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LiCAS Project: FSI Overview

LiCAS Project: FSI Overview. Richard Bingham, Edward Botcherby, Paul Coe, John Green, Grzegorz Grzelak, Ankush Mitra , John Nixon, Armin Reichold University of Oxford Andreas Herty, Wolfgang Liebl, Johannes Prenting, Markus Schloesser Applied Geodesy Group, DESY. Contents. FSI Concept

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LiCAS Project: FSI Overview

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  1. LiCAS Project: FSI Overview Richard Bingham, Edward Botcherby, Paul Coe, John Green, Grzegorz Grzelak, Ankush Mitra, John Nixon, Armin Reichold University of Oxford Andreas Herty, Wolfgang Liebl, Johannes Prenting, Markus Schloesser Applied Geodesy Group, DESY

  2. Contents • FSI Concept • ATLAS FSI • Implementation • Results • LiCAS Extensions FSI Overview

  3. IRef time Change of phase: DFRef IGLI time Frequency Scanning Interferometry • Interferometric length measurement technique • Require precision of 1mm over 5m • Originally developed for online alignment of the ATLAS SCT tracker Tunable Laser Reference Interferometer: L Measurement Interferometer: D (Grid Line Interferometer (GLI)) Change of phase: DFGLI FSI Overview

  4. FSI: Length Measurement DFGLI DFRef FSI Overview

  5. FSI: Thermal Drift Cancellation • Thermal effects add subtle systematic errors to FSI • Nanometre movements can contribute micron errors (µ (n/Dn) ) • Use two lasers tuning in opposite directions to cancel thermal drift FSI Overview

  6. FSI: Thermal Drift Cancellation DFGLI DQ(+) True Gradient DQ(-) Measured Gradient with Laser Tuning Up Measured Gradient with Laser Tuning Down DFRef FSI Overview

  7. FSI: 2-Laser Thermal Drift Cancellation FSI Overview

  8. FSI: ATLAS Implementation FSI Overview

  9. 6 simultaneous length measurements made between four corners of the square. +7th interferometer to measure stage position. Displacements of one corner of the square can then be reconstructed. FSI: ATLAS Test Grid FSI Overview

  10. FSI: ATLAS Resolution FSI Overview

  11. 1mm FSI: ATLAS Resolution • Stage is kept stationary • RMS 3D Scatter < 1 mm FSI Overview

  12. Amplitude Modulation @ f1 C-Band Amplifier (1520-1570 nm) f1 L-Band Amplifier (1572-1630 nm) f2 Amplitude Modulation @ f2 APD Demodulator @ f1 , F1 Demodulator @ fn , Fn ADC +AMPS RAM Detectors Demodulator @ f2 , F2 Retro Reflector To PC LiCAS FSI System ATLAS FSI System Laser 1 Laser 2 Reference Interferometer Splitter Tree piezo detector Uncollimated Quill Collimated Quill 1m GLI 5m GLI FSI Overview

  13. l2 wavelength l1 time Amplitude Modulation @ f1 f1 Volts Demodulator @ f1 , F1 f2 Amplitude Modulation @ f2 l2 time wavelength Demodulator @ f2 , F1 Volts l0 time time Two Laser AM Demodulation • Need 2 lasers for drift cancellation • Have both lasers present & use AM demodulation to electronically separate signals M1 t0 t1 M2 Laser 1 Laser 2 Detector t0 t1 FSI Overview

  14. Two Laser AM Demodulation 15% mod. Volts 15% mod. Time Volts Time • Amplitude Modulation on FSI fringe • @ 40 & 80 kHz (now) 0.5 & 1MHz (later) • High Pass Filter • FSI fringe stored as amplitude on • Carrier (àla AM radio) • Demodulation reproduces FSI Fringes FSI Overview

  15. Results of Demodulation Both signals have same frequency !! Demodulation of modulated laser does not effect interferometer signal FSI Overview

  16. Carré Algorithm Reference Interferometer Phase Extraction • Reference Interferometer is FSI’s “yard-stick” • Must measure interferometer phase precisely • Uses standard technique of Phase-Stepping Step1: I(ftrue-1.5Df) Step2: I(ftrue-0.5Df) Step3: I(ftrue+0.5Df) Step4: I(ftrue+1.5Df) Reference Interferometer mirror moved in 4 equal sized steps ftrue FSI Overview

  17. Raw Data Reconstructed Interferometer Signal Software Phase Extraction • Telecoms laser tunes linearly • Extract phase with software “phase-stepping” FSI Overview

  18. FSI Challenges for LiCAS • Telecoms wavelength (1520nm – 1640nm) • Cheap, high quality tuneable lasers • 120nm (15THz) continuous tuning range • 40nm/s (5THz/s) continuous tuning speed • Use Erbium Doped Fibre Amplifiers • Output power between 4mW and 125mW • Reduced cost • Increased flexibility and reusability FSI Overview

  19. g g Incoming Single Photon g Outgoing Photons Erbium Doped Fibre Amplifiers • EDFA are optical power amplifiers • Used to amplify low power tunable laser • Standard equipment for Telecoms • but will it work for FSI ? 4I11/2 Pump 980nm Decay Single Telecoms Channel 4I13/2 Signal ~1550nm fluorescence 4I15/2 1610 Wavelength / nm 1530 FSI Overview

  20. EDFA System FSI Overview

  21. Fibre end Collimation lens Retroreflector Quill Collimation • Refractive • Reflective Fibre Retroreflector Reflective, off-axis paraboloid FSI Overview

  22. Commercial Collimation FSI Overview

  23. FSI Overview

  24. Fibre Retro Custom Collimation • 0.8% Return for on-axis Retro • 0.04% return for 7mm off axis retro FSI Overview

  25. Reference Interferometer 1 • Michelson style interferometer Beam Splitter Photodetector Long Arm Short Arm FSI Overview

  26. Reference Interferometer 2 • GLI Style interferometer Retroreflector Collimatior FSI Overview

  27. Summary • FSI provides abosoluted length measurements • Multiple laser system using amplitude modulation • Software phase extraction • Telecommunications wavelengths: cheap, high quality equipment • Erbium Doped Fibre Amplifiers provide scalable power output • Collimation optics used for longest distance FSI Overview

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