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DIRECTED ENERGY PROFESSIONAL SOCIETY

Abstract: 25-Symp-160

UNCLASSIFIED, PUBLIC RELEASE

Scalable Coherent Laser Beam Combining suing Stimulated Brillouin scattering

ABSTRACT:
We propose and experimentally explore a simplified coherent beam combining approach scalable to thousands of lasers, consistent with unverified claims by other researchers, that overlaps beams to reflect from the stimulated Brillouin scattering (SBS) acoustic grating formed by their convolution. This simplified approach uses confocal stimulated Brillouin scattering (SBS) to force phase coherence between multiple laser sources, opening a new avenue to high-power laser sources in the kilowatt to megawatt levels. This greatly simplified geometry should allow generation of high energy laser systems that can scale to meet future requirements in the fields of fusion research, power beaming and remote sensing. Potential application includes very compact lasers sources for: space power beaming; high energy lasers for fusion research; Z-Next and the National Ignition Facility in the Department of Energy; and replacing the aging Sandia Laser Applications Project ruby laser used in orbital sensor calibrations.

SUMMARY OF RESULTS:
Most research programs based on high-energy lasers would benefit from increased coherent energy delivered to the receiver. Two ways to increase energy at the receiver are increasing the laser output power or improving beam propagation to the receiver. Maximum achievable laser output power is limited by thermal management or material properties and cannot be easily scaled to higher power when using a single laser source. Coherent beam combination (CBC) using a phase conjugating mirror (PCM) by SBS can both avert the thermal management limit by arraying individual lasers and improve propagation to receiver. Whereas linear beam combining methods such as passive mirrors or beam splitters do not drive mutual coherence of among arrayed lasers, the SBS nonlinearity drives coherence, resulting in a beam with good wavefront quality that leads to increased total pulse energy delivered at the receiver.
In recent years, a variety of technologies have been used for coherent beam combination (CBC): polarization beam combination; wavelength multiplexing; phase modulators; and complex SBS schemes [1-4].

These techniques combined small numbers of beams and achieved output powers on the order of 100s of kilowatts. In contrast, researchers at Russia’s All-Soviet Scientific Research Institute of Experimental Physics laboratory (known as VNIIEF) have used a phase-conjugate CBC technology in nuclear pumped lasers [5] to achieve ~ 9 MW output power by combining thousands of laser beams, an improvement of nearly 100X in output power and ~1000X in number of beams combined. The current state of the art for phase-conjugate mirror (PCM) CBC is stimulated Brillouin scattering (SBS) and is typically applied to correct optical wavefront phase in laser amplifiers. Prior to VNIEEF’s recent claim of combining thousands of lasers, SBS-PCM research had been limited to combining a small number of lasers [3, 4, 6] and required a complex scheme of cascaded piezoelectric control. Instead of using cascaded SBS-PCM to coherently combine the beam, we propose a simplified approach scalable to thousands of lasers, consistent with VNIIEF’s claim, by overlapping beams to reflect from the SBS acoustic grating formed by their convolution. This greatly simplified geometry should allow generation of high energy laser systems that can scale to meet future requirements in the fields of fusion research, power beaming and remote sensing [7-12].
Beams reflected from a SBS-PCM without phase mixing show a relative phase stability of ~ /5. We achieved best stabilization with a 2.3 degree crossing angle. In this case, we were able to achieve phase stability between beams of lambda/18 in FC-72, lambda/31.5 in FC-77, and, in one extremely good run, a stabilization of /150. Increasing the crossing angle even slightly to 4.3 degrees reduced the phase stability to lambda/7, nearly as low as completely independent beams. While we were able to achieve stability of /30 as desired, it was only in the case of the smallest possible crossing angle which was 2.3 degrees, or 8mm (1 diameter) beam separation. We presume that the small interaction distance, which is a function of the input lens focal length and the beam diameters, as well as heating of the SBS material, are the primary cause of this limited success. By increasing the focal length, we expect that the acceptance angle could be increased and potentially lead to a system that would scale to much larger pulse energies. Because of supply chain issues and the short length of the project, we were not able to procure enough quantity of the SBS liquid samples to lengthen the sample cell (as required when going to a longer focal length lens), nor get approval for a high-pressure Xe gas cell which would likely have overcome issues with heating.

REFERENCES:
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14. Zel'Dovich, B.Y., et al., Connection between the wave fronts of the reflected and exciting light in stimulated Mandel'shtam-Brillouin scattering. Sov. Phys. JETP, 1972. 15: p. 109.

UNCLASSIFIED, PUBLIC RELEASE