End Facet Roughness and Coupling Loss Hadi Baghsiahi, Kai Wang, David R. Selviah Photonics Research Group, Department of Electronic & Electrical Engineering University College London, UCL, UK International Electrotechnics Commission IEC Annual International Conference, Charlotte USA 29 th October 2013

2 Waveguide End Facet Roughness Aims and Motivations: To find the relationship between the waveguide end facet roughness and the optical coupling loss. Best method to cut a polymer waveguides based on the surface quality, cost and reliability of the method. Optimise the cutting parameters of the selected cutting method to reduce the waveguide end facet roughness. To find and apply a range of methods for polishing, reducing and minimising the end facet roughness.

3 The white rectangles are the cut-out connector sites where the daughter boards are plugged into the backplane End Facet Roughness

4 Schematic diagram of a one flute router and the cutting procedure used for the roughness investigation. (RLG 615 Drill/Router from Ernst Wessel Machinebau Gmbh). End Facet Roughness Milling Router:

5 Photomicrograph of the end of the waveguide after cutting with a one- flute router entering the sample from (a) the waveguide and (b) the copper coated FR4 PCB side. End Facet Roughness

6 A sample of the roughnesses of the surfaces cut by router cutters with different numbers of flutes rpm, cutting speed = 0.25 mm/min End Facet Roughness Number of cutting edges on the router:

7 End facet roughness for different rotation and feed speeds. Blue indicates low roughness and red high roughness End Facet Roughness One flute router Rotation and translation speed optimization:

8 Photomicrograph taken through a Nomarski microscope of a back illuminated waveguide cut at a rotation speed of 34,000 rpm and a translation speed of 0.75 m/min High Rotation Speed Cutting speed of 0.50 m/min and A: 60,000 rpm, B: 70,000 rpm. The surface roughness in case A is 395 ±18 nm and case B: 432 ± 21 nm

9 The recommended chip load from the router manufacturer is 8 µm/revolution. A minimum on the roughness was observed in the chip load of 8 µm/revolution. Minimum roughness was obtained at a chip load of 16 µm/revolution for the best surface quality. Stevenage Circuits use this one flute router to cut FR4 glass fibre reinforced epoxy PCBs and they have optimised that to operate at 17 µm/revolution Chip Load:

10 Confidential to the Partners of the IeMRC Flagship OPCB Project Experiment Configuration:

11 Experimental results for the optical input and output coupling loss due to the roughness at the end of the waveguide End Facet Roughness Measurement Results: Coupling loss measurements for different magnitude of roughness: Optical loss for several samples with different end facet roughness was measured.

12 Experimental result of coupling loss due to the roughness at the end of the waveguide. End Facet Roughness Measurement Results: Coupling loss measurements for different magnitude of roughness:

13 Confidential to the Partners of the IeMRC Flagship OPCB Project Optical insertion loss plotted versus, σ/T, ratio of RMS roughness to autocorrelation length of the waveguide core end facet roughness. Theoretical/Statistical Investigation:

14 Confidential to the Partners of the IeMRC Flagship OPCB Project Roughness Comparison:

End Facet Roughness Polishing: Surface of the waveguide after hand polishing. A: AFM scanned data. B: The scanned surface of the waveguide by Zygo interferometry microscope

16 End Facet Roughness Polishing (Future Work): Schematic diagram of polishing router

17 End Facet Roughness Treatment: New technique for coating the ends of an array of cut waveguides with core polymer and curing to leave a flat smooth surface.

18 The improvement of the coupling loss after applying index fluid matching on average is 2.23 ± 1.2 and after applying Truemode TM acrylate polymer is 2.60 ± 1.3. End Facet Roughness Treatment Results: