An uplink and downlink wavelength locking method for TFDM-PON

Abstract

This invention discloses an uplink and downlink wavelength locking method and system for Time Division Frequency Division Multiplexing Passive Optical Network (TFDM-PON), including a downlink wavelength difference estimation and compensation stage; an uplink carrier construction and transmission stage; and an optical line terminal (OLT) side reception and demodulation stage. In the downlink wavelength difference estimation and compensation stage, the frequency offset between the OLT transmitting source and the ONU local laser is estimated through the reception and processing of the downlink coherent optical signal by the optical network unit (ONU), and the local laser frequency is compensated. In the uplink carrier construction and transmission stage, an uplink coherent optical signal with controllable spectrum is generated based on the local laser source locked in the downlink wavelength difference estimation and compensation stage. In the OLT side reception and demodulation stage, the OLT receives the uplink coherent optical signals from each ONU, utilizing the predictability of the uplink optical carrier frequency. This invention provides an uplink and downlink wavelength locking method and system for TFDM-PON, effectively reducing the complexity of digital signal processing on the OLT side and improving the stability, reliability, and scalability of the system in scenarios with multiple ONUs concurrent access.

Description

Technical Field

[0001] This invention relates to the field of optical fiber communication technology, and in particular to an uplink and downlink wavelength locking method for TFDM-PON. Background Technology

[0002] With the rapid growth in access network bandwidth demand, coherent PON has become an important development direction for next-generation high-speed access networks due to its high spectral efficiency, high receiver sensitivity, and good scalability. In TFDM-PON systems, multiple ONUs share fiber resources in the same passive optical distribution network, and uplink and downlink signals are typically multiplexed using a combination of time division and frequency division.

[0003] In coherent PON systems, the ONU side typically uses a local laser as the uplink carrier light source, while the OLT side uses a highly stable light source. Due to cost constraints, the ONU-side laser has significant initial wavelength error, drift, and temperature sensitivity, which can easily lead to a large frequency offset between the ONU uplink carrier and the OLT local oscillator. This increases the complexity of digital signal processing and may even affect the normal reception of the system.

[0004] Existing solutions often rely on highly complex frequency offset estimation and carrier phase recovery algorithms, or introduce additional pilot signals, which can lead to problems such as complex implementation, slow convergence speed, or reduced spectral efficiency. Summary of the Invention

[0005] In view of the aforementioned shortcomings of the prior art, the technical problem to be solved by the present invention is that the prior art usually relies on highly complex frequency offset estimation and carrier phase recovery algorithms, or introduces additional pilot signals, which results in problems such as complex implementation, slow convergence speed, or reduced spectral efficiency. Therefore, the present invention provides an uplink and downlink wavelength locking method and system for TFDM-PON, which effectively reduces the complexity of digital signal processing on the OLT side and improves the stability, reliability, and scalability of the system in scenarios with multiple ONUs concurrent access.

[0006] To achieve the above objectives, this invention provides an uplink and downlink wavelength locking method for TFDM-PON, including a downlink wavelength difference estimation and compensation stage; an uplink carrier construction and transmission stage; and an OLT-side reception and demodulation stage.

[0007] In the downlink wavelength difference estimation and compensation stage, the frequency offset between the OLT transmitting source and the ONU local laser is estimated by the ONU receiving and processing the downlink coherent optical signal, and the frequency of the local laser is compensated.

[0008] During the uplink carrier construction and transmission phase, a spectrum-controllable uplink coherent optical signal is generated based on the local laser source locked after the downlink wavelength difference estimation and compensation phase.

[0009] During the reception and demodulation phase on the OLT side, the OLT receives uplink coherent optical signals from each ONU, taking advantage of the predictability of the uplink optical carrier frequency.

[0010] Furthermore, the downlink wavelength difference estimation and compensation stage includes the following steps:

[0011] Step S101: The transmitter of the optical line terminal uses a frequency of The optical carrier transmits downlink coherent optical signals, and the optical network unit receives downlink coherent optical signals from the OLT. It uses its local laser as a local oscillator source and performs photoelectric conversion and analog-to-digital conversion on the downlink coherent optical signals based on the coherent receiving structure to obtain the corresponding downlink digital signals.

[0012] Step S102: The ONU performs digital signal processing on the downlink digital signal and uses a carrier frequency offset estimation algorithm to determine the frequency of the OLT's transmitting light source. With ONU local laser frequency The frequency offset between the two is estimated to obtain the frequency offset between them. or equivalent wavelength difference ;

[0013] Step S103: ONU based on frequency offset Initial frequency compensation adjustment is performed on its local tunable laser;

[0014] Step S104: Through the above compensation adjustment, the local laser output frequency of the ONU is initially aligned with the downlink optical carrier frequency of the OLT.

[0015] Furthermore, carrier frequency offset estimation algorithms may include, but are not limited to: FFT algorithms based on fourth power spectrum analysis, frequency offset estimation algorithms based on pilots, and correlation algorithms based on training sequences.

[0016] Furthermore, the ONU adjusts the frequency offset accordingly. The local tunable laser is initially compensated and adjusted so that the adjusted output frequency satisfies:

[0017]

[0018] in, This indicates the compensated output frequency of the local laser.

[0019] Furthermore, the uplink carrier construction and transmission phase specifically includes the following steps:

[0020] Step S201: After the downlink wavelength difference estimation and compensation adjustment, the ONU obtains a local laser precisely aligned with the downlink carrier frequency of the OLT, with an output frequency of... ;

[0021] Step S202: The ONU performs frequency shift modulation on the output light of the local laser, and loads the uplink data signal onto the target uplink optical carrier frequency. This allows for the construction of an uplink coherent optical signal at the target's uplink wavelength.

[0022] Step S203: The ONU sends the generated uplink coherent optical signal to the OLT to realize the transmission of the uplink signal.

[0023] Furthermore, by pre-allocating different target uplink carrier frequencies to different ONUs... This allows the uplink signals of multiple ONUs to be arranged in a non-overlapping manner on the spectrum, thereby enabling frequency-based multiple access.

[0024] Furthermore, the OLT-side reception and demodulation stage specifically includes the following steps:

[0025] Step S301: The OLT receives the uplink signal from the ONU through the coherent receiving module;

[0026] Step S302: The DSP unit of the OLT performs split filtering and demodulation based on the known frequency.

[0027] Furthermore, the digital signal processing unit of the OLT performs demultiplexing demodulation on the received mixed uplink signal based on the known uplink carrier frequencies of each ONU, and extracts the uplink data of each ONU in sequence.

[0028] In a preferred embodiment of the present invention, a TFDM-PON system is provided for implementing the above-mentioned uplink and downlink wavelength locking method, including an optical line terminal (OLT) and multiple optical network units (ONUs), wherein the OLT and each ONU are connected via an ODN.

[0029] Furthermore, the OLT side includes a downlink signal transmission module, an uplink coherent reception module, and an uplink signal demodulation module; wherein, the downlink signal transmission module is used to employ a fixed frequency The system generates and transmits downlink coherent optical signals; the uplink coherent receiving module receives uplink coherent optical signals transmitted by each ONU through the fiber optic link, completing the initial reception and photoelectric conversion of the optical signals; the uplink signal demodulation module is used to demodulate the signals based on the preset uplink optical carrier frequency of each ONU. The received mixed uplink signal is demodulated and the uplink data of each ONU is extracted without the need for additional frequency offset estimation and correction.

[0030] The ONU side includes a downlink coherent receiver module, a frequency offset estimation module, a local laser compensation control module, an uplink carrier generation module, and an uplink signal transmission module. The downlink coherent receiver module receives the downlink coherent optical signal from the OLT and performs mixing, photoelectric conversion, and analog-to-digital conversion using a local oscillator. The frequency offset estimation module uses digital signal processing algorithms to estimate the frequency offset between the OLT's transmitting source and the ONU's local laser. or equivalent wavelength difference The local laser compensation control module performs initial frequency compensation on the local tunable laser based on the frequency offset, achieving precise locking with the downlink wavelength of the OLT. The uplink carrier generation module, while keeping the output wavelength of the local laser constant, encodes the uplink data electrical signal to construct the target uplink optical carrier and loads it onto the preset uplink optical carrier frequency. The uplink coherent optical signal is generated; the uplink signal transmission module is used to transmit the generated uplink coherent optical signal to the OLT through the optical fiber link.

[0031] Technical effect

[0032] This invention provides an uplink and downlink wavelength locking method for TFDM-PON, which uses the downlink signal as a frequency reference to achieve frequency alignment between ONUs and OLTs without requiring additional wavelength monitoring hardware. It constructs the target uplink carrier through single-sideband modulation and other methods, rather than directly tuning the laser, significantly improving the speed and stability of uplink wavelength generation. The uplink carrier frequency of each ONU is predictable, eliminating the need for a large-scale frequency offset search on the OLT side and reducing DSP complexity. It is suitable for burst-mode access scenarios, improving system stability when multiple ONUs are connected concurrently. It also has good scalability and is suitable for TFDM-PON architectures.

[0033] The following will further explain the concept, specific structure, and technical effects of the present invention in conjunction with the accompanying drawings, so as to fully understand the purpose, features, and effects of the present invention. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of a TFDM-PON system structure according to a preferred embodiment of the present invention;

[0035] Figure 2 This is a schematic diagram of the internal structure of an ONU according to a preferred embodiment of the present invention;

[0036] Figure 3 This is a schematic diagram of the internal structure of an OLT according to a preferred embodiment of the present invention;

[0037] Figure 4This is a flowchart of an uplink / downlink wavelength locking method according to a preferred embodiment of the present invention;

[0038] Figure 5 This is a fourth power spectrum analysis result diagram of an ONU terminal according to a preferred embodiment of the present invention;

[0039] Figure 6 This is a preferred embodiment of the ONU-side LO frequency offset correction received signal constellation diagram;

[0040] Figure 7 This is a constellation diagram of the received signal after ONU-side LO frequency offset correction according to a preferred embodiment of the present invention;

[0041] Figure 8 This is a preferred embodiment of the received signal constellation diagram for frequency offset estimation at the OLT end of the present invention;

[0042] Figure 9 This is a constellation diagram after carrier phase recovery at the OLT end, which is a preferred embodiment of the present invention. Detailed Implementation

[0043] To make the technical problems, solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0044] In the following description, specific details, such as particular internal procedures and techniques, are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of the invention. However, those skilled in the art will appreciate that the invention may be practiced in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods have been omitted so as not to obscure the description of the invention with unnecessary detail.

[0045] like Figure 1 As shown, this embodiment of the invention provides a TFDM-PON system, which includes an optical line terminal (OLT) and multiple optical network units (ONUs) connected to the OLT via a passive optical distribution network. The OLT is configured to transmit downlink coherent optical signals to each ONU and receive uplink coherent optical signals from each ONU; the ONUs are configured to receive downlink coherent optical signals and perform frequency alignment and uplink carrier construction based on the downlink signals, thereby transmitting uplink coherent optical signals.

[0046] like Figure 2As shown, the ONU side includes: a downlink coherent receiving module for receiving downlink coherent optical signals from the OLT and performing photoelectric conversion and analog-to-digital conversion; a frequency offset estimation module for processing the downlink digital signal and estimating the frequency offset between the OLT's transmitting source and the ONU's local laser; a local laser compensation control module for performing initial frequency compensation on the local tunable laser based on the frequency offset; an uplink carrier generation module for constructing a target uplink optical carrier while keeping the output wavelength of the local laser constant; and an uplink signal transmission module for transmitting uplink coherent optical signals.

[0047] like Figure 3 As shown, the OLT side includes: a downlink signal transmission module, used to transmit signals at a fixed frequency. The system generates and transmits downlink coherent optical signals; the uplink coherent receiving module receives uplink coherent optical signals transmitted by each ONU through the fiber optic link, completing the initial reception and photoelectric conversion of the optical signals; the uplink signal demodulation module is used to demodulate the signals based on the preset uplink optical carrier frequency of each ONU. The received mixed uplink signal is demodulated and the uplink data of each ONU is extracted without the need for additional frequency offset estimation and correction.

[0048] like Figure 4 As shown, this invention provides an uplink and downlink wavelength locking method in a TFDM-PON system. This method is based on a bidirectional communication architecture between the OLT and ONU, and is implemented through three stages: downlink wavelength difference estimation and compensation on the ONU side, uplink signal generation and transmission, and OLT-side reception and demodulation. The specific steps are as follows:

[0049] (1) Phase 1: Downlink wavelength difference estimation and compensation (ONU side)

[0050] In this stage, the ONU receives and processes the downlink coherent optical signal, estimates the frequency offset between the OLT transmitting source and the ONU local laser, and compensates for the frequency of the local laser to achieve initial locking with the downlink wavelength of the OLT.

[0051] Step S101: The transmitter of the optical line terminal uses a frequency of The optical carrier transmits downlink coherent optical signals. Without considering polarization multiplexing (PM), the downlink coherent optical signal of the OLT is represented as:

[0052]

[0053] in For the amplitude information of the modulated signal, This refers to the phase information of the modulated signal. The optical network unit (ONU) receives the downlink coherent optical signal from the OLT and uses its local laser (LO) as the local oscillator source. Based on the coherent receiving structure, it performs photoelectric conversion and analog-to-digital conversion on the downlink coherent optical signal to obtain the corresponding downlink digital signal. The output optical field of the ONU's local laser is:

[0054]

[0055] The baseband electrical signal obtained after coherent reception and (complex signal form) This can be represented as:

[0056]

[0057]

[0058] in For receiver noise, This represents the frequency offset.

[0059] Step S102: The ONU performs digital signal processing on the downlink digital signal and uses a carrier frequency offset estimation algorithm to determine the frequency of the OLT's transmitting light source. With ONU local laser frequency The frequency offset between the two is estimated to obtain the frequency offset between them. or equivalent wavelength difference The carrier frequency offset estimation algorithm may include, but is not limited to: the FFT algorithm based on fourth power spectrum analysis, the frequency offset estimation algorithm based on pilots, and the correlation algorithm based on training sequences.

[0060] Step S103: ONU based on frequency offset The local tunable laser is initially compensated and adjusted so that the adjusted output frequency satisfies:

[0061]

[0062] in, This indicates the compensated output frequency of the local laser.

[0063] Step S104: Through the above compensation adjustment, the local laser output frequency of the ONU is initially aligned with the downlink optical carrier frequency of the OLT, so that the ONU can receive the downlink coherent signal with a smaller frequency deviation, reduce the complexity of the subsequent digital signal processing module, and improve the stability of downlink reception.

[0064] (2) Phase 2: Uplink signal generation and transmission phase (ONU side)

[0065] This stage, based on the local laser source locked in stage one, generates a spectrum-controllable uplink coherent optical signal, enabling the orderly arrangement of the uplink signals of multiple ONUs and avoiding interference.

[0066] Step S201: After the compensation adjustment in Phase 1, the ONU obtains a local laser precisely aligned with the downlink carrier frequency of the OLT, with an output frequency of... At this time, the local laser frequency It can be represented as:

[0067]

[0068] in, It is the residual frequency offset after locking (usually very small, on the order of MHz, or even close to zero).

[0069] Step S202: The ONU does not directly perform further physical wavelength tuning on the local laser. Instead, it uses methods such as Single Sideband (SSB) modulation to frequency-shift the output light of the local laser, loading the uplink data signal onto the target uplink optical carrier frequency. This allows for the construction of an uplink coherent optical signal at the target's uplink wavelength. It is assumed that the center frequency of the uplink is offset relative to the local laser. That is, uplink optical carrier frequency for

[0070]

[0071] in, The frequency shift is pre-allocated. Therefore, a complex electrical signal is constructed:

[0072]

[0073] in This is baseband data. It is decomposed into real and imaginary parts:

[0074]

[0075]

[0076] Will and The I-arm and Q-arm of the IQ modulator of the ONU are respectively loaded. The output optical field of the IQ modulator is:

[0077]

[0078] As can be seen from the formula, the center frequency of the output optical signal is precisely equal to... It also carries data By pre-allocating different target uplink carrier frequencies to different ONUs. This allows the uplink signals of multiple ONUs to be arranged in a non-overlapping manner on the spectrum, thereby realizing frequency-based multiple access.

[0079] Step S203: The ONU sends the generated uplink coherent optical signal to the OLT to realize the transmission of the uplink signal.

[0080] (3) Phase 3: OLT side receiving and demodulating

[0081] This stage utilizes the predictability of the uplink optical carrier frequency to simplify the signal processing flow on the OLT side and achieve efficient demodulation.

[0082] Step S301: The OLT receives uplink coherent optical signals from each ONU. After multiplexing by the optical distribution network (ODN), the input optical field to the OLT's coherent receiver is:

[0083]

[0084] in, It is the first The amplitude of the uplink signal of each ONU (including data modulation information, such as amplitude changes in QPSK / 16QAM). It is the first The uplink carrier frequency of each ONU is determined in the second stage: ; It is the first The phase of each ONU uplink signal (including data phase and possible residual phase noise).

[0085] Because the first phase ensured (Locking the OLT downlink frequency), and the second phase It is pre-allocated and known by the OLT, therefore:

[0086]

[0087] For OLT, It is the frequency of the downlink laser emitted by the OLT itself, and it is a precisely known internal parameter. It was OLT that gave the first The frequency shift allocated to each ONU is stored in the OLT's scheduling table. Therefore, for the OLT receiver, the uplink carrier frequency of each ONU is a predictable and stable frequency.

[0088] Step S302: The digital signal processing unit of the OLT performs split demodulation on the received mixed uplink signal based on the known uplink carrier frequencies of each ONU, and extracts the uplink data of each ONU in sequence. Compared with the traditional method, no additional frequency offset estimation and correction is required, which greatly reduces the complexity of the receiving processing on the OLT side and improves the demodulation efficiency and accuracy.

[0089] Tunable laser at the ONU end

[0090] A variety of commercial or custom-made tunable light sources can be used, including but not limited to the following typical types and their tuning mechanisms:

[0091] Tunable distributed feedback (DFB) laser arrays: By integrating multiple DFB lasers with different center wavelengths and coordinating with temperature control, wavelength tuning can be achieved. The typical tuning range is 10-15 nm, with tuning accuracy within ±0.02 nm. Wavelength fine-tuning is mainly achieved by selecting and switching different lasers and adjusting their injection current.

[0092] Sampled grating distributed Bragg reflector (SG-DBR) lasers: By adjusting the injection current (e.g., 0 mA to 50 mA) in the front and rear grating regions, the refractive index of the grating is changed, thereby achieving wide-range wavelength tuning. Its tuning range typically covers 40-50 nm or even wider, and mode-hopping tuning speeds can reach the nanosecond level, making it suitable for fast wavelength switching scenarios.

[0093] External cavity lasers (ECLs): High-precision, narrow-linewidth wavelength tuning is achieved by mechanically or through MEMS adjusting the reflective structure of the external cavity (such as the angle of a diffraction grating or the position of a filter), i.e., changing the effective cavity length. Their tuning range can cover the C-band or L-band (typically > 40 nm), and the linewidth can usually be controlled below 100 kHz.

[0094] MEMS tunable vertical-cavity surface-emitting lasers (VCSELs): By changing the position of the suspended mirror (i.e., adjusting the cavity length) through a MEMS structure, single-mode, low-power wavelength tuning can be achieved. Its tuning range is typically 20-30 nm, and it has the advantages of high modulation rate and low power consumption.

[0095] Furthermore, the wavelength locking and fine-tuning of the aforementioned lasers can be supplemented by temperature control. By changing the operating temperature of the laser chip (typically ranging from 15°C to 45°C), continuous and precise wavelength tuning can be achieved by utilizing the temperature-dependent refractive index of the material, with a tuning factor of approximately 0.1 nm / °C. The various tuning methods mentioned above (current, temperature, cavity length) can be used individually or in combination to achieve rapid, wide-range, and high-precision wavelength locking and switching.

[0096] The following will use specific examples to illustrate an uplink and downlink wavelength locking method for TFDM-PON according to the present invention.

[0097] (1) Phase 1: Downlink wavelength difference estimation and compensation (ONU side)

[0098] Step S101: The OLT transmitter adopts a frequency THz transmits a downlink coherent optical signal; each ONU receives this downlink signal via an optical fiber link, and the initial frequency of the local oscillator... THz signals are coherently received via a 90-degree mixer, converted into analog electrical signals by a photodetector, and then converted into downlink digital signals by an ADC chip.

[0099] Step S102: The ONU uses a pilot-based estimation algorithm to estimate the carrier frequency offset. A pilot sequence is inserted into the downlink signal, and frequency offset features are extracted through correlation calculation to calculate the frequency offset. MHz, corresponding equivalent wavelength difference nm. The carrier frequency offset estimate can also be obtained using an FFT algorithm based on fourth-order spectral analysis. Figure 5 This shows the spectrum of the signal after performing a fourth-power spectral analysis. Through this fourth-power operation, in... A single-frequency signal can be observed at that location.

[0100] Step S103: The ONU controls the local tunable laser to adjust its frequency. After adjustment, the local laser frequency... THz, perfectly aligned with the frequency of the OLT's emitting light source.

[0101] Step S104: After adjustment, the bit error rate of the ONU receiving downlink signals is reduced, the receiving sensitivity is improved, and stable reception is achieved. Figure 6 This is a constellation diagram of the received signal before frequency adjustment of the locally tunable laser. It can be observed that due to... and There is a frequency shift, which causes the constellation points to rotate and diverge. Figure 7 It is a constellation diagram of the received signal after the frequency of the locally tunable laser is adjusted, and the QPSK signal can be accurately demodulated in the end.

[0102] (2) Phase 2: Uplink signal generation and transmission (ONU side)

[0103] Step S201: ONU after locking THz is used as the frequency reference to prepare for generating the uplink signal.

[0104] Step S202: The system allocates uplink optical carrier frequency to the ONU. The ONU encodes the uplink data electrical signal using RS encoding and then modulates it using SSB to load it onto the corresponding input. Uplink coherent optical signals are generated and transmitted to the OLT via the fiber optic link.

[0105] (3) Phase 3: OLT side reception and demodulation

[0106] Step S301: The OLT receives the uplink signal from the ONU through the coherent receiving module;

[0107] Step S302: The DSP unit of the OLT performs split filtering and demodulation based on the known frequency. Figure 8 It is a received signal constellation diagram, which does not require additional frequency offset estimation and correction. Figure 9 This is the signal constellation diagram after carrier phase recovery, proving the feasibility of the scheme.

[0108] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for uplink and downlink wavelength locking in TFDM-PON, characterized in that, This includes the downlink wavelength difference estimation and compensation stage; the uplink carrier construction and transmission stage; and the OLT-side reception and demodulation stage. In the downlink wavelength difference estimation and compensation stage, the frequency offset between the OLT transmitting source and the ONU local laser is estimated by the ONU receiving and processing the downlink coherent optical signal, and the frequency of the local laser is compensated. During the uplink carrier construction and transmission phase, a spectrum-controllable uplink coherent optical signal is generated based on the local laser source locked after the downlink wavelength difference estimation and compensation phase. During the reception and demodulation phase on the OLT side, the OLT receives uplink coherent optical signals from each ONU, taking advantage of the predictability of the uplink optical carrier frequency.

2. The uplink / downlink wavelength locking method for TFDM-PON as described in claim 1, characterized in that, The downlink wavelength difference estimation and compensation stage includes the following steps: Step S101: The transmitter of the optical line terminal uses a frequency of The optical carrier transmits downlink coherent optical signals, and the optical network unit receives the downlink coherent optical signals from the OLT. It uses its local laser as a local oscillator light source and performs photoelectric conversion and analog-to-digital conversion on the downlink coherent optical signals based on the coherent receiving structure to obtain the corresponding downlink digital signals. Step S102: The ONU performs digital signal processing on the downlink digital signal and uses a carrier frequency offset estimation algorithm to determine the frequency of the OLT's transmitting light source. With ONU local laser frequency The frequency offset between the two is estimated to obtain the frequency offset between them. or equivalent wavelength difference ; Step S103: The ONU adjusts the frequency offset accordingly. Initial frequency compensation adjustment is performed on its local tunable laser; Step S104: Through the above compensation adjustment, the local laser output frequency of the ONU is initially aligned with the downlink optical carrier frequency of the OLT.

3. The uplink / downlink wavelength locking method for TFDM-PON as described in claim 2, characterized in that, The carrier frequency offset estimation algorithm may include, but is not limited to: the Fast Fourier Transform (FFT) algorithm based on fourth power spectrum analysis, the frequency offset estimation algorithm based on pilots, and the correlation algorithm based on training sequences.

4. The uplink / downlink wavelength locking method for TFDM-PON as described in claim 2, characterized in that, ONU based on the frequency offset The local tunable laser is initially compensated and adjusted so that the adjusted output frequency satisfies: ， in, This indicates the compensated output frequency of the local laser.

5. The uplink / downlink wavelength locking method for TFDM-PON as described in claim 1, characterized in that, The uplink carrier construction and transmission phase specifically includes the following steps: Step S201: After the downlink wavelength difference estimation and compensation adjustment, the ONU obtains a local laser precisely aligned with the downlink carrier frequency of the OLT, with an output frequency of... ; Step S202: The ONU performs frequency shift modulation on the output light of the local laser, and loads the uplink data signal onto the target uplink optical carrier frequency. This allows for the construction of an uplink coherent optical signal at the uplink wavelength of the target; Step S203: The ONU sends the generated uplink coherent optical signal to the OLT to realize the transmission of the uplink signal.

6. The uplink / downlink wavelength locking method for TFDM-PON as described in claim 5, characterized in that, By pre-allocating different target uplink carrier frequencies to different ONUs This allows the uplink signals of multiple ONUs to be arranged in a non-overlapping manner on the spectrum, thereby realizing frequency-based multiple access.

7. The uplink / downlink wavelength locking method for TFDM-PON as described in claim 1, characterized in that, The OLT-side reception and demodulation phase specifically includes the following steps: Step S301: The OLT receives the uplink signal from the ONU through the coherent receiving module; Step S302: The DSP unit of the OLT performs split filtering and demodulation based on the known frequency.

8. The uplink / downlink wavelength locking method for TFDM-PON as described in claim 7, characterized in that, The digital signal processing unit of the OLT demodulates the received mixed uplink signal based on the known uplink carrier frequencies of each ONU, and extracts the uplink data of each ONU in sequence.

9. A TFDM-PON system, characterized in that, The method for implementing the above uplink and downlink wavelength locking includes an optical line terminal (OLT) and multiple optical network units (ONUs), wherein the OLT and each ONU are connected through an optical distribution network (ODN).

10. A TFDM-PON system as described in claim 9, characterized in that, The OLT side includes a downlink signal transmission module, an uplink coherent reception module, and an uplink signal demodulation module; wherein, the downlink signal transmission module is used to employ a fixed frequency. The system generates and transmits downlink coherent optical signals; the uplink coherent receiving module receives uplink coherent optical signals transmitted by each ONU through the optical fiber link, completing the initial reception and photoelectric conversion of the optical signals; the uplink signal demodulation module is used to demodulate the signals based on a preset uplink optical carrier frequency of each ONU. The received mixed uplink signal is demodulated and the uplink data of each ONU is extracted without the need for additional frequency offset estimation and correction. The ONU side includes a downlink coherent receiving module, a frequency offset estimation module, a local laser compensation control module, an uplink carrier generation module, and an uplink signal transmission module. The downlink coherent receiving module receives the downlink coherent optical signal from the OLT and performs mixing, photoelectric conversion, and analog-to-digital conversion using a local oscillator. The frequency offset estimation module uses digital signal processing algorithms to estimate the frequency offset between the OLT's transmitting source and the ONU's local laser. or equivalent wavelength difference The local laser compensation control module is used to perform initial frequency compensation on the local tunable laser according to the frequency offset, so as to achieve precise locking with the downlink wavelength of the OLT; the uplink carrier generation module is used to construct a target uplink optical carrier by encoding the uplink data electrical signal while keeping the output wavelength of the local laser unchanged, and load it onto the preset uplink optical carrier frequency. The uplink coherent optical signal is generated; the uplink signal transmission module is used to transmit the generated uplink coherent optical signal to the OLT through the optical fiber link.

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