Real-time holographic lensless micro-endoscopy through flexible fibers via fiber bundle distal holography

Flexible fiber-based micro-endoscopes are crucial for minimally invasive deep-tissue imaging, but current technologies often rely on bulky distal optical elements (lenses, scanners) that limit flexibility and increase tissue damage. While various lensless endoscopy approaches have been explored, they often struggle with dynamic optical wavefront distortions in flexible fibers, restricting their use to nearly-static or inflexible fibers. This paper addresses this challenge by developing a method that allows holographic imaging through dynamically bent fibers without the need for bulky distal components. The demand for a flexible, video-rate, label-free micro-endoscope with a minimal footprint is high in various fields such as clinical procedures and biomedical research, particularly for imaging micron-scale structures deep within tissues. Current methods, based on single-mode fibers or conventional fiber bundles, have significant drawbacks. Single-mode fiber endoscopes, while bend-insensitive, require distal optical elements increasing the instrument size and tissue damage. Multi-core fiber (MCF) based systems offer simplicity, but suffer from limited resolution, pixelation, fixed working distance, and poor axial sectioning. Emerging transmission matrix-based methods improve upon these limitations but are typically limited to non-flexible fibers due to the sensitivity of the transmission matrix to bending. Speckle-based incoherent imaging has low contrast. Recent advances using a partially reflecting mirror and spatial light modulators for phase correction are limited by the difficulty of correcting dynamic distortions and the requirement for fluorescence labeling. Existing label-free holographic approaches using MCFs often involve additional illumination fibers and suffer from poor axial sectioning and limited field of view. The presented FiDHo technique aims to overcome these limitations by providing a simple, robust solution for high-quality, flexible micro-endoscopy.

The paper reviews existing micro-endoscopy techniques, categorizing them into those using single-mode fibers and those using multi-core fibers (MCFs). Single-mode fiber approaches, while bend-insensitive, necessitate distal optical elements, impacting the overall size and invasiveness. Conventional MCF-based endoscopes, though simple to implement, suffer from poor resolution, pixelation, limited working distance, and axial sectioning capabilities. Transmission matrix-based approaches, capable of compensating for fiber transfer functions, are reviewed, highlighting their limitations when dealing with flexible fibers where bending distorts the transmission matrix. Speckle-based incoherent imaging, while offering lensless capabilities, is criticized for its low image contrast. Methods employing a partially reflecting mirror at the distal end for in-situ phase distortion measurement and correction with a spatial light modulator are discussed, noting their limitations in handling dynamic distortions and reliance on fluorescence labeling. Finally, the authors discuss existing holographic methods, pointing out their limitations, including the use of additional illumination fibers, poor axial sectioning, and limited field of view.

FiDHo uses a commercially available MCF with a partially reflecting mirror added to the distal tip. A low-coherence laser beam is split into two delayed replicas: an illumination beam and a reference beam, both coupled into a single MCF core. At the distal end, the illumination beam reflects off the target, while the reference beam reflects off the mirror. The interference pattern, created by the superposition of these reflected beams, is relayed back to the proximal end by the MCF and captured by a camera. Low-coherence phase-shifting holography is used to reconstruct the object's complex field from N ≥ 3 camera frames by controlling the phase of the reference beam. The time delay between the two beams is set to match the object distance from the mirror (τ = 2zo/c) for coherence gating to reject spurious reflections. The object's complex field is reconstructed using phase-shifting interferometry and normalized with the conjugate of the known reference field. 3D reconstruction is achieved through back-propagation of the recorded field to the desired distance, normalizing by the expected illumination field. Digital refocusing allows sharp image generation at different depths. The MCF pixelation is removed by digital filtering, suppressing k-space aliasing. The system's resolution and field-of-view (FoV) are characterized using a USAF resolution target and knife-edge measurements. Axial resolution is determined by coherence gating and the numerical aperture (NA) of the imaging system. Phase-contrast imaging is demonstrated by imaging cheek cells. Video-rate imaging and insensitivity to fiber bending are shown through experiments with moving targets and dynamically bent fibers. Axially sectioned imaging is demonstrated using stacked and tilted USAF targets. The method uses cross-polarized detection to minimize proximal facet reflections and a detailed analysis of unwanted reflection suppression using coherence gating is provided. Digital filtering of MCF pixelation is described in detail. The experimental setup includes a Mach-Zehnder interferometer for beam splitting and delay control, telescopes for beam coupling and imaging, and polarizers for suppressing reflections. A distal camera is used for ground truth images. The partially reflecting mirror is fabricated by E-beam evaporation of Titanium on a glass slide. The impact of various parameters such as fiber diameter, NA, distal mirror distance, and reflectivity are discussed.

FiDHo achieves video-rate imaging (50 FPS) with diffraction-limited transverse resolution (better than 2.7 µm), exceeding the core-to-core pitch of conventional MCF endoscopes. The system demonstrated an unpixelated, full-field reconstruction requiring only three frames. The technique is inherently insensitive to fiber bending, enabling imaging through dynamically bent fibers. The low-coherence gating provides effective axial sectioning, allowing 3D imaging and depth measurement. Phase-contrast imaging capabilities, crucial for visualizing biological samples with low reflection contrast, are demonstrated. Experimental results show that the resolution and FoV are independent of imaging distance within a certain range. The system's effective NA is approximately 0.15, and space-bandwidth product is approximately 5300. The axial resolution, determined by coherence length and NA, is improved via computational reconstruction. The minimal working distance is determined by the coherence length of the laser (400 µm in the experiments but potentially shorter), and the object reflectivity. Experiments with stacked and tilted USAF targets show successful 3D imaging and depth measurement. The insensitivity to fiber bending is demonstrated experimentally with moving targets and dynamically bent fibers.

FiDHo offers a unique combination of advantages not found in existing lensless endoscopy techniques: video-rate imaging, diffraction-limited resolution, axial sectioning, label-free imaging, bend insensitivity, and calibration-free operation, all within a simple system using readily available components and non-iterative reconstruction. The optimal parameters for the MCF, distal mirror, and power ratios are analyzed, emphasizing the trade-off between field of view, resolution, and bend sensitivity. The impact of higher-order fiber modes and their temporal separation are addressed. Image contrast limitations, similar to OCT, are discussed, acknowledging speckle noise inherent to coherent imaging. Strategies to mitigate speckle noise, such as incoherent compounding and computational denoising, are suggested. The use of MCFs with disordered core positions is explored, suggesting the potential for improved reconstruction using compressed-sensing algorithms. Future improvements, such as single-shot implementation using off-axis holography and using GRIN lenses, are discussed. The potential applications in biomedical diagnostics using endogenous reflection contrast are highlighted.

This work successfully demonstrates FiDHo, a novel lensless endoscopic imaging technique combining several desirable features: high speed, diffraction-limited resolution, bend insensitivity, and 3D imaging capabilities. The method uses readily available components and simple reconstruction algorithms, making it a promising approach for various applications requiring minimally invasive deep-tissue imaging. Future research will focus on improving axial resolution, minimizing speckle noise, and exploring applications in biomedical diagnostics.

The current implementation uses a relatively long coherence length laser, limiting the axial resolution. Speckle noise, inherent in coherent imaging, is present in the results. While insensitivity to fiber bending is demonstrated, extreme, rapid bending within the three phase-shifting frames could affect image quality. The current setup uses a significant number of bulk optical components; a more compact design is needed for practical applications. The minimal working distance could be improved by using a shorter coherence length laser.