Low-cost coherent passive optical network carrier recovery method based on residual optical carrier

Abstract

The invention belongs to the technical field of optical fiber communication, and particularly relates to a low-cost coherent passive optical network carrier recovery method based on residual optical carriers, which comprises a sending end and a receiving end, the transmitting end comprises a light source, a light modulator, an input module and an amplifier; the light source outputs continuous light waves; the optical modulator modulates the baseband modulation signal to an optical carrier and outputs a dual-polarization signal; the input module is used for generating a baseband modulation signal; the amplifier is used for amplifying optical signal power; the receiving end mainly comprises a variable optical attenuator, a coherent receiver and an oscilloscope; the variable optical attenuator is used for adjusting the receiving optical power; the coherent receiver is used for coherent detection and outputting a baseband signal; the oscilloscope is used for storing baseband signals; according to the method, an optical carrier is reserved as a pilot frequency for frequency offset estimation and carrier phase recovery, and is combined with digital subcarrier multiplexing, so that the burden and complexity of subsequent blind phase search are reduced; the cost and the architecture efficiency are effectively improved, and the phase noise related to the DFB with MHz-level line width is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber communication, and in particular is a low-cost 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 50Gb its / s / λ. However, for applications of 100Gbits / s / λ and above, studies have shown that 50Gb its / s / λ IM / DD PON may have reached its theoretical sensitivity limit, making it challenging to meet the power budget requirements of next-generation 100Gb its / s / λ systems. In addition, dispersion tolerance also faces major challenges when upgrading IM / DD PON to higher rates, as its performance degrades 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 leveraging advanced modulation formats and powerful digital signal processing (DSP). However, traditional coherent detection incurs significant costs due to the need for laser sources, including optical transmitters, optical receivers, and DSP modules, which together lead to 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 the PON scenario, the cost rises significantly because there must be a laser source 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 expensive ECL with cheaper options, such as distributed feedback (DFB) lasers, and accept a certain degree of power loss in the access network. Regarding the simplification and optimization of DSP, the relatively short transmission distance in the PON system may allow the removal of static dispersion compensation and polarization mode dispersion compensation. 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

[0003] Purpose of the invention: To provide a low-cost 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 100Gb its / s / λDP-QPSK PON system, DFB laser is used instead of ECL to improve cost and architecture efficiency. This method significantly reduces the total cost of coherent PON system, while effectively mitigating the phase noise associated with DFB with MHz-level linewidth.

[0004] To solve the above problems, the technical solution of the present invention provides a low-cost coherent passive optical network carrier recovery method based on residual optical carrier, which is described in the order of signal transmission direction, including: a signal transmitting end and a signal receiving end:

[0005] The signal sending end comprises:

[0006] An external cavity laser (ECL) 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;

[0007] A distributed feedback (DFB) laser with a line width of 0.01-10 MHz and a wavelength in the C band or other bands;

[0008] An arbitrary waveform generator (AWG) or digital-to-analog converter that outputs a baseband modulation signal of arbitrary waveform;

[0009] two electrical amplifiers (EAs) to amplify the baseband modulated signals;

[0010] A polarization multiplexed I / Q modulator (IQM) that modulates the amplified baseband signal onto an optical carrier;

[0011] An erbium-doped fiber amplifier (EDFA) to enhance the optically modulated signal

[0012] A single-mode standard optical fiber (SSMF);

[0013] The signal receiving end comprises:

[0014] a variable optical attenuator that divides the loss and adjusts the received optical power (ROP) for sensitivity measurements to estimate the power budget;

[0015] An external cavity laser 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 a dual-polarization integrated coherent receiver (DP-I CR) with a bandwidth of 1-100 GHz;

[0016] A distributed feedback laser is used as a local oscillator light source of a coherent receiver to beat the received optical dual-polarization signal;

[0017] a DP integrated coherent receiver (DP-I CR) for intradyne coherent detection with a bandwidth of tens of GHz;

[0018] A digital-to-analog converter, used to convert analog signals into digital signals for subsequent digital signal processing

[0019] In a further embodiment, the erbium-doped fiber amplifier in the signal transmitting end cooperates with the optical attenuator in the signal receiving end to control the optical signal power to find the optical power most suitable for system transmission.

[0020] In a further embodiment, a pilot is generated in the optical domain by adjusting the optical modulator bias away from the null point to manage the phase noise introduced by the distributed feedback (DFB) laser linewidth, and digital subcarrier multiplexing (DScM) with two subbands is used to create a 2 GHz guard interval to mitigate crosstalk between the signal and the inserted optical pilot.

[0021] In a further embodiment, the residual carrier undergoes the same conditions as the signal band after beating with the local oscillator at the receiver and then performs FOE based on fast Fourier transform to determine the peaks in the electrical spectrum; when the residual carrier is extracted by a digital low-pass filter, it is conjugated and multiplied with the signal to eliminate phase noise.

[0022] In a further embodiment, all operations are performed in the digital domain.

[0023] In a further embodiment, the recovery method comprises the following steps:

[0024] Step 1: At the transmitting end, the light source is intensity modulated using a digital baseband signal;

[0025] The I / Q modulator using polarization multiplexing outputs dual-polarized optical signals;

[0026] Control signal power using erbium-doped fiber amplifiers;

[0027] Step 2: At the receiving end, after passing through 20 kilometers of standard single-mode optical fiber, the received optical power is adjusted using a variable optical attenuator. The received optical dual-polarization signal beats with the C or L band light wave generated by the laser. After the integrated coherent receiver performs intra-heterodyne coherent detection, the baseband signal is observed and stored through an oscilloscope, and the signal is restored using an offline DSP.

[0028] Step 3: The phase noise increase associated with low-cost DFB lasers is mitigated by retaining an optical carrier as a pilot for frequency offset estimation (FOE) and carrier phase recovery (CPR).

[0029] Beneficial effects: The low-cost coherent passive optical network carrier recovery method based on residual optical carrier proposed in the present invention retains an optical carrier as a pilot for frequency offset estimation and carrier phase recovery, and combined with digital subcarrier multiplexing, reduces the burden and complexity of subsequent blind phase search. In addition, this method can not only effectively improve cost and architecture efficiency, but also effectively reduce the phase noise associated with DFB with MHz-level linewidth, providing a feasible solution for future large-capacity PON. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of a low-cost coherent passive optical network carrier recovery method based on residual optical carrier proposed by the present invention.

[0031] Figure 1 (a) shows that the transmitter uses a digital subcarrier multiplexing signal (DScM) to generate a residual optical carrier (ROC). 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.

[0032] Figure 1 (b) shows a description of 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).

[0033] Figure 1 (c) shows the residual carrier going through the same conditions as the signal band after beating the LO at the receiver. A fast Fourier transform based FOE is then 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 remove the phase noise. All operations are performed in the digital domain, reducing the hardware burden.

[0034] Figure 2 The invention discloses a system architecture of a low-cost coherent passive optical network carrier recovery method based on residual optical carrier.

[0035] Figure numerals: 1 and 10 are external cavity modulated lasers ECL; 2 and 11 are distributed feedback lasers DFB; 3 is an I / Q modulator IQM for polarization multiplexing; 4 is an arbitrary waveform generator AWG; 5 and 6 are electrical amplifiers EA; 7 is an erbium-doped fiber amplifier EDFA; 8 is a 20km single-mode standard optical fiber SSMF; 9 is a variable optical attenuator VOA; 12 is a DP integrated coherent receiver DP-ICR; 13 is a digital phosphor oscilloscope DPO. DETAILED DESCRIPTION

[0036] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] A low-cost coherent passive optical network carrier recovery method based on residual optical carrier, comprising:

[0038] Signal sending end:

[0039] An external cavity laser (ECL) 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;

[0040] A distributed feedback (DFB) laser with a linewidth of several MHz and a wavelength in the C-band or other bands;

[0041] An arbitrary waveform generator (AWG) or digital-to-analog converter that outputs a baseband modulation signal of arbitrary waveform;

[0042] two electrical amplifiers (EAs) to amplify the baseband modulated signals;

[0043] A polarization multiplexed I / Q modulator (IQM) that modulates the amplified baseband signal onto an optical carrier;

[0044] An erbium-doped fiber amplifier (EDFA) to enhance the optically modulated signal

[0045] A single-mode standard optical fiber (SSMF) with an average loss of 0.2dB / km at 1550nm;

[0046] Signal receiving end

[0047] a variable optical attenuator that divides the loss and adjusts the received optical power (ROP) for sensitivity measurements to estimate the power budget;

[0048] An external cavity laser 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 a dual-polarization integrated coherent receiver (DP-ICR) with a bandwidth of tens of GHz.

[0049] A distributed feedback laser is used as the local oscillator light source of the coherent receiver to beat the received optical dual-polarization signal;

[0050] A DP integrated coherent receiver (DP-ICR) for intradyne coherent detection with a bandwidth of tens of GHz;

[0051] A digital-to-analog converter is used to convert analog signals into digital signals for subsequent digital signal processing.

[0052] In one embodiment, Figure 1 and Figure 2 As shown, an erbium-doped fiber amplifier and an optical attenuator are used to control the optical signal power, thereby finding the optical power most suitable for system transmission.

[0053] In one embodiment, Figure 1 and Figure 2 As shown, in order 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.

[0054] In one embodiment, Figure 1 and Figure 2 As shown, the residual carrier undergoes the same conditions as the signal band after beating 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 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.

[0055] In one embodiment, Figure 1 and Figure 2 As shown, the following steps are included:

[0056] At the transmitting end, the intensity of the light source is modulated using a digital baseband signal;

[0057] The I / Q modulator using polarization multiplexing outputs dual-polarized optical signals;

[0058] Control signal power using erbium-doped fiber amplifiers;

[0059] At the receiving end, after passing through 20 kilometers of standard single-mode optical fiber, the received optical power is adjusted using a variable optical attenuator. The received optical dual-polarization signal beats with the C or L band light wave generated by the laser. After intra-heterodyne coherent detection is performed in an integrated coherent receiver, the baseband signal is observed and stored through an oscilloscope, and the signal is restored using an offline DSP.

[0060] To mitigate the increase in phase noise 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).

[0061] So far, this system has completed the function of the low-cost coherent passive optical network system.

[0062] Working principle: At the signal transmitting end, 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 the electrical amplifier EA (5) (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).

[0063] At the signal receiving end, a variable optical attenuator VOA (9) is used to adjust the optical power of the received signal. Another external cavity modulated laser ECL (10) or distributed feedback laser DFB (11) has a constant output power of 11 dBm and is used as a 1536.8 nm local oscillator to perform differential beat frequency with the received optical dual-polarization signal. The DP integrated coherent receiver DP-ICR (12) with a bandwidth of 34 GHz performs intra-differential coherent detection, and the baseband signal is stored in a digital phosphor oscilloscope DPO (13).

[0064] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. A low-cost coherent passive optical network carrier recovery method based on residual optical carrier, characterized in that: Including signal sending end and signal receiving end: The signal sending end comprises: An external cavity laser (ECL) 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 (DFB) laser with a line width of 0.01-10 MHz and a wavelength in the C band or other bands; An arbitrary waveform generator (AWG) or digital-to-analog converter that outputs a baseband modulation signal of arbitrary waveform; two electrical amplifiers (EAs) to amplify the baseband modulated signals; A polarization multiplexed I / Q modulator (IQM) that modulates the amplified baseband signal onto an optical carrier; An erbium-doped fiber amplifier (EDFA) to enhance the optically modulated signal A single-mode standard optical fiber (SSMF); The signal receiving end comprises: a variable optical attenuator that divides the loss and adjusts the received optical power (ROP) for sensitivity measurements to estimate the power budget; An external cavity laser 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 a dual-polarization integrated coherent receiver (DP-ICR) with a bandwidth of tens of GHz. A distributed feedback laser 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) for intradyne coherent detection with a bandwidth of 1-100 GHz; A digital-to-analog converter is used to convert analog signals into digital signals for subsequent digital signal processing.

2. According to claim 1, a low-cost coherent passive optical network carrier recovery method based on residual optical carrier is characterized in that: The erbium-doped fiber amplifier in the signal transmitting end cooperates with the optical attenuator in the signal receiving end to control the optical signal power to find the optical power most suitable for system transmission.

3. According to claim 1, a low-cost coherent passive optical network carrier recovery method based on residual optical carrier is characterized in that: A pilot is generated in the optical domain by adjusting the optical modulator bias away from the null point to manage the phase noise introduced by the distributed feedback (DFB) laser linewidth, and digital subcarrier multiplexing (DScM) with two subbands is used to create a 2 GHz guard interval to mitigate the crosstalk between the signal and the inserted optical pilot.

4. The low-cost coherent passive optical network carrier recovery method based on residual optical carrier according to claim 1 is characterized in that: The residual carrier undergoes the same conditions as the signal band after beating with the local oscillator at the receiver, and then performs FOE based on fast Fourier transform to determine the peak in the electrical spectrum; when the residual carrier is extracted by the digital low-pass filter, it is conjugated and multiplied with the signal to eliminate phase noise.

5. The low-cost coherent passive optical network carrier recovery method based on residual optical carrier according to claim 4 is characterized in that: All operations are performed in the digital domain.

6. The low-cost coherent passive optical network carrier recovery method based on residual optical carrier according to claim 1 is characterized in that: The recovery method comprises the following steps: Step 1: At the transmitting end, the light source is intensity modulated using a digital baseband signal; The I / Q modulator using polarization multiplexing outputs dual-polarized optical signals; Control signal power using erbium-doped fiber amplifiers; Step 2: At the receiving end, after passing through 20 kilometers of standard single-mode optical fiber, the received optical power is adjusted using a variable optical attenuator. The received optical dual-polarization signal beats with the C or L band light wave generated by the laser. After the integrated coherent receiver performs intra-heterodyne coherent detection, the baseband signal is observed and stored through an oscilloscope, and the signal is restored using an offline DSP. Step 3: The phase noise increase associated with low-cost DFB lasers is mitigated by retaining an optical carrier as a pilot for frequency offset estimation (FOE) and carrier phase recovery (CPR).

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