CN109586145B - Double-color terahertz source and method for outputting double-color terahertz pulses by using same - Google Patents
Double-color terahertz source and method for outputting double-color terahertz pulses by using same Download PDFInfo
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Abstract
The invention discloses a bicolor terahertz source and a method for outputting bicolor terahertz pulses by the bicolor terahertz source.
Description
Technical Field
The invention relates to the technical field of ultrafast optics and femtosecond lasers, in particular to a bicolor terahertz source and a method for outputting bicolor terahertz pulses by using the same.
Background
The terahertz wave band is an electromagnetic wave with the frequency of 0.1-10THz, and has a great significance in the fields of material characteristic research, imaging, biomedicine, astronomy and the like due to the advantages and unique advantages of penetrability, low photon energy, high bandwidth, spectral fingerprint characteristics, ultrafast characteristics and the like.
The terahertz is used for measuring TPTP (THz pump/THz probe), and the method has unique identification capability for the dynamic collective behavior of particles with characteristic responses falling in a terahertz waveband, such as carrier dynamics, resonance of a magnetic vibrator, electronic transient loss and the like, so that the method is very important for researching the characteristic physical properties of functional materials and understanding deep physical mechanisms. The basic mode of TPTP measurement is to utilize two beams of homologous terahertz light, wherein one beam is used for ultrafast excitation, and the other beam is used for ultrafast detection. Taking TPTP measurements as an example for the study of nonlinear THz field-induced carrier dynamics for condensed materials, THz pump light can excite doped semiconductor conduction band electrons, and homologous THz probe light is used to probe the characteristics of the field-induced carriers.
The existing terahertz source for TPTP measurement is mostly a single-frequency terahertz source, namely the frequency of pump light and probe light is consistent, and the adjustability is poor.
Disclosure of Invention
In view of this, the technical solution of the present invention provides a bi-color terahertz source and a method for outputting bi-color terahertz pulses by the bi-color terahertz source, so that bi-color terahertz light output by a terahertz generation module is bi-color terahertz pulses including two sub-pulses with different frequencies.
In order to achieve the above purpose, the invention provides the following technical scheme:
a bi-color terahertz source, comprising:
a light source module for outputting laser pulses;
the laser pulse train forming module is used for performing chirped broadening on the laser pulses output by the light source module to form two paths of chirped pulses, performing beat frequency on the two paths of chirped pulses respectively to form a first laser pulse comprising an ultrashort laser sub-pulse train and a second laser pulse comprising the ultrashort laser sub-pulse train, and combining the first laser pulse and the second laser pulse to form a third laser pulse, wherein the third laser pulse comprises a plurality of ultrashort laser pulse trains with different frequency sub-pulse trains;
a terahertz generation module for outputting bi-color terahertz light based on the third laser pulse.
Preferably, in the above-mentioned dual-color terahertz source, the laser pulse train shaping module includes:
the chirped stretching unit is used for performing chirped stretching on the laser pulse output by the light source module and outputting a chirped pulse;
the beam splitting unit is used for splitting the chirped pulses output by the chirped stretching unit into two paths of chirped pulses;
the first beat frequency unit is used for carrying out beat frequency on one path of chirped pulse output by the beam splitting unit and outputting a first laser pulse comprising an ultrashort laser sub-pulse train; the second beat frequency unit is used for carrying out beat frequency on the other path of chirped pulse output by the beam splitting unit and outputting a second laser pulse comprising an ultrashort laser sub-pulse train;
and the beam combination unit is used for combining the first laser pulse and the second laser pulse and outputting a third laser pulse, and the third laser pulse comprises an ultrashort laser pulse string of a plurality of different-frequency sub-pulse strings.
Preferably, in the above-mentioned bichromatic terahertz source, the chirp stretching unit includes: the device comprises a first beam splitter, a parallel grating pair and a back reflection element;
laser pulses output by the light source module sequentially pass through the first beam splitter and the parallel grating pair to be incident to the backward reflecting element, and are reflected and sequentially pass through the parallel grating pair and the first beam splitter to be incident to the beam splitting unit, and the parallel grating pair is used for chirp broadening.
Preferably, in the dual-color terahertz source, the parallel grating pair includes two gratings that are oppositely disposed and parallel to each other, and the gratings are holographic diffraction gratings.
Preferably, in the bichromal terahertz source, the retroreflective element is a retroreflective prism.
Preferably, in the bichromal terahertz source, the beam splitting unit is a second beam splitter, and the reflection transmittance of the second beam splitter is adjustable.
Preferably, in the dual-color terahertz source, the first beat frequency unit and the second beat frequency unit each include a plate beam splitter and two plane mirrors;
for the first beat frequency unit, one chirp pulse output by the beam splitting unit is divided into two chirp sub-pulses by a flat plate beam splitter of the first beat frequency unit, the two chirp sub-pulses are respectively reflected back to the flat plate beam splitter of the first beat frequency unit by a plane mirror of the first beat frequency unit, and the two chirp sub-pulses are input to the beam combining unit after being coupled by the flat plate beam splitter;
for the second beat frequency unit, the other path of chirped pulse output by the beam splitting unit is split into two paths of chirped sub-pulses by a flat beam splitter of the second beat frequency unit, the two paths of chirped sub-pulses are respectively reflected back to the flat beam splitter of the second beat frequency unit by a plane mirror of the second beat frequency unit, and the chirped sub-pulses are input to the beam combining unit after being coupled by the flat beam splitter.
Preferably, in the bichromatic terahertz source, in the same beat frequency unit, the mirror surface of the plane mirror is perpendicular to the transmission direction of the chirped sub-pulse output by the flat plate beam splitter.
Preferably, in the dual-color terahertz source, the beam combining unit is a beam combining mirror, the reflection transmittance of the beam combining mirror is adjustable, and the included angles between the laser pulse incident to the beam combining mirror and the medium surface of the beam combining mirror and the normal of the medium surface are both 45 °.
Preferably, in the above-mentioned bichromal terahertz source, the terahertz generating module is a photoconductive antenna or a nonlinear crystal.
Preferably, in the above-mentioned two-color terahertz source, further includes: and the included angle between the direction of the mirror surface of the reflecting mirror and the transmission direction of the light pulse incident to the reflecting mirror is larger than 0 degree and smaller than 90 degrees.
The invention also provides a method for outputting the bicolor terahertz pulse by the bicolor terahertz source, which comprises the following steps:
outputting laser pulses through a light source module;
performing chirped broadening on the laser pulse output by the light source module through a laser pulse train forming module to form two paths of chirped pulses, performing beat frequency on the two paths of chirped pulses respectively to form a first laser pulse comprising an ultrashort laser sub-pulse train and a second laser pulse comprising the ultrashort laser sub-pulse train, and combining the first laser pulse and the second laser pulse to form a third laser pulse, wherein the third laser pulse comprises a plurality of ultrashort laser pulse trains with different frequency sub-pulse trains;
outputting, by a terahertz generation module, bi-color terahertz light based on the third laser pulse.
As can be seen from the above description, in the bi-color terahertz source and the method for outputting bi-color terahertz pulses by using the bi-color terahertz source provided by the invention, the laser pulse train forming module can form third laser pulses based on the laser pulse width output by the light source module, where the third laser pulses include ultrashort laser pulse trains with a plurality of sub-pulse trains with different frequencies, so that bi-color terahertz light output by the terahertz generating module is bi-color terahertz pulses including two sub-pulses with different frequencies.
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 schematic structural diagram of a bicolor terahertz source provided by an embodiment of the present invention;
fig. 2 is a schematic flow chart of a method for outputting a dual-color terahertz pulse according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of two-color terahertz pulses obtained by the two-color terahertz source shown in FIG. 1 under the condition that the beat frequencies of the two paths are 1.0THz and 0.5THz respectively;
FIG. 4 is a schematic diagram of two-color terahertz pulses obtained by the two-color terahertz source shown in FIG. 1 under the condition that the beat frequencies of the two paths are 1.5THz and 2.0THz respectively;
fig. 5 is a schematic diagram of two-color terahertz pulses obtained by the two-color terahertz source shown in fig. 1 under the condition that the beat frequencies of the two paths of the two-color terahertz sources are 2.5THz and 3.0THz respectively.
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 art, most of the existing terahertz sources for TPTP measurement are single-frequency terahertz sources, i.e., the frequencies of pump light and probe light are consistent, and the adjustability is poor.
Because the characteristic response frequency coverage range of a system to be measured can be very large, a single-frequency terahertz source forms great limitation on the application range of TPTP measurement. However, if two independent terahertz sources are used to realize continuous adjustment of frequency, the requirement on synchronization of two beams of terahertz light is extremely high, which cannot be realized by the prior art. A dual-frequency terahertz source which can be used for TPTP measurement is in urgent need of development.
In order to solve the above problem, an embodiment of the present invention provides a dual-color terahertz source, which is simple in apparatus, and capable of conveniently and efficiently generating a dual-color terahertz pulse including two sub-pulses with different frequencies, and the frequency of each sub-pulse can be independently and continuously adjusted.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
Referring to fig. 1, fig. 1 is a schematic structural diagram of a bi-color terahertz source provided in an embodiment of the present invention, where the bi-color terahertz source includes: the terahertz wave generating device comprises a light source module 1, a laser pulse train forming module 3 and a terahertz wave generating module 2.
Wherein, the light source module 1 is used for outputting laser pulses. The laser pulse train forming module 3 is used for performing chirped broadening on the laser pulses output by the light source module 1 to form two chirped pulses, performing beat frequency on the two chirped pulses respectively to form first laser pulses comprising ultrashort laser sub-pulse trains and second laser pulses comprising ultrashort laser sub-pulse trains, and combining the first laser pulses and the second laser pulses to form third laser pulses, wherein the third laser pulses comprise ultrashort laser pulse trains with a plurality of different frequency sub-pulse trains. The terahertz generation module 2 is configured to output the two-color terahertz light based on the third laser pulse.
As shown in fig. 1, the laser pulse train forming module 3 includes: a chirped stretching unit 31, where the chirped stretching unit 31 is configured to perform chirped stretching on the laser pulse output by the light source module 1, and output a chirped pulse; the beam splitting unit 32 is configured to split the chirped pulses output by the chirped stretching unit 31 into two chirped pulses; the first beat frequency unit 33a is configured to beat frequency of one path of chirped pulses output by the beam splitting unit 32, and output a first laser pulse including an ultrashort laser sub-pulse train; the second beat frequency unit 33b is configured to beat frequency of the other path of chirped pulses output by the beam splitting unit 32, and output a second laser pulse including an ultrashort laser sub-pulse train; a beam combining unit 34, where the beam combining unit 34 is configured to combine the first laser pulse and the second laser pulse and output a third laser pulse, where the third laser pulse includes an ultra-short laser pulse train of a plurality of sub-pulse trains with different frequencies. The two chirped pulses are respectively input into the independent beat frequency units for beat frequency, then input into the beam combination unit 34, and finally output into the ultrashort laser pulse train comprising two sub-pulse trains with independently adjustable frequencies.
The chirp widening unit 31 includes: a first beam splitter 313, a parallel grating pair 311 and a retroreflective element 312. The laser pulse output by the light source module 1 sequentially passes through the first beam splitter 313 and the parallel grating pair 311 to enter the retro-reflection element 312, and after reflection, the laser pulse sequentially passes through the parallel grating pair and the first beam splitter 313 to enter the beam splitting unit 32, where the parallel grating pair is used for chirp broadening. The first beam splitter 313 has a semi-reflective and semi-transparent interface, laser pulses output by the light source module 1 are divided into two paths by the first beam splitter 313, one path of the laser pulses is reflected downwards, the other path of the laser pulses penetrates through the first beam splitter 313 and enters the parallel grating pair 311, the parallel grating pair 311 enters the retro-reflection element 312 after undergoing first chirp broadening, the parallel grating pair 312 enters the parallel grating pair 311 again, the parallel grating pair 311 undergoes second chirp broadening, the laser pulses enter the first beam splitter 313 and are reflected to the beam splitting unit 32 by the first beam splitter 313.
Optionally, the parallel grating pair 311 includes two oppositely disposed and mutually parallel gratings, and the gratings are holographic diffraction gratings, so as to generate linearly chirped pulses by using group delay dispersion thereof. The laser pulse output from the light source module 1 is expanded into a laser pulse having a linear chirp characteristic (the temporal frequency of the light field changes as a linear function of time) after passing through the parallel grating pair 311. The wavelength λ of the laser pulse is 800nm, and the full width at half maximum FWHM of the pulse is 160 fs. The parameters of the parallel grating pair 311 are as follows: the reticle density D is 1800 (lines/mm), the incident light angle γ is 39.7 °, and the grating pitch b is 2.9 cm.
The retroreflective elements 312 are retroreflective prisms. The retroreflective element 312 is used to change the transmission direction of the laser pulse with chirp characteristics output by the parallel grating pair 311 to make it return to the parallel grating pair 311 to be widened twice, so as to increase the widening amount. The first beam splitter 313 is a prism beam splitter, and the chirped pulse whose secondary broadening is completed by the parallel grating pair 311 is reflected to the beam splitting unit 32 via the first beam splitter 313.
The beam splitting unit 32 is a second beam splitter, and the reflection transmittance of the second beam splitter is adjustable. By adjusting the reflection transmittance of the laser, the intensity ratio of two paths of beam splitting laser pulses can be changed, so that the finally generated bicolor terahertz radiation power is also adjustable.
As shown in fig. 1, each of the first beat unit 33a and the second beat unit 33b includes a plate beam splitter 331 and two plane mirrors 332. In the same beat frequency unit, it is composed of a plate beam splitter 331 and two plane mirrors 332 with different reflection optical paths and capable of being independently and continuously adjusted. The plate beam splitter 331 is configured to split an input chirped pulse into chirped sub-pulses, and perform beat frequency on the chirped sub-pulses returned by the plane mirror 332 to form an ultrashort laser sub-pulse train, where the beat frequency can be adjusted by adjusting an optical path of the plane mirror 332. Here, a plate beam splitter 331 is used instead of a prism beam splitter to reduce the optical path difference error.
For the first beat frequency unit 33a, one chirp pulse output by the beam splitting unit 32 is split into two chirp sub-pulses by the flat beam splitter 331 of the first beat frequency unit 33a, the two chirp sub-pulses are reflected back to the flat beam splitter 331 of the first beat frequency unit 33a by one plane mirror 332 of the first beat frequency unit 33a, and are coupled by the flat beam splitter 331 and then input to the beam combining unit 34. For the second beat frequency unit 33b, the other chirped pulse output by the beam splitting unit 32 is split into two chirped sub-pulses by the plate beam splitter 331 of the second beat frequency unit 33b, the two chirped sub-pulses are respectively reflected back to the plate beam splitter 331 of the second beat frequency unit 33b by one plane mirror 332 of the second beat frequency unit 33b, and are coupled by the plate beam splitter 331 and then input to the beam combining unit 34.
In the same beat frequency unit, the mirror surface of the plane mirror 332 is perpendicular to the transmission direction of the chirped sub-pulse output by the plate beam splitter 331. That is, in the first beat frequency unit 33a, the mirror surfaces of the two corresponding plane mirrors 332 are perpendicular to the transmission direction of the chirped sub-pulse correspondingly output by the plate beam splitter 331, and in the second beat frequency unit 33b, the mirror surfaces of the two corresponding plane mirrors 332 are perpendicular to the transmission direction of the chirped sub-pulse correspondingly output by the plate beam splitter 331.
Optionally, the dual-color terahertz source may further include a plurality of reflectors 35 according to a light path layout requirement, and an included angle greater than 0 ° and less than 90 ° exists between a mirror surface direction of the reflector 35 and a transmission direction of the light pulse incident to the reflector 35. As shown in the figure, one mirror 35 is provided between the first beat unit 33a and the beam splitting unit 32, and another mirror 35 is provided between the first beat unit 33a and the beam combining unit 34. The reflecting mirror 35 is used for flexibly changing the transmission direction of the light path, and in practical application, the included angle between the mirror surface direction of the reflecting mirror 35 and the incident pulse transmission direction can be adjusted at will within the range of (0 degrees and 90 degrees). It is also possible to reduce the number of mirrors 35 or to eliminate the mirrors 35 by changing the placement direction of the beam splitting unit 32 and the position of the beat unit.
Optionally, the beam combining unit 34 is a beam combining mirror, the reflection transmittance of the beam combining mirror is adjustable, and the included angles between the laser pulse incident to the beam combining mirror and the normal lines of the medium surface and the medium surface of the beam combining mirror are both 45 °, so as to ensure that the transmission directions of the two beams of pulses after passing through the beam combining mirror are on the same straight line. The terahertz generation module 2 includes but is not limited to a photoconductive antenna or a nonlinear crystal.
It should be noted that, in the embodiment of the present invention, light transmission between each component may be implemented by an optical fiber, or by a free optical field, and may be set according to an application scenario of the apparatus, which is not specifically limited in the embodiment of the present invention.
The light source module 1 emits laser pulses as fourier-change limit pulses. Between the first beam splitter 313 and the parallel grating pair 311 is a pulse which is output by widening the current chirp, the pulse is a Gaussian pulse, the pulse width of the pulse is widened, the bandwidth is unchanged, and the limit chirp represents that the instantaneous carrier frequency of the pulse is changed linearly with time. The laser pulse types between the beam splitting unit 32 and the first chirp unit 33a, and between the beam splitting unit 32 and the second chirp unit 33b are identical to the laser pulse types between the first beam splitter 313 and the parallel grating pair 311, but the light intensity is half of the laser pulse between the first beam splitter 313 and the parallel grating pair 311. The laser pulse train is transmitted between the two beat frequency units and the beam combination unit 34, comprises micro pulses which are Gaussian pulses, the bandwidth of the micro pulses is reduced, and the ratio of the reduced bandwidth to the initial Gaussian Fourier change limit bandwidth is
The envelope of the pulse train is gaussian in shape and the distribution is consistent with the distribution of the chirped and spread light pulses transmitted by the first beam splitter 313.
The double-color terahertz source is simple in structure, can conveniently and efficiently generate double-color terahertz pulses comprising two sub-pulses with different frequencies, and the frequency of each sub-pulse can be independently and continuously adjusted.
By adjusting the beat frequency to make the micro-pulse repetition frequencies in the sub-pulse train different from each other, an ultrashort laser pulse train including a plurality of sub-pulse trains with different frequencies can be obtained, so that the ultrashort laser pulse train excites the photoconductive antenna and radiates electromagnetic waves according to the displacement current equation
When the pulse width of the exciting light pulse is in a subpicosecond range, the electromagnetic wave pulse is a terahertz pulse, and the radiation frequency of the terahertz pulse is determined by the repetition frequency of the micro-pulse in the ultra-short pulse train, so that the ultra-short laser pulse train excited photoconductive antenna can generate high-power terahertz pulses with various different wave bands, and the frequency of the terahertz pulse is continuously adjustable. The method is applied to the TPTP measurement field, can cover more measurement systems and enlarge the measurement range; the device of the invention also has great application potential in the field of THz tomography.
Based on the above embodiment, another embodiment of the present invention further provides a method for outputting a dual-color terahertz pulse by using the dual-color terahertz source in the above embodiment, where the method is shown in fig. 2, and fig. 2 is a schematic flow chart of the method for outputting a dual-color terahertz pulse provided in the embodiment of the present invention, and the method includes:
step S11: and outputting laser pulses through the light source module.
Typically the laser pulses may be gaussian fourier transform laser pulses. The light source module includes a semiconductor laser. The light source module can output a single laser pulse
Step S12: the laser pulse train forming module is used for carrying out chirp broadening on the laser pulse output by the light source module to form two paths of chirp pulses, the two paths of chirp pulses are respectively subjected to beat frequency to form a first laser pulse and a second laser pulse, and the first laser pulse and the second laser pulse are combined to form a third laser pulse. The laser pulse can be subjected to chirp broadening twice through the laser pulse shaping module.
The first laser pulse comprises a train of ultrashort laser sub-pulses. The second laser pulse comprises a train of ultrashort laser sub-pulses. The third laser pulse comprises an ultrashort laser pulse train of a plurality of different frequency sub-pulse trains. E.g. the third laser pulse may comprise an ultra-short laser pulse train of two sub-pulse trains with independently adjustable frequencies. Each sub-burst contains a plurality of micro-pulses.
Step S13: outputting, by a terahertz generation module, bi-color terahertz light based on the third laser pulse.
According to the method provided by the embodiment of the invention, the double-color terahertz source can output the double-color terahertz pulse, the double-color terahertz pulse comprising two sub-pulses with different frequencies can be conveniently and efficiently generated, and the frequency of each sub-pulse can be independently and continuously adjusted.
The following describes, with reference to a simulation result, an effect of the dual-color terahertz source and the method for outputting the dual-color terahertz pulse according to the embodiment of the present invention when outputting the dual-color terahertz pulse:
referring to fig. 3 to 5, fig. 3 is a schematic diagram of two-color terahertz pulses obtained by the two-color terahertz source shown in fig. 1 under the condition that the beat frequencies are 1.0THz and 0.5THz, respectively, fig. 4 is a schematic diagram of two-color terahertz pulses obtained by the two-color terahertz source shown in fig. 1 under the condition that the beat frequencies are 1.5THz and 2.0THz, respectively, and fig. 5 is a schematic diagram of two-color terahertz pulses obtained by the two-color terahertz source shown in fig. 1 under the condition that the beat frequencies are 2.5THz and 3.0THz, respectively.
As can be seen from fig. 3 to 5, the bicolor terahertz pulse generated by the bicolor terahertz source of the present invention includes two sub-pulse trains with different frequencies, and by adjusting the beat frequency, the frequencies of the sub-pulse trains are also continuously adjustable and independent of each other, and the time interval between the two sub-pulse trains is also continuously adjustable.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. For the method disclosed by the embodiment, since the method corresponds to the bicolor terahertz source disclosed by the embodiment, the description is simple, and the relevant points can be described by referring to the corresponding part of the bicolor terahertz source.
It is further noted that, herein, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that an article or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such article or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other like elements in an article or device that comprises the element.
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 (10)
1. A bi-color terahertz source, comprising:
a light source module for outputting laser pulses;
the laser pulse train forming module is used for performing chirped broadening on the laser pulses output by the light source module to form two paths of chirped pulses, performing beat frequency on the two paths of chirped pulses respectively to form a first laser pulse comprising an ultrashort laser sub-pulse train and a second laser pulse comprising the ultrashort laser sub-pulse train, and combining the first laser pulse and the second laser pulse to form a third laser pulse, wherein the third laser pulse comprises a plurality of ultrashort laser pulse trains with different frequency sub-pulse trains;
a terahertz generation module for outputting bi-color terahertz light based on the third laser pulse.
2. The bi-color terahertz source of claim 1, wherein the laser pulse train shaping module comprises:
the chirped stretching unit is used for performing chirped stretching on the laser pulse output by the light source module and outputting a chirped pulse;
the beam splitting unit is used for splitting the chirped pulses output by the chirped stretching unit into two paths of chirped pulses;
the first beat frequency unit is used for carrying out beat frequency on one path of chirped pulse output by the beam splitting unit and outputting a first laser pulse comprising an ultrashort laser sub-pulse train; the second beat frequency unit is used for carrying out beat frequency on the other path of chirped pulse output by the beam splitting unit and outputting a second laser pulse comprising an ultrashort laser sub-pulse train;
and the beam combination unit is used for combining the first laser pulse and the second laser pulse and outputting a third laser pulse, and the third laser pulse comprises an ultrashort laser pulse string of a plurality of different-frequency sub-pulse strings.
3. The bi-color terahertz source of claim 2, wherein the chirp stretching unit comprises: the device comprises a first beam splitter, a parallel grating pair and a back reflection element;
laser pulses output by the light source module sequentially pass through the first beam splitter and the parallel grating pair to be incident to the backward reflecting element, and are reflected and sequentially pass through the parallel grating pair and the first beam splitter to be incident to the beam splitting unit, and the parallel grating pair is used for chirp broadening.
4. The bi-color terahertz source of claim 3, wherein the parallel grating pair comprises two oppositely disposed and mutually parallel gratings, the gratings being holographic diffraction gratings.
5. The bi-color terahertz source of claim 3, wherein the retro-reflective element is a retro-reflective prism.
6. The bi-color terahertz source of claim 2, wherein the beam splitting unit is a second beam splitter, and a reflection transmittance of the second beam splitter is adjustable.
7. The bi-color terahertz source of claim 2, wherein the first and second beat frequency units each comprise one plate beam splitter and two plane mirrors;
for the first beat frequency unit, one chirp pulse output by the beam splitting unit is divided into two chirp sub-pulses by a flat plate beam splitter of the first beat frequency unit, the two chirp sub-pulses are respectively reflected back to the flat plate beam splitter of the first beat frequency unit by a plane mirror of the first beat frequency unit, and the two chirp sub-pulses are input to the beam combining unit after being coupled by the flat plate beam splitter;
for the second beat frequency unit, the other path of chirped pulse output by the beam splitting unit is split into two paths of chirped sub-pulses by a flat beam splitter of the second beat frequency unit, the two paths of chirped sub-pulses are respectively reflected back to the flat beam splitter of the second beat frequency unit by a plane mirror of the second beat frequency unit, and the chirped sub-pulses are input to the beam combining unit after being coupled by the flat beam splitter.
8. The bichromatic terahertz source according to claim 7, wherein in the same beat frequency unit, the mirror surface of the plane mirror is perpendicular to the transmission direction of the chirped sub-pulse output by the flat-plate beam splitter.
9. The bicolor terahertz source according to claim 2, wherein the beam combining unit is a beam combining mirror, the reflection transmittance of the beam combining mirror is adjustable, and the included angles between the laser pulse incident to the beam combining mirror and the normals of the medium surface and the medium surface of the beam combining mirror are both 45 °.
10. A method for outputting bicolor terahertz pulses by a bicolor terahertz source is characterized by comprising the following steps:
outputting laser pulses through a light source module;
performing chirped broadening on the laser pulse output by the light source module through a laser pulse train forming module to form two paths of chirped pulses, performing beat frequency on the two paths of chirped pulses respectively to form a first laser pulse comprising an ultrashort laser sub-pulse train and a second laser pulse comprising the ultrashort laser sub-pulse train, and combining the first laser pulse and the second laser pulse to form a third laser pulse, wherein the third laser pulse comprises a plurality of ultrashort laser pulse trains with different frequency sub-pulse trains;
outputting, by a terahertz generation module, bi-color terahertz light based on the third laser pulse.
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