Transmission Studies of a Long Single-Mode Fiber - Measurements and Consideration for Bandwidth Optimization

The high bandwidths and low losses of single-mode fibers make them leading contenders for use in future wideband undersea cable systems. 1 These systems are expected to transmit at 274 M b / s between repeater stations that will be approximately 35 km apart. In anticipation of future need for long lengths of single-mode fiber, the modified chemical vapor deposition (MCVD) preform fabrication process has been scaled up. By using 19- by 25-mm support tubes 4 ft 1713 long, and glass deposition rates of 0.5 g/min, very large preforms can be made in reasonable times. Each preform yields up to 40 km of fiber. 2 The purpose of this paper is to present the results of transmission measurements made on a 21.7-km fiber--the longest continuous MCVD fiber drawn to date. Improved automated test set-ups were used to measure loss and dispersion spectra in the 1.06- to 1.7-jum wavelength region.3'4 Group-delay measurements were used to determine the minimum dispersion wavelength of the fiber. Bandwidth spectra, calculated from group-delay measurements, were compared to direct measurements of pulse broadening due to light sources with 4-nm emission linewidths. 5 Potential system performance was estimated by using the baseband frequency response of the fiber to calculate dispersion power penalties and chromatic-dispersion-limited repeater spacings for 274and 548-Mb/s data rates. 6,7 Finally, results from a numerical study were used to suggest more optimal waveguide parameters for future fibers that could have higher band widths in the vicinity of 1.3-jum v avelength.