Patent Publication Number: US-2010111533-A1

Title: Wdm pon system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on, and claims benefit of U.S. Provisional Patent Application Ser. No. 61/111,757 filed Nov. 6, 2008. 
    
    
     FIELD OF THE INVENTION 
     The present application relates generally to Wavelength Division Multiplexed Passive Optical Networks (WDM PON) and, more specifically, to a WDM PON System With Distribution Via Cyclic Array Waveguide Grating. 
     BACKGROUND OF THE INVENTION 
     A passive optical network (PON) is a point-to-multipoint network architecture in which unpowered optical splitters are used to enable a single optical fibre to serve multiple premises. A PON typically includes an Optical Line Terminal (OLT) at the service provider&#39;s central office connected to a number (typically 32-128) of Optical Network Terminals (ONTs), each of which provides an interface to customer equipment. 
     In operation, downstream signals are broadcast from the OLT to the ONTs on a shared fibre network. Various techniques, such as encryption, can be used to ensure that each ONT can only receive signals that are addressed to it. Upstream signals are transmitted from each ONT to the OLT, using a multiple access protocol, such as time division multiple access (TDMA), to prevent “collisions”. 
     A Wavelength Division Multiplexing PON, or WDM-PON, is a type of passive optical network in which multiple optical wavelengths are used to increase the upstream and/or downstream bandwidth available to end users.  FIG. 1  is a block diagram illustrating a typical WDM-PON system. As may be seen in  FIG. 1 , the OLT  4  comprises a plurality of transceivers  6 , each of which includes a light source  8  and a receiver  10  for sending and receiving optical signals on respective wavelength channels, and an optical combiner/splitter  12  for combining light from/to the light source  8  and receiver  10  onto a single optical fibre  14 . The light source  8  may be a conventional laser diode such as, for example, a distributed feed-back (DFB) laser, for transmitting data on the desired wavelength using either direct laser modulation, or an external modulator (not shown) as desired. The receiver  10  may, for example, comprise a PIN diode for detecting optical signal received through the network. An optical mux/demux  16  (such as, for example, a Thin-Film Filter (TFF) or an Array Waveguide Grating (AWG)) is used to couple light between each transceiver  6  and an optical fibre trunk  18 , which may include one or more passive optical power splitters (not shown). 
     A passive remote node  20  serving one or more customer sites includes an optical mux/demux  22  for demultiplexing wavelength channels (λ 1  . . . λn) from the optical trunk fibre  18 . Each wavelength channel is then routed to an appropriate branch port  24  which supports a respective WDM-PON branch  26  comprising one or more Optical Network Terminals (ONTs)  28  at respective customer premises. Typically, each ONT  28  includes a light source  30 , detector  32  and combiner/splitter  34 , all of which are typically configured and operate in a manner mirroring that of the corresponding transceiver  6  in the OLT  4 . 
     Typically, the wavelength channels (λ 1  . . . λn) of the WDM-PON are divided into respective channel groups, or bands, each of which is designated for signalling in a given direction. For example, C-band (e.g. 1530-1565 nm) channels may be allocated to uplink signals transmitted from each ONT  28  to the OLT  4 , while L-band (e.g. 1565-1625 nm) channels may be allocated to downlink signals from the OLT  4  to the ONT(s)  26  on each branch  26 . In such cases, the respective optical combiner/splitters  12 ,  34  in the OLT transceivers  6  and ONTs  28  are commonly provided as passive optical filters well known in the art. 
     The WDM-PON illustrated in  FIG. 1  is known, for example, from “Low Cost WDM PON With Colorless Bidirectional Transceivers”, Shin, D J et al, Journal of Lightwave Technology, Vol. 24, No. 1, January 2006. With this arrangement, each branch  26  is allocated a predetermined pair of wavelength channels, comprising an L-band channel for downlink signals transmitted from the OLT  4  to the branch  26 , and a C-band channel for uplink signals transmitted from the ONT(s)  28  of the branch  26  to the OLT  4 . The MUX/DEMUX  16  in the OLT  4  couples the selected channels of each branch  26  to a respective one of the transceivers  6 . Consequently, each transceiver  6  of the ONT is associated with one of the branches  26 , and controls uplink and downlink signalling between the ONT  4  and the ONT(s)  28  of that branch  26 . Each transceiver  6  and ONT  28  is rendered “colorless”, by using reflective light sources  8 ,  30 , such as reflective semi-conductor optical amplifiers; injection-locked Fabry-Perot lasers; reflective electro-absorptive modulators; and reflective Mach-Zehnder modulators. With this arrangement, each light source  8 ,  30  requires a “seed” light which is used to produce the respective downlink/uplink optical signals. In the system of  FIG. 1 , the seed light for downlink signals is provided by an L-band seed light source (SLS)  36  via an L-band optical circulator  38 . Similarly, the seed light for uplink signals is provided by a C-band seed light source (SLS)  40  via a C-band optical circulator  42 . 
     A limitation of the system of  FIG. 1  is that signal reach is dependent on the optical power of the seed light that is injected into each light source  8 ,  30 , and the modulated power that can be derived from that seed light by each light source. This issue is particularly important in the up-link direction, because the C-band seed light must be transmitted from the OLT  4  to each ONT  26 , injected into each light source  30 , and the resulting modulated uplink signals must then traverse the WDM-PON to the transceivers  6  in the OLT  4 . The noise and signal attenuation associated with traversing the WDM-PON twice imposes significant limitations in the signal reach and bandwidth of the uplink signals. 
     A further limitation of this system is that the bandwidth of the light generated by reflective light sources seeded with a spectrally-sliced broadband seed light source tends to be broad. This means that, as data rates rise above about 1 Gb/s, dispersion penalties can significantly degrade system performance. 
     A further limitation of this system is that the use of spectral slicing of the BLS  36 ,  40  imposes a noise-related bit-error-rate floor that increases proportionally with decreasing channel-width. This noise-related floor limits both data transmission rate and the maximum channel-count by preventing more, narrower, channels within a band. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention provides a Wavelength Division Multiplexed Passive Optical Network (WDM-PON) including: a respective Optical Network Terminal (ONT) at each one of a plurality of customer sites, each ONT comprising an ONT Fabry Perot (F-P) laser for generating a respective broadband multi-mode uplink optical signal; and an Array Waveguide Grating (AWG) for receiving each broadband multi-mode uplink optical signal through a respective branch port, and for multiplexing a portion of each received broadband multi-mode uplink optical signal into a Wavelength Division Multiplexed (EDFM) signal. Each ONT F-P laser is non-injection locked. A gain of each ONT F-P laser is sufficiently inhomogeneous that the modes of the respective broadband multi-mode uplink optical signal are independent. A filter function of the AWG includes a pass band that encompasses at least one mode of a broadband multi-mode uplink optical signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
         FIG. 1  schematically illustrates a conventional WDM-PON known in the prior art; 
         FIG. 2  illustrates an output spectrum of a non-injection-locked Fabry-Perot laser; 
         FIG. 3  schematically illustrates a representative embodiment of the present invention; and 
         FIG. 4  schematically illustrates a WDM-PON implementing the embodiment of  FIG. 3 . 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides techniques for enabling low-cost high performance WDM-PON operation with increased signal reach as compared to conventional systems. A representative embodiment is described below with reference to  FIGS. 2-5 . 
     In very general terms, the present invention exploits the characteristics of AWGs and non-injection locked F-P lasers to facilitate low-cost high performance WDM-PON operation with increased signal reach as compared to conventional systems. 
     As is also known in the art, an injection-locked Fabry-Perot (F-P) laser produces an output light that is frequency-locked to the frequency of the seed light injected into the F-P laser. In conventional WDM-PON systems of the type described above with reference to  FIG. 1 , this characteristic is used to generate uplink signals that are centered at desired wavelengths. Thus, the C-band seed light source (SLS)  40  (see  FIG. 1 ) generates a “comb” of narrow-band continuous seed lights, each centered at a desired uplink channel wavelength. The MUX  22  couples each seed light into a respective one of the branch ports  24 , so that each ONT  28  receives one of the seed lights. At each ONT  28 , the seed light is injected into the light source  30 , which outputs an uplink signal that is frequency-locked with the injected seed light and modulated with data received from customer premise equipment (not shown). 
     In the absence of an injected seed light, an F-P laser will output a multi-mode optical signal having a broad optical spectrum.  FIG. 2  illustrates a typical spectrum of an F-P laser without seed light injection. As may be seen in  FIG. 2 , the non-injection seeded F-P laser output spectrum  44  is not continuous, but rather is composed of a plurality of narrow “spikes” (or modes)  46  at a spacing that is determined by the design and construction of the laser. The strongest modes  46  are concentrated within a band  48  having a width that is also determined by the construction of the laser. Thus, for example, the laser can be constructed such that the band  48  corresponds with a channel band  50  (e.g. either the C-band or L-Band) of a WDM-PON. Modern semiconductor lasers and Quantum Dot lasers can be constructed to exhibit desired output band width and mode spacing parameters, and have a sufficiently inhomogeneous gain that the modes  46  are effectively independent of one another. 
     As is known in the art, an Array-Waveguide Grating (AWG) is capable of demultiplexing a plurality of wavelength channels from Wavelength Division Multiplexed (WDM) signal received through a WDM fibre, and outputting each demultiplexed wavelength channel though a respective one of a plurality of branch fibres. Within the free spectral range (FSR) of the AWG, the AWG implements a filter function characterised by a respective pass-band centered at each channel wavelength of the WDM-PON. Each pass-band is associated with a respective branch fiber, so that light of a given WDM PON channel is coupled between the WDM fiber and the associated branch fibre, while out-or-band (for that branch fibre) noise is suppressed. 
     Referring to  FIG. 3 , a representative technique for utilizing non-injection seeded, directly modulated F-P lasers and AWGs to enable WDM-PON communications is schematically illustrated. 
     As may be seen in  FIG. 3 , in the uplink direction, uplink data D UL  from a given customer site (not shown) is supplied to the customer&#39;s ONT  28   x  (where “x” is an index), and used to drive a non-injection seeded F-P laser  30 . The optical signal  52   x  output by the laser  30  is a broadband multimode optical signal having a spectrum  44  similar to that described above with reference to  FIG. 2 , and which is intensity modulated with the uplink data D UL . This optical signal  52   x  is supplied to a respective branch  24   x  of the Remote node&#39;s AWG  22 . The AWG  22  implements a band pass filter function  54  characterised by a pass-band  56   x  uniquely associated with the respective branch  24   x,  as described above. Consequently, the AWG  22  operates to couple a narrow band of wavelengths within the pass-band  56  from the branch port  24   x  to the trunk fibre  18 . As may be seen in  FIG. 3 , this operation also has the effect of filtering the broadband signal  52  generated by the F-P laser  30  to produce a narrow uplink channel signal  58  which is Wavelength Division Multiplexed with uplink channel signals from other ONTs within the trunk fibre  18 . In order to successfully convey uplink data D UL  to the OLT  4 , the pass-band  50  of the AWG filter function  52  must be wide enough to encompass at least one mode of the broadband signal  46  generated by the F-P laser  30 . In addition, it is important that each of the modes are sufficiently independent that attenuation of out-of-band modes by the AWG filter function  54  does not seriously degrade in-band modes that are coupled into the trunk fibre  18 . Modern F-P lasers and AWGs can be designed and manufactured to satisfy both of these conditions. 
     As shown in  FIG. 4 , the OLT AWG  16  implements a band pass filter function  54  closely matching that of the Remote node&#39;s AWG  22 , and thus having a pass-band  56   x  uniquely associated with a respective branch  14   x  associated with the transmitting ONT  28   x,  as described above. Consequently, the AWG  16  demultiplexes the narrow uplink channel signal  58   x  from the trunk fibre  18 , and the demultiplexed channel signal  58   x  is supplied to the receiver  10  for detection of the uplink data D UL , all in a conventional manner. As may be appreciated, the spectrum  60  of the demultiplexed channel signal  58   x  input to the receiver  10  will be the product of the respective filter functions of both AWGs  22  and  16 . 
     In order to successfully convey uplink data D UL  to the OLT  4 , the pass-band  56   x  of the filter functions  54  implemented in both AWGs  16  and  22  must have a passband width  62  that is wide enough to encompass at least one mode of the broadband signal  44  generated by the F-P laser  30 . In embodiments in which the AWG filter function pass-band width  62  are broad enough to encompass two or more modes  46 , precise alignment between any given modes and the center wavelength of the pass-band  56  may not be essential. However, in some cases, it will be desirable to construct the AWGs  16  and  22  such that the pass-band  56  encompasses a single mode  46  of the F-P laser  30  output spectrum  44 . In such cases, misalignment between the pass-band  56  and the modes  46  of the F-P laser  30  output spectrum  44  can significantly degrade performance of the WDM PON, and it is therefore desirable to implement a control loop to prevent any such drift.  FIG. 3  illustrates a possible feedback control loop  64  for this purpose. 
     In the feedback control loop  64  of  FIG. 4 , a detector  66  monitors the recovered signal  68  output from the receiver  10  to detect one or more parameters indicative of the quality of the received channel signal  58 . For example, the detector  52  may operate to detect any one or more of: a power level received channel signal  58 ; a bit error rate; or a FEC error rate. Other suitable signal quality parameters may equally be detected. Information indicative of the detected parameter (which may be a value of the parameter itself, or a value derived from it) is then sent to a control unit  70  of the F-P laser  30 . In some embodiments, a control channel of the WDM-PON may be used for this purpose. Based on the information received from the detector  66 , the control unit  70  can control the F-P laser  30  so as to optimize the quality of the of the channel signal  58  at the receiver  10 . For example, the control unit  70  may operate to adjust the laser temperature and/or the drive current, both of can be used (alone or in combination) to adjust the laser output spectrum  44 , and thereby ‘tune” the laser output for a given WDM PON channel by aligning a mode to the center wavelength of the AWG pass-band  56  corresponding to that channel. 
     In some embodiments, the laser  30  can be constructed such that the mode spacing in the laser output spectrum  44  corresponds with the channel spacing of the WDM-PON. In such cases, a one-to-one correspondence will exist between each mode  46  and each channel of the WDM PON. 
     In other embodiments, the laser  30  can be constructed such that the mode spacing in the laser output spectrum  44  differs from the channel spacing of the WDM-PON. For example, a laser cavity length of 400 um will yield an output spectrum  44  having approximately 30 modes within the frequency range of a channel band having 32 channels. In such cases, the feedback loop  64  described above can be used to tune the nearest mode  46  to any desired channel. However, once this has been done, it will be seen that the remaining modes  46  of the laser spectrum  44  will be misaligned with the other channels  58  of the WDM PON channel band. In some cases, this is advantageous, in that it reduces crosstalk between channels. 
     More particularly, referring back to  FIG. 3 , the band-pass filter function  54  implemented by the AWGs  16  and  22  comprises a pass band  56  corresponding to a specific channel of the WDM-PON, and a noise floor  72  which represents leakage of output-of-band light through the AWG. As such, the filtered channel signal  58   x  multiplexed into the trunk fibre  18  by the Remote Node&#39;s AWG  22  is accompanied by out-of-band light from each of the other modes of the laser output signal  52 . However, in a case where the mode spacing corresponds with the channel spacing, these out-of-band modes are aligned with the filter pass-band(s) of the OLT AWG  16 , and thus will be coupled into each OLT receiver  10 , where it will appear as cross-talk in the received channel signal  60 . On the other hand, by ensuring that the mode spacing does not correspond with the channel spacing, the out-of-band modes from channel  58   x  will not be aligned with the pass-band(s) of the OLT AWG  16 , and thus will be attenuated. 
     An advantage of the embodiment of  FIG. 3  is that effectively colorless operation of the ONTs  28  is enables without the use of seed light. As a result, the C-band SLS  40  and circulator  42  can be eliminated, thereby reducing cost of the OLT  4 . If desired, the arrangement of  FIG. 3  can be mirrored in the downlink direction, thereby enabling elimination of the L-Band SLS  36  and circulator  38 , as may be seen in  FIG. 4 . Furthermore, the round-trip attenuation and data-transmission-rate impairments associated with conventional seed light injection systems of the type described above with reference to  FIG. 1  are avoided. At high-speed line rates (for example of about 1 GHz and higher) the technique of the present invention offers a significant performance advantage over prior art systems. 
     As may be seen in  FIGS. 2 and 3 , the bandwidth of the multi-mode signal  44  generated by an F-P laser is typically broader than any one channel band  50  of the WDM PON. This is advantageous because all of the ONTs  28  can be provided with identical F-P lasers  30 , and all of the OLT  4  transceivers  6  can be provided with identical F-P lasers  8 , allowing economies of scale to reduce unit costs. 
     The embodiments of the invention described above are intended to be illustrative only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.