Investigation and cancellation of residual amplitude modulation in fiber electro-optic modulator based frequency modulation gas sensing technique

Frequency modulation spectroscopy (FMS) is most commonly used for trace gas detection, but its sensitivity is usually limited by the background noise attributed to residual amplitude modulation (RAM). In this paper, comprehensive theoretical analyses of FMS with and without RAM are presented. Then the measurements of RAM are experimentally verified utilizing a fiber electro-optic modulator based FMS setup. In order to reduce the RAM, a novel servo control method with its correction signal obtained from a polarization detection scheme was proposed and pure FMS signals of C2H2 gas at wavenumber of 6544.4419 cm−1 are obtained. Furtherly, long-term measurements of the dispersion background signal with feedback loop on and off are performed to evaluate the stability of the instrument. Finally, this servo control method is extended into the application of Pound–Drever–Hall frequency stabilization technique to improve the locking accuracy and stability.