CN114301524B - Method and system for reducing crosstalk in high-order OAM mode group - Google Patents
Method and system for reducing crosstalk in high-order OAM mode group Download PDFInfo
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Abstract
The invention discloses a method for reducing crosstalk in a high-order OAM mode group, which comprises the following steps: (1) generating 4 types of vortex light in the mode group; (2) establishing a fibre channel of vortex rotation within the transmission mode group; (3) demodulating the eddy current rotation based on the inverse transmission matrix method. The invention also discloses a system for reducing the crosstalk in the high-order OAM mode group. The invention provides an inverse matrix compensation method for reducing crosstalk in a high-order OAM mode group, improving the number of OAM modes available for space division multiplexing and improving the channel capacity and the frequency spectrum efficiency of vortex communication.
Description
Technical Field
The present invention relates to a method and a system for reducing crosstalk, and in particular, to a method and a system for reducing crosstalk in a high-order OAM mode group.
Background
Vortex beams, which carry different integers of OAM, exhibit a new physical spatial degree of freedom due to orthogonality between them, have been consistently considered as a promising candidate for spatial multiplexing (SDM). Since the OAM-based SDM technique is compatible with the other multiplexing techniques described above, it can further improve the channel capacity and spectrum effectiveness of Optical Fiber Communication (OFC) or Free Space Optical Communication (FSOC).
Although OAM-based SDMs have great potential in improving channel capacity and spectral efficiency, cross-talk between intra/inter-mode groups between different OAM modes severely hampers OAM-based SDM technology development and application. To reduce the impact of crosstalk on system performance, many researchers have employed various approaches and have made significant efforts. In 2019, yellow et al proposed a crosstalk mitigation method using a single amplified spontaneous emission source in a mode and wavelength multiplexed OFC system. In 2016, wang et al adopted a low density parity check code (LDPC) based channel coding method to increase the ability to combat channel crosstalk, where two OAM modes in the x and y polarization directions were adopted by OFC systems. In 2018, in et al adopted an adaptive turbulence compensation method in an OAM-based FSOC system that uses a hybrid output algorithm to overcome crosstalk caused by atmospheric turbulence, wherein two OAM modes, a wavefront phase sensor, and a deformable mirror were adopted. In 2020, wang et al propose a low crosstalk propagation scheme by multiplexing mode groups, designed a low transmission loss annular optical fiber, obtained a large effective refractive index difference, improved isolation between two adjacent mode groups, effectively blocked mode coupling between modes, and realized the purpose of reducing crosstalk. In 2019, chen et al proposed a simplified multiple-input multiple-output (MIMO) concept, which employs a low-complexity and small-scale equalization algorithm for multiple MIMO to mitigate the impact of crosstalk on system performance.
However, for the OFC system based on the SDM of the OAM, since there are large mode coupling and crosstalk between OAM modes in the mode group, the above-mentioned research mainly adopts a mode group multiplexing method to suppress the crosstalk between OAM mode groups, so that the number of reusable OAM modes in the OFC is severely limited, and the channel capacity and the spectral efficiency cannot be further improved.
Disclosure of Invention
The invention aims to: the invention aims to provide a method and a system for reducing crosstalk in a high-order OAM mode group, which solve the problem of larger mode coupling and crosstalk between OAM modes in the mode group.
The technical scheme is as follows: the invention discloses a method for reducing crosstalk in a high-order OAM mode group, which comprises the following steps:
(1) Generating 4 vortex lights in the mode group;
(2) Establishing a fiber channel transmission vortex rotation of vortex rotation in a transmission mode group;
(3) Eddy current is demodulated based on an inverse transmission matrix method.
Step (1) comprises the steps of:
(11) The spatial light modulator generates linear polarization vortex rotation L1 carrying l=2, and is divided into two equal branches L3 and L5 through an optical coupler OC 1; branch L3 is adjusted to carry left circularly polarized eddy current L with l=2 by half wave plate HWP1 and quarter wave plate QWP1 in sequence 1 The method comprises the steps of carrying out a first treatment on the surface of the The leg L5 is adjusted to carry right circularly polarized eddy current L of l=2 by the half wave plate HWP2 and the quarter wave plate QWP2 in this order 2 ；
(12) The linear polarization vortex rotation L2 carrying l= -2 generated by the spatial light modulator is divided into two equal branches L4 and L6 by an optical coupler OC 2; the leg L4 is adjusted to carry l= -2 left circularly polarized eddy current L by the half wave plate HWP3 and the quarter wave plate QWP3 in sequence 3 The method comprises the steps of carrying out a first treatment on the surface of the The leg L6 is adjusted to carry right circularly polarized eddy current L of l= -2 by the half wave plate HWP4 and the quarter wave plate QWP4 in sequence 4 。
The optical fiber in the step (2) comprises a three-layer structure of an inner core, a ring core and a cladding, wherein the contrast ratio between the ring core and the cladding is more than 2%; under the condition of fixed cladding radius, the sizes of the inner diameter and the outer diameter of the ring core layer are adjusted to ensure that the equivalent coefficient difference between the second-order mode group and the mode group is respectively more than 10 -3 And 10 -4 。
Step (3) comprises the following steps:
(31) Obtaining a channel transmission matrix PM model
Wherein each coefficient a in the matrix ij Is a complex number, which represents OAM mode i For OAM mode l j Is a cross-talk of (1).
(32) The channel transmission matrix PM model is expressed as:
wherein, respectively represent l i Channel coupling in j The optical amplitude of the channel;
P 0 ,P π/2 ,P π and P 3π/2 Indicating that l with phase differences of 0, pi/2, pi and 3 pi/2 are sent, respectively i And l j After the combined vortex probe light, the receiving end detects l j Optical power of the mode;
(33) Obtaining the inverse of the transmission matrix PM
(34) And demodulating the original carrier light through the spatial light modulator according to the inverse matrix.
Demodulating the hologram used in step (34)
l i ＝b i1 ×QXT 1 +b i2 ×QXT 2 +b i3 ×QXT 3 +b i4 ×QXT 4 ，
Wherein QXT 1 ……QXT 4 Respectively, that demodulation is not affected by crosstalkAnd->Is a hologram of (2).
The system for reducing the crosstalk in the high-order OAM mode group comprises a modulation module, a transmission module and a demodulation module which are connected in sequence;
the modulation module generates 4 vortex lights in the mode group; the transmission module adopts the vortex rotation in the designed fiber channel transmission mode group; the demodulation module demodulates the received vortex rotation by adopting a method based on an inverse transmission matrix.
The modulation module comprises a spatial light modulator, a coupler OC1, a half-wave plate HWP1, a quarter-wave plate QWP1, a half-wave plate HWP2, a quarter-wave plate QWP2, a coupler OC2, a half-wave plate HWP3, a quarter-wave plate QWP3, a half-wave plate HWP4 and a quarter-wave plate QWP4; the half-wave plate HWP1 and the quarter-wave plate QWP1 are sequentially arranged at the upper rear part of the coupler OC 1; the half-wave plate HWP2 and the quarter-wave plate QWP2 are sequentially arranged below and behind the coupler OC 1; the half-wave plate HWP3 and the quarter-wave plate QWP3 are sequentially arranged at the upper rear part of the coupler OC 2; a half wave plate HWP4 and a quarter wave plate QWP4 are shown disposed in sequence below and behind the coupler OC 2.
The optical fiber of the transmission module comprises a three-layer structure of an inner core, a ring core and a cladding, wherein the contrast ratio between the ring core and the cladding is more than 2%; under the condition of fixed cladding radius, the sizes of the inner diameter and the outer diameter of the ring core layer are adjusted to ensure that the equivalent coefficient difference between the second-order mode group and the mode group is respectively more than 10 -3 And 10 -4 。
Inverse transmission matrix of the demodulation module
Wherein,each coefficient a in the matrix ij Is a complex number, representing OAM mode i For OAM mode l j Cross-talk of->
Respectively represent l i Channel coupling in j The optical amplitude of the channel;
P 0 ,P π/2 ,P π and P 3π2 Indicating that l with phase differences of 0, pi/2, pi and 3 pi/2 are sent, respectively i And l j After the combined vortex probe light, the receiving end detects l j Optical power of the mode.
Demodulating the hologram used in the demodulation module
l i ＝b i1 ×QXT 1 +b i2 ×QXT 2 +b i3 ×QXT 3 +b i4 ×QXT 4 ，
Wherein b ij QXT based on the weight coefficient obtained by inverse matrix IM 1 ……QXT 4 Respectively, that demodulation is not affected by crosstalkAnd->Is a hologram of (2).
The beneficial effects are that: compared with the prior art, the invention has the following remarkable advantages: the inverse matrix compensation method reduces crosstalk in a high-order OAM mode group, improves the number of OAM modes available for space division multiplexing, and improves channel capacity and spectrum efficiency of vortex communication.
Drawings
FIG. 1 is a schematic diagram of the present invention for reducing crosstalk in a higher order mode group;
fig. 2 is a schematic diagram of 4 OAM optical modulation schemes within a second order mode group according to the present invention;
fig. 3 is a diagram of OAM fiber structure and performance according to the present invention;
FIG. 4 shows an inverse matrix based on a transmission matrix according to the present inventionChannel demodulation and cross-talk reduction holograms;
FIG. 5 is a schematic diagram of an experimental platform of the present invention;
FIG. 6 is a captured interferogram and measured polarization profile of the present invention;
FIG. 7 is a pattern group crosstalk profile of the present invention;
FIG. 8 is a graph showing the distribution of channel error rate and constellation diagram in the mode group of the present invention;
FIG. 9 is a graph showing the effect of different transmission rates on BER and CD in accordance with the present invention;
fig. 10 is a graph showing BER and CD distribution effects for different OAM fiber lengths according to the present invention.
Detailed Description
The technical scheme of the invention is further described below with reference to the accompanying drawings.
As shown in fig. 1, the system for reducing crosstalk in a high-order OAM mode group according to the present invention includes a modulation module, a transmission module, and a demodulation module, which are sequentially connected, where the modulation module generates 4 kinds of vortex light in the mode group; the transmission module adopts the vortex rotation in a designed fiber channel transmission mode group; the demodulation module demodulates the received vortex rotation by adopting a method based on an inverse transmission matrix.
The invention relates to a method for reducing crosstalk in a high-order OAM mode group, which comprises the following steps:
(1) Generating 4 vortex lights in a second-order mode group;
(2) Establishing a fiber channel transmission vortex rotation of vortex rotation in a transmission mode group;
(3) And constructing a vortex optical demodulation and crosstalk alleviation module based on an inverse transmission matrix method.
Step 1: the generation of 4 vortex lights within the second order mode group.
Within each higher order OAM mode group |l| there are 4 OAM modes, which are respectively @ When |l|=2, a total of 4 OAM modes are contained in the second order mode group, which are l respectively 1 And->
As shown in fig. 1, loading a first digital hologram with a spatial light modulator produces left circularly polarized vortex light carrying l=2, respectivelyAnd right circularly polarized vortex rotation +.>Loading a second digital hologram with a spatial light modulator generates left circularly polarized eddy current +_ carrying l = -2, respectively>And right circularly polarized vortex rotation +.>The objective beam of the first digital hologram is vortex light with topological charge number of l=2, and the objective beam of the second digital hologram is vortex light with topological charge number of l= -2.
FIG. 2 depicts l 1 -l 4 Detailed modulation process of 4-way vortex rotation. In fig. 2, according to a first digital hologram, a linear polarization vortex rotation L1 carrying l=2 generated by a spatial light modulator is split into two equal branches L3 and L5 by one optical coupler OC 1;the L3 and L5 light paths are respectively adjusted to carry left circular polarized vortex light with l=2 by a half wave plate HWP and a quarter wave plate in sequenceAnd right circularly polarized vortex rotation +.>According to the second digital hologram, the linear polarization vortex rotation L2 carrying l= -2 generated by the spatial light modulator is divided into two equal branches L4 and L6 by an optical coupler OC; the L2 and L4 light paths are respectively adjusted to carry l= -2 left circular polarization eddy optical rotation +.>And right circularly polarized vortex rotation +.>
Step 2: a fibre channel suitable for the transmission of 4 OAM lights within the second order mode group is established.
An OAM optical fiber with a refractive index contrast ratio of more than 2% is designed, an optical fiber channel suitable for 4 vortex optical transmission and multiplexing in a second-order mode group is constructed, and the equivalent refractive index contrast ratio between vector modes in the second-order mode group in the optical fiber is ensured to be more than 10 -4 The energy coupling between modes is reduced, and the crosstalk between modes is reduced. Although the OAM fiber is taken to have reduced the probability of energy coupling between channels as much as possible, energy coupling between 4 channels may also occur, thereby causing unavoidable crosstalk.
In the process of designing an optical fiber, as shown in fig. 3, firstly, in order to meet the requirements of the energy profile characteristics of vortex rotation and the improvement of the equivalent refractive index contrast between modes, a three-layer structure with an inner core, a ring core and a cladding is determined, wherein the three-layer structure is shown in fig. 3; forming optical fibers with contrast ratio between the ring core layer and the cladding layer being more than 2% through different degrees of ion chemical vapor deposition and doping; next, under the condition that the radius of the fixed cladding is 62.5umContinuously adjusting the sizes of the inner diameter (nr) and the outer diameter (wr) of the ring core layer (the inner diameter size of the ring core layer should be smaller than the outer diameter size), calculating and measuring the equivalent coefficient separation degree between the vector mode groups and in the mode groups in the optical fiber by using a full-vector finite element analysis method, and ensuring that the equivalent coefficient difference between the second-order mode groups and in the mode groups is respectively larger than 10 -3 And 10 -4 . Finally, according to the above conditions, the ring core inner diameter and outer diameter dimensions of the optical fiber are determined to be 4.2um and 7.9um respectively in this embodiment, and the designed OAM optical fiber is prepared by adopting an improved chemical vapor deposition method.
Step 3: eddy current is demodulated based on an inverse transmission matrix method.
First, a channel transmission matrix (PM) model is measured using a plurality of probe lights,
wherein each coefficient a in the matrix ij Is a complex number, representing OAM mode i For OAM mode l j Is a cross-talk of (1).
Next, the formula (1) can be further expressed as the form of the equation (2),
wherein,
wherein |a ij Sum of IRespectively represent the coefficient a in equation (1) ij Amplitude and phase of (a) are provided. The diagonal matrix in equation (2) is a matrix of +.>And->An identity matrix is formed which represents the phases of only 4 channel coefficients in the transmission matrix. Phi for easy calculation 11 ，φ 22 ，φ 33 And phi 44 The value of (2) is typically set to 0. Thus, by measuring |a ij Value sum phi ij Acquiring complex coefficient a of transmission matrix ij 。
Amplitude |a according to the received optical power of each OAM channel ij The value of can be further expressed in the form of equation (3), where P ij Andrespectively represent l i Channel coupling in j The optical power and amplitude of the channel.
Each phase difference is shown as formula (4), P 0 ,P π/2 ,P π And P 3π/2 Indicating that l with phase differences of 0, pi/2, pi and 3 pi/2 are sent, respectively i And l j After the combined vortex probe light, the detected l is received j Optical power of the mode.
To obtain each phase difference in equation (2), it is necessary to construct 4 kinds of vortex probe light having fixed phase differences (0, pi/2, pi and 3 pi/2) in pairs according to equation (4), and measure the optical power P obtained after the probe light is transmitted through the channel 0 ,P π2 ,P π And P 3π2 . In the present embodiment, θ is calculated 12 -θ 11 Is composed ofAnd->Composition is prepared. Sequentially obtaining other phase difference values to obtain 4 patterns from the two-level pattern group +.> And->A4 x 4 transmission matrix PM is formed.
Again, the inverse matrix IM of the transmission matrix PM is calculated using the matrix inversion method, where b 1 T ,b 2 T ,b 3 T And b 4 T The row vector of the inverse matrix IM is shown in equation (5).
Finally, based on the obtained inverse matrix, an IM digital hologram for demodulating various vortex lights and reducing channel crosstalk is generated.
The digital hologram of the final IM is generated in dependence on the weighting coefficients of the rows in the IM. By superimposing different weighting coefficients (b i1 ，b i2 ，b i3 And b i4 ) 4 kinds of vortex lightAnd->) The hologram generating method of (1) is as follows:
l i ＝b i1 ×QXT 1 +b i2 ×QXT 2 +b i3 ×QXT 3 +b i4 ×QXT 4 ，
wherein QXT 1 ……QXT 4 Respectively, that demodulation is not affected by crosstalkAnd->Is a hologram of (2).
On the first channelFor example, then is l 1 ＝b 11 ×QXT 1 +b 12 ×QXT 2 +b 13 ×QXT 3 +b 14 ×QXT 4 。
Wherein, the hologram of each of the 4 kinds of vortex rotation is obtained by the following method:
first, an interference pattern needs to be generated using a computer-encoded method, and a holographic grating needs to be generated by writing the interference pattern into an appropriate medium, and then light is incident on the holographic grating to finally generate a hologram of a desired vortex beam.
From the interference theory, it can be known that when the two wave functions are A 1 exp(il 1 θ) and A 2 exp(il 2 θ) the light intensity distribution of the interference pattern is:
wherein 2A 1 A 2 cos(il 1 θ-il 2 θ) describes the spatial distribution characteristics of the interference light intensity. Now suppose that there is a vortex beam a propagating along the z-axis 1 exp(il 1 θ), where l is the number of topological charges of the vortex beam, a planar lightwave propagating along a direction with a z-axis inclusion angle α, the planar lightwave function may be written as E 2 exp (ikxsin α+ikzcos α). Assuming that the beam waist planes of the two beams are on the z=0 plane, when the two beams interfere, the interference light intensity distribution is:
when both beams are of unit amplitude, i.e. R 1 ＝R 2 When=1:
I＝2+2cos(lθ-kxsinα)
from the above formula, when lθ -kxsin α=2npi (n=0, 1,2, …), the interference intensity is maximized whenThe interference intensity is minimized. Thus, by computer numerical encoding of lθ -kxsin α, an interference pattern, i.e. a hologram, can be obtained.
To demodulate the first channelMode light, the first row vector of the IM matrix calculated in this embodiment is b 11 ＝1，b 12 ＝1,/>Loading by a spatial light modulator is not affected by crosstalk>And->Demodulating the original carrier light after crosstalk reduction. A phase diagram as shown in fig. 4 is obtained.
The system for reducing the crosstalk in the high-order OAM mode group comprises a modulation module, a transmission module and a demodulation module which are connected in sequence; the modulation module generates 4 vortex lights in the mode group; the transmission module adopts the vortex rotation in the designed fiber channel transmission mode group; the demodulation module demodulates the received vortex rotation by adopting a method based on an inverse transmission matrix.
The modulation module comprises a spatial light modulator, a coupler OC1, a half-wave plate HWP1, a quarter-wave plate QWP1, a half-wave plate HWP2, a quarter-wave plate QWP2, a coupler OC2, a half-wave plate HWP3, a quarter-wave plate QWP3, a half-wave plate HWP4 and a quarter-wave plate QWP4; the half-wave plate HWP1 and the quarter-wave plate QWP1 are sequentially arranged at the upper rear part of the coupler OC 1; the half-wave plate HWP2 and the quarter-wave plate QWP2 are sequentially arranged below and behind the coupler OC 1; the half-wave plate HWP3 and the quarter-wave plate QWP3 are sequentially arranged at the upper rear part of the coupler OC 2; a half wave plate HWP4 and a quarter wave plate QWP4 are shown disposed in sequence below and behind the coupler OC 2.
The optical fiber of the transmission module comprises a three-layer structure of an inner core, a ring core and a cladding, wherein the contrast ratio between the ring core and the cladding is more than 2%; under the condition of fixed cladding radius, the sizes of the inner diameter and the outer diameter of the ring core layer are adjusted to ensure that the equivalent coefficient difference between the second-order mode group and the mode group is respectively more than 10 -3 And 10 -4 。
Inverse transmission matrix of demodulation module
Wherein,each coefficient a in the matrix ij Is a complex number, representing OAM mode i For OAM mode l j Cross-talk of-> Respectively represent l i Channel coupling in j The optical amplitude of the channel;
P 0 ,P π/2 ,P π and P 3π/2 Indicating that l with phase differences of 0, pi/2, pi and 3 pi/2 are sent, respectively i And l j After the combined vortex probe light, the receiving end detects l j Optical power of the mode.
Hologram for demodulation in demodulation module
l i ＝b i1 ×QXT 1 +b i2 ×QXT 2 +b i3 ×QXT 3 +b i4 ×QXT 4 ，
Wherein b ij QXT based on the weight coefficient obtained by inverse matrix IM 1 ……QXT 4 Respectively, that demodulation is not affected by crosstalkAnd->Is a hologram of (2).
As shown in fig. 5, in order to verify the feasibility and correctness of the method and system of the present invention, an experimental verification platform is established, and specific experimental steps are as follows:
first, a polynomial is x 31 +x 28 The +1 pseudo random sequence generating module PRBS-31 is used to generate test data or user data required in the experimental process; the pseudo-random sequence data is mapped into corresponding digital QPSK symbols by a Quadrature Phase Shift Keying (QPSK) module. The resulting digital QPSK signal is then converted to an analog QPSK signal by an Arbitrary Waveform Generator (AWG). An External Cavity Laser (ECL) was used to generate 1550nm carrier gaussian light. After modulation by the polarization controller, carrier light transmitted by a Single Mode Fiber (SMF) and an analog QPSK signal drive a quadrature modulator (I/Q) together to realize modulation of test data to an optical carrier. After being modulated by an lug-doped optical fiber amplifier (EDFA) and a band-pass filter (BPF), the power and spontaneous noise of the optical signal are respectively amplified and filtered, and the quality of the optical signal is improved. The optical signal is split into three equal branches by means of an optical coupler (OC 1). After 3 different lengths of SMF transmission, the correlation between the three optical paths is reduced, so that the three optical paths can be regarded as independent and mutually noninterfere channels, wherein PC2/3/4 is used for adjusting each branch light to be linear polarized light respectively.
Secondly, in the three light paths, the three collimators C1-C3 respectively convert the optical fiber light into free space light. In order to increase the irradiation area of a Spatial Light Modulator (SLM), three light paths of the three light paths are provided with a 3f beam expander consisting of two lenses (L1/3/5 and L2/4/6)The dimensions are enlarged by a factor of 2. Since the SLM is only efficient for linearly polarized light, a combination of 1 polarizer (P1/P2/P3) and 1 half wave plate (HWP 1/HWP2/HWP 3) is used to adjust the polarization of each branch light to linearly polarized gaussian light, respectively. The upper two light branches SLM1 and SLM2 respectively generate corresponding two eddy currents carrying l=2 and l= -2, wherein mirror (M1/3 and M2/4) combinations are used for adjusting the transmission direction of the light path respectively. The two generated vortex lights are respectively divided into two equal vortex rotations by two optical splitters (BS 1 and BS 3). After adjustment by half wave plate (HWP 2/3/5/6) and quarter wave plate (QWP/1/2/3/4), the four vortex lights are Left Circular Polarization (LCP) vortex light and Right Circular Polarization (RCP) vortex light modulated to carry topological charges l=2 and l= -2, respectively, which are respectivelyAnd->Wherein the subscript + -2 represents the topological charge l and the superscript + -represents LCP and RCP, respectively. The 4-way vortex light described above is multiplexed into 1-way light by BS2, BS4 and BS5, where M3/8/9/10/11 is used to modulate the optical path transmission direction. The vortex light after multiplexing by an objective lens OL1 and 4 paths is coupled into a laboratory self-made OAM fiber with the length of 1km for transmission. At the other end of the OAM fiber, the multiplexed vortex light is coupled into free space through a C4.
Again, after passing through a beam expander consisting of L7 and L8, the size of the vortex light coupled into free space expands by a factor of 2. A QWP5 and HWP7 combination was used to adjust the LCP and RCP to linearly polarized light. The adjusted linearly polarized light is equally divided into two paths by one BS 7. After one vortex of light interferes with the reference gaussian light through BS6, a camera (CCD) is used to capture the received interferogram and analyze the polarization of the received light, with the leading P4 used to adjust the polarization direction of the light. The other light, after passing through the spatial light modulator SLM3 carrying the hologram inverse-transformed to the transmission matrix, is vortex-optically modulated into various original gaussian lights, where M5 and M6 are used to adjust the light transmission direction.
Finally, after passing through an objective lens OL2 and optical coupling OC2, the demodulated gaussian light is coupled into SMF and equally split into 2 branch lights. An optical Power Meter (PM) is used to measure the power of a light and is used to generate a corresponding inverse transmission matrix transformed digital hologram, wherein a computer (Com) is used to calculate and generate the hologram. The other optical path is adjusted by EDFA2 and a variable power attenuator (VOA), and a coherent demodulator is used to extract the transmitted original QPSK data from the Gaussian optical carrier. The demodulated QPSK data is used to analyze the performance of the communication system via an off-line data processing module.
In order to verify the correctness of the proposed scheme, at the transmitting end, four-way vortex optical rotation is performed Modulated and transmitted sequentially, the CCD captures the corresponding interferograms and measures the polarization of the light. As shown in fig. 6, a1-a4 in fig. 6 represent cross-sectional views of the light received in 4, respectively, which take on a doughnut shape, indicating that the vortex rotation is properly received. B1-b4 in FIG. 6 represent the captured interferograms of a1-a4 in FIG. 6, respectively, where the number of interference fringes represents the value of |l|, and the counter-clockwise/clockwise rotation direction represents the sign of the topological charge as +or-, respectively. In fig. 6 c shows the measured polarization of the medium light of a1-a4 in fig. 6, where the energy is located in the polarization direction-150 ° to 50 °, or-50 ° to 150 °, respectively LCP and RCP. According to the above measurement method, the received vortex light shown in fig. 6a1-a4 is respectivelyAnd->Which is consistent with the swirling light being sent, thus proving the correctness of the method.
To evaluate the effect of the method on the improvement of channel crosstalk, crosstalk profiles with or without the method were measured and analyzed, respectively. As shown in fig. 7, the normalized crosstalk profile measured using the present method is shown on the left and the normalized crosstalk profile measured using the present method is shown on the right. The maximum crosstalk and the minimum crosstalk which are not measured by the method are-2.88 dB and-16.92 dB respectively, and the normalized values of the maximum crosstalk and the minimum crosstalk are 0.72 and 0.14 respectively. When the method is adopted, the maximum value and the minimum value of the crosstalk measured by the method are respectively-6.23 dB and-21.75 dB, and the normalized values corresponding to the maximum value and the minimum value are respectively 0.48 and 0.08. Compared with the method which is not used, the method has the advantages that the crosstalk in the second-order OAM mode group is obviously reduced, so that the effectiveness and the correctness of the method are further proved.
In addition, in order to investigate the effect of the present method and not the present method on the system performance, the bit error rate BER and the constellation CD were measured and analyzed, respectively. Fig. 8 depicts BER and CD distribution for 4 channels in a second order mode group with and without the present method. Compared with the traditional back-to-back case, four channels with/without the method are adopted And->At the hard forward error correction threshold with 7% overhead (FEC, 3.8x10) -3 ) The optical signal to noise ratio (OSNR) distribution was reduced by 2.38/6.63dB,2.58/6.81dB,2.90/7.10dB, and 3.04/7.40dB. Compared with the method without the use of the method, the method is adopted to treat the disease>And->BER performance of the channel is improved by 4.25db,4.23db,4.20db and 4.36, respectively. Further, in FIG. 8, when the OSNR is set to 21dB, it is compared with the measured non-measured valueThe QPSK signal CD using the method is more concentrated in the center position than the CD using the method, which further indicates the result of consistency with the bit error rate distribution. Thus, with the present method, 4 channel errors can be significantly improved, further proving that the present method is correct and reliable.
In a communication system, transmission rate and fibre channel length are key elements affecting system performance. To simplify the measurement process, onlyBER and CD performance of the channel are measured. To analyze the impact of transmission rate and fibre channel length on system performance, fig. 9 and 10 depict different transmission rate and fibre channel length pairs, respectively, after the present method is employedBER of the channel and CD performance. In fig. 9, BER and CD performance gradually decrease as the rate gradually increases. Transmission rates of s=20gbps and s=40gbps compared to transmission rate of s=10gbps +.>The error rate distribution drops by 0.51dB and 1.31dB. Furthermore, under osnr=16.8 dB condition, with increasing rate, the +_on>The QPSK constellation of (c) becomes increasingly blurred and its surface performance decreases. In fig. 10, as the length of OAM fiber is programmed gradually, +.>BER and CD performance of the channel gradually decrease. Compared with the OAM fiber length l=1 Km, the transmission through the fiber lengths l=10 Km and l=20 Km is +.>The bit error rate performance distribution drops by 1.30dB and 2.97dB. Further, at osnr=Under 18dB condition, with increasing rate, < +.>The QPSK constellation of (c) also becomes increasingly ambiguous, which indicates a gradual decrease in performance. In summary, it is shown that even though the transmission rate and the optical fiber length shown in FIGS. 9 and 10 are increased as compared with BER and CD performance without using the present method in FIG. 8, the present method is used +.>The BER and CD performance of the channel are anyway superior to those of channels not using the present method. />
Claims (6)
1. A method for reducing crosstalk in a high order OAM mode group, comprising: the method comprises the following steps:
(1) Generating 4 vortex lights in the mode group;
(2) Establishing a fiber channel transmission vortex rotation of vortex rotation in a transmission mode group;
(3) Eddy current rotation is demodulated based on an inverse transmission matrix method,
the step (1) comprises the following steps:
(11) The spatial light modulator generates linear polarization vortex rotation L1 carrying l=2, and is divided into two equal branches L3 and L5 through an optical coupler OC 1; branch L3 is adjusted to carry left circularly polarized eddy current L with l=2 by half wave plate HWP1 and quarter wave plate QWP1 in sequence 1 The method comprises the steps of carrying out a first treatment on the surface of the The leg L5 is adjusted to carry right circularly polarized eddy current L of l=2 by the half wave plate HWP2 and the quarter wave plate QWP2 in this order 2 ；
(12) The linear polarization vortex rotation L2 carrying l= -2 generated by the spatial light modulator is divided into two equal branches L4 and L6 by an optical coupler OC 2; the leg L4 is adjusted to carry l= -2 left circularly polarized eddy current L by the half wave plate HWP3 and the quarter wave plate QWP3 in sequence 3 The method comprises the steps of carrying out a first treatment on the surface of the The leg L6 is adjusted to carry right circularly polarized eddy current L of l= -2 by the half wave plate HWP4 and the quarter wave plate QWP4 in sequence 4 ，
The step (3) comprises the following steps:
(31) Obtaining a channel transmission matrix PM model
Wherein each coefficient a in the matrix ij Is a complex number, representing OAM mode i For OAM mode l j Is used in the optical system,
(32) The channel transmission matrix PM model is expressed as:
wherein, respectively represent l i Channel coupling in j The optical amplitude of the channel;
P 0 ，P π/2 ，P π and P 3π/2 Indicating that l with phase differences of 0, pi/2, pi and 3 pi/2 are sent, respectively i And l j After the combined vortex probe light, the receiving end detects l j Optical power of the mode;
(33) Obtaining the inverse of the transmission matrix PM
(34) And demodulating the original carrier light through the spatial light modulator according to the inverse matrix.
2. The method for reducing intra-group crosstalk in a higher order OAM mode as recited in claim 1, wherein: the step (2) is thatThe optical fiber comprises a three-layer structure of an inner core, a ring core and a cladding, wherein the contrast ratio between the ring core and the cladding is more than 2%; under the condition of fixed cladding radius, the sizes of the inner diameter and the outer diameter of the ring core layer are adjusted to ensure that the equivalent coefficient difference between the second-order mode group and the mode group is respectively more than 10 -3 And 10 -4 。
3. The method for reducing intra-group crosstalk in a higher order OAM mode as recited in claim 1, wherein: demodulating the hologram used in step (34)
l i ＝b i1 ×QXT 1 +b i2 ×QXT 2 +b i3 ×QXT 3 +b i4 ×QXT 4 ，
Wherein QXT 1 ......QXT 4 Respectively, that demodulation is not affected by crosstalkAnd->Is a hologram of (2).
4. A system for reducing crosstalk in a high order OAM mode group, comprising: the device comprises a modulation module, a transmission module and a demodulation module which are connected in sequence; the modulation module generates 4 vortex lights in the mode group; the transmission module adopts vortex rotation in a designed fiber channel transmission mode group; the demodulation module demodulates the received eddy current by adopting an inverse transmission matrix method, and comprises a space light modulator, a coupler OC1, a half-wave plate HWP1, a quarter-wave plate QWP1, a half-wave plate HWP2, a quarter-wave plate QWP2, a coupler OC2, a half-wave plate HWP3, a quarter-wave plate QWP3, a half-wave plate HWP4 and a quarter-wave plate QWP4;
the half-wave plate HWP1 and the quarter-wave plate QWP1 are sequentially arranged at the upper rear part of the coupler OC 1;
the half-wave plate HWP2 and the quarter-wave plate QWP2 are sequentially arranged below and behind the coupler OC 1;
the half-wave plate HWP3 and the quarter-wave plate QWP3 are sequentially arranged at the upper rear part of the coupler OC 2;
the half wave plate HWP4 and the quarter wave plate QWP4 are arranged in sequence below and behind the coupler OC2,
inverse transmission matrix of the demodulation module
Wherein,each coefficient a in the matrix ij Is a complex number, representing OAM mode i For OAM mode l j Cross-talk of-> Respectively represent l i Channel coupling in j The optical amplitude of the channel;
P 0 ，P π/2 ，P π and P 3π/2 Indicating that l with phase differences of 0, pi/2, pi and 3 pi/2 are sent, respectively i And l j After the combined vortex probe light, the receiving end detects l j Optical power of the mode.
5. The system for reducing crosstalk in a high-order OAM mode set as recited in claim 4, wherein: the optical fiber of the transmission module comprises a three-layer structure of an inner core, a ring core and a cladding, wherein the contrast ratio between the ring core and the cladding is more than 2%; under the condition of fixed cladding radius, the sizes of the inner diameter and the outer diameter of the ring core layer are adjusted to ensure that the equivalent coefficient difference between the second-order mode group and the mode group is respectively more than 10 -3 And 10 -4 。
6. The system for reducing crosstalk in a high-order OAM mode set as recited in claim 4, wherein: demodulating the hologram used in the demodulation module
l i ＝b i1 ×QXT 1 +b i2 ×QXT 2 +b i3 ×QXT 3 +b i4 ×QXT 4 ，
Wherein b ij QXT based on the weight coefficient obtained by inverse matrix IM 1 ......QXT 4 Respectively, that demodulation is not affected by crosstalkAnd->Is a hologram of (2).
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