Abstract:
An optical network system can be used to update legacy passive optical networks by adding an optical transmitter, blocking filter, and/or pluggable or unpluggable optics. In one embodiment, an optical network system, including several optical transmitters and receivers, multiplexers, demultiplexers, erbium-doped fiber amplifier, and blocking filter, may be employed. The additional transmitter increases available bandwidth, while the blocking filter allows existing customers&#39; service(s) to not be impacted. Another embodiment uses pluggable or unpluggable optics, instead of the aforementioned blocking filter, to receive and modulate optical signals to transmit services to end users. In one embodiment, an optical network system can be employed that allows for simultaneous upgrading of the system and providing of legacy services, while allowing for the of removal existing optical network components over time.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Passive optical networks are currently used in telecommunications to provide services to end users. Example services include telephone, cable television, and the Internet. Passive optical networks, as used in current practice, typically include a service provider network, optical line terminal, multiplexer/demultiplexer, optical network unit(s), and end user equipment connected via an architecture of optical fiber. Implementation of the currently used passive optical network has been costly and is widely used. 
       SUMMARY OF THE INVENTION 
       [0002]    An example embodiment of the invention provides a method, or corresponding apparatus, of upgrading existing optical networks. The method includes adding a supplemental optical communications band, normally used in optical transport networks to carry identical forms of data traffic as carried in other optical communications bands, to at least a subset of multiple existing optical access networks having at least one existing communications band. Over time, a radio frequency video overlay is removed from at least one of the existing optical communications bands in a subset of the multiple existing optical access networks. Multi-cast channels may be applied to the supplemental optical communications bands to carry forms of data traffic previously carried by the radio frequency overlay. 
         [0003]    Another example embodiment of the invention provides an optical receiver. The optical receiver includes a filter configured (i) to reflect a first optical signal traveling in a forward direction along a first optical path onto a second optical path and (ii) to pass a second optical signal traveling in a forward direction along the first optical path to a third optical path and, in a reverse direction, from the third optical path to the first optical path. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0005]      FIG. 1  is a schematic diagram of network components of an example Gigabit Passive Optical Network (GPON) with video overlay as configured under current practice in which embodiments of the invention may be employed. 
           [0006]      FIG. 2A  is a schematic representation of network components of an example gigabit passive optical network with video overlay that includes an upgrade in accordance with example embodiments of the present invention. 
           [0007]      FIG. 2B  is a first of two flow diagrams illustrating upgrading existing optical networks in accordance with example embodiments of the present invention. 
           [0008]      FIG. 2C  is a second of two flow diagrams illustrating upgrading existing optical networks in accordance with example embodiments of the present invention. 
           [0009]      FIG. 3  is a graphical representation of an optical design of an example quadplexer in accordance with an embodiment of the present invention. 
           [0010]      FIG. 4  is a network schematic diagram of network components of an example wavelength division multiplexing passive optical network with video overlay that supports a cost-effective transparent upgrade in accordance with an example embodiment of the present invention. 
           [0011]      FIG. 5  is a graphical representation of an integrated reflective semiconductor optical amplifier (RSOA)-based diplexer that may be used in accordance with an example embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    A description of example embodiments of the invention follows. 
         [0013]    In current practice, Gigabit Passive Optical Network (GPON) deployments have an optical line terminal (OLT) communicating with multiple optical network terminals (ONTs) with, for example, 2.4 Gbps downstream and 1.2 Gbps upstream links. Optical loss that can be tolerated in some networks is, for example, 28 dB bidirectionally. 
         [0014]      FIG. 1  shows a central office (CO)  105  with OLT equipment that has an S-band transmitter and O-band receiver. A video signal may be overlaid through the use of an externally-modulated C-Band radio frequency transmitter (EMCT)  115 , an Erbium-Doped Fiber Amplifier (EDFA)  120  to boost the optical power, and a Wavelength Division Multiplexer (WDM) to combine the video signal with the OLT signal. A GPON Optical Network Unit (ONU) (GONU) separates the two downstream signals and isolates them from the transmitter O-Band signal. 
         [0015]    Continuing to refer to  FIG. 1 , a subsystem  110  includes an S-Band transmitter (S-Band TX), which transmits optical wavelengths at approximately 1490 nm, and an O-Band receiver (O-Band RX), which receives optical wavelengths at approximately 1310 nm. The CO  105  may also include a coarse multiplexer  125 . Optical paths and connections represented herein may be optical fibers or, in some embodiments, free space optical paths and span between corresponding optical communications components. Additionally, multiplexers and demultiplexers represented herein may employ arrayed waveguide gratings or other forms of optical components understood in the art to support multiplexing and demultiplexing operations. 
         [0016]    The subsystem  110  with the S-Band TX and O-Band RX may be connected via an optical path  113  to the multiplexer  125 . The EMCT  115  may be connected via an optical path  117  to the EDFA  120 , which may then be connected via an optical path  123  to the multiplexer  125 . The multiplexer  125  may then be connected via an optical path  128  to a 20 Km single mode optical fiber (SMF)  130 , for example. The SMF  130  may be connected via an optical path  133  to a power splitter/combiner (power splitter)  135 , which, in turn, may be connected to gigabit optical network units (GONU), represented as GONU  140  via an optical path  137  and GONU  145  via a different optical path  143 . 
         [0017]    Each GONU, as used in current practice, may contain a triplexer  146  with a filter to separate the different optical bands of wavelengths, such as C-, S-, and O-Band wavelengths. The triplexer  146  may be connected via an internal optical path  147  to a C-Band detector  148  connected via an electrical link  149  to a C-Band radio frequency receiver (C-Band RF RX)  150 . The triplexer  146  may also be connected via an optical path  151  to an S-Band detector  152  connected via an electrical link  153  to an S-Band RX  154 . The GONU  145  may also contain an O-Band TX  155  connected via an electrical link  156  to an O-Band laser source  157  connected via an optical path  158  to the triplexer  146 . 
         [0018]    Due to demand for bandwidth by consumers, there is a need in the communications industry to increase data rate in access networks. As service providers transition from video delivery to Internet Protocol Television (IPTV) and as high-definition television (HDTV) channels receive an upsurge in popularity, data rates supported in current broadband passive optical networks (BPON) or gigabit passive optical networks (GPON) may be insufficient to support the commercial demand. Due to the large capital investments made by service providers in the current BPON/GPON, a smooth and cost-effective transition to the next generation of network(s) is expected to be useful. Additionally, the next generation network(s) are unlikely to replace existing networks, so there is expected to be a period where next generation and legacy (i.e., existing) networks will coexist so that existing customers are not impacted. 
         [0019]    An example embodiment of the invention is directed to a migration path from the currently deployed GPON networks to next generation networks that support advanced services along with the existing video overlay through the transition to a full wavelength division multiplexing network. The transition is transparent to existing GPON customers while providing a cost-effective method to upgrade the existing GPON customers. The upgraded passive optical network employed in accordance with an example embodiment of the invention may work within current Optical Distribution Network (ODN) constraints of a 28 dB budget in some optical networks. 
         [0020]    The first upgrade may be in the form of adding another transmitter at the central office to carry new advanced services, such as multi-cast channels for IPTV. One example of a proposed wavelength according to an example embodiment of this invention for that transmitter is greater than 1600 nm, which is in the optical L-Band. This may enable service providers to employ a 45° beamsplitter and wavelength division multiplexing (WDM) blocking filters to provide enough isolation between this new wavelength and the GPON wavelengths. The new ONTs may receive the new wavelength through a new optical device that has photodiode for the L-Band and internal optical filters to provide enough isolation between the different bands. The resulting device is more of a quadplexer, where it has an L-Band RX, C-Band video RX, S-Band RX, and O-Band TX. ONTs already deployed in the field may be equipped with an in-line blocking filter so the traffic carried on the new wavelength does not interfere with the legacy services on other wavelengths. 
         [0021]    An example embodiment of the invention provides a method, or corresponding apparatus, of upgrading existing optical networks. The method includes adding a supplemental optical communications band, normally used in optical transport networks to carry identical forms of data traffic as carried in other optical communications bands, to at least a subset of multiple existing optical access networks having at least one existing communications band. Over time, a radio frequency video overlay is removed from at least one of the existing optical communications bands in a subset of the multiple existing optical access networks. Multi-cast channels may be applied to the supplemental optical communications bands to carry forms of data traffic previously carried by the radio frequency overlay. 
         [0022]    The supplemental optical communications band may be defined as at least a portion of the optical L-Band, and the other optical communications bands may be defined as at least a portion of at least two of the optical C-, O-, and S-Bands. The supplemental optical communications band and the other optical communications bands may be directing onto respective optical paths. 
         [0023]    The method or corresponding apparatus may also include adding a wavelength in at least a portion of the optical C-Band or O-Band, transmitting at least one continuous wavelength optical signal in at least one respective subband of the optical C-Band or O-Band in a downstream direction, modulating the at least one continuous wavelength optical signal to produce a modulated optical signal, and directing the modulated optical signal in an upstream direction. 
         [0024]    The method or corresponding apparatus may also include repurposing at least one erbium-doped fiber amplifier (EDFA) previously used to amplify optical signals carrying the radio frequency video overlay. The repurposing of the at least one EDFA may include amplifying digital optical signals to extend reach and density of the existing passive optical network or increasing a density of digital optical signals of the supplemental optical communications band. 
         [0025]    The method or corresponding apparatus may also include applying data traffic for emerging services to the supplemental optical communications band. 
         [0026]    Another example embodiment of the invention provides an optical receiver or corresponding method. The optical receiver includes a filter configured (i) to reflect a first optical signal traveling in a forward direction along a first optical path onto a second optical path and (ii) to pass a second optical signal traveling in a forward direction along the first optical path to a second optical path and, in a reverse direction, from the second optical path to the first optical path. 
         [0027]    The optical receiver or corresponding method may also include an optical detector to detect the first optical signal and a reflective semiconductor optical amplifier (RSOA) configured to modulate the second optical signal and direct the second optical signal from the forward direction to the reverse direction in the second optical path. The RSOA may include an integrated photodiode and may be configured to be a pluggable device. 
         [0028]      FIG. 2  illustrates a GPON-VO  200  as used in accordance with an example embodiment of the invention. The central office (CO)  205  may include: a subsystem  210  with an S-Band TX and an O-Band RX, EMCT  215 , L-Band TX  225 , EDFA  220 , and multiplexer  230 . The subsystem  210 , EMCT  215 , EDFA  220 , and multiplexer  230  may be connected as illustrated in  FIG. 1 . The L-Band TX  225  of  FIG. 2  may also be connected via an optical path  227  to the multiplexer  230 . The L-Band may be used because it is least intrusive and useful because separation into subbands is easy. For example, a 45° optical filter in an optical receiver (discussed below in reference to  FIG. 3 ) may be used based upon separation, and expenses may be reduced because to separate the optical L-Band from other bands may not require additional coating of a filter. The multiplexer  230  may be connected via an optical path  233  to a 20 Km SMF  235 , which may be connected via an optical path  237  to a power splitter  240 . The power splitter may be connected to a next generation GONU (NG-GONU)  245  and a legacy GONU  265 . 
         [0029]    The NG-GONU  245  may contain a quadplexer  246 , L-Band detector  248  and L-Band RX  250 , C-Band detector  252  and C-Band RF RX  254 , S-Band detector  256  and S-Band RX  258 , and O-Band detector  260  and O-Band TX  262 . The power splitter  240  may be connected via an optical path  243  to the quadplexer  246 , which, in turn, may be connected to detectors and a laser source for the bands, as applicable (L-Band detector  248  by an optical path  247 , C-Band detector  252  by an optical path  251 , S-Band detector  256  by an optical path  255 , and O-Band laser source  260  by an optical fiber  259 ). Each detector  248 ,  252 ,  256  may then be connected to its respective receiver: L-Band RX  250  by electrical link  249 , C-Band RF RX  254  by an electrical link  253 ; and S-Band RX  258  by an electrical link  257 . The O-Band laser source  260  may be connected via an electrical link  261  to an O-Band TX  262 . 
         [0030]    For each existing customer, an L-Band blocking filter  266  may be added to the GONU  265  to allow for changing and upgrading other GONUs  245  in a manner that may be transparent to a current user. The power splitter  240  may be connected via an optical path  263  to the L-Band blocking filter  266 . Optical signals (not shown) destined for receivers in the GONU  265  may then be transmitted via an optical path  267  to a triplexer  268 . The triplexer  268  may be connected to a detector for each band: C-Band detector  270  connected by an optical path  269 , S-Band detector  274  connected by an optical path  273 , and O-Band laser source  278  connected by an optical path  277 . The detectors and laser source may then be connected to respective receivers or transmitter: C-Band RF RX  272  connected by an electrical link  271 , S-Band RX  276  connected by an electrical link  275 , and O-Band TX  280  connected by an electrical link  279 . 
         [0031]      FIG. 2B  is a first of two flow diagrams  283  illustrating upgrading existing optical networks in accordance with example embodiments of the present invention. After the process is initiated ( 284 ), a subset (or all) of the multiple existing optical networks may be upgraded ( 285 ), which may include adding a supplemental optical communications band ( 286 ). Next, over a period of time, radio frequency video overlay may be removed ( 287 ) from the existing optical networks. Then, multi-cast channels may be applied ( 289 ) to the supplemental optical communications band. Upon applying the multi-cast channels, the upgrading process terminates ( 290 ). 
         [0032]      FIG. 2C  is a second of two flow diagrams  293  of upgrading existing optical networks in accordance with example embodiments of the present invention. After the process is initiated ( 294 ), a subset (or all) of the multiple existing optical networks may be upgraded ( 295 ), which may include adding a supplemental optical communications band to existing other communications bands ( 296 ). Next, over a period of time, radio frequency video overlay may be removed ( 297 ) from the existing optical networks. Then, data traffic for emerging services may be applied ( 298 ) to the supplemental optical communications band. Upon applying the multi-cast channels, the upgrading process terminates ( 299 ). 
         [0033]      FIG. 3  is a diagram of the optical design of an example quadplexer  300  in accordance with an example embodiment of the present invention. The quadplexer  300 , which corresponds to the quadplexer  247  of  FIG. 2  and may be the same or different in design or construction, includes units (e.g., O-Band unit  380 , L-Band unit  335 , C-Band unit  345 , S-Band unit  360 ) configured for each band transmitted within a passive optical network in which the example embodiment of the present invention is deployed. As described in detail immediately below, each unit may be configured to receive and transmit its respective band to a corresponding receiver. 
         [0034]    Continuing to refer to  FIG. 3 , an optical input/output port  325  permits the optical signals  327  to enter the quadplexer  300 . As the optical signals  327  travel through the quadplexer  300 , the optical signals  327  encounter a series of filters (L-Band filter  330 , C-Band RF filter  340 , S-Band filter  355 , and O-Band filter  370 ). As the optical signals  327  reach each filter, the appropriate wavelength for each band is filtered. For example, L-Band wavelength  328  is reflected by the L-Band filter  330  and emerges from the optical signals  327  as an L-Band filtered wavelength  333  received by an L-Band lens  334   a , then an L-Band opto-electronic chip  334   b  used to convert optical signals to a corresponding electrical signal. Similar filtering occurs for the C-, S-, and O-Bands: C-Band wavelength  338 , C-Band filter  340 , C-Band filtered wavelength  343 , C-Band preamplifier  344   a , C-band amplifier  344   b ; S-Band wavelength  353 , S-Band filter  355 , S-Band filtered wavelength  358 , S-Band lens  359   a , S-Band opto-electronic chip  359   b ; O-Band wavelength  305 , O-Band filter  370 , O-Band filtered wavelength  365 , O-Band lens  374   b , O-Band opto-electronic chip  374   a ). Each unit contained within the quadplexer  300  may be configured to receive optical signals and direct electrical signals to the appropriate receiver. For example, the L-Band unit  335  directs the L-Band electrical signal  336  to the L-Band receiver  250  (see  FIG. 2 ); the C-Band unit  345  directs the C-Band electrical signal  350  to the C-Band receiver  254  (see  FIG. 2 ); the S-Band unit  360  directs the S-Band electrical signal  365  to the S-Band receiver  258  (see  FIG. 2 ); and the O-Band unit  380  directs the O-Band optical signal  385  to the O-Band receiver  210  (see  FIG. 2 ) via the input/output port  325 . 
         [0035]    The second upgrade may be in the form of upgrading the entire network into a full coarse or dense WDM (CWDM/DWDM) network. To do so, the power splitters in the field are replaced with array waveguide modules, and a reflective device is installed at each ONT to make it wavelength-agnostic. An integrated photodiode, filter, and reflective semiconductor optical amplifier (RSOA) may be employed as a diplexer. The new WDM-PON network may be overlaid over the already-deployed time division multiplexing (TDM) GPON network without impacting any of the legacy services. The EDFAs, currently used for video overlay, may be repurposed to increase the C-band CWDM/DWDM signals to overcome the ODN 28 dB budget or extend optical transmission distances beyond 20 Km. 
         [0036]      FIG. 4  is a schematic diagram of a WDM PON  400  in accordance with another embodiment of the present invention. The CO  405  may contain an S-Band TX  413 , O-Band TXs  420  (represented herein as the O-Band TX  421 ,  423 , . . . ,  425 ), first fine multiplexer  430 , EDFA  435 , circulator  437 , second fine multiplexer  440 , O-Band RXs  441  (represented as the O-Band RX  443 ,  445 , . . . ,  447 ), and first (near end) coarse multiplexer  450 . It should be understood that the naming convention “coarse” and “fine” multiplexers as used herein refers to multiplexing “bands” and “subbands within a band,” respectively. 
         [0037]    The S-Band TX  410  may be connected via an optical path  415  to the first coarse multiplexer  450 . The O-Band TXs  420  may be connected via optical paths ( 422 ,  424 , . . . ,  426 , respectively) to the first fine multiplexer  430 , which may, in turn, be connected via an optical path  433  to the EDFAs  435 . The EDFAs  435  transmit optically amplified signals via an optical path  436 , circulator  437 , and optical path  438  to the first coarse multiplexer  450 . The circulator  437  returns optical signals from an optical path  438  to an optical path  439  to the second fine multiplexer (demultiplexer)  440 . The first coarse multiplexer  450  is configured to multiplex the optical signals received from the S-Band TX  410  and the O-Band TX  420 . 
         [0038]    The first coarse multiplexer  450  is connected via the optical path  453  to a 20 Km SMF  455 , which may then connect via an optical path  458  to a second (far end) coarse demultiplexer  460 . The second coarse demultiplexer  460  may be connected via an optical path  463  to a power splitter  465  and GONUs  470 ,  475 . The second coarse demultiplexer  460  may also be connected via an optical path  478  to a third fine demultiplexer  480 . The third fine demultiplexer  480  may be connected via an optical path  482  to a power splitter  485 , which may, in turn, be connected via optical paths  487  . . .  490  to reflective optical network units (RONUs)  489  . . .  491 , respectively. 
         [0039]    Each RONU may contain a diplexer  492  that may be connected to an S-Band RX  496  and a RSOA  498 . In particular, in this example embodiment, the diplexer  492  may be connected via an electrical link  493  to an S-Band detector  494  that may be connected by an optical path  495  to the S-Band RX  496 . The diplexer  492  may also be connected via an optical path  497  to the RSOA  490  (see  FIG. 5 ), which may have an uplink connection  499 . 
         [0040]      FIG. 5  is a schematic diagram of components of an integrated RSOA-based diplexer  500  that may support cost-effective and transparent upgrading of a passive optical network in accordance with example embodiments of the present invention. An optical signal  524  may enter the integrated RSOA-based diplexer  500  via an input/output port  525 , where optical signals in the L-Band  528  may travel to an L-Band unit  535  by reflection of an L-Band filter  530 . An L-Band “filtered” signal  533  may reach the L-Band lens  534   a , and then the L-Band opto-electronic chip  534   b , by which the optical signal is converted to a corresponding electrical signal. The L-Band unit  535  may then transmit the corresponding L-Band electrical signal  540  to an L-Band RX (not shown). 
         [0041]    Continuing to refer to  FIG. 5 , the RSOA  510  may receive an electrical (or optical) signal  505  and convert it by a light modulating chip (LMC)  506   a  with an integrated photodiode. The LMC has a reflective coating on the back facet that reflects and directs an amplified optical signal to its respective optical device. The LMC  506   a  modulates a continuous wave (CW) optical carrier signal, passed by an O-Band filter  520  in a downstream (i.e., forward) direction, with an optical signal corresponding to the electrical (or optical) signal  505  to produce a modulated O-Band signal. The LMC  506   a  directs the modulated O-Band optical signal to a coupling lens  506   b , which further directs the modulated O-Band optical signal to the O-Band filter  520 , which passes the O-Band optical signal to the input/output port  525  for upstream transmission (i.e., reverse direction) to the O-Band RXs  441  (see  FIG. 4 ). 
         [0042]    Based on the configuration illustrated in  FIG. 5 , an optical path to the left of the filters  530 ,  520  may be referred to herein as a first optical path, an optical path between the L-Band filter  530  and L-Band RX  535  may be referred to herein as a second optical path, and an optical path between the O-Band filter  520  and RSOA  510  may be referred to herein as a third optical path. 
         [0043]    Further referring to  FIG. 5 , the RSOA may include mechanical connectors  503   a  and  503   b  (mating connection not shown) that allow the RSOA to be plugged and unplugged into and from a chassis that is configured with at least one other receiver (not shown). “Mechanical connectors” includes pin socket, screw fitting, or other mechanical connectors known in the art. 
         [0044]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.