Patent Publication Number: US-2020304209-A1

Title: Transceiving With a Predetermined Frequency Spacing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of Int&#39;l Patent App. No. PCT/CN2019/099092 filed on Aug. 2, 2019 by Huawei Technologies Co., Ltd. and titled “Transceiving With a Predetermined Frequency Spacing,” which claims priority to U.S. Prov. Patent App. No. 62/739,997 filed on Oct. 2, 2018 by Futurewei Technologies, Inc. and titled “Transceiving With a Predetermined Frequency Spacing,” both of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The disclosed embodiments relate to optical networks in general and transceiving in optical networks in particular. 
     BACKGROUND 
     Optical networks are networks that use light waves, or optical signals, to carry data. Light sources such as lasers and LEDs generate the optical signals, modulators modulate the optical signals with the data to generate modulated optical signals, and various components transmit, propagate, amplify, receive, and process the modulated optical signals. Optical networks may implement WDM or other forms of multiplexing to achieve high bandwidths. 
     SUMMARY 
     In an embodiment, an apparatus comprises: a receiver; a transmitter; a laser device coupled to the receiver and the transmitter and comprising: a first laser configured to provide to the receiver a first optical wave centered at a first frequency, and a second laser configured to provide to the transmitter a second optical wave centered at a second frequency, the first frequency and the second frequency have a predetermined frequency spacing; and a processor coupled to the receiver, the transmitter, and the laser device, with the processor configured to control the first laser and the second laser to maintain the predetermined frequency spacing. 
     In any of the preceding embodiments, the first laser is a local oscillator (LO) laser, wherein the first optical wave is an LO wave. 
     In any of the preceding embodiments, the receiver is configured to: receive a downstream optical signal centered at a third frequency; receive the LO wave from the first laser; determine a frequency offset between the first frequency and the third frequency; and provide to the processor a feedback signal based on the frequency offset. 
     In any of the preceding embodiments, the receiver is a coherent optical receiver. 
     In any of the preceding embodiments, the second laser is a carrier laser, and the second optical wave is a carrier wave. 
     In any of the preceding embodiments, the transmitter is configured to: receive the carrier wave from the second laser; receive a data signal from the processor; modulate the carrier wave using the data signal to create an upstream optical signal; and provide the upstream optical signal. 
     In any of the preceding embodiments, the transmitter is further configured to further modulate the carrier wave using OOK modulation. 
     In any of the preceding embodiments, the transmitter is further configured to further modulate the carrier wave using PAM. 
     In any of the preceding embodiments, the apparatus further comprises a splitter coupled to the receiver and the transmitter and configured to: provide the downstream optical signal to the receiver; and receive the upstream optical signal from the transmitter. 
     In any of the preceding embodiments, the apparatus further comprises a port coupled to the splitter and configured to: receive the downstream optical signal from a second apparatus over an optical fiber, provide the downstream optical signal to the splitter, receive the upstream optical signal from the splitter, and transmit the upstream optical signal towards the second apparatus over the optical fiber. 
     In any of the preceding embodiments, the port is further configured to provide bidirectional communication over the optical fiber. 
     In any of the preceding embodiments, the port is the only communications port in the apparatus. 
     In any of the preceding embodiments, the laser device further comprises a controller coupled to the processor and configured to: receive a control signal from the processor; and perform a control action on both the first laser and the second laser in response to the control signal. 
     In any of the preceding embodiments, the controller is a heater, wherein the control action is heating. 
     In any of the preceding embodiments, the controller is a TEC, wherein the control action is cooling. 
     In any of the preceding embodiments, the controller is a bias current controller, wherein the control action is a bias current. 
     In any of the preceding embodiments, the predetermined frequency spacing is set by a design of the laser device. 
     In any of the preceding embodiments, the processor is further configured to further maintain the predetermined frequency spacing independent of an ambient temperature. 
     In any of the preceding embodiments, the predetermined frequency spacing is about 100 GHz. 
     In any of the preceding embodiments, the apparatus is an ONU. 
     In any of the preceding embodiments, the apparatus is part of a PTMP network. 
     In an embodiment, a method comprises: providing, by a first laser of a laser device and to a receiver, a first optical wave centered at a first frequency; providing, by a second laser of the laser device and to a transmitter, a second optical wave centered at a second frequency, the first frequency and the second frequency have a predetermined frequency spacing; and maintaining, by a processor coupled to the laser device, the predetermined frequency spacing. 
     In any of the preceding embodiments, the first optical wave is an LO wave. 
     In any of the preceding embodiments, the method further comprises: receiving a downstream optical signal centered at a third frequency; determining a frequency offset between the first frequency and the third frequency; and providing to the processor a feedback signal based on the frequency offset. 
     In any of the preceding embodiments, the second optical wave is a carrier wave. 
     In any of the preceding embodiments, the method further comprises: receiving a data signal from the processor; and modulating the carrier wave using the data signal to create an upstream optical signal. 
     In any of the preceding embodiments, the method further comprises further modulating the carrier wave using OOK modulation. 
     In any of the preceding embodiments, the method further comprises further modulating the carrier wave using PAM. 
     In any of the preceding embodiments, the method further comprises: receiving a control signal from the processor; and performing a control action on both the first laser and the second laser in response to the control signal. 
     In any of the preceding embodiments, the control action is heating. 
     In any of the preceding embodiments, the control action is cooling. 
     In any of the preceding embodiments, the control action is a bias current. 
     In any of the preceding embodiments, the predetermined frequency spacing is set by a design of the laser device. 
     In any of the preceding embodiments, the method further comprises further maintaining the predetermined frequency spacing independent of an ambient temperature. 
     In any of the preceding embodiments, the predetermined frequency spacing is about 100 GHz. 
     In an embodiment, an ONU comprises: a receiver; a laser device coupled to the receiver and comprising: a first laser configured to provide to the receiver a first optical wave centered at a first frequency, and a second laser configured to provide an upstream optical signal centered at a second frequency, the first frequency and the second frequency have a predetermined frequency spacing, and the second laser is a DML; and a processor coupled to the receiver and the laser device, with the processor configured to control the first laser and the second laser to maintain the predetermined frequency spacing. 
     In any of the preceding embodiments, the first laser is an LO, wherein the first optical wave is an LO wave. 
     In any of the preceding embodiments, the receiver is configured to: receive a downstream optical signal centered at a third frequency; receive the LO wave from the first laser; determine a frequency offset between the first frequency and the third frequency; and provide to the processor a feedback signal based on the frequency offset. 
     In any of the preceding embodiments, the receiver is a coherent optical receiver. 
     In any of the preceding embodiments, the second laser is further configured to: receive a data signal from the processor; and generate the upstream optical signal through direct modulation of the data signal. 
     In any of the preceding embodiments, the second laser is further configured to further generate the upstream optical signal through OOK modulation. 
     In any of the preceding embodiments, the second laser is further configured to further generate the upstream optical signal through PAM. 
     In any of the preceding embodiments, the ONU further comprises a splitter coupled to the receiver and the second laser and configured to: provide the downstream optical signal to the receiver; and receive the upstream optical signal from the second laser. 
     In any of the preceding embodiments, the ONU further comprises a port coupled to the splitter and configured to: receive the downstream optical signal from an OLT over an optical fiber, provide the downstream optical signal to the splitter, receive the upstream optical signal from the splitter, and transmit the upstream optical signal towards the OLT over the optical fiber. 
     In any of the preceding embodiments, the port is further configured to provide bidirectional communication over the optical fiber. 
     In any of the preceding embodiments, the port is the only communications port in the apparatus. 
     In any of the preceding embodiments, the laser device further comprises a controller coupled to the processor and configured to: receive a control signal from the processor; and perform a control action on both the first laser and the second laser in response to the control signal. 
     In any of the preceding embodiments, the controller is a heater, wherein the control action is heating. 
     In any of the preceding embodiments, the controller is a TEC, wherein the control action is cooling. 
     In any of the preceding embodiments, the controller is a bias current controller, wherein the control action is a bias current. 
     In any of the preceding embodiments, the predetermined frequency spacing is set by a design of the laser device. 
     In any of the preceding embodiments, the processor is further configured to further maintain the predetermined frequency spacing independent of an ambient temperature. 
     In any of the preceding embodiments, the predetermined frequency spacing is about 100 GHz. 
     In any of the preceding embodiments, the ONU is part of a PTMP network. 
     Any of the above embodiments may be combined with any of the other above embodiments to create a new embodiment. These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG. 1  is a schematic diagram of a network. 
         FIG. 2  is a schematic diagram of an ONU according to an embodiment of the disclosure. 
         FIG. 3  is a schematic diagram of an ONU according to another embodiment of the disclosure. 
         FIG. 4A  is a graph of a channel scheme according to an embodiment of the disclosure. 
         FIG. 4B  is a graph of a channel scheme according to another embodiment of the disclosure. 
         FIG. 5  is a flowchart illustrating a method of implementing transceiving with a predetermined frequency spacing according to an embodiment of the disclosure. 
         FIG. 6  is a schematic diagram of an apparatus according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that, although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     The following abbreviations apply: 
     ADC: analog-to-digital conver(sion,ter) 
     ASIC: application-specific integrated circuit 
     BNG: broadband network gateway 
     CPU: central processing unit 
     DFB: distributed feedback 
     DML: directly-modulated laser 
     DSP: digital signal processor 
     EBE: electrical bandwidth efficiency 
     EO: electrical-to-optical 
     FPGA: field-programmable gate array 
     GBd: gigabaud 
     Gb/s: gigabit(s) per second 
     GHz: gigahertz 
     GS/s: gigasamples(s) per second 
     Hz: hertz 
     LED: light-emitting diode 
     LO: local oscillator 
     MHZ: megahertz 
     m/s: meter(s) per second 
     NRZ: non-return-to-zero 
     OADM: optical add-drop multiplexer 
     ODN: optical distribution network 
     OE: optical-to-electrical 
     ONU: optical network unit 
     OOK: on-off keying 
     OSE: optical spectral efficiency 
     PAM: pulse-amplitude modulation 
     PAM-4: 4-level PAM 
     PDM: polarization-division multiplexing 
     PTMP: point-to-multipoint 
     QPSK: quadrature phase-shift keying 
     RAM: random-access memory 
     RF: radio frequency 
     ROM: read-only memory 
     RX: receiver unit 
     SRAM: static RAM 
     TCAM: ternary content-addressable memory 
     TEC: thermoelectric cooler 
     TX: transmitter unit 
     WDM: wavelength-division multiplexing 
     16-QAM: 16-level quadrature amplitude modulation. 
       FIG. 1  is a schematic diagram of a network  100 . The network  100  comprises data centers  110 , BNGs  120 , an OADM  130 , an optical fiber  140 , an ODN  150 , optical fibers  160 , and ONUs  170 . The data centers  110  are facilities that house computer systems, communications systems, and storage systems for communicating data with the BNGs  120 . The BNGs  120  provide access points for the OADM  130  to communicate with the data centers  110 . The OADM  130  dynamically implements WDM by adding and dropping wavelength channels. The OADM  130  communicates with the ONUs  170  through the optical fiber  140 , the ODN  150 , and the optical fibers  160  and using those wavelength channels. The ODN  150  comprises passive optical components such as couplers, splitters, and distributors in order to facilitate that communication. The ONUs  170  are endpoints associated with customers. Together, the OADM  130 , the optical fiber  140 , the ODN  150 , and the optical fibers  160  form a PTMP network. 
     The ONUs  170  receive downstream optical signals from the ODN  150  at first wavelengths, transmit upstream optical signals to the ODN  150  at second wavelengths, and may lock the second wavelengths to the first wavelengths using heterodyne detection or homodyne detection. However, heterodyne detection and homodyne detection suffer from low OSE, difficulty in separating downstream channels from upstream channels, and low EBE. 
     Disclosed herein are embodiments for transceiving with a predetermined frequency spacing. An ONU provides bidirectional communication, which in this context means both downstream reception and upstream transmission, through a single port and over a single optical fiber. The ONU transmits upstream optical signals in sub-channels that are well aligned in frequency, meaning with minimal spectral gap between adjacent sub-channels, thus increasing an OSE. An increased OSE allows for a reduced receiver electronic bandwidth needed to simultaneously detect and recover the upstream optical signals, thus increasing an EBE. The ONU comprises a laser (such as a laser device or laser chip) that provides an LO signal for a receiver and a laser that provides a carrier signal for a transmitter. The laser implements a predetermined frequency spacing between a frequency of the LO signal, and thus a downstream optical signal, and a frequency of the carrier signal, and thus an upstream optical signal. The predetermined frequency allows for easier separation of the downstream optical signal and the upstream optical signal and also reduces or eliminates crosstalk between the downstream optical signal and the upstream optical signal. In addition, the laser maintains the predetermined frequency spacing by adjusting the frequency of the LO signal and the frequency of the carrier signal by the same amount, so the predetermined frequency spacing is insensitive to, or independent of, an ambient temperature. Though ONUs are discussed, the embodiments apply to any apparatus implementing a transceiver in an optical network. 
       FIG. 2  is a schematic diagram of an ONU  200  according to an embodiment of the disclosure. The ONU  200  implements the ONUs  170  in  FIG. 1  in some embodiments. The ONU  200  comprises a laser device  210 , a receiver  250 , a processor  260 , a transmitter  270 , a splitter  280 , and a port  290 . In some embodiments, the laser device  210  comprises a laser chip or laser sub-assembly, for example. The receiver  250  is communicatively coupled to the laser device  210 , the processor  260 , and the splitter  280  in the embodiment shown. The transmitter  270  is similarly communicatively coupled to the laser device  210 , the processor  260 , and the splitter  280 . The splitter  280  is further communicatively coupled to the port  290 . 
     The laser device  210  may also be referred to as a laser substrate or a laser semiconductor. The laser device  210  comprises a laser  220 , a controller  230 , and a laser  240 . The laser  220  may be referred to as a receiver laser, an LO, or an optical LO, and the laser  240  may be referred to as a transmitter laser or a carrier laser. The lasers  220 ,  240  may be distributed feedback (DFB) lasers. The laser  220  generates and emits an LO wave centered at a first frequency, and the laser  240  generates and emits a carrier wave centered at a second frequency. The LO wave and the carrier wave are optical waves. The controller  230  is a temperature controller in the form of a heater or a TEC, a bias current controller, or another suitable controller. A manufacturer of the laser device  210  designs the first frequency and the second frequency as defaults and therefore designs a predetermined frequency spacing between the first frequency and the second frequency. For instance, the lasers  220 ,  240  are DFB lasers and the manufacturer designs a first grating reflector for the laser  220  to have a reflection band center at the first frequency and a second grating reflector for the laser  240  to have a reflection band center at the second frequency. 
     The receiver  250  may be referred to as a coherent optical receiver. Together, the receiver  250  and the transmitter  270  form a transceiver to implement transceiving. The port  290  is a communications port and provides bidirectional communication via an optical fiber or such as one of the optical fibers  160  or via another optical medium. Though the ONU  200  may further include a power port (not shown), the port  290  may be the only communications port in the ONU  200 . 
     In a downstream direction, the port  290  receives a downstream optical signal from the OADM  130  and through the optical fiber  140 , the ODN  150 , and an optical fiber  160  in  FIG. 1 . The port  290  provides the downstream optical signal to the splitter  280 . The splitter  280  provides the downstream optical signal to the receiver  250 . Meanwhile, in response to a power instruction from the processor  260 , the laser  220  powers on, generates an LO wave, and provides the LO wave to the receiver  250 . The LO wave may also be referred to as an optical LO wave. 
     The receiver  250  receives the downstream optical signal from the splitter  280  and the LO wave from the laser  220 , beats together the downstream optical signal and the LO wave to create a beat signal, and determines a frequency of the beat signal. The frequency of the beat signal is the same or about the same as a frequency offset, or frequency difference, between a frequency of the downstream optical signal and a frequency of the LO wave. The receiver  250  provides to the processor  260  a feedback signal based on the frequency offset. The feedback signal may indicate the frequency offset. 
     In response to the feedback signal, the processor  260  generates a control signal to reduce the frequency offset and provides the control signal to the controller  230 . The controller  230  responds to the control signal by performing a control action. For instance, the controller  230  is a heater and the control action is heating up, which heats up the laser  220  and shifts the frequency of the LO wave. Alternatively, the controller  230  is a TEC and the control action is cooling or the controller  230  is a bias controller current controller and the control action is a bias current. The receiver  250  continues providing feedback signals to the processor  260  and the processor  260  continues providing control signals to the controller  230  in a feedback loop until the receiver  250  locks the LO wave to the downstream optical signal, which occurs when the frequency offset is less than a threshold, for instance about 100 MHz. After the locking occurs, the receiver  250  performs coherent detection of the downstream optical signal using the LO wave. 
     In an upstream direction, in response to a power instruction from the processor  260 , the laser  240  powers on, generates a carrier wave, and provides the carrier wave to the transmitter  270 . The transmitter  270  receives the carrier wave from the laser  240 , receives a data signal from the processor  260 , modulates the carrier wave using the data signal to create an upstream optical signal, and provides the upstream optical signal to the splitter  280 . The transmitter  270  uses OOK modulation, PAM, or another suitable modulation format. The splitter  280  provides the upstream optical signal to the port  290 . The port  290  transmits the upstream optical signal towards the OADM  130  and through an optical fiber  160 , the ODN  150 , and the optical fiber  140  in  FIG. 1 . 
     As mentioned above, the manufacturer of the laser device  210  designs the predetermined frequency spacing between the first frequency of the LO wave and the second frequency of the carrier wave. Because the LO wave is locked to the downstream optical signal, like the LO wave, the downstream optical signal is also centered at the first frequency. Because the upstream optical signal is based on the carrier wave, like the carrier wave, the upstream optical signal is also centered at the second frequency. Thus, like the LO wave and the carrier wave, the downstream optical signal and the upstream optical signal also have the predetermined frequency spacing. The processor  260  and the controller  230  maintain the predetermined frequency spacing. Specifically, the control signal from the processor  260  to the controller  230  and the resulting control action of the controller  230  affect both the laser  220  and the laser  240  the same or substantially the same so that the first frequency and the second frequency shift by the same or substantially the same amount. The predetermined frequency spacing is therefore independent of an ambient temperature of the laser device  210  specifically, and the ONU  200  generally. 
     As an example, the predetermined frequency spacing is 100 GHz, the downstream optical signal is centered at a first frequency of 0 GHz, and the upstream optical signal is centered at a second frequency of 100 GHz. Though frequencies are described, one may determine a corresponding wavelength based on the following relationship: 
       λ= c/ν   (1)
 
     λ is wavelength, c is the speed of light, and ν is frequency. c is approximately 3×10 8  m/s in a vacuum. 
       FIG. 3  is a schematic diagram of an ONU  300  according to another embodiment of the disclosure. The ONU  300  is similar to the ONU  200 . Specifically, like the ONU  200 , the ONU  300  comprises a laser device  310 , a receiver  350 , a processor  360 , a splitter  380 , and a port  390 . Like the laser device  210  in the ONU  200 , the laser device  310  comprises a laser  320 , a controller  330 , and a laser  340 . However, unlike the ONU  200 , which comprises the transmitter  270 , the ONU  300  does not comprise a transmitter. Instead, the laser  340  may be referred to as a transmitter laser or a DML. In addition, the laser  340  receives a data signal from the processor  360 , generates an upstream optical signal through direct modulation of the data signal, and provides the upstream optical signal directly to the splitter  380 . 
       FIG. 4A  is a graph of a channel scheme  400  according to an embodiment of the disclosure. The channel scheme  400  may apply to both the downstream optical signal and the upstream optical signal in  FIGS. 2-3 . The channel scheme  400  shows 8 sub-channels, which combine to form a single channel. 
     As a first example, for the downstream optical signal, each sub-channel has a bandwidth of 8 GHz and comprises a 6.25 GBd QPSK signal to provide a total data rate of 100 Gb/s since QPSK provides 2 bits per symbol. For the upstream optical signal, each sub-channel has a bandwidth of 8 GHz and comprises a 6.25 GBd NRZ signal to provide a total data rate of 50 Gb/s since NRZ provides 1 bit per symbol or comprises a 6.25 GBd PAM-4 signal to provide a total data rate of 100 Gb/s since PAM-4 provides 2 bits per symbol. The downstream optical signal and the upstream optical signal have a frequency spacing of 100 GHz. 
     As a second example, for the downstream optical signal, each sub-channel has a bandwidth of 8 GHz and comprises a 6.25 GBd QPSK signal to provide a total data rate of 100 Gb/s since QPSK provides 2 bits per symbol. In addition, the receivers  250 ,  350  implement intradyne detection. The receivers  250 ,  350  may therefore achieve an ADC sampling speed of about 14 GS/s or about 28 GS/s and an RF bandwidth of about 3.5 GHz or about 7 GHz. 
     As a third example, for the downstream optical signal, each sub-channel has a bandwidth of 8 GHz and comprises a 6.25 GBd 16-QAM signal to provide a total data rate of 200 Gb/s since 16-QAM provides 4 bits per symbol. In addition, the receivers  250 ,  350  implement intradyne detection. The receivers  250 ,  350  may therefore achieve an ADC sampling speed of about 14 GS/s or about 28 GS/s and an RF bandwidth of about 3.5 GHz or about 7 GHz. 
     As a fourth example, for the downstream optical signal, each sub-channel has a bandwidth of 8 GHz and comprises a 6.25 GBd PDM 16-QAM signal to provide a total data rate of 400 Gb/s since PDM 16-QAM provides 8 bits per symbol. In addition, the receivers  250 ,  350  implement PDM intradyne detection. The receivers  250 ,  350  may therefore achieve an ADC sampling speed of about 14 GS/s or about 28 GS/s and an RF bandwidth of about 3.5 GHz or about 7 GHz. 
       FIG. 4B  is a graph of a channel scheme  410  according to another embodiment of the disclosure. The channel scheme  410  may apply to both the downstream optical signal and the upstream optical signal in  FIGS. 2-3 . The channel scheme  410  shows 4 sub-channels, which combine to form a single channel. 
     As a first example, for the downstream optical signal, each sub-channel has a bandwidth of 16 GHz and comprises a 12.5 GBd QPSK signal to provide a total data rate of 100 Gb/s since QPSK provides 2 bits per symbol. For the upstream optical signal, each sub-channel has a bandwidth of 16 GHz and comprises a 12.5 GBd NRZ signal to provide a total data rate of 50 Gb/s since NRZ provides 1 bit per symbol or comprises a 12.5 GBd PAM-4 signal to provide a total data rate of 100 Gb/s since PAM-4 provides 2 bits per symbol. The downstream optical signal and the upstream optical signal have a frequency spacing of 100 GHz. 
     As a second example, for the downstream optical signal, each sub-channel has a bandwidth of 16 GHz and comprises a 12.5 GBd QPSK signal to provide a total data rate of 100 Gb/s since QPSK provides 2 bits per symbol. In addition, the receivers  250 ,  350  implement intradyne detection. The receivers  250 ,  350  may therefore achieve an ADC sampling speed of about 14 GS/s or about 28 GS/s and an RF bandwidth of about 3.5 GHz or about 7 GHz. 
     As a third example, for the downstream optical signal, each sub-channel has a bandwidth of 16 GHz and comprises a 12.5 GBd 16-QAM signal to provide a total data rate of 200 Gb/s since 16-QAM provides 4 bits per symbol. In addition, the receivers  250 ,  350  implement intradyne detection. The receivers  250 ,  350  may therefore achieve an ADC sampling speed of about 14 GS/s or about 28 GS/s and an RF bandwidth of about 3.5 GHz or about 7 GHz. 
     As a fourth example, for the downstream optical signal, each sub-channel has a bandwidth of 16 GHz and comprises a 12.5 GBd PDM 16-QAM signal to provide a total data rate of 400 Gb/s since PDM 16-QAM provides 8 bits per symbol. In addition, the receivers  250 ,  350  implement PDM intradyne detection. The receivers  250 ,  350  may therefore achieve an ADC sampling speed of about 14 GS/s or about 28 GS/s and an RF bandwidth of about 3.5 GHz or about 7 GHz. 
       FIG. 5  is a flowchart illustrating a method  500  of implementing transceiving with a predetermined frequency spacing according to an embodiment of the disclosure. The ONU  200 ,  300  may implement the method. At step  510 , a first optical wave centered at a first frequency is provided by a first laser of a laser device and to a receiver. For instance, the laser  220  provides the first optical wave to the receiver  250 . At step  520 , a second optical wave centered at a second frequency is provided by a second laser of the laser device and to a transmitter. For instance, the laser  240  provides the second optical wave to the transmitter  270 . The first frequency and the second frequency have a predetermined frequency spacing. Finally, at step  530 , the predetermined frequency spacing is maintained by a processor coupled to the laser device. For instance, the processor  260  provides a control signal to the controller  230 , the controller  230  responds to the control signal by performing a control action, and the control action affects both the first laser and the second laser the same or substantially the same. 
       FIG. 6  is a schematic diagram of an apparatus  600  according to an embodiment of the disclosure. The apparatus  600  may implement the disclosed embodiments. The apparatus  600  comprises ingress ports  610  and an RX  620  coupled to the ingress ports  610  to receive data; a processor, logic unit, baseband unit, or CPU  630  coupled to the RX  620  to process the data; a TX  640  coupled to the processor  630  and egress ports  650  coupled to the TX  640  to transmit the data; and a memory  660  coupled to the processor  630  and configured to store the data. The apparatus  600  may also comprise OE components, EO components, or RF components coupled to the ingress ports  610 , the RX  620 , the TX  640 , and the egress ports  650  to provide ingress or egress of optical signals, electrical signals, or RF signals. 
     The processor  630  is any combination of hardware, middleware, firmware, or software. The processor  630  comprises any combination of one or more CPU chips, cores, FPGAs, ASICs, or DSPs. The processor  630  communicates with the ingress ports  610 , the RX  620 , the TX  640 , the egress ports  650 , and the memory  660 . The processor  630  comprises a transceiving component  670 , which implements the disclosed embodiments. The inclusion of the transceiving component  670  therefore provides a substantial improvement to the functionality of the apparatus  600  and effects a transformation of the apparatus  600  to a different state. Alternatively, the memory  660  stores the transceiving component  670  as instructions, and the processor  630  executes those instructions. 
     The memory  660  comprises any combination of disks, tape drives, or solid-state drives. The apparatus  600  may use the memory  660  as an over-flow data storage device to store programs when the apparatus  600  selects those programs for execution and to store instructions and data that the apparatus  600  reads during execution of those programs. The memory  660  may be volatile or non-volatile and may be any combination of ROM, RAM, TCAM, or SRAM. 
     An apparatus comprises: a receiver element; a transmitter element; a laser element coupled to the receiver element and the transmitter element and comprising: a first laser element configured to provide to the receiver element a first optical wave centered at a first frequency, and a second laser element configured to provide to the transmitter element a second optical wave centered at a second frequency, the first frequency and the second frequency have a predetermined frequency spacing; and a processor element coupled to the receiver element, the transmitter element, and the laser element and configured to control the first laser and the second laser to maintain the predetermined frequency spacing. 
     In an example embodiment, the apparatus  600  includes a first optical wave module providing to a receiver a first optical wave centered at a first frequency, a second optical wave module providing to a transmitter a second optical wave centered at a second frequency, the first frequency and the second frequency have a predetermined frequency spacing, and a spacing module maintaining the predetermined frequency spacing. In some embodiments, the apparatus  600  may include other or additional modules for performing any one of or combination of steps described in the embodiments. Further, any of the additional or alternative embodiments or aspects of the method, as shown in any of the figures or recited in any of the claims, are also contemplated to include similar modules. 
     The term “about” means a range including ±10% of the subsequent number unless otherwise stated. The term “substantially” means within ±10%. While several embodiments have been provided in the present disclosure, it may be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, components, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled may be directly coupled or may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and may be made without departing from the spirit and scope disclosed herein.