Abstract:
A passive optical network which employs multiple wavelengths to increase overall system bandwidth, with each wavelength being shared by multiple optical network units (ONUs) according to a time division multiple access (TDMA) protocol. The upstream TDMA traffic therefore includes multiple TDMA data streams at different wavelengths. An optical line terminal (OLT) preferably receives the multiple TDMA data streams and separates them to different detectors before ultimately combining all data into a single data stream using a multiplexer after performing clock and data recovery functions. In this manner, the upstream bandwidth in a passive optical network can be markedly increased without requiring an increase in data transmit speeds, and while using low cost/low speed detectors in the OLT, and low cost/low speed transceivers in the ONUs. System bandwidth can be further improved by using higher cost, higher speed components.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application No. 60/155,892 filed Sep. 27, 1999, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates primarily to passive optical networks and, more particularly, to passive optical networks utilizing multiple TDMA data streams of different wavelengths over the same optical transmission line for increased bandwidth and/or reduced system complexity and cost. 
     BACKGROUND OF THE INVENTION 
     Point-to-multipoint passive optical networks are commonly deployed in telecommunications and cable television local access networks. As shown in  FIG. 1A , a typical passive optical network includes an optical line terminal (OLT), often located in a Central Office, and multiple optical network units (ONUs) which are usually located near end-users/subscribers for delivering information (such as voice, data and/or video) services. One or more passive optical splitters are commonly used to distribute signals to/from the OLT from/to the ONUs. Because these optical networks do not require an optical-to-electrical-to-optical conversion at each node, but rather rely on passive optical components, they are considered more reliable than active optical networks. 
     In the downstream direction (i.e., from the OLT to the ONUs), the passive optical network is a broadcast system; traffic inserted at the OLT is sent to all ONUs. In the upstream direction (i.e., from the ONUs to the OLT), a time division multiple access (TDMA) protocol is frequently used to coordinate the sending of multiple signals to the same OLT point. This is shown generally in  FIG. 1B , which illustrates the TDMA data stream received at the OLT. 
     In general, increasing the upstream communication bandwidth for a fully-loaded passive optical network requires an increase in data transmit speeds. However, higher data speeds typically require more complex and thus expensive electronics for managing packet alignment and performing packet tracking and synchronization. Further, higher data speeds usually require higher speed, higher cost optical transmitters in the ONUs. For these reasons, there are practical limitations on the extent to which bandwidth can be increased in passive optical networks employing current TDMA technology. 
     As an alternative to TDMA protocols, wavelength division multiplexing (WDM) can be used to provide an ONU with increased upstream bandwidth. However, a system employing WDM requires multiple optical transmitters with closely spaced wavelengths, with each ONU being assigned a unique wavelength. WDM is therefore an expensive solution to bandwidth problems because of the high cost of wavelength specific lasers and multi-channel demultiplexing equipment. 
     SUMMARY OF THE INVENTION 
     In order to solve these and other needs in the art, the inventor hereof has designed an advanced, low-cost, high bandwidth, multiple access technology for passive optical networks. Rather than sharing the bandwidth available over a single wavelength among all ONUs, or assigning each ONU its own unique wavelength, the present invention provides a passive optical network in which multiple wavelengths are used to increase overall system bandwidth, with each wavelength being shared by multiple ONUs according to a TDMA protocol. The upstream TDMA traffic therefore includes multiple TDMA data streams at different wavelengths. The OLT preferably receives the multiple TDMA data streams and separates them to different detectors before ultimately combining all data into a single data stream using a multiplexer after performing clock and data recovery functions. In this manner, the upstream bandwidth in a passive optical network can be markedly increased without requiring an increase in data transmit speeds. It therefore follows that upstream bandwidth in a passive optical network can be increased significantly while using low cost/low speed detectors, and low cost/low speed transceivers in the ONUs, although system bandwidth can be further improved by using higher cost, higher speed components, as apparent. 
     According to one aspect of the present invention, an optical communication network includes an optical transmission line, an optical line terminal connected to the optical transmission line, a first plurality of optical network units each connected to the optical line terminal and configured for optically transmitting TDMA signals of a first wavelength to the optical line terminal through the optical transmission line, and at least a second plurality of optical network units configured for optically transmitting TDMA signals of a second wavelength different than the first wavelength to the optical line terminal through the optical transmission line. 
     According to another aspect of the present invention, an optical communication network includes an optical transmission line, an optical line terminal connected to the optical transmission line, and N optical network units. Each optical network unit is connected to and communicates with the optical line terminal through the optical transmission line at one of M different wavelengths using a TDMA protocol, where N and Mare integers and 1&lt;M&lt;N. 
     According to yet another aspect of the present invention, in a passive optical network having an optical line terminal and a first plurality of optical network units passively connected to the optical line terminal through an optical transmission line, the first plurality of optical network units communicating with the optical line terminal at a first wavelength using a TDMA protocol, the improvement comprising a second plurality of optical network units passively connected to the optical line terminal through the same optical transmission line as the first plurality of optical network units, the second plurality of optical network units communicating with the optical line terminal at a second wavelength different than the first wavelength using a TDMA protocol. 
     According to still another aspect of the present invention, a method of grouping optical transmitters for use in an optical communication network includes the steps of providing a plurality of optical transmitters having the same nominal wavelength, determining the operating wavelength for each of the optical transmitters, and separating the plurality of optical transmitters into a plurality of different groups according to the determined operating wavelengths. 
     Other features and advantages of the present invention will be in part apparent and in part pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram of a prior art passive optical network using a TDMA protocol for upstream communications. 
         FIG. 1B  illustrates the TDMA data stream that is sent upstream to the optical line terminal (OLT) in the optical network of  FIG. 1A . 
         FIG. 2A  is a block diagram of an exemplary passive optical network using two TDMA data streams at different wavelengths according to the present invention. 
         FIGS. 2B and 2C  illustrate the two TDMA data streams that are sent upstream through the same optical transmission line in the exemplary optical network of  FIG. 2A . 
         FIG. 3  is a block diagram of the optical line terminal used in the exemplary network of  FIG. 2A . 
         FIG. 4  is a block diagram of an exemplary optical network employing a fault tolerant redundant ring architecture and using two TDMA data streams at different wavelengths according to the present invention. 
         FIGS. 5A and 5B  are spectral diagrams for illustrating how several laser diodes having the same nominal wavelength can be separated into two or more different wavelength groups according to the actual operating wavelengths of such diodes. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A multiple wavelength TDMA optical network according to one exemplary embodiment of the invention is shown in  FIG. 2A  and indicated generally by reference character  100 . Similar to the prior art optical network shown in  FIG. 1A , the exemplary optical network  100  shown in  FIG. 2A  includes an optical line terminal (OLT)  102  connected to a passive splitter  104  through an optical transmission line  106  (e.g., optical fiber). The optical network  100  also includes several exemplary optical network units (ONUs)  108 ,  110 ,  112 ,  114  each connected to the passive splitter  104  through an associated optical transmission line  118 ,  120 ,  122 ,  124 . 
     Each ONU is configured to send data packets PK 1 , PK 2 , . . . , or PKN in the upstream direction (i.e., toward the OLT  102 ) at an appropriate time using a TDMA protocol. These data packets pass through the passive optical splitter  104  and ultimately arrive at the OLT  102  through the optical transmission line  106 . In contrast to the prior art, the exemplary ONUs  108 - 114  do not all communicate with the OLT  102  using the same wavelength. Instead, each ONU is assigned to one of two distinct TDMA groups, with each TDMA group using a wavelength different than that of the other TDMA group. As shown in  FIG. 2A , some ONUs  108 ,  112  communicate with the OLT  102  at a wavelength λ−, while other ONUs  110 ,  114  communicate with the OLT  102  at a wavelength λ+ that is longer than the wavelength λ−. As a result, two TDMA data streams travel over the same optical transmission line  106  to the OLT  102 ; one at the wavelength λ− as illustrated in  FIG. 2B , and one at the wavelength λ+as illustrated in  FIG. 2C . While, in the exemplary network  100  shown in  FIG. 2A , only two ONUs are assigned to each TDMA group, those skilled in the art will recognize that this is for illustrative purposes only and that, in many specific implementations of the invention, greater than two ONUs will be assigned to one or more of the TDMA groups. 
     As further explained below, the OLT  102  receives both TDMA data streams and separates them by wavelength to two different detectors. By using this inventive approach which involves two (or more) TDMA groups operating at different wavelengths, the bandwidth of the exemplary network  100 , and thus the bandwidth allocated to each ONU, can be increased without increasing data transmit speeds between the ONUs and the OLT. This is because multiple ONUs can now send data packets to the OLT  102  at the same time, provided that they transmit data at different wavelengths. Synchronization guard bands may also be made long enough in time to permit detector signal recovery using low cost, low speed electronic circuitry in the OLT  102 , as well as low cost, low speed optical transmitters in the ONUs  108 - 114 . In this manner, the present invention provides for a passive optical network having an increased bandwidth while using low cost components. As should be apparent, the invention can also be used to increase the bandwidth of higher cost, higher speed networks. 
     The internal functions of the preferred OLT  102  will now be described with reference to  FIG. 3 . As shown therein, the OLT  102  preferably includes a wavelength demultiplexer (WDM)  130 , which is preferably a passive optical component, a pair of optical receivers (also called optical detectors)  132 ,  134 , a pair of clock/data recovery circuits  136 ,  138 , and a multiplexer  140 . The WDM  130  preferably receives the TDMA data streams shown in  FIGS. 2B and 2C  through the optical transmission line  106 , and separates these data streams into two separate channels. The TDMA data stream of wavelength λ− is routed to the optical receiver  132 , while the TDMA data steam of wavelength λ+ is routed to the optical receiver  134 , as shown in  FIG. 3 . Preferably, the wavelengths λ− and λ+ are sufficiently separated from one another (e.g., by greater than 30 dB) so that the WDM  130  can provide each of the TDMA data streams to one of the optical receivers  132 ,  134  without harmful noise. Each independent TDMA data stream is processed by one of the optical receivers  132 ,  134  and one of the clock/data recovery circuits  136 ,  138  in a conventional manner. The data steams are then multiplexed together to a common data receiving point  142  using the multiplexer  140 . As will be apparent to those skilled in the art, the multiplexer  140  may include buffers for delaying any data packets received out of order by the multiplexer  140 . 
     Although only two TDMA groups are utilized in the exemplary optical network  100  described above, it should be apparent that more than two groups can likewise be used. Additional TDMA groups at additional wavelengths are preferably supported by replacing the WDM  130  and the multiplexer  140  shown in  FIG. 3  with similar components capable of demultiplexing and multiplexing, respectively, more than two data channels, and by adding an additional optical receiver and an additional clock/data recovery circuit for each additional TDMA wavelength group beyond the two groups described above. 
     It should also be noted that although every other ONU in  FIG. 2A  is assigned to the same TDMA group, this was done for illustrative purposes only and is not a requirement or limitation of the present invention. 
     As will be apparent to those skilled in the art, while the exemplary optical network  100  described above employs a tree architecture, the teachings of the present invention are applicable to all point-to-multipoint topologies including ring and bus architectures. An exemplary implementation of the present invention in an optical network having a fault tolerant redundant ring architecture is illustrated in  FIG. 4  and indicated generally by reference character  200 . As shown in  FIG. 4 , the network  200  includes an OLT  202  with which several ONUs  204 - 216  communicate redundantly and bidirectionally through an optical transmission line  220 . Similar to the optical network  100  described above with reference to  FIG. 2A , each ONU of the optical network  200  is configured to communicate with the OLT  202  using a TDMA protocol at one of two different wavelengths λ− and λ+. Thus, in the optical network  200  of  FIG. 4 , two (or more) TDMA data streams are preferably used to communicate data from the ONUs  204 - 216  to a common OLT data entrance point. For purposes of redundancy, the OLT  202  shown in  FIG. 4  preferably includes two sets of the components shown in  FIG. 3 , with each set connected to one end of the optical transmission line  220 . Additional details of the optical network  200  and the preferred manner in which the ONUs  204 - 216  are passively coupled to the optical transmission line  220  are described in U.S. application Ser. No. 09/568,358 filed May 10, 2000, the entire disclosure of which is incorporated herein by reference. 
     The ONUs included in the exemplary optical networks  100 ,  200  described above are preferably provided with low cost laser diodes for communicating upstream using a TDMA protocol at one of the wavelengths λ− and λ+. This can be accomplished by providing laser diodes having a nominal (i.e., rated) wavelength of λ− and laser diodes having a nominal wavelength of λ+, as should be apparent. Alternatively, a number of low cost laser diodes having the same nominal wavelength can be tested and separated into two or more distinct wavelength groups according to the present invention. This is because the actual operating wavelength of a laser diode often varies by as much as 20 nm or more from its nominal wavelength. For example, the range of actual operating wavelengths for laser diodes rated at 1310 nm exceeds 1290 nm to 1330 mm. 
     As shown in  FIG. 5A , a laser diode having a nominal wavelength of λn may have an operating wavelength anywhere in the spectral window extending from λlow to λhigh. Thus, once the actual operating wavelength of the laser diode is determined, that diode can be separated into one of two different wavelength groups λ− or λ+ according to whether its actual operating wavelength is greater or less than some reference wavelength, such as the nominal wavelength λn as shown in  FIG. 5A . 
     In the case where some degree of isolation is needed between the wavelength groups λ− and λ+, the actual operating wavelength of a particular laser diode can be compared with two reference wavelengths λr 1  and λr 2 , as shown in  FIG. 5B . Thus, a laser diode having an operating wavelength less than both reference wavelengths λr 1  and λr 2  can be separated into wavelength group λ−, and a laser diode having an operating wavelength greater than both reference wavelengths λr 1  and λr 2  can be separated into wavelength group λ+. Laser diodes having operating wavelengths falling between the reference wavelengths λr 1  and λr 2  are preferably separated into a third wavelength group, and such diodes are preferably not used in combination with the diodes separated into wavelength group λ− or λ+ for optical networks requiring a degree of isolation (represented by the difference of the reference wavelengths λr 1  and λr 2 ) between each wavelength group. 
     As used herein, wavelength λ+ refers to a wavelength that is longer than wavelength λ−, and wavelength λ− refers to a wavelength that is shorter than wavelength λ+; no specific wavelength values are intended. The particular values that should be assigned to wavelengths λ− and λ+ in any given implementation of the invention is a matter of design choice. It should also be understood that wavelengths in a relatively narrow spectral range (e.g., wavelengths that would be grouped together by the WDM  130  shown in  FIG. 3 ) are considered the same wavelength for purposes of this disclosure. Thus, wavelength λ− (also called a “first wavelength”) may represent a group of wavelengths in a relatively narrow spectral range that does not overlap with another relatively narrow spectral range of wavelengths that may comprise the wavelength λ+ (also called a “second wavelength”). 
     When introducing elements of the present invention or the preferred embodiments thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.