Instrumented Research Of Gas Sensing System Based On Intra-cavity Optical Fiber Ring Laser Technology

The gas sensing method based on intra-cavity optical fiber ring laser （ICOFRL） technology is subjected to wide attention at home and abroad because of its high sensitivity, low detectable concentration and other features. But there is no instrument based on this sensing technology now, so the sensing effect is far less than ideal due to system instability, noise and other issues.The topic for the problems carried out the following main tasks:1、The characteristic of EDFA and the theory of EDF energy levels have been studied. The theoretical model of ICOFRL was constructed based on rate equation and the practical optical fiber ring laser system was built. The relationships between laser output and pump current, system loss, output coupling ratio were obtained based on the model. The optimized parameters of the system were also gained.2、The theoretical model of gas sensing system was built based on the theory and experiment of ICOFRL technology. Prototype was designed from following aspects:（a） Optical design: By using F-P etalon the non-linearity of tunable optical filter was calibrated. Optimizing the optical system made the sensitivity enhancement coefficient increase by 5 times.（b） Mechanical design: A series of gas cells which are 10cm, 20cm and 50cm were desiged, and the designing of instrument shell was completed.（c） Circuit design: Low-noise optical detection circuit was designed. The optical power measurement and display circuit were designed by using the STC SCM as core. The intelligent frequerncy selection circuit was designed.（d） Algorithm design: The wavelet denoising was used on the absorption spectrum. Lorentzian fitting algorithm was adopted to realize gas demodulation and absorption lines extraction algorithm was developed.（e） Software system: Built the labview-based software system which can realize prototype controlling, data acquisition and processing, and result display.3、Tested the prototype. The spectral range can be measured is 1525nm-1565nm. Sampling error, wavelength positioning error and gas concentration reproducibility are less than 11×10^-7, 80pm and 0.015 respectively. The minimum detectable concentration of acetylene is 200ppm. Using the prototype studied the differences of the cavity method and external cavity method, and also studied the relationship between the sensitivity enhancement and gas cell length, system loss, gas concentration. Test results consitented with the theoretical results.The gas sensing prototype base on fiber ring cavity laser technology laid the foundation for studying the technology and pratical use of the technology as early as possible.