Electric field poled organic electro-optic materials: state of the art and future prospects.

Long-range surface plasmon polaritons (LRSPPs) are optical surface waves that
 propagate along a thin symmetric metal slab or stripe over an appreciable length
 (centimeters). Vigorous interest in LRSPPs has stimulated a large number of studies
 over three decades spanning a broad topical landscape. Naturally, a good segment of
 the literature covers fundamentals such as modal characteristics, excitation, and
 field enhancement. But a large portion also involves the LRSPP in diverse phenomena,
 including nonlinear interactions, molecular scattering, fluorescence,
 surface-enhanced Raman spectroscopy, transmission through opaque metal films and
 emission extraction, amplification and lasing, surface characterization, metal
 roughness and islandization, optical interconnects and integrated structures,
 gratings, thermo-, electro- and magneto-optics, and (bio)chemical sensing. Despite
 the breadth and depth of the research conducted to date, much remains to be
 uncovered, and the scope for future investigations is broad. We review the properties
 of the LRSPP, survey the literature involving this wave, and discuss the prospects
 for applications. Avenues for further work are suggested.

Long-range surface plasmon polaritons

Optical microresonators have recently attracted growing attention in the photonics community1. Their applications range from quantum electrodynamics to sensors and filtering devices for optical telecommunication systems, where they will probably become an essential building block2. Integration of nonlinear and electro–optical properties in resonators represents a very stimulating challenge, as it would incorporate new and more advanced functionality. Lithium niobate is an excellent candidate material, being an established choice for electro–optic and nonlinear optical applications. Here we report on the first realization of optical microring resonators in submicrometre thin films of lithium niobate. High-index-contrast films are produced by an improved crystal-ion-slicing and bonding technique using benzocyclobutene. The rings have radius R = 100 µm, and their transmission spectrum has been tuned using the electro–optic effect. These results open new possibilities for the use of lithium niobate in chip-scale integrated optical devices and nonlinear optical microcavities.

Electro–optically tunable microring resonators in lithium niobate

This paper is divided into two major parts. Following a brief introduction that establishes some definitions and assumptions, Section II updates our earlier study on the limits of the RF performance of optical links. Section III reviews progress since our 1997 review paper in the development of devices enabling link performance closer to these limits, including (but not limited to): 1) cascade lasers that permit broad-band direct modulation links with gain >0 dB; 2) injection-locked edge- and surface-emitting lasers at 1300 and 1550 nm with modulation frequency responses as great as 40 GHz; 3) modulators with improved performance, especially electroabsorption modulators that now have switching voltages as low as 0.36 V, or handle optical powers as great as 60 mW, or have bandwidths as great as 50 GHz (but not all three of these in one device yet); and 4) high-speed photodetectors with high saturation currents, e.g., a 20-GHz device with a saturation current of 90 mA and a 55-GHz device with saturation at 50 mA. We conclude in Section IV by summarizing the component developments necessary for higher performance RF-over-fiber links, i.e.: 1) semiconductor lasers (for direct modulation) that have higher slope efficiency and bandwidth and lower relative intensity noise (RIN) at reasonable bias current levels; 2) continuous wave (CW) lasers (for external modulation) with higher fiber-coupled power and lower RIN; 3) higher frequency lower loss external modulators with more linear transfer functions and lower V/sub /spl pi// that can withstand larger CW optical powers; and 4) photodetectors with higher responsivity and bandwidth that respond linearly even when illuminated by greater average optical powers.

Limits on the performance of RF-over-fiber links and their impact on device design

Electro–optic (EO) modulators are typically made from inorganic materials such as LiNbO3, but replacing them with organic EO materials, that is, ones with optical properties that change in response to an electric field, could be a promising alternative because they offer large bandwidth, ease of processing and relatively low cost. Here we incorporate a doped, crosslinked organic EO polymer into hybrid polymer/sol–gel waveguide modulator devices with exceptional performance. The half-wave voltages of the resulting Mach–Zehnder (MZ) and phase modulators at 1550 nm are 1 V and 2.5 V, respectively. The unique properties of the sol–gel cladding materials used in the hybrid structure result in a 100% device poling efficiency, leading to respective in-device EO coefficients of 138 pm V–1 and 170 pm V–1 in the MZ and phase modulators. These results are the first to show in-device EO coefficients that are five to six times larger than those of the benchmark inorganic material.

Hybrid polymer/sol–gel waveguide modulators with exceptionally large electro–optic coefficients

Based on a nonlinear optical polymer with a highly nonlinear chromophore (CLD) dispersed in an amorphous polycarbonate (APC), we have developed electrooptic (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 that 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 the EO polymer modulator made of CLD/APC polymer were investigated. When the modulator was hermetically sealed in an inert gas, the V/sub /spl pi// change of a Mach-Zehnder modulator was negligible over 30 d of operation with 20-mW exposure to the waveguide input. In the thermal stability measurement, 25% V/sub /spl pi// increase was observed from the sample heated to 60/spl deg/C over 40 d, though the sample left at room temperature showed no decay of nonlinearity.

Recent advances in electrooptic polymer modulators incorporating highly nonlinear chromophore

The authors report the bandpass-modulation characteristics of a ring resonator-based traveling-wave modulator using an electrooptic polymer AJL8/APC. The use of the traveling-wave electrode makes it possible to achieve an efficient modulation around 28 GHz, which is the free spectral range of the resonator, with a 3-dB bandwidth of 7 GHz. The modulation sensitivity is evaluated by defining an equivalent half-wave voltage of ring modulators. It is shown that the modulator has a potential in the microwave and millimeter-wave photonics applications

Ring resonator-based electrooptic polymer traveling-wave modulator

Polymeric APC-CPW electrooptic modulators incorporating an inverted-rib waveguide structure were fabricated and tested for the first time. The inverted structure greatly simplifies fabrication procedures, and additionally improves propagation loss performance. Mach-Zehnder electrooptic modulators fabricated using this structure exhibited 6 dB of fiber-to-fiber insertion loss for 2.3-cm length and 6 V of driving voltage for a one-arm micro-strip electrode of 1.5-cm interaction length, at a wavelength of 1.55 /spl mu/m.

Electrooptic polymer modulators with an inverted-rib waveguide structure

This paper demonstrates a four-element integrated photonic radio-frequency (RF) phase shifter array in a single chip with an advanced configuration. These devices are integrated using electrooptic polymer materials and involve several novel technologies. Measurements of this configuration showed that our four outputs were independent and had highly linear RF phases over 360/spl deg/ with negligible RF power fluctuation at the modulation frequency of 20 GHz. This significant improvement is capable of removing one of the major problems in using this type of phase shifter architecture.

Single-chip integrated electro-optic polymer photonic RF phase shifter array

Electro-optic (EO) polymer modulators with a wide range of thermal stability from −46°Cto95°C for fiber-optic gyroscope applications are reported. The synthesized EO side-chain polymer used has a glass transient temperature (Tg) of 200°C and a large EO coefficient of 25.2pm∕V in a real device measurement. Mach–Zehnder (MZ) intensity and optical phase modulators are implemented based on this high-Tg side-chain EO polymer, exhibiting ∼3.75V half-wave voltage with 1.5cm interaction length and 2.3cm total length at 1.55μm wavelength. The optical fiber-to-lens insertion loss is ∼7.5dB in the MZ interferometers and ∼6dB in the straight waveguides. We examine the long-term thermal stability of these devices and demonstrate their ability to meet the strict requirements of various EO device applications, particularly fiber-optic gyroscopes.

H.R. Fetterman

Papers

Recent advances in electrooptic polymer modulators incorporating highly nonlinear chromophore

Ring resonator-based electrooptic polymer traveling-wave modulator

Electrooptic polymer modulators with an inverted-rib waveguide structure

Single-chip integrated electro-optic polymer photonic RF phase shifter array

Side-chain electro-optic polymer modulator with wide thermal stability ranging from −46°Cto95°C for fiber-optic gyroscope applications