A phonon parametric oscillator is provided. The phonon parametric oscillator comprises a laser for periodically emitting brief optical pulses (IL), an assembly for generating acoustic pulses (IA) and a medium for coupling the acoustic pulses to an object (O), the assembly for generating acoustic pulses comprising an entrance face, an exit face, a conversion medium for converting the brief optical pulses into acoustic pulses and a propagation medium for propagating said acoustic pulses, the entrance and exit faces being reflective to the acoustic pulses, the propagation medium having a defined thickness, the exit face making contact with the coupling medium. In the phonon parametric oscillator according to the invention, the round-trip time of an acoustic pulse due to reflection from the entrance and exit faces, is equal to the emission period (τ) of the laser, so that the reflected acoustic pulse is in phase with the following acoustic pulse.