Optic Fiber Methane Gas Sensor Based on Tunable Diode Laser Absorption Spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) is a widely used technique in measuring the concentration of trace gas due to its high sensitivity, high selectivity, and fast time response. The fluctuations of temperature in the gas cell can cause the characteristics of the absorption spectra to change in the TDLAS methane sensing system. The three absorption lines in the R(3) transitions of 2v₃ band of methane at 1653.72 nm have been studied, and the influence of the temperature fluctuations on the spectral absorption coefficient and the amplitude of second harmonic is analyzed. A simple piece of equipment with temperature acquisition devices is developed for measuring methane gas concentration, temperature signals are obtained for eliminating the impact of temperature fluctuations. Using calibration coefficient, the amplitude of second harmonic is transformed into standard signal at reference temperature in order to restrain the influence of temperature fluctuations. The results show that this optic fiber methane gas sensor using tunable diode laser absorption spectroscopy can restrains the influence of temperature fluctuations and improves detection accuracy effectively.

I. Introduction

Methane is a flammable and explosive gas, with a lower explosion limit of 4.90% and an upper explosion limit of 15.4% [1]. It is the second most important greenhouse gas after CO₂. Methane concentration in the atmosphere is about 1.6ppm and much lower than CO₂. However, it is responsible for 26% of the total greenhouse effect, as its capability as a greenhouse gas is 22 times larger than that of CO₂. Spectral absorption method combined with harmonic detection is widely used in the measurement of trace gas concentration. Tunable diode laser absorption spectroscopy (TDLAS) has been widely employed in detecting atmospheric trace gases due to its high sensitivity, high selectivity, and fast time response [2]. Wavelength modulation spectroscopy (WMS) is based on the modulation of the light emitted by a laser that is slowly tuned through an absorption feature of the species to be detected. The signal of second harmonic can be measured with lock-in amplifier and is proportional to the concentration. An important advantage of this technique is to shift the detection to higher frequencies, at which the laser excess noise ( noise) is reduced [3]. The device uses an opening absorption cell and the characteristics of the absorption lines are vulnerable to the impact of ambient temperature fluctuations. In different temperature conditions, for the same concentration of gas, the amplitude of second harmonic detected by lock in amplifier fluctuates with the variation of ambient temperature. For the retrieval of the trace gas concentration, the influence of temperature fluctuations must be taken into account.