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

1 April 2003 FSI Overview2 Contents  FSI Concept  ATLAS FSI Implementation Results  LiCAS Extensions

1 April 2003 FSI Overview3 Frequency Scanning Interferometry  Interferometric length measurement technique  Require precision of 1  m over 5m  Originally developed for online alignment of the ATLAS SCT tracker Tunable Laser Reference Interferometer: L Measurement Interferometer: D Change of phase:  GLI Change of phase:  Ref time I Ref time I GLI (Grid Line Interferometer (GLI))

1 April 2003 FSI Overview4 FSI: Length Measurement  GLI  Ref

1 April 2003 FSI Overview5 FSI: Thermal Drift Cancellation  Thermal effects add subtle systematic errors to FSI −Nanometre movements can contribute micron errors (   Use two lasers tuning in opposite directions to cancel thermal drift

1 April 2003 FSI Overview6 FSI: Thermal Drift Cancellation  GLI  Ref   True Gradient Measured Gradient with Laser Tuning Up Measured Gradient with Laser Tuning Down

1 April 2003 FSI Overview7 FSI: 2-Laser Thermal Drift Cancellation

1 April 2003 FSI Overview8 FSI: ATLAS Implementation

1 April 2003 FSI Overview9 FSI: ATLAS Test Grid  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.

1 April 2003 FSI Overview10 FSI: ATLAS Resolution

1 April 2003 FSI Overview11 1m1m FSI: ATLAS Resolution  Stage is kept stationary  RMS 3D Scatter < 1  m

1 April 2003 FSI Overview12 Retro Reflector ATLAS FSI System Laser 1 Laser 2 Reference Interferometer piezo detector C-Band Amplifier ( nm) L-Band Amplifier ( nm) Splitter Tree LiCAS FSI System 1m GLI Uncollimated Quill APD Collimated Quill 5m GLI ADC + AMPS RAMRAM To PC f1f1 f2f2 Amplitude f 1 Amplitude f 2 Detectors f 1,  1 f 2,  2 f n,  n

1 April 2003 FSI Overview13 Laser 1 M1 M2 Detector Laser 2 f 1,  1 f 2,  1 wavelength time 1 2 wavelength time 0 2 Volts time Volts time t0t0 t1t1 t0t0 t1t1 Amplitude f 1 Amplitude f 2 f1f1 f2f2 Two Laser AM Demodulation  Need 2 lasers for drift cancellation  Have both lasers present & use AM demodulation to electronically separate signals

1 April 2003 FSI Overview14 Volts Time 15% mod. Time Volts Amplitude Modulation on FSI 40 & 80 kHz (now) 0.5 & 1MHz (later) FSI fringe stored as amplitude on Carrier (à la AM radio) Demodulation reproduces FSI Fringes High Pass Filter Two Laser AM Demodulation

1 April 2003 FSI Overview15 Results of Demodulation Demodulation of modulated laser does not effect interferometer signal Both signals have same frequency !!

1 April 2003 FSI Overview16 Reference Interferometer Phase Extraction  Reference Interferometer is FSI’s “yard-stick” Must measure interferometer phase precisely  Uses standard technique of Phase-Stepping Step1: I(  true -1.5  ) Step2: I(  true -0.5  ) Step3: I(  true +0.5  ) Step4: I(  true +1.5  ) Carré Algorithm  true Reference Interferometer mirror moved in 4 equal sized steps

1 April 2003 FSI Overview17 Raw DataReconstructed Interferometer Signal Software Phase Extraction  Telecoms laser tunes linearly  Extract phase with software “phase-stepping”

1 April 2003 FSI Overview18 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

1 April 2003 FSI Overview19 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 ? Decay Signal ~1550nm Pump 980nm 4 I 15/2 4 I 11/2 4 I 13/2  Incoming Single Photon   Outgoing Photons fluorescence Wavelength / nm Single Telecoms Channel

1 April 2003 FSI Overview20 EDFA System

1 April 2003 FSI Overview21 Quill Collimation  Refractive  Reflective Fibre end Retroreflector Collimation lens Retroreflector Reflective, off- axis paraboloid Fibre

1 April 2003 FSI Overview22 Commercial Collimation

1 April 2003 FSI Overview23

1 April 2003 FSI Overview24 Custom Collimation  0.8% Return for on-axis Retro  0.04% return for 7mm off axis retro

1 April 2003 FSI Overview25 Reference Interferometer 1  Michelson style interferometer Long Arm Short Arm Beam Splitter Photodetector

1 April 2003 FSI Overview26 Reference Interferometer 2  GLI Style interferometer Retroreflector Collimatior

1 April 2003 FSI Overview27 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