Design method of Bragg grating with multi-mode wavelength selection function

Abstract

The invention discloses a Bragg grating design method with a multi-mode wavelength selection function, and the method comprises the steps: determining target modes of two target wavelengths reflected by a Bragg grating to be set, namely a long wave LP01 mode of a first target wavelength and a short wave LP11 mode of a second target wavelength; determining a grating period corresponding to the target mode; reflection areas corresponding to the target modes of the two target wavelengths are arranged at different axial length positions of a grating fiber core; and designing radial refractive index distribution of reflection regions corresponding to the target modes of the two target wavelengths, so that mode field separation is realized in the target modes of the two target wavelengths. Through the method provided by the invention, the mode field Bragg grating capable of effectively separating the fundamental mode and the high-order mode can be designed.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber Bragg grating design, in particular to a method for designing a Bragg grating with a multi-mode wavelength selection function. Background Art

[0002] According to the traditional Bragg condition, the wavelength spacing of the traditional Bragg grating (FBG) is By the grating period and mode effective refractive index difference The design is determined based on a single grating period and the inherent material structure properties of the optical fiber. This principle leads to the following limitations: since the grating period cannot be adjusted after manufacturing, and the effective refractive index difference between different modes is limited by the optical fiber material and geometric structure parameters (such as core diameter, doping distribution, etc.), the wavelength spacing of traditional gratings is strictly bound to the initial design value. It is impossible to dynamically expand the wavelength spacing according to actual needs to suppress mode crosstalk, nor can it compress the wavelength spacing to achieve high-density wavelength allocation. For example, in multimode optical fiber, the fundamental mode (LP 01 ) and higher-order modes (LP 11 ) wavelength interval is often due to The tiny size of less than 1nm makes it difficult to separate optical signals in the spectral overlap region at the receiver, causing inter-mode energy coupling and signal distortion. Therefore, a new Bragg grating design that breaks through the traditional design paradigm is needed to solve the problems of fixed wavelength spacing and insufficient mode selectivity. Summary of the Invention

[0003] In view of the defects of the prior art, the present invention proposes a Bragg grating design method with multi-mode wavelength selection function.

[0004] In order to achieve the above technical objectives, the technical solutions adopted by the present invention are as follows: On the one hand, a method for designing a Bragg grating with a multi-mode wavelength selection function is provided, comprising: Step 1: Determine the target mode of the two target wavelengths reflected by the Lager grating to be designed, which is the long-wave LP of the first target wavelength. 01 mode and the second target wavelength shortwave LP 11 model; Step 2: determine the grating period corresponding to the target pattern; Step 3, setting the reflection areas corresponding to the target modes of the two target wavelengths at different axial length positions of the grating fiber core; Step 4: Design the radial refractive index distribution of the reflection area corresponding to the target modes of the two target wavelengths so that the target modes of the two target wavelengths can achieve mode field separation.

[0005] Furthermore, a Bragg grating designed based on the above-mentioned Bragg grating design method with multi-mode wavelength selection function is provided.

[0006] Compared with the prior art, the present invention can achieve the following beneficial technical effects: The Bragg grating design method with multi-mode wavelength selection function provided by the present invention first determines the target modes of the two target wavelengths reflected by the Bragg grating to be designed, which is the long-wave LP of the first target wavelength. 01 mode and the second target wavelength shortwave LP 11 Mode, the method provided by the present invention can be used to design a Bragg grating that can effectively separate the fundamental mode and the high-order mode. Specifically, the reflection areas corresponding to the target modes of the two target wavelengths are set at different axial length positions of the grating core, that is, the reflection areas corresponding to the target modes of the two target wavelengths are located at different positions in the axial direction of the optical fiber core and have different grating periods. Compared with traditional gratings, the sensitivity of the grating designed with differentiated grating periods is greatly improved. After the fiber structure and fiber parameters of the Bragg grating to be designed are known and the target mode of the grating target wavelength and the grating period are determined, the Bragg grating to be designed is simulated using COMSOL software. The radial refractive index distribution of the reflection areas corresponding to the target modes of the two target wavelengths is simulated and designed, so that the target modes of the two target wavelengths can achieve mode field separation. The Bragg grating finally designed can output different modes at different wavelengths. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0008] Figure 1 is a flow chart of a method for designing a Bragg grating with multi-mode wavelength selection function in one embodiment; Figure 2 LP propagating in a conventional grating 01 LP 11 LP 21 LP 02 Mode field distribution diagram of the mode, where Figure 2 (a) is LP 01 Mode field distribution diagram, Figure 2 (b) for LP 11 Mode field distribution diagram, Figure 2 (c) LP 21 Mode field distribution diagram, Figure 2(d) for LP 02 Mode field distribution diagram; Figure 3 For LP 11 Schematic diagram of the regional division of the radial cross section of the fiber core in the reflection area corresponding to the mode.

[0009] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0010] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0011] In one embodiment, a method for designing a Bragg grating with a multi-mode wavelength selection function is provided, comprising: Step 1: Determine the target mode of the two target wavelengths reflected by the Lager grating to be designed, which is the long-wave LP of the first target wavelength. 01 mode and the second target wavelength shortwave LP 11 model; Step 2: determine the grating period corresponding to the target pattern; Step 3, setting the reflection areas corresponding to the target modes of the two target wavelengths at different axial length positions of the grating fiber core; Step 4: Design the radial refractive index distribution of the reflection area corresponding to the target modes of the two target wavelengths so that the target modes of the two target wavelengths can achieve mode field separation.

[0012] For the fiber core 、 NA Conventional grating of 0.061, LP 01 and LP 11 The effective refractive index difference is 0.00035, so when the grating period length is limited, the wavelength interval is difficult to separate. Active regulation, the preferred core diameter is The fiber is used to control the 1080nm and 1050nm modes. The modes propagating in a conventional grating with NA of 0.061 are LP 01 LP 11 LP 21 LP 02 Mode, such as Figure 2 As shown. Figure 2It can be seen that LP 01 Mode and LP 11 Mode, LP 11 Model and LP 21 The mode fields of the modes have a large degree of overlap. 01 Mode and LP 11 Mode, LP 11 Model and LP 21 The mode field of the mode can realize LP 01 Mode, LP 11 Selective reflection of the mode. To achieve the purpose of mode selection, the mode propagating in the fiber core must first be separated so that the refractive index modulation and the mode field integral of the mode are The grating has a high reflectivity that is significantly higher than other modes. Different modes correspond to different reflection areas of the grating, and different wavelengths correspond to different grating periods. In summary, by setting gratings of different periods in different reflection areas along the fiber core axis and designing the radial refractive index distribution of the reflection area to achieve the purpose of mode separation, both mode selection and wavelength separation can be achieved.

[0013] The Bragg grating selectively modulates the refractive index distribution of the fiber core to control the self-coupling coefficients of different modes: ; in, is the wave vector, is the effective propagation constant, is the radial refractive index modulation variation of the independent grating, is the mode field distribution function of the mode.

[0014] According to the reflection characteristics of the Bragg grating, the reflectivity R It can be approximately expressed by the following formula: ; in, is the coupling coefficient, L is the effective length of the grating. In the case of a smaller value), the reflectivity can be approximately proportional to Differentiated grating periods are written in different regions of the fiber core for different transmission wavelengths and desired modes ( ), realize the active expansion or compression of wavelength interval and mode selection, thereby solving the problem of fixed wavelength interval of traditional grating.

[0015] In one embodiment, a Bragg grating design method with multi-mode wavelength selection function is proposed, the goal is to achieve 1080nm LP 01 mode and 1050nm LP 11The strong self-coupling of the mode can achieve the purpose of wavelength separation and mode selection. That is, in step 1, the target modes of the two target wavelengths are LP at 1050nm and LP at 1050nm. 11 mode, 1080nm LP 01 model.

[0016] In order to achieve 1080nm LP 01 mode and 1050nm LP 11 The strong self-coupling of the mode is known from the grating Bragg condition. In step 2, the 1050nm LP 11 The grating period of the mode is 362nm, and the LP of 1080nm 01 The grating period corresponding to the pattern is 372nm.

[0017] In this embodiment, in step 4, designing the radial refractive index distribution of the reflection area corresponding to the target modes of the two target wavelengths includes: (1) Design of 1050nm LP 11 The radial refractive index distribution of the reflection area corresponding to the mode is as follows: For 1050nm LP 11 The radial cross section of the core of the reflection area corresponding to the mode is divided into regions: Figure 3 As shown, with the core center as the center of the circle and also the origin of the xy coordinate system of the core radial section, determine the central circle area in the core radial section, and determine four arc areas with the core center as the center of the circle, symmetrically distributed about the x axis and symmetrically distributed about the y axis: Let is the radius of the center circle with the core center as the center, r1, r2, r3, r4, and r5 are respectively the core radius, the distance from the center to the outer ring, the distance from the center to the inner ring, the distance from the center to the outer diameter of the arc area, and the distance from the center to the inner diameter of the arc area. is the angle between the edge of the arc area close to the x-axis and the x-axis, is the angle between the edge of the arc area away from the x-axis and the x-axis, ; Preferred parameter settings of this embodiment: They are , angle They are 、 .

[0018] 1050nm LP 11 The four arc regions in the radial cross section of the core corresponding to the mode are used as the radial refractive index modulation regions of the grating. The distance from the four arc regions to the core center is The refractive index of the point is Determine, and thus obtain the radial refractive index distribution of the arc area, where is the refractive index of SiO2, the fiber matrix material, which is 1.45; is an intrinsic refractive index added to the fiber core, is an intrinsic refractive index added to the arc area, The distance from the center of the arc region to the center of the fiber core. The center of the arc region is the point in the arc region where the distances to the inner arc and outer arc of the arc region are equal. represents a Gaussian function with a standard deviation of 0.3 and a total integral value of 1. Represents the distance from any point in the four arc areas to the center of the fiber core; the second item is the refractive index modulation term of the arc grating area. Through the Gaussian modulation of the second term, the Gaussian peak is located at Pick When the arc modulation area matches the 1050nm LP 11 mode field distribution, while suppressing the 1080nm LP 01 Mode coupling to achieve mode selective reflection.

[0019] 1050nm LP 11 The central circle area in the radial cross section of the core of the reflection area corresponding to the mode is the radial intrinsic refractive index area of ​​the grating. The distance from the central circle area to the core center is The refractive index of the point is Determine, thus obtaining the radial refractive index distribution of the central circle area, where Indicates the distance from any point in the central circle area to the center of the fiber core.

[0020] 1050nm LP 11 The circular area between the outer ring and the inner ring in the radial cross section of the core corresponding to the reflection area of ​​the mode, the distance from the circular area to the core center is The refractive index of the point is Determine, thus obtaining the radial refractive index distribution of the annular region, where Indicates the distance from the center of the outer ring to the center of the circle. The center of the outer ring is the point in the ring area where the distances to the inner and outer rings are equal. represents the refractive index modulation of the outer ring.

[0021] 1050nm LP 11 The distance from the core center to the core center in the radial cross section of the core corresponding to the mode, except for the four arc areas, the central circle area, and the ring area between the outer ring and the inner ring, is The refractive index of the point is Determine, thereby determining the radial refractive index distribution in other areas of the core area except the four arc areas, the central circle area, and the ring area between the outer ring and the inner ring. It represents the distance from any point in the core area except the four arc areas, the central circle area, and the circular area between the outer and inner rings to the core center.

[0022] (2) Design of 1080nm LP 01 The radial refractive index distribution of the reflection area corresponding to the mode is as follows: For 1080nm LP 01 The core radial section of the reflection area corresponding to the mode is divided into regions: the core center is taken as the center of the circle and also as the origin of the xy coordinate system of the core radial section, and the central circle area is determined in the core radial section. is the radius of the center circle with the core center as the center, r1, r2, and r3 are the core radius, the distance from the center to the outer ring, and the distance from the center to the inner ring, respectively. .

[0023] 1080nm LP 01 The central circle area of ​​the radial cross section of the core in the reflection area corresponding to the mode is the radial refractive index modulation area of ​​the grating, and the distance from the central circle area to the core center is The refractive index of the point is Determine, thereby determining the radial refractive index distribution of the central circle area, where Indicates the distance from any point in the central circle area to the center of the fiber core. The fourth term is the grating refractive index modulation term, and through the fourth Gaussian modulation, the Gaussian peak is located at When 0 is taken, the gate area and 1080nm LP are realized 01 The mode field overlap of the mode is much higher than that of LP at 1050nm 11 mode field to reduce high-order mode self-coupling.

[0024] 1080nm LP 01 The circular area between the outer and inner rings of the radial cross section of the core corresponding to the mode is the radial intrinsic refractive index area of ​​the core, and the distance from the circular area between the outer and inner rings to the center of the core is The refractive index of the point is Determine the LP of 1080nm 01 The radial refractive index distribution of the circular area between the outer circular area and the inner circular area of ​​the radial cross section of the fiber core in the reflection area corresponding to the mode.

[0025] 1080nm LP 01In the radial cross section of the core of the reflection area corresponding to the mode, except for the central circle area and the ring area between the outer ring and the inner ring, the rest of the core area is the inherent refractive index area of ​​the optical fiber. For the 1080nm LP 01 The distance from the core center to the core center in the radial cross section of the core corresponding to the mode, except for the central circle area and the ring area between the outer ring and the inner ring, is The refractive index of the point is Determine the LP of 1080nm 01 The radial refractive index distribution of other areas of the core region except the central circle area and the annular area between the outer ring and the inner ring in the radial cross section of the core of the reflection area corresponding to the mode.

[0026] Preferred parameter settings for this embodiment: is 1.45, is 0.0031, is 0.0005, is 0.01, for , for .

[0027] Reflects 1050nm LP 11 The lengths L1 and L2 of the symmetrical arc grating of the mode are 0.198m and 0.0216m respectively, reflecting the LP of 1080nm 01 The lengths of the small circular gratings L3 and L4 in the center of the pattern are 0.0245m and 0.00267m respectively.

[0028] For LP reflecting 1050nm 11 Symmetrical arc grating refractive index modulation area of ​​the mode, LP 11 The mode self-coupling coefficient is about 15.12, LP 01 The mode self-coupling coefficient is about 1.80, LP at 1050nm 02 The mode self-coupling coefficient is about 4.01, LP at 1050nm 21 The mode self-coupling coefficient is about 4.04. According to the reflectivity formula, when L1 is 0.198 m, the LP at 1050 nm is 11 Mode reflectivity is 99.0%, 1050nm LP 01 The reflectivity of the mode is 11.7%, 1050nm LP 02 The reflectivity of the mode is 43.7%, 1050nm LP 21 The reflectivity of the mode is 44.1%. When L1 is 0.0216 m, the LP at 1050 nm 11 Mode reflectivity is 10.0%, 1050nm LP01 Mode reflectivity is 0.15%, 1050nm LP 02 Mode reflectivity is 0.75%, 1050nm LP 21 The reflectivity of the mode is 0.76%. For the LP reflecting 1080nm 01 The refractive index modulation area of ​​the small circular grating in the center of the mode, 1080nm LP 01 The mode self-coupling coefficient is about 122.61, LP at 1080nm 11 The mode self-coupling coefficient is about 2.19, LP at 1080nm 02 The mode self-coupling coefficient is about 12.25.

[0029] According to the reflectivity formula, when L3 is 0.0245 m, the LP of 1080 nm is 01 Mode reflectivity is 99.0%, 1080nm LP 11 The reflectivity of the mode is 0.287%, 1080nm LP 02 The reflectivity of the mode is 8.41%. When L4 is 0.00267 m, the LP at 1080 nm 01 Mode reflectivity is 10.0%, 1080nm LP 11 The reflectivity of the mode is 0.00342%, 1080nm LP 02 The reflectivity of the mode is 0.1056%.

[0030] In summary, compared with 25 Conventional gratings, through differential grating period and refractive index modulation, make 1080nm LP 01 mode and 1050nm LP 11 The modes undergo strong self-coupling, which not only achieves wavelength spacing expansion but also mode-selective reflection.

[0031] In another embodiment, a Bragg grating design method with multi-mode wavelength selection function is proposed, the goal is to achieve 1080nm LP 11 mode and 1050nm LP 01 The strong self-coupling of the mode can achieve the purpose of wavelength separation and mode selection. That is, in step 1, the target modes of the two target wavelengths are LP at 1080nm and LP at 1080nm. 11 mode, 1050nm LP 01 model.

[0032] In order to achieve 1080nm LP 11 mode and 1050nm LP 01 The strong self-coupling of the mode is known from the grating Bragg condition. In step 2, the 1080nm LP11 The grating period of the mode is 372nm, and the LP is 1050nm. 01 The grating period corresponding to the pattern is 362nm.

[0033] In this embodiment, in step 4, designing the radial refractive index distribution of the reflection area corresponding to the target modes of the two target wavelengths includes: (1) Design of 1080nm LP 11 The radial refractive index distribution of the reflection area corresponding to the mode is as follows: For 1080nm LP 11 The radial cross section of the core of the reflection area corresponding to the mode is divided into regions: Figure 3 As shown, with the core center as the center of the circle and also the origin of the xy coordinate system of the core radial section, determine the central circle area in the core radial section, and determine four arc areas with the core center as the center of the circle, symmetrically distributed about the x axis and symmetrically distributed about the y axis: Let is the radius of the center circle with the core center as the center, r1, r2, r3, r4, and r5 are respectively the core radius, the distance from the center to the outer ring, the distance from the center to the inner ring, the distance from the center to the outer diameter of the arc area, and the distance from the center to the inner diameter of the arc area. is the angle between the edge of the arc area close to the x-axis and the x-axis, is the angle between the edge of the arc area away from the x-axis and the x-axis, ; Preferred parameter settings of this embodiment: They are , angle They are 、 .

[0034] 1080nm LP 11 The four arc regions in the radial cross section of the core corresponding to the mode are used as the radial refractive index modulation regions of the grating. The distance from the four arc regions to the core center is The refractive index of the point is Determine, and thus obtain the radial refractive index distribution of the arc area, where is the refractive index of SiO2, the fiber matrix material, which is 1.45; is an intrinsic refractive index added to the fiber core, is an intrinsic refractive index added to the arc area, The distance from the center of the arc region to the center of the fiber core. The center of the arc region is the point in the arc region where the distances to the inner arc and outer arc of the arc region are equal. represents a Gaussian function with a standard deviation of 0.3 and a total integral value of 1. Represents the distance from any point in the four arc areas to the center of the fiber core; the second item is the refractive index modulation term of the arc grating area. Through the Gaussian modulation of the second term, the Gaussian peak is located at Pick When the arc modulation area matches the 1080nm LP 11 mode field distribution of the mode, while suppressing the 1050nm LP 01 Mode coupling to achieve mode selective reflection; 1080nm LP 11 The central circle area in the radial cross section of the core of the reflection area corresponding to the mode is the radial intrinsic refractive index area of ​​the grating. The distance from the central circle area to the core center is The refractive index of the point is Determine, thus obtaining the radial refractive index distribution of the central circle area, where Indicates the distance from any point in the central circle area to the center of the fiber core; 1080nm LP 11 The circular area between the outer ring and the inner ring in the radial cross section of the core corresponding to the reflection area of ​​the mode, the distance from the circular area to the core center is The refractive index of the point is Determine, thus obtaining the radial refractive index distribution of the annular region, where Indicates the distance from the center of the outer ring to the center of the circle. The center of the outer ring is the point in the ring area where the distances to the inner and outer rings are equal. Indicates the refractive index modulation of the outer ring, increasing the refractive index of the outer ring to make LP 21 The mode is bounded in the outer ring, preventing it from interfering with the LP 11 Patterns overlap.

[0035] 1080nm LP 11 The distance from the core center to the core center in the radial cross section of the core corresponding to the mode, except for the four arc areas, the central circle area, and the ring area between the outer ring and the inner ring, is The refractive index of the point is Determine, thereby determining the radial refractive index distribution in other areas of the core area except the four arc areas, the central circle area, and the ring area between the outer ring and the inner ring.

[0036] (2) Design of 1050nm LP 01 The radial refractive index distribution of the reflection area corresponding to the mode is as follows: For 1050nm LP 01The core radial section of the reflection area corresponding to the mode is divided into regions: the core center is taken as the center of the circle and also as the origin of the xy coordinate system of the core radial section, and the central circle area is determined in the core radial section. is the radius of the center circle with the core center as the center, r1, r2, and r3 are the core radius, the distance from the center to the outer ring, and the distance from the center to the inner ring, respectively. .

[0037] 1050nm LP 01 The central circle area of ​​the radial cross section of the core in the reflection area corresponding to the mode is the radial refractive index modulation area of ​​the grating, and the distance from the central circle area to the core center is The refractive index of the point is Determine, thereby determining the radial refractive index distribution of the central circle area, where Indicates the distance from any point in the central circle area to the center of the fiber core. The fourth term is the grating refractive index modulation term, and through the fourth Gaussian modulation, the Gaussian peak is located at When 0 is taken, the gate area and 1050nm LP are realized 01 The mode field overlap of the mode is much higher than that of 1080nm LP 11 mode field to reduce high-order mode self-coupling.

[0038] 1050nm LP 01 The circular area between the outer and inner rings of the radial cross section of the core corresponding to the mode is the radial intrinsic refractive index area of ​​the core, and the distance from the circular area between the outer and inner rings to the center of the core is The refractive index of the point is Determine the LP of 1050nm 01 The radial refractive index distribution of the annular region between the outer annular region and the inner annular region of the radial cross section of the fiber core corresponding to the mode, where It represents the distance from any point in the circular area between the outer ring and the inner ring to the center of the fiber core.

[0039] 1050nm LP 01 In the radial cross section of the core of the reflection area corresponding to the mode, except for the central circle area and the ring area between the outer ring and the inner ring, the rest of the core area is the inherent refractive index area of ​​the optical fiber. For the LP of 1050nm 01 The distance from the core center to the core center in the radial cross section of the core corresponding to the mode, except for the central circle area and the ring area between the outer ring and the inner ring, is The refractive index of the point is Determine the LP of 1050nm 01The radial refractive index distribution of the core region other than the central circle region and the annular region between the outer ring and the inner ring in the radial cross section of the core in the reflection region corresponding to the mode is: It represents the distance from any point in the core area except the central circle area and the circular area between the outer and inner rings to the core center.

[0040] Preferred parameter settings for this embodiment: is 1.45, is 0.0031, is 0.0005, is 0.01, for , for .

[0041] Reflective 1080nm LP 11 The lengths L5 and L6 of the symmetrical arc grating of the mode are 0.205m and 0.0224m respectively, reflecting the LP of 1050nm 01 The lengths of the small circular gratings L7 and L8 in the center of the pattern are 0.0237m and 0.0026m respectively.

[0042] For 1080nm LP 11 Symmetrical arc grating refractive index modulation area of ​​the mode, 1080nm LP 11 The mode self-coupling coefficient is about 14.60, LP at 1080nm 01 The mode self-coupling coefficient is about 1.75, LP at 1080nm 02 The mode self-coupling coefficient is about 3.90, 1080nm LP 21 The mode self-coupling coefficient is about 3.93. According to the reflectivity formula, when L5 is 0.205 m, the LP at 1080 nm is 11 Mode reflectivity is 99.0%, 1080nm LP 01 The reflectivity of the mode is 11.8%, 1080nm LP 02 The reflectivity of the mode is 44.1%, 1080nm LP 21 The reflectivity of the mode is 44.6%. When L6 is 0.0224m, the LP at 1080nm 11 Mode reflectivity is 10.0%, 1080nm LP 01 The reflectivity of the mode is 0.154%, 1080nm LP 02 The reflectivity of the mode is 0.762%, 1080nm LP 21 The reflectivity of the mode is 0.771%.

[0043] For 1050nm LP 01 The refractive index modulation area of ​​the small circular grating in the center of the mode, 1050nm LP 01 The mode self-coupling coefficient is about 126.12, LP at 1050nm 11 The mode self-coupling coefficient is about 2.26, LP at 1050nm 02 The mode self-coupling coefficient is about 12.61. According to the reflectivity formula, when L7 is 0.0237m, the LP at 1050nm is 01 Mode reflectivity is 99.0%, 1050nm LP 11 The reflectivity of the mode is 0.286%, 1050nm LP 02 The reflectivity of the mode is 8.35%. When L8 is 0.0026m, the LP at 1050nm 01 Mode reflectivity is 10.0%, 1050nm LP 11 The reflectivity of the mode is 0.00345%, 1050nm LP 02 The reflectivity of the mode is 0.107%.

[0044] In summary, compared with 25 Conventional gratings, through differential grating period and refractive index modulation, make 1050nm LP 01 mode and 1080nm LP 11 The modes undergo strong self-coupling, which not only achieves wavelength spacing expansion but also mode-selective reflection.

[0045] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions based on the principles of the present invention are within the scope of protection of the present invention. It should be noted that for those skilled in the art, various improvements and modifications that do not depart from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A Bragg grating design method with multi-mode wavelength selection function, characterized in that: include: Step 1: Determine the target mode of the two target wavelengths reflected by the Lager grating to be designed, which is the long-wave LP of the first target wavelength. 01 mode and the second target wavelength shortwave LP 11 model; Step 2: determine the grating period corresponding to the target pattern; Step 3, setting the reflection areas corresponding to the target modes of the two target wavelengths at different axial length positions of the grating fiber core; Step 4: Design the radial refractive index distribution of the reflection area corresponding to the target modes of the two target wavelengths so that the target modes of the two target wavelengths can achieve mode field separation.

2. The Bragg grating design method with multi-mode wavelength selection function according to claim 1, characterized in that: In step 1, the target modes of the two target wavelengths are LP at 1050nm and LP at 1050nm. 11 mode, 1080nm LP 01 model.

3. The Bragg grating design method with multi-mode wavelength selection function according to claim 2, characterized in that: In step 2, 1050nm LP 11 The grating period of the mode is 362 nm, and the LP of 1080 nm 01 The grating period of the pattern is 372 nm.

4. The Bragg grating design method with multi-mode wavelength selection function according to claim 3, characterized in that: In step 4, the radial refractive index distribution of the reflection area corresponding to the target modes of the two target wavelengths is designed, including: (1) Design of 1050nm LP 11 The radial refractive index distribution of the reflection area corresponding to the mode is as follows: For 1050nm LP 11 The core radial section of the reflection area corresponding to the mode is divided into regions: the core center is taken as the center of the circle and is also the origin of the xy coordinate system of the core radial section. The central circle area is determined in the core radial section, and four arc areas are determined with the core center as the center of the circle and are symmetrically distributed about the x axis and symmetrically distributed about the y axis: Let is the radius of the center circle with the core center as the center, r1, r2, r3, r4, and r5 are respectively the core radius, the distance from the center to the outer ring, the distance from the center to the inner ring, the distance from the center to the outer diameter of the arc area, and the distance from the center to the inner diameter of the arc area. is the angle between the edge of the arc area close to the x-axis and the x-axis, is the angle between the edge of the arc area away from the x-axis and the x-axis, ; 1050nm LP 11 The four arc regions in the radial cross section of the core corresponding to the mode are used as the radial refractive index modulation regions of the grating. The distance from the four arc regions to the core center is The refractive index of the point is Determine, and thus obtain the radial refractive index distribution of the arc area, where is the refractive index of SiO2, the fiber matrix material, which is 1.45; is an intrinsic refractive index added to the fiber core, is an intrinsic refractive index added to the arc area, The distance from the center of the arc region to the center of the fiber core. The center of the arc region is the point in the arc region where the distances to the inner arc and outer arc of the arc region are equal. represents a Gaussian function with a standard deviation of 0.3 and a total integral value of 1; 1050nm LP 11 The central circle area in the radial cross section of the core of the reflection area corresponding to the mode is the radial intrinsic refractive index area of ​​the grating. The distance from the central circle area to the core center is The refractive index of the point is Determine, thereby obtaining the radial refractive index distribution of the central circle area; 1050nm LP 11 The circular area between the outer ring and the inner ring in the radial cross section of the core corresponding to the reflection area of ​​the mode, the distance from the circular area to the core center is The refractive index of the point is Determine, thus obtaining the radial refractive index distribution of the annular region, where Indicates the distance from the center of the outer ring to the center of the circle. The center of the outer ring is the point in the ring area where the distances to the inner and outer rings are equal. represents the refractive index modulation of the outer ring; 1050nm LP 11 The distance from the core center to the core center in the radial cross section of the core corresponding to the mode, except for the four arc areas, the central circle area, and the ring area between the outer ring and the inner ring, is The refractive index of the point is Determine, thereby determining the radial refractive index distribution in other areas of the core region except the four arc areas, the central circle area, and the annular area between the outer ring and the inner ring; (2) Design of 1080nm LP 01 The radial refractive index distribution of the reflection area corresponding to the mode is as follows: For 1080nm LP 01 The core radial section of the reflection area corresponding to the mode is divided into regions: the core center is taken as the center of the circle and also as the origin of the xy coordinate system of the core radial section, and the central circle area is determined in the core radial section. is the radius of the center circle with the core center as the center, r1, r2, and r3 are the core radius, the distance from the center to the outer ring, and the distance from the center to the inner ring, respectively. ; 1080nm LP 01 The central circle area of ​​the radial cross section of the core in the reflection area corresponding to the mode is the radial refractive index modulation area of ​​the grating, and the distance from the central circle area to the core center is The refractive index of the point is Determine, thereby determining the radial refractive index distribution of the central circle area; 1080nm LP 01 The circular area between the outer and inner rings of the radial cross section of the core corresponding to the mode is the radial intrinsic refractive index area of ​​the core, and the distance from the circular area between the outer and inner rings to the center of the core is The refractive index of the point is Determine the LP of 1080nm 01 The radial refractive index distribution of the annular region between the outer annular region and the inner annular region of the radial cross section of the fiber core in the reflection region corresponding to the mode; 1080nm LP 01 In the radial cross section of the core of the reflection area corresponding to the mode, except for the central circle area and the ring area between the outer ring and the inner ring, the rest of the core area is the inherent refractive index area of ​​the optical fiber. For the 1080nm LP 01 The distance from the core center to the core center in the radial cross section of the core corresponding to the mode, except for the central circle area and the ring area between the outer ring and the inner ring, is The refractive index of the point is Determine the LP of 1080nm 01 The radial refractive index distribution of other areas of the core region except the central circle area and the annular area between the outer ring and the inner ring in the radial cross section of the core of the reflection area corresponding to the mode.

5. The Bragg grating design method with multi-mode wavelength selection function according to claim 1, characterized in that: In step 1, the target modes of the two target wavelengths are LP at 1080nm and LP at 1080nm. 11 mode, 1050nm LP 01 model.

6. The method for designing a Bragg grating with a multi-mode wavelength selection function according to claim 5, wherein: In step 2, 1080nm LP 11 The corresponding grating period of the mode is 372 nm, and the LP of 1050 nm 01 The grating period of the pattern is 362 nm.

7. The method for designing a Bragg grating with a multi-mode wavelength selection function according to claim 6, wherein: In step 4, the radial refractive index distribution of the reflection area corresponding to the target modes of the two target wavelengths is designed, including: (1) Design of 1080nm LP 11 The radial refractive index distribution of the reflection area corresponding to the mode is as follows: For 1080nm LP 11 The core radial section of the reflection area corresponding to the mode is divided into regions: the core center is taken as the center of the circle and is also the origin of the xy coordinate system of the core radial section. The central circle area is determined in the core radial section, and four arc areas are determined with the core center as the center of the circle and are symmetrically distributed about the x axis and symmetrically distributed about the y axis: Let is the radius of the center circle with the core center as the center, r1, r2, r3, r4, and r5 are respectively the core radius, the distance from the center to the outer ring, the distance from the center to the inner ring, the distance from the center to the outer diameter of the arc area, and the distance from the center to the inner diameter of the arc area. is the angle between the edge of the arc area close to the x-axis and the x-axis, is the angle between the edge of the arc area away from the x-axis and the x-axis, ; 1080nm LP 11 The four arc regions in the radial cross section of the core corresponding to the mode are used as the radial refractive index modulation regions of the grating. The distance from the four arc regions to the core center is The refractive index of the point is Determine, and thus obtain the radial refractive index distribution of the arc area, where is the refractive index of SiO2, the fiber matrix material, which is 1.45; is an intrinsic refractive index added to the fiber core, is an intrinsic refractive index added to the arc area, The distance from the center of the arc region to the center of the fiber core. The center of the arc region is the point in the arc region where the distances to the inner arc and outer arc of the arc region are equal. represents a Gaussian function with a standard deviation of 0.3 and a total integral value of 1; 1080nm LP 11 The central circle area in the radial cross section of the core of the reflection area corresponding to the mode is the radial intrinsic refractive index area of ​​the grating. The distance from the central circle area to the core center is The refractive index of the point is Determine, thereby obtaining the radial refractive index distribution of the central circle area; 1080nm LP 11 The circular area between the outer ring and the inner ring in the radial cross section of the core corresponding to the reflection area of ​​the mode, the distance from the circular area to the core center is The refractive index of the point is Determine, thus obtaining the radial refractive index distribution of the annular region, where Indicates the distance from the center of the outer ring to the center of the circle. The center of the outer ring is the point in the ring area where the distances to the inner and outer rings are equal. represents the refractive index modulation of the outer ring; 1080nm LP 11 The distance from the core center to the core center in the radial cross section of the core corresponding to the mode, except for the four arc areas, the central circle area, and the ring area between the outer ring and the inner ring, is The refractive index of the point is Determine, thereby determining the radial refractive index distribution in other areas of the core region except the four arc areas, the central circle area, and the annular area between the outer ring and the inner ring; (2) Design of 1050nm LP 01 The radial refractive index distribution of the reflection area corresponding to the mode is as follows: For 1050nm LP 01 The core radial section of the reflection area corresponding to the mode is divided into regions: the core center is taken as the center of the circle and also as the origin of the xy coordinate system of the core radial section, and the central circle area is determined in the core radial section. is the radius of the center circle with the core center as the center, r1, r2, and r3 are the core radius, the distance from the center to the outer ring, and the distance from the center to the inner ring, respectively. ; 1050nm LP 01 The central circle area of ​​the radial cross section of the core in the reflection area corresponding to the mode is the radial refractive index modulation area of ​​the grating, and the distance from the central circle area to the core center is The refractive index of the point is Determine, thereby determining the radial refractive index distribution of the central circle area; 1050nm LP 01 The circular area between the outer and inner rings of the radial cross section of the core corresponding to the mode is the radial intrinsic refractive index area of ​​the core, and the distance from the circular area between the outer and inner rings to the center of the core is The refractive index of the point is Determine the LP of 1050nm 01 The radial refractive index distribution of the annular region between the outer annular region and the inner annular region of the radial cross section of the fiber core in the reflection region corresponding to the mode; 1050nm LP 01 In the radial cross section of the core of the reflection area corresponding to the mode, except for the central circle area and the ring area between the outer ring and the inner ring, the rest of the core area is the inherent refractive index area of ​​the optical fiber. For the LP of 1050nm 01 The distance from the core center to the core center in the radial cross section of the core corresponding to the mode, except for the central circle area and the ring area between the outer ring and the inner ring, is The refractive index of the point is Determine the LP of 1050nm 01 The radial refractive index distribution of other areas of the core region except the central circle area and the annular area between the outer ring and the inner ring in the radial cross section of the core of the reflection area corresponding to the mode.

8. The Bragg grating design method with multi-mode wavelength selection function according to claim 4 or 7, characterized in that: They are , angle They are 、 .

9. The method for designing a Bragg grating with a multi-mode wavelength selection function according to claim 8, wherein: is 1.45, is 0.0031, is 0.0005, is 0.01, for , for .

10. Bragg grating, characterized in that The method is designed based on the Bragg grating design method with multi-mode wavelength selection function as claimed in claim 1.

Images