In an opto-acoustic measuring device for thin films and solid surfaces, the probe beam (103) is split into a first probe beam portion and a second reference beam portion. The splitting of the probe beam is achieved using a phase mask (5) that also splits the excitation beam (102). The probe beam is aligned using a retro-reflector on a motorized stage to control the beam angle. Excitation and probe/reference beams are overlapped at the sample surface (109). The first probe beam portion gets diffracted by material disturbances generated by excitation beams. The diffracted part of the first probe beam portion is collinear with the second reference beam portion, resulting in heterodyning. The heterodyne signal measured by the detector (206) is analyzed in order to determine thickness and/or other properties of a thin film or solid surface. The invention improves magnitude and reproducibility of the opto-acoustic signal which results in enhanced precision of measurements.
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