Design of a gas sensor based on asynchronous double beam structure and balanced photodetector

In this paper, we report on the utilization of a novel background-free approach for trace gas sensing. The classical collocation of wavelength modulation spectroscopy and phase sensitive detection is replaced by asynchronous double beam structure in which a balanced amplified photodetector is introduced. Via a beam splitter, the laser beam passing through the gas medium is divided into the real-time and the delayed branches. An optic fiber delay line is taken to create a time-delay between the separated branches, and the following differential detection yields the first derivative of absorption spectrum that is equivalent to the first harmonic in wavelength modulation spectroscopy. By means of the straightforward method, the quantitative analysis of gas is performed without the disturbance that is caused by optical intensity modulation. The gas sensor is validated using ammonia and nitrogen mixture in a laboratory controlled environment (296 K in temperature and 1.01e5 Pa in total pressure). In the case of an absorption path with the effective length of 15.4 cm, the detection limit of 6.4 ppm is predicted from the assumption that the signal is weakened to be equal with the noise. Absorption spectroscopy for 500 ppm ammonia is recovered by integrating the output of balanced photodetector.