# Fabrication of Proton-Exchange Waveguide Using  StoichiometricLiTaO3 for Guided Wave Electrooptic  Modulators with Polarization-Reversed Structure

Authors: Hiroshi Murata; Yasuyuki Okamura
Journal: Advances in OptoElectronics
 (2008)

---

## Abstract

Optical waveguides were fabricated on z-cut stoichiometricLiTaO3 (SLT) by using the proton-exchange method. The surface index change for the extraordinary ray on the SLT
substrate resulting from the proton exchange was 0.017, which coincided well with congruent
LiTaO3 substrates.  The proton exchange coefficient in the SLT was
0.25×10−12cm2/s. The application of the SLT waveguide to a quasi-velocity-matched travelling-wave electrooptic modulator with periodically polarization-reversed structure is also reported.


---

## Body

## 1. Introduction

Recently,
optical-quality  stoichiometricLiNbO3 (SLN) and stoichiometric LiTaO3 (SLT) crystals have been attracting a lot of interest due to their excellent
characteristics   as a material for optical functional devices; small coercive
electric field for   polarization reversal, excellent nonlinear optic (NLO) and
electrooptic (EO) characteristics, and small defect density
[1–5]. Several
studies on the optical wavelength conversion devices using NLO effects in SLN
and SLT have been reported, however, there are few reports on their applications
to EO devices. In particular, a guided-wave optical modulator based on SLT has
not been yet reported, as far as we know. One main reason, we believe, is that waveguide
fabrication methods have not been established for SLT. SLT exhibits a large
tolerance for optical damage and small birefringence which can be controlled precisely.
These features are attractive for applications to advanced NLO and EO devices.In this
report, we  present the fabrication of proton exchange waveguides in z-cut SLT substrates.
The measured surface refractive index change for an extraordinary ray,Δne, was
Δne=0.017,
which was in good agreement with the reported value in congruent LiTaO3 [6]. The application to high-speed traveling-wave EO modulators using SLT with periodic
polarization reversal for quasi-velocity-matching is also reported.


## 2. Waveguide Fabrication

Z-cut stoichiometricLiTaO3 (SLT) wafers from Oxide Corporation were used in this study.  For the fabrication of the proton exchange
waveguides, the standard exchange technique using melted benzoic acid was used
[6]. The temperature of the melted benzoic acid for the proton exchange was set
at 240 degrees centigrade. During the proton exchange process, the temperature
of the melted benzoic acid was kept within 240±0.1 degrees centigrade by use of a proportional-integral-derivative (PID) temperature controller. Several slab waveguides were
fabricated using SLT with three different proton exchange times set at 4, 9,
and 24 hours.The
effective indices of the TM-guided modes in the fabricated slab waveguides were
measured by using the prism coupling method with a standard rutile prism
coupler at a wavelength of 633 nm. The measured results are plotted in
Figure1
with the dispersion curves of TM-guided modes.
Figure 2 shows the relationship between the proton exchange thickness
and the square root of the exchange time. From the measurement results, we
obtained the surface index change, Δne, for
the  extraordinary ray and the exchange coefficient, Dex, in
the SLT by the proton exchange with benzoic acid at 240 degrees centigrade. We defined
the exchange coefficient Dex
by use of the depth d of
the proton-exchanged layer from the surface and the proton exchange time tex
as
the following equation: (1)d=2Dextex.The obtained results are summarized
in Table 1. For comparison, the reported surface index change value and
exchange coefficient in CLT [6] are also shown in Table 1. The surface index
change value in SLT coincided with the reported one of CLT by the proton
exchange with benzoic acid at 249 degrees centigrade [6]. On the other hand, the
exchange coefficient in SLT at 240 degrees centigrade was about 30% larger than
that in CLT at 249 degrees centigrade. In other words, the velocity of the
proton exchange in SLT was slightly faster compared with CLT. This might come
from the small defect density of SLT. From the measured surface refractive
index change and the depth of the exchanged layer, we can derive the proton
exchange condition for SLT in order to obtain a single-mode channel optical waveguide
at a designed wavelength.Table 1
The
surface refractive index changeΔne and
the exchange coefficient Dex in the
proton exchange slab waveguides of SLT and CLT.
ΔneDex(10−12cm2/s)StoichiometricLiTaO3 (benzoic acid, 240 degrees)0.0170.25CongruentLiTaO3 [6]
(benzoic acid, 249 degrees)0.0170.184Figure 1
Measured effective index values with TM-guided wave mode dispersion
curves.Figure 2
Proton exchange thickness as a function of the
square root of exchange time in the measured samples.


## 3. Fabrication of Quasi-Velocity-Matched Electrooptic Modulator

Utilizing the proton exchange single-mode optical
waveguide of SLT, we tried to fabricate the quasi-velocity-matched (QVM)
electrooptic (EO) modulators with traveling-wave electrodes and periodically polarization-reversed
structure [7]. The basic structure of the device is shown in
Figure 3.  It consists of a single-mode channel waveguide
formed by the proton exchange method and traveling-wave coplanar electrodes
fabricated on a z-cut SLT substrate with an SiO2 buffer layer. A periodically
polarization-reversed structure is also fabricated through the substrate for
the quasi-velocity-matching between the lightwaves propagating in the optical waveguide
and the modulation microwave traveling along the coplanar electrodes.Figure 3
Structure of QVM EO modulator with
periodically polarization-reversed structure.In the device design, we set the peak modulation
frequency as 15 GHz and the operational light wavelength as 633 nm for the
prototype device. The required length for each polarization-reversed and
nonreversed regionL for the quasi-velocity-matching is given by the following
equation [7]: (2)L=c2fm(nm−ng),where ng
is the
group index of the lightwaves propagating in the waveguide, nm
is the
effective index of the modulation microwave traveling along the electrodes, and c is the
lightwave velocity in vacuum.  In order
to obtain a single-mode channel waveguide at 633 nm, we designed the waveguide core
width as 3 μm and the waveguide core depth
as 0.7 μm. From the reported wavelength
dispersion characteristics of the refractive index of SLT [4] and calculated waveguide
dispersion characteristics, we obtained the group index value of the lightwaves
propagating in the waveguide as ng=2.30 at a wavelength of
633 nm. The effective index of the modulation microwave was also calculated as nm=4.55
from the dielectric constants of SLT and the structure of the coplanar asymmetric
electrodes with a hot electrode of 14 μm in width and an electrode
separation of 33 μm. As a result, the length for
the polarization-reversed and nonreversed region for the quasi-velocity-matching
was obtained as L=4.44mm for the peak modulation frequency at 15 GHz
from (2). The calculated frequency response of the QVM modulator is shown in
Figure 4 when the electrode length for modulation Lt is set
as 7 times that of L(Lt=31mm).Figure 4
Calculated modulation frequency
dependence of the QVM modulator when the polarization reversal period is2L=8.88mm and the total electrode length is Lt=31mm.The designed device was fabricated using z-cut SLT as
shown in Figure5. Firstly, the periodic polarization reversal pattern with a
period of 2L=8.88mm was fabricated on a 0.4 mm thick z-cut SLT
substrate by use of the pulse voltage applying method. The electric field required
for the polarization reversal was rather small (~3.3 kV/mm) compared with
standard CLT substrates (~22 kV/mm). Next, the single-mode channel waveguide was
fabricated on the periodically poled SLT by using the proton exchange method.
The waveguide core width was 3 μm and the core depth was set as
0.7 μm. The periodically-poled Cr-masked
SLT substrate for the fabrication of the channel waveguide with a width of 3 μm was immersed into the melted
benzoic acid at 240 degree centigrade for 90 minutes. After the removal of the
Cr film, a 0.1 μm thick SiO2 buffer
layer was deposited on the waveguide by sputtering. Finally, 2 μm thick Al asymmetric coplanar
electrodes were fabricated onto the waveguide by use of EB deposition and a standard
photolithography technique. The hot electrode width was set as 14 μm and the electrodes separation
was set as 33 μm, where the intrinsic
impedance of the electrodes became 50Ω.  The electrode
length for modulation Lt
was 31 mm, which corresponded with 3.5 times the polarization reversal period 2L.Figure 5
Fabrication sequence of the QVM EO modulator.Both
ends of the waveguides were cut and polished for light-beam coupling. The total
device length was 42 mm and the optical insertion loss of the device was about
25 dB including the coupling losses at both ends. We believe that the
relatively large optical loss will be reduced by a thermal annealing process as
with the proton exchange CLT waveguides (annealed proton exchange process).  Microwave characteristics of the fabricated
electrodes were measured by use of a network analyzer and good microwave
responses of SLT (almost the same as CLT) were confirmed.The optical spectrum of the modulated lightwave from
the fabricated device was measured by use of a scanning Fabry-Perot
interferometer. The measured modulation frequency dependence of the fabricated
QVM EO modulator is shown in Figure6. The band modulation characteristic was
confirmed. The peak modulation frequency was in good agreement with the
designed frequency of 15 GHz. We think that the dip in the modulation index at
14 GHz in Figure 5 was due to the effect of the substrate resonance mode, which
could be reduced by changing the size of the substrate and the coupling of the
microwave signal to the electrodes.Figure 6
Measured modulation frequency dependence of
the fabricated modulator.


## 4. Discussion and Conclusion

Basic characteristics of the proton exchange waveguide in SLT and the
fabrication condition of a single-mode waveguide were obtained. However, the optical
loss in the fabricated waveguide was large (~25 dB in the 42 mm waveguide with
coupling loss). In addition, it is well known that the proton exchange process
might degrade the Pockels effect. Thermal annealing is rather effective in reducing
the optical loss and recovering the Pockels effect. We have also tried to do
the thermal annealing of the fabricated proton exchange SLT waveguides with the
standard annealing condition for the proton exchange CLT waveguides (250∼400
degrees centigrade, ~1 hour). However, after annealing, the guiding
characteristics became poor and the   output beam spot from the end of the
waveguide could not be oberved. It might come from the large diffusion velocity, and some specific techniques like rapid thermal
annealing might be necessary to realize good annealing
conditions.In conclusion, we fabricated the proton-exchanged waveguide on z-cut
stoichiometric SLT.  The measured surface
index change for the extraordinary ray wasΔne=0.017, which coincided well with congruent LiTaO3 substrates. The proton exchange coefficient in SLT was Dex=0.25×10−12cm2/s.  Some interesting applications include EO
modulators with advanced functions using the polarization reversal and optical
waveguide technologies.



---

*Source: 654280-2008-11-16.xml*