interferometery basics

8/12/2019 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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interferometery basics

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