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.
The relationship between polymer waveguide optical interconnection end facet roughness and the optical input and output coupling losses
The RMS surface roughness of an optical polymer waveguide end facet cut by a milling router and measured by AFM is investigated for a range of rotation speeds and translation speeds of the router. It was found that 1 flute (cutting edge) routers gave significantly less rough surfaces than 2 or 3 flute routers. The best results were achieved for a 1 flute router when the milling bit was inserted from the copper layer side of the board with a rotation speed of 15,000 rpm and a translation speed of 0.25 m/min which minimized the waveguide core end facet RMS roughness to 183 ± 8 nm and gave input optical coupling loss of 1.7 dB ± 0.5 dB and output optical coupling loss of 2.0 dB ± 0.7 dB. The relationship between optical coupling loss at the input and output of the waveguides and waveguide end facet roughness is also investigated in this paper. The ratio of RMS roughness to autocorrelation length of the roughness is shown to have a quantified linear relationship with experimental measurements of optical insertion loss, input optical coupling loss and output optical coupling loss. A new fabrication technique for cut waveguide end facet treatment has been proposed and demonstrated which reduces the insertion loss by 2.60 dB ± 1.3 dB which is more than that achieved by the closest available index matching fluid which gave 2.23 dB ± 1.2 dB and which is far more robust for use in commercial products.
Insertion Loss and Misalignment Tolerance in Multimode Tapered Waveguide Bends
Experimental measurements of laterally tapered multimode waveguide bends fabricated photolithographically on FR4 printed circuit board establish that the product of the mean insertion loss (in linear units) and the mean source misalignment tolerance is a constant which depends only on the taper ratio TR (input width/output width) and not on the radius of curvature R. The minimum loss of 0.78 dB occurs in the special case of TR = 0.8, R = 14 mm. Together, these form waveguide layout design rules for board-to-board and chip-to-chip optical interconnects.