Optical position sensor using coherent detection and polarization

Abstract

A precision optical distance measuring device utilizes coherent optical detection for extreme precision, and polarization preserving fiber to enable its use in limited access environments. A laser diode provides a linearly polarized source light beam which is directed to a polarizing beam splitter. The source beam passes through the beam splitter and is focused by a lens into a polarization preserving fiber where it travels along one optical axis toward a probe head disposed at the target location. In the probe head, the source beam is focused by a lens onto the target. After focusing, the source beam is passed through a quarter-wave plate. The interface between ambient air and the target-side of the quarter-wave plate reflects a local oscillator beam back through the quarter-wave plate. Meanwhile, the source beam is reflected from the target as a return beam. Mixing of the two beams occurs at the interface and the mixed beam is focused back into the polarization preserving fiber where it is transmitted along a second transmission axis of the fiber. The mixed beam emerges from the fiber and is reflected by the polarizing beam splitter along a second optical axis. The mixed beam is then focused by a detection lens onto a photo detector which coherently detects the mixed beam. The source laser beam is frequency swept so that the photo detector provides an output signal regarding the precise distance between the probe head and the target.