The development of an in-fiber whitelight interferometric distance sensor based on the mode coupling effect of a Long Period Fiber Grating (LPFG) for absolute distance measurement has been investigated. Unlike the Fabry-Perot interferometric distance sensor that uses a regular single mode fiber as the sensor probe, the LPFG-based distance sensor employs a single mode fiber whose cladding region is coated with a reflective film and having a LPFG located at a distance from the cleaved fiber end. The LPFG serves as a beam splitter that separates the light into a core mode and a cladding mode and recombines the two light waves to generate interference, from which the absolute distance can be deducted. Because the optical path difference of the LPFG-based interferometric distance sensor is contributed by both the free-space cavity distance and the distance between the LPFG and the fiber end, it does not impose a limit on the minimum measurable distance. In this paper, the experimental setup for mechanically inducing LPFG in the optical fiber is first described. It is demonstrated that the transmission characteristics of the LPFG can be easily controlled by adjusting the grating period and the applied pressure. The implementation and data analysis of an LPFG-based whitelight interferometric distance sensor are then discussed in detail. Experimental results verified that the distance sensor is capable of measuring small distances that can not be measured by a Fabry-Perot interferometric distance sensor.

Date Published: 8 April 2008
PDF: 8 pages
Proc. SPIE 6932, Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2008, 69321Q (8 April 2008); doi: 10.1117/12.775871

Published in SPIE Proceedings Vol. 6932:
Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2008
Masayoshi Tomizuka, Editor(s)