A comprehensive finite element physics model is constructed to analyze the thermal effects of a thin disk medium. Based on this physical model, the temperature and stress distribution inside the medium are obtained. According to the corresponding theory, the internal optical path difference (OPD) and thermal focal length of the medium can be calculated when subjected to thermal load. According to the numerical model of the thermal effect, which has been verified by experiments conducted on commercial thin disk purchased from Dausinger + Giesen GmbH (DG), the study investigated the influence of various factors on the thermal effect of the thin disk medium. Additionally, a thin disk medium based on photoadhesion technology has been developed. The thermal focal length of the thin disk medium has been compensated by adjusting the surface shape in advance to match the desired focal length, which is significantly higher than that of the uncompensated medium. Below a pump density of 1.27 kW/cm2, the thermal focal length of the self-developed thin disk medium is comparable to that of the DG's thin disk medium.
To achieve high-precision dispersion measurement of optical components, a new method based on spectral shaping and frequency-to-time mapping was proposed, which was experimentally demonstrated and verified. In this research, a modelocked laser and a home-made spectral shaper were used to realize the spectral shaping of laser pulse with fingerprint-like characteristic. After transmitted through a dispersive medium, the temporal profile of the laser pulse became a scaled replica profile of the spectrum owing to the frequency-to-time mapping of chirped pulse. A Taylor-expanded chromatic dispersion model and least-square method were adopted for data analysis, which realized a measurement of 1-km fiber to the second-order dispersion with an error of 0.43%. The precision of the dispersion measurement was greatly enhanced by constructing a fingerprint-like characteristic spectrum. The experimental results showed that this method has better robustness and fidelity for different types of optical fiber, as well as various dispersive components in high-power laser system.
The thin-disk laser is promising in high average and peak power laser systems. Due to its high surface-to-volume ratio, the thin-disk laser has many advantages: low thermal lens, low brightness constraints for pump diodes, average and peak power scaling by increasing the beam cross section and negligible nonlinearities. In this letter, we report on our latest development of key technologies of the thin-disk laser, including the thin-disk crystal module, multi-pass pump scheme, thermal management and large-mode resonator technology. The small diameter thin-disk module with a 24 pump passes system and an efficient cooling module is realized. The optical path difference of the thin-disk module is measured. The key large-mode resonator technology is developed in an Nd:glass large-mode rod regenerative amplifier which delivers 125 mJ pulses at 1053 nm.
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.
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.
A fiber-based,high precision long-term stable time synchronization system for multi-channel laser pulses is presented,using fiber pulse stacker combined with high-speed optical-electrical conversion and electronics processing technology. This scheme is used to synchronize two individual lasers including a mode-lock laser and a time shaping pulse laser system. The relative timing jitter between two laser pulses achieved with this system is 970 fs (rms) in five minutes and 3.5 ps (rms) in five hours. The synchronization system is low cost and can work at over several tens of MHz repetition rate.
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.
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.
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.
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