Abstract
A novel strain sensor based on polarisation-maintaining optical fibre and FMCW laser diode technology is described. The sensor consists of a l00m length of fibre, a frequency-modulated laser diode light source and a PIN photodiode detector. The principle of operation relies on that facts that the optical frequency of the laser light is modulated by approximately 1GHz during a current modulated cycle of 1ms, and that the fibre has a twist inserted to induce coupling between the eigenmodes HEx 11 or HEy 11 due to circular birefringence. By selectively launching the modulated laser light into one of the eigenmodes and retaining the other for detection of coupled modes, the length of fibre from the launch end to the twist was rendered insensitive to changes in environmental conditions such as temperature. The system was shown to have a sensitivity of approximately 2πrad per 1000 microstrain with a resolution of 2 microstrain within a dynamic range of 5000 microstrain.
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