OPTICAL COMMUNICATION SYSTEM AND METHOD

Abstract

It are provided an optical communication system and an optical communication method. The system comprising at least two optical channels for communicating optical data signals; at least one optical filter arrangement for compensating distortions of the optical data signals communicated via the optical channels and/or crosstalk between the optical channels. The optical filter arrangement comprises at least one optical filter assigned to one of the optical channels and at least one optical filter assigned to the other one of the optical channels, wherein each one of the optical filters is configurable in such a way that different wavelength components of an incoming optical signal will be modified individually.

Claims

1. An optical communication system comprising at least two optical channels for communicating optical data signals; at least one optical filter arrangement for compensating distortions of the optical data signals communicated via the optical channels and/or crosstalk between the optical channels, wherein the optical filter arrangement comprises at least one optical filter assigned to one of the optical channels and at least one optical filter assigned to the other one of the optical channels, wherein each one of the optical filters is configurable in such a way that different wavelength components of an incoming optical signal will be modified individually.

2. The optical communication system as claimed in claim 1, wherein the at least two optical channels are provided a) by spatially separated optical paths, b) by using at least two optical carrier waves having different polarizations and/or c) by using at least two optical carrier waves having different orbital angular momentum states.

3. The optical communication system as claimed in claim 2, wherein the spatially separated optical paths are realized using at least two optical free space waves, at least two spatially separated optical waveguides, at least two cores of a multicore fiber and/or at least two modes of an optical fiber.

4. The optical communication system as claimed in claim 1, wherein the optical filters are configurable in such a way that the phase and/or the amplitude of the different wavelength components of the incoming optical signal will be modified individually.

5. The optical communication system as claimed in claim 1, wherein the optical filters are programmable optical filters.

6. The optical communication system as claimed in claim 5, wherein the programmable optical filters are provided by a device that is configured to carry out a spatial Fourier transform of the incoming light.

7. The optical communication system as claimed in claim 1, further comprising a multiplexer for multiplexing at least two input signals, wherein the optical filter arrangement is used for pre-compensation, wherein output signals of the optical filter arrangement are used as input signals supplied to the multiplexer.

8. The optical communication system as claimed in claim 1, further comprising a demultiplexer, wherein the optical filter arrangement is used for compensating the demultiplexed signals, wherein output signals of the demultiplexer are supplied to the optical filter arrangement.

9. The optical communication system as claimed in claim 1, wherein the optical filter arrangement is configured for receiving L input signals and for generating M output signals, wherein the optical filter arrangement comprises L?M optical filters.

10. The optical communication system as claimed in claim 9, wherein the optical filter arrangement comprises L optical splitters, and wherein each optical splitter splits an incoming optical signal into M optical partial signals, each optical partial signal being supplied to one of the optical filters.

11. The optical communication system as claimed in claim 9, wherein the optical filter arrangement comprises M optical combiners wherein each optical combiner combines the output of a plurality of the optical filters.

12. An optical communication method comprising the steps of: communicating optical data signals via at least two optical channels; compensating distortions of the data signals communicated via the optical channels and/or crosstalk between the optical channels using at least one optical filter arrangement, wherein the optical filter arrangement comprises at least one optical filter assigned to one of the optical channels and at least one optical filter assigned to the other one of the optical channels, and wherein each one of the optical filters is configurable in such a way that different wavelength components of an incoming optical signal will be modified individually.

13. The optical communication method as claimed in claim 12, wherein the optical characteristics of the optical filters are set using the results of a calibration measurement with respect to the at least two optical channels.

14. The optical communication method as claimed in claim 13, wherein the calibration measurement is carried out using a channel estimation scheme with respect to the at least two optical channels.

15. The optical communication method as claimed in claim 14, wherein the channel estimation scheme comprises transmitting a channel estimation sequence via the two optical channels.

16. The method as claimed in claim 15, wherein a first channel estimation sequence is transmitted via one of the optical channels and a second channel estimation sequence is transmitted via the other one of the optical channels, wherein the first channel estimation sequence is distinguishable from the second channel estimation sequence.

17. The optical communication method as claimed in claim 12, wherein the optical characteristics of the optical filters are set adaptively using an output signal of at least one of the optical filters and/or by carrying out a calibration measurement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Embodiments are explained in more detail with reference to the drawings.

[0032] FIG. 1 shows a diagram illustrating a receiver portion of an optical communication system.

[0033] FIG. 2 shows a diagram of an optical communication system.

[0034] FIG. 3 shows a diagram further illustrating the optical communication system depicted in FIG. 2.

DETAILED DESCRIPTION

[0035] FIG. 1 depicts a receiver portion 1 of an optical communication system according to an embodiment. The optical communication system comprises a few mode fiber 2, wherein optical data signals are communicated via different modes of the few mode fiber 2. The few mode fiber 2 thus provides a plurality of spatially separated optical paths which form part of different optical communications channels provided by the optical communication system. Of course, instead of or in addition to the few mode fiber 2 other means could be used for providing the optical data channels such as at least one multicore fiber having at least two spatially separated cores. Also free space transmission could be employed instead of few mode fiber 2 or polarization division multiplexing is used, wherein polarization beam splitters might be provided for generating carrier waves of different polarization. Possible means for providing the optical communication channels of the optical communication system have been already discussed above. Further, wavelength division multiplexed signals might be communicated via the spatially separated optical paths, thereby combining space division and wavelength division multiplexing.

[0036] The optical data signals communicated via the few mode fiber 2 are demultiplexed using a spatial demultiplexer 3, wherein the demultiplexed signals are output to three single mode fibers 41 to 43. Of course, more than three optical channels and thus more than three different optical data signals could be communicated such that more than the three single mode fibers 41 to 43 would be employed.

[0037] Note, that not necessarily single mode fibers need to be used as interconnection between the optical equalizer and the multiplexer and the receiver, respectively. Rather, free space, waveguide or multimode coupling arrangements might be suited as well as mentioned above.

[0038] The output signals of the demultiplexer 3 are supplied to an optical filter arrangement in the form of an optical MIMO equalizer 5 via the single mode fibers 41 to 43. The optical MIMO equalizer 5 comprises three optical splitters 51 to 53, wherein each one of the optical splitters 51 to 53 receives the output of one of the single mode fibers 41 to 43 and splits the received light into partial signals, Each one of the partial signals, in turn, is supplied to one of a plurality of optical filters OF.sub.ij of the optical MIMO equalizer 5. The output signals of the optical filters OF.sub.ij are fed into combiners 61 to 63, wherein output ports of the combiners 61 to 63 are connected to further single mode fibers 71 to 73. The single mode fibers 71 to 73 supply the output signals of the combiners 61 to 63 to receivers Rx1 to Rx3. In case wavelength division multiplexed signals are communicated via the few mode fiber 2, the receivers Rx1 to Rx3 might be wavelength division multiplex receivers.

[0039] The optical filters OF.sub.ij are programmable filters and as such part of a wave shaper device as already set forth above. More particularly, the transmission characteristic, i.e. amplitude and/or phase, of each one of the optical filters OF.sub.ij can be adjusted independently, wherein wavelength components of the light supplied to the optical filters OF.sub.ij can be modified individually. In particular, the characteristics of the optical filters OF.sub.ij are set dependent on MIMO matrix coefficients H.sub.ij underlying the communication system. The MIMO matrix describes the transmission characteristics of the communication system, i.e. a functional relationship between multiple input (sent) signals and multiple output (received) signals. The MIMO matrix coefficients H.sub.ij may be estimated using a channel estimation scheme as described above.

[0040] Thus, by adjusting the filter characteristics of each one of the optical filters OF.sub.ij individually it is possible to compensate distortions and/or crosstalk between the optical channels. For example, cross talk components of the MIMO matrix (i.e. MIMO matrix coefficients H.sub.ij, i?j) are compensated by adjusting the optical filters OF.sub.ij, i?j accordingly. For example, the characteristics of the optical filters OF.sub.ij may be set dependent on coefficients of the inverse of the MIMO matrix.

[0041] It is noted that the optical communication system according to FIG. 1 comprises three input signals (generated by three transmitters not shown in FIG. 1) and three output signals (the signals received by the receivers Rx1 to Rx3), wherein three optical communications channels are provided (disregarding the cross talk channels, which are not considered as communication channels). Accordingly, the optical communication system could be described as a 3?3 MIMO system. However, the embodiment is of course not restricted to a particular number of input or output signals. Rather, the embodiment could be implemented in any MIMO system having any number of input and output signals (i.e. a N?M MIMO system), see e.g. FIG. 2.

[0042] FIG. 2 shows an embodiment of an optical Communication system 10. The system 10 comprises a receiver portion 1, which might be configured identically to the receiver portion illustrated in FIG. 1 (i.e. comprising a MIMO equalizer 5). However, the receiver portion 1 may comprise any number of receivers Rxj, j=1, . . . , M.

[0043] The optical system 10 further comprises a transmitter portion 10 that has a plurality of transmitters Txi, I=1, . . . , N. The data signals generated by the transmitters Txi are multiplexed into a few mode fiber 2 by means of a spatial multiplexer 30. The transmitters Txi might further be configured for transmitting wavelength division multiplexed signals, which are communicated via the spatially separated optical paths provided by the few mode fiber 2. Note, that other optical transmission media could be used as described forth above.

[0044] Between the transmitters Txi and the multiplexer 30 a further optical filter arrangement in form of a MIMO pre-equalizer 50 is arranged. The MIMO pre-equalizer 50 is configured similarly (or even identically) to the MIMO equalizer 5 of the receiving portion 1, i.e. the pre-equalizer 50 comprises a plurality of programmable optical filters used for individually filtering the optical data signals. The programmable optical filters of the pre-equalizer 50 may also be provided by a wave shaper unit.

[0045] The programmable optical filters of the pre-equalizer 50 are used for individually filtering the optical signals generated by the transmitters Txi, wherein the characteristics of the programmable optical filters are set dependent on the MIMO matrix coefficients H.sub.ij related to the optical communication system 100 as described above with respect to MIMO equalizer 5 depicted in FIG. 1. For example, MIMO processing based on a singular value decomposition of the MIMO matrix coefficients H.sub.ij could be utilized to determine the required characteristics of the programmable optical filters. Thus, the distortions and/or the cross talk between the optical data signals communicated by means of the optical communication system 100 are not only compensated in the receiver portion 1, but are also be compensated in the transmitter portion 10.

[0046] It is, of course, also conceivable that the optical communication system 100 comprises a single MIMO equalizer only (i.e. the MIMO pre-equalizer 50 of the transmitter portion 10 or the MIMO equalizer 5 of the receiver portion 1). For example, MIMO pre-equalizer 50 may obviate the need for MIMO equalizer 5 at the receiver portion 1 if the coherence time of the optical channels is greater than the feedback time to transmit channel state information from the receivers Rxj to the transmitters Txi.

[0047] As illustrated in FIG. 3, the MIMO pre-equalizer 50 might be configured for mapping the N signals generated by the transmitters Txi onto K signals supplied to the spatial multiplexer 30. The spatial demultiplexer 3 may be configured to generate L signals such that the optical communication link between the spatial multiplexer 30 and the demultiplexer 3 may be regarded as a K?L MIMO system.

[0048] Moreover, the MIMO equalizer 5 of the receiver portion 1 maps the incoming L signals onto M signals fed to the receivers Rxj. Accordingly, the MIMO equalizer 5 may comprise L?M optical filters, while the MIMO pre-equalizer 50 may comprise N?K optical filters.