Fiber-Optic Pressure Sensor Based on $\pi$-Phase-Shifted Fiber Bragg Grating on Side-Hole Fiber

We present a fiber-optic pressure sensor based on a
$\pi$
-phase-shifted fiber Bragg grating
$(\pi{\rm FBG})$
fabricated on a side-hole fiber. Due to the resonance effect of a
$\pi{\rm FBG}$
, its reflection spectrum features two notches that are dramatically narrower than the linewidth of a regular FBG of similar length. The narrow spectral notches allow high-resolution measurement of their spectral separation, significantly improving the pressure detection limit (defined herein as the minimum detectable pressure change) compared to sensors based on a regular FBG of a similar length and on the same fiber. The
$\pi{\rm FBG}$
demonstrated in this letter is 8.3 mm long and the linewidth of each spectral notch is only 3.6 pm, corresponding to a quality factor of 4.3
$\,\times 10^{5}$
. The spectral notch separation exhibited a sensitivity of 20 pm/kpsi to pressure, which was limited by the geometry of the fiber holes, and little sensitivity to temperature. The Bragg wavelength shift exhibited a sensitivity of 11.4 pm/
$^{\circ}{\rm C}$
to temperature. In practice, a spectral resolution of 0.028 pm can be easily achieved for the
$\pi{\rm FBG}$
demonstrated in this letter, leading to a pressure detection limit of 1.4 psi and a temperature detection limit of 0.0025
$^{\circ}{\rm C}$
.