Optical Network

The invention relates to an optical network, the transfer of a signal from a central station to subscriber stations taking place via of optical fibres, and the signal transferred by the central station being split, by a signal splitter in a splitting stage, into several output signals.

So far it has been customary, in the event of such networks not to go beyond one splitting stage which splits the signal into, e.g., 16 or 32 output signals. Further signal splitting would result in a low signal strength which would require sensitive, expensive optical receivers at the subscribers.

The invention provides for a considerable increase in the number of output signals without, however, the optical receivers at the subscribers requiring a great sensitivity. According to the invention, such may be achieved if the output signals are split, in a second splitting stage, by one or more second signal splitters after, however, having been amplified by an optical amplifier. There may possibly be added still further splitting stages. It is essential for the present invention that the power reduction occurring in each splitting stage be compensated by optical amplifiers.

As amplifiers, there may be used EDFAs (Erbium Doped Fiber Amplifiers). These consist of a piece of erbium-doped glass fiber to which there is presented, by way of a coupler, the signal to be amplified, as well as a supply signal. The supply signal is generated in a "pumping laser". The supply signal has a wavelength which does not lie that far below the wavelength of the signal carrier.

The EDFAs may be supplied by one or more pumping lasers in the immediate vicinity of the EDFAs, in the local distributor ("curb"). A drawback hereof is that the curbs must then be connected to the electricity grid in order to electrically supply the pumping laser(s).

Preferably, therefore, the pumping lasers are placed in the central station and the "pumping powers" for the EDFAs are supplied, by way of optical "pumping fibres", to the curbs. The required power is so great (approx. 100 mW per EDFA), however, that the pumping signals, as a result of "Stimulated Brillouin Scattering" (SBS) in the pumping fibres, are not capable of being transferred over the relatively large distance from the central station to the curbs.

This problem may be solved according to a preferred embodiment of the invention, however, by making use of, on the one hand, the discovery that SBS in the event of transferring a large power over a large distance does not occur if the supply signal is broken down into a number of supply signals with reduced powers having, within a certain wavelength range, different wavelengths. On the other hand, it was discovered that EDFAs are not very critical as far as the wavelength of the supply signal is concerned. It has turned out that EDFAs which are included in a transmission system having a carrier wavelength of 1550 nm, which are normally supplied with a supply signal having a wavelength of 1480 nm, function just as well if they are supplied by a number of supply signals with a wavelength of roughly 1480 nm having a mutual wavelength difference of some 10 to 100 MHz or over, each having a reduced power. Use is made of, e.g., 10 supply signals, each having a power of 10 mW, with which a combined power of 100 mW is transferred. The supply sources are, e.g., 10 lasers, each giving off 100 mW--each with somewhat different wavelengths 1.sub.1 . . . 1.sub.10 --to an optical 10.times.10 splitter which on the one hand combines the 10 supply signals to a signal having a power of 10.times.100 mW and on the other hand splits it into 10 supply signals, each composed of 10 signals having a power of 10 mW and wavelengths 1.sub.1 . . . 1.sub.10, respectively, as a result of which the total power per composite supply signal is 100 mW.

FIG. 1 depicts an exemplary embodiment of the invention; and

FIG. 2 depicts a modification of the embodiment shown in FIG. 1.

FIG. 1 shows an exemplary embodiment of the invention. In a signal distributor ("curb"), an optical signal originating from a central station and modulated by information having a wavelength of 1550 nm is distributed, by way of a splitter, among 16 outputs. Resulting output signals are amplified by 16 EDFAs. The EDFAs are supplied, by way of 16 optical couplers, with an optical supply signal of approx. 1480 nm. In order to avoid "scattering" (SBS), the supply signal is composed of 16 signals having wavelengths of around 1480 nm and a mutual wavelength differential of 0.1 GHz (in practice this differential, depending on the fibre properties, may be even lower, down to approx. 0.01 GHz). The table below gives the laser wavelengths and frequencies.

The supply signals generated by the various pumping lasers--each with a power of 100 mW--are combined with one another in a 16.times.16coupler (total power 16.times.100 mW) and subsequently split into a number of supply signals to which each pumping laser did supply its contribution. The power of each of the supply signals is 100 mW, composed of 16 signals of different wavelengths, each having a power of 100/16 mW.

Each of the outputs of the EDFAs is connected to a 1.times.32splitter, which splits the signal amplified by the EDFA into 32 output signals which are presented, by way of 32 optical fibres, to 32 subscribers. In total, therefore, 16 * 32=512 subscribers are served. Since the optical power of the signals presented, as a result of the amplification by the EDFAs, is relatively large, the subscribers may convert the signals into electric signals, making use of simple linear optical receivers which are very inexpensive.

FIG. 2 shows a modification as compared to FIG. 1 regarding the supply of the EDFAs. By supplying the 16 EDFAs with a supply signal formed by 8 instead of 16 signals having different wavelengths, e.g., supplied by the pumping lasers 1 up to and including 8 from the above table, there may be obtained a reduction in the number (now 8) of "supply couplers", namely, if there are coupled to each of said couplers not one but two EDFAs, as shown in FIG. 2.

Finally, it should be noted that the system described above provides for "downstream" signal transfer, from the central station to the subscribers, as this takes place, e.g., in CATV distribution. With a view to interactive TV and "video on demand" etc., there should also be, however, provisions for "upstream" signal transfer. In addition, subscribers must be provided with provisions for bidirectional signal transfer for the benefit of telephone, telefax and data traffic. Although it would appear to be obvious, for the construction of new networks ("green-field" locations), to opt for bi-directional (completely or largely) optical connections, it is proposed by the applicant in green-field situations to jointly install, simultaneously with the optical fibres, conventional "twisted-pair" copper veins. In this connection, the optical fibres are intended for downstream distribution of wide-band signals for, in particular, CATV, while the twisted-pair veins are used for telephone, telefax and (private and small-business) data traffic. The upstream signals for the benefit of "video on demand" and interactive TV (selection or "televoting" commands), too, are transferred by way of the twisted-pair veins. This hybrid configuration, consisting of a fully optical distribution network for downstream signals combined with a twisted-pair network for upstream and bi-directional transfer of narrow-band signals, turns out to have an unexpectedly favourable performance/price ratio. For locations where twisted-pair networks are already present, the hybrid solution is very suitable as well. For clarity's sake, in FIGS. 1 and 2 the twisted-pair network was omitted. Generally known is the setup of such a conventional network, which usually consists of copper cables with a great number of twisted-pair copper veins, which run to different curbs, from where, thin copper cables are run, usually having two twisted pairs, to the subscribers. In green-field situations, a choice will be made for common curbs for the optical and the copper cables; in the event of existing networks, there will often be placed new curbs which are only used by the new optical network. Having said this, it is obvious that not only twisted-pair copper networks are applicable, but possibly other nonoptical bidirectional networks as well.