A resonant cavity and high power terahertz radiation source combining optimized cascaded difference frequency

Abstract

The application aims to provide a resonant cavity and high-power terahertz radiation source combining optimization of cascade difference frequency, comprising a first pump source and a second pump source, a non-periodic polarization APPLN crystal, a beam combining mirror, a phase delay system, a fifth mirror, a sixth mirror, a seventh mirror, an eighth mirror and a beamsplitter constituting a resonant cavity; the intensity of the terahertz wave can be enhanced, and the energy conversion efficiency of the terahertz wave can be improved. Through setting of the reflectivity of the cavity mirror, the Stokes light wave can be oscillated in the cavity, and the energy can be repeatedly transferred from the pump wave to the high-order Stokes wave.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of terahertz wave application, and particularly relates to a resonant cavity and a high-power terahertz radiation source combining resonant cavity and optimized cascade difference frequency. BACKGROUND

[0002] Terahertz (THz) is electromagnetic wave with frequency in the range of 0.1-10 THz (1 THz = 10 12 THz), which is located between millimeter wave and infrared in the electromagnetic spectrum, and is the transition area from macroscopic theory to microscopic theory. The special position of THz makes it have great scientific research value and broad application prospect in the fields of basic research such as physics, chemistry, astronomy, molecular spectroscopy, life science and medical science, and application research such as medical imaging, environmental monitoring, material detection, food detection, radio astronomy, mobile communication, satellite communication and military radar. THz is mainly applied in the following fields:

[0003] (1) Imaging field

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

[0005] (2) Biomedical technology field

[0006] The "fingerprint" characteristics of the THz band make it contain rich physical and chemical information when interacting with the medium, and the low energy ensures that it can be applied to the field of biomedical imaging. Since the THz light source is composed of different complex polarized light waves, different polarized light can be used to collect information of the medium to obtain more accurate diagnostic information of pathological tissues, so THz wave has a significant role in clinical diagnosis and treatment of cancer.

[0007] (3) Non-destructive testing field

[0008] The penetration of THz wave can be well applied to the fields of non-destructive testing and THz imaging. Using THz time-domain spectroscopy technology, non-contact and non-destructive conditions can be realized, and non-polar dielectric materials such as clothes, cartons and plastics can be penetrated, so not only the chemical properties of the medium can be detected, but also the contour and shape of the object can be determined.

[0009] (4) Communication field

[0010] THz wave has wide bandwidth, good directivity and high transmission rate, so it has great potential in space high-speed communication and radar application, and has the application prospect of military and civilian combination and balanced and coordinated development. Because THz wave is sensitive to water molecules, it can realize secret communication in the atmosphere. Compared with visible light and infrared, THz wave has better directivity due to long wavelength, so it can realize space communication with high bandwidth in clouds and fog.

[0011] (5) National security field

[0012] THz wave has good directivity and narrow beam, and has strong cloud and smoke penetration ability. In military application, THz wave is used for correction of missile terminal precise guidance to improve guidance accuracy, which has high military application value.

[0013] The main problem at present is that there is a lack of a THz source capable of generating high-power, high-quality and high-efficiency THz wave, and low cost and operating at room temperature. At present, the generation methods of THz wave mainly include electronic method and photonics method. The electronic method is generally to extend the wavelength of electromagnetic radiation from millimeter wave to THz wave band, which is equivalent to a process of increasing frequency. However, when the frequency is greater than 1 THz, great obstacles will be encountered, so that the efficiency is very low. At the same time, the THz wave radiation source generated by the electronic method is large in size, which limits its application in many fields. The main direction of the photonics method is to convert visible light or infrared light into THz wave. The advantage of this method is that the THz radiation source generated has high coherence and directivity. However, the power and efficiency of the THz wave generated at present are low. SUMMARY

[0014] The purpose of the present application is to provide a resonant cavity and high-power THz radiation source combined with optimized cascade difference frequency, which can enhance the intensity of THz wave and improve the energy conversion efficiency of THz wave.

[0015] The purpose of the present application is achieved in the following manner: a resonant cavity and high-power THz radiation source combined with optimized cascade difference frequency, characterized by comprising a first pump source and a second pump source, a non-periodic polarization APPLN crystal, a beam combining mirror, a phase delay system, a fifth mirror, a sixth mirror, a seventh mirror, an eighth mirror and a beam splitter which constitute a resonant cavity.

[0016] The first pump light emitted from the first pump source enters the beam combiner; the second pump light emitted from the second pump source enters the beam combiner through the phase delay system; the first pump light and the second pump light are combined into a first mixed light in the beam combiner; the first mixed light enters the APPLN crystal through the fifth mirror, and the second mixed light is generated through the cascaded optical difference frequency effect; the second mixed light transmits the third mixed light through the sixth mirror; the Stokes light wave is reflected by the sixth mirror; the Stokes light wave enters the APPLN crystal in the direction parallel to the first mixed light through the seventh mirror, the eighth mirror and the fifth mirror with high reflection, and the Stokes light wave completes the circulating oscillation in the resonant cavity; the third mixed light is divided into the optical wave and the terahertz wave through the beam splitter; the optical wave is obtained by reflection, and the terahertz wave is obtained by transmission;

[0017] The plane of the light beam propagation is the plane determined by the X-axis and the Y-axis, the Z-axis is perpendicular to the plane of the light beam propagation, 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 direction of the second pump light emitted from the second pump source is the negative direction of the Y-axis, the propagation direction of the first mixed light is the positive direction of the X-axis, the propagation direction of the second mixed light is the positive direction of the X-axis, the propagation direction of the third mixed light is the positive direction of the X-axis, the propagation direction of the terahertz wave is the positive direction of the X-axis, and the propagation direction of the optical wave is the positive direction of the Y-axis.

[0018] The first pump source adopts a pulse laser, the second pump source adopts a pulse laser, and the polarization directions of the pump light emitted by the above pump sources are all the Z-axis; the frequency difference between the first pump light and the second pump light is equal to the frequency of the terahertz wave.

[0019] The phase delay system is composed of a first mirror, a second mirror, a third mirror and a fourth mirror, and the pump light is reflected by the first mirror, the second mirror, the third mirror and the fourth mirror in sequence to complete the phase delay.

[0020] The first mirror, the second mirror, the third mirror and the fourth mirror are all plane mirrors; the first mirror, the second mirror, the third mirror and the fourth mirror fully reflect the second pump light.

[0021] The APPLN crystal is a cuboid, which is 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 non-polarization period distribution of the APPLN crystal is divided into two parts along the length direction of the APPLN crystal, the first part satisfies the cascade difference frequency from the first-order Stokes to the N-order, the N-order refers to any order in the range greater than the first order and less than {the frequency of the first pump light-60THz / the frequency of the terahertz wave}, the phase mismatch of the Stokes cascade difference frequency is equal to 0 step by step along the length of the crystal, and the second part satisfies the cascade difference frequency from the N-order to the M-order, the M-order refers to any order in the range greater than the N-order and less than {the frequency of the first pump light-60THz / the frequency of the terahertz wave}, and the phase mismatch of the Stokes cascade difference frequency is equal to 0 step by step along the length of the crystal. MThe phase mismatch of the Kth-order anti-Stokes cascade difference frequency is equal to 0 along the crystal length, wherein the difference frequency light corresponding to the Mth-order anti-Stokes cascade difference frequency is the anti-Stokes light with the highest power density, K represents any order in the range greater than the first order and less than the { (frequency of the first pump light - 60 THz) / frequency of the terahertz wave} order, and K > N.

[0022] The first mixed frequency light comprises the first pump light and the second pump light, the second mixed frequency light comprises the Stokes light wave, the light wave and the terahertz wave, the third mixed frequency light comprises the light wave and the terahertz wave, the light wave comprises the first pump light, the second pump light and the anti-Stokes light wave, and the frequency difference of the adjacent cascade light waves is equal to the frequency of the terahertz wave.

[0023] The fifth mirror, the sixth mirror, the seventh mirror and the eighth mirror are all concave lenses. The fifth mirror and the sixth mirror are both highly transmissive to the first mixed frequency light with a transmittance of 0.99 and highly reflective to the Stokes light wave with a reflectivity of 0.99. The sixth mirror is highly transmissive to the third mixed frequency light with a transmittance of 0.99. The seventh mirror and the eighth mirror are both highly reflective to the Stokes light wave with a reflectivity of 0.99.

[0024] The Stokes light wave circulates and oscillates in the resonant cavity, and the third mixed frequency light transmits out of the resonant cavity.

[0025] The sixth mirror and the beam splitter are made of silicon material and have a small absorption coefficient for the terahertz wave. The beam splitter has a high transmittance for the terahertz wave, and the transmittance is greater than 0.9. The beam splitter has a high reflectivity for the light wave, and the reflectivity is 0.99.

[0026] Compared with the prior art, the high-power terahertz radiation source provided by the application combines the resonant cavity and the optimized cascade difference frequency, and has the following advantages compared with the existing terahertz radiation source based on the optical difference frequency effect:

[0027] (1) By setting the reflectivity of the cavity mirror, the Stokes light wave oscillates in the cavity, and the energy can be repeatedly transferred from the pump wave to the high-order Stokes wave.

[0028] (2) By setting the non-polarization period of the APPLN crystal, the energy can be continuously transferred from the pump wave to the high-order Stokes wave.

[0029] (3) The non-polarization period distribution of the APPLN crystal and the Stokes light wave circulating and oscillating in the resonant cavity enable the low-order Stokes photon to continuously transfer to the high-order Stokes photon. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is the structural schematic diagram of the embodiment of the present application.

[0031] Figure 2 is I P = I S =1000 MW / cm 2 Below, the polarization period of the APPLN crystal changes with the crystal length diagram;

[0032] Figure 3 is in I P = I S =1000 MW / cm 2 , the cascade light wave evolution diagram.

[0033] Figure 4 is in I P = I S =1000 MW / cm 2 , the terahertz wave intensity changes with the crystal length. DETAILED DESCRIPTION

[0034] The present application will be described in detail below in conjunction with specific embodiments, it is necessary to point out here that the embodiments are only used to further illustrate the present application, and cannot be understood as limiting the protection scope of the present application, and the skilled in the art can make some non-essential improvements and adjustments according to the above content of the present application.

[0035] As shown in the accompanying Figure 1 , a resonant cavity and high-power terahertz radiation source combined with optimized cascade difference frequency, characterized by: comprising a first pump source 1 and a second pump source 4, a non-periodic polarization APPLN crystal 15, a beam combination mirror 3, a first mirror 6 used as a phase delay system, a second mirror 7, a third mirror 8 and a fourth mirror 9, a fifth mirror 11, a sixth mirror 12, a seventh mirror 13, an eighth mirror 14, a beam splitter 19, which constitute a resonant cavity.

[0036] The phase delay system is used for phase delay, which can be realized for the four mirrors described in the embodiments of the present application, or the phase delay can be realized through other ways.

[0037] The first pump light 2 emitted from the first pump source 1 enters the beam combiner 3. The second pump light 5 emitted from the second pump source 4 enters the beam combiner 3 through the phase delay system composed of the first mirror 6, the second mirror 7, the third mirror 8 and the fourth mirror 9 in sequence. The first pump light 2 and the second pump light 5 are combined into the first mixed light 10 in the beam combiner 3. The first mixed light 10 enters the APPLN crystal 15 through the fifth mirror 11, and the second mixed light 17 is generated through the cascaded optical difference frequency effect. The second mixed light 17 transmits through the sixth mirror 12 to obtain the third mixed light 18. The second mixed light 17 reflects off the sixth mirror 12 to obtain the Stokes light wave 16. The Stokes light wave 16 enters the APPLN crystal 15 through the seventh mirror 13, the eighth mirror 14 and the fifth mirror 11, and completes the cyclic oscillation in the resonant cavity. The third mixed light 18 is split into the optical wave 21 and the terahertz wave 20 through the beam splitter 19. The optical wave 21 is obtained by reflecting through the beam splitter 19, and the terahertz wave 20 is obtained by transmitting through the beam splitter 19.

[0038] The plane of the light beam propagation is the plane determined by the X axis and the Y axis, and the Z axis is perpendicular to the plane of the 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, the initial propagation direction of the second pump light 5 emitted from the second pump source 4 is the negative direction of the Y axis, the propagation direction of the first mixed light 10 is the positive direction of the X axis, the propagation direction of the second mixed light 17 is the positive direction of the X axis, the propagation direction of the third mixed light 18 is the positive direction of the X axis, the propagation direction of the terahertz wave 20 is the positive direction of the X axis, and the propagation direction of the optical wave 21 is the positive direction of the Y axis.

[0039] In the embodiment, the first pump source 1 is a Yb:YAG pulse laser, and the frequency is 291.5 THz. The second pump source 4 is a Yb:YAG pulse laser, and the frequency is 291 THz. The frequency difference between the first pump light 2 and the second pump light 5 is 0.5 THz. The power density of the two pump sources is 1000 MW / cm 2 , and the polarization direction is the Z axis. The frequency of the terahertz wave 20 is 0.5 THz.

[0040] In the embodiment, the first mirror 6, the second mirror 7, the third mirror 8 and the fourth mirror 9 are all plane mirrors, and the first mirror 6, the second mirror 7, the third mirror 8 and the fourth mirror 9 fully reflect the second pump light 5.

[0041] The APPLN crystal 15 is 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 distribution of the non-polarization period is divided into two parts along the length direction of the APPLN crystal, the first part satisfies the difference frequency from the first-order Stokes cascade to the N-order, the N-order refers to any order in the range greater than the first order and less than the frequency of {the frequency of the first pump light 2-60 THz / the frequency of the 20th terahertz wave}, and the phase mismatch of the Stokes cascade difference frequency is equal to 0 along the crystal length step by step, and the second part satisfies the difference frequency from the K-order anti-Stokes cascade to the K-order, the K-order refers to any order in the range greater than the first order and less than the frequency of {the frequency of the first pump light 2-60 THz / the frequency of the 20th terahertz wave}, and K>N. M order

[0042] Specifically, in the embodiment, the APPLN crystal 15 is a cuboid, and the size XxYxZ of the APPLN crystal 15 is 2 mmx5 mmx2 mm. In the embodiment, the first pump light 2 is a 2.52 μm wavelength laser, the second pump light 3 is a 1.06 μm wavelength laser, and the third pump light 4 is a 0.65 μm wavelength laser. I P I S =1000 MW / cm 2 At this time, the distribution of the non-polarization period of the APPLN crystal 15 along the length direction of the APPLN crystal is as shown in the accompanying drawings. Figure 2 The first part is reduced from 237.81 μm to 237.22 μm, and the second part is reduced from 238.18 μm to 235.72 μm. The first part satisfies the phase mismatch of the difference frequency from the first-order Stokes cascade to the 60th-order Stokes cascade, and the phase mismatch of the Stokes cascade difference frequency is equal to 0 along the crystal length step by step, and the second part satisfies the phase mismatch of the difference frequency from the 16th-order anti-Stokes cascade to the 280th-order Stokes cascade, and the phase mismatch of the Stokes cascade difference frequency is equal to 0 along the crystal length step by step. At this time, the cascade light wave evolution diagram is as shown in the accompanying drawings. Figure 3

[0043] The terahertz wave intensity along the length direction of the APPLN crystal is as shown in the accompanying drawings. Figure 4 The terahertz intensity gradually increases with the increase of the crystal length. At the crystal length of 2 mm, the maximum power density is 745.37 MW / cm 2 .

[0044] ​​​The frequency difference of adjacent cascade light waves is equal to the frequency of the terahertz wave 20. In this embodiment, the frequency difference of adjacent cascade light waves contained in the second mixed light 17 is equal to 0.5 THz.

[0045] In this embodiment, the fifth mirror 11, the sixth mirror 12, the seventh mirror 13, and the eighth mirror 14 are all concave lenses. The fifth mirror 11 and the sixth mirror 12 are both highly transmissive to the first mixed light 10 with a transmittance of 0.99, and are highly reflective to the Stokes light wave 16 with a reflectance of 0.99. The sixth mirror 12 is highly transmissive to the third mixed light 18 with a transmittance of 0.99. The seventh mirror 13 and the eighth mirror 14 are both highly reflective to the Stokes light wave 16 with a reflectance of 0.99.

[0046] In this embodiment, the sixth mirror 12 and the beam splitter 19 are made of silicon material and have a small absorption to the terahertz wave 20. The beam splitter 19 has a high transmittance to the terahertz wave 20, and the transmittance is greater than 0.9. The beam splitter 19 has a high reflectance to the light wave 21, and the reflectance is 0.99.

[0047] The above gives a specific embodiment, but the present application is not limited to the described embodiment. The basic idea of the present application is the above basic scheme, and according to the teaching of the present application, a person skilled in the art can design various modified models, formulas, and parameters without creative labor. Changes, modifications, replacements, and variations of the embodiments without departing from the principles and spirits of the present application still fall within the protection scope of the present application.

Claims

1. A resonant cavity and optimized cascade difference frequency combined high power terahertz radiation source, characterized in that: The first pump source (1) and the second pump source (4), the non-periodic polarization APPLN crystal (15), the beam combining mirror (3), the phase delay system, the fifth mirror (11), the sixth mirror (12), the seventh mirror (13), the eighth mirror (14), and the beam splitter (19) constitute a resonant cavity. The first pump light (2) emitted from the first pump source (1) enters the beam combining mirror (3); the second pump light (5) emitted from the second pump source (4) enters the beam combining mirror (3) through the phase delay system; the first pump light (2) and the second pump light (5) are combined into the first mixed light (10) in the beam combining mirror (3); the first mixed light (10) enters the APPLN crystal (15) through the fifth mirror (11), and the second mixed light (17) is generated through the cascaded optical difference frequency effect; the second mixed light (17) transmits the third mixed light (18) through the sixth mirror (12); the second mixed light (17) reflects the Stokes light wave (16) through the sixth mirror (12); the Stokes light wave (16) enters the APPLN crystal (15) in the direction parallel to the first mixed light (10) after being reflected by the seventh mirror (13), the eighth mirror (14), and the fifth mirror (11), and the Stokes light wave (16) completes the cyclic oscillation in the resonant cavity; the third mixed light (18) is divided into the light wave (21) and the terahertz wave (20) after passing through the beam splitter (19); the light wave (21) is obtained by reflection, and the terahertz wave (20) is obtained by transmission. The plane of the light beam propagation is the plane determined by the X-axis and the Y-axis, the Z-axis is perpendicular to the plane of the 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, the initial propagation direction of the second pump light (5) emitted from the second pump source (4) is the negative direction of the Y-axis, the propagation direction of the first mixed light (10) is the positive direction of the X-axis, the propagation direction of the second mixed light (17) is the positive direction of the X-axis, the propagation direction of the third mixed light (18) is the positive direction of the X-axis, the propagation direction of the terahertz wave (20) is the positive direction of the X-axis, and the propagation direction of the light wave (21) is the positive direction of the Y-axis.

2. The resonator and optimized cascaded difference frequency combined high power terahertz radiation source of claim 1, wherein: The first pump source (1) is a pulse laser, the second pump source (4) is a pulse laser, the polarization directions of the pump lights emitted by the above pump sources are all the Z-axis, and the frequency difference between the first pump light (2) and the second pump light (5) is equal to the frequency of the terahertz wave (20).

3. The resonator and optimized cascaded difference frequency combined high power terahertz radiation source of claim 1, wherein: The phase delay system is composed of the first mirror (6), the second mirror (7), the third mirror (8), and the fourth mirror (9), and the pump light (5) is reflected by the first mirror (6), the second mirror (7), the third mirror (8), and the fourth mirror (9) in sequence to complete the phase delay.

4. The resonator and optimized cascaded difference frequency combined high power terahertz radiation source of claim 3, wherein: The first mirror (6), the second mirror (7), the third mirror (8), and the fourth mirror (9) are all plane mirrors, and the first mirror (6), the second mirror (7), the third mirror (8), and the fourth mirror (9) fully reflect the second pump light (5).

5. The resonator and optimized cascaded difference frequency combined high power terahertz radiation source of claim 1, wherein: The APPLN crystal (15) is a cuboid, 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 non-polarization period distribution of the APPLN crystal (15) is divided into two parts along the length direction of the APPLN crystal, the first part satisfies from the first-order Stokes cascade difference frequency to the N-order, the N-order refers to any order in the range greater than the first order and less than {the frequency of the first pump light - 60 THz / the frequency of the terahertz wave}, and the phase mismatch of the Stokes cascade difference frequency is equal to 0 along the crystal length step by step, and the second part satisfies from the M-order anti-Stokes cascade difference frequency to the K-order, the phase mismatch of the Stokes cascade difference frequency is equal to 0 along the crystal length step by step, wherein the difference frequency light corresponding to the M-order anti-Stokes cascade difference frequency is the anti-Stokes light with the highest power density, the K-order refers to any order in the range greater than the first order and less than {the frequency of the first pump light - 60 THz / the frequency of the terahertz wave}, and K>N. M The APPLN crystal (15) is a cuboid, 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 non-polarization period distribution of the APPLN crystal (15) is divided into two parts along the length direction of the APPLN crystal, the first part satisfies from the first-order Stokes cascade difference frequency to the N-order, the N-order refers to any order in the range greater than the first order and less than {the frequency of the first pump light - 60 THz / the frequency of the terahertz wave}, and the phase mismatch of the Stokes cascade difference frequency is equal to 0 along the crystal length step by step, and the second part satisfies from the M-order anti-Stokes cascade difference frequency to the K-order, the phase mismatch of the Stokes cascade difference frequency is equal to 0 along the crystal length step by step, wherein the difference frequency light corresponding to the M-order anti-Stokes cascade difference frequency is the anti-Stokes light with the highest power density, the K-order refers to any order in the range greater than the first order and less than {the frequency of the first pump light - 60 THz / the frequency of the terahertz wave}, and K>N.

6. The resonator and optimized cascaded difference frequency combined high power terahertz radiation source of claim 1, wherein: The first mixed light (10) comprises a first pump light (2) and a second pump light (5), the second mixed light (17) comprises a Stokes light wave (16), a light wave (21) and a terahertz wave (20); the third mixed light (18) comprises the light wave (21) and the terahertz wave (20); the light wave (21) comprises the first pump light (2), the second pump light (5) and an anti-Stokes light wave; the frequency difference of adjacent cascade light waves is equal to the frequency of the terahertz wave (20).

7. The resonator and optimized cascaded difference frequency combined high power terahertz radiation source of claim 1, wherein: The fifth mirror (11), the sixth mirror (12), the seventh mirror (13) and the eighth mirror (14) are all concave lenses. The fifth mirror (11) and the sixth mirror (12) are both highly transmissive to the first mixed light (10) with a transmittance of 0.99 and highly reflective to the Stokes light wave (16) with a reflectance of 0.99; the sixth mirror (12) is highly transmissive to the third mixed light (18) with a transmittance of 0.99; the seventh mirror (13) and the eighth mirror (14) are both highly reflective to the Stokes light wave (16) with a reflectance of 0.

99.

8. The resonator and optimized cascaded difference frequency combined high power terahertz radiation source of claim 1, wherein: The Stokes light wave (16) is cyclically oscillated in the resonant cavity, and the third mixed light (18) is transmitted out of the resonant cavity.

9. The resonator and optimized cascaded difference frequency combined high power terahertz radiation source of claim 1, wherein: The sixth mirror (12) and the beam splitter (19) are made of silicon material and have a small absorption coefficient to the terahertz wave (20); the beam splitter (19) has a high transmittance to the terahertz wave (20) with a transmittance greater than 0.9 and a high reflectance to the light wave (21) with a reflectance of 0.99.

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