Systems and methods are provided, which use at least two coherent light sources with known phase relations between them, which are configured to illuminate a target with at least two corresponding spots, an optical unit comprising a mask and configured to focus, onto a sensor, interfered scattered illumination from the spots of the target, passing through the mask, to yield a signal, at least one shifter configured to shift a frequency of at least one of the coherent light sources to provide a carrier frequency in the signal, and a processing unit configured to derive a vibration frequency of the target from the sensor signal with respect to the carrier frequency. The vibration frequency of the target is separated from the carrier frequency and speckle disturbances may be attenuated or avoided.
展开▼
