A high magnification diffraction-limited laser beam expander

Abstract

The present invention provides a high-magnification diffraction-limited laser beam expander, which includes a first beam expansion lens group and a second beam expansion lens group. The first beam expansion lens group and the second beam expansion lens group are arranged at intervals and are located on the same optical axis. The first beam expansion lens group includes a first lens with negative refractive power, and the focal length F1 of the first lens satisfies: 5.5 mm ≤ |F1| ≤ 6 mm. The second beam expansion lens group includes a second lens and a third lens both with positive refractive power. The second lens and the third lens are adjacent and arranged at intervals. The focal length F4 of the second beam expansion lens group satisfies: 60 mm ≤ F4 ≤ 66 mm. The ratio of the focal length of the second beam expansion lens group to the focal length of the first beam expansion lens group satisfies: 10 < |F4 / F1| < 12. This beam expander is for high-magnification beam expansion, well compensates for aberration, can also achieve a good beam expansion effect on a beam with a certain inclination angle, and can reach the diffraction limit after focusing after expanding ultraviolet light or green light by adjusting the lens spacing.

Description

Technical Field

[0001] The present invention relates to the field of laser technology, and in particular, to a high-magnification diffraction-limited laser beam expander. Background Art

[0002] Laser beam expansion systems mainly have two structures: Keplerian and Galilean. The Galilean type has no real focus and a short overall length, so the Galilean type is mostly used in high-power laser systems; the laser beam expander has at least two lenses, and can also have three or more lenses, which can better correct aberration.

[0003] A dual-wavelength high-magnification continuously variable laser beam expander with the publication number of CN113204122A includes a first lens, a second lens, a third lens, and a fourth lens arranged coaxially in sequence along the optical axis in the laser incident direction; the first lens and the second lens are plano-concave negative lenses, and the concave surfaces face the laser incident direction; the third lens is a meniscus positive lens, and the fourth lens is a double-convex positive lens, and the adjustable range of the beam expansion ratio is 20-40 times.

[0004] The optical structure of the laser beam expander in the above solution is relatively complex and has a poor spherical aberration correction effect on the beam expander, and cannot perform good beam expansion on a beam with a certain inclination angle, and cannot focus the ultraviolet light and green light after beam expansion to reach the diffraction limit, thus reducing the performance of the laser beam expansion. Summary of the Invention

[0005] In view of this, the present invention proposes a high-magnification diffraction-limited laser beam expander, which is aimed at high-magnification beam expansion and has a different configuration, well compensates for aberration, can also achieve good beam expansion effect on a beam with a certain inclination angle, and can reach the diffraction limit by adjusting the lens spacing for focusing after expanding the ultraviolet light and green light, improving the performance of the laser beam expansion.

[0006] The technical solution of the present invention is realized as follows: The present invention provides a high-magnification diffraction-limited laser beam expander, including a first beam expansion lens group and a second beam expansion lens group, wherein,

[0007] The first beam expansion lens group and the second beam expansion lens group are arranged at intervals, and the first beam expansion lens group and the second beam expansion lens group are located on the same optical axis;

[0008] The first beam expansion lens group includes a first lens with negative refractive power, and the focal length F1 of the first lens satisfies: 5.5mm ≤ |F1| ≤ 6mm;

[0009] The second beam expansion lens group includes a second lens and a third lens both having positive refractive power, and the second lens and the third lens are adjacent and arranged at intervals;

[0010] The focal length F4 of the second beam expander lens group satisfies: 60 mm ≤ F4 ≤ 66 mm;

[0011] The ratio of the focal length of the second beam expander lens group to the focal length of the first beam expander lens group satisfies 10 < |F4 / F1| < 12.

[0012] Based on the above technical solutions, preferably, the first lens is a double concave negative lens, and both the light incident side and the light exiting side of the first lens are concave arcs facing the optical axis side, and the curvature radius of the light incident side of the first lens is not equal to the curvature radius of the light exiting side.

[0013] Based on the above technical solutions, preferably, the second lens is a meniscus positive lens, and the shapes of both the light incident side and the light exiting side of the second lens are convex arcs facing away from the first lens.

[0014] Based on the above technical solutions, preferably, the third lens is a double convex positive lens, and the shapes of both the light incident side and the light exiting side of the second lens are convex arcs facing away from its axis.

[0015] Based on the above technical solutions, preferably, the high magnification diffraction-limited laser beam expander satisfies the following conditions: 115 mm ≤ F2 < F3 ≤ 135 mm;

[0016] wherein, F2 is the focal length of the second lens 21, and F3 is the focal length of the third lens 22;

[0017] The ratio of the focal length of the third lens to the focal length of the second lens satisfies: 1 < F3 / F2 < 1.1;

[0018] The ratio of the focal length of the third lens to the focal length of the first lens satisfies: 18 < |F3 / F1| < 25.

[0019] Based on the above technical solutions, preferably, the high magnification diffraction-limited laser beam expander satisfies the following conditions:

[0020] 1.5|R2| < |R1| < 2|R2|;

[0021] 2.5|R4| < |R3| < 3.5|R4|;

[0022] |R5| > 2|R6|;

[0023] wherein, R1 is the curvature radius of the light incident side S1 of the first lens, R2 is the curvature radius of the light exiting side S2 of the first lens; R3 is the curvature radius of the light incident side S3 of the second lens, R4 is the curvature radius of the light exiting side S4 of the second lens, R5 is the curvature radius of the light incident side S5 of the third lens, and R6 is the curvature radius of the light exiting side S6 of the third lens.

[0024] On the basis of the above technical solutions, preferably, the length L of the entire high-magnification diffraction-limited laser beam expander satisfies: 55 mm < L < 70 mm, the central thickness d1 of the first lens is 2 mm, the central thickness d3 of the second lens is 4.4 mm, the distance d4 between the axes of the light-emitting side surface of the second lens and the light-incident side surface of the third lens is 1 mm, the central thickness d5 of the third lens is 4 mm, and the distance d2 between the axes of the light-emitting side surface of the first lens and the light-incident side surface of the second lens satisfies: 43.6 mm < d2 < 58.6 mm.

[0025] On the basis of the above technical solutions, preferably, the distance between the axes of the light-emitting side surface of the first lens and the light-incident side surface of the second lens is adjustable, so as to change the divergence angle of the laser passing through the beam expander;

[0026] Let the divergence angle of the incident light beam be θ1, the spot size be ω1, the divergence angle of the light beam emitted after passing through the beam expander be θ2, and the spot size be ω2;

[0027] After adjusting d2 to reach the corresponding distance, the following conditions are satisfied:

[0028] ω2 / ω1 = |F4 / F1|;

[0029] θ1 / θ2 ≥ |F4 / F1|.

[0030] On the basis of the above technical solutions, preferably, adjusting the distance between the axes of the light-emitting side surface of the first lens and the light-incident side surface of the second lens includes the following steps:

[0031] S1. Install the first beam expander lens group and the second beam expander lens group so that the first beam expander lens group and the second beam expander lens group are on the same optical axis, and use a laser measurement device to test the divergence angle and spot shape of the output laser;

[0032] S2. Adjust the distance between the axes of the light-emitting side surface of the first lens and the light-incident side surface of the second lens according to the divergence angle and spot shape of the output laser;

[0033] If the divergence angle of the output laser is less than the preset far-field divergence angle and the spot state is normal, the adjustment can be directly ended;

[0034] If the divergence angle of the output laser is greater than the preset far-field divergence angle, then gradually increase the distance between the axes of the light-emitting side surface of the first lens and the light-incident side surface of the second lens at a certain step length. After each increase in the distance, retest the divergence angle of the output laser. If the divergence angle of the output laser decreases to within the preset value, end the adjustment. If the divergence angle of the output laser increases, decrease the just-increased distance, and then enter step S3;

[0035] S3. If the divergence angle increases after increasing the distance, then decrease the distance between the optical axis of the light-emitting side of the first lens and the optical axis of the light-incident side of the second lens in a certain step, and retest the divergence angle of the output laser. If the divergence angle of the output laser decreases within the preset value, then end the adjustment. If the divergence angle has not decreased to the preset value, then continue to decrease the distance between the optical axis of the light-emitting side of the first lens and the optical axis of the light-incident side of the second lens until the divergence angle of the output laser decreases within the preset value.

[0036] Based on the above technical solutions, preferably, the materials of the first lens, the second lens, and the third lens are all fused silica glass, with a refractive index Nd = 1.46 and an Abbe number V = 68.

[0037] The high-magnification diffraction-limited laser beam expander of the present invention has the following beneficial effects compared with the prior art:

[0038] (1) Through this beam expander, high-magnification laser beam expansion can be achieved, increasing the beam diameter of the incident laser, effectively compressing the divergence angle of the beam, thereby greatly increasing the collimation of the beam, that is, effectively compressing the divergence angle of the beam, which helps to form a smaller spot on the rear focal plane of the focusing lens, thereby improving the power density and processing accuracy of the laser. And this beam expander works under the diffraction limit, that is, the divergence angle of the beam is compressed to the maximum extent, and at the same time, the introduced aberration is extremely small, and the beam quality hardly deteriorates, which helps to ensure the stability and consistency of the laser beam during transmission and focusing.

[0039] (2) By adjusting the distance d2 between the optical axis of the light-emitting side of the first lens and the optical axis of the light-incident side of the second lens, the divergence angle of the beam expander for the incident beam can be changed, which can meet the requirements of laser beam expansion in a relatively wide wavelength band, can be used for ultraviolet or visible light beam expansion, providing additional flexibility and adaptability for the laser beam expander to meet the needs of different application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0041] Figure 1 It is a schematic optical path diagram of the high-magnification diffraction-limited laser beam expander of the present invention;

[0042] Figure 2 It is a schematic structural diagram of the high-magnification diffraction-limited laser beam expander of the present invention;

[0043] Figure 3Optical spot diagram of 0-degree parallel light incident on the high-magnification diffraction-limited laser beam expander of the present invention;

[0044] Figure 4 Optical spot diagram of 0.34-degree incident light on the high-magnification diffraction-limited laser beam expander of the present invention;

[0045] Figure 5 MTF curve graph of the high-magnification diffraction-limited laser beam expander of the present invention;

[0046] Figure 6 Wavefront aberration diagram of 0-degree incident light on the high-magnification diffraction-limited laser beam expander of the present invention;

[0047] Figure 7 Wavefront aberration diagram of 0.34-degree incident light on the high-magnification diffraction-limited laser beam expander of the present invention. Detailed implementation manners

[0048] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0049] As Figure 1 shown, a high-magnification diffraction-limited laser beam expander of the present invention includes a first beam-expanding lens group 1 and a second beam-expanding lens group 2. Among them, the first beam-expanding lens group 1 and the second beam-expanding lens group 2 are arranged at intervals, and the first beam-expanding lens group 1 and the second beam-expanding lens group 2 are located on the same optical axis; the first beam-expanding lens group 1 includes a first lens 11 with negative refractive power, and the focal length F1 of the first lens satisfies: 5.5 mm ≤ |F1| ≤ 6 mm; the second beam-expanding lens group 2 includes a second lens 21 and a third lens 22 both with positive refractive power, and the second lens 21 and the third lens 22 are adjacent and arranged at intervals; the focal length F4 of the second beam-expanding lens group 2 satisfies: 60 mm ≤ F4 ≤ 66 mm; the ratio of the focal length of the second beam-expanding lens group 2 to the focal length of the first beam-expanding lens group 1 satisfies: 10 < |F4 / F1| < 12.

[0050] It should be noted that the high-magnification diffraction-limited laser beam expander mainly consists of two lens groups: the first beam-expanding lens group 1 and the second beam-expanding lens group 2. The two lens groups are located on the same optical axis and are spaced apart. The first beam-expanding lens group 1 is a concave lens used to diverge the incident laser beam, thereby forming a virtual focus. The second beam-expanding lens group 2 is a convex lens group used to converge the beam diverged by the first lens. Its object-side 2 focus coincides with the image-side focus of lens group 1. Lens group 2 contains two lenses that respectively bear the optical power, reducing the degree of light bending, thereby reducing the introduced spherical aberration, which is beneficial to improving the beam-expanding performance. The proportional relationship ensures the high-magnification performance of the beam expander, and the curvature design ensures that the focusing after beam expansion can reach the diffraction limit.

[0051] This beam expander can achieve high-magnification laser beam expansion, increase the diameter of the incident laser beam, and maintain the collimation of the beam at the same time. Due to the longer focal length of the second beam-expanding lens group, it can effectively compress the divergence angle of the beam, helping to form a smaller spot on the focusing lens, thereby improving the power density and processing accuracy of the laser. And this beam expander works under the diffraction limit, that is, the divergence angle of the beam is compressed to the maximum extent while keeping the waveform and energy distribution of the beam unchanged, which helps to ensure the stability and consistency of the laser beam during transmission and focusing.

[0052] The first lens 11 in this embodiment is a double-concave negative lens. The light-incident side and the light-emitting side of the first lens 11 are both concave arcs facing the optical axis center side, and the curvature radius of the light-incident side of the first lens 11 is not equal to the curvature radius of the light-emitting side.

[0053] It should be noted that the double-concave negative lens design of the first lens 11 and the characteristic of unequal curvature radii together constitute an accurate control mechanism for the incident laser beam, which helps to achieve low spherical aberration under high-magnification laser beam expansion, enabling the focusing to reach the diffraction limit, increasing the tolerance, and improving the optical performance and stability of the system; by further optimizing the parameters and shape of the lens, the performance and application range of the beam expander can be further improved.

[0054] The second lens 21 in this embodiment is a meniscus positive lens. The shapes of the light-incident side and the light-emitting side of the second lens 21 are both convex arcs facing away from the first lens 11.

[0055] The third lens 22 in this embodiment is a double-convex positive lens. The shapes of the light-incident side and the light-emitting side of the second lens 21 are both convex arcs facing away from its axis center.

[0056] In this embodiment, by combining the diverging effect of the first lens 11, the converging effect of the second lens 21, and the further enhanced converging effect of the third lens 22, the entire beam expander system can achieve a high-magnification laser beam expansion effect, which not only improves the imaging quality and stability of the system, but also makes the finally output beam have a higher power density and a smaller focused spot.

[0057] As Figure 2 shown, the high-magnification diffraction-limited laser beam expander in this embodiment satisfies the following conditions: 115 mm ≤ F2 < F3 ≤ 135 mm;

[0058] wherein, F2 is the focal length of the second lens 21, and F3 is the focal length of the third lens 22;

[0059] It should be noted that the focal lengths of the second lens 21 and the third lens 22 are both relatively long, and the focal length of the third lens 22 is slightly larger than that of the second lens 21; this helps to further compress the divergence angle of the beam while maintaining the beam collimation.

[0060] The ratio of the focal length of the third lens 22 to that of the second lens 21 satisfies: 1 < F3 / F2 < 1.1;

[0061] It should be noted that this ratio indicates that the focal lengths of the third lens 22 and the second lens 21 are very close, but the focal length of the third lens 22 is still slightly larger. This small difference helps to fine-tune the focusing performance of the beam while maintaining the stability of the system.

[0062] The ratio of the focal length of the third lens 22 to that of the first lens 11 satisfies: 18 < |F3 / F1| < 25.

[0063] It should be noted that this ratio indicates that the focal length of the third lens 22 is much larger than that of the first lens 11; this helps to achieve a high-magnification laser beam expansion effect because a lens with a long focal length can more effectively compress the divergence angle of the beam.

[0064] The high-magnification diffraction-limited laser beam expander in this embodiment satisfies the following conditions:

[0065] 1.5|R2| < |R1| < 2|R2|;

[0066] 2.5|R4| < |R3| < 3.5|R4|;

[0067] |R5| > 2|R6|;

[0068] Wherein, R1 is the radius of curvature of the light incident side S1 of the first lens 11, R2 is the radius of curvature of the light exiting side S2 of the first lens 11; R3 is the radius of curvature of the light incident side S3 of the second lens 21, R4 is the radius of curvature of the light exiting side S4 of the second lens 21, R5 is the radius of curvature of the light incident side S5 of the third lens 22, and R6 is the radius of curvature of the light exiting side S6 of the third lens 22.

[0069] It should be noted that in this embodiment, the relationship between the radius of curvature of the lenses of the high magnification diffraction-limited laser beam expander is clearly defined. Among them, according to the radius of curvature relationship of the first lens 11, since the first lens 11 is a negative lens, its light incident side is usually more concave towards the optical axis, and the light exiting side is relatively flat or slightly concave inward, which helps to achieve the divergence effect on the incident laser beam; according to the radius of curvature relationship of the second lens 21, it shows that the light incident side of the third lens 22 is more convex towards the optical axis than the light incident side of the second lens 21, which helps to further converge the light rays diverged by the first lens 11; through the radius of curvature relationship of the first lens 11, the second lens 21 and the third lens 22 and their relative position relationship, the high magnification diffraction-limited laser beam expander realizes the precise control and beam expansion effect on the incident laser beam; not only improves the imaging quality and stability of the system, but also makes the finally output beam have a higher power density and a smaller focused spot.

[0070] In this embodiment, the length L of the entire high magnification diffraction-limited laser beam expander satisfies: 55mm < L < 70mm, the central thickness d1 of the first lens 11 is 2mm, the central thickness d3 of the second lens 21 is 4.4mm, the distance d4 between the optical axes of the light exiting side of the second lens 21 and the light incident side of the third lens 22 is 1mm, the central thickness d5 of the third lens 22 is 4mm, and the distance d2 between the optical axes of the light exiting side of the first lens 11 and the light incident side of the second lens 21 satisfies: 43.6mm < d2 < 58.6mm.

[0071] In this embodiment, the distance between the optical axes of the light exiting side of the first lens 11 and the light incident side of the second lens 21 is adjustable, which changes the divergence angle of the laser beam passing through the beam expander;

[0072] Let the divergence angle of the incident beam be θ1, the spot size be ω1, the divergence angle of the output beam after passing through the beam expander be θ2, and the spot size be ω2;

[0073] After adjusting d2 to reach the corresponding distance, the following conditions are satisfied:

[0074] ω2 / ω1 = |F4 / F1|;

[0075] θ1 / θ2 ≥ |F4 / F1|.

[0076] It should be noted that by adjusting the distance d2 between the optical axis of the light-emitting side of the first lens 11 and the light-incident side of the second lens 21, the adjustment effect of the beam expander on the divergence angle and spot size of the incident light beam can be changed, providing additional flexibility and adaptability for the laser beam expander to meet the requirements of different application scenarios. For example, in application scenarios where a larger spot and a smaller divergence angle are required, the value of d2 can be appropriately increased; while in application scenarios where a smaller spot and a larger divergence angle are required, the value of d2 can be appropriately decreased.

[0077] Regarding the wavelength adaptability in this embodiment, since the design does not eliminate chromatic aberration, in principle, it is only applicable to narrow-band laser beam expansion and not applicable to wide-band laser beam expansion. The actual applicability is affected by the coating wavelength of the lens. For example, if a single-wavelength antireflection coating such as a 355nm antireflection coating is applied, it is applicable to 355nm beam expansion; if a 532nm antireflection coating is applied, it is applicable to 532nm beam expansion. If a wide-band laser film is coated, it is applicable to narrow-band lasers with wavelengths within the film band and not applicable to wide-band laser beam expansion, and different wavelengths can be adapted by adjusting d2.

[0078] In this embodiment, the distance between the optical axis of the light-emitting side of the first lens 11 and the light-incident side of the second lens 21 is adjusted, including the following steps:

[0079] S1, Install the first beam-expanding lens group 1 and the second beam-expanding lens group 2 so that the first beam-expanding lens group 1 and the second beam-expanding lens group 2 are on the same optical axis, and use a laser measurement device to measure the divergence angle of the output laser.

[0080] S2, Adjust the distance between the optical axis of the light-emitting side of the first lens 11 and the light-incident side of the second lens 21 according to the divergence angle of the output laser.

[0081] If the divergence angle of the output laser is less than the preset far-field divergence angle, the adjustment can be directly ended.

[0082] If the divergence angle of the output laser is greater than the preset far-field divergence angle, the distance between the optical axis of the light-emitting side of the first lens 11 and the light-incident side of the second lens 21 is gradually increased in a certain step. After each increase in the distance, the divergence angle of the output laser is measured again. If the divergence angle of the output laser decreases to within the preset value, the adjustment ends. If the divergence angle of the output laser increases, the just-increased distance is decreased, and then enter step S3.

[0083] S3. If the divergence angle increases after increasing the distance, then decrease the distance between the optical axis of the light-emitting side of the first lens 11 and the light-incident side of the second lens 21 in a certain step, and retest the divergence angle of the output laser. If the divergence angle of the output laser decreases within the preset value, the adjustment ends. If the divergence angle has not decreased to the preset value, continue to decrease the distance between the optical axis of the light-emitting side of the first lens 11 and the light-incident side of the second lens 21 until the divergence angle of the output laser decreases within the preset value.

[0084] It should be noted that, first, the first beam expander lens group 1 and the second beam expander lens group 2 are installed on the same optical axis to ensure that the light beam can pass through smoothly and be affected by the lens group together. Use a laser measurement device to test the divergence angle and spot shape of the output laser in the initial state, providing reference data for subsequent adjustment. According to the initial test data, change the divergence angle and spot shape of the laser beam by gradually adjusting the value of d2.

[0085] In this embodiment, the materials of the first lens 11, the second lens 21, and the third lens 22 are all fused silica glass, with a refractive index Nd = 1.46 and an Abbe number V = 68.

[0086] It should be noted that due to the low dispersion and high light transmittance of fused silica glass, it can ensure that the laser beam maintains high quality when passing through the lens. This helps to reduce beam distortion and scattering, and improve the collimation and focusing performance of the beam.

[0087] As Figure 2 shown, the parameters of each lens of a high-magnification diffraction-limited laser beam expander in this embodiment are F1 = -5.5 mm, F2 = 128 mm, F3 = 131 mm, F4 = 65.2 mm, d1 = 2 mm, d2 = 53.9 mm, d3 = 4.4 mm, d4 = 1 mm, d5 = 4 mm, the total optical length L = 65.3 mm, the curvature radii of S1 to S6 are R1 = -8.1 mm, R2 = 4.2 mm, R3 = -110.9 mm, R4 = -39.9 mm, R5 = 454.8 mm, R6 = -72 mm, the glass refractive index Nd = 1.46, the Abbe number V = 68, the designed incident spot is within 1 mm, the divergence angle is within 6 mrad, and the beam expansion multiple is 12 times.

[0088] As Figure 3 and Figure 4As shown in the figure, the imaging quality of the optical system is demonstrated by the spot diagram. The spot diagram shows the positions of the object and the image, as well as the corresponding angular deviations, at different incident angles. The results of the spot diagram indicate that it can be seen that the spot diagram of the 0-field of view is much smaller than the Airy disk diameter (i.e., the black circle represents the diffraction diameter and the blue dots represent the geometric trace points), indicating that the 0-field of view can reach the diffraction limit after focusing. The geometric trace points of the 0.34-degree field of view (corresponding to the full divergence angle of 6 mrad) are also within the Airy disk diameter, indicating that the incident light can also reach the diffraction limit with a slight tilt.

[0089] As Figure 5 shown, the MTF curve of the 0-field of view in the MTF coincides with the diffraction limit MTF curve, and the 0.34-field of view also almost coincides, indicating that perfect focusing can be achieved at a small field of view.

[0090] As Figure 6 and Figure 7 shown, it is generally considered that the wavefront aberration within 1 / 4 wavelength is acceptable. In this design, the wavefront aberration is extremely small at the 0-field of view. As the light beam tilts, the wavefront aberration will deteriorate, but the wavefront aberration is still less than 1 / 4 wavelength at a beam divergence angle of 6 mrad, meeting the design requirements.

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A high magnification diffraction-limited laser beam expander, characterized in that, It includes a first beam expander lens group (1) and a second beam expander lens group (2), where the first beam expander lens group (1) and the second beam expander lens group (2) are arranged at intervals, and the first beam expander lens group (1) and the second beam expander lens group (2) are located on the same optical axis; the first beam expander lens group (1) includes a first lens (11) with negative refractive power, and the focal length F1 of the first lens (11) satisfies: 5.5mm ≤ |F1| ≤ 6mm; the second beam expander lens group (2) includes a second lens (21) and a third lens (22) both with positive refractive power, and the second lens (21) and the third lens (22) are adjacent and arranged at intervals; the focal length F4 of the second beam expander lens group (2) satisfies: 60mm ≤ F4 ≤ 66mm; the ratio of the focal length of the second beam expander lens group (2) to the focal length of the first beam expander lens group (1) satisfies: 10 < |F4 / F1| < 12.

2. The high-magnification diffraction-limited laser beam expander according to claim 1, wherein: The first lens (11) is a biconcave negative lens, and both the incident light side and the outgoing light side of the first lens (11) are concave arcs facing the optical axis side, and the curvature radius of the incident light side of the first lens (11) is not equal to the curvature radius of the outgoing light side.

3. The high-magnification diffraction-limited laser beam expander according to claim 2, characterized in that: The second lens (21) is a meniscus positive lens, and the shapes of both the incident light side and the outgoing light side of the second lens (21) are convex arcs facing away from the first lens (11).

4. The high-magnification diffraction-limited laser beam expander according to claim 3, wherein: The third lens (22) is a biconvex positive lens, and the shapes of both the incident light side and the outgoing light side of the second lens (21) are convex arcs facing away from its axis.

5. The high-magnification diffraction-limited laser beam expander according to claim 4, wherein: The high-magnification diffraction-limited laser beam expander satisfies the following conditions: 115mm ≤ F2 < F3 ≤ 135mm; where, F2 is the focal length of the second lens (21), and F3 is the focal length of the third lens (22); the ratio of the focal length of the third lens (22) to the focal length of the second lens (21) satisfies: 1 < F3 / F2 < 1.1; the ratio of the focal length of the third lens (22) to the focal length of the first lens (11) satisfies: 18 < |F3 / F1| < 25.

6. The high-magnification diffraction-limited laser beam expander according to claim 5, wherein: The high-magnification diffraction-limited laser beam expander satisfies the following conditions: 1.5|R2| < |R1| < 2|R2|; 2.5|R4| < |R3| < 3.5|R4|; |R5| > 2|R6|; where, R1 is the curvature radius of the incident light side S1 of the first lens (11), R2 is the curvature radius of the outgoing light side S2 of the first lens (11); R3 is the curvature radius of the incident light side S3 of the second lens (21), R4 is the curvature radius of the outgoing light side S4 of the second lens (21), R5 is the curvature radius of the incident light side S5 of the third lens (22), and R6 is the curvature radius of the outgoing light side S6 of the third lens (22).

7. The high-magnification diffraction-limited laser beam expander according to claim 6, characterized in that: The length L of the entire high-magnification diffraction-limited laser beam expander satisfies: 55 mm < L < 70 mm. The central thickness d1 of the first lens (11) is 2 mm, the central thickness d3 of the second lens (21) is 4.4 mm, the distance d4 between the optical axes of the light-emitting side of the second lens (21) and the light-incident side of the third lens (22) is 1 mm, the central thickness d5 of the third lens (22) is 4 mm, and the distance d2 between the optical axes of the light-emitting side of the first lens (11) and the light-incident side of the second lens (21) satisfies: 43.6 mm < d2 < 58.6 mm.

8. The high-magnification diffraction-limited laser beam expander according to claim 7, wherein: The distance between the optical axes of the light-emitting side of the first lens (11) and the light-incident side of the second lens (21) is adjustable, which changes the divergence angle of the laser beam passing through the beam expander. Let the divergence angle of the incident light beam be θ1, the spot size be ω1, the divergence angle of the output light beam after passing through the beam expander be θ2, and the spot size be ω2. After adjusting d2 to the corresponding distance, the following conditions are satisfied: ω2 / ω1 = |F4 / F1|; θ1 / θ2 ≥ |F4 / F1|.

9. The high-magnification diffraction-limited laser beam expander according to claim 8, characterized in that: Adjusting the distance between the optical axes of the light-emitting side of the first lens (11) and the light-incident side of the second lens (21) includes the following steps: S1. Install the first beam expander lens group (1) and the second beam expander lens group (2) so that the first beam expander lens group (1) and the second beam expander lens group (2) are on the same optical axis, and use a laser measurement device to measure the divergence angle of the output laser. S2. Adjust the distance between the optical axes of the light-emitting side of the first lens (11) and the light-incident side of the second lens (21) according to the divergence angle of the output laser. If the divergence angle of the output laser is less than the preset far-field divergence angle, the adjustment can be directly ended. If the divergence angle of the output laser is greater than the preset far-field divergence angle, then gradually increase the distance between the optical axes of the light-emitting side of the first lens (11) and the light-incident side of the second lens (21) in a certain step. After each increase in distance, re-measure the divergence angle of the output laser. If the divergence angle of the output laser decreases to within the preset value, end the adjustment. If the divergence angle of the output laser increases, reduce the just-increased distance, and then enter step S3. S3. If the divergence angle increases after increasing the distance, then decrease the distance between the optical axes of the light-emitting side of the first lens (11) and the light-incident side of the second lens (21) in a certain step, and re-measure the divergence angle of the output laser. If the divergence angle of the output laser decreases to within the preset value, end the adjustment. If the divergence angle has not decreased to the preset value, continue to decrease the distance between the optical axes of the light-emitting side of the first lens (11) and the light-incident side of the second lens (21) until the divergence angle of the output laser decreases to within the preset value.

10. The high-magnification diffraction-limited laser beam expander according to claim 1, wherein: The materials of the first lens (11), the second lens (21), and the third lens (22) are all fused silica glass, with a refractive index Nd = 1.46 and an Abbe number V = 68.

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