This paper presents direct field measurements of acoustic modes guided by the interface between two transparent solids. The measurement technique is based on the acousto-optical interaction inside the solid between the acoustic field and the probe laser beam of an interferometer. The main advantage of the method is its ability to measure acoustic strain fields in areas of difficult access with the classic detection methods. Moreover, it gives complete information about the dilatation strain field inside the solid, e.g., amplitude and phase. The propagation of a real velocity mode (Stoneley wave) is first illustrated. Then the situation of complex velocity modes is investigated for a Plexiglas–fused quartz slip interface. This material combination supports two possible interface modes theoretically. These modes are simultaneously observed and the differences between their behavior are measured.