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.