Abstract:
A multiplexing or demultiplexing system comprises an optical fiber interface and a concentric spectrometer coupled to the optical fiber interface. The concentric spectrometer multiplexes or demultiplexes optical signals and subjects such optical signals to a relatively low amount of optical aberrations. Thus, using the concentric spectrometer to multiplex or demultiplex optical signals within an optical network helps to reduce spreading of the channels within the optical network.

Description:
BACKGROUND  
       [0001]     Optical networks often employ wavelength division multiplexing (WDM) to transmit multiple optical signals using a single optical transmission path. In WDM, an optical multiplexing system spatially overlaps several quasi-monochromatic component optical signals to form a multi-wavelength optical signal. Each of the component optical signals occupies a wavelength range that does not overlap with the wavelength ranges occupied by the other component optical signals. Each non-overlapping wavelength range occupied by a component optical signal is often referred to as a “channel,” and the wavelength range occupied by the multi-wavelength optical signal is often referred to as the “system bandwidth.” 
         [0002]     If each of the component optical signals were truly monochromatic and had zero wavelength tolerance, then it would theoretically be possible to provide an infinite number of channels within a finite system bandwidth. However, in reality, since each component optical signal is quasi-monochromatic and occupies a range of wavelengths, the system bandwidth can only be divided into a finite number of channels. Additionally, optical scattering properties of light in a transmission medium further increase the wavelength range of a component optical signal thereby increasing the channel bandwidth required to accommodate the component optical signal. An increase in the channel bandwidths to accommodate the increased bandwidths of component optical signals is often referred to as spreading of the channels. Such spreading of the channels further reduces the number of channels available in a given system bandwidth.  
         [0003]     Moreover, in a typical optical network, a conventional optical multiplexing system at a network node spatially overlaps component optical signals in different channels into a multi-wavelength optical signal. The multiplexing system then transmits the multi-wavelength optical signal through an optical transmission path, such as an optical fiber, to another network node. At the other network node, an optical demultiplexing system normally receives the multi-wavelength optical signal and spatially separates the multi-wavelength optical signal into its constituent component optical signals. Note that the term “wavelength division multiplexing,” in a general sense, may be used to refer either to the multiplexing performed at the one network node or the demultiplexing performed at the other network node.  
         [0004]     Unfortunately, conventional wavelength division multiplexing systems introduce optical aberrations, which further increase the wavelength range occupied by each component optical signal. Such increase in the wavelength ranges of the component optical signals necessitates additional spreading of the channels and a further reduction in the number of available channels.  
       SUMMARY  
       [0005]     The present invention generally pertains to a system and method for using a concentric spectrometer to multiplex or demultiplex optical signals.  
         [0006]     Briefly described, one embodiment of a multiplexing or demultiplexing system comprises an optical fiber interface and a concentric spectrometer coupled to the optical fiber interface. The concentric spectrometer multiplexes or demultiplexes optical signals and subjects such optical signals to a relatively low amount of optical aberration. Thus, using the concentric spectrometer to multiplex or demultiplex optical signals within an optical network helps to reduce spreading of the channels within the network. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The invention can be better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the invention. Furthermore, like reference numerals designate corresponding parts throughout the several views.  
         [0008]      FIG. 1  is a block diagram illustrating an optical network.  
         [0009]      FIG. 2  is a block diagram illustrating two of the network nodes depicted in  FIG. 1 .  
         [0010]      FIG. 3  is a flowchart illustrating a multiplexing process performed by a multiplexing system depicted in  FIG. 2 .  
         [0011]      FIG. 4  is a flowchart illustrating a demultiplexing process performed by a demultiplexing system depicted in  FIG. 2 .  
         [0012]      FIG. 5  is a diagram illustrating an exemplary concentric spectrometer that may be used in the multiplexing or demultiplexing systems depicted in  FIG. 2 .  
         [0013]      FIG. 6  is a diagram illustrating a detailed view of the demultiplexing system depicted in  FIG. 2 .  
         [0014]      FIG. 7  is a diagram illustrating a portion of an aberration-corrected concentric diffraction grating for use in the concentric spectrometer depicted in  FIG. 5 .  
         [0015]      FIG. 8  is a diagram illustrating a more detailed view of the multiplexing system depicted in  FIG. 2 .  
     
    
     DETAILED DESCRIPTION  
       [0016]     The present invention generally pertains to a system and method for performing wavelength division multiplexing (WDM). In this regard, a WDM system receives optical signals and uses a concentric spectrometer, which will be described in more detail below, to multiplex or demultiplex the optical signals. Using a concentric spectrometer helps to reduce optical aberrations (e.g., spherical aberrations, astigmatism, distortions due to field curvature, coma, etc.) to which the component optical signals are subject, thereby helping to reduce spreading of the channels. A reduction in the spreading of the channels permits division of the system bandwidth into a greater number of channels.  
         [0017]      FIG. 1  depicts an exemplary optical network  10 . The network  10  has multiple nodes  52  interconnected via optical fibers  110 . Each node  52  may receive an optical signal from any optical fiber  110  coupled to it, and each node  52  may transmit an optical signal to another node  52  through an optical fiber  110  optically coupled to both nodes  52 .  
         [0018]     To increase the information carrying capacity of the network, the nodes  52  incorporate optical multiplexing and demultiplexing systems for performing wave division multiplexing (WDM). As an example, to enable the communication of optical signals between exemplary nodes  53  and  54  using WDM, one of the nodes  54  incorporates a demultiplexing (DEMUX) system  30 , and the other node  53  incorporates a multiplexing (MUX) system  60 , as shown by  FIG. 2 . Both MUX system  60  and DEMUX system  30  incorporate a concentric spectrometer. As shown by  FIG. 3 , the optical multiplexing system  60  receives multiple component optical signals (block  220 ) and spatially overlaps the multiple component optical signals into a single multi-wavelength optical signal (block  230 ) using a concentric spectrometer. After forming the multi-wavelength optical signal, the multiplexing system  60  then directs the multi-wavelength optical signal toward an optical fiber  110 , which transmits the multi-wavelength optical signal to the other node  54 . As shown by  FIG. 4 , the demultiplexing system  30  at this other node  52  receives (block  320 ) and spatially separates (block  340 ) the original component optical signals from one another using a concentric spectrometer. If desired, the node  54  may transmit, using similar techniques, one or more of the component optical signals to one or more other nodes  52  in the network  10 . Moreover, the network  10  routes each optical signal in the foregoing manner until the signal arrives at its destination.  
         [0019]     As will be described in more detail below, the DEMUX system  30  and the MUX system  60  both incorporate a concentric spectrometer for, respectively, spatially separating and spatially overlapping optical signals. A concentric spectrometer is a well-known device commonly used to divide incident light into its spectral components for enabling analysis of the incident light.  
         [0020]     As shown by  FIG. 5 , a concentric spectrometer  100  is composed of a concave reflective device  120  and a convex diffraction grating  130 . The spectrometer  100  is concentric in the sense that the center of curvature of the reflective device  120  and the center of curvature of a convex diffraction grating  130  are substantially co-located. In this regard,  FIG. 5  depicts the radius of curvature  134  of the reflective device  120  and the radius of curvature  136  of the diffraction grating  130 . Point  138  represents the common centers of curvature of the reflective device  120  and the diffraction grating  130 . As shown by  FIG. 5 , each of the radii of curvature  134  and  136  ends at point  138 .  
         [0021]     Substantially co-located centers of curvature distinguish concentric spectrometers  100  from other types of spectrometers (i.e., non-concentric spectrometers). Such co-locating significantly reduces optical aberrations in the spectra generated by a concentric spectrometer compared to spectra generated by non-concentric spectrometers, as will be further explained below.  
         [0022]     The reflective device  120  and diffraction grating  130  are arranged such that light traveling along an optical path  139  is reflected between the reflection device  120  and the diffraction grating  130  multiple times. In particular, the light strikes the concave surface of the reflective device  120 , which reflects (reflection  140 ) and focuses the light toward the diffraction grating  130 . Note that the focal point of the light being reflected by the reflection device  20  may be beyond the diffraction grating  130  such that the light reaches the diffraction grating  130  before reaching the focal point. The diffraction grating  130  diffracts the light toward the reflection device  120 , which again reflects the light (reflection  141 ).  
         [0023]     The reflection of light by any reflective surface introduces optical aberrations. However, when the centers of curvature of the reflective device  120  and the diffraction grating  130  are substantially co-located, the optical aberrations introduced by reflections  140  and  141  tend to cancel each other out. Indeed, depending on various factors, such as the angular relationship between reflective device  120  and diffraction grating  130 , the focal points of reflections  140  and  141 , the distance between reflective device  120  and diffraction grating  130 , and the locations of the centers of curvature of the reflective device  120  and the diffraction grating  130 , it is possible for the optical aberrations introduced by a first reflection  140  to be substantially canceled by the optical aberrations introduced by a second reflection  141 .  
         [0024]     Moreover, to minimize the total optical aberrations introduced by the concentric spectrometer  100 , the reflective device  120  and the diffraction grating  130  may be arranged to maximize the degree to which the optical aberrations introduced by reflection  141  cancels the optical aberrations introduced by reflection  140 . Techniques for arranging two reflective devices within a concentric spectrometer to enhance optical aberration cancellation are generally known in the art.  
         [0025]     For example, it is generally well-known that having approximately a 1:2 ratio between the radii of curvatures of the two reflective devices within a concentric spectrometer helps to enhance cancellation of optical aberrations. Thus, to increase cancellation of optical aberrations, thereby reducing the total optical aberrations introduced by concentric spectrometer  100 , the reflective device  120  and the diffraction grating  130  may be arranged such that the radius of curvature  134  is approximately twice as long as the radius of curvature  136 . However, if desired, other ratios between the radius of curvature of the diffraction grating  130  and the radius of curvature of the reflective device  120  are possible.  
         [0026]      FIG. 6  illustrates a detailed view of an exemplary embodiment of the optical demultiplexing system  30  described above. The demultiplexing system  30  is composed of a concentric spectrometer  100  optically coupled to at least one optical fiber  110  of the network  10  ( FIG. 1 ) and to at least one array  197  of optical fibers of the network  10 . In the embodiment depicted by  FIG. 6 , the concentric spectrometer  100  is coupled to optical fiber interfaces  146  and  147 . The optical fiber interface  146  is coupled to an end of the optical fiber  110  and holds this fiber in a fixed position with respect to diffraction grating  130 , reflective device  140 , and optical fiber interface  147 . The optical fiber interface  147  is coupled to ends of the optical fiber array  197  and holds this array  197  in a fixed position with respect to diffraction grating  130 , reflective device  140 , and optical fiber interface  146 . Note that various known or future-developed devices for receiving and holding ends of optical fibers may be used to implement optical fiber interfaces  146  and  147 . As a mere example, a device commonly referred to as as a “V groove array” may be used to implement optical fiber interfaces  146  and  147 .  
         [0027]     When the optical fiber  110  and the array  197  are respectively coupled to optical fiber interfaces  146  and  147 , as shown by  FIG. 6 , the fiber  110  and the array  197  are precisely aligned with respect to each other and with respect to the concentric spectrometer  100  such that a multi-wavelength optical signal emitted from the fiber  110  is demultiplexed by the concentric spectrometer  100 , as will be described in more detail below, and received by the array  197 . In this regard, the concentric spectrometer  100  receives a multi-wavelength optical signal  155  from the optical fiber  110  coupled to the optical interface  146 . The multi-wavelength optical signal  155  is composed of multiple quasi-monochromatic component optical signals, and the concentric spectrometer  100  spatially separates the received multi-wavelength optical signal  155  into its component optical signals  180 .  
         [0028]     The concave surface  142  of the reflective device  120  is mirrored. The mirrored surface  142  reflects and focuses the multi-wavelength optical signal  155  diverging from the end of fiber  110  toward the diffraction grating  130 . The convex surface  143  of the diffraction grating  130  has a plurality of grooves (not shown in  FIG. 6 ) patterned, as will be described in more detail below, to disperse the multi-wavelength optical signal  155  striking the grating&#39;s surface  143 . Thus, in reflecting the multi-wavelength optical signal  155 , the convex surface  143  of the diffraction grating.  130  disperses the multi-wavelength optical signal  155  thereby angularly separating the multi-wavelength optical signal  155  into its component optical signals  180 . These component optical signals  180  strike the reflective device  120 , which reflects and focuses the component optical signals  180  toward the array  197  of optical fibers  110  coupled to the optical interface  147 . The reflective device  120  and diffraction grating  130  are arranged such that optical aberrations introduced by the reflection of the component optical signals  180  by the reflective device  120  substantially cancel the optical aberrations introduced by the reflection of the multi-wavelength optical signal  155  by the reflective device  120 .  
         [0029]     As described above, by canceling optical aberrations introduced by the reflection of optical signals by the reflective device  120 , the concentric spectrometer  100  introduces significantly less aberration (e.g., less spherical aberration, less astigmatism, less distortion due to field curvature, and less coma) than the non-concentric spectrometers that are conventionally used in optical demultiplexers. By introducing less aberration, the concentric spectrometer  100  can be used to multiplex or demultiplex optical signals of an optical network  10  with less spreading the channels of the network  10 . Thus, using the concentric spectrometer  100  instead of a non-concentric spectrometer to demultiplex or multiplex optical signals within a network  10  permits the fibers of the network  10  to carry a greater number of channels within a given bandwidth.  
         [0030]     Note that, if desired, an optical interface device (not shown) may be positioned to receive the component optical signals  180 . This optical interface device may be structured to selectively route different component optical signals  180  to different optical fibers  110 . For example, such an optical interface device may initially route a component optical signal  180  of a particular wavelength to a first optical fiber  110  of the array  197 . However, at some point, it may be desirable to route the component optical signal  180  of the same wavelength to a different fiber  110  of the array  197 . The optical interface device may comprise an optical switch (not shown) to control which of the optical fibers  110  of the array  197  receives the foregoing component optical signal  180 . In addition, the optical interface device may comprise an optical multiplexer (not shown) to spatially overlap selected ones of the component optical signals  180  into another multi-wavelength optical signal for transmission through optical network  10 .  
         [0031]     Various types of diffraction gratings are known in the art and may be used as the diffraction grating  130  to separate the multi-wavelength optical signal  155  into its component optical signals  180 . Many conventional diffraction gratings have straight grooves that are linearly spaced across the grating&#39;s surface such that the distance between adjacent grooves is substantially constant across the grating&#39;s surface. However, by using a grating having curved grooves instead of straight grooves to diffract the light, it is possible to enhance the optical power of the diffracted light and to reduce aberrations in the diffracted light. Aberrations may be further reduced by spacing the grating grooves non-linearly such that the distance between adjacent grooves varies across the grating surface. Diffraction gratings having curved grooves and/or non-linearly spaced grooves to help reduce optical aberrations shall be referred to herein as “aberration-corrected” gratings. Aberration-corrected gratings and techniques for manufacturing such gratings are described in more detail in U.S. Pat. No. 6,266,140, which is incorporated herein by reference.  
         [0032]     In one exemplary embodiment, the diffraction grating  130  is aberration-corrected and, as shown by  FIG. 7 , has curved grooves  145  etched, milled, or otherwise formed in the surface  143 . As described above, the curved grooves  145  provide additional optical power and help to improve correction of aberrations, compared to diffraction grating surfaces with substantially straight grooves.  
         [0033]      FIG. 8  illustrates a detailed view of an exemplary embodiment of the optical multiplexing system  60  described above with reference to  FIG. 2 . As shown by  FIG. 8 , the optical multiplexing system  60  is composed of a concentric spectrometer  100  optically coupled to an optical fiber  110  and array  197  of optical fibers of the network  10  ( FIG. 1 ). Similar to the optical demultiplexing system  30  of  FIG. 6 , the concentric spectrometer  100  of the multiplexing system  60  is coupled to optical interfaces  146  and  147 , which respectively hold an end of the optical fiber  110  and ends of the array in a fixed position relative to each other and relative to the concentric spectrometer  100 .  
         [0034]     The concentric spectrometer  100  spatially overlaps component optical signals  680  to form a multi-wavelength optical signal  655 . Note that the components of the optical multiplexing system  60  of  FIG. 8  are identical to the components of the optical demultiplexing system  30  of  FIG. 6 . However, the direction of light propagation is reversed for the optical multiplexing system  60 .  
         [0035]     Thus, in operation, an array  197  of optical fibers  110  of the network  10  ( FIG. 1 ) transmits each of the component optical signals  680  towards the reflective device  120 . Note that it is possible for at least one of the fibers  110  of the array  197  to transmit a multi-wavelength optical signal. In such a case, an optical demultiplexer (not shown) may be used to spatially separate the multi-wavelength optical signal into its component optical signals  680 , which are then transmitted to the reflective device  120 .  
         [0036]     The reflective device  120  reflects and focuses the component optical signals  680  toward diffraction grating  130 . In one exemplary embodiment, the diffraction grating  130  is aberration-corrected to further reduce optical aberrations. The patterning of the grooves of the diffraction grating  130  is such that diffraction of the component optical signals  680  by the grating&#39;s surface  143  spatially overlaps the signals  680  into a multi-wavelength optical signal  655 . The multi-wavelength optical signal  655  strikes the reflective device  120 , which reflects this multi-wavelength optical signal  655  toward optical fiber  110 . The optical fiber  110  then transmits the multi-wavelength optical signal  655  to another node  52  of the network  10 . At this network node  52 , an optical demultiplexing system  30  ( FIG. 6 ) may demultiplex the multi-wavelength optical signal  655 , using the techniques previously described above or otherwise, to recover the component optical signals  680 .  
         [0037]     The reflective device  120  and diffraction grating  130  are arranged such that optical aberrations introduced by the reflection of the multi-wavelength optical signal  655  by the reflective device  120  substantially cancel the optical aberrations introduced by the reflection of the component optical signals  680  by the reflective device  120 . Thus, the concentric spectrometer  100  in the optical multiplexing system  60  helps to reduce optical aberrations to which the component optical signals are subject by the demultiplexing process performed by the system.  60 . By reducing aberrations, the concentric spectrometer  100  reduces the spreading of channels, thereby permitting a greater number of channels within the system bandwidth of the optical transmission path, such as the optical fiber  110 , carrying the multi-wavelength optical signal  655 .  
         [0038]     It should be noted that the embodiments of a concentric spectrometer described above have a reflective device  120  that is concave and a diffraction grating  130  that is convex. However, in other embodiments, the concentric spectrometer may have a convex reflective device and a concave diffraction grating.