Method for improving performance of KDP and DKDP crystal in ultraviolet optical frequency conversion and laser device

Abstract

The application discloses a method for improving the ultraviolet optical frequency conversion performance of KDP and DKDP crystals and a laser device, and the method comprises the following steps: monitoring the working temperature of KDP crystals, wherein the KDP crystals comprise KDP crystals or DKDP crystals; and stabilizing the monitored working temperature of the KDP crystals at a set sub-phase transition temperature. Through the method and the laser device, the ultraviolet optical frequency conversion efficiency of the crystals can be effectively improved, and high-energy ultraviolet laser can be output.

Description

Technical Field

[0001] The present invention relates to the technical field of crystal performance improvement testing, and more particularly to a method and a laser device for improving the ultraviolet optical frequency conversion performance of KDP and DKDP crystals. Background Art

[0002] Inertial confinement fusion (ICF), one of the most promising approaches to achieving controlled thermonuclear fusion, promises to provide humanity with virtually unlimited clean energy. Potassium dihydrogen phosphate (KH2PO4, KDP) and its deuteride (KDxH(2-x)PO4, DKDP) exhibit excellent properties, including a wide transmission range, a high laser-induced damage threshold (LITD), a large nonlinear optical coefficient, the ability to grow to large sizes, and good machinability. Currently, they are the only nonlinear optical crystal materials that meet the optical aperture requirements of ICF laser driver devices. In ICF laser devices, Z-cut KDP crystals serve as optical switches, Type I-cut KDP crystals serve as frequency-doubling devices, and Type II-cut DKDP crystals with approximately 70% deuterium content serve as frequency mixers. These latter two types of crystals, in the terminal optical assembly, convert the fundamental laser (1053 nm wavelength) into a frequency-tripled laser (351 nm wavelength).

[0003] With the continuous development of ICF technology, the overall quality of KDP and DKDP crystals has reached a very high level. Both Z-cut KDP crystals used as optical switches and Class I KDP crystals used as frequency-doubling devices meet the current ICF design flux requirements in terms of laser-induced damage threshold and frequency conversion efficiency. However, laser damage to KDP crystals in the UV band seriously affects the energy output of the tripled frequency light and the crystal lifespan, making them unable to fully meet the operating flux requirements of ICF laser drivers.

[0004] Numerous studies have shown that defect-assisted multiphoton absorption dominates the laser damage problem in KDP-type crystals under high-flux UV laser conditions. However, a direct relationship between electronic defects and crystal structural properties cannot currently be established, making it difficult to provide the most direct guidance for controlling the crystal growth process. Despite this, several laser pretreatment methods have been developed that can effectively suppress the nonlinear absorption of KDP-type crystal materials, improve the crystal's resistance to UV laser damage, and enhance the triple frequency performance of KDP and DKDP crystals. Currently, with the exception of thermal annealing pretreatment, the effect of temperature on the crystal's photodamage threshold during nonlinear frequency conversion has not been reported.

[0005] Therefore, how to improve the frequency tripling performance of KDP and DKDP crystals is an urgent problem that needs to be solved by those skilled in the art. Summary of the Invention

[0006] In view of this, the present invention provides a method and laser device for improving the ultraviolet optical frequency conversion performance of KDP and DKDP crystals, which are used to improve the triple frequency performance of KDP and DKDP crystals and further output high-energy ultraviolet laser.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] In one aspect, the present invention discloses a method for improving the ultraviolet optical frequency conversion performance of a KDP crystal, comprising the following steps:

[0009] Monitor the operating temperature of the KDP crystal;

[0010] The monitored operating temperature of the KDP crystal is stabilized at a set sub-phase transition temperature.

[0011] In the above embodiment, the sub-phase transition temperature is set to 130°C.

[0012] In one aspect, the present invention further discloses a method for improving the ultraviolet optical frequency conversion performance of a DKDP crystal, comprising the following steps:

[0013] Monitor the operating temperature of the DKDP crystal;

[0014] The monitored operating temperature of the DKDP crystal is stabilized at a set sub-phase transition temperature.

[0015] In the above embodiment, the sub-phase transition temperature is set to 130°C.

[0016] Another aspect of the present invention discloses a laser device for improving the ultraviolet optical frequency conversion performance of a crystal, comprising a short pulse laser, and a front mirror of a beam reduction system, a rear mirror of a beam reduction system, a double frequency crystal, a focusing mirror, a triple frequency crystal, and a triple frequency ultraviolet filter arranged in sequence along the optical path of the short pulse laser;

[0017] Among them, the tripled frequency crystal is a KDP crystal or DKDP crystal to be upgraded, and is arranged in a temperature control device, and the laser incident angle of the tripled frequency crystal matches the processing angle of the tripled frequency crystal; the temperature control device is a sealed temperature-controlled container with light passing through both ends, which can regulate the temperature inside the sealed temperature-controlled container and allow the incident light to pass through the incident end of the sealed temperature-controlled container, the tripled frequency crystal inside the sealed temperature-controlled container, and the output end of the sealed temperature-controlled container in sequence.

[0018] Preferably, the temperature control device is arranged on an angle rotating table.

[0019] Preferably, the short pulse laser includes a 1064 nm Nd:YAG pulse laser or a 1053 nm Nd:YLF pulse laser.

[0020] Preferably, the focal length of the focusing mirror is 500 mm, and the focus is located in front of the frequency tripling crystal.

[0021] Preferably, the front mirror of the shrinking system and the rear mirror of the shrinking system constitute a shrinking system, and the shrinking ratio of the shrinking system is 3:1.

[0022] It can be seen from the above technical solution that, compared with the prior art, the present invention discloses a method for improving the ultraviolet optical frequency conversion performance of KDP and DKDP crystals and a corresponding laser device, which has the following beneficial effects:

[0023] Previous thermal annealing pretreatments were mainly used to eliminate crystal stress and reduce crystal defects, and were an offline processing method. Their impact on the crystal is permanent and has nothing to do with the frequency conversion temperature of the crystal. During actual operation, the crystal temperature is usually set below 30°C. This invention, for the first time, proposes that KDP and DKDP crystals undergo frequency conversion at a preset subphase transition temperature, particularly 130°C. This is an online working method at the subphase transition temperature, which eliminates stress and reduces defects while reducing the Raman scattering cross section and two-photon absorption coefficient, making it more effective in improving the crystal's light damage resistance threshold. Its impact on the crystal is temporary, and as the crystal temperature decreases, this optimization effect gradually weakens until it disappears. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0025] Figure 1 This is a schematic diagram of the structure of the laser device for improving the tripled frequency performance of KDP-type crystals as described in Examples 1 and 2 of the present invention, wherein 1 is a short pulse laser, 2 is the front mirror of the beam reduction system, 3 is the rear mirror of the beam reduction system, 4 is the doubled frequency crystal, 5 is the focusing mirror, 6 is the tripled frequency crystal, 7 is the temperature control device, 8 is the angle rotation stage, and 9 is the tripled frequency UV filter.

[0026] Figure 2 These are experimental data obtained for the solution to improve the triple frequency performance of KDP crystals in Example 1 of the present invention.

[0027] Figure 3 These are experimental data obtained for the solution to improve the triple frequency performance of the DKDP crystal by 70% in Example 2 of the present invention.

[0028] Figure 4 This is the variable temperature open-aperture Z-scan data of KDP and DKDP crystals at 355 nm in Example 3 of the present invention.

[0029] Figure 5 This is the temperature-dependent spontaneous Raman test data of the KDP crystal in Example 4 of the present invention. DETAILED DESCRIPTION

[0030] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative efforts are within the scope of protection of the present invention.

[0031] Example 1

[0032] Example 1 first discloses a method for improving the triple frequency performance of a KDP crystal, comprising the following steps:

[0033] Monitor the operating temperature of the KDP crystal; use the temperature control system to stabilize the monitored operating temperature of the KDP crystal at the set sub-phase transition temperature, specifically 130°C, that is, stabilize the KDP crystal at 130°C to improve the triple frequency performance of the KDP crystal.

[0034] Based on the above method, this embodiment also discloses a laser device for improving the ultraviolet optical frequency conversion performance of KDP crystals, such as Figure 1 As shown, it includes a short pulse laser 1, and a front mirror 2 of a beam reduction system, a rear mirror 3 of a beam reduction system, a double frequency crystal 4, a focusing mirror 5, a triple frequency crystal 6, and a triple frequency ultraviolet filter 9, which are arranged in sequence along the optical path of the short pulse laser. The triple frequency crystal 6 is a KDP crystal whose triple frequency performance needs to be improved and is arranged in a temperature control device 7. The temperature control device 7 is a sealed temperature control container with light passing through both ends. It can regulate the temperature within the sealed temperature control container and allow incident light to pass through the incident end of the sealed temperature control container, the triple frequency crystal 6 inside the sealed temperature control container, and the exit end of the sealed temperature control container in sequence. Specifically, the incident end face and the exit end face of the temperature control device can be selected as high-transmittance quartz windows. The internal working space of the temperature control device is heated by a heating element inside the temperature control device.

[0035] The temperature control range of the temperature control device in the above embodiment is from room temperature to 180°C, and the temperature control accuracy is 0.1°C.

[0036] In the above embodiment, the short pulse laser 1, which serves as a pump source, emits fundamental frequency light that is spatially shaped by a beam reduction system composed of a front mirror 2 and a rear mirror 3 of the beam reduction system to increase the power density of the fundamental frequency light. The beam-reduced fundamental frequency light enters the doubled frequency crystal 4 to generate doubled frequency light. The fundamental frequency light and the doubled frequency light are focused by a focusing lens 5 to increase the power density of the two. The focus is located in front of the tripled frequency crystal 6. The tripled frequency crystal 6 is placed in a temperature control device 7, and the laser incident angle of the tripled frequency crystal 6 (the angle between the laser and the incident surface of the tripled frequency crystal) matches the processing angle of the tripled frequency crystal; the temperature control device 7 is a sealed temperature-controlled container with light passing through both ends, which can regulate the temperature inside the sealed temperature-controlled container and allow the incident light to pass through the incident end of the sealed temperature-controlled container, the tripled frequency crystal 6 inside the sealed temperature-controlled container, and the exit end of the sealed temperature-controlled container in sequence.

[0037] When the operating temperature of the tripled frequency crystal (KDP crystal) 6 is set at the sub-phase transition temperature, especially 130°C, the mixed light generated by the tripled frequency crystal (KDP crystal) 6 is filtered out by the tripled frequency ultraviolet filter to remove the fundamental frequency light and the doubled frequency light, and the final output performance is improved and optimized tripled frequency light.

[0038] In the above embodiment, the ultraviolet filter is used to observe ultraviolet light energy, and both ends of the ultraviolet filter are coated with dielectric films that are highly reflective of pump light and doubled frequency light and highly transparent to ultraviolet light.

[0039] In the above embodiment, the tripled frequency crystal 6 is specifically a type II KDP crystal. The output lasers of the short pulse laser 1 are different, and the processing angles of the tripled frequency crystal 6 are slightly different.

[0040] When the short pulse laser 1 selects an Nd:YAG short pulse laser with a central wavelength of 1064 nm, its processing angle is (θ=59.4°, φ=0°), and the corresponding doubled frequency crystal 4 is specifically selected as a KDP crystal that meets type I phase matching, and the processing angle is θ=41°, φ=45°.

[0041] When the short pulse laser 1 selects a Nd:YLF pulse laser with a central wavelength of 1053 nm, the processing angle of the type II KDP crystal 6 is (θ=60.1°, φ=0°), and the doubled frequency crystal 4 is also selected as a KDP crystal that meets type I phase matching, with a processing angle of θ=41°, φ=45°.

[0042] Furthermore, the temperature control device 7 is placed on the angle rotation table 8. The angle resolution of the angle rotation table 8 is 0.1°, which can fine-tune the angle of the tripled frequency crystal 6. Since errors are inevitable during the processing of the tripled frequency crystal, the angle rotation table 8 is required to fine-tune the tripled frequency crystal (incident angle) so that the angle can match the processing angle (phase matching angle) of the crystal (the incident angle and the crystal processing angle are aligned with each other).

[0043] In the above embodiment, the reduction ratio of the reduction system is set to 3: 1, that is, the ratio of the initial laser spot size to the laser spot size after focusing is 3: 1. In the above embodiment, the focal length of the focusing lens 5 is 500 mm.

[0044] In order to verify the superiority of the present invention, the performance of the triple frequency laser output by the laser device was compared and verified at the sub-phase transition temperature of 130°C and the room temperature of 21°C. The verification results are as follows: Figure 2 As shown:

[0045] It can be seen that when the operating temperature of the KDP crystal 6 is controlled by the temperature control device 7 at the sub-phase transition temperature of 130°C, its maximum triple frequency output energy and the highest triple frequency conversion efficiency are 1.91mJ and 33.67% respectively, and the laser-induced damage threshold is 3.39GW / cm 2 Compared with room temperature (21°C), when the KDP crystal temperature is 130°C, its maximum tripled output energy, highest tripled conversion efficiency and laser-induced damage threshold are increased by 16.5%, 4.8% and 17.3% respectively.

[0046] Example 2

[0047] Example 2 first discloses a method for improving the triple frequency performance of a DKDP crystal, comprising the following steps:

[0048] Monitor the operating temperature of the KDP crystal; use the temperature control system to stabilize the monitored operating temperature of the DKDP crystal at the set sub-phase transition temperature, specifically 130°C, that is, stabilize the KDP crystal at 130°C to improve the triple frequency performance of the DKDP crystal.

[0049] Based on the above method, this embodiment also discloses a laser device for improving the ultraviolet optical frequency conversion performance of DKDP crystals. The overall structure of the device is similar to the structure of the laser device for improving the ultraviolet optical frequency conversion performance of KDP in Example 1, except that the doubled frequency crystal 4 and the tripled frequency crystal 6 in the temperature control device are both DKDP crystals.

[0050] like Figure 1As shown, the laser device for improving the ultraviolet optical frequency conversion performance of the DKDP crystal comprises a short pulse laser 1, and a pre-beam-reducing system front mirror 2, a pre-beam-reducing system rear mirror 3, a second harmonic generation crystal 4, a focusing mirror 5, a third harmonic generation crystal 6 and a third harmonic generation ultraviolet filter 9 arranged in sequence along the light path direction of the short pulse laser; wherein the third harmonic generation crystal 6 is a KDP crystal with a third harmonic generation performance to be improved and is arranged in a temperature control device 7. The temperature control device 7 is a sealed temperature control container with light passing through both ends, capable of controlling the temperature inside the sealed temperature control container, and enabling the incident light to pass through the incident end of the sealed temperature control container, the third harmonic generation crystal 6 inside the sealed temperature control container and the exit end of the sealed temperature control container in sequence. The incident end face and the exit end face of the specific temperature control device can be selected as high-transparency quartz windows, and the inside of the temperature control device is heated by a heating element to heat the working space inside.

[0051] In the above device, when the short pulse laser 1 is a Nd:YAG short pulse laser with a center wavelength of 1064 nm, the third harmonic generation crystal 6 is a type II DKDP crystal with a processing angle of (θ = 60.2°, φ = 0°).

[0052] When the short pulse laser 1 is a Nd:YLF pulse laser with a center wavelength of 1053 nm, the third harmonic generation crystal 6 is a type II DKDP crystal with a processing angle of (θ = 61°, φ = 0°).

[0053] Also to verify the superiority of the present application, the performance of the third harmonic generation laser output by the laser device is compared and verified at a sub-transition temperature of 130°C and a room temperature of 21°C, and the verification results are as shown in Figure 3

[0054] As can be seen from the above, when the working temperature of the DKDP crystal 6 is controlled by the temperature control device 7 at a sub-transition temperature of 130°C, the maximum third harmonic generation output energy and the highest third harmonic generation conversion efficiency are 2 mJ and 32.33%, respectively, and the laser-induced damage threshold is 2.44 GW / cm 2 . Compared with the room temperature of 21°C, the maximum third harmonic generation output energy, the highest third harmonic generation conversion efficiency and the laser-induced damage threshold of the DKDP crystal at 130°C are increased by 4.2%, 2.7% and 13.5%, respectively.

[0055] Long-term verification from multiple aspects shows that setting the working temperature of the KDP crystal or the DKDP crystal at a sub-transition temperature of 130°C can greatly improve the performance of the crystal. In order to further verify the performance verification effect of the sub-transition temperature on the crystal, the present application also discloses that examples 3 and 4 are used to verify the performance improvement effect, as follows.

[0056] Example 3

[0057] ​Example 3 discloses a verification method and system for reducing the two-photon absorption coefficient of KDP and DKDP crystals at a sub-phase transition temperature of 130°C.

[0058] Type II KDP and Type II 70% DKDP thin wafers were taken from adjacent positions of the original crystal cut from the tripled frequency crystal in Example 1. The wafer thickness was 2 mm, which was much smaller than the Rayleigh length of the Z scanning device. The temperature-dependent nonlinear absorption performance was tested using the open-aperture Z scanning method under a short-pulse laser with a wavelength of 355 nm. The specific testing method is as follows:

[0059] The laser wavelength is 355nm, the laser pulse is 20ps, the pulse repetition frequency is 10Hz, and the input energy range is 0-100μJ. After the laser beam passes through a beam splitter (50:50), a comparison beam enters the energy meter, and the other experimental beam is focused by a focusing lens onto a sample that can move back and forth. The light that passes through the sample enters another identical energy meter. During the measurement process, the sample to be measured is placed in a temperature-controlled furnace 7 and moves from in front of the focus of the focusing lens (-Z) along the optical axis toward the focus (Z=0), and then continues to move behind the focus (+Z). Because the nonlinear optical properties of the material can cause changes in transmittance, a curve of the normalized transmittance changing with the Z axis can be measured. Through data analysis, the nonlinear optical properties of the device can be verified.

[0060] Attachment Figure 4 (a) shows the open-aperture Z-scan data of a type II KDP crystal sheet at 355 nm under variable temperature. The horizontal axis represents the sample position, and the vertical axis represents the normalized transmittance. The two-photon absorption coefficients of the type II KDP crystal sheet are 21.81 ± 0.32 × 10-1 at a fixed incident energy when the temperatures are 21°C, 70°C, 90°C, and 130°C, respectively. -2 cm / GW, 17.75±0.38×10 - 2 cm / GW, 14.79±0.31×10 -2 cm / GW and 12.08±0.28×10 -2 cm / GW.

[0061] Attachment Figure 4 (b) Temperature-dependent open-aperture Z-scan data of a Type II 70% DKDP crystal sheet at 355 nm. The horizontal axis represents the sample position, and the vertical axis represents the normalized transmittance. The two-photon absorption coefficients of the Type II 70% DKDP crystal sheet are 35.41 ± 0.62 × 10-1 at fixed incident energy and temperatures of 21°C, 70°C, 90°C, and 130°C, respectively. -2 cm / GW, 21.57±0.79×10 -2 cm / GW, 16.73±0.26×10 -2 cm / GW, 11.67±0.31×10-2 cm / GW.

[0062] Compared with room temperature, the two-photon absorption coefficient of the type II KDP crystal sheet at 130℃ is reduced by 44.65%, and the two-photon absorption coefficient of the type II DKDP crystal sheet is reduced by 67%, which indicates that the increase of temperature can effectively inhibit the nonlinear absorption effect of the two crystals in the ultraviolet band, and is beneficial to enhance the anti-laser damage ability of the crystal.

[0063] Embodiment 4

[0064] Embodiment 4 discloses the detection process of reducing the Raman gain coefficient of KDP crystal at sub-transition temperature 130℃.

[0065] Take the type II KDP thin crystal sheet of the original crystal cut by the third harmonic generation crystal in embodiment 1, and the thickness of the crystal sheet is 1mm. The KDP crystal sheet is placed in the spontaneous Raman temperature changing table, and the temperature changing spontaneous Raman test is carried out under the laser with a wavelength of 632.8nm. The detection result is shown in FIG. 6. Figure 5

[0066] From room temperature to 130℃, the P-O stretching characteristic peak frequency and intensity of the KDP crystal gradually decrease, from 915.9cm -1 at 21℃ to 912.7cm -1 at 130℃, and the intensity value decreases from 11278 at 21℃ to 8108 at 130℃, and the relative intensity decreases by 28.1%. According to the principle of steady-state stimulated Raman scattering, the Raman scattering cross section decreases with the decrease of peak intensity, and then the Raman gain coefficient is reduced. This indicates that the stimulated Raman scattering effect of the type II KDP crystal will be inhibited in the high temperature working state, which is beneficial to enhance the anti-laser damage ability of the crystal.

[0067] Each embodiment in the specification is described in a progressive manner, and each embodiment focuses on the difference from other embodiments. The same and similar parts between each embodiment can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the related parts can be referred to the method part.

[0068] The above description of the disclosed embodiments enables a person skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.​

Claims

1. A method for improving the ultraviolet optical frequency conversion performance of KDP crystals, characterized in that: The following steps are involved: Monitor the operating temperature of the KDP crystal; The monitored operating temperature of the KDP crystal is stabilized at a set sub-phase transition temperature, which is 130°C.

2. A method for improving the ultraviolet optical frequency conversion performance of DKDP crystals, characterized in that: The following steps are involved: Monitor the operating temperature of the DKDP crystal; The monitored operating temperature of the DKDP crystal is stabilized at a set sub-phase transition temperature, which is 130°C.

3. A laser device for improving the ultraviolet optical frequency conversion performance of a crystal, characterized in that: It comprises a short pulse laser (1), and a front mirror of a beam reduction system (2), a rear mirror of a beam reduction system (3), a double frequency crystal (4), a focusing mirror (5), a triple frequency crystal (6), and a triple frequency ultraviolet filter (9) which are sequentially arranged along the optical path of the short pulse laser. The tripled frequency crystal (6) is a KDP crystal or a DKDP crystal and is arranged in a temperature control device (7), and the laser incident angle of the tripled frequency crystal (6) matches the processing angle of the tripled frequency crystal; the temperature control device is a sealed temperature control container with light passing through both ends, which can regulate the temperature inside the sealed temperature control container and allow the incident light to pass through the incident end of the sealed temperature control container, the tripled frequency crystal inside the sealed temperature control container, and the exit end of the sealed temperature control container in sequence; the operating temperature of the tripled frequency crystal (6) is stabilized at a set sub-phase transition temperature, which is 130°C.

4. A laser device for improving the ultraviolet optical frequency conversion performance of a crystal according to claim 3, characterized in that: The temperature control device (7) is arranged on the angle rotating platform (8).

5. The laser device for improving the ultraviolet optical frequency conversion performance of a crystal according to claim 3, characterized in that: The short pulse laser (1) comprises a 1064nm Nd:YAG pulse laser or a 1053nm Nd:YLF pulse laser.

6. The laser device for improving the ultraviolet optical frequency conversion performance of a crystal according to claim 3, characterized in that: The focal length of the focusing mirror (5) is 500 mm, and the focus is located in front of the triple frequency crystal (6).

7. The laser device for improving the ultraviolet optical frequency conversion performance of a crystal according to claim 3, characterized in that: The front mirror (2) of the shrinking system and the rear mirror (3) of the shrinking system form a shrinking system, and the shrinking ratio of the shrinking system is 3:1.

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