[Theoretical analysis of the residual amplitude modulation of frequency modulation strong absorption spectroscopy].

ABSTRACT

The application of frequency modulation (FM) spectroscopy technology in the trace gas detection is greatly restricted by the residual amplitude modulation (RAM) which induced by the birefringence of the electro-optic modulator (EOM) and the misaligned linear polarization direction of laser to the EOM. Based on the interaction between the laser field and crystal, a line-shape expression of the FM spectroscopy with RAM is obtained. The angle between the polarization direction of input light and the principle axis of EOM, theta, the phase difference between the two principle axes of electro-optic crystal, delta phi, and the modulation index FM, beta, are the major factors to influence the lineshape. The larger theta and larger delta phi are, the stronger distortion of the lineshape is. Meanwhile a DC offset exists in the FM dispersion spectroscopy which is influenced by theta and delta phi. Finally a servo control of theta and delta phi is suggested to reduce the RAM. These phenomenon and the analysis of the lineshape provide a necessary technical support for the fiber components based FM spectroscopy.