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 31st October 2013

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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.

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

End Facet Roughness

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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:

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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

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A sample of the roughnesses of the surfaces cut by router cutters with different numbers of flutes.15000 rpm, cutting speed = 0.25 mm/min

End Facet Roughness

Number of cutting edges on the router:

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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:

300

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400

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200

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400

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Rotation Speed (rpm)

Tra

nsla

tion

Spee

d (m

/min

)

1 2 3 4 5

x 104

0.1

0.2

0.3

0.4

0.5

0.6

0.7

200

250

300

350

400

450Rotation speed

(rpm) 5000 15000 34000 50000

Translation Speed (m/min)

0.10

Roughness (nm)

389 ± 61 310 ± 12 384 ± 10 342 ± 23

Chip load (µm/revolution)

20 6.6 2.9 2

0.25

Roughness (nm)

376 ± 30 183 ± 13 358 ± 12 339 ± 42

Chip load (µm/revolution)

50 16 7.4 5

0.50

Roughness (nm)

474 ± 32 381 ± 18 205 ± 24 410 ± 15

Chip load (µm/revolution)

100 33 15 10

0.75

Roughness (nm)

434 ± 54 386 ± 20 407 ± 43 296 ± 12

Chip load (µm/revolution)

150 50 22 14

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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

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• 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:

10Confidential to the Partners of the IeMRC Flagship OPCB Project

Experiment Configuration:

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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.

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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:

13Confidential 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:

14Confidential 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

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End Facet Roughness Polishing (Future Work):

Schematic diagram of polishing router

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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.

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The improvement of the coupling loss after applying index fluid matching on average is 2.23 ± 1.2 and after applying TruemodeTM acrylate polymer is 2.60 ± 1.3.

End Facet Roughness Treatment Results: