Method for accurately and synchronously exciting optical signal by space division multiplexer based on time calibration

Abstract

The invention discloses a method for accurately and synchronously exciting optical signals by a space division multiplexer based on time calibration. A calibration system mainly comprises a tunable laser, an M-path optical splitter, an adjustable delay line array and the space division multiplexer, one delay line is connected with a reference port of the space division multiplexer to form a reference branch, and the other delay lines are respectively connected with ports to be calibrated to form a plurality of branches to be calibrated. During each time of calibration, light transmission of the reference branch and one branch to be calibrated is maintained, data is collected and Fourier transform is carried out, and a peak value is extracted to obtain relative time delay. And adjusting the corresponding delay line according to the time delay until the time delay is zero, and completing the calibration of the current branch. According to the invention, accurate excitation of the mode can be realized by using the space division multiplexer, and the delay of the tail fiber of the space division multiplexer is effectively calibrated through the adjustable delay line array, so that different optical signals are synchronously and accurately excited.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of calibration methods for mode multiplexer tail fiber length differences, and particularly relates to a method for realizing accurate synchronization excitation of space division multiplexer light signals based on time calibration. BACKGROUND

[0002] In recent years, with the rapid development of data-intensive services such as the Internet of Things, artificial intelligence, virtual reality, and the like, traditional optical communication systems are facing severe challenges in terms of transmission capacity. In order to break through this bottleneck, mode division multiplexing (MDM) technology based on few-mode fiber (FMF) has become an important research direction for the next generation of high-capacity optical transmission systems because it can significantly improve the transmission capacity of the system. However, during MDM transmission, factors such as differential mode group delay (DMGD) and chromatic dispersion (CD) can cause signal pulse broadening, which in turn leads to problems such as increased bit error rate and crosstalk between modes. Therefore, accurately characterizing the optical performance of few-mode fiber is crucial for optimizing transmission quality.

[0003] As the number of modes supported by few-mode fiber continues to increase, the complexity of the fiber characterization system also significantly increases, especially in long-distance MDM systems. In the widely used graded-index (GI) few-mode fiber, the problem of mutual interference between modes is particularly prominent. This interference can lead to misjudgment between the fundamental mode and high-order modes, as well as between different high-order modes, which in turn makes it impossible for the system to accurately characterize key parameters such as DMGD between modes. Therefore, accurate excitation of modes is particularly important during the performance characterization of few-mode fiber.

[0004] In addition, time error calibration caused by fiber length differences and time delay introduced by different space division multiplexing signals also play an important role in characterization and optical communication applications. Therefore, it is of great significance to propose effective solutions to these problems in order to improve the accuracy of fiber characterization, the performance and reliability of optical communication systems.

[0005] In a few-mode fiber characterization system, in order to reduce the interference between multiple modes, the S 2 The solution to the problem of multiple mode interference in the microwave photonics link is as follows: first, bias excitation is performed at the input end of the fiber to be tested. Bias excitation makes the proportion of the fundamental mode at the input end much larger than that of high-order modes, so that the interference between high-order modes is submerged in noise, only the interference between the fundamental mode and high-order modes is retained, and accurate testing of DMGD is realized. Second, a multiplexer (MUX) is used to accurately excite the modes. Based on the mode selectivity of the MUX, the light output by the tunable laser is connected to two ports of the mode multiplexer to accurately excite the modes. In this solution, because there is a length difference in the tail fiber of the mode multiplexer, an additional time delay value is introduced during the characterization process, which seriously affects the final test results.

[0006] In the aspect of mode excitation, the following researches are currently available:

[0007] (1) Bias excitation technology is a spatial spectrum method (S 2 ) and a characterization system of microwave photonics link, etc. When the few-mode fiber to be measured supports more than two modes, a solution is to avoid complex interference between modes. When the multi-mode interference occurs during the characterization process, the Fourier transform is performed on the obtained experimental data, and many peak values are obtained. The light field corresponding to these peak values may overlap with each other, and therefore it is difficult to obtain the exact DMGD information between two modes. Therefore, in the characterization test, bias excitation is usually performed at the input end of the fiber to be measured, so that the proportion of the fundamental mode at the input end is much larger than that of the high-order mode. In this way, the interference between the high-order modes is submerged in the noise, and only the interference between the fundamental mode and the high-order mode is reserved, thereby realizing accurate testing of DMGD [1] .

[0008] (2) A few-mode fiber comprehensive tester and a testing method are disclosed in a patent document with publication date of February 9, 2018 and publication number CN107677452A. The output light is loaded on a mode selection module (photonic lantern), different modes are excited by relying on the mode selection characteristics, and the time domain waveform is obtained by an oscilloscope. Based on the oscilloscope data and the length of the fiber to be measured, the DGD of the fiber to be measured is further obtained.

[0009] In the aspect of time compensation of the characterization system, a few-mode fiber differential mode group delay measurement method, system and device are disclosed in a patent document with publication date of July 9, 2021 and publication number CN113098595A. The frequency-modulated continuous light signal is split into two coherent different mode light signals, and the light signal is subjected to mode division multiplexing by a mode division multiplexing module, so as to excite the fundamental mode and the high-order mode light signal in the few-mode fiber. The signal is demultiplexed by a mode demultiplexing module, so as to generate inter-mode interference light signal, and then the DMGD is measured. The delay calibration module is introduced in the device to reduce the delay error introduced by the non-measured fiber.

[0010] In the aspect of fiber characterization accuracy, the following researches are currently available:

[0011] (1) The time-of-flight (ToF) technology transmits a narrow pulse signal into a long FMF with a certain offset, simultaneously excites multiple modes, and detects the delay between different modes to calculate the DMGD. The ToF technology can support the characterization of DMGD of up to 9 LP modes [2] , and the absolute CD measurement of up to 4 LP modes [3] , but its accuracy is relatively low, and expensive equipment is required.

[0012] (2) Optical low coherence interferometry (OLCI) based on Michelson interferometer and broadband incoherent light source, can effectively improve the spatial resolution and sensitivity, suitable for high-precision time-domain and frequency-domain measurement. Although OLCI technology has made breakthroughs in characterizing the DMGD and absolute CD of FMF, the equipment required by OLCI is expensive and complex to set up, and its highest achievement is limited to measuring 4 LP modes [4] .

[0013] [References]

[0014] [1] Nicholson, J. W.; Meng, L.; Fini, J. M.; Windeler, R. S.; DeSantolo, A.; Monberg, E.; DiMarcello, F.; Dulashko, Y.; Hassan, M.; Ortiz, R. Measuring Higher-Order Modes in a Low-Loss, Hollow-Core, Photonic-Bandgap Fiber. Opt. Express 2012, 20, 20494, doi:10.1364 / OE.20.020494.

[0015] [2] Sillard, P.; Molin, D.; Bigot-Astruc, M.; De Jongh, K.; Achten, F.; Velazquez-Benitez, A. M.; Amezcua-Correa, R.; Okonkwo, C. M. Low-Differential-Mode-Group-Delay 9-LP-Mode Fiber. J. Light. Technol. 2016, 34, 425-430, doi:10.1109 / JLT.2015.2463715.

[0016] [3] Cheng, J.; Pedersen, M. E. V.; Wang, K.; Xu, C.; Gruner-Nielsen, L.; Jakobsen, D. Time-Domain Multimode Dispersion Measurement in a Higher-Order-Mode Fiber. Opt. Lett. 2012, 37, 347, doi:10.1364 / OL.37.000347.

[0017] [4] Gabet, R.; Le Cren, E.; Jin, C.; Gadonna, M.; Ung, B.; Sillard, P.; Nguyen, H. G.; Jaouen, Y.; Thual, M.; LaRochelle, S. Complete Dispersion Characterization of Few Mode Fibers by OLCI Technique. J. Light. Technol. 2015, 33, 1155-1160, doi:10.1109 / JLT.2014.2376702. SUMMARY

[0018] In view of the prior art, the present application provides a method for precise synchronization excitation of light signals of space division multiplexer based on time calibration. The method combines adjustable delay line array, space division multiplexing fiber (including few-mode fiber and multi-mode fiber) and mode multiplexing device. By using mode multiplexing device, precise excitation of modes can be achieved, and by adjusting the delay of the tail fiber of the mode multiplexing device with the adjustable delay line array, different modes in the fiber can be precisely excited synchronously, and finally the precise measurement of differential modal group delay can be achieved.

[0019] In order to solve the above technical problems, the present application provides a method for precise synchronization excitation of light signals of space division multiplexer based on time calibration. The calibration system for realizing the method comprises a tunable laser, a first M-way optical splitter, an adjustable delay line array and a space division multiplexer. The space division multiplexer comprises N input ports, N≥2, one of which is a reference port, and the remaining ports are all to-be-calibrated ports. The adjustable delay line array is composed of N delay lines. One of the delay lines is connected with the reference port of the space division multiplexer to form a reference branch, and the remaining delay lines are respectively connected with the to-be-calibrated ports of the space division multiplexer one by one to form N-1 to-be-calibrated branches. The output end of the tunable laser is connected with the input end of the first M-way optical splitter, M≥2. The optical path of one of the output ports of the first M-way optical splitter is connected with the reference branch, and the optical paths of the remaining output ports of the first M-way optical splitter are respectively connected with the N-1 to-be-calibrated branches through flanges, and the method comprises the following steps:

[0020] Step 1) In each calibration, the reference branch and one of the to-be-calibrated branches are always kept in optical communication.

[0021] Step 2) The tunable laser performs data acquisition according to the set frequency sweep interval Δf. Fourier transform is performed on the acquired data, and the peak value is extracted. The peak value represents the relative time delay between the reference branch and the current to-be-calibrated branch.

[0022] Step 3) adjusting the time delay of the current branch to be calibrated in the current adjustable delay line array through software of the adjustable delay line array according to the current relative time delay;

[0023] Step 4) judging whether the current time delay is zero, if not, returning to step 2), otherwise, ending the adjustment of the time delay of the current branch to be calibrated;

[0024] Step 5) realizing the switching connection of the optical paths of the remaining output ports of the first M-way optical splitter with N-1 branches to be calibrated through the flange, and realizing the adjustment of the time delays of all branches to be calibrated according to steps 1)-4).

[0025] Further, the method for realizing the precise synchronization excitation of the light signal of the space division multiplexer based on the time calibration comprises the following steps:

[0026] The length difference of the tail fiber of the reference branch and the current branch to be calibrated of the space division multiplexer is measured by using a ruler with a precision of millimeter level to obtain a coarse measurement length difference, and the time delay value introduced by the coarse measurement length difference is estimated according to formula (1), and the precision of the time delay value is picosecond level,

[0027]

[0028] Wherein, C is the speed of light, L is the coarse measurement length difference, n eff is the effective refractive index of the optical fiber, and t is the time delay value;

[0029] The scanning interval Δf of the tunable laser is set according to formula (2) according to the time delay value t,

[0030]

[0031] Wherein, Δf is the scanning interval, P is the number of collected data, and t is the time delay value.

[0032] Further to the above basic structure, a second M-way optical splitter is arranged between the output end of the tunable laser and the input end of the first M-way optical splitter; one of the output ports of the second M-way optical splitter is connected with a variable optical attenuator, and the other output port of the second M-way optical splitter is connected with the input port of the first M-way optical splitter; a first polarization controller and a first variable optical attenuator are arranged on the optical path of one of the output ports of the first M-way optical splitter in sequence according to the direction of the light signal, and a second polarization controller and a second variable optical attenuator are arranged on the optical paths of the remaining output ports of the first M-way optical splitter in sequence according to the direction of the light signal; and the method comprises the following steps:

[0033] Step 1) At each calibration, always keep the reference branch and one of the branches to be calibrated in light, observe the signal light mode spot of the reference branch and the current branch to be calibrated, and restore the signal light mode spot to a standard mode spot by adjusting the first polarization controller and the second polarization controller;

[0034] Step 2) The tunable laser performs data acquisition according to the set sweep interval Df;

[0035] Step 3) Fourier transform is performed on the collected data, and a peak value is extracted, the peak value representing the relative time delay of the reference branch and the current branch to be calibrated, if the numerical difference between the peak value and the noise floor is less than 5dB, the light intensity of the reference branch and the current branch to be calibrated is adjusted using the first variable optical attenuator and the second variable optical attenuator respectively, so that the numerical difference between the peak value and the noise floor is greater than or equal to 5dB, and the current relative time delay is obtained;

[0036] Step 4) The time delay of the current branch to be calibrated in the adjustable delay line array is adjusted through the software of the adjustable delay line array according to the current relative time delay;

[0037] Step 5) It is judged whether the current time delay is zero, if not, return to step 2), otherwise, end the adjustment of the time delay of the current branch to be calibrated;

[0038] Step 6) The light paths of the remaining output ports of the first M-way optical splitter are switched and connected to the N-1 branches to be calibrated through the flanges, and the time delays of all the branches to be calibrated are adjusted according to steps 1)-5).

[0039] In the application, the space division multiplexer is any one of a mode multiplexer and a core division multiplexer.

[0040] Compared with the prior art, the application has the following beneficial effects:

[0041] In documents [2] to [4], the characterization techniques used have limitations in the measurement of FMF, most of which can only characterize up to 4 LP modes, and it is still a challenge to simultaneously obtain DMGD and CD information. The limitations of these techniques make it more complex to achieve accurate and comprehensive optical performance characterization in large-scale multi-mode fiber systems.

[0042] In document [1], the bias excitation technique used will simultaneously excite multiple high-order modes through bias excitation for a graded-index type few-mode fiber with a large number of modes, and the mode spots of each other are superimposed together, so that there is no corresponding peak value after Fourier transform of the results collected by the characterization system, but a region with energy higher than noise, which will lead to the inability of the experiment to obtain DMGD information.

[0043] The patent document with publication number CN107677452A discloses a technical solution in which a photonic lantern is used as a mode selection module to realize mode selection excitation. However, due to the inconsistency of the length of the tail fiber in the preparation process of the photonic lantern, unnecessary time errors are introduced in the experiment. At the same time, the tail fiber of the photonic lantern is usually long, which is not conducive to the integration of the system in practical application. The system proposed in the technical solution calculates the differential group delay of the optical fiber through the time domain signal and the length of the optical fiber, but due to the low precision, the performance of the system is limited.

[0044] The patent document with publication number CN113098595A discloses a technical solution in which a delay calibration module is used in the characterization process to reduce the time delay error introduced by the non-measured optical fiber. However, this method cannot completely eliminate the delay caused by the non-measured optical fiber quantitatively.

[0045] The present application proposes a time calibration method for precise mode excitation of few-mode optical fiber, which uses a tunable optical delay line and a space division multiplexer in cascade, combines a mode multiplexer and a few-mode / multi-mode optical fiber, and realizes multi-mode high-precision synchronous excitation. This method flexibly switches different mode gears through an optical coupler, and uses micrometer-level and picosecond-level high-precision delay control to effectively eliminate the system error introduced by the non-measured optical fiber, and ensure the precise measurement of the differential mode group delay. The tunable delay line used has a simple structure, low cost, high integration and cost performance, and is suitable for building a compact and high-performance optical fiber interference measurement system. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 is a system schematic diagram for realizing accurate synchronous excitation of optical signals by space division multiplexer based on time calibration;

[0047] Figure 2 Flowchart of the time calibration method;

[0048] Figure 3 Based on Figure 1 Further regulate the light signal quality to improve the effect schematic diagram of realizing accurate synchronous excitation of optical signals by space division multiplexer based on time calibration

[0049] Figure 4 S 2 Test system schematic diagram;

[0050] Figure 5 Fundamental mode and LP 11The calibration process of the inter-mode relative time delay, wherein: (a) the time delay caused by the tail fiber length difference when not time-calibrated; (b) in the calibration process, the introduced time delay gradually decreases; (c) after the calibration is completed, the time delay is adjusted to the calibration completion state without peak value; (d) after the time calibration method, the accurately measured LP 11 The mode DMGD is 44.91 ps / km.

[0051] Figure 6 The S2 test system using the precise mode excitation and time calibration method of the few-mode fiber characterization system is used to measure the absolute chromatic dispersion;

[0052] Figure 7 Schematic diagram of a multi-mode excitation and power regulation system based on a variable optical attenuator;

[0053] Figure 8 Schematic diagram of a system for regulating mode switching and excitation time;

[0054] Figure 9 Schematic diagram of an experimental system for mode-ordered transmission based on a time calibration method. DETAILED DESCRIPTION

[0055] The present application proposes a method for accurately synchronously exciting light signals of a space division multiplexer based on time calibration, and the basic structure of a calibration system for implementing the method is shown in Figure 1 The calibration system includes a tunable laser, a first M-way optical splitter, an adjustable delay line array, and a space division multiplexer; the space division multiplexer includes N input ports, N≥2, one of which is a reference port, and the remaining ports are all to-be-calibrated ports; the adjustable delay line array is composed of N delay lines; one of the delay lines is connected to the reference port of the space division multiplexer to form a reference branch, and the remaining delay lines are respectively connected to the to-be-calibrated ports of the space division multiplexer one by one to form N-1 to-be-calibrated branches; the output end of the tunable laser is connected to the input end of the first M-way optical splitter, M≥2, the optical path of one of the output ports of the first M-way optical splitter is connected to the reference branch, and the optical paths of the remaining output ports of the first M-way optical splitter are respectively connected to the N-1 to-be-calibrated branches through flanges.

[0056] As shown in Figure 2 The method for accurately synchronously exciting light signals of a space division multiplexer using the above calibration system includes the following steps:

[0057] Step 1) At each calibration, always keep the reference branch and one of the to-be-calibrated branches open;

[0058] Step 2) The tunable laser performs data acquisition according to the set sweep interval Δf. The method for setting the sweep interval Δf of the tunable laser is to use a ruler with a precision of millimeters to measure the length difference between the reference branch of the space division multiplexer and the current branch to be calibrated to obtain a rough length difference, and estimate the delay value introduced by the rough length difference according to formula (1). The accuracy of the delay value is in the picosecond level.

[0059]

[0060] Where C is the speed of light, L is the rough length difference, n eff is the effective refractive index of the optical fiber, and t is the delay value;

[0061] According to the delay value t, the frequency sweep interval Δf of the tunable laser is set according to formula (2):

[0062]

[0063] Where Δf is the frequency sweep interval, P is the number of collected data, and t is the delay value.

[0064] Then, Fourier transform is performed on the collected data and a peak value is extracted, where the peak value represents the relative time delay between the reference branch and the current branch to be calibrated;

[0065] Step 3) adjusting the delay of the current branch to be calibrated in the current adjustable delay line array through the software of the adjustable delay line array according to the current relative delay;

[0066] Step 4) Determine whether the current delay is zero. If not, return to step 2). Otherwise, end the adjustment of the delay of the current branch to be calibrated. Step 5) Use the flange to switch the optical paths of the remaining output ports of the first M-way splitter to the N-1 branches to be calibrated, and adjust the delays of all branches to be calibrated according to steps 1)-4).

[0067] In order to better control the quality of the optical signal required for calibration, such as Figure 3 As shown, a further optimized structure of the calibration system of the above basic structural form is as follows: a second M-way optical splitter is provided between the output end of the tunable laser and the input end of the first M-way optical splitter; the output end of the tunable laser is connected to the input end of the second M-way optical splitter, one of the output ports of the second M-way optical splitter is connected to a variable optical attenuator, and the other output port of the second M-way optical splitter is connected to the input port of the first M-way optical splitter; a first polarization controller and a first variable optical attenuator are provided in sequence on the optical path of one of the output ports of the first M-way optical splitter according to the direction of the optical signal, and a second polarization controller and a second variable optical attenuator are provided in sequence on the optical paths of the remaining output ports of the first M-way optical splitter according to the direction of the optical signal.

[0068] The method for realizing accurate synchronization excitation of light signals of the space division multiplexer by using the calibration system with the above-mentioned optimized structure form comprises the following steps.

[0069] Step 1) In each calibration, the reference branch and one of the branches to be calibrated are always kept in light transmission, the signal light mode spots of the reference branch and the branch to be calibrated are observed, and the signal light mode spots are restored to standard mode spots by adjusting the first polarization controller and the second polarization controller.

[0070] Step 2) The tunable laser performs data acquisition according to the set sweep interval Δf.

[0071] Step 3) The acquired data are subjected to Fourier transform and the peak value is extracted, the peak value representing the relative time delay of the reference branch and the branch to be calibrated, if the numerical difference between the peak value and the noise floor is < 5 dB, the light intensity of the reference branch and the branch to be calibrated is adjusted by using the first variable optical attenuator and the second variable optical attenuator respectively, so that the numerical difference between the peak value and the noise floor is ≥ 5 dB, and the current relative time delay is obtained.

[0072] Step 4) The time delay of the branch to be calibrated in the adjustable delay line array is adjusted according to the current relative time delay by using the software of the adjustable delay line array.

[0073] Step 5) It is judged whether the current time delay is zero, if not, returning to step 2), otherwise, ending the adjustment of the time delay of the branch to be calibrated.

[0074] Step 6) The light paths of the remaining output ports of the first M-way optical splitter are switched and connected to the N-1 branches to be calibrated by using the flange, and the adjustment of the time delays of all the branches to be calibrated is realized according to steps 1)-5).

[0075] The present application will be further described in conjunction with the drawings and specific embodiments, but the following embodiments are by no means limiting to the present application.

[0076] Embodiment 1

[0077] The calibration system with the optimized structure form as shown in Figure 3 In this embodiment, the first M-way optical splitter and the second M-way optical splitter each adopt a 1:2 optical coupler, wherein the three ports of the first 1:2 optical coupler are denoted as port A, port A1 and port A2, wherein the port A is the light inlet port, and the ports A1 and A2 are the light output ports. The three ports of the second 1:2 optical coupler are denoted as port B, port B1 and port B2, wherein the port B is the light inlet port, and the ports B1 and B2 are the light output ports. The adjustable delay line array is composed of N delay lines, which are denoted as E, F1, …, FN respectively. N-2, F N-1 , the space division multiplexer adopts an N mode multiplexer, and the N input ports of the space division multiplexer are port G, port H1 to H N-1 . The port G is a base mode port, and the other ports are high-order mode ports. The adjustable delay line E is connected with the base mode port G of the mode multiplexer to form a base mode branch, the adjustable delay line F1 is connected with the port H1 of the mode multiplexer, and the other delay lines are connected with each port of the mode multiplexer in the same way to form high-order mode branches, such as the adjustable delay line F2 connected with the port H2 of the mode multiplexer, the adjustable delay line F3 connected with the port H3 of the mode multiplexer, and so on, and the adjustable delay line F N-1 connected with the port H N-1 of the mode multiplexer, so as to form an adjustable delay line array. In the embodiment, the output light of the tunable laser is divided into two by the second optical coupler 1, one beam of light is taken as reference light for absolute CD characterization after passing through a single mode fiber and a variable optical attenuator, and the other beam of light is input into the port A of the first optical coupler, sequentially passes through the port A1, the first polarization controller, the first variable optical attenuator, and then is connected in series with the single mode branch, and then sequentially passes through the port A2 of the first optical coupler, the second polarization controller, the second variable optical attenuator, and then is connected in series with the high-order mode branch. The output port of the mode multiplexer is connected with the fiber to be measured, thereby forming a precise mode excitation and self-calibration system.

[0078] The calibration method effectively solves the problems of inaccurate mode excitation and time delay error caused by the length difference of the tail fiber in the prior art, and provides a simple and accurate solution for accurate characterization of few-mode fibers, and has important practical application value. Since there is a length difference between the tail fibers of the mode multiplexer, the difference will bring an error that cannot be ignored to the characterization system, and the flowchart of the calibration method is shown in Figure 2 .

[0079] In each calibration, the reference branch and one of the branches to be calibrated are always kept in light, and the signal light mode spot is observed, and the signal light mode spot is restored to a standard mode spot by adjusting the first polarization controller and the second polarization controller. In the embodiment, the output light 1 of the tunable laser is divided into two by the second optical coupler, one beam of light is connected to the port A of the first optical coupler, and the light is further divided into two paths. Before the next connection, the length difference of the tail fibers of the base mode port G and each high-order mode port F i of the mode multiplexer is roughly measured by using a ruler to estimate the time delay value introduced by the length difference, and a suitable sweep interval is set based on this. The tunable laser performs data acquisition according to the set sweep interval Δf.

[0080] In the present application, the method for setting the sweep interval Δf of the tunable laser is that a ruler with millimeter level precision is used to measure the length difference of the tail fiber of the reference branch and the current branch to be calibrated of the space division multiplexer to obtain a rough measurement length difference, and the time delay value introduced by the rough measurement length difference is estimated according to formula (1), and the precision of the time delay value is picosecond level,

[0081]

[0082] Wherein, C is the speed of light, L is the rough measurement length difference, n eff is the effective refractive index of the optical fiber, and t is the time delay value;

[0083] According to the time delay value t, the sweep interval Δf of the tunable laser is set according to formula (2),

[0084]

[0085] Wherein, Δf is the sweep interval, P is the number of collected data, and t is the time delay value.

[0086] Then, the collected data is subjected to Fourier transform and the peak value is extracted, the peak value represents the relative time delay between the fundamental mode and the high-order mode, if the numerical difference between the peak value and the noise floor at this time is < 5dB, the light intensity of the reference branch and the current branch to be calibrated is adjusted by using the first variable optical attenuator and the second variable optical attenuator respectively, so that the numerical difference between the peak value and the noise floor is ≥ 5dB, and the current relative time delay is obtained.

[0087] According to the current relative time delay, the time delay of the corresponding high-order mode path in the TDLA (adjustable delay line array) is adjusted through the software of the adjustable delay line array, and the experiment is repeated. The relative time delay between the fundamental mode and the high-order mode gradually decreases. The delay of the TDLA corresponding to the high-order mode path is adjusted again until the numerical difference between the peak value and the noise floor after the collected data is subjected to Fourier transform is < 5dB, which means that the current time delay is zero, and the calibration of the mode is completed. Then, the flange control optical coupler is connected to the fundamental mode and other high-order modes, and the above experimental steps are repeated until there is no relative time delay between all high-order modes and the fundamental mode, and the adjustment of the time delay of all branches to be calibrated is realized.

[0088] After all the calibrations are completed, the measured optical fiber is connected to the output end of the mode multiplexer, the fundamental mode and the selected mode port of the mode multiplexer are connected through the optical coupler, the mode is precisely excited, and the DMGD of each mode in the measured optical fiber is precisely characterized.

[0089] After completing the DMGD characterization, port B2 of the second optical coupler outputs plane light as a reference path, port A1 of the first optical coupler continues to be connected to the fundamental mode branch, and A2 is disconnected. The plane reference light is combined with the fundamental mode light beam to interfere, the interference image is collected, and a Fourier transform is performed to obtain the relative delay peak of the fundamental mode and the plane light. By changing the center wavelength of the tunable laser, the relationship between the delay value and the center wavelength in the C band is obtained. By performing a second-order polynomial fitting, the functional relationship between the delay value and the wavelength is obtained, and the absolute CD parameter of the fundamental mode of the optical fiber to be tested in the C band is further obtained. The relationship between the higher-order mode and the fundamental mode obtained previously is again fitted with a second-order polynomial and differentiated to further obtain the CD difference between the higher-order mode and the fundamental mode. This data is added to the CD value of the fundamental mode to finally obtain the CD value of all modes.

[0090] Example 2

[0091] S 2 It is an effective method to characterize the differential mode group delay and multipath interference (MPI) in few-mode fibers from both spatial and spectral dimensions. It has been widely used in relevant research and engineering tests in recent years. However, graded-index few-mode fibers are widely used in long-distance MDM systems at present. In this type of fiber, the mode coupling between high-order modes is strong, which gives S 2 The measurement pattern differentiation and feature extraction bring great challenges.

[0092] This embodiment is based on the traditional S 2 System, such as Figure 4 As shown, a mode multiplexer and an adjustable delay line array are added before the optical fiber to be tested to accurately excite the mode to obtain the exact DMGD and other information between two modes. The optical fiber characterized by the embodiment is a 6-mode optical fiber. First, through self-calibration technology, TDLA is used to effectively solve the system error problem caused by the inconsistent optical path of each single-mode input end of MUX. Then, the ports of the fundamental mode and different high-order modes are connected by an optical coupler, and each mode in the few-mode optical fiber is accurately excited. It is achieved that only the fundamental mode interferes with the selected high-order mode at the receiving end, and the test data is recorded by a CCD camera. Then, the data is Fourier transformed, and the relative delay between the selected mode and the fundamental mode is accurately obtained, and further information such as its DMGD is obtained.

[0093] Time calibration method: LP 11 For example, through the above test system, the collected data is Fourier transformed and the peak value is extracted. The delay value corresponding to the peak value is the delay between the fundamental mode and LP 11 The relative delay between modes, Figure 5 (a) shows the time delay caused by the difference in pigtail length when time calibration is not performed; Figure 5Fig. 2(b) shows that the introduced time delay is gradually reduced in the calibration process; then the corresponding LP 11 mode path is adjusted and the experiment is repeated, as shown in Figure 5 Fig. 2(c), the calibration is completed, and the time delay is adjusted to the calibration completion state without peaks. It can be seen that after the TDLA adjustment, the relative time delay between the fundamental mode and the LP 11 mode is reduced from 15.9 ps to 1.2 ps. Then the time delay of the corresponding LP 11 mode path of the TDLA is adjusted again until there is no peak corresponding to the time delay after Fourier transform of the collected data, as shown in Figure 5 Fig. 2(d), at this time it is considered that the calibration between the fundamental mode and the LP 11 mode is completed; then, a 200 m long few-mode fiber supporting 6 LP modes with low DMGD is connected at the end of the mode multiplexer, and the DMGD of the LP 11 modes is tested, and after the time calibration method, the accurately measured LP 11 mode DMGD is 44.91 ps / km.

[0094] Example 2

[0095] After completing the measurement of the DMGD of the tested fiber, in order to further measure the CD parameters of each mode in the few-mode fiber, a reference light path is introduced based on the system shown in Figure 6 , which is used to construct a planar wave interference structure, as shown in Figure 6 , the S2 test system using the accurate mode excitation and time calibration method for the few-mode fiber characterization system is used to measure the absolute chromatic dispersion.

[0096] The reference light path is connected to the fundamental mode channel of the mode multiplexer through an optical coupler, so that the system only interferes the reference path with the few-mode fiber path. In this structure, the tunable laser output is directly irradiated to the CCD through the reference path, and the interference fringes image is formed after the interference of the fundamental mode output by the tested fiber. After collecting the interference image, Fourier transform is performed to obtain the relative time delay peak value between the fundamental mode and the reference wave. By adjusting the center wavelength of the tunable laser, the change of the relative time delay τ at different wavelengths is recorded, and the discrete data points are obtained. The discrete data points are fitted by a second-order polynomial, and the fitting function is obtained and the wavelength is derived, so that the absolute CD parameter of the fundamental mode is obtained. Further, based on the DMGD data between each high-order mode and the fundamental mode in the C band obtained by the system shown in Figure 4 , the time delay difference between each high-order mode and the fundamental mode is fitted by a second-order polynomial and derived, and the CD difference value of the high-order mode relative to the fundamental mode is obtained. The difference value is superimposed with the CD value of the fundamental mode, and the absolute CD value of each high-order mode is obtained. Finally, the absolute CD parameters of all supported modes in the tested fiber are obtained, and the quantitative extraction of the full modal chromatic dispersion characteristics is realized.

[0097] Example 3

[0098] As Figure 7 shown, the multi-mode excitation and power regulation system based on variable optical attenuator uses a 1 / 6 optical coupler to connect each mode port of the mode multiplexer, so that each mode signal can be independently accessed to the control module. In order to further flexibly adjust the excitation intensity of each mode, a variable optical attenuator (VOA) is connected in series at the output port of each MUX and the 1 / 6 coupler. By adjusting the attenuation of each VOA, the optical power ratio of each mode channel can be accurately controlled, thereby realizing flexible setting of the power distribution of different modes. In actual operation, first, the power ratio corresponding to each mode is set according to the target experimental requirements, and then the VOA is adjusted step by step until the output power of each mode reaches the preset value. In addition, in order to eliminate the group delay error caused by the inconsistent tail fiber length of each port of the mode multiplexer, an adjustable delay line array is introduced in the system for compensation. By calibrating and dynamically adjusting the adjustable delay line array, the time delay consistency of each channel is ensured. Finally, at the receiving end, multi-mode excitation with different ratios and accurate group delay matching can be realized at the same time, effectively supporting subsequent multi-mode interference, characteristic measurement and performance evaluation. This method not only improves the flexibility and adjustability of the system, but also provides a reliable experimental basis for the comprehensive testing of complex multi-mode optical communication systems and devices.

[0099] Example 4

[0100] The experimental system of mode ordered transmission based on time calibration method improvement, as Figure 9 shown, this embodiment is based on Figure 9After initially completing delay compensation, the experimental system shown further adjusts the TDLA to introduce specific delay values ​​for each mode. This step precisely controls the delay differences between different modes, ensuring accurate synchronization of different modes during signal propagation. The TDLA adjustment process involves comparing measurement results and gradually adjusting the delays so that the relative delays between modes meet the desired standard. Next, the 1-to-2 coupler used in the experiment is replaced with a 1-to-6 coupler, enabling connection to each port of the six-mode multiplexer. With this connection method, the six-mode multiplexer ensures that the light intensity of each mode is equal and that the signals of each mode are transmitted in an orderly manner. The key to this process lies in precisely controlling the excitation and transmission path of each mode to ensure that each mode propagates in the fiber system in a predetermined manner, thereby eliminating mutual interference between different modes. In this embodiment, all operational steps combine the use of a MUX, TDLA, and a new optical coupler structure to successfully achieve precise excitation and transmission of different modes in a multimode fiber system. This greatly facilitates the subsequent performance characterization of few-mode fibers and provides technical support for high-precision mode testing and signal transmission.

[0101] Example 5

[0102] like Figure 8 As shown, a system for regulating mode excitation time for automatic mode switching is introduced in this system, which is used to achieve fast and automatic switching between the fundamental mode and any higher-order mode. Traditional characterization systems usually require manual replacement of optical path connections when switching between different mode groups, which is cumbersome and prone to errors. However, this embodiment connects the OSW after each mode port of the mode multiplexer and combines it with an adjustable delay line array to dynamically compensate for the length differences of the pigtails in each path. It can quickly switch between different mode combinations without interrupting the test process. The specific process is: the switching sequence and time window are preset, and the OSW automatically switches between the fundamental mode and the target high-order mode according to the instructions to excite the required mode combination; the system synchronously adjusts the delay of the adjustable delay line of the corresponding path to achieve accurate delay compensation. After each switch, the interference signal is immediately collected and Fourier transform analysis is performed to extract the differential mode group delay information between the fundamental mode and the higher-order mode. There is no need to rebuild the optical path or recalibrate during the entire process, which greatly improves the test efficiency and data consistency. This solution is particularly suitable for scenarios with multi-mode and large-scale characterization requirements. It can significantly reduce manual operation errors, improve the automation level and reliability of the test system, and provide an efficient means for rapid performance evaluation of complex few-mode optical fibers or multi-mode devices.

[0103] Example 6

[0104] The embodiment identifies the peak shift after Fourier transform by introducing a time delay value to the port of the adjustable delay line, so as to realize the determination of the speed of the reference branch and the branch to be calibrated. The specific steps are as follows: after the time calibration is completed, first, a positive time delay value is introduced to the reference branch through the adjustable delay line, and the optical signal is introduced into the optical fiber to be measured and Fourier transformed. By observing the data after Fourier transform, it can be seen that the interference peak of the two beams of light changes with the adjustment of the introduced time delay value. When the time delay value is introduced, the interference peak of the two beams of light will form obvious shift. Through these shift data, the system can judge the speed difference of the optical signal propagation between the reference branch and the branch to be calibrated. By further analyzing the peak change after Fourier transform, the speed ratio between the reference branch and the branch to be calibrated can be accurately judged, and the relative propagation speed is obtained.

[0105] In this process, the fine adjustment of the delay line enables the measurement system to respond very sensitively to the speed difference between the fundamental mode and the high-order mode, and to judge through the peak value after Fourier transform. Through this method, the measurement deviation caused by system error or other external factors can be effectively avoided, and the high precision and reliability of the measurement result are ensured. This embodiment of the present application provides an effective scheme for the accurate measurement of the speed difference between the fundamental mode and the high-order mode in the few-mode optical fiber

[0106] Although the present application is described above in conjunction with the drawings, the present application is not limited to the specific embodiments described above, and the specific embodiments described above are only illustrative but not limiting, and those skilled in the art can make many improvements and changes under the inspiration of the present application without departing from the purpose of the present application, and these all belong to the protection of the present application.

Claims

1. A method for achieving precise synchronization of excitation light signals in a space division multiplexer based on time calibration, characterized in that: The calibration system for implementing the method includes a tunable laser, a first M-way optical splitter, an adjustable delay line array, and a space division multiplexer; the space division multiplexer includes N input ports, N≥2, one of which is a reference port, and the remaining ports are ports to be calibrated; the adjustable delay line array consists of N delay lines; one of the delay lines is connected to the reference port of the space division multiplexer to form a reference branch, and the remaining delay lines are respectively connected to the ports to be calibrated of the space division multiplexer in a one-to-one correspondence to form N-1 branches to be calibrated; the output end of the tunable laser is connected to the input end of the first M-way optical splitter, M≥2, the optical path of one of the output ports of the first M-way optical splitter is connected to the reference branch, and the optical paths of the remaining output ports of the first M-way optical splitter are respectively switched and connected to the N-1 branches to be calibrated via flanges, and the method includes the following steps: Step 1) During each calibration, always keep the reference branch and one of the branches to be calibrated light-free; Step 2) The tunable laser collects data according to a set sweep interval Δf; the collected data is Fourier transformed and a peak value is extracted, where the peak value represents the relative delay between the reference branch and the current branch to be calibrated; Step 3) adjusting the delay of the current branch to be calibrated in the current adjustable delay line array through the software of the adjustable delay line array according to the current relative delay; Step 4) Determine whether the current delay is zero. If not, return to step 2). Otherwise, end the adjustment of the current branch delay to be calibrated. Step 5) The optical paths of the remaining output ports of the first M-way optical splitter are switched and connected to the N-1 branches to be calibrated through the flange, and the delays of all branches to be calibrated are adjusted according to steps 1) to 4).

2. The method for realizing accurate synchronization of excitation light signals of a space division multiplexer based on time calibration according to claim 1, characterized in that: The method for setting the tunable laser frequency sweep interval Δf is as follows: A ruler with millimeter-level precision is used to measure the length difference between the reference branch of the space division multiplexer and the pigtail of the branch to be calibrated to obtain a rough length difference. The delay value introduced by the rough length difference is estimated according to formula (1). The accuracy of the delay value is in the picosecond level. Where C is the speed of light, L is the rough length difference, n eff is the effective refractive index of the optical fiber, and t is the delay value; According to the delay value t, the frequency sweep interval Δf of the tunable laser is set according to formula (2): Where Δf is the frequency sweep interval, P is the number of collected data, and t is the delay value.

3. The method for realizing accurate synchronization of excitation light signals of a space division multiplexer based on time calibration according to claim 2, characterized in that: A second M-way optical splitter is provided between the output end of the tunable laser and the input end of the first M-way optical splitter; the output end of the tunable laser is connected to the input end of the second M-way optical splitter, one of the output ports of the second M-way optical splitter is connected to a variable optical attenuator, and the other output port of the second M-way optical splitter is connected to the input port of the first M-way optical splitter; a first polarization controller and a first variable optical attenuator are provided in sequence on the optical path of one of the output ports of the first M-way optical splitter according to the direction of the optical signal, and a second polarization controller and a second variable optical attenuator are provided in sequence on the optical paths of the remaining output ports of the first M-way optical splitter according to the direction of the optical signal; and the steps are included: Step 1) During each calibration, the reference branch and one of the branches to be calibrated are always kept open, the signal light pattern of the reference branch and the branch to be calibrated is observed, and the signal light pattern is restored to the standard pattern by adjusting the first polarization controller and the second polarization controller; Step 2) The tunable laser performs data acquisition according to a set frequency sweep interval Δf; Step 3) Performing a Fourier transform on the collected data and extracting a peak value, where the peak value represents the relative delay between the reference branch and the current branch to be calibrated. If the difference between the peak value and the noise floor is less than 5 dB, adjusting the light intensities of the reference branch and the current branch to be calibrated using the first and second variable optical attenuators, respectively, until the difference between the peak value and the noise floor is greater than or equal to 5 dB, and obtaining the current relative delay. Step 4) adjusting the delay of the current branch to be calibrated in the adjustable delay line array through the software of the adjustable delay line array according to the current relative delay; Step 5) Determine whether the current delay is zero. If not, return to step 2). Otherwise, end the adjustment of the current branch delay to be calibrated. Step 6) The optical paths of the remaining output ports of the first M-way optical splitter are switched and connected to the N-1 branches to be calibrated through the flange, and the delays of all branches to be calibrated are adjusted according to steps 1) to 5).

4. The method for realizing accurate synchronization of excitation light signals of a space division multiplexer based on time calibration according to any one of claims 1 to 3, characterized in that: The space division multiplexer is one of a mode multiplexer and a core division multiplexer.

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