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Transcript of Fsi pacman meeting
Frequency Scanning Interferometry (FSI) measurements
PACMAN internal meeting28/04/2014
Solomon William Kamugasa
Frequency Scanning Interferometry (FSI) Measurements
1. Background concepts2. Interferometry: (Displacement & FSI)3. Sources of error in FSI4. System components5. How to determine 3D position using FSI6. FSI in CLIC7. Other applications of FSI8. Ongoing work
PACMAN internal meeting, 28/04/2014
Contents
Frequency Scanning Interferometry (FSI) Measurements
• Superposition: Resultant displacement produced by a number of waves at a point is the algebraic sum of displacements of the individual waves.• Interference: Combination of 2 waves to form composite waves.
PACMAN internal meeting, 28/04/2014
Constructive interference
Background
Destructive interference
In phase
Out of phase
Frequency Scanning Interferometry (FSI) Measurements
Phase: Used to describe a specific location within a cycle of a periodic wave. * OPL Phase ambiguity when OPL > Phase difference: Describes how far out of sync two waves are. * OPD OPD is Optical Path Difference; OPL is Optical Path Length.
PACMAN internal meeting, 28/04/2014
2𝜋
Background
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0 2𝜋δ 𝜋
0λ
Frequency Scanning Interferometry (FSI) Measurements
• Technique based on interference that measures properties of light waves such as wave length and optical path length.
Requires:• Coherent light source.• Monochromatic light.• Measures displacement by observing
fringes.
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Interferometry
Coherent light source
Detector
Beam SplitterMovableReflector
Michelson Interferometer Fixed
Reflector
Frequency Scanning Interferometry (FSI) Measurements
• Light of fixed wavelength.• Precise (fraction of wavelength).• Cannot measure absolute distance
directly.• To measure displacements > requires
fringe counting.• Target needs to be physically moved.• Measurements have to be repeated if
system loses count of cycles.
PACMAN internal meeting, 28/04/2014
Displacement Interferometry
Intensity minima
Intensity maxima
Frequency Scanning Interferometry (FSI) Measurements
• Absolute distance measuring interferometric technique.• Measures phase changes in a
measurement and reference interferometer as frequency is scanned.
• (when no drift)
PACMAN internal meeting, 28/04/2014
FSI
PACMAN internal meeting, 28/04/2014
Tuna
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lase
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Measurement interferometer OPD:
Reference interferometer OPD:
Frequency Scanning Interferometry (FSI) Measurements
Advantages• Measures absolute distance.• Beams can be interrupted as does not rely on fringe counting.• Ability to measure several interferometers simultaneously.
Disadvantages• Tend to be less accurate than displacement interferometry (because it
measurements are made relative to a physical reference).• Accuracy reduced by drift errors (dominant source of error in FSI).
PACMAN internal meeting, 28/04/2014
FSI
Frequency Scanning Interferometry (FSI) Measurements
Drift: Change of interferometer length during measurement typically through thermal expansion or contraction.In the presence of drift the phase ratio, q is
=
Where,
PACMAN internal meeting, 28/04/2014
where: is the average frequency.
is the scanned frequency.
& are reference & measurement interferometer drifts.
is a magnifying factor that causes much greater error than the drift itself (typically >100).
is the relative drift error.
Frequency Scanning Interferometry (FSI) Measurements
Drift management• Limited by control of environmental factors that cause drift.
• Using faster electronics that make measurements quicker.
• Using a stable reference such as Invar (nickel-iron alloy) with low CTE ≈ 1.2 ppm per °C in range of 20-100 °C.
• Reference interferometer drift can be eliminated by replacing physical length reference.
PACMAN internal meeting, 28/04/2014
Frequency Scanning Interferometry (FSI) Measurements
Corrected by 2 lasers scanning simultaneouslyin opposite directions.
• is the drift error & magnification factorSince beam travel same path, = where, =2 similar lasers scanning in opposite directionssimultaneously ≈ -1.0.
PACMAN internal meeting, 28/04/2014
Measurement interferometer
Reference interferometer
Tunable laser
Tunable laser
Drift cancellation
Frequency Scanning Interferometry (FSI) Measurements
• Modelled using refractivity, -1• Refractometer: Directly measures • Alternatively by measuring the parameters on which depends i.e.
temperature, pressure, humidity and .• Relies on homogeneity of air.• Use of air tight container (not very practical, severe engineering and
high cost).
PACMAN internal meeting, 28/04/2014
Refractive index,
Frequency Scanning Interferometry (FSI) Measurements
• Position of launchers is required in order to determine that of the fiducials.• Distance from launcher to fiducials determined
using FSI.• Fiducial coordinates previously measured by
CMM.• Coordinates of fibre launcher determined using
the equation below:
• Whereis the fiducial number.
PACMAN internal meeting, 28/04/2014
Determining the position of fibre launchers
Frequency Scanning Interferometry (FSI) Measurements
• Requires at least 3 launchers with known position.• Unknown coordinates of fiducial (determined using the equation below.
where is the fibre number.• Networks will be resolved using Least Squares
Adjustments.
PACMAN internal meeting, 28/04/2014
Determination of fiducial coordinates
Frequency Scanning Interferometry (FSI) Measurements
Physical length standard• Reference needs to be calibrated in order to be traceable.• Measurements rely on stability of reference.Alternative• Wavelength of a He-Ne laser determined by energy levels in the gas
atoms in the laser cavity (little dependence on ambient conditions).• Atomic transitions lines in an absorption gas cell (weak dependence on
ambient conditions).• Benefit: Requires a single calibration, valid for the life of the instrument.
PACMAN internal meeting, 28/04/2014
Reference Interferometer
Frequency Scanning Interferometry (FSI) Measurements
PACMAN internal meeting, 28/04/2014
Measurement interferometer• Each measurement interferometer consists of a quill (two parallel
optical fibres and a beam splitter) and a reflector.
Return fibre
Delivery fibre
Beam SplitterRetroreflector
Frequency Scanning Interferometry (FSI) Measurements
Laser• Narrow line width: Provides good fringe visibility. • Wide tuning range: Better measurement precision.• Diode laser most commonly used for FSI.• Replaced dye lasers.
PACMAN internal meeting, 28/04/2014
Main System components
Frequency Scanning Interferometry (FSI) Measurements
Optical fibres • Used to deliver light waves from laser to interferometer and to deliver
the return signal from interferometer to photo detector.• Light propagated through fibres by total internal reflection.• Single mode fibres (D=8-10μm).• Sharper light longer distances.
PACMAN internal meeting, 28/04/2014
Main system components
Frequency Scanning Interferometry (FSI) Measurements
Retroreflectors Incident light is reflected exactly in the direction of origin.Typically corner cube (3 mutually orthogonal surfaces).
Retroreflectors are made of various materials.• Aluminium pellets coated with gold to enhance reflectivity
(ATLAS).• Commercially - Glass prisms (utilise total internal reflection).• Mirror reflectors are also available.
PACMAN internal meeting, 28/04/2014
Main system components
Frequency Scanning Interferometry (FSI) Measurements
FSI in CLICCMM: Most accurate (0.3 μm + 1ppm) for Leitz at CERN.
Loses accuracy beyond measurement volume of 1.2*1.0*0.7.Impossible to use when dimensional control measurements are required
on site.
Existing portable means don’t perform to the required accuracy.• Faro Romer Arm: Accuracy (5-10 μm at 1σ)• Leica AT401 Laser Tracker (7-10 μm at 1σ)• Photogrammetry (12 μm at 1σ)
Therefore need to develop a portable means that is more accurate.• Microtriangulation (1.3)• FSI (1.2)PACMAN internal meeting, 28/04/2014
Frequency Scanning Interferometry (FSI) Measurements
• CMMs show systematic deviations resulting from aging & elastic deformations etc.• FSI will be used to continuously
monitor the CMM to provide control of systematic deviations.
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Monitoring and Control
Frequency Scanning Interferometry (FSI) Measurements
FSI at ATLASUsed to remotely monitor shape changes of the SCT.• Measurement precision of 1μm. • 3D coordinate reconstruction to 10μm.• 842 measurement interferometers.• Radiation-hard low mass components.• No maintenance over 10 years.• Fibres 100m long from interferometer to detector/laser.
PACMAN internal meeting, 28/04/2014
Other FSI applications
ATLAS Experiment © 2014 CERN
Frequency Scanning Interferometry (FSI) Measurements
Work in progressDevelop fiducials measureable by FSI, Micro-triangulation & CMM.Why?More redundancy and ability to detect faults.Study the mechanics of fibre ends and targets in order to determine their systematic offsets/errors and those of the target holders.Study various configurations of the FSI network in order to select the best one through simulations.Extrapolate to a portable solution.
PACMAN internal meeting, 28/04/2014
Project 1.2
Frequency Scanning Interferometry (FSI) Measurements
• Coe P. A (2001) An Investigation of Frequency Scanning Interferometry for the alignment of the ATLAS semiconductor tracker. Dphil thesis, University of Oxford.
• Dale J. (2009) A Study of Interferometric Distance Measurement Systems on a Prototype Rapid Tunnel Reference Surveyor and the Effects of Reference Network Errors at the International Linear Collider. Dphil thesis, University of Oxford.
• Griffet S., Cherif A., Kemppinen J., Mainaud Durand H., Rude V., Sterbini G. Strategy and validation of fiducialisation for the pre-alignment of CLIC components, CERN Geneva, Switzerland.
• Griffet S (2010) Fiducialisation and Dimensional Control: Study of existing means and expected performances. EDMS 1097661.
• Warden M.S (2011) Absolute distance metrology using frequency swept lasers. DPhil thesis, University of Oxford.
• Absolute Multiline®-Technology A revolution in length metrology. Available on Indico, Presented by Mainaud Durand H.
PACMAN internal meeting, 28/04/2014
References