Polymer waveguide end facet roughness and optical input/output coupling loss for OPCB applications
Electro-optical printed circuit board technology (EOCB) based on integrated planar polymer optical waveguides has been the subject of research and development for many years to provide a cost viable, fully integrated system embedded optical interconnect solution, however a number of constraints of this technology have yet to be overcome. Optical coupling loss at the input and output of the waveguides is one of the major issues and waveguide end facet roughness is one of the main sources of the coupling loss which is investigated in this paper. The results of a comprehensive investigation of the end facet roughness of multimode polymer waveguides, fabricated on FR4 printed circuit boards, PCBs, and its effect of optical loss are presented theoretically and experimentally. The waveguide end facet roughness was measured using an atomic force microscope, AFM, when the waveguides were cut using a milling router with various numbers of cutting edges called flutes. The optimized cutting parameters are derived and the optical coupling loss, between the laser source and the waveguide, due to the different roughness magnitudes is measured by experiment for the first time. To improve the surface quality and decrease the waveguide optical loss, a new fabrication technique for reducing the end facet roughness after cutting is proposed and demonstrated. The insertion loss was reduced by 2.60 dB ± 1.3 dB which is more than that achieved by other conventional methods such as index matching fluid.
Cost-effective integration of plastic optical fiber and total internal reflection mirrors in printed circuit boards for parallel optical interconnects
A novel method of integrating a total internal reflection (TIR) mirror into an optical waveguide embedded in a printed circuit board (PCB) was demonstrated for application in chip-to-chip optical interconnects. Plastic optical fiber (POF) was placed into channels that were mechanically machined into FR-4 composite plates. The TIR mirror was created by a second mechanical machining. The mirror loss was measured to be approximately −1.6 dB per reflection. The use of POF resulted in attenuation losses in the waveguide over an order of magnitude lower than what is obtainable with typical planar waveguides that are integrated into PCBs. Monte Carlo ray tracing was used to determine the theoretical efficiency of the system as well as cross talk between channels due to optical device alignment tolerances.
Direct laser-writing of polymer structures for optical interconnects on backplane printed circuit boards
We have previously developed a direct UV-laser-writing technique and custom photo-polymer optimised to form multimode polymer waveguides and embedded 45deg out-of-plane mirrors. In the IeMRC funded ldquoIntegrated Optical and Electronic Interconnect PCB Manufacturingrdquo (OPCB) project the key aim is to explore how these techniques can be extended to suit real-world optical backplane applications - both in the context of scale and manufacturability. In particular, we are assessing the viability of the technique for writing polymer optical waveguides of approximately 50 times 50 mum in cross-section over large metre-scale areas and the compatibility of this optical interconnect approach with existing PCB manufacturing processes.