WO2000064078A1

WO2000064078A1 - Method and apparatus for implementing an optical cross-connection

Info

Publication number: WO2000064078A1
Application number: PCT/FI2000/000226
Authority: WO
Inventors: Markku Oksanen
Original assignee: Nokia Networks Oy
Current assignee: Nokia Oyj
Priority date: 1999-04-15
Filing date: 2000-03-17
Publication date: 2000-10-26
Anticipated expiration: 2001-10-15
Also published as: AU3559900A; FI990836A0; FI990836A7

Abstract

The invention concerns a very high-capacity optical cross-connection method and apparatus, which are intended for use in an optical telecommunications system based on WDM technology, especially in the network node of the telecommunications system. The implementation according to the invention is intended to utilise a coherent transmission layer and broadcast and select architecture. In the solution, opto-electric-opto converters are placed both on the input side and on the output side of the cross-connection area. In addition, narrow-band modulation is utilised; the signal supplied to the cross-connection apparatus is modulated in a single-sideband modulator SSB, whereby the band of the optical signal can be narrowed. Owing to the single-sideband emission, the signals can be packed very close to each other, whereby 2 - 10 times more wavelengths can be placed within the same wavelength range than in earlier cross-connection solutions. For example, in the wavelength range of 1530-1560 nm, which is often used in optical data transmission, a cross-connection apparatus of several terabits can be constructed, wherein thousands of wavelengths can be transmitted. However, the solution according to the invention is not restricted to any certain wavelength range, but besides fibre optics, it may also be based on free-space optics.

Description

Method and apparatus for implementing an optical cross-connection

Field of the invention

The invention relates generally to optical transmission systems using WDM, Wavelength Division Multiplexing, and especially to an optical cross-connection for implementation in such a system.

Technical background

In optical transmission systems the data flow to be transmitted is used for modulating an optical signal, and the modulated optical signal is supplied to an optical fibre. The capacity of the transmission system can be increased in two different ways: by increasing the band width of the data flow or by increasing the number of wavelengths transported in the fibre. The latter alternative is efficiently implemented by WDM, Wavelength Division Multiplexing. At the present time, a 1550 nm window is a typical wavelength range for optical data transmission. The light to be transmitted is coherent light, that is, the light signal to be transmitted contains only a certain frequency spectrum of a regular shape. Major characteristics of the components of the optical fibre's output end are related not only to optical power but also to the width of the resulting spectrum and to the degree of modulation of the signal to be transmitted. The quality and structure of components that can be used depend on the purpose of use. Lasers and filters functioning at an exact optical wavelength are essential components in wavelength division multiplexing. The window to be used determines what kind of laser is the most suitable in the system to be implemented. At the optical signal output end there is an optical transmitter, usually a laser suitable for the purpose, to generate a coherent light signal. At the reception end of the fibre there is an optical receiver, e.g. an APD, avalanche photo diode, or a PIN diode, which is simpler than the one mentioned above. Broadcast and select architecture is a part of the WDM network architecture. The above-mentioned architecture is described with the aid of a simple diagrammatic plan in Figure 1. In the figure, A, B and C illustrate network nodes, which transmit signals at different wavelengths: node A transmits at wavelength λ_{1 f} node B at wavelength λ₂ and node C at wave- length λ₃. At the centre of the network there is a passive unit, an optical combiner (the figure shows a star combiner, but it could also be a bus com- biner), the duty of which is to collect signals transmitted by the nodes and to broadcast them further to all nodes in the network. Each node has an optical transmitter and an optical receiver. With the aid of an adjustable filter element at the receiving end the node can select the wavelength it desires from the arriving signal. Transmitters and receivers operate at the same speed. The broadcast and select network does not route signals, but it sends further to all nodes in the network the signals it receives from the nodes. It is a problem with the system presented above that it is not able to distinguish from each other two signals arriving at the same wavelength. For example, if node C, too, transmits a signal at wavelength λ.,, then the wavelengths of nodes A and C will overlap. In addition, a great many wavelengths are transmitted unnecessarily in the system, since the receiving end does not usually make use of all wavelengths that are transmitted to it.

Known structures of the optical cross-connection are based on a cross-connection function in an optical space switch, wherein incoming wavelengths are switched over to outgoing fibres. It is also known to implement a cross-connection by an opto-elect c-opto converter located between the inlet and the exit. Problems with known cross-connection implementations are e.g. the small number of channels and also the circumstance that two (or more) same wavelengths can not be placed in the same fibre without a wavelength conversion, because the same wavelengths will otherwise be superimposed. The wavelength conversion is generally implemented by using a transponder converting the wavelength. It is difficult to implement an electric cross-connection matrix having a high penetration capacity. Another problem as yet unsolved is how to implement such a cross-connection, which would broadcast the wavelengths of the WDM signal from the incoming fibre and would later rearrange them on the selected outgoing optical fibres. A possible solution could be e.g. a pure switch solution, but it is extremely difficult to implement one, because the switch ought to be able to deal with hundreds of signals. Even dealing with e.g. 256 signals would be difficult, if it must be possible freely to select a certain signal from them and lead it out of a certain gate. In fact, one of the greatest problem of the solution would be the rearrangement of wavelengths. Brief summary of the invention

The invention concerns an optical cross-connection, which is intended for use in an optical telecommunications system based on WDM technology, especially in a network node of a telecommunications system. The objective of the invention is to implement a very high-capacity cross- connection architecture by utilising a coherent optical transmission layer and broadcast and select architecture. The established objective is achieved in the manner presented in the independent claims.

The invention aims at implementing a cross-connection apparatus with a high packing density, that is, the objective is to maximise the data transmission capacity of the cross-connection system. In the solution according to the invention, opto-electric-opto converters are placed both at the input end and the output end of the cross-connection area and, in addition, narrow-band modulation is utilised, with the aid of which wavelengths arriving in the device are converted into wavelengths inside the apparatus. The signal brought into the cross-connection apparatus is modulated in a single- sideband modulator SSB allowing narrowing of the band of the optical signal. Thanks to the single-sideband emission, the signals can be packed very close to each other, whereby 2 - 10 times more wavelengths can be placed within the same wavelength range compared with earlier cross-connection solutions. For example, in the wavelength range of 1530 -1560 nm generally used in optical data transmission it is possible to construct a cross- connection apparatus of several terabits, in which thousands of wavelengths can be transmitted. In addition, owing to the opto-electric-opto conversion, the cross- connection method in accordance with the invention is very flexible; within the optical cross-connection area the cross-connection is not restricted to any certain wavelength range, but it may be based also on free-space optics, besides fibre optics. The solution according to the invention can thus be applied e.g. in a wavelength range of visible light, besides the so-called telecom wavelengths generally used in optical telecommunications.

List of figures

In the following the invention will be described in greater detail with the aid of the appended diagrammatic figures, of which Figure 1 illustrates the principle of the known broadcast and select architecture in a WDM network, Figure 2 shows an embodiment of cross-connection architecture in accordance with the invention.

Detailed description of the invention

In the following the invention will be examined by way of example with the aid of Figure 2. According to the figure, the cross-connection implementation includes several optical incoming fibres (11 IN) and the optical signals arriving from these are conducted to input gates of the cross- connection apparatus. The input gates include elements for converting the signal from an optical into an electric form (A,), components for forming an optical carrier wave (C,), and components for modulating the carrier wave by a useful signal (B,). From the input gates the different signals, of which there may be hundreds or even thousands, are conducted to an entirely optical broadcast and select cross-connection area, wherein the signals are combined (D) and from which the said signals are broadcast further (E) to several output gates. The output gates include components for detecting the signal and for converting the optical signal into electric form (F,), as well as compo- nents for converting the signal from electric into optical form (H,). A typical practical example could be a system, wherein the signals are within a wavelength range of 1530 -1560 nm. The wavelengths arriving in the device need not necessarily be different, but e.g. several different signals may arrive at the same wavelength, because the signals are converted onto wavelengths of their own inside the device, whereby they can be distinguished from each other on the output side.

The signal's propagation in the embodiment shown in the figure is studied in detail in the following. It is assumed that optical signals containing several different wavelengths arrive along the optical fibres (11 ,..., IN) shown at the left in the figure to the cross-connection device, so that there is only one separate optical signal at one input gate. At gate 1 there could be e.g. wavelength 1530.334 nm (195.9 THz) and at the following gates wavelengths at intervals of approximately 0.8 nm (100 GHz) up and all the way to 1563.863 nm. In practice, the cross-connection apparatus in the example would allow connecting of 43 different wavelengths from the input gates to the output gates. Alternatively, when using a separation of 50 GHz, the num- ber of different channels would be 85. Each optical fibre arriving at the input gate is connected functionally to its own optical-to-electric converter A„ wherein the incoming signal is detected and converted into a broadband electric signal. When required, 2-R- or 3-R regeneration is carried out for the signal. 2-R regeneration is a technique carried out for digital data, wherein the data is strengthened and reshaped, whereas in 3-R regeneration the data is strengthened, reshaped and timed. This layer may be an entirely broadband one, especially when linear modulation signals, such as e.g. CA - or Cellular RF signals are concerned. Each optical signal which has been converted into electric form is conducted into an optical single- sideband modulator SSB, which is indicated by B, in the figure. A new optical carrier wave OC„ is also conducted into the modulator, which carrier is obtained from laser C, operating exactly at a certain wavelength. Each laser C, may function at a certain wavelength of its own and the lasers may be e.g. of the Heterodyne Injection Locked Laser type. The interval between carrier wave frequencies is preferably equal to the double bandwidth of each electric broadband signal. In single-sideband modulator B, a digital amplitude modulation is performed on the carrier wave. In single-sideband modulator SSB the band of the optical signal can be narrowed and the signals can be packed very close to each other, whereby 2 - 10 times more wavelengths can be placed in the same optical spectrum than in conventional cross- connection solutions.

The signal is conducted from single-sideband modulator B„ now in optical form, to optical combiner D, to which it is connected through optical fibre OF. In an alternative embodiment free-space optics FS may be used, whereby the entire optical area presented in the following is implemented by free-space optics in the manner shown in the figure. A rearrangement of the signals takes place in this area. Optical combiner D may be e.g. a combiner combining the incoming optical signals. In this example, the average power of signals is 1 mW. There may be hundreds of signals at the same time, even thousands of signals. From combiner D the signals are conducted to optical divider E, to which the combiner is functionally connected by way of an optical transmission path. Combiner D and divider E together form a broadcast component. The said devices may be separate as in this example, but such an embodiment where one device contains the functionality of combiner D and divider E is also possible. Optical divider E once again sepa- rates the signals combined by combiner D. If there are e.g. 1000 channels in the embodiment, the signal power is hereby 1 mW at each output gate. Important advantages are achieved when dealing with sideband signals only: firstly, so-called telecom wavelengths are not processed directly, but since all incoming wavelengths will undergo wavelength conversion, it is possible within the said entirely optical area to process wavelengths within any wavelength range which can be freely selected. With the system according to the invention it is possible to process thousands of wavelengths, which is not possible with the present systems based on switch matrices. The number of signals to be processed depends only on the technology used in the optical area. As was mentioned above, the same wavelengths are not superimposed in the cross-connection, because all wavelengths undergo a wavelength conversion. The output gates of divider E are functionally connected over an optical cable to the input gate of each detector F, in the manner shown in Figure 2. Detector F, is a so-called heterodyne receiver. Each detector F, 'sees' all signals in divider E, in other words, all signals are visible in one medium, which in the embodiment can be either a device, a wire, an optical fibre or an optical route. It is hereby possible that e.g. three different receivers F, can receive the same signal and transmit it further, if they so desire. The same signal can thus be distributed to several different places, but some signals are not necessarily transmitted further. Several advantages are achieved with the heterodyne system, the very accurate selection of channels being one of the most important advantages.

In a first preferable embodiment, there is a set of lasers on the in- put gate side, and with the aid of a local oscillator signal G available from the input side the desired wavelength is selected for a certain detector F, In other words, detector F, may select the desired wavelength from any laser C,. In a second preferable embodiment, there is the said set of lasers on the input gate side, and in addition there is also a set of lasers on the output gate side, which may be implemented either as a separate set of lasers or in such a way that each heterodyne detector F, contains its own adjustable laser C,' . With the aid of an adjustable laser and by adjusting the wavelength it is possible to select the correct channel very precisely. In heterodyne detector F, the optical signal is converted into electric form. From component F, the signals go out arranged in electric form, so that e.g. a signal arrived in the system from input gate 11 is got out from the output gate of component F₄₆. The output gate of detector F, is functionally connected to the input gate of component H,. H, is e.g. a modulator transponder, which with the aid of an electric broadband signal modulates from the outgoing optical wavelength. Low-pass filters FC, may be installed on the transmission path between the said components F, and H, in the manner shown by the figure. The output gates (01 ON) of the above-mentioned components H, are output for the entire cross-connection apparatus, from which output gates the desired optical signal can be transmitted at any wavelength used in optical telecommunications. The cross-connection according to the invention can be implemented e.g. in the telecommunications node of a WDM network. The bit flow is processed at wavelength level, that is, each bit flow is rearranged and routed at the wavelength level. An important advantage achieved with the solution according to the invention is that the rearrangement of signals does not necessarily take place at typical wavelengths used in telecommunications; in the optical area of the cross-connection embodiment shown in the middle of the figure the optics may function at any wavelength, that is, it need not be fibre optics at all, but the implementation may be with any optical system, e.g. with so-called free space optics. In the solution according to the invention, the number of signals to be cross-connected may vary greatly. This advantage, too, is achieved by using single-sideband signals.

Thus, fibre optical and free-space optics are alternative solutions for the invention. The wave range used may also be another than the 1550 nm window mentioned in the example, e.g. a 1300 nm or 1700 nm or any other range, wherein optical sources may be used. The drawings used and their related explanation are only intended to illustrate the inventive idea. Different variations are of course possible in the practical implementation. There are quite many different implementation alternatives. The laser set according to the example can be implemented in very many different ways; firstly, separate lasers are not necessarily needed, but any way of producing such comb wavelengths can be used, from which suitable wavelengths can be selected. The lasers may be of any kind, e.g. semiconductor lasers or solid-state lasers. The components to be used are selected individually for the application, in other words, the invention is not limited to the use of certain lasers, since the use of free-space optics demands its own special ar- rangements. The most important matter for the invention is the maximising of the cross-connection's data transmission capacity, which is carried out in the solution according to the invention in the optical area located in the middle of Figure 2. Within the wavelength range of 1530 -1560 nm used in the exam- pie, it is possible to form a cross-connection apparatus of several terabits, wherein there can be thousands of wavelengths, that is, in practice thousands of bit flows within the said area. The connections and components presented in the example are only one manner of embodying the invention, and they do not restrict the invention in any way. Nor are the distances be- tween connections shown in Figure 2 restricted, in other words, the shown solution is also suitable e.g. as the solution for a star network.

Claims

1. Method for implementing a high-capacity optical cross- connection in a cross-connection apparatus, which cross-connection includes several optical input gates and output gates, and between the input gates and output gates there are components for connecting to output gates the optical signals arriving from input gates, characterized in that

- several optical signals are received at the input gates, of which each is at a certain incoming wavelength, - the received signals are converted into electric form in the input gates,

- each electric signal is converted into optical form for a certain internal wavelength of the apparatus with the aid of a narrow-band modulation method known as such, so that all signals are at their own internal wavelengths of the apparatus,

- the signals at internal wavelengths are combined with the aid of an optical combiner,

- the combined signal is divided to all output gates,

- at each output gate the desired internal wavelength is selected and the signal at the said wavelength is converted into electric form, and

- the obtained electric signal is converted back into optical form at the desired outgoing wavelength.

2. Method as defined in claim 1, c h a racte rized in that each input gate includes several optical incoming fibres (11 IN), of which incoming fibres each is functionally connected to its own optical-to-electric converter (A,), which converter is used for detecting the optical signals arriving from the optical incoming fibre and for converting them into the form of a broadband electric signal.

3. Method as defined in claim 2, characterized in that the output of each optical-to-electric converter (A,) is connected to the first input

(IN1,) of an own optical single-sideband modulator (B,), which said optical single-sideband modulator (B,) is used for narrowing the band width of the received signal, whereby the signal packing density can be increased.

4. Method as defined in claim 3, cha racterized in that at each input gate components (C,) producing an optical carrier wave are used for producing an optical carrier wave (OC, ) and the output of each compo- nent producing an optical carrier wave is functionally connected to the carrier wave input (IN2,) of the optical single-sideband modulator (B,).

5. Method as defined in claim 4, characterized in that for combining the optical output signals of several single-sideband modulators (B,) a signal combiner (D) is used, which includes several inputs for receiving optical signals from the single-sideband modulators.

6. Method as defined in claim 5, characterized in that the output of combiner (D) is connected to the input of optical divider (E), which divider includes several output gates for dividing the combined signal to the output gates.

7. Method as defined in claim 6, ch a ra cte ri zed in that from at least one output of the optical divider (E) an optical signal is conducted to the input of at least one optical detector (F,), which detector is preferably a heterodyne detector, and wherein the optical signal is detected and converted into electric form.

8. Method as defined in claim 7, characterized in that the output of each optical detector (F,) is functionally connected to the input of its own modulation and conversion component (H,), which component converts the signal back into optical form at the desired outgoing wavelength by modulating the optical carrier wave with the aid of the electric signal.

9. A high-capacity optical cross-connection system, which system includes several optical input gates and output gates, and between the input gates and output gates there are components for connecting to the output gates the optical signals arriving from the input gates, c h a ra ct e r i z e d in that the cross-connection system includes

- several reception components, which are adapted to receive several optical signals, each of which is at a certain incoming wavelength, and which reception components convert the received signals into electric form,

- several narrow-band modulators known as such, which convert each electric signal into optical form for a certain internal wavelength of the apparatus, so that all signals are at their own internal wavelengths of the apparatus, - a combiner, which combines the signals at their internal wavelengths and divides the combined signal to all output gates, - at each output gate components, which select the desired internal wavelength and convert the signal at the said wavelength into electric form, and

- components converting the selected electric signal back into op- tical form at the desired outgoing wavelength.

10. Cross-connection system as defined in claim 9, cha racte r i z e d in that each input gate includes several optical incoming fibres (I1,...,IN), of which incoming fibres each is functionally connected to its own optical-to-electric converter (A,), which detects the optical signals arriving from the optical incoming fibre and converts them into the form of a broadband electric signal.

11. Cross-connection system as defined in claim 10, characte rized in that the output of each optical-to-electric converter (A,) is connected to the first input (IN1, ) of its own optical single-sideband modulator (B,), and the band width of the received signal is narrowed with the aid of the said single-sideband modulator (B,), whereby the signal packing density can be increased.

12. Cross-connection system as defined in claim 11, characterized in that the system at the input gates includes several components (C,) producing an optical carrier wave (OC,), whereby the output of each component is functionally connected to the carrier wave input (IN2,) of its own single-sideband modulator (B,).

13. Cross-connection system as defined in claim 12, characte rized in that the system includes a signal combiner (D) which is com- mon for the input gates and which contains several inputs for receiving optical signals from the input gates, and which combiner combines the optical signals it receives from several own single-sideband modulators (B,).

14. Cross-connection system as defined in claim 13, characte rized in that the output of the signal combiner (D) is functionally con- nected to the input of the optical divider (E), which divider includes several output gates for dividing the combined signal to the output gates.

15. Cross-connection system as defined in claim 14, characterized in that at least one output of the optical divider (E) conducts the optical signal to the input of at least one optical detector (F,), which detector detects the signal it receives and converts it into electric form.

16. Cross-connection system as defined in claim 15, characte rized in that the output of each optical detector (F,) is functionally connected to the input of its own modulation and conversion component (H,), which component converts the signal back into optical form at the desired outgoing wavelength by modulating the optical carrier wave with the aid of the electric signal.