MULTI-RATE AND MULTI-MODULATION ONT AND OLT

An optical network unit may include a light sensitive unit suitable for receiving NRZ and/or PAM signals effectively. The optical network unit may include a laser transmission unit for providing NRZ and/or PAM signals effectively.

BACKGROUND

The subject matter of this application relates to optical network components.

Many telecommunications networks include Passive Optical Networks (“PONs”). In PONs, generally most to all components which require power (“active components”), e.g., repeaters, relays, memory chips, processors, between the Central Office exchange and termination points at the customer premises are eliminated, and passive optical components are put into the network to guide traffic based on splitting the power of optical wavelengths to endpoints along the way. The passive splitters or couplers are devices working to pass or restrict light, and as such, have no power or processing requirements thereby lowering overall maintenance costs for the service provider.

FIG.1shows a typical PON100for an optical access architecture. The PON100includes an optical line terminator (“OLT”)110located at a Central Office (“CO”) and a set of optical network units (“ONU”)120, or optical network terminals, located at the customer premise. Each of the ONUs120is connected to the OLT110through feeder fiber130, e.g., an outside fiber plant, optical power splitter140, and individual distribution fibers150. The feeder fiber130may transmit optical signals at 125 Megabits per second (“Mbps”), 155 Mbps, 622 Mbps, 1.25 Gigabits per second (“Gbps”), 2.5 Gbps, 10 Gbps, or 50 Gbps, or otherwise, in accordance with standards used for various access platforms. Various access platforms, including various transmission formats, and communication and control protocols, e.g., Ethernet based PON (“EPON”), Broadband PON (“BPON”), Gigabit PON (“GPON”), and ATM based PON (“APON”), have been developed to deliver information, e.g., data, voice, and video, from the Central Office to each of the customer premises.

Access platforms, e.g., EPON, BPON, or GPON, use light having a wavelength of 1.49 microns (“um”), or otherwise, to transmit information in downstream160direction and light having the wavelength of 1.31 um, or otherwise, to transmit information in upstream170direction between the Central Office and the customer premises. The OLT110contains a high power distributed feedback (“DFB”) laser to produce the light at 1.49 um in downstream160direction, which is shared by a plurality, e.g., 16, 32, or more of ONUs120.

For example, BPON (ITU-T G.983 (01/2005) “Broadband optical access systems based on Passive Optical Networks (PON)”, incorporated by reference herein) operates at generally 155/622/1200 Mbps downstream and 155/622/1200/2500 Mbps upstream, with laser wavelength of 1490 downstream and laser wavelength of 1310 nm upstream. BPON transmits downstream in a broadcast manner and upstream in a time division multiple access manner.

For example, GPON (ITU-T G.984.1 (03/2008) “Gigabit-capable passive optical networks (GPON): General characteristics”, incorporated by reference herein) operates at generally 155 Mbps/622 Mbps/1.2 Gbps/2.5 Gbps downstream and 1.244 Gb/s upstream, with laser wavelength of 1490 nm downstream and laser wavelength of 1310 nm upstream. GPON transmits downstream in a broadcast manner and upstream in a time division multiple access manner.

The OLT110may service the plurality of ONUs120through the use of one or more optical power splitters140and access platform PON protocols to control the sending and transmission of signal across the shared access facility. Data may be transmitted downstream160from OLT110to each of ONU120, and each ONU120processes the data destined to it by matching the address at the access protocol transmission unit header. Upstream170data from each of the ONUs120to the OLT110is transmitted according to access control mechanisms and protocols in the OLT110, which include a time division multiplexing scheme, in which dedicated transmission time slots are granted to each individual ONU120, to avoid data collision. As such, transport of information between the Central Office and customer premises depends on the type of the access platform used by the Central Office and customer premises. Further, each OLT110at the Central Office requires its own feeder fiber130to provide data transmission to and from the plurality of ONUs120. In addition, a timing algorithm may be used in existing access platforms, which limits the distance between the OLT110and the ONU120.

In this manner, depending on the particular architecture implemented, the OLT and/or the ONUs are configured to include the appropriate lasers together with the appropriate modulation scheme, and appropriate optical sensors.

DETAILED DESCRIPTION

Referring toFIG.2, one network implementation may include 10G-PON, where the OLT is configured to send and receive 10G-PON signals, and each ONU is configured to send and receive 10G-PON signals. For 10G-PON the data is transmitted as a binary code of 1's and 0's based upon a non-return to zero (NRZ) signaling, where ones are represented by one significant condition (normally a higher value) while zeros are represented by some other significant condition, usually a lower value, with no other neutral or rest condition.

Referring toFIG.3, one network implementation may include 25G-PON, where the OLT is configured to send and receive 25G-PON signals, and each ONU is configured to send and receive 25G-PON signals. For 25G-PON the data is transmitted as a binary code of 1's and 0's based upon a PAM4 signaling, where the message information is encoded in the amplitude of a series of signal pulses. PAM4 uses four signal levels for transmission, such that within each clock period, where two bits of logic information can be transmitted (i.e., 0,0; 0,1; 1,0; and 1,1). Therefore, under the same rate, the bit rate of a PAM4 signal is twice that of a NRZ signal.

Referring toFIG.4, one network implementation may include 50G-PON, where the OLT is configured to send and receive 50G-PON signals, and each ONU is configured to send and receive 50G-PON signals. For 505G-PON the data is transmitted as a binary code of 1's and 0's based upon a PAM8 signaling, where the message information is encoded in the amplitude of a series of signal pulses. PAM8 uses eight signal levels for transmission, such that within each clock period, where four bits of logic information can be transmitted (i.e., 0,0,0,0; 0,0,0,1; . . . , 1,1, 1,0; 1,1,1,1). Therefore, under the same rate, the bit rate of a PAM8 signal is twice that of a PAM4 signal. Other pulse amplitude modulation signaling may be used.

It is noted that with each of the different modulation schemes, the timing between bits may stay the same while the throughput of the data is different. Also, the timing between the bits may be changed, if desired. Also, the signal to noise ratio for each of the modulation schemes is different.

In many environments it is desirable to deploy a network architecture that includes 10G-PON, at the OLT and the ONU, because it provides sufficient data throughput at a lower complexity and expense than 25G-PON and/or 50G-PON. Over time as the customers consume increasing amounts of data, it may be desirable to upgrade from 10G-PON to 25G-PON, which traditionally requires changing out the OLT and the ONUs at each customer's premise, which is burdensome for the customer and burdensome for the service provider. Over time as the customers consume increasing amounts of data, it may be desirable to upgrade from 25G-PON to 50G-PON, which traditionally requires changing out the OLT and the ONUs at each customer's premise, which is burdensome for the customer and burdensome for the service provider. As it may be observed, it is burdensome to change out the OLT and/or the ONUs of the network over time, which often involves a service technician arranging to change out the ONUs one at time at each customer's premises. Also, PAM8 signaling is more sensitive to noise than PAM4, which in turn is more sensitive to noise than NRZ. Higher order PAM signaling may likewise be used.

After further consideration, it was determined it would be desirable to include a set of optics within the OLT and/or ONUs where the laser (e.g., light source), the photo diode (e.g., light detector), and its associated optics (e.g., lens) may be reused in a manner that enables the OLT and/or the ONUs to be upgraded based upon controlling software (e.g., firmware) from 10G-PON to 25G-PON and/or 50G-PON, or from 25G-PON to 50G-PON. In this manner, the customer ONU may be upgraded with improved data capacity without the need to replace the customer premise equipment. In this manner, the OLT may be upgraded with improved data capacity without the need to replace the equipment. By way of example, the same optical receiver may be used for the different configurations, such as a positive-intrinsic-negative diode or an avalanche photodiode. In this manner, the same interconnection to the optical fibers may be used for the different configurations. By way of example, the same laser transmitter may be used for the different configurations, such as an indium gallium arsenide based laser.

Referring toFIG.5, the ONU (or OLT) may include a first stage that receives the optical signal from the fiber, such as using a diode. The diode receives the optical signal and in response provides a current output. The ONU may include a second stage that receives the current output from the diode and in response provides a voltage output, such as using a transimpedance amplifier typically implemented using one or more operational amplifiers and/or a current mirror. Accordingly, the two stages of the ONU converts the received optical signal to a corresponding voltage level. The same may be applied to the OLT, as desired. The first and second stages may be combined within a single stage, as desired.

Referring toFIG.6, the voltage level from stage2tends to be a relatively small signal that should be amplified by a third stage so that it may be more readily processed by a digital processor, such as a field programmable gate array or an application specific integrated circuit. Upon further review, it was determined that signals that are NRZ tend to be generally rectangular and/or sinusoidal in nature to indicate the binary levels, while signals that are PAM4 and/or PAM8 tend to be multi-level in nature to indicate the different values. A switch600may be included, such as one that is controllable by software, to provide the output of stage2to a different amplification stage based upon the type of signals that are being amplified. For the NRZ signals, the switch600sends the voltage signals to a limiting amplifier610. Referring also toFIG.7, the limiting amplifier610generally allows signals below a level to pass mostly unaffected while attenuating the signals above the level, or also or alternatively, the limiting amplifier610generally allows signals above a level to pass mostly unaffected while attenuating the signals below the level, or otherwise those signals in the middle band to pass. In general, the limiting amplifier tends to pass the middle region of the signals while attenuating the upper and/or lower signals. As described, a limiting amplifier has extremely variable and non-linear gain, and this gain may be a function of the amplitude of the input signal. Typically, low amplitude signals see a lot of gain, which increases signal edge rate and “squares up” the signal. Typically, large amplitude signals see effectively way less gain because the limiting amplifier has a maximum high output level and a maximum low output level that it can achieve. For the PAM4 and/or PAM8 signals, the switch600sends the voltage signals to a linear amplifier620(or substantially linear or non-linear, and more generally a non-limiting amplifier). In general, a linear amplifier maintains the eye pattern of the PAM4 and/or PAM8 signals unlike a limiting amplifier, and a limiting amplifier accentuates the transitions between 0 and 1 of the NRZ signals unlike a linear amplifier.

The output of the limiting amplifier610or the linear amplifier620is provided to a digital processor630, such as a field programmable gate array or an application specific integrated circuit, for receiving the input signal, decoding the signal levels, and processing the resulting data. The processor630may include a de-serializer640that receives the serialized 0's and 1's and forms a set of bytes or otherwise which are parallel in nature. The processor630may include an analog-to-digital converter650that receives each of the amplitudes of the different levels, converts the level to an associated digital signal, and forms a set of bytes or otherwise which are parallel in nature. Accordingly, when the input signal is switched by the switch600between a NRZ signals and a PAM4 and/or PAM8 signal, the FPGA630is likewise switched between the de-serializer640and the analog to digital converter650, so that the appropriate signals are processed.

Referring toFIG.8, in another embodiment, the output of stage2may be processed by a preamplifier800(e.g., linear or non-linear) the output of which is provided to a FPGA810. The FPGA810may include a switch820which selectively provides the pre-amplified signals to either a de-serializer830or an analog-to-digital converter840, in a manner akin toFIG.6.

In another embodiment, a preamplifier may be included together with an analog to digital converter that is used for both the NRZ and the PAM4 and/or PAM8 signaling. In this case, the sensitivity may tend to be lower than desired for the NRZ. Depending on the particular implementation, a link budget that is available for the Optical Distribution Network (ODN) loss, which includes fiber and passive splitting losses, may be sufficient to support lower sensitivity for the NRZ while also supporting the PAM4 and/or PAM8.

The FPGA may include digital processing, that includes a clock recovery, as desired. Further, the OLT may include the same type of configuration as the ONU, described above.

Referring toFIG.9, the OLT and/or the ONU may include a laser900together with a laser driver910that modulate an optical signal920that is transmitted through the optical fiber930. Referring also toFIG.10, the laser900is operated based upon being provided a current input1000which includes a corresponding optical power output1010from the laser900. The response curve of the laser900is typically relatively flat until a knee1020is reached, corresponding to a bias current level1030. With a bias current being applied at the bias current level1030the output of the laser remains zero or substantially zero. A modulation current1040may be selectively provided in addition to the bias current level1030. With a selected modulation current1040being provided a desired power output1010may be selected. In this manner, with the bias current “on” and the modulation current “on”, a high-power output1010is achieve normally referred to as a binary “1”. In this manner, with the bias current “on” and the modulation current “off”, a lower power output1010is achieve normally referred to as a binary “0”. Also, with the bias current “off” and the modulation current “off”′, a lower power output1010is achieve normally referred to as a binary “0”, though for stable operation often the bias current remains on while optical modulation is occurring.

Referring again toFIG.9, the laser driver910may include a bias current driver940that selectively provides a bias current942to the laser900. The bias current driver940may be selectively enabled by one or more control signals944from the FPGA950. The laser driver910may include a modulation current driver960that selectively provides a modulation current962to the laser900. The modulation current driver940may be selectively enabled by one or more control signals964from the FPGA950. Based upon the selective enabling of the bias current driver940and the modulation current driver960, a suitable current level may be provided to the laser900to modulate the optical signal920in a manner to provide a NRZ signal. By way of example, the laser driver910may be suitable for 10G-PON.

To provide a multi-level signal, such as one suitable for PAM4 and/or PAM8 signaling, the modulation current driver could be designed to be suitable for providing multiple levels of output. However, including a multi-level current driver tends to require relatively complicated electronics, with the current driver being tuned to provide relatively accurate signal at a plurality of different levels, which is more prone to error than providing a more binary set of outputs.

Referring toFIG.11, the OLT and/or the ONU may include a laser1100together with a laser driver A1110that modulate an optical signal A1120that is transmitted through the optical fiber1130. The laser driver A1110may include a bias current driver A1140that selectively provides a bias current A1142to the laser1100. The bias current driver A1140may be selectively enabled by one or more control signals A1144from the FPGA1150. The laser driver A1110may include a modulation current driver A1160that selectively provides a modulation current A1162to the laser1100. The modulation current driver A1140may be selectively enabled by one or more control signals A1164from the FPGA1150. Based upon the selective enabling of the bias current driver A1140and the modulation current driver A1160, a suitable current level may be provided to the laser1100to modulate the optical signal1120in a manner to provide a NRZ signal. By way of example, the laser driver A1110may be suitable for 10G-PON.

The OLT and/or the ONU may include the laser1100together with a laser driver B1112that modulate the optical signal1120that is transmitted through the optical fiber1130. The laser driver B1112may include a bias current driver B1170that selectively provides a bias current B1172to the laser1100. The bias current driver B1170may be selectively enabled by one or more control signals B1174from the FPGA1150. The laser driver B1112may include a modulation current driver B1180that selectively provides a modulation current B1182to the laser1100. The modulation current driver B1180may be selectively enabled by one or more control signals B1184from the FPGA1150. Based upon the selective enabling of the bias current driver B1170and the modulation current driver B1180, a suitable current level may be provided to the laser1100to modulate the optical signal1120in a manner to provide a NRZ signal. By way of example, the laser driver B1112may be suitable for 10G-PON.

The FPGA may selectively use either the laser driver A1110or the laser driver B1112to provide 10G-PON optical signals, with a backup laser driver in the event one of the laser drivers become non-operational. In addition, by the selective use of the laser driver A1110in combination with the laser driver B1112, a set of four different currents may be provided to the laser1110.

Referring also toFIG.12, the four different currents that may be selectively provided to the laser1110include, (1) the bias current A1142; (2) the modulation current A1162; (3) the bias current B1172; and (4) the modulation current B1182. Various combination of the four different currents levels may be selectively provided to the laser1110, such as for example, to provide four different output levels from the laser1110which in turn provide four different power levels for the optical signal1120,0,0 optical signal level:the bias current A1142“on”,the modulation current A1162“off”,(3) the bias current B1172“off”,(4) the modulation current B1182“off”.0,1 optical signal level:the bias current A1142“on”,the modulation current A1162“on”,(3) the bias current B1172“off”,(4) the modulation current B1182“off”.1,0 optical signal level:the bias current A1142“on”,the modulation current A1162“on”,(3) the bias current B1172“on”,(4) the modulation current B1182“off”.1,1 optical signal level:the bias current A1142“on”,the modulation current A1162“on”,(3) the bias current B1172“on”,(4) the modulation current B1182“on”.

The values of the respective modulation and bias currents are selected so that they provided the desired power output from the laser. For example, the bias current A may be different than the bias current B. For example, the modulation current A may be different than the modulation current B. For example, the bias current A may be different than the modulation current B. For example, the bias current B may be different than the modulation current A. For example, each of the currents may be different than any of the others. Moreover, the selection of the current levels is preferably based upon optical power output profile of the laser, which is especially suitable for a non-linear profile. Additional laser drivers may be included for additional levels of PAM modulation. For example, with three laser drivers PAM8 modulation may be achieved.

It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of a claim beyond its literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated. The word “comprise” or a derivative thereof, when used in a claim, is used in a nonexclusive sense that is not intended to exclude the presence of other elements or steps in a claimed structure or method.