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3D-Holographic Microendoscopy
1. Objective: microendoscopic 3D-imaging
Holographic microendoscopic topography measurement is a cutting-edge technique designed for high-resolution 3D-imaging of microscopic surfaces within confined or hard-to-reach areas. By combining holography and endoscopy, this method enables precise, non-invasive analysis of delicate structures, such as biological tissues, or small industrial components. With the ability to access spaces that traditional tools cannot, it provides accurate surface topography in real-time, making it invaluable for both medical diagnostics and industrial quality control.
2. Background
High accuracy 3D-printing
3D-printed micro-optics using two-photon polymerization (2PP) represents a breakthrough in the fabrication of highly precise and intricate optical components at the microscale. This advanced technique leverages the nonlinear absorption of two-photon polymerization to create complex 3D-structures with sub-micron resolution, enabling the production of micro-optics with unparalleled detail and accuracy. Ideal for applications in photonics, medical devices, and advanced imaging systems, 2PP allows for the rapid prototyping and customization of micro-optical elements, pushing the boundaries of what is possible in modern optics.
Courtesy of Dr. Simon Thiele, Printoptix GmbH.
Digital holography, imaging fibers and 3D-printed micro-optics
Holographic microendoscopy combines the principles of two-wavelength digital holography with 3D-printed micro-optics and ultra-thin imaging fiber bundles. This integration enables the creation of highly compact endoscopic systems with an overall diameter of less than 500 microns. Such miniaturization allows for high-resolution, 3D-imaging in extremely confined spaces, making it ideal for applications in biomedical diagnostics and advanced micro-scale inspections.
3. System development
Optical design
The optical design of the two-component lens system printed onto the imaging fiber is crafted to optimize light coupling into the fiber cores. This lens system features a planar surface on both the front optical surface and the last surface, where it connects to the entrance facet of the imaging fiber. The two intermediate surfaces are aspheric, in order to correct aberrations and focus light effectively. The design is image-sided telecentric, ensuring that the light enters the imaging fiber cores at consistent angles, maximizing the efficiency and accuracy of light transmission through the fiber. This telecentricity is crucial for maintaining proper alignment and coupling, leading to high-quality imaging performance.
Mechanical design
The 3D-printed lens system is securely held in place and protected by a metal sleeve, ensuring its stability and alignment within the microendoscope assembly. Parallel to the imaging fiber, a single-mode fiber is integrated for illumination, which is equipped with a 3D-printed beam-shaping lens designed to deliver uniform illumination across the target area. The mechanical configuration is meticulously crafted to maintain a small overall diameter while enabling high-resolution imaging, making the microendoscope suitable for applications in confined and challenging environments.
Optical performance validation
The optical performance validation of the microendoscope is a critical step to ensure the system’s capability to deliver high-resolution imaging. To evaluate the resolving power of the 3D-printed micro lens system, a USAF resolution target is employed. This target allows for precise measurement of the system’s ability to distinguish fine details. The performance of the lens system is considered adequate when it resolves details down to the limit imposed by the individual imaging fiber cores or pixels. This ensures that the lens system is optimized to its fullest potential, providing the highest possible resolution before the inherent pixelation of the fiber bundle becomes the limiting factor.
Prototype setup
The prototype setup of the holographic microendoscope is designed to perform high-resolution 3D-imaging using two-wavelength holography. The setup employs two stabilized lasers operating at around 780 nm. An off-axis holographic setup is used to achieve spatial phase-shifting. The optical wave fields of both wavelengths are reconstructed and computationally combined, generating a synthetic wavelength in the millimeter range due to the slight difference in optical wavelengths. This synthetic wavelength is ideal for probing the test object. With phase unwrapping algorithms, the topography of the test object can be accurately reconstructed.
In this setup, a pyramid with a known geometry serves as the test object, allowing for the validation of the system’s ability to capture detailed surface features and reconstruct the 3D-shape with high fidelity.
4. Results
The results demonstrate the accurate reconstruction of the pyramid’s surface, confirming the effectiveness of the holographic microendoscope setup. The system successfully captured the fine details of the pyramid’s geometry, with the reconstructed surface closely matching the known dimensions and angles. This precision highlights the capability of the optical system and phase unwrapping algorithms to deliver high-resolution topographic measurements.
