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The near field of the probe beam after passing through the gain module is measured. The “streamer effect” is first discovered in the experiment, which is characterized as continuity in the flow direction and non-uniformity in the lateral direction with high spatial frequency. To explain the “streamer effect”, a fluid–solid coupling model and an unstable resonator oscillation model of the direct-liquid-cooled thin-disk laser are established. The simulation results show that the non-uniformity of the coolant along the width of the flow channel gives rise to the streamer effect. A feasible cooling channel to achieve a high uniformity coolant is proposed and experimentally verified. At CW pumping of 9.6 kW, the influence of coolant on the output beam quality can be controlled at β

A direct-liquid-cooled Nd:YLF thin disk laser resonator is presented, which features the use of refractive index matching liquid (RIML) as coolant. Highly uniform pump intensity distribution with rectangular shape is realized by using metallic planar waveguides. Much attention has been paid on the design of the gain module, including how to achieve excellent cooling ability with multi-channel coolers and how to choose the doping levels of the crystals for realizing well-distributed pump absorption. The flow velocity of the coolant is found to be a key parameter for laser performance and optimized to keep it in laminar flow status for dissipating unwanted heat load. A single channel device is used to measure the convective heat transfer coefficient (CHTC) at different flow velocities. Accordingly, the thermal stress in the disk is analyzed numerically and the maximum permissible thermal load is estimated. Experimentally, with ten pieces of a-cut Nd:YLF thin disks of different doping levels, a linear polarized laser with an average output power of 1120 W is achieved at the pump power of 5202 W, corresponding to an optical-optical efficiency of 21.5%, and a slope efficiency of 30.8%. Furthermore, the wavefront aberration of the gain module is measured to be quite weak, with a peak to valley (PV) value of 4.0 μm when it is pumped at 5202 W, which enables the feasibility of its application in an unstable resonator. To the best of our knowledge, this is the first demonstration of kilowatt-level direct-‘refractive index matching liquid’-cooled Nd:YLF thin disk laser resonator.

The fiber coupled system based on mini-bars is an effective way of avoiding complex beam shaping to reduce costs and improve brightness. Forty mini-bars mounted on copper microchannel heatsink are arranged in two stacks to collimate the output beam of the stacks with the aspherical cylindrical lens and cylindrical lens array in fast and slow axis directions in this paper. For increasing the fill factor of the beams of the stacks, the beams are combined spatially in the fast axis direction. The combined beam could be coupled into the fiber of 800 m core diameter and 0.22 numerical aperture. The opto-optical efficiency of the fiber coupled module is about 58% and the maximum output power is 1360 W with the power density of 1.73105 W/cm2 at fiber output facet.

A large aperture tapered fused silica fiber phase conjugate mirror with a maximum 50.7% stimulated Brillouin scattering (SBS) reflectivity is presented, which is operated with 400 Hz pulse repetition rate and 36.5 mJ input pulse energy. To the best of our knowledge, it is the first time that over 50% SBS reflectivity is achieved by using solid-state phase conjugate mirror under such high pulse repetition rate and high pulse energy. With much higher pulse repetition rate of 500 and 1000 Hz, the maximum SBS reflectivity is 41.2% and 33.3%, respectively. A single-longitudinal-mode Nd:YAG laser is experimentally studied with master oscillator power amplifier (MOPA) scheme using such a tapered fiber as a phase conjugate mirror. A 101 mJ pulse energy is achieved at 400 Hz repetition rate, with a pulse width of 6 ns and a M 2 factor of less than 2. The corresponding peak power reaches 16.8 MW.

The relation between macro-properties and micro-quantities of the large-grained polycrystalline chemical vapour deposited diamond films was investigated.Considering the growth non-uniformity,the micro-Raman spectra measured on the top growth surface show large variation from spot to spot.Whereas,the micro-Raman spectra taken on the fine-grained smooth interface of the diamond films show that small deviation and generally can represent the overall quality of the large-grained polycrystaline diamond films,and correlate well with hydrogen content of the film measured by Fourier-transform infrared spectroscopy.It can be concluded that a diamond film with darker color or lower transparency has lower quality and lower phase purity characterized by the larger full width at half maximum(FWHM) and lower intensity of the diamond Raman peak around 1 332 cm-1.

A continuous-wave-operation laser amplifier chain consisting of three multi-concentration-doped Yb:YAG slab gain modules (GMs) at room temperature is presented. The output power of 22.3 kW with the beam quality of 3.3 times the diffraction limit is achieved from this chain. To the best of our knowledge, based on a Yb:YAG slab at room temperature, the highest power to date while maintaining excellent beam quality laser output. An extraction efficiency of 36% from the single slab GM is obtained and can be further enhanced to 46% by optimizing the parameters of GM. These results have confirmed that the Yb:YAG slab has an excellent scaling performance and is suitable for the development of high-average-power lasers.

We demonstrate a master oscillator power amplifier (MOPA) architecture based on Yb:YAG amplifiers and adaptive optics (AO) systems with a high power and high beam quality laser output. With two conduction cooled, dual-end-pumped Yb:YAG zigzag-slab amplifiers at room temperature, the fiber laser of 300 W was scaled to 11.9 kW. Moreover, AO system positioned downstream was utilized to correct wavefront of amplified laser. The beam quality β at maximum output power was 2.8 times diffraction limited with closed-loop AO system.

A spectral beam combination(SBC) system with dual Multi-Layer Dielectric(MLD) grating dispersion compensation configuration, which can combine multiple fiber lasers into a common aperture beam with high beam quality and relax the linewidth requirement of the individual fiber laser, has become gradually one of the most important technique route of fiber laser beam combining. The basic principle and the key technologies of the SBC system with dual-MLD-grating configuration were introduced and analyzed briefly. The recent progress of the main key technologies about high power beam combinable narrow linewidth fiber laser, high power high efficiency short wavelength narrow linewidth fiber laser, large dispersion high efficiency MLD grating and high integration dense beam combining were reviewed. The recent progress on the key technologies of SBC with dual-MLD-configuration in Institute of Applied Electronics, China Academy of Engineering Physics, were also presented. The potential of the SBC with dual-MLD-configuration was prospected.

A model for describing laser-induced damage in optical materials by nanosecond laser pulses is investigated. The laser-damage critical fluence is obtained based on calculating the light absorption of nanoabsorbers by using Mie theory and solving the heat equation. Considering a power law distribution of nano-absorbers, we calculated the damage probability at the surface of fused silica including Pt particles. The theoretical results calculated with appropriate parameters are applied to fit the experimental data in order to identify the properties of nanodefects.

A phase conjugating mirror combining a fused silica rod and an optical fiber is presented. This configuration combines a large diameter fused silica rod with a high laser damage threshold and an optical fiber with a low stimulated Brillouin scattering (SBS) threshold. The composed SBS generator-amplifier construction, similar to the two-cell type makes a forced SBS process, in which the Stokes beam reflected by the fiber is injected into the fused silica rod. Using this combined phase conjugating mirror in a laser-diode-pumped laser master oscillator power amplifier system, we obtained 42% SBS reflectivity at most at 100 Hz repetition rate. At the same time, the inverse beam quality parameter M(2) is reduced from 3 to 1.6, while at 400 Hz repetition rate, M(2) is reduced from 4 to 1.7.

A large aperture fused silica tapered fiber phase conjugate mirror is presented with a maximum 70% stimulated Brillouin scattering (SBS) reflectivity, which is obtained with 1 kHz repetition rate, 15 ns pulse width and 38 mJ input pulse energy. To the best of our knowledge, this is the highest SBS reflectivity ever reported by using optical fiber as a phase conjugate mirror for such high pulse repetition rate (1 kHz) and several tens of millijoule (mJ) input pulse energy. The influences of fiber end surface quality and pump pulse widths on SBS reflectivity are investigated experimentally. The results show that finer fiber end surface quality and longer input pulse widths are preferred for obtaining higher SBS reflectivity with higher input pulse energy. Double passing amplification experiments are also performed. 52 mJ pulse energy is achieved at 1 kHz repetition rate, with a reflected SBS pulse width of 1.5 ns and a M(2) factor of 2.3. The corresponding peak power reaches 34.6 MW. Obvious beam quality improvement is observed.