Study On Optical Fiber Gas Sensors

Large-scale power factories and mine exhaust a great deal of flammable, explosive and poisonous gas into the air, human's living environment and safety are seriously menaced, so rapid and real-time detection of the gas is very important for secure production of mine and environment protection.,especially low concentration of the poisonous gas should be detected. Conventional gas sensors work by the sense organ's resistance and capacitance's change, so their sensitivity is law and anti-jamming ability is bad. Rapid, sensitive and effective gas concentration detection methods are urgent. Recently, optical fiber gas sensors are focused on all over the world, for they are inherently secure, anti-electronagnetic, sensitive, have large dynamic range, respond rapidly, measure precisely and remotely.In this paper, an optical fiber gas measurement system is set up based on spectrum absorption method. Each kind of gas has absorptive near-infrared spectra, if a light source's center wavelength is locked on the gas's absorptive spectrum peak, then absorption will happen, the output light intensity decrement is related to the gas's concentration, so we analyze the output light, we will know the concentration.The absorption spectra of methane and ethyne are confirmed and the spectra characteristics which is fit for low-loss transmission window of optical fiber is also discussed. Distributed feedback laser diode (DFB LD) is used as a light source and light source modulation harmonic measurement is presented in this paper. The ratio of the fundamental and second harmonic signal can be used as output for eliminating the interference owing to light power changing;the difference absorption method can also be used.The detailed research contents are as following:Study on spectrum absorption measurement principle of optical fiber gas sensor,especially analyze sine wave and triangle wave modulation harmonic measurement and compare them, then theory and experiment show that triangle wave modulation is better.Build up lab research system of optical fiber gas sensor using DFB LD as light sourceand high speed data acquisition card, debug the circuit, design special demodulation software GasSensor which output directly data and wave, also make research and test experiment conveniently.Study on the difference absorption method and design a system with reference circuit, do some primary experiments.Using methane and ethyne as sample, do some experiments with GasSensor, get theirconcentration measurement curves, and validate the system.Improve on controlling the light source's working temperature and do the system's temperature controlling experiments, because the changing of DFB LD surface and kernel temperature will affect directly the light source's center wavelength.