Light input device

Abstract

A light input device (1000) for inputting laser light into a nonlinear optical crystal in order to generate terahertz waves through terahertz wave parametric generation comprises: a laser light source (101) that outputs laser light; a first optical component (102) that branches the output laser light (200) which was output from the laser light source (101) into first laser light (201) which propagates in the direction of the nonlinear optical crystal (105) and second laser light (202) which propagates in a direction different from the nonlinear optical crystal (105); and a second optical component (103) that changes the propagation direction of the second laser light (202) so as to be directed toward the nonlinear optical crystal (105). The first laser light (201) and the second laser light (202) propagate through the nonlinear optical crystal (105) in mutually different directions so as to interpose the propagation direction of Stokes light (300) which is paired with the terahertz waves of a target wavelength. The angle formed by the propagation direction of the first laser light (201) and the propagation direction of the Stokes light (300) is substantially equal to the angle formed by the propagation direction of the second laser light (202) and the propagation direction of the Stokes light (300).

Description

light incidence device Cross-reference of related applications

[0001] This application is based on Japanese Patent Application No. 2025-26733, filed on 21 February 2025, and claims the benefit of that priority, the entirety of which is incorporated herein by reference.

[0002] This disclosure relates to an optical injector.

[0003] Terahertz waves are electromagnetic waves classified in the millimeter wave to far-infrared region, with frequencies ranging from approximately 0.3 THz to 10 THz and wavelengths from approximately 1 mm to 30 μm. These terahertz waves possess excellent properties such as directivity, material penetration, non-invasiveness to living organisms, safety for the human body, and unique absorption spectra for reagents, making them promising for applications in a wide range of fields.

[0004] One technique for generating terahertz waves with a narrow spectral width and high output is the injection-seeded terahertz-wave parametric generator (hereinafter also called "is-TPG" (injection-seeded THz-wave parametric generator)) (see, for example, Patent Document 1). is-TPG is a technique that excites a nonlinear optical crystal by injecting it with a single-frequency laser beam (excitation beam) and simultaneously injects light with a laser beam (injection beam) that is in the same direction and of the same frequency as the Stokes beam.

[0005] Japanese Patent Publication No. 2002-72269

[0006] S. Hayashi, K. Nawata, T. Taira, J. Shikata, K. Kawase, H. Minamide "Ultrabright continuously tunable terahertz-wave generation at room temperature," Scientific Reports, Vol. 4, Article number 5045 (2014). https: / / doi.org / 10.1038 / srep05045

[0007] For example, the is-TPG system described in Patent Document 1 requires at least two laser light sources: one for excitation light and one for injection light. However, physically providing multiple laser light sources leads to larger and more complex equipment, as well as higher costs. Therefore, it is desirable to be able to realize is-TPG with a single laser light source.

[0008] This disclosure is made in view of these circumstances, and its purpose is to provide a technology that enables is-TPG using a single laser light source.

[0009] To solve the above problems, an optical incident device according to one embodiment of the present invention is an optical incident device that incidents laser light onto a nonlinear optical crystal in order to generate terahertz waves by parametric generation of terahertz waves, and comprises a laser light source that outputs output laser light, a first optical component that splits the output laser light output from the laser light source into a first laser light that propagates in the direction of the nonlinear optical crystal and a second laser light that propagates in a direction other than that of the nonlinear optical crystal, and a second optical component that changes the propagation direction of the second laser light so that it is directed toward the nonlinear optical crystal. The first laser light and the second laser light propagate within the nonlinear optical crystal in different directions, straddling the propagation direction of Stokes light which is paired with a terahertz wave of a target wavelength, and the angle between the propagation direction of the first laser light and the propagation direction of Stokes light is approximately equal to the angle between the propagation direction of the second laser light and the propagation direction of Stokes light.

[0010] Another aspect of the present invention is also a light injector. This light injector is used to inject laser light into a nonlinear optical crystal in order to generate terahertz waves by parametric generation of terahertz waves, and comprises a laser light source that outputs output laser light in a first direction, an incident surface into which the output laser light is incident on the nonlinear optical crystal, and an optical component installed near the intersection line of the incident surface and one surface of the nonlinear optical crystal that intersects the incident surface. The optical component reflects the output laser light propagating in a first direction in a second direction different from the first direction. A portion of the output laser light output in the first direction is directly incident on the incident surface of the nonlinear optical crystal and propagates within the nonlinear optical crystal in the first direction. Another portion of the output laser light output in the first direction is reflected by the optical component in the second direction, then incident on the incident surface of the nonlinear optical crystal, refracted at the incident surface, and propagates within the nonlinear optical crystal in a third direction. The first and third directions are different from each other, with the propagation direction of Stokes light, which is paired with the terahertz wave of the target wavelength, in between. The angle between the first direction and the propagation direction of Stokes light is approximately equal to the angle between the third direction and the propagation direction of Stokes light.

[0011] According to this disclosure, is-TPG can be realized using a single laser light source.

[0012] LiNbO as a nonlinear crystal ３This is the dispersion curve of polaritons when using [this method]. This is a schematic diagram showing the principle of parametric terahertz wave generation. This is a schematic diagram showing the principle of light-injection type parametric terahertz wave generation. This is a diagram showing how Stokes light is generated when excitation light is incident on a nonlinear optical crystal. The upper left shows the Stokes light generated when the first excitation light is incident on the nonlinear optical crystal. The upper right shows the Stokes light generated when the second excitation light is incident on the nonlinear optical crystal. The lower shows the situation when the first excitation light and the second excitation light are incident on the nonlinear optical crystal simultaneously. This is a schematic diagram of the light incident device according to the first embodiment. This is a schematic diagram of another embodiment of the light incident device according to the first embodiment. This is a schematic diagram of yet another embodiment of the light incident device according to the first embodiment. This is a diagram showing the state when the first optical component of the light incident device in Figure 8 is rotated by a predetermined angle. This is a diagram showing the state of the support rod of the light incident device in Figure 8 before and after rotation by a predetermined angle, superimposed on each other. This is a schematic diagram of yet another embodiment of the light incident device according to the first embodiment. This figure shows the state of the second optical component of the light injector in Figure 11 when it is rotated by a predetermined angle. This figure shows the state of the support rod of the light injector in Figure 11 before and after rotation, superimposed on each other. This is a schematic diagram of another embodiment of the light injector according to the second embodiment. This is an enlarged view of Figure 14. This is a schematic diagram of the experimental setup for the verification experiment. Figures 17(a), (b), (c), and (d) show the incident positions of excitation light 1 and excitation light 2 at rotation angles of 0°, 0.8°, 1.6°, and 2.4°, respectively. This figure shows the graph of the intensity distribution of interference fringes in Figure 17(a). Figure 19(a) shows the results of measuring the generated terahertz waves with a pyroelectric detector. Figure 19(b) shows the results of spectroscopic measurement of a reagent using this light source.

[0013] Before describing specific embodiments, let's explain the underlying knowledge. First, we will explain terahertz wave parametric generation (see, for example, Non-Patent Document 1).

[0014] When an electromagnetic wave with a strong electromagnetic field such as a pulsed laser passes through a nonlinear optical crystal having a transverse optical phonon mode, photons and phonons are coupled to form an elementary excitation state called a polariton. In Fig. 1, the dispersion curve of polaritons when using LiNbO ３ is shown. Polaritons behave like phonons near the resonance frequency ω ＴＯ , but behave like photons in the low-frequency region sufficiently separated from the resonance frequency. Therefore, broadband terahertz wave generation can be performed by induced Raman scattering (induced polariton scattering) via polaritons. Induced polariton scattering is observed in polar crystals such as LiNbO ３ , LiTaO ３ , and GaP. In particular, LiNbO ３ has no strong absorption in the broadband (0.4 - 5.5 μm) in the visible to infrared region, has high resistance to optical damage (damage threshold > several 100 MW / cm ２ ), can obtain a high gain by induced scattering, and has excellent characteristics such as a smaller loss of terahertz waves in the crystal compared to other materials. Therefore, it is useful for high-power and broadband terahertz wave generation.

[0015] Fig. 2 is a schematic diagram showing the principle of terahertz wave parametric generation. 1 is a nonlinear optical crystal (for example, LiNbO ３ ), 2 is the excitation light (for example, YAG laser light), 3 is the terahertz wave, and 4 is the Stokes light. The excitation light is sometimes called the pump light, the terahertz wave is sometimes called the signal light, and the Stokes light is sometimes called the idler light.

[0016] The nonlinear optical crystal 1 is a crystal such that the passing electromagnetic wave has a transverse optical phonon mode as described above. When the excitation light 2 (usually near-infrared light or visible light) is incident in a certain direction, a terahertz wave 3 and Stokes light 4 are generated through the elementary excitation wave (polariton) of the substance by the induced Raman effect (or parametric interaction). Let the frequency of the excitation light 2 be ω Ｐ and the wave vector be k Ｐ . Let the frequency of the terahertz wave 3 be ω Ｔ and the wave vector be k ＴLet's assume the frequency of Stokes light 4 is ω Ｓ , the wave vector is k Ｓ Let's assume that the following conditions apply between the excitation light 2, the terahertz wave 3, and the Stokes light 4: the energy conservation law shown in equation (1) and the momentum conservation law shown in equation (2) (non-collinear phase matching condition). The non-collinear phase matching condition is represented by the vector diagram in the upper right of Figure 2.

[0017] ω Ｐ = ω Ｔ +ω Ｓ ... (1) k Ｐ = k Ｔ +k Ｓ ... (2)

[0018] The terahertz waves 3 and Stokes light 4 generated at this time have a spatial extent. Their wavelengths change continuously depending on the emission angle. Figure 2 illustrates terahertz waves 3 at 1.0 THz, 1.5 THz, and 2.0 THz, and their corresponding Stokes light 4. The generation mechanism of terahertz waves and Stokes light in this single-pass configuration is called terahertz-wave parametric generation (THz-wave Parametric Generation) or parametric fluorescence.

[0019] A basic optical parametric process is defined by the annihilation of one excited photon and the simultaneous creation of one terahertz photon and one Stokes photon. Parametric oscillation occurs when the excitation light intensity exceeds a certain threshold. Stimulated Raman scattering is the annihilation of one excited photon and the simultaneous creation of one Stokes photon and one polariton. These are included in parametric interactions in a broad sense. Thus, terahertz wave parametric generation converts near-infrared or visible light to terahertz waves.

[0020] However, the broadband terahertz waves generated by the single-pass terahertz wave generator shown in Figure 2 are weak, and most of them are absorbed while traveling several hundred micrometers through the nonlinear optical crystal. These weak, broadband terahertz waves generated by this parametric terahertz wave generation mechanism are also called parametric fluorescence.

[0021] Note that in Figure 2, for the sake of clarity, only Stokes light 4, which is generated on the right side of the direction of propagation of excitation light 2, is shown. However, please be aware that in reality, Stokes light is also generated on the left side of the direction of propagation of excitation light 2.

[0022] Figure 3 is a schematic diagram of an optical injection type terahertz wave parametric generator (is-TPG) that solves the parametric fluorescence problem. In this device, in addition to excitation light 2, a laser beam (injection light 5) with the same frequency as the Stokes light 4, which is paired with the target wavelength terahertz wave 3 (in the example of Figure 3, a terahertz wave with a frequency of 1.5 THz), is incident onto the nonlinear optical crystal 1 in the same direction as the generation direction of the Stokes light 4.

[0023] As shown in Figure 3, the inventors have for the first time in the world confirmed that in parametric oscillation under non-collinear phase matching conditions in a nonlinear optical crystal, by exciting with a single-frequency laser beam (excitation beam) and injecting with a laser beam (injection beam) of the same direction and frequency as the Stokes beam, the spectral width of the generated terahertz wave can be narrowed to approximately the sum of the spectral widths of the excitation beam and the injection beam, and the terahertz wave output is significantly increased.

[0024] The generation mechanism of terahertz waves and Stokes light in the system shown in Figure 3 is called injection-seeded THz-wave parametric generator (is-TPG). Because the light-injection-seeded terahertz-wave parametric generator has high peak power (~100 kW), wide frequency tunability (0.4-5 THz), and Fourier-limited linewidth, it is extremely useful as a monochromatic terahertz wave light source.

[0025] Figure 4 shows the generation of Stokes light 4 and Stokes light 4' when the excitation light 2 is incident on the nonlinear optical crystal 1. In Figure 4, Stokes light 4' generated on the left side of the propagation direction of the excitation light 2, which was not shown in Figure 2, is also shown. That is, it can be seen that Stokes light is generated on both sides of the excitation light. Here, it should be noted that Stokes light 4 and Stokes light 4' have angular dispersion in opposite directions.

[0026] In the upper left of Figure 5, Stokes light 41 generated when the first excitation light 21 is incident on a nonlinear optical crystal (not shown) is shown. However, only the Stokes light 41 generated on the right side of the propagation direction of the first excitation light 21 is shown here, and the Stokes light generated on the left side of the propagation direction of the first excitation light 21 is not shown.

[0027] In the upper right of Figure 5, Stokes light 42' generated when the second excitation light 22 is incident on a nonlinear optical crystal (not shown) is shown. However, only the Stokes light 42' generated on the left side of the propagation direction of the second excitation light 22 is shown here, and the Stokes light generated on the right side of the propagation direction of the second excitation light 22 is not shown.

[0028] Here, let the angle formed by the propagation direction of the first excitation light 21 and the propagation direction of the Stokes light that pairs with the 1.5 THz terahertz wave among the Stokes lights 41 be θ1. Also, let the angle formed by the propagation direction of the second excitation light 22 and the propagation direction of the Stokes light that pairs with the 1.5 THz terahertz wave among the Stokes lights 42' be θ2. At this time, θ1 = θ2. That is, "the angle formed by the propagation direction of the first excitation light 21 and the propagation direction of the Stokes light that pairs with the 1.5 THz terahertz wave among the Stokes lights 41" and "the angle formed by the propagation direction of the second excitation light 22 and the propagation direction of the Stokes light that pairs with the 1.5 THz terahertz wave among the Stokes lights 42'" are always equal.

[0029] As shown in the upper left and upper right of FIG. 5, when comparing the first excitation light 21 and the second excitation light 22, the Stokes lights of both have angular dispersions in opposite directions. Therefore, when the first excitation light 21 and the second excitation light 22 simultaneously enter the nonlinear optical crystal at an angle of θ1 + θ2 (= 2θ1 = 2θ2), as shown in the lower part of FIG. 5, only the Stokes light exactly in the middle between the first excitation light 21 and the second excitation light 22 (in this example, the Stokes light paired with the 1.5 THz terahertz wave 31) is enhanced. As a result, only the Stokes light paired with the 1.5 THz terahertz wave 31 automatically becomes the seed light. Thereby, the spectral width of the 1.5 THz terahertz wave 31 is narrowed and its output increases.

[0030] Hereinafter, the Stokes light paired with the terahertz wave of the target wavelength is referred to as the "target Stokes light". The Stokes light exactly in the middle of the propagation directions of the first excitation light and the second excitation light becomes the target Stokes light, and is-TPG can be realized.

[0031] Furthermore, by changing the angle formed by the first excitation light and the second excitation light, the wavelength of the target Stokes light and the wavelength of the target terahertz wave can be made variable. At that time, it is important for the efficient generation of the terahertz wave to make the intersection point of the first excitation light and the second excitation light move as little as possible.

[0032] Based on the principle described above, the embodiments of the present disclosure will be described below.

[0033] [First Embodiment] FIG. 6 is a functional block diagram of an optical incident device 1000 according to the first embodiment. The optical incident device 1000 includes a laser light source 101, a first optical component 102, and a second optical component 103.

[0034] The laser light source 101 outputs output laser light 200. The output laser light 200 output from the laser light source 101 enters the first optical component 102.

[0035] The first optical component 102 splits the output laser light 200 into a first laser beam 201 that propagates to the nonlinear optical crystal 105 and a second laser beam 202 that propagates in a direction different from the nonlinear optical crystal 105. In the example in Figure 6, the first optical component 102 is formed from a half-mirror.

[0036] The first laser beam 201 is incident on the nonlinear optical crystal 105.

[0037] The second laser beam 202 is incident on the second optical component 103.

[0038] The second optical component 103 changes the propagation direction of the second laser beam 203 so that it is directed toward the nonlinear optical crystal 105. In the example shown in Figure 6, the second optical component 103 is formed of a mirror (for example, a total internal reflection mirror).

[0039] The second laser beam 202, whose propagation direction has been changed by the second optical component 103, is incident on the nonlinear optical crystal 105.

[0040] The nonlinear optical crystal 105 generates terahertz waves and Stokes light from excitation light through terahertz wave parametric generation. The nonlinear optical crystal 105 is, for example, LiNbO ３ ya LiTaO ３ , a polar crystal such as GaP. Particularly preferably, the nonlinear optical crystal 105 is LiNbO ３ (LiNboO ３ (In some cases, MgO is used for doping.) The mechanism of terahertz wave parametric generation by the nonlinear optical crystal 105 is as described above.

[0041] Two output laser beams (the first laser beam 201 and the second laser beam 202) are incident on the nonlinear optical crystal 105 from two directions. The first laser beam 201 and the second laser beam 202 (i.e., the output laser beam 200) have, for example, a light intensity of 100 MW / cm². ２ It is a nanosecond pulsed laser of a certain magnitude or greater (for example, a YAG laser). However, it is not limited to this; the output laser light 200 can be any suitable light capable of achieving terahertz wave parametric generation.

[0042] The first laser beam 201 and the second laser beam 202 intersect at point O in the nonlinear optical crystal 105, respectively, generating a terahertz wave and Stokes light. However, the rotation and translation of optical components 102 and 103 are used to control the position of point O so that it does not move as much as possible when the wavelength of the terahertz wave is tuned.

[0043] The first laser beam 201 and the second laser beam 202 incident on the nonlinear optical crystal 105 propagate within the nonlinear optical crystal 105 in opposite directions, straddling the propagation direction of the Stokes light 300 which is paired with the terahertz wave of the target wavelength.

[0044] The angle θ1 between the propagation direction of the first laser beam 201 and the propagation direction of the Stokes light 300 is equal to the angle θ2 between the propagation direction of the second laser beam 202 and the propagation direction of the Stokes light 300.

[0045] With the above configuration, the output laser beam 200 emitted by the laser light source 101 is split into two laser beams (the first laser beam 201 and the second laser beam 202), and these are incident on the nonlinear optical crystal 105 from different directions. At this time, the Stokes beam 300 is generated in a direction that satisfies the condition that the angle θ1 between the propagation direction of the first laser beam 201 and the propagation direction of the Stokes beam 300 is equal to the angle θ2 between the propagation direction of the second laser beam 202 and the propagation direction of the Stokes beam 300 (θ1 = θ2). Using this Stokes beam 300 as a seed light, is-TPG can be realized. In other words, is-TPG can be realized with a single laser light source.

[0046] As described above, Stokes rays 300 are generated most strongly in the direction that satisfies the condition θ1 = θ2. However, even if there is an error of about 10% in θ1 + θ2, the purpose of this disclosure is still sufficiently achieved.

[0047] Therefore, in this specification, expressions such as "the angle between the propagation direction of the first laser beam and the propagation direction of the Stokes beam is approximately equal to the angle between the propagation direction of the second laser beam and the propagation direction of the Stokes beam" may be used. This means that "the angular error between the angle between the propagation direction of the first laser beam and the propagation direction of the Stokes beam and the angle between the propagation direction of the second laser beam and the propagation direction of the Stokes beam is within ±10%."

[0048] (Modification 1) Figure 7 is a schematic diagram of the light injector 1001 according to the first embodiment. The light injector 1001 comprises a laser light source 101, a first optical component 104, and a second optical component 103. The light injector 1001 is a modification of the light injector 1000 in Figure 5, and differs from the light injector 1000 in that the first optical component 104 is formed by a beam splitter instead of a half mirror. The other components of the light injector 1001 are the same as those of the light injector 1000. The operation of the light injector 1001 is the same as that of the light injector 1000, so redundant explanations are omitted.

[0049] As explained above, the first optical component may be formed from a half-mirror or a beam splitter, and the second optical component may be formed from a mirror. However, it goes without saying that the first and second optical components may be formed from any suitable optical components, and are not limited to these.

[0050] In the light incident device 1000 of Figure 6, the first optical component 102 may be configured to be movable. Similarly, in the light incident device 1001 of Figure 7, the first optical component 104 may be configured to be movable.

[0051] In particular, the first optical component 102 and the first optical component 104 may be rotatable around the rotation axis 106. Alternatively, the first optical component 102 and the first optical component 104 may be translationally responsive.

[0052] By configuring the first optical component 102 and the first optical component 104 to be movable in this way, the direction in which the Stokes light 300 is generated can be varied. Therefore, the wavelength of the desired terahertz wave can be varied.

[0053] In the light incident device 1000 of Figure 6, the second optical component 103 may be configured to be movable. Similarly, in the light incident device 1001 of Figure 7, the second optical component 103 may be configured to be movable.

[0054] In particular, the second optical component 103 may be rotatable around the rotation axis 107. Alternatively, the second optical component 103 may be translatable.

[0055] By configuring the second optical component 103 to be movable in this way, the direction in which the Stokes rays 300 are generated can be varied. Therefore, the wavelength of the desired terahertz wave can be varied.

[0056] (Modification 2) Figure 8 is a schematic diagram of a light injector 1002 according to the first embodiment. The light injector 1002 comprises a laser light source 101, a first optical component 102, and a second optical component 103. The light injector 1001 is a modification of the light injector 1000 in Figure 6.

[0057] The first optical component 102 is formed, for example, from a half-mirror. The second optical component 103 is formed, for example, from a total reflection mirror. The first optical component 102 reflects a portion of the output laser light 200 output from the laser light source 101 and transmits a portion of it. The output laser light 200 reflected by the first optical component 102 is incident on the nonlinear optical crystal 105 as the first laser light 201. On the other hand, the output laser light 200 transmitted by the first optical component 102 is reflected by the second optical component 103 and incident on the nonlinear optical crystal 105 as the second laser light 202.

[0058] The first optical component 102 is attached near the tip of the support rod 1021. The support rod 1021 is configured to be rotatable about an axis passing through point P. As shown in Figure 8, point P lies on the circumference of a circle C centered at the intersection point O of the first laser beam 201 and the second laser beam 202.

[0059] Near point O of the nonlinear optical crystal 105, terahertz waves (not shown) and Stokes rays 300 are generated. Specifically, the Stokes rays 300 are generated in a direction that satisfies the condition that θ1 and θ2 are equal (θ1 = θ2), where θ1 is the angle between the first laser beam 201 and the Stokes rays 300, and θ2 is the angle between the second laser beam 202 and the Stokes rays 300. This achieves narrowing of the spectrum and increased output of the terahertz waves paired with the Stokes rays 300. In other words, is-TPG is realized.

[0060] Figure 9 is a schematic diagram of the light injector 1002, similar to Figure 8. Figure 9 shows the state when the support rod 1021 is rotated counterclockwise by an angle φ around an axis passing through point P, from the state shown in Figure 8.

[0061] As the support rod 1021 and the first optical component 102 attached thereto rotate, the propagation direction of the first laser beam 201 changes from the state shown in Figure 8. In this case, when the angle between the first laser beam 201 and the Stokes beam 300 is θ1', and the angle between the second laser beam 202 and the Stokes beam 300 is θ2', the Stokes beam 300 is generated in a direction that satisfies the condition that θ1' and θ2' are equal (θ1' = θ2'). Therefore, in this case as well, narrowing of the spectrum of the terahertz wave paired with the Stokes beam 300 and an increase in output are achieved, and is-TPG is realized.

[0062] However, because the propagation direction of the first laser beam 201 changes due to the rotation of the first optical component 102, the wavelength of the generated terahertz wave also changes. In this way, by configuring the first optical component 102 to be rotatable around an axis passing through point P, the wavelength of the desired terahertz wave can be made variable.

[0063] Figure 10 is a diagram that superimposes the state of the support rod 1021 and the first optical component 102 attached thereto before rotation (state in Figure 8) and after rotation (state in Figure 9). The points of incidence of the output laser light 200 onto the first optical component 102 are Q1 and Q2, respectively, before and after rotation. Then, the optical paths of the first laser light 201 emitted from the first optical component 102 become straight lines Q1O and straight lines Q2O, respectively, before and after rotation. At this time, since the axis of rotation of the support rod 1021 passes through point P on the circumference of a circle centered at point O, the angle between the straight line Q1O and the straight line Q2O is twice the angle between the straight line Q1P and the straight line Q2P, according to the relationship between the central angle and the inscribed angle. In Figure 10, the support rod 1021 is rotated by an angle φ around an axis passing through point P, so the angle between the line Q1O and the line Q2O (i.e., the angle between the optical path of the first laser beam 201 before rotation and the optical path of the first laser beam 201 after rotation) is 2φ.

[0064] In this way, by configuring the rotation axis of the support rod 1021 to pass through point P on the circumference of a circle C centered at point O in the nonlinear optical crystal 105, the first laser beam will always pass through point O regardless of the angle at which the support rod 1021 is rotated. Needless to say, the second laser beam 202 also passes through point O, so regardless of the angle at which the support rod 1021 is rotated, the first laser beam 201 and the second laser beam 202 will always pass through point O in the nonlinear optical crystal 105. The target terahertz waves are then generated near point O.

[0065] As explained above, the wavelength of the target terahertz wave can be varied by arranging the first optical component 102 so that it can rotate around a point P on the circumference of a circle C centered on point O (the point where the target terahertz wave is to be generated) within the nonlinear optical crystal 105, so that the propagation direction of the first laser beam 201 can be varied. Furthermore, no matter how much the first optical component 102 is rotated, the first laser beam 201 and the second laser beam 202 will automatically pass through point O, eliminating the need for troublesome and complicated optical adjustments.

[0066] (Modification 3) Figure 11 is a schematic diagram of a light injector 1003 according to the first embodiment. The light injector 1003 comprises a laser light source 101, a first optical component 102, and a second optical component 103. The light injector 1003 is a modification of the light injector 1000 in Figure 5.

[0067] The first optical component 102 is formed, for example, from a half-mirror. The second optical component 103 is formed, for example, from a total reflection mirror. The first optical component 102 reflects a portion of the output laser light 200 output from the laser light source 101 and transmits a portion of it. The output laser light 200 reflected by the first optical component 102 is incident directly on the nonlinear optical crystal 105 as the first laser light 201. On the other hand, the output laser light 200 transmitted by the first optical component 102 is reflected by the second optical component 103 and incident on the nonlinear optical crystal 105 as the second laser light 202.

[0068] The second optical component 103 is attached near the tip of the support rod 1031. The support rod 1031 is configured to be rotatable about an axis passing through point P. As shown in Figure 11, point P lies on the circumference of a circle C centered at the intersection point O of the first laser beam 201 and the second laser beam 202.

[0069] Near point O of the nonlinear optical crystal 105, terahertz waves (not shown) and Stokes rays 300 are generated. Specifically, the Stokes rays 300 are generated in a direction that satisfies the condition that θ1 and θ2 are equal (θ1 = θ2), where θ1 is the angle between the first laser beam 201 and the Stokes rays 300, and θ2 is the angle between the second laser beam 202 and the Stokes rays 300. This achieves narrowing of the spectrum and increased output of the terahertz waves paired with the Stokes rays 300. In other words, is-TPG is realized.

[0070] Figure 12 is a schematic diagram of the light injector 1003, similar to Figure 11. Figure 12 shows the state when the support rod 1031 is rotated counterclockwise by an angle ψ around an axis passing through point P, from the state shown in Figure 11.

[0071] As the support rod 1031 and the second optical component 103 attached thereto rotate, the propagation direction of the second laser beam 202 changes from the state shown in Figure 11. In this case, when the angle between the first laser beam 201 and the Stokes beam 300 is θ1', and the angle between the second laser beam 202 and the Stokes beam 300 is θ2', the Stokes beam 300 is generated in a direction that satisfies the condition that θ1' and θ2' are equal (θ1' = θ2'). Therefore, in this case as well, narrowing of the spectrum of the terahertz wave paired with the Stokes beam 300 and an increase in output are achieved, and is-TPG is realized.

[0072] However, because the propagation direction of the first laser beam 201 changes due to the rotation of the second optical component 103, the wavelength of the generated terahertz wave also changes. In this way, by configuring the second optical component 103 to be rotatable around an axis passing through point P, the wavelength of the desired terahertz wave can be made variable.

[0073] Figure 13 is a diagram that superimposes the state of the support rod 1031 and the second optical component 103 attached thereto before rotation (state in Figure 11) and after rotation (state in Figure 12). The points of incidence of the output laser beam 200 onto the second optical component 103 are Q3 and Q4, respectively, before and after rotation. Then, the optical paths of the second laser beam 202 emitted from the second optical component 103 become straight lines Q3O and straight lines Q4O, respectively, before and after rotation. At this time, since the axis of rotation of the support rod 1031 passes through point P on the circumference of a circle centered at point O, from the relationship between the central angle and the inscribed angle, the angle between the straight line Q3O and the straight line Q4O is twice the angle between the straight line Q3P and the straight line Q4P. In Figure 13, the support rod 1031 is rotated by an angle ψ around an axis passing through point P, so the angle between the line Q3O and the line Q4O (i.e., the angle between the optical path of the second laser beam 202 before rotation and the optical path of the second laser beam 202 after rotation) is 2ψ.

[0074] In this way, by configuring the rotation axis of the support rod 1031 to pass through point P on the circumference of a circle C centered at point O in the nonlinear optical crystal 105, the first laser beam will always pass through point O regardless of the angle at which the support rod 1031 is rotated. Needless to say, the first laser beam 201 also passes through point O, so regardless of the angle at which the support rod 1031 is rotated, the first laser beam 201 and the second laser beam 202 will always pass through point O in the nonlinear optical crystal 105. The target terahertz waves are then generated near point O.

[0075] As explained above, the wavelength of the target terahertz wave can be varied by arranging the second optical component 103 so that it can rotate around a point P on the circumference of a circle C centered on point O (the point where the target terahertz wave is to be generated) within the nonlinear optical crystal 105, so that the propagation direction of the second laser beam 202 can be varied. Furthermore, no matter how much the second optical component 103 is rotated, the first laser beam 201 and the second laser beam 202 will automatically pass through point O, eliminating the need for troublesome and complicated optical adjustments.

[0076] The following are points to note that apply to both the light injection device 1002 and the light injection device 1003.

[0077] In the above explanation, point O was located inside the nonlinear optical crystal 105. However, this is not the only option; point O may be located outside the nonlinear optical crystal 105, as long as it is the point where the first laser beam 201 and the second laser beam 202 are to intersect.

[0078] LiNbO ３ When using this method, it is desirable that the variable ranges of the rotation angle φ of the first optical component 102 and the rotation angle ψ of the second optical component 103 are 0° < φ < 10° and 0° < ψ < 10°, respectively.

[0079] Point P can be anywhere on the circumference of the circle C centered at point O.

[0080] The first optical component 102 and its support rod 1021 do not have to be parallel. Similarly, the second optical component 103 and its support rod 1031 do not have to be parallel.

[0081] The propagation direction of the output laser beam 200 is preferably such that it passes near the tangent to circle C.

[0082] Any suitable rotating mechanism, such as a rotating stage or a single-axis actuator, may be used to rotate the support rods 1021 and 1031.

[0083] [Second Embodiment] The light injection device according to the second embodiment will be described with reference to Figures 14 and 15. Figure 14 is a schematic diagram of the light injection device 1004. Figure 15 is an enlarged view of the vicinity of the incident surface S1 of the nonlinear optical crystal 105 in Figure 14.

[0084] The light injector 1004 comprises a laser light source 101 and an optical component 110.

[0085] The laser light source 101 outputs output laser light 400 toward the nonlinear optical crystal 105. In Figures 14 and 15, the output laser light 400 is represented as a beam of light with a constant diameter.

[0086] The optical component 110 is placed near the intersection line L1 of the incident surface S1 into which the laser light is incident on the nonlinear optical crystal 105 and one surface S2 that intersects with the incident surface S1.

[0087] The optical component 110 reflects the output laser light 400 propagating in the first direction D1 in a second direction D2 that is different from the first direction D1.

[0088] The optical component 110 may be a mirror (for example, a total internal reflection mirror).

[0089] A portion of the output laser light 400 output in the first direction D1 (the lower half of the output laser light 400 in Figure 15) is directly incident on the incident surface S1 of the nonlinear optical crystal 105 and propagates through the nonlinear optical crystal 105 in the first direction D1 as the first laser light 401.

[0090] Another portion of the output laser beam 400, which is output in the first direction D1 (the upper half of the output laser beam 400 in Figure 15), is reflected in the second direction D2 by the optical component 110 and then incident on the incident surface S1 of the nonlinear optical crystal 105. This laser beam is refracted at the incident surface S1 and propagates as the second laser beam 402 through the nonlinear optical crystal 105 in the third direction D3.

[0091] The nonlinear optical crystal 105 generates terahertz waves and Stokes light from excitation light through terahertz wave parametric generation. The nonlinear optical crystal 105 is, for example, LiNbO ３ ya LiTaO ３ , a polar crystal such as GaP. Particularly preferably, the nonlinear optical crystal 105 is LiNbO ３ (LiNboO ３ (In some cases, MgO is used for doping.) The mechanism of terahertz wave parametric generation by the nonlinear optical crystal 105 is as previously explained.

[0092] Two output laser beams (a first laser beam 401 propagating in the first direction D1 and a second laser beam 402 propagating in the second direction D2) are incident on the nonlinear optical crystal 105 from two directions. The first laser beam 401 and the second laser beam 402 (i.e., the output laser beam 400) are, for example, nanosecond pulsed lasers (e.g., YAG lasers) with an optical intensity of about 100 MW / cm² or more. However, the output laser beam 400 is not limited to this, and can be any suitable light capable of realizing terahertz wave parametric generation.

[0093] The first laser beam 401 and the second laser beam 402 generate terahertz waves and Stokes light, respectively, within the nonlinear optical crystal 105. Specifically, the first laser beam 401 and the second laser beam 402 overlap in the gray region R1 shown in Figure 15, where terahertz waves and Stokes light are generated.

[0094] The first direction D1 and the third direction D3 are different from each other, with the propagation direction D4 of Stokes light 500, which is paired with the terahertz wave of the target wavelength, in between them.

[0095] Let Θ1 be the angle between the first direction D1 and the propagation direction D4 of the Stokes light 500. Let Θ2 be the angle between the third direction D3 and the propagation direction D4 of the Stokes light 500. Θ1 and Θ2 are equal.

[0096] With the above configuration, a portion of the output laser light 400 emitted by the laser light source 101 is incident on the nonlinear optical crystal 105, and another portion is reflected in a different direction by the optical component 110 before being incident on the nonlinear optical crystal 105. At this time, Stokes light 500 is generated in a direction that satisfies the condition that the angle Θ1 between the propagation direction of the first laser light 401 and the propagation direction of the Stokes light 500 is equal to the angle Θ2 between the propagation direction of the second laser light 402 and the propagation direction of the Stokes light 500 (Θ1 = Θ12). Using this Stokes light 500 as a seed light, is-TPG can be realized. In other words, is-TPG can be realized with a single laser light source.

[0097] As described above, Stokes light 500 is generated most strongly in the direction that satisfies the condition Θ1 = Θ2. However, even if there is an error of about 10% in Θ1 + Θ2, the purpose of this disclosure is sufficiently achieved.

[0098] Therefore, in this specification, expressions such as "the angle between the first direction and the direction of propagation of Stokes light is approximately equal to the angle between the third direction and the direction of propagation of Stokes light" may be used, which means that "the angular error between the angle between the first direction and the direction of propagation of Stokes light and the angle between the third direction and the direction of propagation of Stokes light is within ±10%."

[0099] In the light incidence device 1004 shown in Figures 14 and 15, the optical component 110 may be configured to be movable.

[0100] In particular, the optical component 110 may be rotatable around the intersection line L1 between the incident surface S1 of the nonlinear optical crystal 105 and one surface S2 that intersects the incident surface S1.

[0101] By configuring the optical component 110 to be movable in this way, the direction in which the Stokes rays 500 are generated can be varied. Therefore, the wavelength of the desired terahertz wave can be varied.

[0102] Ideally, the rotation axis 111 of the optical component 110 should be the intersection line L1 of the incident surface S1 of the nonlinear optical crystal 105 and one surface S2 that intersects the incident surface S1, but the rotation axis does not need to precisely coincide with the intersection line L1. Specifically, when the size of the incident surface S1 of the nonlinear optical crystal 105 is taken as 100%, it is desirable that the positional error between the rotation axis 111 and the intersection line L1 be about ±10%. However, even if the positional error exceeds ±10%, although the intensity of the terahertz waves of the target wavelength will decrease, it will not hinder the purpose of this invention.

[0103] Therefore, in this specification, expressions such as "the optical component is rotatable around an axis that substantially coincides with the intersection line" may be used, which means that "when the size of the incident surface S1 of the nonlinear optical crystal 105 is taken as 100%, the positional error between the rotation axis and the intersection line is within ±10%."

[0104] [Verification Experiment] To demonstrate the effectiveness of this disclosure, the inventors conducted a verification experiment. The results are shown below. Figure 16 is a schematic diagram of the experimental setup for this verification experiment. In this experiment, an angle adjustment mechanism consisting of one rotation stage is introduced in an is-TPG composed only of excitation light in order to more easily obtain wavelength tunability.

[0105] An optical system is arranged on the circumference of a circle of radius r centered on point O, which corresponds to the incident plane of the crystal. The excitation light is split into excitation light 1 and excitation light 2 by a half-mirror. Excitation light 2 travels tangentially to the circle and is reflected by a mirror with its center of rotation on the arc and directed towards point O. In this setup, when the angle between excitation light 1 and excitation light 2 is small, excitation light 2 is likely to overlap with excitation light 1 near point O. In an is-TPG composed only of excitation light, the difference in incident angles of the excitation light is approximately 2 to 6 degrees, and because θ is small, the overlap of the excitation light is maintained even when the rotation stage is rotated. According to the inscribed angle theorem, the incident angle of the excitation light changes by 2Δθ for every rotation angle Δθ of the rotation stage. In this case, the larger the radius r of the circle, the smaller the error in the incident position can be kept.

[0106] The 1.4 mJ excitation light output from the microchip Nd:YAG laser was amplified to 18 mJ by an amplifier, and 3 mJ of this was used for the experiment. This 3 mJ excitation light was split into excitation light 1 and excitation light 2 by a 50 / 50 half mirror, and the two were connected via an angle adjustment mechanism to LiNbO2. ３ The light was incident on the crystal. At this time, the excitation light beam diameter was 0.8 mm (FWHM), and the collimated light was flattened by a cylindrical lens (focal length f = 75 mm) and LiNbO ３ The excitation light was incident on a crystal. The reason for flattening the excitation light was to increase the efficiency of terahertz wave generation with limited excitation energy. The beam diameter was 0.8 mm in the y-axis direction, and under these conditions, the linewidth of the terahertz wave was expected to be around 30 GHz. The generated terahertz wave was focused with a terahertz lens and detected with a pyroelectric detector. The distance from the crystal incidence surface to the half-mirror was set to 20 cm, and the rotation stage was placed on a circular arc with a radius of 20 cm centered on the crystal incidence surface.

[0107] First, to evaluate the change in the incident position of excitation light due to the rotation of the rotation stage, the shift of the incident position O when the angle of the rotation stage was changed was measured using a near-infrared camera. LiNbO ３A near-infrared camera was placed at the crystal's incident position, and the rotation stage was rotated to a predetermined angle by mechanical control. Figures 17(a), (b), (c), and (d) show the incident positions of excitation light 1 and excitation light 2 at rotation angles of 0°, 0.8°, 1.6°, and 2.4°, respectively. The rightward direction in the figures corresponds to the downward direction in Figure 16. When the angle was rotated by 2.4°, although there was a shift of approximately 1 mm in the incident position, it was confirmed that excitation light 1 and excitation light 2 still overlapped spatially. Furthermore, Figure 17(e) shows the results of measuring the change in the incident position of excitation light 2 by changing the angle in 0.1° increments. 0 mm on the vertical axis indicates a state where excitation light 1 and excitation light 2 completely overlap, and it was found that at a rotation angle of 2.8°, excitation light 2 shifted approximately 2 mm to the right. When obtaining wavelength tunability of 1 to 3 THz using is-TPG with excitation light alone, the rotation stage is adjusted within a range of approximately 2°, and it was confirmed that the incident position shift can be kept to about 1 mm. Furthermore, since excitation light 1 and excitation light 2 only need to overlap within the crystal, monochromatic terahertz waves are possible even if the overlapping position is slightly shifted from the incident end face.

[0108] Furthermore, when the excitation light overlapped spatially, interference fringes parallel to the crystal's Z-axis direction were observed. Figure 18 shows a graph of the intensity distribution of the interference fringes in Figure 17(a). Due to the large pixel pitch of the camera (5.5 μm), accurate measurement of the period of the interference fringes was difficult. However, since the angle between excitation light 1 and excitation light 2 is 3°, theoretically, the period of the interference fringes is predicted to be around 20 μm. In an is-TPG composed only of excitation light, monochromatic terahertz waves are obtained due to the constructive interference of Stokes light generated at intermediate angles of the excitation light. From another perspective, it can be considered that the interference fringes caused by the excitation light selectively enhance the generation of specific terahertz waves and Stokes light, similar to a DFB laser in the optical wave band. For this reason, good beam quality of the excitation light is required in an is-TPG using only excitation light.

[0109] Next, Figure 19(a) shows the results of measuring the generated terahertz waves with a pyroelectric detector. The vertical axis represents the energy of the terahertz waves, and the horizontal axis represents the frequency of the terahertz waves. The rotation stage was rotated in 0.005° increments, and data was sampled approximately every 5 GHz. The frequency of the terahertz waves was calibrated by measuring the wavelength of the Stokes light generated in pairs with a spectrometer. As a result, it was confirmed that broadband wavelength tunability was obtained in the range of approximately 1 to 2.6 THz. The peak of the terahertz waves was located around 1.5 THz, and a maximum energy of 10 nJ / pulse was obtained. The linewidth of the terahertz waves was narrowed to about 30 GHz, and water vapor absorption lines around 1.7 THz, which are difficult to observe with broadband light sources, were also confirmed. Finally, Figure 19(b) shows the results of spectroscopic measurements of reagents using this light source. The sample was lactose approximately 1 mm thick sealed in a small plastic bag. The measurement results showed a clear absorption around 1.37 THz, which is a characteristic absorption peak of lactose, indicating that spectroscopic measurements of reagents using this light source are possible.

[0110] [Each Embodiment of the Disclosure] The following summarizes various embodiments of the Disclosure. In one embodiment of the Disclosure, a light injector device injects laser light into a nonlinear optical crystal to generate terahertz waves by parametric generation of terahertz waves. This light injector device comprises a laser light source that outputs output laser light, a first optical component that splits the output laser light output from the laser light source into a first laser light that propagates in the direction of the nonlinear optical crystal and a second laser light that propagates in a direction other than the nonlinear optical crystal, and a second optical component that changes the propagation direction of the second laser light so that it is directed toward the nonlinear optical crystal. The first laser light and the second laser light propagate within the nonlinear optical crystal in different directions, straddling the propagation direction of Stokes light which is paired with a terahertz wave of the target wavelength. The angle between the propagation direction of the first laser light and the propagation direction of Stokes light is approximately equal to the angle between the propagation direction of the second laser light and the propagation direction of Stokes light.

[0111] According to this embodiment, is-TPG can be realized using a single laser light source.

[0112] In one embodiment, the first optical component is a beam splitter or a half-mirror, and the second optical component is a mirror.

[0113] According to this embodiment, a light injection device can be manufactured using a beam splitter, a half mirror, a mirror, etc.

[0114] In one embodiment, the first optical component is configured to be movable so that the propagation direction of the first laser beam can be varied.

[0115] According to this embodiment, the first optical component can be made movable.

[0116] In one embodiment, the second optical component is configured to be movable so that the propagation direction of the second laser beam can be varied.

[0117] According to this embodiment, the second optical component can be made movable.

[0118] In one embodiment, the first optical component is arranged to be rotatable about a point on the circumference of a circle centered at the intersection of the first and second laser beams, so that the propagation direction of the first laser beam is variable.

[0119] According to this embodiment, no matter how much the first optical component is rotated, the first laser beam and the second laser beam will automatically pass near a predetermined point, thus eliminating the need for troublesome and complicated optical adjustments.

[0120] In one embodiment, the second optical component is arranged to be rotatable about a point on the circumference of a circle centered at the intersection of the first and second laser beams, so that the propagation direction of the second laser beam is variable.

[0121] According to this embodiment, no matter how much the second optical component is rotated, the first laser beam and the second laser beam will automatically pass near a predetermined point, thus eliminating the need for troublesome and complicated optical adjustments.

[0122] One embodiment of the present disclosure is an optical injector that injects laser light into a nonlinear optical crystal to generate terahertz waves by parametric generation of terahertz waves. The optical injector comprises a laser light source that outputs output laser light in a first direction, an incident surface into which the output laser light is incident on the nonlinear optical crystal, and an optical component installed near the intersection line of the incident surface and one surface of the nonlinear optical crystal that intersects the incident surface. The optical component reflects the output laser light propagating in the first direction in a second direction different from the first direction. A portion of the output laser light output in the first direction is directly incident on the incident surface to the nonlinear optical crystal and propagates within the nonlinear optical crystal in the first direction. Another portion of the output laser light output in the first direction is reflected in the second direction by the optical component, then incident on the incident surface to the nonlinear optical crystal, refracted at the incident surface, and propagates within the nonlinear optical crystal in a third direction. The first direction and the third direction are different from each other, with the propagation direction of Stokes light, which is paired with the terahertz wave of the target wavelength, in between. The angle between the first direction and the direction of propagation of Stokes light is approximately equal to the angle between the third direction and the direction of propagation of Stokes light.

[0123] According to this embodiment, is-TPG can be realized using a single laser light source.

[0124] In one embodiment, the optical component is a mirror.

[0125] According to this embodiment, a light injection device can be manufactured using mirrors or the like.

[0126] In one embodiment, the optical component is rotatable around an axis that substantially coincides with the line of intersection.

[0127] According to this embodiment, the optical components can be made movable.

[0128] In a certain type of light injector, the wavelength of the terahertz wave is variable.

[0129] According to this embodiment, the wavelength of the target terahertz wave can be varied.

[0130] In one embodiment of the light incidence device, the nonlinear optical crystal is LiNbO ３ That is the case.

[0131] This embodiment of the light incidence device, LiNbO ３ This can be applied to nonlinear optical crystals using the like.

[0132] The present disclosure has been described above based on embodiments. These embodiments are illustrative, and it will be understood by those skilled in the art that various modifications are possible in combinations of their components and processing processes, and that such modifications are also within the scope of the present disclosure.

[0133] This disclosure relates to an optical injector.

[0134] 1...Nonlinear optical crystal, 2...Excitation light, 3...Terahertz wave, 4...Stokes light, 4'...Stokes light, 5...Probe light, 21...Excitation light, 22...Excitation light, 31...Terahertz wave, 41...Stokes light, 42'...Stokes light, 1000...Light injector, 1001...Light injector, 1002...Light injector, 1003...Light injector, 1004...Light injector, 101...Laser light source, 102...First optical component, 103...Second optical component, 104...First optical component, 105...Nonlinear optical crystal, 106...Rotation axis, 107...Rotation axis, 110...Optical component, 111...Rotation axis 200... Output laser beam, 201... First laser beam, 202... Second laser beam, 300... Stokes beam, 400... Output laser beam, 500... Stokes beam, 1021... Support rod, 1031... Support rod, 401... First laser beam, 402... First laser beam, O... Intersection of the first laser beam and the second laser beam, C... Circle, P... Point on the circumference, Q1... Incident point of the output laser beam to the first optical component, Q2... Incident point of the output laser beam to the first optical component, S1... Incident plane on which the laser beam is incident on the nonlinear optical crystal, S2... One plane that intersects with the incident plane of the nonlinear optical crystal, L1... Intersection line of the incident plane on which the laser beam is incident on the nonlinear optical crystal and one plane that intersects with the incident plane.

Claims

1. An optical incident device for injecting laser light into a nonlinear optical crystal in order to generate terahertz waves by parametric generation of terahertz waves, comprising: a laser light source that outputs output laser light; a first optical component that splits the output laser light output from the laser light source into a first laser light that propagates in the direction of the nonlinear optical crystal and a second laser light that propagates in a direction other than the nonlinear optical crystal; and a second optical component that changes the propagation direction of the second laser light so as to be toward the nonlinear optical crystal, wherein the first laser light and the second laser light propagate within the nonlinear optical crystal in different directions, straddling the propagation direction of Stokes light which is paired with a terahertz wave of a target wavelength, and the angle between the propagation direction of the first laser light and the propagation direction of Stokes light and the angle between the propagation direction of the second laser light and the propagation direction of Stokes light are substantially equal.

2. The light incident device according to claim 1, characterized in that the first optical component is a beam splitter or a half mirror, and the second optical component is a mirror.

3. The light incident device according to claim 1, characterized in that the first optical component is configured to be movable so that the propagation direction of the first laser light can be varied.

4. The light incident device according to claim 1, characterized in that the second optical component is configured to be movable so that the propagation direction of the second laser light can be varied.

5. The light incident device according to claim 1, characterized in that the first optical component is arranged to be rotatable about a point on the circumference of a circle centered at the intersection of the first laser beam and the second laser beam, so that the propagation direction of the first laser beam is variable.

6. The light incident device according to claim 1, characterized in that the second optical component is arranged to be rotatable about a point on the circumference of a circle centered at the intersection of the first laser beam and the second laser beam, so that the propagation direction of the second laser beam is variable.

7. An optical injector for injecting laser light into a nonlinear optical crystal in order to generate terahertz waves by parametric generation of terahertz waves, comprising: a laser light source that outputs output laser light in a first direction; an incident surface into which the output laser light is incident on the nonlinear optical crystal; and an optical component installed near the intersection line of the incident surface and one surface of the nonlinear optical crystal that intersects the incident surface, wherein the optical component reflects the output laser light propagating in the first direction in a second direction different from the first direction; a portion of the output laser light output in the first direction is directly incident on the incident surface into the nonlinear optical crystal and propagates within the nonlinear optical crystal in the first direction; another portion of the output laser light output in the first direction is reflected in the second direction by the optical component and then incident on the incident surface into the nonlinear optical crystal, refracted at the incident surface and propagates within the nonlinear optical crystal in a third direction; the first direction and the third direction differ from each other, straddling the propagation direction of Stokes light which is paired with a terahertz wave of the target wavelength. An optical incident device characterized in that the angle between the first direction and the propagation direction of the Stokes light is approximately equal to the angle between the third direction and the propagation direction of the Stokes light.

8. The light incident device according to claim 7, characterized in that the optical component is a mirror.

9. The light incident device according to claim 7, characterized in that the optical component is rotatable about an axis substantially coinciding with the intersection line.

10. The light incidence device according to any one of 3 to 6 or 9, characterized in that the wavelength of the terahertz wave is variable.

11. The nonlinear optical crystal is LiNbO ３ The light incidence device according to claim 1 or 7, characterized in that it is the same as the one described in claim 1 or 7.

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