Spectroscopic Gas Sensing Based on a MEMS-SOA Swept Fiber Laser Source

In this work, we report on real-time gas sensing using a swept fiber laser source (SFLS) based on a micro-electro-mechanical-system (MEMS) tunable filter, a semiconductor optical amplifier (SOA) gain medium and a fiber ring. The MEMS filter is implemented using deeply-etched silicon-on-insulator (SOI) silicon/air Bragg mirrors attached to a micro-actuator enabling a relatively high-speed tunability and compatibility with single-mode fibers. A theoretical analysis is presented relating the required laser output power and spectral linewidth to the gas detection limits taking into account the signal-to-noise ratio (SNR). The minimum detectable concentrations of nitrous oxide, carbon dioxide, carbon monoxide and acetylene gases were determined. The dependence of the gas spectrum in terms of its absorption line strength, spectral width and spacing on the laser source was emphasized in the analysis, revealing the limits of distinguishing between different gases. The fiber-laser source is then characterized under static and dynamic conditions. An experimental parametric study was carried out varying the fiber cavity length from 20 m to 2 km, the sweep rate up to 2 kHz and the pumping current up to 6 times the threshold current. Finally, the SFLS is exploited to measure the transmission spectrum of carbon dioxide gas - as an example - at various pressures. The effects of the pumping current, the sweeping speed, and the sweeping direction on the measured spectrum were studied in order to determine the optimal conditions. The measured spectrum is compared with the theoretical one taking into consideration the FWHM of the laser source. The comparison shows a good match.