TDLAS using FPGA-based lock-in detection for multi-channel Chemical Species Tomography

In this paper, a large scale, multi-channel optical Chemical Species Tomography (CST) system is presented to aid research into aero jet-engine performance. Through high-speed, non-intrusive measurement and spatio-temporal imaging of gas concentration dynamics; fuel efficiency, emissions, combustion diagnostics and novel engine designs, can be tested to achieve greener aviation. A 126-channel tunable diode laser absorption spectroscopy (TDLAS) system is proposed, utilizing wavelength modulation spectroscopy (WMS) for robust noise performance in harsh industrial environments. Narrow CO2 absorption spectra, in the region of 1997.2nm, can be obtained from first (1f) and second (2f) harmonics of WMS signals by an FPGA-based digital lock-in amplifier (DLIA). Comparative single-channel gas-cell experiments, using the DLIA and a commercial rack-mounted system, demonstrate the clear prospect of all-digital, scalable, multi-projection CST of jet-engine exhaust-plumes. Further, the low cost and compact nature of FPGAs indicate the feasibility of reduced image reconstruction distortion, through massively multi-channel diagnostic tomography instrumentation.