject to receiving back- propagating residual cladding light that was not fully dissipated by elements 101 and 102.
  • an intervening cladding light stripper (CLS) 120 is between cladding light dissipation elements 101 and 102.
  • CLS 120 is to filter residual cladding light from laser system 100.
  • CLS 120 may therefore be operable to at least partially mitigate any adverse effects of incomplete cladding light energy dissipation within elements 101, 102.
  • CLS 120 may comprise one or more materials or structures that reduce an amount of cladding light that is coupled from a first dissipation element (e.g., 101 or 102) into a second dissipation element (e.g., 102 or 101).
  • CLS 120 is to dissipate residual cladding light 121 not dissipated within element 101, thereby reducing cladding light propagating from element 101 into element 102.
  • CLS 120 is to dissipate residual cladding light 122 not dissipated within element 102, thereby reducing cladding light propagating from element 102 into fiber segment 101.
  • CLS 120 can therefore remove residual cladding light before it has an opportunity to back propagate through either cladding light combiner 105, 106, which may, for example, reduce a heat load placed on combiners 105, 106.
  • FIG. 1 further illustrates some exemplary embodiments where each of elements 101 and 102 comprises an active fiber segment 110.
  • Active fiber segment 110 may include any gain medium known to be suitable for the purpose of coupling optical energy from a cladding light beam propagating through a cladding layer of active fiber segment 110 into a signal light beam propagating through a core of active fiber segment 110, which is referred to as “cladding pumping.”
  • cladding pumping For such systems, cladding light combiners 105 and 106 may be referred to as “pump combiners.” Combiners 105 and 106 are illustrated to be pointing at each other with one combiner in a reverse pump configuration.
  • CLS 120 may filter out residual cladding light that is not absorbed by an active fiber segment 110. Since residual cladding light 121, 122 that is filtered by CLS 120 will also not enter into a second one of active fiber segments 110 as pump light, the combined efficiency of active fiber segments 110 may be slightly less than for an architecture where opposite ends of one continuous active fiber having a length equivalent to that of segments 110 is pumped with forward and backward propagating pump light. However, the protection afforded to a reversed cladding light combiner may enable fiber system 100 to operate with a significantly greater number of pump light sources and/or significantly higher operating powers without any need for increasing the complexity and/or cost of a reversed cladding light combiner.
  • Cladding light combiner 105 is, more specifically, a fiber pump combiner having a multi-fiber side coupled to a plurality of pump fibers 106, and a single-fiber side 111 coupled into an end of a first active fiber segment 110.
  • cladding light combiner 105 is spliced in an optical/laser oscillator that further includes a fiber Bragg grating 115 on a first side of active fiber segment 110 and an output coupler 116 on a second side of active fiber segment 110.
  • An additional active fiber segment may be further included between output coupler 116 and CLS 120, for example in a MOPA configuration as further described below.
  • Cladding light combiner 106 is likewise more specifically a fiber pump combiner having a multi-fiber side coupled to another plurality of pump fibers 106, and a single-fiber side 114 coupled into an end of a second active fiber segment 110.
  • Individual pump fibers 106 may each be any optical fiber known to be suitable for supporting the propagation of a beam of pump light from any suitable pump light source.
  • pump fibers 106 may be single clad or multi-clad fiber suitable for single-mode or multi- mode beam propagation.
  • individual ones of pump fibers 106 are single clad, multimode (MM) fibers suitable for propagating pump light within their fiber core.
  • a first end of each of the pump fibers may be suitable for coupling to a pigtail of any known pump light source.
  • Combiner 105, 106 may each be any fiber pump combiner suitable for coupling pump light conveyed by pump fibers 106 into a cladding layer of the single-fiber side of the combiners.
  • pump combiners 105, 106 are multimode fiber pump combiners that optically couple cores of pump input fibers 106 into a“pump” cladding layer that is to guide pump light.
  • Fiber pump combiners 105, 106 may have any architecture, such as, but not limited to, an end pumping architecture typically characterized by a tapered combiner fiber profile, or a side pumping architecture typically characterized by an non-tapered fiber combiner profile, or some hybrid architecture.
  • Pump combiners 105, 106 may have any integer number of pump input ports N, for a Nxl architecture. N may vary, for example ranging from 2-24.
  • a signal fiber 150 may be further coupled into the multi -fiber side of cladding light combiner 105, for example to input/output signal light to/from fiber system 101.
  • a signal fiber 160 may be further coupled into the multi-fiber side of cladding light combiner 106, for example to input/output signal light to/from fiber system 101.
  • the combiner has a (N+l)xl architecture, and the combiner may be referred to as “pump-signal” combiner in recognition of the signal pass-through.
  • a pump-signal combiner may be a multimode fiber pump-signal combiner, and may also have an end-coupling, a side-coupling, or an alternative architecture.
  • the active fiber segments 110 may each include any gain medium, such as rare-earth impurity dopant(s).
  • impurity dopants e.g., Ytterbium
  • each of the active fiber segments 110 is a multi-clad fiber comprising a single core and two cladding layers (double-clad fiber) or three cladding layers (triple-clad fiber).
  • At least one cladding layer e.g., an“inner” cladding layer
  • a“pump cladding” cladding layer is capable of guiding a pump light beam, and may also be referred to as a“pump cladding.”
  • Additional core and/or cladding layers are also possible (e.g., multi-core fiber, quad-clad fiber, etc.).
  • the active fiber segments 110 have substantially equal longitudinal lengths.
  • each of the active fiber segments 110 have substantially the same pumping efficiency, equal segment lengths position CLS 120 at a center point where residual cladding light 121 and 122 should be at nearly equal minimums.
  • Longitudinal lengths of active fiber segments 110 can differ however, for example to compensate for different active fiber pumping efficiencies that might occur in asymmetrical forward-reverse pumping architectures.
  • CLS 120 comprises a fiber that has one or more materials or structures that reduce an amount of the light propagated through the fiber pump cladding.
  • CLS 120 may include a region within which fiber cladding is more lossy and/or where cladding light becomes unguided.
  • CLS 120 may have any suitable architecture.
  • CLS 120 includes one or more materials that act to remove cladding light. For example, a layer of a high-index material (e.g., exceeding the index of a pump cladding material, or exceeding the index of a material index-matched to the pump cladding material) may be applied onto an outer surface of the pump cladding over a length of CLS 120.
  • a high-index material e.g., exceeding the index of a pump cladding material, or exceeding the index of a material index-matched to the pump cladding material
  • the high-index material e.g., a polymer
  • the high-index material has a suitable index contrast with the pump cladding to "un-guide" light propagating within the fiber cladding.
  • a suitable index contrast with the pump cladding to "un-guide" light propagating within the fiber cladding Over a longitudinal length of CLS 120 more than one high- index material may be employed, or the material index may otherwise vary, for example to distribute the thermal load associated with stripping residual cladding light 121, 122 from system 100
  • CLS 120 may include one or more structures operable to remove cladding light.
  • an outer surface of a fiber pump cladding may be roughened, or much larger features (e.g., on the order of the fiber pump cladding diameter) may be milled into the cladding for the purpose of scattering, reflecting, refracting and/or diffracting out light.
  • CLS 120 includes one or more recessed surface regions within a fiber cladding.
  • the recessed surface regions extend into the cladding to some desired depth, and are to induce optical perturbations in the fiber waveguide that allow light propagating within the pump cladding (originating, e.g., from either active fiber segment 110) to escape out of the cladding within CLS 120.
  • FIG. 2 is block diagram illustrating methods 201 of fabricating fiber system 100, in accordance with some embodiments. While methods 201 illustrate the fabrication of system 100 for the sake of clarity of description, it is noted that the principles taught in the context of methods 201 may be applied to the fabrication of other fiber systems that share salient attributes with fiber system 100. No order is required by FIG. 2, with numerical ordering of the blocks being representative of only one possible sequence for fabricating a fiber system.
  • Methods 201 begin at block 205 where a first plurality of pump fibers is joined to a multi- fiber side of a first fiber combiner, and a second plurality of pump fibers is joined to a multi -fiber side of a second fiber combiner.
  • the joining may be by end-to-end fusing, side-to-side fusing, or a by a hybrid technique where tapered surfaces of the pump fibers and/or fiber combiners are fused, for example.
  • Methods 201 continue at block 210 where a single-fiber side of the combiner is joined to a segment of active fiber.
  • a combiner output fiber may be directly fused to an end of one segment of active fiber, for example with a pump cladding of the fiber combiner fused to a pump cladding of a multi-clad active fiber segment.
  • a cladding of the fiber pump-signal combiner may be fused to a pump cladding of a multi-clad active fiber segment, while a core of the fiber pump-signal combiner may be fused to a core of the active fiber segment.
  • Methods 201 continue at block 215 where second ends of the active fiber segments are each joined to opposite ends of a CLS.
  • the pump combiners are to propagate cladding light toward each other, through the active fiber segments, and into opposite ends of the intervening CLS.
  • the joining at block 215 may be through end-to-end thermal fusing, for example, with a core of each active fiber segment facing a core of the CLS, and a cladding of each active fiber segment facing a cladding of the CLS.
  • Methods 201 complete at block 220 where an output fiber is joined to one of the fiber combiners (i.e., the pump-signal combiner).
  • the output fiber may be joined directly to the combiner, for example with a core of the output fiber facing a fiber core on the multi-fiber side of the combiner.
  • the CLS may be fabricated directly into an active fiber that has opposite ends, which are joined to the combiner (before or after fabricating the CLS).
  • the active fiber is effectively bifurcated into two segments of substantially identical active fiber, one of the segments at opposite ends of the CLS.
  • the resulting fiber assembly will be substantially the same regardless of whether two segments are thermally fused to opposite ends of a CLS fiber, or the CLS is fabricated within an active fiber.
  • FIG. 3 is block diagram illustrating methods 301 for operating fabricating fiber system 100, in accordance with some embodiments. While methods 301 illustrate the operation of system 100 for the sake of clarity of description, it is noted that the principles taught in the context of methods 301 may be applied to the operation of other fiber systems that share salient attributes with system 100. No order is required by FIG. 3, with numerical ordering of the blocks being representative of only one possible sequence for operating a fiber system.
  • Methods 301 begin at block 305 with combining a plurality of pump light beams to a first fiber pump combiner, and propagating the resulting first pump light beam into a pump cladding of a first active fiber segment.
  • a signal light beam propagating within the first active fiber segment may be amplified by the first pump light beam, for example according to any optical pumping phenomena.
  • Methods 301 continue at block 310 with combining another plurality of pump light beams to a second pump combiner, and propagating the resulting second pump light beam into a pump cladding of a second active fiber segment.
  • the signal light beam propagating within the second active fiber segment may be amplified by the second pump light beam, for example according to any optical pump phenomena.
  • Methods 301 continue at block 315 where the signal light beam propagating through the first active segment and/or the second active segment is conveyed across a CLS.
  • a signal light beam propagating through a core of the first active fiber segment may be conveyed through a core of a fiber CLS to continue propagating through a core of the second active fiber segment.
  • Methods 301 complete at block 320 where residual first pump light propagating between the pump claddings of the first and second segments is stripped as it propagates through the CLS, thereby reducing cladding light entering ends of the active fiber segments that are opposite the fiber combiners.
  • Fiber laser systems including a cladding light stripper separating segments of active fiber between reversed fiber combiners may further include other components, for example to implement a reverse pumped fiber laser system suitable for specific applications and/or use cases.
  • FIG. 4 and FIG. 5 depict reverse pumped fiber laser systems further illustrating some exemplary
  • FIG. 1 Reference numbers previously introduced are repeated in FIG. 4 and FIG. 5 where a component or feature shares the same salient attribute(s) (e.g., composition, structure, function, etc.) with a component or feature previously described in the context of fiber laser system 100.
  • salient attribute(s) e.g., composition, structure, function, etc.
  • a reverse pumped fiber laser system 400 includes a fiber pump combiner 405, and a fiber pump-signal combiner 406 arranged in a reverse/counter pumping architecture in which the multi-fiber ends combine into single-fiber ends that face each other.
  • fiber pump combiner 405 may have any of the attributes of fiber pump combiners 105, 106 (FIG. 1).
  • Fiber pump-signal combiner 406 may have any of the attributes of fiber pump combiner 405, with the addition of an output signal fiber 450 on the multi-fiber side, which is absent from the fiber pump combiner 405 (or is otherwise un-utilized).
  • Fiber laser system 400 includes pump fibers 106 that optically couple a plurality of pump light sources 401 into combiners 405, 406.
  • Each pump light source 401 may be any source known to be suitable for laser pumping, such as, but not limited to, a laser diode module.
  • the laser diode modules may each output a pump light beam of any center frequency with some exemplary center wavelengths being 920 nm and 976 nm.
  • pump light sources 401 may all output at substantially the same center wavelength, the center wavelength of pump sources coupled into combiner 405 may instead differ from the center wavelength of pump sources coupled into combiner 406.
  • there are four pump light sources 401 are coupled into fiber combiner 405 and fiber combiner 406. However, more or fewer pump light sources 401 may be coupled into each of combiners 405 and 406, and/or the number of pump light sources 401 may differ between fiber combiner 405 and fiber combiner 406.
  • system 400 includes a master oscillator-power amplifier (MOP A) 410 optically coupled to the single-fiber side of pump combiner 405.
  • a master oscillator may comprise a resonant length of fiber defined by a fiber Bragg grating (not depicted) proximal to pump combiner 405, and an output coupler (not depicted).
  • the active fiber includes a multi-clad fiber to propagate a pump light beam received from pump combiner 405, and a gain medium to transfer energy from the pump light beam propagated in the cladding to a signal beam propagated within a core of the active fiber.
  • the power amplifier of MOP A 410 may include a length of active fiber, such as active fiber segment 110 (FIG.
  • MOPA 410 may further comprise a second length of active fiber (an amplifier stage) between the output coupler and CLS 120.
  • a master fiber oscillator may further comprise a seed laser (not depicted) that is to inject a signal into the resonant length of fiber, and that is to be amplified by pump light within the power amplifier of MOPA 410.
  • System 400 further includes a fiber power amplifier 430 optically coupled to the single- fiber side of pump-signal combiner 406.
  • amplifier 430 includes another length of active fiber, such as active fiber segment 110 (FIG. 1).
  • the active fiber may include a multi-clad fiber to propagate a pump light beam received from pump-signal combiner 406, and a gain medium to transfer energy from the pump light beam propagated in the cladding to a signal beam propagated within a core of the active fiber.
  • Fiber power amplifier 430 is coupled to MOPA 410 through CLS 120.
  • CLS 120 is therefore to strip residual pump light 121 output from MOPA 410 that would otherwise enter power amplifier 430 and potentially bleed back into (reverse) pump-signal combiner 406.
  • CLS 120 is also to strip residual cladding (pump) 122 output from fiber amplifier 430 that would otherwise enter MOPA 410 and potentially bleed back into (forward) pump combiner 405.
  • CLS 120 may be fabricated directly within active fiber of either MOPA 410 or fiber amplifier 430, or may comprise passive fiber joined to opposite ends of active fibers of MOPA 410 and fiber amplifier 430.
  • Output signal fiber 450 may be a single clad or multi-clad, SM or MM optical fiber.
  • output signal fiber 450 is a double-clad, single core MM fiber that is to propagate a signal beam, which has been forward and reverse pumped by cladding light within active fiber segments of MOPA 410 and fiber amplifier 430, respectively.
  • Output fiber 450 may propagate a signal beam to any destination external of system 400 as embodiments are not limited in this respect.
  • FIG. 5 illustrates a reverse pumped fiber laser system 500 that includes two fiber signal- pump combiners.
  • dashed lines represent pump light while solid lines represent signal light with arrowheads depicting directions of beam propagation.
  • a forward fiber signal-pump combiner 406 that includes a signal fiber on the multi-fiber side that may receive a signal input.
  • the input signal fiber is optically coupled to receive an output of an aiming laser 540.
  • Aiming laser 540 may output a signal beam within the visible spectrum that is suitable for a user of system 500 to locate and position a signal output of system 500 propagated through a feeding fiber 570, for example.
  • the forward fiber signal-pump combiner 406 may instead be optically coupled to a signal seed laser, or otherwise coupled receive an injected signal beam.
  • output fiber 450 is coupled into a CLS 560 that may, for example, strip from the fiber cladding some portion of back-propagating beam energy that has been reflected through feeding fiber 570.
  • FIG. 6A and 6B depict orthogonal cross-sectional views of a single-clad fiber 601, in accordance with some embodiments.
  • Single-clad fiber 601 may be employed within any of the fiber laser systems described above.
  • pump fiber 106 (FIG. 1, FIG. 4, FIG. 5) may utilize single-clad fiber 601.
  • FIG. 6A is a cross-sectional view of fiber 601 with the longitudinal fiber axis in the plane of the page.
  • FIG. 6B is a cross-sectional view of fiber 601 with the optical fiber axis perpendicular to the plane of the page.
  • Fiber 601 may have any refractive index profile (RIP) suitable for a single-clad fiber.
  • RIP refractive index profile
  • RIP refers to the refractive index as a function of position along a line (e.g., x or y axis in FIG. 6B) or in a plane (e.g. x-y plane in FIG. 6B) perpendicular to the fiber axis (e.g., z-axis in FIG. 6A).
  • fiber 601 is azimuthally symmetric, with the ID RIP being identical for any azimuthal angle.
  • Core 605 and cladding 615 can each have a step- index or graded-index, for example.
  • a step-index fiber has a RIP that is substantially flat (refractive index independent of position) within fiber core 605.
  • Cladding 615 may also have a substantially flat RI over Dciad, with a RIP of fiber 601 then being stepped at the interface between core 605 and cladding 615.
  • An example of one illustrative stepped RIP suitable for a pump fiber is shown in FIG. 6A.
  • Core 605 may have any suitable composition (e.g., glass, fused silica).
  • Cladding 615 may be a polymer or glass, for example.
  • core 605 cladding 615 may be a variety of other shapes, such as, but not limited to annular, polygonal, arcuate, elliptical, or irregular.
  • Core 605 may be suitable for SM or MM propagation of light.
  • cladding 615 has a diameter Dciad while core 605 has a diameter Dcore.
  • Dciad and Dcore are illustrated to be constants about a central fiber axis in the longitudinal direction (z-axis in FIG. 6A).
  • the diameters Dciad and Dcore may instead vary over a longitudinal length of fiber 601.
  • fiber 601 is multimode fiber
  • the core diameter Dcore is in the range of 50-200 micron (pm)
  • the cladding diameter Dciad is in the range of 125-220pm. Other diameters for each are also possible.
  • FIG. 7A and 7B depict orthogonal cross-sectional views of a double-clad fiber 701, in accordance with some active fiber embodiments.
  • Double-clad fiber 701 may be employed within any of the fiber laser systems described above.
  • active fiber segments 110 (FIG. 1) may utilize double-clad fiber 701.
  • each of MOP A 410, fiber amplifier 430, output fiber 450, and feeding fiber 570 may utilize double-clad fiber 701, for example.
  • FIG. 7A is a cross-sectional view of fiber 701 with the longitudinal fiber axis in the plane of the page.
  • FIG. 7B is a cross-sectional view of fiber 701 with the optical fiber axis perpendicular to the plane of the page.
  • Fiber 701 may have any RIP suitable for a multi-clad fiber.
  • fiber 701 has a radially symmetric RIP with a central core 705, and an inner (pump) cladding 710, which is annular and encompasses core 705.
  • An annular outer cladding 715 further surrounds inner cladding 710.
  • Core 705, inner cladding 710, and outer cladding 715 can each have step-index or graded-index, for example.
  • RIP may be substantially flat within fiber core 705.
  • Inner cladding 710 may also have a substantially flat RI over Dciad.i, with a RIP of fiber 701 then being stepped at the interface between core 705 and inner cladding 710, and stepped again at the interface between inner cladding 710 and outer cladding 715.
  • An example of one illustrative stepped RIP suitable for a fiber laser is shown in FIG. 7 A.
  • core 705 and inner cladding 710 may have a "graded-index" in which the RI varies (e.g., decreases) with increasing radial position (i.e., with increasing distance from the core and/or cladding axis).
  • Core 705 may be suitable for single-mode or multimode propagation of light.
  • Inner cladding 710 may have an area larger than that of the core, may also have a higher NA (numerical aperture), and may support a large number of propagation modes.
  • Core 705 and inner cladding 710 may have any suitable composition (e.g., glass, fused silica).
  • Outer cladding 715 may be a polymer or also glass, for example.
  • core 705 and inner cladding 710 is illustrated as being concentric (i.e., a centered core), it need not be.
  • core 705 and inner cladding 710 may also be a variety of shapes other than circular, such as, but not limited to annular, polygonal, arcuate, elliptical, or irregular.
  • Core 705 and inner cladding 710 in the illustrated embodiments are co-axial, but may alternatively have axes offset with respect to one another.
  • inner cladding 710 has a diameter Dciacu while core 705 has a diameter Dcore.
  • Dciad,i and Dcore are illustrated to be constants about a central fiber axis in the longitudinal direction (z-axis in FIG. 7A).
  • the diameters Dciad.i and Dcore may instead vary over a longitudinal length of fiber 701, and/or vary between different segments of fiber within a fiber system.
  • fiber 701 is multimode MM fiber and the core diameter Dcore is in the range of 10-100 micron (pm) and the inner cladding diameter Dciad.i is in the range of 200-1000pm, although other values for each are possible.
  • FIG. 8A is an isometric view of fiber pump-signal combiner 406, in accordance with some embodiments.
  • fiber combiner 406 includes a single-fiber side that may be spliced onto an output end of active fiber 110.
  • core 705 has a core diameter Dcore, s that is greater than, or approximately equal to, a core diameter Dcore of the active fiber 110, with one example being around 25 pm.
  • the diameter of fiber combiner 406 tapers up from the single-fiber end to have a core diameter Dcore, m sufficient to accommodate output fiber 450 having an approximately equal, or slightly larger core diameter Dcore, with one example being around 50 pm.
  • a fiber pump combiner (e.g., pump combiner 405 in FIG. 4-5) may have similar tapered profile with smallest core diameter advantageously being no greater than a core diameter of the first end of the active fiber 110 to which it is coupled.
  • the core diameters may be substantially constant or monotonically increase within a forward and reverse pumped fiber laser system.
  • the NA of the fiber cores may also be substantially constant or monotonically increase in the signal propagation direction.
  • FIG. 8B and 8C depict cross-sectional views of the multi-fiber side of the fiber pump-signal combiner 406, in accordance with some embodiments.
  • the large cladding diameter e.g., 500 pm
  • a single ring of pump fiber cores 605 e.g., each with a diameter Dcore of 200 pm
  • a double ring of pump fiber core 605 e.g., having a diameter Dcore of 125 pm
  • FIG. 8A-8C illustrate an end coupling combiner embodiment that the inventors have found to be sensitive to thermal loading
  • other combiner architectures may also benefit from the fiber system architectures described herein.
  • a side-coupling combiner architecture may be employed in the fiber system architectures described herein.
  • Another combiner architecture disclosed in U.S. Patent 9,871,338, titled“Pump Combiner for Multi-Clad Fibers,” and under common ownership, may also be suitable for the system architectures described herein.
  • FIG. 9 is a cross-sectional view of further illustrating two ends of double-clad active fiber segments 110 directly coupled to opposite ends of CLS 120, in accordance with some
  • One or more of the features and/or attributes of the fiber CLS architecture illustrated in FIG. 9 may be incorporated into any of embodiment of CLS 120 described above (e.g., in the context of FIG. 1-5).
  • CLS 120 comprises a double-clad fiber segment 903.
  • core 705 and inner cladding 710 are continuous between the active fiber segments 110 and fiber segment 903 such that fiber segment 903 is merely a portion of active fiber segments 110.
  • fiber segment 903 is spliced (along dashed lines), or otherwise joined end-to-end to the active fiber segments 110 with the axes of the core 705 and inner cladding 710 substantially aligned with the corresponding axes of fiber segment 903.
  • Each splice may be a single fusion of two fiber-end faces that have compatible cleave angles (e.g., substantially orthogonal to the longitudinal fiber axis, or z-dimension in FIG. 9).
  • fiber segment 903 has a core diameter that is no larger than a core diameter of at least one active fiber segment 110. Fiber segment 903 may therefore have a core diameter that is less than, or substantially equal to, the core diameter of at least one active fiber segment 110. In some further embodiments, fiber segment 903 has a core diameter that is no smaller than a core diameter of a second of active fiber segments 110. Fiber segment 903 may therefore have a core diameter that is greater than, or substantially equal to, the core diameter of the other active fiber segment 110 such that core diameter increases in a desired signal propagation direction (e.g., increases from MOPA 410 to amplifier 430).
  • a desired signal propagation direction e.g., increases from MOPA 410 to amplifier 430.
  • the NA of fiber segment 903 may have a similar relationship, for example being no smaller than one active fiber segment 110, and no larger than the other active fiber segment 110 such that NA increases in a desired signal propagation direction (e.g., from MOPA 410 to output fiber 450 in FIG. 4 or 5).
  • Fiber segment 903 may further have an inner diameter that is at least substantially equal to the inner cladding diameter of at least one, and advantageously both, active fiber segments 110.
  • Fiber segment 903 may comprise any finite length of fiber within which features, such as recessed surface regions 950, are located.
  • fiber segment 903 lacks an outer cladding material (e.g., lacks outer cladding 715), which for example may have been stripped away during a CLS fabrication process.
  • an outer surface including recessed surface regions 950
  • outer cladding 715 may be exposed to free space (i.e., recessed surface regions 150 are free surfaces).
  • outer cladding 715 may be present within at least some regions or locations of fiber segment 903.
  • fiber segment 903 comprises an outer cladding material (not depicted).
  • a suitable material may be applied as an outer coating over inner cladding 710 within fiber segment 903, and such a material may be different than that of cladding 715.
  • recessed surface regions 950 are to strip light propagating with inner cladding 910. Such light may be propagating to/from active fiber segments 110 and would propagate through fiber segment 903 as well, but for waveguide perturbations associated with recessed surface regions 950 and/or a lack of outer cladding 715. Cladding light denoted by arrow 911 in FIG. 9 may experience total internal reflection, or nearly so, until encountering recessed surface regions 950 where cladding light 911 loss from the waveguide increases. It is generally desirable to remove cladding light gradually over the length fiber segment 903, for example to control the density of power dissipation, and to provide sufficient heat sinking as light stripped from inner cladding 710 may be predominantly converted to heat.
  • Some fiber system components have temperature limits below 100° C, and where an outer cladding 715 comprises a polymer, the upper limit on continuous operating temperature can be less than 85° C, for example.
  • CLS 120 may be thermally coupled to any suitable heat sink, such as but not limited to, a passive heat exchanger operable to transfer heat generated from cladding light to a fluid medium, such as ambient air or a liquid coolant.
  • a passive heat exchanger operable to transfer heat generated from cladding light to a fluid medium, such as ambient air or a liquid coolant.
  • an absorptive mass or block 920 surrounds fiber segment 903 with free space 916 between an interior surface of block 920 and recessed surface regions 950.
  • Block 920 may comprise one or more materials that are absorbing within a band of the cladding light.
  • Block 920 advantageously also has high thermal conductivity to spread heat to an exterior surface where it may interface with an external fluid medium (e.g., air).
  • Block 920 may be a metal, such as, but not limited to, a stainless steel.
  • heat sink block 920 surrounds the entire fiber segment 903 and overlaps a portion of active fiber segments 110.
  • fiber segment 903 may have any number of bends, and may for example comprise a one or more wraps about a mandrel having a suitable radius of curvature, which may for example induce bend losses from inner cladding 710.
  • recessed surface regions 950 do not significantly perturb light propagating within core 705.
  • recessed surface regions 950 may have a depth D, along a ray perpendicular to the cladding axis, that is less than the annular thickness of inner cladding 710 (e.g., D ⁇ Rdad-Rcore).
  • Recessed surface regions 950 may have any topology. In the example illustrated in FIG. 9, recessed surface regions 950 have a depth that varies over the longitudinal fiber length. Variation in the depth may be a result of a beam profile of a laser employed to form the recessed surface regions, or may be the result of modulations in power of a laser employed to form the recessed surface regions, for example. In FIG.
  • recessed surface regions 950 illustrated in dashed line are at a different angular position than those shown in solid- line profile, and further illustrate a polygonal boundary that intersects a cylindrical surface of inner cladding 710.
  • the boundary of an individual recessed surface region 950 is an ellipse, for example associated with the minimum laser spot diameter and the nominal outer circumference of the inner cladding.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Optics & Photonics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • General Physics & Mathematics (AREA)
  • Lasers (AREA)

Abstract

L'invention concerne un système à fibre optique, tel qu'un laser à fibre à pompage inverse, comprenant une paire de combineurs de pompe à fibre. Un suppresseur de lumière de gaine, qui réduit une quantité de la lumière de pompe couplée entre les segments de fibre active, peut joindre deux segments de fibre active entre les combineurs de pompe. Le suppresseur de lumière de gaine peut éliminer l'énergie de pompe avant qu'elle ne soit rétropropagée jusqu'à une sortie des combineurs pour réduire une charge thermique sur les combineurs de pompe.
PCT/US2020/039570 2019-06-27 2020-06-25 Amplificateur à fibre à pompage inverse avec suppresseur de lumière de gaine entre segments de fibre active WO2020264126A1 (fr)

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US62/867,694 2019-06-27

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20140168756A1 (en) * 2011-12-13 2014-06-19 Ofs Fitel, Llc Multi-core erbium-doped fiber amplifier
CN105140763A (zh) * 2015-09-02 2015-12-09 北京航天控制仪器研究所 一种全光纤高功率光纤激光器
CN107732641A (zh) * 2017-11-10 2018-02-23 山东大学 高功率光纤激光器
CN109599740A (zh) * 2019-01-31 2019-04-09 天津大学 具有抑制sbs作用的双向泵浦双包层光纤激光放大器

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20140168756A1 (en) * 2011-12-13 2014-06-19 Ofs Fitel, Llc Multi-core erbium-doped fiber amplifier
CN105140763A (zh) * 2015-09-02 2015-12-09 北京航天控制仪器研究所 一种全光纤高功率光纤激光器
CN107732641A (zh) * 2017-11-10 2018-02-23 山东大学 高功率光纤激光器
CN109599740A (zh) * 2019-01-31 2019-04-09 天津大学 具有抑制sbs作用的双向泵浦双包层光纤激光放大器

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