High-power narrow-linewidth 589 nm lasers are able to excite the sodium layer with 10 km thick. ness at the altitude of 90 km and emit the backward scattering. Sodium guide star generated by 589 nm laser is con. sidered as a bright beacon for ground-based optical telescope adaptive optical system. The basic principles of sodi.um atomic transitions, the approaches to improving the sodium beacon brightness, the applications of sodium lasersand the evolution of 589 nm sodium laser systems are presented.

In military laser countermeasure, reconnaissance and detection systems and guided weapons usually use image sensors to achieve precise control. In recent years, how to use high-energy laser weapons to accurately strike enemy sensing systems has become a research hotspot. From the damage effect of short pulse lasers with highpeak power on the charge coupled device (CCD) and complementary metal oxide semiconductor (CMOS) sensor, us.ing the hundreds picosecond laser obtained by stimulated Brillouin scattering (SBS) pulse width compression tech.nology as an electro-optical countermeasure weapon is proposed. The principle and research progress of SBS pulse width compression are introduced and the application prospects of hundreds picosecond lasers in electro-optical countermeasures are analyzed.

The development history of quantum cascade lasers (QCL) is reviewed. The research progress of ma. terials and devices in quantum cascade lasers with typical wavebands such as mid-infrared, long-wave infrared, and terahertz is summarized. Meanwhile, the applications of QCL in the fields of composition detection, free-space opti. cal communication and directional infrared countermeasure are introduced. Finally, the development trend of QCL technology is summarized.

Ever since the first free-electron laser (FEL) is both conceived and demonstrated in the 1970s at Stanford, it has attracted much attention as a high average power light source. Since its light is produced from free electrons, an FEL can be designed to laser at virtually any wavelength. Furthermore, its gain medium cannot be damaged by any conventional means. Accordingly, as there have been several ambitious attempts to design and con.struct high average power FELs over the past several decades. Additionally, an overview of FEL theory is provided,the key techniques and research in the military are summarized, and the future development is prospected.

Since the thin-disc laser is proposed in the 1990 s, it has been widely considered as a sort of high power light source. Compared with the traditional solid-state laser, the thin-disc laser works in a very thin sheet, so it can significantly reduce the thermal effect of the crystal, and has the advantages of excellent beam quality, high conversion efficiency, and high output power and so on. An overview of the principle of the thin-disc laser is provid.ed, its key technologies and the applications such as industrial, military and scientific research are summarized, andits future development is forecasted.

Optical frequency combs are known to output uniformly-spaced ultrashort optical pulse trains in the time domain, which correspond to a series of phase-locked discrete comb lines in the frequency domain. Such unique feature makes them capable of high-precision distance measurement as a standard time-frequency reference, which is extremely promising in scientific research and industrial applications. Firstly, recent advances of high-preci. sion distance metrology based on optical frequency combs are reviewed. Secondly, basic principles and representa. tive results of single-comb time-of-flight method, dual-comb optical down-sampling method and dispersive interfer. ometry method are presented in detail. Advantages, limitations and appropriate applications of each method are also analyzed. Finally, enhancement of metrology performance based on dual-comb lasers and micro-resonator frequency combs is discussed.

The energy level structure of the Cr,Er:YAG crystal is studied, the energy resonance transfer pro. cess from the Cr3+ to Er3+ , and the energy level transition process of the 3 μm laser are also discussed. The xenon lamp pump source and the Cr,Er:YAG laser are designed and constructed at room temperature. The relationships be.tween the output energy and the pump energy, the output energy and the crystal temperature are analyzed. In the sin. gle-shot free running operation, the maximum output energy, the pulse width and the slope efficiency are 529 mJ, 103 μs and 0.26%, respectively. The spectrum range is 2 923 nm to 2 940 nm, and the two peak wavelengths are 2 930 nm and 2 933 nm respectively. Experimental results show that the Cr, Er:YAG crystal is an efficient candi. date for the 3 μm laser operation.

The solid state lasers with high power and high beam quality at 2 μm band have important applica.tions in laser medical treatment, remote sensing, high precision spectral measurement and military. Compared with traditional doped laser crystal such as YAG, YLF and YAP, Tm doped laser crystal such as Y2O3, Lu2O3 and Sc2O3 has the advantages of low phonon energy, high thermal conductivity and wide spectral range. A high efficiency con. tinuous wave Tm:Y2O3mid-infrared solid laser is reported. Using 1 678 nm Er-doped Raman fiber laser as pump source, the highest slope efficiency of 63.3% is obtained. The maximum output power of the laser is 0.4 W, and the central wavelength is located at 2 051 nm. In addition, the beam quality factors in the horizontal and vertical direc.tions under the maximum output power are estimated to be 1.2 and 1.1 respectively. Finally, the future technologi.cal development of doped Tm solid laser is prospected, aiming to provide some guidance and inspiration for techni.cians in related fields.

Stimulated Brillouin scattering (SBS) is one of the most efficient methods to achieve optical phase conjugation. However, practical SBS phase conjugation applications is always accompanied pulse compression, re. sulting pulse waveform change. To obtain better waveform fidelity while applying the stimulated Brillouin scattering phase conjugate mirror, pulse control in stimulated Brillouin scattering process is investigated experimentally. By raising the energy density of the injected pump pulse, a higher transient gain is provided, and the stokes signal is generated earlier at time domain. With the control of pump intensity, the stokes pulse with high waveform fidelity is achieved.

An all-solid-state passive mode-locked oscillator has the characteristics of high pulse energy and good stability. A high-power neodymium-doped yttrium vanadate (Nd:YVO4) parametric Kerr lens mode-locked os. cillator is researched, in which the parametric Kerr effect is from the cascaded second-order nonlinear effect pro. duced by light in lithium triborate(LBO). The results show that the oscillator can be self-started with a total aver. age power of 20 W, repetition rate of 79.3 MHz and pulse duration of 22 ps.

External cavity tunable semiconductor lasers have wide applications in fiber sensing, cold atom physics, laser radar, etc. To achieve wide mode-hopping free tuning range, the length of laser cavity, parameter of grating, the position of optical elements in the cavity and the position of pivot point should be carefully designed. The Littman-Metcalf type external cavity tunable semiconductor laser is modeled to study the longitudinal allowanceerror for mode-hopping free tuning. The influence of the distance from pivot point to motion axis of piezoelectric ce.ramic motor and the diffracting point of the blazed grating, the installation angle of the blazed grating, and the grat.ing groove density are analyzed. The results show that mode-hopping can be effectively avoided with careful choos. ing the parameters above.

High-order Laguerre-Gaussian (LG) mode output, the angular indices m of which can be tuned con. veniently, is generated in an end-pumped Nd:YVO4 laser which has strong spherical aberration (SA) in its cavity. The experimental results are explained based on the beam radius and ring-shaped beam width of the LG mode beam. It is found that the SA-induced actual focal point displacement between different modes is essential for single high-order mode operation, while the SA and the beam width determine the loss on a certain mode and whether it can oscillate. For high-order mode operation, long-focal lens is used to decrease the SA and resultant loss on high-order modes and increase the pump power to enhance the laser gain. A maximum angular index m of ±95 is ob. tained with radial index of p of 0. With short-focal lens to enhance the SA and corresponding mode-selecting capa. bility, LG mode output with non-zero radial indices is obtained.

It is an effective method to obtain high beam quality and high power laser output by coherently com.bining each sub-laser source of the laser array. According to the international standard of beam quality, the bucketradius of coherent array laser is calculated by the energy or power ratio in the bucket. And the beam quality factor M2 of coherent array laser is calculated by fitting the hyperbolic equation of laser beam transmission. The results of calculation show that the beam quality of coherent array laser is close to that of fundamental mode Gaussian beam.