Multi-path receiving fusion method for reducing signal outage probability and verification system thereof

Abstract

The invention discloses a multi-path receiving fusion method for reducing the signal outage probability and a verification system thereof. The method comprises the steps of DMT signal modulation and OAM multi-path simultaneous modulation. Multiplexing the OAM mode group; performing multi-channel demodulation, fusion receiving and data analysis in an OAM mode group; the verification system comprises a user data modulation module on a carrier, an OAM mode modulation and propagation module and an OAM demodulation and offline processing module. The WCOF with a three-layer structure is designed and prepared, the RIC between the core layer and the cladding is smaller than 1%, the propagation loss of a supported vector mode is reduced, and the minimum effective refractive index difference between different OMGs can be obtained; the WCOF provided by the invention can be used for OMG multiplexing based on OAM, and has relatively low propagation loss; and the channel capacity of a multiplexing optical fiber system based on the OMG is improved, and the interruption probability of signals is reduced.

Description

Technical Field

[0001] The present invention relates to the field of signal transmission, and in particular to a multi-path receiving fusion method and a verification system thereof for reducing the probability of signal interruption. Background Art

[0002] As the demand for channel capacity increases for a large number of new applications (such as unmanned technology, artificial intelligence, online interaction, etc.), traditional multiplexing methods (such as time, polarization, mode, wavelength, etc.) will not be able to meet these needs in the near future. In order to alleviate the above concerns, the most effective way is to explore a new dimension of independent free-space multiplexing, which is expected to improve channel capacity (CC) and spectral efficiency (SE). Due to the orthogonality between different OAM modes and the infinite number of OAM modes estimated by Hilbert, each OAM mode can be regarded as an independent channel and then used through space division multiplexing (SDM). In addition, OAM modes are independent of other traditional physical spaces, which means that OAM-based SDM is compatible with other traditional multiplexing methods, thereby significantly improving channel capacity and spectral efficiency. Although data rates exceeding existing Tbit / s have been achieved by combining OAM-based SDM with traditional multiplexing technology, signal interruption or fading caused by crosstalk, turbulence, fiber loss, etc. are inevitable in OAM-based communication systems. When the received bit error rate (BER) reaches a high level and exceeds the error correction capability, communication may still be interrupted.

[0003] Especially for OAM-based SDM and long-distance communication systems, weakly coupled OAM fibers are usually used as propagation media with low propagation losses, in which the refractive index contrast (RIC) between the core and cladding is designed to be low. Although in OMG-based SDM, a large effective refractive index difference (Δn) can be achieved between different OAM mode groups (OMGs), eff ), but it is inevitable that the effective refractive index difference between OAM modes within the same OMG is small. This easily leads to severe crosstalk between OAM modes within the same OMG, especially in low-order OMGs, thereby increasing the probability of signal interruption. To alleviate the signal interruption problem, there are three main approaches, including adaptive optics, MIMO-based digital signal processing, and deep learning. However, when OAM-based SDM is jointly multiplexed with traditional multiplexing technology, the complexity of MIMO increases significantly as the number of multiplexed channels in digital signal processing increases. In addition, in traditional OMG multiplexing systems, only one OAM mode is selected and received for demodulation, and the propagated signal may be interrupted due to crosstalk between different OAM modes. Therefore, a concise and effective method is needed to reduce signal interruption in long-distance OAM communication systems. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a multi-channel reception fusion method and a verification system thereof for reducing the probability of signal interruption.

[0005] Technical solution: The multi-channel receiving fusion method for reducing the probability of signal interruption of the present invention comprises the following steps:

[0006] (1) DMT signal modulation and OAM multi-channel simultaneous modulation;

[0007] (2) OAM mode group multiplexing;

[0008] (3) Multi-channel demodulation, fusion reception and data analysis within the OAM mode group.

[0009] Furthermore, the DMT signal modulation in step (1) is implemented by a DMT signal radio frequency signal modulation module, completing QAM modulation and DMT encapsulation of the user's original random data.

[0010] Furthermore, the DMT signal modulation in step (1) includes:

[0011] (1.1) Using binary pseudo-random polynomial x 20 +x 3 +1 Generates a random length of 2 20 -1 bit serial data, simulating the actual user data;

[0012] (1.2) converting the generated serial user data into a series of continuous parallel data groups with a length of 4 bits;

[0013] (1.3) Mapping the generated 4-bit pseudo-random data into a hexadecimal symbol and using quadrature amplitude modulation to implement QAM-16 modulation of the 4-bit data;

[0014] (1.4) The QAM-16 modulated signal is divided into two equal paths through a beam splitter;

[0015] (1.5) Using the conjugate conversion method, a QAM-16 adjusted signal is converted into its conjugate signal;

[0016] (1.6) Using an adder, QAM-16 and QAM-16' are added to cancel out the imaginary part of the signal, obtaining a signal containing only twice the real part, thus forming a 64-channel discrete multi-tone signal;

[0017] (1.7) Convert the DMT signal into a time domain signal through inverse Fourier transform;

[0018] (1.8) A fixed overhead cyclic prefix byte (CP) is inserted in front of the generated DMT signal according to certain timing requirements to achieve isolation between different symbols;

[0019] (1.9) Insert a long training sequence directly in front of the CP to facilitate signal synchronization and channel estimation;

[0020] (1.10) The encapsulated DMT signal is converted into a serial signal through a parallel-to-serial conversion module;

[0021] (1.11) Using a vector signal generator, modulate the DMT signal into an analog RF signal;

[0022] The RF signal carrying the DMT signal generated by (1.12) and a laser together drive a Mach-Zehnder modulator, thereby modulating the user signal onto a Gaussian optical carrier and realizing optical transmission of the user signal.

[0023] Furthermore, the OAM multi-channel simultaneous modulation in step (1) includes:

[0024] (2.1) The transmitted GB light is incident on a multi-helical phase modulator;

[0025] (2.2) Design a multi-spiral phase modulator, which includes three OAM spiral phases of l = 1, TC = 1, l = 2, TC = 2, and l = 3, TC = 3, arranged at diffraction angles of 0 degrees, 90 degrees, and 180 degrees, thereby constructing a triangularly arranged multi-spiral phase modulator;

[0026] (2.3) The modulated OAM light is output along different diffraction angles, completing the simultaneous modulation of different OAM mode lights.

[0027] Furthermore, the OAM mode group multiplexing in step (2) includes:

[0028] (3.1) The three OAM mode group lights modulated by the objective lens are coupled into the three input port fibers of the fiber array coupler respectively;

[0029] (3.2) Using a fiber array coupler to couple or multiplex three different OAM mode lights into one OAM light;

[0030] (3.3) The fiber array coupler mainly consists of two parts, one is the input part and the other is the tapered coupling part. The main function of the input part is to receive the free-space OAM light and couple it into the three-way interface fiber of the fiber array coupler. The function of the tapered part is to multiplex the three-way OAM light into one-way OAM light. The diameters of the three-way OAM fibers that are tapered by the fiber tapering method are Micrometer, Micrometer, Micrometer, where n eff It represents the equivalent refractive index of different modes and is fused with an OAM fiber to achieve OAM optical coupling and multiplexing of different transmission paths.

[0031] Furthermore, the OAM optical fiber has a three-layer structure, and their radii are r1=1.82um, r2=7.80um and r3=62.5um, and their refractive indexes are n1, n1=1.448, n2, n2=1.455 and n3, n3=1.444.

[0032] Furthermore, the verification of the OAM optical fiber performance includes:

[0033] The finite element method is used to analyze the equivalent refractive index distribution of the OAM mode supported by the optical fiber;

[0034] Analyze the group delay between different mode groups based on the equivalent refractive index distribution;

[0035] The optical time domain reflectometry is used to analyze the transmission loss of the mode groups supported by OAM optical fibers.

[0036] Furthermore, in the step (3), the multi-channel demodulation, fusion reception and data analysis within the OAM mode group are implemented by a multi-channel OAM mode demodulation and fusion module within the OAM mode group. The demodulation and fusion module demodulates a pair of conjugate modes within the mode group, and adopts a maximum ratio fusion method to formulate weight coefficients between different OAM modes to recover user signals and avoid signal interruption. The transmission OAM mode can be expressed as or Wherein the superscript ± indicates left / right circular polarization, the subscript ± indicates left / right rotation phase, m indicates radial coefficient, and |l| indicates the absolute value of TC.

[0037] Furthermore, the step (3) comprises:

[0038] (4.1) An OAM mode transmitted in optical fiber Optical OAM l，m The energy of This results in energy splitting, resulting in four OAM modes actually being transmitted in the channel;

[0039] (4.2) Through a quarter-wave plate, the left circular / right circularly polarized OAM light is converted into X horizontal / Y vertically polarized OAM combined light OAM respectively. |l|，m +OAM -|l|，m ;

[0040] (4.3) Through a half-wave plate, the OAM combined light of the Y vertical polarization branch is converted into the OAM combined light of the X horizontal polarization branch;

[0041] (4.4) Through a beam combiner, the two X-horizontally polarized lights are combined into one OAM light containing only X-horizontally polarized light. |l|，m +OAM -|l|，m , and is equally divided into 6 paths;

[0042] (4.5) When the six beams are irradiated into the helical plates 1 to 6 carrying the helical beam phases of l = ±1 to ±3, the six transmitted OAM beams, P1 to 6, are converted into Gaussian beams 1 to 6 accordingly;

[0043] (4.6) When the six Gaussian beams illuminate the six photodetectors respectively, the radio frequency signals carried by the six Gaussian beams are converted into electrical signals 1 to 6 respectively, where the photodetectors only work on the radio frequency signals and are ineffective on the six optical carrier signals;

[0044] (4.7) According to the received 6 electrical signals 1 to 6 in the 3 mode groups |l|=1, 2, 3, DMT de-framing is performed respectively and the original user data carried by l=±1 to ±3 is obtained, which mainly includes serial-to-parallel conversion, frame header search and positioning, training sequence removal, redundant cyclic prefix removal, Fourier transform, and QAM demodulation.

[0045] (4.8) Using the error vector magnitude method, combine the two conjugates in each mode group, l = ±1 / ±2 / ±3, and the input electrical signals 1-2 / 3-4 / 5-6, measure their combined EVM, and determine the percentage of the electrical signal 1 / 3 / 5, l = +1 / +2 / +3, and the electrical signal 2 / 4 / 6, l = -1 / -2 / -3 at the minimum EVM point, and use this percentage as the weight coefficient of the two electrical signals in the mode group;

[0046] (4.9) According to the obtained weight coefficient, the DSP method is used to propose the data carried by each initial pattern group, |l| = 1, 2, 3;

[0047] (4.10) According to the received data, the bit error rate BER analysis and the constellation diagram CP analysis are performed respectively;

[0048] (4.11) Based on the obtained BER and CP diagrams, the signal interruption probability is analyzed and the system performance is evaluated. When the bit error rate is greater than 3.8×10 -3 When the bit error is uncorrectable and causes signal interruption, it is determined whether the system can transmit data reliably.

[0049] The verification system of the multi-channel receiving fusion method for reducing the probability of signal interruption of the present invention comprises: a user data modulation module on the carrier, an OAM mode modulation and propagation module, and an OAM demodulation and offline processing module;

[0050] The user data modulation module on the carrier includes EA: electrical amplifier, PC: polarization controller, Q: quarter wave plate, H: half wave plate, BPF: bandpass filter, OL: objective lens, OC: optical coupler, C: collimator, P: polarizer, BS: beam splitter, M: reflector, SMF: single mode fiber, ECL: external cavity laser;

[0051] The OAM mode modulation and propagation module includes M: reflector, FAC: fiber array coupler, EDFA: erbium-doped fiber amplifier;

[0052] The OAM demodulation and offline processing module includes VOA: variable optical attenuator, CCD: charge coupled device, VPP: eddy current phase plate, AWG: arbitrary waveform generator, and L: lens.

[0053] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: a pair of conjugated OAM modes are received in the OMG to demodulate the multiplexed OMG propagated by the WCOF designed by us; a WCOF with a three-layer structure is designed and prepared, in which the RIC between the core layer and the cladding is less than 1%, which reduces the propagation loss of the supported vector mode and can obtain the minimum effective refractive index difference between different OMGs; in the supported mode, a maximum propagation loss of 0.289dB / Km is achieved, and the proposed WCOF can be used for OAM-based OMG multiplexing with lower propagation loss; and the channel capacity of the OMG-based multiplexed optical fiber system is improved, reducing signal interruption. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is a structural diagram of the present invention;

[0055] Figure 2 This is the flow chart of radio frequency modulation of DMT signal;

[0056] Figure 3 Schematic diagram of multi-spiral phase and multi-diffraction angle modulation to generate multi-path OAM light simultaneously; 3(a) is the input Gaussian light, 3(b) is the complex spiral phase plate, and 3(c) is the OAM light field distribution after modulation;

[0057] Figure 4 This is a diagram of the fiber array coupling structure;

[0058] Figure 5 is the structure diagram of OAM fiber; 5(a) is the structure distribution, 5(b) is the refractive index distribution, and 5(c) is the equivalent refractive index distribution of different mode groups;

[0059] Figure 6 Transmission loss diagram for different mode groups;

[0060] Figure 7 This is a schematic diagram of the experimental platform structure;

[0061] Figure 8 8(a)-8(c) are intensity profiles and interference images detected; 8(d)-8(f) are interference images;

[0062] Fig. 9 Schematic diagram of the received power and normalized power of OAM modes between different OMGs and within the same OMG for crosstalk distribution; 9(a) is the measured crosstalk between different OMGs, 9(b) is the measured crosstalk between OAM modes within OMG1, 9(c) is the measured crosstalk between OAM modes within OMG2, and 9(d) is the measured crosstalk between OAM modes within OMG3;

[0063] Fig.10 The comparison diagram of measured optical power and received optical power when using / not using the proposed method;

[0064] Fig.11 The relationship between the bit error rate and wavelength measured by MPR when the WCOF length is set to 40km, 60km and 80km respectively;

[0065] Fig.12 The relationship between the bit error rate and wavelength measured using MPR when the data rate is set to 10, 20 and 40Gbit / s respectively;

[0066] Fig.13 The figure shows the relationship between the bit error rate and wavelength measured using MPR when the received optical power is set to -21dBm. DETAILED DESCRIPTION

[0067] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0068] like Figure 1 As shown, the multi-channel reception fusion method for reducing the probability of signal interruption described in the present invention mainly includes DMT signal modulation, OAM multi-channel simultaneous modulation, OAM mode group multiplexing, multi-channel demodulation within the OAM mode group, fusion reception and data analysis.

[0069] Step 1: DMT signal modulation, OAM multi-channel simultaneous modulation

[0070] 1. DMT signal modulation

[0071] like Figure 2 As shown, the DMT signal RF signal modulation module mainly completes the QAM modulation and DMT encapsulation of the user's original random data, which is suitable for optical fiber transmission. The implementation steps are as follows:

[0072] (1) Using binary pseudo-random polynomial x 20 +x 3 +1 Generates a random length of 2 20 -1 bit serial data, simulating the actual user data, to facilitate testing and system performance analysis.

[0073] (2) The generated serial user data is converted into a series of continuous parallel data groups with a length of 4 bits.

[0074] (3) The generated 4-bit pseudo-random data is mapped into a hexadecimal symbol (0 to F), and quadrature amplitude modulation is used to implement QAM-16 modulation of the 4-bit data.

[0075] (5) The QAM-16 modulated signal is divided into two equal paths through a beam splitter.

[0076] (6) A conjugate conversion method is used to convert a QAM-16 adjusted signal into its conjugate signal.

[0077] (7) Using an adder, QAM-16 (original signal) and QAM-16' (conjugate signal) are added to cancel out the imaginary part of the signal, obtaining a signal containing only twice the real part, forming a 64-channel discrete multi-tone signal (DMT).

[0078] (8) Convert the DMT signal into a time domain signal through inverse Fourier transform.

[0079] (9) A fixed overhead cyclic prefix (CP) byte is inserted in front of the generated DMT model according to certain timing requirements to achieve isolation between different symbols.

[0080] (10) A long training sequence (TS) is directly inserted in front of the CP to facilitate signal synchronization and channel estimation.

[0081] (11) Through a parallel-to-serial conversion module, the encapsulated DMT signal is converted into a serial signal, which facilitates data signal transmission and has higher reliability and anti-interference capabilities.

[0082] (12) A vector signal generator is used to modulate the DMT signal into an analog RF signal, which facilitates the modulation of the user signal onto the optical carrier.

[0083] (13) The radio frequency signal carrying the DMT signal and a laser together drive a Mach-Zehnder (MZM) modulator, thereby modulating the user signal onto a Gaussian beam carrier and realizing optical transmission of the user signal.

[0084] 2. OAM multi-channel simultaneous modulation

[0085] like Figure 3 As shown in the figure, the main function of the multi-OAM simultaneous module modulation module is to realize the simultaneous modulation of multiple OAM modes, effectively reducing the need for multiple spatial modulators in the traditional modulation process and effectively reducing the equipment cost. It can be divided into the following steps:

[0086] (1) Figure 3 As shown in (a), the transmitted GB light is incident on a retrospective multi-spiral phase modulator.

[0087] (2) Figure 3 As shown in (b), a multi-spiral phase modulator is designed, which includes three OAM spiral phases of l=1 (TC=1), l=2 (TC=2) and l=3 (TC=3), arranged at diffraction angles of 0 degrees, 90 degrees and 180 degrees, thereby constructing a multi-spiral phase modulator. The phase modulator can be laser pulsed etched to achieve complex phase patterns loaded on the phase plate.

[0088] (3) Figure 3 As shown in (c), the modulated OAM light is output along different diffraction angles, thereby completing the simultaneous modulation of different OAM mode lights, effectively reducing the OAM modulation cost based on traditional modulation methods.

[0089] Step 2: OAM mode group multiplexing

[0090] like Figure 4 As shown, the main function of the OAM mode light multiplexing method is to realize the multiplexing of multiple OAM light channels into one OAM light channel, and the steps are as follows:

[0091] (1) Figure 3 As shown in (a), the three OAM mode group lights modulated by the objective lens are coupled into the three input ports of the fiber array coupler respectively.

[0092] (2) Figure 3 As shown in (b), a fiber array coupler is used to couple three different OAM mode lights into one OAM light.

[0093] (3) Figure 3 In (b), the fiber array coupler mainly consists of two parts, one is the input part and the other is the tapered coupling part. The main function of the input part is to receive the free-space OAM light and couple it into the three-way interface fiber of the fiber array coupler. The function of the tapered part is to multiplex the three-way OAM light into one-way OAM light. The diameters of the three OAM optical fibers that are tapered by the fiber tapering method are Micrometer, Micrometer, Micrometer (n effRepresents the equivalent refractive index of different modes) and is fused with an OAM fiber to achieve OAM optical coupling and multiplexing of different transmission paths.

[0094] like Figure 5 As shown in Figure 1, the main function of OAM fiber is to transmit multiplexed OAM light, reduce transmission loss and improve isolation between OAM modes. The specific design steps of OAM fiber are as follows:

[0095] (1) Design a Figure 5 The OAM optical fiber shown in (a) has a three-layer structure. Figure 5 As shown in (b), their radii are r1 (1.82um), r2 (7.80um) and r3 (62.5um), and their refractive indexes are n1 (n1=1.448), n2 (n2=1.455) and n3 (n3=1.444), respectively. In the traditional optical fiber design, the lower the refractive index (n1) of the innermost layer, the better. However, the purpose of deliberately increasing the refractive index (n1) of the innermost layer in this design is to improve the equivalent refractive index separation between low-order OAM mode groups and reduce the crosstalk between OAM mode groups.

[0096] (2) The finite element method is used to analyze the equivalent refractive index distribution of the OAM mode supported by the optical fiber. Figure 5 As shown in (c), the results show that the fiber can support up to three OAM mode groups, and the isolation between mode groups is 1.89×10 -3 (Mode groups 0 and 1), 2.20×10 -3 (Mode Groups 1 and 2), 3.12×10 -3 (Mode groups 2 and 3), and the supported OAM mode transmission range includes C+L bands.

[0097] (3) According to the distribution of equivalent refractive index, the group delay between different mode groups is analyzed. Compared with mode group 0, the group delays of mode groups 1, 2, and 3 are 4.3, 9.2, and 14.1 ps / kmm, respectively. The measurement results show that a better transmission delay between mode groups is conducive to reducing the difference in transmission between different OAM mode groups.

[0098] (4) Using optical time domain reflectometry, the transmission loss of the mode group supported by OAM optical fiber is analyzed. Its low transmission loss is an important supporting condition for cladding OAM long-distance communication. Figure 6 As shown, the measured transmission losses of mode groups 0 to 3 are 0.243 dB / Km, 0.252 dB / Km, 0.269 dB / Km and 0.289 dB / Km respectively.

[0099] Step 3: Multi-channel demodulation, fusion reception and data analysis within the OAM mode group

[0100] The main function of the multi-channel OAM mode demodulation and fusion module in the OAM mode group is to demodulate a pair of conjugate modes in the mode group and use the maximum ratio fusion method to formulate the weight coefficients between different OAM modes, so as to recover the user signal and avoid signal interruption. The transmission OAM mode can be expressed as or The superscript ± indicates left / right circular polarization, the subscript ± indicates left / right rotation phase, m indicates radial coefficient, and |l| indicates the absolute value of TC. The specific implementation steps are as follows:

[0101] (1) Due to the influence of factors such as crosstalk in the channel, an OAM mode light (OAM) transmitted in the optical fiber l，m ) is more likely to leak to other OAM modes in other mode groups. This results in energy splitting, resulting in four OAM modes actually being transmitted in the channel.

[0102] (2) Through a quarter-wave plate, the left circular / right circularly polarized OAM light is converted into X horizontal / Y vertically polarized OAM combined light (OAM |l|,m +OAM -|l|，m) .

[0103] (3) Through a half-wave plate, the OAM combined light of the Y vertical polarization branch is converted into the OAM combined light of the X horizontal polarization branch.

[0104] (4) Through a beam combiner, the two X-horizontally polarized lights are combined into an OAM light combination containing only X-horizontally polarized light (OAM |l|，m +OAM -|l|，m ) and is equally divided into 6 paths.

[0105] (5) When the six beams are respectively irradiated into the helical plates 1 to 6 carrying the helical beam phases of l=±1 to ±3, the six transmitted OAM beams (P1 to 6) are converted into Gaussian beams 1 to 6 accordingly.

[0106] (6) When the six Gaussian beams illuminate the six photodetectors (PD1 to 6), the radio frequency signals carried by the six Gaussian beams are converted into electrical signals 1 to 6. The photodetectors only work on radio frequency signals and are ineffective on the six optical carrier signals.

[0107] (7) According to the received six electrical signals 1 to 6 in the three mode groups (|l|=1, 2, 3), DMT de-framing is performed respectively and the original user data carried by l=±1 to ±3 is obtained, which mainly includes serial-to-parallel conversion, frame header search and positioning, training sequence removal, redundant cyclic prefix removal, Fourier transform, and QAM demodulation.

[0108] (8) The error vector magnitude (EVM) method is used to combine the two conjugate (l = ±1 / ±2 / ±3) input electrical signals 1-2 / 3-4 / 5-6 in each mode group, measure their combined EVM, and determine the percentage of electrical signals 1 / 3 / 5 (l = +1 / +2 / +3) and electrical signals 2 / 4 / 6 at the minimum EVM point, and use this percentage as the weight coefficient of the two electrical signals in the mode group.

[0109] (9) According to the obtained weight coefficient, the DSP method is used to propose the initial data carried by each pattern group (|l|=1, 2, 3).

[0110] (10) Based on the received data, bit error rate (BER) analysis and constellation diagram (CP) distribution are performed respectively.

[0111] (11) Based on the obtained BER and CP diagrams, the signal interruption probability is analyzed and the system performance is evaluated. When the bit error rate is greater than 3.8×10 -3 When the bit error is not correctable and causes signal interruption, it is difficult to determine whether the system can transmit data reliably.

[0112] 5. Verification Platform

[0113] In order to study the validity of the proposed concepts, Figure 7 A demonstration platform is designed and constructed as shown in the figure. The proposed experimental configuration can be divided into three parts: user data generation and modulation, OAM beam modulation, multiplexing and fiber transmission, OAM demodulation and signal analysis. In addition, due to the limited number of spiral phase plates, Figure 7 The system shown can only demodulate light of one OAM mode group at a time, and other mode groups can be demodulated by using the same method.

[0114] In order to verify the effectiveness of the proposed scheme, a proof scheme is designed and constructed, such as Figure 3 As shown. Figure 3 The experimental platform we proposed can be divided into three parts: user data modulation on the carrier, OAM mode modulation and propagation, OAM demodulation and offline processing. First, a pseudo-random binary sequence (PRBS) module is used to generate serial random data that simulates the actual user data. 31 +x 28 +1 PRBS polynomial generation length is 2 32 -1bit PRBS. In order to map the generated PRBS into quadrature amplitude modulation (QAM-16) symbols, the generated serial PRBS is converted to a length of The modulated OFDM signal (QAM-16) carries the user data and drives the arbitrary waveform generator (AWG) to generate a transmittable analog electrical signal. The modulated analog electrical signal is amplified by an electrical amplifier (EA) to enhance the power of the propagated signal. In order to transmit user data using a Gaussian beam (GB), an in-phase / quadrature (I / Q) modulator is used to modulate the electrical signal generated on the Gaussian beam, which is generated by a tunable external cavity laser (ECL) with a wavelength of 1550nm and propagated through a single mode fiber (SMF). In addition, a fiber polarization controller (PCs) is added in the SMF between the ECL and the I / Q modulator to adjust the GB output from the ECL to a linearly polarized (LP) GB.

[0115] Secondly, the modulated GB carrying user data is amplified and filtered by an erbium-doped fiber amplifier (EDFA1) and a bandpass filter (BPF1) in sequence; the adopted BPF1 is mainly used to filter the amplified spontaneous emission noise (ASEN). Subsequently, the GB output from BPF1 is divided into four equal copies by an optical coupler (OC), which are marked as paths 1 / 2 / 3 / 4, respectively. Four delay-SMFs of different lengths and four PCs (i.e., PC1 to 4) are inserted into the four paths after the OC to derive the correlation between the four copies and adjust the polarization of the GB propagating in the SMF. The GB propagating in the four paths is illuminated by four collimators (C1 to 4) to the free step of OAM modulation. Subsequently, the size of the illuminated GB is enlarged by a 3f system composed of a pair of lenses (L1 and 2, L3 and 4, L5 and L, L7 and 8) on each path to increase the illumination area on the vortex phase plate (VPP), which is beneficial to improve the OAM mode conversion efficiency of the VPP between the GB and the OAM beam. In addition, a combination of a polarizer (P1 or 2 or 3 or 4) and a half-wave plate (H1 or 2 or 3 or 4) is inserted after the expansion system to adjust the polarization of the irradiated GB to LP, which is beneficial to improving the conversion efficiency between the OAM beam and the GB and also to the expansion system. Through three optical separators BS1~3 and two mirrors M1~2, the three lights output by the optical polarizers H2~4 are combined into one light (GB). Subsequently, the multiplexed three-way GB light is irradiated into a comprehensive spiral phase plate VPP1, thereby realizing the modulation of the three-way OAM mode (l=1 / 2 / 3). The converted three OAM beams are multiplexed and combined into one beam through a fiber array coupler. Then, the multiplexed OAM beam is coupled to the WCOF through an objective lens (OL1), and an H5 and Q1 are inserted before OL1 to adjust the multiplexed OAM beam to a left-right circularly polarized OAM beam to improve the propagation efficiency.

[0116] Finally, the OAM beam output by WOCF is coupled to the free space through C5. A combination of L9, L10, P5, H6 and Q2 is inserted after C5 to expand the size of the multiplexed OAM beam, and the output OAM beams are adjusted to LP OAM beams respectively. BS3 is used to split the OAM beam into two parts. BS4 interferes with the reference GB in path 4, and distinguishes the received OAM mode according to the interference fringes detected by the charge coupled device (CCD), where M1 is inserted in path 4 to adjust the propagation direction of the reference GB. Another OAM beam is also split into two by BS5 for demodulating the received OAM beam into the corresponding GB, where the combination of P6, H7 and the combination of P7, H8 are inserted before the spatial light modulator (SLM1) and SLM2 to adjust the irradiated OAM beam to a linearly polarized OAM beam. The received OAM mode is demodulated into the corresponding GB using a pair of SLM1 and SLM2, where a pair of conjugate holograms (l and -l) are loaded on SLM1 and SLM2 to demodulate the transmitted OAM beam (l or -l) and the corresponding crosstalk mode (-l or l). The two demodulated GBs are recoupled into two SMFs through C6 and 7 respectively, where M4 is used to adjust the propagation direction of the GB demodulated by SLM2. The power of the two coupled GBs is orderly amplified and adjusted by the combination of EDFA2 and variable optical attenuator (VOA1) and another set of EDFA3 and VOA2. The adjusted two GBs are demodulated into a pair of electrical signals by the coherent demodulator module. The user data propagated by the transmitter is recovered by the offline processing module, and the system performance is analyzed, where the maximum ratio combining (MRC) method between the two received data signals is used to improve the bit error rate performance. It is worth noting that due to the limited SLM in our laboratory, only one multiplexed OAM can be demodulated at a time. But other multiplexed OAM beams can also be demodulated by changing the two conjugate holograms of the OAM modes transmitted on SLM1 and SLM2.

[0117] 2. Experimental results and performance analysis

[0118] According to the proposed experimental platform, three OAM modes carrying l1=1, l2=2 and l3=3 supported by the proposed WOCF are selected as representatives of OMG1 to OMG3, which are generated by VPP1 to VPP3 in our experiment. In addition, without other special instructions, the data rate, wavelength and WCOF length are set to 10Gbps, 1550nm and 40km respectively.

[0119] In order to verify that the adopted OAM mode can be supported and propagated by the proposed WCOF, the intensity distribution map of the propagating OAM mode and the corresponding interference pattern are recorded by CCD, respectively, as shown in Figure 8 shown. Figure 88(a)-8(c) in Figure 8 show the intensity images detected when the three OAM modes propagate one by one on the proposed WCOF. Figure 8 Three hollow sections can be observed in (a)-8(c), which means that a singular point appears at the center of each beam received at 1550nm. By interfering each detected OAM beam with a reference Gaussian beam (GB), Figure 8 (d)-8(e) show Figure 8 Interference patterns of the detected beams in (a)-8(c), where the number of fringes and the direction of rotation (left or right) indicate the order / topological charge (TC, |l|) and sign (±) of l, respectively. Figure 8 In (d), clear interference fringes rotating in the counterclockwise direction can be observed and marked with dotted circles, which indicates that Figure 8 The beam detected in (a) is an OAM beam carrying l = 1. Similarly, Figure 8 (e) and Figure 8 (f) shows two counterclockwise interference fringes, marked by two dotted circles, and three counterclockwise interference fringes, marked by three dotted circles, which indicates Figure 8 (b) and Figure 8 The OAM modes detected in (c) are l = 2 and l = 3 respectively. In addition, in order to facilitate the description of OAM modes, OAM and Notation, where “+” and “-” in the superscripts denote SAM, and m denotes the order of the radial mode.

[0120] In addition, in OAM-based SDM systems, crosstalk between different OMGs and crosstalk between OAM modes within the same OMG also play a crucial role. In this case, the propagated power may leak into other OAM modes / mode groups, resulting in signal interruption or fading of the received target OAM mode. In order to study the above crosstalk, the three adopted OAM modes are propagated in turn, and the power of all corresponding OAM modes / OMGs is measured to describe the crosstalk distribution between the three OMGs and between OAM modes within an OMG. Fig. 9 The figure shows the received power and normalized power of OAM modes between different OMGs and within the same OMG for describing the crosstalk distribution. Fig. 9 In (a), the crosstalk between the three OMGs is studied due to Figure 1 Δn between the three OMGs eff (greater than 1.0×10 -3 ) is larger, so a smaller crosstalk (less than 4.5%) can be achieved between the three OMGs (l=1, l=2, l=3). Fig. 9 (b) shows two OAM modes ( and )Measured and normalized power in OMG1. Fig. 9 (b) In and More severe crosstalk can be observed between the two OMGs, which can be attributed to the proposed WCOF, which has a smaller RIC between the core and cladding to reduce the PL, thereby reducing the Δn between the two adjacent OMGs (OMG0 and OMG1). eff In addition, Fig. 9 In (b), only two OAM modes consisting of HE modes are studied in OMG1 ( and ), this is because the OAM mode composed of TE and TM modes has a large Δn between TE and TM. eff , but cannot be stably propagated on WOCF. In addition, Fig. 9 (c) shows the HE mode ( and ) and EH( and ) mode consists of four OAM modes ( and ). Fig. 9 In (c), due to the Δn between HE and EH in OMG2 eff It is smaller, and it can be found that the crosstalk between the four OAM modes in OMG2 is larger than that between OMGs. However, compared with the crosstalk within OMG1, the crosstalk within OMG2 is significantly less, which may be due to the larger Δn between OMG2 and OMG3, and between OMG2 and OMG1. eff This results in a larger Δn between HE and EH in OMG2. eff Similarly, Fig. 9 (d) shows four OAM modes ( and ) are measured and normalized by the corresponding HE modes ( and ) and EH mode ( and ) Fig. 9 In (d), a slight increase in crosstalk inside OMG3 can be observed, which can be attributed to the fact that large intensity profiles are easily interfered on the proposed WCOF system. Therefore, OMG-based SDM is beneficial to improve the system performance to higher Δn compared with the OAM modes in each OMG. eff , rather than the OAM modes per OMG.

[0121] In addition to the crosstalk mentioned above, the bit error rate performance is also measured and displayed as Fig.10As shown in the figure, the performance of the proposed scheme is studied. In order to evaluate the bit error rate, the verification schemes are divided into three types: "BTB", "OMG with MPR" and "field = ±1 / 2 / 3". The "BTB" mentioned above means that only GBs are used to transmit user data on the proposed system. The symbol "OMG1 / 2 / 3with MPR" indicates that when two conjugate OAM beams carrying l = +1 / +2 / +3 and l = -1 / -2 / -3 are received simultaneously, the proposed MPR is used to propagate the signal in an OMG-based multiplexing manner in the proposed system. In the third verification scheme, the symbol "l = ±1 / ±2 / ±3" indicates that when three OAM modes are used to represent the corresponding three OMGs to be propagated and multiplexed simultaneously, only one OAM mode in the corresponding OMG1 / 2 / 3 is detected without using the proposed MPR method. Fig.10 The figure shows the bit error rate distribution measured when implementing the above three verification schemes respectively. Fig.10 Compared with the “BTB” verification scheme, the -3 [threshold of hardware forward error correction (FEC) with 7% overhead], when 3 OMGs are multiplexed through SDM and receive only one of l=+1 / +2 / +3 in turn, the measured BERs of l=+1, l=+2, l=+3 are reduced by 2.59dBm, 3.10dBm and 3.87dBm respectively. In addition, three OAM beams of l=-1, l=-2, and l=-3 are used to represent the corresponding three OMGs 1 to 3 to study the bit error rate of the system when the proposed MPR is not used. Fig.10 In the figure, compared with the "BTB" verification, when the three OMGs represented by l=-1, l=-2, and l=-3 are multiplexed and one OMG among l=-1, l=-2, and l=-3 is detected in turn, the measured BER distributions of l=-1, l=-2, and l=-3 are reduced by 2.91dBm, 3.45dBm, and 4.20dBm, respectively. When two different groups of OAM (l=+1, l=+2, l=+3 or l=-1, l=-2, and l=-3) are selected to represent the corresponding OMGs, slight BER performance differences can be observed. And slight attenuations of 0.32dBm, 0.35dBm, and 0.33dBm can be obtained, which can be ignored even if the proposed MPR is not used. Therefore, these two groups can be arbitrarily selected to represent the corresponding OMGs for studying the bit error rate performance of the proposed scheme. In order to evaluate the performance of the proposed method, two OAM modes consisting of l = +1 / +2 / +3 and l = -1 / -2 / -3 conjugate pairs are received simultaneously using MRC, as shown in Fig.10 As shown, in our experiment, three OAM modes l = +1 / +2 / +3 are used to represent three OMGs. Fig.10In the example, by combining two simultaneously received OAM modes according to the MRC rule, the measured BER distribution of |l|=1 / 2 / 3 with MPR is 3.8×10 -3 A slight BER degradation of 0.29 / 0.71 / 0.89 dBm can be observed at 100 / 200 dBm and 100 / 200 dBm respectively. However, a significant improvement of 2.30 / 2.39 / 2.98 dBm can be achieved between using MPR (receiving and combining a pair of conjugated OAM modes) and not using MPR (receiving only one OAM mode). Therefore, the proposed concept effectively improves the system performance of OMG-based SDM systems on WCOF and improves the ability to resist signal interruption, thereby increasing the propagation distance in the future.

[0122] Although the validity and feasibility of the concept have been demonstrated by studying the bit error rate distribution of each OMG and OAM mode with or without MPR, the achievable propagation distance, propagation rate and wavelength are also key factors in the communication system. For simplicity, when using the proposed MPR, only OMG3 (|l|=3) is used for monitoring to evaluate the bit error rate distribution for different fiber lengths, different DRs and different λ. Fig.11 As shown in Figure 1, when the WOCF length increases from 40km to 60km, the measured BER curve moves to the right, which means that the measured BER performance decreases as the WOCF length increases. Fig.11 In the figure, when the multiplexed OMG is transmitted in a WCOF with a length of 60km and 80km, the bit error rate performance of |l|=3 decreases by 2.12dBm and 4.98dBm respectively compared with the case of 40km. As the length of WOCF increases, the bit error rate decreases significantly, indicating that the bit error rate is more sensitive to the fiber length.

[0123] like Fig.12 As shown, the bit error rate distribution when |l|=3 is also calculated when DR changes from low speed to high speed. Fig.12 It is described that when DR increases from 10 Gbit / s to 40 Gbit / s, the received BER performance becomes worse. Compared with "DR = 10 Gbit / s", when user data is transmitted at DR of 20 and 40 Gbit / s, the received BER performance is 3.8×10 -3 The BER loss measured at 0 and 1 is right-shifted by 0.48 and 1.28 dBm, respectively, which shows that a large increase in data rate can be achieved at the expense of a small received optical power. However, compared with the effect of WCOF length on BER performance, DR is less sensitive to BER performance and can be further improved.

[0124] In addition, the influence of different λ on the bit error rate performance is also discussed to verify the achievable working wavelength of the proposed scheme. Fig.13The figure shows the BER distribution when the measurement wavelength changes from 1530nm to 1630nm and the received power is adjusted to -21dBm. Fig.13 In the example, when the received optical power is set to -21dBm, all the measured bit error rate (BER) performances are lower than the penalty level of hardware forward error correction (3.8×10 -3 ), which indicates that all the found errors can be corrected. Therefore, the proposed concept is effective and feasible to effectively resist signal outages in the entire C+L band, thereby propagating user information through the proposed scheme.

Claims

1. A multi-channel receiving fusion method for reducing the probability of signal interruption, characterized in that: The steps include: (1) DMT signal modulation and OAM multi-channel simultaneous modulation; (2) OAM mode group multiplexing; (3) Multi-channel demodulation, fusion reception and data analysis within the OAM mode group.

2. The multi-channel receiving fusion method for reducing the probability of signal interruption according to claim 1, characterized in that: The DMT signal modulation in step (1) is implemented by a DMT signal radio frequency signal modulation module, completing QAM modulation and DMT encapsulation of the user's original random data.

3. The multi-channel receiving fusion method for reducing the probability of signal interruption according to claim 1, characterized in that: The DMT signal modulation in step (1) includes: (1.1) Using binary pseudo-random polynomial x 20 +x 3 +1 Generates a random length of 2 20 -1 bit serial data, simulating the actual user data; (1.2) converting the generated serial user data into a series of continuous parallel data groups with a length of 4 bits; (1.3) Mapping the generated 4-bit pseudo-random data into a hexadecimal symbol and using quadrature amplitude modulation to implement QAM-16 modulation of the 4-bit data; (1.4) The QAM-16 modulated signal is divided into two equal paths through a beam splitter; (1.5) Using the conjugate conversion method, a QAM-16 adjusted signal is converted into its conjugate signal; (1.6) Using an adder, QAM-16 and QAM-16' are added to cancel out the imaginary part of the signal, obtaining a signal containing only twice the real part, thus forming a 64-channel discrete multi-tone signal; (1.7) Convert the DMT signal into a time domain signal through inverse Fourier transform; (1.8) In front of the generated DMT model, a fixed overhead cyclic prefix byte (CP) is inserted according to certain timing requirements to achieve isolation between different symbols; (1.9) Insert a long training sequence directly in front of the CP to facilitate signal synchronization and channel estimation; (1.10) The encapsulated DMT signal is converted into a serial signal through a parallel-to-serial conversion module; (1.11) Using a vector signal generator, modulate the DMT signal into an analog RF signal; The RF signal carrying the DMT signal generated by (1.12) and a laser together drive a Mach-Zehnder modulator, thereby modulating the user signal onto a Gaussian optical carrier and realizing optical transmission of the user signal.

4. The multi-channel receiving fusion method for reducing the probability of signal interruption according to claim 1, characterized in that: The OAM multi-channel simultaneous modulation in step (1) includes: (2.1) The transmitted GB light is incident on a multi-helical phase modulator; (2.2) Design a multi-spiral phase modulator, which includes three OAM spiral phases of l = 1, TC = 1, l = 2, TC = 2, and l = 3, TC = 3, arranged at diffraction angles of 0 degrees, 90 degrees, and 180 degrees, thereby constructing a triangularly arranged multi-spiral phase modulator; (2.3) The modulated OAM light is output along different diffraction angles, completing the simultaneous modulation of different OAM mode lights.

5. The multi-channel receiving fusion method for reducing the probability of signal interruption according to claim 1, characterized in that: The OAM mode group multiplexing in step (2) includes: (3.1) The three OAM mode group lights modulated by the objective lens are coupled into the three input port fibers of the fiber array coupler respectively; (3.2) Using a fiber array coupler, three different OAM mode lights are coupled into one OAM light; (3.3) The fiber array coupler mainly consists of two parts, one is the input part and the other is the tapered coupling part. The main function of the input part is to receive the free-space OAM light and couple it into the three-way interface fiber of the fiber array coupler; the function of the tapered part is to multiplex the three-way OAM light into one-way OAM light; the diameter of the three-way OAM fiber tapered by the fiber tapering method is Micrometer, Micrometer, Micrometer, where n eff It represents the equivalent refractive index of different modes and is fused with an OAM fiber to achieve OAM optical coupling and multiplexing of different transmission paths.

6. The multi-channel receiving fusion method for reducing the probability of signal interruption according to claim 5, characterized in that: The OAM optical fiber is a three-layer structure, whose radii are r1=1.82um, r2=7.80um and r3=62.5um, and whose refractive indexes are n1=1.448, n2=1.455 and n3=1.

444.

7. The multi-channel receiving fusion method for reducing the probability of signal interruption according to claim 6, characterized in that: The verification of the OAM fiber performance includes: The finite element method is used to analyze the equivalent refractive index distribution of the OAM mode supported by the optical fiber; Analyze the group delay between different mode groups based on the equivalent refractive index distribution; The optical time domain reflectometry is used to analyze the transmission loss of the mode groups supported by OAM optical fibers.

8. The multi-channel receiving fusion method for reducing the probability of signal interruption according to claim 1, characterized in that: In the step (3), the multi-channel demodulation, fusion reception and data analysis within the OAM mode group are implemented by a multi-channel OAM mode demodulation and fusion module within the OAM mode group. The demodulation and fusion module demodulates a pair of conjugate modes within the mode group, and adopts a maximum ratio fusion method to formulate weight coefficients between different OAM modes to recover user signals and avoid signal interruption. The transmission OAM mode can be expressed as or Wherein the superscript ± indicates left / right circular polarization, the subscript ± indicates left / right rotation phase, m indicates radial coefficient, and |l| indicates the absolute value of TC.

9. The multi-channel receiving fusion method for reducing the probability of signal interruption according to claim 1, characterized in that: The step (3) comprises: (4.1) An OAM mode transmitted in optical fiber Optical OAM l，m The energy of This results in energy splitting, resulting in four OAM modes actually being transmitted in the channel; (4.2) Through a quarter-wave plate, the left circular / right circularly polarized OAM light is converted into X horizontal / Y vertically polarized OAM combined light OAM respectively. |l|,m +OAM -| / |，m ; (4.3) Through a half-wave plate, the OAM combined light of the Y vertical polarization branch is converted into the OAM combined light of the X horizontal polarization branch; (4.4) Through a beam combiner, the two X-horizontally polarized lights are combined into one OAM light containing only X-horizontally polarized light. |l|，m +OAM -|l|，m , and is equally divided into 6 paths; (4.5) When the six beams are irradiated into the helical sheets 1 to 6 carrying the helical beam phases of l = ±1 to ±3, the six transmitted OAM beams, P1N6, are converted into Gaussian beams 1 to 6 accordingly; (4.6) When the six Gaussian beams illuminate the six photodetectors respectively, the radio frequency signals carried by the six Gaussian beams are converted into electrical signals 1 to 6 respectively, where the photodetectors only work on the radio frequency signals and are ineffective on the six optical carrier signals; (4.7) According to the received 6 electrical signals 1 to 6 in the 3 mode groups |l|=1, 2, 3, DMT de-framing is performed respectively and the original user data carried by l=±1 to ±3 is obtained, which mainly includes serial-to-parallel conversion, frame header search and positioning, training sequence removal, redundant cyclic prefix removal, Fourier transform, and QAM demodulation. (4.8) Using the error vector magnitude method, combine the two conjugates in each mode group, l = ±1 / ±2 / ±3, and the input electrical signals 1-2 / 3-4 / 5-6, measure their combined EVM, and determine the percentage of the electrical signal 1 / 3 / 5, l = +1 / +2 / +3, and the electrical signal 2 / 4 / 6, l = -1 / -2 / -3 at the minimum EVM point, and use this percentage as the weight coefficient of the two electrical signals in the mode group; (4.9) According to the obtained weight coefficient, the DSP method is used to propose the data carried by each initial pattern group, |l| = 1, 2, 3; (4.10) According to the received data, the bit error rate BER analysis and the constellation diagram CP analysis are performed respectively; (4.11) Based on the obtained BER and CP diagrams, the signal interruption probability is analyzed and the system performance is evaluated. When the bit error rate is greater than 3.8×10 -3 When bit errors occur, they are uncorrectable and result in signal interruption, making it unclear whether the system can transmit data reliably.

10. A verification system for a multi-channel receiving fusion method for reducing the probability of signal interruption, characterized in that: include: User data modulation module on the carrier, OAM mode modulation and propagation module, and OAM demodulation and offline processing module; The user data modulation module on the carrier includes EA: electrical amplifier, PC: polarization controller, Q: quarter wave plate, H: half wave plate, BPF: bandpass filter, OL: objective lens, OC: optical coupler, C: collimator, P: polarizer, BS: beam splitter, M: reflector, SMF: single mode fiber, ECL: external cavity laser; The OAM mode modulation and propagation module includes M: reflector, FAC: fiber array coupler, EDFA: erbium-doped fiber amplifier; The OAM demodulation and offline processing module includes VOA: variable optical attenuator, CCD: charge coupled device, VPP: eddy current phase plate, AWG: arbitrary waveform generator, and L: lens.

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