Device and method for improving pointing stability of low-repetition-frequency laser

Abstract

The invention discloses a low-repetition-frequency laser pointing stability improving device and method, and belongs to the field of lasers. According to the invention, the seed laser pulse of the MHz magnitude is used as the reference light to adjust the directivity of the light path, and compared with the main laser repetition frequency of the Hz magnitude, the sampling frequency of the subsequent directivity adjustment is greatly improved, so that the laser auto-collimation precision is improved. The use of the first shutter, the second shutter and the third shutter ensures the safety of the first detector and the second detector in the transmission process of the main laser when the energy difference between the main laser and the reference light is 9 orders of magnitude. The first detector images the first grating of the compressor to ensure that the positions of laser entering the compressor are the same, so that the consistency of laser pulse width and directivity can be ensured to a great extent, the second detector images the acting target to ensure that the acting positions are the same, and the stability of a subsequent radiation source is ensured. The laser directivity correction method is applied to laser directivity correction of hundred TW-level and PW-level lasers.

Description

Technical Field

[0001] This invention relates to lasers, and more specifically to a device and method for improving the pointing stability of low-repetition-rate lasers. Background Technology

[0002] With the development of laser technology, pursuing higher intensity lasers has always been one of the goals of laser development. Currently, it is difficult to break through the repetition frequency of high-intensity lasers to exceed 100 Hz. For example, the repetition frequency of 100 TW-level lasers is around 10 Hz, and the highest repetition frequency of PW-level lasers is in the Hz range. For such low-repetition-frequency lasers, when the laser only needs to perform stability self-calibration, the sampling rate of the detector when collecting laser pointing information is limited by the laser repetition frequency, resulting in an extremely low sampling rate, which greatly reduces the quality of pointing correction. Summary of the Invention

[0003] To address the problems existing in the prior art, this invention proposes a device and method for improving the pointing stability of low-repetition-rate lasers.

[0004] One object of the present invention is to provide a device for improving the pointing stability of low-repetition-rate lasers.

[0005] The low-repetition-rate laser pointing stability improvement device of the present invention includes: a laser and an action target; wherein, the laser includes: a seed source, a pulse down-converter, a stretcher, an amplifier group, a first shutter, a polarization beam combiner, a spatial filter, a second Pockel cell, a main amplifier, a compressor, an autocollimation system, and a controller; the pulse down-converter, the first shutter, the second Pockel cell, and the autocollimation system are respectively connected to the controller; The seed source emits a seed laser pulse, which is input to the pulse downconverter. The pulse downconverter periodically changes the polarization of the seed laser pulse. The first laser pulse is downconverted after passing through the pulse downconverter and becomes the main laser. The laser pulses after the first laser pulse become the reference light. The reference light is perpendicular to the polarization of the main laser and their transmission directions are separate. The master laser undergoes time stretching via a stretcher to obtain a long laser pulse. An amplifier group amplifies the energy of this stretched pulse without altering its polarization. The amplified master laser passes through a first shutter to a polarization combiner, where it is combined with a reference beam, with both beams perfectly aligned in axis and direction. The combined laser then passes through a spatial filter and enters a second Pockel cell. The second Pockel cell periodically changes the laser's polarization. After passing through the second Pockel cell, the amplified master laser has the same polarization as the reference beam and enters the master amplifier for further energy amplification. The high-energy laser, amplified by the master amplifier, enters a compressor, which compresses the high-energy laser in the time domain. The compressor outputs a laser beam which enters a focusing lens. The focusing lens focuses the laser beam onto the target, which is a gas target. The controller adjusts the laser beam after beam combining to enter the same position of the compressor through the self-collimation system, and adjusts the laser output from the compressor to enter the same position of the target.

[0006] The pulse downconversion device includes a first polarizing beam splitter, a first Pockel cell, and a second polarizing beam splitter. The first Pockel cell is connected to a controller. Initially, the first Pockel cell is a full-wave plate. The controller periodically applies a half-wave voltage to the first Pockel cell via a high-voltage power supply. The frequency of this half-wave voltage is the same as the frequency of the subsequent amplifier group, ranging from 10Hz to 1000Hz. The period of the half-wave voltage is greater than the seed pulse width but less than the seed period. The seed laser pulse is vertically polarized when input to the first polarizing beam splitter. After being reflected by the first polarizing beam splitter, the first laser pulse enters the first Pockel cell. At this point, the laser is in a half-wave voltage state. After the first laser pulse passes through the first Pockel cell, its polarization changes to horizontal. After being transmitted through the second polarization beam splitter, it becomes the main laser and enters the stretcher. The laser pulses following the first laser pulse are reflected by the first polarization beam splitter and enter the first Pockel cell, which is in a 0-voltage state. After passing through the first Pockel cell, the polarization of the laser pulses following the first laser pulse does not change and remains vertical. After being reflected by the second polarization beam splitter, they become the reference light output. The reference light is perpendicular to the polarization of the main laser and their transmission directions are separate.

[0007] The amplifier array obtains a high-energy pulse, amplifying the pulse energy to greater than 100 mJ but less than 1 J. The reference light, after being reflected by the fourth mirror, enters the polarization combiner and is combined with the amplified main laser beam, ensuring complete alignment of their axes and directions. The main amplifier amplifies the energy to greater than 5 J. The compressor compresses the laser pulse in the time domain to obtain a laser pulse of less than 200 fs.

[0008] The second Pockel cell is initially a full-wave plate. The controller periodically applies a half-wave voltage to the second Pockel cell via a high-voltage power supply. The frequency of this half-wave voltage is the frequency of the subsequent main amplifier, which ranges from 1Hz to 10Hz. The period of the half-wave voltage is greater than the pulse width amplified by the main amplifier but less than the seed period. The laser after passing through the spatial filter consists of an amplified main laser and a reference light. The amplified main laser is horizontally polarized, and the reference light is vertically polarized. The second Pockel cell is initially a full-wave plate. The first laser pulse enters the second Pockel cell, which is then in a half-wave voltage state. After passing through the second Pockel cell, the polarization of the first laser pulse (the amplified main laser) changes to vertical polarization. The laser pulse following the first laser pulse enters the second Pockel cell, which is then in a zero-voltage state. The reference light following the first laser pulse does not change polarization and remains vertically polarized. The reference light has the same polarization as the amplified main laser, allowing both the amplified main laser and the reference light to pass through the compressor efficiently.

[0009] The compressor includes a first grating and a second grating. The high-energy laser, amplified by the main amplifier, is input into the compressor and first incident on the upper half of the first grating at an incident angle of α. The first grating diffracts the incident laser, and the first-order diffracted light with a diffraction angle of β is reflected onto the second grating. The first and second gratings are parallel, and the reflection angle of the 0th-order reflected light on the first grating is equal to the incident angle of the laser beam. The 0th-order reflected light and the 1st-order diffracted light are spatially separated.

[0010] The autocollimation system includes: first to third reflectors, second and third shutters, first and second imaging lenses, and first and second detectors; wherein, the compressor, second and third shutters, first and second imaging lenses, first and second detectors, second and third reflectors, focusing lens, and target are all placed in a vacuum; the first and second detectors, second and third shutters, and first and second reflectors are respectively connected to a controller; the first reflector is placed in front of the second Pockel cell, and the combined laser beam is reflected by the first reflector to the second Pockel cell after passing through a spatial filter.

[0011] The zero-order reflected light from the first grating of the compressor is used as the diagnostic light. A first detector is set in the optical path of the diagnostic light. The first detector receives the zero-order reflected light from the first grating. A second shutter and a first imaging lens are placed sequentially between the first detector and the first grating. The first imaging lens enables the first detector to image the surface of the first grating. The spot information received by the first detector is transmitted to the controller. The controller outputs a control signal to adjust the first reflector so that the laser always enters the first grating at the same position.

[0012] The laser beam is focused onto the target by a focusing lens, generating radiation with a divergence angle smaller than that of the laser beam. A second reflecting mirror is placed after the compressor, reflecting the laser output from the compressor to the focusing lens. A third reflecting mirror is placed around the laser beam behind the target, reflecting a portion of the laser beam from the outer edge into the second detector. The radiation generated by the laser and the target is not blocked by the third reflecting mirror. A third shutter and a second imaging lens are placed sequentially between the third reflecting mirror and the second detector. The second imaging lens enables the second detector to image the target. The spot information received by the second detector is transmitted to the controller, which sends a control signal to adjust the second reflecting mirror so that the laser always enters the target at the same position.

[0013] The first shutter is connected to the controller and is normally closed. When the user needs the laser to interact with the target, the controller sends a signal to the first shutter, causing it to open and close rapidly. This allows a laser pulse to pass through the shutter and enter the subsequent optical path, interacting with the target. When the first shutter is open, the second and third shutters are closed to protect the first and second detectors. When the first shutter is closed, the reference light enters the subsequent system through the polarization combiner and is received by the first and second detectors. The controller controls the first and second reflectors in real time based on the signals from the first and second detectors, ensuring the reference light remains at the same point of interaction on the target. Because the reference light is coaxial and in the same direction as the main laser, the main laser remains at the same point of interaction on the target. When the controller signals to open the first shutter, the second and third shutters are closed beforehand. The main laser then passes through the subsequent system without entering the first and second detectors, protecting them. After the first shutter closes, the second and third shutters reopen, and the controller continues to correct the subsequent optical path in real time based on the signals from the first and second detectors.

[0014] Another objective of this invention is to propose a method for improving the pointing stability of low-repetition-rate lasers.

[0015] The method for improving the pointing stability of low-repetition-rate lasers of the present invention includes the following steps: 1) Perform laser calibration. During laser calibration, the controller keeps the first shutter open and the second and third shutters closed. 2) After the laser is properly adjusted, the controller keeps the first shutter closed. The first and second detectors collect the spot information to obtain the initial position information and transmit it to the controller. The controller stores the initial position information as the initial adjustment result. Subsequently, the controller adjusts the first and second reflectors in real time according to the spot information transmitted by the first and second detectors to change the laser directivity and ensure that the laser pointing performance is consistent with the initial adjustment result. 3) When the user needs to use the laser to interact with the target, the controller sends a control signal to close the second and third shutters, and then sends a control signal to quickly open the first shutter, so that a laser pulse passes through the first shutter; 4) After the first shutter closes, the controller sends a control signal to open the second and third shutters. The controller adjusts the first and second reflectors in real time according to the spot information transmitted by the first and second detectors to change the laser pointing direction and ensure that the laser pointing performance is consistent with the initial adjustment result.

[0016] In step 3), the opening time of the first shutter is less than the seed period; the closing time of the second and third shutters is more than 1 second earlier than the opening time of the first shutter.

[0017] In step 4), the opening time of the second and third shutters is more than 1 second later than the closing time of the first shutter.

[0018] Advantages of this invention: This invention uses a seed laser pulse on the order of MHz as a reference light to adjust the optical path directivity. Compared with the main laser's repetition rate on the order of Hz, this significantly increases the sampling frequency for subsequent directivity adjustment, thereby improving the accuracy of laser self-collimation. The use of the first to third shutters ensures the safety of the first and second detectors during the transmission of the main laser, even when the energy difference between the main laser and the reference light is nine orders of magnitude. The first detector images onto the first grating of the compressor, ensuring that the laser enters the compressor at the same position, thus greatly ensuring the consistency of laser pulse width and directivity. The second detector images onto the target, ensuring that the position of each action is the same, providing a guarantee for the stability of the subsequent radiation source. Attached Figure Description

[0019] Figure 1 This is an overall structural block diagram of an embodiment of the low-repetition-rate laser pointing stability improvement device of the present invention; Figure 2 This is a structural block diagram of a pulse down-conversion device, which is an embodiment of the low-repetition-rate laser pointing stability improvement device of the present invention. Figure 3 A half-wave voltage is applied to the first Pockel cell in one embodiment of the low-repetition-rate laser pointing stability improvement device of the present invention. Timing diagram; Figure 4 This is a schematic diagram illustrating the adjustment of the position of the laser entering the compressor, as shown in one embodiment of the low-repetition-rate laser pointing stability improvement device of the present invention. Detailed Implementation

[0020] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0021] A Pockel cell is an electro-optic device, essentially a voltage-controlled waveplate. It consists of an electro-optic crystal and a high-voltage power supply. The optical properties of the crystal are controlled by applying a voltage to it. In its zero-voltage state, the Pockel cell is a full-wave plate. When a voltage is applied to the electro-optic crystal, its optical properties are superimposed on its initial state. If the superimposed optical properties exhibited after applying a voltage result in a half-wave plate, this applied voltage is called the half-wave voltage. By controlling the voltage of the Pockel cell, the polarization state of the laser can be modulated, causing the laser to be either reflected or transmitted when passing through a polarizing beam splitter, thus achieving time-domain beam splitting.

[0022] A polarization beam combiner allows horizontally polarized laser light to pass through and vertically polarized laser light to be reflected; a polarization beam splitter allows horizontally polarized laser light to pass through and vertically polarized laser light to be reflected.

[0023] like Figure 1 As shown, the low repetition rate laser pointing stability improvement device of this embodiment includes: a laser and an action target; wherein, the laser includes: a seed source, a pulse down-converter, a stretcher, an amplifier group, a first shutter, a polarization beam combiner, a spatial filter, a second Pockel cell, a main amplifier, a compressor, a self-collimation system, and a controller; the pulse down-converter, the first shutter, the second Pockel cell, and the self-collimation system are respectively connected to the controller; the seed source emits a seed laser pulse which is input to the pulse down-converter.

[0024] like Figure 2 As shown, the pulse downconverter includes a first polarizing beam splitter, a first Pockel cell, and a second polarizing beam splitter. The first Pockel cell is connected to a controller. Initially, the first Pockel cell is a full-wave plate. The controller periodically applies a half-wave voltage to the first Pockel cell via a high-voltage power supply. The frequency of this half-wave voltage is the frequency of the subsequent amplifier group, ranging from 10Hz to 1000Hz. The period of the half-wave voltage is greater than the seed pulse width but less than the seed period. The seed laser pulse is vertically polarized when input to the first polarizing beam splitter. After being reflected by the first polarizing beam splitter, the first laser pulse enters the first Pockel cell, as shown... Figure 3 As shown, the first Pockel cell is currently in a half-wave voltage state. After the first laser pulse passes through the first Pockel cell, its polarization changes to horizontal polarization. After being transmitted through the second polarization beam splitter, it enters the stretcher as the main laser. The laser pulse following the first laser pulse is reflected by the first polarization beam splitter and enters the first Pockel cell, which is currently in a 0-voltage state. After the laser pulse following the first laser pulse passes through the first Pockel cell, its polarization does not change and it remains vertically polarized. After being reflected by the second polarization beam splitter, it enters the fourth reflecting mirror M4 as a reference beam. The reference beam is perpendicular to the polarization of the main laser and their transmission directions are separate.

[0025] The main laser is time-stretched by a stretcher to obtain a long laser pulse. The amplifier group amplifies the energy of the stretched long laser pulse to an energy greater than 100mJ and less than 1J, obtaining a high-energy laser pulse. The amplifier group does not change the polarization of the laser. The horizontally polarized amplified main laser is transmitted through the first shutter and then through a polarization beam combiner. It is then combined with the vertically polarized reference light reflected by the fourth mirror M4 and reflected by the polarization beam combiner. The two beams are completely aligned in axis and direction. The combined laser beam passes through a spatial filter and is reflected by the first mirror M1 into the second Pockel cell.

[0026] The second Pockel cell is initially a full-wave plate. The controller periodically applies a half-wave voltage to the second Pockel cell via a high-voltage power supply. The frequency of this half-wave voltage is the frequency of the subsequent main amplifier, which ranges from 1Hz to 10Hz. The period of the half-wave voltage is greater than the pulse width amplified by the main amplifier but less than the seed period. The laser after passing through the spatial filter consists of an amplified main laser and a reference beam, where the amplified main laser is horizontally polarized and the reference beam is vertically polarized. The second Pockel cell is initially a full-wave plate. The first laser pulse passes through and enters the second Pockel cell, which is then in a half-wave voltage state. In the process, the first laser pulse, i.e., the amplified master laser, undergoes a polarization change after passing through the second Pockel cell, becoming vertically polarized. The laser pulse following the first laser pulse enters the second Pockel cell, which is currently in a 0-voltage state. The reference laser pulse following the first laser pulse does not undergo a polarization change after passing through the second Pockel cell, remaining vertically polarized. After passing through the second Pockel cell, the reference laser has the same polarization as the amplified master laser and enters the main amplifier for further energy amplification. The high-energy laser amplified by the main amplifier then enters the compressor, where both the amplified master laser and the reference laser can pass through the compressor efficiently.

[0027] like Figure 4 As shown, the compressor includes a first grating and a second grating. The high-energy laser, amplified by the main amplifier, is input into the compressor and first incident on the upper half of the first grating at an incident angle of α. The first grating diffracts the incident laser, and the first-order diffracted light with a diffraction angle of β is reflected onto the second grating. The first and second gratings are parallel, and the reflection angle of the 0th-order reflected light on the first grating is equal to the incident angle of the laser beam. The 0th-order reflected light and the 1st-order diffracted light are spatially separated. The compressor compresses the high-energy laser in the time domain.

[0028] The laser output from the compressor is reflected by the second mirror and enters the focusing mirror, which focuses the laser onto the target, which is a gas target.

[0029] The autocollimation system includes: first to third reflectors M1~M3, second and third shutters, first and second imaging lenses, and first and second detectors; wherein, the compressor, second and third shutters, first and second imaging lenses, first and second detectors, second and third reflectors, focusing lens, and target are all placed in a vacuum; the first and second detectors, second and third shutters, and first and second reflectors are respectively connected to a controller; the first reflector M1 is placed in front of the second Pockel cell, and the laser beam after beam combining is reflected by the first reflector M1 to the second Pockel cell after passing through a spatial filter.

[0030] The zero-order reflected light from the first grating of the compressor is used as the diagnostic light. A first detector is set in the optical path of the diagnostic light. The first detector receives the zero-order reflected light from the first grating. A second shutter and a first imaging lens are placed sequentially between the first detector and the first grating. The first imaging lens enables the first detector to image the surface of the first grating. The spot information received by the first detector is transmitted to the controller. The controller outputs a control signal to adjust the first reflector so that the laser always enters the first grating at the same position.

[0031] Here's a brief explanation of the incident angle and diffraction angle: The diffraction formula for a grating is mλ = d(sinα ± sinβ), where m is the order of the diffracted light, λ is the diffraction wavelength, α is the incident angle, β is the diffraction angle, and d is the grating constant. In this formula, m can take values ​​of 0, ±1, ±2, etc., and the resulting spectra are called the 0th-order spectrum, 1st-order spectrum, 2nd-order spectrum, etc. The + and - signs indicate whether the incident angle and diffraction angle are on the same or opposite sides of the normal, respectively. For 1st-order diffraction, we have λ = d(sinα ± sinβ). For a beam with an incident angle of α, its 1st-order diffraction angle is β; for a beam with an incident angle of β, its 1st-order diffraction angle is α.

[0032] The laser beam is focused onto the target by a focusing lens, generating radiation with a divergence angle smaller than that of the laser beam. A second reflecting mirror M2 is placed after the compressor, reflecting the laser beam output from the compressor to the focusing lens. A third reflecting mirror M3 is placed around the laser beam behind the target, reflecting a portion of the laser beam from the outer edge into the second detector. The radiation generated by the laser beam and the target is not blocked by the third reflecting mirror M3. A third shutter and a second imaging lens are placed sequentially between the third reflecting mirror M3 and the second detector. The second imaging lens enables the second detector to image the target. The spot information received by the second detector is transmitted to the controller, which sends a control signal to adjust the second reflecting mirror M2 so that the laser beam always enters the same position on the target.

[0033] The first shutter is connected to the controller and is normally closed. When the user needs the laser to interact with the target, the controller sends a signal to the first shutter, causing it to open and close rapidly. This allows a laser pulse to pass through the shutter and enter the subsequent optical path, interacting with the target. When the first shutter is open, the second and third shutters are closed to protect the first and second detectors. When the first shutter is closed, the reference light enters the subsequent system through the polarization combiner and is received by the first and second detectors. The controller controls the first and second mirrors in real time based on the signals from the first and second detectors, ensuring that the reference light remains at the same point of interaction on the target. Because the reference light is coaxial and in the same direction as the main laser, the main laser remains at the same point of interaction on the target. When the controller signals to open the first shutter, the second and third shutters are closed in advance. At this time, the main laser passes through the subsequent system and will not enter the first and second detectors, thus protecting them. After the first shutter closes, the second and third shutters reopen, and the controller continues to correct the subsequent optical path in real time based on the signals from the first and second detectors.

[0034] The method for improving the pointing stability of low-repetition-rate lasers in this embodiment includes the following steps: 1) Perform laser calibration. During laser calibration, the controller keeps the first shutter open and the second and third shutters closed. 2) After the laser is properly adjusted, the controller keeps the first shutter closed. The first and second detectors collect the spot information to obtain the initial position information and transmit it to the controller. The controller stores the initial position information as the initial adjustment result. Subsequently, the controller adjusts the first and second reflectors in real time according to the spot information transmitted by the first and second detectors to change the laser directivity and ensure that the laser pointing performance is consistent with the initial adjustment result. 3) When the user needs to use the laser to interact with the target, the controller sends a control signal to close the second and third shutters, and then sends a control signal to quickly open the first shutter, so that a laser pulse passes through the first shutter. The opening time of the first shutter is less than the seed period, and the closing time of the second and third shutters is more than 1 second earlier than the opening time of the first shutter. 4) After the first shutter closes, the controller sends a control signal to open the second and third shutters. The opening time of the second and third shutters is more than 1 second later than the closing time of the first shutter. The controller adjusts the first and second reflectors in real time according to the spot information transmitted by the first and second detectors to change the laser directionality and ensure that the laser directionality is consistent with the initial adjustment result.

[0035] Finally, it should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the present invention is defined by the claims.

Claims

1. A device for improving the pointing stability of low-repetition-rate lasers, characterized in that, The device includes a laser and a target; wherein the laser includes a seed source, a pulse down-converter, a stretcher, an amplifier group, a first shutter, a polarization combiner, a spatial filter, a second Pockel cell, a main amplifier, a compressor, an autocollimation system, and a controller; the pulse down-converter, the first shutter, the second Pockel cell, and the autocollimation system are respectively connected to the controller; The seed source emits a seed laser pulse, which is input to the pulse downconverter. The pulse downconverter periodically changes the polarization of the seed laser pulse. The first laser pulse is downconverted after passing through the pulse downconverter and becomes the main laser. The laser pulses after the first laser pulse become the reference light. The reference light is perpendicular to the polarization of the main laser and their transmission directions are separate. The master laser undergoes time stretching via a stretcher to obtain a long laser pulse. An amplifier group amplifies the energy of this stretched pulse without altering its polarization. The amplified master laser passes through a first shutter to a polarization combiner, where it is combined with a reference beam, with both beams perfectly aligned in axis and direction. The combined laser then passes through a spatial filter and enters a second Pockel cell. The second Pockel cell periodically changes the laser's polarization. After passing through the second Pockel cell, the amplified master laser has the same polarization as the reference beam and enters the master amplifier for further energy amplification. The high-energy laser, amplified by the master amplifier, enters a compressor, which compresses the high-energy laser in the time domain. The compressor outputs a laser beam which enters a focusing lens. The focusing lens focuses the laser beam onto the target, which is a gas target. The controller adjusts the laser beam after beam combining to enter the same position of the compressor through the self-collimation system, and adjusts the laser output from the compressor to enter the same position of the target.

2. The apparatus according to claim 1, characterized in that, The pulse downconversion device includes a first polarizing beam splitter, a first Pockel cell, and a second polarizing beam splitter. The first Pockel cell is connected to a controller. Initially, the first Pockel cell is a full-wave plate, and a half-wave voltage is periodically applied to it. When the seed laser pulse is input to the first polarizing beam splitter, it is vertically polarized. After being reflected by the first polarizing beam splitter, the first laser pulse enters the first Pockel cell, which is then in a half-wave voltage state. After passing through the first Pockel cell, the polarization of the first laser pulse changes to horizontal polarization. It is then transmitted through the second polarizing beam splitter and becomes the main laser beam entering the stretcher. Subsequent laser pulses, after being reflected by the first polarizing beam splitter, enter the first Pockel cell, which is then in a zero-voltage state. Subsequent laser pulses, after passing through the first Pockel cell, do not change polarization and remain vertically polarized. They are then reflected by the second polarizing beam splitter and output as reference light.

3. The apparatus according to claim 1, characterized in that, The second Pockel cell is initially a full-wave plate, and a half-wave voltage is periodically applied to it. The laser beam after passing through the spatial filter consists of an amplified master laser beam and a reference beam, where the amplified master laser beam is horizontally polarized and the reference beam is vertically polarized. The second Pockel cell is initially a full-wave plate. The first laser pulse enters the second Pockel cell, which is then in a half-wave voltage state. After the first laser pulse, i.e., the amplified master laser beam, passes through the second Pockel cell, its polarization changes to vertical. The laser pulse following the first laser pulse enters the second Pockel cell, which is then in a zero-voltage state. After the laser pulse following the first laser pulse, i.e., the reference beam, passes through the second Pockel cell, its polarization does not change and remains vertically polarized.

4. The apparatus according to claim 1, characterized in that, The autocollimation system includes: first to third reflecting mirrors, second and third shutters, first and second imaging lenses, and first and second detectors; wherein, the compressor, second and third shutters, first and second imaging lenses, first and second detectors, second and third reflecting mirrors, focusing lens, and target are all placed in a vacuum; the first and second detectors, second and third shutters, and first and second reflecting mirrors are respectively connected to a controller; the first reflecting mirror is placed in front of the second Pockel cell, and the combined laser beam is reflected by the first reflecting mirror to the second Pockel cell after passing through a spatial filter.

5. The apparatus according to claim 4, characterized in that, The zero-order reflected light from the first grating of the compressor is used as the diagnostic light. A first detector is set in the optical path of the diagnostic light. The first detector receives the zero-order reflected light from the first grating. A second shutter and a first imaging lens are placed sequentially between the first detector and the first grating. The first imaging lens enables the first detector to image the surface of the first grating. The spot information received by the first detector is transmitted to the controller. The controller outputs a control signal to adjust the first reflector so that the laser always enters the first grating at the same position.

6. The apparatus according to claim 4, characterized in that, A second reflector is placed after the compressor. The laser output from the compressor is reflected by the second reflector to the focusing lens. A third reflector is placed around the laser behind the target. The third reflector reflects the peripheral portion of the laser behind the target into the second detector. The radiation generated by the laser and the target is not blocked by the third reflector. A third shutter and a second imaging lens are placed sequentially between the third reflector and the second detector. The second imaging lens enables the second detector to image the target. The spot information received by the second detector is transmitted to the controller. The controller gives a control signal to adjust the second reflector so that the laser always enters the target at the same position.

7. A method for improving the pointing stability of a low-repetition-rate laser according to claim 1, characterized in that, The method includes the following steps: 1) Perform laser calibration. During laser calibration, the controller keeps the first shutter open and the second and third shutters closed. 2) After the laser is properly adjusted, the controller keeps the first shutter closed. The first and second detectors collect the spot information to obtain the initial position information and transmit it to the controller. The controller stores the initial position information as the initial adjustment result. Subsequently, the controller adjusts the first and second reflectors in real time according to the spot information transmitted by the first and second detectors to change the laser directivity and ensure that the laser pointing performance is consistent with the initial adjustment result. 3) When the user needs to use the laser to interact with the target, the controller sends a control signal to close the second and third shutters, and then sends a control signal to quickly open the first shutter, so that a laser pulse passes through the first shutter; 4) After the first shutter closes, the controller sends a control signal to open the second and third shutters. The controller adjusts the first and second reflectors in real time according to the spot information transmitted by the first and second detectors to change the laser pointing direction and ensure that the laser pointing performance is consistent with the initial adjustment result.

8. The method according to claim 7, characterized in that, In step 3), the opening time of the first shutter is less than the seed period; the closing time of the second and third shutters is more than 1 second earlier than the opening time of the first shutter.

9. The method according to claim 7, characterized in that, The opening time of the second and third shutters is more than 1 second later than the closing time of the first shutter.

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