Self-mixing interference effects of microchip Nd:YAG laser with a wave plate in the external cavity

We present the optical feedback characteristics of a single-mode Nd:YAG laser with a wave plate in the external cavity. The laser intensities of the two orthogonal directions, which are both modulated by the change of external cavity length, have a phase difference due to the birefringence effect of the wave plate. When threshold intensity is introduced, a period of intensity fringe can be divided into four equal zones. Each zone corresponds to lambda/8 displacement of the external feedback reflector. The direction of displacement can be discriminated by the sequence of these four zones. This phenomenon provides a potential displacement sensor with directional discrimination and high resolution of eighth wavelength compared with the traditional optical feedback.