Brillouin Scattering Phase Conjugate Mirrors Research Articles

Characteristics and suppression of beam distortion in a high repetition rate nanosecond stimulated Brillouin scattering phase conjugation mirror

Abstract The stimulated Brillouin scattering phase conjugation mirror (SBS-PCM) based on liquid media is widely used in high-power laser systems due to its robust thermal load capacity, high energy conversion efficiency and improved beam quality. Nevertheless, with an increase in the pump repetition rate, thermally-induced blooming and optical breakdown can emerge, leading to distortions in the Stokes beam. In this study, we delved into the thermal effects in liquid SBS-PCMs employing hydrodynamic analysis, establishing a relationship between beam profile distortion and the thermal convection field. We calculated the temperature and convection velocity distribution based on the pump light parameters and recorded the corresponding beam profiles. The intensities of the beam profiles were modulated in alignment with the convection directions, reaching a velocity peak of 2.85 mm/s at a pump pulse repetition rate of 250 Hz. The residual sum of squares (RSS) was employed to quantify the extent of beam profile distortion relative to a Gaussian distribution. The RSS escalated to 7.8, in contrast to 0.7 of the pump light at a pump pulse repetition rate of 500 Hz. By suppressing thermal convection using a high-viscosity medium, we effectively mitigated beam distortion. The RSS was reduced to 0.7 at a pump pulse repetition rate of 500 Hz, coinciding with a twentyfold increase in viscosity, thereby enhancing the beam quality. By integrating hydrodynamic analysis, we elucidated and mitigated distortion with targeted solutions. Our research offers an interdisciplinary perspective on studying thermal effects and contributes to the application of SBS-PCMs in high-repetition-rate laser systems by unveiling the mechanism of photothermal effects.

High temporal waveform fidelity stimulated Brillouin scattering phase conjugate mirror using Novec-7500.

A high temporal waveform fidelity stimulated Brillouin scattering phase conjugate mirror (SBS-PCM) with high energy efficiency, based on a novel medium, Novec-7500, is proposed and practically achieved in this study. A theoretical analysis reveals that the temporal-domain waveform distortion is caused by the inherent pulse duration compression effect of the SBS, and this undesirable phenomenon can be significantly suppressed by decreasing the compression coefficient (CC afterwards), which is defined as the gain coefficient divided by the phonon lifetime, which coefficient and is identified as the key parameter for high waveform-fidelity in SBS-PCM. The feasibility of this approach was demonstrated experimentally, in which a reflected pulse with waveform symmetry equals to the pump and an average pulse duration of 0.974 τp (τp is the duration of pump) with an energy efficiency of over 90% was achieved using Novec-7500.

SBS-PCM characteristic of sub-nanosecond laser based on rotating wedge

One major setback in the applications of high-power solid-state lasers in areas like high energy density physics is the beam quality that deteriorates with increasing pump power. To solve this problem, we proposed to develop a high beam quality and stable stimulated Brillouin scattering phase conjugation mirror (SBS-PCM) using a filtration approach and a carefully designed geometry that includes a rotating wedge to operate under a high repetition rate laser. Upon reducing the micro-impurities to 100 nm, the accumulated heat at the focus reduced greatly, resulting in improved beam quality and better phase fidelity. At 6.5 mJ input energy and below 100 Hz repetition rates, there is no observable optical breakdown or obvious thermal effects as opposed to the unfiltered medium that experience optical breakdown at 0.5 mJ and 50 Hz repetition rate. The simulated temperature gradient at the focus shows that the material absorbed the laser within a short distance from the irradiation side, and the temperature probe reveals that the temperature increases with pumping power and time due to heat accumulation at the focus. To disperse the heat accumulation through a large area within the SBS cell, we design and introduce a rotating wedge. The impact of the rotating wedge was much more noticeable when the system works at higher pump power.

Application progress of the stimulated Brillouin scattering phase conjugate mirror in high power nanosecond lasers

Stimulated Brillouin Scattering (SBS) is a third-order nonlinear optical effect, the reflected beam of which has the property of phase conjugation. The reflected beam is time inversed on amplitude, phase and polarization with the incident beam and maintains the same wavefront as the incident light. In the Master Oscillator Power Amplifier (MOPA) nanosecond laser system, the thermal effect in the laser amplifiers and a large number of optical elements in the optical path cause serious wavefront distortion of the transmitted beam, which not only deteriorates the beam quality, but also limits the possibility of further power improvement. Stimulated Brillouin Scattering Phase Conjugate Mirror (SBS-PCM) is utilized widely in this kind of laser system for the fact that using SBS-PCM by making the beam transmit along a round trip in amplifiers which bring in serious wavefront distortion compensates the distortion in real time, therefore is capable of optimizing the beam quality and improving the output power, so as to promote the development of nanosecond laser system in the direction of both high power and high beam quality. In this paper, the basic principle and phase conjugation characteristics of SBS-PCM were briefly introduced theoretically. Then the characteristics and application scope of different SBS-PCM media were compared. The research progress by domestic and foreign institutions on SBS-PCM and the typical application and development process of SBS-PCM in high power nanosecond lasers were summarized. Finally, the development trend of SBS-PCM was prospected.

High beam quality of nanosecond Nd:YAG slab laser system with SBS-PCM

All-solid-state nanosecond laser has a wide application prospect, but it remains an outstanding challenge to achieve high-power output with excellent beam quality. An optimized slab-amplifier-based laser system was demonstrated with 600 mJ output energy, 9.3 ns pulse duration and 200 Hz repetition rate. Most importantly, the beam quality M2 reached to approximately 2, which can be attributed to the improvement of spatial profile of near/far-field by the stimulated Brillouin scattering phase conjugation mirror (SBS-PCM). The comprehensive performance shows that the slab laser adopting SBS-PCM is a promising technical solution for high power laser system.

1.5 J high-beam-quality Nd:LuAG ceramic active mirror laser amplifier

A 1.5 J Nd:LuAG ceramic active mirror laser amplifier with a high beam quality is demonstrated in which a 0.8% (atomic fraction) Nd-doped Nd:LuAG ceramic disk with a diameter of 64 mm and a thickness of 5.5 mm is used as a laser gain medium. A maximum single-pass small-signal gain of 2.59 is measured when the pump energy is 11.5 J, with an injected seed energy of 0.4 J; a maximum output energy of 1.5 J is obtained at the repetition rate of 10 Hz. A far-field beam spot 1.25 times the diffraction limit (DL) is achieved by using a stimulated Brillouin scattering phase conjugation mirror (SBS-PCM) for wavefront correction.

Rotating off-centered lens in SBS phase conjugation mirror for high-repetition-rate operation.

A new method using a rotating off-centered lens is proposed to reduce the heat accumulation at the focal spot of a stimulated Brillouin scattering phase conjugation mirror at high-repetition-rate operation. Theoretical simulation of the beam intensity pattern at the focal point indicates there is less coma-aberration using a rotating off-centered focusing lens than with a rotating wedge and a conventional lens. The resultant SBS output parameters using this new method are substantially improved comparable to those of a non-rotating conventional method for high-repetition-rate operation, while the former operates quite well for higher power and the latter operates only for lower input power. High reflected energy and a good beam pattern are demonstrated using the proposed method in the present experimental conditions of 50 mJ at 1 kHz.

Experimental study on SBS-PCM at 200 Hz repetition rate pumped with joule-level energy

In this paper, an experiment is designed to research the properties of the stimulated Brillouin scattering phase conjugation mirror (SBS-PCM) working under joule-level pump energy and 200 Hz repetition rate. The nonlinear medium FC-770 is chosen as the SBS medium working in high-power condition. Due to the excellent preparation process and ultra-filtration technology for SBS-PCM, there is no optical breakdown phenomenon under the reflectivity maintained at 98% under 200 Hz and pump energy from 500 mJ to 1100 mJ. Under 800 mJ pump energy, the diffraction fringes of the near-field pattern are reduced and the beam quality of the laser beam has been improved to 1.42 times diffraction limited after the double-pass amplification from the single-pass 2.01 times diffraction limited. The incident and reflected pulse width are becoming wider with pump energy increasing. When the pump energy reaches a certain extent, the reflected pulse width will become nearly approximate to the incident pulse width. When the pump energy is 800 mJ, the pump pulse width is 40.3 ns and the output pulse width is 38.8 ns.

Coherent beam combination using self-phase locked stimulated Brillouin scattering phase conjugate mirrors with a rotating wedge for high power laser generation.

The self-phase locking of a stimulated Brillouin scattering-phase conjugate mirror (SBS-PCM) allows a simple and scalable coherent beam combination of existing lasers. We propose a simple optical system composed of a rotating wedge and a concave mirror to overcome the power limit of the SBS-PCM. Its phase locking ability and the usefulness on the beam-combination laser are demonstrated experimentally. A four-beam combination is demonstrated using this SBS-PCM scheme. The relative phases between the beams were measured to be less than λ/24.7.

Conceptual design of the Kumgang laser: a high-power coherent beam combination laser using SC-SBS-PCMs towards a Dream laser

In this review paper, we introduce a self-phase controlled stimulated Brillouin scattering phase conjugate mirror (SC-SBS-PCM) and the Kumgang laser. The SC-SBS-PCM was proposed and demonstrated its success at the academic low power level, ${\sim}100~\text{mJ}@10~\text{Hz}$ . The Kumgang laser is under development to verify whether the SC-SBS-PCM is operable at the kW level. It is a 4 kW beam combination laser combining four 1 kW beams using the SC-SBS-PCM. If the Kumgang laser functions successfully, it will be the most important step towards a Dream laser, a hypothetical laser with unlimited power and a high repetition rate.

高平均功率光纤受激布里渊散射相位共轭镜的研究与探讨

高平均功率光纤受激布里渊散射相位共轭镜的研究与探讨

Current status of the development of the Kumgang laser

The Kumgang laser, a 4 kW (4 x 0.1 J @ 10 kHz / 8.5 ns) coherent beam combination laser using self-controlled stimulated Brillouin scattering phase conjugation mirrors (SBS-PCMs), is being developed. The front-end (FE) and the pre-amplifier (PA) are completed. The FE produces pulse energy of 0.51mJ and a pulse width of 8.5 ns with a 10 kHz repetition rate (0.51mJ @ 10 kHz / 8.5 ns) and a 95 MHz linewidth. The PA amplifies up to 200 W (20 mJ @ 10 kHz / 8.5 ns) with an input power 5.1 W.

0.4 J/10 ns/10 kHz‐4 kW coherent beam combined laser using stimulated Brillouin scattering phase conjugation mirrors for industrial applications

AbstractA laser system that can produce 0.4 J/10 ns/10 kHz‐4 kW using a coherent beam combination with stimulated Brillouin scattering phase conjugate mirrors (SBS‐PCMs) is proposed. The 4 kW coherent beam combination laser system consists of a front‐end system, a pre‐amplifier module, a coherent beam divider/combiner, and 4 main amplifier modules. The front‐end system produces 1064 nm @ 10 kHz laser pulses, and it is amplified by the pre‐amplifier. After the pre‐amplifier module, the laser beam is divided into 4 sub‐beams. Each sub‐beam is amplified by the main amplifier module in a double‐pass configuration and reflected by SBS‐PCMs. Sub‐beams are combined by a coherent beam combiner, and the output of the total system is designed to be 4 kW. With this 4 kW laser module, high‐speed 2D holographic laser processing will be utilized. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Four-Beam Coherent Combination by Stimulated Brillouin Scattering with Wavefront Division

Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea(Received 1 February 2010, in final form 10 July 2010)Using stimulated Brillouin scattering phase conjugate mirrors (SBS-PCMs), we conducted a co-herent beam combination of wavefront-divided beams. “Self-phase-controlled” SBS-PCMs wereused to lock and control the phases of the SBS wave. Using an interferometer, we monitored therelative phase differences of the four spatially divided beams in real time. With long-term phasestabilization, the phase differences between the reference beam and the other three beams were wellstabilized at standard deviations of λ/36.3, λ/35.8, and λ/43.4 for a duration of 2500 shots (250 s).

Phase stabilization of a wave-front dividing four-beam combined amplifier with stimulated Brillouin scattering phase conjugate mirrors

Beam combining method is a promising technique for achieving energy scaling of solid-state lasers. The key technique of the coherent beam combining is the phase control of the stimulated Brillouin scattering wave. In previous works, the amplitude dividing scheme has shown more effective phase control than the wave-front dividing scheme. A recent experiment demonstrated that the phase stabilization in the wave-front dividing scheme can be improved by reducing the beam pointing fluctuation. In this work, a phase stabilization experiment is performed with the wave-front dividing four-beam combined amplifier. The phases are stabilized within λ/25 with an amplified gain of 5.25.

Investigation on high power phase compensation of strong aberrations via stimulated Brillouin scattering

The strong wave-front aberrations compensated by stimulated Brillouin scattering phase conjugate mirror (SBS-PCM) in a high power laser were investigated experimentally. The wave-front fluctuation of the 10 Hz 500 mJ Nd:YAG laser is 0.5λ. Transmitting through a random phase plate (RPP), it increases to 8.6λ. While using SBS-PCM in place of a high reflection mirror, the distortion induced by the RPP is compensated, and the wave-front fluctuation becomes 0.9λ. At the same time, obvious breakdown phenomena in the SBS cell were observed, and the reflectivity is unstable. A rotating wedge plate is introduced into the phase conjugating mirror to make the focus of the SBS cell rotate and optical breakdown is avoided effectively. The reflectivity becomes stable around about 70% and the instability is changed from ±4.1% to ±0.9%. Besides, the compensated wave-front fluctuation is only 0.6λ, which is near to the original wave-front distribution. The results show that the reflectivity of the SBS-PCM is very high and stable when there are no other nonlinear effects such as optical breakdown. Thus the compensating effects for strong wave-front errors are perfect.

Compensation of the thermally induced depolarization in a double-pass Nd:YAG rod amplifier with a stimulated Brillouin scattering phase conjugate mirror

Stimulated Brillouin scattering phase conjugate mirrors (SBS-PCMs) are extensively used to compensate the phase distortion of active media in a high-energy laser system with double-pass amplification. However, the depolarization loss from the thermally induced birefringence of the active medium by a photoelastic effect cannot be avoided despite the uses of phase conjugate mirrors. In this work, the depolarization loss in a double-pass Nd:YAG rod amplifier with a SBS-PCM has been studied with Jones matrix calculations. In addition, the depolarization ratios and the leak beam patterns are obtained experimentally and theoretically for four possible optical schemes. Both experiment and theoretical results show that depolarization is effectively compensated when a Faraday rotator is located after the amplifier.

Current Trends in Laser Fusion Driver and Beam Combination Laser System using Stimulated Brillouin Scattering Phase Conjugate Mirrors for a Fusion Driver

Laser facilities in the world have been developing flash-lamp-pumped ultrahigh-energy solid-state lasers for fusion research and high-repetition diode-pumped solid-state lasers to act as commercial fusion drivers. A commercial laser fusion driver requires a high-energy beam with a total energy of several megajoules per pulse in several nanoseconds with a ∼10-Hz repetition rate. However, current laser technologies have limitations in raising the beam energy when operating with a high repetition rate, which is necessary for a commercial fusion driver to function properly. The beam combination laser system, which that uses stimulated Brillouin scattering phase conjugate mirrors, is a promising candidate for a fusion driver because it can obtain both a high energy and a high repetition rate with separate amplifications. For the realization of the beam combination laser system, a self-phase control technique was proposed for the coherent beam combined output, and its principle was demonstrated experimentally.

A new circulating two-cell structure for stimulated Brillouin scattering phase conjugation mirrors with 1-J load and 10-Hz repetition rate

A new approach to realize high-energy and high-power stimulated Brillouin scattering phase conjugation mirrors (SBS-PCMs) is described. The reflectivity of SBS-PCM is investigated under a 10-Hz repetition rate and a high energy load. The relationship between reflectivity and input energy is examined experimentally with different PCM structures, focus lengths, and medium cell structures. A medium cell with a circulating structure is designed, and its advantage is demonstrated through an experimental comparison with traditional PCM structures. The 30-cm focus lens and 150-cm collimation lens are optimized when the input energy reaches 1010 mJ at 10-Hz repetition rate. Therefore, a reflectivity of 84.7% and a higher energy load using the circulating two-cell structure are achieved.

Wave-front dividing beam combined laser fusion driver using stimulated Brillouin scattering phase conjugation mirrors

The beam combination method using stimulated Brillouin scattering phase conjugation mirrors (SBS-PCMs) is a promising technique for a commercial laser fusion driver operated with a high repetition rate around 10 Hz. The phase control of the SBS wave is essential for realization of the coherent beam combined output in the beam combination. For this reason, we proposed the ‘self-phase control technique’ of the stimulated Brillouin scattering wave, and its principle has been demonstrated experimentally over the past several years. In this work, the experiments are performed for the phase stabilization of wave-front dividing two-beam and four-beam combined systems by reducing the beam pointing fluctuation of the laser source using a beam expansion.