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WP3 Frequency Scanning Interferometry Analysis Techniques for the LiCAS RTRS

WP3 Frequency Scanning Interferometry Analysis Techniques for the LiCAS RTRS. John Dale. Introduction. Aims and Requirements LiCAS-Rapid Tunnel Reference Surveyor (RTRS) Overview Current Status Frequency Scanning Interferometry (FSI). Aims and Requirements.

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WP3 Frequency Scanning Interferometry Analysis Techniques for the LiCAS RTRS

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  1. WP3Frequency Scanning Interferometry Analysis Techniques for the LiCAS RTRS John Dale

  2. Introduction • Aims and Requirements • LiCAS-Rapid Tunnel Reference Surveyor (RTRS) Overview • Current Status • Frequency Scanning Interferometry (FSI)

  3. Aims and Requirements • ILC will have ~70km of beam lines. • LiCAS will survey straight sections. • Keep machine downtime to an acceptable level. • 5m of tunnel per minute (7km of tunnel per day) • ~ 30 times faster than a team of three surveyors using a laser tracker. • Fully remote controlled • 1 operator for multiple RTRS’s • Required to enable alignment to • 200 μm over 600m vertically (a betatron wavelength) • 500 μm over 600m horizontally

  4. internal FSI wall markers external FSI LSM beam collider component LiCAS Concept Tunnel Wall Reconstructed tunnel shapes (relative co-ordinates) produced by Dr Armin Reichold

  5. LiCAS RTRS Train

  6. Current Status • Prototype shipped to DESY in spring • Calibration to begin in May • Prototype running and data taking over summer 2007

  7. Tunable Laser Reference Interferometer Measurement Interferometer FSI Sub-System • FSI sub-system uses 2 interferometers, with the same tuneable laser • Reference Interferometer has a precisely known optical path length. • As laser tunes interference fringes are produced. • ωgli/ ωref =Dgli/Dref 35.5 fringes Reference Length 6m 11.5 fringes Measurement Length 6*11.5/35.5= 1.94m

  8. Reference Interferometers External FSI Splitter Tree EDFA Laser FSI Sub-System

  9. Current Analysis Steps • Reference Interferometer Phase Extraction using Carre algorithm • Analytical method to determine phase • Requires 4 points equally spaced in phase • Problem 1: points are evenly spaced in time not in phase, causes errors in the extracted phase. • Problem 2: Extracted phase in range 0-2π, requires unwrapping, can lead to unwrapping errors • Spectral Analysis of Intensity vs. Extracted Phase • using the Lomb Periodogram • Peak Fitting to give frequencies • Gaussian peak fitting

  10. Analysis Techniques Under Development • Extended Kalman Filtering (EKF) techniques for phase extraction • Recursive filter which estimates the state of a dynamic system. • Initial have φi and Δφi • Can guess by φi+1=φi + Δφi and Δφi+1 = Δφi • Improves guess by looking at residuals in data and sin(φi+1) • Leads in incorrect improvement in region of π/2 and 3π/2 • Improved by running filter forwards and then backwards • Advantage 1: No phase unwrapping required • Advantage 2: Computationally quicker

  11. Comparison between Carre and EKF 0.01% Noise 0.005% Noise

  12. Preliminary comparison of length analysis between Carre and EKF • Two analysis chains set up 1) Carre, Lomb, peak fitting, length calculation 2) EKF, Lomb, peak fitting, length calculation • Preliminary results show EKF increases the length measurement precision by ~20%

  13. Thank You

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