Multiple-wavelength optical communication system with optical amplifiers

#The present invention relates to a multiple-wavelength optical communication system with optical amplifiers having optimized performance, and a method for optimizing the said performance.

#There are known multiple-wavelength (Wavelength Division Multiplexing, WDM) optical communication systems in which the information to be transmitted is carried by a plurality of signals, each at a predetermined wavelength (channels). In long-distance WDM systems, use is increasingly made of optical amplifiers which are connected between sections of optical fibre to amplify the signals which are attenuated during propagation in the said sections, and thus to increase the distances which can be covered with these WDM systems. However, conventional optical amplifiers, for example those of the rare earth doped fibre type or the semiconductor type, have the disadvantage that they do not have a flat gain spectrum (gain as a function of the wavelength). Instead, they have a gain spectrum which varies with the wavelength according to a predetermined curve. Consequently, the different channels of a WDM system are not amplified uniformly along the transmission line, and therefore their optical signal to noise ratios (SNR) differ from each other (non-equalized optical SNR) at the receiving point at the end of the cascade of optical amplifiers.

#In the present description and in the attached claims, the expression “optical signal to noise ratio” is used to denote the ratio, at the receiving point (at the end of the cascade of optical amplifiers), between the optical power of a channel and the optical noise power due to the spontaneous emission (Amplified Spontaneous Emission, ASE) of the optical amplifiers which is present in the optical band about the said channel. In its turn, the expression “optical band of a channel” is used to denote the band of the optical filter which is used in the receiving equipment of an optical communication system to filter the optical noise and to separate this channel from the others with respect to wavelength. Typically, this optical band ranges from 0.2 nm to 1 nm.

#The characteristics and advantages of the invention will now be explained with reference to embodiments represented by way of example and without restriction in the attached drawings, in which:

#FIG. 1 shows a first embodiment of an optical communication system according to the invention;

#FIG. 2 shows another embodiment of an optical communication system according to the invention;

#In a first embodiment shown in FIG. 1, an optical communication system 1 according to the invention comprises a transmitting apparatus, a receiving apparatus (neither of which is shown) and an optical transmission line 1000.

#The said transmitting apparatus is a conventional WDM apparatus capable of supplying m optical signals (channels) having a predetermined power and a predetermined wavelength λ1, λ2 . . . λm, each being different from the others. Additionally, according to the prior art, the said transmitting apparatus wavelength multiplexes the said channels and sends them along the optical transmission line 1000.

#The wavelengths of the said channels are selected, according to the invention, from a first wavelength range Δλ having a width of at least 3 nm. In its turn, this first wavelength range Δλ is selected from a second wavelength range extending from 1300 nm to 1700 nm.