Active optical fibres

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 was flash-pumped in the form of an optical fibre, a configuration which was used to overcome the difficulties of obtaining high-quality glass in bulk form. Apart from a report 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 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, polarisers, filters) were available which allowed construction of complex, all-fibre ring and Fabry-Perot resonators' to form a unique and powerful new fibre-laser technology. Even so, it was only the announcement in 1987 of a high-gain, erbium-doped fibre amplifier' (EDFA) operating in the third telecommunications wavelength-window at 1.54µm 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 recently resulted in several commercial products appearing, a time-lag of only three years after the first research announcement. The fibre laser, on the other hand, is only now beginning to receive widespread attention as a possible contender for a well-controlled, stable light source for telecommunications, despite its obvious advantages of high-power pulsed operation, single-frequency capability, ease of access to the resonator and compatibility with communications fibre