Variational coupled-mode theory of optical couplers

A scalar coupled-mode theory is developed from the variational principle. The formulation is variational in the sense that the mode parameters are adjustable and can be optimized subject to the coupling between the guides. The theory is applied to slab and fiber couplers. In comparison with the conventional scalar coupled-mode theory in which only the amplitudes of the modes in the individual guides are adjustable, the variational scalar coupled-mode theory predicts more accurate propagation constants and field patterns of the normal modes of the couplers     

光方向耦合器汉明函数之波导结构影响

在本文章中研究光方向耦合器汉明函数之波导结构影响.传统的光方向耦合器之理论是由绝热的条件下所获得之结果,因此在波道耦合长度不夠长时则变得可疑,有鑑于此需要依靠光束模拟软体.在文章中有两个主要的指标:第一个为理论所适用的范围,第二个为最佳化低串扰、宽频宽和较短的耦合器长度.最后由结果得知我们所提出使用的汉明权重函数在此架构下能获得较佳的设计结果.     

Optical wavelength filter with tapered couplers

Optical wavelength filters based on tapered directional couplers are analyzed by nonorthogonal coupled-mode theory. The nonorthogonal coupled-mode formulation in general yields more accurate results than the conventional orthogonal coupled-mode theory. The focus is on the novel features of the tapered couplers used as wavelength filters. It is demonstrated that, in addition to reducing the crosstalk and suppressing the sidelobes, the tapered structure can also make the filtering response independent of the device length beyond a critical coupling length,.<>     

A scalar finite-difference time-domain approach to guided-wave optics

A finite-difference time-domain approach that solves the scalar wave equations is proposed and validated. The propagation, reflection, scattering, and radiation of electromagnetic waves in weakly guiding optical devices are described explicitly in the time domain. The method is applied to the simulation of guided-wave devices such as directional couplers and distributed feedback reflectors. A comparison to known analytical solutions shows good agreement.<>     

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.     

Space-wavelength tradeoff in the design of nonblockingdirectional-coupler-based networks under crosstalk constraint

Directional couplers are electrooptical switching devices that are capable of switching multiple wavelength signals. Once the state of a coupler is set up, optical signals can pass through the coupler with the rate of terabits per second. A directional coupler-based photonic switching system, however, suffers from the intrinsic crosstalk problem. This shortcoming has been the most limiting factor in building a large switching network of this kind. It has been shown that by using more hardware (couplers) we can reduce the crosstalk inside a switching system. In this paper, we explore the wavelength approach to achieve the same goal. We show that if two wavelengths are available for sending data, the number of couplers suffering the first-order crosstalk can be reduced to zero while the total number of couplers remains the same as in a network without any crosstalk constraint     

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     

田口稳健设计优化之绝热式光方向完全耦合器

本研究利用田口实验优化法配合光束传播法设计优化光方向完全耦合器 sin-square 函数,能夠达到低串扰与短耦合器长度的目标,在田口实验优化法中我们选择直交表 L9 做为我们的实验平台.研究结果显示,使用 BPM 辅助设计方式配合田口方法可有效的减少设计优化光方向完全耦合器的实验数.在实验过程中我们尝试去找出并设定符合低串扰与短耦合器长度的绝热式光方向耦合器 sin-square 函数的结构参数范围,并且在田口实验优化法中的确认实验可以达到95%的信心指数.在这份研究中,我们证实利用田口实验优化法配合 BPM 辅助设计优化光方向完全耦合器 sin-square 函数是确实可行的.     

作波分复用AWG同频串扰的相关特性及理论研究

本文讨论了光信号在阵列波导光栅（AWG）中同频光电流串扰的自相关函数,从理论上探讨了同频串扰的自相关系数,研究了相关系数与同频串扰引起延时的关系,及同频串扰噪声功率和串扰的极化方向的关系,得到了相关系数R_F与AWG延时时间τ、记录值n之间的关系曲线.数值计算表明:串扰延时使AWG的传输信号存在负相关、弱相关、正相关,其串扰延时对记录长度的影响呈周期性变化.     

基于正交定向耦合器组合的微波功率测量方法

定向耦合器广泛应用于微波系统的功率测量.但定向耦合器的方向隔离度有限,两个方向的耦合输出信号存在一定程度地串扰,影响功率测量的准确性.为了解决这一问题,本文提出一种正交耦合测量方法.该方法采用一对相位差为90°的定向耦合器同时测量功率,通过对测量结果的计算处理,可消除方向串扰的影响.推导了正反向传输功率与正交耦合器组合功率测量值的关系,分析了正交偏差所产生的误差.为了确定耦合器的正交状态,提出了一种准确测量两个定向耦合器之间相位差的方法.实测表明正交耦合方法可有效提高功率和驻波比测量的准确性.     

Single-mode fused biconical couplers for wavelength divisionmultiplexing with channel spacing between 100 nd 300 nm

The authors report on single-mode fused biconical couplers tailored to the requirements of wavelength-division-multiplexing systems. The wavelengths of the two channels can be chosen arbitrarily for wavelength separations between 100 and 30 nm. For separations between 150 and 300 nm, the couplers can be manufactured with an accuracy of ±5 nm at the two operating wavelengths. For decreasing wavelength separation of the channels, the influence of the state of polarization results in an increased inaccuracy of the two wavelengths, increased far-end crosstalk, and power fluctuations after the coupler. This sets a lower limit for channel separation. Good agreement between theory and experiment is achieved     

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     

Comparison between strong coupling theory and experiment forthree-arm directional couplers in Ti:LiNbO3

The derivation of scalar coupled mode theory for TEM waveguide arrays is examined. It is demonstrated that four previously published methods are identical at this level of approximation. The accuracy of the theory and its application to a slab-waveguide model of a three-arm coupler is considered. It is shown that the coupled mode equations require modification for slowly varying arrays. Analytic expressions for the matrix coefficients are presented, and it is demonstrated that device input-output transitions have significant effect on the response. The model is compared with experimental measurements of three-arm couplers fabricated in Ti:LiNbO₃, and excellent qualitative agreement is obtained     

Three-dimensional electromagnetic modeling of fiber-core effects onthe coupling characteristics of weakly fused tapered fiber-opticcouplers

Based on the full-wave formulations, which combine the finite element method add the boundary element technique, and the step-like approximation method, a three-dimensional (3-D) electromagnetic modeling approach is proposed in this work to investigate the fiber-core effects on the coupling characteristics of the weakly fused tapered couplers. We find the fiber cores have significant effects on the mode field patterns, coupling coefficients, and birefringence for the coupler with large normalized frequencies. The influence of the fiber cores on the coupling behavior for the real couplers, such as 3 dB power dividers, polarization beam splitters, and wavelength demultiplexing/multiplexing couplers, are also studied in 3-D view. It is found that the effects of fiber-cores in the weakly fused tapered couplers have to be considered for accurately modelling their coupling behavior, although the model with no core assumption can predict the trend of the coupling characteristics for the real couplers     

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     

Further evidence of strong coupling effects in three-arm Ti:LiNbO3 directional couplers

The switch characteristics of three-arm integrated optic directional couplers with both symmetric and antisymmetric drive voltages are examined. The symmetry properties of these devices, predicted by previous theory, are shown to be violated and the reasons for this are discussed in terms of scalar strong coupling coefficients normally neglected as small. It is also shown that the inclusion of modal overlaps introduces additional coupling terms and causes significant power redistribution in the bend sections. Experimental measurements for the antisymmetric configuration are presented, and it is shown that different electrode loading of the center and outer guides affects device operation     

Interference filter all-fibre directional coupler for w.d.m.

Directional couplers for wavelength division multiplexing using l.e.d.s as transmitters have been realised without imaging elements. Their total insertion loss (MUX + DEMUX) is of the order of 4 dB; their crosstalk attenuation and near-end crosstalk attenuation are 30 dB and >60 dB, respectively.     

A new approach to grating-assisted couplers

A novel approach to the analysis and design of grating-assisted directional couplers is proposed. Power exchange between the waveguides is maximized through phase matching of two power-orthogonal modes of the parallel coupler. It is shown that either complete power transfer or zero crosstalk can be achieved at two different coupling lengths even when the two wavelengths are strongly coupled and/or close to synchronism. An analytical solution to the coupled-mode equations is obtained for the grating of rectangular shape. A grating-assisted coupler made of two slab waveguides is examined as an example to illustrate the salient features of the scheme     

A multiple-scale analysis of grating-assisted couplers

Optical directional couplers with longitudinal periodic perturbations or gratings are analyzed by a multiple-scale solution to the coupled-mode equations. The use of two length scales in the analysis becomes the key to obtaining globally valid analytic solutions, which are shown to be in excellent agreement with the exact numerical solutions. Because of the nonorthogonality of the coupled modes in the structure, two different coupling lengths, a maximum power transfer length and a zero crosstalk length, are predicted for the power exchange. The spectral properties of the coupler are also studied through a generalized multiple scale method     

Development of compact and low-crosstalk PLC-WDM filters for hybrid-integrated bidirectional optical transceivers

This paper reports on the development of planar-lightwave-circuit wavelength-division-multiplexing (PLC-WDM) filters based on double polynomial curve directional couplers, which will be integrated into compact and low-cost bidirectional optical transceivers for FTTx systems. Silica-based PLC-WDM filters smaller than 5 mm/spl times/100 /spl mu/m for Ethernet passive optical network (E-PON) applications were designed and fabricated to have low bidirectional crosstalk (lower than -40 dB) as well as low insertion losses (lower than 1 dB) within the operating temperatures of -40 to 80/spl deg/C. Several features and design guides that can reduce the size of directional couplers and improve the level of bidirectional crosstalk are discussed.