P. Di Vita

Mode field diameter measurements in single-mode optical fibers

The role of the mode field diameter in the characterization of single-mode fibers is examined. The most relevant definitions of this parameter are reviewed, and a comparative analysis of methods for its measurement is performed. All the discussed measurement methods have reached a repeatability and reproducibility which are quite satisfying. Emphasis is given to the requirements posed by the new fiber designs, such as the polarization-maintaining structures. Most of the discussed techniques have been industrialized, and a number of instruments based on them are commercially available; however, it is predicted that the evolution of fiber design will impose new requirements on some of these instruments. >

The backscattering technique: its field of applicability in fibre diagnostics and attenuation measurements

The evaluation of power decay along the fibre by means of the backscattering technique may be altered by the presence of unknown irregularities. In the present paper the effects of such imperfections are estimated; it is demonstrated that mainly local inhomogeneities and numerical aperture fluctuations influence backscattered power. Finally an efficient and simple method for the separation of the power decay contribution from the irregularity contribution is proposed.

Theory of power coupling between multimode optical fibres

In this paper power coupling between two multimode optical fibres is investigated. A formalism (based on a geometrical approach) suitable for analysing any coupling configuration between fibres with any index profile and any radiance distribution is presented. The model is applied to obtain coupling losses both in uniformly excited parabolic- and step-index fibres, in the presence of all kinds of coupling errors. Numerical results of extensive utility are shown and useful asymptotic expressions of coupling efficiency, valid for small values of coupling error parameters, are derived together with some practical rules for error combination. Mention is also made of some experimental results we have obtained, which corroborate some previous assertions. The work can supply a useful tool in the design of joints and connectors between single fibres or optical cables.

Geometrical theory of coupling errors in dielectric optical waveguides

Abstract The power losses due to coupling errors between source and fibre or two fibres are evaluated by means of a geometrical analysis of the coupling between an emitting and a detecting surfac. The theory is apllied to actual cases of separation, displacement and misalignment between a uniformly emitting lambertian source and fibre, and between two fibres. Good agreement is obtained with experimental results found in the literature.

Optical fibre amplifiers: physical model and design issues

The development of optical fibre amplifiers has caused an impressive evolution in optical telecommunications systems since the end of the 1980s. The widespread application of active fibre amplifiers requires accurate tools to design and simulate these devices in very different operating conditions. The present work describes some physical aspects of active fibre amplifiers and a numerical model for the analysis of active optical fibres that can be applied to investigate spatial and spectral properties of amplifiers for the transmission windows of optical fibre telecommunications. Finally, various design issues relevant to the investigation of optimal fibre design of silica, fluoride erbium-doped and fluoride praseodymium-doped fibres are discussed and illustrated by means of examples.

Theory of scattering in multimode optical fibres

The problem of scattering in multimode optical fibres is treated in a rigorous way by means of both a ray and a statistical approach. These two methods are interlaced and harmonized. Applications to microbending, ellipticity, core radius and maximum numerical aperture variations, and fluctuations of index profile shape are performed. Useful results about power distribution and fibre attenuation are derived in each case.

Single-Mode Optical Fiber Characterization

The measurement of the relevant parameters of single-mode optical fibers (namely, cutoff wavelength, mode field diameter, chromatic dispersion, and refractive index profile) is discussed, with explicit reference both to the work carried out within the Comite Consultatif International Telephonique et Telegraphique (CCITT) and to original techniques developed by the authors. The main research evolution trends are pointed out.

Processing of Near-Field Intensity Measurements in Optical Fibres

The processing of near-field intensity measurements is discussed. In multimode fibres mode distributions can be derived; in monomode fibres the index profile and all the related quantities can be calculated.

The “radiance law” in radiation transfer processes

The conservation law of radiance as a suitable tool for investigation of radiation transfer processes between two systems is treated. Original proofs of this law are given and its deep thermodynamical foundation is pointed out. Finally, applications to general problems of power coupling and particularly to the radiation transfer from incoherent sources (LED) into multimode optical fibres are carried out.

Group velocity of modes and pulse distortion in dielectric optical waveguides

After a brief summary of the main factors causing pulse distortion in optical fibres, expressions are derived for phase velocity, group velocity and dispersion, that is velocity variation with frequency, of each mode. The analysis is carried out by making extensive use of the functiony(x) (y andx being the arguments of the first and second kind Bessel functions that appear in the solutions of Maxwell equations), that we call characteristic function. By using this function we obtain particularly simple and compact formulas that make easier the analysis of the behaviour of velocity and dispersion near and far from cut-off. It will also be seen that the ratio between the power propagating in the core,W1, and the power in the cladding,W2, for each mode takes a very simple form if expressed by means of the characteristic function: this fact permits a direct relation to be found between group velocity and power flow in an optical fibre. The study of the ratioW1/W2 will show that, contrary to expectation, for some modes, a substantial fraction of energy travels inside the core, even near cut-off. Expressions are given forW1/W2 at cut-off, and far from cut-off. Plots of velocity and dispersion are shown. On the grounds of the obtained results pulse distortion in optical fibres is discussed.