Carrier modulation method of optical fiber communication link

Abstract

The invention provides a carrier modulation method for an optical fiber communication link, and relates to the technical field of optical fiber communication. A high-speed data stream to be transmitted is divided into a plurality of low-speed data streams at a sending end, the plurality of low-speed data streams are respectively modulated to a plurality of mutually independent subcarriers, and a signal to be transmitted of each subcarrier is transmitted after a preamble is added to the signal; and modulating and synthesizing the received signals transmitted by the plurality of subcarriers in a coupling orthogonal modulation mode at a receiving end, and performing dispersion compensation and nonlinear compensation on the synthesized total carrier signal. The method is of great significance in improving the performance of an optical fiber communication system, prolonging the transmission distance, increasing the system capacity and reducing the cost.

Description

Technical Field

[0001] The invention provides a carrier modulation method for an optical fiber communication link, and relates to the technical field of optical fiber communication. Background Art

[0002] With the rapid development of international Internet business and communication industry, informatization has greatly promoted the development of world productivity and human society. Fiber optic communication system is a communication system that uses light as carrier, uses extremely pure glass to draw extremely fine optical fiber as transmission medium, and uses light to transmit information through photoelectric conversion.

[0003] like Figure 1 As shown in the figure, the optical fiber communication system includes basic functional modules such as emission, transmission, and reception. The information source converts the user information (voice, image, data, etc.) into the original electrical signal, which is called the baseband signal. The electrical transmitter converts the baseband signal into an electrical signal suitable for channel transmission.

[0004] The electrical signal with information is input into the optical transmitter, which converts it into an optical signal through modulation. The optical carrier is transmitted via optical fiber to a distant optical receiver, and then the information source required by the user is extracted from the carrier through demodulation by the electrical transmitter.

[0005] As Internet traffic continues to grow, fiber-optic communication technology will continue to pursue higher transmission rates, larger system capacities, and longer transmission distances, involving more advanced modulation techniques, greater spectrum utilization, and the development of new optical fibers; at the same time, in high-power and long-distance transmission, the nonlinear effects of optical fibers will become more significant, requiring new technologies and methods to manage and suppress; as networks become more complex and dynamic, management and security issues become more prominent.

[0006] For example, in the prior art, coherent optical communication technology can effectively compensate for the dispersion and nonlinear effects in optical fibers by using coherent detection and digital signal processing (DSP) technology, thereby improving the transmission rate and distance.

[0007] As another example, in the prior art, as the transmission rate increases, the nonlinear effects in the optical fiber become more significant, and new nonlinear compensation technologies are being developed, such as digital post-processing algorithms, which can improve transmission performance and extend transmission distances.

[0008] However, the carrier modulation method of the existing optical fiber communication link still has the following technical problems. With the growth of communication capacity demand, how to further improve spectrum utilization is the focus of continuous research in the field of optical fiber communication; cost and energy efficiency: with the increase of system complexity, how to control costs and improve energy efficiency while ensuring performance is also an issue that needs to be considered in the development of optical fiber communication technology.

[0009] In addition, in long-distance transmission, the nonlinear effects of optical fiber can cause damage to the signal; however, how to achieve effective dispersion compensation under different transmission distances and conditions remains a challenge. Summary of the invention

[0010] In order to solve the above technical problems, the present invention proposes a carrier modulation method for an optical fiber communication link, comprising the following steps:

[0011] At the transmitting end, the high-speed data stream to be transmitted is divided into multiple low-speed data streams, and the multiple low-speed data streams are modulated onto multiple independent subcarriers, and a preamble signal is added to the signal to be transmitted on each subcarrier before transmission;

[0012] At the receiving end, the received signals transmitted by multiple sub-carriers are modulated and synthesized by coupled orthogonal modulation, and dispersion compensation and nonlinear compensation are performed on the synthesized total carrier signal.

[0013] Furthermore, the preamble signal is a copy of a signal added at the beginning of the signal to be transmitted by each subcarrier, and the copy is the signal from the tth time period to the end time period of the signal to be transmitted by each subcarrier, and the tth time period of the signal to be transmitted by the subcarrier is the starting time period of the preamble signal.

[0014] Furthermore, the confirmation method of the starting time period of the preamble signal includes:

[0015] (1) F(k) represents the signal sequence of the kth time period of the signal to be transmitted by the subcarrier, where k is a natural number, k = 1, 2, 3…T; T represents the total number of time periods for the signal to be transmitted by the subcarrier;

[0016] The phase difference P(k) between the signal sequence in the kth time period and the signal sequence in the (k+Δk)th time period is calculated.

[0017] (2) Calculate the amplitude ratio M(k) of the signal sequence in the kth time period and the signal sequence in the (k+Δk)th time period;

[0018] (3) M(k) and P(k) are used to determine whether the kth sampling point belongs to the starting time period of the preceding signal. If M(k) is less than the amplitude ratio threshold T1 and P(k) is less than the phase difference threshold T2, then the current time period k is the starting time period of the preceding signal.

[0019] Furthermore, the method of coupling orthogonal modulation is to modulate the signal by coupling the modulation end, specifically including:

[0020] The subcarrier signal S(I) received in the Ith time period includes the phase of the same-direction branch optical signal Phase of the orthogonal branch optical signal

[0021] Optical signal of the same-direction branch Z The optical signal of the orthogonal branch J Respectively expressed as:

[0022]

[0023] in, are the input modulation signal voltages of the Z and J branches respectively; are the DC bias signal voltages of the Z and J branches respectively; V Z 、V Q They are the half-wave voltages of the Z and J branch modulators respectively.

[0024] Furthermore, the coupled modulation end outputs each subcarrier modulation signal as:

[0025]

[0026] In the Ith period, the final synthesized signal H(I) of the N subcarrier modulation signals is expressed as:

[0027]

[0028] Among them, F n (I) is the nth subcarrier modulation signal in the Ith time period, N is the number of subcarriers, ω n is the frequency of the nth subcarrier modulation signal.

[0029] Furthermore, the signal finally synthesized by the N subcarrier modulation signals is compensated alternately by multiple nonlinear compensation modules and multiple dispersion compensation modules, so as to correct the signal changes caused by nonlinear effects during transmission and the signal distortion caused by optical fiber dispersion.

[0030] Furthermore, the transfer function of the nonlinear effect of the synthesized total carrier signal H(I) in the time domain is subjected to a fractional Fourier transform FRFT of order P. P , the transfer function of the nonlinear effect of the synthesized total carrier signal H(I) in the fractional Fourier domain is obtained

[0031]

[0032] Where: h NL (I) is the time domain transfer function of the nonlinear effect, i is the imaginary unit, α is the loss coefficient of the optical fiber, γ is the nonlinear coefficient of the optical fiber, |H(I)| is the signal power amplitude, and h is the step size of the compensation algorithm.

[0033] Furthermore, the transfer function of the dispersion of the total carrier signal after nonlinear compensation in the frequency domain is subjected to a fractional Fourier transform of order 1-p to obtain the transfer function of the dispersion of the total carrier signal after nonlinear compensation in the fractional Fourier domain:

[0034]

[0035] Where: H CD (ω) is the transfer function of dispersion in the frequency domain, i is the imaginary unit, β is the group velocity dispersion of the fiber, ω is the angular frequency of the total carrier signal, and h is the step size of the compensation algorithm; is the transfer function of dispersion in the P-order fractional Fourier domain, FRFT 1-P Represents the fractional Fourier transform of order 1-P.

[0036] Furthermore, the transfer function of the dispersion in the P-order fractional Fourier domain is convolved with the transfer function of the nonlinear effect in the P-order fractional Fourier domain to obtain the compensated total signal X(I,ω). The convolution process can be implemented by the following formula:

[0037]

[0038] Suppose the signal after m-times dispersion compensation and m-times nonlinear compensation is expressed as X (m) (I,ω).

[0039] Compared with the prior art, the present invention has the following beneficial technical effects:

[0040] (1) The carrier modulation method of the optical fiber communication link of the present invention divides the high-speed data stream to be transmitted into multiple low-speed data streams at the transmitting end, and modulates the multiple low-speed data streams onto multiple independent subcarriers. Through the subcarrier multiplexing technology, more data can be transmitted in limited spectrum resources because each subcarrier can be modulated independently, thereby improving the utilization efficiency of the spectrum.

[0041] (2) The carrier modulation method of the optical fiber communication link of the present invention adds a preamble signal to the signal to be transmitted by each subcarrier before transmission; each subcarrier can be modulated and demodulated independently, so that even if a subcarrier is interfered, it will not affect other subcarriers, thereby improving the reliability of the entire system.

[0042] (3) The carrier modulation method of the optical fiber communication link of the present invention modulates and synthesizes the received signals transmitted by multiple subcarriers by coupled orthogonal modulation at the receiving end. It can effectively reduce or eliminate inter-symbol interference and is suitable for high-speed data transmission, such as 100Gbit / s high-speed optical access.

[0043] (4) The carrier modulation method of the optical fiber communication link of the present invention performs dispersion compensation and nonlinear compensation on the synthesized total carrier signal. It can effectively suppress the signal damage caused by the nonlinear effects of optical fiber (such as self-phase modulation, cross-phase modulation and four-wave mixing, etc.), thereby improving the maximum capacity of the optical fiber transmission network. It improves the Q value and bit error rate (BER) performance of the system, so that the system can still maintain high performance at higher fiber input power. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative labor.

[0045] Figure 1 It is a schematic diagram of the structure of an optical fiber communication system in the prior art;

[0046] Figure 2 It is a flow chart of a carrier modulation method of an optical fiber communication link of the present invention;

[0047] Figure 3 Schematic diagram of sine wave and cosine wave with a phase difference of 90 degrees;

[0048] Figure 4 Schematic diagram of different dispersion compensation performance corresponding to different incident light powers. DETAILED DESCRIPTION

[0049] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below by coupling the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0050] In the drawings of the specific embodiments of the present invention, in order to better and more clearly describe the working principles of the various components in the system, the connection relationship of the various parts in the device is shown, which only clearly distinguishes the relative position relationship between the various components, and cannot constitute a limitation on the signal transmission direction, connection sequence and structural size, size and shape of each part within the component or structure.

[0051] like Figure 2 FIG. 1 is a flow chart of a carrier modulation method for an optical fiber communication link of the present invention, and the carrier modulation method comprises the following steps:

[0052] First, at the transmitting end, a high-speed data stream to be transmitted is divided into multiple low-speed data streams, and the multiple low-speed data streams are modulated onto multiple independent subcarriers respectively.

[0053] Subcarrier is the basic transmission unit, each subcarrier carries a part of information and is transmitted through modulation technology. For each subcarrier, the signal on each subcarrier is modulated independently, so they can make full use of their own bandwidth resources, and can also choose the modulation method that best suits their transmission characteristics (such as QPSK, 16-QAM, etc.) to achieve the highest baud rate.

[0054] During the specific modulation process, the following parameters need to be confirmed:

[0055] Baud rate: Baud rate is also called modulation rate or symbol rate, which indicates the number of code elements (or symbols) transmitted through the subcarrier channel per unit time. Each subcarrier can be regarded as an independent transmission channel, and the signal on each channel is transmitted at a certain baud rate.

[0056] Subcarrier spacing: Subcarrier spacing determines the density and number of subcarriers. A smaller subcarrier spacing means that more subcarrier channels can be accommodated within the same bandwidth, but it may also cause interference between carriers.

[0057] Increased disturbance.

[0058] Subcarrier length: The subcarrier length determines how many code elements can be transmitted in one symbol period. A longer symbol length can provide better resistance to multipath interference, but it will also reduce the data transmission rate. Therefore, when designing, it is necessary to make a trade-off between symbol length and data transmission rate.

[0059] Modulation method: Different modulation methods affect the number of bits carried by each symbol.

[0060] For example, QPSK (quadrature phase shift keying) carries 2 bits of information per symbol, while 16-QAM (hexadecimal quadrature amplitude modulation) carries 4 bits of information per symbol. Therefore, at the same baud rate, using a higher-order modulation method can increase the data transmission rate.

[0061] Secondly, a preamble signal is added to the signal to be transmitted on each subcarrier before transmission.

[0062] The preamble signal is to add a copy of a signal at the beginning of the signal to be transmitted by the subcarrier, and the copy of this signal is selected from the signal from the tth time period to the last time period of the signal to be transmitted by each subcarrier. Therefore, it is necessary to determine the tth time period of the signal to be transmitted by the subcarrier, that is, the time period where the preamble signal starts.

[0063] This embodiment identifies the starting time period of the preamble signal by combining the changes in the amplitude ratio and the phase difference, and specifically includes the following steps:

[0064] (1) F(k) represents the signal sequence of the kth time period of the signal to be transmitted by the subcarrier, where k is a natural number, k = 1, 2, 3…T; T represents the total number of time periods for the signal to be transmitted by the subcarrier;

[0065] The phase difference P(k) between the signal sequence in the kth time period and the signal sequence in the (k+Δk)th time period is calculated.

[0066] (2) Calculate the amplitude ratio M(k) of the signal sequence in the kth time period and the signal sequence in the (k+Δk)th time period;

[0067] (3) M(k) and P(k) are used to determine whether the kth sampling point belongs to the starting time period of the preceding signal. If M(k) is less than the amplitude ratio threshold T1 and P(k) is less than the phase difference threshold T2, then the current time period k is the starting time period of the preceding signal.

[0068] Preferably, the phase difference threshold T2 and the amplitude ratio threshold T1 are adjusted according to actual signal characteristics and requirements of the optical fiber communication link.

[0069] Each subcarrier channel transmits the signal to be transmitted after adding the preamble signal. At the receiving end, the received signals transmitted by multiple subcarriers are modulated and synthesized by coupled orthogonal modulation. Here, the number of received subcarrier signals is assumed to be N.

[0070] It needs to be explained that the method of coupled orthogonal modulation is to transmit data by changing the amplitude of two orthogonal waves. Figure 3 As shown, the two orthogonal waves are a sine wave and a cosine wave with a phase difference of 90 degrees. In coupled orthogonal modulation mapping, the data signal is mapped onto a complex plane to form a complex modulation symbol, and then the real part and the imaginary part of the complex modulation symbol are modulated onto the two orthogonal waves respectively.

[0071] The coupled orthogonal modulation method can simultaneously use the amplitude and phase of the signal wave to transmit information bits, and represent the transmitted information by changing the amplitude of the pulse. Therefore, under the same bandwidth, the coupled orthogonal modulation method can achieve a higher data transmission rate than AM and PM. The more channels there are in the coupled orthogonal modulation method, the more information each symbol can transmit, but at the same time, the signal-to-noise ratio requirement is also higher, so it is more susceptible to noise. Coupled orthogonal modulation can transmit more data within the same bandwidth, thereby increasing the data transmission rate.

[0072] In a preferred embodiment, the coupled orthogonal modulation is implemented through a coupled modulation end. The coupled modulation end serves the optical fiber data transmission system better with its high spectral efficiency performance, thereby greatly improving the spectrum utilization, effectively combating frequency selective fading and narrowband interference, and improving the data transmission rate and the signal's ability to resist inter-code interference, and has been widely used in current optical fiber communication systems.

[0073] Specifically, the subcarrier signal S(I) received in the Ith time period includes the phase of the same-direction branch optical signal Phase of the orthogonal branch optical signal like Figure 3 As shown, it is.

[0074] Optical signal of the same-direction branch Z The optical signal of the orthogonal branch J Respectively expressed as:

[0075]

[0076] in, are the input modulation signal voltages of the Z and J branches respectively; are the DC bias signal voltages of the Z and J branches respectively; V Z 、V Q They are the half-wave voltages of the Z and J branch modulators respectively.

[0077] The coupled modulation end outputs each subcarrier modulation signal F(I) as:

[0078]

[0079] In the Ith period, the signal H(I) finally synthesized by the N subcarrier modulation signals is expressed as follows:

[0080]

[0081] Since a preamble signal is added to the signal to be transmitted on each subcarrier, the time interval of the preamble signal is [t, T], so the value of I is [0, 2T-t].

[0082] Among them, F n (I) is the nth subcarrier modulation signal in the Ith time period, N is the number of subcarriers, ω n is the frequency of the nth subcarrier modulation signal.

[0083] Finally, the synthesized total carrier signal is subjected to dispersion compensation and nonlinear compensation.

[0084] In order to improve the security during signal transmission, it is further necessary to compensate the synthesized total carrier signal at the receiving end.

[0085] Intra-subcarrier channel cross-phase modulation and intra-channel four-wave mixing are two main nonlinear disturbance terms that can have a significant impact on signal transmission quality.

[0086] In a preferred embodiment, the receiving end has a plurality of nonlinear compensation modules and a plurality of dispersion compensation modules, and the synthesized total carrier signal passes through the nonlinear compensation modules and the dispersion compensation modules alternately.

[0087] The function of the nonlinear compensation module is to correct the changes in the signal caused by nonlinear effects during transmission. The dispersion compensation module is used to correct the signal distortion caused by fiber dispersion. These compensation modules can be physical devices or digital signal processing algorithms.

[0088] Specifically, the nonlinear compensation module has a nonlinear compensation algorithm, and the process of performing a nonlinear compensation includes the following steps:

[0089] The transfer function of the nonlinear effect of the synthesized total carrier signal H(I) in the time domain is subjected to a fractional Fourier transform FRFT of order P P , the transfer function of the nonlinear effect of the synthesized total carrier signal H(I) in the fractional Fourier domain is obtained

[0090]

[0091] Where: h NL (I) is the time domain transfer function of the nonlinear effect, i is the imaginary unit, α is the loss coefficient of the optical fiber, γ is the nonlinear coefficient of the optical fiber, |H(I)| is the signal power amplitude, and h is the step size of the compensation algorithm.

[0092] The total carrier signal after nonlinear compensation enters the dispersion compensation module and performs a dispersion compensation algorithm.

[0093] Specifically, a dispersion compensation process includes the following steps:

[0094] The transfer function of the dispersion of the total carrier signal after nonlinear compensation in the frequency domain is transformed by a fractional Fourier transform of order 1-p to obtain the transfer function of the dispersion of the total carrier signal after nonlinear compensation in the fractional Fourier domain:

[0095]

[0096] Where: H CD (ω) is the transfer function of dispersion in the frequency domain, i is the imaginary unit, β is the group velocity dispersion of the fiber, ω is the angular frequency of the total carrier signal, and h is the step size of the compensation algorithm;

[0097] is the transfer function of dispersion in the P-order fractional Fourier domain, FRFT 1-P Represents the fractional Fourier transform of order 1-P.

[0098] The transfer function of dispersion in the P-order fractional Fourier domain is convolved with the transfer function of nonlinear effect in the P-order fractional Fourier domain to obtain the compensated total signal X(I,ω). The convolution process can be achieved by the following formula:

[0099]

[0100] Suppose the signal after m-times dispersion compensation and m-times nonlinear compensation is expressed as X (m) (I,ω).

[0101] In a preferred embodiment, the dispersion compensation module can also perform pre-compensation at the transmitting end as needed. The dispersion compensation module can compensate for signals of multiple wavelengths and has the characteristics of low insertion loss, low polarization mode dispersion and low polarization-dependent loss. Figure 4 As shown in the figure, different dispersion compensation performances corresponding to different incident optical powers are demonstrated. The optical power is set to scan in the range of -10 to 10dBm. Under three different dispersion compensation times at a transmission rate of 10Gbit / s, the Q value changes with the change of the input optical power.

[0102] The combination of nonlinear compensation and dispersion compensation can significantly improve the transmission quality of signals, especially in high-speed optical fiber communication systems. Through these compensations, inter-symbol interference can be reduced and the integrity and reliability of signals can be improved.

[0103] It should be understood that the above is only an example and does not constitute any limitation on the technical solution of the present invention. In specific applications, technicians in this field can make settings as needed, and the present invention does not limit this.

[0104] It should be understood that, although the various steps in the flowchart in the embodiment of the present application are displayed in sequence according to the indication of the arrows, these steps are not necessarily performed in sequence according to the order indicated by the arrows. Unless there is a clear explanation in this article, the execution of these steps does not have a strict order restriction, and it can be performed in other orders. Moreover, at least a portion of the steps in the figure may include a plurality of sub-steps or a plurality of stages, and these sub-steps or stages are not necessarily performed at the same time, but can be performed at different times, and their execution order is not necessarily performed in sequence, but can be performed in turn or alternately with other steps or at least a portion of the sub-steps or stages of other steps.

[0105] It should be noted that the workflow described above is merely illustrative and does not limit the scope of protection of the present invention. In practical applications, technicians in this field can select part or all of them according to actual needs to achieve the purpose of the present embodiment, and no limitation is made here.

[0106] In addition, it should be noted that, in this article, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or system including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or system. In the absence of further restrictions, an element defined by the sentence "comprises a ..." does not exclude the existence of other identical elements in the process, method, article or system including the element.

[0107] The serial numbers of the above embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0108] The above are only preferred embodiments of the present invention, and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A carrier modulation method for an optical fiber communication link, characterized in that: The steps include: At the transmitting end, the high-speed data stream to be transmitted is divided into multiple low-speed data streams, and the multiple low-speed data streams are modulated onto multiple independent subcarriers, and a preamble signal is added to the signal to be transmitted on each subcarrier before transmission; At the receiving end, the received signals transmitted by multiple sub-carriers are modulated and synthesized by coupled orthogonal modulation, and dispersion compensation and nonlinear compensation are performed on the synthesized total carrier signal.

2. The carrier modulation method of the optical fiber communication link according to claim 1, characterized in that: The preamble signal is a copy of a signal added at the beginning of the signal to be transmitted by each subcarrier, and the copy is the signal from the tth time period to the end time period of the signal to be transmitted by each subcarrier, and the tth time period of the signal to be transmitted by the subcarrier is the starting time period of the preamble signal.

3. The carrier modulation method of the optical fiber communication link according to claim 2, characterized in that: The confirmation methods of the starting period of the preamble signal include: (1) F(k) represents the signal sequence of the kth time period of the signal to be transmitted by the subcarrier, where k is a natural number, k = 1, 2, 3…T; T represents the total number of time periods for the signal to be transmitted by the subcarrier; The phase difference P(k) between the signal sequence in the kth time period and the signal sequence in the (k+Δk)th time period is calculated. (2) Calculate the amplitude ratio M(k) of the signal sequence in the kth time period and the signal sequence in the (k+Δk)th time period; (3) M(k) and P(k) are used to determine whether the kth sampling point belongs to the starting time period of the preceding signal. If M(k) is less than the amplitude ratio threshold T1 and P(k) is less than the phase difference threshold T2, then the current time period k is the starting time period of the preceding signal.

4. The carrier modulation method of the optical fiber communication link according to claim 1, characterized in that: The coupled orthogonal modulation method is to modulate the signal through the coupled modulation end, specifically including: The subcarrier signal S(I) received in the Ith time period includes the phase of the same-direction branch optical signal Phase of the orthogonal branch optical signal Optical signal of the same-direction branch Z The optical signal of the orthogonal branch J Respectively expressed as: in, are the input modulation signal voltages of the Z and J branches respectively; are the DC bias signal voltages of the Z and J branches respectively; V Z 、V Q They are the half-wave voltages of the Z and J branch modulators respectively.

5. The carrier modulation method of the optical fiber communication link according to claim 4, characterized in that: The coupled modulation end outputs each subcarrier modulation signal as: In the Ith period, the final synthesized signal H(I) of the N subcarrier modulation signals is expressed as: Among them, F n (I) is the nth subcarrier modulation signal in the Ith time period, N is the number of subcarriers, ω n is the frequency of the nth subcarrier modulation signal.

6. The carrier modulation method of the optical fiber communication link according to claim 5, characterized in that: The final synthesized signal of the N subcarrier modulation signals is alternately compensated by multiple nonlinear compensation modules and multiple dispersion compensation modules to correct the changes in the signal caused by nonlinear effects during transmission and the signal distortion caused by fiber dispersion.

7. The carrier modulation method of the optical fiber communication link according to claim 6, characterized in that: The transfer function of the nonlinear effect of the synthesized total carrier signal H(I) in the time domain is subjected to a fractional Fourier transform FRFT of order P P , the transfer function of the nonlinear effect of the synthesized total carrier signal H(I) in the fractional Fourier domain is obtained Where: h NL (I) is the time domain transfer function of the nonlinear effect, i is the imaginary unit, α is the loss coefficient of the optical fiber, γ is the nonlinear coefficient of the optical fiber, |H(I)| is the signal power amplitude, and h is the step size of the compensation algorithm.

8. The carrier modulation method of the optical fiber communication link according to claim 7, characterized in that: The transfer function of the dispersion of the total carrier signal after nonlinear compensation in the frequency domain is transformed by a fractional Fourier transform of order 1-p to obtain the transfer function of the dispersion of the total carrier signal after nonlinear compensation in the fractional Fourier domain: Where: H CD (ω) is the transfer function of dispersion in the frequency domain, i is the imaginary unit, β is the group velocity dispersion of the fiber, ω is the angular frequency of the total carrier signal, and h is the step size of the compensation algorithm; is the transfer function of dispersion in the P-order fractional Fourier domain, FRFT 1-P Represents the fractional Fourier transform of order 1-P.

9. The carrier modulation method of the optical fiber communication link according to claim 8, characterized in that: The transfer function of dispersion in the P-order fractional Fourier domain is convolved with the transfer function of nonlinear effect in the P-order fractional Fourier domain to obtain the compensated total signal X(I,ω). The convolution process can be achieved by the following formula: Suppose the signal after m-times dispersion compensation and m-times nonlinear compensation is expressed as X (m) (I,ω).

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