Open-type resonator

Abstract

To provide an open-type resonator which can measure the dielectric property of a sample more accurately by removing an unnecessary resonance of a higher mode.SOLUTION: An open-type resonator 100 includes: a first spherical reflection mirror 11 having a first reflection sphere 13; and a second sphere reflection mirror 12 having a second reflection sphere 14 facing the first reflection sphere 13. At least one of the diameters M of opening surfaces exposed in the space between a first reflection sphere 13 and a second reflection sphere 13 of the first reflection sphere 13 and the second reflection sphere 14 is not larger than a half of the distance D between the first reflection sphere 13 and the second reflection sphere 14.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an open-type resonator suitable for measuring the dielectric properties of a dielectric (the real part of the complex relative permittivity (relative permittivity ε') and the dielectric loss tangent (tan δ)). [Background technology]

[0002] Millimeter-wave frequencies are used in applications such as automotive radar, optical communications, and high-speed digital devices. Improved position resolution in radar, increased communication speeds in optical communications, and faster processing speeds in digital devices are essential challenges, and these demands are expected to drive millimeter-wave frequencies even higher. Currently, cutting-edge devices use the 75-80 GHz, 50 GHz, and 40 GHz bands, respectively. Going forward, frequencies above 100 GHz are expected. Furthermore, discussions are underway regarding the use of frequencies up to 330 GHz for the sixth-generation (6G) communications network, which will replace the fifth-generation (5G) communications network. This trend is driving the need for higher-frequency measurements of the materials used in these devices. Among these material properties, the energy loss of millimeter-wave waves associated with higher frequencies poses a significant problem, making it essential to measure the dielectric properties of materials.

[0003] In measuring dielectric properties in the millimeter wave band, reducing energy loss is a particularly important development issue, making measurement of the dielectric loss tangent (loss angle, tanδ) crucial, and traditionally, measurements using resonators have been the norm. Split cylinder resonators are a typical example, and are used to measure the dielectric loss tangent of low-loss materials up to approximately 60 GHz. However, it has become difficult to accurately measure dielectric properties at higher frequencies using split cylinder resonators, and open-type resonators (Fabry-Perot resonators) are more suitable for higher frequencies (Non-Patent Document 1).

[0004] In a Fabry-Perot resonator, a film-like sample is inserted between two spherical reflecting mirrors arranged opposite each other. An input signal with a frequency of, for example, 100 GHz or higher is input, and resonance measurements are performed to obtain the resonance waveform and measure the dielectric properties of the sample. A network analyzer is often used for resonance measurements. The network analyzer is connected to the Fabry-Perot resonator to obtain a graph (resonance waveform) with frequency on the horizontal axis and transmitted signal strength (transmission coefficient) on the vertical axis, and the resonance characteristics are measured. Here, "resonance characteristics" refers to the center frequency of the resonance (resonance frequency) and the Q value (in this specification, the ratio of the center frequency to the 3 dB bandwidth). The relative permittivity and dielectric loss tangent of the sample are generally calculated or simulated from the resonance characteristics with and without the sample.

[0005] Non-Patent Document 1 above discloses an open resonator in which the distance D between the two spherical reflecting mirrors is 50 to 70 cm, the radius of curvature R of the two spherical reflecting mirrors is 127.0 cm, and the aperture diameter (diameter) of the reflecting spherical surfaces of the two spherical reflecting mirrors is 36.8 cm. Furthermore, Japanese Industrial Standards (JIS R1660-2) exemplifies an open resonator in which the aperture diameter of the reflecting spherical surfaces of the two spherical reflecting mirrors is 80 to 205 mm, the radius of curvature R of the two spherical reflecting mirrors is the same as the aperture diameter of the reflecting spherical surfaces, and the distance D between the two spherical reflecting mirrors is usually 1.2 times (1.1 to 1.3 times depending on the value of tan δ) the aperture diameter of the reflecting spherical surfaces.

[0006] When measuring the dielectric properties of a sample using a Fabry-Perot resonator, the resonance used for measurement is the TEM00q mode. However, it is known that unwanted higher-order TEMmnq mode resonances occur. The order m is the rotational order, and the order n is the radial order. The order q is the z-direction order (the direction connecting the centers of the two spherical reflectors) and corresponds to the number of standing waves present between the two spherical reflectors (number of standing waves = q + 1). TEM00q mode resonances can be symmetric (with the anti-symmetric mode) or anti-symmetric (with the anti-node of the standing wave at the center). The resonance used to measure dielectric properties is the symmetric mode, and the order q is an even number. In practical Fabry-Perot resonators, higher-order mode resonances rarely occur with an order m other than zero, resulting in the TEM0nq mode, where the order n is a natural number between 1 and 5. The resonance of the TEM0nq mode occurs at a frequency slightly higher than that of the TEM00q mode.

[0007] Figure 10 is a conceptual diagram for explaining the resonance modes in a Fabry-Perot resonator, where (a) of Figure 10 shows the resonance of the TEM00q mode, (b-1) of Figure 10 shows the resonance of the TEM01q mode of the TEM0nq mode, and (b-2) of Figure 10 shows the resonance of the TEM02q mode of the TEM0nq mode. As shown in (a) of Figure 10, the resonance of the TEM00q mode has a spread that is approximated by a Gaussian distribution around the central axis connecting the centers of the opposing spherical reflecting mirrors 60. In contrast, the resonance of the TEM0nq mode has a wider spread from the central axis than the TEM00q mode, as shown in (b-1) and (b-2) of Figure 10.

[0008] When a sample is attached to a Fabry-Perot resonator to measure its dielectric properties, the resonant frequencies of each mode change (shift) from the state without the sample attached. Depending on the sample, the resonant frequency of the TEM00q mode used in the measurement may shift, causing the TEM00q mode resonant waveform to overlap with the TEM0nq mode resonant waveform, resulting in measurement errors or even failure. Therefore, the JIS standard recommends inserting an aperture (optical diaphragm) with a variable hole diameter (diaphragm diameter) made of polyvinyl chloride (PVC) or similar between the sample stage and one of the spherical reflectors 60 to reduce unwanted TEM0nq mode resonance. Figure 11 is a conceptual diagram illustrating the function of an aperture in a Fabry-Perot resonator. As shown in Figure 11(a), the TEM00q mode resonance has little divergence from the central axis connecting the centers of the opposing spherical reflectors 60, so it is hardly affected by the aperture diaphragm of the aperture 61. In contrast, the resonance of the TEM0nq mode has a spread in the radial direction of the spherical reflecting mirror 60, as shown in FIG. 11(b), and is therefore blocked by the aperture 61 and suppressed. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] AL CULLEN and PK YU, The accurate measurement of permittivity by means of open resonator, Proc. R. Soc. Lond. A. 325, 493-509 (1971) Summary of the Invention [Problem to be solved by the invention]

[0010] To provide an open-type resonator that can measure the dielectric properties of a sample with higher accuracy by suppressing the resonance of unnecessary higher modes (particularly, TEM04q mode) while minimizing the effect on the resonance mode (TEM00q mode) used to measure the dielectric properties more efficiently than when an aperture is used. [Means for solving the problem]

[0011] The open resonator of the present disclosure includes a first spherical reflecting mirror having the first reflective spherical surface, and a second spherical reflecting mirror having a second reflective spherical surface disposed opposite the first reflective spherical surface, wherein at least one of the diameters of the opening surfaces of the first and second reflective spherical surfaces exposed to the space between the first and second reflective spherical surfaces is equal to or less than half the inter-spherical distance between the first and second reflective spherical surfaces. [Effects of the Invention]

[0012] The open-type resonator of the present disclosure can suppress resonance of unnecessary higher-order modes while minimizing the effect on the resonance mode used to measure the dielectric properties more efficiently than when an aperture is used, thereby enabling the dielectric properties of the sample to be measured with higher accuracy. [Brief explanation of the drawings]

[0013] [Figure 1] Schematic diagram of a Fabry-Perot resonator according to the first embodiment [Figure 2] 1A and 1B are schematic diagrams showing the aperture diameter M, the distance D between the spherical surfaces, and the radius of curvature R of the two reflecting spherical surfaces of the Fabry-Perot resonator according to the first embodiment ((a): Comparative Example 1a, (b): Example 1). [Figure 3] Schematic diagram to explain the resonant modes measured in a Fabry-Perot resonator [Figure 4] 1A and 1B are diagrams showing the resonance waveform measured by the Fabry-Perot resonator according to the first embodiment when no measurement sample is attached ((a): Comparative Example 1a, (b): Example 1). [Figure 5]1A and 1B are diagrams showing the resonance waveform measured by the Fabry-Perot resonator according to the first embodiment when a measurement sample is attached ((a): Comparative Example 1a, (b): Example 1). [Figure 6] 1A and 1B are diagrams showing the dielectric loss tangent (tanδ) measured in the Fabry-Perot resonator according to the first embodiment ((a): Comparative Example 1a, (b): Example 1). [Figure 7] 1A and 1B are schematic diagrams showing the aperture diameter M, the distance D between the spherical surfaces, and the radius of curvature R of the two reflecting spherical surfaces of a Fabry-Perot resonator according to the second embodiment ((a): Comparative Example 2a, (b): Example 2). [Figure 8] 10A and 10B are diagrams showing the resonance waveform measured by the Fabry-Perot resonator according to the second embodiment when no measurement sample is attached ((a): Comparative Example 2a, (b): Example 2). [Figure 9] FIG. 11 is a diagram summarizing the results of resonance measurements using a Fabry-Perot resonator as a third embodiment. [Figure 10] Conceptual diagram to explain the resonant modes in a Fabry-Perot resonator ((a): TEM00q mode, (b-1): TEM0nq (TEM01q) mode, (b-2): TEM0nq (TEM02q) mode) [Figure 11] Conceptual diagram to explain the effect of an aperture in a Fabry-Perot cavity ((a): TEM00q mode, (b): TEM0nq mode) DETAILED DESCRIPTION OF THE INVENTION

[0014] (Embodiment 1) FIG. 1 is a schematic diagram of a Fabry-Perot resonator according to the first embodiment. As shown in FIG. 1, the Fabry-Perot resonator 100 according to the first embodiment includes a fixed base 10, a first spherical reflecting mirror 11, a second spherical reflecting mirror 12, a sample stage 20, a position adjustment mechanism 35, and a cover 50. The Fabry-Perot resonator 100 is an example of an open-type resonator. In the following description, an XYZ Cartesian coordinate system shown in FIG. 1 is used, in which the X direction corresponds to the up-down direction, the Y direction corresponds to the front-back direction, and the Z direction corresponds to the left-right direction.

[0015] As shown in FIG. 1 , a first spherical reflecting mirror 11 having a first reflecting spherical surface 13 and a second spherical reflecting mirror 12 having a second reflecting spherical surface 14 are arranged facing each other on a fixed base 10. The center of the first reflecting spherical surface 13 and the center of the second reflecting spherical surface 14 have a predetermined inter-spherical distance D. A first waveguide 41 and a second waveguide 42 are arranged at the centers of the first reflecting spherical surface 13 and the second reflecting spherical surface 14, respectively. A first coupling hole 15 and a second coupling hole 16 having a minute diameter are formed at the tip openings of the first waveguide 41 and the second waveguide 42 on the spherical surface side, respectively, to form a coupling state that obtains desired resonance characteristics. The first waveguide 41 of the first spherical reflecting mirror 11 is a signal injection section to which an input signal for measuring the dielectric properties of a sample is input, and the second waveguide 42 of the second spherical reflecting mirror 12 is a signal detection section to which a detection signal is output.

[0016] As shown in Fig. 1, the sample stage 20 has a through-hole 24 and is disposed between the first spherical reflecting mirror 11 and the second spherical reflecting mirror 12 which face each other. A sample 25, the dielectric properties of which are to be measured, is mounted on the sample stage 20. The sample 25 mounted on the sample stage 20 is exposed through the through-hole 24. The diameter of the through-hole 24 is set smaller than the opening diameter M of the first and second reflecting spherical surfaces 13 and 14.

[0017] As shown in FIG. 1, the position adjustment mechanism 35 has a pedestal 36 and a micrometer 37. The pedestal 36 is installed so as to be movable (slidable) in the Z direction relative to the fixed base 10 (i.e., the first spherical reflecting mirror 11 and the second spherical reflecting mirror 12). An operator can move the pedestal 36 by operating the micrometer 37. A sample stage 20 is fixed to the pedestal 36. That is, an operator can adjust the position of the sample 25 attached to the sample stage 20 in the Z direction by operating the micrometer 37 to move the sample stage 20 fixed to the pedestal 36 via the pedestal 36.

[0018] The cover 50 is U-shaped and includes a front plate, a back plate, and a top plate connecting the front plate and the back plate, which are made of transparent acrylic plates. As shown in Fig. 1, when measuring dielectric properties, the front plate, back plate, and top plate of the cover 50 respectively cover the front side, back side, and top side of the Fabry-Perot resonator 100. When the sample 25 is mounted on the sample stage 20, the cover 50 is slid upward and removed from the Fabry-Perot resonator 100, exposing the space between the first spherical reflecting mirror 11 and the second spherical reflecting mirror 12 (i.e., the space where the sample stage 20 is disposed).

[0019] 2 is a schematic diagram showing the relationship between the aperture diameter M, the distance D between the spherical surfaces, and the radius of curvature R of the two reflecting spherical surfaces of the Fabry-Perot resonator according to the first embodiment, with (a) in FIG. 2 showing Comparative Example 1a and (b) in FIG. 2 showing Example 1. In the first embodiment, as shown in FIGS. 1 and 2, the entire first reflecting spherical surfaces 13, 13a and the entire second reflecting spherical surfaces 14, 14a are exposed in the space between the first and second reflecting spherical surfaces, so that the diameter of the reflecting spherical surfaces matches the aperture diameter M (corresponding to the diameter of the aperture surface) of the reflecting spherical surfaces.

[0020] The aperture diameter M, inter-sphere distance D, and curvature radius R of the reflecting spherical surfaces of Comparative Example 1a and Example 1 shown in Fig. 2 are as follows: In the Fabry-Perot resonator 100 according to the first embodiment, the inter-sphere distance D is set to 80 mm to accommodate an input signal with a relatively high measurement frequency bandwidth (J band: 220 to 330 GHz). Reflective spherical surface opening diameter M: 50 mm (Comparative Example 1a), 28 mm (Example 1) Curvature radius R: 64 mm (Comparative Example 1a and Example 1) Distance between spherical surfaces D: 80 mm (Comparative Example 1a and Example 1)

[0021] That is, the opening diameter M of the reflecting spherical surface in Comparative Example 1a is larger than half the inter-sphere distance D and larger than half the radius of curvature R. In contrast to this, the opening diameter M of the reflecting spherical surface in Example 1 is equal to or smaller than half the inter-sphere distance D and equal to or smaller than half the radius of curvature R. Note that the configuration of the Fabry-Perot resonator in Comparative Example 1a is the same as that of the Fabry-Perot resonator 100 in Example 1, except for the first and second spherical reflecting mirrors 11a and 12a and the first and second reflecting spherical surfaces 13a and 14a, and therefore description thereof will be omitted.

[0022] (Measurement of dielectric properties) The procedure (steps) for measuring the dielectric properties using the Fabry-Perot resonator 100 are as follows. 1) Connect the Fabry-Perot resonator 100, the network analyzer, and the controller with cables. 2) With no sample attached (no sample), the resonance characteristic (first resonance characteristic) is measured at the resonance frequency to be measured, and the Q value Qempty is calculated from the bandwidth of the resonance waveform. 3) Measure five resonance frequencies, including the resonance at the frequency to be measured and the resonances before and after it, and calculate the inter-spherical distance D between the reflecting spherical surfaces from the five resonance frequencies. 4) After removing the cover 50 and mounting the sample 25 on the sample stage 20, the cover 50 is used to cover the space between the first spherical reflecting mirror 11 and the second spherical reflecting mirror 12. 5) The micrometer 37 is operated to adjust the position of the sample 25 (the sample 25 is positioned at the position where the resonance frequency is at its minimum value). 6) With the position of the sample 25 adjusted (sample present), measure the resonance characteristics (second resonance characteristics) at the resonance frequency to be measured, and determine the center frequency of the resonance (resonance frequency Fsample) and the Q value Qsample that have shifted due to the insertion of the sample 25. 7) The relative permittivity ε' and dielectric loss tangent tanδ of the sample are calculated from the thickness t of the sample 25, the distance between spheres D, the Q value Qempty without the sample, the resonant frequency Fsample with the sample, and the Q value Qsample with the sample. When measuring the dielectric properties of the same sample 25 at multiple frequencies, after step 1), steps 2) and 3) are performed at all frequencies to be measured, then steps 4) and 5) are performed, and then steps 6) and 7) are performed at all frequencies to be measured.

[0023] Next, we will explain the resonance modes excited in the Fabry-Perot resonator. Figure 3 is a conceptual diagram for explaining the resonance modes excited in the Fabry-Perot resonator. Figure 3 shows the resonance waveforms measured when the resonance characteristics are measured without a sample in step 2) above, along with the corresponding resonance modes. The arrows in Figure 3 also indicate the predicted direction and amount of change (amount of movement) of the resonance frequency of each resonance mode when the resonance characteristics are measured with a sample in step 6) above.

[0024] The resonance used to measure dielectric properties is the TEM00q mode. Resonance of the TEM00(q-1) mode is observed at lower frequencies than the resonance frequency of the TEM00q mode, and resonance of the TEM00(q+1) mode is observed at higher frequencies. The TEM00q mode, TEM00(q-1) mode, and TEM00(q+1) mode are resonances in which the signal intensity spreads in a Gaussian distribution around the central axis connecting the centers of two opposing reflective spherical surfaces. The TEM00q mode has a standing wave antinode located at the center of its central axis, while the TEM00(q-1) mode and TEM00(q+1) mode have a standing wave node located at the center of their central axes. Resonances of the TEM0n(q-1) mode and TEM0nq mode appear at frequencies between the TEM00q mode and the TEM00(q-1) mode, and between the TEM00q mode and the TEM00(q+1) mode, respectively. FIG. 3 shows resonances in the TEM0n(q-1) mode and the TEM0nq mode when the order n is 1 to 4.

[0025] When the sample 25 is mounted in step 6 above and positioned at the antinode of the TEM00q mode resonance (the center of the resonator), the measured resonant frequency shift alternates between relatively large and small resonance modes, as shown by the arrows in Figure 3. As shown by the arrows in Figure 3, the large-shift resonance mode is the resonance whose antinode is located at the center of the resonator where the sample is located, while the small-shift resonance mode is the resonance whose node is located at the center. Because the TEM00q mode resonance is used to measure dielectric properties, the sample 25 is positioned at the antinode of the resonance (the center of the resonator), and the resonant frequency shift is relatively large. This shift varies depending on the type and thickness (t) of the sample 25. For example, comparing a modified polyimide (MPI) with a thickness (t) of 50 μm and a polycarbonate (PC) with a thickness (t) of 98 μm as sample 25, the resonant frequency shift of the polycarbonate is larger. When the resonant frequency of the TEM00q mode used to measure dielectric properties shifts significantly, the resonant frequency of the TEM04(q-1) mode, which exists on the lower frequency side of the TEM00q mode, shifts relatively little. Therefore, when measuring the resonant characteristics of a sample 25 mounted on the sample, the resonant waveform of the TEM00q mode is superimposed on the resonant waveform of the TEM04(q-1) mode, distorting the resonant waveform and potentially preventing accurate measurement of the dielectric properties of the sample 25. In a Fabry-Perot resonator with a realistic design, TEM0n(q-1) mode resonances with order n of 5 or greater rarely occur. Furthermore, as shown in Figure 3, the resonant frequency of the TEM03(q-1) mode shifts in the same direction as the TEM00q mode, while the resonant frequency of the TEM02(q-1) mode is significantly different from the TEM00q mode, so no effect occurs in actual measurements. Therefore, to more accurately measure the dielectric properties of the sample 25, it is necessary to sufficiently reduce the resonant frequency of the TEM04(q-1) mode.

[0026] The amplitude (intensity) of each resonance, such as the TEM00q mode and the TEM04(q-1) mode, tends to be relatively large on the high-frequency side of the measurement frequency bandwidth (220 to 330 GHz in the first embodiment) and small on the low-frequency side. On the high-frequency side, the wavelength of the input signal is short, so the input signal easily passes through the coupling hole. Furthermore, the spread due to diffraction is small, so the effect of reducing the opening diameter M of the reflective spherical surface (reducing the aperture diameter if aperture 61 is used) is less pronounced, resulting in a large amplitude of each resonance mode. On the low-frequency side, the wavelength of the input signal is long, so the input signal does not easily pass through the coupling hole. Furthermore, the spread due to diffraction is large, so the effect of reducing the opening diameter M of the reflective spherical surface (reducing the aperture diameter if aperture 61 is used) is more pronounced, resulting in a small amplitude of each resonance and a deterioration in the Q value. Therefore, in order to measure the dielectric properties of sample 25 more accurately, it is necessary to sufficiently reduce the resonance of the TEM04(q-1) mode, which has a large amplitude at high frequencies, and to sufficiently secure the amplitude and Q value of the resonance of the TEM00q mode, which has a small amplitude at low frequencies and a degraded Q value.

[0027] Next, we will explain the results of resonance measurements using Fabry-Perot resonators having the aperture diameter M, inter-sphere distance D, and curvature radius R shown in Comparative Example 1a and Example 1 in FIG. 2. FIG. 4 shows resonance waveforms measured without a measurement sample attached, with FIG. 4(a) showing Comparative Example 1a and FIG. 4(b) showing Example 1. In Comparative Example 1a shown in FIG. 4(a), a vinyl chloride aperture 61 was inserted between the sample stage 20 and the second reflective spherical surface 14a, and the aperture diameter of the aperture 61 was adjusted to minimize the resonance of the unnecessary higher-order TEM04(q-1) mode without significantly affecting the resonance of the TEM00q mode required for measurement. In Example 1 shown in FIG. 4(b), no aperture was used.

[0028] As shown in FIG. 4, in both Comparative Example 1a and Example 1, an input signal of 299 GHz was used as the measurement frequency, and resonance in the TEM00q mode used to measure dielectric properties was observed near 299 GHz. Furthermore, in Comparative Example 1a, as shown in FIG. 4(a), resonance in the TEM04(q-1) mode, a higher-order mode, was observed near a frequency of 298.3 GHz. That is, in Comparative Example 1a, even if the aperture diameter of aperture 61 was optimized, if an attempt was made to maintain the amplitude and Q value of the TEM00q mode resonance at a level required for measurement, the TEM04(q-1) mode resonance, which interferes with measurement, inevitably remained. In contrast, in Example 1, as shown in FIG. 4(b), resonance was not observed near a frequency of 298.3 GHz. That is, in the Fabry-Perot resonator 100 of Example 1, the aperture diameter M (28 mm) of the reflective spherical surface is set smaller than the aperture diameter M (50 mm) in Comparative Example 1a, so that the resonance of the TEM04(q-1) mode, which interferes with measurement, is efficiently eliminated without significantly affecting the resonance of the TEM00q mode.

[0029] 5A and 5B show resonance waveforms measured with a measurement sample attached, with (a) in FIG. 5 showing Comparative Example 1a and (b) in FIG. 5 showing Example 1. Polycarbonate with a thickness t of 98 μm was used as sample 25. In Comparative Example 1a shown in (a) in FIG. 5, an aperture 61 was inserted for measurement, as in Comparative Example 1a shown in (a) in FIG. 4, while no aperture was used in Example 1 shown in (b) in FIG.

[0030] As shown in FIG. 5, in both Comparative Example 1a and Example 1, the insertion of sample 25 shifts the resonant frequency of the TEM00q mode toward lower frequencies, and a TEM00q mode resonance is observed near a frequency of 298.45 GHz. In Comparative Example 1a, as shown in FIG. 5(a), when sample 25 is inserted, a resonance of the higher-order TEM04(q-1) mode is observed near a frequency of 298.3 GHz. Furthermore, as described with reference to FIG. 3, the amount of shift of the resonant frequency of the TEM04(q-1) mode toward lower frequencies due to the insertion of sample 25 is generally smaller than that of the TEM00q mode. However, when polycarbonate with a thickness t of 98 μm is used as sample 25 as in the present embodiment, the resonant frequency of the TEM04(q-1) mode hardly shifts at all. In contrast, in Example 1, as shown in FIG. 5(b), no resonance is observed near a frequency of 298.3 GHz. That is, in the Fabry-Perot resonator 100 of Example 1, the aperture diameter M (28 mm) of the reflective spherical surface is set to be smaller than half the distance D (80 mm) between the spherical surfaces, which is considered to eliminate the resonance of the higher-order TEM04(q-1) mode. As shown in Comparative Example 1a in Figure 5(a), if a higher-order mode resonance having a resonance frequency near the resonance frequency of the TEM00q mode used to measure the dielectric properties appears, it will cause distortion in the TEM00q mode resonance waveform. If the TEM00q mode resonance waveform is distorted, an error will occur in the 3 dB bandwidth, making it impossible to accurately measure the Q value (i.e., tan δ).

[0031] 4(a) of Comparative Example 1a (no sample) with that of FIG. 5(a) shows that the amplitude of the TEM00q mode resonance used to measure the dielectric properties is reduced due to the influence of the dielectric loss of the sample, but the TEM04(q-1) mode resonance, which is detrimental to the measurement, is less affected by the sample and its amplitude is not reduced. Therefore, if the resonance of the TEM04(q-1) mode, which is an unnecessary higher-order mode, is not sufficiently reduced and remains, the amplitude of the TEM04(q-1) mode resonance becomes relatively large compared to the TEM00q mode, significantly affecting the measurement of the dielectric properties.

[0032] 6A and 6B are graphs showing the dielectric loss tangent (tanδ) measured for the Fabry-Perot resonators of Comparative Example 1a and Example 1, with FIG. 6A showing Comparative Example 1a and FIG. 6B showing Example 1. The measured sample 25 was made of polycarbonate with a thickness t of 98 μm. In Comparative Example 1a shown in FIG. 6A, an aperture 61 was inserted as in Comparative Example 1a shown in FIG. 4A, while no aperture was used in Example 1 shown in FIG. 6B.

[0033] As shown in Fig. 6, the measured values ​​of tan δ according to Comparative Example 1a shown in Fig. 6(a) vary more than those according to Example 1 shown in Fig. 6(b). This is because, in the Fabry-Perot resonator of Comparative Example 1a, the opening diameter M of the first and second reflecting spherical surfaces 13a and 14a is larger than half the distance D between the spherical surfaces, and therefore the resonance of higher modes is not sufficiently reduced, and the resonance waveform of the higher mode TEM04(q-1) is superimposed on the resonance waveform of the TEM00q mode used to measure the dielectric properties, causing distortion in the resonance waveform of the TEM00q mode.

[0034] (Embodiment 2) 7A and 7B are schematic diagrams showing the relationship between the aperture diameter M, the distance D between the spherical surfaces, and the radius of curvature R of the two reflecting spherical surfaces of the Fabry-Perot resonator according to the second embodiment, where (a) in FIG. 7A shows Comparative Example 2a, and (b) in FIG. 7B shows Example 2.

[0035] 7, the aperture diameter M, the distance D between the spheres, and the radius of curvature R of the reflective spherical surfaces of Comparative Example 2a and Example 2 are as follows: In the Fabry-Perot resonator according to the second embodiment, the distance D between the spheres is set to 120 mm to accommodate input signals with a measurement frequency bandwidth (D band: 110 to 170 GHz) lower than that of the Fabry-Perot resonator according to the first embodiment. Reflective spherical surface opening diameter M: 61 mm (Comparative Example 2a), 45 mm (Example 2) Curvature radius R: 96 mm (Comparative Example 2a and Example 2) Distance between spherical surfaces D: 120 mm (Comparative Example 2a and Example 2)

[0036] That is, the opening diameter M of the reflecting spherical surface in Comparative Example 2a is larger than half the inter-sphere distance D and larger than half the radius of curvature R. In contrast to this, the opening diameter M of the reflecting spherical surface in Example 2 is equal to or smaller than half the inter-sphere distance D and equal to or smaller than half the radius of curvature R. Note that the configurations of the Fabry-Perot resonators of Comparative Example 2a and Example 2 other than first spherical reflecting mirrors 111a, 111, second spherical reflecting mirrors 112a, 112, first reflecting spherical surfaces 113a, 113, and second reflecting spherical surfaces 114a, 114 are the same as those of Fabry-Perot resonator 100 of Embodiment 1 (Example 1), and therefore description thereof will be omitted.

[0037] Next, we will explain the results of resonance measurements using Fabry-Perot resonators having the aperture diameter M, inter-sphere distance D, and curvature radius R shown in Comparative Example 2a and Example 2 in FIG. 7. FIG. 8 shows resonance waveforms measured without a measurement sample attached, with FIG. 8(a) showing Comparative Example 2a and FIG. 8(b) showing Example 2. In Comparative Example 2a shown in FIG. 8(a), an aperture 61 was inserted between the sample stage 20 and the second reflecting spherical surface 114a, and the aperture diameter of the aperture 61 was adjusted to minimize the resonance of the unnecessary higher-order TEM0nq mode without affecting the resonance of the TEM00q mode required for measurement. In Example 2 shown in FIG. 8(b), no aperture was used.

[0038] As shown in Figure 8, in both Comparative Example 2a and Example 2, an input signal of 167 GHz was used as the measurement frequency, and resonance in the TEM00q mode used to measure dielectric properties was observed near 167 GHz. Furthermore, in Comparative Example 2a, as shown in Figure 8(a), resonance in the TEM04(q-1) mode, which is a higher-order mode, was observed near a frequency of 166.6 GHz. In contrast, in Example 2, as shown in Figure 8(b), resonance was not observed near a frequency of 166.6 GHz. That is, in the Fabry-Perot resonator of Example 2, the aperture diameter M (45 mm) of the reflective spherical surface was set smaller than the aperture diameter M (61 mm) in Comparative Example 2a, and therefore it is believed that resonance in the TEM04(q-1) mode, which is a higher-order mode, was eliminated.

[0039] (Embodiment 3) Next, the relationship between the aperture diameter M of the reflective spherical surface and the measurement frequency (i.e., the corresponding wavelength λ) is described. Comparative Examples 1a and 2a of Embodiments 1 and 2 show measurement results obtained by adjusting the aperture diameter of the aperture 61 to minimize the unwanted high-order TEM04(q-1) mode resonance without substantially affecting the TEM00q mode resonance used in the resonance measurement. In Embodiment 3, measurement results are shown for low and high frequencies for Examples 1 and 2 of Embodiments 1 and 2, Comparative Examples 1a and 2a (large aperture diameter) in which the aperture 61 was adjusted to minimize the TEM04(q-1) mode resonance without affecting the TEM00q mode resonance, and Comparative Examples 1b and 2b (small aperture diameter) in which the aperture 61 was further narrowed compared to Comparative Examples 1a and 2a to the extent that the TEM04(q-1) mode resonance was eliminated. Figure 9 summarizes the results of these resonance measurements. Figure 9 shows the wavelength λ corresponding to the measurement frequency range and the ratio M / λ of the aperture diameter M of the reflective sphere to the wavelength λ, as well as whether the TEM04(q-1) mode resonance that interferes with measurement is sufficiently eliminated, and whether the amplitude and Q value of the TEM00q mode resonance used in measurement are sufficient. A "◯" symbol in Figure 9 indicates that the elimination of the TEM04(q-1) mode resonance or the amplitude and Q value of the TEM00q mode resonance are sufficient, and an "×" symbol indicates that the elimination of the TEM04(q-1) mode resonance or the amplitude or Q value of the TEM00q mode resonance is insufficient.

[0040] As mentioned above, in resonance measurements, the amplitude of each resonance mode is larger on the high-frequency side of the measurement frequency bandwidth than on the low-frequency side. As mentioned above, the reduction in amplitude when a sample is attached is greater for the TEM00q mode used in the measurement than for the TEM04(q-1) mode, which interferes with the measurement. Therefore, on the high-frequency side, the amplitude of the TEM04(q-1) mode resonance must be sufficiently reduced when measuring resonance without a sample. As shown in Figure 9, the amplitude of the TEM04(q-1) mode resonance was sufficiently reduced for the Fabry-Perot resonator of Example 1 at both the high-frequency side (330 GHz) and the low-frequency side (220 GHz), and for the Fabry-Perot resonator of Example 2 at both the high-frequency side (170 GHz) and the low-frequency side (110 GHz). It was confirmed that the amplitude and Q value of the TEM00q mode resonance were sufficient.

[0041] In contrast, the Fabry-Perot resonator of Comparative Example 1a did not sufficiently reduce the TEM04(q-1) mode resonance at the measurement frequency of 330 GHz on the high frequency side, and the Fabry-Perot resonator of Comparative Example 2a did not sufficiently reduce the TEM04(q-1) mode resonance at the measurement frequency of 170 GHz on the high frequency side. In Comparative Example 1b, in which the aperture diameter of the aperture 61 was narrowed so that the TEM04(q-1) mode resonance was sufficiently reduced at 330 GHz on the high frequency side, and in Comparative Example 2b, in which the aperture diameter of the aperture 61 was narrowed so that the TEM04(q-1) mode resonance was sufficiently reduced at 170 GHz on the high frequency side, the TEM04(q-1) mode resonance was sufficiently reduced, but the amplitude or Q value of the TEM00q mode resonance was insufficient. That is, in each comparative example, even if the aperture diameter of the aperture 61 is adjusted, it is difficult to achieve both a sufficient reduction in the TEM04(q-1) mode resonance and a sufficient amplitude and Q value of the TEM00q mode resonance across the entire measurement frequency bandwidth.

[0042] Furthermore, as mentioned above, the resonance amplitude and Q value of each resonance mode become relatively small at the low-frequency side of the measurement frequency bandwidth. If the resonance amplitude and Q value of the TEM00q mode used to measure dielectric properties become too small, measurement of the resonance characteristics becomes difficult. In particular, for samples with large dielectric loss, the resonance amplitude and Q value of the TEM00q mode tend to become too small, making it difficult to distinguish from noise and making it difficult to obtain accurate resonance measurement results. In resonance measurements using polycarbonate (PC) with a thickness t of 98 μm, which has a relatively large dielectric loss, the Fabry-Perot resonator of Example 1 and the Fabry-Perot resonator of Example 2 were able to obtain the amplitude and Q value of the TEM00q mode itself required for resonance measurement without becoming too small, even at the low-frequency side of 220 GHz, and at the low-frequency side of 110 GHz, respectively. Furthermore, when the aperture diameter M of the reflective spherical surface of the Fabry-Perot resonators of Examples 1 and 2 was further reduced, a tendency for the resonance characteristics of the TEM00q mode to deteriorate, particularly on the low frequency side. From the above results, for accurate resonance measurement, it is necessary to set the aperture diameter M of the reflective spherical surface to 15 times or more the wavelength of the measurement frequency at a measurement frequency of 100 GHz or more, and to 20 times or more the wavelength of the measurement frequency at a measurement frequency of 220 GHz or more.

[0043] (Effects, etc.) As described above, the Fabry-Perot resonators of Examples 1 and 2 include first spherical reflecting mirrors 11 and 111 having first reflecting spherical surfaces 13 and 113, and second spherical reflecting mirrors 12 and 112 having second reflecting spherical surfaces 14 and 114 arranged opposite the first reflecting spherical surfaces 13 and 113. The opening diameter M of the first reflecting spherical surfaces 13 and 113 and the second reflecting spherical surfaces 14 and 114 is equal to or less than half the inter-spherical distance D between the first reflecting spherical surfaces 13 and 113 and the second reflecting spherical surfaces 14 and 114.

[0044] This allows the Fabry-Perot resonator to eliminate unnecessary higher-order mode resonances without substantially affecting the TEM00q mode resonance used in the measurement, thereby enabling more accurate measurement of the dielectric properties of the sample.

[0045] Furthermore, the opening diameter M of the first reflecting spherical surface 13, 113 and the second reflecting spherical surface 14, 114 in the Fabry-Perot resonators according to Examples 1 and 2 is equal to or less than half the radius of curvature R of the first reflecting spherical surface 13, 113 and the second reflecting spherical surface 14, 114. This allows the Fabry-Perot resonator to more sufficiently reduce resonance of unnecessary higher-order modes.

[0046] It is also desirable to set the aperture diameter M of the reflective spherical surface to at least 15 times the wavelength of the measurement frequency, so that the Fabry-Perot resonator can measure the resonance of the TEM00q mode used in the measurement with a sufficiently large amplitude and Q value.

[0047] Furthermore, the diameter of the through-hole 24 of the sample stage 20 in the Fabry-Perot resonators according to the first and second embodiments is smaller than the opening diameter M of the first reflecting spherical surfaces 13, 113 and the second reflecting spherical surfaces 14, 114. This allows the Fabry-Perot resonators to more sufficiently reduce resonances of unnecessary higher-order modes.

[0048] (Other embodiments) In the first to third embodiments, a Fabry-Perot resonator has been described in which the opening diameters M of the first and second spherical reflecting surfaces 13, 113 and 14, 114 are the same. Since the measured resonance characteristics depend on the smaller opening diameter, if the opening diameter M of at least one of the first and second spherical reflecting surfaces is equal to or less than half the inter-sphere distance D, resonances of higher-order modes unnecessary for measuring dielectric characteristics can be sufficiently reduced, as in the first to third embodiments. For example, resonances of higher-order modes unnecessary for measuring dielectric characteristics can also be sufficiently reduced in a Fabry-Perot resonator using the first spherical reflecting mirror 11a of Comparative Example 1a of the first embodiment (opening diameter M of the first spherical reflecting surface 13a: 50 mm) and the second spherical reflecting mirror 12 of Example 1 (opening diameter M of the second spherical reflecting surface 14: 28 mm).

[0049] In the first to third embodiments, a Fabry-Perot resonator has been described in which the entire first reflecting spherical surface 13, 113 and the entire second reflecting spherical surface 14, 114 are exposed in the space between the first and second reflecting spherical surfaces, and the diameter of the reflecting spherical surface is the same as the aperture diameter M of the reflecting spherical surface. Instead of reducing the diameter of the reflecting spherical surface itself, a limiter (e.g., an aluminum or resin plate) having a circular aperture smaller than the diameter of the reflecting spherical surface may be attached to the reflecting spherical surface so that the centers of the reflecting spherical surface and the circular aperture are coaxial, thereby substantially reducing the aperture diameter M of the reflecting spherical surface. In this case, too, if the diameter (aperture diameter M) of the aperture surface of the reflecting spherical surface that contributes to the measured resonance characteristics is equal to or smaller than half the inter-sphere distance D, it is possible to sufficiently reduce high-order mode resonances that are not required for measuring dielectric characteristics, as in the first to third embodiments. For example, in the Fabry-Perot resonator of comparative example 1a of embodiment 1, if a limiter having a circular opening (diameter: 28 mm) is attached to first spherical reflecting mirror 11a (opening diameter M of first reflecting spherical surface 13a: 50 mm), the opening diameter M of first reflecting spherical surface 13a will effectively be 28 mm, and therefore, resonance of unnecessary higher-order modes can be sufficiently reduced.

[0050] In the Fabry-Perot resonators of the first to third embodiments, examples have been described in which the diameter of through-hole 24 of sample stage 20 is smaller than the opening diameter M of first reflecting spherical surfaces 13, 113 and second reflecting spherical surfaces 14, 114. If opening diameter M is equal to or smaller than half of the inter-sphere distance D, it is possible to sufficiently reduce resonances of higher-order modes that are not required for measuring dielectric properties, and therefore the diameter of through-hole 24 may be set larger than opening diameter M. However, setting the diameter of through-hole 24 smaller than opening diameter M allows for a greater reduction in resonances of higher-order modes.

[0051] In the Fabry-Perot resonators of the first to third embodiments, the first waveguide 41 and the second waveguide 42 are used in the signal injection section and the signal detection section, respectively. Depending on the measurement frequency, the first waveguide 41 and the second waveguide 42 may be replaced with a coaxial cable having a loop antenna at its tip. [Industrial Applicability]

[0052] The open resonator of the present invention is suitable for measuring the dielectric properties of a sample with high precision by eliminating unnecessary higher-order mode resonances in the millimeter-wave frequency band exceeding 100 GHz. [Explanation of symbols]

[0053] 10 Fixed base 11, 11a, 111, 111a: First spherical reflecting mirror 12, 12a, 112, 112a Second spherical reflecting mirror 13, 13a, 113, 113a First reflecting sphere 14, 14a, 114, 114a Second reflecting sphere 15 First coupling hole 16 Second connecting hole 20 Sample stage 24 through holes 25 samples 35 Position adjustment mechanism 36 Pedestal 37 micrometer 41 First Waveguide 42 Second Waveguide 50 Cover 60 Spherical reflector 61 aperture 100 Fabry-Perot resonator D Distance between spheres M Opening diameter R radius of curvature λ wavelength

Claims

1. a first spherical reflector having a first reflective spherical surface; a second spherical reflecting mirror having a second reflecting spherical surface disposed opposite the first reflecting spherical surface, wherein the open resonator is used for measuring dielectric characteristics of a dielectric at a measurement frequency of 100 GHz or more, At least one of the opening diameter of the first reflecting spherical surface and the opening diameter of the second reflecting spherical surface is equal to or less than half the inter-spherical distance between the first reflecting spherical surface and the second reflecting spherical surface, and is equal to or less than half the radius of curvature of the first reflecting spherical surface and the second reflecting spherical surface. Open resonator.

2. At least one of the opening diameter of the first reflecting spherical surface and the opening diameter of the second reflecting spherical surface is 15 times or more the wavelength of the measurement frequency.

2. The open resonator according to claim 1.

3. a sample stage disposed between the first and second reflective spherical surfaces and having a through-hole for exposing a sample; the diameter of the through hole is smaller than at least one of the opening diameter of the first reflecting spherical surface and the opening diameter of the second reflecting spherical surface; 3. The open-type resonator according to claim 1 or 2.

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