New calculations for coupling laser diode to multimode fiber

A simple and useful method is presented for calculating characteristics of a laser diode to a multimode fiber coupling. Attenuation of higher order modes in multimode fiber is introduced into ray tracing. Maximum coupling efficiencies and misalignment tolerances of optical circuit components are computed for utilization of a relatively large spherical lens and combination of two relatively large spherical lenses. It is clarified that attenuation of higher order modes strongly affects both maximum coupling efficiency and misalignment tolerances. Agreement between theoretical and experimental coupling characteristics is greatly improved compared with the case for conventional ray tracing.     

Nonlinear switching of optical pulses in fiber Bragg gratings

We study numerically the nonlinear switching characteristics of optical pulses transmitted though fiber Bragg gratings. We consider both the uniform and phase-shifted gratings and compare their performance as a nonlinear switch. The nonlinear coupled-mode equations were solved numerically to obtain the pulse-switching characteristics. The steady-state behavior known to occur for continuous-wave optical beams is realized only for pulses wider than 10 ns with long tails. For pulsewidths in the range 0.1-1 ns, the use of phase-shifted gratings reduces the switching threshold, but the on-off contrast is generally better for uniform gratings. We also quantify the effects of rise and fall times associated with an optical pulse on nonlinear switching by considering the Gaussian pulses with smooth tails and nearly rectangular pulses with sharp leading and trailing edges.     

一种多模-细芯-多模结构的干涉型光纤传感器

设计并制作了一种基于马赫-曾德干涉仪(Mach -Zehnder interferometer,MZI)原理的温度与压力同 时测量的光纤传感器,该传感器采用多模-细芯-多模(Multimode fiber -Thin core fib er- Multimode fiber,MTM)结构,其原理是,当多模光纤(Multimode fiber,MMF)中的光到达第 1个熔接点时 一部分进入细芯光纤(Thin core fiber,TCF)的纤芯,激发起纤芯模;另一部分进入TCF的 包层,激发起 包层模。当这两部分光到达第2个熔接点处时,TCF纤芯中的光进入MMF的纤芯,TCF包层中 的光一部分进入 到MMF的包层,另一部分进入MMF的纤芯,与纤芯模式发生干涉。外界环境如温度、压力等发 生改变时干涉 谱将随之漂移,因此可以通过观察光谱中干涉谷的漂移来测量外界温度、压力的变化。当温 度变化范围为 30℃时,实验测得Dip1、Dip2的温度 灵敏度分别为0.042nm/℃、0.051nm/℃。当 压力变化范围为 0N－9.8N时,实验测得Dip1、Dip2的压力灵敏度分别为－0.240nm/N、－0.524nm/N。由于 Dip1和Dip2对温度 和压力的灵敏度分别不同,因此可以利用矩阵实现对温度和压力的同时测量。该传感器制造 简单精巧,成本较低,具有一定的研究以及应用价值。     

Nonlinear interaction of light propagation in a birefringentoptical fiber with external perturbations

Nonlinear interaction between two polarization modes by the optical Kerr effect is investigated when the light propagates in a birefringent optical fiber, with consideration of random external perturbations. A pair of nonlinear coupled differential equations are derived, which include both the linear birefringence fluctuation and the polarization mode coupling caused by the random external perturbations     

超短脉冲强激光大气传输的非线性效应

超短脉冲强激光大气传输在光学遥感、电子对抗、以及人工放电等方面具有重要的应用．例如,利用超短脉冲强激光使得远距离处的大气局域电离,其所产生的紫外辐射可以为大气组分主动荧光光谱学研究提供光源。对短脉冲强激光在大气中的传输进行了理论分析,对超短脉冲强激光大气传输中的各种线性效应及非线性效应,诸如色散效应、非线性自聚焦效应、受激拉曼散射效应、多光子电离及隧道电离效应、电离引起的能量损耗效应、相对论聚焦、有质动力激发的等离子体尾波场等效应进行了讨论,给出了一组描述超短脉冲强激光大气传输的三维非线性传输方程,为此类问题的进一步研究提供了理论基础。     

Efficient power coupling between an MH LED and a taper-ended multimode fiber

A more than 50 percent high power coupling efficiency has been demonstrated between a multiheterojunction GaAlAs LED (MH LED) and a taper-ended fiber. Optical power values coupled from the MH LED into a Corning silica fiber [numerical aperture (NA) = 0.18] and a Toshiba multicomponent glass fiber (NA = 0.27) were 360 and 560 μW, respectively, at 100 mA drive current. Maximum improvement factor beyond the simple butt joint was 5.2, which is larger than any reported values for edge emitting LED's. Modal power distributions in multimode fiber excited by MH LED with a taper-ended fiber launcher were measured to show the leaky mode contribution negligibly small. Coupling mechanical tolerance and the cladding effect in the tapered section on coupling efficiency are discussed.     

光纤中皮秒高斯脉冲的传输特性研究

对描述具有初始频率啁啾的皮秒高斯脉冲传输的微分方程组用龙格-库塔算法进行了求解,得到了高斯脉冲各参数随距离演化的数值解并据此画出了高斯脉冲包络的演化。数值结果表明：二阶色散与光纤非线性的比值为0.72时,脉冲的展宽可以被光纤非线性有效地补偿,且脉冲的频率抖动和啁啾也可以得到较好的抑制,但是过强的三阶色散和初始啁啾将导致脉冲能量的迅速衰减和波形的快速畸变。     

Mode excitation theory and experiment of single-mode fiber directional coupler with strong coupling

The mode excitation theory is presented to analyze single-mode optical fiber directional couplers with transverse strong coupling, which either the weakly coupling theory or the ray theory fails to expound. By using the equivalence between the waveguides with square and circular cross sections, regarding the coupling zone of the coupler as a single waveguide, the excitation coefficients of the propagation modes in the coupling zone excited by the modes in the input waveguide have been evaluated. The theoretical coupling amount and insertion loss have been studied. Using a special monitoring technique to control the coupling length, several couplers with various coupling amounts have been made by the polishing method. The spectral characteristic of the coupling amount has been measured and compared with the theoretically inferred result. The two are found to agree with each other. The offset effect on the coupling amount has also been measured and it is different from that of the weak coupling device-the Stanford coupler.     

Low-noise amplification of high-power pulses in multimode fibers

Low-noise amplification of high-power pulses in multimode erbium/ytterbium-doped fibers is demonstrated. The use of multimode fiber amplifiers allows to overcome the peak power limitations of single-mode amplifiers. As a result femtosecond pulses can be efficiently amplified to peak powers >50 kW in a high-power chirped pulse amplification system using chirped PPLN for frequency-doubling. With an average power level of 1.2 W at the fundamental wavelength of 1.55-μm average powers up to 420 mW are generated at 775 mm     

Pulse propagation in multimode optical fibres

Coupled mode theory is unnecessary for the analysis of pulse propagation in multimode optic waveguides. Instead, a very simple ray or continuum equation is derived from elementary plane-wave concepts to account for the effects of absorption loss and scattering. The formalism reduces to a diffusion equation for highly forward directed scatterers.     

矩形隧道中的多波模传播特性

分别推导出矩形隧道内多模式电磁波综合作用下的隧道壁反射衰减公式,以及由隧道壁粗糙度、倾斜度引起的散射衰减公式,建立了多波模传输理论。据此分析的电磁波传播特性显示:频率越高,高次模的影响越大,隧道壁粗糙度和倾斜度的影响亦随之增大;且水平极化波和垂直极化波的衰减值之差逐渐减小,收敛在稳定值。所得结论与传统只分析基模衰减公式的电磁波传输理论相比,更接近于实际测量数据和几何光学的仿真结果。     

Nonlinear interaction between optical pulses from high-repetition gain-switched DFB-LD in optical fiber

Interaction forces between propagating pulse pairs from a gain-switched distributed feedback laser diode (DFB-LD) were investigated. The authors discuss the experimental observation of a nonlinear interaction force between the fiber-compressed, gain-switched optical pulses. Optical pulses were generated by a gain-switched 1.55 mu m DFB-LD at a 10 GHz repetition rate, and compressed by compensating for the red-shifted chirping in the optical pulses using a polarization-holding fiber with a 18.7 ps/km/nm normal dispersion at a 1.55 mu m wavelength. The measured interaction force, which was greater than that theoretically predicted, was caused by the wings and/or pedestal associated with copropagating pulses.<>     

Wave propagation and coupling in linear arrays with application to the analysis of large arrays

Waves inside linear arrays are analyzed for the case of single-port excitation. A frequency-domain method is proposed for the extraction of the amplitude decay and phase velocity of the waves, based on simulation results obtained for terminal excitation at one end and near the middle of the array. The waves reflected by the array ends are also extracted, under a single-reflection assumption. This model is then exploited for estimating the port currents when other elements of the array are excited, as well as for larger arrays. Simulation results are shown for port currents and element patterns in arrays of broad-plate dipoles.     

A multimode CMOS PA with a single propagation path

Analog Integrated Circuits and Signal Processing - In this paper a four mode 130&nbsp;nm RF CMOS power amplifier (PA) in a single propagation path topology is presented and evaluated with IEEE...     

Optical power distribution in multimode fibers with angular-dependent mode coupling

An expression for the power flow, in multimode optical fibers, is derived, which is mathematically simple and allows a clear physical interpretation of the effects of mode coupling. Coupling strength is assumed to decrease with increasing axial angle. The solution obtained under this assumption is supported by experimental observations. Following the analysis, an experimental procedure to determine the coupling constant is devised. The theoretical results enable the prediction of the loss due to the mode coupling and the transmission bandwidth of the angular multiplexed channels in step-index optical fibers.     

高功率线阵半导体激光器光纤耦合设计

本文通过分析高功率线阵半导体激光器的激光输出特性，设计了一种微台阶反射镜阵列对其进行光束变换，并分别采用几何光线追迹法和高斯光束的ABCD定律，模拟计算了光束变换情况。计算结果表明，本文设计的光束变换系统，可将10mm宽的线阵半导体激光器的输出激光耦合进一根芯径为800μm，数值孔径NA≥0.37的光纤中。     

Orthogonal frequency-division multiplexing in wirelesscommunication systems with multimode fiber feeds

The feasibility of using multimode fiber as an inexpensive cell feed in broad-band indoor picocellular systems is investigated in this paper. The performance of coded orthogonal frequency-division multiplexing (OFDM) for a variety of multimode fiber profiles, including stepped index and α-profile graded index fibers, is assessed. In addition to its ability to perform well in a frequency-selective multipath environment, OFDM is shown to offer good protection against the frequency selectivity of a dispersive multimode fiber. Data rates in excess of 100 Mb/s (without equalization) over a multimode fiber channel are possible, whereas they may be limited to some 20-30 Mb/s using conventional ASK modulation     

Free-field propagation of localized pulses

Incident transient fields obey homogeneous Maxwell equations and are assumed to be a known input to the integral equations of electromagnetic scattering. These fields can be specified by two solenoidal fields foroverrightarrow{E}andoverrightarrow{B}at the initial time, and they can be computed at other times using the Green function for the scalar wave equation. Equations are derived that are then used in a computer program to do the integrations over the information-collecting sphere. Sample calculations show that a modulated pulse keeps its shape much better than a simple pulse.     

用光线追迹法计算半锥形多模光纤的耦合效率

为了计算半锥形多模光纤和半导体激光器的耦合效率,并分析影响耦合效率的因素,在考虑光线在锥面上多次反射、反射损耗、透射损耗以及波导的耦合条件的情况下,采用改进后的光线追迹法计算了不同锥角或不同锥长、以及两者发生相对平移或倾斜的情况下,无包层半锥形多模光纤和半导体激光器之间的耦合效率.模拟结果表明,当半锥宽度取适当值时,锥角对耦合效率的影响弱化了;耦合效率对相对平移比较敏感,而对相对倾斜有着很高的冗余度.这对实际工作中提高耦合效率具有指导意义.     

耦合相互作用对光纤中类明孤子传输特性的影响

本文采用变分法，导出了双全同耦合光纤中类明孤子参数的演化方程组，在此基础上讨论了类明孤子在耦合光纤中的传输特性，分析了耦合相互作用对类明孤子参数的影响。