<title>High-silica optical fibers and trends in lightwave communications: a twenty-five year-retrospective</title>

Abstract

Low-loss high-silica fibers were first reported in 1970 and since then these glass fibers have proven to be the most viable transmission media for lightwave communication. Even though the first low loss fiber was a single-mode fiber, initial systems revolved around multimode fibers from the point of view of practical reasons like easier fiber to fiber splicing and coupling from off-the-shelf available LEDs. Since early 1980s, single-mode fibers became the preferred media for optical transmission and single-mode fibers overtook the multimode fibers for lightwave communication. Networks which operate with single-mode fibers at the 1.31 /mm low-loss wavelength window are now taken for granted. Operations with advanced fiber designs at the silica fiber’s lowest loss window of 1.55 /mm are now gearing to take over the long-haul routes; these are contemplated to be based on erbium doped fiber amplifiers (EDFA) and dispersion shifted fibers. An alternative and promising scheme for this wavelength window involving use of already laid fibers optimized for 1.31 /an requires insertion of a dispersion compensating fiber(DCF) as an additional component in the link. In the immediate future, lightwave systems would operate with EDFA at the OC-48 level, which amounts to a bit rate of — 2.5 Gb/s and are compatible with SDH (STM-16) networks. For WDM transmission with EDFAs, one would require a variety of in-line fiber components like wavelength selective couplers, and bandpass/bandstop filters to add/drop channels. Until recently, optical fibers were considered to be optically linear. However, with the introduction of EDFAs, which provide large and broadband (* 30-35 nm) optical gains, a large number of WDM information channels can be transmitted and simultaneously provided gains across this broad gain spectrum of EDFAs. Furthermore, amplifier spacings could be much longer than the present day regenerator spacings. These two fall outs of EDFA namely, large optical power density and long interaction lengths have elevated optical fiber nonlinearities to an important design issue in lightwave systems. In this presentation we would aim to trace the evolutionary trends in lightwave communications seen along the above lines since the reporting of the first low-loss fibers a quarter century ago. INTRODUCTION