CN109755847B - Method for generating ultrashort laser pulse train - Google Patents
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Abstract
The invention discloses a method for generating an ultrashort laser pulse train, which comprises the steps of performing beat frequency on the laser pulse train comprising a plurality of sub-pulses with chirp characteristics to obtain the ultrashort laser pulse train comprising a plurality of micro-pulses with quasi-sinusoidal modulation characteristics, and finally generating the ultrashort laser pulse train to realize a stacking effect by adjusting and optimizing the time intervals of the plurality of sub-pulses so as to drive a photocathode electron gun to generate corresponding electron beam groups. The technical scheme provided by the invention can effectively reduce the charge amount in a single micro-pulse, thereby effectively reducing the space charge repulsive force to obtain a short-length micro-pulse, effectively reducing the length of an electron beam group, further improving the clustering factor of the electron beam group, and correspondingly effectively improving the terahertz coherent radiation power.
Description
Technical Field
The invention relates to the technical field of accelerators and free electron lasers, in particular to a method for generating an ultrashort laser pulse train.
Background
The terahertz wave band is an electromagnetic wave with the frequency of 0.1THz-10THz, and has a great significance for researching material science and material science due to the advantages and unique advantages of penetrability, low photon energy, high bandwidth, spectral fingerprint characteristics, ultrafast characteristics and the like. The development of terahertz radiation sources meeting application requirements is a research hotspot in related fields. The generation of terahertz light at present comprises broadband radiation generated by ultrafast pulse excitation, tunable single-frequency radiation based on electron beam excitation and the like. Among them, broadband radiation generated by ultrafast pulses is receiving more and more attention and becoming mature; among Terahertz radiation sources based on Electron beam excitation, a radiation source generated by periodic torsional pendulum motion of a relativistic Electron beam by a undulator is called a Terahertz Free Electron Laser (THz-FEL).
The THz-FEL can obtain coherent terahertz light with narrow bandwidth, is a radiation source capable of obtaining THz high output power at present, has the outstanding advantages of continuously adjustable wavelength in a large range, good beam quality, fine and adjustable optical pulse time structure and the like, plays an important role in the field of terahertz technology and application research thereof, and is a research hotspot at home and abroad. THz-FEL can be classified into an Oscillator type (Oscillator) and a Single-pass type (Single-pass). Among them, the single-pass THz-FEL directly generates coherent radiation in the undulator by using a single ultra-short electron beam (or beam bunch), so as to overcome the adverse effects of physical effects such as diffraction and slippage, etc. encountered by the oscillator type THz-FEL in generating long-wavelength THz radiation, and is receiving more and more attention.
The power of the terahertz radiation source of the single-pass THz-FEL is proportional to the charge amount of the electronic pulse and the square of the clustering factor. For a single-pulse electron beam, taking a gaussian distribution as an example, the cluster factor is expressed as:
where ω is the angular frequency of radiation, σtIs the electronic pulse time length. It can be seen that to produce a high clustering factor: the electron pulse length should be as short as possible, the higher the angular frequency ω of the radiation, the shorter the electron pulse length should be.
Producing high power radiation requires that the electron beam has as high a charge as possible and as short a length as possible. But the larger the charge amount, the longer the electron beam tends to be due to the space charge repulsion. Instead of a single pulse, the electron pulse train effectively reduces the space charge repulsion limitation for obtaining a short pulse length electron beam cluster by apportioning the amount of charge into the electron beam cluster consisting of a train of equally spaced micropulses. For an electronic pulse train consisting of a train of equally spaced micropulses, also exemplified by the gaussian distribution of the micropulses, the clustering factor can be expressed as:
where at is the time interval between micropulses. The bunching factor will resonate at a series of resonant frequencies ωm2m pi/Δ t, the first order resonance frequency when m is 1, the second order resonance frequency when m is 2, and so on. The more the number of the micro-pulses in the electron beam cluster is, the smaller the charge amount of a single micro-pulse is, the shorter the micro-pulse length can be, and finally, the higher clustering factor can be obtained.
In the THz-FEL device, an electron beam cluster can be generated by driving a laser pulse train to excite a photocathode electron gun, initial parameters of the electron beam cluster are determined by parameters of the driving laser pulse train, and the evolution of subsequent electron beam cluster parameters is closely related to space charge effect, electric field parameters in the electron gun and the like. Wherein the influence of space charge effects dominates during acceleration of the electron beam cluster to relativistic velocity. Therefore, an ultrashort electron beam is desired to be obtained, the root of which is to obtain an ultrashort laser pulse train, and the method for obtaining the ultrashort laser pulse train is also based on the principle of "sharing the charge amount", and the charge amount contained in the laser pulse is shared in the laser pulse train containing a plurality of micropulses, so that the charge amount of a single micropulse is reduced to overcome the increase of the electron beam micropulse length caused by the space charge effect. The mainstream methods for generating the ultrashort laser pulse train include a pulse pile-up method and a beat frequency method. The pulse accumulation method is complex in light path and numerous in influencing factors; meanwhile, the dispersion effect is strong, the laser pulse width is widened, and the dispersion compensation difficulty is high. The beat frequency method can effectively overcome the defects of the pulse accumulation method, but the number of micro-pulses obtained by the beat frequency method is related to the broadening quantity and the beat frequency, and the problem of small number of micro-pulses exists, especially the number of the micro-pulses in the low frequency band is obviously insufficient; if a larger number of micropulses are obtained by increasing the broadening amount, the negative effect of broadening the micropulse width to reduce the clustering factor is caused, and the effective reduction of the electron beam cluster length is difficult.
Disclosure of Invention
In view of the above, the invention provides a method for generating an ultrashort laser pulse train, which effectively solves the problem that the number of micropulses of the ultrashort laser pulse train generated by the existing beat frequency method is relatively small, and can effectively reduce the length of an electron beam group, thereby effectively improving the terahertz coherent radiation power.
In order to achieve the purpose, the technical scheme provided by the invention is as follows:
a method of generating an ultrashort laser pulse train, comprising:
generating a laser pulse train including a plurality of sub-pulses having a chirp characteristic, wherein a pulse width of each of the sub-pulses is uniform;
and performing beat frequency on the laser pulse train to obtain an ultrashort laser pulse train containing a plurality of micropulses.
Compared with the prior art, the technical scheme provided by the invention at least has the following advantages:
the invention provides a method for generating an ultrashort laser pulse train, which comprises the steps of performing beat frequency on the laser pulse train comprising a plurality of sub-pulses with chirp characteristics to obtain the ultrashort laser pulse train comprising a plurality of micro-pulses with quasi-sinusoidal modulation characteristics, and finally generating the ultrashort laser pulse train to realize a stacking effect by adjusting and optimizing the time intervals of the plurality of sub-pulses so as to drive a photocathode electron gun to generate corresponding electron beam groups. The technical scheme provided by the invention can effectively reduce the charge amount in a single micro-pulse, thereby effectively reducing the space charge repulsive force to obtain a short-length micro-pulse, effectively reducing the length of an electron beam group, further improving the clustering factor of the electron beam group, and correspondingly effectively improving the terahertz coherent radiation power.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.
Fig. 1 is a flowchart of a method for generating an ultrashort laser pulse train according to an embodiment of the present disclosure;
FIG. 2 is a flowchart of another method for generating an ultrashort laser pulse train according to an embodiment of the present disclosure;
FIG. 3 is a flowchart of a method for generating an ultra-short laser pulse train according to an embodiment of the present disclosure;
FIG. 4 is a flowchart of a method for generating an ultra-short laser pulse train according to an embodiment of the present disclosure;
FIG. 5 is a flowchart of a method for generating an ultra-short laser pulse train according to an embodiment of the present disclosure;
fig. 6 is a schematic diagram of an ultra-short laser pulse train obtained under the condition that the beat frequency is 0.7THz according to an embodiment of the present disclosure;
fig. 7 is a schematic diagram of an ultra-short laser pulse train obtained under the condition that the beat frequency is 1.0THz according to an embodiment of the present disclosure;
fig. 8 is a schematic diagram of an ultra-short laser pulse train obtained under the condition that the beat frequency is 1.5THz according to an embodiment of the present disclosure;
FIG. 9 is a schematic diagram of an ultra-short laser pulse train obtained under the conditions of beat frequencies of 0.7THz, 1.0THz and 1.5THz provided by the prior art;
fig. 10 is a schematic structural diagram of an apparatus for generating an ultrashort laser pulse train according to an embodiment of the present disclosure;
FIG. 11 is a schematic structural diagram of another apparatus for generating ultrashort laser pulses according to an embodiment of the present disclosure;
fig. 12 is a schematic structural diagram of another apparatus for generating an ultrashort laser pulse train according to an embodiment of the present application.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As described in the background, in the THz-FEL apparatus, an electron beam cluster may be generated by exciting a photocathode electron gun with a driving laser pulse train, initial parameters of the electron beam cluster are determined by parameters of the driving laser pulse train, and the evolution of subsequent electron beam cluster parameters is closely related to space charge effects, electric field parameters in the electron gun, and the like. Wherein the influence of space charge effects dominates during acceleration of the electron beam cluster to relativistic velocity. Therefore, an ultrashort electron beam is desired to be obtained, the root of which is to obtain an ultrashort laser pulse train, and the method for obtaining the ultrashort laser pulse train is also based on the principle of "sharing the charge amount", and the charge amount contained in the laser pulse is shared in the laser pulse train containing a plurality of micropulses, so that the charge amount of a single micropulse is reduced to overcome the increase of the electron beam micropulse length caused by the space charge effect. The mainstream methods for generating the ultrashort laser pulse train include a pulse pile-up method and a beat frequency method. The pulse accumulation method is complex in light path and numerous in influencing factors; meanwhile, the dispersion effect is strong, the laser pulse width is widened, and the dispersion compensation difficulty is high. The beat frequency method can effectively overcome the defects of the pulse accumulation method, but the number of micro-pulses obtained by the beat frequency method is related to the broadening quantity and the beat frequency, and the problem of small number of micro-pulses exists, especially the number of the micro-pulses in the low frequency band is obviously insufficient; if a larger number of micropulses are obtained by increasing the broadening amount, the negative effect of broadening the micropulse width to reduce the clustering factor is caused, and the effective reduction of the electron beam cluster length is difficult.
Based on this, the embodiment of the application provides a method and a device for generating an ultrashort laser pulse train, which effectively overcome the problem that the number of micropulses of the ultrashort laser pulse train generated by the existing beat frequency method is relatively small, can effectively reduce the length of an electron beam group, and further effectively improve the terahertz coherent radiation power. In order to achieve the above object, the technical solutions provided by the embodiments of the present application are described in detail below, specifically with reference to fig. 1 to 12.
Referring to fig. 1, a flowchart of a method for generating an ultrashort laser pulse train according to an embodiment of the present application is shown, where the method for generating an ultrashort laser pulse train includes:
s1, generating a laser pulse train comprising a plurality of sub-pulses with chirp characteristics, wherein the pulse width of each sub-pulse is consistent;
and S2, performing beat frequency on the laser pulse train to obtain an ultrashort laser pulse train containing a plurality of micropulses.
From the above, the laser pulse train including the plurality of sub-pulses with the chirp characteristic is subjected to beat frequency to obtain the ultra-short laser pulse train including the plurality of micro-pulses with the quasi-sinusoidal modulation characteristic, and the time interval of the plurality of sub-pulses is adjusted and optimized, so that the finally generated ultra-short laser pulse train can realize the stacking effect, and the photo-cathode electron gun is driven to generate the corresponding electron beam group. The technical scheme provided by the embodiment of the application can effectively reduce the charge amount in a single micropulse, thereby effectively reducing the space charge repulsive force to obtain the micropulse with a short length, effectively reducing the length of an electron beam group, further improving the clustering factor of the electron beam group, and correspondingly effectively improving the terahertz coherent radiation power.
Referring to fig. 2, a flowchart of another method for generating an ultrashort laser pulse train according to an embodiment of the present application is shown, where generating a laser pulse train including a plurality of sub-pulses with chirp characteristics includes:
s11, generating a second laser pulse comprising a plurality of sub-pulses;
and S12, performing chirp broadening on the second laser pulse to form a laser pulse train comprising a plurality of sub-pulses with chirp characteristics.
It is understood that the laser pulse train provided by the embodiment of the present application, which includes a plurality of sub-pulses having chirp characteristics, can be obtained by chirping and stretching a second laser pulse that includes a plurality of sub-pulses. The chirped stretching of the second laser pulse provided in the embodiment of the present application includes: and inputting the second laser pulse into a parallel grating pair for chirp broadening.
In an embodiment of the present application, the second laser pulse including a plurality of sub-pulses provided herein may be obtained by splitting a laser source. Referring to fig. 3, a flowchart of a method for generating a further ultrashort laser pulse train according to an embodiment of the present application is shown, where S11 for generating a second laser pulse including a plurality of sub-pulses according to an embodiment of the present application includes:
s101, outputting a first laser pulse by a laser source;
s102, carrying out beam splitting processing on the first laser pulse to obtain a plurality of sub-pulses;
and S103, combining all the sub-pulses to form a second laser pulse comprising a plurality of sub-pulses.
It can be understood that the second laser pulse provided in the embodiments of the present application obtains a plurality of sub-pulses by performing beam splitting on the first laser pulse output by one laser source, and then combines the plurality of sub-pulses to form the second laser pulse including the plurality of sub-pulses. The first laser pulse provided by the embodiment of the application is an ultrafast femtosecond pulse, the pulse width range of the ultrafast femtosecond pulse is not more than 150fs, and the pulse is a Fourier transform limit pulse. And, the laser source provided by the embodiment of the present application may be an oscillator type titanium sapphire laser, and the present application is not particularly limited.
In an embodiment of the present application, the splitting processing of the first laser pulse to obtain a plurality of sub-pulses provided in the present application may be to directly split the first laser pulse into a plurality of sub-pulses, where the splitting processing of the first laser pulse to obtain a plurality of sub-pulses includes:
at least one beam splitting is performed on the first laser pulse.
It can be understood that, when the number of the plurality of sub-pulses is 2, the first laser pulse may be split once, and the obtained two paths of sub-first laser pulses are 2 sub-pulses. Or when the number of the plurality of sub-pulses is greater than 2, two paths of sub-first laser pulses are obtained after the first laser pulse is subjected to beam splitting for the first time, then one or two paths of the two paths of sub-first laser pulses are subjected to beam splitting again respectively, and the analogy is carried out according to the rule, so that the beam splitting times are optimized to obtain the sub-pulses with the preset number.
Or, in an embodiment of the present application, the splitting processing of the first laser pulse to obtain a plurality of sub-pulses provided in the present application may be to indirectly split the first laser pulse into a plurality of sub-pulses, where the splitting processing of the first laser pulse to obtain a plurality of sub-pulses includes:
and splitting the beam again after at least one beam splitting and combining operation is carried out on the first laser pulse.
It can be understood that, when the number of the plurality of sub-pulses is greater than 2, after the first laser pulse is subjected to the first beam splitting and combining operation, the obtained first laser pulse after the first operation includes two sub-pulses; and then, carrying out secondary beam splitting and beam combining on the second laser pulse after the first operation, wherein the obtained first laser pulse after the second operation comprises four sub-pulses, so as to analogize the rule, the times of beam splitting and beam combining operations are optimized, and finally, the sub-pulses with the preset number are indirectly obtained after the beam splitting is carried out again.
In an embodiment of the present application, the second laser pulse including a plurality of sub-pulses provided in the present application may be obtained by a plurality of independent laser sources. Referring to fig. 4, a flowchart of a method for generating a further ultrashort laser pulse train according to an embodiment of the present application is shown, where S11 for generating a second laser pulse including a plurality of sub-pulses according to an embodiment of the present application includes:
s111, outputting corresponding first laser pulses by a plurality of laser sources;
s112, adjusting the phase of the first laser pulse to synchronize the phase of each first laser pulse into a plurality of sub-pulses;
s113, all the sub-pulses are combined to form a second laser pulse including a plurality of sub-pulses.
It can be understood that, when the second laser pulse is acquired in the embodiment of the present application, the phase of the first laser pulse output by each of the multiple independent laser sources is adjusted, so that the phases of all the first laser pulses are synchronized, each first laser pulse is equivalent to one sub-pulse, and then multiple sub-pulses are obtained; and then combining the plurality of sub-pulses to obtain a second laser pulse comprising a plurality of sub-pulses. The first laser pulse provided by the embodiment of the application is an ultrafast femtosecond pulse, the pulse width range of the ultrafast femtosecond pulse is not more than 150fs, and the pulse is a Fourier transform limit pulse. And, the laser source provided by the embodiment of the present application may be an oscillator type titanium sapphire laser, and the present application is not particularly limited.
It should be noted that, when the method provided in the embodiment of the present application includes a plurality of laser sources, at least one of the plurality of laser sources may be further split, and the phases of the split pulse and the pulse emitted from the laser source that is not split are simultaneously adjusted to obtain a plurality of first laser pulses, which is not limited in this application.
Referring to fig. 5, a flowchart of a method for generating a further ultrashort laser pulse train according to an embodiment of the present application is shown, where S2 for performing beat frequency on the laser pulse train according to the embodiment of the present application includes:
s21, splitting the laser pulse train into two sub-laser pulse trains;
s22, reflecting the two sub laser pulse trains with different optical paths respectively;
and S23, beating the reflected two paths of sub-laser pulse trains to obtain an ultra-short laser pulse train containing a plurality of micro-pulses.
It can be understood that in the beat frequency process provided by the embodiment of the present application, the beat frequency can be changed by adjusting and optimizing the reflection optical lengths of the two sub-laser pulse trains, and finally, the widths and intervals of the micropulses in the ultrashort laser pulse train are changed.
In an embodiment of the present application, the center-to-center distance between adjacent sub-pulses in the laser pulse train provided by the present application is greater than the pulse width of the micro-pulse in the ultra-short laser pulse train, and is less than the pulse width of the sub-pulse in the laser pulse train.
The technical solution provided by the present application is compared with the prior art with reference to the simulation result of the ultrashort laser pulse train shown in the attached drawing. Fig. 6 is a schematic diagram of an ultra-short laser pulse train obtained under the condition that the beat frequency is 0.7THz provided in the embodiment of the present application, fig. 7 is a schematic diagram of an ultra-short laser pulse train obtained under the condition that the beat frequency is 1.0THz provided in the embodiment of the present application, fig. 8 is a schematic diagram of an ultra-short laser pulse train obtained under the condition that the beat frequency is 1.5THz provided in the embodiment of the present application, and fig. 9 is a schematic diagram of an ultra-short laser pulse train obtained under the conditions that the beat frequency is 0.7THz, 1.0THz, and 1.5THz provided in the prior art. It should be noted that the ultrashort laser pulse trains shown in fig. 6-8 are based on data obtained by simulation under the condition of a laser pulse train including two sub-pulses with chirp characteristics.
In the schematic diagram (simulation result) of the ultrashort laser pulse train shown in fig. 6 provided in this embodiment of the present application, the beat frequency is 0.7THz, the spectral delay (time interval between two sub-pulses after being split) in fig. 6(a) is adjusted to τ 'of 12.5ps, and the spectral delay in fig. 6(b) is adjusted to τ' of 5.5 ps.
Fig. 7 shows a schematic diagram of an ultrashort laser pulse train (simulation result) with a beat frequency of 1.0THz, where the spectral delay in fig. 7(a) is adjusted to τ 'of 12.5ps and the spectral delay in fig. 7(b) is adjusted to τ' of 5.5 ps.
Fig. 8 shows a schematic diagram of an ultrashort laser pulse train (simulation result) with a beat frequency of 1.5THz, where the spectral delay in fig. 8(a) is adjusted to τ 'of 12.5ps and the spectral delay in fig. 8(b) is adjusted to τ' of 4 ps.
As can be seen from fig. 6 to 8, the ultrashort laser pulse train generated by the ultrashort laser pulse train generating method provided by the embodiment of the present application has a stacking effect, and the stacking effect is realized by the light splitting delay τ', so that the total length of the pulse train can be equivalent to that in the pulse stacking scheme.
Fig. 9 is a schematic diagram of ultra-short laser pulse trains obtained under different beat frequency conditions by using the existing beat frequency method (simulation result). Wherein, the condition of fig. 9(a) is consistent with fig. 6(b), the condition of fig. 9(b) is consistent with fig. 7(b), and the condition of fig. 9(c) is consistent with fig. 8(b), and the ratio of the number of micropulses included in the ultra-short laser pulse train obtained by the conventional beat frequency method and the method provided by the embodiment of the present application is shown in table 1:
| experimental group | Number of micropulses | Control group | Number of micropulses | Rate of increase |
| FIG. 4(a) | 7 | FIG. 1(b) | 11 | 57.14％ |
| FIG. 4(b) | 11 | FIG. 2(b) | 16 | 45.45％ |
| FIG. 4(c) | 15 | FIG. 3(b) | 22 | 46.67％ |
TABLE 1
Therefore, by the ultrashort laser pulse train generating method provided by the embodiment of the application, the number of micro-pulses contained in a single ultrashort laser pulse train generated is increased by about 50% compared with that of the existing beat frequency method.
The charge amount of a single micro-pulse in the ultra-short laser pulse train obtained by the conventional beat frequency method and the method provided by the embodiment of the application is shown in table 2 (the total charge amount is 240 pC):
TABLE 2
Therefore, by the ultrashort laser pulse train generating method provided by the embodiment of the application, the electric quantity of the micro-pulse contained in the generated ultrashort laser pulse train is reduced by more than 30% compared with that of the existing beat frequency method, so that the space charge repulsive force is effectively reduced to obtain the micro-pulse with short length, the clustering factor of an electron beam group is further improved, and the terahertz coherent radiation power is correspondingly effectively improved.
Correspondingly, an embodiment of the present application further provides a device for generating an ultrashort laser pulse train, which is shown in fig. 10 and is a schematic structural diagram of the device for generating an ultrashort laser pulse train provided in the embodiment of the present application, where the device for generating an ultrashort laser pulse train includes:
a laser pulse train generation system 10, wherein the laser pulse train generation system 10 is configured to generate a laser pulse train including a plurality of sub-pulses having chirp characteristics, and a pulse width of each of the sub-pulses is uniform;
and a beat frequency system 20, wherein the beat frequency system 20 is configured to beat frequency of the laser pulse train to obtain an ultra-short laser pulse train including a plurality of micro pulses.
From the above, the laser pulse train including the plurality of sub-pulses with the chirp characteristic is subjected to beat frequency by the beat frequency system to obtain the ultrashort laser pulse train including the plurality of micro-pulses with the quasi-sinusoidal modulation characteristic, and the finally generated ultrashort laser pulse train can realize the stacking effect by adjusting and optimizing the time interval of the plurality of sub-pulses, so as to drive the photocathode electron gun to generate the corresponding electron beam group. The technical scheme provided by the embodiment of the application can effectively reduce the charge amount in a single micropulse, thereby effectively reducing the space charge repulsive force to obtain the micropulse with a short length, effectively reducing the length of an electron beam group, further improving the clustering factor of the electron beam group, and correspondingly effectively improving the terahertz coherent radiation power.
Referring to fig. 10, embodiments of the present application provide that the laser pulse train generation system 10 may include:
a second laser pulse generating unit 110, the second laser pulse generating unit 110 being configured to generate a second laser pulse comprising a plurality of sub-pulses;
and a chirped stretching unit 120, wherein the chirped stretching unit 120 is configured to perform chirped stretching on the second laser pulse to form a laser pulse train including a plurality of sub-pulses with chirping characteristics.
It is understood that the laser pulse train provided by the embodiment of the present application, which includes a plurality of sub-pulses having chirp characteristics, can be obtained by chirping and stretching a second laser pulse that includes a plurality of sub-pulses. As shown in fig. 10, the chirped stretching unit 120 provided in the embodiment of the present application includes:
a second reflection module 121, where the second reflection module 121 is configured to receive and output the second laser pulse;
a chirp stretching module 122 disposed on the optical path of the second reflection module 121, where the chirp stretching module 122 is configured to perform first chirp stretching on a second laser pulse output by the second reflection module;
the third reflection module 123 is disposed on the optical path of the chirp widening module 122, where the third reflection module 123 is configured to reflect the second laser pulse after the first chirp widening to the chirp widening module 122, and the chirp widening module 122 performs second chirp widening on the second laser pulse after the first chirp widening and outputs the second laser pulse to the second reflection module 121, and the second reflection module 121 outputs the second laser pulse after the second chirp widening to the beat frequency system 20.
It is understood that the amount of broadening of the second laser pulses can be increased by chirping the second laser pulses twice. Optionally, the chirped stretching module provided in this embodiment of the present application is a parallel grating pair, where the parallel grating pair is composed of two gratings that are oppositely disposed and parallel to each other, and the grating provided in this embodiment of the present application may be a holographic diffraction grating, so as to generate the second laser pulse with a linear chirp characteristic (an instantaneous frequency of an optical field changes as a linear function of time) by using a group delay dispersion of the grating. In addition, the embodiments of the present application may also be replaced with optical elements having similar characteristics, such as Gires-Tournois interference mirrors or prisms.
As shown in fig. 10, the second reflective module 121 provided in this embodiment of the present application may be a prism beam splitter (the prism beam splitter may be a half mirror with T: R being 50%: 50%, or a half mirror with total transmission toward the beam combining module side and total reflection on the other side). The third reflection module 123 provided in this embodiment of the present application may be a retro-reflection prism, configured to change a transmission direction of the second laser pulse output by the chirp stretching module 122 and subjected to the first chirp stretching, so as to return the second laser pulse to the chirp stretching module 122 for the second chirp stretching, so as to increase the stretching amount.
In addition, the third reflection module 123 provided in this embodiment of the present application may further include a planar mirror, as shown in fig. 12, which is a schematic structural diagram of a device for generating a further ultrashort laser pulse train provided in this embodiment of the present application, where the third reflection module 123 may include two planar mirrors disposed perpendicular to each other, and a mirror surface direction and a normal direction of any one of the planar mirrors form an included angle of 45 degrees with a direction of a second laser pulse incident to the planar mirror. And the second reflecting module can also be a flat plate beam splitter.
In an embodiment of the present application, the second laser pulse including a plurality of sub-pulses provided herein may be obtained by splitting a laser source. As shown in fig. 10, the second laser pulse generating unit 110 provided in the embodiment of the present application includes:
a laser source 111, wherein the laser source 111 is configured to output a first laser pulse;
the laser beam splitting module 112 is disposed on an optical path of the laser source 111, and the laser beam splitting module 112 is configured to split the first laser pulse to obtain a plurality of sub-pulses;
and a beam combining module 113 disposed on the optical path of the laser beam splitting module 112, where the beam combining module 113 is configured to combine all the sub-pulses to form a second laser pulse including a plurality of sub-pulses.
It can be understood that the second laser pulse provided in the embodiments of the present application obtains a plurality of sub-pulses by performing beam splitting on the first laser pulse output by one laser source, and then combines the plurality of sub-pulses to form the second laser pulse including the plurality of sub-pulses. The first laser pulse provided by the embodiment of the application is an ultrafast femtosecond pulse, the pulse width range of the ultrafast femtosecond pulse is not more than 150fs, and the pulse is a Fourier transform limit pulse. The laser source provided by the embodiment of the present application may be an oscillator type titanium sapphire laser, and the present application is not particularly limited thereto. And the beam combining module provided by the embodiment of the application is a beam combining mirror.
In an embodiment of the present application, beam splitting processing of a first laser pulse provided by the present application to obtain a plurality of sub-pulses may be performed to directly split a first laser pulse into a plurality of sub-pulses, where the laser beam splitting module provided by the embodiment of the present application is configured to split a first laser pulse at least once, where the laser beam splitting module includes:
the beam splitting subsystem comprises a first-stage beam splitting sub-module to an Nth-stage beam splitting sub-module, each stage of beam splitting sub-module comprises at least one beam splitting mirror, and an i +1 th-stage beam splitting sub-module beam splitting mirror is correspondingly arranged on at least one path of beam splitting light path of at least one beam splitting mirror of the ith-stage beam splitting sub-module, N is an integer larger than 0, and i is a positive integer not larger than N;
and the first reflection module is used for transmitting the plurality of sub-pulses output by the beam splitting subsystem to the beam combining module.
It can be understood that, when the number of the plurality of sub-pulses is 2, the first laser pulse may be split once, and the obtained two paths of sub-first laser pulses are 2 sub-pulses. Or when the number of the plurality of sub-pulses is greater than 2, two paths of sub-first laser pulses are obtained after the first laser pulse is subjected to beam splitting for the first time, then one or two paths of the two paths of sub-first laser pulses are subjected to beam splitting again respectively, and the analogy is carried out according to the rule, so that the beam splitting times are optimized to obtain the sub-pulses with the preset number.
As shown in fig. 10, the number of the sub-pulses is 2 for example, wherein the laser beam splitting module 112 includes a beam splitting subsystem and a first reflection module 1122, wherein the beam splitting subsystem includes a primary beam splitting sub-module, and the beam splitting sub-module includes a beam splitter 1121(T: R is 50%: 50%, i.e., transmission equals reflection). The beam splitter 1121 is connected to the first laser pulse output by the laser source 111, and splits the first laser pulse into two sub-pulses with mutually perpendicular transmission directions and polarization directions. The two sub-pulses have different polarization directions, so that the coherence effect caused by the consistent polarization directions of the sub-pulses in beat frequency can be avoided.
Then, the two sub-pulses output by the beam splitter 1121 are guided by the first reflection module 1122 to be transmitted to the beam combining module 113. The first reflective module 1122 provided in this embodiment of the present application may include a plurality of reflective mirrors. As shown in fig. 10, the first reflection module 1122 may include 4 reflectors, where a mirror surface direction and a normal direction of any one reflector form an angle of 45 degrees with a transmission direction of the sub-pulse incident to the reflector, and the transmission direction and the polarization direction of the two sub-pulses are still perpendicular to each other after passing through the first reflection module 1122.
As shown in fig. 10, the beam combining module 113 provided in this embodiment of the application may be indirectly disposed on the optical path of the beam splitter 1121 through the first reflection module 1122, that is, two sub-pulses of the beam splitter 1121 respectively correspond to two reflective mirrors, and are respectively transmitted to the beam combining module 113 after being reflected by the two respective reflective mirrors. In addition, the positions of the beam splitter 1121 and the beam combining module 113 may also be adjusted, so as to reduce the number of the reflective mirrors, as shown in fig. 11, which is a schematic structural diagram of another apparatus for generating an ultrashort laser pulse train provided in this embodiment of the present disclosure, where the beam combining module 113 is directly disposed on a reflection light path of the beam splitter 1121, and the first reflective module 1122 is disposed on a transmission light path of the beam splitter 1121, where the first reflective module 1122 may include two reflective mirrors (e.g., planar reflective mirrors) disposed perpendicular to each other, a mirror surface direction and a normal direction of any one of the reflective mirrors form an included angle of 45 degrees with a transmission direction of a sub-pulse incident to the reflective mirror, and the sub-pulse output from the transmission light path of the beam splitter 1121 is guided and transmitted to the beam combining module 113 through the two.
In an embodiment of the present application, a beam splitter in the beam splitting sub-module provided by the present application may also be replaced by a combination of a half-wave plate and a polarization beam splitter, which is not specifically limited in this application.
Or, in an embodiment of the present application, the beam splitting processing on the first laser pulse to obtain a plurality of sub-pulses provided in the present application may be to indirectly split the first laser pulse into the plurality of sub-pulses, where the laser beam splitting module provided in the embodiment of the present application is configured to split the first laser pulse again after performing at least one beam splitting and combining operation on the first laser pulse, where the laser beam splitting module includes:
the beam splitting and combining sub-module comprises a first-stage beam splitting and combining sub-module, an Nth-stage beam splitting and combining sub-module and a beam splitting sub-module, wherein the beam splitting and combining sub-module is arranged on the optical path of the Nth-stage beam splitting and combining sub-module, each stage of beam splitting and combining sub-module comprises a beam splitter and a beam combining mirror arranged on the optical path of the beam splitter, the beam splitting sub-module is a beam splitter, and N is an integer greater than 0;
and the first reflection module is used for transmitting the two paths of pulses output by the beam splitting sub-module to the beam combining module.
It can be understood that, when the number of the plurality of sub-pulses is greater than 2, after the first laser pulse is subjected to the first beam splitting and combining operation, the obtained first laser pulse after the first operation includes two sub-pulses; and then, carrying out secondary beam splitting and beam combining on the second laser pulse after the first operation, wherein the obtained first laser pulse after the second operation comprises four sub-pulses, so as to analogize the rule, the times of beam splitting and beam combining operations are optimized, and finally, the sub-pulses with the preset number are indirectly obtained after the beam splitting is carried out again.
In an embodiment of the present application, the second laser pulse including a plurality of sub-pulses provided by the present application may also be obtained by a plurality of independent laser sources, that is, the second laser pulse generating unit provided by the embodiment of the present application includes:
a plurality of laser sources, each for outputting a respective first laser pulse;
the phase adjusting module is arranged on the optical paths of the laser sources and used for adjusting the phase of the first laser pulse to enable the phase of each first laser pulse to be synchronous into a plurality of sub-pulses;
and the beam combining module is arranged on the optical path of the phase adjusting module and is used for combining all the sub-pulses to form a second laser pulse comprising a plurality of sub-pulses.
As shown in fig. 10, the beat frequency system 20 provided by the embodiment of the present application includes:
a beam splitting interference module 210, where the beam splitting interference module 210 is configured to split the laser pulse train into two sub-laser pulse trains;
the two reflectors 220 are respectively disposed on the two optical paths of the beam splitting interference module 210, a mirror surface of each reflector 220 is perpendicular to a direction of the corresponding sub-laser pulse train, and the two reflectors 220 are used for respectively reflecting the two sub-laser pulse trains with different optical lengths, so that beat frequencies occur in the two reflected sub-laser pulse trains.
Further, in order to reduce an error of an optical path difference, the beam splitting interference module provided in the embodiment of the present application is a flat beam splitter (T: R is 50%: 50%, that is, transmission is equal to reflection), wherein after the flat beam splitter splits a laser pulse train into two sub-laser pulse trains, each sub-laser pulse train is reflected by a corresponding mirror, and then a beat frequency is performed at the flat beam splitter to form an ultrashort laser pulse train including a plurality of micropulses having a quasi-sinusoidal modulation characteristic.
It can be understood that in the beat frequency process provided by the embodiment of the present application, the beat frequency can be changed by adjusting and optimizing the reflection optical lengths of the two sub-laser pulse trains, and finally, the widths and intervals of the micropulses in the ultrashort laser pulse train are changed.
In an embodiment of the present application, the center-to-center distance between adjacent sub-pulses in the laser pulse train provided by the present application is greater than the pulse width of the micro-pulse in the ultra-short laser pulse train, and is less than the pulse width of the sub-pulse in the laser pulse train.
The embodiment of the application provides a method and a device for generating an ultrashort laser pulse train, wherein the laser pulse train comprising a plurality of sub-pulses with chirp characteristics is subjected to beat frequency to obtain the ultrashort laser pulse train comprising a plurality of micro-pulses with quasi-sinusoidal modulation characteristics, and the finally generated ultrashort laser pulse train can realize a stacking effect by adjusting and optimizing the time interval of the plurality of sub-pulses, so that a photocathode electron gun is driven to generate a corresponding electron beam group. The technical scheme provided by the embodiment of the application can effectively reduce the charge amount in a single micropulse, thereby effectively reducing the space charge repulsive force to obtain the micropulse with a short length, effectively reducing the length of an electron beam group, further improving the clustering factor of the electron beam group, and correspondingly effectively improving the terahertz coherent radiation power.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (8)
1. A method for generating an ultrashort laser pulse train, comprising:
generating a laser pulse train including a plurality of sub-pulses having a chirp characteristic, wherein a pulse width of each of the sub-pulses is uniform;
performing beat frequency on the laser pulse train to obtain an ultrashort laser pulse train containing a plurality of micropulses, and performing beat frequency on the laser pulse train, wherein the beat frequency comprises the following steps:
splitting the laser pulse train into two paths of sub-laser pulse trains;
reflecting the two sub laser pulse trains with different optical paths respectively;
and enabling the reflected two paths of sub laser pulse trains to generate beat frequency.
2. The method of generating an ultrashort laser pulse train according to claim 1, wherein generating a laser pulse train including a plurality of sub-pulses having chirp characteristics includes:
generating a second laser pulse comprising a plurality of sub-pulses;
and performing chirp broadening on the second laser pulse to form a laser pulse train comprising a plurality of sub-pulses with chirp characteristics.
3. The method of generating an ultrashort laser pulse train according to claim 2, wherein generating a second laser pulse comprising a plurality of sub-pulses comprises:
outputting a first laser pulse by a laser source;
carrying out beam splitting processing on the first laser pulse to obtain a plurality of sub-pulses;
and combining all the sub-pulses to form a second laser pulse comprising a plurality of sub-pulses.
4. The method of generating ultrashort laser pulse train according to claim 3, wherein splitting the first laser pulse into a plurality of sub-pulses comprises:
at least one beam splitting is performed on the first laser pulse.
5. The method of generating ultrashort laser pulse train according to claim 3, wherein splitting the first laser pulse into a plurality of sub-pulses comprises:
and splitting the beam again after at least one beam splitting and combining operation is carried out on the first laser pulse.
6. The method of generating an ultrashort laser pulse train according to claim 2, wherein generating a second laser pulse comprising a plurality of sub-pulses comprises:
outputting, by a plurality of laser sources, respective first laser pulses;
adjusting the phase of the first laser pulses to synchronize the phase of each first laser pulse into a plurality of sub-pulses;
and combining all the sub-pulses to form a second laser pulse comprising a plurality of sub-pulses.
7. The method of generating an ultrashort laser pulse train of claim 2, wherein the chirped stretching of the second laser pulse comprises:
and inputting the second laser pulse into a parallel grating pair for chirp broadening.
8. The method of claim 1, wherein the center-to-center distance between adjacent sub-pulses in the laser pulse train is greater than the pulse width of the micro-pulse in the ultrashort laser pulse train and less than the pulse width of the sub-pulse in the laser pulse train.
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