Gradient-assisted focusing light through scattering media

Wavefront shaping improves the transparency of the scattering media: a review

Significance

Wavefront shaping (WFS) can compensate for distortions by optimizing the wavefront of the input light or reversing the transmission matrix of the media. It is a promising field of research. A thorough understanding of principles and developments of WFS is important for optical research.

Aim

To provide insight into WFS for researchers who deal with scattering in biomedicine, imaging, and optical communication, our study summarizes the basic principles and methods of WFS and reviews recent progress.

Approach

The basic principles, methods of WFS, and the latest applications of WFS in focusing, imaging, and multimode fiber (MMF) endoscopy are described. The practical challenges and prospects of future development are also discussed.

Results

Data-driven learning-based methods are opening up new possibilities for WFS. High-resolution imaging through MMFs can support small-diameter endoscopy in the future.

Conclusion

The rapid development of WFS over the past decade has shown that the best solution is not to avoid scattering but to find ways to correct it or even use it. WFS with faster speed, more optical modes, and more modulation degrees of freedom will continue to drive exciting developments in various fields.

Progressive acceleration in the genetic algorithm by intragenerational mutation

The intragenerational mutation of the genetic algorithm (IMGA) is proposed to actively broaden the searching space during the optimization process. The searching space is aggressively increased by expanding the variation of mutation rates of all individuals within each generation, leading to the reduction of the required number of iterations, improving the convergence speed and the enhancement factor.

Formation of multiple complex light structures simultaneously in 3D volume using a single binary phase mask

Complex structure formation inside or through turbid media is a challenging task due to refractive index inhomogeneity, random light scattering, and speckle noise formation. In this article, we have coupled the data regression model in the R-squared metric and used its advantages as a fitness function in the genetic algorithm to advance the resolution and structural uniformity. As a compatible system with the binary genetic algorithm, we have presented a cost-effective iterative wavefront shaping system-design with binary phase modulation using an affordable ferroelectric liquid crystal (FLC) based binary-phase spatial light modulator (SLM). R-squared metric in the genetic algorithm is analyzed to optimize the binary phase mask, and the prototype system based on iterative binary phase modulation has been validated with a 120-grit ground glass diffuser and fresh chicken tissues of thickness 307 [Formula: see text] and 812 [Formula: see text]. The detailed results show that the proposed cost-effective wavefront shaping system with data regression model assisted R-squared fitness function can construct high-resolution multiple complex hetero-structures simultaneously in 3D volume using an optimized single phase-mask.

Speedy light focusing through scattering media by a cooperatively FPGA-parameterized genetic algorithm

We developed an accelerated Genetic Algorithm (GA) system based on the cooperation of a field-programmable gate array (FPGA) and the optimized parameters that enables fast light focusing through scattering media. Starting at the searching space, which influences the convergence of the optimization algorithms, we manipulated the mutation rate that defines the number of mutated pixels on the spatial light modulator to accelerate the GA process. We found that the enhanced decay ratio of the mutation rate leads to a much faster convergence of the GA. A convergence-efficiency function was defined to gauge the tradeoff between the processing time and the enhancement of the focal spot. This function allowed us to adopt the shorter iteration number of the GA that still achieves applicable light focusing. Furthermore, the accelerated GA configuration was programmed in FPGA to boost processing speed at the hardware level. It shows the ability to focus light through scattering media within a few seconds, 150 times faster than the PC-based GA. The processing cycle could be further promoted to a millisecond-level with the advanced FPGA processor chips. This study makes the evolution-based optimization approach adaptable in dynamic scattering media, showing the capability to tackle wavefront shaping in biological material.

Long-distance pattern projection through an unfixed multimode fiber with natural evolution strategy-based wavefront shaping

Focusing light into an arbitrary pattern through complex media is desired in energy delivery-related scenarios and has been demonstrated feasible with the assistance of wavefront shaping. However, it still encounters challenges in terms of pattern fidelity and focusing contrast, especially in a noisy and perturbed environment. In this work, we show that the strategy relying on natural gradient ascent-based parameter optimization can help to resist noise and disturbance, enabling rapid wavefront optimization towards high-quality pattern projection through complex media. It is revealed that faster convergence and better robustness can be achieved compared with existing phase control algorithms. Meanwhile, a new fitness function based on cosine similarity is adopted for the algorithm, leading to higher focusing contrast without sacrificing similarity to the target pattern. As a result, long-distance projection of an arbitrary pattern can be accomplished with considerably enhanced performance through a 15-meter multimode fiber that is not fixed and susceptible to perturbation. With further engineering, the approach may find special interests for many biomedical applications, such as deep-tissue photon therapy and optogenetics, where free-space localized optical delivery encounters challenges.

Wavefront shaping: A versatile tool to conquer multiple scattering in multidisciplinary fields

Optical techniques offer a wide variety of applications as light-matter interactions provide extremely sensitive mechanisms to probe or treat target media. Most of these implementations rely on the usage of ballistic or quasi-ballistic photons to achieve high spatial resolution. However, the inherent scattering nature of light in biological tissues or tissue-like scattering media constitutes a critical obstacle that has restricted the penetration depth of non-scattered photons and hence limited the implementation of most optical techniques for wider applications. In addition, the components of an optical system are usually designed and manufactured for a fixed function or performance. Recent advances in wavefront shaping have demonstrated that scattering- or component-induced phase distortions can be compensated by optimizing the wavefront of the input light pattern through iteration or by conjugating the transmission matrix of the scattering medium. This offers unprecedented opportunities in many applications to achieve controllable optical delivery or detection at depths or dynamically configurable functionalities by using scattering media to substitute conventional optical components. In this article, the recent progress of wavefront shaping in multidisciplinary fields is reviewed, from optical focusing and imaging with scattering media, functionalized devices, modulation of mode coupling, and nonlinearity in multimode fiber to multimode fiber-based applications. Apart from insights into the underlying principles and recent advances in wavefront shaping implementations, practical limitations and roadmap for future development are discussed in depth. Looking back and looking forward, it is believed that wavefront shaping holds a bright future that will open new avenues for noninvasive or minimally invasive optical interactions and arbitrary control inside deep tissues. The high degree of freedom with multiple scattering will also provide unprecedented opportunities to develop novel optical devices based on a single scattering medium (generic or customized) that can outperform traditional optical components.