The study of measurement method of sweep interference absolute distance based on phase shifted frequency sampling method

Abstract

The laser scanning interference method requires the optical frequency emitted by the tunable laser be ideally linear with time. However, the laser is affected by the mechanical control precision and the influence of carriers in the gain medium. It is difficult to meet this requirement. The spectrum of the target spectrum will be widened, leading to a serious decrease in measurement accuracy and resolution. The optical frequency sampling method can eliminate the influence of the time domain nonlinearity of the frequency sweep in principle by sampling the frequency sweeping interferometric signal in the optical frequency domain. However, this method is limited by the Nyquist Sampling Theorem. It requires that the optical path difference of the system's auxiliary interferometer should be at least 2 times of the measuring optical path, which seriously reduces the theoretical measurement range of the frequency sweep interference determined by the coherence length of the laser. In response to this problem, the paper puts forward the phase shift frequency sampling method. When the optical path exceeds 1/2 of the optical path of the auxiliary interferometer, by introducing the phase-shifting factor, the measured signal is shifted according to the phase of the auxiliary interferometer signal, so that the frequency of the measured beat signal after phase-shifting is less than 1/2 of the frequency of the auxiliary beat signal, which satisfies the Nyquist sampling theorem in principle, and breaking through the measurement range limitation based on the frequency sampling method. Through simulation experiments, comparing the traditional frequency sampling method and phase shift frequency sampling method, the effectiveness of the method is verified and the condition of the phase-shift factor is given.