Abstract:
A method and apparatus is provided for reformatting or interleaving a WDM signal that includes a plurality of optical channels having a first bandwidth and a first channel spacing. The method begins by receiving the WDM signal and dividing it into first and second subsets of optical channels each having a second channel spacing. Next, the first subset of optical channels are divided into third and fourth subsets of optical channels each having a third channel spacing. In addition, the second subset of optical channels is divided into fifth and sixth subsets of optical channels each having a fourth channel spacing. The third and fifth subsets of optical channels are combined to generate a first output WDM signal, while the fourth and sixth subsets of optical channels are combined to generate a second output WDM signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to WDM and DWDM communication systems, and more generally to an optical interleaver employed in such systems. 
   BACKGROUND OF THE INVENTION 
   Optical wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) have gradually become the standard backbone networks for fiber optic communication systems. WDM and DWDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM and DWDM technology. 
   One approach to increasing fiber optic capacity is to use more closely spaced channels. For example, at one point in time, 200 GHz spacing was common for optical channels. At that time optical components were designed to operate on 200 GHz spaced channels. As the state of the art improved, 100 GHz spacing was used for optical channels. Optical components were then designed to operate on 100 GHz spaced channels and devices designed to operate on 200 GHz spaced channels had to be replaced of modified to operate on the 100 GHz spaced channels. This upgrade requirement can be very expensive for parties with an extensive amount of fiber optic equipment that is already deployed. 
   An optical device that can be used for interfacing between different channel spacing schemes is known as an interleaver/deinterleaver, which is essentially an optical router that allows systems designed for operation at a wide channel spacing to be extended to systems designed for narrow channel spacings. In its simplest form, an interleaver combines two sets of channels into one densely packed set with half the channel spacing. Interleavers/deinterleavers are also used for other purposes, such as to add/drop channels at a node in such a way that one interleaver output adds/drops local channels while the other interleaver output forwards express channels to another node. 
   Interleavers that can provide a series of channels with wide passbands are important for increasing the spectral efficiency of optical communication systems. In particular, it is important to increase the ratio of the interleaver&#39;s passband width to the channel spacing. Unfortunately, when the passband of a conventional interleaver is increased, the fall-off (i.e., the slope of the passband sidewalls) also increases. 
   Accordingly, it would be desirable to provide an improved optical interleaver that has an increased passband width relative to its channel spacing. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a method and apparatus are provided for reformatting a WDM signal that includes a plurality of optical channels having a first bandwidth and a first channel spacing. An important advantage of the present invention is that it provides an interleaver arrangement in which the ratio of the passband width relative to the channel spacing is approximately doubled. 
   The method begins by receiving the WDM signal and dividing it into first and second subsets of optical channels each having a second channel spacing. Next, the first subset of optical channels are divided into third and fourth subsets of optical channels each having a third channel spacing. In addition, the second subset of optical channels is divided into fifth and sixth subsets of optical channels each having a fourth channel spacing. The third and fifth subsets of optical channels are combined to generate a first output WDM signal, while the fourth and sixth subsets of optical channels are combined to generate a second output WDM signal. 
   In accordance with one aspect of the invention, the first and second subsets of optical channels are even and odd channels, respectively, of the plurality of optical channels of the WDM signal. Likewise, the third and fourth subsets of optical channels may be even and odd channels, respectively, of the first subset of optical channels, whereas the fifth and six subsets of optical channels may be even and odd channels, respectively, of the second subset of optical channels. 
   In accordance with another aspect of the invention, the second channel spacing is approximately equal to twice the first channel spacing. Also, the third and fourth channel spacings are approximately equal to twice the second channel spacing. 
   In accordance with another aspect of the invention, an interleaver arrangement is provided. The arrangement includes an input interleaver having an input port and at least a pair of output ports and a second interleaver having a second input port coupled to a first of the two output ports of the input interleaver. The second interleaver also has at least a second pair of output ports. The arrangement also includes a third interleaver having a third input port coupled to a second of the two output ports of the input interleaver. The third interleaver also has at least a third pair of output ports. A first optical combiner has a first combiner input port coupled to a first of the second pair of output ports of the second interleaver. The first optical combiner also has a second combiner input port coupled to a first of the third pair of output ports of the third interleaver. A second optical combiner has third and fourth combiner input ports and a second combiner output port. The third combiner input port is coupled to a second of the second pair of output ports of the second interleaver. The fourth combiner input port is coupled to a second of the third pair of output ports of the third interleaver. 
   In accordance with another aspect of the invention, the input interleaver is configured to receive on the input port a WDM signal that includes a plurality of optical channels having a first bandwidth and a first channel spacing and to divide the WDM signal into first and second subsets of optical channels each having a second channel spacing. Likewise, the second interleaver may be configured to receive on the second input port the first subset of optical channels and to divide the first subset of optical channels into third and fourth subsets of optical channels each having a third channel spacing. The third interleaver may be configured to receive on the third input port the second subset of optical channels and to divide the second subset of optical channels into fifth and sixth subsets of optical channels each having a fourth channel spacing. 
   In accordance with yet another aspect of the invention, the first optical combiner is configured to combine the third and fifth subsets of optical channels to generate a first output WDM signal and the second optical combiner may be configured to combine the fourth and sixth subsets of optical channels to generate a second output WDM signal. 
   In accordance with another aspect of the invention, the first and second subsets of optical channels are even and odd channels, respectively, of the plurality of optical channels of the WDM signal. Additionally, the third and fourth subsets of optical channels may be even and odd channels, respectively, of the first subset of optical channels. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates the functionality of a conventional single-stage optical interleaver. 
       FIG. 2  shows an example of a conventional interleaver that is based on a birefringent crystal. 
       FIG. 3  is a block diagram of one embodiment of an optical interleaver arrangement constructed in accordance with the present invention. 
       FIGS. 4 and 5  shows the results of a simulation that was performed in connection with the interleaver arrangement shown in FIG.  3 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates the functionality of a conventional single-stage 1×2 optical interleaver. As shown, interleaver  150  receives on input port  156  a WDM optical signal having a channel spacing of 100 GHz and converts it two signals each having a channel spacing of 200 GHz, which are directed to output ports  152  and  154 . While the device shown in  FIG. 1  can function as both an interleaver and a deinterleaver, it will generally be referred to simply as an interleaver. The deinterleaver separates a subsets of channels. The interleaver mixes subsets of channels. In this example, when functioning as a deinterleaver, the odd channels have a 200 GHz spacing and the even channels have a 200 GHz spacing. When functioning as an interleaver, the even and odd channels having 200 GHz spacing are combined into a signal stream having 100 GHz signal spacing. Similar interleaving can be provided to convert between 50 GHz spaced channels and 100 GHz spaced channels, as well as between other sets of channel spacing schemes. 
   The conversion process depicted in  FIG. 1  is useful, for example, to allow devices designed to operate with an optical channel scheme having 200 GHz channel spacing to interact with other devices or a network designed to operate with an optical channel scheme having 100 GHz channel spacing. Conversion between 100 GHz channel spacing and 200 GHz channel spacing allows, for example, network bandwidth to be increased without upgrading all of the devices that interact with the network. 
   As described in more detail below, the present invention concatenates three or more interleavers to provide an interleaver arrangement that not only converts between channel schemes having different channel spacings, but which also converts between channel schemes having different passbands. The individual interleavers, such as intereaver  100 , that are employed in the invention may be conventional interleavers that are commercially available. While conventional interleavers may be based on a variety of different technologies, all operate on the general principal of an interferometric overlap of two optical beams. The interference produces a periodic repeating output as different integral multiples of wavelengths pass through the device. The desired channel spacing of the device is set by controlling the fringe pattern that is produced. 
   In some embodiments of the invention the individual interleavers that are employed are based on Mach-Zehnder interferometers or Fourier filters, which advantageously can be produced in the form of a planar lightguide circuit. An example of an interleaver employing a Fourier filter is a nonlinear Fourier Filter Flat-top (F 3 T) interleaver such as the WaveProcessor™ Interleaver available from WaveSplitter Technologies. Of course, the invention may also employ interleavers based on more conventional technologies such as the interleaver depicted in  FIG. 2 , which is based on a birefringent crystal. 
     FIG. 2  shows a conventional interleaver that is shown in U.S. Pat. Nos. 5,694,233 and 6,215,923 and which employs a birefringent crystal. As shown, a WDM signal  500  containing two different channels  501  and  502  enters interleaver  999  at an input port  11 . A first birefringent element  30  spatially separates WDM signal  500  into horizontal and vertically polarized components  101  and  102  by a horizontal walk-off. Component signals  101  and  102  both carry the full frequency spectrum of the WDM signal  500 . 
   Components  101  and  102  are coupled to a polarization rotator  40 . The rotator  40  selectively rotates the polarization state of either signal  101  or  102  by a predefined amount. By way of example, in  FIG. 2  signal  102  is rotated by 90 degrees so that signals  103  and  104  exiting rotator  40  are both horizontally polarized when they enter a wavelength filter  61 . 
   Wavelength filter  61  selectively rotates the polarization of wavelengths in either the first or second channel to produce filtered signals  105  and  106 . For example wavelength filter  61  rotates wavelengths in the first channel  501  by 90 degrees but does not rotate wavelengths in the second channel  502  at all. 
   The filtered signals  105  and  106  enter a second birefringent element  50  that vertically walks off the first channel into beams  107  and  108 . The second channel forms beams  109  and  110 . 
   A second wavelength filter  62  then selectively rotates the polarizations of signals  107  and  108  but not signals  109  and  110 , thereby producing signals  111 ,  112 ,  113  and  114 , having polarizations that are parallel each other. A second polarization rotator  41  then rotates the polarizations of signals  111  and  113 , but not  112  and  114 . The resulting signals  115 ,  116 ,  117 , and  118  then enter a third birefringent element  70 . Note that second wavelength filter  62  may alternatively be replaced by a polarization rotator  41  suitably configured to rotate the polarizations of signals  111  and  113  but not  112  and  114 . 
   Third birefringent element  70  combines signals  115  and  116 , into the first channel, which is coupled to output port  14 . Birefringent element  70  also combines signals  117  and  118  into the second channel, which is coupled into output port  13 . 
   As previously mentioned, for many applications it is important to have an interleaver that has a wide passband. Generally, when the passband of a conventional interleaver is increased, the fall-off (i.e., the slope of the passband sidewalls) also increases. The present inventors have found that a series of interleavers can be cascaded in such a way that the passband is increased without a commensurate increase in the falloff of the passband. In particular, as will be depicted below, if three interleavers are employed, the present invention can increase the passband while only increasing the falloff by the amount imparted by the first input interleaver. 
     FIG. 3  is a block diagram of an optical interleaver arrangement  300  constructed in accordance with the present invention for converting an optical channel scheme having 25 GHz spacing to an optical channel scheme having 50 GHz spacing and twice the passband of the original optical channel scheme. In general, interleaver arrangement  300  includes interleaver  310  to convert from one set of 25 GHz spaced channels to two sets of 50 GHz spaced channels. Interleaver arrangement  300  also includes two interleavers  312  and  314 , each of which convert one of the sets of 50 GHz spaced channels to two sets of 100 GHz spaced channels. Individual interleavers  310 ,  312  and  314  may be, for example, any conventional interleavers based on the aforementioned technologies. Interleaver arrangement  300  allows devices designed for 25 GHz spaced channels to interact with devices or networks designed for 50 GHz spaced channels. 
   In operation, optical fiber  302  carries a set of optical channels λ 1  having 25 GHz spacing. Interleaver arrangement  300  separates the set of optical channels into sets of even (2(j+1)) and odd (2j+1) channels. The even channels are input to interleaver  312  and the odd channels are input to interleaver  314 . The even and the odd channels have 50 GHz spacing. 
   Interleavers  312  and  314  operate to further separate the set of optical channels. Conceptually, interleaver  312  and  314  operate on the respective 50 GHz spaced channels to separate the input channels into “even” and “odd” channels. The sets of channels output by interleavers  312  and  314  have 100 GHz spacing. Interleaver  312  separates the even channels into two sets of channels (4J+2) and 4(J+1) output by optical fibers  303  and  304 , respectively. Interleaver  314  separates the odd channels into two sets of channels (4J+1) and (4J+3) output by optical fibers  305  and  306 , respectively. The four sets of channels output by interleavers  312  and  314  are 100 GHz spaced channels. 
   The set of channels output by interleaver  312  on optical fiber  303  and the set of channels output by interleaver  314  on optical fiber  305  and are input to a first combiner  320 , which adds the two sets of channels. Likewise, the set of channels output by interleaver  312  on optical fiber  304  and the set of channels output by interleaver  314  on optical fiber  306  and are input to a second combiner  322 , which also adds the two sets of channels. That is, combiner  320  adds channels (4J+2) and channels (4J+1), while combiner  322  adds channels 4(J+1) and (4J+3). First and second combiners  320  and  322  each may be, for example, a Y-branch or a Fourier filter. 
   Because combiner  320  adds adjacent channels (4J+2) and (4J+1), the resulting channels output by combiner  320  on optical fiber will have twice the passband of the set of optical channels initially received by interleaver  310 . Similarly, the channels output by combiner  322  will also have twice the passband of the set of optical channels initially received by interleaver  310 . Moreover, the channel spacing of the sets of channels output by combiners  320  and  322  on optical fibers  307  and  308  is 50 GHz. That is, the channel spacing is equal to the spacing that was achieved by the first interleaver  310 . 
   One important advantage of the present invention is that the passband of the output signal is increased without increasing the fall-off as much as a conventional interleaver arrangement that would otherwise be used to increase the passband of the channels. For example,  FIGS. 4 and 5  shows the results of a simulation that was performed in connection with the interleaver arrangement  300  shown in  FIG. 3  in which the individual interleavers employ Fourier filters. In  FIG. 4 , the spectra generated on the individual output ports  303 - 306  of interleavers  312  and  314  are shown. In  FIG. 5 , the spectra generated on the outputs  307  and  308  of combiners  320  and  322  are shown. As seen in  FIG. 5 , the passband and the channel spacing have increased by almost a factor of two. 
   In some embodiments of the invention the inventive interleaving arrangement may be formed on a single planar light-guide circuit. Alternatively, the inventive interleaving arrangement may be formed from discrete components. 
   While the interleaving arrangement shown in  FIG. 3  has been described as performing a deinterleaving process, those of ordinary skill in the art will recognize that it can also be used in reverse to perform an interleaving process. In this regard it should be noted because the device is operational in a reciprocal manner, the terms input port and output port as used herein in connection with  FIG. 3  are not limited to ports that transmit a WDM signal or channel in a single direction relative to the interleaver arrangement. In other words, when a WDM signal enters the arrangement from any so-called output port, this output port serves as an input port, and similarly, any so-called input port can equally serve as an output port. 
   One of ordinary skill in the art will also recognize that the present invention is not limited to the use of 1×2 interleavers such as shown in the figures and described above. For example, the principals of the present invention may be readily extended to an interleaver arrangement employing 1×4 interleavers or other interleavers having any number of output ports. Moreover, the inventive arrangement may be cascaded with additional interleavers and combiners to further increase the channel spacing and passband of the resulting output WDM signals.