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 $^{\circ}{\rm C}$ .

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