Abstract:
The transmission of multiple signals over multimode fiber is accomplished using single-mode transmission lasers and single-mode DWDM (Dense Wave Division Multiplexing) and CWDM (Coarse Wave Division Multiplexing) multiplexers. It also allows for any datarate communication, including high datarate (10 Gbps and faster) signals to be transported over any distance of multimode fiber. This ability will allow institutions that currently have multimode fiber in place, to extend the useful life of the fiber by increasing multimode fiber transmission capacity and thereby reducing overall infrastructure costs.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to fiber optic communications and more specifically, to a system of transmitting and receiving multiple fiber optic signals over multimode optical fiber using DWDM or CWDM multiplexers. 
       BACKGROUND OF THE INVENTION 
       [0002]    Dense wavelength division multiplexing (DWDM) is a method of combining multiple signals that use lasers that use specific wavelengths for transmission along optical fiber. DWDM systems are a popular choice for metro and long-haul access networks on single-mode fiber, and major telecoms have a significant capital investment in the DWDM infrastructure. DWDM is widely deployed by major carriers due to the high density of channels per fiber, and because the distance can be greatly extended by amplifiers. As used herein, DWDM refers to an ITU standard in which the channel spacing of 200, 100, or 50 Ghz is used. 
         [0003]    As used herein, a DWDM signal then would be any modulated optical signal at any datarate at a specific wavelength designed for transport through a DWDM multiplexer. This is different from a DWDM channel which as used herein, is an optical path that a specific wavelength of light travels through a DWDM multiplexer. 
         [0004]    Course wavelength division multiplexing (CWDM) is a method of combining multiple signals that use lasers that use specific wavelengths for transmission along optical fiber, like DWDM. Also like DWDM, CWDM systems are a popular choice for metro networks on a single-mode fiber, and many smaller, regional, networks have a significant investment in CWDM infrastructure. CWDM is widely deployed because it has a lower initial capital investment over DWDM, but its disadvantages are that not as many channels can be deployed on a single fiber, and amplification is difficult, thus they are not used on longer-haul networks. As used herein, CWDM refers to an ITU standard in which the channel spacing is 20 nm from 1271 nm to 1611 nm. 
         [0005]    As used herein, a CWDM signal then would be any modulated optical signal at any datarate at a specific wavelength designed for transport through a CWDM multiplexer. This is different from a CWDM channel which as used herein, is an optical path that a specific wavelength of light travels through a CWDM multiplexer. 
         [0006]    DWDM and CWDM multiplexers are units that are capable of combining or separating numerous signals into and from a common aggregate fiber. There are four current methods of creating a DWDM or CWDM multiplexer. Those methods use: thin film filters (TFF), fiber Bragg gratings (FBG), array waveguide gratings (AWG), and diffraction grating filters (DF). 
         [0007]    The most common is the thin film filter method. These filters allow a single band, or channel, to pass through them. Thus a DWDM or CWDM multiplexer is created by cascading a number of thin film filters together with the desired channel number. This works well for up to around 40 channels and is completely passive. 
         [0008]    The second method of creating a DWDM or CWDM is to use a fiber Bragg grating. This method uses the fiber Bragg gratings and circulators in a similar cascaded method as with the TFF. But this is not as common as thin film filters because of the added cost of putting circulators. FBGs work well with up to 40 channels and are completely passive. 
         [0009]    The third method of creating a DWDM or CWDM multiplexer is to use an array waveguide grating. This method uses an input coupler that splits the optical signal from the common fiber onto an arrayed waveguide. Here the optical signals experience a phase shift that creates an interference pattern at the output coupler, allowing individual channels to be directed to a single fiber. This technology is expensive, but it allows for very narrow channel spacing and high channel counts, and it is completely passive. 
         [0010]    The fourth method of creating a DWDM or CWDM multiplexer is to use a diffraction grating filter. This method uses a diffraction grating to spatially separate out and combine the different wavelengths using the different angles of refraction that different wavelengths make at the grating. This method is expensive as well, but works well for high channel counts, and is completely passive 
         [0011]    Single-mode fiber has been deployed in optical networks since the early 1980s, and it has become the most popular type of fiber for communications further than a kilometer. It serves as a waveguide to the signal, allowing only the first mode of the laser to propagate down its core. Single-mode fiber is distinguished because of its small core size between 7 and 10 micrometers in diameter. 
         [0012]    Multimode fiber has been deployed in networks since 1977 and was the first fiber optic technology to be introduced to the industry. It still maintains popularity among networks where the distances are between 0 and 2000 meters because of the inexpensive nature of the equipment associated with it. Multimode fiber is distinguished by its large core size from 100 to 50 micrometers in diameter. 
         [0013]    Bandwidth requirements have forced network administrators who have multimode networks to scramble to meet those demands. Their only options were to increase the datarate or install more fiber. But both options can be unfeasible due to limitations on distance for higher data rates, or the high costs associated with new installations and the possible destruction of landscaping, parking lots, etc. 
         [0014]    What is needed is a method of multiplexing DWDM or CWDM signals onto a multimode fiber at all data rates. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Examples of the present invention are illustrated in the accompanying drawings. The accompanying drawings, however, do not limit the scope of the present invention. Similar references in the drawings indicate similar elements. 
           [0016]      FIG. 1  is a block diagram of how the DWDM or CWDM multiplexer would be connected to a single multimode fiber to achieve uni-directional communication. 
           [0017]      FIG. 2  is a block diagram of how the DWDM or CWDM multiplexer would be connected to a single multimode fiber to achieve bi-directional communication. 
           [0018]      FIG. 3  is a block diagram of how the DWDM or CWDM multiplexer would be connected to two multimode fibers to achieve bi-directional communication. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The invention described involves optical communications, specifically, it involves a system that integrates single-mode CWDM or DWDM multiplexers onto multimode fiber that will provide for the ability to increase bandwidth without having to replace existing multimode fiber infrastructure. 
         [0020]    In this description, references will be made to the drawings to illustrate particular elements of this invention and the method in which it is implemented. It is to be understood that minor changes in the configuration can be made to the following system without deviating from the scope of this invention. 
         [0021]      FIG. 1  illustrates a simple uni-directional DWDM optical system  100  that includes a single-mode fiber DWDM multiplexer  101  with a common port  102 . At the other end is a single-mode fiber DWDM demultiplexer  103  with a common port  104 . Joining the two common ports  102  and  104  is a multimode fiber  105 . This multimode fiber can be of any core size greater than  10  micrometers in diameter, including 50, 62.5, and 100 micrometers in diameter. The plurality of DWDM signals is generally indicated by  110 , and represents input channels into the DWDM, at any spacing, including 200, 100, 50, or 25 Ghz as defined by ITU documents, but not restricted to these channel spacings. 
         [0022]    In  FIG. 1 , each signal is generated by a single-mode DWDM modulated laser and propagates from  110  at a specific, unique, wavelength. This signal then propagates into the DWDM multiplexer  101  into a port specific to that wavelength over a single-mode fiber. In multiplexer  101 , the signal is multiplexed with the other signals connected to other multiplexer ports at different wavelengths onto the common fiber at the common port  102 . The signal then travels over any length of multimode fiber  105  into common port  104 . Here the signals in the common fiber are broken up into individual signals and exit the de-multiplexer  103  as signal  111  over single-mode fiber in separate ports specific to that signal&#39;s wavelength, where it is received by an optical receiver. 
         [0023]    This concept can be extended to create a bi-directional DWDM optical system as indicated by  FIG. 2 . In this figure, a simple bi-directional DWDM optical system  200  is shown that includes a single-mode fiber DWDM multiplexer  201 , henceforth referred to as “West”, with a common port  202 . At the other end is an identical single-mode fiber DWDM multiplexer  203 , henceforth referred to as “East”, with a common port  204 . Joining the two common ports is any length of multimode fiber  205 . This multimode fiber can be of any core size greater than 10 micrometers in diameter, including 50, 62.5, and 100 micrometers in diameter. The plurality of DWDM signals traveling from “West to East” are generally indicated by  210  and the plurality of DWDM signals traveling from “East to West” are generally indicated by  212 . These signals represent channels spaced at 200, 100, 50, or 25 Ghz as defined by ITU documents, but not restricted to these channel spacings. 
         [0024]    In  FIG. 2 , each signal is generated by a single-mode DWDM modulated laser and propagates from  210  at a specific, unique, wavelength. This signal then propagates into the DWDM multiplexer  201  into a port specific to that wavelength over a single-mode fiber. In multiplexer  201 , the signal is multiplexed with the other signals connected to other multiplexer ports at different wavelengths onto the common fiber at the common port  202 . The signal then travels over multimode fiber  205  into common port  204 . Here the signals in the common fiber are broken up into individual channels and exit the de-multiplexer  203  as signal  211  over single-mode fiber, where it is received by an optical receiver. Then in the other direction, each signal is generated by a single-mode DWDM modulated laser and propagates from  212  at a specific, unique, wavelength. This signal then propagates into the DWDM multiplexer  203  into a port specific to that wavelength over a single-mode fiber. In multiplexer  203 , the signal is multiplexed with the other signals connected to other multiplexer ports at different wavelengths onto the common fiber at the common port  204 . The signal then travels over multimode fiber  205  into common port  202 . Here the signals in the common fiber are broken up into individual channels and exit the de-multiplexer  201  as signal  213  over single-mode fiber, where it is received by an optical receiver. 
         [0025]    It should be noted here that bi-directional communication is possible over one fiber because the light traveling in the fiber is able to travel in both directions. 
         [0026]    Taking the bi-directional system and extending it to a system with two available fibers is indicated by  FIG. 3 . In this figure, a simple bi-directional DWDM optical system  300  is shown using two fibers.  301  is a single-mode fiber DWDM multiplexer and  302  is a single-mode fiber DWDM de-multiplexer. Both of these will be on the “West” side of the system.  301  has a common port  303  and  302  has a common port  304 . On the “East” side is  305 , a DWDM de-multiplexer and  306 , a DWDM multiplexer.  305  has a common port  307 , and  306  has a common port  308 . Joining common ports  303  and  307  is a multimode fiber  309 ; and joining common ports  304  and  308  is a multimode fiber  310 . These multimode fibers can be of any core size greater than 10 micrometers in diameter, including 50, 62.5, and 100 micrometers in diameter. 
         [0027]    The plurality of DWDM signals traveling from “West to East” are generally indicated by  320 , and the plurality of DWDM signals traveling from “East to West” are generally indicated by  321 . These signals represent channels spaced at 200, 100, 50, or 25 Ghz as defined by ITU documents, but not restricted to these channel spacings. 
         [0028]    In  FIG. 3 , each signal is generated by a single-mode DWDM modulated laser and propagates from  320  at a specific, unique, wavelength. This signal then propagates into the DWDM multiplexer  301  into a port specific to that wavelength over a single-mode fiber. In multiplexer  301 , the signal is multiplexed with the other signals connected to other multiplexer ports at different wavelengths onto the common fiber at the common port  302 . The signal then travels over multimode fiber  309  into common port  307 . Here the signals in the common fiber are broken up into individual channels and exit the de-multiplexer  305  as signal  321  over single-mode fiber, where it is received by an optical receiver. Then in the other direction, each signal is generated by a single-mode DWDM modulated laser and propagates from  322  at a specific, unique, wavelength. This signal then propagates into the DWDM multiplexer  306  into a port specific to that wavelength over a single-mode fiber. In multiplexer  306 , the signal is multiplexed with the other signals connected to other multiplexer ports at different wavelengths onto the common fiber at the common port  308 . The signal then travels over multimode fiber  310  into common port  302 . Here the signals in the common fiber are broken up into individual channels and exit the de-multiplexer  301  as signal  323  over single-mode fiber, where it is received by an optical receiver. 
         [0029]    In  FIG. 3 . where a multiplexer and de-multiplexer are on the same side to achieve bi-directional communication over two fibers, it can save space to place the multiplexer and de-multiplexer in the same contained unit. 
         [0030]    It should also be noted that in all three figures, the DWDM multiplexers and de-multiplexers can be replaced with single-mode CWDM multiplexers and de-multiplexers. And as long as the signals entering the multiplexers are of CWDM wavelengths (or certain DWDM wavelengths that work on CWDM channels), the systems will work just as described above for DWDMs.