Abstract:
A reversible optical circulator has an optical switch that includes: an arm composed of piezoelectric material with first and second faces and first and second ends, an electrode coupled to the arm for providing a voltage difference between the first and second faces of the arm, a support coupled to the first end of the arm for fixedly supporting the first end, an object with a convex surface coupled to the second end of the arm, a polarization rotation element coupled to the second face of the arm, a first magnet proximately located to the object and the first face of the arm, and a second magnet proximately located to the object and the second face of the arm. By using this optical switch, the optical circulator has stable and reproducible operation, high switching speeds, and low sensitivity to slight optical mis-alignments.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a DIVISIONAL of U.S. Patent Application titled, “Method And Apparatus For Optical Switching Devices Utilizing A Bi-Morphic Piezoelectric Apparatus”, Ser. No. 09/513,777, filed on Feb. 24, 2000 now U.S. Pat. No. 6,463,189. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to optical devices, and more particularly to optical switching and routing devices. 
     BACKGROUND OF THE INVENTION 
     The use of optical fiber for long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity continues to increase, there is a continuing need to utilize the bandwidth of existing fiber-optic cable more efficiently. An established method for increasing the carrying capacity of existing fiber cable is Wavelength Division Multiplexing (WDM) in which multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light. In this practice, each light-wave-propagated information channel corresponds to light within a specific wavelength range or “band.” 
     Because of the increased network traffic resulting from the use of the WDM technique, there is an increasing need for sophisticated optical switching and routing devices which can quickly route numerous channels among various optical communications lines and which can reliably divert network traffic to alternative routes in the event of network failures. Routine network traffic routing requires optical switching devices that can perform reproducibly over many thousands of switching operations. Network failure restoration requires a switching device that must instantaneously perform according to specification after long periods of non-use. The present invention addresses these needs. 
     SUMMARY OF THE INVENTION 
     A reversible optical circulator has an optical switch that includes: an arm composed of piezoelectric material with first and second faces and first and second ends, an electrode coupled to the arm for providing a voltage difference between the first and second faces of the arm, a support coupled to the first end of the arm for fixedly supporting the first end, an object with a convex surface coupled to the second end of the arm, a polarization rotation element coupled to the second face of the arm, a first magnet proximately located to the object and the first face of the arm, and a second magnet proximately located to the object and the second face of the arm. By using this optical switch, the optical circulator has stable and reproducible operation, high switching speeds, and low sensitivity to slight optical mis-alignments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a  and  1   b  are side and top views, respectively, of a first preferred embodiment of a bimorphic piezoelectric deflection and latching apparatus in accordance with the present invention. 
     FIG. 1 c  illustrates a second preferred embodiment of a bimorphic piezoelectric deflection and latching apparatus in accordance with the present invention. 
     FIGS. 2 a  and  2   b  are side views of the two stable operating positions of the second preferred embodiment of the bimorphic piezoelectric optical switch in accordance with the present invention. 
     FIGS. 3 a  and  3   b  illustrate a first preferred embodiment of an optical switch in accordance with the present invention. 
     FIG. 3 c  is a diagram of the optical pathway of a signal or composite signal through the glass prism in accordance with the present invention. 
     FIG. 3 d  is a graph of total deflection and difference between incidence and exit angles through the glass prism of the optical switch in accordance with the present invention. 
     FIG. 4 a  illustrates a second preferred embodiment of an optical switch in accordance with the present invention. 
     FIGS. 4 b  and  4   c  illustrate two alternative dispositions of the optically slow direction and optically fast direction of a half-wave plate in the differential phase retardance switch in accordance with the present invention. 
     FIGS. 5 a  and  5   b  illustrate a third and a fourth preferred embodiment of an optical switch in accordance with the present invention. 
     FIGS. 6 a ,  6   b  and  6   c  are, respectively, a side view, a top view and an end view of a preferred embodiment of a reversible circulator in accordance the present invention. 
     FIG. 7 is an end view of the port configuration of the input and output ports of the reversible circulator in accordance with the present invention. 
     FIGS. 8 and 9 are sequences of cross sections through the preferred embodiment of the reversible circulator in accordance with the present invention. 
     FIG. 10 a  illustrates the operation of a conventional 4-port optical circulator. 
     FIG. 10 b  illustrates the operation of a preferred embodiment of a reversible circulator in accordance with the present invention. 
     FIGS. 11 a ,  11   b  and  11   c  are, respectively, a side view, a top view and an end view of a preferred embodiment of a switchable optical channel separator in accordance the present invention. 
     FIGS. 12-15 are sequences of cross sections through the preferred embodiment of the switchable optical channel separator in accordance with the present invention. 
     FIGS. 16 a  and  16   b  illustrate two operational states of the switchable optical channel separator in accordance with the present invention. 
     FIG. 17 is an illustration of a preferred embodiment of a self-switching optical line restoration switch in accordance with the present invention. 
     FIG. 18 is an illustration of a preferred embodiment of an optical bypass switch in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides method and apparatus for optical switching devices utilizing a bi-morphic piezoelectric apparatus. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     To more particularly describe the features of the present invention, please refer to FIGS. 1 through 18 in conjunction with the discussion below. 
     FIGS. 1 a  and  1   b  are side and top views, respectively, of a first preferred embodiment  100  of a bimorphic piezoelectric deflection and latching apparatus in accordance with the present invention. The apparatus  100  comprises two elongate plates  102   a - 102   b  comprised of a piezoelectric material such as quartz and securely bonded in parallel to one another and mounted in support members  104   a  and  104   b . A first electrode  101   a  is disposed between the piezoelectric plates  102   a - 102   b  along their bonded faces. Also, a second  101   b  and a third  101   c  electrode is disposed along the side of the first  102   a  and the second  102   b  plate opposite to its bonded face. When so bonded and mounted, the pair of piezoelectric plates  102   a - 102   b  comprise a single cantilever arm  103  comprised of a first end  103   a , which is rigidly physically supported by support elements  104   a - 104   b , and a second opposing “free” end  103   b  which is not permanently physically mounted. Disposed to either side of the free end  103   b  of cantilever arm  103  are a first  110   a  and second  110   b  permanent magnet. Also, a solid object with a rounded convex surface  108 , such as a metallic sphere or spheroid, is mounted at the free end  103   b  of cantilever arm  103 . The metallic sphere or spheroid  108  is comprised of a material such as iron, steel, or nickel that experiences a magnetic force of attraction towards either permanent magnet  110   a  or  110   b . Finally, an optical element  106 , such as a glass prism, is mounted to cantilever arm  103  along a free length of the arm  103  near the metallic sphere  108 . 
     FIG. 1 c  illustrates a second preferred embodiment  150  of the bimorphic piezoelectric deflection and latching apparatus in accordance with the present invention. In the apparatus  150 , the single sphere  108  is replaced by a pair of opposing hemispheres  108   a - 108   b , where the first metallic hemisphere  108   a  is mounted on the first plate  102   a  at the free end  103   b  of arm  103  so as to face the first magnet  110   a , and the second metallic hemisphere  108   b  is mounted on the second plate  102   b  at the free end  103   b  of arm  103  so as to face the second magnet  110   b . The operation of apparatus  150  is not significantly different from that of apparatus  100  described above. 
     When at rest precisely between the two magnets  110   a - 110   b , as shown in FIG. 1 a , the free end  103   b  of cantilever arm  103  is in a hypothetical metastable physical state since the upward force of attraction between sphere or spheroid  108  and the first magnet  110   a  exactly balances the downward force of attraction between sphere or spheroid  108  and the second magnet  110   b . However, such an intermediate metastable state cannot physically exist for any finite period of time because slight perturbations of the position of the arm  103  will create situations in which the upward and downward magnetic forces are unbalanced and where the free end  103   b  of arm  103  will either be pulled upward until sphere/spheroid  108  comes into contact with the first magnet  110   a  or else will be pulled downward until sphere/spheroid  108  comes into contact with the second magnet  110   b . These two alternative positions comprise a pair of stable, “latched” positions. 
     In operation, differential voltages are placed across the faces of the two bonded piezoelectric plates  102   a - 102   b  via the electrode  101   a - 101   c  such that the resulting differential piezoelectric expansion and/or contraction causes flexure of the cantilever arm  103 . Electrode  101   b  maintains a constant voltage and electrode  101   c  is electrically grounded. A variable signal voltage is applied to the central electrode  101   a  so as to create the differential voltages across the two piezoelectric plates  102   a - 102   b . The direction of flexure for cantilever arm  103  is controlled by the magnitude of the signal voltage on electrode  101   a  and can be either upward or downward. Because support members  104   a - 104   b  rigidly support the first end  103   a  of cantilever arm  103 , all such flexure is taken up by the second end  103   b  of arm  103  disposed between magnets  110   a - 110   b . By this means, it is possible to achieve precise, rapid and reproducible bi-stable control of the deflection of the second end  103   b  of cantilever arm  103 , and, more particularly, of the position of the prism  106 . As shown in FIG. 2 a , when apparatus  150  is in the upward latched position or “off” state, prism  106  does not intercept an optical signal  202 . However, as shown in FIG. 2 b , when apparatus  150  is in the downward latched position or “on” state, prism  106  is disposed so as to intercept, and thereby deflect, the optical signal  202 . 
     FIGS. 3 a  and  3   b  illustrate a first preferred embodiment of an optical switch which utilizes the deflection and latching apparatus in accordance with the present invention. This optical switch  300  is a 1×2 optical switch. FIG. 3 a  illustrates the “off” switch position in which apparatus  150  is latched in its upward state such that prism  106  does not intercept signal light pathways. Conversely, FIG. 3 b  illustrates the “on” switch position in which apparatus  150  is latched in its downward state such that prism  106  intercepts signal light pathways. In both FIG. 3 a  and FIG. 3 b , an optical signal or composite optical signal  202  emanates from an input fiber  302  and is collimated by a collimating lens  303  so that the resulting collimated light beam crosses the “on” position of prism  106 . As shown in FIG. 3 a , with the switch  300  in the “off” state, the optical signal or composite optical signal  202  passes in a straight line past the position of apparatus  150  so as to be intercepted by focusing lens  305   a  and thereby focused into first output fiber  304   a . However, with the switch  300  latched in the “on” state, as shown in FIG. 3 b , the signal or composite signal  202  intercepts the prism  106  and is thereby deflected from a straight line path. The deflection is such that the signal or composite signal  202  is intercepted by focusing lens  305   b  and thereby focused into second output fiber  304   b.    
     FIG. 3 c  is a diagram of the optical pathway of a signal or composite signal  202  through the glass prism  106  of the optical switch  300  in accordance with the present invention. The angle φ 1  is the angle of incidence, with respect to the surface normal to the entrance face of prism  106 , of signal  202  upon the prism  106  and the angle φ 2  is the exit angle, with respect to the surface normal to the exit face of prism  106 , of signal  202  upon leaving the prism  106 . 
     For maximum stability of the output ray path against slight angular mis-alignments or vibrations, the angle δ between the projections of the incoming and outgoing segments of signal  202  must vary as little as possible with the angle of incidence φ 1 . This condition is true when δ is at a minimum value and, consequently, when the sum φ 1 +φ 2  is at a minimum value. Simple geometric analysis shows that this condition is true when the angle of incidence φ 1  is chosen such that φ 1 =φ 2 . FIG. 3 d  is a graph of δ and φ 2 −φ 1  versus φ 1  showing that, for a typical prism, the minimum in δ occurs when the incidence and exit angles are identical. The stability and reproducibility of the preferred embodiment is greatest with such a configuration. 
     FIG. 4 a  illustrates a second preferred embodiment of an optical switch in accordance with the present invention. The second preferred embodiment is a differential phase retardance switch  180  which is based upon the bimorphic piezoelectric deflection and latching apparatus  150 , described above. In the switch  180 , instead of a prism, an optical half-wave plate  186  is mounted to the arm  103 . When the switch  180  is in its “off” position, the half-wave plate  186  is not in the path of an optical signal or beam. When the switch  180  is in its “on” position, however, the half-wave plate  186  is disposed so as to intercept the path of an optical signal or beam and to be in a particular optical orientation. Other aspects of the operation of the switch  180  are similar to those of apparatus  150 . The differential phase retardance switch  180  may be utilized in complex switching devices as subsequently described herein in more detail. 
     FIG. 4 b  and FIG. 4 c  illustrate two alternative dispositions of the optically slow direction  188   a  and optically fast direction  188   b  of half-wave plate  186  in the switch  180  in accordance with the present invention. The ellipses in FIGS. 4 b  and  4   c  are representations of the refractive indices experienced by plane polarized light passing through plate  186  with a variety of polarization plane orientations. The orientations of direction  188   a  and direction  188   b  may be interchanged—that is plate  186  may be rotated by 90°—in the configuration of either FIG. 4 b  or FIG. 4 c  without changing the operation of the differential phase retardance switch  180 . In FIGS. 4 b  and  4   c , the angle α  190  represents the rotation angle that the plate  186  undergoes during rotation of the switch  180  from its latched “off” to its latched “on” position. The orientation of either the fast direction  188   b  or the slow direction  188   a  of half-wave plate  186  makes an angle β  192  with the horizontal when apparatus  180  is in the “off” position. In the preferred configuration illustrated in FIGS. 4 b - 4   c , the angle  190  comprises the angle between the base of plate  186  and the horizontal, but this need not be the case. 
     The configuration illustrated in FIG. 4 b  is such that the slow and fast directions of half-wave plate  186  are disposed horizontal and vertical, or vice versa, when switch  180  is in its “on” position and half-wave plate  186  is disposed so as to intercept an optical path. The configuration illustrated in FIG. 4 b  is suitable for rotating the polarization plane of plane polarized light from a first to a second orientation where the first and second orientations are both at 45° to the horizontal or vertical. The configuration illustrated in FIG. 4 c  is such that the slow and fast directions of half-wave plate  186  are disposed at 45° to the horizontal and vertical when the switch  180  is in its “on” position. The configuration illustrated in FIG. 4 c  is suitable for rotating the polarization plane of plane polarized light from horizontal to vertical or vice versa. 
     FIGS. 5 a  and  5   b  illustrate a third and a fourth preferred embodiment of optical switches in accordance with the present invention. These embodiments are 1×4 optical switches, each comprising a cascaded arrangement of a set of bimorphic piezoelectric apparatuses in accordance with the present invention. Both the parallel cascaded 1×4 optical switch  500  of FIG. 5 a  and the serial cascaded 1×4 optical switch  550  of FIG. 5 b  comprise a single input optical fiber  502  disposed adjacent to a collimating lens  503  and a set of four output optical fibers  504   a - 504   d , each of which is disposed adjacent to its own focusing lens  505   a - 505   d , respectively. It is to be kept in mind, however, that the illustrated optical pathways in either switch  500  or switch  550  may be reversed so as to comprise a 4×1 optical switch with four input fibers  504   a - 504   d  and a single output fiber  502 . In switch  500  (FIG. 5 a ), three bimorphic piezoelectric deflection/latching apparatuses  150   a - 150   c  in accordance with the present invention are disposed adjacent to one another so that the optical signal or beam pathways  506   a - 506   d  cross the positions of the apparatuses  150   a - 150   c  in sequence and either pass by each apparatus in a straight line or are deflected depending upon whether the apparatus is in its “off” or “on” position, respectively. In switch  550  (FIG. 5 b ), two bimorphic piezoelectric deflection/latching apparatuses  150   d - 150   e  in accordance with the present invention are disposed similarly. 
     For instance, in FIG. 5 a , an optical beam or signal  508  is output from fiber  502  and, after being collimated by lens  503 , initially follows path segment  506   a , which crosses the position of first deflection/latching apparatus  150   a . Depending upon whether apparatus  150   a  is in the “off” or “on” state, signal  508  either passes the position of apparatus  150   a  undeflected, thereby remaining on path  506   a , or else is deflected onto path  506   b , respectively. The path  506   a  and the path  506   b  continue on so as to cross the positions of deflection/latching apparatuses  150   c  and  150   b , respectively. If apparatus  150   a  is “off”, then, depending upon whether apparatus  150   c  is in the “off” or “on” state, signal  508  either continues on along path  506   a  so as to be focused by lens  505   a  into fiber  504   a  or else is deflected onto path  506   c  so as to be focused by lens  505   c  into fiber  504   c , respectively. Likewise, if apparatus  150   a  is “on”, then, depending upon whether apparatus  150   b  is in the “off” or “on” state, signal  508  either continues on along path  506   b  so as to be focused by lens  505   b  into fiber  504   b  or else is deflected onto path  506   d  so as to be focused by lens  505   d  into fiber  504   d , respectively. By this means, the apparatus  500  functions as a 1×4 optical switch. 
     In the 1×4 switch  550 , only two deflection and latching apparatuses,  150   d - 150   e  are utilized. The prism  106   e  of apparatus  150   e  is larger and formed with a wider apex angle than that of the prism  106   d  of apparatus  150   d . The apparatus  150   d  either passes optical signal  512  straight through along optical path  510   a  without deflection, or else deflects it onto path  510   b  depending upon the state of apparatus  100   d . Both optical pathways  510   a  and  510   b  are subsequently intercepted by the prism  106   e  comprising deflection/latching apparatus  150   e . Depending upon whether deflection/latching apparatus  150   e  is “off” or “on”, it respectively either passes signal  512  straight through along one of the paths  504   a  or  504   b  without deflection, or else deflects signal  512  onto one of the paths  504   c  or  504   d . The signal  512  is then focused by one of the lenses  505   a - 505   d  into one of the output fibers  504   a - 504   d . By this means, the apparatus  550  functions as a 1×4 optical switch. 
     Either of the switch embodiments illustrated in FIGS. 5 a  and  5   b  may be expanded to a greater number of output ports by adding more deflection and latching apparatuses in accordance with the present invention in either the parallel cascade (FIG. 5 a ) or the serial cascade (FIG. 5 b ) arrangement. Moreover, the separate deflection and latching apparatuses comprising either the switch  500  or the switch  550  may be disposed to as to cause successive signal deflections about respective axes that are not parallel to one another. This latter arrangement produces a switch capable of directing signals to outputs disposed within three dimensions, thereby saving space and increasing usage flexibility. The switch  550  has the advantage over switch  500  of utilizing fewer components, thereby facilitating alignment and fabrication ease, and producing the advantage of compactness. However, the switch  500  has the potential advantage over switch  550  of not requiring ever-larger deflection prisms for the second and subsequent deflection/latching apparatuses of which it is comprised. 
     FIGS. 6 a  and  6   b  respectively show a side view and a top view of a preferred embodiment of a reversible optical circulator  600  which utilizes the differential phase retardance switch  180  in accordance with the present invention to switch optical circulation between logical “clockwise” and “counterclockwise” directions. In the reversible optical circulator  600  shown in FIGS. 6 a  and  6   b , reference numeral  615  is a ferrule and reference numerals  601 ,  602 ,  603  and  604  are four optical ports contained within or secured by ferrule  615 . Preferably, such optical ports comprise optical fibers although they may comprise any type or combination of types of optical inputting and outputting device, such as windows. 
     FIG. 7 shows an end view of the configuration of the four ports—Port A  601 , Port B  602 , Port C  603  and Port D  604 —as viewed from the left side of the device  600  of FIGS. 6 a  and  6   b . As also shown in FIGS. 6 a  and  6   b , four collimator lenses  605 - 608  are disposed at the end of ferrule  615  such that each collimator receives light from and directs light to exactly one of the ports, specifically Port  601 ,  602 ,  603  and  604 , respectively. Collimated light rays emanating from any of these four ports  601 - 604  are parallel to one another and define the direction of the main axis of reversible circulator  600 . 
     In this specification, the positive or forward direction of the main axis of the reversible circulator  600  is defined as extending from left to right as viewed in either FIG. 6 a  or  6   b . Consequently, as used in this document, the term “emanating from” refers to light or signal propagation along the positive main axis, from left to right, of circulator  600 , and the term “destined for” refers to light propagation in the reverse direction, from right to left, along the negative direction of the main axis of the circulator  600 . 
     Disposed adjacent to the end of ferrule  615  is a first birefringent walk-off plate  609  which has the property of separating any signal light ray emanating from any of the ports  601 - 604  into two physically separated linearly polarized sub-signal rays—one innermost and one outermost sub-signal ray. This separation of signals into sub-signals is accomplished by deflection or offset of the path of one—the e-ray—of each pair of sub-signals in a first direction perpendicular to the circulator main axis. Because four ports exist, eight separate sub-signals are so defined and are comprised of four outermost and four innermost sub-signals. The outermost and innermost sub-signals from both Port A  601  and Port B  602  comprise e-rays and o-rays, respectively, in their traverse through birefringent plate  609 . Conversely, the outermost and innermost sub-signals from both Port C  603  and Port D  604  comprise o-rays and e-rays, respectively, in their traverse through birefringent plate  609 . 
     Disposed adjacent to the first birefringent plate  609  and on the side of plate  609  opposite to ferrule  615  are both a first  610  and a second  611  optical rotator, respectively. These two optical rotators,  610  and  611 , have the property of rotating the orientation of the plane of polarized light passing therethrough by 90° around or about the light propagation direction. In the preferred embodiment, both optical rotators  610  and  611  comprise half wave plates, although either or both may comprise some other type of optically active element such as a liquid crystal device. Optical rotator  610  is disposed so as to intercept only the two outermost sub-signals arising from or destined for Port A  601  and Port B  602 . Likewise, optical rotator  611  is disposed so as to intercept only the two outermost sub-signals arising from or destined for Port C  603  and Port D  604 . 
     A second birefringent walk-off plate  612  is disposed adjacent to the two reciprocal optical rotators  610  and  611  on the side opposite to the first birefringent plate  609 . The thickness and optical orientation of birefringent plate  612  are chosen so as to provide an offset in the first direction of one of the rays propagating therethrough by a distance equivalent to the common center-to-center inter-port separation distance. 
     A pair of 45° optical polarization rotation elements—a reciprocal optical rotator  616  and a non-reciprocal optical rotator  617 —are disposed to the side of the second birefringent walk-off plate  612  opposite to the 90° optical rotators  610  and  611 . As shown in FIG. 6 b , the reciprocal optical rotator  616  is disposed so as to intercept all and only those sub-signal light rays either emanating from or destined for Port A  601  and Port C  603 . The polarization plane direction of linearly polarized light of sub-signals propagating through reciprocal optical rotator  616  is reversibly rotated by 45° in the clockwise (CW) direction. The non-reciprocal optical rotator  617  is disposed so as to intercept all and only those sub-signal light rays either emanating from or destined for Port B  602  and Port D  604 . The polarization plane direction of linearly polarized light of sub-signals propagating through non-reciprocal optical rotator  617  is non-reversibly rotated by 45° in the counter-clockwise (CCW) direction. 
     A switchable 90° optical rotation element  618  is disposed to the side of either the reciprocal optical rotator  616  or the non-reciprocal optical rotator  617  opposite to that of the plate  612 . The switchable 90° optical rotation element  618  is controlled so as to rotate or not rotate the polarization plane of all light either emanating from or destined for exactly two of the optical ports. If, as in the example illustrated in FIG. 6 b , the switchable rotation element  618  is disposed adjacent to non-reciprocal rotator  617 , then the switchable rotation element  618  can rotate the polarization plane of all and only those sub-signal light rays either emanating from or destined for Port B  602  and Port D  604 . If, on the other hand, the switchable rotation element  618  is disposed adjacent to reciprocal rotator  616 , then the switchable rotation element  618  is capable of rotating the polarization plane of all and only those sub-signal light rays either emanating from or destined for Port A  601  and Port C  603 . 
     A lens or lens assembly  613  is disposed to the side of element  618  opposite to rotation elements  616  and  617 . Finally, a mirror  614  is disposed at the focal point of lens  613  opposite to the rotation elements  616 - 618 . 
     The two states of switchable 90° optical rotation element  618  comprise a first state in which the orientation of the plane of polarized light either emanating from or destined for the two ports in question is rotated by 90° and a second state in which the orientation is not rotated. In the preferred embodiment of the present invention, the switchable 90° optical rotation element  618  comprises the half wave plate  186  of a differential phase retardance switch  180 , as shown in FIG. 4 a . FIG. 6 c  shows an end view of the circulator  600  illustrating the disposition of element  618  in relation to the differential phase retardance switch  180  and a few other selected components of reversible circulator  600 . In the preferred embodiment of reversible circulator  600 , the first and second state of switchable 90° optical rotation element  618  respectively correspond to the situation in which the element  618  is disposed so as to not intercept and so as to intercept optical ray paths emanating from or destined for the two ports in question. As further illustrated in FIG. 6 c , the two states of element  618  are controlled by the latching state of switch  180  and the fast and slow optical orientations of the wave plate comprising element  618  are disposed horizontal and vertical or vice versa. In an alternative embodiment, the two-state 90° optical rotation element  618  may comprise a liquid crystal device, wherein the two polarizing states of the liquid crystal device are controlled by a voltage applied across the device. 
     As used in this specification, the terms “reciprocal optical rotator” or equivalently “reversible optical rotator” or “reciprocally rotating optical element” refer to optical components having the property such that the direction of rotation about the axis of light propagation, either clockwise (CW) or counter-clockwise (CCW), of the plane of polarization of linearly polarized light propagated therethrough is always the same when viewed facing the rotator towards the side at which the linearly polarized light beam enters the component. Conversely, the terms “non-reciprocal optical rotator” or equivalently “non-reversible optical rotator” or “non reciprocally rotating optical element” refer to optical components having the property such that the direction of rotation about the axis of light propagation, either clockwise (CW) or counter-clockwise (CCW), of the plane of polarization of linearly polarized light propagated therethrough is always the same when viewed facing the rotator from a fixed reference point in a fixed direction, regardless of the propagation direction of the light ray through the element. Non-reciprocal rotators typically comprise Faraday rotators, which rotate polarization planes of polarized light passing therethrough in response to or under the influence of an external magnetic field. A magnet or magnets in close proximity to the Faraday rotator usually produce the external magnetic field. In the case in which the non-reciprocal rotator  617  comprises a Faraday rotator, the optical circulator  600  also comprises such magnets but, for clarity, these magnets are not shown in FIGS. 6 a - 6   c.    
     The operation of circulator  600  is described herein below with reference to FIG.  8  and FIG.  9 . FIGS. 8 and 9 are both sequences of cross sections through the reversible circulator  600  illustrating the locations and polarization states of port images created by the light of signals and sub-signals propagating therethrough in accordance with the present invention. The cross sections of FIG. 8 represent operation of the reversible circulator  600  in which the switchable 90° optical rotation element  618  is in its first, “off,” or no-rotation state. Conversely, the cross sections of FIG. 9 represent operation of the reversible circulator  600  in which the switchable 90° optical rotation element  618  is in its second or 90°-rotation state. 
     The cross-sections of FIGS. 8-9 are all drawn as viewed from the left side of the device  600  of FIGS. 6 a  and  6   b  and are taken at the labeled cross-sectional planes U-U′, V-V′, W-W′, X-X′, and Y-Y′. These cross-sections correspond to locations similarly labeled on FIGS. 6 a  and  6   b . In the cross sections of FIGS. 8-9, the centers of labeled circles denote the positions of port images created by sub-signals propagating through circulator  600  as projected onto the respective cross section. Concentric circles of different sizes indicate overlapping or co-propagating sub-signals. The sizes of these circles in the diagrams of FIGS. 8-9 have no physical significance. Barbs on the circles of FIGS. 8-9 indicate the orientations of polarization planes of the linearly polarized sub-signals that the respective circles represent. Circles with two pairs of barbs represent unpolarized or randomly polarized light or else superimposition of two lights with differing linear polarization orientations. A cross (“+”) in each cross-section of FIGS. 8-9 represents the projection of the center of the lens  613  onto the cross section along a line parallel to the circulator main axis. 
     As will be evident from the discussion following, all sub-signal light is reflected by the mirror  614  of the reversible circulator  600  so as to make one complete forward and one complete return traverse through reversible circulator  600 . Therefore, each cross-section of sub-signal port images is shown twice, one time labeled with capital letters to denote forward propagation (FIGS. 8-9, upper rows) along the positive direction of the circulator main axis and one time labeled with small letters (FIGS. 8-9, lower rows) to denote reverse propagation along the negative direction of the circulator main axis. Heavy arrows indicate the sequence of images produced by light signals propagating through the reversible circulator  600 . 
     The paths of signals and sub-signals propagating through reversible circulator  600  in its first state are now described with reference to FIG.  8 . As seen in cross section U-U′  800  of FIG. 8, signals emanating from each of the four ports—Port A  601 , Port B  602 , Port C  603  and Port D  604 —are comprised of unpolarized light. After emanating from one of the four ports and passing through one of the collimator lenses  605 - 608 , signal light enters and passes through the first birefringent plate  609  which separates it into physically separated horizontally and vertically polarized sub-signal components. In FIG. 8, sub-signal A  810 , sub-signal B  812 , sub-signal C  814  and sub-signal D  816  represent the images of horizontally polarized sub-signal light emanating, respectively, from Port A  601 , Port B  602 , Port C  603  and Port D  604 . Likewise, sub-signal A′  811 , sub-signal B′  813 , sub-signal C′  815  and sub-signal D′  817  represent the images of vertically polarized sub-signal light emanating, respectively, from Port A  601 , Port B  602 , Port C  603  and Port D  604 . It is to be noted the terms “vertical” and “horizontal” are used in this specification in a relative sense only and do not necessarily imply any particular spatial orientation of the referred-to apparatus or component. 
     The four vertically polarized sub-signals A′  811 , B′  813 , C′  815  and D′  817  all comprise e-rays during their traverse through the first birefringent plate  609 . Therefore, as shown in cross-section V-V′  801 , sub-signals  811 ,  813 ,  815  and  817  are all shifted in the first direction with respect to the corresponding horizontally polarized sub-signals  810 ,  812 ,  814  and  816 , respectively. After passing through the first birefringent plate  609 , the four outermost sub-signals A′  811 , B′  813 , C  814  and D  816  pass through one of the two 90° optical rotators,  610  and  611 , and therefore their light rays incur 90° rotations of the orientations of their polarization planes. Thus, as shown in cross section W-W′  802 , the polarization plane directions of sub-signals A′  811  and B′  813  change from vertical to horizontal while those of sub-signals C  814  and D  816  change from horizontal to vertical. 
     After passing the positions of the reciprocal optical rotators  610  and  611 , all sub-signals enter and pass through the second birefringent walk-off plate  612 . The four vertically polarized sub-signals C′  815 , D′  817 , C  814  and D  816  traverse birefringent plate  612  as e-rays and are thus deflected in the first direction while the four horizontally polarized sub-signals A′  811 , B′  813 , A  810  and B  812  traverse birefringent plate  612  as undeflected o-rays. The optical orientation and thickness of birefringent plate  612  are chosen such that the lateral deflection of e-rays upon traversing therethrough is exactly equal to the center-to-center inter-port separation distance. For this reason, after passing through birefringent plate  612 , the two sub-signal images C′  815  and C  814  become superimposed on the sub-signal images A′  811  and A  810 , respectively and the two sub-signal images D′  817  and D  816  become superimposed on the sub-signal images B′  813  and B  812 , respectively. Furthermore, the two sub-signals comprising each pair of superimposed sub-signals each follow identical paths until later separated during their return paths. This superimposition of sub-signals is shown in cross section X-X′  803  of FIG.  8 . 
     After exiting plate  612 , each pair of superimposed sub-signals, A′  811  and C′  815 , A  810  and C  814 , B′  813  and D′  817 , and B  812  and D  816  each travels along its own path with the two sub-signals comprising each pair remaining superimposed, one upon the other. The two pairs of sub-signals A′  811  and C′  815  and A  610  and C  614 , which comprise all and only that light originating from Port A and Port C, pass through the 45° reciprocal optical rotator  616 . In passing through reciprocal optical rotator  616 , the polarization plane directions of light comprising these four sub-signals are all rotated by an angle of 45° CW around or about their propagation directions. The two pairs of sub-signals B′  813  and D′  817  and B  812  and D  816 , which comprise all and only that light originating from Port B and Port D, pass through the non-reciprocal optical rotator  617 . In passing through non-reciprocal optical rotator  617 , the polarization plane directions of light comprising these four sub-signals are all rotated by an angle of 45° CCW around or about their propagation directions. Barbs in cross section Y-Y′  804  show the orientations of the polarization planes of light of the various sub-signals after exiting elements  816  and  817 . 
     The four pairs of sub-signals travel to and through the lens  613 , which brings them all to a common focal point at mirror  614 . The mirror  614  immediately reflects all sub-signals back along their return paths through circulator  600 . Because the focal point of the lens  613  is on the plane of mirror  614 , the four pairs of sub-signals immediately diverge from one another after being reflected by the mirror  614  and pass through lens  613  a second time in the reverse direction. The diverging pathways of the four pairs of returning sub-signals are set once again parallel to one another by lens  613 . Because the projection of the center of lens  613  onto cross-section Y-Y′  804  is centrally located between the four pairs of port images and because the focal point of lens  613  is on mirror  614 , the four pairs of sub-signals are directed back towards reciprocal optical rotator  616  and non-reciprocal optical rotator  617  along pathways which exactly superimpose upon those of forward propagating pairs of sub-signals. 
     Cross section y-y′  805  shows the locations of the pairs of superimposed sub-signal images at their points of return entry into reciprocal optical rotator  616  and non-reciprocal optical rotator  617 . The focusing and re-collimation of sub-signal images by lens  613  causes the inversion of image positions about the center of the lens as projected onto cross-section y-y′  805 . This inversion causes interchange of the positions of opposing pairs of sub-signals as projected onto cross-section y-y′  805 . Thus, upon re-entry into either reciprocal optical rotator  616  or non-reciprocal optical rotator  617 , as shown in cross-section y-y′  805 , the location of the returning pair of sub-signal images B  812  and D  816  is the same as that of the forward propagating pair of sub-signals A′  811  and C′  815 . Likewise, in cross-section y-y′  805 , the locations of returning pairs of sub-signals A  810  and C  814 , B′  813  and D′  817 , and A′  811  and C′  815  are identical to those of forward propagating pairs of sub-signals B′  813  and D′  817 , A  810  and C  814 , and B  812  and D  816 , respectively. 
     Because of the inversion properties of lens  613 , each of the returning sub-signals within reversible circulator  600  encounters an optical rotation element—either the reciprocal optical rotator  616  or the non-reciprocal optical rotator  617 —through which it did not pass during its forward path through reversible circulator  600 . Thus, after passing through lens  613  on their return traverse through reversible circulator  600 , the sub-signals B  812 , B′  813 , D  816  and D′  817  all pass through reciprocal optical rotator  616  and thus their light rays incur 45° CW rotations of the directions of their polarization planes. Because reciprocal optical rotator  616  is a reversible optical rotator and the sub-signal propagation in question is in the return direction, this rotation has an apparent CCW direction as viewed from the left side of the device  600  and as indicated in FIG.  8 . The sub-signals A  810 , A′  811 , C  814  and C′  815  all pass through non-reciprocal optical rotator  617  and thus their light rays incur 45° CCW rotations of the directions of their polarization planes after passing through lens  613  on their return traverse through reversible circulator  600 . Because non-reciprocal optical rotator  617  is a non-reversible optical rotator, the rotation of the polarization planes of sub-signals passing therethrough is always in the CCW direction as viewed from the left side of the device  600 . The polarization state of each of the sub-signals after passing through either reciprocal optical rotator  616  or non-reciprocal optical rotator  617  in the return direction is therefore either horizontal or vertical as indicated in cross section x-x′  806  of FIG.  8 . With the circulator  600  in its first state, as shown in FIG. 8, the optical rotation element  618  causes no additional polarization plane rotation of sub-signals passing between cross section y-y′  805  and cross section x-x′. 
     During return passage through the second birefringent plate  612 , the vertically polarized sub-signals B  812 , C  814 , B′  813  and C′  815  pass therethrough as deflected e-rays while the horizontally polarized sub-signals D  816 , A  810 , D′  817  and A′  811  pass therethrough as undeflected o-rays. For this reason, the two sub-signals comprising each pair of superimposed sub-signals become re-separated one from another upon passing through birefringent plate  612  a second time. The deflection of sub-signals B  812 , C  814 , B′  813  and C′  815  upon their second traverse through birefringent plate  612  is exactly equal and opposite to the deflection of sub-signals C′  815 , D′  817 , C  814 , and D  816  and during their first traverse through this plate  612 . Therefore, the locations of the images of the various sub-signals after the second traverse of these sub-signals through birefringent plate  612  are as shown in cross section w-w′  807  of FIG.  8 . 
     After exiting the second birefringent plate  612 , the outermost returning sub-signals D  816 , A  810 , B′  813  and C′  815  pass through one of the two 90° optical rotators,  610  and  611 , and therefore their light rays incur 90° rotations of the orientations of their polarization planes. As a result of these rotations, the polarization plane directions of light of sub-signals D  816  and A  810  become vertical and those of the light of sub-signals B′  813  and C′  815  become horizontal. The positions and polarization states of the various sub-signals are thus as shown in cross section v-v′  808  after passing, in the return direction, the positions of the 90° reciprocal optical rotators,  610  and  611 . 
     Finally, all sub-signals enter the first birefringent walk-off plate  609  in the return direction. The vertically polarized sub-signals D  816 , A  810 , B  812  and C  814  pass through plate  609  as deflected e-rays whilst the horizontally polarized sub-signals D′  817 , A′  811 , B′  813  and C′  815  pass through plate  609  as undeflected o-rays. The deflection of sub-signals D  816 , A  810 , B  812  and C  814  during return passage through plate  609  is exactly equal and opposite to the deflection of sub-signals A′  811 , B′  813 , C′  815  and D′  817  during their forward passage through this plate  609 . Therefore, the vertically and horizontally polarized pairs of sub-signals A  810  and A′  811 , B  812  and B′  813 , C  814  and C′  815 , and D  816  and D′  817  become recombined at the positions of the collimator lenses  605 - 608 . Each of the collimator lenses focuses the return-path signal impinging thereon into the immediately adjacent port. As shown in cross section u-u′  809 , therefore, the recombined signals are located such that the signals originally from Port A, from Port B, from Port C and from Port D are directed into Port B, Port C, Port D and Port A, respectively. In this fashion, when reversible circulator  600  is in its first or “off” state, it functions as a logical “clockwise” optical circulator. 
     FIG. 9 illustrates the operation of reversible circulator  600  in its second or “on” state. In this first state, the switchable 90° optical rotation element  618  imposes a 90° rotation upon the polarization plane orientation of plane polarized light passing therethrough. The manifestation of this 90° rotation is illustrated in the sequence of cross sections  903 - 904  and in the sequence of cross sections  905 - 906  in FIG.  9 . In passing from cross section X-X′  903  to Y-Y′  904 , the sub-signals B  812 , B′  813 , D  816  and D′  817  all pass through the non-reciprocal optical rotator  617  as well as through switchable 90° optical rotation element  618 . The polarization planes of these four sub-signals are first rotated 45° CCW by non-reciprocal optical rotator  617  and then rotated an additional 90° by element  618 . The net effect of these two rotations in sequence is equivalent to a 45° CW rotation of the polarization planes of sub-signals B  812 , B′  813 , D  816  and D′  817  between cross section X-X′  903  and cross section Y-Y′  904 . The polarization plane orientation of light of sub-signals A  810 , A′  811 , C  814  and C′  815  only undergoes a single 45° CW rotation from passage through reciprocal optical rotator  616  as previously described in the discussion to FIG.  8 . 
     In passing from cross section y-y′  905  to x-x′  906 , the sub-signals A  810 , A′  811 , C  814  and C′  815  all pass through the switchable 90° optical rotation element  618  followed by the non-reciprocal optical rotator  617 . Thus, the polarization planes of these four sub-signals are first rotated by 90° by element  618  and then rotated an additional 45° CCW (as viewed from the left side of FIGS. 6 a  and  6   b  according to the convention of FIGS. 8-9) by element  617 . The net effect of these two rotations in sequence is equivalent to a 45° CW rotation (as viewed from the left of FIGS. 6 a - 6   b ) of the polarization planes of sub-signals A  810 , A′  811 , C  814  and C′ between cross section y-y′  905  and cross section x-x′  906 . 
     Each of the sub-signals  810 - 817  incurs an additional 90° rotation of its polarization plane orientation when reversible circulator  600  is in its second or “on” state relative to the situation in which reversible circulator  600  in its first or “off” state. This additional 90° rotation is illustrated by comparison of cross sections  907 ,  908  and  909  with cross sections  807 ,  808  and  809 , respectively. Because of this additional 90° rotation in the “on” state of reversible circulator  600 , the identities of o-rays and e-rays are interchanged from those in the “off” state during the return passage of sub-signals through second birefringent plate  612 . Thus, in the “on” state, the paths of sub-signals D  816 , A  810 , D′  817  and A′  811  are deflected during the return passage through second birefringent plate  612  (FIG.  9 ), but, in the “off” state, those of B  812 , C  814 , B′  813  and C′  815  are instead deflected (FIG.  8 ). As a final result, with reversible circulator  600  in the first or “on” state, the light signals from Port A, Port B, Port C and Port D are respectively directed to Port D, Port A, Port B and Port C. Thus, in this fashion, when reversible circulator  600  is in its second or “on” state, it functions as a logical “counterclockwise” optical circulator. 
     FIG. 10 a  illustrates the operation of a conventional four-port optical circulator  1000 . In the circulator  1000 , light input to Port A  1002  is output from Port B  1004 , light input to Port B  1004  is output from Port C  1006 , light input to Port C  1006  is output from Port D  1006  and light input to Port D  1008  is output from Port A  1002 . This operation is termed herein as “clockwise” optical circulation. 
     By contrast, FIG. 10 b  illustrates the operation of the preferred embodiment of the reversible optical circulator  600  in accordance with the present invention. In its “off” state, the reversible circulator  600  operates with “clockwise” optical circulation. However, in its “on” state, the reversible circulator  600  operates with “counterclockwise” optical circulation, which is exactly opposite to “clockwise” circulation. The “clockwise” or “counterclockwise” state of reversible circulator  600  is controlled by the state of the switchable 90° optical rotation element  618 . When switchable 90° optical rotation element  618  is in its “on” state such that there is effected a 90° rotation of the polarization plane of plane polarized light passing therethrough or there-past, then reversible circulator  600  operates in the “counterclockwise” state. However, when switchable 90° optical rotation element  618  is in its “off” state such that there is no polarization plane rotation of plane polarized light passing therethrough or there-past, then the operation of reversible circulator  600  is “clockwise”. When the switchable 90° optical rotation element  618  comprises the half-wave plate of a differential phase retardance switch  180  in accordance with the present invention, then the reversible circulator  600  can be switched between its two circulatory states in approximately one millisecond. 
     FIGS. 11 a  and  11   b  are side and top views, respectively, of a preferred embodiment of a switchable optical channel separator in accordance with the present invention which utilizes the differential phase retardance switch  180 . Most of the components comprising the switchable optical channel separator  1100  illustrated in FIGS. 11 a - 11   b  are identical in type and disposition to their counterparts in the reversible circulator  600  and are therefore numbered similarly to those counterparts as shown in FIGS. 6 a  and  6   b . However, the switchable optical channel separator  1100  does not comprise the reciprocal rotator  616  or the non-reciprocal rotator  617 , and comprises a non-linear interferometer  1114  in place of the mirror  614 . The switching capability of switchable optical channel separator  1100  is derived from the operation of the switchable 90° optical rotation element  618  which, in the preferred embodiment, comprises the half-wave plate of a differential phase retardance switch  180  of the present invention, as is illustrated in FIG. 11 c.    
     The non-linear interferometer  1114  is an instance of an invention disclosed in a co-pending U.S. Patent Application, incorporated herein by reference, entitled “Nonlinear Interferometer for Fiber Optic Wavelength Division Multiplexer Utilizing a Phase Differential Method of Wavelength Separation,” Ser. No. 09/247,253, filed on Feb. 10, 1999, and also in a second co-pending U.S. Patent Application, also incorporated herein by reference, entitled “Dense Wavelength Division Multiplexer Utilizing an Asymmetric Pass Band Interferometer”, Ser. No. 09/388,350 filed on Sep. 1, 1999. The non-linear interferometer  1114  has the property such that, if the light beam reflected therefrom is an optical signal comprised of a plurality of channels and the light of each channel is linearly polarized, then the light of every member of a second set of channels is reflected with a 90° rotation of its polarization plane direction while the light of every member of a first set of channels, wherein the first and second channel sets are interleaved with one another, is reflected with unchanged polarization. In the following discussion, the channels whose light rays experience the 90° polarization-plane rotation upon interaction with non-linear interferometer  1114  are arbitrarily referred to as “even” channels and the remaining channels are referred to as “odd” channels. The patent application with Ser. No. 09/247,253 teaches the operation of an interferometer in which all channels have identical channel spacings and channel band widths. The patent application with Ser. No. 09/388,350 teaches the operation of an interferometer in which the channel bandwidths of the first interleaved set of channels are not the same as those of the second interleaved set of channels and the channel spacing is not uniform. 
     FIGS. 12-15 illustrate the operation of the switchable optical channel separator  1100  and, similarly to FIGS. 8-9, comprise sequences of cross sections through separator  1100  illustrating the locations and polarization states of fiber images. FIGS. 12 and 13 illustrate the propagation of signals of odd and even channels, respectively, through the separator  1100  in its first state. This first state is such that the switchable 90° optical rotation element  618  does not rotate the polarization plane of polarized light passing therethrough. FIGS. 14 and 15 illustrate the propagation of signals of odd and even channels, respectively, through the separator  1100  in its second state. This second state is such that the switchable 90° optical rotation element  618  rotates the polarization plane of polarized light passing therethrough. 
     The basic principles of operation of channel separator  1100 , as illustrated in FIGS. 12-15, are similar to those of the reversible circulator  600 , as previously illustrated in FIGS. 8-9, and are not repeated here. However, it is to be kept in mind that, in FIGS. 12 and 13, the switchable 90° optical rotation element  618  is not disposed so as to rotate signal light polarization and thus the two members of each of the pairs of cross sections  1203 - 1204  and  1205 - 1206  (FIG.  12 ), or the pairs of cross sections  1303 - 1304  and  1305 - 1306  (FIG. 13) are identical. Furthermore, in FIGS. 14 and 15, the switchable 90° optical rotation element  618  is disposed so as to rotate by 90° the light polarization planes of signals disposed to the right side of the appropriate cross sections. The effects of these rotations are seen by comparison of the pairs of cross sections  1403 - 1404  and  1405 - 1406  (FIG.  14 ), or the pairs of cross sections  1503 - 1504  and  1505 - 1506  (FIG.  15 ). It is also to be kept in mind that, in FIG.  13  and FIG. 15, the polarization planes of even-channel signals are rotated by 90° between cross section  1304  and cross section  1305  (FIG. 13) and also between cross section  1504  and  1505  (FIG.  15 ). The effect of each such rotation of signal light polarization is propagated along the remainder of the optical path until the signal is outputted from the channel separator  1100  through one of its four input and output ports. 
     FIGS. 16 a  and  16   b  respectively depict the two operational states of the switchable optical channel separator  1100  in accordance the present invention. In the first such operational state illustrated in FIG. 16 a , a first set of wavelengths consonant with a first set of interleaved channels are routed from Port A to Port B and from Port C to Port D and a second set of wavelengths consonant with a second set of interleaved channels are routed from Port A to Port D and from Port C to Port B. For convenience, the first and second sets of interleaved channels are herein termed “odd” and “even” channels, respectively. 
     For instance, if a set of n wavelength-division multiplexed channels denoted by λ 1 , λ 2 , λ 3 , . . . , λ n  are input to Port A of the switchable optical channel separator  1100  in its first operational state, then the first or “odd” channels λ 1 , λ 3 , λ 5 , . . . are routed to Port B and the second or “even” channels λ 2 , λ 4 , λ 6 , . . . are routed to Port D. Similarly, if a second set of n wavelength-division multiplexed channels denoted by λ′ 1 , λ′ 2 , λ′ 3 , . . . , λ′ n  are input to Port C of the switchable optical channel separator  1100  in the same first operational state, then the first or “odd” channels λ′ 1 , λ′ 3 , λ′ 5 , . . . are routed to Port D and the second or “even” channels λ′ 2 , λ′ 4 , λ′ 6 , . . . are routed to Port B. Thus, with the switchable optical channel separator  1100  in its first operational state, the output at Port B comprises the odd channels λ 1 , λ 3 , λ 5 , . . . originally from Port A multiplexed together with the even channels λ′ 2 , λ′ 4 , λ′ 6 , . . . originally from Port C, and the output at Port D comprises the odd channels λ′ 1 , λ′ 3 , λ′ 5 , . . . originally from Port C multiplexed together with the even channels λ 2 , λ 4 , λ 6 , . . . originally from Port A. The channel separator operates similarly in the reverse direction—that is, when Ports B and D are utilized for input and Ports A and C are utilized for output. In other words, the path of each and every channel is reversible. 
     In FIG. 16 b , the switchable optical channel separator  1100  is illustrated in its second operational state. In this state, the output at Port B comprises the even channels λ 2 , λ 4 , λ 6 , . . . originally from Port A multiplexed together with the odd channels λ′ 1 , λ′ 3 , λ′ 5 , . . . originally from Port C, and the output at Port D comprises the even channels λ′ 2 , λ′ 4 , λ′ 6 , . . . originally from Port C multiplexed together with the odd channels λ 1 , λ 3 , λ 5 , . . . originally from Port A. The channel separator operates similarly in the reverse direction. 
     The operational state of switchable optical channel separator  1100  is controlled by the state of the switchable 90° optical rotation element  618 . When switchable 90° optical rotation element  618  is in its “on” state such that there is effected a 90° rotation of the polarization plane of plane polarized light passing therethrough or there-past, then the switchable optical channel separator  1100  is in its second state. However, when switchable 90° optical rotation element  618  is in its “off” state such that there is no polarization plane rotation of plane polarized light passing therethrough or there-past, then the switchable optical channel separator  1100  is in its first state. When the switchable 90° optical rotation element  618  comprises the half-wave plate of a differential phase retardance switch  180  of the present invention, then the channel separator  1100  can be switched between its two routing states in approximately one millisecond. 
     FIG. 17 is an illustration of a preferred embodiment of a self-switching optical line restoration switch  1700  in accordance with the present invention. The switch  1700  utilizes a reversible optical circulator  600  of the present invention. The reversible optical circulator  600  is optically coupled to an input telecommunications line  1702 , an output telecommunications line  1704 , an auxiliary telecommunications line  1710 , and a detector link  1708  through its Port A, Port B, Port D and Port C, respectively. The detector link is optically coupled to a photo-detector  1712  at its end opposite reversible circulator  600 . The photo-detector  1712  is electrically coupled to the switchable 90° optical rotation element  618  component (not shown) of reversible circulator  600  through an electrical or electronic link  1714 . In normal operation, the reversible circulator  600  of self-switching optical line restoration switch  1700  is in its “off” position, and thus signals input to Port A from input line  1702  are directed in the “clockwise” circulation direction to Port B and thence to output line  1704 . In this situation, the auxiliary telecommunications line  1702  remains unused and no optical signal is directed to the photo-detector  1712 . 
     If there should be a line break within output telecommunications line  1704  and there is no optical isolator between the device  1700  and the line break, then signals will be reflected at the break point and will propagate backwards through line  1704  back to reversible circulator  600 . These reflected signals and/or other lights will then be input to reversible circulator  600  through its Port B. Since the reversible circulator  600  will be in its “off” state immediately after such a line break occurs, these reflected signals and/or other lights will be directed in a “clockwise” circulation direction so as to be output from Port C to link  1708  and thence to photo-detector  1712 . When photo-detector  1712  senses the presence of the reflected signals or other lights, it sends an electrical or electronic signal, via line  1714 , which is sufficient to switch the switchable 90° optical rotation element  618  to its “on” state, thereby transforming reversible circulator  600  into its “on” state. Once this switching has occurred, signals or other lights inputted to Port A from input line  1702  will be directed in a “counter-clockwise” circulatory direction to Port D and thence to auxiliary telecommunications line  1710 . In this fashion, the self-switching optical line restoration switch  1700  automatically switches signals and/or other lights away from the broken primary output line  1704  and into the auxiliary line  1710 . 
     FIG. 18 is an illustration of a preferred embodiment of an optical cut-in or bypass switch  1800  in accordance with the present invention. The cut-in or bypass switch  1800  is suitable for automated insertion or removal of a network component  1812  into or out of a telecommunications line. The switch  1800  comprises a reversible optical circulator  600  in accordance with the present invention respectively optically coupled to an input telecommunications line  1802  through its Port A, to an output telecommunications line  1804  through its Port B, and to a first  1808  and a second  1810  optical link through its Port C and Port D. The optical links  1808 - 1810  are each optically coupled to the network component  1812 . The network component  1812  may comprise any one or a combination of a variety of optical or electro-optical components such as optical filters, optical attenuators, optical amplifiers, optical add/drops, dispersion compensators, transponders, wavelength shifters, etc. 
     In a first or “off” state, the reversible circulator  600  of switch  1800  receives optical signal input from input line  1802  through its port A and re-directs this signal in a “clockwise” circulatory direction so as to be output from Port B to output line  1804 . In this state of operation, signals completely bypass the component  1812 . In a second state of operation, the reversible circulator  600  is placed in its “on” state such that signals input at Port A are re-directed in a “counter-clockwise” circulatory direction to Port D and thence to the second optical link  1810  and network component  1812 . The network component  1812  performs one or more of signal conditioning, signal addition or signal deletion operations upon the signal or signals received from the second link  1810  and then outputs the conditioned, modified or substituted signals to the first optical link  1808 . The signal(s) output from component  1812  to the first link  1808  need not be the same signal or signals received by component  1812  from the second link  1810 . The signals received by the first optical link  1808  from component  1812  are then delivered to Port C of the reversible circulator  600  from which they are directed to Port B and subsequently output to the output line  1804 . In this fashion, the network component can be automatically switched in or out of an optical transmission within a millisecond as changing needs require. 
     Although the present invention has been described with an optical switching device utilizing a bi-morphic piezoelectric material, one of ordinary skill in the art will understand that other suitable materials may be used without departing from the spirit and scope of the present invention. 
     A method and apparatus for optical switching devices utilizing a bi-morphic piezoelectric electro-mechanical deflection and latching apparatus has been disclosed. The optical switching devices include a 1×2 optical switch utilizing a single electro-mechanical apparatus, various 1×N optical switches utilizing a plurality of electro-mechanical apparatus in a cascade arrangement, a reversible optical circulator, and a switchable optical channel separator. The optical devices in accordance with the present invention posseses the advantages of stable and reproducible operation, high switching speeds relative to other mechanical devices and low sensitivity to slight optical mis-alignments or vibrations. The optical devices in accordance with the present invention are of a compact modular design that allows the construction of more complex optical devices through utilization of a cascading arrangement, where an optical beam or signal can be deflected about axes in more than one spatial dimension. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.