The refractive index distribution in the core of a multimode optical-fiber waveguide plays an important role in determining the transmission properties of the guide. The closer the index profile is to the required ideal distribution, the greater the resulting information carrying capacity of the fiber. This review paper discusses methods for measuring the refractive index distribution in optical fibers and for predicting their impulse response and signal bandwidth from the measured profiles. Some attention is also given to preform and single-mode fiber profiling.

https://ieeexplore.ieee.org/iel5/5/31290/01455989.pdf

Index profile measurements of fibers and their evaluation

A simple perturbation theory, based on the scalar wave analysis, has been used to study the propagation characteristics of radially inhomogeneous lossless fibres. The formalism has been developed with a truncated parabolic-index profile as the zero-order solution and the perturbation approach is valid for azimuthally symmetric or anti-symmetric modes near cut-off as well as far from cut-off. In addition, numerical calculations have been carried out forα-profiles and have been found to be in good agreement with more rigorous calculations reported by previous authors.

Analyse perturbative des caractéristiques de propagation des fibres optiques à gradient d'indice quasi-parabolique

The refractive index profiles of several multimode optical fibers were measured by four of the current state-of-the-art techniques. These include interference microscopy on slab samples, transverse interference microscopy on whole fiber samples, the focusing method, and the refracted near-field method. The profile of the parent preform of one of the fibers was also measured by both the focusing method and by a ray-tracing approach. Comparisons of the results and the measurement methods are made emphasizing the applicability and use of the various techniques.

http://ieeexplore.ieee.org/iel7/6731005/6771169/06771175.pdf

Profile characterization of optical fibers — A comparative study

The information carrying capacity of multimode optical fibers is mainly determined by the quality of the refractive index distribution of their cores which, in turn, is determined by the distribution existing in the fiber's parent--the preform. This paper reviews methods that can be used to measure the index profiles of preforms--both multi-mode and single mode. Attention is also given to the effects of profile perturbations and means of eliminating them, as well as to applications of the techniques to fiber profiling.

The index-profile characterization of fiber preforms and drawn fibers

Optical fiber transmission systems with extremely long repeater spacing are very attractive for various applications both in land and undersea communication systems. This paper describes the results of transmission experiments, using low-loss fibers in the 1.2–1.6 μm wavelength region. Graded-index optical fibers having an optimum profile at 1.27 μm were manufactured in cables. Average bandwidth was 1,275 MHz-km, and the average optical loss was 0.6 dB/km. The 52.6-km and 62.3-km repeater spacings were realized at 100 Mb/s and 32 Mb/s transmission, respectively, using InGaAsP/InP LD and Ge-APD. The 21.5-km and 12.0-km repeater spacings at 32 Mb/s were also realized by 1.2μm and 1.5μm LEDS, respectively.

Transmission experiments in the 1.2-1.6 μm wavelength region using graded-index optical fiber cables

The use of graded-index fibers for optical communication systems is now under consideration for numerous civil and military applications, and in these either LED or laser sources can be developed. The bandwidth of the system depends on the source linewidth, the dispersion of refractive index in the fiber, the strength of excitation of the modes by the source, and the extent of mode mixing caused, by example, by microbending. In fibers with refractive-index profiles that have a nearly parabolic dependence upon radius, pulse broadening is a slight, but small departures from an optimum profile can cause a dramatic decrease in bandwidth. It is of considerable importance to predict the bandwidth of fibers from a knowledge of the refractive-index profile and source geometry. The purpose of the first part of this paper is to compare two computational methods by which the propagation characteristics of a fiber, hence the impulse response and system bandwidth, may be determined. In the second part, we sha...

P. J. B. Clarricoats

Papers

Transmission characteristics of graded-index fibers