© Fraunhofer INT Fibre Optic Sensors at Accelerators – Considerations and Pitfalls Jochen Kuhnhenn

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 2 Fibre Optic Sensors at Accelerators Overview Radiation effects in optical fibres Radiation detection with optical fibres Fibre-optic beam-loss monitors Fibre-optic integrating dosimeters Fibre optic temperature and strain sensors at accelerators

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 3 Introduction of radiation effects group at Fraunhofer INT Background of experience Investigation of radiation effects in electronic and opto-electronic components since 25 years Operating several dedicated irradiation facilities (Co-60, Neutrons, X-Ray, …) Supports manufacturers and users (space, accelerators, medicine, nuclear facilities, …) Specialised knowledge led to the development of several unique radiation detection systems Fraunhofer Locations in Germany Thanks to our collaborators: DESY (M. Körfer, K. Wittenburg) HMI (F. Wulf, W. Goettmann) BESSY (J. Bahrdt) CERN (T. Wijnands, D. Ricci, Elisa Guillermain)

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 4 Fibre Optic Sensors at Accelerators Overview Radiation effects in optical fibres Radiation detection with optical fibres Fibre-optic beam-loss monitors Fibre-optic integrating dosimeters Fibre optic temperature and strain sensors at accelerators

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide Radiation effects in optical fibres Overview Throughout this presentation “Radiation” means ionising radiation (X-rays,  -rays, particles) Radiation changes all properties of optical fibres, but some are only relevant at high doses with small (practical) influence Change of refractive index Change of bandwidth Change of mechanical properties (e.g. dimension, strength) Radiation-induced luminescence light Most important effect in this context Cherenkov radiation Most obvious and disturbing effect is an increase of their attenuation (RIA) Strongly depending on actual fibre and radiation environment

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 6 Parameter dependencies of RIA Experimentally observed effects Manufacturing influences Fibre type (Single mode, graded index, step index) Doping of core/ Doping of cladding (for SM fibres) Preform manufacturer and used processes Core material manufacturer OH Content Cladding core diameter ratio (CCDR) Coating material Drawing conditions Operation conditions Wavelength Light power Launch conditions Environment Total dose Dose rate Annealing periods / Duty cycle Temperature In combination with each other: Differences of many orders of Magnitude possible!

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 7 Wavelength dependence Example of Ge-doped GI fibre

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 8 Example of dependencies Core doping effects (~830 nm) What does that mean for injected light of 1 mW: Wavelength: ~830 nm Fibre length: 100 m Pure silica fibre: 0.89 mW F-doped fibre: 0.17 mW Ge-doped fibre: 3  mW P-doped fibre: mW

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 9 Differences between manufacturers GI fibres and SI fibres

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 10 Radiation effects in optical fibres: Short summary Huge (orders of magnitude) differences between different fibres, environments and operation conditions Reliable and application specific radiation testing requires experience Difficult to transfer or even compare results of different tests No predictive theoretical model available, some extrapolations possible There are only very few “rules of thumb” you can trust! Carefully review sales information, question simplified statements

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 11 Fibre Optic Sensors at Accelerators Overview Radiation effects in optical fibres Radiation detection with optical fibres Fibre-optic beam-loss monitors Fibre-optic integrating dosimeters Fibre optic temperature and strain sensors at accelerators

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 12 Main advantages of optical fibres as radiation sensors General and for accelerator applications Immune against external electro-magnetic-fields Do not disturb external high precision magnetic fields, e.g. in the undulator section of free electron lasers Environmental conditions (temperature, vacuum, …) usually no major problem Capable of monitoring extended areas Extremely small sensors: diameter of much less than 1 mm

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 13 Introduction to light guiding in step-index optical fibres Total reflection of light if angle below critical value Different possible light paths cause dispersion Parameters of interest: Difference of refractive index between core and cladding Launch conditions into fibre (angle of incident)

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 14 Wavelength dependencies for Cherenkov detection Light guiding and signal detection dependent on the following contributions Fibre attenuation as a function of wavelength Photon efficiency of selected photodetector Wavelengths of interest: 400 nm to 800 nm

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 15 Detection efficiency of fibre optic Cherenkov sensor Influence of fibre length

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 16 Detected signals as a function of fibre length Decrease of signal due to (intrinsic) attenuation in the fibre Comparable signals at different locations if events are within ~20 m BUT: No influence of radiation- induced attenuation considered

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 17 Photon collection efficiency Full light cone has to be taken into account Possibility for grazing incident and spiral light propagation G. Anzivino et. al., NIMA(357)380

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 18 Lots of 3-D simulation results Meas. Sci. Technol. 18 (2007) 3257–3262 Vol. 45, No. 36 APPLIED OPTICS 9151 NIM A 357 (1995) 380 P. Gorodetzky et. al., NIMA(361)161 Radiat. Phys. Chem. Vol. 41, pp. 253, 1993 CERN-ATS

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 19 Experimental angular dependence NIM A 357 (1995) 369 NIM A 357 (1995) 380 NIM A 360 (1995) 237 DOI: / ° NIM A 367 (1995) 271

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 20 Installation at accelerators Version 2: PMT looks downstream Beam pipe Beam PMT Beam pipe Beam PMT Version 1: PMT looks upstream Advantages: Higher signal due to better geometry Advantages: Better resolution (“velocity” for time scaling: 0.4 c) Always correct order of events recorded in PMT

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 21 Selection of a Cherenkov fibre General considerations Manufacturer: Really know who has drawn the fibre, who has made the preform Selecting the fibre type: Established recommendation: High OH pure-silica core step-index fibre with F-doped silica cladding Alternatives might become more interesting soon (see below) Selecting the core diameter: The larger the core, usually the higher the price Minor dependence of bandwidth and core diameter (if any) Selecting the NA: Compromise between efficiency and bandwidth:  t ≈ L/(2nc)*(NA)² Shield ambient light with buffer, e.g. black nylon Perform dedicated, meaningful radiation tests!

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 22 Examples of uses at accelerators: DELTA Installation at DELTA (Uni Dortmund) Injection efficiency was poor DELTA Dortmund

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 23 Examples of results at DELTA: Injection efficiency Kuhnhenn, doi: /

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 24 Radial arrangement of 4 sensor fibres Beam pipe Asymmetric signals can detect directed losses Fibres

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 25 Results of the Cherenkov system at FLASH undulators F. Wulf, 2009 IEEE Nuclear Science Symposium

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 26 Other examples: University Lund and DESY Zeuthen J. Bahrdt, FEL 2008 Grabosch, SEI Herbst 2007

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 27 Advantages of fibre optic Cherenkov detectors “Real time” commissioning and optimisation Prevents damages due to high undetected beam losses Simple to install and covers whole accelerator areas Proven and used routinely at several installations

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 28 Considerations for fibre optic Cherenkov detectors Selection between “equal” pure-silica core fibres At 850 nm At 660 nm

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 29 Considerations for fibre optic Cherenkov detectors New optical fibres with better (UV) radiation resistance Solarisiation-optimised optimised fibres F-doped core optical fibres DOI: /TNS A. Alessi, presented at RADECS nm214 nm 310 nm551 nm

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 30 Fibre Optic Sensors at Accelerators Overview Radiation effects in optical fibres Radiation detection with optical fibres Fibre-optic beam-loss monitors Fibre-optic integrating dosimeters Fibre optic temperature and strain sensors at accelerators

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 31 Fibre optic dosimetry Historical perspective S. Kronenberg and C. Siebentritt, Nucl.Instr.Meth. 175 (1980)

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 32 Fibre optic dosimetry A long way from the idea to the real application Gaebler, 1983: „... fibres exhibit properties, which are excellent suited for their application as radiation detectors.“ Lyons, 1985: „... P-doped fibers... might be... suitable for... dosimetry.“ Henschel, 1992: „... radiation induced loss... has been investigated with respect to the suitablility for radiation dosimetry purposes.“ Borgermans, 1999: „The... fibre may be used for dosimetry applications...“ West, 2001: „ response of P-doped fibres is reviewed... [for] their possible use in dosimetry.“ van Uffelen, 2002: „ Feasibility study for distributed dose monitoring...“

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 33 Fibre optic dosimetry Principle: Measuring RIA in P-doped optical fibres Properties of radiation induced attenuation (RIA) in P-co-doped optical fibres: Strong effect  High sensitivity Linear dose response  Quantitative results Slow annealing  Dose rate independence High reproducibility  Only one calibration per fibre sample necessary

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 34 Fibre optic dosimetry Calibration and dose rate dependency One fit covers all dose rates (>4 orders of magnitude difference) Nearly linear dose-attenuation function Saturation of induced attenuation only above ~1000 Gy Calibration for this fibre: D[Gy]  A[dB/m] * 27

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 35 Power meter system for FLASH

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 36 Exemplary results for power meter measurements at TTF1 Dose [Gy] Date Accumulated dose since compared to TLD measurements Easter

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 37 Power meter system at MAXlab, Sweden: Results

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 38 Power meter system at MAXlab, Sweden: Results

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 39 Fibre optic dosimeters based on RIA measurements Simple principle requires sophisticated measurement techniques for reliable and accurate data Main advantages: Integrating (even if no readout takes place) Small sensor size (< 0.5 mm if necessary) Quantitative dose data (tested for different dose rates and radiation energies) High sensitivity (~ some 10 mGy/hour)

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 40 Fibre Optic Sensors at Accelerators Overview Radiation effects in optical fibres Radiation detection with optical fibres Fibre-optic beam-loss monitors Fibre-optic integrating dosimeters Fibre optic temperature and strain sensors at accelerators

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 41 Using Fibre-Bragg-Gratings in radiation environments Principle Applications for FBGs: Temperature sensors Strain sensors „Mirrors“ for fibre lasers Advantages: Distributed system Passive = 2 n  Light sourceTransmitted spectrum Reflected spectrum  Change of refractive index due to radiation

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 42 Radiation effects in Fibre-Bragg-Gratings Differences in manufacturing method and used fibre CLPG ; ~5000 pm Very sensitive radiation sensor UV FBG ; 100 pm High dose radiation sensor Fs-IR FBG ; 5 pm Radiation „hard“ temperature and strain sensor  B [pm] Dosis [kGy]

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 43 Using other fibre optic sensors in radiation environments The above mentioned advantages of fibre optic sensors are attractive for other measurements at accelerators Widely used fibre optic sensors in conventional environments Strain (bridges, buildings, tunnels, …) Temperature (tunnels, dams, …) Moisture (tunnels, dams, …) Application in radiation environments can be challenging

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 44 Example: Fibre optic temperature sensor Principle: Similar to OTDR measurement but not the original backscattered signal is analysed but two spectrally shifted peaks (stokes and anti-stokes) Temperature information is derived by comparing the amplitudes of the two signals Problem in radiation environment: Radiation induced loss strongly depends on wavelength  One peak (at lower wavelength) gets more attenuated than the other on (at higher wavelength) Radiation leads to apparent temperature change

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 45 Last slide Overview of radiation effects in optical fibres was given This presentation introduced different radiation sensors using optical fibres Cherenkov systems Power meter systems Fibre-Bragg-Gratings Finally some other aspects of using optical fibres at accelerators were presented, such as using optical fibre sensors in radiation environments Unfortunately not covered Telecommunication applications in radiation environments (e.g. CERN) Stimulated annealing of radiation induced attenuation

© Fraunhofer INT 3rd oPAC Topical Workshop, , Jochen Kuhnhenn, Slide 46 Thank you for your attention! Contact: Jochen Kuhnhenn Fraunhofer INT Appelsgarten Euskirchen Tel.: Fax: