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.