A device and method for measuring the nonlinear coefficient of an optical fiber based on cross-phase modulation

Abstract

The application relates to the technical field of optical fiber measurement, in particular to an optical fiber nonlinear coefficient measuring device and method based on cross phase modulation. The device comprises a signal generating module, a to-be-measured optical fiber and a data acquisition and analysis module, the to-be-measured optical fiber is a single-mode optical fiber, the signal generating module mainly comprises a Mach-Zehnder modulator and an erbium-doped fiber amplifier, and the data acquisition and analysis module is a high-resolution spectrum analyzer. The continuous optical signal used for measurement is modulated by using the Mach-Zehnder modulator, the pump light is obtained, the power of the pump light and the probe light is limited, the cross phase modulation nonlinear effect of the to-be-measured optical fiber is excited, the self-phase modulation nonlinear effect is avoided, the spectrum presented by the nonlinear effect is recorded and calculated, the measurement precision is improved, and the nonlinear coefficient measurement of a few-mode optical fiber can be realized by adding a mode conversion module.

Description

Technical Field

[0001] This invention relates to the field of fiber optic measurement technology, and in particular to a device and method for measuring the nonlinear coefficient of fiber optics based on cross-phase modulation. Background Technology

[0002] With the rapid development of technologies such as the Internet of Things, cloud computing, cloud storage, and artificial intelligence, global data traffic and the number of global internet users have increased dramatically, leading to a rapid increase in the demand for transmission capacity in optical fiber communication systems. To improve the transmission capacity of single-mode fiber (SMF) communication systems, various optical signal multiplexing technologies have been proposed, including time-division multiplexing, wavelength-division multiplexing, polarization multiplexing, and quadrature amplitude modulation. Although these capacity-enhancing technologies have been fully explored, bringing the transmission capacity of single-mode fiber communication systems close to the nonlinear Shannon limit of single-mode fiber, they still cannot meet the ever-increasing demand for communication capacity. Therefore, mode-division multiplexing (MDM) technology based on few-mode fiber (FMF) has attracted considerable attention. Mode division multiplexing (MDM) technology utilizes a finite number of mutually orthogonal spatial modes within a few-mode fiber to transmit information in parallel, thereby significantly improving the transmission capacity of optical fiber communication systems. However, as the number of modes increases, the nonlinear effects within the few-mode fiber also increase significantly. The stronger the nonlinear effects, the lower the performance of the optical fiber system. Therefore, in order to ensure the transmission performance of the MDM system, accurate measurement of the intra-mode and inter-mode nonlinear coefficients of the few-mode fiber is particularly important. Existing technologies for measuring the nonlinear coefficients of single-mode fibers are often inaccurate and cannot measure the nonlinear coefficients of few-mode fibers.

[0003] Existing technology discloses an in-situ measurement device and method for the nonlinear coefficient and nonlinear refractive index coefficient of an optical fiber. The device includes a tunable laser module for outputting laser with adjustable power, amplitude modulation, and frequency modulation depth; a beam splitter for splitting the laser into two paths, one leading to a power meter and the other to a spectrometer; and a processor for receiving measurement results and acquiring measured values ​​of the nonlinear coefficient and nonlinear refractive index coefficient of the optical fiber when the tunable laser module compresses the laser spectrum to its narrowest point. The method includes: instrument calibration to obtain the frequency modulation depth when the spectrum is compressed to its narrowest point; and measurement: connecting the optical fiber under test and adjusting... The tunable laser module is compressed to its narrowest possible spectrum, and the nonlinear coefficient and nonlinear refractive index coefficient are calculated based on the measured parameters. However, this device and method for measuring fiber nonlinearity does not process the laser used for measurement, thus failing to excite intramode nonlinear effects in the fiber. Consequently, the measurement accuracy of the nonlinear coefficient is low, and it cannot measure the nonlinear coefficient of few-mode fibers. Summary of the Invention

[0004] The purpose of this invention is to solve the problems of low accuracy in measuring the nonlinear coefficient of single-mode fiber and inability to measure the nonlinear coefficient of few-mode fiber in the prior art, and to provide a fiber nonlinear coefficient measurement device and method based on cross-phase modulation.

[0005] To achieve the above objectives, the present invention provides a fiber optic nonlinear coefficient measurement device based on cross-phase modulation, comprising: a signal generation module, an optical fiber under test, and a data acquisition and analysis module, wherein the signal generation module, the optical fiber under test, and the data acquisition and analysis module are connected sequentially. The signal generation module is used to generate an optical signal that can excite a nonlinear effect of cross-phase modulation of the optical fiber under test. The optical fiber under test is a single-mode optical fiber. When the optical signal passes through the single-mode optical fiber, it will excite the nonlinear effect of cross-phase modulation of the single-mode optical fiber. The data acquisition and analysis module is used to acquire the output spectrum of the optical signal when it passes through the single-mode optical fiber. The output spectrum is determined by the 0th and 1st harmonic sidebands presented by the nonlinear effect of the cross-phase modulation. The nonlinear coefficient is obtained by calculating the power ratio of the 0th and 1st harmonic sidebands.

[0006] Preferably, if the optical fiber under test is a few-mode fiber, the device further includes: a mode conversion module, which includes a first mode conversion module and a second mode conversion module. The input of the first mode conversion module is connected to the output of the signal generation module, and the output of the first mode conversion module is connected to the input of the few-mode fiber. The first mode conversion mode is used to convert the optical signal from the fundamental mode to a higher-order mode. The higher-order mode optical signal can excite the nonlinear effect of cross-phase modulation of the few-mode fiber after passing through the few-mode fiber. The input of the second mode conversion module is connected to the output of the few-mode fiber, and the output of the second mode conversion module is connected to the input of the data acquisition and analysis module. The second mode conversion module is used to convert the higher-order modes of the optical signal into the fundamental mode.

[0007] Preferably, the signal generation module includes: a first branch, a second branch, and a coupler, wherein the first branch is used to generate the pump light, the second branch is used to generate the probe light, and the coupler is used to couple the pump light and the probe light to obtain the optical signal.

[0008] Preferably, the first branch includes: a semiconductor laser, a Mach-Zehnder modulator, a polarization controller, an erbium-doped fiber amplifier, and a filter, wherein the semiconductor laser, Mach-Zehnder modulator, polarization controller, erbium-doped fiber amplifier, and filter are connected in sequence. The semiconductor laser is used to generate a continuous optical signal and transmit the continuous optical signal to the Mach-Zehnder modulator. The Mach-Zehnder modulator is used to modulate the continuous optical signal and obtain intensity-modulated pump light, and transmit the intensity-modulated pump light to the polarization controller; The polarization controller is used to adjust the polarization state of the intensity modulated pump light so that the intensity modulated pump light is linearly polarized, and to transmit the linearly polarized intensity modulated pump light to the erbium-doped fiber amplifier. The erbium-doped fiber amplifier is used to enhance the power of the intensity-modulated pump light; The filter is used to filter out the noise generated by the erbium-doped fiber amplifier during the process of enhancing the power of the intensity-modulated pump light, obtain the pump light, and transmit the pump light to the coupler.

[0009] Preferably, the first branch further includes: an arbitrary waveform generator, an electrical amplifier, and a bias point controller, wherein the arbitrary waveform generator, the electrical amplifier, and the Mach-Zehnder modulator are connected in sequence, and the bias point controller is connected to the Mach-Zehnder modulator. The arbitrary waveform generator is used to generate a sinusoidal drive signal and transmit the sinusoidal drive signal to the electrical amplifier; The electrical amplifier is used to amplify the sinusoidal drive signal and transmit the amplified sinusoidal drive signal to the Mach-Zehnder modulator. The amplified sinusoidal drive signal is used to modulate the continuous optical signal to obtain the intensity-modulated pump light. The bias point controller is used to control the bias point of the Mach-Zehnder modulator, so that the Mach-Zehnder modulator can achieve intensity modulation.

[0010] Preferably, the second branch includes a semiconductor laser and a polarization controller, wherein the semiconductor laser and the polarization controller are connected in sequence. The semiconductor laser is used to generate a continuous optical signal and transmit the continuous optical signal to the polarization controller; The polarization controller is used to adjust the polarization state of the continuous optical signal so that the continuous optical signal is linearly polarized. The linearly polarized continuous optical signal is the probe light. The probe light is transmitted to the coupler, and the coupler couples the pump light and the probe light to obtain the optical signal.

[0011] Preferably, the data acquisition and analysis module is a high-resolution spectrometer.

[0012] Preferably, the mode conversion module is a photon lantern.

[0013] In addition, the present invention also provides a method for measuring the nonlinear coefficient of optical fiber based on cross-phase modulation, comprising: The signal generation module generates two signals: one is a pump light and the other is a probe light. The power of the pump light is 24dBm and the power of the probe light is 1dBm. The pump light and the probe light are coupled together using a coupler to obtain the optical signal. The optical signal is input to the optical fiber under test, which is a single-mode optical fiber, and the optical signal excites the nonlinear effect of cross-phase modulation in the single-mode optical fiber. The output spectrum of the optical signal when it passes through the single-mode optical fiber is obtained. The output spectrum is determined by the 0th and 1st harmonic sidebands presented by the nonlinear effect of the cross-phase modulation. The power ratio of the 0th and 1st harmonic sidebands is calculated to obtain the nonlinear coefficient.

[0014] Preferably, if the optical fiber under test is a few-mode fiber, the method further includes: a mode conversion module, wherein the mode conversion module includes a first mode conversion module and a second mode conversion module. The first mode conversion mode is used to convert the optical signal from the fundamental mode to a higher-order mode. The higher-order mode optical signal can excite a nonlinear effect of cross-phase modulation in the few-mode fiber after passing through the few-mode fiber. The second mode conversion module is used to convert the higher-order mode of the optical signal into the fundamental mode.

[0015] Compared with the prior art, the advantages of this invention are as follows: This invention modulates the continuous optical signal used for measurement using a Mach-Zehnder modulator to obtain pump light, and limits the power of the pump light and probe light to excite the nonlinear effect of cross-phase modulation of the fiber under test, thus avoiding the nonlinear effect of self-phase modulation. The spectrum exhibited by the nonlinear effect is then recorded and calculated, improving the measurement accuracy. Furthermore, by adding a mode conversion module, this invention can realize the measurement of the nonlinear coefficient of few-mode fibers. Attached Figure Description

[0016] Figure 1 This is an embodiment of the present invention: a single-mode fiber nonlinear coefficient measurement device based on cross-phase modulation. Figure 2 This is an embodiment of the invention: a few-mode fiber nonlinear coefficient measurement device based on cross-phase modulation. Figure 3 This is a detailed structural diagram of the device for obtaining fiber mode-correlated transmission constants according to an embodiment of the present invention; Figure 4 This is the transmission constant of the optical fiber under test used in this embodiment of the invention; Figure 5 This is a graph showing the relationship between the accuracy of standard single-mode fiber measurement results and modulation frequency under different wavelength differences according to an embodiment of the present invention. Figure 6 This is a graph showing the measurement results of an embodiment of the present invention; Figure 7 This is the injected LP of an embodiment of the present invention. 01 Spectrum of the probe light before it enters the fiber in the mode; Figure 8 This is the injected LP of an embodiment of the present invention. 01 The spectrum of the probe light after it enters the fiber in the mode; Figure 9 This is a flowchart of a single-mode fiber nonlinear coefficient measurement method based on cross-phase modulation according to an embodiment of the present invention.

[0017] The following are the labeling instructions in the diagram: Box 1, Signal Transmission Module; Box 2, Mode Conversion Module; Box 3, Fiber Under Test; Box 4, Data Acquisition and Analysis Module; 111, Semiconductor Laser; 112, Arbitrary Waveform Generator; 113, Electrical Amplifier; 114, Mach-Zehnder Modulator; 115, Bias Point Controller; 116, Polarization Controller; 117, Erbium-Doped Fiber Amplifier; 118, Filter; 121, Photonic Lantern; 141, High-Resolution Spectrometer; 119, Coupler; 211, Photodetector; 212, Oscilloscope. Detailed Implementation

[0018] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0019] Example 1: Please see Figure 1-2 A preferred embodiment of the present invention provides an optical fiber nonlinear coefficient measurement device based on cross-phase modulation, comprising: a signal generation module 1, an optical fiber under test 3, and a data acquisition and analysis module 4, wherein the signal generation module 1, the optical fiber under test 3, and the data acquisition and analysis module 4 are connected sequentially. Signal generation module 1 is used to generate an optical signal that can excite a nonlinear effect of cross-phase modulation of the fiber under test 3; The fiber under test 3 is a single-mode fiber. When an optical signal passes through a single-mode fiber, it will excite a nonlinear effect of cross-phase modulation in the single-mode fiber. Data acquisition and analysis module 4 is used to acquire the output spectrum of the optical signal passing through a single-mode fiber. The output spectrum is determined by the 0th and 1st harmonic sidebands exhibited by the nonlinear effect of cross-phase modulation. The power ratio of the 0th and 1st harmonic sidebands is calculated and denoted as... According to the power ratio The nonlinear coefficients can be obtained through calculation. The calculation formula is as follows: If the pump light uses carrier-suppressed double-sideband modulation within the signal generation module, then the power ratio ； If the pump light uses carrier double-sideband modulation within the signal generation module, then the power ratio ； in, It is an i-th order Bessel function of the first kind; This is the nonlinear phase shift caused by the cross-phase modulation effect; The nonlinear coefficient of the optical fiber under test is denoted as . For amplitude modulation coefficients, The effective operating length of the optical fiber under test. For cross-phase modulation efficiency, the Depending on the dispersion correlation characteristics of the fiber under test, if the fiber under test 3 is a few-mode fiber, the device further includes: a mode conversion module 2, which includes a first mode conversion module and a second mode conversion module. The input of the first mode conversion module is connected to the output of the signal generation module 1, and the output of the first mode conversion module is connected to the input of the few-mode fiber. The first mode conversion module is used to convert the optical signal from the fundamental mode to a higher-order mode. The higher-order mode of the optical signal can excite the nonlinear effect of cross-phase modulation of the few-mode fiber through the few-mode fiber. The input of the second mode conversion module is connected to the output of the few-mode fiber, and the output of the second mode conversion module is connected to the input of the data acquisition and analysis module 4. The second mode conversion module is used to convert the higher-order modes of the optical signal into the fundamental mode. This invention uses a Mach-Zehnder modulator to modulate the continuous optical signal used for measurement to obtain pump light, and limits the power of the pump light and probe light to excite the nonlinear effect of cross-phase modulation in the fiber under test, avoiding the nonlinear effect of self-phase modulation. The spectrum exhibited by the nonlinear effect is then recorded and calculated, improving the measurement accuracy. Furthermore, by adding a mode conversion module, this invention can realize the measurement of the nonlinear coefficient of the few-mode fiber.

[0020] Example 2 Please see Figure 1-8This embodiment provides a process for measuring the nonlinear coefficient of an optical fiber, including: the intramodal nonlinear coefficient of a few-mode fiber and the nonlinear coefficient of a single-mode fiber. The difference between the two is that in a few-mode fiber, after being multiplexed by a photonic lantern, the two optical paths coupled and injected into the fiber are in the same mode. Here, the mode can be any mode in the fiber, while in a single-mode fiber, the mode is only the fundamental mode. Here, the probe wavelength is selected as 1550 nm, and the pump wavelength is selected as 1550.2 nm, according to... Figure 5 A suitable modulation frequency was selected; here, 500MHz was chosen. Finally, a 2km standard single-mode fiber with a nonlinear coefficient of approximately 1.2 / km / W was connected. The pump light and its corresponding wavelength were adjusted to -1dBm and 24dBm, respectively, to measure the nonlinear coefficient. Unlike the traditional CW-SPM method, the measurement method based on cross-phase modulation shows that the probe light, under the influence of the modulated pump light, generates sidebands with a period related to the modulation frequency. For details, please refer to [link to relevant documentation]. Figure 6 For the measurement of intermodal nonlinearity coefficients in few-mode fibers, please refer to [link / reference needed]. Figure 3 By generating a pulse signal for each mode and using the time-of-flight method, the delay time after transmission through the optical fiber under test is observed on an oscilloscope, and the relevant transmission constants for each mode are calculated using the following formula.

[0021] Where c is the speed of light in a vacuum. The effective refractive index of mode a, Let the group velocity be for pattern a. Let L be the group delay of mode a after transmission through an optical fiber of length L. Let be the chromatic dispersion coefficient of mode a.

[0022] Please refer to the delay characteristic curve of the fiber group under test. Figure 4 Two specific wavelengths are selected such that the probe and pump lights have the same transmission constant under these conditions. Here, the probe wavelength is chosen to be 1545 nm, and the pump wavelength is chosen to be 1548 nm. Then, according to... Figure 2 The diagram shows the setup for a measurement system, where the probe light is directed into the LP. 01 The modulated pump light is injected into the LP 11 In this mode, after the pump light is modulated by the signal transmission module, it generates harmonic sidebands of various orders. Finally, a 500m section of the two-mode fiber under test with an intermodal nonlinearity coefficient of approximately 3 / km / W is connected to measure its intermodal nonlinearity coefficient. The pump light and the pump beam are adjusted to -1dBm and 24dBm respectively. On the spectrometer, it is observed that the probe light output generates a pattern due to induced nonlinear phase modulation, as shown in the image. Figure 8The sidebands shown are used to record the power ratio between the main peak and the side peaks. The corresponding nonlinear phase shift is then calculated using the following formula. in, It is an i-th order Bessel function of the first kind; Finally, substitute the values ​​into the following formula to calculate. and The nonlinear coefficient between them is 3 / km / W. in, For the resulting nonlinear phase shift, The nonlinear coefficient to be measured is... The pump power is for inductive cross-phase modulation effect. The effective working length of the optical fiber under test.

[0023] Example 3: Please see Figure 9 This invention also provides a method for measuring the nonlinear coefficient of a single-mode fiber based on cross-phase modulation, comprising: The signal generation module generates two signals: one is a pump light and the other is a probe light. The power of the pump light is 24dBm and the power of the probe light is 1dBm. The pump light and the probe light are coupled together using a coupler to obtain the optical signal. The optical signal is input to the optical fiber under test, which is a single-mode optical fiber, and the optical signal excites the nonlinear effect of cross-phase modulation in the single-mode optical fiber. The output spectrum of the optical signal passing through the single-mode fiber is obtained. The output spectrum is determined by the 0th and 1st order harmonic sidebands exhibited by the nonlinear effect of the cross-phase modulation. The nonlinear coefficient is obtained by calculating the power ratio of the 0th and 1st order harmonic sidebands. This invention modulates the continuous optical signal used for measurement using a Mach-Zehnder modulator to obtain pump light, and limits the power of the pump light and probe light to excite the nonlinear effect of cross-phase modulation in the fiber under test, thus avoiding the nonlinear effect of self-phase modulation. The spectrum exhibited by the nonlinear effect is then recorded and calculated, improving the measurement accuracy. Furthermore, by adding a mode conversion module, this invention can realize the measurement of the nonlinear coefficient of few-mode fibers.

[0024] In summary, this invention provides a device and method for measuring the nonlinear coefficient of optical fibers based on cross-phase modulation. By using a Mach-Zehnder modulator to modulate the continuous optical signal used for measurement to obtain pump light, and limiting the power of the pump light and probe light, the nonlinear effect of cross-phase modulation of the fiber under test is excited, avoiding the nonlinear effect of self-phase modulation. The spectrum exhibited by the nonlinear effect is then recorded and calculated, improving the measurement accuracy. Furthermore, by adding a mode conversion module, this invention can realize the measurement of the nonlinear coefficient of few-mode fibers.

[0025] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A fiber optic nonlinear coefficient measurement device based on cross-phase modulation, characterized in that, include: The system comprises a signal generation module (1), an optical fiber under test (3), and a data acquisition and analysis module (4), which are connected sequentially. The signal generation module (1) is used to generate an optical signal that can excite the nonlinear effect of cross-phase modulation of the optical fiber under test (3); The optical fiber under test (3) is a few-mode optical fiber; The data acquisition and analysis module (4) is used to acquire the output spectrum of the optical signal when it passes through the few-mode fiber. The output spectrum is determined by the 0th and 1st harmonic sidebands presented by the nonlinear effect of the cross-phase modulation. The power ratio of the 0th and 1st harmonic sidebands is calculated to obtain the nonlinear coefficient. The device further includes: a mode conversion module (2), the mode conversion module (2) includes a first mode conversion module and a second mode conversion module, the input of the first mode conversion module is connected to the output of the signal generation module (1), the output of the first mode conversion module is connected to the input of the few-mode fiber, the first mode conversion module is used to convert the optical signal from the fundamental mode to a higher-order mode, and the optical signal of the higher-order mode can excite the nonlinear effect of cross-phase modulation of the few-mode fiber through the few-mode fiber; The input of the second mode conversion module is connected to the output of the few-mode fiber, and the output of the second mode conversion module is connected to the input of the data acquisition and analysis module (4). The second mode conversion module is used to convert the higher-order mode of the optical signal into the fundamental mode.

2. The fiber optic nonlinear coefficient measuring device based on cross-phase modulation according to claim 1, characterized in that, The signal generation module (1) includes: a first branch, a second branch and a coupler (119). The first branch is used to generate pump light, the second branch is used to generate probe light, and the coupler (119) is used to couple the pump light and the probe light to obtain the optical signal.

3. The fiber optic nonlinear coefficient measuring device based on cross-phase modulation according to claim 2, characterized in that, The first branch includes: a semiconductor laser (111), a Mach-Zehnder modulator (114), a polarization controller (116), an erbium-doped fiber amplifier (117), and a filter (118), wherein the semiconductor laser (111), the Mach-Zehnder modulator (114), the polarization controller (116), the erbium-doped fiber amplifier (117), and the filter (118) are connected in sequence. The semiconductor laser (111) is used to generate a continuous optical signal and transmit the continuous optical signal to the Mach-Zehnder modulator (114). The Mach-Zehnder modulator (114) is used to modulate the continuous optical signal and obtain intensity-modulated pump light, and transmit the intensity-modulated pump light to the polarization controller (116). The polarization controller (116) is used to adjust the polarization state of the intensity modulated pump light so that the intensity modulated pump light is linearly polarized, and transmit the linearly polarized intensity modulated pump light to the erbium-doped fiber amplifier. The erbium-doped fiber amplifier (117) is used to enhance the power of the intensity-modulated pump light; The filter (118) is used to filter out the noise generated by the erbium-doped fiber amplifier (117) during the process of enhancing the power of the intensity-modulated pump light, obtain the pump light, and transmit the pump light to the coupler (119).

4. The fiber optic nonlinear coefficient measuring device based on cross-phase modulation according to claim 3, characterized in that, The first branch further includes: an arbitrary waveform generator (112), an electrical amplifier (113), and a bias point controller (115), wherein the arbitrary waveform generator (112), the electrical amplifier, and the Mach-Zehnder modulator (114) are connected in sequence, and the bias point controller (115) is connected to the Mach-Zehnder modulator (114). The arbitrary waveform generator (112) is used to generate a sinusoidal drive signal and transmit the sinusoidal drive signal to the electrical amplifier (113). The electrical amplifier (113) is used to amplify the sinusoidal driving signal and transmit the amplified sinusoidal driving signal to the Mach-Zehnder modulator (114). The amplified sinusoidal driving signal is used to modulate the continuous optical signal to obtain the intensity-modulated pump light. The bias point controller (115) is used to control the bias point of the Mach-Zehnder modulator (114) so ​​that the Mach-Zehnder modulator (114) can achieve intensity modulation.

5. The fiber optic nonlinear coefficient measuring device based on cross-phase modulation according to claim 4, characterized in that, The second branch includes a semiconductor laser (111) and a polarization controller (116), wherein the semiconductor laser (111) and the polarization controller (116) are connected in sequence. The semiconductor laser (111) is used to generate a continuous optical signal and transmit the continuous optical signal to the polarization controller (116). The polarization controller (116) is used to adjust the polarization state of the continuous optical signal so that the continuous optical signal is linearly polarized. The linearly polarized continuous optical signal is the probe light, and the probe light is transmitted to the coupler (119). The coupler (119) couples the pump light and the probe light to obtain the optical signal.

6. The fiber optic nonlinear coefficient measuring device based on cross-phase modulation according to claim 5, characterized in that, The data acquisition and analysis module (4) is a high-resolution spectrometer (141).

7. The fiber optic nonlinear coefficient measuring device based on cross-phase modulation according to claim 2, characterized in that, The mode conversion module (2) is a photon lantern (121).

8. A method for measuring the nonlinear coefficient of optical fiber based on cross-phase modulation, characterized in that, include: The signal generation module generates two signals: one is a pump light and the other is a probe light. The power of the pump light is 24dBm and the power of the probe light is 1dBm. The pump light and the probe light are coupled together using a coupler to obtain an optical signal. The optical signal is input to the optical fiber under test, which is a few-mode fiber. The output spectrum of the optical signal when it passes through the few-mode fiber is obtained. The output spectrum is determined by the 0th and 1st harmonic sidebands presented by the nonlinear effect of the cross-phase modulation. The power ratio of the 0th and 1st harmonic sidebands is calculated to obtain the nonlinear coefficient. The method further includes: a mode conversion module (2), the mode conversion module (2) including a first mode conversion module and a second mode conversion module, The first mode conversion module is used to convert the optical signal from the fundamental mode to a higher-order mode. The higher-order mode optical signal can excite the nonlinear effect of cross-phase modulation of the fewer-mode fiber after passing through the fewer-mode fiber. The second mode conversion module is used to convert the higher-order mode of the optical signal into the fundamental mode.

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