|Title:||Focusing and manipulation of diffused light with wavefront engineering : techniques and applications|
|Advisors:||Lai, Puxiang (BME)|
|Subject:||Light -- Scattering|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Biomedical Engineering|
|Pages:||xviii, 125 pages : color illustrations|
|Abstract:||Optical techniques have been playing an important role in modern biomedicine. Their applications, however, have been constrained to superficial layers of biological tissue due to strong scattering of light in tissue. Manipulating and focusing light deep within or through biological tissue and tissue-like complex media has been sought after for long yet considered challenging. One promising strategy is via optical wavefront engineering, where scattering-induced phase distortions are time reversed or pre-compensated so that photons travelling along different optical paths interfere constructively at the targeted position. In the past decade, various implementations of wavefront engineering, such as digital optical phase conjugation (DOPC) and wavefront shaping (WFS), have been developed to tackle different situations. In this thesis, these two approaches were used for turbidity suppression, and their functionalities were explored from intuitive to abstract for biomedicine and optical computing because of the modulation flexibility. To start with, a plain yet reliable DOPC platform with an embedded four-phase non-iterative approach was presented that can rapidly compensate for the wavefront modulator's surface curvature. The phase conjugation is implemented with a non-phase-shifting in-line holography method in the absence of an electro-optic modulator (EOM). The platform was optimized to obtain robust and superior performance as measured by optimization speed and peak-to-background ratio (PBR). Based on this platform, two essential applications of DOPC were proposed. The first one is time-reversed magnetically controlled perturbation (TRMCP) optical focusing inside scattering media. Sharp optical focus within scattering media through time-reversing the scattered light perturbed by magnetic microspheres was obtained, where magnetically controlled optical absorbing microspheres were used as the internal guidestar for diffused light. As the object is magnetically controlled, dynamic optical focusing can be achieved in a relatively large field-of-view with a high precision. In addition, the magnetic microspheres, which can be packaged with organic membranes, can potentially serve as drug carriers. The second application is related to image transmission. It is object edge enhancement through scattering media, which is enabled by adjusting the intensity ratio between the sample and reference beams in the DOPC system. The capability is demonstrated experimentally, and furtherly the performance, as measured by the edge enhancement index (EI) and enhancement-to-noise ratio (ENR), can be controlled easily through tuning the beam ratio. EI and ENR can be reinforced by ~8.5 and ~263 folds, respectively in current system. This is the first demonstration that edges of a spatial pattern can be extracted through strong scattering medium, which can potentially broaden the comprehension and development of image transmission in a complex environment such as inside/through a biological tissue.|
At last, to further broaden the scope of wavefront engineering, a diffusive optical logic (DOL) assisted by wavefront shaping to achieve reconfigurable and multifunctional logic operations on one platform was proposed: light was firstly encoded by a digital micromirror device displayed with a precalculated wavefront and then the encoded light was diffused and decoded by a scattering medium to form logical states. As a proof of concept, five basic logic functions (AND, OR, NOT, NAND, NOR) through experiment were demonstrated, with a ground glass as the scattering medium. This is the first demonstration that a scattering medium in combination with wavefront shaping can be used as optical logic gates. As the transmission matrix of strong scattering media has huge ranks and provides enormous degrees of freedom, the concept of DOL shows great potential in optical computing with many advantages including simple fabrication process, scalability, and reconfigurability. In summary, this thesis aims to extend the functionality and scope of wavefront engineering against optical scattering in complex media. After the general introduction of the field in Chapter 1, four topics are organized with increasing complexity of wavefront manipulation and information transmission, from optical focusing, image edge enhancement, to multifunctional diffusive optical logic gates. In the first two cases (optical focusing and image edge enhancement), wavefront modulator, as a whole, contributes to one output channel and the all output channels, respectively, where the wavefront modulation processes are overall intuitive. In the last case (diffusive optical logic gates), the wavefront modulator is divided into many control units which are tailored independently. It requires more advanced manipulation procedures, and different control units contribute to diverse output channels at the output plane. Although a lot need to be further developed, these explorations provide promising solutions to a wide range of optical applications that desire highly confined and intense optical delivery in deep biological tissue and advanced optical computation in a complex environment
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