A broadband mode filter based on a three-core photonic crystal fiber

Abstract

The application discloses a broadband mode filter based on a three-core photonic crystal fiber, and belongs to the technical field of fiber communication. The base is internally provided with a plurality of cladding air holes. The first side core and the second side core are respectively located on the left side and the right side of the main core. In the application, the main core of the mode filter can support LP 01 , LP 11 and LP 21 modes. By adjusting the radius, spacing and inner wall gold film thickness of the circular air holes of the two side core claddings, and selecting the optimal device length, the LP 01 and LP 11 modes in the main core can be coupled with the LP 01 mode in the side core. The mode coupled to the side core is lost after transmitting a short distance in the side core. Finally, the mode filter can output only a single LP 21 mode in a specific wavelength range.

Description

Technical Field

[0001] This invention relates to the field of optical fiber communication technology, specifically to a broadband mode filter based on a three-core photonic crystal fiber. Background Technology

[0002] With the continuous development of technologies such as Wavelength Division Multiplexing (WDM), Polarization Division Multiplexing (PDM), Mode Division Multiplexing (MDM), and higher-order modulation, the transmission rate and capacity of optical fiber communication systems have been greatly improved. However, due to the existence of fiber dispersion and nonlinear effects, the transmission capacity of single-mode fiber is gradually approaching the Shannon limit, making it difficult for traditional single-mode fiber to meet the network data traffic demands of various bandwidth-enhanced Internet services. Mode division multiplexing technology, as an effective method to improve the capacity of optical fiber communication systems, has attracted the attention of many researchers. It greatly improves the data transmission capability of optical fibers by simultaneously transmitting data using multiple modes in the same fiber. However, high inter-mode crosstalk is one of the main challenges limiting the performance improvement of MDM systems. Efficient mode filtering technology is considered an effective method to reduce inter-mode crosstalk. Furthermore, with the deepening application of mode division multiplexing technology in quantum communication, optical fiber sensing, and other fields, the demand for mode control devices such as mode dividers, mode converters, and mode filters is also increasing. Among these, mode filters with high bandwidth, low insertion loss, low crosstalk, and compact structure have become 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 scope. Simultaneously, low insertion loss reduces the power consumption of the fiber optic system, and lower crosstalk ensures effective suppression of non-target modes, thus improving the overall performance of the fiber optic communication / fiber optic sensing system. Therefore, research on high-performance mode filters is of great significance for promoting the development of fiber optic communication, fiber optic sensing, and related quantum information technologies, and has become a research hotspot in the field of information photonics technology.

[0003] Reported mode filters are mainly classified into two types according to their operating methods: direct filtering and indirect filtering. Direct filtering is based on the difference in loss between different modes (typically, lower-order modes have lower losses, while higher-order modes have higher losses). It effectively filters out specific modes by significantly increasing the loss of higher-order modes through specific fiber (or waveguide) structures, bending the fiber (or waveguide), or introducing additional loss mechanisms. This method is simple and direct, but it easily introduces significant losses, thus increasing the overall power consumption of the system. Indirect filtering relies on finely designed waveguide structures to achieve energy conversion between modes. This typically involves mode-changing devices (such as waveguide- or grating-based mode converters), which not only convert unwanted modes to other modes but also guide them to branch waveguide paths that do not participate in transmission, effectively filtering out unwanted modes by adding specific loss mechanisms.

[0004] Structurally, mode filters can be divided into two types: grating type and fiber type. Grating type mode filters primarily achieve energy conversion between modes through indirect mode removal, relying on complex waveguide structures. In 2014, Hu et al. proposed a mode filter based on a dual-mode fiber Bragg grating. This device can transmit LP signals normally at a wavelength of 1065.7 nm. 11 Pattern, while also targeting LP 01 The mode caused a loss of 5.98 dB. In 2016, Chen et al. designed a mode filter based on a long-period fiber grating. By cascading few-mode long-period fiber gratings, LP mode filtering was effectively achieved within a length of 140 mm. 01 The device supports both TE0 and TE1 modes and provides a bandwidth exceeding 20 nm near a wavelength of 1.55 μm. In 2018, Yu et al. designed a mode filter using a cascaded plasmonic-bridged subwavelength grating. This device supports both TE0 and TE1 modes, filtering out the TE0 mode within a bandwidth of 35 nm and the TE1 mode within a bandwidth of 26 nm, with insertion losses exceeding 20 dB for both modes. In 2020, Ling et al. designed a mode filter based on a few-mode long-period fiber grating. This device supports both LP01 and LP1 modes and provides a bandwidth exceeding 20 nm near a wavelength of 1.55 μm. 11 Two modes are available, and LP can be effectively filtered within a device length of 34-46mm. 01 The mode achieves a bandwidth of 70nm. Although the above-mentioned grating-type filter realizes the filter function, it still has drawbacks such as long device length or narrow bandwidth.

[0005] In recent years, photonic crystal fiber (PCF) mode filters have attracted widespread attention from researchers. PCF is a microstructured optical fiber with a nanometer-scale periodic distribution of air holes in its cross-section. The flexible degrees of freedom in cross-sectional design give PCF unique advantages in adjusting its optical transmission characteristics. By rationally designing the arrangement of micro-air holes in the fiber cross-section, it can exhibit extraordinary light-guiding properties such as large mode area, high nonlinearity, high birefringence, and ultra-flat dispersion. Furthermore, research shows that by rationally designing the microstructure of PCF, mode-selective transmission can also be achieved. In 2015, Chen et al. proposed a high-refractive-index core photonic bandgap fiber mode filter, which supports LP... 01 LP 11 LP 21 and LP 02 Four modes are used to couple unwanted modes to the cladding for filtering through bandgap and refractive index guiding effects. This device can achieve a bandwidth of 200 nm. However, this design requires three materials with different refractive index coefficients, making the fabrication process complex. In 2019, Yue et al. proposed a surface plasmon-based mode-filtering fiber. This structure incorporates gold wires outside the cladding to generate high leakage loss in the cladding modes. By coupling the cladding modes with the modes to be filtered in the fiber core, the filtering effect is achieved. This device can effectively filter out LP modes in the core within the 1534-1570 nm wavelength range. 11 The first method involved a mode filter with a device length of only 50 mm. However, this design still required materials with different refractive index coefficients, resulting in a complex fabrication process. In the same year, Dai et al. proposed a petal-shaped microstructured mode filter fiber based on high-refractive-index liquid filling. This device incorporates liquid material into the cladding air holes, utilizing the thermo-optic effect of the liquid material. By adjusting the material temperature, the cladding refractive index is changed, allowing the mode to be filtered to couple with the cladding mode to achieve a mode filtering effect. This device can effectively filter modes in the 1.53-1.57 μm wavelength range with a length of 71.4 mm. However, the performance of this device depends on the thermal stability of the liquid material, severely limiting its reliability in engineering applications. Therefore, the development of a mode filter with wide bandwidth, compact structure, low fabrication cost, stable performance, and ease of engineering application is particularly urgent. Summary of the Invention

[0006] To address the problems of existing technologies, this invention provides a broadband mode filter based on a three-core photonic crystal fiber, comprising:

[0007] A substrate having multiple air holes arranged therein, the air holes surrounding a main core, a first side core and a second side core, as well as 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 region of the substrate, and the first side core and the second side core are located in the regions to the left and right of the main core, respectively.

[0009] Both the first and second side cores 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 layer and a first inner cladding layer formed by air holes respectively;

[0012] The second side core includes: a second outer cladding layer and a second inner cladding layer formed by air holes, respectively;

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

[0014] A gold film is provided in the air pores of both the first outer layer and the second outer layer.

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

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

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

[0018] The second outer cladding layer contains thirteen eighth air holes, eight of which contain a tenth gold film. These air holes containing the tenth gold film are designated as fifth air holes.

[0019] Three of the thirteen eighth air holes coincide with the air holes in the first layer of air holes, and four of the thirteen eighth air holes coincide with the air holes in the first layer of air holes; the second inner cladding layer includes nine fourth air holes and one seventh air hole, which coincides 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 size of the eighth air hole, the second air hole, the fourth air hole, the first air hole, the seventh air hole, and the sixth air hole decreases in sequence, and the thickness of the gold film on the first outer layer and the second outer layer decreases in sequence.

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

[0022] The second outer cladding layer 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 layer is a second sector composed of the fourth air hole and the seventh air hole;

[0023] The circular area formed 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 is d1 = 0.35 μm, the radius of the seventh air hole is d2 = 0.45 μm, the radius of the first air hole is r0 = 0.45 μm, and the center-to-center distance between any two adjacent air holes on the main core is Λ1 = 4 μm.

[0025] Furthermore, the radius of the fourth air hole is r1 = 0.65 μm, and the center-to-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 hole includes multiple fourth air holes and one seventh air hole. The line connecting the centers of the air holes forms a fan shape. The radius of the second air hole is r2 = 0.49 μm, and the distance between the centers of any two adjacent air holes on the second side core is Λ4 = 2.1 μm.

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

[0028] The main chip supports LP 01 LP 11 and LP 21 Three modes, each with different paths and mode field distributions during transmission;

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

[0030]

[0031] In the formula, MSR 01 MSR 11 and MSR total These represent the filter modulators for LP. 01 LP 11 Patterns and LPs 01 +LP 11 The degree of suppression of the pattern, and The output terminals LP in the main core are respectively 21 LP 01 LP 11 The 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 10dB.

[0033] The insertion loss is defined as the core LP. 21 The ratio of the mode input energy to its output energy is ten times the logarithm, specifically expressed as:

[0034]

[0035] In the formula, Indicates the output terminal LP in the main core 21 Mode power, Indicates the input terminal LP in the main core 21 The power of the mode.

[0036] The beneficial effects of this invention are:

[0037] 1. This invention is a three-core photonic crystal mode filter based on coupled modes. Three cores—a main core, a first side core, and a second side core—with different cladding air hole structures are arranged in the photonic crystal fiber to support local optical transmission. By employing a design scheme of side core modulation and loss light, and main core coupling light, the device length is effectively shortened while improving mode rejection ratio, bandwidth, and reducing main core mode insertion loss.

[0038] 2. This invention utilizes the flexibility of photonic crystal fiber design. By simply adjusting the radius and spacing of the side cores and the air hole cladding of the side cores, the LP in the main core can be adjusted during transmission. 01 Model and LP 11 LP converted from model to core 01 The pattern is filtered out after it has been transmitted a certain distance.

[0039] 3. This invention can support LP. 01 LP 11 and LP 21 Three modes, and the LP in the main core01 and LP 11 The mode can be used with the LP in the side core. 01 The modes are coupled together, and because a gold film of a specific thickness is added to the inner wall of the air hole in the cladding of the side core, the modes coupled from the main core to the side core are allowed to travel a shorter distance in the side core before being lost. Ultimately, this allows the filter to output only a single LP within a specific wavelength range. 21 Mode. Numerical simulations show that this device has dual operating bands with bandwidths of 125 nm and 53 nm (1.5-1.625 μm and 1.766-1.819 μm, including S, C, and L bands), respectively. At 1550 nm, the insertion loss is only 0.024 dB, and the mode rejection ratio is 19.84 dB.

[0040] 4. This invention fully leverages the flexibility of photonic crystal fiber design. Without doping or using liquid crystals, it achieves three modes of transmission in the core by designing the air holes in the fiber cladding. It also boasts advantages such as high bandwidth, low insertion loss, high mode rejection ratio, and compact structure. The device is only 46.5mm long, and its output port is easily fused and combined with existing fiber optic communication systems, making it more suitable for miniaturized and integrated modern fiber optic communication systems. Attached Figure Description

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

[0042] Figure 2 (a) is the LP in the main core 01 The model field diagram;

[0043] Figure 2 (b) is the LP in the main core 11 The model field diagram;

[0044] Figure 2 (c) is the LP in the main core 21 The model field diagram;

[0045] Figure 2 (d) is the LP in the main core 02 The model field diagram;

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

[0047] Figure 4 (a) is the LP in the first side core of the present invention. 01 Graph showing 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 model varies 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 Graph showing the relationship between the effective refractive index of the model and the air hole spacing Λ2 of the first side core cladding;

[0050] Figure 4 (d) is the LP in the first side core. 01 LP in the mold and main core 11 Graph showing 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 this invention. 01 Graph showing 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 model varies 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 Graph showing 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 the mold and main core 01 Graph showing 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 this invention. 01 Mold and central core LP 01 Modulus Δ neff Relationship with RI variation graph;

[0056] Figure 6 (b) shows the left-side core LP with different gold film thicknesses. 01 Mold and central core LP 11 Modulus Δ neff Relationship with RI variation graph;

[0057] Figure 7 (a) represents the LP corresponding to different gold film thicknesses t in this invention. 01 Mode coupling efficiency varies with device length;

[0058] Figure 7 (b) represents the LP corresponding to different gold film thicknesses t. 11 Mode coupling efficiency varies with device length;

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

[0060] Figure 8 (b) shows the right-side core LP corresponding to different gold film thicknesses t. 01 Mode loss;

[0061] Figure 9 (a) In this invention, the filter modulator is used for LP 01 LP 11 Patterns and LPs 01 +LP 11 A graph showing the degree of mode suppression as a function of wavelength;

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

[0063] Figure 10 It is the main chip LP 21 Graph showing the variation of mode insertion loss with wavelength.

[0064] Figure label:

[0065] In the diagram: 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 Implementation

[0066] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0067] Please see Figure 1This 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 cores), and multiple layers of air holes; all three cores and the multiple layers of air holes are disposed within the substrate; the air holes in the main core are circular; the first side core is located to the left of the main core and consists of a semi-closed hexagonal first outer cladding and a fan-shaped first inner cladding; the second side core is located to the right of the main core and consists of a semi-closed hexagonal second outer cladding and a fan-shaped second inner cladding; the multiple layers of air holes are composed of regular hexagons excluding the main core, the first side core, and the second side core, and sequentially include 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 out, wherein the first layer of air holes a surrounds the main core, the first side core, and the second side core.

[0068] In this 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 layer of the first side core includes seven eighth air holes 8, of which five eighth air holes 8 are plated with a ninth gold film 9, and the air holes with the ninth gold film 9 are designated as third air holes 3; the first inner cladding layer 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 layer of the second side core includes thirteen eighth air holes, of which eight eighth air holes are provided with a tenth gold film 10, and the air holes with the tenth gold film are designated as fifth air holes 5; three of the thirteen eighth air holes coincide with the air holes in the first layer of air holes, and four of the thirteen eighth air holes coincide with the air holes in the first layer of air holes;

[0069] The second inner cladding in the second side core includes nine fourth air holes 4 and one seventh air hole 7, which 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 contain a plurality of eighth air holes. The size of the eighth air hole, the second air hole, the fourth air hole, the first air hole, the seventh air hole, and the sixth air hole decreases in sequence. The thickness of the gold film on the first outer layer and the second outer layer decreases in sequence, and the thickness of the ninth gold film and the tenth gold film decreases in sequence.

[0071] In this invention, the first outer cladding layer is a semi-closed hexagonal structure composed of an eighth air hole and a ninth gold film plated on the inner wall of the eighth air hole, and the first inner cladding layer is a first sector composed of a second air hole and a sixth air hole.

[0072] The second outer cladding layer 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 layer is a second sector composed of the fourth air hole and the seventh air hole;

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

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

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

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

[0077] In this invention, the circular area formed by the first air hole, the sixth air hole, and the seventh air hole constitutes the core of the filter.

[0078] In this invention, the first side core, the second side core, the gold film of a specific thickness outside the side core, and the cladding air holes serve to confine light within the side core and adjust the LP of the side core. 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] In this invention, the selection of the filter length takes into account both the optimal coupling length for the transition from the first and second side cores to the main core mode and the shortest distance for high loss in the side core mode. With the selected filter length, the LP of the main core... 01 Model and LP 11 The modules can be fully coupled to the side core and lose power.

[0080] It should be noted that mode filters, as a research hotspot in the field of photonics, have wide-ranging applications in fiber optic communication, fiber optic sensing, cloud computing, big data, the Internet of Things, and telemedicine. Photonic crystal fiber mode filters, characterized by high bandwidth, low loss, high mode suppression ratio, and compact structure, represent an important direction for the future development of optical quantum information technology.

[0081] In the broadband filter based on a three-core photonic crystal fiber proposed in this invention, all air holes adopt a circular structure and do not require doping with materials such as liquid crystals, making fabrication convenient and highly feasible. Due to the design of the side-core structure in this invention, the LP in the side core... 01The effective refractive index of the mode is matched with the effective refractive index of the higher-order modes in the core over a wide bandwidth, enabling the mode filter to output 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) refers to the LP present in the main core of this invention. 01 LP 11 LP 21 LP 02 The modulus field diagrams for these four modes, Figure 3 This graph shows the effective refractive index of the four modes and the difference in effective refractive index between adjacent modes as a function of wavelength. In a mode-division multiplexing system, the effective refractive index difference Δ between different modes is also shown. neff When the spectral density is >0.001, simultaneous transmission without interference is possible within the same few-mode fiber. As shown in the figure, in the wavelength range of 1.5μm-1.9μm, LP... 01 LP 11 LP 21 The difference Δ in the effective refractive index of the three modes neff Δ is satisfied near the center wavelength of 1.55 μm. neff >0.001, and Δ neff The extinction rate increases with increasing operating wavelength, indicating that intermode crosstalk is well suppressed, which is beneficial for improving the mode extinction ratio and thus achieving stable transmission; LP 21 With LP 02 The effective refractive index difference does not satisfy Δ at and near the center wavelength of 1.55 μm. neff A value >0.001 may indicate inter-mode crosstalk. Therefore, we discard LP. 02 Mode, only keep L P01 LP 11 LP 21 All three modes are used for transmission.

[0083] This invention employs a two-core structure. From an industrial manufacturing perspective, we should reduce the number of core parameters to facilitate fabrication. Λ1 is uniformly set to 4μm, and each parameter is accurate to two decimal places to meet industrial manufacturing precision requirements. The following section analyzes each core individually.

[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 model and the air hole radius r1 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 model varies 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) indicate that in the first side core, LP 01 The effective refractive index coefficient of the mode decreases with increasing radius r1; furthermore, its effective refractive index coefficient decreases with increasing d1; conversely, when the spacing between adjacent circles in the first side core decreases... Λ2 As the value increases, its effective refractive index coefficient also increases accordingly. (Comparison) Figure 4 (c) and Figure 3 It can be seen that when Λ2 = 2.5 μm, the LP in its first side core 01 The effective refractive index coefficient of the mode and the LP in the main core 11 The effective refractive index coefficients of the two modes are roughly similar. To further ensure phase matching between the two modes, Λ2 is adjusted to 2.6 μm. Furthermore, r1 and d1 are adjusted to allow the side-core LP to achieve different center wavelengths. 01 The effective refractive index coefficient of the mode and the LP in the main core 11 The effective refractive index coefficients of the modes are made as similar as possible to achieve phase matching. After analyzing different structural parameters one by one, we determined that the optimal air hole radius of the first side core is r1 = 0.65 μm, and the air hole radius at the connection with the main core is d1 = 0.35 μm. Figure 4 (d) is the LP in the first side core of this invention. 01 LP in the mold and main core 11 The graph showing the relationship between the effective refractive index difference of the modes and the wavelength of the input light is as follows: 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 coefficient of the mode decreases with increasing wavelength, and simultaneously, it differs from the LP in the main core. 11 The effective refractive index difference of the modes fluctuates with increasing wavelength. Near a wavelength of 1.55 μm, 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 this invention. 01 The relationship between the effective refractive index of the model and the air hole radius r2 of the second side core; Figure 5 (b) In this invention, the LP is in the second side core. 01 The effective refractive index of the model varies 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 this invention. 01The graph shows the relationship between the effective refractive index of the mode and the air hole spacing Λ4 of the second side core cladding; from Figure 5 From (a)(b)(c), it can be seen that LP in the second side core 01 The effective refractive index coefficient of the mode decreases with increasing cladding air hole radius r2 and air hole radius d2 at the connection with the main core, and increases with increasing distance Λ4 between adjacent centers. This is to ensure that LP in the second side core... 01 LP in mode and main core 01 By achieving phase matching in the mode, we obtained the optimal second side core air hole radius r2 = 0.49 μm, adjacent center distance Λ4 = 2.1 μm, and d2 = 0.45 μm. Figure 5 (d) is the LP in the second side core of this invention. 01 LP in the mold and main core 01 The graph showing the relationship between the effective refractive index difference of the modes and the wavelength of the input light is as follows: Figure 5 (d) shows that the effective refractive index coefficients of the two modes are basically equal in the band around 1.55 μm, thus achieving phase matching.

[0086] Figure 6 (a) is the second side core LP with different gold film thicknesses in this invention. 01 Mold and central core LP 01 Modulus Δ neff The variation of RI with wavelength; Figure 6 (b) is the left-side core LP under different gold film thicknesses in this invention. 01 Mold and central core LP 11 Modulus Δ neff The variation of RI with wavelength; by Figure 6 (a) and (b) show that, within the wavelength range of 1.5 μm to 1.9 μm, for different gold film thicknesses, the LP values ​​in the first and second side cores are... 01 The effective refractive index of the mode deviates only slightly, and its effective refractive index coefficient is similar to that of the LP in the main core. 01 LP 11 The effective refractive index coefficients of the modes are similar, and the difference in their effective refractive index coefficients is Δ. neff >0.001, therefore the phase matching between the main core and the secondary core modes is satisfied in the wavelength range of 1.5-1.9μm.

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

[0088] Figure 8 (a) is the first side core LP corresponding to different gold film thicknesses t in this invention. 01 Mode loss; Figure 8 (b) is the second side core LP corresponding to different gold film thicknesses t in this invention. 01 Mode loss; by Figure 8 (a) and (b) show that within the range of 1.5-1.9 μm, the LP in the first and second side cores 01 The limiting loss of the mode generally increases with the increase of the gold film thickness. To ensure a high coupling efficiency between the modes to be filtered in the main core and the modes in the auxiliary core, while also considering the limiting loss (LP) in the auxiliary core... 01 The filter has relatively high losses. Considering all factors, the thickness of the gold film on the first side core is set to 30nm, and the thickness of the gold film on the second side core is set to 50nm. Simultaneously, to ensure that the filter fully removes LP from the first and second side cores... 01 The filter length is selected as 46.5mm.

[0089] Figure 9 (a) is the filter of the present invention for LP 01 LP 11 Patterns and LPs 01 +LP 11 A graph showing the degree of mode suppression as a function of wavelength; Figure 9 (b) is the filter core LP of the present invention. 21 Mode and LP 01 The graph shows the variation of normalized mode power with transmission distance; from Figure 9 (a) It can be seen that in the wavelength ranges of 1.5–1.625 μm and 1.766–1.819 μm, this device exhibits good performance for LP wavelengths. 01 LP11 LP 01 +LP 11 The mode suppression ratios for all modes are greater than 10 dB. At a wavelength of 1550 nm, for LP... 01 LP 11 LP 01 +LP 11 The mode suppression ratios (PSRs) for the proposed filters were 20.8 dB, 26.6 dB, and 19.84 dB, respectively, indicating that the proposed filter exhibits good stray mode suppression performance. Furthermore, the LP values ​​in the core were calculated. 21 Mode and LP in the side core 01 The normalized mode power of the mode varies with transmission distance, as shown in the following results. Figure 9 As shown in (b). Figure 9 (b) It can be seen that after transmitting 46.5mm, the LP in the first and second side cores of the three modes... 01 The mode power was reduced to less than 1% of the initial input power, while the LP in the main core... 21 The mode power remains essentially unchanged.

[0090] Figure 10 The filter core LP of this invention 21 The mode insertion loss varies with wavelength; by Figure 10 It can be seen that within the wavelength range of 1.5-1.9 μm, the insertion loss gradually increases with increasing wavelength, but remains less than 0.057 dB. Specifically, at 1550 nm, the insertion loss of the main core LP... 21 The insertion loss of the mode is only 0.024 dB. This indicates that the main core LP 21 The mode can achieve 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 within the protection scope of the present invention.

Claims

1. A broadband mode filter based on a three-core photonic crystal fiber, characterized in that, include: A substrate having multiple air holes arranged therein, the air holes surrounding a main core, a first side core and a second side core, as well as 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 region of the substrate, and the first side core and the second side core are located in the regions to the left and right of the main core, respectively. Both the first and second side cores 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 layer and a first inner cladding layer formed by air holes respectively; The second side core includes: a second outer cladding layer and a second inner cladding layer formed by air holes, respectively; The first inner cladding layer and the second inner cladding layer are located on the left and right sides of the main core, respectively. The first outer cladding layer is located outside the first inner cladding layer, and the second outer cladding layer is located outside the second inner cladding layer. Both the first outer cladding layer and the second outer cladding layer have gold films deposited in their air pores; The core is surrounded by circular air holes. The first outer layer and the second outer layer are both semi-closed hexagonal structures, and the first inner layer and the second inner layer are both fan-shaped structures. The main core includes 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 have a ninth gold film plated on their inner walls. The air holes plated with the ninth gold film are referred to as third air holes. The first inner cladding layer includes eight second air holes and one sixth air hole, which coincides with the sixth air hole of the main core. The second outer cladding layer contains thirteen eighth air holes, eight of which have a tenth gold film plated on their inner walls. These air holes with the tenth gold film plated are designated as fifth air holes. Three of the thirteen eighth air holes coincide with the air holes in the first layer of air holes, and four of the thirteen eighth air holes coincide with the air holes in the first layer of air holes; the second inner cladding layer includes nine fourth air holes and one seventh air hole, which coincides 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 size of the eighth air hole, the second air hole, the fourth air hole, the first air hole, the seventh air hole, and the sixth air hole decreases in sequence, and the thickness of the gold film on the first outer layer and the second outer layer decreases in sequence. The first outer cladding layer is a semi-closed hexagonal structure composed of an eighth air hole and a ninth gold film plated on the inner wall of the eighth air hole, and the first inner cladding layer is a first sector composed of a second air hole and a sixth air hole. The second outer cladding layer 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 layer is a second sector composed of the fourth air hole and the seventh air hole; The circular area formed by the first air hole, the sixth air hole, and the seventh air hole constitutes the main core.

2. The broadband mode filter based on a three-core photonic crystal fiber according to claim 1, characterized in that, The radius of the sixth air hole is d1=0.35μm, the radius of the seventh air hole is d2=0.45μm, the radius of the first air hole is r0=0.45μm, and the center-to-center distance between any two adjacent air holes on the main core is Λ1=4μm.

3. The broadband mode filter based on a three-core photonic crystal fiber according to claim 1, characterized in that, The radius of the second air hole in the first inner cladding is r1=0.65μm, and the center-to-center distance between any two adjacent second air holes in the first inner cladding is Λ2=2.6μm.

4. The broadband mode filter based on a three-core photonic crystal fiber according to claim 1, characterized in that, The radius of the fourth air hole in the second inner cladding is r2 = 0.49 μm, and the center-to-center distance between any two adjacent fourth air holes in the second inner cladding is Λ4 = 2.1 μm.

5. The broadband mode filter based on a three-core photonic crystal fiber according to claim 1, characterized in that, The performance of a mode filter is determined by its mode rejection ratio, insertion loss, and bandwidth. The main chip supports LP 01 LP 11 and LP 21 Three modes, each with different paths and mode field distributions during transmission; The Mode Rejection Ratio (MSR) is defined as follows: at the input of the mode filter, multiple modes are transmitted simultaneously in the main core, and at the output, only the target mode LP is obtained. 21 Therefore, the mode suppression ratio is used to characterize the filtering effect of the filter, and its corresponding expression is as follows: ； In the formula, MSR 01 MSR 11 and MSR total These represent the filter modulators for LP. 01 LP 11 Patterns and LPs 01 +LP 11 The degree of suppression of the pattern, , ,and The output terminals LP in the main core are respectively 21 LP 01 LP 11 The optical power of the mode; The LP 01 + 11 LP 01 Patterns and LPs 11 Combination of patterns P out-main -LP 01 + 11 Main core output terminal LP 01 Mode optical power and LP 11 The sum of the optical power of the modes; The bandwidth is defined as the wavelength range in which the mode-division multiplexer operates when the MSR is greater than 10dB. The insertion loss is defined as the core LP. 21 The ratio of the mode input energy to its output energy is ten times the logarithm, specifically expressed as: ； In the formula, Indicates the output terminal LP in the main core 21 Mode power, Indicates the input terminal LP in the main core 21 The power of the mode.

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