Abstract:
A method and apparatus for executing narrow channel spaced dense wavelength division multiplexing (“DWDM”) includes an optical multiplexor/demultiplexor with channel add/drop having a first circulator and a second circulator. A first fiber Bragg grating couples with the first circulator and the second circulator. A second fiber Bragg grating couples with the second circulator. The fiber Bragg gratings separate optical signals in an interleaved manner. The optical multiplexor/demultiplexor with channel add/drop further includes at least one optical filter in communication with the first circulator. The system can further include at least one optical filter in communication with the second circulator as well. The optical filters are spaced apart a greater distance with respect to channel spacing than the channels passing through the system. However, the unique combination of the circulators and fiber Bragg gratings allow the filters to function at the wider spacing to add/drop channel signals.

Description:
FIELD OF THE INVENTION 
     The invention relates to a system and method for narrow channel spaced dense wavelength division multiplexing, and more particularly relates to a narrow channel spaced dense wavelength division multiplexing system and method using circulators, fiber Bragg gratings, and staggered optical filters to operate with greater efficiency. 
     BACKGROUND OF THE INVENTION 
     Conventional optical communications systems employ optical fibers as transmission mediums. Each optical fiber can carry more than one optical signal at a time. In order to maximize the amount of information that is sent over such optical fibers, conventional optical communications systems typically transmit multiple optical signals concurrently over a single optical fiber. Each optical signal is a modulated signal at a particular wavelength. As will be described below, conventional optical communications systems use multiplexing and demultiplexing to transmit the multiple optical signals. 
     Typical optical communications first multiplex a collection of separate signal channels (e.g., wavelengths) into a single transmission medium (e.g., optical fiber). The medium then carries the multiplexed signal channels from an origination point to a destination point. The systems then demultiplex, or separate each of the signal channels back into their original state, at the destination point of the transmission medium. 
     Conventional optical communications systems often employ add and drop capabilities. Individual channels may be added or dropped from the multiplexed transmission medium at any point between the origination of the signal channels and the destination point. Channel add/drop must be able to add individual signal channels and remove individual signal channels as desired. 
     The technology of wavelength division multiplexing experiences some level of through traffic signal loss. One additional desire in multiplexing technology is to multiplex/demultiplex, and add/drop channels in the most efficient manner possible, so as to reduce the level of through traffic signal loss as much as possible. 
     SUMMARY OF THE INVENTION 
     There exists in the art a need for a system and method to perform narrow channel spaced dense wavelength division multiplexing (“DWDM”) in a more efficient manner. The present invention provides an efficient mechanism for DWDM that is especially useful at add/drop nodes of an optical communications network. An optical multiplexor/demultiplexor with channel add/drop, in accordance with one aspect of the present invention, includes a first circulator and a second circulator. A first fiber Bragg grating couples with the first circulator and the second circulator. A second fiber Bragg grating couples with the second circulator. The fiber Bragg gratings separate optical signals in an interleaved manner. 
     The optical multiplexor/demultiplexor with channel add/drop, according to another aspect of the present invention, further includes at least one optical filter in communication with the first circulator. The system can further include at least one optical filter in communication with the second circulator as well. The optical filters are spaced apart a greater distance with respect to channel spacing than the channels passing through the system. However, the unique combination of the circulators and fiber Bragg gratings allow the filters to function at the wider spacing to add/drop channel signals. 
     The system, according to further aspects of the present invention, has 50 GHz fiber Bragg gratings, in combination with 100 GHz optical filters, as one embodiment. The arrangement of the fiber Bragg gratings and the circulators enables the use of the 100 GHz optical filters for more narrowly spaced signal channels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned features and advantages, and other features and aspects of the present invention, will become better understood with regard to the following description and accompanying drawings, wherein: 
     FIG. 1 is a schematic illustration of a narrow channel spaced DWDM demultiplexor with channel drop according to one aspect of the present invention; 
     FIG. 2 is a schematic illustration of the structure of FIG. 1 serving as a narrow channel spaced DWDM multiplexor with channel add according to one aspect of the present invention; 
     FIG. 3 is a narrow channel spaced DWDM channel add/drop according to one aspect of the present invention; and 
     FIG. 4 is a narrow channel spaced DWDM multiplexor/demultiplexor with channel add/drop according to one aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention generally relates to the use of circulators, fiber Bragg gratings, and staggered optical filters to combine in a unique and efficient manner enabling the configuration of a channel multiplexor/demultiplexor and channel add/drop device. The system and method provides for the separation of channels within the feasible limit of fiber Bragg grating technology (i.e., 50 GHz—a grating suitable for filtering channels spaced 50 GHz apart), while utilizing wider optical filters (i.e., thin film filters at 100 GHz—a filter suitable for filtering channels spaced 100 GHz apart), which are less expensive. The combination of the circulators with the fiber Bragg gratings reduces the overall number of optical circulators required. The result is reduced loss of through traffic while dropping/adding a number of channels and maintaining narrow channel spacing. The configuration utilizes relatively wider band optical filters combined with fiber Bragg grating technology to separate channels in an interleaved fashion. This enables the use of the less expensive filters because the signals are separated in an alternating/interleaving fashion to provide greater spacing between adjacent signals as they enter the filters. The use of wider optical filters also contributes to the reduced amount of signal loss. 
     FIGS. 1 through 4, wherein like parts are designated by like reference numerals throughout, illustrate example embodiments of circulators combined with fiber Bragg gratings and staggered optical filters according to the present invention. Although the present invention will be described with reference to the example embodiments illustrated in the figures, it should be understood that many alternative forms can embody the present invention. One of ordinary skill in the art will additionally appreciate different ways to alter the parameters of the embodiments disclosed, such as the size, shape, or type of elements or materials, in a manner still in keeping with the spirit and scope of the present invention. 
     FIG. 1 illustrates a channel drop/terminal demultiplexor  10  according to one embodiment of the present invention. The input channel signal enters at arrow I. The channel signal proceeds to the first circulator  12 . The first circulator  12 , in this arrangement, is a three-port circulator. Circulators, in general have several ports. Each circulator can take in a signal at any one port. The signal travels around the circulator to the next port along the perimeter, and exits through that port. The signals travel around the circulator in either a clockwise or a counterclockwise direction, depending on the circulator design. 
     The signal, in the illustrated embodiment, proceeds around the circulator  12  and out toward the fiber Bragg gratings  14 . The fiber Bragg gratings reflect predetermined channel signals back in the direction from which they entered, and other channel signals pass through the fiber Bragg gratings  14  and proceed in the original direction along the path. The reflected channel signals reflect back to re-enter the circulator  12 , travel around the circulator  12  and exit the circulator  12  at the next port. 
     In FIG. 1, signals with wavelengths λ 1 , λ 3 , and λ 5  are the signals that are reflected back in the direction of arrow R 1 . The fiber Bragg gratings  14 , in accordance with aspects of the present invention, reflect back channel signals in an interleaved fashion. The odd “λ” wavelengths are an indication that every other channel signal wavelength reflects back, while intermediate interleaved channel signals, i.e., between those reflected back (the even “λ” wavelengths), proceed along the path. The result of taking out every other wavelength is that there is greater spacing between the remaining wavelengths (λ 2 , λ 4 , and λ 6 ), and there is greater spacing between the reflected wavelengths (λ 1 , λ 3 , and λ 5 ). The greater spacing between each wavelength in each group provides for better channel separation and the ability to pass the signals through larger, less costly, filters. 
     It should be noted that the “λx” notation does not refer to the actual wavelength number and its condition of being odd or even, but rather the notation is an indication of the interleaving, or alternating manner by which otherwise neighboring wavelengths are separated out from their multiplexed state. 
     The reflected channel signals (having wavelengths λ 1 , λ 3 , and λ 5 ) enter the circulator  12  and exit the circulator once again toward the filters  16  and  17 . A first filter  16  receives the signals and pulls out only the λ 1  wavelength channel signal, allowing the λ 3 , and λ 5  wavelength channel signal to pass through. A second filter  17  receives the remaining channel signal and pulls out the signal having wavelength λ 3 , allowing the λ 5  wavelength channel signal to pass through. The λ 5  channel signal is all that remains at this point, and thus there is no need for a third filter. 
     While the channel signals having wavelengths λ 1 , λ 3 , and λ 5  are reflecting back toward the circulator  12 , those predetermined channel signals that pass through the fiber Bragg gratings  14  (those having wavelengths other than λ 1 , λ 3 , or λ 5 ) continue to the second circulator  18 . These channel signals exit the circulator  18  and channel signals with predetermine wavelengths reflect off of the second set of fiber Bragg gratings  20 . The fiber Bragg gratings  20 , in this instance, reflect back those signals having wavelengths of λ 2 , λ 4 , and λ 6 , while allowing any remaining signals to pass through in the direction of through traffic arrow T. 
     The reflected signals (with wavelengths λ 2 , λ 4 , and λ 6  in this embodiment) reflect back in the direction of arrow R 2  through the second circulator  18  and exit the circulator  18  in the direction of the filters  22 . The first filter  22  receives the signals and pulls out the signal having wavelength λ 2 . The signals continue on and the second filter  23  receives the signals and pulls out the signal having wavelength λ 4 , leaving the remaining signal having wavelength λ 6  to proceed. Again, there is no need for a third filter because all that is left at this point is the signal having a wavelength of λ 6 . 
     Those of ordinary skill in the art are aware of the circulators  12  and  18  utilized herein. An example circulator appropriate for this arrangement is the CR 5500 series 3 port optical circulator manufactured by JDS-Uniphase, but the present invention is not limited to only this form of circulator. 
     FIG. 2 illustrates a channel add or terminal multiplexor  11 . The channel add/terminal multiplexor  11  has the same structure as the channel drop/terminal demultiplexor  10 , however the signals are routed in the reverse direction. In FIG. 2, signals enter through the first set of filters  16  and  17  and thus have wavelengths of λ 1 , λ 3 , and λ 5 . The illustration does not indicate a filter for the λ 5  wavelength, but one may be included if necessary. Alternatively, the signals can enter at the same point as the illustrated filters, but already in the desired wavelengths thus omitting the need for the filters. 
     Other signals enter through filters  22  and  23 , and thus have respective wavelengths λ 2 , λ 4 , and λ 6 , as shown in FIG.  2 . Again, the wavelengths indicated in the illustrated embodiments are merely representative of possible wavelengths. The actual wavelengths may vary, but the relationship of the wavelengths to each other, i.e., the interleaved arrangements, is maintained. 
     Signals entering through, and from the direction of, filters  22  and  23  enter the circulator  18  and exit through the next port to the fiber Bragg gratings  20 , which reflect those signals having wavelengths λ 2 , λ 4 , λ 6  back in the direction of arrow R 2 . At this point, the reflected signals are traveling in the same direction as through traffic T back into the circulator  18  and exiting the circulator toward the direction of the second set of fiber Bragg gratings  14 . The signals having wavelengths λ 2 , λ 4 , and λ 6 , in addition to any additional through traffic signals, pass through the fiber Bragg gratings  14  and enter the circulator  12 , which they then exit in the direction of output arrow O. 
     Simultaneously, the signals having wavelengths λ 1 , λ 3 , and λ 5  enter the circulator  12  from the direction of filters  16  and  17 , and exit the circulator  12  at the next port toward the fiber Bragg gratings  14 . The signals having wavelengths λ 1 , λ 3 , and λ 5  are reflected back by the fiber Bragg gratings  14  in the direction of arrow R 1  into the circulator  12 . The signals travel around the circulator  12  and exit in the direction of output arrow O. 
     Any through traffic passes through the fiber Bragg gratings  20 , the circulator  18 , the fiber Bragg gratings  14 , and the circulator  12  before exiting in the direction of output arrow O. The added signals having wavelengths λ 1 , λ 2 , λ 3 , λ 4 , λ 5 , and λ 6  are all added, via the mechanism described, to the through traffic. 
     FIG. 3 illustrates a combination of the arrangements of FIGS. 1 and 2 to form a simultaneous add/drop functionality. Through traffic enters in the direction of T 1  into circulator  70 . The signal exits the circulator  70  to the fiber Bragg grating  72  where, in this instance, channel signals with odd wavelengths reflect back in the direction of arrow R 1  to the circulator  70  while the channel signals with even wavelengths pass through the fiber Bragg grating  72 . The reflected channel signals with odd wavelengths enter the circulator  70  and re-exit the circulator  70  at the next port toward the odd drop filters  74 . The odd drop filters  74  remove the channel signals with odd wavelengths. 
     The channel signals with even wavelengths, as previously mentioned, pass through the fiber Bragg grating  72  into the circulator  76 . The channel signals with even wavelengths then exit the circulator  76  at the next port and in the direction of the fiber Bragg gratings  78 . The fiber Bragg gratings  78 , in this instance, reflect channel signals with even wavelengths back in the direction of arrow R 2  toward the circulator  76 . The channel signals with even wavelengths exit the circulator  76  in the direction of the even drop filters  80 . The even drop filters  80  then remove the channel signals with even wavelengths. 
     Channel signals with odd wavelengths enter through the odd add filters  82  into the circulator  76 . The channel signals with odd wavelengths continue around the circulator  76 , exiting toward the fiber Bragg gratings  72 . The fiber Bragg gratings  72 , in this instance, are odd channel gratings and they reflect the channel signals with odd wavelengths back in the direction of the circulator  76 . The channel signals with odd wavelengths continue around the circulator  76 , exiting at the next port in the direction of the fiber Bragg gratings  78 . The fiber Bragg gratings  78 , in this instance, reflect channel signals with even wavelengths. Therefore the channel signals with odd wavelengths pass through the fiber Bragg gratings  78  and into the circulator  84 . 
     Channel signals with even wavelengths enter through the even add filters  86  into the circulator  84 . The channel signals with even wavelengths exit the circulator  84  in the direction of the fiber Bragg gratings  78 . The fiber Bragg gratings  78  reflect the channel signals with even wavelengths back into the circulator  84 , combining the signals with the channel signals of odd wavelengths, both of which exit in the direction of through traffic arrow T 2 . 
     Through use of the interleaving fiber Bragg gratings  72  and  78 , and the circulators  70 ,  76 , and  84 , this arrangement as taught by the present invention provides for an efficient channel add/drop device with minimal through loss and relatively low cost. The system utilizes commonly available components in a unique arrangement to manipulate narrow spaced signals with wider spaced filters. 
     FIG. 4 illustrates the use of a three-port odd/even interleaving device to further decrease channel spacing. The through traffic signal enters in the direction of T 1  to the odd/even interleaver  24 , which separates the signal into two categories of wavelengths. The two categories are illustrated as whole-number and half-number wavelengths to demonstrate the ability to handle signals of even narrower spacing than in the previous embodiments. Again, the actual notation of a whole-number or a half-number does not directly correlate to characteristics of the actual signals. These are merely illustrative tools to indicate different wavelength spacings. 
     The whole-number wavelength channel signals proceed in the direction of arrow A and the half-number wavelength channel signals proceed in the direction of arrow B. 
     The whole-number wavelength channel signals first enter the circulator  26  and exit through the next port toward the fiber Bragg gratings  28 , which are odd channel fiber Bragg gratings. The channel signals with odd wavelengths reflect back toward the circulator  26  and all other channel signals proceed toward circulator  32 . Those odd channels that are reflected back toward circulator  26  enter the circulator  26  and exit toward the odd drop filters  30 . The odd drop filters  30  remove the channel signals with odd wavelengths. 
     Those signals not reflected by the fiber Bragg gratings  28  enter the next circulator  32  and exit through the next port toward the fiber Bragg gratings  34 , which reflect channel signals with even wavelengths. The channel signals with even wavelengths pass back through the circulator  32  and exit toward the even drop filters  36 . The even drop filters  36  filter and remove the channel signals with even wavelengths. 
     As with the removal of the channel signals with even wavelengths, any channel signals with odd wavelengths can be added through the odd add filters  38  into the circulator  32 . The channel signals with odd wavelengths exit the circulator  32  toward the fiber Bragg gratings  28 , which reflect the odd channels back to the circulator  32 . The channel signals then exit the circulator  32  through the fiber Bragg gratings  34 , which reflect channel signals with even wavelengths. The channel signals with odd wavelengths pass through the fiber Bragg gratings  34  and enter the circulator  40 . 
     The even add filters  42  add channel signals with even wavelengths to the circulator  40 . The channel signals with even wavelengths proceed around the circulator  40  to the next port to exit toward the fiber Bragg gratings  34  and reflect back to the circulator  40 . The channel signals with even wavelengths then combine with the channel signals having odd wavelengths and all signals exit the circulator  40  toward the odd/even interleaver  44 . 
     The channel signals with half-number wavelengths, which exit the odd/even interleaver  24  in the direction of arrow B proceed to the circulator  46  and exit the circulator toward the fiber Bragg gratings  48 . The fiber Bragg gratings  48  reflect the channel signals with odd wavelengths back into the circulator  46 , allowing channel signals with even wavelengths to proceed through. The channel signals with odd wavelengths enter the circulator  46  and exit toward the odd drop filters  50 , which subsequently remove the channel signals with odd wavelengths. The channel signals with even wavelengths continue on and enter the circulator  52 , exiting toward the fiber Bragg gratings  54 . The fiber Bragg gratings  54 , in this instance, reflect the channel signals with even wavelengths back into the circulator  52 . The channel signals with even wavelengths one again enter the circulator  52  and exit toward the even drop filters  56 , which remove the channel signals with even wavelengths. 
     The odd channel filters  58  add channel signals with odd wavelengths to the circulator  52 , which exit the circulator  52  toward the fiber Bragg gratings  48  and reflect back to re-enter the circulator  52 . The channel signals with odd wavelengths exit the circulator  52  again, this time toward the fiber Bragg gratings  54 , and pass through the gratings  54  to the circulator  60 . 
     Even add filters  62  provide channel signals with even wavelengths to the circulator  60 , which then exit the circulator  60  in the direction of the fiber Bragg gratings  54 . The channel signals with even wavelengths reflect back from the fiber Bragg gratings  54  to re-enter the circulator  60  and combine with the channel signals having odd wavelengths to exit the circulator  60  toward the odd/even interleaver  44 . 
     These half-number signals combine with the previous whole-number signals from the other side of the system and exit in the form of through traffic in the direction of arrow T 2 . 
     This approach utilizes the wider band filters (e.g., 100 GHz) combined with fiber Bragg grating technology to separate channels in an interleaved fashion. The channels can then be further separated utilizing standard filter technology (e.g., thin film, AWG). After passing through an optical circulator, channels are selected in a staggered or noncontiguous order utilizing a series of narrow fiber Bragg gratings suitable for a given channel plan. This means the gratings have high adjacent channel isolation but are spaced relatively far apart in the frequency domain. For example, a 50 GHz channel plan can use a series of very narrow gratings spaced 100 GHz apart. The reflected channels then pass backward through the circulator to be separated by wider band optical filters. Utilizing this example, the filters are intended to work on a 100 GHz channel plan. Through traffic is then fed into a second circulator, followed by a similar series of gratings. These gratings are offset from the previous series in order to select the remaining channels. The reflected channels then pass backward through the second circulator to be separated by appropriate wider band filters. Typical configurations use an odd/even type splitting of the channels. This approach is useful for channel add/drop of terminal multiplexing/demultiplexing, and provides an efficient and cost effective solution. 
     Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode for carrying out the invention. Details of the structure may vary substantially without departing from the spirit of the invention, and exclusive use of all modifications that come within the scope of the appended claims is reserved. It is intended that the invention be limited only to the extent required by the appended claims and the applicable rules of law.