Download - Interferometery Basics
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BASICS OF INTERFEROMETRY
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CCDCCD
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Interferometer
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Typical Interferometer
The expanded beamThe expanded beamexiting from the lightexiting from the lightsource is divided by asource is divided by a
BeamsplitterBeamsplitterinto twointo twobeams.beams.
One beam is reflectedOne beam is reflectedfrom the Referencefrom the Reference
Mirror, and the other oneMirror, and the other onefrom the Sample.from the Sample.
These two beams areThese two beams arerecombined by therecombined by theBeamsplitterBeamsplitterto interfere.to interfere.
The imaging lensThe imaging lensimages the interferogramimages the interferogram
onto the CCD camera.onto the CCD camera.
CCDCCD
Sample
ReferenceMirror
BeamsplitterTest arm
Reference arm
Optical Path Difference (OPD)- difference in optical path lengths that
beams travel in Reference and Test arms.
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Tilt of one of Mirrors in Interferometer
If one of the mirrors isIf one of the mirrors isslightly tilted,slightly tilted,
then the reflected beamthen the reflected beam(wavefront) also is tilted.(wavefront) also is tilted.
For two tilted and flat wavefronts, aninterferogram of straight, parallel, light and
dark bands will be formed.
CCDCCD
ReferenceMirror
Beamsplitter
Sample
If mirror and flatIf mirror and flatsample are perfectlysample are perfectly
perpendicular, thenperpendicular, thenreflected wavefrontsreflected wavefrontsare parallel.are parallel.
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Interferogram
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Interferogram for Flat Wavefronts with Tilt
Two interferingTwo interferingwavefrontswavefronts
InterferogramInterferogram,,(interference pattern(interference patternor fringe pattern)or fringe pattern)
Interference between
two wavefronts is
constructive at thesemultiple points,destructive at others,forming an
interferogram.
Intensity profileIntensity profile
of interferogram.of interferogram.
Fringe spacing
corresponds to pathdifference between
wavefronts.
Tested beam
(wavefront)
Reference beam (wavefront).
Multiple distancesbetween wavefronts,
where is the wavelengthof the source. 44
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Change in Tilt Causes Change in Number of Fringes.
Reference
Test
The number and spacing of fringes
changes with tilt.
NULL FRINGES
When wavefronts are parallel thenWhen wavefronts are parallel thenthe fringes arethe fringes are nullednulled and almostand almost
uniform intensity is visible in theuniform intensity is visible in thefield of view.field of view.
Press EnterPress Enter
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Shape of fringes
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When one wavefront is spherical and the other is flat, and in adWhen one wavefront is spherical and the other is flat, and in additionditionthere is some tilt between interfering wavefronts, then the frinthere is some tilt between interfering wavefronts, then the fringesges
will be curved. When tilt is not present, the fringes are circuwill be curved. When tilt is not present, the fringes are circular.lar.
Interferograms forSpherical Sample
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The fringes can represent a concave wavefront instead convexThe fringes can represent a concave wavefront instead convexwavefront as on previous slide.wavefront as on previous slide.
Interferograms forSpherical Sample
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Typical Interferogram for
Fringes Phase map
Flat Surface
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Typical Interferogram for
Fringes Phase map
Cylindrical Surfaces
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Interference Microscope
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# of fringes 2/
Filter Bandwidthand Number of Fringes
Narrow bandwidth filter
(3nm) (in PSI)
Medium bandwidth filter
(40nm)
Wide bandwidth filter
(300nm) - white light (inVSI)
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Microscope Diagram
Digitized IntensityData
Beamsplitter
Detector Array
Illuminator
MicroscopeObjective
Translator
MirauInterferometer
Light Source
ApertureStop
FieldStop
Filter
Interference
Filters all but the red
light from white lightof halogen lamp
Sample
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Michelson Interferometer
1.5X, 2.5X, 5Xsmall divergence of beam
long working distance Microscope
Objective
Sample
Beamsplitter
Cube
Reference
Mirror
Workingdistance
New workingdistance
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Mirau Interferometer
Reference
Microscope
Objective
Sample
BeamsplitterPlate
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Mirau Interferometer for Small Magnification?
Reference
Microscope
Objective
Sample
BeamsplitterPlate
1.5X, 2.5X, 5Xsmall divergence of beam
long working distance
LARGE central obscuration
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Rt: 2.62 umRa: 255.14 nmRq: 344.18 nmSurface Stats:
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PV: 23.64 um
RMS: 7.23 um
Surface Stats:
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