FIELD OF THE INVENTION 
     The present invention relates to optical fiber based communication networks, and more particularly, to couplers for selectively transferring data between two such networks. 
     BACKGROUND OF THE INVENTION 
     Communication networks based on optical fibers for transferring data between terminals are attractive because of the high bandwidth of the optical fibers. Data is transmitted on these networks by modulating a light source, usually a laser. A plurality of users may share a single fiber either via time or wave division multiplexing (WDM). Wave division multiplexing is typically implemented by assigning a different wavelength to each user or channel. 
     A limited number of stations may be accommodated on any given fiber ring. Accordingly, a coupling device is used to selectively couple data from a first fiber to a second fiber. The data to be coupled is typically isolated to a sub-set of the channels on the first fiber. In some cases, it is advantageous to simultaneously remove the transferred channel from the first fiber after the channel is connected to the second fiber. Such coupling devices include optical circulators. 
     In a telecommunications network each subscriber communicates with a central office over a fiber that is arranged in a ring with the subscriber and central office stations disposed along the ring. If the fiber is broken, communication between one or more of the users and the central office will be interrupted. In principle, these users can still communicate with the central office by sending messages along the uninterrupted portion of the loop. However, this requires that the direction of propagation along the fiber be reversed over a portion of the fiber. 
     Unfortunately, the fiber ring typically includes components that are unidirectional in nature such as the optical circulators used to couple the fibers discussed above. To reverse the direction of propagation in response to a fiber break, duplicate optical circulators configured to propagate signals in the opposite direction are included in the network. These components are inserted into the fiber in place of the corresponding components by utilizing bypass switches. Such bypass arrangements substantially increase the cost and complexity of the optical network, and hence, it would be advantageous to avoid these bypass arrangements. 
     Broadly, it is the object of the present invention to provide an improved optical circulator. 
     It is another object of the present invention to provide an optical circulator whose direction of light transmission can be reversed by applying a control signal to the optical circulator without the need to utilize bypass switches and additional optical circulators. 
     It is yet another object of the present invention to provide an optical coupling arrangement in which the direction of propagation of the coupled signals may be switched without the need to utilize bypass switches and other circulators. 
     These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings. 
     SUMMARY OF THE INVENTION 
     The present invention is a reversible optical circulator and a coupling device constructed therefrom. The optical circulator has first, second, and third ports for receiving and transmitting light signals. A light separator resolves an incoming light signal on one of the ports into first and second light signals having orthogonal polarizations with respect to one another. A first non-reciprocal optical element that includes a first magnetic field generator for generating a first magnetic field operates on the first optical signal. The first magnetic field has a direction that is determined by a first control signal. The first non-reciprocal optical element rotates the polarization of the first optical signal by either 90 degrees or 0 degrees depending on the direction of the first magnetic field. A second non-reciprocal optical element that includes a second magnetic field generator for generating a second magnetic field operates on the second optical signal. The second magnetic field has a direction determined by a second control signal. The second non-reciprocal optical element rotates the polarization of the second optical signal by either 90 degrees or 0 degrees depending on the direction of the second magnetic field. A light collector combines the first and second optical signals after the first and second optical signals have traversed the first and second non-reciprocal optical elements, respectively, to create a combined light signal, the combined light signal leaving the optical circulator by another of the first and second ports. The non-reciprocal optical elements are preferably constructed from Faraday rotators and a half-wave plate. 
     The light coupling device adds a first light signal having a wavelength of λ 1  to a second light signal of wavelength λ 2  traveling in an optical channel. The coupling includes first and second optical channel ports, the second light signal entering one of the first and second optical channel ports and leaving by the other of the first and second optical channel ports and an optical channel input port for receiving the first light signal. The coupler includes an optical circulator and first and second reflectors. The optical circulator has first, second, and third ports and a circulation direction determined by the circulator input signal. Each of the reflectors has first and second ports, light entering one of the ports exits the other of the ports unless the light is reflected by the reflector. Each reflector has first and second reflection states, the reflector reflecting light of wavelength λ 1  in the first reflection state, and passing light of wavelength λ in the second reflection state. The reflection state of the reflector is set in response to a reflector input signal associated with that reflector. Only one of the first and second reflectors is set to the second reflection state at any one time, the identity of the reflector in the second reflection state depending on the circulator input signal which also determines the direction of travel of the first light signal in the optical channel. The optical circulator preferably includes a Faraday rotator having a magnetic field direction that is determined by the circulator input signal. The reflectors are preferably constructed from variable wavelength fiber Bragg reflectors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a Faraday rotator that rotates the polarization angle of the light that passes therethrough ±45° depending on the direction of the magnetic field. 
     FIG. 2 is a schematic view of a reversible circulator according to the present invention. 
     FIGS. 3 and 4 illustrate the manner in which light traverses the optical circulator shown in FIG. 2 depending on the port of entry of that light. 
     FIG. 5 is a block diagram of a cross-coupler according to the present invention. 
     FIG. 6 is a schematic view of the cross-coupler shown in FIG. 5 in a configuration for coupling light into the optical channel in the opposite direction to that shown in FIG.  5 . 
     FIGS. 7-10 are block diagrams of a 3-port circulator according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides its advantages by utilizing an optical circulator whose direction of propagation may be altered via a signal applied thereto. For the purposes of this discussion, an optical circulator will be defined to be any device having an ordered array of ports and a direction of circulation such that light entering any port of the optical circulator leaves the circulator by the next port encountered in the direction of circulation. 
     The present invention makes use of a Faraday rotator having a magnetic field whose direction is determined by the application of an external signal. A Faraday rotator is an element that is composed of an optically active compound such as yttrium-iron-garnet Y 3 Fe 5 O 12  which turns the direction of the polarization vector of the light passing therethrough by an angle of 45°. The direction of rotation is determined by the direction of an applied magnetic field. The direction of rotation of the polarization vector is independent of the direction of travel of the light through the element. By altering the direction of the magnetic field, the rotation of the polarization vector changes from 45° to −45° independent of the direction of travel of the light through the device. 
     Refer now to FIG. 1 which is a schematic view of a Faraday rotator  123  that rotates the polarization vector of the light that passes therethrough ±45° depending on the direction of the magnetic field. The direction of the magnetic field is determined by the current passing through coil  110 , which generates a magnetic field having a component that is parallel to the direction of travel of the light. The current is provided by current source  109 , which sets the direction of the current in response to a direction control signal. The direction control signal may be electrical or optical. Other embodiments in which the Faraday rotator utilizes a latching material may also be employed in the present invention. In such embodiments a current pulse sets the direction of magnetization. The direction remains the same until another current pulse is applied. Similarly, a permanent magnet together with a device for flipping the direction of the magnet in response to a control signal could also be utilized. 
     The present invention is based on non-reciprocal polarization rotators. Each such rotator is constructed from a Faraday rotator having a reversible magnetic field and a half-wave plate. The direction of rotation of the polarization vector provided by the Faraday rotator is the same regardless of the direction of travel of the light therethrough. The half-wave plate, in contrast, provides either a 45° or −45° rotation depending on the direction of travel of the light. As a result, in one direction the polarization vector is rotated through 90°, and in the other, it is rotated through 0°. 
     Refer now to FIG. 2 which is a schematic view of a reversible circulator  150  according to the present invention. Circulator  150  utilizes two non-reciprocal polarization rotators. 
     Consider the case in which the reversible Faraday rotators  162  and  164  are set such that circulator  150  passes light from port  152  to port  155  which will be referred to as ports A and B. Light entering port  152  is collimated by lens  153 . A polarization beam splitter  181  decomposes the polarization vector  191  of the incoming light into orthogonal components shown at  192  and  194 . The polarization beam splitters are constructed from prisms that selectively reflect light of a predetermined polarization while passing light of the orthogonal polarization. The polarization component reflected by splitter  181  is reflected by mirror  171  into reversible Faraday rotator  162 , which rotates the polarization by 45°. Half-wave plate  163  rotates the polarization vector by another 45° as shown at  195 . This polarization passes through polarization splitter  182 . 
     The component of the input light that passed through splitter  181  is shown at  192 . This component is likewise rotated through a total of 90° by reversible Faraday rotator  164  and half-wave plate  165 . The output of half-wave plate  165  is reflected into splitter  182  by mirror  172 . This polarization is reflected by splitter  182  into lens  154  where it is combined with the output of half-wave plate  163  discussed above to reconstitute the input light as shown at  196 . Hence, light entering port A leaves by port B. 
     Refer now to FIG. 3, which illustrates the manner in which light entering port  155  is routed to port  130  which will be referred to as port C. The combination of half-wave plate  163  and Faraday rotator  162  leaves the polarization vector unchanged for this direction of travel. Hence, the component of the polarization that is passed by splitter  182  leaves reversible Faraday rotator  162  with the same polarization as shown at  197 . This component is reflected by mirror  171  into splitter  181  and passes through splitter  181 . The component of the input light reflected by splitter  182  and mirror  172  passes unchanged through half-wave plate  165  and reversible Faraday rotator  164  since Faraday rotator  164  reverses the 45° rotation introduced by half-wave plate  165 . This component is reflected by splitter  181  and is recombined with component  197 . The reconstituted light then leaves via the bottom of splitter  181  as shown at  199 , and enters port  130 . A similar analysis can be applied to show that light entering port  130  leaves via port  120 , referred to as port D. Accordingly, the direction of circulation for the orientation of the magnetic fields shown in FIGS. 1 and 2 is A-B-C-D. 
     Now consider the case in which the magnetic fields in the two Faraday rotators are reversed. Referring to FIG. 4, light entering port  155  is once again decomposed into the orthogonal components shown at  195  and  193 . However, component  195  is now rotated through 90° by the combination of half-wave plate  163  and Faraday rotator  201 . Similarly, component  193  is rotated through 90° by half-wave plate  165  and Faraday rotator  164  as shown at  202 . Component  201  is reflected by mirror  171  and beam splitter  181  while component  202  passes through beam splitter  181 . Hence, the two components are combined and exit via port  152 . That is, light entering port B now exits via port A. A similar analysis can be used to confirm that light entering port C now exits through port B, and light entering port D now exits through port C. That is, the direction of circulation is now D-C-B-A. Accordingly, reversing the magnetic fields in the Faraday rotators reverses the direction of circulation. 
     A reversible coupler according to the present invention utilizes a reversible circulator as described above in combination with a number of variable wavelength fiber Bragg reflectors (VFBRs). Since fiber Bragg reflectors are well known to the art, they will not be discussed in detail here. For the purpose of the present discussion, it is sufficient to note that a Bragg reflector may be viewed as a grating that has been induced in the core of an optical fiber. The grating consists of periodic alterations in the index of refraction of the core of the fiber. Such alterations may be induced by illuminating the core with a UV light pattern having regularly spaced maxima of sufficient intensity to damage the core. The pattern is typically generated by the interference of two UV light beams. 
     When light having a wavelength twice the spacing of the grating strikes the grating, the light is reflected because of the coherent interference of the various partial reflections created by the alterations in the index of refraction of the fiber core. The wavelength at which the reflection occurs may be varied by varying the optical path length between the periodic alterations in the index of refraction. This may be accomplished by heating the fiber or by stretching the fiber. The input signal used to activate the heating or stretching system is not shown in the figures to simplify the figures; however, it is to be understood that each VFBR includes a signal input and the appropriate hardware for altering the reflection wavelength. 
     In a typical wavelength division multiplexed (WDM) optical network, the channels have closely spaced wavelengths. The spacing between any two channels is sufficient to provide channel isolation. However, the total wavelength range across all of the channels is preferably kept to a minimum to allow a single optical amplifier to be used to maintain the signal strength in the network at each amplification station. 
     In general, the degree of variation in the reflection wavelength of the VFBR is sufficient to move the reflection wavelength from a value between two channels to a value on one of the adjacent channels. Hence, the light in a single channel can be selectively reflected by heating or stretching a corresponding VFBR to move its reflection wavelength from a value between two channels to the channel value. When the wavelength is set between the two channels, no reflections occur, since there is no light at that wavelength. 
     The manner in which a coupler according to the present invention operates may be more easily understood with reference to FIG.  5 . FIG. 5 is a block diagram of a cross-coupler  300  according to the present invention. Cross-coupler  300  transfers signals from a first optical network operating on an optical channel  302  to a second optical network operating on an optical channel  304 . For simplicity, it will be assumed that optical channels  302  and  304  are optical fibers; however, it will be apparent to those skilled in the art that other forms of optical transmission may be utilized. For the purpose of the present discussion, it will be assumed that light signals transmitted on wavelengths λ 1  and λ 2  enter cross-coupler  300  on optical channel  302 . The signals transmitted on λ 2  are to be transferred to optical channel  304  and eliminated from optical channel  302 . It will also be assumed for the purposes of this example that optical channel  304  currently has signals transmitted on λ 3  which is different from λ 2 . 
     Cross-coupler  300  may be viewed as being constructed from a first interface  350  that is inserted into optical channel  302  and a second interface  360  that is inserted into optical channel  304 . Interface  350  includes an optical circulator  310  and a reflector  322  comprising one or more VFBRs. There is one VFBR corresponding to each channel wavelength that is to be switchable between optical channels  302  and  304 . Light that is not reflected by one of the VFBRs  322  is injected back into optical channel  302 . 
     Interface  360  has first and second ports that receive and transmit the light signals travelling in optical channel  304  as shown at  361  and  362 . A third port  363  receives the signals that are to be added to the signals currently traversing optical channel  304 . 
     To simplify the following discussion, it will be assumed that optical channel  302  has light of wavelengths λ 1  and λ 2 . The light at wavelength λ 2  is to be switched to optical channel  304  which is currently occupied by light at λ 3 . The direction of travel of light in optical channels  302  and  304  is indicated by the arrows at  371  and  372 , respectively. 
     To switch the light at λ 2  from optical channel  302  to optical channel  304 , one of the VFBRs in reflector  322  is tuned to reflect light of this wavelength. The remaining VFBRs are set to frequencies between the wavelengths that are present in the system, and hence, light of wavelength λ 1  is passed by reflector  322  and remains in optical channel  302 . The light at wavelength of λ 2  is reflected back into port B of circulator  310 , where it exits through port C on an optical connecting channel  311 . 
     The control of reflector  322  is provided by a signal to a device included in the reflector which shifts the reflection wavelength of one of the VFBRs from a “parked wavelength” that does not match either λ 1  or λ 2  to λ 2 . For the purposes of this discussion, a “parked wavelength” is defined to be a wavelength at which no signal is present or a wavelength at which the VFBR does not interact with the light. When optical channel  302  is configured to allow light of both λ 1  and λ 2  to pass, reflector  322  is adjusted such that all of the VFBRs are set to parked wavelengths. Since no light is present at the parked wavelengths, reflector  322  does not alter the operation of the network on optical channel  302  in this configuration. 
     The light present on connecting channel  311  is integrated into the signals on optical channel  304  in an analogous manner by interface  360 . Light on connecting channel  311  enters port A of optical circulator  312  and exits through port B of that circulator. Optical circulator  312  must have a minimum of three ports; however, an optical circulator having more ports such as the 4-ported circulator described above may be utilized. In such cases only the first three ports are actually used. A second reflector  332  comprising a plurality of VFBRs is set to selectively reflect the light of λ 2  by adjusting the reflection wavelength of one of the VFBRs in the reflector. Hence, the light of λ 2  leaving port B is reflected back to port B of circulator  12  and exits via port C onto optical channel  304 . Since reflector  332  reflects only light of λ 2 , the light at λ 3  that is already traveling through optical channel  304  is not altered. This light also enters circulator  312  and exits back onto optical channel  304 . A third reflector  330  is set to pass all wavelengths in this configuration. 
     Now consider the case in which the direction of travel in optical channel  304  is to be reversed as shown in FIG.  6 . FIG. 6 is a schematic view of cross-coupler  300  in a configuration for coupling light into optical channel  304  in the reverse direction to that shown in FIG.  5 . Circulator  312  is a reversible optical circulator of the type discussed above. To reverse the direction of injection of light of wavelength λ 2 , the direction of circulation in circulator  312  is reversed by reversing the direction of the magnetic fields in the Faraday rotators contained in circulator  312 . In addition, reflector  330  is now tuned to reflect light of λ 2 , and reflector  332  is tuned to pass light having a wavelength of either λ 2  or λ 3 . When light of λ 2  enters circulator  312 , the light will now exit port C where it is reflected by reflector  330  back into port C. The light entering port C of circulator  312  exits via port B and passes through reflector  332 . 
     The above-described embodiment of a reversible circulator according to the present invention discussed with reference to FIGS. 2-4 utilized a particular 4-port circulator design having two Faraday rotators. However, the observation that reversing the direction of the magnetic fields of the Faraday rotators in the circulator can reverse the direction of circulation of a circulator is true of other circulator designs that utilize Faraday rotators. Refer now to FIGS. 7-10, which are block diagrams of a 3-port circulator  400  according to the present invention. To simplify the drawing, the various imaging lenses have been omitted. Circulator  400  is constructed from a polarization beam splitter (PBS)  401 , a reversible Faraday rotator  402 , and a walk-off crystal (WOC)  403 . 
     The light entering and exiting the ports of circulator  400  is most conveniently viewed as being composed of two orthogonal polarizations. Referring to FIG. 7, the light entering port  1  is composed of polarization components  1 A and  1 B. Similarly, light entering port  3  is composed of polarization components  3 A and  3 B as shown at  410 . PBS  401  causes the polarization vectors of the light to be rotated by 45° and also causes one of the components to be displaced as shown at  411 . PBS  401  can be constructed from a Rutile crystal as taught in U.S. Pat. No. 5,734,763 which is hereby incorporated by reference. Faraday rotator  402  is set so as to rotate all of the polarization vectors through 45°. Hence, the light beams are rotated such that the polarization vectors are as shown at  412 . WOC  403  causes the vertical polarization components, i.e.,  1 A and  3 A, to be displaced in the direction of the arrow shown in WOC  403 . The walk-off distance is determined by the thickness of the crystal and chosen such that component  1 A is displaced to the location of component  1 B as shown at  413 . Port  2  is positioned to collect light from this position. Hence, light entering port  1  exits through port  2 , and light entering port  3  is lost. 
     Referring to FIG. 8, it can be seen that light entering port  2  exits through port  3  when Faraday rotator  402  is set to provide 45° of rotation. The light entering port  2  is composed of polarization components  2 A and  2 B as shown at  423 . WOC  403  decomposes the light into two polarized beams as shown at  422 . These beams are rotated through 45° by Faraday rotator  402  as shown at  421 . The polarizations of these beams are rotated by PBS  401  and displaced to the location of port  3  as shown at  420 . 
     Now consider the case in which the magnetic field in Faraday rotator  402  is set to provide a rotation of −45°. Refer first to FIG. 9, which illustrates the paths traversed by light entering ports  1  and  3 . Once again, the entering light is assumed to consist of two orthogonal linear polarizations. PBS  401  rotates the polarizations and mixes the polarizations as shown at  431 . The polarizations are then rotated through −45° by Faraday rotator  402  as shown at  432 . It should be noted that components  3 A and  3 B are now rotated by 90° from their positions in FIG.  7 . Hence, WOC  403  recombines  3 A and  3 B at the location of port  2  and the components from port  1  are lost. That is, port  3  is now routed to port  2 . 
     Referring to FIG. 10, it can be seen that light entering port  2  now exits through port  1 . Once again, WOC  403  decomposes the input light shown at  443  into two polarized beams shown at  442 . The polarization directions of these beams are rotated through −45° by Faraday rotator  402 , as shown at  441 . PBS  401  then recombines these beams at the location of port  1  as shown in  440 . 
     Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.
A reversible optical circulator and a coupling device constructed therefrom. The optical circulator includes first and second non-reciprocal optical elements having magnetic field generators for generating magnetic fields that determine the rotation of the polarization vector of light signals passing therethrough. The non-reciprocal optical elements rotate the polarization of the light signals by either 90 degrees or 0 degrees depending on the direction of the magnetic field associated with that non-reciprocal element. The reversible circulator can be combined with wavelength selective reflectors to form a light coupling device that adds a first light signal having a wavelength of l 1 to a second light signal of wavelength l 2 traveling in an optical channel. The direction of the added light signal in the optical channel is controlled by the direction of the magnetic field in the optical circulator and the choice of which selective reflector is active.