Abstract:
A conventional interleaver, based on a stack of waveplates, relies on the orientation and the birefringence of the waveplates to differentiate the polarizations of one set of channels from another, so that the one set of channels can be separated from the other. The present invention relates to a virtual waveplate that is used to replace a birefringent waveplate. A virtual waveplate imposes a phase delay between the extraordinary ray and the ordinary ray by separating one from the other and differentiating the actual path lengths taken thereby, before recombining them. An interleaver constructed with the virtual waveplates of the present invention can be substantially a-thermal and potentially chromatic dispersion free.

Description:
RELATED APPLICATIONS 
     The present application claims priority from the provisionally filed U.S. patent application Ser. No. 60/222,288, filed Aug. 1, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates a device for imposing a retardance between orthogonally polarized components of an optical signal without the use of birefringent material, and in particular to a virtual waveplate that can be used in an optical channel interleaver. 
     BACKGROUND OF THE INVENTION 
     Optical multiplexers are used in the wavelength division multiplexing (WDM) of a plurality of optical channels for transmission via a single carrier fiber. At the receiving end of the fiber, the same general techniques are used to demultiplex the optical channels back into individual channels once again. Adding more channels to an optical signal increases the amount of data that can be sent down an optical network without the laying down of any additional fiber. The demand for additional capacity on current fiber networks keeps increasing, along with the demand for lower cost equipment to minimize initial capital costs, and less complicated equipment to reduce installation and maintenance costs. 
     One solution to the aforementioned problem includes the use of interleaver technology, and in particular to the birefringent waveplate-based interleaver technology disclosed in U.S. Pat. No. 4,566,761 issued Jan. 28, 1986; and U.S. Pat. No. 4,685,773 issued Aug. 11, 1987 both to Carlsen et al. Birefringent materials are relatively costly and temperature sensitive, moreover, there is a limit to the size that a single crystal can be grown, which could eventually restrict the capacity of the system. Other interleaver technologies are based on the Michelson interferometer, and include a Gires Tournois etalon in one arm (EP 933,657, filed Jan. 8, 1999 in the name of Dingel et al) or in both arms (U.S. Pat. No. 6,169,626 issued Jan. 2, 2001 in the name of Chen et al). Michelson Gires Tournois (MGT) interleavers are complicated to manufacture due to the high degree of accuracy required to match both of the arms thereof and to tune one or more cavities, which are temperature sensitive. 
     An object of the present invention is to overcome the shortcomings of the prior art by providing a less costly virtual waveplate that uses passive elements, which can be used in an interleaver device. Another object of the present invention is to increase the temperature stability of the waveplate assembly and to enable reflection losses to be minimized. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention relates to a device for introducing a relative delay between orthogonally polarized components of an input signal comprising: 
     an input port for launching the input signal; 
     beam splitting means for dividing the input signal into first and second orthogonally polarized sub-beams, and for directing the first sub-beam along a first path and the second sub-beam along a second path; 
     first reflecting means in the first path for redirecting the first sub-beam back towards the beam splitting means; 
     delay means in the first path for imposing a relative delay between the first and second sub-beams; 
     second reflecting means in the second path for redirecting the second sub-beam back towards the beam splitting means for recombination with the first sub-beam; and 
     an output port for outputting the recombined first and second sub-beams. 
     Another aspect of the present invention relates to an optical filter comprising: 
     an first port for launching an input optical signal, which comprises a first and a second set of channels; 
     a second port for outputting the first set of channels; 
     a third port for outputting the second set of channels; 
     a first virtual waveplate; and 
     polarized beam separating means for directing the first set of channels to the second port, and for directing the second set of channels to the third port. The virtual waveplate comprises: 
     first beam splitting means for dividing the input signal into first and second orthogonally polarized sub-beams, and for directing the first sub-beam along a first path and the second sub-beam along a second path; 
     first reflecting means in the first path for redirecting the first sub-beam back towards the first beam splitting means; 
     second reflecting means in the second path for redirecting the second sub-beam back towards the first beam splitting means for recombination with the first sub-beam forming a first recombined signal; and 
     first delay means in the first path for imposing a first relative delay between the first and second sub-beams, whereby when the first and second sub-beams recombine, the first set of channels is orthogonally polarized relative to the second set of channels. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     This present invention will be more fully described with reference to the accompanying drawings, which illustrate preferred embodiments of the invention, wherein: 
     FIG. 1 is a schematic representation of a conventional birefringent waveplate-based interleaver; 
     FIG. 2 is a schematic representation of a virtual waveplate according to the present invention; 
     FIG. 3 is a schematic representation of the dual-pass interleaver including the virtual waveplate of FIG. 2 illustrating the beam paths; 
     FIG. 4 is a schematic representation of another embodiment of an interleaver, which includes the virtual waveplate according to FIG. 2; and 
     FIG. 5 is a schematic representation of a dual-input interleaver, which includes the virtual waveplate according to FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1, a conventional birefringent waveplate interleaver includes a first birefringent element  1  of length L and oriented with the optical axis at 45° with respect to the polarization of the input beam (vertical), and a second birefringent element  2  of length  2 L and oriented with the optical axis at 105° with respect to the polarization of the input beam (vertical). The use of birefringent elements of length L and  2 L is the optical version of adding the first harmonic frequency to the fundamental frequency to build a filter with a more square-shaped channel response. Of course, if an infinite number of the harmonics were added the result would be an ideal square shape. When in use as a de-interleaver, a beam of light  3  with mixed polarization and a series of channels is launched through collimating lens  4  into a polarization beam splitter  6 , e.g. a rutile crystal. The polarization beam splitter  6  divides the beam of light  3  into two orthogonally polarized sub-beams  7 , only one of which can be seen in FIG. 1. A half wave plate  8  is positioned in the path of one of the sub-beams, so that both sub-beams  7  will enter the first birefringent element  1  with the same polarization (e.g. vertical as shown in FIG.  1 ). After passing through the first and second birefringent elements  1  and  2 , the state of polarization of the even channels in the sub-beams  7  effectively remains the same, while the state of polarization of the odd channels is rotated by 90°. Accordingly, when the sub-beams  7  pass through a middle section  9  of a stacked polarization beam splitter  11 , the odd channels sub-beams  12  (only one shown) are reflected towards the bottom section  13 , while the even channel sub-beams  14  (only one shown) are passed through middle section  9  to quarter wave plate mirror  16 . The quarter wave plate mirror  16  rotates the polarization of the even channels  14  (e.g. vertical to horizontal) and reflects them back towards the middle section  9 , whereby they are reflected to the top section  17  of the stacked polarization beam splitter  11 . The even channels are reflected in the top section  17 , and pass through the second and first birefringent elements  2  and  1  for a second time. Again, the even channels  14  exit the birefringent elements  1  and  2  with the same polarization as when they entered. A half wave plate  18  and a beam combiner  19 , e.g. a rutile crystal, are used to combine the two even channel sub-beams  14  for output via focusing lens  21 . Similarly, the odd channel sub-beams  12  get reflected in the bottom section  13  towards the quarter wave plate mirror  16 , which rotates the polarization of the odd channel sub-beams  12  (e.g. from horizontal to vertical), whereby they pass through the bottom section  13  and back through the second and first birefringent elements  2  and  1  for a second time. Again, the odd channel sub-beams  12  exit the birefringent elements  1  and  2  with polarizations orthogonal to each other when they entered, e.g. vertical to horizontal. A half wave plate  22  and a beam combiner  23 , e.g. a rutile crystal, are used to combine the odd channel sub-beams  12  for output via lens  24 . 
     With reference to FIG. 2, the virtual waveplate according to the present invention includes a half waveplate  31  oriented at an angle of 22.5° relative to an input beam  32  of polarized light providing a rotation of 45° thereto. The input beam  32  can be either horizontally or vertically polarized; however in the illustrated example the input beam  32  is vertically polarized. Moreover, the waveplate  31  can be eliminated if the light is input in the appropriate state of polarization. A polarization beam splitter (PBS)  33  splits the input beam  32  into an s-polarized (horizontal) sub-beam  34 , which passes straight through the PBS  33 , and a p-polarized (vertical) sub-beam  36 , which is reflected in the PBS  33 . The PBS is preferably comprised of two triangular prisms with a polarization sensitive coating therebetween. The s-polarized (horizontal) sub-beam  34  traverses a gap  37  of length d 1  until being intercepted by a quarter wave plate  38  with a reflective rear surface  39 . Two passes through the quarter wave plate  38  result in a 90° rotation of the polarization of the sub-beam  34 , e.g. from horizontal to vertical, whereby when the sub-beam  34  re-enters the PBS  33  it gets reflected. Similarly, p-polarized sub-beam  36  traverses a gap  41  of length d 2  until being intercepted by a quarter wave plate  42  with a rear reflective surface  43 . Accordingly, the p-polarized sub-beam  36  is reflected back towards the PBS  33  after a cumulative rotation of 90°, so that the PBS  33  will pass the sub-beam  36  therethrough for interference with the reflected sub-beam  34  forming combined beam  44 . Therefore, the s-polarized (vertical) sub-beam and the p-polarized (horizontal) sub-beam have retardance between them defined by the difference between d 1  and d 2 , thereby providing the virtual equivalent of a waveplate. In practice d 1  or d 2  can be equal to zero, whereby the difference would be d 1  or d 2 . 
     Since this device provides different paths for the o and e waves, surfaces in the path of the o waves can be coated with a different antireflective coating than the surfaces in the path of the e waves. Therefore, the effectiveness of the anti-reflective coatings can be maximized in each path, rather than having to compromise the effectiveness of the coating when both types of waves travel both paths. Moreover, this system is athermal, since the path difference is provided in air, not in some other material substance more susceptible to thermal expansion. 
     With reference to FIG. 3, two of the aforementioned virtual wave plates  48  and  49  of FIG. 2 are arranged in an interleaver device as substitutes for the birefringent elements  1  and  2  from FIG.  1 . This configuration yields a flattop interleaver because the path lengths d 1  and d 2  of this system are set to give the fundamental and first harmonic from lengths L and  2 L , which are selected to provide the desired channel spacing. It is well known that the summation of a fundamental and the odd harmonics of the fundamental, yields a square wave response, and that a summation of the fundamental and first harmonic gives a good approximation of a square wave response, i.e. a flattop filter. 
     When the illustrated interleaver device is used for de-interleaving optical channels, a beam of light  50  of mixed polarization comprising a plurality of channels is launched through input port  51 . Port  51  comprises a lens  52 , preferably a graded index (GRIN) lens, a beam splitter  53 , preferably a rutile crystal, and a polarization rotator  54 , preferably a half wave plate. Please note: in drawings  3 , 4  and  5  polarization rotators  54  are shown in side view for clarity, while the remainder of the figure is in plan view. The beam splitter  53  separates the input beam  50  into two orthogonally polarized sub-beams  56  (only one of which can be seen in the Figure), and the polarization rotator  54  rotates the polarization of one of the sub-beams  56  so that both of the sub-beams  56  have the same polarization. As described above, the input sub-beams  56  are passed through a half-wave plate  57  oriented at an angle of 22.5° thereto, resulting in the state of polarization of the sub-beams  56  being rotated by 45°. A PBS  58  splits the sub-beams  56  into horizontally polarized sub-beams  59 , which get passed therethrough, and vertically polarized sub-beams  60 , which get reflected thereby. The horizontally polarized sub-beams  59  are directed through a quarter wave plate  62 , and get reflected back by a reflective surface  63 . The double pass through the quarter wave plate  62  results in the horizontally polarized sub-beams becoming vertically polarized, whereby they get reflected by the PBS  58  rather than passed therethrough. The quarter wave plate  62  is fixed to the side of the PBS  58  making the length d 1 =0. The vertically polarized sub-beams  60  pass through a quarter wave plate  64  after traversing a gap  66  of length d 2 =L defined by spacers  67 . A shear plate  68  is positioned in the gap  66  to enable minor adjustments to the optical path length. The sub-beams  60  get directed back through the PBS  58  by a reflective coating  69  on the quarter wave plate  64  after the state of polarization has been rotated by 90°. Accordingly, the two sets of sub-beams  59  and  60  recombine into sub-beams  71  and are directed to the second virtual wave plate  49 . 
     The recombined sub-beams  71  pass through a half wave plate  73 , which is oriented at an angle of 52.5°, before entering a PBS  74 . The PBS  74  again splits the sub-beams  71  into orthogonally polarized sets of sub-beams  76  and  77 . Sub-beams  76  pass through quarter wave plate  78  and are directed back by reflective surface  79  after a 90° rotation. Sub-beams  77  traverse a gap  80  of length  2 L , defined by spacers  81 , and pass through quarter wave plate  82 . A reflective surface  83  on the quarter wave plate  82  directs the sub-beams  77  back through the PBS  74  for recombination with sub-beams  76 . A sheer plate  84  positioned in the gap  80  enables minor adjustments to be made to the optical path length of the sub-beams  77 . A half wave plate  96 , oriented at an angle of 3.5°, is positioned between the PBS  87  and the second virtual waveplate  49  for making a minor adjustment to the state of polarization of the sub-beams passing therethrough. 
     Due to the appropriate phase delays caused by the gaps  66  and  80 , and the orientations of the waveplates  57 ,  73  and  96 , the channels in the recombined sub-beams  85  have states of polarization that alternate between vertical and horizontal. Accordingly, by passing the recombined beam  85  through the middle section  86  of a stacked PBS  87 , the even number channels  88  can be separated from the odd number channels  89 . 
     The odd channels  89 , which have become horizontally polarized, pass through the middle section  86  of the PBS  87 . A quarter wave plate  91 , with a reflective coating  92 , is positioned in the path of the odd channels  89  for redirecting them back through the middle section  86  of the PBS  87  after a 90° rotation of their state of polarization. The odd channels  89 , which are now vertically polarized, are reflected by the middle section  86  towards a lower section  93  of PBS  87 , which in turn reflects the odd channels  89  back through the half wave plate  96  into the second virtual waveplate  49 . 
     The even channels  88 , the polarization of which has not been effectively changed, are initially vertically polarized, and get reflected by the middle section  86  of the PBS  87  towards an upper section  94 . The upper section  94  reflects the even channels  88  towards the quarter wave plate mirror  91 , which reflects the even channels back through the upper section  94  after a 90° rotation. The even channels  88 , now horizontally polarized, pass straight through the upper section  94  and back through the waveplate  96  into the second virtual waveplate  49 . 
     In the preferred embodiment illustrated in FIG. 3, the odd channels  89  (represented by the three headed arrows) and the even channels  88  (represented by the two headed arrows) make a second pass through the second and first virtual waveplates  49  and  48  along paths separate from each other and from the path taken during the first pass. By rotating the state of polarization of the odd channels  89  and the even channels  88  between passes, the system becomes chromatic dispersion free. The odd channels  89  travel from the second virtual waveplate  49  to the first virtual waveplate  48 , and undergo a 90° change in their state of polarization, whereby both of the odd channel sub-beams  89  exit the first virtual waveplate  48  horizontally polarized. A half wave plate  97  is positioned in the path of one of the odd channel sub-beams  89  for rotating the state of polarization thereof by 90°. The, now, orthogonally polarized odd channel sub-beams exit a first output port, during which they are combined in a beam combining walk-off crystal  98 , and focused by lens  99 . Similarly, the even number channels  88  are routed from the second virtual waveplate to the first virtual waveplate; however, as before, this does not result in a change in their state of polarization. In a second output port, a half-wave plate  101  rotates the polarization of one of the even channel sub-beams  88 , so that a beam-combining walk-off crystal  102  can combine the pair of sub-beams  88  for output via focusing lens  103 . 
     When used for interleaving channels the opposite occurs, whereby odd channels input through lens  99  are mixed with even channels input via lens  103  for output through port  51 . 
     The structure of the single pass embodiment illustrated in FIG. 4 is identical to the aforementioned double pass embodiment illustrated in FIG. 3, except that a pair of PBS  104  and  105  replaces the stacked PBS  87 . As above, a signal with mixed polarization is launched via lens  52 , and divided into orthogonally polarized sub-beams by beam separating walk-off crystal  53 . The polarization of one of the sub-beams is rotated by 90° in half-wave plate  54 , and the two similarly polarized sub-beams are directed through the first and second virtual waveplates  48  and  49 . A signal having odd channels orthogonally polarized to the even channels exits the second virtual waveplate  49  through the waveplate  96  and penetrates the PBS  104 . The odd channels pass to the beam combiner  98  for output via lens  99 , after the polarization of one of the odd channel sub-beams is rotated by the half-wave plate  97 . The even channels are reflected by the PBS  104  towards the other PBS  105 , which directs the even channels through the half-wave plate  101  and beam combining walk-off crystal  102  for output via lens  103 . 
     The embodiment illustrated in FIG. 5 includes an additional input port  110 , which includes a collimating lens  111 , and beam separating walk-off crystal  112 , and a half-wave plate  113 . The half-wave plate  113  is positioned in the path of the separated sub-beams from the walk-off crystal  112  so that both sub-beams have the same polarization and so that the polarization of both sub-beams is orthogonal to the sub-beams entering from the first input port  51 . This arrangement enables the sub-beams entering the second port  110  to be reflected by a PBS  114  towards another PBS  115 . PBS  115  is positioned between the first input port  51  and the first virtual waveplate  48 , whereby the sub-beams from the first input port  51  are combined with the orthogonally polarized sub-beams from the second input port  110 . Since the channels from the first input port  51  are orthogonal to the channels from the second input port  110 , and since the polarization of the odd channels is rotated by 90° and the polarization of the even channels is not, this arrangement enables the odd channels input via the first input port  51  to be mixed with the even channels input via the second input port  110 . Of course, various other scenarios are possible dependant upon which channels are input via which input port. 
     The virtual waveplate assemblies, constructed in the above-described preferred embodiments, allow for ITU tuning; however, other assemblies with different waveplate arrangements are possible. To ensure that the assembly is not temperature sensitive, it is preferable to construct the spacers  67  and  81  from an ultra low expansion material, e.g. Zerodur™, and to make the PBS&#39;s balanced fused silica beam splitter cubes. 
     Furthermore, since the above-identified assembly splits the ordinary wave (o-wave) and the extraordinary wave (e-wave), different anti-reflective coatings can be used on the surfaces of the different materials in accordance with the characteristics of the appropriate sub-beam to maximize the through put of light intensity.