DESIGN CHALLENGES OF A TUNABLE LASER INTERROGATOR FOR GEO-STATIONARY COMMUNICATION SATELLITES

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1 DESIGN CHALLENGES OF A TUNABLE LASER INTERROGATOR FOR GEO-STATIONARY COMMUNICATION SATELLITES Selwan K. Ibrahim 1, Arthur Honniball 1, Raymond McCue 1, Michael Todd 1, John A. O Dowd 1, David Sheils 1, Liberis Voudouris 1, Martin Farnan 1, Andreas Hurni 2, Philipp Putzer 2, Norbert Lemke 2, Markus Roner 2 (1) FAZ Technology Ltd., 9C Beckett Way, West Business Park, Dublin 12, Ireland (2) OHB System AG, Manfred-Fuchs-Straße 1, Weßling, Oberpfaffenhofen, Germany

2 Optical vs. Electronic Sensing Systems Why use fiber optic sensing? Immunity to electromagnetic interference (EMI). Allows multiplexing and distributing several sensors on a single fiber over long distances. GUI DATA Display + Software + Data formats + Connectivity Light-weight when compared to the electric sensors and their cable harnesses. Could be used in high temperature and harsh environments with careful packaging. Interrogator (HW, SW, Optics) Transmission line (Fiber type, Coating, Spec.) Multi-meter (ADC + Electronic circuit) Wires (twisted) Transducer (Pressure, Acoustics, Acceleration) Sensing element FBG Wavlength λ ~ fn ( Temp T, Strain ɛ ) Strain gauge/temperature probe Resistive element 2

3 Reflectivity Time Transmission Fiber Bragg Grating (FBG) Wavelength Light IN Reflected Light FBG Reflection λ peak = fn (Strain, Temp) SENSING ELEMENT: FBG TRANSDUCER Bragg wavelength λ= 2 n eff Λ Λ Single FBG (sensor) (typically 8mm long) λ trough Wavelength Transmitted LIGHT OUT Compression Δλ α ΔΛ Elongation λ peak Wavelength Temperature sensitivity (~10pm/ C) Strain sensitivity (~1.2 pm/με) 3

4 Reflectivity Performance Overview Definitions Temperature sensitivity (~10pm/ C) Strain sensitivity (~1.2 pm/με) λ peak = fn (Strain, Temp) Short term Repeatability/Sensitivity = fn (Noise) Long term Precision = fn (Drift) Peak detection λ peak Abs. Accuracy = fn (True λ location) λ peak Wavelength Time Precision: the minimum resolvable change in wavelength Characterized by deviation of measurements over a timeframe Reported as peak-to-peak or 1σ standard deviation. 1pm λ accuracy ~ 0.1 C or 1με 0.01pm λ precision ~ C or 0.01με Accuracy/Trueness/Systematic Bias: The offset from of the mean of measurements from the standard (NIST HCN gas cell) Once characterized, can be offset / corrected from the measurements, if sufficiently stable over time 4

ESTEC Contract: Status: 15 year lifetime 5 4000106145/11/NL/PC Engineering Model (Use FAZT tuneable laser

optical components for Gamma Radiation testing to 100,000 rad) Sampling speeds 1Hz over 16 sensor array

5 ARTES4.2: Hybrid Sensor Bus (HSB) optical sensing for Heinrich geostationary Hertz Satellite satellites ( ) ESTEC Contract: Status: 15 year lifetime /11/NL/PC Engineering Model (Use FAZT tuneable laser calibration, operation and wavelength referencing techniques for interrogator) Rad Hard components (Test optical components for Gamma Radiation testing to 100,000 rad) Sampling speeds 1Hz over 16 sensor array (Demonstrate functionality with EM board) Target Temperature accuracy < ±0.5 C (±5pm) (Test using FS-FBG and FAZT PDFS mitigation technique) To be on-board Heinrich Hertz Geo-Stationary Satellite Heinrich Hertz Satellite

6 15 year lifetime? Use FAZ Technology Tunable Laser Interrogation Architecture

Tunable Laser Receiver and Referencing ADC & Data Processing Fiber/Sensor Array λ

7 Intensity Time Tunable Laser Interrogator System Time Etalon Reference wavelengths Wavelength Tunable Laser Segmented Sweeps Wavelength Wavelength Tunable Laser Receiver and Referencing ADC & Data Processing Fiber/Sensor Array λ peak = fn (Strain, Temp) Data Client Tunable Laser Interrogator FBG Sensors detected Wavelength 7

FAZT Interrogator Architecture FAZT V4 FAZT I4 High Speed Data

Data communication Transmitter Polarization Controller Optics (Rx and ref.

Temp controller Lasers Control Electronics Polarization Switch

Cell Electronics ADC/FPGA Photo receivers Array of FBG fibre sensors

8 FAZT Interrogator Architecture FAZT V4 FAZT I4 High Speed Data Communications Sub-System COB Control and configuration Data processing Data communication Transmitter Polarization Controller Optics (Rx and ref.) Receiver Sensor Array Electronics Laser MCU FPGA/DAC Current Sources Temp controller Lasers Control Electronics Polarization Switch Polarization Scrambler Splitters Couplers Circulators Etalon Filters Gas Cell Electronics ADC/FPGA Photo receivers Array of FBG fibre sensors *Presented at SPIE Photonics Europe 2016 invited paper 98990Z-3 Enabling Technologies for Fiber Optic Sensing 8

7/35/40 nm Multiplexing & Synchronization GNSS Capability Wavelength Spectral Mode (@1pm resolution) 50 4 n/a Hz Relative Accuracy/Repeatability (short term) 0.01 0.05 1 (0.001 C or 0.

9 Optical Interrogator Systems Parameters FAZT V4 GSTP4 FAZT I4 GSTP5.1 OHB HSB ARTES 5.2 (RAD Hard) Number of Channels Ch Total FBG sensors capacity (+/-1nm) FBG's Sample rate (Fs) 1000/3000/ /4000/ Hz Wavelength Range 40/10/1 40/18/8 27.7/35/40 nm Multiplexing & Synchronization GNSS Capability Wavelength Spectral Mode (@1pm resolution) 50 4 n/a Hz Relative Accuracy/Repeatability (short term) (0.001 C or 0.01με) (0.005 C or 0.05με) (0.1 C or 1με) pm Absolute Accuracy (long term) ±1 ±1 ±5 (±0.1 C or ±1με) (±0.1 C or ±1με) (±0.5 C or ±5με) pm Power Consumption Watt System Dimensions 3U VPX 1U Rack 3U Eurocard 395*395* *330*44 160*100 mm Units FAZT V4 FAZT I4 HSB Commercial Product 9

10 ADC FPGA (ICM) HCM Controller Schematic diagram of the HSB Fiber Optic Interrogator EM FIM-AFE Tunable Laser PM Isolator 10% 1:4 Splitter Reserved Reserved HCN Gas Cell 25GHz Etalon 90% (50:50) Pol. SW (50:50) (50:50) OFE FBG ch1 FBG ch2 10

11 Rad Hardened? Radiation Testing of Optical components

RADIATION TESTING FOR INTERROGATOR COMPONENTS Critical optical components (MG-Y tunable laser, Etalon, Gas Cell, Iso., Pol. SW, etc.) were tested up to 100krad.

<±5pm) after 100krad, and 1E12 neutrons/cm² on certain fixed laser output wavelength points.

12 RADIATION TESTING FOR INTERROGATOR COMPONENTS Critical optical components (MG-Y tunable laser, Etalon, Gas Cell, Iso., Pol. SW, etc.) were tested up to 100krad. Radiation test set-up at Fraunhofer INT Previous laser radiation test results showed a shift up to 50pm (target spec. <±5pm) after 100krad, and 1E12 neutrons/cm² on certain fixed laser output wavelength points. This was addressed by calibrating the laser to sweep over multiple overlapping sweep segments and wavelength referencing based on FAZ Technology interrogator architecture. The wavelength references (Athermal Etalon and Gas Call) were tested when exposed to the radiation environment. The pol SW was introduced to mitigate polarization dependent frequency shift in FBGs and was tested for the same radiation conditions.. 12

13 HCN Gas Cell Line Radiation Test Results Over All Stages Channel 3 Gas Cell Channel 3 Gas Cell 3500 Start of Testing 3500 Start of Testing 3400 Closed Chamber 3400 Closed Chamber Start of 5 krad/hr End of 5 krad/hr Start of 20 krad/hr End of 20 krad/hr End of Testing Start of 5 krad/hr End of 5 krad/hr Start of 20 krad/hr End of 20 krad/hr End of Testing Wavelength / nm Wavelength / nm Chamber Closed (pm) Before Test Started During 5krad/hr (pm) Difference (pm) After 5krad/hr (pm) Difference (pm) During 20krad/hr (pm) Difference (pm) After 20krad/hr (pm) Difference (pm) The results show a maximum change of 0.32pm for the worst absorption line throughout all the radiation test stages. 13

14 Athermal Etalon Radiation Test Results Over All Stages Channel 2 Etalon Channel 2 Etalon 4500 Start of Testing 4500 Start of Testing 4000 Closed Chamber Start of 5 krad/hr 4000 Closed Chambe Start of 5 krad/h 3500 End of 5 krad/hr Start of 20 krad/hr 3500 End of 5 krad/hr Start of 20 krad/ 3000 End of 20 krad/hr 3000 End of 20 krad/h 2500 End of Testing 2500 End of Testing Wavelength / nm Wavelength / nm Chamber Open Pico-meter (pm) Wavelength Spacing Between Etalon Peaks Chamber During During After 5krad/hr Closed 5krad/hr 20krad/hr After 20krad/hr Etalon Peak Difference Mean Std Mean Std Mean Std Mean Std Mean Std Mean Std Etalon Etalon Etalon Etalon Etalon Etalon The results show that all the spacings between neighboring Etalon peaks remain at 200pm throughout the test with a standard deviation (Std) of < 0.06pm. 14

15 FBG Temp accuracy < ±0.5 C (±5pm) Achieved by using a polarization switch with the interrogator to mitigate for PDFS of FBG

) PDFS mitigation for FS-FBGs using a Polarization

5 1548.75 1549 1549.25 1549.5 1549.75 1550 1550.

25 Wavelength (nm) PDFS~13.5pm (p-p) PDFS~18.

16 Linear amplitude (a.u.) PDFS mitigation for FS-FBGs using a Polarization Switch Linear amplitude (a.u.) FBG1 FBG Wavelength (nm) Wavelength (nm) PDFS~13.5pm (p-p) PDFS~18.5pm (p-p) 1.2pm (p-p) PDFS mitigation ON 1.75pm (p-p) PDFS mitigation ON 16

Temperature measurements using FBG Temperature sensors (-30 C +70 C -30 C ) Set Temperature ( C) Lakeshore (#2631) Mean Temp ( C) Offset from Set Temperature ( C) Lakeshore (#2634) Mean Temp ( C)

1541 50 49.9288 0.0712 50.0393-0.0393 30 29.4936 0.5064 29.6307 0.3693 10 9.42949 0.57051 9.38622 0.61378-10 -10.7599 0.7599-10.6658 0.6658-30 -30.0566 0.0566-30.0724 0.

17 Temperature measurements using FBG Temperature sensors (-30 C +70 C -30 C ) Set Temperature ( C) Lakeshore (#2631) Mean Temp ( C) Offset from Set Temperature ( C) Lakeshore (#2634) Mean Temp ( C) Offset from Set Temperature ( C) The sensors were placed in a thermal chamber and the temperature of the chamber was ramped from -30 C to +70 C and back to -30 C in 20 C steps. The max offset for an individual sensor from the oven set temperature was <0.76 C which is due to the oven specified stability/accuracy of ± 0.5 C. The two temperature sensors measured a maximum differential error <0.25 C 17

Tracking of a total of 16 FBG sensors @ 1Hz for ~4.

18 Tracking of a total of 16 FBG 1Hz for ~4.8 hours with 16 sensors (2 channels) The 16 FBG sensors were distributed across the two channels (8 per channel) and captured at 1Hz highlighting the stability of the system for ~4.8hours The polarization switch was active during the full duration and the wavelength scan range was ~27.7nm

19 Conclusion The design and operation a fiber optic interrogator system as part of a hybrid sensor bus system (HSB) engineering model has been demonstrated with 16 FBG sensors scanned at 1Hz. A polarization mitigation technique using a polarization switch was also introduced in the system to achieve the desired specifications of ± 0.5pm (~±0.5 C) using radiation tolerant femto-second inscribed FBGs (fs-fbg). The performance of the system was also evaluated using commercial FBG temperatures sensors operating from -30 C +70 C. The HSB system is planned to be used as a flight demonstrator on-board the German Heinrich Hertz geo-stationary satellite.

20 THANK YOU

Proc. of SPIE Vol Y-1

Proc. of SPIE Vol Y-1 International Conference on Space Optics 18 21 October 2016 Edited by Bruno Cugny, Nikos Karafolas and Zoran Sodnik ente on Space Optics 31. 18-21 October www.icso2016.com Design challenges of a tunable