The issue of laser-induced damage on critical components emerges as a bottleneck that limits the high-power or high-energy laser systems, especially for the fused silica optics used in ultraviolet light. Sub-surface defects such as microcracks and impurities on fused silica optics have been discovered as damage precursors and determine the laser-induced damage threshold (LIDT) of the optics. Under the state-of-the-art advanced mitigation processes (AMP) and laser conditioning, only a few destructive damage sites that grow rapidly with successive shots still exist on a large-aperture fused silica optic. Therefore, we propose a method of selectively eliminating the destructive damage sites on fused silica optics by laser micromachining and consequently lead to a significant enhancement of LIDT in this paper. The removal of a damage site is implemented by precisely shaping the destructive damage site into an optically benign cone of special design using a femtosecond laser, with a subsequent CO2-laser-polishing process to reduce the roughness. Compared with previous methods, the thermal effect on the processed region is dramatically reduced because of the nonthermal ablation by a femtosecond laser. Through optimizing the parameters of laser micromachining, a typical damage site is eliminated and replaced with a designed cone of excellent quality. The manufactured cone typically has a smooth wall with a slope angle of 12°, a diameter of 800 μm, and a negligible raised rim with a height of 14.5 nm (∼ λ/25 @ 355 nm). By employing the raster scan LIDT test procedure, several fused silica optics processed by laser micromachining are investigated and a laser-induced damage threshold (@ 355 nm, 1.6 ns) higher than 14 J/cm2 and 10 J/cm2 on the input surface and output surface are discovered, respectively. Furthermore, the downstream light intensification is proven to be trivial in the absence of a detrimental high-intensity central spot, owing to the ultra-low raised rim. These results demonstrate that rapid laser micromachining is an effective way to improve laser-induced damage resistance of fused silica optics and eventually enhance the performance of high-power or high-energy laser systems.
This study presents the hydrodynamic simulation for high energy large-size Nd:YAG liquid-cooled laser amplifier. Based on hydrodynamic equations, the heat transfer coefficient as a function of cooling fluid velocity is derived. The velocity of cooling fluid could be chosen as 0.8m/s by considering critical Reynolds numbers for laminar and turbulent flows. In order to assess the uniformity of the cooling fluid, a four-channel fluid model is established. The uniformity of four D2O fluid channel in gain region are 98.5%, 98.7%, 98.5%, 98.6%, respectively. Besides, A simple Nd:YAG heat transfer model is built for assessing the cooling capacity of fluid, which is based on the calculation of heat transfer coefficients equation. The temperature differences of central and marginal gain medium planes is 3.451K and 1.951K, respectively, which is close to 3.255K and 1.778K calculated in the heat-fluid-solid coupling model. Based on the heat fluid-solid coupling model, the total wavefront aberration is 0.266λ cooled by D2O. Finally, another cooling fluid FC770 has been compared with D2O. In terms of fluid uniformity, the uniformity of four FC770 fluid channels in the gain region are 98.5%, 98.7%, 98.5%, 98.6%, respectively, which is close to D2O. In terms of heat effect, the heat transfer coefficients of D2O is larger than FC770. And total wavefront aberration of Nd:YAG cooled by FC770 is 0.840λ, which is larger than D2O. Eventually, the thermally induced wavefront aberrations of D2O and FC770 are 0.0475λ and 0.6092λ, respectively. The calculated results showed that D2O is a better cooling fluid than FC770.
Laser induced damage on dielectric mirrors and its rapid growth with successive shots have been and continue to be an important barrier to high power laser systems. Here the morphology of mitigation pit is optimized theoretically, and an ultrashort laser is utilized to totally remove damage on both high-reflective (HR) and anti-reflective (AR) coating. At the same time, the substrate is handled carefully and free of laser ablation, which lower the scattering loss and the amount of debris during laser machining process. Then, using R-on-1 test procedure, several mitigated sites with size of 1mm× 1mm are investigated by a Nd:YAG laser system with a flat-top spatial distribution of fully covering the mitigated site. The experimental results show even at the average fluence of 18J/cm2@6ns, there’s no damage initiation on AR coatings and no damage growth on HR coatings. It demonstrates that ultrashort laser machining is an effective and robust way to mitigate laser damage and a promising way to improve dielectric mirror performance of high power laser system in volume production.
A kind of defects on the incident surfaces of fused silica optics are reported having the potential to initiate the damages on the exit surfaces in the final optical assembly in high power lasers. In this light, the new safe criterions for defects on the incident surfaces are proposed to avoid the detrimental modulation effects in downstream.
In this work, we evaluated the feasibility of sapphire conductive cooling for short-pulse Ti:Sapphire (Ti:sa) laser amplifiers which suffers from thermal issues under high-repetition-rate and high-energy operation. Numerical heat transfer simulations of 100-TW class sapphire face-cooled Ti:sa gain modules operated around 300 W average powers are presented. The distributions of temperature, stress, strain, and birefringence in liquid cooled sapphire/Ti:sa/sapphire assembly are calculated by a finite element analysis. Based on these data, the thermal induced wave front distortions and depolarization are investigated for different repetition rates. We determine that sapphire face cooling concept holds a promise of achieving higher energies and repetition-rates in Ti:Sa amplifiers.
This paper introduces the recent development of our integrated optical addressed spatial light modulator and its applications in the high power laser systems. It can be used to convert the incident beam into uniform beam for high energy effiency, or it can realize special distribution to meet the requirements of physical experiment. The optical addressing method can avoid the problem of the black matrix effect of the electric addressing device. Its transmittance for 1053nm light is about 85% and the aperture of our device has reached 22mm× 22mm. As a transmissive device, it can be inserted into the system without affecting the original optical path. The applications of the device in the three laser systems are introduced in detail in this paper.
In the SGII-Up laser facility, this device demonstrates its ability to shape the output laser beam of the fundamental frequency when the output energy reaches about 2000J. Meanwhile, there’s no change in the time waveform and far field distribution. This means that it can effectively improve the capacity of the maximum output energy.
In the 1J1Hz Nd-glass laser system, this device has been used to improve the uniformity of the output beam. As a result, the PV value reduces from 1.4 to 1.2, which means the beam quality has been improved effectively.
In the 9th beam of SGII laser facility, the device has been used to meet the requirements of sampling the probe light. As the transmittance distribution of the laser beam can be adjusted, the sampling spot can be realized in real time. As a result, it’s easy to make the sampled spot meet the requirements of physics experiment.
A new high power laser facility with 8 beams and maximum output energy of one beam 5kJ/3.4ns/3ω has been performed and operated since 2015. Combined together the existing facilities have constructed a multifunction experimental platform including multi-pulse width of ns, ps and fs and active probing beam, which is an effective tool for Inertial Confinement Fusion (ICF) and High Energy Density (HED) researches. In addition another peculiar high power laser prototype pushes 1ω maximum output energy to 16kJ in 5ns and 17.5kJ in 20ns in flat-in-time pulse, this system is based on large aperture four-pass main amplifier architecture with 310mm×310mm output beam aperture. Meanwhile the near field and far field have good quality spanning large energy scope by use of a wide range of technologies, such as reasonable overall design technique, the integrated front end, cleanness class control, nonlinear laser propagation control, wave-front adaptive optics and precision measurement. Based on this excellent backup, 3ω damage research project is planning to be implemented. To realize the above aims, the beam expanding scheme in final transport spatial filter could be adopted considering tradeoff between the efficient utilization of 1ω output and 3ω damage threshold. Besides for deeply dissecting conversion process for beam characteristic influence of 1ω beam, WCI (Wave-front Code Image) instrument with refined structure would be used to measure optical field with simultaneous high precision amplitude and phase information, and what’s more WCI can measure the 1ω, 2ω and 3ω optical field in the same time at same position, so we can analyze the 3ω beam quality evolution systematically, and ultimately to improve the 3ω limited output.
In a word, we need pay attention to some aspects contents with emphasis for future huger laser facility development. The first is to focus the new technology application. The second is to solve the matching problem between 1ω beam and the 3ω beam. The last is to build the whole effective design in order to improve efficiency and cost performance.
In the high-power laser facility, frequency modulation to amplitude modulation (FM-to-AM) effects has seriously affected the power balance between beams and restricted the laser flux levels of safe operation in the system. For FM-to- AM effects produced by gain-narrowing effects, according to the amplifier gain-narrowing function model, after simulating and analyzing the properties of FM-to-AM effects, a corresponding compensation function is designed. Using sinusoidal compensation function, with the use of a birefringent crystal and liquid crystal modulator, adjusting the crystal angle in the range of 45 °, the center wavelength could be reduced in the magnitude of the range from 0 to 30dBm. By changing the voltage of the liquid crystal, the center wavelength could be adjusted within 1051.5-1054.5nm freely. For the regenerative amplifier with the gain of 70dB and input center wavelength of 1053nm and bandwidth of 0.7nm, the output FM-to-AM magnitude could be controlled within ~11% by this compensation system.
As the development of the laser-driven technology, the next generation of laser-driven device sets higher requirement for the repetition frequency. The higher repetition gives rise to thermal deposition, which induces thermo-optical effect, elasto-optical effect and bulk displacement. The thermal efficient management is an important approach to dissolve the thermal deposition. The quasi uniform distribution of heat medium is realized by helium cooling Nd:glass slab and the control of edge temperature. In the case, wavefront distortion and depolarization losses is obtained in experiment. Results said that both of them are improved greatly. At the same time, the distribution of temperature, stress and strain and stress birefringence in Nd:glass are analyzed by using finite element numerical simulation method. And the calculation results show that the wavefront distortion and depolarization losses match with the experimental results very well.
In order to broaden the spectrum of laser pulse and reduce the gain narrowing effect in Nd:glass regenerative amplifier to
realize the ambition of inhibiting amplitude and frequency modulation, proper quartz birefringence crystal plate is inserted
into the cavity. The influence factors of central wavelength, depth of modulation and range of modulation are obtained
theoretically. The width of the spectrum is broadened by controlling all the factors. Two kinds of thickness, 5mm and
6mm, are inserted into the regenerative amplifier cavity. The results of theoretical calculation and experiment both show
that the effect of spectrum widening is evident, which reduces the gain narrowing effect to some extent. The amplitude and
frequency modulation resulted from gain narrowing effect is inhibited when the central wavelength deflects. The simulated
results show that inhibited effect of amplitude and frequency modulation is remarkable. And the method is a potential
effective technique for amplitude and frequency modulation inhibition.
Pre-amplifier between the frontend and main power amplifier is the key unit of high power laser divers. The recent
progresses on the off-axis quadruple pass amplifier are presented, which include the beam path design, parasitic
oscillation research and experimental results. A single longitudinal mode, temporally shaped laser pulse with 5ns pulse
duration at 1053nm is injected into a Nd: Glass regenerative amplifier, which can provide a 12mJ energy output with
0.5% long term energy stability. The quadruple pass amplifier is designed as an off-axis pattern. With 1.3mJ energy
injection, amplified pulse with 16.5J can be achieved, and the measured output energy stability of the amplifier is 7.3%
(PV) at this output energy level, corresponding to a 21 shot result. The total gain of the amplifier is more than 10,000.
The parasitic oscillation was analyzed and discussed, and the parasitic mode and pencil beam are neither observed in the
Experimental performance of one-dimensional (1D) smoothing by spectral dispersion (SSD) combined with distributed
phase plate (DPP) on the ninth beam of SG-II is presented. Without the application of SSD, normalized focal-spot
non-uniformity of an 85% energy concentration is about 60%. Then, spectral bandwidth of the 3-ns, 1053-nm laser pulse
is broadened to 0.3 nm (as 270GHz in 3ω) by a 3-GHz modulator and a 10-GHz modulator integrated in the front-end
system. Spectral dispersion of 236 μrad/Å is achieved by a Littrow-configuration, 1480-l/mm grating placed between the
Φ40mm faraday isolator and the third Φ40mm rod-amplifier. By using such SSD, normalized non-uniformity with the
same energy concentration is decreased to 16%. A scheme of spatial power spectral density (PSD) in different directions
is adopted to analyze the intensity distribution of the far-field irradiation. Based on the spatial PSD analysis, theoretical
predictions of spectral peak caused by SSD’s color cycles is in excellent agreement with the experimental result. With
double-frequency modulation, the amplitude of the spectral peak is reduced by ~10dB. The temporal waveform of the 1ω
laser is measured. Waveform distortion criterion defining the frequency modulation to amplitude modulation conversion
(FM-to-AM) is about 6% with 1ω laser energy of ~1.8kJ.
We present an optically addressed liquid crystal light valve based on a twisted nematic liquid crystal layer associated to a photoconductive BSO layer. Based on the optical addressing of a continuous layer of liquid crystal, the spatial transmittance distribution of 1053nm coherent light through the light valve has a corresponding relationship with the intensity distribution of 470nm incoherent light projected onto the photoconductive BSO layer. This relationship has been studied experimently. As a transmissive device, it has the advantage of high transmittance and it can overcome the problem of black-matrix effect. The aperture of our device has reached 22mm× 22mm.
In high-power laser system, in order to extend the components' service life and reduce the operation costs, more
attentions should be pay at the research for damages ablation at multi-layer optical components and other high load
optical components. 240ps, 35ps, 6ps 1053nm laser pulses has been used to investigate damage ablation and damage
resistant experiments at 0° high reflection films. By comparing the damage morphology and damage resistant threshold
of the ablation pits at different pulses width, it was superior to use ultra-short pulse to repair multi-layers optical
components. It was found that the shorter pulse width has been used, the higher the damage resistant threshold and the
lower the laser modulation. Furthermore, the finite-difference time-domain method was used to simulate the
electric-field intensification within the large size damage region of multilayer films.
The spectrum characteristic of cryogenic has been investigated and the cryogenic
Yb:YAG amplification has been developing. As the temperature decreases, the
stimulated emission cross section increasing rapidly with the center wavelength
becoming short and the gain spectrum bandwidth narrowed. A diode-pumped
cryogenic Yb:YAG regenerative amplifier at 10Hz repetition rate has been carrying
out. Temperature of the Yb: YAG crystal has been controlled between 185K and 190K.
A ~100 pJ optical pulse with 10 ns time duration and 10 Hz repetition rate at 1030 nm
wavelength is inject into the regenerative amplifier. ~10.5 mJ output energy at 10 Hz
from the regenerative amplifier with a square-pulse distortion of ~1.5 and an
output-pulse-energy fluctuation of 7% was achieved.
? phase-shifted distributed feedback fiber lasers , ofwhich the power would focus on the phase-shift region ifthe coupling coefficient was high enough, was often adopted in order to acquire single-frequency output. But if the phase shift region was far away from the center of the grating, the output wavelength would be different that the fundamental mode would focus on the port which was near the phase shift region while the ±1 modes would focus on the other port due to the spatial hole burning. Both the theoretical analysis and experiments were presented.
In this paper the principle of CLA-target system to obtain focal line with homogenous intensity distribution was described. And tow new structure modal CLA used for improving the homogeneous of focal line was described and the numerical results of the classical CLA and new type CLA was also given. Those result showed that the focal line long-range intensity distribution can be improved greatly by using CLA with optimized unequal cylindrical lens element for beam with Gaussian intensity distribution and by using hybrid element CLA for the case of super-Gaussian distribution. The optimal process was treated by simulated annealing method. The intensity modulation decreased to 0.7 percent for optimized 4-element unequal width CLA system when incident laser with Gaussian section distribution.
This paper introduces a new type of reflector mount array, which is designed for main amplifier of ICF Laser Driver in China. The whole system utilizes a special array structure, in compliance with the requirement of a small distance among each beam of the laser system. The one-body joint drive structure and a flexible structure ensure the system with high adjusting accuracy and stability. In addition, according to the characteristic of this system, the paper calculates the turning angle and accuracy of the flexible, which shows that the theoretical accuracy can reach 0.017', and the experiment results confirm this accuracy. The transmission error of one- body joint drive structure is analyzed in detail, and Finite Elements Method is used to optimize the support structure in order to lessen the deformation of the reflector.