Modified laser scanning technique in wavelength modulation spectroscopy for advanced TDLAS gas sensing

Abstract

• Modified laser scanning technique, i.e. half-wave scan, allows the unique in-situ monitoring and regulation of laser wavelength in the TDLAS gas sensor. • Nonlinear half-wave scan can maintain the sensing stability with a minimized sampling rate beyond the conventional linear scan for data volume and time saving. • Minimum detecting limit of 3 ppm and measurement accuracy of 9.6 ppm @400 ppm in methane sensors. Tunable diode laser absorption spectroscopy (TDLAS) for trace gas sensing has been investigated widely. Conventional wavelength-modulation spectroscopy (WMS) in TDLAS usually involves linear triangle patterns for tunable laser scanning. Here, a novel modified laser scanning technique is proposed and experimentally demonstrated in the WMS-based TDLAS gas sensor. The half-wave scan is primarily introduced to drive the laser diode to cover 1/2 range of the Lorentz-like gas absorption line. By monitoring waveform derivation of second harmonics, real-time status of the laser output could be exhibited and the output wavelength constancy can be controlled. It is also verified that second harmonics feature basically linearly with wavelength drifts, allowing the in-situ regulation of laser wavelength stability. Moreover, the nonlinear half-wave scan is introduced and designs a nonlinear function across the scanning pattern. It enlarges the temporal window around strong absorption and consequently acquires additional valuable waveform data. With the limited sampling rate for TDLAS signal processing, by using the nonlinear half-wave scan, second harmonics fluctuate merely 1/3 of that in the conventional WMS-2f linear scan. The proposed modified laser scanning technique is employed in a methane sensor, realizing about 3 ppm minimum detection limit and 9.6 ppm measurement accuracy. This technique is beneficial and universal for real-time monitoring and in-situ control of laser sources and improvement in accuracy and stability for current WMS-based TDLAS gas detection.