Optimized cascaded difference frequency terahertz wave radiation source

Abstract

The application aims to provide a kind of optimized cascade difference frequency terahertz wave radiation source, which can enhance the intensity of terahertz wave and improve the energy conversion efficiency of terahertz wave.Compared with the traditional cascade difference frequency needing two pump sources, the scheme only needs one pump source, the polarization period of AFB-KTP crystal determines the wavelength of generated signal light and idler light, and through the setting of the polarization period of AFB-KTP crystal, the purpose of different frequency THz wave tuning is achieved.Through the setting of the non-polarization period of APPLN crystal, the signal light photon is repeatedly transferred to high-order Stokes light photon, and the conversion efficiency of terahertz wave is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of terahertz wave application, and particularly relates to an optimized cascade difference frequency terahertz wave radiation source. 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 region 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 through 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 the interaction with the medium contain rich physical and chemical information, 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 the clinical diagnosis and treatment of cancer.

[0007] (3) Non-destructive testing field

[0008] The penetration of THz wave can be well applied to the field 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 it can not only detect the chemical properties of the medium, but also judge the contour and shape of the object.

[0009] (4) Communication field

[0010] THz waves have a wide bandwidth, good directionality, and high transmission rate, making them highly promising for high-speed space communication and radar applications, with a promising future for both military and civilian applications and balanced, coordinated development. Because THz waves are sensitive to water molecules, secure communication can be achieved through atmospheric transmission. Compared to visible light and infrared light, THz waves, due to their longer wavelength and better directionality, enable space communication with extremely high bandwidth even in clouds and fog.

[0011] (5) Homeland security

[0012] THz waves have good directionality and narrow beams, giving them strong penetration capabilities through clouds and smoke. In military applications, THz waves are used for terminal precision guidance of missiles to improve guidance accuracy, making them extremely valuable for military applications.

[0013] The lack of a terahertz source capable of generating high-power, high-quality, and high-efficiency terahertz waves at low cost and operating at room temperature is a major problem, limiting its application in many fields. The advantage of this method is that the generated terahertz radiation source has high coherence and directionality; however, the power and efficiency of the terahertz waves produced at this stage are relatively low. Summary of the Invention

[0014] The purpose of this invention is to provide an optimized cascaded difference frequency terahertz wave radiation source that can enhance the intensity of terahertz waves and improve the energy conversion efficiency of terahertz waves.

[0015] The object of the present invention is achieved in the following manner: an optimized cascaded difference frequency terahertz wave radiation source, comprising a pump source, an AFB-KTP crystal, an APPLN crystal, a polarizer, a parabolic mirror, a first mirror, a second mirror, a third mirror, and a fourth mirror constituting a resonant cavity;

[0016] Pump light emitted from the pump source is incident on the AFB-KTP crystal through the first reflector to generate the first mixed light. The first mixed light is split into two beams by a polarizer, namely the second mixed light and the third mixed light. The third mixed light is reflected by the second reflector and then by the third reflector before being incident on the APPLN crystal to generate terahertz waves and cascaded light waves. The terahertz waves are reflected by the parabolic mirror and output. The cascaded light waves are reflected by the fourth reflector back to the first reflector, and the anti-Stokes light waves in them are transmitted out through the first reflector. The cascaded light waves are incident on the AFB-KTP crystal in a direction parallel to the first mixed light. After passing through the polarizer, and then being reflected by the second and third reflectors, they are incident on the APPLN crystal again in a direction parallel to the third mixed light, completing the cycle in the resonant cavity.

[0017] Pump light incident on the AFB-KTP crystal generates a pair of signal lights. and a pair of idle frequency lights ; the first mixed light contains pump light, signal light , idler light and cascade light wave; the second mixed light contains pump light and idler light; the third mixed light contains signal light and cascade light wave;

[0018] The plane of light beam propagation is the plane determined by X axis and Y axis, and Z axis is perpendicular to the plane of light beam propagation; the initial propagation direction of pump light emitted from the pump source is the positive direction of X axis, the propagation direction of the first mixed light is the positive direction of X axis, the propagation direction of the third mixed light out of the polarizer is the positive direction of X axis, the propagation direction of the second mixed light is the negative direction of Y axis, and the propagation direction of the terahertz wave is the positive direction of Y axis.

[0019] The pump source adopts a pulse laser, and the polarization directions of the pump light and the idler light are parallel to Y axis, and the polarization direction of the signal light is parallel to Z axis.

[0020] The direction of the third mixed light incident on the APPLN crystal is the positive direction of X axis.

[0021] The third mixed light incident on the APPLN crystal generates cascade light wave and terahertz wave through cascade optical difference frequency effect; the polarization direction of the cascade light wave is parallel to Z axis.

[0022] The first mirror, the second mirror, the third mirror and the fourth mirror are all concave lenses; the first mirror and the second mirror are high-transmissive to pump light and anti-Stokes light wave, with a transmittance of 0.99, and are high-reflective to the third mixed light, with a reflectivity of 0.99; the third mirror and the fourth mirror are high-reflective to the third mixed light and the cascade light wave, with a reflectivity of 0.99.

[0023] The polarizer does not change the propagation directions of the signal light and the cascade light wave, so that the second mixed light is emitted along the negative direction of Y axis.

[0024] The parabolic mirror is provided with a small hole in the center, which allows only the cascade light wave and the signal light to pass through.

[0025] The AFB-KTP crystal and the APPLN crystal are both cuboids, rectangular in the X-Y plane, and the length direction of the crystal is consistent with the positive direction of X axis; the crystal optical axis of the APPLN crystal is parallel to Z axis; the AFB-KTP crystal is a binder-free KTP crystal.

[0026] The polarization period distribution of the APPLN crystal satisfies the first-order Stokes cascade difference frequency to high-order, the high-order refers to any order in the range of greater than the first order and less than { (the frequency of the pump light - 60 THz) / the frequency of the terahertz wave} order, the phase mismatch of the Stokes cascade difference frequency along the crystal length is equal to 0 step by step; the frequency difference of the adjacent order light waves of the cascade light waves, the frequency difference of the signal light and the frequency of the terahertz wave are equal.

[0027] Compared with the prior art, the cascade difference frequency terahertz wave radiation source provided by the application has the following advantages:

[0028] (1) Compared with the traditional cascade difference frequency which needs two pump sources, the scheme only needs one pump source, and the polarization period of the AFB-KTP crystal determines the wavelength of the generated signal light and the idler light , and through the setting of the polarization period of the AFB-KTP crystal, the purpose of tuning different frequency THz waves is achieved.

[0029] (2) Through the setting of the non-polarization period of the APPLN crystal, the signal light photon is repeatedly transferred to the high-order Stokes photon, and the terahertz wave conversion efficiency is improved. BRIEF DESCRIPTION OF DRAWINGS

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

[0031] Figure 2 is the wavelength output characteristic of the signal light and the idler light with the change of the KTP polarization period when the pump light wavelength is 532 nm.

[0032] Figure 3 is the change of the polarization period of the APPLN crystal with the change of the crystal length.

[0033] Figure 4 is the change of the terahertz wave intensity with the change of the APPLN crystal length. DETAILED DESCRIPTION

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

[0035] As shown in the accompanying Figure 1As shown, an optimized cascade difference frequency terahertz wave radiation source comprises a pump source 1, an AFB-KTP crystal 7, an APPLN crystal 14, a polarizer 9, a parabolic mirror 15, a first mirror 3, a second mirror 4, a third mirror 5 and a fourth mirror 6.

[0036] The pump light 2 emitted from the pump source 1 is incident into the AFB-KTP crystal 7 through the first mirror 3 to generate the first mixed frequency light 8, which is divided into the second mixed frequency light 10 and the third mixed frequency light 11 through the polarizer 9. The third mixed frequency light 11 is reflected by the second mirror 4 and then by the third mirror 5 to be incident into the APPLN crystal 14 to generate the terahertz wave 16 and the cascade light wave 17, which is reflected by the parabolic mirror 15 to be output. The cascade light wave 17 is reflected by the fourth mirror 6 to the first mirror 3, and the anti-Stokes light wave 13 in the cascade light wave 17 is transmitted through the first mirror 3. The cascade light wave 17 is incident into the AFB-KTP crystal 7 in a direction parallel to the first mixed frequency light 8, and then is reflected by the second mirror 4 and the third mirror 5 to be incident into the APPLN crystal 14 in a direction parallel to the third mixed frequency light 11, thereby completing the circulation in the resonant cavity.

[0037] The plane of the light beam propagation is 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 pump light 2 emitted from the pump source 1 is the positive direction of the X-axis, the propagation direction of the first mixed frequency light 8 is the positive direction of the X-axis, the propagation direction of the third mixed frequency light 11 from the polarizer 9 is the positive direction of the X-axis, the propagation direction of the second mixed frequency light 10 is the negative direction of the Y-axis, and the propagation direction of the terahertz wave 16 is the positive direction of the Y-axis.

[0038] In this embodiment, the pump source 1 is a pulse laser, the wavelength is 532 nm, the power density is 1000 MW / cm 2 , the repetition frequency is 10 Hz, the beam diameter is 1 mm, and the pulse width is 10 ns. The pump light 2 is incident into the AFB-KTP crystal 7 to generate a pair of signal light (Signal 2) and a pair of idler light (Idler 2). The polarization directions of the pump light 2 and the idler light are parallel to the Y-axis, and the polarization direction of the signal light is parallel to the Z-axis. When the wavelength of the pump light is 532 nm, the wavelength output characteristics of the signal light and the idler light varying with the polarization period of the KTP are as shown in Figure 2 .

[0039] In this embodiment, the first mixed frequency light 8 comprises the pump light 2, the signal light , the idler light and the cascade light wave 17. The second mixed frequency light 10 comprises the pump light 2 and the idler light . The third mixed frequency light 11 comprises the signal light and the cascade light wave 17.

[0040] In this embodiment, the third mixing light 11 incident on the APPLN crystal 14 generates the cascade light wave 17 and the terahertz wave 16 through the cascade optical difference frequency effect, and the polarization direction of the cascade light wave 17 is parallel to the Z axis. Further, in this embodiment, the direction of the third mixing light 11 incident on the APPLN crystal 14 is the positive direction of the X axis.

[0041] In this embodiment, the first mirror 3, the second mirror 4, the third mirror 5, and the fourth mirror 6 are all concave lenses. The first mirror 3 and the second mirror 4 are highly transmissive to the pump light 2 and the anti-Stokes light wave 13, with a transmittance of 0.99, and are highly reflective to the third mixing light 11, with a reflectance of 0.99. The third mirror 5 and the fourth mirror 6 are highly reflective to the third mixing light 11 and the cascade light wave 17, with a reflectance of 0.99.

[0042] In this embodiment, the polarizer 9 does not change the propagation direction of the signal light and the cascade light wave 17, so that the second mixing light 10 is emitted along the negative direction of the Y axis.

[0043] In this embodiment, the parabolic mirror 15 has a small hole in the center that only allows the cascade light wave 17 and the signal light to pass through, and the diameter of the hole is 0.1 mm.

[0044] The polarization period of the AFB-KTP crystal 7 determines the wavelength of the generated signal light and the idler light; the polarization period distribution of the APPLN crystal 14 satisfies the cascade difference frequency from the first-order Stokes to high-order, where high-order refers to any order within the range of greater than the first order and less than the order of { (the frequency of the pump light 2 - 60 THz) / the frequency of the terahertz wave 16}, and the phase mismatch of the Stokes cascade difference frequency along the crystal length is equal to 0 at each order; the frequency difference between adjacent order light waves of the cascade light wave 17, the frequency difference between the signal light and the idler light, and the frequency of the terahertz wave 16 are equal.

[0045] Specifically, in this embodiment, the AFB-KTP crystal 7 and the APPLN crystal 14 are both cuboids, rectangular in the X-Y plane, and the length direction of the crystal is consistent with the positive direction of the X axis. The AFB-KTP crystal 7 is a KTP crystal without adhesive bonding. The size of the AFB-KTP crystal 7 is X × Y × Z = 50 mm × 3 mm × 5 mm. The size of the APPLN crystal 14 is X × Y × Z = 5 mm × 2 mm × 2 mm, and the optical axis is parallel to the Z axis.

[0046] In this embodiment, when the wavelength of the pump light is 532 nm, the wavelength output characteristics of the signal light and the idler light with the KTP polarization period are as shown in Figure 2As shown, when the polarization period of the AFB-KTP crystal 7 is 6087 μm, the wavelengths of the idler light are 1.0913 μm and 1.0874 μm respectively, and the wavelengths of the signal light are 1.0415 μm and 1.038 μm respectively, the intensity of the signal light generated after the pump light 2 is incident on the AFB-KTP crystal is calculated to be 151 MW / cm 2 and 132 MW / cm 2 respectively.

[0047] In this embodiment, the polarization period of the APPLN crystal varies with the length of the crystal as shown in Figure 3 . The intensity of the terahertz wave varies with the length of the crystal as shown in Figure 4 . The polarization period distribution of the APPLN crystal 14 satisfies that the phase mismatch along the crystal length for the difference frequency from the 1st Stokes cascade to the 200th Stokes cascade is equal to 0 step by step. The frequency difference between adjacent order light waves of the cascade light wave 17, the frequency difference of the signal light , and the frequency of the terahertz wave 16 are all equal to 0.1 THz. Since the length of the APPLN crystal is 5 mm, the intensity of the generated terahertz wave (the intensity of the terahertz wave refers to the peak intensity in the Figure 4 ) is 2.82 MW / cm 2 , and the conversion efficiency is 1.1%.

[0048] The above only describes the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and it should be noted that, for those skilled in the art and any person skilled in the technical field, under the premise of not departing from the overall concept of the present application, according to the technical solution and the inventive concept of the present application, some equivalent replacements or changes, and several changes and improvements are made, which should also be considered as the protection scope of the present application.

Claims

1. An optimized cascaded difference frequency terahertz wave radiation source, characterized in that: The resonant cavity comprises a pump source (1), an AFB-KTP crystal (7), an APPLN crystal (14), a polarizer (9), a parabolic mirror (15), a first mirror (3), a second mirror (4), a third mirror (5) and a fourth mirror (6). The pump light (2) emitted from the pump source (1) is incident into the AFB-KTP crystal (7) through the first mirror (3) to generate the first mixed light (8), the first mixed light (8) is divided into two beams, the second mixed light (10) and the third mixed light (11) through the polarizer (9); the third mixed light (11) is reflected by the second mirror (4) and then reflected by the third mirror (5) to be incident into the APPLN crystal (14) to generate the terahertz wave (16) and the cascade light wave (17), the terahertz wave (16) is reflected by the parabolic mirror (15) and output; the cascade light wave (17) is reflected by the fourth mirror (6) to the first mirror (3), the anti-Stokes light wave (13) in the cascade light wave (17) is transmitted through the first mirror (3); the cascade light wave (17) is incident into the AFB-KTP crystal (7) in a direction parallel to the first mixed light (8), passes through the polarizer (9), is reflected by the second mirror (4) and the third mirror (5) and then is incident into the APPLN crystal (14) in a direction parallel to the third mixed light (11), and the circulation is completed in the resonant cavity. Pump light (2) is incident on AFB-KTP crystal (7) to generate a pair of signal lights and a pair of idler lights ; the first mixed light (8) comprises pump light (2), signal light , idler light and cascade light wave (17); the second mixed light (10) comprises pump light (2) and idler light ; The third mixed light (11) contains signal light and a cascade light wave (17); 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 pump light (2) emitted from the pump source (1) is the positive direction of the X-axis, the propagation direction of the first mixed light (8) is the positive direction of the X-axis, the propagation direction of the third mixed light (11) from the polarizer (9) is the positive direction of the X-axis, the propagation direction of the second mixed light (10) is the negative direction of the Y-axis, and the propagation direction of the terahertz wave (16) is the positive direction of the Y-axis.

2. The optimized difference frequency terahertz wave radiation source according to claim 1, characterized in that: The pump source (1) adopts a pulse laser, the polarization direction of pump light (2) and idler light is parallel to Y axis, and the polarization direction of signal light is parallel to Z axis.

3. The optimized difference frequency terahertz wave radiation source according to claim 1, wherein: The direction of the third mixed light (11) incident into the APPLN crystal (14) is the positive direction of the X-axis.

4. The optimized difference frequency terahertz wave radiation source according to claim 1, wherein: The third mixed light (11) incident into the APPLN crystal (14) generates the cascade light wave (17) and the terahertz wave (16) through the cascade optical difference frequency effect, and the polarization direction of the cascade light wave (17) is parallel to the Z-axis.

5. The optimized difference frequency terahertz wave radiation source according to claim 1, wherein: The first mirror (3), the second mirror (4), the third mirror (5) and the fourth mirror (6) are all concave lenses, the first mirror (3) and the second mirror (4) have high transmittance for the pump light (2) and the anti-Stokes light wave (13) with a transmittance of 0.99 and high reflectivity for the third mixed light (11) with a reflectivity of 0.99, and the third mirror (5) and the fourth mirror (6) have high reflectivity for the third mixed light (11) and the cascade light wave (17) with a reflectivity of 0.

99.

6. The optimized cascaded difference frequency terahertz wave radiation source according to claim 1, wherein: The polarizer (9) does not change the signal light and the propagation direction of the cascade light wave (17), so that the second mixed light (10) is emitted in the negative direction of the Y axis.

7. The optimized cascaded difference frequency terahertz wave radiation source according to claim 1, wherein: The parabolic mirror (15) is centrally provided with a small hole only allowing the cascade light wave (17) and the signal light to pass through.

8. The optimized cascaded difference frequency terahertz wave radiation source according to claim 1, wherein: The AFB-KTP crystal (7) and the APPLN crystal (14) 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 crystal optical axis of the APPLN crystal (14) is parallel to the Z-axis, and the AFB-KTP crystal (7) is a KTP crystal without adhesive bonding.

9. The optimized cascaded difference frequency terahertz wave radiation source according to claim 1, characterized in that: The polarization period distribution of the APPLN crystal (14) satisfies that from the first-order Stokes cascade difference frequency to a high order, the high order refers to any order in a range greater than the first order and less than { (frequency of pump light - 60 THz) / frequency of terahertz wave} order, and the phase mismatch of the Stokes cascade difference frequency along the crystal length is equal to 0 step by step; the frequency difference of adjacent order light waves of the cascade light wave (17), the frequency difference of the signal light , and the terahertz wave frequency (16) are equal.

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