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We demonstrate an improved laser-based receiver for the detection of small ultrasonic surface displacements. The receiver is based on a homodyne interferometer using a photorefractive polymer as an adaptive beam combiner. The interferometer requires no path-length stabilization and can process speckled beams from rough surfaces. In experiments using a 10 mW diode laser at 676 nm, we have measured a shot noise-limited surface displacement sensitivity that is within a factor of three of the ideal limit for a lossless, plane wave interferometer.