We introduce a new method to increase the sensitivity of an optical ultrasound receiver by utilizing a waveguide between the mirrors of the Fabry-Perot resonator. This approach eliminates diffraction loss from the cavity and therefore the finesse is only limited by mirror loss and absorption. Recently, we have reported our measurements of high finesse (695) and quality factor (59000) in a waveguide Fabry-Perot film resonator of 100 μm thickness. Here we describe the implementation of this method for the design of a thin film ultrasound detector. By applying this method, we have achieved noise equivalent pressure of 178 Pa over a bandwidth of 30 MHz which is 20 folds better than the sensitivity of our previous non-waveguide detector.