Fast bidirectional vector wave propagation method showcased on targeted noise reduction in imaging fiber bundles using 3D-printed micro optics.

In order to extend simulation capabilities for reflective and catadioptric 3D-printed micro optics, we present a fast bidirectional vector wave propagation method (BWPM). Contrary to established fast simulation methods like the wave propagation method (WPM), the BWPM allows for the additional consideration of reflected and backwards propagating electric fields. We study the convergence of the BWPM and investigate relevant simulation examples. Especially, the BWPM is used for evaluation of 3D-printed index matching caps (IMCs) in order to suppress back reflected light in imaging fibers, used for keyhole access endoscopy. Simulations studying the viability of IMCs are followed up with experimental investigations. We demonstrate that 3D-printed IMCs can be used to suppress noise caused by back reflected light, that otherwise would prohibit the use of imaging fibers in an epi-illumination configuration.

Fast algorithm for the simulation of 3D-printed microoptics based on the vector wave propagation method.

In this work, we propose the Fast Polarized Wave Propagation Method (FPWPM), which is an efficient method for vector wave optical simulations of microoptics. The FPWPM is capable of handling comparably large simulation volumes while maintaining quick runtime. This allows for real-world application of this method for the rapid development process of 3D-printed microoptics. By comparison to established routines like the rigorous coupled wave analysis (RCWA) or the Richards-Wolf-Integral, accuracy and superior runtime efficiency of the FPWPM are demonstrated by simulation of interfaces, gratings, and lenses. By considering polarization in simulations, the FPWPM facilitates the analysis of optical elements which employ this property of electromagnetic waves as a feature in their optical design, e.g., diffractive elements, gratings, or optics with high angle of incidence like high numerical aperture lenses.

3D-Printed Micro Lens-in-Lens for In Vivo Multimodal Microendoscopy.

Multimodal microendoscopes enable co-located structural and molecular measurements in vivo, thus providing useful insights into the pathological changes associated with disease. However, different optical imaging modalities often have conflicting optical requirements for optimal lens design. For example, a high numerical aperture (NA) lens is needed to realize high-sensitivity fluorescence measurements. In contrast, optical coherence tomography (OCT) demands a low NA to achieve a large depth of focus. These competing requirements present a significant challenge in the design and fabrication of miniaturized imaging probes that are capable of supporting high-quality multiple modalities simultaneously. An optical design is demonstrated which uses two-photon 3D printing to create a miniaturized lens that is simultaneously optimized for these conflicting imaging modalities. The lens-in-lens design contains distinct but connected optical surfaces that separately address the needs of both fluorescence and OCT imaging within a lens of 330 µm diameter. This design shows an improvement in fluorescence sensitivity of >10x in contrast to more conventional fiber-optic design approaches. This lens-in-lens is then integrated into an intravascular catheter probe with a diameter of 520 µm. The first simultaneous intravascular OCT and fluorescence imaging of a mouse artery in vivo is reported.

Ultra-compact 3D-printed wide-angle cameras realized by multi-aperture freeform optical design.

Simultaneous realization of ultra-large field of view (FOV), large lateral image size, and a small form factor is one of the challenges in imaging lens design and fabrication. All combined this yields an extensive flow of information while conserving ease of integration where space is limited. Here, we present concepts, correction methods and realizations towards freeform multi-aperture wide-angle cameras fabricated by femtosecond direct laser writing (fsDLW). The 3D printing process gives us the design freedom to create 180° × 360° cameras with a flat form factor in the micrometer range by splitting the FOV into several apertures. Highly tilted and decentered non-rotational lens shapes as well as catadioptric elements are used in the optical design to map the FOV onto a flat surface in a Scheimpflug manner. We present methods to measure and correct freeform surfaces with up to 180° surface normals by confocal measurements, and iterative fabrication via fsDLW. Finally, approaches for digital distortion correction and image stitching are demonstrated and two realizations of freeform multi-aperture wide-angle cameras are presented.

Ultrathin monolithic 3D printed optical coherence tomography endoscopy for preclinical and clinical use.

Preclinical and clinical diagnostics increasingly rely on techniques to visualize internal organs at high resolution via endoscopes. Miniaturized endoscopic probes are necessary for imaging small luminal or delicate organs without causing trauma to tissue. However, current fabrication methods limit the imaging performance of highly miniaturized probes, restricting their widespread application. To overcome this limitation, we developed a novel ultrathin probe fabrication technique that utilizes 3D microprinting to reliably create side-facing freeform micro-optics (<130 µm diameter) on single-mode fibers. Using this technique, we built a fully functional ultrathin aberration-corrected optical coherence tomography probe. This is the smallest freeform 3D imaging probe yet reported, with a diameter of 0.457 mm, including the catheter sheath. We demonstrated image quality and mechanical flexibility by imaging atherosclerotic human and mouse arteries. The ability to provide microstructural information with the smallest optical coherence tomography catheter opens a gateway for novel minimally invasive applications in disease.

Mass-producible micro-optical elements by injection compression molding and focused ion beam structured titanium molding tools.

We demonstrate mass production compatible fabrication of polymer-based micro Fresnel lenses by injection compression molding. The extremely robust titanium-molding tool is structured with high precision by focused ion beam milling. In order to achieve optimal shape accuracy in the titanium we use an iterative design optimization. The inverse Fresnel lens structured into the titanium is transferred to polymers by injection compression molding, enabling rapid mass replication. We show that the optical performance of the molded diffractive Fresnel lenses is in good agreement with simulations, rendering our approach suitable for applications that require compact and high-quality optical elements in large numbers.

3D printed stacked diffractive microlenses.

Planar lenses such as metalenses and diffractive lenses exhibit severe field-dependent aberrations when imaging extended objects with high numerical aperture. This problem can be overcome by stacking at least two of such devices on top of each other. In this work, we present such stacked imaging systems, namely doublets and triplets of diffractive optical elements. They are fabricated by femtosecond direct laser writing in one single step without the need for alignment in sizes of below 200 µm in diameter and 100 µm in height. The lenses allow for efficient sub µm resolution imaging at visible wavelengths combined with a full field-of-view of up to 60°. As additional benefit, our approach dramatically reduces the writing times of 3D printed lens systems to below 15 minutes.

Alignment-free integration of apertures and nontransparent hulls into 3D-printed micro-optics.

The fabrication of 3D-printed micro-optical systems by femtosecond direct laser writing is state of the art. However, the inherent transparency of the lens mount, which is also made of photopolymer, causes a degradation of the image contrast due to stray light and scattering. Furthermore, apertures play a key role in optical design but cannot be directly integrated during 3D printing. Here, we present a superfine inkjet process for targeted filling of 3D-printed cavities in order to integrate apertures and nontransparent hulls without any alignment. Considerable contrast improvement and micro-optical systems with increased functionality are demonstrated.

3D-printed eagle eye: Compound microlens system for foveated imaging.

We present a highly miniaturized camera, mimicking the natural vision of predators, by 3D-printing different multilens objectives directly onto a complementary metal-oxide semiconductor (CMOS) image sensor. Our system combines four printed doublet lenses with different focal lengths (equivalent to f = 31 to 123 mm for a 35-mm film) in a 2 × 2 arrangement to achieve a full field of view of 70° with an increasing angular resolution of up to 2 cycles/deg field of view in the center of the image. The footprint of the optics on the chip is below 300 µm × 300 µm, whereas their height is <200 µm. Because the four lenses are printed in one single step without the necessity for any further assembling or alignment, this approach allows for fast design iterations and can lead to a plethora of different miniaturized multiaperture imaging systems with applications in fields such as endoscopy, optical metrology, optical sensing, surveillance drones, or security.

Alternate optical designs for head-mounted displays with a wide field of view.

The most widely applied design form for mixed reality head-mounted display (HMD) systems is generally a prism with one surface in total internal reflection (TIR). This, however, limits the angle of the incident rays, and thus decreases the design freedom and affects the performance. To obtain better performance of the HMD optics, in this paper two seldom used design forms of HMD systems are presented and compared to the standard TIR HMD optics. One of them is a catadioptric HMD system, consisting of one lens and two mirrors; the other is a prism HMD with a different folding geometry. The designs are compared for a field of view of 40°×30°; however, they are also investigated for an increased field of view of 50°×30°. The evaluation indicates good performance of our systems. In particular, the prism with an alternate folding geometry has advantages in both performance and size.

Generalized Aldis theorem for calculating aberration contributions in freeform systems.

Compact folded imaging systems often require freeform surfaces to correct astigmatic and other off-axis aberrations. However, aberration theory for non-rotational symmetric systems is quite complex and it is especially hard to quantify individual surface aberration contributions. In this paper we develop a matrix method based on the propagation of a differential ray pair, which allows determining the aberration contribution of each individual surface for any ray. We can mathematically prove that the sum of the aberrations is identical to the exact ray-tracing result at the image plane. A head-mounted display lens is employed for testing and verification of this method. As will be shown, the method proves to be a universal tool for aberration calculations within freeform system.

Ultra-compact on-chip LED collimation optics by 3D femtosecond direct laser writing.

By using two-photon lithographic 3D printing, we demonstrate additive manufacturing of a dielectric concentrator directly on a LED chip. With a size of below 200 µm in diameter and length, light output is increased by a factor of 6.2 in collimation direction, while the emission half-angle is reduced by 50%. We measure excellent form fidelity and irradiance patterns close to simulation. Additionally, a more complex shape design is presented, which exhibits a nonconventional triangular illumination pattern. The introduced method features exceptional design freedoms which can be used to tailor high-quality miniature illumination optics for specific lighting tasks, for example, endoscopy.

High order surface aberration contributions from phase space analysis of differential rays.

Phase space methods are very popular for illumination systems or paraxial system analysis. In this paper it will be shown that it is also a promising tool to visualize and quantify surface aberration contributions, including all orders. The method is based on the calculation and propagation of a differential ray pair. In order to validate the method we compare to Aldis calculus, an exact method to determine high order aberrations in rotational symmetric systems. A triplet lens is used as an example to visualize the results. The analysis indicates that the phase space method is a very good approximation to Aldis calculus and moreover it is not limited to any symmetry assumptions.

In situ microscopy using adjustment-free optics.

In the past years, in situ microscopy has been demonstrated as a technique for monitoring the concentration and morphology of moving microparticles in agitated suspensions. However, up until now, this technique can only achieve a high resolution if a certain manual or automated effort is established for continuous precise focusing. Therefore, the application of in situ microscopes (ISMs) as sensors is inhibited in the cases where unattended operation is required. Here, we demonstrate a high-resolution ISM which, unlike others, is built as an entirely rigid construction, requiring no adjustments at all. This ISM is based on a specially designed water immersion objective with numerical aperture = 0.75 and a working distance of 15 µm. The objective can be built exclusively from off-the-shelf parts and the front surface directly interfaces with the moving suspension. We show various applications of the system and demonstrate the imaging performance with submicron resolution within moving suspensions of microorganisms.