High sensitive scheme for methane remote sensor based on tunable diode laser absorption spectroscopy

Abstract

Methane is an important raw material for various petrochemicals in industrial fields and as also a clean fuel in daily life. However, as an inflammable and explosive material, methane leak can lead to disastrous consequences such as fire and explosion. Furthermore, as a kind of greenhouse gas, methane has stronger influence on global warming than carbon dioxide. In this paper, we present a new high sensitive scheme for methane remote sensing, which can facilitate detection and location of methane leakage. And the 2v3 band (near 1653.7 nm) of methane is chosen as the target transition which is free from the absorption of the other molecule in atmosphere. A tunable distributed-feedback diode laser is adapted to scan across the target transition. A Fresnel lens with a diameter of 150 mm is employed to collect the ambient backscattering light from natural features such as the buildings. The first harmonic signal is used to normalize the second harmonic signal to remove the influence introduced by the unknown reflectance factor of the actual target, therefore no retro-reflector is needed. Traditional tunable diode laser absorption spectroscopy (TDLAS) method has difficulty in locating the second harmonic signal peak position in low concentration conditions because of low signal-noise-ratio (SNR). To improve the SNR especially in low concentration environment, a scheme named baseline-offset TDLAS is presented in the paper, in which a reference cell filled with standard methane sample is inserted into the measuring optical path. The reference cell can also be used to calibrate the sensor. Furthermore, the reference cell can be used to lock the central frequency of the diode laser to the absorption peak position to monitor concentration fluctuation continuously. In the peak-locking mode, the sensor demodulates the third harmonic signal as error signal to control the injection current of the laser source with PID control. Moreover, one advantage of peak-locking mode is that the measurement frequency is about two orders of magnitude higher than the traditional TDLAS method. With baseline-offset TDLAS, the remote sensor described in this paper obtains SNRs as high as 19 and 16 at a stand-off distance of 10 m and 20 m, respectively. With such a high SNR, there is no necessity for complex algorithm in absorption peak position location. By defining the standard deviation of the measuring concentration as the detection limit, experimental results show that the proposed methane remote sensor has detection limits of 5 ppm m at a distance of 10 m and 16 ppmm for 20 m, respectively, while measuring the ambient methane. In peak-locked mode, the experimental system has a detection limit of 22 ppmm at a distance up to 37 m and can monitor rapid concentration fluctuation in.