JP5616714B2 - All-fiber combined high-power coherent beam combining - Google Patents

Description

  [0001] The present disclosure relates generally to high power fiber laser amplifiers, and more specifically, tapered ends of fiber amplifiers for combining beams with improved fill factors. A high power fiber laser amplifier that couples into a fiber bundle.

  [0002] High power laser amplifiers have many applications including industrial, commercial, military and the like. Laser amplifier designers are continually investigating ways to increase the power of laser amplifiers for these applications. One known type of laser amplifier is a fiber laser amplifier that uses a doped fiber and a pump beam to generate a laser beam. Typically, high power fiber laser amplifiers use fibers with an active core of about 10-20 μm or larger diameter. Current fiber laser amplifier designs achieve single fiber power levels up to 5 kW. Some fiber laser systems use multiple fiber laser amplifiers and somehow combine them into higher power ones.

  [0003] The challenge of fiber laser amplifier design is to provide coherent (coherent) coherent (coherent) so that the beam provides a single beam output with a uniform phase across the beam diameter so that the beam can be focused to a small focal point. ) Method to combine the beams from each fiber.

  By concentrating the combined beam into a small spot at long distance (far field), the beam quality of the beam is defined and the more coherent the individual fiber beams, the more uniform the combined phase, Quality will be better. Improvements in fiber laser amplifier design increase the output power and coherency of the fiber beam to approach the theoretical power limit and theoretical beam quality limit of the laser system.

[0004] FIG. 1 is a schematic plan view of a known fiber laser amplifier including a fiber lens array. [0005] FIG. 2 is a cross-sectional view of a fiber lens array used in the fiber amplifier shown in FIG. [0006] FIG. 1 is a schematic plan view of a known fiber laser amplifier including a DOE coupler; [0007] FIG. 1 is a schematic plan view of a fiber laser amplifier including a tapered fiber bundle and a beam phase detector. [0008] FIG. 2 is a view of a tapered fiber bundle and an end cap. [0009] FIG. 6 is a cross-sectional view of the input end of the tapered fiber bundle shown in FIG. [0010] FIG. 6 is a cross-sectional view of the output end of the tapered fiber bundle shown in FIG. [0011] FIG. 6 is a near-field beam intensity profile of the output beam from the tapered fiber bundle shown in FIG. [0012] FIG. 6 is a graph with a core diameter on the horizontal axis and an effective mode diameter on the vertical axis showing the effective diameter of the mode of the step index fiber. [0013] FIG. 6 is a profile of the near-field beam intensity distribution of seven closely packed fiber bundles before tapering. [0014] FIG. 11 is a profile of the near-field beam intensity distribution of the seven fiber bundles shown in FIG. 10 after tapering. [0015] FIG. 6 is a profile of the near-field beam intensity distribution of 19 closely packed fiber bundles. [0016] FIG. 6 is a cross-sectional view of the input end of a tapered fiber bundle including a low index glass cladding. [0017] FIG. 14 is a cross-sectional view of the output end of the tapered fiber bundle shown in FIG. 13 including a low index glass cladding. [0018] FIG. 5 is a perspective view of an end cap for a tapered fiber bundle including a negative GRIN lens. [0019] FIG. 5 is a perspective view of a segmented end cap for a tapered fiber bundle. [0020] FIG. 5 is a perspective view of a tapered end cap for a tapered fiber bundle. [0021] FIG. 7 is a perspective view of a segmented end cap for a tapered fiber bundle including a positive GRIN lens. [0022] FIG. 6 is a schematic plan view of a fiber laser amplifier including a tapered fiber bundle, a phase detector, and a fiber polarization controller. [0023] FIG. 3 is a cross-sectional view of a multi-core fiber. [0024] FIG. 21 is an illustration of the multi-core fiber shown in FIG. [0025] FIG. 2 is a schematic plan view of a fiber laser amplifier including a plurality of master oscillators, a tapered fiber bundle and a phase detector. [0026] FIG. 2 is a schematic plan view of a fiber laser amplifier including a plurality of master oscillators, an SBC grating and a plurality of phase detectors. [0027] FIG. 2 is a schematic plan view of a fiber laser amplifier including a plurality of master oscillators, an SBC grating, a phase detector, and a fiber polarization controller. [0028] FIG. 6 is a schematic plan view of a fiber laser amplifier including a plurality of master oscillators, a plurality of pre-dispersion gratings and an SBC grating. [0029] FIG. 6 is a schematic plan view of a fiber laser amplifier including a plurality of master oscillators, SBC gratings and staircase mirrors. [0030] FIG. 1 is a schematic plan view of a known fiber laser amplifier including a feedback single mode fiber and a preamplifier. [0031] FIG. 6 is a schematic plan view of a fiber laser amplifier including a tapered fiber bundle, a feedback single mode fiber, a preamplifier and a beam sampler. [0032] FIG. 6 is a schematic plan view of a fiber laser amplifier including a tapered fiber bundle, a feedback single mode fiber, a preamplifier, and a fiber polarization controller. [0033] FIG. 6 is a schematic plan view of a fiber laser amplifier including a tapered fiber bundle, a feedback single mode fiber, a preamplifier, and a sampling grating.

  [0034] The following description of embodiments of the present disclosure directed to a fiber laser amplifier including a tapered fiber bundle is merely exemplary in nature and is intended to limit the invention or its application or use. Not.

  FIG. 1 is a schematic plan view of a known fiber laser amplifier system 10 that includes a master oscillator (MO) 12 that generates a signal beam on an optical fiber 14. A fiber laser amplifier system of the type shown in FIG. 1 was issued to Wickham et al. On March 16, 2004, entitled Optical Energy Transmission System utilizing precise phase and amplitude control, and assigned to the assignee of the present application. See assigned US Pat. No. 6,708,003, which is incorporated herein by reference. The signal beam is split into a number of split beams (split beams) by a splitter and phase modulator 16 and a separate phase modulator 16 is provided for each split beam. The splitter and the phase modulator are actually two separate devices, but since they can be implemented on a single chip, here is a single object. As discussed in more detail below, the phase modulator 16 adjusts the phase of each split beam so that all the beams are in phase with each other in the combined output beam 26. The split beam from phase modulator 16 is then sent to fiber amplifier 18 and amplifier 20 represents the doped amplifier portion of fiber amplifier 18 that receives the optical pump beam (not shown). Thereafter, the amplified fiber beams from the fiber amplifiers 18 are sent to a fiber lens array 22 including a cylindrical fiber lens 24 of each fiber amplifier 18 where all the fiber beams are combined together as an output beam 26 combined. Thus, each of the lenses 24 are coupled together as an array 22. The fiber lens array 22 collimates each of the fiber beams to form a tiled array of collimated beams and accurately aligns them with each other. The combined output beam 26 is sent to a beam sampler 28 that splits the beam 26 and the majority of the beam intensity is provided as the output beam of the system 10.

  [0036] ΜO12 also provides a reference beam on fiber 30 that is amplified by fiber amplifier 32 and collimated by lens 34. The collimated reference beam from lens 34 is sent to beam sampler 28, where the reference beam interferes with each of the fiber beams in the combined beam 26, and interference between the reference beam and each separate fiber beam. Bring a pattern. The interference beam is directed by lens 36 to an array 38 of phase detectors, and a separate phase detector 38 is provided for each separate fiber beam. The electrical signal defining the interference pattern between the beams from the detector 38 is sent to the phase processor and the controller 40 that provides a phase correction signal to each of the phase modulators 16 so that all of the split beams are mutually connected. And the phase of the split beam from MO 12 is adjusted so that the output beam 26 is one coherent beam that can be tightly focused in the far field.

  FIG. 2 is a cross-sectional view of a lens array 22 that includes individual lenses 24. As is apparent from this configuration, the cylindrical lens 24 creates a dead space 42 between the lenses 24 that has a reduced fill factor, defined as a fraction of the combined beam area occupied by the high power beam. Come back. As used herein, improved fill factor means greater fill factor and better beam quality or focusability for smaller diffraction limited spots. By making the beams in phase with each other and continuous, the beam quality of the output beam 26 is improved and can be concentrated in a small spot. Therefore, it is desirable to make the lens 24 so that it is packed together as tightly as possible. Further, the actual beam propagating through the core in each fiber is a Gaussian beam having a bell-shaped beam profile with higher center intensity and reduced peripheral intensity. When a closely packed array of Gaussian beams is concentrated, the central focus lobe is typically only about 60% of the combined beam output as a result of the Gaussian shape and the dead space interposed between the beams. Does not include. Thus, the reduced fill factor of the combined beam array is due to the combination of both the Gaussian shape of the individual beams and the intervening dead space 42, and the coupling concentrated in the central far-field focus lobe. The output power given is given by the fill factor, which is about 60% of the total beam power.

  [0038] FIG. 3 is a schematic plan view of a known fiber laser amplifier system 50 that eliminates the fill factor described above, and elements similar to system 10 are identified by the same reference numerals. This type of fiber amplifier is a U.S. Pat. No. 7, issued to Oct. 21, 2008 issued to Rice et al. And assigned to the assignee of the present application, entitled coherent fiber diffractive optical element beam combiner. 440,174, which is incorporated herein by reference. In this embodiment, the fiber amplifier 18 is spliced into a single fiber array 52 to produce an array of closely spaced output beams 54. The output beam 54 is collimated by an optical system 56 and then sent to a diffractive optical element (DOE) 58 that combines the beams when the beam is accurately aligned and adjusted. The diffracted beam from DOE 58 provided at the same angle is directed to a beam sampler 60 that splits the beam, with the majority of the combined beam being the output beam from system 50.

  [0039] The split portion of the combined beam from DOE 58 is collected by focusing optics 52 and sent to phase detector 64. The phase detector 64 measures the phase of the combined beam and sends a measurement signal to the synchronous phase processor 66. By detecting the phase of the beams thus combined, the reference beam can be removed and a single phase detector can be used. The phase of the constituent beam is a different frequency for frequency modulation (FM) or amplitude modulation (AM), different codes for code division multiple access (CDMA) or time division multiple access (TDＴMA), etc. Can be distinguished in a single output phase detector 64 by independently dithering or encoding for phase or amplitude so that a synchronous detector scheme can be used for each fiber beam in the combined beam. The constituent phase signals can be distinguished. Such a technique is described in US Pat. No. 7,346,085 entitled Multistage Method and System for Coherent Diffraction Beam Coupling, issued to Rothenberg et al. On Mar. 18, 2008 and assigned to the assignee of the present application. And is incorporated herein by reference. Synchronous phase processor 66 decodes the different constituent phases in the measurement signal from phase detector 64 and adjusts to the phase of the individual fiber beams in fiber amplifier 18 so that all of the constituent fiber beams in the output beam are in phase. A phase error correction signal is generated for each fiber beam sent to the corresponding phase modulator 16 to be locked. Since the array of fiber beams 52 is combined into a single output beam, the fill factor problem is eliminated, and the output beam can be concentrated in a nearly diffraction limited spot, provided by the total combined power of the beam. The theoretical limit of brightness is almost reached.

   [0040] Diffracted beams 68 from DOE 58 other than the combined output beam limit angular separation, and therefore require a fairly large path length to adequately separate the diffracted output beam, and therefore Let system 50 be less compact. In addition, the array of output fibers must be very accurately aligned with each other and the output optics of the system 50 in order to achieve high beam coupling efficiency. Such accurate alignment is even more difficult in the presence of inevitable heat dissipation with the presence of multi-kW laser beams. Thus, while it would be desirable to provide a fiber amplifier system with excellent beam quality, it avoids the need for the fiber array to be accurately aligned with a large free space optical element. In addition, combining beams in all-fiber format can provide an ideal packaging solution for power scaling at high power within a single fiber aperture, which is then injected directly into the telescope Or can be used as a component for further beam combining.

  [0041] FIG. 4 is a schematic plan view of a fiber laser amplifier system 70 that provides an improvement over the systems 10 and 50 described above by providing beam combining with higher fill factor and beam quality in the fiber material itself. In system 70, elements similar to systems 10 and 50 are identified by the same reference numbers. In this embodiment, the end of the fiber amplifier 18 is connected to the input end of a tapered fiber bundle 72 that couples the fiber amplifier 18 into a single fiber mass. Thereafter, the end cap 74 is attached to the output end of the tapered fiber bundle 72. The output beam from the end cap 74 is collected and concentrated by a telescope 76 that includes a receiving optical system 78 and a parallel optical system 80. The beam from the telescope 76 is sampled by the beam sampler 82 and most of the beam is directed from the system 70 as an output beam. In the manner as described above, the sampled portion of the combined beam from the beam sampler 82 measures the phase of the combined beam and sends a phase measurement electrical signal to the synchronous N-beam phase processor 88. It is concentrated by concentrating the optical system 84 on 86. Thereafter, in order to control the phase of the beam at the fiber amplifier 18 so that all constituent fiber beams in the combined output beam are locked together at the same phase in the manner described above, the processor 88 performs phase modulation. A phase error correction signal is sent to the device 16. Similar to the method described in laser system 50, phase modulator 16 is coupled at the output of system 70 so that an appropriate phase control signal can be determined for the individual beams split by splitter 16. In each split beam present in the beam, a different dithering frequency for FM or AM or a different code for CDMA or TDMA is applied. The phase detector 86 can detect different dithering frequencies or codes, and the processor 88 uses that information to determine the phase error of each split beam and for each of the split beams. An error correction signal can be provided to the corresponding phase modulator 16 to properly phase lock all constituent beams at the combined output.

   [0042] FIG. 5 shows an example of seven fibers 100 each having an outer cladding layer 102 and an inner core 104 through which the beam propagates, coupled to a tapered fiber bundle 106 of the type referred to above. Show. FIG. 6 shows a cross-sectional view of a plurality of clad fibers 108 formed around the input end of a tapered fiber bundle 106 with seven fibers 100 in the inner portion of the bundle 106 and around the bundle of fibers 100. FIG. 7 is a cross-sectional view of the output end of the tapered fiber bundle 106 showing that the combination of fiber 100 and clad fiber 108 has been formed into a single fiber mass 110, with point 112 being the core 104 of the fiber 100. Represents. FIG. 8 is a cross-sectional view of the near-field beam profile 116 of the beam output from the end cap 114.

   [0043] The tapered fiber bundle 106 can be made by any known technique for producing a tapered fiber bundle, where the fibers 100 and 108 are collected into a bundle, and the bundle is a carefully controlled hot melt process. It is pulled out with decreasing diameter. The final result is a reduced version of the first closely packed fiber amplifier bundle, with the final core diameter 2a and spacing b determining the final output fill factor of the combined beam. Because these beams are so intense, it is necessary to join the end cap 114 to the output end of the tapered fiber bundle 106 to avoid damage at the exit surface of the tapered fiber bundle 106. The combined beam expands by diffraction at the end cap 114 until the peak intensity is sufficiently reduced so that surface damage is avoided. A gradient index (GRIN) lens with negative focal length can be incorporated into the end cap 114 to increase the divergence of the output beam, as described below.

  [0044] Once the output beam exits end cap 114, the output beam is collimated to the desired beam size and collimation by telescope 76 by a single lens or curved mirror and / Or imaged. There is no need for a lens array or other precise fiber-to-fiber alignment. External optics are just collimation and / or telescope optics used to expand the beam to the desired size, which is commonly used in many high power laser systems and beam directors. This is in contrast to systems 10 and 50, which both require a very precise alignment of each individual fiber with respect to each external free space optical system. There is no need in the system 70 to exceed the normal alignment requirements for all single beams in the final telescope. Furthermore, there is no requirement for the exact spacing of the fiber cores within the tapered fiber bundle 72 other than that they be spaced as closely as possible, and the divergence of each individual fiber is a few ten mrad. Since (10's of mrad), the co-alignment of the core is considerably relaxed. Thus, this approach provides an output beam that combines a combined output beam with N times the beam power from a single fiber aperture and minimal free space optics, where N is the number of combined fiber beams. The laser system 70 thereby provides a breakthrough in integration, simplicity and durability compared to the systems 10 and 50.

  [0045] The tapered fiber bundle 106 maximizes the fill factor by bringing the fiber cores close together so that the individual fiber modes overlap. Once the modes overlap, there is cross coupling and interference between fiber modes. By locking the fiber phases together, as described above, the formation of an in-phase super-mode can be ensured, which shows constructive interference between all the fibers and between the beams The strength of the gap is greatly improved. In this way, a combined beam with a continuous intensity profile and little or no intervening dead space can be obtained. The challenge is to make a tapered fiber bundle to ensure that the loss in the bundle is negligible. Therefore, the input fiber to the tapered fiber bundle 106 must have a sufficiently large diameter cladding so that little power appears at the cladding surface. In general, this requires the cladding diameter b to be about 2-3 times the core diameter 2a, limiting the power at the cladding surface to a total of 1 PPM for large mode region fibers of interest. Since the core is separated by the cladding diameter b, this ratio mainly determines the pre-tapered fill factor. The fill factor can be quite low for the ratio b / 2a = 3, where only about 20% of the power is concentrated in the central lobe with a 25 μm core and NA = 0.06, where NA is the numerical aperture It is. As the bundle tapers, both the core and cladding diameters generally decrease proportionally as the fiber melts together, so that this ratio of cladding diameter to core diameter is approximately maintained by the taper, Thus, the fill factor appears to be unchanged. In addition, tapering the core diameter appears to reduce the mode diameter so that the peak intensity increases, which may be limited to very high power amplifiers. However, the surprising result is that when the core diameter decreases during the taper, the mode shape changes so that the reduction of the mode region is limited to a minimum and the tails of the mode field distribution are significantly broadened. . This behavior helps to limit the peak intensity in the tapered fiber bundle 72 and ensure a better overlap of the modes, and therefore the fill factor increases significantly even more because it remains constant due to the taper. It can happen.

[0046] FIG. 9 is a graph with core diameter on the horizontal axis and effective mode diameter on the vertical axis showing the effective diameter of the mode of the step index fiber with NA = 0.06 as a function of core diameter. is there. The effective diameter in this plot is defined as the 1 / e ² intensity diameter of Gaussian with the same peak intensity as the fiber mode. It can be seen that as the core diameter decreases, the mode diameter reaches a minimum of approximately 13 μm and then increases rapidly. It should be noted that if the core diameter is less than about 14 μm (V # = NA × 2πa / λ <2.4 for λ = 1080 nm), the fiber is strictly single mode. This sharp increase in mode diameter for smaller cores is the result of an increase in mode tail. Starting with an initial 25 μm core diameter, the mode is well confined to negligible power beyond approximately 2.5 times the core diameter, but as the core diameter and thus V # decreases, the mode tail decreases. This limitation is shown to increase the effective mode diameter and thus increase the mode overlap in the tapered fiber bundle 72. With further reduction of the core diameter, the tail approaches a very limited limit as V # approaches approximately 1, thus allowing arbitrarily large mode overlap, but increased from the cladding layer This is also true for losses. If the phase of the individual beams is locked to ensure in-phase (constructive) interference in the formation of the supermode, the fill factor penalty can be greatly reduced. By optimizing the core size at the output of the tapered fiber bundle 72, while still allowing excellent limits within the now larger cladding of reasonably sized bundles such as 400 μm diameter, the A lap can be achieved.

  [0047] It should be noted that the process can be improved over a simple tapered process in which both core size and spacing are reduced commensurately. Using carefully adapted temperatures in the tapering process leads to improved diffusion of dopants around the core, and thus the effective core size is increased by diffusion for a proportional change in core-to-core spacing. Can do. The effect of this process can further improve the tapered fiber bundle output mode fill factor.

  [0048] As an example of a combined output beam obtained from end cap 74, a tapered fiber bundle closely packed in a hexagonal shape that takes 7 fibers with a core / cladding diameter of 25 μm / 62.5 μm as input think of. Here, the initial core-to-core spacing is also about 62.5 μm. The input is tapered by about 3.6 times to 6.9 μm, V # is reduced to about 1.2, and the core-to-core spacing is reduced to 17.2 μm. The input fiber mode has a negligible portion of the fiber output (approximate signal 1PPΜM) at the non-tapered cladding interface, but once they are fused and tapered together, the mode has a large overlap with the adjacent core. Propagation simulations show that proper adiabatic tapering of the core limits the out-coupling from the lowest order mode to 10-thousand PPM in each core. It is assumed that all mode fields are phased so that they join coherently and thus sufficiently maximize the fill factor.

[0049] A near-field intensity distribution 120 of seven closely packed fiber bundles with a core / cladding diameter of 25 μm / 62.5 μm before tapering is shown in FIG. 10 and the ring 122 is about 190 μm. An assumed reference aperture diameter D _ref , used to define the far field diffraction limited radius λ / D _ref . It can be shown that the LMA mode is well limited and does not overlap, and due to the large spacing between the input cores, the fill factor is quite low. The calculated power in the bucket (PIB) of the combined beam based on this geometry is within a diffraction-limited far-view with an angular radius of 1.2λ / D _ref It can be further shown that it is only about 17%. In comparison, a diffraction-limited flattop beam that completely fills the reference aperture achieves about 84% in this diffraction-limited angle bucket.

[0050] By tapering the seven fiber input bundles to a core diameter of 6.9 μm assumed at a core-to-core spacing of 17.2 μm, an emergency as shown by the near-field intensity distribution 124 in FIG. Resulting in different combined output beams, where ring 126 is the assumed reference diameter. The reference aperture diameter of the near field in this case is chosen to be 69 μm and contains more than 99% of the combined power. Due to the greatly increased mode overlap and fill factor, the combined tapered fiber bundle output now has a very efficient concentration into the diffraction-limited far-field bucket. It can be shown that the PIB of the seven ideally phased beams is about 92% to a diffraction-limited angular radius of 1.2λ / D _ref . Note that this PIB exceeds the 84% achieved by a flat top beam that is diffraction limited and fully filled. The FIB of the seven combined beams increases to about 95% within a radius of 1.5λ / D _ref . Thus, it may be shown that the effect of the tapered fiber bundle 72 is to dramatically increase the fill factor and PIB compared to the input fiber bundle.

[0051] For a given core shape, the effective area of the combined beam can be defined based on the peak intensity, and used to define the maximum power before the loss caused by the intensity becomes a problem. Where the peak intensity of the combined beam of power P is defined as I _peak = P / A _eff . For the seven beams coupled with the fiber bundle of tapered above, but A _{eff =} 630μm ^2, single component beam at the tapered fiber bundle output has an effective area of 80 [mu] m ^2, therefore, the effective area single Increased by 7.8 times for one beam.

[0052] A higher number of input fibers can be used for higher power. For a hexagonal close packing, the next magic number with an additional fiber ring is 19, yielding an effective area of about 1860 μm ² based on the core diameter of the above spacing, and thus about Assuming 3 kW allows more than 60 kW at a single tapered fiber bundle output. The near-field intensity distribution 130 of the output of the tapered fiber bundle with the ring aperture 132 is shown in FIG. 12, where the aperture reference diameter is 96 μm.

  [0053] As described above, it was assumed that supermodes simply occurred in coherent in-phase superposition of individual fiber modes. Due to symmetry, if the six external beams have the same phase locked, there are only two modes of interest and the central beam is in phase or out of phase, which is in phase I +> and out of phase respectively. I-> Super mode. Therefore, the above results depend on the suppression of the phase shift I-> super mode by proper phase matching of the input beam. The use of a phase locking system can ensure that the central beam at the output has the appropriate relative phase with respect to the external beam. However, due to the large mode overlap between the constituent fibers and the tapered fiber bundle 72, there is considerable power exchange between the cores. Simulation shows that for the above example core diameter at the end of the taper, the power initiated at the central core couples from the central beam to the external beam at a propagation distance of about 2.5 mm. Thus, in addition to proper phase matching of the input field, the length and taper of the tapered fiber bundle 72 must be adjusted to ensure the desired uniform power of the beam. In fact, the central peak in the assumed output beam has about 30% higher power than the peak outside the beam. Thus, by designing the length of the tapered fiber bundle 72 so that the power coupling between the cores reduces some of the central core power, the peaks can be made uniform and the peaks for a given total array output. A reduction in intensity can be provided, thereby increasing the total power limit for a given damaged threshold. The required design accuracy for a few percent power balance, based on a simulated 2.5 mm coupling distance, is a few hundred microns and is easily achieved.

  [0054] Current commercial high power tapered fiber bundle packages have a dissipation capability of about 100 W, which will grow as the development of these devices progresses. Reports on multimode pump couplers used in fiber amplifiers that combine powers in excess of 1 kW are reasonable for pump throughput achieved at values greater than 98%. These commercial devices generally attempt to maximize pump brightness by coupling a tapered fiber bundle to the output fiber by receiving an angle that is only slightly larger than the effective cumulative reception of the input. Therefore, these devices generally have significant, i.e. greater than 1% coupling loss. In a tapered fiber bundle of the type proposed in this application, only end caps are used, so there is no loss due to coupling to the output fiber. The inherent absorption loss of high quality transmission fibers used in tapered fiber bundles is very low, i.e. less than 10 PPΜM / cm and is therefore not considered to be a limiting factor.

  [0055] The remaining loss results from large angular mode conversion and scattering during propagation or near the tapered end of the tapered fiber bundle. This of course depends on the design and quality of the production of the tapered fiber bundle. However, the LΜA input fiber of interest has a fairly low, i.e., approximately 0.06 NA, and the angular divergence of this input light is limited, including the residual power in mode wings. For example, an LMA fiber mode of 25 μm / 0.06 NA has a residual power of less than 100 PPM propagating at an angle greater than ± 10 mrad. Even the small 6.9 μm core mode at the end of the tapered fiber bundle described above has a residual power of less than 100 PPM outside an angle of ± 0.2 rad. Heating in a tapered fiber bundle package is likely to be dominated by large angles of externally coupled light absorbed by the tapered fiber bundle cladding material. Thus, the use of a modest NA glass cladding material in a tapered fiber bundle is virtually not absorbing and will greatly mitigate heating from all but a very large angle dispersed within the tapered fiber bundle. For example, fluorine-doped glass can be used as a cladding material with an NA limit of approximately 0.3, thus preventing absorption into a tapered fiber bundle package and allowing avoidance by end caps. Can limit any lower angle scattered light.

  [0056] FIG. 13 is a cross-sectional view of the input end of a tapered fiber bundle 140 including an external low index refractive glass tube 142, and as described above, FIG. 14 shows a tapered tube tube 142. It is sectional drawing of the output edge part of the fiber bundle 140 of.

[0057] As described above, the end cap 74 is used to obtain a high power beam combined from glass without beam quality damage or degradation. As described, the purpose of the end cap 74 is to allow the beam to expand sufficiently so that the strength at the exit surface is below the damaged threshold. Second, it must be ensured that the power reflected from its surface does not adversely affect the fiber amplifier performance. Therefore, it is usually desirable to provide an anti-reflective (AR) coating on the output surface of end cap 74 to minimize reflection. For the small beams described here, it has been reported that damage thresholds greater than about 1 MW / cm ² can be achieved. For a 20 kW output beam, this suggests that the beam must expand to an effective area of about 2 mm ² . For the seven beam combinations described above, the effective 1 / e ² diameter of the combined beam is about 45 μm when it enters the end cap 74, and the total divergence angle is therefore quite small, ie the angle Is approximately ± 0.01 at 1 / e ² of glass, and as a result, a long propagation distance is required to reduce peak intensity. Calculations show that after about 11 cm of propagation such that the beam is an approximate Gaussian with a FWHM of approximately 1.3 mm, the peak intensity is reduced to about 1 MW / cm ² for seven beam outputs of 20 kW. It is. Therefore, as described below, the lens cap diameter needs to be increased to about 5 mm in either the tapered method or segment to provide a diverging beam at the output facet.

  [0058] Even with end caps that have very low absorption, long propagation distances in glass cause difficulties from accumulated thermal optical path distortion (OPD). However, since the beam is maximum at about 1 mm, this is mitigated by the high aspect ratio of the end cap 74. Although surface cooling of the end cap 74 should be adequate, due to the inherent absorption in the end cap 74, there are still inevitable secondary temperature changes. Estimating that heat deposition is uniform over the range of the beam, the temperature difference caused by absorption across the beam width is approximately ΔT = Pα / 4πk = Pα / 180 ° C., where P is the total kW Is the beam power, α is the intrinsic glass absorption in PPM / cm, and the glass conductivity is κ = 1.4 W / m- ° C. The OPD in the glass is about 1.3 waves per cm in length and a temperature difference of 0 ° C., so for a 20 kW beam and 10 cm path length, the maximum OPD is about α / 7 waves. Very low absorption fused silica is reported to be α <1 PPM / cm, therefore OPD is not overwhelming and almost spherical, but this problem is critical for power scaling with this method Limitations may be presented. This shows that thermal management of the end cap 74 for fiber schemes operating at 10+ kW power levels is quite important to minimize OPD.

[0059] As the number of beams expands, this problem becomes more serious as the combined beams have a larger effective diameter, i.e., about 70 [mu] m, and thus have a smaller divergence. For the 19 beam combinations described above, the divergence angle calculated at 1 / e ² is about ± 7.2 mrad in the glass, combined with a larger total power of 60 kW, resulting in an exit intensity of about 1 MW / cm ² . An end cap about 27 cm long would be required to reduce. The dominant problem is the very small divergence of the combined beam.

  [0060] One approach to alleviating this problem is to produce an end cap that includes a negative gradient index (GRIN) lens close to a tapered fiber bundle junction. FIG. 15 is a perspective view of an end cap 150 that includes a negative GRIN lens 152 coupled to a tapered fiber bundle. The remaining portion of the end cap 150 is a uniform glass rod 154, and the GRIN lens 152 and glass rod 154 are optically coupled by a suitable joint 156. A negative focal length lens can significantly increase the divergence of the combined beam, thus reducing the required end cap length to a few centimeters, thereby greatly reducing the accumulated OPD in the end cap 150. Can be reduced. For example, a GRIN lens with a focal length of −0.8 mm approximately increases the divergence of the 7-fiber tapered fiber bundle output by approximately 3 times, thus approximately proportional to OPD for a 20 kW output beam. Reduce to α / 20 waves. With such an approach, the scaling of this scheme for a single aperture power down to 100 kW can be made accessible.

  [0061] The diameter of the end cap 74 can be increased in the segment or by a taper to provide an expanding beam. FIG. 16 is a perspective view of an end cap 160 that includes a stepped segment, in which a negative GRIN lens 162 is coupled by a joint 168 to a tapered fiber bundle and a uniform glass rod 164 of approximately the same diameter. Opposing ends of the glass rod 164 are joined to a larger diameter glass rod 166, which in turn is joined to an even larger diameter glass rod 170 to provide a segment for beam expansion. An anti-reflective coating 172 can be provided on the output surface of the glass rod 170 opposite the GRIN lens 162.

  [0062] FIG. 17 shows an end cap 180 including a negative GRIN lens 182 coupled to a tapered fiber bundle at one end and coupled to a uniform glass rod 184 of approximately the same diameter by a junction 186 at the opposite end. FIG. Thereafter, the tapered glass rod 188 is bonded to the uniform glass rod 184 and the wide end of the tapered glass rod 188 includes an anti-reflective coating 190.

  [0063] According to the standard AR coating reflectivity of 0.2%, the reflected power of 20 kW is only 40 W in the expanded beam, and therefore this reflection reenters the small tapered fiber bundle output fiber. Some should be easily limited to small and safe power.

[0064] For large aperture beam directors, it is desirable that the near field magnified image of the tapered fiber bundle be relayed to the aperture of the beam director. This is achieved by the telescope 76, the lens 78 has a focal length _{f 1,} the lens 80 has a focal length _{f 2,} the lens 78 and 80 are separated by _f 1 + _{f 2,} the length _f 2 / _{f 1} To enlarge the image.

  [0065] It is possible to integrate the lens 78 into the end cap 74 by using a spherical exit surface on the end cap 74 or by joining a focusing GRIN lens at the end cap output. FIG. 18 is a perspective view of an end cap 192 similar to the end cap 160, where like elements are identified by the same reference numbers. End cap 192 includes a positive GRIN lens 194 attached to an anti-reflective coating 172 that operates as lens 78. Such an optical arrangement can be integrated directly into the beam director telescope as well. A more compact telescope with a standard design for high magnification using both positive and negative lenses can also be implemented to optimize the footprint of the magnifying optics.

  [0066] In order to maintain proper beam quality, it is necessary that the polarization of the fiber beam in each of the fiber amplifiers 18 has the same direction. For system 70, the fiber used in fiber amplifier 18 is a polarization maintaining fiber so that all beams in all fibers have the same polarization direction. In some applications, such as high power applications, it may not be appropriate to use polarization maintaining fibers, and thus it is necessary to align the polarization of each of the fiber beams in the fiber amplifier 18.

  [0067] FIG. 19 is a schematic plan view of a fiber laser amplifier system 200 that does not use polarization maintaining fibers, where elements similar to system 70 are identified by the same reference numbers. System 200 uses polarizer 202 to determine the polarization of the fiber beam in the sampled beam from sampler 82. Because the polarizations in the fiber change relative to each other, the polarizer 202 directs more or less light to the polarization detector 204. In order to determine the polarization of each beam in the output beam, the polarization detector 204 uses different frequency dithering or tags in the individual beams. The polarization measurement is provided to a synchronous N-beam polarization processor 206 that determines the relative direction of the polarization in the beam. The processor 206 uses different dithering frequencies or tags to identify the fibers for all measured polarization changes, so that they are the same to control the polarization direction in each fiber. The signal is provided to the polarization controller 208 for the corresponding fiber amplifier 18. Such a polarization control system is disclosed in U.S. Pat. No. 6,317,257 entitled “Technology for Polarization Locked Optical Output” issued to Upton et al. On Nov. 13, 2001 and assigned to the assignee of the present application. And is hereby incorporated by reference.

  [0068] Forming the fiber amplifier 18 into a tapered fiber bundle 72 provides many challenges. It is desirable to provide a certain ratio of fiber core to fiber diameter and provide the fiber cores as closely spaced as possible. Further, for the diameter fibers described herein, fiber flexibility limits fiber handling capabilities. Multi-core fibers are known in the art that include a plurality of cores coupled together in a bundle surrounded by a common cladding layer. Such multi-core fibers are easier to handle and form into a tapered fiber bundle as described above. However, it is necessary to put fiber beams into the individual cores in the multi-fiber core. Furthermore, it is known in the art to provide an outer air cladding around individual cores in a multi-core fiber to provide high NA limited pump light within the cladding around each core. ing.

  [0069] FIG. 20 is a cross-sectional view of a multi-core fiber 210 of the type described above. Multi-core fiber 210 effectively includes a plurality of individual fibers 212 each including a core 214 and an inner cladding layer 216. In addition, each individual core 214 and inner cladding layer 216 has an outer air cladding formed by a number of small glass air bridges 226 that effectively make the air cladding all air in a manner well understood by those skilled in the art. Surrounded by 222. By providing an air cladding 222 around the individual cores 214, the individual fibers 212 are multi-core fiber bodies 224 by chemically etching the air cladding 222 and the air bridges 226 in the glass in the multi-core fiber bodies 224. Can be separated from

  [0070] FIG. 21 is a top view of multi-core fiber 210, with individual fibers 212 separated to form a pigtail extending from multi-core fiber portion 218. FIG. In one embodiment, the multi-core fiber body 224 and the air cladding 222 are etched using hydrofluoric acid or another suitable chemical agent, where the individual fibers 212 are coupled to the fiber amplifier 18. The individual fibers 212 are separated from the portion 218 so that Because the multi-core fiber portion 218 has a significantly larger diameter than the individual fibers 212, it can be more easily handled to form a tapered fiber bundle of the type described above. In the taper process, a suitably high temperature and possibly a vacuum must be applied so that the bridge in the air cladding 222 collapses so that the fiber cladding layer 216 is continuous between the core and the multi-core fiber body 224. Is noticed. This widens the restricted mode in each core 214 and allows it to overlap with other modes in the tapered region of the multi-core fiber 210.

[0071] The embodiments described above can be extended to other types of fiber laser amplifier systems to further increase the output power of the system. FIG. 22 is a schematic plan view of a fiber laser amplifier system 230 that combines multiple beams using spectral beam combining (SBC) to increase beam power. In system 230, a plurality of N master oscillators 232 individually provide beams on fiber 234 at different wavelengths (λ ₁ , λ ₂ ,... Λ _N ). Each master oscillator wavelength is then split into M fibers by a number of splitters and phase modulators 236 in the manner described above. A separate fiber from each splitter and phase modulator 236 is then coupled to a fiber amplifier 238 represented by amplifier 240. The fiber amplifier 238 is then coupled to the tapered fiber bundle 242, which is coupled to the end cap 244 in the manner described above. The tapered fiber bundle 242 and the end cap 244 may be any of the above-described tapered fiber bundles and / or end caps.

  [0072] The N tapered fiber bundles are arranged in a linear array, which is arranged in the back focal plane of a common parallel optical system 248. The output from each end cap 244 is concentrated by the telescope lens 246 and the combined beam for all master oscillator wavelengths is collimated by the parallel optics 248. Thereafter, the parallel beam from the parallel optics 248 is sampled by the beam sampler 250 and most of the beam is sent to the SBC grating 252. The SBC grating 252 is disposed on the opposite focal plane of the parallel optical system 248, and its dispersion with the master oscillator wavelength, the spacing between adjacent tapered fiber bundles, and the focal length of the parallel optical system are determined by the SBC grating 252 for each beam. Are chosen so that they propagate exactly together with all other beams after diffraction by. Thus, all the beams of each master oscillator wavelength are concentrated in the same spot as all of the other master oscillator beam wavelengths.

  [0073] The beam sampler 250 provides a small sample of a group of N beams incident on the grating 252, each of which propagates at a slightly different angle. Focusing optics 254 focuses the combined beams onto N separate phase detectors, and each detector 256 measures the phase relationship between the M beams at each distinct master oscillator wavelength. As described above, the frequency tag may receive each distinct master oscillator wavelength such that the measurement signal from detector 256 is received by a synchronous phase processor 258 that adjusts phase modulator 236 in each wavelength group as described above. For each individual fiber beam. Thus, the signal from each of the N phase detectors 256 is used to phase lock each group of M beams combined by the tapered fiber bundle 242 at each of the N respective wavelengths. . The phase signal is processed synchronously to identify which of the M fibers in the group cause the phase error, and an appropriate modulator is used so that the beam in each wavelength group is optimally phase locked. A correction signal is provided to 236. In this embodiment, the fiber amplifier 238 is a polarization maintaining fiber to ensure a coherent and polarized output beam, and therefore can efficiently obtain maximum diffraction from the SBC grating 252; This is usually much more efficient for one polarization state than the other.

  [0074] FIG. 23 is a schematic plan view of a fiber laser amplifier system 260 similar to system 230, where like elements are identified by the same reference numerals. System 260 is a simplified design for system 230 that utilizes zero order reflection from SBC grating 252. The primary reflection from the SBC grating 252 is the main beam focused to the desired location, and a partial portion of the beam is reflected from the SBC grating 252 as the zero order. Since the zero order reflection from the SBC grating 252 is slightly different for each distinct wavelength group, the focusing optics 254 can focus a distinct beam on a particular detector 256 as described above. Thus, the system 260 does not require the beam sampler 250.

  [0075] FIG. 24 is a schematic plan view of a fiber laser amplifier system 270 similar to system 260, where like elements are identified by the same reference numerals. System 260 uses polarization maintaining fiber, which may or may not be feasible at high power. The system 270 does not use polarization maintaining fiber in the fiber amplifier 256, and therefore needs to use techniques to provide a polarization direction between the fiber beams in each distinct master oscillator wavelength group. To do this, the system 270 is between the focusing optics 254 and a detector 256 that directs a portion of the beam to N polarization detectors 274 that measure the polarization of each distinct wavelength group. A polarizer 272 is used. The sampled beam may be provided by a zero order grating reflection as shown in system 270, or may be provided by a separate sampling optics as shown in system 230. The measured signal from detector 274 is provided to N polarization processors 276 that determine the relative polarization direction between the M fiber beams in each of the N wavelength groups, and the M fibers An appropriate signal is provided to the M polarization controllers 278 on the low power side of each of the amplifiers 238.

  [0076] If the beam from the master oscillator 232 has a very narrow beam bandwidth, the SBC grating 252 provides better beam quality and less divergence. However, by providing a narrow beam bandwidth from the main oscillator 232, acoustic effects in various fibers and other optical components cause stimulated Brillouin scattering (SBS) that tends to damage the optical components. Therefore, it is desirable to increase the beam bandwidth of the master oscillator signal to prevent SBS resulting in the low beam quality as described.

  [0077] FIG. 25 is a schematic plan view of a fiber laser amplifier system 280 that allows a wider beam bandwidth master oscillator but provides a narrower beam bandwidth in the SBC grating 25, with similar elements the same Identified by reference number. To provide this feature, system 280 includes N pre-dispersion gratings 282, one for each wavelength group. The dispersion grating 282 provides dispersion compensation that has substantially the same dispersion as the SBC grating 252 but is oriented in the opposite direction to cancel the net dispersion of each wavelength group beam. The dispersion grating 282 is oriented so that the beams overlap on the SBC grating 252 and are incident at the correct angle to provide co-propagation of the diffracted beam. When the beam from the distributed grating 282 is imaged on the SBC grating 252 using an image relay telescope 284, the beam quality is optimized. The relay telescope optics may be cylindrical to allow a large beam width in a direction perpendicular to the dispersion direction so that the intensity on the grating surface is maintained below the optical damage threshold. .

  [0078] In the system 280, the distributed grating 282 must be accurately and individually aligned with the SBC grating 252 in the manner described above, which can be cumbersome and complex. FIG. 26 shows an alternative embodiment of a fiber laser amplifier system 290 that serves this problem, where elements similar to system 280 are identified by the same reference numerals. In system 290, each distributed grating 282 is replaced with a single pre-dispersed grating 292 that operates in the same manner. Individual beam wavelengths are reflected from the pre-dispersion grating 292 at different angles, and they need to be modified so that all beams are directed to the beam spot before impacting the SBC grating 252. A stair mirror 294 is provided with individual step steps for each beam wavelength group, allowing the beams to have the proper angle so that all beams are aligned together after diffraction from the SBC grating 252. The steps are appropriately selected to have step heights and widths to do. For high power applications, the cylindrical optics 296 and 298 are used to power each beam to a line focus or short focus at different steps of the step mirror 294 to limit the peak intensity below the optics damage threshold. In order to increase the density, it is provided in the beam path between the pre-dispersion grating 292 and the SBC grating 252. The characteristics and incidence angle of the pre-dispersion grating are selected to offset the dispersion of the SBC grating 252. One design that has substantially no net dispersion is to use the same grating with opposite directions for the pre-dispersion grating and the SBC grating.

  [0079] FIG. 27 is disclosed in US Pat. No. 7,130,113 entitled Passive Tuning of Fiber Amplifiers, issued to Shakir et al. On Oct. 31, 2006 and assigned to the assignee of the present application. FIG. 2 is a schematic plan view of a known fiber laser amplifier system 300, such as the type incorporated herein by reference. The system 300 differs from the amplifier system 10 described above and others because it does not use a master oscillator but instead uses an optical feedback loop. The amplifier system 300 includes a fiber amplifier 302 represented by an amplifier 304 that is pumped by a pump beam (not shown) to produce optical amplification. The amplified signal from the fiber amplifier 302 is then sent to a lens array 306 of the type described above that collimates the fiber beam. In order for all fiber beams to propagate together in the same direction, the individual lenses in lens array 306 must be accurately aligned. Co-propagating beams from lens array 306 are sampled by beam sampler 308, with most beams passing through beam sampler 308 as the system output beam. The sampled portion of the beam from the beam sampler 308 is concentrated by the coupling lens 310 and collected by the collector 312 for transmission through a single mode fiber 314 that provides beam feedback. Because fiber 314 is single mode, it passively provides phase alignment of the fiber beam in fiber amplifier 302 as opposed to active control provided by electrical feedback to the phase modulator described above. An optical isolator 16 is provided in the single mode fiber 314 so that the light simply propagates in the feedback direction. The feedback beam is amplified by a preamplifier 318 and split by a beam splitter to provide a fiber beam to several fiber amplifiers 302. Although this technique has been shown to be effective in passively locking the phase of the fiber amplifier 302, it still suffers from the fill factor problem described above with respect to the system 10.

  [0080] The system 300 is also more compact in design and can be improved to reduce optical components requiring alignment by using tapered fiber bundles in the same manner as described above. FIG. 28 is a schematic plan view of a fiber laser amplifier system 330 illustrating this embodiment, where elements similar to system 300 are identified by the same reference numbers. The system 330 includes a tapered fiber bundle 332 that couples the fiber amplifier 302 in the manner described above to provide beam overlap at the output of the tapered fiber bundle 332. The end cap 334 is coupled to the tapered fiber bundle 332 and may be any of the various end cap embodiments described above. The output beam from the end cap 334 is collected by a collimating and magnifying telescope 336 that includes focusing optics 338 and parallel optics 340. Thus, the system 330 solves the fill factor problem of the system 300 with a compact design. As described above, the focusing optical system 338 may be part of an end cap 334 such as a positive GRIN lens.

  [0081] It is possible for the systems 300 and 330 to be passively self-polarizing, which means that all fiber beams have the same polarization state and are coherent. Necessary for beam combining. This can be done passively using the single mode fiber 314 or the polarization of the fiber amplifier 302 can be forced to have all the same polarization by including a polarization maintaining fiber. . Alternatively, a polarization controller can be provided in the system to maintain the polarization direction in the fiber amplifier 302 in the manner described above. FIG. 29 is a schematic plan view of a fiber laser amplifier system 350 that provides polarization control, where elements similar to systems 300 and 330 are identified by the same reference numerals. In this embodiment, the polarizer 352 collects a portion of the beam toward the coupling lens 310 and a polarization detector 354 that measures the polarization difference in the combined beam from the output beam of the tapered fiber bundle 332. Provided to the device 312. The synchronized N beam polarization processors 356 receive the polarization signal measured from the polarization detector 354 and maintain the polarization direction in each fiber amplifier 302 such that the polarization direction in each fiber amplifier 302 is maintained. The control unit 358 is controlled. In order for the polarization processor 356 to identify which of the N beams needs correction, each of the polarization controllers 358 has its own, similar to the method described for phase control in the previous embodiment. A dithering frequency or code must be provided.

  [0082] FIG. 30 is a schematic plan view of a fiber laser amplifier system 360 similar to systems 300, 330 and 350, where like elements are identified by the same reference numerals. In this embodiment, the collimating and expanding telescope includes a combined lens and a sampling grating assembly 362 that includes a lens 364 and a sampling grating 366. Lens 364 collimates the output beam from end cap 334, and sampling grating 366 redirects a small portion of the output beam onto coupling lens 310. The sampling grating 366 can provide any small sample of output without introducing additional separate optics. Magnifying telescopes can also use mirrors instead of lenses.

  [0083] The above discussion merely discloses and describes exemplary embodiments. Those skilled in the art will recognize from such discussion and various modifications, without departing from the spirit and scope of the present invention, as defined in the following claims, from the accompanying drawings and claims. It will be readily appreciated that modifications and changes can be made.

Claims (21)

A main oscillator that generates a signal beam;
A splitter for splitting the signal beam into a plurality of fiber beams;
A plurality of phase modulators each receiving one of said fiber beams and providing phase modulation;
A plurality of fiber amplifiers each receiving a fiber beam from one of said phase modulators, amplifying said fiber beam, each including an output end;
A tapered fiber bundle including an input end and an output end, wherein the input end is coupled to the output end of all the fiber amplifiers, and the output end of the tapered fiber bundle is a single A combined fiber that includes all fiber amplifier portions with fiber cores in a fiber mass, wherein the tapered fiber bundle outputs a combined output beam, and the tapered fiber bundle propagates through each fiber amplifier. A tapered fiber bundle formed and configured such that individual fiber modes to be combined coherently in the tapered fiber bundle into a single combined fiber mode in the combined output beam ; ,
An end cap optically coupled to the output end of the tapered fiber bundle and extending the output beam from the tapered fiber bundle;
A fiber amplifier system comprising a beam sampler that samples a portion of the output beam from the end cap and provides a sample beam.   A phase detector and a synchronous phase processor, wherein the phase detector detects a phase of the fiber beam in the sample beam and provides a phase measurement signal to the phase processor; The system of claim 1, wherein the phase modulator is controlled to control an optical phase of the fiber beam in the fiber amplifier in response to. The phase detector detects the own dithering the phase or amplitude for said fiber beam using different codes for different frequency or code division multiple access or time division multiple access for frequency modulation or amplitude modulation The system of claim 2, wherein the phase of the fiber beam is detected.   A polarization detector, a synchronous polarization processor, and a plurality of polarization controllers each receiving one of the fiber beams, the polarization detector comprising a polarization of the fiber beam in the sample beam. Detecting a wave and providing a polarization measurement error signal to the polarization processor, wherein the polarization processor controls the polarization of the fiber beam in the fiber amplifier in response to the polarization measurement error signal. The system according to claim 1, wherein the polarization control unit is controlled to make the polarization directions of the fiber beams the same.   The polarization detector detects unique dithering in phase or amplitude for the fiber beam using different frequencies for frequency modulation or amplitude modulation or different codes for code division multiple access or time division multiple access The system according to claim 4, wherein the polarization of the fiber beam is detected.   The system of claim 1, further comprising a telescope that receives the output beam from the end cap before the beam sampler, expands and collimates the output beam, and collimates.   The system of claim 1, wherein the tapered fiber bundle includes a plurality of coreless clad fibers disposed around the fiber amplifier.   The system of claim 1, wherein the tapered fiber bundle includes a low index refractive glass tube provided around the fiber amplifier.   The fiber amplifier is coupled together into a multicore fiber, each fiber in the multicore fiber includes an air cladding layer, the multicore fiber is chemically etched at one end into a separate constituent fiber, and the individual core The system of claim 1, wherein a fiber amplifier is coupled to the multi-core fiber by bonding to the constituent fiber, and the other end of the multi-core fiber is tapered to form the tapered fiber bundle.   The end cap includes a negative gradient index lens and a uniform glass rod that are optically coupled together, and the negative gradient index lens is coupled to the output end of the tapered fiber bundle. The system according to claim 1.   The end cap includes an anti-reflective coating provided at the output end of the end cap and a positive gradient index lens coupled to the output end of the end cap or the end cap. The system of claim 1 including a positive lens formed by a curved surface at the output end.   The system of claim 11, wherein the positive lens is part of a collimating and expanding telescope.   The system of claim 1, wherein the end cap surface is curved to form a surface lens. A plurality of master oscillators each generating a signal beam at a different wavelength;
A plurality of splitters each receiving one of the signal beams and splitting the signal beam into a plurality of fiber beams;
A plurality of phase modulators, each including a separate phase modulator for each fiber beam split by each splitter, each receiving one of said fiber beams and providing phase modulation;
A plurality of fiber amplifiers, wherein a separate fiber amplifier is provided for each fiber beam, each fiber amplifier receiving a fiber beam from one of the phase modulators, and the fiber amplifier amplifying the fiber beam A plurality of fiber amplifiers, each including an output end; and
A plurality of tapered fiber bundles, wherein separate tapered fiber bundles are provided for each master oscillator wavelength group, each tapered fiber bundle including an input end and an output end, wherein the input of each tapered fiber bundle The ends are coupled to the output ends of all the fiber amplifiers that receive the same wavelength signal beam, and the output ends of each tapered fiber bundle all receive the same wavelength signal beam as the fiber core in a single fiber mass It said a combined fiber comprises a portion of a fiber amplifier, a fiber bundle of each tapered outputs a beam that is coupled at a wavelength received by said fiber amplifier, a fiber bundle of each tapered, each fiber amplifier Propagating individual fiber modes are coherently combined in the tapered fiber bundle to form a single in the combined output beam. As will be engaged fiber mode, and is formed, a plurality of tapered fiber bundle,
A plurality of end caps, wherein a separate end cap is optically coupled to the output end of each tapered fiber bundle, and the end cap is configured to couple the combined beam from the tapered fiber bundle. Multiple end caps to expand;
Before SL tapered takes accept the combined beam from the fiber bundle, and outputs the pre-Symbol combined beam as a single beam directed in the same direction, and a spectral beam combining (SBC) Grating Fiber amplifier system.   A plurality of phase detectors, wherein a separate phase detector is provided for each distinct signal beam wavelength, further comprising a plurality of synchronous phase processors, and a separate phase processor for each distinct signal beam wavelength. And each phase detector detects a phase of the fiber beam for each signal beam wavelength and provides a phase measurement signal to one of the phase processors, the phase processor responsive to the phase measurement signal. 15. The system of claim 14, wherein the phase modulator is controlled to control the optical phase of the fiber beam for each signal beam wavelength in the fiber amplifier.   A plurality of polarization detectors, wherein a separate polarization detector is provided for each distinct signal beam wavelength, further comprising a plurality of synchronous polarization processors, each comprising a separate polarization processor; A polarization controller provided for a signal beam wavelength, wherein the polarization detector receives a portion of the combined beam and provides a polarization measurement error signal; 15. The system of claim 14, wherein each wave processor receives the measurement error signal from the polarization detector, and each polarization processor controls a polarization controller for one fiber beam.   And further comprising a plurality of pre-dispersion gratings, wherein the separate pre-dispersion grating receives one of the combined beams for a single master oscillator wavelength, the pre-dispersion grating being dispersed with respect to the SBC grating The system of claim 14, wherein compensation is provided, each of the pre-dispersion gratings directs the dispersion-compensated combined beam to the SBC grating.   The system of claim 14, further comprising a single pre-dispersion grating that receives all the combined beams from the tapered fiber bundle and provides dispersion compensation for the combined beams. A splitter that splits the feedback beam into multiple fiber beams;
A plurality of fiber amplifiers each receiving and amplifying a fiber beam, each including an output end;
A tapered fiber bundle including an input end and an output end, wherein the input end is coupled to the output end of all the fiber amplifiers, and the output end of the tapered fiber bundle is a single A combined fiber comprising a portion of all the fiber amplifiers with fiber cores in a fiber mass, wherein the tapered fiber bundles output a combined output beam, and the tapered fiber bundles are each fiber amplifier Tapered fiber formed and configured such that individual fiber modes propagating through the fiber are coherently combined in the tapered fiber bundle into a single combined fiber mode in the combined output beam A bunch,
An end cap optically coupled to the output end of the tapered fiber bundle and extending the output beam from the tapered fiber bundle;
A beam sampler that samples a portion of the output beam from the end cap and provides a focused sample beam;
A fiber amplifier system comprising a single mode fiber that receives the focused sample beam from the beam sampler and provides the feedback beam.   A polarization detector; a synchronous polarization processor; and a plurality of polarization controllers each receiving one of the fiber beams, wherein the polarization detector is a polarization of the fiber beam in the sample beam. And detecting a polarization measurement error signal to the polarization processor, wherein the polarization processor controls the polarization of the fiber beam in the fiber amplifier in response to the polarization measurement error signal. The system according to claim 19, wherein the direction of polarization of the fiber beam is made the same by controlling a wave control unit.   20. The system of claim 19, further comprising a preamplifier that receives the feedback beam in the single mode fiber and amplifies the feedback beam before being sent to the splitter.

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