| A fiber optic sensor base on intermodal interference is studied in this paper. Compare with traditional sensor, this sensor has a series of advantages: small, light, high sensitivity, easy to shape, large scale of frequency, large area detecting, immune to electromagnetic effects, strong anti-corrosive, etc. So in some application areas it has unmatchable advantages, such as auto control in industry under complicated environment, and some monitor and security in important installation.In classical electro-magnetic theory, the incident light is coherent different modes will have spatial overlap and result in a pattern of interference speckles. When the optical fiber is perturbed, different modes have different effects with the perturbation. The intermodal interference will be reconstructed. Though the total light intensity remains unchanged, the distributing of speckles has changed. The change of speckle can get by gray image from CCD or the intensity in a small area. The frequency of speckle is equal to the perturbation's frequency. For small perturbations the amplitude will also be linear. In this paper the theory is analyzed in detail, and a mathematical model is built. Simulation is executed by Matlab and expected results are gotten.In this paper we using a single mode optical fiber connected to the multimode optical fiber to get a small area of speckles pattern. This design will reduce the cost, simplify the system and enable long distance detection. We designed SMS-A and SMS-B fiber optic sensor base on intermodal interference. Take cost and practice operate in account we designed another SMS-A system. In signal processing part, A real-time vibration signal time and frequency analysis tool is designed with virtual instrument technology of NI Company. Image Acquisition Toolbox of Matlab also been employed.A set of experiments corresponding to different application areas are designed to test the system's performance and the outcome of tests is given. |
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