Photoacoustic imaging, especially for intravascular and endoscopic applications, requires ultrasound probes with miniature size and high sensitivity. In this paper, we present a new photoacoustic sensor based on a small-sized fiber laser. Incident ultrasound waves exert pressures on the optical fiber laser and induce harmonic vibrations of the fiber, which is detected by the frequency shift of the beating signal between the two orthogonal polarization modes in the fiber laser. This ultrasound sensor presents a noise-equivalent pressure of 40 Pa over a 50-MHz bandwidth. We demonstrate this new ultrasound sensor on an optical-resolution photoacoustic microscope. The axial and lateral resolutions are 48 μm and 3.3 μm. The field of view is up to 1.57 mm 2 . The sensor exhibits strong resistance to environmental perturbations, such as temperature changes, due to common-mode cancellation between the two orthogonal modes. The present fiber laser ultrasound sensor offers a new tool for alloptical photoacoustic imaging.Photoacoustic tomography (PAT) is a phenomenally growing imaging technology which offers high-contrast, high-resolution and non-invasive imaging in deep tissue 1,2 . PAT has received great interests due to promising applications in disease diagnoses and life science researches [3][4][5][6][7] . PAT detects optically induced ultrasound signals to form three-dimensional images [8][9][10] . Further development of PAT demands high-performance ultrasound sensors with high sensitivity, small size, broad bandwidth and great stability. Specifically, intravascular and endoscopic photoacoustic imaging requires ultrasound probes with miniature sizes and high sensitivity 11 . However, piezoelectric detectors are limited by the tradeoff between sensor size and sensitivity. For instance, a polyvinylidene (PVDF) needle hydrophone with a diameter of 75 μ m presents much lower sensitivity than that with ordinary size, with a noise-equivalent pressure (NEP) of 6 kPa over 100 MHz 12 . In recent years, a number of photonic ultrasound sensors have been developed with outstanding sensitivity, broad bandwidth and compact size [13][14][15] , optical transparency, and even possibility for non-contact measurement [16][17][18] . Photonic ultrasound sensors usually employ high-finesse optical resonators to detect incident elastic waves. For instance, a polymer micro-ring resonator (60 μ m in diameter) has presented 105 Pa NEP over a bandwidth of 350 MHz 19,20 . Ultrasound transducers based on planar Fabry-Perot (FP) cavity have reached a NEP of 210 Pa over 20 MHz bandwidth 21 . A reflection-mode PAM system using FP etalons for optical ultrasound detection has been developed 22,23 , which has achieved a NEP of 80 Pa and bandwidth of 18 MHz. The high sensitivity, however, raises its susceptibility to environmental disturbances. Complicated stabilization strategies have to be employed to enhance the resistance to external perturbations.Advanced fiber laser techniques have been developed for optical communication, sensing, measurement, ...