频率调制强吸收光谱中残余幅度调制的理论分析

Theoretical Analysis of the Residual Amplitude Modulation of Frequency Modulation Strong Absorption Spectroscopy

由电光调制器(EOM)中双折射效应及线偏光不完全沿EOM调制方向诱发的残余幅度调制(RAM)使频率调制(FM)光谱技术在微量气体检测中的应用受到极大的限制。基于光场与晶体相互作用及光学干涉原理推导出存在RAM时FM光谱的线型表达式,确定出输入线偏光角度、EOM中双折射效应、FM系数等是影响线型的主要因素,且当入射EOM光的偏振角度偏离调制方向越大,双折射效应引起的特征偏振方向相位差越大,线型扭曲越严重;同时在FM色散光谱中存在一个受两者影响的直流偏置;最后给出通过伺服控制这两过程可以达到抑制RAM的目的。这些现象及线型的分析将为基于光纤器件的FM光谱提供必要的理论支持。

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 lineshape expression of the FM spectroscopy with RAM is obtained.The angle between the polarization direction of input light and the principle axis of EOM,θ,the phase difference between the two principle axes of electro-optic crystal,Δ,and the modulation index FM,β,are the major factors to influence the lineshape.The larger θ and larger Δ are,the stronger distortion of the lineshape is.Meanwhile a DC offset exists in the FM dispersion spectroscopy which is influenced by θ and Δ.Finally a servo control of θ and Δ 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.