Light source with orthogonally linear polarized two‐frequency beam from laser diode and surface profile measurement

Abstract

A novel light source using a laser diode is proposed for optical heterodyne interferometry. The light from this source has a two- frequency beam with mutually orthogonal polarization directions. The light source consists of a laser diode and some optical elements. A Mach-Zehnder interferometer using polarizing beam splitters and a right-angle prism creates orthogonal linearly polarized two-frequency light. The laser diode is easily available to give frequency modulation by current injection. A two-frequency beam is created by giving the time difference due to different optical paths in the Mach-Zehnder interferom- eter. The frequency difference between two beams remains constant regardless of frequency modulation of the laser diode. Moreover, in order to compensate for the power fluctuation by injection current modulation and to keep the total power constant, another incoherent laser diode is added. This light source is suitable for practical applications and requires a smaller and less expensive unit than such conventional light sources as a Zeeman laser or an acousto-optic modulator. This source is applied to a differential interferometer to demonstrate utility for three-dimensional surface profilometry. © 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: optical heterodyne interferometry; laser diode; frequency modulation; differential interferometer; surface profilometry.