A method for transmitting sub-terahertz data using optical fiber and a verification system thereof

Abstract

The present invention discloses a method for transmitting sub-terahertz data via optical fiber and a verification system thereof, the method comprising: data framing and generation of sub-terahertz waves; OAM mode light field modulation and multiplexing; OAM mode group optical fiber transmission; OAM mode signal demodulation and data recovery; the verification system comprising: user data generation and modulation module, OAM beam modulation and propagation module, and signal demodulation and processing module. The present invention can effectively improve CC and SE and extend the transmission distance; the designed OAM optical fiber supports 6 OMG multiplexing, has a large OMG isolation, can effectively reduce the crosstalk between OMGs, realizes large-capacity, long-distance optical transmission of sub-terahertz signals, improves the transmission distance of STB signals, reduces the density of base station construction, and reduces the construction and operation and maintenance costs.

Description

Technical Field

[0001] The present invention relates to the field of signal transmission, and in particular to a method for transmitting sub-terahertz data through optical fiber and a verification system thereof. Background Art

[0002] In recent years, many novel and popular applications, such as video, live broadcast, smart driving, real-time online video conferencing, etc., use mobile communication technology of radio frequency (RF) signals to achieve transmission, which continuously enriches people's lives. In order to improve the user experience, information transmission requires higher channel capacity (CC), spectrum efficiency (SE) and lower latency. However, the current methods to improve CC and SE mainly use various physical spaces, namely wavelength, polarization, time and other multiplexing methods and advanced modulation technologies, such as quadrature amplitude modulation (QAM), quadrature phase shift keying (QPSK), pulse amplitude modulation (PAM), etc., which will not be able to meet the growing demand for CP and SE of a large number of new applications in the near future. In order to improve CC and SE, two methods are usually used: new available spectrum sources and new multiplexing spaces. Compared with the traditional RF used in the fifth generation (5G) communication technology, from 700MHz to 4.9GHz, etc., the sub-terahertz band (STB) is considered to be a new radio frequency (NR) in 5G, which can effectively increase the bandwidth. Since the new RF STB wave has a wider bandwidth (100 to 300GHz), it can be used for high-speed communication and improved CC. However, due to the large propagation loss (PL) of RF signals, including traditional RF and NR in free space, RF signals cannot achieve and transmit to cover a larger range. Especially for STB waves with higher frequencies, it is limited to a shorter transmission distance (DD). Therefore, in order to achieve wide coverage, the technical communication scheme using STB transmission signals must densely build a large number of base stations, which will inevitably bring heavy construction and operation costs (equipment, operation and maintenance, electricity, etc.) to operators. In order to overcome the above problems, the transmission of traditional RF signals mainly adopts the method of long-distance propagation of optical fiber radio (ROF), in which single-mode fiber (SMF) and Gaussian beam (GB) are usually used as propagation medium and carrier, respectively. However, due to the limitations of Shannon and nonlinearity, the CC and SE of ROF systems based on GB and SMF cannot be further expanded by traditional multiplexing methods. Summary of the invention

[0003] Purpose of the invention: The purpose of the present invention is to provide a method for transmitting sub-terahertz data through optical fiber and a verification system thereof.

[0004] Technical solution: The method for transmitting sub-terahertz data via optical fiber of the present invention comprises the following steps:

[0005] (1) Data framing and sub-terahertz wave generation;

[0006] (2) OAM mode optical field modulation and multiplexing;

[0007] (3) OAM mode group fiber transmission;

[0008] (4) OAM mode signal demodulation and data recovery.

[0009] Furthermore, the step (1) includes QAM modulation of user data and OFDM framing and modulation of sub-terahertz waves.

[0010] Furthermore, the user data QAM modulation and OFDM framing include:

[0011] (1.1) Using the pseudo-random sequence polynomial x 23 +x 18 +1, resulting in a length of 2 23 -1-bit random binary sequence, used to simulate random user data actually transmitted;

[0012] (1.2) Convert the serial PRBS sequence into a 4-bit parallel array through serial-to-parallel conversion to form 32 data streams;

[0013] (1.3) Using the QAM mapping module, the binary data is mapped into a quadrature amplitude modulation signal (QAM-16) in the complex domain;

[0014] (1.4) The generated 32-channel QAM data is converted into its conjugate 32-channel QAM data by a conjugate conversion method, and combined with the previous 32-channel original QAM data to form 64-channel real data;

[0015] (1.5) Performing inverse Fourier transform on the 64-channel data generated above by an inverse Fourier transform method to form real OFDM payload data suitable for intensity modulation;

[0016] (1.6) Insert or add cyclic prefix data at a fixed starting position in OFDM data through the cyclic prefix byte insertion module to isolate different OFDM symbols and reduce interference between OFDM symbols;

[0017] (1.7) Through the training sequence insertion module, a long training sequence is inserted before every 80 OFDM data carrying cyclic prefix data for synchronization and channel estimation;

[0018] (1.8) Through the parallel-to-serial conversion module, parallel OFDM data is converted into serial OFDM data, which is suitable for channel modulation and transmission.

[0019] Furthermore, the modulation of the sub-terahertz wave comprises the following steps:

[0020] (1.9) A distributed feedback laser with a central wavelength of 1550 nanometers is used as a Gaussian light source for generating sub-terahertz waves;

[0021] (1.10) Through the phase modulator 1, the transmitted 1550 nm GB light is subjected to a certain phase delay, and the interval of the phase delay is mainly determined by the integer multiple n of the 25 GHz RF signal source, n = 1;

[0022] (1.11) Through the phase modulator 2, the transmitted 1550 nm GB light is phase delayed to a certain extent, and the interval of the phase delay is also mainly determined by the integer multiple m of the 25 GHz RF signal source, m = 1;

[0023] (1.12) Through the Mach-Zehnder modulator, 25 GHz RF signal, and two-time phase modulator, the GB light generated by DFB is modulated into multiple coherent optical carriers.

[0024] (1.13) Through a 25 GHz delay interferometer, the optical carrier is modulated into a periodic pulse wave with a delay interval of 25 GHz.

[0025] (1.14) The power of GB transmission light in a single-mode optical fiber is increased by an erbium-doped optical fiber amplifier;

[0026] (1.15) Design a bandpass filter to select the wavelength 4× from the generated pulse. 25 , 5× 25 , 6× 25 , 7× 25 GHz pulses to achieve modulation of sub-terahertz waves of 100, 125, 150, and 175 GHz.

[0027] Furthermore, the step (2) comprises the following steps:

[0028] (2.1) Through a fiber array coupler, the sub-terahertz Gaussian light is divided into five branch lights, arranged in a "five-tube" shape;

[0029] (2.2) generating a comprehensive OAM array hologram according to the flame fork grating patterns of l = -1, l = -2, l = -3, l = -4, l = -5 and different partition intervals;

[0030] (2.3) Through an OAM array hologram, the five branch lights are modulated into five vortex lights, where each mode represents their mode group OMG1-5;

[0031] (2.4) Using the same fiber array coupler, the five vortex lights generated in the array are multiplexed into one vortex light, thereby completing the multiplexing of the OAM mode group.

[0032] Furthermore, the step (3) comprises the following steps:

[0033] (3.1) Design an OAM fiber with a 5-layer structure;

[0034] (3.2) The radius of each layer of the optical fiber is designed to be 1.55 μm, 9.22 μm, 11.20 μm, 14.30 μm, and 62.50 μm respectively;

[0035] (3.3) The contrast ratio of the refraction system of each layer of the optical fiber is set to 0.8%, 1.28%, 0%, -0.8%, and 0% respectively;

[0036] (3.4) Analyze the equivalent refractive index distribution of different modes supported by optical fiber;

[0037] (3.5) At 1550nm, the transmission losses of various OMG0-5 supported by optical fibers measured by OTDR method are 0.24dB / km, 0.23dB / km, 0.25dB / km, 0.27dB / km, 0.29dB / km, and 0.31dB / km respectively.

[0038] Furthermore, in the step (3.1), the innermost layer of the 5-layer structure has a radius of r1=1.55 μm and a refractive index contrast ratio of 0.8%; the second layer has a radius of r2=9.22 μm, and the middle layer has a core of an OAM optical fiber with 1.28% CRIC; the third layer is a trench layer with a radius between r3=11.20 μm and r4=14.3 μm, which is formed by doping fluoride.

[0039] Further, the step (3.4) includes using a finite element method to analyze the n of the OAM mode or OAMG supported by the OAM fiber. eff , OAM fiber supports 6 mode groups from l = |0| to l = |5|, which are composed of even and odd vector mode combinations with π / 2 mode phase difference. The Δn between different OMGs is eff It also increases with the increase of OMG order. The Δn between two adjacent OAMGs eff All greater than 1.0×10 -3 .

[0040] Furthermore, the step (4) comprises the following steps:

[0041] (4.1) The multiplexed light coupled into the OAM optical fiber transmission is the free space OAM multiplexed light;

[0042] (4.2) Design a grating that simultaneously demodulates the vortex light, which can simultaneously demodulate the incident OAM multiplexed light into Gaussian basis mode light and is located at different spatial coordinates;

[0043] (4.3) The free-space OAM multiplexed light is irradiated into a spatial light modulator loaded with a simultaneously demodulated vortex light grating, thereby simultaneously demodulating different OAM lights into Gaussian lights with different spatial coordinates;

[0044] (4.4) Using a direct detection method with photoelectric detectors of different sensitive frequency bands to convert optical signals carrying different THz data into corresponding electrical signals;

[0045] (4.5) Use offline digital signal processing module to analyze the performance of system transmission.

[0046] The verification system of the optical fiber transmission sub-terahertz data method of the present invention comprises: a user data generation and modulation module, an OAM beam modulation and propagation module and a signal demodulation and processing module;

[0047] The user data generation and modulation module includes a PRBS data module generated by FPGA, a serial-to-parallel conversion module, a QAM modulation mapping module, a QAM modulation module, and an OFDM framing module. The OFDM framing module includes an inverse fast Fourier transform, a cyclic prefix insertion, a training sequence insertion, a parallel-to-serial conversion module, an arbitrary waveform RF signal generator, and a terahertz wave modulator. The terahertz wave modulator includes a distributed feedback laser, a phase modulator, a 25GHz RF source, a Mach-Zehnder, a phase delay interferometer, a fiber amplifier, a bandpass filter, a single-mode relationship, a Mach-Zehnder modulator, and a polarization controller;

[0048] Beneficial effect: Compared with the prior art, the present invention has the following significant advantages: it adopts the method of OAM fiber and OMG multiplexing to transmit 5G NR signals based on STB, which can effectively improve CC and SE and extend the transmission distance; the designed OAM fiber supports 6 OMG multiplexing, has a large OMG isolation, can effectively reduce the crosstalk between OMGs, and realize large-capacity and long-distance optical transmission of sub-terahertz signals, which improves the transmission distance of STB signals, reduces the density of base station construction, and reduces construction and operation and maintenance costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a framework diagram of the present invention;

[0050] Figure 2 This is a diagram of the user data modulation and OFDM framing structure;

[0051] Figure 3 This is a diagram of the sub-terahertz wave modulation structure;

[0052] Figure 4 This is the effect diagram of the multi-pulse sub-terahertz wave generated;

[0053] Figure 5It is a flow chart of OAM mode modulation and multiplexing process;

[0054] Figure 6 Schematic diagram of OAM fiber structure, where 6(a) is the fiber structure, 6(b) is the structural parameters, 6(c) is the simulated and measured refractive index distribution, and 6(d) is the equivalent refractive index distribution diagram of different vector modes;

[0055] Figure 7 Schematic diagram of transmission loss of mode groups supported by OAM optical fiber;

[0056] Figure 8 This is a structural diagram of the simultaneous demodulation scheme for multiple OAM mode groups;

[0057] Fig. 9 This is a schematic diagram of the experimental platform structure;

[0058] Fig.10 (a1)-(a5) intensity distributions and (b1)-(b5) interference patterns captured when the OAM mode propagates alone;

[0059] Fig.11 Schematic diagram of crosstalk between different OAM mode groups during independent propagation;

[0060] Fig.12 This is the electrical signal diagram after the oscillator demodulates the 100GHz STB signal;

[0061] Fig.13 Measured BER distribution diagram of OAM beam propagation with / without SDM and received OSNR and CP;

[0062] Fig.14 The BER distribution diagram of the received OSNR measured when the user data propagates on 25km OAM optical fiber;

[0063] Fig.15 Figure 2 is the relationship between the measured BER distribution and OSNR when the STB signal propagates on OAM optical fibers with lengths of 25 km, 50 km, and 75 km, respectively. DETAILED DESCRIPTION

[0064] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0065] like Figure 1 As shown, the present invention proposes a method for transmitting sub-terahertz wave signals via optical fiber, which can be divided into four parts: data framing and sub-terahertz modulation, OAM modulation and multiplexing, OAM optical fiber transmission, and signal demodulation and recovery.

[0066] Step 1: Data framing and sub-terahertz wave generation.

[0067] This step mainly includes four sub-modules, namely user data QAM modulation and OFDM framing; sub-terahertz wave modulation; intensity modulation; laser.

[0068] 1. User data QAM modulation and OFDM framing module

[0069] like Figure 2 The above describes the user data QAM modulation and OFDM framing, and the process can be divided into the following steps:

[0070] (1.1) Using the pseudo-random sequence (PRBS) polynomial x 23 +x 18 +1, resulting in a length of 2 23 -1-bit random binary sequence, used to simulate actual transmitted user data;

[0071] (1.2) Convert the serial PRBS sequence into a 4-bit parallel array through serial-to-parallel conversion to form 32 data streams;

[0072] (1.3) Using the QAM mapping module, the binary data is mapped into a quadrature amplitude modulation signal (QAM-16) in the complex domain;

[0073] (1.4) by a conjugate conversion method, the generated 32-channel data is converted into its conjugate 32-channel data, and 64-channel complex data is formed;

[0074] (1.5) The 64-channel data generated above are subjected to inverse Fourier transform by an inverse Fourier transform method to form real OFDM payload data (valid data) suitable for intensity modulation;

[0075] (1.6) Through the cyclic prefix byte insertion module, the cyclic prefix data is inserted or added at a fixed starting position in the OFDM data to isolate different OFDM symbols and reduce interference between OFDM symbols.

[0076] (1.7) Through the training sequence insertion module, a long training sequence is inserted before every 80 OFDM data carrying cyclic prefix data for synchronization and channel estimation.

[0077] (1.8) Through the parallel-to-serial conversion module, parallel OFDM data is converted into serial OFDM data, which is suitable for channel modulation and transmission.

[0078] 2. Modulation of sub-terahertz waves

[0079] like Figure 3 As shown in Figure 1, the modulation process of sub-terahertz is described, and the specific process can be divided into the following steps:

[0080] (2.1) A distributed feedback laser (DFB) with a center frequency of 1550 nm is used as the Gaussian (GB) light source for THz wave generation;

[0081] (2.2) Through the phase modulator 1, a certain phase delay is performed on the transmitted 1550 nm GB light, and the interval of the phase delay is mainly determined by the integer multiple n of the 25 GHz RF signal source, and n=1 in this patent;

[0082] (2.3) Through the phase modulator 2, a certain phase delay is performed on the transmitted 1550 nm GB light, and the interval of the phase delay is also mainly determined by the integer multiple m of the 25 GHz RF signal source, where m=1 in the patent;

[0083] (2.4) Through the Mach-Zehnder modulator, 25 GHz RF signal, and two-time phase modulator, the GB light generated by DFB is modulated into multiple coherent optical carriers.

[0084] (2.5) Figure 4 As shown, the optical carrier is modulated into a periodic pulse wave with a delay interval of 25 GHz through a 25 GHz delay interferometer.

[0085] (2.6) The power of light transmitted in a single-mode optical fiber is increased by using an erbium-doped fiber amplifier.

[0086] (2.7) Design a bandpass filter to select pulses that deviate from the central wavelength by 4×25, 5×25, 6×25, and 7×25 GHz from the generated pulses, thereby achieving modulation of sub-terahertz waves of 100, 125, 150, and 175 GHz.

[0087] Step 2: OAM modulation and multiplexing

[0088] This step is mainly divided into the following parts:

[0089] 1. If Figure 5 As shown, through a fiber array coupler, the Gaussian light carrying sub-terahertz is divided into five branch lights arranged like a "five-cake".

[0090] 2. Based on the flame grating patterns of l=-1, l=-2, l=-3, l=-4, l=-5 and different separation intervals, a comprehensive OAM array hologram is generated.

[0091] 3. Through an OAM array hologram, the 5 day branch lights are converted into 5 vortex lights (l = -1, l = -2, l = -3, l = -4, l = -5), each of which represents their mode group OMG1-5. Through an OAM array hologram, the 5 day branch lights are converted into 5 vortex lights (l = -1, l = -2, l = -3, l = -4, l = -5), each of which represents their mode group OMG1-5.

[0092] 4. Using the same fiber array coupler, the five array-shaped vortex lights generated are multiplexed into one vortex light, thereby completing the multiplexing of the OAM mode group.

[0093] Step 3: OAM fiber transmission

[0094] 1. If Figure 6 As shown in (a), an OAM fiber with a 5-layer structure is designed.

[0095] Although OMG multiplexing can significantly increase the number and improvement of independent channels, the OAM light in OMG cannot be transmitted in traditional SMF. Therefore, the transmission of OAM light requires special optical fiber. In the 5G new radio frequency over optical fiber (NROF) transmission system based on STB-OAM, OAM fiber with lower transmission loss plays an important role, which determines the transmission distance of 5G NR based on STB. In order to reduce transmission loss, support more OMG transmission and ensure a larger effective refractive index difference () between OAMG, this patent proposes a 5-layer OAM fiber.

[0096] like Figure 6 (a) describes the physical structure of the proposed OAM fiber, in which the innermost layer (first layer) with a radius of r1 = 1.55 μm marked in green has a refractive index contrast ratio (CRIC) of 0.8% (compared to the cladding), and its main purpose is to increase the low-order OAMG. The middle layer (second layer) with a radius of r2 = 9.22 μm marked in orange is designed as the core of the OAM fiber with a CRIC of 1.28%, and its main function is to propagate the OAM mode. The outer layer (third layer) marked in yellow with a radius between r3 = 11.20 μm and r4 = 14.3 μm is the trench layer, which is mainly composed of fluoride doping, causing the CRIC to become negative, which is beneficial to improve the bending resistance of the OAM fiber. The OAM fiber proposed in this patent can be prepared by plasma chemical vapor deposition technology (IPCVDT), and its process mainly involves process control such as doping, stretching, temperature, speed, and power.

[0097] 2. If Figure 6As shown in (b), the radius of each layer of the optical fiber (from the inner band to the outside) is designed to be 1.55, 9.22, 11.20, 14.30, and 62.50 microns respectively.

[0098] 3. As shown in 6(c), the contrast ratio of the refractive system of each layer of the optical fiber (from the inner band to the outer band) is 0.8%, 1.28%, 0%, -0.8%, and 0%, respectively. The distribution of the refractive index of each layer of the optical fiber obtained by simulation and experiment is described respectively. The simulation results are basically consistent with the experimental results and can be ignored. The distribution of the refractive index of each layer of the optical fiber obtained by simulation and experiment is described respectively. The simulation results are basically consistent with the experimental results and can be ignored.

[0099] 4. If Figure 6 As shown in (d), the equivalent refractive index (n eff )distributed.

[0100] In order to analyze the distribution of refractive index of each layer of the optical fiber and evaluate the performance of OAM fiber, we use finite element method (FEA) to analyze the OAM mode supported by OAM fiber or OAMG. The OAM fiber can support 6 mode groups, where each OAM mode in OMG can be expressed as ) mode form and or Mode form. The above modes are composed of even and odd modes (HE / EH) with π mode phase difference. In addition, the difference between different OMGs also increases with the increase of OMG order. The measured values ​​between OMG1 to OMG0, OMG2 to OMG1, OMG3 to OMG2, OMG4 to OMG3, and OMG5 to OMG4 are 1.31×10 -3 , 2.22×10 -3 , 2.89×10 -3 , 3.41×10 -3 , 3.86×10 -3 The measurement values ​​between two adjacent OAMGs are all less than 1.0×10 -3 , so it can be used in OMG-based SDM to improve CC and SE of optical communication systems.

[0101] 5. If Figure 7 As shown, at 1550nm, the transmission losses of various OMG0-5 supported by the optical fiber measured by the OTDR method are 0.24, 0.23, 0.25, 0.27, 0.29, and 0.31dB / km respectively.

[0102] Compared with the conventional RCF with a larger CRIC, the optical fiber proposed in this patent has a smaller core and cladding CRIC, which is beneficial to reduce the propagation loss of the OAM mode. However, compared with the conventional RCF, the OAM fiber proposed in this patent supports fewer mode groups. In order to study the proposed propagation loss, we used an optical time domain reflectometer (OTDR), and the measurement results are shown in Figure 6 As shown in Figure 1, a 10.2 km long OAM fiber was used as a sample. Figure 7 In the results, the propagation losses measured at 1550 nm are 0.24, 0.23, 0.25, 0.27, 0.29, and 0.31 dB / km, respectively, which indicates that due to the small CRIC between the core and cladding, all OMGs supported by the proposed OAM fiber have lower propagation losses and can be used for longer distance propagation.

[0103] Step 4: Signal demodulation and recovery

[0104] like Figure 8 As shown, this step adopts a method of simultaneously demodulating multiple OAM mode groups, which is beneficial to improving the OAM mode demodulation efficiency. The specific steps are as follows:

[0105] 1. The coupled OAM fiber transmission multiplexed light is free space OAM multiplexed light.

[0106] On the receiving side, the OAM light groups transmitted in the OAM fiber are converted into OAM multiplexed light for free space transmission through devices such as collimators and polarization controllers.

[0107] 2. A Dammann vortex grating is designed, which can simultaneously demodulate the incident OAM multiplexed light into Gaussian base mode light and locate it at different spatial coordinates.

[0108] 3. Free-space OAM multiplexed light illuminates the spatial light modulator to simultaneously demodulate different OAM lights into Gaussian lights of different spatial coordinates.

[0109] 4. Use the direct detection method of photoelectric detector to convert optical signals into electrical signals.

[0110] When the converted GB light is irradiated onto a photodetector (PD), the OFDM electrical signal carrying the user's information is demodulated from the GB.

[0111] 5. Use offline digital signal processing module to analyze the performance of system transmission.

[0112] In the offline processing module, OFDM deframing, QAM demodulation and other methods are used in turn to demodulate the original user data signal and analyze the system performance.

[0113] 5. Verification Platform

[0114] In order to investigate the validity of the proposed concept, we Fig. 9 A demonstration platform is designed and built in . The proposed experimental configuration can be divided into three parts: user data generation and modulation, OAM beam modulation and propagation, and signal demodulation and processing.

[0115] First, in order to simulate real user data, the PRBS data module uses a polynomial as a random sequence polynomial to generate a serial pseudo-random bit sequence (PRBS) of length. In order to convert the user data into symbols of orthogonal amplitude modulation 16QAM, the generated serial PRBS is converted into a corresponding parallel PRBS sequence of length 4 bits through the serial-parallel conversion (S / P) module. Then, the converted 4-bit length PRBS is mapped into the corresponding "radio" into the corresponding symbol using the mapping module. All mapped symbols are modulated into corresponding data of 4 bits in length and form 16 QAM signals. Then, the modulated 16 QAM signal is modulated into an orthogonal frequency division multiplexing (OFDM) signal through the OFDM module, where the modulation process of OFDM mainly includes inverse fast Fourier transform (IFFT), cyclic prefix (CP) insertion, training sequence insertion, etc. The generated OFDM signal is transmitted through the parallel-serial (P / S) conversion module and the arbitrary wave The signal is converted back into a serial signal by an AWG module or a vector signal generator (VSG), and modulated to generate an analog signal. Sub-terahertz waves are generated by devices such as distributed feedback (DFB) lasers, phase modulators, and Mach-Zehnder (MZM). A polarization controller (PC) is inserted after the STB to adjust the polarization of the input Gaussian beam carrier (GBC) to a linearly polarized GBC. Subsequently, the modulated OFDM data is used together with the sub-terahertz wave to drive the MZM to modulate the user data onto the GBC and propagate on the single-mode fiber (SMF).

[0116] Secondly, the modulated GBC carrying PRBS data is sequentially amplified and filtered by an erbium-doped fiber amplifier (EDFA1) and a bandpass filter (BPF1) to increase the propagation power of GBs and reduce the interference of amplified spontaneous noise (ASEN), respectively. An optical coupler (OC1) is used to split the adjusted GB into 6 copies, generating 5 OAM beams and a reference GB. Six SMFs of different lengths are used to decorrelate the 6 copies in the 6 paths, and then each GB in the path is regarded as an independent channel for propagating user data. In each of the 6 paths, collimation (C1 / C2 / C3 / C4 / C5 / C6) is used to couple the GB in the SMF into free space, where 5 paths are used to generate the carrying beam. The above collimators C1-6 can be arranged in a certain order to form a fiber array coupler (OFAC). In order to enlarge the GB size in free space, a "successful" system consisting of a pair of lenses (L1&L2 / L3&L4 / L5&L6 / L7&L8 / L9&L10 / L11&L12) is used in each of the six paths, which is conducive to improving the conversion efficiency of GB to OAM beam. Subsequently, on each path, the polarization of the amplified GB light is adjusted to linearly polarized GB light using a combination of polarizers (P1 / P2 / P3 / P4 / P5 / P6) and half-wave plates (H1 / H2 / H3 / H4 / H5 / H6). Five paths are used to correspond to the SPP of the hologram (SPP1 / SPP2 / SPP3 / SPP4 / SPP5), and paths 1 / 2 / 3 / 4 / 5 are converted into corresponding OAM modes (l=-1 to -5) as representatives of OAMG1 to OAMG5, respectively. In free space, an OAM array hologram loaded with a spatial light modulator (SLM) is used to convert the incident 5-channel GBC into spatial array OAM light. Another OFAC is used to multiplex the generated array OAM light into 1-channel OAM light, where M1 is used to control the transmission direction of the light. An inverse system consisting of L13 and L14 is inserted into the optical path to reduce the beam size of the multiplexed OAM beam coupled into the proposed OAM fiber. Meanwhile, H8 and a quarter-wave plate (Q1) are used to adjust the linearly polarized OAM beam to circular polarization so as to propagate the OAM beam on the proposed OAM fiber. Then, an objective lens (OL1) is inserted after Q1 for coupling the multiplexed OAM beam into the proposed OAM fiber from free space. After the multiplexed OAM beam propagates through the proposed OAM fiber, PC2 and C7 are used to adjust the polarization of the propagated OAM beam and couple the OAM beam into the free space in the proposed OAM fiber. Then, the size of the multiplexed OAM beam in the free space is expanded by a pair of L15 and L16 to improve the illustrated area of ​​the spatial light modulator (SLM) and the mode conversion efficiency between the OAM beam and the GB.The OAM beam is adjusted to a linearly polarized OAM beam in free space by the combination of Q2, P7 and H7. The OAM beam adjusted by H7 is split into two by BS6. One path is used to observe the received intensity distribution and interfere with the reference GB obtained from H6 through BS5, thereby identifying the detected OAM mode through a charge coupled device (CCD1). The other path is to demodulate the propagated OAM beam into GB by illuminating an anti-hologram (LM) equipped with a aiming OAM beam on the corresponding SPP, where M2 and M3 are inserted between the SLMs to adjust the directions of the incident and outgoing light, respectively. It is worth noting that since SLM2 is loaded with the designed Dammann OAM grating, all the incident OAM light can be demodulated into GB at the same time. The demodulated GB wave is reselected and coupled into the SMF through another fiber array switch (OFAS) and OL2.

[0117] Finally, another EDFA2, variable optical attenuator (VOA), and PC3 are used after OL2 to amplify the optical power, adjust the optical power, and adjust the polarization of the demodulated GB in the SMF, respectively. Then, the input GB is converted into electrical signals (ESs) using a photodetector (PD). The system performance of the proposed concept is analyzed by measuring the bit error rate, CP, etc. using an offline processing module.

[0118] 2. Experimental results and performance analysis

[0119] Without stating other instructions, the wavelength, data rate, and OAM fiber length are configured as 1550nm, 10Gbit / s, and 25km, respectively, in this experiment. In order to verify the successful propagation of the adopted OAM mode, Fig. 9 The CCD in the method captures the intensity distribution and the corresponding interference pattern. Fig.10 In the example, when one of the adopted OAM modes (l1=-1, l2=-2, l3=-3, l4=-4, l5=-5) is independently generated and propagates on the proposed OAM fiber, Fig.10 (a1)-(a5) show the captured intensity profiles, where five hollow rings are observed that are consistent with the intensity characteristics of the OAM beam. Fig.10 The OAM beam received in Fig.10 (a1)-(a5)) are disturbed by the reference GBs, and the CCD1 used can capture Fig.10 The corresponding interference patterns in (b1)-(b5), where the number of interference fringes and the clockwise or counterclockwise rotation direction of the phase can be used to determine the value of |l| and the sign (- / +) of l. Fig.10 In (b1), we can observe an interference fringe marked by a red dotted circle and a phase clockwise rotation direction marked by a red dotted curve with an arrow, which indicates that Fig.10 As shown in (a1), the topological charge of the detected OAM beam is l = -1. Fig.10 In (b2)-(b5), 2, 3, 4, and 5 clockwise stripes can be observed, which indicates that Fig.10 The OAM light beams detected in (a2)-(a5) are l=-2, l=-3, l=-4, and l=-5 respectively. Fig.10 The measurement results in show that the OAM mode generated at the transmitting end can be successfully propagated and received at the receiving end, further demonstrating the effectiveness and feasibility of the proposed method.

[0120] Due to the imperfect manufacturing process of the proposed OAM fiber, crosstalk between different OAM modes is inevitable. Part of the power of the propagated OAM mode can easily leak into other adjacent OAM modes, causing crosstalk and significantly degrading the system performance of the proposed concept. In order to evaluate the crosstalk between different OAM modes, the OAM modes are propagated separately at the transmitting end and Fig.11 The power of all OAM modes is detected in , where , respectively, represent OAMGs 1 to 5. Fig.11 As shown in the figure, the crosstalk mainly occurs between two adjacent OAMGs, in which the most serious crosstalk occurs between and. Compared with the low-order OAMGs, the crosstalk between high-order OAMGs is weaker, which may be related to Figure 6 (d) is related to the larger one in the higher-order OAMG shown.

[0121] In addition, in order to evaluate the transmission performance, the transmitted data needs to be demodulated into an electrical signal through PD photoelectric conversion and then demodulated into an intermediate frequency (IF) signal. Fig.12 As shown in FIG. 1 , the intermediate frequency demodulated electrical signal after mixing the converted signal in the PD with the local GHz oscillator is described, which shows that the transmitted STB signal can be successfully received at the receiving end. Fig.12 As shown, a propagating user data signal with a fluctuation of can be detected at a center frequency of 25 GHz. The slight fluctuation can be attributed to the large interference between carriers.

[0122] In order to explore the communication performance of the system, we use BER and CP to analyze the user data received in the offline processing module. When each OAM beam propagates alone without using SDM or all OAM beams propagate together with SDM, Fig.13As shown, the measured BER distribution of the used OAM beams (l = -1, l = -2, l = -3, l = -4, and l = -5) is shown, with a red dashed line inserted to describe the hard forward error correction (FEC) threshold with an overhead of 7%. In addition, the symbol "BTB" is used to describe the "back-to-back" case, where only GB is used as a carrier to transmit user data, which then propagates on the proposed OAM fiber. Fig.13 In the figure, compared with the propagation on “, when the five OAM beams propagate independently without SDM, l = -1, l = -2, l = -3, l = -4 and l = -5 are 3.8 × 10 3 The BER measured at the locations decreased by 0.61, 0.83, 1.27, 1.63 and 1.98 dB, respectively. Compared with “98”, there is a slight decrease in the BER performance of each OAM mode from 0.61 dB to 1.98 dB, which can be attributed to the fact that the intensity distribution of the OAM beam is larger than that of the GB. In addition, when OAM beams with l=-1, l=-2, l=-3, l=-4 and l=-5 are co-propagated on the system through SDM, the measured BER is reduced by 2.8, 3.21, 3.64, 4.05, 4.39 dB, respectively, compared with “98”. It can be observed that the BER of the five OAM beams is reduced by 2.19, 2.38, 2.37, 2.42, and 2.41 dB when l = -1, l = -2, l = -3, l = -4, and l = -5, respectively, which can be attributed to the crosstalk caused by OAM-based SDM when all OAM beams co-propagate on the system. In addition, when the OSNR is set to 15.2 dB, the measured CPs of the 16QAM signal are as follows: Fig.13 As shown, the first row represents the “shown” state, and the second and third rows represent each OAM channel without SDM and with SDM, respectively. In the case with / without SDM, the measured CP becomes more and more blurred as the order of the OAM mode increases from -1 to -5, which is consistent with the measured BER distribution. Compared with the case with / without SDM, the blur level is more serious in the case of W / SDM, which may be due to the increase in crosstalk caused by SDM.

[0123] Although the adopted 10 Gb / s data signal has been successfully propagated and received by the proposed OAMG-based SDM communication system for transmitting STB-based 5G NR data, higher data rates still need to be explored to evaluate the impact of different data rates on system performance. In addition, for simplicity, since the BER performance of different OAM modes is similar, only the OAM beam with l = -5 is used for monitoring to evaluate the impact of different data rates on the system BER distribution when the five OAM beams are propagated through SDM. Fig.14Figure 1 depicts the measurement of BERs at l = -5 when five OAM beams co-propagate at different speeds through the proposed SDM system. Fig.14 As shown in the figure, compared with v = 10 Gb / s, when the user data is transmitted at the speed of v = 20 Gb / s and v = 40 Gb / s, the -3 The BER measured under the HFEC penalty of 0.18 and 0.42 dB respectively. A slight decrease in BER can be observed as the data rate increases from 10 Gb / s to 40 Gb / s, which also shows that the proposed concept has the potential to transmit user data at higher speeds with an acceptable increase in OSNR cost.

[0124] In this experiment, the fabricated OAM fiber with a length of 25 km has been successfully used to propagate 5 OAM beams carrying STB signals, and the BER of each channel is acceptable. However, the impact of different OAM fiber lengths on the system still needs further study to evaluate the performance of the system, so as to evaluate the long-distance propagation capability of the proposed system. Fig.15 Figure 2 shows the measured BER distribution for l=-5 when the five adopted OAM beams co-propagate on the proposed system with different fiber lengths from 25km to 75km. As the OAM fiber length increases from 25km to 75km, the detected BER performance degrades. Compared with the quantitative statistics, when the multiplexed OAM beams transmitted at a transmission rate of 10Gb / s propagate on OAM fibers with lengths of 50km and 75km, the measured BER for l=-5 decreases by 0.83 and 2.14dB, respectively. Fig.14 In comparison, the effect of OAM fiber length on BER is Fig.15 This is more serious at data rates of 100 Mbps, because longer OAM fiber lengths may result in greater transmission losses and greater crosstalk. In order to achieve long-distance transmission on OAM fibers, the fiber structure and the CRIC between the core and cladding can be optimized. A thinner core layer is beneficial for suppressing high-order radial modes and reducing crosstalk between different OAM modes. Although a lower CRIC between the core and cladding may be beneficial for reducing transmission losses, the number of OAM modes supported may also be reduced. Therefore, in the future, a comprehensive balance should be made between the propagation distance and the number of supported OAM modes.

Claims

1. A method for transmitting sub-terahertz data via optical fiber, characterized in that: The steps include: (1) Data framing and sub-terahertz wave generation; (2) OAM mode optical field modulation and multiplexing; (3) OAM mode group fiber transmission; (4) OAM mode signal demodulation and data recovery, The step (2) comprises the following steps: (2.1) Through a fiber array coupler, the sub-terahertz Gaussian light is split into five branch lights; (2.2) generating a comprehensive OAM array hologram according to the flame fork grating patterns of l = -1, l = -2, l = -3, l = -4, l = -5 and different partition intervals; (2.3) Through an OAM array hologram, the five branch lights are modulated into five vortex lights, where each mode represents their mode group OMG1-5; (2.4) Using the same fiber array coupler, the five vortex lights generated in the array are multiplexed into one vortex light, thereby completing the multiplexing of the OAM mode group; The step (3) comprises the following steps: (3.1) Design an OAM fiber with a 5-layer structure; (3.2) The radius of each layer of the optical fiber is designed to be 1.55 μm, 9.22 μm, 11.20 μm, 14.30 μm, and 62.50 μm respectively; (3.3) The contrast ratio of the refraction system of each layer of the optical fiber is set to 0.8%, 1.28%, 0%, -0.8%, and 0% respectively; (3.4) Analyze the equivalent refractive index distribution of different modes supported by optical fiber; (3.5) At 1550nm, the transmission losses of various OMG0-5 supported by optical fibers measured by OTDR method are 0.24dB / km, 0.23dB / km, 0.25dB / km, 0.27dB / km, 0.29dB / km, and 0.31dB / km respectively.

2. The method for transmitting sub-terahertz data via optical fiber according to claim 1, characterized in that: The step (1) includes QAM modulation of user data and OFDM framing and modulation of sub-terahertz waves.

3. The method for transmitting sub-terahertz data via optical fiber according to claim 2, characterized in that: The user data QAM modulation and OFDM framing include: (1.1) Using the pseudo-random sequence polynomial x 23 +x 18 +1, resulting in a length of 2 23 -1-bit random binary sequence, used to simulate random user data actually transmitted; (1.2) Convert the serial PRBS sequence into a 4-bit parallel array through serial-to-parallel conversion to form 32 data streams; (1.3) Using the QAM mapping module, the binary data is mapped into a complex domain quadrature amplitude modulation signal QAM-16; (1.4) The generated 32-channel QAM data is converted into its conjugate 32-channel QAM data by a conjugate conversion method, and combined with the previous 32-channel original QAM data to form 64-channel real data; (1.5) Performing inverse Fourier transform on the 64-channel data generated above by an inverse Fourier transform method to form real number OFDM payload data suitable for intensity modulation; (1.6) Insert or add cyclic prefix data at a fixed starting position in OFDM data through the cyclic prefix byte insertion module to isolate different OFDM symbols and reduce interference between OFDM symbols; (1.7) Through the training sequence insertion module, a long training sequence is inserted before every 80 OFDM data carrying cyclic prefix data for synchronization and channel estimation; (1.8) Through the parallel-to-serial conversion module, parallel OFDM data is converted into serial OFDM data, which is suitable for channel modulation and transmission.

4. The method for transmitting sub-terahertz data via optical fiber according to claim 2, characterized in that: The modulation of the sub-terahertz wave comprises the following steps: (1.9) A distributed feedback laser with a central wavelength of 1550 nanometers is used as a Gaussian light source for generating sub-terahertz waves; (1.10) The transmitted 1550 nm Gaussian light is subjected to a certain phase delay through the phase modulator 1, and the interval of the phase delay is mainly determined by the integer multiple n of the 25 GHz RF signal source, where n=1; (1.11) Through the phase modulator 2, the transmitted 1550 nm Gaussian light is subjected to a certain phase delay, and the interval of the phase delay is also mainly determined by the integer multiple m of the 25 GHz RF signal source, m = 1; (1.12) The Gaussian light generated by the DFB is modulated into multiple coherent optical carriers through the Mach-Zehnder modulator, 25 GHz RF signal, and two-time phase modulator; (1.13) Through a 25 GHz delay interferometer, the optical carrier is modulated into a periodic pulse wave with a delay interval of 25 GHz; (1.14) The power of Gaussian transmission light in a single-mode optical fiber is increased by an erbium-doped optical fiber amplifier; (1.15) Design a bandpass filter to select the wavelength 4× from the generated pulse. 25 , 5× 25 , 6× 25 , 7× 25 GHz pulses to achieve modulation of sub-terahertz waves of 100, 125, 150, and 175 GHz.

5. The method for transmitting sub-terahertz data via optical fiber according to claim 1, characterized in that: The first layer of the five-layer structure in step (3.1) is the innermost layer with a radius of r1=1.55 microns and a refractive index contrast ratio of 0.8%; the second layer is an intermediate layer with a radius between r1=1.55 microns and r2=9.22 microns and a refractive index contrast ratio of 1.28%; the third layer is a cladding layer with a radius between r2=9.22 microns and r3=11.20 microns; the fourth layer is a trench layer with a radius between r3=11.20 microns and r4=14.3 microns, which is formed by doping fluoride; and the fifth layer is the outermost cladding layer.

6. The method for transmitting sub-terahertz data via optical fiber according to claim 1, characterized in that: The step (3.4) includes using the finite element method to analyze the OAM mode supported by the OAM fiber or the n of the OAMG. eff , OAM fiber supports 6 mode groups from l = |0| to l = |5|, which are composed of even and odd vector mode combinations with π / 2 mode phase difference. The Δn between different OMGs is eff It also increases with the increase of OMG order. The Δn between two adjacent OAMGs eff All are greater than 1.0×10 -3 .

7. The method for transmitting sub-terahertz data via optical fiber according to claim 1, characterized in that: The step (4) comprises the following steps: (4.1) The multiplexed light coupled into the OAM optical fiber transmission is free space OAM multiplexed light; (4.2) Design a grating that simultaneously demodulates the vortex light, which can simultaneously demodulate the incident OAM multiplexed light into Gaussian base mode light and is located at different spatial coordinates; (4.3) The free-space OAM multiplexed light is irradiated into a spatial light modulator loaded with a simultaneously demodulated vortex light grating, thereby simultaneously demodulating different OAM lights into Gaussian lights with different spatial coordinates; (4.4) Using a direct detection method with photoelectric detectors of different sensitive frequency bands to convert optical signals carrying different THz data into corresponding electrical signals; (4.5) Use offline digital signal processing module to analyze the performance of system transmission.

8. A verification system for a method for transmitting sub-terahertz data via optical fiber, used for verifying the method for transmitting sub-terahertz data via optical fiber according to any one of claims 1 to 7, characterized in that: include: User data generation and modulation module, OAM beam modulation and propagation module, and signal demodulation and processing module; The user data generation and modulation module includes a PRBS data module generated by FPGA, a serial-to-parallel conversion module, a QAM mapping module, a QAM modulation module, and an OFDM framing module. The OFDM framing module includes an inverse fast Fourier transform, a cyclic prefix insertion, a training sequence insertion, a parallel-to-serial conversion module, an arbitrary waveform RF signal generator, and a terahertz wave modulator. The terahertz wave modulator includes a distributed feedback laser, a phase modulator, a 25GHz RF source, a Mach-Zehnder modulator, a phase delay interferometer, a fiber amplifier, a bandpass filter, a single-mode fiber, and a polarization controller. The OAM beam modulation and propagation module includes an erbium-doped fiber amplifier, a bandpass filter, a delayed single-mode fiber, a collimator, a fiber array coupler, a lens, a 3f system, a polarizer, a half-wave plate, a spiral phase plate, an objective lens, a spatial light modulator, a charge coupler, and a mirror; The signal demodulation and processing modules include OL, EDFA, variable optical attenuator, polarization controller, photodetector, oscilloscope, and FPGA development board.

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