A frequency-modulated continuous wave birefringent fibre-optic strain sensor based on a Sagnac ring configuration

Two-mode fibre interferometers have been shown to be well suited as optical fibre strain sensors because (i) they have a higher resolution than do intensity modulation-based strain sensors and (ii) they have a simple geometry since the reference and signal light beams propagate along a single fibre core. This paper discusses a Sagnac ring configuration of polarization-maintaining fibre strain sensor based on frequency-modulation continuous wave technology wherein the basic sensor layout consists of a frequency-modulated single-mode laser, a 100 metre ring of single-mode birefringent fibre and a PIN photodiode detector. By launching light into both ends of the fibre, propagating and counter-propagating beams may be employed to sense the variations in length due to strain. The sensor exhibits good characteristics including temperature insensitivity over a limited temperature range (similar or equal to +/- 10 degrees C), high resolution (similar or equal to 4 microstrain), large dynamic measurement range (similar or equal to 5000 microstrain), large signal intensity and good signal contrast.