Abstract:
Programmable wavelength line switches and routers based on a complementary wavelength switch (CWS) building block are described for switching optical signals of different wavelengths between signal lines, with each carrying multiple wavelengths. The CWS building block is based on a complementary bandpass filter structure. The reconfigurable wavelength routers described allow any of a plurality of wavelengths on any line to be switched to any output line by programming the filters accordingly. The various implementations described are useful for wavelength division multiplexing (WDM), dense WDM, and ultra dense WDM optical communications systems, as well as for on-chip interconnects and optical signal processing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of optical communications, and more specifically to apparatus and methods related to optical wavelength switching. 
     BACKGROUND INFORMATION 
     In Wavelength Division Multiplexing (WDM) optical communications networks, optical signals are transmitted at predetermined wavelengths in which each wavelength forms a communication channel in the network and the wavelength of the optical signal is used to control the destination of the signal through the network. In Dense Wavelength Division Multiplexing (DWDM) networks, the number of wavelength channels is increased by reducing the channel wavelength separation. In a standard DWDM network, the separation between communication channels is 100 GHz, and 50 GHz in more advanced systems. 
     In WDM and DWDM networks, switches are used to select paths for optical signals through the optical fibers forming the networks, i.e., to direct optical signals from one optical fiber to another and from one wavelength channel to another. Switches tend to be large and complex, interconnecting many inputs and outputs over multiple wavelengths. 
     One type of switch implementation, such as arrayed waveguide grating (AWG) implementations, entails complicated routing and wavelength sequencing. Moreover, for AWG implementations with large numbers of inputs and outputs, it is not clear that such implementations are truly reconfigurable on any wavelength and between any input and any output. 
     SUMMARY OF THE INVENTION 
     In an exemplary embodiment, the present invention provides an optical 2×2 complementary wavelength switch comprising an optical bandpass filter. In operation, the switch can selectively switch optical signals of two different wavelengths at either of two inputs to either of two outputs. The 2×2 complementary wavelength switch of the present invention can be used as a building block for more complex wavelength switches, with larger numbers of inputs and outputs, and/or capable of handling larger numbers of wavelengths. Wavelength overlap is not a concern because the operation of the switch in the frequency domain is complementary. 
     Moreover, the switch of the present invention enjoys fine wavelength resolution and can be implemented on a single substrate using, for example a complementary metal oxide semiconductor (CMOS) process. 
     The present invention has wide applicability, including, for example in Wavelength Division Multiplexing (WDM), dense WDM and ultra dense WDM switching applications. 
     The aforementioned and other features and aspects of the present invention are described in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an exemplary embodiment of a two-by-two complementary wavelength switch (CWS) in accordance with the present invention. 
         FIGS. 2A and 2B  illustrate a first switching configuration of the CWS of  FIG. 1 . 
         FIGS. 3A and 3B  illustrate a second switching configuration of the CWS of  FIG. 1 . 
         FIGS. 4A and 4B  illustrate a third switching configuration of the CWS of  FIG. 1 . 
         FIGS. 5A and 5B  illustrate a fourth switching configuration of the CWS of  FIG. 1 . 
         FIG. 6  is a schematic representation summarizing the possible switching configurations of the 2×2 CWS of  FIG. 1 . 
         FIG. 7A  is a schematic representation of an exemplary embodiment of a 2×2 wavelength switch for handling four wavelengths;  FIG. 7B  is a schematic representation of a four-wavelength configurable switch for use in the 2×2 four-wavelength switch of  FIG. 7A  as a 1×2 splitter;  FIG. 7C  is a schematic representation of a four-wavelength configurable switch for use in the 2×2 four-wavelength switch of  FIG. 7A  as a 2×1 combiner; and  FIG. 7D  is a table listing the combinations of wavelengths at crossing points of the 2×2 four-wavelength switch of  FIG. 7A . 
         FIG. 8  is a schematic representation of an exemplary embodiment of a 4×4 wavelength router comprising multiple 2×2 CWSs. 
         FIG. 9  is an exemplary butterfly interconnect diagram illustrating one of a plurality of switching configurations of the 4×4 wavelength router of  FIG. 8 . 
         FIG. 10  is a schematic representation of an exemplary embodiment of an 8×8 wavelength router comprising multiple 2×2 CWSs. 
         FIG. 11  is an exemplary butterfly interconnect diagram illustrating one of a plurality of switching configurations of the 8×8 wavelength router of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic representation of an exemplary embodiment of a two-input, two-output or “2×2” complementary wavelength switch (CWS)  100  in accordance with the present invention. The CWS  100  comprises a complementary optical bandpass filter structure comprising a first coupler  101  with input arms receiving the inputs of the CWS  100 , In 1  and In 2 . The first coupler  101  has a first output arm connected to a first series arrangement including a phase shifter  111 , a ring resonator  121 , and a ring resonator  122 , and a second output arm connected to a second series arrangement including a phase shifter  112 , a ring resonator  123 , and a ring resonator  124 . The two series arrangements are connected to respective input arms of a second coupler  102  with output arms providing the outputs of the CWS  100 , Out 1  and Out 2 . 
     The ring resonators  121 - 124  can be implemented in a conventional manner, each with a thermally controlled phase shifter and a thermally controlled coupler, allowing the CWS  100  to be controlled to provide various switching configurations, as described below. Tuning the resonance frequencies of the ring resonators  121 - 124  will alter the phase therethrough which can be compensated for using the phase shifters  111  and  112  to equalize the total phase through the two arms of the filter structure. 
     It should be noted that while a four-ring filter structure is used in the exemplary embodiment shown, other filter configurations can be used depending on the desired filter characteristics to be achieved. For example, the four-ring structure produces a four-pole filter and can provide, for example, at most 1 dB loss at the center of the passband and at least 30 dB suppression with 50 GHz wavelength channel spacing. If, however, the two wavelengths are spaced further apart and the filter characteristics need not be as sharp, then a configuration with one ring resonator in each arm may be satisfactory. Conversely, for sharper cut-offs with closer separation of wavelengths, a configuration with six or more rings may be needed to achieve the desired filter characteristic. 
     The 2×2 CWS  100  can be controlled to provide any one of four switching configurations for two different wavelengths, λ 1  and λ 2 , at its two inputs, In 1  and In 2 , to its two outputs, Out 1  and Out 2 .  FIGS. 2A and 2B  illustrate a first such configuration in which the bandpass filter structure of CWS  100  is tuned to λ 1 . As shown in  FIG. 2A , optical signals of wavelengths λ 1  and λ 2  applied at In 1 , will be subjected to the filter characteristics shown so that the signal of wavelength λ 1  will be output at Out 2 , and the signal of wavelength λ 2  will be output at Out 1 . Conversely, the signal of wavelength λ 1  will be suppressed at Out 1 , and the signal of wavelength λ 2  will be suppressed at Out 2  for optical signals of wavelengths λ 1  and λ 2  applied at In 1 . As shown in  FIG. 2B , optical signals of wavelengths λ 1  and λ 2  applied at In 2 , will be subjected to the filter characteristics shown so that the signal of wavelength λ 1  will be output at Out 1 , and the signal of wavelength λ 2  will be output at Out 2 . Conversely, the signal of wavelength λ 1  will be suppressed at Out 2 , and the signal of wavelength λ 2  will be suppressed at Out 1  for optical signals of wavelengths λ 1  and λ 2  applied at In 2 . As such, the configuration of  FIGS. 2A and 2B  provides cross-over routing of wavelength λ 1  from In 1  to Out 2  and In 2  to Out 1 , and pass-through routing of wavelength λ 2  from In 1  to Out 1  and In 2  to Out 2 . 
       FIGS. 3A and 3B  illustrate a second switching configuration in which the bandpass filter structure of CWS  100  is tuned to λ 2 . As shown in  FIG. 3A , optical signals of wavelengths λ 1  and λ 2  applied at In 1 , will be subjected to the filter characteristics shown so that the signal of wavelength λ 1  will be output at Out 1 , and the signal of wavelength λ 2  will be output at Out 2 . Conversely, the signal of wavelength λ 1  will be suppressed at Out 2 , and the signal of wavelength λ 2  will be suppressed at Out 1  for optical signals of wavelengths λ 1  and λ 2  applied at In 1 . As shown in  FIG. 3B , optical signals of wavelengths λ 1  and λ 2  applied at In 2 , will be subjected to the filter characteristics shown so that the signal of wavelength λ 1  will be output at Out 2 , and the signal of wavelength λ 2  will be output at Out 1 . Conversely, the signal of wavelength λ 1  will be suppressed at Out 1 , and the signal of wavelength λ 2  will be suppressed at Out 2  for optical signals of wavelengths λ 1  and λ 2  applied at In 2 . As such, the configuration of  FIGS. 3A and 3B  provides pass-through routing of wavelength λ 1  from In 1  to Out 1  and In 2  to Out 2  and cross-over routing of wavelength λ 2  from In 1  to Out 2  and In 2  to Out 1 . 
       FIGS. 4A and 4B  illustrate a third switching configuration in which the tuning and coupling of the bandpass filter structure of CWS  100  are turned off. As shown in  FIG. 4A , optical signals of wavelengths λ 1  and λ 2  applied at In 1 , will be subjected to the filter characteristics shown so that the signal of wavelength λ 1  will be output at Out 1 , and the signal of wavelength λ 2  will be output at Out 1 . Conversely, the signal of wavelength λ 1  will be suppressed at Out 2 , and the signal of wavelength λ 2  will be suppressed at Out 2  for optical signals of wavelengths λ 1  and λ 2  applied at In 1 . As shown in  FIG. 4B , optical signals of wavelengths λ 1  and λ 2  applied at In 2 , will be subjected to the filter characteristics shown so that the signal of wavelength λ 1  will be suppressed at Out 1 , and the signal of wavelength λ 2  will be suppressed at Out 1 . Conversely, the signal of wavelength λ 1  will be output at Out 2 , and the signal of wavelength λ 2  will be output at Out 2  for optical signals of wavelengths λ 1  and λ 2  applied at In 2 . As such, the configuration of  FIGS. 4A and 4B  provides pass-through routing of both wavelengths from In 1  to Out 1  and In 2  to Out 2 . 
       FIGS. 5A and 5B  illustrate a fourth switching configuration in which the tuning and one of the two couplers ( 101 ,  102 ) of the bandpass filter structure of CWS  100  are turned off, while the other of the two couplers ( 101 ,  102 ) provides full coupling. As shown in  FIG. 5A , optical signals of wavelengths λ 1  and λ 2  applied at In 1 , will be subjected to the filter characteristics shown so that the signal of wavelength λ 1  will be output at Out 2 , and the signal of wavelength λ 2  will be output at Out 2 . Conversely, the signal of wavelength λ 1  will be suppressed at Out 1 , and the signal of wavelength λ 2  will be suppressed at Out 1  for optical signals of wavelengths λ 1  and λ 2  applied at In 1 . As shown in  FIG. 5B , optical signals of wavelengths λ 1  and λ 2  applied at In 2 , will be subjected to the filter characteristics shown so that the signal of wavelength λ 1  will be output at Out 1 , and the signal of wavelength λ 2  will be output at Out 1 . Conversely, the signal of wavelength λ 1  will be suppressed at Out 2 , and the signal of wavelength λ 2  will be suppressed at Out 2  for optical signals of wavelengths λ 1  and λ 2  applied at In 2 . As such, the configuration of  FIGS. 5A and 5B  provides cross-over routing of both wavelengths from In 1  to Out 2  and In 2  to Out 1 . 
     Note that each pair of figures described above (i.e.,  FIGS. 2A  and B,  3 A and B,  4 A and B, and  5 A and B) pertain to the same filter configuration. Two figures are used for each configuration to illustrate the two different (but complementary) filter characteristics between each of the two inputs and the two outputs. 
     As can be seen from the various configurations described above, the exemplary CWS  100  of the present invention allows two identical sets of wavelengths from two input ports to be selected with arbitrary combinations for routing to two output ports without overlapping or interference. Note that in each configuration, the filter characteristics at the inputs are complementary. In other words, the filter characteristic to which a signal applied at In 1  is subjected is complementary to that to which a signal applied at In 2  is subjected. 
     The various configurations of the CWS  100  described above are summarized schematically in  FIG. 6 . As depicted in  FIG. 6 , the CWS  100  can be configured so that signals of either wavelength λ 1  or λ 2  applied at either input, In 1  and In 2 , can be switched to either output, Out 1  and Out 2 , independently. As shown in  FIG. 6 , the solid lines represent cross-over switching from In 1  to Out 2  or In 2  to Out 1  and the dotted lines represent straight-through routing from In 1  to Out 1  or In 2  to Out 2 . 
     The CWS  100  can be used as a unit cell or building block to form more complex switching structures. For example, as shown in  FIG. 7A , an exemplary embodiment of a 2×2 four-wavelength switch  700  for switching four wavelengths between two inputs (In 1 , In 2 ) and two outputs (Out 1 , Out 2 ) can be implemented using 2×2 CWSs  710  and  720 , implemented as described above. The four-wavelength switch  700  also comprises two four-wavelength 1×2 splitters  721  and  722 , and two four-wavelength 2×1 combiners  723  and  724 . Each of the 1×2 splitters  721  and  722  can be implemented with a complementary bandpass filter structure such as that shown in  FIG. 7B . The complementary bandpass filter structure of  FIG. 7B  is similar to that shown in  FIG. 1  but tuned to handle four wavelengths. The 2×1 combiners  723  and  724  can be implemented using the complementary bandpass filter structure of  FIG. 7C . Note that the combiner structure of  FIG. 7C  can be implemented as a mirror image of the splitter structure of  FIG. 7B . 
       FIG. 7D  shows a table listing the combinations of wavelengths at crossing points b 1 , b 2 , b 3 , b 4 , c 1 , and c 2 , of the switch  700  of  FIG. 7A . The 2×2 four-wavelength switch  700  is able to select any combination of four identical wavelength signals from two lines without causing interference. In other words, the switch  700  will not allow signals of the same wavelength from different inputs to go to the same output. 
     Even more complex wavelength switching structures can be readily implemented by scaling-up the exemplary arrangement of  FIG. 7A  with 2×2 CWSs as building blocks, in accordance with the present invention. For example, a 2×2 switch that can handle eight wavelengths can be implemented using two one-to-four splitters, four 2×2 complementary wavelength switches and two four-to-one combiners, etc. The one-to-four wavelength splitters and four-to-one combiners can be implemented with a complementary bandpass filter structure, similar to that shown in  FIG. 7B , but tuned accordingly to accommodate the additional wavelengths. In this fashion, 2×2 wavelength switches with two inputs and two outputs handling 2, 4, 8, 16, . . . , 2 n  wavelengths can be implemented. 
     In addition to implementing 2×2 switches of greater wavelength handling complexity, the 2×2 CWS building block of the present invention can be used to implement more complex switching and routing systems with more inputs and outputs. For example,  FIG. 8  shows a schematic representation of an exemplary embodiment of a 4×4, two-wavelength router  800  comprising multiple 2×2 CWSs  801 - 806 . Each of the CWSs  801 - 806  can be implemented as shown in  FIG. 1 . 
     Note that for 4×4 implementations handling four wavelengths, each of the 2×2 two-wavelength CWSs  801 - 806  can be replaced by a four-wavelength 2×2 switch such as that shown in  FIG. 7A . In similar fashion, routers of greater wavelength handling ability can be implemented with 2×2 switches of commensurate wavelength handling ability. As described above, each of the 2×2 CWS can be designed to handle 2, 4, 8, . . . , 2 n  numbers of wavelengths. 
       FIG. 9  is an exemplary 4×4 butterfly interconnect diagram applicable to the 4×4 wavelength router of  FIG. 8 . 
       FIG. 10  shows a schematic representation of an exemplary embodiment of an 8×8 router  1000  comprising multiple 2×2 CWSs  1001 - 1020 . Each of the CWSs  1001 - 1020  can be implemented as shown in  FIG. 1 , for a two-wavelength implementation, or can be scaled-up to handle 2, 4, 8, . . . , 2 n  numbers of wavelengths, as described above. 
       FIG. 11  is an exemplary 8×8 butterfly interconnect diagram applicable to the 8×8 wavelength router of  FIG. 10 . The wavelength routings that prevent congestion (i.e. same wavelengths at different inputs connecting to the same output) are controlled like a conventional butterfly interconnect and applied on each of the wavelengths. Thus, each wavelength can be switched on its own following its own butterfly interconnecting rules. Systems with larger numbers of inputs and outputs can be expanded accordingly. The present invention thus provides a switching building block and associated architecture that is readily scalable. 
     Router implementations in accordance with the present invention can be particularly useful in integrated platforms. Moreover, such routers can be implemented with minimal waveguide crossings as compared to conventional approaches which use wavelength splitters for each input and which route the outputs with the same wavelength from the splitters to a dedicated switch for each wavelength and then decide to which output port each wavelength should go. With such conventional implementations, the number of waveguide crossings grows tremendously as the number of input and output ports increases. 
     It is understood that the above-described embodiments are illustrative of only a few of the possible specific embodiments which can represent applications of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.