A method and device for quickly measuring ultra-long-distance optical fiber transmission matrix

Abstract

The present invention discloses a method and device for quickly measuring an ultra-long-distance optical fiber transmission matrix, which belongs to the field of information optics. The solution couples a spatial mode into an ultra-long-distance few-mode / multi-mode optical fiber for transmission, obtains a common-path reference light at the output end of the ultra-long-distance few-mode / multi-mode optical fiber and interferes with the signal light at the output end, reconstructs the complex amplitude distribution of the signal light field output by the optical fiber, calculates the overlap integral of the complex amplitude distribution and the ideal complex amplitude distribution of different spatial modes to obtain the weight coefficients of different modes, quickly switches the mode incident on the ultra-long-distance few-mode / multi-mode optical fiber and calculates the weight coefficients of different modes at the output end in turn to obtain the transmission matrix of the ultra-long-distance few-mode / multi-mode optical fiber. The technical solution of the present invention breaks the coherence limitation of the measurement of the few-mode / multi-mode optical fiber transmission matrix, and provides a feasible solution for the measurement of the few-mode / multi-mode optical fiber transmission matrix over ultra-long distances (e.g., hundreds of kilometers).

Description

Technical Field

[0001] The present invention belongs to the field of information optics, and more specifically, relates to a method and device for quickly measuring an ultra-long-distance optical fiber transmission matrix. Background Art

[0002] In recent years, mode division multiplexing (MDM) technology, which loads different signals onto different spatial modes in few-mode / multimode optical fibers, has significantly increased the communication capacity limit within a single fiber and promoted further development in the field of optical fiber communications. Compared to traditional single-mode optical fiber communication systems, MDM communication technologies based on few-mode / multimode optical fibers are limited by the problem of mode crosstalk between different spatial modes transmitted within the fiber. Specifically, after long-distance transmission through the fiber, the input spatial mode couples with other modes, resulting in the light beam reaching the receiver as a superposition of multiple modes rather than the input spatial mode, which can cause bit errors at the receiver. Therefore, rapid measurement of the crosstalk matrix of few-mode / multimode optical fibers to measure the weight of coupling from different input modes to different output modes is of great significance for MDM communication applications based on few-mode / multimode optical fibers. The measured MDM matrix provides a clearer understanding of the coupling and evolution of spatial modes within the fiber, enabling mode crosstalk compensation within the fiber and improving the communication performance of optical fiber MDM communication systems.

[0003] To date, many techniques have been developed to measure the mode crosstalk matrix within few-mode / multimode fibers, including methods that use a demodulation hologram at the output end to demodulate the fiber's output modes, measurement methods based on the rotational Doppler effect, and mode crosstalk matrix measurement methods based on complex amplitude reconstruction algorithms. Among these, the method that uses a demodulation hologram at the output end to demodulate the fiber's output modes requires demodulating each output mode individually, requiring N×N measurements to measure an N×N mode crosstalk matrix, resulting in high complexity. The measurement method based on the rotational Doppler effect places extremely high demands on optical path alignment, resulting in high system loss and susceptibility to noise. Compared to these two methods, the mode crosstalk matrix measurement method based on the complex amplitude reconstruction algorithm offers advantages such as low complexity (measuring an N×N mode crosstalk matrix requires only N measurements) and a simple optical path. Therefore, it has been widely used in recent years for measuring the transmission matrix of few-mode / multimode fibers. Common complex amplitude reconstruction algorithms include the phase-shift interference complex amplitude reconstruction algorithm based on coaxial interference and the off-axis digital holography technology based on off-axis interference. The former reconstructs the complex amplitude distribution of the light field based on the interference light field measured after multiple phase shifts between the coaxial interference signal path and the reference path, while the latter reconstructs the complex amplitude information of the light field based on the off-axis interference hologram of a single signal path and the reference path. However, although both interference methods are highly efficient, they are also limited by the coherence of the light field: considering that there is a long-distance path mismatch in the signal path after the light beam is transmitted in a long-distance few-mode / multi-mode optical fiber, in order to reconstruct the complex amplitude information of the light field through interference at the receiving end, the laser source needs to have very high coherence. In 2019, Bell Labs achieved rapid measurement of a 26.5km multimode optical fiber transmission matrix based on a sub-hertz linewidth laser with excellent coherence.

[0004] However, for practical mode-division multiplexing (MDM) fiber-optic communication systems, it is difficult to achieve linewidths below the kHz level with commercial lasers. Therefore, transmission matrix measurements for practical MDM communication systems over ultra-long-haul few-mode / multimode fibers are difficult to implement due to such coherence limitations. Therefore, developing a method and device for rapid measurement of the transmission matrix of ultra-long-haul optical fibers, freeing transmission matrix measurements for few-mode / multimode fibers from the coherence limitations imposed by ultra-long-haul fiber transmission, has become an urgent issue to address. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention provides a method and device for rapidly measuring the transmission matrix of an ultra-long-distance optical fiber. The goal is to obtain a stable and high-performance common-path reference light based on a common-path reference light acquisition method, thereby achieving high-contrast interference at the output end of an ultra-long-distance few-mode / multimode fiber. Based on the recorded high-contrast interference pattern, the complex amplitude of the light field at the output end of the few-mode / multimode fiber is reconstructed, thereby obtaining the transmission matrix of the ultra-long-distance few-mode / multimode fiber.

[0006] To achieve the above object, the present invention provides a method for quickly measuring an ultra-long-distance optical fiber transmission matrix, which is used to measure the transmission matrix of N modes, where N is a positive integer, and includes the following steps:

[0007] Taking any one of the N modes as a reference mode, preprocessing the light beam of the reference mode to obtain the field distribution of the output light field of the reference mode;

[0008] The preprocessing includes: coupling the light beam into the optical fiber to be tested, and after transmission through the optical fiber to be tested, dividing the output light beam of the optical fiber to be tested into two paths, one for signal light and the other for reference light; detecting the intensity of the signal light and feeding it back to the reference light; converting the reference light into a reference light with uniform phase, and limiting the reference light to a preset polarization, and then adjusting the power ratio of the signal light and the reference light according to the intensity of the signal light to ensure interference contrast; combining the reference light and the signal light, causing the reference light to interfere with the signal light, and dividing the obtained interference light into two polarized light beams, respectively obtaining a first interference pattern and a second interference pattern after imaging the two polarized light beams, and reconstructing the field distribution of the output light field of the light beam based on the first interference pattern and the second interference pattern;

[0009] The reference mode is sequentially subjected to polarization control and mode control to be controlled into N-1 modes other than the reference mode, and the N-1 modes other than the reference mode are respectively subjected to the preprocessing to obtain field distributions of output light fields of the N-1 modes;

[0010] Calculating weight coefficients of N modes according to the field distribution of the output light fields of the reference mode and N-1 modes, that is, obtaining N rows of the optical fiber transmission matrix to be tested;

[0011] After superimposing any mode other than the reference mode onto the reference mode to obtain N-1 superimposed modes, the light beams are preprocessed to obtain the field distribution of the output light fields of the N-1 superimposed modes; the phase difference between the field distribution of each superimposed mode and the reference mode is calculated, and the initial phase difference between different rows is obtained based on the phase difference to determine the arrangement of the N rows, thereby obtaining the optical fiber transmission matrix to be tested.

[0012] Optionally, when measuring an ultra-long-distance few-mode / multimode optical fiber transmission matrix, the selected mode basis is a linearly polarized vortex mode, an elliptical polarization vortex mode, a circularly polarized vortex mode, a linear polarization mode, or a vector mode. The optical field distribution at the optical fiber output end is obtained by interferometry between a signal path and a reference path, using a phase-shift interferometry method or off-axis digital holography.

[0013] Optionally, when the phase transmission matrix of an ultra-long-distance few-mode / multimode fiber needs to be measured, the first and second modes can be respectively input into the fiber under test at the input end through polarization control and mode control to obtain a first interference light field and a second interference light field. Then, the superposition state of the first and second modes is input into the fiber under test to obtain a third interference light field at the fiber output end. The global phase difference of the common-path reference light obtained when the first and second modes are respectively input is derived from the first, second, and third interference light fields, thereby obtaining the phase transmission matrix of the long-distance fiber.

[0014] Optionally, the reference light is converted into a reference light with uniform phase by spatial filtering, and the reference light is limited to a preset polarization, where the preset polarization is 45°.

[0015] Optionally, polarization control is achieved through piezoelectric polarization modulation or electro-optical polarization modulation; mode control is achieved by controlling the two-dimensional phase and amplitude distribution of the light field; and the reference light is confined to a preset polarization to achieve tracking and control of the polarization state through piezoelectric control, lithium niobate polarization control, lithium niobate thin film polarization control or silicon-based polarization control.

[0016] The present invention also provides a fast measurement device for ultra-long-distance optical fiber transmission matrix, comprising: a laser, a control module, a pre-processing module and a processor.

[0017] The control module is used to perform polarization control and mode control on the reference mode generated by the laser to obtain N-1 modes other than the reference mode;

[0018] The preprocessing module includes an objective lens, an optical fiber to be tested, a beam splitter, a reference light acquisition module, a first power matching module, a reference light acquisition module, a polarization tracking module, a second power matching module, a beam combiner, a polarization beam splitter, and a camera; the light beam is coupled to the optical fiber to be tested through the objective lens, and after being transmitted through the optical fiber to be tested, it is divided into two paths through the beam splitter, one for signal light and the other for reference light; the first power matching module is used to detect the intensity of the signal light and feed it back to the reference light; the reference light acquisition module is used to convert the reference light into a reference light with uniform phase, the polarization tracking module is used to limit the reference light to a preset polarization, and the second power matching module is used to adjust the power ratio of the signal light and the reference light according to the intensity of the fed-back signal light to ensure interference contrast; the reference light and the signal light are combined through the beam combiner, and interference occurs to obtain interference light, and the interference light is divided into two polarized light beams through the polarization beam splitter, and the two polarized light beams are imaged by the camera to obtain a first interference pattern and a second interference pattern, respectively, and the field distribution of the output light field of the light beam is reconstructed based on the first interference pattern and the second interference pattern;

[0019] The preprocessing module is used to take any one of the N modes as a reference mode, preprocess the light beam of the reference mode to obtain the field distribution of the output light field of the reference mode, and perform the preprocessing on N-1 modes other than the reference mode to obtain the field distribution of the output light field of the N-1 modes; after superimposing any other mode other than the reference mode on the reference mode to obtain N-1 superimposed modes, perform the preprocessing on the light beam of each mode to obtain the field distribution of the output light field of the N-1 superimposed modes;

[0020] The processor is used to calculate the weight coefficients of N modes based on the field distribution of the output light fields of the reference mode and N-1 modes, that is, to obtain N rows of the optical fiber transmission matrix to be tested; and calculate the phase difference between the field distribution of each superimposed mode and the reference mode, calculate the weight coefficient of each superimposed mode based on the phase difference, obtain the initial phase difference between different rows, and determine the arrangement of the N rows, thereby obtaining the optical fiber transmission matrix to be tested.

[0021] Optionally, the reference light acquisition module connects a multimode fiber to a single-mode fiber. The multimode reference light split by the beam splitter is coupled into the multimode fiber. The single-mode fiber connected to the multimode fiber acts as a spatial mode filter, filtering the multimode reference light into a Gaussian beam. To ensure that the light beam from the multimode fiber is always coupled into the single-mode fiber, the reference light acquisition module utilizes a fiber perturbation module. This module rapidly perturbs the single-mode fiber to ensure that the optical power in the single-mode fiber is non-zero when no light enters the single-mode fiber from the multimode fiber.

[0022] Optionally, the polarization control module is a piezoelectric polarization controller or an electro-optic polarization modulator; the mode control module is a digital micromirror device, a high-speed spatial light modulator or other high-speed mode modulator; the polarization tracking module is a piezoelectric polarization controller, a lithium niobate polarization modulator, a thin-film lithium niobate polarization modulator or a silicon-based polarization modulator; the second power matching module is an optical amplifier operating in automatic power control mode APC and a programmable optical attenuator.

[0023] Optionally, the optical fiber to be tested is a solid core few-mode / multimode optical fiber, a ring core few-mode / multimode optical fiber, a full-vector multimode optical fiber, an air core few-mode / multimode optical fiber, an antiresonant few-mode / multimode optical fiber, or a photonic bandgap few-mode / multimode optical fiber.

[0024] Furthermore, the present invention provides a method for mode division multiplexing communication in ultra-long-haul multimode optical fibers. This method and apparatus rapidly measures the intensity transmission matrix of the ultra-long-haul multimode optical fiber, quickly compensating for power crosstalk between different modes, improving communication system performance and reducing the bit error rate.

[0025] Furthermore, the present invention provides an ultra-long-distance multimode fiber imaging method. By rapidly measuring the intensity and phase transmission matrices of the ultra-long-distance multimode fiber, a mapping relationship between the input and output light fields is established, thereby reconstructing the input image from the complex output field. Alternatively, the chaotic light field calculated by pre-compensating the input at the multimode fiber input is used to obtain the target image output at the output. Alternatively, the light beam is converged to a single focus at the multimode fiber output to achieve endoscopic scanning imaging.

[0026] Compared with the prior art, the above technical solutions proposed by the present invention can achieve the following:

[0027] Beneficial effects:

[0028] 1. The solution provided by the present invention breaks the coherence limitation of the measurement of the few-mode / multimode optical fiber transmission matrix, greatly reducing the line width requirement of the laser for the measurement of the few-mode / multimode optical fiber transmission matrix, and provides a feasible solution for the measurement of the few-mode / multimode optical fiber transmission matrix over ultra-long distances (e.g., hundreds of kilometers).

[0029] 2. Compared to existing solutions that use single-mode fibers of the same length to balance signal light in few-mode / multimode fibers and reference light in single-mode fibers, the reference light in this invention shares a common path with the signal light. Therefore, the optical path differences between different order modes in the signal light and the reference light are extremely small, ensuring that after ultra-long-distance transmission, all order modes can achieve high-contrast interference with the reference light. Existing single-mode fiber balancing solutions, however, cannot balance the optical path differences between all signal light modes and the reference light because different order modes have different effective refractive indices when transmitted in multimode fibers.

[0030] 3. The present invention provides a solution to solve the problem of unknown global phase variation of common-path reference light by inputting mode superposition states. This solution realizes the phase transmission matrix measurement of ultra-long-distance few-mode / multi-mode optical fibers, thereby providing a basis for ultra-long-distance optical fiber imaging technology based on phase transmission matrix.

[0031] 4. The present invention can still realize the measurement of the ultra-long-distance few-mode / multi-mode optical fiber transmission matrix even when the light source coherence is poor, and can further realize the measurement of the ultra-long-distance few-mode / multi-mode optical fiber transmission matrix based on the decoherent light beam loaded with the signal, thereby realizing the rapid compensation of mode crosstalk in the ultra-long-distance optical fiber mode division multiplexing communication system, so as to improve the performance of the mode division multiplexing communication system. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a principle flow chart of the ultra-long-distance optical fiber transmission matrix rapid measurement method provided by the present invention.

[0033] Figure 2This is a comparison of the ultra-long-distance optical fiber transmission matrix rapid measurement solution provided by the present invention and the traditional technical solution.

[0034] Figure 3 The present invention provides a schematic diagram of the principle and device for rapid measurement of ultra-long-distance optical fiber transmission matrix.

[0035] Figure 4 This is the ideal field distribution of all circularly polarized vortex beam modes supported in the ring-core optical fiber to be tested provided by the present invention.

[0036] Figure 5 These are the experimental results of measuring the complex amplitude distribution and mode weight coefficient of the light field in free space and ring-core optical fiber using the off-axis holographic technology provided by the present invention.

[0037] Figure 6 This is the efficiency verification experimental device and results of the polarization tracking module in the embodiment of the present invention.

[0038] Figure 7 This is the working principle of the optical fiber rapid disturbance module provided by the present invention.

[0039] Figure 8 This is the experimental device and results for measuring the laser line width in the device of the embodiment of the present invention.

[0040] Figure 9 It is a principle verification experimental device for the phase transfer matrix measurement method provided by the present invention.

[0041] Figure 10 This is the experimental result of the principle verification of the phase transfer matrix measurement method provided by the present invention.

[0042] Figure 11 These are the experimental results of measuring different optical fiber transmission matrices using the device according to the embodiment of the present invention. DETAILED DESCRIPTION

[0043] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0044] like Figure 1 As shown, the present invention provides a principle flow chart of a method for rapid measurement of ultra-long-distance optical fiber transmission matrix, and the specific implementation method is as follows:

[0045] The incident light beam output by the laser is modulated into different modes through rapid polarization control and rapid mode control, and then the different modes are coupled into the optical fiber to be tested. After being transmitted through the optical fiber to be tested, at the output end of the optical fiber to be tested, the output light beam is divided into two paths, one for the signal path and the other for the reference path. These two light beams are two light beams transmitted through the same common path of the optical fiber to be tested. Among them, the signal path is polarized and split into two orthogonal polarization channels, and the reference path converts the mode output by the optical fiber to be tested into a reference light that is approximately a plane wave, and fixes the polarization of the reference light at a specific polarization state through polarization tracking. A power feedback adjustment device is introduced between the signal path and the reference path to match the power of the two paths. The reference path light beam is also polarized and split into two orthogonal polarization channels consistent with the signal light, and interferes with the two polarizations of the signal light beam respectively, obtaining polarization under two different polarization channels. Figure 1 and polarization Figure 2 After the optical fiber to be tested inputs a single mode, the interference Figure 1 and interference Figure 2 The field distribution of the output light field of the optical fiber to be tested is reconstructed, and the weight coefficients of different modes at the output end are obtained by overlapping integral with the ideal mode field distribution, that is,

[0046]

[0047] Among them, C l is the weight coefficient of the l-order mode, E S (x,y) is the complex amplitude distribution of the reconstructed signal light field, The complex conjugate of the complex amplitude distribution of the optical field in the ideal order mode is obtained by overlapping the reconstructed complex amplitude distribution of the optical field in the two polarization channels with all the different order modes supported by the optical fiber to obtain the weight coefficients of the output modes. The measured mode weight coefficient is a complex number, whose real part corresponds to the power weight of the mode and whose imaginary part corresponds to the global phase of the mode. Through the initial rapid polarization control and mode control, the modes supported by all the optical fibers under test at the input end are quickly switched. The above-mentioned transmission matrix measurement process is quickly performed at the output end of the optical fiber under test to obtain the values ​​of different rows of the transmission matrix, and finally complete the rapid measurement of the ultra-long-distance optical fiber transmission matrix.

[0048] like Figure 2 As shown in the figure, the ultra-long distance optical fiber transmission matrix fast measurement solution provided by the present invention is compared with the traditional technical solution. The specific implementation method is as follows:

[0049] Figure 2(a) is a schematic diagram of the device of the traditional technical solution. In the traditional solution, the light beam output by the laser is divided into two paths: the signal path and the reference path. Among them, the signal path is adjusted to the required mode and then coupled into the multimode optical fiber to be tested for transmission. At the output end, it is combined with the reference path and interferes. The interference pattern recorded by the CCD is used to reconstruct the complex amplitude of the light field and then calculate the weight coefficients of different modes in the output field. In the traditional technical solution, the signal light is transmitted through the optical fiber to be tested, while the reference path does not pass through such a path. Therefore, there is an optical path difference of the length of the optical fiber to be tested between the two. This puts forward requirements for the coherence of the laser. Considering the coherence time and coherence length definitions of the laser, for the optical path difference of the optical fiber to be tested with a length of L, the line width of the laser needs to meet at least the following requirements to achieve interference between the signal path and the reference path:

[0050]

[0051] Among them, is the coherence length of the laser, c is the speed of light in vacuum, n is the refractive index of the fiber to be measured, and Δυ is the line width of the laser. Therefore, there is a certain relationship between the laser line width and the measurable length of the fiber to be measured, such as Figure 2 As shown in (b), this traditional technical solution can only measure the part below the orange coherence boundary. Taking a 10kHz linewidth laser as an example, it is obviously impossible to measure a 10km fiber transmission matrix. In fact, considering the influence of interference contrast on complex amplitude reconstruction, Figure 2 The coherence boundary in (b) will be more stringent. In contrast, the technical solution provided by the present invention is as follows Figure 2 As shown in (c), the light beam output by the laser is regulated to the required mode and coupled to the multimode optical fiber to be measured for transmission. At the output end, the light field is split into a signal path and a reference path by BS1. The reference path obtains a common-path reference light through the spatial filtering measure of the multimode fiber to single-mode fiber (an example of obtaining the common-path reference light, which does not constitute a limitation of the present invention). The signal path and the common-path reference path light are combined by BS2 and interfere with each other. The interference field is recorded by the CCD to reconstruct the complex amplitude of the signal path light field to obtain different mode weight coefficients. In the technical solution provided by the present invention, the signal path and the reference path share the same path, so there is no coherence limitation in the traditional technical solution. Therefore, the laser line width is independent of the measurable optical fiber length, such as Figure 2 As shown in (d) in .

[0052] like Figure 3 As shown, the present invention provides a schematic diagram of a fast measurement principle and device for ultra-long-distance optical fiber transmission matrix, and the specific implementation method is as follows:

[0053] Figure 3(a) is a schematic diagram of the principle of rapid measurement of the ultra-long-distance optical fiber transmission matrix provided by the present invention, with left-handed and right-handed circularly polarized vortex beams carrying orbital angular momentum as the measurement mode basis. The incident orbital angular momentum beam is coupled into the optical fiber to be measured for transmission, and the beam output at the output end of the optical fiber to be measured is split by BS1 into two paths, the signal path and the reference path. Among them, the reference path obtains the common-path Gaussian reference light through the common-path reference light acquisition scheme of multi-mode optical fiber to single-mode optical fiber, and the polarizer Pol. is used to limit the polarization of the reference light. The signal path and the reference path beams are combined by BS2 and off-axis interference occurs. The beam displacer BD is used to split the two orthogonal linear polarizations, and the QWP of the signal path maps the left and right circular polarizations to two orthogonal linear polarizations, respectively, so as to measure the off-axis interference light field under the two orthogonal circular polarization channels on the CCD. When off-axis interference occurs, the light field distribution of the signal light and the reference light can be written as,

[0054]

[0055] Among them, E S (x,y) is the complex amplitude distribution of the signal light, is the phase distribution of the signal light, E R (x,y) is the complex amplitude distribution of the reference light, is the phase distribution of the reference light, where the term -2πu0x represents the off-axis phase of the reference light. The interference field of these two light fields after they are combined is written as:

[0056]

[0057] Where a(x,y)=|E S (x,y)| 2 +|E R (x,y)| 2 is the DC term of off-axis interference, and the other two terms are the AC terms of off-axis interference, and the two are complex conjugates of each other. Such off-axis interference AC terms are written as Perform Fourier transform on the off-axis interference field represented by equation (5), taking the one-dimensional Fourier transform in the x direction as an example:

[0058]

[0059] That is, in the spatial frequency domain after Fourier transform, the interference DC term and AC term are separated. Then, the interference AC term B(u-u0,y) is filtered out and moved to the center of the spatial frequency domain. Then, an inverse Fourier transform is performed to obtain the complex amplitude distribution of the signal light:

[0060]

[0061] In this way, the complex amplitude distribution of the light field can be obtained through the off-axis interference hologram. Figure 3(a) shows the measurement results of the left-handed channel -5th order vortex beam and the right-handed channel 5th order vortex beam. In this measurement, the light field is subjected to a two-dimensional Fourier transform to obtain the spatial frequency domain as shown in the figure. The interference AC term CC is separated from the interference DC term DC. The CC term is filtered out and moved to the center of the spatial frequency domain and an inverse Fourier transform is performed to obtain the light field intensity and phase distribution of the 5th and -5th order vortex beams as shown in the figure.

[0062] Figure 3(b) is a schematic diagram of the ultra-long-distance fiber transmission matrix rapid measurement device provided by the present invention. The laser output by the 1550nm laser is connected to the erbium-doped fiber amplifier EDFA through the single-mode fiber SMF to achieve power amplification. The output end of the EDFA is connected to the polarization controller PC for controlling the polarization of the light beam in the single-mode fiber. The PC is connected to the collimator Col.1 to achieve the fundamental mode Gaussian light output in free space. The polarizer Pol.1 limits the polarization of the light beam to the working polarization of the spatial light modulator SLM. The spatial light modulator corresponds to the fast mode control module (which can be replaced with a digital micromirror device to achieve higher-speed control). The quarter-wave plate QWP is used for fast polarization control. The light beam after passing through the SLM and QWP is the required specific beam mode. This beam mode is coupled through the objective lens OL into the optical fiber to be tested placed on the fiber coupling platform FCP for transmission. At the output end of the optical fiber to be tested, the output light field is collimated by the objective lens OL and then divided into a signal path and a reference path by the beam splitter BS1. Among them, the signal path passes through a quarter-wave plate QWP, which maps the left and right circular polarization into linearly polarized beams in the x-direction and y-direction respectively. A beam splitter BS2 is added to the signal path to split a portion of the signal path light. This light is then lens-coupled to a detector PD, where it is received and used as reference information for power matching between the signal and reference paths. The reference path beam is coupled into a multimode fiber (MMF) by collimator Col.2 and connected to a single-mode fiber (SMF) for spatial filtering to obtain the Gaussian base mode beam required for the reference light. The output of the single-mode fiber is connected to an erbium-doped fiber amplifier (EDFA) operating in constant power mode. The EDFA output is connected to a bandpass filter (BPF) to filter out the EDFA's spontaneous emission (ASE) noise and enhance off-axis interference contrast. The BPF output is connected to a programmable variable optical attenuator (VOA). The VOA receives power feedback from the signal path and rapidly adjusts the applied attenuation to achieve power matching between the signal and reference paths. The VOA output is connected to a polarization tracking module, which in turn is connected to collimator Col.3 to generate a Gaussian beam with a large free-space waist for use as the reference light. Polarizer Pol.2 limits the polarization of the reference light to 45°, ensuring that both orthogonal polarization measurement channels have the same reference light intensity. The signal and reference beams are combined by beam splitter BS3 to generate off-axis interference. Beam displacer BD is used to separate the off-axis interference into two orthogonal polarization channels, which are then received by the CCD. The off-axis interference holograms of the two polarization channels received by the CCD can be used to reconstruct the complex amplitude distribution of the light field in both polarization channels. Overlap integrals of this complex amplitude distribution with the complex amplitude distributions of the ideal modes are used to determine the power coefficients of the different modes. Rapidly switching the incident beam mode enables measurement of the mode transmission matrix of the fiber under test.

[0063] Because the reference light in this device is obtained by splitting the beam after being transmitted through the ultra-long-distance optical fiber to be tested and performing spatial filtering processing to convert the multimode fiber to a single-mode fiber, when different beam modes are input into the optical fiber to be tested, the global phase of the reference light obtained by the above measures is a different unknown global phase. Since the light field phase information measured by the interference method is the phase difference between the signal light and the reference light, there is an unknown phase difference between the different rows of the phase transmission matrix obtained under different beam mode input conditions. To address this problem, the present invention provides a method for measuring the phase transmission matrix of ultra-long-distance few-mode / multimode optical fibers, the specific implementation of which is as follows:

[0064] The input end inputs the first mode and the second mode into the optical fiber to be tested respectively through the fast polarization control and mode control to obtain the first interference light field and the second interference light field. The light field of the first mode after long-distance transmission through the optical fiber to be tested is written as Writing the light field of the second mode after long-distance optical fiber transmission When the first mode is input, the global phase of the first reference light is When the second mode is input, the global phase of the second reference light is The light field distributions of the first mode and the second mode obtained by measuring the first interference light field and the second interference light field can be written as and Afterwards, the superposition state of the first mode and the second mode is input into the optical fiber to be tested to obtain a third interference light field. The light field intensity distribution of the superposition state of the first mode and the second mode after transmission through the optical fiber to be tested can be obtained through the third interference light field and can be expressed as:

[0065]

[0066]

[0067] Because |E 1S |、|E 2S |、 and It is known that the global phase difference of the reference light under the first mode input and the second mode input can be obtained through the three interference light fields. Therefore, the phase difference between the two different rows of the phase transfer matrix obtained under the first mode and the second input condition becomes a known term. By rapidly switching the input mode and the input mode superposition state, the phase transfer matrix of different rows of the fiber under test can be measured and the phase difference between different rows can be obtained, thereby obtaining the entire phase transfer matrix of the fiber under test.

[0068] like Figure 4 As shown, the ideal field distribution of all circularly polarized vortex beam modes supported in the ring-core optical fiber to be tested provided by the present invention is specifically implemented as follows:

[0069] The results show all the circularly polarized vortex beam modes supported by the ring-core fiber under test in the embodiment, which are divided into 6 mode groups and 22 vortex modes. Except for the 0th-order mode group, the remaining five mode groups each have four vortex beams with opposite topological charges and orthogonal polarizations.

[0070] like Figure 5 As shown in FIG, the experimental results of measuring the complex amplitude distribution and mode weight coefficient of the light field using the off-axis holographic technology in free space and ring core fiber provided by the present invention are as follows:

[0071] Figure 5 (a) and (b) are the results of reconstructing the complex amplitude distribution of the light field using off-axis digital holography in free space. Figure 5 (a) shows the off-axis interference hologram obtained by off-axis interference of a pure -5th order vortex beam in free space in simulation and experiment, the spatial frequency domain after Fourier transform, the reconstructed intensity and phase distribution, and the power weights of different order vortex beams calculated by overlap integral. Figure 5 (b) shows the measurement results of similar -4th-order and 5th-order vortex beams superimposed with a power ratio of 1:2. Figure 5 (c) and (d) in the figure show the Figure 3 In (b), the device measures the crosstalk matrix when a +5th-order left-handed circularly polarized vortex beam is incident on a 100 km ring-core optical fiber. Figure 5 (c) shows the measurement results of the left-handed circularly polarized channel at the receiving end. The light field distribution of the left-handed circularly polarized channel is reconstructed by the recorded off-axis interference hologram, and the power weights of different-order vortex modes are calculated by overlapping integrals. Figure 5 (d) shows the measurement results of the left-handed circularly polarized channel at the receiving end. The light field distribution of the right-handed circularly polarized channel is reconstructed by recording the off-axis interference hologram, and the power weights of the different order vortex modes are calculated by overlap integral. The power weights calculated in both channels are a 1×11 crosstalk matrix. Then, they are normalized according to the ratio of the signal light intensities in the two polarization channels, and the two 1×11 crosstalk matrices are normalized to a 1×22 crosstalk matrix. By quickly switching the mode at the input end and performing a similar process, it is possible to achieve a 22×22 ring core fiber crosstalk matrix measurement.

[0072] like Figure 6 As shown, the present invention provides Figure 3 (b) The experimental device and results for verifying the efficiency of the polarization tracking module in the embodiment are as follows:

[0073] Figure 6(a) in the figure is an experimental device for verifying the efficiency of the polarization tracking module. The incident light beam is a rapidly switching vortex beam. After the BS splitting, one path is a signal path and the other is a reference path. Here, the polarization tracking effect of the reference path is investigated. The reference path light beam is connected to the single-mode fiber through a multimode fiber to realize the spatial filtering mode from the high-order mode to the zero-order Gaussian beam, thereby obtaining a common-path reference Gaussian beam. Since the vortex beam incident on the multimode fiber is a rapidly changing vortex beam, the polarization state of the Gaussian reference light output by the single-mode fiber will change rapidly, and it is not a stable polarization state. The off-axis holographic technology requires that the light field intensities of the two orthogonal polarization channels of the reference light are similar, that is, about 45° linear polarization beams. Therefore, a polarization tracking device is added. Here, a polarization beam splitter PBS is used after the polarization tracking device to split the light beam into two polarization paths, and the power ratio of the two orthogonal polarizations is measured to investigate the degree of polarization jitter. Figure 6 (b) shows the polarization jitter before polarization tracking is enabled. The range of polarization jitter exceeds 10dB. After polarization tracking is enabled, the range of polarization jitter is limited to within 2.5dB.

[0074] like Figure 7 As shown, the working principle of the optical fiber rapid disturbance module provided by the present invention is as follows:

[0075] When the reference light acquisition module is a device that connects a multimode fiber to a single-mode fiber, since the light beam incident on the multimode fiber switches rapidly, the light field transmitted in the multimode fiber also changes rapidly. Therefore, it is possible that the light beam in the multimode fiber cannot be coupled into the single-mode fiber. In this case, the following method is introduced: Figure 7 The piezoelectric ceramic tube shown is a fiber-optic rapid perturbation module. The field output from the multimode fiber to be measured is split into two paths by the BS, of which the reference path is coupled into the multimode fiber by a collimating lens. The multimode fiber is connected to the single-mode fiber to achieve spatial filtering to obtain the reference common-path Gaussian light. A piezoelectric ceramic tube is added to the multimode fiber as a fiber-optic rapid perturbation module. A 10:90 fiber beam splitter is used at the output end of the single-mode fiber to split the reference light. The 90% energy port passes through an erbium-doped fiber amplifier (EDFA), a bandpass filter (BPF), a variable optical attenuator (VOA), and a polarization tracking module as the reference light for off-axis interferometry. The 10% energy port is used to build a feedback device connected to the PD. When the power is detected to be zero, the piezoelectric ceramic tube is controlled to rapidly dither to ensure that the reference light intensity meets the off-axis holographic requirements.

[0076] like Figure 8 As shown, the present invention provides Figure 3 The experimental apparatus and results for measuring the laser line width in the embodiment (b) are as follows:

[0077] Figure 8(a) is the experimental setup for measuring the laser linewidth. The laser output signal is split into two paths by a 50:50 fiber coupler. One path passes through the variable optical attenuator (VOA) and the polarization controller (PC) before entering the coherent detector. The other path, by contrast, passes through a 20km single-mode fiber delay before entering the coherent detector from the other port. These two beams pass through a 90° hybrid in the coherent detector to obtain I / Q signal outputs. The oscilloscope samples the I / Q signals and uses the I / Q signal ratio to obtain the frequency modulation noise spectrum of the laser, thereby calculating the laser linewidth. The laser linewidth should be π times the frequency of the white noise range in the frequency modulation noise spectrum. Figure 8 (b) shows the principle diagram of the 90° Hybrid inside the coherent detector. After the two incident light beams pass through the 90° Hybrid, I / Q signal outputs are obtained. Figure 8 (c) shows the results of two different measurements. The white noise range of π times the frequency of the two measurements is about 10 kHz, that is, the line width of the laser used is 10 kHz.

[0078] like Figure 9 As shown, the principle verification experimental device of the phase transfer matrix measurement method provided by the present invention is specifically implemented as follows:

[0079] The collimated Gaussian beam output by the collimator (Col.) in free space is modulated into a specific pattern after passing through a polarizer (Pol.) and a spatial light modulator (SLM). The SLM's output beam is split into a signal path and a reference path by a spatial light modulator (BS). The reference path is subjected to a spatial filtering setup using a multimode fiber-to-singlemode fiber converter to generate a fundamental-mode Gaussian reference beam. The signal and reference paths are then combined by another BS to generate off-axis interference, which is recorded by a CCD. In this proof-of-principle experiment, the SLM modulated beam is a superposition of different modes to simulate the field generated after long-distance fiber transmission. In this proof-of-principle experiment, the field corresponding to the 5th-order beam is set to a superposition of -5th-order, -4th-order, +4th-order, and +5th-order vortex beams with power ratios of [46%, 4%, 4%, 46%] and a global phase of [0 0.5ππ 1.5π]. The field corresponding to the 4th-order beam is set as a beam superimposed by -5th-order, -4th-order, +4th-order, and +5th-order vortex beams with power ratios of [5%, 15%, 75%, 5%] and global phases of [0.7π, 1.8π, 1.3π, 1.3π]. First, the 5th-order superposition field (corresponding to the first mode of the above-mentioned invention scheme) is input and its off-axis interference hologram is recorded; then the 4th-order superposition field (corresponding to the second mode of the above-mentioned invention scheme) is input and its off-axis interference hologram is recorded; finally, the superposition beam of these two superposition fields is input (corresponding to the superposition state of the first and second modes of the above-mentioned invention scheme) to obtain its off-axis interference hologram. Based on these three off-axis interference holograms, the 4th-order and 5th-order phase transfer matrices can be calculated using the technical solution provided in the present invention.

[0080] like Figure 10 As shown in FIG, the principle verification experimental results of the phase transfer matrix measurement method provided by the present invention are as follows:

[0081] Figure 10 (a) and (b) in the figure respectively show the set values ​​of the intensity matrix and phase matrix of the set 5th-order and 4th-order light fields, as well as the intensity matrix and phase matrix of the 5th-order and 4th-order light fields measured in simulation and experiments based on the technical solution proposed in the present invention. Figure 10 (c) in the figure shows the intermediate results of the simulation, including the off-axis interference hologram of the simulated 5th-order light field and the intensity and phase distribution of the 5th-order light field reconstructed based on it (first row), the off-axis interference hologram of the simulated 4th-order light field and the intensity and phase distribution of the 4th-order light field reconstructed based on it (second row), as well as the off-axis interference hologram of the simulated 5th-order and 4th-order light field superposition state, the intensity distribution of the superposition state light field reconstructed based on the off-axis interference hologram, and the superposition state light field intensity distribution reconstructed according to the measured intensity matrix and phase matrix (third row). It can be seen that the measured intensity matrix and phase matrix can well restore the superposition state light field due to their high accuracy. Figure 10 (d) in the figure shows the intermediate results of the experiment, including the off-axis interference hologram of the experimental 5th-order light field and the intensity and phase distribution of the 5th-order light field reconstructed based on it (first row), the off-axis interference hologram of the experimental 4th-order light field and the intensity and phase distribution of the 4th-order light field reconstructed based on it (second row), as well as the off-axis interference hologram of the experimental 5th-order and 4th-order light field superposition state, the intensity distribution of the superposition state light field reconstructed based on the off-axis interference hologram, and the superposition state light field intensity distribution reconstructed according to the measured intensity matrix and phase matrix (third row). It can be seen that the measured intensity matrix and phase matrix can well restore the superposition state light field due to their high accuracy.

[0082] like Figure 11 As shown, the present invention provides Figure 3 The experimental results of the embodiment (b) of the present invention for measuring different optical fiber transmission matrices include:

[0083] from Figure 11 (a)-€ in the figure are the measured intensity transmission matrices of 5m air-core fiber, 5m ring-core fiber, 2km ring-core fiber, and the intensity transmission matrix and phase transmission matrix of 100km ring-core fiber, respectively.

[0084] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A fast measurement method for ultra-long-distance optical fiber transmission matrix, for measuring the transmission matrix of N modes, where N is a positive integer, characterized in that: The following steps are involved: Taking any one of the N modes as a reference mode, preprocessing the light beam of the reference mode to obtain the field distribution of the output light field of the reference mode; The preprocessing includes: coupling the light beam into the optical fiber to be tested, and after transmission through the optical fiber to be tested, dividing the output light beam of the optical fiber to be tested into two paths, one for signal light and the other for reference light; detecting the intensity of the signal light and feeding it back to the reference light; converting the reference light into a reference light with uniform phase, and limiting the reference light to a preset polarization, and then adjusting the power ratio of the signal light and the reference light according to the intensity of the signal light to ensure interference contrast; combining the reference light and the signal light, causing the reference light to interfere with the signal light, and dividing the obtained interference light into two polarized light beams, respectively obtaining a first interference pattern and a second interference pattern after imaging the two polarized light beams, and reconstructing the field distribution of the output light field of the light beam based on the first interference pattern and the second interference pattern; The reference mode is sequentially subjected to polarization control and mode control to be controlled into N-1 modes other than the reference mode, and the N-1 modes other than the reference mode are respectively subjected to the preprocessing to obtain field distributions of output light fields of the N-1 modes; Calculating weight coefficients of N modes according to the field distribution of the output light fields of the reference mode and N-1 modes, that is, obtaining N rows of the optical fiber transmission matrix to be tested; After superimposing any mode other than the reference mode onto the reference mode to obtain N-1 superimposed modes, the light beams are preprocessed to obtain the field distribution of the output light fields of the N-1 superimposed modes; the phase difference between the field distribution of each superimposed mode and the reference mode is calculated, and the initial phase difference between different rows is obtained based on the phase difference to determine the arrangement of the N rows, thereby obtaining the optical fiber transmission matrix to be tested.

2. The ultra-long distance optical fiber transmission matrix rapid measurement method according to claim 1, characterized in that: The reference light is converted into phase-uniform reference light through spatial filtering.

3. The ultra-long distance optical fiber transmission matrix rapid measurement method according to claim 2, characterized in that: The reference light is converted into a reference light with uniform phase, and the reference light is limited to a preset polarization, where the preset polarization is 45°.

4. The ultra-long distance optical fiber transmission matrix rapid measurement method according to claim 1, characterized in that: The polarization control is achieved through piezoelectric polarization modulation or electro-optical polarization modulation; the mode control is achieved by controlling the two-dimensional phase and amplitude distribution of the light field; and the limiting of the reference light to a preset polarization is achieved through piezoelectric control, lithium niobate polarization control, lithium niobate thin film polarization control or silicon-based polarization control.

5. Ultra-long distance optical fiber transmission matrix rapid measurement device, characterized in that: include: Laser, control module, pre-processing module and processor, The control module is used to perform polarization control and mode control on the reference mode generated by the laser to obtain N-1 modes other than the reference mode; The preprocessing module includes an objective lens, an optical fiber to be tested, a beam splitter, a reference light acquisition module, a first power matching module, a reference light acquisition module, a polarization tracking module, a second power matching module, a beam combiner, a polarization beam splitter, and a camera; the light beam is coupled to the optical fiber to be tested through the objective lens, and after being transmitted through the optical fiber to be tested, it is split into two paths by the beam splitter, one path being the signal light and the other being the reference light; The first power matching module is used to detect the intensity of the signal light and feed it back to the reference light; the reference light acquisition module is used to convert the reference light into a reference light with uniform phase; the polarization tracking module is used to limit the reference light to a preset polarization; the second power matching module is used to adjust the power ratio of the signal light and the reference light according to the intensity of the feedback signal light to ensure interference contrast; the reference light and the signal light are combined by a beam combiner, and interference is generated to obtain interference light, and the interference light is divided into two polarized light beams by a polarization beam splitter. The two polarized light beams are imaged by a camera to obtain a first interference pattern and a second interference pattern, respectively. The field distribution of the output light field of the light beam is reconstructed based on the first interference pattern and the second interference pattern; The preprocessing module is used to take any one of the N modes as a reference mode, preprocess the light beam of the reference mode to obtain the field distribution of the output light field of the reference mode, and perform the preprocessing on N-1 modes other than the reference mode to obtain the field distribution of the output light field of the N-1 modes; after superimposing any other mode other than the reference mode on the reference mode to obtain N-1 superimposed modes, perform the preprocessing on the light beam of each mode to obtain the field distribution of the output light field of the N-1 superimposed modes; The processor is used to calculate the weight coefficients of N modes based on the field distribution of the output light fields of the reference mode and N-1 modes, that is, to obtain N rows of the optical fiber transmission matrix to be tested; and calculate the phase difference between the field distribution of each superimposed mode and the reference mode, calculate the weight coefficient of each superimposed mode based on the phase difference, obtain the initial phase difference between different rows, and determine the arrangement of the N rows, thereby obtaining the optical fiber transmission matrix to be tested.

6. The ultra-long distance optical fiber transmission matrix rapid measurement device according to claim 5, characterized in that: The reference light acquisition module includes a multimode optical fiber and a single-mode optical fiber connected to each other. The multimode optical fiber is used to couple the reference light, and the single-mode optical fiber is used to spatially filter the reference light output by the multimode optical fiber to obtain a Gaussian beam.

7. The ultra-long distance optical fiber transmission matrix rapid measurement device according to claim 6, characterized in that: The reference light acquisition module further includes an optical fiber perturbation module, which is arranged on the multimode optical fiber and is used to ensure that the optical power in the single-mode optical fiber is not zero by perturbation when no light enters the single-mode optical fiber from the multimode optical fiber; The optical fiber disturbance module is a piezoelectric ceramic tube or a piezoelectric bimorph disturber.

8. The ultra-long distance optical fiber transmission matrix rapid measurement device according to claim 5, characterized in that: The polarization control module is a piezoelectric polarization controller or an electro-optical polarization modulator; the mode control module is a digital micromirror device or a spatial light modulator; the polarization tracking module is a piezoelectric polarization controller, a lithium niobate polarization modulator, a thin-film lithium niobate polarization modulator or a silicon-based polarization modulator; the second power matching module is an optical amplifier and an optical attenuator working in the automatic power control mode APC.

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