Joachim Guttmann

10 Gb/s data transmission with TO-packaged multimode GaAs VCSELs over 1 m long polymer waveguides for optical backplane applications

Abstract TO-packaged GaAs vertical-cavity surface-emitting lasers (VCSELs) are being investigated for high-bit-rate data transmission over 103 cm of a 250×200 μm 2 core size polymer-based optical waveguide to be employed in optical backplanes. The bit-error rates for 5 and 10 Gb/s transmission over the waveguides incorporating 45° deflection mirrors are better than 10−12. For the first time, the measured bandwidth-length-product for a polymer-based optical backplane exceeds 10 GHz × m .

Polymer waveguides for 100-cm (40) optical backplanes

Optical backplanes are attractive components for systems with high data rates between subsystems and a large number of interconnects. An optical backplane which uses multimode polymer waveguides was originally developed for avionic applications but can be used in telecom switching systems as well. For transmission distances in the range of 100cm and data rates up to 10Gbps, the modal dispersion can be negelected. The waveguides are fabricated on large substrates (aluminum, FR4 and others) by a direct writing technique. Splitters and couplers can be fabricated with the same technique. The waveguides have a low loss (0.04dB/cm) and high temperature stability (up to 250°C) and are used with 840nm vertical cavity lasers. The waveguide cross section can be chosen between approx. 250μm x 250μm and 50μm x 50μm. We have successfully transmitted up to 10Gbit/s over multimode polymer waveguides with lengths of 100cm. A free space, expanded beam coupling is used for the board-backplane transition, resulting in high alignment tolerances. The overall insertion loss for a backplane connection is typically between 2 and 8 dB, depending on waveguide length, radius of curvature, number of waveguide crossings etc. A typical transceiver power budget of 15-20dB allows the integration of star couplers with up to 16 ports. Several test systems with different interconnection schemes have been realized and tested. Tests include mechanical stability (vibration), thermal stability (cycles, shocks and accelerated aging) and gamma irradiation as well as optical power levels, signal integrity and bit error rates.

Optical backplane in planar technology

Optical interconnects are expected to overcome the limitations imposed by electrical interconnects. For board- to-board and board-to-multiboard communication we have developed an optical backplane for applications in mobile systems. Compared to existing realizations it is compact, rugged and has the potential to be fabricated at low cost. The main features of the optical backplane in planar technology are free space expanded beam transmission between boards and backplane and guided wave transmission within the backplane. No optical connectors are required. Due to the expanded beams and highly multimode waveguides large coupling tolerances of several 100 micrometers are achieved. Low loss polymer backplane waveguides allow transmission length of more than 19 inches. Demonstrators for board-to-board interconnections and for ring and star networks have been realized. Transmission experiments at 1GBit/s have been successfully performed. First environmental test with respect to dust, moisture, temperature and vibrations showed the feasibility of the concept.