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The authors demonstrate 40 Gbit/s RZ signal transmission over a 240 km conventional singlemode fibre, using a time-division alternating polarisation format, a dispersion compensation fibre, and a polarisation independent receiver without an additional polariser. Alternating polarisation is compared to single polarisation for 200 km SMF transmission.

A 3 dB gain bandwidth of 75 nm (1531-1606 nm) is realised in a novel configuration of a two-stage erbium-doped fluoride fibre amplifier with a discrete fibre Raman amplifier. Error-free operation in an in-line amplifier configuration is also confirmed through a WDM (2.5 Gbit/s/spl times/9 channel) transmission experiment.

A high-gain (33 dB) and wideband (over 40 nm) 1.65 µm fibre Raman amplifier is realised using a novel pump light source. The light source emits intense amplified spontaneous emission light with a tunable wavelength (1540–1563 nm) and broad linewidth.

3 dB bandwidths of 52 nm (1556 – 1608 nm) for a silicate erbium-doped fibre amplifier (EDFA) and 50 nm (1554 – 1604 nm) for a fluoride EDFA have been obtained by a novel scheme with two-stage erbium-doped fibres and an intermediate equaliser. Low noise figures and high pump efficiencies of the EDFAs have been confirmed.

The successful two-channel WDM transmission of optically multiplexed 40 Gbit/s RZ signals with properly managed fibre dispersion is demonstrated. The dispersion dependence of the signal-to-noise-ratio for each signal channel is measured and it is found that the optimum total dispersion value, including dispersion compensation fibres, is not zero but is anomalous, which is slightly smaller than the value predicted by the soliton condition with uniform dispersion.

The widest, seamless, 76 nm (1531–1607 nm) 3 dB gain-band bidirectional amplifier is realised by combining an erbium-doped fluoride fibre amplifier (1531–1562 nm) and a gain-flattened erbium-doped silicate fibre amplifier (1562–1607 nm) in a novel configuration of gain-band separation between upstream and downstream signals. Bidirectional 10-channel WDM transmission at 2.5 Gbit/s over a 250 km conventional singlemode fibre is successfully demonstrated using our amplifiers as two bidirectional repeaters.

A 3 dB gain-reduction bandwidth up to 65 nm (1549-1614 nm) is achieved using a two-stage erbium-doped fibre amplifier, together with an intermediate equaliser and Raman amplification in the transmission fibre.