Hybridized wavefront shaping for high-speed, high-efficiency focusing through dynamic diffusive media

Performance enhancement in wavefront shaping of multiply scattered light: a review

Significance

In nonballistic regime, optical scattering impedes high-resolution imaging through/inside complex media, such as milky liquid, fog, multimode fiber, and biological tissues, where confocal and multiphoton modalities fail. The significant tissue inhomogeneity-induced distortions need to be overcome and a technique referred as optical wavefront shaping (WFS), first proposed in 2007, has been becoming a promising solution, allowing for flexible and powerful light control. Understanding the principle and development of WFS may inspire exciting innovations for effective optical manipulation, imaging, stimulation, and therapy at depths in tissue or tissue-like complex media.

Aim

We aim to provide insights about what limits the WFS towards biomedical applications, and how recent efforts advance the performance of WFS among different trade-offs.

Approach

By differentiating the two implementation directions in the field, i.e., precompensation WFS and optical phase conjugation (OPC), improvement strategies are summarized and discussed.

Results

For biomedical applications, improving the speed of WFS is most essential in both directions, and a system-compatible wavefront modulator driven by fast apparatus is desired. In addition to that, algorithm efficiency and adaptability to perturbations/noise is of concern in precompensation WFS, while for OPC significant improvements rely heavily on integrating physical mechanisms and delicate system design for faster response and higher energy gain.

Conclusions

Substantial improvements in WFS implementations, from the aspects of physics, engineering, and computing, have inspired many novel and exciting optical applications that used to be optically inaccessible. It is envisioned that continuous efforts in the field can further advance WFS towards biomedical applications and guide our vision into deep biological tissues.

Focusing through scattering media by a single polarization transmission matrix with binary polarization modulation

Wavefront distortion induced by scattering media seriously affects optical focusing. Wavefront shaping based on a transmission matrix (TM) is useful in controlling light propagation in highly scattering media. Traditional TM generally studies amplitude and phase, but the stochastic nature of the light propagation in the scattering medium also affects its polarization. Based on the binary polarization modulation, we propose a single polarization transmission matrix (SPTM) and achieve single-spot focusing through scattering media. We anticipate that the SPTM will be widely used in wavefront shaping.

High-speed single-exposure time-reversed ultrasonically encoded optical focusing against dynamic scattering

Focusing light deep inside live scattering tissue promises to revolutionize biophotonics by enabling deep tissue noninvasive optical imaging, manipulation, and therapy. By combining with guide stars, wavefront shaping is emerging as a powerful tool to make scattering media optically transparent. However, for in vivo biomedical applications, the speeds of existing techniques are still too slow to accommodate the fast speckle decorrelation of live tissue. To address this key bottleneck, we develop a quaternary phase encoding scheme to enable single-exposure time-reversed ultrasonically encode optical focusing with full-phase modulations. Specifically, we focus light inside dynamic scattering media with an average mode time down to 29 ns, which indicates that more than 104 effective spatial modes can be controlled within 1 millisecond. With this technique, we demonstrate in vivo light focusing in between a highly opaque adult zebrafish of 5.1 millimeters in thickness and a ground glass diffuser. Our work presents an important step toward in vivo deep tissue applications of wavefront shaping.

Wavefront Shaping Concepts for Application in Optical Coherence Tomography-A Review

Optical coherence tomography (OCT) enables three-dimensional imaging with resolution on the micrometer scale. The technique relies on the time-of-flight gated detection of light scattered from a sample and has received enormous interest in applications as versatile as non-destructive testing, metrology and non-invasive medical diagnostics. However, in strongly scattering media such as biological tissue, the penetration depth and imaging resolution are limited. Combining OCT imaging with wavefront shaping approaches significantly leverages the capabilities of the technique by controlling the scattered light field through manipulation of the field incident on the sample. This article reviews the main concepts developed so far in the field and discusses the latest results achieved with a focus on signal enhancement and imaging.

Increasing the enhancement factor for DMD-based wavefront shaping

Focusing through scattering media is a subject of great interest due to its direct impact in the field of biomedical optics. However, the greatest barrier currently limiting direct applications is the fact that most scattering media that we wish to deliver light through are dynamic. To focus or deliver light through dynamic scattering media, using a digital micromirror device (DMD) has been demonstrated to be a potential solution, as it enables fast modulation speeds. However, since a DMD is a binary amplitude modulator, the large number of controlled modes needed to acquire adequate focus enhancement has limited optimal usage. Here we demonstrate a novel (to the best of our knowledge) scheme to use the "thrown-away" components of light to effectively use a binary amplitude DMD as a binary phase modulator, thereby increasing the correction efficiency by a factor of two. Our concept can be applied to any iterative optimization algorithm and can speed up the iterative optimization process by increasing the enhancement factor, rather than the measurement or modulation speeds.

Recovery of wave-mixing conversion efficiency in weakly scattering nonlinear crystals

Nonlinear optical wave mixing is a widely used method to produce light with new frequencies that has a significant impact on laser technology and optical imaging. The most important figure of merit in wave-mixing processes, i.e., high conversion efficiency, is always required in laser applications. We demonstrate a method to recover high conversion efficiency of second harmonic generation in a BaMgF₄ single crystal with weakly scattering defects via feedback-based wavefront shaping under birefringent phase-matching condition. By optimizing the fundamental wavefront, a typical second harmonic output with an enhancement factor of 1.14 and a corresponding recovery efficiency of 86.3% is displayed. This investigation may modify the wide understanding of scattering in crystals and provide an avenue to recover the nonlinear optical conversion efficiency in crystals with various defects without special fabrications.

High-speed alignment optimization of digital optical phase conjugation systems based on autocovariance analysis in conjunction with orthonormal rectangular polynomials

Digital optical phase conjugation (DOPC) enables many optical applications by permitting focusing of light through scattering media. However, DOPC systems require precise alignment of all optical components, particularly of the spatial light modulator (SLM) and camera, in order to accurately record the wavefront and perform playback through the use of time-reversal symmetry. We present a digital compensation technique to optimize the alignment of the SLM in five degrees of freedom, permitting focusing through thick scattering media with a thickness of 5 mm and transport scattering coefficient of 2.5  mm  -  1 while simultaneously improving focal quality, as quantified by the peak-to-background ratio, by several orders of magnitude over an unoptimized alignment.

Adaptive pumping for spectral control of broadband second-harmonic generation

Second-harmonic generation (SHG) is always a significant frequency conversion process in nonlinear optics for many great applications but can be limited when broadband spectral laser sources are involved, e.g., femtosecond pulses. The conversion efficiency can be high, but the spectral control is hard because of the phase-matching (PM) limitation. Recently, a random quasi-phase-matching (QPM) scheme was proposed to make use of highly nonlinear materials that are difficult to be phase matched under traditional configurations. The spectral control is even harder in anisotropic random materials, and the coherence is completely lost. Here, we proposed an approach to solve this problem by coherent light control via feedback-based wavefront shaping. We utilized this method for spectral control of broadband SHG, which can be efficient even in strongly scattering media. Randomly selected wavelengths in the broadband spectra were enhanced with a good selectivity, and the direction was also controlled in a three-dimensional (3D) configuration. This technique paves the way for convenient spatial and spectral control of both linear and nonlinear emissions and a local enhancement of their conversion efficiency, indicating great progress in both random and ultrafast nonlinear optics.

Time-reversed ultrasonically encoded optical focusing through highly scattering ex vivo human cataractous lenses

Normal development of the visual system in infants relies on clear images being projected onto the retina, which can be disrupted by lens opacity caused by congenital cataract. This disruption, if uncorrected in early life, results in amblyopia (permanently decreased vision even after removal of the cataract). Doctors are able to prevent amblyopia by removing the cataract during the first several weeks of life, but this surgery risks a host of complications, which can be equally visually disabling. Here, we investigated the feasibility of focusing light noninvasively through highly scattering cataractous lenses to stimulate the retina, thereby preventing amblyopia. This approach would allow the cataractous lens removal surgery to be delayed and hence greatly reduce the risk of complications from early surgery. Employing a wavefront shaping technique named time-reversed ultrasonically encoded optical focusing in reflection mode, we focused 532-nm light through a highly scattering ex vivo adult human cataractous lens. This work demonstrates a potential clinical application of wavefront shaping techniques.

Focusing light through dynamical samples using fast continuous wavefront optimization

We describe a fast continuous optimization wavefront shaping system able to focus light through dynamic scattering media. A micro-electro-mechanical system-based spatial light modulator, a fast photodetector, and field programmable gate array electronics are combined to implement a continuous optimization of a wavefront with a single-mode optimization rate of 4.1 kHz. The system performances are demonstrated by focusing light through colloidal solutions of TiO₂ particles in glycerol with tunable temporal stability.

High-speed single-shot optical focusing through dynamic scattering media with full-phase wavefront shaping

In biological applications, optical focusing is limited by the diffusion of light, which prevents focusing at depths greater than ∼1 mm in soft tissue. Wavefront shaping extends the depth by compensating for phase distortions induced by scattering and thus allows for focusing light through biological tissue beyond the optical diffusion limit by using constructive interference. However, due to physiological motion, light scattering in tissue is deterministic only within a brief speckle correlation time. In in vivo tissue, this speckle correlation time is on the order of milliseconds, and so the wavefront must be optimized within this brief period. The speed of digital wavefront shaping has typically been limited by the relatively long time required to measure and display the optimal phase pattern. This limitation stems from the low speeds of cameras, data transfer and processing, and spatial light modulators. While binary-phase modulation requiring only two images for the phase measurement has recently been reported, most techniques require at least three frames for the full-phase measurement. Here, we present a full-phase digital optical phase conjugation method based on off-axis holography for single-shot optical focusing through scattering media. By using off-axis holography in conjunction with graphics processing unit based processing, we take advantage of the single-shot full-phase measurement while using parallel computation to quickly reconstruct the phase map. With this system, we can focus light through scattering media with a system latency of approximately 9 ms, on the order of the in vivo speckle correlation time.

High-speed single-shot optical focusing through dynamic scattering media with full-phase wavefront shaping

In biological applications, optical focusing is limited by the diffusion of light, which prevents focusing at depths greater than ∼1 mm in soft tissue. Wavefront shaping extends the depth by compensating for phase distortions induced by scattering and thus allows for focusing light through biological tissue beyond the optical diffusion limit by using constructive interference. However, due to physiological motion, light scattering in tissue is deterministic only within a brief speckle correlation time. In in vivo tissue, this speckle correlation time is on the order of milliseconds, and so the wavefront must be optimized within this brief period. The speed of digital wavefront shaping has typically been limited by the relatively long time required to measure and display the optimal phase pattern. This limitation stems from the low speeds of cameras, data transfer and processing, and spatial light modulators. While binary-phase modulation requiring only two images for the phase measurement has recently been reported, most techniques require at least three frames for the full-phase measurement. Here, we present a full-phase digital optical phase conjugation method based on off-axis holography for single-shot optical focusing through scattering media. By using off-axis holography in conjunction with graphics processing unit based processing, we take advantage of the single-shot full-phase measurement while using parallel computation to quickly reconstruct the phase map. With this system, we can focus light through scattering media with a system latency of approximately 9 ms, on the order of the in vivo speckle correlation time.

Focusing light through scattering media by polarization modulation based generalized digital optical phase conjugation

Optical scattering prevents light from being focused through thick biological tissue at depths greater than ∼1 mm. To break this optical diffusion limit, digital optical phase conjugation (DOPC) based wavefront shaping techniques are being actively developed. Previous DOPC systems employed spatial light modulators that modulated either the phase or the amplitude of the conjugate light field. Here, we achieve optical focusing through scattering media by using polarization modulation based generalized DOPC. First, we describe an algorithm to extract the polarization map from the measured scattered field. Then, we validate the algorithm through numerical simulations and find that the focusing contrast achieved by polarization modulation is similar to that achieved by phase modulation. Finally, we build a system using an inexpensive twisted nematic liquid crystal based spatial light modulator (SLM) and experimentally demonstrate light focusing through 3-mm thick chicken breast tissue. Since the polarization modulation based SLMs are widely used in displays and are having more and more pixel counts with the prevalence of 4 K displays, these SLMs are inexpensive and valuable devices for wavefront shaping.

Second-harmonic focusing by a nonlinear turbid medium via feedback-based wavefront shaping

Scattering has usually been considered detrimental for optical focusing or imaging. Recently, more and more research has shown that strongly scattering materials can be utilized to focus coherent light by controlling or shaping the incident light. Here, purposeful focusing of second-harmonic waves, which are generated and scattered from nonlinear turbid media via feedback-based wavefront shaping, is presented. This Letter shows a flexible manipulation of both disordered linear and nonlinear scattering signals, indicating more controllable degrees of freedom for the description of turbid media. This technique also provides a possible way to an efficient transmission of nonlinear signal at a desired location in the form of a focal point or other patterns. With the combination of random nonlinear optics and wavefront shaping methods, more interesting applications can be expected in the future, such as nonlinear transmission matrix, multi-frequency imaging, and phase-matching-free nonlinear optics.

Focusing light inside dynamic scattering media with millisecond digital optical phase conjugation

Wavefront shaping based on digital optical phase conjugation (DOPC) focuses light through or inside scattering media, but the low speed of DOPC prevents it from being applied to thick, living biological tissue. Although a fast DOPC approach was recently developed, the reported single-shot wavefront measurement method does not work when the goal is to focus light inside, instead of through, highly scattering media. Here, using a ferroelectric liquid crystal based spatial light modulator, we develop a simpler but faster DOPC system that focuses light not only through, but also inside scattering media. By controlling 2.6 × 105 optical degrees of freedom, our system focused light through 3 mm thick moving chicken tissue, with a system latency of 3.0 ms. Using ultrasound-guided DOPC, along with a binary wavefront measurement method, our system focused light inside a scattering medium comprising moving tissue with a latency of 6.0 ms, which is one to two orders of magnitude shorter than those of previous digital wavefront shaping systems. Since the demonstrated speed approaches tissue decorrelation rates, this work is an important step toward in vivo deep-tissue non-invasive optical imaging, manipulation, and therapy.