Mode Coupling And Power Transfer In A Coaxial Sector Waveguide With A Sector Angle Taper

We report a theoretical study of mode coupling and power transfer in a coaxial sector taper. The power transferred from the desired TE /sub 01/ mode into other propagating modes is calculated as a function of taper length and operating frequency. Power transfer via mode coupling involves at least three other modes: TE/sub 21/, TE/sub22/, and TM/sub21/. Power transfer as a function of final sector angle is also shown. At sector angles greater than 180° the taper is highly over-moded. This type of waveguide taper is utilized to feed a wide-band input coupler for gyrotron traveling wave amplifiers.     

Modal Inversion in Circular Waveguides--- Part I: Theory and Phenomenology

Part I of this work investigates the phenomenon of mode reordering in circular waveguides containing one or more dielectrics. Specifically, conditions are established, be they ranges of dimensions, dielectric constants, or frequency, under which the sequence, TM/sub 01/, TE/sub 01/, TE/sub 11/, and TE/sub 21/ is obtained instead of the conventional TE/sub 11/, TM/sub 01/, TE/sub 21/, and TE/sub 01/. The desired effect is the reversal of the order of propagation between the normally dominant TE/sub 11/ mode and the circularly symmetric low-loss TE/sub 01/. Bandwidth regimes are drawn and the credibility of the dielectric model is examined and found to be satisfactory for the application of the phenomenon to the design of digital ferrite phase shifters operating in the TE/sub 01/ mode.     

Ridge-Shaped Narrow Wall Directional Coupler Using TE/sub 10/, TE/sub 20/ , and TE/sub 30/ Modes

A new type of compact narrow wall directional coupler, whose coupling region is a ridge waveguide having dimensions such that TE/sub 10/, TE/sub 20/, and TE/sub 30/ modes can exist, is proposed and analyzed. Coupling ratio and nonreflecting condition are determined by the degree of interference between TE/sub 20/ and TE/sub 30/ modes, and between TE/sub 10/ and TE/sub 30/ modes, respectively. The coupler can be used as a variable power divider by varying the coupling slot height. For 3-dB coupling over the 16.0 to 19.0-GHz frequency range, power equality within /spl plusmn/0.5 dB, more than 30-dB isolation and more than 30-dB return loss can be obtained. Experimental coupling results have shown good agreement with the theory.     

Influence of the fiber index profile on vectorial fiber modes and application to tapered fiber devices

The correspondence between the scalar modes LP and the nearly degenerate true vectorial modes in circular two-layer fibers is well established. For example, LP/sub 21/ corresponds to a combination of EH/sub 11/ and HE/sub 31/, while LP/sub 02/ is composed only of HE/sub 12/. For the multilayer fibers, inversion of the correspondences for certain modes can occur. We illustrate this inversion for a three-layer fiber. We propose a method permitting to distinguish the hybrid modes HE/sub /spl nu//spl mu// and EH/sub /spl nu//spl mu// in such fibers and we show the possibility of major taper-induced power coupling between these modes in SMF-28 fiber.     

Circular TE/sub 0n/ Mode Filters for Guided Millimeter-Wave Transmission

A millimeter-wave circular TE/sub 01/ mode waveguide generates undesired circularly symmetric modes (TE/sub 02/, TE/sub 03/ modes, etc.) in bends or at discontinuities along a waveguide line. This paper describes the theory and experiment on the TE/sub 02/ and TE/sub 03/ mode filters developed for guided millimeter-wave transmission. The experimental results of two improved TE/sub 03/ mode filters show that the attenuation of the TE/sub 03/ mode is more than 16 dB for one type over the 40-70-GHz range. The TE/sub 01/-mode insertion loss of another type is about 0.2 dB over the 40-80-GHz range. The present mode filters can be applied to various high-speed guided rnillimeter-wave systems currently under development.     

Efficient selection of high-order Laguerre-Gaussian modes in a Q-switched Nd:YAG laser

Intra-cavity binary phase elements are incorporated into a Q-switched Nd:YAG laser resonator to obtain efficient high-order transverse mode selection. The resonator configuration is analyzed using the propagation-matrix diagonalization method and the Fox-Li algorithm, and a simple model for predicting the relative output powers of the selected modes is developed. The predicted results are verified experimentally with binary phase elements for selecting the TEM/sub 01/, TEM/sub 02/ and TEM/sub 03/, degenerate Laguerre-Gaussian modes. The output energy per pulse was 15 mJ for the TEM/sub 01/, 16.5 mJ for TEM/sub 02/ and 18.3 mJ for TEM/sub 03/, all higher than the 10 mJ for the TEM/sub 00/. The performance in Q-switched operation was found to be similar to that in free-running operation. The numerical calculations, experimental procedure and experimental results are presented.     

一种基于三芯光纤的模式复用器/解复用器的设计

提出了一种基于三芯光纤的模式复用器/解复用器,三个纤芯包括两个相同的单模纤芯和一个少模纤芯,在少模纤芯中写入长周期光纤光栅以实现单模纤芯中LP01模与少模纤芯中LP11模间的转换。同时从两个单模纤芯输入LP01模,能激发少模纤芯中两个简并的LP11,a模和LP11,b模。采用光束传播法分析了该模式复用器/解复用器的耦合特性及串扰。数值模拟结果显示,若以少模纤芯中LP11模输出能量大于-1dB的波长范围作为该模式复用器/解复用器的工作带宽,则能够达到23nm,并且少模纤芯中生成的其他模式的输出能量皆比LP11模的输出能量小-21dB以上。该模式复用器/解复用器能够实现对少模光纤的模式复用与解复用,并具有极低的串扰。     

Design Considerations of Broadband Dual-Mode Optical Fibers

We propose and present design data for a new type of graded index fiber which has a profile and radius such that only two mode groups (LP/sub 01/ and LP/sub 11/) propagate and both propagate with virtually identical group velocities. This dual-mode fiber has a core diameter approximately twice that of a conventional step index single-mode fiber. For example, a core diameter of 16.3 mu m is attainable with relative index difference Delta=0.3 percent at 1.25-mu m wavelength. Fabrication tolerances securing a group delay difference below 100 ps/km are given by a power-law profile parameter alpha=4.85/spl plusmn/0.25 and a normalized frequency upsilon=4.45/spl plusmn/0.11. The allowable upsilon-value deviation range to keep the group delay difference within 100 ps/km is about five times as large as that of a step-index fiber, in which group delays of two mode groups are matched. Comparison with a multimode graded-index fiber, with respect to group delay characteristics and bending loss of the dual-mode fiber, are also discussed.     

Improved Selectivity in Cylindrical TE/sub 011/ Filters by TE/sub 211/TE/sub 311/ Mode Control

A new method is presented for the design of low loss cylindrical TE/sub 011/-mode resonators whereby transmission nulls can be placed near the TE/sub 011/, resonance by controlling the TE/sub 211/, and TE/sub 311/, modes that are naturally excited in the same resonator. The frequencies at which the nulls occur are controlled by the angular offset of the sidewall coupling apertures and the relative amplitude of the TE/sub 011/ mode compared to the TE/sub 211/ and TE/sub 311/ modes. It is also shown that a lumped constant circuit model can be used to accurately represent the multimode response of the resonator.     

Single high-order transverse mode surface-emitting laser with controlled far-field pattern

Single transverse mode vertical-cavity surface-emitting lasers (VCSELs) with high output power are important for high-speed data links, optical recording, laser processing, and so on. We demonstrate a single high-order transverse mode VCSEL with narrow trenches formed on its top surface by using a focused ion beam. The formation of four straight cross trenches resulted in the selection of a stable LP/sub 41/ mode oscillation. A large far-field angle of the LP/sub 41/ mode, which causes a low coupling efficiency even with a multimode fiber, was drastically reduced by loading a phase shift layer. A single-lobe far-field pattern with low scattering loss was realized by loading a spatial phase shift of SiO/sub 2/. We evaluated a coupling efficiency with a single-mode fiber including its alignment tolerance. The controlled far-field pattern enables lens-free direct coupling with a single-mode fiber even for large active-area VCSELs.     

Multiple-wavelength up-conversion laser in Tm/sup 3+/-doped ZBLAN glass fiber

In this letter, we report on a multiple-wavelength up-conversion laser in Tm/sup 3+/-doped ZrF/sub 4/--BaF/sub 2/--LaF/sub 3/--AlF/sub 3/--NaF glass fiber pumped by an 1120-nm fiber laser. Single-wavelength, two-wavelength, and three-wavelength up-conversion lasers can be obtained by adjusting the distance between the coupler mirror and the fiber end, which is attributed to the frequency-filter effect in the etalon composed of the coupler mirror and the fiber end. The effects of the pump power on the performance of dual-wavelength laser were also discussed.     

Gyrotron-traveling wave-tube circuits based on lossy ceramics

The gyro-traveling wave tube (gyro-TWT) is a microwave amplifier with simultaneous high power, high frequency, and broad bandwidth capabilities. Techniques for providing a controlled loading of the TE/sub 01/ cylindrical-guide operating mode of a 35 GHz gyro-TWT using monolithic, lossy ceramic structures are presented. The loading scheme, which also suppresses spurious backward-wave oscillations in the TE/sub 11/, TE/sub 21/, and TE/sub 02/ modes, is based on a sequence of alternating ceramic cylindrical shells and metal rings to form the electron beam tunnel. Design techniques for achieving optimal performance and methods for reducing the sensitivity to temperature-induced variations in ceramic dielectric properties are presented.     

TE/sub 01/ Mode Components in the 3-mm Region

In the 3-mm region (94 Gc) it is desirable to use waveguide components operating in the low loss TE/sub 01/ mode in circular waveguide rather than in fundamental-mode rectangular waveguide. Because this is a higher mode, mode purity is of major concern. A method of identifying undesired modes and their amplitudes is by means of radiation patterns from the end of the waveguide. Components developed to operate in this mode include a transition from rectangular to circular waveguide, standing wave detector, variable attenuator, directional coupler, flexible waveguide, fixed 90/spl deg/ bend and rotary joint.     

Arbitrary dual-band components using composite right/left-handed transmission lines

Arbitrary dual-band microstrip components using composite right/left-handed (CRLH) transmission lines (TLs) are presented. Theory, synthesis procedure, and implementation of the dual-band quarter-wave (/spl lambda//4) CRLH TL are presented. Arbitrary dual-band operation is achieved by the frequency offset and the phase slope of the CRLH TL. The frequency ratio of the two operating frequencies can be a noninteger. The dual-band /spl lambda//4 open/short-circuit stub, dual-band branch-line coupler (BLC), and dual-band rat-race coupler (RRC) are also demonstrated. The performances of these dual-band components are demonstrated by both simulated and measured results. Insertion loss is larger than 23 dB for the shunt /spl lambda//4 CRLH TL open-circuit stub and less than 0.25 dB for the shunt /spl lambda//4 CRLH TL short-circuit stub at each passband. The dual-band BLC exhibits S/sub 21/ and S/sub 31/ larger than -4.034 dB, return losses larger than 17 dB, isolations larger than 13 dB, phase differences 90/spl deg//spl plusmn/1.5/spl deg/, and gain imbalance less than 0.5 dB at each passband. The dual-band RRC exhibits S/sub 21/ and S/sub 31/ larger than -4.126 dB, return losses larger than 12 dB, isolations larger than 30 dB, phase difference 180/spl deg//spl plusmn/4/spl deg/, and gain imbalance less than 0.2 dB at each passband.     

An Analysis of a Hybrid-Mode in a Twisted Rectangular Waveguide

Analytic expressions of electromagnetic fields for the dominant hybrid-mode in a twisted rectangular waveguide are obtained. The fields exactly satisfy the boundary conditions on the guide walls in a helicoidal shape. By expanding these expressions for the fields in terms of the eigenfunctions of a straight waveguide, the hybrid-mode is found to be composed of a fundamental TE/sub 10/ -mode component, accompanied with TE/sub 01/, TM/sub 21/,TE/sub 21/, and TE/sub 03/ modes, as successive higher order components. The result of the modal power calculation reveals that there exists a frequency at which the transmitting power carried in the cross-polarized TE/sub 01/-mode component just vanishes. As a limiting case of the twisted waveguide, fields in a twisted strip line are discussed also, and the existence of a propeller-like equiphase surface is shown.     

Multiple Branch-Guide Directional Coupler Using TE/sub 01/-Mode Semicircular Waveguide (Short Papers)

This short paper describes experimental results derived from a newly devised branch-guide directional coupler using a TE/sub 01/-mode semicircular waveguide. The length of an experimental 0-dB coupler is about 150 mm, which is one-third shorter than the conventional coupled wave-type 0-dB coupler, and the loss is decreased in proportion to the reduction in length. Using this coupler, it is possible to manufacture more compact millimeter-wave diplexers with reduced insertion loss.     

Transmission Characteristic Measurement of Two-Mode Optical Fiber with a Nearly Optimum Index-Profile

Two-mode optical fibers are studied experimentally and theoretically. A fiber with a nearly optimum index profile, designed for the material dispersion-free spectral region of 1.3 mu m has been fabricated. The fiber core diameter is 20 mu m with a 0.26 percent of relative-index difference between the core and cladding. Group delay time differences Delta tau between LP/sub 01/ and LP/sub 11/ modes are measured in the spectral region 1.06-1.44 mu m by using a fiber Raman laser. As a result, the zero-modal dispersion characteristic is confirmed to occur near 1.4 mu m. It is shown that a gently sloping Delta tau characteristic against V-value is obtained when the central index dip in core index profile is eliminated. In the present test fiber, Delta tau = 230 ps/km, caused by a V-value deviation of 5 percent from the optimum V/sub 0/ at which Delta tau = 0. The theory predicts 200 ps/km with an optimum index profile.     

Comparison of wavelength splitting for selectively oxidized, ion implanted, and hybrid vertical-cavity surface-emitting lasers

The wavelength splitting between the LP/sub 01/ and LP/sub 11/ modes of selectively oxidized, ion implanted, and hybrid ion implanted/selectively oxidized vertical-cavity surface-emitting lasers is studied by experiment and theory. Measured splittings at threshold show marked differences between the different laser structures due to the effects of index guiding and thermal lensing. Theoretical results were obtained using a vector optical mode solver and show good agreement with experimental results. The hybrid lasers exhibited behavior intermediate between the ion implanted and selectively oxidized lasers and could be optimized for high power single transverse mode emission.     

光纤激光及其相干合成光场的准确分析

为了准确地研究光纤激光及其相干合成光束的远场传输特性,采用将LP01模分解为许多不同拉盖尔-高斯模叠加的方法,分析了LP01模以及多个光纤激光相干合成光束的传输光场分布和光束质量,并与用高斯函数表示的光纤激光的光场进行了比较.分析中发现,基模高斯光束与LP01模的初始场分布基本相同,但传输后的光场分布会产生较大误差,在光纤激光相干合成光场的分析中两者间的差别也非常明显.结果表明,为更准确地分析光纤激光及其相干合成光束的传输特性,光纤激光的模场表达式应采用拉盖尔-高斯模的叠加式.     

基于少模光纤布拉格光栅的模分复用系统实验研究

本文提出了一种基于少模光纤布拉格光栅(Few-mode FBG)的模分复用通信系统,阐述了基于少模光纤布拉格光栅的模分复用/解复用原理,建立了2×2的模分复用实验系统,分别利用LP01和LP11模作为独立信道,实现了1.25 Gbps和622Mbps两路伪随机序列(PRBS)的10km传输实验,给出了传输后的眼图,分析了当激光器波长为1549.228nm时,实验系统的误码特性.实验验证了基于少模光纤布拉格光栅的模分复用通信系统的可行性,为进一步实现长距离高速率的模分复用通信奠定了实验基础.