Side-pumped signal combiner for realizing flat-top beam and preparation method thereof

Abstract

The application provides a side-pumped signal combiner for realizing flat-top beam and a preparation method thereof. The side-pumped signal combiner comprises a main signal optical fiber and at least one side arm input optical fiber. The side arm input optical fiber is attached to a waveguide structure outside the innermost waveguide structure of the main signal optical fiber. The mode field distribution in the side arm input optical fiber is cladding multimode by controlling the values of the three parameters of fusion depth H, fusion length L and fusion area diameter D. The cladding and air of the side arm input optical fiber respectively constitute the core and cladding structure of the original side arm input optical fiber. When the signal light enters the main signal optical fiber through the side arm input optical fiber, the corresponding high-order mode is excited. By changing the optical waveguide mode of the signal light entering the main signal optical fiber in the side arm input optical fiber, the output of the flat-top mode light spot in the main signal optical fiber is realized.

Description

[0001] This application is a divisional application of the invention patent filed on December 20, 2018, entitled "Side-pumped signal beam combiner for realizing a flat-top beam and its preparation method", application number 201811564754.4. Technical Field

[0002] This invention relates to the field of laser beam shaping technology, and in particular to a side-pumped signal beam combiner for realizing a flat-top beam and its fabrication method. Background Technology

[0003] Laser beams are generally Gaussian beams, characterized by high intensity in the center and a gradual decrease in intensity along a Gaussian profile in the radial direction. A flat-top beam, on the other hand, is a laser beam with a uniform intensity distribution. In high-power and high-energy systems, the energy of a Gaussian beam is mostly concentrated in the center, easily damaging optical components. Furthermore, Gaussian beams can cause non-uniform heating, affecting beam quality. In practical applications, Gaussian beams, due to their extremely high central intensity, easily penetrate materials. Flat-top beams, used in laser engraving, offer uniform intensity and are less likely to damage materials. Additionally, flat-top beams produce smoother and more uniform weld seams compared to Gaussian beams in laser welding. Therefore, shaping Gaussian beams into flat-top beams has become a pressing technological challenge.

[0004] Traditional beam shaping methods mostly utilize spatial optical shaping. These methods include, but are not limited to, the following: aperture method, multi-lens array, binary optical elements, long depth of field shaping, liquid crystal spatial light modulator, birefringent lens group, and aspherical lens group, etc.

[0005] In the field of fiber optic waveguides, there are several ways to generate flat-top light.

[0006] Cylindrical vector beam focusing and shaping: Cylindrical vector beams are four precise modes under the linear polarization mode LP01 in fiber lasers, including angular cylindrical vector polarized light, radial cylindrical vector polarized light, and mixed-state cylindrical vector polarized light. Unlike traditional linearly polarized or elliptically polarized beams, which have uniformly distributed polarization, cylindrical vector beams are non-uniformly polarized beams. The polarization states on the beam cross-section satisfy an axisymmetric distribution, and the polarization state at each point is related to its position. Under strong focusing, cylindrical vector beams can achieve a uniform intensity distribution at the focal plane. When strongly focusing a radial cylindrical vector beam, a strong longitudinal component and a radial component with a ring distribution will be generated at the focal plane, while angularly polarized light will only produce a ring-distributed angular component in this case. Mixed-state cylindrical vector polarized light, since it can be considered as a superposition of the previous two types of cylindrical vector beams, will produce components in three polarization directions after strong focusing. By controlling the ratio of radial to angular cylindrical vector beams, a flat-top beam can be constructed at the focal plane. [Reference "Focus shaping using cylindrical vector beams"]

[0007] Vortex beam focusing and shaping: A vortex beam is a beam carrying a certain orbital angular momentum, with a non-uniform phase distribution across its cross-section. The expression for a vortex beam includes a phase factor exp(ilθ), where 1 is the orbital angular quantum number of the vortex beam. Researchers have found that strong focusing of a vortex beam with l = -1 can produce a solid intensity distribution in the longitudinal direction and a ring-shaped distribution in the transverse direction. Since these two components are independent, their intensities can be directly superimposed, resulting in a flat-top beam. [Reference: "Construction of a Flat-Top Beam Based on Strong Focusing of a Circularly Polarized Vortex Beam"]

[0008] The advantages of the above two focusing and shaping methods are high energy utilization, but the disadvantages are that they can only form a flat-top beam on the focal plane, which is inconvenient to operate in actual use. They are also not conducive to all-fiber integration for spatial shaping and cannot be widely used in the industrial applications of high-power fiber lasers.

[0009] A flat-top mode is obtained by changing the optical fiber structure: on the basis of an ordinary two-layer step-index optical fiber, a recessed portion is introduced at the center of the fiber core, the refractive index of this portion is made different from that of other portions, and through theoretical calculation, different flat-top fundamental mode field distributions can be obtained by adopting different refractive indices for the recessed portion. [Document "Design of Waveguide Refractive Index Profile to Obtain Flat Modal Field"] Since then, different optimized designs for flat-top mode field optical fiber structures have been carried out both domestically and internationally. However, after light of different modes is introduced into the optical fiber, a specific refractive index distribution is required to obtain a flat-top spot energy distribution, and the same optical fiber is not universal for different incident lights. In addition, the fabrication of an optical fiber preform with a specifically distributed and continuously varying refractive index requires relatively high process requirements and costs.

[0010] At present, solutions for side coupling of optical fibers have been proposed, but they are currently all used for pump light transmission in fiber lasers, where signal light is transmitted in the main signal optical fiber and pump light is transmitted in the side-arm input optical fiber, and there is no requirement for the energy distribution of the light beam entering the main signal fiber from the side arm. Content of the Invention

[0011] In view of the above problems, the purpose of the present invention is to provide a side-pump signal combiner and a fabrication method thereof, which change the optical waveguide mode of the signal light in the side-arm input optical fiber entering the main signal optical fiber by controlling the fusion length, diameter and depth of the tapered side-arm input optical fiber, so as to realize the output of a flat-top mode spot in the main signal optical fiber.

[0012] A side-pump signal combiner for realizing a flat-top light beam, comprising one main signal optical fiber and at least one side-arm input optical fiber. The main signal optical fiber has at least two layers of waveguide structures, the refractive index of the inner waveguide structure is higher than that of the outer waveguide structure, and the main signal optical fiber is a multimode optical fiber for laser of the transmission wavelength. The side-arm input optical fiber is attached inside the waveguide structure outside the innermost waveguide structure of the main signal optical fiber, and the fusion length L of the side-arm input optical fiber is: (0mm<L≤200mm); the fusion depth H is: (0μm<H≤500μm); the diameter D of the fusion region is: (5μm≤D≤500μm).

[0013] Fusion length L: taking the position where the side-arm input optical fiber starts to fit with the main signal optical fiber as the starting point, and the position where the fitting between the side-arm input optical fiber and the main signal optical fiber ends as the end point, the distance between the two is the fusion length.

[0014] Fusion depth H: taking the cladding of the main signal optical fiber as the starting point, and the deepest position where the side-arm input optical fiber fits into the main signal optical fiber as the end point, the distance between the two is the fusion depth.

[0015] Fusion region diameter D: When manufacturing the side pump combiner, the side arm input fiber needs to be tapered. The side arm input fiber will change into the original fiber-transition region-conical waist structure, and the diameter of the conical waist is the fusion region diameter.

[0016] The outermost waveguide structure of the main signal fiber has a diameter of 250μm to 1100μm, and the innermost waveguide structure has a diameter of 20μm to 1000μm.

[0017] In another preferred embodiment, the diameter of the outermost waveguide structure of the main signal fiber is 250μm, 300μm, 400μm, 600μm, or 1100μm.

[0018] In another preferred embodiment, the diameter of the innermost waveguide structure of the main signal fiber is 20μm, 25μm, 30μm, 50μm or 1000μm.

[0019] The numerical aperture of the main signal fiber is 0.03 to 0.5.

[0020] In another preferred embodiment, the numerical aperture of the main signal fiber is 0.06, 0.065, 0.09, 0.11, 0.15 or 0.22.

[0021] The numerical aperture of the input fiber in the side arm is 0.03 to 0.5.

[0022] In another preferred embodiment, the numerical aperture of the side arm input fiber is 0.065, 0.075, 0.08, 0.12, or 0.22.

[0023] The side-arm input fiber has at least two waveguide layers.

[0024] The outermost waveguide structure of the side-arm input fiber has a diameter of 125μm to 1100μm, and the innermost waveguide structure has a diameter of 10μm to 1000μm.

[0025] In another preferred embodiment, the diameter of the outermost waveguide structure of the side arm input fiber is 125μm, 130μm, 600μm, or 1100μm.

[0026] In another preferred embodiment, the diameter of the innermost waveguide structure of the side arm input fiber is 10 μm, 15 μm, 20 μm, 30 μm, 50 μm or 1000 μm.

[0027] The innermost waveguide structure of the main signal fiber has a circular or polygonal cross-section.

[0028] A method for fabricating a side-pumped signal beam combiner for realizing a flat-top beam, characterized by comprising the following steps:

[0029] Heating the side-arm input optical fiber to perform tapering processing, wherein the taper waist diameter D after tapering satisfies: 5μm ≤ D ≤ 500μm;

[0030] Fitting the side-arm input optical fiber and the main signal optical fiber such that the fusion length L satisfies: 0mm < L ≤ 200mm; performing side fusion on the side-arm input optical fiber and the main signal optical fiber such that the fusion depth H satisfies: 0μm < H ≤ 500μm;

[0031] The main signal optical fiber has at least two layers of waveguide structures, the refractive index of the inner waveguide structure is higher than that of the outer waveguide structure, and the main signal optical fiber is a multimode optical fiber for laser light of the transmission wavelength.

[0032] The heating source used for the heating and fusion is: oxyhydrogen flame, CO2 laser, electrode discharge or high-frequency induction.

[0033] The technical effects of the present invention are as follows: by controlling the length, diameter and depth of tapering and fusion of the side-arm input optical fiber in the side-pump signal combiner, the optical waveguide mode of the signal light entering the main signal optical fiber from the side-arm input optical fiber is changed, so as to realize the output of a flat-top mode spot in the main signal optical fiber, which solves the problem that when a flat-top mode is obtained by changing the optical fiber structure, a specific refractive index distribution is required, and the same optical fiber has no universality for different incident lights. The advantages of the present invention compared with the prior art are as follows:

[0034] 1) High light energy utilization rate;

[0035] 2) Wide effective range of the flat-top spot;

[0036] 3) Mass production is achievable;

[0037] 4) Products can be customized according to actual spot requirements;

[0038] 5) Short manufacturing cycle;

[0039] 6) Easy for all-fiber integration. Brief Description of the Drawings

[0040] Figure 1 is a schematic diagram of the signal transmission principle;

[0041] Figure 2 is a schematic diagram of the side-pump signal combiner;

[0042] Figure 3 is a schematic diagram of parameters for controlling the fusion region;

[0043] Figure 4 is a schematic diagram of the energy distribution of a Gaussian beam;

[0044] Figure 5 is a schematic diagram of the energy distribution of a flat-top light. Detailed Description of the Embodiments

[0045] Working principle:

[0046] The basic principle of this invention is as follows: Figure 1 As shown.

[0047] Optical fiber is a weak waveguide structure, and the modes of the optical waveguide within the fiber are usually represented by linearly polarized modes LPmn. m represents a lobe-like distribution, and n represents a ring-like distribution. For example, the fundamental mode LP... 01 The light spot is distributed in a circular shape without lobes. For the mode energy distribution of an optical fiber, the intensity distribution of the fundamental mode is a standard Gaussian distribution. Generally, the energy distribution of higher-order modes is closer to the edges, so increasing the proportion of higher-order modes helps to homogenize the light spot energy distribution.

[0048] During fiber tapering, the diameters of the fiber core and cladding decrease proportionally, leading to changes in the mode field distribution. Based on the mode field distribution, this can be divided into four stages: core multimode, core single-mode, cladding multimode, and cladding single-mode. In the core multimode and core single-mode stages, the diameter of the fundamental mode field in the core decreases accordingly, but the mode field is still confined within the core. However, with further tapering, the mode field diffuses into the cladding, reaching the third stage—cladding multimode. The cladding and air together constitute the original fiber core and cladding structure, exciting higher-order modes that meet the fiber transmission requirements. These higher-order modes typically include LP11, LP21, LP02, LP12, LP31, LP41, LP03, LP22, LP51, LP32, LP13, and LP61. Higher-order mode lasers from the side-arm input fiber pass through the fusion region into the main signal fiber, significantly altering the energy distribution of the beam spot and achieving the effect of shaping a Gaussian beam into a flat-top beam.

[0049] During the tapering process, the values ​​of three parameters—fusion depth H, fusion length L, and fusion region diameter D—are controlled. When the signal light enters the main signal fiber through the side arm input fiber, it excites corresponding higher-order modes.

[0050] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0051] Example 1:

[0052] like Figure 2 As shown, a side-pump signal combiner includes a main signal fiber and at least one side-arm input fiber. (Two side arms are shown in the figure.)

[0053] The main signal fiber must have at least two waveguide layers. The refractive index of the inner waveguide layer must be higher than that of the outer waveguide layer. Furthermore, the main signal fiber must be a multimode fiber for the transmission wavelength of the laser.

[0054] The main signal optical fiber preferably has a two-layer waveguide structure, the outer waveguide structure is a cladding, and the inner waveguide structure is a core, and the specific parameters can be selected from any of the following:

[0055]

[0056] The side arm input optical fiber preferably has a two-layer waveguide structure, the outer waveguide structure is a cladding, and the inner waveguide structure is a core, and the specific parameters can be selected from any of the following:

[0057]

[0058] The side arm input optical fiber is attached to the outer waveguide structure of the main signal optical fiber by means of fused tapering. When preparing the side pump combiner, it is necessary to control the fusion length, fusion depth and fusion region diameter of the side arm input optical fiber.

[0059] As shown in Figure 3 which is a schematic diagram of the parameters for controlling the fusion region:

[0060] Fusion length L: The starting point is where the side arm input optical fiber begins to attach to the main signal optical fiber, and the end point is where the attachment between the side arm input optical fiber and the main signal optical fiber ends. The distance between the two points is the fusion length. The requirement for fusion length L is: (0mm<L≤200mm).

[0061] Fusion depth H: During the fused tapering process, part of the side arm input optical fiber will be attached into the interior of the main signal optical fiber. Taking the cladding of the main signal optical fiber as the starting point and the deepest position of the side arm input optical fiber as the end point, the distance between the two points is the fusion depth. The requirement for fusion depth is: (0μm<H≤500μm), and the fusion depth is less than or equal to the diameter of the main signal optical fiber.

[0062] Fusion region diameter D: When manufacturing the side pump combiner, the side arm input optical fiber needs to be tapered, and the side arm input optical fiber will be changed into a structure of original optical fiber - transition region - taper waist, and the diameter of the taper waist is the fusion region diameter. The requirement for fusion region diameter D is: (5μm≤D≤500μm), the fusion region diameter is less than or equal to the cladding diameter of the side arm input optical fiber, and is less than or equal to the cladding diameter of the main signal optical fiber.

[0063] Example 2

[0064] Both the main signal optical fiber and the side arm signal optical fiber have a two-layer waveguide structure, the main signal optical fiber is 50 / 400 with NA: 0.15 / 0.46, and the side arm input optical fiber is 50 / 125 with NA: 0.12;

[0065] A hydrogen-oxygen flame is used as the heating source;

[0066] Pretreatment of the side arm input optical fiber: The side arm input optical fiber is subjected to tapering treatment, and the diameter of the taper waist after tapering is 50μm;

[0067] Bonding of side arm input fiber and main signal fiber: Both the main signal fiber with the coating removed and the side arm input fiber are fixed with special clamps. The side arm input fiber is stretched and bonded to the surface of the main signal fiber without deformation or sagging. The fusion length L = 2mm.

[0068] Side fusion of the side arm input fiber and the main signal fiber: The fusion depth of the side arm input fiber and the main signal fiber is controlled by controlling the temperature of the oxyhydrogen flame and the fusion time. The fusion depth H = 5 μm.

[0069] The final example parameters are: fusion depth H = 5 μm, fusion length L = 2 mm, and fusion region diameter D = 50 μm;

[0070] Figure 4 The figure shows the Gaussian spot energy distribution of the signal light without being shaped by the sidewall signal combiner. Figure 5 The image shows the energy distribution of the flat-top light spot after being shaped by the side arm signal combiner.

[0071] Example 3

[0072] Both the main signal fiber and the side arm signal fiber have a three-layer waveguide structure. The main signal fiber is 50 / 70 / 660 with NA: 0.22, and the side arm input fiber is 100 / 120 / 360 with NA: 0.22.

[0073] An oxyhydrogen flame is used as the heating source;

[0074] Pre-processing of side-arm input fiber: Tapering is performed on the side-arm input fiber, and the diameter of the tapered waist is 100μm after tapering.

[0075] Bonding of side arm input fiber and main signal fiber: Both the main signal fiber with the coating removed and the side arm input fiber are fixed with special clamps. The side arm input fiber is stretched and bonded to the surface of the main signal fiber without deformation or sagging. The fusion length L = 5mm.

[0076] Side fusion of the side arm input fiber and the main signal fiber: The fusion depth of the side arm input fiber and the main signal fiber is controlled by controlling the temperature of the oxyhydrogen flame and the fusion time. The fusion depth H = 10 μm.

[0077] The final example parameters are: fusion depth H = 10 μm, fusion length L = 5 mm, and fusion region diameter D = 100 μm.

Claims

1. A side-pumped signal combiner for realizing a flat-top beam, comprising a main signal fiber and at least one side-arm input fiber, characterized in that... : The side arm input optical fiber is used to transmit signal light; The main signal fiber has at least two waveguide structures, with the refractive index of the inner waveguide structure being higher than that of the outer waveguide structure. The side-arm input fiber is bonded to the waveguide structure outside the innermost waveguide structure of the main signal fiber. By controlling the values ​​of the three parameters of the side-arm input fiber, namely the fusion depth H, fusion length L, and fusion region diameter D, the mode field distribution in the side-arm input fiber is made to be cladding multimode. The cladding and air of the side-arm input fiber constitute the core and cladding structure of the original side-arm input fiber, respectively. This allows the signal light to excite the corresponding higher-order modes when it enters the main signal fiber through the side-arm input fiber, and to shape the light spot output by the main signal fiber from a Gaussian beam into a flat-top beam. The fusion length L is 0mm≤L≤200mm; the fusion depth H is 0μm≤H≤500μm; and the fusion region diameter D is 5μm≤D≤500μm.

2. The side pump signal combiner according to claim 1, characterized in that, The higher-order mode is one or more of the following modes that meet the optical fiber transmission requirements: LP11, LP21, LP02, LP12, LP31, LP41, LP03, LP22, LP51, LP32, LP13, and LP61.

3. The side pump signal combiner according to claim 1 or 2, characterized in that, The high-order mode laser in the side arm input fiber passes through the fusion region into the main signal fiber, changing the energy distribution of the light spot in the main signal fiber and shaping the Gaussian beam in the main signal fiber into a flat-top beam.

4. The side pump signal combiner according to claim 1 or 2, characterized in that, The main signal fiber has a two-layer waveguide structure, with an outer waveguide structure being the cladding and an inner waveguide structure being the fiber core. The diameter of the outer waveguide structure is 250μm~1100μm, and the diameter of the inner waveguide structure is 20μm~1000μm. The numerical aperture of the main signal fiber is 0.06~0.

22.

5. The side pump signal combiner according to claim 1 or 2, characterized in that, The side-arm input fiber has a two-layer waveguide structure, with an outer waveguide structure being the cladding and an inner waveguide structure being the fiber core. The diameter of the outer waveguide structure is 125μm~1100μm, and the diameter of the inner waveguide structure is 10μm~1000μm. The numerical aperture of the input optical fiber in the side arm is 0.03~0.

22.

6. The side pump signal combiner according to claim 1 or 2, characterized in that, When both the main signal fiber and the side arm input fiber have a two-layer waveguide structure, the main signal fiber is 50 / 400 with a numerical aperture (NA) of 0.15 / 0.46, and the side arm input fiber is 50 / 125 with a numerical aperture (NA) of 0.12, the fusion length L of the side arm input fiber is: L = 2 mm; the fusion depth H is: H = 5 μm; and the fusion region diameter D is: D = 50 μm. or When both the main signal fiber and the side arm input fiber have a three-layer waveguide structure, and the main signal fiber is 50 / 70 / 660 with a numerical aperture (NA) of 0.22, and the side arm input fiber is 100 / 120 / 360 with a NA of 0.22, the fusion length L of the side arm input fiber is: L = 5 mm; the fusion depth H is: H = 10 μm; and the fusion region diameter D is: D = 100 μm.

7. The side pump signal combiner according to claim 1, characterized in that, The fusion length L is the distance between the starting point where the side arm input fiber begins to attach with the main signal fiber and the ending point where the side arm input fiber and the main signal fiber end to attach. The fusion depth H is the radial projection distance between the main signal fiber cladding as the starting point and the side arm input fiber as the deepest point where it is attached to the main signal fiber as the ending point. The diameter D of the fusion region is the diameter of the fusion region when the side-arm input fiber is tapered during the fabrication of the side-pump combiner. The side-arm input fiber is changed into a tapered waist structure.

8. The side pump signal combiner according to claim 1 or 2, characterized in that, The main signal fiber has at least two waveguide layers, and the refractive index of the inner waveguide layer must be higher than that of the outer waveguide layer. The main signal fiber is a multimode fiber for transmitting laser wavelengths.

9. A method for fabricating a side-pumped signal beam combiner to realize a flat-top beam, characterized in that, The method includes: A heated side arm input optical fiber is used to transmit signal light. A tapered process is performed to fit the side-arm input fiber into the waveguide structure outside the innermost waveguide structure of the main signal fiber. By controlling the values ​​of the fusion depth H, fusion length L, and fusion region diameter D of the side-arm input fiber within the ranges of 0mm≤L≤200mm; 0μm≤H≤500μm; and 5μm≤D≤500μm, the mode field distribution within the side-arm input fiber is cladding multimode. The cladding and air of the side-arm input fiber constitute the core and cladding structures of the original side-arm input fiber, respectively. This allows the signal light to excite corresponding higher-order modes when it enters the main signal fiber through the side-arm input fiber, and to shape the light spot output from the main signal fiber from a Gaussian beam into a flat-top beam. The main signal fiber has at least two waveguide layers, with the refractive index of the inner waveguide layer being higher than that of the outer waveguide layer.

10. The method according to claim 9, characterized in that, The higher-order mode laser in the side arm input fiber passes through the fusion region and enters the main signal fiber; The energy distribution of the light spot within the main signal fiber is altered to shape the Gaussian beam within the main signal fiber into a flat-top beam for output.

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