WO2021136041A1 - 一种通信系统 - Google Patents

一种通信系统 Download PDF

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Publication number
WO2021136041A1
WO2021136041A1 PCT/CN2020/138718 CN2020138718W WO2021136041A1 WO 2021136041 A1 WO2021136041 A1 WO 2021136041A1 CN 2020138718 W CN2020138718 W CN 2020138718W WO 2021136041 A1 WO2021136041 A1 WO 2021136041A1
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Prior art keywords
mode
group
few
groups
fiber
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PCT/CN2020/138718
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English (en)
French (fr)
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郭强
周锐
杨志群
张�林
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华为技术有限公司
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Priority to EP20910532.9A priority Critical patent/EP4080788A4/en
Priority to JP2022540507A priority patent/JP7438367B2/ja
Publication of WO2021136041A1 publication Critical patent/WO2021136041A1/zh
Priority to US17/854,585 priority patent/US20220342151A1/en

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/14—Mode converters
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
    • G02B6/02085—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the grating profile, e.g. chirped, apodised, tilted, helical
    • G02B6/02095—Long period gratings, i.e. transmission gratings coupling light between core and cladding modes
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
    • G02B6/021—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the core or cladding or coating, e.g. materials, radial refractive index profiles, cladding shape
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03638—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03638—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only
    • G02B6/0365—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only arranged - - +
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2581—Multimode transmission
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/04—Mode multiplex systems

Definitions

  • This application relates to the field of communication technology, and in particular to a communication system.
  • Multi-mode optical fiber is proposed. Although multi-mode optical fiber can meet the demand of communication capacity (supporting 100-200 modes), the dispersion between modes is serious, which leads to serious signal distortion. Therefore, the application of few-mode optical fiber communication was born. The few-mode optical fiber can not only reduce the inter-mode dispersion, but also increase the capacity of the communication system. Therefore, it is widely used in optical fiber communication systems.
  • the signals carried on different modes will have crosstalk during transmission. Therefore, the receiving end needs to use the multi-input-multi-output (MIMO) algorithm to transfer the signals carried on the different modes.
  • MIMO multi-input-multi-output
  • the signal is demodulated.
  • the complexity of the MIMO algorithm is positively correlated with the differential mode group delay (DMGD) difference of each mode, that is, the larger the DMGD difference between modes, the more complex the MIMO algorithm, which leads to the receiving end
  • DMGD differential mode group delay
  • a section of positive-negative mode dispersion small-mode optical fiber is usually cascaded after each span of the small-mode optical fiber to reduce DMGD.
  • the preparation process of the positive-negative mode dispersion low-mode fiber is complicated, and it is difficult to match the low-mode fiber with the positive-negative mode dispersion low-mode fiber, and the cascade process is more difficult.
  • the present application provides a communication system for reducing the group delay when different module groups arrive at the receiving end.
  • the present application provides a communication system, which may include at least two spans of few-mode optical fibers and a mode converter.
  • the few-mode fiber is used to transmit the received M mode groups.
  • the group delay of the M mode groups transmitted in the few-mode fiber is centrally symmetrically distributed.
  • the center of the group delay of the M mode groups is the group delay of the M mode groups.
  • each of the M mode groups carries a signal, and M is an integer greater than 1;
  • the mode converter is used to receive M mode groups from the few-mode fiber, and
  • the first mode group and the second mode group in the M module groups are exchanged to obtain M exchanged module groups, and the M exchanged mode groups are coupled to the few-mode fiber of the next span, the first
  • the group delay of the module group and the group delay of the second module group are symmetric about the center, wherein the first module group and the second module group are at least two module groups among the M module groups.
  • M mode groups are transmitted via a few-mode fiber.
  • the group delays of the M mode groups are distributed symmetrically, and the group delays of the M mode groups are distributed center-symmetrically into the mode converter.
  • the mode converter exchanges the mode groups of the first mode group and the second mode group that are centrally symmetrical.
  • the mode group with the maximum group delay is exchanged with the mode group with the minimum group delay, and the group delay is
  • the next-largest mode group is exchanged with the next-lowest group delay, and so on, and the exchanged M mode groups are coupled to the next span of few-mode fiber for transmission.
  • the mode group with the maximum group delay is switched for the mode group with the minimum group delay, and the mode group with the minimum group delay is switched for the mode group with the maximum group delay.
  • the group delay of the M mode groups is zero. In this way, it helps to reduce the complexity of the MIMO algorithm at the receiving end.
  • the sum of the group delay of any first module group that is symmetric about the center among the M module groups is equal to the sum of the group delay of the second module group.
  • the M group delays include M/2 pairs of group delays symmetric about the center, that is, the M module groups include M/2 first module groups and M/2 first module groups.
  • the few-mode fiber includes a core, an inner cladding, a trench layer, and an outer cladding in order from the inside to the outside.
  • the group refractive index of the M mode groups is centrally symmetrically distributed; among them, the group refractive index of each mode group in the M mode groups is determined according to the effective refractive index of the mode group, and the group refractive index of the mode group is determined by the effective refractive index of the mode group.
  • the effective refractive index is based on the core refractive index of the few mode fiber, the refractive index of the inner cladding, the refractive index of the outer cladding, the refractive index of the trench layer, the radius of the core, the width of the inner cladding, the width of the trench layer, and the outer cladding layer. The width is determined.
  • the mode converter may include Fiber gratings with different periods, one period corresponds to a first mode group and a second mode group, Indicates rounding down; Each of the two different periods is determined according to the communication wavelength, the effective refractive index of the corresponding first mode group, and the effective refractive index of the corresponding second mode group.
  • the difference between the effective refractive index of the first mode group and the effective refractive index of the second mode group corresponding to the grating period multiplied by the grating period is equal to the communication wavelength.
  • the fiber gratings of different periods can be a cascade structure or a stacked grating structure. when two fiber gratings with different periods are in a stacked-grid structure, it helps to avoid the influence of the cross-section of fiber gratings in different regions on the coupling of other mode groups with phase mismatch.
  • the mode converter may include a reflective phase plate or a transmissive phase plate.
  • the communication system also includes an amplifier.
  • the mode converter can be located between the few-mode fiber and the amplifier; alternatively, the mode converter can be integrated in the amplifier. By integrating the mode converter in the amplifier, the insertion loss introduced by the mode converter can be compensated without deteriorating the signal-to-noise ratio of the transmission signal.
  • Figure 1 is a schematic diagram of a centrally symmetrically distributed value provided by this application.
  • FIG. 2 is a schematic diagram of the architecture of a communication system provided by this application.
  • FIG. 3 is a schematic structural diagram of a cross-section of a few-mode optical fiber provided by this application.
  • 4a is a schematic structural diagram of a step-type few-mode fiber provided by this application.
  • 4b is a schematic diagram of the group refractive index distribution of a step-type few-mode fiber provided by this application;
  • 4c is a schematic diagram of the group refractive index distribution of another step-type few-mode fiber provided by this application.
  • FIG. 5a is a schematic structural diagram of a graded few-mode optical fiber provided by this application.
  • FIG. 5b is a schematic diagram of the group refractive index distribution of a graded few-mode optical fiber provided by this application.
  • FIG. 5c is a schematic diagram of the group refractive index distribution of another graded few-mode optical fiber provided by this application.
  • FIG. 6a is a schematic diagram of a cascaded structure of a fiber grating provided by this application.
  • FIG. 6b is a schematic diagram of a stacked grating structure of a fiber grating provided by this application.
  • Fig. 7a is a schematic diagram of a mode converter based on a phase plate for mode group conversion provided by this application;
  • FIG. 7b is a schematic structural diagram of another mode converter based on a phase plate for mode group conversion provided by this application;
  • FIG. 7c is a schematic structural diagram of a mode converter provided by this application as a reflective phase plate
  • FIG. 7d is a schematic structural diagram of a mode converter provided by this application as a transmissive phase plate
  • FIG. 8a is a schematic diagram of the position of a mode converter in the communication system provided by this application.
  • Fig. 8b is a schematic diagram of the position of another mode converter provided in the application in the communication system
  • Fig. 8c is a schematic diagram of the position of another mode converter provided in the application in the communication system.
  • FIG. 9 is a schematic diagram of the architecture of another communication system provided by this application.
  • FMF is a single-core fiber with a large mode field area, allowing multiple independent modes to transmit signals in parallel.
  • a few-mode fiber can support 2 to 50 modes.
  • a few-mode fiber is a fiber that has a relatively large core area and supports parallel transmission of signals in multiple modes at the same time.
  • Mode can be understood as a form of field distribution formed when light is transmitted as an electromagnetic wave in an optical fiber. That is, the light travels in the few-mode fiber to form a hybrid mode, which is generally called a linearly polarized (LP) mode.
  • LP linearly polarized
  • Few-mode fiber is based on mode division multiplexing technology to transmit signals.
  • the different modes are orthogonal to each other.
  • the few-mode fiber can support multiple mutually orthogonal independent modes, and these modes are used as independent channels, and these modes can carry multiple signals at the same time.
  • a module group refers to a group of multiple modes with the same propagation constant. In other words, the group rates of the multiple modes included in one module group are equal.
  • One fiber can transmit multiple mode groups.
  • ⁇ represents the communication wavelength
  • v g represents the group velocity
  • c represents the speed of light in vacuum
  • N eff represents the effective refractive index of this mode group.
  • the effective refractive index is a quantity that can quantitatively describe the phase delay per unit length in a waveguide (such as a few-mode fiber), and is relative to the phase delay per unit length in vacuum.
  • the refractive index n can be used to quantitatively describe the increase in the wave number (phase change per unit length) caused by the medium: its wave number is n times the wave number in vacuum.
  • the effective refractive index n eff has a similar meaning: in a waveguide (such as a few-mode fiber), the ⁇ value of a certain wavelength is n eff times the vacuum wave number, where,
  • the effective refractive index depends not only on the wavelength, but also on the mode of transmission. Therefore, the effective refractive index is also called the mode refractive index.
  • the refractive index profile refers to the relationship between the refractive index or relative refractive index and the radius of the optical fiber.
  • n(r) represents the refractive index at radius r.
  • the relative refractive index difference refers to the ratio of the difference between two refractive indexes to the relative refractive index.
  • ⁇ n TR (n TR- n CL )/n CL , where n CL represents the refractive index of the outer cladding layer, and n TR represents the refractive index of the trench layer.
  • the number axis it is customary to call two value points with the same distance from the origin 0 and opposite directions as symmetric points. For example, 7 and -7 are called central symmetric points. The sum of the two points of the symmetry point with 0 as the center is 0. In the symmetric point interval [-a, +a] centered on 0 point, excluding 0 itself, the effective number of points is 2a, and at most a pair of symmetric points can be formed.
  • Fig. 1 a schematic diagram of centrally symmetrically distributed values provided by this application.
  • the M numerical values are arranged in order as ⁇ a 1 , a 2 , a 3 , a 4 , a 5 , a 6 , a 7 , a 8 , a 9 ⁇
  • a 1 and a 9 are at the same distance from the center
  • a 2 and a 8 are at the same distance from the center
  • a 3 and a 7 are at the same distance from the center
  • a 1 and a 9 are symmetric about the center
  • a 2 and a 8 Regarding center symmetry
  • a 3 and a 7 are symmetric about the center
  • a 4 and a 6 are symmetric about the center.
  • the distance between the largest value and the smallest value from the center is equal
  • the distance between the second largest value and the second smallest value from the center is also the same
  • the sum of the largest value and the smallest value is equal to the second The sum of the largest value and the second smallest value, and so on.
  • the orthogonal mode supported by the few-mode fiber is used as an independent channel for parallel transmission of signals, which can increase the transmission capacity of the communication system.
  • the mode of the few-mode fiber has a larger mode field area, the nonlinearity tolerance is also very high. That is to say, a communication system including a few-mode optical fiber can not only increase the transmission capacity of the communication system, but also help avoid the interference of the nonlinear effect on the system, but there will be problems described in the background art.
  • this application proposes a communication system.
  • the communication system can reduce the group delay between different module groups in the communication system, thereby helping to reduce the complexity of the MIMO algorithm at the receiving end and improve the efficiency of the transmission system.
  • FIG. 2 it is a schematic diagram of the architecture of a communication system provided by this application.
  • the communication system may include at least two spans of few-mode optical fibers and mode converters.
  • Figure 2 uses a few-mode fiber with two spans and a mode converter as an example.
  • the few-mode fiber is used to transmit the received M mode groups, and the group delays of the M mode groups transmitted in the few-mode fiber are centrally symmetrically distributed.
  • the mode converter is used to receive M mode groups from the few-mode fiber, and perform mode group exchange between the first mode group and the second mode group in the M mode groups to obtain M exchanged mode groups, and exchange the M
  • the latter mode group is coupled to the few-mode fiber of the next span.
  • the group delay of the first mode group and the group delay of the second mode group are symmetrical about the center.
  • the first mode group and the second mode group are M
  • the center of the group delay of the M module groups is the middle value between the maximum and minimum group delays of the M module groups, and each module group of the M module groups bears There is a signal
  • M is an integer greater than 1.
  • each of the M module groups corresponds to one group delay
  • the M module groups correspond to M group delays.
  • the M group delays are respectively t 1 , t 2 ,..., t M-1 , t M , where t 1 >t 2 >...>t M.
  • one of the module group corresponding to t 1 and the module group corresponding to t M is the first module group, and the other is the second module group; one of the module group corresponding to t 2 and the module group corresponding to t M-1 is The first module group, the other is the second module group.
  • the module group corresponding to t 1 is the first module group
  • the module group corresponding to t M is the second module group
  • the module group corresponding to t 2 is the first module group
  • the module group corresponding to t M-1 is the second module group. Group, and so on.
  • the module group corresponding to t 1 is the second module group
  • the module group corresponding to t M is the first module group
  • the module group corresponding to t 2 is the second module group
  • the module group corresponding to t M-1 is the first module group.
  • the mode converter is used to convert the first mode group into the second mode group, and convert the second mode group into the first mode group. That is, the mode converter is used to gradually convert the mode field of the first mode group into the mode field of the second mode group, and gradually convert the mode field of the second mode group into the mode field of the first mode group.
  • At least two spans of the few-mode fiber include a mode converter between each two spans of the few-mode fiber, that is, a span of the few-mode fiber + mode converter + down A span of few-mode fiber.
  • the few-mode fiber with at least two spans can be the few-mode fiber with an even number of spans, or the few-mode fiber with an odd number of spans.
  • M mode groups can be grouped between M mode groups after passing through one span of few-mode fiber + mode converter + next span of few-mode fiber The delay is zero.
  • the group delay between the M mode groups may be the group delay of the few-mode fiber of the last span.
  • M mode groups are transmitted via a few-mode fiber.
  • the group delays of the M mode groups are distributed centrally symmetrically, and the group delays are distributed centrally symmetrically.
  • the mode group enters the mode converter, and the mode converter exchanges the mode groups of the first mode group and the second mode group that are centrally symmetrical. For example, the mode group with the maximum group delay and the mode group with the minimum group delay occur After switching, the mode group with the second largest group delay is exchanged with the mode group with the second smallest group delay, and so on, and the exchanged M mode groups are coupled to the next span of few-mode fiber for transmission.
  • the mode group with the maximum group delay is switched for the mode group with the minimum group delay, and the mode group with the minimum group delay is switched for the mode group with the maximum group delay.
  • the group delay of the M mode groups is zero. In this way, it helps to reduce the complexity of the MIMO algorithm at the receiving end.
  • FIG. 2 The respective structures shown in FIG. 2 are respectively introduced and explained below to provide exemplary specific implementation solutions.
  • the few-mode optical fiber may include a core, an inner cladding, a trench, and an outer cladding in order from the inside to the outside.
  • FIG. 3 a schematic structural diagram of a cross-section of a few-mode optical fiber provided in this application.
  • the few-mode fiber includes a core and a cladding.
  • the cladding refers to a layer of glass or other transparent material that covers the core carrying light waves. It has a slightly lower refractive index than the core, so it can confine light to the fiber. In-core transmission.
  • the cladding layer includes an inner cladding layer and an outer cladding layer.
  • the inner cladding layer of the few-mode fiber in Figure 3 is a ring structure wrapped around the core, and the structure surrounding the inner cladding layer is a trench layer (also called a depressed layer), the outermost ring The structure is the outer layer.
  • few-mode fibers can be divided into step-index few-mode fibers and graded-index few-mode fibers.
  • step-type few-mode fiber and the graded-type few-mode fiber respectively, the following describes in detail the realization of the group delay distribution of the M mode groups in the two types of the two types of the few-mode fiber when the group delay is distributed in a center symmetrical manner.
  • the parameters of the few-mode fiber can be designed, such as the refractive index distribution of the few-mode fiber and the core of the fiber.
  • Type one, step-type few-mode fiber is
  • FIG. 4a it is a schematic structural diagram of a step-type few-mode optical fiber provided in this application.
  • the refractive index distribution function of this step-type few-mode fiber is as follows:
  • n CO represents the refractive index of the core
  • n IC represents the refractive index of the inner cladding
  • n CL represents the refractive index of the outer cladding.
  • ⁇ n TR represents the relative refractive index difference between the trench layer and the outer cladding layer
  • ⁇ n TR (n TR -n CL )/n CL
  • R CO represents the core radius
  • R IC represents the width of the inner cladding layer
  • R TR represents the width of the gully layer
  • R CL represents the width of the outer cladding layer.
  • the gully layer is a trench dug between the inner cladding layer and the outer cladding layer
  • n IC n CL .
  • the number of mode groups transmitted by the few-mode fiber is M, and the corresponding normalized frequency parameter is V.
  • the range of each parameter of the few-mode fiber can be determined by the following relationship:
  • NA numerical aperture
  • ⁇ communication wavelength, which is also called working wavelength.
  • NA is an empirical value and the value range is 0.12 ⁇ 0.02.
  • the value range of ⁇ can be 1.5-1.6 ⁇ m.
  • ⁇ n TR is an empirical value, and the value range is -0.0045 ⁇ n TR ⁇ 0.
  • the trench layer In order to confine the mode group in the core, the trench layer should not be too far away from the core.
  • the value range of R IC is 0 ⁇ R IC ⁇ R CO /2. Considering the manufacturing process of the few-mode fiber, the wider the trench layer, the higher the drawing cost.
  • the value range of R TR is usually 0 ⁇ R TR ⁇ R CO , and R CL is usually 62.5 ⁇ m.
  • the parameter ranges of the step-type few-mode fiber can be obtained as follows:
  • R CO can be determined according to the number of mode groups transmitted by the step-type few-mode fiber.
  • the maximum error of the relevant refractive index parameters of the few-mode fiber can be 1e-4, and the maximum relative error of the structure-related parameters R CO , R IC and R TR of the few-mode fiber is 2.5%.
  • the range of the normalized frequency parameter V corresponding to 3.8 ⁇ V ⁇ 5.1.
  • the parameter ranges of the step-type few-mode fiber are as follows:
  • the group delays are respectively among them, with Are the group refractive index corresponding to the mode group, and c is the speed of light in vacuum. It can be determined that the group delay of the mode group propagating in the few-mode fiber is proportional to the group refractive index of the mode group.
  • step-type few-mode fiber Based on the parameter ranges of the above-mentioned step-type few-mode fiber, through the joint simulation of simulation software COMSOL and MATLAB, it can be determined that the corresponding step-type few-mode fiber parameters when the group refractive index of the M mode groups are distributed in center symmetry .
  • the radius of the core R CO , the refractive index of the core n CO , the width of the inner cladding layer R IC , the refractive index of the inner cladding layer n IC , the width of the trench layer R TR , and the ravine can be scanned across the board.
  • the relative refractive index difference ⁇ n TR between the layer and the outer cladding layer is used to obtain the relationship between the parameters of the few-mode fiber and the M effective refractive indexes.
  • the effective refractive index of the above-mentioned mode group and the group refractive index of the mode group the The relationship between the parameters of the mode fiber and the M group refractive indices.
  • one of the parameters can be used as a variable, and the scanning step is 0.1 ⁇ m to determine the group refractive index of the M mode groups respectively.
  • the parameters of a group of few-mode fibers ⁇ n CO , n CL , n IC , ⁇ n TR , R CO , R IC , R TR ⁇ can determine M effective refractive indexes, and further determine M A group refractive index.
  • the mode group of a step-type few-mode fiber transmission is a schematic diagram of the group refractive index distribution of LP 01 , LP 11 , and LP 21 in the communication wavelength range of 1.5-1.6 ⁇ m.
  • the group refractive index of the three mode groups shown in Figure 4b is determined based on the effective refractive index obtained from the joint simulation of COMSOL and MATLAB. It can be determined from Fig. 4b that the group refractive indices of the three mode groups LP 01 , LP 11 , and LP 21 all increase with the increase of the communication wavelength, and the trend of change is the same.
  • the group refractive index, group velocity and group delay of the three mode groups are shown in Table 2 respectively.
  • the group refractive index of LP 01 is 1.46993, and the group delay is 0;
  • the group refractive index of LP 11 is 1.47100, and the group delay is 3.5200 ps/m;
  • the M group delays distributed in a centrally symmetrical manner may be in an asymmetric relationship (as shown in Table 2), or may be in an unequal relationship.
  • Table 2 asymmetric relationship
  • the simulation process for determining the refractive index distribution and structural parameters of the few-mode fiber is more complicated; on the other hand, the production cost of the few-mode fiber is higher.
  • the few-mode fiber can be designed such that the group delays of the M mode groups transmitted in the few-mode fiber are centrally symmetrically distributed with unequal differences, as shown in the following example
  • the ground shows the refractive index distribution and structural parameters of a step-type few-mode fiber, which can realize that the M group delays corresponding to the M-mode groups transmitted in the step-type few-mode fiber are centrally symmetrically distributed and not Arithmetic.
  • the group delay transmitted in this step-type few-mode fiber is centrally symmetrical with unequal differences.
  • ⁇ is in the range of 1.530-1.565 ⁇ m
  • the group delay of LP 01 is 0, and the group delay of LP 11 is 2.344 ps/m
  • the group delay of LP 02 is 5.925 ps/m
  • the group delay of LP 31 is 8.221 ps/m.
  • Type two, graded less-mode fiber
  • FIG. 5a it is a schematic structural diagram of a graded-type few-mode optical fiber provided in this application.
  • the refractive index distribution function of the graded few-mode fiber is as follows:
  • ⁇ n CO represents the relative refractive index of the core and the outer cladding layer
  • ⁇ n CO (n CO -n CL )/n CO
  • ⁇ is the index of the refractive index profile shape, which is an empirical value, and the value range is 2 ⁇ 0.5.
  • n CO represents the refractive index of the core
  • n IC represents the refractive index of the inner cladding
  • n CL represents the refractive index of the outer cladding
  • ⁇ n TR represents the relative refractive index difference between the trench layer and the outer cladding layer
  • ⁇ n TR (n TR -n CL )/n CL
  • R CO represents the core radius
  • R IC represents the width of the inner cladding layer
  • R TR represents the width of the gully layer
  • R CL represents the width of the outer cladding layer.
  • the corresponding parameter of the graded few-mode fiber can be determined when the group refractive index of the M mode groups is distributed in a center symmetrical manner.
  • the specific process please refer to the introduction of the above-mentioned step-type few-mode fiber, which will not be repeated here.
  • the parameters of the corresponding few-mode fiber are determined as the parameters of the graded-type few-mode fiber.
  • FIG. 5b a schematic diagram of the group refractive index distribution of the mode LP 01 , LP 11 , and LP 21 in the range of 1.5-1.6 ⁇ m for the transmission mode of a graded few-mode optical fiber provided by this application.
  • the group refractive index of the three mode groups shown in Figure 5b is determined based on the effective refractive index obtained from the joint simulation of COMSOL and MATLAB. It can be seen from Fig. 5b that the group refractive indices of the three mode groups LP 01 , LP 11 , and LP 21 all increase with the increase of wavelength, and the trend of change is the same.
  • the group refractive index, group velocity and group delay of the three mode groups LP 01 , LP 11 , and LP 21 are shown in Table 3.
  • the group refractive index of LP 01 is 1.46929, and the group delay is 0; the group refractive index of LP 11 is 1.46944, and the group delay is 0.49176 ps/m; the group refractive index of LP 21 is 1.46958, and the group delay is 0.98218 ps/m; it can be determined
  • the few-mode fiber can be designed such that the group delays of the M mode groups transmitted in the few-mode fiber are centrally symmetrically distributed and unequal, as shown in the following example
  • the refractive index distribution and structural parameters of a graded-type few-mode fiber are shown, which can realize that the M group delays corresponding to the M mode groups transmitted in the graded-type few-mode fiber are centrally symmetrically distributed with unequal differences.
  • the group delay of LP 31 transmitted in the graded few-mode fiber is symmetrically distributed with unequal differences. Refer to Figure 5c.
  • the group delay of LP 01 is 0, and the group delay of LP 11 is 0.
  • the delay is 0.3372 ps/m
  • the group delay of LP 02 is 0.6842 ps/m
  • the group delay of LP 31 is 1.021 ps/m.
  • the mode converter is used to realize the conversion between different module groups, that is, to realize the conversion from one module group to another module group.
  • the mode converter can be used to exchange the first mode group and the second mode group whose group delays are centrally symmetric among the received M mode groups.
  • the mode converter can exchange the module group with the fastest transmission speed (that is, the smallest group delay) with the module group with the slowest transmission speed (that is, the largest group delay), that is, Exchange the module group with the second fastest transmission speed (that is, the second smallest group delay) and the module group with the second slowest transmission speed (that is, the second largest group delay), namely And so on.
  • the group delays of M module groups are respectively t 1 , t 2 ,..., t M-1 , t M , where t 1 and t M are symmetric about the center, and t 2 and t M-1 are symmetric about the center.
  • the mode converter may be t 1 corresponding to the mode group and t M corresponding modular group exchange, i.e. t converting a corresponding mode group is t M corresponding to the mode group, the t mod group converting M corresponds to The module group corresponding to t 1 ; the module group corresponding to t 2 is converted to the module group corresponding to t M-1 , and the module group corresponding to t M-1 is converted to the module group corresponding to t 2.
  • the module group corresponding to t 1 carries signal 1
  • the module group corresponding to t M carries signal 2.
  • the module group corresponding to t 1 is converted to the module group corresponding to t M
  • the module group corresponding to t M is converted to After the module group corresponding to t 1
  • signal 1 is carried by the module group corresponding to t M
  • signal 2 is carried by the module group corresponding to t 1.
  • the mode converter may receive a bundle of optical signals from a few-mode optical fiber, and the optical signal includes M mode groups.
  • the input to the mode converter is a beam of optical signals
  • the output of the mode converter is also a beam of optical signals.
  • the mode converter can be implemented based on the principle of multi-plane light conversion, such as a mode converter based on a phase plate, or a mode converter based on spatial light modulation, or a mode converter based on a metasurface structure; the mode converter is also It can be realized based on waveguide mode coupling theory, such as a mode converter based on fiber grating or a mode converter based on fiber coupler.
  • waveguide mode coupling theory such as a mode converter based on fiber grating or a mode converter based on fiber coupler.
  • the following takes the phase plate-based mode converter and the fiber grating-based mode converter as examples to introduce in detail.
  • Fiber grating can be regarded as a kind of fiber whose core refractive index changes periodically.
  • a fiber grating is made of an optical fiber with a photosensitive core, and the refractive index of the outer cladding of the optical fiber does not change. It should be understood that when the fiber grating is used as a mode converter, the period of the fiber grating is relatively long, so it is also called a mode converter based on a long-period fiber grating.
  • the fiber grating can realize the coupling between the mode groups. If the two mode groups meet the phase matching condition at the communication wavelength, the two mode groups can undergo mode group conversion.
  • the mode field of LP 01 gradually changes, and finally it can be converted to LP 21 at the other end of the fiber grating.
  • the mode field of LP 21 also changes gradually, and finally it can be converted to LP 01 at the other end of the fiber grating.
  • the mode converter may include Fiber gratings of different periods, Indicates rounding down.
  • a period corresponds to a first module group and a second module group, that is, a period corresponds to a pair of module groups whose group delay is symmetric about the center.
  • the exchange of the first mode group and the second mode group that are symmetric about the center among the M mode groups can be realized by controlling the period of each fiber grating.
  • the group delays of M module groups are t 1 , t 2 ,..., t M-1 , t M , t 1 and t M are symmetric about the center, t 2 and t M-1 are symmetric about the center, and so on ;
  • the period of the fiber grating that realizes the exchange of the mode group corresponding to t 1 and the mode group corresponding to t M is ⁇ 1 , which realizes the exchange of the mode group corresponding to t 2 into the mode group corresponding to t M-1
  • the period of the fiber grating is ⁇ 2 , and so on.
  • ⁇ 1 ⁇ /(the effective refractive index of the mode group corresponding to t 1- the effective refractive index of the mode group corresponding to t M )
  • ⁇ 2 ⁇ /(the effective refractive index of the mode group corresponding to t 2- t M -1 corresponds to the effective refractive index of the mode group), and so on.
  • the M group delays include M/2 pairs of group delays symmetric about the center, that is, the M module groups include M/2 first module groups and M/2 first module groups.
  • Two-mode group The mode converter can exchange the first mode group and the second mode group whose group delays are symmetric about the center in the M mode groups.
  • M group delays include For the group delay symmetric about the center, that is, the M module groups include First module group and A second module group and a single module group.
  • the mode converter can exchange the first module group and the second module group whose group delays are symmetric about the center of the M module groups. exchange.
  • carbon dioxide (CO 2 ) laser or femtosecond laser can be used to write on the few-mode fiber Fiber gratings of different periods.
  • CO 2 carbon dioxide
  • Two fiber gratings with different periods can be in a cascade structure. See Figure 6a. Take two fiber gratings as an example (fiber grating I and fiber grating II); specifically, it can be written continuously in the axial direction of the few-mode fiber Fiber gratings of different periods. or Two fiber gratings with different periods can be a stacked grating structure. See Figure 6b.
  • Fiber grating I and fiber grating II Inscribed at different angles in a radial direction (any direction perpendicular to the axial direction, Figure 6b shows two possible radial directions) Fiber gratings of different periods.
  • the stacked grating structure helps to prevent the cross-section of fiber gratings in different regions from affecting the coupling of other mode groups with phase mismatches. It should be noted that the refractive index modulation of the stacked structure is approximately the linear superposition of the refractive index modulations of two fiber gratings, where the change in the refractive index of the core is:
  • I the average refractive index change of the k-th fiber grating
  • ⁇ 1 and ⁇ 2 are the periods of fiber grating I and fiber grating II, respectively, with These are the additional phases related to the phase shift or chirp of the two fiber gratings.
  • the total length of each fiber grating is greater than the threshold, so that the efficiency of mode group conversion can be improved.
  • the second structure is a mode converter based on a phase plate (or called a phase plate).
  • Phase plate-based mode converters are also called multi-plane light conversion (MPLC).
  • the phase plate is a local area (usually an annular zone) on a glass plate or lens, coated with a film with a certain thickness and refractive index, so that the light passing through this area is ahead of the phase of the light passing through the non-coated area Or lag.
  • There are different pixels on each phase plate and the pixels on the phase plate can introduce different phases to modulate the incident light field, which can realize the conversion between module groups.
  • the mode converter based on the phase plate has a strong ability to control the spatial light field, and theoretically can complete the conversion between any mode group. For example, the mode converter based on the phase plate can realize the round-robin conversion of the mode group. As shown in Fig.
  • the incident mode groups are LP 01 , LP 11 , and LP 21 , and the incident mode groups are sequentially converted to LP 11 , LP 21 , LP 01 .
  • the mode converter based on the phase plate can realize the exchange of the group delay about the center symmetrical mode group.
  • the incident mode groups are LP 01 , LP 11 , LP 21 and LP 02 respectively , which can realize LP 01 exchanges with LP 02 , LP 11 exchanges with LP 21.
  • the mode converter may include a reflective phase plate or a transmissive phase plate.
  • it may include one or more transmissive phase plates, or at least two reflective phase plates up and down, or at least one reflective phase plate and at least one high-reflection mirror.
  • the phase plate-based mode converter can realize the mode group exchange between the first mode group and the second mode group whose group delay is symmetric about the center.
  • the switching strategy of the mode converter based on the phase plate is: the first mode group and the second mode group whose group delay is symmetric about the center are exchanged. It should be understood that the greater the number of phase plates included in the mode converter, the higher the fineness of the phases that can be adjusted.
  • FIG. 7c a schematic structural diagram of a mode converter provided in this application as a reflective phase plate.
  • the mode converter in Fig. 7c can be two reflective phase plates, or a combination of a reflective phase plate and a high-reflection mirror.
  • LP 01 , LP 11 and LP 21 have multiple reflections in the reflective phase plate to realize the conversion of LP 01 to LP 21 , Convert LP 21 to LP 01 , LP 11 does not change.
  • the mode converter provided in this application is a structural schematic diagram of a transmissive phase plate.
  • Figure 7d takes the mode converter including 4 transmissive phase plates as an example. Taking the mode converter to receive 3 module groups (LP 01 , LP 11 and LP 21 ), for example, after LP 01 , LP 11 and LP 21 pass through 4 transmissive phase plates in turn, LP 01 can be converted to LP 21 , LP 21 is converted to LP 01 , and LP 11 is not converted.
  • the communication system may further include an amplifier, which is used to amplify the received M mode groups.
  • the amplifier can be a doped fiber amplifier.
  • the mode converter and the amplifier may include the following two positional relationships.
  • the mode converter is located between the few-mode fiber and the amplifier.
  • Case 1 is a schematic diagram of the position of a mode converter provided in this application in a communication system.
  • the mode converter can be after the few-mode fiber and before the amplifier.
  • the mode converter is used to convert the first mode group and the second mode group whose group delays are distributed symmetrically, and the converted M mode groups are transmitted to the amplifier, and the converted M modes are amplified by the amplifier. After the group, it is transmitted to the few-mode fiber of the next span.
  • Case 2 is a schematic diagram of the position of another mode converter provided in this application in the communication system.
  • the mode converter is before the few-mode fiber and after the amplifier.
  • the M module groups are amplified by the amplifier and then enter the mode converter.
  • the mode converter converts the first and second module groups whose group delays are symmetric about the center among the amplified M module groups.
  • the latter M mode groups are transmitted to the few-mode fiber of the next span.
  • the mode converter is integrated in the amplifier.
  • FIG. 8c it is a schematic diagram of the position of another mode converter provided in this application in the communication system.
  • the mode converter can be integrated in the amplifier.
  • the mode converter can be located between the two ends of doped (for example, bait-doped) fibers.
  • the M mode groups enter the amplifier, they enter the Variable Optical Attenuator (VOA) after being amplified by the first doped fiber. ), after being attenuated by the VOA signal (to achieve adjustable gain of the amplifier), it is transmitted to the mode converter.
  • VOA Variable Optical Attenuator
  • the mode converter converts the first mode group and the second mode group whose group delays are centrally symmetrically distributed in the M mode group, and
  • the converted M mode groups are transmitted to the second segment of doped fiber, and after being amplified by the second segment of doped fiber, they are coupled to the next span of few-mode fiber to continue transmission. Since the mode converter has a certain insertion loss, by integrating the mode converter into the amplifier, the insertion loss introduced by the mode converter can be compensated without deteriorating the signal-to-noise ratio of the transmission signal.
  • the communication system may include a transmitting module, a mode multiplexer, a few-mode fiber, a mode converter, an amplifier, a mode demultiplexer, and a receiving module.
  • the transmitting module can be M single-mode transmitting modules, and each single-mode transmitting module is used to transmit one signal to obtain M signals (such as wavelength division signals), and transmit the M signals to the mode multiplexer, where, Each signal is carried by a specific module group.
  • the mode multiplexer is used to convert each signal from the fundamental mode to a different high-order mode group, then multiplex the M mode groups carrying different signals into a signal, and couple the multiplexed signal to the at least mode fiber for transmission.
  • the mode demultiplexer is used to separate the received multiplexed signal into M module groups carrying different signals and transmit them to the receiving module.
  • the receiving module is used to receive M module groups.
  • the receiving module may include M single-mode receiving modules, and each single-mode receiving module may receive a corresponding module group.
  • the transmitting module may include a light source, and the light source may be used to transmit optical signals of the same wavelength, or may be used to transmit optical signals of different wavelengths.
  • each light source may include at least one laser, and the laser may emit a signal in a wavelength range between 1.5-1.6 ⁇ m.
  • the character “/” generally indicates that the associated objects before and after are in an “or” relationship. In the formula of this application, the character “/” indicates that the associated objects before and after are in a “division” relationship.

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Abstract

一种通信系统，用于解决现有技术中通信系统中传输的不同模群间的群延迟较大的问题。本申请中，通信系统可包括至少两个跨段的少模光纤和模式转换器，少模光纤用于传输接收到的M个模群，M个模群在少模光纤中传输的群延时呈中心对称分布；模式转换器用于接收来自少模光纤的M个模群，并将M个模群中的第一模群和第二模群进行模群交换，得到M个交换后的模群，将M个交换后的模群耦合至下一跨段的少模光纤，第一模群的群延时与第二模群的群延时关于中心对称。通过将群延迟关于中心对称的第一模群和第二模群交换模群后，耦合至下一跨段的少模光纤中传输，有助于减小接收端接收到的M个模群间的群延迟。

Description

一种通信系统
本申请要求于2019年12月31日提交中国国家知识产权局、申请号为201911417874.6、发明名称为“一种通信系统”的中国专利申请的优先权，其全部内容通过引用结合在本申请中。
技术领域
本申请涉及通信技术领域，尤其涉及一种通信系统。
背景技术
随着光纤通信技术的发展，单模光纤通信系统已经无法满足日益增长的通信容量的需求。提出了多模光纤，多模光纤虽然可以满足通信容量的需求(可支持100-200个模式)，但是模式间色散严重，导致信号失真也比较严重。因此，少模光纤通信应用而生，少模光纤既可以减小模式间色散，又可以提升通信系统的容量，因此，被广泛应用在光纤通信系统中。
在光纤通信系统中，承载在不同模式上的信号在传输过程中会发生串扰，因此，接收端需要利用多入多出技术(multi-input-multi-output，MIMO)算法将不同模式上承载的信号进行解调，其中，MIMO算法的复杂度与各个模式的差分模式群延时(differential mode group delay，DMGD)差成正相关，即模式间的DMGD差越大，MIMO算法越复杂，导致接收端解调信号的时间越长，从而造成接收端解调信号的实时性较差。
为了降低接收端解调信号的复杂度，现有技术中，通常是在每个跨段的少模光纤后级联一段正负模式色散少模光纤来减少DMGD。然而，正负模式色散少模光纤的制备工艺复杂，而且少模光纤与正负模式色散少模光纤较难匹配，级联过程较困难。
发明内容
本申请提供一种通信系统，用于减小不同模群到达接收端时的群延迟。
第一方面，本申请提供一种通信系统，该通信系统可包括至少两个跨段的少模光纤和模式转换器。少模光纤用于传输接收到的M个模群，M个模群在少模光纤中传输的群延时呈中心对称分布，M个模群的群延迟的中心为M个模群的群延迟的最大值与最小值之间的中间值，M个模群的每个模群上承载有信号，M为大于1的整数；模式转换器用于接收来自少模光纤的M个模群，并将M个模群中的第一模群和第二模群进行模群交换，得到M个交换后的模群，将M个交换后的模群耦合至下一跨段的少模光纤，第一模群的群延时与第二模群的群延时关于中心对称，其中，第一模群和第二模群为M个模群中的至少两个模群。
基于该方案，M个模群经少模光纤传输，在到达少模光纤的末端时，M个模群的群延迟呈中心对称分布，群延迟呈中心对称分布的M个模群进入模式转换器，模式转换器将呈中心对称的第一模群和第二模群的模群进行交换，例如，群延迟为最大值的模群与群延迟为最小值的模群发生了交换，群延迟为次大的模群与群延迟为次小的模群发生了交换，以此类推，再将交换后的M个模群耦合至下一跨少模光纤中传输。在下一跨少模光纤中传输时，群延迟 为最大值的模群交换为了群延迟为最小值的模群，群延迟为最小值的模群交换为了群延迟为最大值的模群，以此类推，M个模群到达下一跨少模光纤末端时，M个模群的群延时为零。如此，有助于降低接收端的MIMO的算法的复杂度。
在一种可能的实现方式中，M个模群中的任意关于中心对称的第一模群的群延迟与第二模群的群延迟之和相等。进一步，可选地，当M为偶数时，M个群延迟中包括M/2对关于中心对称的群延迟，即M个模群中包括M/2个第一模群和M/2个第二模群；当M为奇数时，M个群延迟中包括
Figure PCTCN2020138718-appb-000001
对关于中心对称的群延迟，即M个模群中包括
Figure PCTCN2020138718-appb-000002
个第一模群和
Figure PCTCN2020138718-appb-000003
Figure PCTCN2020138718-appb-000004
个第二模群，以及1个单独的模群。
在一种可能的实现方式中，少模光纤从内向外依次包括纤芯、内包层、沟壑层和外包层。在该少模光纤中，M个模群的群折射率呈中心对称分布；其中，M个模群中的每个模群的群折射率是根据模群的有效折射率确定的，模群的有效折射率是根据少模光纤的纤芯的折射率、内包层的折射率、外包层的折射率、沟壑层的折射率、纤芯的半径、内包层的宽度、沟壑层的宽度以及外包层的宽度确定的。
为了实现将M个模群中的群延迟关于中心对称的第一模群与第二模群进行交换，模式转换器可包括
Figure PCTCN2020138718-appb-000005
个不同周期的光纤光栅，一个周期对应一个第一模群和一个第二模群，
Figure PCTCN2020138718-appb-000006
表示向下取整；
Figure PCTCN2020138718-appb-000007
个不同周期中的每个周期是根据通信波长、对应的第一模群的有效折射率和对应的第二模群的有效折射率确定的。
进一步，可选地，所述光栅周期对应的第一模群的有效折射率与第二模群的有效折射率之差乘以所述光栅周期等于通信波长。
在一种可能的实现方式中，
Figure PCTCN2020138718-appb-000008
个不同周期的光纤光栅可为级联结构，也可为叠栅结构。当
Figure PCTCN2020138718-appb-000009
个不同周期的光纤光栅为叠栅结构时，有助于避免不同区域的光纤光栅的横截面对相位不匹配的其它模群的耦合造成影响。
在一种可能的实现方式中，模式转换器可包括反射式相位板或透射式相位板。
本申请中，通信系统还包括放大器。模式转换器可位于少模光纤与放大器之间；或者，模式转换器可集成于放大器内。通过将模式转换器集成于放大器内，可以弥补模式转换器引入的插损且可不恶化传输信号的信噪比。
附图说明
图1为本申请提供的一种呈中心对称分布的值的示意图；
图2为本申请提供的一种通信系统的架构示意图；
图3为本申请提供的一种少模光纤的横截面的结构示意图；
图4a为本申请提供的一种阶跃型少模光纤的结构示意图；
图4b为本申请提供的一种阶跃型少模光纤的群折射率分布示意图；
图4c为本申请提供的另一种阶跃型少模光纤的群折射率分布示意图；
图5a为本申请提供的一种渐变型少模光纤的结构示意图；
图5b为本申请提供的一种渐变型少模光纤的群折射率分布示意图；
图5c为本申请提供的另一种渐变型少模光纤的群折射率分布示意图；
图6a为本申请提供的一种光纤光栅的级联结构示意图；
图6b为本申请提供的一种光纤光栅的叠栅结构示意图；
图7a为本申请提供的一种基于相位板的模式转换器进行模群转换的示意图；
图7b为本申请提供的另一种基于相位板的模式转换器进行模群转换的结构示意图；
图7c为本申请提供的一种模式转换器为反射式相位板的结构示意图；
图7d为本申请提供的一种模式转换器为透射式相位板的结构示意图；
图8a为本申请提供的一种模式转换器在通信系统的位置示意图；
图8b为本申请提供的另一种模式转换器在通信系统的位置示意图；
图8c为本申请提供的又一种模式转换器在通信系统的位置示意图；
图9为本申请提供的另一种通信系统的架构示意图。
具体实施方式
以下，对本申请中的部分用语进行解释说明，以便于本领域技术人员理解本发明。
一、少模光纤(few mode fiber，FMF)
FMF是一种单芯光纤，具有较大的模场面积，允许多个独立的模式并行传输信号。一般情况下，少模光纤可支持2至50个模式。也就是说，少模光纤是一种纤芯面积比较大，支持同时在多个模式上并行传输信号的光纤。模式可以理解为是光作为电磁波在光纤中传输时形成的一种场型分布形式。即在少模光纤内光行进形成混合型模，该混合型模一般被称为线性偏振(linearly polarized，LP)模式。
少模光纤是基于模分复用技术传输信号的。在理想情况下，不同模式之间是相互正交的。也可以理解为，少模光纤可支持多个相互正交的独立模式，将这些模式作为独立信道，这些模式可以同时承载多路信号。
二、模群
模群是指传播常数一致的多个模式组成的群组。也就是说，一个模群中包括的多个模式的群速率相等。一根光纤可以传输多个模群。
三、差分模式群延时(differential mode group delay，DMGD)
少模光纤中传输的模群具有不同的有效折射率，由于v＝c/n，即不同的模群在少模光纤中的传输的速率不同，因此，不同模群在少模光纤中传输一段距离后，模群间会产生的延迟差即为DMGD，数学表达式如下：
Figure PCTCN2020138718-appb-000010
其中，λ表示通信波长，v g表示群速度，c表示真空中的光速，光信号的传输常数β＝n eff*k 0，k 0＝2π/λ表示自由化开放空间中存在的光波得到数量，n eff表示这一模群的有效折射率。
四、有效折射率n eff
有效折射率是一个可以定量描述波导(如少模光纤)中单位长度相位延迟的量，是相对于真空中单位长度相位延迟而言的。在均匀透明介质中，折射率n可以用于定量描述由介质引起的波数(单位长度的相位变化)的增加程度：其波数是在真空中波数的n倍。有效折射率n eff有着类似的含义：在波导(如少模光纤)中，其某一波长的β值是真空波数的n eff倍，其中，
Figure PCTCN2020138718-appb-000011
需要说明的是，有效折射率不仅依赖于波长，还依赖于传输的模式，因此，有效折射率也被称为模式折射率。
五、折射率分布
折射率分布是指折射率或相对折射率与光纤的半径之间的关系。例如，n(r)表示在半径r处的折射率。
六、相对折射率差
相对折射率差指两个折射率之差与相对的折射率的比值。例如，沟壑层与外包层的相对折射率差Δn TR，Δn TR＝(n TR-n CL)/n CL，其中，n CL表示外包层的折射率，n TR表示沟壑层折射率。
七、中心对称分布
在数轴上，习惯把距离原心0距离相等、方向相反的两个值点互称为对称点，例如，7和-7互称为中心对称点。以0为中心的对称点两点值之和为0。在以0点为中心的对称点区间[-a，+a]内，不包括0本身，有效点数为2a个，最多可组成a对对称点。
针对M个数值，按从小到大或从大到小的顺序排列，距离中心距离相等的每对数值的和相等，把这对数值称为关于中心对称。如图1所示，为本申请提供的一种呈中心对称分布的值的示意图。以M＝9为例，M个数值按顺序排列为{a 1，a 2，a 3，a 4，a 5，a 6，a 7，a 8，a 9}，M个数值的中心值为a 5＝(a 1+a 9)/2，a 1和a 9距离中心的距离相等，a 2和a 8距离中心的距离相等，a 3和a 7距离中心的距离相等，a 4和a 6距离中心的距离相等；且a 1+a 9＝a 2+a 8＝a 3+a 7＝a 4+a 6＝2a 5，则a 1和a 9关于中心对称，a 2和a 8关于中心对称，a 3和a 7关于中心对称，a 4和a 6关于中心对称。
也可以理解为，呈中心对称分布的M个值中，最大值与最小值距离中心的距离相等，次大值与次小值距离中心的距离也相等，且最大值与最小值的和等于次大值与次小值的和，以此类推。
基于模分复用的少模光纤的通信系统中，利用少模光纤支持的正交模式作为独立信道进行信号的并行传输，可提升通信系统的传输容量。而且由于少模光纤的模式具有较大的模场面积，因此非线性容限也很高。也就是说，包括少模光纤的通信系统，既可提高了通信系统的传输容量，又有助于避免了非线性效应对系统的干扰，但会存在背景技术中所描述的问题。
鉴于背景技术中存在的技术问题，本申请提出一种通信系统。该通信系统可降低通信系统中不同模群间的群延迟，从而有助于降低接收端的MIMO的算法的复杂度，提高传输系统效率。
下面结合附图2至附图9，对本申请提出的通信系统进行详细阐述。
如图2所示，为本申请提供的一种通信系统的架构示意图。该通信系统可包括至少两个跨段的少模光纤和模式转换器。图2以包括两个跨段的少模光纤和一个模式转换器为例。少模光纤用于传输接收到的M个模群，M个模群在少模光纤中传输的群延时呈中心对称分布。模式转换器用于接收来自少模光纤的M个模群，并将M个模群中的第一模群和第二模群进行模群交换，得到M个交换后的模群，将M个交换后的模群耦合至下一跨段的少模光纤，第一模群的群延时与第二模群的群延时关于中心对称，其中，第一模群和第二模群为M个模 群中的至少两个模群，M个模群的群延迟的中心为M个模群的群延迟的最大值与最小值之间的中间值，M个模群的每个模群上承载有信号，M为大于1的整数。
本申请中，M个模群中的每个模群对应一个群延迟，M个模群对应M个群延迟。M个群延迟分别为t 1，t 2，…，t M-1，t M，其中，t 1＞t 2＞…＞t M。t 1，t 2，…，t M-1，t M呈中心对称分布，则t 1+t M＝t 2+t M-1＝…，即t 1和t M关于中心对称，t 2和t M-1关于中心对称，以此类推。相应地，t 1对应的模群和t M对应的模群中一个为第一模群，另一个为第二模群；t 2对应的模群和t M-1对应的模群中一个为第一模群，另一个为第二模群。例如，t 1对应的模群为第一模群，t M对应的模群为第二模群，t 2对应的模群为第一模群，t M-1对应的模群为第二模群，以此类推。再比如，t 1对应的模群为第二模群，t M对应的模群为第一模群，t 2对应的模群为第二模群，t M-1对应的模群为第一模群，以此类推。应理解，模式转换器用于将第一模群转换为第二模群，将第二模群转换为第一模群。也就是说，模式转换器用于将第一模群的模场逐步转换为第二模群的模场，将第二模群的模场逐步转换为第一模群的模场。
在一种可能的实现方式中，至少两个跨段的少模光纤的每两个跨段的少模光纤之间包括一个模式转换器，即一个跨段的少模光纤+模式转换器+下一跨段的少模光纤。需要说明的是，至少两个跨段的少模光纤可以是偶数个跨段的少模光纤，可以是奇数个跨段的少模光纤。当至少两个跨段的少模光纤为偶数个跨段时，M个模群经过一个跨段少模光纤+模式转换器+下一个跨段的少模光纤后，可实现M个模群间的群延迟为零。当至少两个跨段的少模光纤为奇数个跨段时，M个模群间的群延迟可为最后一个跨段的少模光纤中的群延迟。
基于上述图2所示的通信系统，M个模群经少模光纤传输，在到达少模光纤的末端时，M个模群的群延迟呈中心对称分布，群延迟呈中心对称分布的M个模群进入模式转换器，模式转换器将呈中心对称的第一模群和第二模群的模群进行交换，例如，群延迟为最大值的模群与群延迟为最小值的模群发生了交换，群延迟为次大的模群与群延迟为次小的模群发生了交换，以此类推，再将交换后的M个模群耦合至下一跨少模光纤中传输。在下一跨少模光纤中传输时，群延迟为最大值的模群交换为了群延迟为最小值的模群，群延迟为最小值的模群交换为了群延迟为最大值的模群，以此类推，M个模群到达下一跨少模光纤末端时，M个模群的群延时为零。如此，有助于降低接收端的MIMO的算法的复杂度。
下面对图2所示的各个结构分别进行介绍说明，以给出示例性的具体实现方案。
一、少模光纤
本申请中，少模光纤从内向外可依次包括纤芯、内包层(inner cladding)、沟壑层(trench)和外包层(outer cladding)。如图3所示，为本申请提供的一种少模光纤的横截面的结构示意图。该少模光纤包括纤芯和包层，包层是指在携带光波的纤芯外面覆盖的一层玻璃或其他的透明材料，具有比纤芯略低的折射系数，因此能将光限制在纤芯内传输。包层包括内包层和外包层，图3中的少模光纤的内包层为包裹于纤芯的环形结构，围绕着内包层的结构为沟壑层(亦称为下陷层)，最外层的环形结构为外包层。
少模光纤基于折射率分布可分为阶跃型(step-index)少模光纤和渐变型(graded-index)少 模光纤。如下分别基于阶跃型少模光纤和渐变型少模光纤，详细介绍M个模群在这两种类型的少模光纤中传输时的群延时呈中心对称分布的实现方式。
在一种可能的实现方式中，为了实现M个模群在少模光纤中传输的群延迟呈中心对称分布，可通过设计少模光纤的各参数，如少模光纤的折射率分布、纤芯半径、沟壑层的宽度、内包层的宽度、外包层的宽度等。
类型一、阶跃型少模光纤。
如图4a所示，为本申请提供的一种阶跃型少模光纤的结构示意图。该阶跃型少模光纤的折射率分布函数如下：
Figure PCTCN2020138718-appb-000012
其中，n CO表示纤芯的折射率，n IC表示内包层的折射率，n CL表示外包层的折射率，
Figure PCTCN2020138718-appb-000013
表示沟壑层的折射率，Δn TR表示沟壑层与外包层的相对折射率差，Δn TR＝(n TR-n CL)/n CL，R CO表示纤芯半径，R IC表示内包层的宽度，R TR表示沟壑层的宽度，R CL表示外包层的宽度。应理解，沟壑层是在内包层和外包层之间挖出的沟壑，因此n IC＝n CL。
本申请中，少模光纤传输的模群的数量为M，对应的归一化频率参数为V，可通过如下关系确定少模光纤的各参数的范围：
V＝2πR CONA/λ
Figure PCTCN2020138718-appb-000014
其中，NA表示数值孔径，λ表示通信波长，也称为工作波长。需要说明的是，NA是经验值，取值范围为0.12±0.02。为了保证少模光纤具有较高的非线性功率阈值，需要较大的模场面积，因此需要较大的纤芯半径，对应的数值孔径也较大，NA可为0.14。另外，λ的取值范围可为1.5-1.6μm。
进一步，外包层的材料主要为二氧化硅，在λ＝1.55μm时，对应的折射率为1.4444。由于少模光纤在制备过程中可能会存在工艺误差等，因此，外包层的折射率通常有±0.006的误差。也就是说，1.4444-0.006≤n CL≤1.4444+0.006，即1.4434≤n CL≤1.4446。根据
Figure PCTCN2020138718-appb-000015
可确定出n CO。Δn TR为经验值，取值范围为-0.0045≤Δn TR＜0。
为了将模群束缚在纤芯内，沟壑层不能距离纤芯太远，通常R IC的取值范围为0≤R IC≤R CO/2。考虑到少模光纤的制作工艺，沟壑层越宽，拉制成本越高，R TR的取值范围通常为0≤R TR≤R CO，R CL通常取62.5μm。
基于上述计算，可得到阶跃型少模光纤的各参数范围如下：
1.4502≤n CO(r)≤1.4514
1.4434≤n CL＝n IC≤1.4446
-0.0045≤Δn TR＜0
0≤R IC≤R CO/2
0≤R TR≤R CO
R CL＝62.5μm
需要说明的是，R CO可根据该阶跃型少模光纤传输的模群的数量确定。少模光纤的相关折射率参数的最大误差可为1e-4，少模光纤的结构相关参数R CO、R IC和R TR的最大相对误差为2.5％。
示例性地，以少模光纤传输的模群的数量M＝3为例，可确定出(例如查表)对应的归一化频率参数V的范围为3.8≤V≤5.1。也就是说，少模光纤传输的模群的数量与归一化频率参数之间有对应关系。进一步，可选地，可根据V＝2πR CONA/λ，确定出R CO的范围。需要说明的是，通过模拟仿真发现，R co的范围对模拟仿真的结果影响较小。为了减少模拟仿真的复杂度，可取R co范围内的一个中间值，例如R co的范围中最大值和最小值的平均值，即R co＝8.5μm。进一步，可选地，可根据
Figure PCTCN2020138718-appb-000016
确定出1.4502≤n CO(r)≤1.4514。又由于0≤R IC≤R CO/2，得到0＜R IC≤4.25μm。R TR的取值范围为0≤R TR≤R CO，得到0＜R TR≤8.5μm。
基于上述计算，可得到该阶跃型少模光纤传输的模群的数量为3时，该阶跃型少模光纤的各参数范围如下：
1.4502≤n CO(r)≤1.4514
1.4434≤n CL＝n IC≤1.4446
-0.0045≤Δn TR＜0
R co＝8.5μm
0＜R IC≤4.25μm
0＜R TR≤8.5μm
R CL＝62.5μm
在一种可能的实现方式中，3个模群经过长度为L的一个跨段的少模光纤的传输后，群延迟分别为
Figure PCTCN2020138718-appb-000017
其中，
Figure PCTCN2020138718-appb-000018
Figure PCTCN2020138718-appb-000019
和
Figure PCTCN2020138718-appb-000020
分别为模群对应的群折射率，c为真空中的光速。由此可以确定，模群在少模光纤中传输的群延迟与模群的群折射率呈正比。示例性地，M个模群的群延迟分别为t 1，t 2，…，t M-1，t M，M个模群的群折射分别为
Figure PCTCN2020138718-appb-000021
要满足t 1+t M＝t 2+t M-1＝…，要求
Figure PCTCN2020138718-appb-000022
进一步，可选地，模群的群折射率与有效折射率之间满足：n gi＝n effi-λ(dn effi/dλ)，其中，n gi表示第i个模群的群折射率，n effi表示第i个模群的有效折射率。
基于上述阶跃型少模光纤的各参数范围，通过仿真软件COMSOL和MATLAB联合仿真， 可确定出M个模群的群折射率呈中心对称分布式时，对应的阶跃型少模光纤的参数。
在一种可能的实现方式中，可全面扫描纤芯的半径R CO、纤芯的折射率n CO、内包层的宽度R IC、内包层的折射率n IC、沟壑层的宽度R TR、沟壑层与外包层的相对折射率差Δn TR，得到少模光纤的参数和M个有效折射率的关系，再根据上述模群的有效折射率与模群的群折射率的关系，可确定出少模光纤的参数与M个群折射率的关系。示例性地，可将其中的某一个参数作为变量，扫描的步进为0.1μm，来分别确定M个模群的群折射率。
如表1所示，一组少模光纤的参数{n CO，n CL，n IC，Δn TR，R CO，R IC，R TR}，可确定出M个有效折射率，进一步可确定出M个群折射率。
表1少模光纤的参数、模群的有效折射率和模群的折射率
Figure PCTCN2020138718-appb-000023
基于上述表1，将满足
Figure PCTCN2020138718-appb-000024
对应的少模光纤的参数{n CO，n CL，n IC，Δn TR，R CO，R IC，R TR}确定为实现在该少模光纤中传输的M个模群的群延迟呈中心对称分布的参数。
示例性地，以该阶跃型少模光纤传输的模群为LP 01、LP 11、LP 21为例，通过仿真可确定出该阶跃型少模光纤的参数为n CO(r)＝1.45，n CL＝n IC＝1.444，R CO＝8.5μm、R IC＝1μm、R TR＝4μm和Δn TR＝-0.001时，三个模群的群折射率满足
Figure PCTCN2020138718-appb-000025
也就是说，三个模群的群延迟满足t 1+t 3＝2t 2，即三个模群的群延迟呈中心对称分布。
参考图4b，为本申请提供的一种阶跃型少模光纤传输的模群为LP 01、LP 11、LP 21在通信波长为1.5-1.6μm范围内的群折射率分布示意图。图4b所示的三个模群的群折射率是基于COMSOL和MATLAB联合仿真得到的有效折射率确定的。由图4b可以确定，三个模群LP 01、LP 11、LP 21的群折射率均随通信波长的增加而增大，且变化的趋势相同。在λ＝1.55μm处，三个模群(LP 01、LP 11、LP 21)的群折射率、群速度和群延迟分别如表2所示。LP 01的群折射率为1.46993，群延迟为0；LP 11的群折射率为1.47100，群延迟为3.5200ps/m；LP 21的群折射率为1.47204，群延迟为7.0431ps/m；从而可确定LP 01、LP 11、LP 21的群折射率呈中心对称分布，即1.46993+1.47204＝2*1.47100，LP 01、LP 11、LP 21的群延迟也呈中心对称分布，即2*3.5200＝0+7.0431。
表2三个模群LP 01、LP 11、LP 21在λ＝1.55μm处的群折射率、群速度和群延迟
模式 LP 01 LP 11 LP 21
群折射率 1.46993 1.47100 1.47204
群速度(m/s) 2.0410×10 8 2.0394×10 8 2.380×10 8
DMGD(ps/m) 0 3.5200 7.0431
在一种可能的实现方式中，呈中心对称分布的M个群延迟可能是等差关系(如表2所示)，也可能是不等差关系。要设计M个模群在少模光纤中传输的群延迟满足等差关系，需要将M个模群中每个模群与前一个模群进行相减来确定是否满足等差关系，一方面，确定少模光纤的折射率分布及结构参数的仿真过程较复杂；另一方，少模光纤的制作成本较高。为了降低少模光纤的制作成本和仿真的复杂度，可将所述少模光纤设计为：在少模光纤中传输的M个模群的群延迟呈中心对称分布且不等差，如下示例性地的示出了一种阶跃型少模光纤的折射率分布和结构参数，可实现在该阶跃型少模光纤中传输的M个模群对应的M个群延迟呈中心对称分布且不等差。示例性地，以该阶跃型少模光纤传输4个模群(LP 01，LP 11，LP 02，LP 31)为例，基于上述相同的过程，该阶跃型少模光纤的参数为n CO(r)＝1.45，n CL＝n IC＝1.444，Δn TR＝0，R CO＝11.9μm，R IC＝0μm，R TR＝0μm，可实现LP 01，LP 11，LP 02，LP 31在该阶跃型少模光纤中传输的群延迟呈中心对称分布，且不等差，参阅图4c，λ在1.530-1.565μm范围内，LP 01的群延迟为0，LP 11的群延迟为2.344ps/m，LP 02的群延迟为5.925ps/m，LP 31的群延迟为8.221ps/m。
类型二、渐变型少模光纤。
如图5a所示，为本申请提供的一种渐变型少模光纤的结构示意图。该渐变型少模光纤的折射率分布函数如下：
Figure PCTCN2020138718-appb-000026
其中，Δn CO表示纤芯与外包层的相对折射率，Δn CO＝(n CO-n CL)/n CO，α为折射率剖面形状指数，为经验值，取值范围为2±0.5。n CO表示纤芯的折射率，n IC表示内包层的折射率，n CL表示外包层的折射率，
Figure PCTCN2020138718-appb-000027
表示沟壑层的折射率，Δn TR表示沟壑层与外包层的相对折射率差，Δn TR＝(n TR-n CL)/n CL，R CO表示纤芯半径，R IC表示内包层的宽度，R TR表示沟壑层的宽度，R CL表示外包层的宽度。
以该渐变型少模光纤传输的模群数量M＝3为例，参考上述计算阶跃型少模光纤的参数过程，可确定出该渐变型少模光纤的参数为范围为：
1.4502≤n CO(r)≤1.4514
1.4434≤n CL＝n IC≤1.4446
-0.0045≤Δn TR＜0
R CO＝14.2μm
1.5≤α≤2.5
0＜R IC≤7.1μm
0＜R TR≤14.2μm
R CL＝62.5μm
基于上述渐变型少模光纤的参数范围，通过仿真软件COMSOL和MATLAB联合仿真，可确定出M个模群的群折射率呈中心对称分布式时，对应的渐变型少模光纤的参数。具体过程可参见上述阶跃型少模光纤的介绍，此处不再一一赘述。可将满足
Figure PCTCN2020138718-appb-000028
Figure PCTCN2020138718-appb-000029
对应的少模光纤的参数确定为该渐变型少模光纤的参数。
示例性地，以该渐变型少模光纤传输的模群为LP 01、LP 11、LP 21为例，通过仿真可确定出该渐变型少模光纤的参数为n CO(r)＝1.45，n CL＝n IC＝1.444，R CO＝14.2μm、R IC＝1μm、R TR＝1μm、α＝2.3、Δn TR＝-0.0045时，三个模群的群折射率满足
Figure PCTCN2020138718-appb-000030
也就是说，三个模群的群延迟满足t 1+t 3＝2t 2，即三个模群的群延迟呈中心对称分布。
参考图5b，为本申请提供的一种渐变型少模光纤传输的模式LP 01、LP 11、LP 21在1.5-1.6μm范围内的群折射率分布示意图。图5b所示的三个模群的群折射率是基于COMSOL和MATLAB联合仿真得到的有效折射率确定的。从图5b可以看出，三个模群LP 01、LP 11、LP 21的群折射率均随波长的增加而增大，且变化的趋势相同。在λ＝1.55μm处，三个模群LP 01、LP 11、LP 21的群折射率、群速度和群延迟如表3所示。LP 01的群折射率为1.46929，群延迟为0；LP 11的群折射率为1.46944，群延迟为0.49176ps/m；LP 21的群折射率为1.46958，群延迟为0.98218ps/m；可确定LP 01、LP 11、LP 21的群折射率呈中心对称分布，即1.46929+1.46958＝2*1.46944，LP 01、LP 11、LP 21的群延迟也呈中心对称分布，即2*0.49176＝0+0.98218。
表3三个模群LP 01、LP 11、LP 21在λ＝1.55μm处的群折射率、群速度和群延迟
模式 LP 01 LP 11 LP 21
群折射率 1.46929 1.46944 1.46958
群速度(m/s) 2.0418×10 8 2.04159×10 8 2.04140×10 8
DMGD(ps/m) 0 0.49176 0.98218
为了降低少模光纤的制作成本和仿真的复杂度，可将少模光纤设计为：在少模光纤中传输的M个模群的群延迟呈中心对称分布且不等差，如下示例性地的示出了一种渐变型少模光纤的折射率分布和结构参数，可实现在该渐变型少模光纤中传输的M个模群对应的M个群延迟呈中心对称分布且不等差。示例性地，以该渐变型少模光纤传输4个模群(LP 01，LP 11，LP 02，LP 31)为例，基于上述相同的过程，该渐变型少模光纤的参数为n CO(r)＝1.45，n CL＝n IC＝1.444，R CO＝19.6μm、R IC＝2μm、R TR＝1μm、α＝2.3、Δn TR＝-0.0045，可实现LP 01，LP 11，LP 02，LP 31在该渐变型少模光纤中传输的群延迟呈中心对称分布，且不等差，参阅图5c，在λ在1.530-1.565μm范围内，LP 01的群延迟为0，LP 11的群延迟为0.3372ps/m，LP 02的群延迟为0.6842ps/m，LP 31的群延迟为1.021ps/m。
二、模式转换器
本申请中，模式转换器用于实现不同模群之间的转换，即可实现从一个模群转换为另一 个模群。模式转换器可用于将接收到的M个模群中群延迟呈中心对称的第一模群和第二模群进行交换。也就是说，模式转换器可实现将传输速度最快(即群延迟最小)的模群与传输速度最慢(即群延迟最大)的模群进行交换，即
Figure PCTCN2020138718-appb-000031
将传输速度次快(即群延迟次小)的模群与传输速度次慢(即群延迟次大)的模群进行交换，即
Figure PCTCN2020138718-appb-000032
以此类推。例如，M个模群的群延迟分别为t 1，t 2，…，t M-1，t M，其中，t 1和t M关于中心对称，t 2和t M-1关于中心对称，以此类推；则模式转换器可将t 1对应的模群与t M对应的模群进行交换，即将t 1对应的模群转换为t M对应的模群，将t M对应的模群转换为t 1对应的模群；将t 2对应的模群转换为t M-1对应的模群，将t M-1对应的模群转换为t 2对应的模群。
示例性地，t 1对应的模群承载有信号1，t M对应的模群承载有信号2，将t 1对应的模群转换为t M对应的模群，t M对应的模群转换为t 1对应的模群后，信号1承载于t M对应的模群，信号2承载于t 1对应的模群。
在一种可能的实现方式中，模式转换器可接收来自少模光纤的一束光信号，该光信号中包括M个模群。也就是说，输入模式转换器的为一束光信号，输出模式转换器的也为一束光信号。
本申请中，模式转换器可基于多平面光转换的原理实现，例如基于相位板的模式转换器、或基于空间光调制的模式转换器、或基于超表面结构的模式转换器；模式转换器也可以是基于波导模式耦合理论实现，例如基于光纤光栅的模式转换器、或基于光纤耦合器的模式转换器等。如下以基于相位板的模式转换器和基于光纤光栅的模式转换器为例详细介绍。
结构一，基于光纤光栅的模式转换器。
光纤光栅可以看成是一种纤芯折射率呈周期性变化的光纤。通常来说，光纤光栅是由具有光敏性纤芯的光纤制成的，光纤的外包层的折射率并不发生变化。应理解，光纤光栅作为模式转换器时，光纤光栅的周期较长，因此也称为基于长周期的光纤光栅的模式转换器。
光纤光栅可实现模群之间的耦合，若两个模群在通信波长上满足相位匹配条件，则这两个模群可以发生模群转换。相位匹配条件为：λ＝Δn eff*Λ，其中，λ是通信波长，Λ是光纤光栅的周期，Δn eff＝n eff1-n eff2，Δn eff1是第一模群的有效折射率，n eff2是第二模群的有效折射率。也就是说，当第一模群和第二模群经过该周期的光纤光栅后，第一模群和第二模群可以发生相互转换。例如，当通信波长λ＝1.55μm，Λ＝290.5μm，光纤光栅长度L＝24.9mm，输入上述三个模群LP 01、LP 11、LP 21，LP 01、LP 11、LP 21进入光纤光栅后，LP 01的模场逐步发生变化，最终可在光纤光栅的另一端转换为LP 21，相应地，LP 21的模场也逐步发生变化，最终可在光纤光栅的另一端转换为LP 01。
本申请中，为了将M个模群中群延迟呈中心对称的第一模群和第二模群进行交换，该模式转换器可包括
Figure PCTCN2020138718-appb-000033
个不同周期的光纤光栅，
Figure PCTCN2020138718-appb-000034
表示向下取整。一个周期对应一个第一模群和一个第二模群，即一个周期对应一对群延迟关于中心对称的模群。
进一步，可选地，可通过控制每个光纤光栅的周期来实现M个模群中关于中心对称的第一模群和第二模群的交换。例如，M个模群的群延迟分别为t 1，t 2，…，t M-1，t M，t 1和t M关于中心对称，t 2和t M-1关于中心对称，以此类推；实现将t 1对应的模群与t M对应的模群的进 行交换的光纤光栅的周期为Λ 1，实现将t 2对应的模群转换为t M-1对应的模群的进行交换的光纤光栅的周期为Λ 2，以此类推。其中，Λ 1＝λ/(t 1对应的模群的有效折射率-t M对应的模群的有效折射率)，Λ 2＝λ/(t 2对应的模群的有效折射率-t M-1对应的模群的有效折射率)，以此类推。
进一步，可选地，当M为偶数时，M个群延迟中包括M/2对关于中心对称的群延迟，即M个模群中包括M/2个第一模群和M/2个第二模群；模式转换器可将M个模群中群延迟关于中心对称的第一模群和第二模群两两进行交换。当M为奇数时，M个群延迟中包括
Figure PCTCN2020138718-appb-000035
Figure PCTCN2020138718-appb-000036
对关于中心对称的群延迟，即M个模群中包括
Figure PCTCN2020138718-appb-000037
个第一模群和
Figure PCTCN2020138718-appb-000038
个第二模群，以及1个单独的模群，模式转换器可将M个模群中群延迟关于中心对称的第一模群和第二模群两两进行交换，单独的模群不进行交换。
在一种可能的实现方式中，可通过二氧化碳(CO 2)激光器或飞秒激光器在少模光纤上刻写
Figure PCTCN2020138718-appb-000039
个不同周期的光纤光栅。进一步，可选地，
Figure PCTCN2020138718-appb-000040
个不同周期的光纤光栅可以为级联结构，参见图6a，以两个光纤光栅为例(光纤光栅Ⅰ和光纤光栅Ⅱ)；具体可在少模光纤轴向上连续刻写
Figure PCTCN2020138718-appb-000041
个不同周期的光纤光栅。或者
Figure PCTCN2020138718-appb-000042
个不同周期的光纤光栅可以为叠栅结构，参见图6b，以两个光纤光栅为例(光纤光栅Ⅰ和光纤光栅Ⅱ)；具体可沿少模光纤的
Figure PCTCN2020138718-appb-000043
个径向(垂直于轴向的任意方向，图6b示例出两个可能的径向)的不同角度刻写
Figure PCTCN2020138718-appb-000044
Figure PCTCN2020138718-appb-000045
个不同周期的光纤光栅。叠栅结构有助于避免不同区域的光纤光栅的横截面对相位不匹配的其它模群的耦合造成影响。需要说明的，叠栅结构的折射率调制近似为两个光纤光栅折射率调制的线性叠加，其中，纤芯折射率的改变量为：
Figure PCTCN2020138718-appb-000046
其中，
Figure PCTCN2020138718-appb-000047
为第k个光纤光栅的平均折射率改变量；Λ 1、Λ 2分别为光纤光栅Ⅰ和光纤光栅Ⅱ的周期，
Figure PCTCN2020138718-appb-000048
和
Figure PCTCN2020138718-appb-000049
分别为两个光纤光栅相移或啁啾相关的附加相位。
在一种可能的实现方式中，每个光纤光栅的总长度大于阈值，如此可提高模群转换的效率。
结构二，基于相位板(或称为相位片)的模式转换器。
基于相位板的模式转换器也称为多平面光转换器(multi-plane light conversion，MPLC)。相位板是在玻璃平板或透镜上的局部区域内(通常是环带)，镀上一层具有一定厚度和折射率的膜层，使透过该区域的光比通过非镀层区的光相位超前或滞后。每个相位板上有不同的像素点，相位板上的像素点可以引入不同的相位，以调制入射光场，可以实现模群之间的转换。基于相位板的模式转换器对空间光场的调控能力较强，理论上可以完成任意模群之间的转换。例如，基于相位板的模式转换器可以实现模群的轮循转换，如图7a所示，入射的模群依次为LP 01，LP 11，LP 21，将入射模群依次转换为LP 11，LP 21，LP 01。再比如，基于相位板的模式转换器可实现群延迟关于中心对称的模群的交换，如图7b所示，入射的模群分别为LP 01，LP 11，LP 21和LP 02，可实现LP 01和LP 02交换，LP 11和LP 21交换。
在一种可能的实现方式中，模式转换器可包括反射式相位板或者透射式相位板。例如可包括一个或多个透射式相位板，或者包括至少上下两个反射式相位板，或者包括至少一个反射式相位板和至少一个高反镜。该基于相位板的模式转换器可实现将群延迟关于中心对称的第一模群与第二模群进行模群交换。也就是说，基于相位板的模式转换器的交换策略为：群 延迟关于中心对称的第一模群和第二模群进行交换。应理解，模式转换器包括的相位板的数量越多，可以调节的相位的精细度越高。
参考图7c，为本申请提供的一种模式转换器为反射式相位板的结构示意图。图7c中模式转换器可为两个反射式相位板，或者也可为一个反射式相位板与一个高反镜的组合。以模式转换器接收到3个模群(LP 01、LP 11和LP 21)为例，LP 01、LP 11和LP 21在反射式相位板内发生多次反射，实现将LP 01转换为LP 21，将LP 21转换为LP 01，LP 11不变换。
参考图7d，为本申请提供的一种模式转换器为透射式相位板的结构示意图。图7d以模式转换器包括4个透射式相位板为例。以模式转换器接收到3个模群(LP 01、LP 11和LP 21)，为例，LP 01、LP 11和LP 21依次经4个透射式相位板后，可实现将LP 01转换为LP 21，将LP 21转换为LP 01，LP 11不变换。
三、放大器
本申请中，通信系统还可包括放大器，放大器用于对接收到的M个模群进行放大。放大器可为掺杂光纤放大器。
在一种可能的实现方式中，模式转换器与放大器的之间可包括如下两种位置关系。
位置关系一，模式转换器位于少模光纤与放大器之间。
基于该位置关系一又可分如下两种情形。
情形1，如图8a所示，为本申请提供的一种模式转换器在通信系统的位置示意图。该模式转换器可在少模光纤之后，放大器之前。
基于该情形1，模式转换器用于将群延迟呈中心对称分布的第一模群和第二模群进行转换，将转换后的M个模群传输至放大器，经放大器放大转换后的M个模群后，传输至下一跨段的少模光纤。
情形2，如图8b所示，为本申请提供的另一种模式转换器在通信系统的位置示意图。该模式转换器在少模光纤之前，在放大器之后。
基于该情形2，M个模群经放大器放大后进入模式转换器，模式转换器将放大后的M个模群中群延迟关于中心对称的第一模群和第二模群进行转换，将转换后的M个模群传输至下一跨段的少模光纤。
位置关系二，模式转换器集成于放大器。
如图8c所示，为本申请提供的又一种模式转换器在通信系统的位置示意图。该模式转换器可集成于放大器内。例如模式转换器可位于两端掺杂(例如掺饵)的光纤之间，M个模群进入放大器后，经第一段掺杂光纤放大后，进入可变光衰减器(Variable Optical Attenuator，VOA)，经VOA信号衰减(以实现放大器的增益可调)后传输至模式转换器，模式转换器将M模群中群延迟呈中心对称分布的第一模群和第二模群进行转换，并将转换后的M个模群传输至第二段掺杂光纤，经第二段掺杂光纤放大后，耦合至下一跨段的少模光纤继续传输。由于模式转换器具有一定的插损，通过将模式转换器集成于放大器，可以弥补模式转换器引入的插损且不恶化传输信号的信噪比。
基于上述内容，下面结合具体的结构，给出上述通信系统的一种具体实现方式。以便于进一步理解上述通信系统的架构及减小接收端接收到的M个模群间的群延迟的实现过程。
如图9所示，为本申请提供的另一种通信系统的架构示意图。该通信系统可包括发射模块、模式复用器、少模光纤、模式转换器、放大器、模式解复用器和接收模块。其中，发射模块可为M个单模发射模块，每个单模发射模块用于发射一路信号，得到M路信号(例如波分信号)，并将M路信号传输至模式复用器，其中，每路信号由一个特定的模群承载。模式复用器用于将每路信号从基模转换成不同的高阶模群，然后将M个承载不同信号的模群复用为一路信号，并将该复用后的信号耦合至少模光纤进行传输。少模光纤、模式转换器、放大器可参见上述关于少模光纤、模式转换器、放大器的介绍，此处不再一一赘述。模式解用器用于将接收到的复用的该路信号分离为M个承载不同信号的模群，并传输至接收模块。接收模块用于接收M个模群。例如，接收模块可包括M个单模接收模块，每个单模接收模块可接收一个对应的模群。
在一种可能的实现方式中，发射模块可包括光源，光源可用于发射相同波长的光信号，也可用于发射不同波长的光信号。示例性地，每个光源可以包括至少一个激光器，该激光器可以发射1.5-1.6μm之间的波长范围的信号。
在本申请的各个实施例中，如果没有特殊说明以及逻辑冲突，不同的实施例之间的术语和/或描述具有一致性、且可以相互引用，不同的实施例中的技术特征根据其内在的逻辑关系可以组合形成新的实施例。
本在本申请的文字描述中，字符“/”，一般表示前后关联对象是一种“或”的关系。在本申请的公式中，字符“/”，表示前后关联对象是一种“相除”的关系。
可以理解的是，在本申请中涉及的各种数字编号仅为描述方便进行的区分，并不用来限制本申请的实施例的范围。上述各过程的序号的大小并不意味着执行顺序的先后，各过程的执行顺序应以其功能和内在逻辑确定。术语“第一”、“第二”等是用于分区别类似的对象，而不必用于描述特定的顺序或先后次序。此外，术语“包括”和“具有”以及他们的任何变形，意图在于覆盖不排他的包含，例如，包含了一系列步骤或单元。方法、系统、产品或设备不必限于清楚地列出的那些步骤或单元，而是可包括没有清楚地列出的或对于这些过程、方法、产品或设备固有的其它步骤或单元。
尽管结合具体特征及其实施例对本申请进行了描述，显而易见的，在不脱离本申请的精神和范围的情况下，可对其进行各种修改和组合。相应地，本说明书和附图仅仅是所附权利要求所界定的方案进行示例性说明，且视为已覆盖本申请范围内的任意和所有修改、变化、组合或等同物。
显然，本领域的技术人员可以对本申请进行各种改动和变型而不脱离本发明的精神和范围。这样，倘若本申请实施例的这些修改和变型属于本申请权利要求及其等同技术的范围之内，则本申请也意图包含这些改动和变型在内。

Claims (8)

  1. 一种通信系统，其特征在于，包括：至少两个跨段的少模光纤，以及模式转换器；其中，所述少模光纤，用于传输接收到的M个模群，所述M个模群在所述少模光纤中传输的群延时呈中心对称分布，所述M个模群的群延迟的中心为所述M个模群的群延迟的最大值与最小值之间的中间值，所述M个模群的每个模群上承载有信号，所述M为大于1的整数；
    所述模式转换器，用于接收来自所述少模光纤的M个模群，并将所述M个模群中的第一模群和第二模群进行模群交换，得到M个交换后的模群，将所述M个交换后的模群耦合至下一跨段的所述少模光纤，所述第一模群的群延时与所述第二模群的群延时关于所述中心对称，其中，所述第一模群和所述第二模群为所述M个模群中的至少两个模群。
  2. 如权利要求1所述的通信系统，其特征在于，所述M个模群中的任意关于所述中心对称的第一模群的群延迟与所述第二模群的群延迟之和相等。
  3. 如权利要求1或2所述的通信系统，其特征在于，所述少模光纤从内向外依次包括纤芯、内包层、沟壑层和外包层；
    在所述少模光纤中，所述M个模群的群折射率呈中心对称分布；
    其中，所述M个模群中的每个模群的群折射率是根据所述模群的有效折射率确定的，所述模群的有效折射率是根据所述少模光纤的所述纤芯的折射率、所述内包层的折射率、所述外包层的折射率、所述沟壑层的折射率、所述纤芯的半径、所述内包层的宽度、所述沟壑层的宽度以及所述外包层的宽度确定的。
  4. 如权利要求1至3任一项所述的通信系统，其特征在于，所述模式转换器包括
    Figure PCTCN2020138718-appb-100001
    个不同周期的光纤光栅，一个周期对应一个第一模群和一个第二模群，
    Figure PCTCN2020138718-appb-100002
    表示向下取整；
    其中，
    Figure PCTCN2020138718-appb-100003
    个不同周期中的每个周期是根据通信波长、对应的第一模群的有效折射率和对应的第二模群的有效折射率确定的。
  5. 如权利要求4所述的通信系统，其特征在于，所述光栅周期对应的第一模群的有效折射率与第二模群的有效折射率之差乘以所述光栅周期等于通信波长。
  6. 如权利要求4或5所述的通信系统，其特征在于，所述
    Figure PCTCN2020138718-appb-100004
    个不同周期的光纤光栅为级联结构或叠栅结构。
  7. 如权利要求1至3任一项所述的通信系统，其特征在于，所述模式转换器包括反射式相位板或透射式相位板。
  8. 如权利要求1至7任一项所述的通信系统，其特征在于，所述通信系统还包括放大器；
    所述模式转换器位于所述少模光纤与所述放大器之间；或者，
    所述模式转换器集成于所述放大器内。
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