The generation of diffractive optical elements often requires time and cost consuming production techniques such as photolithography. Especially in research and development, small series of diffractive microstructures are needed and flexible and cost effective fabrication techniques are desirable to enable the fabrication of versatile optical elements on a short time scale. In this work, we introduce a novel process chain for fabrication of diffractive optical elements in various polymers. It is based on a maskless lithography process step, where a computer generated image of the optical element is projected via a digital mirror device and a microscope setup onto a silicon wafer coated with photosensitive resist. In addition, a stitching process allows us to microstructure a large area on the wafer. After development, a soft stamp of the microstructure is made from Polydimethylsiloxane, which is used as a mold for the subsequent hot embossing process, where the final diffractive optical element is replicated into thermoplastic polymer. Experimental results are presented, which demonstrate the applicability of the process.
Dermoscopy systems that do not require direct skin contact open up the possibility to overcome limitations of established dermoscopes and promote application of this diagnostic tool beyond skin cancer screening. In this work, we present a non-contact remote digital dermoscope with large field of view, high resolution and true color imaging. We report preliminary clinical results for imaging of melanocytic nevi and two common inflammatory skin diseases, psoriasis and lichen ruber planus, obtained in non-contact mode demonstrating the potential of this method for skin cancer screening and for diagnosis of inflammatory disease. In the future, the system can be applied for whole-body automated cancer screening and support diagnostics of inflammatory skin diseases in the form of a hand-held device.
We propose an architecture with a remote phosphor-based modular and compact light-emitting diode (LED) light source in a noncontact dermoscope prototype for skin cancer screening. The spectrum and color temperature of the output light can easily and significantly be changed depending on spectral absorption characteristics of the tissues being imaged. The new system has several advantages compared to state-of-the-art phosphor converted ultrabright white LEDs, used in a wide range of medical imaging devices, which have a fixed spectrum and color temperature at a given operating point. In particular, the system can more easily be adapted to the requirements originating from different tissues in the human body, which have wavelength-dependent absorption and reflectivity. This leads to improved contrast for different kinds of imaged tissue components. The concept of such a lighting architecture can be vastly utilized in many other medical imaging devices including endoscopic systems.
Abstract. 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.
Long-period fiber gratings (LPGs) are well known for their sensitivity to external influences, which make them interesting for a large number of sensing applications. For these applications, fibers with a high numerical aperture (i.e., fibers with highly germanium (Ge)-doped fused silica fiber cores) are more attractive since they are intrinsically photosensitive, as well as less sensitive to bend- and microbend-induced light attenuations. In this work, we introduce a novel method to inscribe LPGs into highly Ge-doped, single-mode fibers. By tapering the optical fiber, and thus, tailoring the effective indices of the core and cladding modes, for the first time, an LPG was inscribed into such fibers using the amplitude mask technique and a KrF excimer laser. Based on this novel method, sensitive LPG-based fiber optic sensors only a few millimeters in length can be incorporated in bend-insensitive fibers for use in various monitoring applications. Moreover, by applying the described inscription method, the LPG spectrum can be influenced and tailored according to the specific demands of a particular application.
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