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137 km, 4*5 Gbit/s optical time division multiplexed unrepeatered system with distributed erbium fibre preamplifier
Electronics Letters
Abstract
A 35 km long distributed erbium fibre preamplifier has been used to provide dispersion equalisation and to maintain the signal above the receiver sensitivity level. Remote supply of pump power to the input of the lightly-doped fibre provides significant improvement in amplifier performance.
We have seen in Chapter 6 that a conventional semiconductor laser does not emit light in a single longitudinal mode. In general, the mode closest to the gain peak is most intense, and a few percent of the output power is carried by other longitudinal modes lying close to the gain peak. Furthermore, even when these side modes are reasonably suppressed under CW operation, their power content increases significantly when the laser is pulsed rapidly. Direct modulation of a semiconductor laser at frequencies in the gigahertz range is commonly employed in optical communication systems. In the presence of chromatic dispersion, the unwanted side modes limit the information transmission rate by reducing the fiber bandwidth. It is therefore desirable to devise means for a semiconductor laser to emit light predominantly in a single longitudinal mode even under high-speed modulation. Such Lasers are referred to as single-frequency or single-longitudinal-mode Lasers and in view of their potential application in optical communication systems were studied extensively during the 1980s.1 They were used in commercial lightwave systems by 1990.
Connecting an input port and an output port of a single-mode fiber coupler to form a fiber-loop, and making the time delay
of the fiber-loop odd multiple of the half-period of the input optical pulse stream, we can obtain a pulse stream with multiplying
repetition rate at the other output port of the coupler. Utilizing three loop-connecting fiber couplers cascaded, a pulse
stream with eight times the repetition rate is achieved.
Connecting an input port and an output port of a single-mode fiber
coupler to form a fiber-loop, and making the time delay of the
fiber-loop odd times half-period of the input optical pulse stream, we
can obtain a pulse stream with multiplying repetition rate at the other
output port of the coupler. Utilizing three loop-connecting fiber
couplers concatenated, a pulse stream with eight times the repetition
rate is achieved
The error ratio performance of soliton like transmission over a 68
km distributed erbium fibre link is reported. The transmission link had
a net gain of only 2 dB, with remarkably low signal power excursions and
a penalty of <1 dB was observed at 40 Gbit/s
A rate equation model is used to calculate the excess noise figure
for erbium-doped fibre amplifiers. It is shown that erbium amplifiers
can operate with high gain and can have noise figures only 0.2 dB above
the quantum limit
Noise measurements on a distributed erbium-doped fibre amplifier are reported for the first time. Data are presented for fibre lengths up to 54 km, and are in good agreement with theoretical predictions.
A 20 Gbit/s optical time division multiplexed transmission system based on components and subsystems operating at not more than 5 GHz repetition rate is reported. Full BER measurements indicate a penalty of only 0.8 dB after transmission through 205 km of dispersion-shifted fibre. The penalty was mostly attributed to the accumulation of noise from erbium-doped fibre repeater amplifiers in the system.