Recent advances in electro-optic polymer modulators incorporating
highly nonlinear chromophore
Min-Cheol Oh, Hua Zhang, Cheng Zhang, Hernan Erlig, Yian Chang, Boris Tsap, Dan Chang,
Attila Szep, William H. Steier, Harold R. Fetterman, and Larry R. Dalton
Abstract
Based on a nonlinear optical polymer with a highly nonlinear chromophore (CLD) dispersed in a
amorphous polycarbonate (APC), we have developed electro-optic (EO) polymer modulators operating at
1550 nm wavelength with low loss and good thermal stability. By incorporating polymer insulation layer,
push-pull poling was successfully performed without film damages. We also demonstrated the propagation
loss of the EO polymer waveguide could be reduced as low as 1.2 dB/cm at 1550 nm when the large core
waveguide structure was incorporated. The long-term reliabilities of EO polymer modulator made of
CLD/APC polymer were investigated. When the modulator was hermetically sealed in an inert gas, the V 
change of Mach-Zehnder modulator was negligible over 30 days of operation with 20-mW exposure to the
waveguide input. In the thermal stability measurement, 25% of V increase was observed from the sample
heated to 60 ºC over 40 days, though the sample left at room temperature showed no decay of nonlinearity.
Min-Cheol Oh, Cheng Zhang, Hernan Erlig, Yian Chang, and Boris Tsap are with Pacific Wave Industries, Los
Angeles, CA 90024. Hua Zhang, Attila Szep, William H. Steier, and Larry Dalton are with the Department of Electrical
Engineering and Chemistry Department, University of Southern California, Los Angeles, CA 90089-0483. Dan Chang
and Harold R. Fetterman are with the Department of Electrical Engineering, University of California Los Angeles, Los
Angeles, CA 90095
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I. Introduction
Polymer electro-optic (EO) waveguide devices have been investigated for over a decade and there have
been various experimental results to demonstrate the merits of EO polymer devices. The demonstrations
have included high-speed modulators based on an inherent velocity match of RF and optical waves [1][2],
integration of electronic circuits with polymer waveguides [3][4], multilevel-stacked integrated optical
circuits [5][6], and polarization controlling waveguide devices [7]. However, to be accepted in system
applications, these EO polymer devices must demonstrate modest insertion loss, good long-term thermal
stability, and sufficient photo-stability at the communication wavelengths.
Typical EO polymers have higher absorption loss than LiNbO3, especially at 1550 nm wavelength, due
to the C-H vibration overtones. There has been recent work which shows the losses can be decreased by the
substitution of fluorine or chlorine for the hydrogen [8]. The thermal stability of the EO polymer materials
has been improved by employing a high Tg backbone polymer, a side chain attached chromophore, and a
crossliked polymer matrix [9-11]. However, in these polymers with an enhanced thermal stability, the
efficiency of poling was reduced and the EO coefficient was sacrificed to improve the thermal stability.
Recently an EO polymer based on a dendritic structure has demonstrated both high EO coefficient and
excellent poled order stability at 85 C [12]. The photo-stability of EO polymer waveguides under 1300 nm
wavelength illumination has been investigated and it was found that some EO polymer materials degrade
due to the chemical reaction with excited oxygen [13]. The dependence of the photostability on the
structure of the EO polymer and its backbone was investigated [14], and a double crosslinked chromophore
was incorporated to demonstrate excellent photostability [15].
To develop an EO polymer material suitable for commercial devices, the material synthesis research
has been directed toward the optimization of several requirements simultaneously. These include reduced
insertion loss, long-term thermal stability at 85 ºC, and long term photo-stability at 10 mW input power at
the communication wavelengths while at the same time maintaining a high electro-optic coefficient. There
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have been significant advances on all of these issues based on a guest-host polymer consisting of highly
nonlinear chromophore with phenyltetraene bridge (CLD) [16] and amorphous polycarbonate (APC) [17].
This chromophore was mixed in a PMMA host to achieve a push-pull modulator with a V of 0.8 V @ 1300
nm [18]. In addition to this highly nonlinear chromophore, APC was identified as a promising host material
because of its high thermal stability and low loss at 1550 nm wavelength [19]. >From the simple guest-host
mixture of CLD and APC, we obtained short term thermal stability of 120 ºC and waveguide optical
propagation loss of 1.7 dB/cm @ 1550nm [20]. In addition, the photo-stability of devices made with
CLD/APC can be significantly improved via a hermatic packaging which removes the reactive oxygen. In
this paper we will review the advances made based on the CLD/APC EO polymer which demonstrate that
the polymer devices are now becoming a commercially viable device.
II. EO polymers and Cladding materials
The recent progress to improve the nonlinearity of the chromophore has been accelerated by the
invention of a novel tricyanobutadiene acceptor with a furan-derivative ring. Fig. 1 shows the structure of
three chromophores with the novel acceptor, and Table 1 summarizes the physical properties and the
measured nonlinearity of the chromophores. FTC was the first chromophore which exhibited the high
nonlinearity induced by the efficient electron acceptor, 2 - (3 – cyano - 4,5,5 – trimethyl - 5H – furan – 2 ylidene) - malononitrile. The three cyano-groups in the acceptor are aligned along the charge-transfer
direction of the chromophore, thus they can pull the electron more efficiently so as to provide relatively
large dipole moment (12-15 Debye). The two butyl groups on the thiophene increase the solubility and
reduce the interchromophore electrostatic interaction which can lead to aggregates. The FTC chromophore
when doped into a PMMA host gives an EO coefficient r33 of 57 pm/V at 1060 nm. By using the FTC
chromophore dispersed into polyurethane, an ordinary MZ modulator with a 2-cm electrode was fabricated
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and V of 4.5 V was obtained at 1300 nm.
The basic structure of the CLD chromophore is obtained by replacing the thiophene unit in FTC with a
diene. The aromatic resonance energy of the bridge is reduced to increase the nonlinearity of CLD. The
two bulky t-butyldimethylsilyl groups used in CLD-1 enhance the molecular cross section so that
aggregates are less likely to form, and they also reduce the interchromophore interaction during the poling
process so as to increase the poling efficiency [21]. As a comparison with FTC, when CLD-1 was doped
into PMMA an r33 of 88 pm/V @ 1060 nm was obtained. When CLD-1 was used in the APC host, r33 of 90
pm/V @ 1060 nm was obtained. In a single-arm 2-cm MZ modulator made of the CLD-1/APC, a V was
reduced to 2.4 V at 1300 nm. The loss at 1550 nm of the CLD/APC polymer was measured using the liquid
immersion technique [22] and found to be 1.2 dB/cm
The effects of chromophore geometry on the optimum loading density and the poling efficiency have
been investigated with another modified chromophore CLD-72. The EO coefficient obtained from CLD-72
is higher than that of CLD-1 by ~ 40% though the two chromophores have the same -conjugate system and
approximately the same absorption peak. It is believed that the two TBDMS bulky groups of CLD-1 while
reducing the interchromophore interaction also restrict the poling efficiency by making it more difficult for
the chromophore to rotate within the host polymer. Due to the limited solubility and poor quality of coated
film, CLD-72 was not evaluated in a MZ modulator.
Cladding materials must satisfy a number of requirements, including higher electrical conductivity than
the core, lower index of refraction than the core, good adhesion to the adjacent materials, low optical loss,
and enough temperature stability to withstand the poling temperatures. An UV curable epoxy available
from Masterbond Co., UV15, was found to be a suitable lower cladding for CLD/APC. The UV15 was
spin-coated on a Au coated silicon wafer and cured by a UV lamp. The films were post baked to improve
the crosslinking and to eliminate small molecules remaining in the UV cured film. The resistivity of the
lower cladding was important in determining the efficiency of the poling. With UV15 lower cladding layer,
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we achieved ~ 75% of the poling efficiency compared to the result of CLD/APC single layer poling. The
loss measured using the liquid immersion technique was 4.2 dB/cm at 1550 nm.
Because CLD/APC can be easily dissolved by organic solvents, the upper cladding should not contain
any solvent. Moreover we found some polymers even without the solvent still can dissolve CLD/APC layer
due to the small molecules. We have identified UFC170 polymer [23] as an appropriate upper cladding
material. It has been designed to preclude small molecules that can penetrate and dissolve the surface of the
CLD/APC core layer. UFC170 is curable with a relatively small dosage of UV light to form a thin film with
good hardness. In addition, cured UFC170 is not attacked by the solvents used in photoresists, is
compatible with Cr-Au upper electrode, and has an electrical resistivity which allows for efficient electrode
poling of the CLD/APC. However, the UFC170 is not appropriate for the lower cladding layer because of
the weak adhesion to the Au lower electrode. The measured loss of UFC170 at 1550nm is 3.1 dB/cm. The
UV15:CLD/APC:UFC170 structure is robust and the end faces can be cut with a dicing saw to provide good
optical coupling to the waveguide.
III. Design of rib waveguide and RF strip line
The waveguides used in polymer modulators have buried rib structure fabricated by dry etching in
oxygen and they must be single mode to provide a high extinction ratio of the modulator. EO polymers have
a relatively high index of refraction compared to the passive cladding polymers. Hence, in order to keep the
waveguide single mode, it has been widely accepted that the thickness of the core layer has to be thin
enough to suppress higher order modes in its slab waveguide. At 1550 nm this requires the typical thickness
of the EO polymer layer to be less than 1.5 m. However, the small vertical mode size in the thin waveguide
leads to a high coupling loss between the optical fiber and the waveguide. Moreover, when the core layer is
thin, the rib height is small for single mode operation and the fabrication tolerance is tight because the
effective index change is so sensitive to the core thickness and the etch depth. As a result, it is difficult to
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maintain the required fabrication tolerance to achieve a high extinction ratio in the modulator.
In case of three dimensional rib waveguides, it has been shown that the single mode condition does not
require the thin core layer [24]. Even if the core layer is so thick that it could support several modes in its
slab waveguide, it is still possible to design the rib waveguide to confine only one mode by radiating the
higher order modes horizontally into the slab modes [25]. Then the oversized rib waveguide will support
only the single mode without additional radiation loss [26].
This approach can be used to design large core single mode waveguides in EO polymer devices. Fig. 2
shows the effective index of the planar waveguide as a function of the thickness of core layer. The
refractive indices used in this calculation are 1.504 for the UV15 lower cladding, 1.612 for the CLD/APC
core layer in the TM polarization, and 1.488 for the UFC170 upper cladding. The thickness of the core
layer effects the EO modulation efficiency by changing the film thickness across the electrodes and also
effects the fiber coupling loss to the waveguide because of the mode size mismatch. In Fig. 2, we have
selected 3 m as a reasonable compromise. The rib height  is selected to obtain sufficient confinement in
the lateral direction due to the effective index contrast eff, which is the difference of the effective index of
waveguide side N0(side) and that of waveguide center N0(center) of the lowest order mode. For 6-8 m
wide waveguides, a eff of ~ 0.003 is required to produce the single mode waveguide with good
confinement. From Fig. 2, one sees that waveguide side needs to be etched by 0.4 m to provide eff of ~
0.003 for the 3-m thick waveguide center.
Also from Fig. 2, it is clear that the 3-m slab waveguide can support higher order modes. However, the
higher order mode will not propagate through the rib waveguide because the effective index of the first
higher order mode in the 3-m slab waveguide, N1(center) is lower than that of the fundamental mode in the
2.6-m slab waveguide N0(side). The higher order mode supported by the 3-m slab waveguide will be
coupled into the fundamental mode of the 2.6-m slab waveguide. In consequence, the over-sized rib
waveguide will satisfy the single mode condition due to the radiation of higher order modes. This design
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has been verified by the high extinction ratio of the fabricated MZ modulators.
The minimum thickness of the cladding layers is determined by setting the metallic absorption loss due
to the surface plasmon coupling to the upper or lower electrodes to an acceptable value. Making the
cladding layers thicker than this minimum only increases V. To find the minimum thickness, the effective
index of the multilayer planar waveguide structure including the two Au electrode layers is calculated. The
imaginary part of the effective index is then used to obtain the metallic absorption loss. As shown in Fig. 3,
the losses are calculated as a function of the upper cladding layer thickness for several core layer
thicknesses. The thickness of the lower cladding layer is fixed at 2.5 m throughout the calculation. From
Fig. 3, one sees that the metallic loss is less than 0.1 dB/cm when the cladding thickness is over than 2 m
for the 3-m core waveguide. It is also shown that for the same cladding thickness the thicker core layer has
the lower metallic loss due to the better mode confinement into the core layer.
The total thickness of the waveguide consisting of lower cladding, core and upper cladding should be as
thin as possible to minimize V. For the polymers used in the modulators, the minimum thickness for
negligible metallic loss is approximately 8 m for the 3-m core waveguide. The upper and lower
claddings are 2.5-m thick. Even if the thickness of the core is reduced less than 3 m, the total thickness of
the device cannot be decreased because the thinner core waveguide requires the thicker cladding layers.
The high-speed electrode structure for the push-pull modulators, shown in Fig. 4, incorporates the
two-arm microwave transmission line [27]. The line is designed using commercial microwave design
software. In the push-pull modulator, the arms of the MZ modulator are poled in the opposite directions and
the rf electrode covers both arms to apply the rf electric field in the same direction on each arm. To achieve
50- characteristic impedance, the width of the electrodes over each arm should be set to give 100-
impedance to each arm. However, the width of strip line cannot be reduced to less than 8 m in order to
maintain reasonable overlap of the rf field to the guided mode in 6-m wide waveguides. In addition, the
tolerance of alignment would be too small to reliably obtain good alignment if the strip line width is close to
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waveguide width. Consequently, the characteristic impedance of the strip lines over the MZ arms with the
8-m width and 7.5-m polymer thickness becomes 70 . The two strip lines on MZ arms are connected to
the wider strip line that has the impedance of 35  with a width of 28 m. Those lines are connected by the
right angle bend without chamfers at the corner. According to the simulation, the reflection from the right
angle bend is negligible and the total reflection is below -25 dB. The predicted loss coefficient for the line
using Au with 4.1x107 S/m conductivity is 0.48 dB/cm GHz . For the 2.6-cm long transmission lines
including the RF feeding lines, the S21 is supposed to be -7.9 dB at 40 GHz. We assumed that the dielectric
loss of the polymers is low [28] and the loss is dominated by metallic losses.
IV. Device Fabrication and Poling
A schematic diagram of the push pull polymer modulator fabrication procedure is shown in Fig. 5. The
bottom electrode is made of Cr and Au with a thickness of 0.1 and 3.0 m, respectively. UV15 was
spin-coated on the bottom electrode as the lower cladding. A disposable filter with a 0.2-m pore was used
to filter the solution. The UV15 was cross-linked and hardened as described earlier.
To prepare the guest-host polymer solution for the core layer coating, the CLD chromophore was mixed
with the APC polymer in the optimum concentration of 25 wt% [29].
It was then dissolved in
dichloroethane to make 10 % solution in a volume ratio. The solvent has a low boiling point (82 ºC) which
requires the spin coating of the polymer solution to be done in a short time to prevent wrinkles on the film
caused by the inhomogeneous drying of the solvent. After spinning, the 3-m core layer was dried in a
vacuum for one hour then baked at 120 ºC for one hour to remove the residual solvent.
The waveguide ridges are patterned by photo-lithography and reactive ion etching (RIE) in oxygen. One
of the difficult issues in the fabrication of multi-layer polymer devices is the solvent attack on the first layer
during the second layer coating. This is difficult to prevent in the guest-host EO polymers which are not
crosslinked and which can be dissolved by the solvents used in the standard photolithography process. In
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order to overcome this problem, we used a sacrificial protective layer of UV15 polymer diluted with
methanol to be 20 wt%. Methanol does not dissolve the CLD/APC polymer. The protective UV15 layer
was spin coated to give ~0.5 m thickness and then UV cured.
Standard photolithography was used on the protective layer to define the waveguide patterns.
The
waveguide patterns were etched by O2 RIE to have a rib height of 0.5 m. The rib width was 6 m. To
minimize the roughness of the etched surface, a largely chemical etch was achieved at 450 mtorr with an RF
power density of ~30 mW/cm2. After removing the residual photoresist using a developer, the entire surface
of the sample was etched again to transfer the waveguide pattern into the CLD/APC core layer and to
remove the remainder of the sacrificial layer. The final rib height on the CLD/APC layer became 0.4 m
due to the slower etch rate of CLD/APC compared to that of UV15. For the upper cladding, UFC170
polymer was filtered and spin coated to obtain 2.5-m cladding thickness on top of the rib. After the UV
exposure to cure the polymer, the polymer was post-baked. No cracking was occurred in the films of the
multi-layer polymer devices.
The Au poling electrodes were patterned by photolithography. For the push-pull operation, each arm of
the MZ modulator must be poled in opposite directions. In this configuration, there is substantial electric
field between the two top electrodes and unless the electrodes are covered there is a likelihood of avalanche
breakdown across the surface at the poling temperatures [30]. To prevent the breakdown, we coated the
surface with a layer of the same APC polymer used for the host material of the EO core layer. With the APC
layer, the current flow through the top electrodes could be kept below 1 A during the entire poling process.
Other steps to prevent the surface breakdown such as poling in SF6 and coating with AZ5214 photoresist
were not successful. The SF6 gas was not effective in preventing the breakdown at the high temperatures
used in the poling. The AZ5214 photoresist was effective but it was difficult to remove after the high
temperature baking during the poling.
When the electrode poling was done in air, we observed a high poling induced loss in the waveguides.
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There was a significant change of the mode confinement after the poling due to the refractive index drop of
the EO polymer. We believe this is caused by a reaction between the dissolved oxygen in the polymer and
the chromophores at high temperature with high electric field. A similar effect was observed during the
corona poling [31]. To find the role of oxygen, straight waveguide samples were prepared and poled in
either air or nitrogen by applying 600 V at 140 ºC for 20 minutes. The insertion loss of the samples was
measured before and after the poling. The poling induced loss measured from 2-cm long waveguides was as
high as 4-7 dB when the sample was poled in air.
No poling induced loss was observed when the sample was poled in nitrogen. We also measured the
insertion loss of the unpoled waveguides that were located adjacent to the poled waveguides on the same
substrate. Even though both waveguides were heated to the same temperature, there was no increase of the
loss in the waveguide without the poling electrodes. It therefore appears that the assistance of poling
current is necessary to initiate the oxidation of CLD chromophore at the poling high temperature.
According to our observations, about 30 min. at 120 °C was sufficient to purge the dissolved oxygen from
the polymer and to prevent the oxygen-current induced degradation.
To determine the optimum electrode poling conditions for the multi-layer device structure we
fabricated several MZ modulators with CLD-1/APC using different poling conditions and the r33 @ 1550nm
was calculated from the measured V of the modulators as shown in Fig. 6 (a). An r33 of ~43 pm/V could be
attained using poling voltages from 300 V to 400 V at poling temperatures of 140-150°C. The r33
dependence on the poling time was also measured from MZ modulators. Fig. 6 (b) shows that at a
temperature of 140 ºC with 400 V the maximum r33 is achieved after 20 min. of poling.
Based on these measurements the poling procedure used in the push-pull modulators is determined as
shown in Fig. 7. The total applied voltage was 800 V (each arm 400 V) to give an estimated poling electric
field of 50 V/m if the poling field is assumed to be uniform across the device. The poling temperature was
145 ºC and the poling time was 30 min. After the poling, the insulating APC polymer layer coated on top of
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the poling electrodes was removed using acetone then the poling electrodes were etched away by a chemical
etching.
To fabricate the modulation RF electrodes, a seed layer of Cr and Au were evaporated using an e-beam
evaporator. On the metal seed layer, 3-m thick photoresist was depositied to pattern the upper electrode of
the RF strip line. To prevent the relaxation of the poling, the resist was dried in vacuum at room temperature
instead of standard high temperature baking. The upper electrode was plated to ~ 3 m thick Au. The
end-facet of the waveguide was diced with a nickel blade. The polymers used in the device were
sufficiently hard for the dicing to provide optical quality end faces.
V. Device Performance
Cut-back measurements at1550 nm were made to measure the propagation loss of the waveguides. The
input single mode fiber was aligned to the waveguide and the output was collected by 20x lens and coupled
to a detector. A fiber-optic polarization controller was used to adjust the input polarization to TM. To
increase the accuracy of the measurement, several waveguides were measured at each length. Fig. 8 shows
the results of the measurement for two waveguides with different core thickness.
The 3-m core
waveguides were identical to those used in the modulators. To obtain single mode waveguides with the
5-m thick core layer, the sample was etched to make a rib with a height of 1.3 m. From the small
scattering of the data, we believe the dicing saw produced reasonable quality end faces repeatedly and the
film quality over the 4-cm long waveguide is quite uniform. The 3-m core guides have a propagation loss
of 1.7 dB/cm and a waveguide coupling loss of 4 dB. The 5-m core guides have a propagation loss of 1.2
dB/cm and a coupling loss of 2.5 dB.
The propagation loss of the 5-m core guides is the same as the 1.2 dB/cm core material loss measured
in a slab waveguide. The 3-m core waveguides are 0.5 dB/cm higher. Some of this additional loss can be
explained by the loss of the claddings. The cladding materials have a measured higher loss than the core
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(4.2 dB/cm for UV15 and 3.1 dB/cm for UFC170) and the larger mode confinement of the thicker core
waveguide therefore results in smaller loss due to the claddings.
Moreover, the scattering at the
cladding-core interfaces may be lower in the 5-m core waveguides because the amplitude of the electric
field is lower at the interface. The lower coupling loss of the 5-m core waveguides is due to the better
mode match to the input fiber.
For the push-pull MZ modulators made of CLD-1, the low frequency half-wave voltage, V and the
extinction ratio were measured by applying a 1-KHz signal with a triangular waveform. As shown in Fig. 9,
the source signal and the output waveform from the photodetector were observed to find the V  of the
device. The V was 1.4 V for 1300 nm and 2.1 V for 1550 nm corresponding to a VL product of 2.8 and 4.2
V-cm, respectively. In case of high-speed LiNbO3 modulators, the electric field strength across the optical
waveguide is reduced by the thick coplanar electrode used to reduce the velocity mismatch and this results
in a VL product of 9 V-cm with z-cut push-pull electrode structure [32] and 6 V-cm under the external
push-pull operation mode where an additional RF circuit is required [33].
The insertion loss of the unpackaged 3-cm long MZ modulator, which includes two Y-branches and
2-cm long EO interaction length, was in the range of 9-10 dB for the most of the fabricated devices. This
includes the coupling loss due to the input fiber and the output lens.
According to the cut-back
measurement, the chip loss of the 3-cm device was ~ 5 dB. Then the coupling loss between the normal
single mode fiber and the waveguide input facet was ~ 3.5 dB, and the extra loss introduced by the lens to
collect the light from the waveguide end would be ~ 0.5 dB. The extinction ratio of the modulator, when the
output is collected by a lens, was greater than 20 dB, which indicates the oversized rib waveguide
suppresses the higher order mode propagation and the two waveguide arms of MZ modulator poled in
opposite direction have about the same loss.
The performance of the RF strip line was measured on an unpackaged chip in the range of 45 MHz to 40
GHz using a network analyzer and probes. The line was terminated in 35  which is close to the
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characteristic impedance of the microstrip line.
>From the S21 measurement the line loss is ~0.7
dB/cm GHz . This is higher than that predicted for very good gold (0.48 dB/cm GHz ) and is an
indication that the quality of the plated gold could be improved. A measurement of the dc resistance of the
upper electrode also shows that the resistivity of the gold is lower than that of bulk gold. The reflection loss
from the line is less than 13 dB over the frequency band.
The optical frequency response of the 2-cm push-pull modulator was measured with a lightwave
component analyzer (HP83420A) from 130 MHz to 20 GHz. Electrical contact to the modulator under test
was established via an air spaced 50  coplanar probe and the transmission line was terminated with a 35 
resistance on the chip. Because of the resistance of the upper electrode the input impedance of the
modulator is dispersive for frequencies below ~2 GHz. The input impedance is higher for the lower
frequencies and this results in an apparent increase in the modulation efficiency at low frequencies. This
artifact can be eliminated by a low frequency matching circuit on the input to make use of the full
bandwidth made available by the polymer modulator. Fig. 10 shows the results of the measurement from 2
to 20 GHz. The dip around 17 GHz may be due to a calibration error in our receiver since we see this
structure in the frequency response of other lightwave components. To extend the 3-dB bandwidth, a 1-cm
interaction length modulator was fabricated. Other than the shorter interaction length the design was
identical to the 2-cm device. The V of the shorter modulator is twice that of the 2-cm modulator. The 1-cm
device was completely packaged with RF connectors and optical fibers to demonstrate the performance.
The optical frequency response, shown in Fig. 10, was measured with an optical network analyzer
(HP8510) with the test set replaced by an HP8511B and a 50 GHz detector (U2T). The packaged 1-cm
device has a 3-dB bandwidth of ~34 GHz.
The bandwidth of these modulators is limited by the rf loss of the microstrip line and, as noted above,
the loss on our lines is about 40% larger than the predicted loss for high quality gold electrodes. The quality
of our plated gold can be improved and if one could approach that of perfect gold the 3-dB bandwidth of the
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polymer modulators could be considerably extended.
The velocity mismatch effect on the bandwidth
should not be a factor until the modulation frequency is less than 100GHz.
VI. Stability of the CLD/APC Polymer Modulator
One of the major concerns for the polymer devices when used in systems is their long term stability.
The devices must demonstrate good long term thermal stability, high photo-stability at 1550 nm, and a
stable and trackable bias point.
Earlier work has shown that some EO polymers show chromphore damage when exposed to infrared
wavelengths even though these wavelengths are well away from the principle absorption peak of the
chromophore. Index of refraction changes and chromophore damage were observed at 1300 nm and in each
case oxygen appeared to play a role [13]. To study these effects, we aligned and attached input and output
fibers to a group of modulators and straight waveguides fabricated with CLD-1/APC polymer. During the
alignment we used 1 mW of TM polarized power at 1550 nm to avoid any initial degradation of the polymer
waveguide. After the fiber attachment, 10 mW was launched into a straight waveguide. Within a few
hours, there was a significant power drop in the waveguide output. After this exposure, a bleached area
could be observed near the input. The pattern of the bleached area indicated that, because of the index drop
in the core, the waveguide mode lost lateral confinement near the input.
After a continuous TM polarization exposure at 10 mW at 1550 nm for 1 day, the change of output
mode profile was observed as shown in Fig. 11. Fig. 11 (a) is the TM mode picture of the unbleached
waveguide adjacent to the exposed waveguide. The TE profile was essentially the same. Fig. 11 (b) and (c)
show the TM and TE output mode profiles respectively after the high power exposure with the TM
polarized input light. Clearly the TM index of refraction has decreased sufficiently to lose the lateral mode
confinement. The TE polarization is reduced to a smaller extent.
The refractive index decrease in the waveguide core is due to a photochemical reaction which damages
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the chromophores and therefore reduces their contribution to the refractive index. At 1300 nm wavelength,
the singlet oxygen generated by the intensive optical power is believed to play a role to this reaction [14].
The excited oxygen attacks the carbon double bond or the nitrogen bond in the chromophore molecule. It
has also been speculated that two photon absorption may play a role in the chromophore degradation. The
CLD-1 chromophore has an absorption peak near 700 nm, which could be excited by absorbing 1550 nm
light through the two-photon absorption process. The excited chromophore could react with the ground
state oxygen to cause damage. The two-photon process could explain the larger index change for the TM
polarization since the high intensity light was TM polarized and tended to preferentially excite
chromphores oriented in the TM direction. As a result, the refractive index for the TM polarization will be
decreased much faster than the index for the TE polarization. A depoling of the material due to two photon
induced trans-cis excition and relaxation is not the dominate process in our experiments because the index
appears to decrease for both polarizations.
To determine the role of oxygen, we operated pigtailed modulators and waveguides in a sealed
container with a continuous Ar gas flow through the container. Before launching the high optical power
into the device, we waited for 1 day at ambient temperature until the absorbed O2 in the polymer diffused
out completely in an Ar environment in which the amount of oxygen was on the order of a few ppm. Fig. 12
shows the monitored output power from the straight waveguide exposed to 10-mW input power at 1550 nm.
The sample in the Ar environment shows no drop in output power for 120 hours while the sample in air
shows a rapid decrease.
Perhaps a more sensitive measure of optical damage is to measure the V of the devices. We monitored
the V of a single arm (not push-pull) modulator kept in Ar at an input power of 20 mW at 1550 nm. Due to
the coupling loss between fiber and waveguide, the power coupled into the waveguide was ~ 8 mW. During
32 days of continuous operation, as shown in Fig. 13, the device exhibited excellent stability with no
measurable change in V.
- 15 -
For the comparison of the thermal stability of materials, the monitoring of the generated second
harmonic signal as the temperature of a poled sample is increased by 10 ºC/min. is a useful measurement.
The poled nonlinearity of the CLD/APC material can be maintained up to 110-120 ºC in this measurement
[19]. Although this short-term measurement can give some guidelines for material selection, it is not a
measure of the long term thermal stability. To get a better assessment of the long term stability, a wafer
containing four modulators was held at 60 ºC in air and the V of the modulator was monitored over 40 days.
A similar wafer was kept at room temperature as a reference. As shown in Fig. 14, we observed the increase
of V from 4.8 V to 6.0 V (~ 25%) from the sample heated at 60 ºC, though the sample kept at room
temperature did not show increase of V. No similar measurements have been made in the Ar atmosphere
so we do not know if oxygen played any role in the increase of V.
VII. Conclusion
This paper reviews the current state-of-the-art of the CLD/APC based high speed modulators. Based
on this material polymer EO modulators are becoming a commercially viable component. The low
frequency V L product @ 1550 nm is 4.2 V-cm. The V @ 1550 nm is 2.1 V and the frequency response is
near 20 GHz for a 2-cm long modulator. By the modification of chromophore structure to reduce the
intermolecular interaction, less than 2 V of V was achieved very recently [34]. Shorter devices with higher
V have nearly 35 GHz of bandwidth. There is still considerable improvement that is possible in the
fabrication to reduce the rf micro-strip line losses which will considerably extend the bandwidths. The
optical loss of the modulators is acceptable and material research to further reduce the loss is underway.
The long term thermal stability of the current devices shows 25% poling relaxation at 60°C and needs to be
improved. We are currently measuring a cross-linked polymer which appears to extend the thermal stability
by ~ 30°C. It appears that the packaging that removes O2 can solve most of the photo-stability problems. To
confirm this, measurements must be made at higher temperatures and for longer times. There appears to be
- 16 -
no problems in fabricating single mode waveguides and therefore in achieving high extinction ratios in the
modulators.
In summary, there has been considerable recent progress in polymer EO modulators and the long years
of materials research by many investigators is now bearing fruit. Although not all of the questions have
been answered regarding the critical long-term stability of the polymer devices, the progress made makes
the outcome look promising
Acknowledgement
The authors would like to appreciate Uray Co., Korea for supplying the UFC polymers and the Advanced
Technology Center at Lockheed Martin for identifying the APC material. This work was supported by
AFOSR, BMDO, ONR, and Pacific Wave Industries.
- 17 -
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Table
Table 1. Physical properties of the FTC and CLD chromophores: the absorption peak wavelength was
measured with two different solvents, dioxane and CHCl3, the decomposition temperature of chromophore
Td was measured in a nitrogen atmosphere with 10 ºC/min temperature ramp, the electro-optic coefficient
r33 was measured from a single layer EO material at 1060 nm, and the V  were measured at 1300 nm from
the fabricated MZ modulators made of each chromophore with single arm 2-cm long modulation electrode.
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Figure captions
Fig. 1. Chemical structures of the high  chromophores, FTC and phenyltraene-bridged CLD
chromophores.
Fig. 2. Effective index of the planar waveguide as a function of the thickness of core layer. The waveguide
consists of UV15 lower cladding (1.504), CLD/APC core layer (1.612), and UFC170 upper cladding
(1.488). In the rib waveguide structure, the waveguide side needs to be etched by  to obtain the effective
index contrast of Neff for the fundamental mode.
Fig. 3. Metallic loss of the planar waveguide structure including top and bottom electrode layers made of Cr
and Au as a function of cladding thickness for the three different core layer thickness. The loss will be less
than 0.1 dB/cm if the cladding thickness is greater than 2 m for the 3-m core waveguide.
Fig. 4. The structure of RF strip line incorporated for the push-pull MZ modulator.
Fig. 5. Schematic diagram of the polymer modulator fabrication procedure. The polarity of electric field
applied for the push-pull poling is denoted.
Fig. 6. Electrooptic coefficients measured from the fabricated MZ modulators with different poling
conditions: (a) Poling efficiency depending on the poling temperature and the applied voltage, (b) Poling
efficiency depending on the poling time with a fixed poling temperature of 140 ºC and an applied voltage of
400 V.
- 23 -
Fig. 7. Typical temperature and voltage profiles used for push-pull poling of the CLD/APC polymer
modulator. The sample is purged with nitrogen at 120 ºC for 30 min. to remove the dissolved oxygen from
the polymer, then the sample is poled at 145 ºC for 30 min. in nitrogen atmosphere.
Fig. 8. Propagation loss of CLD/APC waveguide measured by the cutback method for the TM polarization,
which shows the propagation loss of 1.2 dB/cm and 1.7 dB/cm and the coupling loss of 4 dB and 2.5 dB for
the waveguide with a core thickness of 3 m and 5 m, respectively.
Fig. 9. Oscilloscope trace of the input and output signal during the V measurement. The result shows the
MZ modulator made of CLD-1/APC has 1.4 V of half wave voltage @ 1300 nm.
Fig. 10. The frequency response of the polymer MZ modulator with 1-cm and 2-cm long electrodes. The
shorter device exhibits the 3-dB electrical bandwidth over 30 GHz from 2 GHz.
Fig. 11. Comparison of guided mode profile exhibiting the effect of waveguide bleaching: (a) is the initial
mode profile before the bleaching, (b) and (c) are the TE and TM polarized mode pictures respectively after
the light exposure.
Fig. 12. The change of waveguide transmission during the high optical power illumination for the
waveguide sealed in Ar and the other waveguide exposed to air.
Fig. 13. Long-term measurement of half-wave voltage from the CLD/APC polymer MZ modulator in Ar
atmosphere with a power of 20 mW exposed at the waveguide input. No considerable change of the EO
coefficient was observed over 30 days.
- 24 -
Fig. 14. The result of long-term thermal stability measured from the CLD/APC polymer MZ modulator.
Although the sample kept in room temperature does not show poling relaxation, we observed 25 % of V 
increase from the sample heated to 60 ºC over 40 days.
- 25 -