Applications of Electro-optic Polymers and Devices:
Breaking the High Frequency, Broad Bandwidth Barrier
White Paper
February 2008
Summary
High frequency, broad bandwidth technology development in both civilian and defense
applications is being driven by the need to quickly process and distribute large amounts of
information. Due to the high cost, complexity, and performance limitations of electronic high
frequency systems, hybrid electrical-optical or all-optical systems are necessary. Current off-theshelf optical components function well at frequencies below 20 GHz, but their performance begins
to degrade quickly above 40 GHz.
This performance degradation results primarily from
limitations in the crystalline electro-optic materials currently used to fabricate optical components.
Optical components and integrated optical devices that operate at high frequency and with high
bandwidth are necessary for next generation applications such as:
•
high capacity optical networks
•
high speed microprocessors
•
high bandwidth satellites and avionics
•
phased array radar and antennae
•
high frequency wireless communications
•
extremely high frequency (THz and MMW) imaging
•
electromagnetic field sensing
To break through the current frequency limitations, adoption of new electro-optic materials and
devices is necessary.
Electro-optic polymers have high electro-optic activity and consistent
frequency response up to at least 200 GHz.
Additionally, electro-optic polymers can be
processed to facilitate integration with other materials such as semiconductor light sources and
detectors, low voltage CMOS drivers, and inorganic and polymeric waveguides.
These EO
polymer properties, either alone or in combination, lead to optical components or integrated
optical devices that can generate, process, and detect optical signals at high frequency with high
data rates and broad bandwidth.
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
-1-
Table of Contents
Background..........................................................................................
3
Electro-optic Polymers.......................................................................
3
Table 1: Comparison of EO Material and Device Properties..........
4
Figure 1: Illustration of an organic chromophore............................
4
Figure 2: The poling process and shaped chromophores..............
5
Figure 3: Some EO polymers devices............................................
6
Optical Components for Fiber Optic Networks................................
Figure 4: Mach-Zehnder and DQPSK modulators..........................
Optical Interconnects for High Speed Computing...........................
Figure 5: Illustration of an Optical Interconnect..............................
Satellites and Avionics.......................................................................
Table 2: Comparison of Coax Cable and Optical Fiber..................
Phased Array Radar and Antennas...................................................
Table 3: Comparison of RF and Optical Switch Array....................
6
7
8
9
9
10
10
11
High Data Rate Wireless Communications.......................................
11
High Frequency Imaging and Sensing..............................................
12
Electromagnetic Field Sensors..........................................................
13
Conclusions.........................................................................................
14
References..........................................................................................
15
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
-2-
Background
The need for increased capacity and speed in transporting and processing information is driving
the development of advanced optical components for applications in broadband communication,
high speed computing, and national security. In general, advanced optical components should
operate at high frequency with broad bandwidth, consume less power with a smaller footprint,
and be integratable with other optical components. A key aspect of any optical component is the
translation of an electrical signal into an optical signal. One way to accomplish this electrical-tooptical translation is by exploiting the electro-optic effect, which changes the refractive index of a
material in response to an applied electric field. One important metric of an electro-optic (EO)
material is the EO activity, commonly measured as r33, which governs the strength of the
electrical field required to change the refractive index. When the EO activity of a material is high,
less electric field strength is required to change the refractive index. One EO material that has
been used to fabricate a variety of optical components is Lithium Niobate (LiNbO3, LN), which has
a relatively high r33 of ~ 32 pm/V. However, LN has several properties that hinder applications
with high data rates, among which are: 1) the dielectric constant is high and increases at higher
frequency and the speed of light waves and electrical waves are mismatched, which limits
operational speed and bandwidth and presents electrode-waveguide design challenges; 2) the
electro-optic coefficient is relatively fixed, which practically limits the operational speed due to the
size and complexity of high voltage, high speed electronic drivers; and 3) the crystal structure is
fragile and fixed, which severely restricts the integration of LN devices with other components.
Thus, there is a need for new materials that allow high speed, low power operation while
providing the opportunity for integration with other optical and electrical components.
Electro-optic Polymers
Electro-optic polymers have fundamental property advantages that enable high data rate
operation, low drive voltage, and broad bandwidth. Such fundamental properties include: 1) very
fast EO response time (less than 10 femtoseconds); 2) very high EO coefficient (r33 up to 300
pm/V); 3) relatively low dielectric constant that shows little dispersion up to 200 GHz; 4) closely
matched refractive indices at optical and RF wavelengths; and 5) intrinsic radiation hardness for
space applications.
Additionally, since electro-optic polymers can be processed using
conventional semi-conductor fabrication techniques, an assortment of sophisticated, novel
devices that may be arrayed or integrated with other components is possible. A comparison of
some properties of LN and EO polymers is shown in Table 1
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
-3-
Table 1: Comparison of EO Material and Device Properties
Property
LiNbO3
Polymers
EO coefficient (pm/V)
31
300
Maximum speed (GHz)
40
>100
V- (V)
5
<1
Manufacturability
Fair
Easy
Radiation hardness
Poor
Very good
Relatively high
Low
Hard
Relatively easy
Cost
Integration
An organic chromophore is the functional element of an EO polymer. The chromophore has an
electron donor coupled to an electron acceptor through a -electron bridge, as shown in Figure
1, and must be non centrosymmetric. The separation of the electron donor and the electron
acceptor gives the chromophore a net dipole moment (μ). The -electron bridge allows electron
density to transfer between the electron donor and the electron acceptor, which allows the
polarization of the chromophore to change in response to an external electrical field (Figure 1),
This change in polarization is referred to as the hyperpolarizability (). The figure of merit for an
organic chromophore in EO applications is measured as the
Acceptor
product of the dipole moment and the hyperpolarizability
(μ). The μ of a chromophore can be optimized by varying
the donor, the -electron bridge, the acceptor, or any
combination thereof. Thus, through molecular design and
organic synthesis, the EO activity of an EO polymer can be
improved to increase the power efficiency of optical
components. Additionally, the donor, -electron bridge, and
the acceptor can be substituted with functional groups that
can
increase
processability,
improve
photo-chemical
stability, or improve thermal stability of the chromophore
and/or the EO polymer as a whole.
Donor
Figure 1: Illustration of an organic
chromophore showing the donor,
acceptor, and the polarization
(right half) that occurs under an
electric field.
In the polymer matrix, the chromophore dipoles pair up with other chromophore dipoles in an antiparallel arrangement (Figure 2a). When the chromophores have these intermolecular dipolar
interactions, polarization change in one direction on a chromophore is offset by a polarization
change in the opposite direction on the paired chromophore, which results in no net change in
polarization. Since a net polarization change is necessary to produce an electro-optic effect, the
chromophore dipoles must be aligned in one direction with a high voltage electric field. Aligning
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
-4-
the chromophores, which is referred to as poling, is accomplished by: 1) heating the polymer to a
temperature where the chromophores have sufficient mobility (at or near the glass transition
temperature (Tg) of the polymer); 2) applying the high voltage electric field; and 3) cooling the
polymer to ambient temperature. Typically, application of the electric field is continuous until the
chromophores lose mobility and are effectively “locked” in alignment.
Poling the polymer to
induce electro-optic activity may be done at various stages of device fabrication. To make poling
more efficient, the chromophores can be “shaped” to help overcome the deleterious
1
intermolecular dipolar interactions. In shaping the chromophore, the -bridge is functionalized to
transform the typically oblate spheroid shape to more of a spherical shape (Figure 2b), which
spatially separates the chromophores and allows the external poling field to more easily
overcome the intermolecular dipolar forces.
High Voltage
Anti-parallel
Unpoled
a)
Poled
b)
Figure 2: a) Poling process; b) shaped chromophores
Electro-optic polymers may be combined with a variety of active or passive clad materials such as
polymers, inorganic materials, or organic/inorganic hybrids to fabricate a variety of devices such
as Mach-Zehnder modulators, phase modulators, directional couplers, and micro-ring resonators
(Figure 3). In particular, polymer clads are useful because properties such as electrical resistivity,
optical loss, refractive index, and mechanical strength can be optimized through organic
synthesis and processing. Devices are fabricated using semiconductor-compatible device
fabrication methods at relatively mild temperatures and environmental conditions. Processes
such as spin coating are used for depositing electro-optic and clad polymer. Plasma and/or wet
etching are typically used to define the optical waveguides. Theses processes are relatively mild
compared to, for example, defining waveguides in LN devices by titanium (Ti) indiffusion, which
o
can require temperatures around 1000 C for over 10 hours. Additionally, the refractive indices of
the electro-optic polymer and various clad materials can be tuned over a relatively wide range to
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
-5-
give high density, high-index contrast, compact waveguiding structures with tight radii of
curvature.
The semiconductor-compatible fabrication and tunable refractive indices enable
integrated systems that meet challenging geometric and physical conditions like those in complex
2D and 3D integrated devices, which combine
photonic devices with semiconductor electronic
circuitry.
Materials such as Si, SiGe, GaAs, InP
and GaN may be used in integrated polymer
b
photonics because amorphous polymers do not
have the lattice mismatch problems that are
frequently
encountered
in
mixed
a
lattice
c
semiconductor devices.
Figure 3: a) Microring resonator; b) Directional
coupler; c) Mach-Zehnder modulator
Optical Components for Fiber Optic
Networks
Fiber optic networks are the backbone of the Internet and telephone infrastructure. Current fiber
optic networks will need greater capacity due to bandwidth exhaustion from the increasing
demand for rich multimedia content, video, and internet telephony. The type of optical network
(ultra-long haul, long-haul, metro core, access, enterprise, and residential) determines what type
of optical components are necessary. Optical components used include a wide variety of data
modulators, amplifiers, switches, filters, and circulators that may be discrete or integrated with
other components. Ultra-long and long haul networks, where component performance is critical
and cost is secondary, typically require amplification at critical points in the network. Amplification
is expensive and must be minimized, therefore it is important to deploy components with the
lowest possible optical loss throughout the network. In addition, dispersion becomes an issue due
to the distances involved, and must be managed very precisely. In metro core networks, cost and
performance are important. Most metro networks do not employ amplification, and thus there is a
strict optical loss budget. In metro access, enterprise, and residential networks, where the
distances are relatively short, the loss and dispersion requirements are relatively relaxed. In these
networks cost is critical due to the large number of components that are needed. So, across the
optical network space, there is a growing need for optical components that increase performance
and decrease system cost, power consumption, and size through integration.
EO polymers can increase performance and decrease system cost by reducing power
consumption, increasing bandwidth and speed, decreasing size, and/or integrating with other
components. One critical property of EO polymers in bit rate applications is the relatively high EO
coefficient, which enables large bandwidths and low power operation with less complex electronic
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
-6-
drivers since swinging high voltage becomes increasingly difficult at higher frequency.
Additionally, simplified electrode design and broad bandwidth are possible because polymers
have closely matched refractive indices at the electrical and optical wavelength and because the
dielectric constant (and loss tangent) is low and does not increase appreciably at higher
frequencies (up to at least 200 GHz).
2
These highly advantageous properties, along with the
semiconductor processing of polymer devices, have driven the development and demonstration
of optical components made from EO polymers. Some of these devices are: 1) phase modulators
3
with operation at 110 GHz and a bandwidth of 40 GHz, and Mach-Zehnder modulators with
some modulation at 1 THz (1000 GHz);
4
2) linear analog modulators for cable TV deployment;
3) micro-ring resonators that are useful as both modulators and tunable filters;
7
6
5
4) Fabry-Perot
8
spatial light modulators; and 5) digital optical switches among others.
Although
many
devices
have
been
demonstrated in EO polymers, perhaps the
most immediately useful is the Mach-Zehnder
a)
interferometer (MZI, Figure 4a). MZIs have
been used in digital data transmission using
quadrature amplitude modulation (QAM) with
return-to-zero (RZ) and non-return-to-zero
Substrate
Waveguide
Electrode
(NRZ) formats; however, as transmission
speed increases to 40 Gbps and beyond,
problems
in
the
optical
fiber
such
as
polarization mode dispersion (PMD) start to
b)
Drive
Bias
Figure 4: a) MZI modulator; b) Nested MZI for DQPSK
modulation. Some device layers are not illustrated for clarity.
The stacking order is substrate, waveguide, electrodes.
cause intolerable bit error rates over long
distances with standard modulators and modulation formats. New modulation techniques have
been developed to address PMD problems; but they have required new and more sophisticated
devices.
One such technique is differential quadrature phase shift keying (DQPSK), which
effectively doubles the bit rate of optical transmission (i.e., 20 Gbps modulators can be used for
40 Gbps transmission). A DQPSK modulator, shown in Figure 4b, typically has nested MZIs
where the arms of the inner MZIs are operated as phase modulators.
9
One of the arms of the
outer MZI is used to set the quadrature bias. These types of devices give higher sensitivity,
higher spectral efficiency, and PMD tolerance; but they are more difficult to manufacture and the
power and footprint requirements increase undesirably. EO polymers are particularly useful for
these applications since the high EO activity can decrease both power requirements and footprint
and the low dielectric constant reduces crosstalk so electrodes can be spaced more closely
together. The needed increase in bit rate and increase in device complexity with smaller footprint
underscores the utility of EO polymer components in next generation networks.
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
-7-
Another important driving force in optical components is integration. Integration of modulators
with lasers and detectors (e.g., for monitoring bias points) is a significant trend in products such
as pluggable optical transceivers and line cards; however, higher integration is also important to
dense wavelength division multiplexing (DWDM) and agile optical networks (AONs). Equipment
for AONs include reconfigurable add drop multiplexers (ROADMs) and wavelength selective
switches (WSS), which are remotely tunable and may combine modulators, switches, variable
optical attenuators, and tunable filters, among others, in a single piece of equipment.
EO
polymers are also useful in these applications because they can be controlled by EO and thermooptic (TO) effects,
10
which can be used for a variety of tunable components. Additionally, EO
polymers can be integrated with a variety of different polymer waveguides
waveguides.
selector
14
12
13
Currently, tandem integrated micro-ring resonators,
11
or even with silicon
and a wavelength channel
have been demonstrated in EO polymers. As the demand transmission capacity and
ease of network administration increases, along with a decrease in power consumption and
footprint, the integratability and high performance of EO polymers will become increasingly
important.
Optical Interconnects for High Speed Computing
Optical interconnects are an extremely promising solution for next-generation high data rate
computing platforms and data storage systems since the increasing clock speed of digital
processors is driving the obsolescence of traditional copper, aluminum, and coax cable
interconnects. Replacing copper or coax with parallel optical links can result in high-speed data
transmission while limiting problems associated with electromagnetic interference such as skew,
noise, and crosstalk.
For high-performance computing platforms (servers, next-generation
laptops, backplanes, etc.), critical issues concerning optical interconnects are: 1) integratability of
the optical component, especially with light sources such as LEDs; 2) robust bit error rate (BER);
3) wide dynamic range (in view of the lack of amplification); and 4) thermal and environmental
stability. Optical interconnects can be configured in serial mode, where the transmitter optically
multiplexes the data streams originating from individual links while the optical demultiplexer
receiver restores the parallel data streams and delivers them to the local system logic; or in
N
parallel mode, where there are usually 2 + 2 parallel channels between communicating devices
(e.g. chips, modules, boards), where N = 1, 2, 3, 4, 5 or 6, for 2-bit, 4-bit, 8-bit, 16-bit, 32-bit or
64-bit architectures, respectively and the two extra channels are used for clock distribution. In
signaling/telemetry, a 400Mz system clock translates into 800 Mbps, which in a 64-bit server
architecture (N = 6) implies a throughput between CPU and memory of 52.8 Gbps, assuming the
2 bps/Hz of spectral efficiency that is readily found in digital systems. Given the size constraints
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
-8-
and high data rates in optical interconnects, integratable optical components are necessary that
have high data rates and low power consumption that match high speed CMOS circuitry.
15
New EO materials and device technologies must be developed to enable on-chip optical
interconnects. In particular, very high EO activity and dense channel spacing is desirable due to
ever-tightening space constraints in microprocessor evolution. Also, compatibility with CMOS
circuitry, laser, and detectors in both fabrication and power consumption is critical. Dense EO
polymer MZI modulator arrays have already been demonstrated and are enabled by the low drive
16
voltage and dielectric constant of EO polymers.
Another type of device that has been
envisioned as an optical interconnect is shown in Figure 5. The device uses electrodes from a
semiconductor transistor to encode data into a waveguide loop of silicon with an EO clad
material. EO polymers would be very useful in
this application because they can be spin
deposited on the silicon waveguide, cured, and
poled under conditions that are compatible with
the microprocessor.
Another useful feature of
EO polymers in this application is that the
refractive index can be modified to match that of
the
waveguide
core
material.
The
characteristics of high EO activity, high speed
modulation,
low
dielectric
constant,
CMOS
integratability, and material adaptability make
EO polymers an attractive material for optical
interconnect
applications
in
future
Figure 5: An optical interconnect employing a
ring resonator waveguide. From: Block, B. A. et
al., US Patent 6,993,212 (Intel Corporation).
microprocessors.
Satellites and Avionics
Optical components are critical for next generation defense and civilian satellites and avionics. In
communication satellites, higher capacity and low launch weight are critical (launch costs can be
$10,000/pound). In avionics, there are an increasing number of flight control sensors and a
demand to deliver high bandwidth multi-media content to each seat while reducing weight to
increase fuel efficiency (e.g., there is around 300 miles of wiring in the A380). Both satellites and
avionics have an increased need for bandwidth for intra-aircraft communications; but using metal
cables to transfer information becomes an increasing problem at high frequencies due to high
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
-9-
weight and high signal loss. One method of replacing the metal cables is to use fiber optic links.
The weight savings and bandwidth increase by using fiber optic links can be seen by comparing a
high quality coax cable with an optical fiber (Table 2); however, the performance of the whole
system determines the feasibility of an optical fiber link, and any performance and weight
advantages gained by replacing cables with optical fiber can be lost quickly if complex and bulky
equipment is needed to generate, process, and detect the optical signal. Thus, in optical fiber
links, optical modulators with low drive voltage and low optical loss are key to translating the high
frequency electrical signals into optical signals in a manner that avoids the use of high frequency
electrical amplifiers, optical amplifiers, and sensitive optical detectors. When modulators have a
drive voltage of less than 1 V, the fiber optic link can act as an amplifier itself (i.e., link gain is
17
achieved).
EO polymers are critical to efficient fiber optic links because the high EO coefficient
leads directly to low drive voltage optical modulators. Additionally, EO polymers are resistant to
18
radiation damage,
and polymer devices can be made flexible to conform to aircraft skins or to
fold or roll into tight spaces during launch.
19
Thus, EO polymer modulators can enable fiber optic
links within satellites and avionic systems that have higher bandwidth and are significantly more
lightweight (over 100x) than coax cable links, which is critical due to high launch costs and aircraft
fuel efficiency goals.
Table 2: Comparison of Coax Cable and Optical Fiber
Property
Low Loss Coax
Optical Fiber
Diameter
1 cm
0.03 cm
Loss (6 GHz)
12.65 dB (94%) per 100’
0.007 dB per 100’
Weight
9.3 lbs per 100’
~1 oz. per 100’
Phased Array Radar and Antennas
Phased array radar and antennas use high frequency RF switch arrays and true time delays to
form beams that are steerable with no moving parts. As the numbers of battlefield sensors and
the amount of communication increases, there is an increasing need for radars and antennas that
can steer high bandwidth beams quickly to multiple points either from the ground or air in
packages that are small, lightweight, and consume relatively little power. All electronic phased
array radars and antennae can require many components per channel such as multiple band
pass filters, low noise amplifiers, and switches, which combined lead to performance limitations,
high weight, and large size. Replacing the electrical components with optical components such
as optical channelizers (common in DWDM), broadband tunable filters, and broadband high
frequency switches and switch arrays can lead to radar and antenna beams that have increased
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
- 10 -
bandwidth and more than a 10x decrease in size and weight (Table 3). Additionally, the use of
optical components provides immunity to electro-magnetic inference (EMI). As with fiber optic
links, low voltage optical modulators and switches are critical to obtaining the desired overall RF
performance. Another advantage is that integrating optical components such as the switches and
delay lines can decrease size and system manufacturing complexity.
20
Again, EO polymer
devices are particularly useful for phased array radar and antennas due to low drive voltage,
integratability with other components and waveguides, and low crosstalk in dense arrays.
Demonstrated EO polymer optical components that may be used to create an integrated optical
processor for phased array radar and antennas include low voltage data modulators, optical
switches integrated with low loss silicon or polymer waveguides delay lines, and micro-ring
resonant tunable filters.
Table 3: Comparison of a 16 x 16 RF and Optical Switch Array (Interconnect)
Parameter
RF Module
Optical Module
Size (in )
400
15
Weight (lbs)
20
1.5
Power (W)
130
13
Bandwidth (GHz)
0.5 – 18
>40
Insertion Loss (dB)
10-15 (RF)
8-12 (optical)
Channel Isolation
70 dB
>75dB
3
21
High Data Rate Wireless Communications
An emerging area of wireless communication is high data rate intra-campus and intra-city pointto-point links. In these links, there is a strong need for high frequency generators because multigigabit data streams require very high frequency carrier signals (e.g., a 10 Gbps wireless link
requires a 100 GHz carrier). Generating such high frequency signals in the electrical domain
requires complex frequency synthesizers that increase system cost, size, and weight. One way
to avoid costly electronic equipment is to use optical processing for encoding and generating the
high frequency carrier. In one method, the first sidebands of a modulated carrier-suppressed
signal are generated and then a second data modulator is used to encode data onto the carriersuppressed sideband signal.
22
The modulated side band signal is then fed into an antenna for
conversion to the wireless frequency. In this way, a carrier frequency double that of the sideband
generating modulator is generated at the antenna (e.g., a 60 GHz modulator produces side bands
separated by 120 GHz, which corresponds to the carrier frequency). In another method, a low
frequency modulator is overdriven to produce higher order sidebands whose difference
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
- 11 -
corresponds to the carrier frequency and is a multiple of the modulating frequency (e.g., a 12
GHz modulator will produce side bands separated by 24 GHz (2 x 12 GHz), 48 GHz (4 x 12
GHz), 72 GHz (6 x 12 GHz), etc.).
23
EO polymer devices are well suited to either of these
methods because of the high bandwidth, high speed operation (for the first method that requires
high frequency operation) and because of low drive voltage (for the second method where
overdriving a modulator by many times (6x) would be difficult if the 1x drive voltage is high). The
integratability and device versatility of EO polymers is also advantageous since optical filters can
be used to clean up the side-band signal. Further integration with, for example, the phased array
antenna capabilities described above can enable point-to-multipoint high data rate wireless
systems.
High Frequency Imaging and Sensing
Terahertz imaging is a relatively new technique that shows potential in applications such as
quality control in manufacturing, imaging of cancerous tissue, and security screening for hidden
metal objects.
24
Materials and methods used in terahertz imaging (1 THz = 1000 GHz) should
efficiently generate and detect THz radiation over a broad spectral range with high
intensity/sensitivity over wide areas if necessary. EO materials can be used for generation of
THz wave radiation through optical rectification (where an optical pulse in converted to a freely
propagating THz through the EO medium) and detection of THz radiation through EO sampling
(where the freely propagating THz wave travels through an EO sample and generates an EO
response). EO polymers are very useful for generation
25
and detection
26
of THz radiation due to
very rapid EO response, high EO activity, closely matched refractive indices at optical and THz
frequencies, and the ability to provide films that cover wide areas. In particular, EO polymers
showed very broadband detection of THz radiation (from 1 THz to 30 THz); however, the EO
polymer used had a relatively low r33.
Using EO polymers with higher EO activity and less
absorptive functional groups should increase efficiency and expand the bandwidth for THz
detection in manufacturing quality control, biomedical imaging, and security and defense
applications.
Passive millimeter wave (MMW) imaging is useful to detect naturally emitted radiation under any
lighting conditions, through haze, smoke, fog, and clouds, and through materials such as clothing.
In many cases, MMW imaging is useful under conditions where infrared imaging fails; however,
efficient wide-area detectors for MMW imaging still need to be developed. One approach is to
use a focal plane array (FPA) of EO modulators. In an FPA, the detection end is an array of
small patches of metal that act as electrodes for an underlying array of EO modulators that are
coupled individually to light sources and detectors on the back end. EO polymers are particularly
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
- 12 -
interesting in this application because high EO activity can lead to small devices with low drive
voltage that do not suffer from crosstalk from adjacent modulators (i.e., pixels). The low drive
voltage increases sensitivity, which is especially important in passive imaging systems where a
target may naturally emit low levels of MMW radiation. The small device size and low crosstalk
can lead to dense, high resolution arrays. Another advantage is that EO polymer device arrays
can be fabricated relatively easily over broad areas and on many different substrates, which
avoids tedious and costly manual placement of the pixels that can lead to decreased image
quality.
Electromagnetic Field Sensors
Electromagnetic (EM) field sensors are used in a variety of applications to measure and map EM
fields; such applications include medical instrumentation, flight guidance systems, and high speed
integrated circuits.
Traditional EM field sensors use metallic probes as part of the sensing
mechanism, which disturbs the EM field being measured and is sensitive to EM field noise. EM
field sensors that employ optical methods offer advantages such as EM interference immunity,
high sensitivity, and broad bandwidth.
27
Many optical EM sensors have been developed using
optical fibers with both “intrinsic” and “extrinsic” EM response mechanisms. For intrinsic EM field
fiber sensors, the response mechanism is inside of or coated onto the optical fiber, whereas in
extrinsic EM field fiber sensors, the response mechanism is separate from the optical fiber. Both
intrinsic and extrinsic sensors have been developed that combine a variety of materials with the
optical fiber including: coated metals and ceramics, polymer dispersed liquid crystals,
electrochromic materials, thermo-optic heaters, piezoelectrics, and EO crystals. In some cases,
the response time is slow (PDLCs and electrochromics) so frequency range is limited, and in
other cases frequency is potentially high (thermo-optic heaters, 30 GHz), but sensitivity is limited
(only 60 V/m). In general, intrinsic EM field fiber sensors are relatively easy to manufacture, but
lack sensitivity and bandwidth. Extrinsic EM field fiber sensors have improved performance, but
are more difficult to manufacture.
Integrated optical devices can overcome the limitations of optical fiber sensors to provide high
frequency operation, broad bandwidth, high sensitivity, and good EM interference immunity.
Integrated optical EM field sensors can include either single or combinations of devices such as
Mach-Zehnder interferometers (MZIs), directional couplers, and microring resonators.
28
Both
lithium niobate (LN) EO and semi-conductor electro-absorptive (EA) modulators have been used
to demonstrate integrated optical EM field sensors. In most of these designs, an antenna is
either coupled to or used as the drive electrode for the modulator. The various devices have
shown improvements in one or some of the performance characteristics such as good sensitivity
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
- 13 -
(0.1 V/m), broad bandwidth (30 GHz), and good linearity. One demonstrated method to increase
sensitivity is to decrease the drive voltage of the modulator and/or increase optical power. A
highly sensitive (0.079 mV/m) EM field sensor has been obtained with 1 GHz bandwidth using an
LN MZI under high optical power; however, increasing the electrode length to decrease drive
voltage in these LN MZI systems will further limit bandwidth, and increasing optical power
generally requires undesirably high power consumption. EO polymers and devices have strong
potential to produce highly sensitive EM field sensors with broad bandwidth, low power
consumption, and small footprint due to the EO polymer’s high EO activity, low dielectric
constant, and good electrical/optical velocity overlap. Additionally, polymer devices can act as a
good substrate for antennas, show good EM interference immunity, and are integratable with
other optical devices, lights sources, and detectors. Such EO polymer devices are particularly
important for high sensitivity, high frequency, broad bandwidth, and compact EM field sensors
that are necessary in the next generations of medical instrumentation, aerial guidance systems,
and integrated circuits,
Conclusions
Electro-optic polymers have a very fast electro-optic response time, very high electro-optic
activity, relatively low dielectric constant, closely matched refractive indices at optical and RF
wavelengths,
intrinsic
radiation
hardness
and
electro-magnetic
interference
immunity.
Additionally, electro-optic polymers can be processed using semiconductor compatible
techniques and can be integrated with other materials such as semiconductor light sources and
detectors, low voltage CMOS drivers, and inorganic and polymeric waveguide materials. These
properties, either alone or in combination, lead to optical components or integrated optical
devices that can generate, process, and detect optical signals with high speed and broad
bandwidth. Such devices can break through the high frequency barrier found with devices made
from current inorganic EO materials.
Thus, high performance EO polymer components and
integrated devices are key to enabling the next generation of high capacity optical networks, high
speed microprocessors, high bandwidth satellites and avionics, and phased array radar and
antennae as well as emerging technologies such as high frequency wireless communications and
extremely high frequency imaging.
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
- 14 -
References
1
Dalton, L.R., et al., From molecules to opto-chips: organic electro-optic materials, J. Mater.
Chem. 9, 1905 (1999).
2
Lee, M., et al., Millimeter-wave dielectric properties of electro-optic polymer materials, Appl.
Phys. Lett. 81(8), 1474 (2006) and Mohapatra, S.K., et al., Microwave loss in nonlinear optical
polymers, J. Appl. Phys, 73(5), 2569 (1993).
3
Chen, D., et al. Demonstration of 110 GHz electro-optic polymer modulators, Appl. Phys. Lett.,
70 (25), 3335 (1997).
4
Lee, M., et al. Broadband modulation of light by using an electro-optic polymer, Science, 298,
1401 (2002).
5
Shi, Y., et al. Fabrication and characterization of high-speed polyurethane-disperse red 19
integrated electrooptic modulators for analog systems, IEEE J. Sel. Top. Quant. Electron., 2(2),
289, (1996).
6
Rabiei, P., et al., Polymer micro-ring filters and modulators, J. Lightwave Technol. 20(11), 1968
(2002)
7
Enami, Y., et al., Low half-wave voltage and high electro-optic effect in hybrid polymer/sol-gel
waveguide modualtors, Appl. Phys. Lett. 89, 143506 (2006).
8
Lee, S.-S., et al., Electro-optic polymer digital optical switch with photobleached waveguides
and self-aligned electrode, Opt. Comm. 138(4-6), 298 (1997).
9
Higuma, K., et al., A bias condition monitor technique for the nested Mach-Zehnder modulator,
IEICE Electron. Exp. 3(11), 238 (2006).
10
Park, S., et al., Thermal bias operation in electro-optic polymer modulators, Appl. Phys. Lett.
83(5), 827 (2003).
11
Oh, M.-C., et al., Low-loss interconnection between electrooptic and passive polymer
waveguides with a vertical taper, IEEE Photon. Tech. Lett. 14(8), 1121 (2002).
12
Nippa, D., et al., US Patent No 7,215,851.
13
Rabiei, P., et al., Tunable double micro-ring filters, IEEE Photon. Tech. Lett. 15(9), 1255
(2003).
14
Ahn, J.T., et al., Polymer wavelength channel selector composed of electrooptic polymer switch
array and two polymer arrayed waveguide gratings, IEEE Photon. Tech. Lett. 16(6) 1567 (2004).
15
Kobrinsky, M.J., et al., On-chip optical interconnects, Intel Tech. J. 8(2), 129 (2004).
16
Udupa, A.H., et al., High frequency, low crosstalk modulator arrays based on FTC polymer
systems Electron. Lett. 35(20), 1702 (1999) and Park, S. 16 Arrayed electrooptic polymer
modulator, IEEE Photon. Tech. Lett. 16(8), 1834 (2004).
17
Cox, C.H.III, et al., High electro-optic sensitivity (r33) polymers: they are not just for low voltage
modulators any more, J. Phys. Chem. B 108, 8540 (2004).
18
Taylor, E.W. et al., Radiation resistance of electro-optic polymer-based modulators, Appl. Phys.
Lett. 86, 201122 (2005).
19
Song, H.-S., et al., Flexible low-voltage electro-optic polymer modulators, Appl. Phys. Lett.
82(25), 4432 (2003) and Huang, Y., et al., Demonstration of flexible freestanding all-polymer
integrated optical ring resonator devices, Adv. Mater. 16(1), 44 (2004).
20
Murphy, E.J., et al., Guided-wave optical time delay network, IEEE Photon. Tech. Lett. 8(4),
545 (1996) and Chang, Y., et al., Optical controlled serially fed phased-array transmitter, IEEE
Microwave Guid. Wave Lett. 7(3), 69 (1997).
21
Wilgus, J. RF Photonics—Key to Information Dominance, Analog Optical Signal Processing
Workshop, December 6, 2000.
22
Hirata, A., et al., Low phase noise photonics millimeter-wave generator using an AWG
integrated 3-dB combiner, IEICE Trans. Electron. E88-C, 1458 (2005).
23
Sun C. K., et al., A Photonic Link Millimeter-Wave Mixer Using Cascaded Optical Modulators
and Harmonic Carrier Generation, IEEE Phot. Tech. Lett. 8(9), 1166 (1996).
24
Ferguson, B., et al., Materials for terahertz science and technology, Nat. Mater. 1, 26 (2002).
25
Nahata, A., et al., Generation of terahertz radiation from a poled polymer, Appl. Phys. Lett.
67(10), 1358 (1995).
26
Cao, H., et al., Electro-optic detection of femtoseconds electromagnetic pulses by use of poled
polymers, Opt. Lett. 27(9), 775 (2002).
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
- 15 -
27
Passaro, et al., Electro-magnetic field photonic sensors, Prog. Quant. Electron. 30, 45-73
(2006).
28
Sun, et al., Broadband electric field sensor with electro-optic polymer micro-ring resonator, on
side-polished optical fiber, Organic Photonic Materials and Devices VIII(Proc. SPIE Vol. 6117)
(2006)
ALL CONTENTS ARE COPYRIGHT © 2008 • LUMERA CORPORATION • ALL RIGHTS RESERVED
- 16 -