Gas detection techniques with fiber optical spectrum absorption at near-IR wavelength

Abstract

Detection of pollution gas is important in environmental and pollution monitoring, which can be used widely in mining and petrochemical industry. Fiber optical spectrum absorption (FOSA) at near-IR wavelength is widely used in gas detection due to its essential advantages. It has attracted considerable attention, and there are several types and methods in FOSA. Wavelength modulation technique (WMT) is one of them, which will improve the gas detection sensitivity dramatically. This technique can be realized by detecting the intensity of the second-harmonic component signal. Intra-cavity laser spectroscopy (ICLS) is another alternative technique for high sensitivity absorption measurement. With an absorber directly placed within the laser cavity, a short absorption cell can be transformed into a high sensitivity system. But the practical sensitivity is obviously less than the theoretical value. The authors did some works in these fields and have obtained some remarkable progress. With broad reflectors instead of FBG as mirror of the cavity and wavelength sweep technique (WST), several absorption spectra of detected gas can be collected. And the detection sensitivity can be enhanced sharply by averaging the results of each spectrum, with acetylene sensitivity less than 100 ppm . When ICLS is used combined with WST and WMT, the detection sensitivity of acetylene can be enhanced further. The sensitivity is less than 75 ppm. By using FBGs as wavelength references, the absorption wavelength of the detected gas is obtained, which can be used to realize gas recognition. The system is capable of accessing into fiber intelligent sensing network.