Terahertz wave generation device

Abstract

To provide a terahertz wave generation device capable of largely rotating a nonlinear optical element without damaging the nonlinear optical element.SOLUTION: A terahertz wave generation device 1 includes a pump light source 2 for generating pump light P, a periodic polarization inversion element 8 as a nonlinear optical element having a periodic structure in which polarization or crystal orientation is inverted in a certain inversion period, and a rotary stage 9 for rotatably supporting the periodic polarization inversion element 8. Signal light L which is a terahertz wave is generated by making pump light P incident on a periodic polarization inversion element 8, and the wavelength of the signal light L is changed by rotating the periodic polarization inversion element 8. The periodic polarization inversion device 8 has an incident end surface 8a on which the pump light P is incident and an emission end surface 8b from which the pump light P is emitted. The rotation axis 8c of the periodic polarization inversion device 8 is closer to the incident end face 8b than to the exit end face 8a.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a terahertz wave generating device. [Background technology]

[0002] Patent Document 1 discloses a technique for generating signal light, which is a terahertz wave, in the opposite direction to the pump light by irradiating pump light into a periodically poled element, which is a nonlinear optical element. Specifically, it discloses that the wavelength of the signal light, which is a terahertz wave, can be adjusted by increasing or decreasing the angle of the inverted structure of the nonlinear optical element relative to the pump light. As a means for increasing or decreasing the angle of the inverted structure of the nonlinear optical element relative to the pump light, it cites, for example, rotating the nonlinear optical element relative to the pump light. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6810954 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when the nonlinear optical element is rotated, the pump light eventually reaches the edge of the input end face or the edge of the output end face of the nonlinear optical element, damaging the nonlinear optical element, so it has been difficult to ensure a wide range of allowable rotation angles for the nonlinear optical element.

[0005] An object of the present disclosure is to provide a terahertz wave generating device that can rotate a nonlinear optical element by a large amount without damaging the nonlinear optical element. [Means for solving the problem]

[0006] a pump light source for generating pump light; a nonlinear optical element having a periodic structure in which polarization or crystal orientation is inverted at a certain inversion period; a rotation stage that rotatably supports the nonlinear optical element; Including, The pump light is incident on the nonlinear optical element to generate signal light which is a terahertz wave, and the wavelength of the signal light is changed by rotating the nonlinear optical element. A terahertz wave generating device, the nonlinear optical element has an incident end surface onto which the pump light is incident and an exit end surface from which the pump light is exited, a rotation axis of the nonlinear optical element is closer to the incident end face than to the output end face; A terahertz wave generating device is provided. [Effects of the Invention]

[0007] According to the present disclosure, a terahertz wave generating device is realized that can rotate a nonlinear optical element by a large amount without damaging the nonlinear optical element. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 2 is a plan cross-sectional view of a terahertz wave generating device. [Figure 2] 1 shows the measurement results of the transmittance spectrum of a wavelength filter. [Figure 3] 10 is a graph showing the relationship between the oscillation frequency of the signal light and the rotation angle of the periodically poled element. [Figure 4] FIG. 2 is a plan view of a periodically poled element, in which the optical path of pump light incident on the incident end face of the periodically poled element is shown by a broken line. [Figure 5] FIG. 2 is a plan view of a periodically poled element, in which the optical path of pump light incident on the incident end face of the periodically poled element is shown by a broken line. [Figure 6] FIG. 2 is a plan view of a periodically poled element. [Figure 7] 10 is a graph showing a change in the wavelength of idler light caused by rotating a periodically poled element. [Figure 8]FIG. 1 is a plan view of a spatial resolution test device. [Figure 9] This is the test result of the spatial resolution test. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention will be described below through embodiments of the invention, but the invention according to the claims is not limited to the following embodiments. Furthermore, not all of the configurations described in the embodiments are necessarily essential means for solving the problems. For clarity of explanation, the following description and drawings have been omitted and simplified as appropriate. In each drawing, the same elements are given the same reference numerals, and repeated explanations are omitted as necessary.

[0010] In each figure, X, Y, and Z represent three spatial axes that are orthogonal to one another. In this specification, the directions along these axes are referred to as the X direction, Y direction, and Z direction. In each figure, the direction indicated by the arrow is referred to as the positive (+) direction, and the direction opposite the arrow is referred to as the negative (-) direction. Furthermore, the directions of the three spatial axes that are not limited to the positive and negative directions are referred to as the X-axis direction, Y-axis direction, and Z-axis direction.

[0011] 1 shows a plan cross-sectional view of a terahertz wave generating device 1. The terahertz wave generating device 1 is also called a BW-TPO (backward terahertz-wave parametric oscillator). The terahertz wave generating device 1 includes a pump light source 2, a light source cooling plate 3, an output control unit 4, a high-reflection laser mirror 5, a wavelength filter 6, a dichroic mirror 7, a periodically poled element 8, a rotation stage 9, a high-reflection laser mirror 10, a beam damper 11, a seed collimator 12, a signal collimator 13, and a housing 14.

[0012] The pump light source 2, light source cooling plate 3, output control unit 4, high-reflection laser mirror 5, wavelength filter 6, dichroic mirror 7, periodically poled element 8, rotation stage 9, high-reflection laser mirror 10, beam damper 11, and seed collimator 12 are housed in a housing 14. The signal collimator 13 is provided on a side wall 14a extending in the X direction, which is the longitudinal direction of the housing 14. As shown in FIG. 1 , a pump-side space 20 and a seed-side space 21 are defined adjacent to each other in the Y direction in the internal space of the housing 14.

[0013] In the pump-side space 20, a pump light source 2, a light source cooling plate 3, an output control unit 4, a high-reflection laser mirror 5, and a beam damper 11 are arranged in this order in the X direction. In the seed-side space 21, a seed collimator 12, a wavelength filter 6, a dichroic mirror 7, a periodically poled element 8, and a high-reflection laser mirror 10 are arranged in this order in the X direction.

[0014] The pump light source 2 generates and emits pump light P of a single wavelength. The pump light source 2 is typically an Nd:YAG laser device or a semiconductor laser device. The wavelength of the pump light P is selected from a wavelength range that is not absorbed by the periodically poled element 8. The wavelength of the pump light P is typically about 0.5 to 5 micrometers. That is, the pump light P is laser light in the infrared or visible range. As an example, the wavelength of the pump light P is 1064 nanometers. The pump light P is typically pulsed laser light, but may alternatively be continuous wave laser light. Another pump light having a wavelength of typically 808 nanometers is supplied to the pump light source 2 from outside the terahertz wave generating device 1 via an optical fiber 2a to generate the pump light P.

[0015] The light source cooling plate 3 is thermally coupled to the pump light source 2 and cools the pump light source 2. The light source cooling plate 3 is formed with a cooling flow path 3b through which cooling water supplied via a water supply tube 3a from outside the terahertz wave generating device 1 flows. The cooling water discharged from the cooling flow path 3b is discharged to the outside of the terahertz wave generating device 1 via a drain tube 3c.

[0016] The output control unit 4 adjusts the output power and beam diameter of the pump light P emitted from the pump light source 2. The output control unit 4 is typically composed of a half-wave plate, a polarizing beam splitter, and a lens pair. The output control unit 4 adjusts the output power of the pump light P to typically 10 megawatts.

[0017] The high-reflection laser mirror 5 reflects the pump light P whose output has been adjusted by the output control unit 4 and makes it incident on the wavelength filter 6 .

[0018] The seed collimator 12 collimates the seed light S supplied via an optical fiber 12a from outside the terahertz wave generating device 1. As an example, the wavelength of the seed light S is 1065 nanometers or more, and typically 1065 to 1066 nanometers.

[0019] The wavelength filter 6 reflects the pump light P and transmits the seed light S. FIG. 2 shows the measurement results of the transmittance spectrum of the wavelength filter 6. In FIG. 2, the horizontal axis represents wavelength, and the vertical axis represents transmittance. As shown in FIG. 2, the wavelength filter 6 is reflective for laser light having wavelengths between 1064.3 and 1064.99 nanometers and transmissive for laser light having other wavelengths. As an example, the wavelength filter 6 may be a bandstop filter BP-1064.3-99 manufactured by OptiGrate (registered trademark). This wavelength filter 6 has specified angles of incidence and reflection when reflecting laser light. That is, the angle of incidence is specified between 13 and 17 degrees, and the angle of reflection is specified between 13 and 17 degrees. Therefore, in this embodiment, as an example, the wavelength filter 6 is positioned so that the pump light P is incident on the wavelength filter 6 at an angle of incidence of 15 degrees. The pump light P reflected by the wavelength filter 6 and the seed light S transmitted through the wavelength filter 6 are transmitted through a dichroic mirror 7 and enter a periodic polarization inversion element 8 .

[0020] The dichroic mirror 7 reflects terahertz waves and transmits light waves. Here, terahertz waves are electromagnetic waves with a frequency in the range of 0.1 to 100 terahertz. Typically, terahertz waves are electromagnetic waves with a frequency in the range of 0.1 to 10 terahertz. More typically, terahertz waves are electromagnetic waves with a frequency in the range of 0.1 to 1.5 terahertz. Furthermore, light waves are electromagnetic waves with a wavelength of 3 micrometers to 300 nanometers. The dichroic mirror 7 includes a crystal layer and two anti-reflection coatings. Either or both of the two anti-reflection coatings can be omitted. The crystal layer is either a lithium niobate crystal layer or a lithium tantalate crystal layer. The thickness of the crystal layer is typically 10 to 50 micrometers. However, the thickness of the crystal layer is not limited thereto, and may be less than 10 micrometers or greater than 50 micrometers. The crystal layer has a first surface 7a as a reflective surface onto which terahertz waves are incident and a second surface 7b opposite the first surface 7a. Both the lithium niobate crystal layer and the lithium tantalate crystal layer are uniaxial crystals. The crystal compositions of the lithium niobate crystal layer and the lithium tantalate crystal layer may be melt-matched or composition-matched. Two antireflection films are provided on both sides of the crystal layer. That is, two antireflection films are provided on the first surface 7a and the second surface 7b of the crystal layer 2, respectively. The thickness of each antireflection film is typically 0.1 to 9 micrometers. The material of the antireflection film is typically, but not limited to, magnesium fluoride or silicon dioxide. With the above configuration, the dichroic mirror 7 exhibits optical characteristics such as a reflectance of 50% or more for terahertz waves and a transmittance of 99% for light waves. That is, as described above, the dichroic mirror 7 exhibits the optical property of reflecting terahertz waves and transmitting light waves. Furthermore, even if one or both of the two antireflection films 3 are omitted, a light wave transmittance of approximately 80 to 90% can be ensured. This is because the crystal layer has the optical property of not absorbing light waves and does not include a metal layer.

[0021] The periodically poled element 8 is a specific example of a nonlinear optical element having a periodic structure in which the polarization or crystal orientation is inverted at a certain inversion period. The periodically poled element 8 of this embodiment has a periodic structure in which the polarization direction is inverted at a certain inversion period. Since a nonlinear optical element having a periodic structure due to inversion of the crystal orientation also has the same function as the periodically poled element 8, any subsequent explanation based on the periodically poled element 8 will also be considered as part of the explanation of a periodic structure due to inversion of the crystal orientation. The periodically poled element 8 is typically PPLN (periodically poled lithium niobate; LiNbO3). A nonlinear optical element having a periodic structure due to inversion of the crystal orientation is typically OP-GaAs (orientation-patterned gallium arsenide). When pump light P with an intensity exceeding a certain threshold is incident on the periodically poled element 8, idler light Q is generated in a direction approximately parallel to the pump light P, and signal light L is generated in a direction approximately opposite to the direction of the pump light P. When the pump light P is 10 megawatts, the signal light L generated by the periodically poled element 8 is, for example, 15 watts. In addition to the pump light P being incident on the periodically poled element 8, by incidenting the seed light S on the periodically poled element 8 so as to satisfy the phase matching condition of the periodically poled element 8, it is possible to generate the idler light Q and the signal light L even if the intensity of the pump light P incident on the periodically poled element 8 is low.

[0022] The periodically poled element 8 is typically a rectangular parallelepiped measuring 50 millimeters in the X direction, 5 millimeters in the Y direction, and 1 millimeter in the Z direction. The periodically poled element 8 has an incident end face 8a onto which the pump light P is incident and an exit end face 8b from which the pump light P exits. The incident end face 8a and the exit end face 8b are end faces facing each other in the X direction.

[0023] Figure 3 shows the relationship between the oscillation frequency of the signal light L and the relative rotation angle of the periodically poled element 8, as well as the effect of refraction of the pump light P. In Figure 3, the horizontal axis represents the oscillation frequency of the signal light L, and the vertical axis represents the relative rotation angle of the periodically poled element 8. In the graph of Figure 3, the relative rotation angle of the periodically poled element 8 is defined so that when the oscillation frequency of the signal light L is 0.5 terahertz, the relative rotation angle of the periodically poled element 8 is zero. Figure 3 also shows a case where the refraction of the pump light P at the incident end face 8a of the periodically poled element 8 is taken into consideration. When the pump light P is incident on the incident end face 8a of the periodically poled element 8, the pump light P is always refracted at the incident end face 8a. Figure 3 indicates that the oscillation frequency of the signal light L increases or decreases by rotating the periodically poled element 8 around the Z axis. This is because the oscillation frequency of the signal light L is determined by the intersection angle between the wave vector of the pump light P and the lattice vector of the periodically poled element 8 within the periodically poled element 8. Therefore, it can be said that it is essential to rotate the periodically poled element 8 by a large amount in order to ensure a wide variable range of the oscillation frequency of the signal light L. Generally, when pump light P is incident from a medium with a low refractive index, such as the air, to a medium with a high refractive index, such as the periodically poled element 8, the refraction angle becomes smaller than the angle of incidence.

[0024] The rotation stage 9 rotates the periodically poled element 8 around the Z axis in accordance with a drive signal supplied from outside the terahertz wave generating device 1 via a signal cable 9a. The rotation stage 9 is typically a stepping motor or a servo motor. As shown in FIG. 1, the rotation axis 8c of the periodically poled element 8 caused by the rotation stage 9 is set so as to be closer to the incident end face 8a than to the emitting end face 8b. Specifically, this is as follows.

[0025] In Figures 4 and 5, the optical path of the pump light P incident on the incident end face 8a of the periodically poled element 8 is indicated by a dashed line. In Figure 4, the rotation axis 8c of the periodically poled element 8 is closer to the incident end face 8a than to the exit end face 8b. In Figure 5, the rotation axis 8c of the periodically poled element 8 is set at an equal distance from the incident end face 8a and the exit end face 8b. Figure 5 shows that when the rotation axis 8c of the periodically poled element 8 is set at an equal distance from the incident end face 8a and the exit end face 8b, even a slight rotation of the periodically poled element 8 causes the pump light P to overlap the edge of the incident end face 8a. If the pump light P overlaps the edge of the incident end face 8a, the periodically poled element 8 will be damaged. This is because if the pump light P overlaps the edge of the incident end face 8a, dielectric breakdown will occur due to extreme electric field concentration caused by the pump light P. 4, by bringing the rotation axis 8c of the periodically poled element 8 closer to the incident end face 8a than to the output end face 8b, it can be seen that the pump light P does not overlap the edge of the incident end face 8a even when the periodically poled element 8 is rotated significantly. From this, it can be said that by bringing the rotation axis 8c of the periodically poled element 8 closer to the incident end face 8a than to the output end face 8b, it is possible to ensure a large rotation angle range for the periodically poled element 8.

[0026] 4, if the rotation axis 8c of the periodically poled element 8 is brought closer to the incident end face 8a than to the output end face 8b, there is a risk that the pump light P will overlap the edge of the output end face 8b. If the pump light P overlaps the edge of the output end face 8b, the periodically poled element 8 will be damaged. The reason for this is as described above. Therefore, the rotation axis 8c of the periodically poled element 8 cannot be set close to the input end face 8a indiscriminately.

[0027] Therefore, in order to maximize the rotation angle range that the periodically poled element 8 can assume without damaging the periodically poled element 8 with the pump light P emitted to the incident end face 8a of the periodically poled element 8, the rotation axis 8c of the periodically poled element 8 is set as follows.

[0028] FIG. 6 is a plan view of the periodically poled element 8 as viewed in the Z direction. In FIG. 6, Θ is the angle of incidence of the pump light P incident on the incident end face 8a of the periodically poled element 8. θ is the refraction angle of the pump light P inside the periodically poled element 8. x is the rotation axis distance, which is the distance from the incident end face 8a to the rotation axis 8c of the periodically poled element 8 in the longitudinal direction of the periodically poled element 8. L is the element length in the plane of rotation of the periodically poled element 8. w is the element width in the plane of rotation of the periodically poled element 8. n is the refractive index of the periodically poled element 8. In FIG. 6, when the rotation axis distance x satisfies the following formula, the rotation angle range of the periodically poled element 8 is maximized without damaging the periodically poled element 8. The derivation of the following formula will be explained below. TIFF2026007365000002.tif11150

[0029] Since the element width w of the periodically poled element 8 on the plane of rotation is finite, the upper limit of the rotation angle of the periodically poled element 8 (=incident angle Θ of the pump light P) is determined by one or both of the following (1) and (2). (1) When the periodically poled element 8 is rotated by Θ, the pump light P incident on the incident end face 8a reaches the edge of the incident end face 8a. (2) When the periodically poled element 8 is rotated by Θ, the pump light P incident on the output end face 8b reaches the edge of the output end face 8b.

[0030] If only the above condition (1) is satisfied but the above condition (2) is not satisfied, this means that the rotation axis 8c of the periodically poled element 8 is too far from the incident end face 8a, as shown in Figure 5. On the other hand, if only the above condition (2) is satisfied but the above condition (1) is not satisfied, this means that the rotation axis 8c of the periodically poled element 8 is too close to the incident end face 8a, as shown in Figure 4. Therefore, it can be said that the rotation angle range of the periodically poled element 8 can be maximized when the above conditions (1) and (2) are satisfied simultaneously.

[0031] The condition for simultaneously satisfying the above (1) and (2) is geometrically expressed by the following formula. TIFF2026007365000003.tif8150 TIFF2026007365000004.tif11150 Furthermore, when the refraction of the pump light P at the incident end face 8a of the periodically poled element 8 is taken into consideration, the following Snell's law holds true. TIFF2026007365000005.tif6150 Combining the above equations, the above-mentioned equation for maximizing the rotation angle range of the periodically poled element 8 without damaging the periodically poled element 8 can be obtained.

[0032] Here, assuming that the wavelength of the pump light P is 1064 nanometers and the refractive index of the periodically poled element 8 is 2.15, for example, if the element length L is 50 millimeters and the element width w is 5 millimeters, the rotation axis distance x will be 11.4 millimeters. Also, for example, if the element length L is 40 millimeters and the element width w is 5 millimeters, the rotation axis distance x will be 9.0 millimeters. Also, for example, if the element length L is 50 millimeters and the element width w is 10 millimeters, the rotation axis distance x will be 10.7 millimeters. However, when the following formula is satisfied, the rotation axis distance x will be 0 millimeters. TIFF2026007365000006.tif6150

[0033] FIG. 7 is a graph showing the change in the wavelength of idler light Q due to rotation of the periodically poled element 8. In FIG. 7, the horizontal axis represents wavelength, and the vertical axis represents normalized intensity. FIG. 7 shows the frequency characteristics of the intensity of idler light Q when the rotation angle of the periodically poled element 8 is +5 degrees, +4 degrees, +3 degrees, +2 degrees, +1 degree, zero degrees, −1 degree, −2 degrees, −3 degrees, −4 degrees, and −5 degrees. FIG. 7 shows that by optimizing the rotation axis distance x as described above, it is possible to ensure a rotation angle width of 10 degrees for the periodically poled element 8, for example, and thereby to freely increase or decrease the wavelength of idler light Q between 1065.63 and 1065.85 nanometers. The wavelength of the terahertz waves generated by the periodically poled element 8 corresponds to the difference between the wavelength of the pump light P and the wavelength of the idler light Q. Therefore, in the example of Figure 7, it can be said that the frequency of the signal light L, which is a terahertz wave, can be tuned by approximately 60 gigahertz between 0.29 and 0.35 terahertz.

[0034] Returning to FIG. 1 , the high-reflection laser mirror 10 reflects the pump light P and idler light Q emitted from the exit end face 8b of the periodically poled element 8 and makes them incident on the beam damper 11. As a result, the pump light P and idler light Q emitted from the exit end face 8b of the periodically poled element 8 are blocked by the beam damper 11. In this way, the pump light P and idler light Q emitted from the exit end face 8b of the periodically poled element 8 are reflected at approximately right angles by the high-reflection laser mirror 10 and then incident on the beam damper 11. By ensuring the optical path length from the exit end face 8b to the beam damper 11, it is possible to prevent the 0.1 megawatt-class pump light P reflected by about 1% by the concave lens of the beam damper 11 from being condensed within the periodically poled element 8 and damaging the periodically poled element 8.

[0035] The signal light L generated by the periodically poled element 8 is emitted in the minus X direction from the incident end face 8a of the periodically poled element 8, reflected by the dichroic mirror 7, and collimated by the signal collimator 13. After emitting from the incident end face 8a of the periodically poled element 8, the signal light L diverges, expanding its beam diameter. Therefore, in order to capture all of the signal light L with an expanding beam diameter and reflect it toward the signal collimator 13, the dichroic mirror 7 is positioned as close as possible to the incident end face 8a of the periodically poled element 8. If the dichroic mirror 7 were positioned between the wavelength filter 6 and the seed collimator 12, the dichroic mirror 7 would only be able to capture a portion of the signal light L with an expanding beam diameter. In fact, if the signal light L is extracted from the terahertz wave generating device 1 after passing through the wavelength filter 6, the signal light L would be attenuated by the wavelength filter 6, preventing high-power signal light L from being obtained. In this sense, it is preferable to place the dichroic mirror 7 between the wavelength filter 6 and the periodically poled element 8 .

[0036] Referring again to FIG. 1 , the pump light P and the seed light S must be incident on the periodically poled element 8 at a slight angle relative to each other to satisfy the phase matching condition of the periodically poled element 8. If the wavelength filter 6 were not used, both the pump light source 2 and the seed collimator 12 would have to be placed in the seed-side space 21. Furthermore, to prevent the pump light source 2 from interfering with the optical path of the seed light S and the seed collimator 12 from interfering with the optical path of the pump light P, the pump light source 2 and the seed collimator 12 placed in the seed-side space 21 would need to be placed far away from the periodically poled element 8. In contrast, if the wavelength filter 6 is used to reflect the pump light P and roughly align the optical paths of the pump light P and the seed light S, the pump light source 2 can be placed in the pump-side space 20 rather than the seed-side space 21, thereby significantly reducing the longitudinal size of the terahertz wave generating device 1. While the wavelength filter 6 is generally used as a beam splitter, in this embodiment, the wavelength filter 6 is used as a beam combiner that aligns the pump light P and the seed light S. It is believed that employing the wavelength filter 6 as a beam combiner rather than a beam splitter in this way is a design concept unique to this embodiment.

[0037] Finally, with reference to FIGS. 8 and 9, a spatial resolution test of the signal light L generated by the terahertz wave generating device 1 will be introduced.

[0038] FIG. 8 is a plan view of the test apparatus 40. In FIG. 8, the terahertz wave generator 1 is mounted on an electric stage that moves the terahertz wave generator 1 in the X and Z directions. Signal light L emitted from the signal collimator 13 of the terahertz wave generator 1 is reflected by the first mirror 30 and the second mirror 31 and focused onto the resolution test chart 33 by a Tsurupica lens 32 (registered trademark). The resolution test chart 33 was made of USAF1951. The signal light L reflected by the resolution test chart 33 was collimated again by the Tsurupica lens 32 and detected by a Schottky barrier diode detector 35 using a beam splitter 34. In the spatial resolution test, a raster scan was performed, moving the terahertz wave generator 1 by 1 millimeter increments in the X and Z directions. FIG. 9 shows the test results using signal light L with a frequency of 0.33 terahertz. The horizontal and vertical axes of FIG. 9 are both in millimeters. 9, it can be seen that the spatial resolution of the terahertz wave generating device 1 is 2 mm or less. In addition, the signal light L is always stable during the raster scan, which proves that the robustness of the terahertz wave generating device 1 as a sensing technology has reached a practical level.

[0039] The preferred embodiments of the present disclosure have been described above. The above embodiments have the following features. Specifically, the terahertz wave generating device 1 includes a pump light source 2 that generates pump light P, a periodically poled element 8 as a nonlinear optical element having a periodic structure in which the polarization or crystal orientation is inverted at a certain inversion period, and a rotation stage 9 that rotatably supports the periodically poled element 8. Signal light L, which is a terahertz wave, is generated by irradiating the pump light P onto the periodically poled element 8, and the wavelength of the signal light L is changed by rotating the periodically poled element 8. The periodically poled element 8 has an incident end face 8a onto which the pump light P is incident and an exit end face 8b from which the pump light P exits. The rotation axis 8c of the periodically poled element 8 is closer to the incident end face 8a than to the exit end face 8b. The above configuration realizes a terahertz wave generating device that can rotate the periodically poled element 8 by a large amount without damaging the periodically poled element 8.

[0040] Furthermore, the rotation axis distance x, which is the distance between the incident end face 8a and the rotation axis 8c of the periodically poled element 8, is set so that when the pump light P enters from the edge of the incident end face 8a, the pump light P exits from the edge of the exit end face 8b. With the above configuration, the rotation angle range of the periodically poled element 8 can be maximized without damaging the periodically poled element 8.

[0041] The terahertz wave generating device 1 also includes a wavelength filter 6 that transmits seed light S supplied from the outside and reflects pump light P. The seed light S is transmitted through the wavelength filter 6 and then incident on the incident end face 8a of the periodically poled element 8. The pump light P is reflected by the wavelength filter 6 and then incident on the incident end face 8a of the periodically poled element 8. With the above configuration, the pump light source 2 can be disposed away from the optical path of the seed light S, thereby realizing a compact size of the terahertz wave generating device 1 in the longitudinal direction.

[0042] The terahertz wave generating device 1 further includes a dichroic mirror 7, which is provided between the periodically poled element 8 and the wavelength filter 6, and transmits the pump light P and the seed light S and reflects the signal light L generated by the periodically poled element 8. With the above configuration, the signal light L generated by the periodically poled element 8 can be extracted from the terahertz wave generating device 1 without attenuation and without omission.

[0043] In addition, the pump light source 2 is water-cooled. According to the above configuration, the cooling performance of the pump light source 2 can be ensured with a small occupied area, which contributes to the miniaturization of the terahertz wave generating device 1. [Explanation of symbols]

[0044] 1. Terahertz wave generator 2. Pump light source 2a Optical Fiber 3 Light source cooling plate 3a Water supply tube 3b Cooling channel 3c Drainage tube 4 Output control section 5 Highly reflective laser mirrors 6 wavelength filters 7 Dichroic Mirror 7a 1st page 7b 2nd side 8 Periodic polarization inversion element 8a Incidence end face 8b Output end face 8c Rotation axis 9 Rotation Stage 9a signal cable 10 Highly reflective laser mirrors 11 Beam dumper 12 Seed Collimator 12a optical fiber 13 Signal collimator 14. Case 14a side wall 20 Pump side space 21 Seed side space 30 1st Mirror 31 Second mirror 33 Resolution Test Chart 34 Beam Splitter 35 Schottky barrier diode detector 40 Test Equipment P pump light S seed light Q Idler Light L signal light x Rotation axis distance w Element width Θ angle of incidence

Claims

1. a pump light source for generating pump light; a nonlinear optical element having a periodic structure in which polarization or crystal orientation is inverted at a certain inversion period; a rotation stage that rotatably supports the nonlinear optical element; Including, The pump light is incident on the nonlinear optical element to generate signal light which is a terahertz wave, and the wavelength of the signal light is changed by rotating the nonlinear optical element. A terahertz wave generating device, the nonlinear optical element has an incident end surface onto which the pump light is incident and an exit end surface from which the pump light is exited, a rotation axis of the nonlinear optical element is closer to the incident end face than to the output end face; Terahertz wave generator.

2. a rotation axis distance, which is the distance between the incident end face and the rotation axis of the nonlinear optical element, is set so that when the pump light is incident on the edge of the incident end face, the pump light is emitted from the edge of the emission end face; The terahertz wave generating device according to claim 1 .

3. The rotation axis distance is set according to the following formula: The terahertz wave generating device according to claim 2 . where x is the rotation axis distance, L is the element length of the nonlinear optical element in the plane of rotation, w is the element width of the nonlinear optical element in the plane of rotation, and n is the refractive index of the nonlinear optical element.

4. a wavelength filter that transmits the seed light supplied from an external source and reflects the pump light; the seed light is transmitted through the wavelength filter and then incident on the incident end surface of the nonlinear optical element; the pump light is reflected by the wavelength filter and then incident on the incident end face of the nonlinear optical element; The terahertz wave generating device according to claim 1 .

5. provided between the nonlinear optical element and the wavelength filter, a dichroic mirror that transmits the pump light and the seed light and reflects the signal light generated by the nonlinear optical element; The terahertz wave generating device according to claim 4 .

6. The pump light source is water-cooled. The terahertz wave generating device according to claim 1 .

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