Abstract:
Tuned lasers in the ONU&#39;s are eliminated in WDM PON by use of a burst mode transmission. An apparatus for communicating over a passive optical network comprises a transmitting portion operable to generate an optical signal comprising a first portion modulated with a first data signal and a second portion that is unmodulated and to transmit the optical signal over the passive optical network and a receiving portion operable to receive an optical signal comprising the second portion of the transmitted optical signal modulated with a second data signal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an apparatus that eliminates the laser in an Optical Network Unit (ONU) in a Wave Division Multiplexed (WDM) Passive Optical Network (PON).  
         [0003]     2. Description of the Related Art  
         [0004]     Optical networks have become a standard technology for the transport of information in the telecommunications industry. A number of different optical network standards have been defined, with each having advantages and disadvantages for different uses. Synchronous optical network (SONET) is one standard for optical telecommunications transport. SONET is often used for long-haul, metro level, and access transport applications.  
         [0005]     Another standard for optical telecommunications transport is passive optical networks (PONs). PONs are commonly used to address the last mile of the communications infrastructure between the service provider&#39;s central office, head end, or point of presence (POP) and business or residential customer locations. Also known as the access network or local loop, the last mile consists predominantly, in residential areas, of copper telephone wires or coaxial cable television (CATV) cables. In metropolitan areas, where there is a high concentration of business customers, the access network often includes high-capacity synchronous optical network (SONET) rings, optical T3 lines, and copper-based T1s.  
         [0006]     Bandwidth is increasing dramatically on long-haul networks through the use of wavelength division multiplexing (WDM) and other new technologies. Recently, WDM technology has even begun to penetrate metropolitan-area networks (MAN), boosting their capacity dramatically. At the same time, enterprise local-area networks (LAN) have moved from 10 Mbps to 100 Mbps, and soon many LANs will be upgraded to gigabit Ethernet speeds. The result is a growing gulf between the capacity of metro networks on one side and end-user needs on the other, with the last-mile bottleneck in between.  
         [0007]     PONs are one solution to this problem in an attempt to break the last-mile bandwidth bottleneck that other access network technologies do not adequately and economically address.  
         [0008]     Important parts of the PON architecture are the Optical Network Unit (ONU) and the Optical Line Termination (OLT), which are active network elements located at end points of a PON. The OLT provides an interface for data to be transmitted over the PON. The ONU provides an interface between the customer&#39;s data, video, and telephony networks and the PON. The primary function of the ONU is to receive traffic in an optical format and convert it to the customer&#39;s desired format. Many PONs use wavelength division multiplexing (WDM) of multiple signals over each optical fiber. WDM PON provides dedicated optical wavelengths in each direction, for each ONU. This provides improved operations over other types of PON, where the same wavelength(s) are shared by up to 32 (or more) ONU&#39;s. However, a typical implementation of WDM PON requires a tuned narrowband laser in the ONU, and a fixed narrowband laser in the OLT dedicated to each ONU. This results in too costly an implementation for access applications. Most PON&#39;s today aren&#39;t based on WDM PON due to cost, they are APON, EPON, etc where ONU&#39;s share wavelengths in both directions. Thus, a need arises for a technique that can both eliminate tuned lasers in the ONU&#39;s and also provide shared optical carrier sources for the OLT&#39;s.  
       SUMMARY OF THE INVENTION  
       [0000]     Shared Multi-Lambda Source for WDM PON  
         [0009]     The present invention eliminates tuned lasers in the ONU&#39;s and also provide shared optical carrier sources for the OLT&#39;s.  
         [0010]     In one embodiment of the present invention, an apparatus comprises a plurality of optical carrier generators, each optical carrier generator outputting an optical carrier at a different wavelength, an optical multiplexer operable to combine the plurality of optical carriers to form a wave division multiplexed optical carrier, and an optical power splitter having a plurality of outputs, each output connectable to an optical line termination unit, the optical power splitter operable to split the wave division multiplexed optical carrier to form a plurality of wave division multiplexed optical carriers.  
         [0011]     In one aspect of the present invention, each optical carrier generator comprises a narrowband laser. The apparatus further comprises an optical amplifier operable to amplify at least one of the plurality of wave division multiplexed optical carriers. The apparatus further comprises a protection switch operable to provide switching between working and protect optical WDM carriers. At least some of the optical line termination unit are in separate physical enclosures.  
         [0000]     Avoiding ONU Laser by Optical Modulation and Remodulation  
         [0012]     The present invention eliminates tuned lasers in the ONU&#39;s and also provide shared optical carrier sources for the OLT&#39;s.  
         [0013]     In one embodiment of the present invention, a method of communicating over a passive optical network comprises generating an optical signal modulated with a first data signal at a first network element, transmitting the modulated optical signal over the passive optical network from the first network element to a second network element, remodulating the received modulated optical signal with a second data signal at the second network element, and transmitting the remodulated optical signal from the second network element to the first network element.  
         [0014]     In one embodiment of the present invention, an apparatus for communicating over a passive optical network comprises a transmitting portion operable to generate an optical signal modulated with a first data signal and to transmit the modulated optical signal over the passive optical network and a receiving portion operable to receive an optical signal comprising the transmitted optical signal remodulated with a second data signal.  
         [0015]     In one aspect of the present invention, the transmitting portion comprises an optical modulator operable to modulate an unmodulated optical signal with the first data signal. The first data signal comprises a line code signal having a symbol rate greater than a symbol rate of the first data. The receiving portion comprises an optical demodulator operable to demodulate the received optical signal to recover the second data signal.  
         [0016]     In one embodiment of the present invention, an apparatus for communicating over a passive optical network comprises a receiving portion operable to receive an optical signal modulated with a first data signal over the passive optical network, a remodulating portion operable to remodulate the received optical signal with a second data signal, and a transmitting portion operable to transmit the remodulated optical signal over the passive optical network.  
         [0017]     In one aspect of the present invention, the receiving portion comprises a power splitter operable to split the received optical signal between the receiving portion and the remodulating portion and a line code demodulator operable to detect the first data signal from the received optical signal. The optical signal modulated with the first data signal comprises a training interval and the line code demodulator further comprises a framing device operable to identify the training interval. The receiving portion further comprises circuitry operable to output a signal phase locked to the training interval signal that is locked to the downstream frame and clock identified by the framing device.  
         [0018]     In one aspect of the present invention, the remodulating portion comprises a line code modulator operable to remodulate the received optical signal with a second data signal based on the signal phase locked to the training interval signal. The remodulating portion comprises a line code modulator operable to remodulate the received optical signal with a second data signal  
         [0019]     In one aspect of the present invention, the transmitting portion comprises an optical amplifier operable to amplify the remodulated optical signal.  
         [0020]     In one embodiment of the present invention, an apparatus for communicating over a passive optical network comprises a beamsplitter operable to split a received optical signal between a receiving portion and a remodulating portion, a remodulating portion operable to remodulate the received optical signal with a second data signal, and a photodetector operable to detect the first data signal from the received optical signal.  
         [0021]     In one aspect of the present invention, the remodulating portion comprises a silicon optical amplifier reflective operable to receive the modulated optical signal from the beamsplitter, to remodulate the received optical signal with the second data signal, and to reflect the remodulated optical signal back to the beamsplitter.  
         [0000]     Eliminating ONU Laser for WDM PON by Burst Mode  
         [0022]     The present invention eliminates tuned lasers in the ONU&#39;s and also provide shared optical carrier sources for the OLT&#39;s.  
         [0023]     In one embodiment of the present invention, a method of communicating over a passive optical network comprises generating at a first network element an optical signal comprising a first portion modulated with a first data signal and a second portion that is unmodulated, transmitting the optical signal over the passive optical network from the first network element to a second network element, modulating the second portion of the received optical signal with a second data signal at the second network element, and transmitting the modulated second portion of the received modulated optical signal from the second network element to the first network element.  
         [0024]     In one embodiment of the present invention, an apparatus for communicating over a passive optical network comprises a transmitting portion operable to generate an optical signal comprising a first portion modulated with a first data signal and a second portion that is unmodulated and to transmit the optical signal over the passive optical network and a receiving portion operable to receive an optical signal comprising the second portion of the transmitted optical signal modulated with a second data signal.  
         [0025]     In one aspect of the present invention, the transmitting portion comprises an optical modulator operable to modulate the first portion of an unmodulated optical signal with the first data signal and to not modulate the second portion of the unmodulated optical signal.  
         [0026]     In one aspect of the present invention, the receiving portion comprises an optical demodulator operable to demodulate the received optical signal to recover the second data signal.  
         [0027]     In one embodiment of the present invention, an apparatus for communicating over a passive optical network comprises a receiving portion operable to receive an optical signal comprising a first portion modulated with a first data signal and a second portion that is unmodulated over the passive optical network, a modulating portion operable to modulate the second portion of the received optical signal with a second data signal to form a second optical signal, and a transmitting portion operable to transmit the second optical signal over the passive optical network.  
         [0028]     In one aspect of the present invention, the receiving portion comprises a power splitter operable to split the received optical signal between the receiving portion and the remodulating portion and a demodulator operable to detect the first data signal from the received optical signal. The demodulator further comprises a framing device operable to identify the second portion of the received optical signal.  
         [0029]     In one aspect of the present invention, the modulating portion comprises a modulator operable to modulate the second portion of the received optical signal with a second data signal based on the identification of the second portion of the received optical signal from the framing device. The transmitting portion comprises an optical amplifier operable to amplify the second optical signal.  
         [0030]     In one embodiment of the present invention, an apparatus for communicating over a passive optical network comprises a beamsplitter operable to split a received optical signal between a receiving portion and a modulating portion, the received optical signal comprising a first portion modulated with a first data signal and a second portion that is unmodulated, a modulating portion operable to modulate the second portion of the received optical signal with a second data signal to form a second optical signal, and a photodetector operable to detect the first data signal from the received optical signal.  
         [0031]     In one aspect of the present invention, the modulating portion comprises a silicon optical amplifier reflective operable to receive the second portion of the optical signal from the beamsplitter, to modulate the second portion of the optical signal with the second data signal to form the second optical signal, and to reflect the second optical signal back to the beamsplitter. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]      FIG. 1   a  is an exemplary block diagram of a WDM PON system, in which the present invention may be implemented.  
         [0033]      FIG. 1   b  is an exemplary block diagram of a WDM PON system, in which the present invention may be implemented.  
         [0034]      FIG. 2  is an exemplary block diagram of a shared lambda source shown in  FIG. 1   b.    
         [0035]      FIG. 3  is an exemplary block diagram of a PON system, in which the present invention may be implemented.  
         [0036]      FIG. 4  is an exemplary format of a line code that may be used in an embodiment of the present invention.  
         [0037]      FIG. 5  is an exemplary block diagram of optical and electrical components in an OLT that that may be used to implement the present invention.  
         [0038]      FIG. 6   a  is an exemplary block diagram of optical and electrical components in an ONU that that may be used to implement the present invention.  
         [0039]      FIG. 6   b  is an exemplary block diagram of optical and electrical components in an ONU that that may be used to implement the present invention.  
         [0040]      FIG. 6   c  is an exemplary illustration of a downstream training signal used for phase locking in an embodiment of the present invention.  
         [0041]      FIG. 6   d  is an exemplary illustration of a downstream training signal used for phase locking in an embodiment of the present invention.  
         [0042]      FIG. 6   e  is an exemplary illustration of a downstream training signal used for phase locking in an embodiment of the present invention.  
         [0043]      FIG. 7  is an exemplary format of signals that may be used in an embodiment of the present invention.  
         [0044]      FIG. 8  is an exemplary block diagram of optical and electrical components in an ONU that that may be used to implement the present invention.  
         [0045]      FIG. 9  is an exemplary block diagram of optical and electrical components in an OLT that that may be used to implement the present invention.  
         [0046]      FIG. 10  is an exemplary block diagram of optical and electrical components in an ONU that that may be used to implement the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]     An exemplary PON system  100 , in which the present invention may be implemented, is shown in  FIG. 12   a.  One or more Optical Line Termination Units (OLTs)  102  provide the interface with data to be transmitted over the Optical Distribution Network (ODN)  104  to the Optical Network Unit (ONU)  106  portion of the PON. The passive elements of the PON are located in ODN  104  and may include single-mode fiber-optic cable, and passive optical devices such as splitters/couplers, connectors, multiplexers, and splices. In the example shown in  FIG. 1   a,  ODN  104  includes lambda multiplexer  105  and a number of optical fibers.  
         [0048]     The ONU  106  portion of the PON includes one or more ONUs that provide the interface between the customer&#39;s data, video, and telephony networks and the PON. The primary function of an ONU is to receive traffic in an optical format and convert it to the end user&#39;s desired format and to receive traffic from the end user and convert it to an optical format. Alternatively, the end user&#39;s format is typically an electrical format, such as Ethernet, IP multicast, POTS, T1, etc., but the end user&#39;s format may be an optical format, such as SONET/SDH.  
         [0049]     The exemplary PON system  100 , in which the present invention may be implemented, is shown in more detail in  FIG. 1   b.  OLT  102  includes or is connected to a shared lambda source  108 . Shared lambda source  108  includes a plurality of single wavelength optical carrier generators such as optical carrier generators  108 - 1  to  108 - 32 . Each optical carrier generator outputs an optical carrier at a different wavelength. In the example shown in  FIG. 1   b,  there are 32 optical carrier generators shown as an example. However, the present invention contemplates usage of any number of optical carrier generators. The optical carrier generators are typically narrowband lasers. Shared lambda source  108  also includes lambda multiplexer  110 , which multiplexes the plurality of optical carriers from optical carrier generators  108 - 1  to  108 - 32  onto a single optical fiber, to form a wavelength division multiplex (WDM) carrier on the optical fiber. Shared lambda source  108  also includes optical power splitter  112 , which splits the WDM carrier into a plurality of WDM carrier, each of which may be used by a particular PON. Optionally, optical amplifiers may be used to amplify the plurality of WDM carrier, if higher WDM carrier amplitude is needed for a particular application. Optical amplifiers are typically used to compensate for losses incurred in the power splitters. In addition, if shared source  108  is used to provide a WDM carrier to multiple OLTs in different physical enclosures, then preferably shared source  108  includes a protection switch to provide switching between the working and protect optical WDM carriers. It is to be noted that the need for protection may apply even if shared source  108  and the OLTs are in the same location. Typically, shared lambda source  108  provides optical carriers having wavelengths in a range from 1525 to 1565 nm. However, this is merely an example. The present invention contemplates operations over any range of optical wavelengths.  
         [0050]     OLT  102  includes a plurality of Semiconductor Optical Amplifier Reflective (SOAR) devices  114 - 1  to  114 - 32 , a plurality of photodetector circuits  116 - 1  to  116 - 32 , lambda multiplexers  118  and  120 , and optical circulators  122  and  124 . A WDM carrier from one tap of optical power splitter  112  is input to input  122 - 1  of optical circulator  122 . The WDM carrier is circulated from input  122 - 1  to input/output  122 - 2 , where the WDM carrier is output to lambda multiplexer/demultiplexer  118 . The WDM carrier is demultiplexed by lambda multiplexer/demultiplexer  118  and separated into a plurality of narrow wavelength carriers. Each narrow wavelength carrier is input to a SOAR device  114 - 1  to  114 - 32 , where it is modulated with data to be transmitted over the PON. A modulated narrow wavelength signal is output from each SOAR device  114 - 1  to  114 - 32  and input to lambda multiplexer/demultiplexer  118 . These may be termed the OLT modulated signals. The input OLT modulated signals are multiplexed in lambda multiplexer/demultiplexer  118  to form a modulated WDM signal. This may be termed the OLT WDM signal. The OLT WDM signal is output from lambda multiplexer/demultiplexer  118  and input to input/output  122 - 2  of optical circulator  122 . The OLT WDM signal is circulated in optical circulator  122  and output from output  122 - 3  of optical circulator  122 . The OLT WDM signal is input to input  124 - 1  of optical circulator  124 , circulated and output from input/output  124 - 2  of optical circulator  124 . The OLT WDM signal is carried via ODN  104  to the ONU  106  portion of the PON.  
         [0051]     The modulation present in the OLT modulated signals varies in different embodiments of the present invention. In some embodiments, the narrow wavelength signal is not modulated 100% of the time, but rather, unmodulated or continuous-wave (CW) portions of the narrow wavelength signal may be output from one or more SOAR devices  114 - 1  to  114 - 32 . For simplicity, the signal output from a SOAR device in the OLT is referred to as an OLT modulated signal, even if it includes unmodulated or CW portions.  
         [0052]     A second modulated WDM signal is also carried via ODN  104  from the ONU  106  portion of the PON to OLT  102 . This second modulated WDM signal is termed the ONU WDM signal. The ONU WDM signal is input to input  124 - 2  of optical circulator  124 , circulated and output from output  124 - 3  of optical circulator  124 . The second ONU WDM signal is demultiplexed by lambda multiplexer/demultiplexer  120  and separated into a plurality of modulated narrow wavelength signals. Each modulated narrow wavelength signal is input to a photodetector  116 - 1  to  116 - 32 , where the data modulated onto the signal is detected. Each photodetector outputs an electrical signal carrying the data stream extracted from its input modulated narrow wavelength signal.  
         [0053]     ODN  104  includes the passive elements of one or more PONs. ODN  104  may include single-mode fiber-optic cable, and passive optical devices such as splitters/couplers, connectors, multiplexers, and splices. Active network elements, such as OLT  102  and the ONU  106  portion of the PON, are located at the end points of the PON. Optical signals traveling across the PON are either split onto multiple fibers or combined onto a single fiber by optical splitters/couplers, depending on whether the light is traveling up or down the PON. The PON is typically deployed in a single-fiber, point-to-multipoint, tree-and-branch configuration for residential applications. OLTs may also be connected in a protected ring architecture for business applications or in a bus architecture for campus environments and multiple-tenant units (MTU).  
         [0054]     As shown in  FIG. 1   b,  ODN  104  includes lambda multiplexer/demultiplexer  105  and a plurality of optical fibers  128 . Lambda multiplexer/demultiplexer  105  receives a modulated WDM signal from OLT  102  and demultiplexes it to from a plurality of modulated narrow wavelength signals, each of which is transmitted over an optical fiber  128 . Likewise, lambda multiplexer/demultiplexer  105  receives a modulated narrow wavelength signal from each optical fiber  128  and multiplexes them to form a modulated WDM signal that is transmitted to OLT  102 . In this way, ODN  104  provides bi-directional optical communications paths.  
         [0055]     The ONU  106  portion of the PON includes one or more ONUs  130 - 1  to  130 - 32 . Each modulator/detector unit, such as modulator detector unit  130 - 1 , includes a beam splitter  132 - 1 , a SOAR device  134 - 1 , and a photodetector  136 - 1 . Beam splitter  132  is an optical device that splits a beam of light in two. In its most common form, it is a cube, made from two triangular glass prisms that are glued together at their base using a resin. The thickness of the resin layer is adjusted such that approximately half of the light incident through one “port” (i.e. face of the cube) is reflected and the other half is transmitted. Another possible design is the use of a “half-silvered mirror”. This is a plate of glass with a thin coating of silver (usually deposited from silver vapor) with the thickness of the silver coated such that of light incident at a 45 degree angle, one half is transmitted and one half it reflected. Instead of a silver coating, a dielectric optical coating may be used instead. In order to be usable with the present invention, beamsplitter  132  must work over the range of optical wavelengths generated by the OLT, since the same ONU may be connected to any of the wavelengths generated by the OLT.  
         [0056]     An OLT modulated signal, which is a modulated narrow wavelength signal generated in OLT  102 , is output from an optical fiber, such as fiber  128  and is input to a beam splitter, such as beam splitter  132 - 1 . A portion of the OLT modulated signal is output to SOAR device  134 - 1  and a portion of the OLT modulated signal is output to photodetector  136 - 1 . The data modulated onto the OLT modulated signal is detected by photodetector  136 - 1 . Each photodetector outputs an electrical signal carrying the data stream extracted from its input OLT modulated signal. Thus, photodetector  136 - 1  extracts the data transmitted over one wavelength of one fiber of the PON from OLT  102  to ONU  106 .  
         [0057]     The OLT modulated signal is also input to SOAR device  134 - 1 . As noted above, the modulated narrow wavelength signal may include some unmodulated or CW portions. These unmodulated or CW portions of the OLT modulated signal are modulated in the SOAR device  134 - 1  based on input electrical signals that carry data to be modulated onto the optical signal. The portions of an OLT modulated signal that are modulated by SOAR device  134 - 1  are termed an ONU modulated signal. The ONU modulated signal is output from SOAR device  134 - 1 , passes through beam splitter  132 - 1 , and is transmitted over optical fiber  128  to lambda multiplexer/demultiplexer  105 . The plurality of ONU modulated signals from modulator detector units  130 - 1  to  130 - 32  are multiplexed by lambda multiplexer/demultiplexer  105  to form a WDM signal termed the ONU WDM signal. As described above, the ONU WDM signal is input to input  124 - 2  of optical circulator  124 , circulated and output from output  124 - 3  of optical circulator  124 . The second ONU WDM signal is demultiplexed by lambda multiplexer/demultiplexer  120  and separated into a plurality of modulated narrow wavelength signals. Each modulated narrow wavelength signal is input to a photodetector  116 - 1  to  116 - 32 , where the data modulated onto the signal is detected. Each photodetector outputs an electrical signal carrying the data stream extracted from its input modulated narrow wavelength signal.  
         [0058]     An example of a shared lambda source  108  is shown in  FIG. 2 . Shared lambda source  108  includes a plurality of single wavelength optical carrier generators such as optical carrier generators  108 - 1  to  108 -K. In the example shown in  FIG. 2 , there are K optical carrier generators shown as an example. However, the present invention contemplates usage of any number of optical carrier generators. The optical carrier generators are typically narrowband lasers. Shared lambda source  108  also includes lambda multiplexer  110 , which multiplexes the plurality of optical carriers from optical carrier generators  108 - 1  to  108 -K onto a single optical fiber  202 , to form a wavelength division multiplexed (WDM) carrier on the optical fiber  202 . Shared lambda source  108  also includes optical power splitter  112 , which splits the WDM carrier into a plurality of WDM carriers, each of which may be routed to an OLT. In the example shown in  FIG. 2 , optical power splitter  112  routes the WDM carriers to N OLTs. However, the present invention contemplates routing to any number of OLTs. Optionally, optical amplifier  204  may be used to amplify the plurality of WDM carriers, if higher WDM carrier amplitude is needed for a particular application. Optical amplifier  204  is typically used to compensate for losses incurred in the power splitters. In addition, if shared source  108  is used to provide a WDM carrier to multiple OLTs in different physical enclosures, then preferably shared source  108  includes a protection switch to provide switching between the working and protect optical WDM carriers. It is to be noted that the need for protection may apply even if shared source  108  and the OLTs are in the same location. Typically, shared lambda source  108  provides optical carriers having wavelengths in a range from 1525 to 1565 nm. However, this is merely an example. The present invention contemplates operations over any range of optical wavelengths.  
         [0059]     An exemplary PON system  300 , in which the present invention may be implemented, is shown in  FIG. 3 . In this example, one or more OLTs  102  provide the interface with data to be transmitted over the Optical Distribution Network (ODN)  104  to the Optical Network Unit (ONU)  106  portion of the PON. OLT  102  provides interconnection with electrical networks, such as Ethernet, optical networks, such as SONET, and receives a plurality of optical carriers from a shared lambda source. The passive elements of the PON are located in ODN  104  and may include single-mode fiber-optic cable, and passive optical devices such as splitters/couplers, connectors, multiplexers, and splices. In the example shown in  FIG. 1   a,  ODN  104  includes a plurality of cascaded lambda multiplexers and a number of optical fibers.  
         [0060]     The ONU  106  portion of the PON includes one or more ONUs that provide the interface between the customer&#39;s data, video, and telephony networks and the PON. The primary function of an ONU is to receive traffic in an optical format and convert it to an electrical signal in the end user&#39;s desired format and to receive traffic as an electrical signal from the end user and convert it to an optical signal and format. Typically, the end user&#39;s format is an electrical format, such as Ethernet, IP multicast, POTS, T1, etc., but alternatively, the end user&#39;s format may be an optical format, such as Ethernet over fiber.  
         [0061]     As described above, the modulation performed in the OLT varies in different embodiments of the present invention. In one embodiment, the optical signal modulated in the OLT is transmitted to the ONU, where the modulated signal is remodulated and transmitted back to the OLT. The operation of this embodiment may be termed “modulation—remodulation”. An example of the operation of modulation—remodulation is shown in  FIG. 4 . In this example, a downstream signal is modulated to carry data in the OLT and transmitted to the ONU. At the ONU, the data carried by the signal is recovered, and the downstream signal is remodulated to carry data to form an upstream signal that is transmitted to the OLT. At the OLT, the data carried by the upstream signal is recovered.  
         [0062]     In the example shown in  FIG. 4 , a data bit “0” is modulated onto the downstream signal using a line code of “01”  402 , while a data bit “1” is modulated onto the downstream signal using a line code of “10”  404 . When the downstream signal is received at the ONU, the ONU remodulates the signal to carry upstream data. In this example, a data bit “0” is remodulated onto the upstream signal using a line code of “00”  406 , while a data bit “1” is remodulated onto the upstream signal using a line code of “01”  408  or “10”  410 . The upstream line code is obtained by multiplying the downstream line code by full bit period “0” (modulator switch off), or full bit period “1” (modulator switch on). Either “01”  408  or “10”  410  is read by the OLT as a “1” from the ONU.  
         [0063]     It is seen that in this example, the downstream line code is twice the frequency of information bit rate. In this example, a 310 MHz line code, which provides a 155 Mbs data rate, is shown. It is to be noted that these rates and line codes are merely examples, the present invention is not limited to these rates and line codes. Rather, the present invention contemplates any and all rates and line codes for data transmission.  
         [0064]     The modulation—remodulation technique may be implemented in the embodiment of the present invention shown in  FIG. 1   b.  Likewise, the modulation—remodulation technique may be implemented in the embodiment of the present invention shown in  FIG. 5 , which is an exemplary block diagram of optical and electrical components in OLT  500 .  
         [0065]     OLT  500  includes an optical power splitter  502 , a lambda demultiplexer  504 , a plurality of optical modulators  506 - 1  to  506 -K, a lambda multiplexer  508 , an optical amplifier  510 , an optical coupler  512 , a lambda demultiplexer  514 , and a plurality of optical demodulators/CDRs  516 - 1  to  516 -K. An unmodulated WDM carrier (including a plurality of optical carriers) is input to optical power splitter  502 , which splits the WDM carrier into a plurality of WDM carriers, each of which may be used by a particular PON. One or more optical amplifiers may be used to amplify the WDM carrier, if higher WDM carrier amplitude is needed for a particular application. In addition, since the WDM carrier is provided to multiple PONs, a protection switch is included to provide switching between the working and protect optical WDM carriers.  
         [0066]     The unmodulated WDM carrier is input to lambda demultiplexer  504 , which separates the signal into a plurality of narrow wavelength carriers. Each narrow wavelength signal is input to an optical modulator  506 - 1  to  506 -K, where it is modulated with data to be transmitted over the PON. A modulated narrow wavelength signal is output from each optical modulator  506 - 1  to  506 -K and input to lambda multiplexer  508 . These may be termed the OLT modulated signals. The input OLT modulated signals are multiplexed in lambda multiplexer  508  to form a modulated WDM signal. This may be termed the OLT WDM signal. The OLT WDM signal is output from lambda multiplexer  508  and input to optical amplifier  510 , where the signal is amplified for transmission over the optical fiber. The amplified signal is input to optical coupler  512 , where it is coupled onto the optical fiber for transmission to the ONU.  
         [0067]     Turning briefly to  FIG. 6   a,  an example of optical and electrical components in an ONU  600 , in which the modulation—remodulation technique may be implemented, is shown. The ONU shown in  FIG. 6   a  operates in conjunction with the embodiment of the OLT shown in  FIG. 5 . ONU  600  includes optical coupler  602 , optical power splitter  604 , line code demodulator  606 , line code modulator  608 , and optical amplifier  610 . The signal from the OLT is received over the optical fiber and input to optical coupler  602 . The signal is input to optical power splitter  604 , which transmits the signal to line code demodulator  606  and line code modulator  608 . Line code demodulator  606  demodulates the optical signal and extracts the downstream data and clock signals from the optical signal. The downstream data and clock signals are output from line code demodulator  606  as electrical signals.  
         [0068]     Line code modulator  608  remodulates the optical signal with upstream data according to the line code modulation scheme shown in  FIG. 4 , or another equivalent scheme. The upstream data is input to line code modulator  608  as an electrical signal. Line code modulator  608  syncs to the downstream optical line code, then multiplies signal by upstream electrical bits (1 or 0). Thus, multiplying the downstream line code (01 or 10) by two periods of “0” (00) results in upstream modulation of “00”. Likewise, multiplying the downstream line code (01 or 10) by two periods of “1” (11) results in upstream modulation of “01” or “10”. Accurate phase alignment is required for upstream modulation.  
         [0069]     The remodulated optical signal is input to optical amplifier  610 , which amplifies the optical signal and outputs the signal to coupler  602 . Coupler  602  couples the amplified remodulated signal onto the optical fiber for transmission to the OLT.  
         [0070]     Returning to  FIG. 5 , the upstream, remodulated signal is received at coupler  512 , which outputs the upstream signal to lambda demultiplexer  514 . Lambda demultiplexer  514  separates the signal into a plurality of narrow wavelength remodulated signals. Each modulated narrow wavelength signal is input to an optical demodulator/CDR  516 - 1  to  516 -K, where the data modulated onto the signal is detected. Each photodetector outputs an electrical signal carrying the data stream extracted from its input modulated narrow wavelength signal.  
         [0071]     There are additional considerations related to the modulation-remodulation example described above. At the end of a received frame, a training interval is provided, which is a fixed downstream 1,0,1,0 . . . pattern. The training interval is a small fraction in bandwidth of the frame payload. Consistent with the normal payload, each “0” is a (0,1) at double line rate and each “1” is a (1,0) at double line rate. During the training interval, the ONU sends upstream a 1,0,1,0 . . . pattern. Consistent with normal payload, each 1 is a full period 1, each 0 is a full period 0. The receiving framer identifies the portion of time dedicated to the training interval.  
         [0072]     An example of ONU circuitry  650  that can provide the accurate phase alignment that is required for upstream modulation is shown in  FIG. 6   b.  ONU circuitry  650  includes optical coupler  652 , optical power splitter  654 , photodetector  656 , line code decoder and framer  658 , ONU transmit electrical circuitry  660 , line code modulator and optical amplifier  662 , optical power splitter  664 , photodetector and amplifier  666 , field-effect transistor (FET)  668 , low pass filter  670 , DC amplifier  672 , and voltage-control crystal oscillator (VCXO)  674 . The operation of ONU  650  is similar to that of ONU circuitry  600 , shown in  FIG. 6   a,  with additional functionality. As shown in  FIG. 6   b,  during the training interval, a sampling FET  668  is turned on, so as to pass the recovered electrical signal from the line code modulator  662  (via the photo detector  666 ). The sampled signal is filtered by a low pass filter  670 , such that the DC output of the filter is a measure of the duty cycle of the optical pulses. The filter&#39;s electrical output is then amplified and fed to a VCXO  674  to create a phase lock loop.  
         [0073]     Referring briefly to  FIG. 6   c,  an example of a downstream training signal  680  is shown. In this example, downstream training signal  680  includes a series of 1s and 0s, which is a square wave of 50% duty cycle. An upstream modulating signal  681 , which, in this example, is in correct phase alignment with the downstream training signal  680 , is shown. The downstream training signal  680  is modulated (anded) with the upstream modulating signal  681  to form upstream modulated signal  682 . With upstream modulating signal  681  in correct phase alignment with the downstream training signal  680 , upstream modulated signal  682  has a duty cycle of 25%.  
         [0074]     The DC output of low pass filter  670  is a measure of the duty cycle of the optical pulses of upstream modulated signal  682 . For example, referring to  FIG. 6   d,  upstream modulating signal  683  is early relative to downstream training signal  680 . When downstream training signal  680  is modulated with upstream modulating signal  683 , the resulting upstream modulated signal has a duty cycle greater than 25%. Alternatively, referring to  FIG. 6   e,  upstream modulating signal  685  is late relative to downstream training signal  680 . When downstream training signal  680  is modulated with upstream modulating signal  683 , the resulting upstream modulated signal has a duty cycle less than 25%. In either case, the feedback loop is designed to drive the duty cycle to ¼ during the training interval, thereby assuring phase alignment for the re-modulation.  
         [0075]     Between training intervals, the FET  668  is turned off, such that the filter retains its DC value during the rest of the frame. The total phase control can optionally utilize the following: analog to digital converter, digital processor, and digital to analog converter. This could be used between the filter  670  output and the VCXO  674 , or between the DC AMP  672  output and the VCXO  674 .  
         [0076]     As described above, the modulation performed in the OLT varies in different embodiments of the present invention. In one embodiment, the narrow wavelength signal is not modulated 100% of the time, but rather, unmodulated or continuous-wave (CW) portions of the narrow wavelength signal may be output from the OLT. The unmodulated portions of the narrow wavelength signal are modulated by the ONU and transmitted to the OLT. The operation of this embodiment may be termed “ping-pong”. An example of the operation of the ping-pong technique is shown in  FIG. 7 . In this example, the OLT transmits a burst of modulated optical signal followed by a period of unmodulated optical signal. The optical signal is received by the ONU, which demodulates the modulated portion of the optical signal and extracts the downstream data, and which modulates the unmodulated portion of the optical signal with upstream data, and transmits the upstream modulated optical signal to the OLT.  
         [0077]     In the example shown in  FIG. 7 , an effective data rate of 310 Mbs in each of the upstream and downstream directions is achieved with the use of transmission bursts at 622 Mbs for one half of the time. It is to be noted that these rates and timings are merely examples, the present invention is not limited to these rates and timings. Rather, the present invention contemplates any and all rates and timings for data transmission. For example, other transmission duty cycles are possible and may be advantageous for various reasons, such as to reduce the effect of reflections on the system performance.  
         [0078]     As shown in  FIG. 7 , the OLT transmits a burst of modulated optical signal  702 . In this example, the burst includes 8 STS3 frames of data transmitted at a 622 Mbs rate. This burst lasts 250 μS. The OLT then transmits a period  704  of unmodulated optical signal, which lasts 250 μS. The unmodulated optical signal  704 A is received at the ONU at a time that is dependent upon the length of the optical fiber connecting the OLT and the ONU, and upon the time delays of the other optical components in the path, such as lambda multiplexers and demultiplexers, optical power splitters, couplers, circulators, etc. The ONU then modulates the unmodulated optical signal  704 A and transmits the modulated upstream signal  704 B to the OLT. There is some time delay in the optical path in the ONU and time delay in the return path back to the OLT. After this total path delay, the ONU burst  706  is received at the OLT.  
         [0079]     This embodiment assumes the optical return loss as seen by an OLT is not severe enough to prevent reliable detection of desired the ONU upstream optical signal, and similarly reflections as seen at the ONU are not severe enough to prevent reliable detection of the OLT downstream signal  
         [0080]     The ping-pong technique may be implemented in the embodiment of the present invention shown in  FIG. 1   b.  Likewise, the ping-pong technique may be implemented in the embodiment of the present invention shown in  FIG. 8 , which is an exemplary block diagram of optical and electrical components in ONU  800 .  
         [0081]     ONU  800  includes coupler  802 , optical power splitter  804 , optical to electrical receiver  806 , downstream framer  808 , upstream framer  810 , electrical transmitter  812 , optical modulator  814 , and optical amplifier  816 . The optical signal from the OLT is input to coupler  802  and thence to optical power splitter  804 , which transmits the signal to optical to electrical receiver  806  and optical modulator  814 . Optical to electrical receiver  806  demodulates the optical signal and extracts the downstream data and clock signals from the optical signal. The downstream data and clock signals are output from optical to electrical receiver  806  as electrical signals. These electrical signals are input to downstream framer  808 , which detects the start and/or end of the downstream frames and outputs a timing signal  818  that is used by upstream framer  810  to set the start of the upstream frames. Upstream data is input to upstream framer  810  and assembled into frames in accordance with the timing indicated by signal  818 . At the appropriate time, the upstream frames are input to electrical transmitter  812 , which drives the electrical input of optical modulator  814 .  
         [0082]     Optical modulator  814  modulates the unmodulated optical signal from optical power splitter  804  with upstream data as framed by and at the time controlled by upstream framer  810 . The upstream data is input to optical modulator  814  as an electrical signal. The modulated optical signal is input to optical amplifier  816 , which amplifies the optical signal and outputs the signal to coupler  802 . Coupler  802  couples the amplified remodulated signal onto the optical fiber for transmission to the OLT.  
         [0083]     An example of optical and electrical components in an OLT  900 , in which the ping-pong technique may be implemented, is shown in  FIG. 9 . OLT  900  includes an optical power splitter  902 , a lambda demultiplexer  904 , a plurality of optical modulators  906 - 1  to  906 -K, a lambda multiplexer  908 , an optical amplifier  910 , an optical coupler  912 , a lambda demultiplexer  914 , a plurality of optical receivers  916 - 1  to  916 -K, and a plurality of upstream framers. An unmodulated WDM signal (including a plurality of optical carriers) is input to optical power splitter  902 , which splits the WDM signal into a plurality of WDM signals, each of which may be used by a particular PON. One or more optical amplifiers may be used to amplify the WDM signal, if higher WDM signal amplitude is needed for a particular application. In addition, since the WDM signal is provided to multiple PONs, a protection switch is included to provide switching between the working and protect optical WDM carriers.  
         [0084]     The unmodulated WDM signal is input to lambda demultiplexer  904 , which separates the signal into a plurality of narrow wavelength signals. Each narrow wavelength signal is input to an optical modulator  906 - 1  to  906 -K, where it is modulated with data to be transmitted over the PON. A modulated narrow wavelength signal is output from each optical modulator  906 - 1  to  906 -K and input to lambda multiplexer  908 . These may be termed the OLT modulated signals. The input OLT modulated signals are multiplexed in lambda multiplexer  908  to form a modulated WDM signal. This may be termed the OLT WDM signal. The OLT WDM signal is output from lambda multiplexer  908  and input to optical amplifier  910 , where the signal is amplified for transmission over the optical fiber. The amplified signal is input to optical coupler  912 , where it is coupled onto the optical fiber for transmission to the ONU.  
         [0085]     The upstream modulated signal is received at coupler  912 , which outputs the upstream signal to lambda demultiplexer  914 . Lambda demultiplexer  914  separates the signal into a plurality of narrow wavelength remodulated signals. Each modulated narrow wavelength signal is input to an optical receiver  916 - 1  to  916 -K, where the data modulated onto the signal is detected. Each photodetector outputs an electrical signal carrying the data stream extracted from its input modulated narrow wavelength signal.  
         [0086]     Each electrical signal carrying the data stream extracted from its input modulated narrow wavelength signal is input to an upstream framer, which detects the start and/or end of the upstream frames and outputs the data in these frames as electrical signals. In the ping-pong technique, the OLT and ONU bursts require preambles for clock recovery and start of burst detection. Once the OLT acquires burst start, it starts looking for burst start in the next frame a few microseconds before the expected start of burst. This reduces the likelihood of false sync detection.  
         [0087]     There are additional considerations related to the ping-pong example described above. In particular, it is preferred that the data clock of the downstream data is recovered at the ONU and used as the data clock for the upstream data as well. In order to accomplish this, a circuit such as that shown in  FIG. 10  may be used. An example of ONU clock recovery and holdover circuitry  1000  is shown in  FIG. 10 . The circuitry shown in  FIG. 10  may be used in conjunction with the ONU circuitry shown in  FIG. 1   b,  or with some minor modifications, with the ONU circuitry shown in  FIG. 9 .  
         [0088]     ONU block diagram including clock recovery and holdover circuitry  1000  includes SOAR device  1002 , transmitter electronics  1004 , first-in, first-out (FIFO) buffer  1006 , photodetector, amplifier, and line clock recovery circuitry  1008 , beam splitter  1010 , phase detector  1012 , FET  1013 , framer  1014 , low pass filter  1016 , amplifier  1018 , and VCXO  1020 .  
         [0089]     Downstream data passes thru beam splitter  1010  to photo detector, amplifier, and line clock recovery circuitry  1008 . The line clock recovery function may be performed, for example, by a wideband phase-locked loop (PLL). Photo detector, amplifier, and line clock recovery circuitry  1008  has electrical outputs including a line data output and a line clock output. The line data output and a line clock output are both input to FIFO  1006  and framer  1014 , while the line clock output alone is input to phase detector  1012 . The output of phase detector  1012  is fed thru a FET  1013  to low pass filter  1016 . FET  1013  is controlled by framer  1014  so that the ONU clock generation loop only functions while a downstream burst is received. Between downstream bursts, FET  1013  is opened to allow holdover of the state of the ONU clock loop. The VCXO  1020  output is a continuous ONU clock that traces its reference to the clock rate of the downstream burst. This clock is used to read out downstream data from the FIFO  1006 . This clock is also clock for transmitter electronics  1004 . It is possible to optionally utilize the following: analog to digital converter, digital processor, and digital to analog converter. This could be used between the low pass filter  1016  output and VCXO  1020 , or between the subsequent amplifier  1018  and the VCXO  1020 . The transmitter electronics  1004 , using the ONU clock, outputs electrical data to modulate the SOAR device  1002 , which sends modulated light upstream via the beam splitter  1010 .  
         [0090]     Although specific embodiments of the present invention have been described, it will be understood by those of skill in the art that there are other embodiments that are equivalent to the described embodiments. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims.