Fig. 1: Principle of UFP optical focusing.
From: Focusing light into scattering media with ultrasound-induced field perturbation
In UFP optical focusing, we detect only the frequency-unshifted scattered photons because their higher energy dominates over that of the extremely weak frequency-shifted photons by an ultrasound field. a The scattered field through the scattering medium is directly measured via interference with a reference beam while the ultrasound is OFF. b Repeat the same measurement as in a while the ultrasound is ON. c The signals measured in a and b are different because of the ultrasound-induced field perturbation. Playing back the phase-conjugated differential field via a DOPC system generates a time-reversed beam that converges to the location where the perturbation occurs
