Rashed, AM

Selviah, DR

English

UCL Discovery

Modelling of Polymer TaperWaveguide for Optical BackplaneAtef M. Rashed and David R. SelviahDepartment of Electronic and Electrical EngineeringUniversity College LondonTorrington Place, London WC1E 7JE, United Kingdoma.rashed@ee.ucl.ac.uk d.selviah@ee.ucl.ac.ukContents• Optical backplane.• Polymer taper waveguide.• FD-BPM technique.• Calculation of lateral near field patterns.• Calculation of lateral far field patterns.• Effect of source horizontal misalignment.• Effect of source angular misalignment.• Conclusions.Optical Backplane• Complex, high capacity data storage units involve different levels of communication suchas board-to-board, rack-to-rack and cabinet-to-cabinet.• With data rates in the range of 10 Gb/s the electrical interconnects are a bottleneckbecause of the cross talk and EMI. • Optical backplanes using optical waveguide interconnects offer many advantages such ashigh distance bandwidth product, immunity to EMI and light weight.• Polymer waveguides are easy to integrate within the FR4 PCBs.Polymer Taper Waveguide• Tapered waveguide with wider input aperture provides the optimum solution for wide tolerance to source misalignment and excellent modal behaviour.• Half taper angle ≈ 1°.• Core input and output cross sections are50x50 and 20×50 μm, respectively.• 2% step index profile with ncore= 1.54.• The fundamental mode of a 20 μm apertureVCSEL emitting at 850 nm is used as thesource located at (0, 0, 0).13 μm@ FWHMxyVCSEL fundamental mode 50 μm20 μm800 μm xzy(0, 0, 0)Schematic of taper waveguide800 μm50 μmWaveguide Model Using FD-BPM Technique⎟⎟⎠⎞⎜⎜⎝⎛⎟ ⎟⎠⎞⎜ ⎜⎝⎛− +∂∂+∂∂=∂∂uz y x nyuxu izu 222 22222 ) , , ( 42βλπβ Helmholtz Equation for the field u:• A 3D mesh is created with step sizes Δx = Δy = 0.2 μm and Δz = 1 μm.• The second order partial derivatives are replaced by theirfinite difference approximation.• Both sides of the equation are then integrated.• The resulting tridiagonal system of linear equations issolved iteratively and the field is determined for eachstep along the z-axis.• β is the propagation constant for the fundamental waveguide mode.xzTop view of the waveguidewith schematic of the mesh• Tapered waveguide provides near field of 19.2 μm at fwhm near, 29% broader than 13.6μm of the straight narrow waveguide. • The near field pattern of the straight wide waveguide exhibits multilobe feature withabout 45% modulation depth.Output Lateral Near FieldDistance across X, μmIntensity (a.u.)-80 -60 -40 -20 0 20 40 60 8000.10.20.30.40.50.60.70.80.91Straight, 50 μmTaperStraight, 20 μmLateral Far Field-15 -10 -5 0 5 10 1500.10.20.30.40.50.60.70.80.91TaperStraight, 20 μmStraight, 50 μmFar Field Angle (deg.)Intensity, (a.u.)• FWHM of the far field patterns are 2.1°, 2.8° and 0.8° for the tapered, narrow and the widestraight waveguides, respectively.• Tapered waveguide shows 25% improvement of the far field over the narrow one with lessthan 15% power in side lobes. Source Horizontal Misalignment40 - 30 - 20 - 10 - 0 10 20 30 400.00.10.20.30.40.50.60.70.80.91.0Offset Launch Position along x-axis (μm)Normalised Output WaveguidePowerTaperStraight, 20 μmStraight, 50 μm• Output power tolerance to misalignment is more dependant of waveguide width than thetaper section.• Taper maintains output power within 90% of its optimum value for ±20 μm misalignment inx direction.10 - 0 100.00.10.20.30.40.50.60.70.80.91.0Launch Angle (deg.)Normalised Output Power-5 5TaperStraight, 50 µmStraight,20 µmSource Angular Misalignment• Launch angle in xz-plane and measured from z-axis clockwise.•Up to 99% of output power could still be achieved within ±2° source misalignment forstraight waveguides.• For the same range taper waveguide is less efficient due to the reflection off the taperedsides.Conclusions• Optical backplanes with polymer waveguides interconnects are viable solution for the EMI problems of the copper tracks in electrical backplanes at 10 Gb/s data rate.• Taper waveguide with wider input aperture is a compromise between narrow and wide straight waveguides in terms of the output power and the modal behaviour.• Taper waveguide provides better modal behaviour and higher coupling efficiency represented by 29% broader near field and 25% narrower far field than the narrow straight waveguide.• The wide straight waveguide exhibits multimodal near field pattern and ≈ 25% of sidelobes power in far field pattern.• The 50 µm input width of the taper waveguide maintains at least 90% of output power for ±20 µm source misalignment in xdirection.• The straight waveguides perform better than the tapered one with respect to angular source misalignment due to reflection off thetapered sides.AcknowledgementThe authors are grateful to the UK EPSRC and DTI for funding via the Storlite LINK project.The fruitful partnership with Xyratex, Havant is highly appreciated.

Modelling of Polymer Taper Waveguide for Optical Backplane

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