APPARATUS FOR SPECTRALLY COMBINING BROADBAND LASER BEAMS BY VOLUME BRAGG GRATINGS
A spectral beam combiner includes at least one transmitting volume chirped Bragg grating (TVCBG) which 1. diffracts a first broadband beam propagating at one central wavelength, which satisfies the Bragg condition, and incident on the TVCBG at one of (+) (−) Bragg angles, and 2. transmits at least one second broadband beam propagating at a second central wavelength, which does not satisfy the Bragg condition. The second broadband beam is incident on the TVCBG at the Bragg angle which is opposite to the one Bragg angle of the first broadband beam. The TVCBG is configured to eliminate divergence of the first broadband beam, which is resulted from dispersion of the one TVCBG, in a plane of diffraction, and combine the first diffracted and second transmitted broadband beams into a first single high-power collimated broadband output beam.
Latest IPG PHOTONICS CORPORATION Patents:
- Ultrahigh power fiber laser system with controllable output beam intensity profile
- Optical parametric device based on random phase matching in polycrystalline medium
- Fiber laser apparatus and method for processing workpiece
- FLEXIBLE MINIATURE ENDOSCOPE
- METHOD AND APPARATUS FOR SAFE AND EFFICACIOUS TREATMENT OF UROLOGICAL CONDITIONS WITH LASER ENERGY
The present disclosure relates to laser-beam combining techniques. In particular, the disclosure relates to a beam combining system including at least one transmission volume Bragg grating (TVBG) which spectrally combines high power broadband laser beams into a high power high-brightness broadband collimated beam.
Background Art DiscussionNumerous 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 in particular demand for a variety of applications. However, nonlinear and thermal effects restrict power of a stand-alone SM laser with narrow bandwidth (up to 3 nm) to about 1 kW, while a SM laser with broad bandwidth (4-10 nm) can output up to 10 kW. The limited scalability has a variety of reasons including, among others, finite pump brightness, limited doping concentrations and non-linear effects. While waveguide designs of SM fiber lasers may somewhat mitigate the detrimental effects of Raman and Brillouin nonlinear effects which increases the scalability limit, the fiber core diameter nonnegotiably curtails this limit.
The high power limit can be much higher if two or more SM outputs from respective fiber lasers are combined together. Various methods of beam combining include, among others, coherent (CBC) and spectral (SBC) beam-combining techniques.
The CBC is realized by mutually coherent beams, i.e., the beams which propagate at the same wavelength such that the phase difference between/among their respective waves is constant. This technique requires controlling relative phases of SM beams from respective sources so as to provide constructive interference between the output beams. The phase control includes active or passive feedback providing for a stable coherent addition. However, the required control increases a structural complexity of the CBC system.
The SBC or wavelength beam combining is an incoherent beam combining technique which does not require controlling the phase. The goal of SBC is to combine two or more high-power laser beams propagating at respective different wavelengths into a combined beam which has not only a high power, but also preserved beam quality which determines spatial brightness. Thus, while the spectral brightness in a spectral combining system based on VBGs decreases because of the spectral broadening, the spatial brightness increases by transmitting a beam at a non-resonant wavelength and diffracting a beam at a resonant (Bragg) wavelength, explained in detail below. Based on foregoing, the main advantage of SBC over CBC is the structural simplicity since there is no need to monitor and adjust the phases of individual beams.
Both CBS and SBC employ spectrally dispersive optical elements including, among others, prisms, surface diffractive gratings and volume Bragg gratings (VBGs). The prisms and surface diffractive gratings require the use of narrow band laser sources because the angular dispersion of these elements results in dramatic increase of divergence in a plane of refraction or diffraction. Yet, as mentioned above, the narrowband lasers output a limited-power beam. To obtain higher powers, for example 2-100 kW, a great number of SM narrow-band lasers should operate simultaneously. However, a multi-laser system has quite a few optical and dimensional problems.
The VBGs can be recorded in a photo-thermo-refractive (PTR) glass which is known to function well under kW-level power loads. The fabrication of these gratings includes holographically recording the interference fringe pattern. The thermal treatment of the exposed glass sample produces a permanent spatial refractive index modulation (RIM) inside it.
The diffraction of light in the VBG occurs only at a resonant (Bragg) wavelength and two specific angles of incidence known as “+” and “−” Bragg angles. These conditions are known as Bragg conditions. The diffraction efficiency of the VBG, depending on the angle of incidence and wavelength, has a central lobe 16 and a number of side lobes 18 separated by zeros, as illustrated in
The graphs of respective
There are two basic types of VBG: transmission VBG (TVBG) and reflection VBG (RVBG) both providing diffraction of light which satisfies the Bragg condition. The TVBG and RVBG have different capabilities of combining beams. The standard RVBGs effectively combine only narrowband beams with a spectral width not exceeding a small fraction of nanometer which limits RVBGs usefulness for broadband high power laser systems.
In contrast to RVBGs, TVBGs can effectively diffract radiation that has a broad spectral width ranging from above 3 nm to 10 μm (and greater) which is quite typical for SM fiber lasers outputting a 2 to 10 kW SM beam in a 1 μm wavelength range. Based on the foregoing, the TVBG's ability to diffract high-power, broad spectral width radiation renders this type of gratings particularly attractive for combining high-power broadband beams. If a TVBG diffracts one beam with maximum efficiency at, for example, a (+) Bragg angle and transmits another beam at the opposite (−) Bragg angle, diffracted and transmitted beams merge into a combined collimated high-brightness beam.
Using a TVBG for combining broadband beams poses a few problems. The root of at least some of these problems is a beam divergence. It causes decreasing the beam's power density and spatial brightness of the combined beam. The following explains physical phenomena causing the beams diffracted by the TVBG to diverge.
As one of ordinary skill knows, any grating introduces angular dispersion illustrated in
However, the above-mentioned feature of the grating is undesirable when it functions as a beam combiner. Yes, the spectral selectivity of the grating is still paramount to the beam combining, but the fact that the diffracted beam diverges is highly undesirable for the purposes of this invention as explained herein below.
The divergent or fan-shaped beam has a lower spatial brightness. Yet many industrial applications require high spatial brightness beams in which its spectral components are all parallel. These beams are known as collimated near diffraction-limited beams. The angular dispersion is not limited to standard periodic TVCBGs but also happens in transmitting volume chirped Bragg gratings (TVCBGs). However, as explained herein below, a TVCBG interacts with a diffracted beam in a manner different from that of the periodic TVBG.
Another problem associated with periodic and chirped TVBGs is a thermal leasing phenomenon affecting the beam's divergence. When a high-power laser beam propagates through a VBG, the latter partially absorbs the beam which then releases light energy heating the VBG. The heating causes a change of refractive index and expansion of PTR glass. The temperature distribution in the VBG is not uniform which leads to the formation of a lens. Hence, this phenomenon is known as thermal lensing.
The thermal lensing distorts the divergence and quality of the diffracted beam which has the highest power density in its central regions and low power density in the wing regions. To compensate for the lens, various phase masks are successfully used. Yet, the known phase masks all are monochromatic and cannot shape effectively broadband beams.
Still another problem associated with TVBGs (and TVCBGs) used for combining broadband beams is leakage between spectral channels or beams. Returning to
Based on the foregoing it is desirable to have a TVCBG- and TVBG-based beam combiner, utilizing a SBC technique for combining broadband beams into a combined high-power, high-brightness collimated beam, which is configured to:
-
- minimize or preferably eliminate the effect of the angular dispersion on diffracted beams,
- minimize the lensing effect, and
- minimize the leakage among multiple broadband beams (or channels).
The disclosed combiner addresses the above-mentioned problems and satisfies the existing needs. The inventive structure includes at least one TVBG which functions as a beam combiner merging high-power broadband beams into a combined broadband multi-KW collimated output beam.
The inventive structure eliminates the angular dispersion of the TV(C)BG. Some of the schematics dealing with the angular dispersion include a single TVCBG. Other schematics relate to a pair of two identical periodic TVBGs.
The TVCBG is a structure with a gradually varying period of the spatial refractive index modulation (RIM). The TVCBG is capable of diffracting different wavelengths from different areas of the CTVBG and of controlling their diffraction angles. Therefore, a single properly designed TVCBG is capable of compensating the angular dispersion in the diffracted beam. As a beam combiner, the TVCBG, recorded in a PTR glass, transmits a first broadband beam incident thereon at one of (+)(−) Bragg angles and diffracts a second broadband beam at the other, opposite Bragg angle. The diffracted and transmitted beams merge with one another into a collimated broadband output beam.
The above-disclosed schematic can include a plurality of TVCBGs which are arranged to combine three and more broadband beams. In this schematic, the first upstream TVCBG outputs a single broadband collimated broadband beam including the first and transmitted second beams. The first collimated broadband beam is incident on at least one downstream TVCBG which transmits this beam without distortion. However, the downstream TVCBG, like the upstream TVCBG, compensates for the angular dispersion of the third broadband incident thereon. As a result, all three beams merge into a high brightness high power broadband collimated beam at the output of the downstream TVCBG. The number of TVCBGs is unlimited. If desired, one or more TVCBGs can effectively operate in combination with one or multiple pairs of periodic TVBGs.
The TVBGs of each pair are spaced apart along a light path and aligned at respective “+” and “−” Bragg angles. The upstream TVBGs diffracts a first beam which is incident thereon at, for example, “+” Bragg angle. Due to the inherent angular dispersion, the beam's spectral components diverge at the output of the TVBG. Upon launching the diffracted first beam to the downstream TVBG at the “−” Bragg angle, its spectral components again are diffracted but in the direction opposite to that provided by the upstream TVBG. Thus, the dispersion effects in respective TVBGs cancel out each other. This allows the downstream TVBG to output a multi-KW collimated broadband output beam at the same angle as the incident Bragg angle of the beam incident on the upstream TVBG.
The downstream TVBG thus functions as a combining TVBG for the diffracted and second transmitted broadband beams. The second beam is centered on a wavelength different from the Bragg wavelength of the first beam and is directly incident on the downstream TVBG which transmits it. The twice-diffracted first and transmitted second beams overlap one another in both near and far optical fields thus merging into an output spatially bright broadband collimated beam. As one of ordinary skill readily understands, the number of aligned pairs of TVBGs is not limited to just a single pair and can include multiple pairs which are positioned so that multiple broadband beams eventually overlap one another in the utmost downstream TVBGs outputting a single high-brightness high-power combined beam.
Still another schematic takes advantage of the PTR glass which allows recording several standard TVBGs in a single glass plate. While the TVBGs completely overlap each other within the plate, they are optically independent. Accordingly, this schematic has two upstream “−” and “+” TVBGs which diffract respective beams at Bragg wavelengths which are offset from one another. The diffracted beams then are incident on respective “+” and “−” downstream TVBGs written in the same glass plate. The twice-diffracted beams overlap one another while exiting the PTR glass so as to form a single high power broadband collimated output beam.
The aspect of the disclosure addressing the leakage problem includes forming a specified (e.g. Gaussian) profile of RIM in the direction perpendicular to the grating vector which results in suppression of side lobes. In particular, the disclosed TVBG is configured with an optimized apodization (or spatially non-uniform coupling) profile enabling almost a complete suppression of side lobes which may lead to the decreased distance between channels and therefore increased number of these channels.
Another aspect of the disclosure deals with a thermal lensing compensation in the above-disclosed broadband beam combiner including TVCBGs and TVBGs. One of the schematics that minimizes the thermal lensing phenomenon includes the inventive combiner configured with the smallest possible thickness. The latter decreases absorption of radiation limiting thus heat generation, accelerates thermal conductivity to the surfaces, reduces the optical path, and increases the focal length of the lens.
Other schematics minimizing the temperature gradient across the TVBG (and TVCBG) are based on shaping a SM broadband beam so that its Gaussian intensity distribution is transformed to a flat top intensity distribution. The lower 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 through the beam's cross-section which is about half the peak power of the Gaussian beam. However, the average power is practically the same.
One of these beam-shaping schematics includes the inventive combiner configured with a holographic achromatic phase mask created in a dedicated PTR glass plate which is positioned along a light path downstream from the combining TVBG (or single TVCBG). In contrast to the known monochromatic masks, the holographic mask effectively operates with broadband beams. It is possible to have a TVBG or TBCBG and mask in the same PTR plate.
In accordance with another schematic of this aspect, the beam combiner disclosed in the above-discussed aspects additionally includes a separate holographic phase mask which transmits a broadband SM beam while converting it to an optical vortex (Laguerre-Gaussian beam). When the converted beam merges with a diffracted beam, the radial intensity distribution of the combined beam assumes the flat top profile.
Still other aspects, embodiments, and advantages of these example aspects and embodiments, are disclosed in detail below. Moreover, both the foregoing information and the following detailed description are merely illustrative examples of various aspects and embodiments, and are intended to provide an overview or framework for understanding the nature and character of the claimed aspects and embodiments. The features of respective schematics illustrating any of the above-disclosed aspects can be fully incorporated in any of the schematics representing two other disclosed-above aspects.
The above and other features will become more apparent with reference to the accompanying figures, which are not drawn to scale. The figures provide an illustration and a further understanding of the various aspects and schematics, and constitute a part of this specification, but do not represent the limits of any particular schematic or aspect. In the drawings, each identical or nearly identical component that appears in various figures is denoted by a like numeral. For purposes of clarity, not every component may have the same reference numeral. In the figures:
Turning to CTVBGs,
This means that diffracted beam 25′ is collimated. The diffraction of different spectral components rests in the induced ellipticity of the diffracted beam.
In particular,
If additional third broadband 70 needs to be combined with the first combined one, a downstream TVCBG 64 is installed downstream from TVCBG 62. The TVCBG 64 intercepts the first combined beam, which is incident thereon at the negative Bragg angle, and transmits this beam without distortion. The broadband beam 70, in turn, is incident on downstream TVCBG 64 at the positive Bragg angle and is diffracted such that all spectral components of this beam exit TVCBG 64 are parallel to one another and parallel to λ1 and λ2 spectral components of the first combined broadband beam. As a result, beam combiner, TVCBG 64 outputs a high power high spatial brightness broadband collimated output beam 72.
The beam combiner 50 configured with two or more TVCBGs 62,64 operates with high diffraction efficiency if the spectral width of the grating is equal or larger than that of a laser beam while the distance between spectral beams/channels is larger than that of the spectral width of teach TVCBG. A plurality of additional TVCBGs can be placed anywhere along a light path. Each TVCBG functions as both a dispersion compensator and beam combiner. Also, one or multiple pairs of periodic TVBGs located before or after a TVCBG can be further added to any of the schemes of respective
The illustrated schematic operates in the following manner. The incident beam 25 is first diffracted upon coupling into upstream TVBG 22. The angular dispersion of TVBG 22 causes a fan of spectral components 26 at the output of the grating. As can be seen, the solid central Bragg wavelength is diffracted at the double Bragg angle, the long/dashed and short/dotted wavelengths (or spectral components) of fan-shaped beam 26 are output at respective angles relative to the Bragg angle. The fan-shaped beam 26 is further incident on downstream TVBG 24 at a negative Bragg angle. Consequently, downstream TVBG 24 reverses the angles at which respective long and short wavelengths have been diffracted in upstream TVBG 22 and outputs collimated broadband beam 28. In other words, the effects of angular dispersion in respective TVBG 22, 24 cancel out each other. The width of collimated output beam 28 in a plane of diffraction is greater than that of incident beam 25 because in TVBGs different spectral components have different lateral displacement. However, the increased width does not notably affect the target since the light power density does not decrease in the far field of beam 28, as explained in detail below.
A more detailed explanation of the angular dispersion and a mechanism for compensating it in periodic TVBGs are discussed in reference to
The positive Bragg diffraction is the diffraction where an angle between KG and KB is positive (counter clockwise). A diffraction angle is determined as a vector sum of a grating vector and a particular wave vector—fan-shape beam. For a symmetric VBG, incident and diffraction angles in medium for a Bragg wavelength (solid lines) are equal, i.e., θim=θdB. Therefore, the exit angle of diffracted central spectral component—Bragg wavelength—is equal to the incident one θi=θeB. For spectral components of incident beam 25 propagating in the same plane wave but detuned from the Bragg wavelength, symmetry is destroyed because of different lengths of wave vectors. Hence, for the detuned wavelengths (shorter λS and longer λL) which is excited at a Bragg angle for wavelength λB, respective diffraction angles are not equal to the incident angle. Accordingly, a fan of spectral components of output beam 26 acquires a divergence in the plane of diffraction where direction of spectral components propagation sweeps in a negative direction (clockwise) while wavelength increases.
As mentioned above, the inventive schematic causes a lateral walk-off (lateral spectral chirp) of spectral components in a twice-diffracted laser beam and ellipticity in near field. For a beam of 10 mm diameter and angular dispersion of 4 mrad, diameter increase after propagation for 50 mm would be 0.2 mm. This effect causes a small decrease of power density of the beam in near field while does not change power density in far field. Hence, the brightness remains unchanged.
The other beam 32 has spectral components λ2 and is incident at “−” Bragg angle for λ1, while λ2 is detuned to one of zeros in the diffraction spectrum shown in
In particular, beam combiner 50 spectrally combines three broadband beams 42,40 and 36 centered at respective λ1, λ2 and λ3 wavelengths. The upstream and downstream TVBGs 22 and 24 respectively operate identically to those of
The other pair of identical TVBGs 22′, 24′ respectively also operates in accordance with the basic schematic of
It is clear from the schematic of
The upstream TVBG2 22 and TVBG3 22′ diffract respective broadband beams 54, 56 including respective groups of spectral components centered at λ3 and λ2. Once diffracted, respective fans of spectral components centered at λ3 and λ2 impinge on downstream PTR plate 35. The downstream TBVGs 24 and 24′ recorded in PTR plate 35 provide diffraction at Bragg angles opposite to those of respective upstream gratings 22 and 22′. As a result, collimated beams at λ3 and λ2, respectively are overlapped in near and far fields. A third broadband beam 52 at λ1 propagates through multiplexed hologram 35 without distortion and overlaps twice-diffracted beams 54, 56 in the near and far fields. Similar to the above-disclosed schematics, the number of TVBGs is not limited to the shown three gratings.
Referring specifically to
The second problem associated with TVBGs and TVCBGs stems from the leakage of radiation between channels, i.e., partial diffraction of a transmitted beam. As discussed above, the spectral width of VBG must be wide enough to diffract a broadband beam, such as one shown in
Referring to
The third problem related to both TVCBGs and TVBGs and dealt with by this invention is the thermal lensing effect induced by heat which is a result of absorption of high-power broadband beams in the inventive combiner. One of deleterious effects of the thermal lens is its detrimental influence on the parallelism and the beam quality of a beam passing through a TVBG or TVCBG. The absorption of laser emission in a 1 μm wavelength range is about 10−4 cm−1 (≈250 ppm/cm). It has a thermo-optic coefficient dn/dT<1 ppm/K and coefficient of thermal expansion CTE=9.5 ppm/K. Therefore, the thermal lens in PTR glass holographic elements is so small that it is of no concern for power densities below 1 kW/cm2.
In contrast to low power densities, higher power densities significantly affect the beam quality. For example, 1 kW SM fiber outputs a 6 mm beam with a beam quality parameter M2=1.1 which is incident on a TVBG at the Bragg angle. The grating, configured with a period Λ 1.17 μm, thickness 1 mm, and aperture 25×25 mm2, diffracts the beam with the average power density of about 3.5 kW/cm2.
One of the structural additions includes using a phase mask. Yet in contrast to monochromatic phase masks that cannot operate with broadband beams, the inventive combiner utilizes a holographic achromatic phase mask (HAPM) that corrects the thermal distortions accumulated in a beam combiner. The thermal lens induced in the combining TVBG (or single TVCBG) of the inventive combiner by high power beams with Gaussian profile of power density is usually a concave lens with a complex shape. Accordingly, to minimize the induced divergence, this beam passes through a negative lens with a complex shape which substantially compensates the spectral aberrations caused by the positive thermal lens. Thus, the HAPM provides the optical effect which is opposite to the effect by the induced thermal lens and therefore compensates for the thermal lens induced in the combining TVBGs. Accordingly, the inventive combiner provided with the HAPM outputs the combined beam with a substantially higher quality and therefore greater brightness than the beam combined in the inventive combiner without it. The HAPM used in this invention is disclosed in detail in a co-pending patent application PCT/US21/16588 which together with the subject matter application is commonly owned and incorporated herein in its entirety by reference.
Referring specifically to
The aspects disclosed herein in accordance with the present invention, are not limited in their application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. These aspects are capable of assuming other embodiments and of being practiced or of being 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 embodiments.
Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. References in the singular or plural form are not intended to limit the presently disclosed systems or methods, their components, acts, or elements. In addition, in the event of inconsistent usages of terms between this document and documents incorporated herein by reference, the term usage in the incorporated reference is supplementary to that of this document; for irreconcilable inconsistencies, the term usage in this document controls.
Having thus described several aspects of at least one example, one of ordinary skill in the art readily appreciates that various alternations, modifications, and improvements will readily occur to those skilled in the art. For instance, examples disclosed herein are applicable in other contexts. Such alterations, modifications, and improvements are part of this disclosure. Accordingly, the foregoing description and drawings are by way of example only.
Claims
1. A spectral beam combiner comprising at least one transmitting volume chirped Bragg grating (TVCBG) which wherein the one TVCBG is configured to:
- diffracts a first broadband beam propagating at one central wavelength, which satisfies the Bragg condition, and incident on the TVCBG at one of (+)(−) Bragg angles, and
- transmits at least one second broadband beam propagating at a second central wavelength, which does not satisfy the Bragg condition, and incident on the TVCBG at the Bragg angle which is opposite to the one Bragg angle of the first broadband beam,
- eliminate divergence of the first broadband beam, which is resulted from dispersion of the one TVCBG, in a plane of diffraction, and
- combine the first diffracted and second transmitted broadband beams into a first single high-power collimated broadband output beam.
2. The spectral beam combiner of claim 1 further comprising at least one or more additional TVCBGs located upstream or downstream from the one TVCBG and diffracting a third broadband beam propagating at the Bragg angle and incident on the additional TVCBG at a central wavelength which satisfies the Bragg condition, wherein the additional TVCBG merges the first collimated output additional beams into a second high-power collimated broadband output beam.
3. The spectral beam combiner of claim 1 further comprising a first thermal lens compensator, wherein a thermal lens is induced in the one TVCBG by one of or both first and second broadband beams.
4. The spectral beam combiner of claim 3, wherein the first thermal lens compensator includes a holographic achromatic broadband phase mask (HAPM) recorded in a designated PTR glass plate which is located downstream from the one TVCBG or recorded in a PTR glass plate with the one TVBG.
5. The spectral beam combiner of claim 3, wherein the first thermal lens compensator includes a pair of holographic phase masks (HPM) which are aligned at respective opposite Bragg angles upstream from the one TVCBG, the HPMs diffracting the second broadband beam while converting a Gaussian intensity profile thereof to a donut-shaped intensity profile.
6. The spectral beam combiner of claim 1 further comprising at least one pair of identical first and second transmitting volume Bragg gratings (TVBGs) in optical communication with the TVCBG and spaced therefrom along a light path, the TVBGs being spaced apart and aligned at respective opposite “+” and “−” Bragg angles.
7. The spectral beam combiner of claim 6, wherein the first and second TVBG sequentially diffract a fourth broadband beam at a fourth central wavelength satisfying the Bragg condition to provide a twice-diffracted fourth collimated broadband beam, the second TVBG transmitting the first broadband output beam which merges with the twice-diffracted fourth collimated beam into a single third broadband collimated output beam.
8. The spectral beam combiner of claim 7 further comprising at least one additional pair of first and second identical TVBGs aligned at respective opposite Bragg angles to provide sequential diffraction of a fifth broadband beam at a fifth central wavelength which satisfies the Bragg condition and is different from the first, second and fourth central wavelengths.
9. The spectral beam combiner of claim 8, wherein the first and second TVBGs of respective pairs each are recorded in a designated PTR glass plates.
10. The spectral beam combiner of claim 8, wherein the first TVBGs of respective pairs each are recorded in a designated PTR glass plate, whereas the second TVBGs of respective pairs both are recorded in a single multiplexing PTR glass plate.
11. The spectral beam combiner of claim 6 further comprising a second thermal lens compensator including a holographic achromatic broadband phase mask (HAPM) recorded in a designated F glass plate which is located downstream from the second TVBG or recorded in the PR glass with the second TVBG.
12. The spectral beam combiner of claim 6 further comprising a second thermal lens compensator which includes a pair of holographic phase masks (HPM) and is located upstream from the first TVBG, the HPMs being aligned at respective opposite Bragg angles so as to twice diffract the first broadband collimated output beam second broadband beam while converting a Gaussian intensity profile thereof to a donut-shaped intensity profile.
13. The spectral beam combiner of claim 12, wherein the first broadband collimated output beam with the donut-shaped intensity profile merges with the fourth twice-diffracted broadband beam in the second TVBG such that the third collimated broadband output beam has a flattop intensity profile.
14. The spectral beam combiner of claim 6, wherein the first and second TVBGs each have a thickness of at most about 1 mm and refractive index modulation (RIM) of about 1000 ppm to provide a 100% diffraction efficiency at a wavelength of the twice-diffracted first broadband beam in a 1 μm range.
15. The spectral beam combiner of claim 6, wherein the first and second TVBGs are apodized to minimize leakage between diffracted and transmitted beams, the apodized TVBGs being configured with a bell-shaped profile of RIM recorded in a direction perpendicular to a grating vector of the TVBG.
16. The spectral beam combiner of claim 1, wherein the first and second broadband beams each have a spectral width ranging between 3 and 10 nm.
17. The spectral beam combiner of claim 1, wherein the first and second broadband beams each are a single transverse mode beam or multimode beam.
18. The spectral beam combiner of claim 6, wherein the first and second TVBGs each are configured with a grating vector (KG) which has an arbitrary orientation relative to the surface.
19. The spectral beam combiner of claim 1, wherein the first and second central wavelengths each are selected from a 205-3500 nm wavelength range.
Type: Application
Filed: Jun 8, 2021
Publication Date: Jan 18, 2024
Applicant: IPG PHOTONICS CORPORATION (Marlborough, MA)
Inventors: Leonid GLEBOV (Oxford, MA), Ivan DIVLIANSKY (Oxford, MA), Oussama MHIBIK (Oxford, MA), Elena SHIRSHNEVA (Oxford, MA), Vadim SMIRNOV (Oxford, MA)
Application Number: 18/034,214