For interferometric topography measurements of optically rough surfaces dual wavelength heterodyne Interferometry (DWHI) is a powerful tool. A DWHI system based on a two-wavelength HeNe laser and a matched grating technique is described. This set-up improves system stability and simple heterodyne frequency generation

Fischer, Edgar

Ittner, Thomas

Sodnik, Zoran

Tiziani, Hans J.

Online Publikationen der Universität Stuttgart

Dual Wavelength Heterodyne Interferometry for Rough Surface Measurements 
E. Flscher, Z. Sodnlk, Th. Ittner and H.J. Tlzlanl 
Instltut fUr Technlsche Optlk, Unlyersltlt Stuttgart 
Pfaffenwaldrlng g, 0 7000 - Stuttgart 80, Fed. Rep. Germany 
ABSTRACT 
For interferometrfc topography measurements of optfcally rough 
dual wayel.ngth h.terodyne Interfero.etry (DWHI) I. a powerful 
DVHI system based on a two-wavelength HeNe laser and a .atched 
technique 1s described. This set-up improves sy s tem stability and 
simple heterodyne frequency generation. 
I. INTRODUCTION 
surfaces 
too 1. A 
gratIng 
allows a 
Applying interferometrfc technique for the analysfs of optfcally rough 
surfaces two serfous problems arfse. Ffrst measurements become difficult 
or fmpossfble due to the speckle effect. Second when using a reflexion set-
up the ambIguIty range of the measurements Is lImIted to a half of the 
laser wavelength . To overcome the disadvantages fnterferometer wfth grazing 
fnc1dence t , as well as wfth fncreased laser wavelength' were developed, 
alternatively two wavelength interferometry fs used. By applyfng two 
wavelengths simultaneously to the object the sensftfvfty is reduced to an 
effective w ... length 3,' given by ~'" = ~t · ~,/ I~t-~,I . Fercher et al.' de-
scrfbed a two wavelength heterodyne speckle 1nterfero_eter where the phase 
of the effectfve wavelength must be evaluated from two 1ndependent detector 
signals. In this paper a scanning dual wavelength heterodyne technique 1s 
described, that generates a low frequency detection s1gnal wfth a phase 
shift that corresponds to the effective wavelength. 
2. PRINCIPLE OF A MATCHED SRATING SET-UP 
Dual wavelength heterodyne 1nterferometrry (DWHI) bases on two independent , 
simultaneously and coaxfal workfng heterodyne interferometers with dif-
ferent frequencies "'t and "'2 and different heterodyne frequencfes ft and f2 • The phase of the beat frequency fl-12 depends on the synthetfc wave ... 
length l~ and can therefore be examined for dfstance evaluatfon 6• 
In fIg. I a multl-wayelength HeNe laser Is 
at 632.8 and 640.1 nm leadIng to a leq of 
Mdulator (AOM) driven with f.t • 40 MHz Is 
shIft part of the lIght to be used as 
reference bea.s. In the first diffraction 
order one gets light with frequencies 
Vl+fct and "'2+fd for the two laser wave-
lengths ~t, ~, • Th. reference light pas_ 
ses first a high spatIal frequency dIs-
persion gratIng S. It Is desIgned such 
that the diffraction angle difference 6a 
between vt+f.t and ..,+f.t Is the .... as 
the first order diffraction angle of a 
following low spatIal frequency grating 
RG. The second motor driven grating 
leads to a frequency shift f. of the 
diffracted lIght In the first order. Due 
to appropriate adjustment, the 1. order 
dIffraction of vt+ft = vt+f.t+f. and 
O. order of VJ;+(, C lJ2+fct become parallel 
after posslng RG as shown In fig. I. The 
reference beat frequency f. = (1-12 is generated by the rotation of an angle 
encoder. In our experi_ent we found 20KHz 
670 / SPlE VoI. '319 ~ic. in CompMll Syn.",. (1990) 
shown 
about 
used 
to 
whIch ._Its slmultoneously 
56 pm. An acusto-optlcal 
to split and frequency 
L __ 
-
-
... 
to 
~~to L..-" 
fig. 1 experl.ental set-up 
to be most appropriate. 
reference signal to a phase 
This 
meter, frequency can be directly applied e.g. lock-In-amplifier (LIA). as 
After passing a lens (L). a polarizing beam splltter (P8S) and a quarter-
wave plate fQWP) the object beams are focused onto the specfmen under test 
by a microscope object1Ye (MO) as shown In fig. 1. The QWP Is passed a 
second time by baCk travelling light thus It Is reflected at the P8 S. The 
beat of the two heterodyne signals can be observed after demodulAting the 
a.plltude modulated detector output. After DC cut off. de. odulatlon and bandpass filtering one gets 
U(I) = Uo . oos ( 2>«(, ...f,)1 - 4r· dfI,q + Ill> ) (1) 
Pha se shift of thi s term corresponds to the effective wavelength. The demo-
dulated and bandpass filtered signal Is finally fed to the LIA. 
3. RESULTS 
The results were obtafned with a lOx miroscope objective and an avalanche 
photodfode detector. The workfng distance to the target was about 7 • • . 
Scanning was achieved by moving the sample with a stepper motor . Figure 2a 
shows two measurements on a milled aluminum sample . On the sample there 
were milled steps with step heights of 5 /lIIIand 10/llll. The milling groves 
are clearly resolved. They are reproduced in both measurements. In fig. 2b 
a measurement of a moving mirror 15 shown. The target distance variation 
was 100 JlUl , unallbfguous dfstance range as well as good linearity are shown. 
••• 
••• 
••• 
•• 
...  
i ... . 
f ... . 
1 -u 
-I'., 
-I'," --...;._7.'--;.,;:;-_;;:;_-;; 
.i"' I .' •.• I . ' • .• M.I 11 .• 
- ..... 1-1 
fig. 2a) line scannes over a sample 
I 
I 
-
-.. 
-. 11 • ,. ..... JI ... '3O 
- ....... '-1 
2b) Profile .easure.ent 
4. CONCLUSION 
Dual wavelength heterodyne fnterfero . etry has proven to be a powerful tool 
for precfsfon interferometrfc _easurements on optical rough as well as on 
SlIooth surfaces. The syste. acts 1fte a heterodyne interfero. eter using 
light with a wavelength of the synthetiC wavelength. 
S REFERENCES 
1. M. Hurty et al., -An Ob11que Incfdence Interfero. eter,-
Opt. Eng. IS. 461 (1976) 10 6/l111' 
2. O. Kwon et al •• -Rough Surface Interferometry at • 
3. ~~p~: e~!~t~9:T!:~~n(I!:~~erlcs Using Two-Wavelength Holography.' 
Appl. Opt . 10. 2113 11971) 1 'R 1 Tl.e Contour Holography Using BSO 4. F. M. KUchel. and H. J . T1zlan I ea-
Crystals.' Opt . Commun . 38. 17 19811 t rfero.etry with a Two-Wavelength 
5. A.F. Fercher et al •• 'Rough Surface, net 24 2181 (1985) 
Heterodyne Speckle Interferometer'h ~PP1~ ~~t;rfe~o.etry Usfng Super-
6. R. Dindlfker et al., -Two .. wavelengt l tas~0.1I1sion for Optics, Quebec, Heterodyne Detection'. 14th Congr. n) . 
August 1987 paper Bl.2 (SPIE Vol . 813 
SPlEVtJI. UtSOpticsinCon¥J/hSyltem,(1990) / 671 


Dual wavelength heterodyne interferometry for rough surface measurements

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Dual wavelength heterodyne interferometry for rough surface measurements

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