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 fiber's 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.

Mode Coupling Effects 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.

Pulse broadening in graded-index optical fibers.

Glass fibers are in general not thick enough to withstand external forces on their own without suffering axial distortion, mode coupling, and loss. Thus, plastic jackets must be carefully designed to provide effective protection. We evaluate jacket designs ranging from the mere use of soft materials to the application of multiple plastic coatings and graphite reinforcement. We compute the distortion loss as a function of dimensional variations and of lateral forces considered typical for cable packaging. The protective quality of a jacket is found to depend on a combination of stiffness and compressibility and on the fiber characteristics.

Optical-fiber packaging and its influence on fiber straightness and loss

This paper investigates experimentally the change of the refractive index, due to forces such as pulling and pure bending, in an optical fiber fabricated by the CVD technique. It is found that this phenomenon can be interpreted in terms of a simple model of the fiber, that is, a mechanically homogeneous circular rod. We compare the effect of the refractive-index change and that of a geometrical deformation of the fiber on transmission characteristics. A new method based on photoelasticity is also proposed to measure the curvature distribution of a fiber whose axis is deformed by external forces.

Change of the refractive index in an optical fiber due to external forces

Distortion losses in cabled optical fibers.

Externally controlled mode coupling was observed in a 0.7-km optical waveguide fiber resulting in a significant pulse narrowing. The method of mode coupling generation requires that care be exercised in any mode coupling measurement. Observations on a 3-km waveguide failed to show evidence of mode coupling, allowing the possibility of practical external control.

D.B. Keck

Papers

Observation of externally controlled mode coupling in optical waveguides