EP2522088A1

EP2522088A1 - Optical signal receiver

Info

Publication number: EP2522088A1
Application number: EP10795670A
Authority: EP
Inventors: Jean-Christophe Antona; Sébastien Bigo
Original assignee: Alcatel Lucent SAS
Current assignee: Alcatel Lucent SAS
Priority date: 2010-01-07
Filing date: 2010-12-16
Publication date: 2012-11-14
Also published as: TW201207457A; WO2011083018A1; FR2955003A1; FR2955003B1

Abstract

An optical signal receiver (30) comprises an input intended to be connected to a multimode waveguide (21) for receiving an incoming optical signal modulated with digital data, said incoming optical signal comprising a plurality of spatial modes, and a mode demultiplexing module (31) arranged between an input and coherent optical detectors (34), said mode demultiplexing module comprising a plurality of mode-selector filters respectively associated with said coherent optical detectors, each of said mode selector filters comprises a monomode waveguide (81) connected to the associated coherent mixer and an optical device (80) capable of coupling the fundamental spatial mode of said monomode waveguide with an essentially one chosen spatial mode of said incoming optical signal.

Description

OPTICAL SIGNAL RECEIVER

The invention pertains to the field of optical communication systems, in particular to devices enabling communications over multimode waveguides.

One experiment in communication over a multimode waveguide is related in

"Fundamentals and Challenges of Optical Multiple-Input Multiple-Output Multimode Fiber Links", by A. Tarighat et al., IEEE Communications Magazine, May 2007. However, the transmission distance has remained very modest. Developments are still needed to apply multimode waveguides to long-range and/or high-capacity transmissions.

According to one embodiment, the invention provides an optical signal receiver comprising:

an input intended to be connected to a multimode waveguide for receiving an incoming optical signal modulated with digital data, said incoming optical signal comprising a plurality of spatial modes,

an optical source for producing a local oscillator signal,

a plurality of coherent optical detectors, each of said coherent optical detectors comprising a coherent mixer for producing an interference signal between said local oscillator signal and an optical signal to be detected coming from said input, and photoelectric detectors for producing detection signals, corresponding, for example, to one in-phase component and one quadrature component of the interference signal,

one or more digital processing modules for processing said detection signals so as to locate the digital data being carried by the incoming optical signal,

and a mode demultiplexing module arranged between said input and the coherent optical detectors, said mode demultiplexing module comprising a plurality of mode- selector filters respectively associated with said coherent optical detectors,

each of said mode selector filters being capable of providing the coherent mixture of the associated coherent optical detector with an optical signal to be detected essentially corresponding to a respective spatial mode of said incoming optical signal.

In other advantageous embodiments, such an optical signal receiver may exhibit one or more of the following characteristics: a beam splitter is provided to distribute said incoming optical signal to said mode-selector filters.

One or each of said mode selector filters comprises a monomode waveguide connected to the associated coherent mixer and an optical device capable of coupling the fundamental spatial mode of said monomode waveguide with essentially one chosen spatial mode of said incoming optical signal.

The receiver may comprise a monomode waveguide connecting said optical source to the coherent mixer of one or each of said coherent optical detectors. Thus, the overlapping of the signals within the coherent mixer may be carried out essentially within a fundamental spatial mode, which ensures that the coherent overlapping is effective, producing an interference signal with satisfactory amplitude. Furthermore, a coherent mixer operating with signals in fundamental mode is easier to construct, which constitutes an advantage in terms of cost and reliability.

the beam splitter may be multimode. In another embodiment, the beam splitter and the mode selector filters may be constructed in the form of an integrated component jointly carrying out the division of the incoming signal and the selection of respective modes.

the mode selector filters select respective modes of the incoming optical signal, said respective modes comprising a fundamental mode and a top mode, the mode selector filters select respective modes of the incoming optical signal, said respective modes comprising multiple top modes.

the top modes belong to the group consisting of LP02, LP1 1 , LP21 , and LP03. the incoming optical signal results from the propagation, over said multimode waveguide, of a plurality of overlapping modal components, each of said modal components having been modulated with a subset of said digital data within one end of said multimode waveguide placed remotely from the receiver,

the digital processing module implements a matrix calculation intended to reverse the couplings that occurred between said respective spatial modes during the propagation within said multimode waveguide.

The waveguide may be weakly multimode, for example with fewer than 1 0 modes at the examined wavelength. In one embodiment, the invention also discloses an optical communication system comprising an aforementioned receiver, a multimode waveguide connected to the input of said receiver, and an optical transmitter connected to one end of said multimode waveguide placed remotely from the receiver, said optical transmitter being capable of transmitting within said multimode waveguide the overlapping of a plurality of modal components, each of said modal components being modulated with a subset of said digital data.

One idea at the basis of the invention is that the use of optical fibers or other waveguides exhibiting a relatively high effective cross-section within an optical communication system is likely to reduce the nonlinear effects affecting the transmitted signals, which could encourage an increase in the power level of the optical signals to extend the transmission distance. Some aspects of the invention derive from the observation that such an optical fiber is likely to render the transmission multimodal, and therefore to cause interference between symbols that must be offset on the receiver's end to rediscover the data. Other aspects of the invention are founded on the idea of separating a received signal into a plurality of multimodal components via a multimodal waveguide and of processing detection signals corresponding to these respective multimodal components in such a way as to reverse the propagation effects. Further aspects of the invention are founded on the idea of constructing a mode adaptation between a local oscillator signal and components of a multimodal signal to reduce effective coherent detection of these components. Further aspects of the invention are founded on the idea of using spatial mode multiplexing at a given wavelength to obtain a total transmission capacity equal to the sum of the capacities of a plurality of spatial modes.

The invention will be better understood, and other purposes, details, characteristics, and advantages thereof will become more clearly apparent upon examining the following description of multiple particular embodiments of the invention, which are given only by way of illustrative and non-limiting examples, with reference to the attached drawings. In these drawings:

Figure 1 is a functional block diagram of an optical transmission system according to one embodiment,

Figure 2 depicts one embodiment of a mode demultiplexer, which may particularly be used in the system of Figure 1 , Figure 3 depicts one embodiment of a mode converter, which may particularly be used in the demultiplexer of Figure 2,

Figure 4 depicts one embodiment of a coherent receiver, which may particularly be used in the system of Figure 1 , and

Figure 5 depicts one embodiment of an optical reception device, which may particularly be used in the system of Figure 1 .

With reference to Figure 1 , an optical communication system is schematically depicted. This system comprises an optical transmission device 1 0, an optical reception device 30, and a transmission line 20 capable of conducting optical signals from the optical transmission device 1 0 to the optical reception device 30. The transmission line 20 comprises a multimode optical fiber 21 . The transmission line may also comprise other monomode or multimode optical elements not shown here, such as optical amplifiers, chromatic dispersion offset devices, connectors, add-drop multiplexers, transparent switching devices, optical fibers of different types, and others. It is not necessary here to describe in greater detail such elements commonly used in optical communication networks.

The multimode optical fiber 21 is an optical fiber whose structure enables, at a wavelength used for communication, the propagation of multiple proper transverse modes. A proper transverse mode is a spatial distribution of the electromagnetic field in a plane orthogonal to the propagation direction, which remains roughly identical in the course of propagation, subject to a longitudinal phase factor and a longitudinal attenuation factor. The proper transverse modes of a waveguide, which will be called spatial modes for brevity's sake, are conventionally designated by the symbol LP (for linearly polarized) followed by two whole numbers. In this format, the first number represents the phase variation of the electromagnetic field along a circle centered on the longitudinal axis of the optical fiber, the unit of measure being In; and the second number represents the phase variation of the electromagnetic field along a radius of the optical fiber, the unit of measure being π.

The multimode fiber 21 may support between two and more than a hundred modes, depending on its design. Preferentially, the multimode optical fiber 21 is weakly multimodal, the number of spatial modes not exceeding 1 0, or even 3. Such an optical fiber may particularly exhibit a relatively broad effective cross-section, particularly one greater than 300μηι², such as 400 or 500μηι². The effective cross- section being examined here may be a mode-based effective cross-section or an equivalent effective cross-section pertaining to the tolerance of total optical power injected with respect to the overall non-linear effects for the propagation of multiple spatial modes.

The optical transmission device 1 0 creates one or more optical signals modulated with data and injects that or those signals within the transmission line 20 that carries them to the reception device 30. For the sake of explanations, the transmission of a monochromatic signal over a given wavelength channel will first be examined. Likewise, a signal emitted, propagated, and received on a single polarization will be considered first. Owing to the multimodal structure of the optical fiber 21 , the optical signal that reaches the reception device 30 comprises multiple spatial modes at the wavelength that it used. These spatial modes propagate at different group speeds, which causes the appearance of interference, particularly interference between symbols, during propagation. Furthermore, couplings between modes are possible during propagation, particularly within multiple fiber seams, accentuating the interference. The reception device 30 implements a coherent optical detection and a digital processing of detection signals particularly to offset the multimodal dispersion and rediscover the transmitted data.

To do so, the reception device 30 comprises a mode demultiplexer 31 , one input 32 of which is connected to the optical fiber 21 for receiving the transported multimodal signal. The mode demultiplexer 31 separates the incoming signal into multiple respective modal signals which are directed via respective outputs 33 to respective coherent detectors 34. At the outputs 33 of the mode demultiplexer 31 , each modal signal essentially corresponds to a respective spatial mode of the incoming signal. In other words, more than 50% of the power of a mobile signal obtained at an output of the mode demultiplexer 31 comes from a given spatial mode of the incoming signal within the mode demultiplexer 31 . These spatial modes strictly exist only within the multimodal fiber 21 . Outside of the multimodal fiber, the distribution of the electromagnetic fields corresponding to such a mode is also designated by the expression "spatial mode".

Each time, a coherent detector 34 receives a mobile signal to be detected from the mode demultiplexer 31 such as through a fiber or waveguide 36, and a local oscillator signal from a local oscillator 37, such as through a fiber or waveguide 38. The local oscillator signal is tuned to the same wavelength as the one used to transport the data. Preferentially, the fibers or guides 36 and 38 are monomode at this wavelength, which makes it possible to use a monomode coherent mixer whose design is relatively simple.

A digital processing module 35 receives from each coherent detector 34 an electrical detection signal I representing an in-phase component and an electrical detection signal Q representing a quadrature component of the modal signal detected by that coherent detector. The processing module 35 samples these signals and applies a processing to all of the detection signals received at a given moment to reverse the propagation effects and rediscover the initially modulated data. To do so, adaptive algorithms commonly used in the field for coherent detection may be modified appropriately. Particularly, it is possible to use for this purpose adaptive filters of the type known for offsetting the polarization mode dispersion. The filter's temporal depth may be adapted to the maximum time shift existing between the spatial modes being examined.

A beam splitter 40 is used in Figure 1 to distribute the local oscillator signal to each coherent detector 34 from the single local oscillator 37. In one variant, multiple respective local oscillators may be connected to the various coherent detectors 34.

With reference to Figure 4, a coherent detector 60, which may be used in the reception device 30 of Figure 1 , comprises a coherent mixer 61 commonly formed of a combination of semi- reflective blades and converging lenses, photoelectric detectors 62 and signal combiners 63. The coherent mixer 61 forms interference signals from optical signals received on its two inputs 64. The photoelectric detectors 62 convert these interference signals into electrical signals. The signal combiners 63 combine these electrical signals to form the in-phase detection signal I and the quadrature detection signal Q.

With reference to Figure 2, a mode demultiplexer 70, which may be used in the reception device 30 of Figure 1 , comprises an input element 71 , for example a length of multimode fiber, for receiving a multimode optical signal 78 to be demultiplexed, a multimode beam splitter 72 for distributing the multimode optical signal to multiple mode selector filters 73, output elements 74 for connecting a modal signal 79 selected by the associated mode selector filter 73 each time. Each mode selector filter 73 selects a respective spatial mode of the incoming signal, that is to say modes LP02, LP1 1 , and LP01 in the example depicted. In other words, a mode selector filter 73 allows into the output element 74 an optical signal more than 50% of whose energy, and preferably more than 66%, comes from the indicated spatial mode.

The multimode beam splitter 72 may be constructed from lenses and semi- transparent blades.

With reference to Figure 3, in a preferred embodiment, the mode selector filter 73 and the output element 74 are constructed in the form of a combination of a mode converter 80 and a monomode waveguide 81 . The mode converter 80 receives the multimode signal to filter from a multimode waveguide 82 and converts a given spatial mode of that signal, for example mode LP02, into a fundamental spatial mode LP01 , which it transmits to the monomode waveguide 81 . Owing to its structure, the monomode waveguide 81 only allows through the fundamental spatial mode, which makes it possible to eliminate any higher-order spatial components.

Such a mode converter 80 may be constructed in accordance with the disclosure found in document US-A-6377726, with lenses 83 and 84 and phase masks 85 and 86. The modal component which is converted to the fundamental mode depends on the precise configuration of the phase masks 85 and 86. Different mode converters may be constructed from this model to select different higher-order modes of the incoming signal, for example LP02, LP1 1 , LP21 , and LP03. To select the fundamental mode LP01 of the incoming signal, this device or a simpler device may be used, for example a converging lens.

Whenever the mode selector filter 73 from Figure 2 is constructed in the form of a mode converter like the one from Figure 3, the selected modal signal is provided to the coherent detector in the form of the fundamental mode of a monomode waveguide. In this situation, the overlapping of the modal signal to be detected with the local oscillator signal in the receiver's coherent mixer may be carried out with very good effectiveness, owing to the mode agreement between the modal signal to be detected and the local oscillator signal. This is why it is preferable to transport the local oscillator signal over a monomode waveguide in this situation. Another advantage resulting from this configuration is the option of using, in each of the coherent detectors 34, a conventional coherent mixer available on the market at low cost.

In one embodiment, the multimode optical signal's distribution function may be carried out in combined fashion with the selection function of the spatial modes to be detected.

Depending on the configuration of transmission and reception devices 1 0 and 30, the optical communication system depicted in Figure 1 may be used in several ways. In a first application, known as SISO (Single Input-Single Output) or SIMO (Single Input Multiple Output), the multimodal nature of the propagation is viewed as the replication of the same data signal on different propagation paths. For such an application, the transmission device 1 0 is designed to inject the optical signal modulated with the data signal into one end of the line 20, in such a way that this signal is coupled with one or more spatial modes of the multimode fiber 21 . At the other end of the line 20, the digital processing module 35 processes the various detection signals as different linear combinations of the multiple time-shifted replicas of that data signal to be rediscovered.

In a second application, known as MIMO (Multiple Input Multiple Output), the multimodal nature of the propagation is viewed as a demultiplication of the transmission channels, thereby making it possible to increase the capacity of the communication system by transmitting multiple respective data flows over multiple spatial flows or sets of respective spatial flows. For such an application, with reference to Figure 1 , the transmission device 1 0 may comprise multiple signal modulators 1 1 configured to modulate optical signals with respective data flows Dl , D2, Dk and a mode demultiplexer 1 2 configured to couple each modulated optical signal SI , S2,... Sk with a respective spatial mode of the multimode fiber 21 or a set of such modes. To carry out this selective coupling of a signal modulated with a particular spatial mode of the fiber 21 , the mode multiplexer 1 2 may comprise mode converters similar to that which is described with reference to Figure 3, the conversion direction being reversed in this case.

In this application, at the other end of the line 20, the digital processing module 35 processes the various detection signals as mixtures of data signals to be rediscovered, given the couplings between modes that occur during propagation. These intermodal couplings may be reversed by numeric methods as long as a sufficient number of distinct detection signals are provided to the digital processing module 35. As each coherent detector 34 serves to essentially capture the information about a given spatial mode, as many coherent detectors 34 are preferably provided as there are signal modulators 1 1 , and the mode demultiplexer 31 selects the respective spatial modes with which the initially transmitted optical signals SI , S2 ... Sk were respectively coupled.

In the event of mode multiplexing during transmission, if the modes do not mix in the multimode fiber, it is also possible to process the detected signals mode by mode, such as by providing a separate digital processing module for each spatial mode detected.

In the embodiments above, the implementation of an optical communication over a multimodal transmission line has been described using only a carrier wavelength. Mode multiplexing makes it possible to envision an increase in the capacity of a wavelength channel. To create a larger-capacity communication system, the methods described above may be combined with wavelength division multiplexing techniques and/or polarization division multiplexing techniques.

To do so, a wavelength combiner, for example a wavelength division multiplexer, may be provided within the transmission device 1 0, and a wavelength separator, for example a wavelength division demultiplexer, may be provided within the reception device 30. In one corresponding embodiment, the elements of the transmission device 1 0 and/or the reception device 30 which were described above with reference to a wavelength may be multiplied into as many copies as there are wavelength channels to process.

Likewise, to implement polarization division multiplexing, one or more polarization combiners may be provided within the transmission device 1 0, and one or more polarization separators may be provided within the reception device 30.

Whether with or without polarization division multiplexing during transmission, it is preferable to construct a diverse-polarization reception device, particularly to be able to offset the modal polarization dispersion. To do so, multiple possibilities exist.

One embodiment of the diverse-polarization reception device is sketched in Figure 1 . Here, the incident optical signal passes into a polarization separator element 25, at whose output two multimodal signals with orthogonal polarizations are separated. Likewise, a polarization separator element 26 separates the local oscillator signal into two orthogonal polarized components. The block 50 depicted in Figure 1 therefore represents the coherent detection chain for a polarization component. A second, identical block 50, not depicted, must be connected to the other ports 51 of the polarization separators 25 and 26 to detect the other polarization component. In this situation, the numeric polarization module 35 may be shared by both blocks 50, in order to simultaneously process the detection signals corresponding to the two polarization components. Instead of the separator 26, a different local oscillator may be provided for each polarization.

In another embodiment of the diverse-polarization reception device, depicted in Figure 5, the separation of optical signals into two orthogonal polarized components is done downstream of the separation of spatial modes. For the rest, elements identical or analogous to those in Figure 1 bear the same reference number plus 1 00.

Some of the elements depicted, particularly the control units and the various modules, may be constructed in various forms, in a stand-alone or distributed fashion, using hardware and/or software components. Hardware components that may be used are application-specific integrated circuits, field-programmable gate arrays, or microprocessors. Software components may be written in various programming languages, such as C, C + + , Java, or VHDL. This list is not exhaustive.

Although the invention has been described in connection with multiple specific embodiments, it is naturally not in any way limited to them, and comprises all technical equivalents of the means described, as well as their combinations, if said combinations fall within the scope of the invention.

The use of the verb "comprise" or "include" and their conjugated forms does not exclude the presence of elements or steps other than those set forth in a claim. The use of the indefinite article "a" or "an" for an element or step does not, unless otherwise stated, excluded the presence of a plurality of such elements or steps. Multiple means or modules may be depicted by a single hardware element.

In the claims, any reference sign within parentheses should not be interpreted as limiting the claim.

Claims

1 . An optical signal receiver (30) comprising:

an input intended to be connected to a multimode waveguide (21 ) for receiving an incoming optical signal modulated with digital data, said incoming optical signal comprising a plurality of spatial modes,

an optical source (37) for producing a local oscillator signal,

a plurality of coherent optical detectors (34), each of said coherent optical detectors comprising a coherent mixer for producing an interference signal between said local oscillator signal and an optical signal to be detected coming from said input, and photoelectric detectors for producing detection signals,

a digital processing modules (35) for processing said detection signals so as to locate the digital data being carried by the incoming optical signal, and a mode demultiplexing module (31 ) arranged between said input and the coherent optical detectors, said mode demultiplexing module comprising a plurality of mode selector filters (73) respectively associated with said coherent optical detectors,

2. A receiver according to claim 1 , characterized by the fact that one or each of said mode selector filters comprises a monomode waveguide (81 ) connected to the associated coherent mixture and an optical device (80) capable of coupling the fundamental spatial mode of said monomode waveguide with an essentially one chosen spatial mode of said incoming optical signal.

3. A receiver according to claim 1 or 2, characterized by the fact that it comprises a monomode waveguide (38) connecting said optical source to the coherent mixer of one or each of said coherent optical detectors.

4. A receiver according to one of the claims 1 to 3, characterized by the fact that it comprises a beam splitter (72) to distribute said incoming optical signal to said mode selector filters.

5. A receiver according to claim 4, characterized by the fact that said beam splitter (72) is multimode.

6. A receiver according to one of the claims 1 to 5, characterized by the fact that said mode selector filters (73) select multiple respective modes of the incoming optical signal, said respective modes comprising a fundamental mode and a top mode.

7. A receiver according to one of the claims 1 to 6, characterized by the fact that said mode selector filters (73) select multiple respective modes of the incoming optical signal, said respective modes comprising multiple top modes.

8. A receiver according to claim 6 or 7, characterized by the fact that said or each top mode belongs to the group consisting of LP02, LPl l , LP21 , and LP03.

9. A receiver according to one of the claims 1 to 8, characterized by the fact that said incoming optical signal results from the propagation, over said multimode waveguide, of a plurality of overlapping modal components, each of said modal components having been modulated with a subset (Dl , D2) of said digital data within one end of said multimode waveguide placed remotely from the receiver.

10. A receiver according to one of the claims 1 to 9, characterized by the fact that said digital processing module (35) implements a matrix calculation intended to reverse the couplings that occurred between said respective spatial nodes during the propagation within said multimode waveguide.

1 1 . A receiver according to one of the claims 1 to 1 0, characterized by the fact that said detection signals produced by a coherent optical detector correspond to an in-phase component (I) and a quadrature component (Q) of the interference signal.

12. An optical communication system comprising a receiver according to one of the claims 1 to 1 1 , a multimode waveguide (21 ) connected to the input of said receiver, and an optical transmitter (1 0) connected to one end of said multimode waveguide placed remotely from the receiver, said optical transmitter being capable of transmitting within said multimode waveguide the overlapping of a plurality of modal components, each of said little components being modulated with a subset (Dl , D2) of said digital data.