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 from 64 GaAs/AlGaAs electro-optic directional couplers using a simplified tree architecture. The devices have a chip size of 26.5*3 mm/sup 2/ and a minimum total loss (excluding coupling loss) of 8.7 dB.<>     

Improved tolerance in optical directional couplers

Tolerance requirements in conventional synchronous-mode directional couplers are examined. It is proposed that the tolerance situation can be improved considerably by tapering one, or both, of the propagation coefficients, so that they intersect at one position. This model is examined numerically, and suggestions are made for the practical realisation of a device.     

The crosstalk in three-waveguide optical directional couplers

An expression for the crosstalk in three-waveguide directional couplers is derived. The crosstalk is found to be due to the uneven spacing in the distribution of the propagation constants of the three eigenmodes of the couplers. Similar to the crosstalk in the two-waveguide directional couplers, it is also proportional to the square of the field overlap of the two neighboring waveguides. This feature seems to be universal in the straight couplers.     

Broad-band optical directional couplers and polarization splitters

A method is described for designing broadband optical directional couplers. The basic idea is to determine an optimum distance between the waveguides where wavelength dependence of the coupling coefficient vanishes at a given wavelength. The method is applied to design directional coupler polarization splitters. As an example the computer design of a polarization splitter made of single-mode twin-core optical fiber is presented. The extinction ratio obtained for waves with undesired direction of polarization is better than -20 dB within a wavelength range of 80 nm. The sensitivity of the design to variations of the main parameters caused by the production process is discussed for characteristic examples     

Photoelastic optical directional couplers in epitaxial GaAs layers

Photoelastic channel optical waveguides showing strong lateral confinement at ?=1.55 ?m have been fabricated by etching a narrow slot through a Au Schottky contact deposited on an n/n+ GaAs layer. Such structures allow electro-optic directional coupler switches of short coupling length (~2 mm) to be easily realised.     

Broad-band high-silica optical waveguide star coupler withasymmetric directional couplers

A broadband optical waveguide star coupler with asymmetric directional couplers is proposed. A coupling or splitting ratio of the coupler is flattened with respect to wavelength by choosing appropriate values for Δβ and complete power transfer length. The wavelength characteristics of uniform and alternating Δ&thetas;b directional couplers are investigated, and their broadband operation is shown theoretically. To demonstrate the feasibility of the new proposal, 8×8 star couplers composed of such asymmetric directional couplers are fabricated by using high-silica waveguides on a silicon chip. A star coupler composed of asymmetric directional couplers exhibited 2-dB loss variation in the wavelength range of 1.3-1.55 μm, and this value was almost one fourth of that of a star coupler composed of symmetric directional couplers     

Novel method for making single-mode optical fibre directional couplers

An optical single-mode directional fibre coupler was made from two lengths of fibre of approximately semicircular cross-section. These new fibres were drawn from a preform ground parallel to the core axis. The coupling factor was measured as a function of wavelength. It is shown that the coupling factor can be influenced by bending the coupler.     

Microwave balanced-mixer circuits using combline directional couplers

The use of 180° combline directional couplers in strip transmission line mixer circuits enables completely planar single and dual balanced devices to be produced. Results are presented for an X-band single-balanced mixer in triplate. A circuit is described for a planar dual balanced mixer.     

Analysis of optical directional couplers that include gain or loss and their application to semiconductor slab dielectric guides

The effect of gain or loss in one arm of a pair of parallel coupled optical guides has been investigated theoretically. The general transfer characteristics have been derived for the weakly coupled case in terms of the individual waveguide characteristics and the coupling coefficient. Examples of strongly coupled dielectric slab guides, appropriate for embodiment in the form of semiconductor heterostructures, have also been examined. Only small deviations from the weakly coupled behavior are found, even at maximum coupling when the core layers come into contact. In particular the interesting and useful condition where the two composite modes of the system become degenerate, with identical phase, attenuation, and field distribution characteristics, still occurs. Applications for integrated optics are discussed.     

Analysis of grating-assisted directional couplers using theFloquet-Bloch theory

The Floquet-Bloch theory is used to develop a theory for grating-assisted directional couplers which predicts the coupled power and coupling lengths and is applicable to lossy waveguides. This theory views grating-assisted directional couplers as conceptually similar to conventional synchronous (nongrating) couplers. In the Floquet-Bloch analysis of the directional coupler, it is necessary to include both proper and improper space harmonics. The determination of which space harmonics are improper is critical to the understanding of the coupler performance. The choice of the improper space harmonics used for the analysis of the coupler is different from that used in contemporary papers     

Multilayer MMIC directional couplers using thin dielectric layers

Low-loss and small-sized MMIC directional couplers utilizing a multilayer structure composed of coupled thin-film transmission lines on a GaAs wafer surface are newly proposed. The fundamental characteristics of the couplers are discussed through calculations by numerical analysis, and the performance of the couplers and an application to reverse-phase hybrid ring are demonstrated. The results show that a 3 dB coupler can be designed within a 0.8 mm×0.8 mm area for a center frequency of 20 GHz. Coupling losses of 3.7 dB±0.2 dB over a 4-GHz bandwidth and isolation of better than 26 dB in the frequency range of 0-30 GHz are achieved. The proposed coupler configurations can be applied to the high-density and multifunction integration of MMIC's     

High-directivity microstrip directional couplers

The letter describes a simple method for improving the directivity of side-coupled microstrip couplers an unmetallised piece of the substrate material is cemented on top of the microstrip structure. The measured directivity ranges from 30 to 40 dB. The couplers are fully compatible with normal microstrip techniques.     

Multi-combline warped-mode directional couplers

A first report of multi-combline warped-mode directional couplers using three lines is presented. The technique outlined can be extended to many different designs and additional lines. The specific example given has been designed as a 3-way equal power splitter with a phase difference of 180° and 0° between the two outer lines and the central line. The coupler is fed from the central line at the input. The device exhibits significant coupling over the range 0.7 to 9 GHz, but with ripples at the lower frequencies and loss at the higher frequencies the most useful range is about 3 to 8 GHz.     

Improved design of multihole directional couplers using aniterative technique

In the design, the widely used first-order polynomial representation of the frequency response of a multihole direction coupler is used. The roots of this polynomial are displaced iteratively until a desired response is achieved. One possible outcome is a Chebyshev response, but the method is capable of improving on that result if all portions of the passband are not equally important. Further improvement can be achieved if the directivity is made to ripple around a somewhat higher level. This causes a minor sacrifice in bandwidth. Examples are given     

Novel directional couplers using broadside-coupled coplanarwaveguides for double-sided printed antennas

Two types of tight-coupling, broadband directional couplers employing open broadside-coupled coplanar waveguides (BC CPWs) are proposed which can easily be integrated with printed strip antennas. The first type uses the BC CPWs with dielectric overlays above and below the center substrate, while the second type uses the nonuniform BC CPWs. The scattering parameters of these directional couplers are derived with the even-odd mode analysis method based on the quasi-TEM wave approximation. Two 3-dB directional couplers, one for each type, are designed and measured characteristics are compared with theoretical results     

Radiation loss of grating-assisted directional couplers using the Floquet-Bloch theory

The Floquet-Bloch theory (FBT) is used to determine the radiation loss of a grating-assisted directional coupler. Improper waves obtained by FBT represent leakage and produce radiation loss along the transverse direction. By summing the fields of all leaky harmonics, the power flow radiated into the superstrate and substrate can be calculated from Poynting's theorem. However, the locations at which these summations occur impact the accuracy. An analytic formula for radiation loss is also derived. Results from both the FBT and the analytic formula are compared with results obtained using the coupled-mode theory.     

Optimal design of grating-assisted directional couplers

In this paper, a rigorous leaky mode propagation method has been used to investigate the influence of the grating period and grating index profile on the design of grating-assisted directional couplers (GADC's). A detailed explanation of resonance condition and radiation loss in terms of electromagnetic field contribution in the grating region as a function of the grating period and profile is given. Optimal design parameters hare been found for well-defined structures in order to achieve either minimum coupling length or maximum coupling efficiency. A very fast method to extract the resonance condition in any grating-assisted structure by using a sinusoidal profile is proposed. Numerical results are presented for both moderately and strongly asymmetric structures in terms of normalized propagation constant, mode radiation loss, coupling length and coupling efficiency. Comparisons with grating period and coupling length predictions obtained by other methods are also shown. The rectangular profile with optimized duty cycle has been demonstrated to be the best choice in order to minimize the GADC coupling length     

Proposed semiconductor narrow-band wavelength filter using directional coupler

A GaInAsP/InP waveguide narrow-band wavelength filter is proposed and analysed. The proposed wavelength filter is based on an asymmetric semiconductor directional coupler having a step-index waveguide and a double clad waveguide. It is numerically shown that a bandwidth as narrow as 5.4 nm is feasible for 1.3 mm long devices at a 1.53 mu m centre wavelength.<>