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The authors constructed two low-noise and high-gain 1.58 µm band Er3+-doped fibre amplifiers (EDFAs) with cascade configurations. One consisted of 0.98 and 1.48 µm band pumped silica-based Er3+-doped fibre amplifier units as first and second units, exhibiting a noise figure of 5.0 dB and a signal gain of 32.8 dB for a 1.58 µm band WDM signal. The other employed a fluoride-based Er3+-doped fibre amplifier unit as a second unit and exhibited a noise figure of 5.1 dB and a signal gain of 25.0 dB. Both EDFAs had low-noise and high-gain characteristics.

The authors demonstrate a gain-flattened Er3+-doped fluoride fibre amplifier with low noise amplification characteristics which employs 0.97 µm laser diode pumping. Excellent flat amplification is achieved with an average signal gain of 12.5 dB and a gain excursion of < 1.0 dB, with a noise figure of 4.7 ± 0.2 dB for an 8 channel WDM signal in the 1532–1560 nm wavelength region. The authors confirm that an Er3+-doped fluoride fibre can be excited effectively to the 4I11/2 level by shifting the pump wavelength from 0.98 to 0.97 µm.

The authors construct a broadband and gain-flattened Er3+-doped fibre amplifier (EDFA) with a flat amplification bandwidth of 54 nm for wavelength division multiplexed signals. This was achieved by employing a 1.58 µm-band EDFA and a 1.55 µm-band EDFA in a parallel configuration. Excellent flat amplification characteristics were obtained for 1530–1560 nm and 1576–1600 nm signal wavelength regions, with a gain non-uniformity of <1.7 dB and a signal gain of 30 dB.

An Er3+-doped fluoride-based fibre amplifier (F-EDFA) with flat amplification characteristics in the range 1565–1600 nm was constructed for the first time. This provided a 5 nm wider flat amplification range than a silica-based Er3+-doped fibre amplifier (S-EDFA). A flattened signal gain of 30 dB was achieved with a gain excursion of 0.9 dB for a WDM signal in this wavelength range. The Er3+-doped fluoride fibre (F-EDF) which achieved a signal gain of over 30 dB without any noise figure degradation had a shorter optimum length than Er3+-doped silica fibre (S-EDF). Furthermore, F-EDF extended the amplification wavelength in the longer wavelength range by 2 nm compared with S-EDF.

The output characteristics of 1.58 µm band gain-flattened Er3+-doped fibre amplifiers (EDFAs) for wavelength division multiplexing (WDM) systems are investigated. It is found that the total output signal power is uniquely determined by the input signal power and the average signal gain of each WDM channel, which is determined by EDF length. It is also found that there is a trade-off between the power conversion efficiency (PCE) and the noise figure. The PCE for low-noise operation is ~45 – 65%.

The amplification characteristics of Er3+-doped fibres for 1.58 and 1.55 µm band amplification are compared. The gain coefficient and power conversion efficiency were higher for the 1.55 µm band than for the 1.58 µm band.

The authors have developed praseodymium (Pr3+)-doped PbF2/InF3-based fluoride optical fibres (PDFs) with relative refracfive index differences (Δn) of 2.5, 3.7 and 4.6% for use as high efficiency optical fibre amplifiers. The small signal gain coefficients of the above fibres were 0.18, 0.26 and 0.30 dB/mW. These values are higher than those of ZrF4-based PDFs.

The amplification characteristics of 1.58 µm band Er3+-doped fibre amplifiers pumped in the 0.98 and 1.48 µm bands were investigated. The noise characteristics were improved by adopting 0.98 µm band pumping. There was > 0.3 dB improvement in both the average noise figure and the flatness of the noise figure.