# Optical Intensity Modulation in  an LiNbO3  Slab-Coupled Waveguide

Authors: Yalin Lu; Kitt Reinhardt
Journal: Advances in OptoElectronics
 (2008)

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## Abstract

Optical intensity modulation has been demonstrated through switching the optical beam between the main core waveguide and a closely attached leaky slab waveguide by applying a low-voltage electrical field. Theory for simulating such an LiNbO3 slab-coupled waveguide structure was suggested, and the result indicates the possibility of making the spatial guiding mode large, circular and symmetric, which further allows the potential to significantly reduce the coupling losses with adjacent lasers and optical networks. Optical intensity modulation using electro-optic effect was experimentally demonstrated in a 5 cm long waveguide fabricated by using a procedure of soft proton exchange and then an overgrowth of thin LN film on top of a c-cut LiNbO3 wafer.


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## Body

## 1. Introduction

High-speedoptical modulation by
microwave and milli-meter-wave signals is useful in many optoelectronics and
microwave photonics applications, including long-haul optical communication
systems, radio frequency-over-fiber (ROF) for transferring mobile RF signals,
optical measurement, and high-resolution coherence spectroscopy. A typical
optical intensity modulation for such ROF applications is the single-sideband
modulation having a basic operation principle through mixing two balanced-phase-modulated
light beams driven by a pair of RF signals with a phase shift ofπ/2 [1].  These used optical modulators usually have
very complicated structures, because of the involvement of at least two fast electro-optic
(EO) modulators, a pair of RF modulation sources, and a set of fine tuning
electronics. Such subsystems’ overall performances are primarily limited by the
used fast EO modulators, mainly characterized by their half-wave voltage and
insertion loss—a combined term of
them is defined as the modulation efficiency.The majority of fast optical
modulators are relying on using EO effect in ferroelectric oxide crystals such
as LiNbO3 (LN), LiTaO3 (LT), and EO ceramics [2],  which
are able to render extremely fast response speeds in the range from nano-to-picosecond
and an ultrawide wavelength bandwidth over a few hundred nanometers [3]. EO guided-wave modulators have been becoming very important in integrated optoelectronics,
following a tremendous number of studies and publications in about the past
four decades. Recently, high-speed and low-voltage LN traveling-wave electrode
intensity modulators operated at 40 Gbps with a half-wave voltage of ~1 V have
been reported [4].In general, a guided-wave LN EO
modulator includes a Mach-Zehnder (MZ) waveguide interferometer composing of
two “Y” branches, a section of parallel waveguides connecting the two “Y”
branches, and an electrode layout suitable for a specific performance
requirement. Waveguides are normally buried supporting a single propagation mode,
and were fabricated by either titanium-indiffusion or proton exchanging (PE)
methods. Pure ridge-type waveguides are less useful in real cases, because of the
high propagation loss associated with the microfabricated ridge’s boundaries.
Electrode layout can be very versatile too. They can be right on top of the
guide, or aside the guide, or using the traveling-wave design, and so forth. No
matter what designs discussed above that will be used, the majority of past EO guide modulators
are relying on the principle of modulating optical phase, which requires the
device to reach the so-called half-wave voltage,Vπ=λd/(n3γL).
Here d  is the electrode spacing, λ the wavelength, γ the EO
coefficient, and L the interaction
length. The full modulation thus corresponds to a  90° rotation of
the incident light’s polarization plane by the applied electrical field.
Apparently, tough challenges will
find the way to further reduce the modulation voltage.High insertion loss is also a “hard-to-avoid” issue,
which is almost intrinsic to currently used waveguide designs and those
associated fabrications. For example, usually both titanium-indiffusion and
proton exchange create a relatively large index difference (δn~0.02), which will confine the fundamental
mode in a small core size of only about a few microns. Because indiffusion/exchanging
methods are normally unidirectional (normal to the wafer’s surface), it clearly
lacks a capability to fabricate a waveguide having circular and symmetric core.
Smallness and asymmetry of common waveguide cores have been found to account
for a large portion of the measured high insertion losses, when coupling them to
the common optical networks or pumping diode lasers.In this research, a new intensity modulation
mechanism, which has the potential to use a very low driving electrical voltage,
was investigated. This modulation principle involves leaking the light from the
main guiding core to an attached leaky slab waveguide through slightly changing
the beam’s polarization orientation by the applied electrical field. Realization
of this function will rely on designing such a slab-coupled waveguide (SCW)
structure. Additionally, this SCW structure has the potential to offer a large,
circular, and symmetric beam profile for propagating the single-spatial-mode,
which therefore further provides the possibility to significantly reduce the
coupling losses. In the following sections of this paper, preliminary SCW
design/simulation, initial fabrication of such SCW on LiNbO3 crystals, and experimental demonstration of the optical intensity modulation will
be introduced.


## 2. The SCW Design Using LN Crystal

In recent years, a new kind of diode lasers,
referred to as slab-coupled optical waveguide lasers, has been demonstrated mainly
for outputting near infrared wavelengths [5]. Characteristics of such diode
lasers are their large and symmetric single-spatial-mode outputs that can reach
a few Watts. The slab-coupled
waveguide (SCW) concept is actually derived from Marcatili’s coupled-mode
analysis, which shows that a large passive multimode guide could be designed to
operate in a single-spatial-mode by coupling all the higher-order modes to an
adjacent slab [6]. In this research, this new SCW concept will be
attempted to be used in the LiNbO3 crystal, in order to expand the
crystal’s potential for more efficient EO applications.Figure1 shows a schematic of a possible SCW design using
a z-cut LN followed by two epilayers.
Inside the SCW, the main channel waveguide will be buried by a slab waveguide,
and the slab is then laterally confined by two etched drains deep into the
second epilayer. Vertically, this SCW has a uniform
LiNbO3   substrate
layer with an index of n3,
a uniform core epilayer with an index of n1  and a layer thickness of h, and a
uniform slab epilayer with an index of n2 and a thickness of  l.  The difference
among the three refractive indices should be small and should satisfy n3<n2<n1.
Figure 2 shows an anticipated index profile for such SCW, in which each
individual profile of  n3,  n1,  and n2 is assumed to be varying (simulating the real PE fabrication
conditions). The refractive index profile of the structure can then be written
as(1)n2(z)={n32,0≤z≤a,n12(z)=(n10−n3)exp[−{(b−a−z)/(b−a)}r],a≤z≤b,n102,b≤z≤c,n202−(n202−n102)[d/c−z/cd/c−1]q,c≤z≤d,1,d≤z, where r and q are profile parameters used to adjust the similarity of the
theoretical profiles and actual index profiles inside each epilayer.Figure 1
Schematic of the LN SCW design.Figure 2
Index profile along thec-axis of the LN SCW structure.By carefully selecting both vertical and lateral
confinement conditions, the waveguide can be designed leaky to all high-order modes
(complex propagation constants), and guiding only to the fundamental spatial
mode. Depending on the cladding index profile, the fundamental spatial mode can
be either slightly leaky (its loss is much smaller than that of high-order
modes. Therefore, it is loosely guided but has a complex propagation constant
too) or fully guided (a real propagation constant). Such propagation constants
can be calculated with the transfer-matrix method (TMM) [7], where the
graded-index profile in (1) is approximated by a stair-case profile.To implement the TMM, the refractive index profile
of the structure is divided into a large number of homogeneous layers. The
electric field in theith layer with a refractive index ni and width di  can be written as (2)Ei={Cicos[ki(z−di+1)]+Disin[ki(z−di+1)],ki2>0,Cicosh[ki(z−di+1)]+Disinh[ki(z−di+1)],ki2<0, where ki=|ki|, ki2=k02(ni2−neff2), k0=2π/λ is the free-space wave number, and neff=β/k0 is the effective index of the mode with β being
the propagation constant. By applying suitable boundary conditions at the
interface between the ith and (i+1)th layers, the
field coefficients, Ci, Di, Ci+1,  and Di+1,
are related by a 2×2  matrix (3)(Ci+1Di+1)=Si(CiDi), where Si is known as the
transfer matrix of the ith layer, and for TM and TE modes, it
is given differently. For the TE mode, for example, Si is given by (4)si={(cosΔi+1−(kiki+1)sinΔi+1sinΔi+1(kiki+1)cosΔi+1),ki2>o,(coshΔi+1(kiki+1)sinhΔi+1sinhΔi+1(kiki+1)coshΔi+1),ki2<0, where Δi=ki(di−di+1). The field
coefficients of the last layer can be related to those of the first layer by
simply multiplying the transfer matrices of all layers. Following those
standard procedures, we can obtain an eigenvalue equation for the propagation
constant β, that is, F(β)=0. In general, the propagation
constant can be expressed as β=βr+βi with βr being the real part and βi the imaginary part. A
plot of 1/|F|2 with β shows a number of resonance peaks, each of
which corresponds to a mode. These peaks are Lorentzian in shape. The value of β corresponding to the resonance peak gives the
real part of the propagation constant and the FWHM of the Lorentzian gives the
imaginary part. In this way, both real and imaginary parts of β can be calculated. The device designing effort
should focus on finding both material and structural parameters for best
understanding dispersion, phase and attenuation constant of the wave supported
in both main and leaky waveguides. However, the optimization procedure is
laborious in simulation, and the results are anticipated to be published
separately.For simplicity and by a few times of fitting with
LiNbO3’s refractive indices and the SCW’s structural parameters, in Figure 3 we show a preliminary single-spatial-mode simulated at the telecommunication
wavelength of 1.55 μm in the LN SCW, when we select
parameters of h=9 μm, t=8.4 μm, l=1.0 μm, w=10.0 μm, n2 = 2.13, and n1 = 2.15.
This mode shows a reasonably symmetric and a large profile of ~15 × 13 μm2, which is actually much
larger than that from a common single mode LN waveguide.Figure 3
A simulated single-spatial-mode
guided in a LN SCW having parameters ofh =
9 μm, t = 8.4 μm, l = 1.0  μm, w = 10.0 μm, n2 = 2.13 and n1 = 2.15.


## 3. Intensity Modulation Using the Leaky Mode

Figure4 shows a schematic of the new intensity modulation
principle, in which at the entrance the single-spatial-mode polarization is aligned vertically (as in
Figure 1) is fully supported by the main guide (guiding). When rotating the
polarization in the incident plane via an applied electrical field, the mode
becomes leaky and flows to the leaky slab guide, due to the weakening optical
confinement after rotating the incident polarization. When measuring the output
from the main guide, an efficient intensity modulation is therefore achieved. In
this case, only a few degrees of the incident polarization rotation would be
needed to complete the switching from the main guide to the leaky slab guide
(to reach one modulation cycle, if the guide is long enough), instead the
90° rotation is required in the case of using the phase modulation (to reach the
half-wave voltage).  Therefore, the
required voltage for this leaky modulation could be much lower.Figure 4
New modulation
mechanism using the main and the leaky waveguides (top); when applying an
electrical field, the polarization rotates along the propagation. The
propagation in the main guide therefore leaks to the leaky guide; and the
voltage requirement for such a leakage is low, and can be much lower than the
common  Vπ in all phase
modulated schemes (bottom).The remaining question will be how to rotate the
polarization in order to realize such an efficient intensity modulation? The answer
will directly relate to the used waveguide fabrication method. It is well known
that the proton exchanging (PE) will generate a significant difference for
ordinary (no) and
extraordinary (ne)
refraction indices inside the LN’s index ellipsoid [8]. Normally the PE
largely increases the extraordinary (ne)
index which makes the δne positive (δne~0.03 to 0.15). It slightly reduces the ordinary (no) index which makes the δne slightly negative (δne~−0.03  to 0.00). Figure 5 shows a typical index variation as the wavelength of
both no and ne, before and after the
proton exchanging [9]. Extraordinary (ne) index corresponds to the light’s polarization
aligned to the LN crystal’s z-axis
(c-cut), and ordinary (no)
to x-axis or y-axis (a-cut or b-cut).Figure 5
Refractive index
variations before and after the proton exchanging in LN crystal.Further to use a c-cut, LN SCW is an
example. When the incident single-spatial-mode polarization is aligned along the crystal’sz-axis, the light sees a large increase
in the extraordinary (ne)
index in the main guiding layer if fabricated by the PE method. The SCW
structure meets the requirement of n3<n2<n1, this single-spatial-mode
is optically confined and thus fully guided. If its polarization is rotated 90° and is parallel to y-axis, the light
now sees an unchanged or even slightly reduced ordinary (no) index, when comparing to the substrate’s ordinary
index. In this case, this designed SCW structure no more exists and the
incident light will be fully leaky to both leaky guide and substrate sides. An
intensity monitoring at the main guide’s output end will thus see a 0-to-100%
variation. In real case, there is no need to rotate the polarization to a full
90° for reaching a full leakage. This is because that as long as it
starts to leak, it will become eventually full when the electrode-applied waveguide
is sufficiently long.


## 4. Experimental Demonstration

Fabrication of such LN SCOWs follows
a procedure of a soft proton exchange (SPE) on c-cut LN crystal and then an
overgrowth of thin LN film on top of the wafer, and it was introduced with
details in a previous publication [10]. The final pattern formation was
done by using drying etching method and standard chromium protection mask to
fabricate two ridges (each ~10 μm wide), which are separated by a slab
with a designed width of ~10 μm. The waveguide was then end-polished
to be ~5 mm long. Gold electrodes were evaporated on top of the slab and the
two side ridges (using the same protection mask), and the dc electrical field was applied via a pair of contacting probes.In order to effectively rotate the
polarization plane of the incident beam that was initially set parallel to the
LiNbO3  crystal’s z-axis, the applied internal electric field must
have an angle to the polarization plane. The electrode alignment used here
(made on top of both the slab and the two ridges aside) actually uses both  γ33 and   γ31 EO coefficients, and is thus effective,
even though it is not the optimized design.Preliminary demonstration of the switching from the
main guide to the leaky guide was done by end-coupling a cw 1.55μm Er-doped fiber laser into the main
guide (vertically polarized by a polarizer), and the output was monitored by a
CCD at about 1 cm away. When applying ~10 V to the device, a clear switching
was seen as shown in Figure 6. Those visible side diffractions indicate a
certain level of imperfection of the fabricated waveguide, which suggests the
needed effort for further improvement over both fabrication and design. The
measured extinction ratio is only about 10 dB, which may be affected by those
side-diffractions. We have to point out that in Figure 6 the images were taken
intentionally off the best extinction status, in order to compare the two-output
spots from the facet. It is also believed that the electrode voltage can be
further reduced if the electrodes are made on top of the slab and inside the
ditches, and by using more optimized designs and fabrications.Figure 6
Experimental image
showing intensity switching between the main and leaky waveguides by applying~10 V electrical field.


## 5. Conclusion

A new design of electro-optic intensity
modulator using LN SCWs was demonstrated, in which the modulation can be
realized by electrically switching the optical beam between the main waveguide
and the closely attached leaky slab waveguide. Low driving voltage can be
expected from using such SCW designs because of using the new leaky mechanism. The
theory for simulating such LN SCWs was suggested, and the result indicates the
possibility of making a large, circular, and symmetric single-spatial-mode
waveguide. This allows the potential to reduce coupling losses with adjacent
fiber networks.  Optical modulation was
experimentally demonstrated in a 5 cm long SCW waveguide fabricated using a
procedure of soft proton exchange and then an overgrowth of thin LN film on top
of the wafer. The used switching voltage is about 1.0 V in this preliminary
design, and the result indicates the potential for further improvement.



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*Source: 373259-2009-02-03.xml*