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


• 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.


Constructive interference

Background

Destructive interference

In phase

Out of phase


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.


2𝜋

Background

𝜋

0 2𝜋δ 𝜋

0λ


• 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.

PACMAN internal meeting, 28/04/2014PACMAN internal meeting, 28/04/2014

Interferometry

Coherent light source

Detector

Beam SplitterMovableReflector

Michelson Interferometer Fixed

Reflector


• 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.


Displacement Interferometry

Intensity minima

Intensity maxima


• Absolute distance measuring interferometric technique.• Measures phase changes in a

measurement and reference interferometer as frequency is scanned.

• (when no drift)


FSI


Tuna

ble

lase

r

Measurement interferometer OPD:

Reference interferometer OPD:


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).


FSI


Drift: Change of interferometer length during measurement typically through thermal expansion or contraction.In the presence of drift the phase ratio, q is

=

Where,


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.


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.



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.


Measurement interferometer

Reference interferometer

Tunable laser

Tunable laser

Drift cancellation


• 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).


Refractive index,


• 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.


Determining the position of fibre launchers


• 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.


Determination of fiducial coordinates


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.


Reference Interferometer



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


Laser• Narrow line width: Provides good fringe visibility. • Wide tuning range: Better measurement precision.• Diode laser most commonly used for FSI.• Replaced dye lasers.


Main System components


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.


Main system components


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.


Main system components


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


• 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.


Monitoring and Control




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.


Other FSI applications

ATLAS Experiment © 2014 CERN


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.


Project 1.2


• 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.


References

Environment

Fsi pacman meeting