Applications of Electro-optic Polymers and Devices: Breaking the High Frequency, Broad Bandwidth Barrier

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1 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

2 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... 6 Figure 4: Mach-Zehnder and DQPSK modulators... 7 Optical Interconnects for High Speed Computing... 8 Figure 5: Illustration of an Optical Interconnect... 9 Satellites and Avionics... 9 Table 2: Comparison of Coax Cable and Optical Fiber Phased Array Radar and Antennas Table 3: Comparison of RF and Optical Switch Array High Data Rate Wireless Communications High Frequency Imaging and Sensing Electromagnetic Field Sensors Conclusions References

3 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 r 33, 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 (LiNbO 3, LN), which has a relatively high r 33 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 (r 33 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 1-3 -

4 Table 1: Comparison of EO Material and Device Properties Property LiNbO 3 Polymers EO coefficient (pm/v) Maximum speed (GHz) 40 >100 V- (V) 5 <1 Manufacturability Fair Easy Radiation hardness Poor Very good Cost Relatively high Low Integration Hard Relatively easy 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 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 Acceptor 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 - 4 -

5 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 (T g ) 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 intermolecular dipolar interactions. 1 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 can require temperatures around 1000 o 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 - 5 -

6 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 photonics because amorphous polymers do not have the lattice mismatch problems that are frequently encountered in mixed lattice semiconductor devices. Optical Components for Fiber Optic Networks a c Figure 3: a) Microring resonator; b) Directional coupler; c) Mach-Zehnder modulator b 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 - 6 -

7 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 with operation at 110 GHz and a bandwidth of 40 GHz, 3 and Mach-Zehnder modulators with some modulation at 1 THz (1000 GHz); 4 2) linear analog modulators for cable TV deployment; 5 3) micro-ring resonators that are useful as both modulators and tunable filters; 6 4) Fabry-Perot spatial light modulators; 7 and 5) digital optical switches 8 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 b) (NRZ) formats; however, as transmission Waveguide Drive Bias Electrode speed increases to 40 Gbps and beyond, problems in the optical fiber such as Figure 4: a) MZI modulator; b) Nested MZI for DQPSK modulation. Some device layers are not illustrated for clarity. polarization mode dispersion (PMD) start to 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

8 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 11 or even with silicon waveguides. 12 Currently, tandem integrated micro-ring resonators, 13 and a wavelength channel selector 14 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 parallel mode, where there are usually 2 N + 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 - 8 -

9 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 voltage and dielectric constant of EO polymers. 16 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 Figure 5: An optical interconnect employing a ring resonator waveguide. From: Block, B. A. et interconnect applications in future 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 - 9 -

10 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 achieved). 17 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 radiation damage, 18 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) db (94%) per 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

11 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) 21 Parameter RF Module Optical Module Size (in 3 ) Weight (lbs) Power (W) Bandwidth (GHz) >40 Insertion Loss (db) (RF) 8-12 (optical) Channel Isolation 70 db >75dB 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

12 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 r 33. 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

13 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

14 (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

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