Dispersion pre-compensation method for bipolar pulse amplitude modulation direct detection system

Abstract

The invention discloses a dispersion pre-compensation method for a bipolar pulse amplitude modulation direct detection system, and relates to the technical field of optical fiber communication. The method comprises the following steps: performing triple oversampling on an original bipolar PAM signal to generate an oversampling signal of three samples per symbol; carrying out iterative processing on the signal based on an improved Gilbert-Saxon algorithm to generate a dispersion pre-compensated transmitting signal, wherein the iterative process comprises receiving end amplitude constraint, inverse dispersion transformation, transmitting end real part constraint and positive dispersion transformation; the compensated signal is modulated to an optical carrier and transmitted through an optical fiber; and recovering amplitude and phase difference information at a receiving end by using a specific sample. The limitation of optical fiber dispersion on the performance of a BPAM direct detection system is effectively overcome through pre-compensation, the transmission distance and the energy efficiency of the system are remarkably improved, and the method is particularly suitable for short-distance optical interconnection application.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical fiber communication technology, and in particular to a dispersion pre-compensation method for a bipolar pulse amplitude modulation direct detection system. BACKGROUND

[0002] In the field of optical fiber communication technology, direct detection systems have become the mainstream technology scheme in short-distance optical interconnection applications due to their simple structure and low cost. However, the traditional direct detection system can only detect the intensity information of the optical signal and cannot obtain the phase information, which leads to the fact that it usually adopts unipolar pulse amplitude modulation (UPAM) with lower energy efficiency. In contrast, bipolar pulse amplitude modulation (BPAM) has higher energy efficiency, and under the same bit error rate condition, the required signal-to-noise ratio can be reduced by at most 6dB compared with UPAM. However, the traditional direct detection system cannot directly utilize this advantage of BPAM.

[0003] To overcome the above problems, the prior art proposes a new direct detection scheme for BPAM signals, the core of which is to use a two-fold symbol rate oversampling technology. This scheme extracts amplitude information through sampling points located at the center of the symbol, and at the same time uses oversampling points located between symbols to detect the interference effect of adjacent pulses, thereby extracting the phase difference information between consecutive symbols, successfully realizing the demodulation of the direct detection system for BPAM signals, and obtaining significant optical signal-to-noise ratio gain.

[0004] However, the performance of this BPAM direct detection system is still severely affected by the group velocity dispersion of the optical fiber. The pulse broadening and inter-symbol interference caused by dispersion not only destroys the sampling values used for amplitude judgment, but also seriously disturbs the precise interference waveform relied on by the phase difference judgment, resulting in a sharp deterioration of the required signal-to-noise ratio of the system with increasing transmission distance. What is particularly critical is that the BPAM transmission system essentially belongs to a one-dimensional modulation and one-dimensional reception architecture, and cannot directly apply the traditional two-dimensional dispersion compensation technology, which makes the optical fiber dispersion a core technical bottleneck that restricts the transmission distance and application range of the system. SUMMARY

[0005] Therefore, the embodiments of the present application provide a dispersion pre-compensation method for a bipolar pulse amplitude modulation direct detection system, which is used to solve the problem of system performance deterioration and transmission distance limitation caused by optical fiber dispersion in the prior art.

[0006] To solve the above technical problems, the embodiments of the present application provide a dispersion pre-compensation method for a bipolar pulse amplitude modulation direct detection system, which comprises: The original bipolar pulse amplitude modulation signal is triple oversampled to generate an oversampled signal with three samples per symbol, wherein the three samples include a first sample, a second sample and a third sample; The oversampled signal is iteratively processed to generate a dispersion pre-compensated bipolar transmission signal, wherein the iterative processing includes: (a): taking the target signal as the initial signal of the receiving end; (b): updating the amplitude of the receiving end signal, replacing the amplitudes corresponding to the first sample and the second sample with target amplitudes, introducing an error inversion factor for error feedback, and optimizing the amplitude corresponding to the third sample as a degree of freedom; (c): transforming the updated receiving end signal to the transmitting end through an inverse dispersion transmission function; (d): taking the real part of the transmitting end signal as the transmitting signal of the current iteration; (e): transforming the transmitting end signal back to the receiving end through a dispersion transmission function; repeating steps (b) to (e) until the iteration converges; converting the dispersion pre-compensated bipolar transmission signal into an analog electrical signal, modulating the analog electrical signal onto an optical carrier, and transmitting through an optical fiber; sampling and decoding the received signal after transmission through the optical fiber and square law detection, recovering amplitude information using the sampling value corresponding to the first sample in the received signal, and recovering phase difference information using the sampling value corresponding to the second sample.

[0007] Preferably, the first sample is the value of the original signal, which is used as the main feature of the recovered amplitude at the receiving end; the second sample is the value of the primary oversampling point of the original signal, which is used as the main feature of the recovered phase difference at the receiving end; The third sample is the value of the secondary oversampling point of the original signal, which is used as the degree of freedom for iterative optimization in step S2.

[0008] Preferably, the method of introducing an error inversion factor for error feedback is: ; ; wherein, is the error of the amplitudes of the first sample and the second sample and the corresponding target amplitude ; is the error inversion factor.

[0009] Preferably, the method of recovering amplitude information using the sampling value corresponding to the first sample in the received signal and recovering phase difference information using the sampling value corresponding to the second sample includes: According to the value of the sampling point in the received signal corresponding to the second sample, a preset threshold is compared to determine whether the phase difference between adjacent symbols is 0 or π, if greater than the preset threshold, it is determined that there is no phase difference between adjacent symbols, otherwise it is determined that the phase difference is π; The sampling point in the received signal corresponding to the first sample is combined with the recovered phase difference information to form a bipolar pulse amplitude modulation signal, and the intersymbol interference is eliminated by an equalizer to recover the original signal amplitude.

[0010] Preferably, the preset threshold is optimized according to the specific modulation format used by the system.

[0011] Preferably, the equalizer is a forward feedback equalizer or a decision feedback equalizer.

[0012] The embodiment of the present application also provides a transmitting end device for a bipolar pulse amplitude modulation direct detection system, comprising: A preprocessing module is configured to triple oversample an original bipolar pulse amplitude modulation signal to generate an oversampled signal with three samples per symbol, wherein the three samples include a first sample, a second sample and a third sample. A dispersion pre-compensation module is configured to perform iterative processing on the oversampled signal to generate a dispersion pre-compensated bipolar transmitting signal, wherein the iterative processing includes: (a) taking a target signal as an initial signal of a receiving end; (b) updating the amplitude of the signal of the receiving end, replacing the amplitudes corresponding to the first sample and the second sample with target amplitudes, introducing an error inversion factor for error feedback, and optimizing the amplitude corresponding to the third sample as a degree of freedom; (c) transforming the updated signal of the receiving end to a transmitting end through an inverse dispersion transmission function; (d) taking the real part of the signal of the transmitting end as the transmitting signal of the current iteration; (e) transforming the signal of the transmitting end back to the receiving end through a dispersion transmission function; Steps (b) to (e) are repeated until the iteration converges. A digital-to-analog conversion module is configured to convert the dispersion pre-compensated bipolar transmitting signal into an analog electrical signal. A modulator is configured to modulate the analog electrical signal onto an optical carrier.

[0013] The embodiment of the present application also provides a receiving end device for a bipolar pulse amplitude modulation direct detection system, comprising: A photodetector is configured to perform square law detection on an optical signal transmitted through an optical fiber and convert it into an electrical signal. an oscilloscope for sampling the electrical signal; a decoding module for recovering amplitude information and phase difference information of the transmission data respectively by using the first sample and the second sample of each symbol in the sampling signal.

[0014] Preferably, the decoding module comprises: a phase difference decision unit for comparing the value of the second sample with a preset threshold to determine the phase difference; an equalizer for equalizing a signal composed of the first sample and the determined phase difference to eliminate inter-symbol interference.

[0015] The embodiment of the present application also provides a bipolar pulse amplitude modulation direct detection system, which comprises a transmitting terminal device, a transmission optical fiber and a receiving terminal device.

[0016] From the above technical solution, the present application has the following beneficial effects: The dispersion pre-compensation method for the bipolar pulse amplitude modulation direct detection system provided by the present application successfully provides an effective dispersion compensation solution for the bipolar pulse amplitude modulation direct detection system with one-dimensional modulation and one-dimensional reception by combining the improved Gerdjikov-Saxton algorithm with a specific three-sample oversampling structure. The method innovatively uses three samples to assume different roles of amplitude characteristics, phase difference characteristics and algorithm freedom, and accurately compensates for dispersion damage in advance at the transmitting terminal through an iterative algorithm, so that the problem of inter-symbol interference caused by fiber dispersion and the problem of rapid deterioration of system performance with transmission distance are solved. Compared with the traditional unipolar pulse amplitude modulation system, the present application significantly improves the transmission distance and system performance under the premise of maintaining the low complexity of the direct detection system, and provides a high energy efficiency and high performance transmission solution for short distance optical interconnection applications. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly described below. The features and advantages of the present application can be more clearly understood by referring to the drawings. The drawings are schematic and should not be understood as any limitation on the present application. Those skilled in the art can obtain other drawings according to these drawings without any creative effort. Among them: Figure 1 is a flow chart of a dispersion pre-compensation method for a bipolar pulse amplitude modulation direct detection system provided by the present application; Figure 2 is a schematic diagram of a dispersion pre-compensation scheme based on an improved Gerdjikov-Saxton algorithm in the present application; Figure 3is the DSP of the BPAM direct detection system in the application and the experimental system diagram. DETAILED DESCRIPTION

[0018] To make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0019] The dispersion pre-compensation method, device and system for the bipolar pulse amplitude modulation (BPAM) direct detection system provided by the present application will be described in detail below with reference to the flow chart and system block diagram shown in Figures 1 to 3

[0020] Embodiment one: in order to solve the problems of system performance deterioration and transmission distance limitation caused by fiber dispersion in the prior art, such as Figure 1 As shown in the figure, the present application proposes a dispersion pre-compensation method for a bipolar pulse amplitude modulation direct detection system, which comprises: S1: triple oversampling the original bipolar pulse amplitude modulation signal to generate an oversampled signal with three samples per symbol, wherein the three samples include a first sample, a second sample and a third sample; S2: performing iterative processing on the oversampled signal to generate a bipolar transmission signal after dispersion pre-compensation; S3: converting the bipolar transmission signal after dispersion pre-compensation into an analog electrical signal, modulating the analog electrical signal onto an optical carrier, and transmitting through an optical fiber; S4: sampling and decoding the received signal after fiber transmission and square-law detection, using the sampling value corresponding to the first sample in the received signal to recover the amplitude information, and using the sampling value corresponding to the second sample to recover the phase difference information.

[0021] Specifically, in step S1, the original digital bit sequence is subjected to Gray code mapping to generate an original bipolar PAM signal (i.e. an electrical domain BPAM signal). Subsequently, the original BPAM signal is oversampled and interpolated to increase the sampling rate to 3 samples per symbol, thereby generating an oversampled signal. In this process, the three samples per symbol generated are assigned different functional roles: First sample: its value is the value of the original BPAM signal in the symbol period, which is used as the main feature for recovering the signal amplitude at the receiving end.

[0022] ​The second sample is the value of the primary oversampling point of the original BPAM signal, which is used as the main feature of the recovered inter-symbol phase difference at the receiving end.

[0023] The third sample is the value of the secondary oversampling point of the original BPAM signal, and the amplitude of the sample will be used as a degree of freedom for iterative optimization in the subsequent dispersion pre-compensation algorithm to accelerate the algorithm convergence.

[0024] In step S2, based on the oversampled signal obtained above, an improved Gershgorin-Saunders (GS) algorithm is applied for iterative processing to generate a dispersion pre-compensated bipolar transmission signal. The goal of the iterative processing is to find an optimal transmission signal that can be as close as possible to the expected target signal after the signal is subjected to fiber dispersion damage and square-law detection at the receiving end.

[0025] As shown in FIG. 2, the iterative processing specifically includes the following steps (a) to (e) and is executed in a loop until convergence: Figure 2 (a) Initialization: the target signal expected to be obtained at the receiving end is taken as the initial signal at the receiving end .

[0026] (b) Receiving end constraint: update the amplitude of the current receiving end signal . Specifically: the amplitudes corresponding to the positions of the first sample and the second sample in the receiving end signal are replaced with the corresponding target amplitudes .

[0027] An error feedback factor is introduced to perform error feedback to accelerate convergence. The specific calculation method is as follows: Calculate the error: ; Update the amplitude: .

[0028] For the amplitude corresponding to the position of the third sample in the receiving end signal, it is optimized as a degree of freedom, that is, it is not forcibly constrained, but the value calculated in the iteration process is retained to improve the solving ability and convergence speed of the algorithm.

[0029] (c) Transform to the transmitting end: the receiving end signal whose amplitude has been updated (and whose phase is kept unchanged) is transformed through the inverse dispersion transfer function (ICDTF) to be back-propagated (pre-compensated) to the transmitting end.

[0030] ​​(d) Transmitter constraint: Take the real part of the transmitter signal obtained in step (c) as the signal to be transmitted in the current iteration cycle. This constraint ensures that the finally generated pre-compensated signal is a physically realizable real-valued signal.

[0031] (e) Transformation to the receiver: The constrained transmitter signal is transformed by the dispersive transfer function (CDTF) to simulate the effect after transmission through optical fiber, and then propagated back to the receiver in the forward direction.

[0032] Repeat steps (b) to (e) until the signal change is less than a preset threshold or the preset number of iterations is reached, at which point the algorithm is considered to have converged. Finally, the transmitter signal obtained in step (d) of the last iteration... This refers to the bipolar transmission signal that has undergone dispersion pre-compensation.

[0033] In step S3, the dispersion-compensated digital transmit signal obtained in step S2 is resampled and then converted into an analog electrical signal by an arbitrary waveform generator at the transmitter. Subsequently, a chirp-free Mach-Zehnder modulator (MZM) modulates this analog electrical signal onto an optical carrier. Crucially, to achieve bipolar modulation, the bias point of the MZM must be set to the null point (i.e., the minimum transmission point), and the optical carrier of the MZM is generated by an external cavity laser. In this way, positive and negative levels are mapped to optical pulses, while zero level is mapped to no light or extremely weak light, thus realizing the BPAM format in the optical domain. Finally, the modulated optical signal is injected into a single-mode fiber for transmission, and the received signal is amplified by an optical amplifier at the receiver. Then, an optical filter removes out-of-band optical noise.

[0034] In step S4, at the receiving end, a high-speed photodetector is first used to perform square-law detection on the received optical signal, converting it into an electrical signal. Then, the electrical signal is sampled using a device such as an oscilloscope (the sampling rate is typically matched to or higher than the oversampling rate at the transmitting end). Next, offline digital signal processing is performed on the sampled signal, including clock recovery, synchronization, and other operations.

[0035] Finally, decoding is performed to recover the original information. The specific steps are as follows: Phase difference estimation: The value of the sample point corresponding to the second sample in the received signal is compared with a preset threshold A. If the sample value is greater than the threshold A, it is determined that there is no phase difference between adjacent symbols (phase difference is 0); otherwise, the phase difference is determined to be π. This threshold A can be optimized according to the specific modulation format used by the system.

[0036] Amplitude recovery: the sampling point in the received signal corresponding to the first sample is combined with the recovered phase difference information in the phase difference estimation to re-form a complete BPAM signal. Subsequently, the signal is equalized by an equalizer (such as a forward feedback equalizer or a decision feedback equalizer) to eliminate residual intersymbol interference in the system, and finally the original signal amplitude is recovered.

[0037] Embodiment two: the embodiment provides a transmitting end device for implementing the above method, and the device comprises: A pre-processing module: for performing step S1 of embodiment one, i.e. tripling the oversampling of the original bipolar PAM signal to generate an oversampled signal containing a first sample, a second sample and a third sample.

[0038] A dispersion pre-compensation module: for performing step S2 of embodiment one, i.e. performing iterative processing on the oversampled signal based on the improved GS algorithm to generate a bipolar transmitting signal after dispersion pre-compensation, wherein the iterative processing comprises: (a): taking the target signal as the initial signal of the receiving end; (b): updating the amplitude of the receiving end signal, replacing the amplitudes corresponding to the first sample and the second sample with the target amplitude, introducing an error reversal factor for error feedback, and optimizing the amplitude corresponding to the third sample as a degree of freedom; (c): transforming the updated receiving end signal to the transmitting end through an inverse dispersion transmission function; (d): taking the real part of the transmitting end signal as the transmitting signal of the current iteration; (e): transforming the transmitting end signal back to the receiving end through a dispersion transmission function; repeating steps (b) to (e) until the iteration converges.

[0039] A digital-to-analog conversion module: for converting the bipolar transmitting signal after dispersion pre-compensation into an analog electrical signal.

[0040] A modulator: preferably a non-chirp Mach-Zehnder modulator, for modulating the analog electrical signal onto an optical carrier, wherein the MZM is biased at the Null point to achieve bipolar modulation, and the optical carrier of the MZM is generated by an external cavity laser.

[0041] Embodiment three: the embodiment provides a receiving end device matched with the above transmitting end device, and the device comprises: A photodetector: for square-law detection of the optical signal after transmission through an optical fiber, and converting it into an electrical signal.

[0042] An oscilloscope: for sampling the electrical signal.

[0043] The decoding module is configured to recover amplitude information and phase difference information of the transmitted data by using the first sample and the second sample of each symbol in the sampling signal, respectively. The phase difference decision unit is configured to compare the value of the second sample with a preset threshold to determine the phase difference.

[0044] The equalizer is configured to equalize a signal composed of the first sample and the determined phase difference to eliminate inter-symbol interference.

[0045] Embodiment four provides a bipolar pulse amplitude modulation direct detection system. The system comprises the transmitting end device as described in embodiment two, a transmission optical fiber (single-mode optical fiber) for optical signal transmission, and the receiving end device as described in embodiment three. The system is particularly suitable for low-complexity and high-energy-efficiency communication scenarios for intelligent center-to-center optical interconnection (transmission distance < 80 km).

[0046] From the above technical solutions, the present application has the following beneficial effects: (1) The improved GS algorithm is innovatively applied to the BPAM direct detection system, which provides an effective dispersion compensation means for the one-dimensional modulation and one-dimensional reception architecture, and solves the problem of rapid deterioration of system performance with distance caused by fiber dispersion.

[0047] (2) By designing a three-sample oversampling structure and clearly dividing the functions of each sample (amplitude feature, phase feature, and optimization degree of freedom), the improved GS algorithm can efficiently converge, thereby generating an optimal dispersion pre-compensation signal at the transmitting end.

[0048] (3) A BPAM transmission solution with lower signal-to-noise ratio requirement and higher energy efficiency than traditional UPAM systems is provided for short-distance optical interconnection applications, which significantly improves the transmission distance and performance of the system.

[0049] Those skilled in the art should understand that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) containing computer usable program code.

[0050] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 one or more flow or blocks

[0051] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 one or more flow or blocks Figure 1 The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flow or blocks

[0052] Obviously, the above-mentioned embodiments are only examples for clearly illustrating the present application and are not intended to limit the implementation modes. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, it is not necessary and also impossible to enumerate all the implementation modes. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A dispersion pre-compensation method for a bipolar pulse amplitude modulation direct detection system, characterized in that, include: The original bipolar pulse amplitude modulation signal is oversampled three times to generate an oversampled signal with three samples per symbol, wherein the three samples include a first sample, a second sample, and a third sample. The oversampled signal is iteratively processed to generate a dispersion-precompensated bipolar emission signal, wherein the iterative processing includes: (a): Use the target signal as the initial signal of the receiver; (b): Update the amplitude of the received signal, replace the amplitudes corresponding to the first and second samples with the target amplitude, introduce an error inversion factor for error feedback, and optimize the amplitude corresponding to the third sample as a degree of freedom. (c): Transform the updated receiver signal to the transmitter using the inverse dispersion transfer function; (d): Take the real part of the transmitted signal as the transmitted signal for the current iteration; (e): The transmitting signal is converted back to the receiving signal using a dispersive transfer function; Repeat steps (b) to (e) until the iteration converges; The dispersion-compensated bipolar transmitted signal is converted into an analog electrical signal, the analog electrical signal is modulated onto an optical carrier, and transmitted through an optical fiber. The received signal, after being transmitted through optical fiber and subjected to square law detection, is sampled and decoded. The amplitude information is recovered using the sampled value corresponding to the first sample in the received signal, and the phase difference information is recovered using the sampled value corresponding to the second sample.

2. The dispersion pre-compensation method for a bipolar pulse amplitude modulation direct detection system according to claim 1, characterized in that, The first sample is the value of the original signal, which is used as the main feature for recovering the amplitude at the receiving end; The second sample is the value of the first oversampled point of the original signal, which is used at the receiving end as the main feature for recovering the phase difference; The third sample is the value of the minor oversampling point of the original signal, which serves as the degree of freedom for iterative optimization in step S2.

3. The dispersion pre-compensation method for a bipolar pulse amplitude modulation direct detection system according to claim 1, characterized in that, The method for introducing an error reversal factor for error feedback is as follows: ； ； in, The amplitude of the first sample and the second sample With the corresponding target amplitude The error, This is the error reversal factor.

4. The dispersion pre-compensation method for a bipolar pulse amplitude modulation direct detection system according to claim 1, characterized in that, The step of recovering amplitude information using the sampled values ​​corresponding to the first sample in the received signal, and recovering phase difference information using the sampled values ​​corresponding to the second sample, includes: The value of the sampling point corresponding to the second sample in the received signal is compared with a preset threshold to determine whether the phase difference between adjacent symbols is 0 or π. If it is greater than the preset threshold, it is determined that there is no phase difference between adjacent symbols; otherwise, the phase difference is determined to be π. The sampling points corresponding to the first sample in the received signal are combined with the recovered phase difference information to form a bipolar pulse amplitude modulation signal, and the inter-symbol interference is eliminated by an equalizer to restore the original signal amplitude.

5. The dispersion pre-compensation method for a bipolar pulse amplitude modulation direct detection system according to claim 4, characterized in that, The preset threshold is optimized based on the specific modulation format used by the system.

6. The dispersion pre-compensation method for a direct detection system for bipolar pulse amplitude modulation according to claim 4, wherein the equalizer is a forward feedback equalizer or a decision feedback equalizer.

7. A transmitter device for a bipolar pulse amplitude modulation direct detection system, characterized in that, include: The preprocessing module is used to perform triple oversampling on the original bipolar pulse amplitude modulation signal to generate an oversampled signal with three samples per symbol, wherein the three samples include a first sample, a second sample, and a third sample. A dispersion pre-compensation module is used to iteratively process the oversampled signal to generate a dispersion-pre-compensated bipolar transmission signal, wherein the iterative processing includes: (a): Use the target signal as the initial signal of the receiver; (b): Update the amplitude of the received signal, replace the amplitudes corresponding to the first and second samples with the target amplitude, introduce an error inversion factor for error feedback, and optimize the amplitude corresponding to the third sample as a degree of freedom. (c): Transform the updated receiver signal to the transmitter using the inverse dispersion transfer function; (d): Take the real part of the transmitted signal as the transmitted signal for the current iteration; (e): The transmitting signal is converted back to the receiving signal using a dispersive transfer function; Repeat steps (b) to (e) until the iteration converges; A digital-to-analog converter module is used to convert the dispersion-compensated bipolar transmission signal into an analog electrical signal; A modulator for modulating the analog electrical signal onto an optical carrier.

8. A receiving device for a bipolar pulse amplitude modulation direct detection system, characterized in that, include: A photodetector is used to perform square-law detection on optical signals transmitted through optical fibers and convert them into electrical signals. An oscilloscope is used to sample the electrical signal; The decoding module is used to recover the amplitude information and phase difference information of the transmitted data using the first and second samples of each symbol in the sampled signal, respectively.

9. The receiving device according to claim 8, characterized in that, The decoding module includes: A phase difference decision unit is used to compare the value of the second sample with a preset threshold to determine the phase difference; An equalizer is used to equalize the signal composed of the first sample and the determined phase difference to eliminate inter-symbol interference.

10. A bipolar pulse amplitude modulation direct detection system, characterized in that, It includes the transmitting device as described in claim 7, the transmission optical fiber, and the receiving device as described in claim 8 or 9.

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