According to the invention, optical amplifiers operate in one direction, whereas the traffic by switches is divided into two groups, which are routed in different directions.

Prior art In the optical transmission networks of today, WDM (Wavelength Division Multiplex) is often used to maximize the number of channels per amplifier. For the price of optical amplifiers are high, and thanks to WDM, the number of amplifiers can be reduced. A large number of channels and high channel power, however, produce crosstalk problems, for instance FWM (Four Wave Mixing). This problem will increase in a progressive way as separation in frequencies between the channels is growing smaller and smaller. At introduction of dense WDM (DWDM) in optical networks, the optical non-linearity of the fiber that can cause crosstalk between the wavelength channels must be taken into consideration. This problem can be reduced in known way, either by specific fiber with adapted dispersion, or by controlled dispersion, when specific compensators are connected in a network consisting of ordinary fiber. However, there still is the fact that fewer channels, and by that increased channel separation, provide

better condition for a problem-free transmission than more channels.

Another problem is to protect the traffic in optical networks against breakdowns in the traffic. Regular protection architectures which, for instance, are put into practice with SDH (Synchronized Digital Hierarchy) in principle imply a doubling of the whole transmission network, and by that the cost of the protection will be very high.

It is in fact also previously known to use two-way transmission in a common fiber. In the known configurations in networks with long spans, however, amplifiers of reversed direction are used along the line, which also demands a doubling of the number of amplifiers.

The present invention solves i. a. above mentioned problems by the two-way traffic that is coming in in opposite directions being routed to the optical amplifiers so that the whole traffic is amplified in common. By that, the number of channels can be doubled without the number of amplifiers having to be increased. The two-way traffic also can be utilized for protection of traffic in optical ring networks by frequency shifted channels carrying a copy of the traffic in the same fiber as the original but, in opposite direction compared with this.

Summary of the invention The invention provides a procedure for two-way communication over a common optical fiber, at which traffic is routed through a connection with channels with a plurality of optical amplifiers in between.

According to the invention, a first channel group that is coming in in one direction at one side of each amplifier is routed to the input of the amplifier, and a second channel group that is coming in in opposite direction at the other side of each amplifier, also to the input of the

amplifier. The traffic from the output of the amplifier is divided so that the first channel group is routed further at the other side of the amplifier in one direction of the connection, and the second channel group is routed further along the connection in opposite direction.

The ordinary traffic in each node can be frequency shifted so that the ordinary traffic constitutes the first channel group, whereas the frequency shifted traffic constitutes the second channel group. The invention also relates to an arrangement to execute the procedure. The procedure and arrangement are defined in patent claim 1, respective 5, whereas preferred embodiments are given in the subclaims.

Brief description of the drawings The invention now will be described in detail with reference to enclosed drawings, of which Figure 1 is a schematic illustration of the present invention; Figure 2 shows an alternative embodiment of the invention, and Figure 3 schematically shows a ring network that can utilize the invention.

Detailed description of preferred embodiments As has been mentioned above, the invention provides a procedure and an arrangement to, in a common transmission fiber, create a division of channels in two-way transmission with common amplification. The invention can be used in an architecture for protection switching of ring networks, where cost reasons speak against a protection built on ordinary architecture.

By the invention, disturbances from optical unlinearity of fibers in amplified links and fiber optical

networks are avoided, without the number of fiber amplifiers having to be increased. In one variant of the invention, the disturbances are not reduced, but the two- way traffic can be utilized for protection switching in ring networks.

The main component to realize two-way communication with common amplification is a number of wavelength selective switches. In the simplest case there are needed four simple, broadband WDM-switches per amplifier, but if FWM-immunization is wanted, more advanced filtering is needed. Such is at present expensive, but the development within for instance MEMS (Integrated Micro Electro- Mechanical Systems) may provide new, cheaper components with wanted functionality. As a simpler but good alternative to interleaving filter, also polarizers are suggested.

In the fully developed invention, which aims at protection switching in ring networks, traffic is reflected in a slightly frequency shifted spectrum. Components for such a frequency shift can be realized. They are based on (crystalline) material that phonon-interacts with the optical signal, which can be shifted by an order of magnitude comparable with the Brillouin shift in optical fiber.

In Figure 1 is shown schematically an arrangement according to the invention. Two amplifiers modified according to the invention are shown interconnected with a fiber link connection. The connection 1 consists of an optical fiber. Along the fiber there are amplifiers 2 located to compensate for attenuation in the fiber. The amplifiers 2 can be ordinary fiber amplifiers, for instance EDFA (Erbium-Doped Fiber Amplifier) with a usable spectrum from about 1530-1565 nm. To make it possible for the amplifier 2 to amplify the signals that are coming from opposite directions, a division is made so that both the signals are routed to the input of the amplifier. This is

effected by switches 3, for instance broadband WDM- switches.

As can be seen, a channel group 5, marked with light arrows, is coming in from the right, and a channel group 6 marked with dark arrows from the left. The channel groups cover together the wave length spectrum of the amplifier.

The switches block one of the channels groups, but allow the other to pass through, so that both the groups 5,6 are routed to the input of the amplifier 2, and are let out in respective opposite direction at the other side of the amplifier.

The functionality of the invention is as follows.

1) Each amplifier 2 amplifies all wavelengths in one and the same direction.

2) Frequency spectrum is divided into two groups of wavelength channels at the output of the amplifier 2.

3) These groups are routed in opposite directions along the transmission fiber 1. By division of the channels in two directions, the same number of channels can be achieved per fiber and direction as at exclusive one-way transmission, so the capacity of the fiber can be doubled.

The groups now can be selected in different ways, depending on wanted system qualities.

4.1) The channel groups are interleaved, i. e. are put together with a certain, small spectral shift. This means that channels of meeting traffic are displaced in wavelength in relation to each other. A suitably adjusted spectral shift minimizes disturbances due to Raleigh and Brillouin dispersion from meeting channels in the transmission fiber.

4.2) Each group is given the same, dense channel distance as in the amplifier 2, but are spectrally divided into two intervals, for instance 1530-1545, respective 1546-1565 nm if the C-band of EDFA is utilized. Single channel distance can be 0,8 nm according to ITU- specification.

In other words, from a channel spectrum G with wavelengths X in rising order, G = k 2x 3r ns these are divided in the interleaved case (4.1) according to G1 = k 3 r N-1 and G2 = A2, A. 4,..., A, N- At single division (4.2) this is instead made according to Gl = X 2r and G2 X i+lt N- The distribution according to (4.1) has an advantage from a transmission point of view: For a given number of channels per direction, the comparatively sparse allocation between wavelength channels running in the same direction results in a potentially low crosstalk because chiefly FWM is reduced, alternatively (4.1) allows more channels than (4.2). The division (4.2) allows that simpler filters with lower selectivity can be used at both the amplifiers 2 and at the add/drop-points.

5) After an initial switch 3 closest to the output of the amplifier, respective group is transmitted along the fiber link 1 and here meets the opposite, completing group from and adjacent amplifier. At the end of the link, the group reaches a switch that routes the group into the amplifier together with opposite, attenuated signal. This process is repeated a number of times in a multiple of fiber links, while channels of the group one by one are dropped off in any drop-point and are replaced by new traffic channels with the same wavelength.

The invention in the variant (4.1)-and with certain limitation (4.2)-provides possibility to, without increasing the number of amplifiers, make possible protection switching of traffic between all nodes in a ring network consisting of one single fiber. The functionality then will have the addition that 6) The whole spectrum with all ordinary channels are frequency shifted, with about the double frequency in relation to the Brillouin-shift of optical fiber (which is about 11 GHz), by means of the energy contribution from phonons produced in previously mentioned component. This

spectrum contains a copy of the information of ordinary traffic, which by that provides complete redundancy if the signal could be reproduced"at the other side of the break". This can be made by returning the signal in a ring all the way between the nodes that are bordering on the break. The traffic in opposite direction, that is the returning signal, will consist of a slightly frequency shifted copy of ordinary traffic. This copy may constitute protected traffic in a network protected by means of the invention.

An interleaved division can cause problems with the wavelength selectivity, i. a. when a large number of filters have to be passed through in the ring network. By replacing the imagined multichannel filter above by (four) polarizers 4, as can be seen in the Figure 2, this problem is avoided because all wavelengths (with right polarization) can pass through the polarizers 4. A polarizer is, in addition, of a simpler construction than a multichannel filter. Otherwise the principle is similar.

The signal"in wrong direction"that reaches the output of an amplifier 2, does not imply any problem since the supposed type of amplifier normally includes a built-in insulator. It is also to be remembered that the signal power in the amplifiers are considerably smaller than the power at the output of the amplifiers. By the location of the polarizers close to the amplifier 2, a polarization retaining fiber in the links between the amplifiers is not needed, only that the polarization of respective signals are retained in the amplifier 2. The attenuation in the optical track originates from four switches (4 x 3dB) and from two polarizers (3 + OdB ideally). This totally results in about 15 dB, which is compensated by the amplifier 2, but which results in increased ASE-noise, Amplified Spontaneous Emission, at higher amplification. Further contribution of noise can occur when the detector window has to be increased.

The principles above apply, irrespective of whether the wavelengths are the same in both directions of the transmission network or not.

Figure 3 shows the intended application of the invention as protection of the traffic in an optically amplified ring at cut off of cable at digging, weighing anchor etc. Figure 3 shows how the protection is applied in a network with ten add-/drop-nodes 9 (smaller arrows) and transmission links in between with a number of amplifiers (not shown) each. The outer track 8 is the route for regular traffic, and the inner track 7 is the opposite route for protected traffic. Both tracks are running in the same fiber. The protected traffic counter-clockwise consists of a slightly frequency shifted reflection (larger arrows) of all traffic clockwise in the node closest to the fiber break 10.

By the larger arrows can be seen that the protected traffic (the copy) can be routed without deviations to the same point that normally should be passed through by the ordinary traffic (the original). The traffic only has taken a detour round the place of the break of traffic.

The principle of the invention can be applied at upgrading of existing links and networks, in order to increase the traffic capacity (number of channels) or increased network functionality (the protection). The application of the invention does not create conflict with valid ITU-standards (International Telecommunications Union) because the signal lasers are allowed to have standard frequencies. The filters of the detectors, however, must be sufficiently broadband to accept both ordinary and reflected (protected) traffic.

By the invention the protection of rings can be realized in a new way: The protected traffic shares both a common fiber and all line amplifiers with the normal traffic.

Upgrading of an existing optically amplified fiber network can be made without installing new fiber, and in certain cases without changing the amplifiers. The physical change consists of a number of components that are connected to the input and output of the amplifier.