Dynamics of an optical signal in a multimode waveguide structure

A mathematical model of linear dynamic distortions of average signal power in a planar optical waveguide stemming from the intermodal coupling and dispersion of waveguide modes caused by macroscopic bends and irregularities of the optical waveguide surface has been developed on the basis of pulse intermodal scattering matrix. It is shown that the elements of this matrix are determined at a waveguide segment whose length greatly exceeds the correlation scale of the field of irregularities but is much smaller than the normalization length of waveguide modes and are described by rectangular functions with a duration proportional to the relative velocity of the corresponding modes. The dependences of the pulse intermodal scattering matrix on the intensities of absorption, Rayleigh scattering, and optical signal scattering from waveguide irregularities have been established. It is shown that, at waveguide lengths exceeding the normalization length, the optical signal shape becomes almost stable and can be expressed in terms of the pulse characteristic of an integrating circuit. It has been established that irregularities of the film in a bent optical waveguide lead not only to additional attenuation but also to enhancement of multimode distortion. A matrix method is proposed for taking into account these distortions, which is based on the effect of equalization (scrambling) of the signal energy over the spectrum of the bent waveguide modes. The model developed was used to process the experimental data on the dynamics of a signal in a weakly guiding multimode polymer optical fiber in order to estimate the parameters of attenuation and irregularities of the waveguide surface.     

High-order mode of polymeric waveguide selective excitation

Conditions for the selective excitation of multimode waveguide high-order modes by power trans-formation of fundamental modes of a tunnel connected waveguide are calculated. It is shown that the power conversion factor can reach 100% under phase-matching conditions, which can be satisfied by the selection of a second waveguide refractive index.     

Parametric interaction of space-charge waves in thin-film semiconductor structures

The general theory of parametric coupling between space-charge waves and drifting charge carriers in thin-film semiconductor structures has been worked out. This theory is applicable, in particular, to n-GaAs and n-InP semiconductors with negative differential conductance due to intervalley electron transitions under high electric fields. We started from the electrodynamic theory of waveguide excitation by extraneous currents, which was extended for arbitrary waveguide structures with composite active media. Our theory makes it possible to study parametric interaction between space-charge waves in semiconductor films with regard for boundary conditions, diffusion, the anisotropy and the frequency dispersion of the differential electron mobility, as well as the multifrequency and multimode nature of a wave process in thin-film structures.     

Coaxial fiber-optic chemical-sensing excitation-emission matrix fluorometer

Great reductions in the overall size and complexity of high throughput multichannel UV-visible fluorometers were achieved by coupling a compact optical fiber array to compact dispersive transmission optics. The coaxial configuration centers on the insertion of a silica/silica optical fiber into the hollow region of a UV-fused silica capillary waveguide. The outer core delivers the maximum power of the narrow wavelength region of the excitation spectrum created by coupling a xenon arc discharge lamp to a compact spectrometer. The molecular fluorescence resulting from the interaction of light emitted at the distal end of the hollow waveguide and the sample matrix is received and transmitted to a CCD via a compact dispersive grating-prism (grism) optical assembly. A linear array of the coaxial optical fibers permits a full excitation-emission matrix spectrum of the analyte matrix to be projected onto the face of the CCD. The in situ identification and monitoring of polycyclic aromatic hydrocarbons was carried out for the initial application testing for this prototype.     

衰减匹配的超声Barker码激励方法

为增加超声穿透高声衰减介质的能力,提出了一种衰减匹配的超声Barker码激励方法。基于换能器高斯响应与材料非频散线性衰减的假设,得到了Barker码激励的信号模型,求解旁瓣抑制滤波后脉冲压缩的信噪比表达式可知,该方法仅需要根据材料衰减特性与轴向分辨率的要求,分别调整Barker码的中心频率与时长,便可以获得更高的信噪比。取衰减系数为1.4 Np/(MHz·cm)、厚度为5 cm的橡胶为试样进行验证。当与方波激励方法的轴向分辨率相近时,衰减匹配的Barker码激励方法比传统Barker码激励方法的信噪比增益提高接近5 dB;当牺牲一定轴向分辨率时,信噪比增益提高接近11 dB。结果表明,衰减匹配的Barker码激励方法可以降低依频率衰减对脉冲压缩的影响,有效提高衰减回波的信噪比。      

Loss calculations in bent multimode optical waveguides

We present a straightforward geometric optics method to calculate the power attenuation induced by a circular bend in the axis of a multimode optical waveguide. The method involves tracing of rays and use of generalized power transmission coefficients. Both slab and fibre waveguides (with either step-index or parabolic core profiles) have been considered; the source has been assumed to be Lambertian.     

Analysis of the dispersion relation of nonlinear slab-guided waves

The dispersion relations of TE-polarized nonlinear guided waves (NGWs) in a symmetrical slab configuration (linear slab embedded between two identical media with a Kerr nonlinearity) are investigated in detail. Among even and odd NGWs the waveguide can support also NGWs, revealing an asymmetrical field pattern. The excitation of these NGWs requires a minimum guided power flux indicating a bifurcation of the corresponding dispersion curves.     

Multifocus confocal Raman microspectroscopy for fast multimode vibrational imaging of living cells

We have developed a multifocus confocal Raman microspectroscopic system for the fast multimode vibrational imaging of living cells. It consists of an inverted microscope equipped with a microlens array, a pinhole array, a fiber bundle, and a multichannel Raman spectrometer. Forty-eight Raman spectra from 48 foci under the microscope are simultaneously obtained by using multifocus excitation and image-compression techniques. The multifocus confocal configuration suppresses the background generated from the cover glass and the cell culturing medium so that high-contrast images are obtainable with a short accumulation time. The system enables us to obtain multimode (10 different vibrational modes) vibrational images of living cells in tens of seconds with only 1 mW laser power at one focal point. This image acquisition time is more than 10 times faster than that in conventional single-focus Raman microspectroscopy.     

Multimode interference effect in plasmonic subwavelength waveguides and an ultra-compact power splitter

The real part of the effective index is enhanced in a metal-insulator-metal (MIM) surface plasmon waveguide when the thickness of the insulator is quite small and this mechanism can be utilized to achieve some ultra-compact photonic devices. The mode properties of an index-guided multimode plasmonic waveguide of subwavelength size are analyzed and the multimode interference (MMI) effect is investigated. A novel ultra-compact MMI power splitter, with the transverse size less than 1/3 of the free space wavelength, is proposed.     

Single mode heat rectifier: controlling energy flow between electronic conductors

We study heat transfer between conductors, mediated by the excitation of a monomodal harmonic oscillator. Using a simple model, we show that the onset of rectification in the system is directly related to the nonlinearity of the electron gas dispersion relation. When the metals have a strictly linear dispersion relation, a Landauer-type expression for the thermal current holds, symmetric with respect to the temperature difference. Rectification becomes prominent when deviations from linear dispersion exist, and the fermionic model cannot be mapped into a harmonic bosonized representation. The effects described here are relevant for understanding radiative heat transfer and vibrational energy flow in electrically insulating molecular junctions.     

220 GHz 折叠波导慢波结构

 优化设计了一种220 GHz的折叠波导慢波结构的尺寸,对其冷测特性如色散、耦合阻抗和衰减进行了分析。理论分析和软件仿真结果表明设计的折叠波导慢波结构在中心频率处具有较平缓的色散关系,较高的耦合阻抗和较低的电路衰减。互作用模拟表明,在电子注电压为20 kV,电流为10 mA时,27 mm（50个周期）的折叠波导慢波结构在220 GHz具有14.5 dB的增益,3 dB带宽为16.3 GHz（211.9～228.2 GHz）。     

TE modes in a slab waveguide bounded by nonlinear media

A theory is presented of guided TE modes in a one-dimensional slabe waveguide bounded by nonlinear media with intensity dependent refractive indices. It is shown that this is a straightforward generalisation of linear slab waveguide theory with the modes parametrised by the power flow down the guide. Illustrative dispersion curves and mode shapes are discussed.     

Dynamic measurement of room impulse responses using a moving microphone

A technique for the recording of large sets of room impulse responses or head-related transfer functions is presented. The technique uses a microphone moving with constant speed. Given a setup (e.g., length of the room impulse response), a careful choice of the recording parameters (excitation signal, speed of movement) leads to the reconstruction of all impulse responses along the trajectory. In the case of a moving microphone along a circle, the maximal angular speed is given as a function of the length of the impulse response, its maximal temporal frequency, the speed of sound propagation, and the radius of the circle. As a result of the presented algorithm, head-related transfer functions sampled at 44.1 kHz can be measured at all angular positions along the horizontal plane in less than 1 s. The presented theory is compared with a real system implementation using a precision moving microphone holder. The practical setup is discussed together with its limitations.     

Nondegenerate mirrorless oscillation in silicon waveguide

We propose nondegenerate four-wave mixing mirrorless oscillation in a multimode silicon nonlinear waveguide. Thanks to the large modal dispersion between two spatial modes caused by the high-index-contrast waveguide structure, two counterpropagating pumps of one spatial mode can generate two new optical waves of the other spatial mode at different frequencies. The phase-matching condition can be satisfied with the higher-order modes involved; therefore, frequencies of the newly generated light can be tuned by simply changing the pump frequency. The threshold power and conversion efficiency of the proposed mirrorless oscillation are investigated under different waveguide parameters.     

Broadband emission from a relativistic electron bunch in a semi-infinite waveguide

A study is made of the excitation of a transition radiation pulse during the injection of a charged particle bunch through the end metal wall into a semi-infinite cylindrical waveguide. Exact analytic expressions for the fields of a thin ring-shaped bunch are obtained in terms of the Lommel functions of two variables. The energy efficiency, power, and spectrum of radiation emitted from a finite-size charged bunch in a vacuum waveguide are calculated numerically with allowance for the multimode nature of the excited field. It is shown that, under certain conditions, the bunch can generate a short, high-intensity electromagnetic pulse with a broad frequency spectrum. The effect of various parameters of the charged bunch-waveguide system (such as the bunch current, bunch duration, and waveguide radius) on the generation efficiency of a transition radiation pulse is investigated.     

All-optical multichannel 2R regeneration in a fiber-based device

We propose the design of an all-optical 2R regenerator capable of handling multiple wavelength-division-multiplexed channels simultaneously. It extends the known concept of off-center filtering of self-phase-modulation-broadened signal spectra. The novel feature of the proposed device is a dispersion map that strongly suppresses interchannel impairments. The map employs several sections of nonlinear fiber with high normal dispersion, separated by dispersion compensators with spectrally periodic group delay. The results of our numerical simulations indicate the feasibility of such a multichannel regenerator.     

The effect of cladding loss in graded-index fibres

We present quantitative results for the effect of cladding absorption on the transmitted power and impulse response of a typical multimode, graded-index fibre that is excited by a Lambertian source. Power attenuation is due to the absorbing cladding material and the radiation loss of the tunnelling rays. Graded-index fibres are shown to be far less susceptible to the effect of cladding absorption than step-index fibres.     

Analysis of optical rib self-imaging multimode interference (MMI) waveguide devices using the discrete spectral index method

We have developed a semianalytical approach to study the propagation characteristics of multimode interference (MMI) waveguide structures. The effect of transverse crosssection geometry on self-imaging length and optical power throughput are investigated. We present results confirming that the nonquadratic modal dispersion encountered in stripe-loaded waveguides impairs self-imaging performance. Results indicate that the image degradation is more predominant for asymmetrically fed MMI waveguides.     

有损金属圆波导中电磁波传输特性的研究

基于电磁模式的色散方程，研究了有损金属圆波导中电磁模式的传输问题.在考虑了损耗层厚度影响的情况下，得到了各模式传输常数的近似解析表达式，此式适用于截止频率附近.通过解析表达式和波导模式色散方程，对衰减常数和相位常数进行了大量数值计算，得到了它们随损耗层厚度、频率以及损耗层电导率的变化规律，两种方法所获得的结果有较好的一致性.研究表明，损耗强度和损耗层厚度的变化会改变波导中电磁模式的色散和简并特性，对于改善高功率回旋行波放大器中注波互作用带宽和模式竞争非常有益. 关键词： 有损圆波导 相位常数 衰减常数 色散方程     

甚短距离光传输(VSR_1)系统色散功率代价的分析

导出了一种用系统10%—90%上升时间所表示的最坏情况下多模光纤色散功率代价的计算方法。将最坏情况下的眼图闭合用光接收器在t0时刻的输出信号波形表示,从而得到了色散功率代价的数学表达式。为了得到光接收器输出信号波形的解析解,假设多模光纤链路是一个低通滤波器,其归一化脉冲响应是高斯形式的。用这种方法对甚短距离光传输系统VSR_1的色散功率代价做了计算分析,所得结果可作为最坏情况下VSR_1链路功率预算分析的参考。