A system and method for measuring laser contrast using chirped pulses

Abstract

The application discloses a system and method for measuring laser contrast by using chirped pulses, comprising a collimating beam splitting device, a main laser energy tuner, a plasma generating device, a probe light delay adjusting device, a probe light spectrum widening device and a monitoring device; laser pulses are divided into two beams by the collimating beam splitting device, one of which is a main laser and the other is a probe light; the main laser passes through the main laser energy tuner, enters the plasma generating device to generate plasma, and then returns to a light return monitoring camera; the probe light enters the probe light delay adjusting device, becomes a chirped pulse through the probe light spectrum widening device, then passes through the plasma generating device, carries information in the plasma, and enters the monitoring device.

Description

Technical Field

[0001] This invention belongs to the field of optical measurement, specifically relating to a system and method for measuring laser contrast using chirped pulses. Background Technology

[0002] Laser light is produced by stimulated emission of atoms. Electrons in atoms absorb energy, transition from a low energy level to a high energy level, and then release energy as photons as they fall back down. Lasers are characterized by monochromaticity, good directionality, and the ability to carry higher energy. With the continuous maturation of pulsed laser amplification technology, especially femtosecond lasers, the single-pulse energy and peak power of ultrashort and ultra-intense pulsed lasers are constantly improving. Existing technologies now include ultrashort pulse amplification systems on the order of milliseconds (PW), and ultrashort pulsed lasers are being applied to experimental research on the interaction between light and matter, driving the rapid development of a series of disciplines such as strong-field physics, atomic and molecular physics, and condensed matter physics.

[0003] Laser contrast ratio is an important parameter for measuring laser quality, defined as the ratio of sub-pulse to main pulse. Current technologies utilize ultrashort laser pulses with widths reaching femtoseconds or even attoseconds, offering the advantage of high instantaneous energy. However, ultrashort laser pulses exhibit strong spontaneous emission amplification at their leading edge, and sub-pulses remain even during pulse selection. These sub-pulses are several orders of magnitude weaker than the main laser pulse in intensity, but their duration is close to the main pulse and long, thus affecting laser quality.

[0004] Methods for measuring laser contrast include third-order autocorrelation and high-speed photoelectric methods, among which third-order autocorrelation can measure a large dynamic range (~10). 13 The laser contrast ratio is limited to a few nanoseconds in the time domain. High-speed photoelectric methods can measure laser contrast ratio over a very large time domain, using ultrafast photodetectors and high-bandwidth oscilloscopes, but the dynamic range is limited. Common methods for measuring laser contrast ratio involve indirect measurement; if the actual intensity of the prepulse is needed, it can only be calculated from the intensity of the main pulse, making direct measurement of the laser prepulse impossible. Summary of the Invention

[0005] This application provides a system and method for measuring laser contrast using chirped pulses. When a high-intensity ultrashort pulse laser acts on a substance such as a metal or gas, plasma is generated. Another beam of light, a probe beam, carries the plasma information, which is then converted into an image for data analysis and inversion to ultimately obtain the corresponding laser pulse intensity. The probe beam carrying different plasma information at different time points carries different plasma information. By using optical color to broaden the laser pulse dispersion, chirped laser pulses are obtained to carry different plasma information. A single chirped laser pulse contains multiple wavelengths. By utilizing the different arrival times of laser pulses of different wavelengths in the plasma, plasma information corresponding to different regions is obtained, thus determining the plasma velocity and energy in different regions. The laser contrast is calculated, and the pre-pulse intensity at different locations is directly obtained. The laser contrast is measured, achieving measurements over an ultra-wide time domain and a high dynamic range.

[0006] The specific technical solution is as follows: A system for measuring laser contrast using chirped pulses, including a collimating beam splitter, a main laser energy tuner, a plasma generating device, a probe light delay adjustment device, a probe light spectral broadening device, and a monitoring device;

[0007] The laser pulse is split into two beams by the collimating and beam splitting device, one of which is the main laser and the other is the probe beam;

[0008] After the main laser passes through the main laser energy tuner and enters the plasma generating device to generate plasma, the main laser returns to the backlight monitoring camera.

[0009] The probe light enters the probe light delay adjustment device, is converted into a chirped pulse by the probe light spectrum broadening device, and then passes through the plasma generation device, carrying the information in the plasma, and enters the monitoring device;

[0010] Preferably, the probe light delay adjustment device includes a translation stage and a sixth total reflection mirror and a seventh total reflection mirror mounted on the translation stage, used to change the optical path of the probe light by the horizontal movement of the translation stage, thereby changing the time when the probe light arrives at the plasma generating device.

[0011] Preferably, the probe light spectral broadening device includes a grating, a concave mirror, a long strip mirror, a climbing mirror group, and a broadening output mirror, used to broaden the probe light spectrally, so that the probe light becomes chirped light;

[0012] The probe light enters the grating through the climbing mirror group. The first-order diffracted light of the probe light returns to the grating after passing through the concave mirror and the elongated mirror. After passing through the climbing mirror group again, it enters the grating, the concave mirror and the elongated mirror, and returns to the grating. It is then reflected by the broadened output mirror and output.

[0013] The monitoring device includes a first 50% beam splitter, a first filter, a first CCD camera, a second 50% beam splitter, a second filter, a second CCD camera, a ninth total reflection mirror, a third filter, and a third CCD camera.

[0014] Preferably, the main laser energy tuner includes a rotating waveplate, a first polarizing mirror, and a second polarizing mirror.

[0015] Preferably, the first polarizing mirror and the second polarizing mirror are wire grating polarizers and are arranged parallel to each other. The polarization direction of the main laser is changed by the rotating waveplate, and the energy of the main laser is changed by the first polarizing mirror and the second polarizing mirror, and the polarization direction of the main laser is changed back to the polarization direction before passing through the rotating waveplate.

[0016] Preferably, it also includes an energy meter, wherein the main laser enters the energy meter through the first 99% reflectivity mirror.

[0017] Preferably, the chirped pulse carrying information in the plasma is split into a first pulse and a second pulse by the first 50% beam splitter, and the first pulse enters the first CCD camera through the first filter;

[0018] The second pulse is then split into a third pulse and a fourth pulse by a second 50% beam splitter, and the third pulse enters the second CCD camera through a second filter.

[0019] The fourth pulse enters the third CCD camera via the ninth total reflection mirror and the third filter.

[0020] Preferably, the transmission wavelength range of the first filter is 405~415nm, the transmission wavelength range of the second filter is 390~400nm, and the transmission wavelength range of the third filter is 375~385nm.

[0021] Preferably, the chirped pulse includes multiple wavelength ranges, including 405~415nm, 390~400nm and 375~385nm.

[0022] Preferably, the chirped pulses of different wavelength ranges pass through the plasma generating device at the same location but at different times, and carry different plasma information.

[0023] A method for measuring laser contrast using a system that measures laser contrast with chirped pulses includes the following steps:

[0024] S01: The laser pulse is split into a main laser and a probe beam. The energy of 1% of the main laser is measured using an energy meter, and the main pulse energy is calculated and recorded as follows: ;

[0025] S02: Adjust the probe light delay adjustment device to delay the probe light so that when the probe light enters the plasma generating device, there is plasma at the plasma generating device;

[0026] S03: The lengths of the plasma shadow region in the expansion direction are measured using the first CCD camera, the second CCD camera, and the third CCD camera, and are denoted as L1, L2, and L3, respectively; the times when the plasma shadow is measured are denoted as t1, t2, and t3, respectively.

[0027] S04: Calculated laser contrast The calculation formula is as follows:

[0028] ;

[0029] ;

[0030] ;

[0031] In the formula, V S The velocity of the high-density plasma layer, and The adiabatic constant and density of the target material, Where A is the laser wavelength, A is the atomic weight of the target material, and Z is the number of nucleons in the target material. For laser contrast, Main pulse energy, The intensity of the laser prepulse.

[0032] Preferably, step S02 includes:

[0033] If λ1, λ2, and λ3 are all less than λ, then t1=T×(λ1-(λ-Δλ)) / Δλ, t2=T×(λ2-(λ-Δλ)) / Δλ, t3=T×(λ3-(λ-Δλ)) / Δλ;

[0034] If λ1, λ2, and λ3 are all greater than or equal to λ, then t1=2T-T×(λ1-(λ-Δλ)) / Δλ, t2=2T-T×(λ2-(λ-Δλ)) / Δλ, t3=2T-T×(λ3-(λ-Δλ)) / Δλ;

[0035] T is the half-width at half-maximum (FWHM) of the chirped pulse time width; 2T is the optical pulse 1 / e 2 The pulse width is given by Δλ, where Δλ is the full width at half maximum (FWHM) of the spectrum; 2Δλ is the spectral 1 / e 2 t0 is the initial time of plasma generation; λ1 is the center wavelength of the laser after filtering with the first filter; λ2 is the center wavelength of the laser after filtering with the second filter; λ3 is the center wavelength of the laser after filtering with the third filter; λ is the center wavelength of the probe light.

[0036] This application provides a system and method for measuring laser contrast using chirped pulses. When a high-intensity ultrashort pulse laser acts on a substance such as a metal or gas, plasma is generated. Another beam of light, a probe beam, carries the plasma information, which is then converted into an image for data analysis and inversion to ultimately obtain the corresponding laser pulse intensity. The probe beam carrying different plasma information at different time points carries different plasma information. By using optical color to broaden the laser pulse dispersion, chirped laser pulses are obtained to carry different plasma information. A single chirped laser pulse contains multiple wavelengths. By utilizing the different arrival times of laser pulses of different wavelengths in the plasma, plasma information corresponding to different regions is obtained, thus determining the plasma velocity and energy in different regions. The laser contrast is calculated, and the pre-pulse intensity at different locations is directly obtained. The laser contrast is measured, achieving measurements over an ultra-wide time domain and a high dynamic range. Attached Figure Description

[0037] Figure 1 A schematic diagram illustrating the working principle of a system for measuring laser contrast using chirped pulses in one embodiment of the present invention is shown.

[0038] Figure 2 A schematic diagram of a system structure for measuring laser contrast using chirped pulses is shown in one embodiment of the present invention;

[0039] Figure 3 A schematic diagram of the spectral broadening of the measurement system in one embodiment of the present invention is shown;

[0040] Figure 4 This diagram illustrates the process of chirped light carrying plasma information in a system for measuring laser contrast using chirped pulses, according to one embodiment of the present invention.

[0041] Figure 5 A schematic diagram of the time delay between the chirped light and the main laser in a system for measuring laser contrast using chirped pulses according to an embodiment of the present invention is shown.

[0042] Among them, 1. First total internal reflection mirror; 2. Second total internal reflection mirror; 3. First beam splitter; 4. Third total internal reflection mirror; 5. Rotating waveplate; 6. First polarizing mirror; 7. Second polarizing mirror; 8. First 99% reflectivity mirror; 9. Second 99% reflectivity mirror; 10. First lens; 11. Target material; 12. 800nm ​​fundamental frequency light filter; 13. Backlight monitoring camera; 14. Energy meter; 15. Second lens; 16. Frequency doubling crystal; 17. Third lens; 18. Fourth total internal reflection mirror; 19. Fifth total internal reflection mirror; 20. Sixth... 21. Total Internal Reflection Mirror; 22. Seventh Total Internal Reflection Mirror; 23. Grating; 24. Concave Mirror; 25. Strip Mirror; 26. Climbing Mirror Group; 27. Widening Output Mirror; 28. Fourth Lens; 29. ​​Eighth Total Internal Reflection Mirror; 30. First 50% Beam Splitter; 31. First CCD Camera; 32. Second 50% Beam Splitter; 33. Second Filter; 34. Second CCD Camera; 35. Ninth Total Internal Reflection Mirror; 36. Third Filter; 37. Third CCD Camera; 38. Second Beam Splitter; 39. Fourth CCD Camera;

[0043] Ⅰ. Collimation and beam splitting device; Ⅱ. Main laser energy tuner; Ⅲ. Plasma generation device; Ⅳ. Probe delay adjustment device; Ⅴ. Probe spectral broadening device; Ⅵ. Monitoring device. Detailed Implementation

[0044] The embodiments of this application will be described in further detail below. Obviously, the described embodiments are only a part of the embodiments of this application, and not an exhaustive list of all embodiments. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other.

[0045] The terms “first,” “second,” etc. (if applicable) in the specification and claims are used to distinguish similar objects and are not necessarily used to describe a particular order or sequence. It should be understood that such data used in this way can be interchanged where appropriate so that the embodiments described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms “comprising” and “having,” and any variations thereof, are intended to cover a non-exclusive inclusion, such as a process, method, system, product, or apparatus that comprises a series of steps or units, not necessarily limited to those explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0046] It should be understood that the term "and / or" used in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship. Example

[0047] Figure 1 This diagram illustrates the working principle of a system for measuring laser contrast using chirped pulses in one embodiment of the present invention. After collimation and beam splitting, the laser beam is divided into two beams, preferably a high-energy femtosecond laser of 800 nm and 30 fs. One beam is the main laser with 90% energy, which, after energy tuning, acts on the target material 11 to generate plasma. The other beam is a probe beam with 10% energy. After delay and spectral broadening, the probe beam becomes a chirped pulse, i.e., chirped light. The chirped light contains different wavelengths, which are used to image the plasma, collect plasma information, and carry this information into monitoring device VI. Then, data inversion is performed on the plasma information to obtain inversion data, and finally, the laser contrast is calculated.

[0048] Figure 2 This diagram illustrates a system structure for measuring laser contrast using chirped pulses according to an embodiment of the present invention. The device for measuring laser contrast using chirped pulses in this embodiment includes a collimating and beam-splitting device I, a main laser energy tuner II, a plasma generation device III, a probe light delay adjustment device IV, a probe light spectral broadening device V, a monitoring device VI, and other optical components. The laser pulse is split into two beams by the collimating and beam-splitting device I, with one beam accounting for 90% of the total laser energy as the main laser. Preferably, the main laser energy range is 10... 18 ~10 20 The main laser beam passes through the third total reflection mirror 4, then through the main laser energy tuner II, and then through the first 99% reflectivity mirror 8 and the second 99% reflectivity mirror 9 into the plasma generation device III, generating plasma. The main laser beam then returns through the second 99% reflectivity mirror 9 and the 800nm ​​fundamental frequency filter 12 to the backlight monitoring camera 13. Another beam, accounting for 10% of the total laser energy, is the probe light. This probe light passes sequentially through the second lens 15, the frequency doubling crystal 16, the third lens 17, the fourth total reflection mirror 18, and the fifth total reflection mirror 19, entering the probe light delay adjustment device IV and the probe light spectrum broadening device V. At this point, the probe light is chirped light with different wavelengths. The chirped light passes through the plasma generation device III, carrying information from the plasma generated by the main laser, and then through the fourth lens 27 and the eighth total reflection mirror 28 into the monitoring device VI.

[0049] Based on the plasma information mentioned above, data inversion is performed and laser contrast is calculated.

[0050] The collimation and beam splitting device I includes: a first total reflection mirror 1, a second total reflection mirror 2, and a first beam splitter 3.

[0051] The main laser energy tuner II includes a rotating waveplate 5, a first polarizing mirror 6, and a second polarizing mirror 7. Preferably, the first polarizing mirror 6 and the second polarizing mirror 7 are linear grating polarizers, using a reflective grating. The polarization direction and energy of the incident laser can be changed by rotating the mirror surfaces. Linearly polarized light has its polarization direction changed by the rotating waveplate 5, and then its energy is changed by the mutually parallel first polarizing mirror 6 and second polarizing mirror 7, while simultaneously changing the polarization direction back to its original direction. The function of the main laser energy tuner II is to achieve automatic adjustment of the main laser energy while keeping parameters such as the main laser pulse width, spot size, and beam divergence angle constant.

[0052] The plasma generating device III includes a first lens 10 and a target 11. Preferably, the target 11 is a micron-sized metal film. The function of the plasma generating device III is to generate plasma when the main laser acts on the target 11, and to carry plasma information when the probe light, i.e., the chirped light, passes through this device.

[0053] The probe light delay adjustment device IV includes a translation stage, a sixth total reflection mirror 20, and a seventh total reflection mirror 21. Preferably, the sixth total reflection mirror 20 and the seventh total reflection mirror 21 are mounted on the translation stage. The function of the probe light delay adjustment device IV is to change the optical path of the probe light by moving the translation stage horizontally, thereby changing the time it takes for the probe light to reach the plasma generation device III, i.e., the plasma region, so that when the probe light passes parallel to the surface of the target material 11, the plasma is generated just in time, and the probe light successfully captures the plasma-carrying information.

[0054] The probe light spectral broadening device V includes: a grating 22, a concave mirror 23, a strip mirror 24, a climbing mirror group 25, and a broadening output mirror 26. The function of the probe light spectral broadening device V is to broaden the probe light spectrum, making the probe light chirped.

[0055] Monitoring device VI consists of a series of mirrors, filters, and a CCD sensor of different wavelengths, including: a first 50% beam splitter 29, a first filter 30, a first CCD camera 31, a second 50% beam splitter 32, a second filter 33, a second CCD camera 34, a ninth total reflection mirror 35, a third filter 36, and a third CCD camera 37. The function of monitoring device VI is to acquire plasma information by imaging the plasma captured by the probe light, facilitating subsequent inversion and measurement of laser contrast using this plasma information.

[0056] Preferably, the positional relationships of the components in the system are as follows: the first total reflection mirror 1, the second total reflection mirror 2, the first beam splitter 3, and the third total reflection mirror 4 are parallel; the third total reflection mirror 4 is coaxial with the rotating waveplate 5; the first polarizing mirror 6 and the second polarizing mirror 7 are parallel; the second 99% reflectivity mirror 9, the first lens 10, the target material 11, the 800nm ​​fundamental frequency filter 12, and the backlight monitoring camera 13 are coaxial. The second total reflection mirror 2, the first beam splitter 3, the second lens 15, the frequency doubling crystal 16, the third lens 17, and the fourth total reflection mirror 18 are coaxial; the fourth total reflection mirror 18 and the fifth total reflection mirror 19 are perpendicular; the sixth total reflection mirror 20 and the seventh total reflection mirror 21 are perpendicular and are jointly mounted on the translation stage. The widening output mirror 26, the fourth lens 27, and the eighth total reflection mirror 28 are coaxial, and the line connecting their axes is parallel to the surface of the target material 11 and 1mm away. The first 50% beam splitter 29, the second 50% beam splitter 32, and the ninth total reflection mirror 35 are coaxial and their surfaces are parallel.

[0057] When the main laser acts on the target 11, plasma is generated on its surface and rapidly overflows. This is because the main laser energy range is 10... 18 ~10 20 The generated plasma density is 10 18 -10 20 pcs / cm 3At this point, the probe light can pass through the plasma. When the probe light passes through the plasma, due to the difference in refractive index between the plasma and the surrounding environment, the probe light will be deflected by refraction effects, resulting in a shadow after the laser passes through the plasma region. The shadow region directly reflects the size of the plasma. The probe light, i.e., the chirped light, penetrates the plasma region perpendicular to the direction of plasma expansion. Different wavelengths take different times to pass through the plasma region, capturing different plasma images, thus carrying plasma information at different times. After passing through the fourth lens 27, the eighth total reflection mirror 28, and the second beam splitter 38, the probe light is split into three beams by the first 50% beam splitter 29, the second 50% beam splitter 32, and the ninth total reflection mirror 35. These beams pass through the first filter 30, the second filter 33, and the third filter 36, respectively, and are then imaged in the corresponding first CCD camera 31, second CCD camera 34, and third CCD camera 37, obtaining the plasma shadow region at different times. Preferably, the chirped pulse carrying information from the plasma is split into a first pulse and a second pulse by the first 50% beam splitter (29). The first pulse passes through the first filter (30) and enters the first CCD camera (31). The second pulse is then split into a third pulse and a fourth pulse by the second 50% beam splitter (32). The third pulse passes through the second filter (33) and enters the second CCD camera (34). The fourth pulse passes through the ninth total reflection mirror (35) and the third filter (36) and enters the third CCD camera (37). Preferably, the first filter 30 transmits wavelengths in the range of 405~415nm, the second filter 33 transmits wavelengths in the range of 390~400nm, and the third filter 36 transmits wavelengths in the range of 375~385nm.

[0058] 1% of the main laser beam is transmitted through the first 99% reflectivity mirror 8 and enters the energy meter 14. The energy meter 14 measures the laser energy in real time and calculates the actual energy acting on the target material 11. The calculation formula is as follows:

[0059] (1)

[0060] In the formula, It is the actual energy of the laser acting on the target 11. It is the laser energy measured by energy meter 14.

[0061] The main laser beam is collimated and its direction is altered by the first 99% reflectivity mirror 8 and the second 99% reflectivity mirror 9. It is then focused by the first lens 10 and finally incident perpendicularly onto the target material 11 at the focal point, i.e., at an incident angle of 0°. 1% of the reflected light from the target material 11 passes through the second 99% reflectivity mirror 9 and the 800nm ​​fundamental frequency filter 12 before entering the backlight monitoring camera 13.

[0062] Preferably, if the main laser acts on the target 11 and no plasma is generated, the wavelength of the returned light from the target 11 is a single wavelength of 800nm. After being filtered by the 800nm ​​fundamental frequency light filter 12, the light signal cannot be detected by the return light monitoring camera 13. If the main laser acts on the target 11 and plasma is generated, the returned light from the target 11 will be mixed with other wavelengths of light. After being filtered by the 800nm ​​fundamental frequency light filter 12, a bright spot can be detected on the return light monitoring camera 13.

[0063] Figure 3 A schematic diagram of the spectral broadening of the measurement system in one embodiment of the present invention is shown, wherein the centers of the grating 22, the concave mirror 23, and the elongated mirror 24 are collinear and parallel. The probe light, i.e., the chirped light, enters the grating 22 through the ascending mirror group 25. Its first-order diffracted light passes through the concave mirror 23 and the elongated mirror 24 and returns to the grating 22. Then it passes through the ascending mirror group 25 again and enters the grating 22, passes through the concave mirror 23 and the elongated mirror 24 again and returns to the grating 22. Finally, it is reflected and output by the broadened output mirror 26.

[0064] Preferably, the probe light passes through the ascending mirror group 25, so that the probe light passes through the grating 22, the concave mirror 23 and the elongated reflector 24 twice, thereby completing the spectral broadening of the probe light, making the probe light a chirped light or a chirped pulse. The chirped light contains pulses of different wavelengths, preferably pulses with wavelength ranges of 405~415nm, 390~400nm and 375~385nm.

[0065] Figure 4 The diagram illustrates the process of chirped light carrying plasma information in a system for measuring laser contrast using chirped pulses according to an embodiment of the present invention. The broadened probe light, i.e., the chirped light, contains laser pulses of different wavelengths with different speeds. They arrive at the same location in the plasma region but at different times.

[0066] Figure 5 This diagram illustrates the time delay between the chirped light and the main laser in a system for measuring laser contrast using chirped pulses, according to an embodiment of the present invention. The full width at half maximum (FWHM) of the probe light time width after passing through the probe light spectral broadening device V is measured using an autocorrelator, and is T. The time delay is 1 / e. 2 The pulse width can be approximated as 2T. The full width at half maximum (FWHM) of the probe light spectrum after broadening by probe light spectral broadening device V is Δλ, and the spectral density is 1 / e. 2 The spectrum at that point can be approximated as 2Δλ. The leading edge of the probe light pulse is 1 / e. 2 The point is defined as the probe light initiation time t0, where the main pulse acts on the target material 11 at time t0, and t0 is the initial time of plasma generation.

[0067] Preferably, the laser center wavelength after filtering by the first filter 30 is λ1, the laser center wavelength after filtering by the second filter 33 is λ2, the laser center wavelength after filtering by the third filter 36 is λ3, and the probe light center wavelength is λ.

[0068] After filtering by the first filter 30, the laser's center wavelength λ1 spectral component is located at position t1 of the probe light. After filtering by the second filter 33, the laser's center wavelength λ2 spectral component is located at position t2 of the probe light. After filtering by the third filter 36, the laser's center wavelength λ3 spectral component is located at position t3 of the probe light. Preferably, if λ1, λ2, and λ3 are less than λ, then t1 = T × (λ1 - (λ - Δλ)) / Δλ, t2 = T × (λ2 - (λ - Δλ)) / Δλ, and t3 = T × (λ3 - (λ - Δλ)) / Δλ. If λ1, λ2, and λ3 are greater than or equal to λ, then t1 = 2T - T × (λ1 - (λ - Δλ)) / Δλ, t2 = 2T - T × (λ2 - (λ - Δλ)) / Δλ, and t3 = 2T - T × (λ3 - (λ - Δλ)) / Δλ.

[0069] The main laser is incident on the target 11. The probe light delay adjustment device IV is adjusted, the backlight monitoring camera 13 detects the signal, and the fourth CCD camera 39 also detects the plasma shadow signal. At this time, the main laser is located at the probe light position t0. The probe light energy is >100mJ, and the pulse width is greater than 50ps and less than 200ps. A probe light pulse width greater than 50ps can cover the critical expansion time of the plasma, while a diagnostic pulse width less than 200ps ensures that the laser is within 1 / e of the laser leading edge. 2 The diagnostic light at this location can be received by the fourth CCD camera 39. At this time, the signals detected by the first CCD camera 31, the second CCD camera 34, and the third CCD camera 37 are the shadow images of the plasma at times t1, t2, and t3. Measuring the plasma shadow region in the plasma shadow images at times t1, t2, and t3 yields the moving velocity of the high-density plasma layer generated by the main laser, obtains the intensity of the laser pre-pulse, and inverts it to obtain the laser contrast and the moving velocity V of the high-density plasma layer. S The formula is shown below:

[0070] (2)

[0071] In the formula, t1 is the time when the first filter 30 obtains the plasma shadow, t2 is the time when the second filter 33 obtains the plasma shadow, t3 is the time when the third filter 36 obtains the plasma shadow, L1 is the length of the plasma shadow region in the expansion direction obtained by the first CCD camera 31, L2 is the length of the plasma shadow region in the expansion direction obtained by the second CCD camera 34, and L3 is the length of the plasma shadow region in the expansion direction obtained by the third CCD camera 37.

[0072] Assuming the intensity of the laser prepulse is Then we have the following formula:

[0073] ;

[0074] In the formula, V S The velocity of the high-density plasma layer, and The adiabatic constant and density of the target material, The wavelength of the laser. Let A be the intensity of the laser prepulse, A be the atomic weight of the target material, and Z be the number of nucleons in the target material.

[0075] Based on the above formula and the main pulse energy measured by energy meter 14 The laser contrast ratio is calculated using the following formula. :

[0076] ;

[0077] In the formula, For laser contrast, Main pulse energy, The intensity of the laser prepulse.

[0078] Therefore, by measuring the plasma shadow region at different times, the velocity of the high-density plasma layer generated by the incident laser can be obtained, and the intensity of the laser prepulse can be obtained.

[0079] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A system for measuring contrast of a laser using chirped pulses, the system comprising: The monitoring device (VI) comprises a first 50% beam splitter (29), a first filter (30), a first CCD camera (31), a second 50% beam splitter (32), a second filter (33), a second CCD camera (34), a ninth total reflection mirror (35), a third filter (36) and a third CCD camera (37); The laser pulse is divided into two beams by the collimating beam splitter (I), one of which is the main laser and the other is the probe light; The main laser passes through the main laser energy tuner (II) and enters the plasma generating device (III) to generate plasma, and then returns to the light monitoring camera (13); The probe light enters the probe light delay adjustment device (IV), becomes a chirped pulse through the probe light spectrum widening device (V), and then passes through the plasma generating device (III) to carry the information in the plasma into the monitoring device (VI); The probe light delay adjustment device (IV) comprises a translation stage, a sixth total reflection mirror (20) and a seventh total reflection mirror (21) installed on the translation stage, which is used to change the optical path of the probe light by horizontal movement of the translation stage, thereby changing the time of the probe light reaching the plasma generating device (III); The probe light spectrum widening device (V) comprises a grating (22), a concave mirror (23), a long mirror (24), a climbing mirror group (25) and a widening output mirror (26), which is used to widen the spectrum of the probe light and make the probe light into a chirped light; The probe light enters the grating (22) through the climbing mirror group (25), the first-order diffracted light of the probe light returns to the grating (22) through the concave mirror (23) and the long mirror (24), and then enters the grating (22), the concave mirror (23) and the long mirror (24) again after passing through the climbing mirror group (25) again, and the returned light of the grating (22) is reflected and output by the widening output mirror (26); The monitoring device (VI) comprises a first 50% beam splitter (29), a first filter (30), a first CCD camera (31), a second 50% beam splitter (32), a second filter (33), a second CCD camera (34), a ninth total reflection mirror (35), a third filter (36) and a third CCD camera (37); The chirped pulse carrying the information in the plasma is divided into a first pulse and a second pulse by the first 50% beam splitter (29), and the first pulse enters the first CCD camera (31) through the first filter (30); The second pulse is divided into a third pulse and a fourth pulse by the second 50% beam splitter (32), and the third pulse enters the second CCD camera (34) through the second filter (33); The fourth pulse enters the third CCD camera (37) through the ninth total reflection mirror (35) and the third filter (36).

2. The system for measuring contrast of a laser using chirped pulses of claim 1, wherein, The main laser energy tuner (II) comprises a rotating wave plate (5), a first polarization mirror (6) and a second polarization mirror (7); The first polarization mirror (6) and the second polarization mirror (7) are wire grid polarizers and are arranged in parallel to each other, the polarization direction of the main laser is changed by the rotating wave plate (5), and then the energy of the main laser is changed by the first polarization mirror (6) and the second polarization mirror (7), and the polarization direction of the main laser is changed back to the polarization direction before passing through the rotating wave plate (5).

3. The system for measuring contrast of a laser using chirped pulses of claim 1, wherein, An energy meter (14) is further included, and the main laser enters the energy meter (14) through a first 99% reflectivity mirror (8).

4. The system for measuring contrast of a laser using chirped pulses of claim 1, wherein, The first filter (30) has a transmission wavelength range of 405-415 nm, the second filter (33) has a transmission wavelength range of 390-400 nm, and the third filter (36) has a transmission wavelength range of 375-385 nm.

5. The system for measuring contrast of a laser using chirped pulses of claim 1, wherein, The chirped pulse contains a plurality of wavelength ranges, and the wavelength ranges include 405-415 nm, 390-400 nm, and 375-385 nm.

6. The system for measuring contrast of a laser using chirped pulses of claim 1, wherein, The chirped pulses of different wavelength ranges pass through the same position of the plasma generation device (III) at different times and carry different plasma information.

7. A method of measuring laser contrast of a system for measuring laser contrast using chirped pulses according to any one of claims 1 to 6, characterized in that, The method comprises the following steps: S01: split the laser pulse into the main laser and probe light, 1% of the main laser energy is measured by the energy meter (14), and the main pulse energy is calculated as ; S02: adjusting the delay of the probe light by the probe light delay adjustment device (IV) so that the plasma generation device (III) has plasma when the probe light enters the plasma generation device (III); S03: measuring the lengths of the plasma shadow area in the expansion direction by the first CCD camera (31), the second CCD camera (34), and the third CCD camera (37), and recording the lengths as L1, L2, and L3, respectively; and measuring the time points of the plasma shadow and recording the time points as t1, t2, and t3, respectively; S04: Calculate the laser contrast The calculation formula is as follows: ； ； ； wherein V S is the velocity of the high-density plasma layer, and is the heat capacity and density of the target material, is the laser wavelength, A is the atomic weight of the target material, and Z is the number of nuclei of the target material, is the laser contrast, is the main pulse energy, is the intensity of the laser pre-pulse.

8. The method of measuring laser contrast of claim 7, wherein, The step S02 comprises: If λ1, λ2, and λ3 are all less than λ, then t1=T×(λ1-(λ-Δλ)) / Δλ, t2=T×(λ2-(λ-Δλ)) / Δλ, and t3=T×(λ3-(λ-Δλ)) / Δλ; If λ1, λ2, and λ3 are all greater than or equal to λ, then t1=2T-T×(λ1-(λ-Δλ)) / Δλ, t2=2T-T×(λ2-(λ-Δλ)) / Δλ, and t3=2T-T×(λ3-(λ-Δλ)) / Δλ; T is the full width at half maximum of the chirped pulse; 2T is the full width at 1 / e of the optical pulse 2 Δλ is the full width at half maximum of the spectrum; 2Δλ is the full width at 1 / e of the spectrum 2 t0 is the initial time of the plasma generation; λ1 is the central wavelength of the laser after filtered by the first filter (30); λ2 is the central wavelength of the laser after filtered by the second filter (33); λ3 is the central wavelength of the laser after filtered by the third filter (36); λ is the central wavelength of the probe light.

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