Hybrid fiber optic sensors use the optical fiber as a means for delivering an interrogating laser beam to a sensor element at the remote end of the fiber. Therefore, measurements can be made in environments where other sensor types cannot function because of adverse temperature, radiation conditions, EMP, RFI, or the presence of metals in the sensor element. The most sensitive hybrid fiber-optic sensors operate in a coherent scheme. Homodyne schemes suffer front phase drift problems, or the system can be sensitive to effects acting on the fiber that delivers the laser beam to the sensor element. We have minimized these problems by using a true heterodyne detection scheme in which two different laser frequencies propagate to the sensor element, but interact differently with it. The returning phase-or polarization-modulated beams are demodulated to provide real-time display of the measurand. Having both laser frequencies that are to be mixed propagating in the same fiber provides common-mode rejection of phase effects caused by the intrinsic sensing properties of the fiber itself.
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