Customizing the speckle field from multimode fiber with wavefront shaping

Significance 

Optical illumination and imaging of target objects hidden behind scattering media are important in numerous applications, including astronomy, remote sensing, and deep-tissue imaging. A typical example of such a scenario is driving through fog at night. However, low image quality could be produced in these cases due to two main reasons: the scattering medium-induced optical wavefront distortion and the glare from unwanted backscattered light by the medium. These factors limit the ability to see or sense through complex environments. Even if the wavefront distortion could be eliminated by an adaptive optics approach, glare remains a challenge that degrades the signal-to-ratio and image quality and hence needs to be suppressed.

Glare suppression allows for sensing or imaging weak objects that would otherwise be overwhelmed by a speckled background. Methods like time-of-flight technique, coherence gating or coherence gated negation can be used to separate and reject the glare component. However, these methods for glare suppression are usually technically demanding. Apart from reflection-mode glare suppression, suppressing scattered light intensity in a transmission geometry also finds great potentials in imaging-related applications.

Recent developments in glare suppression through scattering media are mainly based on wavefront shaping to control the propagation of diffused light. Wavefront shaping is widely used to focus diffused light for high-resolution point-scanning imaging or localized optical energy delivery at depths. It has also been employed to suppress scattered glare through scattering media. There have been several efforts using feedback-based iterative optimization, which were, however, time and computationally consuming. Not long ago, an aperture-target transmission matrix (TM) method was proposed for constructing the desired low-transmittance eigenchannel, which necessitated the amplitude-phase modulation ability and was not sufficiently generic.

On this account, PhD candidate (Mr.) Shengfu Cheng, Dr. Tianting Zhong, Miss. Chi Man Woo, and Professor Puxiang Lai from The Hong Kong Polytechnic University developed an alternating projection method to fully exploit the TM of the scattering medium for fast and effective glare suppression. Numerical simulations were carried out to validate the feasibility of the proposed approach. Their main objective was to determine a generic way to obtain the required phase mask for glare suppression in arbitrary target region using a single TM. Particularly, two speckle-domain constraints, namely, error reduction (ER) and hybrid input-output (HIO), were explored by such TM-based phase optimization method. Besides, the commonly used genetic algorithm (GA) was included for performance comparison. The work is recently published in the research journal, Optics and Lasers in Engineering.

The authors showed that phase optimization based on alternating projection could facilitate arbitrary and fast glare suppression through scattering media. With the knowledge of TM, it was possible to computationally optimize multiple phase masks corresponding to various target regions in parallel without entailing iterative hardware feedback. The operation was further accelerated by running on a graphics processing unit. As a result, a suppression factor of ∼10-3 could be achieved numerically with only 30 iterations, even for large target regions containing over 100 speckle grains.

The experimental results further revealed the superiority of the alternating projection method optimized with HIO constraint regarding suppression result and convergence property, in comparison to GA. For example, a remarkable suppression factor of 0.17 for a large target region with about 625 speckles in the output field was experimentally achieved by the proposed method with HIO constraint. Moreover, fast (about 50 fps) and effective glare suppression was realized in target regions of various shapes and sizes at the distal end of a multimode fiber.

In summary, the study proposed a TM-based optimization method for rapid glare suppression under phase-only modulation. The advantages of the present approach included fast and generic phase computation as well as effective glare suppression for large target regions. In a statement to Advances in Engineering, the lead and corresponding author, Professor Puxiang Lai noted that insights revealed in the study, like using multimode fiber to customize speckle field, would gain special interest for many applications like flexible and minimally invasive speckle-based optical imaging and tweezer at depths in complex environments.

Customizing the speckle field from multimode fiber with wavefront shaping - Advances in Engineering Customizing the speckle field from multimode fiber with wavefront shaping - Advances in Engineering Customizing the speckle field from multimode fiber with wavefront shaping - Advances in Engineering Customizing the speckle field from multimode fiber with wavefront shaping - Advances in Engineering Customizing the speckle field from multimode fiber with wavefront shaping - Advances in Engineering

About the author

Dr. Puxiang Lai received his Bachelor from Tsinghua University in 2002, Master from Chinese Academy of Sciences in 2005, and PhD from Boston University in 2011. In September 2015, he joined Department of Biomedical Engineering at the Hong Kong Polytechnic University as a Tenure-track Assistant Professor and was promoted to Tenured Associate Professor in July 2021.

Dr. Lai’s research interests focus on deep-tissue optical focusing, imaging, stimulation, and treatment. Current research projects include, but are not limited to, wavefront shaping, photoacoustic imaging, fiber imaging, computational optics, and artificial intelligence. His research has fueled more than 80 top journal publications, such as Nature Photonics, Nature Communications, Light: Science and Applications, The Innovation, and Advanced Science. He has been invited to give more than 80 seminars or invited talks worldwide. Since 2015, his research has been continuously supported by various national and local funding agencies. Dr. Lai was awarded the 2016-2017 Hong Kong RGC Early Career Award and as a recognition for his contribution to the field, currently Puxiang serves as Associate Editor or Editor for a few premium academic journals, such as Journal of Visual Computing for Industry, Biomedicine, and Art (VCIBA), Journal of Innovative Optics in Health and Science (JIOHS), Medicine in Novel Technology and Devices (MEDNTD), Advanced Photonics Nexus, and The Innovation.

Reference

Cheng, S., Zhong, T., Man Woo, C., & Lai, P. (2023). Alternating projection-based phase optimization for arbitrary glare suppression through multimode fiber. Optics and Lasers in Engineering, 161, 107368.

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