ATTENUATION-FREE OPTICAL CONNECTION

Abstract: The invention relates to an attenuation-free optical connection which includes a fibre optical signal fibre (1) for transmission of an effective signal. According to the invention amplification elements (2) are placed at intervals along the signal fibre for amplification of the effective signal and compensation for the signal fibre intrinsic attenuation. A drive element (3) is common for the amplificatio elements (2) which include a length of doped fibre (8) which is fed via a feeding fibre (4) by means of a ray distributor (6) and a coupler (7). The drive element includes a pump station (3) with one or more pump lasers (5) for feeding of one or more signal fibres (1). Thanks to the passive amplification elements (2) an attenuation-free connection with a minimal noise contribution is obtained.

Description: TITLE OF THE INVENTION: ATTENUATION-FREE OPTICAL CONNECTION FIELD OF APPLICATION OF THE INVENTION The present invention relates to a device at an optical communications network which supplies a virtual attenuation-free optical connection by utilizing a driver laser which transmits an optical pump signal which activates amplifying elements which are placed along a signal line. The amplifying elements give an essentially noiseless amplification by utilizing a doped fibre, and by that a compensation of the in the signal line intrinsic attenuation is obtained in a way that a virtually attenuation-free optical connection is obtained. PRIOR ART A problem at transmission of optical signals in an optical communications network is that often too much noise is obtained from the big and besides sparsely located signal amplifiers. This results in a restricted transmission capacity at the transmission of optical signals and also requires a complex and expensive terminal equipment both on the amplifier side and the receiver side. A technology which is used today is to place fibre optical amplifiers with high amplification in special pump stations at certain points along the signal fibre. The pump stations contains the pump laser and an active fibre in the same place. The amplifiers are placed comparatively sparsely, usually at a distance of 50-100 km. The high amplification results in that the amplifiers produce much noise and that they must be specially equipped with isolators, filters etc to avoid disturbances like self-oscillations and saturation. It is too expensive to place these amplifiers close, because of installation costs and supervision and maintenance. US 5 115 338 describes a system of this type. Also US 4 699 452 describes a similar system which utilizes the Raman principle instead of doped fibre as amplifying element. The big noise contribution from the amplifiers results in that the optical transmitter laser must produce high output power, often via a special transmitter booster. High power levels results in that the fibre becomes unlinear as signal carrier. Such an unlinear transmission channel leads to strong restrictions of the range, how high data rates that can be used, and the number of optical channels which can be transmitted in the signal fibre at the same time. Another method is to distribute the amplification along the signal fibre by applying doped fibres at points along the signal fibre and in this way reduce the noise contributions. At this the power of the pump laser is driven directly into one end of the signal fibre. The high pump power which is required, results in further problems with unlinear transmission, as in the case above. An example of this method is described in US 5 039 199. A similar method is to make the whole signal fibre as a low-doped fibre. This method is however impaired by the same problem. An example of this method is given in US 5 058 974 and a combination of these methods is given in EP 0 408 394. EP 0 387 075 shows that a doped fibre can be used in optical transmitters as an alternative to the semiconductor laser. The document shows a way to more efficiently utilize the pump power necessary to drive the doped fibre. US 4 546 476 describes a fibre optical amplifier where , the coupler and the active fibre are arranged as one component. The present invention solves the problems of the previously known technology by placing passive amplifying elements at regular intervals along the signal fibre. The amplifying elements are fed via a feeding fibre by a common pump laser. This makes the signal fibre attenuation-free and gives minimal noise contribution. CONCLUSION OF THE INVENTION The present invention consequently supplies an attenuation-free optical connection comprising a fibre optical signal fibre for transmission of an effective signal. According to the invention amplifying elements are placed at intervals along the signal fibre for amplification of the effective signal and compensation for the in the signal fibre intrinsic attenuation. A drive element is in common for the amplification elements for the driving of these. Each amplification element includes preferably a lenght of doped fibre which is fed via a feeding fibre by means of a ray distributor and a coupler. The drive element includes a pump station with one ore more pump lasers for supplying of one or more signal fibres. The invention is defined in details in the enclosed patent claims. SHORT DESCRIPTION OF THE DRAWINGS The invention will now be described in details with reference to enclosed drawings where. Figure 1 is a block diagram over an attenuation-free optical connection according to the invention. Figure 2 is a diagram over an amplifying element in details, and Figure 3 is a diagram over an alternative amplifying element in details. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The invention is based on the principle of amplifying an optical signal in a fibre by introducing a bit of doped fibre. By introducing a pump signal in the fibre the doped fibre amplifies the optical, effective signal in the signal fibre making the attenuation in the signal fibre being compensated for. In Figure 1 is shown an embodiment of the present invention. The optical connection includes an optical fibre 1, here called the signal fibre, for transmission of the effective signal. Along the signal fibre 1, a number of amplifying elements 2 are placed. The distance between the amplifying elements is a few kilometres. The amplifying elements are driven by a pump station 3 which provides the amplifying elements with optical power via a feeding fibre 4. The signal fibre 1 and the feeding fibre 4 are preferably parts of a common fibre cable. In the pump station there are one or more pump lasers 5. The optical power from the pump lasers is transmitted into the feeding fibre 4. At each amplifying element a fraction of the power in the feeding fibre is drawn off to the amplifying element via a ray distributor. In Figure 2 the amplifying element is shown. It consists of passive optical components: a ray distributor 6, a coupler 7, and a doped fibre 8. The drawn pump power is via the coupler 7 brought to the signal fibre 1 and is made to act on the specially doped, signal amplifying fibre 8, which is spliced into the signal fibre 1. The pump power excites the doping ions in the signal amplifying fibre 8 which results in that these amplify the optical effective signal. The length of the signal amplifying fibre 8 is typically about a metre. All the amplifying elements 2 can be of the same kind. The part of the pump power which the ray distributor 5 drains depends on how many separations that shall be made from the pump station. In the solution according to Figure 1 the pump power is reduced on one hand due to losses in the feeding fibre and on the other due to the used power in the coupler 7. The ray distributor 6 consequently shall make use of different size of the power which is fed to the amplifying element depending on where in the chain it is located. The amplifying elements according to the presented invention in Figure 2 consequently implies that the amplifying elements are different in so far that the ray parts shall draw different large parts of the available power. An alternative construction of the amplifying elements is shown in Figure 3. The effective signal and the optical power is introduced from the left in the figure. The optical signal is transferred from the pump station in the cable 4 to the coupler 7'. In the coupler 7' the optical power is transferred to the signal fibre 1. The transferred pump power in the fibre 1 is after that active in the specially doped signal amplifying fibre 8 which as previously is spliced in the signal fibre 1. The pump power excites the doping ions in the signal amplifying fibre 8 which results in that these amplify the optical, effective signal. The length of the signal amplifying fibre 8 is for instance about a metre. The signal is after that brought to a second coupler 7". In the coupler 7" that part of the pump power which is not used in the feeding fibre is brought back. The amplified effective signal is after that forwarded out into the signal fibre to the next amplifying element or receiver. The pump power which remains is also forwarded into the feeding fibre to the next amplifying element. At the next amplifying element the feeding fibre, however, beside the used power in the first amplifying element 2 will also lose some power at the transmission in the fibre. Amplification in the following amplifying element is however made in a corresponding way as has recently been described. In the solution according to Figure 3 all amplifying elements can be identically designed. No consideration need in this case to be taken to the fact that the optical power is reduced through drainage in the amplifying elements and through losses in the feeding fibre 4. As is indicated in Figure 1 the pump station 3 also can provide a number of signal fibres with pump power by ray distribution in the pump station or by different pump lasers. It is convenient that an amplifying element is placed in the pump station 3 itself, as is shown with the doped fibre 8' in Figure 1. It is first of all the intention that the amplifying elements 2 should be placed at the same distance between them along the signal fibre 1, and the drained pump power be adjusted in a way that the attentuation in the signal fibre 1 is just compensated. Another variant is to have a somewhat higher amplification making the connection slowly amplifying the optical signal as this is travelling along the fibre cable. The present invention consequently solves the problem at an attenuation-free optical connection and has more advantages compared with the previously known technology. The fibre cable becomes attenuation-free and gives minimal noise contribution. This results in that one can reduce the transmission power with 20-30 dB (100-1000 times). One then has at least a margin of 20 dB to the threshold where unlinearity starts to appear. Elimination of unlinearity results in that one can use dispersion shifted fibre as signal fibre. One then evades complex installations for dispersion compensation. No filters or isolators are needed, which makes the fibre cable transparent in both directions. One no longer has need for high power in the transmitter equipment. This results in strongly changed conditions for construction and optimization of optical high capacity systems. With the present invention the optical transmission equipment can be simplified and becomes cheaper. Solely passive optical components result in simple, robust and cheap implementation. The distance between pump stations can be increased compared with the case where optical amplifiers at certain points are used. Signal format, data rate, wavelenghts etc can be changed also after installation of the optical cable. Further embodiments of the invention are evident for any person with expert knowlewdge. The invention is only restricted by the following patent claims.

Claims: PATENT CLAIMS 1. Attenuation-free optical connection including a fibre optical signal fibre (1) for transmission of an effective signal; c h a r a c t e r i z e d in amplification elements (2) placed at intervals along the signal fibre (1) for amplification of the effective signal and compensation of the in the signal fibre (1) intrinsic attenuation and in a drive element (3) which is common for the amplification elements (2) for the driving of these. 2. Optical connection according to patent claim 1, c h a r a c t e r i z e d in that each amplification element (2) includes a lenght of doped fibre (8) which is fed via a feeding fibre (4) by means of a ray distributor (6) and a coupler (7). 3. Optical connection according to patent claim 1, c h a r a c t e r i z e d in that the amplification elements (2) includes a first coupler (7') , and a length of doped fibre (8) and a second coupler (7"). 4. Optical connection according to patent claim 3, c h a r a c t e r i z e d in that in the first coupler (7') is transferred at least a part of one in the feeding fibre (4) provided power to the signal fibre (1), and that the effective signal is amplified in the doped fibre (8) . 5. Optical connection according to patent claims 3 and 4, c h a r a c t e r i z e d in that the optical power which is not utilized at the amplification of the effective signal is fed back to the feeding fibre (4) in the second coupler (7"). 6. Optical connection according to patent claims 2 or 3, c h a r a c t e r i z e d in that the drive element includes a pump station (3) with one or more pump lasers (5) for the feeding of one or more signal fibres (1). 7. Optical connection according to patent claims 1, 2, 3 or 6, c h a r a c t e r i z e d in that the drive element includes an amplifying element (8) of its own. 8. Optical connection according to patent claims 1, 2, 3, 6 or 7, c h a r a c t e r i z e d in that the amplification elements (2) are placed at essentially the same distance along the signal fibre (1) and are fed with the same size of power from the drive element (3). 9. Optical connection according to patent claims 8, c h a r a c t e r i z e d in that the distance between the amplification elements (2) is of the size kilometres and the length of the doped fibre is of the size metres.

Inventors: KAELLGREN DAN, KAELLGREN, DAN
Applicants: KAELLGREN DAN, TELIA AB, KAELLGREN, DAN
Application year: 1996
Main classification: H04B10
IPC codes: H04B10