Ba1-xB2-y-zO4SixAlyGaz晶体和频可调谐深紫外飞秒激光器

介绍了一种基于新型非线性晶体Ba_1-xB_2-y-zO₄Si_xAl_yGa_z 的可调谐深紫外飞秒激光光源. 从理论上分析了基频光和倍频光在通过非线性晶体时所造成的空间走离和群速度失配, 为了补偿空间走离以及波长调谐过程中晶体折射造成的光束偏离现象, 将两块相同的倍频晶体成镜像放置来产生二次谐波. 并调节延迟线的长度来补偿基频光和倍频光之间的群速度失配, 从而提高和频转换效率. 然后通过和频方式进行三倍频和四倍频来突破晶体相位匹配条件的限制, 产生了波长低于200 nm的深紫外飞秒激光. 利用钛宝石激光器提供基频光光源, 最终在250–300 nm, 192.5–210 nm 范围内获得了高重频、可调谐超短脉冲紫外和深紫外激光. 并在基频光波长为800 nm时, 得到的二倍频、三倍频和四倍频的功率分别为1.28 W, 194 mW和5.8 mW, 相对于前一级的转换效率依次为46.14%, 15.16%和3%. 采用互相关法测量得到266.7 nm紫外激光的脉冲宽度约为640.4 fs.

Tunable deep ultraviolet femtosecond sum frequency laser based on Ba1-xB2-y-zO4SixAlyGaz crystal

Tunable coherent deep ultraviolet (DUV) light sources, especially ultrashort pulse DUV lasers have great applications in the fields of time-resolved, material processing, spectroscopy, laser spectroscopy and laser fusion. In the UV region, the best choice of generating the laser pulses in the femtosecond or picosecond regime is the frequency up-conversation technique based on second order nonlinearities. Over the past three decades, quite a lot of nonlinear crystals, such as LiB₃O₅, βup-BaB₂O₄, KBe₂BO₃F₂ and Ba_1-xB_{2-y- z}O₄Si_xAl_yGa_z have been developed and employed for generating the femtosecond pulses in the blue, ultraviolet, and even the deep-ultraviolet region. A tunable deep ultraviolet femtosecond laser is experimentally studied based on the new nonlinear crystal Ba_1-xB_2-y-zO₄Si_xAl_yGa_z It is a kind of low-temperature phase barium metaborate single crystal belonging to a trigonal system, doped with one or more elements selected from Si, Al and Ga. As an optimized β-BaB₂O₄ crystal, Ba_1-xB_2-y-zO₄Si_xAl_yGa_z completely overcomes the shortcomings of deliquescence compared with β-BaB₂O₄, and its nonlinear efficiency and optical damage threshold have also been greatly improved. Using two crystals as second harmonic generation is to compensate for the spatial walk-off effect and the light path walk-off due to refraction effect The optical axis of the second Ba_1-xB_2-y-zO₄Si_xAl_yGa_z is twice the phase matching angle with respect to the first one. In a femtosecond regime, short pulse provides high efficient frequency conversation due to their high peak powers, but the group velocity mismatch is a cognitive factor to limit conversion efficiency. It is obvious that after the frequency doubling, the second harmonic pulse and fundamental pulse separate from each other. The second harmonic pulse lags behind the fundamental pulse as they propagate through the crystal and the second harmonic pulse is broadened into a longer pulse duration than the fundamental pulse The method to compensate for the group velocity mismatch is to adjust the path length between the fundamental and second harmonic pulse by means of time delay line. It consists of beam splitters and mirrors. Tunable deep ultraviolet pulse within a wavelength range from 192.5 to 210 nm is produced, with a maximum average power of 5.8 mW, under a 2.78 W fundamental power. The average power of second harmonic, third harmonic and fourth harmonic are 1.28 W, 194 mW and 5.8 mW at the fundamental wavelength of 800 nm, corresponding to conversion efficiencies of 46.14%, 15.16% and 3% from the previous stage, respectively. The duration of the third harmonic pulse is 640.4 fs at 266.7 nm as measured by the cross-correlation technique.