Abstract:
An improved optical circulator transfers light from a first optical port to a second optical port, and from the second port to a third port. The circulator has non-reciprocal polarization rotators, birefringent beam splitters and combiners, and a polarization-dependent light bending device comprising two tapered birefringent plates. The light bending device compensates for an angle between a first light beam emanating from the first port and a second light beam propagating to the third port. The existence of this angle allows the first and third fibers to be coupled to the light beams using a single lens.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of patent application Ser. No. 09/025,526 filed on Feb. 18, 1998 now U.S. Pat. No. 5,930,039. Patent application Ser. No. 09/025,526 is a continuation-in-part of patent application Ser. No. 08/986,064 filed Dec. 8, 1997 now U.S. Pat. No. 5,909,310. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to non-reciprocal couplings for optical fibers, and in particular, to optical circulators. 
     BACKGROUND 
     An optical circulator is a device that has at least three ports for accepting optical fibers. Light that enters the circulator through the first port exits through the second port; light that enters through the second port exits through the third. The optical circulator is an inherently non-reciprocal device, since if light enters through the first port it exits through the second, but if that light is subsequently reflected back into the second port, it does not retrace its path back to the first port, but exits through the third port instead. 
     Circulators are necessary, for example, to use the same fiber for both receiving and transmitting data. The first port may be connected to a data transmitter, and the second port to a long distance optical fiber. In that case, data can be sent from the transmitter to the fiber. At the same time, incoming optical data from the long distance fiber enters the circulator through the second port and is directed to the third port where a receiver may be connected. 
     One prior art optical circulator is described in U.S. Pat. No. 4,650,289 by Kuwahara; see FIG.  1 . In this circulator, the labels A, B, and C correspond to the first, second, and third ports described above (port D need not be used). This circulator suffers from the following disadvantages: it requires two spatially separated optical paths, and the ports A and C are perpendicular. This means that the circulator will be bulky when a more compact size is desirable. 
     A more compact circulator is described in U.S. Pat. No. 5,204,771 by Koga; see FIG.  2 . This circulator shows an improvement over the previous one since the two optical paths can be in close proximity, and the first and third ports (designated  27  and  28  in the drawing) are parallel. Unfortunately, this device still suffers from a disadvantage. A lens must be placed between the first optical fiber and the circulator to collimate light coming from the first fiber. A lens must also be placed between the third fiber and the circulator to focus light onto the third fiber. If the first and third fibers are far enough apart that there is room to insert two lenses side by side (one for each fiber), the circulator will have to be quite large. Such a circulator will also be expensive, since the cost increases with the size of the components. 
     If the first and third ports ( 27  and  28  in FIG. 2) are very close together, the first and third fibers will have to share a common lens for collimating and focusing. However, it is impossible for a single lens to perform adequately for both fibers. The difficulty can be traced to the fact that the light beams coupled to the first and third ports are parallel, and a single lens cannot focus two parallel beams to two different points (i.e., to two different fibers). This prior art therefore suffers from the shortcoming that it cannot be manufactured economically when the circulator is large, and it cannot be efficiently coupled to optical fibers when the circulator is small. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     In view of the above, it is an object of the present invention to provide a compact and economical optical circulator that can be efficiently coupled to optical fibers. 
     The invention consists of an optical circulator having at least three ports for optical fibers. Light beams coupled to the first and third fibers are not parallel; there is a slight angle between the two beams. Because of this angle, a single lens may be used for coupling both the first and the third fibers to the circulator. 
     The invention further consists of a light-bending device comprising two tapered birefringent plates, situated to compensate for the angle between the light beams coupled to the first and third fibers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art optical circulator by Kuwahara. 
     FIG. 2 shows a prior art optical circulator by Koga. 
     FIG. 3 shows how light is transmitted from a first optical fiber to a second optical fiber in a first embodiment of a circulator according to the invention. 
     FIG. 4 shows how light is transmitted from the second optical fiber to a third optical fiber in the circulator of FIG.  4 . 
     FIG. 5 a  shows various embodiments of a polarization-dependent light guiding device when n o &gt;n e . 
     FIG. 5 b  shows various embodiments of the polarization-dependent light guiding device when n o &lt;n e . 
     FIG. 6 shows a three dimensional view of a second embodiment of the circulator with a light beam propagating from a first fiber to a second fiber. 
     FIG. 7 a  is a top plan view of the circulator of FIG. 6 showing a light beam propagating from the first fiber to the second fiber. 
     FIG. 7 b  is a side view of the circulator of FIG. 6 showing the light beam propagating from the first fiber to the second fiber. 
     FIG. 8 a  is a top plan view of the circulator of FIG. 6 showing a light beam propagating from the second fiber to the third fiber. 
     FIG. 8 b  is a side view of the circulator of FIG. 6 showing the light beam propagating from the second fiber to the third fiber. 
     FIG. 9 shows a three dimensional view of a third embodiment of the circulator with a light beam propagating from the first fiber to the second fiber. 
     FIG. 10 a  is a top plan view of a fourth embodiment of the circulator with a light beam propagating from the first fiber to the second fiber. 
     FIG. 10 b  is a side view of the circulator of FIG. 10 a  showing the light beam propagating from the first fiber to the second fiber. 
     FIG. 11 a  is a top plan view of the circulator of FIG. 10 a  showing a light beam propagating from the second fiber to the third fiber. 
     FIG. 11 b  is a side view of the circulator of FIG. 10 a  showing the light beam propagating from the second fiber to the third fiber. 
     FIG. 12 a  is a top plan view of a fifth embodiment of the circulator with a light beam propagating from the second fiber to the third fiber. 
     FIG. 12 b  is a side view of the circulator of FIG. 12 a  showing the light beam propagating from the second fiber to the third fiber. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 shows a circulator  100  according to a first embodiment of the invention. A first optical fiber  1  is inserted into a first glass capillary  10 A. A second optical fiber  2  is inserted into a second glass capillary  10 B opposite first fiber  1 . A third optical fiber  3  is inserted into first glass capillary  10 A adjacent to fiber  1 , so that fiber  3  and fiber  1  are parallel. 
     A set of orthogonal reference axes is arranged so that the y-axis is parallel to fibers  1 ,  2 , and  3 . A reference point P is located near the first glass capillary  10 A. 
     Fiber  1  emits a light beam  30  that is collimated by a first lens  12 A. Lens  12 A also causes beam  30  to make an angle θ with respect to the y-axis. Preferably, lens  12 A is a graded index (GRIN) lens. 
     Beam  30  then passes through a first birefringent block  14 A. Beam  30  is thereby divided into two beams having orthogonal polarizations, specifically beams  30 A and  30 B, corresponding to the ordinary and extraordinary rays in birefringent block  14 A. Beam  30 A is polarized along the x-axis (out of the page); this polarization is indicated by a dot in FIG.  3 . Beam  30 B is polarized in the y-z plane; this polarization is indicated by a line segment. The length of birefringent block  14 A is adjusted to obtain a spatial separation between beams  30 A and  30 B which permits to pass them through independent optical elements. 
     Thus, beam  30 A enters a first half wave plate  18 A which rotates the plane of polarization by 45° in the counterclockwise direction as seen from point P in FIG.  3 . Beam  30 A then enters a first Faraday rotator  20 A which rotates the plane of polarization by 45° in the clockwise direction as seen from point P. The net effect of half wave plate  18 A and Faraday rotator  20 A (the first a reciprocal device and the second non-reciprocal), therefore, is to leave the polarization of beam  30 A unaltered. 
     Beam  30 B, meanwhile, enters a second half wave plate  16 A positioned above first half wave plate  18 A. Second half wave plate  16 A rotates the plane of polarization of beam  30 B by 45° clockwise as seen from P; i.e., half wave plate  16 A effects a rotation in the opposite direction to half wave plate  18 A. Beam  30 B then passes through a Faraday rotator  20 A, which again rotates the plane of polarization by 45° clockwise as seen from P. Therefore, after passing through half wave plate  16 A and Faraday rotator  20 A, the polarization of beam  30 B is in the x-direction, or parallel to the polarization of beam  30 A. 
     Half wave plates  16 A and  18 A, together with Faraday rotator  20 A, make up a first compound polarization rotator  40 A that renders two orthogonal polarizations parallel to each other. 
     At this point beams  30 A and  30 B still propagate at angle θ with respect to the y-axis as they exit rotator  20 A. This angle of propagation is changed by a polarization-dependent light guiding device  42 . Device  42  consists of a first tapered birefringent plate  22  and a second tapered birefringent plate  24 . The tapering of plate  22  is complementary to the tapering of plate  24 , and each plate is tapered by an angle α. Plates  22  and  24  are made from the same birefringent material and each plate has two indices of refraction: n e  and n o , corresponding to the extraordinary and ordinary rays. In the embodiment illustrated in FIG. 3, n o &gt;n e . 
     The index of refraction in general determines how much a light ray will bend, or refract, upon entering a material. When the index of refraction is known, the amount of refraction can be determined by Snell&#39;s law. A birefringent material has two indices of refraction, indicating that different polarizations of light will refract by different amounts. 
     Plate  22  has an optic axis OA 1  that is oriented parallel to the x-axis. Therefore beams  30 A and  30 B are viewed as extraordinary rays in plate  22 , and are therefore refracted according to the extraordinary index of refraction n e . Plate  24  has an optic axis OA 2  that is parallel to the z-axis, so beams  30 A and  30 B are ordinary rays within plate  24 . Therefore beams  30 A and  30 B are refracted upon passing from plate  22  to plate  24  because of the difference between indices of refraction n e  and n o . 
     The angle α is adjusted so that beams  30 A and  30 B are rendered parallel to the y-axis by light guiding device  42 . Using Snell&#39;s law at each interface, the relationship between the angles α and θ is: 
     
       
         sinθ= n   e sin{sin −1 [( n   o   /n   e )sin α]−α}.  (1) 
       
     
     Beams  30 A and  30 B exit plate  24  and enter a second birefringent block  26 . The optical axis of block  26  is oriented such that beams  30 A and  30 B are ordinary rays in block  26  and thus remain undeflected. 
     Next, beam  30 A enters a half wave plate  18 B which rotates the plane of polarization of beam  30 A by 45° counterclockwise as seen from point P. Beam  30 A then passes through a Faraday rotator  20 B which rotates the polarization by another 45° counterclockwise as seen from P. Beam  30 A is now polarized in the z-direction (indicated by a line segment in the figure). 
     Meanwhile, beam  30 B passes through a half wave plate  16 B which rotates the polarization 45° clockwise as seen from P. Beam  30 B then enters Faraday rotator  20 B which rotates the plane of polarization of beam  30 B by 45° counterclockwise as seen from point P. Consequently, half wave plate  16 B and rotator  20 B together have no net effect on the polarization of beam  30 B. 
     Half wave plates  16 B and  18 B and Faraday rotator  20 B together comprise a second compound polarization rotator  40 B that renders two parallel polarizations perpendicular to each other. 
     Beams  30 A and  30 B subsequently pass through a third birefringent block  14 B, where beam  30 A is the extraordinary ray and beam  30 B is ordinary. Block  14 B combines beams  30 A and  30 B to form a single beam  31  that is in general unpolarized since it combines the two orthogonal polarizations of beams  30 A and  30 B. 
     Beam  31  is focused by a second lens  12 B (preferably a GRIN lens) and enters optical fiber  2  mounted in glass capillary  10 B. 
     The description so far shows how light starting from fiber  1  is guided into fiber  2 . For circulator  100  to work properly, light entering the circulator from fiber  2  must be channeled into fiber  3 . In other words, circulator  100  has the property of channeling light from fiber  1  to fiber  2  and from fiber  2  to fiber  3  without any light being channeled from fiber  2  back to fiber  1 . This second step is shown in FIG.  4 . 
     Thus, a beam  32  exits fiber  2  and is collimated by lens  12 B. Beam  32  then enters birefringent block  14 B and is split into two beams,  32 A and  32 B, having orthogonal polarizations. Beam  32 A is ordinary, beam  32 B extraordinary in block  14 B. Upon leaving block  14 B, beam  32 A is polarized in the x-direction and beam  32 B is polarized in the y-direction, as indicated in FIG.  4 . 
     Beams  32 A and  32 B next enter compound polarization rotator  40 B. Beam  32 A enters Faraday rotator  20 B, which rotates the polarization of beam  32 A by 45° counterclockwise as seen from point P. Then beam  32 A enters half wave plate  16 B, which rotates the polarization of beam  32 A by another 45° counterclockwise as seen from P. 
     Meanwhile the polarization of beam  32 B is rotated by 45° counterclockwise as seen from point P by Faraday rotator  20 B. The polarization of beam  32 B is then rotated back 45° clockwise as seen from P by half wave plate  18 B. 
     Therefore, just before beams  32 A and  32 B enter birefringent block  26 , they are both polarized in the z-direction. Here the non-reciprocal nature of circulator  100  is already clear, since if beams  32 A and  32 B were to exactly retrace the paths of beams  30 A and  30 B (FIG.  3 ), they should be polarized in the x-direction. The origin of the non-reciprocity is the Faraday rotator  20 B, whose direction of polarization rotation does not reverse with the change in direction of light propagation. 
     Beams  32 A and  32 B enter birefringent block  26 , where they are extraordinary rays and are offset by a distance f. Both beams  32 A,  32 B then enter light guiding device  42 , which causes beams  32 A and  32 B to each make an angle φ with respect to the y-axis. 
     Beams  32 A and  32 B now enter birefringent plate  24 . The optic axis OA 2  of plate  24  is parallel to the polarizations of beams  32 A and  32 B. Beams  32 A and  32 B are therefore extraordinary rays within plate  24 , but are undeflected since they are normally incident upon plate  24 . 
     Upon leaving plate  24  and entering plate  22 , however, beams  32 A and  32 B become ordinary rays, since their polarizations are perpendicular to the optic axis OA 1  of plate  22 . Beams  32 A and  32 B therefore refract upon entering plate  22  due to the difference between refraction indices n e  and n o . When beams  32 A and  32 B exit plate  22 , they refract again to exit at angle φ with respect to the y-axis. Using Snell&#39;s law, the relationship between angle φ and angle α is as follows: 
     
       
         sin 1000+= n   o sin {α−sin  −1 [( n   e   /n   o )sin α]}.  (2) 
       
     
     After leaving light guiding device  42 , beam  32 A then passes through Faraday rotator  20 A and half wave plate  16 A with no net effect on its polarization. Beam  32 B passes through Faraday rotator  20 A and half wave plate  18 A; the result is a rotation of the polarization of beam  32 A by 90° clockwise as seen from point P. Beams  32 A and  32 B now have orthogonal polarizations and are combined into a single beam  33  by birefringent block  14 A. Beam  33  is subsequently focused by lens  12 A onto fiber  3 . 
     Birefringent block  26  is a polarization-dependent beam deflector that offsets beams  32 A and  32 B but does not offset beams  30 A and  30 B. Birefringent block  26  plays an important role in guiding light from fiber  2  to fiber  3 . Since light guiding device  42  bends beams  32 A and  32 B by the angle φ, beams  32 A and  32 B travel laterally (in the negative z-direction) as well as longitudinally (in the negative y-direction) after they leave device  42 . This lateral travel is compensated by block  26 . 
     To be precise, beams  32 A and  32 B are offset a distance f by birefringent block  26 . The distance along the z-axis between the point where beam  32 B enters plate  24  and fiber  3  is d 2  (see FIG.  4 ). The distance along the z-axis between fiber  1  and the point where beam  30 A leaves plate  24  is d 1  (see FIG.  3 ). The vertical or z-axis distance between fiber  1  and fiber  3  is t. The relation between these quantities is: 
     
       
           f=d   1   +d   2   −t.   (3) 
       
     
     This equation teaches how to design block  26  to have the correct offset f given the other parameters of circulator  100 , i.e., when d 1 , d 2 , and t are known. 
     In an alternative embodiment, the apparatus is designed in such a way that d 1 +d 2 =t. Eq. (3) then implies that f=0, which means that birefringent block  26  can be eliminated completely from the design. 
     If angle θ were exactly equal to angle φ, fibers  1  and  3  would be placed symmetrically with respect to the center of lens  12 A. That is, the lateral distance (distance measured along the z-axis) from fiber  1  to the center of lens  12 A would equal the lateral distance from fiber  3  to the center of lens  12 A. However, angles θ and φ are only approximately equal: if angles θ, φ, and α are all small, then equations (1) and (2) yield to a first approximation: 
     
       
         θ≈( n   o   −n   e )α≈φ 
       
     
     To a better approximation, angle θ differs slightly from angle φ. This difference can be accommodated in at least two ways. The first option is to adjust the lateral positions (i.e. z-coordinates) of fibers  1  and  3  so that the fibers are asymmetric with respect to the center of lens  12 A. The second, preferred option is to place fibers  1  and  3  symmetrically with respect to the center of lens  12 A, and to rotate light guiding device  42  slightly about an axis parallel to the x-axis, thereby altering equations (1) and (2) to ensure that θ=φ. Either approach represents a minor adjustment of the overall apparatus. In practice, angles φ and θ are between 1° and 3°, and light guiding device  42  is rotated a fraction of a degree. 
     Birefringent elements  14 A,  14 B,  22 ,  24 , and  26  can be made of any birefringent material, such as rutile, calcite, or yttrium orthovanadate. 
     It should be clear that several variations of the above embodiment are possible and remain within the scope of the invention. For example, the polarizations of beams  30 A and  30 B need not be exactly as shown. It is only important that the polarizations of beams  30 A and  30 B are orthogonal or perpendicular to each other when the beams exit block  14 A, and that the polarizations are parallel after leaving rotator  20 A. When the polarizations of beams  30 A and  30 B are not as described above, the optic axes of the birefringent elements  14 A,  14 B,  22 ,  24 , and  26  are adjusted accordingly. This adjustment changes the polarizations of beams  32 A and  32 B. However, as is apparent to a person of average skill in the art, the principles of circulator  100  remain unchanged. 
     Thus, in another embodiment, beam  30 A is extraordinary and beam  30 B is ordinary in block  14 A. In this embodiment, the beams have complementary properties in block  14 B: beam  30 A is ordinary and beam  30 B is extraordinary. This arrangement ensures, as does the embodiment of circulator  100 , that beams  30 A and  30 B both traverse approximately the same optical path, and therefore the overall phase relation between them is maintained. 
     Variations of light guiding device  42  are also possible. FIG. 5 a  shows different shapes and orientations of optic axes OA 1  and OA 2  that plates  22  and  24  can have when n o &gt;n e . If plates  22  and  24  are made of some birefringent material with n o &lt;n e , other geometries are used, as shown in FIG. 5 b . Still other variations are possible: in the examples of FIG. 5 a  and FIG. 5 b , plates  22  and  24  each have one face parallel to the z-axis. However, a more general trapezoidal shape can be used for either or both of plates  22  and  24 , with no faces parallel to the z-axis. Furthermore, plate  22  need not be made of the same material as plate  24 . 
     In a second embodiment a circulator  200  is designed such that angles θ and φ lie in the same plane while the walk-off in the birefringent blocks takes place in a perpendicular plane. The general construction and operation of this embodiment is analogous to that of circulator  100  and is illustrated in the three dimensional view of FIG.  6 . 
     First and third fibers  202 ,  204  are inserted in parallel and adjacent to each other into a glass capillary  206 A which is followed by a first lens  208 A. A first block of birefringent material  210 A, a first compound polarization rotator  230 A, a light guiding device  250  comprising first and second tapered birefringent plates  252  and  254 , a second birefringent block  256 , a second compound polarization rotator  230 B and a third block of birefringent material  210 B are located along a longitudinal axis L of circulator  200 . A second lens  208 B and a second glass capillary  206 B holding a second fiber  258  are found at the opposite end of device  200 . 
     Longitudinal axis L is parallel to the y-axis. In distinction to circulator  100  where first and third fibers  1 ,  3  are inserted one below the other (along the z-axis) fibers  202 ,  204  are arranged next to each other (along the x-axis). 
     In circulator  200 , first compound polarization rotator  230 A comprises first and second half-wave plates  220 A and  222 A, and a first Faraday rotator  224 A. Second compound polarization rotator  230 B comprises third and fourth half-wave plates  220 B and  222 A, and a second Faraday rotator  224 B. 
     A first light beam  240  propagating from first fiber  202  enters first block  210 A and the two orthogonal polarizations  240 A and  240 B are walked off within block  210 A as shown. These polarizations continue propagating through the elements of circulator  200  until they are recombined by third block  210 B and focused by second lens  208 B into second fiber  258 . 
     The top view of FIG. 7 a  also shows first light beam  240  propagating from fiber  202  to fiber  258  through the elements of circulator  200 . Angle θ, made by beam  240  with respect to longitudinal axis L when exiting through first lens  208 A, lies in the x-y plane. Meanwhile, as shown in the side view of FIG. 7 b , the walk off of the two orthogonal polarizations  240 A and  240 B in birefringent block  210 A occurs in the y-z plane. 
     When a second light beam  270  propagates from second fiber  258  to third fiber  204 , as illustrated in FIGS. 8 a-b , it is offset by distance f in second block  256 . Note that offset distance f is in the x-y plane (FIG. 8 a ). Next, in light guiding device  250  beam  270  is bent at angle φ with respect to longitudinal axis L. In other words, beam  270  exits light guiding device  250  at angle φ. Angle φ also lies in the x-y plane. Thus, angles φ and θ lie in planes which are parallel while the walk-off occurs in a plane perpendicular to them. 
     The advantage of having angles φ and θ lie in an x-y plane while the walk-off takes place in the y-z plane is that it is easier to adjust angles φ and θ independently of the walk-off. Specifically, in practice it is easier to adjust the positions of the elements of circulator  200  to obtain proper coupling of beams  240  and  270  between fibers  202 ,  258  and  204  when the walk-off and the compensating angles φ, θ are in perpendicular planes. Also, in this configuration the elements of circulator  200  can be made smaller and the entire circulator is easier to manufacture. 
     Because circulators  100  and  200  comprise half-wave plates, the efficiencies of circulators  100  and  200  are sensitive to the wavelength of light transmitted. A circulator  300  according to a third embodiment is shown in FIG.  9 . Circulator  300  is nearly identical to circulator  200  except that first and second compound polarization rotators  330 A and  330 B of circulator  300  comprise only non-reciprocal elements. Circulator  300  is preferred over circulator  200  because circulator  300  is insensitive to the wavelength of light used, and has fewer parts. 
     In FIG. 9, light beam  240  emerges from first fiber  202  and enters a first birefringent block  310 A. Beam  240  then diverges into two beams  301  and  302  corresponding to the ordinary and extraordinary rays in block  310 A. Beams  301  and  302  have orthogonal polarizations  340 A and  340 B, respectively, in block  310 A. Block  310 A has an optic axis along a direction such that polarizations  340 A and  340 B each make a 45° angle with the z-axis. 
     First compound polarization rotator  330 A comprises a first Faraday rotator  320 A and a second Faraday rotator  322 A. Faraday rotator  320 A rotates polarization  340 B by 45° clockwise. Faraday rotator  322 A rotates polarization  340 A by 45° counter-clockwise. Therefore, beams  301  and  302  emerge from compound polarization rotator  330 A with polarizations parallel to the z-axis, as shown in FIG.  9 . 
     Beams  301  and  302  then propagate through light guiding device  250  and second birefringent block  256  just as in circulator  200 . Beams  301  and  302  then reach second compound polarization rotator  330 B. Compound polarization rotator  330 B comprises a third Faraday rotator  320 B and a fourth Faraday rotator  322 B. Faraday rotator  320 B rotates the polarization of beam  301  by 45° clockwise, and Faraday rotator  322 B rotates the polarization of beam  302  by 45° counter-clockwise. 
     Beams  301  and  302  therefore emerge from compound polarization rotator  330 B with polarizations  340 D and  340 C, respectively. Polarizations  340 C and  340 D are orthogonal, and each makes a 45° angle with respect to the z-axis. 
     Beams  301  and  302  are subsequently recombined by a birefringent block  310 B, and focused by lens  208 B onto second fiber  258 . 
     When light is emitted from second fiber  258  in circulator  300 , the light is split into polarizations  340 C and  340 D by block  310 B. Polarizations  340 C and  340 D are then rendered parallel to the x-axis by compound polarization rotator  330 B. The light is then guided into third fiber  204  according to the principles outlined above. 
     A circulator  400  according to a fourth embodiment is shown in FIGS. 10 a  and  10   b . Circulator  400  is identical to circulator  200  with the following exceptions: birefringent block  256  is omitted, and light guiding device  250  is replaced by a light guiding device  450 . 
     Light guiding device  450  comprises first and second tapered birefringent plates  452  and  454 . Tapered plate  452  has an optic axis OA 3  parallel to the z-axis; tapered plate  454  has an optic axis OA 4  that lies in the xy-plane, as shown in FIG. 10 a . Optic axis OA 4  is neither parallel to nor perpendicular to longitudinal axis L. That is, optic axis OA 4  is skewed with respect to longitudinal axis L. 
     When beam  240  emerges from first fiber  202 , block  210 A divides beam  240  into two beams  401  and  402  having orthogonal polarizations; see FIG. 10 b . Upon exiting first compound polarization rotator  230 A, beams  401  and  402  have polarizations  440  parallel to the z-axis. Beams  401  and  402  then propagate through light guiding device  450  to enter second fiber  258  as in circulator  200 . 
     When beam  270  is emitted from second fiber  258 , block  210 B divides beam  270  into two beams  403  and  404  having orthogonal polarizations, as shown in FIGS. 11 a  and  11   b . Compound polarization rotator  230 B causes beams  403  and  404  to have polarizations  441  parallel to the x-axis. 
     When beams  403  and  404  enter tapered plate  454 , the beams are offset in the x-direction by a distance f′, as shown in FIG. 11 a . This offset occurs because optic axis OA 4  is neither perpendicular to nor parallel to polarizations  441  of beams  403  and  404 . In circulator  400 , therefore, tapered plate  454  performs the functions of both birefringent block  256  and tapered plate  254  of circulator  200 . 
     Beams  403  and  404  are offset by tapered plate  454 , and are then refracted by tapered plate  452 . Beams  403  and  404  exit tapered plate  452  at an angle φ with respect to the longitudinal axis L, as shown in FIG. 11 a . Beams  403  and  404  are then directed to third fiber  204 , as in circulator  200 . 
     The exact angle that optic axis OA 4  makes with respect to longitudinal axis L, as well as the precise length (in the y-direction) of tapered plate  454 , can be easily determined by one skilled in the art. 
     Circulator  400  is preferred over circulator  200  since circulator  400  eliminates the need for birefringent block  256 . Relative to circulator  200 , circulator  400  is smaller, cheaper, and has a lower light loss from reflections off the surfaces of components. 
     A circulator  500  according to a fifth embodiment is shown in FIGS. 12 a  and  12   b . In the fifth embodiment, compound polarization devices  230 A and  230 B of circulator  400  are replaced by compound polarization devices  330 A and  330 B of circulator  300 . Accordingly, blocks  210 A and  210 B are replaced by blocks  310 A and  310 B. The fifth embodiment combines the advantages of both circulators  300  and  400 : the fifth embodiment comprises compound polarization rotators having Faraday rotators but not half-wave plates, and the fifth embodiment does not contain birefringent block  256 . 
     When beam  270  is emitted from second fiber  258  of circulator  500 , beam  270  is divided into beams  503  and  504  by birefringent block  310 B, as shown in FIGS. 12 a  and  12   b . Beams  503  and  504  enter compound polarization rotator  330 B comprising Faraday rotators  320 B and  322 B. Upon exiting compound polarization rotator  330 B, beams  503  and  504  have polarizations  541  parallel to the x-axis. Beams  503  and  504  then enter third fiber  204  after passing through light guiding device  450 , compound polarization rotator  330 A, birefringent block  310 A, and lens  208 A. 
     Many variations of circulator  500  are possible and remain within the scope of the invention. For example, the directions of polarizations  540  and  541  may be changed if the directions of optic axes OA 3  and OA 4  are correspondingly altered. Furthermore, the shapes of tapered plates  452  and  454  are subject to the same variation as shown in FIG. 5 for tapered plates  22  and  24 . 
     In the broad sense, the circulator can be used to couple light between three optical ports. The ports can include optical fibers as in the embodiments above or other optical elements. 
     It will be apparent to a person of average skill in the art that many variations of the circulator are possible within the scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.