Ultra-sensitive remote heterodyne fiber magnetic field detector

The most sensitive hybrid fiber-optic sensors operate in a coherent scheme. Homodyne schemes suffer from 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 magnetic field 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