CN104269720B - A kind of terahertz emission source based on inner chamber optical parameter and beat effect - Google Patents

Abstract

The present invention relates to a kind of terahertz emission source based on inner chamber optical parameter and beat effect, including pump light source, the first ktp crystal 9, the second ktp crystal 10, periodical poled crystal 12.The optical parametric oscillator that pumping optical pumping is made up of double ktp crystals, produces two beam difference frequency lights, two beam difference frequency lights incidence periodical poled crystal 12 to produce THz wave based on optical difference frequency effect in difference frequency optical cavity.Two beam difference frequency lights can occur cascaded optical parametric effect generation THz wave respectively as pump light difference Energizing cycle polarized crystal 12 in difference frequency optical cavity again.During the generation of whole THz wave, difference frequency light can be with Energizing cycle polarized crystal 12 through cascaded optical parametric effect generation THz wave, and a pump photon can produce multiple Terahertz photons, effectively improve THz wave output energy and optical conversion efficiencies.

Description

A Terahertz Radiation Source Based on Cavity Optical Parameters and Difference Frequency Effect

technical field

The invention relates to a terahertz radiation source based on an inner cavity optical parameter and a difference frequency effect, and belongs to the application field of terahertz wave technology.

Background technique

Terahertz (Terahertz, THz for short, 1 THz=10 ¹² Hz) wave refers to an electromagnetic wave with a frequency in the range of 0.1-10 THz. The frequency range of THz waves is in the transition region from electronics to photonics. In the long-wave direction, it overlaps with millimeter waves; in the short-wave direction, it overlaps with infrared rays. In terms of frequency, THz waves are in the transition zone from macroscopic classical theory to microscopic quantum theory. Due to its special location, THz waves have broad application prospects: (1) THz photons have lower energy, which is 10 ⁷ -10 ⁸ times weaker than X-ray photons, and will not cause photodamage and actinization in biological tissues Ionization; (2) The vibration and rotation frequencies of many material macromolecules, such as biological macromolecules, are in the THz band, so they show strong absorption and resonance in the THz band; (3) THz waves are electromagnetic waves with quantum characteristics, which have Penetration ability similar to microwave, but also has directionality similar to light wave; (4) The typical pulse width of THz pulse is on the sub-picosecond level, which can treat various materials including liquids, semiconductors, superconductors, biological samples, etc. Perform sub-picosecond and femtosecond time-resolved transient spectroscopy studies.

One of the main technical bottlenecks currently limiting the rapid development of terahertz wave technology is the lack of coherent terahertz radiation sources with high power, tunable, narrow linewidth, and room temperature operation. The method of generating terahertz waves based on the optical parametric effect and the optical difference frequency effect has the characteristics of high power, high efficiency, tunability, narrow linewidth, and room temperature operation, but the current terahertz radiation source based on the optical parametric effect and the optical difference frequency effect The non-collinear phase matching method is mainly used, and the pump light, signal light and idler light are separated in space, which seriously limits the interaction of the three waves. The efficiency of the terahertz wave generated by the pump light is very low, and the pump light is not used efficiently. Pu light to generate terahertz waves. Moreover, nonlinear optical crystals have a large absorption of terahertz waves, and the terahertz waves generated in the crystals are severely absorbed, so the output terahertz wave energy is severely limited.

Contents of the invention

The purpose of the present invention is to provide a terahertz radiation source based on the optical parameters of the cavity and the difference frequency effect, so as to solve the problems of low terahertz wave output energy and optical conversion efficiency of the current terahertz radiation source.

In order to achieve the above object, the solution of the present invention includes a terahertz radiation source based on the optical parameters of the cavity and the difference frequency effect, including a pumping light source, and the pumping light source includes a first plane mirror 1, a KD ^* P crystal 2, Polarizer 3, Nd:YAG laser pumping module 4, second plane mirror 14. The terahertz radiation source also includes a first KTP crystal 9 and a second KTP crystal 10 for generating difference frequency light arranged in sequence on the pump light optical path between the Nd:YAG laser pump module 4 and the second plane mirror 14, The first periodically polarized crystal 12; the first KTP crystal 9 and the second KTP crystal 10 can rotate in opposite directions synchronously through the axis perpendicular to the optical path of the pumping light, and are close to the two ends of the first periodically polarized crystal 12 on the optical path of the pumping light A first parabolic mirror 11 and a second parabolic mirror 13 are respectively provided; the concave surfaces of the first parabolic mirror 11 and the second parabolic mirror 13 face the first periodically polarized crystal 12 .

Further, the first KTP crystal 9 and the second KTP crystal 10 are identical and arranged symmetrically.

Further, a harmonic mirror 8 for transmitting pump light is arranged on the pump light optical path between the Nd:YAG laser pump module 4 and the first KTP crystal 9 .

Further, the harmonic mirror 8 is also used to reflect the difference frequency light.

Further, a second periodically polarized crystal 6 is arranged on the pump light optical path between the Nd:YAG laser pumping module 4 and the harmonic mirror 8, and is close to the second periodically polarized crystal 6 on the pump light optical path. A third parabolic mirror 5 and a fourth parabolic mirror 7 are arranged at the end; the concave surfaces of the third parabolic mirror 5 and the fourth parabolic mirror 7 face the second periodically polarized crystal 6 .

Further, a small hole is opened at the intersection of each parabolic mirror and the optical path of the pumping light.

The purpose of the present invention is to provide a terahertz radiation source based on the optical parameters of the cavity and the difference frequency effect, which has the following advantages compared with the existing terahertz radiation sources:

(1) Using the fundamental frequency pump light to excite the periodically polarized crystal, through the cascade optical parametric effect in the cavity, collinearly generate terahertz waves, one pump photon can generate multiple terahertz photons, and improve the optical conversion efficiency of terahertz waves .

(2) Using the difference frequency light λ1 and λ2 to excite the periodically polarized crystal, the terahertz wave is generated collinearly through the optical difference frequency effect. One difference frequency photon λ1 can generate multiple terahertz photons, which effectively improves the optical conversion efficiency of the terahertz wave. The wavelength-tuned light waves λ1 and λ2 are realized by synchronously adjusting the azimuth angles of the dual KTP crystals, so that the wavelength-tuned output terahertz wave can be realized.

(3) The whole cascade parametric process adopts collinear phase matching, which improves the interaction volume of pump light, Stokes light and terahertz wave, and effectively improves the optical conversion efficiency of terahertz wave. The cascade parametric process all adopts double resonance in the cavity, which can obtain terahertz waves propagating forward and backward, effectively improving the utilization efficiency of difference frequency light, and further improving the output energy and conversion efficiency of terahertz waves.

(4) The present invention has simple structure and low cost. The whole set of devices only needs a self-made pump source, two ordinary KTP crystals, a periodically polarized crystal and some optical lenses.

Description of drawings

Fig. 1 is a schematic structural view of Embodiment 1 of the present invention;

Fig. 2 is a schematic structural diagram of a second embodiment of the present invention;

Fig. 3 is a schematic diagram of the relationship between λ ₁ , the length of the polarization period of the periodically poled crystal 12, and the frequency of the terahertz wave;

FIG. 4 is a schematic diagram of the relationship between the frequency of the terahertz wave and the length of the polarization period of the periodically poled crystal 6 .

detailed description

The present invention will be described in further detail below in conjunction with the accompanying drawings.

A terahertz radiation source based on intracavity optical parameters and difference frequency effects, including a pumping light source, the pumping light source includes a first plane mirror 1, a KD ^* P crystal 2, a polarizer 3, and a Nd:YAG laser pumped in sequence Module 4 , second plane mirror 14 . The terahertz radiation source also includes a first KTP crystal 9, a second KTP crystal 10, and a first periodically polarized crystal 12 arranged sequentially on the pump light optical path between the Nd:YAG laser pump module 4 and the second plane mirror 14. ; The first KTP crystal 9 and the second KTP crystal 10 are used to generate difference frequency light, and the first KTP crystal 9 and the second KTP crystal 10 can rotate in opposite directions synchronously through an axis perpendicular to the pumping light path, on the pumping light path A first parabolic mirror 11 and a second parabolic mirror 13 are arranged near both ends of the first periodically polarized crystal 12 ; the concave surfaces of the first parabolic mirror 11 and the second parabolic mirror 13 face the first periodically polarized crystal 12 .

Based on the above technical solutions and in conjunction with the accompanying drawings, the following specific implementation methods are given.

Implementation Mode 1

The technical solution of the present invention is to provide a terahertz radiation source based on the optical parameters of the cavity and the difference frequency effect, as shown in Fig ^. Sheet 3, Nd:YAG laser pumping module pumping module 4, plane mirror 14. A harmonic mirror 8 for transmitting pump light and reflecting difference frequency light, a KTP crystal 9, a KTP crystal 10, and a periodically polarized crystal are sequentially arranged on the pump light optical path after the pump module 4 of the Nd:YAG laser pump module. 12. A parabolic mirror 11 and a parabolic mirror 13 are arranged near both ends of the periodically polarized crystal 12 on the optical path of the pump light, and the concave surfaces of the parabolic mirror 11 and the parabolic mirror 13 face the periodically polarized crystal 12 . All the above components are arranged between the plane mirror 1 and the plane mirror 14 .

The azimuth of this embodiment is set as shown in Figure 1, and the pumping light source is made up of plane mirror 1,14, KD ^* P crystal 2, polarizer 3 and Nd:YAG laser pumping module pumping module 4, and single pulse energy is in 10- In the range of 1000mJ, the repetition frequency is in the range of 1-200Hz, and the pulse width is in the range of 1-100ns. This embodiment adopts an electro-optic Q-switched pulse Nd:YAG laser pump module with a single pulse energy of 150mJ, a wavelength of 1064nm, a pulse width of 10ns, a repetition frequency of 10Hz, a diameter of pump light of 2mm, and a polarization direction along the Z-axis.

The 1064nm pump light generated by the pump light source enters two identical KTP crystals 9 and 10 through the harmonic mirror 8, and adopts a type II phase matching method. The pump light excites the KTP crystal to obtain two beams of difference frequency light λ ₁ and λ ₂ near the near degeneracy point. The KTP crystal 9 and the KTP crystal 10 are symmetrically placed, that is, the KTP crystal 9 rotates 180° along the Z axis to obtain the orientation of the KTP crystal 10, so that placement can eliminate the walk-off of the difference frequency light λ ₁ and λ ₂ in the KTP crystal. KTP crystals 9 and 10 rotate in opposite directions synchronously through the axis perpendicular to the pump light path, and adjust the azimuth angles of KTP crystals 9 and 10 synchronously Wavelength tunable difference frequency light λ ₁ and λ ₂ can be obtained, the wavelength range of λ ₁ is 1.82-2.128 μm, and the wavelength range of λ ₂ is 2.128-2.56 μm. The difference frequency light λ ₁ and λ ₂ oscillate and amplify in the resonant cavity composed of mirrors 8 and 14, excite the periodically polarized crystal 12, and generate terahertz waves through the optical difference frequency effect. Oscillation, so backward and forward propagating terahertz waves will be generated in the periodically polarized crystal 12, which are coupled out through the parabolic mirrors 11 and 13 respectively. _In the process of generating terahertz waves by the difference frequency light λ1 and _λ2 through the optical difference frequency effect, the energy of the light wave λ1 is reduced, and the energy of the light wave _λ2 is amplified, because _{one λ1} photon, one _λ2 photon, and one terahertz wave The single-photon energy relationship among the three photons is that the energy of a λ ₁ photon is equal to the sum of the energy of a λ ₂ photon and the energy of a terahertz wave photon, that is, the consumption of a λ ₁ photon will produce a λ ₂ photon and a terahertz wave photon. Hertzian photons. The energy-amplified light wave _λ2 continues to excite the periodically polarized crystal 12, and the terahertz wave and the first-order Stokes light propagating forward and backward are generated through the optical parametric effect, and the first-order Stokes light can be generated forward and backward propagating through the second-order optical parametric effect. Terahertz wave and second-order Stokes light, so the cascading process will continue forever. One λ ₁ photon can generate multiple terahertz photons, and the generated terahertz waves are all coupled out by parabolic mirrors 11 and 13, effectively improving the terahertz wave Energy and optical conversion efficiency. During this whole process, the wavelength-tuned light waves λ ₁ and λ ₂ can be realized by synchronously adjusting the azimuth angles of the KTP crystal 9 and the KTP crystal 10 , so that the wavelength-tunable output terahertz wave can be realized.

As shown in FIG. ₃ , when the wavelength of λ1 is in the range of 2.05-2.125 μm and the polarization period of the periodically poled crystal 12 is in the range of 8.1-2527 μm, terahertz waves in the range of 0.4-10.7 THz can be obtained. In this embodiment, the wavelength of λ1 is 2113nm, and the wavelength of _λ2 is _2143.2nm . Correspondingly, the polarization period of the periodically poled crystal 12 is 487 μm, which can generate terahertz waves with a frequency of 2THz.

Implementation Mode Two

As shown in Figure 2, a periodically polarized crystal 6 is also arranged between the pump module 4 of the Nd:YAG laser pump module and the harmonic mirror 8, and is arranged near the two ends of the periodically polarized crystal 6 on the optical path of the pump light There are a parabolic mirror 5 and a parabolic mirror 7 , and the concave surfaces of the parabolic mirror 5 and the parabolic mirror 7 face the periodically polarized crystal 6 . The configuration of the rest of the terahertz radiation source in this embodiment is the same as that in the first embodiment.

The 1064nm pump light emitted by the pump light source excites the periodically polarized crystal 6 to generate terahertz waves and first-order Stokes light through optical parametric oscillation. Since the wavelengths of the first-order Stokes light and the pump light are very close, the first-order Stokes light generated from the periodically polarized crystal 6, like the pump light, will oscillate in the resonant cavity formed by the mirrors 1 and 14, so the optical parameter process is The inner cavity is collinearly double-resonant, and terahertz waves propagating backward and forward can be generated in the crystal 6, which are coupled out through the parabolic mirrors 5 and 7, respectively. The first-order Stokes light continues to excite the periodically polarized crystal 6, through the second-order optical parametric effect, the terahertz wave propagating forward and backward and the second-order Stokes light can be generated, and the generated terahertz wave is coupled out by the parabolic mirrors 5 and 7. The second-order Stokes light is very close to the wavelength of the first-order Stokes light, and it can also oscillate in the resonant cavity. Continue to excite the periodically polarized crystal 6 to generate terahertz waves and third-order Stokes light propagating forward and backward. Similarly, the third-order Stokes light also Higher-order Stokes light can be generated through the optical parametric effect, and the parametric process will continue forever. In this way, through the cascaded optical parametric effect, one pump photon can generate multiple terahertz photons, effectively improving the terahertz output energy and optical conversion efficiency.

Figure 4 simulates the relationship between the frequency of the terahertz wave and the length of the period of the period of the periodic poled crystal 6 when the pump wavelength is 1064nm. From the figure, it can be concluded that the frequency tuning output can be obtained by changing the length of the period of the period of the period of the period of polarization of the crystal 6. of terahertz waves. The periodic poled crystal 6 has a polarization period of 526 μm and can generate terahertz waves with a frequency of 2 THz.

The remaining pumping light emitted after the reaction in the periodically polarized crystal 6 to generate terahertz waves is incident on two identical KTP crystals 9 and 10 through the harmonic mirror 8, and a type II phase matching method is adopted. The pump light excites the KTP crystal to obtain two beams of difference frequency light λ ₁ and λ ₂ near the near degeneracy point. The KTP crystal 9 and the KTP crystal 10 are symmetrically placed, that is, the KTP crystal 9 rotates 180° along the Z axis to obtain the orientation of the KTP crystal 10, so that placement can eliminate the walk-off of the difference frequency light λ ₁ and λ ₂ in the KTP crystal. KTP crystals 9 and 10 rotate in opposite directions synchronously through the axis perpendicular to the pump light path, and adjust the azimuth angles of KTP crystals 9 and 10 synchronously Wavelength tunable difference frequency light λ ₁ and λ ₂ can be obtained, the wavelength range of λ ₁ is 1.82-2.128 μm, and the wavelength range of λ ₂ is 2.128-2.56 μm. The difference frequency light λ ₁ and λ ₂ oscillate and amplify in the resonant cavity composed of mirrors 8 and 14, excite the periodically polarized crystal 12, and generate terahertz waves through the optical difference frequency effect. Oscillation, so backward and forward propagating terahertz waves will be generated in the periodically polarized crystal 12, which are coupled out through the parabolic mirrors 11 and 13 respectively. _In the process of generating terahertz waves by the difference frequency light λ1 and _λ2 through the optical difference frequency effect, the energy of the light wave λ1 is reduced, and the energy of the light wave _λ2 is amplified, because _{one λ1} photon, one _λ2 photon, and one terahertz wave The single-photon energy relationship among the three photons is that the energy of a λ ₁ photon is equal to the sum of the energy of a λ ₂ photon and the energy of a terahertz wave photon, that is, the consumption of a λ ₁ photon will produce a λ ₂ photon and a terahertz wave photon. Hertzian photons. The energy-amplified light wave _λ2 continues to excite the periodically polarized crystal 12, and the terahertz wave and the first-order Stokes light propagating forward and backward are generated through the optical parametric effect, and the first-order Stokes light can be generated forward and backward propagating through the second-order optical parametric effect. Terahertz wave and second-order Stokes light, so the cascading process will continue forever. One λ ₁ photon can generate multiple terahertz photons, and the generated terahertz waves are all coupled out by parabolic mirrors 11 and 13, effectively improving the terahertz wave Energy and optical conversion efficiency. During this whole process, the wavelength-tuned light waves λ ₁ and λ ₂ can be realized by synchronously adjusting the azimuth angles of the KTP crystal 9 and the KTP crystal 10 , so that the wavelength-tunable output terahertz wave can be realized.

In this embodiment, terahertz waves can be generated simultaneously by periodically polarizing the crystals 6 and 12, which increases the power of the terahertz waves and improves the utilization rate of pump light and the optical conversion efficiency.

The periodically polarized crystal used in the above two embodiments is a PPGaP crystal, which is formed by stacking five GaP wafers. Adjacent GaP wafers form a second-order nonlinear optical constant array with positive and negative periodic inversions. The thickness of the wafer is equal to the coherence of the crystal. The length is half of the crystal polarization period. The wafer thickness of the PPGaP crystal 6, that is, half of the polarization period, is 263 μm, and the wafer size is 263 μm (X)×20 mm (Y)×20 mm (Z). The wafer thickness of PPGaP crystal 12, which is half of the polarization period, is 243.5 μm, and the wafer size is 243.5 μm (X) × 20 mm (Y) × 20 mm (Z). The direction is the X-axis direction of propagation.

The size of the KTP crystals 9 and 10 used is 20mm (X axis) × 8mm (Y axis) × 8mm (Z axis), the cutting angle θ is equal to 49.5°, equal to 0°.

The plane mirrors 1 and 14 used are highly reflective to 1064nm light, the harmonic mirror 8 is highly transparent to 1064nm light, and the harmonic mirror 8 and plane mirror 14 are highly reflective to 1800-2500nm light.

The parabolic mirrors 5 and 7 used are placed on both sides of the crystal 6, and a small hole is opened in the center of the parabolic mirrors 5 and 7. The diameter of the small hole is 2 mm, and only the pump light can pass through. The parabolic mirrors 11 and 13 are placed on both sides of the crystal 12, and a small hole is opened in the center of the parabolic mirrors 11 and 13. The diameter of the small hole is 2mm, and only the pump light and the two beams of difference frequency light can pass through.

Specific embodiments have been given above, but the present invention is not limited to the described embodiments. The basic idea of the present invention lies in the above-mentioned basic scheme. For those of ordinary skill in the art, according to the teaching of the present invention, it does not need to spend creative labor to design various deformation models, formulas, and parameters. Changes, modifications, substitutions and variations to the implementation without departing from the principle and spirit of the present invention still fall within the protection scope of the present invention.

Claims (4)

1. a kind of terahertz emission source based on inner chamber optical parameter and beat effect, including pump light source, pump light source includes The first level crossing (1), the KD for setting gradually^*P crystal (2), polarizer (3), Nd:It is YAG laser pump module (4), second flat Face mirror (14)；Characterized in that, the terahertz emission source is additionally included in Nd:YAG laser pump module (4) and the second plane The first ktp crystal (9) and the second ktp crystal for producing difference frequency light set gradually in pump light light path between mirror (14) (10), period 1 polarized crystal (12)；First ktp crystal (9) and the second ktp crystal (10) can by with pump light The synchronous opposite rotation of the vertical axis of light path, sets respectively in pump light light path at period 1 polarized crystal (12) two ends It is equipped with the first paraboloidal mirror (11) and the second paraboloidal mirror (13)；First paraboloidal mirror (11) and the second paraboloidal mirror (13) Concave surface towards period 1 polarized crystal (12)；In Nd:Between YAG laser pump module (4) and the first ktp crystal (9) Pump light light path on be provided with one for transmiting the harmonic wave mirror (8) of pump light；In Nd:YAG laser pump module (4) with Second round polarized crystal (6) is provided with pump light light path between harmonic wave mirror (8), near second week in pump light light path The 3rd paraboloidal mirror (5) and the 4th paraboloidal mirror (7) are provided with phase polarized crystal (6) two ends；3rd paraboloidal mirror (5) With the concave surface of the 4th paraboloidal mirror (7) towards second round polarized crystal (6).

2. the terahertz emission source based on inner chamber optical parameter and beat effect according to claim 1, it is characterised in that First ktp crystal (9) is identical with the second ktp crystal (10) and is symmetrical arranged.

3. the terahertz emission source based on inner chamber optical parameter and beat effect according to claim 1, it is characterised in that The harmonic wave mirror (8) is additionally operable to reflect the difference frequency light.

4. the terahertz emission source based on inner chamber optical parameter and beat effect according to claim 1, it is characterised in that Each paraboloidal mirror opens an aperture with the point of intersection of pump light light path.