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Publication Detail
Photolithographically Manufactured Acrylate Multimode Optical Waveguide Loss Design Rules
  • Publication Type:
  • Authors:
    Wang K, Selviah DR, Papakonstantinou I, Yu G, Baghsiahi H, Fernández FA
  • Publication date:
  • Status:
  • Name of conference:
    2nd Electronics System-Integration Technolgy Conference (ESTC) Greenwich, UK, 4th September 2008, Session Th-P-9 Optoelectronics: Interconnect Design and Applications
The paper describes how design rules are established for optical multimode polymer waveguide components by comparing modelled simulation results for the waveguide component with optical experimental measurements made on the manufactured waveguide component. The loss of individual waveguide components, such as straight sections, 90° bends, crossings, tapers and bent tapers must be known so that the combined loss of a cascade of such elements can be found to determine whether the interconnection’s optical power budget is sufficient to achieve a good bit error rate. However, the loss depends on several factors: the materials: the polymer used for the core and for the cladding, the fabrication technique: the photolithographic procedure and the precise temperature baking regime used, and the measurement technique: the optical source lateral size and angular divergence and precise position relative to the entrance of the waveguide, the output detector lateral size, its angular acceptance angle (if any) and its precise position relative to the exit of the waveguide. Ray trace modelling is suitable for large optical components and for modelling a complete waveguide path. Beam Propagation Method (BPM) is more accurate particularly for smaller waveguide structures or corners of the order of a few wavelengths of light. Both methods can be time consuming if high accuracy is required. Figure 1. The BPM modelled optical field profiles across the waveguide for two segments around a waveguide 90° bend. In the left hand photo light can be seen exiting the guide toward the left hand side due to the mismatch between the incoming modes of a straight waveguide and the leaky modes present in a bend. Once the waveguide pattern had been layed out by UCL an electron beam mask was made and used by a commercial polymer company, Exxelis, to photolithographically fabricate waveguides in Truemode® acrylate low loss polymer. The experimental techniques developed for reproducible measurement of insertion loss, propagation loss, coupling loss, transition loss and radiation loss are described. The measurement of waveguide control or calibration patterns allows the performance of the waveguide component of interest to be isolated. Figure 2. The experimentally measured loss of a 90° waveguide bend as function of bend radius compared to the modelled loss using Beam Propagation Method for two cases. Surprisingly although the experiment used an input multimode fibre the modelled results show that the short straight waveguide section between the input fibre and the bend changes the modes to those of a waveguide before the bend. If the modelled and experimental results do not agree to within experimental accuracy then improved waveguide layouts must be designed to avoid possible problems which may have corrupted the experimental results, the experimental procedure must be revised and numerous further measurement taken, and, the assumptions and approximations used in the modelling must be revisited and improved modelling techniques developed to overcome their limitations often ultimately revealing a deeper physical understanding of the behaviour of light in these waveguide structures allowing design rules to be established.
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