| In biological tissue where light is strongly scattered, direct optical focusing beyond one optical transport mean free path (～1mm for human skin) becomes infeasible. To overcome the scattering, time-reversed ultrasonically encoded (TRUE) optical focusing technique has been developed to achieve light focusing inside a scattering medium.In this technique, a noninvasive focused ultrasound beam is employed to modulate (or encode, tag) those randomly scattered photons propagating through the ultrasonic focus. The ultrasonically encoded photons experience a frequency shift with respect to those that bypass the acoustic focal region. The encoded light is collected to interfere with a reference beam, so that a stationary interference is formed and is recorded as a hologram inside a photorefractive material. This material also serves as a phase conjugation mirror (PCM) when a reading beam travelling in the direction opposite to the reference beam reads out the hologram from the material. In this way, a phase conjugated copy (or the time-reversed counterpart) of the initial encoded light can be generated and eventually converges back, although tortuously, to the ultrasonic focal zone where the "guide star" is located and forms an optical focusing.In previous TRUE optical focusing systems, a photorefractive crystal (PR), or a digital phase-only spatial light modulator (SLM) was adopted as the phase conjugated mirror (PCM) to generate the time-revered counterpart of the original wavefront for optical focusing. In the PR material scheme, the continuous wave of ultrasound forms a relatively long acousto-optic modulation area, resulting in poor focusing resolution along the acoustic axis. In the phase-only SLM scheme, only the phase item of light was modulated by the SLM while the amplitude information is lost, which decreases the fidelity of time-reversed wavefront as well as the quality of time-reversed optical focsuing.To address these issues existed in previous TRUE optical focusing techniques, the procedures as shown below have been executed in this thesis (corresponding to the published first-author papers):(1) Adopted two ultrasound transducers, emitting two intersecting ultrasound beams at two slightly different frequencies, to modulate the diffuse light within their intersection volume at the two beams’beat frequency. We showed that light encoded at the beat frequency can be time-reversed and converge to the two ultrasound beams’intersection volume, which was significantly shrinked compared with the single transducer’s focal zone. Experimentally, TRUE focusing with an (acoustic) axial resolution of～1.1mm was demonstrated inside turbid media, agreeing with the theoretical estimation. This resolution makes an improvement of2.4times compared with the single-transducer scheme.(2) Modeled the ultrasound intensity profile generated by one single focused transducer, and the intensity profile at the intersection of two ultrasound beams emitted from two focused transducers. These ultrasound intensity distributions were calculated by borrowing the wave theory which was used to obtain the optical field distribution nearby a convex lens. The ultrasound distribution, acting as the point spread function of the imaging system, was then convoluted with the target function, resulting in the output imaging signals, which matched well with our experimental results.(3) Optimized the TRUE system’s performance by maximize the optical power for both holographic recording and reading. For this purpose, the polarization of the laser beam was switched between horizontal and vertical states by an Electro-Optic Modulator (EOM), which was placed before a polarized beam splitter in the optical path. A digital delay generator was also employed to synchronize the EOM and the two shutters.(4) Proposed a new method of numerically reconstruction of the full phase and amplitude of a simulated speckle field, to obtain time-reversed optical focusing through scattering media, by using one single phase-only SLM. The speckle pattern of coherent light passing through a diffuser was simulated at first. The speckle pattern’s amplitude was then applied in the G-S algorithm for iterative process, to retrieve the phase profile displayed at the speckle pattern’s Fourier plane. Simulation showed that the computed phase numerically reconstructed the original amplitude at the phase pattern’s Forier plane with2%errors within6803iterations under coherent illumination of a plane wave. This is the first time, to the best of our knowledge, that a wavefront full reconstruction (with both amplitude and phase) of speckle patterns was demonstrated in simulationbased on one single phase modulater. Based on this, a proposed digital time-reversed (TR) optical focuing setup was discussed for potential biomedical applications. Because this method can reconstruct both phase and amplitude, it affords a way to improve the fidelity of TR wavefront as well as the quality of TR optical focusing. |
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