基于BN晶体的2~2.5微米宽调谐超短脉冲固体激光研究

2~2.5μm wavelength bands laser sources, covering absorption spectrum of many important gases and being in the atmospheric window, have promising applications in the fields of spectral analysis and atmospheric monitoring. In order to meet the requirement of gas detection for ultrashort pulse and wide band tuning laser, we intend to insert a Raman crystal into the passively mode-locked laser cavity, and to achieve a 2~2.5μm wavelength bands laser with ultrashort pulse and broad tuning range. We first use the BN crystal as the saturated absorption materials of 2 micron ultrashort pulse passively Q-switched and mode-locked laser. By using the BN materials with obviously wide bandgap energy (~6eV), high thermal conductivity, and transparent ranges, we explore a new technology way to realize a 2μm ultrashort pulsed laser. We chose a 1.94 μm fiber-coupled diode laser end pumped Ho:CaF2 for the first time, allowing us to obtain a compact structure laser. The short phonon spectra of Ho:CaF2 crystal results in much weaker multiphonon relaxation, which can reduce the thermal losses and improve the optical efficiency and oscillator stability of passively Q-switched and mode-locked laser. By using BaWO4 crystal with large Raman gain in 2μm as the nonlinear frequency conversion medium, we study the key technical issues of the 2~2.5μm laser with ultrashort pulse and broad tuning range. The research results will promote the two micron laser into the new investigation and application fields of ultrashort pulse laser with wide tuning range.

2~2.5μm波段覆盖了许多重要气体的吸收谱且处于大气窗口，在光谱分析和大气监测等领域具有重要的应用价值。为满足气体探测对激光器超短脉冲和宽调谐的要求，本项目拟将被动锁模和拉曼激光器相结合，实现2~2.5μm波段宽调谐的超短脉冲固体激光输出。本项目首次选用BN晶体作为2μm超短脉冲全固态激光器的被动调Q/锁模材料。充分利用该材料禁带宽度（~6eV）宽、热导率高和透光范围宽等优点，探索2μm波段实现超短脉冲输出的新技术途径。首次选用1940nm光纤耦合半导体激光器端面泵浦短声子谱的氟化物晶体Ho:CaF2的设计方案，此方案将大大降低激光介质的热负载，提高被动调Q/锁模激光器的效率和稳定性，且使激光器的结构更加紧凑。利用在2μm有较大拉曼增益的BaWO4晶体为非线性频率变换介质，研究实现2~2.5μm宽调谐超短脉冲激光的关键技术问题，推动2μm激光进入超短脉冲宽调谐的研究和应用新领域。

2~2.5μm波段覆盖了许多重要气体的吸收谱且处于大气窗口，在光谱分析和大气监测等领域具有重要的应用价值。本项目利用声波降解法、液体剥离法及高能激光溅射法等多种方法成功制作了多片BN饱和吸收体，并测试了相关各项性能参数，成功实现了2微米的被动调Q激光和锁模激光输出。以BN为饱和吸收体，研究了被动调Q的Tm:LGGG激光器，获得了平均输出功率约为845mW的2009.7 nm脉冲激光，脉冲宽度约为736 ns；研究了被动调Q的Tm,Ho:YAP被动调Q激光器，获得了649 mW的2000.5 nm的被动调Q激光，脉冲宽度约为5.8 μs，重频45.1 kHz。以BN为饱和吸收体，研究了被动调Q和被动调Q锁模的Tm:YAP激光器，获得了平均输出功率为1.54 W的1937.4 nm的被动调Q脉冲激光，最高脉冲重复频率和最窄脉冲宽度分别为301.2 kHz和349 ns；获得了平均输出功率880 mW的1937.0 nm的调Q锁模激光，脉冲宽度为478.8 ps，重复频率为96.5 MHz，单脉冲能量为9.11 nJ，峰值功率为18.68 W，信噪比为45.6 dB。利用1940 nm半导体激光器泵浦Ho:CaF2晶体，获得了4.2 W的2085.2 nm连续激光，激光线宽（FWHM）为1.5 nm，斜率效率为48.4%；以Cr2+:ZnS为饱和吸收体获得了3.6 W的2084.1 nm被动调Q激光，斜率效率为42.2%，脉冲宽度约为92 ns，重复频率为5.88 kHz。基于申报书的实验装置，研究了连续和被动连续锁模运转的Tm:YAG激光器，获得了平均输出功率为497 mW的2017.7 nm的连续激光，基于GaAs的SESAM作为饱和吸收镜，获得了平均输出功率为112 mW的2012.2 nm被动连续锁模激光，脉冲宽度47.9 ps，重复频率97.7 MHz，单脉冲能量1.2 nJ，信噪比72 dB，被动连续锁模Tm:YAG激光器的时间带宽积计算为4.3。上述2微米被动调Q、被动调Q锁模和被动连续锁模激光的获得，对工业检测和基础研究领域所需的光源提供了更多的选择，将极大的推动相关科学领域的进一步发展。