A device for generating high-power terahertz waves based on triple-optimized cascaded difference frequency generation

Abstract

The object of the present invention is to provide a device for generating high-power terahertz waves based on triple-optimized cascaded difference frequency, which includes a first pump source, a second pump source, a third pump source and a fourth pump source, a first APPLN crystal, a second APPLN crystal and a third APPLN crystal with different polarization period distributions, a first parabolic mirror, a second parabolic mirror, a third parabolic mirror, a first beam combiner, a second beam combiner, a third beam combiner, and a phase delay system; it can generate multiple terahertz waves with the same frequency simultaneously, improving the terahertz wave conversion efficiency. By setting the distribution of the polarization period of the APPLN crystal, the Stokes cascaded difference frequency can be enhanced while suppressing the anti-Stokes cascaded difference frequency, improving the terahertz wave optical conversion efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz wave applications, and particularly relates to a device for generating high-power terahertz waves based on triple-optimized cascaded difference frequency generation. Background Art

[0002] Terahertz waves (abbreviated as THz), refer to electromagnetic waves with frequencies in the range of 0.1 - 10 THz (1 THz = 10 12 Hz). Its band is located between millimeter waves and infrared rays in the electromagnetic spectrum, and it is the transition region from photonics and electronics to microscopic theory from macroscopic theory. The special position of terahertz waves makes them of great scientific research value and broad application prospects in basic research fields such as physics, chemistry, astronomy, molecular spectroscopy, life science, and medical science, as well as in applied research fields such as medical imaging, environmental monitoring, material detection, food detection, radio astronomy, mobile communication, satellite communication, and military radar. Terahertz waves are mainly applied in the following fields:

[0003] (1) Imaging field

[0004] THz wave imaging is different from ordinary optical images or X-ray images. Each pixel in a pulsed THz wave image contains the entire THz waveform, rather than just the intensity of the light beam. The Fourier transform of the THz waveform can also extract the spectral information of this pixel. Therefore, THz wave imaging not only identifies the target through its contour, but also obtains the composite information of the target.

[0005] (2) Biomedical technology field

[0006] Due to the "fingerprint" characteristics of the THz band, it contains rich physical and chemical information when interacting with the medium, and its low energy ensures its application in fields such as biomedical imaging. Since the THz light source consists of different composite polarized light waves, information can be collected from the medium through different polarized lights to obtain more accurate diagnostic information about pathological tissues. Therefore, terahertz waves play a significant role in the clinical diagnosis and treatment of cancer.

[0007] (3) Nondestructive testing field

[0008] The penetrability of THz waves can be well applied in the fields of non-destructive testing and THz imaging. By using THz time-domain spectroscopy technology, under non-contact and non-destructive conditions, it can penetrate non-polar dielectric materials such as clothes, cardboard boxes, and plastics. Therefore, it can detect the chemical properties of the medium, provide new biological characteristic information and identification basis for stored-grain pests in the terahertz band, establish a new method of identification and research that complements the advantages of other detection technologies, so as to quickly and conveniently detect stored-grain pests and their species, which has important practical significance for the investigation and monitoring of stored-grain pest infestations, accelerating the rapid detection and quarantine of pests at ports and the accurate detection of stored-grain pests in granaries.

[0009] (4)Communication field

[0010] THz waves have a relatively wide bandwidth, good directivity, and high transmission rate, so they have great potential in the applications of high-speed space communication and radar, and have the application prospect of combining military and civilian use and developing in a balanced and coordinated manner. Since THz waves are sensitive to water molecules, secure communication can be achieved during transmission in the atmosphere. Compared with visible light and infrared rays, THz waves have better directivity due to their longer wavelengths, enabling them to achieve space communication with extremely high bandwidth in clouds and fog.

[0011] (5)Homeland security field

[0012] THz waves have good directivity, narrow beamwidth, and strong cloud and smoke penetration ability. In military applications, THz waves are used for correction in the terminal precision guidance of missiles to improve the guidance accuracy, which has extremely high military application value.

[0013] The main problem currently faced is the lack of a terahertz source that can generate high-power, high-quality, and high-efficiency terahertz waves, and is low-cost and can operate at room temperature. Summary of the invention

[0014] The purpose of the present invention is to provide a device for generating high-power terahertz waves based on triple optimized cascaded difference frequency, which can generate multiple terahertz waves with the same frequency simultaneously and improve the terahertz wave conversion efficiency.

[0015] The purpose of the present invention is achieved in the following way: A device for generating high-power terahertz waves based on triple optimized cascaded difference frequency includes a first pump source, a second pump source, a third pump source, and a fourth pump source, a first APPLN crystal, a second APPLN crystal, and a third APPLN crystal with different polarization period distributions, a first parabolic mirror, a second parabolic mirror, a third parabolic mirror, a first beam combiner, a second beam combiner, a third beam combiner, and a phase delay system;

[0016] The first pump light emitted from the first pump source enters the first beam combiner; the second pump light emitted from the second pump source passes through the phase delay system and then enters the first beam combiner; the first pump light and the second pump light are combined into a mixed-frequency light in the first beam combiner;

[0017] The mixed-frequency light is incident on the first APPLN crystal, and the first cascaded light and the first terahertz wave are generated through the cascaded optical difference frequency effect; the first terahertz wave is reflected and output by the first parabolic mirror, and the first cascaded light enters the second beam combiner after passing through the first parabolic mirror; meanwhile, the third pump light emitted from the third pump source enters the second beam combiner, is combined with the first cascaded light into a beam and then incident on the second APPLN crystal, and the third pump light and the first cascaded light generate the second cascaded light and the second terahertz wave through the cascaded optical difference frequency effect in the second APPLN crystal; the second terahertz wave is reflected and output by the second parabolic mirror, and the second cascaded light enters the third beam combiner after passing through the second parabolic mirror; meanwhile, the fourth pump light emitted from the fourth pump source enters the third beam combiner, is combined with the second cascaded light into a beam and then incident on the third APPLN crystal; the fourth pump light and the second cascaded light generate the third cascaded light and the third terahertz wave through the cascaded optical difference frequency effect in the third APPLN crystal, and the third terahertz wave is reflected and output by the third parabolic mirror;

[0018] The frequency difference between the first pump light and the second pump light is 0.5 - 2 THz, the frequency of the third pump light is equal to the frequency of the light wave with the strongest energy in the first cascaded light, and the frequency of the fourth pump light is equal to the frequency of the light wave with the strongest energy in the second cascaded light; the generated first terahertz wave, second terahertz wave, and third terahertz wave have the same frequency, which is the frequency difference between the first pump light and the second pump light;

[0019] The plane in which the light beam propagates is the plane determined by the X-axis and the Y-axis, the Z-axis is perpendicular to the plane in which the light beam propagates, the initial propagation direction of the first pump light emitted from the first pump source is the positive direction of the X-axis, the initial propagation directions of the second pump light emitted from the second pump source, the third pump light emitted from the third pump source, and the fourth pump light emitted from the fourth pump source are all the positive direction of the Y-axis, the propagation direction of the mixed-frequency light is the positive direction of the X-axis, the propagation directions of all cascaded lights are the positive direction of the X-axis, and the propagation directions of all terahertz waves are the positive direction of the Y-axis; the polarization directions of the first pump light, the second pump light, the third pump light, and the fourth pump light are all the Z-axis.

[0020] The phase delay system is composed of a first reflector, a second reflector, a third reflector, and a fourth reflector, and the pump light sequentially passes through the first reflector, the second reflector, the third reflector, and the fourth reflector for reflection to complete the phase delay; the first reflector, the second reflector, the third reflector, and the fourth reflector are all plane mirrors; the first reflector, the second reflector, the third reflector, and the fourth reflector totally reflect the second pump light.

[0021] The first APPLN crystal, the second APPLN crystal, and the third APPLN crystal are all cuboids, rectangular in the X-Y plane. The length direction of the crystal is consistent with the positive direction of the X axis, and the optical axis of the crystal is along the Z axis. The first APPLN crystal, the second APPLN crystal, and the third APPLN crystal are all non-periodically poled crystals. The non-periodic polarization distribution of the first APPLN crystal has a phase mismatch that gradually equals 0 along the crystal length from the 1st order redshift to the mth order redshift, where the mth order is any order within the range greater than the first order and less than { (frequency of the first pump light - 60 THz) / frequency of terahertz wave T1}; the non-periodic polarization distribution of the second APPLN crystal has a phase mismatch that gradually equals 0 along the crystal length from the nth order redshift to the pth order redshift, where the two difference-frequency lights included in the nth order difference frequency are the third pump light and its adjacent and lower-frequency cascaded light, and the pth order is any order within the range greater than the nth order and less than { (frequency of the first pump light - 60 THz) / frequency of terahertz wave T1}; the non-periodic polarization distribution of the third APPLN crystal (APPLN3) has a phase mismatch that gradually equals 0 along the crystal length from the qth order redshift to the sth order redshift, where the two difference-frequency lights included in the qth order difference frequency are the fourth pump light and its adjacent and lower-frequency cascaded light, and the sth order is any order within the range greater than the nth order and less than { (frequency of the first pump light - 60 THz) / frequency of terahertz wave T1}.

[0022] A small hole that only allows the cascaded lights of each order to pass through is opened at the center of the first parabolic mirror, the second parabolic mirror, and the third parabolic mirror.

[0023] The first cascaded light, the second cascaded light, and the third cascaded light passing through the first parabolic mirror, the second parabolic mirror, and the third parabolic mirror are mixed-frequency lights formed by mixing the cascaded lights of each order, and the frequency difference between adjacent cascaded lights is the frequency difference between the first pump light and the second pump light.

[0024] Compared with the prior art, a device for generating high-power terahertz waves based on triple-optimized cascaded difference frequency provided by the present invention has the following advantages compared with the existing terahertz radiation source based on optical difference frequency effect:

[0025] (1) By setting the distribution of the polarization period of the APPLN crystal, Stokes cascaded difference frequency can be enhanced while anti-Stokes cascaded difference frequency is suppressed, improving the optical conversion efficiency of terahertz waves.

[0026] (2) By setting the frequencies of the third pump light and the fourth pump light, the optical conversion efficiency of terahertz waves can be further improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic structural diagram of an embodiment of the present invention.

[0028] Figure 2(a) is the energy intensity distribution diagram of the first cascaded light C1 when the power densities of both the first pump source 1 and the second pump source 3 are 700 MW / cm 2 , and the length of the first APPLN crystal APPLN1 is 13.1 mm.

[0029] Figure 2(b) is the diagram showing the variation of the intensity of the first terahertz wave T1 generated when the power densities of both the first pump source 1 and the second pump source 3 are 700 MW / cm 2 , and the length of the first APPLN crystal APPLN1 is 13.1 mm, with respect to the length of the first APPLN crystal.

[0030] Figure 2(c) is the polarization period distribution diagram of the first APPLN crystal APPLN1 when the power densities of both the first pump source 1 and the second pump source 3 are 700 MW / cm 2 , and the length of the first APPLN crystal APPLN1 is 13.1 mm.

[0031] Figure 3(a) is the energy intensity distribution diagram of the second cascaded light C2 when the power densities of both the first pump source 1 and the second pump source 3 are 700 MW / cm 2 , the length of the first APPLN crystal APPLN1 is 13.1 mm, the power density of the third pump source 11 is 100 MW / cm 2 , and the length of the second APPLN crystal APPLN2 is 18.5 mm.

[0032] Figure 3(b) is the diagram showing the variation of the intensity of the second terahertz wave T2 generated when the power densities of both the first pump source 1 and the second pump source 3 are 700 MW / cm 2 , the length of the first APPLN crystal APPLN1 is 13.1 mm, the power density of the third pump source 11 is 100 MW / cm 2 , and the length of the second APPLN crystal APPLN2 is 18.5 mm, with respect to the length of the second APPLN crystal.

[0033] Figure 3(c) is the polarization period distribution diagram of the second APPLN crystal APPLN2 when the power densities of both the first pump source 1 and the second pump source 3 are 700 MW / cm 2 , the length of the first APPLN crystal APPLN1 is 13.1 mm, the power density of the third pump source 11 is 100 MW / cm 2 , and the length of the second APPLN crystal APPLN2 is 18.5 mm.

[0034] Figure 4(a) is the energy intensity distribution diagram of the third - stage cascaded light C3 when the power densities of both the first pump source 1 and the second pump source 3 are 700 MW / cm 2 , the length of the first APPLN crystal APPLN1 is 13.1 mm, the power density of the third pump source 11 is 100 MW / cm 2 , the length of the second APPLN crystal APPLN2 is 18.5 mm, and the power density of the fourth pump source 14 is 100 MW / cm 2 , and the length of the third APPLN crystal APPLN3 is 11.6 mm.

[0035] Figure 4(b) is a graph showing the variation of the intensity of the third terahertz wave T3 with the length of the third APPLN crystal when the power densities of both the first pump source 1 and the second pump source 3 are 700 MW / cm 2 , the length of the first APPLN crystal APPLN1 is 13.1 mm, the power density of the third pump source 11 is 100 MW / cm 2 , the length of the second APPLN crystal APPLN2 is 18.5 mm, and the power density of the fourth pump source 14 is 100 MW / cm 2 , and the length of the third APPLN crystal APPLN3 is 11.6 mm.

[0036] Figure 4(c) is the polarization period distribution diagram of the third APPLN crystal APPLN3 when the power densities of both the first pump source 1 and the second pump source 3 are 700 MW / cm 2 , the length of the first APPLN crystal APPLN1 is 13.1 mm, the power density of the third pump source 11 is 100 MW / cm 2 , the length of the second APPLN crystal APPLN2 is 18.5 mm, and the power density of the fourth pump source 14 is 100 MW / cm 2 , and the length of the third APPLN crystal APPLN3 is 11.6 mm. Detailed implementation manners

[0037] The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention and should not be construed as limiting the protection scope of the present invention. Those skilled in the art can make some non - essential improvements and adjustments based on the content of the present invention below. In the present invention, unless otherwise clearly specified and limited, the technical terms used in this application should have the ordinary meaning understood by those skilled in the art of the present invention.

[0038] As shown in the attached Figure 1As shown in the figure, a device for generating high-power terahertz waves based on triple-optimized cascaded difference frequency includes a first pump source 1, a second pump source 3, a third pump source 11, and a fourth pump source 14, a first APPLN crystal APPLN1, a second APPLN crystal APPLN2, and a third APPLN crystal APPLN3 with different polarization period distributions, a first parabolic mirror M1, a second parabolic mirror M2, a third parabolic mirror M3, a first beam combiner 9, a second beam combiner 13, a third beam combiner 16, and a phase delay system.

[0039] The first pump light 2 emitted from the first pump source 1 enters the first beam combiner 9; the second pump light 4 emitted from the second pump source 3 passes through a phase delay system composed of a first reflector 5, a second reflector 6, a third reflector 7, and a fourth reflector 8 and then enters the first beam combiner 9; the first pump light 2 and the second pump light 4 are combined into a mixed-frequency light 10 in the first beam combiner 9.

[0040] The mixed-frequency light 10 is incident on the first APPLN crystal APPLN1, and a first cascaded light C1 and a first terahertz wave T1 are generated through the cascaded optical difference frequency effect; the first terahertz wave T1 is reflected and output by the first parabolic mirror M1, and the first cascaded light C1 enters the second beam combiner 13 after passing through the first parabolic mirror M1. At the same time, the third pump light 12 emitted from the third pump source 11 enters the second beam combiner 13, is combined with the first cascaded light C1 into a beam, and is incident on the second APPLN crystal APPLN2. The third pump light 12 and the first cascaded light C1 generate a second cascaded light C2 and a second terahertz wave T2 through the cascaded optical difference frequency effect in the second APPLN crystal APPLN2; the second terahertz wave T2 is reflected and output by the second parabolic mirror M2, and the second cascaded light C2 enters the third beam combiner 16 after passing through the second parabolic mirror M2. At the same time, the fourth pump light 15 emitted from the fourth pump source 14 enters the third beam combiner 16, is combined with the second cascaded light C2 into a beam, and is incident on the third APPLN crystal APPLN3. The fourth pump light 15 and the second cascaded light C2 generate a third cascaded light C3 and a third terahertz wave T3 through the cascaded optical difference frequency effect in the third APPLN crystal APPLN3, and the third terahertz wave T3 is reflected and output by the third parabolic mirror M3.

[0041] The frequency difference between the first pump light 2 and the second pump light 4 is 0.5 - 2 THz. The frequency of the third pump light 12 is equal to the frequency of the light wave with the strongest energy in the first cascaded light, and the frequency of the fourth pump light 15 is equal to the frequency of the light wave with the strongest energy in the second cascaded light. The generated first terahertz wave, second terahertz wave, and third terahertz wave have the same frequency, which is the frequency difference between the first pump light and the second pump light.

[0042] Specifically, in this embodiment, the frequency difference between the first pump light 2 and the second pump light 4 is 0.5 THz. The frequency of the third pump light 12 is equal to the frequency of the light wave with the strongest energy in the first cascaded light C1, which is 205.76 THz. The frequency of the fourth pump light 15 is equal to the frequency of the light wave with the strongest energy in the second cascaded light C2, which is 111.76 THz. The first terahertz wave T1, the second terahertz wave T2, and the third terahertz wave T3 generated have the same frequency, which is the frequency difference between the first pump light 2 and the second pump light 4, and is 0.5 THz.

[0043] In this embodiment, the plane in which the light beam propagates is the plane determined by the X-axis and the Y-axis. The Z-axis is perpendicular to the plane in which the light beam propagates. The initial propagation direction of the first pump light 2 emitted from the first pump source 1 is the positive direction of the X-axis. The initial propagation directions of the second pump light 4 emitted from the second pump source 3, the third pump light 12 emitted from the third pump source 11, and the fourth pump light 15 emitted from the fourth pump source 14 are all the positive direction of the Y-axis. The propagation direction of the mixed-frequency light 10 is the positive direction of the X-axis. The propagation directions of all the cascaded lights are the positive direction of the X-axis. The propagation directions of all the terahertz waves are the positive direction of the Y-axis. The polarization directions of the first pump light 2, the second pump light 4, the third pump light 12, and the fourth pump light 15 are all the Z-axis.

[0044] The purpose of passing a pump light through the phase delay system is to synchronize the phases of the two pump lights. Passing the pump light through the phase delay system does not change the propagation direction of the light, and the pump light can also pass through more than one of the above-mentioned phase delay systems.

[0045] In this embodiment, the first pump source 1 uses a Yb:YAG pulsed laser. The frequency of the first pump light 2 is 291.26 THz, and the pump power density is 700 MW / cm 2 ; the second pump source 3 uses a Yb:YAG pulsed laser. The frequency of the second pump light 4 is 290.76 THz, and the pump power density is 700 MW / cm 2 ; the third pump source 11 uses a pulsed laser. The frequency of the third pump light 12 is 205.76 THz, and the pump power density is 100 MW / cm 2 ; the fourth pump source 14 uses a pulsed laser. The frequency of the fourth pump light 15 is 111.76 THz, and the pump power density is 100 MW / cm 2 . The repetition frequencies of the above four pump lights are all 10 Hz, the beam diameters are all 1 mm, and the polarization directions are all the Z-axis.

[0046] In this embodiment, the first reflector 5, the second reflector 6, the third reflector 7, and the fourth reflector 8 are all plane mirrors; the first reflector 5, the second reflector 6, the third reflector 7, and the fourth reflector 8 totally reflect the second pump light 4.

[0047] In this embodiment, the first APPLN crystal APPLN1, the second APPLN crystal APPLN2, and the third APPLN crystal APPLN3 are all cuboids, rectangular in the X-Y plane, with the length direction of the crystal consistent with the positive direction of the X axis and the optical axis of the crystal along the Z axis; the first APPLN crystal APPLN1, the second APPLN crystal APPLN2, and the third APPLN crystal APPLN3 are all non-periodically poled crystals. The non-periodic polarization distribution of the first APPLN crystal APPLN1 has the phase mismatch gradually equal to 0 along the crystal length from the first-order redshift to the m-order redshift, where the m-order is any order within the range greater than the first order and less than { (frequency of the first pump light 2 - 60 THz) / frequency of the terahertz wave T1}; the non-periodic polarization distribution of the second APPLN crystal APPLN2 has the phase mismatch gradually equal to 0 along the crystal length from the n-order redshift to the p-order redshift, where the two difference-frequency lights included in the n-order difference frequency are the third pump light 12 and its adjacent and lower-frequency cascaded light, the p-order is any order within the range greater than the n-order and less than { (frequency of the first pump light 2 - 60 THz) / frequency of the terahertz wave T1}, and n depends on the energy transfer situation in the first section of the crystal; the non-periodic polarization distribution of the third APPLN crystal APPLN3 has the phase mismatch gradually equal to 0 along the crystal length from the q-order redshift to the s-order redshift, where the two difference-frequency lights included in the q-order difference frequency are the fourth pump light 15 and its adjacent and lower-frequency cascaded light, depending on the energy transfer situation in the second section of the crystal, the s-order is any order within the range greater than the n-order and less than { (frequency of the first pump light 2 - 60 THz) / frequency of the terahertz wave T1}. In this paragraph, the lower frequency means that there are two adjacent cascaded lights, one with a frequency higher than this cascaded light and one with a frequency lower than this cascaded light. Therefore, after determining the cascaded light with the maximum energy, the adjacent and lower-frequency lights are determined simultaneously. The redshift phenomenon refers to the phenomenon of decreasing frequency and increasing wavelength.

[0048] In this embodiment, the first cascaded light C1, the second cascaded light C2, and the third cascaded light C3 passing through the first parabolic mirror M1, the second parabolic mirror M2, and the third parabolic mirror M3 are mixed-frequency lights composed of cascaded lights of each order, and the frequency difference between adjacent-order cascaded lights is 0.5 THz; the diameters of the small holes opened at the centers of the first parabolic mirror M1, the second parabolic mirror M2, and the third parabolic mirror M3, which only allow the cascaded lights of each order to pass through, are 1 mm.

[0049] The size of the first APPLN crystal APPLN1 is X×Y×Z = 13.1 mm × 4 mm × 2 mm. Its polarization period is as shown in Fig. 2(c), which decreases from 237.14 μm to 234.85 μm. That is, the phase mismatch of the non-periodic polarization distribution of the first APPLN crystal along the crystal length redshifts from the 1st order to the 132nd order, and each order is equal to 0. After the first pump light 2 and the second pump light 4 are input into the first APPLN crystal APPLN1, the intensity distribution of the first cascaded light C1 is as shown in Fig. 2(a), and the variation of the first terahertz wave T1 with the crystal length is as shown in Fig. 2(b). When the length of APPLN1 on the X-axis is 13.1 mm, the intensity of the first terahertz wave T1 obtained is 126 MW / cm 2 , at this time, the intensity of the cascaded light at the 171st order, that is, 205.76 THz, in the first cascaded light C1 is the largest. Therefore, the frequency of the input third pump light 12 is set to 205.76 THz, and the intensity is 100 MW / cm 2 , and the phase mismatch of the non-periodic polarization distribution of the second APPLN crystal APPLN2 along the crystal length redshifts from the 171st order to the 365th order, and each order is equal to 0.

[0050] The size of the second APPLN crystal APPLN2 is X×Y×Z = 18.5 mm × 4 mm × 2 mm. Its polarization period is as shown in Fig. 3(c), which decreases from 234.37 μm to 234.25 μm. After the first cascaded light C1 is input into the second APPLN crystal APPLN2, the intensity distribution of the second cascaded light C2 is as shown in Fig. 3(a), and the variation of the second terahertz wave T2 with the crystal length is as shown in Fig. 3(b). When the length of APPLN2 on the X-axis is 18.5 mm, the intensity of the second terahertz wave T2 obtained is 157 MW / cm 2 , at this time, the intensity of the cascaded light at the 359th order, that is, 111.76 THz, in the second cascaded light C2 is the largest. Therefore, the frequency of the input fourth pump light 15 is set to 111.76 THz, and the intensity is 100 MW / cm 2 , and the phase mismatch of the non-periodic polarization distribution of the third APPLN crystal APPLN2 along the crystal length redshifts from the 359th order to the 476th order, and each order is equal to 0.

[0051] The size of the third APPLN crystal APPLN3 is X×Y×Z = 11.6 mm×4 mm×2 mm, and its polarization period increases from 234.787 μm to 234.829 μm as shown in Figure 4(c). After the second cascaded light C2 is input into the third APPLN crystal APPLN3, the intensity distribution of the third cascaded light C3 is as shown in Figure 4(a), and the variation of the third terahertz wave T3 with the crystal length is as shown in Figure 4(b). When the length of APPLN3 on the X-axis is 11.6 mm, the intensity of the terahertz wave T3 obtained is 65 MW / cm 2 .

[0052] The change in the polarization period of the APPLN crystal is along the length direction of each APPLN crystal. In the crystal length direction, for each APPLN crystal, the crystal length of the plane where light enters each APPLN crystal is 0, and along the positive X-axis direction, the APPLN crystal length gradually increases.

[0053] The above-described embodiments are only examples and illustrations of the technical solutions of the present invention, which are convenient for those skilled in the art to understand the technical solutions of the present application, rather than all implementation manners. The protection scope of the present invention is not limited thereto. The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as the combination of these technical features does not conflict, it should be considered as the scope described in this specification. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the protection scope required by the present invention. The basic idea of the present invention lies in the above basic solution. For those skilled in the art and any person familiar with the technical field, without departing from the overall concept and the spirit of the principle of the present invention, according to the teachings of the present invention, designing various deformed models, formulas, and parameters does not require creative labor. Changes, modifications, substitutions, equivalent substitutions, and variations made to the embodiments without departing from the principle and spirit of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A device for generating high-power terahertz waves based on triple-optimized cascaded difference frequency, characterized in that: It includes a first pump source (1), a second pump source (3), a third pump source (11), and a fourth pump source (14), a first APPLN crystal (APPLN1), a second APPLN crystal (APPLN2), a third APPLN crystal (APPLN3) with different polarization period distributions, a first parabolic mirror (M1), a second parabolic mirror (M2), a third parabolic mirror (M3), a first beam combiner (9), a second beam combiner (13), a third beam combiner (16), and a phase delay system; The first pump light (2) emitted from the first pump source (1) enters the first beam combiner (9); the second pump light (4) emitted from the second pump source (3) enters the first beam combiner (9) after passing through the phase delay system; the first pump light (2) and the second pump light (4) are combined into a mixed-frequency light (10) in the first beam combiner (9); The mixed-frequency light (10) is incident on the first APPLN crystal (APPLN1), and a first cascaded light (C1) and a first terahertz wave (T1) are generated through the cascaded optical difference frequency effect; the first terahertz wave (T1) is reflected and output by the first parabolic mirror (M1), and the first cascaded light (C1) enters the second beam combiner (13) after passing through the first parabolic mirror (M1); meanwhile, the third pump light (12) emitted from the third pump source (11) enters the second beam combiner (13), is combined with the first cascaded light (C1) into a beam and is incident on the second APPLN crystal (APPLN2), and the third pump light (12) and the first cascaded light (C1) generate a second cascaded light (C2) and a second terahertz wave (T2) in the second APPLN crystal (APPLN2) through the cascaded optical difference frequency effect; the second terahertz wave (T2) is reflected and output by the second parabolic mirror (M2), and the second cascaded light (C2) enters the third beam combiner (16) after passing through the second parabolic mirror (M2); meanwhile, the fourth pump light (15) emitted from the fourth pump source (14) enters the third beam combiner (16), is combined with the second cascaded light (C2) into a beam and is incident on the third APPLN crystal (APPLN3); the fourth pump light (15) and the second cascaded light (C2) generate a third cascaded light (C3) and a third terahertz wave (T3) in the third APPLN crystal (APPLN3) through the cascaded optical difference frequency effect, and the third terahertz wave (T3) is reflected and output by the third parabolic mirror (M3); The frequency difference between the first pump light (2) and the second pump light (4) is 0.5 - 2 THz, the frequency of the third pump light (12) is equal to the frequency of the light wave with the strongest energy in the first cascaded light (C1), and the frequency of the fourth pump light (15) is equal to the frequency of the light wave with the strongest energy in the second cascaded light (C2); the generated first terahertz wave (T1), second terahertz wave (T2), and third terahertz wave (T3) have the same frequency, which is the frequency difference between the first pump light (2) and the second pump light (4); The plane in which the light beam propagates is the plane determined by the X-axis and the Y-axis, the Z-axis is perpendicular to the plane of light beam propagation, the initial propagation direction of the first pump light (2) emitted from the first pump source (1) is the positive direction of the X-axis, and the initial propagation directions of the second pump light (4) emitted from the second pump source (3), the third pump light (12) emitted from the third pump source (11), and the fourth pump light (15) emitted from the fourth pump source (14) are all the positive direction of the Y-axis. The propagation direction of the mixed-frequency light (10) is the positive direction of the X-axis, the propagation directions of all cascaded lights are the positive direction of the X-axis, and the propagation directions of all terahertz waves are the positive direction of the Y-axis; the polarization directions of the first pump light (2), the second pump light (4), the third pump light (12), and the fourth pump light (15) are all the Z-axis.

2. The device for generating high-power terahertz waves based on triple-optimized cascaded difference frequency according to claim 1, characterized in that: The phase delay system consists of a first mirror (5), a second mirror (6), a third mirror (7), and a fourth mirror (8). The pump light (4) sequentially passes through the first mirror (5), the second mirror (6), the third mirror (7), and the fourth mirror (8) for reflection to complete the phase delay; the first mirror (5), the second mirror (6), the third mirror (7), and the fourth mirror (8) are all plane mirrors; the first mirror (5), the second mirror (6), the third mirror (7), and the fourth mirror (8) totally reflect the second pump light (4).

3. The device for generating high-power terahertz waves based on triple-optimized cascaded difference frequency according to claim 1, wherein: The first APPLN crystal (APPLN1), the second APPLN crystal (APPLN2), and the third APPLN crystal (APPLN3) are all cuboids, rectangular in the X-Y plane, the length direction of the crystal is consistent with the positive direction of the X-axis, and the optical axis of the crystal is along the Z-axis; the first APPLN crystal (APPLN1), the second APPLN crystal (APPLN2), and the third APPLN crystal (APPLN3) are all periodically poled crystals; the periodic polarization distribution of the first APPLN crystal (APPLN1) has the phase mismatch gradually equal to 0 from the 1st order redshift to the mth order redshift along the crystal length, and the mth order is any order within the range greater than the first order and less than {(frequency of the first pump light - 60 THz) / frequency of terahertz wave T1}; the periodic polarization distribution of the second APPLN crystal (APPLN2) has the phase mismatch gradually equal to 0 from the nth order redshift to the pth order redshift along the crystal length, where the two difference-frequency lights included in the nth order difference frequency are the third pump light (12) and its adjacent and lower-frequency cascaded light, and the pth order is any order within the range greater than the nth order and less than {(frequency of the first pump light - 60 THz) / frequency of terahertz wave T1}; the periodic polarization distribution of the third APPLN crystal (APPLN3) has the phase mismatch gradually equal to 0 from the qth order redshift to the sth order redshift along the crystal length, where the two difference-frequency lights included in the qth order difference frequency are the fourth pump light (15) and its adjacent and lower-frequency cascaded light, and the sth order is any order within the range greater than the qth order and less than {(frequency of the first pump light - 60 THz) / frequency of terahertz wave T1}.

4. The device for generating high-power terahertz waves based on triple-optimized cascaded difference frequency generation according to claim 1, wherein: A small hole that only allows the cascaded light of each stage to pass through is opened at the centers of the first parabolic mirror (M1), the second parabolic mirror (M2), and the third parabolic mirror (M3).

5. The device for generating high-power terahertz waves based on triple-optimized cascaded difference frequency according to claim 1, wherein: The first cascaded light (C1), the second cascaded light (C2), and the third cascaded light (C3) passing through the first parabolic mirror (M1), the second parabolic mirror (M2), and the third parabolic mirror (M3) are mixed-frequency lights formed by mixing the cascaded lights of each stage, and the frequency difference between adjacent-stage cascaded lights is the frequency difference between the first pump light (2) and the second pump light (4).

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