On grating-coupled radiation from waveguides

We show that grating coupled radiation from a waveguide is a very sensitive oscillatory function of waveguide thickness when substrate and guide indices differ greatly. Because of this phenomenon, computed results are strongly dependent on the choice of waveguide geometry used to calculate the unperturbed modes.     

Analysis of grating-coupled radiation in GaAs:GaAlAs lasers and waveguides

Grating-coupled radiation in GaAs:GaAlAs lasers and waveguides is analyzed. A general formulation is developed for arbitrary-shaped gratings which need not be small in size. Two methods are used to solve the resulting equations in the case of rectangular-shaped gratings. The first is a perturbation technique and the second is iterative in nature. The iterative procedure converges to a numerical exact solution in many cases of practical interest and indicates that the perturbation results are quite accurate. Curves are presented for radiated power from traveling waves as a function of grating tooth height, tooth width, refractive index, waveguide thickness, and period for rectangular gratings in heterostructure waveguiding geometries. It is shown that radiation is not a monotonically increasing function of tooth height, but rather maxima occur when the teeth are half the optical wavelength in the material. Also, in particular geometries with an air:GaAs grating interface, radiated power of a mode can exceed 100 cm^-1.     

Ultrafast all-optical modulation of infrared radiation viametal-semiconductor waveguide structures

We present a novel optical-optical semiconductor switching technique for application to infrared laser beam modulation and ultrashort infrared laser pulse switching. This method relies on the ultrafast optical excitation, with femtosecond above-bandgap laser radiation, of an air-filled metal-clad semiconductor waveguide. Guided electromagnetic wave analysis combined with time-varying dielectric properties of the semiconductor layer are used to investigate the ultrafast switching speed of the structure. The device is capable of modulation at various infrared wavelengths. In particular, we investigate intensity modulation of the quasi-TE₁₀ mode for 10.6-μm laser radiation. At an electron-hole photoinjection density of ~1.8×10¹⁸ cm^-3, an extinction ratio of 83 dB is demonstrated. This ratio is significantly higher than that exhibited by current optical-optical semiconductor switches. Potential applications to all-optical Mach-Zehnder metal-clad semiconductor modulators and self-limiting switches are also discussed     

Analysis of grating-coupled radiation in GaAs:GaAlAs lasers and waveguides--II: Blazing effects

Grating-coupled radiation in lasers and waveguides is analyzed for trapezoidal-shaped grating teeth which includes all triangular shapes. A perturbation technique is used which involves calculating the modes in a geometry with an inhomogeneous refractive index profile in the grating region. Curves are presented for radiated power from traveling waves as a function of blazing angle, grating tooth height, refractive index, and grating period. It is shown that the ratio of power radiated into the outer media depends greatly on tooth shape and size. By judiciously selecting parameters this ratio can exceed 50:1.     

Slotted waveguide directional coupler characteristics

A moment method (MoM) solution for slotted waveguide directional couplers which is sufficiently accurate for predicting the couplers' characteristics is demonstrated. The slot length in the waveguide directional couplers considered is about four wavelengths and the wall thickness is about 0.27 λ. To simplify the numerical computation, the formulation assumes a uniform field across the slot aperture and utilizes Galerkin's method with the entire domain sinusoidal basis. A self-consistent method taking the finite wall thickness into account avoids an additional matrix equation, as in other MoM formulations. The final solution forms the basis for an efficient computer code applicable to dielectric-loaded slotted waveguide directional couplers. The uniform field assumption restricts the present solution to narrow slots. Numerical results verify, by comparisons with published analytical and experimental data, that this solution is valid for slot lengths as long as 4λ. A detailed discussion of the characteristics of narrow-wall and broad-wall slotted waveguide directional couplers is presented     

Spontaneous emission coupling to radiation and guided modes ofplanar waveguide structures

A general analysis of spontaneous emission in the vicinity of a symmetric slab dielectric waveguide is presented using explicit energy-normalized functions for the complete set of bound and radiating TE and TM modes. Expressions for the spontaneous emission rates are derived for each mode type, and calculated results are presented illustrating the emission behavior as a function of the position of the radiating atom, for various slab waveguides. The model is used to analyze the spontaneous emission coupling efficiency to guided modes in planar erbium-doped amplifier structures. Spontaneous emission factors for the TE and TM modes are calculated for the cases of erbium doping of the core and of the cladding     

Low-loss fibre-coupled waveguide directional coupler modulator

We report a fibre-coupled Ti: LiNbO3 directional coupler modulator with total fibre-device-fibre insertion loss as low as 1.5 dB for ?=1.32 ?m. Low loss has been achieved by using diffusion parameters to give good mode match to the fibre and low propagation loss, an effective SiO2 buffer layer to eliminate electrode loading loss and a directional coupler geometry which minimises the effect of bend loss.     

The Bragg reflection waveguide directional coupler

A directional coupler formed by two Bragg reflection waveguides (BRWs) with a common finite periodic region between them is analyzed. The coupling coefficient is a sensitive function of the index difference in the periodic coupling section. This feature can lead to a novel kind of electrooptic switch. In addition, the BRW directional coupler exhibits dispersion properties characteristic of the periodic bounding media, which can be used to implement a narrowband wavelength filter and a TE polarizer     

多孔耦合型太赫兹波导定向耦合器的设计

设计了一种结构紧凑、工作频带较宽、耦合平稳、高方向性的十字形多孔耦合的太赫兹波导定向耦合器。基于多孔耦合原理,利用HFSS软件对太赫兹波导定向耦合器进行了模型仿真和结构优化。仿真结果表明:在325~475 GHz带宽范围内,该多孔耦合太赫兹波导定向耦合器耦合度达到7.50.8 dB,隔离度达到30 dB,即方向性优于20 dB,各端口回波损耗小于-20 dB。通过对该波导定向耦合器进行高温高压模拟仿真,确定了使用负性光刻胶SU-8作为结构材料的可行性,提出应用MEMS工艺在硅衬底上进行加工,将牺牲层工艺应用到波导腔结构的制作中。利用光刻在直通波导和耦合波导公共宽壁上形成的十字形等间距排列耦合孔结构,可以实现较宽的带宽和良好的耦合平坦度。该方法提高了耦合孔尺寸和位置的精度,减小了反射损耗,为太赫兹波导结构的加工提供了新思路。     

0.5THz波导双定向耦合器设计

介绍了0.5 THz波导双定向耦合器的原理分析及仿真设计.采用双排多孔等间距不等孔径的方案,设计了0.325～0.5 THz宽带波导双定向耦合器,并给出了仿真及测试曲线.测试结果显示,0.5 THz波导双定向耦合器在0.325～0.5 THz全波导带宽内耦合度为(7.6±1)dB,方向性大于18 dB,输入端口回波损耗...     

Photonic crystal line defect waveguide directional coupler

A three-port directional coupler based on photonic crystal line defect waveguides is demonstrated. The directional coupler has only a 21 μm long coupling region. Wavelength-selecting operation around 1.5 μm wavelength was confirmed     

Improved directional couplers for overmoded waveguide systems

The performance of multihole directional couplers can be considerably improved by placing a second hole structure to the existing one. The improvement may be aimed at directivity or at suppression of certain unwanted modes propagating either in forward or backward direction.     

Analysis of waveguide slot-based structures using wide-band equivalent-circuit model

Analysis of geometrically complicated waveguide-based slotted arrays and filters is performed using a simple equivalent-circuit model. First, the circuit parameters (inductance and capacitance) of a simple waveguide slot-coupler problem are obtained through moment-method (MoM) analysis. The values of the lumped LC elements are virtually constant over the frequency range of interest (the X-band) for specific waveguide and slot dimensions. Based on the equivalent-circuit model of a single slot of two coupled waveguides, more complicated structures are then analyzed, such as slot coupler arrays and slot-based waveguide filters. The scattering parameters of these structures are obtained through circuit analysis, and are verified using the MoM and finite-difference time-domain method. Excellent agreement is observed over a wide band of frequencies and is confirmed by experimental results.     

SOI矩形光波导定向耦合器的研究

提出一种利用有效折射率方法分析SOI矩形光波导定向耦合器的理论,并且取得了与精确解非常吻合的结果,最后得出了一些有用的结论,对分析和设计光波导定向耦合器有一定指导意义.     

Resonant grating waveguide structures

Under certain conditions, a resonance phenomenon can occur in waveguide grating structures. Such structures have multilayer configuration, the most basic of which is comprised of a substrate, a thin dielectric layer or semiconductor waveguide layer, and an additional transparent layer in which a grating is etched. When such a structure is illuminated with an incident light beam, part of the beam is directly transmitted and part is diffracted and subsequently trapped in the waveguide layer. Some of the trapped light is then rediffracted outwards, so that it interferes destructively with the transmitted part of the light beam. At a specific wavelength and angular orientation of the incident beam, the structure “resonates”; namely, complete interference occurs and no light is transmitted. This paper reviews previous investigations on the resonance phenomena and presents analytic and numerical models for evaluating the resonance as a function of the geometric and optical parameters of the structures and incident radiation     

Mode selective directional couplers for overmoded waveguide systems

Two types of directional couplers for transverse electric (TE) modes are described: short and multihole couplers, respectively. They selectively pick one mode out of a mode mixture in an overmoded circular waveguide system. Unwanted modes are either statistically kept at low level or are suppressed by destructive interference in the coupling waveguide. Mode selectivity and directivity in multihole couplers oscillate up and down with an increasing number of holes, finally reaching a minimum of approximately 20 dB, unless there are competing modes with rational fractions of the beat wavelength. A multihole coupler for the TE₀₂ mode (28 GHz, 63.4 mm waveguide diameter, 41 holes) and a length of 1.6 m shows a calculated directivity of 68 dB and suppresses the unwanted modes TE₀₁ with 34 dB (24 dB), TE₂₂ with 37 dB (45 dB), and further modes TE_?m (?<5, m<6) with 17 dB to 34 dB in forward direction (figures in parentheses are for unwanted modes propagating in backward direction). A short directional coupler for the TE₀₁ mode (28 GHz, 63.4 mm waveguide diameter) with 16 holes and a length of 230 mm shows a directivity of 55 to 100 dB between 27.9 and 28.1 GHz, suppressing the TE₀₂ mode with 35 to 80 dB, the TE₀₃ mode with 30 to 65 dB, and the TE₂₂ mode with 30 to 70 dB.     

Modified 1×2 directional coupler waveguide modulator

A novel 1×2 Ti:LiNbO3 modified directional coupler modulator is demonstrated. The 3 dB operating point requires no bias voltage. The device has a lower switching voltage requirement than a conventional directional coupler.     

A new multiple-tuned six-port Riblet-type directional coupler in rectangular waveguide

Directional couplers are fundamental components for the realization of power splitting and combining networks. A basic component is the four-port coupler realized in a wide range of different electrical and geometrical configurations. Using multiport directional couplers, with six or more ports, compact power splitting and combining networks can be designed. Most of the multiport directional couplers in a rectangular waveguide found in literature use an E-plane branch-line layout. A special configuration of a six-port narrow-wall short-slot directional coupler (Riblet type) is presented here. The coupler has only one central coupling region, following the Riblet concept based on the differences among the propagation constants of the modes through the coupling region. A number of different solutions have been investigated and some examples are presented here. Moreover, a multituning realized by using input resonators permits one to significantly enlarge the bandwidth. A six-port coupler equally splitting on the output ports the power injected at each input port has been designed for a 8.4% working band. The theoretical and measured responses presented here prove the effectiveness of the multituning concept.     

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