Optical fiber gas sensor development and application

Abstract

ABSTRACT This paper reports recent development and application of optical fiber gas sensors using absorption spectroscopy, including open-path gas sensors using fiber coupled micro-optic cells and photonic bandgap (PBG) fibers. A fiber-optic sensor system capable of detecting dissolved fault gases in oil-insulated equipment in power industry is presented. The gases include methane (CH 4 ), acetylene (C 2 H 2 ) and ethylene (C 2 H 4 ). In addition, the development of gas sensor using PBG fiber will be reported. Keywords: optical fiber gas sensor, dissolved gas measurement, photonic bandgap fiber. 1. INTRODUCTION Absorption based optical spectroscopy [1] is widely used in conventional gas analysis. As the rapid development of optical fiber technology, the optical fiber sensors based on these techniques have attracted significant attention because of their importance in applications such as environmental monitoring, biomedical sensing, and industrial process control. The advantages of fiber sensors include their remote detection capability, safety in hazardous environments and immunity to electromagnetic fields. A number of important gases which possess overtone or combination absorption lines in the low-loss transmission window of silica fibers may be detected with fiber optic systems. Early fiber optic gas sensors made use of open path, bulk gas cells and LED sources with optical fibers for light transmission to and from the cell where light absorption took place whic h resulting in relative low detection se nsitivity. Sensitivity and selectivity can be significantly enhanced by use of single frequency laser sources such as distributed feedba ck (DFB) lasers and external cavity diode (ECD) lasers [2-5]. The laser sources have very high spectral power densities within a spectral width less than that of a single absorpti on line in the absorption spectrum. Selectivity can be assured by tuning the laser wavelength to an absorption line of the gas being measured through the us e of a reference gas cell [3,5]. The use of the high spectral density DFB/ECD lasers allowed the demonstration of high se nsitivity gas detection with relatively simple micro-optic gas cells [6-8]. For such kind of gas sensors, there is a variety of applications in industry. Recently, we have developed a fiber optic multi-gas detection system for monitoring the dissolved fault gases in oil-filled power equipment. In power systems, there is a number of oil-insulated power equipment such as power transformers which are im portant and valuable assets. Suitable and frequent maintenance is a prerequisite to ensure the reliability. However, in order to save the maintenance .