Fiber grating laser sensors have been attracting great interest because of their high signal-to-noise ratio and narrow linewidth that permit high resolution sensing. According to the working principle, fiber grating laser sensors can be classified into two types: wavelength encoding sensor and polarimetric heterodyning sensor. The former responds to external perturbations in terms of shift in the operation wavelength of the fiber laser, which is similar to that of fiber grating sensor. The latter converts measurand into change in beat frequency between the two orthogonal polarization modes from the fiber laser. The polarimetric fiber grating laser sensor not only has almost all advantages of passive fiber grating sensors, but also has a distinctive advantage of ease of interrogation. This is because the beat frequency is in the RF domain, which avoids the employment of expensive wavelength measurement devices. This type of sensor has been demonstrated for measurement of temperature, axial strain, lateral force, hydrostatic pressure, bending, displacement, acceleration, electric current, and acoustic and ultrasonic signal. In this paper, we review the principle, fabrication, characterization, and implementation of the polarimetric heterodyning fiber grating laser sensors, and the sensor multiplexing in the RF domain.
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