Optical Focusing and Imaging through Scattering Media

Optical techniques, which have been widely used in various fields including bio-medicine, remote sensing, astronomy, and industrial production, play an important role in modern life. Optical focusing and imaging, which correspond to the basic methods of utilizing light, are key to the implementation of optical techniques. In free space or a nearly transparent medium, optical imaging and focusing can be easily realized by using conventional optical elements, such as lenses and mirrors, due to the ballistic propagation of light in these media. However, in scattering media like biological tissue and fog, refractive index inhomogeneities cause diffusive propagation of light that increases with depth, which restricts the use of optical methods in thick, scattering media. Generally speaking, scattering media poses three challenges to optical focusing and imaging: wavefront aberrations, glare, and decorrelation. Wavefront aberrations can randomize light traveling through a scattering medium, disrupt the formation of focus, and break the conjugate relation in imaging. Glare caused by backscattering will largely impair the visibility of imaging, and decorrelation in dynamic media requires systems that counter the effect of scattering to operate faster than the decorrelation time. In this thesis, we explored solutions to the problem of scattering from different aspects. We presented Time Reversal by Analysis of Changing wavefronts from Kinetic targets (TRACK) technique to realize noninvasive optical focusing through a scattering medium. We showed that by taking the difference between time-varying scattering fields caused by a moving object and applying optical phase conjugation, light can be focused back to the location previously occupied by the object. To tackle the decorrelation of living tissue, we built up a fast digital optical phase conjugation (DOPC) system based on FPGA and DMD, which has a response time of 5.3 ms and was the fastest DOPC system in the world before 2017. We demonstrated that the system is fast enough to focus light through 2.3mm-thick living mouse skin. As for glare, inspired by noise canceling headphones, we invented an optical analogue termed coherence gated negation (CGN) technique. CGN can optically cancel out the glare in an active illumination imaging scenario to realize imaging through scattering media, like fog. In the experiment, we suppressed the glare by an order of magnitude and allowed improved imaging of a weak target. Finally, we demonstrated a method to image a moving target through scattering media noninvasively. Its principle roots are in the speckle-correlation-based imaging (SCI) invented by Ori Katz. We improved the technique and extended its application to bright field imaging of a moving target.</p

Imaging moving targets through scattering media

Imaging in turbid media such as biological tissue is challenging primarily due to light scattering, which degrades resolution and limits the depths at which we can reliably image objects. There are two main approaches for realizing non-destructive optical imaging through scattering tissue: gated approaches, which serve to distinguish and reject the multiply scattered photons; and non-gated approaches, which detect both the unscattered and scattered light contributions, and leverage the information from the scattering process in order to image the object1. In terms of non-gated approaches, both wavefront shaping (WFS) and speckle-correlation-based imaging (SCI) techniques can achieve high-resolution imaging of objects hidden within scattering media1,2. WFS techniques exploit the principles of time-reversal to undo the effects of scattering, whereas SCI methods exploit the angular correlations inherent within the scattering process to reconstruct the hidden object. In contrast with WFS approaches, SCI methods do not need long acquisition times or the presence of a guide star2. However, SCI methods are currently limited to imaging sparsely tagged objects in a dark-field scenario, and are strongly impacted by noise from other sources.2 In this work, we establish a technique that allows SCI to image obscured objects in a bright-field scenario.3 Our technique leverages the temporal correlations inherent in the scattering process to distinguish the object signal from the remaining, undesired ‘background’ light contributions. By using a deterministic phase modulator to generate a spatially incoherent light source, the background light contribution is kept constant between different acquisitions and can subsequently be subtracted out. As long as the object moves between acquisitions, the signal from the object can be isolated. The object can be reconstructed from this signal with high fidelity. Using this technique, we experimentally demonstrate successful reconstruction of moving objects hidden behind and between optically translucent materials. Due to the ability to effectively isolate the object signal, our work is not limited to imaging objects in the dark-field case, but also works in bright-field scenarios, with non-emitting objects. This ability opens up many potential applications for imaging in scattering media, such as through turbulent atmosphere or biological tissue, and makes this work relevant to the technical session on ‘Biophotonics in scattering tissue.’ References 1 R. Horstmeyer et al, “Guidestar-assisted wavefront-shaping methods for focusing light into biological tissue,” Nat. Photon. 9, 563-571 (2015). 2O. Katz et al, “Non-invasive single-shot imaging through scattering layers and around corners via speckle correlations,” Nat. Photon. 8, 784-790 (2014). 3M.Cua et al, “Imaging moving targets through scattering media,” O.E. 25(4), 3935-3945 (2017) Please click Additional Files below to see the full abstract

Focusing on moving targets through scattering samples

Focusing light through scattering media has been a longstanding goal of biomedical optics. While wavefront shaping and optical time-reversal techniques can in principle be used to focus light across scattering media, achieving this within a scattering medium with a noninvasive and efficient reference beacon, or guide star, remains an important challenge. Here, we show optical time-reversal focusing using a new technique termed Time Reversal by Analysis of Changing wavefronts from Kinetic targets (TRACK). By taking the difference between time-varying scattering fields caused by a moving object and applying optical time reversal, light can be focused back to the location previously occupied by the object. We demonstrate this approach with discretely moved objects as well as with particles in an aqueous flow, and obtain a focal peak-to-background strength of 204 in our demonstration experiments. We further demonstrate that the generated focus can be used to noninvasively count particles in a flow-cytometry configuration—even when the particles are hidden behind a strong diffuser. By achieving optical time reversal and focusing noninvasively without any external guide stars, using just the intrinsic characteristics of the sample, this work paves the way to a range of scattering media imaging applications, including underwater and atmospheric focusing as well as noninvasive in vivo flow cytometry

Glare suppression by coherence gated negation

Imaging of a weak target hidden behind a scattering medium can be significantly confounded by glare. We report a method, termed coherence gated negation (CGN), that uses destructive optical interference to suppress glare and allow improved imaging of a weak target. As a demonstration, we show that by permuting through a set range of amplitude and phase values for a reference beam interfering with the optical field from the glare and target reflection, we can suppress glare by an order of magnitude, even when the optical wavefront is highly disordered. This strategy significantly departs from conventional coherence gating methods in that CGN actively 'gates out' the unwanted optical contributions while conventional methods 'gate in' the target optical signal. We further show that the CGN method can outperform conventional coherence gating image quality in certain scenarios by more effectively rejecting unwanted optical contributions.Comment: main article (14 pages) and appendices (3 pages

Focusing through dynamic tissue with millisecond digital optical phase conjugation

Digital optical phase conjugation (DOPC) is a new technique employed in wavefront shaping and phase conjugation for focusing light through or within scattering media such as biological tissues. DOPC is particularly attractive as it intrinsically achieves a high fluence reflectivity in comparison to nonlinear optical approaches. However, the slow refresh rate of liquid crystal spatial light modulators and limitations imposed by computer data transfer speeds have thus far made it difficult for DOPC to achieve a playback latency of shorter than ∼200  ms and, therefore, prevented DOPC from being practically applied to thick living samples. In this paper, we report a novel DOPC system that is capable of 5.3 ms playback latency. This speed improvement of almost 2 orders of magnitude is achieved by using a digital micromirror device, field programmable gate array (FPGA) processing, and a single-shot binary phase retrieval technique. With this system, we are able to focus through 2.3 mm living mouse skin with blood flowing through it (decorrelation time ∼30  ms) and demonstrate that the focus can be maintained indefinitely—an important technological milestone that has not been previously reported, to the best of our knowledge

Wide-angular-range and high-resolution beam steering by a metasurface-coupled phased array

Optical beam steering has broad applications in lidar, optical communications, optical interconnects, and spatially resolved optical sensors. For high-speed applications, phased-array-based beam-steering methods are favored over mechanical methods, as they are unconstrained by inertia and can inherently operate at a higher speed. However, phased-array systems exhibit a tradeoff between angular range and beam divergence, making it difficult to achieve both a large steering angle and a narrow beam divergence. Here, we present a beam-steering method based on wavefront shaping through a disorder-engineered metasurface that circumvents this range-resolution tradeoff. We experimentally demonstrate that, through this technique, one can continuously steer an optical beam within a range of 160° (80° from normal incidence) with an angular resolution of about 0.01° at the cost of beam throughput

Wide-angular-range and high-resolution beam steering by a metasurface-coupled phased array

Optical beam steering has broad applications in lidar, optical communications, optical interconnects, and spatially resolved optical sensors. For high-speed applications, phased-array-based beam-steering methods are favored over mechanical methods, as they are unconstrained by inertia and can inherently operate at a higher speed. However, phased-array systems exhibit a tradeoff between angular range and beam divergence, making it difficult to achieve both a large steering angle and a narrow beam divergence. Here, we present a beam-steering method based on wavefront shaping through a disorder-engineered metasurface that circumvents this range-resolution tradeoff. We experimentally demonstrate that, through this technique, one can continuously steer an optical beam within a range of 160° (80° from normal incidence) with an angular resolution of about 0.01° at the cost of beam throughput

Supplement 1: Glare suppression by coherence gated negation

Originally published in Optica on 20 October 2016 (optica-3-10-1107

Supplement 1: Focusing through dynamic tissue with millisecond digital optical phase conjugation

Supplemental Document Originally published in Optica on 20 August 2015 (optica-2-8-728