The detection sensitivity of fiber-optic ultrasound sensors is crucial for image quality in fiber-based photoacoustic imaging. In this study， we employed a dual-polarization fiber laser as the acoustic sensing element and introduced radio-frequency heterodyne demodulation for sensitivity enhancement. Experimental results demonstrate that this method effectively eliminates noise from additional local radio-frequency oscillation sources. The proposed technique achieved an improved sensitivity， evidenced by a noise-equivalent pressure （NEP） of 9.0 Pa， compared to the 5.7 Pa offered by previous approaches within the frequency range of 8-32 MHz. Furthermore， applying this sensor to photoacoustic microscopy resulted in a signal-to-noise ratio enhancement of over 4 dB. The heterodyne detection technology significantly improves the sensitivity and image signal-to-noise ratio of fiber-optic sensors， facilitating the application of fiber-based photoacoustic imaging.