JP4402901B2 - Terahertz wave generator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a coherent terahertz wave generator.
[0002]
[Prior art]
A terahertz wave generator using a polariton mode in a dielectric LiNbO ₃ crystal is known to be used as a spectral light source using a wavelength-tunable terahertz wave for identifying large molecules and macromolecules such as biological materials and cancer cells. It has been. That is, one or two pump lights (the latter may be called signal light or idler light) are incident on the LiNbO ₃ crystal, and a terahertz wave is generated by difference frequency generation or parametric oscillation. The frequency of the terahertz wave thus obtained is coherent light in the range of about 0.7 THz to 2.5 THz. However, the frequency range is too narrow as a spectrum variable range in order to identify various biological materials using the difference in terahertz wave spectrum. By measuring up to higher frequencies, the difference in the spectral pattern becomes clear and the molecule can be identified.
[0003]
[Problems to be solved by the invention]
The absorption coefficient in the terahertz wave band of the LiNbO ₃ crystal becomes extremely large when it exceeds 3 THz, so that the output is remarkably lowered, which is one factor that limits the frequency band. Further, since the degree of phase mismatch in the parallel direction of the two pump light beams is large, an angle matching method in which the angle between the pump light and the extraction angle of the terahertz wave is increased as shown in FIG. The overlap between the terahertz wave and the pump light beam is poor, which causes a decrease in both frequency band and efficiency.
[0004]
On the other hand, in the parametric oscillation type LiNbO ₃ terahertz generator, the line width of the generated terahertz wave can be narrowed to about 100 MHz by performing injection seeding with a continuous wave (CW) laser diode. On the contrary, because of the mode hopping of the diode, it is difficult to continuously sweep the frequency over a wide range. On the other hand, if injection seeding is not performed, the line width is extremely large and exceeds 100 GHz (0.1 THz), so the resolution is remarkably lowered and the performance of substance identification is not sufficient in combination with the narrow frequency band.
[0005]
An object of the present invention is to provide a frequency variable terahertz generator that overcomes the above-mentioned drawbacks and has a wide frequency band, high output, high resolution, and no mode hop.
[0006]
[Means for Solving the Problems]
As a typical example of the present invention, GaP is used as a crystal that generates a terahertz wave (THz wave). We have shown that GaP has a higher Raman efficiency than LiNbO ₃ . Furthermore, since the pure vibration frequency of the transverse wave constituting the phonon polariton is as extremely high as 11 THz, a polariton mode in a wide frequency band can be obtained. Furthermore, it is already known that the terahertz band absorption coefficient is smaller than that of LiNbO ₃ crystal. In the range where the wavelengths of the two pump lights are smaller than about 1.0 μm, the phase matching relationship between the difference frequency light and the nonlinear polarization can be obtained in the parallel arrangement (collinear). However, in the present invention, contrary to the wavelength range that satisfies the collinear phase matching condition, the wavelengths of the two pump lights are made slightly larger than 1.0 μm. As a result, phase matching cannot be obtained in the parallel direction, but angular phase matching can be obtained by providing a minute angle between the pump lights. Angular phase matching is also performed in the LiNbO ₃ crystal, but a large angle must be given, and the overlapping of pump light and terahertz light is poor and the efficiency is low. In GaP, if the pump light has a wavelength slightly longer than 1.0 μm, the phase matching angle is very small, so that such disadvantages can be overcome and the effect of expanding the frequency band is great. In order to sweep the terahertz wave frequency, the rotation angle of the mirror or beam splitter close to the GaP crystal is slightly changed, and at the same time, the frequency of the second pump light source is swept. Precise control of minute change in angle can be easily achieved by a rotating stage, so that terahertz frequency can be precisely controlled.
[0007]
As a first method for operating the terahertz wave with a narrow line width and without mode hops over a wide frequency, a YAG laser having a wavelength of 1.064 μm is used as the first pump light source, and the second pump light source, that is, the wavelength variable. An optical parametric oscillator (OPO) equipped with an injection seeding device is used as a pump light source. Such an OPO avoids wavelength degeneration that causes the line width to be increased by being excited by the third harmonic of the YAG laser, that is, the wavelength of 355 nm, and further includes a master oscillator in the OPO, which is used for injection seeding. Since the line width of the OPO can be narrowed by performing the above, the line width of the THz wave generated as the difference frequency is similarly narrowed. A YAG laser having a narrow line width subjected to injection seeding is also used.
[0008]
As a second method, a Cr: FORSTERITE (Cr-added olivine) laser is used as a pump light source. Since this laser uses the Cr level, the line width is extremely narrow compared to OPO without injection seeding. It is excited using a YAG laser having a wavelength of 1.064 μm, and does not use the third harmonic of YAG unlike OPO, so that the efficiency is high. The wavelength variable range of Cr: FORSTERITE is a range from 1.15 μm to 1.35 μm. Two Cr: FORSTERITE lasers are used as pump light sources, one is a fixed wavelength and the other is a tunable pump light source, and the line width of the terahertz wave as a difference frequency can be reduced without injection seeding.
[0009]
[Action]
The principle of phonon polariton minute angle matching in the case where the crystal generating the terahertz wave is GaP will be described with reference to FIG. k _L , k _S , and q are wave vector vectors of phonon polariton that are sources of the first and second pump lights and the terahertz wave, and v _L , v _S , and v are respective frequencies. In GaP, phase matching (Δq = 0) is obtained in the parallel direction when the wavelength of the pump light is 1.0 μm or less, but phase mismatch Δq is generated at wavelengths longer than 1.0 μm. When the angle θ _in formed by the two pump lights in the crystal is sufficiently small, the following equation is obtained from the geometric relationship of the wave vector in the figure.
[0010]
[Expression 1]
θ _in = (2Δq / q) ^1/2 (n _I / n _L ) (v / v _L )
[0011]
However, Δq is the magnitude of the mismatch of wave numbers generated because the phase matching cannot be obtained in the parallel direction because the frequency of the pump light is longer than 1.0 μm, and Δq = k _L −k _S −q. The phase matching in the parallel direction is calculated to be a critical wavelength of about 1.0 μm, and it is known experimentally that it is close to this. When the pump light wavelength is YAG laser wavelength 1.064 μm, the wavelength is 6.4% longer than the limit wavelength, but the phase mismatch degree Δq / q is calculated to be about 3% from the dispersion relation of the THz wave. Therefore, from the equation (1), the angle θ _in is required to have a small value of 3.2 minutes at v = 1 THz.
On the other hand, the traveling direction theta _I of the THz wave the following equation is obtained from Fig.
[0012]
[Expression 2]
sin θ _I = k _s / q sin θ _in
Therefore, the following formula is obtained from the formulas [1] and [2].
[0013]
[Equation 3]
sin θ _I = (2Δq / q) ^1/2
[0014]
That is, since the angle θ _I is multiplied by k _s / q, it is considerably larger than the angle θ _in , but at 0.5 THz to 4 THz, θ _I is about 10 degrees to 14 degrees. Since Tan14 ° = 0.25, the overlap between the THz wave and the beam of pop light is approximately 80% or more as shown in FIG. 3, and the effective interaction distance is close to the crystal length, so that the high output has a wide frequency range. Can be obtained over.
[0015]
When the frequency of the terahertz wave is increased to 7 THz, the dispersion of the THz wave increases, so 2Δq / q increases. According to the calculation of the dispersion characteristic, Δq / q increases to 12% at 7 THz, but even in this case, Δq Is still in a small range. That is, θ _in = 46 minutes is calculated from the equation (1) and is 1 degree or less. Outside the crystal, it is about 142 minutes, that is, 2.4 degrees. At this time, the direction of the terahertz wave, θ _I is about 30 degrees from the equation (3). When the beam diameter is 3 mm and the crystal length is 5 mm as shown in FIG. 4, a beam overlap of about 50% or more can be obtained. For this reason, the efficiency reduction is not significant even at 7 THz. However, since θ _I exceeds the total reflection critical angle of 17 degrees in the crystal, when it is close to normal incidence as shown in FIG. 3, it cannot be extracted from the output surface due to total reflection. In this case, as shown in FIG. 4, if the incident angle α ^ext of the pump light is not 0 but is more than 13 degrees, the critical angle can be made 17 degrees or less.
[0016]
In order to make the output terahertz wave narrow with no mode hop, it is necessary to select an appropriate pump light source, which has already been described in the previous section.
[0017]
【Example】
Example 1
As shown in FIG. 4, a pulse YAG laser having a wavelength of 1.064 μm is used as the first pump light source 16. As the second pump light source, an injection seeded wavelength tunable parametric oscillator (OPO) 17 that is excited by 355 nm light obtained by multiplying the fundamental wave of the first pump light by three is used. If the output light wavelength of the OPO is selected in the range of 1.038 μm to 1.0635 μm, the difference frequency is in the range of 0.15 THz to 7 THz. Further, the wavelength of OPO may be selected in the range of 1.0646 μm to 1.091 μm. The first pump light and the second pump light have orthogonal polarization directions, and the beams are superposed in parallel by the polarization beam splitter 18. The beam splitter 18 or the mirror 18 ′ is slightly rotated, and the beam direction of the first or second pump light is adjusted so that the angle θ _in ^ext is extremely small according to the terahertz wave frequency v. The angle θ _in ^ext is an angle outside the crystal and has a relationship of approximately θ _in ^ext = n _L θ _in , where n _L is the refractive index at the pump light frequency, and n _L = 3.105. is there. For example, when v is 1 THz, θ _in ^ext is 10 minutes (3.2 minutes _{in the} crystal) outside the crystal.
[0018]
The optical axis direction of the GaP crystal 19 is 110 directions, and has an input surface and an output surface, and both the THz wave 22 and the pump lights 20 and 21 are extracted from the output surface. From 0.15 THz to 5 THz, θ _I is about 10-17 degrees, and even at 7 THz is as small as 30 degrees or less, so that the overlap between pump light and terahertz light is large, and long-distance interaction is performed. As a result, a terahertz pulse output of 100 mW or more can be obtained over a wide frequency range of 0.15 to 7 THz.
[0019]
Since the line width of the YAG laser that is the first pump light is sufficiently narrow, the line width of the terahertz wave that is the difference frequency is determined by the line width of the OPO. Since OPO is injection seeded, the line width is about 4 GHz, and the line width of the terahertz wave that is the difference frequency is also about 4 GHz. Since the spectral line width of a solid or liquid substance in the terahertz band is generally 50 GHz or more, this embodiment enables spectroscopic measurement with sufficiently high resolution.
[0020]
(Example 2)
As shown in FIG. 5, it is the GaP crystal 27 that generates the terahertz wave as in the first embodiment. As the first and second pump light sources 24 and 25, a Cr: FORSTERITE (Cr-added olivine) laser excited by the YAG laser 23 is used. Cr: FORSTERITE is a tunable laser that can obtain a wavelength in the range of 1.15 μm to 1.35 μm by exciting with a YAG laser with a wavelength of 1.064 μm. By using the Cr level in FORSTERITE, injection is performed. Line width values of 10 GHz to 15 GHz can be obtained without seeding. Further, since it is not necessary to use YAG harmonics unlike OPO, it can be excited by a fundamental wave of 1.064 mm, and is characterized by high output and low cost.
[0021]
The wavelength of the first pump light laser is fixed at 1.20 μm, for example. By installing an etalon inside the first pump light source, the line width can be reduced to about 1 GHz at a fixed wavelength. The second pump light source uses the same kind of Cr: FORSTERITE laser and sweeps the wavelength. Since the latter is a continuous wavelength sweep, no etalon is inserted, so the line width is 10 GHz to 15 GHz. As a result, the line width of the THz wave generated as the difference frequency can be approximately 10 GHz to 15 GHz. By changing the wavelength of the second pump light source from 1.2024 μm to 1.2346 μm, the frequency range where the terahertz wave is generated is between 0.5 THz and 7 THz as in the first embodiment. The output is the same as in the first embodiment.
[0022]
The Cr: FORSTERITE laser as the pump light can be excited with a fundamental wave without using the third harmonic of the YAG laser, and a relatively narrow line width can be obtained without injection seeding. Therefore, it is an advantage that it is extremely inexpensive. As a terahertz wave generating crystal, not only when performing phase matching by angle like GaP, but also by applying it to a crystal that performs collinear phase matching using crystal anisotropy, it is extremely inexpensive and can be used as a wavelength tunable terahertz wave. A generator can be obtained. Examples of such an optically anisotropic crystal include ZnGeP ₂ and GaSe.
[0023]
(Example 3)
In the first and second embodiments, when the THz wave frequency exceeds 6 THz, the shape of the GaP crystal 31 is approximately 30 degrees and a right angle triangle of 60 degrees as shown in FIG. 6, and the pump light 32, 33 is incident. For example, in the case of 7 THz, the θ _I of the THz wave is approximately 30 degrees, but the THz wave detection optical system is easily arranged as 34 from the output end face 36, and the crystal arrangement is 1 -2 degrees will not cause a problem. On the other hand, the pump light is totally reflected at the surface 36 but is emitted from the surface 37 to the outside. Further, unlike the first embodiment, the present embodiment can extract the output without totally reflecting the THz wave even when the frequency of the THz wave becomes 7 THz or more.
[0024]
【The invention's effect】
According to the present invention, a frequency variable THz wave single frequency coherent terahertz light source from 0.5 THz to 7 THz can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a conventional terahertz wave generation method.
FIG. 2 is a diagram showing directions of wave vectors of pump light and terahertz waves in the present invention.
FIG. 3 is a diagram showing an intra-crystal propagation optical path of pump light and terahertz waves in Example 1.
4 is a diagram illustrating a configuration of a terahertz wave generation device according to Embodiment 1. FIG.
FIG. 5 is a diagram illustrating a configuration of a terahertz wave generation device according to a second embodiment.
6 is a diagram showing the shape and beam direction of a GaP crystal in Example 3. FIG.
[Explanation of symbols]
1 ... LiNbO ₃ crystal 2 ... first pump light 3 ... second pump light (also called idler light or signal light)
4 ... terahertz wave 5 ... silicon prism 6 ... wave vector 7 of first pump light ... wave vector 8 of second pump light ... wave vector 9 of terahertz wave ... angle θin formed by pump light in crystal
10 ... Terahertz wave internal angle θ _I
DESCRIPTION OF SYMBOLS 11 ... GaP crystal 12 ... Optical path 13 of 1st pump light ... Optical path 14 of 2nd pump light ... Optical path 15 of terahertz wave ... Overlapping of terahertz wave and pump beam 16 ... YAG laser 17 ... Optical parametric oscillator (OPO) )
DESCRIPTION OF SYMBOLS 18 ... Polarizing beam splitter 18 '... Mira 19 ... GaP crystal 20 ... 1st pump light traveling direction 21 ... 2nd pump light traveling direction 22 ... Terahertz wave traveling direction 23 ... YAG laser 24 ... 1st Cr: FORSTERITE Laser 25 ... Second Cr: FORSTERITE laser 26 ... Polarizing beam splitter 26 '... Mira 27 ... GaP crystal 28 ... First pump light beam 29 ... Second pump light beam 30 ... Terahertz wave output 31 ... GaP crystal 32 ... First pump light 33 ... second pump light 34 ... terahertz wave 35 ... pump light incident surface 36 ... terahertz wave output surface 37 ... pump light exit surface 38 ... angle 30 degrees 39 ... angle 60 degrees

Claims (5)

A first pump light source that generates first pump light that is laser light having a wavelength longer than 1.0 μm ;
Generating a second pump light which is a laser light having a wavelength longer than 1.0 μm, and a second pump light source having a variable wavelength;
The cross-section is a right triangle with a minimum apex angle of 30 degrees, and the surface that is the longest side of the right triangle is the input surface, and the two beams of the first and second pump lights are incident on the input surface. Crystals,
The two beams is incident on the crystal at each other small angle, and means for exciting the phonon polariton tons in the crystal, by changing the frequency and the minute angle of the second pump light at the same time, the A terahertz wave generator characterized by generating a wavelength-tunable, high-power, broadband terahertz wave from the crystal in a range where the difference frequency between the frequencies of the first and second pump lights ranges from 0.15 THz to 7 THz. The terahertz wave generator according to claim 1, wherein the crystal is a GaP crystal. The first pump light source is a YAG laser having a wavelength of 1.064 μm;
The second pump source is excited by the third harmonic of the first pump light source is the optical parametric oscillator (OPO) which includes a injection seeding means
The terahertz wave generator according to claim 1 or 2, characterized in that. A first pump light source that generates first pump light that is laser light having a wavelength longer than 1.0 μm;
Generating a second pump light which is a laser light having a wavelength longer than 1.0 μm, and a second pump light source having a variable wavelength;
A crystal on which the two beams of the first and second pump lights are incident;
Means for injecting the two beams into the crystal at a minute angle to excite phonon polaritons in the crystal;
Has at least one of said first and second pump light sources for generating laser light of a wavelength tunable excited by YAG laser with a wavelength of 1.064 .mu.m Cr: a FORSTERITE laser, before Symbol second pump light By simultaneously changing the frequency and the minute angle, a tunable, high-power, broadband terahertz wave is generated from the crystal when the difference frequency between the first and second pump light frequencies is in the range of 0.15 THz to 7 THz. The terahertz wave generator characterized by the above-mentioned.   At least one of the first and second pump light sources is a Cr: FORSTERITE laser that is pumped by a YAG laser having a wavelength of 1.064 μm and generates a wavelength-tunable laser beam. A terahertz wave generator characterized by generating a broadband and high-output coherent terahertz wave as a difference frequency by entering light.

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