High-power laser fiber coupling method

Abstract

The invention discloses a high-power laser fiber coupling method. The method comprises the following steps: firstly, Gaussian light emitted by a laser is converted into flat-topped light through an aspheric negative lens; the flat-topped light is shrunk through a lens group, the depth of field of the light beam is prolonged, the end face of the optical fiber is adjusted to couple a far-end laser beam into the optical fiber, and the lens group comprises two high-refractive-index spherical positive lenses, a high-refractive-index spherical negative lens and a low-refractive-index spherical positive lens; the high-refractive-index spherical positive lens is used for shrinking beams and reducing spherical aberration, and the high-refractive-index spherical negative lens and the low-refractive-index spherical positive lens are used for deepening the depth of field and optimizing the spherical aberration. According to the method, the incident laser is homogenized, the depth of field length is prolonged, the debugging process of coupling of the laser and the optical fiber can be greatly simplified, the beam shrinking coupling lens is separated from the optical fiber, replacement is convenient, the later cost is reduced, the energy density of the homogenized laser is more uniform, the risk of device breakdown is reduced, and higher safety is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of laser fiber coupling, and in particular relates to a high-power laser fiber coupling method. Background Art

[0002] In order to change the direction of the laser more conveniently, after coupling the laser into the optical fiber, the laser direction is changed by changing the light output direction of the optical fiber. Compared with directly changing the direction of the laser to change the direction of the laser, changing the laser direction through optical fiber coupling is more convenient and flexible.

[0003] Optical fiber is a slender tubular structure with glass or plastic as the base that can transmit laser. Laser is a high-intensity, highly directional, and highly monochromatic light beam, and high-power laser energy is mostly Gaussian distributed. The coupling between laser and optical fiber mainly relies on two methods: end coupling and internal coupling. End coupling refers to transmitting the laser directly to the end face of the optical fiber, while internal coupling attempts to transmit the laser to the inside of the optical fiber, and achieves energy transmission through refraction and reflection in the optical fiber. End coupling is to collimate and focus the laser beam and align it with the end face of the optical fiber as much as possible. Common methods include using lenses, collimating lenses, and beam reduction lenses to converge or reduce the laser and then irradiate it to the end face of the optical fiber to achieve the purpose of laser coupling.

[0004] Patent CN 213338103U provides a laser coupling optical fiber device, involving related principles and the basic idea of ​​coupling high-power laser into optical fiber. Patent CN114935828A provides a method for debugging the focal plane of an optical fiber coupling lens, which involves a method for debugging a coupling lens, but does not involve efficiency issues.

[0005] The existing high-power laser coupling into optical fiber mainly uses lenses, collimating lenses, and beam reducing lenses to converge or reduce the laser beam and then irradiate it to the end face of the optical fiber to achieve the purpose of laser coupling. Since the depth of field of the spot after the lens converges or the beam reducing lens is short, the position of the optical fiber end face in the focal plane attachment debugging is extremely short, which adds great challenges to the debugging process.

[0006] Lens convergence or beam reduction is to reduce the spot size. The energy density of the laser after convergence and reduction is extremely large, especially for Gaussian beams. The energy distribution of Gaussian lasers determines that the laser intensity trends from high to low from the center to the edge, that is, the energy density at the center is higher. After beam reduction, a beam with a very small diameter is formed, and the energy distribution is still mainly concentrated in the center. The uneven energy distribution increases the risk of ionization.

[0007] In the prior art, the light guide cone is fused to the end face of the optical fiber. This method increases the contact area between the laser and the optical fiber, but the integrated design increases the cost of replacing the optical fiber. Once damaged, the light guide cone and the optical fiber need to be re-fused or replaced as a whole, which is costly.

[0008] At present, the commonly used method for coupling high-power laser into optical fiber is to reduce the high-power laser beam. After the beam is reduced, the laser aperture is smaller than the optical fiber core diameter. By adjusting the position of the optical fiber end face to the coupling focal plane, the laser is coupled into the optical fiber and then transmitted. The debugging process of this method is difficult, and the position of the optical fiber and the beam waist after the beam is reduced is difficult to determine. Alternatively, by designing a light guide cone on the end face of the optical fiber, increasing the coupling end face of the laser coupled into the optical fiber, the surface density of the high-power laser is reduced to reduce the ionization phenomenon during the laser coupling process. This method is costly and it is difficult to replace the light guide cone. Summary of the invention

[0009] In view of the current problems of difficulty in debugging, high replacement cost and safety risks when coupling high-power lasers into optical fibers, the present invention proposes a high-power laser fiber coupling method. This method can greatly simplify the debugging process of laser and optical fiber coupling by homogenizing the incident laser and extending the depth of field. The beam shrinking coupling lens is separated from the optical fiber for easy replacement, which reduces the later cost. The energy density of the laser after homogenization is more uniform, which reduces the risk of device breakdown and is safer.

[0010] The present invention achieves this object through the following technical solutions:

[0011] A high-power laser fiber coupling method, comprising the following contents:

[0012] Firstly, the Gaussian light emitted by the laser is converted into flat-top light by an aspheric negative lens;

[0013] The flat-top light is then beam-contracted and the depth of field of the light beam is extended through a lens group, and the end face of the optical fiber is adjusted to couple the distal laser beam into the optical fiber. The lens group comprises two high-refractive-index spherical positive lenses, a high-refractive-index spherical negative lens and a low-refractive-index spherical positive lens. The two high-refractive-index spherical positive lenses are placed adjacent to each other, and the high-refractive-index spherical negative lens is located between the high-refractive-index spherical positive lens and the low-refractive-index spherical positive lens. The high-refractive-index spherical positive lens is used for beam contraction and reduction of spherical aberration, and the high-refractive-index spherical negative lens and the low-refractive-index spherical positive lens are used for deepening the depth of field and optimizing spherical aberration.

[0014] Furthermore, the lens group is fixed in a beam reduction coupling lens.

[0015] Furthermore, the refractive index, size and spacing of the lenses in the lens group are adjusted according to the diameter of the optical fiber.

[0016] Furthermore, the transmittance of the aspheric negative lens and the lens of the lens group is ≥99.8%, and the damage resistance threshold is 30J / cm 2 It is also coated with a 1064nm damage-resistant anti-reflection film.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. Optimizing and improving the beam-shrink coupling lens and increasing the depth of field length can increase the debugging distance of the coupling end face, greatly simplifying the debugging process, and also helping to improve the coupling efficiency;

[0019] 2. The beam coupling lens does not directly contact the optical fiber, which is convenient for maintenance and replacement and reduces costs;

[0020] 3. Use an aspheric negative lens to flatten the Gaussian light, homogenize the intensity of the light spot distribution at the focus, and thus reduce the energy density of the optical coupling to avoid ionization.

[0021] The coupling optical path designed in this technical solution homogenizes Gaussian light and reduces energy density, which can effectively avoid the air ionization phenomenon caused by laser convergence during the beam reduction process. The effect of the depth of field length on the debugging process is taken into consideration when designing the coupling optical path. The depth of field length of the coupling optical path is extended, making the debugging position of the optical fiber in the coupling debugging process more diverse, and it is very convenient to find the highest coupling efficiency point. While reducing the difficulty of debugging, the coupling efficiency of high-power lasers is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. Obviously, the drawings in the following description are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0023] Figure 1 A schematic diagram of a lens structure for optimizing Gaussian light into flat-top light according to an embodiment of the present invention;

[0024] Figure 2 A schematic diagram of the imaging effect of optimizing Gaussian light into flat-top light according to an embodiment of the present invention;

[0025] Figure 3 A schematic diagram comparing the light intensity distribution of Gaussian light and flat top light according to an embodiment of the present invention;

[0026] Figure 4 3D schematic diagram of the light intensity distribution of Gaussian light and flat top light according to an embodiment of the present invention;

[0027] Figure 5Schematic diagram of the structure of a beam-shrink coupling lens according to an embodiment of the present invention.

[0028] The reference numerals in the figure are as follows: 1-aspherical plano-concave lens, 2-spherical bi-convex lens, 3-spherical bi-concave lens, 4-spherical plano-convex lens. DETAILED DESCRIPTION

[0029] The exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present invention and to be able to fully convey the scope of the present invention to those skilled in the art. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with the embodiments.

[0030] The present invention provides a method for coupling laser into optical fiber, which includes two parts, namely, optimization of Gaussian light to flat-top light and optical design method for increasing depth of field of flat-top light. The method for optimizing Gaussian light to flat-top light uses aspheric surface for optimization. Aspheric lens is usually divided into aspheric positive lens and aspheric negative lens. Aspheric positive lens converges Gaussian light and images it as evenly distributed flat-top light after focus. This structure forms a focus in the center of lens group, which ionizes laser and increases optical cavity length. Therefore, aspheric negative lens is used to optimize flat-top light; flat-top light increases depth of field by completing laser beam reduction through four-piece lens group. The functions of four lenses are: using two high-refractive index spherical positive lenses to reduce spherical aberration while reducing beam, and high-refractive index spherical negative lens and low-refractive index spherical positive lens are used to adjust depth of field while reducing spherical aberration of outgoing light.

[0031] like Figure 1 As shown, firstly, the Gaussian light intensity distribution function and the flat-top light intensity distribution function are clarified, and an aspheric negative lens is designed. The incident light is Gaussian light. The Gaussian light is optimized into a flat-top light through the ZPL macro instruction in the optical simulation software. The schematic diagram of the optimized flat-top light spot is shown in Figure 2 In order to make the description of light intensity distribution more intuitive, the light intensity distribution curve is called as shown in Figure 3 As shown, Figure 3 (a) and Figure 3 (b) are the intensity distribution curves of Gaussian light and flat-top light after optimization using aspheric surfaces. Compared with Gaussian light, the optical power density of flat-top light is more uniform in the entire spot distribution area. This design is conducive to uniform distribution of laser energy at different focal positions within the depth of field. 3D views of the intensity distribution of Gaussian light and flat-top light are shown in Figure 1. Figure 4 As shown, Figure 4 (a) is the 3D view of Gaussian light. Figure 4 (b) is the 3D view of the top light.

[0032] In order to improve the high-power laser coupling efficiency, the present invention designs a beam reduction coupling lens, such as Figure 5 As shown, the lens consists of four lenses, including two high-refractive-index spherical biconvex lenses 2 for beam reduction and reducing spherical aberration, a high-refractive-index spherical biconcave lens 3 and a low-refractive-index spherical plano-convex lens 4 for deepening the depth of field and optimizing spherical aberration. The four lenses are installed in a customized lens barrel according to the designed spacing to be fixed without shaking.

[0033] In order to compare the coupling efficiency of Gaussian light and optimized flat-top light, a comparison group of spherical negative lenses is added on the basis of this embodiment. The key process is to build the test table, assemble the coupling lens in the lens barrel, and process the spherical negative lens and the aspherical negative lens to be consistent in diameter for easy replacement. The distance from the spherical negative lens to the coupling lens is fine-tuned to find a suitable focusing point. After fixing the laser position, the spherical / aspherical negative lens is replaced to complete the comparison test. In order to ensure the consistency of the beam reduction lens, a spherical negative lens is used to expand the Gaussian light to the same spot diameter as the aspherical negative lens, and then the beam reduction coupling is performed.

[0034] The invention uses an optical system of a single lens plus a four-piece lens group to improve the high-power laser coupling efficiency, fixes the position of the laser, uses red light to pass through a beam reduction coupling lens, a spherical negative lens and a laser light outlet, makes the three coaxial, controls the laser light emission, passes through an attenuation sheet, and images the distribution of a Gaussian light spot on a CCD, tests the energy X1=98.72 mJ at a position 40 cm before the focus after removing the attenuation sheet, places the end face of the optical fiber at a position where the diameter of the focal spot is 2 mm, repeatedly adjusts the height and pitch of the lens and the optical fiber through red light, makes the center of the laser light outlet, the aspherical negative lens and the beam reduction coupling lens coincide with the center of the fiber core at the incident end of the optical fiber as much as possible, controls the laser light emission so that the laser is coupled into the optical fiber, tests the laser energy X2=84.63 mJ at a position 40 cm away from the output end of the optical fiber, and calculates the X2 / X1 ratio to obtain the laser coupling efficiency φ1=85.73%.

[0035] The spherical negative lens in the control group is replaced with an aspherical plano-concave lens 1, such as Figure 5As shown, red light is used to pass through the beam reduction coupling lens, the aspheric plano-concave lens 1 and the laser light outlet. After fine-tuning, the three are kept coaxial. An attenuation plate is added near the light outlet and then imaged onto the CCD to check whether the light spot is a uniform flat-top light distribution. After removing the attenuation plate, the energy X3=97.25mJ at a position 40cm before the focus is tested. The fiber input end face is placed at a position where the focal spot diameter is 2mm. The height and pitch of the lens and the fiber are repeatedly adjusted with red light to make the center of the laser light outlet, the aspheric plano-concave lens 1, and the beam reduction coupling lens coincide with the center of the fiber core at the incident end of the fiber as much as possible. The laser light is controlled to couple the laser into the fiber. The laser energy X4=70.18mJ is tested at a position 40cm away from the fiber output end. The X4 / X3 ratio is calculated to obtain a laser coupling efficiency φ2=72.16%. By comparing the coupling efficiency φ1>φ2, when using optical fibers of the same caliber, the use of an aspheric negative lens can couple in flat-top light with greater energy.

[0036] The test light in the embodiment and the control group is a general pulse laser with a spot diameter of 8mm, good collimation, a divergence angle of 2mrad, and a single pulse laser energy of 100mJ±3mJ; the lenses used are all coated with a 1064nm anti-damage anti-reflection film, with a transmittance of ≥99.8% and an anti-damage threshold of 30J / cm 2 Both the spherical negative lens and the aspherical negative lens are mounted using a lens frame, which is convenient for adjusting the position and pitch. The beam-shrink coupling lens is assembled using a customized lens barrel, which has higher stability and coaxiality.

[0037] In summary, the method of improving the coupling efficiency of high-power laser optical fiber of the present invention can reduce the ionization effect in the laser beam shrinking coupling process by adopting the technology of the present invention, and can evenly distribute the energy in the depth of field range by increasing the length of the depth of field, which not only improves the coupling efficiency of the laser, but also ensures the safety during the operation.

[0038] The key point of the present invention is to homogenize Gaussian light through the design of laser coupling optical path, and at the same time extend the depth of field length, so as to simplify the debugging process, improve safety, and reduce costs. The main method is to homogenize Gaussian laser into flat-top laser by optimizing the coupling optical path, reduce the energy density after laser convergence, and reduce the risk of device breakdown. At the same time, by extending the depth of field length through the coupling optical path, it is convenient for subsequent optical fiber position debugging, which can greatly improve the coupling efficiency.

[0039] In this technical solution, by optimizing the optical path and further extending the depth of field, the difficulty of debugging can be reduced; by using different aspherical lenses, the coupling optical path can be further optimized to improve the coupling efficiency. By replacing the large core diameter optical fiber, a higher energy laser can be coupled.

[0040] The present invention has been described in detail above through embodiments, but the contents described are only exemplary embodiments of the present invention and cannot be considered to limit the scope of implementation of the present invention. The protection scope of the present invention is defined by the claims. Anyone who utilizes the technical solution described in the present invention, or a technician in the field, inspired by the technical solution of the present invention, designs a similar technical solution within the essence and protection scope of the present invention to achieve the above technical effects, or makes equal changes and improvements to the scope of application, shall still belong to the scope of protection covered by the patent of the present invention. It should be noted that in order to make a clear statement, the description of some components and processes that have no direct and obvious connection with the scope of protection of the present invention but are known to those skilled in the art are omitted in the description of the present invention.

Claims

1. A high-power laser fiber coupling method, characterized in that: Includes the following: Firstly, the Gaussian light emitted by the laser is converted into flat-top light by an aspheric negative lens; The flat-top light is then beam-contracted and the depth of field of the light beam is extended through a lens group, and the end face of the optical fiber is adjusted to couple the distal laser beam into the optical fiber. The lens group comprises two high-refractive-index spherical positive lenses, a high-refractive-index spherical negative lens and a low-refractive-index spherical positive lens. The two high-refractive-index spherical positive lenses are placed adjacent to each other, and the high-refractive-index spherical negative lens is located between the high-refractive-index spherical positive lens and the low-refractive-index spherical positive lens. The high-refractive-index spherical positive lens is used for beam contraction and reduction of spherical aberration, and the high-refractive-index spherical negative lens and the low-refractive-index spherical positive lens are used for deepening the depth of field and optimizing spherical aberration.

2. A high-power laser fiber coupling method according to claim 1, characterized in that: The lens group is fixed in a beam reduction coupling lens.

3. A high-power laser fiber coupling method according to claim 1, characterized in that: The refractive index, size and spacing of the lenses in the lens group are adjusted according to the diameter of the optical fiber.

4. A high-power laser fiber coupling method according to claim 1, characterized in that: The transmittance of the aspheric negative lens and the lens of the lens group is ≥99.8%, and the damage resistance threshold is 30J / cm 2 It is also coated with a 1064nm damage-resistant anti-reflection film.

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