Directional Coupler Switch Research Articles

Design of Vertical Directional Coupler Switch using Ferroelectric Liquid Crystal (FLC)

To decrease the switching length of a directional coupler switch and to make use of the electro-optic properties of FLC, we propose a novel vertical directional coupler switch using FLC and present the calculated results relevant to the design considerations. Because of the large birefringence of FLC, a very short switching length less than 40 μm is possible. Besides, a N x N switch is also proposed for practical applications.

SSFLC optical directional coupler switch with a short device length: a proposal

A novel vertical directional coupler waveguide switch composed of ferroelectric liquid crystal (FLC) is proposed and analyzed. The surface stabilized ferroelectric liquid crystal break (SSFLC) is used as an active separation layer between passive waveguides. To decrease the switching length, the waveguide switch is based on the power crossing the separation layer rather than coupling between waveguides. The calculated results relevant to the switch parameters such as the switching length, switching time, capacitance, and extinction ratio are presented. Because of the large birefringence of FLC, a very short switching length, less than 60 /spl mu/m, is possible.

Performance of field-induced directional coupler switches

Switching in a GaAs field-induced, three-waveguide straight directional coupler is studied theoretically. This device can be tuned externally by changing the voltage across the waveguides. Because of this tunability, the device has some very attractive features as a switching element. The performance change due to variations in the device parameters such as length, waveguide separation, waveguide width, and wavelength of operation is numerically computed. The effect of material absorption, input and output coupling, and asymmetric excitation are included in the performance evaluation. For a device length of 1400 /spl mu/m, a crosstalk of -34 dB and a power transfer efficiency of -2 dB is predicted, while for the same device size a two-guide directional coupler is predicted a crosstalk of -13.4 dB. The device has a 400 nm voltage tunable bandwidth with a maximum crosstalk penalty of 3 dB.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Finite element analysis of optically controlled non-linear directional coupler switches

Optically controlled two-way optical switches are useful for high-speed photonic switches and rapidly reconfigurable optical interconnection networks. A directional coupler forms such a switch because the field can be switched from one waveguide to the other by altering the optical length of the waveguide. Nonlinear materials are incorporated in the waveguide so that the optical path length may be altered by using an optical control beam to vary the intensity of light. A finite element method is described that enables the modelling of the non-linear dual waveguide structure for arbitrary waveguide shapes. Spurious modes are avoided in the transverse magnetic field method and non-linearity is handled by iteration and assuming an equivalent non-linear relative permittivity.

InGaAsP/InP MQW directional coupler switch with small and low-loss bends for fiber-array coupling

We report on a compact InGaAsP/InP MQW directional coupler switch with spacing between the waveguides at the input and the output facets larger than 125 μm, required for the fiber-array coupling, by employing small low-loss bends. The total device length, including both the coupling and the bending regions, with the different optical confinements in the lateral direction is 2 mm. The coupling region of the switch is 420 μm, and the voltage-length product at switching is around 6 Vmm.

High-extinction directional coupler switches by compensation and elimination methods

A conventional guided-wave optical directional coupler switch always presents a low-extinction switching property less than 30 dB. One main reason is that coupling occurs between input and output lead regions of the coupler, causing low extinction in the through switching state despite the coupling strength being very weak. Another reason may be due to the asymmetric structural designs of couplers in optical integrating circuits, making low extinction result in the cross switching state. In this paper, we propose two methods which can solve this crosstalk problem easily. One is by the compensation technique in which we design an optimum structural construction for the reversal /spl Delta//spl beta/-directional coupler switch, and for which theoretical prediction shows that infinity extinction can be achieved where the fabricated switch element gave above 36 dB extinction ratio for both the through and cross switching states. In another method, coupling effect between lead waveguides are eliminated by means of refractive index control of the waveguides. Theoretical prediction shows that if the change of refractive index /spl delta/n is greater than 3/spl times/10/sup -3/, over 60 dB extinction can result, where the fabricated switch element also gave more than 38 dB extinction in both the through and cross switching states. Fabrication error, existence of propagation loss, and stray lights due to scattering may be the causes of difference between predicted and experimental values. However, both proposed methods show evidence that it is possible to obtain an extinction ratio as high as nearly 40 dB. >

Y-cut LiNbO/sub 3/ directional coupler with a self-aligned electrode

A 2/spl times/2 Y-cut LiNbO/sub 3/ directional coupler switch using a self-aligned uniform /spl Delta//spl beta/ electrode is fabricated by the proton diffusion method. Our design method makes it possible to form the electrode without the delicate alignment of the electrode pattern mask and optical waveguide. The switching voltage is also reduced due to the enhanced electro-optic efficiency by using the self-aligned electrode. To achieve the initial cross state of the uniform /spl Delta//spl beta/ switch that is essential for low crosstalk, we control the proton diffusion time successively after the proton exchange. The measured crosstalk level of the directional coupler switch is -23 dB/spl sim/-29 dB and its voltage-length product is as small as 9.6 Vmm.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pump controlled all-optical switching by using high-concentrationEr 3+ doped nonlinear waveguides

A 3-D split-step finite-element method is used for the numerical investigation of optical switching in two-core nonlinear active directional couplers made of high concentration erbium doped silica waveguides. The Letter shows that optical switching at 1532 nm can be controlled by changing the input pump power at 980 nm.

A coupled mode theory for electron wave directional couplers

In this paper, a coupled mode theory for electron wave directional couplers is presented. This theory includes the dephasing effects on electrons due to temporally random collisions with lattice ions and other electrons in the system. This is done by considering the phase coherence length L/sub c/ as a single lumped parameter of the material. Using this model, the effects of temperature and other random dephasing collisions in electron wave devices can be studied easily. As an example, this model is applied to study the switching behavior of the electron wave directional coupler switch.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Design and fabrication of monolithic optical spot size transformers (MOSTs) for highly efficient fiber-chip coupling

A novel type of monolithic optical spot size transformer realized in the InP/InGaAsP material system combines many desirable features like high fiber-chip coupling efficiency, large spot size transformation, high tolerances for fiber alignment, simple fabrication, and straightforward integrability with other devices. The spot size transformation is accomplished by two waveguide sections tapered in the vertical and lateral directions, respectively. We discuss design issues, describe the fabrication process, give experimental results, and present the monolithic integration of the spot size transformers with a current controlled directional coupler switch.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fabrication-tolerant Δκ directional coupler switch using annealing in proton exchange

A fabrication-tolerant Δκ directional coupler switch was fabricated in z-cut LiNbO 3 substrate by the annealed proton exchange method. In order to improve fabrication tolerance of the device, the metal mask was used for selective proton exchange as a self-aligned electrode, and the desired overlap of electrode and waveguide mode was accomplished by annealing process. Performing successive annealing of the device, at each annealing step the width mode size of the waveguide, the switching voltage of the device, and the crosstalk of two output waveguides were measured. The voltage-length product of the device is 28 Vmm at the wavelength of 633 nm and the crosstalk is smaller than -20 dB.

Elimination of crosstalk in directional coupler switches

We propose a tapered electrode structure to make the ratio of the coupling coefficient to the difference of propagation constants between two coupled waveguides in a directional coupler switch a constant throughout it. In this way, the crosstalk caused by the existence of tapered sections is entirely eliminated.

Nonlinear switching in InGaAsP laser amplifier directional couplers biased at transparency

Ridge waveguide laser amplifier directional couplers have been developed to exploit recent observations of ultrafast refractive index changes in amplifiers biased at transparency. All-optical self-switching has been observed in a device with a bulk active region.

4×4 directional coupler switch matrix with an InGaAlAs/InAlAs multiquantum well structure

A 4×4 directional coupler switch matrix is developed which uses, for the first time, the quantum confined Stark effect of InGaAlAs/InAlAs multiquantum well structures. The rearrangeable nonblocking 4×4 network with six 2×2 switches is shown to be perfectly functional with switching voltages between 5 and 6 V and crosstalk below −17 dB in all the operation states.

Ultrafast all-optical switching in semiconductor nonlinear directional couplers at half the band gap

Efficient ultrafast all-optical switching in nonlinear directional couplers made of AlGaAs and AlGaAs/GaAs quantum wells near half the band gap is reported. The switching is limited by multiphoton absorption which is dominated by three-photon absorption in this spectral range. The three-photon absorption in the quantum well nonlinear directional coupler is stronger than that of bulk AlGaAs. Autocorrelations of the output pulses in the bar and cross states confirm pulse breakup through nonlinear coupling, and illustrate the effects of multiphoton absorption. All sets of experimental data are fitted well by a theoretical model.

Vertical multiple-quantum-well directional-coupler switch with low switching voltage

A novel device structure of the vertical directional-coupler optical switch with multiple quantum wells (MQWs) is proposed and fabricated. All the clad, guide, and separation layers are superlattices of Al/sub 0.3/Ga/sub 0.7/As and GaAs. This leads to a better control of the complete coupling length. The main feature of the proposed structure is that the refractive index modulation due to the quantum confined Stark effect (QCSE) is applied only to the undoped central region of the separation layer. It is found that this leads to reduction of the switching voltage. In an actual sample 138 mu m long, a switching voltage of 5 V has been obtained.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Implementation of a fiber-optic delay-line memory

The construction and operation of a 50-MHz 64 x 16 bit fiber-optic bit-serial delay-line memory is described. It consists of LiNbO(3) directional coupler switches, fused-fiber couplers, and a 4.17-km fiber loop. It is a subsystem of a bit-serial optical computer under construction by our group. We discuss delay and clock source stability requirements for the long delay line in the face of a limited phase error tolerance. The reliability testing of the memory subsystem is described. The degradation of data in the memory loop as the phase error tolerance is exceeded by a small amount is studied through the temperature dependence of the memory loop. Data are presented for the memory-loop stability with respect to temperature variations. The memory subsystem design and construction is presented. The results of these experiments support the feasibility of a 100-MHz 128 x 16 bit memory.

Switch energy analysis of quantum interference directional couplers

Electronic quantum interference (QI) directional coupler switches are analyzed in terms of switching voltages, energies, and speeds and shown to be superior to present electronic switches. Comparisons with thermal and quantum limits are carried out. It is shown that the QI directional coupler (and smaller devices) is superior to present electronic devices in regard to switch energy: the latter have E/sub SW/ on the order of 60000 eV compared to the approximately=4 eV derived for the QI directional coupler.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dynamic switching characteristics of a 4×4 InP/InGaAsP matrix switch

High-speed switching capabilities are demonstrated for an InP/InGaAsP optical injection-type directional coupler switch. Equal rise and fall times of 2 ns are achieved at low switching currents of only 10 mA. Modulation of the switch with pulses between 200 Mbit/s and 400 Mbit/ shows its suitability for asynchronous transfer mode (ATM) switching systems.

New microwave components with inductively coupled superconducting striplines

A theoretical study is made of the inductive coupling of two superconducting striplines. The study takes advantage of the kinetic inductance, which is remarkable in superconducting films where the thickness is less than the London penetration depth. In the case of weak coupling where the coupled-mode description is most effective, it has been found that the forward-wave coupling leads to power exchange between two striplines with the efficiency of m= kappa 2/( kappa 2+ Delta 2) at a distance of I= pi /2( kappa 2+ Delta 2)12/. Here kappa 2= beta 1 beta 2M2/4L1L2 is the coupling coefficient and delta =(1/2)(B1-B2) is the phase mismatch parameter, where beta i, Li (i=1,2) and M are propagation constants, self-inductances and the mutual inductance of the two striplines, respectively. Numerical studies are also made for new types of kinetic inductance devices for microwave components, i.e. a directional coupler and microwave switch using the scattering coefficients in the general case.