Abstract

Fiber laser sensors have found strong interest due to inherent advantages such as a frequency-encoded output, high resolution, and a compact inline design.^1,2 Here, we consider a polarimetric fiber laser sensor for fluid pressure³ with highly biréfringent fiber grating reflectors and show that the grating dispersion can be employed to significantly improve the resolution into the range of a few ppm of the full scale signal. The laser consists of two identical sections of Er-doped elliptical- core fiber with a 90° offset in the angular orientation of their cores (Fig. la). The gratings are written into the birefringent fibers with equal Bragg wavelengths near 1535 nnt and a width of 320 pm (for polarized light). The reflection maxima for the two biréfringent axes are separated by Δλ_biref = 60 pm. Due to the 90°-splice the reflection peaks for x and y polarized light are interchanged in wavelength at the two gratings. For both polarizations the laser oscillates on a few longitudinal modes with wavelengths in the central dip between the reflection maxima. Fluid pressure applied to one of the two fiber sections introduces a differential phase shift between the two orthogonal polarizations⁴ which can be observed as a change in the polarization mode beat (PMB) frequencies.³ Phase shifts in the two fiber sections caused by a common temperature change cancel each other.

© 2002 Optical Society of America