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

Abstract

The invention 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 steps: monitoring the working temperature of KDP type crystals, the KDP type crystals comprise KDP crystals or DKDP crystals; and the monitored working temperature of the KDP crystal is stabilized at the set sub-phase transition temperature. By means of the method and the laser device, the crystal ultraviolet optical frequency conversion efficiency can be effectively improved, and large-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] As one of the most likely ways to achieve controlled thermonuclear fusion, inertial confinement fusion (ICF) is expected to provide almost unlimited clean energy for mankind. Potassium dihydrogen phosphate (chemical formula KH2PO4, abbreviated as KDP) and its deuteride (chemical formula KDxH(2-x)PO4, abbreviated as DKDP) have excellent properties such as wide transmission range, large laser-induced damage threshold (LITD), large nonlinear optical coefficient, can be grown to large size and good processability. It is currently the only nonlinear optical crystal material that can meet the light aperture of ICF laser drive device. In the ICF laser device, Z-cut KDP crystal is used as an optical switch, type I cut KDP crystal is used as a frequency doubling device, and type II cut DKDP crystal with a deuterium content of about 70% is used as a mixing device. The latter two types of crystals jointly complete the process of converting the fundamental frequency laser (wavelength of 1053nm) into the triple frequency laser (wavelength of 351nm) in the terminal optical component.

[0003] With the continuous development of ICF engineering, the overall quality of KDP and DKDP crystals has reached a very high level. Both the Z-cut KDP crystals used as optical switches and the Class I KDP crystals used as frequency doubling devices have met the current ICF design flux requirements in terms of laser-induced damage threshold and frequency conversion efficiency. However, the laser damage problem of KDP crystals in the ultraviolet band seriously affects the energy output of tripled frequency light and the crystal life, and cannot fully meet the ICF laser driver operating flux requirements.

[0004] A large number of studies have shown that under high-flux UV laser conditions, defect-assisted multiphoton absorption dominates the laser damage problem of KDP-type crystals. However, it is currently impossible to establish a direct relationship between electronic defects and crystal structure characteristics, so it is difficult to provide the most direct guidance for crystal growth process control. Despite this, there are already some laser pretreatment methods that can effectively suppress the nonlinear absorption of KDP-type crystal materials, improve the crystal's resistance to UV laser light damage, and improve the triple frequency performance of KDP and DKDP crystals. At present, except for thermal annealing pretreatment, the effect of temperature on the crystal's light damage 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 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 a 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 lasers.

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

[0008] 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] The present invention also 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] On the other hand, the present invention discloses a laser device for improving the frequency conversion performance of crystal ultraviolet optics, 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 direction of the short pulse laser;

[0017] Among them, the tripled frequency crystal is a KDP crystal or a 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 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.

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

[0019] Preferably, the short pulse laser comprises 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 triple frequency 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 solutions 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 pretreatment mainly eliminates crystal stress and reduces crystal defects, and is an offline processing method. Its impact on the crystal is permanent and has nothing to do with the frequency conversion temperature of the crystal. The crystal temperature is usually set below 30°C during actual operation. The present invention proposes for the first time that KDP and DKDP crystals are frequency-converted at a preset sub-phase transition temperature, especially 130°C. This is an online working method at the sub-phase transition temperature. While eliminating stress and reducing defects, it reduces the Raman scattering cross section and two-photon absorption coefficient, which is more effective for improving the light damage resistance threshold of the crystal. Its impact on the crystal is temporary. 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 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 embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0025] Figure 1 Schematic diagram of the structure of the laser device for improving the triple 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 a front mirror of the beam reduction system, 3 is a rear mirror of the beam reduction system, 4 is a double frequency crystal, 5 is a focusing mirror, 6 is a triple frequency crystal, 7 is a temperature control device, 8 is an angle rotation table, and 9 is a triple frequency UV filter.

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

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

[0028] Figure 4 The Z scan data of the KDP and DKDP crystals at 355 nm with variable temperature and opening in Example 3 of the present invention.

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

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

[0031] Example 1

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

[0033] Monitor the working temperature of the KDP crystal; use the temperature control system to stabilize the monitored working temperature of the KDP crystal at a set sub-phase transition temperature, and the sub-phase transition temperature is specifically selected to be 130°C, that is, the problem of the KDP crystal is stabilized 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 crystal, 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 sequentially arranged along the optical path direction of the short pulse laser; wherein 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, which can regulate the temperature in the sealed temperature control container and allow the incident light to sequentially 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. The incident end face and the exit end face of the specific temperature control device can be selected as a high-transmittance quartz window, and 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 which 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 fundamental frequency light after beam reduction 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 control container with light passing through both ends, which can regulate the temperature in 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 6 inside the sealed temperature control container, and the exit end of the sealed temperature control 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 the 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 triple frequency crystal 6 is specifically a type II KDP crystal. The output laser of the short pulse laser 1 is different, and the processing angle of the triple frequency crystal 6 is slightly different.

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

[0042] Furthermore, the temperature control device 7 is placed on an angle rotation table 8, and 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 a sub-phase transition temperature of 130°C and a room temperature of 21°C. The verification results are as follows: Figure 2 As shown:

[0045] It can be seen that when the temperature control device 7 is used to control the working temperature of the KDP crystal 6 at the sub-phase transition temperature of 130°C, its maximum triple frequency output energy and maximum 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 triple frequency output energy, maximum triple frequency conversion efficiency and laser-induced damage threshold are increased by 16.5%, 4.8% and 17.3% respectively.

[0046] Example 2

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

[0048] Monitor the working temperature of the KDP crystal; use the temperature control system to stabilize the monitored working temperature of the DKDP crystal at the set sub-phase transition temperature, and the sub-phase transition temperature is specifically selected to be 130°C, that is, the problem of the KDP crystal is stabilized 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 crystal. 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 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 arranged in sequence along the optical path direction of the short pulse laser; wherein the triple frequency crystal 6 is a KDP crystal whose triple frequency performance is 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, which can regulate the temperature in the sealed temperature control container and allow the 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. The incident end face and the exit end face of the specific temperature control device can be selected as a high-transmittance quartz window, and the internal working space of the temperature control device is heated by a heating element inside the temperature control device.

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

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

[0053] 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 a sub-phase transition temperature of 130°C and a room temperature of 21°C. The verification results are as follows: Figure 3 As shown:

[0054] It can be seen that when the temperature control device 7 is used to control the operating temperature of the DKDP crystal 6 at the sub-phase transition temperature of 130°C, its maximum triple frequency output energy and maximum triple frequency conversion efficiency are 2mJ and 32.33% respectively, and the laser induced damage threshold is 2.44GW / cm 2 Compared with room temperature of 21°C, when the DKDP crystal temperature is 130°C, its maximum triple frequency output energy, maximum triple frequency conversion efficiency and laser-induced damage threshold are increased by 4.2%, 2.7% and 13.5% respectively.

[0055] Long-term verification in many aspects has shown that setting the operating temperature of KDP crystal or DKDP crystal at the sub-phase transition temperature of 130°C can significantly improve the performance of the crystal. In order to further verify the effect of the sub-phase transition temperature on the performance of the crystal, the present invention also discloses Examples 3 and 4 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] Take the type II KDP and type II 70% DKDP thin wafers adjacent to the original crystal cut from the triple frequency crystal in Example 1. The wafer thickness is 2 mm, which is much smaller than the Rayleigh length of the Z scanning device. The open hole Z scanning method is used to detect the variable temperature nonlinear absorption performance under a short pulse laser with a wavelength of 355 nm. The specific detection 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 the beam splitter (50:50), a beam of comparison light enters the energy meter, and the other beam of experimental light is focused by the focusing lens onto a sample that can move back and forth, and 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 the front of the focus of the focusing lens (-Z) along the optical axis to the focus (Z=0), and continues to move to the back of the focus (+Z). Since the nonlinear optical properties of the material will cause changes in transmittance, the normalized transmittance curve that changes with the Z axis can be measured, and the nonlinear optical properties of the device can be tested through data analysis.

[0060] Attached Figure 4 (a) is the temperature-dependent open-aperture Z-scan data of type II KDP crystal slice at 355 nm. The horizontal axis represents the sample position and the vertical axis represents the normalized transmittance. When the incident energy is fixed and the temperature is 21℃, 70℃, 90℃ and 130℃, the two-photon absorption coefficient of type II KDP crystal slice is 21.81±0.32×10 -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] Attached Figure 4 (b) is the temperature-dependent open-aperture Z-scan data of type II 70% DKDP crystal slice at 355 nm. The horizontal axis represents the sample position and the vertical axis represents the normalized transmittance. When the incident energy is fixed and the temperature is 21℃, 70℃, 90℃ and 130℃, the two-photon absorption coefficient of type II 70% DKDP crystal slice is 35.41±0.62×10 -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 type II KDP crystal thin film decreased by 44.65% at 130°C, and the two-photon absorption coefficient of type II DKDP crystal thin film decreased by 67%, indicating that increasing temperature can effectively inhibit the nonlinear absorption effect of these two crystals in the ultraviolet band, which is beneficial to enhancing the crystal's resistance to light damage.

[0063] Example 4

[0064] Example 4 discloses the detection process of reducing the Raman gain coefficient of KDP crystal at a sub-phase transition temperature of 130°C.

[0065] Take a Type II KDP thin wafer adjacent to the original crystal cut from the triple frequency crystal in Example 1, the wafer thickness is 1 mm, put the KDP wafer in a spontaneous Raman temperature change table, and perform a temperature change spontaneous Raman test under a laser with a wavelength of 632.8 nm. The test results are shown in the attached figure. Figure 5 shown.

[0066] As the temperature rises from room temperature to 130℃, the frequency and intensity of the PO stretching characteristic peak of KDP crystal gradually decreases from 915.9cm at 21℃ to -1 Drop to 912.7cm at 130℃ -1 , the intensity value dropped from 11278 at 21℃ to 8108 at 130℃, and the relative intensity decreased 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, thereby reducing the Raman gain coefficient. This shows that the stimulated Raman scattering effect of type II KDP crystal will be suppressed under high temperature working conditions, which is beneficial to enhance the crystal's ability to resist laser damage.

[0067] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments 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 relevant parts can be referred to the method part.

[0068] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather 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 crystal, 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.

2. A method for improving the ultraviolet optical frequency conversion performance of KDP crystal according to claim 1, characterized in that: The sub-phase transition temperature was set to 130 °C.

3. A method for improving the ultraviolet optical frequency conversion performance of a DKDP crystal, 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.

4. A method for improving the ultraviolet optical frequency conversion performance of KDP crystal according to claim 3, characterized in that: The sub-phase transition temperature was set to 130 °C.

5. 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 direction 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 control the temperature in 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.

6. A laser device for improving the ultraviolet optical frequency conversion performance of a crystal according to claim 5, characterized in that: The temperature control device (7) is arranged on the angle rotating platform (8).

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

8. A laser device for improving the ultraviolet optical frequency conversion performance of KDP and DKDP crystals according to claim 5, 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).

9. A laser device for improving the ultraviolet optical frequency conversion performance of KDP and DKDP crystals according to claim 5, 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.

Images