The Research Of High Stability Fiber Frequency Modulation Spectroscopy Technology

As a high sensitivity laser spectroscopy, Frequency Modulation (FM) spectroscopy technology has been widely used in the field of gas detection. There will induce the Residual Amplitude Modulation (RAM) in the FM spectroscopy by using the fiber devices. The RAM will cause the instability of the FM spectroscopy signal and affect the spectral lineshape of the FM spectroscopy and finally influence the sensitivity for the gas detection.The thesis firstly analyzed the spectral lineshape of the FM spectroscopy in the different detection phase and deduced the expression of the fiber FM spectroscopy. Then, theoretical analyzed the FM spectral lineshape for wavelength in6544.4419cm-1of C2H2with absorption band v1+v3of P5(e) by using the Matlab. According to the analysis of FM spectroscopy with RAM, it identified two main factors, the angle between the polarization direction of the incident light of fiber EOM and the extraordinary axis of it and the phase difference of the ordinary axis and the extraordinary axis of the fiber EOM, which affect the spectral lineshape of the FM spectroscopy. Though the theory analysis, it put forward to using Hansch-Couillaud (H-C) scheme to restrain the RAM of the FM spectroscopy by controlling the phase difference of the ordinary axis and the extraordinary axis of the fiber EOM. In the experiment, it measured the FM spectroscopy signal of the C2H2with the wavelength of6544.4419cm-1of Vi+v3absorption band of P5(e) and making least square fitting between the experimental result and the theoretical analysis. The fitting result is less than4%compared with the signal peak value. It is not only demonstrates the Correctness of the theoretical analysis, but also illustrates the validity of the H-C scheme for restraint the RAM of the FM spectroscopy.