Patent Publication Number: US-2015086200-A1

Title: Space To Wavelength Superchannel Conversion

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 communications systems are approaching the capacity limits imposed by single mode optical fibers. To increase the capacity of optical networks to meet expected increases of traffic demand, space division multiplexing (SDM) may be used with optical fibers having multiple spatial propagation modes. Such optical fibers include multi-mode fibers (MMF) and multi-core fibers (MCF). Increasing attention has been directed to development of multiple spatial mode fibers (both MMF and MCF), resulting in significant progress on fundamental issues of SDM transmission. For example, transmission capacity using SDM has been demonstrated having a factor of ten improvement compared with single mode fibers (SMFs). 
     SUMMARY 
     One embodiment provides an apparatus, e.g. configured to convert spatial modes of an input superchannel to spatial modes of an input superchannel. The apparatus has an input configured to receive an input optical signal and an output configured to output an output optical signal. A superchannel converter is configured to convert N spatial modes of the input optical signal to M spatial modes of the output optical signal. 
     In any embodiment of the apparatus the superchannel converter may be configured to convert an N-mode space superchannel to a wavelength superchannel that includes a corresponding plurality of wavelength channels. In any embodiment the superchannel converter may be configured to convert an N-mode space superchannel to an M-mode space/wavelength superchannel, with at least one mode of the M-mode space/wavelength superchannel including a plurality of wavelength channels. In any of the above embodiments the wavelengths of the converted space/wavelength superchannel may be mode-locked. 
     Another embodiment provides a method, e.g. for forming a superchannel converter. The method includes configuring a superchannel converter to receive an input superchannel having N spatial modes and to output at least one output superchannel having M spatial modes. The method further includes configuring the superchannel converter to convert the N spatial modes of the input superchannel to the M spatial modes of the at least one output superchannel. 
     In any embodiment the superchannel converter may be configured to perform optical-electrical-optical conversion of the input optical signal to the output optical signal. In any such embodiment the superchannel converter may be further configured to frequency-shift a quadrature signal. 
     In any embodiment the superchannel converter may be configured to optically shift each of a plurality of the N spatial modes of the input optical signal from an input frequency to a corresponding output frequency. In any such embodiment the optical frequency shift may be performed by four-wave mixing (FWM) or parametric amplification. 
     In any embodiment the superchannel converter may further be configured to convert a plurality of wavelength channels of a spatial superchannel to a wavelength superchannel. In such embodiments the wavelength channels may have different center wavelengths. In any embodiment the apparatus may further include an input optical waveguide coupled to the input and an output optical waveguide coupled to the output. 
    
    
     
       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. of a system, detailing inter-connection of a multimode fiber (MMF), multi-core fiber (MCF) and a single-mode fiber (SMF) via a space and wavelength switch; 
         FIG. 2  illustrates an embodiment of a space/wavelength superchannel converter coupled to an input fiber with N spatial modes and an output fiber with M spatial modes; 
         FIG. 3  illustrates schematically the conversion of a space superchannel with N spatial modes to a wavelength superchannel with a single spatial mode (M=1); 
         FIG. 4  illustrates schematically the conversion of a space superchannel with N spatial modes to a space and wavelength superchannel with M modes (M&gt;1 and M&lt;N); 
         FIG. 5A  illustrates an embodiment of a space superchannel to wavelength superchannel convertor based on an optical-electrical-optical structure (OEO), with individual lasers providing source light for some frequency channels; 
         FIG. 5B  illustrates an embodiment of a single optical to electrical convertor (O/E), such as may be used in the embodiment of  FIG. 5A ; 
         FIG. 5C  illustrates an embodiment of the convertor of  FIG. 5A , in which a comb generator provides source light for some frequency channels such that the frequency channels are mode-locked; 
         FIG. 6A  illustrates a space superchannel to wavelength superchannel convertor that uses wavelength shifters; 
         FIG. 6B  illustrates a single wavelength shifter based on I/Q modulators, such as may be used in the embodiment of  FIG. 6A ; 
         FIG. 6C  illustrates a single wavelength shifter such as may be used in the embodiment of  FIG. 6A , wherein the wavelength shifter is based on four wave mixing (FWM) or parametric amplification; 
         FIG. 7A  illustrates an embodiment of space superchannel conversion to a wavelength superchannel with a single spatial mode (M=1), such as may be implemented by the embodiment of  FIG. 8A , wherein each input spatial mode is assigned a different center wavelength; 
         FIG. 7B  illustrates an embodiment of space superchannel conversion to a combined space-wavelength superchannel, such as may be implemented by the embodiment of  FIG. 8B ; 
         FIG. 8A  illustrates space superchannel to wavelength superchannel conversion using the wavelength/spatial plan shown in  FIG. 7A ; and 
         FIG. 8B  illustrates an embodiment of space superchannel to wavelength superchannel conversion using the plan of  FIG. 7B , in which a frequency shift may be applied to a group of wavelength channels. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure is directed to, e.g. methods and systems for converting superchannels of a first (e.g. input) optical communication signal propagating on a first fiber to different superchannels of a second (e.g. output) optical communication signal propagating on a second fiber. 
     Deployment of SDM transmission in existing optical networks will in many cases necessarily include integrating spatial mode fibers and single mode fibers in the system. For example, mixed systems may integrate multi-core and/or multi-mode fibers with single-mode fibers, or multi-core fibers and/or multi-mode fibers with other multi-core fibers and/or multi-mode fibers having different numbers of cores/modes. 
     Embodiments described herein and otherwise within the scope of the disclosure and the claims reflect the recognition that novel switching devices and methods will be needed to implement spatial mode-diverse optical transmission systems. Furthermore embodiments reflect the further recognition that when such devices and methods include the ability to remap frequency channels among superchannels, routing of signals in a spatial mode-diverse optical network is advantageously simplified. 
       FIG. 1  illustrates an embodiment, e.g. of a network  100 , detailing inter-connection between several optical fibers via a space and wavelength switch (SWS)  110 . The SWS  110  provides interconnection between a first MCF  120 , an MMF  130 , an SMF  140  and a second MCF  150 . In the illustrated embodiment the number of modes of the MMF  130 , e.g. &gt;1, is different from the number of mode of the SMF  140 . Moreover, the number of cores of the MCF  130 , e.g. seven, is different from the number of cores of the MCF  150 , e.g. three. 
     Transmission via a MCF or a MMF may include the use of one or more space superchannels. As used herein, a “space superchannel” includes a number of sub-channels, wherein sub-channels of the space superchannel may have a same wavelength and may additionally occupy a same bandwidth. When inter-connecting multi-core fibers having a different number of cores, multi-mode fibers having a different number of modes, or a multi-core fiber or multi-mode fiber to a single-mode fiber, one technical issue is the conversion of superchannels of one fiber to other fibers, for example converting space superchannels of a MMF/MCF to a SMF. Embodiments described herein and otherwise consistent with the description provide various solutions to these technical challenges. 
     In some embodiments, described further below, and illustrated at a high level in  FIG. 2 , a function module, e.g. a superchannel converter  210 , switches a superchannel from one propagation type to another propagation type to interface a fiber  220  to a fiber  230 . For example, the converter  210  may receive N spatial modes from the fiber  220 , and convert the N spatial modes to M spatial modes on the fiber  230 . In another example, the converter  210  may receive N spatial modes from the fiber  220 , and convert the N spatial modes to M frequency channels on the fiber  230 . In another example, the converter  210  may convert the N received spatial modes to M spatial modes, each having one or more frequency channels propagating thereon. Those skilled in the pertinent art will appreciate that these examples are not exhaustive of the possible conversions performed by the converter  210 . It is noted that the conversion performed by the converted  210  may not be needed when N≦M. 
       FIG. 3  illustrates schematically the conversion of a space superchannel  310  with N modes to a wavelength superchannel  320  with N wavelengths, with M equal to one. A received signal has N modes, each of which may propagate a signal modulated independently of the other modes. The center wavelength of each of the N received signals may be about equal to each other. Each of the N received signals is transferred, e.g. by the converter  210 , to one of N output signals at corresponding signals having non-overlapping center wavelengths, e.g. λ 1 , λ 2  . . . λ N . By “non-overlapping” it is meant that the center wavelengths of the N output signals are not about equal, e.g. such that the spectral peaks intensity of each of the output signals may be clearly resolved. The N output signals may be combined to a wavelength-division multiplexed (WDM) signal propagating on a single propagation mode. As used herein, the group of N signals propagating via different modes is referred to as a space superchannel. As used herein, a “wavelength superchannel” refers to a group of N signals propagating via different frequencies on a single propagating mode. A “space/wavelength superchannel” is a signal having characteristics of both a space superchannel and a wavelength superchannel, e.g. channel signals propagating by multiple spatial modes and multiple frequencies. 
       FIG. 4  illustrates schematically the conversion of the space superchannel  310  with N modes to M wavelength superchannels  410   1 ,  410   2  . . .  410   M , e.g. by the converter  210 . The set of M wavelength superchannels is denoted space/wavelength superchannel  420 . Each of the M wavelength superchannels  410  occupies a single spatial mode and has a number of multiplexed wavelength channels k 1 , k 2  . . . k M  such that k 1 +k 2 + . . . +k M =N. Thus the N signals received at the converter  210  via the N propagation modes are distributed among the M propagation modes of the converted optical signal. 
     FIGS.  5 A/B and  6 A/B illustrate embodiments that may be used to implement portions of the converter  210 . While these embodiments provide specific examples of implementation of the converter  210 , those skilled in the optical arts will appreciate that there are numerous alternative embodiments that may provide substantially similar functionality. Such embodiments are expressly included in the scope of the description and claims. Moreover, while the embodiments in FIGS.  5 A/B and  6 A/B are described for embodiments in which M=1, those skilled in the pertinent art can easily extend the described principles to embodiments in which M&gt;1. 
     The embodiment of  FIG. 5A  illustrates, without limitation, use of an optical-electrical-optical (OEO) technique for converting N spatial modes to a wavelength superchannel. A spatial mode demultiplexer  510  receives an optical signal that includes a space superchannel, e.g. a plurality of signals propagating via different orthogonal spatial modes, such as the space superchannel  310  ( FIG. 3 ). The space superchannel  310  may be received, e.g. from a multi-mode optical fiber or a multi-core optical fiber. The demultiplexer  510  may be implemented in any conventional or future discovered manner. One nonlimiting example of the demultiplexer  510  is provided by the M3 Modal MUX/DEMUX, available from Kylia, Paris, France, www.kylia.com/Kylia.modal.mux.pdf, incorporated herein by reference. 
     The demultiplexer  510 , e.g. an SDM demultiplexer, separates the N spatial modes and provides these at N corresponding outputs. Each output provides the corresponding spatial mode signal to a corresponding optical/electrical (O/E) converter  520   1 ,  520   2  . . .  520   N . Each of modulators  530   1 ,  530   2  . . .  530   N , e.g. Mach-Zehnder modulators, receives the electrical-domain output of a corresponding one of the O/E converters  520   1 ,  520   2  . . .  520   N , and modulates the output of a corresponding unreferenced laser source. The laser sources may have wavelengths λ 1 , λ 2  . . . λ N . The modulators  530   1 ,  530   2  . . .  530   N  thereby produce N modulated signals with center wavelengths λ 1 , λ 2  . . . λ N . A multiplexer  540 , e.g. a WDM multiplexer, receives the outputs of the modulators  530   1 ,  530   2  . . .  530   N , and produces a combined output, e.g. a WDM signal, exemplified by wavelength superchannel  320  of  FIG. 3 . 
       FIG. 5B  illustrates a nonlimiting embodiment of one instance of the O/E converter  520 . The illustrated embodiment assumes without limitation thereto that the signals output at the outputs of the demultiplexer  510  are polarization multiplexed, e.g. with horizontal (H) and vertical (V) components. A polarization beam splitter (PBS)  550  receives the corresponding output of the demultiplexer  510  and separates the H and V polarizations. Ninety degree optical hybrids  555   h ,  555   v  respectively receive the H and V components of the received signal from the PBS  550 . A second PBS  560  receives the output of a local oscillator (LO)  565 , and provides H and V components thereof respectively to the optical hybrids  555   h ,  555 V. Detectors  570   h   I  and  570   h   Q  respectively convert I and Q outputs of the optical hybrid  555   h  to analog-electrical signals, which are filtered by filters  575   h  and converted to digital-electrical signals by analog-to-digital converters (ADCs)  580   h . Similarly, detectors  570   v   I  and  570   v   Q  respectively convert I and Q outputs of the optical hybrid  555   v  to analog-electrical signals, which are filtered by filters  575   v  and converted to digital-electrical signals by ADCs  580   v . Skilled practitioners of the optical arts will recognize that the described embodiment of the O/E  520  may have numerous variants with equivalent functionality within the scope of the description and the claims. 
       FIG. 5C  illustrates an alternate embodiment of the converter illustrated in  FIG. 5A . In the embodiment of  FIG. 5C , a comb generator  590  provides unmodulated outputs at λ 1 , λ 2  . . . λ N  to corresponding ones of the modulators  530   1 ,  530   2  . . .  530   N . It is believed that if the input signal  310  received from an MMF or an MCF has crosstalk between spatial modes, it may be preferable that the λ 1 , λ 2  . . . λ N  signals be mode-locked. The outputs of the comb generator  590  provide this feature. In this case the wavelength channels of the wavelength channels in the output of the multiplexer  540  are also mode-locked. 
       FIG. 6A  illustrates another embodiment that may implement the conversion of N spatial modes to a wavelength superchannel. In this embodiment wavelength shifters are used to effect the conversion. In this embodiment, a spatial mode demultiplexer  610  receives an optical signal that includes a space superchannel, e.g. the space superchannel  310  ( FIG. 3 ). As described previously the demultiplexer  610  separates the N spatial modes and provides these at N corresponding outputs. Each of N wavelength shifters  620   1  . . .  620   N  receives a corresponding output of the demultiplexer  610 . The wavelength shifters  620  convert signals in the N different spatial modes to wavelength sub-channels having different center wavelengths λ 1 , λ 2  . . . λ N . A multiplexer  630 , e.g. a WDM multiplexer, combines the N wavelength sub-channels into a single WDM signal to form a wavelength superchannel, e.g. the wavelength superchannel  320 . 
     The wavelength shifters  620  may be implemented by one of a number of techniques, one of which is illustrated in  FIG. 6B  based on I/Q modulation, without limitation thereto. Other techniques that may be used include, e.g., four-wave mixing based techniques, parametric amplification, and in-phase/quadrature (I/Q) modulator-based techniques. 
     A PBS  640  receives a signal denoted E in  from one of the outputs of the demultiplexer  610 . As before this embodiment assumes without limitation thereto that the E in  signal is polarization multiplexed, e.g. with H and V components. The PBS  640  splits the E in  signal into the H and V polarized components which are routed to I/Q modulators  650 H and  650 V. Considering the modulator  650 H, the H signal component is split between an I modulator  650   h   I  and a Q modulator  650   h   Q  and recombined. The modulator  650 H is driven by sinusoidal signals having frequency f m . An unreferenced I/Q bias may be adjusted, thereby shifting the frequency of the modulated signal up or down by kf m  where k is unity or an integer greater than unity. The modulator  650 V operated in analogous fashion. A polarization beam combiner (PBC)  660  combines the outputs of the modulators  650 H and  650 V to produce a signal E out  with the desired wavelength to be received by the multiplexer  320 . 
       FIG. 6C  illustrates an embodiment of one instance of the wavelength shifter  620 . In this embodiment the wavelength shifter  620  is based on four wave mixing (FWM) or parametric amplification. As will be appreciated by those skilled in the optical arts an input signal E in  with one wavelength, such as from the demultiplexer  610 , is wavelength shifted to another wavelength at the output E out  with the use of a pump laser. As described earlier, in some embodiments the input signal  310  from an MMF or an MCF may be subject to crosstalk between spatial modes. In such cases is may be preferable that the pumps lasers of the N wavelength shifters  620   1  . . .  620   N  be mode locked. In such embodiments the pumps may be provided by a single source such as an optical comb generator, e.g. the comb generator  590 . 
       FIG. 7A  illustrates aspects of an alternate embodiment in which each spatial mode of a space superchannel may be associated with a different wavelength. In the illustrated embodiment a space superchannel  710  that includes N spatial modes each having a different wavelength λ 1 , λ 2  . . . λ N  is converted to a wavelength superchannel  720 . Such embodiments may provide an advantage over some other embodiments in that the space superchannels can be transformed into wavelength superchannels without the need of a wavelength shifter. 
       FIG. 8A  illustrates an embodiment that may implement the superchannel conversion illustrated in  FIG. 7A . In  FIG. 8A  a spatial demultiplexer  810  is coupled to a WDM multiplexer  820 . The spatial demultiplexer  810  receives an input signal, e.g. the space superchannel  710 , and outputs N signals corresponding to each of the wavelength sub-channels λ 1 , λ 2  . . . λ N  of the input signal. The WDM multiplexer  820  then combines the separate wavelength sub-channels into a single wavelength superchannel, e.g. the wavelength superchannel  720 . 
       FIG. 7B  illustrates aspects of an alternate embodiment in which a space superchannel  730  includes K spatial propagation modes (K=4), each mode having N wavelength channels (N=4), wavelength channels being designated λ KN . The space superchannel is converted to a combined space-wavelength superchannel  740 , in which the wavelength channels are reordered. Such a conversion may be implemented using, e.g. the embodiment illustrated in  FIG. 8B . 
       FIG. 8B  illustrates an embodiment of conversion of K space superchannels each having N wavelength channels to a combined space-wavelength superchannel. The embodiment of  FIG. 8B  is illustrated without limitation for the case that K=N=4. 
     An SDM demultiplexer  830  receives the space superchannel  730  ( FIG. 7B ). The demultiplexer  830  provides at outputs separated spatial superchannels {λ 11 , λ 12 , λ 13 , λ 14 }, {λ 21 , λ 22 , λ 23 , λ 24 }, {λ 31 , λ 32 , λ 33 , λ 34 }, {λ 41 , λ 42 , λ 43 , λ 44 } corresponding to modes designated 1-4. A wavelength-selective K×K switch  840  is configured to provide at its outputs superchannels having a desired reconfigurable reordering of the wavelength channels among the separated spatial superchannels. Wavelength selective switches are described more fully in, e.g. N. K. Fontaine, et al., “N×M wavelength selective crossconnect with flexible passbands,” PDP5B.2, OFC/NFOEC (2012). In the illustrated embodiment the reordered superchannels are {λ 11 , λ 22 , λ 33 , λ 44  }, {λ 34 , λ 12 , λ 23 , λ 41 }, {λ 24 , λ 31 , λ 42 , λ 13 }, {λ 14 , λ 21 , λ 32 , λ 43 }. Each of the latter three superchannels is wavelength-shifted by an instance of a wavelength shifter,  850   a ,  850   b  or  850   c . The wavelength shifters  850  respectively provide wavelength shifts of Δλ, 2Δλ, 3Δλ such that the reordered superchannels do not overlap. A K×1 switch  860  combines the reordered superchannels to a produce combined space-wavelength superchannel exemplified by the signal  740  ( FIG. 7B ). In this manner the wavelength channels may be redirected to a desired space superchannel to implement a desired signal routing. 
     Those skilled in the pertinent art will appreciate that the embodiments of FIG.  7 A/B and FIG.  8 A/B are only two of many embodiments of conversion between space superchannels and wavelength superchannels. 
     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.