Abstract:
An optical system and method includes a source-free optical network unit coupled to an optical fiber for receiving an original carrier signal with downstream data over the optical fiber. The optical network unit includes a modulator configured to remodulate the original carrier signal with upstream data to produce an upstream data signal for transmission back down the optical fiber in a direction opposite to a direction in which that original carrier signal was received.

Description:
RELATED APPLICATION INFORMATION 
     This application claims priority to provisional application Ser. No. 60/868,567 filed on Dec. 5, 2006 incorporated herein by reference. 
     The present application is related to U.S. application Ser. No. 11/832,075 filed Aug. 1, 2007, entitled “SYSTEM AND METHOD FOR PROVIDING WIRELESS OVER A PASSIVE OPTICAL NETWORK (PON)” and incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to wavelength division multiplexing passive optical network (WDM-PON) architecture and more particularly to a WDM-PON system and method to simultaneously provide video, voice and data services with a source-free optical network unit (ONU). 
     2. Description of the Related Art 
     With the growing amount of Passive Optical Network (PON) subscribers, annual sales of the PON equipment and sales are projected to grow accordingly. Transmission over such networks may be limited by the increasing data demand on existing passive optical networks. Limited bandwidth often results in limited services being provided to customers. Overcoming bandwidth issues by deploying additional fiber is often undesirable due to the large expenses associated therewith. 
     Furthermore, additional interfaces and/or equipment needed for additional optical fiber branches will further introduce ongoing management costs. Such costs are detrimental to providing broadband and other services in a competitive service provider market. 
     It would be advantageous to employ pre-constructed PON networks with increased bandwidth and reduced cost, in which traffic is terminated at the PON ONUs (Optical Network Units). Therefore, a need exists for providing service options using existing network hardware with improved multiplexing and modulation schemes that optimize resources and bandwidth. 
     SUMMARY 
     In accordance with illustrative embodiments, wavelength division multiplexing passive optical networks (WDM-PON) are employed, which are capable of handling large data bandwidth demands, provide enhanced security, and scalability to support several local subscribers. In addition, there is no need for time-multiplexing and ranging protocols in WDM-PONs. In the implementation of practical WDM-PON networks, one major issue is cost reduction. 
     Wavelength reuse with source-free optical network units (ONUs) permits a reduction in cost for a whole network. A source-free ONUs is employed in accordance with the present principles to return upstream data without an optical source. In accordance with one embodiment, a carrier signal is reused and employed to carry information both to and from the source-free ONU. This optical carrier is advantageously employed to use all optical power effectively and in a highly efficient operation. 
     Wavelength division multiplexing passive optical networks (WDM-PON) in accordance with the present principles can be utilized for broadcasting video service or providing triple play services (TPS) including data, video, and voice transmission. A WDM-PON can provide these services without the need to use a new lightwave source for upstream signal modulation. A WDM-PON architecture in accordance with the present principles provides services using centralized lightwave sources to reduce the cost of the system and improve efficiency. High capacity, symmetric data at 10 Gbit/s per channel for both downstream and upstream data, and 2.5 Gbit/s video broadcast have been successfully demonstrated in accordance with the present embodiments. 
     An optical system and method includes a transceiver configured to modulate a downstream data signal for transmission on an original carrier signal and a first data signal on a sub-carrier signal. A source-free optical network unit is coupled to the transceiver by an optical fiber. The optical network unit has a modulator configured to remodulate the original carrier signal with upstream data to produce an upstream data signal for transmission back to the transceiver. 
     An optical system includes a source-free optical network unit coupled to an optical fiber for receiving an original carrier signal with downstream data over the optical fiber. The optical network unit includes a modulator configured to remodulate the original carrier signal with upstream data to produce an upstream data signal for transmission back down the optical fiber in a direction opposite to a direction in which that original carrier signal was received. 
     An optical system includes a transceiver configured to modulate a downstream data signal for transmission on an original carrier signal and modulate a first data signal on a sub-carrier signal. A source-free optical network unit is coupled to the transceiver by an optical fiber. The optical network unit has a modulator configured to remodulate the original carrier signal with upstream data to produce an upstream data signal for transmission back to the transceiver. 
     An optical system includes a transceiver configured to phase modulate a downstream data signal for transmission on an original carrier signal and intensity modulate a first data signal on a sub-carrier signal. A source-free optical network unit is coupled to the transceiver by an optical fiber. The optical network unit has a modulator configured to remodulate the original carrier signal with upstream data to produce an upstream data signal for transmission back to the transceiver. 
     A method for providing a centralized lightwave source includes intensity modulating a first data signal for transmission on at least one subcarrier signal, phase modulating a downstream data signal for transmission on an original carrier signal, receiving the original carrier signal and the sub-carrier signal by a source-free optical network unit coupled to the transceiver by an optical fiber, remodulating the original carrier signal with upstream data to produce an upstream data signal for transmission, and transmitting the upstream data signal back to the transceiver. 
     These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein: 
         FIG. 1  is a block/flow diagram showing an optical network system in accordance with the present principles; 
         FIG. 2  is a block/flow diagram showing an optical network system which shares a video data signal over multiple wavelengths in accordance with the present principles; 
         FIG. 3  is a block/flow diagram showing an optical network system which includes one video data signal over a wavelength in accordance with the present principles; 
         FIG. 4  is a block/flow diagram showing an experimental step for demonstrating the present principles; and 
         FIG. 5  is a graph showing receiver sensitivity versus power ratio between downstream signals and video signals. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Present embodiments include wavelength division multiplexing passive optical network (WDM-PON) architectures capable of providing a large bandwidth and reduced costs. In one embodiment, video, voice and data services are simultaneously provided with a source-free optical network unit (ONU). In a particularly useful embodiment, service has been provided with 2.5 Gbit/s video signals, 10 Gbit/s downstream signals, and 10 Gbit/s upstream signals per channel. 
     Embodiments of the present invention can take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment including both hardware and software elements. In a preferred embodiment, the present invention is implemented in hardware having software elements, which include but are not limited to firmware, resident software, microcode, etc. 
     It is to be understood that the present embodiments are described in terms of a passive optical network (PON); however, other optical networks are contemplated and may benefit for the present teachings. While the FIGS. show illustrative optical hardware configurations, these configuration may be reconfigured or combined to provide functionality within the scope of the present principles. 
     Referring now to the drawings in which like numerals represent the same or similar elements and initially to  FIG. 1 , an illustrative system  10  includes a transceiver (transmitter/receiver)  12  connected to an optical fiber  14 . The optical fiber  14  preferably connects a source-free optical network unit (ONU)  16  to the transceiver  12  to permit two-way lightwave propagation through the fiber  14  between the transmitter/receiver  12  and the ONU  16 . 
     In accordance with the present principles, a carrier signal is generated for the transmission of data (e.g., downstream data) to the ONU  16  from the transceiver  12 . A sub-carrier signal is also generated to carry second data signals (e.g., video) to the ONU  16  from the transceiver  12 . In one embodiment, the sub-carrier carries the video signals at at least 2.5 Gbit/s, and at least 10 Gbit/s downstream signals are carried by the optical carrier, which are phase modulated signals. 
     The carrier signal and sub-carrier(s) are preferably multiplexed using wavelength division multiplexing. The carrier and sub-carrier are transmitted through fiber  14 , which is preferably a single mode fiber (SMF). The transmitted signal is received by the ONU  16 , and the carrier and subcarrier are separated and the data is removed from each. The phase modulated downstream optical carrier is re-modulated by intensity modulated upstream signals, and returned through fiber  14  to the transmitter/receiver  12 . In this way, the carrier signal is reused for bidirectional transmission of data over a same fiber. The ONU  16  does not need an independent light source (hence is source-free). 
     Referring to  FIG. 2 , an illustrative WDM-PON architecture  100  is shown in greater detail for an exemplary implementation in accordance with the present principles. Architecture  100  includes a network architecture for providing a broadcasting video service, although other broadband services and data types may be employed. Lightwaves  102  are input to a multiplexer  104  on channels (e.g., Ch 1 -Chn). Channels Ch 1 -Chn may each have their own laser source  102  or share a laser source depending on the design. Laser source  102  may include a laser, a laser diode, a light emitting diode or any other suitable light source. The channels Ch 1 -Chn are preferably multiplexed by a multiplexer  104 . After multiplexing, all lightwaves are modulated by an external modulator  112  to generate sub-carrier multiplexing signals. Modulator  112  includes a local oscillator  106  and a mixer  110  which mixes video or other data  108  with sub-carrier frequencies to modulate the light. 
       FIG. 2  shows optical subcarrier multiplexing modulation. When the lightwave carrier is modulated by a subcarrier multiplexing signal, there are subcarrier signals (smaller peaks on opposite sides of the center carrier peak) generated by the intensity modulator  112 , which enter an optical interleaver  114 . The signals are carried by the subcarrier, and the carrier will be able to carry less information or signal. In this way, the carrier the large center peak) will be more easily re-modulated. Optical carriers and sub-carriers are separated using the optical interleaver  114 . 
     A demultiplexer  116  is employed to separate the carriers before a phase modulator(s) (PM)  120  driven by downstream data  121  modulates each optical carrier. Phase modulation (PM) is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave. Unlike intensity modulation performed by, e.g., intensity modulator  112 , the amplitude of the carrier does not change. 
     Suppose that the signal to be sent, the modulating signal with frequency ω m  and phase φ m , is: m(t)=M sin(ω m t+φ m ), and the carrier onto which the signal is to be modulated is c(t)=C sin(ω c t+φ c ). Then, the modulated signal is y(t)=C sin(ω c t+m(t)+φ c ), which shows how m(t) modulates the phase. It can also be viewed as a change of the frequency of the carrier signal. PM can thus be considered a special case of frequency modulation (FM) in which the carrier frequency modulation is given by the time derivative of the phase modulation. 
     Then, all downstream phase signals at different wavelengths are multiplexed by a multiplexer  118 , which may include an arrayed waveguide grating (AWG), before the carriers are combined with the sub-carriers using a second optical interleaver  122 . Arrayed waveguide grating (AWG)  118  is employed as an optical multiplexer for wavelength division multiplexing (WDM). AWG  118  device is capable of multiplexing a large number of wavelengths into a single optical fiber  128 , thereby increasing the transmission capacity the optical network. 
     The downstream data  121  and video  108  signals are delivered to an ONU  160  through an optical circulator  126  to an optical fiber  128 . In the ONU  160 , an interleaver  130  is employed to separate the sub-carriers and phase modulated downstream signals. The sub-carriers at different wavelengths are demultiplexed by a demultiplexer  134  before a detector (e.g., a receiver)  138  directly detects them with a low-pass filter. The phase modulated downstream signals, after being demultiplexed by demultiplexer  132 , are sent to two paths. One part is converted to intensity signals by a demodulator  144  before it is detected by a photodiode  142  to realize optical to electrical conversion. The other part is re-modulated by an intensity modulator  140  driven by upstream data  141 . The re-modulated signal is fed back to an optical circulator  136  and can be returned back over fiber  128  by demultiplexing the signal with multiplexer  132  and deinterleaving the signal with interleaver  130 . 
     A centralized lightwave is realized in an optical line terminal (OLT)  156 . The upstream data are sent back to the OLT  156  by a same fiber  128 . In the OLT  156 , the upstream data, at different wavelengths, are demultiplexed by demultiplexer  152  before they are optic-electrically converted for each channel using receivers  154 . 
     Advantageously, the carrier lightwave is reused by sending the carrier wave back to the OLT  156  from the ONU  160 . The ONU therefore does not need an optical signal source, which would otherwise require power and introduce cost and complexity to the system. Instead, the carrier lightwave is employed to carry video and downstream data in one direction and upstream data in the opposite direction. 
     Referring to  FIG. 3 , if different wavelengths need to carry different video signals  208 , architecture  200  may be employed to realize this function. Similar to  FIG. 2 , only a transmitter configuration  202  needs to be changed. Each lightwave is separately modulated by modulator  112  to generate sub-carrier modulation (SCM) signals. Then, an interleaver  114  separates the carrier and sub-carriers. A phase modulator  120  driven by the downstream data  121  modulates the separated carrier. Then, another interleaver  122  combines the carrier and sub-carrier before all channels are multiplexed by multiplexer  218 . 
     Comparing the configurations of  FIG. 2  and  FIG. 3 , the transmitter of  FIG. 2  employs one high-speed intensity modulator (IM)  112  and two interleavers (IL)  114  and  122 , three multiplexers  104 ,  116  and  118 , while  FIG. 3  employs N high-speed intensity modulators (IM)  112  (one for each video signal), 2N inter-leavers ( 14 ,  122 ) and one multiplexer  218  in the transmitter when the channel number is N. The transmitter in  FIG. 3  may be more expensive. 
     Referring to  FIG. 4 , an experimental setup  300  is illustratively shown for demonstration of the present principles. While  FIG. 4  and the description herein provide specific equipment, magnitudes and settings, this information is for illustrative purposes and should not be construed as limiting the present invention. Variations and combinations of the equipment, magnitudes and settings as described here can be modified depending of the design application and preferences of the implementer. 
     2.5 Gbit/s video signals  308  generated from a pattern generator (not shown) with a pseudo-random bit sequence (PRBS) word length of 2 31 −1 were mixed with a 20 GHz sinusoidal wave  306 . The signals were mixed in a mixer  110  and used to drive an intensity modulator  112 , e.g., a LiNbO 3  modulator, after amplification by an electrical amplifier  305 . 
     The optical spectrum after the intensity modulator  112  is inserted in  FIG. 4  as inset (i). A carrier suppression ratio (the ratio of the optical carrier to subcarrier at the first-order mode) is 12 dB as indicated in inset (i). An optical interleaver  114  with 50/25 GHz and two output ports to separate the optical carrier and the sub-carriers was employed. The optical spectra are shown in insets (ii) and (iii). The separated optical carrier was modulated by a phase modulator  120  driven at 10 Gbit/s electrical signals (downstream phase signals  309 ) generated from another pattern generator (not shown) with a PRBS word length of 2 31 −1. The optical spectrum after phase modulation is shown in inset (iv) of  FIG. 4 . Then, the phase downstream signals were combined with the video signals using a 3 dB optical coupler (OC)  310 . The optical spectrum of the combined the signals is shown in inset (v) of  FIG. 4 . 
     Here, the power levels of the video signals  308  and downstream phase signals  309  have to be chosen properly because the video signals  308  and downstream phase signals  309  have to be separated in an GNU  320  and there may be some linear cross-talk between the video and phase signals. We measured the receiver sensitivity of the video and phase modulated downstream signals with different ratios, which are defined as the power of phase downstream signals divided by the power of video signals. The measured results without transmission fiber are illustratively shown in  FIG. 5 . When the ratio is 5 dB, the video and downstream signals have good receiver sensitivities. So, we set the power of the downstream signals to be 5 dB larger than the video signals with two sidebands in this experiment. 
     The combined signals were sent to the ONU  320  after passing through one optical circulator  126  to a fiber  128  (e.g., over a single mode fiber, in this case, 20 km SMF-28), and another optical circulator  322 . To overcome the effect of the Rayleigh reflection scattering, the total power for the video signals and downstream signals into the fiber was 6 dBm. In the ONU  320 , one delay line Mach-Zehnder interferometer (DI-MZ)  310  with 44 GHz free spectral range (FSR) was employed to separate the phase downstream signals and video signals. A commercial 2.5 GHz receiver  138  directly detected the video signals with an APD receiver and 2 GHz low-pass filter. The separate optical spectrum is shown in  FIG. 4  as inset (vi). 
     The power penalty caused by the transmission fiber was 0.4 dB at a BER of 10 −9 . The separated phase downstream signals were separated into two parts by a 3 dB optical coupler  312 . One part was converted into the intensity signals by using a DI-MZ interferometer  144  with FSR of 20 GHz. For the 10 Gbit/s downstream ( 309 ) and upstream ( 325 ) signals, we use PIN receivers to detect these signals. The power penalty caused by the transmission fiber is negligible. The other part was re-modulated driven by another 10 Gbit/s electrical signal with a PRBS length of 2 31 −1. The optical spectrum after re-modulation is shown in inset (vii) of  FIG. 4 . An integrated semiconductor optical amplifier (SOA) and electro-absorption modulator (EAM)  140  was employed to amplify and modulate the signals. The pure gain of the integrated SOA and EAM is 4 dB when the dc bias of the SOA is 120 mA and EAM dc bias is −1.4 V. The upstream signals  325  were delivered back to OLT  330  after passing through the circulator  126 , the fiber (e.g., 20 km SMF-28), and the second circulator  322 . The power penalty after transmission was negligible. The receiver sensitivity due to the intensity noise may be degraded a small amount. The PIN receiver sensitivity at a BER of 10 −9  is −15 dBm. 
     A novel WDM-PON configuration with centralized lightwaves in the OLT is provided. Illustrative embodiments provide sufficient bandwidth to provide services with at least 2.5 Gbit/s video, 10 Gbit/s downstream and 10 Gbit/s upstream service. In one network embodiment, a sub-carrier carries the video signals at 2.5 Gbit/s, and the 10 Gbit/s downstream signals are carried by the optical carrier, which are phase modulated signals. The phase modulated downstream optical carrier is re-modulated by intensity modulated upstream signals. The power penalty for video signals after transmission was 0.4 dB at a BER of 10 −9 , while the power penalty is negligible for the downstream and upstream signals after transmission over 20 km SMF-28. 
     Having described preferred embodiments of a wavelength division multiplexing passive optical network architecture with source-free optical network units (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.