The internally generated laser beam is split into two: a probe beam and a reference beam. The probe beam travels through the system before coming into contact with the surface of the sample. From there, it is reflected back into the system. The reference beam never leaves the Tempo and is instead redirected towards a photorefractive crystal. In the photorefractive crystal (a Non-Linear Material), the two beams are combined, or mixed to create a dynamic hologram. If there is a perturbation at the surface of the sample, the system measures the phase variation between the reference and probe beams. This variation is then converted into an electrical signal by the photodetector. The response time of the process allows the Tempo to compensate for slow phase changes in the signal beam. The diffraction of the reference beam on the dynamic hologram creates a local oscillator adapted to the signal i.e. same wavefront and same direction. The two beams are incident on the detector. Two-wave mixing in a photorefractive material is equivalent to an adaptive beam splitter. It allows the Tempo to achieve perfect homodyne detection on a highly speckled beam.

When the surface of the work-piece or sample is displaced as a result of ultrasonic excitation, there is a transient phase change induced onto the signal beam. The coherent detection process converts this phase change into an amplitude change that is proportional to the phase change. To provide linear detection of the transient phase-modulated signal beam, the phases of the two combined beams must be biased in quadrature. In the Tempo, quadrature is achieved by the application of a high voltage dc electric field to the Photorefractive Crystal. The dc field also serves to enhance the strength of the real-time hologram.