image: Figure 1. Schematic of MMF scanning endoscope based on wavefront shaping principle. The different technical routes, imaging modalities and some key imaging performance metrics of the MMF scanning endoscope are also illustrated.
Credit: OES
A new publication from Opto-Electronic Science; DOI 10.29026/oes.2024.230041 discusses optical scanning endoscope via a single multimode optical fiber.
Endoscopes are widely employed visual instruments in biomedicine, allowing in-depth, rapid and minimally invasive imaging of objects that cannot be directly accessed from surfaces. To alleviate the discomfort of patients in clinical applications and enhance the reliability of diagnosis, medical endoscopes inherently demand continuous miniaturization, excellent spatiotemporal resolution, and high image quality. Among numerous endoscopes, optical fiber endoscopes provide some unique advantages, such as high flexibility, compact structure, ease of high-temperature sterilization, and resistance to electromagnetic interference. Regarding most imaging applications for relatively large tubular or cavity structures within the human body, the current commercialized fiber bundle endoscopes and single-mode fiber scanning endoscopes already meet the demands effectively. However, when the target application involves some extreme scenarios, such as high-resolution in vivo imaging of deep brain areas, it is still challenging for the current fiber endoscopes. Their footprints are relatively large, preventing them from accessing the desired area of samples for imaging or creating substantial mechanical lesions of the tissues.
Over the past decade, optical fiber endoscopes based on a single multimode optical fiber (MMF) have attracted widespread research interest due to their potential to significantly reduce the footprint of optical fiber endoscopes and enhance imaging capabilities. In comparison with other imaging principles of MMF endoscopes, the scanning imaging method based on the wavefront shaping technique is highly developed and provides benefits including excellent imaging contrast, broad applicability to complex imaging scenarios, and good compatibility with various well-established scanning imaging modalities. A timely and comprehensive review of MMF scanning endoscopes can offer a valuable resource for researchers in this field and facilitate the development of this technique.
The authors of this article review the MMF scanning endoscope based on the wavefront shaping principle and summarize the fundamental focusing and scanning imaging mechanisms, key performance metrics, diverse imaging modalities, and applications of MMF scanning endoscopes, while also offering insights into the prospects of this field.
This review introduces the speckle output characteristics of MMF caused by intermodal dispersion and mode coupling effects. To overcome the disorderly nature of the output light field and generate focused light spots for scanning imaging, this article summarizes three techniques that can counteract the modal scrambling properties of MMF based on the wavefront shaping principle. These techniques include the transmission matrix measurement method, digital phase conjugation method and phase optimization algorithms. This article provides a detailed introduction to the physical principles, experimental implementations, and the latest research progress of these different techniques. The focusing performances of MMFs based on these techniques are also compared. Besides, the all-optical scanning imaging procedures of MMF scanning endoscopes are elaborated.
Subsequently, this article analyzes several key scanning imaging performance metrics of MMF scanning endoscopes and discusses advancements in improving these performance metrics, including imaging resolution, imaging contrast, scanning imaging speed, working distance, field-of-view and stability of the system. So far, the MMF scanning endoscopes have achieved imaging resolution of 250 nm, imaging contrast of 96% and imaging speed of 3.5 fps for 7-kilopixel images. These performance metrics can well satisfy the requirements of many applications. However, the imaging performance of MMF scanning endoscopes is susceptible to external dynamic disturbances. Specifically, when MMFs are subjected to external disturbances, the effectiveness of pre-calibrated wavefront shaping masks is diminished, resulting in a significant deterioration of the generated focal spots and thus affecting the imaging performance. The article summarized the measures proposed in recent years to mitigate this challenge. These measures can generally be divided into two categories: one strategy is to shield the MMF from external disturbances or develop MMFs that are insensitive to disturbances, while the other one is to perform the real-time recalibration of the transmission matrix for MMFs. These methods have greatly enhanced the feasibility of MMF scanning endoscopes in practical application scenarios.
Finally, this article introduces the integration of MMF scanning endoscopes with numerous advanced imaging modalities, including confocal imaging, two-photon imaging, Raman imaging and photoacoustic imaging modalities. The integration of MMF scanning endoscopes with these imaging modalities not only enhances their imaging performance but also significantly expands their application scenarios.
Over the past decade, significant progress has been made in improving the MMF scanning endoscopes. On this basis, some studies have begun to attempt the implementation of this technology in practical settings, such as conducting in vivo imaging of live mouse brains and guiding femtosecond laser ablation in biological samples for selective modifications. Nevertheless, we have to acknowledge that this technique is still in development and is not yet operational in practical applications. Before MMF scanning endoscopes can be widely commercialized, several issues need to be addressed, such as further improving the system’s resistance to external disturbances and promoting imaging speed. With the breakthrough of these bottlenecks, it is believed that MMF scanning endoscopes will find widespread applications in biomedical and other fields, thereby substantially driving the progress of these fields.
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Dr. Fei Xu is currently a professor at the College of Engineering and Applied Sciences, Nanjing University, China. He is a winner of the National Science Foundation for Distinguished Young Scholars (2019) from the National Natural Science Foundation of China. He is also the Fellow of SPIE and the Senior Member of Optica/IEEE. He focuses his research on optical and electrical intelligent sensing technologies. To date, he has authored or co-authored 9 book chapters, > 20 invited reviews, granted >50 patents (China and abroad), granted 4 PCT and US patents, and > 160 peer-reviewed articles in academic journals including Science Advances, Optica, Light: Science & Application, Advanced Photonics, Advanced Materials, etc. in the previously mentioned areas. Professor Fei Xu’s home page: https://eng.nju.edu.cn/xf/main.htm
With optical fiber as the carrier, the laboratory has independently developed optical fiber integrated devices and systems with ultra-high precision and excellent performance after over ten years accumulated high-precision processing technologies such as micro/nano processing technologies and femtosecond laser processing technologies, and developed the frontier applications in aerospace, medical diagnosis, optical fiber communication, safety monitoring, biochemical analysis, and other fields. With the support of major national requirements and key projects, the laboratory focuses on scientific issues in next-generation optical fiber integration, human-computer interaction, computational sensing and medical imaging, and innovates in advanced optoelectronic devices, integration methods, as well as algorithms and architectures of novel models through cross-merging physical optics, materials science, electronics, machine learning, micro/nano-optics, and computer vision.
The Laser Application Center of Xiamen University is a scientific research team with 30+ members, consisting of 2 postdocs, 9 PhDs, 12 Master’s degree students, and 5 Engineers. The team leader, Professor Hong Minghui, joins Xiamen University as a full-time staff in August 2022 and is appointed as the Tan Kah Kee Chair Professor of Xiamen University. He is presently the Engineering Technology Division Chairman of Xiamen University, and the Dean of the Pen-TungSah Institute of Micro-Nano Science and Technology, as well as Chief Scientist of TanKah Kee Innovation Laboratory. He is also Fellow of Academy of Engineering, Singapore (FSEng), Optical Society of America (OSA), International Society for Optics and Photonics (SPlE), International Academy of Photonics and Laser Engineering (IAPLE), and Institution of Engineers, Singapore (lES).
Prof. Hong Minghui specializes in laser micro-processing & nano-fabrication, optical engineering and their applications, as well as explores the physical mechanisms and dynamic processes behind the laser materials interaction. Prof. Hong has co-authored 15 book chapters, 42 patents granted and 500+ academic papers being published in Nature, Chemical Reviews, Nature Nanotechnology, Advanced Materials, Light: Science and Applications, Science Advances, Nature Communications, ACS Nano, Nano Letters, etc. Technology commercialization and industrial applications of his research outcomes have also been realized.
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Wu GX, Zhu RZ, Lu YQ et al. Optical scanning endoscope via a single multimode optical fiber. Opto-Electron Sci 3, 230041 (2024). doi: 10.29026/oes.2024.230041
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Opto-Electronic Science