Gas Absorption Center-Based Wavelength Calibration Technique in QEPAS System for SNR Improvement

Abstract

A simple and effective wavelength calibration scheme is proposed in a quartz enhanced photoacoustic spectroscopy (QEPAS) system for trace gas detection. A reference gas cell is connected an InGaAs photodetector for detecting the absorption intensity peak caused by the gas to calibrate the gas absorption center using distributed feedback laser diode (DFB-LD) with sawtooth wave driver current. The gas absorption wavelength calibration and gas sensing operations are conducted at a special internal to eliminate the wavelength shift of DFB-LD caused by the ambient fluctuations. Compared with the conventional wavelength modulation spectroscopy (WMS), this method uses a lower lock-in amplifier bandwidth and averaging algorithm to improve signal noise ratio (SNR). Water vapor is chosen as a sample gas to evaluate its performance. In the experiments, the impact of sawtooth wave frequency and lock-in amplifier bandwidth on the harmonic signal is analyzed, and the wavelength-calibration technique-based system achieves a minimum detection limit (MDL) of 790 ppbv and SNR with 13.4 improvement factor compared with the conventional WMS system.

Improvement of the Detection Sensitivity for Tunable Diode Laser Absorption Spectroscopy: A Review

“…Wei Y et al employed the 1.62 μm fiber-coupled DFB diode laser as the light source to achieve a ppm-level high sensitivity detecting system for the multi-point ethylene concentration detection, meeting the needs of fire warning in coal mine goafs [30]. Wang Z et al chose the absorption line of water vapor at 1,368.5597 nm for detection and achieved the lowest detection limit of 790 ppb [31]. Using a single DFB diode laser emitting at 2.33 μm, Shao L et al presented a spectrometer for constantly measuring atmospheric CO and CH 4 [32].…”