A NOVEL FIBER OPTIC STRAIN SENSOR

A novel strain sensor based on polarisation-maintaining optical fibre and FMCW laser diode technology is described. The sensor consists of a 100 m length of fibre, a frequency-modulated laser diode light source and a PIN photodiode detector. The principle of operation relies on t hat facts that the optical frequency of the laser light is modulated b y approximately 1 GHz during a current modulated cycle of 1 ms, and th at the fibre has a twist inserted to induce coupling between the eigen modes HE11x or HE11y due to circular birefringence. By selectively lau nching the modulated laser light into one of the eigenmodes and retain ing the other for detection of coupled modes, the length of fibre from the launch end to the twist was rendered insensitive to changes in en vironmental conditions such as temperature. The system was shown to ha ve a sensitivity of approximately 2 pi rad per 1000 microstrain with a resolution of 2 microstrain within a dynamic range of 5000 microstrai n.