A long bandwidth, high extinction ratio photonic crystal fiber filter

Abstract

This invention discloses a long-bandwidth, high-extinction-ratio photonic crystal fiber filter, belonging to the field of fiber optic filtering technology. The technical solution includes a photonic crystal fiber, comprising a core and air holes disposed around the core. The air holes include cladding air holes and SPR air holes. The cladding air holes include three different diameters: large circular air holes, medium circular air holes, and small circular air holes. The medium circular air holes are periodically distributed in a regular hexagonal pattern. Four large circular air holes are symmetrically distributed in a rhomboid pattern around the core. The small circular air holes are close to the core and distributed on both sides of the core. The SPR air holes are two symmetrically distributed elliptical air holes on both sides of the core, with gold and graphene layers sequentially deposited on their inner surfaces. The major axis of the elliptical air holes is parallel to the vertical axis of the photonic crystal fiber. This invention's filter has the advantages of long bandwidth and high extinction ratio, with a bandwidth exceeding 800 nm and a maximum extinction ratio exceeding 250 dB.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber filtering, and particularly relates to a long-bandwidth high-extinction-ratio photonic crystal fiber filter. BACKGROUND

[0002] The optical fiber filter is a device for selecting specific polarized light through a special structure, and can select or filter out specific wavelength polarized light from multiple polarized light as needed, and has great application prospects in the fields of optical fiber communication, optical fiber sensing and spectrum analysis.

[0003] SPR is a resonance oscillation phenomenon of electrons at the interface of positive and negative dielectric constant materials under the excitation of incident light. Specifically, when a monochromatic polarized plane wave is incident on the glass surface coated with a metal layer or a metal wire filled position at a certain angle and the wave vector and plasmon oscillation frequency are matched, the optical energy can be coupled to the metal surface to cause the SPR phenomenon. The interaction of surface charge oscillation and photo electromagnetic field makes SPP have many unique and meaningful properties. Among them, the photonic crystal fiber polarization filter based on SPR effect is a very important functional application.

[0004] Since foreign scholars proposed in 1993 that photonic crystal fiber (PCF) is used as a carrier to excite surface plasmon mode, many fiber functional devices based on SPR effect have been developed. The PCF polarization filter based on SPR effect has been widely concerned by scholars at home and abroad. Most of the existing related researches are to change the structure of the photonic crystal fiber to obtain a relatively good extinction ratio. Although these structures have good polarization filtering characteristics, most of the PCF structures are designed very complicatedly, which increases the difficulty of preparing the filter and reduces the feasibility of engineering application. Since the PCF has high freedom, it is very easy to combine with metal materials, and the combination of the two can greatly reduce the size of the device and has excellent performance. Based on the photonic crystal fiber and the SPR effect, the existing disclosed inventions have two characteristics. One is that the cladding is a circular hole, and the hole is filled or plated with metal, such as CN211293338U and CN110568545B. The other is to process the structure of the photonic crystal fiber and combine it with metal materials, such as CN112444914B and CN111208601B. The former is easily affected by the shape of the hole, and the latter is more likely to break the glass fiber due to the structural processing. Therefore, the optical fiber polarization filter scheme of introducing holes other than circular holes at the end face of the optical fiber and not processing the structure of the optical fiber is more popular. SUMMARY

[0005] The long-bandwidth high-extinction-ratio photonic crystal fiber filter has the advantages of long bandwidth and high extinction ratio, and has excellent performance of covering the commonly used communication window of 1550nm.

[0006] To achieve the above object, the application adopts the following technical scheme: a long-bandwidth high-extinction-ratio photonic crystal fiber filter, comprising a photonic crystal fiber, the photonic crystal fiber comprising a fiber core and air holes arranged around the fiber core, the air holes being arranged in a radial cross section of the photonic crystal fiber, the air holes comprising cladding air holes and SPR air holes, the cladding air holes comprising three different diameters of large circular air holes, medium circular air holes and small circular air holes, the medium circular air holes being periodically arranged in a regular hexagon, four large circular air holes being symmetrically arranged in a diamond around the fiber core, the small circular air holes being close to the fiber core and arranged on both sides of the fiber core, the SPR air holes being two elliptical air holes symmetrically arranged on both sides of the fiber core and comprising a gold layer and a graphene layer arranged on the inner surface of the elliptical air holes in sequence, the SPR air holes being between adjacent large circular air holes and small circular air holes, and the long axis of the elliptical air hole being parallel to the vertical axis of the photonic crystal fiber.

[0007] Further, the four small circular air holes are arranged in a rectangle around the fiber core, and the small circular air holes are located at the vertices of the rectangle.

[0008] Further, the material of the photonic crystal fiber is N-SF57 glass.

[0009] Further, the center distance between the adjacent large circular air holes and the medium circular air holes and the center distance between two adjacent medium circular air holes are the same, and the center distance is 2μm.

[0010] Further, the diameter ratio of the large circular air holes to the medium circular air holes is 2, and the diameter of the small circular air hole is 0.2μm.

[0011] Further, the length of the long semi-axis of the elliptical air hole is 1.3μm, and the length of the short semi-axis is 0.4μm.

[0012] Further, the thickness of the gold layer is 0.05μm, and the thickness of the graphene layer is 0.02μm.

[0013] Compared with the prior art, the application has the following beneficial effects:

[0014] (1) Strong energy coupling occurs between the fiber core conduction mode and the plasmonic mode excited by the metal, thereby enhancing the polarization effect in the x direction;

[0015] (2) The filter has the advantages of long bandwidth and high extinction ratio, when the length of the optical fiber is 2mm, the bandwidth exceeds 800nm, the maximum extinction ratio is more than 250dB, and the excellent performance can cover the commonly used communication window 1550nm. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 It is a schematic diagram of the radial cross-sectional structure of the embodiment 1 of the application;

[0017] Figure 2 It is a schematic diagram of the structure parameters of the embodiment 1 of the application;

[0018] Figure 3 It is a mode field distribution diagram of the embodiment 1 of the application;

[0019] Figure 4 It is a graph of the relationship between the limited loss and the wavelength of the embodiment 1 of the application and the comparative example 1 and the comparative example 2;

[0020] Figure 5 It is a graph of the relationship between the limited loss and the wavelength of the embodiment 1 of the application and the comparative example 3 and the comparative example 4;

[0021] Figure 6 It is a graph of the relationship between the limited loss and the wavelength of the embodiment 1 of the application and the comparative example 5 and the comparative example 6;

[0022] Figure 7 It is a graph of the relationship between the limited loss and the wavelength of the embodiment 1 of the application and the comparative example 7 and the comparative example 8;

[0023] Figure 8 It is a graph of the relationship between the limited loss and the wavelength of the embodiment 1 of the application and the comparative example 9 and the comparative example 10;

[0024] Figure 9 It is a graph of the relationship between the limited loss and the wavelength of the embodiment 1 of the application and the comparative example 11 and the comparative example 12;

[0025] Figure 10 It is a graph of the normalized power spectrum of the embodiment 1 of the application under different optical fiber lengths;

[0026] Figure 11 It is a graph of the extinction ratio spectrum of the embodiment 1 of the application under different optical fiber lengths;

[0027] Wherein, 1 is a photonic crystal fiber, 2 is a medium circular air hole, 3 is a large circular air hole, 4 is a small circular air hole, 5 is an elliptical air hole, 6 is a gold layer, and 7 is a graphene layer. DETAILED DESCRIPTION

[0028] The application will be further described below in combination with the drawings and examples.

[0029] The application provides a photonic crystal fiber filter with long bandwidth and high extinction ratio. Figure 1 As shown in the figure, the filter comprises a photonic crystal fiber 1, the photonic crystal fiber 1 comprises a fiber core and air holes arranged outside the fiber core, the air holes are arranged in a radial cross section of the photonic crystal fiber, the air holes comprise cladding air holes and SPR air holes, the cladding air holes comprise three different diameters of large circular air holes 3, medium circular air holes 2 and small circular air holes 4, the medium circular air holes 2 are periodically and regularly hexagonally distributed, four large circular air holes 3 are regularly and symmetrically distributed in a rhombus around the fiber core, the small circular air holes 4 are close to the fiber core and are distributed on both sides of the fiber core, the SPR air holes are two elliptical air holes 5 symmetrically distributed on both sides of the fiber core and are sequentially provided with a gold layer 6 and a graphene layer 7 on inner surfaces of the elliptical air holes 5, the SPR air holes are between the adjacent large circular air holes 3 and the small circular air holes 4, and a long axis of the elliptical air hole 5 is parallel to a vertical axis of the photonic crystal fiber.

[0030] The filter is made of N-SF57 glass, the material is environmentally friendly, is suitable for a visible light to near infrared light wave band, is commonly used as a base medium of a PCF and is used for generating extremely high nonlinearity. When the wavelength is 1.06 microns, the nonlinearity coefficient is 4.1x10 -19 m 2 / W; at 1550 nm, the higher refractive index is 1.81, and at the same time, the loss can be as low as 0.3 dB / m. The N-SF57 glass also has a low softening temperature of 520 DEG C, is suitable for using an extrusion method in the preparation of the optical fiber, and compared with a traditional stacking technology, the extrusion method is simpler and easier to realize.

[0031] The application introduces the elliptical air hole and the gold layer and the graphene layer arranged on the inner surface of the elliptical air hole to form the SPR air hole, the SPR air hole generates SPR effect with the unidirectional polarization light wave of the fiber core, the device reaches the good filtering performance that the extinction ratio is more than 20 dB in the whole investigation wave band at 2 mm and does not need to make complex design on the structure of the photonic crystal fiber, reduces the difficulty of device preparation, and avoids the problem that the designed fiber structure cannot be realized in actual preparation.

[0032] In order to quantify the phenomenon of loss limitation, the following theory is introduced: when the effective refractive index of the fiber core mode and the surface plasmon polariton (SPP) mode of the metal surface is matched, that is, the phase matching condition is met, strong energy coupling will occur between the x-polarized mode of the fiber core and the excited SPP mode, thereby expanding the polarization difference in the x and y directions, that is, enhancing the birefringence characteristics. Based on the enhancement of birefringence, the filter in the present application is plated with a gold layer in the elliptical air hole to produce SPR with single-mode single-polarized light in the core, and the polarization light at a specific wavelength is filtered out based on this characteristic.

[0033] In the radial cross-sectional structure of the filter in the present application, the gold layer is arranged in the elliptical air hole to produce SPR effect with unidirectional polarized light in the core. The critical surface of gold and dielectric can produce special SPP mode under the stimulating action of light, and SPR effect will be produced when certain conditions are met. This special optical effect can be used to regulate the optical transmission characteristics of the PCF.

[0034] The relative dielectric constant of gold can be given by the Drude-Lorentz model as formula (1):

[0035]

[0036] In the formula: ε (m) is the relative dielectric constant of gold; ε ∞ = 5.9673 is the dielectric constant of metal at high frequency; Δε = 1.09 is the weighting factor; ω is the angular frequency of incident light; ω D and γ d are the plasma frequency and damping frequency of gold, wherein ω D / 2π = 2113.6 THz, γ d / 2π = 15.92 THz; Ω L and Г L represent the frequency and spectral width of Lorentz oscillation, wherein Ω L / 2π = 650.07 THz, Г L / 2π = 104.86 THz; j represents the complex imaginary symbol.

[0037] The refractive index n g (λ) of graphene is given by formula (2):

[0038]

[0039] In the formula: C1 = 5446 μm -1 , λ is the wavelength (μm), and i represents the complex imaginary symbol.

[0040] For SPR, resonance phenomenon occurs when the energy and momentum of the incident light match the surface plasmon wavelength (SPW), in other words, when the phase matching condition (PMC) is satisfied. The necessary condition for SPR is expressed as formula (3) as follows:

[0041]

[0042] wherein λ is the wavelength of the transmitted light, θ is the angle of the incident light, ε p is the dielectric constant of the material in which the metal is deposited, ε m and ε s are the dielectric constants of the metal and the sensing medium, respectively.

[0043] The confinement loss L loss in the photonic crystal fiber is proportional to the imaginary part Im(n eff ) of the effective refractive index, and the specific relationship is formula (4) as follows:

[0044]

[0045] wherein n eff is the effective refractive index of the core mode.

[0046] Embodiment 1

[0047] A filter comprising a photonic crystal fiber, the photonic crystal fiber comprising a core and air holes arranged around the core, the air holes being arranged in a radial cross-section of the photonic crystal fiber, the air holes comprising cladding air holes and SPR air holes, the cladding air holes comprising three different diameters of large circular air holes 3, medium circular air holes 2 and small circular air holes 4, the medium circular air holes 2 being arranged in a periodic hexagonal distribution, four of the large circular air holes 3 being arranged in a diamond symmetry around the core, the small circular air holes 4 being arranged close to the core and on both sides of the core, the SPR air holes being two elliptical air holes 5 arranged symmetrically on both sides of the core and comprising a gold layer 6 and a graphene layer 7 arranged in sequence on the inner surface thereof, the SPR air holes being located between the adjacent large circular air holes 3 and the small circular air holes 4, the major axis of the elliptical air holes 5 being parallel to the vertical axis of the photonic crystal fiber; the material of the photonic crystal fiber being N-SF57 glass.

[0048] The four small circular air holes 4 are arranged in a rectangular distribution around the core, and the small circular air holes 4 are located at the vertices of the rectangle.

[0049] The diameter ratio δ = d2 / d1 = 2 of the large circular air holes to the medium circular air holes, the diameter d3 of the small circular air holes is 0.2 μm, the diameter d2 of the large circular air holes is 1.2 μm, the diameter d1 of the medium circular air holes is 0.6 μm,

[0050] The center distance between the adjacent large circular air holes and the middle circular air holes and the center distance between two adjacent middle circular air holes are the same, and the center distance Λ is 2 μm. a The long semi-axis length l b of the elliptical air hole is 1.3 μm, and the short semi-axis length l Au is 0.4 μm;

[0051] The thickness t G of the gold layer is 0.05 μm; and the thickness t Au of the graphene layer is 0.02 μm.

[0052] As shown in the figure, the structure outer layer array of the present application adopts three different diameter circular air holes as cladding air holes, which are periodically arranged in a hexagonal shape, and the three different diameter circular air holes are referred to as large circular air holes, middle circular air holes and small circular air holes. The center distance between the adjacent large circular air holes and the middle circular air holes and the center distance between two adjacent middle circular air holes are the same, and the elliptical air holes are symmetrically arranged on both sides of the center of the optical fiber. The elliptical air hole structure has a large ratio of long axis to short axis, forming a long and narrow elliptical shape. The cladding air holes are plated with a gold layer and a graphene layer, the long and narrow shape and the symmetric distribution can not only limit the light wave path in the center of the optical fiber, but also form a double-layer metal layer structure with a small pore, and N-SF57 is used as the base material of the whole optical fiber structure, and the whole is designed according to the photonic crystal fiber structure.

[0053] Figure 2 is a structural parameter diagram of the embodiment 1 of the present application, wherein d1, d2 and d3 respectively represent the diameters of different circular air holes, Λ represents the center distance between the adjacent middle circular air holes and the center distance between the adjacent large circular air holes and the middle circular air holes, t b and t a respectively represent the thicknesses of the gold layer and the graphene layer, and the a axis and the b axis respectively represent the long axis and the short axis of the ellipse. The vertical axis of the photonic crystal fiber is the y axis as shown. Figure 2

[0054] As shown in the figure, the structure outer layer array of the present application adopts three different diameter circular air holes as cladding air holes, which are periodically arranged in a hexagonal shape, and the three different diameter circular air holes are referred to as large circular air holes, middle circular air holes and small circular air holes. The center distance between the adjacent large circular air holes and the middle circular air holes and the center distance between two adjacent middle circular air holes are the same, and the elliptical air holes are symmetrically arranged on both sides of the center of the optical fiber. The elliptical air hole structure has a large ratio of long axis to short axis, forming a long and narrow elliptical shape. The cladding air holes are plated with a gold layer and a graphene layer, the long and narrow shape and the symmetric distribution can not only limit the light wave path in the center of the optical fiber, but also form a double-layer metal layer structure with a small pore, and N-SF57 is used as the base material of the whole optical fiber structure, and the whole is designed according to the photonic crystal fiber structure. Figure 3 is the mode field distribution diagram of the embodiment 1 of the present application, and the arrow represents the electric field direction under different modes, wherein, Figure 3 (a) is the x polarization mode; Figure 3 (b) is the y polarization mode; Figure 3 (c) is the resonance mode field generated by the x polarization mode and the SPP mode. Due to the existence of the resonance mode field generated by the x polarization mode and the SPP mode, a great energy will be released on the x polarization, which causes a great difference in energy between the x polarization and the y polarization. The size of the difference directly determines the performance of the filter.

[0055] ​By adjusting the signal intensity of two polarization directions and the working waveband, the polarization filtering function of the filter at specific wavelength can be realized. Since the signal investigated in the application is around 1550 nm, the working wavelength and signal intensity change regularly with the structure parameters, and considering the preparation error and material loss during use, the basic structure parameters of the filter are determined as δ = d2 / d1 = 2, d2 = 1.2 μm, d1 = 0.6 μm, Λ = 2.0 μm, l b = 0.4 μm, l a = 1.3 μm, t Au = 0.05 μm, t G = 0.02 μm.

[0056] Comparative Example 1

[0057] The same as Example 1, except that the diameter ratio δ of the large circular air hole and the middle circular air hole is 1.9.

[0058] Comparative Example 2

[0059] The same as Example 1, except that the diameter ratio δ of the large circular air hole and the middle circular air hole is 2.1.

[0060] Comparative Example 3

[0061] The same as Example 1, except that the center distance Λ is 1.9 μm.

[0062] Comparative Example 4

[0063] The same as Example 1, except that the center distance Λ is 2.1 μm.

[0064] Comparative Example 5

[0065] The same as Example 1, except that the long semi-axis length l a of the elliptical air hole is 1.25 μm.

[0066] Comparative Example 6

[0067] The same as Example 1, except that the long semi-axis length l a of the elliptical air hole is 1.35 μm.

[0068] Comparative Example 7

[0069] The same as Example 1, except that the short semi-axis length l b of the elliptical air hole is 0.35 μm.

[0070] Comparative Example 8

[0071] The same as Example 1, except that the short semi-axis length lb It is 0.45μm.

[0072] Comparative Example 9

[0073] Same as Example 1, except for the gold layer thickness t Au It is 0.045μm.

[0074] Comparative Example 10

[0075] Same as Example 1, except for the gold layer thickness t Au It is 0.055μm.

[0076] Comparative Example 11

[0077] Same as Example 1, except for the thickness t of the graphene layer. G It is 0.015μm.

[0078] Comparative Example 12

[0079] Same as Example 1, except for the thickness t of the graphene layer. G It is 0.025μm.

[0080] The relationship between the confinement loss of Example 1 and Comparative Examples 1-12 and wavelength is as follows: Figures 4 to 9 As shown. Figures 4 to 9 As shown, these represent the limited loss conditions of devices under different structural parameters, revealing the magnitude and wavelength of the peak value. Figure 4 As shown, in Example 1, Comparative Example 1, and Comparative Example 2, changing the diameter ratio δ = d2 / d1 of the large circular air hole and the medium circular air hole can change the wavelength band where the loss peak is located. When the ratio δ is equal to 1.9 to 2.1, the operating wavelength is near the commonly used communication window of 1550nm. Preferably, δ = 2.0. Figure 5 As shown, in Example 1, Comparative Example 3, and Comparative Example 4, changing the center-to-center distance Λ can also change the wavelength band of the loss peak and thus change the peak value. When the center-to-center distance Λ is between 1.9 and 2.1 μm, the wavelength band of the peak value is also within the commonly used communication window of 1550 nm. Preferably, Λ = 2.0 μm. Figure 6 and Figure 7 As shown, in Examples 1 and Comparative Examples 5 to 8, by changing the length of the major semi-axis 1a and the length of the minor semi-axis 1b of the elliptical air hole, the loss difference in the two polarization directions can be increased by increasing the length of the dual axes. Preferably, 1 b =0.4μm, l a =1.3μm; such as Figure 8 As shown, in Example 1, Comparative Example 9, and Comparative Example 10, the gold layer thickness t was varied. AuThis can significantly alter the signal intensity in both polarization directions. Preferably, t Au =0.05μm; such as Figure 9 As shown, in Example 1, Comparative Example 11, and Comparative Example 12, the thickness t of the graphene layer was varied. G This can slightly change the signal intensity in the two polarization directions. Preferably, t G =0.02μm.

[0081] By changing the fiber length L, the normalized output power and extinction ratio can be adjusted. Under the filter structure parameters of Example 1, the fiber length L of the filter is 2mm, the maximum extinction ratio is as high as 250dB, and an ultra-long bandwidth of over 800nm ​​is obtained, which can cover the O+E+S+C+L bands.

[0082] like Figures 10 to 11 As shown, the graphs represent the normalized power and extinction ratio for different fiber lengths L. This invention uses normalized output power (NOP) and extinction ratio (ER) to evaluate the filter performance. Normalized power and extinction ratio are closely related numerically and are quantified by a calculation formula, as shown in formula (7):

[0083] Normalized output power P out It can be obtained through equation (7):

[0084]

[0085] Where the input power P is assumed in α is 1, L represents the fiber length, and α(x,y) is the limiting loss.

[0086] The extinction ratio ER can be calculated using formula (8):

[0087]

[0088] Where P out (x) and P out (y) is the output power in the x and y polarization directions.

[0089] Figure 10 The NOP spectral lines are shown at different lengths. In Example 1, according to Formula (7), the NOP in the x polarization direction is always close to 0, while the NOP in the y polarization direction is larger. There is a very obvious energy difference between the two polarization directions. As the filter length increases, the change in energy difference becomes more and more obvious.

[0090] Figure 11 The area marked by the red dotted line represents an extinction ratio of 20 dB. Figure 11As can be seen, when the filter length of the application is greater than or equal to 2 mm, the extinction ratio is above 20 dB in the entire investigation band, not only having good filtering performance, but also having an ultra-long bandwidth, and the working waveband covers O+E+S+C+L wavebands.

Claims

1. A long bandwidth, high extinction photonic crystal fiber filter, characterized in that, The photonic crystal fiber comprises a core and air holes arranged outside the core, the air holes are arranged in the radial cross section of the photonic crystal fiber, the air holes comprise cladding air holes and SPR air holes, the cladding air holes comprise three different diameters of large circular air holes, medium circular air holes and small circular air holes, a plurality of the medium circular air holes are periodically arranged in a regular hexagon, four large circular air holes are symmetrically arranged in a diamond around the core, the small circular air holes are close to the core and are arranged on both sides of the core, the SPR air holes are two elliptical air holes symmetrically arranged on both sides of the core and are sequentially provided with a gold layer and a graphene layer on the inner surface thereof, the SPR air holes are between the adjacent large circular air holes and the small circular air holes, and the major axis of the elliptical air hole is parallel to the vertical axis of the photonic crystal fiber. Four small circular air holes are arranged in a rectangle around the core, and the small circular air holes are located at the vertices of the rectangle. The center distance between the adjacent large circular air holes and the medium circular air holes and the center distance between two adjacent medium circular air holes are the same, and the center distance is 2μm.

2. The long-bandwidth high- extinction-ratio photonic crystal fiber filter according to claim 1, wherein, The material of the photonic crystal fiber is N-SF57 glass.

3. The long-bandwidth high- extinction-ratio photonic crystal fiber filter according to claim 1, wherein, The diameter ratio of the large circular air hole to the medium circular air hole is 2, and the diameter of the small circular air hole is 0.2μm.

4. The long-bandwidth high- extinction-ratio photonic crystal fiber filter according to claim 1, wherein, The length of the major axis of the elliptical air hole is 1.3μm, and the length of the minor axis is 0.4μm.

5. The long-bandwidth high- extinction-ratio photonic crystal fiber filter according to claim 1, wherein, The thickness of the gold layer is 0.05μm, and the thickness of the graphene layer is 0.02μm.

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