Amplifier gain error

#The invention relates to optical networks and in particular to amplifying modules for such networks.

#In optical telecommunications networks, encoded optical signals are transported generally between transmitters and receivers by optical fibre links. At various locations along the links, usually at nodes or link junctions, the optical signals are amplified so as to compensate for losses incurred during transportation. Typically, the signals are amplified by optical amplifiers such as erbium doped fibre amplifiers (EDFAs).

#An EDFA or other fibre amplifier module typically has an input to which an incoming signal for amplification is applied, and an output from which an amplified signals emerges. The EDFA has an active fibre part which comprises an optical fibre doped with erbium and coupled to a source of pump light for exciting the erbium dopant ions. The pump light wavelength is chosen so as to excite electrons within the erbium ions to a level from which, when they transit downwards to a lower energy level, photons of wavelength which is the same as the input signals are emitted. These photons are in phase with and travel in the same direction as the input signal photons, and the net result is an amplified output signal.

#FIG. 1 is a schematic diagram of a prior art optical amplifying system;

#FIG. 2 is a schematic diagram of an exemplary amplifying system according to the invention; and

#FIG. 3 is a table of comparative values for a prior art system and a system according to the invention.

#With reference to FIG. 2, an amplifier module 101 according to the invention is situated at the node N in an optical fibre link between a transmitter (not shown) and a receiver (not shown). The module comprises all EDFA 102 having an erbium doped fibre component including a length of erbium doped fibre (not shown) pumped with light of appropriate wavelength from a semiconductor laser (not shown). The module 101 also comprises control electronics constituted by a signal processor 108 which controls the operating conditions of the module 101. In FIG. 2, some of the control electronics have been omitted for ease of understanding, but these are the same as the control electronics described with reference to FIG. 1.

#An input fibre 114 carries encoded optical signals for amplification transmitted in dense wavelength division multiplexed form. That is to say, the transmission band, for example the C band, is divided into a number of closely spaced wavelength channels and each signal is assigned to one of these particular channels. The channel signals are combined at a multiplexer (not shown) into a multiplexed stream and the stream is sent along the fibre 114. At the EDFA 102 the stream is amplified. The EDFA has a target or set point gain, that is, a target amount by which the stream is preferably amplified. Some degree of variation to either side of the set point gain is acceptable. However, limits are put on the extent of acceptable deviation. At any one time, the stream may consist of any possible configuration of channels, or channel loading condition. Each loading condition may affect the set point gain variation and, in the case of some conditions, may result in an unacceptably large deviation.

#Also sent along the fibre is an optical service channel (OSC) signal. This signal, carried on an out-of-band channel, is encoded with link servicing information. The OSC signal may also be encoded with channel information, in particular the in-band channels present at the node N. In other words, the OSC signal nay include information representative of the channel loading condition.