A fiber grating based on a ferrofluid and a single mode optical fiber with a hole array

Abstract

The application provides a fiber grating based on magnetic fluid and single-mode optical fiber with hole array. The fiber grating is composed of a uniform magnetic field source 1, a single-mode optical fiber with hole array 2, a magnetic fluid 3 and a UV sealing glue 4. By adjusting the magnetic field intensity of the uniform magnetic field source, the refractive index of the fiber core of the single-mode optical fiber with axial hole can be tuned periodically, so that a fiber grating with tunable central wavelength can be obtained. The application can be used in the fields of fiber sensing and fiber communication, and has the advantages of small volume, low cost and compact structure.

Description

Technical Field

[0001] The present invention relates to an optical fiber grating based on magnetic fluid and single-mode optical fiber with hole array, belonging to the technical field of magnetic fluid-based optical fiber devices and optical fiber gratings. Background Art

[0002] Fiber Bragg Grating (FBG) is a reflective passive fiber component that uses a special method to create a periodic refractive index within the fiber core. It essentially forms a narrowband reflection filter within the core. When incident light enters the fiber core and propagates, it is reflected when the Bragg condition is met and returns along the original transmission fiber path. Light waves of other wavelengths can pass through without loss. The transmitted light waves can then be transmitted to other fiber Bragg Grating arrays with different central wavelengths, where narrowband light of the corresponding central wavelengths is reflected one by one and all are reflected back along the original path.

[0003] Fiber Bragg gratings (FBGs) offer a compact size, excellent wavelength selectivity, immunity to nonlinear effects, low cost, and integration with other fiber optic components, resulting in excellent practicality. FBGs are used in fiber optic sensing, fiber optic communications, and optical information processing. FBGs are perfectly compatible with fiber optic systems and hold significant research value in the field of fiber-optic magnetic field sensing.

[0004] There are many types of fiber Bragg gratings, which are mainly divided into uniform fiber Bragg gratings and non-uniform fiber Bragg gratings.

[0005] The refractive index variation period of a common uniform fiber Bragg grating (FBG) is typically on the order of 0.1 μm, with a narrow reflection bandwidth. It is widely used in sensors and optical communications. A fiber Bragg grating (FBG) is a reflective structure in the core of an optical fiber that periodically or non-periodically perturbs the effective refractive index. It is a core component in a variety of systems, including optical fiber sensors, optical communication systems, optical information processing systems, and precision detection devices. It modifies or controls the propagation behavior and mode of light in that region by establishing a periodic refractive index distribution along the optical axis. Fiber Bragg grating fabrication methods include traditional standing wave writing, point-by-point writing, phase masking, and holographic writing. The refractive index variation period of a uniform long-period fiber grating (LPFBG) is typically on the order of 100 μm. It can couple the forward-propagating guided mode of incident light within a certain wavelength range into the cladding mode, reducing losses. Common fabrication methods for long-period fiber gratings include metal masking and point-by-point writing.

[0006] In 2016, Chen Daru et al. proposed a method for manufacturing fiber Bragg grating based on coreless optical fiber. A femtosecond laser and a focusing system are controlled by a computer system to write on the coreless optical fiber. The coreless optical fiber is used as the core, and the surrounding air acts as the cladding to form a waveguide. A section of coreless optical fiber with Bragg grating is formed, which overcomes the difficulty of laser writing on the fiber core. This method needs to determine the reflection center wavelength of the Bragg grating and the grating period.

[0007] In 2017, Xu Feng et al. proposed a low-cost method for manufacturing long-period fiber grating. The method mainly proposes etching solution gas-phase etching of optical fiber to form periodic distribution of etching deep grooves on the surface of the optical fiber. The periodic distribution of etching deep grooves forms a periodic stress change, and the photoelastic effect of the optical fiber causes a periodic modulation of the refractive index of the fiber core, thereby forming a long-period grating. This method is improved compared to the above method, and the grating period size can be changed.

[0008] In 2020, Deng Shijie et al. proposed a large-range tunable filter of programmable fiber grating based on magnetic fluid. The method takes advantage of the characteristics of the internal refractive index change of the magnetic fluid programmable optical fiber under the influence of external magnetic field to control the grating period of the fiber grating so that light of different wavelengths can pass through, thereby expanding the filtering range and realizing controllable grating period. By controlling the energization state of the micro electromagnet, different grating periods of the fiber grating are generated, the center wavelength is obtained, and different wavelength filtering is obtained. This invention controls the grating period of the fiber grating through an electrode array.

[0009] The present invention proposes a fiber grating with adjustable center wavelength based on magnetic fluid filled hole array single mode fiber. The single mode fiber array hole is formed by a femtosecond laser on a single mode fiber. The small holes of the single mode fiber with hole array in the hole array are filled with magnetic fluid and sealed with sealing glue. By adjusting the magnetic field intensity of the uniform magnetic field source, the magnetic nanoparticles of the magnetic fluid are rearranged, the effective refractive index of the waveguide composed of the fiber and the magnetic fluid changes, and the center wavelength of the grating changes, thereby forming a fiber grating with adjustable center wavelength. This structure has the characteristics of strong flexibility, adjustable center wavelength, simple structure, etc., and has good application prospect. SUMMARY

[0010] The present invention aims to provide a fiber grating based on magnetic fluid and hole array single mode fiber with simple and compact structure, easy operation and adjustment.

[0011] The purpose of the present invention is achieved as follows:

[0012] The fiber Bragg grating (FBG) based on magnetic fluid and a single-mode fiber with a hole array comprises a uniform magnetic field source 1, a single-mode fiber with a hole array 2, a magnetic fluid 3, and a UV sealant 4. The small holes in the single-mode fiber with a hole array 2 are filled with magnetic fluid 3. UV sealant 4 is then applied to both ends of the holes in the hole array region filled with magnetic fluid to seal the array holes. The sealed single-mode fiber is placed in the uniform magnetic field source 1. When the uniform magnetic field strength is changed, the magnetic fluid in the hole array is polarized by the magnetic field and redirected to form magnetic flux chains, causing the refractive index of the magnetic fluid in the array holes to change. This results in a periodic variation in the refractive index of the single-mode fiber along its core, making the core refractive index of the single-mode fiber with the axial holes filled with magnetic fluid tunable.

[0013] The single-mode optical fiber with a hole array is obtained by using a femtosecond laser to scan and punch holes at equal intervals along the longitudinal direction of the single-mode optical fiber. The cross-sectional shape of each hole in the array is a small hole that penetrates the core of the single-mode optical fiber or is parallel to the core of the single-mode optical fiber (see the attached figure in the specification). Before using the femtosecond laser for scanning and punching, the single-mode optical fiber is first fixed on a three-dimensional object displacement platform using an optical fiber clamp, and the single-mode optical fiber is placed in the center of the femtosecond laser three-dimensional displacement platform. The femtosecond laser is then controlled by operating the femtosecond laser, such as adjusting and setting the femtosecond laser drilling parameters (drilling distance, hole cross-sectional size, etc.) to obtain the required single-mode optical fiber hole array parameters.

[0014] Perfusion of magnetic fluid in single-mode optical fiber with hole array: Since the small holes of the hole array are invisible to the naked eye, injecting magnetic fluid into the small holes can use the capillary action of the small holes to absorb the magnetic fluid into the small holes. Therefore, immersing the single-mode optical fiber in the magnetic fluid for filling the magnetic fluid is conducive to completely filling the small holes with magnetic fluid.

[0015] Sealing of magnetic fluid in small holes of single-mode optical fiber with hole array: The small holes of the single-mode optical fiber with hole array are filled with magnetic fluid and the small hole area filled with magnetic fluid is coated and sealed with ultraviolet sealant along the optical fiber core on both sides of the hole array.

[0016] The hole size in the hole array single-mode optical fiber is small. The magnetic fluid is immersed in the array hole, which actually forms a magnetic fluid film microcavity or an approximate Fabry-Perot cavity with equal spacing arranged along the single-mode optical fiber. The transmittance of the magnetic fluid film is

[0017]

[0018] is the effective density of the liquid phase under the action of an external perpendicular magnetic field, A is the cross-sectional area of ​​the magnetic fluid film, and A col is the cross-sectional area occupied by the magnetic flux in area A, I and I0 represent the transmitted light power intensity and the initial light power intensity, respectively. When the external uniform magnetic field intensity increases, the weak flocculation state of the magnetic particles increases, and the equivalent liquid phase density decreases, affecting the light transmittance.

[0019] Each aperture in the array forms a Fabry-Perot cavity. The thickness of the magnetic fluid film is δ, which is calculated based on the aperture size. Since the absorption coefficient spectrum is ln(1 / T) = αδ, where T is the transmittance of the magnetic fluid film, α is the absorption coefficient, and δ is the film thickness, the reflectivity R of the magnetic fluid film can be calculated. When incident light enters a single-mode optical fiber, the number of magnetic fluid film microcavities it passes through is equal to the number of apertures in the array (1, 2, 3, ..., N). If the transmitted light power is significantly greater than the reflected light power and coupling occurs between the core fundamental mode and cladding modes propagating in the same direction, a long-period fiber grating (LPG) is formed, passing through N magnetic fluid films. If the reflected light power is significantly greater than the transmitted light power and coupling occurs between two core modes propagating in opposite directions, a fiber Bragg grating (FBG) is formed, passing through 2N magnetic fluid films. To improve grating efficiency, a special coating is applied to the apertures in the array to enhance reflection efficiency.

[0020] When no magnetic field is applied, the refractive index of the magnetic fluid is 1.40. The magnetic nanoparticles in the magnetic fluid are evenly distributed around the single-mode optical fiber, and the refractive index of the magnetic fluid is isotropic. When an external magnetic field is applied, the refractive index of the magnetic fluid changes with the change of the magnetic field according to the Langevin formula, that is,

[0021]

[0022] Where n0 is the external magnetic field less than a critical magnetic field strength H c,n is the refractive index of the magnetic fluid at saturation, ns is the refractive index of the magnetic fluid at saturation, T is the thermodynamic temperature, H is the strength of the applied magnetic field, and α is the adjustment parameter. From a microscopic perspective, the magnetic nanoparticles in the magnetic fluid rearrange to form magnetic nanochains due to the applied magnetic field. However, the distribution of these magnetic nanochains within the optical fiber array aperture is uneven and exhibits anisotropy.

[0023] The effective refractive index n can be calculated based on the magnetic field change and the single-mode fiber array hole size parameters. eff , the fiber Bragg grating formed by the single-mode fiber array hole has a reflection center wavelength of where n eff is the effective refractive index of the waveguide composed of a single-mode optical fiber with array holes and a magnetic fluid, and ∧ is the period of the fiber grating. When the grating period ∧ is less than 1 μm, it is a Bragg fiber grating, and when ∧ is greater than 1 μm, it is a long-period fiber grating.

[0024] The array of magnetic fluid thin film microcavities within the apertures of a single-mode fiber array can be viewed as a structure similar to a Fabry-Perot cavity. When incident light enters a single-mode fiber, each time it passes through a magnetic fluid thin film microcavity, some light is reflected back, some is transmitted forward, and some is absorbed and lost. Besides the light energy absorbed by the magnetic fluid, if the reflected light power is significantly greater than the transmitted light power and coupling occurs between two core modes propagating in opposite directions, a structure similar to a Fiber Bragg Grating (FBG) is formed. If the transmitted light power is significantly greater than the reflected light power and coupling occurs between the core fundamental mode and cladding mode propagating in the same direction, a structure similar to a Long-Period Fiber Grating (LPG) is formed.

[0025] The ratio and concentration of the magnetic fluid are different. When the magnetic field changes, the effective refractive index of the waveguide composed of the magnetic fluid in the array holes also changes.

[0026] The array holes can be in the shape of circular holes, rectangular holes or irregular holes. When magnetic fluid enters the holes, a magnetic fluid film is formed. The size of the array holes ranges from 8.0 to 10.0 μm.

[0027] Under the influence of an external uniform magnetic field, the arrangement of the magnetic nanoparticles in the magnetic fluid array changes, altering the effective refractive index formed with the fiber core, thereby controlling the effective refractive index of the grating. Specifically, given a fixed grating period, when a specific grating wavelength is required, the desired effective refractive index can be calculated. By varying the intensity of the uniform magnetic field, which influences the arrangement of the magnetic nanoparticles in the magnetic fluid and, consequently, the effective refractive index of the grating, the core refractive index of a single-mode optical fiber with an axial hole filled with magnetic fluid can be made tunable, thereby achieving a fiber Bragg grating with a tunable center wavelength.

[0028] Compared with the prior art, the advantages of the present invention are:

[0029] The present invention has a simple structure and strong applicability. By utilizing the characteristic that the refractive index of the grating inside the array hole optical fiber changes with the change of the external uniform magnetic field, a fiber grating with adjustable center wavelength is obtained. The center wavelength of this grating has a large tunable range. By controlling the strength of the external uniform magnetic field, the requirements for tuning the center wavelength of the grating over a wide range can be met, and it can be widely used in the fields of optical fiber sensing and optical fiber communication. Femtosecond laser technology is mature, easy to operate, highly stable, short in pulse width, high in power generation, and does not generate excess heat, thus avoiding damage, melting, and fracture of the material during processing. By adjusting the magnetic field strength of the uniform magnetic field source, the core refractive index of the single-mode optical fiber with the axial hole filled with magnetic fluid can be made to have tunable periodicity, thereby obtaining a fiber grating with adjustable center wavelength. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1This is a schematic diagram of a fiber Bragg grating based on magnetic fluid and single-mode optical fiber with hole array proposed in the present invention.

[0031] Numbers in the figure: 1 uniform magnetic field source, 2 single-mode optical fiber with hole array, 3 magnetic fluid, 4 ultraviolet sealant. DETAILED DESCRIPTION

[0032] The present invention will be further described below in conjunction with specific implementations.

[0033] The fabrication of fiber Bragg gratings based on magnetic fluid and single-mode fiber with hole arrays includes the following steps:

[0034] A 100cm section of ordinary single-mode optical fiber is scanned and punched longitudinally with a femtosecond laser at equal intervals. The cross-sectional shape of each hole in the array is a small hole that penetrates or is parallel to the core of the single-mode fiber. Before using the femtosecond laser for scanning and drilling, the single-mode fiber is first fixed to a three-dimensional displacement platform using a fiber clamp. The single-mode fiber is then placed in the center of the femtosecond laser three-dimensional displacement platform. The femtosecond laser is then controlled by adjusting the femtosecond laser drilling parameters (drilling distance, hole cross-sectional dimensions, etc.) to obtain the desired single-mode fiber hole array parameters.

[0035] Perfusion of magnetic fluid in single-mode optical fiber with hole array: Since the small holes of the hole array are invisible to the naked eye, injecting magnetic fluid into the small holes can use the capillary action of the small holes to absorb the magnetic fluid into the small holes. Therefore, immersing the single-mode optical fiber in the magnetic fluid for filling the magnetic fluid is conducive to completely filling the small holes with magnetic fluid.

[0036] Sealing of magnetic fluid in the small holes of a single-mode optical fiber with a hole array: The small holes of a single-mode optical fiber with a hole array are filled with magnetic fluid, and the small hole area filled with magnetic fluid is coated and sealed with sealant along the optical fiber core on both sides of the hole array to obtain a single-mode optical fiber with an array of holes and magnetic fluid sealed in the holes.

[0037] The prepared single-mode optical fiber with a hole array and a magnetic fluid in the holes was placed in a uniform magnetic field. The strength of the uniform magnetic field was varied. The magnetic nanoparticles in the magnetic fluid were rearranged by the external magnetic field to form magnetic nanochains. The distribution of the magnetic nanochains within the holes of the optical fiber array was uneven, exhibiting anisotropy. The refractive index of the magnetic fluid changed, and the effective refractive index of the optical fiber array holes also changed.

[0038] The hole size in the hole array single-mode optical fiber is small. The magnetic fluid is immersed in the array hole, which actually forms a magnetic fluid film microcavity or an approximate Fabry-Perot cavity with equal spacing arranged along the single-mode optical fiber. The transmittance of the magnetic fluid film is

[0039]

[0040] is the effective density of the liquid phase under the action of an external perpendicular magnetic field, A is the cross-sectional area of ​​the magnetic fluid film, and A col is the cross-sectional area occupied by the magnetic flux in area A, I and I0 represent the transmitted light power intensity and the initial light power intensity, respectively. When the external uniform magnetic field intensity increases, the weak flocculation state of the magnetic particles increases, and the equivalent liquid phase density decreases, affecting the light transmittance.

[0041] Each aperture in the array forms a Fabry-Perot cavity. The thickness of the magnetic fluid film is δ, which is calculated based on the aperture size. Since the absorption coefficient spectrum is ln(1 / T) = αδ, where T is the transmittance of the magnetic fluid film, α is the absorption coefficient, and δ is the film thickness, the reflectivity R of the magnetic fluid film can be calculated. When incident light enters a single-mode optical fiber, the number of magnetic fluid film microcavities it passes through is equal to the number of apertures in the array (1, 2, 3, ..., N). If the transmitted light power is significantly greater than the reflected light power and coupling occurs between the core fundamental mode and cladding modes propagating in the same direction, a long-period fiber grating (LPG) is formed, passing through N magnetic fluid films. If the reflected light power is significantly greater than the transmitted light power and coupling occurs between two core modes propagating in opposite directions, a fiber Bragg grating (FBG) is formed, passing through 2N magnetic fluid films. To improve grating efficiency, a special coating is applied to the apertures in the array to enhance reflection efficiency.

[0042] When no magnetic field is applied, the refractive index of the magnetic fluid is 1.40. The magnetic nanoparticles in the magnetic fluid are evenly distributed around the single-mode optical fiber, and the refractive index of the magnetic fluid is isotropic. When an external magnetic field is applied, the refractive index of the magnetic fluid changes with the change of the magnetic field according to the Langevin formula, that is,

[0043]

[0044] Where n0 is the external magnetic field less than a critical magnetic field strength H c,n is the refractive index of the magnetic fluid at saturation, ns is the refractive index of the magnetic fluid at saturation, T is the thermodynamic temperature, H is the strength of the applied magnetic field, and α is the adjustment parameter. From a microscopic perspective, the magnetic nanoparticles in the magnetic fluid rearrange to form magnetic nanochains due to the applied magnetic field. However, the distribution of these magnetic nanochains within the optical fiber array aperture is uneven and exhibits anisotropy.

[0045] The effective refractive index n can be calculated based on the magnetic field change and the single-mode fiber array hole size parameters. eff , the fiber Bragg grating formed by the single-mode fiber array hole has a reflection center wavelength of where n effThe effective refractive index of the waveguide composed of the single-mode fiber with arrayed holes and the magnetic fluid, and Λ is the period of the fiber grating, wherein when the grating period Λ < 1 μm, it is a Bragg fiber grating, and when Λ > 1 μm, it is a long-period fiber grating.

[0046] The arrayed magnetic fluid microcavity array in the arrayed holes of the single-mode fiber can be regarded as a structure similar to a Fabry-Perot cavity, the incident light enters the single-mode fiber, and after passing through each magnetic fluid microcavity, the light is reflected back, transmitted forward, and absorbed. Except for the light energy absorbed by the magnetic fluid, if the reflected light power is much greater than the transmitted light power and coupling occurs between two core modes in the opposite transmission direction, a Bragg fiber grating (FBG) is formed; if the transmitted light power is much greater than the reflected light power and coupling occurs between the core fundamental mode and the cladding mode in the same transmission direction, a long-period fiber grating (LPG) is formed.

[0047] When the magnetic field changes, the effective refractive index of the waveguide composed of the magnetic fluid in the arrayed holes also changes.

[0048] The small hole shape of the arrayed holes can be a circular hole, a rectangular hole or an irregular hole, and the magnetic fluid enters the small hole to form a magnetic fluid film. The size of the arrayed holes ranges from 8.0 μm to 10.0 μm.

[0049] Under the influence of the external uniform magnetic field, the arrangement of the magnetic nanoparticles in the magnetic fluid in the arrayed holes changes, and the effective refractive index formed with the core changes, so as to achieve the purpose of controlling the effective refractive index of the grating. Specifically, when a certain grating wavelength is needed, the effective refractive index required can be obtained through calculation when the grating period is certain. By changing the strength of the uniform magnetic field, the arrangement of the magnetic nanoparticles of the magnetic fluid is affected, and then the effective refractive index of the grating is affected, so that the core refractive index of the single-mode fiber with the axial hole filled with the magnetic fluid has a tunable periodicity, and thus a fiber grating with a tunable central wavelength can be obtained.

[0050] The working process of the present application has been described in detail above. For those skilled in the art, according to the idea provided by the present application, there can be non-essential changes in the specific implementation mode, such as changing the type of fiber, changing the type of magnetic field source, etc. Such changes should also be regarded as the protection scope of the present application.

Claims

1. A fiber Bragg grating based on magnetic fluid and single-mode optical fiber with hole array, characterized by: The fiber grating is composed of a uniform magnetic field source (1), a single-mode optical fiber with a hole array (2), a magnetic fluid (3), and an ultraviolet sealant (4); A single-mode fiber with a hole array is a single-mode fiber with small holes arranged at equal intervals along the length of the single-mode fiber. The size of the small holes is consistent with the core diameter of the single-mode fiber and ranges from 8.0 to 10.0 μm. The small holes of the hole array single-mode optical fiber (2) are filled with magnetic fluid (3), and then ultraviolet sealant (4) is applied to both ends of the holes in the hole array area filled with magnetic fluid to seal the array holes. The sealed single-mode optical fiber is placed in a uniform magnetic field source (1). When the uniform magnetic field strength is changed, the magnetic fluid in the hole array is polarized by the magnetic field and redirected to form magnetic chains, causing the refractive index of the magnetic fluid in the array holes to change. In this way, the refractive index of the single-mode optical fiber along the core changes periodically, making the core refractive index of the single-mode optical fiber with the axial hole filled with magnetic fluid tunable.

2. The fiber Bragg grating based on magnetic fluid and hole array single-mode optical fiber according to claim 1, characterized in that: The small holes in the single-mode optical fiber with hole array are punched by femtosecond laser drilling.

3. The fiber Bragg grating based on magnetic fluid and hole array single-mode optical fiber according to claim 1, characterized in that: The small holes of the single-mode optical fiber with a hole array are filled with magnetic fluid, and the small hole area filled with magnetic fluid is coated and sealed with ultraviolet sealant along the optical fiber core on both sides of the hole array.

4. The fiber Bragg grating based on magnetic fluid and hole array single-mode optical fiber according to claim 1, characterized in that: The single-mode optical fiber is placed in a uniform magnetic field source. By adjusting the magnetic field strength of the uniform magnetic field source, the refractive index of the magnetic fluid in the hole array along the core direction of the single-mode optical fiber can be tuned within a certain range, thereby obtaining a fiber grating with tunable central wavelength.

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