Active Phase-Shifting Interferometer Using Current Modulation of a Laser Diode

Abstract

An active phase-shifting interferometer using current modulation of a laser diode is proposed. Fringe movement in the instrument is detected by a spatial filtering detector whose output is fed back to the injection current of the laser diode to lock the fringes and shift them correctly for phase shifting analysis. Frequency modulation characteristics of the laser diode and frequency response of the feedback system are first investigated. We constructed a portable interferometer head set apart from a mirror under test then measured the shape of a spherical mirror 130 mm in diameter, which was placed on a wooden table subject to vibration. Measurement repeatability of λ/35 was achieved.