MOPA laser system for improving output efficiency, light field distribution and light beam quality

Abstract

The invention provides an MOPA laser system capable of improving output efficiency, light field distribution and light beam quality, the MOPA laser system comprises a nanosecond laser oscillator, an isolator, a concave lens and a two-way amplifier which are connected in sequence, the centers of the nanosecond laser oscillator, the isolator and the concave lens are arranged on the same straight line, and the concave lens is matched with the two-way amplifier; low-energy nanosecond pulse laser generated by the nanosecond laser oscillator enters the isolator, then the size of laser spots is adjusted through the concave lens, then the low-energy nanosecond pulse laser enters the two-way amplifier from one end of the two-way amplifier to be amplified, the low-energy nanosecond pulse laser is reflected at the other end of the two-way amplifier to be amplified again, and then the low-energy nanosecond pulse laser is output from one end of the two-way amplifier. The light beam quality of amplified light is remarkably improved through the two-way amplifier, deterioration of the light beam quality is avoided, upper energy level particles of the gain medium are fully extracted, and therefore the amplification efficiency of the first-stage amplifier is greatly improved. According to the invention, the light beam quality of the amplified light is effectively improved, and the light field distortion is optimized, so that the high-quality amplified light output is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of laser amplifiers, and in particular to a laser system for improving output efficiency, light field distribution and beam quality. Background Art

[0002] The goal of solid-state laser development is to obtain high beam quality and high-power lasers. In particular, it is widely used in the fields of medical treatment, lidar, laser ranging, laser processing, etc. The MOPA system has shown significant advantages in the pursuit of high-power lasers with its unique ability to amplify in stages and control the beam quality step by step. However, during the amplification process of high-power solid-state lasers, the beam quality and light field distribution deteriorate due to the thermal accumulation effect, which greatly limits the application of lasers. Therefore, further improving the beam quality of high-power solid-state lasers while improving the light field distribution is the goal of solid-state laser development.

[0003] In the prior art, the technology of improving the beam quality of high-power solid-state lasers and improving the light field distribution is, on the one hand, to suppress the thermal effect of the gain medium to improve the beam quality of the laser, although there are many methods for alleviating the thermal effect of the crystal, such as increasing the heat exchange area between the crystal and the cooling device to improve the heat dissipation effect, using a temperature control system to control the temperature around the crystal, and adjusting the distribution of the pump light and the seed light to improve the beam quality of the amplified light. However, under high-power pumping, the gain crystal will still produce adverse effects such as thermal lensing, thermal depolarization, and thermally induced birefringence, causing it to transform from a plane to a lens with strong spherical aberration, thereby causing a decrease in beam quality and a decrease in the utilization rate of the pump light. On the other hand, the spherical aberration of the laser is compensated by introducing a special optical device in the optical path, thereby improving the beam quality of the amplified light. However, the spherical aberration in the MOPA system will change with the increase of the pump power, while the spherical aberration introduced outside the cavity is a constant, so a series of spherical aberration correctors with different parameters are required to compensate, which leads to the complexity of the laser system.

[0004] The invention patent with application number 201310228520.3 discloses a gas laser amplification system using a MOPA structure, including a master oscillator cavity (MO), a power amplifier cavity (PA), a line width narrowing module (LNM), an output coupler, a first reflector, and a second reflector. The gas laser amplification system also includes a third reflector and a fourth reflector. The seed light output from the master oscillator cavity (MO) via the output coupler is reflected by the first reflector and the second reflector into the power amplifier cavity (PA); the third reflector and the fourth reflector are respectively located at the two ends of the outside of the power amplifier cavity (PA), and their reflection surfaces are perpendicular to the length direction of the power amplifier cavity (PA); the seed light entering the power amplifier cavity (PA) is reflected multiple times between the third reflector and the fourth reflector, and output from the notch of the third reflector. The invention has stable performance, low cost and easy adjustment of the optical path. However, the amplification system has the problem of decreased sensitivity caused by misalignment and spherical aberration caused by the thermal lens effect in the gain medium. This application integrates the 4f system and the 0° reflector into the amplification system. This configuration not only solves the problem of decreased sensitivity caused by misalignment of the amplification system and spherical aberration caused by the thermal lens effect in the gain medium, but also solves the problem of deterioration of the spot distribution during laser amplification. Therefore, it significantly optimizes the beam quality and light field distribution of the amplified laser, so that the first-stage amplifier 10 obtains a laser pulse output that is very similar to the Gaussian laser field output by the oscillator, with high conversion efficiency and high beam quality. Summary of the invention

[0005] In view of the technical problem that a series of spherical aberration correctors with different parameters are needed to compensate for the existing MOPA system, which leads to a complex system, the present invention proposes a device for improving the output efficiency, light field distribution and beam quality of the MOPA laser system. The device has a simple structure, effectively improves the beam quality of the amplified light, optimizes the light field distortion, and thus obtains high-quality amplified light output.

[0006] In order to achieve the above object, the technical solution of the present invention is implemented as follows: a MOPA laser system for improving output efficiency, light field distribution and beam quality, comprising a nanosecond laser oscillator, an isolator, a concave lens and a two-way amplifier connected in sequence, the centers of the nanosecond laser oscillator, the isolator and the concave lens are arranged on the same straight line, and the concave lens and the two-way amplifier are matched;

[0007] The low-energy nanosecond pulse laser generated by the nanosecond laser oscillator enters the isolator and passes through a concave lens to adjust the laser spot size. It then enters the two-way amplifier from one end of the two-way amplifier for amplification, is reflected at the other end of the two-way amplifier, and is amplified again before being output from one end of the two-way amplifier.

[0008] The isolator includes a polarization beam splitter I, a half wave plate, a Faraday rotator and a polarization beam splitter II which are connected in sequence. The centers of the polarization beam splitter I, the half wave plate, the Faraday rotator and the polarization beam splitter II are arranged on the same straight line. The centers of the nanosecond laser oscillator and the polarization beam splitter I are arranged on the same straight line. The centers of the polarization beam splitter II and the concave lens are arranged on the same straight line.

[0009] The double-pass amplifier comprises a polarization beam splitter III, a primary amplifier, a 4f optical imaging system, a quarter wave plate and a 0° reflector which are arranged in sequence. The center points of the polarization beam splitter III, the primary amplifier, the 4f optical imaging system, the quarter wave plate and the 0° reflector are arranged on the same straight line. The polarization beam splitter III is matched with a concave lens through a reflection module.

[0010] The 4f optical imaging system comprises a convex lens I, a vacuum tube and a convex lens II which are arranged in sequence. The centers of the convex lens I, the vacuum tube and the convex lens II are arranged on the same straight line. The center point of the convex lens I and the first-stage amplifier are arranged on the same straight line.

[0011] A gain medium is arranged in the first-stage amplifier, one end of the gain medium matches the output end of the polarization beam splitter III, and the other end of the gain medium matches one end of the convex lens I.

[0012] The 0° reflector is arranged at the focus of the laser after the laser is amplified by the first-stage amplifier in one pass.

[0013] The reflection module comprises a reflector I and a reflector II, one end of the reflector I matches with a concave lens, the other end of the reflector I matches with the reflector II, and the reflector II matches with a polarization beam splitter III.

[0014] The reflector I and the reflector II are both 45° reflectors.

[0015] The angle between the reflector I and the reflector II is 90°.

[0016] The gain medium is Nd:YAG crystal.

[0017] The beneficial effects of the present invention are as follows: the present invention places a 4f optical imaging system, a quarter wave plate and a 0° reflector placed at the focal position of the one-way amplified light of the first-stage amplifier after the first-stage amplifier, which not only utilizes the spherical aberration caused by the thermal lens effect in the gain medium for self-compensation, thereby significantly improving the beam quality of the amplified light and avoiding the deterioration of the beam quality, but also can more fully extract the upper energy level particles of the gain medium, thereby greatly improving the amplification efficiency of the first-stage amplifier. In addition, the 4f optical imaging system placed in the amplification system also improves the dislocation sensitivity of the laser system, reduces the spatial frequency spectrum of the light beam, corrects the uniformity of the light spot of the light beam, and maintains the coherence of the light beam. Not only that, it also optimizes the mode structure of the amplified light and compensates for the distortion of the light beam. These characteristics effectively improve the beam quality of the amplified light and optimize the light field distortion, thereby obtaining high-quality amplified light output. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0019] Figure 1 This is a graph showing the relationship between the dimensionless aberration coefficient of the lens and the lens shape factor and laser imaging parameters.

[0020] Figure 2 It is a schematic diagram of the structure of the present invention.

[0021] Figure 3 This is a diagram showing the change in the light field diameter of the light amplified in one pass by the first-stage amplifier.

[0022] Figure 4 Figure 2 shows the beam quality of the single-pass amplified light of the first-stage amplifier, the double-pass amplified light with the 0° reflector placed at position A, and the ordinary double-pass amplified light without the 4f optical imaging system under different pump powers. (a) is the beam quality of the laser on the y-axis after amplification by the first-stage amplifier, and (b) is the beam quality of the laser on the x-axis after amplification by the first-stage amplifier.

[0023] Figure 5 is the single pulse energy of the double-pass amplified light with the 0° reflector placed at position A and the ordinary double-pass amplified light without the 4f optical imaging system under different pump powers.

[0024] Figure 6Figure 2 shows the y-axis beam quality and x-axis beam quality of the single-pass amplified light of the first-stage amplifier under different pump powers, as well as the y-axis beam quality and x-axis beam quality of the double-pass amplified light with the 0° reflector placed at positions A, B, and C. (a) The y-axis beam quality of the double-pass amplified light with the single-pass amplification and the 0° reflector at different positions, (b) The x-axis beam quality of the double-pass amplified light with the single-pass amplification and the 0° reflector at different positions.

[0025] Figure 7 This is the light field shape of the double-pass amplified light of the first-stage amplifier 10 at 25 cm and 120 cm from the light outlet when the 0° reflector is placed at positions A, B, and C respectively under the condition of a pump power of 60.7 W.

[0026] Figure 8 When the 0° reflector is placed at positions A, B, and C, the focal position of the first-stage amplifier 10 after the single-pass amplified light passes through a convex lens with a focal length of 100.

[0027] Fig. 9 (a) The relationship between different pump powers and amplified light energy when the laser is amplified by a first-stage amplifier in single-pass and double-pass; (b) The relationship between different pump powers and light-to-light conversion efficiency.

[0028] In the figure, 1 is a nanosecond laser oscillator, 2 is a polarization beam splitter I, 3 is a half wave plate, 4 is a Faraday rotator, 5 is a polarization beam splitter II, 6 is a concave lens, 7 is a 45° reflector I, 8 is a 45° reflector II, 9 is a polarization beam splitter III, 10 is a first-stage amplifier, 11 is a convex lens I, 12 is a vacuum tube, 13 is a convex lens II, 14 is a quarter wave plate, and 15 is a 0° reflector. DETAILED DESCRIPTION

[0029] 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.

[0030] A MOPA (Master Oscillator Power Amplifier) ​​laser system that improves output efficiency, light field distribution and beam quality. There is an optical element with spherical aberration in the optical path, and the beam quality M of the output laser 2 The initial beam quality factor M r0 2 The additional factor M that causes the beam quality to deteriorate due to spherical aberration rq 2Therefore, when the seed light remains unchanged, the M of the amplifier output laser 2 Finally, the additional factor M rq 2 Determine, additional factor M rq 2 for:

[0031]

[0032] Among them, C4 is the fourth-order aberration coefficient caused by spherical aberration, λ is the laser wavelength, r is the laser light field radius, β r for in is the radial moment on the transverse intensity profile. From formula (1), we can see that the size of the fourth-order aberration coefficient C4 determines the additional factor M of the laser. rq 2 During the MOPA amplification process, the thermal effect of the gain medium makes the Nd:YAG crystal rod equivalent to a lens. At this time, the fourth-order aberration coefficient C4 is:

[0033]

[0034] Where f is the focal length and the dimensionless aberration coefficient C is 4f for

[0035]

[0036] Where n is the refractive index of the medium, p is the laser imaging parameter, and the positive and negative values ​​of the parameter represent the front and rear positions of the laser focus. Figure 1 The dimensionless aberration coefficient C is given when p is 0, -1, and +1 respectively. 4f The relationship between the lens shape factor q (q = 0 is a symmetrical lens, when q = ± 1 is a plane-curved lens and a curved-plane lens). Figure 1 It can be seen that when the laser passes through the first-stage amplifier for double-pass amplification, if a 0° reflector is placed at the focal position of the single-pass amplified light, the heat generated by the LD is the same, the laser imaging parameters p are opposite to each other, and the lens shape factors q are consistent or opposite to each other, and then the dimensionless aberration coefficient C at the same distance on both sides of the laser focus is 4f The values ​​are consistent. From formulas (1), (2), and (3), it can be seen that the fourth-order aberration coefficient C4 at the same distance on both sides of the laser focus is the opposite of each other. During the amplification process, due to the thermal lens effect of the crystal, if a 0° reflector is placed at the focus position of the single-pass amplified light, when the light beam passes through the Nd:YAG crystal for two amplifications, the additional factor M that causes the beam quality to deteriorate due to spherical aberration rq 2 Theoretically, this method can alleviate the problem of the beam quality M of the amplified light caused by thermal lensing. 2deterioration problem, thereby improving the beam quality of the amplified light.

[0037] like Figure 2 As shown, a device for improving the output efficiency, light field distribution and beam quality of a MOPA laser system comprises a nanosecond laser oscillator 1, an isolator, a concave lens 6 and a two-way amplifier connected in sequence, wherein the centers of the nanosecond laser oscillator 1, the isolator and the concave lens 6 are arranged on the same straight line, and the concave lens 6 matches the two-way amplifier. Among them, the nanosecond laser oscillator 1 is mainly used to output low-energy nanosecond pulse lasers, and the isolator is mainly used to reduce the generation of self-excited oscillations during the amplification process, while protecting the optical elements in the device from the influence of residual depolarization radiation. The concave lens 6 is mainly used to adjust the spot size of the low-energy nanosecond pulse laser so that the low-energy nanosecond pulse laser can smoothly enter the two-way amplifier, thereby increasing the overlapping area between the laser and the gain medium and improving the amplification efficiency. The two-way amplifier is mainly used to amplify the low-energy nanosecond pulse laser and output the amplified laser.

[0038] The low-energy nanosecond pulse laser generated by the nanosecond laser oscillator 1 enters the isolator and passes through the concave lens 6 to adjust the laser spot size. It then enters the two-way amplifier from one end of the two-way amplifier for amplification, is reflected from the other end of the two-way amplifier, and is amplified again, and then output from one end of the two-way amplifier.

[0039] The isolator includes a polarization beam splitter Ⅰ2, a half wave plate 3, a Faraday rotator 4 and a polarization beam splitter Ⅱ5 connected in sequence. The centers of the polarization beam splitter Ⅰ2, the half wave plate 3, the Faraday rotator 4 and the polarization beam splitter Ⅱ5 are arranged on the same straight line, the centers of the nanosecond laser oscillator 1 and the polarization beam splitter Ⅰ2 are arranged on the same straight line, and the centers of the polarization beam splitter 5 and the concave lens 6 are arranged on the same straight line. Among them, the polarization beam splitter Ⅰ2 is mainly used to guide the incident laser into the isolator, the half wave plate 3 is mainly used to rotate the polarization direction of the laser so that it can correctly pass through the subsequent polarization beam splitter when returning, and the Faraday rotator 4 is mainly used to rotate the polarization direction of the laser so that the light can smoothly pass through the polarization beam splitter Ⅱ5. In the reverse transmission, due to the non-reciprocity of the Faraday rotator 4, the light will be blocked by the polarization beam splitter Ⅰ2, thereby realizing unidirectional transmission.

[0040] The double-pass amplifier includes a polarization beam splitter III9, a primary amplifier 10, a 4f optical imaging system, a quarter wave plate 14 and a 0° reflector 15, which are arranged in sequence. The center points of the polarization beam splitter III9, the primary amplifier 10, the 4f optical imaging system, the quarter wave plate 14 and the 0° reflector 15 are arranged on the same straight line, and the polarization beam splitter III9 is ​​matched with the concave lens 6 through a reflection module. Among them, the polarization beam splitter III9 is ​​mainly used for outputting the amplified light after the laser is amplified twice by the primary amplifier 10.

[0041] The first-stage amplifier 10 is mainly used to amplify the input laser. A gain medium is provided in the first-stage amplifier 10, one end of the gain medium matches the output end of the polarization beam splitter III9, and the other end of the gain medium matches one end of the convex lens I11. Among them, the gain medium is Nd:YAG crystal. The quarter-wave plate 14 is mainly used to change the polarization direction of the laser, so that the polarization direction of the laser after the second amplification by the first-stage amplifier is rotated 90° relative to the polarization direction of the laser after a single amplification by the first-stage amplifier. The 0° reflector 15 is mainly used to reflect the laser that has been amplified once, so that the laser returns to the first-stage amplifier 10 for secondary reflection, thereby realizing secondary amplification of the laser. The 0° reflector 15 is set at the focus of the laser after the laser is amplified by the first-stage amplifier 10 in a single pass, and uses the spherical aberration generated by the thermal lens effect in the gain medium for self-compensation, thereby significantly improving the beam quality of the amplified light and avoiding the deterioration of the beam quality. It can also more fully extract the upper energy level particles of the gain medium, thereby greatly improving the amplification efficiency of the first-stage amplifier.

[0042] The 4f optical imaging system is mainly used to improve the misalignment sensitivity of the laser system, eliminate laser inhomogeneity and astigmatism, and thus further improve the beam quality and light field distribution of the output laser.

[0043] Specifically, the 4f optical imaging system includes a convex lens Ⅰ11, a vacuum tube 12 and a convex lens Ⅱ13 arranged in sequence, the centers of the convex lens Ⅰ11, the vacuum tube 12 and the convex lens Ⅱ13 are arranged on the same straight line, and the center points of the convex lens Ⅰ11 and the first-stage amplifier 10 are arranged on the same straight line. The convex lens Ⅰ11 is mainly used to focus the laser to the middle plane in the vacuum tube. The convex lens Ⅱ13 is mainly used to refocus the focused laser from the middle plane to the output surface. The vacuum tube 12 is mainly used to prevent the laser focused by the convex lens 11 and the convex lens Ⅱ13 from breaking through the air.

[0044] The reflection module includes a reflector I7 and a reflector II8. The input end of the reflector I7 matches the output end of the concave lens 6, the output end of the reflector I7 matches the input end of the reflector II8, and the output end of the reflector II8 matches the input end of the polarization beam splitter III9. Both the reflector I7 and the reflector II8 are 45° reflectors. When set, the reflector I7 and the reflector II8 form an angle of 90°, so that the laser is input from the concave lens 6 to the polarization beam splitter III9 through reflection.

[0045] Figure 3 The change of the diameter of the light field of a single-pass amplified light is shown. When the laser passes through the first-stage amplifier 10 once, due to the thermal lens effect of the Nd:YAG crystal, the light beam is focused first and then diverges after passing the focus. This phenomenon is consistent with the theory that the spherical aberration before and after the focus is opposite.

[0046] The performance of the device proposed in the present invention was experimentally verified:

[0047] The output repetition frequency of the nanosecond laser oscillator 1 is 300 Hz, the single pulse energy is 9.46 mJ, the RMS is 1.6% (30 min), and the beam quality M of the y-axis and x-axis is y 2 , M x 2 The laser pulses are 1.21 and 1.30 respectively, with a pulse width of 30ns and a central wavelength of 1064.7nm, providing high-quality seed light for subsequent amplification.

[0048] In order to avoid problems such as beam distortion and gain loss of the amplified light caused by the low beam overlapping efficiency of the first-stage amplifier 10, a 2000W LD side pump module is used as the first-stage amplifier 10 in the MOPA system in the experiment, and a φ4×78mm Nd:YAG crystal rod (1064nm anti-reflection) doped with 0.6% is used as the gain medium. An isolation device with a central wavelength of 1064nm is placed after the nanosecond laser oscillator 1 to reduce the generation of self-oscillation during the amplification process, while protecting the optical components in the nanosecond laser oscillator 1 from the influence of residual depolarization radiation.

[0049] Since the output light of the nanosecond laser oscillator 1 is a reduced beam, a concave lens 6 (1064nm anti-reflection) with a focal length of 786mm is used in the experiment to match the output light of the nanosecond laser oscillator 1 with the crystal of the first-stage amplifier 10, thereby increasing the overlapping area of ​​the seed light and the gain medium and improving the amplification efficiency of the amplifier. After the seed light output by the nanosecond laser oscillator 1 is introduced into the first-stage amplifier 10 for single-pass amplification, a 4f optical imaging system is placed after the first-stage amplifier 10 in order to improve the stability of the amplification system and its misalignment sensitivity. After the light beam passes through the 4f optical imaging system (1064nm anti-reflection), it passes through the quarter-wave plate 14 (1064nm anti-reflection), and is reflected by the 0° reflector 15 (1064nm 0° high reflection) located at the single-pass laser focus position B, and then returns to the original path. The laser passes through the quarter-wave plate 14 again, and the circular polarization is changed to S polarization. After passing through the 4f imaging system again, it enters the first-stage amplifier 10 for secondary amplification. In the experiment, a water cooling system was set up outside the MOPA system. Finally, under the conditions of cooling water temperature of 21°C, flow rate of 14.7L / min, and pump power of 60.7W, the MOPA system outputted a repetition frequency of 300Hz, a single pulse energy of 73.7mJ, a pulse width of 30ns, an RMS (30min) of 1.17%, and a beam quality M of y-axis and x-axis through polarization beam splitter III9. y 2 , M x 2 They are 1.21 and 1.30 respectively.

[0050] In order to analyze the influence of the 4f optical imaging system on the amplified light energy and beam quality in the present invention, in the experiment, the following three cases of M after amplification by the first-stage amplifier 10 are first discussed. 2 : One-way magnification, 0° reflector 15 is placed at position A ( Figure 2 ) and ordinary double-pass amplification without the 4f optical imaging system (i.e., double-pass amplification with a quarter-wave plate and a 0° reflector placed in sequence near the first-stage amplifier 10). The results are as follows Figure 4 As shown. Then, further analysis is made on the 0° reflector 15 placed at position A ( Figure 2 The relationship between pump power and amplified light energy in the double-pass amplification of the 4f optical imaging system and the ordinary double-pass amplification without 4f optical imaging system is shown in the figure. Figure 5 As shown. Figure 4 and Figure 5 It can be seen that, taking the pump power of 60.7W as an example, when the 4f optical imaging system is added to the double-pass amplification system, the single pulse energy of the amplified light remains almost unchanged compared with the ordinary double-pass amplification system, and the beam quality M of the y-axis and x-axis is y 2 , M x 2 They are improved by 0.3 and 0.2 respectively. This is because the 4f optical imaging system can improve the misalignment sensitivity of the laser system, eliminate the inhomogeneity and astigmatism of the laser, and thus improve the beam quality of the amplified light. However, since the spherical aberration introduced by the thermal effect of the Nd:YAG crystal in the first-stage amplifier 10 is not compensated, the beam quality of the double-pass amplification with the 0° reflector 15 placed at position A is still lower than that of the single-pass amplification.

[0051] In order to verify the effect of passive spherical aberration compensation using the heat of the gain medium (Nd:YAG) in the present invention, in the experiment, the position of the 0° reflector 15 under different pump powers was first adjusted using a manual translation stage so that it was accurately placed at the focus position B of the single-pass amplified light of the first-stage amplifier 10. Then, M 2 Beam U3 measures the M corresponding to the amplified light of the first-stage amplifier 10 in single-pass amplification under different pump powers. 2 , and the 0° reflector 15 is respectively placed at the focal position B ( Figure 2 ), position A 1m in front of the focus ( Figure 2 ) and the position C( Figure 2 ) when the light is amplified 2 ,like Figure 6 As shown. Figure 6It can be seen that when double-pass amplification is used and the 0° reflector 15 is placed at position A or C, the beam quality at different pump powers is worse than that of single-pass amplified light. This is because the absolute value of the laser parameter p is quite different after passing through the Nd:YAG crystal and the convex lens Ⅰ11 and convex lens Ⅱ13 in the 4f optical imaging system twice during the amplification process. Therefore, the spherical aberration introduced by the thermal lens of the crystal and the position error of the convex lens Ⅰ11 and convex lens Ⅱ13 in the 4f optical imaging system when the laser is amplified by the first-stage amplifier 10 cannot offset the spherical aberration during the second pass, resulting in the M of the amplified light. rq 2 increases, and the beam quality deteriorates. However, when the 0° reflector 15 is placed at the focal position B, the beam quality of the double-pass amplified light is better than that of the single-pass amplified light. This is because the spherical aberration introduced by the two passes through the Nd:YAG crystal with the same thermal effect before and after the placement of the 0° reflector 15 can be fully compensated. Similarly, in the 4f optical imaging system, the spherical aberration introduced by the position error of the convex lens Ⅰ11 and the convex lens Ⅱ13 can also be fully compensated. Therefore, in the double-pass amplification process of the first-stage amplifier 10, only when the 0° reflector 15 is placed at the focal position B of the single-pass amplified light of the first-stage amplifier 10 can the problem of the degradation of the amplified light beam quality caused by the thermal lens effect of the Nd:YAG crystal be effectively compensated.

[0052] Next, the light field distribution of the amplified light when the 0° reflector 15 is placed at positions A, B, and C, respectively, is tested. Figure 7 This is the light field shape of the double-pass amplified light at 25cm and 120cm from the light outlet when the 0° reflector 15 is placed at positions A, B, and C respectively under the condition of a pump power of 60.7W. It can be seen that when the 0° reflector 15 is placed at positions A and C, the light field at a distance of 25cm from the light outlet is severely distorted; and when the 0° reflector 15 is placed at position B, the light field at a distance of 25cm from the light outlet presents an approximate Gaussian distribution. At the same time, no matter where the 0° reflector 15 is placed among A, B, and C, the light field distribution at a distance of 120cm from the light outlet has no obvious distortion. This is because when the 0° reflector 15 is placed at position A or C, the focus of the returning laser is at position 3 or position 2 of the vacuum tube 12 ( Figure 8 ) is located at a position away from the common focal position of the convex lens Ⅰ11 and the convex lens Ⅱ13 in the 4f optical imaging system ( Figure 8 The laser cannot be focused correctly, which leads to different degrees of distortion in the light field shape of the output laser. When the 0° reflector 15 is at position B, the focus of the return light is exactly at position 1, and the 4f optical imaging system can effectively perform spatial filtering, thereby achieving optimal optical imaging and wavefront modulation, and obtaining the amplified light effect of a near-Gaussian light field.

[0053] The new double-pass amplifier can not only significantly improve the beam quality and light field distribution of the amplified light, but also effectively improve the amplification efficiency of the first-stage amplifier module 10. Because the heat of the Nd:YAG crystal is almost the same when the laser passes through the first-stage amplifier module 10 twice, the phase difference caused by the laser passing through the crystal for the first time can be compensated for the second time, thereby effectively reducing the depolarization effect caused by the thermal stress birefringence of the crystal. The proportion of depolarization in the amplified light to the total energy is reduced from 7.4% to 1.2%. Finally, as Fig. 9 As shown in Figure 2, when the pump power is 60.7W, compared with the single-pass amplification, the new double-pass amplification (i.e. Figure 4 When the 0° reflector 15 is placed in position B), the energy of the output light increases from 47.1 mJ to 73.7 mJ, and the light-to-light conversion efficiency also increases from 18.1% to 31.4%.

[0054] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A MOPA laser system for improving output efficiency, light field distribution and beam quality, characterized in that: It comprises a nanosecond laser oscillator (1), an isolator, a concave lens (6) and a two-way amplifier which are connected in sequence, wherein the centers of the nanosecond laser oscillator (1), the isolator and the concave lens (6) are arranged on the same straight line, and the concave lens (6) and the two-way amplifier are matched; The low-energy nanosecond pulse laser generated by the nanosecond laser oscillator (1) enters the isolator and passes through the concave lens (6) to adjust the laser spot size. It then enters the dual-pass amplifier from one end of the dual-pass amplifier for amplification, is reflected from the other end of the dual-pass amplifier, is amplified again, and is output from one end of the dual-pass amplifier.

2. The MOPA laser system for improving output efficiency, light field distribution and beam quality according to claim 1, characterized in that: The isolator comprises a polarization beam splitter I (2), a half wave plate (3), a Faraday rotator (4) and a polarization beam splitter II (5) which are connected in sequence; the centers of the polarization beam splitter I (2), the half wave plate (3), the Faraday rotator (4) and the polarization beam splitter II (5) are arranged on the same straight line; the centers of the nanosecond laser oscillator (1) and the polarization beam splitter I (2) are arranged on the same straight line; and the centers of the polarization beam splitter II (5) and the concave lens (6) are arranged on the same straight line.

3. The MOPA laser system for improving output efficiency, light field distribution and beam quality according to claim 2, characterized in that: The double-pass amplifier comprises a polarization beam splitter III (9), a first-stage amplifier (10), 4 f Optical imaging system, quarter wave plate (14) and 0° reflector (15), polarization beam splitter III (9), first stage amplifier (10), 4 f The center points of the optical imaging system, the quarter wave plate (14) and the 0° reflector (15) are arranged on the same straight line, and the polarization beam splitter III (9) is matched with the concave lens (6) through a reflection module.

4. The device for improving the output efficiency, light field distribution and beam quality of a MOPA laser system according to claim 3, characterized in that: 4 f The optical imaging system comprises a convex lens I (11), a vacuum tube (12) and a convex lens II (13) which are arranged in sequence. The centers of the convex lens I (11), the vacuum tube (12) and the convex lens II (13) are arranged on the same straight line. The center point of the convex lens I (11) and the first-stage amplifier (10) are arranged on the same straight line.

5. The MOPA laser system for improving output efficiency, light field distribution and beam quality according to claim 3 or 4, characterized in that: A gain medium is provided in the first-stage amplifier (10), one end of the gain medium matches the output end of the polarization beam splitter III (9), and the other end of the gain medium matches one end of the convex lens I (11).

6. The MOPA laser system for improving output efficiency, light field distribution and beam quality according to claim 5, characterized in that: The 0° reflector (15) is arranged at the focus of the laser light after the laser light is amplified in one pass by the first-stage amplifier (10).

7. The MOPA laser system for improving output efficiency, light field distribution and beam quality according to claim 6, characterized in that: The reflection module comprises a reflector I (7) and a reflector II (8), one end of the reflector I (7) matches the concave lens (6), the other end of the reflector I (7) matches the reflector II (8), and the reflector II (8) matches the polarization beam splitter III (9).

8. The MOPA laser system for improving output efficiency, light field distribution and beam quality according to claim 7, characterized in that: The reflector I (7) and the reflector II (8) are both 45° reflectors.

9. The MOPA laser system for improving output efficiency, light field distribution and beam quality according to claim 8, characterized in that: The angle between the reflector I (7) and the reflector II (8) is 90°.

10. The MOPA laser system for improving output efficiency, light field distribution and beam quality according to any one of claims 7 to 9, characterized in that: The gain medium is Nd:YAG crystal.

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