The principle of TEMPO is relatively simple: The probe laser beam is divided into 2 beams: Probe beam and Reference beam. The probe beam is incident on the sample. The light scattered by the surface of the sample is the signal beam collected by TEMPO and whose phase contains the surface displacement information d(t). The signal beam and the reference beam are mixed in the photorefractive crystal where a local oscillator beam adapted to the signal beam is generated. Both beam (signal and local oscillator) are interfering on the photodetector that delivers a photocurrent proportional to the surface displacement d(t) of the sample (homodyne detection).
The two-wave mixing in the photorefractive crystal creates a local oscillator perfectly adapted to the signal, allowing a perfect homodyne detection with multispeckled beam. Due to the nature of the photorefractive two-wave mixing process, the upper frequency detection is limited by the photodetector performances.

The surface displacement of the sample can be created by various methods: laser-generated ultrasound, transducer-generated ultrasound, acoustic vibration, shock-wave, etc.

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