Differential phase-shift keying for coherent signal processing

Abstract

An efficient structure and methodology are provided for coherent signal processing in optical access networks. Differential phase-shift keying (DPSK) modulation is used for coherent passive optical networks. The use of DPSK modulation can reduce the complexity of a receiver of an optical network unit by improving the receiver tolerance to frequency offsets. A system can comprise an optical line terminal having a coherent transmitter configured to transmit an optical signal over an optical fiber channel to a coherent receiver in an optical network unit of a passive optical network, and a DPSK modulator coupled to the coherent transmitter to code the optical signal transmitted. The coherent receiver of the optical network unit of the passive optical network can be coupled to a DPSK demodulator.

Description

DIFFERENTIAL PHASE-SHIFT KEYING FOR COHERENTSIGNAL PROCESSINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to U.S. Provisional Application No. 63 / 713,509, filed on October 29, 2024, entitled “Differential Phase-Shift Keying (DPSK) for Coherent Signal Processing in Optical Access Networks,” which provisional application is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The present disclosure is related to signal processing and, in particular, to differential phase-shift keying for coherent signal processing in optical access networks.BACKGROUND

[0003] A coherent system comprises a transmitter that modulates an optical carrier with an electrical signal of a given bandwidth, and a receiver capable of detecting and processing an optical signal with a corresponding bandwidth. These are linked through an optical channel. In point-to-point systems, the transmitter and receiver bandwidths are matched to maximize efficiency and avoid wasting resources.

[0004] Coherent transmission technology is expected to be adopted in passive optical networks (PONs). This has been included in the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) G.Sup.VHSP for a very-high-speed PON (VHSP) project and G.HSP.TWDM for a high-speed PON (HSP) time- and wavelength-division multiplexing (TWDM) project. In the ITU-T G.Sup.VHSP project, PON systems based on coherent technologies are identified as candidate solutions to provide 100 Gb / s or 200 Gb / s optical access.

[0005] When using coherent transmission in a PON, especially in the upstream, an optical line terminal (OLT) needs to rapidly adapt fromtransmission from one optical network unit (ONU) to transmission from another ONU. This requires the OLT receiver to precisely estimate the optical carrier frequency, phase, and polarization state of the incoming signal in real-time.SUMMARY

[0006] It is an object of various embodiments to provide an efficient architecture and methodology for coherent signal processing in optical access networks. Differential phase-shift keying (DPSK) modulation can be used for coherent PONs. Since information is encoded based on the phase difference between consecutive signal elements in DPSK coding, rather than relying on the absolute phase of the signal, no reference signal is needed, which can improve resilience to phase noise and other frequency-offset impairments. In a PON, DPSK can be implemented with a DPSK modulator coupled to a transmitting means and a DPSK demodulator coupled to a receiver means to receive a signal generated from the transmitting means.

[0007] According to a first aspect of the present disclosure, there is provided a system comprising: an optical line terminal having a coherent transmitter configured to transmit an optical signal over an optical fiber channel to a coherent receiver in an optical network unit of a passive optical network, a differential phase-shift keying modulator coupled to the coherent transmitter to code the optical signal transmitted, and a local coherent receiver to receive an optical transmission from the optical network unit, where the optical transmission is differential phase-shift keying modulated.

[0008] In a first implementation form of the system according to the first aspect as such, the differential phase-shift keying modulator includes a Mach- Zehnder modulator.

[0009] In a second implementation form of the system according to the first aspect as such or any preceding implementation form of the first aspect, the optical line terminal is structured to receive optical transmissions from additional optical network units of the passive optical network, the optical transmissions being differential phase-shift keying modulated.

[0010] In a third implementation form of the system according to the first aspect as such or any preceding implementation form of the first aspect, the optical line terminal includes a single local oscillator arranged to provide a localoptical signal to demodulate the optical transmissions from the optical network unit and from the additional optical network units.

[0011] In a fourth implementation form of the system according to the first aspect as such or any preceding implementation form of the first aspect, the system includes the coherent receiver of the optical network unit of the passive optical network coupled to a differential phase-shift keying demodulator.

[0012] In a fifth implementation form of the system according to the first aspect as such or any preceding implementation form of the first aspect, the differential phase-shift keying modulator is structured to provide differential binary phase-shift keying.

[0013] In a sixth implementation form of the system according to the first aspect as such or any preceding implementation form of the first aspect, the differential phase-shift keying modulator is structured to provide differential quaternary phase-shift keying.

[0014] In a seventh implementation form of the system according to the first aspect as such or any preceding implementation form of the first aspect, the differential phase-shift keying modulator is structured to provide differential eight phase-shift keying.

[0015] According to a second aspect of the present disclosure, there is provided a method of communicating in a passive optical network, the method comprising: performing differential phase-shift keying modulation on an optical signal being transmitted by a coherent transmitter of an optical line terminal to code the optical signal; transmitting, using the coherent transmitter, the optical signal over an optical fiber channel to a coherent receiver in an optical network unit of the passive optical network; and receiving an optical transmission from the optical network unit at a local coherent receiver of the optical line terminal, the received optical transmission being differential phase-shift keying modulated

[0016] In a first implementation form of the method of communicating in a passive optical network according to the second aspect as such, performing differential phase-shift keying modulation includes performing differential binary' phase-shift keying.

[0017] In a second implementation form of the method of communicating in a passive optical network according to the second aspect as such or any precedingimplementation form of the second aspect, performing differential phase-shift keying modulation includes performing differential quaternary phase-shift keying.

[0018] In a third implementation form of the method of communicating in a passive optical network according to the second aspect as such or any preceding implementation form of the second aspect, performing differential phase-shift keying modulation includes performing differential eight phase-shift keying.

[0019] In a fourth implementation form of the method of communicating in a passive optical network according to the second aspect as such or any preceding implementation form of the second aspect, performing differential phase-shift keying modulation includes using a Mach-Zehnder modulator.

[0020] In a fifth implementation form of the method of communicating in a passive optical network according to the second aspect as such or any preceding implementation form of the second aspect, the method includes receiving optical transmissions at the optical line terminal from additional optical network units of the passive optical network.

[0021] In a sixth implementation form of the method of communicating in a passive optical network according to the second aspect as such or any preceding implementation form of the second aspect, the method includes using a single local oscillator of the optical line terminal to provide a local optical signal to demodulate the optical transmissions from the optical network unit and the additional optical network units.

[0022] In a seventh implementation form of the method of communicating in a passive optical network according to the second aspect as such or any preceding implementation form of the second aspect, the method includes demodulating the optical signal transmitted by the coherent transmitter of the optical line terminal by using a differential phase-shift keying demodulator at the coherent receiver of the optical network unit of the passive optical network.

[0023] According to a third aspect of the present disclosure, there is provided a passive optical network comprising an optical line terminal and one or more optical network units. The optical line terminal has: a coherent transmitter configured to transmit an optical signal over an optical fiber channel; a differential phase-shift keying modulator coupled to the coherent transmitter tocode the optical signal transmitted; and a coherent receiver to receive upstream optical signals, where the upstream optical signals is differential phase-shift keying modulated. Each optical network unit of the one or more optical network units includes: a local coherent receiver; a differential phase-shift keying demodulator coupled to the local coherent receiver to demodulate the optical signal from the optical fiber channel; and a local coherent transmitter, the local coherent transmitter to transmit individual upstream optical signals to the optical line terminal.

[0024] In a first implementation form of the passive optical network according to the third aspect as such, the optical line terminal includes a single local oscillator arranged to provide a local optical signal to demodulate the upstream optical signals from the one or more optical network units.

[0025] In a second implementation form of the passive optical network according to the third aspect as such or any preceding implementation form of the third aspect, the differential phase-shift keying modulator and the differential phase-shift keying demodulator are structured to operate with respect to differential binary phase-shift keying.

[0026] In a third implementation form of the passive optical network according to the third aspect as such or any preceding implementation form of the third aspect, the differential phase-shift keying modulator and the differential phase-shift keying demodulator are structured to operate with respect to differential quaternary phase-shift keying.

[0027] In a fourth implementation form of the passive optical network according to the third aspect as such or any preceding implementation form of the third aspect, the differential phase-shift keying modulator and the differential phase-shift keying demodulator are structured to operate with respect to differential eight phase-shift keying.

[0028] According to a fourth aspect of the present disclosure, there is provided a non-transitory computer-readable storage medium storing instructions for communicating in a passive optical network, which, when executed by one or more processors, cause a system to perform operations. The operations compnse performing differential phase-shift keying modulation on an optical signal being transmitted by a coherent transmitter of an optical line terminal to code the optical signal; and transmitting, using the coherent transmitter, the optical signalover an optical fiber channel to a coherent receiver in an optical network unit of the passive optical network.

[0029] In a first implementation form of the non-transitory computer-readable medium according to the fourth aspect as such, performing differential phaseshift keying modulation includes performing differential binary phase-shift keying.

[0030] In a second implementation form of the non-transitory computer- readable medium according to the fourth aspect as such or any preceding implementation form of the fourth aspect, performing differential phase-shift keying modulation includes performing differential quaternary phase-shift keying.

[0031] In a third implementation form of the non-transitory computer-readable medium according to the fourth aspect as such or any preceding implementation form of the fourth aspect, performing differential phase-shift keying modulation includes performing differential eight phase-shift keying.

[0032] In a fourth implementation form of the non-transitory7computer- readable medium according to the fourth aspect as such or any preceding implementation form of the fourth aspect, the operations include receiving an optical transmission from the optical network unit at a local coherent receiver of the optical line terminal.

[0033] In a fifth implementation form of the non-transitory computer-readable medium according to the fourth aspect as such or any preceding implementation form of the fourth aspect, the operations include receiving optical transmissions at the optical line terminal from additional optical network units of the passive optical network.

[0034] In a sixth implementation form of the non-transitory computer-readable medium according to the fourth aspect as such or any preceding implementation form of the fourth aspect, the operations include using a single local oscillator of the optical line terminal to provide a local optical signal to demodulate the optical transmissions from the optical network unit and the additional optical network units.

[0035] In a seventh implementation form of the non-transi tory computer- readable medium according to the fourth aspect as such or any precedingimplementation form of the fourth aspect, the operations include demodulating the optical signal transmitted by the coherent transmitter of the optical line terminal by using a differential phase-shift keying demodulator at the coherent receiver of the optical network unit of the passive optical network.

[0036] Any one of the foregoing examples may be combined with any one or more of the other foregoing examples to create anew embodiment in accordance with the present disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.

[0038] Figure 1 shows function blocks of an example arrangement of a coherent transmitter front-end and a coherent receiver front-end, according to various embodiments.

[0039] Figure 2 illustrates the principle of differential phase-shift keying of a modulated signal, according to various embodiments.

[0040] Figure 3 shows main function blocks of an example differential phaseshift keying modulation arrangement, according to various embodiments.

[0041] Figure 4 shows an example coherent passive optical network using differential phase-shift keying modulation, according to various embodiments.

[0042] Figure 5 illustrates components of an example coherent 50G time- and wavelength-division multiplexing architecture in which differential phase-shift keying modulation can be implemented, according to various embodiments.

[0043] Figure 6 is a schematic diagram of an apparatus that can implement differential phase-shift keying coding, according to various embodiments.

[0044] Figure 7 is a flow7diagram of an example method of communicating in a passive optical netw ork, according to various embodiments.DETAILED DESCRIPTION

[0045] In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized,and that structural, logical, mechanical, and electrical changes may be made. The following description of example embodiments is, therefore, not to be taken in a limited sense.

[0046] The functions or algorithms described herein may be implemented in software in an embodiment. The software may comprise computer-executable instructions stored on computer-readable media or computer-readable storage device such as one or more non-transitory memories or other type of hardwarebased storage devices, either local or netw orked. Further, such functions correspond to modules, which may be software, hardware, firmware, or any combination thereof. Multiple functions may be performed in one or more modules as desired, and the embodiments described are merely examples. The software may be executed on a digital signal processor, application-specific integrated circuit (ASIC), a microprocessor, or other type of processor operating on a computer system, such as a personal computer, server, or other computer system, turning such computer system into a specifically programmed machine.

[0047] Computer-readable non-transitory media include all types of computer- readable media, including magnetic storage media, optical storage media, and solid state storage media and specifically excludes signals. It should be understood that the software can be installed in and sold with the devices that implement arrangements of compute clusters and storage clusters for artificial intelligence training or other data intense operations as taught herein. Alternatively, the software can be obtained and loaded into such devices, including obtaining the software via a disc medium or from any manner of network or distribution system, including, for example, from a server owned by the software creator or from a server not owned but used by the software creator. The software can be stored on a server for distribution over the Internet, for example.

[0048] The following abbreviations apply in this disclosure:ADC: analog-to-digital converter ASIC : application-specific integrated circuit CPU: central processing unit DAC: digital -to-analog converter dB: decibel(s)DP: dual-polarizationDPSK: differential phase-shift keyingDQPSK: differential quadrature phase-shift keyingDSP: digital signal processing, digital signal processorEO: electrical-to-opticalFPGA: field-programmable gate arrayGBd: gigabaud(s)GHz: gigahertzG, Gb / s: gigabit(s) per secondI: in-phaseIM-DD: intensity modulation with direct detectionITU-T: International Telecommunication UnionTelecommunication Standardization Sector kHz: kilohertzLO: local oscillatorMHz: megahertz mrad: milliradian(s)MZ: Mach-Zehnder modulatorOE: optical-to-electricalOLT: optical line terminalONU: optical network unitOTN: optical transport networkPBC: polarization beam combinerPBS: polarization beam splitterPMD: polarization mode dispersionPON: passive optical networkPSK: phase-shift keyingP2P: point-to-pointQ: quadratureQPSK: quadrature phase-shift keyingRAM: random-access memory7RF : radio frequencyROM: read-only memoryRX: receiver unitSNR: signal-to-noise ratioSRAM: static RAMTCAM: ternary content-addressable memory TDMA: time-division multiple access TIA: transimpedance amplifierTWDM: time- and wavelength-division multiplexing TX: transmitter unit.

[0049] Figure 1 show-s function blocks of an arrangement 100 of a coherent transmitter front-end 105 and a coherent receiver front-end 120. Arrangement 100 can be implemented for conventional coherent signal processing in a PON. A DSP 103 can provide signals to a set of DACs 104-1, 104-2, 104-3. and 104-4 whose outputs are coupled to coherent transmitter front-end 105. Though four DACs are shown, an arrangement such as arrangement 100 can include more or fewer than four DACs. The DACs can be arranged in pairs, to handle an in- phase (I) component and quadrature (Q) component of a signal. These components are two orthogonal signals used to represent complex modulation formats. Coherent transmitter front-end 105 can include a laser 110, as a coherent transmitting source, that provides an optical carrier that are modulated by MZs 107-1, 107-2, 107-3, and 107-4, where MZ 107-2 provides an output that is orthogonal to the output of MZ 107-1 and MZ 107-4 provides an output that is orthogonal to the output of MZ 107-3. Outputs from MZs 107-1, 107-2, 107-3, and 107-4 are provided to a PBC 115 to be transmitted from coherent transmitter front-end 105 onto a communication medium such as an optical fiber.

[0050] Coherent receiver front-end 120 can include a PBS 125-1 to receive an optical input and redirect optical components of the optical input. One component of the optical input is provided from PBS 125-1 to a 90-degree optical hybrid mixing 127-1 and another component of the optical input is provided from PBS 125-1 to another 90-degree optical hybrid mixing 127-2. Coherent receiver front-end 120 includes a LO 122 to coherently detect the optical components of the optical input. LO 122 can be a laser. The output from LO 122 is provided to a PBS 125-2 to send the output from LO 122 to 90-degree optical hybrid mixing 127-1 and to 90-degree optical hybrid mixing 127-2. Optical outputs from 90-degree optical hybrid mixing 127-1 and 90-degree optical hybrid mixing 127-2 are converted to electrical signals, which are input to TIAs 128-1 and 128-2 and to TIAs 128-3 and 128-4, respectively. Outputsfrom TIAs 128-1. 128-2, 128-3 and 128-4 can be input to ADCs 124-1, 124-2, 124-3, and 124-4, respectively, that provide inputs to a DSP 123. DSP 123 recovers the signal provided to DSP 103.

[0051] In the coherent transmitter front-end 105, the optical signal is modulated over four components: I and Q for X-polarization and Y- polarizations. In coherent receiver front-end 120, LO 122 provides the carrier wave for a 90-degree optical hybrid mixing. The signal is detected coherently by causing interference between the received signal and the signal from LO 122. A group of balanced photodetectors converts the four components to electrical signals. They are further converted into digital sampling data by the ADCs.With the help of a DSP, signal impairments, including polarization mode dispersion, fiber dispersion, and inter-symbol interference, can be effectively compensated for at the receiver side. All dimensions of the signal are recovered by the coherent receiver.

[0052] In a coherent PON, in the downstream direction, the OLT continuously transmits a coherent signal to all ONUs, which receive the signal in a standard manner. The signal is always present, which can allow the use of frequency and phase tracking algorithms from optical transport networks. These algorithms, with phase control loops operating over time constants spanning thousands of bit periods, function effectively because the signal remains constant.

[0053] In the upstream direction, multiple ONU transmitters send signals to a single OLT receiver, typically managed through a TDMA scheme to flexibly allocate bandwidth. This flexibility’ comes with the challenge of requiring the OLT receiver to rapidly adapt from one ONU’s transmission to another. In IM- DD systems, this involves fast estimation of DC offsets and symbol clock recovery’. However, with coherent transmission, the OLT receiver additionally estimates the optical carrier frequency, phase, and polarization state of the incoming signal in real time.

[0054] Several design approaches can address this challenge. One option is to synchronize all ONU transmitters to the OLT’s local oscillator with high precision, achievable by transmitting a frequency pilot signal downstream from the OLT. However, this approach increases both cost and power consumption, making it less desirable for PON design. It is desired to have a coherent PON solution with low-cost RX or TX at the ONUs.

[0055] Disclosed herein are embodiments for DPSK for coherent signal processing in optical access networks. The embodiments use DPSK modulation (and its advanced forms) for coherent PON. The use of DPSK modulation can reduce the complexity of the RX of the ONU and cost by improving the RX tolerance to frequency offsets. Alternatively, expensive ONU components such as those in OTNs can be used. There can be ONU data rate degradation due to signal frequency offsets between the OLT TX and the ONU RX.

[0056] To achieve a more economical design, the OLT must tolerate sizable frequency shifts, both when receiving signals from a single ONU and when transitioning between transmissions from different ONUs. The most effective approach to manage this is by employing differential modulation, where the payload information is encoded not in the absolute value of each symbol, but in the change between consecutive symbols.

[0057] Differential keying effectively mitigates phase offset between the transmitter and the local oscillator of the receiver, converting frequency offset into a constant phase offset proportional to the ratio of carrier frequency offset to symbol baud rate. Given that typical ONU transmitter linewidths are in the tens of MHz, while the symbol baud rate in future PON systems is expected to reach around 100 GBd, the ratio exceeds four orders of magnitude. This indicates that DPSK can effectively resolve the frequency control challenge for ONU transmitters.

[0058] Figure 2 illustrates the principle of DPSK of a modulated signal 200. The phase of the modulated signal 200 is shifted relative to the previous signal element. In DPSK, information is encoded based on the phase difference between consecutive signal elements, rather than relying on the absolute phase of the signal. As no reference signal is needed, this method improves resilience to phase noise and other frequency -offset impairments.

[0059] Figure 3 shows main function blocks of an example DPSK modulation arrangement 300. A signal is input to an XNOR logic device 306, whose output is input to a one-bit delay unit 308 that provides feedback to XNOR logic device 306. Output of XNOR logic device 306 is provided to a balanced modulator 309 that also receives a carrier input. The output of balanced modulator 309 provides a DPSK coded signal. The carrier signal phase is modulated according to the differentially encoded data. The phase of the carrier signal wall shiftdepending on whether the current bit is 1 or 0. In this example, if the bit is 1, the phase of the carrier signal is shifted by n radians (180°), and if the bit is 0, the phase of the carrier signal is maintained as the previous symbol.

[0060] The carrier can be described as C(f) = A ■ si n( 2TT / Z ). where A is the amplitude of the signal. / is the carrier frequency, and t is time. The modulated DPSK signal for a bit bnat time tncan be expressed as DPSK(t) = A ■ sin(27i / r + ^n), where <j>nis the phase at timedetermined by the previous symbol.

[0061] Since DPSK does not require precise phase synchronization between the transmitter and receiver, the receiver exhibits a high tolerance to frequency offset, making it more resilient in environments with phase noise. This characteristic is highly advantageous for coherent PON design, as it simplifies the ONU receiver architecture and reduces both complexity and cost. Consequently, low-cost or sub-optimal components can be effectively utilized at the ONUs.

[0062] Figure 4 shows an embodiment of an example coherent PON 400 using DPSK modulation. The illustrated architecture of coherent PON 400 emphasizes the dow nstream transmission from an OLT 402 to one or more of ONUs 412-1, 412-2 . . . 412-N. OLT 402 includes a DPSK modulation 417 that operates on a signal. DPSK modulation 417 provides an input to coherent TX 405 and can be coupled to coherent TX 405 to code the optical signal transmitted. Coherent TX 405 can be structured similar to coherent transmitter front-end 105 with modulation provided by DPSK modulation 417. Coherent TX 405 of OLT 402 can be configured to transmit an optical signal over an optical fiber channel to a coherent receiver in at least one optical network unit of coherent PON 400. ONUs 412-1, 412-2 . . . 412-N can include coherent RXs 420-1, 420-2 . . . 420- N coupled to DPSK demodulations 427-1, 427-2 ... 427-N, respectively. Each of ONUs 412-1, 412-2 . . . 412-N can receive an optical input from OLT 402 via a PBS 411.

[0063] Associated with differential signaling methods, compared to other signaling methods, is a 3 dB worse signal-to-noise ratio. This is due to the fact that each symbol received is the arithmetic difference of two detected signals, and so the noise is therefore doubled. However, in the PON application, the loss budget is 30 dB and the signal is being transmitted over a single hop; hence a 3 dB loss in sensitivity is tolerable.

[0064] DPSK and its advanced variants, such as DQPSK, were initially proposed for optical transport solutions like 40G OTN. Their implementation in optical access network, particularly in coherent PONs as disclosed herein, is an important development.

[0065] Figure 5 illustrates components of an embodiment of an example coherent 50G TWDM architecture 500 in which DPSK modulation can be implemented. The components include an OLT 502 and an ONU 512 that illustrate OLT 502 having a coherent transmitter section and a coherent receiver section and ONU 512 having a coherent receiver section and a coherent transmitter section. The coherent receiver section of OLT 502 and the coherent transmitter section of ONU 512 are not shown in the architecture of coherent PON 400 of Figure 4. OLT 502 can include a coherent transistor 510, such as a laser, coupled to a PBS 513-1 that is coupled to optical modulators 507-1 and a 507-2. Optical modulators 507-1 and a 507-2 can be, but are not limited to. MZ modulators. Optical modulators 507-1 and a 507-2 are coupled to a PBC 519 that combines the optical signals operated on by optical modulators 507-1 and a 507-2 and sends the combined signal to a directional coupler 521. Logic 503-1 and DAC 504-1 can be implemented to generate DPSK coding to optical modulator 507-1 to provide a DPSK signal at the output of optical modulator 507-1. Logic 503-2 and DAC 504-2 can be implemented to generate DPSK coding to optical modulator 507-2 to provide a DPSK signal at the output of optical modulator 507-2. PBC 519 provides the combined DPSK signals to a directional coupler 521 that directs the combined DPSK signals to a multiplexer 511 that directs the combined DPSK signals to a number of ONUs. In this example, a DPSK signal can be directed to ONU 512 via an optical fiber.

[0066] Multiplexer 511 can be arranged to direct optical signals from a number of ONUs to a receiver section of OLT 502 via directional coupler 521. The optical signals can be DPSK coded signals. In this non-limiting illustrated example, the receiver section of OLT 502 is arranged to receive DPSK signals from two ONUs. The receiver section of OLT 502 can include a PBS 513-2 to direct an optical signal to one input of a first I / Q mixer 531 and direct another optical signal to one input of a second I / Q mixer 532. A LO 514, which can be a laser, provides a signal to PBS 513-3 that directs the signal to another input of first I / Q mixer 531 and to another input of second I / Q mixer 532. LO 514 can beused from optical signals from the number of ONUs. First I / Q mixer 531 and second I / Q mixer 532 can coherently detect the received signal by causing interference between the received signals and the signal from LO 514. A group of balanced photodetectors can convert the components from the combination of the first mixer and the second mixer to electrical signals. The balanced photodetectors coupled to first I / Q mixer 53 lean provide input to an ADC 518-1 and an ADC 518-2. The balanced photodetectors coupled to second I / Q mixer 532 can provide input to an ADC 518-3 and an ADC 518-4. ADC 518-1, ADC 518-2, ADC 518-3 and ADC 518-4 can be coupled to logic 503-3 to decode the DPSK coded signals received at OLT 502.

[0067] Optical DPSK signals received at multiplexer 511 can be directed to a number of ONUs. For ease of presentation, only ONU 512 is shown coupled to multiplexer 511. ONU 512 can be coupled to multiplexer 511 by an optical fiber. ONU 512 can include directional coupler 539 to output an optical from the transmitter section of ONU 512. Directional coupler 539 can be coupled to modulator 516, which can be, but are not limited to, a MZ modulator to output a DKSP signal. A logic 523-2 in conjunction with a DAC 534 can be implemented to provide DPSK coding to an optical signal generated by a coherent source 530, such as a laser, for input to modulator 516.

[0068] DPSK signals received at directional coupler 539 from OLT 502 can be directed to an input of an I / Q mixer 541. A LO 522, which can be a laser, provides a signal to another input of I / Q mixer 541. The mixer can coherently detect the received signal by causing interference between the received signals and the signal from LO 522. A group of balanced photodetectors can convert the components from the mixer to electrical signals. The balanced photodetectors coupled to the mixer can provide input to an ADC 528-1 and an ADC 528-2. ADC 528-1 and ADC 528-2 can be coupled to logic 523-1 to decode the DPSK coded signal received at ONU 512 from OLT 502.

[0069] Figure 6 is a schematic diagram of an apparatus 600 that can implement DPSK coding as presented in this disclosure. Apparatus 600 can implement DPSK for coherent signal processing in optical access networks, as taught herein. Apparatus 600 can comprise ingress ports 610 and an RX 620 or receiving means to receive data: a processor 630 or processing means, or logic unit, baseband unit, or CPU, to process the data; a TX 640 or transmitting meansand egress ports 650 to transmit the data; and a memory 660 or data storing means to store the data. RX 620 is coupled or connected to ingress ports 610 and processor 630, processor 630 is coupled or connected to RX 620, memory 660, and TX 640, and TX 640 is coupled to or connected to processor 630 and egress ports 650. Apparatus 600 may also comprise OE components, EO components, or RF components coupled to ingress ports 610, RX 620, TX 640, and egress ports 650 to provide ingress or egress of optical signals, electrical signals, or RF signals. Apparatus 600 or components of apparatus 600 can be implemented for use as in DPSK communication in OLTs in arrangements such as coherent PON 400 of Figure 4 or coherent 50G TWDM architecture 500. Apparatus 600 or components of apparatus 600 can be implemented for use as in DPSK communication in ONUs in arrangements such as coherent PON 400 of Figure 4 or coherent 50G TWDM architecture 500.

[0070] Processor 630 is any combination of hardware, middleware, firmware, or software. Processor 630 can comprise any combination of one or more CPU chips, cores, FPGAs, ASICs, or DSPs. Processor 630 can communicate with ingress ports 610, RX 620, TX 640, egress ports 650, and memory 660. The processor 630 comprises a DPSK component 670, which implements DPSK coding disclosed herein. The inclusion of the DPSK component 670 therefore provides a substantial improvement to the functionality of apparatus 600 and effects a transformation of apparatus 600 to a different state. Alternatively, memory 660 stores DPSK component 670 as instructions, and processor 630 executes those instructions.

[0071] Memory 660 comprises any combination of disks, tape drives, or solid- state drives. Apparatus 600 may use memory 660 as an overflow data storage device to store programs when apparatus 600 selects those programs for execution and to store instructions and data that apparatus 600 reads during execution of those programs. Memory 660 may be volatile or non-volatile and may be any combination of ROM, RAM, TCAM, or SRAM, for example. Other memories or memory' types are contemplated and fall within the scope of this disclosure.

[0072] A computer program product may comprise computer-executable instructions that are stored on a computer-readable medium and that, when executed by a processor, cause an apparatus to perform any of the embodiments.The non-transitory medium may be memory 660, the processor may be processor 630, and the apparatus may be apparatus 600.

[0073] Figure 7 is a flow diagram of an embodiment of an example method 700 of communicating in a passive optical network. At 710, DPSK modulation is performed on an optical signal being transmitted by a coherent transmitter of an optical line terminal to code the optical signal. At 720. using the coherent transmitter, the optical signal is transmitted over an optical fiber channel to a coherent receiver in an optical network unit of the passive optical network.The optical line terminal can receive an optical transmission from the optical network unit at a local coherent receiver of the optical line terminal.

[0074] Variations of method 700 or methods similar to the method 700 can include a number of different embodiments that may be combined depending on the application of such methods and / or the architecture of devices or systems in which such methods are implemented. Such methods can include performing DPSK modulation by performing differential binary phase-shift keying.Variations can include performing DPSK modulation by performing differential quaternary phase-shift keying. Variations can include performing DPSK modulation by performing differential eight phase-shift keying.

[0075] Variations of method 700 or methods similar to the method 700 can include receiving optical transmissions at the optical line terminal from additional optical network units of the passive optical network. A single local oscillator of the optical line terminal can be used to provide a local optical signal to demodulate the optical transmissions from the optical network unit and the additional optical network units.

[0076] Variations of method 700 or methods similar to the method 700 can include demodulating the optical signal transmitted by the coherent transmitter of the optical line terminal by using a DPSK demodulator at the coherent receiver of the optical network unit of the passive optical network.

[0077] In various embodiments, a non-transitory machine-readable storage device, such as computer-readable non-transitory medium, can comprise instructions stored thereon, which, when performed by a machine, cause the machine to perform operations, where the operations comprise one or more features similar to or identical to features of methods and techniques describedwith respect to method 700, variations thereof, and / or features of other methods taught herein such as associated with Figures 1-6. The physical structures of such instructions may be operated on by one or more storage processors. For example, executing these physical structures can cause the machine to perform operations comprising performing DPSK modulation on an optical signal being transmitted by a coherent transmitter of an optical line terminal to code the optical signal; and transmitting, using the coherent transmitter, the optical signal over an optical fiber channel to a coherent receiver in an optical network unit of the passive optical network.

[0078] Variations of the operations can include a number of different embodiments that may be combined depending on the application of such operations and / or the architecture of devices or systems in which such operations are executed. Such operations can perform DPSK modulation by performing differential binary’ phase-shift keying. Variations of the operations can include performing DPSK modulation by performing differential quaternary phase-shift keying. Variations of the operations can include performing DPSK modulation by performing differential eight phase-shift keying.

[0079] Variations of the operations can include receiving an optical transmission from the optical network unit at a local coherent receiver of the optical line terminal. Variations of the operations can include receiving optical transmissions at the optical line terminal from additional optical network units of the passive optical network. The operations can use a single local oscillator of the optical line terminal to provide a local optical signal to demodulate the optical transmissions from the optical network unit and the additional optical network units.

[0080] Variations of the operations can include demodulating the optical signal transmitted by the coherent transmitter of the optical line terminal by using a DPSK demodulator at the coherent receiver of the optical network unit of the passive optical network.

[0081] In various embodiments, a passive optical network can include an optical line terminal having a coherent transmitter configured to transmit an optical signal over an optical fiber channel, a DPSK modulator coupled to the coherent transmitter to code the optical signal transmitted, and a coherent receiver to receive upstream optical signals. The optical line terminal canoperate with respect to one or more optical network units. Each optical network unit can include a local coherent receiver, a DPSK demodulator coupled to the local coherent receiver to demodulate the optical signal from the optical fiber channel, and a local coherent transmitter, where the local coherent transmitter is structured to transmit individual upstream optical signals to the optical line terminal.

[0082] Variations of such a passive optical network and its features, as taught herein, can include a number of different embodiments and features that can be combined depending on the application of such passive optical networks, the format of such passive optical network, and / or the architecture in which such passive optical network are implemented. Features of such passive optical network can include the optical line terminal having a single local oscillator arranged to provide a local optical signal to demodulate the upstream optical signals from the one or more optical network units. Variations can include the DPSK modulator and the DPSK demodulator structured to operate with respect to differential binary phase-shift keying, differential quaternary phase-shift keying, or differential eight phase-shift keying.

[0083] The components of the illustrative devices, systems, and methods employed in accordance with the illustrated embodiments can be implemented, at least in part, in digital electronic circuitry, analog electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. These components can be implemented, for example, as a computer program product such as a computer program, program code or computer instructions tangibly embodied in a machine-readable storage device, for execution by, or to control the operation of, data processing apparatus such as a programmable processor, a computer, or multiple computers.

[0084] The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general-purpose processor, a digital signal processor (DSP), an ASIC, a FPGA (field-programmable gate array) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, orstate machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

[0085] Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random-access memory or both. The elements of a computer include a processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. The processor and the memory can be supplemented by, or incorporated in special purpose logic circuitry.

[0086] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary' skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Various embodiments use permutations and / or combinations of embodiments described herein. The above description is intended to be illustrative, and not restrictive, and that the phraseology' or terminology employed herein is for the purpose of description. Combinations of the above embodiments and other embodiments will be apparent to those of skill in the art upon studying the above description.

Claims

CLAIMSWhat is claimed is:

1. A system comprising: an optical line terminal (OLT) comprising: a coherent optical transmitter configured to transmit an optical signal over an optical fiber channel to a coherent optical receiver in an optical network unit (ONU) of a passive optical network (PON); a differential phase-shift keying (DPSK) modulator coupled to the coherent optical transmitter, the DPSK modulator configured to code the optical signal being transmitted; and a local coherent receiver to receive an optical transmission from the optical network unit (ONU), the optical transmission being differential phase-shift keying (DPSK) modulated.

2. The system of claim 1, wherein the differential phase-shift keying modulator includes a Mach-Zehnder modulator.

3. The system of claim 2, wherein the optical line terminal (OLT) is structured to receive optical transmissions from additional optical network units (ONUs) of the passive optical network (PON), the optical transmissions being differential phase-shift keying (DPSK) modulated.

4. The system of claim 3, wherein the optical line terminal (OLT) includes a single local oscillator (LO) arranged to provide a local optical signal to demodulate the optical transmissions from the optical network unit (ONU) and from the additional optical network units (ONUs).

5. The system of claim 1, wherein the system includes the coherent optical receiver of the optical network unit (ONU) of the passive optical network (PON) coupled to the differential phase-shift keying (DPSK) demodulator.

6. The system of any one of claims 1-5, wherein the differential phase-shift keying (DPSK) modulator is structured to provide differential binary phase-shift keying (DBPSK).

7. The system of any one of claims 1-5, wherein the differential phase-shift keying (DPSK) modulator is structured to provide differential quaternary phaseshift keying (DQPSK) .

8. The system of any one of claims 1-5, wherein the differential phase-shift keying (DPSK) modulator is structured to provide differential eight phase-shift keying (D8PSK).

9. A method of communicating in a passive optical network (PON), the method compnsing: performing differential phase-shift keying (DPSK) modulation on an optical signal being transmitted by a coherent optical transmitter of an optical line terminal (OLT) to code the optical signal; transmitting, using the coherent optical transmitter, the optical signal over an optical fiber channel to a coherent optical receiver in an optical network unit (ONU) of the passive optical network (PON); and receiving an optical transmission from the optical network unit (ONU) at a local coherent receiver of the optical line terminal (OLT), the received optical transmission being differential phase-shift keying (DPSK) modulated.

10. The method of claim 9, wherein performing the differential phase-shift keying (DPSK) modulation includes performing differential binary phase-shift keying (DBPSK).

11. The method of claim 9, wherein performing the differential phase-shift keying (DPSK) modulation includes performing differential quaternary phaseshift keying (DQPSK).

12. The method of claim 9, wherein performing the differential phase-shift keying (DPSK) modulation includes performing differential eight phase-shift keying (D8PSK).1 . The method of claim 9, wherein performing differential phase-shift keying (DPSK) modulation includes using a Mach-Zehnder modulator.

14. The method of claim 13, wherein the method includes receiving optical transmissions at the optical line terminal (OLT) from additional optical network units (ONUs) of the passive optical network (PON).

15. The method of claim 14, wherein the method includes using a single local oscillator (LO) of the optical line terminal (OLT) to provide a local optical signal to demodulate the optical transmissions from the optical network unit (ONU) and the additional optical network units (ONUs).

16. The method of claim 9. wherein the method includes demodulating the optical signal transmitted by the coherent optical transmitter of the optical line terminal (OLT) using a differential phase-shift keying (DPSK) demodulator at the coherent optical receiver of the optical network unit (ONU) of the passive optical network (PON).

17. A passive optical network (PON), comprising: an optical line terminal (OLT) comprising: a coherent optical transmitter configured to transmit an optical signal over an optical fiber channel; a differential phase-shift keying (DPSK) modulator coupled to the coherent optical transmitter to code the optical signal being transmitted; and a coherent optical receiver to receive upstream optical signals, the upstream optical signals being differential phase-shift keying (DPSK) modulated; and one or more optical network units (ONUs), each optical network unit (ONU) of the one or more optical network units (ONUs) including: a local coherent optical receiver; a differential phase-shift keying (DPSK) demodulator coupled to the local coherent optical receiver to demodulate the optical signal received from the optical fiber channel; and a local coherent optical transmitter, the local coherent optical transmitter to transmit individual upstream optical signals to the optical line terminal (OLT).

18. The passive optical network of claim 17, wherein the optical line terminal (OLT) includes a single local oscillator (LO) arranged to provide a local optical signal to demodulate the upstream optical signals from the one or more optical network units (ONUs).

19. The passive optical network of claim 17, wherein the differential phase-shift keying (DPSK) modulator and the differential phase-shift keying (DPSK) demodulator are structured to operate with respect to differential binary phaseshift keying (DBPSK).

20. The passive optical network of claim 17, wherein the differential phase-shift keying (DPSK) modulator and the differential phase-shift keying (DPSK) demodulator are structured to operate with respect to differential quaternary phase-shift keying (DPSK).

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