In this paper, a novel photoacoustic (PA) sensing probe design consisting of single optical fiber is reported. The same optical fiber is used for light delivery, which also serves as an acoustic delay line to relay the PA signal. As the key feature of the design, the ultrasound transducer is made optically-transparent to allow excitation light to pass through. This probe design provides three major benefits, including miniaturization, co-registered optical excitation and acoustic detection, and clear separation of PA signal from interference signals. Testing results show that PA probe provides good sensitivity and high linearity.