Heat-shrinkable endoscopes with meta-optic fibers

Heat-shrinkable endoscopes with meta-optic fibers

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A meta-optics is optimized for integration with the coherent fiber bundle, while individual fiber cores are considered as imaging limitation. The MOFIE achieves a short tip length while maintaining a large 22.5 field of view and a large depth of field of more than 30mm, compared to a traditional GRIN lens. Credits: Johannes E. Frch, Luocheng Huang, Quentin AA Tanguy, Shane Colburn, Alan Zhan, Andrea Ravagli, Eric J. Seibel, Karl Bhringer, Arka Majumdar

Ultra-compact and agile endoscopes with a large field of view (FoV), long depth of field (DoF) and short rigid tip are essential for the development of minimally invasive operations and new experimental surgeries. As these fields develop, the requirements for miniaturization and higher accuracy become progressively demanding.

In existing endoscopes, the rigid length of the tip is a key limitation of the device’s agility inside small, tortuous conduits, such as an artery. It is mainly constrained by the size of the optical elements needed for imaging. Therefore, workarounds to shorten the tip length are urgently needed.

In a new article published in eLighta team of scientists led by Dr. Johannes Frch and Prof Arka Majumdar of the University of Washington has developed a new technique to reduce the length of the stiff tip.

Existing solutions include lensless and computational imaging with single fibers or bundles of coherent fibers. However, these are typically limited to a short working distance and often extremely sensitive to bending and twisting of the optical fiber, affecting or even precluding accurate computational reconstruction.

Flat meta-optics is an emerging and versatile idea in the photonics community to create miniaturized optical elements. These are sub-wavelength diffractive optical elements composed of nanoscale scatterer arrays. They are designed to model the phase, amplitude and spectral response of an incident wavefront. Such ultra-thin flat optics not only drastically reduce the size of traditional optics, but can also combine multiple functions into a single surface.

Flat meta-optics are compatible with high-volume semiconductor manufacturing technology and can create disposable optics. These properties have already inspired researchers to explore the potential of meta-optics for endoscopy, including integrated fiber endoscopy, side-view single-fiber scanning endoscopy, and forward-viewing fiber endoscopy. scanning.

A light microscope image of the fabricated meta-optics (left) placed in front of the bundle of coherent fibers. The scanning electron microscope images of the meta-optics (right) show the single scatterer, which covers the entire aperture of the device. Credits: Johannes E. Frch, Luocheng Huang, Quentin AA Tanguy, Shane Colburn, Alan Zhan, Andrea Ravagli, Eric J. Seibel, Karl Bhringer, Arka Majumdar

Unfortunately, meta-optics traditionally suffer from severe aberrations, making wide FoV and color imaging challenging. Several works have shown that the standard metalens design is not suitable for simultaneously capturing color information across the visible spectrum.

Typically produces sharp images for the design wavelength (e.g. green) but severely aberrated/blurred for other colors (red and blue). While some approaches such as dispersion engineering and computational imaging techniques can reduce chromatic aberration, they suffer from small apertures, low numerical apertures, or require a computational post-processing step, complicating real-time video capture.

Likewise, an additional aperture before the meta-optics can provide a larger field of view. However, this results in less light gathering and thicker optics. Until now, these limitations have limited most meta-optical endoscopes to single wavelength operation.

Although, recently, a meta-optical doublet has been demonstrated together with a coherent fiber bundle for polychromatic imaging. Such polychromatic imaging is not suitable for broadband illumination, which is often the case in clinical endoscopy. Furthermore, the frontal aperture was limited to 125m, with a short operating distance of 200m.

The research team noted the desire for broadband and ultrathin meta-optics for endoscopy. However, making it smaller than the diameter of the optical fiber is not favorable and severely limits light collection. Therefore, color meta-optic endoscopy with acceptable FoV, DoF and a large enough aperture has not yet been achieved.

In this work, the research team demonstrated an inverse design meta-optics optimized to capture real-time color scenes with a 1 mm diameter coherent fiber bundle. The meta-optics allows for operation at a FoV of 22.5, a DoF of >30 mm (greater than 300% of the nominal design working distance), and a minimum rigid tip length of only ~2.5 mm.

This is a 33% reduction in tip length compared to a traditional commercial Gradient Index Lens (GRIN) integrated fiber bundle endoscope. This is due to the shorter focal length and ultra-thin nature of meta-optics.

The images above show scenes on an OLED screen and captured through the MOFIE, allowing researchers to directly assess image quality. The bottom three images show images of a caterpillar, taken under real-time environmental imaging and life capture conditions, with no computational deconvolution applied. Credits: Johannes E. Frch, Luocheng Huang, Quentin AA Tanguy, Shane Colburn, Alan Zhan, Andrea Ravagli, Eric J. Seibel, Karl Bhringer, Arka Majumdar

At the same time, comparable imaging performance and working distance are maintained. To achieve outstanding FoV, DoF, and color performance of the meta-optic fiber endoscope (MOFIE), the research team approached this design issue from a system-level perspective.

They believed that the diameter and spacing of individual fiber cores within the bundle limited the achievable image quality, which also limits the achievable FoV and modulation transfer function (MTF). This aspect is implemented in an automatic differentiation framework using the average loudness under the multichromatic modulation transfer function (MTF) curve as a figure of merit.

By ensuring that the meta-optics has an MTF within the boundaries of the fiber bundle, the research team achieved color operation without requiring a computational reconstruction step, thus facilitating real-time operation. The team emphasized that its design approach differs fundamentally from traditional design efforts of achromatic metalens.

Researchers formulated an optimization problem to find the best solution for color imaging. This is instead of trying to achieve diffraction-limited performance in all wavelengths, which could be a physically unsolvable problem.

This approach is important because it is not limited to this particular system. It can be extended to larger aperture sizes and support computational post-processing steps. To highlight this, they also demonstrated an example of meta-optics with an aperture of 1cm and color imaging under ambient light conditions.

More information:
Johannes E. Frch et al, Real-time color imaging in a meta-optic fiber endoscope, eLight (2023). DOI: 10.1186/s43593-023-00044-4

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