Shaping%20Nanowire%20Tapers

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Shaping Nanowire Tapers With A CO2 Laser Powered
Micro-Furnace

• By David Kedziora

Supervisors Eric Magi, Ben Eggleton
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The Project

• Aim
• To test a CO2 Laser Powered Micro-Furnace in
  tapering fibres.
• Method
• Stretching heated fibres so that they thin down.
• Pass light in, detect light out, work out optical
  loss.

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Definitions

• Fibre Optic Cable Flexible, optically
  transparent wire through which light can be
  transmitted by total internal reflection.

• Taper A section of optical fibre that has a
  continuously changing outer dimension along its
  length.
• Tapers waist included!

Centre for Photonics and Photonic Materials,
University of Bath 2007
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Nanowires!

• Nanowire Optical fibre with diameter on the
  micron or, ideally, submicron scale.
• Why do we care?
• Nanowires transmit some of the light energy
  OUTSIDE cladding!
• This is called Evanescent Field.
• Other fibres and crystals can interact with this
  field.

Grillet, CUDOS 2006

• Examples of uses
• Photonics circuitry
• Photonic sensing

Centre for Photonics and Photonic Materials,
University of Bath 2007
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Outline

• Tapering The Basic Concept
• Brushing With Fire
• The Limitations
• The CO2 Laser Powered Furnace
• Tapering In Action
• Results And Conclusion

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Tapering The Basic Concept

• Heat a section of the fibre.
• Stretch the fibre.
• Conservation of mass implies increase in length
  is matched with decrease in diameter.
• Variable elongation velocity determines taper
  shape.
• Theory states taper adiabaticity (gradual
  decrease) required for low optical loss.

Birks and Li, Journal Of Lightwave Technology,
1992
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Brushing With Fire
Centre for Photonics and Photonic Materials,
University of Bath 2007

• Standard tapering procedure flame brushing
  method.
• Flame sweeps over a section (heated to gt1600
  degrees C).
• Two clamps stretch fibre.
• Note It works. Dimensions can be reduced by up
  to 100 times.

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The Limitations

• Problem 1 Increasing Viscous Forces
• Problem encapsulated by Reynolds Number theory.
• Fibre diameter decreases, friction of fibre
  increases.
• The expelled gas literally pushes fibre out of
  flame.
• Not optimal due to irregularities in taper shape.
• Problem 2 Contaminants
• OH- ions are a by-product of flame process.
• The ions are absorbed into fibre over time.
• Contamination results in optical loss.

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CO2 Laser Powered Furnace (1)

• Solution CO2 Laser
• Beam of EM-radiation in infrared.
• Operates at 10.6 micrometer wavelength.

• Why CO2 Laser?
• No chemical by-products (contaminants).
• The Silica fibres are opaque to infrared.
  Therefore absorption.
• Highest-power continuous wave gas lasers
  currently available.
• Can have as large as 20 efficiency. (This is
  among the best.)
• Gives system greatest flexibility with heat-zone
  temperature.

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CO2 Laser Powered Furnace (2)

• Problem Laser by itself wont do.
• Criticism of Local Heating
• (Energy Loss)/(Energy Input) e-a(Diameter)
• Solution Sapphire Furnace

• Consists of two Sapphire Tubes heated by laser.
• Allows radiative heating, which leads to more
  absorption.
• Also aids in disrupting friction-inducing air
  convection currents.

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Tapering In Action

• Clamp fibre down to two stages.
• Turn on laser.
• Stages move left and right allowing heating
  section to sweep over fibre.
• Over time, elongates fibre.

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Results (1)
0.3 dB loss 6.7 loss
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Results (2)
Before Tapering
After Tapering
125 Microns
24.8 Microns
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Results (3)
Profile Of Outer Diameter
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Conclusion

• There is a lot left we can do with the system.
• Have additional degree of freedom laser.
• Potential applications of local heating
• Example Fibre bending.
• Ultimately though success! The system works!
• Will lead to higher yield for CUDOS.
• Smaller nanowires with higher performance.