Robert Olshansky

Pulse broadening in graded-index optical fibers

This paper reports on some theoretical and experimental investigations of the radial refractive index gradient that maximizes the information-carrying capacity of a multimode optical waveguide. The primary difference between this work and previous studies is that the dispersive nature of core and cladding materials is taken into consideration. This leads to a new expression for the index gradient parameter alpha(c) which characterizes the optimal profile. Using the best available refractive index data, it is found that in high-silica waveguides, the dispersive properties of the glasses significantly influence the pulse broadening of near-parabolic fibers, and that the parameter alpha(c) must be altered by 10-20% to compensate for dispersion differences between core and cladding glasses. These predictions are supported by pulse broadening measurements of two graded-index fibers. A comparison is made between the widths and shapes of measured pulses and pulses calculated using the WKB approximation and the near-field measurement of the index profiles. The good agreement found between theory and experiment not only supports the predictions made for the value of alpha(c), but demonstrates an ability to predict pulse broadening in fibers having general index gradients.

Mode Coupling Effects in Graded-Index Optical Fibers

Mode coupling theory is applied to the study of multimode optical fibers with graded-index cores. For coupling caused by random bends in the waveguide axis, the results predict the dependence of the induced losses on the fibers characteristics. The impulse response is determined for fibers with random bends having several different power spectra. The results are used to predict the transmitted power, the delay time, and the rms pulse width in fibers with graded-index cores.

Tensile strength and fatigue of optical fibers

A method is presented for analyzing laboratory measurements of tensile strength and fatigue failure rates of glass fibers. This method provides the physical basis for extrapolating laboratory data to the multikilometer lengths and long lifetimes which will be characteristic of field use of optical fibers in telecommunication systems.

Differential mode attenuation measurements in graded-index fibers

An automated apparatus is described for measuring mode-dependent scattering and total attenuation in graded-index waveguides. Measurements are reported for two low-loss waveguides and several high-loss waveguides. The results illustrate the ability of this technique to identify different types of loss mechanisms.

Pulse broadening caused by deviations from the optimal index profile

The excess pulse broadening caused by deviations from the optimal index profile is analyzed for multimode optical fibers. First-order perturbation theory is used to evaluate the effects of arbitrary index perturbations having longitudinal spatial frequencies which are too low to cause mode coupling. A simplified expression is found for the excess pulse broadening in terms of a double integral over a weighting function. Weighting functions are evaluated and used to calculate the effects of two kinds of circularly symmetric perturbations, a Gaussian-shaped bump or dip in the index profile and an oscillating perturbation. These perturbations are found to affect the pulse broadening significantly according to the position and width of the bump and the transverse spatial frequency of the oscillating perturbation. The more narrow bumps and those located at larger core radii are found to have the greatest effect. For a typical multimode fiber, perturbations making 2-18 oscillations along the radius are found to cause significant increases in pulse broadening, while more rapid oscillations are found to have negligible effect. These results indicate the type and degree of index profile control required to keep the pulse broadening below a specified level.

Mode-dependent attenuation of optical fibers: excess loss

A theory is presented for calculating the excess loss produced by random perturbations of optical fibers. The theory is applicable to perturbations whose longitudinal spatial frequencies are below the range required for mode coupling. To illustrate the method, losses due to diameter variations are calculated for the case of a step-index optical fiber. The diameter variations are found to produce a strong attenuation of the higher order modes. The total excess loss is approximately wavelength independent.

Multiple-α index profiles

A new class of index profiles is analyzed. These profiles afford new degrees of freedom in profile design, which make it possible to obtain maximum bandwidth over an extended range of wavelengths or at several different wavelengths. A method is developed for calculating pulse dispersion vs wavelength for this generalized class of index profiles.

Effect of the cladding on pulse broadening in graded-index optical waveguides

The WKB analysis is extended to take into account the discontinuity in index profile shape that occurs at the core-cladding boundary of graded-index optical waveguides. The presence of the cladding introduces a large correction to the delay times of the highest 5% of the guided modes and greatly increases the predicted rms pulse broadening of a parabolic profile fiber. Using a model of differential mode attenuation, it is shown that these highest order modes are very susceptible to attenuation. A small degree of waveguide irregularity restores the low pulse broadening originally predicted for a parabolic fiber.

Leaky modes in graded index optical fibers

Leaky mode attenuation coefficients for graded index optical fibers are derived using the WKB approximation. For the case of a parabolic index profile, the attenuation coefficients for explicitly evaluated and used to calculate the leaky mode contribution to near field and attenuation measurements. Failure to observe the expected leaky mode contributions in several graded index fibers is interpreted as evidence for the presence of an additional loss mechanism.

Mode dependent attenuation in parabolic optical fibers

A previous study of excess attenuation caused by slowly varying perturbations is extended to optical fibers with parabolic index profiles. For random diameter variations, total losses are found to be essentially the same in step and parabolic fibers.