Shaping Nanowire Tapers… With A CO2 Laser Powered

Micro-Furnace

By David Kedziora

Supervisors: Eric Magi, Ben Eggleton

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.

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.”

•Taper’s waist included!

Centre for Photonics and Photonic Materials, University of Bath 2007

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.

Examples of uses:

•Photonics circuitry

•Photonic sensing

Grillet, CUDOS 2006

Centre for Photonics and Photonic Materials, University of Bath 2007

Outline

• Tapering: The Basic Concept

• Brushing With Fire

• The Limitations…

• The CO2 Laser Powered Furnace

• Tapering: In Action

• Results And Conclusion

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

Brushing With Fire

• Standard tapering procedure: flame brushing method.

• Flame sweeps over a section (heated to >1600 degrees C).

• Two clamps stretch fibre.

• Note: It works. Dimensions can be reduced by up to 100 times.

Centre for Photonics and Photonic Materials, University of Bath 2007

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.

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.

CO2 Laser Powered Furnace (2)

• Problem: Laser by itself won’t do.

• Criticism of Local Heating:(Energy Loss)/(Energy Input) = e-α(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.

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.

Results (1)

0.3 dB loss = 6.7% loss

Results (2)

125 Microns 24.8 Microns

Before Tapering After Tapering

Results (3)

Profile Of Outer Diameter

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.