Abstract:
A demultiplexer system having an interleaver for dividing the incoming wavelength-division multiplexed (WDM) signal into interleaved WDM signals and a single demultiplexing device for demultiplexing the de-interleaved WDM signals into single-channel signals is disclosed. Because a single demultiplexing device is used, size and cost savings can be realized.

Description:
BACKGROUND  
         [0001]    The present invention relates to optical communication devices. More specifically, the present invention relates to optical multiplexer and demultipiexer.  
           [0002]    A wavelength demultiplexer  10 , illustrated in FIG. 1, is a device with one input port and N output ports, N being a number greater than one. The input port receives, from an input fiber (transmission medium) an N-channel wavelength-division multiplexed (WDM) signal  11 . That is, the input WDM signal  11  includes N different channels, each channel modulated onto an optical carrier with a different wavelength. For convenience, each channel is represented as L i  where i ranges from 1 to N. For example, N can be eight, in which case the demultiplexer  10  is referred to as an 8-channel demultiplexer.  
           [0003]    The channels of a WDM signal are spaced in frequency, and the channel spacing is designated, for convenience, as D. The demultiplexer  10  directs each of the N channels (wavelengths) to a different output port, thus separating, or demultiplexing, the input channels.  
           [0004]    A wavelength multiplexer (not illustrated) performs the reverse operation. That is, a multiplexer combines N channels onto a single WDM signal. For simplicity, this document discusses demultiplexers; however, it is understood that the following discussions also apply to multiplexers.  
           [0005]    The demultiplexer  10  is described as an N-channel device because it can demultiplex (separate) a WDM signal having N channels with wavelength spacing D between the channels. Properties (for example, cost, size, and optical loss) of demultiplexers often scale strongly with number of channels and spacing requirement. That is, a multiplexer having larger number of channels or requiring demultiplexing of closely spaced channels require more real estate (larger size), has higher cost, and introduces more signal loss than a multiplexer having relatively smaller number of channels, greater spacing between channels, or both.  
           [0006]    To improve the Properties of a demultiplexing system, interleavers are often used reduce the channel-number requirements and tight-spacing requirements of demultiplexers. An interleaver  20  of FIG. 2 is a three-port wavelength-separation device that divides a single input WDM signal  21  into two de-interleaved WDM signals  23  and  25 , first de-interleaved output signal  23  having the odd channels of the input WDM signal  21  and the second de-interleaved output signal  25  having the even channels of the input WDM signal  21 . For instance, if the input WDM signal  21  includes channels L 1  through L 8 , inclusive, then the first de-interleaved output signal  23  is a WDM signal including the odd channels L 1 , L 3 , L 5 , and L 7 , and the second de-interleaved output signal  25  is a WDM signal including the even channels L 2 , L 4 , L 6 , and L 8 . It is possible to make compact, low-loss interleavers at relatively low cost. For example, one embodiment of an interleaver is described in B. N. Dingel and T. Aruga, “Properties of a novel, noncascaded-type easy-to-design, ripple-free optical bandpass filter,” IEEE Journal of Lightwave Technology, Vol. 7, No. 8, August 1999, pp. 1461-1469.  
           [0007]    One technique to build a wavelength demultiplexing system for large N, small D, or a combination of large N and small D is to use one or more stages of interleavers followed by demultiplexers as shown in FIG. 3. In FIG. 3, a demultiplexer system  30  is illustrated. An interleaver  30  divides an input WDM signal  31  having 8 channels with a first channel spacing D 1  into two 4-channel WDM signals  33  and  35  each having four channels with spacing D 2  where D 2  is twice that of D 1 . As illustrated, each of the 4-channel WDM signals  33  and  35  are demultiplexed by one of the 4-channel demultiplexers  34  and  36 , respectively.  
           [0008]    Here, the number of channels of the input signals (the 4-channel WDM signals  33  and  35 ) for each of the 4-channel demultiplexers  34  and  36  are relatively small compared to the number of channels of the WDM input DJC file  314 - 042  signal  31  (or input signal  11  for the 8-channel demultiplexer  10  of FIG. 1). Moreover, the spacing D 2  between the channels of the input signals (the 4-channel WDM signals  33  and  35 ) is wider than the spacing D 1  of the input WDM 8-channel signal  31  (or input signal  11  of FIG. 1). For these reasons, the demultiplexer system  30  can be fabricated having more desirable properties than the properties of the demultiplexer  10  of FIG. 1.  
           [0009]    However, there is a continuing demand and need for even better demultiplexer systems having even more desirable properties, for example, smaller size requirement.  
         SUMMARY  
         [0010]    The need is met by the present invention.  
           [0011]    According to a first aspect of the present invention, a demultiplexer system includes an interleaver for dividing an input multiplexed signal (having a plurality of channels) into two de-interleaved multiplexed signals (each de-interleaved multiplexed signal having a subset of the channels of the input signal). A single demultiplexing device separates both of the de-interleaved multiplexed signals into single-channel output signals.  
           [0012]    According to a second aspect of the present invention, a method for demultiplexing an N-channel multiplexed signal is disclosed. First, the N-channel multiplexed signal is divided into two M-channel multiplexed signals where M is one half of M. Then, both of the M-channel signals are separated into single-channel output signals using a single demultiplexing device.  
           [0013]    According to a third aspect of the present invention, a demultiplexer system includes a first-stage interleaver dividing an input multiplexed signal (having a plurality of channels) into two de-interleaved multiplexed signals. Then, each of two second-stage interleavers divides one of the de-interleaved multiplexed signals into two twice-de-interleaved multiplexed signals. Finally, a single demultiplexing device demultiplexes the twice-de-interleaved multiplexed signals into single-channel output signals.  
           [0014]    According to a third aspect of the present invention, a demultiplexer system includes a first-stage interleaver dividing an input multiplexed signal (having a plurality of channels) into two de-interleaved multiplexed signals. Then, each of two second-stage interleavers divides one of the de-interleaved multiplexed signals into two twice-de-interleaved multiplexed signals. Finally, a single demultiplexing device demultiplexes the twice-de-interleaved multiplexed signals into single-channel output signals.  
           [0015]    According to a fourth aspect of the present invention, a multiplexer system includes a single multiplexing device for multiplexing a first set of single-channel signals into a first multiplexed signal and a second set of singled-channel signals into a second multiplexed signal. An interleaver combines the first multiplexed signal and the second multiplexed signal into a third multiplexed signal.  
           [0016]    Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in combination with the accompanying drawings, illustrating by way of example the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a diagram of an N-channel demultiplexer;  
         [0018]    [0018]FIG. 2 is a diagram of an interleaver;  
         [0019]    [0019]FIG. 3 is a diagram of an 8-channel demultiplexer system;  
         [0020]    [0020]FIG. 4 is a diagram of an 8-channel demultiplexer system according to one embodiment of the present invention;  
         [0021]    [0021]FIG. 5 is a side view of a single demultiplexing device of FIG. 4;  
         [0022]    [0022]FIG. 6 is a diagram of an 16-channel demultiplexer system according to one embodiment of the present invention; and  
         [0023]    [0023]FIG. 7 is a diagram of an 8-channel multiplexer system according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]    As shown in the drawings for purposes of illustration, the present invention is embodied in apparatus and techniques for fabricating demultiplexer ad multiplexer systems while minimizing the number and coost of the required components.  
         [0025]    [0025]FIG. 4 is a diagram of an 8-channel demultiplexer system  40  according to one embodiment of the present invention. Portions of the system  40  are similar to portions of the demultiplexer system  30  of FIG. 3. For convenience, components in FIG. 4 that are similar to components in FIG. 3 are assigned the same reference numerals and different components are assigned different reference numerals.  
         [0026]    The system  40  includes an interleaver  32  for dividing an input multiplexed signal  31  having a plurality of channels into two de-interleaved multiplexed signals  33  and  35 , each de-interleaved multiplexed signal  33  and  35  having a subset of said plurality of channels. In the illustrated example, the input signal is an 8-channel WDM signal having channels L 1  through L 8 . It is divided into the first de-interleaved multiplexed signal  33  having the odd channels (four channels) including channels L 1 , L 3 , L 5 , and L 7  and the second de-interleaved multiplexed signal  35  having the even channels (four channels) including channels L 2 , L 4 , L 6 , and L 8 . Typically, the number of channels of the de-interleaved signal, referable as M, is half of the number of channels of the input signal  31 .  
         [0027]    Both of the de-interleaved multiplexed signals  33  and  35  are demultiplexed using a single demultiplexing device  42 . The device  42  separates both de-interleaved multiplexed signals  33  and  35  into single-channel output signals. In the demultiplexer system  40 , the single demultiplexing device  42  replaces two demultiplexers  34  and  36  of system  30  of FIG. 3.  
         [0028]    [0028]FIG. 5 is a side view of a single demultiplexing device  42  of FIG. 4 as viewed from line A-A of FIG. 4. As illustrated, the device  42  is implemented using such devices as an Offner spectrograph. The device  42  can be considered as a stacked demultiplexer where its inputs are a one-dimensional array (a vertical column) of multiplexed signals such as signals  33  and  35  and its output is a two-dimensional array of demultiplexed, single-channel signals.  
         [0029]    [0029]FIG. 6 is a diagram of a  16 -channel demultiplexer system  50  according to another embodiment of the present invention. Referring to FIG. 6, input 16-channel WDM signal  51  having wavelength spacing D 3  is demultiplexed by the system  50 . The input signal  51  is divided into two de-interleaved multiplexed signals  61  and  71  by a first-stage interleaver  52 . Each of the de-interleaved multiplexed signals  61  and  71  are 8-channel WDM signals having channel spacing of D 4  where D 4  is twice the value of D 3 .  
         [0030]    A first second-stage interleaver  60  divides the first de-interleaved multiplexed signal  61  into two twice-de-interleaved multiplexed signals  63  and  65 . Each of the twice-de-interleaved multiplexed signals  63  and  65  are 4-channel WDM signals having channel spacing of D 5  that is twice the value of D 4 .  
         [0031]    Likewise, a second second-stage interleaver  70  divides the second de-interleaved multiplexed signal  71  into two twice-de-interleaved multiplexed signals  73  and  75 . Each of the twice-de-interleaved multiplexed signals  73  and  75  are 4-channel WDM signals having channel spacing of D 5  that is twice the value of D 4 .  
         [0032]    A single demultiplexing device  80  demultiplexes all four twice-de-interleaved multiplexed signals  63 ,  65 ,  73 , and  75  into single-channel output signals. Again, the device  80  is, for example, an Offner spectrograph.  
         [0033]    In the above sample embodiments, the present invention is discussed in the context of demultiplexing systems. However, the present invention and techniques are applicable in other context such as a multiplexer.  
         [0034]    In FIG. 7, a multiplexer system  90  is illustrated.  
         [0035]    In the system  90 , a single multiplexing device  92  multiplexes a first set of single-channel signals L 1 , L 3 , L 5 , and L 7  into a first multiplexed signal  93 . The same multiplexing device  92  multiplexes a second set of single-channel signals L 2 , L 4 , L 6 , and L 8  into a second multiplexed signal  95 . The multiplexed signals  93  and  95  are combined by an interleaver  94  into a third multiplexed signal  97  having all eight channels L 1 , L 2 , . . . L 8 .  
         [0036]    From the foregoing, it will be appreciated that the present invention is novel and offers advantages over the current art. Although a specific embodiment of the invention is described and illustrated above, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. For example, differing configurations, sizes, or materials may be used to practice the present invention. The invention is limited by the claims that follow.