High Electro-optic Coefficient Research Articles

Mode-hop-free electro-optically tuned external-cavity diode laser using volume Bragg grating and PLZT ceramic

We present a novel electro-optically tuned external-cavity diode laser (ECDL) that utilizes a volume Bragg grating (VBG) as the frequency selective feedback element and a piece of high electro-optic coefficient Lead Lanthanum Zirconate Titanate (PLZT) transparent ceramic as the frequency tuning element. By adjusting the voltage applied onto the PLZT, a single-mode frequency tuning range of 2.5GHz without mode hoping is achieved. The laser wavelength is around 810.0nm with the line-width of 19MHz and the side mode suppression ratio (SMSR) of 37dB. The advantages of combining a VBG and PLZT transparent electro-optic ceramic is that the laser frequency can be tuned finely and rapidly in a range of longitudinal mode spacing without mode hoping. Moreover, the wide-range coarse frequency tuning of 32.7 GHz can also be realized by changing the angle of incident light beam.

Organic electro-optic thin films by simultaneous vacuum deposition and laser-assisted poling

Organic materials with a high second-order optical nonlinearity have an important application for the next generation of computing, telecommunications, and other industries. Because of a high electro-optic coefficient and low dielectric constant, N-benzyl-2-methyl-4-nitroaniline (BNA) single crystals have been grown and their optical properties have been extensively studied. In this Letter, a poled BNA thin film material was prepared through simultaneous vacuum evaporation and laser-assisted electrical poling. The linear electro-optic coefficients of BNA were measured by a Young's two-slit interference electro-optic measurement method. The dependence of the resulting electro-optic properties on optical power was explored. Enhancement induced by laser illumination was demonstrated. The BNA thin film can exhibit an r(33) value comparable to that of BNA single crystals.

Electric-field sensors utilizing coupling between a D-fiber and an electro-optic polymer slab

This paper provides a detailed analysis of electric field sensing using a slab-coupled optical fiber sensor (SCOS). This analysis explains that the best material for the slab waveguide is an inorganic material because of the low RF permittivity combined with the high electro-optic coefficient. The paper also describes the fabrication and testing of a SCOS using an AJL chromophore in amorphous polycarbonate. The high uniform polymer slab waveguide is fabricated using a hot embossing process to create a slab with a thickness of 50 μm. The fabricated polymer SCOS was characterized to have a resonance slope of ΔP/Δλ=6.83E5 W/m and a resonance shift of Δλ/E=1.47E-16 m(2)/V.

Novel photo-cross-linkable polymer bearing spindle-type chromophores for second-order non-linear optical materials

A new strategy to tackle the orientation-relaxation of the chromophores was developed and used to prepare a film which was consisted of polymethyl methacrylate (PMMA) and photo-cross-linkable polyurethane oligomer (PU). The PU contained spindle-type chromophore and terminated with methacrylolyl groups as cross-linkable group. The chemical reaction process was characterized by infrared spectroscopy and the structure of reaction products were characterized by NMR spectroscopy. The surface morphology of the films was investigated by atomic force microscopy (AFM) images. The electro-optic (EO) coefficient of the prepared film was measured by using Teng-Man technique. The cross-linked films containing 15 wt% of the chromophore units displayed a high EO coefficient up to 41 pm/V. The 5% weight loss temperature was 228 °C for the film after photo-cross-linking. Compared with the uncross-linked films, their thermal and temporal stability were greatly improved.

Calcium barium niobate ridge waveguide on silicon substrate

A novel optical ridge waveguide using calcium barium niobate (CBN) with a high electro-optic coefficient on silicon substrate is proposed for the design and realization of the next-generation hybrid modulators with short length and low driving voltage. To overcome the etching difficulty of CBN, the design of a waveguide, including an elevated CBN layer based on etched silicon dioxide, is presented. Fabrication of a thick 2.5-µm CBN layer is performed using pulsed laser deposition. Deposited silicon dioxide on the sides and top of the CBN reduces the waveguide loss as low as 3.44 dB/cm, measured using a cutback method. Good agreement between the mode profiles of the simulation and measurement is obtained.

Synthesis and Nonlinear Optical Properties of Fluorinated Polyimides Containing Benzothiazole

Hetarylazo chromophores were incorporated into fluorinated polyimide matrices to prepare novel nonlinear optical (NLO) side-chain fluorinated polyimides by the Mitsunobu reaction. Their structures were verified by Fourier transform infrared (FT-IR) spectra, UV-visible (UV-Vis) spectra, 1H nuclear magnetic resonance (1H NMR) spectra, and gel permeation chromatography (GPC). Thermal analysis results indicate that the side-chain polyimides have high glass transition temperatures (Tg) between 184 and 188 ℃ and thermal decomposition temperatures (Td) up to 322 ℃. The electro-optic coefficients (γ33) of the side-chain polyimides were measured by a simple reflection technique at 1550 nm. The fluorinated polyimide PI1 containing a benzothiazolylazoaniline chromophore possesses a high electro-optic coefficient of 15 pm·V-1.

The synthesis and optical properties of novel fluorinated polyimides incorporated with highly electro-optic active thiazole and benzothiazole based chromophores

Novel, non-linear optical fluorinated polyimides were prepared by the Mitsunobu reaction of hydroxyl-containing hetarylazo chromophores with hydroxyl polyimides. Their structures were verified by FT-IR, 1H NMR, UV–Vis spectra, elemental analysis and gel permeation chromatography. The resulting amorphous polyimides exhibited good solubility and can be easily spin-coated into thin films with good optical quality. The high electro-optic coefficients (r 33) obtained for the films of poled polyimides were attributed to the introduction of hetarylazo chromophores with large hyperpolarizability within the polyimides. The polyimides possess high glass transition temperature (201–210 °C) and therefore showed excellent temporal stability, retaining 88% of their initial non-linear optical response at 110 °C for >200 h. Low optical losses in the range of 1.9–2.3 dB cm −1 at 1550 nm were observed for the polyimides and the values of thermooptic coefficients ranged from −1.680 × 10 −4 to −2.008 × 10 −4/°C.

Optical Signal Processor Using Electro-Optic Polymer Waveguides

We have investigated an optical signal processor using electro-optic polymer waveguides operating at a wavelength of 1.55 mum. Due to recent developments, many useful optical devices have become available such as optical filters, modulators, switches, and multiplexers. It will be useful to have a single optical device, which is reconfigurable to implement all of these functions. We call such a device an ldquooptical signal processor,rdquo which will play a similar role to digital signal processors in electrical circuits. We realize such an optical device in a planar lightwave circuit. Since the planar lightwave circuits are based on the multiple interference of coherent light and can be integrated with significant complexity, they have been implemented for various purposes of optical processing such as optical filters. However, their guiding waveguides are mostly passive, and the only viable mechanism to reconfigure their functions is thermal effects, which is slow and cannot be used for high-speed applications such as optical modulators or optical packet switches. On the other hand, electro-optic polymer has a very high electro-optic coefficient and a good velocity match between electrical and optical signals, thus, permitting the creation of high-speed optical devices with high efficiency. Therefore, we have implemented a planar lightwave circuit using the electro-optic polymer waveguides. As a result, the structure is complex enough to generate arbitrary functions and fast enough to obtain high data rates. Using the optical signal processor, we investigate interesting applications including arbitrary waveform generators.

On-Chip Optical Interconnect

We describe a cost-effective and low-power-consumption approach for on-chip optical interconnection. This approach includes an investigation into architectures, devices, and materials. We have proposed and fabricated a bonded structure of an Si-based optical layer on a large-scale integration (LSI) chip. The fabricated optical layer contains Si nanophotodiodes for optical detectors, which are coupled with SiON waveguides using surface-plasmon antennas. Optical signals were introduced to the optical layer and distributed to the Si nanophotodiodes. The output signals from the photodiodes were sent electrically to the transimpedance-amplifier circuitries in the LSI. The signals from the photodiodes triggered of the circuitries at 5 GHz. Since electrooptical modulators consume the most power in on-chip optical interconnect systems and require a large footprint, they are critical to establish on-chip optical interconnection. Two approaches are investigated: 1) an architecture using a fewer number of modulators and 2) high electrooptical coefficient materials.

Synthesis and nonlinear optical properties of soluble fluorinated polyimides containing hetarylazo chromophores with large hyperpolarizability

The novel fluorinated polyimides with side-chain nonlinear optical (NLO) chromophores were synthesized from hydroxyl polyimides, followed by the Mitsunobu reaction with NLO chromophores. Molecular structural characterization for the resulting polymers was achieved by 1H NMR, FT-IR, UV–Vis spectra, elemental analysis and gel permeation chromatography (GPC). The polymers exhibit excellent solubility in common organic solvents, good film-forming properties, high glass transition temperature (Tg) in the range from 193 to 200 °C and thermal stability up to 290 °C. The polyimides P1 and P2 containing hetarylazo chromophores with large hyperpolarizability possess a high electro-optic (EO) coefficient (r33), which is larger than that of the polyimide P3 attached DR1. Excellent temporal stability and low optical losses in the range of 1.9–2.1 dB/cm at 1.55 μm were observed for these polymers. Such new NLO fluorinated polyimides are distinguished by an excellent combination of NLO activity, temporal stability, and optical loss.

Hybrid electro-optic polymer/sol-gel waveguide directional coupler switches

We report electro-optic (EO) switches using transversely tapered hybrid EO/sol-gel waveguide directional couplers. The length of the active region critically reduces to 1.5 mm with transverse tapers, which is shorter by a factor of 5 than the active region with vertical tapers. The transitions are performed from a sol-gel core to two EO polymer cores with gradual index changes using gray-scaled photobleaching. The device shows the lowest switching voltage and an active length product (VsLa) of 2.9 V cm with a guest–host EO polymer AJLS102, compared to hybrid waveguide switches and modulators with vertical taper and higher EO coefficients.

A Strip-Loading Optical Waveguide Using Well Poled Stability Organic/Inorganic Hybrid Materials

An optical waveguide for a polymer modulator based on organic/inorganic hybrid electro-optic (EO) materials is designed and fabricated by utilizing a strip-loading structure. This hybrid material has a controllable refractive index, high EO coefficient and good poled stability, which are suitable for the EO modulators and switches. The embedded waveguide made of the above EO material can reduce the coupling loss. The light is coupled into the guided-core layer and then undergoes a transition from the buried waveguide into the EO material. Obvious modulation is observed by application of ac voltage to the EO material. The measured Vπ of co-planar waveguide (CPW) is 5 V for the Mach–Zehnder (MZ) modulator in length of 3.5 cm.

Effect of solvent and temperature on the preparation of potassium niobate by hydrothermal-assisted sol–gel processing

In this study the preparation of potassium niobate powder, a material with high electro-optic and nonlinear optical coefficients, by hydrothermal-assisted sol–gel processing from potassium-niobium ethoxide [KNb(OC 2H 5) 6] have been performed. The development of potassium niobate phases by hydrothermal hydrolysis of potassium niobium ethoxide at 100 and 200 °C in ethanol, toluene, and a mixture of benzene and ethanol was pursued by X-ray diffraction. Selected powders obtained from hydrothermal-assisted sol–gel processing were characterized by X-ray powder diffraction, thermal analysis (TGA/DSC), and scanning electron microscopy (SEM). X-ray diffraction results showed that the hydrothermal hydrolysis of potassium-niobium ethoxide in ethanol at 100 and 200 °C results in the formation of cubic phase KNbO 3. Hydrolysis in a mixture of benzene/ethanol and toluene at 200 °C produces an orthorhombic phase KNbO 3, which is a mixture of the potassium deficient phase, K 4Nb 6O 17, and cubic and orthorhombic KNbO 3 phases. The potassium niobate powder prepared by the hydrothermal method had the advantage of low weight loss during calcination. SEM micrographs showed that the shape of particles prepared by hydrothermal-assisted sol–gel processing is more uniform when compared with the sample prepared by conventional sol–gel processing.

High-Speed Photonic Functional Circuits Using Electrically Controllable PLZT Waveguides

We fabricated a high-speed wavelength tunable arrayed-waveguide grating (AWG) and an AWG integrated with optical switches using (Pb,La)(Zr,Ti)0 3 (PLZT). PLZT has a high electro-optic (EO) coefficient, which means these devices have considerable potential for use in reconfigurable optical add drop multiplexers (ROADMs). The PLZT waveguides in this work have a rib waveguide structure with an effective relative index difference (Δ) of 0.65%. Both AWGs have 8 channels with a frequency spacing of 500 GHz. The fabricated wavelength tunable AWGs allows us to freely shift the output at a particular wavelength to an arbitrary port by applying voltages to 3 mm long electrodes formed on each of the waveguides. We confirmed that the maximum tuning range with driving voltage of 22 V was approximately 32 nm at 1.55 μm. With the integrated 8-ch PLZT waveguide switch array, we could also select the output port by setting the drive voltage applied to the switch array. 2 x 2 directional coupler switches were used for the switch array. The two devices exhibited insertion losses of 17 dB and 19 dB, adjacent crosstalk of -18.5 dB and -19.7 dB, and a maximum extinction ratio of 19.6 dB and 12.6 dB, respectively. The tuning speed of both devices was 15 ns and their physical sizes were 9.0 x 23.0 mm and 8.0 x 29.5 mm, respectively.

High-speed polymer/silicon on insulator ring resonator switch

We design a high-speed tunable electro-optical (EO) polymer-clad silicon over insulator (SOI) racetrack resonator with a 80-µm length and 5-µm radius that exhibits a switching speed of 100 GHz, a tuning voltage of 6.81 V, and an extinction ratio of 109.55 dB. This work employs high electro-optical coefficient polymers (r33=1000 pm/V), which are currently under development, to implement very high speed switches.

Fabrication and Electro-optic Properties of Ferroelectric Nanocrystal/Polymer Composite Films

BaTiO3 (BTO), Pb(Zr0.52Ti0.48)O3 (PZT), and Sr1-xBaxNb2O6 (SBN; x = 0.5, 0.4, and 0.25) nanocrystal-doped polycarbonate (PC) composite thin films were prepared by a spin-coating method. Ultrafine BTO and PZT nanocrystals of pure tetragonal perovskite phase were synthesized by a hydrothermal method, while the rodlike SBN nanocrystals of pure tetragonal tungsten−bronze structure were prepared by the Pechini method. To produce better electro-optic (EO) properties through creation of noncentrosymmetry in the samples, the composite films were poled under a high electric field at suitable temperatures. Their EO coefficients as well as transmittance spectra were measured. Both BTO/PC and PZT/PC composite films showed excellent EO properties with high EO coefficients and large figures of merit. However, SBN/PC composite films established relatively small EO coefficients due to the small transverse EO coefficients of SBN single crystals. On the basis of our results, we demonstrate that these ferroelectric nanocrys...

Organic Broadband TeraHertz Sources and Sensors

We review recent research using organic materials for generation and detection of broadband terahertz radiation (0.3 THz−30 THz). The main focus is on amorphous electrooptic (EO) polymers, with semiconducting polymers, molecular salt EO crystals, and molecular solutions briefly discussed. The advantages of amorphous EO polymers over other materials for broadband THz generation (via optical rectification) and detection (via EO sampling) include a lack of phonon absorption (good transparency) in the THz regime, high EO coefficient and good phase-matching properties, and, of course, easy fabrication (low cost). Our ∼12-THz, spectral gap-free THz system based on a polymer emitter-sensor pair is an excellent demonstration of the advantages using of EO polymers. We also present a model that can predict the performance of a polymer-based THz system. Both the dielectric properties of an EO polymer and laser pulse related parameters are included in the model, making the simulations close to real conditions. From our modeling work, the roles the dielectric properties play in the THz generation and detection are clearly seen, providing us with a good guide to select and design suitable EO polymers in the future.

All-Dielectric Electrooptic Sensor Based on a Polymer Microresonator Coupled Side-Polished Optical Fiber

A novel electrooptic (EO) electric field (E-field) sensor based on side-polished fiber coupled with an EO polymer microring resonator is proposed and demonstrated. An EO polymer waveguide with a ring shape is fabricated on the polished flat of an optical fiber. Light in the fiber evanescently couples into the resonator and forms resonant modes for certain wavelengths and produces notches in the output intensity of the fiber. External electric fields change the index of refraction of the ring waveguide and therefore dither its resonant wavelengths. For light of wavelength on the slope of a resonance notch, a change in the output intensity can be detected. The sensor is all dielectric without metal layers to distort the measured E-field. The resonant structure allows the sensor to potentially have much higher sensitivity than other electrooptic sensors based on Mach-Zehnder or polarization modulation. Since electrooptic polymers have higher electrooptic coefficients, lower dielectric constants and faster electrooptic responses than inorganic crystals, higher sensitivity, lower invasiveness, and higher bandwidth of E-field sensing can be expected. This sensor eliminates unreliable fiber-to-waveguide butt coupling as well as the high propagation loss encountered in the long straight EO polymer waveguides of sensors based on Mach-Zehnder structures. By using the fiber itself as the supporting substrate of the ring waveguide, the sensor can have small size and low disturbance to the measured electric field. The concept is demonstrated using AJLS103 EO polymer. A sensitivity of 100 mV/m has been achieved at frequencies up to 550 MHz (limited by the measurement system)

Optical electric-field sensors

Based on the electro-optic (EO) polymer Mach–Zehnder interferometer (MZI) technology, IPITEK develops optical E-field sensor devices. As a receive antenna, the present device exhibits wide and flat bandwidth, up to 10 GHz. Testing the E-field sensor response was performed using a transverse electromagnetic (TEM) cell at frequencies from 0.2 to 1 GHz, and a set of 4 horn antennas at frequencies from 2.6 to 12 GHz. The minimum detectable E-field, Emin, was about 70 mV/(m) for an all-dielectric field sensor and was about 7 mV/(m) for a sensor with electrodes and a short wire loop antenna. A photonic down-conversion technique was developed to address bandwidth and receiving power limitations of the receiver photodetector. The down-conversion experimental results agree well with the theoretical heterodyne predictions. The EO polymer sensor sensitivity can be further improved by reducing the device optical insertion loss, optimizing the photodetector and detection circuitry, and using recently developed higher EO coefficients polymers.

Electrical control of the linear optical properties of particulate composite materials

The Bruggeman formalism for the homogenization of particulate composite materials is used to predict the effective permittivity dyadic of a two constituent composite material with one constituent having the ability to display the Pockels effect. Scenarios wherein the constituent particles are randomly oriented, oriented spheres and oriented spheroids are numerically explored. Thereby, homogenized composite materials (HCMs) are envisaged whose constitutive parameters may be continuously varied through the application of a low-frequency (dc) electric field. The greatest degree of control over the HCM constitutive parameters is achievable when the constituents comprise oriented and highly aspherical particles and have high electro-optic coefficients.