Bragg reflection waveguide composite structures

The Bragg reflection waveguide directional coupler is analyzed rigorously in terms of the normal modes of the complete structure. A perturbation analysis leading to an explicit expression for the coupling constant in a general configuration with nearly degenerate modes is performed. The results are compared to those for a conventional directional coupler structure. It is shown that the bandlike dispersion diagram arising from the periodic lateral confining media is responsible for the unique features of the coupling constant and coupling length that are observed     

Non-evanescent adiabatic directional coupler

A directional coupling mechanism based on an adiabatic coupling between three optical modes is suggested. The optical power transfer between two waveguides which are far apart is mediated by adiabatic coupling between zero-order optical modes of the individual waveguides and a high-order intermediate mode. The analytical model for an adiabatic three-mode coupling based on a scalar wave equation is presented. The directional coupling via the adiabatic mode coupling between copropagating modes is described and compared with a nonadiabatic directional coupling assisted by periodic perturbation. It is shown that adiabatic directional coupling has much less sensitivity to the mode parameters and to the wavelength     

Modélisation par méthode de propagation de faisceau à partir d’un coupleur à champ évanescent

We present numerical results obtained by 3D-BPM concerning evanescent field coupling in monomode fibres. At first we study the coupling between a monomode fibre and a mode sink. We show the existence or the absence of guided modes. BPM, is also applied to the study of directional couplers consisting of two fibres separated by an index liquid. The transmission coefficient is computed for various geometries and the electric field intensity is visualised by plotting isovalues.     

Analysis of optical fiber directional coupling based on theHE11 modes. I. The identical-core case

For pt.II see ibid., vol.8, no.6, p.832-7 (1990). The directional coupling between two identical single-mode optical fiber cores is analyzed by using the coupled-mode theory in the vectorial form based on the exact HE₁₁ modes. Analytical expressions for the coupling coefficients and the butt coupling coefficient appearing in the conventional and the recently formulated new coupled-mode equations are presented. The effect of the different polarizations on the coupling is considered. The accuracy of the conventional and new theories as applied to the two-core system is examined by comparing the coupling coefficients with exact numerical values. It is shown that as long as the exact HE₁₁ modes are used, the coupled-mode calculations behave well in predicting the coupling strength beyond the weakly guiding regime, and that the simpler conventional theory can provide satisfactory results when applied in the vectorial form     

Coupling of Degenerate Modes on Curved Dielectric Slab Sections and Application to Directional Couplers

Approximate expressions are derived for the coupling of degenerate modes on two curved dielectric slab sections. From this analysis a directional coupler is designed in which a finite length coupler is joined to terminal lengths via curved structure sections. The reverse coupling (directivity) and reflection, as well as corrections to the coupling length, are studied. The propagation characteristics and the reflection coefficient due to coupling, as well as the correct 3-dB coupling length are calculated, numerically. Second order effects, that determine the band-width as well as the coupling, have been considered and found to be very substantial. In the examples considered the reflection and directivity due to the coupling process were both more than 35 dB down, and the 3-dB outputs were exactly in quadrature, correct to the first order of approximation.     

Novel acoustic-surface-wave directional coupler with diverse applications

An acoustic-surface-wave directional coupler is proposed that may be used on materials having a large electromechanical coupling constant to couple two acoustic beams which are many wavelengths wide. The normal modes of the coupler are identified and used with an equivalent-circuit representation of the device to predict the length of the structure needed for complete energy transfer. The theory indicates that the device can have an intrinsic bandwidth near to 100%.     

Ultrashort 2-D photonic crystal directional couplers

A novel approach to reduce the coupling length and to increase the coupling coefficient in waveguide directional couplers implemented on two-dimensional (2-D) photonic crystals (PhC) is proposed. To increase the coupling coefficient of a 2-D PhC directional coupler, which is composed of two parallel PhC waveguides separated by one or several rows of rods, the radius of the rods between both waveguides is reduced. The application of this feature to ultrasmall channel interleavers and wavelength demultiplexers is addressed.     

三分支可变阻抗任意分贝集总元件定向耦合器

通过奇偶模分析方法,得到了可变阻抗任意分贝二分支集总元件定向耦合器的设计公式,并给出了8种拓扑结构,然后通过级联得到了三分支可变阻抗任意分贝集总元件定向耦合器的设计公式及拓扑结构.经ADS仿真验证,设计所得结果与仿真结果完全一致,隔离度和反射都在20 dB以上,(3±0.5)dB带宽达到了20 MHz.     

The design of mode evolution couplers

The design of an optical switch based on a directional coupler with tapered velocity and tapered coupling is investigated. The physical principle is based on power transfer by the evolution of a single normal mode. We consider cases where the coupling coefficient and guide mismatch variation are determined by optimized shape functions related to rectangle, raised cosine, Hamming, and Blackman window functions. Following simplified analysis by coupled mode theory, the physical parameters of the device are determined and the predicted performance is verified by the method of lines. Significant improvement in sidelobe levels and coupling length ran be obtained by careful design     

Switching dynamics of short optical pulses in a nonlineardirectional coupler

It has been shown recently that the propagation of short optical pulses in a nonlinear directional coupler consisting of two parallel waveguides or fibers should be described more generally by a pair of coupled nonlinear equations that take into account the dispersion properties of the coupling coefficient. In this paper, we use the Fourier series analysis method to solve these equations, and study the effects of a dispersive coupling coefficient on the switching dynamics of short optical pulses in a nonlinear directional coupler. We demonstrate, with numerical examples, that a dispersive coupling coefficient could cause significant pulse distortion and affect the threshold switching intensity. The effects of introducing frequency chirps in the input pulses are also discussed     

一种Q波段多孔耦合型定向耦合器

论述了一种宽带多孔耦合型定向耦合器,工作频段中心频率为40 GHz,相对带宽50%,拟用于射电望远镜Q波段接收机系统。该定向耦合器为一款结构紧凑的器件,采用对称的等间距多孔耦合形式,将主通道圆波导TE11模式信号耦合到耦合通道的矩形波导中,实现了小于-20 dB 耦合特性。为了对设计进行验证,对该定向耦合器进行了加工和测试。测试结果表明,在30~50 GHz 工作频段内,主通道反射系数小于-24.2 dB,且全频段内的端口耦合度保持在-20~-21 dB 之间。仿真与测试结果具有较高的吻合度,进一步证实了该设计可用于射电天文望远镜的Q波段接收机系统。     

Intermodal coupling by periodic microbending in dual-corefibers-comparison of experiment and theory

Grating coupling (caused by periodic microbending) between the quasi-even and quasi-odd normal modes of dual-core fibers is treated, and an accurate design formula is derived for the coupling coefficient. An analytical form for the intrinsic coupling coefficient of two dissimilar cores is also given. Good agreement is found between theory and experimental data on fibers perturbed either by acoustic flexural waves or fixed mechanical gratings. The paper provides useful design rules for dual-core fiber devices     

An optical filter of adjustable finesse using an amplified fiberring resonator

We consider an optical filter constructed from a simple ring resonator with two electro-optical directional couplers and a fiber amplifier in the ring. By adjusting the coupling coefficient of the electro-optical directional couplers, the finesse of this optical filter can be controlled over a specified range. The variation in the insertion loss of the optical filter can be overcome by adjusting the gain of the fiber amplifier simultaneously. This paper analyzes theoretically the performance of the optical filter, as a function of the coupling coefficient of the electro-optical directional coupler and the gain of the fiber amplifier. We also show that it is possible to design an optical filter of wide adjustable finesse by changing only the coupling coefficient of the electro-optical directional couplers, leaving the gain of the fiber amplifier unchanged. This simpler method results only in a slight change in the insertion loss if an appropriate condition is chosen. The filter will be useful in coherent optical systems, where the bandwidth of the optical filter needs to be appropriately adjusted under varying biasing and signaling conditions, or as an optical discriminator with an adjustable Q-factor     

Analysis of optical fiber directional coupling based on theHE11 modes. II. The nonidentical-core case

For pt.I see ibid., vol.8, no.6, p.823-31 (1990). The analysis of the directional coupling between two single-mode optical fiber cores based on the exact HE₁₁ modes is extended to the cases involving nonidentical fibers. The coupled-mode theory in the vectorial form is used, and analytical expressions for the coupling coefficients and the butt coupling coefficient appearing in the conventional and the coupled-mode equations are presented. The accuracy of the conventional and new theories as applied to the two-core system is examined by comparing the coupled-mode predictions with the exact numerical values for different core-radius ratios, waveguiding strengths, polarization states, and core separations. It is shown that the errors in the coupled-mode theories increase as the two cores become more dissimilar. As long as the dissimilarity between the cores is kept small, the coupled-mode calculations using the HE₁₁ modes in predicting the coupling strength can be of satisfactory accuracy even when the individual guides are not weakly guiding ones. It is found that the new theory may give relatively large errors in the touching-core case with distinctly different core radii     

Scalar modes and coupling characteristics of eight-port waveguidecouplers

The scalar modes and the coupling characteristics of an eight-port waveguide coupler consisting of four parallel single-mode waveguides are discussed. The analysis is based on a rectangular waveguide model which has been known to give accurate results in the case of two waveguide directional couplers. A classification of its various scalar modes is also given     

On a Class of Multiple-Line Directional Couplers

Multiple-line directional couplers that utilize only two linearly independent modes of propagation are possible, provided certain restrictions on the maximum coupling are not exceeded. This paper discusses a class of multiple-line directional couplers which may be considered as a generalization of the familiar double-stub four-port directional coupler. The basic design relations for the symmetrical case are developed, and frequency behavior is investigated. Experimental results for an L-band six-port hybrid junction are presented and compared with theoretical curves. Limitations on maximum coupling and fabrication problems will probably confine the number of lines in practical circuits to a small value. Bandwidths of couplers of this class tend to become narrower for the same total coupling off the main line as the number of lines is increased; however, standard techniques for broadbanding are applicable. Experimental results on the six-port hybrid junction agreed well with theory, and the circuit proved to be relatively compact.     

超薄金属膜V-型槽等离子波导的定向耦合研究

分析了在 1 550 nm 波长下超薄金属膜(金膜厚t=10 nm)V-型槽等离子波导间长程沟道等离激元导模的定向耦合。通过计算不同波导间距下的模式分布,得出了定向耦合器中奇、偶模有效折射率实部和传播长度随波导间距的变化情况,并进一步计算了相邻波导间的耦合长度、最大串扰与波导间距的关系曲线。计算结果表明:在波导间距较小时,耦合长度小于各模式的传播长度,随着波导间距的增加,耦合长度随之增加,最大串扰随之减小。对超薄金属膜V-型槽等离子波导的定向耦研究在集成光路的实际应用中具有重要的价值。     

Effectively nonreciprocal evanescent-wave optical-fibre directional coupler

An effectively nonreciprocal directional coupler has been demonstrated using evanescent coupling between a single-mode and a multimode optical fibre. At least 17 dB difference in coupling efficiency between transmit and receive branches has been measured. The near-field mode patterns show coupling between the single-mode fibre and a few high-order modes of the multimode fibre.     

Mode Selective Coaxial Directional Couplers for Harmonic Measurements

Bethe's small-hole coupling theory is applied to coaxial transmission line to design mode selective directional couplers. By independently sampling the TEM and TE11 modes in coaxial line, harmonic measurements can be performed which would require five or more directional couplers if made in rectangular waveguide. Mode selectivity on the order of 30 dB has been achieved.     

Experimental Demonstration of a Linearized Polymeric Directional Coupler Modulator

A linearized directional coupler modulator based on electrooptic polymer was fabricated and tested. The linearity was obtained by tailoring the coupling coefficient of the device using the photobleaching technique. A two-tone test of the device was presented and it demonstrated an enhancement in intermodulation distortion compared to a Mach-Zehnder modulator.