Abstract
This paper investigates the end facet roughness of multimode polymer channel waveguides fabricated on FR4 printed circuit boards, PCBs, when cut at right angles to their optical axis by milling routers for optical butt-coupling connectors and compares it with that resulting from dicing saws and polishing and proposes a novel end facet treatment. RMS surface roughness of waveguide end facets, measured by AFMs, are compared for a range of rotation speeds and translation speeds of a milling router. It was found that one-flute routers gave significantly less rough surfaces than two or three-flute routers. The best results were achieved for a one-flute router when the milling bit was inserted from the PCB 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 ± 13 nm and gave input optical coupling loss of 1.7 dB ± 0.5 B and output optical coupling loss of 2.0 dB ± 0.7 dB. The lowest RMS roughness was obtained at chip loads of 16 μm/revolution. High rotation speeds should be avoided as smearing of the end facet occurs possibly due to polymer heating and softening. For the first time to our knowledge, channel waveguide optical insertion loss is shown to be linearly proportional to the ratio of the waveguide core end facet RMS roughness to its autocorrelation length. A new fabrication technique for cut waveguide end facet treatment is 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. The new fabrication method gives a more robust end facet for use in commercial products.
Highlights
A DVANCED computers, supercomputers, storage arrays, switching fabrics and measurement instruments often use a rack chassis configuration
Optical circuit boards have been demonstrated using layers of overlapping optical fibres glued onto the PCB or glued together into a film which can be applied to the PCB [2], optical waveguide technology offers a lower cost approach that can be fabricated using existing technology used to fabricate PCBs [3]
We report its use when applied to nine waveguides terminating in a mid-board aperture for a commercial optical interconnection demonstrator
Summary
A DVANCED computers, supercomputers, storage arrays, switching fabrics and measurement instruments often use a rack chassis configuration. This paper is concerned with the use of a milling router as a practical production line technique, which is already readily available in PCB manufacturers, to cut the waveguide ends. This has the advantages that it is easy to use and control CNC machines and a wide range of sizes of cuts and shapes are possible without damage to other parts of the OPCB. The divergence angle and polar scattering diagram can be found by calculating the two-dimensional Fourier Transform of the roughness profile assuming uniform intensity and plane wave illumination This assumption may be valid for laser illuminated waveguide inputs for collimated laser beams but breaks down at the waveguide exit where the modes interfere to give an almost random phase distribution and speckle.
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