Research study on development of a CW all-solid-state coherent light source in mid-infrared region

在工业过程监测、环境痕量气体传感、光谱学等领域，需要工作在2-5-μm中红外(MID-IR)范围内的宽调谐相干光源。这项工作的目的是开发一种紧凑、可广泛调谐的相干光源，用于灵敏和选择性地检测环境空气中的污染物和对环境重要的痕量气体。虽然有激光二极管可用，但它们需要低温工作，调谐范围也很窄且不连续。另一方面，也可以考虑基于近红外可调谐激光光源的差频产生(DFG)。在本研究中，基于两个单频激光泵浦的准相位匹配PPLN晶体中的差频产生技术，发展了一种紧凑的、宽调谐的相干光源，用于检测各种气体。泵浦激光器为外腔半导体激光器，可调谐范围为825~870 nm。信号激光器为DFB半导体激光器，波长1060.6 nm。差频输出在3.7~4.8μm范围内可调，单周期为A=2 2.5 8μm。初步研究表明，该光源可用于基于吸收光谱的大气CO_2检测。还研究了激光二极管泵浦、光纤耦合的Cr3+：LiSrAlF-6激光器，它可以在818-900 nm范围内调谐，以取代可调谐的激光二极管，进一步扩大可调谐的DFG输出。这种可调光源还可用于监测几种痕量空气污染物，如一氧化碳(CO)、一氧化二氮(N_2O)、甲醛(H_2CO)和甲烷(CH_4)。

There are various applications, such as industrial process monitoring, environmental trace-gas sensing, spectroscopy, where widely tunable coherent light sources operating in the 2-5-μm mid-infrared(mid-IR) region are necessary. The motivation of this work is to develop a compact and widely tunable, coherent optical source for the sensitive and selective detection of pollutants and environmentally important trace gases in ambient air. Although laser diodes are available, they require low-temperature operation and tuning range is also narrow and discontinuous. On the other hand, difference frequency generation(DFG) based on near-infrared, tunable laser sources can also be considered.In the present study, a compact, widely tunable coherent optical source based on a difference frequency generation technique in a quasi-phase-matched PPLN crystal pumped by two single-frequency lasers for the detection of various gases is developed. The pump laser was an external-cavity laser diode tunable from 825 to 870 nm. And the signal laser was a DFB laser diode at 1060.6 nm. The difference-frequency output was tunable from 3.7 to 4.8 μm with a single PPLN period A=22.58 μm. The tunable DFG source was used for absorption measurement of CO_2 in open air. Preliminary studies show that the source can be used for the detection of ambient CO_2 based on the absorption spectroscopy. A laser diode pumped, fiber-coupled Cr^<3+> : LiSrAlF-6 laser which can be tuned from 818-900 nm is also investigated for substituting the tunable laser diode for further widening the tunable DFG output. The widely tunable light source should also be useful in monitoring several trace air contaminants, such as carbon monoxide (CO), nitrous oxide (N_2O), formaldehyde (H_2CO), and methane (CH_4).