Abstract:
The present claimed invention improves on the conventional comb and multiple router approach by making multiple use of a single router by using interleaved groups of wavelengths that enter through different entrance ports and exit through different groups of exit ports of the same router. This permits better separation (lower crosstalk) between adjacent channels, greatly reduced temperature sensitivity, small physical size and lower cost. With these improvements, a large number of wavelength add/drop sites may be cascaded in WAD/WDM systems.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to optical fiber communications transmission networks, and more particularly, relates to a comb splitting system and a method for multiplexing or demultiplexing a plurality of optical bands each containing a plurality of individual channels at different wavelengths. 
     2. Description of the Related Art 
     There are several network approaches for implementing optical communication networks between central offices and individual subscribers. These include for example point-to-point networks, power splitting networks, and wavelength division multiplexing networks. In a point-to-point network, one or more optical fibers directly link the central office to each subscriber. In a power splitting network, a transmitter, receiver, and part of an optical fiber in the subscriber loop is shared by many subscribers using power splitters, a wide variety of which are well known in the art. See for an example, U.S. Pat. No. 4,904,042 to Dragone that describes a star coupler, which is a typical power splitter (PS). 
     A third approach is a wavelength division multiplexing network that employs wavelength division multiplexers (WDM). In this approach, a particular channel (wavelength of carrier) is allocated to each subscriber. Various channels are multiplexed over a single optical fiber and are demultiplexed onto individual fibers associated with each subscriber to create a virtual point-to-point network. A WDM sometimes referred to as a filter or router, is well known in the art and is generally a device that can multiplex and/or demultiplex optical signal wavelengths. 
     In general, a WDM is usually a passive optical network (PON) element or device with multiple optical paths, each of which exhibits a particular passband, similar to an electrical signal processing filter. The passband permits passage of one or more particular wavelengths along the respective optical path, to the substantial exclusion of others. Thus, the WDM can be used to divide wavelengths of incoming light from a multichannel optical signal or to combine various wavelengths on respective optical paths into one multichannel optical signal on one optical path. For an example of a WDM, see C. Dragone et al., “Integrated Optics N×N Multiplexer on Silicon,” IEEE Photon. Techno. Lettr., Vol. 3, p. 896 (1989), the disclosure of which is also set forth in U.S. Pat. No. 5,136,671 to Dragone. 
     As shown in FIG. 1, a Wavelength add and drop (WAD) site consists of an optical path  3  that carries a multichannel optical signal (light)  5 . The multichannel optical signal is sent to an optical demultiplexing unit (ODU)  8  which separates the light into various wavelengths (channels) and outputs optical signals into individual predetermined wavelengths (channels)  9 ,  11 ,  13 . In reverse, the optical multiplexing unit (OMU)  7  combines the multiple incoming signals (channels) into a single multichannel optical signal  5  in a single optical path  3 . Various WAD sites  1  may be provided in a cascade network, as shown in FIG. 1, to provide a virtual point-to-point or a ring system. 
     FIGS. 2 and 3 show a conventional OMU  7  and ODU  8 , also referred to as a waveguide grating router (WGR), with an input side  17  and an output side  19 . The ODU  8  is composed of an array of single mode waveguides  29  and input  21  and output  23  waveguides connected by two planar slab waveguides  18  and  20 . The principles of multiplexing and demultiplexing of the OMU  7  and the ODU  8  are the same, except that light propagation direction is reversed. Light  5  with a plurality of wavelengths  9 ,  11 ,  13  is received by the single mode input waveguide  21  and diffracts horizontally in the slab waveguide region  18 . Each wavelength propagates through the individual array waveguides  29  and experiences a constant and wavelength-dependent phase shift caused by the path difference. Thus, the phase shift produces a wavelength dependent wavefront tilting, so that light convergence in the output slab waveguide  20  is wavelength dependent. Since every output waveguide  23  is arranged on a circle with radius R/2 (R is shown as  33 ) and is directed at the center of the output array waveguides, the different wavelength channels in the input light are focused along the focal plane of the output aperture and couple into different output waveguides  23 . 
     Light coupled into the input waveguide  21  and emitted from the WGR must satisfy the grating equation 
     
       
           dn   s sinθ i   +dn   s sinθ o   +n   c   ΔL=mλ   
       
     
     where θ i  and θ o  are the diffraction angles at the input  18  and output  20  slabs, respectively; ΔL is the constant optical path length difference between neighboring array waveguides  29 ; n s  and n c  are effective refractive index of the slab and channel waveguide, respectively; d is the grating pitch  31 ; and m is the diffraction order and is an integer. 
     The spatial dispersion is given by               x          λ       =       Rm       n   s        d                           n   g          (   λ   )         n   c                                
     where the group refractive index n g  is defined according to 
     
       
           n   g (λ)= n   c   −λ·dn   c   /dλ   
       
     
     Since the arrayed grating provides liner dispersion in the wavelength along the focal plane of the output aperture, WDM wavelengths are separated by a distance Δλ dx/dλ along the focal plane at the output angular spacing          Δ                   θ   o       =         Δ                 λ       dn   s          m                            
     where Δλ is the channel spacing. 
     The waveguide grating device (WGR)  7  may contain different input and output angular spacings (i.e., Δθ i ≠Δθ o ), which means that asymmetrical I/O-port design will yield different demultiplexed wavelengths when a signal is input from different input port and is output from different output port. Under this design, the center wavelength of a WGR device can be adjusted by inputting the multiplexed signal at an off-center port. This is referred as the Vernier effect. The proper I/O-port angular ratio and a number of dummy input and output ports  22  can be chosen to compensate the center wavelength offset due to material and processing variations. When the multiplexed signal is input at the i-th port, the demultiplexed wavelength at j-th port is given by 
     
       
         λ i→j =λ o +( i+j/R   v )Δλ 
       
     
     where R v  is defined as Vernier ratio between the two angular separations at the output port and input port by 
     
       
           R   v =Δθ o /Δθ i   
       
     
     When the output port j=−i, the center wavelength can be shifted by 
     
       
         Δλ o   =i (1−1 /R   v )Δλ 
       
     
     Δλ is unchanged, but the center wavelength and the all-wavelength comb are tuned, based on the designed R v . 
     A practical WDM network, favors a two stage split of channels that first splits a multichannel optical signal into two channels. Each channel is subsequently demultiplexed into individual channels using a WGR. As shown in U.S. Pat. 5,680,490, by Cohen and Li, a comb splitting system and method for implementing a multistage WDM network are provided for a two-stage split. 
     The problem with such conventional two-stage splitting systems is that each splitting and combining device (WGR) requires separate environmental controls. This would cause each set of wavelength shifts from each other, which would result in crossed traffic. Thus, while a two stage split is preferable, it is not preferable to have multiple WGRs, one for each secondary split. Additional devices result in increased cost and complexity. 
     The present claimed invention is directed to further improving such devices by providing an interleaved multi-channel WGR that may be used in a two-stage split. 
     SUMMARY OF THE INVENTION 
     The present claimed invention improves on the conventional comb and multiple router approach by making multiple use of a single router. 
     The present claimed invention provides a practical interleaved router that has at least two input ports and multiple (N) outputs. A 2×N implementation of this integrated device can be used in the second stage of a two-stage splitting system, but is not limited thereto. 
     Compared with the conventional systems, the integrated 2×N WGR and its based WAD systems have at least the following advantages: 
     1. By dividing the optical spectrum on one fiber into two interleaved sets of N/2 channels (N/2 odd channels on one fiber and N/2 even channels on another fiber), an integrated two input port and N output-port (2×N) WGR is capable of providing accesses to N channels. 
     2. This approach provides a N/2 channel start up capability with a growth increment for the next N/2 channels. The second set of N/2 channels can be installed for expansion later without affecting the transmission of the first set of N/2 channels. In addition, the WAD site can be provisioned to provide access to either N/2 or N channels. 
     3. The present invention has an increased bandwidth when compared to convention 1×N designs and thus is capable of an increased number of cascades. 
     4. Because the integrated 2×N WGR shares one arrayed-grating the odd channels and even channels do not shift from each other. 
     5. Similarly, N channels can be divided into N/m channels (where m=2,4, . . . ) Thus, providing for a modular and growable system that has the capacity for upgrade. 
     6. The same or even better performance can be obtained with smaller physical size and lower cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages and features of the invention will become more apparent from the detailed description of the preferred embodiments of the invention given below with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic diagram of a prior art wavelength add and drop (WAD) site; 
     FIG. 2 is a schematic diagram of an optical demultiplexing unit (ODU) used in a WAD site; 
     FIG. 3 is a schematic diagram showing in the operations of the ODU shown in FIG. 2; 
     FIG. 4 is a schematic diagram of a two-stage WAD site using a 2×N waveguide grating (WGR) constructed in accordance with a first embodiment of the present invention; 
     FIGS. 5 a,    5   b  and  5   c  are schematic diagrams showing the operation of the 2×N WGR of FIG. 4; 
     FIG. 6 is a chart comparing the bandwidth differences between a 1×N ODU and a 2 33  N WGR; 
     FIGS. 7 a ,  7   b  and  7   c  are schematic diagrams showing the application and operation of a second embodiment 2×N WGR; 
     FIGS. 8 a ,  8   b  and  8   c  are schematic diagrams showing the application and operation of a third embodiment 2×N WGR; and 
     FIG. 9 a,    9   b  and  9   c  are schematic diagrams showing the application and operation of fourth embodiment 2×N WGR. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides an interleaved WGR (router) with at least two input ports and multiple (N) output ports. The present invention may be used in a two-stage wavelength demultiplexing multiplexing (WDM) system shown in FIG. 4, but is not limited thereto. 
     As shown in FIG. 4, a wavelength add/drop site (WAD)  35  is provided that receive multichannel optical signals  37  over a single optical line  39  which are separated using a wavelength demultiplexer (WDM)  41  into two interleaved sets of wavelengths  45  and  49 , which are respectively carried on two optical lines  43  and  47 . The two interleaved multichannel signals  43  and  47  are input into a 2×N WGR integrated device  51  and demultiplexed into channels  53 ,  55  and  57  respectively. The WAD site using a 2×N WGR integrated device multiplex in a similar manner, as those of ordinary skill in the art will appreciate. Although the present invention will be described using a 2×N design, those of ordinary skill in the art will recognize that an N×N design is possible, including a 4×N, an 8×N design and so on. 
     As shown in greater detail in FIGS. 5 a ,  5   b  and  5   c  the WGR integrated device has two sets of the input waveguide ports  73  and  74  on opposite sides  69  and  71  of the same arrayed grating waveguides  67 . Each input waveguide port  73  and  74  has respective output waveguide ports  85 ,  87 ,  89  and  86 ,  88  and  90  on an opposite side of the arrayed grating waveguides  67  relative to its respective input port. The input and output waveguide ports  73 ,  74 ,  85 ,  86 ,  87 ,  88 ,  89  and  90  are arranged on a circle with radius R  64 . In addition, a first slab waveguide coupler  60  and a second slab waveguide coupler  62  are provided. Vernier tuning ports  22  are also provided. 
     Light  75  with a plurality of odd wavelengths λ 1 , λ 3 , λ 5  . . . (with channel spacing 2Δλ) is launched into the first input waveguide port  73 . The light  75  is demultiplexed into individual output ports  85 ,  87  and  89  with wavelengths λ 1 , λ 3 , λ 5  . . . 
     Similarly, light  76  with a plurality of even wavelengths λ 2 , λ 4 , λ 6  . . . (with channel spacing 2Δλ) may be launched into the second input waveguide port  74  and demultiplxexed into individual output waveguide ports  86 ,  88  and  90  with wavelengths λ 2 , λ 4 , λ 6  . . . Thus, a 2×N WGR (router) with two input waveguide ports can be used to demultiplex two sets of interleaved wavelengths  75  and  76  into two N/2 channels with channel space 2Δλ. 
     Although only three output waveguide ports and wavelengths per side are shown herein, the present invention may use multiple combinations of wavelengths and output waveguide ports. In addition, the use of the terms odd wavelengths and even wavelengths is not meant to be limited to any particular wavelength or group of wavelengths but is meant to express interleaved wavelengths. 
     For the first input waveguide port  73 , the input waveguide has θ i =(−pΔθ i ) degree diffraction angle at the slab waveguide coupler  60 , where p is an integer and is the input number relative to the central port, and Δν i  is the constant angular spacing value  95 . The sign, whether negative (−), zero (0) or positive (+), is defined with respect to which side of the centerline of the waveguide array  67  the angle is being formed in relationship to. Those of ordinary skill in the art will recognize that this may be changed and that any equation modified accordingly. A set of output waveguide ports  85 ,  87  and  89  have the constant angular spacing Δθ o    91  which corresponds to a set of output odd wavelengths λ 1 , λ 3 , λ 5  . . . For the first input port  73  and its corresponding output ports  85 ,  87  and  89  the following grating equation can be obtained as: 
       dn   s sin(− pΔθ   i )+ dn   s sin( jΔθ   o )+ n   c   ΔL=mλ   odd   Eq. (1) 
     and constant angular spacing Δθ o   91  satisfies                Δ                   θ   o       =         2      Δ                 λ       dn   s          m             Eq. (2)                                
     where j is output port numbering relative to the central port; n s  and n c  are effective refractive index of the slab and channel waveguide, respectively; ΔL is the constant optical path length difference between neighboring array waveguides  67 ; d  66  is the grating pitch; and m is the diffraction order and is an integer. Equation (2) indicates that the each output port corresponds to the double channel spacing. 
     In the first embodiment shown in FIGS. 5 a ,  5   b  and  5   c , the same input port design is used for the second input port  74  but all of the output ports corresponding thereto  86 ,  88  and  90  are shifted by a constant angle (which corresponds to a channel spacing Δλ in the wavelength domain). For the second input port  74 , the input waveguide has the same θ i  (=−pΔθ i ) degree diffraction angle at the second slab-coupler  62 , where p is an integer and Δθ i   95  is the constant angular spacing value. A set of output waveguide ports  86 ,  88  and  90  still have the constant angular spacing Δθ o   91  which corresponds to a set of output even wavelengths λ 2 , λ 4 , λ 6  . . . , but each output has a Δθ o /2 constant angular shift  97  to the positive channel side relative to the central zero line of the grating. The second input waveguide port  74  and its corresponding output waveguide ports  86 ,  88  and  90  satisfy the following grating equation 
       dn   s sin(− pΔθ   i )+ dn   s sin( jΔθ   o +Δθ o /2)+ n   c   ΔL=mλ   even   Eq. 3 
     and constant angular spacing Δθ o   91  satisfies the same equation (2). −pΔθ i  is illustrated at  97  in FIGS. 5 b  and  5   c . jθ o +Δθ o /2 is illustrated at  105 . jΔθ o  is illustrated at  93 . Δθ o /2 is illustrated at  97 . 
     The integrated device shown in FIGS. 5 a ,  5   b  and  5   c  may be manufactured in accordance with known techniques and of known materials for integrated devices. As the present claimed invention is direct to a unique design that provides numerous advantages over prior art designs, the material and method of manufacture is not material to the present invention. 
     The advantages of the integrated device shown in FIGS. 5 a ,  5   b  and  5   c  are as follows: 
     For a router, channel spacing can also be obtained as                Δ                 λ     =         Δ                   x   o       R                         n   s        d     m               Eq. (7)                                
     Bandwidth Δf 3dB  can be estimated as                Δ                   f     3      dB         =     2          ln                 2            ω   0            dn   s     Rm               Eq. (8)                                
     Since the entire spectral range is unchanged for 2×N WGR, the free spectral range and grating order m remains unchanged. Compared with the conventional 1×N WGR, the interleaved 2×N WGR has 
     
       
         Δλ 2×N =2·Δλ 1×N   
       
     
     SO 
     
       
           R   2×N =0.5 R   1×N   , Δf   3dB,2×N =2 ·Δf   3dB,1×N   
       
     
     Thus, the integrated 2×N WGR has small focal length R (resulting in small chip size) and bandwidth that is twice as wide (resulting in less filter narrowing and more cascaded WAD sites), as shown in FIG.  6 . This permits better separation (lower crosstalk) between adjacent channels and reduced temperature sensitivity. Compared with a single input port using the same router technology, it is possible to obtain much better channel separation and cascadablity permitting many more add/drop stages in a system. 
     FIGS. 7 a ,  7   b  and  7   c  shows a second embodiment which includes an alternate design for the second input waveguide port  74 . The alternate design uses the same waveguide output port design for all waveguide output ports but shifts the waveguide input port  74  by a constant angle which corresponds to a channel spacing Δλ in the wavelength domain. For the second waveguide input port  74 , the corresponding input waveguide has the θ i  degree diffraction angle  99  at the second slab coupler, where θ i =−(pΔθ i )+Δθ o /2 degree diffraction angle  99  at the second slab coupler  62 , which shifts −Δθ o /2 constant angular value  97  relative to the central zero line of the grating. A set of output waveguide ports  86 ,  88  and  90  have the same constant angular spacing Δθ o    91  which corresponds to a set of output even wavelengths λ 2 , λ 4 , λ 6  . . . 
     The second waveguide input port  74  and its corresponding waveguide output ports  86 ,  88  and  90  satisfy the following grating equation 
     
       
           dn   s sin(− pΔθ   i +Δθ o /2)+ dn   s sin( jΔθ   o )+ n   c   ΔL=mλ   even   Eq. (9) 
       
     
     and constant angular spacing Δθ o    91  satisfies the same equation (2). 
     The same Vernier tuning principle is applied to the wavelength interleaved 2×N router of the invention. A few of additional input waveguide ports  22  and output waveguide ports  22  are designed to compensate for device processing variations. For example, the second input waveguide port has a few additional waveguide input ports and the corresponding waveguide output ports in FIG. 7 c . When light with odd wavelengths is launched into the first waveguide input port located at −p-th input port and output from the j-th output port, from equation (4), 
     
       
           dn   s sin(−( pΔθ   i +Δθ o /2))+ dn   s sin( jΔθ   o )+ n   c   ΔL=mλ   even   Eq. (13) 
       
     
     When light with odd wavelengths is launched into the second waveguide input port located at −(p+1)-th input port and exits from (j+1)-th output port, the grating equation is 
     
       
           dn   s sin(−( p+ 1)Δθ i −Δθ o /2)+ dn   s sin(( j+ 1)Δθ o )+ n   c   ΔL=m λ′   even   Eq. (14) 
       
     
     By combining the above two equations, the wavelength shift is determined as 
     
       
         δλ even =λ′ even −λ even   =dn   s Δθ o   /m (1−Δθ i /Δθ o )=2Δλ(1−1 /R   v )  Eq. (15) 
       
     
     The same Vernier tuning principle is also applied to the second waveguide input port. Thus, the entire odd wavelength comb and even wavelength comb can be tuned by an incremental amount in a unit of 2(1−1/R v ) Δλ through choosing a pair of different I/O-port. 
     A third and fourth embodiment of the invention is shown in FIGS. 8 a,    8   b  and  8   c  and FIGS. 9 a,    9   b  and  9   c,  respectively, in which two sets of the waveguide inputs  73  and  74  are provided on the same side  69  and share the same arrayed grating waveguides  67 . The designs for the first input port and second input port are the same as that described above. For the odd wavelength channels, the first input port and its corresponding output ports can be placed based on a constant angular spacing. For the even wavelength channels, either the second input port (as shown in FIGS. 8) or the all output ports (as shown in FIGS. 9) are shifted by Δθ o /2  125 . 
     In FIGS. 8 a,    8   b  and  8   c,  a signal at the odd wavelength  75   
     
       
         λ odd =λ 1 +2 jΔλ   
       
     
     is inputted at the first input port  73  with input angle −pΔθ i    97  and will output from ports located at the angle jΔθ o    93  A signal at the even wavelength  76   
     
       
         λ even =λ 2 +2 jΔλ   
       
     
     is inputted at the second input port  74  with angle pΔθ i +Δθ o /2  127  and will output from ports located at the angle −jΔθ o    129 . 
     In FIGS. 9 a,    9   b  and  9   c,  for the first input port  73 , angular spacing Δθ o    95  is designed to satisfy the equation (2). A signal at the odd wavelength  75   
     
       
         λ odd =λ 1 +2 jΔλ   
       
     
     is inputted at the first input port  73  with input angle −pΔθ i    97  and will output from ports located at the angle jΔθ o    93 . A signal at the even wavelength  76   
     
       
         λ even =λ 2 +2 jΔλ   
       
     
     is inputted at the second input port  74  with angle pΔθ i    153  and will output from ports located at the angle −(jΔθ o +Δθ o /2)  157 . 
     While the invention has been described in detail in connection with the preferred embodiments know at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.