A surface plasmon optical fiber with high thermal radiation performance

Abstract

The present invention provides a surface plasmon optical fiber with high thermal radiation performance, comprising an optical fiber core and a cladding. A metal layer arranged in a porous array is provided outside the optical fiber cladding. A thermal emissivity layer is provided on the side of the metal layer away from the optical fiber core, and a cooling layer is provided on the side of the thermal emissivity layer away from the optical fiber core. The optical fiber of the present invention utilizes the metal layer arranged in a porous array so that the cladding light excites surface plasmons at the interface between the metal layer and the cladding; the heat carried by the surface plasmon wave is transmitted through the thermal emissivity layer mainly by thermal radiation, in combination with heat conduction and heat convection. The optical fiber of the present invention can dissipate the heat generated by the cladding light while filtering out the cladding light, effectively improving the beam quality of the output laser and solving the problem of excessively high local temperature of the optical fiber.

Description

Technical Field

[0001] The present invention belongs to the field of optical fibers, and in particular relates to a surface plasmon optical fiber with high thermal radiation performance, which can be applied to scenarios such as optical fiber lasers, communication optical fibers, and optical fiber sensing. Background Art

[0002] Double-clad fiber lasers are beneficial for improving laser output power and pump coupling efficiency. Their cladding pumping scheme enables multimode pumping and single-mode output. However, the output laser contains a certain amount of cladding light, which seriously affects the beam quality and monochromaticity of the laser and requires timely filtering. In actual optical systems, the main factors that produce residual cladding light include: residual unabsorbed pump light at the end of the gain fiber; signal light and amplified spontaneous emission (ASE) that leak into the cladding during the propagation of the core light; core light leakage into the cladding due to splice loss; mode mismatch at the fiber splice point; and leakage of higher-order modes in the core due to fiber bending. Light that escapes the fiber transmission line interacts with other materials and generates heat. In high-power fiber lasers, the residual cladding light power can reach hundreds of watts, generating a large amount of heat that accumulates in a small area. Residual cladding light will affect the output beam quality of the fiber laser and pose a significant threat to the output equipment at the end of the fiber laser, such as the collimator, and in severe cases, it can cause damage or even burn out.

[0003] However, existing methods for filtering cladding light fail to effectively integrate cladding light removal with heat dissipation. Extensive cladding light filtering places significant strain on heat dissipation. Common methods for filtering cladding light include coating the inner cladding with a high-refractive-index adhesive and etching the optical fiber with hydrofluoric acid. These methods can easily lead to localized heat accumulation and excessively high temperatures. Traditional heat dissipation methods, such as air cooling and indirect water cooling, are complex, bulky, and expensive to maintain.

[0004] Surface plasmons (SPs) are near-field electromagnetic waves that propagate along the metal surface when a light wave (electromagnetic wave) is incident on the interface between a metal and a dielectric, causing the free electrons on the metal surface to collectively oscillate. The electromagnetic wave couples with the free electrons on the metal surface, forming a near-field electromagnetic wave. SPs exhibit excellent local field confinement and field enhancement properties. Existing fiber-optic surface plasmon resonance sensors remove a section of the optical fiber's cladding and then wrap the core with a metal layer. The evanescent wave generated by total internal reflection of the incident light compensates for the wave vector of the free-space light wave, exciting SPs on the metal surface and enabling research in sensing and filtering. However, in fiber lasers, filtering the cladding light requires disrupting the total internal reflection condition, causing the cladding light to refract out of the cladding. Directly depositing a metal layer on the surface of the optical fiber cladding cannot guarantee the confinement and filtering of the cladding light. This is because, in order for a fiber-optic surface plasmon resonance sensor to excite SPs on the metal surface for sensing, it is necessary to match the free-space light wave vector with the surface plasmon wave vector. This requires a comprehensive consideration of the incident angle of the incident wave and the refractive index of the medium on either side of the metal layer. In practice, the incident angle of the cladding light is unknown and complex. Except at certain angles, the cladding light cannot excite SPs on the metal surface and the transmittance is low. Therefore, achieving the desired effect of confinement and filtering of the cladding light requires redesign and reconsideration. Summary of the Invention

[0005] Purpose of the Invention: To address the problem that existing methods cannot effectively balance filtering cladding light and heat dissipation, the present invention aims to provide a surface plasmon fiber with high thermal radiation performance. While effectively confining and filtering cladding light, it can effectively dissipate the heat of the cladding light, reducing the problem of excessive fiber temperature. This is of great significance for ensuring the high reliability, high beam quality, high power, and good spectral characteristics of the fiber.

[0006] Technical solution: To achieve the above-mentioned purpose, the present invention proposes a surface plasmon optical fiber with high thermal radiation performance, which includes an optical fiber core (1), an optical fiber cladding (2) is provided on the outside of the optical fiber core (1), a metal layer (3) is provided on the outside of the optical fiber cladding (2), and a hole array is arranged on the surface of the metal layer (3), the refractive index of the metal layer (3) is greater than the refractive index of the optical fiber cladding (2), and the cladding light forms plasmons at the interface between the optical fiber cladding (2) and the metal layer (3), and the surface plasmon waves excited by the cladding light propagate along the direction of the metal surface and decay exponentially in the direction perpendicular to the metal surface; a thermal emissivity layer (4) is provided on the outside of the metal layer (3), and the emissivity of the thermal emissivity layer (4) is greater than the emissivity of the metal layer (3).

[0007] Preferably, the hole array on the metal layer (3) extends along the axial direction of the optical fiber and is evenly arranged around the optical fiber axis.

[0008] Preferably, the size of the holes in the hole array and the arrangement period of the holes are related to the wavelength of the cladding light, the refractive index of the optical fiber cladding, and the refractive index of the metal layer, so that the surface plasmon waves within the specified wavelength range excited by the cladding light are confined to an area inside the metal layer and with a spatial size smaller than its free space wavelength.

[0009] Preferably, the shape of the holes on the hole array is any one of rectangular, circular, triangular and polygonal.

[0010] Preferably, the thickness of the metal layer (3) is greater than the penetration depth of the surface plasmon wave in the metal layer.

[0011] Preferably, the metal layer (3) is any one of aluminum, titanium, gold, silver, and copper.

[0012] Preferably, the thermal emissivity layer (4) is any one of carbon, silicon carbide, silicon dioxide, and silicon nitride coatings.

[0013] Preferably, a cooling layer (5) is provided outside the thermal emissivity layer (4).

[0014] Preferably, the cooling layer (5) is any one of a nanoporous polyethylene layer and a silica gel layer.

[0015] The present invention also proposes a working method for a surface plasmon optical fiber with high thermal radiation performance, the working method comprising the following steps: incident light enters the optical fiber, generating cladding light, the cladding light is incident on the interface between the optical fiber cladding (2) and the metal layer (3), forming a surface plasmon wave; the surface plasmon wave propagates along the direction of the metal surface and decays exponentially in the direction perpendicular to the metal surface; the heat carried by the surface plasmon wave is transmitted through the thermal emissivity layer mainly in the form of thermal radiation, coordinated heat conduction and heat convection.

[0016] Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0017] (1) The residual light of the cladding is incident on the metal layer with the hole array. Under the action of surface plasmons, it is well confined to the subwavelength spatial size, effectively limiting the energy and propagation path of the residual light of the cladding.

[0018] (2) By utilizing the structure with contrasting thermal emissivity, the heat carried by the surface plasmon wave is transmitted through the thermal emissivity layer mainly by thermal radiation, synergistically by heat conduction and heat convection, thereby improving the heat dissipation performance, avoiding the problem of excessive temperature, and having a small size and being relatively stable.

[0019] (3) It takes into account both the effects of filtering out the residual light in the cladding and effectively dissipating the heat of the residual light in the cladding. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the cross-sectional structure of a surface plasmon optical fiber with high thermal radiation performance according to the present invention;

[0021] Figure 2 A schematic diagram of the outer surface of the metal layer of a surface plasmon optical fiber with high thermal radiation performance according to the present invention;

[0022] Figure 3 Schematic diagram of the SP propagation and thermal radiation principle of a surface plasmon optical fiber with high thermal radiation performance according to the present invention;

[0023] Figure 4 Schematic diagram of the local radial SP electric field (within one hole array period) of Example 1;

[0024] Figure 5 is a graph showing the relationship between the transmission coefficient and wavelength of Example 1;

[0025] Figure 6 is a graph showing the relationship between transmittance and reflectance and incident angle in Example 1;

[0026] Figure 7 Graph showing the relationship between transmittance and reflectance and incident angle for Comparative Example 1;

[0027] Reference numerals: 1. optical fiber core; 2. optical fiber cladding; 3. metal layer; 4. thermal emissivity layer; 5. cooling layer. DETAILED DESCRIPTION

[0028] The present invention proposes a surface plasmon optical fiber with high thermal radiation performance. The optical fiber comprises an optical fiber core (1), an optical fiber cladding (2) is arranged on the outside of the optical fiber core (1), a metal layer (3) is arranged on the outside of the optical fiber cladding (2), and a hole array is arranged on the surface of the metal layer (3). The refractive index of the metal layer (3) is greater than the refractive index of the optical fiber cladding (2). Cladding light forms plasmons at the interface between the optical fiber cladding (2) and the metal layer (3). The surface plasmon waves excited by the cladding light propagate along the direction of the metal surface and decay exponentially in the direction perpendicular to the metal surface. A thermal emissivity layer (4) is arranged on the outside of the metal layer (3), and the emissivity of the thermal emissivity layer (4) is greater than the emissivity of the metal layer (3).

[0029] Exemplarily, the metal layer (3) is deposited onto the optical fiber cladding (2) by magnetron sputtering, and the thickness of the metal layer (3) is controlled by the power and time of sputtering; the structure of the porous array is made by an ion beam method; the thermal emissivity layer (4) is coated with a slurry onto the metal layer (3) by a scraper, and the thickness of the thermal emissivity layer (4) can be changed by replacing the gauge of the scraper; finally, the cooling layer (5) is coated onto the outside of the thermal emissivity layer (4).

[0030] The present invention proposes a method for operating a surface plasmon optical fiber with high thermal radiation performance, comprising the following steps: incident light enters the optical fiber, generating cladding light, the cladding light being incident on the interface between the optical fiber cladding (2) and the metal layer (3), forming a surface plasmon wave; the surface plasmon wave propagates along the direction of the metal surface and decays exponentially in a direction perpendicular to the metal surface; and the heat carried by the surface plasmon wave is transmitted through the thermal emissivity layer in a manner mainly of thermal radiation, coordinated with heat conduction and heat convection.

[0031] The principle of filtering out cladding light is that when light is incident on a metal film with a sub-wavelength periodic hole array, the light transmission efficiency is greatly enhanced, breaking through the limits of traditional aperture diffraction theory. Figures 1 to 3 As shown, the metal layer (3) is a structure with a multi-hole array arrangement, so that the wave vector of the cladding light freely emitted from the optical fiber cladding (2) satisfies the phase matching condition and achieves wave vector matching with the SP, thereby being able to excite surface plasmons at the interface between the optical fiber cladding (2) and the metal layer (3), causing local field confinement and field enhancement. The excited surface plasmon wave propagates along the direction of the metal surface and decays exponentially in the direction perpendicular to the metal surface; due to the unique properties of the periodic hole array metal film, the surface plasmon wave can be confined to an area inside the metal layer, where the spatial size is much smaller than its free space wavelength, effectively limiting the propagation path of light; thereby avoiding the free scattering of the filtered cladding light, reducing the cladding light remaining in the cladding, and improving the beam quality of the output laser.

[0032] The principle of effectively dissipating the heat of the cladding light is as follows: the metal layer (3) arranged in the hole array structure and the waveguide structure of the optical fiber cladding (2) can excite surface plasmons, excite the cladding residual light into surface plasmon waves, and effectively limit the propagation path of the cladding light. However, the filtered cladding residual light will generate a large amount of heat. On the basis of the above technical solution, a thermal emissivity layer (4) is provided on the outside of the metal layer (3). Since the thermal emissivity of the metal layer (3) is lower than that of the thermal emissivity layer (4), the use of such a structure with a large difference in thermal emissivity can effectively dissipate heat in a manner mainly based on thermal radiation, in conjunction with heat conduction and heat convection, and ultimately organically combine the dual needs of filtering out the cladding residual light and heat dissipation.

[0033] In the present invention, the metal layer (3) is any one of aluminum, titanium, gold, silver, and copper. The metal layer (3) with a hole array structure made of these materials can excite surface plasmon waves. The thermal emissivity layer (4) is any one of carbon, silicon carbide, silicon dioxide, and silicon nitride coating. Such materials have a higher thermal emissivity than the metal layer (3) and form a structure with a large difference in thermal emissivity with the metal layer (3), which can effectively dissipate heat. It should be understood that the materials that can be used for the metal layer (3) and the thermal emissivity layer (4) include but are not limited to the above materials.

[0034] Preferably, the thickness of the metal layer (3) is greater than the penetration depth of the surface plasmon in the metal, so that the surface plasmon wave can be more fully coupled out of the cladding light.

[0035] In terms of the structure of the holes, the present invention does not require any limitation on the shape of the holes. For example, the hole shape can be any of rectangular, circular, triangular, and polygonal. The size of the holes on the hole array and the hole arrangement period are related to the wavelength of the cladding light, the refractive index of the optical fiber cladding (2), and the refractive index of the metal layer (3). As long as the surface plasmon waves within the specified wavelength range excited by the cladding light can be confined to the inner side of the metal layer and the spatial size is smaller than its free space wavelength. Preferably, the hole array on the metal layer (3) extends along the axial direction of the optical fiber and is evenly arranged around the optical fiber axis, so that the heat dissipation of the cladding light is more uniform.

[0036] In order to obtain a better heat dissipation effect, a cooling layer (5) is provided outside the thermal emissivity layer (4). The cooling layer (5) is used to reduce the temperature of the thermal emissivity layer so that the heat carried by the surface plasmon wave inside the metal layer (3) can be better conducted out. Preferably, the cooling layer (5) is any one of a nanoporous polyethylene layer and a silica gel layer.

[0037] Example 1

[0038] This embodiment provides a surface plasmon fiber with high thermal radiation performance. The surface plasmon fiber comprises a fiber core, a cladding, a metal layer with an array of holes, and a thermal emissivity layer. The core is made of ytterbium-doped silica glass with a refractive index of 1.3-1.4; the cladding is made of silica glass with a refractive index of 1.2-1.3; the metal layer is an aluminum coating with a refractive index of 1.3641 and an emissivity of 0.35 at a wavelength of 1080 nm; the holes are rectangular in shape, with a length and width of 675 nm and 150 nm, respectively, and a hole arrangement period of 750 nm in both the axial and radial directions of the fiber; and the thermal emissivity layer is a carbon coating with an emissivity of 0.9.

[0039] Example 2

[0040] This embodiment provides a surface plasmon fiber with high thermal radiation performance. The surface plasmon fiber comprises a fiber core, a cladding, a metal layer with an array of holes, and a thermal emissivity layer. The core is made of ytterbium-doped silica glass with a refractive index of 1.3-1.4; the cladding is made of silica glass with a refractive index of 1.2-1.3; the metal layer is an aluminum coating with a refractive index of 1.3641 and an emissivity of 0.35 at a wavelength of 1080 nm; the holes are shaped like isosceles triangles with a base and height of 200 nm and 800 nm, respectively, and the hole arrangement period is 750 nm in both the axial and radial directions of the fiber; and the thermal emissivity layer is a carbon coating with an emissivity of 0.9.

[0041] Example 3

[0042] This embodiment provides a surface plasmon fiber with high thermal radiation performance. The surface plasmon fiber comprises a fiber core, a cladding, a metal layer with an array of holes, and a thermal emissivity layer. The core is made of ytterbium-doped silica glass with a refractive index of 1.3-1.4; the cladding is made of silica glass with a refractive index of 1.2-1.3; the metal layer is a titanium coating with a refractive index of 3.3489 and an emissivity of 0.20 at a wavelength of 1080 nm; the holes are rectangular in shape, with a length and width of 675 nm and 150 nm, respectively, and a period of 750 nm in both the axial and radial directions of the fiber; and the thermal emissivity layer is a carbon coating with an emissivity of 0.9.

[0043] Example 4

[0044] This embodiment provides a surface plasmon fiber with high thermal radiation performance. The surface plasmon fiber comprises a fiber core, a cladding, a metal layer with an array of holes, and a thermal emissivity layer. The core is made of ytterbium-doped silica glass with a refractive index of 1.3-1.4; the cladding is made of silica glass with a refractive index of 1.2-1.3; the metal layer is an aluminum coating with a refractive index of 1.3641 and an emissivity of 0.35 at a wavelength of 1080 nm; the holes are rectangular in shape, with a length and width of 675 nm and 150 nm, respectively, and a hole arrangement period of 750 nm in both the axial and radial directions of the fiber; and the thermal emissivity layer is a silicon carbide coating with an emissivity of 0.8.

[0045] Comparative Example 1

[0046] This comparative example provides a surface plasmon fiber. The surface plasmon fiber comprises a fiber core, a cladding, and a metal layer. The metal layer has no hole array and no thermal emissivity layer. The fiber core is made of ytterbium-doped silica glass with a refractive index of 1.3-1.4; the cladding is made of silica glass with a refractive index of 1.2-1.3; and the metal layer is an aluminum coating with a refractive index of 1.3641 at a wavelength of 1080 nm.

[0047] Comparative Example 2

[0048] This comparative example provides a surface plasmon fiber. The surface plasmon fiber comprises a fiber core, a cladding, and a metal layer with an array of holes, without a thermal emissivity layer. The fiber core is made of ytterbium-doped silica glass with a refractive index of 1.3-1.4; the cladding is made of silica glass with a refractive index of 1.2-1.3; the metal layer is an aluminum coating with a refractive index of 1.3641 at a wavelength of 1080 nm; the holes are rectangular in shape, with a length and width of 675 nm and 150 nm, respectively, and the hole arrangement period is 750 nm in both the axial and radial directions of the fiber.

[0049] Table 1 shows the temperature simulation results of the surface plasmon fibers of Examples 1-4 and Comparative Examples 1-2 under the conditions of natural convection and a generalized heat source.

[0050] Table 1 Temperature simulation results of surface plasmon fibers of Examples 1-4 and Comparative Examples 1-2

[0051]

[0052]

[0053] Simulation results analysis:

[0054] The partial schematic diagram of the SP electric field of Example 1 is shown in Figure 4 The relationship between transmission coefficient and wavelength is shown in Figure 5 , the electric field intensity at the four corners of the hole array is significantly enhanced, indicating that surface plasmons are excited.

[0055] Figure 6 and Figure 7 Graphs showing the relationship between transmittance T0 and reflectance R0 as a function of incident angle at the same wavelength for Example 1 and Comparative Example 1 demonstrate that the designed hole array structure significantly enhances the transmittance of cladding light. In Comparative Example 1, cladding light is unable to enter the metal layer; in Example 1, the cladding light excites surface plasmons using the hole array, coupling out more cladding light energy in the mid-infrared band, thereby enhancing transmittance. Compared to Comparative Example 1, the structure of Example 1 effectively filters out cladding light.

[0056] The temperature simulation results for Example 1 and Comparative Example 1 in Table 1 demonstrate that the combined structure of the metal layer and the thermal emissivity layer, with their contrasting emissivity, enables uniform temperature distribution and output, alleviating the pressure on heat dissipation caused by the significant filtering of cladding light, resulting in high thermal radiation performance. The temperature simulation results for Comparative Examples 1 and 2 demonstrate that even if surface plasmon waves are excited using only a metal layer without a thermal emissivity layer, the heat carried by the surface plasmon waves cannot be dissipated due to their confinement to the metal layer.

[0057] The temperature simulation results of Example 1 and Example 2 in Table 1 show that different hole shapes can excite surface plasmon waves, achieving the effect of high heat dissipation performance.

[0058] The temperature simulation results of Example 1 and Example 3 in Table 1 show that both aluminum and titanium can achieve high heat dissipation performance. In practical applications, aluminum is preferred for preparing the metal layer to reduce cost and process difficulty.

[0059] The temperature simulation results of Example 1 and Example 4 in Table 1 show that different materials of the thermal emissivity layer can achieve high heat dissipation performance.

[0060] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, without departing from the principles of the present invention, several anticipated improvements and modifications can be made, including but not limited to changes in thickness and coating materials. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A surface plasmon fiber with high thermal radiation performance, characterized in that: The optical fiber comprises an optical fiber core (1), an optical fiber cladding (2) is provided on the outside of the optical fiber core (1), a metal layer (3) is provided on the outside of the optical fiber cladding (2), and a hole array is arranged on the surface of the metal layer (3), the refractive index of the metal layer (3) is greater than the refractive index of the optical fiber cladding (2), and cladding light forms plasmons on the interface between the optical fiber cladding (2) and the metal layer (3), and the surface plasmon waves excited by the cladding light propagate along the direction of the metal surface and decay exponentially in the direction perpendicular to the metal surface; a thermal emissivity layer (4) is provided on the outside of the metal layer (3), and the emissivity of the thermal emissivity layer (4) is greater than the emissivity of the metal layer (3).

2. The surface plasmon fiber with high thermal radiation performance according to claim 1, characterized in that: The hole array on the metal layer (3) extends along the axial direction of the optical fiber and is evenly arranged around the optical fiber axis.

3. The surface plasmon fiber with high thermal radiation performance according to claim 1, characterized in that: The size of the holes in the hole array and the arrangement period of the holes are related to the wavelength of the cladding light, the refractive index of the optical fiber cladding (2), and the refractive index of the metal layer (3), so that the surface plasmon waves within the specified wavelength range excited by the cladding light are confined to an area inside the metal layer with a spatial size smaller than its free space wavelength.

4. The surface plasmon fiber with high thermal radiation performance according to claim 1, characterized in that: The hole shapes on the hole array are any one of circular and polygonal.

5. The surface plasmon optical fiber with high thermal radiation performance according to claim 1, characterized in that: The thickness of the metal layer (3) is greater than the penetration depth of the surface plasmon wave in the metal layer.

6. The surface plasmon optical fiber with high thermal radiation performance according to claim 1, characterized in that: The metal layer (3) is any one of aluminum, titanium, gold, silver, and copper.

7. The surface plasmon fiber with high thermal radiation performance according to claim 1, characterized in that: The thermal emissivity layer (4) is any one of carbon, silicon carbide, silicon dioxide, and silicon nitride coatings.

8. A surface plasmon optical fiber with high thermal radiation performance according to any one of claims 1 to 7, characterized in that: A cooling layer (5) is provided outside the thermal emissivity layer (4).

9. The surface plasmon optical fiber with high thermal radiation performance according to claim 8, characterized in that: The cooling layer (5) is any one of a nanoporous polyethylene layer and a silica gel layer.

10. The method for operating a surface plasmon fiber with high thermal radiation performance according to any one of claims 1 to 7, characterized in that: The working method comprises the following steps: incident light enters the optical fiber, generating cladding light, the cladding light is incident on the interface between the optical fiber cladding (2) and the metal layer (3), and a surface plasmon wave is formed; the surface plasmon wave propagates along the direction of the metal surface and decays exponentially in the direction perpendicular to the metal surface; and the heat carried by the surface plasmon wave is transmitted through the thermal emissivity layer (4) mainly in the form of thermal radiation, in conjunction with thermal conduction and thermal convection.

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