Pulse broadening in graded-index optical fibers

Abstract

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.

The future of plastic optical fiber

“…Th e bandwidth properties of GI POF with refractive index profiles expressed by equation (1) have been analyzed by the Wentzel-Kramers-Brillouin (WKB) method [36][37][38]. Generally, the bandwidth of optical fiber that excites many modes, such as POF, is dominantly influenced by modal dispersion.…”

Influence of core diameter on the 3-dB bandwidth of graded-index optical fibers

“…3 as functions of the refractive index exponent for the three standard wavelengths. These plots are based on Olshansky and Keck's theory which neglects distributed attenuation and mode-coupling effects [6]. The characteristic feature of the grading is the well-known peaked bandwidth due to intermodal dispersion.…”

“…We believe that this slight discrepancy is to be connected to the approximations made by Olshansky and Keck, rather than being a cause of an eventual inaccuracy of the present model. As a matter of fact, the results given in [6] neglect a term proportional to the square spectral width in the intermodal dispersion formula. This term which is the source contribution to intermodal dispersion is quite small when the first term in (25) of [6] is high enough, but it becomes significant for refractive index exponents approaching the optimum value.…”

“…So far, different factors have clearly been identified to influence the information-carrying capacity, namely, the material dispersion (in combination with the spectrum of the exciting source) [6], [36], [7], the launching conditions [8], [9], as well as the mode-dependent characteristics (i.e., delay [6], [36] attenuation [10], and coupling-coefficient [10]- [12]). Unfortunately, the achievements, so far accomplished, are not quite complete to enable precise bandwidth prediction if an arbitrary operating condition is to be considered.…”

“…This might explain, among others, why measured bandwidths in MMF's seldom match exactly with theory. The most interesting description of pulse broadening in graded-index (GRIN) MMF's was certainly that of Olshansky and Keck [6]. This dispersion model provided essential guidelines for the development of GRIN fibers.…”