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William N MacPherson, Robert RJ Maier, James S Barton, Julian DC Jones

Heriot-Watt University, EPS, Applied Optics and Photonics, Edinburgh, UK

Alberto Fernandez Fernandez, Benoît Brichard, Francis BerghmansSCK-CEN, Belgian Nuclear Research Centre, Mol, Belgium

Jonathan C Knight and Philip St J RussellUniversity of Bath, Opto-electronics Group, UK

Lance Farr BlazePhotonics Ltd., Bath, UK

Dispersion and refractive index in Ge, B-Ge doped and photonic crystal fibre

following irradiation at MGy levels

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Introduction

Instrumentation for use in nuclear radiation environments:-

• Nuclear reactors • Nuclear waste storage• High energy physics research • Fusion research

• Require systems for Data communications Remote monitoring Safety

Electronic / semiconductor based systems can be incompatible with harsh environment

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Fibre-optics in radiation environment

Data Links can use radiation hardened fibre:

compatible with radiation levels to MGy levelse.g. Sol gel derived material

fibres typically multi modei.e fibre core un-doped, cladding “depressed”

Fibre optic sensing

Interferometric sensing techniques require: single mode fibre (maintain phase information)

i.e. fibre core doped, cladding “pure SiO2”

Doped core is more susceptible to radiation damage

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Single-mode fibres

Complex dynamics

Radiation chemistry in doped core known to generate absorption bands

Loss of Transmission

Question:

Do other fibre parameters change?

i.e. refractive index and

dispersion ?Critical parameters for interferometric sensing techniques

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PCF fibres

Photonic crystal fibres

Pure SiO2 core

Cladding formed byarray of air channels

Structural dimensions define wave-guiding characteristics:

• Single mode operation over wide spectral range• Control of dispersion possible

How do PCF fibres perform under ionising radiation ?

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High dose rate irradiation facility

Irradiation trials have been carried out at Belgian Nuclear Research Centre (SCK-CEN, Mol)in the “Brigitte” facility

60Co radiation source Dose rate 10 to 30 kGy/hr

Sample compartment:

• Oven assembly• Temp. cont. @ 55oC• Dry N2 atmosphere

Sample holder

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Sample holder

• Fibre samples 100mm long in 300m wide slots in 2mm thick glass plates

• Diameter of SM fibre small vs. range of secondary particles

• Embedding ensures homogeneous energy distribution [Secondary electron equilibrium SEE ]

• PCF is “non homogeneous”: SEE cannot be verified

But:comparison to SMF28 still valid

3mm thick glass plateswith 260 / 300 m slots

tie rods

150m

m

60mm

Glass plate darkened after irradiation

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Samples

• Conventional single-mode fibre: Corning SMF-28

• PCF - manufactured by Blaze Photonics, UK

• Photo sensitised fibres: (B/Ge co-doped fibre)manufactured by Nortel (1998)provided by Fibrecore

• All fibre samples cleaved to 100mm length (+/-100m)- using two cleavers fixed to optical bench - fibres tensioned during cutting stages

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Optical set-up I

• Interferometric determination of refractive index and dispersion

- Use low coherence scanning interferometry- Based upon Michelson Interferometer- Dispersive fourier transform interferometry

DFTS

Distributed Fourier Transform spectrometer

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Optical set-up II

• Interferogram analysis:

- scan OPD to obtain two interferrograms associated with front and back face of the fibre

- distance between these gives optical length

- interferrogram shape change gives dispersion

dispersion

OPD

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Irradiation details

• Dose level- series of 5 sample plates irradiated with total dose :

100, 499, 2308, 4943 and 7046 kGy (@10kGy/hr)

• Range of dose level varied by placing samples in radiation field for different lengths of time

• Location of samples subject to 10 kGy/hr

• After irradiation, samples shipped to HW for post-irradiation measurement

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Experimental results

Effect of gamma irradiation• on dispersion (at 1550nm)

- nothing systematic noted for any of the fibres

• on mean refractive index- nothing systematic noted for any of the fibres

NULL result is still important -

Defines regimes under which interferometric fibre optic sensors can be used

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Conclusions

• No significant change of dispersion or mean refractive index for gamma irradiated samples to 7MGy

• Results in agreement with independent tests on SMF-28 from SCK-CEN

(A. F. Fernandez, IEEE PTL Oct. 2003)

• Result useful because identifies regimes in which these fibres can be used for sensor systems

• Further testing required:- higher irradiation doses and dose rates- attenuation testing