Erbium-doped fibre amplifiers operating at 1.5µm

Abstract

The field of rare-earth-doped fibre lasers and amplifiers has expanded rapidly in recent years and there are now several groups working actively in the area. A variety of glass hosts, dopants and pump sources have been used, with the goal of achieving low-threshold, diode-laser-pumped operation of fibre lasers and amplifiers, particularly those operating in either the second or third telecommunication windows. The small core size of the single-mode fibre allows high pump intensities for modest (~mW) pump powers. Moreover, the intensity can be maintained over long lengths and this leads to ultra-low lasing thresholds and even permits CW diode-laser-pumped operation of three-level lasers. In conjunction with the long fluorescent lifetime of rare-earths in glass, the high pump intensity allows high-gain (>30dB) operation of fibre amplifiers with excellent saturation properties. In addition, compatibility with existing fibre components is excellent, allowing all-optical fibre circuitry to be assembled with both active and passive components. This is particularly beneficial for the fibre amplifier, where splicing of the active fibre into the telecommunication link virtually eliminates troublesome Fresnel-reflection feedback which normally limits the gain in semiconductor laser amplifiers.
The configuration of a rare-earth-doped fibre amplifier is shown in Fig. 1. Using Er³⁺ as a dopant, exceptionally high gains (26dB) have been obtained at a wavelength of 1.536µm, for modulation rates up to 400MHz. The amplifier is optically pumped using any one of a variety of wavelengths. The input equivalent noise power has been measured at -42dBm at a bit rate of 140Mb/s which compares favourably with state-of-the-art APD detectors at 1.54µm. A maximum output power of +13dBm has been achieved before onset of saturation. These early results demonstrate that Er³⁺-doped fibre amplifiers possess excellent gain and noise characteristics which make them attractive as wideband amplifiers and repeaters for multi-channel optical transmission systems.

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