激光二极管自混合干涉信号随外腔长度衰减

Attenuation of Self-Mixing Interference Signal of Laser Diode with External-Cavity Length

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。

Characteristics and laws of the variation of self-mixing interference signal of laser diode along with the increase of external-cavity length are analyzed theoretically. The self-mixing interference signal attenuating exponentially along with increase of external-cavity length is deduced from the composite cavity interference function. The attenuation coefficient is direct ratio to imaginary part of light frequency. A group of transcendental equations for imaginary part and real part of light frequency are also presented. By solving the equations, at external- cavity length more than a critical value, along with the increase of external-cavity length, the imaginary part is inversely proportional to attenuation, so the self-mixing interference signal attenuates very slowly. When external- cavity length is less than the critical value, feedback light increases the imaginary part, moreover, the imaginary part oscillates along with external-cavity length. The self-mixing interference signal appears as a complicated state. The greater the intensity of the external cavity feedback or the single-mode line-width of laser is, the smaller the critical value is. Analysis of the laser diode self-mixing interference also needs to consider the effect of feedback light on the imaginary part.