End Facet Roughness and Coupling Loss

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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
29th 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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End Facet Roughness
The white rectangles are the cut-out connector
sites where the daughter boards are plugged into
the backplane
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End Facet Roughness
Milling Router
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).
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End Facet Roughness
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.
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End Facet Roughness
Number of cutting edges on the router
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
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End Facet Roughness

• One flute router Rotation and
• translation speed optimization

End facet roughness for different rotation and
feed speeds. Blue indicates low roughness and red
high roughness
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High Rotation Speed
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
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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Chip Load

• 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

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Experiment Configuration
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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.

Experimental results for the optical input and
output coupling loss due to the roughness at the
end of the waveguide
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End Facet Roughness Measurement Results
Coupling loss measurements for different
magnitude of roughness
Experimental result of coupling loss due to the
roughness at the end of the waveguide.
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Theoretical/Statistical Investigation
Optical insertion loss plotted versus, s/T, ratio
of RMS roughness to autocorrelation length of the
waveguide core end facet roughness.
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Roughness Comparison
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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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End Facet Roughness Treatment Results
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.