The Generation、Restraint And Application For The Residual Amplitude Modulation Of Frequency Modulation Spectroscopy In Fiber

Due to its high detection sensitivity and simultaneously detecting the absorption and dispersion of the gas, Frequency Modulation Spectroscopy (FMS) technology has been widely applied in atomic and molecular spectroscopy, trace gas detection, etc. Development of all-fiber FMS can avoid light path to expose in the external environment, improve anti-interference of the system, and realize the miniaturization of device. However, the FMS is a polarization sensitive technology, the introduction of the optical fiber makes the polarization of light changes which induced Residual Amplitude Modulation (RAM), and then influence the gas detection sensitivity.In the thesis, the theory about the general FMS lineshape and the mechanism of RAM is analyzed, the expression of RAM background signal without absorption is deduced, and the characteristics of influence factors with the RAM are studied. Based on dependence of detection phase and the RAM, a fast method of determining the FMS detecting phase is presented. Compared with the traditional method, the deviation is controlled within 0.04rad(2°), and the scheme is confirmed feasibility. Our technology has the advantages of rapid detection, simple and reliable setup, which could provide the necessary technical reserves for the fiber FMS technology in the practice applications.In the optical fiber FMS technology, RAM will lead to an unstable dc bias and distortion of the form, which affect the sensitivity of gas concentration measurement, it is a key problem to be overcomed. This paper proposes a scheme to reduce the RAM by Hansch-Couillaud (H-C) polarization detection technology. The scheme utilizes the H-C polarization detection technology to test the output polarization of optical fiber electro-optic modulator (f-EOM), as the error signal feedback to f-EOM low frequency control port to eliminate RAM.First, we realize the high quality linear polarized light with the Polarization-Maintaining fiber; Second, on acetylene gas at 6544.4419cm-1 absorption line, the standard FMS lines is obtained by using H-C feedback control. and FMS dispersion stability with and without the H-C signal feedback is assessed with the Allan variance. In addition, Unbiased and symmetry of PDH error signal is obtained experimentally by the method of H-C feedback restraint. In this way, the situations of frequency locking point drift and the long-term stability for the frequency locking is compressed. The RAM restraint scheme greatly increase the performance of frequency locking, and realize the precision, long-term locking of laser frequency.