Coherent passive optical network carrier recovery method based on residual optical carrier

Abstract

The invention relates to the technical field of optical fiber communication, and discloses a coherent passive optical network carrier recovery method based on residual optical carriers. The coherent passive optical network carrier recovery system based on the residual optical carrier comprises: (1) a signal transmitting end: an external cavity modulation laser ECL 1 as a light source to output continuous light waves, the line width is less than 1MHz, and the working wavelength is a C wave band or other wave bands; the line width of the first distributed feedback laser DFB is several MHz, and the wavelength of the first distributed feedback laser DFB is a C wave band or other wave bands. According to the residual optical carrier-based coherent passive optical network carrier recovery method provided by the invention, a residual optical carrier (ROC) technology and digital subcarrier multiplexing (DScM) are introduced into a 100 Gbits / s / lambda DP-QPSKPON system, and a DFB laser is used for replacing an ECL to improve the cost and the architecture efficiency. According to the method, the total cost of the coherent PON system is remarkably reduced, and meanwhile, the phase noise related to the DFB with the MHz-level line width is effectively reduced.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber communication, and in particular to a coherent passive optical network carrier recovery method based on residual optical carrier. Background Art

[0002] With the growing demand for services such as 5G mobile internet, 4K / 8K high-definition video streaming, the Internet of Things (IoT), and cloud networks, the optical access network industry is seeking solutions for a new wave of bandwidth acceleration. Passive optical networks (PONs), as point-to-multipoint (P2MP) broadband fiber access technologies, have shown cost-effectiveness in the last-mile connection between service providers and customers and have been proven in various passive optical distribution networks. However, the growing demand for bandwidth indicates that PONs capable of supporting 100Gbits / s / λ and above will soon become necessary. Currently, there are two main schemes and their simplified variants that dominate PON architectures: intensity modulation with direct detection (IM / DD) and fully coherent structures. Due to the advantages of cost-effectiveness, simple structure, and small size, the IM / DD scheme is widely used in PONs up to 50Gbits / s / λ.

[0003] However, for applications of 100Gbits / s / λ and above, studies have shown that 50Gbits / s / λ IM / DDPON may have reached its theoretical sensitivity limit, making it challenging to meet the power budget requirements of next-generation 100Gbits / s / λ systems. In addition, dispersion tolerance also faces major challenges when upgrading IM / DDPON to higher rates, as its performance decreases dramatically as the symbol rate increases. In this context, coherent optics has been shown to be a promising candidate technology for realizing single-wavelength PON (100Gbits / s / λ and above) over thousands of kilometers of fiber transmission using phase and polarization diversity, known as coherent sinking. This technology represents a major advancement in achieving ultra-high-speed data transmission for long-haul and metropolitan area networks. By introducing a local oscillator (LO) that coherently beats with the received signal, the receiver sensitivity is improved, thereby extending the power budget and supporting high-capacity, long-distance and large split ratio access. In addition, it facilitates flexible deployment and adaptive access by utilizing advanced modulation formats and powerful digital signal processing (DSP).

[0004] However, conventional coherent detection incurs significant costs due to the need for laser sources, including optical transmitters, optical receivers, and DSP modules, which together result in a significant increase in expenses. This cost barrier has hindered the widespread adoption of PONs, where the ideal design goals are simplicity, economy, and energy efficiency. The light source, usually a laser, is a key component for optimizing system cost and performance. In PON scenarios, the cost rises significantly because a laser source must be used as the LO in each optical network unit (ONU). In addition, the computational complexity and power consumption of distance-dependent equalization also pose challenges, making DSP optimization in coherent PONs an active research area. A large number of studies have been explored to reduce the cost and power consumption of coherent PONs. One potential solution is to utilize a remote laser source from a centralized optical line terminal (OLT) for each ONU. Another approach is to replace the expensive ECL with a cheaper option, such as a distributed feedback (DFB) laser, accepting a certain degree of power loss in the access network. Regarding DSP simplification and optimization, the relatively short transmission distance in PON systems may allow the removal of static dispersion compensation and polarization mode dispersion compensation.

[0005] Therefore, it is necessary to study how to achieve low-cost, high-speed, and large-capacity signal transmission based on PON scenarios to meet the needs of future broadband mobile communications. Summary of the invention

[0006] The purpose of the present invention is to provide a coherent passive optical network carrier recovery method based on residual optical carrier, by introducing residual optical carrier (ROC) technology and digital subcarrier multiplexing (DScM) into the 100Gbits / s / λDP-QPSKPON system, using DFB laser instead of ECL to improve cost and architecture efficiency. This method significantly reduces the total cost of the coherent PON system, while effectively mitigating the phase noise associated with DFB with MHz-level linewidth.

[0007] The coherent passive optical network carrier recovery system based on residual optical carrier provided by the present invention comprises:

[0008] (1) Signal sending end:

[0009] An external cavity modulated laser ECL1 is used as a light source to output continuous light waves with a line width of less than 1MHz and an operating wavelength of C band or other bands;

[0010] A distributed feedback laser DFB1, with a line width of several MHz and a wavelength in the C band or other bands;

[0011] An arbitrary waveform generator AWG, outputting a baseband modulation signal of an arbitrary waveform;

[0012] Two electrical amplifiers EA, namely electrical amplifier EA-5 and electrical amplifier EA-6, for amplifying baseband modulation signals;

[0013] A polarization multiplexing I / Q modulator IQM, used to modulate the amplified baseband signal onto an optical carrier;

[0014] An erbium-doped fiber amplifier (EDFA) is used to enhance the optical modulated signal.

[0015] A 20km single-mode standard optical fiber SSMF has an average loss of 0.2dB / km at 1550nm;

[0016] (2) Signal receiving end:

[0017] A variable optical attenuator (VOA) to split the loss and adjust the received optical power (ROP) for sensitivity measurement and thus estimate the power budget;

[0018] An external cavity modulated laser (ECL) is used as the local oscillator light source of the coherent receiver to beat the received optical dual-polarization signal and perform intraheterodyne coherent detection through the dual-polarization integrated coherent receiver DP-ICR with a bandwidth of tens of GHz.

[0019] A distributed feedback laser (DFB) is used as the local oscillator light source of the coherent receiver to beat the received optical dual-polarization signal;

[0020] A DP integrated coherent receiver DP-ICR for intradyne coherent detection with a bandwidth of tens of GHz;

[0021] A digital phosphor oscilloscope (DPO) is used to convert analog signals into digital signals for subsequent digital signal processing.

[0022] Preferably, the system uses an erbium-doped fiber amplifier 7 and a variable optical attenuator 9 to control the optical signal power, thereby finding the optical power most suitable for system transmission.

[0023] Preferably, in order to effectively manage the phase noise introduced by the larger linewidth of the DFB laser, a pilot is generated in the optical domain by slightly adjusting the optical modulator bias away from the zero point; digital subcarrier multiplexing (DScM) with two subbands is used to create a 2 GHz guard interval, which helps to mitigate the crosstalk between the signal and the inserted optical pilot.

[0024] Preferably, the residual carrier undergoes the same conditions as the signal band after beating with the local oscillator at the receiver, and then, a fast Fourier transform-based FOE is performed to determine the peaks in the electrical spectrum; once the residual carrier is extracted by the digital low-pass filter, it is conjugated and multiplied with the signal to eliminate phase noise; all operations are performed in the digital domain, reducing the hardware burden.

[0025] The present invention proposes a coherent passive optical network carrier recovery method based on residual optical carrier, the method comprising the following steps:

[0026] S1. At the transmitting end, the light source is intensity modulated using a digital baseband signal;

[0027] S2, using the polarization multiplexing I / Q modulator to output a dual-polarized optical signal;

[0028] S3, using erbium-doped fiber amplifier to control signal power;

[0029] S4. At the receiving end, after passing through 20 km of standard single-mode optical fiber, the received optical power is adjusted by a variable optical attenuator, and the received optical dual-polarization signal beats with the C or L band light wave generated by the laser. After the DP integrated coherent receiver DP-ICR performs intraheterodyne coherent detection, the baseband signal is observed and stored by a digital phosphor oscilloscope, and the signal is restored by an offline DSP.

[0030] S5. To mitigate the phase noise increase associated with low-cost DFB lasers (which typically have large linewidths), an optical carrier is retained as a pilot for frequency offset estimation (FOE) and carrier phase recovery (CPR); this approach, combined with digital subcarrier multiplexing (DScM), reduces the burden and complexity of the subsequent blind phase search (BPS);

[0031] So far, this system has completed the function of the low-cost coherent passive optical network system.

[0032] Compared with the related art, the coherent passive optical network carrier recovery method based on residual optical carrier provided by the present invention has the following beneficial effects:

[0033] This scheme reduces the burden and complexity of the subsequent blind phase search (BPS) by reserving an optical carrier as a pilot for frequency offset estimation (FOE) and carrier phase recovery (CPR) combined with digital subcarrier multiplexing (DScM);

[0034] In addition, this solution can not only effectively improve cost and architectural efficiency, but also effectively mitigate the phase noise associated with DFB with MHz-level linewidth, providing a feasible solution for future large-capacity PON. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a schematic diagram of the low-cost coherent passive optical network carrier recovery method based on residual optical carrier proposed by the present invention:

[0036] Figure 1(a) shows the transmitter using digital subcarrier multiplexing (DScM) to generate a residual optical carrier (ROC). DScM with two subbands is used to create a 2 GHz guard interval, which helps mitigate crosstalk between the signal and the inserted optical pilot.

[0037] Figure 1 (b) shows a diagram describing the various noise sources present in the channel, including the phase noise of the lasers at the transmitter and receiver (LD), and the amplified spontaneous emission (ASE) noise of the erbium-doped fiber amplifier (EDFA);

[0038] Figure 1 (c) shows that the residual carrier undergoes the same situation as the signal band after beating with the LO at the receiver.

[0039] Figure 2 The invention discloses a system architecture of a low-cost coherent passive optical network carrier recovery method based on residual optical carrier.

[0040] Figure numerals: 1. External cavity modulated laser ECL-1; 2. Distributed feedback laser DFB-1; 3. Polarization multiplexing I / Q modulator IQM; 4. Arbitrary waveform generator AWG; 5. Electrical amplifier EA-1; 6. Electrical amplifier EA-2; 7. Erbium-doped fiber amplifier EDFA; 8. Single-mode standard optical fiber SSMF; 9. Variable optical attenuator VOA; 10. External cavity modulated laser ECL-2; 11. Distributed feedback laser DFB; 12. DP integrated coherent receiver DP-ICR; 13. Digital phosphor oscilloscope DPO. DETAILED DESCRIPTION

[0041] The present invention will be further described below in conjunction with the accompanying drawings and implementation modes.

[0042] Figure 2 The figure shows a coherent passive optical network carrier recovery system architecture based on residual optical carrier proposed by the present invention, including:

[0043] (1) Signal sending end:

[0044] An external cavity modulated laser ECL-1 is used as a light source to output continuous light waves with a line width of less than 1MHz and an operating wavelength of C-band or other bands;

[0045] A distributed feedback laser DFB-2, with a line width of several MHz and a wavelength in the C band or other bands;

[0046] An arbitrary waveform generator AWG 4, outputting a baseband modulation signal of an arbitrary waveform;

[0047] Two electrical amplifiers EA, namely electrical amplifier EA-5 and electrical amplifier EA-6, for amplifying baseband modulation signals;

[0048] a polarization multiplexing I / Q modulator IQM 3, for modulating the amplified baseband signal onto an optical carrier;

[0049] An erbium-doped fiber amplifier EDFA 7, used to enhance the optically modulated signal

[0050] A 20km single-mode standard optical fiber SSMF 8 has an average loss of 0.2dB / km at 1550nm;

[0051] (2) Signal receiving end:

[0052] a variable optical attenuator VOA 9 for dividing the loss and adjusting the received optical power ROP for sensitivity measurement and thus estimating the power budget;

[0053] An external cavity modulated laser ECL10 is used as the local oscillator light source of the coherent receiver to beat the received optical dual-polarization signal and perform intraheterodyne coherent detection through the dual-polarization integrated coherent receiver DP-ICR with a bandwidth of tens of GHz;

[0054] A distributed feedback laser DFB11 is used as the local oscillator light source of the coherent receiver to beat the received optical dual-polarization signal;

[0055] a DP integrated coherent receiver DP-ICR 12 for intradyne coherent detection with a bandwidth of tens of GHz;

[0056] A digital phosphor oscilloscope DPO 13 is used to convert analog signals into digital signals for subsequent digital signal processing.

[0057] At the signal transmitting end, firstly, an external cavity modulated laser ECL-1 or a distributed feedback laser DFB-2 is used as a light source to generate a light wave with an operating wavelength of 1536.8nm, and the optical power is maintained at 10dBm. In the polarization multiplexing I / Q modulator IQM 3, the baseband modulation signal generated by the arbitrary waveform generator AWG 4 is amplified by two electrical amplifiers EA (electrical amplifier EA-5 and electrical amplifier EA-6) to modulate the coupled optical signal and output a dual-polarized optical signal. The signal passes through the erbium-doped fiber amplifier EDFA 7 to fix the optical signal power at 8dBm and then enters the 20km single-mode standard optical fiber SSMF 8.

[0058] At the signal receiving end, the variable optical attenuator VOA 9 is used to adjust the optical power of the received signal. Another external cavity modulated laser ECL2 10 or distributed feedback laser DFB2 11 has a constant output power of 11dBm and is used as a 1536.8nm local oscillator to perform differential beat frequency with the received optical dual-polarization signal. Intra-differential coherent detection is performed through the DP integrated coherent receiver DP-ICR12 with a bandwidth of 34GHz, and the baseband signal is stored in the digital phosphor oscilloscope DPO 13.

[0059] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A coherent passive optical network carrier recovery system based on residual optical carrier, characterized in that: include: Signal sending end: An external cavity modulated laser ECL-(1) is used as a light source to output continuous light waves with a line width of less than 1 MHz and an operating wavelength in the C band or other bands; A distributed feedback laser DFB-(2) with a line width of several MHz and a wavelength in the C-band or other bands; an arbitrary waveform generator AWG (4), outputting a baseband modulation signal of an arbitrary waveform; Two electrical amplifiers EA, namely electrical amplifier EA 1 (5) and electrical amplifier EA 2 (6), for amplifying the baseband modulated signal; a polarization multiplexing I / Q modulator IQM (3) for modulating the amplified baseband signal onto an optical carrier; An erbium-doped fiber amplifier (EDFA) (7) is used to enhance the optical modulated signal. A 20km single-mode standard optical fiber SSMF (8) has an average loss of 0.2dB / km at 1550nm; Signal receiving end: a variable optical attenuator VOA (9) for dividing the loss and adjusting the received optical power ROP for sensitivity measurement and thus estimating the power budget; An external cavity modulated laser (ECL) (10) is used as a local oscillator light source of a coherent receiver, beats with the received optical dual-polarization signal, and performs intraheterodyne coherent detection through a dual-polarization integrated coherent receiver DP-ICR with a bandwidth of tens of GHz; A distributed feedback laser DFB (11) is used as a local oscillator light source of a coherent receiver to beat the received optical dual-polarization signal; a DP integrated coherent receiver DP-ICR (12) for intradyne coherent detection; A digital phosphor oscilloscope DPO (13) is used to convert analog signals into digital signals for subsequent digital signal processing.

2. The coherent passive optical network carrier recovery system based on residual optical carrier according to claim 1, characterized in that: The system uses an erbium-doped optical fiber amplifier (7) and a variable optical attenuator (9) to control the optical signal power, thereby finding the optical power most suitable for system transmission.

3. The coherent passive optical network carrier recovery system based on residual optical carrier according to claim 1, characterized in that: To effectively manage the phase noise introduced by the larger linewidth of the DFB laser, the pilot is generated in the optical domain by slightly adjusting the optical modulator bias away from the null point; digital subcarrier multiplexing (DScM) with two subbands is used to create a 2GHz guard interval, which helps to mitigate the crosstalk between the signal and the inserted optical pilot.

4. The coherent passive optical network carrier recovery system based on residual optical carrier according to claim 1, characterized in that: The residual carrier undergoes the same conditions as the signal band after beating with the local oscillator at the receiver. Then, a fast Fourier transform-based FOE is performed to determine the peaks in the electrical spectrum. Once the residual carrier is extracted by a digital low-pass filter, it is conjugated and multiplied with the signal to eliminate phase noise. All operations are performed in the digital domain, reducing the hardware burden.

5. A coherent passive optical network carrier recovery method based on residual optical carrier, characterized in that: The method comprises the following steps: S1. At the transmitting end, the light source is intensity modulated using a digital baseband signal; S2, using the polarization multiplexing I / Q modulator (3) to output a dual-polarized optical signal; S3, using erbium-doped fiber amplifier to control signal power; S4. At the receiving end, after passing through 20 km of standard single-mode optical fiber (8), the received optical power is adjusted by using a variable optical attenuator (9), and the received optical dual-polarization signal beats with the C or L band optical wave generated by the laser. After the DP integrated coherent receiver DP-ICR (12) performs intraheterodyne coherent detection, the baseband signal is observed and stored by a digital phosphor oscilloscope (13), and the signal is restored by using an offline DSP; S5. To mitigate the phase noise increase associated with low-cost DFB lasers (which typically have large linewidths), an optical carrier is retained as a pilot for frequency offset estimation (FOE) and carrier phase recovery (CPR); this approach, combined with digital subcarrier multiplexing (DScM), reduces the burden and complexity of the subsequent blind phase search (BPS); So far, this system has completed the function of the low-cost coherent passive optical network system.

Images