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理论分析了激光二极管自混合干涉信号随外腔长度增大的变化特点及规律。从复合谐振腔干涉函数出发,推导出激光二极管自混合干涉信号随外腔长度增加指数衰减,衰减系数与光频虚部成正比,并给出光频虚部和实部满足的超越方程组。求解该超越方程组得到,当外腔长度超过一个临界值时,光频虚部随外腔长度近似反比衰减,则激光二极管自混合干涉信号随外腔长度增加衰减很缓慢;当外腔长度小于该临界值时,外腔反馈会使光频虚部增大,而且光频虚部随外腔长度变化而振荡,激光二极管自混合干涉信号出现复杂的状态。外腔反馈强度越大,或激光单模线宽越大,这一外腔长度的临界值则越小。分析激光二极管自混合干涉还需要考虑外腔反馈对光频虚部的影响。
The characteristics and regularities of the laser diode self-mixing interference signal increasing with the length of the external cavity are theoretically analyzed. Based on the composite resonator interference function, the self-mixing interference signal of laser diode is exponentially decayed with the increase of the external cavity length. The attenuation coefficient is proportional to the imaginary part of the optical frequency, and the transcendental equations of the imaginary part and the real part of the optical frequency are given. When the length of external cavity exceeds a critical value, the imaginary part of optical frequency decays inversely with the length of external cavity, then the attenuation of self-mixing laser diode signal decreases with the increase of external cavity length. When the external cavity length is less than The critical value, the external cavity feedback will increase the imaginary part of the optical frequency, and the imaginary part of the optical frequency changes with the length of the external cavity oscillation, laser diode self-mixing interference signal complex state. The larger the intensity of the external cavity feedback, or the larger the single-mode laser line width, the smaller the critical value of the external cavity length. Analysis of laser diode self-mixing interference also need to consider the impact of external feedback on the imaginary part of the optical frequency.