Optical network using multiple Erbium Doped Fibre Amplifiers with a Common Supply

Abstract: Optical network for signal transfer from a central station to subscriber stations. The signal to be distributed is split in at least two stages, with the signal being amplified by optical amplifiers. The Erbium Doped Fiber Amplifiers (EDFAs) to be used for this purpose are fed by an optical supply signal. By building up the supply signal from various signals with wavelengths lying closely together, Stimulated Brillouin Scattering (SBS) is avoided. As a result, by utilising centrally supplied EDPAs more splitting stages may be applied, as a result of which many more subscribers may be reached from one signal source than before. The subscribers may utilise inexpensive, low-sensitivity receivers. The optical network is particularly suited to form, in conjunction with a conventional non-optical (twisted-pair) network, a hybrid bi-directional transmission system. In this connection, the optical network is intended for the distribution of wide-band signals (CATV) and the non-optical network for bi-directional transfer of narrow-band signals.

Description: A. BACKGROUND OF THE INVENTION 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. A fiber-optic access network having only one splitting stage which splits the signal into a limited number of output signals, e.g., 16, 32 or 64. output signals, is e.g. disclosed in a paper of Okada et al, titled "Introduction approach toward full fiber-optical access network", Internationals Conference on Communications, June 23-26, 1991, Denver US, pp. 903-907. Further signal splitting would result in a low signal strength which would require sensitive, expensive optical receivers at the subscribers. In order to increase the number of output signals without the need of such sensitive and expensive optical receivers at the subscribers, it is known, e.g. from the American patent US-A-5,321,707, that said 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. As amplifiers, there may be used EDFAs (Erbium Doped Fiber Amplifiers). These consist of a piece of erbium-doped glass fibre to which there is presented, by way of a coupler, the signal to be amplified, as well as a supply signal. Said 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, either the curbs must be connected to the electricity grid in order to electrically supply said pumping laser(s), or the pumping lasers should be placed in the central station and the "pumping powers" for the EDFAs should be supplied, by way of optical "pumping fibres", to the curbs. The first case should be avoided, whereas in the latter case the required powers are 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. B. SUMMARY OF THE INVENTION An object of the invention is to provide an optical network in which the pumping lasers can still be located in the central station and the "pumping powers" for the EDFAs may still be limited. The invention achieves this and other things 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 smaller 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 lower 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 l 1 ...l 10 - to an optical 10x10 splitter which on the one hand combines the 10 supply signals to a signal having a power of 10x100 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 l 1 ...l 10 , respectively, as a result of which the total power per composite supply signal is 100 mW. The invention allows the addition of still further splitting stages in a similar way as described for the second splitting stage. C. EXEMPLARY EMBODIMENT 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. Said 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 said 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. Pumping laser 1 l=1479,992 nm f=135135,8 GHz Pumping laser 2 l=1479,993 nm f=135135,7 GHz Pumping laser 3 l=1479,995 nm f=135135,6 GHz Pumping laser 4 l=1479,996 nm f=135135,5 GHz Pumping laser 5 l=1479,997 nm f=135135,4 GHz Pumping laser 6 l=1479,998 nm f=135135,3 GHz Pumping laser 7 l=1479,999 nm f=135135,2 GHz Pumping laser 8 l=1480,000 nm f=135135,1 GHz Pumping laser 9 l=1480,001 nm f=135135,0 GHz Pumping laser 10 l=1480,002 nm f=135134,9 GHz Pumping laser 11 l=1480,003 nm f=135134,8 GHz Pumping laser 12 l=1480,004 nm f=135134,7 GHz Pumping laser 13 l=1480,005 nm f=135134,6 GHz Pumpinglaser 14 l=1480,007 nm f=135134,5GHz Pumping laser 15 l=1480,008 nm f=135134,4 GHz Pumping laser 16 l=1480,009 nm f=135134,3 GHz The supply signals generated by the various pumping lasers -- each with a power of 100 mW -- are combined with one another in a 16x16coupler (total power 16 x 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 said supply signals is 100 mW, composed of 16 signals of different wavelengths, each having a power of 100/16mW. Each of the outputs of the EDFAs is connected to a 1x32splitter, 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 said 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 bi-directional 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. Said 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, said hybrid solution is very suitable as well. For clarity's sake, in figures 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 there run thin copper cables, 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 non-optical bi-directional networks as well. D. REFERENCES None.

Claims: Optical network, wherein the transfer of a signal from a central station to subscriber stations is carried out via optical fibres, and the signal transferred by the central station being split by a first signal splitter in a first splitting stage, into several output signals, said output signals being split, by one or more second signal splitters in a second splitting stage, after, however, having been amplified by an optical amplifier, said optical amplifiers are supplied from a common optical supply source which outputs a supply signal, characterised in that said output supply signal comprises a plurality of supply signals of mutually different wavelengths. Optical network according to claim 1, characterized in that a supply signal of the plurality of supply signals supplies two optical amplifiers via an optical coupler. Optical network according to claim 2, characterized in that each supply signal of the plurality of supply signals supplies two optical amplifiers via an optical coupler. Optical network according to any of the claims 1,-,3, characterised by one or more further splitting stages, wherein signals supplied thereto are split into one or more signal splitters, after, however, having been amplified by an optical amplifier. Optical network according to any of the claims 1,-,4, characterised in that said supply signals maintain a mutual frequency distance from one another of at least 10 MHz. Optical network according to any of the claims 1,-,5, characterised in that the powers of said supply signals of different wavelengths are substantially equal. Optical network according to any of the claims 1,-,6, characterised in that said supply source is formed by a series of optical signal generators which give off optical signals of mutually differing wavelengths, of which optical signals are supplied to the inputs of an NxN coupler which combines the supplied signals with one another and distributes the combined signal among its outputs, with each of the said optical amplifiers being connected to one of the outputs of the NxN coupler. Transmission system, comprising an optical network according to any of the claims 1,-,7, additionally comprising a non-optical bi-directional network between a central station and said subscriber stations.

Inventors: TAN HAN HIONG, TAN, HAN HIONG, Tan, Han Hiong, KPN Research
Applicants: KONINKL KPN NV, KONINKLIJKE KPN N.V.
Application year: 1996
Main classification: H04B10
IPC codes: H04B10