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A method of step height and plate thickness measurement wit
A method of step height and plate thickness measurement wit

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Naoyui Koyama Michelson interferometer Use two lasers simultaneously Laser a wavelength λ a Laser b wavelength λ b Laser a and laser b do not interfere each other ID: 512662 Download Presentation

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variation plate laser intensity plate variation intensity laser interferometer wavelength beam thickness measurement optical fig measure detector prism michelson

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Slide1

A method of step height and plate thickness measurement within the unambiguous range of two laser wavelength interferometer

Naoyui

Koyama Slide2

Michelson interferometer

Use two lasers simultaneously

Laser

a wavelength :λa Laser b wavelength :λb

Laser a and laser b do not interfere each otherSlide3

Example 1

λ

a

/

λ

b

=11/13

λ

a

λ

b

λ

a+λb

Moire pattern

Intensity variation within “beat” B is inverse of “beat” AIntensity varies periodicallyA → B → A → BSlide4

Example 2

A

B

A

Intensity variation within “beat” B is inverse of “beat” A

λ

a

=635nm

λ

b

=785nmSlide5

Modifications of Michelson interferometer

DETECTOR : single detector

→  CMOS(or CCD) detectorMIRROR : perpendicular to the reference beam → slightly inclined

No part of the instrument is mechanically movable, making possible precise measurements free of backlash.  Slide6

Optical path length of reference beam of column ris w(s-r)(sin2θ

longer than column s

CMOS detectorSlide7

Step height is (1/2)(s-r)w(sin2θ)

CMOS detectorSlide8

How to measure thickness of a plate

No plate is placed in the pass of measurement beam, or infinitesimal thin plate is set at zero point of optical path length difference, both case get same intensity variation as Fig. 1.Slide9

How to measure thickness of a plate

Intensity variation is periodic, when infinitesimal thin plate is placed at ,

same intensity variation as placed at

is seen (Fig. 1).Slide10

How to measure thickness of a plate

Measurement beam passed through the prism is reflected by the plate.

 

When the plate is placed appropriately, intensity variation is the same as Fig. 1. Intensity variation of measurement beam not pass through the prism is shifted as seen in Fig. 2. Thickness of the plate d is calculable from this shift.Slide11

An example instrument to measure intensity variation and corresponding optical path length difference.Slide12

How optical axes of two wavelength laser diode coincide with each other by prism.

Compensation plateSlide13

Two beams partially overlap each other.Two wavelength Michelson interferometer is usable as one wavelength Michelson interferometer simultaneously.Slide14

ReferencesN. Koyama, A method of step height measurement within the unambiguous range of two laser wavelengths interferometer,

Optik

, 126 (2015) 313-316.

All slides shown here can be seen in my website.N. Koyama, http://www12.plala.or.jp/sokkyo/prism.html

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