双频激光器用以产生一束含有两个频率，偏振态相互垂直的激光。当其相互发生干涉时，便生成干涉信号，通过对该外差信号的处理即可测量出被测物体的位移。测量系统利用激光的波长作为长度的基准，因此要求具备极高的波长稳定性。同时，激光器的频差决定了测量系统的最大可测量速度。
The dual frequency He Ne laser generates two linearly polarized light with close frequency and orthogonal polarization direction. The wavelength of the outgoing laser beam is taken as the length reference of the laser interference measurement system. 
