Abstract:
An optical add/drop utilizes an interferometer having a plurality of phase modulators in one of two optical paths. Each phase modulator is selectively operable on wavelength components at a corresponding one wavelength of a plurality of predetermined wavelengths. Each phase modulator selectively provides a first or a second phase shift to such wavelength components to produce corresponding first or second phase shift differentials between corresponding wavelength components propagating on the two optical paths. The interferometer produces corresponding first or second interference results on a wavelength component by wavelength component basis. The first interference result causes the wavelength component of a first optical signal at a first input port to be coupled to a first output port. The second interference result causes the wavelength component of the first optical signal to be coupled to a second or drop port and the corresponding wavelength component of second or add optical signals to be coupled to the first output port.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims the benefit of prior U.S. Provisional Patent application Ser. No. 60/240,623 filed Oct. 16, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention pertains to optical communications systems, in general, and to optical switching networks, in particular.  
         BACKGROUND OF THE INVENTION  
         [0003]    An optical cross-connect device is functionally a four port device that works with optical signals comprising a plurality of different wavelengths. An optical cross-connect has an input port, a through port, an add port, and a drop port. Multiplexed wavelength optical signals at the input port are coupled to the through port. The use of add and drop ports allow optical signals at specific wavelengths to be “added” in place of the corresponding wavelength optical signals in the input port signals that in turn are switched to the drop port. This enables optical wavelength components signals to be added and dropped to/from multiplexed wavelength optical signals. An ideal optical cross-connect device is capable of dropping any combination of wavelengths from the input port to the drop port and adding any wavelengths combinations from an add port to the through port.  
           [0004]    Wavelength routing optical cross-connect arrangements presently available separate incoming wavelengths received at inputs by utilizing DWDM demultiplexing. Typically large-scale optical switch matrices are utilized to switch and route the demultiplexed single wavelength signals. In one arrangement micro-machined mirrors are utilized in what is referred to as MEM technology. In other arrangements, total internal reflection techniques are utilized with bubble or liquid crystal displays. These prior arrangements combine out-going wavelengths using DWDM multiplexers.  
           [0005]    Optical switch matrices based on wavelength routing optical cross-connects have severe limitations. To provide for switching of multiplexed optical signals having “n” wavelengths, a complex n×n optical switch matrix must be utilized. Where “n” is a large number, the size of the matrix becomes very large and the cost to provide such a matrix is high. In addition, the insertion loss is also very high-typically in excess of 10 dB for a 64 wavelength optical cross-connect. Because the size of the matrix increases in accordance with the square of “n” it is also difficult to scale up for a matrix to handles larger numbers of wavelength channels. To provide a 256 wavelength optical cross-connect requires over 64,000 switching elements. In addition, such matrices typically operate at a relatively slow speed, on the order of 10 milliseconds. The slow speed is a result of utilizing some sort of mechanical movement. The mechanical movement itself leads to reliability issues.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the principles of the invention, an optical add/drop utilizes an interferometer having a plurality of phase modulators in one of two optical paths. Each phase modulator is selectively operable on wavelength components at a corresponding one wavelength of a plurality of predetermined wavelengths. Each phase modulator selectively provides a first or a second phase shift to such wavelength components to produce corresponding first or second phase shift differentials between corresponding wavelength components propagating on the two optical paths. The interferometer produces corresponding first or second interference results on a wavelength component by wavelength component basis. The first interference result causes the wavelength component of a first optical signal at a first input port to be coupled to a first output port. The second interference result causes the wavelength component of the first optical signal to be coupled to a second or drop port and the corresponding wavelength component of second or add optical signals to be coupled to the first output port. Interferometer wavelength router technology is provided in which only one interferometer and n phase shifters are utilized to provide n wavelength optical cross-connect functionality.  
           [0007]    In accordance with the principles of the invention, an interferometer is coupled to first and second input ports and first and second output ports. The interferometer receives at the first input port first optical signals comprising a plurality of multiplexed first wavelength components, and receives at the second input port second optical signals comprising at least one second wavelength component, each first and second wavelength components have a wavelength corresponding to one wavelength of a plurality of predetermined wavelengths. The interferometer includes first and second optical paths carrying each said first and second wavelength component. A plurality of phase modulators is disposed in the first path. The phase modulators are responsive to control signals to cause, on a wavelength by wavelength basis, each first wavelength component to be coupled to either the first output port or the second output port and further selectively causes each corresponding second wavelength component to be coupled to the first output port when the corresponding first wavelength component is coupled to the second output port.  
           [0008]    Further in accordance with the invention, a controller is coupled to each phase modulator. The controller selectively controls each phase modulator to cause each phase modulator to provide a predetermined phase shift such that either the corresponding first wavelength component or the second wavelength component is coupled to the first output port.  
           [0009]    In one embodiment of the invention, a de-multiplexer is included in the first path and couples each first and second wavelength component to a corresponding one phase modulator.  
           [0010]    In that embodiment, a multiplexer, is disposed in the first path and couples each first and second wavelength component from each phase modulator to the first optical path.  
           [0011]    In another embodiment of the invention a multiplexer/de-multiplexer is disposed in the first path and couples each first wavelength component and each second wavelength component between the first optical path and each corresponding phase modulator.  
           [0012]    In an embodiment of the invention each phase modulator is controlled to selectively operate on a first wavelength component and a second wavelength component so as to selectively cause either the first wavelength component or the second wavelength component to be coupled to the first output port.  
           [0013]    Still further in accordance with the invention each said predetermined phase shift is a first phase shift or a second phase shift. Each first phase shift is selected so that a first or second wavelength component propagated on the first path interferes with the corresponding first or second wavelength component propagated on the second path to produce a first interference result. The first interference result is a coupling of the corresponding first wavelength component from the first input port to the first output port. The second phase shift is selected so that the first or second wavelength component propagated on the first path interferes with the corresponding said first or second wavelength component on the second path to produce a second interference result. The second Interference result is that the second wavelength component is coupled to the first output port and the first wavelength component is coupled to the second output port.  
           [0014]    Still further in accordance with the invention the first optical signals comprise the first wavelength components as wavelength division multiplexed signals.  
           [0015]    Yet further in accordance with one aspect of the invention the second optical signals comprise the second wavelength components as wavelength division multiplexed signals.  
           [0016]    A method for use with wavelength multiplexed optical signals includes receiving first optical signals at a first port, receiving second optical signals at a second port and coupling the first and second optical signals to an interferometer. Further steps include providing in the interferometer a first optical path and a second optical path, coupling the first and second optical signals to the first and second optical paths and providing in the first optical path a plurality of phase modulators. Still further, the method includes coupling to each phase modulator wavelength components of the first and second optical signals at a single predetermined wavelength selected from a plurality of predetermined wavelengths and selectively operating each phase modulator to provide a first or second phase shift to the corresponding wavelength components at the single predetermined wavelength propagating in the first optical path relative to the phase of the corresponding wavelength components propagating in the second optical path. First and second optical signals propagating on the first and second optical paths are combined such that the first optical signals are coupled on a wavelength component by wavelength component basis to either a first output port or a second output port and the second optical signals are selectively coupled on a wavelength by wavelength component basis to the first output port. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0017]    The invention will be better understood from a reading of the following detailed description in conjunction with the drawing in which like reference designations are used in the various drawing figures to identify like elements, and in which:  
         [0018]    [0018]FIG. 1 is a block diagram illustrating wavelength routing optical cross-connect functions;  
         [0019]    [0019]FIG. 2 is a block diagram illustrating a wavelength routing optical cross-connect utilizing prior art switch matrix technology;  
         [0020]    [0020]FIG. 3 illustrates a prior art Sagnac interferometer;  
         [0021]    [0021]FIG. 4 is a diagram of a Sagnac interferometer wavelength router or optical cross-connect in accordance with the principles of the invention;  
         [0022]    [0022]FIG. 5 illustrates the Sagnac interferometer wavelength router of FIG. 4 in greater detail;  
         [0023]    [0023]FIG. 6 illustrates the add/drop of two wavelengths in the router of FIG. 5;  
         [0024]    [0024]FIG. 7 shows a Michelson interferometer structure;  
         [0025]    [0025]FIG. 8 is a diagram of a Michelson interferometer wavelength router or optical cross-connect in accordance with the principles of the invention;  
         [0026]    [0026]FIG. 9 illustrates the Michelson interferometer wavelength router or optical cross-connect of FIG. 8 in greater detail;  
         [0027]    [0027]FIG. 10 illustrates add/drop of two wavelengths in the structure of FIG. 9;  
         [0028]    [0028]FIG. 11 is a diagram of a Mach-Zehnder interferometer wavelength router or optical cross-connect in accordance with the principles of the invention;  
         [0029]    [0029]FIG. 12 illustrates the Mach-Zehnder interferometer router or optical cross-connect of FIG. 11 in greater detail;  
         [0030]    [0030]FIG. 13 illustrates add/drop of two wavelengths in the structure of FIG. 12; and  
         [0031]    [0031]FIG. 14 illustrates a non-reciprocal phase shifter that may be advantageously utilized in the invention.  
     
    
     DETAILED DESCRIPTION  
       [0032]    [0032]FIG. 1 illustrates the functionality of a wavelength routing optical cross-connect  100 . Optical cross-connect  100  has an input port  101  that can receive a number, n, optical wavelength components λ 1 , λ 2 , . . . , λn−1, λn. Optical cross-connect  100  can couple all of the wavelength components λ 1 , λ 2 , . . . ,λn−1, λn to a through port  103 . Selected wavelength components may be substituted for the wavelength components at through port  103  by via add port  107 . In addition, any one or more of the wavelength components λ 1 , λ 2 , . . . , λn−1, λn may be “dropped” from the wavelength components transferred from input port  101  to through port  103  and outputted at drop port  105 . Wavelength optical cross-connect  100  is capable of dropping any combination of wavelength components from input port  101  to drop port  105  and is capable of adding any wavelength component combinations from add port  107  to through port  103 . Typically, when wavelength components are added, the corresponding wavelength components in the input optical signals are dropped.  
         [0033]    [0033]FIG. 2 illustrates wavelength routing optical cross-connect  200  utilizing prior art switch matrix technology. An optical switch matrix  210  is utilized. To provide for “n” multiplexed wavelengths, a complex n×n optical switch matrix density is utilized. Accordingly, n 2  matrix elements must be provided in such prior art arrangements. To provide for optical cross-connect functionality requires that a 1×n DWDM demultiplexer  202  be utilized to de-multiplex n wavelength components from the multiplexed input  201  for coupling to switch matrix  210 . A 1×n DWDM de-multiplexer  208  is also necessary to de-multiplex the multiplexed add wavelength components from add input  207  for coupling to switch matrix  210  for the add wavelength input  207 . An n×1 DWDM multiplexer  206  is used to multiplex the switched wavelength components from switch matrix  210  to multiplexed output  203 . Another n×1 multiplexer  204  is used to multiplex together switched wavelength components from switched matrix  210  to drop output  205 . Each switch matrix element  220  of switch  210  may be in either one or the other of two switched states. As shown in FIG. 2, switch element  211  and switch element  213 , are activated to drop wavelength components λ 1 , λn and output the dropped wavelength components to drop output  203 . In addition wavelengths λ 1 , λn received at input  207  are added and outputted at through output  205 . All the remaining matrix elements pass wavelength components directly from input de-multiplexer  202  to output de-multiplexer  204 . Switch element  211  blocks λ 1  from passing from input de-multiplexer  201  to output multiplexer  204 , allowing add wavelength component λ 1  to traverse path  216  from add de-multiplexer  208  to through multiplexer  204 , while rerouting λ 1  from input de-multiplexer  202  to drop multiplexer  206  via path  218 . Similarly, matrix element  213  allows λn from input de-multiplexer  202  to be routed to drop multiplexer  206  via path  222 .  
         [0034]    Although the example shown drops and adds two wavelengths, it will be understood by those skilled in the art, that any number of wavelengths up to number n may be dropped and added.  
         [0035]    As described above, optical switch matrices such as switch  200  are complex and extremely expensive. They typically have high insertion loss, typically over 10 dB for 64 wavelength components and are relatively slow in switching, i.e. 10 ms. In addition, it is difficult to increase the scale of the switch. By way of example, increasing the number of wavelength components requires an exponential increase in the number of switch matrix elements. By way of example, increasing the number of wavelength components to 256 requires 64,000 switching elements.  
         [0036]    The present invention overcomes the shortcomings of the prior arrangements by utilizing a newly developed interferometer wavelength router technology. With this technology, only one interferometer having n phase modulators or phase shifters is used to achieve the functionality of an n wavelength optical cross-connect. The use of interferometer wavelength router technology leads to very specific advantages. Namely, a very low cost optical cross-connect can be provided that has low insertion loss, on the order of 1-2 dB. The switching speed obtainable is significantly faster, in the microsecond range. The optical router or cross-connect is easy to scale up in size. In addition, an optical cross-connect in accordance with the principles of the invention is highly reliable because it has no moving parts. An optical cross-connect in accordance with the invention is an all optical fiber device.  
         [0037]    [0037]FIG. 3 illustrates a prior art Sagnac type interferometer  300 . Interferometer  300  includes a 2×2 optical coupler  301  that includes optical ports  302 ,  304 ,  306 ,  308 . Ports  306 ,  308  are coupled to a fiber loop  303  to form the well-known configuration of a Sagnac interferometer. Input signals at either port  302  or port  304  produce equal intensity counter-propagating beams in loop  303 . The counter-propagating beams interfere at coupler  301 . Sagnac interferometer principles are well known, and for purposes of succinctness, a description of the operation of the Sagnac interferometer is not presented in this patent.  
         [0038]    [0038]FIG. 4 illustrates an interferometer wavelength router  400  that is based upon a Sagnac interferometer such as that shown in FIG. 3. The Sagnac interferometer configuration is provided by coupler  401  having ports  402 ,  404 ,  406 ,  408 . An optical fiber loop  403  is provided between ports  406 ,  408 . A phase modulator  410  is inserted into the Sagnac loop  403 . A circulator  420  having ports  422 ,  424 ,  426  and a circulator  420  having ports  432 ,  434 ,  436  are each coupled to coupler  401 . Circulators  420 ,  430  have circulation directions indicated by arrows  421 ,  431 , respectively. Circulator  420  has port  424  coupled to port  402  of coupler  401 . Circulator  430  has port  434  coupled to coupler  401  port  404 . Circulator port  430  port  432  functions as an input port and port  436  functions as a through port. Ports  432 ,  436  function as add and drop ports, respectively. Phase modulator  410  has a control input  411  that is utilized to control the operation of phase modulator  410 . More specifically, by controlling the phase shift in Sagnac loop  403 , optical signals may be switched or routed. In the illustrative embodiment shown in FIG. 4, phase modulator  410  is a non-reciprocal phase shifter. A non-reciprocal phase shifter provides a first phase shift in optical signals flowing in one direction and a different phase shift in optical signals flowing in the opposite direction through the phase shifter.  
         [0039]    The Sagnac loop configuration is such that input signals I(ωt) at either port  402  or port  404  produce corresponding counter-propagating beams ½ I(ωt), represented by arrows  441 ,  443 , that propagate from coupler  401  through fiber loop  403 . Non-reciprocal phase shifter  410  provides a non-reciprocal phase shift to the counter propagating beams. In the phase shifter  410  utilized in the illustrative embodiment, an equal magnitude of phase shift Φ is provided to signals in both directions, but the phase shifts are of opposite sign to produce signals ½ I(ωt+Φ), and ½ I(ωt−Φ). When the phase shift Φ of non-reciprocal phase shifter  410  is set to 0°, or the non-reciprocal phase shifter  410  is turned off, Φ=0°, and the phase difference between the two counter-propagating beams after passing through non-reciprocal phase shifter  410  as represented by arrows  441   a ,  443   a  is 0°. In other words, the two beams are in phase. When the two beams recombine at coupler  201  the beams interfere and produce switching such that the optical signals at input port  432  are coupled to through port  436 , and the optical signals at add port  422  are coupled to drop port  426 .  
         [0040]    When the phase shift Φ of non-reciprocal phase shifter  410  is set to 90°, the phase between counter propagating beams  441   a ,  443   a  becomes 180°. In other words, the counter-propagating beams are completely out of phase. When the two counter-propagating, phase shifted beams recombine at coupler  401  the two beams interfere and produce an optical cross-connect such that the optical signals that were at input port  432  are coupled to drop port  426  and optical signals at add port  422  are coupled to through port  436 . Control bus  411  is utilized to provide control signals to determine the phase shift Φ provided by non-reciprocal phase shifter  410 . The structure shown in FIG. 4 will switch/route all wavelengths.  
         [0041]    Turning now to FIG. 5, a Sagnac interferometer wavelength router  400  is shown in more detail to show how a multiple wavelength selective phase shifter is used to separately selectively switch/route a plurality or multiple wavelengths. The structure  400  is identical to that shown in FIG. 4 except that a multiple wavelength non-reciprocal phase shifter  510  is utilized to selectively switch/route individual wavelength components of wavelength multiplexed signals. Multiple wavelength non-reciprocal phase shifter  510  includes multiplexer/de-multiplexer  502  and multiplexer/de-multiplexer  504  and a plurality of non-reciprocal phase shifters  550 . The number of non-reciprocal phase shifters  550  corresponds in number to the number, n, of wavelength components in the multiplexed wavelength component signals at input port  432  and output port  434 . Each non-reciprocal phase shifter  550  is coupled between the corresponding wavelength input/output of multiplexer/de-multiplexer  502  and multiplexer/de-multiplexer  504 . Control bus  511  is utilized to control the operation of each of phase shifters  550  so that the phase shift of each non-reciprocal phase shifter  550  may be controlled independently of all other non-reciprocal phase shifters  550 .  
         [0042]    [0042]FIG. 6 illustrates the operation of the optical cross-connect or router  500  of FIG. 5 for the case where two wavelength components λ 2 , λn are added from add port  422  to input wavelength components λ 1 , λ 2 , . . . ,λn−1, λn received at input port  432 . Wavelength components λ 2 , λn received at port  432  are dropped to drop port  426 . Electrical control signals from a micro controller  1009  are used to individually control the phase shift of non-reciprocal phase shifters  550 . In the illustrative embodiment shown, the magnitude of the phase shift produced by each non-reciprocal phase shifter  550  will be the same for light traveling in a clockwise direction or counter clockwise direction through loop  403 , but the phase shifts will be of opposite sign. The normal or quiescent state for each non-reciprocal phase shifter  550  is to provide a zero phase shift. Input light signals at coupler  401  are split into two counter-propagating light beams. If the non-reciprocal phase shifter  550  for a particular wavelength component does not provide a phase shift, the counter-propagating light beams will be in phase when they reach coupler  401  and will interfere. The result is that the wavelength component is reflected back to the same port  402 ,  404  at which it was supplied to coupler  401 . If the non-reciprocal phase shifter  550  for a wavelength component is set to provide a phase shift of 90°, the clockwise propagating portion of the wavelength component is phase shifted by −90°, and the counter-clockwise propagating portion is phase shifted by +90°. When the counter-propagating wavelength component portions recombine at coupler  401 , they do not interfere and reflect back to the originating port  402  or  404 , but instead interfere and combine and propagate to the other port  404 , or  402 , respectively. In the example shown, non-reciprocal phase shifters  550  for wavelengths λ 2 , and λn are set to provide a 90° phase shift, all other non-reciprocal phase shifters are set to provide a 0° phase shift. Optical wavelength signals λ 1 , λ 2 , . . . ,λn−1, λn at port  432  are applied to port  404  of coupler  401  and each wavelength component is split into two equal counter-propagating beams  441 ,  443  in loop  403 . For wavelength components λ 2  and λn, the corresponding non-reciprocal phase shifters operate so that the wavelength components are switched to port  402 . From port  402 , wavelength components λ 2 , λn are coupled by circulator  420  to drop port  426 . Similarly, add wavelength components λ 2 , λn at add port  422  are split into counter-propagating beams  406 ,  408  on loop  403  by coupler  401 . The same corresponding non-reciprocal phase shifters  550  assigned to the wavelength switch the add wavelength components λ 2 , λn to port  402  of coupler  401 . The add wavelength components are coupled to port  434  of circulator  430 . Circulator  430  couples the add wavelength components to port  436 . All remaining wavelength components at input port  432 , are reflected back by coupler  401  and circulate to port  434  of circulator  430 . The phase shifts for each of wavelength components λ 1 , λ 2 , . . . ,λn−1, λn after passing through non-reciprocal phase shifters  550  for each direction after passing through the non-reciprocal phase shifters is shown in conjunction with arrows  516 ,  518 . For wavelength λ 2 , λ n , the difference is 180°, i.e., these two wavelength components in light beams  526 ,  518  are out of phase. When counter propagating portions of wavelength components λ 2 , λ n  recombine at coupler  401  the counter-propagating portions of the wavelength components will interfere and produce cross-connect. The result is that the two wavelength components λ 2 , λn at input port  432  are automatically transferred to drop port  426  and the two wavelength components λ 2 , λn at add port  422  are coupled to through port  436 . For all other wavelength components, the difference is 0° and those components at input port  432  appear at through port  436 .  
         [0043]    Although the foregoing example utilizes two wavelength components to be added, any number of wavelength components may be added and dropped.  
         [0044]    Turning now to FIG. 7, a prior art Michelson Interferometer  700  is shown. In Michelson interferometer  700 , a 2×2 coupler  701  has ports  702 ,  704 ,  706 ,  708 . Ports  702 ,  704  are used as input/output ports. Port  706  has an optical fiber arm  703  coupled to it and port  708  is coupled to optical fiber arm  707 . Arm  703  terminates in a reflector  705 . Arm  707  terminates in a reflector  709 . The operation Michelson interferometers are known and a description of the operation of such an interferometer is not provided herein.  
         [0045]    [0045]FIG. 8 illustrates an interferometer wavelength router  800  that is based upon a Michelson interferometer such as that shown in FIG. 7. A phase modulator is utilized in a Michelson interferometer configuration. The phase modulator  810  is implemented as a phase shifter  810  coupled into one arm  807  of the interferometer. It should be apparent to those skilled in the art that although only on arm  807  of the structure of FIG. 8 includes a phase modulator or phase shifter, a phase modulator or phase shifter may be also disposed in the other arm  803 . In such a structure, one of the pair of phase modulators could be a non-reciprocal phase shifter and the other could be a reciprocal phase shifter. Each arm  803 ,  807  terminates in a reflective surface or mirror  805 ,  809 , respectively. Reciprocal phase shifter  811  creates a phase shift Φ that is the same regardless of the direction of the light. The phase shifter, or in the case where a pair of phase shifters are utilized, provide switching and routing.  
         [0046]    Input optical signals at ports  802 ,  804  are switched or routed in much the same way that optical signals are switched or routed in the Sagnac interferometer structures described above. Coupler  801  has ports  802 ,  804 ,  806 ,  808 . A circulator  820  having ports  822 ,  824 ,  826  and a circulator  830  having ports  832 ,  834 ,  836  are coupled to coupler  801 . Circulators  820 ,  830  have circulation directions indicated by arrows  821 ,  831 , respectively. Circulator  820  has port  824  coupled to port  802  of coupler  801 . Circulator  830  has port  834  coupled to coupler  801  port  804 . Circulator port  830  port  832  functions as an input port and port  836  functions as a through port. Ports  832 ,  836  function as add and drop ports, respectively. Phase modulator  810  has a control input  811  that is utilized to control the operation of phase modulator  810 . More specifically, by controlling the phase shift in arm  807 , optical signals may be switched or routed. In the illustrative embodiment shown in FIG. 8, phase modulator  810  is a reciprocal phase shifter. A reciprocal phase shifter provides the same amount of phase shift in optical signals flowing in either direction.  
         [0047]    The Michelson interferometer configuration is such that coupler  801  couples a light beam at input port  804  as two equal intensity light beams ½I(ωt) to both arms  807 ,  803 , respectively. The light beam  843  in arm  803  is reflected by reflector  805  to produce return beam  843   a  that is shifted by some amount Φ1. In the specific example shown, Φ1=0°. Light beam  841  passes through phase shifter  810  and is shifted by a phase amount Φ. The shifted beam is reflected by reflector  809  and passes back through phase shifter  810  in the opposite direction. The reflected beam is again shifted by a phase amount Φ. Thus the total amount of phase shift in the return signal  841   a  is 2×Φ=Φ2. By using control signals on bus  811 , the phase shift Φ is selected as either 0° or 90°.  
         [0048]    By selecting the phase shift Φ to be 0°, the beam portions  843   a  and  841   a  are completely in phase. When recombined at coupler  801  these two beams will interfere and cause optical signals at a port  802 ,  804  to reflect back to that same port. By selecting the phase shift to be 90°, the total amount of phase shift Φ2=180°. With a 180° phase shift in the beam  841   a , and no phase shift in beam  843   a , the two beams when combined at coupler  801  interfere and produce a cross-connect of ports  802  and  804 . In other words, when the two beams recombine at coupler  801  the beams interfere and produce switching such that the optical signals at input port  832  are coupled to through port  826 , and the optical signals at add port  822  are coupled to drop port  836 .  
         [0049]    Turning now to FIG. 9, a Michelson interferometer wavelength router  900  that separately switches/routes a plurality or multiple of wavelengths is shown. The structure is identical to that shown in FIG. 8 except that a multiple wavelength phase shifter  810  is utilized to selectively switch/route individual wavelength components of wavelength multiplexed signals. Multiple wavelength phase shifter  810  includes multiplexer/demultiplexer  902 , a plurality of non-reciprocal phase shifters  950 , and a plurality of reflectors  809 . The number of non-reciprocal phase shifters  850  and the number of reflectors  809  each corresponds in number to the number, n, of wavelength components in the multiplexed wavelength component signals at input port  832  and output port  834 . Each phase shifter  950  is coupled between the corresponding wavelength input/output of multiplexer/de-multiplexer  902  and a corresponding one of reflectors  809 . Control bus  811  is utilized to control the operation of each of phase shifters  950  so that the phase shift of each phase shifter  950  may be controlled independently of all other phase shifters  950 .  
         [0050]    [0050]FIG. 10 illustrates the operation of the optical cross-connect or router  800  of FIG. 8 for the case where two wavelength components λ 2 , λn are added from add port  822  to input wavelength components λ 1 , λ 2 , . . . ,λn−1, λn received at input port  832 . Wavelength components λ 2 , λn received at port  832  are dropped to drop port  826 . Electrical control signals from a micro controller  1009  are used to individually control the phase shift of phase shifters  950 . The normal or quiescent state for each non-reciprocal phase shifter  950  is to provide a zero phase shift. Input light signals at coupler  801  are split into two light beams. If phase shifter  950  for a particular wavelength component does not provide a phase shift, the reflected light beams will be in phase when they reach coupler  801  and will interfere. The result is that the wavelength component is reflected back to the same port  802 ,  804  at which it was supplied to coupler  801 . If phase shifter  950  for a wavelength component is set to provide a phase shift of 90°, the reflected portion  841   a  of the wavelength component in that arm is phase shifted by 180°. When two reflected wavelength component portions  841  a,  843   a  recombine at coupler  801 , they interfere to produce a cross-connect and propagate to the other port  804 , or  802 , respectively. In the example shown, phase shifters  850 - 2 ,  850 - n  for wavelengths λ 2 , and λn are set to provide a 90° phase shift, all other phase shifters are set to provide a 0° phase shift. Optical wavelength signals λ 1 , λ 2 , . . . ,λn−1, λn at port  832  are applied to port  804  of coupler  801  and each wavelength component is split into two equal counter-propagating beams in loop  803 . For wavelength components λ 2  and λn, the corresponding phase shifters  850 - 2 ,  850 - n  operate so that the wavelength components are switched to port  802 . From port  802 , wavelength components λ 2 , λn are coupled by circulator  820  to drop port  826 . Similarly, add wavelength components λ 2 , λn at add port  822  are split into beams  906 ,  908  on arms  803 ,  807  by coupler  801 . The same corresponding phase shifters  950  assigned to the wavelength switch the add wavelength components λ 2 , λn to port  802  of coupler  801 . The add wavelength components are coupled to port  834  of circulator  830 . Circulator  830  couples the add wavelength components to port  836 . All remaining wavelength components at input port  832 , are reflected back by coupler  801  and circulate to port  834  of circulator  830 . When reflected portions of wavelength components λ 2 , λ n  recombine at coupler  801  the 180° phase shifted portions of the wavelength components will interfere with the unshifted portions and produce cross-connect. The result is that the two wavelength components λ 2 , λn at input port  832  are automatically transferred to drop port  826  and the two wavelength components λ 2 , λn at add port  822  are coupled to through port  836 . For all other wavelength components, the difference is 0° and those components at input port  832  appear at through port  836 .  
         [0051]    Although the foregoing example utilizes two wavelength components to be added, any number of wavelength components may be added and dropped.  
         [0052]    [0052]FIG. 11 illustrates a Mach-Zehnder interferometer  1100  with phase modulator  1110  in accordance with the invention. A reciprocal phase shifter IS utilized as phase modulator  1110 to provide switching and routing. The Mach-Zehnder configuration utilizes two 2×2 couplers  1101 ,  1103 . Coupler  1101  has four ports  1102 ,  1104 ,  1106 ,  1108  and coupler  1103  has four ports  1112 ,  1114 ,  1116 ,  1118 . A first waveguide arm  1105  couples port  1106  to port  1112  and a second waveguide arm  1107  couples port  1108  to port  1114 . Phase shifter  1110  is disposed in one arm  1107 . Phase shifter  1110  provides switching and routing. Phase shifter  1110  is switchable so as to provide a phase shift of either 0° or 180°. When the phase difference between the beams propagating on arms  1105 ,  1107  is 0°, the beam portions interfere when recombined at coupler  1103  and produce switching such that the input port  1102  is coupled to through port  1116  and add port  1104  is coupled to drop port  1118 . When the phase difference between the beams propagating on arms  1105 ,  1107  is 180°, the beam portions interfere when recombined at coupler  1103  and produce a cross-connect such that signals at input port  1102  are coupled to drop port  1118  and signals at add port  1104  are coupled to through port  1116 .  
         [0053]    Turning now to FIG. 12, a Mach-Zehnder interferometer wavelength router  1100  that separately switches/routes a plurality or multiple of wavelengths is shown. The structure is identical to that shown in FIG. 11 except that a multiple wavelength phase shifter  1210  is utilized to selectively switch/route individual wavelength components of wavelength multiplexed signals. Multiple wavelength phase shifter  1210  includes multiplexer/demultiplexer  1202 , a plurality of phase shifters  1250 , and a second multiplexer/de-multiplexer  1204 . The number of non-reciprocal phase shifters  1250  corresponds in number to the number, n, of wavelength components in the multiplexed wavelength component signals at input port  1102  and output through port  1116 . Each phase shifter  1250  is coupled between the corresponding wavelength input/outputs of multiplexer/de-multiplexers  1204 ,  1204 . Control bus  1111  is utilized to control the operation of each of phase shifters  1250  so that the phase shift of each phase shifter  1250  may be controlled independently of all other phase shifters  1250 .  
         [0054]    [0054]FIG. 13 illustrates the operation of the optical cross-connect or router  1100  of FIG. 11 for the case where two wavelength components λ 2 , λn are added from add port  1104  to input wavelength components λ 1 , λ 2 , . . . ,λn−1, λn received at input port  1102 . Wavelength components λ 2 , λn received at port  1102  are dropped to drop port  1118 . Electrical control signals from a micro controller  1109  are used to individually control the phase shift of phase shifters  1250 . The normal or quiescent state for each phase shifter  1250  is to provide a zero phase shift. Input light signals at coupler  1101  are split into two light beams. If phase shifter  1250  for a particular wavelength component does not provide a phase shift, relative to the wavelength component portion propagating in arm  1105 , the light beams portions propagating in arms  1105  and  1107  will be in phase when they reach coupler  1103 . The result is that the wavelength component from input port  1102  is coupled to through port  1116  and the wavelength component at add port  1101  is coupled to drop port  1118 . If phase shifter  1250  for a wavelength component is set to provide a phase shift of 180°, the portion of the wavelength component in arm  1107  is phase shifted by 180° relative to the portion of the wavelength component in arm  1105 . When the two wavelength component portions recombine at coupler  1103 , they interfere to produce a cross-connect such that the wavelength component from input port  1102  is coupled to drop port  1118  and the wavelength component at add port  1104  is coupled to through port  1116 . In the example shown, phase shifters  1250  for wavelengths λ 2 , and λn are set to provide a 180° phase shift, all other phase shifters  1250  are set to provide a 0° phase shift. Optical wavelength signals λ 1 , λ 2 , . . . ,λn−1, λn at port  1102  of coupler  801  are each split into two equal portions, one propagating on each arm  1105 ,  1107 . For wavelength components λ 2  and λn, the corresponding phase shifters  1250  operate so that the wavelength components from input port  1102  are switched to drop port  1118 . All other wavelength components at input port  1102  are coupled to through port  1116 . Similarly add wavelength components λ 2 , λn at add port  1104  are split into beams on arms  1105 ,  1107  by coupler  1101 . The same corresponding phase shifters  1250  assigned to the wavelength switch the add wavelength components λ 2 , λn to port  1116 . Although the foregoing example utilizes two wavelength components to be added, any number of wavelength components may be added and dropped.  
         [0055]    Reciprocal phase shifter types are known in the prior art and include both waveguide type phase modulators, such as LiNbO 3 , including electro-optic phase modulators and thermal optic modulators, and fiber type phase shifters, including pzt based fiber stretcher type phase shifters.  
         [0056]    One particularly advantageous non-reciprocal phase shifter  1400  that is useable in the structures of the invention is shown in FIG. 14. Optical signals are coupled to and from the non-reciprocal phase shifter  1400  via optical waveguides  1401 ,  1403 , which in the particular embodiment shown are optical fiber. However, in other embodiments, one or both of the waveguides  1401 ,  1403  may be waveguides formed on a substrate and the non-reciprocal phase shifter may be formed on the substrate also as an integrated optic device. Non-reciprocal phase shifter  1400  comprises a Faraday rotator crystal  1405  which may be a crystal or thin-film device. A graded index lens  1407  is attached to the end of optical fiber  1401  and is attached to Faraday rotator crystal  1405 . A second graded index lens  1409  is coupled to optical fiber  1403  and to Faraday rotator crystal  1405 . Lenses  1407 ,  1409  are bonded to optical fibers  1401 ,  1403 , respectively and to Faraday rotator crystal  1405  with epoxy cement. Graded index lenses  1401 ,  1403  are each of a type known in the trade as Sel-Foc lenses.  
         [0057]    Faraday rotator crystal  1405  may be any magneto-optic material that demonstrates Faraday rotation such as Yttrium Iron Garnet or Bismuth Iron Garnet.  
         [0058]    An electromagnet  1425  disposed proximate Faraday rotator crystal  1405  includes a coil assembly  1413 . Electromagnet  1425  provides a magnetic field indicated by field lines  1435  when current flows through coil  1413 . Non-reciprocal phase shifter  1400  operates with optical waves of a single polarization. The polarization, i.e., TE or TM, is determined by the selected crystal orientation. Optical signals in one direction through non-reciprocal phase shifter  1400  are designated as forward beam signals Ifw, and optical signals in the opposite direction are designated as backward beam signals Ibk. For forward beam signals Ifw, non-reciprocal phase shifter  1400  provides a phase shift of ωt+Φ. For backward beam signals Ibw, non-reciprocal phase shifter  1400  provides a reciprocal phase shift of ωt−Φ.  
         [0059]    In the above description reference is made to various directions signal propagation directions. It will be understood that the directional orientations are with reference to the particular drawing layout and are not intended to be limiting or restrictive.  
         [0060]    As will be appreciated by those skilled in the art, various modifications can be made to the embodiments shown in the various drawing figures and described above without departing from the spirit or scope of the invention. It is intended that the invention include all such modifications. It is not intended that the invention be limited to the illustrative embodiments shown and described. It is intended that the invention be limited in scope only by the claims appended hereto.