A ladder-type graded-index positive-branch confocal unstable resonator

Abstract

The present application belongs to the field of pulse laser technology, and particularly relates to a stepped gradient amplification positive branch confocal unstable cavity. The cavity comprises a concave mirror, a laser module, an electro-optic Q-switching device and a Gauss mirror arranged coaxially in sequence, wherein the concave mirror and the Gauss mirror are both of stepped gradient curvature structure from the center to the edge, and the concave mirror and the Gauss mirror form the stepped gradient amplification positive branch confocal unstable cavity. Laser oscillates in the positive branch confocal unstable cavity, and the beam aperture is stepped and uniformly enlarged from the center region of the crystal rod until the entire crystal rod is filled, and finally the Gauss mirror outputs pulse laser with large pulse energy and high beam quality. The present application can obtain laser with high beam quality and large energy pulse, and has the characteristics of high beam quality, high efficiency, simple structure, good amplification, reliable use and the like.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of pulsed laser technology, and particularly relates to a stepped gradually-varying-magnification positive-branch confocal unstable resonator. BACKGROUND

[0002] High-beam-quality high-energy pulsed laser has very important applications in scientific research, nonlinear frequency conversion, and industrial processing. Q-switching technology is the most commonly used technology to obtain high-energy pulsed laser. Using Q-switching technology can achieve pulsed laser output with a pulse width of hundreds of picoseconds to hundreds of nanoseconds and a pulse repetition frequency of several kHz to several hundred kHz. The application of Q-switching technology enables people to obtain laser pulses with a peak power of more than megawatt, making laser a very strong coherent light source, and thus generating new branches of optics such as nonlinear optics, and promoting the development of application technologies such as laser radar, laser ranging, high-speed photography, and nuclear fusion.

[0003] The main pumping methods of solid-state pulsed lasers are end-pumping and side-pumping, and the shapes of laser crystals are mainly rod, slab, and disc. The side-pumping structure has the advantage that high-power pumping radiation can be coupled into the laser medium through a simple structure to achieve high-power laser output. Compared with a side-pumped slab gain medium, a side-pumped rod gain medium has the advantages of simple cooling structure and low price. Therefore, a side-pumped laser module is an important research means to obtain high-energy pulsed laser. However, due to the differences in the cooling structure of the rod crystal and the radial heat generation of the crystal, the thermal focal length of the crystal rod is different along the radial direction, which affects the final beam quality. SUMMARY

[0004] In view of the defects of the prior art, the purpose of the present application is to provide a stepped gradually-varying-magnification positive-branch confocal unstable resonator to solve the problems of poor beam quality and low energy of the pulsed laser output of the existing side-pumped laser module, and to obtain high-beam-quality high-energy pulsed laser with the characteristics of high beam quality, high efficiency, simple structure, good amplification, and reliable use, which can be widely used in the fields of scientific research, nonlinear frequency conversion, and industrial processing.

[0005] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0006] The present application provides a stepped gradually-varying-magnification positive-branch confocal unstable resonator, which comprises a concave mirror, a laser module, an electro-optic Q-switching device, and a Gauss mirror arranged coaxially in sequence, wherein the concave mirror and the Gauss mirror are both of a stepped gradually-varying-curvature structure from the center to the edge, and the concave mirror and the Gauss mirror form a stepped gradually-varying-magnification positive-branch confocal unstable resonator. Laser oscillates in the positive-branch confocal unstable resonator, and finally a pulsed laser with large pulse energy and high beam quality is output by the Gauss mirror.

[0007] The concave mirror and the Gauss mirror are both radially stepped up from the center to the edge; the magnification of the positive branch confocal unstable cavity is stepped down radially from the center to the outside.

[0008] The laser module is a side-pumped laser structure, and the center is a laser crystal rod.

[0009] The diameter of the laser crystal rod is D, and the laser crystal rod is uniformly divided into m concentric rings along the diameter direction. n-1 The diameter of the second ring is d1=d0+(D-d0) / (m-1), the diameter of the nth ring is d m n-1) / (m-1), where m and n are integers greater than or equal to 2, and m is greater than or equal to n; the area between adjacent rings corresponds to different magnifications of the positive branch confocal unstable cavity.

[0010] The laser crystal rod is a crystal, glass or ceramic doped with Nd ions; or the laser crystal rod is a crystal, glass or ceramic doped with Yb ions; or the laser crystal rod is a crystal, glass or ceramic doped with Er ions; or the laser crystal rod is a crystal, glass or ceramic doped with Tm ions.

[0011] The Nd-doped crystal is neodymium-doped yttrium aluminum garnet, neodymium-doped yttrium vanadate, neodymium-doped gadolinium gallium garnet, neodymium-doped yttrium lithium fluoride, neodymium-doped yttrium aluminum or neodymium-doped strontium fluorophosphate.

[0012] In the positive branch confocal unstable cavity, the magnification is stepped down from the center of the laser crystal rod to the outside, and the center region with a diameter of d0 corresponds to a magnification M1=r 20 / r 10 , the ring between the inner diameter d0 and the outer diameter d1 of the laser crystal rod corresponds to a magnification M2=r 21 / r 11 , the ring between the inner diameter d n-2 and the outer diameter d n-1 corresponds to a magnification M n =r 2(n-1) / r 1(n-1) , the outermost ring between the inner diameter d m-2 and the outer diameter D corresponds to a magnification M m =r 2(m-1) / r 1(m-1) , and the final total magnification is M=M1·M2·…M m =[r 10 +(n-1)·Δr] / r 10 =D / d0, where m and n are integers greater than or equal to 2, and m is greater than or equal to n.

[0013] The Gaussian mirror is a concave-convex output mirror, and the concave and convex surfaces of the mirror have the same radius of curvature at the same position along the radial direction;

[0014] The radius of curvature of the concave and convex mirror increases uniformly in a stepwise manner Δr from the center outwards. The radius of curvature of the concave and convex mirror located at the center is r. 10 The perpendicular distance from the edge of the central curvature to the center point is d0 / 2; the radius of curvature of the second concave-convex mirror is r. 11 =r 10 +Δr, the interval of curvature position is the annulus between diameter d0 and diameter d1 on the Gaussian mirror; the radius of curvature of the nth concave-convex mirror is r. 1(n-1) =r 10 +(n-1)·Δr, the interval of curvature position is the diameter d on the Gaussian mirror. n-2 To diameter d n-1 The innermost ring is a ring; the radius of curvature of the outermost ring is r. 1(m-1) =r 10 +(m-1)·Δr, the interval of curvature position is the diameter d m-1 The ring between the lens edge and the lens edge, where m and n are integers ≥ 2, and m ≥ n.

[0015] The concave surface of the Gaussian mirror is coated with a dielectric film that has a gradually varying reflectivity for the laser wavelength, with reflectivity R(w) = R max ·exp[-2·(w / w m ) 2 ], where w is the radial position on the Gaussian mirror, and the central peak reflectivity is R. max Width is w m The convex surface of the Gaussian mirror is coated with a dielectric film that enhances the light transmittance of laser wavelengths.

[0016] The concave mirror is a plano-concave high-reflectivity mirror, with the concave surface coated with a dielectric film that is highly reflective to the laser wavelength.

[0017] The radius of curvature of the concave mirror is increased uniformly in a stepwise manner by Δr from radially outwards, with the radius of curvature of the small concave mirror at the center being r. 20 The perpendicular distance from the edge of the central curvature to the center point is d1 / 2; the radius of curvature of the second concave mirror is r. 21 =r 20 +Δr, the interval of curvature position is the annulus between diameter d1 and diameter d2 on the concave mirror; the radius of curvature of the nth concave mirror is r. 2(n-1) =r 20 +(n-1)·Δr, the interval of curvature position is the diameter d on the concave mirror. n-1 To diameter d n The innermost ring is a ring; the radius of curvature of the outermost ring is r. 2(m-1) =r20 +(m-1)·Δr, the interval of curvature position is on the concave mirror with diameter d m-1 The annular ring between the concave mirror edge and the edge of the laser crystal rod.

[0018] The cavity length L of the stepped gradient amplification positive branch confocal unstable cavity is (r 20 -r 10 ) / 2, and r 20 =r 11 , r 21 =r 12 , r 2(n-1) =r 1n , wherein m and n are integers greater than or equal to 2, and m is greater than or equal to n.

[0019] The electro-optical Q-switching device comprises a polarization plate, a quarter-wave plate and a Pockels cell arranged on the same axis, and the electro-optical Q-switching device can realize pulse laser output with nanosecond pulse width.

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

[0021] The present application provides a stepped gradient amplification positive branch confocal unstable cavity to solve the problems of poor beam quality, low energy and the like of the existing side-pumped laser module pulse laser output, can better homogenize the thermal inhomogeneity of the crystal rod, make the focal length difference always fixed, and the amplification of the outer ring smaller, so that the energy of the outer crystal rod can be better extracted. The geometric loss of the stepped gradient curvature concave mirror combined with the Gaussian mirror can be reduced to be basically ignored. Therefore, high beam quality and large energy pulse laser output can be obtained, which can meet the important applications in the fields of scientific research, nonlinear frequency conversion, industrial processing and the like. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 It is a structure schematic diagram of a stepped gradient amplification positive branch confocal unstable cavity of the present application.

[0023] Figure 2 It is a cavity mirror curvature schematic diagram corresponding to the stepped gradient amplification positive branch confocal unstable cavity of the present application.

[0024] Figure 3 It is a cross-sectional schematic diagram of a laser crystal rod in the present application.

[0025] In the figure: 1, concave mirror, 2, laser module, 21, laser crystal rod, 3, polarization plate, 4, quarter-wave plate, 5, Pockels cell, 6, Gaussian mirror. DETAILED DESCRIPTION

[0026] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort belong to the scope of the present application.

[0027] As shown in Figure 1 The present application provides a stepped magnification positive-branch confocal unstable cavity, which comprises a concave mirror 1, a laser module 2, an electro-optic Q-switching device and a Gauss mirror 6 arranged in sequence along a same axis (z-axis), wherein the concave mirror 1 and the Gauss mirror 6 are both of a stepped magnification structure with a curvature gradually increasing from the center to the edge, and the concave mirror 1 and the Gauss mirror 6 form the stepped magnification positive-branch confocal unstable cavity; the laser oscillates in the positive-branch confocal unstable cavity, and finally a pulse laser with large pulse energy and high beam quality is output by the Gauss mirror 6.

[0028] In the embodiment of the present application, the electro-optic Q-switching device comprises a polarizer 3, a quarter-wave plate 4 and a Pockels cell 5 arranged in sequence along the same axis (z-axis), and the electro-optic Q-switching device can realize the output of pulse laser with nanosecond pulse width.

[0029] In the embodiment of the present application, the curvature of the concave mirror 1 and the Gauss mirror 6 gradually increases from the center to the edge, so that the magnification of the positive-branch confocal unstable cavity gradually decreases from the center to the outside along the radial direction.

[0030] As shown in Figures 1-2 In the embodiment of the present application, the laser module 2 is of a side-pumped laser structure, and the center is a laser crystal rod 21. The laser oscillates in the stepped magnification positive-branch confocal unstable cavity, the beam aperture of the laser gradually and uniformly increases in a stepped manner from the center of the laser crystal rod 21, until the entire laser crystal rod 21 is filled, and finally a pulse laser with large pulse energy and high beam quality is output.

[0031] As shown in Figure 3 In the embodiment of the present application, the diameter of the laser crystal rod 21 is D, the laser crystal rod 21 is uniformly divided into m concentric annular rings along the diameter direction, the diameter of the middle annular ring is d0, the diameter of the second annular ring is d1=d0+(D-d0) / (m-1), the diameter of the nth annular ring is d n-1 n-1) / (m-1), wherein m and n are integers greater than or equal to 2, and m is greater than or equal to n; the area between adjacent annular rings corresponds to different magnifications of the positive-branch confocal unstable cavity.

[0032] Preferably, the laser crystal rod 21 is a Nd-ion-doped crystal, glass or ceramic; or the laser crystal rod 21 is a Yb-ion-doped crystal, glass or ceramic; or the laser crystal rod 21 is an Er-ion-doped crystal, glass or ceramic; or the laser crystal rod 21 is a Tm-ion-doped crystal, glass or ceramic.

[0033] Preferably, the Nd-ion-doped crystal is neodymium-doped yttrium aluminum garnet, neodymium-doped yttrium vanadate, neodymium-doped gadolinium gallium garnet, neodymium-doped yttrium lithium fluoride, neodymium-doped yttrium aluminate or neodymium-doped strontium fluorophosphate.

[0034] As shown in FIG. 1, the embodiment of the present application comprises a laser crystal rod 21, a Gaussian mirror 6, a concave mirror 1 and a laser cavity 2. Figure 2 As shown in FIG. 1, the embodiment of the present application comprises a laser crystal rod 21, a Gaussian mirror 6, a concave mirror 1 and a laser cavity 2. 10 , the vertical distance between the edge of the central curvature and the center point is d0 / 2; the curvature radius of the second ring of the concave-convex mirror is r 11 = r 10 + Δr, the interval of the curvature position is the annulus between the diameter d0 and the diameter d1 on the Gaussian mirror 6; the curvature radius of the nth ring of the concave-convex mirror is r 1(n-1) = r 10 + (n-1)·Δr, the interval of the curvature position is the annulus between the diameter d n-2 and the diameter d n-1 on the Gaussian mirror 6; the curvature radius of the outermost ring of the concave-convex mirror is r 1(m-1) = r 10 + (m-1)·Δr, the interval of the curvature position is the annulus between the diameter d m-1 and the edge of the mirror on the Gaussian mirror 6, wherein m and n are integers greater than or equal to 2, and m is greater than or equal to n.

[0035] As shown in FIG. 1, the embodiment of the present application comprises a laser crystal rod 21, a Gaussian mirror 6, a concave mirror 1 and a laser cavity 2. 20 , the vertical distance between the edge of the central curvature and the center point is d1 / 2; the curvature radius of the second ring of the concave-convex mirror is r 21 = r 20 + Δr, the interval of the curvature position is the annulus between the diameter d1 and the diameter d2 on the Gaussian mirror 6; the curvature radius of the nth ring of the concave-convex mirror is r 2(n-1) = r 20 + (n-1)·Δr, the interval of the curvature position is the annulus between the diameter d n-1 and the diameter d n on the Gaussian mirror 6; the curvature radius of the outermost ring of the concave-convex mirror is r 2(m-1) = r20 +(m-1)·Δr, the interval of curvature position is the diameter d on concave mirror 1. m-1 The annulus between the edge of concave mirror 1; furthermore, it can be deduced that the cavity length L of the step-gradient magnification positive branch confocal unstable cavity is (r 20 -r 10 ) / 2, and r 20 =r 11 r 21 =r 12 r 2(n-1) =r 1n , where m and n are integers ≥ 2, and m ≥ n.

[0036] In an embodiment of the present invention, within the positive branch confocal unstable cavity, the amplification of the laser decreases stepwise from the center of the laser crystal rod 21 outwards, with the amplification M1 = r in the central region (diameter d0 portion). 20 / r 10 The magnification M2 = r of the annulus between the inner diameter d0 and the diameter d1 of the laser crystal rod 21 21 / r 11 The nth ring, i.e., the diameter d n-2 To diameter d n-1 The magnification M corresponding to the two rings n =r 2(n-1) / r 1(n-1) The outermost ring (diameter d) m-2 The magnification M corresponding to the diameter D) m =r 2(m-1) / r 1(m-1) The final magnification is M = M1·M2·…M m =[r 10 +(n-1)·Δr] / r 10 =D / d0, where m and n are integers ≥2, and m ≥ n.

[0037] Furthermore, the concave surface of Gaussian mirror 6 is coated with a dielectric film with a gradually varying reflectivity to the laser wavelength, the reflectivity R(w) = R max ·exp[-2·(w / w m ) 2 ], where w is the radial position on the Gaussian mirror, and the central peak reflectivity is R. max Width is w m The convex surface is coated with a dielectric film that enhances the light transmittance of laser wavelengths.

[0038] Example 1

[0039] The first embodiment of the present invention is as follows: Figure 1As shown, the present application provides a step-graded magnification positive-branch confocal unstable cavity, comprising: a step-graded curvature concave mirror 1, a laser module 2, a polarizer 3, a quarter-wave plate 4, a Pockels cell 5 and a step-graded curvature Gauss mirror 6, which are placed along a straight line (z-axis); the polarizer 3, the quarter-wave plate 4 and the Pockels cell 5 constitute an electro-optic Q-switching device;

[0040] The step-graded curvature concave mirror 1 and the step-graded curvature Gauss mirror 6 constitute the step-graded magnification positive-branch confocal unstable cavity; the laser module 2 is a side-pumped laser structure, the center of which is a laser crystal rod 21, which is made of Nd:YAG crystal, has a diameter D = 5 mm, a length of 125 mm and is doped with 0.6 at.%; the laser crystal rod 21 is uniformly divided into five concentric rings along the diameter direction, the diameter of the centermost circle is d0 = 1 mm, the diameter of the second ring is d1 = 2 mm, the diameter of the third ring is d2 = 3 mm, the diameter of the fourth ring is d3 = 4 mm, and the diameter of the fifth ring is d4 = 5 mm.

[0041] The step-graded curvature Gauss mirror 6 is a concave-convex mirror, the radii of curvature of the concave surface and the convex surface at the same position are equal; the radii of curvature of the concave-convex mirror change in turn along the radial direction and increase uniformly by Δr = 1 m; the radius of curvature at the center is r 10 = 1 m, the vertical distance between the curvature edge and the center point is d0 / 2 = 0.5 mm; the radius of curvature of the second ring is r 11 = r 10 + Δr = 2 m, the interval of the curvature position is between the circle with a diameter of d0 = 1 mm and the circle with a diameter of d1 = 2 mm; the radius of curvature of the third ring is r 12 = r 11 + Δr = 3 m, the interval of the curvature position is between the circle with a diameter of d1 = 2 mm and the circle with a diameter of d2 = 3 mm; the radius of curvature of the fourth ring is r 13 = r 12 + Δr = 4 m, the interval of the curvature position is between the circle with a diameter of d2 = 3 mm and the circle with a diameter of d3 = 4 mm; the radius of curvature of the fifth ring is r 14 = r 13 + Δr = 5 m, the interval of the curvature position is between the circle with a diameter of d3 = 4 mm and the edge of the lens.

[0042] The step-graded curvature concave mirror 1 is a flat concave mirror, the radii of curvature of the concave mirror change in turn along the radial direction and increase uniformly by Δr = 1 m; the radius of curvature at the center is r 20 = 2 m, the vertical distance between the curvature edge and the center point is d1 / 2 = 1 mm; the radius of curvature of the second ring is r 21 = r 20+ Δr = 3m, the interval of curvature position is a circular ring between d1 = 2mm and d2 = 3mm; the curvature radius of the third circular ring is r 22 = r 21 + Δr = 4m, the interval of curvature position is a circular ring between d2 = 3mm and d3 = 4mm; the curvature radius of the fourth circular ring is r 23 = r 22 + Δr = 5m, the interval of curvature position is a circular ring between d3 = 4mm and the edge of the lens, and it can be concluded that the cavity length of the step-graded magnification positive branch confocal unstable cavity is 1m.

[0043] The concave mirror 1 and the Gauss mirror 6 constitute a step-graded magnification positive branch confocal unstable cavity, and the magnification decreases step by step from the center of the laser crystal rod 21 to the outside, and the central region (diameter d0 = 1mm) corresponds to the magnification M1 = r 20 / r 10 = 2, the circular ring between d0 and d1 corresponds to the magnification M2 = r 21 / r 11 = 3 / 2, the circular ring between d1 and d2 corresponds to the magnification M3 = r 22 / r 12 = 4 / 3, the circular ring between d3 and d4 corresponds to the magnification M4 = r 23 / r 13 = 5 / 4, and the final magnification is M = M1·M2·M3·M4 = [r 10 + (m-1)· Δr] / r 10 = D / d0 = 5. The focal points F1, F2, F3 and F4 of the circular rings are equidistant.

[0044] The Gauss mirror 6 is a concave-convex output mirror, and the concave surface is coated with a dielectric film with a gradually changing reflectivity for the laser wavelength, and the reflectivity R(w) = R max · exp[-2· (w / w m ) 2 ], wherein w is the radial position on the Gauss mirror, the central peak reflectivity is R max = 30%, the width is w m = 1.5mm, and the convex surface is coated with a dielectric film with an antireflection for the laser wavelength (transmittance T ≥ 99.9%).

[0045] The concave mirror 1 with stepped gradient curvature is a flat concave high reflection mirror, and the concave surface is coated with a dielectric film with high reflection to laser wavelength (reflectivity R >= 99.9%). The stepped gradient magnification positive branch confocal unstable cavity has a magnification that decreases step by step from the center to the outside along the diameter direction of the lens, and the magnification is 2, 3 / 2, 4 / 3, 5 / 4 in turn. Laser oscillates in the stepped gradient magnification positive branch confocal unstable cavity, and the beam aperture increases step by step uniformly from the center area of the laser crystal rod 21, and the diameter is 1mm, 2mm, 3mm, 4mm in turn until filling the whole crystal rod (5mm), and finally a pulse laser with large pulse energy and high beam quality is output.

[0046] In summary, in order to solve the problems of poor beam quality, low energy and the like of the existing side-pumped laser module, the stepped gradient magnification positive branch confocal unstable cavity structure is adopted, the thermal inhomogeneity of the crystal rod can be better homogenized, the focal length difference is always fixed, the magnification of the outer ring is smaller, so the energy of the outer crystal rod can be better extracted, the geometric loss when combined with the Gaussian mirror can be reduced to be basically ignored, and therefore high beam quality and large energy pulse laser output can be expected. The present application has the characteristics of high efficiency, simple structure, good beam quality, high output energy and the like, and can be widely applied in the fields of scientific research, nonlinear frequency conversion, industrial processing and the like.

[0047] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited to this, any skilled person in the art can make equivalent replacement or change according to the technical scheme and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered in the protection scope of the present application.

Claims

1. A ladder-anamorphic positive-branch confocal unstable resonator, characterized in that, The concave mirror (1), the laser module (2), the electro-optic Q-switching device and the Gauss mirror (6) are arranged in sequence along the same axis, wherein the concave mirror (1) and the Gauss mirror (6) are both of the stepped gradient curvature structure from the center to the edge, and the concave mirror (1) and the Gauss mirror (6) form a positive-branch confocal unstable cavity with a stepped gradient magnification; the laser oscillates in the positive-branch confocal unstable cavity, and finally the pulse laser with large pulse energy and high beam quality is output by the Gauss mirror (6).

2. The step-magnification positive-branch confocal unstable resonator according to claim 1, characterized in that, The concave mirror (1) and the Gauss mirror (6) are both of the stepped gradient curvature from the center to the edge along the radial direction; and the magnification of the positive-branch confocal unstable cavity is stepped gradient smaller from the center to the edge along the radial direction.

3. The step-magnification positive-branch confocal unstable resonator of claim 1, wherein, The laser module (2) is of a side-pumped laser structure, and the center is a laser crystal rod (21); the laser beam aperture is stepped and uniformly enlarged from the center region of the laser crystal rod (21) until the entire laser crystal rod (21) is filled.

4. The step-magnification positive-branch confocal unstable resonator according to claim 3, characterized in that, The diameter of the laser crystal rod (21) is D, the laser crystal rod (21) is evenly divided into m concentric circular rings in the diameter direction, the diameter of the middle circular ring is d0, the diameter of the second circular ring is d1=d0+(D-d0) / (m-1), the diameter of the nth circular ring is d n-1 =d0+(D-d0)·(n-1) / (m-1), wherein m and n are integers greater than or equal to 2, and m is greater than or equal to n; the area between adjacent circular rings corresponds to different magnifications of the positive branch confocal unstable cavity.

5. The step-magnification positive-branch confocal unstable resonator according to claim 4, characterized in that, The laser crystal rod (21) is a crystal, glass or ceramic doped with Nd ions, or a crystal, glass or ceramic doped with Yb ions, or a crystal, glass or ceramic doped with Er ions, or a crystal, glass or ceramic doped with Tm ions. The crystal doped with Nd ions is neodymium-doped yttrium aluminum garnet, neodymium-doped yttrium vanadate, neodymium-doped gadolinium gallium garnet, neodymium-doped yttrium lithium fluoride, neodymium-doped yttrium aluminum, or neodymium-doped strontium fluorophosphate.

6. The step-magnification positive-branch confocal unstable resonator of claim 4, wherein, Within the positive branch confocal unstable cavity, the laser amplification decreases stepwise from the center of the laser crystal rod (21) outwards, with the amplification M1 = r corresponding to the central region with a diameter of d0. 20 / r 10 The magnification M2 = r corresponds to the annulus between the inner diameter d0 and the diameter d1 of the laser crystal rod (21). 21 / r 11 , diameter d n-2 To diameter d n-1 The magnification M corresponding to the two rings n =r 2(n-1) / r 1(n-1) , diameter d m-2 The magnification M corresponding to the outermost ring between the diameter D and the outermost ring is... m =r 2(m-1) / r 1(m-1) The final total magnification is M = M1·M2·…M m =[r 10 +(n-1)·Δr] / r 10 =D / d0, where m and n are integers ≥2, and m ≥ n.

7. The step-magnification positive-branch confocal unstable resonator of claim 2, wherein, The Gauss mirror (6) is a concave-convex output mirror, and the curvature radii of the concave surface and the convex surface of the concave-convex mirror at the same position along the radial direction are equal. The radius of curvature of the concave-convex mirror increases uniformly by a step of Δr from the center to the outside in a radial direction, and the radius of curvature of the concave-convex mirror located at the center is r 10 , the edge of the center curvature is vertically away from the center point by d0 / 2; the radius of curvature of the concave-convex mirror located at the second circle is r 11 = r 10 + Δr, the interval of the curvature position is a circular ring between the diameter d0 and the diameter d1 on the Gauss mirror (6); the radius of curvature of the concave-convex mirror located at the nth circle is r 1(n-1) = r 10 + (n-1)·Δr, the interval of the curvature position is a circular ring between the diameter d n-2 and the diameter d n-1 on the Gauss mirror (6); the radius of curvature of the concave-convex mirror located at the outermost circle is r 1(m-1) = r 10 + (m-1)·Δr, the interval of the curvature position is a circular ring between the diameter d m-1 and the edge of the lens, wherein m and n are integers greater than or equal to 2, and m is greater than or equal to n.

8. The step-magnification positive-branch confocal unstable resonator according to claim 7, characterized in that, The concave surface of the Gaussian mirror (6) is coated with a dielectric film with a graded reflectivity for the laser wavelength, the reflectivity R(w) = R max · exp[-2·(w / w m ) 2 ], where w is the position in the radial direction on the Gaussian mirror, the central peak reflectivity is R max , and the width is w m , and the convex surface of the Gaussian mirror (6) is coated with a dielectric film with an antireflection for the laser wavelength.

9. The step-magnification positive-branch confocal unstable resonator of claim 2, wherein, The concave mirror (1) is a flat-concave high-reflection mirror, and the concave surface is coated with a dielectric film with high reflection to the laser wavelength. The radius of curvature of the concave mirror (1) is increased radially outwards in steps by Δr, the radius of curvature of the small concave mirror at the center is r 20 , the vertical distance from the center point to the center curvature edge is d1 / 2; the radius of curvature of the second small concave mirror is r 21 = r 20 + Δr, the interval of the curvature position is the annulus between the diameter d1 and the diameter d2 on the concave mirror (1); the radius of curvature of the nth small concave mirror is r 2(n-1) = r 20 + (n-1)·Δr, the interval of the curvature position is the annulus between the diameter d n-1 and the diameter d n on the concave mirror (1); the radius of curvature of the outermost annulus is r 2(m-1) = r 20 + (m-1)·Δr, the interval of the curvature position is the annulus between the diameter d m-1 and the edge of the concave mirror (1); The cavity length L of the ladder-graded positive-branch confocal unstable cavity is (r 20 -r 10 ) / 2, and r 20 =r 11 , r 21 =r 12 , r 2(n-1) =r 1n , where m and n are integers greater than or equal to 2, and m is greater than or equal to n.

10. The step-magnification positive-branch confocal unstable resonator of claim 1, wherein, The electro-optic Q-switching device comprises a polarizer (3), a quarter-wave plate (4) and a Pockels cell (5) arranged in sequence along the same axis, and the electro-optic Q-switching device can realize the output of pulse laser with nanosecond pulse width.

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