Patent Publication Number: US-8977121-B2

Title: Spatial domain based multi dimensional coded modulation for multi Tb per second serial optical transport networks

Description:
The present application claims priority to Provisional Application Ser. No. 61/442,854, filed Feb. 15, 2012, the content of which is incorporated by reference. 
    
    
     BACKGROUND 
     The present application relates to a serial optical transport networks based on multimode/multicore fibers. 
     The optical communication systems have been rapidly evolving recently in order to meet continuously increasing demands on transmission capacity, originating mostly from the Internet and multimedia applications. In order to satisfy high capacity demands, according to some industry experts, the 1 TbE standard should be completed in several years. Coherent optical OFDM is one possible pathway towards achieving multi-Tb/s optical transport. Another approach is based on multidimensional coded modulation. Namely, by increasing the number of dimensions (i.e., the number of orthonormal basis functions), a system can increase the aggregate data rate of the system without degrading the bit error rate (BER) performance as long as orthogonality among basis functions is preserved. Most of the papers on multidimensional signal constellations for optical communications so far have been related to single carrier and SMF-based systems. 
     The MMF is traditionally considered as the medium for short-reach applications. However, one recent experiment, in which 21.4 Gb/s polarization-multiplexed coherent OFDM transmission over 200 km of MMF has been demonstrated, indicates that MMFs can be used for metro and medium-haul applications, as well. Today&#39;s MMFs are however far away from being a medium suitable for long-haul transmission. The main issues are related to high loss and excessive number of modes that overwhelms the computational capabilities of currently existing silicon chips. Interestingly enough, the theory behind dual-mode optical fiber is well developed. For example, the group delay experiences a difference between LP 01  and LP 11  modes vanishes at the normalized frequency of V 0 =6.8, which is just below the cutoff frequency of the third mode. The corresponding core diameter and the refractive index difference at λ=1380 nm are 24.8 μm and Δ=(n 1   2 −n 2   2 )/2n 1   2 =0.3% [where n 1  (n 2 ) is the refractive index of the core (cladding)], respectively. The similar approach is applicable to few-mode fibers. 
     SUMMARY 
     Systems and methods are disclosed for an optical transport network based on multimode/multicore fibers. The network includes a mode multiplexer to receive inputs from one or more transmitters; a multimode erbium-doped fiber amplifier (MMEDFA) coupled to the mode multiplexer; a multimode optical add-drop multiplexer (MMOADM) coupled to the MMEDFA; an optical cross-connect coupled to the MMOADM; and a mode demultiplexer coupled to the optical cross-connect to provide data to one or more receivers. 
     Implementations of the foregoing may include one or more of the following. The spatial-domain-based networking scenario is explored. Key components required for spatial-domain-based networking are identified and corresponding implementations are proposed as well. The key optical components for spatial-domain based networking are: MMF multiplexers/demultiplexers, MMF amplifiers, MMF optical add-drop multiplexers, and modes-detector. The operation principle of passive devices is described as follows: the spatial-mode signal is first split using a modes-demultiplexer, and then each mode is processed by the corresponding SMF component, and finally, the resulting SMF signals are combined using a mode-multiplexer. The operation principle of MMF amplifiers is described as follows: the weak spatial-mode signal and the corresponding pump signals are combined together by using a spatial-mode WDM combiner. To avoid the mode-dependent gain problem, the corresponding pumps are independently adjusted so that different spatial-modes have the same output power level. The integration principle can be described as follows: (i) integrate the power combiner, a series of electro-optical modulators (EO MODS) and mode-multiplexer, shown in  FIG. 1(   b ), on a single chip; and (ii) to integrate modes-demultiplexer and series of photodetectors or alternatively donut-shape-based photodetector on a single chip. 
     Advantages of proposed spatial-domain-based multidimensional coded-modulation scheme may include one or more of the following. The system improves dramatically OSNR sensitivity by using N-dimensional (ND) signal constellations instead of conventional 2D constellations (QAM or PSK). The system uses N-dimensional constellations by employing orbital angular momentum (OAM) states as basis functions, therefore, for the same average symbol energy of 2D constellation, the Euclidean distance is much larger resulting in much better BER performance. In other words, for the same target BER, OSNR penalty improvement of conventional 2D coded-modulation is large (&gt;8 dB for large constellations). In combination with quasi-cyclic binary/nonbinary LDPC codes of large girth, the system can achieve multi-Tb/s serial optical transmission for long-haul applications. In combination with in-phase (I) and quadrature (Q) channels, the corresponding space is 2N-dimensional. If further polarization is used, the corresponding space will be 4N-dimensional. When compared to SMFs, MMFs offer (i) easier installation, maintenance and handling, which leads to lower-cost systems; (ii) larger effective cross-sectional area, which increases the immunity to fiber nonlinearities; and (iii) many modes for transmission, which improves the spectral efficiency when an MMF is treated as a multiple-input multiple-output (MIMO) channel in a similar methodology used in wireless communications. By employing OAM states in combination with spin angular momentum (SAM) the dimensionality of OAM-space can be doubled. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an exemplary MMF-supported network, while  FIG. 1B  illustrates a spatial-domain-based N-dimensional modulation. 
         FIG. 2A  shows an exemplary block diagram of a passive multi-mode compatible optical device, while  FIG. 2B  shows an exemplary multi-mode EDFA. 
         FIG. 3A  shows an exemplary frame organization for a mode multiplexing/demultiplexing for 10 Tb/s Ethernet, while  FIG. 3B  shows an exemplary three-layer hierarchy. 
         FIGS. 4A-4B  show exemplary transmitter and receiver configurations for imposing I/Q coordinates of 4N-dimensional signal constellation for an optical transceiver for MMF-supported networks. 
         FIG. 5A-5D  show an exemplary multidimensional LDPC-coded OAM-based modulation system for transmission over MMFs. Only N-dimensions (OAM-dimensions) are shown. 
     
    
    
     DESCRIPTION 
     One exemplary diagram of an MMF-based optical network is shown in  FIGS. 1A-1B . The network contains at least the following MMF devices: MMF multiplexers/demultiplexers  12 / 30 , MMF amplifiers  14 , MMF optical add-drop multiplexers  16 , and modes-detectors (portion of receivers).  FIG. 1A  shows an exemplary MMF supported network. In  FIG. 1A , transceivers  10  forward data to mode-multiplexers  12 , which in turn multiplexes independent data streams for transmission through a multimode fiber system of interest. To compensate for loss due to mode-multiplexing the boost multimode erbium-doped fiber amplifier (MM-EDFA)  14  is used. During propagation the multimode signal may enter the multimode optical add-drop multiplexer (MM OADM)  16 . After various streams are dropped and/or added in MM OADM, the output of MM OADM  16  is boosted by amplifier  18 . The MM signal can also enter the MM optical cross-connect (MM OXC)  20 . Various MM input streams are pre-amplified by a plurality of amplifiers  22  that compensate for MMF propagation loss. The output of MM OXC is amplified by amplifier  22  to compensate for losses due to cross-connect operation. The signal is further periodically amplified, to compensate for propagation losses, until it reaches the desired destination. On destination side, the MM signal is first amplified by  22 , and MM signal is further mode-demultiplexed  30 . Various independent streams after OAM demultiplexing are converted into electrical domain by a plurality of receivers  32 . 
     The proposed system/network increases the photon efficiency by increasing the number of degrees of freedom. Shannon&#39;s theory establishes that the information capacity is given by C(X,Y)=(N/2)log(1+σ 2   x /σ 2   z ) [bits/channel use], where σ 2   x  is the variance of the source and σ 2   z  is the variance of the noise. Therefore, the information capacity is a logarithmic function of the signal-to-noise ratio (SNR) and a linear function of the number of dimensions N. The channel capacity formula indicates that the photon efficiency can be dramatically improved by increasing the number of dimensions. The total aggregate data rate can be significantly increased as long as the orthogonality among OAM basis functions is preserved. The system uses OAM modes in MMF as multidimensional basis functions. 
     The operation principle for spatial-domain-based modulation/multiplexing is illustrated in  FIG. 1B . The spatial-domain-based modulator/multiplexer includes a laser diode  40  driving a power splitter  42 . The power splitter in turn provides unmodulated signal to a plurality of electro-optic modulators (EO MODS)  44 . During operation, the continuous wave laser diode  40  signal is split into N branches by using the power splitter (1:N star coupler)  42 , the outputs are used as input of N electro-optic modulators (EO MODS)  44 , such as Mach-Zehnder modulators. In mode-multiplexing, the N independent electrical data streams are modulated and used as input of mode-multiplexer  45 , implemented by using N waveguides and taper-core fiber  46  within mode-multiplexer  45 , as shown in  FIG. 1B . In N-dimensional modulation instead, the input m bits are used as an address in look-up table (LUT) in which N-dimensional coordinates are stored, which are further used, after conversion into optical domain by EP MODS  44 , as inputs to mode-multiplexer  45 . The tapper-core fiber is properly designed so that orthogonal OAM modes are excited in MMF  48  and sent towards remote destination. At the end of MMF the corresponding MM signal  50  is modes-demultiplexed. The modes-demultiplexer can be implemented in similar fashion as mode multiplexer  44 . Such a solution requires using the orthogonal demultiplexed modes as inputs of corresponding set of N photodetectors. Instead of using N different photodetectors for different modes, one embodiment uses one or more donut-shaped photodetectors which are able simultaneously to detect all separate OAM modes of interest. 
     The current passive devices are single-mode based and are highly wavelength dependent, and will not work as desired when a multimode signal is present. One embodiment for a spatial-mode scenario is illustrated in  FIG. 2A . The spatial-mode signal from MMF  50 A is first split using a splitter  60  or modes-demultiplexer, and then each mode is processed by the corresponding SMF component such as single-mode devices  62 , and finally, the resulting SMF signals are combined using a mode-multiplexer  64  and sent over MMF  50 B. The MM optical amplifier is a critical component, and can be implemented as illustrated in  FIG. 2B  where a plurality of pump lasers  80  signals, after isolators  82 , are used as input to input of spatial-mode WDM combiner  84 . The weak spatial-mode signal  85  and the corresponding pump signals are combined together by using a spatial-mode WDM combiner  84 , and the amplified MMF signal is sent over a multimode Erbium doped fiber  86 . To avoid the mode-dependent gain problem, the corresponding pumps are independently adjusted so that different spatial-modes have the same output power level. 
       FIGS. 3A-3B  illustrate how the spatial-mode-multiplexing can be used to enable multi-Tb/s optical transport. The frame corresponding to 10 Tb/s is organized into five band-groups with center frequencies being orthogonal to each other, each carrying 1 Tb/s traffic. The center frequency of the bands within one band-group is also orthogonal to each other to simplify the band detection. One embodiment employs a three-layer architecture with the baseband being either a 40 Gb/s signal originating from 10 GbE or 40 GbE, respectively. If 10 Gb/s baseband is used, the system performs 4:1 RF multiplexing. This RF layer is optional and as such is not shown in  FIG. 3A . The second layer is mode-multiplexing layer in which 5 orthogonal OAM modes are used to multiplex the corresponding 40 Gb/s streams resulting in 200 Gb/s aggregate data rate per band. The third layer, or the photonic layer, is implemented by combining the signals from five-frequency locked lasers into a 5 Tb/s serial optical transport signal. With polarization-multiplexing, the system can achieve 10 Tb/s serial optical transmissions. The RF layer, which is used to perform RF multiplexing, can quite readily be implemented in mixed-circuit CMOS ASICs. The photonics layer, on the other hand, can be implemented using photonic integrated circuit (PIC) technology. 
     The corresponding transmitter ( 10 ) and receiver configurations ( 32 ), for the system shown in  FIG. 1A , are given in  FIGS. 4A  and B, respectively.  FIG. 4A  shows a mixed signal CMOS device  100  and a silicon photonics device  120 . The CMOS device  100  includes a coded modulation block  102 , whose output is converted to analog domain by digital-to-analog converter (DAC)  104  and low-pass filter (LPF)  106 . Two sets of DAC and LPF are need for in-phase and quadrature channels, respectively. The CMOS device  100  and a DFB  110  are connected to the silicon photonics device  120 , composed of 3 dB-coupler  122  driving Mach-Zehnder modulators (MZMs)  124  and  126 , and the π/2-phase shifter  128 . The phase shifter  128  is employed to ensure the orthogonality of quadrature channel with respect to in-phase channel. The I/Q components are combined by 3 dB-coupler  130  and used as input to mode-multiplexer block  12 . 
       FIG. 4B  shows an exemplary optical receiver ( 32 ), which receives signals from modes demultiplexer. The mode-demultiplexer signal local laser signal is used as input to a balanced coherent detector  130 . The output of the balanced detector  130  is supplied to a mixed signal CMOS detector and decoder  140  that includes LPF circuits  142  connected to digital-to-analog converters (DACs)  144 . The outputs of the DACs are processes by a residual dispersion compensator and decoder  146 . The RF building blocks can be implemented using the mixed-signal CMOS technology. The photonic building blocks can be implemented using CMOS photonics (or silicon photonics), except for lasers that must be flip-chip bonded on top of the silicon die, and germanium photodetectors that should be placed on top of the silicon waveguides. One embodiment integrates the power combiner, a series of EP MODS and mode-multiplexer, shown in  FIG. 4B , on a single chip; and integrates modes-demultiplexer and series of photodetectors or alternatively donut-based photodetector on a single chip. 
     Implementations of mode-multiplexed and polarization-multiplexed, coded, multiband optical system enabling 10 Tb/s optical transport can include one or more of the following: (i) fabrication of MMFs with low attenuation and optimum profile index enabling zero multimode dispersion, (ii) development of both passive and active MMF devices discussed above, (iii) analysis of the nonlinear interaction among different modes from both polarizations, and (iv) development of efficient modulation and coding systems enabling multi-Tb/s optical transport over MMFs. 
     Among various optical beams that can carry OAM, the Laguerre-Gauss (LG) vortex beams can be easily be implemented. For example, the field distribution of an LG beam traveling along the z-axis can be expressed in cylindrical coordinates (r,φ,z) (r denotes the radial distance from propagation axis, φ denotes the azimuthal angle and z denotes the propagation distance) as follows: 
                         u     m   ,   p       ⁡     (     r   ,   ϕ   ,   z     )       =           2   ⁢     p   !           π   ⁡     (     p   ⁢        m          )       !         ⁢         1     w   ⁡     (   z   )         [       r   ⁢     2         w   ⁡     (   z   )         ]          m          ⁢       L   p   m     (       2   ⁢     r   2           w   2     ⁡     (   z   )         )     ⁢     ⅇ     -       r   2         w   2     ⁡     (   z   )             ⁢   ⅇ   ⁢       j   ⁢           ⁢     kr   2     ⁢   z       2   ⁢     (       z   2     +     z   R   2       )         ⁢     ⅇ       j   ⁡     (       2   ⁢   p     +        m        +   1     )       ⁢     tan     -   1       ⁢     z     z   R           ⁢     ⅇ       -   j     ⁢           ⁢   m   ⁢           ⁢   ϕ           ,           (   1   )               
where w(z)=w 0 √{square root over (1+(z+z R ) 2 )} (w 0  is the zero-order Gaussian radius at the waist), z R =πw 0   2 /λ is the Rayleigh range (with λ being the wavelength), k=2π/λ is the propagation constant, and L m   p (·) is the associated Laguerre polynomial, with p and m representing the radial and angular mode numbers, respectively. It can be seen from (1) that the mth mode of the LG beam has the azimuthal angular dependence of the form exp(−jmφ), and consequently, m is also called the azimuthal mode number (index). For m=0, u(r,φ,z) becomes a zero-order Gaussian beam, that is the TEM 00  mode. For p=0, L m   p (·)=1 for all ms, so that the intensity of an LG mode is a ring of radius proportional to |m|.
 
For a fixed p, the following principle of orthogonality is satisfied:
 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     Therefore, different OAM “states” corresponding to a fixed p are all orthogonal with one another, and hence, they can be used as basis functions for an OAM modulation. Spatial Division Multiplexing (SDM) in MMFs relies on skew rays or helical rays that spiral around the fiber&#39;s axis as they propagate along the fiber. Light following a helical path forms an optical vortex. The topological charge of the vortex depends on the number of complete turns around the axis in one wavelength. In other words, the integration of the phase around a closed path yields an integer multiple of 2π. Thus we can deduce that skew rays indeed possess an orbital angular momentum (OAM). For an axially symmetric multimode fiber with a refractive index of n 2 (R)=n 1   2 [1−2Δf(R)] [f(R) is the refractive index profile as a function of radial distance R], the solution for the electric filed distribution takes the form:
 
 e=ê   ± exp(± jm φ) F   m ( R )exp( jβz ),  (3)
 
where ê ±  is the unit vector of the right (+)/left (−) circular polarization, φ is the azimuthal angle, m is the azimuthal index (m=0, 1, 2, . . . ), and β is the propagation constant. The exp(±jmφ) component denotes the azimuthal phase dependence. The radial function F m (R) can be found from the eigenvalue equation. The azimuthal dependence of (3) is the same as that of the LG modes, indicating that skew rays indeed possess the OAM.
 
     Next, one embodiment supporting multi-Tb/s optical transport that is based on multidimensional (N-dimensional) coded modulation is detailed. By increasing the number of dimensions (i.e., the number of orthonormal OAM basis functions), the system can increase the aggregate rate of the system while enabling reliable transmission at these ultra-high speeds using low-density parity-check (LDPC) codes at each level. At the same time, for the same symbol energy, with multidimensional signal constellation we can increase the Euclidean distance between signal constellation points resulting in better BER performance. 
     The overall system configuration is shown in  FIG. 5A  where an N-dimensional OAM based transmitter  200  receives data streams  1  to k, modulates them using N-dimensional OAM modulator, and transport to remote destination. During propagation periodically multimode EDFAs  202  are deployed to compensate for MMF fiber  204  loss. The first MM EDFA is used as boost amplifier, the last as pre-amplifier and the medium ones as inline MM amplifiers. On receiver side, the signal transported over MMF demodulated, detected and decoded in N-dimensional OAM Rx  210 . 
       FIG. 5B  shows an exemplary LDPC-coding based transmitter configuration, while  FIG. 5C  shows an exemplary LDPC coding based receiver configuration with direct detection. Turning now to  FIG. 5B , a plurality of source channels  1  to K are provided to LDPC encoders  220 , whose outputs written to block-interleaver  222  row-wise. K bits are taken from block-interleaver column-wise an used to select an ND-signal constellation point in mapper  224 , implemented as an LUT. The coordinates from LUT are used as input to N MZMs  226 . The MZM outputs are combined together in mode-multiplexer  228  and transmitted to remote destination by MMF. 
       FIG. 5C  shows the receiver configuration with direct detection. The signal transported over MMF is used as input to a modes detector  230 . The modes detector  230  provides the projection alon OAM modes, this projections are after ADCs used as input of an N-dimensional a posteriori probability (APP) demapper  232 , whose output (symbol LLR) is used for bit LLR calculator  234 . The bit LLRs are provided to a plurality of LDPC decoders  236 . 
     As shown in  FIG. 5B , K different bit streams coming from different information sources are encoded using different binary LDPC codes. The outputs of the encoders are then interleaved by a (K×n) block interleaver. The block interleaver accepts data from the encoders row-wise, and outputs the data column-wise to the mapper that accepts K bits at the time instance i. The mapper  224  determines the corresponding Q-ary (Q=M N , M is the number of amplitude levels per dimension) signal constellation point 
     
       
         
           
             
               
                 
                   
                     
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     The N-dimensional mapper outputs are used as the RF inputs into N Mach-Zehnder modulators (MZMs) integrated on the same chip. Finally, the modulated signals are sent over the MMF system of interest after being combined into an optical wave via a modes-multiplexer. In (5), which represents the general formula applicable to any N-dimensional constellation, the set {Φ 1 , Φ 2 , . . . , Φ N } represents a set of N orthonormal OAM basis functions. The number of bits per signal constellation point is determined by log 2 (M N )(=K). The dimensionality, N, and number of amplitude levels per dimension, M, is determined by the desired final rate. The band-group data rate of 1 Tb/s can be achieved in many different ways. For example, if the traffic originates from 40 GbE, selecting M=2 and N=5 results in an aggregate data rate of 200 Gb/s per subcarrier. When binary LDPC codes are used, the receiver side with direct detection can be depicted as in  FIG. 5C . The outputs of the N branches of the modes-detector  230 , are sampled at the symbol rate and the corresponding samples are forwarded to a posteriori probability (APP) demapper  232 . The demapper  232  provides the symbol log-likelihood ratios (LLRs), which are used by the bit LLR calculator  234  to compute bit LLRs required for iterative LDPC decoding. To improve the overall system performance, the system iterates extrinsic information between LDPC decoders  236  and APP demapper  234  until convergence or until a predetermined number of iterations has been reached. From the description of the transmitter and the receiver setups, the system is scalable to any number of dimensions with small penalty in terms of BER performance, as long as the orthonormality between OAM states is preserved. Increasing the number of dimensions leads to an increased complexity, and hence, a compromise between the desired aggregate rate and the complexity of the system should be made in practice. 
     The receiver configuration with coherent detection is shown in  FIG. 5D . A modes demultiplexer  240  provides signals to a plurality of coherent detectors  244  that are driven by a local laser  242 . The coherent detectors  244  are connected to an N-dimensional BCJR equalizer  246 , whose output drives a bit LLR calculator  248 . The results are provided to LDPC decoders  250 . The balanced coherent detector  244  configuration is already provided in  FIG. 4B . The simplest spatial-domain based coded modulation system with direct detection can be described by the following set of constellation points for N=3 and M=2: {(0,0,0), (0,0,1), (0,1,0), (0,1,1), (1,0,0), (1,0,1), (1,1,0), (1,1,1)}, while the corresponding coherent detection counterpart is obtained substituting zeros with −1s. (The normalization factor C 3  in (5) is omitted to simplify presentation.) The signal constellation for coherent detection with N=3 and M=4 is given by: {(−3,−3,−3), (−3,−3,−1), (−3,−3,1), (−3,−3,3), . . . , (3,3,−3), (3,3,−1), (3,3,1), (3,3,3)}. These modulation formats can be straightforwardly extended to higher dimensions. For example, N-dimensional Cartesian product of one-dimensional pulse-amplitude modulation (PAM) can be employed. 
     Because OAM eigenstates are orthogonal, in principle, an arbitrary number of bits per photon can be transmitted. The ability to generate the OAM modes in multimode fibers (MMFs) will allow realization of fiber-optic communication networks with ultra-high bits-per-photon efficiencies. A spatial-domain-based multidimensional coded-modulation system can serve as an enabling technology for multi-Tb/s serial optical transport. Simulations indicate that an eight-dimensional (8D) spatial-domain-based coded modulation system outperforms its corresponding prior-art 4D counterpart by 3.88 dB at BER of 10-8, while outperforming its conventional polarization multiplexed QAM counterpart by even a larger margin of 8.39 dB (also at the BER of 10 −8 ). 
     In addition to proposed spatial-domain-based multidimensional code-modulation system, spatial-domain-based networking scenario can be done. The major optical components for spatial-domain based networking include MMF multiplexers/demultiplexers, MMF amplifiers, MMF optical add-drop multiplexers, and modes-detector. The spatial-mode signal is first split using a modes-demultiplexer, and then each mode is processed by the corresponding SMF component, and finally, the resulting SMF signals are combined using a mode-multiplexer. In the MMF amplifiers, the weak spatial-mode signal and the corresponding pump signals are combined together by using a spatial-mode WDM combiner. To avoid the mode-dependent gain problem, the corresponding pumps are independently adjusted so that different spatial-modes have the same output power level. One embodiment integrates various devices as follows: (i) integrate the power combiner, a series of EO MODS and mode-multiplexer, shown in  FIG. 1B , on a single chip; and (ii) integrate modes-demultiplexer and series of photodetectors or alternatively donut-shaped-based photodetector on a single chip. The operation of the mode multiplexing/demultiplexing can be done for 10 Tb/s Ethernet. A three-layer hierarchy for 10 Tb/s Ethernet is also disclosed. 
     The foregoing detailed description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the description of the invention, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.