English:
[0002] Fiber-to-the-home (FTTH) has been regarded as the ultimate form of broadband access offering very high bandwidth to end-users. Today's FTTH systems are mostly offered through point-to-multipoint time division multiplexed (TDM) passive optical networks (PONs) using a 1:N power splitter (e.g., Ethernet- PON, Gigabit-PON, and 10G versions of these systems) at a remote node (RN) in the field to share a common transceiver at the central office (CO), or through point-to- point (pt-2-pt) optical Ethernets with individual home-run fibers.
[0003] The upstream and downstream signals of a TDM-PON are transmitted using different optical wavelengths (usually 1310 nm for upstream transmission and 1490 nm for downstream transmission). The TDM-PON media access controller (MAC) within the CO schedules the transmission between the CO transceiver (TRX) and the end users by assigning appropriate time slots to each end user. A TDM-PON provides beneficial savings in the number of trunk fibers (between RN and CO) and optical transceiver counts at the CO while saving patch panel space to terminate fibers, but does not scale well with bandwidth growth. The bandwidth per household is often oversubscribed as the bandwidth per optical line terminal (OLT) TRX at the CO is shared among all optical network units (ONUs) connected to the given OLT TRX. To support Gb/s per user transmission speeds on a TDM-PON can require > 10 Gb/s transceivers at each ONU. Thus, high-speed transmissions can be both technologically challenging and expensive.
[0004] Pt-2-pt optical networks provide very high band widths to end users, but do not scale well with optical fiber termination at the CO and fiber counts. transceivers and fiber terminations in the CO. This usually results in greater space requirements, higher power, and increased capital expenses.
[0005] A wavelength division multiplexed (WDM) PON is another approach, which provides the benefit of fiber consolidation and pt-2-pt virtual links to end-users by assigning separate wavelengths between the CO and individual users. It can offer the benefits of both TDM-PON and pt-2-pt architectures. Traditional WDM-PON systems use a wavelength demultiplexer (as opposed to the power splitter used in TDM-PON) at the RN in the field to distribute a separate wavelength to end-users. To upgrade a conventional TDM-PON to a WDM-PON currently involves replacing the power splitter in the RN with the wavelength multiplexer and replacing all TDM-ONUs at user premises with WDM-ONUs. This all or nothing upgrade is a sort of fork-lifting upgrade that is not only cumbersome but also disruptive to current subscribers and difficult to coordinate. In addition, current WDM wavelength-multiplexers fix the wavelength spacing and optical spectrum at deployment time and constrain future spectral flexibility. In other words, conventional WDM-PON systems use a fixed wavelength plan which is difficult to change after deployment.
[0006] Despite its promise, WDM-PON technologies are still maturing and have not yet achieved mainstream adoption. As such, it is important to have a migration strategy to upgrade TDM-PON systems to WDM-PONs seamlessly with minimum disruption to the existing TDM-PON users. Such a system should support coexistence of TDM-PON and WDM-PON architectures during the migration period.
[0007] Non-limiting and non-exhaustive embodiments/implementations of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
[0008] FIG. 1 is a functional block diagram illustrating a hybrid passive optical network ("PON") system, in accordance with an implementation of the disclosure.
[0009] FIG. 2A is a chart illustrating time division multiplexing ("TDM") PON wavelengths and wavelength division multiplexing ("WDM") PON wavelengths of a hybrid PON, in accordance with an implementation of the disclosure.
[0010] FIG. 2B is a chart illustrating pass-bands of an optical combiner, in accordance with an implementation of the disclosure.
[0011] FIGS. 3A & 3B are flow charts illustrating a process of operating a hybrid PON including TDM optical network units ("ONUs") and WDM ONUs, in accordance with an implementation of the disclosure.
[0012] FIG. 4 is a functional block diagram illustrating a hybrid PON implemented with a 2:N remote node power splitter, in accordance with an implementation of the disclosure.
[0013] FIG. 5 is a functional block diagram illustrating a WDM optical line terminal ("OLT"), in accordance with implementations of the disclosure.
[0014] FIG. 6 is a functional block diagram illustrating the interleaving of WDM signals and how component integration at the WDM OLT provides a scalable architecture, in accordance with implementations of the disclosure.
[0015] FIG. 7 is a functional block diagram illustrating an external phase modulator for generating downstream WDM signals of a WDM PON, in accordance with an implementation of the disclosure.
[0016] FIG. 8 is a functional block diagram illustrating a colorless ONU of a WDM PON, in accordance with an implementation of the disclosure.
[0018] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[0019] Fiber-to-the-home ("FTTH") is a broadband access network infrastructure, capable of supporting long-term economic growth. One option for implementing an FTTH access network uses a passive optical network ("PON") architecture. A PON is a point-to-multipoint optical network that uses unpowered or passive optical splitters/multiplexers to enable one or two fibers to server multiple customer premises ("CPs") (e.g., 32 to 128 customer premises). Another option for FTTH is to string a separate homerun fiber from a carrier central office ("CO") all the way to each CP. Such architecture is called a point-2-point architecture (pt-2-pt) as opposed to PON's point-to-multipoint architecture.
[0020] FTTH deployment is so capital intensive that many broadband access infrastructure providers are feverishly looking for methods to lower the deployment cost and migration from existing PON technologies (e.g., TDM-PON) to more advanced PON technologies (e.g., WDM-PON). FIG. 1 is a functional block diagram illustrating a hybrid PON system 100 capable of facilitating a seamless migration from an existing TDM-PON access network infrastructure to a WDM- PON access network infrastructure, in accordance with an implementation of the disclosure. Hybrid PON system 100 is capable of concurrently servicing TDM optical network units ("ONU") and WDM ONUs over a single fiber trunk line and remote node ("RN") power splitter. Hybrid PON system 100 facilitates a virtually seamless transition from TDM-PON architecture to WDM PON architecture.
[0021] The illustrated implementation of hybrid PON system 100 includes a CO 105, a trunk cable 110, a trunk line or trunk fiber strand 115 inside the trunk cable 110, and a RN power splitter 120, access lines 125, demarcation points 135, and TDM ONUs 140 and WDM ONUs 145 at CPs 150. The illustrated implementation of CO 105 includes a WDM optical line terminal ("OLT") 155, a TDM OLT 160, an optical combiner 165, and an aggregation node 170. The illustrated implementation of WDM OLT 155 includes a pt-2-pt PON media access controller ("MAC") 175, an optical transceiver array 180, and a wavelength interleaving multiplexer/demultiplexer ("MUX/DEMUX") 185. The illustrated implementation of TDM OLT 160 includes a TDM-PON MAC 190 and a TDM- PON transceiver 195.
[0022] Trunk line 115, RN PS 120, and access lines 125 that couple to WDM ONUs 145 may be considered to make up a WDM PON or WDM sub-PON, while the trunk line 115, RN PS 120, and access lines 125 that coupled to TDM ONUs 140 may be considered to make up a TDM PON or TDM sub-PON, and collectively trunk line 115, RN PS 120 and all access lines 125 may be referred to as a hybrid PON. WDM OLT 155 and TDM OLT 160 serve as the service provider's endpoint at CO 105 for their respect sub-PONs. Each OLT serves two primary functions: 1) performing optical-to-electrical and electrical-to-optical conversions between the service provider' s equipment and their respective sub-PON and 2) coordinating multiplexing with the ONUs at the other end of their respective sub- PONs. Of course, WDM OLT 155 performs wavelength division multiplexing between ONUs 145 and TDM OLT 160 performs time division multiplexing between ONUs 140. Each WDM OLT 155 and TDM OLT 160 may service a number of CPs 150 (e.g., 32, 64, 128, etc.) while a single CO 105 may services thousands of CPs 150 (e.g., 50,000). Thus, while FIG. 1 illustrates CO 105 as including just one WMD OLT 155 and one TDM OLT 160, in practice, CO 105 may include many paired WDM OLTs 155 and TDM OLTs 160 with their respective MACs 175 or 190 multiplexed via aggregation node 170.
[0023] Trunk cable 110 including trunk fiber strand 115 extends from CO 105 to RN power splitter 120 located within a neighborhood of CPs 150, which represent potential or existing customers of the service provider. From RN power splitter 120, individual fiber access lines 125 extend to corresponding CPs 150. Access lines 125 may be terminated at demarcation points 135. The demarcation point 135 may be located on a side of the CP where the fiber access line enters the building or in an access box or hand hole near the customers property line (e.g., in a utility right-of-way). Demarcation points 135 provide a well-defined demarcation identification point between the customer premise equipment ("CPE") including TDM ONU 140 and/or WDM ONU145 and the carrier's fiber plant. Demarcation points 135 may serve to demark the terminal point of maintenance responsibility for the carrier. Both trunk cable 110 and access lines 125 may run along a utility right- of-way running adjacent to a neighborhood street. Trunk cable 110 and access lines 125 may be a subterranean fiber plant, a suspended fiber plant (e.g., suspended along telephone poles), or otherwise.
[0024] ONUs 140 and 145 serve as the CPE endpoint at CPs 150 and perform the primary function of performing optical-to-electrical and electrical-to- optical conversions. ONUs 140 and 145 link to a paired set of WDM OLT 155 and TDM OLT 160 at CO 105 via a single RN power splitter 120, trunk fiber strand 115 and optical combiner 165. All downstream signals (signals propagating along the optical direction extending from CO to CP) received from both WDM OLT 155 and TDM OLT 160 are broadcast on all downstream ports (ports optically facing CPs) of RN power splitter 120 towards CPs 150. All upstream signals (signals propagating along the optical direction extending from CP to CO) received from either TDM ONUs 140 or WDM ONUs 145 are combined by RN power splitter 120 onto a single upstream port (ports optically facing the CO) for delivery to CO 105 via trunk fiber strand 115 and optical combiner/splitter 165. As discussed in further detail below, although the downstream WDM signals are broadcast to all access lines 125, each WDM ONU 145 includes tunable optical coherent transceiver circuitry that selects only the wavelength(s) it is intended to receive. The tunable optical transceiver circuitry avoids the need for unique, per user, tunable filters on each access line 125 linked to a different WDM ONU 145. These tunable filters can be expensive. Since the TDM signals are time multiplexed broadcast signals, each TDM ONU 140 includes the same optical filter as all other TDM ONUs 140, which passes the upstream and downstream TDM wavelengths (typically 1310 nm upstream and 1490 nm downstream). The WDM ONUs 145 may also include an inexpensive common filter that simply blocks the TDM wavelengths, or may exclude even these filters due to their inherent wavelength selectivity.
[0025] FIG. 2A is a chart 205 illustrating an example wavelength allocation or hybrid PON wavelength grid for the TDM and WDM signals. It should be appreciated that FIG. 2A is merely demonstrative and other wavelengths or bands may be allocated to the TDM and WDM signals than those illustrated. The TDM signals are allocated according to a TDM-PON wavelength grid 215 while the WDM signals are allocated according to a WDM-PON wavelength grid 220. As illustrated, TDM-PON wavelength grid 215 includes just two wavelengths— an upstream wavelength and a downstream wavelength. The downstream wavelength is broadcast to all TDM ONUs 140. In contrast, the WDM-PON wavelength grid 220 includes many upstream and downstream wavelengths (e.g., 32 upstream wavelengths and 32 downstream wavelengths having a 50 GHz wavelength spacing between upstream and downstream wavelengths, and 100GHz wavelength spacing between neighboring upstream or neighboring downstream wavelengths). In one implementation, the WDM-PON wavelength grid 220 allocates the wavelengths such that consecutive communication wavelengths are alternatively assigned between downstream and upstream WDM signals. In one implementation, the WDM-PON wavelength grid 220 is further allocated such that each WDM ONU 145 is allocated adjacent communication wavelengths. For example, adjacent upstream/downstream wavelengths 225 may be allocated to a single CP 150. Of course, a single CP 150 may be allocated additional bandwidth by allocating more than a block of two communication wavelengths (e.g., by allocating 4, 6, etc.).
[0026] In the implementation of FIG. 1, the TDM and WDM signals are delivered to RN power splitter 120 over a single trunk fiber strand 115. As such, an optical combiner 165 is located at the CO 105 to combine the TDM and WDM signals arriving at and departing from the WDM OLT 155 and TDM OLT 160. FIG. 2B is a chart 210 illustrating pass-bands of optical combiner 165, in accordance with an implementation of the disclosure. In the illustrated implementation, optical combiner 165 is a band combiner that includes a first pass-band 230 for directing TDM signals and a second pass-band 235 for directing WDM signals. TDM signals that fall within pass-band 230 are permitted to pass through optical combiner 165 in either the upstream or downstream direction to travel between TDM OLT 160 and trunk fiber strand 115. Conversely, downstream WDM signals that fall within pass- band 235 are coupled into trunk fiber strand 115 in the downstream direction and upstream WDM signals within pass-band 235 are separated from the TDM signals and reflected towards WDM OLT 155. In one embodiment, optical combiner 165 is implemented using a dichroic filter that is substantially transparent in pass-band 230 and substantially reflective in pass-band 235. In another embodiment, optical combiner 165 is substantially reflective in pass-band 230 and substantially transparent in pass-band 235.
[0027] FIGS. 3 A & 3B are flow charts illustrating processes of operation of hybrid PON system 100, in accordance with an implementation of the disclosure. FIG. 3A illustrates a process 301 for downstream flowing communications, while FIG. 3B illustrates a process 302 for upstream flowing communications. The order in which some or all of the process blocks appear in process 301 or 302 should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated, or even in parallel.
[0028] In a process block 305, each OLT within CO 105 receives electrical signals from their respective MACs (e.g., pt-2-pt PON MAC 175 or TDM-PON MAC 190) and generates optical signals for launching into their respect sub-PONs. For example, WDM OLT 155 generates WDM signals (discussed in greater detail below in connection with FIG. 5) while TDM OLT 160 generates TDM signals. In a process block 310, optical combiner 165 combines the downstream WDM signals on WDM-PON wavelength grid 220 with the downstream TDM signal from TDM-PON wavelength grid 215 on trunk fiber strand 115. Trunk fiber strand 115 guides the downstream WDM and TDM signals from CO 105 to RN power splitter 120 (process block 315). In one implementation, RN power splitter 120 is a passive optical element that splits the power of the downstream optical signals received on its upstream port across all of its downstream optical ports. Thus, in a process block 320, RN power splitter 120 broadcasts all the downstream signals onto all of its downstream facing optical ports. In other words, both the downstream TDM signal and the downstream WDM signals are broadcast onto all downstream facing optical ports of RN power splitter 120.
[0029] Since the downstream WDM signals each support a virtual pt-2-pt link between CO 105 and WDM ONUs 145, each WDM ONU 145 includes a tunable optical receiver that selects the specific wavelength intended for the particular CP 150. In one implementation, the downstream WDM signals are phase modulated signals and the WDM ONUs 145 use coherent detection.
[0030] Correspondingly, since the downstream TDM signal is inherently a point-to-multipoint broadcast signal, a common TDM optical filter (not illustrated) is provided for all TDM ONUs 140 to pass the downstream TDM signal (process block 325). The common TDM optical filter is configured to pass the downstream (and upstream) TDM signal, but block all WDM-PON signals. The common TDM optical filters may also be positioned anywhere between TDM ONUs 140 and the downstream ports of RN power splitter 120; however, these filters are typically included within the TDM ONUs 140 themselves.
[0031] In a process block 330, the respective WDM ONUs 145 and TDM ONUs 140 receive and convert the downstream optical signals from the optical realm into the electrical realm for use by CPE.
[0032] Upstream communications are now described in connection with process 302 (FIG. 3B). In a process block 350, each individual CP 150 generates upstream data signals. For CPs 150 having a WDM ONU 145, the upstream data signals are converted to the optical realm using direct amplitude modulation onto a tunable carrier wavelength allocated to the particular CP 150 by the user's WDM ONU 145. Accordingly, WDM ONUs 145 should be capable of outputting upstream optical signals on the particular wavelength allocated to the user's upstream traffic, as allocated from the WDM-PON wavelength grid 220 (WDM ONU operation is discussed in greater detail below in connection with FIG. 8). For CPs 150 having a
[0033] The upstream TDM and WDM signals are delivered to RN power splitter 120 via their respective access lines 125. In a process block 360, the optical power of each of these upstream signals is combined onto trunk fiber strand 115. Accordingly, RN power splitter 120 operates as a power combiner in the upstream direction.
[0034] The combined TDM and WDM signals are delivered to CO 105 along trunk fiber strand 115. At CO 105, optical combiner 165 operates to demultiplex the upstream TDM signal from the upstream WDM signals (process block 365). In the illustrated implementation, the WDM signals are separated from the TDM signal and reflected towards WDM OLT 155, while the TDM signal passes through optical combiner 165 towards TDM OLT 160. Of course, in another implementation, the pass-bands of optical combiner 165 may be configured such that the WDM signals pass through optical combiner 165 while the TDM signals are reflected.
[0035] Finally, in a process block 370, WDM OLT 155 de-multiplexes the individual upstream WMD signals and converts them to the electrical realm using direct detection techniques. Similarly, TDM OLT 160 converts the upstream TDM signal to the electrical realm. Thus, the WDM PON uses an asymmetric modulation scheme. In the downstream direction phase modulation and coherent detection is used while in the upstream direction amplitude modulation and direct detection is used. This asymmetric modulation scheme leverages the benefits of increased sensitivity associated with coherent detection in the downstream direction with the benefits of reduced cost associated with direction detection in the upstream direction. This asymmetric modulation scheme is advantageous because it is economical to place optical amplifiers (e.g., Erbium Doped Fiber Amplifiers) in the upstream and downstream paths at CO 105 where they can be cost shared across multiple CPs 150, as opposed to including two such amplifiers at each CP 150. Since the amplification resides on the CO side, the asymmetric modulation scheme places the more sensitive coherent detectors on the CP side and the less expensive direct detection on the CO side. The asymmetric modulation scheme proposed herein enables a TDM PON and a WDM PON to co-exist within a single hybrid PON during a technological migration from TDM to WDM and strikes a cost efficient balance for operating the WDM PON.
[0036] FIG. 4 is a functional block diagram illustrating a hybrid PON 400 implemented with a 2:N RN power splitter 420, in accordance with an implementation of the disclosure. Hybrid PON 400 is similar to hybrid PON system 100 with the following exceptions. Hybrid PON 400 is implemented with a 2:N RN power splitter 420 instead of a 1:N RN power splitter 120. By using a 2:N power splitter, two fiber trunk lines 416 and 417 are run to RN power splitter 420. Trunk line 416 links WDM OLT 155 to a first upstream port of RN power splitter 420 while the second trunk line 417 links TDM OLT 160 to a second upstream port of RN power splitter 420. While two trunk lines must be run between CO 405 and RN power splitter 420 within trunk cable 410 per pair of WDM OLT 155 and TDM OLT 160, this configuration avoids the need for optical combiner 165 by paying the penalty of doubling the fiber counts of trunk fiber 410. Both RN power splitter 120 and RN power splitter 420 may be implemented using a passive planar lightwave circuit ("PLC"); however, in the case of RN power splitter 120 the PLC is a 1:N optical device while in the case of RN power splitter 420 the PLC is a 2:N optical device.
[0037] FIG. 5 is a functional block diagram illustrating a WDM OLT 500, in accordance with an implementation of the disclosure. WDM OLT 500 is one possible implementation of WDM OLT 155 illustrated in FIG. 1. The illustrated implementation of WDM OLT 500 includes a PT-2-PT PON MAC 505, an optical transmitter array 510, an optical MUX 515, an optical receiver array 520, an optical DEMUX 525, optional optical amplifiers 530 and 535, and an optical diplexer 540.
[0038] Collectively, optical MUX 515, optical DEMUX 525, and optical diplexer 540 functionally form a wavelength MUX/DEMUX for combining the downstream WDM signals onto the I/O port for launching into a PON (e.g., hybrid PON system 100) and for routing the upstream WDM signals received from a PON to the appropriate optical receiver (e.g., PD 11, PD12...PD In). These components collectively represent one possible implementation of wavelength MUX/DEMUX 185, illustrated in FIGs. 1 and 4.
[0039] The illustrated implementation of optical transmitter array 510 includes a plurality of laser sources (e.g., LS 11, LS 12... LS In) each coupled to an external modulator 545. Each external modulator 545 is coupled to receive a data signal (Din) from PT-2-PT PON MAC 505 and to modulate the respective data signal onto the carrier wavelength received from its respective laser source in phase or amplitude or both. Phase modulation may be preferred because of its higher sensitivity. Optical MUX 515 includes a plurality of upstream ports each coupled to a corresponding one of the external modulators 545. Optical MUX 515 combines the various downstream WDM signals that are modulated onto different carrier wavelengths on a single optical link coupled to optical diplexer 540. In one implementation, this optical link includes optical amplifier 530 (e.g., EDFA) to amplify all of the downstream WDM signals together. Optical diplexer 540 directs the downstream WDM signals received from optical MUX 515 to its I/O port for delivery to RN PS 120 (or 420) as described in connection with FIGs. 1-4. Optical diplexer 540 may be implemented using a 3-dB power splitter, a three port optical circulator, a 2: 1 interleaver, a band filter that separates upstream and downstream signals, or otherwise.
[0040] The illustrated implementation of optical receiver array 520 includes a plurality of photo-detectors (e.g., PD 11, PD 12...PD In) each coupled to a respective output port of DEMUX 525. Optical diplexer 540 routes upstream WDM signals received on different carrier wavelengths to optical DEMUX 525, which in turn de-multiplexes the upstream WDM signals based upon their respective carrier wavelengths to the appropriate photo-detector. In one implementation, optical amplifier 535 (e.g., EDFA) is linked between optical diplexer 540 and optical DEMUX 525 to collectively amplify all the upstream WDM signals routed to optical DEMUX 525. In the illustrated implementation, the photo-detectors perform direct detection of information modulated on the amplitude of the upstream WDM signals. In one implementation, photo-detectors are implemented as photo-diodes (e.g., PIN or avalanche photo-diodes) coupled to trans-impedance amplifiers ("TIAs"). The upstream WDM signals are demodulated and converted to the electrical realm and output as data out (Dout) signals, which are coupled into PT-2-PT PON MAC 505.
[0041] Since WDM OLT 500 uses a coherent transmitter and phase modulation in the downstream direction, but direct detection circuitry and amplitude modulation in the upstream direction, WDM OLT 500 implements an asymmetric modulation scheme. The asymmetric modulation scheme leverages the respective advantages of coherent detection in the downstream direction and direct detection in the upstream direction. In implementations where the fiber runs between CO 105 and CPs 150 are sufficiently long so as to substantially benefit from in-line optical amplifiers (e.g., optical amplifiers 530 and 535), these amplifiers are positioned at CO 105 where they can be shared amongst all CPs 150— instead of a one or two amplifiers included in each ONU 145 on the CP side. Since it is often more beneficial to place optical amplification on the received side, the more expensive and sensitive coherent detection circuitry is saved for the CP side (where optical amplification is omitted since it cannot easily be shared across multiple CPs on the CP side), while the less expensive direction detection is used on the CO side, which can include shared optical amplification when necessary.
[0042] In one implementation, the optical transmitter array 510, optical receiver array 520, optical MUX 515, and optical DEMUX 525 may be combined into an integrated module 550 to reduce the manufacturing costs of the WDM OLT and to simplify in-field deployment. In one implementation, integrated module 550 is a single physical package that includes a photonic integrated circuit ("PIC"). The PIC may include both optical and electrical devices, such as laser diodes, photodiodes, driver circuits, TIAs, phase modulators, arrayed waveguide gratings ("AWG"), etc. In one implementation, optical MUX 515 and optical DEMUX 525 are implemented as a passive optical device, such as an AWG.
[0044] FIG. 7 is a functional block diagram illustrating an external phase modulator 700 for generating downstream WDM signals of a WDM PON, in accordance with an implementation of the disclosure. External phase modulator 700 is one possible implementation of external modulators 545. The illustrated implementation of external phase modulator 700 includes an optical phase modulator 705 and drivers 710 and 715. Optical phase modulator 705 may be implemented using an electro-optic modulator to encode a phase modulation onto the carrier wavelength output by the laser source. In one implementation, optical phase modulator 705 is a Mach-Zhender ("MZ") modulator that uses binary phase shift keyed ("BPSK") digital modulation for coherent transmission of downstream WDM signals. The MZ and BPSK combination reduces the circuit complexity relative to a full blown coherent optical transmitter used for long haul WDM transmission systems, which typically include a pair of polarization multiplexed I-Q modulators and digital encoders. Such systems achieve increased spectral efficiency by using both polarizations and both quadratures in each polarization for signal encoding. However, such systems are also complex and currently expensive for large scale integration. The illustrated implementation of external phase modulator 700 trades off spectral efficiency for cost, power, and space savings. However, it is contemplated that in the future, arrays of dual-polarization quadrature amplitude modulation ("QAM") modulators may eventually be realized at competitive costs, power, and space requirements to be incorporated into the architecture disclosed herein.
[0045] FIG. 8 is a functional block diagram illustrating a colorless WDM ONU 800, in accordance with an implementation of the disclosure. WDM ONU 800 represents one possible implementation of WDM ONUs 145 illustrated in FIGs. 1 and 4. The illustrated implementation of WDM ONU 800 includes an optical diplexer 805, a tunable optical receiver 810, a tunable optical transmitter 815, a MAC 820, a switch 825, and one or more local area network ("LAN") physical units (PHYs) 830. The illustrated implementation of tunable optical receive 810 includes a tunable laser source LS I, a polarization beam splitter ("PBS") 840, two 90 degree hybrids 845, balanced photo-receivers 850, analog-to-digital converters A/D, and receive digital signal processing logic 860. The illustrated implementation of tunable optical transmitter 815 includes a tunable laser source LS2 and driver circuitry 860. Optical diplexer 805 may be implemented as 3-dB power splitter, a three port optical circulator, a cyclical comb filter if the upstream and downstream wavelength are interleaved, a red/blue band filter if the upstream and downstream WDM signals use different bands (e.g., C-band for upstream and L-band for downstream), or otherwise.
[0046] WDM ONU 800 includes a coherent receiver that uses coherent detection of a phase modulated signal in the downstream direction and includes a directly modulated transmitter that uses amplitude modulation for upstream signals. In the upstream direction, tunable optical transmitter 815 is capable of launching upstream WDM signals with a selectable carrier wavelength via appropriate tuning of tunable laser source LS2. The information is then directly amplitude modulated onto the selected carrier wavelength. WDM ONU 800 also includes a colorless or wavelength agnostic optical receiver that is compatible with a TDM sub-PON in a hybrid PON system by using a broadcast-and-select approach in the downstream direction. In other words, tunable optical receiver 810 is capable of tuning to a specific carrier wavelength and rejecting the other carrier wavelengths allocated to other WDM ONUs in the PON. The illustrated implementation of tunable optical receiver 810 implements its tunable functionality using laser source LSI as a local oscillator (LO) to create a selectable beat wavelength for down mixing the selected carrier wavelength. Tunable optical receiver 810 includes coherent receiver circuitry that may implement homodyne, heterodyne, or intradyne coherent detection. In one implementation, LSI and LS2 are identical laser sources. In one implementation, LSI and LS2 are identical tunable laser sources integrated into a single package for space and cost efficiency.
[0047] It is also noteworthy that the LO at the tunable receiver may also serve as an amplifier to amplify the received signal from the OLT at the ONU, thus obviating the need for an EDFA at the CP.
[0048] Tunable optical receiver 810 is a coherent optical receiver that uses PBS 840 to split received downstream WDM signals into X and Y polarizations. Each polarization is mixed with the LO signal in the 90 degree hybrid 845. This mixing generates an in-phase and quadrature phase outputs that are detected by pairs of balanced receivers 850. Even though only one polarization may be used by the transmitter in the WDM OLT, the polarization of the downstream WDM signals in unpredictable at tunable optical receiver 810 due to polarization rotation, which occurs during fiber transmission. Therefore, polarization diversity is used at the coherent receiver. Nevertheless, since just one receiver is used on the CP side, as opposed to an array of receivers on the CO side, integration complexity is manageable. Following the 4 balanced receivers 850 are A/D converters and receiver DSP logic 860 to recover the transmitted phase and/or amplitude information depending on the modulation scheme used at the OLT. In one implementation, a simple coherent modulator which can be easily manufactured into arrays at the OLT and has a high signal to noise ratio (SNR) tolerance is a binary phase modulator.
[0049] The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
[0050] These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.

中文:
[0002]光纤到户(FTTH)已被认为是向终端用户提供非常高的带宽的宽带接入的最终形式。当今的FTTH系统大多通过以下网络来提供:在现场的远程节点(RN)处使用1:N的功率分配器以共享中心局(CO)处的公共收发器的点对多点时分复用(TDM)无源光网络(PON)(例如,以太网PON、千兆比特PON和这些系统的10G版本),或者具有各个分布式光纤(home-run fiber)的点对点(pt-2-pt)光以太网。
[0003]使用不同的光波长来传输TDM-PON的上行和下行信号(通常,上行传输为1310nm,且下行传输为1490nm)。CO内的TDM-PON媒体访问控制器(MAC)通过向每个终端用户分配适当的时间片来调度CO收发器(TRX)和终端用户之间的传输。TDM-PON在节省用于终结光纤的接线板空间的同时,有益地节省了(在RN和CO之间的)干线光纤的数量以及在CO处的光收发器计数,但其不能随带宽的增长而很好地缩放。由于在连接到给定的光线路终端(OLT)TRX的所有光网络单元(ONU)之间共享在CO处的每个OLT TRX的带宽,每户带宽通常是超额认购的。在TDM-PON上支持每用户Gb/s的传输速度可能需要在每个ONU处的大于10Gb/s的收发器。因此,高速度传输不仅在技术上具有挑战还很昂贵。
[0004]pt-2-pt光网络向终端用户提供非常高的带宽,但不能随CO处的光纤终端和光纤支数的变化而很好地缩放。相反,pt-2-pt光网络导致大量的干线线路以及CO中的收发器和光纤终端。这样通常会导致更大的空间需求、更高的功率和增加的资本费用。
[0005]波分复用(WDM)PON是另一种方法,其通过在CO和各个用户之间分配不同的波长来提供光纤合并以及到终端用户的pt-2-pt虚拟链路的好处。其可以提供TDM-PON和pt-2-pt两种架构的好处。传统的WDM-PON系统使用在现场的RN处的波长解复用器(与在TDM-PON中使用的功率分配器不同),来向终端用户分发不同的波长。目前,将传统TDM-PON升级到WDM-PON涉及用波长复用器替代RN中的功率分配器,以及用WDM-ONU代替在用户驻地(user premise)处的所有TDM-ONU。这种全部升级或者完全不升级是一种叉车式升级(fork-lifting upgrade),其对于当前用户来说不仅笨重还具有破坏性,并且难以协调。此外,当前WDM波长复用器在部署时间确定波长间隔和光谱,并且限制将来光谱的灵活性。换句话说,传统的WDM-PON系统使用固定的波长计划,其在部署后难以更改。
[0006]尽管具有美好的前景,WDM-PON技术仍然在发展中,且还没有实现主流应用。这样,具有用于将TDM-PON系统无缝升级到WDM-PON且对现有的TDM-PON用户干扰最小的迁移策略很重要。这样的系统应在迁移期间支持TDM-PON和WDM-PON架构共存。
[0007]根据下面的附图描述本发明的非限制性和非穷尽实施例/实施方式,其中除非另有规定,各个附图中的相同参考数字指示相同的部分。
[0008]图1是根据本公开的一个实施方式描述混合无源光网络(“PON”)系统的功能框图;
[0009]图2A是根据本公开的一个实施方式描述混合PON的时分复用(“TDM”)PON波长和波分复用(“WDM”)PON波长的示意图;
[0010]图2B是根据本公开的一个实施方式描述光组合器(optical combiner)的通带的示意图;
[0011]图3A和3B是根据本公开的一个实施方式描述操作包括TDM光网络单元(“ONU”)和WDM PNU的混合PON的过程的流程图;
[0012]图4是根据本公开的一个实施方式描述用2:N远程节点功率分配器实现的混合PON的功能框图;
[0013]图5是根据本公开的一个实施方式描述WDM光线路终端(“OLT”)的功能框图;
[0014]图6是根据本公开的一个实施方式描述WDM信号的交织以及在WDM OLT处的元件集成如何提供可伸缩架构的功能框图;
[0015]图7是根据本公开的一个实施方式描述用于生成WDM PON的下行WDM信号的外相位调制器的功能框图;
[0016]图8是根据本公开的一个实施方式描述WDM PON的无色(colorless)ONU的功能框图。
[0018]整个本说明书中提及的“一个实施例”或“实施例”意味着结合该实施例描述的特定的特征、结构或特性包含在本发明的至少一个实施例中。因此,整个本说明书中各处出现的短语“在一个实施例中”或“在实施例中”不一定都涉及相同的实施例。此外,可以一个或多个实施例中以任何适当的方式组合该特定的特征、结构或特性。
[0019]光纤到户(“FTTH”)是能够支持长期经济增长的宽带接入网络基础设施。用于实现FTTH接入网络的一个选择是使用无源光网络(“PON”)架构。PON是使用不加电的或者无源光分配器(splitter)/复用器以使一个或多个光纤能够服务多个用户驻地(“CP”)(例如32到128个用户驻地)的点对多点光网络。FTTH的另一个选择是将独立的分布式光纤从运营商中心局(“CO”)一直串接到每个CP。与PON的点对多点架构相反,该架构称作点对点架构(pt-2-pt)。
[0020]FTTH部署是资本密集型部署,因此很多宽带接入基础设施提供商都在不遗余力地寻找降低部署成本,以及从现有PON技术(例如TDM-PON)迁移到更高级的PON技术(例如WDM-PON)的方法。图1是根据本公开的一个实施方式描述混合PON系统100的功能框图,该混合PON系统100能够促成从现有TDM-PON接入网络基础设施向WDM-PON接入网络基础设施的无缝迁移。混合PON系统100能够通过单个光纤干线和远程节点(“RN”)功率分配器同时服务TDM光网络单元(“ONU”)和WDM ONU。混合PON系统100促成从TDM-PON架构到WDM PON架构的虚拟无缝的转换。在添加新的CP并给予其更高级的WDM ONU的同时,现有的TDM CP可以继续使用它们现有的用户驻地设备(例如TDMONU)来进行操作。可在混合PON系统100内无限期地同时操作两种技术,或者可在混合PON系统100内同时操作两种技术,直到以受控制和有组织的方式换出传统TDM ONU,其中该方式限制了对现有终端用户的干扰并且提供了时间灵活性。
[0021]所示的混合PON系统100的实施方式包括CO105、干线电缆110、干线电缆110内的干线线路或干线光纤束115,和RN功率分配器120、接入线路125、分界点(demarcation point)135,以及在CP150处的TDMONU140和WDM ONU145。所示的CO105的实施方式包括WDM光线路终端(“OLT”)155、TDM OLT160、光组合器165和聚合节点170。所示的WDM OLT155的实施方式包括pt-2-pt PON媒体访问控制器(“MAC”)175、光收发器阵列180和波长交织复用器/解复用器(“MUX/DEMUX”)185。所示的TDM OLT160的实施方式包括TDM-PON MAC190和TDM-PON收发器195。
[0022]可认为由干线线路115、RN PS120和耦合到WDM ONU145的接入线路125组成了WDM PON或WDM子PON,而可以认为由干线线路115、RN PS120和耦合到TDM ONU140的接入线路125组成了TDMPON或TDM子PON,并且可将干线线路115、RN PS120和所有接入线路125一起称作混合PON。对于相应的子PON,WDM OLT155和TDMOLT160充当服务提供商在CO105处的端点。每个OLT提供两个主要功能:1)执行在服务提供商的设备和它们相应的子PON之间的光电和电光转换,以及2)协调与在它们相应的子PON的另一端处的ONU的复用。当然,WDM OLT155执行ONU145之间的波分复用,并且TDM OLT160执行ONU140之间的时分复用。每个WDM OLT155和TDM OLT160可服务多个(例如32、64、128等)CP150,而单个CO105可服务数千个(例如50000)个CP150。因此,尽管图1将CO105显示为仅包括一个WMD OLT155和一个TDM OLT160,但在实践中,CO105可包括具有经由聚合节点170复用的相应的MAC175或190的多个成对的WDM OLT155和TDM OLT160。
[0023]包括干线光纤束115的干线电缆110从CO105延伸到位于CP150附近的RN功率分配器120,其中CP150代表服务提供商的潜在的或者现有的客户。各个光纤接入线路125从RN功率分配器120延伸到对应的CP150。接入线路125可在分界点135处终止。分界点135可位于CP的一侧(其中光纤接入线路从该处进入建筑),或者位于靠近客户地界线(例如在公用事业路权中)的接入盒或手孔中。分界点135提供了在包括TDM ONU140和/或WDM ONU145的用户驻地设备(“CPE”)与运营商的光纤设备之间的明确界定的分界标识点。分界点135可用来区别运营商的维护责任的终点。干线电缆110和接入线路125可沿着靠近附近街道的公用事业路权延伸。干线电缆110和接入线路125可以是地下光纤设备、悬挂光纤设备(例如沿电线杆悬挂),或者其他设备。
[0024]ONU140和145充当CP150处的CPE端点,并且执行进行光电和电光转换的主要功能。ONU140和145经由单个RN功率分配器120、干线光纤束115和光组合器165链接到在CO105处的一组成对的WDM OLT155和TDM OLT160。在RN功率分配器120的所有下行端口(在光学上面向CP的端口)上,将从WDM OLT155和TDM OLT160接收的所有下行信号(沿从CO向CP延伸的光方向传播的信号)广播到CP150。由RN功率分配器120将从TDM ONU140或WDM ONU145接收的所有上行信号(沿从CP到CO延伸的光方向传播的信号)组合到单个上行端口(在光学上面向CO的端口)上,以经由干线光纤束115和光组合器/分配器165传送到CO105。如在下文中更详细讨论的,尽管将下行WDM信号广播到所有接入线路125,但每个WDM ONU145包括可调光相干收发器电路,该电路仅选择其想要接收的波长。可调光收发器电路避免了对于链接到不同的WDM ONU145的每条接入线路125上的每用户独特的可调滤波器的需求。这些可调滤波器可能很昂贵。由于TDM信号是在时间上复用的广播信号,因此每个TDM ONU140包括与所有其他的TDM ONU140相同的光滤波器,其中该光滤波器通过上行和下行TDM波长(通常上行为1310nm且下行为1490nm)。WDM ONU145还可以包括简单地阻塞TDM波长的便宜的通用滤波器,或者由于其固有的波长选择性,甚至可以排除这些滤波器。
[0025]图2A是描述用于TDM和WDM信号的示例的波长分配或混合PON波长栅格(grid)的示意图205。应理解,图2A仅是说明性的,并且可将与所示的那些不同的其他波长或频带分配给TDM和WDM信号。根据TDM-PON波长栅格215来分配TDM信号,而根据WDM-PON波长栅格220来分配WDM信号。如图所示,TDM-PON波长栅格215仅包括两个波长-上行波长和下行波长。下行波长被广播到所有的TDM ONU140。相反地,WDM-PON波长栅格220包括很多个上行和下行波长(例如32个上行波长和32个下行波长,其中在上行和下行波长之间具有50GHz的波长间隔,并且在相邻的上行波长或相邻的下行波长之间具有100GHz的波长间隔)。在一个实施方式中,WDM-PON波长栅格220分配波长以使得在下行和上行WDM信号之间交替地分配连续的通信波长。在一个实施方式中,还分配WDM-PON波长栅格220,以使得对每个WDM ONU145分配相邻的通信波长。例如,可将相邻的上行/下行波长225分配给单个CP150。当然,可以通过分配不止由两个通信波长构成的块(例如分配4、6等),来为单个CP150分配额外的带宽。
[0026]在图1的实施方式中,通过单个干线光纤束115将TDM和WDM信号传送给RN功率分配器120。这样,将光组合器165布置在CO105处,以组合在WDM OLT155和TDM OLT160处到达的或者从WDM OLT155和TDM OLT160离开的TDM和WDM信号。图2B是根据本公开的一个实施方式描述光组合器165的通带的示意图210。在所示的实施方式中,光组合器165是包括用于指引TDM信号的第一通带230和用于指引WDM信号的第二通带235的频带组合器。允许属于通带230的TDM信号朝上行方向或者下行方向通过光组合器165,以使该信号在TDM OLT160和干线光纤束115之间传输。相反,将属于通带235的下行WDM信号朝下行方向耦合到干线光纤束115,以及将在通带235内的上行WDM信号与TMD信号分开,并将其反射到WDM OLT155。在一个实施例中,使用在通带230中实质上透明的并且在通带235中实质上可反射的二向色滤波器实现光组合器165。在另一个实施例中,光组合器165在通带230中实质上可反射,并且在通带235中实质上是透明的。
[0027]图3A和3B是根据本公开的一个实施方式描述混合PON系统100的操作过程的流程图。图3A描述用于下行流通信的过程301,而图3B描述用于上行流通信的过程302。在过程301或302中出现的一些或所有过程块的顺序不应被认为是限制性的。相反,从本公开受益的本领域普通技术人员应理解,可以按未示出的各种顺序来执行一些过程块,或者甚至并行地执行一些过程块。
[0028]在过程块305中,CO105内的每个OLT接收来自其相应的MAC(例如,pt-2-pt PON MAC175或者TDM-PON MAC190)的电信号,并且生成光信号以发射到其相应的子PON内。例如,WDM OLT155生成WDM信号(将在下文中结合图5更详细地进行讨论),而TDM OLT160生成TDM信号。在过程块310中,光组合器165将WDM-PON波长栅格220上的下行WDM信号与来自TDM-PON波长栅格215的下行TDM信号组合到干线光纤束115上。干线光纤束115将下行WDM和TDM信号从CO105引导到RN功率分配器120(过程块315)。在一个实施方式中,RN功率分配器120是无源光元件,其将在其上行端口上接收的下行光信号的功率分到所有其下行光端口上。因此,在过程块320中,RN功率分配器120将所有下行信号广播到该RN功率分配器120的所有下行面向光端口上。换句话说,将下行TDM信号和下行WDM信号广播到RN功率分配器120的所有下行面向光端口上。
[0029]由于每个下行WDM信号支持CO105和WDM ONU145之间的虚拟pt-2-pt链路,则每个WDM ONU145包括可调光接收器,该可调光接收器选择以特定CP150为目的的特定波长。在一个实施方式中,下行WDM信号是相位调制的信号,且WDM ONU145使用相干检测。
[0030]相应地,由于下行TDM信号本质上是点对多点广播信号,因此向所有TDM ONU140提供通用TDM光滤波器(未示出),以通过下行TDM信号(过程块325)。该通用TDM光滤波器被配置为通过下行(以及上行)TDM信号,但阻塞所有WDM-PON信号。还可以将该通用TDM光滤波器布置在TDM ONU140与RN功率分配器120的下行端口之间的任何位置;然而,通常将这些滤波器包括在TDM ONU140自身之内。
[0031]在过程块330中,相应的WDM ONU145和TDM ONU140接收下行光信号,并且将该下行光信号从光领域转换到电领域,以便由CPE使用。
[0032]现结合过程302(图3B)描述上行通信。在过程块350中,每个单独的CP150生成上行数据信号。对于具有WDM ONU145的CP150,由用户的WDM ONU145使用直接幅度调制将上行数据信号在分配给特定CP150的可调载波波长上转换到光领域。相应地,WDM ONU145应能够输出在分配给用户的上行流量的特定波长上的上行光信号,如根据WDM-PON波长栅格220分配的特定波长(将在下文中结合图8更详细地讨论WDM ONU的操作)。对于具有TDM ONU140的CP150,将上行数据信号转换到光领域,并且将来自不同TDM ONU140的信号在时间上复用(调度)到上行TDM波长上。将光上行TDM和WDM信号发射到接入线路125内(过程块355)。
[0033]经由相应的接入线路125将上行TDM和WDN信号传送到RN功率分配器120。在过程块360中,将这些上行信号中的每个信号的光功率组合到干线光纤束115上。相应地,RN功率分配器120充当上行方向的功率组合器。
[0034]沿干线光纤束115将组合的TDM和WDM信号传送到CO105。在CO105处,光组合器165操作以将上行TDM信号从上行WDM信号中解复用(过程块365)。在所示的实施方式中,将WDM信号与TDM信号分离,并将其反射到WDM OLT155,而TDM信号通过光组合器165去往TDM OLT160。当然,在另一个实施方式中,可将光组合器165的通带配置为使得WDM信号通过光组合器165而反射TDM信号。
[0035]最终,在过程块370中,WDM OLT155使用直接检测技术对各个上行WDM信号进行解复用,并且将它们转换到电领域。类似地,TDM OLT160将上行TDM信号转换到电领域。因此,WDM PON使用不对称调制方案。在下行方向上使用相位调制和相干检测,而在上行方向上使用幅度调制和直接检测。这种不对称调制方案利用了与下行方向上的相干检测关联的增加的灵敏度的益处、以及与上行方向上的直接检测关联的减少的成本的益处。该不对称调制方案是有利的,因为与在每个CP150处包括两个这样的放大器相比,在CO105处布置上行和下行路径中的光放大器(例如,掺铒光纤放大器)是比较经济的,其中该光放大器可在多个CP150之间被成本共享。由于在CO侧执行放大,不对称调制方案将更多的感测相干检测器布置在CP侧,并且将较为便宜的直接检测布置在CO侧。本文提供的不对称调制方案在从TDM到WDM的技术迁移期间使TDMPON和WDM PON能够在单个混合PON内共存,并且取得了用于操作WDM PON的成本效率平衡。
[0036]图4是根据本公开的一个实施方式描述用2:N RN功率分配器420实现的混合PON400的功能框图。除了以下情况,混合PON400类似于混和PON系统100。用2:N RN功率分配器420代替1:N RN功率分配器120来实现混合PON400。通过使用2:N功率分配器,两个光纤干线线路416和417延伸到RN功率分配器420。干线线路416将WDM OLT155链接到RN功率分配器420的第一上行端口,而第二干线线路417将TDMOLT160链接到RN功率分配器420的第二上行端口。尽管必须在CO405和RN功率分配器420之间在干线电缆410内为每对WDM OLT155和TDM OLT160运行两个干线线路,但这种配置通过以干线光纤410的光纤计数加倍为代价避免了对光组合器165的需求。可使用无源平面光波导电路(“PLC”)来实现RN功率分配器120和RN功率分配器420;然而,对于RN功率分配器120,该PLC是1:N光装置;而对于RN功率分配器420,该PLC是2:N光装置。
[0037]图5是根据本公开的一个实施方式描述WDM OLT500的功能框图。WDM OLT500是图1所示的WDM OLT155的一个可能的实施方式。所示的WDM OLT500的实施方式包括pt-2-pt PON MAC505、光发射器阵列510、光MUX515、光接收器阵列520、光DEMUX525、可选的光放大器530和535,以及光双工器540。
[0038]光MUX515、光DEMUX525和光双工器540共同在功能上形成波长MUX/DEMUX,用于将下行WDM信号组合到I/O端口上,以便发射到PON(例如混合PON系统100)内,以及用于将从PON接收的上行WDM信号路由到合适的光接收器(例如PD11、PD12...PD1n)。这些元件共同代表了在图1和4中所示的波长MUX/DEMUX的一种可能的实施方式。
[0039]所示的光发射器阵列510的实施方式包括多个激光源(例如LS11、LS12...LS1n),其中每个激光源耦合到外调制器545。耦合每个外调制器545以接收来自pt-2-pt PON MAC505的数据信号(Din),以及采用相位或幅度或者两者将相应的数据信号调制到从其相应的激光源接收的载波波长上。优选相位调制,因为其灵敏度较高。光MUX515包括多个上行端口,每个上行端口耦合到外调制器545中对应的一个。光MUX515将调制到不同的载波波长上的各个下行WDM信号组合到耦合到光双工器540的单个光链路上。在一个实施方式中,该光链路包括光放大器530(例如EDFA),以将所有下行WDM信号一起放大。光双工器540将从光MUX515接收的下行WDM信号引导到该光双工器540的I/O端口,以便将该信号传送到结合图1-4描述的RN PS120(或420)。可使用3-dB功率分配器、三端口光环行器、2:1交织器、分离上行和下行信号的频带滤波器或者其他设备来实现光双工器540。
[0040]所示的光接收器阵列520的实施方式包括多个光检测器(例如PD11、PD12...PD1n),其中每个光检测器耦合到DEMUX525的相应的输出端口。光双工器540将在不同载波波长上接收的上行WDM信号路由到光DEMUX525,该光DEMUX525继而基于它们相应的载波波长将上行WDM信号解复用到合适的光检测器。在一个实施方式中,在光双工器540和光DEMUX525之间链接光放大器535(例如EDFA),以便一起放大路由到光DEMUX525的所有上行WDM信号。在所示的实施方式中,光检测器对在上行WDM信号的幅度上调制的信息进行直接检测。在一个实施方式中,光检测器被实现为耦合到跨阻抗放大器(“TIA”)的光二极管(例如PIN或者雪崩光二极管)。将上行WDM信号解调和转换到电领域,并且输出为耦合到pt-2-pt PON MAC505的数据输出(Dout)信号。
[0041]由于WDM OLT500在下行方向上使用相干发射器和相位调制,而在上行方向上使用直接检测电路和幅度调制,因此WDM OLT500实现了一种不对称调制方案。该不对称调制方案利用了下行方向上的相干检测以及上行方向上的直接检测的相应益处。在一些实施方式中,其中在CO105和CP150之间延伸的光纤足够长以至于能充分受益于线上光放大器(例如光放大器530和535),则在(其中可在所有CP150之间共享这些放大器的)CO105处布置这些放大器,以代替在CP侧的每个ONU145中包括一个或两个放大器。由于在接收侧进行光放大通常更有益处,因此将更昂贵和灵敏的相干检测电路保留在CP侧(其中,由于不能在CP侧的多个CP之间容易地共享光放大而省略该光放大),而在CO侧使用较为便宜的直接检测,在必要时该CO侧可以包括共享的光放大。
[0042]在一个实施方式中,可将光发射器阵列510、光接收器阵列520、光MUX515和光DEMUX525组合到集成模块550中,以减少WDM OLT的制造成本并且简化现场部署。在一个实施方式中,集成模块550是包括光子集成电路(“PIC”)的单个物理封装。PIC可包括光和电装置,例如激光二极管、光二极管、驱动电路、TIA、相位调制器、阵列波导光栅(“AWG”)等。在一个实施方式中,光MUX515和光DEMUX525被实现为无源光装置,例如AWG。
[0044]图7是根据本公开的一个实施方式描述用于生成WDM PON的下行WDM信号的外相位调制器700的功能框图。外相位调制器700是外调制器545的一种可能的实现方式。所示的外相位调制器700的实施方式包括光相位调制器705以及驱动器710和715。可使用电光调制器来实现光相位调制器705,以在由激光源输出的载波波长上编码相位调制。在一个实施方式中,光相位调制器705是Mach-Zhender(“MZ”)调制器,其针对下行WDM信号的相干传输使用二进制相移键控(“BPSK”)数字调制。相对于用于长距WDM传输系统的成熟的相干光发射器(其通常包括一对偏振复用的I-Q调制器和数字编码器),MZ和BPSK的组合减少了电路复杂性。这样的系统通过使用两偏振和在每一偏振中的两正交来执行信号编码,实现了频谱效率的提高。然而,对于大规模的集成来说,这样系统也是复杂的并且目前来说也是昂贵的。所示的外相位调制器700的实施方式关于成本、功率和空间节省来权衡频谱效率。然而可以预见的是,在将来,可以以具有竞争力的成本、功率和空间需求来最终实现包含在本文公开的架构中的双偏振正交幅度调制(“QAM”)调制器的阵列。
[0045]图8是根据本公开的一个实施方式描述无色WDM ONU800的功能框图。WDM ONU800代表在图1和4中所示的WDM ONU145的一种可能的实施方式。所示的WDM ONU800的实施方式包括光双工器805、可调光接收器810、可调光发射器815、MAC820、开关825,以及一个或多个局域网(“LAN”)物理单元(PHY)830。所示的可调光接收器810的实施方式包括可调激光源LS1、偏振分束器(PBS)840、两个90度混合器845、平衡光接收器850、模数转换器A/D,以及接收数字信号处理逻辑860。所示的可调光发射器815的实施方式包括可调激光源LS2和驱动电路860。可将光双工器805实现为3-dB功率分配器、三端口光环形器、在交织上行和下行波长时的周期性梳状滤波器、在上行和下行WDM信号使用不同频带(例如C频带用于上行,以及L频带用于下行)时的红/蓝频带滤波器,或者其他设备。
[0046]WDM ONU800包括对在下行方向上的相位调制的信号使用相干检测的相干接收器,以及包括对于上行信号使用幅度调制的直接调制的发射器。在上行方向上,可调光发射器815能够通过对可调激光源LS2的适当调整,用可选择的载波波长发射上行WDM信号。接着,将该信息直接幅度调制到所选择的载波波长上。WDM ONU800还包括无色或波长无关的光接收器,其通过在下行方向上使用广播和选择方式与混合PON系统中的TDM子PON相兼容。换句话说,可调光接收器810能够调整到特定的载波波长,并且能拒绝分配给PON中的其他WDN ONU的其他载波波长。所示的可调光接收器810的实施方式通过将激光源LS1用作本地振荡器(LO)来创建可选择的差频波长以向下混合所选择的载波波长,从而实现其可调功能。可调光接收器810包括相干接收器电路,其可以实现零差、外差或内差相干检测。在一个实施方式中,LS1和LS2是相同的激光源。在一个实施方式中,LS1和LS2是为了空间和成本效率而集成到单个封装内的相同的可调激光源。
[0047]另外值得注意的是,在可调接收器处的LO还可以充当放大器,用来在ONU处放大所接收的来自OLT的信号,从而避免了在CP处对EDFA的需求。
[0048]可调光接收器810是相干光接收器,其使用PBS840将所接收的下行WDM信号分为X和Y偏振。在90度混合器845中混合每个偏振与LO信号。该混合生成由成对的平衡接收器850检测的同相和正交相位的输出。尽管WDM OLT中的发射器可能仅使用一个偏振,但由于偏振旋转,下行WDM信号的偏振在可调光接收器810处不可预测,其中偏振旋转在光纤传输的过程中出现。因此,在相干接收器处使用偏振分集。然而,由于不同于在CO侧的接收器阵列,在CP侧仅使用一个接收器,因此可控制集成复杂度。4个平衡接收器850的后面是A/D转换器和接收器DSP逻辑860,用于根据在OLT处使用的调制方案恢复所发送的相位和/或幅度信息。在一个实施方式中,可以简单地制造成OLT处的阵列并且具有高信噪比(SNR)抗性的简单相干调制器是二进制相位调制器。
[0049]上文中对本发明所示实施例的描述(包括在摘要中描述的那些)并非意在穷尽本发明或者将本发明限制为所公开的精确形式。尽管为了说明的目的描述了本发明的特定实施例和示例,但本领域技术人员应理解,在本发明的范围内的各种修改是可能的。
[0050]可根据上文详细的描述对发明进行这些修改。不应将在所附权利要求中使用的术语解释为将本发明限制为在说明书中公开的具体实施例。而是由所附权利要求来完全确定本发明的范围,其中根据权利要求解释的已制定的原则来解释该权利要求。