CN116592996A - System and method for measuring laser contrast by using chirped pulse - Google Patents
System and method for measuring laser contrast by using chirped pulse Download PDFInfo
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- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
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- G—PHYSICS
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- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
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Abstract
The application discloses a system and a method for measuring laser contrast by using chirped pulses, wherein the system comprises a collimation beam splitting device, a main laser energy tuner, a plasma generating device, a detection light delay adjusting device, a detection light spectrum widening device and a monitoring device; the laser pulse is divided into two beams of light by the collimation beam-splitting device, wherein one beam is main laser, and the other beam is detection light; the main laser enters the plasma generating device to generate plasma through the main laser energy tuner, and then returns to the return light monitoring camera; the detection light enters the detection light delay adjustment device, is changed into chirped pulses through the detection light spectrum widening device, then passes through the plasma generation device, carries information in the plasma, and enters the monitoring device.
Description
Technical Field
The application belongs to the field of optical measurement, and particularly relates to a system and a method for measuring laser contrast by using chirped pulses.
Background
Laser light is light generated by stimulated radiation of atoms, electrons in the atoms absorb energy and then transition from a low energy level to a high energy level, and when the electrons fall back from the high energy level to the low energy level, the energy is released in a photon form. The laser has the characteristics of good monochromaticity and directivity and higher carrying energy. Ultrashort laser, especially femtosecond laser, pulse laser amplification technology is mature continuously, single pulse energy and peak power of ultrashort ultrastrong pulse laser are improved continuously, an ultrashort pulse amplification system of PW magnitude already appears in the prior art, and ultrashort pulse laser is applied to experimental study of interaction of light and substances, so that a series of subjects such as strong field physics, atomic molecular physics, condensed state physics and the like are also driven to develop rapidly.
Laser contrast is an important parameter for measuring the quality of a laser, and is defined as the ratio of sub-pulses to main pulses. In the prior art, the ultra-short laser pulse width reaches the femtosecond or even attosecond level, has the advantage of high instantaneous energy, but the front edge of the ultra-short laser pulse has strong spontaneous radiation amplification, and sub-pulses remain at the same time of pulse menu, and compared with the main laser pulse, the ultra-short laser pulse width is several orders of magnitude smaller in intensity, but is close to the main pulse in time and long in duration, and affects the laser quality.
The laser contrast measuring method includes three-order autocorrelation instrument, high-speed photoelectric method, etc. with the three-order autocorrelation instrument being capable of measuring large dynamic range (-10) 13 ) Is limited to a contrast in the range of a few nanoseconds. The high-speed photoelectric method can measure the laser contrast in an ultra-large time domain range, and the ultra-fast photoelectric detector and the high-bandwidth oscilloscope are used for matching measurement, but the dynamic measurement range is limited. The method for measuring the laser contrast is usually carried out by indirect measurement, if the actual intensity of the pre-pulse needs to be obtained, the actual intensity of the pre-pulse can only be obtained by calculating the intensity of the main pulse, and the direct measurement of the laser pre-pulse can not be realized.
Disclosure of Invention
The system and the method for measuring the laser contrast by using the chirped pulse provided by the application utilize high-intensity ultrashort pulse laser to generate plasmas when the high-intensity ultrashort pulse laser acts on substances such as metal, gas and the like, then utilize another beam of light, namely detection light, to carry generated plasma information, convert the information into an image to perform data analysis and data inversion on the plasmas, and finally obtain the corresponding laser pulse intensity. The method comprises the steps of carrying out pulse dispersion broadening on laser by utilizing optical color to obtain chirped pulse laser to carry different plasma information through different plasma information carried by detection light of plasmas in different time periods, obtaining plasma information corresponding to different areas by utilizing different time of arrival of laser pulses of different wavelengths at the plasmas in a plurality of wavelength periods, obtaining plasma speeds and energies of the different areas, obtaining laser contrast by calculation, directly obtaining pre-pulse intensity at different positions, obtaining the laser contrast by measurement, and simultaneously realizing ultra-wide time domain range and high dynamic range measurement.
The specific technical scheme is as follows: a system for measuring laser contrast by using chirped pulses comprises a collimation beam splitting device, a main laser energy tuner, a plasma generating device, a detection light delay adjusting device, a detection light spectrum widening device and a monitoring device;
the laser pulse is divided into two beams of light by the collimation beam-splitting device, wherein one beam is main laser, and the other beam is detection light;
the main laser enters the plasma generating device to generate plasma through the main laser energy tuner, and then returns to the return light monitoring camera;
the detection light enters the detection light delay adjustment device, is changed into chirped pulses through the detection light spectrum widening device, then passes through the plasma generation device, carries information in the plasma, and enters the monitoring device;
preferably, the probe light delay adjustment device comprises a translation stage, and a sixth total reflection mirror and a seventh total reflection mirror which are arranged on the translation stage and are used for changing the optical path length of the probe light through the horizontal movement of the translation stage so as to change the time of the probe light reaching the plasma generation device.
Preferably, the detected light spectrum widening device comprises a grating, a concave mirror, a long-strip reflecting mirror, a climbing mirror group and a widening output reflecting mirror, and is used for performing spectrum widening on detected light to change the detected light into chirped light;
the first-order diffraction light of the detection light returns to the grating through the concave mirror and the long-strip reflecting mirror, enters the grating, the concave mirror and the long-strip reflecting mirror after passing through the climbing mirror group again, and returns to the grating to be reflected and output through the widening output reflecting mirror.
The monitoring device comprises a first 50% beam splitting piece, a first optical filter, a first CCD camera, a second 50% beam splitting piece, a second optical filter, a second CCD camera, a ninth total reflection mirror, a third optical filter and a third CCD camera.
Preferably, the primary laser energy tuner comprises a rotating waveplate, a first polarizing mirror and a second polarizing mirror.
Preferably, the first polarizing mirror and the second polarizing mirror are wire grid polarizers and are arranged in parallel, the polarization direction of the main laser is changed through the rotating wave plate, the energy of the main laser is changed through 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 the main laser passes through the rotating wave plate.
Preferably, the laser device further comprises an energy meter, said main laser light entering said energy meter via said first 99% reflectivity mirror.
Preferably, the information in the chirped pulse carried plasma is split into a first pulse and a second pulse by the first 50% beam splitting piece, and the first pulse enters the first CCD camera through the first optical filter;
the second pulse is split into a third pulse and a fourth pulse by a second 50% beam splitting piece, and the third pulse enters a second CCD camera through a second optical filter;
and the fourth pulse enters a third CCD camera through a ninth total reflecting mirror and a third optical filter.
Preferably, the first filter has a transmission wavelength range of 405 to 415nm, the second filter has a transmission wavelength range of 390 to 400nm, and the third filter has a transmission wavelength range of 375 to 385nm.
Preferably, the chirped pulses comprise a plurality of wavelength ranges including 405 to 415nm, 390 to 400nm, and 375 to 385nm.
Preferably, the chirped pulses in different wavelength ranges pass through the plasma generating device at the same position and at different times, and carry different plasma information.
A method of measuring laser contrast using a chirped pulse system for measuring laser contrast, comprising the steps of:
s01: splitting laser pulse into main laser and detection light, measuring 1% of main laser energy by energy meter, and calculating main pulse energy as I;
s02: the detection light delay adjustment device is adjusted to delay detection light, so that when the detection light enters the plasma generation device, the plasma generation device is provided with plasma;
s03: the lengths of the plasma shadow areas in the expansion direction are measured by the first CCD camera, the second CCD camera and the third CCD camera and are respectively marked as L 1 、L 2 And L 3 The method comprises the steps of carrying out a first treatment on the surface of the The time when the plasma shadows were measured was denoted t 1 、t 2 And t 3 ；
S04: the laser contrast eta is calculated, and the calculation formula is as follows:
η＝I：I p ；
wherein V is S For the movement speed of the high density plasma layer, gamma and rho 0 Is the adiabatic constant and density of the target, lambda is the laser wavelength, A is the atomic weight of the target, Z is the number of target nuclei, eta is the laser contrast, I is the main pulse energy, I p Is the intensity of the laser pre-pulse.
Preferably, the step S02 includes:
if lambda is 1 ，λ 2 ，λ 3 Are all smaller than lambda, t 1 ＝T×(λ 1 -(λ-Δλ))/Δλ，t 2 ＝T×(λ 2 -(λ-Δλ))/Δλ，t 3 ＝T×(λ 3 -(λ-Δλ))/Δλ；
If lambda is 1 ，λ 2 ，λ 3 Are all greater than or equal to lambda, t 1 ＝2T-T×(λ 1 -(λ-Δλ))/Δλ，t 2 ＝2T-T×(λ 2 -(λ-Δλ))/Δλ，t 3 ＝2T-T×(λ 3 -(λ-Δλ))/Δλ；
T is the half-width, half-width and full-width of the chirped pulse time width; 2T is the light pulse 1/e 2 Pulse width is measured; Δλ is the full width at half maximum of the spectrum; 2 delta lambda is the spectrum 1/e 2 A place; t is t 0 An initial time for plasma generation; lambda (lambda) 1 The center wavelength of the laser after being filtered by the first optical filter; lambda (lambda) 2 The center wavelength of the laser after being filtered by the second optical filter; lambda (lambda) 3 The center wavelength of the laser after being filtered by the third optical filter; lambda is the probe light center wavelength.
The beneficial effects of the application are as follows:
the system and the method for measuring the laser contrast by using the chirped pulse provided by the application utilize high-intensity ultrashort pulse laser to generate plasmas when the high-intensity ultrashort pulse laser acts on substances such as metal, gas and the like, then utilize another beam of light, namely detection light, to carry generated plasma information, convert the information into an image to perform data analysis and data inversion on the plasmas, and finally obtain the corresponding laser pulse intensity. The method comprises the steps of carrying out pulse dispersion broadening on laser by utilizing optical color to obtain chirped pulse laser to carry different plasma information through different plasma information carried by detection light of plasmas in different time periods, obtaining plasma information corresponding to different areas by utilizing different time of arrival of laser pulses of different wavelengths at the plasmas in a plurality of wavelength periods, obtaining plasma speeds and energies of the different areas, obtaining laser contrast by calculation, directly obtaining pre-pulse intensity at different positions, obtaining the laser contrast by measurement, and simultaneously realizing ultra-wide time domain range and high dynamic range measurement.
Drawings
FIG. 1 is a schematic diagram showing the operation of a system for measuring laser contrast using chirped pulses in accordance with one embodiment of the present application;
FIG. 2 is a schematic diagram of a system for measuring laser contrast using chirped pulses in accordance with one embodiment of the present application;
FIG. 3 shows a schematic diagram of spectral broadening of a measurement system in an embodiment of the present application;
FIG. 4 is a schematic diagram showing a chirped light carrying plasma information process in a system for measuring laser contrast using chirped pulses according to one embodiment of the present application;
FIG. 5 is a schematic diagram showing time delay of chirped light and main laser in a system for measuring laser contrast using chirped pulses according to an embodiment of the present application;
1, a first total reflection mirror; 2. a second total reflection mirror; 3. a first beam splitting sheet; 4. a third total reflection mirror; 5. rotating the wave plate; 6. a first polarizing mirror; 7. a second polarizing mirror; 8. a first 99% reflectance mirror; 9. a second 99% reflectance mirror; 10. a first lens; 11. a target material; 12. 800nm fundamental frequency optical filter; 13. a return light monitoring camera; 14. an energy meter; 15. a second lens; 16. a frequency doubling crystal; 17. a third lens; 18. a fourth total reflection mirror; 19. a fifth total reflection mirror; 20. a sixth total reflection mirror; 21. a seventh total reflection mirror; 22. a grating; 23. a concave mirror; 24. a long-strip reflector; 25. climbing the mirror group; 26. widening the output mirror; 27. a fourth lens; 28. an eighth total reflection mirror; 29. a first 50% beam splitting sheet; 30. a first optical filter; 31. a first CCD camera; 32. a second 50% beam splitting sheet; 33. a second optical filter; 34. a second CCD camera; 35. a ninth total reflection mirror; 36. a third filter; 37. a third CCD camera; 38. a second beam splitting sheet; 39. a fourth CCD camera;
i, a collimation beam splitting device; II, a main laser energy tuner; III, a plasma generating device; IV, detecting light delay adjusting device; v, a detection light spectrum widening device; VI, a monitoring device.
Detailed Description
The following detailed description of embodiments of the application provides further details of the embodiments described, and it should be apparent that the embodiments described are merely some, rather than all, examples of the application. It should be noted that, without conflict, the embodiments of the present application and features of the embodiments may be combined with each other.
The terms first, second, and the like in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used may be interchanged where appropriate such that the embodiments described herein may be implemented in other sequences than those illustrated or otherwise described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements that are expressly listed or inherent to such process, method, article, or apparatus.
It should be understood that the term "and/or" as used herein is merely one relationship describing the association of the associated objects, meaning that there may be three relationships, e.g., a and/or B, may represent: a exists alone, A and B exist together, and B exists alone. In addition, the character "/" herein generally indicates that the front and rear associated objects are an "or" relationship.
Examples
Fig. 1 is a schematic diagram of the system operating principle of measuring laser contrast by using chirped pulses in one embodiment of the present application. After collimation and beam splitting, the laser is split into two beams, and the preferable laser is high-energy femtosecond laser with the wavelength of 800nm and 30 fs. One of the two beams is divided into two beams of light which is used as main laser, the energy is 90 percent, and the two beams of light are acted on the target 11 to generate plasma after energy tuning. The other beam of light is detection light, the energy is 10%, the detection light is converted into chirped pulses, namely chirped light after being subjected to delay device and spectrum broadening, the chirped light comprises different wavelength bands, plasma is imaged through plasma, plasma information is collected, the plasma information is carried into a monitoring device VI, then inversion data are obtained through data inversion of the plasma information, and finally the laser contrast is calculated.
Fig. 2 is a schematic diagram of a system for measuring laser contrast by using chirped pulses according to an embodiment of the present application, where the device for measuring laser contrast by using chirped pulses includes a collimation beam splitting device i, a main laser energy tuner ii, a plasma generating device iii, a probe light delay adjusting device iv, a probe light spectrum widening device v, a monitoring device vi, and other optical components. The laser pulse is divided into two beams by a collimation beam-splitting device I, wherein one beam of energy accounts for 90% of the total energy of the laser and is the main laser, and the energy range of the main laser is preferably 10 18 ～10 20 . The main laser passes through the main laser energy tuner II by the third total reflection mirror 4, enters the plasma generating device III by the first 99% reflection mirror 8 and the second 99% reflection mirror 9 to generate plasma, and returns to the back light monitoring camera 13 by the second 99% reflection mirror 9 and the 800nm fundamental frequency optical filter 12. The other beam of energy accounts for 10% of the total energy of the laser and is probe light, the probe light sequentially passes 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, and enters the probe light delay adjustment device IV and the probe light spectrum widening device V, at the moment, the probe light is chirped light with different wavelength bands, the chirped light passes through the plasma generating device III and carries information in the ion body generated by the main laser, and the information enters the monitoring device VI through the fourth lens 27 and the eighth total reflection mirror 28.
And carrying out data inversion and calculating laser contrast according to the plasma information.
Wherein collimation beam splitting device I includes: a first total reflection mirror 1, a second total reflection mirror 2 and a first beam splitter 3.
The main laser energy tuner II comprises; a rotating wave plate 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 wire grid polarizers, and a piece of reflective grating is selected, so that the polarization direction and energy of the incident laser light can be changed by rotating the reflecting mirror. The linearly polarized light changes its polarization direction by rotating the wave plate 5, and then changes its energy by the first polarizing mirror 6 and the second polarizing mirror 7 parallel to each other, and at the same time changes its polarization direction back to the original direction. The main laser energy tuner II has the function of realizing the automatic adjustment of the main laser energy, and simultaneously keeping parameters such as the pulse width, the light spot, the beam divergence angle and the like of the main laser unchanged.
The plasma generating apparatus III includes: a first lens 10 and a target 11. Preferably, the target 11 is a micrometer metal film. The plasma generating device III is used for generating plasma when main laser acts on the target 11, and carrying plasma information when probe light, namely chirped light, passes through the device.
The probe light delay adjusting device IV comprises: the translation stage, the sixth total reflection mirror 20 and the seventh total reflection mirror 21, and preferably, the sixth total reflection mirror 20 and the seventh total reflection mirror 21 are mounted on the translation stage. The function of the detecting light delay adjusting device IV is to change the optical path of the detecting light through the horizontal movement of the translation stage, so as to change the time of the detecting light reaching the plasma generating device III, namely the ion body area, and enable the detecting light to precisely generate plasma when the detecting light passes through the surface of the target 11 in parallel, and the detecting light is used for capturing plasma to carry plasma information.
The probe light spectrum widening device v includes: a grating 22, a concave mirror 23, a long mirror 24, a set of climbing mirrors 25 and a widening output mirror 26. The spectrum widening device V of the detection light is used for performing spectrum widening on the detection light, so that the detection light is chirped light.
The monitoring device VI is composed of a series of reflectors with different wavelengths, an optical filter and an acquisition CCD, and comprises: a first 50% beam splitting sheet 29, a first optical filter 30, a first CCD camera 31, a second 50% beam splitting sheet 32, a second optical filter 33, a second CCD camera 34, a ninth total reflection mirror 35, a third optical filter 36, and a third CCD camera 37. The monitoring device VI has the function of acquiring plasma information by imaging the plasma captured by the detection light, so that the subsequent inversion measurement of laser contrast by using the plasma information is facilitated.
Preferably, the position relationship of each component in the system is as follows, and the first total reflection mirror 1, the second total reflection mirror 2, the first beam splitting piece 3 and the third total reflection mirror 4 are in mirror surface parallel; the third total reflecting mirror 4 is coaxial with the rotary wave plate 5; the first polarizing mirror 6 and the second polarizing mirror 7 are in mirror parallel; the second 99% reflectivity reflecting mirror 9, the first lens 10, the target 11, the 800nm fundamental frequency optical filter 12 and the return light monitoring camera 13 are coaxial. The second total reflecting mirror 2, the first beam splitting piece 3, the second lens 15, the frequency doubling crystal 16, the third lens 17 and the fourth total reflecting mirror 18 are coaxial; the fourth total reflection mirror 18 and the fifth total reflection mirror 19 are perpendicular in mirror surface; the sixth total reflection mirror 20 and the seventh total reflection mirror 21 are arranged on the translation stage in a manner that the mirror surfaces are perpendicular. The broadening output reflector 26, the fourth lens 27 and the eighth total reflector 28 are coaxial, and the axis connecting line is parallel to the surface of the target 11 and has a distance of 1mm. The first 50% beam splitting sheet 29, the second 50% beam splitting sheet 32 and the ninth total reflection mirror 35 are coaxial and mirror-parallel.
When the main laser is applied to the target 11, the surface thereof generates plasma and rapidly overflows, due to the energy range 10 of the main laser 18 ～10 20 The density of the generated plasma is 10 18 -10 20 Individual/cm 3 At this time, the probe light may pass through the plasma. When the detection light passes through the plasma, the detection light is deflected by refraction effect and the like because the refraction index of the plasma is different from that of the surrounding environment, so that shadow exists after the laser passes through a plasma region, and the shadow region directly reflects the size of the plasma. The detection light, namely chirped light, penetrates through the plasma region perpendicular to the plasma expansion direction, the time that different wavelengths pass through the plasma region is different, the captured plasma images are different, and therefore plasma information with different times is carried, the detection light is divided into three beams by the first 50% beam splitting piece 29, the second 50% beam splitting piece 32 and the ninth total reflecting piece 35 through the fourth lens 27, the eighth total reflecting piece 28 and the second beam splitting piece 38, and the detection light reaches the corresponding first CCD camera 31, the second CCD camera 34 and the third CCD camera 37 through the first optical filter 30, the second optical filter 33 and the third optical filter 36 respectively to form images, so that plasma shadow regions with different moments are obtained. Preferably, the information in the chirped pulse-carried plasma is split into a first pulse and a second pulse by the first 50% beam splitter (29), the first pulse being fed through a first filter (30)Entering a first CCD camera (31); the second pulse is split into a third pulse and a fourth pulse by a second 50% beam splitting sheet (32), and the third pulse enters a second CCD camera (34) through a second optical filter (33); the fourth pulse enters a third CCD camera (37) through a ninth total reflecting mirror (35) and a third optical filter (36). Preferably, the first filter 30 has a transmission wavelength range of 405 to 415nm, the second filter 33 has a transmission wavelength range of 390 to 400nm, and the third filter 36 has a transmission wavelength range of 375 to 385nm.
The main laser with 1% transmission through the first 99% reflectivity reflector 8 enters the energy meter 14, the energy meter 14 measures the laser energy in real time, and the actual energy acting on the target 11 is inverted, and the calculation formula is as follows:
wherein I is the actual energy of the laser acting on the target 11, I 0 Is the laser energy measured by the energy meter 14.
The main laser beam changes the direction of the laser beam and is collimated by the first 99% reflectivity mirror 8 and the second 99% reflectivity mirror 9, is focused by the first lens 10, and finally is vertically incident on the target 11 at the focal point, i.e. the incident angle is 0 °. The return light of the target 11 passes through the second 99% reflectivity reflecting mirror 9 and the 800nm fundamental frequency light filter 12 to enter the return light monitoring camera 13 by 1%.
Preferably, if the main laser acts on the target 11 and no plasma is generated, the wavelength of the returned light passing through the target 11 is 800nm, the returned light is filtered by an 800nm fundamental frequency optical filter 12, and the returned light monitoring camera 13 cannot monitor the optical signal; if the main laser acts on the target 11, if plasma is generated, the returned light passing through the target 11 is mixed with other wavelength light, and is filtered by the 800nm fundamental frequency light filter 12, and bright spots can be monitored on the returned light monitoring camera 13.
Fig. 3 shows a schematic view of the spectral broadening of the measurement system according to an embodiment of the present application, wherein the grating 22, the concave mirror 23 and the elongated mirror 24 are centered on the same straight line and parallel to each other. The detection light, namely chirped light, enters the grating 22 through the climbing mirror group 25, the first-order diffraction light returns to the grating 22 through the concave mirror 23 and the long-strip reflecting mirror 24, then enters the grating 22 again through the climbing mirror group 25, returns to the grating 22 again through the concave mirror 23 and the long-strip reflecting mirror 24, and finally is reflected and output by the broadening output reflecting mirror 26.
Preferably, the probe light passes through the climbing mirror group 25, so that the probe light passes through the grating 22, the concave mirror 23 and the long-strip reflecting mirror 24, and the spectrum broadening of the probe light is completed, so that the probe light is chirped light, namely chirped pulses, and the chirped light contains pulses with different wavelengths, preferably pulses with the wavelengths ranging from 405 nm to 415nm, 390 nm to 400nm and 375 nm to 385nm.
FIG. 4 is a schematic diagram showing a chirped light carrying plasma information process in a system for measuring laser contrast using chirped pulses according to one embodiment of the present application; the stretched probe light, i.e., chirped light, contains laser pulses of different wavelengths at different speeds and at the same location but at different times in the plasma region.
FIG. 5 shows a schematic diagram of time delay of chirped light and main laser in a system for measuring laser contrast by chirped pulse according to an embodiment of the present application, wherein the full width at half maximum of the time width of the probe light passing through the probe light spectrum widening device V is measured to be T by an autocorrelation instrument, and the time width of the probe light is measured to be 1/e of the light pulse 2 The pulse width may be approximately 2T. The half-width of the spectrum of the detected light stretched by the detected light spectrum stretching device V is delta lambda, and the spectrum is 1/e 2 The spectrum can be approximated as 2Δλ. Detection light pulse front 1/e 2 Defined as the probe light start time t 0 The main pulse is at t 0 Is acted on the target 11 at any time, t 0 The initial time of plasma generation.
Preferably, the laser center wavelength after the first filter 30 filters is lambda 1 The central wavelength of the laser after being filtered by the second filter 33 is lambda 2 The center wavelength of the laser light after being filtered by the third filter 36 is lambda 3 The probe light center wavelength is lambda.
The central wavelength of the laser after being filtered by the first filter 30 is lambda 1 The spectral component being located in the probe light t 1 The position of the second filter 33 is the laser after filteringHeart wavelength lambda 2 The spectral component being located in the probe light t 2 The position, the center wavelength of the laser light after being filtered by the third filter 36 is lambda 3 The spectral component being located in the probe light t 3 Position. Preferably, if lambda 1 ，λ 2 ，λ 3 Less than lambda, t 1 ＝T×(λ 1 -(λ-Δλ))/Δλ，t 2 ＝T×(λ 2 -(λ-Δλ))/Δλ，t 3 ＝T×(λ 3 - (lambda-delta lambda))/delta lambda. If lambda is 1 ，λ 2 ，λ 3 Lambda, t is greater than or equal to 1 ＝2T-T×(λ 1 -(λ-Δλ))/Δλ，t 2 ＝2T-T×(λ 2 -(λ-Δλ))/Δλ，t 3 ＝2T-T×(λ 3 -(λ-Δλ))/Δλ。
The main laser is incident on the target 11, the detection light delay adjusting device IV is adjusted, the back light monitoring camera 13 detects the signal, the fourth CCD camera 39 also just detects the plasma shadow signal, and the main laser is positioned at the detection light t 0 Position. The detected light energy is more than 100mJ, and the broadening pulse width is more than 50ps and less than 200ps. The key expansion time of the plasma can be covered when the detection light is more than 50ps, and the diagnostic light is less than 200ps, so that the laser front edge is ensured to be 1/e 2 The diagnostic light at that point can be received by the fourth CDD camera 39. At this time, the signals detected in the first, second and third CCD cameras 31, 34 and 37 are at t 1 ，t 2 ，t 3 Shadow image of the plasma at the moment. Measurement of t 1 ，t 2 ，t 3 The moving speed of the high-density plasma layer generated by the main laser is obtained from the plasma shadow area in the plasma shadow image at moment, the intensity of the laser pre-pulse is obtained, and the laser contrast and the moving speed V of the high-density plasma layer can be obtained by inversion S The formula is as follows:
wherein t is 1 Obtaining a plasma shadow time, t, for the first filter 30 2 Obtaining a plasma shadow time, t, for the second filter 33 3 Obtaining a plasma shadow for the third filter 36Time, L 1 Obtaining the length L of the plasma shadow region in the expansion direction for the first CCD camera 31 2 Obtaining the length L of the plasma shadow region in the expansion direction for the second CCD camera 34 3 The length of the plasma shadow region in the expansion direction is obtained for the third CCD camera 37.
Let the intensity of the laser pre-pulse be I p The following formula is given:
wherein V is S For the movement speed of the high density plasma layer, gamma and rho 0 Is the adiabatic constant and density of the target material, lambda is the laser wavelength, I p The intensity of the laser pre-pulse is A, the weight of target atoms is A, and Z is the number of target nuclei.
The main pulse energy I measured by the energy meter 14 according to the above formula is calculated as the laser contrast η according to the following formula:
η＝I：I p
wherein eta is laser contrast, I is main pulse energy, I p Is the intensity of the laser pre-pulse.
Therefore, by measuring the plasma shadow areas at different moments, the speed of the high-density plasma layer generated by the incident laser is obtained, and the intensity of the laser pre-pulse is obtained.
It should be understood that the foregoing examples of the present application are provided merely for clearly illustrating the present application and are not intended to limit the embodiments of the present application, and that various other changes and modifications may be made therein by one skilled in the art without departing from the spirit and scope of the present application as defined by the appended claims.
Claims (10)
1. The system for measuring the laser contrast by using the chirped pulse is characterized by comprising a collimation beam splitting device (I), a main laser energy tuner (II), a plasma generating device (III), a detection light delay adjusting device (IV), a detection light spectrum widening device (V) and a monitoring device (VI);
the laser pulse is divided into two beams of light by the collimation beam-splitting device (I), wherein one beam is main laser, and the other beam is detection light;
the main laser enters the plasma generating device (III) to generate plasma through the main laser energy tuner (II), and returns to the return light monitoring camera (13);
the detection light enters the detection light delay adjustment device (IV), is changed into chirped pulses through the detection light spectrum widening device (V), then passes through the plasma generation device (III), carries information in the plasma, and enters the monitoring device (VI);
the detection light delay adjustment device (IV) comprises a translation stage, a sixth total reflection mirror (20) and a seventh total reflection mirror (21) which are arranged on the translation stage, and the detection light delay adjustment device is used for changing the optical path of detection light through the horizontal movement of the translation stage so as to change the time of the detection light reaching the plasma generation device (III).
The detection light spectrum widening device (V) comprises a grating (22), a concave mirror (23), a long-strip reflecting mirror (24), a climbing mirror group (25) and a widening output reflecting mirror (26), and is used for performing spectrum widening on detection light to change the detection light into chirped light;
the detection light enters the grating (22) through the climbing mirror group (25), first-order diffraction light of the detection light returns to the grating (22) through the concave mirror (23) and the long-strip reflecting mirror (24), enters the grating (22), the concave mirror (23) and the long-strip reflecting mirror (24) after passing through the climbing mirror group (25), and returns to the grating (22) to be reflected and output through the widening output reflecting mirror (26);
the monitoring device (VI) comprises a first 50% beam splitting sheet (29), a first optical filter (30), a first CCD camera (31), a second 50% beam splitting sheet (32), a second optical filter (33), a second CCD camera (34), a ninth total reflection mirror (35), a third optical filter (36) and a third CCD camera (37).
2. The system for measuring laser contrast using chirped pulses of claim 1 wherein the main laser energy tuner (ii) comprises a rotating waveplate (5), a first polarizing mirror (6) and a second polarizing mirror (7).
3. A system for measuring laser contrast with chirped pulses according to claim 2, characterized in that the first polarizing mirror (6) and the second polarizing mirror (7) are wire grid polarizers and are arranged parallel to each other, the polarization direction of the main laser light is changed by the rotation wave plate (5), the energy of the main laser light is changed by the first polarizing mirror (6) and the second polarizing mirror (7), and the polarization direction of the main laser light is changed back to the polarization direction before passing the rotation wave plate (5).
4. The system for measuring laser contrast with chirped pulses of claim 1 further comprising an energy meter (14), the main laser light entering the energy meter (14) via the first 99% reflectivity mirror (8).
5. The system for measuring laser contrast using chirped pulses of claim 1 wherein,
the information in the chirped pulse plasma is split into a first pulse and a second pulse by the first 50% beam splitter (29), and the first pulse enters a first CCD camera (31) through a first optical filter (30);
the second pulse is split into a third pulse and a fourth pulse by a second 50% beam splitting sheet (32), and the third pulse enters a second CCD camera (34) through a second optical filter (33);
the fourth pulse enters a third CCD camera (37) through a ninth total reflecting mirror (35) and a third optical filter (36).
6. The system for measuring laser contrast using chirped pulses according to claim 1, wherein the first filter (30) has a transmission wavelength range of 405 to 415nm, the second filter (33) has a transmission wavelength range of 390 to 400nm, and the third filter (36) has a transmission wavelength range of 375 to 385nm.
7. The system for measuring laser contrast using chirped pulses of claim 1 wherein the chirped pulses comprise a plurality of wavelength ranges including 405-415 nm, 390-400 nm, and 375-385 nm.
8. The system for measuring laser contrast using chirped pulses of claim 1 wherein the chirped pulses of different wavelength ranges are positioned at the same location across the plasma generating device (iii) at different times and carry different plasma information.
9. A method of measuring laser contrast using a chirped pulse laser contrast measuring system according to any one of claims 1 to 8, comprising the steps of:
s01: dividing laser pulse into main laser and detection light, measuring 1% of main laser energy by an energy meter (14), and calculating main pulse energy to be recorded as I;
s02: the detection light delay adjusting device (IV) is adjusted to delay detection light, so that when the detection light enters the plasma generating device (III), the plasma generating device (III) is provided with plasma;
s03: the lengths of the plasma shadow areas in the expansion direction are measured by the first CCD camera (31), the second CCD camera (34) and the third CCD camera (37) and are respectively marked as L 1 、L 2 And L 3 The method comprises the steps of carrying out a first treatment on the surface of the The time when the plasma shadows were measured was denoted t 1 、t 2 And t 3 ；
S04: the laser contrast eta is calculated, and the calculation formula is as follows:
η＝I：I p ；
wherein V is S For the movement speed of the high density plasma layer, gamma and rho o Is the adiabatic constant and density of the target, lambda is the laser wavelength, A is the atomic weight of the target, Z is the number of target nuclei, eta is the laser contrast, I is the main pulse energy, I p Is the intensity of the laser pre-pulse.
10. The method of measuring laser contrast according to claim 9, wherein the step S02 includes:
if lambda is 1 ，λ 2 ，λ 3 Are all smaller than lambda, t 1 ＝T×(λ 1 -(λ-Δλ))/Δλ，t 2 ＝T×(λ 2 -(λ-Δλ))/Δλ，t 3 ＝T×(λ 3 -(λ-Δλ))/Δλ；
If lambda is 1 ，λ 2 ，λ 3 Are all greater than or equal to lambda, t 1 ＝2T-T×(λ 1 -(λ-Δλ))/Δλ，t 2 ＝2T-T×(λ 2 -(λ-Δλ))/Δλ，t 3 ＝2T-T×(λ 3 -(λ-Δλ))/Δλ；
T is the half-width, half-width and full-width of the chirped pulse time width; 2T is the light pulse 1/e 2 Pulse width is measured; Δλ is the full width at half maximum of the spectrum; 2 delta lambda is the spectrum 1/e 2 A place; t is t 0 An initial time for plasma generation; lambda (lambda) 1 The center wavelength of the laser after being filtered by the first optical filter (30); lambda (lambda) 2 The center wavelength of the laser after being filtered by the second optical filter (33); lambda (lambda) 3 The center wavelength of the laser after being filtered by the third optical filter (36); lambda is the probe light center wavelength.
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