Abstract:
Methods and apparatus are disclosed for increasing downstream bandwidth of a passive optical network using integrated WDM/power spitting devices, such as the 2P1 devices, and one or more tunable lasers. An optical multiplexing/demultiplexing system is disclosed that comprises an integrated wavelength division multiplexing (WDM)/power spitting device having a WDM passive optical network (PON) and a power splitting PON; and one or more tunable lasers for selectively generating an optical signal of a desired wavelength for at least one subscriber, wherein the optical signal of a desired wavelength is communicated using the WDM PON. A method is also disclosed for communicating optical signals. One or more signals are broadcast to a plurality of subscribers using a power splitting passive optical network (PON). In addition, one or more private signals for at least one subscriber are generated using one or more tunable lasers; and are communicated to the at least one subscriber using a wavelength division multiplexing (WDM) PON.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to optical communication networks, and more particularly, to optical communication networks that include passive components for routing and distributing optical signals.  
       BACKGROUND OF INVENTION  
       [0002]     Optical fiber networks are increasingly important for the distribution of voice, video, and data signals. Such systems generally involve a number of feeder fibers that emanate from a head-end office, and terminate at respective remote terminals. In a Fiber-To-The-Home or a Fiber-To-The-Curb system, optical signals are transmitted from each of these remote terminals to a number of optical network units over distribution fiber. Signals are transmitted optically or electrically to each optical network unit.  
         [0003]     Network architectures have been proposed for transmitting signals between the head-end office and the optical network units.  FIG. 1  illustrates a conventional “2-PONs-In-1” (2P 1) device  100  that uses a passive optical branching architecture to exchange signals between a feeder  10  and distribution fibers  20 - 1  through  20 -n. The 2P1 device  100  is said to combine signal broadcasting with signal distribution. The device  100  is referred to as 2-PONs-In-1 because each function is generally handled by a separate passive optical network (PON).  
         [0004]     U.S. Pat. No. 5,321,541 to Cohen, incorporated by reference herein, discloses a 2P1 device  100 , shown in  FIG. 2 . The disclosed 2P1 device  100  functions transparently as a dense wavelength division multiplexer (DWDM), for example, at 1550 nanometers (nm) and as a power splitter, for example, at 1310 nm. These two wavelength regions are first separated by a coarse wavelength division multiplexer (WDM)  30 . Generally, the 2P1 device  100  overlays a power splitter (PS) PON  50  and a WDM PON  40  on the same optical integrated circuit. The WDM PON  40  can be used to send private signals to each subscriber, while the PS PON  50  can be used simultaneously to broadcast signals. Thus, optical signals in the 1550 nm region are routed around the power splitter  50 , which broadcasts optical signals in the 1310 nm region. These parallel signals are then recombined by coarse WDMs  70  at each output port of the 2P1 device.  
         [0005]     While these disclosed 2P1 devices effectively allow a WDM PON to send private signals to each subscriber, while the PS PON can be used simultaneously to broadcast signals, no practical solution has been proposed for upgrading a power splitting PON to a WDM PON using the 2P1 devices in a practical and scalable manner. A need therefore exists for methods and apparatus for increasing downstream bandwidth of a passive optical network using integrated WDM/power spitting devices, such as the 2P 1 devices, and tunable lasers. A tunable laser allows individual users to be addressed on the PON such that the system cost scales with cumulative bandwidth demand and not each individual user.  
       SUMMARY OF THE INVENTION  
       [0006]     Generally, methods and apparatus are disclosed for increasing downstream bandwidth of a passive optical network using integrated WDM/power spitting devices, such as the 2P1 devices, and one or more tunable lasers. According to one aspect of the invention, an optical multiplexing/demultiplexing system is disclosed that comprises an integrated wavelength division multiplexing /power spitting device having a WDM passive optical network and a power splitting PON; and one or more tunable lasers for selectively generating an optical signal of a desired wavelength for at least one subscriber, wherein the optical signal of a desired wavelength is communicated using the WDM PON.  
         [0007]     According to another aspect of the invention, a method is disclosed for communicating optical signals. One or more signals are broadcast to a plurality of subscribers using a power splitting passive optical network. In addition, one or more private signals for at least one subscriber are generated using one or more tunable lasers; and are communicated to the at least one subscriber using a WDM PON.  
         [0008]     A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS FIG.  1  illustrates a conventional 2P1 device;  
       [0009]      FIG. 2  illustrates a conventional 2P1 device, as disclosed in U.S. Pat. No. 5,321,541 to Cohen;  
         [0010]      FIG. 3  is a schematic block diagram of a conventional power splitter optical multiplexing/demultiplexing system;  
         [0011]      FIG. 4  is a schematic block diagram of a conventional wavelength splitting optical multiplexing/demultiplexing system;  
         [0012]      FIG. 5  is a schematic block diagram of a conventional power splitting optical multiplexing/demultiplexing system;  
         [0013]      FIGS. 6 and 7  illustrate the conventional 2P1 device of  FIGS. 1 and 2  in further detail;  
         [0014]      FIG. 8  illustrates a wavelength allocation map in accordance with the ITU-T standard;  
         [0015]      FIG. 9  illustrates exemplary filter passbands for the 2P 1 devices discussed herein;  
         [0016]      FIG. 10  is a schematic block diagram of an optical multiplexing/demultiplexing system incorporating features of the present invention; and  
         [0017]      FIG. 11  is a sample table illustrating an exemplary DWDM Grid over a given wavelength range. 
     
    
     DETAILED DESCRIPTION  
       [0018]     The present invention provides methods and apparatus for increasing downstream bandwidth of a passive optical network using integrated WDM/power spitting devices and tunable lasers. Among other benefits, the present invention provides wavelength-on-demand to individual subscribers. According to one aspect of the invention, tunable lasers allow one or more subscribers to be selectively addressed using an associated wavelength or range of wavelengths.  
         [0019]      FIG. 3  is a schematic block diagram of a conventional power splitter optical multiplexing/demultiplexing system  300  employing DWDM channels. As shown in  FIG. 3 , the optical demultiplexing system  300  comprises a switch  310  having a wavelength division multiplexing input/output, such as a laser operating at a wavelength, λ i . A power splitter  320  separates the optical signal, λ i , into N different copies of the signal, λ i , and a filter  330  associated with each subscriber filters the optical signal, λ i , to isolate a passband, λ j , that is associated with the subscriber. Note that such an upgrade of DWDM signals onto an existing power splitting PON requires adding such a filter during upgrade.  
         [0020]      FIG. 4  is a schematic block diagram of a conventional wavelength splitting optical multiplexing/demultiplexing system  400 , also referred to as a WDMPON or a wavelength splitter/combiner. Multiple wavelengths, shown in  FIG. 4  as λ i , are launched each with its own time division multiplexed (TDM) traffic (e.g., 1 Gbps) and combined onto a single fiber at a switch  410 . Signals travel over a distribution fiber  415  (SMF, typically up to 20 km) and are then split through a wavelength splitter/router  420 . The wavelength splitter  420  separates the optical signal, λ i , into N different passbands, λ n , each associated with a different subscriber, such as the subscriber  430 . Likewise, signals originating from the end user  430  and matching the filter passband of his or her port of the wavelength splitter/router  420  can be sent upstream through the system to the OLT. The insertion loss of such a wavelength splitter/router is typically 5-6 dB. A Note that because the wavelength splitter  420  separates the wavelengths, no additional filter is required at the user premises. However, use of the more traditional wavelengths present in commercial PONs (e.g., 1310 and 1490 nm) is difficult because of the DWDM splitter.  
         [0021]      FIG. 5  is a schematic block diagram of a conventional power splitting optical multiplexing/demultiplexing system  500 , also referred to as a TDMPON or a power splitter/combiner. As shown in  FIG. 5 , a single wavelength is launched with the combined/aggregate TDM traffic of all users (e.g., 1 Gbps) into a single fiber  515  at the TDM OLT  510 . Signals travel over the distribution fiber  515  (SMF, typically up to 20 km) and are then split through a power splitter/router  520  sending copies of the downstream wavelength to each end user  530 . Likewise, signals originating from the end user  530  and time multiplexed with traffic from other end users via Time Division Multiple Access (TDMA) can be sent upstream through the system  500  to the OLT  510 . The insertion loss of a power splitter can be 3-20 dB depending on split ratio.  
         [0022]      FIGS. 6 and 7  illustrate the conventional 2P1 device  100  of  FIGS. 1 and 2  in further detail. As shown in  FIG. 6 , the exemplary optical demultiplexing system  100  comprises a PON head-end  610  having an associated laser  615  operating at a wavelength of 1490 nm for downstream TDM communications. The head-end  610  also receives upstream TDMA communications of 1310 nm. A 2P1 device  620  separates the optical signal into N different passbands, λ n , each associated with a different subscriber, such as the subscriber  630 .  
         [0023]     As shown in  FIG. 7 , the exemplary 2P1 device  100  functions as a dense wavelength division multiplexer (WDM) around 1550 nanometers, for example, and thus a wavelength range of 1538-1563 nm is applied to the wavelength division multiplexer (WDM) PON  40 , such as a 32 channel Array Waveguide Grating. The 2P1 device also functions as a power splitter at 1310 nm and thus wavelengths of 1310 and 1490 nm (upstream and downstream bands, respectively) are applied to the PS PON  50 . These two wavelength regions are first separated by a coarse WDM  30 , such as a passband filter. The parallel signals are then recombined by coarse WDMs  70  at each output port of the 2P1 device.  
         [0024]     The International Telecommunication Union (ITU) has established a number of standards for PONs, including standards G.983.y and G.984.y. These standards support an “enhancement band” in addition to the upstream/downstream digital bands at 1310 and 1490 nm. For example, the G.983.3 standard provides the following options for the “enhancement band”:  
                                                           G.983.3   λ lower  (nm)   λ upper  (nm)   Service                           Option 1   1550   1560   Analog video           Option 2   1539   1565   DWDM                      
 
 Thus, because of the shared and limited bandwidth of the channels at 1310 and 1490 nm, the present invention recognizes that a bi-directional or unidirectional DWDM overlay can dramatically improve services and security over PON systems. 
 
         [0025]      FIG. 8  illustrates a wavelength allocation map in accordance with the ITU-T standard. As shown in  FIG. 8 , the exemplary upstream band  810  is centered around 1310 nm and the exemplary downstream band  820  is centered around 1490 nm. The present invention recognizes that the enhancement band  830  is available, for example, to provide a WDM function. The potential wavelength ranges for the enhancement band  830  are set forth in Table 2 in  FIG. 8 .  
         [0026]      FIG. 9  illustrates exemplary filter passbands for the 2P 1 devices discussed herein. As shown in  FIG. 9 , the upstream band  810 , downstream band  820  and enhancement band  830  of  FIG. 8  are set forth. The enhancement band  830  is used for WDM communications, in accordance with the present invention. In addition, the passband filter  30  of the 2P1 device ensures that all subscribers receive (and in principle send) the communications having wavelengths below 1519 nm and the wavelength division multiplexer (WDM) PON  40  can provide given wavelength channels to the appropriate subscribers in the enhancement band  830 .  
         [0027]      FIG. 10  is a schematic block diagram of an optical multiplexing/demultiplexing system  1000  incorporating features of the present invention. As shown in  FIG. 10 , the exemplary optical system  1000  comprises a PON head-end  1010  having an associated tunable laser  1015  selectively operating at wavelengths in the enhancement band  830  in the exemplary embodiment for WDM communications. According to one aspect of the invention, the tunable laser  1015  allows one or more subscribers to be selectively addressed using an associated wavelength. The tunable lasers  1015  can be shared over time (TDM) by all subscribers. There is also a filter  1012  that can combine wavelengths from the tunable laser  1015  in the enhancement band with the downstream 1490 nm light. A 2P1 device  1020  separates the optical signal into N different passbands, λ n , each associated with a different subscriber, such as the subscriber  1030 .  
         [0028]     During normal operation, the tunable laser  1015  is off and all end users receive TDM broadcast traffic from the service provider head-end  1010 , while transmitting 1310 nm light upstream using TDMA and scheduling to avoid collisions. When a subscriber requests bandwidth, the tunable laser  1015  (in accordance with a look-up table) transmits at the wavelength associated with the subscriber using the WDM/PS splitter.  
         [0029]     It is noted that, because the subscriber cannot filter the combined enhancement wavelengths and 1490 nm downstream traffic, complete noise would result in a typical WDM/PON overlay system. Due to the nature of the WDM/PS filter, however, which necessarily has unequal insertion losses for the WDM signals versus the PS signals, the DWDM signal will always be approximately 10 dB stronger than the PS signal, thereby overpowering the 1490 nm downstream light (without the need for a filter at each subscriber). Furthermore, the large dynamic range of the subscriber receivers is advantageous, which are designed to specifically for cascaded splitter PONs which have widely varying path losses (up to 20 dB).  
         [0030]     In general, the difference in insertion loss can be obtained as follows: 
 
lossdiff=10 log  N+α   PS    L   eff,PS  −α WDM    L   eff,WDM  
 
 where N is the number of splits, α PS  is the additional path loss per length of power splitter, L eff,PS  is the effective length of the power splitter, α WDM  is the additional path loss per length of WDM splitter, and L eff,WDM  is the effective length of the WDM splitter. 
 
         [0031]     Once the user demand has been fulfilled, the tunable laser  1015  can then serve another subscriber in accordance with his or her demand. It is noted that any subscriber can have up to the full TDM channel capacity without reducing (and, in fact, increasing) the traffic load offered to other users. Even with an additional tunable laser  1015 , each tunable laser  1015  signal would be approximately 7 dB larger than the power splitting traffic. Of notable economic and scaling interest, the tunable laser  1015  does not need to be inserted into a given system until the service provider sees a need for an upgrade, thereby saving deployment costs.  
         [0032]     Among other benefits, the present invention allows power splitting PONs to be upgraded to include WDM PON solutions (without changes to customer premises equipment). In this manner, the disclosed passive optical networks can provide wavelength-on-demand to individual subscribers. It is noted that the data rate of the WDM is equal to or less than that of PS, thus allowing the use of the same (wideband) optical receiver. Furthermore, the tunable lasers  1015  allow the temperature drift normally associated with WDM passband at the power splitter to be countered by actively adjusting the wavelength. In addition, the tunable lasers  1015  can be shared over time by all subscribers.  
         [0033]      FIG. 11  is a sample table  1100  illustrating an exemplary DWDM Grid over 1539-1565 nm. As shown in  FIG. 11 , the table  1100  identifies, for a number of different channel spacings, the number of potential channels, as well as the starting and stopping wavelengths.  
         [0034]     It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention. For example, multiple tunable lasers and cascaded splitters can be employed in the disclosed optical multiplexing/demultiplexing systems, as would be apparent to a person of ordinary skill in the art.