Abstract:
An architecture for a fiber optic communication system that uses only two levels of switches, Tier 1 and Tier 3, is described. The architecture allows one to omit the conventional Top of Rack switch level and the conventional Tier 2 switch level while maintaining performance and throughput. The cost to construct and install the improved switch architecture is lower than the cost of the conventional architecture. There are also described a number of transceivers that are suitable for use in the architecture disclosed. The transceivers employ silicon PIC chips that include high contrast silicon waveguides ion the chip and that connect to various configurations of optical fibers. The transceivers provide enhanced switching capacity with fewer devices.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to signal switching architectures for data communication in general and particularly to signal switching architectures for communication over optical fibers. 
       BACKGROUND OF THE INVENTION 
       [0002]    Signal switching architectures for use in very large data centers have employed four levels (or tiers) of switches and require the use of very large amounts of optical fiber to interconnect them. This results in switching architectures that are complex and that are quite expensive. The switching equipment is expensive and the large amount of optical fiber is also a big expense. 
         [0003]    A significant problem in the data center is the cost of running a very large number of fibers from point to point. While single-mode transceivers have existed for many years, they have generally either transmitted light into one fiber and received it in a second, separate fiber (i.e., a fiber pair), or they have made use of very widely separated wavelengths for transmission and reception (i.e., 1310 nm and 1550 nm). 
         [0004]    There is a need for signal switching architectures that are more compact and that are less costly to construct and install. 
       SUMMARY OF THE INVENTION 
       [0005]    According to one aspect, there is provided a switching architecture for an optical fiber communication system of a data center, the switching architecture comprising: at least one first-level switch of a first level of switches configured to directly communicate optically with a plurality of servers; and at least one second-level switch of a second level of switches configured to directly communicate optically with the at least one first-level switch and with an optical fiber based communication device external to the data center, each first-level switch and each second-level switch comprising a respective at least one photonic integrated circuit chip comprising a transceiver. 
         [0006]    In some embodiments each second-level switch comprises photonic integrated circuit based connections. In some embodiments each second-level switch comprises a plurality of chassis, each chassis connected one to another by said photonic integrated circuit based connections. 
         [0007]    In some embodiments, each first-level switch is optically coupled to at least two of the plurality of servers with use of at least one multi-fiber connector and a plurality of optical fibers. In some embodiments, at least one transceiver of each first-level switch is optically coupled to at least one optical fiber, and configured to simultaneously transmit and receive over each optical fiber. In some embodiments, at least one transceiver of each first-level switch is at least one of a parallel multi-transceiver module and a wavelength division multiplexing transceiver. 
         [0008]    In some embodiments, at least one transceiver of each first-level switch is configured to simultaneously transmit and receive over each optical fiber of the plurality of optical fibers. 
         [0009]    In some embodiments, each second-level switch is optically coupled to at least one of the first-level switches with use of at least one multi-fiber connector and a plurality of optical fibers. In some embodiments, at least one transceiver of each second-level switch is optically coupled to at least one optical fiber, and configured to simultaneously transmit and receive over each optical fiber. In some embodiments, at least one transceiver of each second-level switch is at least one of a parallel multi-transceiver module and a wavelength division multiplexing transceiver. 
         [0010]    In some embodiments, at least one transceiver of each second-level switch is configured to simultaneously transmit and receive over each optical fiber of the plurality of optical fibers. 
         [0011]    In some embodiments, each first-level switch is optically coupled to at least two of the plurality of servers with use of a first at least one multi-fiber connector and a first plurality of optical fibers, and wherein each second-level switch is optically coupled to at least one of the first-level switches with use of a second at least one multi-fiber connector and a second plurality of optical fibers. In some embodiments, at least one transceiver of each first-level switch is optically coupled to a first at least one optical fiber, and configured to simultaneously transmit and receive over each optical fiber of the first at least one optical fiber, and wherein at least one transceiver of each second-level switch is optically coupled to a second at least one optical fiber, and configured to simultaneously transmit and receive over each optical fiber of the second at least one optical fiber. In some embodiments, at least one transceiver of each first-level switch is at least one of a parallel multi-transceiver module and a wavelength division multiplexing transceiver, and wherein at least one transceiver of each second-level switch is at least one of a parallel multi-transceiver module and a wavelength division multiplexing transceiver. 
         [0012]    In some embodiments, said at least one transceiver of each first-level switch is configured to simultaneously transmit and receive over each optical fiber of the first plurality of optical fibers, and wherein said at last one transceiver of each second-level switch is configured to simultaneously transmit and receive over each optical fiber of the second plurality of optical fibers. 
         [0013]    The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The objects and features of the invention can be better understood with reference to the drawings described below, and the claims. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views. 
           [0015]      FIG. 1  is a schematic diagram of the switching and communication architecture of a data center, showing the components and their interconnection in the data center. 
           [0016]      FIG. 2  is a schematic diagram of the data center of  FIG. 1  at a high level. 
           [0017]      FIG. 3  is a schematic diagram of one embodiment of a transceiver. 
           [0018]      FIG. 4  is a schematic diagram of an alternative embodiment of a transceiver. 
           [0019]      FIG. 5  is a schematic diagram of is an embodiment of an array of transceivers. 
           [0020]      FIG. 6  is a schematic diagram of is yet another embodiment of a transceiver. 
           [0021]      FIG. 7  is a schematic diagram of a link between two data centers. 
           [0022]      FIG. 8  is a schematic diagram of another embodiment of a transceiver. 
           [0023]      FIG. 9  is a schematic diagram of another embodiment of a transceiver. 
           [0024]      FIG. 10  is a schematic diagram of another embodiment of a transceiver. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    In the following description, various acronyms are used, each of which is defined hereafter in the section headed ACRONYMS AND TRADE NAMES. 
         [0026]    The data center architecture and the various transceiver implementations allow the use of just two levels of switches, Tier 1 and Tier 3, rather than the more common 4-level data center architecture. The new architecture provides at least comparable performance in an arrangement having fewer devices and at lower overall installed cost. 
         [0027]      FIG. 1  is a schematic diagram of the switching architecture  100  of a data center, showing the components and their interconnection in the data center. In  FIG. 1  there are only two switch levels, Tier 1  110  and Tier 3  150 , as compared to a four-level architecture that is commonly used today, in which there is a Top of Rack (TOR) level, a Tier 1 level, a Tier 2 level, and a Tier 3 level. The architecture that is described in the present application is therefore a simpler, less expensive to construct, architecture in which the switches have increased capacity per level. As will be seen, each of Tier 1 and Tier 3 includes one or more switches. The required capacity for a given data center is provided by using at least one switch or a plurality of switches. 
         [0028]    At the lower left of  FIG. 1  there are illustrated designs for connection to one or more servers, which are not shown, but which are conventional servers such as rack mounted servers in an electronics rack. In one embodiment, the connection is between a server and a line card  112  of a Tier 1 switch  130  of Tier 1  110 . Although for clarity only one Tier 1 switch  130  is illustrated, as mentioned hereinabove, Tier 1  110  may include one or more Tier 1 switches  130 . A plurality of line cards  112  which include portions that are server-facing are shown in the lower center of  FIG. 1 . In one embodiment, the connection between a server and a line card  112  is provided by transceivers at each end communicating over bi-directional optical fibers  115  each transceiver having one transmitter that operates at a first wavelength (λ 1 ) with a capacity of R Gb/s, and having one receiver that operates at a second wavelength (λ 2 ) with a capacity of R Gb/s. At the line card  112  end, the connector  117  is a multi-fiber connector having a plurality of fibers, for example X 1  fibers, each of which has the capacity just recited. Each line card  112  may include one or more connectors  117  and includes transceivers  114  such as a transceiver implemented in a photonic integrated circuit (PIC) such as a parallel multi-transceiver module according to the design of transceiver B described in association with  FIG. 5 . 
         [0029]    Turning to the line cards  112  of  FIG. 1 , which comprise a portion of a Tier 1 switch  130 , there can be one or more line cards  112 , each having a substantially similar capacity. For example, in one embodiment of a line card  112 , there can be N 1  fiber optic input/output (I/O) ports connected to transceivers  114  which are electrically connected, directly or indirectly, to one or more entirely electronic switch chips. In another embodiment, a Tier 1 switch  130  can be comprised of an array of similar M 1  line cards (some facing the servers, some facing Tier  3   150 ) and an array of switch-fabric cards (not shown), with a total capacity of N 1 ×M 1 ×R Gb/s. 
         [0030]    As illustrated on the right hand side of  FIG. 1  at the Tier 1 level  110 , a second line card  120  embodiment which includes portions which are Tier 3-facing according to principles of the invention has one or more (W 1 ) entirely electronic switch chips  122  (which using conventional technology can be one or more electronic switching chips which are commercially available from Broadcom, Cavium, and Barefoot Networks). The line card  120  has in-card connections from the switch chips  122  to one or more (S 1 ) transceivers  124  implemented in photonic integrated circuit chips (such as parallel multi-transceiver modules according to the design of transceiver B or transceiver C described in association with  FIG. 5  or  FIG. 6 , respectively) to one or more (Y 1 ) multi-fiber connectors  127  such as an MTP connector, each connector  127  having a capacity of V 1  optical fibers  125  (V 1 ), such as fibers having a capacity of one or more streams of R Gb/s in each direction. In general, optical fibers used in devices according to the principles of the invention are bi-directional SM fibers or unidirectional SM fibers unless otherwise described. In some embodiments the capacity V 1  of each multi-fiber connector  127  is equal to 12 or more. Although not illustrated, serializer-deserializers are utilized within the switch chips  122  to interface with the transceivers  124 . It should be understood that line cards including both the server facing portions and the Tier 3 facing portions described above are contemplated. It is contemplated that in some embodiments transceivers  124  comprise WDM transceivers configured to transmit and/or receive optical signals with use of multiple wavelengths, and in some embodiments each transceiver  124  is a transceiver module including multiple parallel WDM transceivers. It should be understood that photonic integrated circuit chips may be silicon-based, indium phosphide-based, or based on any other compatible material platform. 
         [0031]    In the middle of  FIG. 1  there are shown a number of connections  141  between Tier 1 switches  130  and Tier 3 switches  152 . Each such connection line  141  is intended to denote an optical fiber bundle or ribbon, in one embodiment having 12 fibers, using an MTP connector to mechanically connect the fibers to the Tier 1 and Tier 3 switches  130 ,  152 , and each bundle having a capacity of 400 G. In other embodiments the numbers and capacities of the various components and connections of the architecture  100  of  FIG. 1  are such that a fiber bundle connecting Tier 1  110  and Tier 3  150  has a capacity of 800 G, as for example in the case of a pair of fibers operating with 32 wavelengths each, unidirectionally. 
         [0032]    As illustrated at the top of  FIG. 1 , there are one or more Tier 3 switches  152  provided for switching of traffic between Tier 1 switches  130  and to enable optical communication  180  from the data center of  FIG. 1  to other data centers (for example a long haul (LH) connection  180   a ), to other communication systems in the vicinity (for example within 80 kilometers, otherwise termed a “Metro” connection  180   b ), or for other types of optical connection. The Tier 3 level  150  of switches  152  in general are comprised of one or more chassis  154  here referred to as a multi-chassis switch  152 . The chassis  154  can be assembled in arrays such as an array of Z Tier 3 chassis  154  (as shown on the leftmost example of a Tier 3 switch  152 ) with all of the chassis  154  interconnected by a switching fabric  158  (of which only one is illustrated). It should be noted that  FIG. 1  illustrates an embodiment with an oversubscription of server bandwidth of Q:1 to the total capacity tying Tier 1  110  to Tier 3  150  which in one embodiment is 3:1. In other embodiments it is contemplated that other oversubscription ratios (the ratio of the total server-facing capacity to the total Tier 3-facing capacity) will be implemented. As shown in  FIG. 1 , since a Tier 1 switch  130  has many more ports than a top-of-rack switch as is used today, the range of oversubscription ratios that are possible in the architecture is greater than that normally possible when using a top-of-rack switch in known architectures. 
         [0033]    In the next example of a Tier 1-Tier 3 architecture, which might be used in connection with a Metro connection  180   b,  there are shown in one embodiment a plurality of Tier 3 linecards  156 , each of which has a capacity of P 1 ×100 G, to provide a capacity of P 2 ×100 G using multiple cards. As shown in the rightmost example switch  152 , for some embodiments, these Tier 3 linecards  156  form part of each chassis  154 . Other embodiments, using different numbers of Tier 3 linecards  156  may be constructed. The primary role of the Tier3 switches  152  is to interconnect all of the Tier1 switches  130 . In addition there will be a smaller number of links to optical devices external to the data center. 
         [0034]    It should be understood that the variable labels used to enumerate and quantify various aspects of the architecture depicted in  FIG. 1  (R, X 1 , M 1 , Z, Q, V 1 , Y 1 , S 1 , W 1 , P 1 , P 2 , N 1 , etc.) are not illustrated as limited to any particular value, as skilled persons in the art will understand each can take on any one of a number of possible values which is compatible with the particular implementation of the architecture being contemplated. It should be understood that variations of the embodiment of the data center shown in  FIG. 1  are contemplated. They include one or more of different electronic switch chips, different capacities, and different data rates on the interconnecting fiber bundles, for example. 
         [0035]      FIG. 2  is a schematic diagram of the architecture  200  of the data center of  FIG. 1  at a high level. As shown in  FIG. 2 , a single server  206  within a server rack  202  containing a plurality of servers is connected by way of a transceiver  204  (such as transceiver A discussed in connection with  FIG. 3 ) to a portion of a transceiver  214  (such as transceiver B discussed in connection with  FIG. 5 ) within a Tier 1 switch  230 . In one embodiment, the distance between the server rack  202  and the Tier 1 switch  230  is typically less than or equal to 100 meters, and experiences an optical loss of no more than approximately 2 dB. A connection  208  is shown between a Tier 1 switch  230  and a Tier 3 switch  252  using transceivers  224  and  253  (such as according to either the Transceiver B or Transceiver C design discussed in connection with  FIG. 5  and  FIG. 6  respectively). In one embodiment, the connection distance is expected to be less than or equal to 2 kilometers, with a loss of no more than 4 dB expected. Each Tier 3 switch  252  may then be optically connected to other (external) optical devices using one or more Metro links  209  utilizing a transceiver  255  (such as Transceiver D discussed in connection with  FIG. 7-10 ) over a distance expected to be less than or equal to 80 kilometers. In other embodiments, the optical connection to external devices is a combination of long haul (LH) connections exceeding 80 km in length and Metro connections (not shown), for example to another data center. 
         [0036]    In the present application, we describe a plurality of transceivers that are built using silicon photonics that will permit a significant reduction in the fiber count (50%) in the data center, by using a single fiber for bidirectional transmission. 
         [0037]      FIG. 3  is a schematic diagram  300  of one embodiment of a transceiver. In  FIG. 3 , one embodiment of a Transceiver A is illustrated, which is based on a silicon PIC chip. In general, the on-chip optical connections on the silicon PIC chip are fabricated using high-contrast silicon optical waveguides, which are well known in the art. The data is encoded using a laser optical carrier, which in some embodiments may be generated using a semiconductor laser built from a gain chip  302  comprising a waveguide in a III-V material, a laser cavity  304 , and a collection of waveguides and other optical elements on the silicon PIC chip. In some embodiments the data is encoded on the optical carrier using a Mach-Zehnder interferometer (MZI)  306  as a modulator. The modulator has an input configured to receive information to be modulated onto a carrier optical signal. The data signal is passed to a waveguide  314  for transmission using a device such as a ring resonator  320  which can be tuned to connect a specific wavelength of light onto the transmission line. In one embodiment, the TE mode is used for transmission. The transmitted signal passes through a polarization splitter and rotator (PSBR)  310  and is sent out to a bi-directional single mode (SM) optical fiber  308 . In the receive direction, the transceiver receives optical signals from the bi-directional single mode optical fiber  308 . In some embodiments, the signals may have an undefined state of polarization. The PBSR  310  splits the TE  316  and TM  318  modes of the incoming signal into two waveguides, each of which is leads to photodetector (PD)  312 , causing the respective portion of the received signal to be detected. The detected signal can be further processed electronically. 
         [0038]    A transceiver as shown in  FIG. 3  may be constructed using any one of several embodiments, or combinations thereof, as are now described. In some embodiments, transmission and reception of optical signals occurs using the same wavelength, making use of directional couplers to separate the two data streams but incurring ˜6 dB of loss in total across the link. In other embodiments, transmission and reception of optical signals occurs using a filter to separate the transmission wavelength from the reception wavelength. In some embodiments, one can transmit modulated data, where the modulation is provided by a Mach-Zehnder interferometer, a ring resonator, or by using an electroabsorption modulator in silicon. In some embodiments, signal transmission may take advantage of polarization diversity to transmit more than one data stream. The receiver may do the same, by integrating a polarization controller into the optical path, along with an appropriate feedback algorithm. In some embodiments, the choice of wavelengths for transmission and reception may be auto-negotiated or may be fixed ab-initio. In some embodiments, the receiver may take advantage of coherent mixing with a local oscillator to enhance sensitivity. In various embodiments, transceivers that operate according to principles of the invention may embody a single device, feeding a single fiber, or an array of such devices, feeding an array of separate fibers. In various embodiments, dispersion compensation may be included as a block within the receiver or transmitter, either on a per-channel basis or as a single block for all channels. 
         [0039]      FIG. 4  is a schematic diagram  400  of an alternative embodiment of a transceiver. In  FIG. 4  there are shown a gain chip  401 , which contains two waveguides which together with further optical elements on the silicon chip (“laser cavity”  402 ,  404 ) generate two laser signals. The laser signals are carriers that can be modulated with information by Mach-Zhender interferometers  406 ,  408  to encode information on each carrier. In one embodiment, each of those laser signals feeds one of two parallel transmitters, one of which will produce a signal to be launched in TE mode on the bi-directional fiber  416 , and the other of which produce a signal to be launched in the TM mode of the fiber  416 . The modulated signals are coupled to one of the two parallel transmitters by way of respective ring resonators  424 ,  426  which can be used as a filter to tune a specific wavelength (or frequency) for passage, and to exclude other wavelengths. In other embodiments one can use one or more of a short wavelength cutoff filter, a long wavelength cutoff filter, and a bandpass filter. In some embodiments the bandpass filter is a narrow pass filter. The outgoing signals are combined in the PBSR 414  and applied to a bi-directional single mode optical fiber  416  for transmission. The incoming signals are handled in a manner similar to what was described for the transceiver of  FIG. 3 , using two photodetectors  418 ,  420 , one for what was the TE mode and one for what was the TM mode of the fiber. However, in addition a polarization controller  422  is used to align the signals in the waveguides in such a way as to de-multiplex the originally polarization multiplexed signals. 
         [0040]      FIG. 5  is a schematic diagram of is an embodiment of an array of transceivers. The transceiver of  FIG. 5  is constructed using an array of N examples of transceiver A of FIG.  3  in a single silicon PIC chip (although only 3 devices are illustrated), where N is a positive integer number greater than 1. N will in general be bounded at a largest finite value constrained by packaging. Each of the three illustrated examples of transceiver A includes a gain chip  502  (which in some embodiments is a common gain chip and in other embodiments can be a separate gain chip for each transceiver), one of the laser cavities  504 ,  506 ,  508 , one of the Mach-Zehnder interferometers  512 ,  510 ,  514 , one of the ring resonators  530 ,  523 ,  534 , one of the photodetectors  516 , 518 ,  520 , one of the PBSRs  540 ,  542 ,  544 , and one of the fibers  522 ,  524 ,  526 . 
         [0041]      FIG. 6  is a schematic diagram  600  of as yet another embodiment of a transceiver. The transceiver of  FIG. 6  is an example of a transceiver C architecture, in which a gain chip  602  comprising an array of waveguides together with the array of silicon optical elements which constitute the remainder of the laser cavities  604 ,  606 ,  608 ,  610  provides signals, which are then encoded using devices (for example, MZIs  612 ,  614 ,  616 ,  618 ) similar to the device of  FIG. 3 . As illustrated in  FIG. 6 , a plurality (in the embodiment shown,  4 ) of transmitted signals are provided, for example having discrete wavelengths, λ 1 , λ 2 , λ 3 , and λ 4 . Each encoded signal at a given wavelength is handled by a respective ring resonator  626 ,  624 ,  622 ,  620 . The transmitted signals are combined using cascaded MZIs  648 ,  652   654  (also sometimes referred to as “interleavers”), each MZI combining two signals onto a single waveguide. In various embodiments one can use a selected one of an arrayed waveguide grating, an echelle grating, a Mach-Zehnder interferometer-based multiplexer and an interleaver in performing the multiplexing. For 4 signals, two levels of cascaded MZIs are used (as is illustrated). For 8 signals, 3 levels of MZIs are used. In general, for N=2 M  signals where M is a positive integer, M levels of MZIs will be required to combine all of the signals onto a single waveguide. The transmitted and received signals are communicated over a bi-directional SM optical fiber  560 . The incoming signals pass through a PBSR  656  and are split into the TE and TM modes of the fiber. The TE signals propagate on the right hand side of the illustrated transceiver, and what had been the incoming TM mode signals on the left side. In one embodiment, the incoming signals are de-multiplexed based on wavelength into eight separate waveguides, each leading to a photodetector. The received electronic signals are processed substantially as described for the other transceivers. The demultiplexing is accomplished in the embodiment illustrated using cascaded MZIs  648 ,  652   654  for the incoming TE mode signal Rx(TE) and cascaded MZIs  650 ,  644   646  for the incoming TM mode signal Rx(TM). Photodetectors  634 ,  632 ,  630  and  628  are used to detect the four incoming Rx(TE) signals. Photodetectors  642 ,  640 ,  638  and  636  are used to detect the four incoming Rx(TM) signals. In various embodiments one can use a selected one of an arrayed waveguide grating, an echelle grating, a Mach-Zehnder interferometer-based demultiplexer and an interleaver in performing the demultiplexing. 
         [0042]    All of the capabilities and methods of operation that are recited for Transceiver A above apply equally to various embodiments of transceiver C. In this embodiment, the transmitter and receiver each transmit and receive, respectively, more than one wavelength, in order to further increase the data transmission capacity of the system. In different embodiments, the wavelengths may be automatically negotiated, or may be fixed to a specific value for each channel in the grid or array. In some embodiments, dual polarization per wavelength is possible using a polarization controller. In cases of low wavelength dependent polarization mode dispersion (PMD) and polarization-dependent loss (PDL), the polarization controller may be shared among multiple wavelength channels (i.e., being located between the wavelength mux and the chip exit/entrance port). For other embodiments, the polarization controller can be provided on a per-channel basis. In various embodiments, arrays of multiple units of the same transceiver can be implemented on a single PIC. In some embodiments, the channel demux/mux capability may be provided by the use of ring resonators. In other embodiments, the channel demux/mux capability may be provided by the use of other filter elements. 
         [0043]      FIG. 7  is a schematic diagram  700  of a link between two data centers. In  FIG. 7 , a detail of the contents of the package of a Transceiver D  720  is shown at the top left. The package for Transceiver D  720  includes a silicon PIC  702  which is connected to a gain chip array  706 , an array of electronic drivers  710  to impress the data to be transmitted on the optical modulators within the PIC and an array of electronic TIA circuits  704  which are used to amplify the signals generated by incoming signals at photodiodes. The PIC  702  is connected to a fiber array  708 . The signals from the fiber array are combined in an array waveguide grating (AWG) router  712  and transferred to a fiber leading to an Erbium-doped fiber amplifier (EDFA)  714  for transmission to a remote location. The EDFA 714  connects to a fiber  724 , such as an 80 Km fiber external to the data center. A second Transceiver D  720  in the remote location is also shown. In one embodiment the remote location is another data center. In some embodiments, a dispersion compensating fiber (DCF)  722  is used to correct for chromatic dispersion that occurs in transmission through the 80 km external fiber  724 . The DCF  722 can be placed at the transmitter data center or at the receiver data center. The incoming signals are amplified in an EDFA  716 , demultiplexed in a second AWG  718 , and processed in the second Transceiver D  720 , which is substantially similar to the Transceiver D first described. 
         [0044]      FIG. 8  is a schematic diagram  800  of another embodiment of a transceiver. In the transceiver of  FIG. 8 , a plurality of transceivers, each operating at 56 Gbaud are provided on a silicon PIC in an array. The number of transmitters and receivers is the same. As illustrated, there are only 6 transmitter section, each having connection to a gain chip  826 , and including a laser cavity  814 ,  816 ,  818 ,  829 ,  822 ,  824  and a respective MZI  802 ,  804 ,  806 ,  808 ,  810 ,  812 . The transmitted signals exit along the illustrated horizontal lines to the left of the drawing. As illustrated, there are 9 receiver sections, each connected to a fiber  870 ,  872 ,  874 ,  876 ,  878 ,  880 ,  882 ,  884 ,  886 , which communicates the incoming signals to respective PBSRs  828 ,  830 ,  832 ,  834836 ,  838 ,  840 ,  842 ,  844  that split the incoming optical signal into TE and TM modes, and that communicate the split signals to photodetectors  85 ,  852 ,  854 ,  856 ,  858 ,  860 ,  862 , 864 ,  866 , which detect the siognals and provide electrical signals representative of the optical signals. 
         [0045]      FIG. 9  is a schematic diagram  900  of another embodiment of a transceiver. The transceiver of  FIG. 9  is an array of transceivers. The embodiment of  FIG. 9  operates using pulse amplitude modulation on each of the two polarization components. One embodiment operates at 28 Gbaud and another embodiment operates at 56 Gbaud. Three transmitters are illustrated. The transmitters comprise a gain chip  936 , respective laser cavities  914 ,  916 ,  918 ,  920 ,  922 ,  924 , MZIs  902 ,  904 ,  906 ,  908 ,  910 ,  912 , and respective PBSRs  930 ,  932 ,  934  that combine the outgoing optical signals and communicate the combined signals on an optical fiber. Three re4ceuivers are also illustrated. The receivers receive optical signals from respective fibers  962 ,  964 ,  966 . The signals are split in PBSRs  938 ,  940  and  942 . The split signals pass through polarization controllers  944 ,  946 ,  948 . The separated optical frequencies and polarizations are then detected by photodetectors  950 ,  952 ,  954 ,  956 ,  958 ,  960 . 
         [0046]      FIG. 10  is a schematic diagram  1000  of another embodiment of a transceiver. The transceiver of  FIG. 10  operates using QPSK. In  FIG. 10  the gain chip  1016  with the “laser cavity” elements  1010 ,  1012 on the PIC provides laser signals for the two branches of the transmitter and a local oscillator (LO) signal  1030  that can be used to analyze the phases in the incoming signals. The transmitter section includes two MZIs  1002 ,  1004  and  1006 ,  1008  per transmitter. The transmitter signals are combined in PBSR  1014  and provided to an optical fiber for transmission. In the receiver, signals enter from fiber  1028  and are split in PBSR  1026 . A polarization controller  1032  provides signals to respective 90 degree hybrid mixers  1034   1036  and thereafter to 4×PD (four-fold photodetectors)  1038 ,  1040 . 
         [0047]    Various options for designs of package D that provide 100 Gb/s per channel can be considered. They include: 56 Gbaud DP 00K, 28 Gbaud DP-PAM4, 28 Gbaud DP-QPSK with LO, and 28 Gbaud DP-DQPSK direct detect. 
       ACRONYMS AND TRADE NAMES 
       [0000]    
       
         OOK is on-off keying. 
         PAM is pulse amplitude modulation. 
         PAM4 is 4-level pulse amplitude modulation. 
         PSK is phase shift keying. 
         QPSK is quadrature or quaternary phase shift keying. 
         DQPSK is differential QPSK. 
         SM fiber is single mode fiber. 
         PBSR is polarization beam splitter and rotator. 
         PD is photodiode. 
         MZI is Mach-Zehnder Interferometer. 
         TE is transverse-electric mode. 
         TM is transverse-magnetic mode. 
         Tx is transmitter. 
         Rx is receiver. 
         TIA is transimpedance amplifier. 
         PIC is photonic integrated circuit. 
         AWG is array waveguide grating. 
         EDFA is Erbium doped fiber amplifier. 
         DCF is dispersion compensating fiber. 
         MTP is multi-termination parallel fiber connector. 
         TOR is top of rack. 
         LH is long haul, meaning a connection of typically more than 80 kilometer distance, up to thousands of kilometers. 
         PMD is polarization mode dispersion. 
         PDL is polarization-dependent loss. 
       
     
         [0072]    Definitions 
         [0073]    Unless otherwise explicitly recited herein, any reference to an electronic signal or an electromagnetic signal (or their equivalents) is to be understood as referring to a non-volatile electronic signal or a non-volatile electromagnetic signal. 
         [0074]    Recording the results from an operation or data acquisition, such as for example, recording results at a particular frequency or wavelength, is understood to mean and is defined herein as writing output data in a non-transitory manner to a storage element, to a machine-readable storage medium, or to a storage device. Non-transitory machine-readable storage media that can be used in the invention include electronic, magnetic and/or optical storage media, such as magnetic floppy disks and hard disks; a DVD drive, a CD drive that in some embodiments can employ DVD disks, any of CD-ROM disks (i.e., read-only optical storage disks), CD-R disks (i.e., write-once, read-many optical storage disks), and CD-RW disks (i.e., rewriteable optical storage disks); and electronic storage media, such as RAM, ROM, EPROM, Compact Flash cards, PCMCIA cards, or alternatively SD or SDIO memory; and the electronic components (e.g., floppy disk drive, DVD drive, CD/CD-R/CD-RW drive, or Compact Flash/PCMCIA/SD adapter) that accommodate and read from and/or write to the storage media. Unless otherwise explicitly recited, any reference herein to “record” or “recording” is understood to refer to a non-transitory record or a non-transitory recording. 
         [0075]    As is known to those of skill in the machine-readable storage media arts, new media and formats for data storage are continually being devised, and any convenient, commercially available storage medium and corresponding read/write device that may become available in the future is likely to be appropriate for use, especially if it provides any of a greater storage capacity, a higher access speed, a smaller size, and a lower cost per bit of stored information. Well known older machine-readable media are also available for use under certain conditions, such as punched paper tape or cards, magnetic recording on tape or wire, optical or magnetic reading of printed characters (e.g., OCR and magnetically encoded symbols) and machine-readable symbols such as one and two dimensional bar codes. Recording image data for later use (e.g., writing an image to memory or to digital memory) can be performed to enable the use of the recorded information as output, as data for display to a user, or as data to be made available for later use. Such digital memory elements or chips can be standalone memory devices, or can be incorporated within a device of interest. “Writing output data” or “writing an image to memory” is defined herein as including writing transformed data to registers within a microcomputer. 
         [0076]    “Microcomputer” is defined herein as synonymous with microprocessor, microcontroller, and digital signal processor (“DSP”). It is understood that memory used by the microcomputer, including for example instructions for data processing coded as “firmware” can reside in memory physically inside of a microcomputer chip or in memory external to the microcomputer or in a combination of internal and external memory. Similarly, analog signals can be digitized by a standalone analog to digital converter (“ADC”) or one or more ADCs or multiplexed ADC channels can reside within a microcomputer package. It is also understood that field programmable array (“FPGA”) chips or application specific integrated circuits (“ASIC”) chips can perform microcomputer functions, either in hardware logic, software emulation of a microcomputer, or by a combination of the two. Apparatus having any of the inventive features described herein can operate entirely on one microcomputer or can include more than one microcomputer. 
         [0077]    General-purpose programmable computers useful for controlling instrumentation, recording signals and analyzing signals or data according to the present description can be any of a personal computer (PC), a microprocessor based computer, a portable computer, or other type of processing device. The general purpose programmable computer typically comprises a central processing unit, a storage or memory unit that can record and read information and programs using machine-readable storage media, a communication terminal such as a wired communication device or a wireless communication device, an output device such as a display terminal, and an input device such as a keyboard. The display terminal can be a touch screen display, in which case it can function as both a display device and an input device. Different and/or additional input devices can be present such as a pointing device, such as a mouse or a joystick, and different or additional output devices can be present such as an enunciator, for example a speaker, a second display, or a printer. The computer can run any one of a variety of operating systems, such as for example, any one of several versions of Windows, or of MacOS, or of UNIX, or of Linux. Computational results obtained in the operation of the general purpose computer can be stored for later use, and/or can be displayed to a user. At the very least, each microprocessor-based general purpose computer has registers that store the results of each computational step within the microprocessor, which results are then commonly stored in cache memory for later use, so that the result can be displayed, recorded to a non-volatile memory, or used in further data processing or analysis. 
         [0078]    Many functions of electrical and electronic apparatus can be implemented in hardware (for example, hard-wired logic), in software (for example, logic encoded in a program operating on a general purpose processor), and in firmware (for example, logic encoded in a non-volatile memory that is invoked for operation on a processor as required). The present invention contemplates the substitution of one implementation of hardware, firmware and software for another implementation of the equivalent functionality using a different one of hardware, firmware and software. To the extent that an implementation can be represented mathematically by a transfer function, that is, a specified response is generated at an output terminal for a specific excitation applied to an input terminal of a “black box” exhibiting the transfer function, any implementation of the transfer function, including any combination of hardware, firmware and software implementations of portions or segments of the transfer function, is contemplated herein, so long as at least some of the implementation is performed in hardware. 
         [0079]    Theoretical Discussion 
         [0080]    Although the theoretical description given herein is thought to be correct, the operation of the devices described and claimed herein does not depend upon the accuracy or validity of the theoretical description. That is, later theoretical developments that may explain the observed results on a basis different from the theory presented herein will not detract from the inventions described herein. 
         [0081]    Any patent, patent application, patent application publication, journal article, book, published paper, or other publicly available material identified in the specification is hereby incorporated by reference herein in its entirety. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material explicitly set forth herein is only incorporated to the extent that no conflict arises between that incorporated material and the present disclosure material. In the event of a conflict, the conflict is to be resolved in favor of the present disclosure as the preferred disclosure. 
         [0082]    While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be affected therein without departing from the spirit and scope of the invention as defined by the claims.