Method and system for compensating thermally induced beam degradation in reflective volume bragg grating

Abstract

A high-power laser system includes at least one high-power laser source outputting a substantially single mode (SM) beam. The SM beam is coupled into a reflective volume Bragg grating (RVBG) made of material which absorbs the SM beam to induce a first thermal lens (TL). Upon reflecting, the beam exits the RVBG through the first TL and is coupled into an optical compensator made from material different from that of the first TL and inducing a second TL. The second TL is configured to have a power opposite to that of the first TL matching to that of the first TL to at least partially compensate heat induced thermal degradation of the reflected beam by the first TL.

Description

[0001] METHOD ANP Syg:rEM M)R€X> M:ra; NSATI G / riHlR Alfi;AJNDlKigiREAM DEGRADATION IN REFLECTIVE VOLUME BRAGG GRATING

[0002] RELATED APPLICATION

[0003] This application claims the benefit of U.S. Patent Application Serial No. 18 / 921,492 filed October 21, 2024 and U.S. Provisional Patent Application No. 63 / 828,570 filed June 23, 2025 the disclosures of which are hereby incorporated by reference in their entirety.

[0004] BACKGROUND OF THE DISCLOSURE

[0005] Field of the Disclosure

[0006] The present disclosure relates to industrial laser systems. In particular, the disclosure relates to a method and system for minimizing detrimental effects of the thermal lensing in reflective volume Bragg gratins (RVBG).

[0007] The Known Art

[0008] The current industrial landscape is dominated by a great variety of lasers including, among others, solid state lasers, fiber lasers etc. For all the structural diversity of industrial lasers, most of them must meet the ever-increasing industrial demand for higher output beam powers. Still another requirement for a large segment of industrial lasers is high beam quality. While singlemode (SM) lasers have better beam quality including a smaller focus beam spot diameter, lower divergence and higher power compared to multimode (MM) lasers, the high-quality beam requirement is generally applicable to both SM and MM lasers.

[0009] Many applications of SM ultrashort (sub-nanosecond) laser poises in industry require high average power and high pulse energy. However, direct amplification of ultrashort pulses can induce detrimental nonlinear effects and / or laser-induced damage in amplifiers due to the extremely high peak power of the amplified pulses. A technique called chirped pulse amplification (CPA) was developed io mitigate these effects. An exemplary CPA system 50 of FIG. 1 generally includes, among other components, a tunable sub-nanosecond SM seed laser, a stretcher which increases the duration of short, typically femtosecond pulses up to a few nanoseconds, one or more pre-amplifying and booster stages. Finally, CPA system 50 necessarily has a compressor which restores the previously amplified stretched pulse to its initial duration using the inverse process of

[0010]

[0011] corresponds to a refractive index distribution A?? which is referred to as the TL. The effect that An takes on light Is called the “thermal lensing” and the minimization or, preferably, complete elimination of its undesirable consequences, discussed herein below, is a primary focus of the inventive system and method disclosed in this application,

[0012] FIGs. 3A - 3C illustrate the TL and its undesirable effects on light pulses compressed in a CVBG 10. Collimated stretched pulses 16 each are incident on a surface 22 of a CVBG 10 which is configured as the pulse compressor. Upon coupling, multiple spectral components λ₁ … λn of the incident beam are reflected from respective planar layers. As a result, a combined reflected beam 18 is compressed and exits CVBG 10 through same surface 22. At some point, any beam, even the collimated one, converges thus forming a waist and is characterized by the Rayleigh length - the distance from the beam waist to the beam region where the beam radius is increased by a factor of the square rout of 2. For Gaussian beams, which are of a particular interest here, the Rayleigh length is Z_R = πω²o / λ where λ is the wavelength and ωo is the Gaussian waist radius. The concept of the Rayleigh length plays an important role for the inventive method and system and is 15 revisited below.

[0013] FIGs. 3B and FIG. 3C illustrate what happens when high intensity pulses interact with the photosensitive material of RCVBG compressor 10. As the pulses sequentially couple into RCVBG compressor 10, each has high optical intensity at the entrance of the compressor and low at the compressors rear end which create a longitudinal and a transverse temperature variation AT. The latter, in turn, creates a heat zone 12 and triggers a TL 14. Many materials used for the CVBG’s production demonstrate the refractive index increase as the AT grows. These materials are known for generating a positive thermal lens, i.e., the lens that converges beams propagating through it in both incident and particularly reflective directions. The higher the thermal index distribution, the higher the power of TL 14, the shorter the focal length of TL 14. In other words, the beam waist 25 of each reflective beam 18 shifts closer to RVBG 10 as the dn / dT increases which causes reflected beams 18 to converge progressively closer to RVBG 10 as the TL power increases.

[0014] The variable waist position represents one of the problems directly stemming from TL 14. For example, the material laser processing often requires that a distance between a laser head, in which compressor 10 is typically mounted, and the object to be processed remain constant. The sQ variation of the TL power causes the deviation of the above-mentioned distance from the desired value. Such a deviation frequently leads to poor-quality products.

[0015]

[0016]

[0017]

[0018]

[0019]

[0020]

[0021] The following table illustrates the advantages of having compensator 32 incorporated in CPA laser system of FIG. 1.

[0022] Table

[0023]

[0024] As indicated in the table, the tests have been conducted on system 50 of FIG. 1 provided with a laser source operating within a 10-100 average power range. First, the system was tested without the disclosed compensator 32 of FIG. 4. As anticipated, the M2factor at low average powers was lower than at high average powers. Taking the beam waist position / Rayleigh length of the reflected beam at 100W as a reference value, the table indicates a gradual shift of the waist of this beam away from RCVBG compressor 10 since the lens power decreases. The waist position at 10W is shifted at 146% relative to the reference value.

[0025] The incorporation of compensator 32 of FIG. 4 in system 50 of FIG. 1 lowers the M2factor at 100W from 1.5 to about 1.4. The compensator also minimizes the waist shift at 10W from the reference value almost in half The increased compensator's length and number of compensators 32 further improve the above-discussed beam characteristics.

[0026] FIG. 5 illustrates an exemplary experimental optical schematic of the disclosed improvement. The laser source of either of systems 50, 52 of respective FIGs. 1 and 2, outputs linearly p-polarized beam 16 propagating through a polarization beam splitter (BPS) 24. λ / 4 waveplate, compensator 32 before it is coupled into RVBG 10. Upon reflecting, combined output beam 30 is coupled to compensator 32 which is selected from material in which TL 20 of FIG. 4 is generated with the dn / d’I value opposite to that of TL 14 in VBG 10 of FIG. 4. Within the specified power range of the laser source, the dimensions of the compensator and power of TL 20 are determined to a. limit the M2of collimated reflected beam 30 to about 1.4 and b. provide the waist shift limited to 60-75 % of the reference value corresponding to the waist position of the reflected beam at the optimal power which is arbitrarily selected from the specified power range.

[0027] Thereafter, reflected beam 30 is guided through the λ / 4 waveplate changing the beams p-polarization to s-polarization which allows BPS 24 to reflect this beam towards the output of the illustrated system. Any suitable device including, for example, the scanner, cat- receive this beam from fee system’s output. Along the light path towards the output, collimated reflected beam 30 can be tapped for measuring its

[0028]

[0029] factor,

[0030] The position of compressor 10 and compensator 32 can be altered. For example, it is possible to place compensator 32 downstream from BPS 24 along the path of reflected beam 30 provided that the distance between these components remains one or two orders of magnitude less than the focal length of positive TL 14. Another example is known system 52 of FIG. 2. As shown

[0031]

[0032]

Claims

CLAIMS:combined beam propagating through the first TL and being coupled into the compensator so that the second TL minimizes the thermally induced beam degradation.

12. The high power laser system of claim 2, wherein the CPA laser source includes a fiber booster extending therefrom and received in the laser head and a pump fiber energizing the fiber booster, the fiber booster and pump fiber having respective downstream ends spliced to a quartz block, the spliced downstream ends and quartz block being tightly fit inside a housing preventing arbitrary displacement thereof.

13. The high power system of claim 12, wherein the laser head is mountable on a robotic arm.

14. A method for compensating thermally induced beam degradation in reflective volume Bragg grating (RVBG) receiving at least one SM or low-mode incidence stretched beam from at least one laser source, comprising:determining a power of a first thermal lens (TL) induced upon absorbing the incidence beam, and configured to compress the incidence beam and reflect the compressed beam; and coupling the reflected beam into a compensator made from a material different from that of the RVBG, the material of the compensator absorbing the reflected beam thereby inducing a second TL which has a lens power value opposite to that of the RVBG, thereby at least partially compensating the degradation of the reflected beam which is thermally induced by the first TL.

15. The method of claim 14 further comprising determining a lens power of the second TL of the compensator and adjusting the lens power to match that of the first TL.

16. The method of claim 15, wherein the power adjustment of the of the second TL includes a step consisting of the group selected from selecting a length of the compensator, controllably healing the compensator or a combination length adjustment and heating.

17. The method of claim 14 further positioning the RVBG and compensator at a distance at least one or two orders of magnitude smaller than a focal length of the first TL.

18. The method of claim 14 further comprising operating the laser source in a range of powers, so that the heat induced thermal degradation of the reflected beam In the second TL of the compensator is represented by an M4beam factor and a waist position of the reflected beam, wherein the M4beam factor is at most equal to or lower than 1.4 at any power of the power range,and a shift of the beam waist, is at most, equal to 75% of a beam waist location which is determined at a reference power selected from the power range.

19. The method of claim 14, wherein the VBG combines a plurality of SM beams at respective different wavelengths output by a plurality of the laser sources at. respective angles which are different from one another.

20. The method of claim 14, wherein the R. VBG is configured with a chirp and incorporated in a CPA laser system to compresses the coupled SM beam.

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