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Correlation between photorefractive index
changes and optical damage thresholds in z-cut
proton-exchanged-LiNbO3 waveguides
F. Luedtke1,*, J. Villarroel2, A. García-Cabañes2, K. Buse1, and M. Carrascosa2
2
1
Physikalisches Institut, Universitaet Bonn, 53115 Bonn, Germany
Departamento de Física de Materiales C-IV, Universidad Autónoma de Madrid, Campus Cantoblanco, E-28049
Madrid, España
*
Corresponding author: [email protected]
Abstract: An interferometric Mach-Zehnder technique very recently
developed has been applied to measure photorefractive index changes in
different types of z-cut proton-exchanged planar waveguides in LiNbO3.
These measurements are complemented by determining the intensitythreshold for the onset of optical damage with a standard single-beam setup.
In the intensity region just below the threshold-intensity obtained in the
single-beam experiment the refractive index change is found to saturate at
values around 1× 10−4 . Furthermore, we measure the dark conductivities of
proton-exchanged waveguides by monitoring the decay of the light-induced
index changes. Via the time constant of the decay we obtain dark
conductivities of the order of about 5 × 10−16 Ω −1 cm −1 , that are negligible
compared with the photoconductivity within the light intensity range used.
The results of the measurements compare well with the predictions of a
recent work, that uses a two-center model to explain the optical damage.
©2009 Optical Society of America
OCIS codes: (160.5320) Photorefractive materials; (160.3730) Lithium niobate; (230.7370)
Waveguides
References and links
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LiNbO3 waveguides," J. Phys. D: Appl. Phys. 22, 611–616 (1988).
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A. Alcázar-de-Velasco, J. Rams, J. M. Cabrera and F. Agulló-López, "Light-induced damage mechanisms
in α-phase proton-exchanged LiNbO3 waveguides," Appl. Phys. B 68, 989–993 (1999).
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O. Caballero-Calero, A. García-Cabañes, J. M. Cabrera, M. Carrascosa, and A. Alcázar, "Optical damage in
x-cut proton exchanged LiNbO3 planar waveguides," J. Appl. Phys. 100, 093103 (2006).
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T. Fujiwara, R. Srivastava, X. Cao, and R. V. Ramaswamy, "Comparison of photorefractive index change
in proton-exchanged and Ti-diffused LiNbO3 waveguides," Opt. Lett. 18, 346–348 (1993).
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M. Carrascosa, J. Villarroel, J. Carnicero, A. García-Cabañes, and J. M. Cabrera, "Understanding light
intensity thresholds for catastrophic optical damage in LiNbO3," Opt. Express 16, 115 (2008).
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J. Rams, A. Alcázar-de-Velasco, M. Carrascosa, J. M. Cabrera, and F. Agulló-López, "Optical damage
inhibition and thresholding effects in lithium niobate above room temperature," Opt. Commun. 178, 211216 (2000)
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I. Breunig, M. Falk, B. Knabe, R. Sowade, K. Buse, P. Rabiei, and D. H. Jundt, "Second harmonic
generation of 2.6 W green light with thermoelectrically oxidized undoped congruent lithium niobate
crystals below 100 °C," Appl. Phys. Lett. 91, 221110 (2007)
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F. Luedtke, J. Villaroel, A. García-Cabañes, M. Carrascosa, H. Steigerwald, and K. Buse "Mach-Zehnder
technique for investigation of optical damage in planar LiNbO3 waveguides," Ferroelectrics, in press
10. A. Méndez, A. García-Cabañes, M. Carrascosa, and J. M. Cabrera, "Photorefractive charge compensation in
α-phase proton-exchanged LiNbO3 waveguides," J. Opt. Soc. Am. B 17, 1412-1419 (2000).
11. M. Falk and K. Buse, "Thermo-electric method for nearly complete oxidization of highly iron-doped
lithium niobate crystals," Appl. Phys. B 81, 853–855 (2005).
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niobate," Adv. Phys. 45, 349-392 (1996)
13. O. Caballero-Calero, A. García-Cabañes, J. M. Cabrera, M. Carrascosa, and A. Alcázar, "Light intensity
measurements in optical wavegudies using prism couplers," J. Appl. Phys. 102, 074509 (2007).
14. F. Agulló-López, G.F. Calvo, and M. Carrascosa, Photorefractive Materials and their Applications 1 –
Basic Effects, P. Günter and J.-P. Huignard, ed. (Springer, 2005), Chap. 3.
15. J. Villarroel, M. Carrascosa, A. García-Cabañes, and J. M. Cabrera, "Light intensity dependence of
holographic response and dark decays in α-phase PE:LiNbO3 waveguides," J. Opt. A: Pure Appl. Opt. 10,
104008 (2008).
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Soc. Am. B 10, 2085-2092 (1993).
19. M. Falk, Th. Woike, and K. Buse, "Reduction of optical damage in lithium niobate crystals by thermoelectric oxidization," Appl. Phys. Lett. 90, 251912 (2007).
20. J. Carnicero, M. Carrascosa, A. Mendez, A. García-Cabañes, and J. M. Cabrera, "Optical damage control
via the Fe2+/Fe3+-ratio in proton-exchanged LiNbO3 waveguides," Opt. Lett. 32, 2294-2296 (2007).
1. Introduction
Photorefractive optical damage in lithium niobate (LiNbO3) waveguides continues to be a
crucial problem for applications at light powers above 10 mW. Although great effort has been
devoted to characterize [1, 2, 3, 4], understand [5, 6] and reduce [7, 8] optical damage, many
experimental and theoretical aspects remain open, and the definite method for optical damage
inhibition is still lacking. For the development of even more efficient methods of optical
damage reduction a reliable and well-understood model of the effect is desired.
So far it is unclear which index changes Δn are acceptable, i.e. do not cause critical beam
distortions, and which values are unacceptable. Also more information is needed about the
origin of the Δn , in particular the role of secondary centers and of the dark conductivity
deserve special attention.
To work on these tasks we take advantage of an interferometric technique we developed
for the measurement of the refractive index change in planar waveguides. The method is
described in detail in [9]. The main advantage of this technique is that, compared to
holographic approaches [10], it measures the change of the effective mode index Δneff for
single-beam propagation, the case more appropriate to investigate optical damage. Moreover,
it is applicable to z-cut guides for which very scarce measurements of Δneff have been
reported because, the z-axis being normal to the guide plane, the typical holographic method
fails. In this work we study planar proton-exchanged (PE) waveguides. The obtained data
for Δneff will be related to complementary measurements of the intensity threshold for the
onset of beam distortion, thus getting more complete information about optical damage.
Furthermore, we study the dark conductivity in these waveguides.
2. Experimental Methods
2.1 Sample preparation
Three different z-cut LiNbO3 substrates have been used: congruently melting undoped,
congruently melting thermo-electrically oxidized [11], and 5.0 % mol Mg-doped material.
Every sample undergoes the same treatment of immersion for 24 h at 300 °C in benzoic acid
buffered with 3 wt. % lithium benzoate in a sealed glass ampoule [10]. This procedure results
in formation of α-phase proton-exchanged LiNbO3 (PE:LiNbO3) waveguides with
extraordinary index jumps at the surface of Δne ≈ 0.01 and smooth Fermi-like index-profiles
with effective depths in the 1-2 µm range [10]. These guides support one or two modes at the
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wavelengths 532 and 633 nm. One of the edges of each sample is polished to optical grade in
order to enable decoupling of light via the waveguide edge in the interferometer setup
described below.
Slight differences with respect to the waveguide fabrication are noticed between the
different substrates. Both, the waveguides in the thermo-electrically oxidized substrate and in
the magnesium-doped substrate tend to guide rather one than two modes at the two
wavelengths mentioned above. For magnesium-doped LiNbO3 it is known that the protonexchange process is slower than in undoped material [12].
2.2 Determination of the threshold intensity for beam degradation
In order to measure the threshold intensity for the onset of appreciable distortion of the outcoupled light we use a simple setup schematically shown in Fig. 1 and described in [3]. While
continuously illuminating the waveguide with light at λ = 532 nm, the output power passing
through a diaphragm placed 30 cm behind the waveguide is monitored. The size of the
diaphragm is chosen such, that at low intensities (no light-induced scattering) about 70% of
the total out-coupled power is transmitted. The light intensity inside the waveguide has been
estimated (to a ±15% accuracy) by following the procedure described in [13].
Nd:YAG laser
532 nm
Powermeter 2
Powermeter 1
Polarizer
Photodetector 1
(total power)
Photodetector 2
and diaphragm
Prisms
Lens
f = 300 mm
10 mm
Beam splitter
Waveguide sample
Fig. 1. Schematic sketch of the single-beam setup that is used for the determination of optical
damage thresholds via the detection of significant beam distortion.
2.3 Measuring optical damage with the interferometric method
The interferometric technique for characterizing the optical damage in waveguides is based on
the measurement of relative phase changes between two weak He-Ne laser beams that
propagate along the two arms of a Mach-Zehnder interferometer (Fig. 2). The key feature of
the setup is the fact that both interferometer beams (represented in Fig. 2 by a dotted and a
dashed line, respectively) propagate separately, but next to each other in the same planar
waveguide. Thus, the setup is insensitive to changes of the external parameters, such as the
ambient temperature, since both interferometer arms are equally influenced. Optical damage is
induced in one of the arms by intense collinear illumination with visible light (532 nm in our
case). The probe beam and the pump beam interact over the whole propagation length from
the incoupling prism to the end face of the waveguide. The measured quantity is the
modulation of the red light-intensity pattern at the output of the interferometer. The use of
chopped probe light and a lock-in amplifier enable us to detect changes in the interference
pattern with a high signal-to-noise ratio. Our realization of the Mach-Zehnder interferometer
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in planar waveguides together with its features and limitations have been previously described
in [9].
HeNe laser,
633 nm
Nd:YAG laser,
532 nm
Lock-in amplifier
and recorder
Collimation
optics
Crossed
polarizers
Reference frequency
Diaphragm
Chopper
Microscope
objective
Mirror
Mirror
Beam splitter
and mirror
Mirror
Beam splitter
Prisms
}
Lens
f = 400 mm
Waveguide sample
on thermally
stabilized mount
Microscope
objective
Mirror
Diaphragm,
bandpass filter
(633 nm) and
photodetector
Fig.
2.
Experimental
setup
for
the
interferometric
measurement
of
photorefractive index changes in planar waveguides. The dashed and the dotted lines on the
lower left-hand side represent the two arms of the interferometer. These two lines are separated
in the direction perpendicular to the plane of the paper.
The experimental procedure is as follows: With the green beam blocked, the coupling
angles for the weak red beams are adjusted to the best interference signal. Afterwards, the
coupling angle for the green beam is adjusted at very low in-coupled power to avoid optical
damage before the actual measurement starts. Then, with the red beam going through and a
green blocking-filter placed in front of the detector, the green intensity is increased up to the
value the measurement is supposed to be conducted with. The measuring time starts when the
shutter for the green light is opened. Depending on the in-coupled intensity, and therefore on
the expected rise time of the photorefractive effect, the modulated intensity is recorded for a
few minutes up to one hour.
In order to compare the relative importance of dark and photoconductivity, we have also
studied dark decays of previously recorded effective index changes ( ≈ 10−4 ). In this case, the
measuring time starts when the green shutter is closed. In order to determine the dark
conductivity σd from these decay measurements, it is necessary to decrease the intensity of
the red probe light to be sure that the induced optical erasure is negligible compared
with σ d ( I red ≈ 0.05 Wcm −2 inside the waveguide). Doubling of the probe intensity does not
accelerate the decay, ensuring that the 0.05 Wcm −2 are small enough to monitor purely the
dark relaxation. Since these dark decays are much slower than the production of optical
damage upon illumination, the typical measurement times here are several hours. All
measurements described in this section are performed with the temperature of the sample
holder being stabilized to ( 25.0 ± 0.1 ) °C.
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3. Experimental results
3.1 Single-beam setup
The results of the measurements of the threshold intensity are summarized in Fig. 3. As long
as the in-coupled beam does not cause appreciable optical damage in the guiding layer, the
out-coupled beam maintains its shape and the power detected behind the diaphragm is
proportional to the in-coupled power. The loss of this proportionality marks the threshold
intensity for the onset of optical damage. It can be seen in Fig. 3 that the waveguides on the
congruent substrate, either untreated or thermoelectrically oxidized, exhibit very similar
thresholds around 270 Wcm −2 . This value is a little higher than the threshold
intensity I thr measured for the same type of waveguide (i.e. produced by the same fabrication
procedure) and with the same experimental method, but fabricated in x-cut congruent
substrates ( I thr ≈ 100 Wcm −2 ) [4]. In the Mg-doped sample no significant degradation of the
out-coupled beam could be measured up to the highest intensity available, 105 Wcm −2 . The
threshold intensities I thr for all the samples used can be found in Table 1. The criterion that
we apply to determine I thr is as follows: I thr is the intensity where for the first time the
derivative dI out / dI in is smaller than 0.15. This value is arbitrarily chosen, but the result is in
good accordance with the intuitive impression given by Fig. 3, thus it provides a reasonable
criterion for quantitative comparison of different waveguides.
105
Iout / Wcm-2
104
103
102
101
100
100
101
102
103
-2
Iin / Wcm
104
105
Fig. 3. Results of the measurements with the single-beam setup: outcoupled intensity Iout versus
incoupled intensity Iin
Table 1. Intensity thresholds for the different types of waveguides
I thr [Wcm −2 ]
Sample
undoped, untreated, z-cut
260
undoped, oxidized, z-cut
280
MgO-doped, z-cut
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3.2 Mach-Zehnder interferometer
In the standard photorefractive model [14], the time dependence of Δneff (t ) involves a
saturating exponential function as follows:
Δneff (t ) = Δneff , sat (1 − e−t / τ ) ,
(1)
where Δneff , sat is the effective index change at saturation and τ is the rise time. This
dependence is valid for our data (see the fit in Fig. 4(a)). Beyond the threshold intensity for
significant beam distortion, the interferometric method becomes problematic. This is because,
due to increasing beam divergence, the intensity inside the waveguide is no longer well
defined.
We measure the development of the change of the effective mode index with
time, Δneff (t ) , for all types of waveguides guides (see Section 2) for an intensity range of
10 – 2000 Wcm −2 . From the interferometer signals we extract the time evolution of the
refractive index changes as described in [9]. Thus we obtain values for the saturation index
change Δneff , sat and the rise constant τ−1 as functions of the intensity inside the waveguide.
With regard to τ−1 , it turns out to be of the same order of magnitude as those values obtained
by the holographic method for congruent samples [15].
Fig. 4. Typical measurement results obtained with the Mach-Zehnder interferometer for the
build-up (a) and dark-decay (b) of optical damage. The solid lines correspond to exponential
fits of Eq. (1) and Eq. (2), respectively.
The dependence of Δneff , sat on the intensity has been plotted in Fig. 5 for all the guides.
All curves show an increase of Δneff , sat with the light intensity I inside the waveguide that
seems to saturate for higher intensities. In the low-intensity-region (< 60 Wcm −2 ) the curves
appear to have also a lower sensitivity to the variation of light intensity. In accordance with
the behavior present in Fig. 3, the thermo-electrically oxidized and the congruent guides
exhibit very similar Δneff , sat ( I ) . It is worthwhile noticing that the intensity thresholds from
Fig. 3 roughly coincide with the region in which Δneff , sat seems to saturate with I. Finally, for
the MgO-doped samples, the measured index changes are too small to detect a
reliable Δneff , sat increase within the intensity range used, in agreement with the much higher
intensity threshold seen in Fig. 3.
Since the competition of dark conductivity σd and photoconductivity σ ph in
photorefractive charge transport was often used in early papers [5] to explain dependences
similar to that of Fig. 5, we have also measured the dark decay time of Δneff in order to
determine σ d . In analogy to Eq. (1), we expect the decay of the index change to behave like
Δneff (t ) = Δneff (t = 0)e −t / τd ,
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where τd is the time constant of the dark decay. It is connected to the dark conductivity via
τd =
εε0
.
σd
(3)
In Eq. (3), ε0 and ε are the vacuum permittivity and the relative permittivity,
respectively. We take ε ≈ 27 [16] to calculate σd from the measured decay times. These are
Δneff, sat / 10-4
typically in the range of a few 1000 s (corresponding to σd of a few 10−16 Ω −1 cm −1 ).
Comparing this to the optical damage rise-time of a few seconds, it becomes unambiguously
clear that the dark conductivity plays no role for the suppression of optical damage for our
experimental conditions. An example of the measured time evolution of the index change
during a dark decay is displayed in Fig. 4(b).
1.0
0.1
1
10
100
Iin / Wcm-2
1000
Fig. 5. Saturation values of the refractive index changes Δneff ,sat for all three types of
waveguides versus incident light intensity Iin. Solid and dashed lines are just guides to the eye,
in analogy to presentations in [6, 17, 18].
5. Discussion
The Mach-Zehnder interferometric technique applied to measure photorefractive index
changes in planar waveguides has proved to be a complementary useful tool for characterizing
the optical damage. First, it has confirmed previous experimental data on a strong increase of
the induced saturation index change Δneff , sat at high intensities obtained by the holographic
method in x-cut planar guides [15] or the Mach-Zehnder method in channel guides [5]. Also
measurements of the photorefractively induced birefringence in x-cut bulk crystals with a
Senarmont compensator setup showed an enhancement of Δn with I up to values of
about 10−4 [19]. Moreover, comparing the results with those from the single-beam technique,
it is observed that the optical damage threshold occurs when the Δneff , sat ( I ) starts to saturate.
We interpret this result as follows: For low enough intensities the photorefractive index
changes, independently of their possible increase with I, are too small to appreciably affect the
light propagation inside the guide. For increasing intensity, Δneff , sat becomes sufficient for
modifying the beam profile and generating noticeable self-defocusing along the propagation
direction causing the sublinear dependence seen in Fig. 3. This should occur
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for Δneff , sat ≈ 10−4 as shown in Fig. 5. For higher in-coupled intensities the actual intensity
inside the guide can hardly increase due to the self defocusing. Hence, in our case the onset of
serious beam distortion is at Δneff , sat of about 1× 10−4 . This threshold, however, may depend
on several parameters like the interaction length and the beam diameter. Finally it is worth to
mention that waveguides fabricated in Mg-doped substrates exhibit Δneff , sat well
below 1× 10−4 even for the highest intensities used. This result is in good accordance with the
absence of measurable optical damage thresholds in the intensity region checked.
The dark conductivity measurements achieved with the Mach-Zehnder technique clearly
indicate that its contribution to charge transport is negligible when compared with the
photoconductivity. Consequently, models based the on the role of dark conductivity to explain
the increase of Δneff , sat with I are not applicable, and different approaches should be invoked.
Conversely, our results are in good semi-quantitative accordance with the intensity
dependence for the Δneff , sat predicted based on a two-center model for undoped LiNbO3
applied to α-phase waveguides [6]. In particular, Δneff starts to grow sharply at similar light
intensities. In this model the intensity dependence of Δneff , sat is attributed to the increasing
importance of the highly bulk photovoltaic defect of niobium on lithium lattice sites (NbLi) in
the charge transport.
Regarding the different crystals used we have fabricated α-phase PE waveguides on
thermoelectrically oxidized LiNbO3 substrates. Unfortunately, the PE treatment seems to alter
the oxidation state of the waveguide layer so that the obtained waveguide has an optical
damage resistance similar to that of standard α-phase guides. This result is indirectly a further
confirmation of the conclusions of a recent work that reports a significant reduction of the
samples during the PE treatment [20]. This effect should cancel the previous strong oxidation
obtained by the thermoelectric technique.
6. Summary
In summary, we applied the recently developed interferometric Mach-Zehnder technique for
characterizing photorefractive optical damage in planar waveguides. The study of
α-phase PE:LiNbO3 guides, fabricated in different z-cut substrates, has shown that the steadystate index change monotonously increases with the light intensity up to a saturating value.
This last value coincides with serious beam divergence, i.e. the threshold for the onset of
optical damage. Dark conductivity appears negligible compared with photoconductivity
within the light intensity range used. Up to saturation, measured values compare well with
results of simulations performed within the two-center model which considers the anti-site or
NbLi-site defect as a secondary photorefractive center.
Acknowledgments
This work was supported by the Ministerio de Educación y Ciencia (MEC) under grant
MEC/MAT2005-06359-C03-01. F. Luedtke acknowledges gratefully financial funding by the
Deutscher Akademischer Austauschdienst (DAAD) and the Deutsche Telekom AG. and J.
Villarroel acknowledges his FPI fellowship from the Spanish MEC. The authors also thank
Prof. J.M. Cabrera for very useful discussions and comments.
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