Hybrid fiber-optic sensor using true heterodyne measurement techniques

Abstract

We describe a hybrid, coherent, fiber-optic sensor for the measurement of phenomena that lead to birefringence, polarization rotation, or differential phase shifts in a remote-sensing element. By utilizing a single fiber that carries two orthogonally linearly polarized laser beams of different frequencies to the sensor and true heterodyne detection, we obtain high common-mode rejection of any unwanted intrinsic sensitivity of the fiber itself. The sensor is resistant to fading and has a high dynamic range. For demodulation we use high-frequency phase-locked loop detection that is insensitive to slowly varying phase shifts.