A high sensitivity, wideband ultrasound sensor based on a high finesse Fabry-Perot (FP) polymer film interferometer has been demonstrated with a bandwidth of 20 MHz and a 50μm diameter active area. Used in conjunction with a balanced photodetector to enable the use of a high intensity interrogating light beam of up to 36mW, the sensor system provided a noise equivalent pressure (NEP) of 0.35kPa over a 20 MHz measured bandwidth. It is shown further that this NEP could, in principle, be reduced to 0.16 kPa by using an interrogating source with a wider wavelength tuning range than was available in the current study to track drift in the phase bias of the FP sensor. The sensitivity achieved is an order of magnitude higher than previously demonstrated with this type of sensors, and is comparable to that of a 1mm diameter PVDF element. The combination of high sensitivity and the small active area (<50μm diameter) makes the FP sensor scheme particularly suitable for photoacoustic imaging applications.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.