A Proof-of-Concept for a multi-channel WDM board-level optical communications link is under development. This
paper is focusing on theoretical and experimental evaluation of thin-glass based nearly single mode graded index optical
waveguides with regard to low loss in the 1310nm regime. Results from waveguide characterization will be reported.
Waveguide modes are determined theoretically from the measured refractive index profiles. Towards improvement of
the robustness of the coupling efficiency against misalignments, investigations on the use of tapered waveguide
structures will be presented too.
Optical interconnects replace electrical links increasingly at shorter distances. At printed circuit board (PCB) level highly
multimodal polymer channel waveguides are the chosen approach to meet bandwidth-length and bandwidth-density
requirements. One important challenge of board integrated waveguides is the coupling problem. The manufacturing
process of PCBs leads to relatively high placement tolerances which cause poor optical coupling efficiency due to
mechanical misalignment between separate components, e.g.: 1) Coupling between a VCSEL and the board integrated waveguides;
2) Coupling between waveguides in two separate boards. This paper deals with the deployment of tapered dielectric multimode waveguides for increasing the optical coupling
robustness towards mechanical misalignments in these two coupling applications.
A coupled mode approach for calculation of the mode coupling and power loss in a taper with decreasing width has been
presented before [5]. In [6], the two above mentioned coupling applications for tapered dielectric waveguides have been
dealt with, but only the coupling efficiency in case of longitudinal misalignment has been calculated.
In this paper, results of advanced analysis of the two applications are presented. The coupling efficiency in case of
transverse misalignment is simulated by a ray-optical approach. Furthermore the results of measurements of the coupling
behaviour of board integrated tapered waveguides are presented.
The results show that tapered multimodal dielectric waveguides have the capability to increase the coupling efficiency
significantly if some conditions are fulfilled.
From long haul, metro access and intersystem links the trend goes to applying optical interconnection technology at increasingly shorter distances. Intrasystem interconnects such as data busses between microprocessors and memory blocks are still based on copper interconnects today. This causes a bottleneck in computer systems since the achievable bandwidth of electrical interconnects is limited through the underlying physical properties. Approaches to solve this problem by embedding optical multimode polymer waveguides into the board (electro-optical circuit board technology, EOCB) have been reported earlier. The principle feasibility of optical interconnection technology in chip-to-chip applications has been validated in a number of projects. For reasons of cost considerations waveguides with large cross sections are used in order to relax alignment requirements and to allow automatic placement and assembly without any active alignment of components necessary. On the other hand the bandwidth of these highly multimodal waveguides is restricted due to mode dispersion. The advance of WDM technology towards intrasystem applications will provide sufficiently high bandwidth which is required for future high-performance computer systems: Assuming that, for example, 8 wavelength-channels with 12Gbps (SDR1) each are given, then optical on-board interconnects with data rates a magnitude higher than the data rates of electrical interconnects for distances typically found at today's computer boards and backplanes can be realized. The data rate will be twice as much, if DDR2 technology is considered towards the optical signals as well. In this paper we discuss an approach for a hybrid integrated optoelectronic WDM package which might enable the application of WDM technology to EOCB.
Modal noise is an undesired modulation of the guided light intensity in a multimode waveguide. Applying the frequency correlation function the frequency dependence of this noise as well as the bandwidth of a multimode waveguide can be estimated. In this paper the existing model of the frequency correlation function for a waveguide with smoothed dielectric interfaces is enhanced to analyze the influence of surface roughness on the achievable bandwidth. This surface roughness is caused by the manufacturing process of the waveguides.
To increase the bandwidth of high-performance chip-to-chip interconnects optical on-board interconnects can be used. Since the design procedure of such optical interconnects has to be widely compatible with current computer aided board design processes, adequate simulation methods are required. In this paper an efficient and design process compatible method for simulating the transmission behavior of optical multimode chip-to-chip interconnects is presented. The approach is based on a time domain description where an optical multimode waveguide is represented by a multiport. The different transfer paths between the input- and output ports describe the transmission behavior of the entire waveguide. The transmission behavior of each individual path can be characterized by its step response, which can be computed by the aid of an extended ray tracing method. Due to some fundamental properties of these step responses, its piecewise approximation by simple exponential functions is possible. As a consequence the pulse responses of each transfer path can be determined analytically and they are also approximated by exponential functions. Finally this procedure enables the application of a semi-analytic recursive convolution method for the computation of the waveguide transmission behavior. The simulation procedure is illustrated and discussed by a set of examples.
Next generation high-speed pc board interconnects will be based on integrated optical multimode waveguides with cross- sectional sizes comparable to those of electrical microstrip lines. The design of such interconnects requires appropriate simulation models of the multimode waveguides and the laser- and photo-diodes, as well. Since single-mode interconnects can be modeled very efficiently by well known numerical methods such as FEM and BPM, these methods are not applicable for optical multimode waveguides with more than 1000 propagating modes. Due to the numerical complexity only methods based on geometrical optics, called ray tracing, can be applied efficiently. This paper deals with a time domain modeling and simulation approach for analyzing the signal behavior of multimode waveguide based electrical-optical interconnection systems as a whole. Modeling of multimode waveguide components as well as macromodeling of laser- and photo-diodes is explained in detail. The modeling approaches are discussed by examples.
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