Abstract:
An apparatus comprising a wavelength division multiplexing (WDM) coupler configured to couple an optical line terminal (OLT) comprising a transmitter and a receiver, wherein the WDM coupler is coupled to the transmitter via a first fiber and to the receiver via a second fiber. An apparatus comprising a WDM coupler for a passive optical network (PON) comprising a plurality of filters and a plurality of ports, wherein the WDM coupler comprises fewer filters than ports. A method comprising receiving a downstream optical signal intended for an optical network terminal (ONT) via a first fiber, and transmitting an upstream optical signal received from the ONT via a second fiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Patent Application 61/157,483, filed Mar. 4, 2009 by Ning Cheng et al., and entitled “Multi-Fiber Ten Gigabit Passive Optical Network Optical Line Terminal for Optical Distribution Network Coexistence with Gigabit Passive Optical Network,” which is incorporated herein by reference as if reproduced in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not applicable. 
     BACKGROUND 
     A passive optical network (PON) is one system for providing network access over “the last mile.” The PON is a point to multi-point network comprised of an optical line terminal (OLT) at the central office, an optical distribution network (ODN), and a plurality of optical network terminals (ONTs) at the customer premises. In some PON systems, such as Gigabit PON (GPON) systems, downstream data is broadcasted at about 2.5 Gigabits per second (Gbps) while upstream data is transmitted at about 1.25 Gbps. However, the bandwidth capability of the PON systems is expected to increase as the demands for services increase. To meet the increased demand in services, some emerging PON systems are being reconfigured to transport the data frames with improved reliability and efficiency at higher bandwidths, for example at about ten Gbps. The integration and compatibility of such emerging systems with current PON systems is one aspect of the system that is being considered. 
     SUMMARY 
     In one embodiment, the disclosure includes an apparatus comprising a wavelength division multiplexing (WDM) coupler configured to couple an OLT comprising a transmitter and a receiver, wherein the WDM coupler is coupled to the transmitter via a first fiber and to the receiver via a second fiber. 
     In another embodiment, the disclosure includes an apparatus comprising a WDM coupler for a PON comprising a plurality of filters and a plurality of ports, wherein the WDM coupler comprises fewer filters than ports. 
     In yet another embodiment, the disclosure includes a method comprising receiving a downstream optical signal intended for an ONT via a first fiber, and transmitting an upstream optical signal received from the ONT via a second fiber. 
     These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG. 1  is a schematic diagram of an embodiment of a PON system. 
         FIG. 2  is a schematic diagram of another embodiment of a PON system. 
         FIG. 3  is an illustration of downstream and upstream wavelength channels. 
         FIG. 4A  is a schematic diagram of an embodiment of a coupler. 
         FIG. 4B  is a schematic diagram of a plurality of transmission spectrums of the coupler in  FIG. 4A . 
         FIG. 5  is a schematic diagram of another embodiment of a coupler. 
         FIG. 6A  is a schematic diagram of another embodiment of a coupler. 
         FIG. 6B  is a schematic diagram of a plurality of transmission spectrums of the coupler in  FIG. 6A . 
         FIG. 7  is a schematic diagram of another embodiment of a coupler. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     One emerging PON system is the ten Gbps GPON (XGPON), which may have a downstream bandwidth of about ten Gbps and an upstream bandwidth of at least about 2.5 Gbps. The XGPON may be deployed to satisfy higher bandwidth demands than may be allowed by current PON systems. However, to reduce cost and facilitate system and bandwidth upgrade, the XGPON may share some network infrastructure and resources with other PON systems, which may include PON systems that have been previously installed and operated. The integrated XGPON/PON system may support the higher bandwidth demands using XGPON wavelength channels, which may be different than the existing PON wavelength channels. 
     Disclosed herein are systems and apparatus for improving the integration between XGPON systems (or other next generation access (NGA) systems) and current PON systems, such as GPONs. Accordingly, an integrated XGPON/PON system may be used, which may comprise two separate OLTs, e.g. a XGPON OLT and a PON OLT, a shared ODN and a plurality of ONTs. The XGPON OLT and the PON OLT may be coupled to a coupler, e.g. located at the central office (CO) or in the ODN, which may comprise a plurality of filters to switch the different downstream and upstream wavelength channels between the XGPON OLT and a XGPON ONT and between the PON OLT and a GPON ONT. The XGPON OLT may be coupled to the coupler by two separate fibers for downstream and upstream transmissions, where a first fiber may be coupled to a transmitter and a second fiber may be coupled to a receiver in the XGPON OLT. Using two separate fibers instead of a shared fiber for downstream and upstream transmissions in the XGPON OLT may reduce the quantity of filters in the coupler, e.g. by about one, and therefore reduce system cost and losses. Additionally, separating downstream and upstream transmissions into two fibers may cancel the need for a filter in the XGPON OLT to split the downstream and upstream transmissions in a shared fiber. 
       FIG. 1  illustrates one embodiment of a PON system  100 , which may be an integrated XGPON (or NGA) and PON system. The PON system  100  may comprise a first OLT  110  (OLT 1 ), a second OLT  115  (OLT 2 ), a plurality of ONTs  120 , and an ODN  130 , which may be coupled to the first OLT  110  and the second OLT  115  on one side and the ONTs  120  on an opposite side. The PON system  100  may be a communications network that does not require any active components to distribute data between the first OLT  110 , the second OLT  115 , and the ONTs  120 . Instead, the PON system  100  may use the passive optical components, such as a coupler  135 , in the ODN  130  to distribute data between the first OLT  110 , the second OLT  115 , and the ONTs  120 . 
     Some components of the PON system  100 , e.g. the first OLT  110 , the ODN  130 , and at least some of the ONTs  120  may be configured to support current PON technology requirements. For instance, such components may be configured for Ethernet PONs (EPONs) defined by the Institute of Electrical and Electronics Engineers (IEEE) 802.3ah standard, 10 Gigabit EPONs as defined by the IEEE 802.3av standard, asynchronous transfer mode PONs (APONs), broadband PONs (BPONs) defined by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T) G.983 standard, GPONs defined by the ITU-T G.984 standard, or WDM PONs (WPONs), all of which are incorporated herein by reference as if reproduced in their entirety. Additionally, the PON system  100  may comprise some components, e.g. the second OLT  115 , the ODN  130 , and at least one of the ONTs  120 , which may be configured to support a plurality of XGPON (or NGA) bandwidths and wavelength channels. 
     In an embodiment, the first OLT  110  and the second OLT  115  may be any devices that are configured to communicate with the ONTs  120  and possibly another network (not shown). Specifically, the first OLT  110  and/or the second OLT  115  may act as an intermediary between the other network and the ONTs  120 . For instance, the first OLT  110  and/or the second OLT  115  may forward data received from the network to the ONTs  120 , and forward data received from the ONTs  120  onto the other network. When the other network is using a network protocol, such as Ethernet or Synchronous Optical Networking/Synchronous Digital Hierarchy (SONET/SDH), that is different from the PON protocol used in the PON system  100 , the first OLT  110  and/or the second OLT  115  may comprise a converter that converts the network protocol into the PON protocol. The converter may also convert the PON protocol into the network protocol. The first OLT  110  and the second OLT  115  may be typically located at a central location, such as a CO, but may be located at other locations as well. In an embodiment, the distributor coupler  135  may be located with the first OLT  110  and the second OLT  115  at the CO. 
     The specific configuration of the first OLT  110  and the second OLT  115  may vary depending on the types of PONs associated with each OLT. Specifically, the second OLT  115  may be configured to support higher downstream and upstream bandwidth (e.g. for the ONTs  120 ) and different wavelength channels than the first OLT  110 . For instance, the second OLT  115  may be configured for XGPON system bandwidth capabilities and the first OLT  110  may be configured for GPON bandwidth capabilities, as described in detail below. The first OLT  110  and second OLT  115  may each comprise a transmitter and a receiver, which may be configured for the corresponding bandwidth and wavelength requirements. 
     In an embodiment, the ONTs  120  may be any devices that are configured to communicate with the first OLT  110  and/or the second OLT  115  and to a customer or user (not shown). Specifically, the ONTs  120  may act as an intermediary between the first OLT  110  and/or the second OLT  115  and the customer. For instance, the ONTs  120  may forward data received from the first OLT  110  and/or the second OLT  115  to the customer, and forward data received from the customer onto the first OLT  110  and/or second OLT  115 . Although the specific configuration of the ONTs  120  may vary depending on the type of PON system  100 , in an embodiment, the ONTs  120  may comprise an optical transmitter configured to send optical signals to the first OLT  110  and/or second OLT  115  and an optical receiver configured to receive optical signals from the first OLT  110  and/or second OLT  115 . Additionally, the ONTs  120  may comprise a converter that converts the optical signal into electrical signals for the customer, such as signals in the Ethernet or asynchronous transfer mode (ATM) protocol, and a second transmitter and/or receiver that may send and/or receive the electrical signals to a customer device. In some embodiments, ONTs  120  and optical network units (ONUs) are similar, and thus the terms are used interchangeably herein. The ONTs  120  may be typically located at distributed locations, such as the customer premises, but may be located at other locations as well. 
     In an embodiment, the ODN  130  may be a data distribution system that may comprise optical fiber cables, couplers, splitters, distributors, and/or other equipment, which may be passive optical components. Specifically, the optical fiber cables, couplers, splitters, distributors, and/or other equipment may be components that do not require any power to distribute data signals between the first OLT  110 , the second OLT  115 , and the ONTs  120 . Alternatively, the ODN  130  may comprise one or a plurality of active components, such as optical amplifiers. Additionally, the ODN  130  may comprise a distributor coupler  135 , which may be configured to switch the different corresponding downstream and upstream wavelength channels between the first OLT  110 , the second OLT  115 , and the ONTs  120 . The ODN  130  may typically extend from the first OLT  110  and the second OLT  115  to the ONTs  120  in a branching configuration as shown in  FIG. 1 , but may be alternatively configured in any other point-to-multi-point configuration. 
       FIG. 2  illustrates another embodiment of a PON system  200 , which may be an integrated XGPON/GPON system. Accordingly, the PON system  200  may support both GPON and XGPON bandwidths, wavelengths, and other requirements. The PON system  200  may comprise a GPON OLT  210 , a XGPON or NGA PON (NGPON) OLT  215 , at least one GPON ONT  220 , and at least one NGPON ONT  226 . The PON system  200  may also comprise a wavelength division multiplexer (WDM) coupler  235 , which may couple the GPON OLT  210  and the NGPON OLT  215  to the GPON ONT  220  and the NGPON ONT  226  via a splitter  236 . The GPON OLT  210  may communicate with the GPON ONT  220  via a first downstream wavelength channel (DCH 1 ) and a first upstream wavelength channel (UCH 1 ). Additionally, the NGPON OLT  215  may communicate with the NGPON ONT  226  via a second downstream wavelength channel (DCH 2 ) and at least a second upstream wavelength channel (UCH 2 ). The WDM coupler  235  may be configured to switch the different upstream and downstream wavelength channels between the corresponding components of the PON system  200 . Additionally, the PON system  200  may comprise a video transmitter  240 , which may receive a video signal, e.g. from a server, convert the video signal from a radio frequency (RF) signal to an optical signal, and transmit the video signal in a third downstream wavelength channel (DCH 3 ), e.g. to the NGPON ONT  226 . In  FIG. 2 , the downstream and upstream channels are represented using different line types, as indicated in the legend of  FIG. 2 . 
     In an embodiment, the GPON OLT  210  may comprise a first OLT transmitter (TX)  211 , a first OLT receiver (RX)  212 , a first OLT media access control (MAC) module  213 , and an OLT diplex filter  214 . Both the first OLT TX  211  and the first OLT RX  212  may be coupled to the first OLT MAC module  213  and the OLT diplex filter  214 , as shown in  FIG. 2 . The OLT diplex filter  214  may be coupled to the WDM coupler  235  via a fiber. The first OLT TX  211  may be configured to transmit downstream data to the GPON ONT  220  at a first OLT downstream bandwidth, e.g. at about 2.5 Gbps, and a first downstream wavelength channel, e.g. from about 1,480 nanometers (nm) to about 1,500 nm. The first downstream wavelength channel may comprise a continuous range of wavelengths or a set of plurality of discrete wavelengths. The first OLT RX  212  may be configured to receive upstream data from the GPON ONT  220  at a first upstream bandwidth, e.g. of about 1.25 Gbps, and a first upstream wavelength channel, e.g. from about 1,290 nm to about 1,330 nm. The first upstream wavelength channel may also comprise a continuous range of wavelengths or a set of plurality of discrete wavelengths. The OLT diplex filter  214  may be configured to direct the first downstream wavelength channel from the first OLT TX  211  to the WDM coupler  235  and redirect the first upstream wavelength channel from the WDM coupler  235  to the first OLT RX  212 . The first OLT MAC module  213  may be configured to send downstream data to the first OLT TX  211 , receive upstream data from the first OLT RX  212 , and process both the downstream and upstream data. 
     In an embodiment, the NGPON OLT  215  may comprise a second OLT TX  216 , a second OLT RX  217 , and a second OLT MAC module  218 . The second OLT TX  216  in the NGPON OLT  215  may be configured to transmit downstream data to the NGPON ONT  226  at a second downstream bandwidth, e.g. at about ten Gbps, and a second downstream wavelength channel, e.g. from about 1,575 nm to about 1,580 nm. The second downstream wavelength channel may comprise a continuous range of wavelengths or a set of plurality of discrete wavelengths. The second OLT RX  217  may be configured to receive upstream data from the NGPON ONT  226  at a second upstream bandwidth, e.g. at about 2.5 Gbps, and a second upstream wavelength channel, e.g. from about 1,260 nm to about 1,280 nm. The second upstream wavelength channel may also comprise a continuous range of wavelengths or a set of plurality of discrete wavelengths. The second OLT MAC module  218  may be configured to send downstream data to the second OLT TX  216 , receive upstream data from the second OLT RX  217 , and process both the downstream and upstream data. 
     Both the second OLT TX  216  and the second OLT RX  217  may be coupled to the second OLT MAC module  214  and to the WDM coupler  235  using a plurality of fibers instead of a single fiber. Specifically, the second OLT TX  216  may be coupled to the WDM coupler  235  via a first fiber and the second OLT RX  217  may be coupled to the WDM coupler  235  via a second fiber. The cost and insertion loss for using an additional fiber in the PON system  200  may be less than the cost and insertion loss for using a single fiber and an OLT filter. Therefore, using separate fibers to transmit downstream data from the second OLT TX  216  and receive upstream data by the second OLT RX  217  may reduce overall system cost and insertion loss in comparison to using a single fiber and an OLT filter for the same purpose. The two separate fiber configurations may also reduce the quantity of filters needed in the WDM coupler  235 , as described in detail below. Hence, the signal quality (e.g. the bit error ratio (BER)) may also be improved, which may be necessary to meet stringent optical power budget requirements in XGPONs or NGPONs. 
     The WDM coupler  235  may be configured to forward the first downstream wavelength channel from the GPON OLT  210  to the GPON ONT  220  and forward the second downstream wavelength channel from the NGPON OLT  215  to the NGPON ONT  226 . Specifically, the WDM coupler  235  may receive the first downstream wavelength channel and the second downstream wavelength channel via different fibers, combine the two downstream wavelength channels, and send a combined downstream wavelength channel to the splitter  236 . The splitter  236  may then split the combined downstream wavelength channel into at least two substantially similar portions that are directed to each of the GPON ONT  220  and the NGPON ONT  226 . As such, both the GPON ONT  220  and the NGPON ONT  226  may initially obtain the first downstream wavelength channel and the second downstream wavelength channel. However, the receivers in the GPON ONT  220  and the NGPON ONT  226  may detect a different downstream wavelength channel from the two downstream wavelength channels, as described below. The WDM coupler  235  may also receive the video signal from the video transmitter  240 , combine the video signal with the first downstream wavelength channel and the second downstream wavelength channel, and send the combined signals downstream. 
     The WDM coupler  235  may also be configured to forward the first upstream wavelength channel from the GPON ONT  220  to the GPON OLT  210  and forward the second upstream wavelength channel from the NGPON ONT  226  to the NGPON OLT  215 . The splitter  236  may receive the first upstream wavelength channel and the second upstream wavelength channel via different fibers, combine the two upstream wavelength channels, and send a combined upstream wavelength channel to the WDM coupler  235 . The WDM coupler  235  may then split (or demultiplex) the combined upstream wavelength channel into a plurality of upstream wavelength channels, e.g. based on the upstream channel wavelength. The upstream wavelength channels may be directed to the corresponding fibers coupled to the GPON ONT  220  and the NGPON ONT  226 . As such, each of the GPON ONT  220  and the NGPON ONT  226  may receive its designated upstream channel via the corresponding fiber. 
     The GPON ONT  220  may comprise a first ONT TX  221 , a first ONT RX  222 , a first ONT MAC module  223 , a first ONT diplex filter  224 , and a first ONT filter  225 . Specifically, the first ONT diplex filter  224  may be coupled to the WDM coupler  235  via a fiber, to the first ONT TX  221 , and to the first ONT RX  222 . The first ONT filter  225  may be a band pass filter coupled to the first ONT diplex filter  224  and the first ONT RX  222 . Both the first ONT TX  221  and the first ONT RX  222  may be coupled to the first ONT MAC module  223 . Based on the GPON ONT  220  configuration in  FIG. 2 , the first upstream wavelength channel may be forwarded in a first path from the first ONT TX  221  to the first ONT diplex filter  224  and then to the WDM coupler  235 . Additionally, the first downstream wavelength channel may be forwarded in a second path from the WDM coupler  235  to the first ONT diplex filter  224 , the first ONT filter  225 , and then the first ONT RX  222 . 
     The first ONT TX  221  may be configured to transmit upstream data to the GPON OLT  210  at the first upstream bandwidth (e.g. of about 1.25 Gbps) and the first upstream wavelength channel (e.g. from about 1,290 nm to about 1,330 nm). The first ONT RX  222  may be configured to receive downstream data from the GPON OLT  210  at the first downstream bandwidth (e.g. of about 2.5 Gbps) and the first downstream wavelength channel (e.g. from about 1,480 nm to about 1,500 nm). The first ONT MAC module  223  may be configured to send upstream data to the first ONT TX  221 , receive downstream data from the first ONT RX  222 , and process both the downstream and upstream data. The first ONT diplex filter  224  may be configured to direct the first upstream wavelength channel from the first ONT TX  221  to the WDM coupler  235  and redirect the first, the second, and the third downstream wavelength channels from the WDM coupler  235  to the first ONT filter  225 . The first ONT filter  225  may be configured to block from the first ONT RX  222  the second downstream wavelength channel, which may be transmitted from the NGPON  215  through the WDM coupler  235 . The first ONT filter  225  may also be configured to block from the first ONT RX  222  the third downstream wavelength channel (e.g. the video signal), which may be transmitted from the video transmitter  240  through the WDM coupler  235 . As such, the GPON ONT  220  may communicate with the GPON OLT  210  but not the NGPON OLT  215  or the video transmitter  240 . Thus, all PON system  200  components other than the GPON OLT  210  may be transparent to the GPON ONT  220 . 
     The NGPON ONT  226  may comprise a second ONT TX  227 , a second ONT RX  228 , a second ONT MAC module  229 , a second ONT diplex filter  230 , and a second ONT filter  231 . Additionally, the NGPON ONT  226  may comprise a video receiver (V-RX)  232  and a filter  233 . The filter  233  may be a band stop filter coupled to the WDM coupler  235 , e.g. via a fiber, and may be coupled to the second ONT diplex filter  230  and the V-RX  232 . The second ONT diplex filter  230  may be coupled to the second ONT filter  231  and the second ONT TX  227 . The second ONT filter  231  may be a band pass filter coupled to the second ONT RX  228 . Both the second ONT TX  227  and the second ONT RX  228  may be coupled to the second ONT MAC module  229 . Based on the NGPON ONT  226  configuration in  FIG. 2 , the second upstream wavelength channel may be forwarded in a first path from the second ONT TX  227  to the second ONT diplex filter  230 , the filter  233 , and then the WDM coupler  235 . Additionally, the second downstream wavelength channel may be forwarded in a second path from the WDM coupler  235  to the filter  233 , the second ONT diplex filter  230 , the second ONT filter  231 , and then the second ONT RX  228 . 
     The second ONT TX  227  may be configured to transmit upstream data to the NGPON OLT  215  at the second upstream bandwidth (e.g. of about 2.5 Gbps) and the second upstream wavelength channel (e.g. from about 1,260 nm to about 1,280 nm). The second ONT RX  228  may be configured to receive downstream data from the NGPON ONT  215  at the second downstream bandwidth (e.g. of about ten Gbps) and the second downstream wavelength channel (e.g. from about 1,575 nm to about 1,580 nm). The second ONT MAC module  229  may be configured to send upstream data to the second ONT TX  227 , receive downstream data from the second ONT RX  228 , and process both the downstream and upstream data. The second ONT diplex filter  230  may be configured to direct the second upstream wavelength channel from the second ONT TX  227  to the WDM coupler  235 . 
     The filter  233  may be configured to reflect the first and second downstream wavelength channel from the WDM coupler  235  to the second ONT diplex filter  230  and may transmit at least a portion of the video signal in the third downstream wavelength channel (from the video transmitter  240 ) to the V-RX  232 . The V-RX  232  may receive the video signal from the filter  233 , covert the video signal from an optical signal to a RF signal, and forward the RF signal at an output. The second ONT diplex filter  230  may redirect the first and second downstream wavelength channels from the filter  233  to the second ONT filter  231  and the second upstream wavelength channel from the second ONT TX  227  to the filter  233 . The second ONT filter  231  may be configured to transmit the second downstream wavelength channel to the second ONT RX  228  and block the first downstream wavelength channel from the second ONT RX  228 . As such, the NGPON ONT  220  may communicate with the NGPON OLT  215  but not the GPON OLT  210 . In some embodiments, the second ONT filter  231  may also block the video signal of the video transmitter  240  from reaching the second ONT RX  228 . Thus, all PON system  200  components other than the NGPON OLT  215  may be transparent to the second ONT RX  228  in the NGPON ONT  226 . 
     As shown in  FIG. 2 , the two separate fiber configuration between the OLT (e.g. NGPON OLT  215 ) and the coupler (e.g. WDM coupler  235 ) may be implemented in an integrated XGPON/PON system to switch the downstream and upstream data without using filters in the OLT, which may reduce system cost and insertion loss and hence improved signal quality. Additionally, using two fibers to separate downstream and upstream transmissions between the OLT and the coupler may reduce the quantity of filters in the coupler and thus further reduce cost and improve signal reliability. In the case of XGPON communications, the power budget for upstream data transmission may be limited due to higher bit rate requirements in the receiver for burst mode signals. For example, a burst mode receiver may have about two decibels (dB) additional power loss in comparison to a continuous burst mode receiver. As such, a conventional coupler, which may be coupled via a single fiber to the OLT, may comprise a quantity of filters that may have an insertion loss from about one to about three dB. Such insertion loss may limit the XGPON power budget, and thus limit the transmission distance or the number of users in the XGPON. Thus, using a coupler configuration that comprises a reduced quantity of filters may be beneficial or necessary. 
       FIG. 3  illustrates an embodiment of a plurality of downstream and upstream wavelength channels  300 , which may be supported in a PON system, such as the PON system  200 . Specifically, the downstream and upstream channels may be allowed based on the transmission spectrum properties of the components in the PON system. The upstream and downstream wavelength channels  300  may comprise a first upstream wavelength channel  302 , a second upstream wavelength channel  304 , a first downstream wavelength channel  306 , a second downstream wavelength channel  308 , and a downstream video wavelength channel  310 . The transmission spectrums of the PON components may comprise a first upstream transmission spectrum  312 , a second upstream transmission spectrum  314 , a first downstream transmission spectrum  316 , a second downstream transmission spectrum  318 , a third downstream transmission spectrum  319 , and a downstream video transmission spectrum  320 . 
     As shown in  FIG. 3 , the first upstream transmission spectrum  312  may overlap with the first upstream wavelength channel  302  and the second upstream wavelength channel  304 . The second upstream transmission spectrum  314  may substantially overlap with the second upstream wavelength channel  304 . The first upstream wavelength channel  302  may be transmitted from a GPON ONT (e.g. GPON ONT  220 ) and received by a GPON OLT (e.g. GPON OLT  210 ), and may comprise at least one optical signal having a wavelength from about 1,290 nm to about 1,330 nm. The second upstream wavelength channel  304  may be transmitted from a NGPON ONT (e.g. NGPON ONT  226 ) and received by a NGPON OLT (e.g. NGPON OLT  215 ), and may comprise at least one optical signal having a wavelength from about 1,260 nm to about 1,280 nm. 
     Specifically, the first upstream wavelength channel  302  may be transmitted using a first ONT TX (e.g. first ONT TX  221 ) in the GPON ONT via a first upstream port of a WDM coupler (e.g. the WDM coupler  235 ), which may support the first upstream transmission spectrum  312 . The second upstream wavelength channel  304  may be transmitted using a second ONT TX (e.g. second ONT TX  227 ) in the NGPON ONT via a second upstream port of the WDM coupler, which may support the second upstream transmission spectrum  314 . Since the first upstream transmission spectrum  312  may overlap with the first upstream wavelength channel  302 , the first upstream port may allow the first upstream wavelength channel  302  to reach the GPON OLT. Additionally, since the second upstream transmission spectrum  314  may substantially overlap with the second upstream wavelength channel  304 , the second upstream port may allow the second upstream wavelength channel  304  to reach the NGPON OLT. The second upstream port may also block the first upstream wavelength channel  302  from the NGPON OLT, since the second upstream transmission spectrum  314  may not overlap with the first upstream wavelength channel  302 . 
     The third downstream transmission spectrum  319  may overlap with the first downstream wavelength channel  306  and the second downstream wavelength channel  308 . Additionally, the first downstream transmission spectrum  316  may substantially overlap with the first downstream wavelength channel  306 , and the second downstream transmission spectrum  318  may substantially overlap with the second downstream wavelength channel  308 . The first downstream wavelength channel  306  may be transmitted from the GPON OLT and received by the GPON ONT, and may comprise at least one optical signal having a wavelength from about 1,480 nm to about 1,500 nm. The second downstream wavelength channel  308  may be transmitted from the NGPON OLT and received by the NGPON ONT, and may comprise at least one optical signal having a wavelength from about 1,575 nm to about 1,580 nm. 
     Specifically, the first downstream wavelength channel  306  may be transmitted using a first OLT TX (e.g. first OLT TX  211 ) in the GPON OLT via a downstream port of the WDM coupler and may be received in the GPON ONT via a first ONT filter (e.g. first ONT filter  225 ). The second downstream wavelength channel  308  may be transmitted using a second OLT TX (e.g. second OLT TX  216 ) in the NGPON OLT via the downstream port of the WDM coupler and may be received in the NGPON ONT via a second ONT filter (e.g. second ONT filter  231 ). The downstream port of the WDM coupler may support the third downstream transmission spectrum  319 , the first ONT filter may support the first downstream transmission spectrum  316 , and the second ONT filter may support the second downstream transmission spectrum  318 . 
     Since both the third downstream transmission spectrum  319  and the first downstream transmission spectrum  316  may overlap with the first downstream wavelength channel  306 , the downstream port and the first ONT filter may allow the first downstream wavelength channel  306  to reach the GPON ONT. Additionally, since both the third downstream transmission spectrum  319  and the second downstream transmission spectrum  318  may overlap with the second downstream wavelength channel  308 , the downstream port and the second ONT filter may allow the second downstream wavelength channel  308  to reach the NGPON ONT. Further, since the first downstream transmission spectrum  316  may not overlap with the second downstream wavelength channel  318 , the first ONT filter may block the second downstream wavelength channel  308  from the GPON ONT. Similarly, since the second downstream transmission spectrum  318  may not overlap with the first downstream wavelength channel  316 , the second ONT filter may block the first downstream wavelength channel  306  from the NGPON ONT. 
     The third downstream transmission spectrum  319  may also overlap with the downstream video wavelength channel  310  and the downstream video transmission spectrum  320  may substantially overlap with the downstream video wavelength channel  310 . The downstream video wavelength channel  310  may be transmitted from a video server (e.g. at a CO) and received by the NGPON ONT, and may comprise at least one optical signal having a wavelength equal to about 1,550 nm. For instance, the downstream video stream may comprise a plurality of video channels that are transmitted at a plurality of wavelengths around 1,550 nm. Specifically, the downstream video wavelength channel  310  may be transmitted using a video transmitter at the CO (e.g. video transmitter  240 ) in the NGPON OLT via the downstream port of the WDM coupler and may be received in the NGPON ONT via a filter (e.g. filter  233 ), which may support the downstream video transmission spectrum  320 . Since both the third downstream transmission spectrum  319  and the downstream video transmission spectrum  320  may overlap with the downstream video wavelength channel  310 , the downstream port and the filter may allow the downstream video wavelength channel  310  to reach a video receiver in the NGPON ONT. Since the downstream video transmission spectrum  320  may not overlap with the first downstream wavelength channel  316  and the second downstream wavelength channel  318 , the filter may also block the two downstream channels from the video receiver. 
       FIG. 4A  illustrates an embodiment of a coupler  400 A, which may be configured substantially similar to the WDM coupler  235  in the PON system  200  or the coupler  135  in the PON system  100 . The coupler  400 A may comprise a first port (Port 1 ) that may be coupled to a video service provider (e.g. in the CO), a second port (Port 2 ) that may be coupled to a XGPON OLT TX, a third port (Port 3 ) that may be coupled to a XGPON OLT RX, a fourth port (Port 4 ) that may be coupled to a GPON OLT, and at least one fifth port (Port 5 ) that may be coupled to a plurality of ONTs via the ODN. The coupler  400 A may comprise a plurality of filters, which may be diplex filters coupled to the ports and may determine the wavelength channels for each port. Specifically, the coupler  400 A may comprise a first filter  402  (Filter 1 ), a second filter  404  (Filter 2 ), and a third filter  406  (Filter 3 ). 
     The first filter  402  may be coupled to the first port, the fifth port, and the second filter  404 , and may be configured to transmit from the first port to the fifth port a downstream video wavelength channel. The first filter  402  may reflect any other incoming wavelengths, e.g. between the fifth port and the second filter  404 . The second filter  404  may be coupled to the second port, the first filter  402 , and the third filter  406 , and may be configured to transmit from the second port to the first filter  402  a XGPON downstream wavelength channel. The second filter  404  may reflect any other incoming wavelengths, e.g. between the first filter  402  and the third filter  406 . The third filter  406  may be coupled to the third port, the second filter  404 , and the fourth port, and may be configured to transmit from the second filter  404  to the third port a XGPON upstream wavelength channel. The third filter  406  may reflect any other incoming wavelengths, e.g. between the second filter  404  and the fourth port. These reflected wavelengths may comprise a GPON upstream wavelength channel and a GPON downstream wavelength channel. 
       FIG. 4B  shows a plurality of transmission spectrums  400 B that correspond to the filters in the coupler  400 A. The transmission spectrums  400 B may comprise a first transmission spectrum  412  for the first filter  402 , a second transmission spectrum  414  for the second filter  404 , and a third transmission spectrum  416  for the third filter  406 . The first transmission spectrum  412  may comprise a plurality of wavelengths from about 1,550 nm to about 1,560 nm, which may overlap with the downstream video wavelength channel without the other wavelength channels. The second transmission spectrum  414  may comprise a plurality of wavelengths greater than about 1,575 nm or a plurality of wavelengths between about 1,575 nm and about 1,580 nm, which may overlap with the XGPON downstream wavelength channel without the other wavelength channels. The third transmission spectrum  416  may comprise a plurality of wavelengths less than about 1,280 nm or a plurality of wavelengths between about 1,260 nm and about 1,280 nm, which may overlap with the XGPON upstream wavelength channel without the other wavelength channels. 
     Accordingly, the first filter  402  may transmit the downstream video wavelength channel from the first port to the fifth port but may reflect other wavelength channels between the fifth port and the second filter  404 . The second filter  404  may transmit the XGPON downstream wavelength channel from the second port to the first filter  402  and may reflect other wavelength channels between the first filter  402  and the third filter  406 . The third filter  406  may transmit the XGPON upstream wavelength channel, which may be reflected from the second filter  404 , to the third port and may reflect other wavelength channels between the second filter  404  and the fourth port. Additionally, the spectrum gap between the first transmission spectrum  412  and the third transmission spectrum  416  may overlap with the GPON upstream and downstream wavelength channels. Consequently, the GPON downstream wavelength channel and the GPON upstream wavelength channel may be reflected by the first filter  402 , the second filter  404 , and the third filter  406  between the fourth port and the fifth port. 
     The quantity of filters in the coupler  400 A may be less than the quantity of filters used in a conventional coupler that is coupled via a single port or fiber to the OLT. For instance, in the case of a single fiber between the coupler and the OLT for both downstream and upstream channels, the coupler may comprise at least about four filters, where at least about two filters may be coupled in series to exchange XGPON downstream and upstream channels via the single fiber. Since two separate fibers are used to connect the coupler  400 A to the XGPON OLT, one filter (e.g. first filter  402 ) instead of two filters may be used in the coupler  400 A to exchange XGPON downstream and upstream channels. Further, using two separate fibers to separate the XGPON downstream and upstream wavelength channels may eliminate the need for using additional optical elements, e.g. a splitter or diplex filter, at the XGPON OLT to achieve the same purpose. Reducing the quantity of filters and/or optical elements in the PON system may substantially reduce system cost and insertion losses. 
       FIG. 5  illustrates another embodiment of a coupler  500 , which may be configured substantially similar to the WDM coupler  235  in the PON system  200  or the coupler  135  in the PON system  100 . The coupler  500  may be an integrated WDM coupler, e.g. on a chip or optical board, which may have lower production cost and may be more suitable for system integration in comparison to other couplers that comprise discrete components, such as the coupler  400 A. Similar to the coupler  400 A, the coupler  500  may comprise a first port (Port 1 ) that may be coupled to a video service provider (e.g. in the CO), a second port (Port 2 ) that may be coupled to a XGPON OLT TX, a third port (Port 3 ) that may be coupled to a XGPON OLT RX, a fourth port (Port 4 ) that may be coupled to a GPON OLT, and at least one fifth port (Port 5 ) that may be coupled to a plurality of ONTs via the ODN. 
     The coupler  500  may comprise a plurality of filters, which may be diplex filters coupled to the ports and may determine the wavelength channels for each port. The coupler  500  may comprise a first filter  502  (Filter 1 ), a second filter  504  (Filter 2 ), and a third filter  506  (Filter 3 ), which may be configured substantially similar to the corresponding components in the coupler  400 A. As such, the first filter  502  may support the first transmission spectrum  412 , transmit the downstream video wavelength channel from the first port to the fifth port, and reflect other wavelength channels between the fifth port and the second filter  504 . The second filter  504  may support the second transmission spectrum  414 , transmit the XGPON downstream wavelength channel from the second port to the first filter  502 , and reflect other wavelength channels between the first filter  502  and the third filter  506 . The third filter  506  may support the third transmission spectrum  416 , transmit the XGPON upstream wavelength channel reflected from the second filter  504  to the third port, and reflect other wavelength channels between the second filter  504  and the fourth port. The first filter  502 , the second filter  504 , and the third filter  506  may also reflect the GPON downstream wavelength channel and the GPON upstream wavelength channel between the fourth port and the fifth port. 
     Additionally, the filters may be coupled to a plurality of lenses  509  that may be arranged to focus the optical beams that correspond to the downstream and upstream wavelength channels onto the filters and align the optical beams with the corresponding ports. The filters  502 ,  504 ,  506  and the lenses  509  may be integrated on a chip  510 , such as a glass or silicon substrate. Although five lenses  509  are shown in  FIG. 5 , other embodiments of the coupler  500  may comprise any quantity of lenses. The filters and the lenses may also be arranged in any suitable order on the chip  510 . 
     In some systems, the XGPON OLT may use a plurality of upstream wavelength channels to communicate with at least one ONT. In such a case, the coupler may be configured to provide about two upstream wavelength channels to the XGPON OLT.  FIG. 6A  illustrates another embodiment of a coupler  600 A, which may be configured substantially similar to the WDM coupler  235  in the PON system  200  or the coupler  135  in the PON system  100  but provides a plurality of upstream wavelength channels. 
     Similar to the coupler  400 A, the coupler  600 A may comprise a first port (Port 1 ) that may be coupled to a video service provider, a second port (Port 2 ) that may be coupled to a XGPON OLT TX, a third port (Port 3 ) that may be coupled to a XGPON OLT RX, a fourth port that may be coupled to a GPON OLT, and at least one fifth port (Port 5 ) that may be coupled to a plurality of ONTs via the ODN. The coupler  600 A may comprise a first filter  602  (Filter 1 ), a second filter  604  (Filter 2 ), and a third filter  606  (Filter 3 ), which may be configured substantially similar to the corresponding components in the coupler  400 A. Additionally, the coupler  600 A may comprise a sixth port (Port 6 ) that may be coupled to the XGPON OLT TX, and a fourth filter  608  (Filter 4 ) that may be coupled to the sixth port, the second filter  604 , and the third filter  606 . The remaining ports and filters of the coupler  600 A may be arranged and coupled to each other as shown in  FIG. 6A . 
     The first filter  602  may be configured to transmit the downstream video wavelength channel from the first port to the fifth port and reflect any other incoming wavelengths between the fifth port and the second filter  604 . The second filter  604  may be configured to transmit the XGPON downstream wavelength channel from the second port to the first filter  602  and reflect any other incoming wavelengths between the first filter  602  and the fourth filter  608 . The third filter  606  may be configured to transmit a first XGPON upstream wavelength channel from the fourth filter  608  to the third port and reflect any other incoming wavelengths between the fourth filter  608  and the fourth port. The fourth filter  608  may be configured to transmit a second XGPON upstream wavelength channel from the second filter  604  to the sixth port and reflect any other incoming wavelengths between the second filter  604  and the third filter  606 . The first filter  602 , the second filter  604 , the third filter  606 , and the fourth filter  608  may reflect the GPON upstream wavelength channel and the GPON downstream wavelength channel between the fourth port and the fifth port. 
       FIG. 6B  shows a plurality of transmission spectrums  600 B that correspond to the filters in the coupler  600 A. The transmission spectrums  600 B may comprise a first transmission spectrum  612  for the first filter  602 , a second transmission spectrum  614  for the second filter  604 , and a third transmission spectrum  616  for the third filter  606 , and a fourth transmission spectrum  618  for the fourth filter  608 . The first transmission spectrum  612 , the second transmission spectrum  614 , and the third transmission spectrum  616  may be substantially similar to the transmission spectrums of the corresponding components of the coupler  400 A. 
     Additionally, the fourth transmission spectrum  618  may comprise a plurality of wavelengths from about 1,340 nm to about 1,360 nm, which may overlap with the second XGPON upstream wavelength channel without the other wavelength channels. Accordingly, the fourth filter  608  may transmit the second XGPON upstream wavelength channel, which may be reflected from the second filter  604 , to the sixth port and may reflect other wavelength channels between the second filter  604  and the third filter  606 . The spectrum gap between the first transmission spectrum  612  and the fourth transmission spectrum  618  may overlap with the GPON downstream wavelength channel, and the spectrum gap between the fourth transmission spectrum  618  and the third transmission spectrum  616  may overlap with the GPON upstream wavelength channel. Thus, the GPON downstream wavelength channel and the GPON upstream wavelength channel may be reflected by the first filter  602 , the second filter  604 , the third filter  606 , and the fourth filter  608  between the fourth port and the fifth port. 
     The quantity of filters in the coupler  600 A may be less than the quantity of filters used in a conventional coupler that connects to the OLT via a single fiber and that supports two XGPON upstream wavelength channels. For instance, in the case of a single fiber between the coupler and the OLT for both upstream channels and downstream channels, the coupler may comprise at least about six filters, where at least about four filters may be used to exchange XGPON downstream and upstream channels via the single fiber. Instead, two separate fibers are used to connect the coupler  600 A to the XGPON OLT and therefore two filters (e.g. first filter  602  and fourth filter  604 ) may be used instead of four filters to exchange XGPON downstream and upstream channels, which may substantially reduce insertion loss and cost. 
       FIG. 7  illustrates another embodiment of a coupler  700 , which may be an integrated WDM coupler configured substantially similar to WDM coupler  235  in the PON system  200  or the coupler  135  in the PON system  100 . Similar to the coupler  600 A, the coupler  700  may comprise about six ports that may be connected to the same or substantially similar PON components. The coupler  700  may also comprise a first filter  702 , a second filter  704 , a third filter  706 , and a fourth filter  708 , which may be configured substantially similar to the filters in the coupler  600 A and may support the same transmission spectrums and wavelength channels. Additionally, the filters may be coupled to a plurality of lenses  709  that may be arranged to focus the optical beams that correspond to the downstream and upstream channels onto the filters and align the optical beams with the corresponding ports, as shown in  FIG. 7 . The filters  702 ,  704 ,  706 , and  708  and the lenses  709  may be integrated on a chip  710 , such as a glass or silicon substrate, which may have lower production cost and may be more suitable for system integration than the coupler  600 A. Although six lenses  709  are shown in  FIG. 7 , other embodiments of the integrated coupler  700  may comprise any quantity of lenses. The filters and the lenses may also be arranged in any suitable order on the chip  710 . 
     At least one embodiment is disclosed and variations, combinations, and/or modifications of the embodiment(s) and/or features of the embodiment(s) made by a person having ordinary skill in the art are within the scope of the disclosure. Alternative embodiments that result from combining, integrating, and/or omitting features of the embodiment(s) are also within the scope of the disclosure. Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a numerical range with a lower limit, R l , and an upper limit, R u , is disclosed, any number falling within the range is specifically disclosed. In particular, the following numbers within the range are specifically disclosed: R=R l +k*(R u −R l ), wherein k is a variable ranging from 1 percent to 100 percent with a 1 percent increment, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . , 50 percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 97 percent, 98 percent, 99 percent, or 100 percent. Moreover, any numerical range defined by two R numbers as defined in the above is also specifically disclosed. Use of the term “optionally” with respect to any element of a claim means that the element is required, or alternatively, the element is not required, both alternatives being within the scope of the claim. Use of broader terms such as comprises, includes, and having should be understood to provide support for narrower terms such as consisting of, consisting essentially of, and comprised substantially of. Accordingly, the scope of protection is not limited by the description set out above but is defined by the claims that follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated as further disclosure into the specification and the claims are embodiment(s) of the present disclosure. The discussion of a reference in the disclosure is not an admission that it is prior art, especially any reference that has a publication date after the priority date of this application. The disclosure of all patents, patent applications, and publications cited in the disclosure are hereby incorporated by reference, to the extent that they provide exemplary, procedural, or other details supplementary to the disclosure. 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.