Abstract:
The present invention relates to an optical interferometer based on a Mach-Zehnder interferometer that can be used as an optical channel interleaver in wavelength division multiplexed (WDM) and dense wavelength division multiplexed (DWDM) optical networks. Optical channel interleavers/de-interleavers combine sets of WDM and DWDM channels for transmission over a network and/or separate WDM and DWDM signals into sets of channels with more convenient channel spacing for further de-multiplexing. The interferometer according to the present invention utilizes a ring resonator in each arm thereof as a phase shifter to provide a flat-top wavelength response with wide pass-bands and stop-bands.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This is the first application filed for the present invention.  
         TECHNICAL FIELD  
         [0002]    The present application relates to an optical interferometer, and in particular to a bulk optical interferometer based on a Mach-Zehnder interferometer (MZI) for use as a wavelength channel interleaver/de-interleaver.  
         BACKGROUND OF THE INVENTION  
         [0003]    Optical interleavers are becoming a popular tool in dense wavelength division multiplexed (DWDM) communications networks as an interface between components designed for signals with a first wavelength channel spacing and components designed for signals with a second wavelength channel spacing. In the past 200 GHz channel spacing was the norm, but as the demand for increased bandwidth grew, 100 GHz channel spacing became the standard. In the next generation of communications networks 50 GHz channels spacing and even 25 GHz channel spacing will become common place. However, conventional de-multiplexing filters, e.g. dichroic filters, do not have the capability to separate channels that are so closely spaced without complex and expensive modifications, and without resulting in significant channel crosstalk. Accordingly, optical interleavers are used to separate the closely spaced channels into two sets of channels, which are twice as far apart. This process can continue until the channels are far enough apart for conventional multiplexing to be effective.  
           [0004]    Interleavers have taken several different forms including: Birefringent Crystal Interleavers (BCI) such as the one disclosed in U.S. Pat. No. 6,301,046 issued Oct. 9, 2001 in the name of Kuochou Tai et al ; Integrated Lattice Filter Interleavers such as the one disclosed in U.S. Pat. No. 5,596,661 issued Jan. 21, 1997 in the name of Charles Henry; and Michelson Gires-Tournois Interleavers (MGTI) such as the ones disclosed in U.S. Pat. Nos. 6,304,689 issued Oct. 16, 2001 in the name of Benjamin Dingel et al., 6,252,716 issued Jun. 26, 2001 in the name of Reza Paiam, and 6,169,828 issued Jan. 2, 2001 in the name of Simon Cao. A polarization based interleaver using a split-mirror ring resonator is disclosed in U.S. Pat. No. 6,243,200 issued Jun. 5, 2001 in the name of Gan Zhou et al.  
           [0005]    An object of the present invention is to overcome the shortcomings of the prior art and provide a simple bulk optical interleaver with very few parts that is easily manufactured at low cost and provides reliable and stable performance.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, the present invention relates to an optical interferometer device comprising:  
           [0007]    a first input port for launching an input optical beam;  
           [0008]    a first beam-splitter for separating the input optical beam into first and second sub-beams traveling along first and second paths, respectively;  
           [0009]    a first ring resonator positioned in the first path including at least two substantially fully reflective surfaces and a first partially reflective surface, the first partially reflective surface for passing a portion of the first sub-beam into the first ring resonator, while reflecting the remainder of the first sub-beam away therefrom, whereby light exiting the first ring resonator is combined with the remainder of the first sub-beam forming a first recombined sub-beam;  
           [0010]    a second ring resonator positioned in the second path including at least two substantially fully reflective surfaces and a second partially reflective surface, the second partially reflective surface for passing a portion of the second sub-beam into the second ring resonator, while reflecting the remainder of the second sub-beam away therefrom, whereby light exiting the second ring resonator is combined with the remainder of the second sub-beam forming a second recombined sub-beam;  
           [0011]    a second beam splitter for receiving the first and second recombined sub-beams resulting in the interference thereof and the production of a first output beam and a second output beam;  
           [0012]    a first output port for outputting the first output beam; and  
           [0013]    a second output port for outputting the second output beam.  
           [0014]    Another aspect of the present invention relates to a Mach-Zehnder interferometer comprising:  
           [0015]    a beam splitter for separating an input beam of light into a first sub-beam and a second sub-beam, and for directing the first and second sub-beams along first and second arms, respectively, of the interferometer;  
           [0016]    a first ring resonator in the first arm of the interferometer having a first resonator delay for effecting the phase response of the first sub-beam;  
           [0017]    a second ring resonator in the second arm of the interferometer having a second resonator delay for effecting the phase response of the second sub-beam;  
           [0018]    a beam combiner/splitter for interfering the first and second sub-beams resulting in first and second output beams. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:  
         [0020]    [0020]FIG. 1 is a schematic representation of one embodiment of an interferometer according to the present invention;  
         [0021]    [0021]FIG. 2 is a schematic representation of another embodiment of an interferometer according to the present invention;  
         [0022]    [0022]FIGS. 3 a  and  3   b  illustrate alternative examples of ring resonators useable in the embodiments of FIGS. 1 and 2;  
         [0023]    [0023]FIG. 4 illustrates a transmission spectrum showing both sets of channels from an optical interleaver in accordance with one embodiment of the present invention;  
         [0024]    [0024]FIG. 5 illustrates a transmission spectrum showing both sets of channels from an optical interleaver in accordance with another embodiment of the present invention; and  
         [0025]    [0025]FIG. 6 is a plot of Phase Difference vs Wavelength for one set of the channels from an optical interleaver in accordance with the one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0026]    The interferometer, generally indicated at  10 , according to the present invention is based on a Mach Zehnder interferometer (MZI) into and out of which light can be launched via one or more of four ports  11 ,  12 ,  13  and  14 . Each port includes a collimating/focusing lens  16  optically coupled to a ferrule  17  encasing an end of an optical fiber waveguide  18  For the sake of convenience and simplicity in our discussion, it will be assumed that the light will be launched into the interferometer  10  via a first port  11  and output via second and third ports  12  and  13 , respectively. However, as would be obvious to one skilled in the art, various other combinations are possible, including: input the second and/or third ports  12  and  13 , respectively and output the first and/or fourth ports  11  and  14 , respectively.  
         [0027]    The input light beam  20 , typically a dense wavelength division multiplexed (DWDM) signal including a plurality of wavelength channels, is launched via the first port  11  and gets split into a first sub-beam  21  and a second sub-beam  22  by a beam-splitter in the form of a first beam splitting coating  23   a  on a portion of one side of a first glass (or other transparent) substrate  24 . Preferably, the first beam splitting coating  23   a  splits the input light beam  20  is half, i.e. the reflectance ranges between 42% and 50%, and ideally 50%. The first sub-beam  21  passes through the first substrate  24  until intersecting a first partially-reflective surface  26  applied to an opposite side of the first substrate  24 . The reflectance of the first partially-reflective surface  26  is preferably between 42% and 50%. A portion of the first sub-beam  21  passes into a first ring resonator  27 , which includes a first mirror  31  and a second mirror  32 . Light exiting from the first ring resonator  27  after traveling a first resonator delay distance is combined with light reflected by the first partially reflective coating  26  forming a recombined first sub-beam  33 . The recombined sub-beam  33  is directed back through the first substrate  24 .  
         [0028]    The second sub-beam  22  is reflected by the first beam splitting coating  23   a  through a second glass (or other transparent) substrate  34  for intersection with a second partially-reflective surface  36  applied thereto. The reflectance of the second partially-reflective surface  36  is preferably between 2.4% and 5.2%. A portion of the second sub-beam  22  passes into a second ring resonator  37 , which includes a first mirror  41  and a second mirror  42 . Light leaving the second ring resonator  37  after traveling a second ring delay distance is combined with light reflected by the second partially reflective surface  36 , and directed towards the first substrate  24  forming a second recombined sub-beam  38 . The second recombined sub-beam  38  interferes with the first recombined sub-beam  33  at a second beam splitting coating  23   b  resulting in a portion of the light, i.e. a first output beam, being output the second port  12  and the remainder of the light, i.e. a second output beam, being output the third port  13 . Preferably, the reflectance of the second beam splitting coating  23   b  also ranges from 43% to 50%, and is ideally 50%. To facilitate manufacture, the first and second beam splitting coatings  23   a  and  23   b  could have the same reflectance, e.g. 50%, and be applied simultaneously.  
         [0029]    The optical path from the first beam splitting coating  23   a  to the first partially-reflective surface  26  back to the second beam splitting coating  22   b  is defined as a first optical path of the Mach-Zehnder interferometer. The optical path from the first beam splitting coating  23   a  to the second partially-reflective surface  26  back to the second beam splitting coating  22   b  is defined as a second optical path of the Mach-Zehnder interferometer. To create interference the first optical path has a different length than the second optical path. This difference is call an optical path length difference. During use as an interleaver, it is preferable that the optical path with the partially reflective surface having the lower reflectivity (e.g. the second optical path) is shorter than the other, and that the optical path length difference between the first and second optical paths is one half of the first resonator delay distance, assuming that the first and second resonator delay distances are equal. In use as an interleaver, one set of wavelength channels, e.g. the even ITU channels, is output the second port  12 , while another set of wavelength channels, e.g. the odd ITU channels, is output the third port  13 .  
         [0030]    [0030]FIG. 2 illustrates an alternative embodiment of the present invention in which a minimum amount of substrate material is used. New first and second substrates  124  and  134 , respectively, are substantially thinner than there counterparts  24  and  34  from FIG. 1. As a consequence, a third substrate  144  is required for supporting the first partially reflective coating  26 . The substrate  124  can also be divided into two separate substrates, each one having one of the beam splitting coatings  23   a  and  23   b.    
         [0031]    [0031]FIGS. 3 a  and  3   b  illustrate two other examples of ring resonators for use in place of the first and second ring resonators  27  and  37 . Ring resonator  137  (FIG. 3 a ) includes the second substrate  34  and the second partially reflective coating  36 , along with three reflective surfaces  141 ,  142  and  143 . As is obvious to one skilled in the art, any number of reflective surfaces could be used. FIG. 3 b  illustrates the second ring resonator  37  with the addition of a wedge-shaped tuning plate  150 . The tuning plate  150 , which has an index of refraction different than air, can be used to make small adjustments to the optical path length of one of the ring resonators to match the two ring resonators appropriately. Lateral adjustment of the wedge-shaped tuning plate  150  will result in the beam of light traveling through more or less thereof, which increases or decreases the optical path length of the ring resonator.  
         [0032]    [0032]FIGS. 4 and 5 illustrate theoretical transmission spectral responses for interleavers according to the present invention. The solid line represents the even ITU wavelength channels, while the dotted line represents the odd ITU wavelength channels. The bandwidth of the pass-band at −0.5 dB is over 85% of the free spectral range (FSR) of the interleaver, and the bandwidth of the stop-band at −25 dB is over 75% of the FSR of the interleaver. The differences between the two response curves is due to the reflectivity of the first and second partially reflective surfaces  26  and  36 . To obtain the plot in FIG. 4, the reflectance of the first and second beam splitting coatings  23   a  and  23   b  are 50% and 48.3, respectively, and the reflectance of the first and second partially-reflective coatings  26  and  36  are 44.8% and 3.4%, respectively. To obtain the plot in FIG. 5, the reflectance of the first and second beam splitting coatings  23   a  and  23   b  are both 50%, and the reflectance of the first and second partially-reflective coatings  26  and  36  are 42.2% and 3.3%, respectively.  
         [0033]    The plot illustrated in FIG. 6 shows the phase difference of the odd ITU wavelength channels for an interleaver according to the present invention. The phase difference alternates between 0 and ±π over consecutive wavelength channels equivalent to the FSR of the optical interleaver. The horizontal segments of the plot with  0  phase difference represent sections of constructive interference, i.e. flattop passbands, while the horizontal segments of the plot with ±π phase differences represent sections of destructive interference, i.e. stop-bands.  
         [0034]    The device according to the present invention can also be used to de-interleave two sets of complementary wavelength channels. Two input beams, each one comprising one of the complimentary sets of wavelength channels, are input the second and third ports  12  and  13 , respectively (or the first and fourth ports  11  and  14 ), and directed at the beam splitter where they are interfered and separated into two sub-beams. Each of the sub-beams travels to a different one of the ring resonators  27  or  37  forming two recombined sub-beams, which are then combined at the beam splitter, and output the first or fourth port,  11  or  14  (or the second or third port,  12  or  13 ).