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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 36, NO. 5, MAY 2000
527
Optical Intensity Modulator Based on a Novel
Electrooptic Polymer Incorporating a
High Chromophore
Sang-Shin Lee, Sean M. Garner, Vadim Chuyanov, Hua Zhang, William H. Steier, Fang Wang, Larry R. Dalton,
Anand H. Udupa, and Harold R. Fetterman
Abstract—We have synthesized a novel electrooptic (EO)
polymer based on a high chromophore incorporating tricyanobutadiene acceptors. A crosslinked polyurethane network
was also adopted to enhance its thermal stability. In order to find
the optimum poling condition for the polymer, the influence of the
electric poling profile on optical characteristics such as EO effect,
thermal stability, and damage was investigated. Then a high-speed
intensity modulator using the EO polymer was designed and
fabricated. The measured half-wave voltage V was 4.5 V at the
wavelength of 1.31 m. Accordingly, the achieved EO coefficient
r33 was as high as 25 pm/V, and the thermal stability of the
poled polymer was as high as 95 C. Finally, the modulator was
successfully operated up to 40 GHz.
Index Terms—Electrooptic modulation, fiber optics, optical
planar waveguide components, optical polymers, optical waveguides.
I. INTRODUCTION
IGH-SPEED optical intensity modulators have been
widely used in analog fiber-optic transmission systems,
community access television (CATV) distribution systems,
radio frequency (RF) phase shifters, and analog/digital microwave links [1], [2]. They are also attractive as an external
modulator in wavelength division multiplexing (WDM)
fiber-optic telecommunication systems [3]. Recently, electrooptic (EO) polymers attracted extensive attention due to
their advantages over inorganic materials. The first advantage
is the low dispersion in the index of refraction between infrared
and millimeter-wave frequencies. The second is that they can
be deposited onto and will adhere to many substrates including
semiconductors. In addition, optical guiding structures and
modulators or optical switches use fabrication techniques that
are compatible with semiconductor electronics. This makes
possible a significant step forward in optoelectronic integration.
The third advantage is the ability to integrate the active polymer
H
Manuscript received November 10, 1999; revised February 3, 2000.
S.-S. Lee was with the Department of Electrical Engineering-Electrophysics,
University of Southern California, Los Angeles, CA 90089-0483 USA. He is
now with the Devices and Materials Laboratory, LG Corporate Institute of Technology, Seoul 137-724, Korea.
S. M. Garner, V. Chuyanov, H. Zhang, and W. H. Steier are with the Department of Electrical Engineering-Electrophysics, University of Southern California, Los Angeles, CA 90089-0483 USA.
F. Wang and L. R. Dalton are with the Department of Chemistry, University
of Southern California, Los Angeles, CA 90089 USA.
A. H. Udupa and H. R. Fetterman are with the Department of Electrical Engineering, University of California at Los Angeles, Los Angeles, CA 90095 USA.
Publisher Item Identifier S 0018-9197(00)03540-5.
materials into an optical circuit, which includes other optical
materials. So far, high-speed EO polymer modulators operating
up to 110 GHz have been demonstrated [4]–[6]. For practical
systems applications, the polymer modulators are required to
have lower half-wave voltage V , higher thermal stability, and
lower loss.
The V of the polymer modulators can be reduced by using
an EO polymer based on chromophores with a high molecular
nonlinearity ( ). Here is the permanent dipole moment in
the ground state and is the first molecular hyperpolarizability,
and so the value is proportional to the EO coefficient of the
polymer. Also, the thermal stability can be enhanced remarkably by using a crosslinkable system instead of the guest-host
or side-chain systems, since the backbone and chromophore of
the crosslinkable polymer are chemically crosslinked to make
the relaxation of the aligned chromophores difficult. Recently,
Shi et al. reported a modulator based on a Polyurethane Disperse
Red (DR) 19 thermoset polymer [7]. In this paper, we have synthesized a new high chromophore, FTC, by using a novel
tricyanobutadiene acceptor incorporating a furan-derative ring.
Its value (15 000210048 esu at 1.06 m) is larger than
that of previous chromophores like DR1 (580210048 esu) [8],
DR19, and DANS [9] by one order. Then a functionalized FTC
was formed to synthesize a thermal crosslinkable EO polymer,
PU-FTC. In order to find the near optimum poling condition for
the new crosslinkable polymer, the influence of the electric poling
profile on the optical characteristics such as EO effect, thermal
stability, and damage was investigated. Finally, we have successfully demonstrated a high-speed optical intensity modulator.
II. SYNTHESIS OF A NEW CROSSLINKABLE EO POLYMER
Recently, we developed a high choromophore based on a
novel tricyanobutadiene acceptor incorporating a furan-derivative ring, FTC (2-dicyanomethylen-3-cyano-4-f2-[trans-(4-N,
N-diacetoxyethyl-amino) phenylene-3,4-dibutylthien-5]vinyl g5,5-dimethyl-2,5-dihydrofuran) [10]. Fig. 1 shows the synthesis procedure for the PU-FTC polymer using the FTC
chromophore. The furan ring plays an important role in keeping
the conjugation planar and stabilizing the acceptor end of the
chromophore. Also, the two methyl groups on the heterocyclic
(oxygen) ring and the two butyl groups on the thiophene ring
should prevent the large dipolar chromophores from aggregating, which is caused by strong electrostatic interactions in
most of the high chromophores. The interaction between
0018–9197/00$10.00 © 2000 IEEE
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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 36, NO. 5, MAY 2000
Fig. 1. The FTC chromophore and the covalent incorporation into a thermal-set polyurethane polymer to from the PU-FTC polymer. TDI and TEA are
commercially available crosslinkers and dioxane is the solvent used.
the chromophores may reduce the achievable EO coefficients.
The FTC chromophore when doped into a PMMA host has
a r33 value of 55 pm/V @ 1.06 m. The chromophore has
excellent solubility essential for materials processibility, high
chromophore thermostability (>300 C), and the guest–host
system has a modest optical loss of 1 dB/cm @ 1.3 m. In this
work, to enhance the thermal stability of the PMMA- doped
FTC, we have synthesized hydroxyl functionalized FTC chromophores by adopting a single-end crosslinked polyurethane
system, because the thermoset polyurethanes have been
widely used to stabilize the polar alignment of a nonlinear
optic polymer. The FTC chromophore is mixed with toluene
diisocyanate (TDI) in a solvent and heated to attach the NCO
groups to the OH groups. Next, the crosslinker, triethanolamine
(TEA), is added which acts to form a 3-D network during the
pre-curing and final hardening during poling. Excess TDI and
TEA can be added to control the density of chromophores.
Fig. 2. The absorption spectrum of the PU-FTC polymer.
III. CHARACTERIZATION OF THE PU-FTC POLYMER
The basic optical properties of the PU-FTC polymer have
been observed first. The absorption spectrum of the polymer
was measured with a spectrophotometer, and Fig. 2 shows the
absorbance as a function of the wavelength. The absorption peak
is at 670 nm, and the absorption beyond = 1000 nm is negligibly small. Also, the propagation loss of the polymer was measured by employing the liquid out-coupling method [11]. A thin
polymer film was prepared on a silicon substrate coated with
a 4-m-thick SiO2 layer to form a planar waveguide structure,
and light was prism-coupled into it. After propagating over a
distance in the polymer layer, the light was coupled from the
polymer film to a high index liquid into which the waveguide
was dipped. The attenuation of the light as a function of the
propagation distance was obtained to provide the propagation
loss of 2 dB/cm @ 1.3 m.
Then, the poling characteristics of the PU-FTC were studied
to find the near optimum poling condition leading to large
EO coefficients without damage. For the electric field-assisted
Fig. 3. The electric poling temperature and voltage profiles for the PU-FTC
polymer.
poling, which is used to align the chromophores for achieving
the EO effect, we adopted the corona poling method. The
typical needle-to-plane distance was about 2 cm and the dc
voltage applied to the needle was 8 kV. The poling schedule for
the thermosetting polymer was designed to achieve a large EO
effect and high thermal stability without causing damage. Fig. 3
LEE et al.: OPTICAL INTENSITY MODULATOR BASED ON A NOVEL ELECTROOPTIC POLYMER
Fig. 4. EO coefficients of the poled polymer as a function of the pre-curing
time.
shows the typical electric poling profile, which consists of two
steps, pre-curing and actual poling. During the pre-curing step,
the crosslinkable nonlinear optical polymer, PU-FTC, is heated
for slight hardening to temperature Tpre for the time tpre to
initiate partial crosslinking prior to applying the high voltage.
This is to prevent surface damage due to the electric charges
induced by the corona needle during poling. There is a tradeoff
between the pre-curing and the poling efficiency. When the
pre-curing is not sufficient, surface damage occurs, while
with excessive pre-curing the rotation of the chromophores is
restricted and the poling efficiency is reduced. Then, during
the actual poling step, a dc voltage Vp is applied to the corona
needle with the film temperature fixed at Tp , so that the aligning
of the chromophores and the crosslinking of the polymers is
completed. Here again there is a tradeoff. Higher Tp may allow
easier rotation of the chromophores and allow more complete
crosslinking, which gives higher poling efficiency and higher
thermal stability. On the other hand, if Tp is too high, the
crosslinking may be achieved before the chromophores have
had time to align to the electric field.
The optical characteristics of the poled polymers were
measured in terms of the EO coefficient r33 , thermal stability,
and damage. First, the effect of the pre-curing time on r33 and
surface damage was examined. Five thin PU-FTC films were
prepared on glass substrates coated with indium–tin–oxide
(ITO) and dried overnight in a vacuum oven. They were
corona-poled on a hot plate by varying the pre-curing time from
0 to 20 min, with the pre-curing temperature fixed at 120 C.
The actual poling temperature and time were fixed at 110 C
and 1 h, respectively. The value of r33 of the poled polymers
was measured by using the attenuated total reflection (ATR)
method [12]. Fig. 4 shows r33 at 1.06 m as a function of the
pre-curing time tpre . The film with no pre-curing was severely
damaged due to insufficient crosslinking and did not provide
any EO effect. However, the films with the tpre of longer
than 3 min were not damaged, indicating that they became
rigid and crosslinked enough to resist the ion bombardment
of the charges. As shown in Fig. 4, the r33 decreases with the
pre-curing time. This is because, as the pre-curing progresses,
the polymer network gradually gets crosslinked. Therefore,
the mobility of the chromophores is reduced to yield smaller
alignment of the dipoles. The EO coefficient of the PU-FTC
is mostly smaller than that of the PMMA-doped FTC. This is
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(a)
(b)
Fig. 5. SEM photographs of the poled PU-FTC polymer films. (a) With no
damage. (b) With damage.
attributed to the lower poling efficiency of the attached PU-FTC
where the chromophores have less freedom than those in the
doped systems. It is noted that, for the thermosetting material,
lattice hardening is taking place during poling and can reduce
poling efficiency by preventing some of the chromophore from
reorienting under the influence of the poling filed. Fig. 5(a)
and (b) show the SEM photographs of the undamaged and
damaged films, respectively. The poling-damaged film reveals
serious surface deformation such as irregular holes, which will
cause serious scattering loss, while the film with no damage
has an optically flat surface. The deformation is attributed
to the strong compressive electrostatic force of the charges
accumulated on the film, since the charged particles may carry
large kinetic energy when they pass through the high electric
potential between the corona needle and the ground electrode.
Then, the effect of the actual poling temperature on the
thermal stability, EO coefficient, and poling damage was
studied. Five polymer films were pre-cured at 120 C for 3 min
and then poled by varying the poling temperature from 80 C
to 120 C. The thermal stability of the poled polymer was
measured with the in situ second harmonic generation (SHG)
temperature ramping method [13]. We applied current through
the ITO to ramp up the temperature of the poled samples,
while monitoring the SHG signal out of them. The thermal
stability was obtained by finding the temperature at which the
SHG signal began to be decreased since the alignment of the
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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 36, NO. 5, MAY 2000
Fig. 6. SHG coefficients of the poled PU-FTC polymer as a function of the
temperature.
Fig. 8. Structure of the Mach–Zehnder intensity modulator based on the
PU-FTC.
Fig. 7. Thermal stability temperatures and EO coefficients of the poled
PU-FTC polymer as a function of the poling temperature.
chromophores was randomized. Fig. 6 shows the normalized
SHG coefficients d33 as a function of the temperature for the
sample poled at 100 C. Tstab can be obtained by finding the
temperature at which the linear lines used for fitting the two
curves with distinct slopes are intersected. As shown in Fig. 6,
the nonlinear optic effect of the poled PU-FTC remains stable
up to the Tstab of 95 C. Also, Fig. 7 shows the thermal stability temperature Tstab and the r33 as a function of the actual
poling temperature. As shown in the figure, Tstab is heightened
when the poling temperature increases from 80 C to 100 C.
However, it is saturated at about 95 C for poling temperatures
beyond 100 C, because the crosslinking density of the polymer
chains is limited. When the poling temperature increases, the
crosslinking density is increased to a certain extent, but then
limited since the uncrosslinked polymer chains cannot find
the partners for crosslinking any more. The film poled at 120
C was severely damaged like the damaged film shown in
Fig. 5(b), since it was too soft at that high temperature. Fig. 7
also shows r33 versus the poling temperature. r33 is as small as
8 pm/V for the Tp of 80 C, and it has been increased up to 40
pm/V for the Tp of 100 C due to the enhanced mobility of the
chromophores. The r33 value @ 1.3 m corresponding to that
of 40 pm/V @ 1.06 m was estimated to be about 25 pm/V
by using the two-level model [14]. However, it is saturated for
the Tp of >100 C. This indicates the saturation of the poling
efficiency: the mobility of the chromophores increases with the
poling temperature, but it is finally limited since the reaction
speed for crosslinking is also increased with the temperature.
Finally, the poling efficiency of the PU-FTC polymer coated
on top of a lower cladding was considered, because for a practical device the electric poling is done across the two polymer
layers. A thermal crosslinkable epoxy, Epoxylite 9653, was
chosen for the lower cladding. We have prepared two samples,
a single PU-FTC layer and a PU-FTC layer stacked on the
lower cladding. The two samples were poled by using the same
poling profile, and the SHG signals from them were measured
and compared. There was no significant difference in the SHG
signals between the two samples. Therefore, it was confirmed
that the PU-FTC polymer could be efficiently poled regardless
of the lower cladding.
IV. DESIGN OF THE POLYMER MODULATOR
The high-speed Mach–Zehnder intensity modulator based on
the proposed PU-FTC is shown in Fig. 8. It combines a traveling wave optical Mach–Zehnder interferometer and a traveling wave microstrip line circuit. First, for vertical confinement
in the optical waveguide, a triple stack structure composed of
the lower cladding, core EO polymer (n = 1:65 @ 1.3 m),
and upper cladding was fabricated. A thermally curable epoxy,
Epoxylite 9653 (n = 1:54 @ 1.3 m), was used for the lower
cladding. It was experimentally confirmed, as mentioned earlier,
that the epoxy has a lower bulk resistivity than PU-FTC, which
facilitates the electric field poling. A UV curable epoxy, NOA73
(n = 1:54 @ 1.3 m), was chosen for the upper cladding. In
the typical device, the waveguide rib width was 6 m, the core
layer thickness was 1.5 m, and the rib height was 0.3 m. The
thickness of the lower and upper claddings was set at 3 m and
3.5 m, respectively, to keep the optical loss due to the electrodes small. The length of the arms of the interferometer where
the modulation interaction occurs was 20 mm and the length of
the linear Y -branch transition was 3 mm at each side. The total
branching angle of the Y -branch was 1 , and the separation
between the two straight waveguides in the interaction region
was 50 m.
For the traveling-wave type microwave electrode, a microstrip line (MSL) structure was adopted. Its characteristic
impedance was designed to be 50 by setting the width of
the upper electrode to be 22 m, with a typical electrode gap
of 10 m. A smaller electrode gap results in a lower driving
voltage. However, this could cause higher microwave loss due
to the ohmic loss. To reduce the mm-wave loss, the thickness of
the upper electrode was increased to 4 m by electroplating
after the electrode shape was defined by lithography. Then the
extra loss due to both the bends and the transitions from the
LEE et al.: OPTICAL INTENSITY MODULATOR BASED ON A NOVEL ELECTROOPTIC POLYMER
531
broad pad to the narrow MSL was considered. It was found
that a simple right-angle bend could be used without any
significant radiation loss. A tapered pad structure, 3 mm2150
m, was formed at each end of the microstrip line to facilitate
the attachment of the coaxial input and output cables during
packaging.
V. DEVICE FABRICATION AND EXPERIMENTAL RESULTS
The fabrication steps for the EO polymer modulator are
described here. The bottom ground electrode was made on a
silicon substrate by depositing Cr/Au. For the lower cladding,
Epoxylite 9653 was spin-coated to be 3 m and thermally cured
by heating at 130 C for 3 h. For the core layer, a 1.5-m-thick
PU-FTC was spun and dried overnight, and then pre-cured at
120 C for 3 min on a hot plate. To induce the EO effect in the
core, corona poling was performed at 100 C by applying 8
kV between the corona needle and the ground electrode. Then,
a standard photolithography and RIE in oxygen were done to
form the core rib for the waveguide. NOA73 was spin-coated
to be 3.5 m for the upper cladding and cured by exposing
under ultraviolet light. Thin Cr/Au layers were deposited on
top of the polymer films as base metals for the electroplating,
and a photoresist 8 m thick was spun on the metal layers and
dried sufficiently at room temperature. Also, we have made
the electrode pattern by aligning the electrode pattern on the
mask with the etched waveguide pattern. Electroplating was
performed at 50 C for 10 min with a pulsed current source.
The thickness and width of the electrode are 4 m and 22 m,
respectively. Finally, for light coupling, the end facets were
prepared by dicing with a nickel blade.
To measure the performance of the modulator, TM-polarized light at 1.31 m was coupled to the device through a
single-mode fiber. The output light was collected by an objective lens and focused onto a photodetector. Fig. 9 shows the
response of the modulator to a low-frequency sawtooth wave
electrical signal. The upper signal is the applied modulating
signal and the lower the optical output. V is obtained by
finding the voltage required to turn the modulator from full on
to full off. The measured V was 4.5 V, which corresponds to
an r33 of about 25 pm/V with an effective field-overlap integral
factor of one. This is consistent with the r33 value measured
at 1.06 m on test samples. The extinction ratio, the ratio of
the light power out during the on state to that of the off state,
was measured to be 18 dB. The extinction ratio should increase
when a single-mode fiber is used to collect the output rather
than the lens. The measured insertion loss was 14 dB for the
device length of 36 mm. The total insertion loss is the sum of
the waveguide propagation loss and the coupling loss at the
facets. There is a 5-dB/facet coupling loss since no attempt
was made to match the 9-m-diameter mode of the fiber to
the elliptical mode (7 m2 2 m) of the polymer waveguide.
Accordingly, the propagation loss of the waveguide was 2.5
dB/cm. The waveguide propagation loss is a collection of the
losses from material intrinsic absorption, waveguide/cladding
layer scattering, and poling-induced scattering [15]. Finally,
Fig. 10 shows the frequency response of the modulator. The
device is working well from 0 to 40 GHz except for some
Fig. 9. Oscilloscope traces for the measurement of V of the polymer
modulator. The upper trace is the 10 Vp-p , 1-kHz sawtooth waveform applied
to the microstrip line. The lower trace is the light output from the device.
Fig. 10. The measured frequency resoponse of the PU-FTC Mach–Zehnder
modulator.
ripples, which are believed to be due to impedance mismatches
at the input and output.
VI. CONCLUSIONS
A novel crosslinkable EO polymer based on a high chromophore incorporating tricyanobutadiene acceptors was proposed and synthesized. We have systematically studied the effect of the electric poling profile on optical characteristics such
as the EO effect, thermal stability, and damage, in order to realize the optimum poling condition leading to a large EO coefficient and high thermal stability. Also, a high-speed intensity modulator was successfully designed and fabricated by employing the polymer and a near optimum poling scheme.
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Sang-Shin Lee was born in Korea on September 5, 1968. He received the B.S.,
M.S., and Ph.D. degree in electrical engineering from Korea Advanced Institute
of Science and Technology, Taejon, in 1991, 1993, and 1997, respectively.
From 1997 to 1998, he worked as a Post-Doctoral Research Associate at
the University of Southern California, Los Angeles. In 1998, he joined the Devices and Materials Laboratory of LG Corporate Institute of Technology, Seoul,
Korea. His current research interests include polymer waveguide devices like
modulators, switches, and variable optical attentuators, and optical MEMS devices.
IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 36, NO. 5, MAY 2000
Sean M. Garner received the B.E. degree in engineering physics from Stevens
Institute of Technology, Hoboken, NJ, in 1993 and the Ph.D. degree in electrical
engineering from the University of Souther California, Los Angeles, in 1998.
Since 1998, he has been a Senior Research Scientist at Corning Incorporated,
and his current research interests include optical waveguide devices.
Vadim Chuyanov, photograph and biography not available at the time of publication.
Hua Zhang was born in China in 1970. He received the B.S. degree in communication engineering from the University of Electronic Science and Technology
of China in 1993 and the M.S. degree in electronic engineering from the University of Science and Technology of China in 1996. He is currently working
toward the Ph.D. degree at the Department of Electrical Engineering of the University of Southern California, Los Angeles.
His research interests are mainly in integrated nonlinear optical devices.
William H. Steier, photograph and biography not available at the time of publication.
Fang Wang, photograph and biography not available at the time of publication.
Larry R. Dalton, photograph and biography not available at the time of publication.
Anand H. Udupa was born in Chennai, India, in 1976. He received the B.Tech.
degree from the Indian Institute of Technology, Chennai, in 1997, and the M.S.
degree in electrical engineering from the University of California, Los Angeles,
in 1999. His graduate research focused on the design and testing of high-frequency polymer modulators.
He co-authored many scientific papers in international journals and conferences. He currently works in the field of mixed signal circuit design at Texas
Instruments, Bangalore, India.
Harold R. Fetterman, photograph and biography not available at the time of
publication.