History Of Optical Fiber Technology

Written by Colby C. Young    Sunday, 08 March 2009

Traditional telegraphy used wire to transmit voice signals. However, the practical frequency limit for metal wire is a few megahertz, since the increase in resistance with frequency causes intolerable loss. Coaxial cables developed in the 1940s enabled transmission up to 10 GHz, and, later, millimeter waveguides allowed transmission to 100 GHz. This technology was abandoned when inexpensive optical fibers were developed as a suitable transmission medium for optical communication. The confinement of light by total internal reflection was well known in the 1850s. Glass fibers based on this principle were developed for endoscopes early in the 1900s. However, even as late as 1966, the best such fibers had losses of 1,000 dB/km compared with 0.2 dB/km today. The use of low-loss glass fiber for communication was first proposed in 1966 by Kao and Hockham,( n4) who suggested that the intrinsic loss of silica-based glass could be low enough to enable use as a lightguide. At the time, absorption was dominated by impurities in the glass, so research throughout the world concentrated on improved purification of conventional multicomponent glasses. Just as these efforts began to succeed, they were supplanted by better processes involving vapor deposition of high-silica glass, first reported by Maurer( n5) in 1970. Vapor deposition processes were refined to compose two categories--outside vapor deposition (OVD) and modified chemical vapor deposition (MCVD), reported by MacChesney et al. in 1974.( n6) Kao, Maurer, and MacChesney received the 1999 Draper Award for their pioneering work. Achieving low loss required: - Pure starting materials provided by high vapor-pressure chlorides of silicon and germanium, - Reaction of those materials with oxygen at high temperature, and - Processing in an ultraclean environment containing chlorine for further purification. The first fibers so produced were multimode guides having a core diameter of 62.5 Mu m to facilitate splicing and connectorizing. Such fibers are still commonly used in local area networks both for these reasons and because light from low-cost light-emitting diodes is readily launched into the large cores. For long-haul applications, however, single-mode fiber having a small core diameter (less than is similar to 10 Mu m) eventually became dominant because of its higher bandwidth and the development of splicing and connector technology. Single-mode fiber designs remained relatively unchanged throughout the 1980s as optical networks were installed around the world. These networks used commercially available lasers operating at 1.3 Mu m, the zero-dispersion point of this standard fiber. As 1.55-Mu m lasers, corresponding to the lowest loss region of silica fiber, became available, new dispersion-shifted fiber, simultaneously offering both zero dispersion and low loss at 1.55 Mu m, was developed. A major revolution in this industry occurred in the early 1990s when erbium-doped optical fiber amplifiers, invented in 1987, became commercially available. These amplifiers permitted, for the first time, the direct amplification of optical signals without conversion to the electronic domain. Now light could continue propagating within a fiber for hundreds of kilometers using periodic optical amplification (every 40 to 80 km) to compensate for the fiber loss. Equally as important, these amplifiers operated independently of data rate, format, and wavelength (within the erbium gain spectrum), thereby making dense WDM (DWDM) practical for the first time. Advances in optical amplifier technology dictated the usable wavelength window, which continues to grow rapidly. New fiber designs emerged to accommodate the broad range of wavelengths required for DWDM, and dispersion-compensating fiber was developed to minimize the deleterious effects of dispersion. To manage the dozens of wavelengths in DWDM systems, optical fiber gratings were developed to control and monitor the transmission of multiple wavelengths in the fiber network, and planar optical waveguides using silica-on-silicon substrates were developed for integration of complex optical devices on a single silicon chip.

Last Updated ( Sunday, 08 March 2009 )