Patent Abstract:
An improved design of beam splitting device is described that is formed by cutting a birefringent material to predetermined dimensions. The beam splitting device resolves input light into mutually orthogonal linearly polarised beams. The mutually orthogonal linearly polarised beams propagate on either side of an axis as defined by the propagation axis of the input light. Such beam splitting devices lends themselves to a reduction in the overall dimensions of an optical system. In particular an optical circulator is described that employs such beam splitting devices so as to allow the transfer of light from a first optical port to a second optical port, and from the second port to a third. Not only does the employment of such beam splitting devices significantly reducing the dimensions of the optical system it also renders the device simpler to align.

Full Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of optics and in particular to a polarisation beam splitting device whose incorporation into optical systems that require polarisation beam splitters, can significantly reduce their overall dimensions. 
     BACKGROUND 
     Beam splitting devices are commonly used in the field of optics when it is required to separate two spatially overlapping beams of light or two polarised components of a single beam. The prior art teaches of various methods for achieving such a result that employ either blocks of birefringent material, polarisation dependent coatings or other polarisation effects. 
     When requiring the polarisation components of a light beam to be resolved, the most efficient manner is to employ a block of birefringent material. A birefringent material is one that is optically anisotropic in that the optical properties it exhibits depend upon the polarisation and propagation direction of the incident light. Many crystalline substances, such as rutile, calcite or yttrium orthovanadate, exhibit such birefringent properties and so provide ideal media from which to develop polarisation beam splitting devices. Such crystal structures are so suited for producing compact birefringent medium, as they comprise high-density structures that lend themselves to cutting, so producing incident surfaces and optic axis of the required predetermined orientations. Nicol prisms and Glan-Foucault prisms are examples taught in the prior art of birefringent crystals employed as beam splitters. 
     FIG. 1 presents a side elevation of a typical block of birefringent material  103  as taught in the prior art. Here an unpolarised incident beam  140  is incident on the block of birefringent material  103 , thereby being resolved into two light beams having orthogonal linear polarisations. For reference a propagation axis L is defined corresponding to the axis of an input beam  140 . With this particular orientation, beam  140   a  corresponds to the ordinary beam while beam  140   b  corresponds to the extraordinary beam. As is typical in optical systems, components are designed such that where possible input and output faces are perpendicular to the central axis L. Therefore, with the incident beam  140  perpendicular to the block of birefringent material  103  the resulting ordinary beam  140   a  passes without deviation through the block  103  while the extraordinary beam  140   b  is refracted as shown. 
     An inherent disadvantage of such a splitting of the ordinary and extraordinary component beams is that when incorporated into an optical system, such blocks of birefringent material  103  introduce an asymmetric beam splitting. It is normally advantageous for the emerging ordinary  140   a  and extraordinary beams  140   b  to be parallel and equidistant from the propagation axis L. The dimensions of the other optical elements of an optical system are then directly dependent on the block of birefringent material  103 . 
     By way of example such blocks of birefringent material  103  are considered herein as incorporated with an optical circulator. However, as will be obvious to those skilled in the art, the problem of reducing the dimensions of an optical system that ernploys such a block of birefringent material  103  as a beam splitter, is not limited solely to optical circulators. Such optical systems also include for example, optical isolators and polarisation beam splitters/combiners. 
     An optical circulator is a device that has at least three ports for accepting optical fibres. 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. 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 fibre 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 fibre. In that case, data can be sent from the transmitter to the fibre. At the same time, incoming optical data from the long distance fibre enters the circulator through the second port and is directed to the third port where a receiver may be connected. 
     An optical circulator found in the prior art is that taught by Li et al in U.S. Pat. No. 5,930,039, see FIG. 2, the contents of which are incorporated herein by reference. 
     This document teaches of an optical circulator  100  that employs reciprocal and non-reciprocal polarisation rotators  130   a  and  130   b , birefringent optical components  103 ,  108  and  111 , and a polarisation dependent refraction element  150  comprising of two tapered birefringent plates  106  and  107 . In the preferred embodiment the optical circulator  100  has its optical components aligned such that effects of the birefringent optical components occur in the vertical plane while the effects of the polarisation dependent refraction element occur in the horizontal plane. 
     The first and third fibres  100   a  and  100   b  are inserted in parallel and adjacent to each other into a glass capillary  101  which is followed by a first lens  102 . Together the glass capillary  101  and the lens  102  comprise a first collimator  120   a . A first block of birefringent material  103 , a first compound polarisation rotator  130   a , a light guiding device  150 , a second birefringent block  108 , a second compound polarisation rotator  130   b  and a third block of birefringent material  111  are then located along a longitudinal axis L of circulator  100 . A second collimator  120   b  comprising a second lens  112  and a second glass capillary  113  which holds the second fibre  114  are found at the opposite end of device  100 . 
     FIG. 3 provides alternative elevations of the optical circulator  100 . In particular FIG. 3 a  presents a side profile of the circulator  100  presenting light propagating in the z-y plane from the first fibre  100   a  to the second fibre  114 . Initially the light propagates through the first lens  102  and into the first birefringent block  103  Walk off within the block  103  in the z-y plane then produces two mutually orthogonal linearly polarised beams,  140   a  and  140   b , as shown. These linearly polarised beams  140   a  and  140   b  then propagate through the first compound polarisation rotator  130   a  before continuing on through the optical circulator  100  until they are recombined by the third birefringent block  111  and focused by second lens  113  into second the fibre  114 . 
     For the optical circulator  100  to work correctly it requires that any light entering the device at the second fibre  114  exits the optical circulator  100  via the third fibre  100   b , and not via the first fibre  100   a . The non-reciprocal nature of the device lies in the inherent properties of the compound polarisation rotators  130   a  and  130   b . To illustrate these features FIG. 3 b  presents a side profile in the z-y plane of the circulator  100  presenting light propagating from the second fibre  114  to the third fibre  100   b.    
     Comparison of the orientations of the linearly polarised electric field components after propagating through the compound polarisation rotators  130   a  and  130   b  shows how the polarisation orientation of an electric field depends on which direction it has propagated through the compound polarisation rotators  130   a  and  130   b . The origin of this non-reciprocity lies in the inherent properties of the Faraday rotators  105  and  110 . Unlike the half wave plates  104   a ,  104   b,   109   a  and  109   b  which reverse the rotation experienced by a linearly polarised electric field on reversal of its propagation direction, a Faraday rotator is designed to always rotate a linearly polarised electric field in the same sense irrespective of propagation direction. 
     FIG. 3 c  shows the x-y plane profile of light propagating from the first fibre  100   a  to the second  114 , along with that propagating from the second fibre  114  to the third  100   b . Initially the light beam from the first fibre exits the first lens  102  at an angle □ to the x-axis. On exiting the compound rotator  130   a  each of the linearly polarised beams,  140   a  and  140   b  propagate at an angle □ relative to the x-axis. The angle of propagation of each of these components is then altered by the light guiding device that exhibits extraordinary refractive index n e  and an ordinary refractive index n o , where n o &gt;n e . 
     Tapered plate  106  has an optic axis OA 1  that is orientated parallel to the z-axis while tapered plate  107  has an optic axis OA 2  parallel to the x-axis. This results in both electric field components of the beams  140   a  and  140   b  exiting the light guiding device  150  parallel to the y-axis. 
     The second block of birefringent material  108  has an optical axis that is also orientated parallel to the z-axis. Therefore, the electric field components of the beams  140   a  and  140   b  are both orientated as ordinary rays relative to the birefringent block  108  and so propagate undeviated through it. 
     The situation is somewhat different in the x-y plane when considering light propagating from the second fibre  114  to the third  100   b . The non-reciprocal nature of the compound polarisation rotators  130   a  and  130   b  is employed by the light guiding device  150  and the second birefringent block  108  in order to translate light from the second fibre  114  to the third  100   b . Before entering the second birefringent block  108  the two electric field components of the light beams are linearly polarised parallel to the z-axis and therefore the beams  140   a  and  140   b  act as extraordinary rays within the second birefringent block  108 . This results in them being spatially translated along the x-axis before propagating through the light guiding device  150 . Translation through the light guiding device  150  imposes an angle φ□ between the linear polarised beams of light and the y-axis. The light then continues on through the optical components being recombined by the first birefringent block  103  before being focused by the first lens  102  into the third fibre  100   b.    
     Analysis of FIG. 3 highlights the inherent disadvantage of employing traditional blocks of birefringent material within this optical circulator  100 . It is seen that the collimators  120   a  and  120   b  are spatially displaced along both the x and z-axes. The result of such an offset in the collimators  120   a  and  120   b  is two fold. In the first instance it makes the optical circulator  100  more difficult to align than if the collimators shared a common axis. Secondly, it restricts the minimum dimensions available for the device. Since cost is directly related to the dimensions of an optical component the offset of the collimators adds additional cost to the manufacture of such an optical system. 
     By redesigning the blocks of birefringent material such that the optical circulator has its collimating elements on a common longitudinal axes the elements of an optical circulator can be made smaller, thus the entire optical circulator is cheaper and easier to manufacture as well as being simpler to align. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     In view of the above, it is an object of the present invention to provide a polarisation beam splitting device formed from a block of birefringent material. The beam splitting device resolves a randomly polarised input beam of light into ordinary and extraordinary linearly polarised beam components that propagate symmetrically about an axis as defined by the input beam. 
     It is a further object of the present invention to provide a compact and economical optical system that employs the aforementioned beam splitting device, such that all the optical elements of the system share a common longitudinal axis. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 presents a side elevation of a block of birefringent material as taught in the prior art. 
     FIG. 2 shows a prior art optical circulator by Li et al, the contents of which are incorporated herein by reference, incorporating blocks of birefringent material as described in FIG.  1 . 
     FIG. 3 presents elevations of the optical circulator of FIG. 2, and in particular presents: 
     (a) a side view showing how light is transmitted from a first optical fibre to a second optical fibre; 
     (b) a side view showing how light is transmitted from the second optical fibre to a third optical fibre; and 
     (c) the elevation, as seen from A in FIGS. 3 a  and  3   b , showing how light propagates around the circulator. 
     FIG. 4 presents a side elevation of: 
     (a) a first embodiment of the beam splitting device according to the present invention; and 
     (b) a second embodiment of the beam splitting device according to the present invention. 
     FIG. 5 presents elevations of an optical circulator incorporating the beam splitting devices of FIG. 4, and in particular presents: 
     (a) a side view showing how light is transmitted from a first optical fibre to a second optical fibre; 
     (b) a side view showing how light is transmitted from the second optical fibre to a third optical fibre; and 
     (c) the elevation, as seen from A in FIGS. 5 a  and  5   b , showing how light propagates around the circulator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to reduce the dimensions of an optical system it is required to have a beam splitting device that provides ordinary and extraordinary output beam components from an input beam, that propagate symmetrically about an axis as defined by the input beam. FIG. 4 presents side elevations of two embodiments of such a beam splitting device that achieves this desired effect. 
     In both Figures light is shown propagating along the y-axis through a birefringent material in the form of crystal structures. The desired symmetry of the ordinary and extraordinary component beams is achieved in the embodiment shown in FIG.  4 ( a ) by cutting the input and output faces of the beam splitting device  203 . The critical factors resulting from the cut are the angles A and B. The angle A lies in the z-y plane and refers to the angle of the front face of the crystal relative to the propagation axis L, as defined by the input beam  240 . The angle B lies in the x-y plane and refers to the angle between the optic axis and the propagation axis L. 
     The angles A and B are defined by the inherent properties of the material from which the beam splitting device  203  is cut. Consideration of Snell&#39;s Law and the equation of the index ellipsoid provides the following expressions for the angles A and B, namely: 
     
       
         A=90°−θ i   (1) 
       
     
     
       
         
           
             
               
                 
                   B 
                   = 
                   
                     θ 
                     - 
                     
                       α 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
                 
         
             
         
      
     
     which can be calculated with the aid of the following equations:                tan                 θ     =       n   e       n   o               (   3   )                 tan                 α     =         n   e   2     -     n   o   2         2        n   o          n   e                 (   4   )                                
     
       
         sin θ i =n o sin θ o   (5) 
       
     
     
       
         
           
             
               
                 
                   
                     θ 
                     o 
                   
                   = 
                   
                     
                       θ 
                       i 
                     
                     - 
                     
                       α 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
                 
         
             
         
      
     
     where 
     θ i —angle of incidence of input beam  240   
     θ o —angle of refraction of the ordinary component  240   a    
     α—angle between the ordinary and extraordinary component  240   a  and  240   b , respectively 
     θ—angle between the ordinary component  240   a  and the optic axis 
     n e —refractive index experienced by the extraordinary component  240   b    
     n o —refractive index experienced by the ordinary component  240   b    
     In the present embodiment the crystal material is formed from Yttrium Vanadate (YVO 4 ) that exhibits n o =1.945 and n e =2.149. Therefore, substitution of these values in equations (1)-(6) provides that the required angles A and B take the values 84.1° and 45.0°, respectively. 
     With this design of the beam splitting device  203  the input beam  240  propagates along the axis L. The input beam  240  is therefore not perpendicularly incident on the beam splitting device  203  and so the ordinary beam  240   a  is refracted to a plane below the axis L. Also as a result of the non-perpendicular angle of incidence of the input beam  240  the extraordinary beam  240   b  does not experience the same level of refraction relative to the axis L as was the case for the extraordinary beam  140   b  in the prior art teachings of FIG.  1 . 
     It should be noted that an alternative cut of the block of the beam splitting device  208  results in a mirror image for the ordinary  240   a  and extraordinary  240   b  beams being produced, relative to the case in FIG. 4 a . FIG. 4 b  shows the critical angles A′ and B′ required such that the ordinary  240   a  and extraordinary  240   b  beams emerge from the beam splitting device  208  on alternative sides of the propagation axis L from the case shown in FIG. 4 a.    
     In order to highlight the advantages of the beam splitting devices  203  and  208 , FIG. 5 presents a more compact and economical optical circulator  200  based on the teachings of Li et al. Alternative profiles of the optical circulator  200  are presented and show that it comprises of: three optical fibres  200   a    214  and  200   b , two collimators  220   a  and  220   b , three beam splitting devices  203 ,  208  and  211 , two compound polarisation rotators  230   a  and  230   b  and a light guiding device  250 . It should be noted at that the third beam splitting device  211  operates in a similar fashion to that outlined above for the first beam splitting device  203 . 
     To aid this description a set of orthogonal reference axes is arranged such that the y-axis is substantially parallel to the three fibres  200   a ,  200   b  and  214 . 
     All but the beam splitting devices  203 ,  208  and  211  are as described in the teachings of Li et al. The collimators  220   a  and  220   b  comprise a glass capillary  201  and  213 , employed to mount the optical fibres and a GRIN lens  202  and  212 , respectively. Each compound polarisation rotator  230   a  and  230   b  comprise two half wave plates ( 204   a ,  204   b    209   a  and  209   b  respectively) and a Faraday rotator ( 205  and  210  respectively). The light guiding device  250  comprises two tapered birefringent plates  206  and  207 . The tapering of plate  206  is complementary to the tapering of plate  207 , each having a tapered angle □. In this embodiment plate  206  has an optic axis OA 1  that is orientated parallel to the z-axis while plate  207  has an optic axis OA 2  that is orientated parallel to the x-axis. Both plates  206  and  207  are made from the same birefringent material exhibiting ordinary (n o ) and extraordinary (n e ) refractive indices such that n o &gt;n e.    
     It should be immediately apparent by comparing FIG.  3  and FIG. 5 that the optical circulator  200  has all of the aforementioned optical components substantially coaxial about a single axis L that lies parallel to the y-axis. However unlike the teachings of Li et al the second optical fibre  214  now also lies on the axis L, while the first  200   a  and third  200   b  fibres lie in a common plane with the this axis. In the teachings of Li et al this is not the case. It is an inherent property of that system that both the collimating elements  101  and  103  are displaced relative to each other along both the x-axis and the z-axis. The more compact design of the present optical circulator  200  is a direct result of the design of the beam splitting devices  203 ,  208  and  211 . 
     FIG. 5 a  presents a side profile of the optical circulator  200  presenting light propagating in the z-y plane from the first fibre  200   a  to the second fibre  214 . Initially the input light  240  propagates along the central axis L through the first lens  202  and into the first beam splitting device  203 . The beam splitting device  203  then resolves the input light  240  into two beams with perpendicular linear polarisation as described above, namely an ordinary beam  240   a  (parallel to the x-axis) and an extraordinary beam  240   b  (parallel to the z-axis). The beams  240   a  and  240   b  emerge from the beam splitting device  203  parallel and equidistant from the axis L. 
     The linearly polarised electric field components  240   a  and  240   b  then propagate through the first compound polarisation rotator  230   a  that acts to rotate the linearly polarised beam  240   b  so as to be parallel the x-axis. The linearly polarised beams  240   a  and  240   b  then propagate undeviated through the light guiding device  250  and the second beam splitting device  208  due to the orientation of their polarisation relative to the optic axes of these optical components. The second compound polarisation rotator  230   b  then acts to rotate the linearly beam  240   a  so as to be parallel the z-axis. At this stage both the linearly polarised beams  240   a  and  240   b  are orientated in the correct sense so as to be recombined by the third beam splitting device  211 . On propagating through the third beam splitting device  211  the light is recombined so as to propagate parallel to the axis L, thereafter being focused by the second lens  212  into the second fibre  214 . It is the design of the first and third beam splitting devices  203  and  211  that overcomes the problem of the spatial offset along the z-axis of the collimators. 
     The non-reciprocal nature of the device is illustrated in FIG. 5 b  and again lies in the inherent properties of the compound polarisation rotators  230   a  and  230   b . FIG. 5 b  presents a side profile in the z-y plane of the circulator  200  presenting light propagating from the second fibre  214  to the third fibre  200   b . As in the prior art, the non-reciprocal nature of the device can be observed by comparing the orientation of polarisation of the linearly polarised beams as they propagate through the optical circulator  200  with those in FIG. 3 a.    
     As with the teachings of Li et al, this inherent non-reciprocal nature of the optical circulator  200  is what is employed in order to guide light entering the device from the second fibre  214  so as to exit via the third  200   b . It is again the combination of the input angles, the light guiding device  250  and the first and second beam splitting devices  203  and  208  that appropriately orientates the propagation angle of the light in the x-y plane so as to arrive at the required fibres. 
     The design of the second beam splitting device  208  overcomes the problem of the spatial offset along the x-axis of the collimators. This is outlined in FIG. 5 c  where the x-y plane profile of light propagating from the first fibre  200   a  to the second  214 , along with that propagating from the second fibre  214  to the third  200   b  is presented. 
     As with the teachings of Li et al a light beam from the first fibre  240  exits the first lens  202  at an angle □ to the x-axis. On exiting the compound rotator  230   a  both the linearly beams,  240   a  and  240   b  propagate at an angle □ relative to the x-axis. The angle of propagation of each of these beams is then altered by the light guiding device  250  that exhibits extraordinary refractive index n e , and an ordinary refractive index n o , where n o &gt;n e . Tapered plate  206  has an optic axis OA 1  that is orientated parallel to the z-axis while tapered plate  207  has an optic axis OA 2  parallel to the x-axis. The result is that each beam exits the light guiding device  250  parallel to the y-axis. The beam splitting device  208  has an optic axis that is also orientated parallel to the z-axis, therefore the linear polarised beams  240   a  and  240   b  are ordinary rays relative to the beam splitting device  208 . However, due to the cut of the beam splitting device  208  both of these beams experience refraction such that they exit propagating along the axis L. The linear polarised beams  240   a  and  240   b  then propagate through the optical circulator  200  as previously describe exiting via the second fibre  214 . 
     A similar situation exists in the x-y plane when considering light propagating from the second fibre  214  to the third  200   b . The non-reciprocal nature of the compound polarisation rotators  230   b  is employed by the light guiding  250  device and the second beam splitting device  208  in order to translate light from the second fibre  214  to the third  200   b.    
     Before entering the beam splitting device  208  the two beams of the light beam are linearly polarised parallel to the z-axis and therefore act as extraordinary rays within the second beam splitting device  208 . This results in them being spatially translated along the x-axis before propagating through the light guiding device  250 . Translation through the light guiding device imposes an angle φ□ between the linear polarised beams of the light beam and the y-axis. The light then continues on through the optical components being recombined by the first beam splitting device  203  before being focused by the first lens  202  into the third fibre  200   b.    
     In a broad sense, the cutting of a block of birefringent material to predetermined characteristics provides a beam splitting device that symmetrically splits linearly polarised ordinary and extraordinary components about an axis as defined by the unpolarised input beam. Incorporating such beam splitting elements in an optical system provides a way of reducing the overall dimensions of the system since this allows all the optical components to share on one common longitudinal axis. These features also make optical systems easier to align. 
     It will be apparent to a person of average skill in the art that variations of the beam splitting devices are possible within the scope of the invention. Also the incorporation of beam splitting devices in an optical system is not restricted solely to the field of optical circulators. Accordingly, the following claims and their legal equivalents should determine the scope of the invention. 
     Teachings of Li et al 
       100  Optical Circulator 
       100   a— First Fibre 
       100   b— Third Fibre 
       101 —First Glass Capillary 
       102 —First Lens 
       103 —First Block of Birefiingent Material 
       130   a— First Compound Polarisation Rotator 
       104   a— Half wave Plate 
       104   b— Half Wave Plate 
       105 —Faraday Rotator 
       150 —Light Guiding Device 
       108 —Second Block of Birefringent Material 
       130   b— Second Compound Polarisation Rotator 
       109   a— Half wave Plate 
       109   b— HalfWave Plate 
       110 —Faraday Rotator 
       111 —Third Block of Birefringent Material 
       112 —Second Lens 
       113 —Second Glass Capillary 
       114 —Second Fibre 
       140 —First Light Field 
     According to the Present Invention 
       200  Optical Circulator 
       200   a— First Fibre 
       200   b— Third Fibre 
       220   a— First Collimator 
       201 —First Glass Capillary 
       202 —First Lens 
       203 —First Beam Splitting Device 
       230   a— First Compound Polarisation Rotator 
       204   a— Half wave Plate 
       204   b— Half Wave Plate 
       205 —Faraday Rotator 
       250 —Light Guiding Device 
       206 —First tapered Plate 
       207 —Second tapered Plate 
       208 —Second Beam Splitting Device 
       230   b— Second Compound Polarisation Rotator 
       209   a— Half wave Plate 
       209   b— Half Wave Plate 
       210 —Faraday Rotator 
       211 —Third Beam Splitting Device 
       220   b— Second Collimator 
       212 —Second Lens 
       213 —Second Glass Capillary 
       214 —Second Fibre 
       240 —First Light Field 
       240 A—Ordinary Component of First Light Field

Technology Classification (CPC): 6