Improvement of angle measurement sensitivity using second harmonic wave interference

This paper presents the angular displacement measurement based on second harmonic generation (SHG). In our previous study, the angular resolution of sub-arcsecond was achieved. In this paper, three kinds of sensors based on SHG are reported. In these three sensors, the angular displacement is determined from the intensity of the SHG signal since the SHG is sensitive to the angle of the incident light. Two of these sensors in this study utilize the interference of SHG to improve the sensitivity performance. Comparing with the sensors which have been studied before, interference type sensors in this study allow the femtosecond laser to pass through the nonlinear optical crystal twice, which leads to a significant sensitivity improvement. The principles were verified by both simulation and experiment. In the simulation part, the sensitivities of these three kinds of sensors are compared. In the experiment, the sensitivities were confirmed to be consistent with the simulation. Additionally, the resolution of the presented systems is evaluated and sub-0.1-arcsecond is achieved.