| A vibration fiber optic sensor base on intermodal interference is studied in this paper. Compare with traditional vibration 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 vibration detection in large engineering structures, large dynamotor group and border security.In classical electro-magnetic theory, light travels through an optical fiber in distinct modes. Each mode has its own light intensity distribution, and if 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 intensity in a small area will vary and its frequency 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. In signal processing part, we employ virtual instrument technology. A real-time vibration signal time and frequency analysis tool is designed by using Labview language.A set of experiments corresponding to different application areas are designed to test the system's performance and the outcome of vibration tests is given. |
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