Active fibres and optical amplifiers

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Abstract
The incorporation of rare-earth ions into glass fibres to form fibre lasers and amplifiers is not a recent development. In fact the first glass laser ever demonstrated [l] was flashpumped in the form of an optical fibre, a configuration that was used to overcome the difficulties of obtaining high-quality glass in bulk form. Apart from a report [2] in 1974 of laser operation in an Nd3+-doped silica multimode fibre, the idea of guided-wave glass lasers attracted little attention for the next 24 years. The idea resurfaced [3] in 1985 because both optical fibre and laser-diode technologies had advanced to a stage where low-loss, rare-earth-doped, single-mode fibres could be made and high-power semiconductor sources were available to pump them. In addition, low-cost fibre components (couplers, polarizers, filters) were available, which allowed construction of complex, all-fibre ring and Fabry-Perot resonators [4] to form a unique and powerful new fibrelaser technology. Even so, it was only the announcement in 1987 of a high-gain, erbiumdoped fibre amplifier (EDFA) [5] operating in the third telecommunications wave length window at 1.54 μm that sparked widespread interest in rare-earth-doped fibres in the optical telecommunications community. From that moment, frenzied worldwide activity has brought numerous new fibre amplifier developments and in 1990 resulted in several commercial products appearing, a time-lag of only three years after the first research announcement.