TDLAS gas sensing system utilizing fiber reflector based round-trip structure: Double absorption path-length, residual amplitude modulation removal

A fiber reflector based round-trip structure is proposed in TDLAS gas sensing system to increase absorption path-length, and a residual amplitude modulation removal (RAMR) technique is introduced, improving the second harmonic profile significantly. Fiber coupled Herriott cell (3 m path-length) is used in experiments, the transmitted light is reflected back by a fiber reflector, realizing double absorption path-length. The double-pass light is split in 1:1 to two photodetectors for second harmonics lock-in, one is for gas concentration demodulation and another for residual amplitude modulation (RAM) measurement by tuning the reference signal in different phases, then a waveform recovery technique based on subtraction is applied to the dual path lock-in for RAMR. The theory is simulated, and experimental results verify that the new structure increases absorption signal efficiently, almost 2 times, the RAM effects are eliminated successfully without increasing the system noise. R-square of 0.9987 is achieved in linearity test of the improved gas sensing system.