Transverse mode formation in longitudinally pumped miniature slab lasers

The formation of transverse modes in longitudinally pumped miniature slab lasers is investigated theoretically and experimentally. The longitudinally non-uniform gain-guiding is studied by expanding the electric field into the Hermite-Gaussian functions that satisfy boundary conditions of the resonator. Non-Gaussian transversal beam profiles in the near field are found and the beam diameter is reduced when the pump spot becomes smaller. The experimental observation agrees with the theoretical calculation.

高功率板条激光器的研究进展

板条激光器，特别是LD抽运的板条激光器，作为高功率同体激光器的一个重要发展方向，在军用和工业应用等领域有着较好的应用前景。综述了板条激光器的抽运、冷却方式以及谐振腔设计方面的进展，并对其应用前景进行展望。

部分抽运的板条激光器的热效应分析

基于热传导方程和高斯-赛德尔数值计算方法，对部分抽运板条激光器的温度分布、热应力和热致折射率变化等热效应进行了详细分析，并与单侧抽运及双侧抽运方式进行了比较。结果表明，设计合理的部分抽运板条激光器，可以获得较高的抽运效率，其热效应相比于均匀抽运的情况并没有显著劣化，不论是采用锯齿形传播方式还是直线传播方式的激光器都可以获得较好的光束质量，当一束高斯激光直线经过板条晶体中部时，光束质量因子为1．4，若采用锯齿形传播方式，则光束质量因子可提高到1．1。

新一代大功率固体板条激光器的技术进展

介绍了固体板条激光器为了获得大功率和高光束质量而采用的新技术, 阐述了新一代大功率固体板条激光器的最新进展, 分析了新一代大功率固体板条激光器的技术特点, 并对其应用前景进行了展望。

边缘泵浦被动调Q的Nd：YAG薄片激光器

文中报道了一台采用激光二极管部分边缘泵浦方式的高功率薄片激光器，晶体尺寸是1 mm×10 mm×60 mm。Cr4+：YAG被用来作为被动调Q晶体，在重复频率高于10kHz时，获得了脉宽10ns，平均功率70W，斜线效率为36\%的激光输出。通过控制泵浦光束直径的大小，我们在厚度方向得到了近似衍射极限的光束输出。整个激光器结构紧凑，大小为60 mm×174 mm×150 mm。

Laser beam control, combining, and propagation in atmospheric turbulence

This dissertation is concerned with the control, combining, and propagation of laser beams through a turbulent atmosphere. In the first part we consider adaptive optics: the process of controlling the beam based on information of the current state of the turbulence. If the target is cooperative and provides a coherent return beam, the phase measured near the beam transmitter and adaptive optics can, in principle, correct these fluctuations. However, for many applications, the target is uncooperative. In this case, we show that an incoherent return from the target can be used instead. Using the principle of reciprocity, we derive a novel relation between the field at the target and the scattered field at a detector. We then demonstrate through simulation that an adaptive optics system can utilize this relation to focus a beam through atmospheric turbulence onto a rough surface. In the second part we consider beam combining. To achieve the power levels needed for directed energy applications it is necessary to combine a large number of lasers into a single beam. The large linewidths inherent in high-power fiber and slab lasers cause random phase and intensity fluctuations occurring on sub-nanosecond time scales. We demonstrate that this presents a challenging problem when attempting to phase-lock high-power lasers. Furthermore, we show that even if instruments are developed that can precisely control the phase of high-power lasers; coherent combining is problematic for DE applications. The dephasing effects of atmospheric turbulence typically encountered in DE applications will degrade the coherent properties of the beam before it reaches the target. Finally, we investigate the propagation of Bessel and Airy beams through atmospheric turbulence. It has been proposed that these quasi-non-diffracting beams could be resistant to the effects of atmospheric turbulence. However, we find that atmospheric turbulence disrupts the quasi-non-diffracting nature of Bessel and Airy beams when the transverse coherence length nears the initial aperture diameter or diagonal respectively. The turbulence induced transverse phase distortion limits the effectiveness of Bessel and Airy beams for applications requiring propagation over long distances in the turbulent atmosphere.

The analysis of the transient temperature distribution of double-slab Nd:YAG laser medium

A novel double-slab Nd:YAG laser, which uses face-pumped slab medium cooled by liquid with different temperatures on both sides, is proposed. The thermal distortion of wavefront caused by the non-uniform temperature distribution in the laser gain media can be self-compensated. According to the method of operation, the models of the temperature distribution and stress are presented, and the analytic solutions for the model are derived. Furthermore, the numerical simulations with pulse pumping energy of 10 J and repetition frequencies of 500 and 1000 Hz are calculated respectively for Nd:YAG laser medium. The simulation results show that the temperature gradient remains the approximative linearity, and the heat stress is within the extreme range. Then the absorption coefficient is also discussed. The result indicates that the doping concentration cannot be too large for the high repetition frequency laser. It has been proved that the high repetition frequency, high laser beam quality, and high average output power of the order of kilowatt of Nd: YAG slab laser can be achieved in this structure.

Design of quantum cascade microcavity lasers based on Q factor versus etching depth

The choice of the etching depth for semiconductor microcavities is a compromise between a high Q factor and a difficult technique in a practical fabricating process. In this paper, the influences of the etching depth on mode Q factors for mid-infrared quantum cascade microcylinder and microsquare lasers around 4.8 and 7.8 mu m are simulated by three-dimensional (3D) finite-difference time-domain (FDTD) techniques. For the microcylinder and the microsquare resonators, the mode Q factors of the whispering-gallery modes (WGMs) increase exponentially and linearly with the increase in the etching depth, respectively Furthermore, the mode Q factors of some higher order transverse WGMs may be larger than that of the fundamental transverse WGM in 3D microsquares. Based on the field distribution of the vertical multilayer slab waveguide and the mode Q factors versus the etching depth, the necessary etching depth is chosen at the position where the field amplitude is 1% of the peak value of the slab waveguide. In addition, the influences of sidewall roughness on the mode Q factors are simulated for microsquare resonators by 2D FDTD simulation. (C) 2009 Optical Society of America

Comparison of modal gain and material gain for strong guiding slab waveguides

The relations between the gain factor, defined as the ratio of modal gain to material gain, and the optical confinement factor are discussed for the TE and TM modes in slab waveguides. For the TE modes, the gain factor is larger than the optical confinement factor, due to the zigzag propagation of the modal light ray in the core layers. For the TM modes, the existence of a nonzero electric field in the propagation direction results in a more complicated relation of the gain factor and the confinement factor. For an air-Si-SiO2 strong slab waveguide, the numerical results show that the modal gain can be larger than the material gain and the higher-order transverse mode can have an even larger modal gain than the fundamental mode, The efficiency of waveguiding photodetectors can be improved by applying the modal gain or loss characteristics in strong waveguides.

Control of the photonic crystal waveguide over the beam profile of vertical-cavity surface-emitting lasers

The control of the photonic crystal waveguide over the beam profile of vertical-cavity surface-emitting lasers is investigated. The symmetric slab waveguide model is adopted to analyze the control parameters, of the beam profile in the photonic-crystal vertical-cavity surface-emitting laser (PC-VCSEL). The filling factor (the ratio of the hole diameter to the lattice constant) and the etching depth control the divergence angle of the PC-VCSEL, and the low filling factor and the shallow etching depth are beneficial to achieve the low-divergence-angle beam. Two types of PC-VCSELs with different filling factors and etching depths are designed and fabricated. The experimental results show that the device with a lower filling factor and a shallower etching depth has a lower divergence angle, which agrees well with the theoretical predictions.