Patent Publication Number: US-2015086201-A1

Title: Data Multiplexing And Mixing Of Optical Signals Across Propagation Modes

Description:
TECHNICAL FIELD 
     The disclosure relates generally to the field of optical communications. 
     BACKGROUND 
     This section introduces aspects that may be helpful to facilitating a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art. 
     Optical fiber nonlinearity may result in degradation of signal fidelity in optical communications systems. Single-mode optical fibers with large effective areas can provide some improvement in nonlinear transmission, but these systems remain limited by fiber nonlinearity. Multicore and multimode fibers can provide additional capacity or reach, but these systems also remain limited by fiber nonlinearity. 
     SUMMARY 
     One embodiment provides an apparatus, e.g. an optical device, that includes an optical transmitter and a mixer. The transmitter is configured to transmit a plurality of optical data channels, each including a spectral component at a same frequency. The mixer is configured to combine a first data channel with a second data channel. The combining is such that first and second optical channels output by the optical transmitter each include contributions from the first and second data channels at the same frequency. In some embodiments of the apparatus a multiple-input-multiple-output (MIMO) module is configured to recover the data channels from the output optical signals. 
     In some embodiments of the apparatus the mixer is configured to optically mix the contributions. In some embodiments the mixer includes an optical coupler having M inputs and N outputs, and is configured to map M optical signals, corresponding to each of the data channels and received at corresponding ones of the M inputs, among N output signals. In some such embodiments M=N. In other such embodiments the mixer is configured to apply a non-equal weighting to each of the N output signals. 
     In some embodiments of the apparatus the mixer includes a mode scrambler. The mode scrambler is configured to remap a plurality of optical signals to a different corresponding optical propagation mode, with each optical signal being received via a corresponding optical propagation mode. In some embodiments of the apparatus the mixer is one of a plurality of optical mixers, with each optical mixer of the plurality having inputs and outputs. In such embodiments each optical mixer is configured to impose a corresponding mixing function on optical signals received at inputs thereof. In some such embodiments the mixing functions are a same mixing function. 
     In some embodiments of the apparatus the mixer is optically coupled to a spatially diverse optical medium and is configured to propagate the output optical signals into a corresponding plurality of spatially diverse optical paths of the optical medium. 
     In some embodiments the mixer is configured to electrically mix the contributions. In some such embodiments the mixer is configured to provide a unitary transformation between the electrical signals. In other such embodiments the mixer is configured to provide an invertible linear transformation of the electrical signals. In some embodiments an inverse transformation module is configured to apply an inverse of the unitary transformation after coherent detection of the first and second optical channels. 
     Another embodiment provides a method, e.g. for reducing the effect of nonlinearities of optical path on data channels propagated via the optical paths. The method includes configuring a signal mixer to receive a plurality of data channels at a plurality of inputs. The method further includes configuring the signal mixer to impose a mixing function on the data channels such that data received at each of the inputs is distributed among output optical signals at the plurality of outputs. The method further includes configuring an optical modulator to modulate an optical signal corresponding to each data channel. 
     In some embodiments, the method includes configuring a multiple-input-multiple-output (MIMO) module to recover the data channels from the output optical signals. In some embodiments the method includes configuring the output optical signals to propagate via a spatially diverse optical medium. 
     Some embodiments of the method include configuring an optical mixer to operate as the signal mixer. The optical mixer includes an N×N optical coupler configured to map N received optical signals among N outputs with a predetermined weighting. In some such embodiments the method includes configuring the optical mixer to remap a plurality of optical signals, each signal being received via a corresponding optical propagation mode, to a different corresponding propagation mode. 
     Some embodiments of the method include configuring an electrical mixer to operate as the signal mixer. The electrical mixer is configured to receive a plurality of electrical data channels at the plurality of inputs. The electrical mixer is further configured to impose a mixing function on the electrical data channels such that data received at each of the inputs is distributed among output electrical signals at the plurality of outputs. The electrical mixer is still further configured to provide each of the output electrical signals to an optical modulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention may be obtained by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  illustrates an embodiment, e.g. a system, in which an optical mixer mixes a plurality of optical channels after optical modulation of the channels, e.g. to reduce the effect of optical channel nonlinearities; 
         FIGS. 2A-2C  illustrate embodiments of transmission media that may be used, e.g. in the embodiments of  FIGS. 1 and 4 , including N single-mode optical fibers ( FIG. 2A ), multi-core optical fibers with uncoupled cores ( FIG. 2B ), and multi-mode optical fibers ( FIG. 2C ); 
         FIG. 3  illustrates an embodiment of the optical mixer of  FIG. 1 , implemented as an N×N optical coupler; 
         FIG. 4  illustrates an embodiment, e.g. a system, in which an electrical mixer mixes a plurality of data channels prior to optical modulation of a corresponding number of optical channels; 
         FIG. 5  illustrates an embodiment of the electrical mixer of  FIG. 4 , implemented including a processor, memory and I/O module; and 
         FIG. 6  illustrates an embodiment, e.g. a system, that provides multistage mixing of optical channels, e.g. to further reduce the effect of optical channel nonlinearities. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure is directed to, e.g. methods and systems for distributing transmitted optical data among two or more optical fiber channels, such that, e.g., the effect of transmission nonlinearities of the fiber channels on the integrity of transmitted data is reduced. 
     The impact of nonlinear distortions on the performance of fiber-optic communication systems caused by optical path nonlinearity is thought to depend on the spatial diversity of propagated waveforms, e.g. how the waveforms are distributed among multiple parallel optical paths. Moreover the distribution of the distortions across the data channels to be transmitted is also expected to affect system performance. 
     Embodiments described herein may mitigate the adverse performance impact caused by these distortions by combining in a transmitter data channels, e.g. first and second data channels, such that optical channels output by the transmitter include contributions from each of first and second data channels. In other words, each data channel is transmitted in parallel among the multiple optical channels. The optical channels are then propagated by corresponding spatially diverse optical paths. The combining, or mixing, distributes the transmitted information among the optical paths in a manner that is expected to average out nonlinear distortions over the data channels. The averaging is expected to reduce the effect of the distortions on the data recovered from the optical channels, e.g. by multiple-input multiple-output (MIMO) processing. In various embodiments a transformation is applied to the data channels in a manner that distributes the channels across all propagating modes. Such embodiments are expected in at least some cases to result in the greatest benefit for a particular system configuration. 
     Turning to  FIG. 1 , a system  100  is illustrated according to one embodiment. The system  100  includes a transmitter  110 , a transmission medium  120 , and a receiver  130 . The transmitter  110  includes a data source  140 , an optical modulator  150 , and a mixer  160 , e.g. an optical mixer. The data source  140  may be any source of data configured to provide a plurality of data channels D 1  . . . D N . The data channels include digital-electrical data representations of data values. The optical modulator  150  receives the data signals at corresponding inputs and modulates a plurality of optical signals corresponding to the data signals, thereby producing modulated optical signals M 1  . . . M N . The optical modulator  150  may include such components as, e.g., digital-to-analog converters, lasers and Mach-Zehnder modulators. The optical channels may be modulated according to any type of modulation scheme, for example and without limitation, on-off keying (OOK), differential phase shift keying (DPSK), quaternary phase shift keying (QPSK), or quadrature amplitude modulation (QAM) format with or without return to zero or nonreturn to zero pulse shaping. Moreover, additional multiplexing schemes may be used to increase the signal capacity, for example and without limitation polarization-division multiplexing (PDM), wavelength-division multiplexing (WDM), or orthogonal frequency-division multiplexing (OFDM). 
     The mixer  160  receives the modulated optical signals and applies a transformation T thereto, as described further below, thereby producing transformed optical signals E 1  . . . E N . The optical transmission medium  120  receives the transformed optical signals, which are propagated via multiple parallel spatially diverse paths. As used herein, spatially diverse paths are optical propagation paths or modes having nominally substantially orthogonal basis sets such that, absent nonlinear effects, there is negligible coupling between any two of the spatially diverse paths. However, as described further below, under some conditions nonlinear effects may become non-negligible, with resulting non-negligible coupling between spatially diverse paths. 
       FIGS. 2A-2C  present three nonlimiting illustrative embodiments of the medium  120 .  FIG. 2A  illustrates N single-mode fibers (SMFs) that may collectively serve as the medium  120 . Each single-mode fiber may operate to support an optical propagation mode, the optical propagation modes of the multiple fibers being spatially diverse propagation modes.  FIG. 2B  illustrates a single fiber having a plurality of N (N=7) optical cores, e.g. a multi-core fiber (MCF). Each of the optical cores may operate to support an optical propagation mode, the optical propagation modes of the multiple cores being spatially diverse propagation modes.  FIG. 2C  illustrates a single fiber having a single optical core capable of supporting a plurality of N propagation modes, e.g. a multi-mode fiber (MMF). The propagation modes of the multi-mode fiber may also considered to be spatially diverse propagation modes. As used herein, the term “spatially diverse optical medium” is inclusive of the N SMFs, the N-core MCF, and N-mode MMF. Each of the optical cores and/or propagation modes of the spatially diverse optical medium may be referred to herein and in the claims as one of a plurality of spatially diverse optical paths. The value of N may correspond to, e.g., the number of spatial modes, the number of vector modes or the number of signal quadratures (such as in coherent optical transmission), according to the context. 
     Returning to  FIG. 1 , each of the optical signals output by the optical modulator  150  may include multiple spectral components λ 1 , λ 2  . . . λ k , e.g. may be wavelength-division multiplexed (WDM) signals. One or more of the spectral components may be organized in one or more superchannels. In some embodiments the outputs may each include one or more same spectral components, e.g. data channels modulated to a same wavelength channel of the WDM signals. The mixer  160  imposes a transformation on the optical signals output by the optical modulator  150 . The transformation distributes the optical channels among the plurality of optical paths. The selection of the transformation may depend on the nature of the medium  120 , as further discussed below. The mixing has the effect of superimposing portions of the received optical signals at one or more same wavelengths of the optical channels. For example, two or more of the signals M 1  . . . M N  may include modulated data on the λ 1  channel of each respective WDM signal. The mixer  160  provides at each of its outputs a portion of the λ 1  signal received via signal M 1 , a portion of the λ 1  signal received via signal M 2 , a portion of the λ 1  signal received via signal M 3 , and so on. In this manner, the spectral components at each frequency of each output of the optical modulator  150  may be mixed with each other at each output of the mixer  160 . Each mixed signals are then coupled to a corresponding spatially diverse optical path of the medium  120 . 
     The medium  120  may impose a distortion signal on the optical signals E 1  . . . E N . The distortion signal may be, e.g. a nonlinear distortion due to intrinsic nonlinearities in the propagation characteristics of the medium  120 . The distortion caused by such nonlinearity in an optical path may be greater for a higher power level of the optical signal propagating within the optical path. The distorted optical signals after propagation by the medium  120  are designated F 1  . . . F N  to reflect the added distortion signal. 
     The receiver  130  receives the signals F 1  . . . F N , and includes a coherent detector  171 , a MIMO module  172  and a data recovery module  175  The coherent detector  171  provides well-known optical and electrical functionality to convert the optical F 1  . . . F N  signals to electrical domain. In a manner analogous to radio frequency (RF) MIMO processing, the MIMO module  172  may apply conventional or novel processing algorithms to account for the spatial separation of the propagated data channels among the propagation paths of the medium  120 . Some aspects of MIMO processing of optical signals are described in Sebastian Randel, et al., “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Opt. Express 19, 16697-16707 (2011), incorporated herein by reference. The data recovery module  175  applies additional computational resources to recover the D 1  . . . D N  signals from the output by the MIMO module  172 . Some embodiments include an inverse transformation module  173  configured to apply an inverse transformation, e.g. an inverse T −1  of the transformation T applied by the optical mixer  160 . One or more of the MIMO module  172 , data recovery module  175  and the transformation module may be provided by a fixed or reconfigurable computational device such as, e.g. a digital signal processor (DSP). 
     Because the data of a particular data channel has propagated via multiple propagation paths and/or modes, the nonlinearity associated with any particular one of the paths and/or modes is expected to be reduced due to an averaging effect. This averaging effect is expected to improve the overall transmission fidelity of the system  100 , as measured by, e.g., a reduced bit error rate (BER). Moreover, this improvement is expected to advantageously allow transmission through the medium  120  with a greater optical power level than would otherwise be possible for a given target BER. 
       FIG. 3  illustrates one embodiment of the mixer  160 . The mixer  160  may include, e.g. an N×N optical coupler or a mode scrambler. In the illustrated embodiment, the mixer  160  includes a N×N optical coupler  310 . While N is not limited to any particular value, in some such embodiments the optical coupler  310  may be a 3×3 optical coupler (N=3). Those skilled in the pertinent art will appreciate that an N×N optical coupler may distribute portions of the optical energy received at each input among each of the outputs according to specific relationships. Of course, the mixer  160  is not limited to using a 3×3 optical coupler. In some embodiments the number of inputs and outputs of an optical coupler used to implement the mixer  160  may be unequal, e.g. M inputs and N outputs. In some embodiments the weighting of the distribution of one or more inputs to the outputs may be equal, while in other embodiments the weighting may be unequal. 
     In some embodiments the optical mixer  160  may include a mode scrambler  320 . As appreciated by those skilled in the pertinent art, a mode scrambler may be used to produce mode coupling between different optical modes of propagating signals. The mixer  160  may also include whatever additional optical functionality is needed to couple the N outputs to a corresponding number of spatially diverse propagation modes of the transmission medium  120 . Such functionality may include any combination of, e.g. mirrors, beam splitters, mode couplers, planar waveguide circuits, multimode interferometers, and 3D-waveguide mode adapters and sorters. 
     In some other embodiments, not shown, data channel mixing may be provided by a multichannel optical component such as, e.g. a few-mode fiber erbium-doped fiber amplifier (EDFA) in a transmission medium that does not exhibit any inherent mixing. In other embodiments, the channel mixing may be provided by combining a section of a propagation medium (e.g. optical fiber) that inherently provides channel mixing with sections of various media that are not characterized by inherent mixing. 
     Turning to  FIG. 4 , a system  400  is illustrated according to another embodiment. In the system  400  a transmitter  410  includes an electrical-domain mixer  420 . The mixer  420  receives the data channels D 1  . . . D N  at corresponding inputs, and electrically applies a mixing function to the data channels such that data received at each of the inputs is distributed among output electrical signals at a plurality of outputs of the mixer  420 . 
       FIG. 5  illustrates without limitation a representative embodiment that may implement the mixer  420 . In the illustrated embodiment a processor  510  is operatively coupled to a memory  520  and an I/O module  530 . The processor  510  may be or include a microprocessor, application-specific integrated circuit (ASIC), digital signal processor (DSP), finite state machine, or any other similar subsystem capable of implementing a fixed or reconfigurable instruction set. The memory  520  may store a fixed or reconfigurable instruction set that is executed by the processor  510 . The I/O module  530  is controllable by the processor to receive the D 1  . . . D N  inputs, and to provide the M 1  . . . M N  outputs. While shown as a separate entity, the memory  520  and/or the I/O module  530  may in some cases be integrated with the processor  510  on a common semiconductor substrate. Similar to the optical-domain mixer  160 , the electrical-domain mixer  420  may also have an unequal number of inputs and outputs, e.g. M×N. 
     In some embodiments the mixer  420  may be configured to perform a unitary transformation between the received data D 1  . . . D N  and the output data M 1  . . . M N . As appreciated by those skilled in the pertinent art, a unitary transformation may provide a linear transformation of basis modes of the input data. In such a transformation, the input data may be mixed without loss of information. The unitary transformation may implemented at the spatial mode, vector mode or signal quadrature levels. Moreover the unitary transformation may also be implemented in the time domain, for instance by introducing multiple time delayed copies of signal portions. In some cases it may be preferable to implement a unitary transformation for which the power content originated from each of the data D 1  . . . D N  at each of the spatial outputs M 1  . . . M N  is about equal, though this feature is not required to realize the benefits of the described embodiments. Those skilled in the pertinent art are able to determine such functions without undue experimentation. 
     In some embodiments the mixer  420  may implement an invertible linear transformation in spatial modes and time. Those skilled in the pertinent art are familiar with such linear transformations. As a non-limiting example, multiple cascaded rotation matrices can be used to mix signals in a reversible way. In some embodiments the transformation is selected according to the nature of the medium  120 . For example the transmission medium may include heterogeneous data channels, such as, for example, embodiments in which the transmission medium includes multiple fibers having different effective areas. 
     Returning to  FIG. 4 , the optical modulator  150  receives the output of the mixer  420 , and produces therefrom the mixed optical signals E 1  . . . E N . After these optical signals propagate via the medium  120 , the MIMO module  170  receives the distorted optical signals F 1  . . . F N  and operates as previously described with respect to  FIG. 1 . In this case, the recovery algorithm may be designed to take into account the specific transformation applied by the mixer  420 , for example by applying an inverse of the transformation applied by the mixer  420 . 
       FIG. 6  illustrates an embodiment, e.g. a system  600 , which includes multiple instances of the mixer  160 . A first instance  160   a  of the mixer receives the output of the optical modulator  150  and provides first transformed signals E 1  . . . E N  to a first instance  120   a  of the transmission medium. A second instance  160   b  of the mixer receives intermediate propagated signals D′ 1  . . . D′ N  from the transmission medium  120   a  and applies a second transformation to the received data to produce second transformed signals E′ 1  . . . E′ N . A second instance  120   b  of the transmission medium  120   b  propagates the signals E′ 1  . . . E′ N  to produce final propagated signal F 1  . . . F N . The receiver  130  then operates as previously described to recover the data D 1  . . . D N . Such multiple placements of the optical mixer  160  is expected in some circumstances to provide additional mixing of the spatial modes at various transmission distances, and therefore further improve performance of the optical transmission system relative to the embodiments of  FIGS. 1 and 2 . While the configuration of the system  600  is described for the case that the mixing is performed in the optical domain, in other embodiments the mixing is performed in the electrical domain. Those skilled in the pertinent art can easily determine the configuration of such embodiments based on the principles described herein. 
     Although multiple embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it should be understood that the present invention is not limited to the disclosed embodiments, but is capable of numerous rearrangements, modifications and substitutions without departing from the invention as set forth and defined by the following claims.