Electro-optic Directional Coupler Research Articles

Plasmonic electro-optic modulators on lead zirconate titanate platform

Abstract The advancement in material platforms exhibiting strong and robust electro-optic effects is crucial for further progress in developing highly efficient and miniaturized optoelectronic components with low power consumption for modern optical communication systems. In this work, we investigate thin-film lead zirconate titanate (PZT) substrates grown by a chemical solution deposition technique as a potential platform for on-chip plasmonic electro-optic modulators. A high modulation depth (>40 %) is achieved with 15 μm-long electro-optic directional coupler modulators. An unusual cutoff in the modulation frequency response at ∼200 kHz is observed and further studied with respect to possible reorientation effects. Second-harmonic generation signals are found influenced by the externally applied electric field, indicating that the domain reorientation effect can be responsible for the unusual frequency response observed.

Plasmonic monolithic lithium niobate directional coupler switches

Lithium niobate (LN) has been the material of choice for electro-optic modulators owing to its excellent physical properties. While conventional LN electro-optic modulators continue to be the workhorse of the modern optoelectronics, they are becoming progressively too bulky, expensive, and power-hungry to fully serve the needs of this industry. Here, we demonstrate plasmonic electro-optic directional coupler switches consisting of two closely spaced nm-thin gold nanostripes on LN substrates that guide both coupled electromagnetic modes and electrical signals that control their coupling, thereby enabling ultra-compact switching and modulation functionalities. Extreme confinement and good spatial overlap of both slow-plasmon modes and electrostatic fields created by the nanostripes allow us to achieve a 90% modulation depth with 20-μm-long switches characterized by a broadband electro-optic modulation efficiency of 0.3 V cm. Our monolithic LN plasmonic platform enables a wide range of cost-effective optical communication applications that demand μm-scale footprints, ultrafast operation and high environmental stability.

Switching behavior engineerable, electro-optic directional couplers in aperiodic optical superlattice waveguides

We report on the first study and a proof-of-principle demonstration of aperiodically alternating-Δβcouplers in LiNbO3 waveguides whose switching behavior can be engineered with a high degree of freedom. A high fabrication-tolerance and broad working-bandwidth electro-optic (EO) coupler is developed based on a unique Δβ scheme derived using a simulated annealing algorithm and realized in Ti-diffused aperiodically poled lithium niobate (APPLN) waveguides. The waveguide fabrication tolerance and the working bandwidth (149.2 nm) of the APPLN EO coupler are found to be increased by 2.7 and 2.4 times, respectively, over that of a conventional periodically alternating-Δβ (PPLN) coupler for ≥99% power transfer efficiency under a fixed switching voltage at the 1550 nm band. The results even hold for a domain poling-width error of ≤9%. An interesting APPLN EO coupler capable of working on a certain coupling state over a broad voltage operating range is also studied. The technology developed in this work should enable the production of coupler devices whose switching characteristics can be tailored and realized with much more relaxed fabrication and bandwidth control, which would be beneficial for the application in efficient and compact integrated-optic circuits/systems.

Evaluation of performance of a hybrid electro-optic directional coupler and a Mach–Zehnder switch

The efficiency of an electro-optic (EO) directional coupler (DC) and a Mach–Zehnder (MZ) switch is discussed. The specific designs employ a passive waveguide core and an EO cladding. The performance of a DC switch was evaluated by numerical methods, namely coupled mode theory (CMT), eigenmode expansion (EME) method, and beam envelope method. Using the CMT and EME methods, it is shown that in the DC with a passive waveguide core and EO cladding at certain waveguide parameters the switching effect does not take place. The effect is demonstrated for the first time and suggests that careful optimization of the DC switch should be done before experimental realization of the device. The efficiency of an optimized DC switch is shown to be of an order lower than that of an MZ switch.

Liquid-Crystal Tunable Long-Range Surface Plasmon Polariton Directional Coupler

A tunable plasmonic directional coupler is proposed based on the electro-optical control of a nematic liquid-crystal layer placed above two side-coupled gold stripe waveguides. It is shown that by proper adjustment of the structural parameters and the control voltage, the coupling length of the structure and the effective modal index of the individual waveguides can be tuned, allowing for the design of electro-optical liquid-crystal plasmonic directional coupler switches.

Integrated fault tolerant connections-scheduling for dilated Benes network

The 2×2 electro-optical directional coupler is a common switching element in dilated optical multistage interconnection networks (DOMINs) and can be applied to photonic switching technology. Because of the undesirable coupling within an optical switch, a portion of the energy of the input signal might be transmitted to the undesired output terminal in the switch. A switch is said to be faulty if it causes excessive crosstalk above a given threshold, and optical crosstalk is a problem commonly found in DOMINs. The problem of connections-scheduling with faulty switches in a dilated re-arrangeable photonic network, say dilated Benes network (DBN), is studied and an integrated fault tolerant DBN architecture is proposed. Based on the faulty switch information given from the fault diagnosis algorithm, the fault-free path generation algorithm and scheduling algorithm are devised to schedule a set of connections avoiding faulty switches with a polynomial time complexity in the DBN. It is desirable to keep the number of rounds as small as possible. The simulation results show that the proposed method outperforms the greedy-scheduling algorithm in terms of routing number, number of failed connections, minimum number of rounds, and connection success probability when the size of the network is large. Furthermore, the proposed methodology can be embedded in the optical switching system to achieve self-testing.

Design considerations for an electrooptic directional coupler modulator

A rigorous numerical model is presented for an electrooptic directional coupler (DC) modulator. It was observed that it is possible for such a device to be optimized by the variation of a number of fabrication parameters. The applied field, the induced refractive index changes, and hence the change in the waveguide optical field due to the antisymmetric refractive index change are calculated for an elected structure. Finally, the product of switching voltage and coupling length, the power transfer efficiency, obtained using the least squares boundary residual (LSBR) method, and the optical loss due to the metal electrodes are also given.

Finite-element analysis of lossy TE–TM modes in metal-clad optical waveguides

Finite-element analysis employing the scalar and vector H-field formulations and with the aid of the perturbation technique is used to calculate the TE-TM complex propagation characteristics of integrated optical devices in gallium arsenide, lithium niobate, and silica fiber, incorporating a lossy metal cladding. The propagation and attenuation properties of several types of metal-clad planar optical waveguide, which exhibit surface-plasmon properties for the TM polarization, are reviewed, and the modal loss caused by the metal cladding in a titanium-diffused lithium niobate electro-optic directional coupler modulator, an indium gallium arsenide phosphide-based TE-TM optical polarizer, and a submicron metal-clad silica fiber suitable for near-field optical scanning microscopy is calculated.

Low-power consumption 1�4 ?cascade switch? for microwave applications

We present the design and realization of a 1×4 switching matrix, based on an electro-optical directional coupler. This matrix is devoted to true time delay systems. It is characterized by an original topology that reduces global power consumption down to 20 mW. Fiber-to-fiber losses are 15 dB, and the crosstalk is lower than −20 dB. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 18: 243–246, 1998.

An optical filter of adjustable finesse using an amplified fiber ring resonator

We consider an optical filter constructed from a simple ring resonator with two electro-optical directional couplers and a fiber amplifier in the ring. By adjusting the coupling coefficient of the electro-optical directional couplers, the finesse of this optical filter can be controlled over a specified range. The variation in the insertion loss of the optical filter can be overcome by adjusting the gain of the fiber amplifier simultaneously. This paper analyzes theoretically the performance of the optical filter, as a function of the coupling coefficient of the electro-optical directional coupler and the gain of the fiber amplifier. We also show that it is possible to design an optical filter of wide adjustable finesse by changing only the coupling coefficient of the electro-optical directional couplers, leaving the gain of the fiber amplifier unchanged. This simpler method results only in a slight change in the insertion loss if an appropriate condition is chosen. The filter will be useful in coherent optical systems, where the bandwidth of the optical filter needs to be appropriately adjusted under varying biasing and signaling conditions, or as an optical discriminator with an adjustable Q-factor.

Electro-optic directional coupler switch characterization

Results are presented on a study of the important parameters of synchronous and nonsynchronous, weakly and strongly coupled optical directional couplers using the finite element method. Accurate propagation constants and field profiles have been obtained for the modes of the isolated guides and the supermodes of the coupled system. The power transfer efficiency between nonidentical coupled optical waveguides has been calculated using the coupled mode, the least squares boundary residual and the finite element-based propagation methods. To show the advantages of the finite element-based approaches, the power transfer efficiency between coupled waveguides with two-dimensional confinement is presented.

First 8×8 semiconductor optical matrix switches using GaAs/AlGaAs electro-optic guided-wave directional couplers

Monolithically integrated 8×8 semiconductor optical matrix switches have been successfully fabricated for the first time. They were constructed on GaAs substrates from 64 GaAs/AlGaAs electro-optic directional couplers using a simplified tree architecture. The devices have a chip size of 26.5×3 mm2 and a minimum total loss (excluding coupling loss) of 8.7 dB.

4*4 GaAs/AlGaAs optical matrix switches with uniform device characteristics using alternating Delta beta electrooptic guided-wave directional couplers

Integrated 4*4 GaAs/AlGaAs optical matrix switches constructed from 12 electrooptic directional couplers have been realized. In order to achieve uniform device characteristics, molecular beam epitaxy and reactive ion beam etching were chosen as the crystal growth technique and waveguide fabrication technique, respectively, in addition to the simplified tree structure as a matrix switch architecture. As a result, matrix switches with quite uniform device characteristics, such as small switching voltage deviation and little path dependence in +or-0.5-dB propagation loss, have been realized. >

Digital optical computing with optically switched directional couplers

Optically switchable directional couplers can function as logically complete building blocks for constructing all-optical computational engines. Logically, such devices operate as all-optical five-terminal gates, where the two output signals are logical functions of two throughgoing input signals and a control input signal. Such an optically controlled exchange element is a promising functional unit for constructing general-purpose digital optical logic circuits. The implementation of such all-optical five-terminal gates using Ti:LiNbO3 electro-optic directional couplers is described. These optical logic gates utilize mature technology developed for the telecommunications industry, facilitating construction of robust optical logic hardware. Single-mode optical fiber is used for all logical interconnections. Effectively dc-coupled control circuitry converts optical pulses into electrical pulses capable of switching the transfer state of low-voltage lumped electrode directional couplers. The use of such Ti:LiNbO3 five-terminal optical logical gates in the construction of various simple circuits such as oscillators and divide-by-N circuits is shown. Such circuits demonstrate many of the issues arising in the construction of all- optical digital computing systems, and are fundamental subsections of an all-optical bit serial computer design currently under construction.

Design of semiconductor electrooptic directional coupler with the beam propagation method

The design of a low-loss switching voltage double-heterostructure electrooptic coupler switch in III-V semiconductor materials is discussed. The optical propagation is analyzed by the effective index method combined with the beam propagation method. The electrostatic field and electron and hole distributions are analyzed by a numerical simulation. This model is in good agreement with experimental results obtained at 1.56 mu m on directional couplers made of GaAs-GaAlAs DH waveguides and Schottky barrier electrodes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Generation of arbitrarily shaped picosecond optical pulses using an integrated electrooptic waveguide modulator

We have produced complex software adjustable laser pulse shapes with ~10-ps resolution, and pulse energies up to 100 microJ for spectroscopic applications. The key devices are a high damage threshold electrooptic directional coupler and a GaAs circuit for synthesizing arbitrarily shaped microwave pulses.

A spatial sampler using integrated optic techniques

A spatial sampler for detecting transient RF and microwave signals is proposed. The sampler consists of a series of electrooptic directional couplers spatially located along an integrated-optic waveguide beneath a coplanar RF stripline. It is theoretically shown that the sampler has the potential of sampling with a temporal resolution of a few picoseconds and a sampling rate in excess of 40 billion samples per second. An electrooptic coupler with a coupling characteristic that is asymmetric with voltage is analyzed and experimentally evaluated for consideration in the sampler.

Three-electrode Ti:LiNbO3optical switch

Modifications of the conventional electrode arrangement of electrooptic directional coupler switches are described. Uniform <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\Delta \Beta</tex> electrodes can be symmetrically shortened to ∼ 40 percent of their length without significant increase in electrical drive voltage. Based on this result, a new three-electrode switch is described which can be used to electrically adjust both the bar and the crossed states of the switch. It is also demonstrated that the three-electrode arrangement can be used to reduce or eliminate optical crosstalk in Ti : LiNbO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> switches.

Gigahertz-bandwidth InGaAsP/InP optical modulators/switches with double-hetero waveguides

A new kind of InGaAsP/InP electro-optic directional coupler modulator/switch with double-hetero plano-convex waveguides, which is suitable for monolithic integrated optics, has been proposed and demonstrated. The fabricated devices have shown low optical loss (4 dB/cm) and high modulation bandwidth of 1 GHz at 1.3 μm wavelength.

InP electro-optic directional coupler

An electro-optic directional coupler switch has been fabricated in InP with an homojunction structure. Each single-mode guide is made in a n layer grown on a n+ substrate, the p+ etched rib used to confine the light being obtained by diffusion. A linear variation of the coupling length with reciprocal wavelength has been found between 1.06 and 1.51 μm. By reversely biasing the ’’stepped Δβ’’ junctions with less than 12 V, both switching states have been achieved at 1.51 μm, with a power isolation better than 16 dB on a 8-mm-long device. This is a new promising step towards InP-integrated optics.