Femtosecond laser modified optical fiber deep cladding light filtering method

Abstract

The invention discloses a femtosecond laser modified optical fiber deep cladding light filtering method, which comprises the following steps of: firstly, stripping a section of a large-mode-field optical fiber to be filtered until an inner cladding is exposed, and placing the large-mode-field optical fiber on a femtosecond laser processing platform; femtosecond laser is focused into the optical fiber, and the refractive index of the large-mode-field optical fiber at the focusing position is increased (I-type modification) by regulating and controlling laser parameters. The relative position of the focus point of the femtosecond laser and the large-mode-field optical fiber is changed, an I-type modification channel from the inside to the surface is formed in the optical fiber, and cladding light is guided out of the optical fiber. Different initial positions of the modification channel can guide cladding light of different NAs out of the large-mode-field fiber. The size of the large-mode-field optical fiber with the modified channel does not change, exported cladding light does not gather on the surface to form heat, and the large-mode-field optical fiber is not prone to damage and can tolerate high power.

Description

Technical Field

[0001] The present invention relates to a method for filtering out large mode field optical fibers, in particular to a method for filtering out deeply cladded light of femtosecond laser modified optical fibers. Background Art

[0002] Fiber lasers have many advantages such as high efficiency, simple structure, and high stability, and are widely used in fields such as precision machining and high-end manufacturing. Among them, cladding light filtering is crucial for improving the beam quality and long-term reliability of the output laser of fiber lasers. This technology filters out the light transmitted in the cladding and only retains the light transmitted in the core, thereby improving the beam quality and long-term reliability of the output laser of fiber lasers.

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

[0004] A common practice is to set a refractive index matching layer on the outer layer of the optical fiber, and by destroying the total reflection condition of the pump light and the cladding laser in the inner cladding, the light is exported from the inner cladding. However, this method has obvious defects. If a gelatinous refractive index matching layer is used, this layer will absorb part of the filtered light, resulting in heat generation and even burning. If a glass refractive index matching layer is used, a matching adhesive is required for fixation, and there is also a risk of heat generation and burning. In addition, during the process of heating and curing the matching glass to the surface of the inner cladding of the optical fiber, stress may be generated inside the optical fiber, thereby affecting the quality of the output beam.

[0005] Another method is to roughen the surface of the inner cladding of the optical fiber, but this will reduce the mechanical strength of the optical fiber, thereby affecting its reliability and service life. Although the cladding light with a low numerical aperture (NA) can be filtered out by reducing the diameter of the inner cladding of the optical fiber and then roughening the surface, this method will further weaken the mechanical strength of the optical fiber, making it unable to withstand high power. At the same time, this method is not applicable to polarization-maintaining optical fibers with stress regions and photonic crystal optical fibers with structural regions.

[0006] Although the above two methods do not damage the inner cladding of the optical fiber, it is difficult to filter out the cladding light with a low NA, and they are not applicable to three-cladding and multi-cladding optical fibers.

[0007] In view of the various defects existing in the above methods, the industry urgently needs a new, more efficient and reliable fiber cladding light filtering technology to solve the deficiencies in the prior art and further improve the performance and stability of fiber lasers. Summary of the Invention

[0008] In view of the deficiencies and defects of the above-mentioned prior art, the present invention proposes a method for filtering deeply cladded light in a femtosecond laser modified optical fiber, which uses femtosecond laser to modify the interior of the optical fiber to form a modified channel extending from the interior of the optical fiber to the surface. This channel can effectively guide the cladded light out of the optical fiber and be absorbed by the external metal encapsulation structure and converted into heat, which is finally dissipated through a heat dissipation device.

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

[0010] A method for filtering deeply cladded light in a femtosecond laser modified optical fiber, characterized by comprising the following steps:

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

[0012] S2. Focus the femtosecond laser inside the optical fiber, and increase the refractive index of the optical fiber at the focal point by adjusting the parameters of the laser to form a type-I modification;

[0013] S3. Change the focal position of the laser along the diameter direction of the optical fiber end face, so as to form a modified channel between the interior of the optical fiber and the surface of the optical fiber for guiding the cladded light out of the optical fiber;

[0014] S4. Prepare modified channels with different depths at different positions along the longitudinal length of the optical fiber to filter cladded light with different numerical apertures.

[0015] Furthermore, it further includes:

[0016] S5. Perform metal encapsulation on the optical fiber to make the cladded light stripping area suspended, and the cladded light guided out of the optical fiber is directly absorbed by the encapsulating metal and converted into heat, and the encapsulating metal is subjected to heat dissipation treatment.

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

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

[0019] Furthermore, step S4 is achieved by controlling the relative movement of the focal point of the femtosecond laser and the optical fiber to achieve modifications with different depths.

[0020] Furthermore, in step S5, the metal encapsulation uses a metal material with good thermal conductivity, and the encapsulating metal is subjected to heat dissipation treatment by water cooling or other heat dissipation methods.

[0021] Further, the depth of the modified channel in the radial direction of the optical fiber varies according to the numerical aperture (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 according to the proportion and power of the cladding light to be stripped.

[0022] Further, the optical fiber includes a large mode field optical fiber and a photonic crystal optical fiber.

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

[0024] 1. By using femtosecond laser to prepare refractive index increased (type I) modified channels in the inner cladding of the optical fiber, the residual pump light, amplified spontaneous emission light, high-order mode signals and other lights remaining in the cladding in the optical fiber can be exported from the optical fiber. By changing the depth of the modified channel in the radial direction of the optical fiber, the cladding light with different numerical apertures can be stripped. By changing 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, the stripping ratio can be adjusted. The modified channel does not generate heat on the surface area of the optical fiber, is not easily damaged, and can withstand high power.

[0025] 2. Without damaging the optical fiber, the power and beam quality of the signal in the fiber core of the optical fiber are maintained. The mechanical strength and service life are not reduced. The filtered light directly exits from the surface of the optical fiber without heat accumulation. It can withstand high power. During the preparation process, the optical fiber does not undergo a heating process, there is no residual stress, and the laser beam quality is not reduced.

[0026] 3. The preparation method is applicable not only to cladding optical fibers (polarization-maintaining, non-polarization-maintaining), but also to photonic crystal optical fibers. It is possible to export the low-NA cladding light effectively without damaging the optical fiber, without reducing the power of the signal in the fiber core of the optical fiber, without damaging the beam quality of the optical fiber, and without generating heat on the surface area of the optical fiber. Description of the Drawings

[0027] Figure 1 It is a flow chart of the femtosecond laser modified optical fiber deep cladding light filtering method of the present invention

[0028] Figure 2 It is a schematic diagram of the optical fiber obtained in the preparation of the femtosecond laser modified large mode field optical fiber deep cladding light filtering method.

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

[0030] In the figure: 1: optical fiber with cladding; 2: optical fiber cladding; 3: optical fiber core; 4: modified filtering channel Detailed Embodiments

[0031] The following further describes the present invention with reference to the drawings and specific embodiments, but the protection scope of the present invention should not be limited thereby.

[0032] This embodiment proposes a method for filtering deeply cladding light in femtosecond laser modified optical fiber, aiming to efficiently and non-destructively filter the cladding light in the optical fiber, especially for the cladding light with a low numerical aperture (NA). The femtosecond laser is used to precisely modify the interior of the optical fiber to form a modified channel extending from the interior of the optical fiber to the surface. This channel can effectively guide the cladding light to be exported from the optical fiber and be absorbed by the external metal encapsulation structure and converted into heat, which is finally dissipated through the heat dissipation device.

[0033] Please refer to Figure 1 , Figure 1 which is the flow chart of the method for filtering deeply cladding light in femtosecond laser modified optical fiber of the present invention. As shown in the figure, it includes the following steps:

[0034] S1. Optical fiber pretreatment: First, strip a section of the coating layer from the large mode field optical fiber 1 whose cladding light is to be filtered to expose the inner cladding 2, so that the femtosecond laser can directly act on the inner cladding of the optical fiber for subsequent modification treatment. Place the optical fiber with the exposed inner cladding 2 on the femtosecond laser processing platform for laser modification operation.

[0035] S2. Femtosecond laser modification: Use the femtosecond laser to focus the focal point on the inner cladding 2 without touching the core 3, and by adjusting the parameters of the laser (such as pulse width, repetition frequency, pulse energy, etc.), achieve an increase in the refractive index of the optical fiber at the focal point, that is, type I modification. This modification changes the optical properties of the optical fiber at the focal point, so as to be able to guide the cladding light to be exported along the modified channel.

[0036] S3. Forming a modified channel: By changing the focal point position of the laser along the diameter direction of the optical fiber end face, form a modified channel 4 extending from the interior of the optical fiber to the surface of the optical fiber. This channel is like an "optical duct" that can guide the cladding light from the interior of the optical fiber to the outside.

[0037] S4. Preparing modified channels with different depths: In order to filter the cladding light with different NAs, it is necessary to prepare modified channels with different depths at different positions along the longitudinal length of the optical fiber. In this way, when the cladding light with different NAs propagates to the modified channels with corresponding depths, it will be guided out of the optical fiber.

[0038] S5. Metal encapsulation and heat dissipation: Metal encapsulate the optical fiber so that the cladding light stripping area is suspended. In this way, the cladding light guided out from the optical fiber will be directly absorbed by the encapsulation metal and converted into heat. For effective heat dissipation, the encapsulation metal can be cooled by passing water to ensure the stable operation of the optical fiber system.

[0039] Figure 2Schematic diagram of the fiber prepared in the method for filtering deeply cladding light of a large mode field fiber modified by femtosecond laser. As shown in the figure, whether it is a double-clad, triple-clad or multi-clad fiber or a photonic crystal fiber, first, strip the coating layer of a section of the fiber 1 whose cladding light is to be filtered to expose the cladding 2, and then place the exposed cladding 2 in a femtosecond laser processing system. Focus the femtosecond laser inside the cladding 2 without touching the core 3. The double-clad silica fiber used in this embodiment has a core of 20 microns and a cladding of 400 microns. The pulse width of the femtosecond laser is not greater than 400 fs, and an increase in the refractive index of the silica glass can be achieved at the focal point. First, focus the femtosecond laser at a position 150 microns from the edge of the core 3, and move the focal point of the femtosecond laser to the surface of the fiber cladding 2 to prepare a cladding light conduction channel. Along the length direction of the fiber, prepare multiple such conduction channels. Focus at a position 100 microns from the edge of the core 3, move the focal point of the femtosecond laser to the surface of the fiber cladding 2 to prepare a cladding light conduction channel. Along the length direction of the fiber, prepare multiple such conduction channels. Focus at a position 50 microns from the edge of the core 3, move the focal point of the femtosecond laser to the surface of the fiber cladding 2 to prepare a cladding light conduction channel. Along the length direction of the fiber, prepare multiple such conduction channels. Rotate the fiber 90 degrees in sequence and repeat the above preparation steps.

[0040] For polarization-maintaining fibers containing stress regions, the prepared cladding light conduction channels should avoid the stress regions. For photonic crystal fibers, the prepared cladding light conduction channels should be controlled so as not to affect their conduction mode characteristics.

[0041] Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. It is recognized that those skilled in the art can make modifications and improvements without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be defined by the claims.

Claims

1. A femtosecond laser modified optical fiber deep cladding light filtering method, characterized in that: The steps include: S1. Stripping a section of the coating layer of the optical fiber to be filtered to expose the inner cladding, and placing the inner cladding on a femtosecond laser processing platform; S2. Use femtosecond laser to focus inside the optical fiber, and increase the refractive index of the optical fiber at the focus by adjusting the parameters of the laser to form type I modification; S3. Changing the focal position of the laser along the diameter direction of the optical fiber end face, thereby forming a modified channel between the inside of the optical fiber and the surface of the optical fiber for guiding the cladding light to be derived from the optical fiber; S4. Prepare modified channels with different depths at different positions along the longitudinal length of the optical fiber to filter out cladding light with different numerical apertures.

2. The method for deep cladding light filtering of femtosecond laser modified optical fiber according to claim 1, characterized in that: Also includes: S5. Metal encapsulate the optical fiber so that the cladding light stripping area is suspended in the air. The cladding light guided out from the optical fiber is directly absorbed by the encapsulation metal and converted into heat, and the encapsulation metal performs heat dissipation treatment.

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

4. The method for deep cladding light filtering of femtosecond laser modified optical fiber 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 removing deep cladding light from femtosecond laser-modified optical fiber according to claim 1, characterized in that: The step S4. realizes modification at different depths by controlling the focusing point of the femtosecond laser and the relative movement of the optical fiber.

6. The method for removing deep cladding light from femtosecond laser-modified optical fiber according to claim 2, characterized in that: The step S5. metal packaging uses metal materials with good thermal conductivity, and the packaging metal is cooled by water cooling or other heat dissipation methods.

7. The method for removing deep cladding light from femtosecond laser-modified optical fiber according to any one of claims 1 to 6, characterized in that: The depth of the modified channel in the radial direction of the optical fiber varies according to 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 according to the proportion and power of the cladding light to be stripped.

8. The method for deep cladding light removal of femtosecond laser modified optical fiber according to any one of claims 1 to 6, characterized in that: The optical fiber includes a large mode field optical fiber and a photonic crystal optical fiber.

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