An all-fiber high-order mode filter and a manufacturing method thereof

Abstract

This invention discloses an all-fiber high-order mode filter and its manufacturing method, comprising: first, determining the parameters of a large-mode-field transmission fiber; second, designing the etching shape on the fiber surface and determining the refractive index material to fill the etched areas; third, placing the decoated power-carrying fiber into an evanescent etching device, controlling the etching depth and width of the cladding by adjusting the etching power and time of the device, and controlling the etching period and total etching length by adjusting the fiber step distance and the number of etching repetitions; fourth, filling the etched areas with a material of the corresponding refractive index and encapsulating the device to complete the fabrication of the high-order mode filter. This invention utilizes evanescent wave mode coupling between the core and cladding etched regions to achieve high-order mode loss, thereby filtering out high-order modes in the core and effectively improving the beam quality of laser output from a large-mode-field fiber.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, and more specifically, relates to an all-fiber high-order mode filter and its manufacturing method. Background Technology

[0002] Due to the wide application of high-power lasers in many civilian and military fields, laser development has been rapid in recent years. High-power fiber lasers have significant advantages in terms of efficiency, size, cooling, and beam quality, attracting increasing interest from researchers both domestically and internationally. However, the main limiting factor for the output power of fiber lasers is the thermal and nonlinear effects that cause optical damage to the fiber material. Currently, the primary method for reducing these thermal and nonlinear effects is to use large-mode-area fibers. To achieve high-power output, the fiber core diameter generally needs to be greater than 10 μm to reduce nonlinear scattering and thermal effects. If the fiber size is too large, multiple modes will exist within the fiber, leading to a decrease in the quality of the output laser beam. Therefore, mode separation technology has become a necessary method for single-mode operation of large-core fibers.

[0003] Currently, single-mode operation of large-core optical fibers is mainly achieved by using fiber winding. However, bending causes deformation of the mode field distribution and reduces the mode field area, which limits the benefits of using large-core optical fibers.

[0004] Due to the inherent nonlinear effects of single-mode optical fibers, the most direct and effective way to reduce these effects is to increase the core diameter. When the core diameter increases to a certain extent, the fiber will support the transmission of higher-order modes, which will affect the laser beam quality. To achieve high-beam-quality laser output, it is necessary to filter out the higher-order mode components in the laser beam. Summary of the Invention

[0005] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides an all-fiber high-order mode filter and its manufacturing method. The filter utilizes an evanescent etching device to etch the cladding of the optical fiber. By coupling the evanescent wave modes between the etched regions of the fiber core and cladding, the loss of high-order modes is reduced, thereby filtering out high-order modes in the fiber core. This effectively improves the beam quality of large-mode-field fiber-based laser output.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for manufacturing an all-fiber high-order mode filter is provided, comprising the following steps:

[0007] The first step is to determine the parameters of the large-mode-field transmission fiber;

[0008] The second step is to design the etching shape on the fiber surface and determine the refractive index material to fill the etched area.

[0009] The third step is to place the power transmission fiber with the coating removed into a device with evanescent etching function. The etching depth and width of the cladding are controlled by adjusting the etching power and time of the device with evanescent etching function. The etching cycle and total etching length of the fiber are controlled by adjusting the fiber step distance and the number of etching repetitions.

[0010] The fourth step is to fill the etched area of ​​the optical fiber with a material of the corresponding refractive index and encapsulate the device to complete the fabrication of the high-order mode filter.

[0011] Furthermore, the method for designing the etching shape on the fiber surface is as follows:

[0012] By setting parameters to change the etching shape and refractive index, the device can achieve greater loss for higher-order modes in the fiber core and less loss for lower-order modes.

[0013] Furthermore, the specific steps for changing the etching shape and refractive index by setting parameters to make the device have greater loss for higher-order modes and less loss for lower-order modes in the fiber core are as follows: keeping the etching shape unchanged, adjusting the refractive index of the groove region, examining the relationship between the loss of higher-order and lower-order modes and the refractive index of the groove region, finding the resonant refractive index range where the mode loss is locally maximized, and finding that the mode loss is smaller outside the resonant refractive index range where the mode loss is locally maximized; since the resonant refractive indices of higher-order modes and lower-order modes are different, adjusting the refractive index of the groove region to the resonant refractive index range of higher-order modes, making the loss of higher-order modes larger and the loss of lower-order modes smaller; further adjusting the etching shape, examining the mode loss, comparing it with the target mode loss, and continuously optimizing and iterating the etching shape parameters until an etching shape that meets the mode loss value requirements is obtained.

[0014] Furthermore, the parameters of the large-mode-field transmission fiber include the core refractive index, cladding refractive index, core diameter, cladding diameter, and the laser wavelength to be used.

[0015] Furthermore, in the second step:

[0016] Along the radial direction of the optical fiber, the cladding is etched in trapezoidal, rectangular, fan-shaped, annular, and semi-annular shapes.

[0017] Along the fiber axis, the cladding is etched in the form of straight lines, spirals, curves, and scattered dots.

[0018] The number of etches on the cladding is an integer greater than or equal to 1.

[0019] Furthermore, in the second step, the filling material at the etched area of ​​the optical fiber is a colloidal, liquid, or solid material with various refractive indices.

[0020] Furthermore, the method for device packaging in the fourth step is as follows:

[0021] The etched optical fiber is passed through a glass tube and straightened so that the etched area is entirely inside the glass tube. Then, UV adhesive is applied to both ends of the glass tube for curing.

[0022] Furthermore, the fiber core diameter is 20μm to 30μm, and the cladding diameter is 125μm to 400μm.

[0023] Furthermore, in the third step, the power of the device with the evanescent etching function is less than 30W;

[0024] The device with the disappearing etching function is a carbon dioxide welding machine.

[0025] According to a second aspect of the present invention, an all-fiber high-order mode filter is provided, which is manufactured using the aforementioned method for manufacturing an all-fiber high-order mode filter.

[0026] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0027] 1. The all-fiber high-order mode filter manufactured by this invention includes a large-mode-field transmission fiber. The cladding of the fiber is etched using a device with evanescent etching function. The loss of high-order modes is achieved by using the evanescent wave mode coupling between the core and the cladding etched region, thereby filtering out the high-order modes in the core. This can effectively improve the beam quality of the laser output from the large-mode-field fiber.

[0028] 2. The method of the present invention, since the field distribution and propagation constants of the fundamental mode and higher-order modes are different, can reduce the coupling of the fundamental mode into another waveguide in the fiber core to a small amount when a specific phase matching condition is met, while the coupling of higher-order modes is large, thereby achieving higher-order mode filtering. Attached Figure Description

[0029] Figure 1 This is a cross-sectional example of a high-order mode filter produced by the manufacturing method of an all-fiber high-order mode filter according to the present invention.

[0030] Figure 2 This is an example diagram of the fiber axial direction of the high-order mode filter, which is a method for manufacturing an all-fiber high-order mode filter according to the present invention.

[0031] Figure 3 This is a schematic diagram of a specific fiber optic high-order mode filter embodiment of the manufacturing method of the all-fiber high-order mode filter of the present invention;

[0032] Figure 4 This is a schematic diagram of a specific embodiment of a fiber optic high-order mode filter, which is a method for manufacturing an all-fiber high-order mode filter according to the present invention. The high-order mode filter is implemented by a spiral etching structure.

[0033] Figure 5A schematic diagram of the laser intensity distribution before using a high-order mode filter;

[0034] Figure 6 A schematic diagram of the laser intensity distribution after using a high-order mode filter;

[0035] Figure 7 This is a schematic flowchart illustrating a method for manufacturing an all-fiber high-order mode filter according to an embodiment of the present invention.

[0036] In all the accompanying drawings, the same reference numerals denote the same technical features, specifically: 1-core, 2-cladding, 3-etched shape. Detailed Implementation

[0037] 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.

[0038] like Figure 1-3 As shown, this embodiment of the invention provides a method for manufacturing an all-fiber high-order mode filter, including the following steps:

[0039] The first step is to determine the parameters of the large mode field transmission fiber, including the core refractive index, cladding refractive index, core diameter, cladding diameter, and the wavelength of the laser to be used.

[0040] The second step is to design the etching shape on the fiber surface and determine the refractive index material to fill the etched area.

[0041] The third step is to place the power transmission fiber with the coating removed into the device with the evanescent etching function. The etching depth and width of the cladding are controlled by adjusting the etching power and time of the device with the evanescent etching function. The etching cycle and total etching length of the fiber are controlled by adjusting the fiber step distance and the number of etching repetitions.

[0042] Along the radial direction of the optical fiber, the cladding is etched in trapezoidal, rectangular, fan-shaped, annular, and semi-annular shapes.

[0043] Along the fiber axis, the cladding is etched in the form of straight lines, spirals, curves, and scattered dots.

[0044] The number of etches on the cladding is an integer greater than or equal to 1.

[0045] The device with the disappearing etching function is a carbon dioxide welding machine;

[0046] The optical fiber is a multimode optical fiber with a core diameter of 20μm to 30μm and a cladding diameter of 125μm to 400μm. The device with evanescent etching function has a power of less than 30W.

[0047] The method for designing the etching shape on the fiber surface is to set parameters through software (such as rsoft, bpm) to change the etching shape and refractive index, so that the device has greater loss for higher-order modes in the fiber core and less loss for lower-order modes.

[0048] The specific steps for designing the etching shape on the fiber surface are as follows: Keeping the etching shape constant, adjust the refractive index of the groove region, examine the relationship between the loss of higher-order and lower-order modes and the refractive index of the groove region, and find the resonant refractive index range where the mode loss is locally maximized. Outside this range, the mode loss is relatively small. Since the resonant refractive indices of higher-order and lower-order modes differ, adjust the refractive index of the groove region to the resonant refractive index of the higher-order mode within the specified range, resulting in a larger higher-order mode loss and a smaller lower-order mode loss. Further adjust the etching shape, examine the mode loss, compare it with the target mode loss, and continuously optimize and iterate the etching shape parameters until an etching shape that meets the mode loss requirements is obtained.

[0049] The fourth step is to fill the etched area of ​​the optical fiber with a material of the corresponding refractive index and encapsulate the device to complete the fabrication of the high-order mode filter.

[0050] The method for encapsulating high-order mode filters involves passing the etched optical fiber through a glass tube, straightening it so that the etched area is entirely inside the glass tube, and then curing the glass tube with UV adhesive at both ends.

[0051] The filling material at the etched part of the optical fiber can be a colloidal, liquid or solid material with different refractive indices.

[0052] In another embodiment of the present invention, an all-fiber high-order mode filter is provided, which is manufactured using the above-described manufacturing method for all-fiber high-order mode filters.

[0053] The fiber core of an optical fiber is equivalent to one waveguide, while the vanishing portion is equivalent to another. According to the directional coupling theory of waveguides, when the distance between two waveguides is small, the refractive index distribution of the waveguides changes. The field in one waveguide is affected by the field in the other, causing the guided modes in the two waveguides to couple with each other. Since the field distribution and propagation constants of the fundamental mode and higher-order modes are different, when certain phase-matching conditions are met, very little of the fundamental mode in the fiber core can couple into the other waveguide, while a lot of higher-order modes can couple, thus achieving higher-order mode filtering.

[0054] Please refer to Figure 1 and Figure 2, where 1 is the fiber core, 2 is the fiber cladding, 3 is the etched shape on the fiber cladding, and 3 is filled with a refractive index matching material.

[0055] Example 1:

[0056] Please refer to Figure 3 A 20 / 400 power transmission fiber is used, where 1 represents the fiber core, 2 represents the fiber cladding, and 3 represents multiple trenches etched on the fiber cladding. The specific fabrication process is as follows: First, the etching shape of the fiber is determined to be semi-circular. By changing the evanescent power and time of the CO2 fusion splicer, and measuring the etching depth and width of the etched fiber under a microscope, the correspondence between the etching depth and width and the evanescent power and time is determined. In this embodiment, the evanescent power of the CO2 laser is set to 10W, the fiber rotation speed is 0.025° / ms, the etching time is 7.2 seconds, and the etching width of a single semi-circular trench is approximately 170µm, with a depth of 186µm. After the etching of a single trench is completed, the axial step distance of the fiber is set to 400µm, i.e., the etching cycle is 400µm. After the fiber step is completed, the above etching steps are repeated. In this embodiment, the total etching length of the fiber is 30mm, meaning the above etching steps are repeated 75 times. After setting the evanescent parameters of the CO2 fusion splicer, the coated optical fiber is inserted, and etching begins. Finally, the etched cladding trench is filled with refractive index material, and the device is encapsulated to complete the fabrication of the high-order mode filter.

[0057] Example 2:

[0058] Please refer to Figure 4 Using 25 / 400 power-transmitting optical fiber, spiral grooves were etched on the fiber cladding. The specific fabrication process was as follows: First, the etching shape of the optical fiber was determined to be spiral. The evanescent power, evanescent time, rotation speed, and step speed of the CO2 fusion splicer were adjusted, and the etched fiber was placed under a microscope to measure its etching depth and width. In this embodiment, the evanescent power of the CO2 laser was set to 30W, the fiber core diameter was 20μm, and the etched grooves were processed on a passive optical fiber with a cladding diameter of 400μm. During the laser etching process, the focusing position of the CO2 laser remained unchanged; only the optical fiber was moved. The fiber rotation speed was 0.1° / ms, and the fiber step speed was 0.1µm / ms. In this embodiment, the total etching length of the optical fiber was 20mm, the spiral period was 360µm, the number of etching cycles was 55, the etching width was 57µm, and the etching depth was 185µm. After setting the evanescent parameters of the CO2 fusion splicer, the power-transmitting optical fiber with the coating removed was inserted, and etching began. Finally, the etched cladding trenches are filled with high-refractive-index material, and the device is encapsulated to complete the fabrication of the high-order mode filter.

[0059] Example 3:

[0060] A fan-shaped groove was etched into the cladding of a 25 / 400 power-transmitting fiber. The specific fabrication process was as follows: First, the etching shape of the fiber was determined to be fan-shaped. The evanescent power, evanescent time, rotation speed, and step speed of the CO2 fusion splicer were adjusted, and the etched fiber was then measured under a microscope to determine its etching depth and width. In this embodiment, the evanescent power of the CO2 laser was set to 5W. The etched grooves were fabricated in a passive fiber with a cladding diameter of 125μm. During the laser etching process, the focusing position of the CO2 laser remained unchanged; only the fiber was moved. The fiber core diameter was 30μm, the fiber rotation speed was 0.2° / ms, and the fiber step speed was 0.2µm / ms. In this embodiment, the total etching length of the fiber was 40mm, the etching spiral period was 360µm, the number of etching cycles was 80, the etching width was 65µm, and the etching depth was 254µm. After setting the evanescent parameters of the CO2 fusion splicer, the power-transmitting fiber with the coating removed was inserted, and etching began. Finally, the etched cladding trenches are filled with high-refractive-index material, and the device is encapsulated to complete the fabrication of the high-order mode filter.

[0061] Please refer to Figures 5-6 , Figure 5 The intensity distribution of the output light spot detected by the camera before using the high-order mode filter. Figure 6 The intensity distribution of the output beam detected by the camera before and after using the higher-order mode filter is shown. Before the higher-order mode filter is used, the beam contains a certain proportion of higher-order modes. After the higher-order modes are filtered out, the output beam becomes a near-Gaussian distribution of the fundamental mode.

[0062] 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. A method for manufacturing an all-fiber high-order mode filter, characterized in that, Includes the following steps: The first step is to determine the parameters of the large mode field transmission fiber, which include the core refractive index, cladding refractive index, core diameter, cladding diameter, and the laser wavelength to be used. The second step is to design the etching shape on the fiber surface and determine the refractive index material to fill the etched areas. The method for designing the etching shape on the fiber surface is as follows: by setting parameters, the etching shape and refractive index are changed to make the device have a higher loss for higher-order modes in the fiber core and a lower loss for lower-order modes. Along the radial direction of the fiber, the etching shape of the cladding can be trapezoidal, rectangular, fan-shaped, circular, or semi-circular. Along the axial direction of the fiber, the etching shape of the cladding can be linear, spiral, curved, or scattered dots. The third step is to place the power transmission fiber with the coating removed into a device with evanescent etching function. The etching depth and width of the cladding are controlled by adjusting the etching power and time of the device with evanescent etching function. The etching cycle and total etching length of the fiber are controlled by adjusting the fiber step distance and the number of etching repetitions. The fourth step is to fill the etched area of ​​the optical fiber with a material of the corresponding refractive index and encapsulate the device to complete the fabrication of the high-order mode filter. The filling material at the etched area of ​​the optical fiber can be a colloidal, liquid or solid material with different refractive indices. The fiber core diameter is 20μm~30μm, and the cladding diameter is 125μm~400μm; Evanescent wave mode coupling between the core and cladding etched regions is used to achieve loss of higher-order modes, thereby filtering out higher-order modes in the core.

2. The manufacturing method of an all-fiber high-order mode filter according to claim 1, characterized in that: The specific steps for changing the etching shape and refractive index by setting parameters to make the device have greater loss for higher-order modes and less loss for lower-order modes in the fiber core are as follows: Keeping the etching shape unchanged, adjust the refractive index of the groove region, examine the relationship between the loss of higher-order and lower-order modes and the refractive index of the groove region, and find the resonant refractive index range where the mode loss is locally maximized. Outside this range, the mode loss is smaller. Since there is a difference in the resonant refractive index between higher-order and lower-order modes, adjust the refractive index of the groove region to the resonant refractive index range of the higher-order mode, thus increasing the loss of higher-order modes and decreasing the loss of lower-order modes. Further adjust the etching shape, examine the mode loss, compare it with the target mode loss, and continuously optimize and iterate the etching shape parameters until an etching shape that meets the mode loss requirements is obtained.

3. The manufacturing method of an all-fiber high-order mode filter according to claim 1, characterized in that: In the second step: The number of etches on the cladding is an integer greater than or equal to 1.

4. The manufacturing method of an all-fiber high-order mode filter according to claim 1, characterized in that: The method for device packaging in the fourth step is as follows: The etched optical fiber is passed through a glass tube and straightened so that the etched area is entirely inside the glass tube. Then, UV adhesive is applied to both ends of the glass tube for curing.

5. The manufacturing method of an all-fiber high-order mode filter according to claim 1, characterized in that: In the third step, the etching power of the device with the evanescent etching function is less than 30W; The device with the disappearing etching function is a carbon dioxide welding machine.

6. An all-fiber high-order mode filter, characterized in that: The all-fiber high-order mode filter is manufactured using the manufacturing method of the all-fiber high-order mode filter described in any one of claims 1-5.

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