Manufacturing of 3D-printed micro optics using two photon lithography (2PL) has been advancing rapidly over the last decade, enabling production of high-performance micro optics. Among many more, 3D-printed miniaturized sensors, imaging optics, OCT systems, spectrometers and optical tweezers appear to be promising for application in the biomedical field. Here, immersion of optical systems into aqueous solutions is required regularly, hence capsulation for protection of the optical system's interior is required. Yet, specific properties of the 2PL fabrication process render capsulation of fabricated optics a delicate task.
In this talk, we outline a wholistic design strategy for 3D-printed immersion micro optics. The optical design and the mechanical manufacturing process are addressed, as well as approaches to combine metrology and simulation techniques for accurate assessment and performance optimization of manufactured systems. The feasibility of the proposed concept is experimentally validated. We discuss current limitations and evaluate the future potential of 3D-printed immersion micro optics.
The smaller the diameter of an endoscope, the greater its potential for minimally invasive surgical treatments. In conventional, flexible (fiber-based) endoscopes with small diameters (<200 µm), imaging is severely limited by the number of fiber cores. Due to this limitation, the image is pixelated. In this work, an engineering approach is used to increase the number of pixels by spectral multiplexing. However, this requires very small color-splitting optical systems at the distal end of the endoscope, i.e., at the body-facing end of the fiber. Such small dispersive optical systems are practically impossible to produce directly on the fiber using conventional techniques. Therefore, the idea is implemented using 3D-printed micro-optics. Preliminary work has shown that multiphoton lithography (fs DLW) is capable of producing imaging and color splitting systems on this size scale.
We present the optical design, fabrication and test of a fiber core multiplexing endoscope with a diameter of only 160 µm. Single-shot resolution enhancement is demonstrated by imaging of a USAF test chart and biological samples.
Additive manufacturing of microoptics using two-photon-lithography has been a rapidly advancing field of technology. Striving for ever more sophisticated optical systems prerequisites the access to appropriately fast and accurate wave-optical simulation methods to predict their optical performance. A simulation routine, which has been proven well suited for simulation of a vast range of 3D-printed microoptical systems, is the wave propagation method (WPM). Nevertheless, limitations in applicability remain due to the restriction on scalar electromagnetic fields, which prohibits consideration of polarization and thereby also the calculation of backward reflection at optical interfaces. Capabilities for design and analyses are, therefore, impaired for 3D-printed optical systems using those properties as key features in their design. As a first step to overcome those limitations, we presented new simulation methods based on the WPM in previous publications, extending its applicability toward simulation of vector electric fields, while maintaining short-simulation runtime. We focus on elaborating the practical application and integration of previously presented simulation methods in the design of complex 3D-printed optical systems. With it, we demonstrate the consideration of polarization and backward reflections in simulations far beyond paraxial and thin element approximations.
Atherosclerosis is characterised by the buildup of plaque in the inner lining of vessels. Unstable atherosclerotic plaques are prone to rupture, which can lead to heart attacks or strokes. Current clinically available imaging techniques cannot accurately identify unstable plaques based on their structural features. It has been previously reported that unstable plaques may emit autofluorescence. Therefore, our team has developed a dual modality optical coherence tomography and fluorescence intravascular imaging system, and a miniaturised fibre-optic probe incorporating a 3D printed lens. The 3D printed lens is designed to correct the astigmatism that arises from the intravascular catheter, and to reduce stray light. Although the 3D printed lens is made by a polymer photoresist, it has low background fluorescence. We are using our novel fibre probe to explore the autofluorescence present in unstable human plaques.
Additive manufacturing of micro-optics using Two-Photon-Lithography (2PL) has been a rapidly advancing field of technology. Striving for ever more sophisticated optical systems prerequisites the access to appropriately fast and accurate wave-optical simulation methods to predict their optical performance. A simulation routine, which has been proven well suited for simulation of a vast range of 3D-printed micro-optical systems, is the Wave Propagation Method (WPM). Nevertheless, limitations in applicability remain due to the restriction on scalar electromagnetic fields, which prohibits e.g. consideration of polarization and thereby also the calculation of backwards reflection at optical interfaces. Capabilities for design and analyses are therefore impaired for 3D-printed optical systems using those properties as key features in their design. As a first step to overcome those limitations, we presented new simulation methods based on the WPM in previous publications, extending its applicability towards simulation of vector electric fields, while maintaining short simulation runtime. In the present manuscript, we focus on elaborating the practical application and integration of the previously presented simulation methods in the design of complex 3D-printed optical systems. With it, we demonstrate the consideration of polarization and backward reflections in simulations far beyond paraxial and thin element approximations.
Aperiodic volume optics have been shown to be multiplexed optical devices that can produce different targeted output distributions from the same volume, only depending on the incoming illumination condition. These structures however have so far been produced inside of rigid glass substrates by introducing targeted microfractures through a focused laser. This makes following preparation steps necessary for any real-world applications, before even considering the illumination setup. We present a step towards the direct, additive manufacturing of multiplexed volume scattering devices by the creation of 3-dimensional structures using two-photon-polymerization, which is capable of creating complex optical microsystems, and their wave-optical simulation.
Preclinical studies and clinical diagnostics increasingly rely on optical techniques to visualize internal organs. Miniaturised catheters or endoscopes are necessary for imaging small and/or delicate arteries. However, current lens fabrication methods limit the performance of these ultrathin devices, resulting in a poor combination of resolution, depth of focus and multimodal imaging capability. This talk will introduce our latest research to address these combined challenges. In particular, we have utilized 3D micro-printing technology to fabricate freeform optics directly onto an optical fiber to achieve freeform designs for aberration-corrected optical coherence tomography (OCT) and to enable highly-sensitive multimodal fluorescence+OCT imaging in vivo.
3D micro-printing enables the formation of complex shapes at a print resolution of tens of nanometers using two-photon lithography. Our team have explored the use of 3D micro-printing to fabricate complex optical geometries directly on the end of a fiber. In this work, we have developed a lens-in-lens design that enables in-vivo high-sensitivity fluorescence and OCT imaging through a single-fiber-based probe, with a diameter <550 microns including a protective catheter sheath. In a parallel work, we have also fabricated a side-facing OCT fiber probe (<300 microns diameter) creating an elongated Bessel beam and simultaneously correct for aberrations.
Femtosecond direct laser writing is a key enabling technology for complex microoptics. Imaging and illumination applications have impressively been demonstrated in the past. Here, we take 3D-printed microoptics one step further and assess the feasibility of complex microoptical systems: From a pinhole camera to a spectrometer.
The first step in successful realization of complex microoptical systems are specialized measurement techniques that match both fabrication and simulation methods. Different setups are presented to reach stray light control, isolate topographic effects, and measure the efficiency of diffractive structures with small lateral extensions. All methods are easy to implement and can be key to targeted optimization of complex systems.
In a second step, the spectrum of corresponding fabrication methods to fsDLW is extended by the microfluidic addition of a functional substance. We show that the incorporation of microfluidic channels into the 3D-printed mounting structures can be used to absorb a non-transparent fluid to create aperutres. Thus, a 3D-printed micro-pinhole camera can be demonstrated.
Finally, all learnings and methods from these studies are combined to create complex microoptical systems. Multiple concepts of ultra-compact 3D-printed wide-angle cameras are examined. A special focus is laid on optical and mechanical design, measurement and optimization of highly tilted refractive and catadioptric freeform surfaces. An iterative correction mechanism is developed to improve shape fidelity to realize first implementations of 180°×360° field of view multi-aperture imaging.
The highest complexity of a 3D-printed microoptical system is finally reached by the realization of an entire measurement system. The feasibility of a monolithic spectrometer in a volume of only 100 × 100 × 300 μm³ is theoretically and experimentally demonstrated. The results represent the first direct spectrometer in this miniature size range and unclose a new era of complex 3D-printed microoptical (measurement) systems, enabled by novel methods for charactarization, optimization and aperture fabrication.
Modern two-photon-polymerization 3D printing technology allows for the creation of almost arbitrary threedimensional structures for the production of complex freeform optical surfaces. While being highly controllable and accurate to below 100 nm some systematic deviation by volumetric changes during the polymerization and development process remains. This can however be corrected for when the surface deviation is known. We present a method to include repeatable measurements and the consequent shape correction during the production process of monolithically created complex freeform lens systems. Measurement concepts as well as consequences to shape improvements are shown. An example for the application of such corrections for the creation of low profile multi-aperture large field of view objectives is presented.
We demonstrate the fabrication of optical elements on the millimeter scale by stitching-free 3D printing via two-photon polymerization. Previous limitations are overcome by the use of a large writing field objective as well as a novel high transparency resist. The printed optical components are free of stitching defects due to a single step exposure and exhibit an unpreceded glass-like appearance due to the low absorption of the resist material throughout the entire visible wavelength range. We print aspherical focusing lenses, characterize and optimize their shape fidelity, and find their optical performance close to the simulated optimum, demonstrating the superior performance of our fabrication. For comparison with commercially available glass lenses we also fabricate spherical half-ball lenses of different sizes. The imaging quality of the lenses is very similar, underpinning the powerfulness of our fabrication strategy.
One of the most important advantages of optical manufacturing by 3D printing is the high degree of freedom in geometry and optical design. This is especially true for fabrication methods like stereolithography which, in contrast to jet or extrusion based methods, usually enable true 3D geometries with undercuts and buried cavities. In case of multiphoton lithography such complex designs can additionally be manufactured with sub-wavelength feature sizes in all three dimensions. This enables optical designs with unmatched complexity combining reflective, refractive and diffractive surfaces as well as structures like photonic crystals in all 3 dimensions.
In this work we introduce different strategies to show how the barely restricted design space can be used to realize compact 3D-printed micro-optics with strong optical performance. Different types of concentration and beam shaping devices for non-imaging purposes are introduced and refractive, diffractive, reflective and hybrid variants are discussed. While these devices serve to transfer one light distribution into another they are not suited for direct imaging.
In order to image extended objects, lenses or lens systems are commonly optimized for a maximum space-bandwidth product which is connected to the product of imaging numerical aperture and image height. The space-bandwidth product is also correlating with the number of distinguishable image points which are transferred through a system. We demonstrate how maximum space-bandwidth product multi-lens optical systems can be designed and realized. Different variants of refractive, purely diffractive and hybrid imaging systems with lens barrel diameters below 500 µm are demonstrated and compared in terms of optical performance and manufacturability.
Modern 3D-printing technologies allow for the production of very small and complex microscale optical systems, which require examination of wave-optical effects within their design process. Common optical design software employs ray-tracing as their basic concept, which is not capable of fully simulating wave-optical effects. Rigorous methods, which solve electromagnetic field equations, are very time consuming and require more proficiency of the users. We present an application of the Wave Propagation Method (WPM), which is able to accurately simulate wave-optical effects in forward direction, while providing a reasonably fast calculation time utilizing both central- and graphics processing units (CPU and GPU) as an easy-to-use plugin for the open-source software ITOM.
The performance of microlenses fabricated via femtosecond direct laser writing is insuperably limited with regards to image contrast within its all-transparent material system. Our proposed super-fine inkjet process for the realization of apertures and non-transparent hulls both shields stray light and adds additional optical design parameters, namely diameter, shape, and position of the aperture. A micropinhole camera, a high-contrast imaging microlens, and a telecentric system are demonstrated. Our method is most suitable for 3D-printed micro-optics due to its one-step integration and auto-adjustment of the aperture position and can be useful for highly precise integration of functional materials beyond mere non-transparency.
We present a concept for virtual reality (VR) headsets which is inspired by the design of the human eye itself. By using a rotatable display system which resembles a mechanical copy of the eye, we achieve a high resolution at the foveal spot and lower resolution in the periphery while maintaining a large field of view. Fast and accurate retinal eye tracking by observing the blind spot on the fovea centralis is possible with this solution. The vergence-accomodation conflict can be solved potentially by integrating an off-the-shelf tunable lens.
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