Abstract:
Interleavers are a useful tool in wavelength division multiplexing (WDM) to separate a signal with closely spaced channels into two signals, e.g. odd and even ITU channels, each with twice the channel spacing. Alternatively, two signals with a large channel spacing can be combined into a single beam with half the channel spacing. The invention relates to an optical interferometer using rhomb prisms as resonant cavities, which, when properly designed, provide the necessary phase shifts for interleaving or de-interleaving sets of optical wavelength channels. The present invention utilizes the differential phase shift between orthogonally polarized components induced by total internal reflection (TIR) off the surfaces of the rhomb prisms. Dispersion reducing techniques are also disclosed, including multiple rhomb interleavers and multi-pass rhomb interleavers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. patent application Ser. Nos. 60/302,900 filed Jul. 2, 2001, and 60/307,149 filed Jul. 24, 2001. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present application relates to optical interleavers, and in particular to a resonator based interleaver using a Fresnel rhomb.  
         BACKGROUND OF THE INVENTION  
         [0003]    It is well known to those skilled in the art of optical dense wavelength division multiplexing (DWDM) that an “interleaving” effect is an efficient and cost-effective means for multiplexing or de-multiplexing optical signals. The interleaving function combines or separates two optical beams, each of which is comprised of signals at a multitude of equally spaced carrier frequencies. With reference to FIG. 1, each of the two beams, beam  1  and beam  2 , is comprised of signals whose carrier frequencies are spaced Δν, which is at twice the spacing of the combined signal. The absolute carrier frequencies of these two beams are offset from one another by one-half the frequency spacing of the carrier frequencies in those two beams. This interleaving functionality is highly desirable when constructing optical communications systems employing DWDM technology. Previously, it has been shown that an interleaving effect can be created by the coherent sum and difference of light reflected from two Gires-Tournois Interferometer (GTI) etalon resonators whose phase responses are offset from each other by ½ of their free spectral range (FSR). Copner et al in U.S. Pat. No. 6,125,220 issued Sep. 26, 2000, describe a polarization-insensitive interleaver, which uses a non-polarizing beam-splitter coupled with offset cavities to generate this effect.  
           [0004]    In another known interleaver device, the required phase offset is produced by a birefringent crystal element inserted into the cavity of a single GTI etalon resonator to create two independent and properly offset optical paths within one resonator structure. U.S. Pat. No. 6,169,604 issued Jan. 2, 2000 to Cao describes an etalon-based interleaver, which derives a 180° phase difference using an intra-cavity phase biasing element. This device has the disadvantages of complexity of construction, fragility, and high manufacturing cost.  
           [0005]    An object of the present invention is to overcome the shortcomings of the prior art by providing a bulk resonator device that does not require complex manufacturing techniques and that does not rely on a birefringent crystal to induce a phase change.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, the present invention relates to a resonator device comprising:  
           [0007]    a first reflective surface on a first end face for receiving an input optical beam having S and P components, for reflecting a reflected portion of the input optical beam, and for passing a transmitted portion of the input optical beam;  
           [0008]    a second reflective surface for receiving the transmitted portion at an angle resulting in total internal reflection of the transmitted portion, which results in a phase shift between S and P components of the transmitted portion;  
           [0009]    a third reflective surface for receiving the transmitted portion from the second reflective surface at an angle resulting in total internal reflection of the transmitted portion, which results in a phase shift between the S and P components of the transmitted portion; and  
           [0010]    a fourth reflective surface for receiving the transmitted portion from the third reflective surface, and for reflecting a returning portion of the transmitted portion back via the third and second reflective surfaces to the first reflective surface.  
           [0011]    Another aspect of the present invention relates to an interferometer device comprising:  
           [0012]    a first port for launching an input optical beam comprising first and second sets of wavelength channels;  
           [0013]    first resonator means;  
           [0014]    a beam splitter for separating the first set of wavelength channels from the second set of wavelength channels;  
           [0015]    a second port for outputting the first set of wavelength channels; and  
           [0016]    a third port for outputting the second set of wavelength channels.  
           [0017]    The first resonator means includes:  
           [0018]    a first reflective surface for reflecting a first portion of the input optical beam, and for transmitting a second portion of the input optical beam;  
           [0019]    a second reflective surface for receiving the second portion of the input optical beam from the first reflective surface at an angle resulting in total internal reflection of the second portion of the input optical beam, which results in a phase shift between first and second components thereof;  
           [0020]    a third reflective surface for receiving the second portion of the input optical beam from the second reflective surface at an angle resulting in total internal reflection of the second portion of the input optical beam, which results in a phase shift between the first and second components thereof; and  
           [0021]    a fourth reflective surface for receiving the second portion of the input optical signal from the third reflective surface, and for reflecting substantially all of the second portion of the input optical beam back to the first reflective surface via the second and third reflective surfaces;  
           [0022]    whereby interference between the first and second components results in the first set of wavelength channels being out of phase with the second set of wavelength channels.  
           [0023]    Another aspect of the present invention relates to an interleaver device comprising:  
           [0024]    an input port for launching an input optical beam comprising odd and even sets of signals;  
           [0025]    beam splitting means for splitting the input optical beam into first and second sub-beams;  
           [0026]    first Fresnel rhomb resonator means;  
           [0027]    second Fresnel rhomb resonator means;  
           [0028]    a first output port for outputting the odd set of signals; and  
           [0029]    a second output port for outputting the even set of signals.  
           [0030]    The first Fresnel rhomb resonator means includes:  
           [0031]    a first reflective surface having a reflectivity R 1  on a first end thereof, which is optically coupled to the beam splitting means, for receiving the first sub-beam, and for passing a first transmitted portion thereof;  
           [0032]    a second reflective surface receiving the first transmitted portion at an angle resulting in total internal reflection thereof causing a phase shift between S and P components of the first transmitted portion;  
           [0033]    a third reflective surface receiving the first transmitted portion from the second reflective surface at an angle resulting in total internal reflection thereof causing a phase shift between the S and P components of the first transmitted portion; and  
           [0034]    a fourth reflective surface having a reflectivity R 2  on a second end thereof for reflecting substantially all of the first transmitted portion back towards the beam splitting means.  
           [0035]    The second Fresnel rhomb resonator means includes:  
           [0036]    a fifth reflective surface having a reflectivity R 3  on a first end thereof, which is optically coupled to the beam splitting means, for receiving the second sub-beam, and passing a second transmitted portion;  
           [0037]    a sixth reflective surface receiving the second transmitted portion at an angle resulting in total internal reflection thereof causing a phase shift between S and P components of the second transmitted portion;  
           [0038]    a seventh reflective surface receiving the second transmitted portion from the sixth reflective surface at an angle resulting in total internal reflection thereof causing a phase shift between the S and P components of the second transmitted portion; and  
           [0039]    an eighth reflective surface having a reflectivity R 4  on a second end thereof for reflecting substantially all of the second transmitted portion back towards the beam splitting means;  
           [0040]    whereby the first and second sub-beams interfere causing the odd set of signals to be out of phase with the even set of signals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:  
         [0042]    [0042]FIG. 1 graphically illustrates the channel spacing of the first and second signals, as well as the combined signal;  
         [0043]    [0043]FIG. 2 is a schematic illustration of the Rhomb resonator according to the present invention;  
         [0044]    [0044]FIG. 3 is a schematic illustration of one embodiment of the interleaver device according to the present invention;  
         [0045]    [0045]FIG. 4 is a schematic illustration of an input port of the interleaver device of FIG. 3;  
         [0046]    [0046]FIG. 5 is a schematic illustration of another embodiment of the interleaver device according, to the present invention;  
         [0047]    [0047]FIG. 6 is a schematic illustration of another embodiment of the interleaver device according to the present invention;  
         [0048]    [0048]FIG. 7 is a schematic illustration of a double rhomb interleaver according to another embodiment of the present invention;  
         [0049]    [0049]FIG. 8 is a schematic illustration of another embodiment of the double rhomb interleaver of FIG. 7;  
         [0050]    [0050]FIGS. 9 a ,  9   b , and  9   c  are the spectral, the group delay and the dispersion responses, respectively, for a single rhomb interleaver according to the present invention;  
         [0051]    [0051]FIGS. 10 a ,  10   b , and  10   c  are the spectral, the group delay and the dispersion responses, respectively, for a double rhomb interleaver according to the present invention;  
         [0052]    [0052]FIGS. 11 a  and  11   b  are the spectral and dispersion responses, respectively, for a double-passed single rhomb interleaver according to the present invention;  
         [0053]    [0053]FIGS. 12 a  and  12   b  illustrate to process steps in the manufacture of a multiple-rhomb interleaver according to the present invention; and  
         [0054]    [0054]FIG. 13 illustrates a triple rhomb interleaver according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0055]    The Fresnel Rhomb is a prismatic optical component that has long been used in applications where achromatic ¼-wave optical retardation is needed. Rather than relying on birefringence to create the differential phase shift between orthogonal polarization states, the Fresnel Rhomb takes advantage of the differential phase shift induced by total internal reflection (TIR). In the present invention, a novel resonator structure based on the Fresnel Rhomb is proposed having a desired FSR and differential phase response for S and P polarization states to create all optical interleaver. Unlike the GTI etalon resonator that requires an auxiliary birefringent crystal or other polarization-sensitive device to create the necessary offset in phase response for the two polarization states, a properly-designed Fresnel rhomb automatically creates a round-trip phase shift of 180° between S and P polarization states without resorting to such auxiliary elements. The superposition of equal parts S and P signals emerging from this novel device results in a wideband interleaving function. This simple architecture overcomes the disadvantages of the GTI etalon resonator by virtue of its monolithic construction, robust mechanical integrity, and low manufacturing cost. The novel contribution of this invention is to create a resonator that is substantially different from traditional etalon-based interleavers; the Fresnel rhomb was chosen specifically to introduce the required phase difference via total internal reflection. Additionally, the rhomb is a particularly manufacturable form of this resonator.  
         [0056]    Multi-cavity etalon interleaver products do not employ destructive interference, and therefore cannot achieve the isolation of this device. The previous GTI interleaver design requires a high-performance non-polarizing beam-splitter and two GT resonators. The new device would consist of a polarizing beam-splitter and a single rhomb, and would therefore cost considerably less. In addition, since the two polarizations use the same resonator, manufacturing tolerances are reduced. Finally, this device may offer superior bandwidth over prior art designs, since it would not require the phase change of a birefringent crystal (which is a more narrow band effect).  
         [0057]    With reference to FIG. 2 the rhomb-resonator interleaver according to the present invention includes a precisely polished Fresnel rhomb  1  having an effective height H, an effective width W, an index of refraction n 1 , and two essentially fully reflective surfaces  5  and  10 . The surface normals of the reflective surfaces  5  and  10  are at an angle beyond the critical angle with respect to the incident beams, thereby producing total internal reflection (TIR) for light normally entering the Rhomb resonator. The total internal reflections at these reflective surfaces  5  and  10  cause a phase shift between the S and P components in the input beam. Front and rear partially reflective surfaces  2  and  3 , respectively, which have reflectivity R 1  and R 2 , are positioned on each end of the Fresnel rhomb at an angle θ from the fully reflective surfaces  5  and  10  to create the resonant space. Preferably, R 1  ranges between 0% and 50%, while R 2  ranges from 90% to 100%. Even more preferably R 1  ranges between 12% and 20%, and ideally R 1  is 17% to 19%. Preferably, R 2  is as close to 100% as possible.  
         [0058]    For light entering the rhomb  1 , as shown by the solid arrow  4 , the rhomb  1  appears to be a resonant cavity. The total optical path length within the rhomb  1  defines the FSR of the rhomb resonator. The amplitude reflection of this cavity is different for S and P polarizations because of the differential phase shifts that occur at the TIR reflections within the rhomb  1 . By analyzing the behavior of the two orthogonal polarization components, one can demonstrate that this device is mathematically equivalent in its response to a GTI etalon resonator, while avoiding the inherent disadvantages of the GTI etalon resonator as discussed above.  
         [0059]    In order for the rhomb-resonator interleaver to produce the desired FSR, loss and finesse, the parameters H, W, θ, R 1  and R 2  must be properly designed. It is also necessary to consider the dimensions of the optical beam that will propagate within the rhomb when finalizing its dimensions. For typical optical glass and a FSR of 100 GHz, one finds that the dimensions of the rhomb prism are quite small, i.e. &lt;1 mm. Calculations indicate that this rhomb would accept an input beam diameter of roughly 400 um.  
         [0060]    The practical implementation of this device would include standard input and output optics for polarization diversity. The preferred optics, illustrated in FIG. 3, includes a first polarization beam splitter (PBS) stack  6 , which includes upper PBS  7  and lower PBS  8 . The upper PBS  7  receives S of P polarized light from an input port  9 , see FIG. 4. Preferably, the input port  9  receives the light from an input fiber  11  via a lens  12  and divides the light into two orthogonally polarized sub-beams  13   a  and  13   b  using a birefringent crystal  14 . A half-wave plate  15  rotates the polarization of one of the sub-beams, e.g.  13   b , so that both sub-beams  13   a  and  13   b  have the same polarization state, e.g. S polarized. Subsequently, the sub-beams  13   a  and  13   b  proceed through a non-reciprocal rotator  16 , a bottom PBS  17  of a PBS stack  18 , and a waveplate  19  before entering the Fresnel rhomb  1 .  
         [0061]    The non-reciprocal rotator  16  preferably comprises a Faraday rotator and a half-wave plate, whereby the state of polarization of sub-beams initially passing therethrough is unaffected, while the polarization of sub-beams passing therethrough on the return trip are rotated by 90°.  
         [0062]    Preferably a waveplate  19  is used for adjusting the polarization states of the two input beams  12  and  13  as they emerge from the polarization diversity optics in order to achieve proper balance of S and P states in each path of the Rhomb. It is also possible to use another auxiliary single TIR prism element to effect this polarization state adjustment. In some cases it is also possible to align the elements of the device at the proper angles to avoid using these elements. Angle tuning can be used to make fine adjustments to the FSR that may be required as a result of fabrication tolerances of the Rhomb.  
         [0063]    In this case R 1  is approximately 19%, while R 2  is approximately 100%. Accordingly, after passing through the Fresnel rhomb  1  a stream of channels with alternating polarizations is created. The P polarized channels  21  (e.g. odd ITU channels) get reflected by the bottom PBS  17  to a top PBS  20 , which reflects the odd channels  21  to a first output port  22 . Output port  22  is similar to input port  9 , whereby the two P polarized channel sub-beams are combined for output onto an output waveguide (not shown). S polarized channels  23  (e.g. even ITU channels) pass through bottom PBS  17 , anti non-reciprocal rotator  16 , which rotates their polarization by 90°. Accordingly, the even channel sub-beams get reflected by the upper PBS  7  to the lower PBS  8 , which reflects the even channel sub-beams to a second output port  24 . The second output port  24  is similar to the first output port  22 . The above description represents the device in use as a de-interleaver; however, one skilled in the art can easily see that this device can work in reverse as an interleaver for channels.  
         [0064]    Preferably, an additional optical path length of half the cavity length of the resonator is incorporated into one of the paths of the S and P components. If we assume the cavity length to be 2L=c/FSR then the delay would be L=c/(2×FSR). FIG. 5 illustrates a device, which is identical to the device of FIG. 3 except for the addition of a thick waveplate  30  for imposing the desired retardance L. In this case the reflectivity R 1  is very low, e.g. 2%, while the reflectivity R 2  remains as high as possible, i.e. approximately 100%.  
         [0065]    [0065]FIG. 6 illustrates another embodiment of the invention in which much of the front end optics can be avoided by providing two Fresnel Rhombs  41  and  42  and a single PBS  43 . Accordingly, the input and output ports will not require birefringent crystals, and a light beam  44  can be input directly from the waveguide  10  via lens  11 . The PBS  43  divides the beam  44  into an S polarized sub-beam  46 , which is directed towards Fresnel Rhomb  41 , and a P polarized sub-beam  47 , which is passed through to Fresnel Rhomb  42 . Again, the polarization of sub-beams  46  and  47  is altered by waveplates  48  and  49 , respectively, before entering their respective resonators  41  and  42 . When the sub-beams  46  and  47  return from the resonators  41  and  42 , respectively, the like polarized even channels  51  are combined in the PBS  43 , which passes them to a first output port  52 , while the like polarized odd channels  53  are combined in the PBS  43 , which reflects them to a second output port  54 .  
         [0066]    It is also possible to replace the PBS  43  with a non-polarizing beam splitter, which would also eliminate the need for the waveplates  48  and  49 .  
         [0067]    In addition, because this is a bulk (vs. air cavity) device, temperature stabilization will be required. This temperature control can additionally be used to fine-tune the alignment of the device&#39;s spectral response to the ITU grid. A possible long-term work-around to this requirement would be the development of a specialty optical glass that provides zero change in optical path length with temperature.  
         [0068]    [0068]FIG. 7 illustrates a preferred embodiment of the present invention, which provides low dispersion of the group delay for interleaving or de-interleaving optical channel wavelengths spaced as close together as 25 GHz or 50 GHz. The double rhomb interleaver  200  includes a first rhomb  201  and a second rhomb  202 . The first rhomb  201  has a first partially reflective surface  203 , which typically ranges between 0% and 5%, preferably ranges between 0.3% and 1.5%, but is ideally 0.7%. A second partially reflective surface  204  is applied between the first and second rhombs  201  and  202 , respectively, which typically ranges between 5% and 25%, preferably ranges between 10% and 18%, but is ideally 14%. The end face of the second rhomb  202  is coated with a substantially fully reflective coating  206  for reflecting light back through the interleaver  200 . In this embodiment a triangular prism  207  is used to provide the initial λ/8 adjustment. Typically, the four surfaces of the rhombs  201  and  202  where the TIR takes place are uncoated glass. However, it is possible to utilize multi-layer thin film interference coatings to provide a phase control different that the λ/8 wave normally provided.  
         [0069]    For the sake of simplicity, we will describe the interleaver  200  in relation to the de-interleaving process. However, the interleaver  200  can be used to interleave wavelength channels by simply reversing the process. The polarization diversity front end illustrated in FIG. 7 includes a first port  211  for launching a beam of light  212 , which includes a series of wavelength division multiplexed (WDM) channels. The first port  211  includes a ferrule tube  213  encasing an end of an optical fiber  214 , and a collimating lens  216 . A birefringent crystal  217  is used to separate the input beam  212  into two orthogonally polarized sub-beams, and a waveplate (not shown) is used to rotate the polarization of one of the sub-beams so both sub-beams have the same first polarization, e.g. vertical. See the description of FIG. 4 above. The sub-beams of input signal  212  reflect off a reflective surface  218  coated onto a triangular prism  219  towards a first polarization beam splitter (PBS)  221 . The first PBS  221  is comprised of two triangular prisms  222  and  223  with a polarization beam splitting coating  224  applied therebetween. The first PBS  221  is designed to pass light of the first polarization therethrough. The sub-beams of the input signal  212  then pass through a non-reciprocal polarization rotator  226  to a second PBS  227 . The non-reciprocal polarization rotator  226  is comprised of a Faraday rotator  228  and a half wave plate  229 . In this example the non-reciprocal polarization rotator  226  is designed to have no resultant effect on the polarization of light passing from the first PBS  221  to the second PBS  227  while rotating the polarization of light passing in the opposite direction by 90°. The second PBS  227  is comprised of two triangular prisms  231  and  232  with a polarization beam splitting coating  233  therebetween. Again, in this example, the second PBS  227  is designed to pass light with the first polarization to the rhomb interleaver  200 . A half wave plate  234  is provided to reorient the sub-beams of the input signal  212  before entering the rhomb interleaver  200 .  
         [0070]    In the preferred embodiment, passage through the rhomb interleaver  200  results in the even set of ITU channels having one polarization, while the odd set of ITU channels having an orthogonal polarization. Accordingly, after the signal reappears from the rhomb interleaver  200 , both sub-beams of one of the sets of channels, i.e. with the second polarization, is reflected by the second PBS  227  towards a second port  242 , while the other set of channels with the first polarization is passed therethrough. The second port  242 , like the first port  211 , includes a half wave plate (not shown) for rotating one of the sub-beams, whereby the two sub-beams of the first set of channels have orthogonal polarizations. A birefringent crystal  243  then recombines the two sub-beams into a single output beam  244 , which is focused onto an end of a fiber  246  by a lens  247 . The end of the fiber  246  is encased in a ferrule tube  248 . The other set of channels passes through the non-reciprocal rotator  226 , which in this direction rotates the polarization of the remaining signal by 90° from the first polarization to the second polarization. As a result, the other set of channels  250  is then reflected by the first PBS  221  to a third port  251 . The third port  251  includes a waveplate (not shown), a birefringent crystal  252 , a lens  253 , and a fiber  254  with a ferrule tube  255  all for the same purposes as in the second port  242 .  
         [0071]    With reference to FIG. 8, to eliminate the need for the first PBS  221  and the non-reciprocal polarization rotator  226 , the light from the first port  211  can be launched at an angle, i.e. non-normal, to the second PBS  227 , which enables the light returning from the rhomb interleaver  200  to follow a different path. Accordingly, light traveling to the third port  251  can simply be redirected using a prism  261 .  
         [0072]    [0072]FIGS. 9 a ,  9   b  and  9   c  represent the spectral response, the group delay, and the chromatic dispersion, respectively of a single rhomb interleaver, while FIGS. 10 a ,  10   b  and  10   c  represent the spectral response, the group delay and the chromatic dispersion, respectively, of a double rhomb interleaver. The single rhomb interleaver has a front partially reflective surface at approximately 18% and a real reflective surface at approximately 100%. The incident angle is 45°, while the critical angle is 41.9°. The index of refraction is approximately 1.554. Clearly, for the double Rhomb interleaver the group delay is lowered by a factor of 16, while the dispersion is lowered by a factor of 7. The low reflectivity front surface of the first rhomb  201  adjusts the shark-finned dispersion response of the single rhomb interleaver (FIG. 9 c ) making it more symmetrical, thereby enabling the dispersion to be compensated for during recombination of the S and P components.  
         [0073]    Another way to adjust the shape of the passband is to reflect the signal through a single rhomb interleaver twice, i.e. double pass. FIGS. 11 a  and  11   b  illustrate the spectral response and dispersion, respectively, for a double passed single rhomb interleaver as defined above. In this example, the ripple is greatly reduced and the dispersion is reduced to approximately 24 ps/nm.  
         [0074]    Fabrication of the double rhomb interleaver  200  can be facilitated by initially optically contacting and bonding two optical flats  301  and  302 , as in FIG. 12 a . The optical flats  301  and  302  are highly polished as close as possible to the precisely matching thickness. The first partially reflective coating  203  and the fully reflective coating  206  can be applied either before or alter the bonding of the optical flats  301  and  302 . Obviously, the second partially reflective surface  204  must be applied prior to bonding. The optical thicknesses of the optical flats  301  and  302  can be tested prior to dicing, and the thicknesses can be adjusted by removing more material or adding a coating layer of the same refractive index. The optical slats  301  and  302  are then diced and polished along lines  303  for use.  
         [0075]    [0075]FIG. 13 illustrates another embodiment of a rhomb interleaver  400  according to the present invention in which three rhombs  401 ,  402  and  403  with three partially reflective surfaces  401 ,  402  and  403  are used to reduce the resultant chromatic dispersion by even more. The λ/8 adjustment is again provided by a triangular prism  404 . The reflectivities R 1 , R 2  and R 3  are again chosen to provide desired spectral and dispersion responses.