Instrumented research of gas sensing system based on intra-cavity optical fiber ring laser technology

学科专业：光学工程
研 究 生：梁霄
指导教师：刘铁根 教授

天津大学精密仪器与光电子工程学院
二零一一年十二月
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中文摘要

基于光纤环腔激光技术的气体传感方法因为具有很高的检测灵敏度，受到了国内外的广泛关注。采用该方法进行气体检测时由于系统不稳定、噪声大等问题使传感效果远不及理想情况。为了使该气体检测方法能够尽快从实验室阶段走向实用化，进行基于光纤环腔激光技术的气体传感仪器化研究意义重大。
本课题针对仪器化气体传感器系统，进行了如下主要工作：
1、对EDFA特性及掺铒光纤的能级理论进行了研究，基于速率方程构建了光纤环腔激光系统的理论模型，搭建了光纤环腔激光系统，利用该系统得到了光纤环腔激光器输出与泵浦电流、系统损耗和输出耦合比的关系，并获得了光纤环腔激光器的优化参数。
2、基于光纤环腔激光技术的理论和实验构建了气体传感系统理论模型，理论模拟了系统损耗、泵浦功率与气体谱线吸收度的关系；在气体传感系统理论模型的基础上，从以下几方面设计了光纤环腔激光气体传感系统的原理样机：
光学设计：采用F-P标准具实现了可调谐光滤波器非线性的实时标定；优化了系统光路使气体传感灵敏度增强系数提高了5倍以上。
机械设计：设计了仪器外壳及10cm、20cm和50cm长度的系列化气室。
电路设计：完成了低噪声的光电探测电路，并以STC单片机为核心设计了光源功率测量显示电路及智能选频电路。
算法设计：利用小波去噪方法对吸收光谱进行了预处理；采用洛伦兹拟合对气体吸收谱线进行了拟合；设计了气体吸收谱线的提取算法。
软件系统构建：以labview为基础构建了集原理样机控制、数据采集与处理和结果显示等功能为一体的软件系统。
3、对设计完成的原理样机进行了测试，该仪器可以实现对1525nm-1565nm气体吸收谱线的测试，采样误差<11×10-7，波长定位误差<80pm，气体浓度测试的累积重复性CV<0.015，可探测乙炔气体的最低浓度达到200ppm。利用该原理样机研究了内腔法与外腔法的不同以及气室长度、系统损耗、气体浓度与灵敏度增强的关系，测试结果与理论结果吻合。
基于光纤环腔激光技术气体传感系统原理样机的设计完成为深入研究该方法以及使该技术尽早实用化奠定了坚实基础。

关键词： 光纤环腔激光技术 气体传感 仪器化研究 原理样机

ABSTRACT

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.

KEY WORDS：ICOFRL Technology, Gas Sensing, Instrumented Reaserch, Prototype