Atmospheric Trace-Gas Detection Using Very High-Sensitive Laser Sensors

点击翻译按钮获取中文摘要

High-sensitive detection of atmospheric trace-gases is becoming more and more important in the environmental issues concerning the earth's atmosphere such as destruction of ozone layrs and acid rain. The purpose of this research is to establish the method of detecting trace gases of ppt order under 1 atm to 10^<-3> atm conditions. The recent development of compact laser sources is fully exploited in the experiments with prototype laboratory models.1.For the spatial propagation experiment we have constructed a vacuum system equipped with appropriate windows. Acetylene is chosen for The specimen that exhibits absorption at the wavelength region of 1.5 mu m. A diode laser of distributed-feedback (DFB) type is mainly used as the source. Fundamental parameters such as the transition moment of the absorption and scanning characteristics of the laser source are measured with the method of highly sensitive absorption spectroscopy using current modulation. A more sensitive method is the one whi … More ch is based on a Fabry-Perot resonator constructed with highly reflective mirrors. Using an absorption cell of 0.25 m in length, we have observed a signal corresponding to an equivalent absorption length of 2 m, verifying the enhancement effect. Further increase of the stability of the resonator structure and that of the light source will lead to development of the sensors of resonator type.2.As an example of the multi-mode scheme, a trace-gas detection experiment has been performed using an all-fiber, ring-laser resonator. An amplifier based on Er-doped fiber is utilized as a light source, and a bandwidth as narrow as 0.04 nm has been attained with band-pass filters. A special type gas cell that can be directly connected with optical fibers is devised to observe the acetylene absorption at 1.5 mu m. Varying ring configurations, we have found that under suitable conditions the change of laser output power with acetylene pressure becomes large when the pumping power is close to the threshold. This sensor exhibits excellent performance also in detecting very slight pressure change of the trace gas in a pressure range determined by the pumping power. Further decrease of the system loss ensures the development of a very compact, all-fiber trace-gas sensor. Less