Merged Sagnac-Michelson interferometer for distributed disturbancedetection

An 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 λ₁ and λ₂, respectively. The mirror reflects λ₁ and transmits λ₂, causing the interferometer to operate as a Michelson at wavelength λ₁ and as a Sagnac at wavelength λ₂. Any time-varying perturbation on, the fiber will, result in a signal at λ₂ 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 λ₁ 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