Abstract:
A dispersion compensated optical signal interleaver having GTI based interferometer for interleaving and deinterleaving sets of odd and even channels and having an additional GTI interferometer for compensating the chromatic dispersion arising from the first interferometer.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to dense wavelength division multiplexed (DWDM) optical communications systems. More particularly the invention relates to interface devices such as interleavers and deinterleavers for use in interfacing between portions of DWDM systems operating at channels spacing differing by a factor of two, between, for example, portions using 50 GHz per channel spacing and portions using 100 GHz per channel spacing.  
         BACKGROUND OF THE INVENTION  
         [0002]    As DWDM optical communications technology has progressed, the channel spacing has decreased over a number of years from 200 GHz to 100 GHz to 50 GHz per channel. When a communications system is being upgraded from say 100 GHz per channel to 50 GHz per channel, it may be expedient to retain some older equipment in the system, e.g., older equipment that was designed for operation at 100 GHz per channel. The older equipment can be retained in the upgraded system by using interleavers and deinterleavers to interface between the two generations of equipment.  
           [0003]    An interleaver combines an optical signal containing even channels with an optical signal containing odd channels. A 50 GHz interleaver, for example, combines an optical signal containing a set of even channels having 100 GHz channel spacing with an optical signal containing a set of odd channels having 100 Ghz spacing and produces an output optical signal containing both sets of channels and having 50 GHz channel spacing.  
           [0004]    A deinterleaver reverses the process of the interleaver. A 50 GHz deinterleaver, for example, receives a signal containing both a set of even channels and a set of odd channels combined and having 50 GHz channel spacing, and separates the set of even channels from the set of odd channels to produce an output signal containing the set of even channels having 100 GHz channel spacing and a separate output signal containing the set of odd channels and having 100 GHz channel spacing.  
           [0005]    The general principle of the interleaver is an interferometric overlap of two light beams. The interference creates a periodic, repeating output as different integral multiples of wavelength pass through the device and the desired channel spacing of the interleaver is set by controlling the fringe pattern. Manufacturers today use fused-fiber Mach-Zehnder interferometers, liquid crystals, birefringent crystals, Gires-Tournois interferometers (GTI) and other devices to build interleavers.  
           [0006]    Of these, the GTI based interleaver has many advantages over the rest. For example, a GTI based interleaver has very low insertion loss, has uniform response over a wide range of wavelengths (flat-top spectrum), and has minimal polarization dependence effect.  
           [0007]    Chromatic dispersion must be considered for 10 Gbit/s and next generation 40 Gbit/s systems. Chromatic dispersion requirements for the higher bit rate systems are extremely tight. While there are currently many technologies being pursued for use in interleaver products, the dispersion performance will probably be the critical factor determining which technology will be successful. To be successful, the interleaver must not only have a low dispersion value at the center ITU wavelength, but over the full useful passband of the device (i.e. the dispersion should not reduce the usable passband). Unfortunately, the GTI based interleaver has a very large dispersion of up to 70-200 ps/nm for a 50 GHz interleaver and up to 250-800 ps/nm for a 25 GHz interleaver.  
           [0008]    U.S. Pat. No. 6,169,604, issued to Cao, discloses a polarization splitting interferometer and an interleaver based on that interferometer. The interferometer is a Gires-Tournois interferometer, with the addition of λ/4 wave-plate between the mirrors and an external λ/8 wave-plate (FIGS. 8 and 9 in the referenced patent). The interleaver consists of a polarization beam splitter and two of these interferometers. This interleaver operates as an interleaver and as a deinterleaver, or in other words, this interleaver performs the two functions of an interleaver and may be called an interleaver/deinterleaver. In both modes of operation the interferometers alter the polarization of the even channels, while leaving the polarization of the odd channels unchanged (FIGS. 10, 11, and  12  in the referenced patent).  
           [0009]    The phase of the output signals from this interleaver is  
             θ   =       -       tan     -   1            [         [       (     1   +     √   R       )     /     (     1   -     √   R       )       ]          tan        (     2      π                   L   /   λ       )         -     π   /   4       ]         -       tan     -   1            [     [           (     1   +     √   R       )     /     (     1   -     √   R       )            tan        (     2      π                   L   /   λ       )         +     π   /   4       )     ]                 (   1   )                               
 
           [0010]    [0010]FIG. 1 shows the group delay and FIG. 2 shows the dispersion as calculated for a 50 GHz interleaver/deinterleaver of the above type, using the following parameters; reflectivity R=18.5% and Length L=1.5 mm. From FIG. 2, the dispersion is +/−60 ps/nm in bandwidth +/−0.11 nm (+/−14 GHz).  
           [0011]    U.S. Pat. No. 6,169,626, issued to Chen et al., discloses an interleaver/ deinterleaver that use a Gires-Tournois-Michelson interferometer (GTMI). The dispersion of a 50 GHz GTMI is calculated by the present inventor to +/−50 ps/nm in the bandwidth +/−10 GHz.  
           [0012]    Neither of the above references disclose any way of compensating the rather large chromatic dispersion associated with the GTI based interferometers.  
           [0013]    An interleaver with such high dispersion is not useful in a high bit rate system. Therefore, there exists a need for a compensation mechanism that would reduce the dispersion of the GTI based interleaver to such a range that the GTI based interleaver becomes a useful device. The present invention addresses this need.  
         OBJECTS AND ADVANTAGES  
         [0014]    It is an object of the present invention to provide an optical signal interleaver and an optical signal deinterleaver that have a much reduced level of chromatic dispersion.  
           [0015]    It is a further object of the present invention to provide an optical signal interleaver and an optical signal deinterleaver that are suitable for use in 50 GHz DWDM systems.  
           [0016]    It is a further object of the present invention to provide an optical signal interleaver and an optical signal deinterleaver that do not require the use of an optical circulator.  
         SUMMARY  
         [0017]    The objects and advantages of the present invention are secured by providing dispersion compensation in a GTI based interleaver and also by providing dispersion compensation in a GTI based deinterleaver. In both the interleaver and the deinterleaver the dispersion compensation is provided by a compensating interferometer that is a GTI interferometer.  
           [0018]    The dispersion compensated interleaver of the present invention includes in its structure a signal processing interferometer and a dispersion compensating interferometer. In the interleaver, the signal processing interferometer performs the function of combining an input signal that contains a set of even channels with an input signal that contains a set of odd channels. The paths of the optical signals through the dispersion compensated interleaver are as follows: the two input signals, one containing a set of even channels and one containing a set of odd channels, are inputted to the signal processing interferometer. The signal processing interferometer combines the input signals and outputs a signal containing both sets of channels. The output signal from the signal processing interferometer is routed to the dispersion compensating interferometer where the chromatic dispersion introduced by the signal processing interferometer is compensated. The output of the dispersion compensating interferometer is the output of the interleaver. essentially all of the light in the ouput signal has been through the signal processing interferometer and through the dispersion compensating interferometer.  
           [0019]    The dispersion compensated deinterleaver of the present invention includes in its structure a signal processing interferometer and a dispersion compensating interferometer. In the deinterleaver the signal processing interferometer performs the function of receiving an input signal containing a set of even channels and a set of odd channels and outputting two separate output signals, one output signal containing the set of odd channels and the other output signal containing the set of even channels. The paths of the optical signals through the dispersion compensated deinterleaver are as follows: the input signal containing a set of even channels and a set of odd channels passes through the dispersion compensating interferometer, where dispersion compensation is applied, and goes to the signal processing interferometer which outputs separate signals one containing the set of even channels and one containing the set of odd channels. all of the light in the output signals passed through the signal processing interferometer and through the dispersion compensation interferometer.  
           [0020]    In one embodiment of the invention, in both the interleaver and the deinterleaver, the signal processing interferometer is a GTI based non-linear interferometer (NLI). This is a GTI interferometer with an internal λ/4 wave-plate and an external λ/8 wave-plate and operates as a polarization splitting interferometer. The NLI changes the polarization of linearly polarized even channels from vertical to horizontal or from horizontal to vertical, while leaving the polarization of linearly polarized odd channels unchanged. The NLI signal processing interferometer receives signals from a polarization beam splitter and sends output signals back to the polarization beam splitter.  
           [0021]    In this embodiment there is one structural difference between the interleaver and the deinterleaver.  
           [0022]    The structural difference is this. In the deinterleaver an optical component that consists of a 22.5° cut half-wave plate with a garnet is located in the optical path between the signal processing interferometer and the dispersion compensating interferometer. A horizontally polarized input signal passes through the half-wave plate and then through the garnet towards the signal processing interferometer with horizontal polarization unchanged, whereas the polarization of the horizontally polarized odd channel output signal that returns along the same path from the signal processor is changed to vertically polarized, thus allowing the odd channel output signal to be reflected to the deinterleaver output by a polarization beam splitter.  
           [0023]    In the interleaver, on the other hand, the respective positions of the half-wave plate and the garnet are interchanged so that a vertically polarized odd channel input signal is changed to a horizontally polarized input signal as it approaches the signal processing interferometer and so that the horizontally polarized signal returning from the signal processing interferometer along the same path retains its polarization unchanged.  
           [0024]    The effect just described that is obtained by interchanging the positions of the the 22.5° half-wave plate and the garnet can also be obtained by reversing the orientation of the garnet.  
           [0025]    The difference in structure between interleaver and deinterleaver applies also the following embodiment of the invention.  
           [0026]    In this embodiment of the invention, in both the interleaver and the deinterleaver, the signal processing intereferometer is a combination of a GTI and a Michelson interferometer (GTMI) where the GTI replaces one of the mirrors of a Michelson interferometer.The dispersion compensation interferometer is a GTI interferometer as before.  
           [0027]    The interleaver of the invention has separate input/output ports so that an optical circulator is not required.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0028]    [0028]FIG. 1 is a graph showing the group delay for a 50 GHz interleaver.  
         [0029]    [0029]FIG. 2 is a graph showing the dispersion for a 50 GHz interleaver.  
         [0030]    [0030]FIG. 3 is a schematic diagram showing a deinterleaver in a first embodiment in accordance with the present invention.  
         [0031]    [0031]FIG. 4 shows a dispersion compensator as used in the deinterleavers of FIG. 3 and FIG. 10.  
         [0032]    [0032]FIG. 5 is a graph showing the group delay of the dispersion compensator of FIG. 4.  
         [0033]    [0033]FIG. 6 shows in cross-section, a nonlinear interferometer (NLI) as used in the interleaver and the deinterleaver of the first embodiment of the invention.  
         [0034]    [0034]FIG. 7 shows the relationship of polarization direction to c axis for the 22.5° cut half-wave plate.  
         [0035]    [0035]FIG. 8 shows a graph of group delay for a 50 GHz dispersion compensated deinterleaver as calculated for the first embodiment shown in FIG. 3.  
         [0036]    [0036]FIG. 9 shows a graph of the dispersion characteristic of a 50 GHz dispersion compensated deinterleaver as calculated for the first embodiment shown in FIG. 3.  
         [0037]    [0037]FIG. 10 shows a schematic diagram of an interleaver in accordance with the present invention.  
         [0038]    [0038]FIG. 11 shows a portion of a deinterleaver showing the arrangement of a garnet and a quarter wave plate.  
         [0039]    [0039]FIG. 12 shows a portion of an deinterleaver showing the orientation of the garnet  32 .  
         [0040]    [0040]FIG. 13 shows, in schematic form, a deinterleaver in a second embodiment in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0041]    [0041]FIG. 3 shows a schematic diagram of a deinterleaver  10  in accordance with the present invention. The deinterleaver  10  receives an input signal at input port  12 . The input signal contains a set of even channels and a set of odd channels, the channel spacing being fixed at, for example, 50 GHz between alternating even and odd channels. Thus, in this example, the spacing between even channels is 100 GHz, and the spacing between odd channels is 100 GHz. A collimator  14  passes the input signal to a translator  16 .  
         [0042]    The translator  16  includes a walk-off crystal  18  and a half-wave plate  20 . The input signal passes through the walk-off crystal  18  and emerges from the walk-off crystal  18  as two separate beams, a vertically polarized beam and a horizontally polarized beam. The horizontally polarized beam passes through a half-wave plate  20  and emerges from the half-wave plate  20  as a vertically polarized beam. Thus the input is translated by the translator  16  into a vertically polarized input signal consisting of two parallel beams, each vertically polarized. The translator  16  outputs this vertically polarized input signal to a polarization beam splitter  22 .  
         [0043]    The translator  16  can be operated in the reverse direction to remove polarization from an output signal. The translator described above adds and removes vertical polarization. A translator for adding and removing horizontal polarization is similar but has the half-wave plate intercepting the other beam. The term translator as used in this application is intended to include any device that performs the functions just described.  
         [0044]    The polarization beam splitter  22  reflects the vertically polarized input signal to a quarter wave plate  24 . The polarized input signal passes through the quarter-wave plate  24 , the polarization of the input signal being transformed to a circular polarization by the quarter-wave plate  24 . The circularly polarized input signal then goes to a dispersion compensating interferometer  26 .  
         [0045]    The dispersion compensating interferometer  26  is a Gires-Tournois interferometer (GTI) as shown in FIG. 2. The dispersion compensating interferometer  26  outputs a dispersion compensated circularly polarized input signal to the quarter-wave plate  24 . The signal passes through the quarter-wave plate  24 , is transformed to a horizontally polarized signal by the quarter wave plate  24 , and emerges from the quarter-wave plate  24  as a dispersion compensated horizontally polarized input signal.  
         [0046]    The horizontally polarized input signal is transmitted through the polarization beam splitter  22 , then through another polarization beam splitter  28 , then through a 22.5° cut half-wave plate  30  which rotates the polarization through forty five degrees positive, and then through a garnet  32  which rotates the polarization through forty five degrees negative. The input signal as it emerges from the garnet  32  remains a horizontally polarized input signal.  
         [0047]    The horizontally polarized input signal then enters the interferometer section  34  of the interleaver  10 . The interferometer section  34  includes a polarization beam splitter  36  and a the signal processing interferometer which in this embodiment of the invention is a nonlinear interferometer NLI  38 . The nonlinear interferometer  38  in this embodiment of the invention is a modified GTI interferometer as shown in FIG.4. The horizontally polarized input signal is transmitted through the polarization beam splitter  36  to the nonlinear interferometer  38 . The nonlinear interferometer  38  outputs a signal to the polarization beam splitter  36  which transmits a horizontally polarized signal containing the set of odd channels (horizontally polarized odd channel signal) and reflects a vertically polarized signal containing the set of even channels (vertically polarized even channel signal).  
         [0048]    The horizontally polarized odd channel signal passes from the polarization beam splitter  36  out of the interferometer section  34  and through the garnet  32  which rotates the polarization forty five degrees, through the 22.5° degree cut half-wave plate  30  which rotates the polarization another forty five degrees and outputs a vertically polarized odd channel signal to the polarization beam splitter  28  where it is reflected to translator  40 .  
         [0049]    The translator  40  receives the vertically polarized odd channel signal, removes the polarization and outputs the odd channel signal. Translator  40  includes a half-wave plate  42  and a walk-off crystal  44 . A part of the vertically polarized odd channel signal enters the walk-off crystal  44  directly and a part passes through the half-wave plate  42  before entering the walk-off crystal  44 , the latter part becoming horizontally polarized. The vertically polarized part and the horizontally polarized part are recombined in the walk-off crystal  44  and outputted from translator  40  as the odd channel output signal which then passes through the collimator  46  to the output port  48 .  
         [0050]    Referring back to the vertically polarized even channel signal that was reflected by the polarization beam splitter  36 , that signal goes to translator  50 . The translator  50  includes a half-wave plate  52  and a walk-off crystal  54  and removes polarization from the polarized even channel signal just as translator  40  does for the odd channel signal. The depolarized even channel signal passes from the translator  50  through a collimator  56  to output port  58 .  
         [0051]    [0051]FIG. 4 shows a schematic cross-section of a dispersion compensating interferometer  60  that is suitable for use as a dispersion compensator in the present invention. The dispersion compensator  60  is a Gires-Tournois interferometer having a first partially reflective mirror  62  spaced apart from and parallel to a highly reflective mirror  64 , with a cavity  66  between the two mirrors. The partially reflective mirror  62  provides a single input/output port  68  to allow light to enter and leave the cavity  66 . The optional spacers  70  are made of ultra-low expansion material. The amplitude response of the Gires-Tournois interferometer is flat (i.e. independent of wavelength),  
         [0052]    the phase response is 
         φ(λ)=2 tan −1 [[(1+{square root} R )/(1−{square root} R )]tan(2π d /λ)],  (2) 
         [0053]    the group delay is 
         τ(λ)=0.01λ 2 /6π[ dφ (λ)/ d λ] (ps),  (3) 
         [0054]    and the dispersion is 
           D (λ)=10 −3   d τ(λ)/ d λ (ps/nm).  (4) 
         [0055]    In the above equations λ is wavelength, R is the power reflectivity of the partially reflective mirror, and d is the length of the cavity  66  from the partially reflective mirror  62  to the highly reflective mirror  64 . The reflectivity of the highly reflective mirror is preferably about 100%.  
         [0056]    [0056]FIG. 5 shows a graph of group delay calculated for a Gires-Tournois interferometer dispersion compensator, having a cavity length d=3 mm and reflectivity R=0.28% for the partially reflective mirror.  
         [0057]    [0057]FIG. 6 shows, in cross section, a nonlinear interferometer suitable for use as the nonlinear interferometer  36  in FIG. 1. The nonlinear interferometer  100  is a modified GTI interferometer having a first partially reflective mirror  62 , spaced apart from and parallel to a highly reflective mirror  64  with a cavity  66  between the two mirrors. The partially reflective mirror  62  provides a single input/output port  68  to allow light to enter and leave the cavity  66 . The partially reflective mirror has a reflectivity preferably of approximately 18.5%. The highly reflective mirror preferably has reflectivity of approximately 100%.  
         [0058]    A quarter-wave plate  70  is located in the cavity  66  and introduces a 180° round trip phase change between an o beam and an e beam of the signal inside the cavity  66 . The external λ/8 plate  72  introduces a round trip phase change of π/2 between the o beam and the e beam.  
         [0059]    [0059]FIG. 7 shows the relationship between polarization direction and the c axis of 22.5° cut half-wave plate as used in the first and second embodiments of the invention.  
         [0060]    [0060]FIG. 8 shows a graph of group delay for a 50 GHz dispersion compensated interleaver and deinterleaver according to the first embodiment of the invention shown in FIG. 3  
         [0061]    [0061]FIG. 9 shows a graph of dispersion for a 50 GHz dispersion compensated interleaver and deinterleaver according to the first embodiment shown in FIG. 3.  
         [0062]    [0062]FIGS. 5, 8 and  9  are the results of calculations wherein the parameters were GTI cavity length=3 mm, and R=0.28%. From FIG. 9 the dispersion is only +/−4.7 ps/nm in bandwidth of +/−0.8 nm (+/−10 GHz). In order to compensate dispersion, the peak of the group delay of the compensator must be aligned with the bottom of the group delay of the interleaver.  
         [0063]    [0063]FIG. 10 shows an interleaver in accordance with the present invention. All of the elements in FIG. 10 are the same locations in FIG. 10 as in FIG. 3 except that the garnet  32  and the 22.5° cut half-wave plate  30  have changed places. In this case the odd channel input signal is changed from vertical polarization to horizontal polarization on the way to the NLI and the horizontally polarized output signal from the NLI to the dispersion compensating interferometer is not changed.  
         [0064]    [0064]FIG. 11 shows the relevant portion of a deinterleaver that is the same as that shown in FIG. 3 except for the garnet  32  which in FIG. 11 is oriented as indicated by the facet in a specific direction, with the “A” face facing the polarization interferometer  36 .  
         [0065]    [0065]FIG. 12 shows an interleaver that has the same elements as the deinterleaver of FIG. Except for the garnet  32  as indicated by the facet  33 .which has the “A” face facing the quarter wave plate  30   
         [0066]    FIG. 13  shows a schematic of a second dispersion compensated deinterleaver in accordance with the present invention. Numeral identifiers used in FIG. 3 are used again in FIG. 13 to identify like elements. All of the parts shown below the dashed line AA in FIG. 13 have like numbered counterparts in FIG. 3. Therefore, the description of the deinterleaver  110  can conveniently begin at the point where the horizontally polarized input signal leaves the garnet  32  and enters the interferometer section  112 . The interferometer section  112  includes a 50/50 beam splitter  114  optically coupled to the garnet  32 , optically coupled to a reflecting mirror  116  and optically coupled to an interferometer  118 . The beam splitter  114  splits the horizontally polarized input signal into a portion which is reflected by the mirror  116  back to the beam splitter  114 , and a portion which is reflected by the interferometer  118  back to the beam splitter  114 .  
         [0067]    The combination of the Gires-Tournois interferometer  118  with the beam splitter  114  and the mirror  116 , is a Gires-Tournois-Michelson interferometer (GTMI). The Gires-Tournois interferometer  118  is shown, in cross-section in FIG. 4.  
         [0068]    The signals reflected from the mirror  116  and from the Gires-Tournois interferometer  118  go back to the beam splitter  114  and interfere within the beam splitter  114  so that the beam splitter  144  outputs a horizontally polarized signal containing the set of odd channels (horizontally polarized odd channel signal) to the garnet  32  and outputs a horizontally polarized signal containing the set of even channels (horizontally polarized even channel signal) to the translator  120 .  
         [0069]    The horizontally polarized odd channel signal passes through the garnet  32  which rotates the polarization through forty five degrees. The signal then passes through the 22.5° cut half-wave plate  30  which rotates the polarization through an additional forty five degrees so that the signal becomes a vertically polarized odd channel signal. As described with regard to FIG. 3 this signal is reflected in polarization beam splitter  28  and is depolarized in translator  40  which sends the odd channel output signal via collimator  46  to output port  48 .  
         [0070]    The horizontally polarized even channel signal goes from the beam splitter  114  to translator  120 . The translator  120  includes a half-wave plate  122  and a walk-off crystal  124 . Part of the horizontally polarized even channel signal enters the walk-off crystal  124  directly and part passes through the half-wave plate  122 , where it becomes vertically polarized, before entering the walk-off crystal  124 . Both parts of the signal are recombined in the walk-off crystal  124  to form a depolarized signal that is the even channel output signal which passes through the collimator  126  to input/output port  128 . The dispersion of a 50 GHz dispersion compensated GTMI based interleaver is calculated to be +/−10 ps/nm as compared to +/−50 ps/nm calculated for a GTMI interleaver without dispersion compensation.  
         [0071]    The above described embodiments of the invention are to be regarded as illustrative of the invention are not intended to be construed as limiting. Accordingly the scope of the invention should be determined by the following claims and their legal equivalents.