Abstract:
Optical switches which take multiple incoming optical signals and switch them to multiple output ports to realize multiple working states. For example, in a four by four switch embodiment, twenty-four working states can be selected. These switches rely on magneto-optically or electro-optically switching the beam polarizations from one state to another to rapidly change the light path. An optical signal is spatially split into two polarized beams by a birefringent element. These beams pass through a series of polarization rotation elements and recombine into output fibers, achieving polarization independent operation. A polarization beam splitter may be used as the key element to establish multi-port switching. Light bending devices that allow two fibers to be coupled to the light beams using a single lens may be used to achieve small fiber separation for compactness.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is related to and claims priority from U.S. provisional patent application 60/509,549, filed Oct. 9, 2003 and entitled “Multi-port optical switches”, and incorporated by reference herein in its entirety. 

   FIELD OF THE INVENTION 
   The present invention is related to non-mechanical optical switches. 
   BACKGROUND 
   Optical switches are devices for directing optical signals along selected fibers of an optical network, in which light signals are transmitted along optical fibers to transfer information from one location to another. Desirable optical switch characteristics include: high speed switching, low optical insertion loss, long operation lifetime, small size, and low cost. Optical switches are key components in present-day optical networks, analogous to electrical switches in electrical networks. However, optical switches have not been widely adopted due to lack of reliability and to high cost associated with fabrication difficulty. 
   In an optical switch, light must be accurately coupled to an optical fiber to reduce loss. The alignment requirements of modern single mode optical fibers are particularly stringent, since their core diameters are typically as small as 2 to 10 microns and their acceptance angle is fairly narrow. Insertion loss due to switch-fiber misalignment reduces the amplitude of the optical signal. Therefore, optical switches which accept light from an input optical fiber, and which selectively couple that light to any of a plurality of output optical fibers, must transfer that light with precise alignment and within the small acceptance angle for light to efficiently couple to the fiber. Most prior art optical switches are based on mechanical movement to switch light beams, and consequently have drawbacks including slow switching time and reduced reliability. To avoid these drawbacks, it is desirable for optical switches to direct light beams without moving parts. Such lack of moving parts is a feature generally associated with high reliability and high speed. 
   Many types of non-mechanical optical switches have been developed for commercial applications, such as switches based on thermal heating, electro-optic phase retardation, and magneto-optic polarization rotation. These devices use various materials and configurations. Thermal heating based switches typically rely on thin film waveguide construction having a long interaction length (e.g., U.S. Pat. No. 5,892,863). This type of switch has a disadvantage of large insertion loss due to fiber to thin film waveguide coupling loss. On the other hand, a micro-optic assembly generally provides low optical loss. Liquid crystal materials have been demonstrated for optical path switching in a micro-optic platform. This type of organic device, however, has disadvantages including slow operation at low temperature and a requirement for a transparent electrode in the optical path (e.g., U.S. Pat. No. 4,917,452). 
   Oxide materials such as magneto-optic and electro-optic materials are particularly attractive for micro-optic devices. Inorganic materials are generally preferred over organic materials in optical network devices, due to their excellent stability. Optical switches based on magneto-optic crystals have been described in several patents (e.g., in U.S. Pat. Nos. 5,724,165, 5,867,291, 5,912,748, 6,097,518, 6,134,358, 6,137,606, 6,166,838, 6,192,174, 6,212,313, and 6,275,312). However, these optical switches are typically limited to a small number of ports (e.g., 1×2 and 2×2 configurations). Furthermore, even for a small number of optical ports, these configurations tend to be costly to manufacture due to tight fiber alignment tolerance requirements and complex configurations that require many optical elements. 
   Accordingly, it would be an advance in the art to provide a simple non-mechanical optical switch that is readily scalable to switches having more than 2 output (or input) ports and is suitable for volume production. It is particularly desirable to provide optical switches having a large number of ports, low optical insertion loss, and high speed switching that are also reliable and require only a small number of components which can be miniaturized and are easy to manufacture. 
   SUMMARY 
   The present invention provides a multi-port optical switch that can be efficiently coupled to multiple optical fibers using fewer parts and having more relaxed assembly tolerance requirements than the prior art. The inventive optical switch is capable of re-directing an incident signal light from an input port to any of multiple output ports, independent of its polarization state and without using moving parts. Key elements in an embodiment of the invention include: a polarization beam splitter (PBS), birefringent blocks, and polarization rotators (e.g., Faraday rotators and electro-optic retarders). 
   The polarization beam splitter generally can be used to separate a laser beam into two beams having orthogonal polarization. A variable beam splitter can be created by passing linearly polarized beams through a group of half wave plates and Faraday rotators in combination with a polarizing beam splitter. The polarization of the light incident on the polarization beam splitter governs the amount of light the beam splitter transmits and reflects. Adjusting the input polarization by changing the working state of the Faraday rotator allows full control of which incoming beams are transmitted and which are reflected by the beam splitter. 
   The birefringent blocks can be used as various functional elements in this invention. They can be used as a beam splitter to split one arbitrarily polarized beam into two orthogonally polarized beams with a certain distance between them. They can also be used as beam walk-off elements which shift one set of the polarized beams laterally to form a second path. They also can be used as beam combiners to re-combine two beams with orthogonal polarization together into a single beam. The inventive switches are based on electrically controllable polarization rotators. Suitable configurations include magneto-optic Faraday crystals or inorganic electro-optic materials as the controllable polarization rotator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an optical switch subassembly using a PBS for beam separation. 
       FIG. 2  shows an optical switch subassembly using a walkoff element for beam separation. 
       FIG. 3  is a plan view of an eight-port four by four optical switch in accordance with an embodiment of the invention. 
       FIG. 4  is an isometric view of one of the input subassemblies of the switch of  FIG. 3 , showing the polarization of light after each component. 
       FIG. 5  is an isometric view of one of the output subassemblies of the switch of  FIG. 3 , showing the polarization of light after each component. 
       FIG. 6  is an isometric view of part of the center subassembly of the switch of  FIG. 3 , showing the polarization of light after each component. 
       FIG. 7   a  is an isometric view of a one by four optical switch in accordance with an embodiment of the invention. 
       FIG. 7   b  is an isometric view of a one by four optical switch in accordance with another embodiment of the invention. 
       FIG. 8   a  is a plan view of a 4 bit optical time delay line using single fiber collimators and PBSs in accordance with an embodiment of the invention. 
       FIG. 8   b  is an isometric view along line A—A on  FIG. 8   a.    
       FIG. 8   c  is an isometric view of part A along line B—B on  FIG. 8   a.    
       FIG. 8   d  is an isometric view of part B along line B—B on  FIG. 8   a.    
       FIG. 9  is a plan view of a 4 bit optical time delay line using single fiber collimators and beam displacers in accordance with an embodiment of the invention. 
       FIG. 10  is a plan view of a 4 bit optical time delay line using dual fiber collimators, PBSs and right angle prisms in accordance with an embodiment of the invention. 
       FIG. 11  is a plan view of an 4 bit optical time delay line using dual fiber collimators and PBSs in accordance with an embodiment of the invention. 
       FIGS. 12   a–b  show two different polarization rotators suitable for use in embodiments of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an optical switch subassembly. A light beam incident on a first birefringent crystal  102  is split into two orthogonally polarized beams  110  and  112 . The length of crystal  102  is selected to provide a spatial separation between beams  110  and  112 , which permits these beams to pass through subsequent optical elements independently. Beams  110  and  112  then pass through a compound half wave plate  104  that rotates the polarization of beams  110  and  112  by 45 degrees in opposite directions. For example, if the polarization of beam  110  is rotated by +45 degrees (i.e., clockwise), then the polarization of beam  112  is rotated by −45 degrees (i.e., counter-clockwise). In the preceding example, +45 degrees and −45 degrees can be exchanged. After passing through wave plate  104 , beams  110  and  112  have the same polarization. 
   Beams  110  and  112  next pass through an electrically controllable polarization rotator  106 , which rotates the state of polarization by +45 degrees or −45 degrees, depending on an applied input signal. Beams  110  and  112  are either horizontally polarized (i.e., x-polarized) or vertically polarized (i.e., y-polarized) after exiting rotator  106 , depending on the input signal to rotator  106 . Beams  110  and  112  are next received by a polarizing beamsplitter (PBS)  108 . If beams  110  and  112  are horizontally polarized, they pass through PBS  108  without a change in propagation direction. If beams  110  and  112  are vertically polarized, they are reflected in PBS  108  and exit PBS  108  as beams  114  and  116  propagating in a different direction than beams  110  and  112 . Thus the input to rotator  106  controls the path the beams take through PBS  108 , making this subassembly useful for optical switching. Splitting the input beam into two orthogonally polarized beams  110  and  112  ensures that this subassembly is applicable for arbitrarily polarized input light. 
     FIG. 2  shows an optical switch subassembly similar to that of  FIG. 1 . A light beam incident on a first birefringent crystal  102  is split into two orthogonally polarized beams  204  and  206 . The length of crystal  102  is selected to provide a spatial separation between beams  204  and  206 , which permits these beams to pass through subsequent optical elements independently. Beams  204  and  206  then pass through a compound half wave plate  104  that rotates the polarization of beams  204  and  206  by 45 degrees in opposite directions. After passing through wave plate  104 , beams  204  and  206  have the same polarization. 
   Beams  204  and  206  next pass through an electrically controllable polarization rotator  106 , which rotates the state of polarization by +45 degrees or −45 degrees, depending on an applied input signal. Beams  204  and  206  are either horizontally polarized or vertically polarized after exiting rotator  106 , depending on the input signal to rotator  106 . Beams  204  and  206  are next received by a second birefringent element  202 . If beams  204  and  206  are vertically polarized, they pass through birefringent element  202  without a change in beam axis position. If beams  204  and  206  are horizontally polarized, they experience walk off and exit birefringent element  202  as beams  208  and  210  which are laterally displaced from beams  204  and  206 . Thus the input to rotator  106  controls the path the beams take through birefringent element  202 , making this subassembly useful for optical switching. Splitting the input beam into two orthogonally polarized beams  204  and  206  ensures that this subassembly is applicable for arbitrarily polarized input light. 
   Appreciation of the switch subassemblies of  FIGS. 1 and 2  is helpful for appreciating the exemplary embodiments of the invention which follow. 
     FIG. 3  shows an eight-port, four by four optical switch according to an exemplary embodiment of the invention. On  FIG. 3 , two input subassemblies (part A) and two output subassemblies (part B) are connected to a central subassembly in a generally cross-like configuration. Details of the input subassemblies are shown on  FIG. 4 , details of the output subassemblies are shown on  FIG. 5 , and details of the central subassembly are shown on  FIG. 6 . In the example of  FIG. 3 , the two input subassemblies have the same optical configuration, as do the two output subassemblies. Thus the following description of the input subassembly is applicable to both input subassemblies on  FIG. 3 , and similarly for the output subassemblies.  FIGS. 4–6  show and provide reference numbers for some optical components which are included in this embodiment of the invention but are not shown on  FIG. 3 . 
     FIG. 4  shows an input subassembly of the switch of  FIG. 3 . A first optical fiber  1  is inserted into a first dual fiber collimator  11  and a second optical fiber  2  is inserted into dual fiber collimator  11  adjacent to fiber  1 , so that fiber  1  and fiber  2  are parallel. Dual fiber collimator  11  allows the outputs of the two fibers to be transformed to collimated beams with a single lens, thereby providing small fiber separation for compactness. 
   Fiber  1  emits an arbitrarily polarized light beam  100  that is collimated by a collimator  11 . Collimator  11  also causes beam  100  to make an angle with respect to the y-axis (since fiber  1  is off-axis with respect to the lens of collimator  11 ). Beam  100  then passes through a first birefringent block  13  and is divided into two beams having orthogonal polarizations, specifically beams  100 A and  100 B. The relative intensity of beams  100 A and  100 B depends on the state of polarization of light emitted from fiber  1 . The length of birefringent block  13  is selected to provide a spatial separation between beams  100 A and  100 B. This spatial separation permits beams  100 A and  100 B to pass through independent optical elements. In this example, beam  100 A enters a first wave plate  15  which rotates its plane of polarization by 90°, while beam  100 B does not pass through wave plate  15 . Thus wave plate  15  makes beams  100 A and  100 B have the same state of polarization (z-axis). 
   Since beam  100  makes an angle with respect to the y-axis, beams  100 A and  100 B also make an angle with respect to the y-axis. This angle is removed by passing beams  100 A and  100 B through a polarization-independent light-bending device  17 . In the example of  FIG. 4 , light bending device  17  is a prism having an angle selected such that beams  100 A and  100 B are parallel to the y axis after passing through device  17 . Beams  100 A and  100 B next pass through a second wave plate  19 , which rotates their plane of polarization by 90°. Thus beams  100 A and  100 B propagate parallel to each other and are x-axis polarized after passing through wave plate  19 . 
   Similarly, fiber  2  emits an arbitrarily polarized light beam  200  that is collimated by collimator  11 . Collimator  11  also causes beam  200  to make an angle with respect to the y-axis (since fiber  2  is off-axis with respect to the lens of collimator  11 ). Beam  200  then passes through first birefringent block  13  and is divided into two beams having orthogonal polarizations, specifically beams  200 A and  200 B. The relative intensity of beams  200 A and  200 B depends on the state of polarization of light emitted from fiber  2 . The length of birefringent block  13  is selected to provide a spatial separation between beams  200 A and  200 B. This spatial separation permits beams  200 A and  200 B to pass through independent optical elements. In this example, beam  200 A enters first wave plate  15  which rotates its plane of polarization by 90°, while beam  200 B does not pass through wave plate  15 . Thus wave plate  15  makes beams  200 A and  200 B have the same state of polarization (z-axis). 
   Since beam  200  makes an angle with respect to the y-axis, beams  200 A and  200 B also make an angle with respect to the y-axis. This angle is removed by passing beams  200 A and  200 B through a polarization-independent light-bending device  17 . In the example of  FIG. 4 , light bending device  17  is a prism having an angle selected such that beams  200 A and  200 B are parallel to the y axis after passing through device  17 . Beams  200 A and  200 B do not pass through second wave plate  19 . Thus beams  200 A and  200 B propagate parallel to each other and are z-axis polarized after passing through device  17 . 
   The four beams  100 A,  100 B and  200 A,  200 B pass through a second birefringent block  21 , where beams  100 A and  200 A are combined into one beam  1000 A and beams  100 B and  200 B are combined into another beam  1000 B. After this combination, a third half wave plate  23  rotates the polarizations of beams  1000 A and  1000 B by 45° clockwise. Thus, beams  100  and  200  from fibers  1  and  2  are mixed with each other to form two parallel beams  1000 A and  1000 B separated along the z-axis. More specifically, beams  1000 A and  1000 B each have two orthogonal polarization components, which can be referred to as +D and −D (in view of the 45 degree rotation of wave plate  23 ) components. Light from fiber  1  is split between the +D components of beams  1000 A and  1000 B, while light from fiber  2  is split between the −D components of beams  1000 A and  1000 B. The roles of +D and −D can be reversed in the preceding sentence. Providing such combined beams  1000 A,B is the main function of the two input subassemblies on  FIG. 3 . 
     FIG. 6  shows a view of the central subassembly of the switch of  FIG. 3  along a line from ports  1  and  2  to ports  5  and  6 . Beams  1000 A and  1000 B (from  FIG. 4 ) are received by a first electrically controllable polarization rotator  25  that rotates the plane of polarization by 45° clockwise (or counter-clockwise), depending on an applied electrical control signal. Beams  1000 A and  1000 B then pass through a birefringent splitter  27  which splits beam  1000 A into beams  100 A′ and  200 A′, and splits beam  1000 B into beams  100 B′ and  200 B′. When rotator  25  performs a 45° clockwise rotation of the plane of polarization, z-polarized beams  100 A′ and  100 B′ come from beam  100  on  FIG. 4 , and x-polarized beams  200 A′ and  200 B′ come from beam  200  on  FIG. 4 . When rotator  25  performs a 45° counter-clockwise rotation of the plane of polarization, z-polarized beams  100 A′ and  100 B′ come from beam  200  on  FIG. 4 , and x-polarized beams  200 A′ and  200 B′ come from beam  100  on  FIG. 4 . Thus the setting of rotator  25  determines the relation between beams  100  and  200  on  FIG. 4  and beams  100 A′ and  100 B′ and beams  200 A′ and  200 B′on  FIG. 6 . In either case, beams  100 A′ and  100 B′ pass through birefringent splitter  27  as ordinary waves, and beams  200 A′ and  200 B′ pass through birefringent splitter  27  as extraordinary waves. Thus beams  200 A′ and  200 B′ experience an x-directed walkoff that spatially separates them from beams  100 A′ and  100 B′. 
   Beams  100 A′ and  100 B′ pass through a half wave plate  29 , which rotates the plane of polarization by 45 degrees clockwise. Beams  200 A′ and  200 B′ pass through a second electrically controllable polarization rotator  31  which rotates the plane of polarization by 45 degrees clockwise or counter-clockwise, depending on an applied control signal. Then beams  100 A′,  100 B′,  200 A′, and  200 B′ pass through a third electrically controllable polarization rotator  33  which rotates the plane of polarization by 45 degrees clockwise or counter-clockwise, depending on an applied control signal. 
   The combination of half wave plate  29  and polarization rotators  31  and  33  acts as a compound polarization rotator that can change the polarization of the beams  100 A′,  100 B′,  200 A′,  200 B′ in four different ways, depending on the applied electrical signals. When rotators  31  and  33  both rotate polarization by +45°, beams  100 A′ and  100 B′ are x-polarized, and beams  200 A′ and  200 B′ are z-polarized (i.e., the polarizations of beams  100 A′,B′ and beams  200  A′,B′ are exchanged). When rotators  31  and  33  both rotate polarization by −45°, beams  100 A′ and  100 B′ are z-polarized, and beams  200 A′ and  200 B′ are z-polarized (i.e., all beams are z-polarized). When rotator  31  rotates polarization by +45° and rotator  33  rotates by −45°, beams  100 A′ and  100 B′ are z-polarized, and beams  200 A′ and  200 B′ are x-polarized (i.e., the polarizations of beams  100 A′,B′ and beams  200  A′,B′ are unchanged). When rotator  31  rotates polarization by −45° and rotator  33  rotates by +45°, beams  100 A′ and  100 B′ are x-polarized, and beams  200 A′ and  200 B′ are x-polarized (i.e., all beams are x-polarized). 
   Beams  100 A′,  100 B′,  200 A′, and  200 B′ are then received by a polarization beamsplitter (PBS)  55 , which in this example transmits x-polarized light and reflects z-polarized light through an angle of 90 degrees. Thus polarization rotator  25  acts as a 2×2 switch to determine which side (left or right) of PBS  55  beams  100  and  200  are directed to. This function can be used to switch between the two ports of a dual fiber collimator (e.g., fibers  1  and  2 ). Rotators  31  and  33  determine whether light on the left side of PBS  55  is transmitted or reflected, and also whether or not light on the right side of PBS  55  is transmitted or reflected. The four cases considered above show that all possibilities are accounted for. 
   The discussion to this point has followed the optical path from input fibers  1  and  2  to PBS  55 . As shown on  FIG. 3 , input fibers  3  and  4  also provide optical beams which are received by PBS  55 . The optical components between fibers  3  and  4  and PBS  55  are the same as between fibers  1  and  2  and PBS  55 . For example, elements  14 ,  18 ,  22 ,  26 ,  28 ,  32 , and  34  correspond to elements  13 ,  17 ,  21 ,  25 ,  27 ,  31 , and  33  respectively. Therefore, the above description in connection with  FIGS. 4 and 6  of the optical elements between fibers  1  and  2  and PBS  55  is also applicable to the optical elements between fibers  3  and  4  and PBS  55 . Accordingly, PBS  55  also receives beams  300 A′,  300 B′,  400 A′, and  400 B′ from fibers  3  and  4 , as shown on  FIG. 6 . Beams  300 A′,  300 B′,  400 A′, and  400 B′ are switchably related to fibers  3  and  4  in the same way that beams  100 A′,  100 B′,  200 A′, and  200 B′ are switchably related to fibers  1  and  2 . Similarly, the polarization of beams  300 A′,  300 B′,  400 A′, and  400 B′ is switchable in the same way as the polarization of beams  100 A′,  100 B′,  200 A′, and  200 B′. 
   These beams then pass through a birefringent combiner  53 , a fourth electrically controllable polarization rotator  51 , a half-wave plate  49 , a birefringent splitter  47 , a half-wave plate  45  and a fifth electrically controllable polarization rotator  43  in succession. The operation of these elements is best appreciated by considering three cases. In case  1 , input fibers  1  and  2  are coupled to output fibers  5  and  6 . In case  2 , input fibers  3  and  4  are coupled to output fibers  5  and  6 . In case  3 , one of output fibers  5  and  6  is coupled to input fiber  1  or  2 , and the other of output fibers  5  and  6  is coupled to input fiber  3  or  4 . 
   In case  1 , beams  100 A′,  100 B′,  200 A′, and  200 B′ are x-polarized as they pass through PBS  55 . These beams remain x-polarized as they pass through combiner  53 , and experience walkoff. For this case, rotator  51  rotates the polarization by +45 degrees, as does half-wave plate  49 , thus making the beams z-polarized when exiting wave plate  49 . These z-polarized beams pass through birefringent splitter  47  without walkoff. Beams  100 A′ and  100 B′ then pass through waveplate  45  which rotates the polarization by +45 degrees, and through rotator  43  which is set to rotate the polarization by −45 degrees. Thus beams  100 A′,  100 B′,  200 A′, and  200 B′ are all z-polarized after rotator  43 . Note that beams  100 A′ and  100 B′ come from fiber  1  and beams  200 A′ and  200 B′ come from fiber  2  (or vice versa) based on the setting of rotator  25 . 
   In case  2 , beams  300 A′,  300 B′,  400 A′, and  400 B′ are z-polarized as they are reflected in PBS  55  toward fibers  5  and  6 . These beams remain z-polarized as they pass through combiner  53 , and do not experience walkoff. The length of combiner  53  is selected to ensure that the beams exiting combiner  53  have the same position for both cases  1  and  2 . For this case, rotator  51  rotates the polarization by −45 degrees, and half-wave plate  49  rotates the polarization by +45 degrees, thus making the beams z-polarized when exiting wave plate  49 . These z-polarized beams pass through birefringent splitter  47  without walkoff. Beams  300 A′ and  300 B′ then pass through waveplate  45  which rotates the polarization by +45 degrees, and through rotator  43  which is set to rotate the polarization by −45 degrees. Thus beams  300 A′,  300 B′,  400 A′, and  400 B′ are all z-polarized after rotator  43 . Note that beams  300 A′ and  300 B′ come from fiber  3  and beams  400 A′ and  400 B′ come from fiber  4  (or vice versa) based on the setting of rotator  26 . 
   In case  3 , beams  200 A′ and  200 B′ are x-polarized as they pass through PBS  55  and beams  400 A′ and  400 B′ are z-polarized as they are reflected in PBS  55  toward fibers  5  and  6 . These beams are combined as they pass through combiner  53 , since beams  200 A′ and  200 B′ experience walkoff relative to beams  400 A′ and  400 B′. For this case, rotator  51  rotates the polarization by −45 degrees, and half-wave plate  49  rotates the polarization by +45 degrees or −45 degrees, thus providing either a 0 degree or a 90 degree polarization rotation through elements  51  and  49 . This combined beam is split by splitter  47  such that beams  200 A′ and  200 B′ are separated from beams  400 A′ and  400 B′. Beams  200 A′ and  200 B′ then pass through waveplate  45  which rotates the polarization by +45 degrees, and through rotator  43  which is set to rotate the polarization by +45 degrees. Thus beams  200 A′,  200 B′,  400 A′, and  400 B′ are all z-polarized after rotator  43 . Note that beams  200 A′ and  200 B′ come from fiber  1  or  2  based on the setting of rotator  25  and beams  400 A′ and  400 B′ come from fiber  3  or  4  based on the setting of rotator  26 . Also note that beams  400 A′, and  400 B′ instead of beams  200 A′ and  200 B′ will walk off in element  47  if elements  51  and  49  provide a 90 degree polarization rotation. Thus beams  200 A′,B′ and  400 A′,B′ exiting from splitter  47  can be laterally exchanged with each other based on the setting of rotator  51 . This degree of freedom permits switchable coupling between fibers  5  and  6  and beams  200 A′,B′ and  400 A′,B′. 
     FIG. 5  shows an output subassembly of the switch of  FIG. 3 . On  FIG. 5 , beams propagate from right to left. Beams  100 A′,  100 B′,  200 A′, and  200 B′ (e.g., case  1  above) exiting from rotator  43  on  FIG. 6  are received by a polarization-independent light-bending device  41 . Light bending device  41  deflects these beams so that they make an angle θ with respect to the y-axis. The angle θ is selected to provide efficient coupling into fibers  5  and  6 . Beams  100 B′ and  200 B′ enter a wave plate  39  which rotates the polarization of these beams by 90 degrees. Orthogonally polarized beams  100 A′ and  100 B′ next enter a birefringent block  37 , which combines these two beams into a single beam that is focused onto fiber  5  by a dual fiber collimator  35 . Similarly, orthogonally polarized beams  200 A′ and  200 B′ also enter birefringent block  37 , which combines these two beams into a single beam that is focused onto fiber  6  by the collimator  35 . Dual fiber collimator  35  allows two collimated beams to be coupled to two fibers with a single lens, thereby providing small fiber separation for compactness. The arrangement of  FIG. 5  operates in the same way for the three switching cases considered above. 
   The discussion in connection with  FIGS. 5 and 6  has followed the optical path from PBS  55  to output fibers  5  and  6 . As shown on  FIG. 3  and discussed above, PBS  55  can also provide beams which are received by output fibers  7  and  8 . The optical components between PBS  55  and fibers  7  and  8  are the same as between PBS  55  and fibers  5  and  6 . For example, elements  54 ,  52 ,  48 ,  44 ,  42 , and  38  correspond to elements  53 ,  51 ,  47 ,  43 ,  41 , and  37  respectively. Therefore, the above description in connection with  FIGS. 6 and 5  of the optical elements between PBS  55  and fibers  5  and  6  is also applicable to the optical elements between PBS  55  and fibers  7  and  8 . 
   Thus optical paths from fiber  1  to fiber  5  and from fiber  2  to fiber  6  (or from fiber  1  to fiber  6  and from fiber  2  to fiber  5 ) are established when appropriate control signals are applied to the electrically controllable Faraday rotators  25 ,  31 ,  33 ,  51  and  43 . Similarly, optical paths from fiber  1  to fiber  7  and from fiber  2  to fiber  8  (or from fiber  1  to fiber  8  and from fiber  2  to fiber  7 ) are established when appropriate control signals are applied to the electrically controllable Faraday rotators  25 ,  31 ,  33 ,  52  and  44 . Likewise, optical paths from fiber  3  to fiber  5  and from fiber  4  to fiber  6  (or from fiber  3  to fiber  6  and from fiber  4  to fiber  5 ) are established when appropriate control signals are applied to the electrically controllable Faraday rotators  26 ,  32 ,  34 ,  51  and  43 . Finally, optical paths from fiber  3  to fiber  7  and from fiber  4  to fiber  8  (or from fiber  3  to fiber  8  and from fiber  4  to fiber  7 ) are established when appropriate control signals are applied to the electrically controllable Faraday rotators  26 ,  32 ,  34 ,  52  and  44 . Thus the inputs  1 , 2 , 3 , 4  can be coupled to the outputs  5 , 6 , 7 , 8  in any of twenty four ways by the switch of  FIG. 3 . 
     FIG. 7   a  shows another embodiment of the invention which is a one by four optical switch. A light beam from a fiber port  1  is incident on a birefringent splitter  702 , which splits the incident beam into two orthogonally polarized beams  740 A and  740 B. These beams then pass through a compound half wave plate  704  that rotates the polarizations of beams  740 A and  740 B by +45 and −45 degrees respectively (or vice versa), so that both beams have the same polarization. The beams then pass through a controllable polarization rotator  706 , which rotates the polarization by +45 degrees or −45 degrees, depending on a control input. Next the beams pass through a walkoff element  708 . If beams  740 A and  740 B are z-polarized, they pass through walkoff element  708  without walkoff. If beams  740 A and  740 B are x-polarized, they pass through walkoff element  708  with walkoff, and exit as beams  750 A and  750 B respectively. The beams then pass through a compound half wave plate  710  that rotates the polarizations of beams  740 A and  740 B by +45 and −45 degrees respectively and rotates the polarizations of beams  750 A and  750 B by +45 and −45 degrees respectively (or vice versa) so that beams  740 A,B have the same polarization, as do beams  750 A,B. Beams  740 A,B and  750 A,B then pass through a controllable polarization rotator  712 , which rotates the polarization by +45 degrees or −45 degrees, depending on a control input. 
   Beams  750 A,B are further separated from beams  740 A,B by passage through a rhomboid prism  714 . A pair of parallel mirrors can also be used to perform the beam separation function of prism  714 . Beams  750 A,B next pass through a walkoff element  716 . If beams  750 A,B are z-polarized, they pass through walkoff element  716  without walkoff. If beams  750 A and  750 B are x-polarized, they pass through walkoff element  716  with walkoff, and exit as beams  770 A and  770 B respectively. Beams  750 A,B and  770 A,B pass through light bending device  718 . Light bending device  718  deflects these beams so that they make an angle θ with respect to the y-axis. The angle θ is selected to provide efficient coupling into fiber ports  4  and  5 , as on  FIG. 5 . 
   If the beams exiting light bending device  718  are beams  770 A,B, the polarization of these beams is rotated by −45 degrees by a controllable polarization rotator  720 . Beams  770 A,B then pass through a compound half wave plate  722  which rotates the polarization of beams  770 A and  770 B by −45 degrees and +45 degrees respectively. Beams  770 A and  770 B are then combined in a birefringent combiner  724  and coupled to fiber port  4 . 
   If the beams exiting light bending device  718  are beams  750 A,B, the polarization of these beams is rotated by +45 degrees by the controllable polarization rotator  720 . Beams  750 A,B then pass through the compound half wave plate  722  which rotates the polarization of beams  750 A and  750 B by −45 degrees and +45 degrees respectively. Beams  750 A and  750 B are then combined in the birefringent combiner  724  and coupled to fiber port  5 . 
   Beams  740 A,B are switchably coupled to fiber port  2  or  3  by splitter  726 , light deflector  728 , rotator  730 , compound half wave plate  732  and combiner  734  in the same way that beams  750 A,B are switchably coupled to fiber port  4  or  5 . Thus the arrangement of  FIG. 7   a  is a one by four optical switch. Rotator  706  determines whether the input is coupled to output  2  or  3 , or to output  4  or  5 . Rotator  712  selects between outputs  2  and  3  (or between outputs  4  and  5 ). 
     FIG. 7   b  shows a one by four optical switch similar to the switch of  FIG. 7   a , where a polarizing beamsplitter (PBS)  740  is used instead of prism  714  to separate the beams. Elements  702 ,  704 ,  706 ,  708 ,  710  and  712  operate as indicated in connection with  FIG. 7   a.    
   If the beams exiting rotator  712  are x-polarized, they are transmitted through PBS  741 . Beams  740 A,B and  750 A,B next pass through light bending device  742 . Light bending device  742  deflects these beams so that they make an angle θ with respect to the y-axis. The angle θ is selected to provide efficient coupling into fiber ports  2  and  3 , as on  FIG. 5 . Beams  740 A and  750 A pass through a half wave plate  744 , which rotates their polarization by 90 degrees. Beams  740 A and  740 B are then combined in a birefringent combiner  746  and coupled to fiber port  2 . Similarly, beams  750 A and  750 B are combined in the birefringent combiner  746  and coupled to fiber port  3 . 
   If the beams exiting rotator  712  are z-polarized, they are reflected in PBS  741 . Beams  780 A,B and beams  790 A,B correspond to beams  740 A,B and beams  750 A,B respectively. Beams  780 A,B and  790 A,B are switchably coupled to fiber ports  4  and  5  by light deflector  748 , wave plate  751  and combiner  752  in the same way that beams  740 A,B and  750 A,B are switchably coupled to fiber ports  2  and  3 . Thus the arrangement of  FIG. 7   b  is also a one by four optical switch. Rotator  706  determines whether the input is coupled to output  2  or  3 , or to output  4  or  5 . Rotator  712  selects between outputs  2  and  3  (or between outputs  4  and  5 ). 
     FIG. 8   a  shows an adjustable time delay element according to an embodiment of the invention. A fiber input is collimated by an input subassembly  802  to provide optical beams  870 A,B. Optical beams  870 A,B pass through PBS  826 , PBS  832 , PBS  838  and PBS  844  and are then coupled to an output fiber by an output subassembly  804 . Polarization control components are placed in beams  870 A,B such that at each PBS the beams either do or do not make a single pass through a corresponding fiber loop. Fiber loops  808 ,  810 ,  812  and  814  correspond to PBSs  826 ,  832 ,  838 , and  844  respectively. It is preferable for the fiber loops to have delays which follow a binary geometric progression, as shown on  FIG. 8   a , where loops  808 ,  810 ,  812 , and  814  have delays ΔT, 2ΔT, 4ΔT and 8ΔT respectively. Further details of the embodiment of  FIG. 8   a  are shown on  FIG. 8   b , which is a view along line A—A on  FIG. 8   a.    
   On  FIG. 8   b , an input beam from an input fiber is collimated and split into beams  870 A and  870 B by a birefringent splitter  820 . Beams  870 A and  870 B are then rotated by 45 degrees in opposite directions by a compound half wave plate  822 . Beams  870 A and  870 B then have the same polarization, and pass through a controllable polarization rotator  824 , which rotates the polarization by +45 degrees or −45 degrees, depending on a control input. If the beams exiting rotator  824  are horizontally polarized, they are transmitted through PBS  826  and do not pass through fiber loop  808 . If the beams exiting rotator  824  are vertically polarized, they are reflected in PBS  826  and pass through fiber loop  808 . 
     FIGS. 8   c  and  8   d  show how fiber loop  808  is coupled to PBS  826 . On  FIG. 8   c  (B—B view of Part B on  FIG. 8   a ), reflection of beams  870 A,B from PBS  826  gives rise to beams  880 A,B. Beam  880 B passes through a half wave plate  854  which rotates its polarization by 90 degrees. Beams  880 A and  880 B are then combined by a birefringent combiner  852  and coupled into the fiber loop. On  FIG. 8   d  (B—B view of Part A on  FIG. 8   a ), light from the fiber loop is split by a birefringent splitter  858  into orthogonally polarized beams  890 A and  890 B. Beam  890 A passes through a half wave plate  856  which rotates its polarization by 90 degrees. Beams  890 A and  890 B now have the appropriate polarization (vertical in this example) to be reflected from PBS  826  toward output assembly  804 . 
   Similarly, 45 degree waveplates  828 ,  834 , and  840  combine with +/−45 degree rotators  830 ,  836 , and  842  respectively to control beam switching at PBSs  832 ,  838 , and  844  respectively into fiber loops  810 ,  812 , and  814  respectively. Beams  870 A,B exiting from PBS  844  can be either horizontally or vertically polarized. A +/−45 degree polarization rotator  846  rotates the polarization by +45 degrees or −45 degrees, depending on a control input. The beams then enter a compound half wave plate  848 , which rotates the polarization of beams  870 A and  870 B by  45  degrees in opposite directions. Rotator  846  is set to ensure that beams  870 A and  870 B are horizontally and vertically polarized, respectively, after exiting from wave plate  848 . Beams  870 A and  870 B are then combined by a birefringent combiner  850  and coupled to an output fiber. 
   Variable time delay is a key function in RF systems. This is presently accomplished by means of electronic time delay circuitry, that is intrinsically limited to a 180 degree phase shift, that is only 50 picoseconds time delay range at 10 GHz operation. Fiber optical time delay offers the solution to overcome this limitation. Fiber is an excellent medium for time delay generation, due to its low loss, independence of operational frequency, and immunity to electromagnetic field interference. However, previous design (e.g., U.S. Pat. No. 6,700,704) uses light travel in free-space to achieve variable optical delay. This type of approach has a very limited delay range (about a few centimeters) due to the fundamental light diffraction induced large loss. Our inventive design is advantageously based on using lowloss optical fiber loops to achieve variable time delay, resulting in significantly extended delay range (kilometers). Therefore, the new design provides a practical solution for a long time delay range device that has not been possible before. 
     FIG. 9  shows a time delay element similar to that of  FIG. 8   a , except that birefringent elements are used instead of PBSs as switching elements for fiber loops  902 ,  904 ,  906 , and  908 . On  FIG. 9 , beams  920 A,B pass through a +/−45 degree rotator  912  and a birefringent walkoff element  910 . Beams  920 A,B entering element  910  are either ordinary waves (no walkoff) or extraordinary waves (walkoff). If walkoff occurs, the beams make a pass through fiber loop  902  guided by prisms  914  and  916  as shown. If no walkoff occurs, the beams do not make a pass through fiber loop  902 . Beams exiting walkoff element  910  pass through a 45 degree half wave plate  922 . A +/−45 degree polarization rotator  918  controls whether or not light passes through fiber loop  904  in the same way that rotator  912  controls fiber loop  902 . Fiber loops  906  and  908  are also controlled in the same way. 
     FIG. 10  shows a time delay element similar to that of  FIG. 8   a , except that dual fiber collimators are employed in the fiber loops. Input assembly  1016  provides beams  1020 A,B, which have their polarization rotated by rotator  1014  to either transmit through PBS  1010  or reflect within PBS  1010 . A prism  1012  provides “same side” coupling for dual fiber collimator  1018 , which can have the structure shown on  FIG. 5 . Fiber loops  1004 ,  1006 , and  1008  are controlled in the same way as fiber loop  1002 . 
     FIG. 11  shows a time delay element similar to that of  FIG. 10 , except that a different PBS configuration is used. Input assembly  1016  provides beams  1120 A,B which have their polarization rotated by rotator  1110  to either transmit through PBS  1112  or reflect from PBS  1112 . The PBS configuration of  FIG. 11  provides “same side” coupling for dual fiber collimator  1018  without the need for separate prisms as on  FIG. 10 . Fiber loops  1104 ,  1106 , and  1108  are controlled in the same way as fiber loop  1102 . 
     FIGS. 12   a–b  shows two ways to implement polarization rotators as used in the above examples.  FIG. 12   a  shows a magneto-optic approach for the polarization rotator. Two orthogonally polarized input beams are received by a compound half wave plate  1202 . Compound half wave plate  1202  rotates the polarization of these beams by 45 degrees in opposite directions, so that they have the same polarization. Next, these beams pass through a +/−45 degree Faraday rotator, which rotates the beam polarization by +45 degrees or −45 degrees, depending on an electrical input to the Faraday rotator  1204 . The beams exiting from Faraday rotator  1204  have the same polarization, which is either horizontal or vertical, depending on the input to Faraday rotator  1204 . 
     FIG. 12   b  shows an electro-optic approach for the polarization rotator. Two orthogonally polarized input beams are received by a compound half wave plate  1202 . Compound half wave plate  1202  rotates the polarization of these beams by 45 degrees in opposite directions, so that they have the same polarization. Next, these beams pass through an electro-optic rotator (or retarder)  1206 , which rotates the beam polarization by 0 degrees or 90 degrees, depending on an electrical input to the rotator  1206 . The beams then pass through a half wave plate  1208 , which rotates the polarization of both beams by 45 degrees (either clockwise or counter-clockwise). The beams exiting from wave plate  1208  have the same polarization, which is either horizontal or vertical, depending on the input to rotator  1206 . 
   Thus the polarization rotators of  FIGS. 12   a  and  12   b  are equivalent (for light traveling from left to right on  FIGS. 12   a–b ), and so either approach can be used interchangeably for any of the polarization rotators in the above examples. More specifically, the combination of electro-optic retarder  1206  and wave plate  1208  is equivalent to Faraday rotator  1204  for the purposes of this invention. 
   The above embodiments are exemplary, and many variations are possible. For example, details of geometrical configuration, polarization direction and polarization rotation sense in the above examples can be varied within the scope of the invention. Also, switches according to the invention (including the above examples) can be unidirectional (if magneto-optic polarization rotators are used) or bidirectional (if electro-optic polarization rotators are used). Another example of such a variation would be a four by one switch analogous to the one by four switches of  FIGS. 7   a–b.