Abstract:
The inventions invention relates to a generic re-configurable WDM optical cross connection device which may be viewed as an optical network element comprising transport interfaces optionally along with tributary interfaces. An switch system includes a plurality of switch modules each essentially including a pair of R-channel input collimators on one side and a pair of R-channel output collimators on the other side with a switching prism moveably positioned therebetween in an existence/active or non-existence/inactive manner. A pair of input fibers are respectively connected to the two input collimators and an pair of output fibers are respectively connected t the two output collimators. A plurality of jumper fibers cascading said switch modules together.

Description:
This is a non-provisional application claiming benefit of the copending provisional application with a Ser. No. of 60/274,949 filed on Mar. 12, 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field of The Invention 
   The invention is related to an optical switch assembly, and particularly to the optical transport interface functioning as a cross connector device in a WDM (Wavelength Division Multiplexed) system. 
   2. The Related Art 
   Wavelength Division Multiplexed (WDM) systems enable existing transport networks to provide increased bandwidth without deploying duplicate overlay networks. The WDM system enables a new level of flexibility in the network—the ability to use the wavelengths for routing, cross connecting or even adding/dropping, etc. Along with these abilities to route wavelengths, the WDM network may also provide the capability to support wavelength survivability. To use WDM systems in this manner requires more sophisticated optical network elements. Thus, a re-configurable WDM cross connection device is one of such elements. 
   Articles titled “Many approaches taken for all-optical switching” and “Matrix optical switches enable wavelength-selective crossconnects” in the August, 2001 issue of  Laser Focus World , disclose several optical switches, of which some require simultaneously precisely positioning the matrix type reflection mirrors relative to the input/output ports for transporting a great number of signals. Anyhow, in some conditions a compact sized switch with few signals transported therein is desired in the industry. 
   SUMMARY OF THE INVENTION 
   The inventions invention relates to a generic re-configurable WDM optical cross connection device which may be viewed as an optical network element comprising transport interfaces optionally along with tributary interfaces. An switch system includes a plurality of switch modules each essentially including a pair of R-channel input collimators on one side and a pair of R-channel output collimators on the other side with a switching prism moveably positioned therebetween in either an existence/active or non-existence/inactive manner. A pair of input fibers are respectively connected to the two input collimators on the one (input) side of the whole system, either of the same switch module or of the two different switch modules on the input side of the whole system. Similarly, a pair of output fibers are respectively connected to the two output collimators on the other (output) side of the whole system. Two sets of input jumper fibers are arranged to respectively start from the corresponding input collimators for cross-connecting to the rest of the input collimators in sequence wherein each set of the input jumper fibers only interconnects one input collimator for each individual switch module and leaves the other input collimator of each individual switch module for the other set of the input jumper fibers. Two sets of output jumper fibers are arranged to connect the output collimators by a same principle except that the last output jumper fiber of each set is terminated at the same output collimator where the output fiber is located. The number (N) of the switch modules corresponds to the number of different WDM channels (i.e., wavelengths at ITU grid) of the transmitted signals, in the input/output fibers, requiring routing flexibilities thereof. 
   Under this arrangement, by respectively having the individual switching prisms in either existence or non-existence position, the first set of input N channels in the first input fiber and the second set of input N channels in the second input fiber can be rearranged into the first set of output N channels in the first output fiber and the second set of output N channels in the second output fiber wherein some channels from the first set of input N channels are switched/exchanged with some of the second set of input N channels so as to substitute each other in the corresponding first set of output N channels and second set of output N channels. The total varieties/possibilities of this rearrangement of these N channels are (M!) N  in this current exemplary system wherein M is the number of the input or output fibers. 
   In one exemplary complete application system, there are two input fibers and two output fibers with different WDM channels in each fiber respectively as the transport interfaces. As controlled by the switching prisms, N 1  channels can be either selectively cross-connected to another output fiber or remaining as direct pass express channels. The system can also selectively add/drop N 2  channels from one transport interface with additional N 2  input(add) fibers and N 2  output(drop) fibers, respectively, as the tributary interfaces, controlled by a different set of switching prisms at a different set of wavelengths from N 1  channels. The system can further additionally selectively add/drop N 3  channels from another transport interface with additional N 3  input fibers and N 3  output fibers as the tributary interfaces, controlled by another different set of switching prisms at another different set of wavelengths from N 1 . All other remaining wavelengths directly pass through the system as express channels in the transport interface. In case N 1 =0, the system functions as a pure reconfigurable add/drop device; in case N 2 =N 3 =0, the system functions as a pure cross connector device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS.  1 ( a )- 1 ( d ) are diagrams to respectively show different outcomes with the same input by an optical switch system according to the invention for switching two different channels. 
     FIGS.  2 ( a )- 2 ( p ) are diagrams to respectively show a plurality of variable outcomes based on anther embodiment for switching four different channels. 
       FIG. 3  shows an optical switching system in a form of the generic re-configurable WDM optical cross connection device. 
       FIG. 4  is a functional block diagram of generic re-configurable WDM optical cross connection device or switching system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   References will now be in detail to the preferred embodiments of the invention. While the present invention has been described in with reference to the specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by appended claims. 
   It will be noted here that for a better understanding, most of like components are designated by like reference numerals throughout the various figures in the embodiments. Attention is directed to FIGS.  1 ( a )- 1 ( d ) wherein an optical switch system  1  includes first and second switch modules  10 ,  12 . The first switch module  10  includes a pair of input/output collimator assemblies arranged with a pair of R-channel first input collimators  14 / 14 ′ on one side, and a pair of R-channel first output collimators  16 / 16 ′ on the other side with a prism  18  moveably positioned therebetween in an existence or non-existence manner, wherein both the first input collimators  14 / 14 ′ and the first output collimators  16 / 16 ′ allow the wavelength λ 1  to pass and reflect others. The second module  12  similarly including another pair of input/output collimator assemblies arranged with a pair of R-channel second input collimators  15 / 15 ′ on the same side with the first input collimators  14 / 14 ′, and a pair of R-channel second output collimators  17 / 17 ′ on the same side with the first output collimators  16 / 16 ′ while being opposite to said pair of second input collimators  15 / 15 ′, a prism  18 ′ moveable positioned between the pair of second input collimators  15 / 15 ′ and the pair of second output collimators  17 / 17 ′, wherein both the second input collimators  15 / 15 ′ and the second output collimators  17 / 17 ′ allow the wavelength λ 2  to pass and reflect others. 
   First and second input fibers  20 ,  22  are respectively connected to the first input collimator  14  of the first switch module  10  and the second input collimator  15 ′ of the second module  12 . Similarly, first and second output fibers  24 ,  26  are respectively connected to the first output collimator  16  of the first module  10  and the second output collimator  17 ′ of the second module  12 . 
   First and second sets of input jumper fibers  28 ,  30  are arranged to respectively start from the input collimators, where the input fibers are connected, for cross-connecting to the rest of the input collimators of other modules in sequence where each set of the input jumper fibers only interconnects one input collimator for each individual module and leaves the other input collimator of each module for the other set. In this embodiment, the first set of input jumper fiber  28  connects the first input collimator  14  of the first module  10 , where the first input fiber  20  is located, to the first input collimator  15  of the second module  12 , where the second input fiber  22  is not located. Similarly, the second set of input jumper fiber  30  connects the second input collimator  15 ′ of the second module  12 , where the second input fiber  22  is located, to the second collimator  14 ′ of the first module  10 , wherein the first input fiber  20  is not located. 
   Similar to the arrangement of the input jumper fibers  28 ,  30 , first and second sets of output jumper fibers  32 ,  34  are configured to respectively start from the output collimators, where the output fibers are connected, for cross-connecting to the rest of the output collimators of other modules in sequence where each set of the output jumper fibers only interconnects one output collimator for each individual module and leaves the other output collimator of each module for the other set. Thus, in this embodiment the first set of output jumper fiber  32  connects the first output collimator  16  of the first module  10 , wherein the first output fiber  24  is located, to the first output collimator  17  of the second module  12 , where the second output fiber  26  is not located. Similarly, the second set of output jumper fiber  34  connects the second output collimator  17 ′ of the second module  12 , where the second output fiber  26  is located, to the second output collimator  16 ′ of the first module  10 , where the first output fiber  24  is not located. 
   Understandably, the existence of the prism  18  switches the light paths of the pair of input/output collimator assemblies in each module, as illustrate in a related copending application Ser. No. 09/750,737 filed on Dec. 29, 2000 now pending having one common inventor with the instant invention. 
   Therefore, with existence or non-existence of the switching prism  18 / 18 ′ in each module  10 ,  12 , there are four variations/possibilities of the outcome of the system  1  when light including both signals of λ 1  and λ 2  appears in each of said first input fiber  20  and the second input fiber  22 . 
   As shown in FIG.  1 ( a ) where both the switching prisms  18  of the first and second modules  10 ,  12  are of non-existence, the light including first group signals of λ 1  and λ 2 , (i.e., the first λ 1  signal and the first λ 2  signal), enters the first input fiber  20  and the light including second group signals of λ 1  and λ 2 , i.e., the second λ 1  signal and the second λ 2  signal), enters the second input fiber  22 . 
   Under this situation,
         (I) the first λ 1  signal in the input fiber  20  passes the first input collimator  14  into and then further passes the first output collimator  16 , and enters the first output fiber  24 ;   (II) the first λ 2  signal in the first input fiber  20  is reflected by the first input collimator  14  and enters the second input collimator  15  via the first input jumper fiber  28 , successively enters the second output collimator  17  further toward the first output collimator  16  via the first output jumper fiber  32 , and finally is reflected by the first output collimator  16  to join the first λ 1  signal in the first output fiber  24 ;   (III) similar to the path pattern of the first λ 1  signal as illustrated in (I), the second λ 2  signal passes the second input collimator  15 ′ into and further passes the second output collimator  17 ′, and enters the second output fiber  26 ;   (IV) similar to the path pattern of the first λ 2  signal as illustrated in (II), the second λ 1  signal of in the second input fiber  22  is first reflected by the second input collimator  15 ′ and enters the first input collimator  14 ′ via the second input jumper fiber  30 , further enters the first output collimator  16 ′ and further enters the second output collimator  17 ′ and finally is reflected by the second output collimator  17 ′ to join the second λ 2  signal in the second.       

   In this arrangement, because none of the switching prisms  18  is of existence, the first output fiber  24  still carries the same first λ 1  signal and first λ 2  signal as those in the first output fiber  20 , and the second output fiber  26  still carries the same second λ 1  signal and second λ 2  signal as those in the second output fiber  20 . In other words, no switching effect is provided. 
   FIG.  1 ( b ) shows the prism  18  of the first module  10  is provided while the prism  18 ′ is not provided therewith, to switch the light paths between the pair of collimator assembly in the first module  10 . Under this situation, the first λ 2  signal in the first input fiber  20  travels the same path as what is shown in FIG.  2 ( a ) and enters the first output fiber  24 , and the second λ 2  signal in the second input fiber  22  travels the same path as what is shown in FIG.  2 ( a ) and enters the second output fiber  24  because the second module  12 , which regulates/controls the signals of λ 2 , has no switching function thereof. Differently, the first λ 1  signal in the first input fiber  20  enters the first input collimator  14  while is switched, by the prism  18  of the first module  10 , to the first output collimator  16 ′ instead of the first output collimator  16 , and finally enters the second output fiber  26  to join the second λ 2  signal in the second output fiber  26 . On the other hand, the second λ 1  signal in the second input fiber  22  entering the first input collimator  14 ′, is switched, by the same prism  18  of the first module  10 , to the first output collimator  16  instead of the first output collimator  16 ′, and finally enters the first output fiber  24  to join the first λ 2  signal in the first output fiber  24 . Therefore, the first output fiber  24  carries the original first λ 2  signal from the first input fiber  20  while with the second λ 1  signal from the second input fiber  22 . Corresponding, the second output fiber  26  carries the original second λ 2  signal from the second input fiber  22  while with the first λ 1  signal from the first input fiber  20 . In conclusion, the signals of λ 1  in the first and second input fibers  20 ,  22  are switched with each other because of existence of the prism  18  in the first module  10  which is intended to control/regulate the signals of λ 1 . 
   Similarly, FIG.  1 ( c ) shows existence of the prism  18 ′ in the second module  12  while the prism  18  in the first module  10  not, where the signals of λ 2  are controlled/regulated, and thus only the λ 2  signals from the input fibers  20 ,  22  are switched in the system. In other words, the first output fiber  24  carries the original first λ 1  signal from the first input fiber  20  while with the second λ 2  signal from the second input fiber  22 . Correspondingly, the second output fiber  26  carries the original second λ 1  signal from the second input fiber  22  while with the first λ 2  signal from the first input fiber  20 . 
   FIG.  1 ( d ) shows existence of the prisms  18  and  18 ′ of both the first module  10  and the second module  12  where signals of λ 1  and λ 2  are respectively controlled/regulated, and thus both the λ 2  signals from the input fibers  20 ,  22  are switched in the system. In other words, the first output fiber  24  carries the original second λ 1  and λ 2  signals from the second input fiber  20 . Correspondingly, the second output fiber  26  carries the original first λ 1  and λ 2  signals from the first input fiber  22 . 
   The above four variations of this system can be calculated from the equation: number of variations=2 N  wherein 2 indicates the number of possibility of switching result of each switching prism, and N represents the number of the modules used in the system. As mentioned before, because each module generally regulates/controls one wavelength signal, in this embodiment there are two wavelengths λ 1  and λ 2  signals are involved, thus requisitely using two modules  10  and  12 . As a result, there are four variations according to the aforementioned formula 2 N  wherein N is equal 2. Understandably, if only one wavelength signal is involved, then only one module is used as shown in the aforementioned copending application Ser. No. 09/750,737. As a result disclosed in such copending application, there are two variations according to the aforementioned formula 2 N  wherein N is equal 1. Similarly, FIGS.  2 ( a )- 2 ( p ) show four different wavelengths λ 1 , λ 2 , λ 3  and λ 4  are involved in the switching arrangement. Thus, there are four modules  10  are required in practice and it results in sixteen variations according to the aforementioned formula 2 N  wherein N is equal 4. 
   As shown in FIGS.  2 ( a ) &amp;  2 ( b ), four switch modules  102 ,  104 ,  106  and  108  are respectively provided with prisms  101 ,  103 ,  105  and  107  and pairs of input collimators  110 / 112 ,  114 / 116 ,  118 / 120  and  122 / 124  on same sides with regard to the corresponding prisms. First and second input fibers  142  and  144  are respectively connected to the input collimator  110  of the switch module  102  and the input collimator  114  of the switch module  104 . A first set of input jumper fibers  146 ,  148  and  150  corresponding to the first input fiber  142 , sequentially connect the input collimator  110  of the switch module  102 , the input collimator  116  of the switch module  104 , the input collimator  120  of the switch module  106 , and the input collimator  124  of the switch module  108  one another in series. Similarly, a second set of input jumper fibers  152 ,  154  and  156  corresponding to the second input fiber  144 , sequentially connect the input collimator  114  of the switch module  104 , the input collimator  112  of the switch module  102 , the input collimator  118  of the switch module  106 , and the input collimator  122  of the switch module  108  one another in series. 
   Oppositely, pairs of output collimators  126 / 128 ,  130 / 132 ,  134 / 136  and  138 / 140  are respectively provided on the same sides of the switch modules  102 ,  104 ,  106  and  108  relative to the corresponding prisms. First and second output fibers  158  and  160  are respectively connected to the output collimator  126  of the switch module  102 , and the output collimator  130  of the switch module  104 . A first set of output jumper fibers  162 ,  164  and  166  sequentially connect the output collimator  140  of the switch module  108 , the output collimator  136  of the switch module  106 , the output collimator  132  of the switch module  104 , and the output collimator  126  of the switch module  102  one another in series. Similarly, the second set of output jumper fibers  168 ,  170  and  172  sequentially connect the output collimator  138  of the switch module  108 , the output collimator  134  of the switch module  106 , the output collimator  128  of the switch module  102 , and the output collimator  130  of the switch module  104  one another in series. 
   As shown in FIG.  2 ( a ), the first input fiber  142  carries signals of λ 1 , λ 2 , λ 3  and λ 4  represented as λ 1 ( 1 ), λ 2 ( 1 ), λ 3 ( 1 ) and λ 4 ( 1 ). Similarly, the second input fiber  144  respectively carries different signals at the same wavelengths λ 1 , λ 2 , λ 3  and λ 4  and represented as λ 1 ( 2 ), λ 2 ( 2 ), λ 3 ( 2 ) and λ 4 ( 2 ). Similar to arrangement of the previous embodiment, the pair of input collimators  110 ,  112  and the pair of output collimators  126 ,  128  of the switch module  102  only transmit the signal of λ 1  and reflect the others. With the same principle, the pair of input collimators  114 ,  116  and the pair of output collimator  130 ,  132  of the switch module  104  only transmit the signal of λ 2  and reflect the others, the pair of input collimators  118 ,  120  and the pair of output collimator  134 ,  136  of the switch module  106  only transmit the signal of λ 3  and reflect the others, and the pair of input collimators  120 ,  122  and the pair of output collimator  138 ,  140  of the switch module  108  only transmit the signal of λ 4  and reflect the others. 
   Under this rule, as shown in FIG.  2 ( a ) where all the prisms  101 ,  103 ,  105  and  107  are in the “inactive” status, the signal of λ 1 ( 1 ) passes the input collimator  110  and the output collimator  126  of the switch module  102  to enter the first output fiber  158 . 
   Signals of λ 2 ( 1 ) with those of λ 3 ( 1 ) and λ 4 ( 1 ) are reflected by the input collimator  110  of the switch module  102  and, via the input jumper fiber  146  of the first set of input jumper fibers, enter the input collimator  116  of the switch module  104  wherein only the signal of λ 2 ( 1 ) passes the input collimator  116  of the switch module  104  and further passes the output collimator  132  thereof and successively enters the output collimator  126  of the switch module  102  via the output jumper fiber  166  of the first set of output jumper fibers, and finally is reflected therefrom toward the first output fiber  158  to join the signal of λ 1 ( 1 ) coming from the other side of the same output collimator  126 . 
   Following the same rule, signals of λ 3 ( 1 ) and λ 4 ( 1 ) which are reflected by and leave from the input collimator  116  of the switch module  104 , enter the input collimator  120  of the switch module  106  via the input jumper fiber  148 , wherein only the signal of λ 3 ( 1 ) passes therethrough and further passes the output collimator  136  thereof and successively enters the output collimator  132  of the switch module  104  via the output jumper  164  of the firs set of output jumper fibers. The signal of λ 3 ( 1 ) is further reflected by and leaves from the output collimator  132  to join the signal of λ 2 ( 1 ) coming from the other side of the same output collimator  132  for entering the output jumper fiber  166  and finally toward the first output fiber  158  by following the aforementioned “outgoing” path of the signal of λ 2 ( 1 ). 
   Signal of λ 4 ( 1 ) reflected by the input collimator  120  of the switch module  106  and separated from signal of λ 3 ( 1 ), enters the input collimator  124  of the switch module  108  via the input jumper fiber  150  of the first set of input jumper fibers. The signal of λ 4 ( 1 ) passes the input collimator  124  and the output collimator  140  and enters the output collimator  136  of the switch module  106  via the output jumper fiber  162  of the first set of output jumper fibers and is reflected by the output collimator  136  to join the signal of λ 3 ( 1 ) coming from the other side of the output collimator  136  for entering the output jumper fiber  164  of the first set of output jumper fibers and finally entering the first output fiber  158  by following the “outgoing” path of the signal of λ 3 ( 1 ). 
   Under this situation, all of the signals of λ 1 ( 1 ), λ 2 ( 1 ), λ 3 ( 1 ) and λ 4 ( 1 ) are leaving from the first output fiber  158 . 
   With the same principle, signals of λ 1 ( 2 ), λ 2 ( 2 ), λ 3 ( 2 ) and λ 4 ( 2 ) coming from the second input fiber  144  allow the signal of λ 2 ( 2 ) to pass through the input collimator  114  and the output collimator  130  of the switch module  104 , and finally enter the second output fiber  160 . 
   Signals of λ 1 ( 2 ), λ 3 ( 2 ) and λ 4 ( 2 ) are reflected by the input collimator  114  of the switch module  104  and, via the input jumper fiber  152  of the first set of input jumper fibers, enter the input collimator  112  of the switch module  102  wherein only the signal of λ 1 ( 2 ) passes the input collimator  112  and further passes the output collimator  128  thereof and successively enters the output collimator  130  of the switch module  104  via the output jumper fiber  172  of the second set of output jumper fibers, and finally is reflected therefrom toward the second output fiber  160  to join the signal of λ 2 ( 2 ) coming from the other side of the same output collimator  130 . 
   Following the same rule, signals of λ 3 ( 2 ) and λ 4 ( 2 ) which are reflected by and leave from the input collimator  112  of the switch module  102 , enter the input collimator  118  of the switch module  106  via the input jumper fiber  154 , wherein only the signal of λ 3 ( 2 ) passes therethrough and further passes the output collimator  134  thereof and successively enters the output collimator  128  of the switch module  102  via the second jumper fiber  170  of the second set of output jumper fibers. The signal of λ 3 ( 2 ) is further reflected by and leaves from the output collimator  128  to join the signal of λ 1 ( 2 ) coming from the other side of the same output collimator  128  for entering the output jumper fiber  172  and finally toward the second output fiber  160  by following the aforementioned “outgoing” path of the signal λ 1 ( 2 ). 
   Signal of λ 4 ( 2 ) reflected by the input collimator  118  of the switch module  106  and separated from the signal of λ 3 ( 2 ), enters the input collimator  122  of the switch module  108  via the input jumper fiber  156  of the first set of input jumper fibers. The signal of λ 4 ( 2 ) passes the input collimator  122  and the output collimator  138  thereof, and enters the output collimator  134  via the output jumper  168  of the second set of output jumper fibers, and is reflected by the output collimator  134  to join the signal of λ 3 ( 2 ) coming from the other side of the output collimator  134  for entering the output jumper fiber  170  of the second set of output jumper fibers and finally entering the output jumper fiber  172  by following the “outgoing” path of the signal λ 3 ( 2 ). 
   Under this situation, all of the signals of λ 1 ( 2 ), λ 2 ( 2 ), λ 3 ( 2 ) and λ 4 ( 2 ) are leaving from the second output fiber  160  in comparison with the signals of λ 1 ( 1 ), λ 2 ( 1 ), λ 3 ( 1 ) and λ 4 ( 1 ) being leaving from the first output fiber  158 . This is one of sixteen variations mentioned earlier. 
   FIG.  2 ( b ) presents the different variation where the prism  101  is active in the switch module  102  which controls λ 1 . Under this situation, signals of λ 1 ( 1 ) and λ 1 ( 2 ) which pass the switch module  102  will exchange with each other. Thus, in comparison with the outgoing result shown in FIG.  2 ( a ) where the first output fiber  158  contains signals of λ 1 ( 1 ), λ 2 ( 1 ), λ 3 ( 1 ) and λ 4 ( 1 ) and the second output fiber  160  contains signals of λ 1 ( 2 ), λ 2 ( 2 ), λ 3 ( 2 ) and λ 4 ( 2 ), the result in FIG.  2 ( b ) shows the first output fiber  158  contains signals of λ 1 ( 2 ), λ 2 ( 1 ), λ 3 ( 1 ) and λ 4 ( 1 ) and the second output fiber  160  contains the signals of λ 1 ( 1 ), λ 2 ( 2 ), λ 3 ( 2 ) and λ 4 ( 2 ) where signals of λ 1 (l) and λ 1 ( 2 ) have exchanged with each other in the first and second output fibers  158 ,  160 . 
   With the same pattern, FIG.  2 ( c ) shows the prism  103  of the switch module  104  controlling λ 2 , is active and the outcome of the first output fiber  158  contains λ 1 ( 1 ), λ 2 ( 2 ), λ 3 ( 1 ) and λ 4 ( 1 ) and that of the second output fiber  160  contains λ 1 ( 2 ), λ 2 ( 1 ), λ 3 ( 2 ) and λ 4 ( 2 ). Similarly, the other thirteen variations can be listed as follows. 
   In FIG.  2 ( d ), the prism  105  of the switch module  106  controlling λ 3  is active and the outcome of the first output fiber  158  contains λ 1 ( 1 ), λ 2 ( 1 ), λ 3 ( 2 ) and λ 4 ( 1 ) and the outcome of the second output fiber  160  contains λ 1 ( 2 ), λ 2 ( 2 ), λ 3 ( 1 ) and λ 4 ( 2 ). 
   In FIG.  2 ( e ), the prism  107  of the switch module  108  controlling λ 4  is active and the outcome of the first output fiber  158  contains λ 1 ( 1 ), λ 2 ( 1 ), λ 3 ( 1 ) and λ 4 ( 2 ) and the outcome of the second output fiber  160  contains λ 1 ( 2 ), λ 2 ( 2 ), λ 3 ( 2 ) and λ 4 ( 1 ). 
   In FIG.  2 ( f ), both the prism  101  of the switch module  102  controlling λ 1  and the prism  103  of the switch module  104  controlling λ 2  are active and the outcome of the first output fiber  158  contains λ 1 ( 2 ), λ 2 ( 2 ), λ 3 ( 1 ) and λ 4 ( 1 ) and the outcome of the second output fiber  160  contains λ 1 ( 1 ), λ 2 ( 1 ), λ 3 ( 2 ) and λ 4 ( 2 ). 
   In FIG.  2 ( g ), both the prism  103  of the switch module  104  controlling λ 2  and the prism  105  of the switch module  106  controlling λ 3  are active and the outcome of the first output fiber  158  contains λ 1 ( 1 ), λ 2 ( 2 ), λ 3 ( 2 ) and λ 4 ( 1 ) and the outcome of the second output fiber  160  contains λ 1 ( 2 ), λ 2 ( 1 ), λ 3 ( 1 ) and λ 4 ( 2 ). 
   In FIG.  2 ( h ), both the prism  105  of the switch module  106  controlling λ 3  and the prism  107  of the switch module  108  controlling λ 4  are active and the outcome of the first output fiber  158  contains λ 1 ( 1 ), λ 2 ( 1 ), λ 3 ( 2 ) and λ 4 ( 2 ) and the outcome of the second output fiber  160  contains λ 1 ( 2 ), λ 2 ( 2 ), λ 3 ( 1 ) and λ 4 ( 1 ). 
   In FIG.  2 ( i ), both the prism  101  of the switch module  102  controlling λ 1  and the prism  105  of the switch module  106  controlling λ 3  are active and the outcome of the first output fiber  158  contains λ 1 ( 2 ), λ 2 ( 1 ), λ 3 ( 2 ) and λ 4 ( 1 ) and the outcome of the second output fiber  160  contains λ 1 ( 1 ), λ 2 ( 2 ), λ 3 ( 1 ) and λ 4 ( 2 ). 
   In FIG.  2 ( j ), both the prism  101  of the switch module  102  controlling λ 1  and the prism  107  of the switch module  108  controlling λ 4  are active and the outcome of the first output fiber  158  contains λ 1 ( 2 ), λ 2 ( 1 ), λ 3 ( 1 ) and λ 4 ( 2 ) and the outcome of the second output fiber  160  contains λ 1 ( 1 ), λ 2 ( 2 ), λ 3 ( 2 ) and λ 4 ( 1 ). 
   In FIG.  2 ( k ), both the prism  103  of the switch module  104  controlling λ 2  and the prism  107  of the switch module  108  controlling λ 4  are active and the outcome of the first output fiber  158  contains λ 1 ( 1 ), λ 2 ( 2 ), λ 3 ( 1 ) and λ 4 ( 2 ) and the outcome of the second output fiber  160  contains λ 1 ( 2 ), λ 2 ( 1 ), λ 3 ( 2 ) and λ 4 ( 1 ). 
   In FIG.  2 ( l ), all three the prism  101  of the switch module  102  controlling λ 1 , the prism  103  of the switch module  104  controlling λ 2  and the prism  105  of the switch module  106  controlling λ 3  are active and the outcome of the first output fiber  158  contains λ 1 ( 2 ), λ 2 ( 2 ), λ 3 ( 2 ) and λ 4 ( 1 ) and the outcome of the second output fiber  160  contains λ 1 ( 1 ), λ 2 ( 1 ), λ 3 ( 1 ) and λ 4 ( 2 ). 
   In FIG.  2 ( m ), the prism  103  of the switch module  104  controlling λ 2 , the prism  105  of the switch module  106  controlling λ 3  and the prism  107  of the switch module  108  controlling λ 4  are active and the outcome of the first output fiber  158  contains λ 1 ( 1 ), λ 2 ( 2 ), λ 3 ( 2 ) and λ 4 ( 2 ) and the outcome of the second output fiber  160  contains λ 1 ( 2 ), λ 2 ( 1 ), λ 3 ( 1 ) and λ 4 ( 1 ). 
   In FIG.  2 ( n ), the prism  101  of the switch module  102  controlling λ 1 , the prism  103  of the switch module  104  controlling λ 2  and the prism  107  of the switch module  108  controlling λ 4  are active and the outcome of the first output fiber  158  contains λ 1 ( 2 ), λ 2 ( 2 ), λ 3 ( 1 ) and λ 4 ( 2 ) and the outcome of the second output fiber  160  contains λ 1 ( 1 ), λ 2 ( 1 ), λ 3 ( 2 ) and λ 4 ( 1 ). 
   In FIG.  2 ( o ), the prism  101  of the switch module  102  controlling λ 1 , the prism  105  of the switch module  106  controlling λ 3  and the prism  107  of the switch module  108  controlling λ 4  are active and the outcome of the first output fiber  158  contains λ 1 ( 2 ), λ 2 ( 1 ), λ 3 ( 2 ) and λ 4 ( 2 ) and the outcome of the second output fiber  160  contains λ 1 ( 1 ), λ 2 ( 2 ), λ 3 ( 1 ) and λ 4 ( 1 ). 
   In FIG.  2 ( p ), all the prism  101  of the switch module  102  controlling λ 1 , the prism  103  of the switch module  104  controlling λ 2 , the prism  105  of the switch module  106  controlling λ 3 , and the prism  107  of the switch module  108  controlling λ 4  are active, the outcome of the first output fiber  158  contains λ 1 ( 2 ), λ 2 ( 2 ), λ 3 ( 2 ) and λ 4 ( 2 ) and the outcome of the second output fiber  160  contains λ 1 ( 1 ), λ 2 ( 1 ), λ 3 ( 1 ) and λ 4 ( 1 ). 
   Understandably, based on each module  102 ,  104 ,  106 ,  108  may provide two variations, the combination of these four modules may result in sixteen possibilities(variations) as listed above. From another viewpoint of the outcome of the output fibers, the signals of the specific wavelength λ may be switched between two fibers by activating the prism of the corresponding switch module controlling said wavelength λ; otherwise, there is no change. It is very simple to operate. On the other hand, because the modules are cascaded with one another, the variation amount for multiple wavelengths can be significantly increased. 
   It is appreciated that in the above embodiments the switch prism of each switch module only switch two signal paths therein. Anyhow, other switching mechanism may also be used in this switching system wherein such switching mechanism may provide three or more than three switching paths. Correspondingly, instead of only two input(output) fibers are involved in the system, three or more than three Input(output) fibers can be implemented therewith. Under this situation, instead of two variations of the switch device as shown in the above embodiments, the total amount of the variations (i.e., of the possible outcome) of such a switching mechanism may follow the equation of M! (i.e., M×(M−1)×(M−2) . . . ×2×1) wherein M is the number of signal paths and is essentially equal to the number of the input(output) fibers in practice. This equation is derived from a math formula  M P M  where P represents the total possibilities of orderly arrangement of M elements. In the above two embodiments, M=2 and then each switching module provides 2!(=2) variations. Alternately, for a switch module providing three paths adapted to cooperate with three input collimators and three output collimators, there would be 3!(=6) variations. Moreover, following the formula disclosed in the aforementioned embodiments, when N wavelengths are involved, N switch modules are required to be used in the system and the number of the total variations may be (M!) N . In the practical sample, M=2 and N=8. Thus, there are 2 8 =256 variations during transmission. 
   It is appreciated that regardless of how many switch modules are used for switching the same corresponding number of channels between two or among more than two fibers, such switch modules should be interconnected one another via jumper fibers to form cascading therebetween. It is also noted there is no absolute sequence for these switch modules either for jumper fiber connection or even the input/output fiber position(s). For example, the input fiber may be located in the third switch, or the jumper fiber may be connected between the first switch module and the fourth switch module. Moreover, it is unnecessary for the input fibers and their associated jumper fibers on one side of the system to be symmetric with regard to the output fibers and their associated jumper fibers on the other side. For example, one input fiber may be located in one switch module while the corresponding output fiber may be located in another switch module. The general rule is that each collimator of each switch module should be connected only once via the jumper fiber(s). 
     FIG. 3  shows another embedment using the basis structure disclosed in the aforementioned two embodiment to complete an optical switch system in practice. The optical switching system  200  comprising three sections wherein the first section is the switching region  202  having the similar arrangement disclosed in the earlier two embodiments wherein there are N 1  (N 1 =8 in this embodiment) switch modules  203 , for switching N 1  channels, in a cross-connection manner with one another via corresponding jumper fibers  222 . 
   It is noted that different from the aforementioned two embodiments where the first input fiber and the second input fiber are respectively connected to the different switch modules, in this embodiment the first input fiber  211  and the second input fiber  212  respectively connects to the pair of input collimators of the first module  2031 . 
   A second section is the first add/drop region  204  wherein there are N 2  (N 2 =2 in this embodiment) switch modules  205 , for adding/dropping N 2  channels, in a cross-connection manner with one another. Different from what are arranged in the first section, (i.e., the switching region  202 ), the subject switch modules  205  do not connect to the signals from the second input fiber  212 . In other words, the signals of the second input fiber  212 , entering the switching region  202  from the first switch module  2031  and leaving out of the last switch module  2032 , bypasses the first add/drop region  204  via the corresponding jumper fiber  2221 . In contrast, without receiving any signals from the second input fiber  212 , for the pair of input collimators of each of the switch modules  205 , one is connected to the incoming signals from the first input fiber transmitted from the switching region  202  or from the front neighboring switch module  205 , while the other is connected to an external add fiber  216 . 
   Similar to what is disclosed in the earlier filed copending application Ser. No. 09/750,737, the signal from the first input fiber with the specific center wavelength compliant with the switch module  205 , will be exchanged with another signal having the same center wavelength from the add fiber  216 . In other words, the original signal will be dropped from the drop fiber  217  while the added signal will join the other signals, passing the switching region  202 , toward the first output fiber  213  located on the output collimator of the first switch module  2031 . Understandably, similar to what is arranged in the switching region  202 , the cascading arrangement among the switch modules  205  also provides multiplexing and demultiplexing for the transmitted combined signals. It is noted that there is a jumper fiber  218  connected between one input collimator of the last switch module in the second section, (i.e., the first add/drop section  204 ) where the cascading arrangement is applied but without the add fiber, and one output collimator which is a partner of that input collimator. With this jumper fiber  218 , the other (remaining) channels (wavelengths) which are not coupled to any of the switch modules in the first add/drop section  204 , will be directed to the output collimator 
   The third section is a second add/drop region  206  wherein there are N 3  (N 3 =1 in this embodiment) switch modules  207  for adding/dropping N 3  channels, in a cascading manner with one another. The second add/drop region  206  is similar to the first add/drop region  204  except that the switch module  207  only receives the signals from the second input fiber  212 , which bypasses the first add/drop region  204 , and is exchanged with the signal from the add fiber  224  and dropped from the drop fiber  219 . On the other hand, the added signal from the add fiber  224  will join the other signals, bypass the first add/drop region  204 , via the switching region  202 , toward the second output fiber  214  which is located on another output collimator of the first switch module  2031 . 
   It is noted that through the cross-connection and add/drop with cascading arrangement, the whole optical switch system can provide not only the switching function but also the add/dropping function. All other remaining wavelengths directly pass through the device as express channels in the transport interface. In this case, when N 1 =0, the device functions as a pure reconfigurable add/drop device; when N 2 =0 and N 3 =0, the device functions as a pure cross connection device. Under this situation, two jumper fibers respectively connect each pair of input collimator and output collimator of the last switch module 
   In summary, as shown in  FIG. 4  where the basic building block (i.e., switch module) comprises two R-channel input dual fiber collimators, and two R-channel output dual fiber collimators and a switching prism, wherein the R-channel dual fiber collimator comprises at least a dual fiber ferrule, a lens and a WDM band pass filter. When a multi-channel WDM signal light is coupled into the collimator from one of its two pigtail fibers, the channel falls in the passband of the WDM filter will be transmitted through the collimator while the remaining channels will be reflected back and coupled out from the other fiber. The switching prism can control the light being transmitted directly pass through from an input collimator to an output collimator, or cross connection transmitted from an input collimator to another output collimator. The cross connection function is realized by cascading both input collimators in each basic building block to the next stage, and cascading the output collimators in preferably the same pattern as the input collimators systematically. The add/drop function can be realized by cascading only one input collimator in each basic building block to the next stage, and cascading one corresponding output collimator in preferably the same pattern as the input collimator, and the remaining, not cascaded, input collimator in each basic building block is used to provide the input pigtail fiber for the add port, and the remaining, not cascaded, output collimator in each basic building block is used to provide the output pigtail fiber for the drop port. In this way, there are two transport interfaces each with a pair of input fiber and output fiber (e.g., input fibers  20 ,  22  and output fibers  24 ,  26  in the first embodiment). An n 1  channels (wavelengths) optical WDM signal transmits through transport interface one, and another n 2  channels (wavelengths) optical WDM signal transmits through transport interface two. In these channels, N 1  of them are cross connected from transport interface one to two, and vice versa. The set of N 1  channels are included within both the n 1  and n 2  channels, respectively. In the mean time, a set of N 2  channels from the n 1  channels can be added/dropped in and from the transport interface one, while a set of N 3  channels from the n 2  channels can be add/dropped in and from the transport interface two, wherein the set of N 2  channels from the n 1  channels is excluded from the set of N 1 , and the set of N 3  from the n 2  channels are excluded from the set of N 1 . Understandably, N 2  and N 3  can either completely or partially overlap with each other. All other channels not belong to N 1 , N 2  and N 3  will transport directly through the transport interface without switching or adding/dropping. It is also appreciated that in the current embodiment, according to the arrangement order among the cross-connection group and the add/drop group, N 2  channels and N 3  channels are excluded from N 1  channels. Anyhow, via the different arrangement order or using the switch prism providing more than two signal paths, N 2  channels and N 3  channels might be included in N 1  channels. In other words, the switch system may provide both the add/drop and cross-connection functions for the same channel in two incoming signals. 
   It is understood that even though the preferred embodiment only disclose the 2×2 switching device, the N transport interfaces by utilizing N×N switches in the base building block can be implemented based on the spirit of the invention. In addition to the disclosed prism, other switching mechanisms, e.g., thermo-optic switch, MEMS, etc, may be utilized in this system. 
   It is noted that the collimators used with the add/drop way may have one of the dual fibers idle as well as those input collimators which do not further connect to another input collimator or those output collimators which do not receive the fibers from another output collimators. For those specific collimators, the associated dual fibers may be simplified as a signal fiber theoretically. It is also noted in the first and second embodiments the input fibers only carry the exact channels the switch modules function to, so it is unnecessary to an additional jumper fiber to connect between the last input collimator and the last output collimator to bypass the channels of the input signals which are not affected by any of the switch modules. Differently, in the third embodiment, such a jumper fiber  218  is provided between the last input collimator and the last output collimator. Moreover, as mentioned before it may not be necessary for the input fibers and the output fibers to be connected to the same switch module(s) either in the cross connection group or in the add/drop group. Moreover, the different input fibers may be respectively connected to the input collimators of the different switch modules, and the output fiber as well. 
   While the present invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims. 
   Therefore, person of ordinary skill in this field are to understand that all such equivalent structures are to be included within the scope of the following claims.