1 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 Dispersion and refractive index in Ge, B-Ge doped and photonic crystal fibre following irradiation at MGy levels

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

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 mode i.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 SiO 2 ” Doped core is more susceptible to radiation damage

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

5 PCF fibres Photonic crystal fibres Pure SiO 2 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 ?

6 High dose rate irradiation facility Irradiation trials have been carried out at Belgian Nuclear Research Centre (SCK-CEN, Mol) in the “Brigitte” facility 60 Co  radiation source Dose rate 10 to 30 kGy/hr Sample compartment: Oven assembly Temp. 55 o C Dry N 2 atmosphere Sample holder

7 Sample holder Fibre samples 100mm long in 300  m 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 Glass plate darkened after irradiation

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 (+/-100  m) - using two cleavers fixed to optical bench - fibres tensioned during cutting stages

9 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

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 dispersion OPD

11 Irradiation details Dose level - series of 5 sample plates irradiated with total dose : 100, 499, 2308, 4943 and 7046 kGy 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

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

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