Fiber optic imaging systems (FOISs) play a critical role in clinical practice and biological research due to their flexibility and small size, such as medical fiber endoscopes for imaging the inside of natural human tubes or surgical incisions, and deep tissue imaging microscopes. A multimode fiber (MMF) supports the simultaneous propagation of over hundreds of optical modes through a hair-thin imaging probe, further reducing the invasiveness of FOIS, which gives MMF-based imaging systems great potential in the field of medical endoscopy. However, the imaging resolution of MMFs is inevitably limited by their inherent physical conditions. Especially for an ultrafine core diameter MMF, the limited spatial mode quantity becomes the bottleneck to achieve high resolution imaging. To break this limitation and fully unlocked the imaging potential of MMF, the team has proposed a mode modulation method based on singular value decomposition. This method can autonomously modulate the excitation modes of MMF during transmission, and significantly improve the imaging quality of MMF-based imaging systems. For the ultrafine 40μm core MMF, the image reconstruction accuracy achieved an increase of up to 7.32dB in peak signal-to-noise ratio (PSNR) and 0.103 in structural similarity (SSIM). This study is expected to promote the further development of MMF imaging towards lower invasiveness and higher resolution, and has great value in the field of MMF-based medical endoscopy imaging. Relevant research results were recently published in Photonics Research, Volume 12, Issue 10, 2024. [Ning Zhan, Zhenming Yu, Liming Cheng, Jingyue Ma, Jiayu Di, Yueheng Lan, Kun Xu, "Enhanced ultrafine multimode fiber imaging based on mode modulation through singular value decomposition," Photonics Res. 12, 2214 (2024)]

To achieve mode modulation of the MMF, the team first obtain a set of associated singular vectors by performing SVD of the MMF's transmission matrix (TM) and then modulate the internal optical modes by displaying the 2-dimension (2D) maps of the singular vectors as a modulation on the spatial light modulator (SLM). As the singular vectors return the eigenmodes of the TM associated with specific values of transmission, they result in specificity in spatial frequency. Different singular vector numbers specifically excite the different order of modes in the MMF, thus customizing the properties (grain size and intensity) of the output speckle. The schematic overview of the mode modulation-enhanced MMF imaging system is illustrated in Figure 1. Each column of V matrix is converted into a discrete 2D map as the input light field displayed on the SLM. Highly transmitting singular vectors (singular vectors corresponding to high singular values) concentrate light in low-order spatial modes of low frequency, resulting in enlarged speckle grain sizes and high intensity. Conversely, singular vectors with lower transmission (singular vectors corresponding to relatively low singular values) tend to excite high-order spatial modes of high frequency in the MMF, leading to smaller speckle grain sizes and lower intensity. Via this process, selective excitation of the transmission modes within the multimode fiber can be achieved to customize the output speckle pattern with the desired properties. In addition, the singular vectors can couple the light field into eigenchannels with extraordinarily high transmission rate for transmission, so that the energy is transmitted more efficiently, making the output speckle more conducive to reconstruction. Optimal imaging-enhanced performance can be achieved by choosing the optimal distribution of the excited modes.

The imaging results are shown in Figure 2(a). In both core diameter MMF imaging systems (40 μm and 105 μm), the image quality with mode modulation has been significantly improved. It is worth noting that the imaging quality of the 40 μm core MMF system (≈ 477 modes) with mode modulation surpassed that of the 105 μm core MMF system (≈ 3288 modes) without mode modulation. These experimental results demonstrate the effectiveness of the mode modulation method in improving the imaging quality of MMF, which is conducive to MMF high-resolution imaging, and highlight the significant advantages of this method in ultrafine MMF imaging, which is advantage for reducing the invasiveness of an MMF-based imaging system. Figure 2(b) shows the output speckle pattern with and without mode modulation, which demonstrates that the mode modulation method can effectively customize the properties of output speckle.

Figure 1. Schematic overview of the mode modulation-enhanced MMF imaging system.

Professor Zhenming Yu, corresponding author of the study, said: "The multimode fibers display significant application potential in the field of medical endoscopy due to their miniaturization and large capacity. However, MMF-based endoscopes have faced the problem of limited resolution because of the uncontrollable spatial modes during propagation. How to achieve high resolution imaging while ensuring low invasiveness has always been a difficult problem to be solved. Therefore, we deeply analyzed the inherent physical transmission characteristics of MMF, and innovatively proposed an effective mode modulation method of MMF. This method can achieve the selective excitation of the inherent transmission modes within the MMF autonomously and controllably during transmission. By setting optimal distribution of the excited modes, the MMF imaging quality can be enhanced to the maximum extent. This method has fully unlocked the imaging potential of MMF and presented a fresh avenue for enhancing the imaging quality of MMF."

Figure 2. Comparison of the MMF systems with two different core diameters. (a) Reconstruction results of both MMF systems w/o mode modulation and w/ mode modulation. (b) Output speckle patterns of the same input pattern captured by both MMF systems w/o mode modulation and w/ mode modulation.

In the next step, the team will further optimize the operational steps of mode modulation and plan to integrate this technology into practical endoscopic devices for application. In addition, the team will explore the potential feasibility of this technology in other MMF applications, with a view to extending this innovation to a wider range of application scenarios.