In short-range interconnect applications, one question arises frequently: When should optical solutions be chosen over
electrical wiring? The answer to this question of course depends on several factors like costs, performance, reliability,
availability of testing equipment and knowledge about optical technologies, and last but not least, it strongly depends on
the application itself. Networking in high performance computing (HPC) is one such example. With bit rates around 10
Gbit/s per channel and cable length above 2 m, the high attenuation of electrical cables leads to a clear preference of
optical or active optical cables (AOC) for most planned HPC systems. For AOCs, the electro-optical conversion is
realized inside the connector housing, while for purely optical cables, the conversion is done at the edge of the board.
Proceeding to 25 Gbit/s and higher, attenuation and loss of signal quality become critical. Therefore, either significantly
more effort has to be spent on the electrical side, or the package for conversion has to be integrated closer to the chip,
thus requiring new packaging technologies. The paper provides a state of the art overview of packaging concepts for
short range interconnects, it describes the main challenges of optical package integration and illustrates new concepts
and trends in this research area.
In the first part of this contribution we present a concept for the fabrication, assembly and alignment of a multichannel
micro optical-coupler and arrayed microelectronic devices placed on a PCB. This concept is based on a micro opticalcoupler
that integrates several optical sub-systems in a monolithic substrate in order to simplify adjustment processes.
The optical-coupler is fabricated by plastic replication of a metal master with the negative shape of the coupler. For the
fabrication on the PCB, only one alignment step is necessary. By placing markers on the PCB it is possible to position
the coupler over the VCSEL or photodiode array. The placement and connections between the electronic devices on the
PCB are taken into account in the design of the coupler. The mechanical assemblies for populating PCBs with electronic
devices have an accuracy of a few micrometers. Using these techniques an optimal position of the coupler relative to the
VCSEL or photodiode array can be found. In the second part we examine with the help of simulations the effect of
misalignment and tilt of the optical surfaces and possible differences between the optical fibers like decentering. Bitrates
of 120 Gbps in a 12-channel system can be reached using this coupler with commercial electronic devices. Applications
for this system are active optical cables and ultra wide-band board to board communication systems. A FPGA-board for
the test of this concept is in the design phase and will be reported.
We propose a method for the characterization of one- and two-dimensional diffraction gratings by means of the
measurement of diffraction efficiencies. The method is based on the comparison of measured and calculated efficiencies.
For the numerical calculation we use the Rigorous Coupled Wave Analysis RCWA and an optimization
algorithm to determine the grating shape that fits best to the measured data. We analyzed in which cases the
method is able to determine the grating shape without ambiguity and which measurement parameters should be
used. By systematically analyzing a given inverse problem, we try to derive the theoretical limits of the method.
A flattened Lorentzian irradiance profile, which was introduced by Brenner in 2003, offers prospects for analytic design of beam shaping optics when transforming a Gaussian beam into a soft-flat-top profile. It is shown that for the flattened Lorentzian profile the output power integral and the ray mapping function are analytic functions. This paper presents a systematic derivation of the flattened Lorentzian profile from the super-Lorentzian profile and exploration of properties of the flattened Lorentzian profile and compares its behavior to the super-Gaussian, flattened Gaussian, and Fermi-Dirac profiles. Details are presented for evaluation of the FL profile normalization, profile matching conditions, ray mapping function, and M2.
The mask structured silver sodium ion exchange in glass (MSI) is a powerful tool for the realization of high precision refractive micro optical GRIN components. Commonly the distribution of the silver ions in GRIN elements and thus the index distribution is determined by the laws of thermal diffusion. By the use of a structured metal mask, which defines the areas of contact between the glass and the silver salt melt, an additional degree of freedom in optical design is introduced. A photolithographic pattern generator provides the accuracy of the mask structure to realize wave front optimized micro lens arrays with 100% filling factor in Cartesian, hexagonal and also in nearly any other arbitrary geometrical arrangements for several applications such as high precision Shack-Hartmann systems.
In this paper we want to discuss the potential and limits of this technique. We report on the family of optical functions, which can be realized with MSI. Furthermore we give an overview over the actual applications.
The Wigner distribution function is viewed as an important tool in the analysis of wave progation and image formation. We discuss why it should be taught in optics courses relating to these topics.
We discuss how we introduce Wigner optics to students of different backgrounds, what sub-topics we find most appropriate, what difficulties we have encountered, and how we motivate an appreciation of the subject by means of suitable examples.
The mask structured ion exchange in glass (MSI) is a powerful tool for realizing general planar phase distributions and in particular custom designed planar GRIN micro lenses with diffraction limited performance and high fill factor. For lens characterization the numerical aperture is a key parameter. However the classical geometrical definition of the N.A. disregards aberrations. Here we suggest an addition to this classical definition, which is based on diffraction limited performance. For a testing of micro lens arrays, global process parameters are assessed by interferometric measurements of a subset of the lenses. Local process variations typically result in small non-symmetric aberrations. These aberrations mainly lead to a lateral shift of the focus. Thus, for rapid quality control of micro lens arrays we analyze all focal positions in parallel. From the lateral deviations of the focal positions a quality criterion for each individual micro lens can be derived.
The task of transforming a given intensity distribution into a different, desired intensity distribution can be achieved either by diffractive optical elements or by beam shaping. The most essential difference between these two approaches is that with diffractive optics, the output amplitude results from a superposition of all contributions of the input plane, while in beam shaping there is a one-to-one correspondence between the Poynting-vectors in both planes. Consequently, beam shaping does not require temporal coherence. Applications of beam shaping are typically in the area of high power lasers, where the Gaussian beam shape leads to an inefficient use of the available laser power due to loss at the focusing lens. In the contribution, different techniques for designing beam shaping elements are discussed and an extension to analytic methods will be presented.
We suggest a new technique for fabricating a wide class of continuous refractive optical elements in glass by combining the technique of ion exchange with high precision structuring of metal masks. We call this technique mask structured ion exchange (MSI). We have demonstrated the potential of this method by fabrication of rectangular shaped microlenses with low numerical aperture for Hartmann-Shack wavefront sensing applications. The lenses, positioned on a 400 micrometers raster, had a fill factor of 100 %, a focal length of 33 mm and diffraction limited performance. Due to the special fabrication conditions, the lens shape, position and even the focal length can be varied spatially within one substrate. For realization of a high aperture microlens array by field assisted exchange process we could reduce proximity effects between adjacent mask apertures by MSI.
A novel technique for the realization of guiding structures for passive / automatic alignment is proposed. The technique is based on a recently available special photoresist. This photoresist can be processed by standard lithographic processes and shows a mechanical stability close to glass. Guiding structures may thus be realized in the same process as the functional structures on a substrate. We discuss the material behavior in terms of processing, change of shape due to shrinking or swelling and shear stability. First results of accuracy experiments are shown. As an example we demonstrate fiber ferrules for horizontal and for vertical mounting. We also show alignment structures for fiber/fiber coupling.
For the next generation of DVD we have designed an 'asymmetric Gaussian to flat-top' converter which uses two refractive optical elements. The design method is based on conservation of energy. This approach leads to an analytically solvable mapping equation which was integrated numerically. The properties of the designed element were analyzed by numerical nonparaxial wave-propagation as well as by experimental characterization. In detail, we have studied the tolerance to phase noise, the tolerance to deviations from the specified form and the tolerance to positioning errors. We observed, that the flat-top-intensity shows a very height sensitivity to phase noise. The sensitivity is increased by rotationally symmetric distributions of phase noise e.g. produced by diamond turning. The influence of these deviations on the intensity distribution in the focal plane of the high NA-focusing lens are investigated. First experimental results of a shaping element fabricated by direct write in resist are presented.
For the transfer of micro optical and mechanical components to an application, packaging currently represents a major problem. The electrical, optical and mechanical components are typically fabricated with lithographic techniques. Thus the relative distances are accurate, but the absolute position on the substrate is defined by the quality of the separation tool. Thus active alignment is usually necessary for the assembly. For the development of standardized MOEM - modules, designed to eliminate the problems of alignment, technological, optical and function considerations have to be addressed.
Angle division multiplexing (ADM) is a multiplexing scheme which is applicable to optical signal transmission through multimode step-index fibers. ADM allows for parallel and high-bandwidth transmission by using passive components, i.e. no external control is necessary. The fiber quality limits the simultaneous optimization of transmission distance, number of channels and cross-talk. This paper reports on first experiments with an ADM based data link over 10 m between workstations with 8 Gigabit-Ethernet channels at a cross-talk of better than -10 dB. For a micro-optical integration of the ADM system design rules and tolerance considerations are discussed.
The propagation angle 9 between the symmetry axis of a fiber and the principle propagation direction of a beam is conserved over short distances within a step-index multimode fiber. This conservation behavior can be used for multiplexed transmission by assigning different channels to different propagation angles. Furthermore, due to the reduction of the angular spread in the fiber, the temporal bandwidth is increased compared to multi-mode transmission. To realize this angular coding, suitable optical setups for multiplexing and demultiplexing operations were designed. The experimental results on the transmission capabilities of an angular multiplexed multimode fiber are presented.
The propagation angle of a beam is conserved when traveling through a multi-mode step-index fiber over short distances. We show that this feature can be used for multiplexed signal transmission thus providing a high bandwidth single-fiber interconnection between chips, boards or racks. With respect to this application the transmission properties of a fiber are analyzed. Concepts for optical multiplexing and de- multiplexing set-ups are outlined. A fiber transmission line consisting of a fiber and an optical de-multiplexing unit is characterized to obtain experimental results on the transmission capabilities in terms of efficiency, channel separation, number of multiplexed channels and cross-talk.
Optical signal transmission benefits, among other features, from its huge potential of multiplexing. Besides WDM or time multiplexing a new kind ofmultiplexing is possible: directional multiplexing. Measurements are presented showing that in short fiber transmission lines the angle 9 between the symmetry axis of a fiber and the principle propagation direction of a beam is conserved over distances in the order of 1 meter. So, provided a beam is coupled into a fiber under a certain off-axis angle 3 it will couple out of the fiber with the same off-axis angle. This conservation behavior can be used for coding different channels for multiplexed transmission and can provide a high bandwidth interconnection between chips or boards, i.e. within computers in a single fiber. Based on the experimental set-up used in this work an estimation on the degree ofmultiplexing will be given. Additionally, suitable components for multiplexing and de-multiplexing have to be designed. That is, on the one hand different signal channels have to be coded into different angles before being coupled into the fiber. On the other hand the directional spectrum of the outcoming beams has to mapped to different positions on a detector plane. Particular attention is paid to the de-multiplexing component: A specially designed diffractive optical element (DOE) serves as a mapping device between directional spectrum and spatial positions on the detector plane.
KEYWORDS: Lenses, Geometrical optics, Free space optics, Optics manufacturing, Integrated optics, Ion exchange, Ray tracing, Optical alignment, Glasses, 3D modeling
Inthe field of optical interconnection research, the topic perhaps closest to widespread applications is the realization of optical board to board and chip to chip interconnections [1]. The inherent limitations of electronic interconnects regarding bandwidth and electromagnetic interference are overcome by waveguide, fiber, or free space optics. With free space optics additionally the high interconnect density ofoptical imaging can be employed. One ofthe basic problems is the addressing of emitted signals to the intended detector. The concept of folding a free-space optical system into a thick transparent planeparallel substrate, was proposed by Jahns and Huang in 1989 [2]. Planar optics, also known as substrate-mode optics, fully employs the three dimensional nature of light propagation and also the fabrication methods known from integrated circuit manufacturing can be adopted. In addition to that the advantage ofthese technologies are compact packaging and simple alignment ofthe optical and electronical elements. The problem involved in folding optical light paths for imaging of extended data fields is the demand for good off-axis imaging properties ofthe optical elements. Besides diffractive solutions i.e. DOEs or CGHs [3] refractive optics offers the possibility for wavelength multiplexing since the wavelength dependence is only due to material dispersion. For off axis imaging in a folded zig-zag-path the optical system also has to be corrected by special astigmatic components.
In order to determine the imaging quality of microlenses, a numerical method was developed. This method, which we called Wave Propagation Method (WPM), was derived from the classical Beam Propagation Method (BPM). In comparison to the classical BPM the new method is not restricted to the paraxial region and therefore can be applied also for large angles and large index steps. Different kinds of microlenses were characterized with the WPM and the theoretical properties of various gradient index lenses were analyzed.
A fully functional optical adder based on Systolic Arrays and Symbolic Substitution (SSL) is demonstrated. It is implemented in a modular, miniaturizable and micro-integrable optical setup. The optical system is designed to fit the requirements of incoherent optical processing with optoelectronic threshold amplifier arrays. Parallel application of four SSL rules is performed in a single, light efficient optical path.
Deep etch lithography of PMMA by the LIGA-technique or H+ lithography is a powerful tool for the fabrication of very high refractive microprisms. We have realized a microintegrated optical imaging system using microprisms fabricated by the LIGA-process and microlenses with a diameter of 250 micrometers . Thus the size of the imaging system is less than 2 mm+.
Current planar integration technologies utilize one or two dimensional light propagation in waveguides. Technologies to achieve microintegration of classical optical components in three dimensions are available.
Diffusion elements are fabricated by locally manipulating the ion composition of a glass substrate. For our experiments we use the silver sodium ion exchange technology. The effort of our work concentrates on the generation of arbitrary index distributions. Therefore we characterized two different types of special glasses--one of them showing linear, the other nonlinear diffusion response. To be able to optimize masks for the generation of arbitrary phase profiles we simulated the diffusion process numerically.
Some of the main advantages of optics in processing are due to the 3-D nature of the optical wavefield. Current planar integration only utilizes one- or two-dimensional light propagation in waveguides. Technologies to achieve an integration of classical optical components in three dimensions are available.
The general goal of integration is to provide enhanced stability and compactness in the construction of complex systems. In the present situation the term 'Integrated Optics' usually denotes two-dimensional planar integration in thin films allowing only the propagation of zero-dimensional optical signals in wave guides. One of the main potentials of optics arises from the fact that the wavefield is three—dimensional, allowing to interconnect a large number of information channels through space with high bandwidth and without crosstalk. An earlier approach to an integrated three-dimensional structure utilized diffractive-reflective components fabricated in dichromated gelatine to connect arrays of optical devices through free space /1/. Diffractive elements by holographic techniques or by etching in glass substrates /2,3/ are one possibility to fabricate three-dimensional integrated optical systems. Diffractive elements however, due to the requirement of constructive interference, are very sensitive to thermal expansion and to wavelenght changes. An alternate concept for 3D-integrating regularly structured digital optical systems based on reflective and refractive components has recently been proposed
A digital optical pipeline adder based on symbolic substitution is presented. We have developed an optoelectronic device as the nonlinear array. Experimental results are demonstrated. 1. OPTICAL ARCHITECTURE We demonstrate a ripple carry adder in a pipeline architecture. A pair of 8 bit numbers is accepted by the system input for every clock cycle. All numbers are dual rail coded. The sum of these numbers reaches the output 8 clock cycles later. The system is designed for active devices i. e. light emitting devices. We use a self-developed optoelectronic inverter array (OEI) as the nonlinearity. The operation is realized by applying four symbolic substitution (Symsub) rules in parallel to a 16 by 16 data plane. This plane contains the pair of input numbers an area with intermediate data and the sum output. The plane is split into four copies one for each rule. These copies enter the recognition stages. The recognition output of the Symsub stages is imaged onto a mask to perform space variant operation. The mask''s output is fed to the substitution parts of the system. The result reaches the OEI where it is amplified and normalized to binary values. The OEI''s output is fed back as the new data plane. 2. THE OPTOELECTRONIC INVERTER ARRAY The OEI is a hybrid optoelectronic system that can process 16x16 pixels at a video frame rate on an area of 13x13 m.
The high data transfer requirements in digital computing demand the use of optics. The next step is to perform data
processing by optical or opto-clectronic devices. As with electronic computing this computer architecture needs computer
skied support. We present a software system which allows interactive design, simulation and evaluation of such computer
architectures. With this system, we have developed arithmetic units based on systolic arrays.
A new technique for the three dimensional integration of optical components is presented. It is based on diffusion techniques and photoinitiated polymerization. Initial experimental results demonstrate the feasibility and efficiency. DIFFUSION jQ PHOTOINITIATED POLYMERISA11ON We want to combine diffusion with photoinitiated polymerization in order to form passive elements which are necessary for three dimensional integration of optical components for microoptical systems1''2. The production of optical elements by photopolymerization consists of two main steps. First the PMMA is sensitized to ultraviolet light by diffusing a photoinitiator into the substrat. Then the exposure by UV light causes an increase in the thickness and in the refractive index. The structuring can be achieved either by local diffusion and global exposure for stabilization or by global diffusion and local exposure. In the first case metal-masks for the diffusion process are necessary whereas in the second case photo-masks can be used. OPTICAL INTERCONNECFION For realizing a flexible optical interconnection plate3''4 the necessary functions are: light collimation light deflection and beam splitting. Collimation can be achieved by microlenses. For deflection we use a miniaturized prism. For beam splitting an index grating can be used. The components are integrated within a planar PMMA plate. EXPERIMENTAL RESULTS By measuring the phase profile of diffused lenses and prisms we demonstrated that phase shifts of several wavelengths can be achieved by polymerization and diffusion techniques. The main effect results from the
A new type of nonlinear image processor based on morphological processing is demonstrated. Morphological image processing is based on the successive binary convolution of an image with a selectable convolution kernel. Its field of application includes numerical optical computing and binary image processing. Simple serial commands from a small electronic computer control the selection of the convolution kernels for a given iteration and consequently the parallel processing of very large optical data planes. 1.
The paper discusses different types of optical implementations of symbolic substitution. A distinction is made between fixed rule systems and dynamic rule systems. The paper also addresses the issue of miniaturization and integration.
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