Design method of super-structure fiber bragg grating single channel filter and filter

Abstract

The application provides a design method of a super-structure few-mode fiber Bragg grating single-channel filter and the filter, and comprises the following steps: calculating the guided modes which can be stably transmitted in the few-mode fiber, selecting two guided modes from the guided modes which can be stably transmitted, and selecting the guided mode with larger effective refractive index as a design guided mode; calculating the Bragg period corresponding to the double-mode filtering and the design guided mode according to the wavelength of the double-mode filtering which needs to be realized, and calculating the displacement between the spectrum reflection spectrums of the two selected guided modes; designing the wavelength of the first reflection peak center of the design guided mode and the wavelength of the second reflection peak center of the design guided mode according to the wavelength of the double-mode filtering which needs to be realized; representing the total reflection spectrum composed of the first reflection peak of the design guided mode and the second reflection peak of the design guided mode by a super-Gaussian function; and calculating the physical structure of the super-structure few-mode fiber Bragg grating according to the total reflection spectrum. The filter adopting the application can be applied to a mode division multiplexing (MDM) communication system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of filter, and particularly relates to a design method of a super-structure few-mode fiber Bragg grating single-channel filter and the filter. BACKGROUND

[0002] At present, the fifth generation mobile communication technology has been widely applied in commercial fields, and its brand-new network architecture brings higher peak rate, millisecond-level latency and trillion-level connection capability, lays a foundation for realizing extensive interconnection of all things and deep interaction between man and machine, but at the same time, puts forward higher demand for the capacity of the communication system. At present, the road of improving the spectral efficiency and transmission rate of the optical fiber communication system through wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) technology has been fully explored, and mode division multiplexing (MDM) will become an important technology for further improving the capacity of the optical fiber communication system and realizing higher speed communication.

[0003] Mode is the inherent electromagnetic resonance property representation of the waveguide structure. For the optical fiber waveguide, the mode can be understood as the state in which the light wave energy can be stably transmitted in the optical fiber waveguide; by giving the boundary condition of the optical fiber waveguide, the eigenvalue solution is obtained by solving the waveguide field equation, each eigenvalue solution corresponds to a mode, also known as a guided mode, and the field distribution corresponding to the mode eigenvalue solution can be obtained. The field distribution between different guided modes is independent of each other, so the signals of different guided modes transmitted at the same wavelength are distinguishable in the transverse mode field, which is the physical prerequisite for realizing the MDM technology. Mode division multiplexing (MDM) technology refers to the technology that multiple orthogonal modes carrying different information are transmitted in the same few-mode or multi-mode optical waveguide. At the sending end, the mode division multiplexer is used to couple the optical carriers transmitted by the base modes to the base mode and different high-order modes and send them into the few-mode optical fiber for transmission; at the receiving end or optical node, the mode division demultiplexer is used to demultiplex the optical signals, so as to realize the transmission of multiple signals through one few-mode optical fiber.

[0004] Based on the above MDM technology principle, the MDM technology can further improve the capacity of the communication system in a new dimension. At present, the design scheme of the two-dimensional multiplexing communication system based on the combination of the MDM technology and the WDM technology has been proposed, and the two-dimensional multiplexing of the combination of the MDM technology and the WDM technology is an important development direction of the future optical fiber communication system, so as to realize the further increase of the capacity of the communication system. For the MDM+WDM two-dimensional multiplexing optical communication system, in order to realize the two-dimensional multiplexing, the problem of parallel processing of the multi-dimensional multiplexing signals at the optical node or the receiving end must be solved, and the super-structure few-mode fiber Bragg grating single-channel filter based on the two-mode division multiplexing can be used as the splitter and the code type converter in the MDM+WDM two-dimensional multiplexing optical fiber communication system.

[0005] The application of MDM+WDM two-dimensional multiplexing fiber communication system must solve the problem of parallel processing of two-dimensional multiplexing signals, however, there is no solution for simultaneously performing optical processing or optical filtering on two or more guided modes at a target wavelength in the MDM+WDM fiber communication system, which is an important problem to be urgently researched and solved. SUMMARY

[0006] In order to overcome the above technical defects, the present application provides a design method of a superstructure few-mode fiber Bragg grating single-channel filter and the filter, which is suitable for a mode division multiplexing (MDM) system with two-mode multiplexing.

[0007] The present application is realized by the following scheme:

[0008] A design method of a superstructure few-mode fiber Bragg grating single-channel filter, comprising the steps of:

[0009] calculating the guided modes that can be stably transmitted in the few-mode fiber, selecting two guided modes as the transmission modes of the information of the MDM system with two-mode multiplexing from the guided modes that can be stably transmitted, and selecting the guided mode with a larger effective refractive index as a design guided mode from the two guided modes;

[0010] calculating the Bragg period corresponding to the design guided mode according to the wavelength of the two-mode filter to be realized ; ;

[0011] setting the wavelength of the center of the first reflection peak as ; ;

[0012] setting the wavelength of the center of the second reflection peak as ; ; ;

[0013] using a super-Gaussian function to represent the total reflection spectrum composed of the first reflection peak and the second reflection peak;

[0014] calculating the physical structure of the superstructure few-mode fiber Bragg grating according to the total reflection spectrum.

[0015] As a further improvement of the present application, the physical structure of the superstructure few-mode fiber Bragg grating is obtained by iterative calculation and solution through the inverse scattering technique in the layer peeling algorithm.

[0016] Furthermore, the application also provides an ultra-structure few-mode fiber Bragg grating single-channel filter, which is designed by using the above-mentioned design method and comprises an ultra-structure few-mode fiber Bragg grating, an optical input end, a first optical output end and a second optical output end.

[0017] The optical input end is physically coincident with the first optical output end, and the first optical output end is connected with one end of the ultra-structure few-mode fiber Bragg grating; the first optical output end serves as an input end of an original optical signal and an output end of an optical signal with two guided modes at a central wavelength after the original optical signal is filtered by the ultra-structure few-mode fiber Bragg grating.

[0018] The second optical output end is connected with the other end of the ultra-structure few-mode fiber Bragg grating; and the second optical output end serves as an output end of an optical signal with two guided modes at a non-central wavelength after the original optical signal is filtered by the ultra-structure few-mode fiber Bragg grating.

[0019] Compared with the prior art, the application has the following beneficial effects:

[0020] The application calculates the effective refractive indexes of two guided modes according to the specifications of the few-mode fiber used, designs a target response spectrum according to the difference between the effective refractive indexes, realizes the peak-misalignment alignment of the response spectra of different guided modes, and finally realizes the bimode filtering at the target wavelength. The application can be used in a mode division multiplexing system in which two modes serve as guided modes. BRIEF DESCRIPTION OF DRAWINGS

[0021] The specific embodiments of the application will be further described in detail below with reference to the accompanying drawings, in which:

[0022] Figure 1 The flow chart of the design method described in the embodiment 1;

[0023] Figure 2 The structure curve diagram of the ultra-structure few-mode fiber Bragg grating single-channel filter described in the embodiment 1;

[0024] Figure 3 The frequency spectrum response curve diagram of the ultra-structure few-mode fiber Bragg grating single-channel filter described in the embodiment 1;

[0025] Figure 4 The structure schematic diagram of the ultra-structure few-mode fiber Bragg grating single-channel filtering and light splitting device described in the embodiment 4;

[0026] Mark description: 1, ultra-structure few-mode fiber Bragg grating; 2, optical input end; 3, first optical output end; 4, second optical output end. DETAILED DESCRIPTION

[0027] The preferred embodiments of the present application will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are merely intended to illustrate and explain the present application, and are not intended to limit the present application.

[0028] It should be noted that similar reference numerals and letters refer to similar items throughout the accompanying drawings, and thus, once an item is defined in one drawing, it need not be further defined and explained in subsequent drawings. Meanwhile, in the description of the present application, the serial numbers of the steps are merely used to distinguish the steps from each other, and do not mean that each step needs to be strictly executed in the order of the serial numbers.

[0029] Embodiment 1

[0030] The present embodiment provides a design method of a super-structure few-mode fiber Bragg grating single-channel filter, as shown in FIG. 1, comprising the steps of: Figure 1

[0031] S1, calculating the guided modes that can be stably transmitted in the few-mode fiber by using formulas (1)-(3), selecting two guided modes as the two-mode division multiplexing (MDM) system information transmission guided modes from the guided modes that can be stably transmitted, and selecting the guided mode with a larger effective refractive index as the design guided mode.

[0032] (1)

[0033] (2)

[0034] (3)

[0035] wherein, is the l-th Bessel function, is the l-th inverse Bessel function, and is the transverse oscillation frequency of the standing wave field of the guided mode in the fiber core, is the core diameter of the few-mode fiber, is the refractive index of the core of the few-mode fiber, is the wave number of light in vacuum, is the effective refractive index of the guided mode in the few-mode fiber.

[0036] S2, calculating the Bragg period corresponding to the design guided mode of the two-mode filter according to the wavelength to be achieved by the two-mode filter, and calculating the displacement difference between the spectral reflection spectra of the two selected guided modes.

[0037] ​​For the guided modes that can be stably transmitted in the optical fiber, the propagation constant and the effective refractive index between different guided modes generally are not the same, so there is mode dispersion between different guided modes in the optical fiber. For the superstructure fiber Bragg grating, because the propagation constant and the effective refractive index between different guided modes are different, the response spectrum of different guided modes through the same superstructure fiber Bragg grating is also different, and the response spectrum of different guided modes for the same superstructure fiber Bragg grating can be regarded as having a displacement, which can be calculated by formula (4) and (5):

[0038] (4)

[0039] (5)

[0040] wherein, is the center wavelength of the first reflection peak, is the center wavelength of the second reflection peak, is the effective refractive index of the designed guided mode, is the effective refractive index of another guided mode, is the Bragg period corresponding to the center wavelength of the double-mode filtering (the first reflection peak).

[0041] S3, according to the wavelength of the double-mode filtering , the center wavelength of the first reflection peak is set as ;

[0042] S4, according to the center wavelength of the first reflection peak , the displacement between the spectral reflection spectrum of the two guided modes , the center wavelength of the second reflection peak is set as ;

[0043] Because there is a difference in the effective refractive index between different guided modes, the reflection spectrum of different modes cannot be completely coincided, to realize the optical filtering of the double guided modes at the target center wavelength, the reflection spectrum must be designed to have two reflection peaks (for convenience of description, the two reflection peaks of the designed guided mode are called the first reflection peak and the second reflection peak), so as to realize the peak shift coincidence. By setting the interval between the two reflection peaks as , the reflection spectrum of the two guided modes at the target wavelength can be as much as possible to shift and coincide, so as to realize the optical filtering of the double guided modes at the target wavelength.

[0044] S5, the total reflection spectrum composed of the first reflection peak and the second reflection peak is represented by a super-Gaussian function.

[0045] S6. According to the total reflection spectrum, the refractive index modulation function and the local chirp function for representing the structure of the super-structured few-mode fiber Bragg grating are obtained by iterative calculation through the inverse scattering technique in the layer stripping algorithm. Subsequently, the transmission spectrum and the reflection spectrum of the fiber grating are simulated by using the transmission matrix method and the calculated refractive index modulation function and the grating chirp function.

[0046] After the physical structure of the super-structured few-mode fiber Bragg grating is determined through the above steps, the grating can be manufactured by using the following known methods, such as the point-by-point writing method, the two-time exposure method, the fiber / mask moving scanning method, the phase mask stretching scanning method, etc. For the super-structured few-mode fiber Bragg grating in the present application, the structure parameters are obtained after the individual design according to the specific filtering requirements, and the structure of the grating is different for different filtering requirements. The specific manufacturing method can be referred to the prior art, which will not be described herein.

[0047] Embodiment 2

[0048] The present embodiment provides another design method of the super-structured few-mode fiber Bragg grating single-channel filter, the two-mode multiplexing is at 1550.12 nm, the channel bandwidth of the super-structured few-mode fiber Bragg grating is 0.4 nm, and the reflectivity is above 95%. The specifications of the few-mode fiber used are: core diameter a = 14 nm, core refractive index = 1.4485, cladding refractive index = 1.44402.

[0049] The design method comprises the following steps:

[0050] S1. The guided modes that can be stably transmitted in the above super-structured few-mode fiber Bragg grating are calculated according to the formulas (1)-(3). According to the given fiber specifications, the guided modes that can be transmitted in the few-mode fiber are: the effective refractive index of the LP01 mode in the few-mode fiber is 1.44804, the effective refractive index of the LP02 mode in the few-mode fiber is 1.44613, the effective refractive index of the LP11 mode in the few-mode fiber is 1.44733, the effective refractive index of the LP12 mode in the few-mode fiber is 1.44479, the effective refractive index of the LP21 mode in the few-mode fiber is 1.44642, the effective refractive index of the LP31 mode in the few-mode fiber is 1.44534, and the effective refractive index of the LP41 mode in the few-mode fiber is 1.44410. According to the actual requirements, the modes LP01 and LP11 are selected as the guided modes for transmitting data.

[0051] From the modes LP01 and LP11, the LP01 with the larger effective refractive index is selected as the design guided mode.

[0052] S2: The double-mode filtering at 1550.12 nm is achieved according to the requirement, and the spectral shift of the mode LP01 and the mode LP11 to the super-structure few-mode fiber Bragg grating is calculated The formula (4) and (5) can be used for calculation.

[0053] According to the calculation of the formula (5), the Bragg period corresponding to the mode LP01 at the double-mode filtering wavelength (1550.12 nm) is 535.25 nm, and the spectral shift difference between the two guided modes due to the intermodal dispersion is The calculation result of the formula (4) is 0.76 nm.

[0054] S2: The double-mode filtering at 1550.12 nm is achieved according to the requirement, and the first reflection peak center is designed at 1550.12 nm. According to the actual requirement of the channel bandwidth of 0.4 nm and the reflectivity of 95%, the super-Gaussian function is used to mathematically describe the first reflection peak as

[0055]

[0056] wherein , is a unit imaginary number.

[0057] S3: The second reflection peak center wavelength can be calculated according to the formula (6).

[0058] The calculation result of the second reflection peak center is The super-Gaussian function is used to mathematically describe the second reflection peak as

[0059]

[0060] wherein .

[0061] S4: The super-Gaussian function is used to mathematically describe the total reflection spectrum.

[0062]

[0063] wherein .

[0064] S5: The grating segmentation number M=1000 is taken, and the layer stripping algorithm is used to calculate and solve the physical structure parameters of the super-structure Bragg grating.

[0065] According to the refractive index modulation function and the local chirp function of the super-structure few-mode fiber Bragg grating obtained by the above steps, the transmission matrix analysis method is used to analyze the reflection spectrum of the mode LP01 and the mode LP11 through the above super-structure few-mode fiber Bragg grating as Figure 2 and Figure 3As shown in the characteristic curve in the figure, the designed super-structured few-mode fiber Bragg grating meets the given requirements, realizes double-mode reflection filtering at 1550.12 nm, and the reflection bandwidth is 0.4 nm, which meets the design requirements.

[0066] After the structure of the super-structured few-mode fiber Bragg grating is determined through the above steps, the grating can be manufactured by the following known methods: point-by-point writing method, secondary exposure method, fiber / mask moving scanning method, phase mask stretching scanning method, etc. For the super-structured few-mode fiber Bragg grating in the present application, the structure parameters are obtained after individual design according to specific filtering requirements. Different filtering requirements result in different grating structures. The specific manufacturing method can be referred to the prior art, which will not be described here.

[0067] Example 3

[0068] The present embodiment provides a super-structured few-mode fiber Bragg grating single-channel filtering and splitting device, which is designed by the design method in Example 1, as shown in Figure 4 including: a super-structured few-mode fiber Bragg grating 1, a light input end 2, a first light output end 3, a second light output end 4, and a multi-port optical directional coupling device; the first end of the multi-port optical directional coupling device is connected with the light input end 2, the second end of the multi-port optical directional coupling device is connected with one end of the super-structured few-mode fiber Bragg grating 1, and the first light output end 3 is connected with one end of the super-structured few-mode fiber Bragg grating 1; the second light output end 4 is connected with the other end of the super-structured few-mode fiber Bragg grating 1; a plurality of wavelength original input signals enter the first end of the multi-port optical directional coupling device and the super-structured few-mode fiber Bragg grating 1 in sequence from the light input end 2, wherein the two central wavelength lights After being filtered and reflected by the super-structured few-mode fiber Bragg grating 1, the lights are output to the multi-port optical directional coupling device through the first light output end 3, and then output from the third port of the multi-port optical directional coupling device after splitting; the non-central wavelength lights are transmitted through the super-structured few-mode fiber Bragg grating 1 and output from the second light output end 4.

[0069] According to the above examples, the filter of the present application can realize the filtering processing of two modes at the target wavelength at the same time, and the two guided modes transmitted in the super-structured few-mode fiber Bragg grating at the target wavelength The reflection spectrum peak values of the filter are uniform, the channel performance is consistent, and the filter has good filtering performance; the overall structure of the filtering scheme is simple, no additional active or passive device is needed to assist filtering, and only the physical structure of the superstructure few-mode fiber Bragg grating needs to be designed; the input signal and the filtered output signal can be further separated by connecting a multi-port optical directional coupling device, facilitating further processing; the grating structure of the filter can be flexibly designed according to specific use scene requirements, and can be applied to the fields of optoelectronics, fiber communication, fiber sensing and the like.

[0070] The above are only preferred embodiments of the present application, and are not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A design method for a superstructure few-mode fiber Bragg grating single-channel filter, characterized in that, Including the following steps: Calculate the stable transmission guided modes in the few-mode fiber, select two guided modes from the stable transmission guided modes as the transmission guided modes of the MDM system information multiplexed by the two modes, and select the guided mode with the larger effective refractive index from the two guided modes as the design guided mode. Based on the wavelength of the dual-mode filter to be implemented Calculate the Bragg period corresponding to the dual-mode filter and the designed guided mode, and use this to calculate the shift between the spectral reflectance spectra of the two selected guided modes. ; Based on the wavelength of the dual-mode filter to be implemented The wavelength of the center of the first reflection peak is set to ; Based on the center wavelength of the first reflection peak The shift between the spectral reflection spectra of the two guided modes The wavelength of the center of the second reflection peak is set to ; The total reflection spectrum composed of the first and second reflection peaks is represented by a super-Gaussian function; Based on the total reflection spectrum, the physical structure of the superstructure few-mode fiber Bragg grating is calculated, including: iteratively calculating and solving for the refractive index modulation function and local chirp function to represent the structure of the superstructure few-mode fiber Bragg grating using the inverse scattering technique in the layer stripping algorithm; subsequently, using the transfer matrix method, the transmission spectrum and reflection spectrum of the fiber grating are simulated from the refractive index modulation function and local chirp function calculated above.

2. A superstructure few-mode fiber Bragg grating single-channel filter, characterized in that, The design, obtained using the design method as described in claim 1, includes: a superstructure few-mode fiber Bragg grating, an optical input end, a first optical output end, and a second optical output end; The optical input end and the first optical output end are physically coincident, and the first optical output end is connected to one end of the superstructure few-mode fiber Bragg grating; the first optical output end serves as the input end of the original optical signal and the output end of the optical signal of the two guided modes at the center wavelength after being filtered by the superstructure few-mode fiber Bragg grating. The second optical output end is connected to the other end of the superstructure few-mode fiber Bragg grating; the second optical output end serves as the output end of the optical signal at the non-center wavelength of the two guided modes after the original signal is filtered by the superstructure few-mode fiber Bragg grating.

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