Device for spectrally combining broadband laser beams by volume Bragg gratings.

Abstract

A beam combiner configured to spectrally combine multiple beams includes at least one pair of identically configured TVBGs. The TVBGs are spaced apart along an optical path and aligned at their respective "+" and "-" Bragg angles. The upstream TVBG diffracts a first beam incident on it at one of the Bragg angles such that the beam's spectral components diverge from each other, thereby defining a fan beam at the output of the upstream TVBG. When the diffracted first beam is emitted to a downstream TVBG at the other Bragg angle, its spectral components are again diffracted, but in a direction opposite to that provided by the upstream TVBG. Thus, the dispersion effects of each TVBG cancel each other out. Another beam is incident on the downstream TVBG and transmitted by it, which combines the twice-diffracted beam and the transmitted beam into a parallel combined beam.

Description

[Technical Field]

[0001] The present disclosure relates to laser beam combining techniques, and more particularly to a system of multiple transmitting volume Bragg gratings that spectrally combine high-power broadband laser beams into a high-power, high-brightness broadband collimated beam. [Background technology]

[0002] Numerous industrial and military applications require high-power laser beams. Typically, high-power single-mode (SM) and low-mode (LM) laser beams, i.e., high-quality laser beams, are particularly sought after for various applications. However, nonlinear and thermal effects limit the output power of standalone SM lasers with narrow bandwidths (up to 3 nm) to approximately 1 kW, while SM lasers with broad bandwidths (4–10 nm) can output up to 10 kW. The limited scalability has various reasons, including finite pump brightness, limited doping concentration, and nonlinear effects, among others. The waveguide design of SM fiber lasers can somewhat mitigate the detrimental effects of Raman and Brillouin nonlinearities, which increases the scalability limit, but the fiber core diameter inevitably lowers this limit.

[0003] The high-power limit can be much higher if two or more SM outputs from individual fiber lasers are combined together. Various methods of beam combining include, among others, coherent beam-combining (CBC) and spectral beam-combining (SBC) techniques.

[0004] CBC is achieved by using mutually coherent beams, i.e., beams propagating at the same wavelength so that the phase difference between each wave is constant. This technique requires controlling the relative phase of the SM beams from each source to provide constructive interference between the output beams. Phase control involves active or passive feedback to provide stable coherent addition. However, the required control increases the structural complexity of the CBC system.

[0005] SBC, or wavelength beam combining, is an incoherent beam combining technique that does not require phase control. The goal of SBC is to combine two or more high-power laser beams propagating at different wavelengths into a combined beam that not only has high power but also retains the beam quality that determines spatial brightness. Thus, while spectral brightness is reduced in volume Bragg grating (VBG)-based spectral combining systems due to spectral broadening, spatial brightness is increased by transmitting beams at non-resonant wavelengths and diffracting beams at resonant (Bragg) wavelengths, as will be explained in detail later. Based on the above, the main advantage of SBC over CBC is its structural simplicity, due to the elimination of the need to monitor and adjust the phase of the individual beams.

[0006] Both CBS and SBC use spectrally dispersive optical elements, including prisms, surface gratings, and volume Bragg gratings (VBGs), among others. Prisms and surface gratings require the use of narrowband laser sources because the angular dispersion of these elements results in a dramatic increase in divergence at the plane of refraction or diffraction. Nevertheless, as mentioned earlier, narrowband lasers output beams of limited power. To obtain higher powers, e.g., 2 to 100 kW, many SM narrowband lasers must operate simultaneously. However, multiple-laser systems present numerous optical and dimensional challenges.

[0007] VBGs can be recorded in photo-thermo-refractive (PTR) glasses, which are known to perform well under kW-level power loads. Fabrication of these gratings involves holographically recording interference fringe patterns. Heat treatment of the exposed glass sample creates a permanent spatial refractive index modulation (RIM) within it.

[0008] Diffraction of light in a VBG occurs only at the resonant (Bragg) wavelength and two specific angles of incidence, known as the "+" and "-" Bragg angles. These conditions are known as the Bragg conditions. The dependence of a VBG's diffraction efficiency on angle of incidence and wavelength is shown in Figures 1A and 1B, with a central lobe 16 and several side lobes 18 separated by zeros. When a beam at a Bragg wavelength (e.g., 1075 nm) strikes the grating at one of the Bragg angles, the VBG diffracts it with maximum diffraction efficiency. When another beam, such as at a wavelength of 1069.3 nm corresponding to one of the spectral zeros, illuminates the VBG at another Bragg angle for the 1075 nm wavelength, the VBG transmits this beam in the same direction as the diffracted 1075 nm beam.

[0009] The graphs in Figures 1A and 1B each show a VBG designed with a period Λ of 1.5 μm and a thickness of 1 mm. As can be seen, the illustrated VBG has 100% diffraction efficiency for radiation at 1075 nm (the Bragg wavelength or resonant wavelength) emitted at 32.6° (the Bragg angle in air). Both the spectral and angular patterns have a central lobe 16 of the diffraction efficiency spectrum and several side lobes 18 separated by zeros corresponding to minimum diffraction efficiencies. The full-width half-maximum (FWHM) wavelength and angular selectivity are Δλ = 5.2 nm and Δθ = 1.8 mrad, respectively. Modeling of the VBG characteristics is based on coupled-wave theory, which is well known to those skilled in the art of laser technology.

[0010] There are two basic types of VBGs: transmission VBGs (TVBGs) and reflection VBGs (RVBGs), both of which provide diffraction of light that satisfies the Bragg condition. TVBGs and RVBGs have different beam combining capabilities. A standard RVBG effectively combines only narrowband beams with spectral widths that do not exceed a small fraction of a nanometer, limiting the usefulness of RVBGs for broadband, high-power laser systems.

[0011] In contrast to RVBGs, TVBGs can effectively diffract radiation with broad spectral widths ranging from over 3 nm to 10 nm (and even larger), which is particularly typical for SM fiber lasers outputting 2-10 kW SM beams in the 1 μm wavelength range. Based on the foregoing, the ability of TVBGs to diffract high-power, broad-spectral-width radiation makes this type of grating particularly attractive for combining high-power broadband beams. When a TVBG diffracts one beam with maximum efficiency at, say, a (+) Bragg angle and transmits another beam at the opposite (-) Bragg angle, the diffracted and transmitted beams combine into a combined, parallel, high-brightness beam.

[0012] FIG. 2 illustrates the operation of a TVBG 10 combining two laser beams 12 and 14 at two different wavelengths, λ1 and λ2, respectively, incident on the same side at Bragg angles "+" and "-" for the wavelength λ1. Beam 12 at a wavelength of λ1, corresponding to 1075 nm in FIG. 1A, is resonant with the grating and is mostly diffracted (deflected by twice the Bragg angle). A second beam 14 at a second wavelength, λ2, such as 1069.3 nm, is at the first minimum (or zero) of the diffraction efficiency curve and passes through the TVBG 10 with minimal loss. In other words, the TVBG 10 does not significantly diffract beam 14 but transmits it. Ideally, the diffracted beam 12 and the transmitted beam 14 are superimposed and collinear in both the near and far optical fields, i.e., they are combined into a single output, collimated, spatially shining beam 20. However, the reality is different as the diffracted beam 12 is fanned out at the output, as indicated by double arrow 15 and explained below.

[0013] However, using a TVBG to combine broadband beams presents several problems. The root of at least some of these problems is beam divergence. Beam divergence reduces the power density of the beams and the spatial brightness of the combined beam. The following describes the physical phenomenon that causes beams diffracted by a TVBG to diverge.

[0014] As those skilled in the art know, all gratings introduce angular dispersion, as shown in Figure 3. In fact, angular dispersion is perhaps one of the most attractive features of any kind of grating, due to its very spectral selectivity. To understand the nature of spectral selectivity, consider a single polychromatic collimated beam at the Bragg wavelength, such as beam 12, incident on TVBG 10 at the Bragg angle. Polychromatic beam 12 has a central wavelength λ 1B (Bragg wavelength) and the slightly shifted spectral component λ 1L and λ 1S(longer and shorter wavelengths, respectively). All spectral components in the incident beam are parallel, but the TVBG 10 diffracts the entire beam, resulting in different wavelengths λ 1L , λ 1S , and λ 1B are deflected at different angles. 1S and λ 1L Both spectral components diverge from their respective resonant Bragg components. This ability of gratings to recognize different wavelengths makes them very attractive for a variety of applications.

[0015] However, the above characteristics of the grating are undesirable when the grating functions as a broadband beam combiner. Indeed, the spectral selectivity of the grating is still important for beam combining, and the fact that the diffracted beam diverges is highly undesirable for the purposes of the present invention for the following reason: diverging or fan-shaped beams have lower spatial brightness. Nevertheless, many industrial applications require beams of high spatial brightness characterized by spectral components that are all parallel, i.e., collimated, nearly diffraction-limited beams.

[0016] Another problem associated with TVBGs is the thermal lensing phenomenon, which affects the divergence of the beam. When a high-power laser beam propagates through a TVBG, the TVBG partially absorbs the beam, thereby releasing optical energy and heating the TVBG. The heating results in a change in the refractive index and expansion of the PTR glass. The temperature distribution in the TVBG is not uniform, which results in the formation of a lens. Therefore, this phenomenon is known as thermal lensing.

[0017] Thermal lenses distort the divergence and quality of the diffracted beam, with the highest power density in the central region and lower power density in the wing-like regions. To compensate for the lenses, various phase masks have been successfully used. However, all known phase masks are monochromatic and cannot effectively shape broadband beams.

[0018] Yet another problem associated with TVBGs used to combine broadband beams is leakage between spectral channels or beams. Returning to Figure 2, leakage occurs when the transmitted beam 14, i.e., the beam that should propagate without diffraction, is still partially diffracted. To avoid this, the spectral width of the TVBG 10 must be wide enough to diffract the broadband beam 12 but not the beam 14. To do this, the side lobes 18 in Figure 1A, which extend into a wider spectral range, should be somewhat suppressed, or better, completely eliminated. The amplitude of the side lobes is inherent in TVBGs with a uniform distribution of refractive index modulation (RIM) in the volume of the PTR medium. To avoid significant leakage between spectral channels, the distance between the spectral channels must be at least 3Δλ. This means that a maximum of three to four channels / laser sources can be used for effective SBC, not only for Yb-doped fiber lasers, which are important for many industrial applications, but also for Cr-doped gas diode lasers. However, the limited number of sources limits the scalability of the TVBG's power output. The same scalability issues are evident for other laser sources, such as fiber, bulk solid-state, semiconductor, and gas emitters. [Prior art documents] [Patent documents]

[0019] [Patent Document 1] PCT / US21 / 16588 Summary of the Invention [Problem to be solved by the invention]

[0020] Based on the foregoing, a beam combiner based on TVBG utilizing SBC technology to multiplex and combine broadband beams into a combined high-power, high-brightness collimated beam, comprising: Minimizing, or preferably eliminating, the effect of angular dispersion in the diffracted beam; Minimize lens effects Minimize leakage between multiple broadband beams (or channels) It is desirable to have a beam combiner configured so that [Means for solving the problem]

[0021] The disclosed combiner addresses the aforementioned problems and fulfills an existing need. The inventive structure comprises at least one pair of identically configured TVBGs that function as a beam combiner to combine high-power broadband beams into a combined broadband, multi-KW, parallel-power beam.

[0022] The inventive structure is configured to compensate for the angular dispersion of TVBGs. Some approaches to addressing angular dispersion are based on one or more pairs of two identically structured standard TVBGs. Each pair of TVBGs is spaced apart along the optical path and aligned at their respective "+" and "-" Bragg angles. The upstream TVBG diffracts a first beam incident at, for example, a "+" Bragg angle. Due to inherent angular dispersion, the beam's spectral components diverge at the output of the TVBG. When the diffracted first beam is directed toward a downstream TVBG at a "-" Bragg angle, the spectral components are again diffracted, but in a direction opposite to that provided by the upstream TVBG. Thus, the dispersion effects of each TVBG cancel each other out. This allows the downstream TVBG to output a multi-kW collimated broadband output beam at the same Bragg angle as the beam incident on the upstream TVBG.

[0023] The downstream TVBG thus functions as a combining TVBG for the diffracted second transmitted broadband beam. The second beam is centered at a wavelength different from the Bragg wavelength of the first beam and is directly incident on the downstream TVBG that transmits the second beam. The twice-diffracted first beam and the transmitted second beam overlap each other in both the near and far optical fields and combine into an output, spatially shining, broadband, collimated beam. As those skilled in the art will readily appreciate, the number of aligned pairs of TVBGs is not limited to just one pair, but can include multiple pairs positioned so that multiple broadband beams ultimately overlap each other in the most downstream TVBG to output a single, high-brightness, high-power combined beam.

[0024] Yet another scheme utilizes PTR glass, which allows several standard TVBGs to be recorded on a single glass plate. The TVBGs completely overlap each other within the plate while remaining optically independent. Thus, this simplest scheme has two upstream "-" and "+" TVBGs that diffract their respective beams at Bragg wavelengths offset from each other. The diffracted beams are then incident on downstream TVBGs, each of which corresponds to a "+" and a "-" written on the same glass plate. A third beam, corresponding to the zeros of both downstream TVBGs, is transmitted by that TVBG. The twice-diffracted and transmitted beams overlap each other upon exiting the PTR glass to form a single high-power, broadband, collimated output beam.

[0025] Aspects of the present disclosure that address the leakage problem include creating a specific (e.g., Gaussian) profile of the RIM in the direction perpendicular to the grating vector that results in suppression of lateral lobes. Specifically, the disclosed TVBGs are configured with an optimized apodization (or spatially non-uniform coupling) profile that allows for almost complete suppression of lateral lobes, which can result in a reduction in the distance between channels and therefore an increase in the number of these channels.

[0026] Another aspect of the present disclosure addresses thermal lens compensation in previously disclosed broadband beam combiners. One approach to minimizing thermal lensing is to have the combiner of the present invention constructed with the minimum possible thickness. The minimum thickness limits heat generation by reducing radiation absorption, accelerating heat conduction to the surface, shortening the optical path, and increasing the focal length of the lens.

[0027] Another approach to minimizing the temperature gradient across the TVBG is based on shaping the SM broadband beam so that its Gaussian intensity distribution is transformed into a flat-top intensity distribution. The smaller the power density gradient between the central and wing regions of the SM beam, the more uniform the heat generation in the grating. The flat-top beam has a substantially constant power density across the cross section of the beam, which is about half the peak power of the Gaussian beam. However, the average power is practically the same.

[0028] One of these beam shaping schemes involves a combiner of the present invention consisting of a holographic achromatic phase mask created in a dedicated PTR glass plate positioned along the optical path downstream from the combining TVBG. In contrast to known monochromatic masks, holographic masks work effectively with broadband beams. It is possible to have both the TVBG and the mask in the same PTR plate.

[0029] According to another outline of this embodiment, the beam combiner disclosed in the previously discussed embodiment additionally comprises another holographic phase mask that transmits and converts the broadband SM beam into an optical vortex (Laguerre-Gaussian beam). When the converted beam is combined with the diffracted beam, the radiant intensity distribution of the combined beam assumes a flat-top profile.

[0030] Still other aspects, embodiments, and advantages of these example aspects and embodiments are disclosed in detail below. Moreover, both the above information and the following detailed description are merely illustrative examples of the various aspects and embodiments and are intended to provide an overview or framework for understanding the nature and characteristics of the claimed aspects and embodiments. Features of each overview illustrating any of the previously disclosed aspects may be fully incorporated into either of the overviews illustrating the two other disclosed above aspects.

[0031] The above and other features will become more apparent with reference to the accompanying drawings, which are not drawn to scale. The drawings provide an illustration and further understanding of various aspects and schematics and constitute a part of this specification, but do not represent limitations of any particular schematic or aspect. In the drawings, each identical or nearly identical component appearing in various figures is designated by a similar reference numeral. For purposes of clarity, not all components have the same reference numeral. [Brief explanation of the drawings]

[0032] [Figure 1A] FIG. 10 illustrates the dependence of the diffraction efficiency of an exemplary TVBG on wavelength. [Figure 1B] FIG. 10 illustrates the dependence of the diffraction efficiency of an exemplary TVBG on the angle of incidence. [Figure 2] Illustrates spectral combining with a single TVBG. [Figure 3] 3 is a diagram of the angular dispersion of the spectral components of the polychromatic beam diffracted by the TVBG of FIG. 2. [Figure 4] FIG. 1 is a schematic diagram of the present invention including two successively positioned TVBGs that in combination eliminate angular dispersion of the spectral components of a polychromatic beam. [Figure 5] FIG. 10 is a ray tracing diagram of multi-wavelength plane wave diffraction with the wave incident on the TVBG at a positive Bragg angle. [Figure 6]FIG. 6 shows the dependence of the diffraction angle in the TVBG of FIG. 5 on the wavelength for excitation at the Bragg angle for a wavelength of 1075 nm. [Figure 7] FIG. 10 is a ray tracing diagram of multi-wavelength plane wave diffraction with the wave incident on the TVBG at a negative Bragg angle. [Figure 8] 8A and 8B are ray tracing diagrams of multi-wavelength plane wave diffraction in two successive TVBGs of FIGS. 5 and 7, respectively. [Figure 9] 1 is a diagram of an exemplary optical schematic configured to combine two broadband, high-power beams into a single broadband, high-power, high-brightness collimated output beam in accordance with concepts of the present invention. [Figure 10] FIG. 10 is a diagram of another exemplary optical schematic configured to combine multiple broadband high-power beams into a single broadband high-power collimated output beam. [Figure 11] FIG. 10 is a diagram of yet another exemplary optical schematic configured for combining multiple broadband, high-power beams into a single broadband, high-power, collimated output beam with a multiplexed TVBG. [Figure 12A] 9 is a diagram of the experimentally obtained divergence of a superluminescent diode beam (M2=1) after Bragg diffraction of the "+" in the first TVBG of FIG. 4 or FIG. 8. [Figure 12B] 9 is a diagram of the experimentally obtained divergence of a superluminescent diode beam (M2=1) after Bragg diffraction of a "+" in the first TVBG and a "-" in the second TVBG in the series of FIG. 4 or FIG. 8. [Figure 13] FIG. 1 shows the emission spectrum of a typical high-power SM beam from a Yb-doped fiber laser modeled with a Gaussian function. [Figure 14A] 11A-11C are diagrams of the inventive beam combiners of FIGS. 4-10 configured to combine beams at wavelengths of 1060 nm, 1070 nm, and 1080 nm, respectively. [Figure 14B] 14B is a diagram of the emission spectra of the three laser sources of FIG. 13 with different center wavelengths and diffraction efficiency spectra of the respective upstream and downstream gratings of the inventive coupler of FIG. 14A. [Figure 14C] FIG. 10 shows the dependence of the diffraction efficiency on the angle of incidence for a uniformly apodized TVBG. [Figure 15] 10 is a diagram of the quality of a 1 kW SM beam diffracted by a TVBG in the horizontal (Mx) and vertical (My) planes of diffraction. [Figure 16] 4-12 and 14A are diagrams of the inventive couplers of FIGS. 4-12 and 14A configured with a holographic achromatic phase mask to compensate for the undesirable effects of thermal lensing produced when combining TVBGs with high-power broadband beams. [Figure 17] 4-12 and 14A are diagrams of the inventive couplers configured with one or more holographic phase masks to transform a Gaussian beam (TEM00) into a parallel optical vortex to minimize the detrimental effects of induced thermal lensing in the coupled TVBG. [Figure 18] FIG. 10 is a diagram of the radiation intensity distribution profile in a combined TVBG that multiplexes a diffracted Gaussian beam and a transmitted vortex beam. [Figure 19] FIG. 10 shows the dependence of the RIM required for 100% diffraction efficiency of the TVBG on the thickness of the TVBG. DETAILED DESCRIPTION OF THE INVENTION

[0033] 4-13 show schematics addressing angular dispersion, i.e., the dependence of diffraction angle on wavelength, and its role in spectral beam combining, respectively. Angular dispersion is an inherent property of VBGs. Physically, angular dispersion is responsible for the increased divergence and therefore reduced spectral brightness of polychromatic beams diffracted in the VBG. When a VBG operates as a beam combiner multiplexing multiple broadband beams, angular dispersion detrimentally affects the brightness of the combined beam. Broadband beams are typically associated with high-power lasers, including, for example, fiber lasers. Nevertheless, the disclosed subject matter is fully applicable to other types of lasers. While SM lasers are of particular interest here, it should be noted that the inventive concepts also encompass multimode (MM) lasers.

[0034] 4, the basic scheme of the present invention for eliminating angular dispersion of VBGs includes two identical periodic "+" and "-" TVBGs 22, 24, respectively. The TVBGs are sequentially aligned along an optical path to output a collimated, i.e., unfanned, broadband beam 28. Note that the output beam 28 propagates parallel to the input polychromatic beam 25, which is incident on the upstream TVBG 22 at a positive Bragg angle.

[0035] The illustrated schematic operates in the following manner: An incident beam 25 is first diffracted and coupled into the upstream TVBG 22. The angular dispersion of the TVBG 22 causes a fan-shaped spread of spectral components 26 at the grating's output. As can be seen, the solid central Bragg wavelength is diffracted at twice the Bragg angle, and the long / dashed and short / dotted wavelengths (or spectral components) of the fan-shaped beam 26 are output at different angles relative to the Bragg angle. The fan-shaped beam 26 then enters the downstream TVBG 24 at a negative Bragg angle. As a result, the downstream TVBG 24 reverses the angles at which the long and short wavelengths were diffracted in the upstream TVBG 22, outputting a collimated broadband beam 28. In other words, the effects of angular dispersion in each TVBG 22, 24 cancel each other out. The width of the collimated output beam 28 in the plane of diffraction is greater than that of the incident beam 25 because different spectral components have different lateral displacements in the TVBG. However, the increased width does not have a noticeable effect on the target because the optical power density is not reduced in the far field of the beam 28, as will be explained in more detail below.

[0036] A more detailed description of angular dispersion and mechanisms for compensating for angular dispersion will be discussed with reference to FIGS. 5-8. Specifically, FIGS. 5 and 7 show ray traces for the diffraction of a multi-wavelength plane wave in TVBGs 22 and 24, respectively, of FIG. 4. For simplicity, TVBGs 22 and 24 are each shown as having a grating vector (K G ) is perpendicular to the surface normal. G where θ represents the grating direction and period. Those skilled in the art will readily understand that the grating vector may not be perpendicular to the surface, but may be tilted at a random angle. The angular dispersion phenomenon in a VBG likely does not follow one of the most well-known laws of reflection, namely, that the angle of incidence is equal to the angle of reflection. In contrast, here, the spectral components in the beam incident on the TVBG are all parallel to each other, but diverge at different angles, providing a fan-shaped form to the output beam.

[0037] Figure 5 shows the Bragg conditions (λ) selected from the wavelength range of 205 to 3500 nm. B ,K B ), longer wavelengths (λ L ,K L ), and shorter wavelengths (λ S ,K S ) shows the ray tracing of the diffraction at positive Bragg angles in the upstream TVBG22. G is the lattice vector, and θ i -Angle of incidence in air, θ im -Angle of incidence in the medium, θ dB - the diffraction angle (Bragg angle) for the resonant wavelength in the medium, θ dL and θ dS - the diffraction angle in the medium for longer and shorter wavelengths, θ e - Exit angle in air, θ tm - angle of transmission in the medium, θ t - the angle of transmission in air, the AA line being a plane of constant refractive index.

[0038] Positive Bragg diffraction is K G and K. B The diffraction angle is determined as the vector sum of the lattice vector and the specific wave vector, i.e., the fan beam. For a symmetric VBG, the angle of incidence and the angle of diffraction in the medium for the Bragg wavelength (solid line) are equal, i.e., θ im =θ dB Therefore, the exit angle of the diffracted central spectral component, i.e., the exit angle of the Bragg wavelength, is equal to the incident angle, θ i =θ eB For spectral components of the incident beam 25 propagating in the same plane wave but detuned from the Bragg wavelength, the symmetry is broken due to the different lengths of the wave vectors. B Detuned wavelengths (shorter λ) pumped at Bragg angles about S and longer λ L), the respective diffraction angles are not equal to the angles of incidence. Thus, the fan of spectral components of output beam 26 acquires divergence in the plane of diffraction, where the direction of propagation of the spectral components expands in the negative direction (clockwise) as the wavelength increases.

[0039] FIG. 6 shows the dependence of the diffraction angle on wavelength for a TVBG 22 recorded with a period Λ of 1 μm. In the graph shown, the vertical axis represents the deviation from the diffraction angle (θ) for a resonant wavelength of 1075 nm. d,λ -θ d,1075 ) As can be seen, detuning on the order of a few nanometers results in a deflection of several milliradians. For high-quality laser beams with diameters exceeding a few millimeters, the diffraction-limited divergence is less than 1 milliradian. The graph clearly shows that the angular dispersion of a TVBG diffracting the broadband SM laser beam 25 of FIG. 5 significantly increases the beam divergence.

[0040] Figure 7 shows a ray trace of diffraction at negative downstream Bragg angles in a TVBG 24. The mechanism illustrated in this figure is identical to that in Figure 5. However, in contrast to Figure 5, here the polychromatic incident SM beam 25 acquires divergence in the diffraction plane, where the direction of propagation of the spectral components 26 widens in the positive direction (counterclockwise) as the wavelength increases.

[0041] Figure 8 shows a structure of the present invention based on a combination of the teachings of Figures 5 and 7, respectively, disclosed above. Figure 8 shows a multi-wavelength plane wave diffracted by two consecutive identical VBGs 22 and 24, corresponding to negative and positive Bragg angles, respectively. The configuration shown eliminates angular dispersion, thus forming a collimated broadband beam 28. In the figure, for a resonant wavelength in the Bragg condition, the wavelength and grating vector are λ B and K. B and for longer wavelengths λ L and K. L and for shorter wavelengths λ S and K. S Instructed to do so, K Gis the lattice vector.

[0042] As mentioned previously, the principles of the present invention cause transverse walk-off of spectral components in a twice-diffracted laser beam (transverse spectral chirp) and ellipticity in the near field. For a 10 mm diameter beam and 4 mrad angular dispersion, the diameter increases by 0.2 mm after 50 mm of propagation. This effect results in a small decrease in the power density of the beam in the near field, while leaving the power density unchanged in the far field. Therefore, the brightness remains unchanged.

[0043] FIG. 9 shows a basic schematic of a two-beam combiner 50 of the present invention based on the teachings of FIGS. 5-8. The combiner 50 spectrally combines two broadband beams 30, 32 emitted by respective high-power laser sources LS1 and LS2 into a parallel combined beam 34 at center wavelengths of λ1 and λ2, respectively. The schematic includes two identical, consecutively positioned upstream and downstream gratings TVBG1B- (22) and TVBG1B+ (24). Beam 30 enters upstream TVBG1 22 at a Bragg angle to acquire divergence in the plane of diffraction and exits TVBG 22 as a fan beam of λ1 spectral components. The fan of λ1 spectral components is further emitted at downstream TVBG1B+ 24, which diffracts beam 30 in the opposite angular direction, canceling the angular dispersion beam 30 acquires at upstream TVBG1B- 22.

[0044] The other beam 32 has a spectral component λ2 and is incident at a Bragg angle of "-" with respect to λ1, where λ2 is detuned to one of the zeros in the diffraction spectrum shown in Figure 1A, so that beam 32 passes through the grating without diffraction.

[0045] Figure 9, like all other similar figures, shows the diffracted beam 30 and the transmitted beam 32, respectively, with lateral shear. However, both beams completely overlap in both the near and far fields. The combined output beam 34 (30 + 32) is characterized by reduced spectral intensity, but doubled spatial intensity.

[0046] 10 shows three beam combiners 50 including multiple TVBGs 22, 24, 22', and 24'. The TVBGs are arranged in respective pairs. One pair includes an upstream "-" TVBG 22 and a downstream "+" TVBG 24, respectively, while TVBG B-22' and TVBG B+24' define the other pair.

[0047] Specifically, beam combiner 50 spectrally combines three broadband beams 42, 40, and 36 centered at wavelengths λ1, λ2, and λ3, respectively. Upstream TVBG 22 and downstream TVBG 24 each operate identically to that in FIG. 9, diffracting the component of beam 40 centered at λ2 twice to cancel out the angular dispersion contribution of beam 40 in each of TVBGs 22 and 24. The twice-diffracted output beam 40' is therefore collimated and propagates downstream from TVBG 24 at the same angle of incidence as beam 40. Beam 42 has a spectral component centered at λ1, which corresponds to one of the zeros in the diffraction spectrum of the TVBG in FIG. 1A, and therefore propagates through TVBG 24 without distortion. Beams 42 and 40' overlap each other in both the near and far optical fields and together define a first broadband high power collimated output beam 34 comprised of wavelengths λ1 and λ2.

[0048] The other pair of identical TVBGs 22', 24' also operates according to the basic schematic of Figure 9. A third beam 36 centered at λ3 impinges on the upstream TVBG 22' at a negative Bragg angle. The once-diffracted fan-shaped beam 36 undergoes another diffraction in the downstream (+) TVBG 24'. The TVBG 24' combines the twice-diffracted parallel beam 36' at λ3 and the broadband parallel beams 34 at λ1 and λ2 into a high-brightness, high-power broadband parallel output beam 44. The first pair of TVBGs 22, 24 and the second pair of TVBGs 22', 24' can all be identical to each other. Alternatively, the second pair of TVBGs can be different from the first pair of TVBGs. For example, the second pair of TVBGs may be slightly skewed relative to the first pair of TVBGs or may have a different period compared to the first pair of TVBGs.

[0049] It is clear from the schematic in Figure 10 that the number of combined channels, and therefore the number of broadband beams, is not limited. Ultimately, all beams overlap in both the near and far fields. The limit on the number of channels is determined by the overall width of the gain spectrum of the laser emission medium compared to the spectral width of the individual lasers.

[0050] FIG. 11 shows yet another schematic of a beam combiner 50 that operates according to the principles of the present invention as described, for example, with respect to FIG. 9. However, in contrast to FIG. 9 (and FIG. 10), this schematic includes an odd number of PTR glass plates. This is because a single PTR glass plate can accommodate multiple gratings. Thus, the illustrated schematic includes two upstream TVBGs 22 and 22′, each recorded on a dedicated PTR plate. However, the downstream TVBGs 24 and 24′ paired with their respective upstream TVBGs 22 and 22′ share the same PTR glass plate 35.

[0051] The upstream TVBGs 222 and 322' diffract respective broadband beams 54, 56, each containing a respective group of spectral components centered at λ3 and λ2. Once diffracted, the respective fans of spectral components centered at λ3 and λ2 impinge on the downstream PTR plate 35. The downstream TBGs 24 and 24' recorded on the PTR plate 35 provide diffraction at Bragg angles opposite those of the respective upstream gratings 22 and 22'. As a result, the collimated beams at λ3 and λ2 are superimposed in the near and far fields, respectively. The third broadband beam 52 at λ1 propagates through the multiplexed hologram 35 without distortion and superimposes with the twice-diffracted beams 54, 56 in the near and far fields. As with the previously disclosed schematics, the number of TVBGs is not limited to the three gratings shown. Because PTR glass can accommodate more than one grating, the number of channels coupled by a single multiplexed TVBG can be increased depending on the thickness and refractive index modulation in the glass plate. As with the previously disclosed schematic, the number of groups containing upstream multiplexed TVBGs is not limited to those shown in Figure 11 and can be increased.

[0052] 12A and 12B show experimental data confirming the ability of a pair of spatially uniform TVBGs (FIGS. 4 and 8) to compensate for the angular dispersion of the TVBGs. The data show the quality parameter M 2 This was obtained using a SM superluminescent diode outputting a collimated beam with λ = 1 (highest quality). The TVBGs have a period Λ of 2.47 μm and a thickness of 1.5 mm, respectively.

[0053] With particular reference to FIG. 12A, when an incident beam in the XZ plane is diffracted by an upstream “+” TVBG, the beam quality is M y 2 =1.03 and M x 2= 2.25. As can be seen, the divergence of the diffracted beam increased slightly in the Y direction due to lattice imperfections. However, in the plane of diffraction (X direction), the diffracted beam exhibited significantly larger divergence, which adversely affected the beam quality. The reason for this deterioration in the X direction is a result of the angular dispersion of the TVBG.

[0054] 12B illustrates the effect a downstream "-" TVBG has on a once-diffracted beam. Specifically, a twice-diffracted beam, such as a single-mode (SM) ytterbium (Yb) beam, upon diffraction at a "-" Bragg angle, exhibits a high M value, which indicates that the beam is of high quality, as will be readily understood by those skilled in the art. y 2 =1.07 and M x 2 = 1.08.

[0055] A second problem associated with TVBGs results from radiation leakage between channels, i.e., diffraction of a portion of the transmitted beam. As discussed previously, the spectral width of the TVBG must be wide enough to diffract a broadband beam, such as that shown in FIG. 13. Specifically, FIG. 13 shows the emission spectrum of the output of a rather typical high-power, single-mode Yb fiber laser. Leakage occurs when the side lobes 18 in FIG. 1A, located to the sides of the main lobe 16 of maximum diffraction efficiency, spectrally broaden and diffract adjacent beams that should be transmitted.

[0056] 14A-14B provide an illustration of the leakage mechanism in a coupler 50 of the present invention, which multiplexes three channels at wavelengths of 1060 nm, 1070 nm, and 1080 nm. This problem is linked to the specific spectrum of the TVBGs. Therefore, the upstream grating (FIG. 10), which provides the initial angular fanning of the beams, is not shown for simplicity. A first downstream TVBG 22 diffracts the 1060 nm channel and transmits the 1070 nm channel, which combine into a first parallel combined beam at 1060 nm and 1070 nm. The 1070 nm channel is incident on a second downstream TVBG 24, which transmits the first combined beam and diffracts the 1080 nm channel, so that all three beams are combined into a fully combined parallel beam. As previously shown, coupler 50 with an upstream TVBG 24 minimizes angular dispersion due to the structure of the present invention. However, angular dispersion is not the only phenomenon observed in the structures shown, as will be explained later.

[0057] Figure 14B shows the diffraction efficiency spectra of the gratings 22 and 24 of Figure 14A. Specifically, the gratings are recorded in PTR glass with a period of 2.05 μm and a thickness of 2.37 mm. The solid curves are the emission spectra of the respective SM Yb lasers with a spectral width of 3.4 nm (FWHM). The gratings have broad spectra to provide efficient diffraction of the respective beams at 1060 nm and 1080 nm. In other words, the 1060 nm and 1080 nm channels are positioned at the maximum diffraction efficiency of the corresponding grating's spectrum. To avoid unnecessary crosstalk between the 1060 nm channel and the 1070 nm channel, and between the 1080 nm and 1070 nm channels, the 1070 nm channel is positioned at the first zero in the diffraction spectrum of each grating 22 or 24. However, as mentioned earlier, due to the nature of VBGs, the spectrum of each grating also has side-lobe groups that extend into the regions of the other channels. Thus, both gratings 22 and 24 transmit the 1070 nm channel with some loss caused by inefficient diffraction at the central lobe and the side-lobe wings, respectively. Also, the combined TVBG 24 of FIG. 14A undesirably diffracts a portion of each 1060 nm channel when the first combined beam propagates through this TVBG. The same result, i.e., unwanted diffraction of the transmitted channels, would be obtained if the relative positions of gratings 22 and 24 were reversed. In this case, TVBG 22 becomes the downstream combined grating, affecting the 1080 nm and 1070 nm channels combined into the first output beam in the upstream grating 24. Based on the foregoing, it is necessary to suppress the side-lobe portions of the spectrum of each grating 22, 24.

[0058] FIG. 14C illustrates the suppression of the side lobes 18 of FIG. 1A in a single TVBG, such as downstream TVBG 24 of FIGS. 4-11, using apodization techniques known to those skilled in the art. Generally, the term apodization refers to a gradual change in RIM in the direction of beam propagation that results in the elimination of undesirable spectral features, such as side lobes 18. Here, apodization refers to the gradual change in RIM in the direction of beam propagation that results in the elimination of undesirable spectral features, such as side lobes 18, of the lattice vector K of FIGS. 5, 7, and 8. G This involves creating a specific profile of RIM in the direction perpendicular to the radiator. Specifically, the apodization technique used here involves gradually erasing the RIM by exposing the surface layer of a PTR glass sheet to short-wavelength UV radiation. The results shown in Figure 14C were obtained using a 4 mm thick TVBG with a 3 μm period Λ and gradually erasing the RIM in a 0.5 mm thick surface layer. Compared to the uniform TVBG 74, the apodized TVBG 76 provides significant suppression of the lateral lobes 18. Full optimization of the apodization profile can practically completely suppress the lateral lobes 18, which can shorten the spectral distance between adjacent channels. Compared to the uniform TVBG of the present invention, which combines four or fewer broadband beams / channels, the apodized TVBG of the present invention can combine five to six channels, significantly increasing overall multi-kW output power.

[0059] The third problem addressed by the present invention is heat-induced thermal lensing, which is a result of the absorption of high-power broadband beams in the coupler of the present invention. One of the adverse effects of thermal lensing is the detrimental effect on the collimation and beam quality of the beam passing through the TVBG. The absorption of laser emission in the 1 μm wavelength range is approximately 10 -4 cm -1 (approximately 250 ppm / cm). It has a thermo-optic coefficient dn / dT<1 ppm / K and a thermal expansion coefficient CTE=9.5 ppm / K. Therefore, the thermal lens in the PTR glass holographic element is 1 kW / cm. 2This is so small that there is no concern about power densities below this level.

[0060] In contrast to low power densities, higher power densities have a significant effect on beam quality. For example, a 1 kW SM fiber has a beam quality parameter M 2 = 1.1, a 6 mm beam with a period Λ of 1.17 μm, a thickness of 1 mm, and a length of 25 × 25 mm. 2 A grid consisting of openings of about 3.5kW / cm 2 diffracts a beam with an average power density of

[0061] Figure 15 shows the beam quality of the diffracted beam obtained during experiments with the previously disclosed 1 kW fiber laser. As can be seen, the M of the diffracted beam in the vertical direction "y" is depicted by a square. 2 is due to the thermal lens effect, M 2 = 1.25. However, the beam quality deteriorates in the horizontal direction "x" due to the cumulative effects of angular dispersion and thermal lensing. 2 =1.57 The coupler of the present invention includes several structural additions to the primary structure disclosed above that solve this problem.

[0062] One of the structural additions involves the use of a phase mask. In contrast to monochromatic phase masks, which still cannot function with broadband beams, the present combiner utilizes a holographic achromatic phase mask (HAPM) to correct thermal distortions accumulated in the beam combiner. The induced thermal lens recorded in the PTR glass with the recorded TVBG of the present combiner by a high-power beam with a Gaussian intensity profile is typically a concave lens with a complex shape. Therefore, to minimize the induced divergence, the beam passes through a negative lens with a complex shape that substantially compensates for the spectral aberration caused by the positive thermal lens. The HAPM therefore provides an optical effect that is opposite to the effect of the induced thermal lens and thus compensates for the thermal lens induced in the combined TVBG. Therefore, the present combiner equipped with the HAPM outputs a combined beam with substantially higher quality and therefore greater brightness than a beam combined by the present combiner without the HAPM. The HAPM used in the present invention is disclosed in detail in co-pending US Patent Application Publication No. 2005 / 0129994, which is commonly owned with the subject application and is incorporated herein by reference in its entirety.

[0063] FIG. 16 shows an example of the inventive combiner 50 of FIGS. 4-11, incorporating a previously discussed HAPM, such as a complex conventional lens or Fresnel lens hologram. A first pair of upstream and downstream TVBGs 122, 24 are aligned at their respective "-" and "+" Bragg angles. The downstream TVBG 124 transmits a broadband high-power beam of spectral components centered at a wavelength of λ1. The upstream TVBG 122 and downstream TVBG 124 diffract another high-power beam with spectral component λ2, such that the combined collimated beam at λ2 and the transmitted beam at λ1 constitute the first combined beam. A second pair of identical upstream and downstream TVBGs 222' and 224' diffracts the group at spectral component λ3 twice. The combined downstream TVBG 242' transmits the first combined beam, outputting an overall combined beam at λ1, λ2, and λ3. The power density radiation gradient in the combined beam induces thermal lensing, which increases the beam divergence. To improve the brightness of the overall combined beam, the overall combined beam is guided through a HAPM 74 configured to compensate for the effect of thermal lensing in TVBG 24'. Another HAPM 74 can be positioned between TVBGs 24 and 24' to increase the brightness of the first combined beam. Alternatively, the HAPM 74 can be written into the same PTR glass as TVBG 24'.

[0064] 17 and 18 show a beam combiner 50 of the present invention that provides another structural solution for compensating for thermal lensing. The added structure minimizes the effect of thermal lensing on the brightness of the combined beam by incorporating a holographic phase mask (HPM). The holographic phase mask (HPM) alters the intensity profile of the transmitted beam, transforming it into a donut-shaped profile. As a result, the fusion of the donut-shaped beam with the Gaussian beam results in a flat-top combined beam.

[0065] 17, a combiner 50 embodying the concepts discussed above combines, for example, three broadband beams having respective groups of spectral components λ1, λ2, and λ3. The upstream pair of "-" HPM 76 and "+" HPM 78 each combines a Gaussian beam (TEM) with a spectral component λ1. 00 ) is a Laguerre-Gaussian beam (LG) which generally has a donut shape, which can be seen better in FIG. 01 The λ1 spectral component of the first broadband Gaussian beam is successively diffracted so that the λ1 spectral component of the first broadband Gaussian beam is transformed into a parallel optical vortex beam such as λ1 . The parallel optical vortex beam has rotational symmetry along its propagation axis and therefore has an inherent rotational orbital angular moment that affects anything along the beam's path. The TVBGs 24 and 24' downstream of each pair of TVBGs 22, 24 and 22', 24', which eliminate the angular dispersion of the grating according to the main inventive concept, are LG 01 Transmits beam. LG 01 The beams are successively combined with each of the diffracted beams of the λ2 and λ3 spectral components in the respective downstream TVBGs 24, 24' to minimize the effects of thermal lensing in each of these gratings. The beam transformation systems of the HPMs 76, 78 may comprise additional HPMs if required, which may be the case if the number of TVBG pairs is increased.

[0066] Figure 18 shows the results obtained using the previously disclosed structure of Figure 17. As can be seen, the doughnut-shaped LG 01 and diffracted Gaussian beam TEM 00 The fusion of the two results in an overall beam with a flattened top intensity profile.

[0067] Figure 19 shows yet another structural consideration, which involves constructing the TVBG with the smallest possible thickness. This approach provides the least absorption of radiation, the least heat generation, the fastest heat transfer to the surface, and the smallest optical path, thus providing the largest focal length for the thermal lens. The determination of the optical thickness is based on the dependence of the grating's efficiency on refractive index modulation (RIM) and thickness.

[0068] The inventive aspects disclosed herein are not limited in their application to the details of construction and the arrangements of components set forth in the following description or illustrated in the accompanying drawings. These aspects are capable of other embodiments and of being practiced or carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, components, elements, and features discussed in connection with any one or more embodiments are not intended to be excluded from a similar role in any other embodiment.

[0069] Additionally, the phraseology and terminology used herein are for purposes of description and should not be construed as limiting. Reference to the singular or plural form is not intended to limit the presently disclosed systems or methods, their components, acts, or elements. Additionally, in the event of inconsistent use of terminology between this document and a document incorporated herein by reference, the use of the term in the incorporated reference is supplementary to the use of the term in this document, and in the event of any inconsistency, the use of the term in this document will control.

[0070] Having thus described several aspects of at least one example, those skilled in the art will readily appreciate that various alternatives, modifications, and improvements will readily occur to those skilled in the art. For example, the examples disclosed herein may be applicable in other contexts. Such alternatives, modifications, and improvements are part of this disclosure. Accordingly, the foregoing description and drawings are by way of example only. [Explanation of symbols]

[0071] 12 Multicolored Beams 16 Major Lobes 18 Lateral lobes 22, 22' upstream transmission volume Bragg grating, TVBG 24, 24' downstream transmission volume Bragg grating, TVBG 25 Multi-wavelength parallel incident beam, input polychromatic beam 26 Fan-shaped beam 28 Broadband beam, parallel output beam 30, 32 Broadband beam 34 Parallel combined beam, combined output beam, broadband parallel beam 35 PTR glass plate, multiplexed hologram 36 Broadband beam, once diffracted fan beam 36' Twice diffracted parallel beam 40, 42 Broadband beam 40' twice diffracted output beam 44 High brightness, high power, broadband parallel output beam 52 Third broadband beam 54, 56 Broadband beam, twice diffracted beam 74 Holographic Achromatic Phase Mask, HAPM K B Bragg condition angle K L Longer wavelength angle K S Shorter wavelength angle K G Lattice Vector LG 01 Laguerre-Gaussian beam LS1, LS2 high power laser sources TEM 00 Gaussian beam λ1, λ2, λ3 spectral components, wavelengths λ B Spectral components or wavelengths of the Bragg condition λ L Longer wavelength spectral components or wavelengths λ S Shorter wavelength spectral components or wavelengths θ i Incidence angle in air θ im Angle of incidence in the medium θ dB Diffraction angle (Bragg angle) for the resonant wavelength in the medium θ dL , θ dSDiffraction angles in the medium for longer and shorter wavelengths θ e Exit angle in air θ tm Transmission angle in the medium θ t Transmission angle in air

Claims

1. at least one pair of first and second transmission volume Bragg gratings (TVBGs) spaced apart along the optical path, the first and second transmission volume Bragg gratings (TVBGs) aligned at respective opposite "+" and "-" Bragg angles to provide continuous diffraction of a first broadband beam incident on the first TVBG at a first center wavelength that satisfies the Bragg condition; the first and second TVBGs are configured identically to one another such that the dispersions of the respective first and second TVBGs compensate each other to eliminate divergence of the twice-diffracted first broadband beam in the plane of diffraction; a spectral beam combiner, wherein the second TVBG transmits a second broadband beam at a second center wavelength that does not satisfy the Bragg condition, and the second broadband beam is thereby emitted at a Bragg angle opposite one of the Bragg angles such that the twice-diffracted first broadband beam and the transmitted second broadband beam combine into a single high-power collimated broadband output beam.

2. at least one additional pair of first and second identical TVBGs aligned at respective opposite Bragg angles to satisfy the Bragg condition and provide continuous diffraction of a third polychromatic broadband beam at a center wavelength different from the first center wavelength; 2. The spectral beam combiner of claim 1, wherein the second TVBG of the additional pair transmits a first parallel broadband beam that combines with the twice-diffracted third polychromatic broadband beam to form an overall combined parallel broadband output.

3. 3. The spectral beam combiner of claim 2, wherein the TVBGs of the second pair have the same arrangement as the TVBGs of the first pair, or the TVBGs of the second pair are configured to be arranged axisymmetrically with respect to the TVBGs of the first pair.

4. further comprising a plurality of additional second pairs of TVBGs, the TVBGs of each additional second pair being aligned at respective opposite Bragg angles to provide successive diffraction of the additional broadband first beam at a center wavelength that satisfies the Bragg condition; 3. The spectral beam combiner of claim 2, wherein the second TVBG of each additional second pair transmits all of the previously collimated broadband beams such that the twice-diffracted first beam of each second additional pair combines with the previously collimated broadband beam to form an overall combined collimated broadband output.

5. 10. The spectral beam combiner of claim 1, wherein the first and second TVBGs are recorded on respective PTR glass plates.

6. 3. The spectral beam combiner of claim 2, wherein the first TVBGs of each first and second pair are each recorded on a PTR glass plate, and the second TVBGs of each pair are both recorded on a single multiplexed PTR glass plate.

7. 10. The spectral beam combiner of claim 1, wherein a thermal lens is induced in one or both of the first and second TVBGs by the first and second broadband beams, and further comprising a thermal lens compensator.

8. 8. The spectral beam combiner of claim 7, wherein the thermal lens compensator comprises a holographic achromatic broadband phase mask (HAPM) recorded on a designated PTR glass plate positioned downstream from the second TVBG or recorded on the PTR glass plate along with the second TVBG.

9. 8. The spectral beam combiner of claim 7, wherein the thermal lens compensator comprises a pair of holographic phase masks (HPMs) arranged at opposite Bragg angles upstream from the first pair of TVBGs, the HPMs diffracting the second broadband beam while converting a Gaussian intensity profile of the second broadband beam into a donut-shaped intensity profile.

10. 10. The spectral beam combiner of claim 9, wherein the second broadband beam with the donut-shaped intensity profile combines with the twice-diffracted second broadband beam to form a collimated broadband output beam with a flat-top intensity profile.

11. 10. The spectral beam combiner of claim 1, wherein the first and second TVBGs each have a thickness of up to about 1 mm and a refractive index modulation (RIM) of about 1000 ppm to provide 100% diffraction efficiency at the wavelength of the twice-diffracted first broadband beam in the 1 μm range.

12. 2. The spectral beam combiner of claim 1, wherein both TVBGs of the pair are apodized to minimize leakage between the diffracted beam and the transmitted beam, and the apodized TVBGs are composed of a bell-shaped profile of RIM recorded in a direction perpendicular to the grating vector of the TVBGs.

13. 10. The spectral beam combiner of claim 1, wherein the first and second broadband beams each have a spectral width ranging between 3 nm and 10 nm.

14. 10. The spectral beam combiner of claim 1, wherein the first and second broadband beams are each a single transverse mode beam or a multimode beam.

15. The first and second TVBGs have lattice vectors (K Ｇ 10. The spectral beam combiner of claim 1, wherein each of said first and second optical fibers comprises:

16. The spectral beam combiner of claim 1, wherein the first center wavelength is selected from a wavelength range of 205 nm to 3500 nm.

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