1. 9 th March 2016 Presented at: ICEC 26 – ICMC 2016 New Delhi Numerical and experimental investigation of FBG strain response at cryogenic temperatures V N Venkatesan, R Ramalingam Institute for Technical Physics (ITEP), Karlsruhe Institute of Technology (KIT), Germany

2. 2 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam Introduction Fiber Bragg Gratings (FBG) FBG is a short segment of the core of an optical fiber which is grated with periodic variation in the refractive index. It is formed by UV phase masking. Figure 2 Formation of FBG by UV phase masking [2] Figure 1 Optical fiber [1]

3. 3 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam FBG Principle FBG reflects a certain wavelength called Bragg wavelength (λ B ) and transmits the others Temperature or strain changes cause a Bragg wavelength shift (Δλ B ) Figure 4 Principle of FBG [3] Application in Measurement Technology

4. 4 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam HTS coated conductors (Mass flow) sensor Prototype toros cyropump (Strain) (Displacement) (FLOW) LTS (Strain) HTS GENO Applications

5. 5 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam Strain Sensitivity Dependence of K Production process:- different types of FBG sensing element Chemical composition and core diameter Sensor installation & Bonding techniques Dependence of pockel coefficient to cryogenic temperatures is unknown Reported K-values - (0.78 – 0.81)

6. 6 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam Numerical Investigation 3D simulation performed using steady state linear elastic solid mechanics model in COMSOL 4.4 r1r1 r2r2 Core Coating FBG core and coating specifications. Core MaterialSilica Shear modulus (G 1 )3.01 x 10 10 Pa Young’s modulus (E 1 )7.33 x 10 10 Pa Thermo-optic coefficient (α e )8.60 x 10 -6 K -1 Thermal expansion coefficient (α s )5.50 x 10 -7 K -1 Strain-optic coefficient (P e )0.22 Bragg wavelength (λ B )1550 nm Diameter (d 1 )9.00 x 10 -6 m Coating MaterialAcrylate polymer Shear modulus (G 2 )1.30 x 10 9 Pa Young’s modulus (E 2 )2.50 x 10 9 Pa Diameter (d 2 )1.95 x 10 -4 m Δl – Displacement l – Initial length of sensor

7. 7 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam Numerical results TemperatureK-value 298 K0.7912 77 K0.7840 10 K0.7806 4 K0.7785

8. 8 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam Experimental investigation Free sensing part (FBG) Glued non-sensing part Sample preparation Sample holding glass tube Programming unit Heating elements Evacuating pump Heating Profiler Temperature Sensor Heating chamber Epoxy-phenol adhesive Heat treatment at 160 0 C, constant for 2 hours Allowed to cool down naturally

9. 9 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam Spectrum characteristics

10. 10 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam Cryostat Displacement bellow FBG sensor 2 Extensometer Bottom load cell FBG sensor 1 Sample installation & experiment Performed for 298 K and 77 K First cooled down to 100 K using LN2 pipe and further to 77 K using compressed He vapor Pressure maintained at 10 -4 mbar

11. 11 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam Experimental result (1)

12. 12 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam Repeatability and hysteresis Repeatability error – 5%

13. 13 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam Fitting and comparison

14. 14 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam Conclusions Strain characteristics of a substrate free acrylate coated FBG sensor have been investigated numerically at 298 K, 77 K, 10 K and 4 K, and experimentally at 298 K and 77 K Results proved that FBG shows a linear behavior towards strain Average values during straining of FBG have been used to estimate the k factor at 298 K and 77 K The K factor has been found to be 0.7732 ± 0.015 at 298 K and 0.7103 ± 0.0245 at 77 K for acrylate coated FBG sensor. As a future work, experiments will be conducted for temperatures down to 4 K with different FBG compositions.

15. 15 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam References 1.http://www.ece.umd.edu/~davis/optfib.html, dt. 26/02/2016. 2.http://www.smartfibres.com/fibre-bragg-grating, dt. 26/02/2016. 3.Ramalingam, R., Neumann, H., “Characterization of Fiber Bragg Grating Sensor in a High Speed Rotating Pipe,” AMA Conferences 2013 - SENSOR 2013, OPTO 2013, 618-622 (2013). 4.Thekkethil, S. R., Venkatesan, V. N., Neumann, H., Ramalingam, R., "Design of cryogenic flow meter using fiber Bragg grating sensors," IEEE SENSORS 2015, 978- 1-4799-8203-5 (2015). 5.Ramalingam, R., "Fiber Bragg grating sensors for localized strain measurements at low temperature and in high magnetic field," Proc. AIP Conf., vol. 1218, no. 1, pp.1197- 1204 (2010). 6.Ramalingam, R., Neumann, H., "Fiber Bragg grating-based temperature distribution evaluation of multilayer insulations between 300 K–77 K," IEEE Sensors Journal, 11(4), 1095-1100 (2011). 7.Ramalingam, R., Kläser, M., Schneider, T., Neumann, H., "Fiber Bragg grating sensors for strain measurement at multiple points in an NbTi superconducting sample coil," IEEE Sensors Journal, 14(3), 873-881 (2014).

16. 16 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam References 8.Bharathwaj, V., Markan, A., Atrey, M., Neumann, H., Ramalingam, R., “Fiber Bragg Gratings for distributed cryogenic temperature measurement in a tube in tube helically coiled heat exchanger,” IEEE sensors 2014, Valencia, Spain, pp.1535-1538 (2014). 9.Ramalingam, R., Nast, R., Neumann, H., "Fiber Bragg grating sensors for distributed torsional strain measurements in a (RE) BCO tape," IEEE Sensors Journal, 15(4), 2023-2030 (2015). 10.Jicheng Li, Neumann, H., Ramalingam, R., “Design, fabrication, and testing of fiber Bragg grating sensors for cryogenic long-range displacement measurement,” Cryogenics, Volume 68, June 2015, Pages 36-43, ISSN 0011-2275 (2015). 11.Freitas, R., Araujo, F., Araujo, J., Neumann, H., Ramalingam, R., “A study on intermediate buffer layer of coated Fiber Bragg Grating cryogenic temperature sensors,” IOP Conf. Series: Materials Science and Engineering, 101, 012154 (2015). 12.Raman Kashyap “Fiber Bragg Gratings,” Academic press (1999). 13.Othonos, A., “Fiber Bragg gratings,” Review of Scientific Instruments, 68, 4309-4341, (1997).

17. 17 Presented at: ICEC 26 – ICMC 2016 New Delhi 9 th March 2016 V N Venkatesan, R Ramalingam THANK YOU for your attention