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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