1. Turbidity Suppression in a Scattering Medium via Digital Optical Phase Conjugation Mehdi Azimipour, Farid Atry, Ramin Pashaie Department of Electrical Engineering, University of Wisconsin-Milwaukee

2. INTRODUCTION AND OBJECTIVE High levels of scattering in biological tissue is the major obstacle in deep tissue imaging. Even though this scattering is a random process, it's shown to be time-reversible [1]. In 1999 it is shown that by using an array of transducers, sound waves can be time reversed and being sent back to its source [2]. The device used for time reversal is called PHASE CONJUGATE MIRROR. Optical phase conjugation is an emerging technique for reversing the effects of multiple scattering which can have many applications as such as:   Deep tissue imaging beyond the ballistic regime.   Laser surgery and laser therapy.   Deep brain photostimulation during optogenetics experiments. (A) (B) FIG. 1: (A) Basic concept of time-reversed acoustic, (B) example of ultrasound time reversal applications: by using an iterative procedure, the ultrasonic power is focused to destroy a kidney stone inside the body [2], FIG. 2: Focusing light through a scattering medium, (A) distortion of a plane wave after passing through a scattering medium, (B) by shaping the wavefront of the incident light, laser beam is focused after passing through a highly scattering medium [1]. [1] I. M. Vellekoop and A. P. Mosk, "Focusing coherent light through opaque strongly scattering media," Opt. Lett. 32, 2309-2311 (2007) [2] M. Fink, "Time-Reversed Acoustics", Scientific American November 1999. pp. 91-97 Time-reversed acoustic

3. METHODOLOGY FIG 2: (A) Experimental DOPC setup for focusing light through and inside a scattering medium, (B) Schematic of the experimental setup, The phase conjugation process to compensate the scattering process is consisting of two steps:   Extracting the phase information of the distorted light: Phase information of the distorted light wave was obtained using phase shifting digital holography technique [3].   Time reversal of the distorted wavefront: At this step, spatial light modulator (SLM), which is registered pixel-by-pixel with the camera, conjugates the measured phase. FIG. 3: (A) HOLOEYE phase-only spatial light modulator, (B) Ground glass diffuser (Thorlabs, DG10-600-MD ) on top of a business card showing the degree of turbulence caused by the scattering medium used during the experiments. [3] Ichirou Yamaguchi and Tong Zhang,"Phase-shifting digital holography", Optics Letters, Vol. 22, Issue 16, pp. 1268-1270 (1997) (A)(B) (A) (B)

4. RESULTS AND CONCLUSION FIG. 3: Focusing Light through a Scattering Medium (Five layers of Scotch tape) FIG. 4: Focusing Light inside a Scattering Medium using Light Perturbation Method.(Between two ground glass diffusers with the distance of 10cm) In this poster, we discussed the design of our custom-made DOPC system and presented our preliminary results focusing light through and inside a highly scattering medium. We demonstrated how this technique is able to focus coherent light through several layers of Scotch tape and/or ground glass diffusers. This method can potentially improve the efficiency of deep tissue fluorescent imaging and deep brain optogenetic photostimulation. FIG 5: Reconstruction of a light absorbing object of arbitrary structure placed between two scattering mediums (Thorlabs, DG10-600-MD ). Here, letter ‘M’ was written on a microscope slide using ink and placed between two ground glass diffusers. By measuring the field before (E 1 ) and after (E 2 ) cover slip insertion and phase information extraction (E 1 –E 2 ), laser was focused in the light perturbing region without the presence of absorbing object.