A continuously variable beam expander suitable for high power density ultraviolet lasers

Abstract

The invention discloses a continuously variable magnification beam expander suitable for high-power density ultraviolet laser, comprising a first lens, a second lens and a third lens arranged in sequence on an optical axis along the laser incident direction; wherein the first lens and the second lens are plano-concave lenses, and the concave surfaces are both facing the laser incident direction; the third lens is a plano-convex lens, and the convex surface is facing the laser emission direction. The beam expander in the present application is specially designed for an incident light spot with a diameter of less than 1 mm, and no additional beam expansion mechanism is required on the optical path of the ultraviolet laser after the triple frequency crystal inside the laser; the first lens and the second lens are both plano-concave lenses with a small curvature radius and the concave surfaces are all facing the laser incident direction, which can make the incident high-power density ultraviolet laser beam diverge quickly without any converging effect, thereby effectively protecting the second lens, and avoiding the incident light being vertically reflected by the front lens surface, causing return light interference to the laser.

Description

Technical Field

[0001] The invention relates to the field of laser technology, in particular to a continuously variable magnification beam expander suitable for high-power density ultraviolet lasers. Background Art

[0002] Lasers have been widely used in cutting, drilling, welding and other material processing fields due to their excellent monochromaticity, coherence, directionality and high brightness. Among them, ultraviolet lasers have been rapidly developed in the field of fine material processing in recent years by virtue of their advantages such as high single photon energy, strong material absorption, small thermal effect and small focusable spot size. Generally, the spot diameter directly output by the laser is small. In order to obtain a smaller spot size after focusing and thus obtain a higher laser power / density at the focus, the laser beam needs to be expanded by a beam expander before focusing, and the divergence angle of the laser beam is compressed at the same time. This is especially true for ultraviolet lasers used in material processing, because most industrial ultraviolet lasers are currently generated by fundamental frequency infrared lasers through nonlinear effects such as frequency doubling and sum frequency. In this process, only when the power density of the fundamental frequency laser beam is high enough can ultraviolet light be efficiently excited, and the power density of the generated ultraviolet beam is also high. Therefore, at the same output power level, the beam spot size directly generated by the ultraviolet laser is smaller than the beam spot size directly output by a laser with a longer wavelength. On the other hand, UV laser beam expanders are more difficult to make than long-wavelength laser beam expanders. This is mainly because most transparent materials have strong absorption of UV wavelengths. Currently, the most widely used and practical UV lens material is only fused quartz, but fused quartz is difficult to process. At present, the output power of UV lasers used in industry is getting higher and higher, which poses an increasing challenge to the production of supporting devices including laser beam expanders. In addition, if the magnification of the laser beam expander is adjustable, it can be used in different application scenarios, which will significantly increase the flexibility of using the same laser and laser processing equipment, and also facilitate the installation and debugging of laser processing equipment.

[0003] At present, although there are a considerable number of patents on the design of ultraviolet laser zoom beam expanders, they all have their own problems. For example, patents CN 101887173 B, CN 102004319 B, CN 206696536 U and CN 105527716B have the following common shortcomings: 1. The first lens along the laser incident direction uses a convex lens, which certainly allows the incident laser beam to obtain a higher divergence after convergence and thus achieve a larger beam expansion ratio, but for ultraviolet laser beams with small spots and high power density, this convergence effect will further enhance the power density of the beam, thereby increasing the risk of damage to the second lens; 2. The design uses a double convex, double concave or meniscus lens - as mentioned above, the processing of ultraviolet fused quartz material is difficult, and adding a curved surface will increase the production cost of the lens. In addition, the design incident spot diameters of patents CN 101887173 B, CN206696536 U and CN 105527716 B are all greater than 2mm. As mentioned above, the spot diameter of the ultraviolet laser beam with a nanosecond pulse width directly generated by the nonlinear crystal is generally no more than 1mm, which requires the laser to add a beam expansion mechanism on the optical path behind the nonlinear crystal, which is not in line with the trend of increasingly miniaturized laser design. Moreover, the plane of the first lens of patent CN 102004319 B faces the incident direction of the laser. This design is dangerous for high-power incident ultraviolet lasers. Even if the lens surface is coated with an anti-reflection film, a certain power of laser will be vertically reflected by the lens surface and return along the original path, which will interfere with the long-term stable operation of the laser. Again, although the design of using four lenses in patent CN 206696536 U can better correct aberrations, this will not only increase the cost of the beam expander and increase the difficulty of calibrating the optical axis of the beam expander, but more importantly, it will also increase the overall insertion loss. In particular, the power loss of high-power ultraviolet lasers after passing through too many lenses may be considerable.

[0004] Based on the above problems, a continuously variable beam expander suitable for high power density ultraviolet laser is now provided. Summary of the invention

[0005] The purpose of the present invention is to provide a continuously variable beam expander suitable for high power density ultraviolet lasers to solve the problems raised in the above background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A continuously variable beam expander suitable for high-power density ultraviolet lasers comprises a first lens, a second lens and a third lens arranged in sequence on an optical axis along the incident direction of the laser; wherein the first lens and the second lens are plano-concave lenses, and the concave surfaces are both facing the incident direction of the laser; and the third lens is a plano-convex lens, and the convex surface is facing the emitting direction of the laser.

[0008] As a further solution of the present invention: the incident light spot diameter of the beam expander is designed to be 0.5-1 mm, the laser wavelength is 355 nm, and it can withstand nanosecond ultraviolet laser with a maximum average power exceeding 20 W.

[0009] As a further solution of the present invention: the first lens includes two surfaces S1 and S2, wherein the S1 surface is a concave surface with a curvature radius R1=-19mm; the S2 surface is a plane with a curvature radius R2=∞; the center thickness d1 between the two surfaces on the optical axis is 2mm; the lens material is fused quartz, with a refractive index Nd1=1.46 and an Abbe number Vd1=68.

[0010] As a further solution of the present invention: the second lens includes two surfaces S3 and S4, wherein the S3 surface is a concave surface with a curvature radius R3=-29mm; the S4 surface is a plane with a curvature radius R4=∞; the center thickness d3 between the two surfaces on the optical axis is 2mm; the material used for the lens is fused quartz, with a refractive index Nd2=1.46 and an Abbe number Vd2=68.

[0011] As a further solution of the present invention: the third lens includes two surfaces S5 and S6, wherein the S5 surface is a plane with a curvature radius R5=∞; the S6 surface is a convex surface with a curvature radius R6=61mm; the center thickness d5 between the two surfaces on the optical axis is 3mm; the material used for the lens is fused quartz, with a refractive index Nd3=1.46 and an Abbe number Vd3=68.

[0012] As a further solution of the present invention: the distance between the first lens and the second lens on the optical axis is d2 = 11.7 ~ 125.8 mm; the distance between the second lens and the third lens on the optical axis is d4 = 79.8 ~ 96.4 mm; by adjusting the values ​​of d2 and d4, the beam expansion ratio of the beam expander can be continuously adjusted, and the beam expansion ratio adjustment range is 6 to 12 times.

[0013] As a further solution of the present invention: when d2=11.7mm and d4=96.4mm, the beam expansion ratio of the ultraviolet laser variable magnification beam expander of the present invention is 6 times;

[0014] When d2=30.7 mm and d4=91.7 mm, the beam expansion ratio of the ultraviolet laser variable-power beam expander of the present invention is 7 times;

[0015] When d2=49.7 mm and d4=88.2 mm, the beam expansion ratio of the ultraviolet laser variable-power beam expander of the present invention is 8 times;

[0016] When d2=68.7mm and d4=85.5mm, the beam expansion ratio of the ultraviolet laser variable magnification beam expander of the present invention is 9 times;

[0017] When d2=87.7 mm and d4=83.2 mm, the beam expansion ratio of the ultraviolet laser variable-power beam expander of the present invention is 10 times;

[0018] When d2=106.8mm and d4=81.4mm, the beam expansion ratio of the ultraviolet laser variable-power beam expander of the present invention is 11 times;

[0019] When d2=125.8 mm and d4=79.8 mm, the beam expansion ratio of the ultraviolet laser variable-power beam expander of the present invention is 12 times.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The beam expander is designed for incident spot diameters smaller than 1 mm, and no additional beam expansion mechanism is required on the optical path of the ultraviolet laser after the triple frequency crystal inside the laser.

[0022] The first lens and the second lens are both plano-concave lenses with a small radius of curvature and the concave surfaces are all facing the incident direction of the laser, which can make the incident high-power density ultraviolet laser beam diverge quickly without any converging effect, thereby effectively protecting the second lens and avoiding the incident light being vertically reflected by the front lens surface, causing return light interference to the laser;

[0023] The surface shapes of the lenses are all plane-spherical designs, which reduces the difficulty of lens processing.

[0024] Only three lenses are used, and the structure is simple and compact, which is convenient for correcting the optical axis inside the beam expander and adjusting the beam expansion ratio. More importantly, it reduces the overall insertion loss, so that the high-power ultraviolet laser loses less power when passing through the beam expander of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention is a schematic diagram of the optical structure of a continuously variable-magnification beam expander suitable for high-power density ultraviolet lasers.

[0026] 2(a) and (b) are the dispersion pattern and wavefront optical path difference distribution diagram of an incident ultraviolet laser beam with a spot diameter of 0.5 mm after being expanded 12 times according to an embodiment of the present invention.

[0027] 3(a) and (b) are the dispersion pattern and wavefront optical path difference distribution diagram of an incident ultraviolet laser beam with a spot diameter of 0.7 mm after being expanded 12 times according to an embodiment of the present invention.

[0028] 4(a) and (b) are the dispersion pattern and wavefront optical path difference distribution diagram of an incident ultraviolet laser beam with a spot diameter of 0.9 mm after being expanded 10 times according to an embodiment of the present invention.

[0029] 5(a) and (b) are the dispersion pattern and wavefront optical path difference distribution diagram of an incident ultraviolet laser beam with a spot diameter of 1.0 mm after being expanded 9 times by an embodiment of the present invention. DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] Example 1

[0032] In an embodiment of the present invention, in order to adapt to different laser processing application scenarios, the present invention provides a high-power density ultraviolet laser continuous variable magnification beam expander, the incident laser wavelength is 355nm, the incident spot size is 0.5-1mm, the beam expansion ratio is continuously adjustable between 6 and 12 times, and the maximum nanosecond ultraviolet laser power that can withstand is more than 20W.

[0033] Reference Figure 1 The ultraviolet laser continuous zoom beam expander of the present invention comprises a first lens L1, a second lens L2 and a third lens L3 which are coaxially arranged in sequence on the optical axis along the incident direction of the laser. Among them, the first lens L1 and the second lens L2 are plano-concave lenses, and the concave surfaces are both facing the incident direction of the laser; the third lens L3 is a plano-convex lens, and the convex surface is facing the laser emission direction. This design ensures that the high-power ultraviolet laser beam can be quickly diverged after passing through the first lens L1 in front, thereby reducing the power density, and there is no convergence effect at all, which can effectively protect the second lens L2 from laser damage. In addition, the concave surfaces of the first lens L1 and the second lens L2 are both facing the incident direction of the laser, thereby preventing the laser from being reflected vertically by the optical surface and returning to the laser along the original path, thereby interfering with the normal and stable operation of the laser.

[0034] The first lens L1 includes two optical surfaces S1 and S2 with curvature radii R1 and R2 respectively, and the center thickness between the two surfaces on the optical axis is d1; the material used for the lens is fused silica, and its refractive index is Nd1, and its Abbe number is Vd1. The second lens L2 includes two optical surfaces S3 and S4 with curvature radii R3 and R4 respectively, and the center thickness between the two surfaces on the optical axis is d3; the material used for the lens is fused silica, and its refractive index is Nd2, and its Abbe number is Vd2. The third lens L3 includes two optical surfaces S5 and S6 with curvature radii R5 and R6 respectively, and the center thickness between the two surfaces on the optical axis is d5; the material used for the lens is fused silica, and its refractive index is Nd3, and its Abbe number is Vd3.

[0035] The distance between the first lens L1 and the second lens L2 on the optical axis is d2, and the distance between the second lens L2 and the third lens L3 on the optical axis is d4.

[0036] The specific data parameters of the ultraviolet laser variable magnification beam expander of this embodiment are shown in Table 1, wherein the incident laser wavelength is 355 nm and the incident spot size is 0.5-1 mm.

[0037] Table 1 Specific parameters of UV laser zoom beam expander

[0038]

[0039] In Table 1, a curvature radius of ∞ represents that the surface is a plane; a curvature radius of negative value represents that the surface is a concave spherical surface; and a curvature radius of positive value represents that the surface is a convex spherical surface.

[0040] By changing the values ​​of d2 and d4 respectively, the beam expansion ratio of the laser beam expander can be continuously adjusted, and the corresponding relationship is shown in Table 2 for example.

[0041] Table 2 The relationship between d2 and d4 corresponding to the beam expansion ratio produced by the beam expander

[0042] d2(mm) d4(mm) Beam expansion ratio 11.7 96.4 6 30.7 91.7 7 49.7 88.2 8 68.7 85.5 9 87.7 83.2 10 106.8 81.4 11 125.8 79.8 12

[0043] It can be seen from Table 2 that increasing the d2 value while decreasing the d4 value will gradually increase the beam expansion ratio, and vice versa. Among them, the range of d2 is 11.7 to 125.8 mm, the range of d4 is 79.8 to 96.4 mm, and the range of beam expansion ratio is 6 to 12 times.

[0044] It should be noted that not all incident laser spot diameters within the range of 0.5 to 1 mm can achieve a high-quality beam expansion effect of 6 to 12 times. Due to the influence of aberrations, the larger the incident spot diameter, the more difficult it is to obtain a larger high-quality beam expansion ratio. The standard for defining "high-quality" beam expansion here is that at an incident angle of 0°, the radius of the diffuse spot of the expanded laser beam after being focused by the ideal lens does not exceed the radius of the diffraction-limited spot (Airy disk), and the peak-to-valley (PV) value of the wavefront optical path difference is less than 0.2 times the laser wavelength. Under this standard, when the incident spot diameter is 0.5-0.7 mm, the maximum multiple of high-quality beam expansion is 12 times, where the spot diffusion diagram and wavefront optical path difference distribution diagram of the 0.5 mm diameter incident beam after 12 times expansion are shown in Figure 2 (a) and (b), respectively; the spot diffusion diagram and wavefront optical path difference distribution diagram of the 0.7 mm diameter incident beam after 12 times expansion are shown in Figure 3 (a) and (b), respectively; when the incident spot diameter is 0.7-0.9 When the incident spot diameter is 0.9-1.0 mm, the maximum multiple of high-quality beam expansion is 10 times, among which the spot diffusion diagram and wavefront optical path difference distribution diagram of the 0.9 mm diameter incident beam after 10 times beam expansion are shown in Figures 4(a) and (b), respectively; when the incident spot diameter is 0.9-1.0 mm, the maximum multiple of high-quality beam expansion is 9 times, among which the spot diffusion diagram and wavefront optical path difference distribution diagram of the 0.9 mm diameter incident beam after 9 times beam expansion are shown in Figures 5(a) and (b), respectively.

[0045] It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

Claims

1. A continuously variable beam expander suitable for high power density ultraviolet laser, characterized in that: It is composed of a first lens, a second lens and a third lens which are sequentially arranged on the optical axis along the incident direction of the laser; wherein the first lens and the second lens are plano-concave lenses, and the concave surfaces are both facing the incident direction of the laser; the third lens is a plano-convex lens, and the convex surface is facing the emitting direction of the laser; The incident light spot diameter of the beam expander is designed to be 0.5 ~ 1 mm, and the laser wavelength is 355 nm; The first lens includes two surfaces, S1 and S2, wherein the S1 surface is a concave surface with a curvature radius of R1 = -19 mm; the S2 surface is a plane with a curvature radius of R2 = ∞; the center thickness d1 between the two surfaces on the optical axis is 2 mm; The second lens includes two surfaces, S3 and S4, wherein the S3 surface is a concave surface with a curvature radius of R3 = -29 mm; the S4 surface is a plane surface with a curvature radius of R4 = ∞; the center thickness d3 between the two surfaces on the optical axis is 2 mm; The third lens includes two surfaces, S5 and S6, wherein the S5 surface is a plane with a curvature radius of R5 = ∞; the S6 surface is a convex surface with a curvature radius of R6 = 61 mm; the center thickness d5 between the two surfaces on the optical axis is 3 mm; The materials used for the first lens, the second lens and the third lens are fused quartz, with a refractive index of Nd3 = 1.46 and an Abbe number of Vd3 = 68; The distance d2 between the first lens and the second lens on the optical axis is 11.7 ~ 125.8 mm; the distance d4 between the second lens and the third lens on the optical axis is 79.8 ~ 96.4 mm; by adjusting the values ​​of d2 and d4, the beam expansion ratio of the beam expander can be continuously adjusted, and the adjustment range of the beam expansion ratio is 6 to 12 times.

2. The continuously variable beam expander suitable for high power density ultraviolet laser according to claim 1, characterized in that: When d2 = 11.7 mm and d4 = 96.4 mm, the beam expansion ratio of the ultraviolet laser variable magnification beam expander is 6 times; When d2 = 30.7 mm and d4 = 91.7 mm, the beam expansion ratio of the ultraviolet laser variable magnification beam expander is 7 times; When d2 = 49.7 mm and d4 = 88.2 mm, the beam expansion ratio of the ultraviolet laser variable magnification beam expander is 8 times; When d2 = 68.7 mm and d4 = 85.5 mm, the beam expansion ratio of the ultraviolet laser variable magnification beam expander is 9 times; When d2 = 87.7 mm and d4 = 83.2 mm, the beam expansion ratio of the ultraviolet laser variable magnification beam expander is 10 times; When d2 = 106.8 mm and d4 = 81.4 mm, the beam expansion ratio of the ultraviolet laser variable magnification beam expander is 11 times; When d2 = 125.8 mm and d4 = 79.8 mm, the beam expansion ratio of the ultraviolet laser variable magnification beam expander is 12 times.

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