Crosstalk of passive directional couplers

Crosstalk in directional couplers (DC's) is an important parameter for device and system performance. Although various crosstalk mechanisms have been studied through the course of integrated optics, the fundamental lower limit for crosstalk is still uncertain. In this paper, crosstalk mechanisms are reviewed and fundamental limits are discussed. A new model for optical scattering is presented, and the effect of bending loss variation between the two propagating modes in the taper region on the crosstalk is discussed     

Limiting characteristics of class II nonsymmetric stepped directional couplers

Limiting characteristics of class II nonsymmetric stepped directional couplers formed by means of cascade connection of alternating sections of coupled and uncoupled transmission lines having different lengths are studied. Fundamental regularities in the distribution of electrodynamic and geometric parameters of m-step structures with m → ∞ are first determined.     

Switching characteristics of directional couplers with alongitudinal electrode gap

The effect of passive phase shift regions on the performance characteristics of parallel waveguide couplers is considered. In particular, a longitudinal electrode gap in addition to phase shifts in the adiabatic input and output taper regions is considered. Both types of phase shifts have a degrading impact on the accessibility of some switching states. However, an electrode gap combined with taper region phase shifts may be beneficial for certain traveling wave electrode configurations. For a certain combination of gap and electrode lengths a through state with better than 30 dB crosstalk over an extended voltage range has been identified     

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.     

Wavelength sensitivity in directional couplers

Measured and calculated crosstalk ratios as a function of wavelength are compared for proton exchanged and annealed LiNbO₃ directional couplers at λ=1550 nm. The analytical technique employed models the experimentally determined wavelength sensitivities reasonably well. The analytical technique is further used to predict comparable functions for titanium indiffused couplers, and these results are compared to other calculations taken from the literature. No significant difference in sensitivity is noted between the two waveguide forming 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.     

Interdigitated coplanar directional couplers

A new coplanar coupler is presented which is suitable both for hybrid and monolithic i.c. applications. Design charts deduced from a theoretical analysis are given and experirmental results concerning X- and J-band quadrature hybrids manufactured on alumina substrates are presented and discussed.     

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 calculation of switching power of symmetric and asymmetric nonlinear directional couplers with variable coupling coefficient

It is proposed that the switching power of symmetric one-coupling-length nonlinear directional couplers (NLDC) with variable coupling coefficient (VCC) can be calculated as the weighted average of the local critical power of the coupler. Further, it is shown that the switching characteristics of a one-coupling-length VCC NLDC are determined mainly by the first half of the coupler and the switching power of an asymmetric VCC NLDC can be calculated as the weighted average of the local critical power over the first half of the coupler. Both symmetric and asymmetric examples are presented.     

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.     

Scattering induced crosstalk in active directional couplers

Scattering from defects in an integrated optical device causes phase randomization and depolarization. Scattered light can be recaptured by the waveguides of the device and perturbs modal fields. This leads to crosstalk in directional coupler switches. A defect-scattering-induced crosstalk model is developed and crosstalk in one-, two-, and three-electrode directional couplers is investigated with the model. The number of independent electrode voltages needed to tune out crosstalk is studied. Simulations show that scattering-induced crosstalk can be tuned out completely in active directional couplers with two independent electrode voltages. When modal differential loss and unequal taper coupling are taken into account, two independent electrode voltages are insufficient to tune out the crosstalk, whereas three independent electrode voltages are sufficient. This agrees with the conclusion from previous three-electrode directional coupler experiments     

Power switching in hybrid coherent couplers

We report on a theoretical and numerical investigation of the switching of power in new hybrid models of nonlinear coherent couplers consisting of optical slab waveguides with various orders of nonlinearity. The first model consists of two guides with second-order instead of the usual third-order susceptibilities as typified by the Jensen coupler. This second-order system is shown to have a power self-trapping transition at a critical power greater than the third-order susceptibility coupler. Next, we consider a mixed coupler composed of a second-order guide coupled to a third-order guide and show that, although it does not display a rigorous self-trapping transition, for a particular choice of parameters it does show a fairly abrupt trapping of power at a lower power than in the third-order coupler. By coupling this mixed nonlinear pair to a third, purely linear guide, the power trapping can be brought to even lower levels and in this way a satisfactory switching profile can be achieved at less than one sixth the input power needed in the Jensen coupler     

Design and performance of groove-guide directional couplers

An approximate closed-form expression for the calculation of the coupling properties of groove-guide directional couplers is deduced from an exact analysis and confirmed by measurements.     

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     

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     

Design data for interdigital directional couplers

The letter presents graphical information relating the performance of interdigital microstrip directional couplers to the geometrical parameters of the circuit. The theoretical background is summarised, and a comparison is made with reported experimental results.     

Transmission-line directional couplers for impedance transforming

Normalized scattering parameters of a two-port circuit terminated in arbitrary impedances are derived and design equations of directional couplers for impedance transforming are derived. To verify the design equations, a microstrip directional coupler is designed at a center frequency of 2GHz, fabricated on a substrate (/spl epsiv//sub r/=3.5,H= 30mil, tan/spl delta/= 0.04) and measured. The measured performance shows good agreement with the simulated one.     

平行耦合线定向耦合器的设计

阐述了平行耦合线定向耦合器的工作原理和设计过程.根据耦合微带线和耦合带状线的主要特征,分别设计了频率为2.5GHz的平行耦合微带线定向耦合器和平行耦合带状线定向耦合器.根据给定耦合器的技术指标,确定耦合器的类型、结构.利用ADS软件环境设计了平行耦合线定向耦合器的电路模型,并对定向耦合器的S参数进行仿真、优化,已达到预期的设计要求.由仿真结果可以看出,在频带范围内,耦合带状线定向耦合器的耦合性能优于耦合微带线定向耦合器.     

Groove-guide directional couplers with improved bandwidth

An exact analysis of the coupling properties of uniformly coupled groove guides with unequal phase constants is developed and confirmed with measurements. The results are applied to the construction of directional couplers with considerably improved bandwidth.