A unique approach to gas detection using optical correlation spectroscopy (OCS) has been investigated. By employing a semiconductor optical amplifier (SOA) with an optical fibre delay line to generate the anti-phase signal, zero point drift errors are minimised and compatibility with single mode optical fibre systems has been achieved. Experimental results, obtained for varying concentrations of acetylene (C<sub>2</sub>H<sub>2</sub>) gas at different pressures, agree well with the theoretical analysis. Signal-to-noise ratio (SNR) analysis was also performed to predict the theoretical minimum detectable concentration for C<sub>2</sub>H<sub>2</sub>.
Near infra-red tuneable diode laser spectroscopy (TDLS) with wavelength modulation spectroscopy (WMS) is a powerful technique for the measurements of gas compositions, and its ability to address multiple sensing points over optical fibre networks is proving to be particularly useful. However, the complexity involved in compensating for errors arising from pressure fluctuations is a weakness. Indeed, it is desirable to develop the technique to be capable of measuring pressure. This requires the extraction of accurate linewidth information from the recovered signals, so far made difficult by the presence of a systematic distortion arising from the laser amplitude modulation. Here we report a simple detection technique to null the effects of laser amplitude modulation and recover undistorted signals from which the gas linewidth can be accurately measured. Firstly we demonstrate that the measurements of accurate gas linewidths and pressure can be made from direct detection TDLS, addressing an atmospheric water absorption line. Finally, we report the accurate and simple measurement of acetylene pressure from TDLS / WMS measurements.
A novel variation in optical correlation spectroscopy is presented. A semiconductor optical amplifier (SOA) is employed as the light source and an optical fibre delay line is used to achieve the 180° phase shift between the reference and measurement signals. The system was tested using acetylene (C2H2) gas. Absorption data from the HITRAN database is used to model the theoretical response of the system. Experimental results for the detection of various acetylene gas concentrations have been obtained and compared with theory.