Abstract:
A four-port wavelength-selective crossbar switch generates an add/drop wavelength signal from a wave division multiplexed (WDM) signal using a plurality of double-sided reflectors that selectively reflects a selected wavelength channel signal of the WDM signal through optical circulators to provide low crosstalk between the dropped and added wavelength signals. The switch also reduces the number of WDM MUX-DEMUX required to one half that compared to a traditional approach. Furthermore, the switch can be designed to be wavelength cyclic with individual free spectral ranges that can be independently set to either through or add/drop states.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application claims priority to provisional U.S. Application Ser. No. 60/276,485, entitled “Four-Port Wavelength-Selective Crossbar Switches (4WCS) Using Reciprocal WDMs and Optical Circulator Combination,” invented by Mark D. Feuer et al., filed Mar. 19, 2001, and is incorporated by reference herein. Additionally, the present application is related to provisional U.S. Patent Application Ser. No. 60/276,495, entitled “Delivering Multicast Services On A Wavelength Division Multiplexed Network Using a Configurable Four-Port Wavelength Crossbar Switch” invented by Mark D. Feuer et al., filed Mar. 19, 2001, and to U.S. patent application Ser. No. ______ (Atty Docket IDS  2000 - 502 ), entitled “Delivering Multicast Services On A Wavelength Division Multiplexed Network Using a Configurable Four-Port Wavelength Selective Crossbar Switch,” invented by Mark D. Feuer et al., filed concurrently with the present application, and each of which is incorporated by reference herein.  
       FIELD OF THE INVENTION  
       [0002]     The invention relates to wavelength division multiplexed (WDM) signals. More particularly, the present invention relates to a crossbar-type switch for generating an added and dropped wavelength signals having low crosstalk between the dropped and added wavelength signals.  
       DESCRIPTION OF THE RELATED ART  
       [0003]      FIG. 1  shows a functional block diagram of a conventional four-port wavelength-selective crossbar switch (4WCS)  100 . Input wavelengths λ 1 , λ 2 , . . . , λ N  are demultiplexed first by a wavelength demultiplexer  101 , which can be formed by, for example, cascaded thin film filters, fiber Bragg gratings, or arrayed waveguide gratings. The demultiplexed signals are connected through an array of 2×2 crossbar switches  105  to a multiplexer  103  prior to the drop port or an output multiplexer  104 . Crossbar switches  105  also connect the wavelengths corresponding to the dropped wavelengths from the add port to output multiplexer  104 . The wavelengths that are to be added/dropped are selected by controlling the respective states of the crossbar switches.  
         [0004]     A critical problem with a conventional 4WCS, such as shown in  FIG. 1 , is the potential for optical crosstalk in the 2×2 crossbar switches  105 , thereby causing an unwanted portion of the dropped signal to coherently interfere with an added signal. The present invention provides a different implementation of a 4WCS switch and still having the functionality shown in  FIG. 1 .  
         [0005]     Consequently, what is needed is a technique for adding/dropping optical signals from a WDM signal that effectively eliminates optical crosstalk between dropped and added optical signals.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a technique for adding/dropping optical signals from a WDM signal that effectively eliminates optical crosstalk between dropped and added optical signals. The advantages of the present invention are provided by an output optical circulator to the output end reciprocal WDM MUX-DEMUX. Each double-sided reflector is disposed in a path of a selected wavelength channel signal between the optical demultiplexer and the optical multiplexer, and is selectably operated so that in a first mode of operation a first side of the double-sided reflector reflects a selected wavelength channel signal corresponding to the wavelength channel signal path in which the double-sided reflector is disposed back to the second port of the input optical circulator. A second side of the doubled-sided reflector in the first mode of operation reflects an add signal having at least one wavelength corresponding to the wavelength channel signal path in which the double-sided reflector is disposed back to the second port of the output optical circulator. The selected reflected wavelength channel signal can be modulated with, for example, multicast data (as described in the provisional U.S. Patent Application Ser. No. 60/276,495, entitled “Delivering Multicast Services on a Wavelength Division Multiplexed Network Using a Configurable Four-Port Wavelength Crossbar Switch), and coupled to the add port of the output optical circulator. In a second mode of operation, each double-sided reflector allows the selected wavelength channel signal corresponding to the wavelength channel signal path in which the double-sided reflector is disposed to pass from the optical demultiplexer to the optical multiplexer. In one embodiment of the present invention, at least one double-sided reflector is a micro-electro-mechanical-system (MEMS) mirror. In an alternative embodiment of the invention, the double-sided reflector is a mechanical anti-reflection switch (MARS). In yet another alternative embodiment, the double-sided reflector is a reflective thin-film interference filter. In a further embodiment, a series of reflective thin-film interference filters corresponding to different FSRs are used in place of the double-sided reflective mirrors. This embodiment allows wavelengths corresponding to different FSRs in each wavelength channel signal to be independently set to the bar (through) or cross (add/drop) state.  
         [0007]     The present invention also reduces the number of WDM MUX-DEMUXs required to achieve the same function by a factor of two compared with the conventional approach. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention is illustrated by way of example and not by limitation in the accompanying figures in which like reference numerals indicate similar elements and in which:  
         [0009]      FIG. 1  shows a functional block diagram of a conventional four-port wavelength-selective crossbar switch (4WCS);  
         [0010]      FIG. 2  shows a functional block diagram of a four-port wavelength-selective crossbar switch (4WCS) according to the present invention;  
         [0011]      FIG. 3  shows a functional block diagram of a four-port wavelength-selective crossbar switch (4WCS) according to the present invention having a free spectral range; and  
         [0012]      FIG. 4  shows a functional block diagram of a four-port wavelength-selective crossbar switch (4WCS)  400  according to the present invention that provides even more flexibility than other embodiments of the present invention without increasing the number of WGR ports. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     The present invention provides a configurable four-port wavelength-selective optical crossbar switch (4WCS) that is capable of dropping any subset of input wavelengths from the input port to a drop port. The same wavelengths dropped at the drop port can be added from an add port to the output port.  
         [0014]      FIG. 2  shows a functional block diagram of a four-port wavelength-selective crossbar switch (4WCS)  200  according to the present invention. Switch  200  includes an input optical circulator  201 , an input bidirectional wavelength demultiplexer  202  (which is the input end reciprocal WDM MUX-DEMUX), a bi-directional wavelength multiplexer  203  (which is the output end reciprocal WDM MUX-DEMUX), and an output optical circulator  204 . Input optical circulator  201  includes a “drop” port, and output optical circulator  204  includes an “add” port. Input demultiplexer  202  and output multiplexer  203  can each be a waveguide grating router (WGR) that separates the different wavelengths of a WDM signal into different channels, or arms, in a well-known manner. Optical circulators  201  and  204  separate the in-coming and outgoing waves, as described in detail below, and reduce the total number of WGR ports and devices to half in comparison to a conventional 4WCS, such as shown in  FIG. 1 . As opposed to the conventional approach, switch  200  operates in a unidirectional manner and is not reversible for bi-directional traffic within a single fiber.  
         [0015]     Switch  200  also includes a plurality of removable, double-sided optical reflectors  205   1 - 205   N  that are each respectively positioned so that an optical reflector can be inserted into a wavelength channel, or arm, between input demultiplexer  202  and output multiplexer  203 . Each reflector  205  provides extremely high isolation between an added and a dropped channel because the reflectivity and the optical thickness of an optical reflector  205  are preferably large. While the embodiment of the present invention shown in  FIG. 2  includes a reflector  205  for each wavelength channel, it should be understood that some wavelength channels might not include a reflector  205 . Accordingly, wavelengths in those channels can only go through switch  200  without being added or dropped.  
         [0016]     Reflectors  205  can use any design that is capable of switching from two-sided back-reflection to a full-transmitting state or mode of operation, that is, an “IN” state and an “OUT” state, respectively. Reflectors  205  can use, for example, micro-electro-mechanical-system (MEMS) technology for selectably inserting or removing a two-sided mirror from an optical beam in a well-known manner. Moreover, because both WGR devices and MEMS devices are fabricated on silicon substrates, WGR devices  202  and  203 , and removable reflectors  205  for an entire 4WCS switch according to the present invention can be fabricated on a single silicon chip.  
         [0017]     WGR devices  202  and  203  provide reciprocal operation, so when a reflector  205  is in the “IN” state, the wavelength corresponding to the reflector is reflected back to a WGR device (input demultiplexer  202  and output multiplexer  203 ), thereby causing a wavelength in a particular arm to be added/dropped. When a reflector  205  is in the “OUT” state, the wavelength corresponding to the reflector is set to the through state, or the express state, and the beam thereby passes through the corresponding arm. For example, when reflector  205   1  is set to the “IN” state, input wavelength λ 1  of an input WDM signal is reflected back through input demultiplexer  202  to input circulator  201 . (For this portion of the wavelength λ 1  signal path, input demultiplexer  202  operates as a multiplexer.) Reflected wavelength λ 1  travels clockwise around optical circulator  201  and is output from the drop port. Dropped wavelength λ, can be modulated with, for example, downstream data from another network node for the local node. Wavelength λ 1  can then be added back to the WDM signal through the add port of output optical circulator  204 . Wavelength λ 1  travels clockwise around output optical circulator  204  and is output from circulator  204  in a direction toward multiplexer  203  (which, for this portion of the signal path of wavelength λ 1 , operates as a demultiplexer). Wavelength λ 1  is reflected by reflector  205   1  back to output multiplexer  203  and is added back to the WDM signal. The added wavelength λ 1  can be modulated with, for example, upstream data from the local node to the next network node.  
         [0018]     There are many ways of implementing reflectors  205 . For example, reflectors  205  can be made similar to MEMS reflectors that are used in an optical MEMS cross-switch. That is, MEMS reflectors  205  can be flipped in a vertical or horizontal position, corresponding to the IN and OUT states of reflectors  205 . Alternatively, rather than physically moving a reflector out of a beam, a reflector may be altered internally so that the reflector becomes non-reflective at the wavelength of interest. Examples of this approach could include a mechanical anti-reflection switch (MARS) or devices that are based on a frustrated total internal reflection.  
         [0019]     Additional system capabilities are provided when an input demultiplexer and an output multiplexer are wavelength-cyclic, that is, have a filter response function that repeats over a period of wavelengths, which is called the free spectral range (FSR). A wavelength cyclic property can be designed into a waveguide grating router, Mach-Zehnder interferometers, Fabry-Perot filters etc., to provide a particular FSR. For example, when a WGR is wavelength cyclic, the output from port i will include wavelength and all wavelengths λ i +m×Λ, where m is an integer and Λ is the free spectral range. Accordingly, a single filter element can provide wavelength routing for many distinct wavelength channels. One important network application might be to use different FSRs for delivering different services and to further separate the different services at each node of an optical network using coarse optical filters.  
         [0020]      FIG. 3  shows a functional block diagram of a four-port wavelength-selective crossbar switch (4WCS)  300  according to the present invention having a free spectral range (FSR). Switch  300  includes an input demultiplexer  302  and/or an output multiplexer  303  that provide an FSR. The bottom of  FIG. 3  illustrates the optical spectrum of the input WDM signal and the FSR of the WDM MUX-DEMUX.  
         [0021]      FIG. 4  shows a functional block diagram of a four-port wavelength-selective crossbar switch (4WCS)  400  according to the present invention that provides even more flexibility than other embodiments of the present invention without increasing the number of WGR ports. Instead of including all-wavelength reflectors between the input demultiplexer and the output multiplexer, such as shown in  FIG. 2 , each reflector can be replaced with a series of reflective filters F, such as thin-film interference filters. For example, in  FIG. 4 , F 1 , F 2  and F 3  represent filters that reflect three independent FSRs and let other optical signals pass. Similar to a double-sided mirror, each filter can be independently set to the IN or OUT position. Consequently, each wavelength in every free spectral range can be independently added/dropped or passed through, extending the functionality and flexibility of the 4WCS.  
         [0022]     The advantage of added/dropped isolation of the alternative embodiment of  FIG. 4  is obtained at the expense of potential self-homodyne interference. The self-homodyne interference is due to imperfect filter reflectivities and scattering at the multiple filter surfaces. This complicated effect is not-related to the claims in the current invention and will not be further described here.  
         [0023]     While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims.