Abstract

Optical techniques for high resolution distance measurement have great potential in industry and medicine, because they are non-intrusive and sensitive, and can be performed in real time. The resolution of existing techniques, such as time-domain reflectometry¹ (OTDR) and coherence-domain reflectometry² (OCDR), is limited to ~10 μm. In OTDR, the resolution is limited by the optical pulse duration, whereas in OCDR, the resolution is limited by the laser bandwidth. We have developed a technique for nanometer-resolution distance measurement using the fact that the wavelength of a broadband laser can be changed by external feedback.³ By making the feedback sensitive to the target position, one can determine that target position from the shift of laser wavelength. In our experiments, this is achieved by placing the target near the focus of a microscope objective. When the target is at the focus, the wavelength shift is largest as a result of maximum optical feedback to the laser cavity. As the target moves away from the focus, feedback is reduced and the wavelength shift becomes smaller. Because laser wavelength can be detected with great sensitivity, 20-nm resolution in target position is achieved. The resolution is 500-fold larger than the best results of OTDR and OCDR. To our best knowledge, this is the first time such a large resolution is achieved optically with a nonincremental (fringe counting) method. In addition to the resolution, the dynamic range of the measurable distance in this technique reaches 16 μm, which is 20-fold larger than ordinary interferometric methods.

© 1993 Optical Society of America