Optical Directional Coupler Research Articles

Matrix coupled mode theory model of strongly-coupled multiwaveguide optical nonlinear directional couplers

This paper discusses an improved coupled mode theory model for the nonlinear directional coupler in the strong-coupling range. A new single-line matrix equation is derived for the multiwaveguide case. For the two-guide case, a new critical power formula and two exact identities are derived. Interrelation between the overlap integral and nonlinear cross-coefficients is demonstrated. It is shown that inconsistent inclusion of these terms in previous approaches leads to inaccuracies. Results obtained by consistent inclusion of these terms and our critical power formula show a great improvement in accuracy over previous analyses in strong-coupling.

Characterization of antiresonant reflecting optical waveguide devices by scanning near-field optical microscopy.

Silicon-based antiresonant reflecting optical waveguide (ARROW) devices were studied by means of a scanning near-field optical microscope. Various structures such as a Y junction of a Mach-Zehnder interferometer and a directional optical coupler were characterized, showing the propagation of the light inside the devices simultaneously with the topography. Scattering on the splitting point of the Y junction was shown, as well as a partial coupling of the light between the two branches of the coupler. Measurements on the decay length of the evanescent field were also performed to study the use of the ARROW waveguide for sensor purposes.

Wavelength-Division-Multiplexed Photonic Switching System Using Integrated Acoustooptic Switches

Collinear acoustooptic (AO) wavelength separators and switches consisting of an optical directional coupler have been investigated near the wavelength of 0.6 µm. The fundamental characteristics of wavelength-selective switching are investigated at the fiber communication wavelength of around 1.55 µm for use in wavelength-division-multiplexed (WDM) networks and photonic switching systems, in comparison with those in the visible wavelength range. As applications to WDM optical systems, optical add/drop multiplexers (ADMs) for WDM network-accessing systems, matrix switches for WDM crossconnects and photonic switching systems are discussed.

Wide Stop-Band Filter Consisting of Multiresonator Based on an Optical Directional Coupler

Recently, the wavelength range of wavelength division multiplex (WDM) systems has been widening. Consequently, optical filters require wide stop bands that can cover this range. Multiresonance filters can have flat pass bands and relatively sharp cutoff characteristics but have narrow stop bands because of the repeated appearance of resonance. In this paper, an optical filter based on a directional coupler is discussed. It may be suitable for integrated optics because of its simple structure. The method of widening the stop band is also reported.

New full-vectorial numerically efficient propagation algorithm based on the finite element method

A new full-vectorial beam propagation algorithm based on the versatile finite element method, in order to accurately characterize three-dimensional (3-D) optical guided-wave devices, is presented. The computationally efficient formulation is based on the two transverse components of the magnetic field without destroying the sparsity of the matrix equation. The robust perfectly matched layer (PML) boundary condition is incorporated into the formulation so as to effectively absorb the unwanted radiation out of the computational domain. The efficiency and precision of the proposed full-vectorial propagation approach is demonstrated through the analysis of single optical waveguide, directional couplers, and electrooptic modulator.

The novel architectures of smart optoelectronic logic circuits based on directional couplers

Novel architectures of the smart optoelectronic logic gate and SR flip-flop are proposed for the first time. In the former, both the AND and OR functions can be integrated in a single circuit. These optoelectronic circuits are composed of integrated optical waveguide directional couplers, optoelectronic converters, EDFAs, a delay line, and a DFB laser. Importantly, linear device operation with low power is the main advantage as compared to the conventional optoelectronic logic elements. Moreover, the characteristics of these optoelectronic circuits, such as optical path, slew-rate limiting, and phase retardation on the different states of operation, are discussed and are found to be informative. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 23: 291–294, 1999.

The novel architectures of smart optoelectronic logic circuits based on directional couplers

Novel architectures of the smart optoelectronic logic gate and SR flip-flop are proposed for the first time. In the former, both the AND and OR functions can be integrated in a single circuit. These optoelectronic circuits are composed of integrated optical waveguide directional couplers, optoelectronic converters, EDFAs, a delay line, and a DFB laser. Importantly, linear device operation with low power is the main advantage as compared to the conventional optoelectronic logic elements. Moreover, the characteristics of these optoelectronic circuits, such as optical path, slew-rate limiting, and phase retardation on the different states of operation, are discussed and are found to be informative. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 23: 291–294, 1999.

A novel wide-band tunable RF phase shifter using a variable optical directional coupler

We present a novel RF phase-shifter design with a usable bandwidth of 80:1. The design is verified through demonstration of a proof of concept device, consisting of a readily available voltage variable optical coupler fabricated from LiNbO/sub 3/, combined with an fiber-optic delay line. The design is analyzed theoretically and measurement of the device confirms the predicted range of operation. Methods of extension of this range of operation are discussed.

On the radiation mode effects in integrated optical directional couplers

In this work we study the effect of radiation modes generated at the input of a directional coupler on its performance. We focus on the splitting ratio and the coupling length of the coupler. The Beam Propagation Method (BPM) is used for numerical simulation. Numerical results are compared to analytical expectations of the coupled mode theory (CMT). We show that, when the coupler is excited by a single mode fibre, ideal 3 dB couplers could be obtained by a simple adjustment of the exciting fibre. On the other hand, when the coupler has input and output bend structures, its coupling length as well as its splitting ratio are affected by the geometry of this bend.

Combination of beam propagation method and finite element method for optical beam propagation analysis

A combined method of the beam propagation method (BPM) and the finite element method (FEM) is proposed for the analysis of reflections of both transverse electric (TE) and transverse magnetic (TM) polarized waves in waveguiding structures containing longitudinal discontinuities. BPM based on FEM is applied to slowly varying regions, and FEM is applied only to regions including abrupt discontinuities. In order to avoid nonphysical reflections from computational window edges, the perfectly matched layer boundary condition is introduced. To show the validity and usefulness of the present approach, numerical examples for an optical directional coupler and a beam splitter are presented.

Combination of beam propagation method and mode expansion propagation method for bidirectional optical beam propagation analysis

A combined method of the beam propagation method (BPM) and the mode expansion propagation method (MEPM) is proposed for the analysis of reflections of both transverse electric (TE) and transverse magnetic (TM) polarized waves in waveguiding structures containing longitudinal discontinuities. BPM based on the finite-element method (FEM) is applied to slowly varying regions and MEPM is applied only to regions including abrupt discontinuities. FEM is also utilized for evaluating the eigenmodes necessary for the MEPM analysis. To show the validity and usefulness of the present approach, numerical examples for a semiconductor laser facet with antireflection coating and an optical directional coupler are presented.

Nonlinear antiresonant reflecting optical waveguide coupler with an optically induced grating

We study externally controlled optical switching in an asymmetric antiresonant reflecting optical waveguide directional coupler. The power exchange between two channels with different propagation constants is obtained by means of an optically induced grating in the spacing region. Compared with a conventional coupler with an optically induced grating, the antiresonant reflecting optical waveguide coupler exhibits reduced switching power and permits remote coupling.

Flame-fused Optical Fiber Directional Couplers: Fabrication and Automated Process Control

A microprocessor controlled system for fabrication of 2 x 2 flame-fused biconical single-mode fiber directional couplers has been designed and realised. The system involves multiple motorized movements of mechanical stages, and an electronic hardware. The associated software developed for executing CPU operation enables presetting key fabrication control parameters like time of flame fusion, fiber pulling speed etc for minimising the excess loss and achieving various target specifications with regard to the coupling ratio. It has almost all the features of industrial fabrication. This automatic fabrication system has been successfully tested in our laboratory by drawing a number of couplers.

Combined beam propagation and bidirectional eigenmode propagation methods for bidirectional optical beam propagation analysis

A combination of the beam propagation method (BPM) and the bidirectional eigenmode propagation method (BEP) is proposed for the analysis of reflections in waveguide structures containing longitudinal discontinuities. The BPM based on the finite element method (FEM) is applied to slowly varying regions and the BEP is applied only to regions that include abrupt discontinuities. The FEM is also utilized for evaluating the eigenmodes necessary in the BEP analysis. To show the validity and usefulness of the approach proposed here, numerical examples for a semiconductor laser facet, an optical waveguide connection, and an optical directional coupler are presented. © 1998 Scripta Technica. Electron Comm Jpn Pt 2, 80(6): 10–18, 1997

Two-section reversed /spl Delta//spl beta/ switch with uniform electrodes and domain reversal

The performance of the reversed-/spl Delta//spl beta/ LiNbO/sub 3/ optical directional coupler (DC) switch with electrodes of opposite polarity is known to be more tolerant to variations in coupling length than that of the single-section DC. However, sectioned electrodes are incompatible to a single microwave source and, therefore, preclude the use of the device in high-speed applications. In this letter, we discuss the operation and results of a modified DC that uses a domain reversal and uniform electrodes that can be readily matched to a microwave source. A crosstalk of -34 dB was measured in an experimental device, fabricated in Z-cut LiNbO/sub 3/. Factors which improve device performance are also discussed.

The design of an optoelectronic arithmetic processor based on permutation networks

This paper introduces a new concept by which it is possible to design and implement arithmetic processors using permutation networks. To demonstrate this concept, several optoelectronic arithmetic units combining optical directional coupler switches and cyclic permutation networks are designed. The designs show that addition, subtraction, and multiplication can all be performed in O(log n) time in residue code domain and using O(n/sup 2/) directional coupler switches and gates, where n=log M, and M is the integer range of interest. These arithmetic units also have the capability of concurrent error detection and fault-tolerance, and they can be used to construct constant time inner product processors.

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.

Guided-wave optics in silicon-on-insulator technology

Silicon-on-insulator (SOI) technology offers tremendous potential for the integration of optoelectronic functions on a silicon substrate. The authors have demonstrated integrated 3 dB optical directional couplers, a 5/spl times/9 star coupler, and a Mach-Zehnder wavelength demultiplexer using SOI rib waveguides. These devices show excellent performance characteristics and represent key components for the realisation of monolithic optoelectronic circuits in a silicon IC-compatible technology.

Polariton propagation in coupled quantum wire waveguides

The propagation of the excitonic polariton along the directional coupler consisting of adjacent parallel GaAs quantum-wire waveguides is simulated by using a finite element method. It is found that the coupling length is as short as 2.5-4 μm, which is three orders of magnitude shorter than the conventional optical directional coupler made of bulk GaAs. Such short coupling length will make it possible to fabricate extremely small optoelectronic devices such as directional-coupler-type switches.

Nonlinear electron-wave directional coupler.

We propose a class of quantum interference device that is an electronic counterpart of the nonlinear optical directional coupler. Nonlinear coupled wave equations including Coulomb charging effects in coupled electron waveguides predict coupling lengths depending on the amplitude of the input electron waves, and also predict that for the initial condition, the electron wave function is equally distributed to two electron waveguides; a small initial fluctuation of the wave function causes the localization of the electron wave in one of the two waveguides by symmetry-breaking instability. These operating characteristics would be useful for constructing ultrafast self-switching devices and logic gates.