Broadband mode filter based on three-core photonic crystal fiber

Abstract

The invention discloses a broadband mode filter based on a three-core photonic crystal fiber, and belongs to the technical field of optical fiber communication, and the broadband mode filter comprises a substrate which is internally provided with a plurality of layers of cladding air holes; the multi-layer cladding air hole comprises a main core, a first bypass core, a second bypass core, a first layer of air holes, a second layer of air holes and a third layer of air holes, wherein the first layer of air holes, the second layer of air holes and the third layer of air holes are of regular hexagon structures from inside to outside. The first bypass core and the second bypass core are located on the left side and the right side of the main core respectively. The main core of the mode filter can support three modes of LP01, LP11 and LP21, and the LP01 and LP11 modes in the main core can be coupled with the LP01 mode in the bypass core by adjusting the radius and the distance of the two bypass core cladding circular air holes and the thickness of the gold films on the inner walls of the bypass core cladding air holes and selecting the optimal device length, so that the coupling between the LP01 mode and the LP11 mode in the main core and the LP21 mode in the bypass core is realized. The mode coupled to the bypass core is lost after being transmitted for a short distance in the bypass core, and finally, the mode filter only outputs a single LP21 mode in a specific wavelength range.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber communication, in particular to a broadband mode filter based on three-core photonic crystal optical fiber. Background Art

[0002] With the continuous advancement of technologies such as wavelength division multiplexing (WDM), polarization division multiplexing (PDM), mode division multiplexing (MDM), and high-order modulation, the transmission rate and capacity of optical fiber communication systems have been greatly improved. However, due to fiber dispersion and nonlinear effects, the transmission capacity of single-mode optical fiber is gradually approaching the Shannon limit, making it difficult for traditional single-mode optical fiber to meet the network data traffic requirements of various bandwidth-enhanced Internet services. Mode division multiplexing (MDM) technology has attracted the attention of numerous researchers as an effective method to increase the capacity of optical fiber communication systems. It utilizes multiple modes to transmit data simultaneously within the same optical fiber, thereby significantly improving the optical fiber's data transmission capacity. However, high intermodal crosstalk (IMC) is one of the major challenges limiting the performance of MDM systems. Efficient mode filtering technology is considered an effective method to reduce IMC. Furthermore, with the in-depth application of MDM technology in fields such as quantum communication and optical fiber sensing, the demand for mode control devices such as MDMs, mode converters, and mode filters is also increasing. Among them, mode filters with large bandwidth, low insertion loss, low crosstalk, and compact structure are becoming particularly important. For example, in fiber-optic communication / fiber-optic sensing systems, wide-bandwidth mode filters can operate over a wide wavelength range, thereby increasing the system's wavelength flexibility and application range. At the same time, low insertion loss can reduce the power consumption of the fiber-optic system, and low crosstalk can ensure that non-target modes are effectively suppressed, thereby improving the overall performance of the fiber-optic communication / fiber-optic sensing system. Therefore, the research on high-performance mode filters is of great significance to promoting the development of fiber-optic communication, fiber-optic sensing, and related optical quantum information technologies, and has become a research hotspot in the field of information photonics technology.

[0003] Reported mode filters are mainly divided into two types according to their working methods: direct filtering and indirect filtering. Direct filtering is based on the difference in loss between different modes (usually low-order modes have lower loss, while high-order modes have larger loss). By designing a specific optical fiber (or optical waveguide) structure, or by bending the optical fiber (or optical waveguide), or introducing additional loss mechanisms, the loss of the high-order mode is significantly increased, thereby effectively filtering out the specific mode. This method is simple and direct, but it is easy to introduce large losses, thereby increasing the overall power consumption of the system. Indirect filtering relies on a carefully designed optical waveguide structure to achieve energy conversion between modes. This usually involves mode conversion devices (such as waveguide- or grating-based mode converters), which can not only convert unwanted modes into other modes, but also guide them into branch waveguide paths that do not participate in transmission, and effectively filter out unwanted modes by adding specific loss mechanisms.

[0004] From a structural perspective, mode filters can be divided into two types: grating type and fiber type. Grating type mode filters mainly rely on complex waveguide structures to achieve energy conversion between modes by indirectly filtering out the mode. In 2014, Hu et al. proposed a mode filter based on a dual-mode fiber Bragg grating. This device can normally transmit LP at a wavelength of 1065.7nm. 11 mode, while LP 01 The mode causes a loss of 5.98dB. In 2016, Chen et al. designed a mode filter based on long-period fiber grating. By cascading few-mode long-period fiber gratings, it can effectively filter out LP within a length of 140mm. 01 mode, and provides a bandwidth of more than 20nm near the wavelength of 1.55μm. In 2018, Yu et al. designed a mode filter using cascaded plasma bridged subwavelength gratings. The device supports both TE0 and TE1 modes, and can filter out the TE0 mode within a bandwidth of 35nm and the TE1 mode within a bandwidth of 26nm, respectively. The insertion loss of the corresponding two modes is more than 20dB. In 2020, Ling et al. designed a mode filter based on few-mode long-period fiber gratings. The device supports LP01 and LP 11 Two modes, and can effectively filter out LP within a device length of 34-46mm 01 Although the grating-type mode filter can achieve the mode filtering function, it still has the disadvantages of long device length or narrow bandwidth.

[0005] In recent years, photonic crystal fiber (PCF) type mode filters have received widespread attention from researchers. PCF is a microstructured optical fiber with a nanometer-scale periodic air hole distribution in its cross section. The flexible cross-sectional design freedom gives PCF a unique advantage in adjusting the light transmission characteristics. By rationally designing the arrangement of micro air holes in the optical fiber cross section, it can exhibit unique light-guiding properties such as large mode area, high nonlinearity, high birefringence, and ultra-flat dispersion. In addition, studies have shown that by rationally designing the microstructure of PCF, it can also achieve mode-selective transmission. In 2015, Chen et al. proposed a high-refractive-index core photonic bandgap fiber mode filter that supports LP 01 LP 11 LP 21 and LP 02 Four modes, through the band gap effect and refractive index guiding effect, the unwanted modes are coupled to the cladding and filtered out. The bandwidth of the device can reach 200nm. However, this design requires the use of three materials with different refractive index coefficients, and the manufacturing process is relatively complicated. In 2019, Yue et al. proposed a surface plasmon-based filter mode fiber. This structure adds gold wire to the outside of the cladding to make the cladding mode produce high leakage loss, and finally achieves the effect of filtering the mode by coupling the cladding mode with the mode to be filtered out of the core. The device can effectively filter out the LP in the main core in the wavelength range of 1534-1570nm. 11 mode, and the device length is only 50mm. However, this design also requires the use of materials with different refractive index coefficients, and the manufacturing process is complicated. In the same year, Dai et al. proposed a petal-shaped microstructured mode filter fiber based on high-refractive index liquid filling. The device adds liquid material to the cladding air hole, uses the thermo-optical effect of the liquid material, and changes the cladding refractive index by adjusting the material temperature, so that the mode to be filtered is coupled with the cladding mode to achieve the mode filtering effect. The device can effectively filter the mode in the wavelength range of 1.53-1.57μm when the length is 71.4mm. However, the performance of the device depends on the thermal stability of the liquid material, which seriously restricts its reliability in engineering applications. Therefore, it is particularly urgent to develop a mode filter with wide bandwidth, compact structure, low production cost, stable performance, and easy engineering application. Summary of the Invention

[0006] In order to solve the problems of the prior art, the present invention provides a broadband mode filter based on a three-core photonic crystal fiber, comprising:

[0007] A substrate, wherein a plurality of air holes are provided in the substrate, wherein the air holes surround a main core, a first side core, a second side core, and a first layer of air holes, a second layer of air holes, and a third layer of air holes, all of which are regular hexagonal structures;

[0008] The main core is located in the central area of the substrate, and the first side core and the second side core are located in the left and right areas of the main core respectively;

[0009] The first side core and the second side core both have overlapping and shared air holes with the main core;

[0010] The first layer of air holes is located outside the main core, the first side core, and the second side core; the second layer of air holes is located outside the first layer of air holes; and the third layer of air holes is located outside the second layer of air holes.

[0011] The first side core includes: a first outer cladding and a first inner cladding respectively surrounded by air holes;

[0012] The second side core comprises: a second outer cladding and a second inner cladding respectively surrounded by air holes;

[0013] The first inner cladding and the second inner cladding are respectively located on the left and right sides of the main core, the first outer cladding is located outside the first inner cladding, and the second outer cladding is located outside the second inner cladding;

[0014] Gold films are provided in the air holes of the first outer cladding and the second outer cladding.

[0015] Furthermore, the main core is surrounded by circular air holes, the first outer cladding and the second outer cladding are both semi-closed hexagonal structures, and the first inner cladding and the second inner cladding are both fan-shaped structures.

[0016] Furthermore, the main core comprises ten first air holes, one sixth air hole and one seventh air hole;

[0017] The first outer cladding layer includes seven eighth air holes, five of which are plated with a ninth gold film. The air holes provided with the ninth gold film are denoted as third air holes. The first inner cladding layer includes eight second air holes and one sixth air hole. The sixth air hole coincides with the sixth air hole of the main core.

[0018] The second outer cladding layer includes thirteen eighth air holes, wherein eight of the eighth air holes are provided with tenth gold films, and the air holes provided with the tenth gold films are recorded as fifth air holes.

[0019] Three of the thirteen eighth air holes overlap with air holes in the first layer of air holes, and four of the thirteen eighth air holes overlap with air holes in the first layer of air holes; the second inner envelope includes nine fourth air holes and one seventh air hole, and the seventh air hole overlaps with the seventh air hole of the main core;

[0020] The first layer of air holes, the second layer of air holes and the third layer of air holes each contain a plurality of eighth air holes, the sizes of the eighth air holes, the second air holes, the fourth air holes, the first air holes, the seventh air holes and the sixth air holes decrease successively, and the thickness of the gold film on the first outer cladding and the second outer cladding decreases successively.

[0021] Furthermore, the first outer cladding is a semi-closed hexagonal structure composed of an eighth air hole and a ninth gold film plated on an inner wall of the eighth air hole, and the first inner cladding is a first fan-shaped structure composed of a second air hole and a sixth air hole;

[0022] The second outer cladding is a semi-closed hexagonal structure composed of the eighth air hole and the tenth gold film plated on the inner wall of the eighth air hole, and the second inner cladding is a second fan-shaped structure composed of the fourth air hole and the seventh air hole;

[0023] A circular area surrounded by the first air hole, the sixth air hole, and the seventh air hole constitutes the main core.

[0024] Furthermore, the radius of the sixth air hole d1 = 0.35 μm, the radius of the seventh air hole d2 = 0.45 μm, the radius of the first air hole r0 = 0.45 μm, and the center distance Λ1 between any two adjacent air holes on the main core = 4 μm.

[0025] Furthermore, the radius r1 of the fourth air hole is 0.65 μm, and the center distance Λ2 between any two adjacent air holes on the first side core is 2.6 μm.

[0026] Furthermore, the second side-core cladding air holes include multiple fourth air holes and one seventh air hole, the lines connecting the centers of the air holes form a sector, the second air hole radius r2 = 0.49 μm, and the distance Λ4 between the centers of any two adjacent air holes on the second side-core = 2.1 μm.

[0027] Furthermore, the performance of the mode filter is determined by the mode suppression ratio, insertion loss and bandwidth;

[0028] The main core supports LP 01 LP 11 and LP 21 Three modes, the three modes have different paths and mode field distributions during transmission;

[0029] The mode suppression ratio MSR is defined as the ratio of the mode at which multiple modes are transmitted simultaneously in the main core at the input of the filter and only the target mode LP is obtained at the output. 21 Therefore, the mode suppression ratio is used to characterize the mode filtering effect of the mode filter, and its corresponding expression is as follows:

[0030]

[0031] Where, MSR 01 、MSR 11 and MSR total Respectively represent the filter mode to LP 01 LP 11 Mode and LP 01 +LP 11 The degree of suppression of the pattern, and They are the output terminals LP in the main core 21 LP 01 LP 11 Optical power of the mode;

[0032] The bandwidth is defined as the wavelength range in which the mode division multiplexer operates when the MSR is greater than 10 dB.

[0033] Among them, the insertion loss is defined as the main core LP 21 Ten times the logarithm of the ratio of the mode input energy to its output energy. The specific expression is:

[0034]

[0035] Where, Indicates the output terminal LP in the main core 21 Mode power, Indicates the input terminal LP in the main core 21 mode power.

[0036] Beneficial effects of the present invention:

[0037] 1. This invention proposes a three-core photonic crystal mode filter based on coupled modes. Three cores supporting localized light transmission are arranged within a photonic crystal fiber: a main core, a first side core, and a second side core with different cladding air hole structures. By utilizing side core modulation and loss light, coupled light from the main core, this design improves the mode suppression ratio and bandwidth, reduces insertion loss in the main core mode, and effectively shortens the device length.

[0038] 2. The present invention utilizes the flexibility of photonic crystal fiber design and can adjust the radius and spacing of the side core and the side core air hole cladding to make the LP in the main core 01 Mod and LP 11 LP for side-core analog conversion 01 mode and completes mode filtering after the mode has transmitted a certain distance.

[0039] 3. The present invention can support LP 01 LP 11 and LP 21 Three modes, and LP in the main core01 and LP 11 Mode can be used with LP in the side core 01 At the same time, due to the addition of a gold film of a specific thickness on the inner wall of the air hole in the side core cladding, the mode coupled from the main core to the side core is lost after a short distance in the side core, and finally the mode filter outputs only a single LP within a specific wavelength range. 21 Numerical simulations show that this device has dual operating bands, with bandwidths of 125nm and 53nm (1.5-1.625μm and 1.766-1.819μm, encompassing the S, C, and L bands). At 1550nm, the insertion loss is only 0.024dB, and the mode suppression ratio is 19.84dB.

[0040] 4. This invention leverages the flexible design of photonic crystal fibers. Without the use of dopants or liquid crystals, the core supports three modes of transmission simply by designing the air holes in the core cladding. This device simultaneously offers advantages such as wide bandwidth, low insertion loss, high mode suppression ratio, and a compact structure. The device is only 46.5 mm long, and its output ports are easily spliced and integrated into existing fiber optic communication systems, making it particularly suitable for modern, miniaturized, and integrated fiber optic communication systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a schematic cross-sectional structural diagram of an ultra-large bandwidth, low-loss mode division multiplexer based on five-core photonic crystal fiber provided by the present invention;

[0042] Figure 2 (a) is the LP in the main core 01 Mode field diagram of the mode;

[0043] Figure 2 (b) is the LP in the main core 11 Mode field diagram of the mode;

[0044] Figure 2 (c) is the LP in the main core 21 Mode field diagram of the mode;

[0045] Figure 2 (d) is the LP in the main core 02 Mode field diagram of the mode;

[0046] Figure 3 This is a graph showing the relationship between the effective refractive index of the fundamental mode and the higher-order mode in the main core of the present invention and the effective refractive index difference between two adjacent modes as a function of the input light wavelength;

[0047] Figure 4 (a) is the LP in the first side core of the present invention 01 The relationship between the effective refractive index of the mode and the radius r1 of the air hole in the first side core cladding;

[0048] Figure 4 (b) is the LP in the first side core 01 The effective refractive index of the mode changes with the radius d1 of the air hole at the connection between the first side core and the main core;

[0049] Figure 4 (c) is the LP in the first side core 01 The relationship between the effective refractive index of the mode and the spacing Λ2 of the air holes in the first side core cladding;

[0050] Figure 4 (d) is the LP in the first side core 01 LP in mold and main core 11 The relationship between the effective refractive index difference of the modes and the wavelength of the input light;

[0051] Figure 5 (a) is the LP in the second side core of the present invention 01 The relationship between the effective refractive index of the mode and the radius r2 of the air hole in the second side core cladding;

[0052] Figure 5 (b) is the LP in the second side core 01 The effective refractive index of the mode changes with the radius d2 of the air hole at the connection between the second side core and the main core;

[0053] Figure 5 (c) is the LP in the second side core 01 The relationship between the effective refractive index of the mode and the air hole spacing Λ4 of the second side core cladding;

[0054] Figure 5 (d) is the LP in the second side core 01 LP in mold and main core 01 The relationship between the effective refractive index difference of the modes and the wavelength of the input light;

[0055] Figure 6 (a) is the right side core LP under different gold film thicknesses in the present invention 01 Mode and center core LP 01 Modulo Δ neff Relationship diagram with RI changes;

[0056] Figure 6 (b) is the left side core LP under different gold film thickness 01 Mode and center core LP 11 Modulo Δ neff Relationship diagram with RI changes;

[0057] Figure 7 (a) is the LP corresponding to different gold film thickness t in the present invention 01 Variation of mode coupling efficiency with device length;

[0058] Figure 7 (b) is the LP corresponding to different gold film thickness t 11 Variation of mode coupling efficiency with device length;

[0059] Figure 8 (a) is the left side core LP corresponding to different gold film thickness t in the present invention 01 Mode loss;

[0060] Figure 8 (b) is the right side core LP corresponding to different gold film thickness t 01 Mode loss;

[0061] Figure 9 (a) is the present invention, the filter mode LP 01 LP 11 Mode and LP 01 +LP 11 A plot of the degree of mode suppression versus wavelength;

[0062] Figure 9 (b) is the filter core LP 21 Mode and Sidecore LP 01 Mode normalized mode power changes with transmission distance

[0063] Figure 10 It is the main core LP 21 Plot of modal insertion loss versus wavelength.

[0064] Reference numerals:

[0065] In the figure: 1 is the first air hole, 2 is the second air hole, 3 is the third air hole, 4 is the fourth air hole, 5 is the fifth air hole, 6 is the sixth air hole, 7 is the seventh air hole, 8 is the eighth air hole, 9 is the ninth gold film, and 10 is the tenth gold film. DETAILED DESCRIPTION

[0066] 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. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0067] See also Figure 1The present invention provides a broadband mode filter based on a three-core photonic crystal fiber, comprising: a substrate, a main core, a first side core, and a second side core, a total of three fiber cores, and a multi-layer air hole; the three fiber cores and the multi-layer air hole are all arranged in the substrate; the main core air hole is a circular structure; the first side core is located on the left side of the main core and is composed of a first outer cladding with a semi-closed hexagonal structure and a first inner cladding with a fan-shaped structure, and the second side core is located on the right side of the main core and is composed of a second outer cladding with a semi-closed hexagonal structure and a second inner cladding with a fan-shaped structure; the multi-layer air hole is composed of regular hexagons except the main core, the first side core, and the second side core, and comprises a first layer of air holes a, a second layer of air holes b, and a third layer of air holes c from the inside to the outside, and the first layer of air holes a surrounds the main core, the first side core, and the second side core.

[0068] In the present invention, the main core air holes include ten first air holes 1, one sixth air hole 6, and one seventh air hole 7; the first outer cladding in the first side core includes seven eighth air holes 8, five of which are plated with a ninth gold film 9, and the air holes provided with the ninth gold film 9 are recorded as third air holes 3; the first inner cladding includes eight second air holes 2 and one sixth air hole 6, and the sixth air hole coincides with the sixth air hole of the main core; the second outer cladding in the second side core includes thirteen eighth air holes, eight of which are provided with a tenth gold film 10, and the air holes provided with the tenth gold film are recorded as fifth air holes 5; three of the thirteen eighth air holes coincide with air holes in the first layer of air holes, and four of the thirteen eighth air holes coincide with air holes in the first layer of air holes;

[0069] The second inner cladding in the second side core comprises nine fourth air holes 4 and one seventh air hole 7, and the seventh air hole coincides with the seventh air hole of the main core;

[0070] The first layer of air holes, the second layer of air holes, and the third layer of air holes each include a plurality of eighth air holes, the sizes of the eighth air holes, the second air holes, the fourth air holes, the first air holes, the seventh air holes, and the sixth air holes decrease in sequence, the thicknesses of the gold films on the first outer cladding and the second outer cladding decrease in sequence, and the thicknesses of the ninth gold film and the tenth gold film decrease in sequence;

[0071] In the present invention, the first outer cladding is a semi-closed hexagonal structure composed of the eighth air hole and the ninth gold film plated on the inner wall of the eighth air hole, and the first inner cladding is a first fan-shaped structure composed of the second air hole and the sixth air hole;

[0072] The second outer cladding is a semi-closed hexagonal structure composed of the eighth air hole and the tenth gold film plated on the inner wall of the eighth air hole, and the second inner cladding is a second fan-shaped structure composed of the fourth air hole and the seventh air hole;

[0073] A circular area surrounded by the first air hole, the sixth air hole, and the seventh air hole constitutes the main core.

[0074] In the present invention, the radius of the sixth air hole d1 = 0.35 μm, the radius of the seventh air hole d2 = 0.45 μm, the radius of the first air hole r0 = 0.45 μm, and the center distance Λ1 between any two adjacent air holes on the main core = 4 μm.

[0075] The radius r1 of the fourth air hole is 0.65 μm, and the center distance Λ2 between any two adjacent air holes on the first side core is 2.6 μm.

[0076] The second side core cladding air holes include multiple fourth air holes and one seventh air hole. The line connecting the centers of the air holes forms a sector. The second air hole radius r2 is 0.49 μm. The distance between the centers of any two adjacent air holes on the second side core is Λ4 = 2.1 μm.

[0077] In the present invention, the circular area surrounded by the first air hole, the sixth air hole and the seventh air hole constitutes the main core of the filter module.

[0078] In the present invention, the first side core, the second side core, the gold film with a specific thickness outside the side core and the cladding air hole play the role of confining light in the side core and adjusting the side core LP. 01 The effective refractive index of the mode and the effect of light loss in the side core after transmission over a certain distance.

[0079] The selection of the filter mode length in the present invention takes into account the optimal coupling length of the first side core and the second side core to the main core mode conversion and the shortest distance of the high loss of the side core mode. Under the selected filter mode length, the LP of the main core is 01 Mod and LP 11 Modes can be fully coupled to the side core and lost.

[0080] It's important to note that mode filters, a research hotspot in information photonics, have broad applications in fiber-optic communications, fiber-optic sensing, cloud computing, big data, the Internet of Things, and telemedicine. Photonic crystal fiber mode filters, with their wide bandwidth, low loss, high mode suppression ratio, and compact structure, are a key area of future development in optical quantum information technology.

[0081] In the broadband mode filter based on three-core photonic crystal fiber proposed in the present invention, all air holes adopt a circular structure and do not need to be doped with liquid crystal and other materials, which is convenient to prepare and has high process feasibility. 01The effective refractive index of the mode matches the effective refractive index of the high-order mode in the main core in a broadband range, so that the filter outputs the LP of the three supported modes at the output end. 21 model.

[0082] Figure 2 (a) Figure 2 (b) Figure 2 (c) Figure 2 (d) is the LP present in the main core of the present invention 01 LP 11 LP 21 LP 02 The mode field diagrams of these four modes are Figure 3 The effective refractive index of the four modes and the difference between the effective refractive index of two adjacent modes vary with wavelength. In the mode division multiplexing system, the effective refractive index difference of different modes Δ neff When the wavelength is greater than 0.001, the transmission can be carried out simultaneously in the same few-mode fiber without interfering with each other. 01 LP 11 LP 21 The difference in effective refractive index between the three modes Δ neff The Δ neff >0.001, and Δ neff It increases with the increase of working wavelength, which shows that the inter-mode crosstalk is well suppressed, which is beneficial to improve the mode extinction ratio and thus achieve stable transmission; LP 21 With LP 02 The effective refractive index difference does not satisfy Δ at the central wavelength of 1.55 μm and its vicinity neff >0.001, there may be inter-mode crosstalk. Therefore, we discard LP 02 Mode, only keep L P01 LP 11 LP 21 The three modes are transmitted together.

[0083] This invention employs a two-side core structure. From an industrial manufacturing perspective, we minimize the number of side core structural parameters to simplify fabrication. Λ1 is uniformly set to 4μm, and each parameter is accurate to a minimum of two decimal places to meet industrial manufacturing precision. Each side core is analyzed below.

[0084] Figure 4 (a) is the LP in the first side core of the present invention 01 The relationship between the effective refractive index of the mode and the radius r1 of the air hole of the first side core; Figure 4 (b) is the LP in the first side core of the present invention 01The effective refractive index of the mode changes with the radius d1 of the air hole at the connection between the first side core and the main core; Figure 4 (c) is the LP in the first side core of the present invention 01 The relationship between the effective refractive index of the mode and the air hole spacing Λ2 of the first side core; Figure 4 (a)(b)(c) show that in the first side core, LP 01 The effective refractive index coefficient of the mode decreases with the increase of radius r1. In addition, its effective refractive index coefficient decreases with the increase of d1. On the contrary, when the spacing between adjacent circles in the first side core is Λ2 When it increases, its effective refractive index coefficient increases accordingly. Figure 4 (c) with Figure 3 It can be seen that when Λ2=2.5μm, the LP in the first side core 01 The effective refractive index coefficient of the mode is related to the LP in the main core 11 The effective refractive index coefficients of the modes are roughly similar. In order to further make the above two modes meet the phase matching, adjust Λ2 to 2.6μm. And on this basis, continue to adjust r1 and d1 so that the side core LP near different central wavelengths 01 The effective refractive index coefficient of the mode is related to the LP in the main core 11 The effective refractive index coefficients of the modes are as similar as possible to achieve phase matching. After analyzing different structural parameters one by one, we determined the optimal air hole radius r1 = 0.65μm in the first side core and the air hole radius d1 = 0.35μm at the connection with the main core. Figure 4 (d) is the LP in the first side core of the present invention 01 LP in mold and main core 11 The relationship between the effective refractive index difference of the two modes and the input light wavelength is shown in the figure. Figure 4 (d) It can be seen that in the wavelength range of 1.5-1.9μm, the LP in the first side core 01 The effective refractive index of the mode decreases with the increase of wavelength. At the same time, it is closely related to the LP in the main core. 11 The effective refractive index coefficient difference of the modes shows a fluctuating trend with increasing wavelength. At a wavelength of around 1.55μm, the effective refractive index difference is less than 0.001, which can achieve quasi-phase matching.

[0085] Figure 5 (a) is the LP in the second side core of the present invention 01 The relationship between the effective refractive index of the mode and the radius r2 of the air hole in the second side core; Figure 5 (b) In the present invention, the LP in the second side core 01 The effective refractive index of the mode changes with the radius d2 of the air hole at the connection between the second side core and the main core; Figure 5 (c) is the LP in the second side core of the present invention 01The relationship between the effective refractive index of the mode and the air hole spacing Λ4 of the second side core cladding; Figure 5 (a)(b)(c) shows that the LP in the second side core 01 The effective refractive index coefficient of the mode decreases with the increase of the radius r2 of the cladding air hole and the radius d2 of the air hole at the connection with the main core, and increases with the increase of the distance Λ4 between the two adjacent circle centers. 01 Mode and main core LP 01 The modes achieve phase matching, and we obtain the optimal second side-core air hole radius r2 = 0.49 μm, the adjacent circle center distance Λ4 = 2.1 μm, and d2 = 0.45 μm. Figure 5 (d) is the LP in the second side core of the present invention 01 LP in mold and main core 01 The relationship between the effective refractive index difference of the two modes and the input light wavelength is shown in the figure. Figure 5 (d) It can be seen that in the band around 1.55μm, the effective refractive index coefficients of the two modes are basically equal, thus achieving phase matching.

[0086] Figure 6 (a) is the second side core LP under different gold film thicknesses in the present invention 01 Mode and center core LP 01 Modulo Δ neff and the variation of RI with wavelength; Figure 6 (b) is the left side core LP under different gold film thicknesses in the present invention 01 Mode and center core LP 11 Modulo Δ neff and RI changes with wavelength; Figure 6 (a) (b) shows that when the wavelength range is 1.5μm-1.9μm, the LP in the first side core and the second side core is different with the thickness of the gold film. 01 The effective refractive index of the mode has only a slight deviation, and its effective refractive index coefficient is the same as that of the LP in the main core. 01 LP 11 The effective refractive index coefficients of the modes are similar, and the effective refractive index difference Δ neff >0.001, so the phase matching between the main core and side core modes is met in the wavelength range of 1.5-1.9μm.

[0087] Figure 7 (a) is the LP corresponding to different gold film thickness t in the present invention 01 Variation of mode coupling efficiency with device length; Figure 7 (b) is the LP corresponding to different gold film thickness t in the present invention 11 Mode coupling efficiency varies with device length; Figure 7 (a) It can be seen that the LP in the main core 01mode and LP in the second side core 01 The mode coupling efficiency increases gradually with the increase of transmission distance. The transmission distance corresponding to the maximum coupling efficiency is slightly different. For different metal film thicknesses, the maximum coupling efficiency is 99.3% (t = 10nm), 98.9% (t = 20nm), and 98% (t = 30nm). Figure 7 (b) It can be seen that within a length of 1900 μm, the LP11 mode in the main core and the LP in the first side core 01 The mode coupling efficiency gradually increases with the increase of device length. When the device length is less than 900μm, the metal film thickness has little effect on the coupling efficiency. When the device length is in the range of 900μm to 1900μm, the coupling efficiency decreases slightly with the increase of metal film thickness. The maximum coupling efficiency is 99.5% (t=40nm), 99.3% (t=45nm), and 97.4% (t=50nm), respectively.

[0088] Figure 8 (a) is the first side core LP corresponding to different gold film thicknesses t of the present invention 01 Mode loss; Figure 8 (b) is the second side core LP corresponding to different gold film thicknesses t of the present invention 01 Mode loss; Figure 8 (a) (b) shows that in the range of 1.5-1.9μm, the LP 01 The mode limitation loss generally increases with the increase of gold film thickness. In order to ensure that the mode to be filtered in the main core has a high coupling efficiency with the mode in the side core, and to take into account the LP in the side core 01 The mode has a high loss. Considering all factors, the thickness of the gold film of the first side core is set to 30nm and the thickness of the gold film of the second side core is set to 50nm. At the same time, in order to ensure that the filter can fully filter out the LP in the first and second side cores 01 mode, select the filter length as 46.5mm.

[0089] Figure 9 (a) is the filter mode of the present invention for LP 01 LP 11 Mode and LP 01 +LP 11 A plot of the degree of mode suppression versus wavelength; Figure 9 (b) is the filter main core LP of the present invention 21 Mode and Sidecore LP 01 The variation of the normalized mode power with the transmission distance; Figure 9 (a) It can be seen that in the wavelength range of 1.5-1.625μm and 1.766-1.819μm, the device has a good performance on LP 01 LP11 LP 01 +LP 11 The mode suppression ratio of several modes is greater than 10dB. At a wavelength of 1550nm, the LP 01 LP 11 LP 01 +LP 11 The mode suppression ratios of the modes are 20.8dB, 26.6dB and 19.84dB respectively, indicating that the proposed mode filter has good spurious mode suppression performance. In addition, the LP 21 Mode and Sidecore LP 01 The normalized mode power of the mode changes with the transmission distance, and the results are as follows Figure 9 (b) is shown. Figure 9 (b) It can be seen that after the three modes have transmitted 46.5 mm, the LP in the first and second side cores 01 The mode power dropped to less than 1% of the initial input power, while the LP 21 The mode power remains basically unchanged.

[0090] Figure 10 The filter module main core LP of the present invention 21 The variation of mode insertion loss with wavelength; Figure 10 It can be seen that within the wavelength range of 1.5-1.9μm, the insertion loss gradually increases with the increase of wavelength, but is less than 0.057dB. 21 The insertion loss of the mode is only 0.024dB. This shows that the main core LP 21 The mode can perform low-loss transmission within this range.

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A broadband mode filter based on three-core photonic crystal fiber, characterized in that: include: A substrate, wherein a plurality of air holes are provided in the substrate, wherein the air holes surround a main core, a first side core, a second side core, and a first layer of air holes, a second layer of air holes, and a third layer of air holes, all of which are regular hexagonal structures; The main core is located in the central area of the substrate, and the first side core and the second side core are located in the left and right areas of the main core respectively; The first side core and the second side core both have overlapping and shared air holes with the main core; The first layer of air holes is located outside the main core, the first side core, and the second side core; the second layer of air holes is located outside the first layer of air holes; and the third layer of air holes is located outside the second layer of air holes. The first side core includes: a first outer cladding and a first inner cladding respectively surrounded by air holes; The second side core comprises: a second outer cladding and a second inner cladding respectively surrounded by air holes; The first inner cladding and the second inner cladding are respectively located on the left and right sides of the main core, the first outer cladding is located outside the first inner cladding, and the second outer cladding is located outside the second inner cladding; Gold films are provided in the air holes of the first outer cladding and the second outer cladding.

2. The broadband mode filter based on three-core photonic crystal fiber according to claim 1, characterized in that: The main core is surrounded by circular air holes, the first outer cladding and the second outer cladding are both semi-closed hexagonal structures, and the first inner cladding and the second inner cladding are both fan-shaped structures.

3. The broadband mode filter based on triple-core photonic crystal fiber according to claim 1, characterized in that: The main core comprises ten first air holes, one sixth air hole and one seventh air hole; The first outer cladding layer includes seven eighth air holes, five of which are plated with a ninth gold film. The air holes provided with the ninth gold film are denoted as third air holes. The first inner cladding layer includes eight second air holes and one sixth air hole. The sixth air hole coincides with the sixth air hole of the main core. The second outer cladding layer includes thirteen eighth air holes, wherein eight of the eighth air holes are provided with tenth gold films, and the air holes provided with the tenth gold films are recorded as fifth air holes. Three of the thirteen eighth air holes overlap with air holes in the first layer of air holes, and four of the thirteen eighth air holes overlap with air holes in the first layer of air holes; the second inner envelope includes nine fourth air holes and one seventh air hole, and the seventh air hole overlaps with the seventh air hole of the main core; The first layer of air holes, the second layer of air holes and the third layer of air holes each contain a plurality of eighth air holes, the sizes of the eighth air holes, the second air holes, the fourth air holes, the first air holes, the seventh air holes and the sixth air holes decrease successively, and the thickness of the gold film on the first outer cladding and the second outer cladding decreases successively.

4. The broadband mode filter based on three-core photonic crystal fiber according to claim 3, characterized in that: The first outer cladding is a semi-closed hexagonal structure composed of the eighth air hole and the ninth gold film plated on the inner wall of the eighth air hole, and the first inner cladding is a first fan-shaped structure composed of the second air hole and the sixth air hole; The second outer cladding is a semi-closed hexagonal structure composed of the eighth air hole and the tenth gold film plated on the inner wall of the eighth air hole, and the second inner cladding is a second fan-shaped structure composed of the fourth air hole and the seventh air hole; A circular area surrounded by the first air hole, the sixth air hole, and the seventh air hole constitutes the main core.

5. The broadband mode filter based on triple-core photonic crystal fiber according to claim 3, characterized in that: The radius of the sixth air hole d1 is 0.35 μm, the radius of the seventh air hole d2 is 0.45 μm, the radius of the first air hole r0 is 0.45 μm, and the center distance Λ1 between any two adjacent air holes on the main core is 4 μm.

6. The broadband mode filter based on three-core photonic crystal fiber according to claim 3, characterized in that: The radius r1 of the fourth air hole is 0.65 μm, and the center distance Λ2 between any two adjacent air holes on the first side core is 2.6 μm.

7. The broadband mode filter based on triple-core photonic crystal fiber according to claim 3, characterized in that: The second side core cladding air holes include multiple fourth air holes and one seventh air hole. The line connecting the centers of the air holes forms a sector. The second air hole radius r2 is 0.49 μm. The distance between the centers of any two adjacent air holes on the second side core is Λ4 = 2.1 μm.

8. The broadband mode filter based on triple-core photonic crystal fiber according to claim 3, characterized in that: The performance of a mode filter is determined by mode suppression ratio, insertion loss, and bandwidth; The main core supports LP 01 LP 11 and LP 21 Three modes, the three modes have different paths and mode field distributions during transmission; The mode suppression ratio MSR is defined as the ratio of the mode at which multiple modes are transmitted simultaneously in the main core at the input of the filter and only the target mode LP is obtained at the output. 21 Therefore, the mode suppression ratio is used to characterize the mode filtering effect of the mode filter, and its corresponding expression is as follows: Where, MSR 01 、MSR 11 and MSR total Respectively represent the filter mode to LP 01 LP 11 Mode and LP 01 +LP 11 The degree of suppression of the pattern, and They are the output terminals LP in the main core 21 LP 01 LP 11 Optical power of the mode; The bandwidth is defined as the wavelength range in which the mode division multiplexer operates when the MSR is greater than 10 dB. Among them, the insertion loss is defined as the main core LP 21 Ten times the logarithm of the ratio of the mode input energy to its output energy. The specific expression is: Where, Indicates the output terminal LP in the main core 21 Mode power, Indicates the input terminal LP in the main core 21 mode power.

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