Abstract:
A non-reciprocal optical device mapping a series of optical input/output signal waveguides to a corresponding series of optical input/output signal waveguides, the device comprising: a series of spaced apart input/output waveguides; a reflective imaging system for reflecting and focussing light emitted from the input/output waveguides; a plurality of crystal elements between the input/output waveguides and the reflective imaging means; at least one non-reciprocal polarization rotation element; wherein light emitted from a first input/output waveguide is transmitted to a second input/output waveguide in a polarization independent manner and light emitted from the second input/output waveguide is transmitted away from the first input/output waveguide.

Description:
This application is a continuation-in-part application Ser. No. 09/345,027 filed on Jul. 2, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of non-reciprocal optical devices such as optical circulators or the like. 
     BACKGROUND OF THE INVENTION 
     Optical circulator devices are well known in the art and normally comprise a series of bi-directional ports and a “non-reciprocal” mapping between ports. For example, in a three-port optical circulator device, the ports may be designated A, B and C and the non-reciprocal nature of the device is such that an input signal at Port A will be output at Port B, an input signal at Port B will be output at Port C and an input signal at Port C will be output at Port A. 
     It is desirable with any circulator type device to manufacture as compact and inexpensive a device as possible. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide for a compact form of optical circulator device having a high level of compactness and flexibility. 
     In accordance with a first aspect of the present invention, there is provided a non-reciprocal optical device mapping a series of optical input/output signal waveguides to a corresponding series of optical input/output signal waveguides, the device comprising: a series of spaced apart input/output waveguides; a reflective imaging system for reflecting and focussing light emitted from the input/output waveguides; a plurality of crystal elements between the input/output waveguides and the reflective imaging means; at least one non-reciprocal polarization rotation element; wherein light emitted from a first input/output waveguide is transmitted to a second input/output waveguide in a polarization independent manner and light emitted from the second input/output waveguide is transmitted away from the first input/output waveguide. 
     Light emitted from the second input/output waveguide can be transmitted to a third input/output waveguide and light emitted from the third input/output waveguide can be transmitted to the first input/output waveguide so as to provide for a fully circulating circulator. 
     The input/output signal waveguides can comprise optical fibres and mode expansion can be provided by utilizing thermally expanded core fibre ends, gradient index fibres, or a separate lensing system, or a combination of thereof. 
     In accordance with a further aspect of the present invention, there is provided a non-reciprocal optical device mapping a series of optical input/output signal waveguides to a series of optical input/output waveguides, the device comprising: a series of spaced apart input/output signal waveguides; a first polarization separation means for spatially separating the optical input signals emitted from the optical input/output signal waveguides into orthogonal polarization components; a first series of reciprocal polarization transformation elements for aligning the polarizations thereby producing aligned polarization components; a non-reciprocal rotator for applying a non-reciprocal rotation to the aligned polarization components; a second polarization separation means for spatially displacing aligned polarization components; at least one reciprocal polarization transformation element for rotating the aligned polarization components emitted from a subset of the input/output signal waveguides; imaging means for imaging the aligned polarization components to produce imaged polarization components; and reflection means for reflecting the polarization components wherein light emitted from a first input/output waveguide is transmitted to a second input/output waveguide in a polarization independent manner and light emitted from the second input/output waveguide is transmitted away from the first input/output waveguide. 
     Again, light emitted from the second input/output waveguide can be transmitted to a third input/output waveguide and light emitted from the third input/output waveguide can be transmitted to the first input/output waveguide so as to provide for a fully circulating circulator. 
     In accordance with a further aspect of the present invention, there is provided a non-reciprocal optical device comprising: at least two spaced apart rows each containing a series of input/output waveguides; a first polarization dependant displacement means spatially displacing orthogonal polarizations of light emitted from the waveguides; a first series of reciprocal polarization transformation elements aligning the orthogonal polarizations emitted from the first polarization displacement means; a non reciprocal-rotator rotating the aligned polarization states in a non reciprocal manner; a second polarization dependant displacement means displacing light emitted from the reciprocal polarization transformation element in a polarization dependant manner; focusing means for focusing light emitted from the waveguides substantially on the waveguides; reflection means reflecting light emitted from a first of the rows back in the direction of a second of the rows; wherein light emitted from a first one of the waveguides in a first row is transmitted to a first one of the waveguides in a second row in a non reciprocal manner. 
     The light emitted from the first one of the waveguides in the second row is preferably transmitted to a second one of the waveguides in the first row. 
     In one embodiment, the number of waveguides in each row can be four and light emitted from any one of the waveguides in a first row can be transmitted to a predetermined waveguide in the second row. The first polarization means preferably translates one orthogonal polarization state substantially perpendicular to the rows. 
     The first series of reciprocal polarization transformation elements can comprise a series of abutted reciprocal rotators which rotate the displaced orthogonal polarizations in an opposite direction. The focusing means can be adjacent the reflection means. The second polarization displacement means can displace one of the polarizations parallel to the rows. 
     In accordance with a further aspect of the present invention, there is provided a method of mapping a first series of optical input/output signal waveguides to a second series of optical input/output waveguides in a non-reciprocal manner, the method comprising the steps of: (a) emitting an optical signal from one of the waveguides; (b) spatially separating substantially orthogonal polarisation states of the emitted light using a first optical element; (c) aligning the substantially orthogonal polarization states using a second optical element; (d) projecting the aligned orthogonal polarization states through a first series of optical elements; and (e) reflecting the light emitted from the step (d) back through the first series of optical elements, the second optical element and the first optical element; wherein light emitted from the first one of the waveguides is transmitted to a second one of the waveguides and light emitted from a second one of the waveguides is transmitted to a third one of the waveguides. 
     The light is ideally transmitted from one waveguide to a second in a polarization independant manner and light emitted from the third one of the waveguides can be transmitted to the first waveguide. 
     Many different uses of the circulator are possible. For example, an add/drop multiplexer or other optical transmission system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Notwithstanding other forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 illustrates schematically in perspective the arrangement of the first embodiment; 
     FIG. 2 illustrates a first polarization transition state diagram of light travelling from fibre A to fibre B; 
     FIG. 3 illustrates a second polarization transition state diagram of light travelling from fibre B to fibre C; 
     FIG. 4 illustrates a third polarization transition state diagram of light travelling from fibre C to fibre A. 
     FIG. 5 illustrates schematically in perspective the arrangement of an alternative embodiment; 
     FIG. 6 to FIG. 9 illustrate polarization transition diagrams for the arrangement of FIG. 5; 
     FIG.  10  and FIG. 11 illustrate modified forms of the present invention; 
     FIG. 12 illustrates schematically in perspective the arrangement of a further alternative embodiment; 
     FIG. 13 illustrates a first polarization transition state diagram of light travelling from fibre A to fibre B for the arrangement of FIG. 12; 
     FIG. 14 illustrates a second polarization transition state diagram of light travelling from fibre B to fibre C for the arrangement of FIG. 12; 
     FIG. 15 illustrates a third polarization transition state diagram of light travelling from fibre C to fibre A for the arrangement of FIG. 12; 
     FIG. 16 illustrates a first use of the embodiments of the invention; and 
     FIG. 17 illustrates a further use of the embodiments of the invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the first embodiment, an optical circulator type device is provided which utilises a series of optical components followed by a mirror which reflects input signals back to a series of outputs so as to provide for a fully circulating three-port circulator. 
     Turning initially to FIG. 1, there is illustrated schematically the arrangement  1  of the first embodiment. The first embodiment includes an initial horizontal array  2  of three single mode fibres (SMF) labelled A, B and C which are approximately 125 microns in diameter and can include a short length of gradient index fiber attached to an end or other forms of mode expansion as discussed hereinafter. The three fibres  2  can be arranged in corresponding V-grooves with approximately 127 microns spacing. The gradient indexed fibre is such that it projects an enlarged image of the SMF fibre mode at distances of approximately 500 microns in air and with a diameter of 30 microns. Such devices are commercially available from Highwave Optical Technologies. 
     The device proper consists of the following components: 
     1. A first walkoff crystal  3  is provided and can comprise a rutile crystal with a width of approximately 1 mm. The walkoff crystal  3  is positioned adjacent to the gradient indexed fibre and is of sufficient dimension to separate the polarisation states projected from the fibres. 
     2. Next, a series of reciprocal rotators  13 ,  14 ,  15 ,  16 ,  17 ,  18  are provided to achieve polarisation state equalisation of the fibres. The middle rotators  15 ,  16  are aligned in an orthogonal manner to the other rotators. 
     3. Next, a Faraday rotator  7  is provided to rotate all polarisation states by 45°. 
     4. Next, a rutile plate  8  is provided to allow 127 microns displacement in the direction  9 . 
     5. Next, a half-wave plate  10  is provided to intercept only the light from the fibre C on the outgoing path. 
     6. Next, a lens  11  can be a gradient indexed lens of pitch such as approximate formation of the fibre images is achieved. 
     7. Finally, a reflective mirror surface  12  is positioned at a distance behind the lens  11  such that parallel rays are returned substantially parallel. 
     The arrangement of FIG. 1 is useful in providing for a fully circulating circulator in that input light from the fibre port A is output at fibre port B, input light from the fibre port B is output at fibre port C and input from fibre port C is output at the fibre port A. 
     Turning now to FIG. 2, there is illustrated an initial polarization transition diagram for light travelling from port A to port B. The two orthogonal polarizations states are initially spatially coincidental and are indicated by the initial cross  20 . Subsequently, after traversing walkoff plate  3 , the polarizations states are separated  21 . The reciprocal rotators  13 ,  14  result in a reciprocal rotation of each of the polarisation states so as to cause the output polarization state  22  to be at 45°. 
     The non-reciprocal rotator  7  provides for a non-reciprocal rotation of the polarisation states which-results in the aligned polarization state  23 . The rutile  8  results in a translation  24  of the two polarisation states. The halfwave plate  10  is positioned so it will have no effect on the polarisation states in this case. The effect of the lensing system  11  and mirror  12  is illustrated by the polarisation state diagram  26  and results in an inversion of the spatial states upon reflection from the mirror. The halfwave plate  10  is positioned not to have an effect on the polarisation state  26  and therefore results in polarisation state  27 . The walkoff plate  8  translates the polarisation states  28 . The Faraday rotator  7  is a non-reciprocal device and results in a non-reciprocal rotation of the polarisation states resulting in aligned polarisation states  29 . The reciprocal rotators  15 ,  16  are positioned to rotate the polarisation states by 45° in a reciprocal manner resulting in the polarisation state  30 . The walkoff plate  3  then combines the polarisation states so as to produce output  31  having spatially combined polarisation states. Hence, the overall result is the transmission of output of fibre A to the fibre port B. 
     Turning now to FIG. 3, there is illustrated the polarisation state transition diagram for light emitted from the fibre port B to the fibre port C. In the initial polarisation state  40 , both orthogonal polarisation states are spatially coincident. The walkoff plates  3  results in a translation of one polarisation state with respect to the other  41 . The reciprocal rotators  15 ,  16  results in the alignment of the two polarisation states  42 . The rutile  8  has no effect  44 , nor does the halfwave plate  10  which results in the polarisation state  45 . The lens and mirror  11 ,  12  again result in an inversion of the polarisation states so as to produce the state  46 . The halfwave plate  10  has no effect  47 . The rutile  8  again has no effect producing polarisation state  48 . The non-reciprocal Faraday rotator  7  rotates the polarisation states  49  as does the reciprocal rotator  17 , 18  which produces the polarisation state  50 . The walkoff plate  3  combines the polarisation states so as to produce output  51  having spatially combined polarisation states. 
     Turning now to FIG. 4, there is illustrated the polarisation state transition diagram for a fibre port C to fibre port A. Starting initially with the polarisation state  60 , the walkoff plate  3  results in a splitting of the polarisation states  61 . Subsequently, the reciprocal rotators  17 , 18  results in a reciprocal rotation of the polarisation states to bring them into alignment. The Faraday rotator  7  in turn rotates the polarisation state to produce polarisation of state  63 . The walkoff plate  8  translates the polarisation state producing the resultant polarisation state  64 . The halfwave plate  10  in turn results in a rotation of the polarisation state producing polarisation state  65 . Subsequently, after passing through lens  11  and being reflected by mirror  12 , the polarisation state  67  is produced. The halfwave plate  10  has no effect  68  nor does the walkoff plate  8  which results in the polarisation of state  69 . The Faraday rotator  7  rotates the polarisation states  70  and the reciprocal rotator further rotates the polarisation states to produce the polarisation state  71  which is combined by walkoff plate  3  to produce output  72  which is output to fibre A. 
     It can therefore be seen from the polarisation transition diagrams of FIG. 2 to FIG. 4 that the first embodiment operates as a fully circulating circulator. 
     Turning now initially to FIG. 5, there is illustrated an alternative embodiment modified so as to provide for multiple input/output ports. In the embodiment of FIG. 5, eight fibres form input/output port  80  as shown, divided into two rows  86 ,  87  each indicated with a corresponding letter A to H. The port  80  can be arranged in two rows and include expanded core fibre ends, or alternatively, gradient index fibre ends of about 125 micron diameter and arranged to the V groove of approximately 127 micron spacing and 200 micron spacing between the two rows of fibres  86 ,  87 . 
     A first rutile crystal  81  is provided with the width of approximately 1 mm to separate the polarisation states projected from the fibres  80 . The walkoff is preferably in the direction  82 . 
     A series of reciprocal rotators or halfwave plates  83 - 85  are positioned to achieve polarisation state equalisation of the light proceeding from the two spaced apart rows  86 ,  87 . The rotator  84  has a 45 degree rotation in the opposite direction of rotation relative to the rotators  83 ,  85 . 
     A non-reciprocal Faraday rotator  89  provides non reciprocal 45° rotation of all polarisation states. 
     Next, a Rutile crystal walkoff plate  90  is provided so as to allow for 63.5 micron displacement of the relevant polarisation state in the direction  91 . Next, a gradient index lens  93  is provided such that proximate collimation of all the fibre images is achieved. This is followed by a reflective mirror surface  94  positioned at a distance behind the lens  93  such that parallel rays are returned substantially parallel. Turning to FIG. 6, there is illustrated a first example polarisation state diagram for the transition from the fibre A to the fibre B. 
     Starting initially with the polarisation state  100  emitted from the fibre A, after the transition through the walkoff plate  81 , the polarisation state is as illustrated  101 . The reciprocal rotators  83 - 85  results in an alignment of the polarisation states  102 . Next, non-reciprocal rotation by the non-reciprocal rotator  89  results in the polarisation states  103 . The rutile crystal  90 , as a result of having its walkoff direction  91  has no effect on the polarisation state  104 . The lens  93  and mirror  94  result in an inversion of the polarisation state  105 . Again the polarization state is unaffected  106  by the walkoff plate  90 . The non-reciprocal rotator  89  provides a rotation  107  in the polarisation state. The reciprocal-rotators  83 - 85  result in polarizations state  108 . This is followed by translation by the walkoff plate  81  which results in an alignment of the polarisation state  109  which is output to the fibre B. 
     Turning now to FIG. 7, there is illustrated the corresponding series of polarisation states in going from fibre B to fibre C. Starting from the initial polarisation state  110 , the walkoff plate  81  separates the polarisation states  111 . The series of reciprocal rotators  83 - 85  result in an alignment  112  to the polarisation state. This is followed by the non-reciprocal rotator  89  rotating the polarisation states  113 . The walkoff plate  90  results in a translation in the direction  91  so as to produce polarisation state  114  which is translated to polarisation state  115  by the lens  93  and mirror  94 . The walkoff plate  90  in turn results in the polarisation state  116  which is rotated to  117  in a non-reciprocal manner by the Faraday rotator  89 . The reciprocal rotators  83 - 85  result in the polarisation state  118  which in turn results in an alignment  119  of the polarisation state which is output to the fibre C. 
     For completeness, FIG. 8 illustrates the corresponding polarisation transition diagrams for the transmission of light from fibre C to fibre D. FIG. 9 further illustrates the polarisation transition diagram for light going from the fibre D to the fibre E. 
     Whilst it will be evident to those skilled in the art that a number of modifications to the disclosed embodiments can be made whilst still utilising the core of the present invention, a number of such modifications will now be discussed. Firstly, the mode expansion can be implemented utilising different techniques. For example, as shown in FIG. 10, graded index fibre imaging can be utilised. In this modification to the arrangement of FIG. 1, the single mode fibre e.g.  120  includes a gradient index (GRIN) fibre  121  which projects a mode enlarged image e.g.  123  part way along the circulator elements which are indicated generally  124 . The image  123  is further again imaged by lens  125  and mirror  126 . 
     Also, the lensing arrangements can be altered in accordance with requirements. For example, FIG. 11 illustrates a re-arrangement which includes two lenses  130 ,  131 . The image from the thermally expanded core fibre e.g.  132  is projected through circulator elements  133  before being imaged by lens  130  along an image plane  135 , which can be part way along further circulator elements. The lens  131  and mirror  137  also act to image and reflect the polarisation components as previously discussed. 
     From the foregoing discussion, it can be seen that the embodiments include a number of significant advantages. These include polarisation independence of operation and good isolation of return paths. Good separation of polarization states is provided by means of mode expansion or offsetting of the mode expanded image such that light is most strongly confined in the region where the polarisation states can to be acted upon separately. 
     The arrangement of FIG. 1, whilst being simple to construct, suffers from a slight disadvantage in that the focal planes for each coupling of light from an input port to an output port may be different due to the walkoff crystals providing different optical path lengths. Turning to FIG. 12, there is illustrated schematically, an alternative embodiment which is free from the disadvantage of the arrangement of FIG.  1 . 
     This preferred embodiment includes an initial array  141  of three single mode fibres (SMF) labelled A, B and C which are approximately 125 microns in diameter. Each fibre can include a short length of gradient index fiber attached to an end (not shown) or other forms of mode expansion as discussed earlier. The three fibres  2  can be arranged in corresponding V-grooves with approximately 127 microns spacing. The gradient indexed fibre is such that it projects an enlarged image of the SMF fibre mode at distances of approximately 500 microns in air and with a diameter of 30 microns. Such devices are commercially available from Highwave Optical Technologies. 
     The device proper consists of the following components: 
     1. A first walkoff crystal  142  is provided and can comprise a rutile crystal with a width of approximately 1 mm. The walkoff crystal  142  is positioned adjacent to the gradient indexed fibre and is of sufficient dimension to separate the polarisation states projected from the fibres. 
     2. Next, a series of reciprocal rotators  143 ,  146 ,  147  abutted with non-rotating glass portions  144 ,  145 ,  148  are provided to achieve polarisation state equalisation of the fibres. 
     3. Next, a Faraday rotator  150  is provided to rotate all polarisation states by 45°. 
     4. Next, a composite of two rutile plates  151 ,  152  is provided to allow displacement in a polarisation dependant manner in the directions indicated  154 ,  155 . 
     5. Next, a half-wave plate  156  is provided to intercept only the light from the fibre C on the outgoing path. 
     6. Next, a lens  157  can be a gradient indexed lens of pitch such as approximate formation of the fibre images is achieved. 
     7. Finally, a reflective mirror surface  158  is positioned at a distance behind the lens  157  such that parallel rays are returned substantially parallel. 
     The arrangement of FIG. 12 is again useful in providing for a fully circulating circulator in that input light from the fibre port A is output at fibre port B, input light from the fibre port B is output at fibre port C and input from fibre port C is output at the fibre port A. 
     Turning now to FIG. 13, there is illustrated an initial polarisation transition diagram for light travelling from port A to port B. The two orthogonal polarisation states are initially spatially coincidental and are indicated by the initial cross  160 . Subsequently, after traversing walkoff plate  142 , the polarisation states are separated  161 . The reciprocal rotator  143  results in a reciprocal rotation of one of the polarisation states so as to cause the output polarisation states to be aligned  162  in vertical direction. 
     The non-reciprocal rotator  150  provides for a non-reciprocal rotation of the polarisation states, which results in the polarisation states  162 . The rutile  151  of the composite has its walkoff axis set so it will have no effect  164  on the two polarisation states in this case, while the rutile  152  results in a translation  165  of the two polarisation states in direction −45° to the horizontal, a distance with a horizontal component of about half the fibre&#39;s separation. The halfwave plate  156  is positioned so it will have no effect on the polarisation states in this case  166 . 
     The effect of the lensing system  157  and mirror  158  is illustrated by the polarisation state diagram  167  and results in an inversion of the spatial states upon reflection from the mirror  158 . 
     The halfwave plate  156  is positioned not to have an effect on the polarisation states and therefore results in polarisation state  168 . The walkoff plate  152  reciprocally translates the polarisation states  169 , while the walkoff plate  151  again does not have any effect. The Faraday rotator  150  is a non-reciprocal device and results in a non-reciprocal rotation of the polarisation states resulting in aligned polarisation states  171 . The reciprocal rotator  146  is positioned to rotate one of the polarisation states by 90° in a reciprocal manner resulting in the polarisation states  172 . The walkoff plate  142  then combines the polarisation states so as to produce output  173  having spatially combined polarisation states. Hence, the overall result is the transmission of output of fibre A to the fibre port B. 
     Turning now to FIG. 14, there is illustrated the polarisation state transition diagram for light emitted from the fibre port B to the fibre port C. In the initial polarisation state  180 , both orthogonal polarisation states are spatially coincident. The walkoff plates  142  results in a translation  181  of one polarisation state with respect to the other. The reciprocal rotator  146  results in the alignment of the two polarisation states  182  in the horizontal direction. The faraday rotator  150  rotates the ploarization states  183 . The rutile  150  translates the polarisation states  184  in direction 45° to the horizontal about halfway through the fibres separation. The rutile  152  has no effect, nor does the halfwave plate  156  which results in the polarisation state  186 . The lens and mirror  157 ,  158  again result in an inversion of the polarisation states so as to produce the state  187 . The halfwave plate  156  has no effect  188 . The rutile  152  again has no effect, while the rutile  151  translates them in reciprocal manner producing polarisation states  190 . The non-reciprocal Faraday rotator  150  rotates the polarisation states  191 . The reciprocal rotator  147  rotates one of the polarization states, which produces the polarisation states  192 . The walkoff plate  142  combines the polarisation states so as to produce output  193  having spatially combined polarisation states. 
     Turning now to FIG. 15, there is illustrated the polarisation state transition diagram for a fibre port C to fibre port A. Starting initially with the polarisation state  180 , the walkoff plate  142  results in a splitting of the polarisation states  181 . Subsequently, the reciprocal rotator  147  results in a reciprocal rotation of one of the polarisation states to bring them into alignment in vertical direction  182 . The Faraday rotator  150  in turn rotates them to produce polarisation states  183 . The rutile  151  of the composite has no effect on the two polarisation states in this case, while the rutile  152  results in a translation  185  of the two polarisation states in direction—45° to the horizontal one about half of the fibres separation. The halfwave plate  156  in turn results in a rotation of the polarisation states producing polarisation states  186 . Subsequently, after passing through lens  157  and being reflected by mirror  158 , the polarisation states  187  are produced. The halfwave plate  156  has no effect  188  nor does the walkoff plate  152  which results in the polarisation states  189 . The walkoff plate  151  reciprocally translates the polarisation states  190  in direction 45° to the horizontal one. The Faraday rotator  150  rotates the polarisation states  191  and the reciprocal rotator  147  further rotates one of the polarisation states to produce the polarisation state  192 , which are combined by walkoff plate  142  to produce output  193 , which is output to fibre A. 
     It can therefore be seen from the polarisation transition diagrams of FIG. 13 to FIG. 15 that the preferred embodiment operates as a fully circulating circulator. 
     The embodiments have a number of uses in optical arrangements. For example, FIG. 16 shows an arrangement  200  including the use of a circulator  201  in conjunction with a bragg grating  202  to isolate a particular frequency. Input frequencies are transmitted by the ciculator from port  1  to  2 . The desired output frequency is reflected by the bragg grating  202  and transmitted back to port  2  wherein it is outputted to port  3 . 
     The arrangement of FIG. 16 can be extended to extracting multiple frequencies as shown in FIG. 17 wherein the elements  200  of FIG. 16 are cascaded together, each tuned to extract a particular frequency. In this manner, the circulator can be incorporated into a telecommunications system. It would be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.