Abstract:
An optical switch system for dropping a ROADM node is presented. The switch system includes an N×M structure having two layers. A first layer includes optical splitters, each splitter receiving a multiplexed input signal and outputting a first multiplexed output signal. A second layer includes switches receiving the first multiplexed output signals from the optical splitters and generating a second multiplexed output signal. The second multiplexed output signal is typically one of the first multiplexed output signals. An optional third layer, which includes optical filters, receives the second multiplexed output signal from the switches and produces a non-multiplexed, single-wavelength output signal.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates generally to optical communication systems and more specifically to optical systems with reconfigurable optical add/drop multiplexers. 
       BACKGROUND 
       [0002]    Reconfigurable optical add-drop multiplexers (ROADMs) are a form of optical add-drop multiplexer that adds the ability to remotely and dynamically switch traffic from a wavelength-division multiplexed (WDM) system at the wavelength layer. ROADMs have a multitude of uses in optical systems. For example, ROADMs may be useful in the field of WDM light wave systems for selective broadcasting, dropping, and monitoring of discrete wavelengths. More specifically, ROADMs allow individual wavelengths carrying data channels to be added and dropped from a fiber without the need to convert the signals on all of the WDM channels to electronic signals and back again to optical signals. 
         [0003]    The flexibility of current ROADM systems is limited because the Drop end is not really directionless, colorless and contentionless. For example, ROADM cannot be configured to freely drop any wavelength from any input ports. A method and apparatus that would allow this type of configuring is desired. 
       SUMMARY 
       [0004]    In one aspect, the invention is an N×M optical switching system that includes N number of 1×M optical splitters and M number of N×1 switches. Each of the 1×M optical splitters receives an input signal, which is multiplexed, and outputs a plurality of first multiplexed output signal. The N×1 switches receive the first multiplexed output signals from the optical splitters and generate a second multiplexed output signal, wherein the second multiplexed output signal is one of the first multiplexed output signals. 
         [0005]    In another aspect, the invention includes a K×(N×M) switch that includes 1×K tunable splitters that pre-split an input signal before it feeds the split input signal to the N×M switching system above. 
         [0006]    In yet another aspect, the invention includes a method of switching optical systems by receiving N optical input signals that are multiplexed, and splitting each of the N optical input signals into M first multiplexed output signals having the same wavelengths as the optical input signals to generate N×M number of first multiplexed output signals. The first multiplexed output signals are fed into M optical switches, each of which selects one of the received multiplexed output signals to generate second multiplexed output signals. Optionally, wavelengths may be selectively dropped from the second multiplexed output signals, resulting in demultiplexed output signals of desired wavelengths. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1A  illustrates an embodiment of a 4×8 Switch Structure capable of producing single-wavelength output signals. 
           [0008]      FIG. 1B  illustrates another embodiment of a 4×8 Switch Structure that produces multiplexed output signals. 
           [0009]      FIG. 2  illustrates wavelengths entering and exiting one of the Tunable Splitters in the 4×8 Switch Structure of  FIG. 1A . 
           [0010]      FIG. 3  illustrates the function of an Optical Switch in the 4×8 Switch Structure of  FIG. 1A . 
           [0011]      FIG. 4  illustrates the function of a Tunable Filter in the 4×8 Switch Structure of  FIG. 1A . 
           [0012]      FIG. 5  depicts an embodiment of an expanded switch structure incorporating the 4×8 Switch Structure of  FIG. 1A . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    In the following description, reference is made to the accompanying drawings which illustrate different embodiments of the present invention. It is understood that other embodiments may be utilized and mechanical, compositional, structural, electrical, and operational changes may be made without departing from the spirit and scope of the present disclosure. The following detailed description is not to be taken in a limiting sense, and the scope of the embodiments of the present invention is defined only by the claims of the issued patent. 
         [0014]    It will be understood that when an element is referred to as being “on”, “connected to” or “coupled to” another element, it can be directly on, connected or coupled to the other element or intervening elements or layers may be present. 
         [0015]      FIG. 1A  shows an optical switch system  10  usable for dropping a ROADM node in an optical network. As shown, the optical switch system  10  has an N×M structure where N denotes the number of input ports and M denotes the number of output ports. In the embodiment of  FIG. 1A , N=4 and M=8. The switch system  10  has three stages: a first stage  20 , a second stage  30 , and a third stage  40 . The first stage  20  includes N number of 1×M optical splitters  22 . Each one of the optical splitters  22  receives a multiplexed input signal  24  and splits the multiplexed input signal  24  into M pieces of multiplexed first output signals  26 . The split ratio in the optical splitters  22  can be either fixed or adjustable (tunable).  FIG. 2 , described below, provides more details about each 1×M splitter  22   
         [0016]    Depending on the embodiment, a regular splitter may be used instead of a Tunable Splitter Tsp in the first stage  20 . A “tunable splitter,” as used herein, includes splitters that allow control over both the number of output ports and the portion of each output port. No wavelength selection is done by a tunable splitter. 
         [0017]    The second stage  30  includes M number of N33 1 switches  32 . The switches  32  receive the first output signals  26  that come out of the first stage  20 . Each switch  32  selects one of the four incoming signals  26  and forwards it to the third stage  40  as a second output signal  36 . Both the signals entering the second stage  30  and exiting the second stage  30  are multiplexed.  FIG. 3 , described below, provides more details about each N33 1 switch  32 . 
         [0018]    The third stage  40  includes a plurality of optical tunable filters  42 . The number of optical tunable filters  42  is the same as that of the N33 1 switches  32 . Each tunable filter  42  selects one wavelength from the received second output signal  36  and passes the selected wavelength out of the switch structure  10  in the form of switch structure output signal  46 . The optical switch system  10  re-routes or switches multiplexed input signals  24  that are fed into the N input ports into M number of single-wavelength (i.e., not multiplexed) switch structure output signals  46 . Different tunable filters  42  may output the same wavelength but these wavelengths originated from different input signals  24 .  FIG. 4 , described below, provides more details about each tunable filter  42 . 
         [0019]    The invention affords more flexibility to the Drop end of the ROADM system. Any wavelength fed into any input port can be freely selected and dropped to any output port. ROADM nodes in the network will become directionless, colorless and contentionless. 
         [0020]      FIG. 1B  illustrates another embodiment of a 4×8 Switch Structure. This switch system  10  of  FIG. 1B  is similar to the embodiment shown in  FIG. 1A  except that it produces multiplexed output signals. The switch system  10  has the first stage  20  and the second stage  30 , but no third stage  40 . Hence, this switch structure functions as an N×M switch but does not provide the wavelength selection option like the embodiment of  FIG. 1A . 
         [0021]      FIG. 2  illustrates wavelengths entering and exiting one of the Tunable Splitters in the 4×8 Switch Structure  10  of  FIG. 1A . In the particular example where M=8, the multiplexed signal  24  entering the 1×8 tunable splitter has 44 wavelengths λ 1  through λ 44 . In many cases, the input signals  24  entering the different tunable splitters in the first stage  20  all carry the same set of wavelengths. However, this is not a limitation of the invention and each port may carry different wavelengths, different number of wavelengths, or a different range of wavelengths as the other input ports. The number of wavelengths entering a single 1×8 splitter  22  is not limited to being 44, and this number could also be 1, i.e., single wavelength signals. 
         [0022]    As shown in  FIG. 2 , there are eight signals  26  exiting the Tunable splitter Tsp. Each of the eight signals  26  contains the same multiplexed wavelengths as the input signal  24  that was fed into the same Tunable splitter Tsp. 
         [0023]      FIG. 3  illustrates the function of an Optical Switch in the 4×8 switching system of  FIG. 1A . As there are four splitters in the first stage  20 , each switch  32  receives four first output signals  26 , one from each splitter. Each switch  32  selects one of the four incoming signals  26  (illustrated as signals a, b, c, and d in  FIG. 3 ) and forwards it to the third stage  40  as a second output signal  36 . In the example of  FIG. 3 , signal b is selected. Signal b is a multiplexed signal as no wavelength selection occurs in stage  30 . 
         [0024]      FIG. 4  illustrates the function of a Tunable Filter in the 4×8 switching system of  FIG. 1A . Exiting each Tunable Filter TF is a single wavelength from the multiplexed input signal  24 . Where different wavelengths are fed into the multiple tunable splitters Tsp, the output signal exiting one of the Tunable Filters TF may be a wavelength from a multiplexed input signal  24  that was fed into any one of the Tunable Filters TF. The Tunable Filters  24  receive multiplexed signals  36  and generate single-wavelength outputs (e.g., λ 3  in  FIG. 4 ). Prior to reaching the Tunable Filters  24 , any one of the four input signals  24  may be redirected to any one of the eight multiplexed wavelength signal paths. 
         [0025]      FIG. 5  shows an embodiment of a K×(N×M) optical switch structure  100  that offers even more flexibility to signal routing, and illustrates how the switching system of  FIG. 1A  can be combined and/or layered to suit an application. The embodiment shown in  FIG. 5  is substantially similar to that shown in  FIG. 1A , with a primary difference being the addition of a fourth stage  50  before the first stage  20 . In the particular embodiment of  FIG. 5 , K=4, such that there are four switch structures  10 . As shown, the fourth stage  50  “ties together” a plurality of switch structures  10 . The addition of the fourth stage  50  makes the optical switch structure  100  a K×(N×M) switch structure. The 1×4 splitter TSp is a Tunable Splitter Tsp. 
         [0026]    The fourth stage  50  includes a group of 1×4 tunable splitters  52 . Each one of the tunable splitters  52  receives an original signal  54  and splits the original signal  54  into up to  4  pieces or branches, depending on the tuning split ratio. If only one N×M switching structure  10  were used, then only one branch of each of the tunable splitters  52  will be set to pass while the others will be blocked to avoid unnecessary splitting. Similarly, if two N×M structures are needed, then two branches of each of the tunable splitters  52  will be set to pass the signals while others will be blocked. The number of N×M structures can keep increasing up to K. 
         [0027]    Depending on the application, the system can adjust the number of N×M structure  10  sets needed to be installed. For example, the user can install one N×M structure  10  first. In this case each tunable splitter  52  in the stage  50  will be tuned so that only one branch goes out (i.e., no splitting). Later, as the network grows, the system user may like to add another N×M structure  10 . At this point, the user will only need to adjust the tunable splitter  52  to make it pass out 2 branches (i.e., 1×2 splitter), and the addition branch will go to the additional N×M structure  10 . The system can keep growing like this up to a plurality (K) of N×M structures  10  together. 
         [0028]    Therefore, it should be understood that the invention can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is not intended to be exhaustive or to limit the invention to the precise form disclosed. It should be understood that the invention can be practiced with modification and alteration and that the invention be limited only by the claims and the equivalents thereof.