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

Dispersion and refractive index in Ge, B-Ge doped and photonic crystal fibre following irradiation at MGy levels. William N MacPherson , Robert RJ Maier , James S Barton, Julian DC Jones Heriot-Watt University, EPS, Applied Optics and Photonics, Edinburgh, UK

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

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  1. Dispersion and refractive index in Ge, B-Ge doped and photonic crystal fibre following irradiation at MGy levels 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 Berghmans SCK-CEN, Belgian Nuclear Research Centre, Mol, Belgium Jonathan C Knight and Philip St J Russell University of Bath, Opto-electronics Group, UK Lance Farr BlazePhotonics Ltd., Bath, UK 1

  2. 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 2

  3. Fibre-optics in radiation environment Data Links can use radiation hardened fibre:  compatible with radiation levels to MGy levels e.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 3

  4. 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 4

  5. PCF fibres • Photonic crystal fibres • Pure SiO2 core • Cladding formed by • array 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? 5

  6. High dose rate irradiation facility Irradiation trials have been carried out at Belgian Nuclear Research Centre (SCK-CEN, Mol) in the “Brigitte” facility Sample holder 60Co g radiation source Dose rate 10 to 30 kGy/hr • Sample compartment: • Oven assembly • Temp. cont. @ 55oC • Dry N2 atmosphere 6

  7. Glass plate darkened after irradiation Sample holder • Fibre samples 100mm long in 300mm 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 7

  8. 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 (+/-100mm) • - using two cleavers fixed to optical bench • - fibres tensioned during cutting stages 8

  9. DFTSDistributed Fourier Transform spectrometer Optical set-up I • Interferometric determination of refractive index and dispersion • - Use low coherence scanning interferometry • - Based upon Michelson Interferometer • - Dispersive fourier transform interferometry 9

  10. 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 OPD dispersion 10

  11. 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 11

  12. 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 12

  13. 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 13

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