CN208045929U - A system that uses 'flying focus' to generate terahertz waves - Google Patents

Abstract

The utility model discloses a system for generating terahertz waves using "flying focus", which comprises: a chirp signal generating device for generating chirped laser pulses; and a "flying focus" device for using diffraction components to Laser pulses are focused to excite an air plasma, which in turn produces terahertz waves. The system for generating terahertz waves using "flying focus" provided by the utility model combines chirped laser pulses and diffraction components to control the moving speed of the peak intensity in the laser focus area and control the propagation of the laser beam in the focus area. And the propagation length is many times the Rayleigh length. Compared with the existing method of using air to generate terahertz waves, the intensity of the terahertz waves generated by the utility model is greatly enhanced, which makes up for the gap in the current high-intensity terahertz wave generation technology field, and has strong scientific research and practical application value .

Description

A system that uses 'flying focus' to generate terahertz waves

technical field

The utility model relates to the field of terahertz wave technology and laser, in particular to a combination of chirped laser pulses and diffraction components to achieve the purpose of controlling the moving speed of air plasma generated by excitation in the focus area and then generate A system of high-intensity terahertz waves.

Background technique

In recent years, with the development of terahertz source technology, the method of using air to generate terahertz waves stands out from many terahertz source technologies due to its advantages of no damage threshold and long-distance generation, and has been well applied in practice. . In order to obtain more high-quality terahertz waves, the system that uses air to generate terahertz waves has been continuously optimized.

The generation of terahertz waves mainly depends on the plasma density gradient or the generation of unsteady current in the optical filament, and the generation efficiency mainly depends on the phase matching between the optical filament and the terahertz wave. For this, please refer to literature 1 "Xu, Xie, Jianming ,Dai,and,X,-C,Zhang.Coherent Control of THz Wave Generation in Ambient Air[J].Phys.Rev.Lett.,2006,96(075005):7-24" and literature 2 "K,Y ,Kim,J,H,Glownia,A,J,Taylor,and,G,Rodriguez.Terahertz emission from ultrafast ionizing air in symmetry-broken laser fields[J].Optics Express, 2007,15(8):4577-4584 ". In traditional schemes, the length of air plasma can only be balanced by plasma refraction and self-focusing, resulting in a limited length and inability to phase match with THz radiation. Due to the limitation of the optical frequency, the maximum intensity of the terahertz wave is limited to the order of 10 ¹⁴ W/cm ² .

Utility model content

In order to solve the above problems, the utility model adopts the following technical means: the optical filament is moved in a certain way, so that a longer high-intensity plasma filament can be obtained and the phase matching with the terahertz wave can be satisfied. In addition, the utility model further separates the intensity of the light filament from the length of the light filament, and separates the moving speed of the light filament from the group velocity (for "group velocity", please refer to the literature 3 "P, A, Cherenkov.Visible emission of cleanliquids by action ofγradiation[J]. Doklady Akademii Nauk SSSR,1934,2(451)"), so as to obtain a terahertz source capable of generating higher intensity terahertz waves.

In order to achieve the above purpose, the utility model provides a system for generating terahertz waves using "flying focus", which includes:

Chirped signal generating means for generating chirped laser pulses; and

The "flying focus" device uses diffraction components to focus chirped laser pulses to excite and generate air plasma, thereby generating terahertz waves.

In an embodiment of the present invention, the system for generating terahertz waves using "flying focus" further includes:

A terahertz wave detection device for detecting the intensity of the generated terahertz waves; or

The terahertz wave scanning device is used for scanning the spatial distribution of the terahertz wave.

In an embodiment of the present invention, the chirp signal generating device includes a laser, an optical parametric amplifier, and a chirped laser pulse generating device sequentially arranged on the optical path, and the laser pulse emitted by the laser is generated by the optical parametric amplifier amplified, and then modulated by the chirped laser pulse generating device to generate chirped laser pulses.

In an embodiment of the present utility model, the "flying focusing" device includes a chopper, a mirror, a diffraction element and a BBO crystal arranged sequentially on the optical path, and the chirped laser pulse passes through the chopper and then passes through the The reflector performs reflection, and then sequentially passes through the diffraction element for focusing and the BBO crystal for frequency doubling, wherein the focus points of lasers of all frequencies in the chirped laser pulse are located on a straight line after focusing.

In an embodiment of the present utility model, the terahertz wave detection device includes a first off-axis parabolic mirror, a first silicon chip, a second off-axis parabolic mirror, a first filter and a first terahertz wave intensity In the detector, the first off-axis parabolic reflector converges the terahertz wave to form a parallel beam, and the parallel beam is filtered by the first silicon chip and projected to the second off-axis parabolic reflector, and then passes through the entering the first terahertz wave intensity detector after being filtered by the first filter;

The terahertz wave scanning device includes a second silicon chip, a second filter, a translation stage and a second terahertz wave intensity detector, and the terahertz wave passes through the second silicon chip and the second filter and enters and is fixed on the translation stage The second terahertz wave intensity detector is a second terahertz wave intensity detector, the translation stage can move in the horizontal plane to detect the distribution of the terahertz wave in space.

In one embodiment of the utility model, the laser is a picosecond laser.

In an embodiment of the present invention, the chirped laser pulse generating device is a diffraction grating or a prism, and the diffraction element is a Fresnel lens.

In an embodiment of the present utility model, the frequency of the chopper is 15-20 Hz.

In an embodiment of the present invention, the first terahertz wave intensity detector and the second terahertz wave intensity detector are pyroelectric detectors or Goulet detectors.

The system for generating terahertz waves using "flying focus" provided by the utility model combines chirped laser pulses and diffraction components to control the moving speed of the peak intensity in the laser focus area and control the propagation of the laser beam in the focus area. And the propagation length is many times the Rayleigh length. Compared with the existing method of using air to generate terahertz waves, the intensity (>10 ¹⁸ W/cm ² ) of the terahertz waves generated by the utility model is greatly enhanced, which makes up for the gap in the current high-intensity terahertz wave generation technology field. It has strong scientific research and practical application value.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description These are only some embodiments of the utility model, and those skilled in the art can also obtain other drawings based on these drawings without creative work.

Fig. 1 is a schematic diagram of the principle of the utility model "flying focus";

Fig. 2a is a schematic diagram of a system for generating terahertz waves using "flying focus" according to an embodiment of the present invention;

Fig. 2b is a schematic diagram of a system for generating terahertz waves using "flying focus" according to another embodiment of the present invention;

Figure 3a is a schematic diagram of the relationship between the focus moving speed and the positive (dotted line) negative (solid line) linearly chirped laser pulse duration when the focus moving range is 4.5mm;

Figure 3b is a schematic diagram of the relationship between the focus moving speed and the positive (dotted line) negative (solid line) linearly chirped laser pulse duration when the focus moving range is 10mm;

Figure 4 shows the minimum spot size at each longitudinal position (z) using an ideal lens (dashed line) and a flying focus (solid line), where r is the distance from the optical axis.

Description of reference signs: A-chirped signal generating device; B-"flying focus" device; C-terahertz wave detection device; 1-laser; 2-optical parametric amplifier; 3-chirped laser pulse generating device; 4- Chopper; 5-mirror; 6-diffraction components; 7-BBO crystal; 8-first off-axis parabolic mirror; 9-first silicon wafer; 10-second off-axis parabolic mirror; 11-th A filter; 12-the first terahertz wave intensity detector; 13-the second silicon chip; 14-the second filter; 15-the second terahertz wave intensity detector; 16-translation stage.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. example. Based on the embodiments of the present utility model, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the scope of protection of the present utility model.

In this utility model, the principle of "flying focus" is as follows:

Diffraction components can make lasers of different frequencies correspond to focal lengths of different lengths, and a chirped laser pulse is a laser pulse whose frequency changes periodically with time. Therefore, lasers of different frequencies in the chirped laser pulse pass through the diffraction components. The focal length will also be different, so as to realize the "flying" of the light filament.

Figure 1 is a schematic diagram of the principle of the "flying focus" of the utility model. As shown in Figure 1, a chirped laser pulse with a bandwidth of Δλ and a pulse length of T is focused by a Fresnel lens to form a periodically moving plasma. Among them, the central wavelength of the chirped laser pulse is λ ₀ , f ₀ is the focal length corresponding to the central wavelength λ ₀ , L is the moving range of the focus along the Z-axis direction, λ _a and λ _b are the minimum wavelengths of the chirped laser pulse and the maximum wavelength, at the same time, λ _a , λ _b correspond to the minimum moving distance and the maximum moving distance of the focus along the Z-axis direction respectively, where, It should be noted that the "movement" here is not a real "movement", but only means that lasers with different frequencies in the chirped laser pulse correspond to different focal lengths, and the relationship between the focal length and the laser frequency is a positive correlation, The chirped laser pulses are continuously propagated, and the focal lengths corresponding to the different frequencies of the laser appear to be "flying" rapidly along the Z axis.

The focal point of the chirped laser pulse after being focused by the Fresnel lens moves along the Z axis at the speed of v _FF (Z)=dz/dt. If the chirped laser pulse is linear, the separation for the diffractive lens is approximately linear, which means that the velocity of focus movement is constant throughout the focal region. In this case, we can get a simple velocity formula:

where T is the pulse duration of the chirped laser pulse, the symbol indicates the direction of the chirp, and c represents the speed of light. Figures 3a–3b show the correspondence between positive (dashed line) and negative (solid line) linearly chirped laser pulse durations and focus movement speeds. Taking L as 4.5mm as an example, when the laser pulse duration is T=300ps and is negatively chirped, the focal point propagates oppositely at the speed of v=-0.05c; when the laser pulse duration is T=30ps and is negatively chirped When chirping, the focal point counterpropagates nearly at the speed of light; when the pulse duration is reduced to 20ps, the counterpropagating hyperlight focal point reaches v=3c. Setting the pulse duration of the laser equal to the propagation time of the light through the focal region (T=L/c=14.9ps) will cause the lasers of all frequencies to focus simultaneously, resulting in a 4.5mm long line focus, as shown in Figure 4 , without considering laser-induced filamentation, the length of the focal region is approximately 100 times the Rayleigh length of the f/7 optical system (Z _R =0.025 mm). From this formula, we can also know that the positive chirp can provide a series of forward-propagating slow light focus velocities.

Figure 2a is a schematic diagram of a system for generating terahertz waves using "flying focus" provided by the present invention. As shown in Figure 2a, the system provided by the present invention for generating terahertz waves using "flying focus" includes:

Chirped signal generating device A, used to generate chirped laser pulses; and

"Flying focusing" device B uses diffraction components to focus chirped laser pulses to excite and generate air plasma to generate terahertz waves. Wherein, the diffractive element may be, for example, a Fresnel lens.

Terahertz wave detection device C, used to detect the intensity of the generated terahertz wave;

In Fig. 2a, the chirp signal generating device A includes a laser 1, an optical parametric amplifier 2, and a chirped laser pulse generating device 3 arranged sequentially on the optical path. The laser pulse emitted by the laser 1 is amplified by the optical parametric amplifier 2, and then passed through The chirped laser pulse generating device 3 is modulated to generate chirped laser pulses. Wherein, the laser 1 may be, for example, a picosecond laser, and the chirped laser pulse generating device 3 may be, for example, a diffraction grating or a prism.

In Fig. 2a, the "flying focus" device B includes a chopper 4, a mirror 5, a diffraction element 6 and a BBO crystal 7 arranged in sequence on the optical path. The frequency of the chopper is 15-20 Hz, and the chirped laser pulse passes through After the chopper 4 is reflected by the reflector 5, it is then focused by the diffraction element 6 and frequency multiplied by the BBO crystal 7 in turn, wherein the laser beams of all frequencies in the chirped laser pulse are focused on a line Straightforward, this principle has been illustrated in Figure 1.

In Fig. 2a, the terahertz wave detection device C includes a first off-axis parabolic mirror 8, a first silicon wafer 9, a second off-axis parabolic mirror 10, a first filter 11 and a first terahertz wave intensity detector 12 , the first off-axis parabolic reflector 8 converges the terahertz wave to form a parallel beam, the parallel beam is filtered by the first silicon chip 9 and projected to the second off-axis parabolic reflector 10, and then passes through the first filter 11 After filtering, enter the first terahertz wave intensity detector 12;

In this embodiment, the laser 1 emits laser light with a wavelength of 800nm, for example, and after passing through the optical parametric amplifier 2, the wavelength becomes laser light with a wavelength of 1200-1600nm. In other embodiments, the wavelength of the laser can be changed according to actual needs. We use a central wavelength λ ₀ =1550nm to generate a linearly chirped laser pulse with a bandwidth of Δλ=35nm through a diffraction grating, and then use a Fresnel lens with a central focal length of 200mm to generate a focal point with a diameter of 7μm, between the extreme wavelengths The longitudinal interval is 4.5mm. As shown in Figure 3a, the focus movement speed is matched with the phase of the terahertz wave by adjusting the pulse duration

Fig. 2b is a schematic diagram of another embodiment of the utility model using "flying focus" to generate a terahertz wave system, as shown in Fig. 2b, this embodiment differs from the embodiment shown in Fig. 2a in that Fig. 2a The terahertz wave detection device C is replaced by a terahertz wave scanning device D, which is used to scan the spatial distribution of the terahertz wave, as shown in Figure 2b, the terahertz wave scanning device D includes a second Silicon chip 13, second filter 14, translation stage 16 and second terahertz wave intensity detector 15, terahertz wave enters second silicon chip 13, second filter 14 and is fixed on translation stage 16 The terahertz wave intensity detector 15 and the translation platform 16 can move in the horizontal plane to detect the distribution of the terahertz wave in space. The remaining parts in FIG. 2b are the same as those in FIG. 2a and will not be repeated here.

The first terahertz wave intensity detector 12 and the second terahertz wave intensity detector 15 in FIG. 2a and FIG. 2b may be, for example, pyroelectric detectors or Goulet detectors.

The chirped laser pulses in the present invention are linear chirps. In addition, non-linear chirped laser pulses can also be considered to periodically generate a moving focus with acceleration to obtain more meaningful physical phenomena.

Since the use of air to generate terahertz waves mainly depends on the plasma density gradient or the generation of unsteady currents in the optical filament, therefore, under this idea, the utility model can enhance the generation of terahertz waves in two aspects:

(1) Based on the influence of the plasma density gradient on the generation of terahertz waves, the "flying focus" system for generating terahertz waves can obtain longer plasma filaments, extending the action length of high-intensity air plasma by about a hundred times.

(2) A system that generates terahertz waves by "flying focus" can obtain a focus propagation velocity faster than its group velocity. According to the angle determined in the Cherenkov radiation theory We can achieve terahertz waves by changing the focus speed phase matching, and further provide a method for measuring the refractive index of materials in the terahertz band.

The system for generating terahertz waves using "flying focus" provided by the utility model combines chirped laser pulses and diffraction components to control the moving speed of the peak intensity in the laser focus area and control the propagation of the laser beam in the focus area. And the propagation length is many times the Rayleigh length. Compared with the existing method of using air to generate terahertz waves, the intensity (>10 ¹⁸ W/cm ² ) of the terahertz waves generated by the utility model is greatly enhanced, which makes up for the gap in the current high-intensity terahertz wave generation technology field. It has strong scientific research and practical application value.

Those of ordinary skill in the art can understand that: the accompanying drawing is only a schematic diagram of an embodiment, and the modules or processes in the accompanying drawing are not necessarily necessary for implementing the utility model.

Those of ordinary skill in the art can understand that: the modules in the device in the embodiment may be distributed in the device in the embodiment according to the description in the embodiment, or may be changed and located in one or more devices different from the embodiment. The modules in the above embodiments can be combined into one module, and can also be further split into multiple sub-modules.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present utility model, and are not intended to limit it; although the utility model has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: It is still possible to modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention .

Claims (9)

1. a kind of system for using " flying focus " to generate THz wave, which is characterized in that including：

Chirp signal generation device, for generating chirped laser pulse；And

" flying focus " device, is focused chirped laser pulse using diffraction component, to excite generation air plasma Body, and then generate THz wave.

2. the system according to claim 1 for using " flying focus " to generate THz wave, which is characterized in that further include：

THz wave detection device, the intensity for detecting the THz wave generated；Or

THz wave scanning means is scanned for the spatial distribution to THz wave.

3. the system according to claim 1 for using " flying focus " to generate THz wave, which is characterized in that the chirp Signal generation device includes the laser being set in turn in light path, photoparametric amplifier and chirped laser pulse generator The laser pulse of part, the laser transmitting is amplified by the photoparametric amplifier, later again via the chirped laser Pulse generator part is modulated, to generate chirped laser pulse.

4. the system according to claim 1 for using " flying focus " to generate THz wave, which is characterized in that described " to fly Line focusing " device includes the chopper being set in turn in light path, speculum, diffraction component and bbo crystal, chirped laser Pulse, through being reflected by the speculum, is gathered by the diffraction component successively again later after the chopper The burnt and process bbo crystal carries out frequency multiplication, wherein the laser of all frequencies is poly- after over-focusing in chirped laser pulse Focus is located on straight line.

5. the system according to claim 2 for using " flying focus " to generate THz wave, which is characterized in that the terahertz Hereby wave detection device includes the first off-axis parabolic mirror, the first silicon chip, the second off-axis parabolic mirror, the first filter plate With the first THz wave intensity detector, the first off-axis parabolic mirror converges THz wave to form a branch of collimated light beam, The collimated light beam is projected to second off-axis parabolic mirror after first silicon chip filtering, later using described Enter the first THz wave intensity detector after the filtering of first filter plate；

The THz wave scanning means includes the second silicon chip, the second filter plate, translation stage and the second THz wave strength investigation Device, THz wave enter the second THz wave strength investigation being fixed on translation stage after the second silicon chip, the second filter plate Device, the translation stage can be in moving, to detect distribution of the THz wave in space in horizontal plane.

6. the system according to claim 3 for using " flying focus " to generate THz wave, which is characterized in that the laser Device is picosecond laser.

7. the system according to claim 1 for using " flying focus " to generate THz wave, which is characterized in that the chirp It is diffraction grating or prism that laser pulse, which generates device, and the diffraction component is Fresnel Lenses.

8. the system according to claim 4 for using " flying focus " to generate THz wave, which is characterized in that the copped wave The frequency of device is 15~20Hz.

9. the system according to claim 5 for using " flying focus " to generate THz wave, which is characterized in that described first THz wave intensity detector and the second THz wave intensity detector are pyroelectric detector or Golay detector.