Ongoing research is actively exploring microendoscopy systems for high-resolution imaging and disease diagnosis. State-of-the-art commercial endoscopes use fiber bundles for relaying the image formed by the lens system in the imaging probe head. However, they are expensive and resolution is limited by the spacing between the cores and cross-talk between the pixels. Fiber scanning-based imaging systems are promising due to their ability to be designed with a single fiber alongside micro-objective lenses. However, these systems face a significant challenge when it comes to the necessity of free space optics and photomultiplier tubes for fluorescence detection, which can be tricky in terms of alignment. Furthermore, the existing imaging systems at near-infrared wavelengths utilize double-clad fiber with larger clad for fluorescence collection. We present a compact, low-cost, and portable system for imaging at visible wavelength that utilizes a double-clad fiber of smaller inner clad (diameter of 15 µm) in conjunction with a bandpass filter and Avalanche Photodetector to detect fluorescence emission. To assess the effectiveness of this system, we conducted a study on the performance of an imaging head consisting of a gradient refractive index (GRIN) lens (diameter of 1 mm) as a micro-objective in a fiber-scanning setup by actuating a piezoelectric tube with a fiber channel inside. We demonstrated the reflectance imaging of a standard resolution chart and fluorescence detection from microspheres at 644 nm.
Fiber-optic microendoscopy imaging systems are actively being researched. They are interesting as they can be designed with a single fiber in conjunction with gradient refractive index (GRIN) micro-lenses. However, these systems face a significant limitation in the form of optical aberrations caused by beam scanning, resulting in reduced resolution at the edges of the imaging field. The current solutions involve bulky refractive optics, combining micro-optics with an aspherical lens that present challenges in fabrication and alignment. To overcome these limitations, we propose the design of a compact metasurface correction element (diameter = 1 mm) that can be seamlessly integrated into the existing optical system alongside the 1 mm diameter GRIN lens. Accurately modeling such a complex system involving nanoscale metasurface and macroscale optics is challenging. We present the interconnection of ray tracing and electromagnetic simulations to simultaneously achieve the desired optical system performance and the required phase profile. In our imaging probe design, the target phase profile of the correction element is optimized using Zemax Optic Studio for multiple beam scan angles to achieve a minimal and uniform spot of 1 μm across the imaging field, which spans approximately 100 μm. The target phase mask obtained from ray tracing simulation and the phase-look-up table obtained from electromagnetic simulation of the unit cell are used to create the metasurface. The simulation of the metasurface of 1mm diameter is performed in Lumerical and the solved near field is propagated in Zemax to assess the imaging system through physical optics propagation and diffraction analysis.
This paper presents the design, fabrication, and testing of a silicon-on-insulator wafer (SOI) based electrothermally actuated MEMS mirror for a micro-endoscope. Finite element analysis (FEA) was conducted using CoventorWare to optimize the design parameters. A fabrication process flow was developed and the process steps were optimized based on the design. The micromirror was fabricated and tested. The results demonstrate the successful design and fabrication of a micromirror suitable for a micro-endoscope application.
Two photon fluorescence imaging microendoscopy is a current research thrust for detailed tissue interrogation and early detection of pre-cancerous lesions. There is a need for miniaturization of endoscopes so that it is minimally invasive for in-vivo optical imaging of cavities in the body inaccessible by existing endoscopes. With the advancements in micro-optics and gradient refractive index (GRIN) lenses, high numerical aperture is achieved at the tissue end with compact lenses. However, GRIN lenses owing to their optical aberrations limit the resolution and thereby image quality. We present the optical design of single fiber microendoscope imaging head with distal beam scanning and diameter of probe optics limited to 1.5mm. We found that positioning of the micro mirror plays a crucial role in controlling the aberrations, diameter of probe and field of view. The effect of gradient constant on aberrations is analyzed with excitation path models at 830 nm. The design is optimized to keep the predominant aberrations such as coma and astigmatism in control when the beam is scanned over the sample by tilting a micro electro mechanical system (MEMS) scan mirror. In addition to the monochromatic aberrations, chromatic focal shift is another major challenge in two photon fluorescence imaging that limits fluorescence collection. The key novelty in this research is the incorporation of phase element in the design for chromatic correction at 830nm and 520nm. A homogenous spot radius of 1.5μm is achieved at multiple tilt angles (±3°) of the scan mirror resulting in a field of view of 142μm.
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