A Study On Dual-wavelength Sagnac Interferometer Based On Wavelength Division Multiplexing Technology With Rayleigh Backscatter Rejection

Distributed disturbance sensing is one of the most attractive features of optical fiber interferometer, of which the sagnac interferometer has been widely used. The sagnac interferometer has a simple structure and is insensitive to the disturbance with low frequency such as the change of temperature. The output of the sagnac interferometer contains the location information of the disturbance applied on the sensing fiber, which makes the sagnac sensor very suitable to be used as the distributed disturbance location system. However, the light path of the sagnac interferometer must be a loop, which can be a limitation when the system is used as a perimeter sensor. As a location system, the monitoring range and location accuracy are the most important performances of the sagnac interferometer. A general belief is that the monitoring range can be increased as the length of the sagnac path. However, random phase noise associated with Rayleigh backscatter is one of the main limits of long-length fiber Sagnac interferometer and the noise makes the location result inaccurate.To overcome the limitation of the traditional sagnac interferometer, we propose a dual-wavelength sagnac interferometer sensor using wavelength division multiplexing technology. Firstly, a sensing fiber is used outside the sagnac loop to sense the disturbance, which means that the interferometer can be used in more areas. Secondly, with the wavelength division multiplexing technology, two sagnac interferometers working at two slightly different wavelengths are combined together. With the outputs of the two sagnac interferometers, the Rayleigh backscatter noise can be rejected which makes a more accurate location result and a larger monitoring range.The main achievements of the article are as follows.(1) The shortcomings of the traditional sagnac interferometer are analysed. And the modified sagnac interferometer with a sensing fiber outside of the sagnac loop is proposed.(2) With the analysis of the polarization state of the single sensing fiber sagnac interferometer, the Faraday Rotator Mirror is used to make the interference output of the sensor more stable.(3) By analyzing the Elasto-Optical Effect, the light path of the interferometer and the 3×3 optical coupler, the relationship between the interference output and the disturbance is provided. And then the demodulation method is proposed to extract the phase difference from the output signal. With the frequency domain analysis of the interferometer, the frequency response is provided, from which the location of the disturbance can be got.(4) With the analysis of the Rayleigh backscatter of the single sensing fiber sagnac interferometer, the interference noise is expressed.(5) By using the wavelength division multiplexing technology, an auxiliary interferometer operating at a slightly different wavelength is combined with the main interferometer to sense the backscattered light. With the outputs of both of the two interferometers, the Rayleigh backscatter noise can be eliminated.(6) In the experiment, the outputs of the single sensing fiber sagnac interferometer and dual-wavelength sagnac interferometer were compared. And the conclusion can be got as follows.① With the wavelength division multiplexing technology, the dual-wavelength sagnac interferometer eliminates the Rayleigh backscatter and offers a larger monitoring range and more accurate location result.② The accuracy of ±50m and the monitoring range of 0～170km of the dual-wavelength sagnac interferometer have been obtained, much better than the single sensing fiber sagnac interferometer.The innovations of the article are shown as follows.(1) The study of the light path of the single sensing fiber sagnac interferometer shows that the main source of the Rayleigh backscatter noise is the sensing fiber outside of the sagnac loop. And then the expression of the interference noise caused by the backscatter light is provided. And the result shows that the backscatter noise limits the location accuracy and the monitoring range of the interferometer.(2) The dual-wavelength sagnac interferometer sensor(DSS) based on the wavelength division multiplexing technology is proposed and the backscatter noise is eliminated. The experimental results on the location accuracy and monitoring range show a better performance of the interferometer.