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
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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