Single-shot time-reversed optical focusing into and through scattering media

Abstract

Optical time reversal can focus light through or into scattering media, which raises a new possibility for conquering optical diffusion. Because optical time reversal must be completed within the correlation time of speckles, enhancing the speed of time-reversed optical focusing is important for practical applications. Although employing faster digital devices for time-reversal helps, more efficient methodologies are also desired. Here, we report a single-shot time-reversed optical focusing method to minimize the wavefront measurement time. In our approach, all information requisite for optical time reversal is extracted from a single-shot hologram, and hence no other preconditions or measurements are required. In particular, we demonstrate the first realization of single-shot time-reversed ultrasonically encoded (TRUE) optical focusing into scattering media. By using the minimum amount of measurement, this work breaks the fundamental speed limit of digitally based time reversal for focusing into and through scattering media, and constitutes an important step toward high-speed wavefront shaping applications.

Keywords: Optical time reversal; Scattering media; Single-shot detection; Spatial light modulator; Ultrasonically encoded optical focusing; Wavefront shaping.

Fig. 1.

Schematic of the optical time reversal system used in this work for focusing through scattering media. AOM, acousto-optic modulator; BS, beam splitter (non-polarizing); CL, camera lens; HWP, half-wave plate; L, lens; M, mirror; PBS, polarizing beam splitter; SLM, spatial light modulator; SM, scattering media.

Fig. 2.

Simulations of the proposed single-shot time-reversed optical focusing through scattering media. (a) Simulated speckle pattern from a laser beam transmitted a scattering medium; (b) Hologram from the interference between the scattered and reference optical fields; (c)-(e) Phase maps (left) and the corresponding time-reversed foci (right) based on Mode 1, Mode 2 and Mode 3, respectively.

Fig. 3.

Experimental verifications for the proposed three modes of single-shot time-reversed optical focusing through scattering media. (a)-(c) Phase maps (left) and the corresponding time-reversed foci (right) for Mode 1, Mode 2, and Mode 3, respectively.

Fig. 4.

Experimental PBRs of the proposed three “real” single-shot modes and the current quasi-single-shot method at different mean intensity ratios between the signal beam and the reference beam.

Fig. 5.

Experimental results for single-shot time-reversed optical focusing into scattering media. (a) Schematic of TRUE optical focusing; (b) Image of the single-shot time-reversed focus achieved by Mode 3; (c) No focus can be seen by the quasi-single-shot method; (d) Line profiles of the central rows in (b) and (c); (e) In a control experiment, no time-reversed focus can be seen when no ultrasound is used. UT, ultrasonic transducer.