Abstract:
A seed light source for use in Wavelength Division Multiplexed Passive Optical Network (WDM-PON) includes a multi-channel quantum dot laser for generating a multi-channel seed light comprising a plurality of respective channel seed lights. Each channel seed light corresponds to a respective channel of the WDM-PON.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on, and claims priority from, U.S. Provisional Patent Application Ser. No. 61/090,644, filed Aug. 21, 2008, the entire contents of which are incorporated herein by reference. This application is a Continuation in Part of U.S. patent application Ser. No. 12/341,012 filed Dec. 22, 2008, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present application relates generally to Wavelength Division Multiplexed Passive Optical Networks (WDM PON) and, more specifically, to seeding a WDM PON system using a quantum dot multi-wavelength laser source 
       BACKGROUND OF THE INVENTION 
       [0003]    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. 
         [0004]    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”. 
         [0005]    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. 
         [0006]    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 detector  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 detector  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 detector  10  may, for example, be 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) 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). 
         [0007]    A passive remote node  20  serving one or more customer sites includes an optical mux/demux  22  for demultiplexing wavelength channels 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 . 
         [0008]    Typically, the wavelength channels 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. 
         [0009]    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 OLT  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 (RSOAs); injection-locked Fabry-Perot lasers; reflective electro-absorptive modulators; and reflective Mach-Zehnder modulators. With this arrangement, each light source  8 ,  30  requires a respective “seed” light which is used to produce the corresponding 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-L)  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-C)  40  via a C-band optical circulator  42 . 
         [0010]    As may be seen in  FIGS. 2   a  and  2   b , each of the seed light sources (SLSs)  36 ,  40  may be constructed in a variety of different ways. In the SLS of  FIG. 2   a , a set of narrow-band lasers  44  are used to generate respective narrow band seed lights  46 , each of which is tuned to the center wavelength of a respective channel of the WDM-PON. A multiplexer  48  combines the narrow-band seed lights  46  to produce a WDM seed light  50 , which is then distributed through the WDM-PON to either the ONTs  26  (in the case of C-band seed light) or the transceivers  6  (in the case of L-Band seed light). If desired, each of the narrow-band lasers  44  may be provided as conventional bulk semiconductor laser diodes. 
         [0011]    In the SLS of  FIG. 2   b , the seed light source (SLS) is provided by a continuous broadband light source (BLS)  52  such as a Superluminescent Light Emitting Diode (SLED) or an Amplified Spontaneous Emission (ASE) source (such as an optical amplifier) that produces a continuous spectrum of light across a wide range of wavelengths. A comb filter  54  generates the desired WDM seed light  50  by filtering the continuous spectrum light emitted by the BLS  52 . 
         [0012]    In both of the SLSs of  FIGS. 2   a  and  2   b , an optical amplifier  58  (for example an Erbium Doped Fiber Amplifier (EDFA)) can be used to amplify the WDM seed light  50 . This arrangement is useful for increasing link budget (and thus signal reach). 
         [0013]    The system of  FIGS. 1 and 2  is advantageous in that the light sources  8 ,  30  are colorless. As a result, a common transceiver configuration can be used for every channel, which facilitates reduced costs via economies of scale. However, in WDM PON systems in which narrow-band lasers  44  are used to generate respective narrow band seed lights  46 , as described above with reference to  FIG. 2   a , the costs of the C-band and L-band SLSs  36 ,  40  may at least partially offset the cost savings obtained by using colorless transceivers. The use of a filtered broadband light source for generating the seed lights (as described with reference to  FIG. 2   b ) lowers the cost of the C-band and L-band SLSs  36 ,  40 , but lowers the seeding efficiency because much of the optical power generated by the BLS  52  is lost in the filter  56 , and results in increased relative intensity noise (RIN) in the output seed light  50 . In addition, filtering a broadband light source  52  to produce individual channel seed lights means that the band-width of each channel seed light is determined by the filter function of the comb filter  56 . Typically, this will result in channel seed lights of increased band width, as compared to the use of semiconductor laser seed light sources  44 , which induces increased noise in the channel signal output by an injection-locked or reflective light source  8 ,  30  due to heterodyne interference between the seed light and the channel signal. 
       SUMMARY OF THE INVENTION 
       [0014]    An aspect of the present invention provides, in a Wavelength Division Multiplexed Passive Optical Network (WDM-PON), a seed light source includes a multi-channel quantum dot laser for generating a multi-channel seed light comprising a plurality of respective channel seed lights. Each channel seed light corresponds to a respective channel of the WDM-PON. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    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: 
           [0016]      FIGS. 1   a  and  1   b  schematically illustrate a conventional WDM-PON known in the prior art; 
           [0017]      FIGS. 2   a  and  2   b  schematically illustrate respective conventional broadband light sources that may be used to general seed light in the WDM-PON of  FIG. 1 ; 
           [0018]      FIGS. 3   a - 3   d  schematically illustrate elements and principal operations of a seed light source in accordance with a representative embodiment of the present invention; and 
           [0019]      FIG. 4  schematically illustrates an Optical Network Terminal of a WDM-PON incorporating the seed light source of  FIGS. 3   a - d.    
       
    
    
       [0020]    It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]    The present invention provides techniques for seeding a Wavelength Division Multiplexing Passive Optical Network (WDM-PON). A representative embodiment is described below with reference to  FIGS. 3-4 . 
         [0022]    Referring to  FIGS. 3-4 , in very general terms, a seed light source utilizes one or more multi-channel quantum dot lasers to generate a WDM seed light for seeding a WDM-PON system. Multi-channel quantum dot based lasers are known in the art. Conveniently, the output spectrum of a Multi-channel quantum dot laser, including the number of channels, and the center wavelength and bandwidth of each channel, can be controlled by the design and construction of the quantum dot laser unit. If desired, known techniques can be used to improve stability of the quantum dot laser, and so reduce jitter in the center wavelength of each channel. For example, known feedback control loop techniques can be used to control temperature and laser drive current to maintain the laser output spectrum within predefined tolerances. 
         [0023]      FIG. 3   a  illustrates a representative embodiment of a Seed Light Source (SLS)  60  which comprises a pair of multi-channel quantum dot lasers  62 . Each laser  62  generates a respective multi-channel seed light  64  which comprises a set of narrow band channel seed lights  66  ( FIG. 3   b ) corresponding to respective channels of the WDM PON. The multi-channel seed lights  64  are combined using a passive optical combiner  68  to generate a WDM seed light  70 . The optical combiner  68  may, for example, be a passive filter based combiner known in the art, although other suitable optical combiner devices may be used, if desired. 
         [0024]    In some embodiments, a single multi-channel single quantum dot laser  62  may be used to generate a WDM seed light  70  encompassing respective channel seed lights  66  for all of the channels of the WDM-PON In such cases, the combiner  68  will clearly not be needed. In other embodiments, two or more lasers  62  may be used, each of which generates a respective multi-channel seed light  64  encompassing a set of channel seed lights  66  corresponding to a respective subset of the channels of the WDM-PON, as may be seen in  FIG. 3   b.    
         [0025]    In some embodiments, a single multi-channel quantum dot laser  62  may be used to generate a respective multi-channel seed light  64  encompassing all of the channel seed lights  66  of a given channel band. For example, in the embodiment of  FIG. 3   c , the multi-channel seed light  64   a  generated by multi-channel quantum dot laser  62   a  encompasses channel seed lights  66  for all of the C-band channels, and the multi-channel seed light  64   b  generated by multi-channel quantum dot laser  62   b  encompasses channel seed lights  66  for all of the L-band channels. In still other embodiments, two or more multi-channel quantum dot lasers  62  may be used for each channel band, if desired. 
         [0026]    In cases where two (or more) multi-channel quantum dot lasers  62  are used to generate seed lights of a given channel band of the WDM-PON, each multi-channel quantum dot laser  62  can be constructed to generate seed lights for a respective set of adjacent channels, as shown in  FIG. 3   b . However, is some cases it may be preferable to design each multi-channel quantum dot laser  62  to generate seed lights for interleaving sets of channels. For example,  FIG. 3   d  shows an embodiment in which multi-channel seed light  64   a  comprises channel seed lights for odd-numbered channels, and multi-channel seed light  64   b  comprises channel seed lights for even-numbered channels. This later arrangement may reduce relative intensity noise (RIN) in the output spectra of each multi-channel quantum dot laser  62 , by increasing the spectral separation between quantum dot emitters of each laser  62 . 
         [0027]    In some embodiments, the SLS  60  comprises two or more multi-channel quantum dot lasers  62  within a single integrated package, such as an Application Specific Integrated Circuit (ASIC), for example. This arrangement is beneficial in that it facilitates low-cost manufacturing of the SLS  60 . Preferably, the seed lights  64  generated by all of the multi-channel quantum dot lasers  62  within such an integrated package are combined, for example using a suitable optical combiner network, to generate a WDM seed light  70  which is output from the integrated package through a common optical fiber “pig-tail”. This arrangement is beneficial in that it eliminates the need for an optical combiner external to the integrated package, and thereby reduces costs and simplifies integration of the SLS  60  with an OLT  4 . 
         [0028]    If desired, an optical amplifier  72 , for example an Erbium Doped Fiber Amplifier (EDFA), can be used to amplify the WDM seed light  70  at the output of the SLS  60 . This arrangement is useful for increasing link budget (and thus signal reach). 
         [0029]    As mentioned above, the OLT transceivers  6  and ONTs  28  comprise reflective reflective light sources  8 ,  30 , such as reflective semi-conductor optical amplifiers (RSOAs); injection-locked Fabry-Perot lasers; reflective electro-absorptive modulators; and reflective Mach-Zehnder modulators. As is known in the art, some reflective light sources (for example RSOAs and injection-locked Fabry-Perot lasers) are polarization dependent. However, the seed lights  64  generated by the multi-channel quantum dot lasers  62  tend to be highly polarized. In such situations, the WDM seed light  70  can be depolarized using a depolarizer  74  as shown in  FIG. 3   a . In the embodiment of  FIG. 3   a , the depolarizer  74  divides the optical signal path into a through-path  76  and a rotation path  78 . Within the rotation path, a polarization rotator  80  (such as, for example, a ¼-wave bi-refringent crystal) is used to rotate the polarization angle by 90-degrees. The two paths  76  and  78  are then combined at the output  82  of the depolarizer  74 . As may be appreciated, known passive optical techniques can be used to implement the various elements of the depolarizer  74 . When the elements of the through-path  76  and a rotation path  78  are suitably matched, the recombined WDM seed light emerging from the output  82  of the depolarizer  74  will contain equal power contributions from both paths  76  and  78 , and thus will be de-polarized. 
         [0030]    In  FIG. 3   a , the depolarizer  74  is shown downstream of the EDFA  72 . However, this is not essential. In fact, the depolarizer  74  can be inserted at any desired location in the signal path. For example, in some embodiments, the depolarizer  74  is integrated into the SLS  60  immediately downstream of the signal combiner  68 . 
         [0031]      FIG. 4  schematically illustrates an OLT  4  incorporating a seed light source  60  in accordance with the present invention. The SLS  60  may be constructed as described above with reference to  FIG. 3 , and generates a WDM seed light  70  comprising channel seed lights  66  for both of the L-band and C-band channels of the WDM-PON. An optical amplifier  72  amplify the WDM seed light  70  as described above. An optical splitter  74 , for example a passive filter-based splitter of a type known in the art is used to separate the L-band and C-band channel seed lights, which are then supplied to the L-band and C-band optical circulators  38  and  42 , respectively. The remainder of the OLT  4  is constructed and operates in a conventional manner, and thus will not be further described. As may be seen in  FIG. 4 , the SLS  60  of the present invention enables a single integrated package to source respective channel seed lights for every channel of the WDM-PON. In so doing, the present invention simplifies integration of seed light sources into the WDM-PON, and reduces costs, as compared to prior art techniques. 
         [0032]    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.