Nonlinear profile dispersion aids optimisation of graded-index fibres

The bandwidth of graded-index fibres is limited not only by distortions of the refractive profile, but also by nonlinear profile dispersion caused by the dependence of the profile shape on wavelength. An efficient method is presented here, which helps to understand the interaction of both effects, shows a new way to minimise total dispersion by matching the two effects and simplifies the calculation of pulse broadening in imperfect real fibres.     

Compensation of profile dispersion in graded-index optical fibres

As the profile dispersion parameter P of graded-index fibres is wavelength-dependent. Marcatili's condition for achieving high bandwidth is usually satisfied only in a narrow wavelength range, if at all. This behaviour can be compensated by a wavelength-dependent refractive-index profile. This letter presents a profile function that compensates for a broad class of profile dispersions and may become a design criterion for fibres in the future, although its practical applicability is presently restricted by the limited variety of available glasses.     

Ray-tracing technique for evaluating the dispersion characteristics of graded-index cylindrical fibres

A method which uses ray-tracing techniques and modal relations is sketched for the determination of the dispersion characteristics of cylindrical fibres with radial graded-index distribution. Examples of the results obtained are described, which show how it is possible, applying this procedure, to fully analyse propagation in optical fibres with any refractive-index distribution.     

Theoretical investigation on the dispersion of graded-index polymeroptical fibers

Various experiments on polymer optical fibers (POF's) suggest that they present a large transmission capacity, but this is not confirmed by the standard theory. In this paper we present a full dispersion model based on evaluating the frequency response of the fiber instead of the direct calculation of the pulse broadening from the moments of the impulse response. The mathematical formalism is firstly displayed and subsequently applied to the graded-index POF made with polymethylmethacrylate (PMMA). The simulation results show that the baseband bandwidth is indeed enhanced due to the strong differential mode attenuation (DMA)     

Prediction of mode dispersion in graded-index single-mode fibres

A Cheby?shev power series method is presented for the calculation of the propagation constant of graded-index fibres. Accurate predictions of the dispersion parameters can be obtained as shown by examples of step and parabolic profiles. Measured index profiles can also be used with local index values found by linear interpolation from nearest reading points. Examples are given to demonstrate the effectiveness of this approach.     

Comprehensive theory of dispersion in graded-index optical fibers

This paper presents a theoretical investigation on the dispersion in graded-index silica glass fibers under overfilled launching with equal excitation of modes. This theory incorporates both chromatic effect and modal contribution which takes not only the modal delay into account but also the distributed loss and mode-coupling. Random microbends are considered to be the most dominant source of coupling. All index perturbations and intrinsic core diameter variations are assumed to be negligible, but they could readily be included without changing the basic structure of the model. The 3-dB bandwidth is analyzed through the study of the fiber transfer function which introduces the wavelength and modal effects as two separate filter functions. The formal derivation of the chromatic transfer function is analytical. On the other hand, the modal transfer function is obtained by numerically solving the power flow equation in the frequency domain using Crank-Nicholson method. As an application, the results are illustrated showing, in particular, the influence of the fiber core/outside diameters, for the first time     

Modal dispersion of optical fibres with a composite ?-profile graded-index core

It is shown that refractive-index profiles with different values of a in different annular regions of the core cross-section make it possible to achieve an almost flat r.m.s. spectral response over a spectral region of several hundred nanometres.     

Simulation investigation on supercontinuum generation and noise characteristics in the normal dispersion photonic crystal fiber with a flattened dispersion profile

The propagation of picosecond pulses in the normal dispersion photonic crystal fiber(PCF) with a flattened dispersion profile is numerically investigated.The characteristics of the amplitude and phase noise in the supercontinuum generation(SCG) are also analyzed through the coherent sliced supercontinuum(SC).The effects of self-phase modulation(SPM) and four-wave mixing(FWM) on broadening of the pulse spectrum are presented,and the best amplitude and phase noise performance with a specific fiber length is o...     

Wavelength dependence of modal dispersion in graded-index optical fibres

We describe a method based on the selective mode excitation at different wavelengths for studying the profile dispersion influence on the modal dispersion. By using this technique, we are able to determine either the optimum wavelength at which the fibre should be operated or the correction needed for optimising the index profile at a given wavelength.     

折射率中心凹陷对GIF色散特性的影响

分析了光纤制造中产生折射率中心凹陷的原因,建立了折射率中心凹陷GIF(渐变折射率光纤)模型,应用射线光学方法得出了光纤传输的时间延迟、自聚焦周期的表达式,数值模拟了中心凹陷GIF的自聚焦周期和时间延迟.仿真结果表明:随着凹陷宽度和凹陷深度的增大,GIF中光束的自聚焦效应变差,时间延迟差增大.     

Multimode fibers for compensating intermodal dispersion of graded-index multimode fibers

This paper proposes a novel type of multimode dispersion compensation fiber (MM-DCF) by which the intermodal dispersion of multimode fibers (MMFs) can be effectively compensated. A theoretical model that precisely calculates the group velocities of a target MMF and its MM-DCF and the power coupling between the two fibers is applied to confirm the proposal presented here. As a demonstration of the operation of the MM-DCF, an MMF optimized for transmission at a wavelength of 850 nm is compensated by the MM-DCF, and its overfilled launch bandwidth at a wavelength of 1300 nm is enhanced from 0.62 to 2.56GHz/spl middot/km.     

Exact analysis of intermodal dispersion compensation in spliced graded-index optical fibers

Intermodal dispersion compensation in spliced graded-index optical fiber is investigated. Group delay times of modes in individual and spliced fibers are exactly analyzed using the Wentzel-Kramers-Brillouin (WKB) method. It is found that in actual fibers whose refractive index profiles are not represented by the power-law function, the compensation effect is less remarkable than that predicted from the power-law approximation. The baseband bandwidth of spliced fiber, composed of under- and over-compensated index profile fibers, is calculated based on the measured index profile. As a result, the measured bandwidth is found to be well predicted with an error less than 5 percent.     

Optimum profile parameter on graded-index optical fibre at 1.27 ?m wavelength

The optimum profile parameter on germanium phosphosilicate graded-index optical fibres at 1.27 ?m wavelength is examined experimentally and is found to be 1.88 ± 0.01. Graded-index optical fibres that simultaneously possess high transmission bandwidth (in excess of 1 GHz km) and attenuation below 1 dB/km are obtained.     

Interference patterns from very thick samples for determining the profile of graded-index fibres

Recent studies have pointed out the need for very thin (10 ?m) fibre samples prepared for interferometric analysis of the refractive index profile of an optical fibre. We show that this is not necessary for all graded-index fibres with reasonable dispersion properties. Sample thicknesses of 125 ?m, or more, can be used for fibres with a core diameter of about 50 ?m.     

Guided-wave characteristics of optical graded-index planarwaveguides with metal cladding: a simple analysis method

The guided mode characteristics of metal-clad graded-index waveguides are generally determined by complex eigenvalue equations or perturbative methods. However, these methods either require several iterations in the complex plane or are only suited to the class of waveguides whose guided field can be described analytically. We describe a method of calculating complex propagation constants for metal-clad graded-index waveguides under very general but weakly guiding conditions. The method, based on Galerkin's formalism using trigonometric basis functions, allows arbitrary inhomogeneous and complex refractive index profiles. Applications to three generally used waveguide models show our approach to be in good agreement with other analytical or numerical methods     

Optimum dispersion profile for compression of fundamental solitonsin dispersion decreasing fibers

In this paper, we have investigated the effect of dispersion profile on the performance of pulse compression in a dispersion decreasing fiber (DDF). Four closely shaped profiles, namely linear, Gaussian, exponential, and hyperbolic, have been considered. We have shown that appropriate shaping of the dispersion profile, particularly when higher order effects are accounted for, is quite effective for adiabatic compression of fundamental solitons, producing high-quality compressed pulses in the sense that the soliton pulse is compressed as a single unit and pedestals do not form. It is concluded that the linear and Gaussian dispersion profiles are nearly optimum in regards to the quality of the compressed pulse and the amount of compression achievable, in particular in the case of subpicosecond soliton pulse compression     

Pulse response of a graded-index optical fibre

An analysis has been made of pulse dispersion in an optical fibre having a continuous radial variation of refractive index. Solutions are presented for Selfoc fibre showing the effects of mode and material dispersion and of group delay. The predicted dispersions range from very low values up to about 1 ns/km, depending on the launching conditions.     

Computation of the dispersion characteristics of a shieldedsuspended substrate microstrip line

The dispersion characteristics of a shielded suspended substrate microstrip line are calculated using five different sets of basis functions for the current distributions. Their comparison leads to the more suitable basis functions for the acquisition of fast and accurate results for frequencies up to 100 GHz