Download SPATIALLY RESOLVED SECOND HARMONIC GENERATION AND

CrySI. Lau. DeJ. and Amorph. Mal,
19117, Vol. 15, pp. 103-11»1
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SPATIALLY RESOLVED SECOND HARMONIC
GENERATION AND INTERFEROMETRIC
INVESTIGATIONS OF LiNb03 CRYSTALSt
N. SCHMIDT, K. BETZLER and S. KAPPHAN
Universität Osnabrück, Fachbereich Physik, Postfach 4469, D 4500 Osnabrück
Two methods of spatially resolved second harmonie generation (SRSHO) - (1) foc:used bc:am and
moving crystal. (2) uniformly ü1uminated crystal and detector array - and an interferometric method
are used to investigate the optical homogencity of various UNbO, sampIes. The experimental results
of a crystal with U-outditJused layer show up typical differentes whieh can bc: describc:d by a bc:am
propagation in a profile with variable refractive index. The detcctor array method is also applied to
characterize imbomogeneities in sampies with different Mg-contents. A c:omparison with Mach­
Zehnder interferometry (MZI) shows that SRSHO yiclds MOre information espccially in very
inbomogencous crystals.
Extrinsic impurities and the Li/Nb ratio are important for optical devices of
LiNb03 crystals because of their inftuence on the refractive indices. While
impurities like Ti or H are used to fabricate waveguides,I-3 Mg greatly reduces
optical damage,· and, together with the Li/Nb ratio varies the phase-matching
conditions for nonlinear optical devices.' In turn, the optical properties of
LiNb03 crystals - e.g. the refractive indices and their variations - can serve as a
measure for other crystal properties - doping homogeneity ete. Index variations
in a crystal can be measured interferometrieally6 or - as LiNb03 shows noneritical
phase matehing for second harmonie generation from Nd-YAG wavelength at
convenient temperatures - by means of temperature dependent spatially resolved
second harmonie generation (SRSHG).7 We appIied both the interferometrie and
the SRSHG method to two types of LiNb03 sampies:
1 undoped congruent sampies with Li-outdüfused surface layer,
2 various Mg-doped sampies grown trom congruent mett.
I
The interferometrie investigations were carried out in a modified Mach­
Zehnder interferometer briefty sketched in Figure 1. By means of the polarizer,
the ordinary or the extraordinary index of the crystal could be seleeted. For the
SRSHG measurements the two experimental setups shown in Figure 2 were used:
(a) the beam of a pulsed Nd-YAG laser was focused onto the sampie crystal
which could be moved perpendieular to the beam direetion by a computer
controlIed translation table, the generated seeond harmonie signal being deteeted
by a photodiode; (b) the seeond harmonie signal generated by the crystal from an
expanded Nd-YAG laser be am was detected spatially resolved by means of an
optical multiehannel analyzer. In both setups the temperature eould be controlled
by Peltier eooling or oven heating within a range of -25 to 20CfC with an
accuracy of about 0.3 degrees.
t Work supported by the Deutsche Forschungsgemeinschaft (SFB 225).
103
104
N. SCHMIDT, K. BElZLER AND S. KAPPHAN
Beam
Sampie Splitter
•
I'
He-Ne
/
Poldrizer
FlOURE 1 Mach-Zehnder interferometer used for the interferometric measurements.
For testing the described methods a Li-outdiffused tayer was prepared by
annealing a LiNb03 crystal in an oxygen atmosphere at llOO"C for 24 hours. Li
outdiffusion causes an increase of the extraordinary refractive index in the surface
tayer of the crystal. This can be visuaiized by Macb-Zehnder interferometry (see
Figure 3). The SRSHG results for this sampIe are shown in Figure 4. where
contour lines for equal second harmortic intensity as a function of temperature
and position in the crystal are plotted. Figure 4a shows the contour plot before
annealing where a rather homogeneous\ phase matching temperature - whieh is
equivalent for constant refractive indices - is found throughout the crystal.
Figures 4b and 4c show the resutts for the annealed erystal measured with the
focused beam method (4b) and the optical multiehannel analyzer method (4c),
respectivety. An exponential-like deerease in the phase-matehing temperature to
the crystal surface is found indicating a corresponding inerease in the extraordi­
nary index of refraction. The reason for the smoother deerease in Figure 4c is a
spatial mixture of ditIerent second harmonie waves in the case of the optieal
multiehannel analyzer method. To check this assumption. a model ealculation8
was carried out simulating the wave propagation in an exponential index profile
and detection by an optical array detector. The result of the calculation is shown
a)
b)
OMA
Micro
Computer
FlOURE 2 Experimental setups used for the SRSHO investigations: a) focused·beam method. b)
detector·array method (SM: steppinl motor. P: Peltier coolinl or oven, T: thermocouple. F: optieal
filter for separatinl the generated harmonie liaht from the fundamental wave. D: photodiode, DMM:
diptal multimeter. OMA: optical multichannel analyzer). Tbe Nd-YAO laser was driven in pulsed
mode with a peak power of about S kW and a repetition rate of 1 kHz.
INTERFEROMETRIC INVESTIGATIONS OF UNbO) CRYSTALS
1..111" L
Et
IDB'
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FIGURE 3 Interferometric view of the U-outdiffused sampie: a) before, b) extraordinary, IOd c)
ordinary Habt polarizatiOD after outdiffusioo.
2
0
I---
--1
crystOI sill
o
o
-2 -4 -
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...
....:::J
GI
E
GI
b
0
-2
-4
~-6
~
-e
500
1000
1500
2000
2500
Position lmicronsl
FJGURE 4 Second harmonie intensity contour plots for the Li-out-diffused sampie, a) before
Outdiffusion, b) IOd c) after outdilJusion measured with focused-beam method (b) IOd detector-array
tnethod (c).
106
N. SCHMIDT, K. BETZLER AND S. KAPPHAN
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U
t.......J
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-4
-6
-8
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0
100
200
300
400
500
Position [mi cronsJ
FlGURE 5 Model calcu1ation ror the measured phase-matching temperature in an exponential index
profile: A) focused-beam method. B) detector-array method.
in Figure S: A) phase matching temperature ~rresponding to the (assumed) real
,index profile (as measured by focused be~m method), B) phase matching
" temperature as 'seen' by optical multichannei analyzer method. The calculation
shows that large deviations occur between the two methods when large index
gradients show up in the crystal. In spite of this disadvantage, for the
measurements on Mg-doped crystals the optical multichannel analyzer method
was applied because of its obvious multiplex advantage.
The magnesium-doped sampies we investigated were grown by Grabmaier et
al. 9 from congruent melt with Mg-concentrations (in mehl between 0 and 9%. In
some of the crystals growth striations can be visually detected, the nature of
whlch is still unclear. A typical crystaJ (6% Mg) with weIl visible striations is
shown in Figure 6a. Mach-Zehnder measurements of this crystaJ (6b for
extraordinary, 6c for ordinary index) show variations in the indices of refraction
but do not give any information about ihe striation lines. SRSHG measurements
of this crystal are plotted in Figure 7 as perspective view for better visualization
and in Figure 8 as intensity contour plot. From these measurements large
variations in the phase-matching temperature can be derived indicating refractive
a)
b)
c)
FlGURE 6 a) Mg-doped LiNbO) sampie with striation Iines and ilS interferometric viewa for
extraordinary (b) and ordinary (c) polarized light.
INTERFEROMETRIC INVESTIGATIONS OF UNbO l CRYSTALS
107
IHG - INTENStTY
T
c
FIOURE 7 Perspcctive plot of tbe temperatun:: and position dependence of tbe eecond-h8nnonic
intensity in sampie VI04 grown from 6% Mg c:ontaining melt.
index variations and - at the horder lines (or better: horder planes) - relatively
targe steps in the index of refraction. From this fact we can interpret the striations
as horder planes between regions of different refractive indices whieh means
regions of different Li/Nb ratio and/or different Mg content. All sampies with
visible striations showed similar results conceming the index inhomogeneities.
The measured phase-matehing temperatures of the different sampies are plotted
in Figure 9. They are in good agreement with resuJts of Zhong Ji-ggo et al.,10
showing a maximum at 5.5% Mg eontent.
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u
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o
L
Q)
Cl..
E
Q)
I-­
500
2000
1500
H'I00
Position [micronsJ
FIOURE 8 Intensity contour plot of Figure 7.
2500
lOS N. SCHMIDT, K. 8ETZLER AND S. KAPPHAN
.,
U
150~-----r----~----~----r---~
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Q)
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I
100
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50
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2
4
6
8
10
[XJ
./ FIGURE 9 Phase matching temperatures of the irivestigated Mg-doped sampies versus melt
concentration (according to Ref. 9 the (mean) Mg contem in the crystals is higher by a factor of 1.2).
Tbe dashed line represents the results of Re!. 10.
Spatially resolved seeond harmonie generation (SRSHG) has been proven to be
a well-suited method for investigating index inhomogeneities in nonlinear
erystals. A eomparison with the interferometrie method shows that by SRSHG
additional information is gained. The method ean be applied to all erystals
exhibiting noneritieal phase matehing and tunable (for instanee by temperature)
phase-matehing eonditions.
ACKNOWLEDGEMENTS
We an gready indebted to Dr. 8. C. Grabmaier for providing the Mg-doped crystals.
REFERENCES
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3
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5
6
7
8
9
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H. Fay. W. J. Alford and H. M. DeIS, Appl. PhYIJ. Lell. U. 89 (1968).
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