ARSAdvances in Radio ScienceARSAdv. Radio Sci.1684-9973Copernicus PublicationsGöttingen, Germany10.5194/ars-15-141-2017Single waveguide silicon-organic hybrid modulatorHoppeNiklasniklas.hoppe@int.uni-stuttgart.dehttps://orcid.org/0000-0001-8818-3144RotheChristianCelikArdaRosaMaría FélixVogelWolfgangWidmannDanielRathgeberLotteRuiz DelgadoM. CarmenVillacampaBelénLudwigsSabineBerrothManfredUniversity of Stuttgart, Institute for Electrical and Optical
Communications Engineering, Stuttgart, GermanyUniversity of Stuttgart, Institute for Polymer Chemistry, Stuttgart,
GermanyUniversity of Málaga, Department of Physical Chemistry,
Málaga, SpainUniversity of Zaragoza, Department of Condensed Matter Physics,
Zaragoza, SpainNiklas Hoppe (niklas.hoppe@int.uni-stuttgart.de)21September20171514114730December201628March20172May2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://ars.copernicus.org/articles/15/141/2017/ars-15-141-2017.htmlThe full text article is available as a PDF file from https://ars.copernicus.org/articles/15/141/2017/ars-15-141-2017.pdf
We present a novel silicon-organic hybrid modulator based
on an integrated dual-mode interferometer. The modulator offers a compact,
simplified design and enhanced robustness to on-chip fluctuations of
temperature compared to conventional Mach-Zehnder based systems. A prototype
modulator showing a voltage dependent transmission spectrum is obtained by
cladding a dual-mode waveguide in a 250 nm silicon-on-insulator technology
with a customized organic electro-optic layer. Estimated phase shifts and
corresponding figures of merit are discussed in this contribution. The used
organic layer is based on the guest-host approach with customized donor-π-acceptor chromophore embedded and poled in a poly(methylmethacrylate)
matrix. The presented prototype is to the best of the authors' knowledge the
first integrated single waveguide silicon-organic hybrid modulator.
Introduction
In the last decades, the integration and parallelization of electro-optic
modulators in combination with higher order modulating schemes led to data
rates up to 54 Tbit s-1 transmitted over a single core fiber and a distance of
9150 km (Cai et al., 2014). One key element is the Mach-Zehnder interferometer
modulator (MZM), which is based on electro-optic phase shifters. Different
electro-optic effects can be used for the realization of silicon-based phase
shifters with electro-optic bandwidths more than 10 GHz (Xiao et al., 2013; Koos et al.,
2016). The plasma dispersion effect offers a refractive index change based
on the free carrier relocation in silicon. The induced absorption and
moderate refractive index change are disadvantages which motivate the
integration of electro-optic polymer compounds showing a strong Pockels
effect. In the so-called silicon-organic hybrid platform this polymer
compound is combined with the silicon-on-insulator (SOI) platform. In-device
electro-optic coefficients up to 230 pm V-1 are measured (Koos et al., 2016). In the
meantime, MZMs based on electro-optic polymer compounds are working even at
elevated temperatures up to 80 ∘C demonstrating their
stability (Lauermann et al., 2016). Whereas the MZM is an established element based
on two waveguide arms, the modulator design can be simplified with the help
of a dual-mode or bimodal interferometer (DMI), which is presented e.g. in
(Zinoviev et al., 2011; Liu et al., 2014; Hoppe et al., 2017a). Using only one dual-mode
waveguide, the novel design is a promising candidate to offer simplified,
compact, parallelizable, broadband and thermal stable silicon modulators.
Utilizing DMIs in combination with organic layers offers a beneficial
thermal stability and robustness to on-chip temperature fluctuations for
sensor applications (Hoppe et al., 2017a). For that reasons, a prototype DMI
modulator is presented in this paper demonstrating the functionality as
electro-optic modulator. The paper is structured as follows: The general
principle is explained in Sect. 2. The tailor-made organic electro-optic
layer is presented in Sect. 3, which is used for the prototype modulator
realization as stated out in Sect. 4. Electro-optic measurements are
depicted in Sect. 5 where different figures of merit are extracted from the
transmission spectra. A conclusion summarizes the achieved results.
(a) Functional principle of a modulator based on a dual-mode
interferometer and (b) a possible schematic structure with two corresponding
electric field profiles of the first and second order TE mode in an
exemplary dual-mode waveguide. The electric field profiles are calculated by
the finite difference method (FDM) mode solver of FIMMWAVE. Here, the two
excited mode profiles at the left converter output can look like the shown
if the refractive indices of the converter and the dual-mode waveguide
claddings are matched.
Device principle
Instead of using two optical paths that are spatially separated like in a
Mach-Zehnder interferometer, a DMI uses two optical paths in a single
waveguide by utilizing two different modes. The working principle is based
on the fact that two different excited modes in one waveguide are confined
differently which results in different penetration depths into the cladding.
Therefore, there is different interaction for the two modes with the
environment which can be used for modulator applications. Different mode
profiles result in different effective refractive indices and finally in a
phase difference Δϕ between the two modes modulating
the output power. Accumulating the phase difference in the same waveguide
reveals two main advantages. Firstly, the device size can be decreased in
comparison to a MZM that has two separated optical paths. And secondly, the
device temperature stability can be increased due to equal environment for
both modes. The principle device consists of a dual-mode waveguide between
two mode converters (Fig. 1a).
Electrodes are placed in the modulation region for applying an electrical
field to an electro-optic organic layer. Two-mode excitation is achieved by
using mode converters, which can be realized e.g. by laterally shifted
tapers (Hoppe et al., 2017a) as depicted in Fig. 1b, by lateral shifted waveguides
(Liu et al., 2014), by waveguide steps (Zinoviev et al., 2011), by waveguide gratings
(Bruck and Hainberger, 2014) or even simpler by a lateral shift of the external fibers with
respect to the center of the grating couplers. Using the last approach, at a
special fiber position, the first and second order modes are excited in the
following dual-mode waveguide with the same power resulting in a balanced
excitation (Hoppe et al., 2015). However, imbalanced excitation can also be
beneficial if the two modes suffer from different losses because it can
increase the extinction ratio (Hoppe et al., 2017a). Similarly, the two modes are
combined at the end of the waveguide by the reverse process using a second
mode converter, which includes a single mode output. For modulation, a
nonlinear electro-optic layer is deposited on the dual-mode waveguide. The
widespread second order mode (see mode profiles in Fig. 1b) with a smaller
fill factor predominantly interacts with this layer causing the desired
phase difference between the two modes. The effect of the cladding can be
described quantitatively by the so-called intrinsic bulk sensitivity in a
DMI, given e.g. by Ramirez et al. (2015) as
ηbulk=∂Δneff∂nc
and describes the influence of the cladding refractive index nc.
Δneff=neff,i-neff,1 is the difference between the
refractive index of the high order mode (i) and the one of the fundamental
mode. The phase shift is proportional to the length L of the waveguide
where the interaction with the layer takes place. Therefore, Ramirez et al. (2015)
defines the bulk sensitivity for the wavelength λ0 as
Sbulk=∂Δϕ∂nc=2πLλ0ηbulk.
To increase the phase shift, the geometrical length has to be increased by a
longer waveguide but it is limited by the maximum device size and losses.
The area can be minimized by using a different structure like spiral or
meander shaped waveguides (Liu et al., 2014) but losses are still a limitation.
(a) Molecular structure of the nonlinear optical active
chromophore with donor (blue), thiophene-linker (black) and acceptor (red).
(b) Top view of the optimized ground state molecular structure in
dichloromethane solution with the corresponding torsion angles between the
building blocks and the dipole moment μ. The simulation was carried out on
the M06-2X/6-31G** level, alkyl chains were truncated to methyl groups. (c)
Molar absorptivity ε of the chromophore in dichloromethane
solution.
Experimental and calculated nonlinear optical properties of the
chromophore, with μβ being the product of dipole moment μ and molecular
first hyperpolarizability β, μg the ground state dipole moment,
μe the excited state dipole moment, Δμge the difference
between the ground state and excited state dipole moments, μge the
transition dipole moment and Emax the energy of the transition.
Experimental Data Theoretical Data μβa (10-48 esu)μβ0b (10-48 esu)μβ0c (10-48 esu)TD-DFT-calculatedd parameters μg (D)μe (D)μge (D)Δμge (D)Emax (eV)77003530690915.739.613.224.02.24
a Measured by EFISH in dichloromethane at 1.9 µm (experimental
accuracy ±10 %).b Experimental μβ0 values calculated using the two
level model.c Calculated at the CPHF level in gas
phase//PCM-M06-2X/6-31G**.d Calculated at the M06-2X/6-31G** level in CH2Cl2.
Electro-optic active layer
The nonlinear optic active layer used in this work is based on the usage of
donor-π-acceptor chromophores which are embedded and poled in a matrix
consisting of polymethylmethacrylate (PMMA). This approach is referred to as
“guest-host approach” (Dalton et al., 2010; Yesodha et al., 2004). The chromophore
consists of a tetrahydroquinoline based unit as donor (Fig. 2a blue) and a
tricyanovinylene (TCV) unit as acceptor (Fig. 2a red) which are linked by
a π-conjugated bridge.
Based on our own experience with branched thiophene polymers (Scheuble et al.,
2015; Richter et al., 2012) we chose a β-branched terthiophene (Fig. 2a
black) as π-conjugated building block. The branching on the thiophene
linker enhances the temperature stability of the molecule and also acts as a
potential hindrance for molecular dipole-dipole stacking of the chromophores
in the polymer matrix. Density functional theory (DFT) simulations suggest
rather planar backbones of the chromophores with the biggest torsion angles
being located between the β-branched thiophene sidechain and the
backbone and between the donor and the first α-linked thiophene of
the terthiophene bridge (see Fig. 2b). The synthesis of the shown
chromophore will be published elsewhere. The molecule shows a broad
absorption of light between 250 and 950 nm with a prominent
charge-transfer absorption band at a wavelength around 660 nm (Fig. 2c)
resulting in a dark blue color in dichloromethane solution. The second order
nonlinear optical properties of the chromophore have been simulated using
the CPHF (coupled perturbed Hartree-Fock) approach and also determined
experimentally by electric field induced second harmonic generation (EFISH)
measurements in dichloromethane solution (see Table 1). The theoretical data
are in good accordance with the experimental data and both also lie in the
range of known materials with comparable acceptors (Cai et al., 1999; Delgado et al.,
2008). Time dependent density functional theory (TD-DFT) calculations reveal
that the nonlinear optical (NLO) response seems to arise from the lowest
energy allowed absorption. Furthermore, the predicted increase of the dipole
moment from 15.7 D in the ground state (μg) to 39.6 D in the first
singlet excited state (μe) suggests that this transition has a
strong charge-transfer nature. Both experimental and theoretical
measurements of the new chromophore therefore confirm the potential use in
NLO applications.
(a) Schematic view of the fabricated modulator prototype. (b)
Corresponding cross section of the cladded dual-mode waveguide section in
(a) after poling of the organic electro-optic film.
For the modulators thin films were prepared by drop-casting chloroform
solutions (V=20µL, c=75 mg mL-1) containing PMMA (weight average
molecular mass: Mw‾=∼350000 g mol-1, purchased from
Sigma Aldrich) and 24 wt % of the chromophore on the chips. The films were
subsequently dried at room temperature under vacuum. The poling of the
chromophores in the films is accomplished directly on the chips.
Prototype realization
The principle in Hoppe et al. (2015) is used to excite the two lowest order
transverse electric (TE) modes in a dual-mode waveguide for building a
prototype modulator. Hereby, two lateral displaced single mode fibers (SMF
28) above the linear input and output grating couplers (see Fig. 3a) act
as mode converters as explained in Sect. 2.
Silicon waveguides, tapers and grating couplers are structured by
reactive-ion etching steps with optical lithography for the waveguides and
electron beam processing for the grating grooves at IMS CHIPS Stuttgart on a
Soitec 250 nm SOI wafer. Etching the original 1 µm thick SiO2
passivating layer with the help of a plasma etching step reduces the oxide
thickness. Further, the 2500 µm long dual-mode waveguide is partly
cladded with the electro-optic film presented in Sect. 3 using drop-casting.
A silver glue electrode is added on the top of the organic film (see Fig. 3b) to
build the top electrode. By adding a second silver glue layer on
the backside of the chip, a contact to the weakly doped Si substrate is
realized. The two silver glue electrodes enable to orientate the
chromophores in the PMMA matrix with the help of a poling procedure. For
this procedure, the chip sample is heated up to 120 ∘C and a
voltage of approximately 400 V is applied to the electrodes for a period of
1 h.
(a) Cross section of a Si waveguide model with SiO2 cladding.
(b) Group index difference for a DMI with the design in (a) versus SiO2
cladding thickness at λ0=1531 nm.
(a) Fiber to fiber transmission spectra of the prototype modulator
for different applied voltages Vapp. (b) Corresponding wavelength shift
with FSR being equal to 2.0 nm at λ0≈1552.5 nm.
Characterization of the waveguide channel
The bimodal waveguide design enables to characterize a geometrical parameter
like the SiO2 cladding thickness with high resolution by evaluating the
measured transmission spectra. The theoretical background is the free
spectral range (FSR) dependence on the group index difference
Δng of the first and second order modes, which can be
expressed with the help of
Δng=ng1-ng2
as
FSR=λ02Δng⋅L,
for wavelengths λ0≫ FSR as
stated out e.g. in Hoppe et al. (2015). If the length L is known and the FSR is
estimated at a center wavelength λ0 with the help of
two minima in the transmission spectrum, the amplitude of Δng can be calculated. Hereby, Δng depends for
example on the SiO2 cladding thickness and shape. Assuming the shape in
Fig. 4a and a width of 550 nm from a waveguide without electro-optic
active cladding, the resulting Δng is depicted in Fig. 4b.
Following this procedure, the measured FSR value of 10.92 nm at λ0=1531nm for a waveguide length of L=2500µm corresponds to a SiO2 thickness of around 180 nm. Note that,
if the waveguide is not homogenously cladded, the result is an averaged
value. Moreover, for specific geometrical parameters the estimated FSR can
correspond to more than one value. This problem can be fixed by evaluating
the transmission spectra at different wavelengths and for slightly different
waveguide geometries.
Knowing the waveguide geometry and the refractive index of the polymer, the
sensitivity to refractive index changes in the electro-optic film can be
simulated with the help of the FIMMWAVE software tool. For the waveguide
used in the prototype an intrinsic bulk sensitivity value of 7 % is
calculated at λ0=1550 nm, whereby the sensitivity of
a DMI results from the difference of both mode sensitivities as explained in
Sect. 2. The intrinsic bulk sensitivity of the presented prototype can be
enhanced to values above 50 % through a better bimodal waveguide design
with reduced SiO2 cladding.
Device functionality
The fabricated device is characterized in an electro-optic measurement setup
to demonstrate the modulating functionality. With the help of a tunable
laser source (Agilent 81682A), an optical power meter and low frequency
probes, the transmission spectrum is measured for different applied voltages
Vapp between the silver glue electrodes. Hereby, the maximum measured
current for the shown voltage range is 0.1 µA which implies a good
electrical insulation. A voltage dependent shift of the transmission
spectrum, which is proportional to the applied voltage, occurs, as depicted
in Fig. 5. Changing the polarity of the applied voltage results in an
opposite shift in the transmission spectrum. The varying transmission value
at a single wavelength demonstrates the modulation of the optical input
power.
The resulting phase shift can be calculated following the formula
Δφ=ΔλFSR⋅2π,
where, Δλ refers to the 0 V spectra and is
determined by the local minima in the transmission spectrum. With an
achieved Δφ of 0.2 π at a voltage of 60 V, the
resulting effective electro-optic coefficient reff can be estimated,
analogous to Koos et al. (2016), as
reff=d⋅λ0⋅ΔφVapp⋅L⋅nEO3⋅π≈7.2pmV-1,
where, the refractive index nEO of the electro-optic layer is assumed
to be 1.7 and the electrode distance d is approximately 13 µm. Using
r33=3⋅r13 (Teng and Man, 1990) and the intrinsic bulk sensitivity from
Sect. 4.1 the on chip r33 of the organic layer can be estimated to
300 pm V-1. However, this value should be treated with caution due to possible
migration of silver particles into the polymer layer during the poling
process and a potentially inhomogeneous oxide cladding shape and thickness
on top of the Si waveguide. The fiber to fiber insertion loss of around 25 dB is mainly caused by a not optimized grating coupler design and large
waveguide losses, which are improved in newer chip generations. E.g. the
waveguide loss for the fundamental TE mode in a 400 nm wide passivated
waveguide is reduced from more than 10 to 3.3 dB cm-1 in the technology
used up to date. The coupling efficiency of the grating couplers can be
enhanced by an aperiodic grating design with back side reflector in the used
technology to -0.62 dB (Sfar Zaoui et al., 2014). To achieve a balanced excitation
of the two TE modes, an additional lateral fiber offset with respect to the
center of the grating coupler is necessary (Hoppe et al., 2015). This can cause an
additional loss, which is dependent on the grating coupler width. E.g. for a
15 µm wide Si grating coupler it is around 3 dB following the
simulations in Hoppe et al. (2015). The extinction ratio of the presented
prototype modulator is around 8 dB (see Fig. 5a).
Conclusions
We present the novel design of a single waveguide silicon-organic hybrid
modulator, which is based on a DMI. The velocities of two optical waveguide
modes are differently influenced by changing the refractive index of an
organic electro-optic active layer. Adding suitable electrodes results in a
voltage dependent transmission, comparable to MZMs. With a fabricated
prototype the modulation of the optical power is demonstrated. A customized
organic donor-π-acceptor chromophore is used for obtaining the Pockels
effect. The fabricated sample shows an effective electro-optic coefficient
of 7 pm V-1 calculated from the voltage dependent transmission spectrum. The
sample is connected by two single mode fibers with the help of grating
couplers at the input and output. The fiber to fiber insertion loss of
around 25 dB at a wavelength of 1552 nm and the extinction ratio of around
8 dB can be improved by a proper converter design and reduced coupling and
waveguide losses.
The measurement data and simulation data that support the findings of this study are available in Zenodo with the identifier 10.5281/zenodo.583020 (Hoppe et al., 2017b).
The authors declare that they have no conflict of interest.
Acknowledgements
We would like to thank Philipp Diersing for the measurements of different
polymer samples in a modified Teng-Man measurement setup. In addition, we
would like to thank IMS CHIPS Stuttgart for fabricating the waveguides used
in the prototype. We further thank Klaus Dirnberger for discussions on
the design of the chromophores and Thomas Föhn and
Wissem Sfar Zaoui for fruitful discussions on the modulator design. The work
at the University of Málaga was supported by MINECO (project reference
CTQ2015–66897) and Junta de Andalucía (P09-FQM-4708). The work at the
University of Zaragoza was supported by Spanish Ministry of Science and
Innovation, MICINN-FEDER (Project CTQ 2014-52331-R) and the Gobierno de
Aragón-Fondo Social Europeo (E39).
Edited by: D. Killat
Reviewed by: R. Kunkel and one anonymous referee
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