WP3 Frequency Scanning Interferometry Analysis Techniques for the LiCAS RTRS
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Transcript of WP3 Frequency Scanning Interferometry Analysis Techniques for the LiCAS RTRS
WP3Frequency Scanning Interferometry
Analysis Techniques for the LiCAS RTRSJohn Dale
Introduction
• Aims and Requirements
• LiCAS-Rapid Tunnel Reference Surveyor (RTRS) Overview
• Current Status
• Frequency Scanning Interferometry (FSI)
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
produced by Dr Armin Reichold
collider component
LiCAS Concept
Tunnel Wall
Reconstructed tunnel shapes(relative co-ordinates)
wall markers external FSIinternal FSI LSM beam
LiCAS RTRS Train
Current Status
• Prototype shipped to DESY in spring
• Calibration to begin in May
• Prototype running and data taking over summer 2007
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
11.5 fringes
Measurement Length 6*11.5/35.5= 1.94m
Reference Length 6m
FSI Sub-System
Reference Interferometers
Laser
EDFA
Splitter Tree
External FSI
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
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
Comparison between Carre and EKF
0.005% Noise 0.01% Noise
Preliminary comparison of length analysis between Carre and EKF
• Two analysis chains set up1) Carre, Lomb, peak fitting, length calculation
2) EKF, Lomb, peak fitting, length calculation
• Preliminary results show EKF increases the length measurement precision by ~20%
Thank You