Compensation of Chromatic Dispersion by Modal Dispersion in MMF- and VCSEL-Based Gigabit Ethernet Transmissions

Multimode vertical-cavity surface-emitting lasers (VCSELs) with large spectral widths are widely used with multimode fibers (MMFs) in Gigabit Ethernet (GbE) systems. MMFs exhibit large chromatic dispersion values that can strongly impact the performance of these systems for high bit rates and/or long distances. Here, we show that in 10-GbE systems using VCSELs and MMFs, the chromatic dispersion can be compensated for by the modal dispersion of the MMF.      

Design of subpicosecond dispersion-flattened fibers

Dispersion flattened (DF) fibers are required for wide-band WDM systems. The DF fibers designed in the past have dispersion in the range of 2.0-3.0 ps/km-nm. In this letter, we define a generalized refractive index profile that can be characterized by few controlling parameters. An optimum refractive index profile is obtained by minimizing the maximum dispersion over the wavelength range of 1300-1600 nm with respect to profile parameters. The designed fiber gives dispersion less than 1.0 ps/km-nm over 1350-1590 nm wavelength range. Sensitivity of the dispersion performance to the profile parameters is also discussed.     

Triple-clad single-mode fibers for dispersion shifting

A design procedure for triple-clad dispersion-shifted single-mode fibers is developed. Following this procedure, fibers are identified which have low dopant concentrations in the core compared to those of the previous dispersion-shifted fiber designs, a second-mode cutoff wavelength close to the operating wavelength, zero dispersion at 1550 nm, a small bending loss at 1550 nm (for a bend radius of 3.75 cm), and a spot size that is large enough not to compromise the splice loss. A significant advantage of these fibers is that low-dispersion is available over a wide wavelength band about the wavelength of zero dispersion     

Improved supercontinuum generation through UV processing of highly nonlinear fibers

We demonstrate that UV exposure of highly nonlinear, germano-silicate fibers can significantly broaden the infrared supercontinuum generated by femtosecond pulses in these fibers. Both simulations and measurements of the fiber chromatic dispersion show that UV-induced refractive index changes increase the waveguide dispersion by up to 5 ps/(nm-km) at 1570 nm and shift the dispersion zero by over 100 nm. We examine fibers with a range of UV exposure levels and show that the short wavelength edge of the supercontinuum can be continuously changed by more than 100 nm. We also show that the long wavelength edge is extended beyond that of the unexposed fiber. The resulting continuum spans from 0.85 to 2.6 /spl mu/m. Cutback measurements show that the supercontinuum in the exposed fiber is generated in as little as 1 cm of fiber. A nonlinear Schro/spl uml/dinger equation (NLSE) model of the supercontinuum generation in the nonlinear fiber shows that the short wavelength behavior of the continuum is primarily controlled by changes in the fiber dispersion caused by the UV-induced change in refractive index of the fiber core.     

Design and optimization of photonic crystal fibers for broad-band dispersion compensation

Design of photonic crystal fibers (PCFs) for application of broad-band dispersion compensation is investigated by using an improved design model based on combination of a rigorous vector solver for modal properties and a scaling approach for dispersion characteristics. The newly designed PCF is shown to exhibit large normal dispersion up to -474.5 ps/nm/km, nearly five times of conventional dispersion compensating fibers, and compensate conventional single-mode fibers within /spl plusmn/0.05 ps/nm/km over a 236-nm wavelength range. Furthermore, the design model and methodology can be applied to design other dispersion-based devices such as dispersion flattened fibers and dispersion shifted fibers.     

A statistical analysis of conditioned launch for gigabit ethernetlinks using multimode fiber

This paper reports, what is believed to be the first statistical analysis of restricted launch schemes. This is achieved by a detailed theoretical study of a large range of perturbed multimode optical fibers (MMF's) that are believed to be representative of those currently in use. It is shown that radial “offset” launches allow a robust and reliable solution to bandwidth collapse that occurs when standard laser sources are used with multimode fibers. Using such an approach, it is shown that gigabit per second link lengths using the offset launch technique can be increased to 550 m at an operating wavelength of 1300 nm using 62.5-μm diameter MMF. These link lengths are greater than those achievable with conventional techniques. The approach is also used to indicate the tolerance on alignment precision required for successful operation and to suggest a simple measurement for the assessment of link performance. Finally, the use of angled launching into MMF is shown to allow enhanced operating bandwidths     

基于保偏光子晶体光纤的高灵敏度曲率传感器

为了能高精度测量小曲率,采用单模-鼓包-保偏光子晶体光纤-多模-单模光纤组成的结构,其中多模光纤和单模光纤之间错位熔接。通过理论分析和实验验证得到在0.04582m-1~0.054776m-1及0.054776m-1~0.06929m-1曲率范围内,谐振波长漂移与曲率变化之间分别存在着线性关系。结果表明,该曲率传感器灵敏度值分别为93.95nm/m-1和30.89nm/m-1,可以应用于健康监测等需要测量小曲率的场合。     

Phase Shift Technique for the Measurement of Chromatic Dispersion in Optical Fibers Using LED's

A sinusoidal technique is reported, which allows simple and accurate measurements of chromatic dispersion in optical fibers. It is based on the phase shift which a sinusoidally modulated light beam undergoes while traveling along a fiber when its wavelength is changed. The choice of a multiple LED's source permits the continuous spectral covering from 750 to 1600 nm; easily available instrumentation and devices are needed for the measurement setup. The technique is reported in detail by showing results obtained in multimode fibers; statistical evaluation of its accuracy and a comparison with conventional methods are carried out. An accuracy of a few picosecond in relative delay and of /spl I.chemc/1 ps/nm /spl dot/ km in chromatic dispersion are demonstrated, that compare very favorably with the existing techniques.     

Antimony oxide-doped silica fibers fabricated by the VAD method

This paper presents a study of antimony oxide as an alternative dopant material for silica optical fibers using the VAD method. It takes a detailed look at fabrication processes and their conditions for graded-index multimode fibers having a 1.0-percent relative refractive index difference. Results reveal a minimum loss value and a residual OH ion content of 7.0 dB/km at 1.65μm and 50 ppb. Furthermore, measurements are made for fundamental properties, such as loss reliability, refractive index dispersion, and transmission loss below the 1.0-μm wavelength due to Sb⁴⁺centers. Finally, the paper discusses the advantages and suitable applications for antimony oxide-doped silica fibers.     

Bragg gratings in multimode and few-mode optical fibers

Bragg gratings in optical fibers in multimode propagation are investigated experimentally and theoretically. Bragg gratings formed in optical fibers in multimode propagation show multiple reflection peaks or multiple transmission dips in the reflection or transmission spectra, respectively. For standard graded-index multimode fiber, the number of reflection peaks of a Bragg grating depends on excitation condition of propagating modes. The number of reflection peaks of a Bragg grating at around 1.55 μm is 19 for highly multimode excitation and 3-4 for lower order mode excitation. We analyze the phase-matching conditions of the propagating modes and identify half of the reflection peaks as the reflection to the same mode and the rest as the reflection to the neighboring modes. In dispersion-shifted fiber, a Bragg grating at around 0.8 μm in three-mode propagation shows three reflection peaks in the reflection spectrum. The temperature dependence of each reflection peak is similar to that of a conventional Bragg grating in single-mode fiber. Polarization dependence measured on a Bragg grating in multimode graded-index fiber is negligible. An advantage of Bragg gratings in multimode fiber (MMF) and the applications are discussed     

Dispersion-compensation module based on one preform design matching arbitrary dispersion and slope requirements

In this article, we propose a method to realize dispersion-compensation modules (DCMs) with a user-defined dispersion in a specified bandwidth for a given tolerance. It is based on the wavelength shift of a characteristic dispersion function by scaling the refractive-index profile. Controlling the fiber diameter during the manufacturing process leads to the desired scaling. In order to get a DCM with the predefined wavelength-dependent dispersion, a specific diameter-versus-position function has to be implemented. To demonstrate the concept, compensators for typical transmission fibers were simulated. For example, the dispersion in the complete C band (1530-1570 nm) can be compensated for 100 km of TeraLight and TrueWave-RS. The results showed a residual dispersion of only /spl plusmn/1 ps/nm and could be realized with overall compensator lengths of 3.54 and 1.97 km, respectively. Furthermore, higher order dispersion in the S, C, and L bands (1490-1610 nm) was compensated for different requirements with a tolerance of only /spl plusmn/0.5 ps/nm, which enables ultrahigh bit-rate transmission at 160 Gb/s. In order to estimate the feasibility of such a DCM, a tolerance analysis is presented, and the guiding properties are approximated.     

Enhanced performance of WDM systems using directly modulated lasers on positive dispersion fibers

Cost-effective directly modulated distributed feedback lasers (DML) have attracted much attention recently for operation at the 1.55 μm wavelength band applications in metropolitan area networks. In this paper, we show by simulation that the effect of directly modulated laser chirp can be compensated by a negative dispersion fiber, but this only occurs in a specific range of DML output power, and that a pulse broadened by the positive dispersion fiber can be equalized using self-phase-modulation (SPM) in optical fiber. The majority of metro and access networks are made up of conventional single-mode fibers (SMF) which are positive dispersion fibers. We demonstrated that optimum compensation is always feasible for such fibers, since the magnitude of the SPM can be controlled by changing the optical power in the fiber. Furthermore, simulation suggests that this technique will enable directly modulated wavelength division multiplexed systems to enhance their performance if the power of each channel is appropriate.     

基于组合包层的微结构光纤的色散特性

为了获得微结构光纤的平坦色散特性,设计了一种圆形排列的微结构光纤,其包层由周期分布的空气孔构成,通过有限元数值分析法对该微结构光纤基模的色散特性进行了数值仿真,研究了色散和纤芯圆孔尺寸以及波长的关系。结果表明:内外空气孔间距和直径对微结构光纤的色散曲线都有影响,但内包层大孔间距和第一圈小空气孔的直径对色散曲线的走向起决定作用。在内圈空气孔直径为3.1μm,其他空气孔直径为1.0μm,内圈空气孔中心间距为5μm,其他空气孔中心间距为4μm时,光纤在1.3μm波长的色散为19.5ps/(nm·km),在1.6μm波长的色散为26.5ps/(nm·km),在1.3～1.6μm波长范围内,其色散值变化范围为7ps/(nm·km)。     

Profile dispersion effects on transmission bandwidths in graded index optical fibers

Transmission bandwidths of optical fibers would be maximized if their refractive index profiles were optimally graded. However, dispersive differences between fiber material constituents make the optimal power law profile exponent α depend on wavelength. This profile dispersion effect is significant for germanium borosilicate fibers and makes their observed transmission bandwidths change by more than 300 percent within a 650-1050 nm wavelength range. Measurements are made in spectrally filtered white light from a xenon arc lamp that is sinusoidally modulated by an electrooptic crystal. Reduction of sine wave envelope intensity due to transmission in a fiber gives its baseband frequency response. The functional dependence of bandwidth on wavelength is used to diagnose whether α is larger or smaller than the optimal value which minimizes intermodal dispersion at particular wavelengths.     

Optical fiber-based dispersion compensation using higher ordermodes near cutoff

Higher order spatial modes in optical fibers exhibit large, negative chromatic dispersion when operated near their cutoff wavelength. By using a spatial mode-converter to selectively excite a higher order mode in specially designed multimode fiber, this dispersion can be used to compensate the positive dispersion in conventional single-mode fiber spans. In this paper, issues related to compensating fiber and mode-converter design are explored. Experimental measurements in specially designed two-mode fibers operated in LP₁₁ mode show negative dispersion as large as -70 ps/nm·km at 1555 nm. Pulse propagation and system experiments employing spatial mode-converters to excite LP₁₁ mode in a two-mode fiber demonstrate the feasibility of this technique for dispersion compensation in lightwave systems     

Design of wavelength-switching erbium-doped fiber lasers with a multimode fiber Bragg grating using spatial-mode excitation and selection techniques

A multimode fiber Bragg grating (MMFBG)-based wavelength-switching erbium-doped fiber laser and its application in displacement sensing is proposed and demonstrated. According to the spatial-mode distribution in the multimode fiber (MMF), the modes reflected by the MMFBG at different wavelengths build a wavelength-dependent mode field on the transverse plane of the MMF. By varying spatial-launching position of the single-mode fiber against the MMFBG to achieve different mode group excitation and selection, single-wavelength lasing at 31 discrete wavelengths has been realized over a wide wavelength range.     

MEMS可调谐平顶窄带光学滤波器

设计了一种基于MEMS技术的可调谐光学滤波器,它通过光栅将输入的宽带光信号色散展开,以一个MEMS扭镜选择将对应滤波器通带的光信号反射至输出端,从而实现光学滤波和波长调谐功能。滤波器的输入端采用单模光纤,输出端采用多模或者少模光纤,可以实现窄带且平顶的通带特性。经过参数优化,仿真分析得结果显示,采用多模/少摸光纤输出的两种滤波器,其0.5 dB和25 dB带宽分别为0.95 nm/0.29 nm和1.39 nm/0.69 nm,分别满足100 GHz和50 GHz信道间隔的DWDM系统要求。由于输出端采用多模或者少摸光纤,从该滤波器输出的光信号不能继续在单模光纤中传输,只能由光探测器接收,因此该滤波器一般应用于全光网络节点中的下载端口。     

A universal fiber-optic (UFO) measurement system based on a near-IR fiber Raman laser

A universal fiber-optic measurement system, which is useful for measuring loss and dispersion in the1.06-1.6 mum wavelength region, is described. The source is a silica fiber Raman laser pumped by a mode-locked andQ-switched Nd:YAG laser at 1.06 μm. Subnanosecond multiple-Stokes pulses in the1.1-1.6 mum wavelength region are generated in a low-loss single-mode silica fiber. The use of this near-infrared fiber Raman laser for characterizing various transmission properties of single and multimode test fibers is demonstrated. Loss spectra, intramodal dispersion, and intermodal dispersion data are obtained in the wavelength region of minimum loss and minimum material dispersion for silica fibers.     

Polarization mode dispersion measurement in elliptical core single-mode fibers by a spatial technique

Polarization mode dispersion in elliptical core single-mode fibers has been measured by a spatial technique based on a visibility maximum position measurement in an interferometer. Using the technique, wavelength dependence of the modal dispersion has been measured by varying optical source wavelength between 821 and 904 nm. As a result, contribution of geometrical and strain birefringences on the modal dispersion has been evaluated, and normalized frequency dependence of the modal dispersion has been clarified. Moreover, the dispersion compensation effect has been observed by interchanging the fast and slow modes of two fibers at a splice point. The experimental results reveal that the spatial technique is very useful for polarization mode dispersion measurement.     

Dispersion compensation optical fiber modules for 40 Gbps WDM communication systems

Dispersion compensation was originally proposed to equalize pulse distortion.With the development of wavelength division multiplexing (WDM) techniques for large capacity optical communication systems,dispersion compensation technologies have been applied into the field.Fiber-based dispersion compensation is an attractive technology for upgrading WDM communication systems because of its dispersion characteristics and good compatibility with transmission optical fibers.Dispersion compensation fibers and the modules are promising technologies,so they have been receiving more and more attention in recent years.In this work,high performance dispersion compensation fiber modules (DCFMs) were developed and applied for the 40 Giga bit-rate systems.First,the design optimization of the dispersion optical fibers was carried out.In theory,the better the refractive index profile is,the larger the negative dispersion we could obtain and the higher the figure of merit (FOM) for the dispersion optical fiber is.Then we manufactured the fiber by using the plasma chemical vapor deposition (PCVD) process of independent intellectual property rights,and a high performance dispersion optical fiber was fabricated.Dispersion compensation fiber modules are made with the dispersion compensating fibers (DCFs) and pigtail fibers at both ends of the DCFs to connect with the transmission fibers.The DCFMs present the following superior characteristics:low insertion loss (IL),low polarization mode dispersion,good matched dispersion for transmission fibers,low nonlinearity,and good stability for environmental variation.The DCFMs have the functions of dispersion compensation and slope compensation in the wavelength range of 1525 to 1625nm.The experiments showed that the dispersion compensation modules (DCMs) met the requirements of the GR-1221-CORE,GR-2854-CORE,and GR-63-CORE standards.The residual dispersions of the G.652 transmission lines compensated for by the DCM in the C-band are less than 3.0ps/nm,and the dispersion slopes are also compensated for by 100%.With the DCFMs,the 8×80km unidirectional transmission experiments in the 48-channel 40Gbps WDM communication system was successfully made,and the results showed that the channel cost was smaller than 1.20dB,without any bit error.