<title>Dual-wavelength Sagnac-Michelson distributed optical fiber sensor</title>

Abstract

A novel interferometric technique is described to detect and locate perturbations along an optical fiber. This distributed sensor has a position dependent response to time-varying disturbances such as strain or temperature. These disturbances cause a phase shift which is detected and converted to spatial information. The sensor consists of a Sagnac interferometer merged with a Michelson interferometer. This is achieved by a frequency selective mirror in the center of the Sagnac-loop. The sensor is illuminated by two light sources with wavelengths lambda<SUB>1</SUB> and lambda<SUB>2</SUB>, respectively. The mirror reflects lambda<SUB>1</SUB> and transmits lambda<SUB>2</SUB>, therefore causing the interferometer to operate as a Michelson at wavelength lambda<SUB>1</SUB> and as a Sagnac at wavelength (lambda) <SUB>2</SUB>. Any time-varying perturbation on the fiber will result in a signal at lambda<SUB>2</SUB> proportional to the product of the rate of phase change caused by the perturbation and the distance of the perturbation relative to the position of the mirror. The output of the Michelson interferometer at wavelength lambda<SUB>1</SUB> is proportional to the phase change caused by the unknown perturbation. By dividing the output of the Sagnac interferometer by the time rate of change of the Michelson interferometer signal, the position of the disturbance relative to the mirror is located. Results obtained with a 200 m distributed fiber sensor are discussed.