Patent Publication Number: US-2016227301-A1

Title: Transponder aggregator photonic chip with common design for both directions

Description:
TECHNICAL FIELD 
     The present invention relates to optical transponder aggregator designs and implementations, and, in particular, to add-drop switch designs and implementations. 
     BACKGROUND 
     An optical transponder aggregator is a flexible optical network element for carrying traffic onto and off an optical network. Each transmitter Tx or receiver Rx in a transponder aggregator can connect to any one of a plurality of wavelengths, and the transponder aggregator includes photonic switches implementing selections of different wavelengths and setting the connection pattern. A transponder aggregator typically includes two optical waveguide modules, a drop switch module and an add switch module. 
     To minimize costs and chip sizes, the drop and add switch modules can be implemented as photonic integrated circuits (PIC), typically silicon photonics. Due to the number of channels existing in the entire optical band and the size of the PIC chip, existing PIC chips do not have broad band operating capability and current solutions involve implementing different sub-bands on different chips. As well, in many applications it is desirable to separate the drop and add switch modules into different chips, due to yield or chip size limitations, or because operators prefer to assemble the networks using separate add and drop switches. These are all factors that contribute to circuit, manufacturing and packaging complexity. 
     Accordingly, while transponder aggregators offer operational flexibility, there is a need to simplify circuit designs for manufacturing and to bring down the costs for market implementations. 
     SUMMARY 
     The following presents a summary of some aspects or embodiments of the invention in order to provide a basic understanding of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some embodiments of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
     In general, the present disclosure relates to an optical transponder aggregator that implements a photonic chip design enabling the chip to be configured to function either in an add role or in a drop role. 
     One inventive aspect of the disclosure is an optical transponder aggregator that includes a plurality of photonic switch cells configured to collectively implement switching functions for either an add transponder aggregator or a drop transponder aggregator. The transponder aggregator also includes a plurality of optical tap-monitors for control of the photonic switch cells. The transponder aggregator further includes a controller configured to change control roles of the optical tap-monitors, depending on whether the plurality of photonic switch cells collectively implements switching functions for the add transponder aggregator or for the drop transponder aggregator. 
     Another inventive aspect of the disclosure is a method for providing an optical transponder aggregator. The method involves providing a plurality of photonic switch cells configured to collectively implement switching functions for either an add transponder aggregator or a drop transponder aggregator, providing a plurality of optical tap-monitors for control of the photonic switch cells; and controlling the plurality of optical tap-monitors to select between a first and a second set of control roles of the photonic switch cells, the first set of control roles corresponding to when the plurality of photonic switch cells collectively implements switching functions for the add transponder aggregator, and the second set of control roles corresponding to when the plurality of photonic switch cells collectively implements switching functions for the drop transponder aggregator. 
     Yet another inventive aspect of the disclosure is an optical transponder aggregator that includes a photonic integrated circuit and a control circuit. The photonic integrated circuit includes a plurality of photonic switch cells for collectively implementing switching functions for either an add transponder aggregator or a drop transponder aggregator, and a plurality of optical tap-monitors for control of the photonic switch cells. The control circuit is configured to change control roles of the optical tap-monitors, depending on whether the plurality of photonic switch cells collectively implements switching functions for the add transponder aggregator or for the drop transponder aggregator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the disclosure will become more apparent from the description in which reference is made to the following appended drawings. 
         FIG. 1  provides a circuit diagram illustrating a drop transponder aggregator according to one embodiment of the present invention. 
         FIG. 2  provides a circuit diagram illustrating an add transponder aggregator according to one embodiment of the present invention. 
         FIG. 3  illustrates a bi-directional tap-monitor according to one embodiment of the present invention. 
         FIG. 4  provides an exemplary example 2×2 photonic switch cell used in connection with the bi-directional tap monitors, according to one embodiment of the present invention. 
         FIG. 5  is a flow chart of providing an optical transponder aggregator, according to one embodiment of the present invention. 
         FIG. 6  illustrates a Dense Wavelength-Division Multiplexing (DWDM) ring using the transponder aggregator, according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description contains, for the purposes of explanation, various illustrative embodiments, implementations, examples and specific details in order to provide a thorough understanding of the invention. It is apparent, however, that the disclosed embodiments may be practiced, in some instances, without these specific details or with an equivalent arrangement. The description should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     An optical transponder aggregator is described hereinafter which is an optical apparatus that generally includes a drop transponder aggregator (also “receiver aggregator”) for carrying traffic off a network (also “drop role”), and an add transponder aggregator (also “transmitter aggregator”) for carrying traffic onto the network (also “add role”). The drop transponder aggregator includes a drop switch module; and the add transponder aggregator includes an add switch module. As will become apparent and depending on the context, the term “transponder aggregator” may refer to either a drop or add transponder aggregator, or both aggregators. For the purposes of this specification, the optical transponder aggregator may or may not include an optical demultiplexer or multiplexer. 
     In various embodiments which follow, each add or drop switch module includes one or more photonic switch cells and one or more optical tap-monitors implemented on a photonic integrated circuit (PIC). Each add or drop switch module includes optical waveguides constructing the photonic switch cells and the optical tap-monitors, typically by way of silicon photonics or other waveguide material systems such as GaAs, InP, LiNbO3, PLZT, SiN, SiON. 
     In a typical but non-limiting structure of the optical waveguide, a silicon core with high refractive index is surrounded by a low refractive index material, for example silicon dioxide. This structure forms an optical waveguide, used at communications wavelengths such as the 1310 nm or 1550 nm bands, wavelengths for which the silicon and silicon dioxide are transparent. Typical silicon PIC chips use a lithographically-defined layout of single mode and multimode waveguide elements, the whole forming a photonic circuit. 
     For Si photonics, Ge or SiGe photodetectors (all of which are group IV semiconductors) can be used for the tap-monitors. For GaAs photonics and InP photonics (III-V semiconductors), InGaAs or a similar III-V semiconductor material photodetectors can be used for the tap-monitors. For non-semiconductor photonics, such as LiNbO3, PLZT, SiN, SiON, the photodetectors can be Ge, SiGe or InGas (or III-V materials similar to InGaAs) photodetectors. 
     In the following description, the add switch module may be referred to as “add switch”, or “PIC performing add role”; and the drop switch module may be referred to as “drop switch”, or “PIC performing drop role”. The add switch and the drop switch may be implemented on the same PIC chip or on separate chips. 
     According to various embodiments, a photonic chip for transponder aggregator is provided that shares the same chip schematic for the add transponder aggregator and the drop transponder aggregator. More specifically, an add-drop photonic chip is provided that shares the same chip schematic for the add switch and the drop switch. If the product requirements call for separate chips to implement the add switch and the drop switch, only one manufacturing run can be used to build both chips. Alternatively, if the product requirements call for one single die containing both the add switch and the drop switch, the same design can be duplicated on the die with reduced design efforts. 
     In either scenario, a transponder aggregator is provided that is configurable to implement the add role or drop role and can be used in either direction, by way of a controller configured to change control roles of the optical tap-monitors, depending on whether the switch cells collectively implements switching functions for the add transponder aggregator or for the drop transponder aggregator. In one direction the transponder aggregator can be used for the add role and in the opposite direction the transponder aggregator can be used for the drop role. 
     As will be explained in more detail hereinafter, the switch cells collectively implementing switching functions for the transponder aggregator refers to the switch cells working as a unit to perform the add or drop role of the transponder aggregator, to either carry traffic onto and off an optical network. The controller may take the form of a control circuit implemented using software or firmware in digital electronics, or alternatively a flip-chip attached smart pixel control chip made in e.g. complementary metal-oxide-semiconductor (CMOS). For the purposes of this specification, the term “controller” will be used to encompass all forms of control devices, control implementations, and control means, be in hardware, software or firmware. 
     The transponder aggregator as designed can be used in various optical networks, such as in a Wavelength-Division Multiplexing (WDM) or other optical edge or core metro area network (MAN), a Passive Optical Network (PON), a wireless aggregation networks or cloud radio access network (C-RAN), or the like. While some embodiments make reference to a ring structure network, it should be understood that the transponder aggregator applies to other network structures, such as a linear structure network. 
       FIG. 1  presents a circuit diagram of a drop transponder aggregator  100 , according to one exemplary embodiment of the present invention. In the embodiment illustrated in  FIG. 1 , light travels from the left to the right in direction  101  for the drop role. 
     As illustrated by way of example in  FIG. 1 , an input line fiber  102 , such as a Dense Wavelength-Division Multiplexing (DWDM) input line fiber, is connected to an optical demultiplexer  104 . The input line fiber  102  carries a group of optical line signals travelling on different wavelengths and the demultiplexer  104  splits the signals onto separate fibers  106  for individual wavelengths. The fibers  106  are connected to optical line input ports  108  of a drop switch  110 . The drop switch  110  is implemented as a PIC and includes a plurality of photonic switch cells  120 , forming a drop selector switch  112  and a receiver aggregator switch  114 , which collectively implements the switching functions for the drop transponder aggregator  100 . In particular, the drop selector switch  112  selects a subset of the incoming line signals to the receiver aggregator switch  114 , which in turn switches the signals to banks of client receivers Rx  116 . The balance of the incoming line signals are passed as optical transit signals via optical transit output ports  118  of the PIC. The optical transit output ports  118  may be connected to an add switch. Some of the transit lines to the add switch may be unoccupied (unused). Each client receiver Rx  116  may be wavelength-agnostic, or tuned to the wavelength that it will receive. 
       FIG. 2  presents a circuit diagram of an add transponder aggregator  200  corresponding to the drop transponder aggregator  100 . As illustrated in this figure, the add transponder aggregator  200  has the same chip schematic as the drop transponder aggregator  100 . As well, an add switch  210 , also implemented as a PIC, has a common chip schematic as the drop switch  110  in  FIG. 1 . When performing the add role, light propagates through the add transponder aggregator  200  in an opposite direction  201  compared to the drop transponder aggregator  100 . 
     In the embodiment depicted in  FIG. 2 , optical transit signals enter the add switch  210  through optical transit input ports  218  where the optical transit input ports  218  may be connected to a drop switch, such as the drop switch  110 . The add switch  210  includes a plurality of photonic switch cells  220 , forming a transmitter aggregator switch  214  and an add selector switch  212 , which collectively implements the switching functions for the add transponder aggregator  200 . In particular, the transmitter aggregator switch  214  selects from client transmitters Tx  216  a subset of signals on wavelengths unoccupied by the optical transit signals and the add selector switch  212  connects the optical transit signals, plus the selected subset of signals from client transmitters Tx  216 , to optical line output ports  208 . The optical line output ports  208  are connected to individual output fibers  206  of different waveguides and an optical multiplexer  204  multiplexes the signals from the optical output ports  208  onto an output line fiber  202 , such as a DWDM output line fiber. Each client transmitter Tx  216  may be wavelength-agile, or tuned to the wavelength that it will occupy. 
     While the embodiment shown in  FIGS. 1 and 2  illustrates a small number of inputs and outputs for purposes of illustration, it should be understood that Rx, Tx, input ports, output ports, and the wavelengths that they carry are not limited to specific numbers and may be equal in some applications or not equal in other applications. As well, while a simple switch matrix is depicted in  FIGS. 1 and 2  to explain the switching functions, any type of switch matrix or suitable combination of switch matrices may be used for the purpose of the switch cells collectively implementing switching functions for the add or drop role. In a typical product, 10 to 64 input/output ports may be implemented. While  FIGS. 1 and 2  illustrate the drop transponder aggregator  100  and the add transponder aggregator  200  as separate circuits, it should be understood that the two circuits may be implemented on a single die depending on product requirements. 
     Both drop switch  110  and add switch  210  comprise optical waveguides that implement photonic switch cells  120 ,  220  and optical tap-monitors  122 ,  124 ,  222 ,  224 . 
     Referring to  FIGS. 1 and 2 , each add-drop switch  110 ,  210  comprises a plurality of photonic switch cells  120 ,  220 . The switch cells  120 ,  220  may be 2×2 switch cells, each of which being used as 1×2, 2×2 or 2×1 switching functionality depending on their respective roles in the switch; or may be 1×2, 2×2 or 2×1 switch cells, each of which implementing their respective 1×2, 2×2 or 2×1 switching functionalities. 
     According to some embodiments, the switching function of each switch cell  120 ,  220  may be realized by a Mach-Zehnder interferometer, containing one or more adjustable phase shifters that are adjustable by means of a switch cell control circuit. Alternatively, the switching function of each switch cell  120 ,  220  may be implemented as a ring resonator interferometer element, splitter plus electro-absorption electro-gain element or any other electrically driven element that can direct light to one of two output ports. 
     According to various embodiments, each add-drop switch  110 ,  210  also includes a plurality of optical tap-monitors  122 ,  124 ,  222 ,  224 . Each tap-monitor  122 ,  124 ,  222 ,  224  comprises an optical tap in the form of a tap waveguide that taps a small fraction of the optical signal power and a photodetector to detect the tapped optical signal power. The tap-monitor  122 ,  124 ,  222 ,  224  may also include a transimpedance amplifier. The output of the photodetector or the transimpedance amplifier may be connected to a receiver circuit to measure the photocurrent or voltage thereby providing a relative indication of the magnitude of the optical signal power. Because tap-monitors usually occupy significant space on the chip, decrease the optical signal power (by tapping some light), and generate heat (mostly in the control circuit), it is desirable to minimize the number of tap-monitors. As well, the input power of each switch cell is generally not monitored. 
     The switching functions themselves are symmetric for the add switch and drop switch. For example, the switching functions can be reversed to make an add switch by operating inputs of a drop switch as outputs, and outputs of the drop switch as inputs. However, in conventional designs, the circuits for an add switch and those for a drop switch are not the same because the structure and arrangement of tap-monitors in the add or drop switch break the symmetry. Accordingly, different chip schematics are used for add and drop switches which in turn doubles the cost of spares and the cost of manufacturing inventory, compared to a solution where the designs for the add switch and drop switch are identical. 
     In conventional designs, a tap-monitor has directionality. For example, the tap-monitor usually includes an optical tap, such as a directional coupler, constructed by a tap waveguide placed near a through waveguide. The through path is carried by the through waveguide, and the tap waveguide is placed in the tail of the optical power carried in the through waveguide, and a small amount of optical power is transferred from the through waveguide to the tap waveguide. The tap waveguide includes a forward-facing port and a backward-facing port. Light going in a forward direction along the through waveguide partially couples out of the forward-facing port and is fed into the input of a waveguide photodetector. In conventional designs the backward-facing port of the tap waveguide may be connected to a beam terminator (e.g. an absorbing medium) to avoid back-reflection and stray light, which may degrade optical signal integrity. 
     According to various embodiments of the present invention, an optical bi-directional tap-monitor is implemented that is symmetric in a forward direction and in a backward direction. The tap-monitor is referred to as “bi-directional” in the sense that the tap-monitor is operable in either a forward-propagating or backward propagating direction. In other words, the tap-monitor has a switchable direction. According to various embodiments, the tap waveguide backward-facing port is connected to the other port (previously-unused) of the same photodetector which is connected to the tap waveguide forward-facing port. The photodetector can be used to detect light signal in either direction. 
       FIG. 3  illustrates an optical tap-monitor  300  according to one embodiment of the present invention. The tap-monitor  300  is designed to couple a fraction of an optical signal power carried by a through waveguide  301 . The tap-monitor  300  comprises an optical tap in the form of a tap waveguide  302 , such as a directional coupler, placed near the through waveguide  316  and taps a small fraction or portion (e.g., 1%, 2% or the like) of the optical signal power carried by the through waveguide  301 . The tap-monitor  300  also includes a waveguide-type photodetector  304  connected to the tap waveguide  302  to detect the tapped fraction of the optical signal power. As illustrated in  FIG. 3 , The tap waveguide  302  includes two ports  306 ,  308  at opposite ends of said tap waveguide. The two ports  306 ,  308  are connected respectively to two ports  310 ,  312  of the photodetector  304 . When light travels in one direction via the through waveguide  301  (from left to right in the embodiment of  FIG. 3 ), the light is coupled out of the tap waveguide  302  via port  306  and enters the photodetector  304  via port  310 . Conversely, when light travels in the opposite direction (from right to left in the embodiment of  FIG. 3 ), the light is coupled out of the tap waveguide  302  via port  308  and enters the photodetector  304  via port  312 . As such, the tap-monitor  300  responds to both the forward-propagating light and the backward-propagating light and the photodetector  304  detects the tapped fraction of the optical signal power in either direction. The photodetector  304  may further be connected to a transimpedance amplifier  314 . The output of the photodetector  304  or the transimpedance amplifier  314  may be connected to an electrical circuit  316  to measure the photocurrent or voltage to provide a relative indication of the magnitude of the optical signal power carried by the through waveguide  301 . The output of the photodetector  304  or the transimpedance amplifier  314  may be used to provide feedback control of a corresponding switch cell. 
     While the tap waveguide  302  in this embodiment is provided in the form of a directional coupler with the through waveguide  301 , other forms of couplers can be implemented for the purpose of the optical tap monitor  300 . While  FIG. 3  illustrates a straight through waveguide  301 , and the waveguides  301 ,  302  each includes a constant cross-section, of the same size, and with a constant gap between the waveguides  301 ,  302 , one or more of those conditions can be changed depending on the desired performance and manufacturability of the couplers. 
       FIG. 4  depicts a 2×2 photonic switch cell  400  used in connection with the optical tap monitors  300 , according to one embodiment of the present invention. The 2×2 photonic switch cell  400  includes two ports  402  on one side and two ports  404  on the other side. When light propagates in a forward direction  401  (from left to right in the embodiment of  FIG. 4 ), ports  402  act as optical inputs and ports  404  act as optical outputs. When light propagates in a backward direction  403  (from right to left in the embodiment of  FIG. 4 ), ports  404  act as optical inputs and ports  404  act as optical outputs. The switch cell  400  may be used in drop role when light propagates one direction, and in add role when light propagates in the opposite direction. The switch cell  400  also includes optical splitter  406  which forms ends of switching element, e.g., Mach-Zehnder interferometer, and tunable optical phase shifter  408  which actuates switching, i.e. performs a switching function. In this 2×2 photonic switch cell  400 , two tap-monitors  300  are provided near ports  404 , each responding to both the forward-propagating light and the backward-propagating light, by way of a single waveguide-type photodetector  304 . 
     The waveguide photodetector  304  may be designed to absorb the light so that optical loop parasitics are suppressed. For example, the waveguide photodetector  304  may be configured to have at least 40 dB optical absorption, by e.g., choosing a suitably long length of photodetecting material. If the PIC comprises silicon waveguides, the photodetecting material may be germanium. Because the leakage through the photodetector is small enough that the two ports  306 ,  308  of the tap waveguide  302  being connected does not cause a significant optical loop, and therefore would not introduce much optical signal integrity penalty. 
     Also, the bi-directional tap-monitor  300  according to the embodiments would not add any additional loss compared to using two directional tap-monitors, because it makes use of the optical tap currently used in the switch cell design and realizes the switchability of directions by connecting the two ports of the tap waveguide  302  with opposite ends of the photodetector  304 . 
     While  FIG. 4  only shows a 2×2 photonic switch cell, it should be appreciated that 1×2 or 2×1 switch cells can be similarly implemented. The unused optical port of the 1×2 or 2×1 switch cell may be terminated to a beam-dump within the switch cell. As well, while tap-monitors  300  are placed near ports  404  in  FIG. 4 , it should be apparent that the tap-monitors  300  can be placed near ports  402  or at other locations, which may be physically laid out within the switch cell  400 , or physically laid out in some other manner. 
     In this particular embodiment for purposes of illustration, light propagates in a forward direction when the transponder aggregator  100  performs the drop role, and in a backward direction when the transponder aggregator  200  performs the add role. It should however be understood that the correspondence between the direction of light and the role of the aggregator can be reversed. In any event, one of the ports  306 ,  308  of the tap waveguide  302  is used to couple out light for the add role, while the other one of the ports  306 ,  308  is used to couple out light for the drop role. 
     According to various embodiments of the present invention, a plurality of tap-monitors  300  are provided for control of the switch cells  120 ,  220 . Each tap-monitor  300  may be used as a switch cell output monitor or an optical input port monitor, depending on its function or role in the add-drop switch  110 ,  210 . 
     Switch cell output monitors are used to implement closed-loop control of the switch cells  120 ,  220 . Each switch cell  120 ,  220  includes a switch cell control circuit to adjust the switching function, e.g., adjusting the adjustable phase shifter(s) within the switch cell to achieve a desired maximum or minimum optical signal power on each output port. Thus, the switch cells  120 ,  220  can be used as a space switch, to direct light in a desired lightpath through the switch circuit. A switch cell output monitor is usually placed to monitor the output power of an output arm of the switch element which performs the switching function in the switch cell  120 ,  220 , e.g., Mach-Zehnder interferometers, and is fed back to the switch cell control circuit of the switch cell to provide closed-loop control. With the Mach-Zehnder interferometer as the switch element, the transfer function with current in and optical power out on each output port is intrinsically analog, but it performs a digital switching role using the closed-loop control assisted by the switch cell output monitors. 
     Optical input port monitors are used for optical inputs of the PIC, which can verify the existence or power level of incoming optical signals arriving at the PIC. This is used for system calibration and fault detection and for where line input fiber comes onto the die, or for where input fiber from Tx comes onto the die. Therefore, for the drop switch  100 , there is usually an optical input port monitor for each optical line input port  108 . For the add switch  200 , there is usually an optical input port monitor for each input from the client transmitter Tx  216  and for each optical transit input port  218 . 
     The last tap-monitor (or any intermediate tap-monitor) before each optical output of the PIC can also be used for optical power leveling, so that each optical output port has a desired optical power level relative to the other optical output ports. 
     In the embodiment illustrated by way of example in  FIG. 1 , the drop switch  110  includes a number of switch cell output monitors  122 , each being used for closed-loop control of an associated photonic switch cell  120 ; and a number of optical input port monitors  124 , each being used for verification of light power arriving at the drop switch  110 . According to the embodiment, the plurality of switch cell output monitors  122  are physically laid out in their associated photonic switch cells  120 , and the plurality of optical input port monitors  124  are physically laid out at associated optical input ports  108 . 
     As apparent from the description, in conventional designs when the light direction is reversed for designing an add transponder aggregator, the physical layouts of tap-monitors, more specifically the physical layouts of the optical input port monitors and the switch cell output monitors would have to be changed. 
     In comparison, in the embodiment illustrated by way of example in  FIG. 2 , tap-monitors  222 ,  224  are physically laid out in the add switch  210  in the same manner as in the drop switch  110 . But when performing the add role, each tap-monitor  222  is given the control role or function of a switch cell output monitor, which is done by setting the association of the respective tap-monitor  222  to its corresponding nearest upstream switch cell  220 , shown by dotted line  230 . Similarly, when performing the add role, each tap-monitor  224  is given the control role or function of an optical input port monitor, by setting the association of the respective tap-monitor  224  to its corresponding nearest upstream device, be it optical transit input port  218  or the transmitter Tx  216 . 
     Accordingly, an add-drop switch  110 ,  210  is provided with the same physical layout of the tap-monitors  122 ,  124 ,  222 ,  224 , that can be used for either the add role or the drop role, where the functions or control roles of the tap-monitors  122 ,  124 ,  222 ,  224  can be set or changed during network installation or during network reconfiguration. 
     The transponder aggregator  100 ,  200  includes a controller (or control circuit)  150 ,  250  configured to set or change the control roles of the tap-monitors  122 ,  124 ,  222 ,  224 . The controller  150 ,  250  may include a closed-loop control cell for each switch cell  120 ,  220 , and an overall system state controller to perform monitoring and control of the state of every switch cell  120 ,  220  in the switch  110 ,  210 . The controller  150 ,  250  is configured to set or change the control roles of the tap-monitors  122 ,  124 ,  222 ,  224  by associating the tap-monitors to their respective devices, in the add-drop switch  110 ,  210 . 
     According to various embodiments, the controller  150 ,  250  provides the capability to change the control roles of the tap-monitors  122 ,  124 ,  222 ,  224  depending on whether to perform the add role or the drop role. The changing of the control roles of the tap-monitors  122 ,  124 ,  222 ,  224  may include changing a switch cell output monitor from being associated with one switch cell  120 ,  220  to another switch cell  120 ,  220 , or switching one tap-monitor  122 ,  124 ,  222 ,  224  between being a switch cell output monitor and being an optical input port monitor. For the purposes of this specification, a control role of a tap-monitor  122 ,  124 ,  222 ,  224  includes the tap-monitor acting as a switch cell output monitor for a corresponding switch cell  120 ,  220 , or acting as an optical input port monitor for a corresponding optical line input port  108 , optical transit input port  218  or client input  216 . The association of a tap-monitor  122 ,  124 ,  222 ,  224  with its corresponding control role includes feeding the tap-monitor output to the corresponding switch cell  120 ,  220 , optical line input port  108 , optical transit input port  218  or client input  216  to complete the feedback loop. 
     Depending on how the transponder aggregator is constructed the controller or control circuit  150 ,  250  can be realized either through software or firmware in digital electronics, or through a flip-chip attached smart pixel control chip made in e.g. CMOS. In both ways, a global switch, e.g., an electrical switch is provided that is selectable between two states, one state for the add role, and the other state for the drop role. The control roles of the tap-monitors  122 ,  124 ,  222 ,  224  may be a Boolean state, and the global switch is provided for the entire circuit  110 ,  210 . 
     If the control circuit  150 ,  250  is implemented using software or firmware in digital electronics, for example, by way of an external controller such as a field programmable gate array (FPGA), central processing unit (CPU), complex programmable logic device (CPLD) or other programmable circuit, the control circuit may include a mapping or look-up table in the firmware or software in digital electronics that maps the tap-monitors  122 ,  124 ,  222 ,  224  to their respective control roles depending on whether to perform the add role or the drop role. An instruction on a digital controller may be sent to implement the change of control roles of the tap-monitors  122 ,  124 ,  222 ,  224 . 
     If the control circuit  150 ,  250  is implemented using analog electronic feedback, then a smart pixel control circuit may be modified to connect current from the photodetector  304  of each tap monitor  300 , or voltage from the transimpedance amplifier  314  of each tap-monitor  300 , to a corresponding one of two possible feedback circuits. 
     In such embodiments, the control circuit  150 ,  250  may be a flip-chip attached smart pixel control chip made in e.g. CMOS. Each switch cell  120 ,  220  may have an associated CMOS micro-control cell, having the same area footprint as the switch cell  120 ,  220  and implemented on one or a few CMOS chips that are attached to the top or bottom surface of the photonic chip  110 ,  210 , by means of die stacking technology (e.g. flip-chip technology or through substrate via technology). In this case, there is no firmware or software that can support a mapping table. Instead, the tap-monitor current or voltage is physically connected to the associated CMOS micro-control cell. 
     In one embodiment, a current-steering circuit is connected to the output of the photodetector  304  of the tap-monitor  300 . The current-steering circuit includes a current directing gate made of transistors, having one input from the tap-monitor  300  and two outputs, one output being used when the design is implemented as an add switch and the other output being used when the design is implemented as a drop switch. A single selector electrode is connected in common to all current directing gates, and the state of the selector electrode determines whether the whole circuit functions as an add switch or as a drop switch. 
     Similarly, in an alternative embodiment, a voltage-steering circuit is connected to the output of the photodetector electrical receiving amplifier, such as a transimpedance amplifier  314 , to connect to two possible feedback circuits. A single selector electrode is connected in common to all voltage directing gates, and the state of the selector electrode determines whether the whole circuit functions as an add switch or as a drop switch. 
     In the embodiments as shown in  FIGS. 1 and 2 , each of the tap-monitors  122 ,  124 , for the drop switch  110  are physically laid out within their associated switch cells  120  or at their associated optical input ports  108 . In an alternative embodiment, each tap-monitors  222 ,  224  for the add switch  210  may be physically laid out within their associated switch cells  220  or at their associated optical input ports  218  and client inputs  216 . In yet another alternative embodiment, tap-monitors  122 ,  124 ,  222 ,  224  may be physically laid out in some other manner, where they are not physically close to their associated switch cells  120 ,  220 , or associated optical input ports  108 ,  218 ,  216  for either drop switch or add switch. It should be understood that various physical layouts of the tap-monitors  122 ,  124 ,  222 ,  224  become possible with the controller or control circuit  150 ,  250  that is adapted to change the control roles of the tap-monitors. 
     In operation, to configure a PIC  110 ,  210  during network installation to implement either an add switch  210  or a drop switch  110 , the following steps are performed. A direction of operation is determined depending on whether the PIC  110 ,  210  is to be used for an add transponder aggregator  200  or a drop transponder aggregator  100 . According to the direction of operation, optical inputs and optical outputs of the PIC can be determined and connected. A global switch may then be set to select between a first state and a second state to indicate whether the PIC is to be used for an add transponder aggregator or for a drop transponder aggregator. 
       FIG. 5  illustrates a method for providing an optical transponder aggregator, according to one embodiment of the invention. The method includes providing, at step  510 , a plurality of photonic switch cells  120 ,  220  configured to collectively implement switching functions for either an add transponder aggregator or a drop transponder aggregator; and providing, at step  520 , a plurality of optical tap-monitors  122 ,  124 ,  222 ,  224  for control of the photonic switch cells  120 ,  220 . 
     The method includes further controlling the tap-monitors  122 ,  124 ,  222 ,  224  to select between a first set and a second set of control roles of the switch cells,  120 ,  220 . The first set of control roles corresponding to when the plurality of switch cells  220  collectively implements switching functions for the add transponder aggregator  200 , and the second set of control roles corresponding to when the plurality of switch cells  120  collectively implements switching functions for the drop transponder aggregator  100 . According to various embodiments, the control roles are a Boolean state, which is set by a global switch selectable between a first state and a second state, wherein the first state causes the tap-monitors  222 ,  224  to select the first set of roles, and the second state causes the optical tap-monitors  122 ,  124  to select the second set of roles. The Boolean state can be set by the state of an electrical control pin or by an instruction on a digital controller. There may be one pin or one instruction for the whole PIC  110 ,  210 . 
     According to the various embodiments described above, the add or drop role is set during network installation or reconfiguration. However, it should be appreciated that the design enables the circuit to be switched during operation from operating in one direction to operating in the other direction. Accordingly, the directionality (i.e. whether the transponder aggregator performs an add role or a drop role) may be changed during operation of the network, and the electrical selection may be reset. Switching direction during operation may involve physically exchanging the Tx and Rx equipment because the Tx function is different than the Rx function. 
       FIG. 6  illustrates a DWDM ring showing a typical application of the transponder aggregator  100 ,  200 , according to one embodiment of the present invention. Each transponder may connect banks of Rx and/or banks of Tx to WDM line fibers. While six wavelengths λ1-λ6 are illustrated in the example, it should be understood that the transponder aggregator is not limited by the number of the wavelengths or the number of inputs and outputs. 
     In a DWDM ring, each transponder aggregator  100 ,  200  is used only for a single direction (depending on whether it performs the add role or drop role), and not in both directions at the same time. 
     It should however be appreciated that the design can be modified such that different paths through the die can operate in different directions at the same time. The modified design can be used in a network structure where light in both directions shares one fiber, for example, in a PON or WDM PON network. In such applications, each switch cell only handles light from one direction at any given moment, and the switch cell does not share a lightpath of both directions. 
     According to the various embodiments described above, the number of tap-monitors are not increased compared to the conventional designs, but their control roles can be changed depending on the direction of the light. 
     In another less compact embodiment, separate backward-facing tap-monitors may be added at the other end of each switch cell (e.g., left end of switch cell in the embodiment of  FIG. 1 ) or at the optical outputs of the switch circuit. Creating separate forward-facing tap-monitors and backward-facing tap-monitors, the same switch chip can perform both the add role and the drop role. The two photodetectors of the two tap-monitors can connect in parallel to one transimpedance amplifier, or can connect to different monitor circuits. The controller will be adapted to connect the correct output to the feedback circuit. 
     However, compared to the embodiments described above which do not increase the number of tap-monitors, this particular embodiment may result in an increase in chip area and in the number of electrical receiver circuits. As well, this embodiment may increase optical loss because each lightpath goes through double the number of tap-monitors, regardless of the direction of light propagation. 
     Any of the methods disclosed herein may be implemented in hardware, software, firmware or any combination thereof. Where implemented as software, the method steps, acts or operations may be programmed or coded as computer-readable instructions and recorded electronically, magnetically or optically on a fixed, permanent, non-volatile or non-transitory computer-readable medium, computer-readable memory, machine-readable memory or computer program product. In other words, the computer-readable memory or computer-readable medium comprises instructions in code which when loaded into a memory and executed on a processor of a computing device cause the computing device to perform one or more of the foregoing method(s). 
     A computer-readable medium can be any means that contain, store, communicate, propagate or transport the program for use by or in connection with the instruction execution system, apparatus or device. The computer-readable medium may be electronic, magnetic, optical, electromagnetic, infrared or any semiconductor system or device. For example, computer executable code to perform the methods disclosed herein may be tangibly recorded on a computer-readable medium including, but not limited to, a CD-ROM, DVD, RAM, ROM, EPROM, Flash Memory or any suitable memory card or data-storage device. The method may also be implemented in hardware. A hardware implementation might employ discrete logic circuits having logic gates for implementing logic functions on data signals, an application-specific integrated circuit (ASIC) having appropriate combinational logic gates, a programmable gate array (PGA), a field programmable gate array (FPGA), or equivalent. 
     It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a device” includes reference to one or more of such devices, i.e. that there is at least one device. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of examples or exemplary language (e.g., “such as”) is intended merely to better illustrate or describe embodiments of the invention and is not intended to limit the scope of the invention unless otherwise claimed. 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.