This PDF file contains the front matter associated with SPIE Proceedings Volume 9130, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Apertures are basic elements which can be found in many optical systems. Since optical systems are continuously being miniaturized and integrated, there is a need for small and inexpensive apertures to control beam shape and light intensity. Current aperture concepts for the micrometer regime rely on moving MEMS lamella or controlling fluids by capillary or electrostatic forces. We demonstrate an aperture concept for single-wavelength operation based on thermal tuning of a segmented thin film resonator. Thermal tuning changes the optical thickness of the elastomer cavity. This allows for adjusting the intensity to any level between constructive and destructive interference in a specific aperture segment. In order to demonstrate aperture operation we simulate thermal, mechanical and optical properties using finite element method and transfer-matrix method. We confirm our simulation results by experimental beam shape measurements and spatially-resolved spectral transmission and light intensity measurements.

We report on the optimized design of a polymer-based actuator that can be directly integrated on a VCSEL for vertical beam scanning. Its operation principle is based on the vertical displacement of a SU-8 membrane including a polymer microlens. Under an applied thermal gradient, the membrane is shifted vertically due to thermal expansion in the actuation arms induced by Joule effect. This leads to a modification of microlens position and thus to a vertical scan of the laser beam. Membrane vertical displacements as high as 8μm for only 3V applied were recently experimentally obtained. To explain these performances, we developed a comprehensive tri-dimensional thermo-mechanical model that takes into account SU-8 material properties and precise MOEMS geometry. Out-of-plane mechanical coefficients and thermal conductivity were thus integrated in our 3D model (COMSOL Multiphysics). Vertical displacements extracted from these data for different actuation powers were successfully compared to experimental values, validating this modelling tool. Thereby, it was exploited to increase MOEMS electrothermal performance by a factor higher than 5.

Continuous optical zoom is an enabling feature for an endoscopic microscope system combining high resolution and large field-of-view (FOV) imaging. Conventional zoom systems utilize translating lenses to achieve optical zoom, but this further reduces the already narrow optical aperture available in the endoscopic probe. Liquid-tunable membrane lenses combine the actuator and optical aperture, and thus allow an efficient use of the available aperture. However, currently, such lenses suffer from significant spherical and other optical aberrations. In this paper, we present a miniaturized continuous zoom microscope design with an optical aperture of 1mm featuring two liquid-tunable aspheric lenses for optical zoom and two fixed lenses for aberration and color dispersion correction. The liquid-tunable aspherical lenses are formed by liquid reservoir sealed with a PDMS membrane of non-uniform thickness. A systematic approach to identify the membrane thickness profile yielding a desired deflection shapes also discussed. The continuous magnification range of the zoom system is from 1.5 to 3.5 using a 1×1mm CCD as the imaging element. With an object distance smaller than 3mm, the total length of the system is less than 9mm. We demonstrate that with the introduction of the aspherical tunable lenses, the optical performance of the zoom system is considerably improved. Keywords: Zoom optics, endomicroscopy, liquid-tunable lenses, aspherical microlenses.

The efficiency of a Bragg reflector design for implementation in optical resonators is highly dependent on the ratio between the high-index material and the low-index material used for the quarter-wavelength (QWOT) layers. A higher contrast implies that fewer layers are required to achieve a specified spectral selectivity over a wider spectral band. In turn, the reduced total thickness of the filter stack reduces the effect of optical absorption in the layers. The research presented here focuses on implementation of filters on top of silicon detectors that are already fabricated in a CMOS process. This implies that the constraints of process compatibility, such as the materials to be used, process temperature and cleanroom reentrance related to contamination, need to be considered. Silicon-dioxide is often used in CMOS-compatible designs, which has an index of refraction n~1.5, thus limiting n_Hi/n_Lo to about 2. This value can be improved by 50% when using air-films as the low-n material. Surface micromachining is used for the fabrication of such mirrors. Multiple layers of Si and SiO₂ were alternatingly deposited, and subsequently the Si layers are selectively removed in a sacrificial etch. The width of the λ/4 air-gaps is about 100 nm, which is narrower as compared to the typical layer thickness that is used in surface micromachining for conventional MEMS applications. Moreover, a demanding optical design requires more layers than typically used in a conventional MEMS device. Since the number of stacked layers is significantly higher as compared to the conventional MEMS, fabricating such filters is a challenge. However, unlike a conventional MEMS, electrical contacting to the structural layers is not required in optical filter application, which, eases the fabrication of such filters. This paper presents the design of several 4-layer structures for use in the visible spectral range, along with the fabrication sequence and preliminary measurement results.

In the last years, the triumphal march of handheld electronics with integrated cameras has opened amazing fields for small high performing optical systems. For this purpose miniaturized iris apertures are of practical importance because they are essential to control both the dynamic range of the imaging system and the depth of focus. Therefore, we invented a micro optical iris based on an electrochromic (EC) material. This material changes its absorption in response to an applied voltage. A coaxial arrangement of annular rings of the EC material is used to establish an iris aperture without need of any mechanical moving parts. The advantages of this device do not only arise from the space-saving design with a thickness of the device layer of 50μm. But it also benefits from low power consumption. In fact, its transmission state is stable in an open circuit, phrased memory effect. Only changes of the absorption require a voltage of up to 2 V. In contrast to mechanical iris apertures the absorption may be controlled on an analog scale offering the opportunity for apodization. These properties make our device the ideal candidate for battery powered and space-saving systems. We present optical measurements concerning control of the transmitted intensity and depth of focus, and studies dealing with switching times, light scattering, and stability. While the EC polymer used in this study still has limitations concerning color and contrast, the presented device features all functions of an iris aperture. In contrast to conventional devices it offers some special features. Owing to the variable chemistry of the EC material, its spectral response may be adjusted to certain applications like color filtering in different spectral regimes (UV, optical range, infrared). Furthermore, all segments may be switched individually to establish functions like spatial Fourier filtering or lateral tunable intensity filters.

Micro-ring resonators are important devices applicable for optical filtering phenomena. The paper provides the detailed description of general characteristics of serially coupled multiple ring resonator (SMRR). The identical perimeters and coupling coefficients provides the pass band characteristics with flatter top. The paper includes the concept of Masons gain formula and its application in order to analyze the transfer characteristics of single and multiple ring resonator structures. The graphical approach provides the fast derivation of transfer function of SMRR. The results are properly verified with the MATLAB.

Functional demonstration of a wide band, wide angular width “wire-grid polarizer” has been made in the framework of a User Project of the European project ACTMOST (Access To Micro-Optics Expertise, Services and Technologies). The polarization function relies upon linear polarizers using the “wire-grid” polarizer principle by means of a metal grating of unusually large period, exhibiting a large extinction of the transmission of the TE polarization in the 850 nm wavelength range. This grating achieves a broadband and especially high angular aperture reflection with low loss and permits resorting to very low cost incoherent light sources of the transmitted TM polarization. The paper will describe the design, the modeling optimization, and the complete technological process chain which has been used: from the photoresist grating printing using phasemask UV-based lithography to the uniform galvanic growth of very shallow gold grating on transparent conductive layer deposited on a glass substrate. Transmission curves for both polarizations on the first demonstrators will be presented.

The silks of Orb-Weaver spiders (family Araneidae) are emerging as fascinating optical materials due to their biocompatibility, ecological sustainability and mechanical robustness. Natural spider silks are mainly spun as double cylinders, with diameters ranging from 0.05 to 10 μm, depending on the species and maturity of the spider. This small size makes the silks difficult to characterize optically with traditional techniques. Here, we present a technique that is capable of measuring both the real and imaginary refractive index components of spider silks. This technique is also a new capability for characterizing micro-optics more generally. It is based on the measurement and analysis of refracted light through the spider silk, or micro-optic, while it is immersed in a liquid of known refractive index. It can be applied at any visible wavelength. Results at 540 nm are reported. Real refractive indices in the range of 1.54-1.58 were measured, consistent with previous studies of spider silks. Large silk-to-silk variability of the p-polarized refractive index was observed of around 0.015, while variability in the s-polarized refractive index was negligible. No discernible difference in the refractive indices of the two cylinders making up the double cylinder silk structure were observed. Measured imaginary refractive indices corresponded to an optical loss of around 14 dB/mm at 540 nm.

We report on variation in the refractive index of amorphous and isotropic TiO₂ thin films grown by Atomic Layer Deposition (ALD) in nano optical devices. ALD-TiO₂ films of thicknesses ≤ 200 nm exhibiting negative thermo-optic coefficient (TOC) due to decrease in refractive index with temperature, owing to inherent hydrophilic nature. While ALD-TiO₂ films with thicknesses > 200 nm show positive TOC due to the predominance of TiO₂ thickness over the very thin surface porosity region. The negative TOC of thin TiO₂ films was controlled by depositing thin ALD-Al₂O₃ diffusion barrier films that showed impermeable behavior to block the evaporation of adsorbed water molecules on TiO₂ surfaces in thermal environments. This approach turns negative sign of TOC of TiO₂ thin films to positive one which is necessary to stabilize the central resonance peak of a guided mode resonance filter (GMRF). The ALD-TiO₂ and ALDAl₂O₃ bi-layer stack was modeled by VASE analysis of spectroscopic ellipsometry using Cauchy Model to extract refractive indices at various temperatures, measured at two different angle of incidence (65° and 75°), covering a wide spectral range 380 ≤ λ ≤ 1800. The temperature dependent index and density of TiO₂ films were calculated from ellipsometric measured data using Lorentz-Lorenz relation.

In the recent years many commercial applications such as optoelectronics, photonic and biomedical devices, as well as image processing require the fabrication of adaptive and adjustable micro-lens array. A lot of attempts have been conducted in order to support the growing interest in the production of lens arrays for sensors or optical communications devices for parallel data transmission. Several fabrication techniques and a large variety of process have been proposed for polymer based microlenses and their incorporation into technological devices with a large area of application, but, the preparation of moulds, masks or metal layers with very accurate dimensions and shapes is generally required. Here we present the application of a pyro-electrohydrodynamic (Pyro-EHD)-dispenser for the fabrication of polymer microlens arrays overpassing the viscosity limit of the conventional ink-jet printing systems and working in a nozzle-less modality. The results regarding the fabrication procedure and the characterization of polymer micro-lens arrays of different shapes and heights are examined.

In this work we report the design and characterization of a Shack-Hartmann wavefront sampling plane based on a microlens array (MLA) composed of 12 X 12 hexagonal contiguous diffractive lenslet, with 355 μm pitch, 4.5 mm focal length, and 4.3 X 4.3 mm lateral dimensions. The device was fabricated by maskless grayscale lithography based on Digital Light Projector (DLP) technology. Optical characterization was performed in order to measure wavefront aberrations in Zernike polynomials terms. Intraocular lenses were used as test elements because they yield well-known optical aberrations, such as defocus and spherical aberration. For the wavefront reconstruction, the modal approach was used, in which the first derivatives of Zernike polynomials are used as the set of orthogonal basis functions. The corresponding polynomial coefficients up to the first 10 Zernike terms were obtained and the resulting reconstructed wavefront presents an RMS reconstruction error compliant to most optical systems of interest.

Microlens Arrays in silicon are suitable for an important wavelength range within the IR spectrum, since silicon features relatively high refractive index and is transparent at the aimed wavelengths, leading to microlenses with a focal length short enough to allow compact systems, offering an alternative for applications where miniaturization and reduction of alignment and packaging costs are necessary. The microlenses are meant to sample and focus an IR beam on a focal plane array, which might be an image sensor, or a dedicated IR sensor, as for instance lab-on-chip, or a selective gas detection system. Nowadays refractive microlenses are manufactured using sophisticated techniques with relatively high costs and complexity of well controlled steps, like thermal reflow, and grayscale lithography. We hereby propose an alternative solution for microfabrication of silicon microlens arrays, with a single-mask step using KOH anisotropic etching of Si. The proposed technique solves many current demands, like achieving high reproducibility, fill-factor close to 100%, and higher precision of focal axes alignment. We have made optical profilometric measurements to estimate the shape, roughness and the focal distance. We have also observed the focal points imaging in the IR spectrum, proving that the silicon microlenses actually yield the results expected.

In this paper we demonstrate the proof-of-concept of an optofluidic module capable of simultaneous laser-induced fluorescence (LIF) and absorbance (ABS) detection based on total internal reflection (TIR) optics. We discuss the design of the optofluidic detection module, its fabrication, and the setup used for the proof-of-concept. The injection of sample under test is done using two 3D printed syringe pumps, managing accurate injection and repeatable sample propagation through the detector module. We discuss the process of development behind these pumps and review their technical specifications. With this demonstrator setup we find that the limits of detection for the ABS and LIF detection of coumarin 480 are 500 nM and 100 nM respectively.

There is an increasing need for reliable authentication for a number of applications such as e commerce. Common authentication methods based on ownership (ID card) or knowledge factors (password, PIN) are often prone to manipulations and may therefore be not safe enough. Various inherence factor based methods like fingerprint, retinal pattern or voice identifications are considered more secure. Retina scanning in particular offers both low false rejection rate (FRR) and low false acceptance rate (FAR) with about one in a million. Images of the retina with its characteristic pattern of blood vessels can be made with either a fundus camera or laser scanning methods. The present work describes the optical design of a new compact retina laser scanner which is based on MEMS (Micro Electric Mechanical System) technology. The use of a dual axis micro scanning mirror for laser beam deflection enables a more compact and robust design compared to classical systems. The scanner exhibits a full field of view of 10° which corresponds to an area of 4 mm² on the retinal surface surrounding the optical disc. The system works in the near infrared and is designed for use under ambient light conditions, which implies a pupil diameter of 1.5 mm. Furthermore it features a long eye relief of 30 mm so that it can be conveniently used by persons wearing glasses. The optical design requirements and the optical performance are discussed in terms of spot diagrams and ray fan plots.

Natural compound-eyes consist of a large number of ommatidia that are arranged on curved surfaces and thus are able to detect signals from a wide field of view. We present an integrated artificial compound-eye sensor system with enhanced field of view of 180° × 60° due to the introduction of curvature. The system bases on an array of adaptive logarithmic wide-dynamic-range photoreceptors for optical flow detection and compound-eye optics for increasing sensitivity and expanding the field of view. Its assembling is mainly done in planar geometry on a flexible printed circuit board. The separation into smaller ommatidia blocks by dicing enables flexibility and finally allows for mounting on curved surfaces. The signal processing electronics of the presented system is placed together with further sensors into the concavity of the photoreceptor array, and facilitates optical flow computation for navigation purposes.

Camera systems become more and more important in everyday life. Some of those systems place special requirements concerning the environmental conditions they are exposed to especially in harsh environment. High temperature and humidity difficult to access areas require individual packaging and joining technologies for the setup of a camera module. Environmental conditions have an influence on optical design and tolerance calculation. In case of high temperatures the different thermal expansion coefficients of the used materials lead to stress in joints, lenses and their fittings. This, in turn, can lead to a loss of adjustment of the mechanical and optical components that have a direct influence on the optical performance of the camera module. The recent work shows the development of miniaturized high resolution camera modules designed for use in harsh environment applications.

There is a steady increase in the demand for internet bandwidth, primarily driven by cloud services and high-definition video streaming. Europe's Digital Agenda states the ambitious objective that by 2020 all Europeans should have access to internet at speeds of 30Mb/s or above, with 50% or more of households subscribing to connections of 100Mb/s. Today however, internet access in Europe is mainly based on the first generation of broadband, meaning internet accessed over legacy telephone copper and TV cable networks. In recent years, Fiber-To-The-Home (FTTH) networks have been adopted as a replacement of traditional electrical connections for the `last mile' transmission of information at bandwidths over 1Gb/s. However, FTTH penetration is still very low (< 5%) in most major Western economies. The main reason for this is the high deployment cost of FTTH networks. Indeed, the success and adoption of optical access networks critically depend on the quality and reliability of connections between optical fibers. In particular a further reduction of insertion loss of field- installable connectors must be achieved without a significant increase in component cost. This requires precise alignment of fibers that can differ in terms of ellipticity, eccentricity or diameter and seems hardly achievable using today's widespread ferrule-based alignment systems. In this paper, we present a field-installable connector based on deflectable/compressible spring structures, providing a self-centering functionality for the fiber. This way, it can accommodate for possible fiber cladding diameter variations (the tolerance on the cladding diameter of G.652 fiber is typically ±0.7μm). The mechanical properties of the cantilever are derived through an analytical approximation and a mathematical model of the spring constant, and finite element-based simulations are carried out to find the maximum first principal stress as well as the stress distribution distribution in the fiber alignment structure. Elastic constants of the order of 104N=m are found to be compatible with a proof stress of 70 M Pa. We show the successful prototyping of 3-spring fiber alignment structures using deep proton writing and investigate their compatibility with replication techniques such as hot embossing and injection moulding. Fiber insertion in our self-centering alignment structures is achieved by means of a dedicated interferometric setup allowing assessment of the fiber facet quality, of the fiber's position in relation to the connector's front and of the spring deformation during fiber insertion. These self-centering structures have the potential to become the basic building blocks for a new generation of field-installable connectors, ultimately breaking the current paradigm of ferrule-based connectivity requiring extensive pre-engineering and highly specialized manpower for field deployment.

Fibre-to-the-home (FTTH) networks provide an ideal means to reach the goal the European Union has set to provide 50 % of the households with a broadband connection faster than 100 Mb/s. Deployment of FTTH networks, which is still costly today, could be significantly boosted by novel ferrule-less connectors which don't require highly skilled personnel and allow installation in the field. We propose a ferrule-less connector in which two single-mode fibres (SMFs) are aligned and maintain physical contact by ensuring that at least one fibre is in a buckled state. To this end, we design a cavity in which a fibre can buckle in a controlled way. Using finite element analysis simulations to investigate the shape of the formed buckle for various buckling cavity lengths, we show that it can be accurately approximated by a cosine function. In addition, the optical performance of a buckled SMF is investigated by bending loss calculations and simulations. We show a good agreement between the analytical and the simulated bending loss results for a G.652 fibre at a wavelength of 1550 nm. Buckling cavity lengths smaller than 20 mm should be avoided to keep the optical bending loss due to buckling below 0.1 dB. In this case the cavity height should at least be 2 mm to avoid mechanical confinement of the fibre.

We present micro polymer optical waveguide elements fabricated using femtosecond laser and two-photon absorption (TPA) process. The POWs are constructed by tightly focusing a laser beam in SU-8 based resists transparent to the laser wavelength for single-photon absorption. The TPA process enables the patterning of the resist in three dimensions at a resolution of 100-200 nm, which provides a high degree of freedom for POW designs. Using this technology, we provide a novel approach to fabricate Three dimensional Polymer Optical Waveguides (3D-POW) and coupling with single mode fibers in the visible wavelength regions. Our research is also focused on fabricating passive micro optical elements such as splitters, combiners and simple logical gates. For this reason we are aiming to achieve optimum coupling efficiency between the 3D-POW and fibers. The technology also facilitates 3D-POW fabrication independent of the substrate material. We present these fabrication techniques and designs, along with supporting numerical simulations and its transmission properties. With a length of 270 μm and polymer core diameter of 9 μm with air cladding, the waveguides possess a total loss of 12 dB. This value also includes the external in and out mode coupling and in continuously being improved upon by design optimization and simulations. We verify the overall feasibility of the design and coupling mechanisms that can be exploited to execute waveguide based optical functions such as filtering and logical operations.

Integrated optics has emerged as a promising solution to the electronic interconnect bottleneck, enabling high bandwidth density and low power consumption. Recently, confining photochemical and physical reactions in a micro-volume has given an extra dimension to optical interconnection using glass or polymer. Three-dimensional waveguides can then connect, combine, or split the optical signal among any blocks in all dimensions. However, the refractive index increase is still a challenge to fabricate free-form, stable and single-mode three-dimensional buried waveguides.

This paper presents a new concept to tackle this challenge using the combination of femtosecond direct laser writing (FsDLW) in polymer and external diffusion of a gaseous monomer. FsDLW with two-photon absorption was used to initiate cross-linking following a programmed trajectory to form the waveguide core. A thermal treatment was then needed to complete cross-linking. Afterwards, a low-index monomer from a gas atmosphere was diffused into the uncross-linked cladding. Since this diffusion hardly occurred in the already cross-linked pattern, the subsequent UV flood exposure only cross-linked the diffused monomer with host oligomer in the cladding. This low-index monomer decreased the refractive index of the cladding and, therefore, created enough refractive index contrast for total internal reflection. Finally, the whole structure was hard-baked for polymerization and stabilization.

The peak refractive index change of 0.012 was revealed using refractive near field method. Measured near-field intensity at the end facet of waveguides showed single-mode Gaussian profiles. We further demonstrated how feature sizes can be linearly adjusted in the range of 5-12 μm by varying scanning speed and laser intensity. Moreover, changing the voxel shape by a field aperture in front of the objective was investigated. Our fabrication method requires only one layer of a single material without masks, contact or wet processing. Free-form waveguides with high index contrast have high potential to improve the density and flexibility of optical interconnects at board level. Some applications of this concept are three-dimensional arrays of optical waveguide network routers, optofans, pitch converters or splitters.

There is a huge demand on miniaturized cameras in the field of mobile consumer electronics. These cameras are currently based on miniaturized single aperture optics. In order to further decrease the thickness of miniaturized camera systems, a multichannel imaging principle needs to be used. These artificial compound eye cameras permit a further decrease in thickness by a factor of two in comparison to miniaturized single aperture optics with same resolution and pixel size. Their fabrication process is currently based on the reflow of photoresist. Due to physical limitations of this technique, only spherical and ellipsoidal surface profiles of the single lenslets are achievable. Consequently, the potential for correcting optical aberrations is restricted leading to limited image quality and resolution. This can be improved significantly by the use of refractive freeform arrays. Due to the non-symmetrical and aspherical surface shapes of the single lenslets, the fabrication by the reflow of photoresist is no longer possible. Therefore, we propose an approach for the fabrication of these structures based on the combination of an ultra-precision machining process together with a microimprinting approach.

In the case of light emitting semiconducting polymers, different techniques have been used for the fabrication of electroluminescent devices. Experiments and characterizations have been carried out at different operating voltages and for voltage dependent emission color also combining the processability of organic materials with efficient luminescence displayed by inorganic nanocrystals (NCs). In fact, the experimental perspective to disperse emitting colloidal NCs into polymers has allowed to further engineer hybrid organic-inorganic materials introducing innovative functionalities as for instance photoluminescence conversion capabilities. This has proved of great interest for novel applications such as the fabrication of photonic crystals and, notably, of innovative solar cells showing enhanced efficiency. Here we report on the fabrication of novel active micro-optical elements made by a mixture of rod-shaped inorganic NCs dispersed into poly-dimethylsiloxane.

This paper reports on an alternative method for precise and uniform fabrication of 100μm-thick SU-8 microstructures on small-sized or non-circular samples. Standard spin-coating of high-viscosity resists is indeed known to induce large edge beads, leading to an air gap between the mask and the SU-8 photo-resist surface during UV photolithography. This results in a non uniform thickness deposition and in a poor pattern definition. This problem becomes highly critical in the case of small-sized samples. To overcome it, we have developed a soft thermal imprint method based on the use of a nano-imprint equipment and applicable whatever sample fragility, shape and size (from 2cm to 6 inches). After final photolithography, the SU8 pattern thickness variation profile is measured. Thickness uniformity is improved from 30% to 5% with a 5μm maximal deviation to the target value over 2cm-long samples.

In micro-optical assembly, the mastering of the steps of passive and active alignment, bonding, and part feeding as well as their interdependencies are crucial to the success of an automation solution. Process development is therefore complex and time consuming. Separation of assembly process planning and assembly execution decouples both phases so that production and process development can take place in parallel and even in spatially separated stations. The work presented in this paper refines the concept of flexible assembly systems by separating the phases of assembly process planning and assembly execution by providing a dedicated process development platform on the one hand and by providing automatisms regarding the transfer from the development platform into industrial production on the other. For this purpose, two key concepts are being developed by the research carried out at Fraunhofer IPT. The paper introduces the overall approach and formalisms as well as a form of notation based on part lists, product features and key characteristics and it shows industrial use cases the approach has been applied to. Key characteristics are constraints on spatial relations and they are expressed in terms of optical functions or geometric constraints which need to be fulfilled. In the paper, special attention is paid to the illustration of the end-user perspective.

This paper presents the design, fabrication and characterization of a linear variable optical filter (LVOF) that operates in the infrared (IR) spectral range. An LVOF-based microspectrometer is a tapered-cavity Fabry-Perot optical filter placed on top of a linear array of detectors. The filter transforms the optical spectrum into a lateral intensity profile, which is recorded by the detectors. The IR LVOF has been fabricated in an IC-compatible process flow using a resist reflow and is followed by the transfer etching of this resist pattern into the optical resonator layer. This technique provides the possibility to fabricate a small, robust and high-resolution micro-spectrometer in the IR spectral range directly on a detector chip. In these designs, the LVOF uses thin-film layers of sputtered Si and SiO₂ as the high and low refractive index materials respectively. By tuning the deposition conditions and analyzing the optical properties with a commercial ellipsometer, the refractive index for Si and SiO₂ thin-films was measured and optimized for the intended spectral range. Two LVOF microspectrometers, one operating in the 1.8-2.8 μm, and the other in the 3.0-4.5 μm wavelength range, have been designed and fabricated on a silicon wafer. The filters consist of a Fabry-Perot structure combined with a band-pass filter to block the out-of-band transmission. Finally, the filters were fully characterized with an FTIR spectrometer and the transmission curve widening was investigated. The measured transmittance curves were in agreement with theory. The characterization shows a spectral resolution of 35-60 nm for the short wavelength range LVOF and 70 nm for the long wavelength range LVOF, which can be further improved using signal processing algorithms.

In this paper, we adapt a technique employed for glass microlenses fabrication in order to obtain matrices of millimeter size lenses for inspection applications. The use of microfabrication processes and Micro-Electro-Mechanical Systems (MEMS) compatible materials allow the integration of lenses larger than usual in microsystems. Since the presented lenses can have 2 mm in diameter or more, some aspects apparently irrelevant when diameters are lower than 500 μm must be reviewed and taken into account. Indeed, when the lenses are in the millimeter range, problems such as size nonuniformities within a matrix and asymmetric shapes of each lens are dependent on parameters as mask design, depth of the silicon cavities and enclosed vacuum control after anodic bonding, glass reflow temperature and even the position of the lenses on the substrate. Issues related to the fabrication flow-chart are addressed in this paper and solutions are proposed. First results are shown to prove the pertinence of this technique to fabricate MEMS-compatible millimetersized lenses to be integrated in miniature inspection systems. We also discuss some of the paths to follow that could help improving the performances.

A polarization rotation is realized by subwavelength binary gratings, where the TE and TM round trip phases of the smallest grating modes are fixed to the smallest possible integer numbers of 2π that allow a straight-through phase difference of π. This results in a subwavelength grating allowing to realize a half-wave element of almost 100% transmission. The principle is applied to a polarization transformation in the 1030-1064 nm wavelength range, using a segmented polarization rotating element converting a linearly polarized incidence to a radial or azimuthal polarization distribution. The elevated costs of such kind of polarization transformers based on assembled birefringent crystals are avoided by using mass-fabrication compatible silicon on insulator technology on a wafer scale. It shows the general potential of microelectronic technology, concerning the batch manufacturing of wavelength-scale diffractive, grating based elements for processing free space waves

In the past, UV lithography has been used extensively for the fabrication of diffractive optical elements (DOEs). The advantage of this technique is that the entire structure can be written at one time, however, the minimum feature size is limited to about 1 μm. Many 1-d and 2-d periodic grating structures may not need such fine details but it is essential for diffractive optics with circular structures. This is because the spacing between features typically decreases towards the edge of the element resulting in the smallest feature falling well below 1 μm. 1-d structures such as sub-wavelength gratings will also have smaller feature sizes throughout the structure. In such cases, advanced techniques such as Focused Ion Beam and Electron-beam Lithography are required for the fabrication of finer structures. In this paper, we present results of DOEs fabricated with a focused ion beam system (Nova Nanolab 600 from FEI) directly on a single mode fibre tip. The ability to write DOEs directly on fibre tip is of great importance in fields such as endoscopy and optical trapping. The DOE itself, transforms the laser beam to a phase and intensity profile that matches the requirement. Because it is located directly on the fibre, no extra alignment is required. In addition, the system becomes more compact, which is especially important for applications in the field of endoscopy. The main goal of the present work was to develop the most accurate method for creating the desired pattern (that is, the DOE structure) into an actually working element. Different exposure strategies for writing test structures directly with the ion beam on the fibre tip have been tested and carefully evaluated. The paper will present in detail the initial fabrication and optical test results for blazed and binary structures of 1-d and circularly symmetric Fresnel axicons on optical fibres.

Electron beam lithography becomes attractive also for the fabrication of large scale diffractive optical elements by the use of the character projection (CP) technique. Even in the comparable fast variable shaped beam (VSB) exposure approach for conventional electron beam writers optical nanostructures may require very long writing times exceeding 24 hours per wafer because of the high density of features, as required by e.g. sub-wavelength nanostructures. Using character projection, the writing time can be reduced by more than one order of magnitude, due to the simultaneous exposure of multiple features. The benefit of character projection increases with increasing complexity of the features and decreasing period. In this contribution we demonstrate the CP technique for a grating of hexagonal symmetry at 350nm period. The pattern is designed to provide antireflective (AR) properties, which can be adapted in their spectral and angular domain for applications from VIS to NIR by changing the feature size and the etching depth of the nanostructure. This AR nanostructure can be used on the backside of optical elements e.g. gratings, when an AR coating stack could not be applied for the reason of climatic conditions or wave front accuracy.

A chromatic confocal microscope is a single point non-contact distance measurement sensor. For three decades the vast majority of the chromatic confocal microscope use refractive-based lenses to code the measurement axis chromatically. However, such an approach is limiting the range of applications. In this paper the performance of refractive, diffractive and Hybrid aspheric diffractive are compared. Hybrid aspheric diffractive lenses combine the low geometric aberration of a diffractive lens with the high optical power of an aspheric lens. Hybrid aspheric diffractive lenses can reduce the number of elements in an imaging system significantly or create large hyper- chromatic lenses for sensing applications. In addition, diffractive lenses can improve the resolution and the dynamic range of a chromatic confocal microscope. However, to be suitable for commercial applications, the diffractive optical power must be significant. Therefore, manufacturing such lenses is a challenge. We show in this paper how a theoretical manufacturing model can demonstrate that the hybrid aspheric diffractive configuration with the best performances is achieved by step diffractive surface. The high optical quality of step diffractive surface is then demonstrated experimentally. Publisher’s Note: This paper, originally published on 5/10/14, was replaced with a corrected/revised version on 5/19/14. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.

Sub-wavelength dielectric gratings can be used to achieve phase retardation. Due to the vector nature of the devices, scalar theory is not applicable and rigorous calculation methods are required. The modal method proves to be a simple but powerful compromise, between rigorous techniques that are computationally expensive and the scalar theory that is inadequate, for design of such elements. As a proof of concept, a quarter wave plate (QWP) was designed and its behaviour compared against previously published data. Wave plate design requires that the orthogonal polarizations travel in the same direction with appropriate phase delay. It is assumed that light is incident normally on the grating. Floquet-Bloch periodicity ensures that discrete modes get excited within the grating. The number of propagating modes and the propagation constant of the modes can be controlled by the angle of incidence, the ratio of period to the incident wavelength and the fill factor. Modal method characterizes the underlying Eigen modes (/ effective indices) of the orthogonal field components in the sub-wavelength structure. Based on the indices obtained by modal method, height of the grating ridge is deduced. The design gives a high aspect ratio of about 8 for a quarter wave phase retarder. The design is also numerically evaluated by the finite element method. The solver COMSOL was used to visualize how polarization direction evolves with time. The designed QWP could convert linearly polarized light into circularly polarized light and vice versa. This result proves the validity of the design procedure.

This presentation reported an approach of glucose derivatives to resist polymers for eco-friendly optical NEMS and MEMS. The material design concept to use the water-soluble resist material with highly efficient crosslinking, water development, and lower film thickness shrinkage was proposed for green lithography. The lithographic properties due to the glucose derivatives, and the low film thickness shrinkage due to distinctive bulky chemical structure were proposed in the resist material, and then demonstrated to be effective for creating high resolution, excellent patterning dimensional accuracy, and low line edge roughness in EB lithography. Mixing or blending of glucose and cellulose derivatives was a valuable approach to the design of resist formulations for eco-friendly optical NEMS and MEMS.

We develop a new technology, which is referred to as progressive phase conjugation (PPC), in which phase conjugation is electrically performed without requiring a coherent reference beam by fusion using a reference-free spatial phase detection and spatial phase modulation. This method enables remote setting of a phase detector from the signal transmitter without an additional transmission line for the reference beam. It also enables realization of high-speed and dynamic wavefront compensation owing to its open-loop architecture using the single-shot phase detection method. Therefore, the PPC is applicable to a wide range of optical communication technologies, including the reconfigurable spatial-mode extraction and conversion of mode transmission in a multi-mode fiber (MMF). In our experiment, spatial modes are generated by directing a laser beam into a MMF with a 50-micron core diameter. At the output side of the optical fiber, the phase distributions of the spatial modes are detected using the reference-free phase detector constructed by combining a spatial filtering method with holographic diversity interferometry using two CCD imagers. Then, the phase conjugate distribution of the detected phase pattern is displayed on a LCOS-type SLM. We confirm that the PPC system can extract a specific mode pattern with a considerably low crosstalk of less than 1% by displaying the corresponding phase-conjugation pattern on the SLM. In addition, we demonstrated a reconfigurable spatial-mode conversion by the phase control technology using the SLM. By applying the spatial phase modulation to an optical beam incident on the SLM, the spatial mode of the output beam is flexibly changed.

We characterize laser-induced damage threshold (LIDT) in transparent photopolymers by a sub-ps laser pulses of 515 nm wavelength representing case of high light intensities. Five different photopolymers (SZ2080, OrmoComp, SU-8, PDMS and PMMA) widely used in the laser lithography are investigated. The relationship of the damage threshold and optical band-gap energy of the polymers indicating possible damage mechanism is considered. Incubation model validating damage threshold dependence on the number of laser pulses is studied as well. The obtained characteristic values of LIDT reveal potential of photopolymers and their possible applications in high power laser systems.

We investigate the use of carbon nano-tubes performed by chemical vapor deposition for photonics applications producing samples of various geometries on the same wafer and performing experiments and numerical modeling. Publisher’s Note: This paper, originally published on 5/2/14, was replaced with a corrected/revised version on 5/19/14. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.

Fiber to planar waveguide interconnects and planar beam modifiers are crucial for the implementing efficient silicon photonic devices for communication applications. In this work, broadband performance in the 1300nm- 1600nm region is ensured by appropriately controlling the spatial variation of the effective index of silicon to realize a beam splitter (1×2) and a quarter wave GRIN lens. Sub wavelength (λ/10) 1D periodic array of low-index air holes in the high index host silicon, along the propagation direction of the optical beam, is repeated, with decreasing or increasing periodicities in the transverse direction to form a 2D photonic crystal(PhC) structure to realize a beam splitter or GRIN lens respectively. The lowest wavelength of 1300nm is used as the design wavelength to ensure less than λ/10 periodicities for all useful wavelength ranges. A decreasing transverse periodicity of 1D hole arrays from the horizontal axis at the centre along the direction of propagation results in formation of 2 high index wave guiding structures towards the edges of the crystal separated by low effective index central region thus leading to formation of a beam splitter. On the other hand increasing transverse periodicity of the defect array leads to lowering of effective index gradually to the edges of the device resulting in the formation of a GRIN lens. FEM analysis of the propagation of electromagnetic field through these structures show that GRIN lens focuses the input beam to a mode field diameter (MFD) of 1.5μm and that MFD of each output arm of the 1×2 splitter is 1.75μm. The decrease in intensities at the focal point of the GRIN lens with increasing input wavelength in the 1300-1600nm is found to be within 6% and that in the two arms of the beam splitter is found to be less than 13%.

Polymers with individually adjusted optical and rheological properties are gaining more and more importance in industrial applications like in information technology. To modify the refractive index n, an electron-rich organic dopant is added to a commercially available polymer based resin. Changes in viscosity for applications like ink-jet printing can be achieved by using a comonomer with suitable properties. Therefore we used a commercially available epoxy acrylate based UV-curable polymer matrix to investigate the influence of ethylene glycol dimethacrylate (EGDMA) on viscosity and phenanthrene on refractive index. Refractive index was measured at a wavelength of 589 nm and 20 °C using an Abbe refractometer. As a result the change in viscosity decreased linearly from 47 Pa·s to 4 mPa·s which is a more suitable region for inkjet printing. However, the refractive index decreased at the same time from 1.548 to 1.514. Adding phenanthrene the refractive index increased linearly from 1.548 up to 1.561. It was shown that both, viscosity and refractive index can be successfully adjusted in a wide range depending on desired properties.

Photoactivation and “optogenetics” require the precise control of the illumination path in optical microscopes. It is equally important to shape the illumination spatially as well as to have control over the intensity and the duration of the illumination. In order to achieve these goals we use programmable, diffractive Micro Mirror Arrays (MMA) as fast spatial light modulators for beam steering. By combining two MMAs with 256×256 mirrors each, our illumination setup allows for fast angular and spatial control at a wide spectral range (260-1000 nm). Illumination pulses can be as short as 50 μs, or can also extend to several seconds. The specific illumination modes of the individual areas results in a precise control over the light dose to the sample, giving significant advantage when investigating dosage dependent activation inasmuch as both the duration and the intensity can be controlled distinctly. The setup is integrated to a microscope and allows selective illumination of regions in the sample, enabling the precise, localized activation of fluorescent probes and the activation and deactivation of cellular and subcellular signaling cascades using photo activated ion channels. The high reflectivity in the UV range (up to 260nm) further allows gene silencing using UV activated constructs (e.g. caged morpholinos).