High-speed endoscopic optical coherence tomography (OCT) imaging in the MHz range has shown great potential in various medical applications ranging from cancer screening to vascular disease monitoring. High-speed imaging always suffers from non-uniform rotational distortion (NURD) due to asynchronous motor rotation with the OCT system. Several research groups have previously attempted to solve this problem, using either an expensive motor with a sensor or numerical correction after data acquisition. However, both techniques pose challenges for practical use. Therefore, in this study, we use an inexpensive sensorless brushless DC motor with a Fourier domain mode-locked (FDML) laser-based MHz OCT system and try to resolve the problem of synchronization using three different modalities, (i) Slave-mode: The FDML frequency serves as a master frequency for the motor, which is phase-locked to the FDML frequency, (ii) Master-mode: The revolution trigger obtained from the motor’s back electromotive force (BEMF) signal serves as a trigger signal for the OCT imaging system, (iii) Both: Fully synchronized setup, where the motor rotation is synchronized with the laser and the imaging system is synchronized with the motor to achieve phase-stable OCT imaging. The first case slightly fluctuates in live preview and imaging due to the absence of a revolution trigger, while the second has varying motor speeds. Therefore, we use the third case to phase-lock the motor with FDML and get a distortion-free live preview and image acquisition. Finally, we demonstrate high-speed SS-OCT structural imaging (at 3.3 MHz A-scan rates) of a finger with a 16 mm diameter probe (at 40,000 rpm).
Straight and bent self-written waveguides (SWWs) are formed within a photomonomer mixture by means of a self-trapping effect when a single laser beam or two laser beams with tilt are propagated inside. These SWWs can be used as optical interconnects in integrated photonic circuits if two laser beams are launched in opposite directions into the photomonomer. In this work, two kinds of photo-polymerization models are implemented to simulate the SWWs. In the phenomenological model, the refractive index increases directly with actinic laser intensity, whereas the diffusion model has a more complex variation of refractive index profile which takes into account the individual redistribution of mixture components. Both these models are linked with a CrankNicholson based Beam Propagation Method (CN-BPM) to simulate the time varying light distribution within the polymer coupling structures. Differences are observed in the numerical simulation results for straight and bent SWWs with respect to the temporal evolution of refractive index within the mixture, corresponding beam intensity profiles and curing time. In addition, we show that a saturation of refractive index change leads to the polymerization of surrounding monomer and, as consequence, to corrupted light guiding. We report on the minimum refractive index modulation that is required for optimal light guiding within the SWW.
Highly-functional photonic sensor networks integrated in thin polymer foils offer great potential for versatile applications in the life sciences, medicine, environmental analytics or production technology. For their realization, suitable low-cost and high-throughput production techniques need to be developed. Here, we describe work towards this goal, i.e. the fabrication of multimode polymer waveguides through a combination of thermal imprint and doctor blading. For imprint master stamp fabrication, a combined Bosch and O2 plasma etching process in silicon is utilized. We also demonstrate stamp fabrication by an additive manufacturing method, i.e. by employing maskless UV lithography, to enhance the flexibility and cost-effectiveness of our approach. We, thus, realize various all-polymer waveguide arrays, beam splitters, and grating couplers which serve as basic elements to create more complex photonic circuits. We also demonstrate polymer based transmission lines comprising semiconductor as well as organic light sources and detectors. We discuss both the integration of semiconductor light sources and detectors such as verticalcavity surface-emitting lasers (VCSEL) and photo detectors as well as organic light emitting diodes (OLEDs) and organic photo detectors. In first applications, we combine these elements to create sensor arrays for measuring temperature, strain or refractive index. We show results of various sensor types utilizing different measurement principles implemented in laboratory environments so far. For example, a waveguide array containing a linear discontinuity which serves as elongation zone for displacement, strain or tilt measurement by detecting the intensity variation of the transmitted light propagating inside the structure is presented. In future, we plan to create more powerful sensor photonics networks for reliable and robust applications in real life, e.g. for point-of-care testing or production monitoring.
While automatic hot embossing systems are available for large- and small-scale productions of polymeric devices, one of the process challenges remains to be the manufacturing of precise, durable, and yet inexpensive hot embossing stamps. The use of metallic stamps manufactured by electroplating a photoresist pattern or by precision milling and their replication into silicone molds with UV-lithography, electroplating, and molding techniques is state of the art. Yet, there have been few, if any, thriving attempts to directly emboss polymers by means of bare photoresists, and in particular polyimide-based photoresists, without transferring the photoresist patterns into a different stamp material. We conduct a proof-of-concept by developing hot embossing stamps based on photosensitive polyimide. We focus primarily on the reliability of the aforementioned stamps throughout the hot embossing cycle and the fidelity of pattern transfer onto polymeric films for different microstructural patterns.
Christian Kelb, Raimund Rother, Anne-Katrin Schuler, Moritz Hinkelmann, Maik Rahlves, Oswald Prucker, Claas Müller, Jürgen Rühe, Eduard Reithmeier, Bernhard Roth
We demonstrate the manufacturing of embedded multimode optical waveguides through linking of polymethylmethacrylate (PMMA) foils and cyclic olefin polymer (COP) filaments based on a lamination process. Since the two polymeric materials cannot be fused together through interdiffusion of polymer chains, we utilize a reactive lamination agent based on PMMA copolymers containing photoreactive 2-acryloyloxyanthraquinone units, which allows the creation of monolithic PMMA-COP substrates through C-H insertion reactions across the interface between the two materials. We elucidate the lamination process and evaluate the chemical link between filament and foils by carrying out extraction tests with a custom-built tensile testing machine. We also show attenuation measurements of the manufactured waveguides for different manufacturing parameters. The lamination process is in particular suited for large-scale and low-cost fabrication of board-level devices with optical waveguides or other micro-optical structures, e.g., optofluidic devices.
Melanoma skin cancer has one of the highest mortality rates of all types of cancer if not detected at an early stage. The survival rate is highly dependent on its penetration depth, which is commonly determined by histopathology. In this work, we aim at combining optical coherence tomography and optoacoustic as a non-invasive all-optical method to measure the penetration depth of melanoma. We present our recent achievements to setup a handheld multimodal device and also results from first in vivo measurements on healthy and cancerous skin tissue, which are compared to measurements obtained by ultrasound and histopathology.
Micro-optical sensors based on optical waveguides are widely used to measure temperature, force and strain but also to detect biological and chemical substances such as explosives or toxins. While optical micro-sensors based on silicon technology require complex and expensive process technologies, a new generation of sensors based completely on polymers offer advantages especially in terms of low-cost and fast production techniques. We have developed a process to integrate micro-optical components such as embedded waveguides and optical interconnects into polymer foils with a thickness well below one millimeter. To enable high throughput production, we employ hot embossing technology, which is capable of reel-to-reel fabrication with a surface roughness in the optical range. For the waveguide fabrication, we used the thermoplastic polymethylmethacrylate (PMMA) as cladding and several optical adhesives as core materials. The waveguides are characterized with respect to refractive indices and propagation losses. We achieved propagation losses are as low as 0.3 dB/cm. Furthermore, we demonstrate coupling structures and their fabrication especially suited to integrate various light sources such as vertical-cavity surface-emitting lasers (VCSEL) and organic light emitting diodes (OLED) into thin polymer foils. Also, we present a concept of an all-polymer and waveguide based deformation sensor based on intensity modulation, which can be fabricated by utilizing our process. For future application, we aim at a low-cost and high-throughput reel-to-reel production process enabling the fabrication of large sensor arrays or disposable single-use sensing structures, which will open optical sensing to a large variety of application fields ranging from medical diagnosis to automotive sensing.
We present our recent results on the fabrication of photonic devices such as single-mode and few-mode waveguides, Ycouplers as well as integrated interferometric sensor devices. The devices were created by means of a fabrication method based on maskless lithography, which allows for fabricating embedded integrated polymer elements on a scale of several square centimeters with a resolution down to one micron. We demonstrate the versatility of our approach by presenting first results on photonic structures created by maskless lithography.
Polymer-based holographic and diffractive optical elements have gained increasing interest due to their potential to be used in a broad range of applications, such as illumination technology, micro-optics, and holography. We present a production process to fabricate polymer-based diffractive optical elements and holograms. The process is based on maskless lithography, which is used to fabricate optical elements in photoresist. We discuss several lab-level lithography setups based on digital mirror devices and liquid crystal devices with respect to illumination efficiency, resolution, and contrast. The entire optical setup is designed with emphasis on low-cost components, which can be easily implemented in an optical research lab. In a first step, a copy of the microstructures is replicated into optical polymeric materials by means of a soft stamp hot embossing process. The soft stamp is made from polydimethylsiloxan, which is coated onto the microstructure in the photoresist. The hot embossing process is carried out by a self-made and low-cost hot embossing machine. We present confocal topography measurements to quantify the replication accuracy of the process and demonstrate diffractive optical elements and holographic structures, which were fabricated using the process presented.
Polymer based diffractive optical elements have gained increasing interest due to their potential to be used in various applications such as illumination technology, micro optics and holography. We present a novel production process to fabricate polymer based diffractive optical elements and holograms. The process is based on maskless lithography, which is used to fabricate optical elements in photoresist. We discuss several lab-made lithography setups based on digital mirror devices and liquid crystal devices with respect to light efficiency, resolution and contrast. The whole optical setup is designed with an emphasis on low-cost setups, which can be easily implemented in an optical research lab. In a subsequent step, a copy of the microstructures is easily replicated into optical polymeric materials by means of a soft stamp hot embossing process step. The soft stamp is made from Polydimethylsiloxan, which is coated onto the microstructure in resist. The hot embossing process is carried out by a self-made and low-cost hot embossing machine. We present confocal topography measurements to quantify the replication accuracy of the process and demonstrate diffractive optical elements and holographic structures, which were fabricated using the process presented.
KEYWORDS: Optical coherence tomography, Calibration, Distortion, Mirrors, Data modeling, Geometrical optics, 3D modeling, Mathematical modeling, 3D metrology, Surgery
This contribution compares two approaches for the geometric calibration of an optical coherence tomography
(OCT) which forms part of a medical navigation system. For this purpose, a one step and a multi step calibration
is performed with a self-produced 3D reference structure and a high-accurate 6 degrees of freedom (DoF)
parallel robot, respectively. These 3D landmark-based geometric calibrations are based on the identification of
a parameterized grey-box OCT model. We show in experimental results that both methods reduce systematic
errors by more than one order of magnitude.
KEYWORDS: Optical coherence tomography, Calibration, Distortion, Data modeling, Mirrors, 3D modeling, 3D metrology, Surgery, Electroluminescent displays, 3D image processing
This article presents a one step geometric calibration for an optical coherence tomography (OCT) which forms part of a medical navigation system. The 3D landmark-based geometric calibration with a self-produced 3D reference structure is based on the identification of a parameterized grey-box OCT model. We show in experimental results by comparing common measurement errors in the field of medical surgery before and after calibration, that the proposed methodology reduces systematic errors by more than one order of magnitude. Due to its simplicity, the calibration can be carried out directly before a surgical intervention enhancing the OCT accuracy.
Rotating machinery parts, e. g. roller bearings or turbine engines, are present in many industrial applications.
Within this paper, a metrology system for determining the out-of-plane and in-plane deformation and vibration of
rotating objects is presented. The system consists of an optomechanical image derotator, which is combined with a
high speed camera and a scanning Laser-Doppler-Vibrometer (LDV). The image derotator is used to measure
deflection and vibration in a coordinate system fixed to the rotating object. To demonstrate the capability of this
measurement system, examples of high relevance in industrial applications are considered.
Electronic Speckle Pattern Interferometry with a light source of short coherence length allows depth-resolved deformation analysis below the surface of light-scattering objects (Low Coherence Speckle Interferometry - LCSI). Interference is tuned to a thin layer - called the coherence layer - by appropriate adjustment of the length of the reference path. The quality of the results is degraded by background light from outside the coherence layer and by de-correlations due to the passage of the useful light through regions that have been altered by the overall deformation field. Basic experimental studies are conducted on a simple two-interface object (two roughened surfaces of glass slides) to determine the effects that one interface (and its deformation) exerts on the quality of the deformation measurement in the other interface. Analytical theoretical calculations of speckle de-correlation on the basis of Fresnel diffraction provide comparative data.
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