Using phase of second-harmonic interference fringes as a position marker for detecting the zero optical path difference in a nonlinear pulse-train interferometer

The possibility of using the phase of second-harmonic interference fringes as a position marker for detecting the zero optical path difference (OPD) in a nonlinear pulse-train interferometer is investigated. A theoretical analysis showed that the phase of second-harmonic fringes can be used as a position marker for detecting the zero OPD, and compared to the phase of the fundamental wave, it provides a higher coefficient of sensitivity to the change in distance. In a nonlinear pulse-train interferometer, the phases of the fundamental and second-harmonic fringes were measured, and the positions of their zero-crossing points near the fringe envelope peak were compared. A difference between these two positions and the effectiveness of the proposed method were confirmed. The concept and analysis reported herein pave the way for the development of higher-accuracy, absolute distance measurements.