CN117559200A

CN117559200A - Laser pulse time domain contrast enhancement method and system

Info

Publication number: CN117559200A
Application number: CN202311586442.4A
Authority: CN
Inventors: 鲁巍; 何运孝; 华剑飞
Original assignee: Tsinghua University
Current assignee: Tsinghua University
Priority date: 2023-11-24
Filing date: 2023-11-24
Publication date: 2024-02-13

Abstract

The present disclosure relates to a laser pulse temporal contrast enhancement method and system, the method comprising: performing frequency shift on the main pulse of the laser pulse to be enhanced by utilizing target plasma matched with the pulse parameters of the main pulse of the laser pulse to be enhanced to obtain a frequency shift laser pulse, wherein the main pulse frequency of the frequency shift laser pulse is different from the pre-pulse frequency; filtering off the pre-pulse of the frequency-shifted laser pulse by using at least one filtering reflector to obtain a target laser pulse with enhanced contrast, wherein the passband of the filtering reflector covers the main pulse frequency of the frequency-shifted laser pulse, and the stopband of the filtering reflector covers the pre-pulse frequency of the frequency-shifted laser pulse; the reflectivity of the filter mirror passband is higher than the reflectivity of the filter mirror stopband. According to the embodiment of the disclosure, the time domain contrast enhancement of the nearest femtosecond scale at the pulse front edge can be realized, the structure is simple, the cost is low, the energy loss is low, the laser pulse distribution is not destroyed, and the time domain contrast enhancement can be applied to femtosecond laser pulses.

Description

Laser pulse time domain contrast enhancement method and system

Technical Field

The disclosure relates to the technical field of laser, in particular to a laser pulse time domain contrast enhancement method and a system.

Background

Since the Chirped-Pulse Amplification (CPA for short) technology has proposed that the development of ultra-short ultra-strong lasers has been greatly promoted, the focused light intensity of ultra-short ultra-strong laser pulses generated by ultra-short ultra-strong lasers has exceeded 10 a ¹⁸ W/cm ² . The interaction between the ultra-short ultra-strong pulse laser and the substance is enabled to enter a relativity interval, and the applications of accelerating electrons, protons and ions, generating X/gamma rays, mid-infrared pulses, terahertz, higher harmonics and the like can be realized. However, with the increase of the focusing light intensity, the time domain contrast problem of the ultrashort ultrastrong pulse laser will be increasingly highlighted, because the substances which interact with the ultrashort ultrastrong pulse laser pulse, such as solid targets, typically have ionization threshold, that is, the ionization threshold of the target molecules or atoms can be ionized when the focusing light intensity of the incident laser exceeds the ionization threshold, and the ionization threshold of the common substances is generally 10 ^11- ¹² W/cm ² Magnitude. Meanwhile, since the resonant cavity structure in the ultra-short ultra-strong laser also introduces spontaneous radiation in the process of realizing laser gain, multiple reflections among cavity mirrors can also form multiple pulses with time difference before and after, and the ultra-short ultra-strong laser pulse generated by the ultra-short ultra-strong laser is accompanied with noise such as pre-pulse of the pulse front edge, spontaneous radiation and the like. The ratio of the peak light intensity between the noise such as the pre-pulse and the spontaneous emission of the laser front and the main pulse, that is, the time domain contrast of the laser pulse, is an index for measuring the cleanliness of the laser pulse on the time domain distribution, which can be defined as that the higher the ratio of the peak light intensity of the pre-pulse to the peak light intensity of the main pulse is, the cleaner the laser pulse on the time domain distribution is, that is, the less the noise such as the pre-pulse and the spontaneous emission of the laser pulse is.

The focusing peak value of the ultrashort ultrastrong laser pulse can reach 10 ²³ W/cm ² Above, if the time domain contrast of the ultrashort ultrastrong laser pulse is low, the focusing light intensity of the pre-pulse in the ultrastrong laser pulse is likely to exceed the ionization threshold of the target, so that the pre-pulse at the pulse front will ionize the target in advance, thereby destroying the interaction between the main pulse and the targetActing to influence the results obtained. In order not to affect the interaction of the main pulse and the target body, the focusing light intensity of the pre-pulse needs to be controlled at 10 ¹² W/cm ² The time domain contrast of the laser pulse is required to reach 10 ¹¹ The above. In addition, it is also critical what time scale reaches a higher temporal contrast at the leading edge of the main pulse. Limited by factors such as spontaneous emission of the laser, it is more difficult to achieve improved temporal contrast closer to the main pulse. Currently, the order of picoseconds reaches 10 at the leading edge of the main pulse ¹⁰ Still having great difficulty in the time domain contrast. However, in terms of the physical process of interaction between the laser pulse and the substance, particularly under the condition of interaction with an ultra-thin target body having a thickness on the nanometer scale, it is necessary to achieve an ultra-high time domain contrast on the time scale of the femtosecond scale of the leading edge of the main pulse in order not to affect the interaction process of the main pulse with the target body.

At present, the existing technology for enhancing the time domain contrast of the pulse laser mainly comprises a cross polarization filter technology (XPW), a plasma mirror technology, ultra-short pulse pump optical parametric amplification and the like. The cross polarization filtering technology mainly utilizes the third-order nonlinear effect of crystals, and in some crystals, when the intensity of incident laser reaches a certain threshold value, the polarization direction of the incident laser rotates. Because the process is related to the light intensity, the light intensity of the pre-pulse is obviously lower than that of the main pulse, the polarization direction of the main pulse can be rotated by 90 degrees while the polarization direction of the pre-pulse is kept unchanged by adjusting the proper light intensity, and orthogonal polarization elements are respectively arranged at the incident end and the emergent end, so that the pre-pulse can be filtered out, and the improvement of the time domain contrast ratio is realized. However, this kind of laser has a large loss of energy, i.e. the energy conversion efficiency is low (generally below 30%), the time domain contrast enhancement effect is limited by the polarization selection efficiency of the polarizing element, and in addition, the laser pulse needs to penetrate the crystal medium (commonly such as barium fluoride BaF ₂ ) This also limits the application of higher power lasers, which are currently used mainly for temporal contrast enhancement of laser pulses with single pulse energy at the millijoule level. The plasma mirror technique is an end contrast enhancement technique, which is based on focusing laser pulses onto the surface of a transparent object, The control of the light intensity of the surface is realized by controlling the light spot size of the physical surface, so that the focus light intensity of the pre-pulse reaches the ionization threshold value of the object and a large amount of plasmas are ionized, when the density of the plasmas exceeds the critical density, the main pulse is reflected at the moment, and the pre-pulse is transmitted, so that the reflected main pulse has high time domain contrast. However, the reflection of the main pulse is closely related to the light intensity control of the plasma mirror surface in this technique, and in extreme cases, the main pulse may penetrate the plasma mirror together with the pre-pulse and nothing is obtained, and in addition, this technique tends to be destructive to the longitudinal distribution of the main pulse. The ultra-short pulse pump optical parametric amplification technology is mainly used for inhibiting spontaneous emission of laser pulses, and the fluorescence relaxation time of the optical parametric technology is equivalent to the pulse width of pump laser, so that the parametric fluorescence can be controlled to be in the picosecond or even femtosecond level by using the ultra-short pulse pump light, and the time domain contrast improvement of a main pulse front edge with a more similar scale is realized. However, the structure of the technology is complex, the pulse width requirement on the pump light is high, and the pump light with the picosecond or even femtosecond scale is required to be used, which can be calculated as a set of ultra-short pulse lasers. And is limited by the ultra-short pumping energy limit, the technology is mainly used for improving the time domain contrast ratio under the condition of front-end low pulse energy, other optical parametric chirped pulse amplification technology (OPCPA) is basically similar to the ultra-short pulse pumping optical parametric amplification principle, and the main difference is that the technology combines the chirped pulse amplification technology, realizes high power output and uses pumping light mainly in picosecond scale in order to realize the improvement of the time domain contrast ratio.

Even if the technologies are comprehensively utilized, such as combining an XPW technology and an ultrashort pulse pumping optical parameter technology, combining an XPW technology, an OPCPA technology and a plasma mirror technology, the effect of improving the time domain contrast of the ultrastrong laser pulse above the clapping level is limited, and the highest time domain contrast of the ultrastrong laser pulse reaches 10 at the nearest 2-3 picoseconds scale at the front edge of the main pulse ¹² Which is enhanced by only 4 orders of magnitude compared to the time domain contrast before being optimized, and is also difficult to achieve in the femtosecond scale. Therefore, how to effectively improve the time domain contrast of the ultrashort ultrastrong laser pulse to meet the physical research requirement is that of the ultrashort ultrastrong laser technologyThe importance of the problems to be solved is increasingly highlighted with the increase of the focusing light intensity of the ultrashort ultrastrong laser pulse.

Disclosure of Invention

In view of this, the disclosure provides a method and a system for enhancing the temporal contrast of laser pulses, which can achieve the temporal contrast enhancement of laser pulses on the scale of up to femto seconds at the front edge of the laser pulses, has a simple structure, low cost and low energy loss, does not destroy the laser pulse distribution, and is applicable to the temporal contrast enhancement of ultra-high peak power laser pulses.

According to an aspect of the present disclosure, there is provided a laser pulse temporal contrast enhancement method, including: performing frequency shift on the main pulse of the laser pulse to be enhanced by utilizing target plasma matched with the pulse parameters of the main pulse of the laser pulse to be enhanced to obtain a frequency shift laser pulse, wherein the main pulse frequency of the frequency shift laser pulse is different from the pre-pulse frequency of the frequency shift laser pulse; filtering the pre-pulse of the frequency-shifted laser pulse by using at least one filtering reflector to obtain a target laser pulse with enhanced contrast, wherein the passband of the filtering reflector covers the main pulse frequency of the frequency-shifted laser pulse, and the stopband of the filtering reflector covers the pre-pulse frequency of the frequency-shifted laser pulse; the reflectivity of the passband of the filter mirror is higher than the reflectivity of the stopband of the filter mirror.

In one possible implementation, the target plasma includes: a first plasma having a first density parameter and a second plasma having a second density parameter; the first density parameter and the second density parameter are determined according to a pulse parameter of the main pulse of the laser pulse to be enhanced and a target frequency to which the main pulse of the laser pulse to be enhanced is frequency-shifted, the first density parameter and the second density parameter are different, the density parameter comprises a density and a length of plasma distribution, and the pulse parameter comprises at least one of the following: light intensity, pulse width, focusing spot radius; the method for obtaining the frequency-shifted laser pulse comprises the following steps of: injecting the laser pulse to be enhanced into the first plasma with the first density parameter to obtain an intermediate laser pulse with a main pulse part frequency shifted to the target frequency, wherein the first plasma is used for enabling the main pulse of the laser pulse to be enhanced to be frequency shifted and generating linear chirp dispersion, and the main pulse of the intermediate laser pulse has linear chirp characteristics; and injecting the intermediate laser pulse into the second plasma with the second density parameter to obtain a frequency-shifted laser pulse with the main pulse frequency shifted to the target frequency, wherein the second plasma is used for shifting the main pulse of the intermediate laser pulse and generating linear chirped dispersion opposite to the effect of the first plasma, and the main pulse of the frequency-shifted laser pulse has the characteristic of no chirp.

In one possible implementation, where the first plasma is a low density plasma and the second plasma is a high density plasma, the first plasma shifts the main pulse of the laser pulse to be enhanced and produces a linear negative chirp dispersion, the main pulse of the intermediate laser pulse has a linear negative chirp characteristic, and the second plasma shifts the main pulse of the intermediate laser pulse and produces a near linear positive chirp dispersion; or, in the case where the first plasma is a high-density plasma and the second plasma is a low-density plasma, the first plasma shifts the main pulse of the laser pulse to be enhanced and generates near-linear positive chirp dispersion, the main pulse of the intermediate laser pulse has near-linear positive chirp characteristics, and the second plasma shifts the main pulse of the intermediate laser pulse and generates linear negative chirp dispersion; wherein the linear negative chirp characteristic is a linear decrease in frequency from the pulse head to the pulse tail; the near-linear positive chirp characteristic is that the frequency increases nearly linearly from the pulse head to the pulse tail, and the non-chirp characteristic is that the frequency is uniform from the pulse head to the pulse tail.

In one possible implementation, the low density plasma includes: center density at 10 ¹⁶ cm ^-3 To 10 ¹⁸ cm ^-3 Is a plasma of (a); the high density plasma includes: center density at 10 ¹⁸ cm ^-3 To 10 ²⁰ cm ^-3 Is a plasma of (a).

In one possible implementation, the filtering, with at least one filtering mirror, the pre-pulse of the frequency-shifted laser pulse to obtain the target laser pulse includes: collimation is carried out on the frequency-shift laser pulse, and a collimated frequency-shift laser pulse is obtained; filtering the pre-pulse of the collimated frequency-shift laser pulse by using at least one filtering reflector to obtain a target laser pulse; wherein the filtering reflector comprises at least one of a bandpass reflector and a lowpass reflector.

In one possible implementation, before frequency shifting the main pulse of the laser pulse to be enhanced with a target plasma that matches the pulse parameters of the main pulse of the laser pulse to be enhanced, the method further comprises: and compressing and focusing the initial laser pulse generated by the laser pulse generating device to obtain the laser pulse to be enhanced after compression and focusing.

In one possible implementation, the laser pulse to be enhanced includes: a femtosecond laser pulse capable of forming a tail wavefield in a plasma, the femtosecond laser pulse comprising: the laser pulses with peak power at or above the level of the tera and pulse width at the femtosecond scale, the wake field comprising a plurality of plasma bubbles that have a frequency shifting effect on the femtosecond laser pulses forming the wake field.

According to another aspect of the present disclosure, there is provided a laser pulse temporal contrast enhancement system comprising: the plasma frequency conversion module is used for carrying out frequency shift on the main pulse of the laser pulse to be enhanced by utilizing target plasma matched with the pulse parameters of the main pulse of the laser pulse to be enhanced to obtain a frequency shift laser pulse, wherein the main pulse frequency of the frequency shift laser pulse is different from the pre-pulse frequency of the frequency shift laser pulse; the reflector filtering module comprises at least one filtering reflector and is used for filtering out the pre-pulse of the frequency-shifted laser pulse to obtain a target laser pulse with enhanced contrast, wherein the passband of the filtering reflector covers the main pulse frequency of the frequency-shifted laser pulse, and the stopband of the filtering reflector covers the pre-pulse frequency of the frequency-shifted laser pulse; the reflectivity of the passband of the filter mirror is higher than the reflectivity of the stopband of the filter mirror.

In one possible implementation, the system further includes: the laser collimation module is used for collimating the frequency-shift laser pulse to obtain a collimated frequency-shift laser pulse; the mirror filter module is further configured to: filtering the pre-pulse of the collimated frequency-shift laser pulse by using at least one filtering reflector to obtain a target laser pulse; wherein the filtering reflector comprises at least one of a bandpass reflector and a lowpass reflector.

In one possible implementation, the system further includes: the laser pulse generation module is used for generating initial laser pulses; and the laser compression focusing module is used for compressing and focusing the initial laser pulse to obtain the laser pulse to be enhanced after compression focusing.

According to the embodiment of the disclosure, the main pulse and the pre-pulse in the laser pulse to be enhanced can be separated on the basis of the target plasma on the frequency attribute, and the pre-pulse in the frequency-shift laser pulse is filtered by utilizing the filtering reflector, so that the filtered target laser pulse mainly comprises the main pulse, thereby realizing the time domain contrast enhancement of the laser pulse to be enhanced.

Other features and aspects of the present disclosure will become apparent from the following detailed description of exemplary embodiments, which proceeds with reference to the accompanying drawings.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments, features and aspects of the present disclosure and together with the description, serve to explain the principles of the disclosure.

Fig. 1 shows a flow chart of a laser pulse temporal contrast enhancement method according to an embodiment of the present disclosure.

Fig. 2a shows a schematic diagram of an electric field distribution of a laser pulse to be enhanced according to an embodiment of the present disclosure.

Fig. 2b shows a schematic diagram of the electric field distribution of a target laser pulse according to an embodiment of the present disclosure.

Fig. 3 shows a flow chart of a laser tuning approach for overall frequency shifting of a main pulse based on a target plasma in accordance with an embodiment of the present disclosure.

Fig. 4 shows a schematic diagram of a femtosecond laser pulse forming a wavefield in a plasma according to an embodiment of the disclosure.

Fig. 5a shows a schematic diagram of spectral intensity contrast of a main pulse and a pre-pulse of a laser pulse to be enhanced according to an embodiment of the present disclosure.

Fig. 5b shows a schematic diagram of spectral intensity contrast of a main pulse versus a pre-pulse of a frequency shifted laser pulse according to an embodiment of the present disclosure.

Fig. 6 illustrates a block diagram of a laser pulse temporal contrast enhancement system according to an embodiment of the present disclosure.

Fig. 7 illustrates a block diagram of a laser pulse temporal contrast enhancement system according to an embodiment of the present disclosure.

Fig. 8 illustrates a block diagram of a laser pulse temporal contrast enhancement system according to an embodiment of the present disclosure.

Detailed Description

Various exemplary embodiments, features and aspects of the disclosure will be described in detail below with reference to the drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Although various aspects of the embodiments are illustrated in the accompanying drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

The word "exemplary" is used herein to mean "serving as an example, embodiment, or illustration. Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

In addition, numerous specific details are set forth in the following detailed description in order to provide a better understanding of the present disclosure. It will be understood by those skilled in the art that the present disclosure may be practiced without some of these specific details. In some instances, methods, means, elements, and circuits well known to those skilled in the art have not been described in detail in order not to obscure the present disclosure.

It will be appreciated that the temporal contrast of the laser pulses is enhanced, essentially separating the main pulse and the pre-pulse of the laser pulses, with the difficulty that separation of the main pulse from the pre-pulse needs to be achieved at time intervals on the order of femtoseconds. To achieve this, different properties need to be introduced into the main pulse and the pre-pulse, and the contrast enhancement technique mentioned in the above prior art achieves separation of the main pulse and the pre-pulse from the properties of polarization state, intensity-related reflection/transmittance, and the like, and further achieves enhancement of the time domain contrast, but all have the problems mentioned above.

In view of the above, the embodiments of the present disclosure provide a method and a system for enhancing the temporal contrast of laser pulses, which can separate a main pulse and a pre-pulse in a laser pulse to be enhanced on the basis of a target plasma on the frequency attribute, and further filter the separated pre-pulse by using a filtering mirror, so as to enhance the temporal contrast of the laser pulse to be enhanced.

The laser pulse temporal contrast enhancement method provided by the embodiments of the present disclosure is described in detail below with reference to fig. 1 to 3.

Fig. 1 shows a flow chart of a laser pulse temporal contrast enhancement method according to an embodiment of the present disclosure. As shown in fig. 1, the laser pulse time domain contrast enhancement method includes:

step S101, utilizing target plasmas matched with pulse parameters of main pulses of laser pulses to be enhanced to carry out frequency shift on the main pulses of the laser pulses to be enhanced to obtain frequency shift laser pulses, wherein the main pulse frequency of the frequency shift laser pulses is different from the pre-pulse frequency of the frequency shift laser pulses;

Step S102, filtering off the pre-pulse of the frequency-shifted laser pulse by using at least one filtering reflector to obtain a target laser pulse with enhanced contrast, wherein the passband of the filtering reflector covers the main pulse frequency of the frequency-shifted laser pulse, and the stopband of the filtering reflector covers the pre-pulse frequency of the frequency-shifted laser pulse; the reflectivity of the pass band of the filter mirror is higher than the reflectivity of the stop band of the filter mirror.

It is known that the plasma as a medium without breakdown threshold can be used for frequency tuning (for short frequency shift, i.e. wavelength tuning) of femtosecond laser pulses, the laser pulses to be enhancedThe punching may include: a femtosecond laser pulse capable of forming a tail wave field in a plasma, the femtosecond laser pulse comprising: peak power is in the range of taiwa (1 taiwa=10 ¹² Tile) level or a level above tai (i.e. at least 10 ¹² Watts), the pulse width being on the femtosecond scale, wherein the wake field comprises a plurality of plasma bubbles that have a frequency shifting effect on the femtosecond laser pulses forming the wake field.

It should be understood that the problem of temporal contrast of the laser pulse may be equivalent to the problem of relative intensity of the main pulse and the pre-pulse of the laser pulse, where the main pulse and the pre-pulse are included in the laser pulse to be enhanced, and where the main pulse frequency and the pre-pulse frequency in the laser pulse to be enhanced are the same or similar, that is, the main pulse frequency and the pre-pulse frequency are the same or similar, that is, the main pulse wavelength and the pre-pulse wavelength are the same or similar. But the focused light intensity of the main pulse (i.e. peak light intensity, normalized light intensity, etc.) in the laser pulse to be enhanced is different from the pre-pulse, and is typically several orders of magnitude higher than the focused light intensity of the pre-pulse. The pulse parameters in the embodiments of the present disclosure may include at least wavelength, pulse width, normalized light intensity, single pulse energy, peak power, focused spot radius, and the like.

Experiments show that the matching relation exists between the focusing light intensity of the laser pulse to be frequency-shifted and the density of plasma, namely, the larger the focusing light intensity is, the lower the required plasma density is, the smaller the focusing light intensity is, and the required plasma density is higher, so that the main pulse of the laser pulse to be enhanced can be frequency-shifted by utilizing the target plasma matched with the pulse parameters (including the focusing light intensity) of the main pulse of the laser pulse to be enhanced, and the focus light intensity of the pre-pulse of the laser pulse to be enhanced is lower than the main pulse by several orders of magnitude, so that the density of the target plasma matched with the focusing light intensity of the main pulse is too low for the pre-pulse, the pre-pulse of the laser pulse to be enhanced has no frequency shift effect, so that the pulse frequency of the pre-pulse of the laser pulse to be enhanced is basically unchanged after the target plasma, and the main pulse of the laser pulse to be enhanced generates larger shift in frequency attribute, so that the main pulse of the laser pulse to be enhanced is separated from the pre-pulse of the laser pulse to be enhanced even though the main pulse frequency of the laser pulse to be different from the pre-pulse frequency of the laser pulse to be frequency shifted. It should be understood that the embodiments of the present disclosure are not limited to a specific frequency shift process for shifting a main pulse of a laser pulse to be enhanced with a target plasma, and are not limited to a center wavelength (i.e., frequency) of the main pulse after the frequency shift, so long as the main pulse and the pre-pulse can be separated in frequency attribute.

For example, when the wavelength of the main pulse and the pre-pulse of the laser pulse to be enhanced is 800nm (nanometers), the pulse width of the main pulse is 30fs (femtoseconds), the pre-pulse advances the main pulse by at least 1 picosecond, the pulse width of the pre-pulse can be 50fs, and the light intensity of the pre-pulse is 10 of the light intensity of the main pulse ^-6 The method comprises the steps of carrying out a first treatment on the surface of the If the main pulse front of the laser pulse to be enhanced has 2 pre-pulses, the first pre-pulse advances by 200fs, and its pulse parameters include 10 of the main pulse intensity ^-2 The pulse width is 50fs, the second pre-pulse advances the main pulse by 5fs, and the pulse parameters comprise the light intensity which is 10 of the light intensity of the main pulse ^-3 Pulse width 50fs; when the main pulse is shifted by the target plasma, the pulse wavelength (i.e., frequency) of the two pre-pulses after passing through the target plasma is substantially unchanged, that is, the center wavelength of the pre-pulse is still 800nm, and the center wavelength of the main pulse may be 1 μm, 1.3 μm, or another specified wavelength.

It can be understood that the improvement of the time domain contrast of the laser pulse is equivalent to filtering the pre-pulse in the laser pulse, so that the laser pulse mainly includes the main pulse, and since the main pulse frequency in the frequency-shifted laser pulse obtained by frequency shifting the main pulse of the laser pulse to be enhanced in the step S101 is different from the pre-pulse frequency, the pre-pulse frequency in the frequency-shifted laser pulse is substantially identical to the pre-pulse frequency of the laser pulse to be enhanced, that is, the target plasma has no frequency shift effect on the pre-pulse, so that the pre-pulse frequency before and after the frequency shift is substantially unchanged, and the main pulse frequency is substantially changed, so that the filtering mirror can be used to filter the pre-pulse of the frequency-shifted laser pulse in the step S102. The main pulse of the laser pulse to be enhanced can be a femtosecond laser pulse, so that the mode that the main pulse penetrates through a material medium is not selected to avoid the widening of the pulse width of the main pulse, and therefore, a filtering reflector which enables the main pulse to be reflected and the pre-pulse to penetrate is adopted, the widening of the pulse width of the main pulse is avoided, and the pre-pulse can be filtered. Even if the passband of the filter reflector covers the main pulse frequency of the frequency-shifted laser pulse, the stopband of the filter reflector covers the pre-pulse frequency of the frequency-shifted laser pulse; the reflectivity of the passband of the filter mirror is higher than the reflectivity of the stopband of the filter mirror, thereby filtering out the pre-pulses of the frequency shifted laser pulses (the pre-pulses are transmitted) while causing the main pulse to be reflected; wherein the pass band of the filter mirror, i.e. the filter mirror, allows the pass band (in reflection) and the stop band, i.e. the filter mirror, does not allow the pass band (in reflection).

The filtering reflector needs to have high reflectivity in the main pulse frequency range of the frequency-shifted laser pulse, and has low reflectivity in the pre-pulse frequency of the frequency-shifted laser pulse (i.e. the original frequency of the laser pulse to be enhanced), so that most of the pre-pulse passes through the filtering reflector after each reflection of at least one filtering reflector, and most of the main pulse is reflected by the filtering reflector, and after multiple reflections, the target laser pulse with the pre-pulse filtered is obtained, namely the target laser pulse with enhanced time domain contrast is obtained.

The filtering reflector can comprise at least one of a band-pass reflector and a low-pass reflector, wherein the passband of the filtering reflector has high reflectivity to the main pulse wave band of the frequency-shift laser pulse, and the stopband of the filtering reflector covers the pre-pulse wavelength of the frequency-shift laser pulse. Currently, conventional dielectric film reflectors in the market generally have bandpass or lowpass characteristics, and can be used as filtering reflectors, and the type or customization of the dielectric film reflector with proper parameters can be selected according to the main pulse frequency of frequency-shifted laser pulses.

Alternatively, the filtering mirror may specifically include: and a film-coated reflecting mirror, a standard reflecting mirror, a short-wave dichroic mirror, a conventional dielectric film reflecting mirror and the like can realize the filtering function. It should be understood that those skilled in the art may also design the type and number of bandpass or lowpass mirrors according to actual needs, and the embodiments of the present disclosure are not limited to the type and number of filtering mirrors. The prior filtering reflector has the effect that the reflectivity of the main pulse wave band is higher than that of the pre-pulse wave band by one order of magnitude under primary reflection, namely, the time domain contrast of the frequency-shift laser pulse can be improved by one order of magnitude through each reflection, and if multiple reflection and filtering are carried out through multiple filtering reflectors, the enhancement of the time domain contrast of the frequency-shift laser pulse by several orders of magnitude can be realized.

Illustratively, if filtering is performed with a standard mirror (i.e., filtering out the pre-pulse), the main pulse of the laser pulse to be enhanced may be shifted to the 930-1200nm band, keeping the pre-pulse around 800nm, since the standard mirror has a reflectivity of up to 99.6% in the 930-1200nm band, while having only a reflectivity of 9.9% around 800 nm; if the short-wave dichroic mirror is used for filtering, the main pulse of the laser pulse to be enhanced can be shifted to the wave band of 1000-1600nm because the short-wave dichroic mirror has the reflectivity of up to 99.4% in the wave band of 1000-1600nm and the reflectivity of only 3.2% near 800nm, and the pre-pulse is kept near 800 nm. The pre-pulse can be filtered obviously through multiple reflections, and the contrast of the frequency-shifted laser pulse is improved.

Considering that the laser pulse to be enhanced enters the target plasma in a focused state in step S101, the frequency-shifted laser pulse emitted from the target plasma will be in a divergent state, and it should be understood that the laser in the divergent state is difficult to be transmitted for a long distance under the size limitation of the optical transmission element, so that the divergent frequency-shifted laser pulse can be collimated, that is, the divergent light is converted into parallel light and then is emitted into the filtering mirror. Based on this, in one possible implementation, filtering the pre-pulse of the frequency shifted laser pulse using at least one filtering mirror in step S102, to obtain the target laser pulse includes:

Collimating the frequency-shifted laser pulse to obtain a collimated frequency-shifted laser pulse; and filtering the pre-pulse of the collimated frequency-shifted laser pulse by using at least one filtering reflector to obtain a target laser pulse. Wherein the collimation of the frequency shifted laser pulses may be accomplished by one skilled in the art using laser collimation techniques known in the art, such as off-axis parabolic mirrors, without limitation to the disclosed embodiments. In this way, the filtering effect of the filter mirror on the pre-pulse in the collimated frequency-shift laser pulse can be improved, and the energy loss of the main pulse can be reduced.

Fig. 2a shows a schematic diagram of an electric field distribution of a laser pulse to be enhanced according to an embodiment of the present disclosure, fig. 2b shows a schematic diagram of an electric field distribution of a target laser pulse according to an embodiment of the present disclosure, fig. 2a shows an electric field waveform distribution of a laser pulse to be enhanced of low temporal contrast, including a main pulse and a pre-pulse; FIG. 2b shows the electric field waveform distribution of a target laser pulse of high temporal contrast, mainly comprising a main pulse, the pre-pulse having been filtered; by using the laser pulse time domain contrast enhancement method of the embodiment of the present disclosure, the pre-pulse in fig. 2a may be transmitted through the filtering mirror, that is, filtering the pre-pulse in fig. 2a may be implemented, so as to separate the pre-pulse from the main pulse reflected by the filtering mirror, and further output the target laser pulse with high time domain contrast in fig. 2 b; wherein, [ a.u ] is a normalization unit.

According to the laser pulse time domain contrast enhancement method disclosed by the embodiment of the invention, the time domain contrast enhancement of the main pulse front edge under the scale of the nearest femto-second can be supported, and the time domain contrast of the laser pulse to be enhanced can be enhanced by a plurality of orders of magnitude; the energy loss of the main pulse of the target laser pulse relative to the main pulse of the laser pulse to be enhanced is less than 30%, namely the energy loss is lower, and the energy conversion efficiency is higher; and enabling the main pulse of the target laser pulse to still keep longitudinal near Gaussian distribution without damaging the main pulse distribution; the time domain contrast enhancement is realized by mainly utilizing low-cost plasmas and a filtering reflector, so that the device has the advantages of simple structure and low cost; because the plasma is adopted to realize frequency shift, the limitation of breakdown threshold is avoided, and the time domain contrast enhancement of the ultra-high peak power laser pulse above the clapping watt can be supported; the main pulse is kept in reflection transmission in the filtering reflector after being subjected to plasma frequency shift, so that the pulse width of the main pulse cannot be widened, namely the pulse width compression effect is achieved; the method has wide applicability and can be applied to the time domain contrast enhancement of femtosecond laser pulses in laser systems including but not limited to a titanium sapphire laser system, an OPCPA system and the like.

In consideration of the fact that frequency shifting of laser pulses by using a single-density target plasma may not enable the laser pulses to be frequency shifted together as a whole, that is, the frequency-shifted laser pulses may show a continuous spectrum from a femtosecond laser band to a mid-infrared band in terms of frequency spectrum distribution, and the energy of the laser pulses after the action is mostly concentrated near the femtosecond laser band (such as 800nm corresponding to titanium sapphire laser), and then laser components near the mid-infrared band need to be filtered before the laser pulses with a specific band need to be utilized, so that energy conversion efficiency and photon conversion efficiency are greatly compromised.

Therefore, the embodiment of the disclosure provides a laser tuning method for performing overall frequency shift on a main pulse based on a target plasma as shown in fig. 3, where the laser tuning method can realize quasi-monochromatic overall frequency shift of the main pulse in the laser pulse to be enhanced, and has the advantages of high energy and photon conversion efficiency, wide frequency tuning range, compatibility with laser pulse frequency shift of ultra-high pulse energy, and pulse compression effect.

As shown in fig. 3, in the step S101, using the target plasma matched with the pulse parameters of the main pulse of the laser pulse to be enhanced, the main pulse of the laser pulse to be enhanced is shifted to obtain a shifted laser pulse, which may include:

Step S1011, injecting the laser pulse to be enhanced into a first plasma with a first density parameter to obtain an intermediate laser pulse with a main pulse part shifted to a target frequency, wherein the first plasma is used for shifting the main pulse of the laser pulse to be enhanced and generating linear chirp dispersion, and the main pulse of the intermediate laser pulse has linear chirp characteristics;

step S1012, the intermediate laser pulse is injected into a second plasma with a second density parameter, so as to obtain a frequency-shifted laser pulse with the main pulse frequency shifted to the target frequency, where the second plasma is used to shift the main pulse of the intermediate laser pulse and generate a linear chirp dispersion opposite to the effect of the first plasma, and the main pulse of the frequency-shifted laser pulse has a chirp-free characteristic. Wherein the target plasma comprises: a first plasma having a first density parameter and a second plasma having a second density parameter; the first density parameter and the second density parameter are determined according to the pulse parameter of the main pulse of the laser pulse to be enhanced and the target frequency to which the main pulse of the laser pulse to be enhanced is shifted, and the first density parameter is different from the second density parameter, and the density parameters comprise the density and the length of the plasma distribution. Wherein the main pulse is shifted to the target frequency.

The first density parameter and the second density parameter are determined according to the pulse parameter of the main pulse of the laser pulse to be enhanced and the target frequency to which the main pulse of the laser pulse to be enhanced is frequency-shifted, so that the first plasma and the second plasma are target plasmas matched with the pulse parameter of the main pulse of the laser pulse to be enhanced, the integral frequency shift of the main pulse can be realized, and meanwhile, the pre-pulse basically has no frequency shift effect, so that the frequency of the pre-pulse of the laser pulse to be enhanced is basically unchanged after the pre-pulse passes through the target plasmas, and the distinction of the main pulse and the pre-pulse in frequency attribute is realized.

In practical application, various target frequencies and various pulse parameters can be preset, and a mapping relation between the target frequencies and the various pulse parameters and between the first density parameter and the second density parameter can be set in advance, so that after the target frequencies and the pulse parameters corresponding to the main pulse of the laser pulse to be enhanced are obtained, the first density parameter which the first plasma should have and the second density parameter which the second plasma should have can be obtained based on the mapping relation, and further the plasma generating device can be controlled to generate the first plasma with the first density parameter and the second plasma with the second density parameter according to the first density parameter and the second density parameter. It should be understood that the above mapping relationship may be in a functional form or a chart form, and it should be understood that a person skilled in the art may obtain the mapping relationship in advance through theoretical analysis and simulation experiment, and the embodiment of the disclosure is not limited to the process of establishing the mapping relationship.

For example, for a main pulse with a center wavelength of 800nm, a pulse width of 30fs, and a peak power of 100 tera, if the target frequency is set to be 1.3 μm corresponding to the target wavelength, the first density parameters of the first plasma may be obtained include: center density of 3×10 ¹⁷ cm ^-3 The distribution length is about 4cm, and the second density parameters of the second plasma include: center density of 1X 10 ¹⁹ cm ^-3 A distribution length of about 0.5mm and having a density rising edge and a density falling edge of 100-300 μm; if the target frequency is set to be 1.6 μm corresponding to the target wavelength, the first density parameter of the first plasma may be obtained including: center density of 3×10 ¹⁷ cm ^-3 The distribution length is about 4cm and the second density parameter of the second plasma may comprise: center density of 8×10 ¹⁸ cm ^-3 The distribution length is about 0.75mm. Wherein, the density and the length of the plasma can be adjusted in a certain range, and when the density is relatively adjusted upwards, the length is relatively shortened; when the density is relatively down-regulated, the length is relatively extended. The plasma generating device may be a device for generating a plasma with adjustable density and adjustable distribution length, which is known in the art, and may be arranged to operate in a vacuum environment, and may generate plasma by ionizing gases, including but not limited to nitrogen, helium, hydrogen, mixed gases, and the like. Specifically, the plasma generating apparatus may generate a gas (including but not limited to nitrogen, helium, hydrogen, and mixed gas, etc.) first, and then use a laser ionization gas or a high-voltage discharge breakdown gas to obtain a plasma having various density parameters. Wherein the longitudinal distribution of the plasma can be controlled by using supersonic nozzle, blade, gas chamber, etc., if the transverse distribution of the plasma is required to be finely adjusted, for example, the plasma with parabolic density distribution having low transverse center density and high edge density can be generated by using single laser ionization gas or high-voltage discharge breakdown gas, that is The transverse distribution structure with approximately parabolic distribution can be obtained through strong laser ionization or high-voltage discharge breakdown gas and the like. The embodiments of the present disclosure are not limited to the structure, model, etc. of the plasma generating apparatus.

Considering that in order to achieve a frequency shift effect on the laser pulses with a low density plasma, the distribution length thereof is typically long (e.g. the 4cm distribution length described above), in order to make the laser pulses propagate more stably in a focused focal spot state in the low density plasma without generating excessive geometrical divergence, in one possible implementation, in case the first plasma is a low density plasma, the first plasma may be parabolic with a low center density and a high edge density in a direction perpendicular to the propagation direction of the laser pulses to be enhanced; alternatively, in the case where the second plasma is a low-density plasma, the second plasma may be parabolic with a low center density and a high edge density in a direction perpendicular to the propagation direction of the intermediate laser pulse.

The plasma generating device can generate plasma with parabolic distribution, including but not limited to plasma with parabolic distribution characteristic formed by laser irradiation of ionized gas and plasma with parabolic distribution characteristic formed by high-pressure discharge of ionized gas. The plasma is in parabolic distribution with low center density and high edge density along the propagation direction of the vertical middle laser pulse, and the plasma is understood to have the characteristics of transverse parabolic distribution, low center density and high edge density, and the longitudinal direction has the characteristics of density rising edge, density platform, density falling edge and the like. By using a plasma with a parabolic profile, a long-distance stable transmission of focused laser pulses in the plasma is possible.

It can be known that the frequency and the wavelength of the laser pulse have a conversion relationship, and the tuning frequency is equal to the tuning wavelength, and because in a normal case, the time domain contrast enhancement of the laser pulse is to adjust the wavelength of the laser pulse to be longer and the frequency is to adjust to be smaller, the target frequency of the embodiment of the disclosure can be smaller than the frequency band of the laser pulse to be enhanced. The target frequency may be a frequency band to which the user desires to tune the main pulse in the laser pulse to be enhanced, and may be a frequency range or a specific frequency value, where the target frequency may be specifically determined according to the type of the filtering mirror used because the filtering mirror of different types has different reflectivities for the laser pulse of different frequency bands, for example, because the short-wave dichroic mirror has a reflectivity of up to 99.4% in a wavelength band of 1000-1600nm and a reflectivity of only 3.2% near 800nm, the target frequency may be set to a frequency range corresponding to a wavelength band of 1000-1600nm, or may be set to a frequency value corresponding to any specific wavelength between the wavelength bands of 1000-1600 nm.

As described above, the laser pulse to be enhanced includes a femtosecond laser pulse capable of forming a tail wave field in plasma, the tail wave field including a plurality of plasma bubbles, the plasma bubbles having a frequency shift effect on the femtosecond laser pulse forming the tail wave field; wherein the femtosecond laser pulse includes: peak power is in the range of taiwa (1 taiwa=10 ¹² Watts) or above the level of teva, with pulse widths on the femtosecond scale. Fig. 4 is a schematic diagram of a femtosecond laser pulse forming a wavefield in a plasma according to an embodiment of the disclosure, as shown in fig. 4, the femtosecond laser pulse includes a plurality of plasma bubbles (i.e., a density bubble structure) in the wavefield formed in the plasma, wherein a leading edge region of a first plasma bubble has a frequency shift effect, i.e., a frequency downshift, also called a red shift, on the femtosecond laser pulse forming the wavefield.

It can be known that, because the scales of the plasma cavitation bubbles generated by the femtosecond laser pulses in the plasmas with different density parameters are different, the frequency shift action modes and the frequency shift action mechanisms of the plasma cavitation bubbles with different scales on the femtosecond laser pulses are different, or the frequency shift action ranges and the frequency shift action directions on the femtosecond laser pulses are different, the embodiment of the disclosure uses the plasmas with different density parameters to perform piecewise linear frequency shift on the main pulse of the laser pulses to be enhanced, for example, the tail frequency shift of the main pulse is realized before the head frequency shift is realized. Specifically, the laser pulse to be enhanced may be first injected into a first plasma with a first density parameter to obtain an intermediate laser pulse with a tail portion linearly shifted to a target frequency, and then the intermediate laser pulse is injected into a second plasma with a second density parameter to perform a head portion linearly shifted to obtain a target laser pulse with a whole main pulse shifted to the target frequency. Therefore, the main pulse of the target laser pulse has the characteristic of no chirp after the frequency shift amounts generated by the tail linear frequency shift and the head linear frequency shift are overlapped, namely, the frequencies at the pulse positions before and after the main pulse of the target laser pulse are consistent. The linear frequency shift is understood to mean that the laser pulse is extended to longer wavelengths (i.e. lower frequencies) in a linear frequency broadening manner, and the effect on the laser pulse is to generate linear chirp dispersion. Because of the linear frequency shift, the frequency shift of the laser pulse can be linearly controlled to ensure that the frequency of the main pulse of the target laser pulse is consistent from the pulse position of the head to the pulse position of the tail after the overall frequency shift, thereby being beneficial to improving the energy and photon conversion efficiency before and after the overall frequency shift.

It should be understood that the laser pulse has a certain length, or from the head of the pulse to the tail of the pulse, the pulse position can be understood as any position between the head and the tail of the laser pulse, and the frequency shift can be understood as the frequency shift. The primary pulse of the initially plasma-inactive laser pulse to be enhanced is generally the same frequency at the different pulse positions. The frequency shift amount of the main pulse of the middle laser pulse subjected to the first plasma effect to the target frequency at different pulse positions and the pulse positions form linear chirp dispersion; and after the main pulse of the middle laser pulse is acted by the second plasma, the frequency shift caused by the linear chirp dispersion opposite to the first plasma is superposed. For example, under the effect of the linear chirped dispersion of the first plasma, the tail frequency shift amount of the main pulse is large and the head frequency shift amount is small or even basically unchanged; the main pulse has a large head shift and a small tail shift even substantially unchanged under the effect of the linear chirped dispersion of the second plasma. Finally, by adjusting the density parameters (density and length) of the first plasma and the second plasma, the same frequency shift amount at each position of the head and the tail of the main pulse can be realized, and thus the frequency shift of the whole main pulse is realized.

As described above, since the femtosecond laser pulses of different laser parameters generate plasma cavitation of different dimensions in plasmas of different density parameters, the frequency shift action modes on the femtosecond laser pulses are different, it is possible to control the linear increase or linear decrease of the main pulse of the intermediate laser pulse relative to the frequency shift amount and pulse position of the main pulse of the laser pulse to be enhanced by controlling the density parameters corresponding to the first plasma and the second plasma, and then frequency shift the main pulse of the intermediate laser pulse to the target frequency as a whole. Further, since the first density parameter and the second density parameter are determined according to the target frequency to which the main pulse is to be tuned, the frequency of the main pulse of the laser pulse to be enhanced (i.e., the wavelength of the main pulse of the laser pulse to be enhanced) can be shifted to the target frequency as a whole, or the frequency of the main pulse of the target laser pulse (i.e., the wavelength of the main pulse of the target laser pulse is at the target wavelength) by linearly shifting the first plasma having the first density parameter and the second plasma having the second density parameter. It should be noted that, the frequency mentioned in the embodiments of the present disclosure may be a center frequency of the laser pulse, and the wavelength may be a center wavelength of the laser pulse.

In one possible implementation, the first plasma with the first density parameter may be controlled to be a low-density plasma, and the second plasma with the second density parameter may be controlled to be a high-density plasma, i.e. the laser pulse to be enhanced may be injected into the low-density plasma first to obtain an intermediate laser pulse; injecting the middle laser pulse into the high-density plasma to obtain a target laser pulse; wherein, optionally, the low density plasma may comprise: center density at 10 ¹⁶ cm ^-3 To 10 ¹⁸ cm ^-3 Is a plasma of (a); the high density plasma may include: center density at 10 ¹⁸ cm ^-3 To 10 ²⁰ cm ^-3 Is a plasma of (a). Based on this, the first plasma is a low density plasma, andin the case that the two plasmas are high-density plasmas, the first plasma shifts the main pulse of the laser pulse to be enhanced and generates linear negative chirp dispersion, the second plasma shifts the main pulse of the middle laser pulse and generates near linear positive chirp dispersion, and the main pulse of the middle laser pulse has linear negative chirp characteristics, wherein the linear negative chirp dispersion can be understood as that the frequency shift amount of the main pulse of the laser pulse to be enhanced from the tail part of the pulse to the head part of the pulse to the target frequency is linearly reduced (the tail part is more in frequency shift), or in other words, the frequency of the main pulse of the laser pulse to be enhanced is linearly reduced along with the position from the head part of the pulse to the tail part of the pulse; and, near-linear positive chirp dispersion is understood to mean that the amount of shift of the main pulse of the intermediate laser pulse from the pulse tail to the pulse head to the target frequency increases nearly linearly (the head shifts much), or that the near-linear positive chirp dispersion increases the frequency of the main pulse of the intermediate laser pulse from the pulse head to the pulse tail nearly linearly with position.

The laser pulse to be enhanced is usually a chirp-free pulse, and the frequencies of all positions before and after the main pulse of the laser pulse to be enhanced are basically consistent; the first plasma (in this case, low-density plasma) shifts the main pulse of the laser pulse to be enhanced and generates a linear negative chirp dispersion, and represents that the first plasma linearly decreases the frequency shift amount of the main pulse of the laser pulse to be enhanced from the pulse tail to the pulse head toward the target frequency, which means that the frequency shift amount of the main pulse tail of the laser pulse to be enhanced is large, and the frequency shift amount of the head is small, that is, the frequency shift amount of the pulse tail to the pulse head linearly decreases, which represents that the main pulse of the laser pulse to be enhanced is red-shifted, and the main pulse of the intermediate laser pulse obtained after the red shift has a linear negative chirp characteristic, which represents that the frequency of the main pulse tail of the intermediate laser pulse is lower than the frequency of the main pulse head of the intermediate laser pulse and the frequency of the main pulse tail to the head increases linearly, which means that the tail of the main pulse of the intermediate laser pulse is first shifted to the target frequency.

The second plasma (in this case, a high-density plasma) causes a main pulse of the intermediate laser pulse to be shifted and a near-linear positive chirp dispersion to be generated, and the second plasma causes a shift amount of the main pulse of the intermediate laser pulse from the tail portion of the pulse to the head portion of the pulse to be linearly increased, which means that a shift amount of the main pulse of the intermediate laser pulse from the tail portion of the pulse to the head portion of the pulse is small, that is, a shift amount of the main pulse from the tail portion of the pulse to the head portion of the pulse is large, that is, a shift amount of the main pulse from the tail portion of the pulse to the head portion of the pulse is nearly linearly increased, and the main pulse having a linear negative chirp characteristic can be changed into a main pulse having no chirp characteristic, that is, an intermediate laser pulse having a shift of the main pulse portion of the main pulse to the target frequency from the head portion of the main pulse to the tail portion of the pulse, and the main pulse of the target laser pulse is uniform in frequency from the head portion to the tail portion of the pulse. The near-linear positive chirp dispersion and the linear negative chirp dispersion can be mutually offset, so that the overall frequency shift of the main pulse of the laser pulse to be enhanced is changed into a frequency shift laser pulse.

In one possible implementation manner, the sequence of the tail frequency shift and the head frequency shift can be exchanged, that is, the head frequency shift can be performed before the tail frequency shift, the first plasma with the first density parameter can be controlled to be high-density plasma, the second plasma with the second density parameter can be controlled to be low-density plasma, that is, the laser pulse to be enhanced can be injected into the high-density plasma to obtain the middle laser pulse; then the middle laser pulse is injected into the low-density plasma to obtain a frequency shift laser pulse; as described above, the low density plasma may include: center density at 10 ¹⁶ cm ^-3 To 10 ¹⁸ cm ^-3 Is a plasma of (a); the high density plasma may include: center density at 10 ¹⁸ cm ^-3 To 10 ²⁰ cm ^-3 Is a plasma of (a). Based on this, in the case where the first plasma is a high-density plasma and the second plasma is a low-density plasma, the first plasma shifts the main pulse of the laser pulse to be enhanced and generates near-linear positive chirp dispersion, the main pulse of the intermediate laser pulse having nearThe second plasma shifts the main pulse of the intermediate laser pulse and produces a linear negative chirp dispersion.

The laser pulse to be enhanced is usually a chirp-free pulse, and the frequencies of the positions before and after the pulse are basically consistent; the first plasma (in this case, high-density plasma) shifts the main pulse of the laser pulse to be enhanced and generates near-linear positive chirp dispersion, and the near-linear positive chirp characteristic represents that the frequency of the main pulse of the laser pulse to be enhanced increases nearly linearly from the pulse tail to the frequency of the main pulse tail, meaning that the frequency shift amount of the main pulse of the laser pulse to be enhanced is large, the frequency shift amount of the tail is small, that is, the frequency shift amount of the pulse head to the pulse tail decreases linearly, so that the main pulse of the intermediate laser pulse after the first plasma (in this case, high-density plasma) acts has near-linear positive chirp characteristic, and the near-linear positive chirp characteristic is that the frequency of the main pulse head of the intermediate laser pulse increases nearly linearly from the laser pulse tail to the frequency of the main pulse tail of the intermediate laser pulse, that the frequency of the main pulse head of the intermediate laser pulse increases nearly linearly from the head to the tail, meaning that the main pulse head of the intermediate laser pulse moves to the target frequency. The main pulse without chirp characteristic in the laser pulse to be enhanced can be changed into the main pulse with near-linear positive chirp characteristic by the action of the high-density plasma on the frequency shift and the linear positive chirp dispersion of the main pulse of the laser pulse to be enhanced.

The second plasma (in this case, low-density plasma) shifts the main pulse of the intermediate laser pulse and generates a linear negative chirp dispersion, and the second plasma linearly reduces the frequency shift amount of the main pulse of the intermediate laser pulse from the tail portion of the pulse to the head portion of the pulse toward the target frequency, which means that the frequency shift amount of the main pulse of the intermediate laser pulse is large, the frequency shift amount of the head portion is small, that is, the frequency shift amount of the tail portion of the pulse to the head portion of the pulse is linearly reduced, which is opposite to the effect of the first plasma on the main pulse of the laser pulse to be enhanced, wherein the second plasma (in this case, low-density plasma) red-shifts the main pulse of the intermediate laser pulse, and the main pulse of the target laser pulse obtained after the red shift has a chirp-free characteristic, that is, the intermediate laser pulse whose main pulse portion is shifted to the target frequency is changed into the target laser pulse whose main pulse is entirely shifted to the target frequency, and the frequencies of the main pulse of the target laser pulse are uniform from the head portion to the tail portion of the pulse. As described above, the linear negative chirp dispersion and the near-linear positive chirp dispersion can cancel each other, thereby realizing the overall frequency shift of the main pulse in the laser pulse to be enhanced to the main pulse in the target laser pulse.

It should be appreciated that the laser tuning process when the first plasma is a low density plasma and the second plasma is a high density plasma is similar to the laser tuning process when the first plasma is a high density plasma and the second plasma is a low density plasma, except that the main pulse of the laser pulse to be enhanced is frequency shifted tail first or head first, but the main pulse of the laser pulse to be enhanced may be tuned as a whole to the target frequency.

FIG. 5a shows a graph of spectral intensity contrast of a main pulse and a pre-pulse of a laser pulse to be enhanced according to an embodiment of the present disclosure, as shown in FIG. 5a, the wavelength centers (i.e., frequency centers) of the main pulse and the pre-pulse before frequency shifting are coincident, and FIG. 5b shows a graph of spectral intensity contrast of a main pulse and a pre-pulse of a frequency shifted laser pulse according to an embodiment of the present disclosure, as shown in FIG. 5b, the wavelength centers (i.e., frequency centers) of the main pulse and the pre-pulse after frequency shifting are separated; the above-mentioned laser tuning method according to the embodiment of the present disclosure may shift the main pulse with the center wavelength of 0.8 μm shown in fig. 5a to the main pulse with the center wavelength of 1 μm shown in fig. 5b, while keeping the center wavelength of the pre-pulse before and after the frequency shift unchanged, so as to distinguish the main pulse from the pre-pulse, that is, even if the frequency of the main pulse in fig. 5a is shifted as a whole, the frequency of the pre-pulse remains unchanged, and finally separation of the main pulse and the pre-pulse is achieved in frequency.

In one possible implementation, the two plasma structures may be used separately in step S1011 and step S1012, or may be combined to use one plasma structure having a step feature in a longitudinal direction, which is not limited to the embodiments of the present disclosure. Specifically, the first plasma and the second plasma may be respectively located in two plasma structures, and for example, other devices required for laser tuning may be located between the two plasmas, which means that the intermediate laser pulse obtained after passing through the first plasma in step S1011 is not directly injected into the second plasma in step S102; the first plasma and the second plasma may be in the same plasma structure with the step characteristic, which means that the intermediate laser pulse obtained after passing through the first plasma in step S1011 may be directly injected into the second plasma in step S1012.

Optionally, a laser focusing device can be added between the first plasma and the second plasma to collimate and focus the laser pulse to be enhanced again, so that the laser pulse has a better frequency shift effect in the second plasma. That is, if the first plasma and the second plasma are disposed in two plasma structures, a laser focusing device may be disposed between the two plasma structures to flexibly adjust the laser pulse, where the laser focusing device may perform focusing optimization on the intermediate laser pulse emitted by the first plasma, so as to obtain a better frequency shift effect in the second plasma. It should be appreciated that one skilled in the art may focus the intermediate laser pulses using laser pulse focusing techniques known in the art, such as off-axis parabolic mirrors, without limitation to the disclosed embodiments.

According to the embodiment of the disclosure, the main pulse of the middle laser pulse after the first plasma is acted can be shifted in a linear chirp dispersion mode by utilizing plasmas with different density parameters, the main pulse of the target laser pulse after the second plasma is acted is shifted in frequency and has a linear chirp characteristic, that is, the main pulse of the laser pulse to be enhanced is subjected to piecewise linear frequency shift, and the linear chirp dispersions of the two sections of frequency shifts are opposite, so that the frequencies of the main pulse before and after the pulse positions of the main pulse obtained after the superposition of the frequency shift amounts generated by the piecewise frequency shifts in the target laser pulse are consistent, that is, the effect of integral frequency shift of the main pulse of the laser pulse to be enhanced is realized, and because the density parameters of the plasmas are determined according to the pulse parameters of the main pulse of the laser pulse to be enhanced and the target frequency to be tuned, the main pulse to be enhanced can be integrally tuned to a required target frequency band (that is, the target band) and the frequency of the main pulse to be enhanced is basically kept unchanged, the main pulse and the pre-pulse to be enhanced can be separated, the high-energy conversion efficiency and the photon conversion efficiency can be realized, and the target pulse energy can be output, and the ultra-high-energy pulse compression is compatible.

In practice, a laser pulse generating device (e.g. a femtosecond laser, an ultra-short super laser, etc.) may be generally used to generate a laser pulse, and the initial laser pulse generated by the laser generating device is not focused and the pulse width is not compressed to the femtosecond level before entering the plasma, which may affect the frequency shift effect of the plasma on the laser pulse, for example, the peak light intensity of the initial laser pulse generated by the laser pulse generating device may not reach the condition of exciting the plasma tail wave field in the target plasma, that is, when the peak light intensity of the femtosecond laser pulse needs to reach a certain intensity, so that the plasma tail wave field is excited in the plasma, based on this, in a possible implementation, before the main pulse of the laser pulse to be enhanced is frequency shifted by using the target plasma matched with the pulse parameters of the main pulse of the laser pulse to be enhanced, the method further includes: and compressing and focusing the initial laser pulse generated by the laser pulse generating device to obtain the laser pulse to be enhanced after compression and focusing.

The compressing and focusing the initial laser pulse generated by the laser pulse generating device to obtain the compressed and focused laser pulse to be enhanced may include: compressing the initial laser pulse to obtain a compressed laser pulse, and focusing the compressed laser pulse according to the focusing light spot radius and the focusing light intensity required by the main pulse to obtain a compressed and focused laser pulse to be enhanced.

The focus light intensity and the focus light spot radius required by the main pulse can be understood as the focus light spot radius and the focus light intensity when a better frequency shift effect can be obtained in the first plasma with the first density parameter. It should be understood that, according to practical experience, a person skilled in the art may preset a correspondence between various first density parameters and various pulse widths, a correspondence between various first density parameters and various focusing light spot radii, and a correspondence between various first density parameters and focusing light intensities, so that, in the case that the first density parameters of the first plasma may be preset, the pulse width, the focusing light spot radius, and the focusing light intensity required to be provided by the main pulse are obtained based on the foregoing correspondence; or the user may directly set the above specified pulse parameters of the main pulse, which is not limited to the embodiment of the present application.

It should be appreciated that those skilled in the art may use laser pulse compression techniques known in the art, such as laser pulse compression gratings, to compress an initial laser pulse according to a pulse width required for a main pulse, resulting in a compressed laser pulse, and embodiments of the present disclosure are not limited. And, a person skilled in the art may use a laser pulse focusing technique known in the art, such as a laser focusing element such as an off-axis parabolic mirror, to focus the compressed laser pulse according to the focusing spot radius and the focusing light intensity required to be possessed by the main pulse, so as to obtain a compressed and focused laser pulse to be enhanced, which is not limited in this embodiment of the disclosure. The initial laser pulse is focused to obtain a femtosecond laser pulse with high focusing light intensity, for example, the laser pulse with peak power reaching the level of Taiwa is focused to reach the normalized light intensity of more than 1, the diameter of a focusing light spot is between 5 and 100 microns, and then a plasma tail wave field can be excited in target plasma, so that the frequency shift effect on the main pulse is realized.

As described above, by adopting the plasma having the transverse parabolic distribution, the focused laser pulse can be stably transmitted in the plasma over a long distance, and in the case where the target plasma is parabolic distribution, the compressed focused laser pulse to be enhanced can be stably transmitted in the target plasma, and at the same time, the target plasma can generate a better frequency shift effect on the main pulse having the specified focusing spot radius, the specified focusing light intensity and the specified pulse width. The step S101, particularly the specific frequency shift manner in the step S1011 to the step S1012, may be referred to for implementing frequency shift of the main pulse of the compressed and concentrated laser pulse to be enhanced by using the target plasma, so as to obtain a frequency shift laser pulse, which is not described herein.

According to the embodiment of the disclosure, the main pulse of the laser pulse to be enhanced after compression and focusing has the characteristics of higher focusing light intensity, smaller focusing light spot, narrower pulse width and the like, so that the target plasma effect matched with the laser pulse to be enhanced is facilitated, the frequency shift of the main pulse of the laser pulse to be enhanced is realized, and a better frequency shift effect is obtained.

The laser pulse temporal contrast enhancement system provided by embodiments of the present disclosure is described in detail below with reference to fig. 6-8.

Fig. 6 illustrates a block diagram of a laser pulse temporal contrast enhancement system, as shown in fig. 6, according to an embodiment of the present disclosure, comprising:

the plasma frequency conversion module 111 is configured to perform frequency shift on the main pulse of the laser pulse to be enhanced by using a target plasma that is matched with a pulse parameter of the main pulse of the laser pulse to be enhanced, so as to obtain a frequency-shifted laser pulse, where the main pulse frequency of the frequency-shifted laser pulse is different from the pre-pulse frequency of the frequency-shifted laser pulse, and the target plasma has only a frequency shift effect on the main pulse of the laser pulse to be enhanced, and has no substantial frequency shift effect on the pre-pulse of the laser pulse to be enhanced;

the mirror filtering module 113 includes at least one filtering mirror, where the at least one filtering mirror is used to filter out the pre-pulse of the frequency-shifted laser pulse to obtain a target laser pulse with enhanced contrast, the passband of the filtering mirror covers the main pulse frequency of the frequency-shifted laser pulse, and the stopband of the filtering mirror covers the pre-pulse frequency of the frequency-shifted laser pulse; the reflectivity of the passband of the filter mirror is higher than the reflectivity of the stopband of the filter mirror.

The plasma frequency conversion module 111 is mainly used for converting the main pulse and keeping the pre-pulse frequency basically unchanged; the mirror filtering module 113 is mainly used for reflecting the main pulse and transmitting the pre-pulse, and is used for filtering the pre-pulse, so as to realize the improvement of the contrast ratio of the main pulse time domain. It should be understood that, the number, the coating and the structure of the filtering mirrors used in the mirror filtering module 113 can be customized by those skilled in the art according to the actual requirements, and the embodiments of the present disclosure are not limited thereto, but the filtering mirrors are required to have a high reflectivity for the main pulse frequency and a lower reflectivity for the pre-pulse frequency, so that the pre-pulse can penetrate the filtering mirrors, and the main pulse can be reflected by the filtering mirrors, especially after being reflected by a plurality of filtering mirrors for multiple times, so as to achieve the filtering of the pre-pulse, thereby obtaining the main pulse with high time domain contrast.

In one possible implementation, the target plasma includes: a first plasma having a first density parameter and a second plasma having a second density parameter; the first density parameter and the second density parameter are determined according to a pulse parameter of the main pulse of the laser pulse to be enhanced and a target frequency to which the main pulse of the laser pulse to be enhanced is frequency shifted, the first density parameter and the second density parameter are different, the density parameters comprise plasma density and length, and the pulse parameters comprise at least one of the following: light intensity, pulse width, focusing spot radius;

The method for obtaining the frequency-shifted laser pulse comprises the following steps of: injecting the laser pulse to be enhanced into the first plasma with the first density parameter to obtain an intermediate laser pulse with a main pulse part frequency shifted to the target frequency, wherein the first plasma is used for enabling the main pulse of the laser pulse to be enhanced to be frequency shifted and generating linear chirp dispersion, and the main pulse of the intermediate laser pulse has linear chirp characteristics; and injecting the intermediate laser pulse into the second plasma with the second density parameter to obtain a frequency-shifted laser pulse with the main pulse frequency shifted to the target frequency, wherein the second plasma is used for shifting the main pulse of the intermediate laser pulse and generating linear chirped dispersion opposite to the effect of the first plasma, and the main pulse of the frequency-shifted laser pulse has the characteristic of no chirp.

In the case where the first plasma is a low-density plasma and the second plasma is a high-density plasma, the first plasma shifts the main pulse of the laser pulse to be enhanced and generates a linear negative chirp dispersion, the main pulse of the intermediate laser pulse has a linear negative chirp characteristic, and the second plasma shifts the main pulse of the intermediate laser pulse and generates a near-linear positive chirp dispersion; or, in the case where the first plasma is a high-density plasma and the second plasma is a low-density plasma, the first plasma shifts the main pulse of the laser pulse to be enhanced and generates near-linear positive chirp dispersion, the main pulse of the intermediate laser pulse has near-linear positive chirp characteristics, and the second plasma shifts the main pulse of the intermediate laser pulse and generates linear negative chirp dispersion; wherein the linear negative chirp characteristic is a linear decrease in frequency from the pulse head to the pulse tail; the near-linear positive chirp characteristic is that the frequency increases nearly linearly from the pulse head to the pulse tail, and the non-chirp characteristic is that the frequency is uniform from the pulse head to the pulse tail.

In one possible implementation manner, the plasma frequency conversion module 111 may include: the plasma generating device can respectively generate first plasmas with first density parameters and second plasmas with second density parameters into the two plasma structures. It should be understood that a person skilled in the art may use a plasma generating device and two plasma structures known in the art to achieve the function of generating the target plasma described above, and the embodiments of the present disclosure are not limited thereto.

Alternatively, the two plasma structures may form a plasma structure having a stepped profile such that an intermediate laser pulse emitted through the first plasma may be directed into the second plasma. Alternatively, the two plasmas may be two independent structures, which means that other devices required for laser tuning may be arranged between the two plasma structures, for example, a laser focusing device may be arranged between the two plasma channels, so that an intermediate laser pulse emitted by the first plasma may be focused after being injected into the laser focusing device, and the focused intermediate laser pulse is obtained and then injected into the second plasma.

In one possible implementation, as shown in fig. 7, the system may further include:

the laser collimation module 112 is configured to collimate the frequency-shifted laser pulse to obtain a collimated frequency-shifted laser pulse;

the mirror filter module 113 is further configured to: filtering the pre-pulse of the collimated frequency-shift laser pulse by using at least one filtering reflector to obtain a target laser pulse; wherein the filtering reflector comprises at least one of a bandpass reflector and a lowpass reflector.

The laser collimation module 112 may have a laser pulse collimation function and a laser pulse transmission function, or the laser collimation module 112 may include a laser pulse collimation device and a laser pulse transmission device, so as to collimate and transmit the frequency-shifted laser pulse. It should be understood that those skilled in the art may employ laser pulse collimation devices and laser pulse delivery devices known in the art to achieve collimation and delivery of laser pulses, and embodiments of the present disclosure are not limited in this regard.

In one possible implementation, as shown in fig. 8, the system may further include:

a laser pulse generation module 109 for generating an initial laser pulse;

And the laser compression focusing module 110 is configured to compress and focus the initial laser pulse to obtain the compressed and focused laser pulse to be enhanced.

The laser pulse generation module 109 may include a laser pulse generation device, among others. It should be appreciated that those skilled in the art may use laser pulse generating devices known in the art, such as femtosecond lasers, ultra-short super lasers, and the like, to generate the laser pulses to be enhanced, and embodiments of the present disclosure are not limited thereto.

The laser compression focusing module 110 may have a laser pulse compression function, a laser pulse focusing function, and a laser pulse transmission function, or the laser compression focusing module 110 may include a laser pulse compression device, a laser pulse focusing device, and a laser pulse transmission device, so as to compress, focus, and transmit the laser pulse to be enhanced. It should be understood that those skilled in the art may employ laser pulse compression means, laser pulse focusing means, and laser pulse transmission means known in the art to effect compression, focusing, and transmission of laser pulses, and embodiments of the present disclosure are not limited in this regard.

In one possible implementation, some of the modules in the laser pulse temporal contrast enhancement system shown in fig. 6-8 described above may be placed in a vacuum environment to avoid interference with the laser pulses during temporal contrast enhancement by air media of a non-vacuum environment.

It should be noted that, each module in the laser pulse time domain contrast enhancement system provided in the embodiments of the present disclosure may be used to perform the method described in the foregoing method embodiments, and a specific implementation manner of the module may refer to the description of the foregoing method embodiments, which is not repeated herein for brevity.

According to the laser pulse time domain contrast enhancement system disclosed by the embodiment of the disclosure, the time domain contrast enhancement of the main pulse front edge which is up to the femto-second scale recently can be supported, the time domain contrast of the laser pulse to be enhanced can be enhanced remarkably, the energy loss of the main pulse of the target laser pulse relative to the main pulse of the laser pulse to be enhanced is less than 30%, namely, the energy loss is lower, and the energy conversion efficiency is higher; and enabling the main pulse of the target laser pulse to still keep longitudinal near Gaussian distribution without damaging the main pulse distribution; the time domain contrast enhancement is realized by mainly utilizing low-cost plasmas and a filtering reflector, so that the device has the advantages of simple structure and low cost; because the plasma is adopted to realize frequency shift, the limitation of breakdown threshold is avoided, and the time domain contrast enhancement of the laser pulse with the ultra-high peak power of the clapping tile and above level can be supported; the main pulse is kept in reflection transmission in the filtering reflector after being subjected to plasma frequency shift, so that the pulse width of the main pulse cannot be widened, namely the pulse width compression effect is achieved; the method has wide applicability and can be applied to the time domain contrast enhancement of femtosecond laser pulses in laser systems including but not limited to a titanium sapphire laser system, an OPCPA system and the like.

According to the laser pulse time domain contrast enhancement method and the system provided by the embodiment of the disclosure, the frequency conversion of the main pulse of the laser pulse to be enhanced is realized by utilizing the target plasma, and meanwhile, the pre-pulse frequency is kept basically unchanged, so that the separation of the main pulse and the pre-pulse on the frequency attribute is realized; and then a filtering reflector matched with bandwidth parameters of the main pulse frequency and the pre-pulse frequency is adopted to realize the filtering of the pre-pulse so as to obtain the target laser pulse with high time domain contrast.

The laser pulse temporal contrast enhancement method and system of the embodiments of the present disclosure are applicable to temporal contrast enhancement of all ultrashort ultrastrong laser pulses capable of exhausting electrons in a plasma to form a tail field, including but not limited to peak power in a terahertz (10 ¹² Tile to clapping tile (10) ¹⁵ Tile) and femto second laser pulses at the level above the tile can realize time domain contrast enhancement; and the method is not limited to performing time domain contrast enhancement on femtosecond laser pulses generated based on titanium sapphire, and can be implemented for femtosecond laser pulses of other wave bandsCurrent domain contrast enhancement; the time domain contrast enhancement magnitude of the laser pulse is not limited, and the method can be applied to Gao Chong-frequency lasers.

It should be noted that the flowcharts and block diagrams in the figures of the disclosed embodiments illustrate the architecture, functionality, and operation of possible implementations of systems and methods according to various embodiments of the present disclosure. In this regard, each block in the flowchart or block diagrams may represent a module or portion of a subsystem, which comprises one or more elements for implementing the specified functions. In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems which perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

The foregoing description of the embodiments of the present disclosure has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the various embodiments described. The terminology used herein was chosen in order to best explain the principles of the embodiments, the practical application, or the technical improvements in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

1. A method of laser pulse temporal contrast enhancement, the method comprising:

performing frequency shift on the main pulse of the laser pulse to be enhanced by utilizing target plasma matched with the pulse parameters of the main pulse of the laser pulse to be enhanced to obtain a frequency shift laser pulse, wherein the main pulse frequency of the frequency shift laser pulse is different from the pre-pulse frequency of the frequency shift laser pulse;

filtering the pre-pulse of the frequency-shifted laser pulse by using at least one filtering reflector to obtain a target laser pulse with enhanced contrast, wherein the passband of the filtering reflector covers the main pulse frequency of the frequency-shifted laser pulse, and the stopband of the filtering reflector covers the pre-pulse frequency of the frequency-shifted laser pulse; the reflectivity of the passband of the filter mirror is higher than the reflectivity of the stopband of the filter mirror.

2. The method of claim 1, wherein the target plasma comprises: a first plasma having a first density parameter and a second plasma having a second density parameter; the first density parameter and the second density parameter are determined according to a pulse parameter of the main pulse of the laser pulse to be enhanced and a target frequency to which the main pulse of the laser pulse to be enhanced is frequency-shifted, the first density parameter and the second density parameter are different, the density parameter comprises a density and a length of plasma distribution, and the pulse parameter comprises at least one of the following: light intensity, pulse width, focusing spot radius;

The method for obtaining the frequency-shifted laser pulse comprises the following steps of:

injecting the laser pulse to be enhanced into the first plasma with the first density parameter to obtain an intermediate laser pulse with a main pulse part frequency shifted to the target frequency, wherein the first plasma is used for enabling the main pulse of the laser pulse to be enhanced to be frequency shifted and generating linear chirp dispersion, and the main pulse of the intermediate laser pulse has linear chirp characteristics;

and injecting the intermediate laser pulse into the second plasma with the second density parameter to obtain a frequency-shifted laser pulse with the main pulse frequency shifted to the target frequency, wherein the second plasma is used for shifting the main pulse of the intermediate laser pulse and generating linear chirped dispersion opposite to the effect of the first plasma, and the main pulse of the frequency-shifted laser pulse has the characteristic of no chirp.

3. The method of claim 2, wherein the step of determining the position of the substrate comprises,

in the case where the first plasma is a low-density plasma and the second plasma is a high-density plasma, the first plasma shifts the main pulse of the laser pulse to be enhanced and generates a linear negative chirp dispersion, the main pulse of the intermediate laser pulse has a linear negative chirp characteristic, and the second plasma shifts the main pulse of the intermediate laser pulse and generates a near-linear positive chirp dispersion; or alternatively, the first and second heat exchangers may be,

In the case where the first plasma is a high-density plasma and the second plasma is a low-density plasma, the first plasma shifts the main pulse of the laser pulse to be enhanced and generates near-linear positive chirp dispersion, the main pulse of the intermediate laser pulse has near-linear positive chirp characteristics, and the second plasma shifts the main pulse of the intermediate laser pulse and generates linear negative chirp dispersion;

wherein the linear negative chirp characteristic is a linear decrease in frequency from the pulse head to the pulse tail; the near-linear positive chirp characteristic is that the frequency increases nearly linearly from the pulse head to the pulse tail, and the non-chirp characteristic is that the frequency is uniform from the pulse head to the pulse tail.

4. The method of claim 3, wherein the low density plasma comprises: center density at 10 ¹⁶ cm ^-3 To 10 ¹⁸ cm ^-3 Is a plasma of (a);

the high density plasma includes: center density at 10 ¹⁸ cm ^-3 To 10 ²⁰ cm ^-3 Is a plasma of (a).

5. The method of claim 1, wherein filtering the pre-pulses of the frequency shifted laser pulses using at least one filter mirror to obtain target laser pulses, comprises:

Collimation is carried out on the frequency-shift laser pulse, and a collimated frequency-shift laser pulse is obtained;

filtering the pre-pulse of the collimated frequency-shift laser pulse by using at least one filtering reflector to obtain a target laser pulse;

wherein the filtering reflector comprises at least one of a bandpass reflector and a lowpass reflector.

6. The method of claim 1, wherein prior to frequency shifting the main pulse of the laser pulse to be enhanced with a target plasma that matches a pulse parameter of the main pulse of the laser pulse to be enhanced, the method further comprises:

and compressing and focusing the initial laser pulse generated by the laser pulse generating device to obtain the laser pulse to be enhanced after compression and focusing.

7. The method according to any one of claim 1 to 6, wherein,

the laser pulse to be enhanced comprises: a femtosecond laser pulse capable of forming a tail wavefield in a plasma, the femtosecond laser pulse comprising: the laser pulses with peak power at or above the level of the tera and pulse width at the femtosecond scale, the wake field comprising a plurality of plasma bubbles that have a frequency shifting effect on the femtosecond laser pulses forming the wake field.

8. A laser pulse temporal contrast enhancement system, the system comprising:

the plasma frequency conversion module is used for carrying out frequency shift on the main pulse of the laser pulse to be enhanced by utilizing target plasma matched with the pulse parameters of the main pulse of the laser pulse to be enhanced to obtain a frequency shift laser pulse, wherein the main pulse frequency of the frequency shift laser pulse is different from the pre-pulse frequency of the frequency shift laser pulse;

the reflector filtering module comprises at least one filtering reflector, wherein the at least one filtering reflector is used for filtering out the pre-pulse of the frequency-shifted laser pulse to obtain a target laser pulse with enhanced contrast, the passband of the filtering reflector covers the main pulse frequency of the frequency-shifted laser pulse, and the stopband of the filtering reflector covers the pre-pulse frequency of the frequency-shifted laser pulse; the reflectivity of the passband of the filter mirror is higher than the reflectivity of the stopband of the filter mirror.

9. The system of claim 8, wherein the system further comprises:

the laser collimation module is used for collimating the frequency-shift laser pulse to obtain a collimated frequency-shift laser pulse;

The mirror filter module is further configured to: filtering the pre-pulse of the collimated frequency-shift laser pulse by using at least one filtering reflector to obtain a target laser pulse; wherein the filtering reflector comprises at least one of a bandpass reflector and a lowpass reflector.

10. The system according to claim 8 or 9, characterized in that the system further comprises:

the laser pulse generation module is used for generating initial laser pulses;

and the laser compression focusing module is used for compressing and focusing the initial laser pulse to obtain the laser pulse to be enhanced after compression focusing.