Ultralow nonlinear gain optical fiber and preparation method thereof

Abstract

The invention belongs to the technical field of special optical fibers, and particularly discloses an ultralow nonlinear gain optical fiber and a preparation method thereof. The optical fiber comprises a fiber core, a core-cladding junction area and an inner cladding which are sequentially arranged from inside to outside in the radial direction, and the fiber core is air; the core package junction region is a rare earth doped region; the inner cladding comprises a plurality of glass thin walls which are periodically distributed around the core cladding junction area, and the glass thin walls are filled with air. The rare earth ions are doped in the core-cladding junction area of the band gap optical fiber, gain amplification of a fiber core fundamental mode is achieved through coupling of a core mode and a surface mode, light is limited in an air fiber core for transmission based on the photonic band gap light guide principle, and the nonlinear effect of the optical fiber can be effectively reduced.

Description

Technical Field

[0001] This invention belongs to the field of special optical fiber technology, and more specifically, relates to an ultra-low nonlinear gain optical fiber and its fabrication method. Background Technology

[0002] A fiber laser is a type of laser that uses optical fiber as the gain medium. It generates laser light by doping the fiber with rare-earth ions (such as erbium-doped fiber). When irradiated by pump light, the rare-earth ions absorb the energy of the pump light, transition to an excited state, and then generate laser light through stimulated emission. Fiber lasers have advantages such as compact structure, high efficiency, and good heat dissipation, and are widely used in industrial processing, communications, and medical fields. In high-power fiber lasers, as the output power increases, nonlinear effects in the fiber become increasingly prominent, becoming a significant factor restricting its further development. The nonlinear effects in the fiber mainly include stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS).

[0003] Traditional fiber lasers mostly use solid-core fibers based on total internal reflection. The core of the solid fiber is filled with a solid material with a high nonlinear coefficient. In order to suppress the nonlinear effect in the fiber, some measures have been taken in the prior art, such as adding magnetostrictive material to the cladding and applying an external magnetic field to suppress the nonlinear effect (e.g., Chinese Patent CN202211717033.9). However, this approach has the following problems: (1) Nonlinear effect is difficult to completely eliminate under high power density: Although the nonlinear effect can be partially suppressed by the external magnetic field, the high power density light energy in the core will still interact with the core material and produce nonlinear effect. Especially in high-power narrow-linewidth fiber lasers, the light energy is highly concentrated and the nonlinear effect is more significant. (2) Complex structure and increased cost: It is necessary to add magnetostrictive material to the cladding and apply an external magnetic field, which makes the structure of the fiber complex and increases the manufacturing cost and system complexity. (3) Negative impact on fiber performance: Adding magnetostrictive material may have a negative impact on other properties of the fiber (such as loss, transmission efficiency, etc.) and affect the overall performance of the fiber laser.

[0004] Hollow-core photonic bandgap fiber is a novel fiber structure whose unique light guiding mechanism is to confine light within an air core through the photonic bandgap effect. The nonlinear coefficient of the air core is extremely low, thus hollow-core photonic bandgap fiber has an extremely high nonlinear threshold, which can effectively reduce nonlinear effects in the fiber. However, current hollow-core photonic bandgap fiber still faces some challenges when applied to fiber lasers. (1) Imperfect gain mechanism: Traditional hollow-core photonic bandgap fiber is mainly used for light transmission and lacks an effective gain mechanism. In fiber lasers, it is necessary to achieve light gain amplification in the fiber, but current hollow-core photonic bandgap fiber has not yet solved this problem well. (2) Complex fiber structure and fabrication process: The structure of hollow-core photonic bandgap fiber is usually quite complex, the fabrication process is demanding, and the cost is high, which limits its large-scale application.

[0005] Therefore, a new technical solution is needed that can effectively reduce fiber nonlinearity while amplifying optical gain to meet the development needs of high-power narrow-linewidth fiber lasers. Summary of the Invention

[0006] To address the above-mentioned deficiencies or improvement needs of existing technologies, this invention provides an ultra-low nonlinear gain optical fiber and its fabrication method. By doping rare earth ions in the core-cladding junction region of the bandgap optical fiber, the gain of the fiber core fundamental mode is amplified by the coupling of the core mode and the surface mode. Furthermore, based on the photonic bandgap guiding principle, light is confined to the air core for transmission, which can effectively reduce the nonlinear effects of the optical fiber.

[0007] To achieve the above objectives, according to one aspect of the present invention, an ultra-low nonlinear gain optical fiber is proposed, comprising: a fiber core, a core-cladding junction region, and an inner cladding arranged sequentially from the inside to the outside along a radial direction, wherein... The fiber core is air; The core-packet boundary region is a rare earth doped region; The inner cladding includes multiple thin-walled periodic structures of glass distributed circumferentially around the core-cladding junction region. The thin-walled periodic structures of glass include multiple thin-walled tubes arranged in a circumferential array and air filling the thin-walled tubes. In this way, light is confined to the air core for transmission, thereby effectively reducing the nonlinear effects of optical fibers.

[0008] As a further preferred embodiment, the inner cladding layer is further provided with an outer support tube on its outer periphery.

[0009] As a further preferred embodiment, the fiber core, core-cladding junction region, inner cladding, and outer support tube all maintain constant refractive index and dimensions along the fiber axis.

[0010] As a further preferred embodiment, the glass thin-walled tube is a glass thin-walled tube doped with rare earth elements, and the types of rare earth elements doped include any one or more of erbium, ytterbium, thulium, holmium, neodymium, and bismuth rare earth ions.

[0011] As a further preferred embodiment, the number of layers of the glass thin-walled periodic structure arranged radially periodically is 2-11.

[0012] As a further preferred embodiment, the diameter of the fiber core is 2-11 times the radial thickness of the glass thin-walled periodic structure.

[0013] As a further preferred embodiment, the radial thickness of the glass thin-walled periodic structure is 2-11 μm, and the ratio of the thickness of the glass thin-wall to the radial thickness of the glass thin-walled periodic structure is less than 15%.

[0014] As a further preferred embodiment, the outer support tube is a glass tube, and the ratio of the inner and outer diameters of the glass tube is greater than 0.1.

[0015] According to another aspect of the present invention, a method for fabricating ultra-low nonlinear gain optical fiber is also provided, comprising the following steps: Step 1: Draw multiple first glass tubes with a duty cycle greater than 0.8 to obtain multiple first capillaries; Step 2: The second glass tube is doped with rare earth elements, and the rare earth-doped second glass tube is stretched to obtain a doped capillary tube. Step 3: Place the doped capillary tube at the core of the formed optical fiber, and stack multiple first capillary tubes on the outer periphery of the doped capillary tube in a periodic arrangement to form a stack. Step 4: Insert the stacked body into a glass outer tube with a suitable inner diameter and duty cycle, and insert a certain number of solid rods into the gap between the stacked body and the glass outer tube for fixation. Step 5: Place the structure obtained in Step 4 in a stretching tower for stretching to obtain a preform. Step six: Place the preform on the drawing tower for drawing to obtain ultra-low nonlinear gain optical fiber.

[0016] As a further preferred embodiment, in step six, inert gas is introduced into the doped tube capillary and the first capillary during the drawing process to maintain a certain structure in the fiber cross-section.

[0017] In summary, compared with the prior art, the above-described technical solutions conceived by this invention mainly possess the following technical advantages: 1. This invention constructs a hollow-core bandgap gain fiber, and simultaneously achieves gain amplification by doping the core-cladding interface region of the bandgap fiber with rare-earth ions. Based on the principle of photonic bandgap, most of the optical energy is confined in the fiber core to form the fundamental mode for transmission. The thickness of the core-cladding interface region controls the energy and number of surface modes present therein. During longitudinal transmission along the fiber, due to the presence of rare-earth ions, the energy of the surface modes at the target wavelength is continuously amplified; the fiber core fundamental mode and the surface modes at the core-cladding interface continuously couple and redistribute energy, achieving amplification of the fiber core fundamental mode at the target wavelength. The entire amplification process utilizes the surface modes at the core-cladding interface to maintain efficient small-signal gain, while the fiber core fundamental mode, which has most of the energy, propagates in the air, effectively reducing the nonlinear effects of the optical fiber.

[0018] 2. This invention constructs an air-core bandgap gain fiber, confining light transmission within an air core with an ultra-low nonlinear coefficient. This fundamentally avoids direct interaction between light and solid materials with high nonlinear coefficients, effectively reducing nonlinear effects such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS). This low nonlinearity characteristic enables fiber lasers to operate stably at higher power levels while maintaining narrow linewidth output characteristics, significantly improving fiber laser performance and meeting the demands of high-power, high-precision applications.

[0019] 3. This invention achieves gain amplification of the fiber core fundamental mode by doping rare-earth ions in the core-cladding interface region and utilizing the coupling mechanism between the core mode and the surface mode. This unique gain mechanism not only solves the problem of insufficient gain in traditional hollow-core fibers but also achieves efficient optical amplification while maintaining low nonlinear effects. This enables fiber lasers to maintain good beam quality and energy conversion efficiency at high power output, further enhancing the output capability of fiber lasers and broadening their application range in industrial processing, communications, medical fields, and other areas.

[0020] 4. The fiber optic structure design of this invention considers several key factors, such as the periodic arrangement of the glass thin walls, the size ratio of the fiber core to the glass thin walls, and the thickness-to-diameter ratio of the glass thin walls. Optimization of these parameters ensures the stability of the fiber during high-power optical transmission and gain amplification. Furthermore, the design of the external support tube further enhances the mechanical strength and structural stability of the fiber, preventing deformation or damage during drawing and use. This optimized structural design not only improves the reliability of the fiber but also extends its service life, reduces system maintenance costs, and provides a guarantee for the long-term stable operation of the fiber laser. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of a rare-earth-doped gain fiber with an ultra-low nonlinear coefficient provided in Embodiment 2 of the present invention; Figure 2 This is a schematic diagram of the structure of a rare-earth-doped gain fiber with an ultra-low nonlinear coefficient provided in Embodiment 3 of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0023] Example 1 This embodiment provides an ultra-low nonlinear gain optical fiber, which consists of four parts: an air core, a rare-earth-doped core-cladding junction region, a periodic inner cladding structure, and an outer support tube. Based on the photonic bandgap principle, the optical fiber confines light within the air core for propagation. The core-cladding junction region maintains a certain thickness to allow for surface modes and is doped with rare-earth elements to amplify the signal. The inner cladding structure consists of periodically arranged thin glass walls and air holes, while the outer support tube is a glass structure of a certain thickness. During optical fiber transmission, energy is concentrated in the core region. Excessive core energy at high power levels can lead to enhanced nonlinear effects in the fiber. Based on the photonic bandgap principle, the glass material in the core region is replaced with air, which has an extremely low nonlinear coefficient, to achieve a high nonlinear threshold. By rationally designing the period of the cladding region and the thickness of the thin glass walls, signal amplification within the core is achieved over a wide wavelength range. Specifically, this embodiment presents an air-core rare-earth ion-doped active optical fiber based on the photonic bandgap principle, providing a technical approach for achieving amplified output of ultra-high signal light. The optical fiber design proposed in this invention has the advantages of excellent performance and flexible design.

[0024] Specifically, the optical fiber includes: a core, a core-cladding junction region, an inner cladding, and an outer support tube arranged in a radial direction from the inside out.

[0025] Preferably, the fiber core, core-cladding junction region, inner cladding, and outer support tube all maintain constant refractive index and size along the fiber axis.

[0026] Preferably, the inner cladding comprises a periodic structure of thin-walled glass arranged layer by layer circumferentially along the core-cladding interface region. The number of radially arranged layers of the periodic glass thin-walled structure is 2-11. The thickness of the periodic glass thin-walled structure is approximately 2-11 μm, and the periodic structure comprises multiple circumferentially arrayed thin-walled glass tubes and air filling the tubes. Generally, the sidewalls of adjacent thin-walled glass tubes are integrally formed. The ratio of the thickness of the thin-walled glass to the thickness of the periodic glass thin-walled structure is less than 15%.

[0027] Preferably, the fiber core is air, and the transverse and longitudinal dimensions of the fiber core cross-section (generally the diameter of the fiber core cross-section) are approximately 2-11 times the radial thickness of the glass thin-walled periodic structure.

[0028] Preferably, the glass thin wall at the core-packet junction region is doped with rare earth elements, and the types of rare earth elements doped include, but are not limited to, rare earth ions such as erbium, ytterbium, thulium, holmium, neodymium, and bismuth. Preferably, the outer support tube is a glass tube, and the ratio of the inner and outer diameters of the glass tube (duty cycle) is greater than 0.1.

[0029] The outer support tube, the core-cladding junction area, and the inner cladding are all made of glass, including but not limited to pure quartz, silicate, phosphate, and other multi-component glasses.

[0030] In this embodiment, an ultra-low nonlinear gain optical fiber is designed for different wavelength bands of signal light. This is achieved by specifically designing the periodicity of the thin-walled periodic structure in the inner cladding (e.g., the number of layers, the array arrangement of the thin-walled glass tubes), thus designing the low-loss transmission band of the low-nonlinear gain fiber to be the same as the signal light's wavelength. Through reasonable design of the fiber core's radial dimensions, the thickness of the core-cladding interface region, the thickness of the thin-walled periodic structure, the refractive index of the core-cladding interface region, and the rare-earth ion doping in the core-cladding interface region, most of the optical energy is confined within the fiber core to form the fundamental mode. The thickness of the core-cladding interface region controls the energy and number of surface modes present within it. During longitudinal transmission along the fiber, the energy of the surface modes at the target wavelength is continuously amplified due to the presence of rare-earth ions. The core fundamental mode and the surface modes at the core-cladding interface continuously couple, redistributing energy and achieving amplification of the core fundamental mode at the target wavelength. The entire amplification process utilizes the surface modes at the core-cladding interface to maintain efficient small-signal gain, while the core fundamental mode, possessing most of the energy, propagates in the air, effectively reducing the nonlinear effects of the optical fiber.

[0031] Example 2 This embodiment provides a rare-earth-doped gain fiber with an ultra-low nonlinear coefficient, as shown in the cross-sectional schematic diagram below. Figure 1As shown, the fiber core 11 is air, the core-cladding junction region 12 is a rare-earth-doped region, the inner cladding region 13 is composed of periodically distributed air 131 and glass thin walls 132, and the outer support tube 14 has a certain thickness to ensure good environmental stability of the optical fiber. This embodiment is a 19-core low-nonlinear rare-earth-doped gain optical fiber suitable for signal amplification in the 1.55μm band. By reasonably designing the periodicity of the inner cladding region 13, the low-loss transmission band of the bandgap fiber is designed to be in the 1.55μm band; by reasonably designing the size of the fiber core region 11, its radial diameter is approximately equal to the thickness of the periodic structure of the 19 inner cladding regions; by reasonably designing the thickness and refractive index of the core-cladding junction region 12, the coupling strength between the core mode and the surface mode is controlled; and by reasonably designing the erbium ion concentration in the core-cladding junction region 12, a certain gain level of signal light amplification is achieved.

[0032] Example 3 This embodiment provides a rare-earth-doped gain fiber with an ultra-low nonlinear coefficient, as shown in the cross-sectional schematic diagram below. Figure 2 As shown, the fiber core 21 is air, the core-cladding junction region 22 is a rare earth doped region, the inner cladding region 23 is composed of periodically distributed air 231 and glass thin wall 232, and the outer support tube 24 has a certain thickness to maintain good environmental stability of the optical fiber.

[0033] This embodiment describes a low-nonlinear rare-earth-doped fiber with a core suitable for signal amplification in the 1μm band. By rationally designing the period of the inner cladding region 23, the low-loss transmission band of the bandgap fiber is designed to be in the 1μm band. By rationally designing the size of the core region 21, its radial diameter is approximately equal to the thickness of seven periodic structures of the inner cladding region. By rationally designing the thickness and refractive index of the core-cladding junction region 22, the coupling strength between the core mode and the surface mode is controlled. By rationally designing the ytterbium ion concentration in the core-cladding junction region 22, a signal amplification of a certain gain level is achieved.

[0034] Example 4 Based on the above embodiments or combinations of embodiments, this embodiment provides a method for fabricating ultra-low nonlinear gain optical fiber, including the following steps: Step 1: Draw multiple first glass tubes with a duty cycle greater than 0.8 to obtain multiple first capillaries; Step 2: The second glass tube is doped with rare earth elements, and the rare earth-doped second glass tube is stretched to obtain a doped capillary tube. Step 3: Place the doped capillary tube at the core of the formed optical fiber, and stack multiple first capillary tubes on the outer periphery of the doped capillary tube in a periodic arrangement to form a stack. Step 4: Insert the stacked body into a glass outer tube with a suitable inner diameter and duty cycle, and insert a certain number of solid rods into the gap between the stacked body and the glass outer tube for fixation. Step 5: Place the structure obtained in Step 4 in a stretching tower for stretching to obtain a preform. Step six: Place the preform on the drawing tower for drawing to obtain ultra-low nonlinear gain optical fiber.

[0035] The ultra-low nonlinear gain optical fiber consists of a core, a core-cladding junction region, and an inner cladding arranged radially from the inside out. The core is air; the core-cladding junction region is a rare-earth-doped region; and the inner cladding includes multiple thin-walled glass periodic structures distributed circumferentially around the core-cladding junction region. Each thin-walled glass periodic structure includes multiple thin-walled glass tubes arranged in a circumferential array and air filling the glass tubes. In this way, light is confined to the air core for transmission, thereby effectively reducing the nonlinear effects of the optical fiber.

[0036] Preferably, in step six, inert gas is introduced into the doped tube capillary and the first capillary during the drawing process to maintain a certain structure of the fiber cross-section.

[0037] Example 5 Based on the above embodiments or combinations of embodiments, this embodiment provides a method for fabricating ultra-low nonlinear gain optical fiber, including the following steps: Step 1: Draw a circular tube with a duty cycle greater than 0.8 to obtain a capillary tube with an outer diameter greater than 1 cm and a duty cycle greater than 0.8; Step 2: Select a circular tube with a duty cycle greater than 0.7 for rare earth doping to obtain a rare earth doped glass tube for preparing rare earth doped capillary tubes. Step 3: Stretch the doped glass tube obtained in Step 2 to obtain a doped capillary tube with an outer diameter between 2 and 11 times that of the capillary obtained in Step 1. Step 4: Stack the doped capillary obtained in Step 3 with the capillary obtained in Step 1, and place the doped capillary obtained in Step 3 at the core of the formed optical fiber to obtain a stack. Step 5: Insert the stacked body obtained in Step 4 into a glass outer tube with a suitable inner diameter and duty cycle, and insert a certain number of solid rods into the gap between the stacked body and the glass outer tube for fixation. Step 6: Place the structure obtained in step 5 in a stretching tower for stretching to obtain a preform. Step 7: Place the preform obtained in step 6 on the drawing tower for drawing. During the drawing process, inert gas is introduced into the fiber core and inner cladding periodic structure to maintain a certain structure of the fiber cross section.

[0038] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An ultra-low nonlinear gain optical fiber, characterized in that, include: The fiber core (11), the core-cladding junction region (12), and the inner cladding layer (13) are arranged sequentially from the inside to the outside along the radial direction. The fiber core (11) is air; The core-packet junction region (12) is a rare earth doped region; The inner cladding (13) includes a plurality of thin-walled periodic structures of glass distributed circumferentially around the core-cladding junction region (12), the thin-walled periodic structures of glass including a plurality of thin-walled tubes (132) arranged in a circumferential array and air filled in the thin-walled tubes (132); In this way, light is confined to the air core for transmission, thereby effectively reducing the nonlinear effects of optical fibers.

2. The ultra-low nonlinear gain optical fiber according to claim 1, characterized in that, The inner cladding (13) is also provided with an outer support tube (14) on its outer periphery.

3. The ultra-low nonlinear gain optical fiber according to claim 2, characterized in that, The fiber core (11), core-cladding junction region (12), inner cladding (13) and outer support tube (14) all maintain the same refractive index and size along the fiber axis.

4. The ultra-low nonlinear gain optical fiber according to claim 1, characterized in that, The glass thin-walled tube (132) is a glass thin-walled tube doped with rare earth elements. The types of rare earth elements doped include any one or more of erbium, ytterbium, thulium, holmium, neodymium, and bismuth rare earth ions.

5. The ultra-low nonlinear gain optical fiber according to claim 1, characterized in that, The number of layers in the radially periodic arrangement of the glass thin-walled periodic structure is 2-11.

6. The ultra-low nonlinear gain optical fiber according to claim 1, characterized in that, The diameter of the fiber core (11) is 2-11 times the radial thickness of the glass thin-walled periodic structure.

7. An ultra-low nonlinear gain optical fiber according to any one of claims 1-6, characterized in that, The radial thickness of the glass thin-walled periodic structure is 2-11 μm, and the ratio of the thickness of the glass thin-wall (132) to the radial thickness of the glass thin-walled periodic structure is less than 15%.

8. An ultra-low nonlinear gain optical fiber according to any one of claims 2-7, characterized in that, The outer support tube (14) is a glass tube, and the ratio of the inner and outer diameters of the glass tube is greater than 0.

1.

9. A method for fabricating ultra-low nonlinear gain optical fiber, characterized in that, Includes the following steps: Step 1: Draw multiple first glass tubes with a duty cycle greater than 0.8 to obtain multiple first capillaries; Step 2: The second glass tube is doped with rare earth elements, and the rare earth-doped second glass tube is stretched to obtain a doped capillary tube. Step 3: Place the doped capillary tube at the core of the formed optical fiber, and stack multiple first capillary tubes on the outer periphery of the doped capillary tube in a periodic arrangement to form a stack. Step 4: Insert the stacked body into a glass outer tube with a suitable inner diameter and duty cycle, and insert a certain number of solid rods into the gap between the stacked body and the glass outer tube for fixation. Step 5: Place the structure obtained in Step 4 in a stretching tower for stretching to obtain a preform. Step six: Place the preform on the drawing tower for drawing to obtain ultra-low nonlinear gain optical fiber.

10. The method for fabricating an ultra-low nonlinear gain optical fiber according to claim 9, characterized in that, In step six, during the drawing process, inert gas is introduced into the doped tube capillary and the first capillary to maintain a certain structure in the fiber cross-section.

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