Fiberscope real-time full-color video imaging

Endoscopy plays a crucial role in the development of minimally invasive surgery. Smaller and less rigid endoscopes allow for more flexible surgical procedures and less stress on surrounding tissues. However, the size of the optics limits the miniaturization of the imaging system. For conventional devices, miniaturization is not only limited by manufacturing, but also degrades in performance as size shrinks. The emergence of metaoptics has opened up an important direction for the miniaturization of devices.

Arka Majumdar’s research team from the University of Washington has reported a reverse-engineered metaoptical device that, combined with a coherent fiber bundle, reduces the rigid tip length by 33 percent compared to conventional gradient refractive index (GRIN) lenses. And full-color real-time video capture is demonstrated by a meta-fiberscope (MOFIE).

The work was published in the new issue of eLight in the High Start of the Excellence Program under the title “Real Time Full-Color Imaging in a Meta-Optical Fiber Endoscope”.

Ultra-compact, flexible endoscopes with large field of view, long depth of field and short rigid tip length are important tools for the development of minimally invasive surgery and novel experimental procedures. With the development of these fields, the requirements for miniaturization and precision are increasing. In existing endoscopes, the device is within a narrow, curved tube (e.g., an artery), and the rigid tip length limits its flexibility, which in turn is limited by the size of the optical element imaging. Therefore, alternative solutions to reduce tip length are urgently needed. Although there are currently some lensless and computational imaging solutions that use single fibers or coherent fiber bundles. However, it is usually limited to short working distances and is extremely sensitive to bending and twisting of optical fibers, making it difficult to accurately calculate reconstructions. In addition, complex computational reconstruction often affects real-time image capture. Meta-optics feature sub-wavelength cell size that adjusts the phase, amplitude, and spectral response of the incident wavefront, dramatically reducing the size of conventional optics and combining multiple functions on a single surface. Therefore, the introduction of metaoptics into the endoscope greatly reduces tip length and optimizes image quality.

In this work, the authors demonstrate a reverse-engineered meta-optics optimized to capture a real-time, full-color scene in visible light in combination with a 1 mm diameter coherent fiber bundle (Figure 1). The device has a field of view of 22.5 degrees, a depth of focus of > 30 mm (more than 300% of the nominal design working distance), and a minimum rigid tip length of only ~2.5 mm (effective numerical aperture of 0.24). Compared to traditional commercial gradient refractive index (GRIN) lens integrated fiber beam endoscopes, metaoptics have a 33% reduction in tip length due to their shorter focal length and ultra-thin characteristics, while the imaging performance is comparable and the working distance remains unchanged.

Figure 1: Schematic diagram of a meta-fiber endoscope. Compared to conventional GRIN lenses, the meta-optics reduce the tip length while maintaining a wide field of view of 22.5° and a large depth of field of more than 30mm

Hyperboloid hyperlenses have diffraction-limited performance at the design wavelength, and the modulation transfer function (MTF) degrades rapidly due to chromatic aberration when operating over the extended spectral range. To solve this problem, the authors use an inverse design approach that maximizes the average volume under the multicolor MTF curve as a figure of merit during optimization and achieves it through an automatic differential framework. The figure of merit also ensures that MTF remains similar over a wide wavelength range. To ensure polarization-insensitive operation and compatibility with high-volume manufacturing processes, the simple SiN square column is designed to achieve phase modulation with a minimum feature size of 75nm and a maximum aspect ratio of 10. The optical microscopic picture and SEM are shown in Figure 2 (a-d). Color images of the OLED screen are taken through a meta-optical fiberscope (MOFIE), as shown in Figure 2(g, h). Complex scenes can be imaged while maintaining resolution, and color quality is fully preserved over the entire visible range.

Figure 2: Characterization of metaoptics. (a) Optical microscope image of a 1 mm aperture meta-optics. (b-d) SEM image. (e-f) A full-color scene displayed on an OLED screen. (g-h) Corresponding images taken at a working distance of 10 mm using a meta-optical fiberscope. The scale bar corresponds to 1 mm. The displayed image is not calculated for deconvolution

First, at a working distance of 10 mm, by measuring the checkerboard pattern (Figure 3 (a,b)), a field of view of approximately 22.5° was determined. View the object area at a field of view of ~5 mm at a working distance of 10 mm (the average diameter of human arteries is about 1-3 mm). Another important indicator of endoscopic imaging is the depth of focus DoF, because in vivo motion, such as heart beating, can significantly change the working distance in tens of milliseconds, so a large DoF is beneficial for imaging. The authors placed OLED screens at different working distances of the corresponding endoscopes and displayed the same images at working distances of 7 mm, 10 mm (design working distance), 18 mm, and 40 mm. It can be clearly seen that the same clarity and color quality are achieved over the entire 33mm range (Figure 3). Figure 3g shows the imaging resolution at different working distances, and the identification of a third set of elements indicates a linear resolution of approximately 50 μm.

Figure 3: Evaluation of MOFIE

Figure 4 shows MOFIE applied to real-time full-color imaging of biological samples. Endoscopes have the ability to image at video speed while maintaining panchromatic information. At a working distance of 10mm, the movement of a live caterpillar on a strawberry leaf was recorded at a video rate of 14 frames per second.

Figure 4: Full-color video-rate imaging clip of a caterpillar on a strawberry leaf

Related paper information: (Source: China Optics WeChat public account)
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