Method for deep cladding light filtering of femtosecond laser modified optical fiber

Abstract

This invention discloses a femtosecond laser-modified fiber deep cladding light filtering method. First, a section of the large-mode-area fiber to be filtered is stripped to expose the inner cladding, and then placed on a femtosecond laser processing platform. The femtosecond laser is focused into the fiber, and by adjusting the laser parameters, the refractive index of the large-mode-area fiber at the focusing position is increased (Type I modification). By changing the relative position of the femtosecond laser's focal point and the large-mode-area fiber, a Type I modified channel is formed on the fiber from the inside to the surface, guiding the cladding light out of the fiber. Different starting positions of the modified channel can guide cladding light with different NAs out of the large-mode-area fiber. The modified channel maintains the same large-mode-area fiber size, and the extracted cladding light does not accumulate on the surface to generate heat, making it less prone to damage and able to withstand high power.

Description

Technical Field

[0001] This invention relates to a method for filtering large-mode-field optical fibers, and more particularly to a method for deep cladding light filtering of femtosecond laser-modified optical fibers. Background Technology

[0002] Fiber lasers possess numerous advantages, including high efficiency, simple structure, and high stability, and are widely used in precision machining, high-end manufacturing, and other fields. Among these, cladding light filtering is crucial for improving the beam quality and long-term reliability of the output laser light. This technology improves the beam quality and long-term reliability of the fiber laser light output by filtering out the light propagating in the cladding and retaining only the light propagating in the fiber core.

[0003] Currently, the main methods for optical fiber cladding stripping are as follows:

[0004] A common approach is to place a refractive index matching layer on the outer layer of the optical fiber. By disrupting the total internal reflection condition of the pump light and cladding laser within the inner cladding, the light is diverted out of the inner cladding. However, this method has significant drawbacks. If a colloidal refractive index matching layer is used, it will absorb some of the filtered light, leading to heat generation or even burnout. If a glass refractive index matching layer is used, matching adhesive is required for fixation, which also carries the risk of heat generation and burnout. Furthermore, during the heat curing process of the matching glass onto the surface of the fiber's inner cladding, stress may be generated inside the fiber, thus affecting the quality of the output beam.

[0005] Another approach is to roughen the surface of the inner cladding of the fiber, but this reduces the mechanical strength of the fiber, thus affecting its reliability and lifespan. Although reducing the diameter of the inner cladding and then roughening the surface can filter out cladding light with low numerical aperture (NA), this method further weakens the mechanical strength of the fiber, making it unable to withstand high power. Furthermore, this method is not suitable for polarization-maintaining fibers with stress regions and photonic crystal fibers with structural regions.

[0006] Although the above two methods do not damage the inner cladding of the optical fiber, they are difficult to filter out cladding light with low NA and are not suitable for triple-clad and multi-clad optical fibers.

[0007] Given the various shortcomings of the above methods, the industry urgently needs a new, more efficient and reliable fiber cladding optical filtering technology to address the deficiencies of existing technologies and further improve the performance and stability of fiber lasers. Summary of the Invention

[0008] To address the shortcomings and defects of the existing technologies, this invention proposes a femtosecond laser-modified fiber deep cladding light filtering method. The method utilizes a femtosecond laser to modify the interior of the fiber, forming a modified channel extending from the inside of the fiber to the surface. This channel can effectively guide the cladding light out of the fiber and be absorbed by the external metal encapsulation structure, converting it into heat, which is then dissipated through a heat dissipation device.

[0009] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0010] A method for deep cladding optical filtering of femtosecond laser-modified optical fiber is characterized by the following steps:

[0011] S1. Strip a section of the coating layer from the optical fiber whose cladding light is to be filtered to expose the inner cladding, and place the inner cladding on a femtosecond laser processing platform;

[0012] S2. A femtosecond laser is focused inside an optical fiber, and the refractive index of the fiber at the focal point is increased by adjusting the parameters of the laser, thus forming a type I modification.

[0013] S3. Change the focal position of the laser along the diameter direction of the fiber end face to form a modified channel between the inside of the fiber and the fiber surface, which is used to guide the cladding light out of the fiber.

[0014] S4. Modified channels with different depths are prepared at different positions along the longitudinal length of the optical fiber to filter out cladding light with different numerical apertures.

[0015] Furthermore, it also includes:

[0016] S5. Metal encapsulation is applied to the optical fiber, suspending the cladding stripping area. The cladding light guided from the optical fiber is directly absorbed by the encapsulation metal and converted into heat, which is then used for heat dissipation.

[0017] Furthermore, the parameters of the laser in step S2 include adjusting the pulse width, repetition frequency, and pulse energy.

[0018] Furthermore, the modified channel formed in step S3 extends continuously from the inside of the optical fiber to the surface of the optical fiber, and the starting position of the modified channel is set according to the numerical aperture of the cladding light to be filtered out.

[0019] Furthermore, step S4 involves controlling the relative movement of the focal point of the femtosecond laser and the optical fiber to achieve modification at different depths.

[0020] Furthermore, in step S5, the metal packaging uses a metal material with good thermal conductivity, and the packaging metal is cooled by water or other heat dissipation methods.

[0021] Furthermore, the depth of the modified channel in the radial direction of the optical fiber varies depending on the NA of the cladding light to be stripped, and the density of the modified channel in the longitudinal direction of the optical fiber and the length of the optical fiber with the modified channel vary depending on the proportion and power of the cladding light to be stripped.

[0022] Furthermore, the optical fiber includes large-mode-field optical fiber and photonic crystal optical fiber.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] 1. By fabricating a type I refractive index-enhanced channel in the inner cladding of an optical fiber using femtosecond lasers, residual pump light, amplified spontaneous emission light, higher-order mode signals, and other residual light within the cladding can be extracted from the fiber. Changing the radial depth of the modified channel allows for the removal of cladding light with different refractive indices (NAs). Adjusting the longitudinal density of the modified channel and the length of the fiber with the modified channel allows for the adjustment of the stripping ratio. The modified channel does not generate heat on the fiber surface, making it less prone to damage and capable of withstanding high power.

[0025] 2. No damage is done to the optical fiber, preserving the signal power and beam quality within the fiber core. Mechanical strength and lifespan are not reduced; filtered light exits directly from the fiber surface without heat accumulation. It can withstand high power. The fiber undergoes no heating during fabrication, resulting in no residual stress and no degradation of the laser beam quality.

[0026] 3. The fabrication method is applicable not only to cladding fibers (polarization-maintaining and non-polarization-maintaining) but also to photonic crystal fibers. It can effectively extract cladding light with low NA without damaging the fiber, without reducing the power of the signal in the fiber core, without compromising the fiber beam quality, and without generating heat on the fiber surface. Attached Figure Description

[0027] Figure 1 This is a flowchart of the femtosecond laser-modified fiber deep cladding optical filtering method of the present invention.

[0028] Figure 2 A schematic diagram of the optical fiber prepared in the deep cladding optical filtering method for femtosecond laser-modified large-mode-field optical fiber.

[0029] Figure 3 This is a simplified diagram of the stripper structure prepared by the femtosecond laser-modified large-mode-field fiber deep cladding optical filtering method of the present invention;

[0030] In the diagram: 1: Cladding fiber; 2: Fiber cladding; 3: Fiber core; 4: Modified filter channel. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this should not be construed as limiting the scope of protection of the present invention.

[0032] This embodiment proposes a femtosecond laser-modified fiber cladding light filtering method, aiming to efficiently and non-destructively filter cladding light from optical fibers, especially for low numerical aperture (NA) cladding light. A femtosecond laser is used to precisely modify the interior of the fiber, forming a modified channel extending from the fiber's interior to its surface. This channel effectively guides the cladding light out of the fiber, where it is absorbed by the external metal encapsulation structure and converted into heat, which is then dissipated through a heat dissipation device.

[0033] Please see Figure 1 , Figure 1 The flowchart of the femtosecond laser-modified fiber deep cladding light filtering method of the present invention is shown in the figure, and includes the following steps:

[0034] S1. Fiber Pretreatment: First, a section of the coating layer is stripped from the large-mode-field fiber 1 (from which cladding light is to be filtered), exposing the inner cladding 2, so that the femtosecond laser can directly act on the inner cladding of the fiber for subsequent modification. The fiber with the exposed inner cladding 2 is placed on a femtosecond laser processing platform for laser modification.

[0035] S2. Femtosecond Laser Modification: A femtosecond laser is used to focus the light on the inner cladding 2 without contacting the fiber core 3. By adjusting the laser parameters (such as pulse width, repetition frequency, and pulse energy), the refractive index of the fiber at the focal point is increased, i.e., Type I modification. This modification changes the optical properties of the fiber at the focal point, thereby guiding the cladding light out along the modified channel.

[0036] S3. Forming a modified channel: By changing the focal position of the laser along the diameter direction of the fiber end face, a modified channel is formed extending from the inside of the fiber to the surface of the fiber. This channel acts like a "light guide," guiding the cladding light from the inside of the fiber to the outside.

[0037] S4. Fabrication of modified channels at different depths: To filter out cladding light from different NAs, modified channels of different depths need to be fabricated at different locations along the longitudinal length of the fiber. In this way, cladding light from different NAs will be guided out of the fiber when it propagates to the corresponding depth of the modified channel.

[0038] S5. Metal Encapsulation and Heat Dissipation: The optical fiber is encapsulated with metal, leaving the cladding stripped area suspended. This allows the cladding light guided from the fiber to be directly absorbed by the encapsulating metal and converted into heat. For effective heat dissipation, the encapsulating metal can be cooled by water, ensuring the stable operation of the optical fiber system.

[0039] Figure 2This is a schematic diagram of the fiber prepared in the femtosecond laser-modified large-mode-field fiber deep cladding light filtering method. As shown, regardless of whether it is a double-clad, triple-clad, multi-clad, or photonic crystal fiber, the coating of a section of the fiber 1 from which the cladding light to be filtered is first stripped to expose the cladding 2. Then, the exposed cladding 2 is placed in the femtosecond laser processing system. The femtosecond laser is focused into the cladding 2, but does not contact the core 3. In this embodiment, a double-clad silica fiber with a core of 20 micrometers and a cladding of 400 micrometers is used. The pulse width of the femtosecond laser is no greater than 400 fs, and the refractive index of the silica glass can be increased at the focal point. First, the femtosecond laser is focused to a position 150 micrometers from the edge of the core 3. The focal point of the femtosecond laser is then moved to the surface of the fiber cladding 2 to create a cladding light transmission channel. Multiple similar transmission channels are then created along the length of the fiber. The focus is then shifted to a position 100 micrometers from the edge of the core 3, and the focal point of the femtosecond laser is moved to the surface of the fiber cladding 2 to create another cladding light transmission channel. Multiple identical transmission channels are fabricated along the length of the optical fiber. Focusing the femtosecond laser at a position 50 micrometers from the edge of fiber core 3, the focal point is moved to the surface of fiber cladding 2 to fabricate a cladding light transmission channel. Multiple identical transmission channels are then fabricated along the length of the optical fiber. The optical fiber is rotated 90 degrees sequentially, and the above fabrication steps are repeated.

[0040] For polarization-maintaining fibers containing stress regions, the fabricated cladding optical transmission channel should avoid the stress regions. For photonic crystal fibers, the fabricated cladding optical transmission channel should be controlled to not affect its transmission mode characteristics.

[0041] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art are encouraged to make modifications and improvements without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention shall be defined by the claims.

Claims

1. A method for deep cladding light filtering of femtosecond laser-modified optical fiber, characterized in that, Includes the following steps: S1. Strip a section of the coating layer from the optical fiber whose cladding light is to be filtered to expose the inner cladding, and place the inner cladding on a femtosecond laser processing platform; S2. A femtosecond laser is focused inside an optical fiber, and the refractive index of the fiber at the focal point is increased by adjusting the parameters of the laser, thus forming a type I modification. S3. Change the focal position of the laser along the diameter direction of the fiber end face to form a modified channel between the inside of the fiber and the fiber surface, which is used to guide the cladding light out of the fiber. S4. Modified channels with different depths are prepared at different positions along the longitudinal length of the optical fiber to filter out cladding light with different numerical apertures.

2. The femtosecond laser-modified fiber deep cladding optical filtering method according to claim 1, characterized in that, Also includes: S5. Metal encapsulation is applied to the optical fiber, suspending the cladding stripping area. The cladding light guided from the optical fiber is directly absorbed by the encapsulation metal and converted into heat, which is then used for heat dissipation.

3. The method for deep cladding light filtering of femtosecond laser-modified optical fiber according to claim 1, characterized in that, In step S2, the parameters of the laser include adjusting the pulse width, repetition frequency, and pulse energy.

4. The femtosecond laser-modified fiber deep cladding optical filtering method according to claim 1, characterized in that, The modified channel formed in step S3 extends continuously from the inside of the optical fiber to the surface of the optical fiber, and the starting position of the modified channel is set according to the numerical aperture of the cladding light to be filtered out.

5. The method for deep cladding light filtering of femtosecond laser-modified optical fiber according to claim 1, characterized in that, Step S4 involves controlling the relative movement of the focal point of the femtosecond laser and the optical fiber to achieve modifications at different depths.

6. The femtosecond laser-modified fiber deep cladding optical filtering method according to claim 2, characterized in that, Step S5. The metal package uses a metal material with good thermal conductivity, and the packaged metal is cooled by water or other heat dissipation methods.

7. The femtosecond laser-modified fiber deep cladding optical filtering method according to any one of claims 1-6, characterized in that, The depth of the modified channel in the radial direction of the optical fiber varies depending on the NA of the cladding light to be stripped, and the density of the modified channel in the longitudinal direction of the optical fiber and the length of the optical fiber with the modified channel vary depending on the proportion and power of the cladding light to be stripped.

8. The method for deep cladding light filtering of femtosecond laser-modified optical fiber according to any one of claims 1-6, characterized in that, The optical fibers include large-mode-field optical fibers and photonic crystal optical fibers.

Images