Differential Absorption Gas Sensing Using Semiconductor Lasers

Abstract

Detection of various gases is very important for safety in mining, laboratories, and manufacturing processes. Current optical methods using lock-in detection and optical absorption spectroscopy have different limitations. Lock-in detection does not distinguish attenuation from absorption, while spectroscopy requires expensive and bulky optical spectrum analyzers. This project aims to measure the absorption with attenuation eliminated so as to reduce the cost of instrument. Two InGaAsP/InP distributed feedback lasers emitting at around 1550 nm are applied as the optical source. One was modulated at 390 kHz with wavelength λ1, while the other was modulated at 440 kHz with wavelength λ2 for differential absorption detection. The modulated signals were injected into an 800-mm C-band hydrogen cyanide (H13C14N) gas cell at 10 Torr. By detuning one of the lasers, the gas absorption and attenuation at different wavelengths are measured on the power spectrum analyzer. The result shows that the absorption peak appears at 1548.884 nm with about 4-dB loss. This method can separate attenuation of optical misalignment from the absorption. Moreover, the instrument cost is reduced by replacing the optical spectrum analyzer. Possibility of employing chaos oscillation for detection was considered.