WO2010064999A1

WO2010064999A1 - Wavelength division multiplexed passive optical network

Info

Publication number: WO2010064999A1
Application number: PCT/SG2009/000469
Authority: WO
Inventors: Yang Jing Wen; Zhaowen Xu; Xiaofei Cheng; Yixin Wang; Yong Kee Yeo
Original assignee: Agency For Science, Technology And Research
Priority date: 2008-12-05
Filing date: 2009-12-04
Publication date: 2010-06-10

Abstract

WDM-PONs and methods of providing WDM-PON services. One method comprises the steps of at a central office (CO), generating a first optical signal comprising N channels such that in each channel data is carried in two sidebands with the respective carriers suppressed; at the CO, generating a second optical signal comprising all carriers; and transmitting, to each of a plurality of optical network units (ONUs) associated with respective ones of the carriers, the data channel of the first optical signal with the two sidebands corresponding to the associated respective carrier, and said associated respective carrier in the second optical signal.

Description

Wavelength Division Multiplexed Passive Optical Network

FIELD OF INVENTION

The invention relates broadly to wavelength division multiplexed passive optical network (WDM-PON) and to a method of providing WDM-PON services.

BACKGROUND

Wavelength division multiplexed passive optical network (WDM-PON) has been demonstrated as a promising solution for future broadband access network, due to its features such as large capacity, privacy, format transparent, network security, and per- customer based flexible upgrade. So far, extensive attention has been paid to WDM- PON R&D, including but not limited to, low-cost light sources, protection and fault localization, novel architectures for further increasing data rate and reach and virtual optical private networking. To increase the usage of WDM-PON, it preferably is capable to provide diverse services like video-on-demand and multimedia broadcasting besides broadband switched services. Typically, WDM-PON utilizes waveguide grating routers (WGRs) at remote nodes (RNs) for the virtual point-to-point connectivity, and this adds complexity to the delivery of broadcast services because of the wavelength-dependent routing property of the remote node.

Early efforts to solve this problem used the WDM overlay on a PON or broadband light-emitting diode (LED) for the broadcast signals [1, 2]. However, the first technique requires a large number of WDM filters and couplers, while the second technique uses not only a pair of fibers but also the frequency up/downconversion of video signals due to the limited modulation bandwidth of LED.

Recently, a bidirectional WDM PON 100 for the transmission of digital broadcast video signals as well as WDM channels, has been demonstrated [3], as shown in Fig. 1. The proposed network utilizes WDM lasers 102 operating in the 1.55μm region for the downstream data (up to 2.5 Gb/s), LEDs 104 operating in the 1.3μm region for the upstream data (155 Mb/s), and a laser 106 operating at 1.53μm for the broadcast. The video signal is delivered to all the optical network units 108, 110 (ONUs) by using the periodic property of WGR.

Each downlink signal is directly modulated to a 1.55μm laser 102 and passed through the 1.3/1.5μm WDM coupler 112, which is used for the separation of upstream and downstream channels, and then are multiplexed by using a WGR 114. The digital broadcast video signal is directly modulated to a 1.53μm DFB laser 106. These multiplexed channels are transmitted to the RN 116 through a feeder fiber 118. At the RN 116, the downstream channels are separated into baseband data and video signal using a 1.53/1.55μm WDM coupler 120. The video signal operating at 1.53 μm is then split by using a 1x16 splitter 122 and sent to the 15 input ports of the 16x16 WGR 124 so that it could be broadcasted to the 15 ONUs 108, 110 via 3 km of SMF 126. The baseband signals, delivered by 15 WDM channels, are demultiplexed by the WGR 124 and sent to each corresponding ONU 108, 110. At the ONU, the video and baseband signals are directed to a PIN photodiode receiver 128 (bandwidth: 1.7 GHz) by the 1.3/1.5μm WDM couplers 130. The output signal of the receiver 128 is split, and sent to an error detector 132 and a TV 134 for the BER and video quality measurements, respectively. For the upstream channels, a directly modulated LED operating at 1.3μm at each ONU 108, 110 is used to transmit 155-Mb/s data. The upstream data are first coupled into the same fiber used for the downstream channels by using 1.3/1.5μm WDM couplers 130, and then sent to the corresponding ports of the WGR 124 at the RN 116. Thus, the upstream channels are automatically spectrum-sliced and multiplexed by the WGR 124 at the RN 116. After transmission over the feeder fiber 118, the multiplexed upstream channels are demultiplexed by the WGR 114 at the CO 134. The demultiplexed upstream channel is then detected by using an APD receiver 136 via a 1.3/1.5μm WDM coupler 112.

Another scheme for broadcasting capable WDM-PON is based on reflective semiconductor optical amplifier (RSOA) where a single optical source is shared for not only up/downlink data service but also broadcasting service [4], as shown in Fig. 2. At CO 200 of the proposed scheme, the baseband digital signal for downstream data service and the subcarrier modulated (SCM) signal for broadcast service are simultaneously modulated using a single distributed feedback (DFB) laser diode (LD) 202. A portion of the downstream source containing the broadcasting signal is detected by a photo detector (PD) 204 in ONU 206 and electrical filters separate the digital and SCM signals. The residue of the downstream source is injected into an RSOA 208 as a seeding source and re-modulated by the RSOA 208 with upstream signal. The unsuppressed SCM signal in the re-modulation process could be removed by a simple electrical low-pass filtering in CO 200.

Recently, another WDM-PON architecture with the capability to provide triple- play services with a source-free optical network unit has been proposed [5], as shown in Fig. 3. All lightwaves after multiplexing are modulated by broadcast signal via an external modulator 300 to generate SCM signals. The optical carriers and subcarriers are separated using an optical interleaver (IL) 302. Then another demultiplexer 304 is employed to separate the carriers, and each optical carrier is modulated with downstream data via a phase modulator 306. Then all the phase-modulated signals at different wavelengths are multiplexed by an AWG 308 before they are combined with the subcarriers using the second optical IL 310. The downstream data and video signals are delivered to the ONU 312 by optical fiber 314 . At the RN 316, another IL 318 is used to separate the subcarriers and phase-modulated downstream signals. The subcarriers at different wavelengths are first demultiplexed, then directly detected by corresponding baseband receivers 320. The phase-modulated downstream signals, after being demultiplexed, are separated into two parts. One part is converted to intensity signals by a demodulator 322 before it is detected by a photodiode 324 to realize optical-electrical (O/E) conversion. The other part is re-modulated by an intensity modulator 326 driven by upstream data; therefore, centralized lightwave in the optical line terminal (OLT) can be realized. The upstream data are sent back to the OLT by a different feeder fiber 328 to reduce Rayleigh backscattering. In the OLT, the upstream data at different wavelengths are demultiplexed before they are O/E converted.

A WDM-PON with an overlay bands for broadcasting signals has also recently been proposed as shown in Fig.4 [7]. The broadcasting signals were imposed on the broadband light source (BLS) 400 by using an external modulator 402. The modulated signals were combined with data signals which were multiplexed at the AWG 1 404. The overlaid broadcasting signals with data signals were transmitted through a feeder fiber 406 and demultiplexed by AWG2 408 located at the remote node (RN) 410. Then, demultiplexed signals were transmitted to optical network units (ONUs) 412 located at subscriber premises. Finally, the video signals and data signals were demultiplexed and received by different receivers 414. The path for upstream data transmission was the same as conventional WDM-PON.

The abovementioned schemes have their drawbacks. The scheme in [3] still involves power splitter at RN, which introduces high loss for broadcast signal. For the method in [4], the extinction ratio (ER) of the downstream signal should be small enough to be suppressed by the high-pass characteristic of an RSOA. Both [3] and [4] requires a high speed photo detector at each ONU once broadcast data rate (as well as subcarrier frequency) is high. Scheme [5] modulates the CW lights first by broadcast signal, then separates subcarrier-modulated (SCM) lights into two parts, one with carrier and the other with subcarriers. The method treats the separated optical carrier from an SCM light as a pure CW light, which is not true for large signal modulation and also induces crosstalk from downlink to uplink. The scheme in [7] requires a separate waveband for broadcasting which reduces the waveband utilization efficiency.

There is therefore a need to solve these problems.

REFERENCES

[1] U. Hilbk, T. Hermes, J. Saniter, and F.-J. Westphal, "High capacity WDM overlay on a passive optical network," Electron. Lett., vol. 32, no. 23, pp. 2162-2163, 1996.

[2] P. P. lannone, K. C. Reichmaηn, and N. J. Frigo, "High-speed point-to point and multiple broadcast services delivered over a WDM passive optical network," IEEE Photon. Technol. Lett., vol. 10, pp. 1328-1330, Sept. 1998. [3] E. S. Son, K. H. Han, J. K. Kim, and Y. C. Chung, "Bidirectional WDM passive optical network for simultaneous transmission of data and digital broadcast video service," J. Lightw. Technol., vol. 21 , no. 8, pp. 1723-1727, Aug. 2003.

[4] T.-Y. Kim, and S.-K. Han, "Reflective SOA-Based Bidirectional WDM-PON

Sharing Optical Source for Up/Downlink Data and Broadcasting Transmission", IEEE IEEE Photon. Technol. Lett., vol. 18, no.22, pp. 2350-2352, Nov. 2006.

[5] J. Yu, O. Akanbi, Y. Luo, L. Zong, T. Wang, Z. Jia, and G.-K. Chang,"

Demonstration of a novel WDM passive optical network architecture with source- free optical network units" IEEE Photon. Technol. Lett., vol. 19, no.8, pp. 571-573, Apr. 2007. [6] Z. Xu, Y. J. Wen, W.-D. Zhong, M. Attygalle, X. Cheng, Y. Wang, and C. Lu, "Carrier-reuse WDM-PON using a shared delay interferometer for separating carriers and subcarriers", IEEE Photon. Technol. Lett., vol. 19, no.11 , pp. 837- 839, Jun. 2007.

[7] J.-H. Moon, K.-M. Choi and C-H. Lee, "Overlay of Broadcasting Signal in a WDM-PON", OFC2006, OThKδ, 2006.

SUMMARY

In accordance with a first aspect of the present invention, there is provided a method of providing WDM-PON services, the method comprising the steps of at a central office (CO), generating a first optical signal comprising N channels such that in each channel data is carried in two sidebands with the respective carriers suppressed; at the CO, generating a second optical signal comprising all carriers; and transmitting, to each of a plurality of optical network units (ONUs) associated with respective ones of the carriers, the data channel of the first optical signal with the two sidebands corresponding to the associated respective carrier, and said associated respective carrier in the second optical signal.

The N channels of the first optical signal may comprise downlink data channels for the respective ONUs, and the second optical signal may comprises a seeding light signal for generation of uplink data channels at the respective ONUs.

The N channels of the first optical signal may comprise video data channels for the respective ONUs, and the second optical signal may comprise downlink data channels for the respective ONUs at the associated respective carriers.

The N channels of the first optical signal may comprise broadcast data channels for the respective ONUs, and the second optical signal may comprise downlink data channels for the respective ONUs at the associated respective ca friers.

The first and second optical signals may be generated from respective source signals of one array of continuous wave (CW) light sources. Generating the first optical signal may comprise multiplexing the source signals from the CW light sources prior to modulating the multiplexed source signals such that in each channel data is carried in the two sidebands with the respective carriers suppressed.

Generating the second optical signal may comprise modulating data onto the source signals from respective ones of the CW sources prior to multiplexing the modulated source signals.

Generating the second optical signal may comprise demultiplexing a multiplexed signal from the CW sources prior to modulating data onto the source signals from respective one of the CW sources, and multiplexing the modulated source signals source signals.

The first and second optical signals may be combined at the CO prior to transmission to a remote node (RN).

Transmitting to each of the plurality of ONUs associated with respective ones of the carriers may comprise using an interferometric filter and an arrayed waveguide grating (AWG) router at the RN.

The first and second optical signals may be transmitted to an RN via first and second feeder fibers respectively.

The transmitting to each of the plurality of ONUs associated with respective ones of the carriers may comprise using an AWG router at the RN.

The transmitting to each of the plurality of ONUs associated with respective ones of the carriers may comprise using a circulator and an AWG router at the RN.

The transmitting to each of the plurality of ONUs associated with respective ones of the carriers may comprise using a circulator, an AWG and a demultiplexer at the RN. The method may further comprise, at the CO, generating a seeding light signal for generation of uplink data channels at the respective ONUs.

The seeding light signal may be combined with the first optical signal prior to transmission to the RN.

The method may further comprise transmitting the first and second optical signals on first and second distribution fibers respectively between an RN and each the ONUs.

In accordance with a second aspect of the present invention, there is provided a method of providing WDM-PON services, the method comprising the steps of at a CO, generating source signals from respective ones of one array of continuous wave (CW) light sources; at the CO, modulating each source signal with respective data signals using IRZ format; at the CO, multiplexing the modulated source signals; at the CO, modulating the multiplexed source signals with broadcast data using FSK format such that for each channel in the multiplexed source signals has twp sidebands; and transmitting, to each of a plurality of ONUs associated with respective ones of the source signals, the modulated multiplexed source signals such that at each ONU, only one of the sidebands for each channel is received at a broadcast receiver of the ONU, and both sidebands for each channel are received for downlink data detection and uplink re-modulation.

In accordance with a third aspect of the present invention, there is provided a WDM-PON comprising a central office (CO) configured for generating a first optical signal comprising N channels such that in each channel data is carried in two sidebands with the respective carriers suppressed, and for generating a second optical signal comprising all carriers; and means for transmitting, to each of a plurality of optical network units (ONUs) associated with respective ones of the carriers, the data channel of the first optical signal with the two sidebands corresponding to the associated respective carrier, and said associated respective carrier in the second optical signal. The N channels of the first optical signal may comprise downlink data channels for the respective ONUs, and the second optical signal may comprise a seeding light signal for generation of uplink data channels at the respective ONUs.

The N channels of the first optical signal may comprise broadcast data channels for the respective ONUs, and the second optical signal may comprise downlink data channels for the respective ONUs at the associated respective carriers.

In accordance with a fourth aspect of the present invention, there is provided a WDM-PON comprising a CO configured for generating source signals from respective ones of one array of continuous wave (CW) light sources, for modulating each source signal with respective data signals using IRZ format, for multiplexing the modulated source signals, and for modulating the multiplexed source signals with broadcast data using FSK format such that for each channel in the multiplexed source signals has twp sidebands; and means for transmitting, to each of a plurality of ONUs associated with respective ones of the source signals, the modulated multiplexed source signals such that at each ONU, only one of the sidebands for each channel is received at a broadcast receiver of the ONU, and both sidebands for each channel are received for downlink data detection and uplink re-modulation.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which: Fig.1 shows a WDM PON for delivering broadcast with splitter and NxN WGR at RN. Where (BPF: bandpass filter; SDHF: synchronous digital hierarchy filter; STB: set top box.)

Fig.2 shows a WDM-PON based on RSOA and subcarrier modulation where up/downlink as well as broadcasting shares a single optical source.

Fig.3 shows a WDM — PON architecture based on carrier separation and recombination for providing the broadcast video service.

Fig.4 shows a WDM — PON architecture with overlay bands for broadcasting signals.

Fig.5 shows a WDM-PON according to an embodiment of the invention.

Fig.6 shows a WDM-PON according to an embodiment of the invention.

Fig.7 shows a WDM-PON according to an embodiment of the invention.

Fig.8 shows a WDM-PON according to an embodiment of the invention.

Fig.9 shows a WDM-PON according to an embodiment of the invention.

Fig.10 shows a WDM-PON according to an embodiment of the invention.

Fig.11 shows a plot of the optical spectra of the Inverted return-to-zero (IRZ) and IRZ/FSK signals in a simulated implementation of the WDM-PON in Fig. 10.

Fig.12 shows a plot of the eye patterns shows the eye patterns of the downlink, broadcast and uplink signals after transmission in a simulated implementation of the WDM-PON in Fig. 10.

Fig.13 shows a plot of the bit-error rate (BER) of the downlink, broadcast and uplink signals in a simulated implementation of the WDM-PON in Fig. 10. Figure 14 shows a flowchart illustrating a method of providing WDM-PON services according to an example embodiment.

Figure 15 shows a flowchart illustrating a method of providing WDM-PON services according to another example embodiment.

DETAILED DESCRIPTION

High-speed wavelength division multiplexed passive optical network (WDM-PON) architecture is proposed that is capable of broadcast delivery and services for different applications. A number of configurations are introduced. In one embodiment, a plurality of carrier wave optical signal or light is separated into two parts at the central office (CO) of the WDM-PON. One part for delivery broadcast services, the other part for carrying downlink channels. The broadcast and downlink channels are sent to remote nodes (RN) via one or two feeder fibers and routed to optical network units (ONUs) via an NxN waveguide grating router and an interferometric filter or a circulator located at the RN.

In the description and claims, the term central office (CO) has been used, however it is to be understood that the term is meant to include synonymous terms in the art, such as optical line terminal (OLT).

The high-speed WDM-PON with broadcasting capability according to our embodiment is shown in Fig. 5. At the CO 500, an array of CW lights 502 are multiplexed together and then separated into two parts by a fiber coupler 504. The first part is sent into a Mach-Zehnder modulator 506 (MZM) driven by the broadcast signal, which is mixed from a local oscillator 508 (LO) with a frequency of f_sc and a baseband data stream 510. The MZM 506 is biased at the transmission null point and its output has two sidebands for each channel with carrier suppressed. In this example, the MZM 506 is biased at the transmission null point" to suppress the carrier. Each pair of sidebands has a mode spacing of twice of the subcarrier frequency f_sc. The other part of CW lights is sent into a WDM demultiplexer 512 to separate the optical carriers, each carrier is modulated via downlink data 514. For the purpose of downlink data re- modulation, inverted return-to-zero (IRZ) format can be used as downlink data format. The optical IRZ signal is realized by driving an MZM around the quadrature point with an electrical RZ signal. All IRZ channels are multiplexed at WDM multiplexes 516 and combined with the broadcast signals via another fiber coupler 518. Each combined channel is a double sideband signal as shown in figure 5 where the optical carrier 520 carries baseband downlink data while the two sidebands 522, 524 carry broadcast service.

All the combined channels are then sent to the remote node 526 via an optical circulator 528 and feeder fiber 530. At the RN 526, an interferometric filter 532 (IF) is used to separate all the carriers and subcarriers [6]. All the subcarriers go to one output port of the IF 532 and are sent into the first input port of an NxN AWG router 534, which is also located at the RN 526. All the carriers go to the other output port of the IF 532 and sent into the (N/2+1 )th input port of the NxN AWG router 534. The λ_sc, -, and λ_c, -, in the Fig. 5 denote the optical subcarrier pair and the optical carrier of the i-th wavelength channel, respectively. Due to the cyclic property of the AWG router, a pair of subcarriers at λ_sc, i and an optical carrier at λ_c, _(i + _N/₂> entering at the two inputs of the AWG, respectively, will emerge on the same output port.

At each ONU 536, the optical carrier and subcarrier pair are then separated via a coarse WDM coupler 538. The subcarriers are sent to a baseband receiver 540 for broadcast detection. The carrier is further divided into two parts, one for downlink signal detection 542, while the other part is seeded into an RSOA 544 for uplink data re- modulation. The re-modulated uplink signal is then sent back to the RN 526 via the same distribution fiber 546 and combined with other uplink channels via the same AWG 534 at its port (N/2+1) of the left side. The combined uplink channels are sent back to the CO 500 for uplink detection via the same IF 532 and feeder fiber 530.

In this embodiment, the interferometric filter 532 can be an interleaver or a delay interferometer. Its free spectral range (FSR) preferably is twice of the subcarrier frequency f_sc, and the WDM channel spacing preferably is multiple integer times of the FSR. For a typical channel spacing of 100 GHz, the FSR can be 10, 20 or 50 GHz, and the corresponding f_sc would be 5, 10 or 25.GHz, respectively. The downlink data rate is preferably smaller than f_sc to make it easy for wavelength separation and typically less than half f_sc would be desirable. In the embodiment described in Fig. 5, all the CW lights are first combined with a WDM multiplexer, then separated into two parts, with one part modulated with broadcast while the other part further separated into individual channel, each modulated with downlink data. Alternatively, as shown in Fig. 6, each CW light can be individually separated into two parts with a fiber coupler e.g. 600. Each lower part is modulated with downlink data and combined with other downlink channels via a WDM multiplexer 602. Each upper part is multiplexed with other channels via a WDM multiplexer 604 and modulated with broadcast service. In one example, each modulator 606 can be in the form of an MZM, biased at the transmission null point to suppress the optical carrier . It is worthwhile to note that each channel can also carry individual video signaf rather than shared broadcast signal. It will be appreciated that individual modulators can be used on the upper parts in such an embodiment, and multiplexing after the individual modulating

Both the previous two example configurations (Fig. 5 & Fig. 6) use an interferometric filter at the RN to separate all the carriers and subcarriers. The IF can be an interleaver or a delay interferometer. An alternative way would be eliminating the IF on the expense of introducing an additional feeder fiber 700, as shown in Fig. 7 for improved stability. Here the broadcast signal is transmitted via the upper feeder fiber, while the downlink channels are transmitted through the lower feeder fiber 702. Similar to the previous two example configurations, at each ONU 704, the broadcast signal and downlink signals are separated via a coarse WDM coupler 706. The downlink signal is further separated into two parts with one part for downlink detection 708, while the other part is used to seed the RSOA 710. The re-modulated uplink signal is sent back to the RN 712 via the same distribution fiber 714 and combined together with other uplink channels via the AWG 716 to the lower feeder fiber 702.

The example embodiments discussed so far use inverted RZ as the downlink data format to facilitate the re-modulation of the downlink signal with uplink data. The generation of IRZ involves the generation of electrical RZ signal which uses a clock signal and high speed AND gate, leading to relatively high cost for the network. Amplitude constant formats like phase shift keying (PSK) and frequency shift keying (FSK) have also been used as downlink signal and reused as uplink light source via direct re-modulation of uplink data, with demodulation of PSK and FSK using optical items which can suffer from stability issue.

In an alternative embodiment, a simple non-return-to-zero (NRZ) is used as downlink format, where uplink optical carriers are distributed from CO to ONUs, as shown in Fig. 8. Here the seeding lights 800 used as uplink carriers are at a different wave band from the downlink channels and are generated at the CO 802, which can be an array of CW lights or from other multiple wavelength light source like supercontinuum and broadband ASE source. They are combined with the broadcast signal via a WDM coupler 804 and sent into the upper feeder fiber 806. At the RN 808, the optical signals from the upper feeder fiber 806 is launched into port 1 of a four port circulator 810, then come out from port 2 and go into input port 1 of the NxN AWG 812. The downlink channels from the lower feeder fiber 814 are sent into port 3 and come out from port 4 of the circulator 810 and are launched into input port (N/2+1) of the AWG 812. Similar to the previous example configurations, a pair of subcarriers at λ_sc, i and the seeding light one FSR away as well as an optical carrier at λ_c, (, + N/₂) entering at the two inputs of the AWG 812, respectively, will emerge on the same output port and go to their corresponding ONU 816 via a single distribution fiber 818. At the ONU 816, the subcarriers at λ_{sCi l} and the optical carrier at λc, <, + N/₂) are first separated from the seeding light via an coarse WDM coupler 820, and are further separated by another WDM coupler 822. The separated subcarriers and carrier are used for broadcast and downlink signal detection, respectively. The seeding light is then sent into an RSOA 824 for uplink data modulation. The modulated uplink signal is then sent back to the AWG 812 via the same distribution fiber and comes out from port 1 of the AWG 812 left side. The signal is then sent back to the CO 802 via port 3 of the circulator 810 and the lower feeder fiber 814.

All the above discussed embodiments use an NxN cyclic AWG at the RN and use a single distribution fiber between RN and each ONU. An additional coarse WDM coupler at each ONU is used to separate broadcast signal and downlink and/or uplink signals. This configuration is e.g. advantageous for the scenario like some already deployed fiber plants where only one distribution fiber is available between RN and each ONU. For the situation where two short distribution fibers are available between RN and each ONU, an alternative configuration can be considered as shown in Fig. 9. In this scheme, a 1xN AWG 900 with cyclic property and an additional WDM demultiplexer (DMUX) 902 are used at the RN. The downlink channels are sent from the CO 904 to the RN 906 via the lower feeder fiber 908 and are launched into the WDM demultiplexer 902 via port 3 to 4 of the circulator 910. After demultiplexing, each downlink channel is sent to the corresponding ONU 912 via the lower distribution fiber 914. The incoming broadcast signal and seeding light wavelengths are launched into the 1xN AWG 900 via port 1 to 2 of the circulator 910. The demultiplexed seeding light and subcarriers are transmitted to each corresponding ONU 912 via the upper distribution fiber 916 and are separated by a coarse WDM coupler 918. The subcarriers are detected by the broadcast receiver 920 and the seeding light is input into the RSOA 922 for uplink data modulation. The uplink signal is then transmitted back the RN 906 and combined with other uplink channels. All the uplink channels are sent back to the CO 904 via port 2 to 3 of the circulator 910 and the lower feeder fiber 908.

The two distribution fiber modification described with reference to Fig. 9 can also be applied to the embodiments described with reference to Fig. 5-8 and combinations based on these configurations. Different embodiments can also be applied to WDM- PONs without broadcast capability. In one such example embodiment of WDM-PONs, an array of CW lights are multiplexed together and then separated into two parts by a fiber coupler. The first part is used as seeding lights for uplink transmission. The other part of CW lights is sent into a WDM demultiplexer to separate the optical carriers, each carrier is modulated by downlink data via an MZM. The MZM is biased at the transmission null point and its output has two sidebands for each channel with carrier suppressed.

A competitive performance analysis is tabulated in Tables I & II. A typical loss was assumed for the following components: WDM multiplexer - 4 dB, AWG - 5.5 dB, 20km feeder fiber - 5 dB (including splicing loss), 1x32 power splitter - 16 dB, 3 dB coupler - 3.3 dB, circulator - 0.6 dB, WDM coupler - 0.4 dB, delay interferometer - 2.5 dB, interleaver - 2 dB, and distribution fiber loss - 1 dB. Table I shows the performance comparison between existing schemes and different example implementation of the configuration shown in Fig. 5 to 9 respectively, which shows that the overall performance is superior to that of the existing schemes.

Table

Table Il summarizes the comparison among the different example implementations.

Table Il

Embodiments of the present invention can exhibit the following advantages:

Improved link power budget for broadcasting signal

Reduced crosstalk from downlink to uplink limitation

Reduced receiver bandwidth requirement

Reduced crosstalk from broadcast to downlink In another embodiment, a WDM-PON architecture 1000 provides broadcast services via Frequency Shift Keying (FSK), as shown in Fig.10. At the CO 1002, each downlink channel is modulated in inverted RZ format and combined with other channels with a WDM multiplexer 1004 (MUX1). The combined channels are further modulated by broadcast signal in FSK format via a shared FSK modulator 1006. The generated IRZ/FSK signals have two sidebands for each channel and are launched into the feeder fiber 1008 (with dispersion compensation fiber 1010 (DCF)) and are separated into two parts via a fiber coupler 1012. After the FSK modulator 1006, each sideband contains a "mixture" of IRZ and FSK data, with the carrier suppressed by the FSK modulation. One part is sent into an interferometric filter (here a delay interferometer 1014(Dl)) for FSK demodulation. The free spectral range (FSR) of the Dl 1014 is twice of the frequency deviation of the FSK signal, therefore only one sideband in each channel is left at any output port of the Dl 1014. The Dl 1014 output is further demultiplexed (by MUX3) 1016 and sent to the individual ONUs 1018 for broadcast detection with a baseband receiver 1020. The other part of the IRZ/FSK channels at the RN 1022 is directly demultiplexed by MUX2 1024 and sent to each ONU 1018 via another distribution fiber 1026. At each ONU 1018, part of the incoming light is used for downlink data detection 1028 and the other part is used for uplink re-modulation (here a reflective semiconductor optical amplifier (RSOA) 1030 is considered) and sent back to the central office 1002 via the same optical path.

An example implementation has been verified via simulation using commercial software tool, VPItransmissionMaker. In the simulation, the downlink data rate is 10Gb/s, the broadcast data rate is 2.5Gbs/ and the uplink date rate is 2.5 Gb/s. The feeder fiber is 20 km single mode fiber with full dispersion compensation. The frequency deviation for the FSK generation is 12.5 GHz and the FSR of the Dl is 25 GHz. The extinction ratio of the IRZ signal is 7.2 dB. At the ONU, an external modulator in adjunction with a circulator rather than RSOA is used due to the lack of proper RSOA module in the software tool.

Fig. 11 shows the optical spectra of the IRZ signal and IRZ/FSK signal. For IRZ, the optical carrier is much stronger than its two clock tones 10 GHZ away alongside the carrier due to the property of the IRZ and the relatively low extinction ratio. After frequency shift keying, the optical carrier disappears with two strong sidebands appear. The frequency deviation of the two sidebands is 12.5 GHz.

Fig. 12 shows the eye patterns of the downlink, broadcast and uplink signals. After transmission, the downlink signal is not degraded due to dispersion compensation which suppressed the FM to AM conversion. Both the broadcast and uplink signals suffer relatively strong crosstalk from downlink signal. The amount of fluctuation at high level due to crosstalk can be suppressed to some extent by using limiting amplifier at receiver in practical system. For uplink, the power fluctuation can be further suppressed if a saturated RSOA is used for uplink modulation.

BER results, as shown in Fig. 13, shows that the downlink, broadcast and uplink signals can all achieve BER free transmission, which demonstrates the feasibility of the example implementation.

Figure 14 shows a flowchart 1400 illustrating a method of providing WDM- PON services according to an example embodiment. At step 1402, at a central office (CO), a first optical signal comprising N channels is generated such that in each channel data is carried in two sidebands with the respective carriers suppressed. At step 1404, at the CO, a second optical signal comprising all carriers is generated. At step 1406, the data channel of the first optical signal with the two sidebands corresponding to the associated respective carrier, and said associated respective carrier in the second optical signal, are transmitted to each of a plurality of optical network units (ONUs) associated with respective ones of the carriers.

Figure 15 shows a flowchart 1500 illustrating a method of providing WDM- PON services according to an example embodiment. At step 1502, at a CO, source signals are generated from respective ones of one array of continuous wave (CW) light sources. At step 1504, at the CO, each source signal is modulated with respective data signals using IRZ format. At step 1506, at the CO, the modulated source signals are multiplexed. At step 1508, at the CO, the multiplexed source signals are modulated with broadcast data using FSK format such that for each channel in the multiplexed source signals has twp sidebands. At step 1510, the modulated multiplexed source signals are transmitted to each of a plurality of ONUs associated with respective ones of the source signals, such that at each ONU, only one of the sidebands for each channel is received at a broadcast receiver of the ONL), and both sidebands for each channel are received for downlink data detection and uplink re-modulation.

Example embodiments of the present invention are applicable in broadband optical access networks; particularly wavelength division multiplexed passive optical networks.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. A method of providing WDM-PON services, the method comprising the steps of: at a central office (CO), generating a first optical signal comprising N channels such that in each channel data is carried in two sidebands with the respective carriers suppressed; at the CO, generating a second optical signal comprising all carriers; and transmitting, to each of a plurality of optical network units (ONUs) associated with respective ones of the carriers, the data channel of the first optical signal with the two sidebands corresponding to the associated respective carrier, and said associated respective carrier in the second optical signal.

2. The method as claimed in claim 1 , wherein the N channels of the first optical signal comprise downlink data channels for the respective ONUs, and the second optical signal comprises a seeding light signal for generation of uplink data channels at the respective ONUs.

3. The method as claimed in claim 1 , wherein the N channels of the first optical signal comprise video data channels for the respective ONUs, and the second optical signal comprises downlink data channels for the respective ONUs at the associated respective carriers.

4. The method as claimed in claim 1 , wherein the N channels of the first optical signal comprise broadcast data channels for the respective ONUs, and the second optical signal comprises downlink data channels for the respective ONUs at the associated respective carriers.

5. The method as claimed in claim 4, wherein the first and second optical signals are generated from respective source signals of one array of continuous wave (CW) light sources.

6. The method as claimed in claim 5, wherein generating the first optical signal comprises multiplexing the source signals from the CW light sources prior to modulating the multiplexed source signals such that in each channel data is carried in the two sidebands with the respective carriers suppressed.

7. The method as claimed in claims 5 or 6, wherein generating the second optical signal comprises modulating data onto the source signals from respective ones of the CW sources prior to multiplexing the modulated source signals.

8. The method as claimed in claims 5 or 6, wherein generating the second optical signal comprises demultiplexing a multiplexed signal from the CW sources prior to modulating data onto the source signals from respective one of the CW sources, and multiplexing the ^"modulated source signals source signals.

9. The method as claimed in any one of the preceding claims, wherein the first and second optical signals are combined at the CO prior to transmission to a remote node (RN).

10. The method as claimed in claim 9, wherein transmitting to each of the plurality of ONUs associated with respective ones of the carriers comprises using an interferometric filter and an arrayed waveguide grating (AWG) router at the RN.

11. The method as claimed in any one of claims 1 to 8, wherein the first and second optical signals are transmitted to an RN via first and second feeder fibers respectively.

12. The method as claimed in claim 11 , wherein the transmitting to each of the plurality of ONUs associated with respective ones of the carriers comprises using an AWG router at the RN.

13. The method as claimed in claim 11 , wherein the transmitting to each of the plurality of ONUs associated with respective ones of the carriers comprises using a circulator and an AWG router at the RN.

14. The method as claimed in claim 1 1 , wherein the transmitting to each of the plurality of ONUs associated with respective ones of the carriers comprises using a circulator, an AWG and a demultiplexer at the RN.

15. The method as claimed in any one of claims 11 to 14, further comprising, at the CO, generating a seeding light signal for generation of uplink data channels at the respective ONUs.

16. The method as claimed in claim 15, wherein the seeding light signal is combined with the first optical signal prior to transmission to the RN.

17. The method as claimed in claim any one of the preceding claims, further comprising transmitting the first and second optical signals on first and second distribution fibers respectively between an RN and each the ONUs.

18. A method of providing WDM-PON services, the method comprising the steps of: at a CO, generating source signals from respective ones of one array of continuous wave (CW) light sources; at the CO, modulating each source signal with respective data signals using

IRZ format; at the CO, multiplexing the modulated source signals; at the CO, modulating the multiplexed source signals with broadcast data using FSK format such that for each channel in the multiplexed source signals has twp sidebands; and transmitting, to each of a plurality of ONUs associated with respective ones of the source signals, the modulated multiplexed source signals such that at each

ONU, only one of the sidebands for each channel is received at a broadcast receiver of the ONU, and both sidebands for each channel are received for downlink data detection and uplink re-modulation.

19. A WDM-PON comprising: a central office (CO) configured for generating a first optical signal comprising N channels such that in each channel data is carried in two sidebands with the respective carriers suppressed, and for generating a second optical signal comprising all carriers; and means for transmitting, to each of a plurality of optical network units (ONUs) associated with respective ones of the carriers, the data channel of the first optical signal with the two sidebands corresponding to the associated respective carrier, and said associated respective carrier in the second optical signal.

20. The WDM-PON as claimed in claim 19, wherein the N channels of the first optical signal comprise downlink data channels for the respective ONUs, and the second optical signal comprises a seeding light signal for generation of uplink data channels at the respective ONUs.

21. The WDM-PON as claimed in claim 19, wherein the N channels of the first optical signal comprise video data channels for the respective ONUs, and the second optical signal comprises downlink data channels for the respective ONUs at the associated respective carriers.

22. The WDM-PON as claimed in claim 19, wherein the N channels of the first optical signal comprise broadcast data channels for the respective ONUs, and the second optical signal comprises downlink data channels for the respective ONUs at the associated respective carriers.

23. A WDM-PON comprising: a CO configured for generating source signals from respective ones of one array of continuous wave (CW) light sources, for modulating each source signal with respective data signals using IRZ format, for multiplexing the modulated source signals, and for modulating the multiplexed source signals with broadcast data using FSK format such that for each channel in the multiplexed source signals has twp sidebands; and means for transmitting, to each of a plurality of ONUs associated with respective ones of the source signals, the modulated multiplexed source signals such that at each ONU, only one of the sidebands for each channel is received at a broadcast receiver of the ONU, and both sidebands for each channel are received for downlink data detection and uplink re-modulation.