Interference measuring device

Abstract

To provide an interference measuring device capable of speedily and suitably measuring interference by using terahertz waves.SOLUTION: An interference measuring device 1A comprises: a light source 10 for outputting terahertz waves L1 and reference light L2 having a wavelength of 3 μm or longer; a beam splitter 22 for branching the terahertz waves L1 and the reference light L2 into branched light La, Lb; an interference optical system 20 having an optical path Pa for reflecting the branched light La by a mirror 23 and allowing it to impinge on the beam splitter 22 again, and an optical path Pb for reflecting the branched light Lb by a rotary mirror 24 and allowing it to impinge on the beam splitter 22 again while a difference in the optical length between the branched light La, Lb is variable; a photomultiplier tube 30; a detector 40; an interference intensity measurement section 51 for measuring intensity of interference light IL1 on the basis of an electric signal outputted from the photomultiplier tube 30; and an analysis section 52 for analyzing an analyzed object S by performing Fourier transform on the basis of the intensity of the interference light IL1 and a detection result of interference light IL2 by the detector 40.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an interferometric measurement apparatus. [Background technology]

[0002] Terahertz waves are light in a band (frequency band of around 1 THz) between light waves and radio waves, and have a unique absorption spectrum for analytes such as pharmaceuticals that is not seen in other wavelength bands, so they are expected to be used for identifying analytes, etc. Various analytical techniques using terahertz waves are known.

[0003] Terahertz time domain spectroscopy (THz-TDS) measures the time waveform of terahertz waves transmitted, reflected, or totally reflected by an object to be analyzed, and performs a Fourier transform on the time waveform of the electric field amplitude of the terahertz waves obtained by this measurement, thereby enabling analysis of the object to be analyzed (Non-Patent Document 1). Hereinafter, this will be referred to as "Prior Art 1." In Prior Art 1, a lock-in amplifier is used to measure the time waveform of the terahertz waves.

[0004] Using a terahertz wave light source with a variable output wavelength, the terahertz waves transmitted, reflected, or totally reflected by the object to be analyzed are spectrally detected, thereby enabling the analysis of the object to be analyzed (Non-Patent Document 2). Hereinafter, this will be referred to as "Prior Art 2." In Prior Art 2, a thermal detector is used to detect the terahertz waves.

[0005] Furthermore, analysis of an object to be analyzed can also be performed by Fourier spectroscopy using interference measurement with terahertz waves, based on the same measurement principle as Fourier Transform Infrared Spectroscopy (FTIR) (Non-Patent Document 3). Hereinafter, this will be referred to as "Prior Art 3." In Prior Art 3, a thermal detector is used to detect the interference of terahertz waves. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6276758 [Non-patent literature]

[0007] [Non-Patent Document 1] JensNeu, et al, "Tutorial: An introduction to terahertz time domain spectroscopy (THz-TDS)," J. Appl. Phys. 124, 231101 (2018). [Non-patent document 2] K. Murate, et al, "Perspective: Terahertz wave parametric generator and its applications," J. Appl. Phys. 124, 160901 (2018). [Non-patent document 3] MasashiYamaguchi, et al, "Terahertz wave generation in nitrogen gas using shaped optical pulses," J. Opt. Soc. Am. B, Vol.26, No.9 (2009). Summary of the Invention [Problem to be solved by the invention]

[0008] In prior art 1, a long integration time is required by a lock-in amplifier when measuring the time waveform of terahertz waves. In prior arts 2 and 3, a thermal detector with slow response is used, which results in a long measurement time. Conventional analytical techniques using terahertz waves, including prior arts 1 to 3, require a long measurement time.

[0009] Therefore, an object of one aspect of the present disclosure is to provide an interferometry apparatus that can perform interferometry using terahertz waves quickly and suitably. [Means for solving the problem]

[0010] The present disclosure includes the following interferometric measurement devices [1] to [9].

[0011] [1] A light source that outputs a terahertz wave for interference measurement and a reference light for measuring an optical path length difference having a wavelength of 3 μm or more; an interference optical system including: a beam splitter that splits the terahertz wave and the reference light output from the light source and incident coaxially into first branched light and second branched light; a first optical path that reflects the first branched light from the beam splitter on a first mirror and causes it to re-enter the beam splitter; and a second optical path that reflects the second branched light from the beam splitter on a second mirror and causes it to re-enter the beam splitter, wherein the first branched light and the second branched light that re-enter the beam splitter are combined, and an optical path length difference between the first branched light and the second branched light is variable; a photomultiplier tube that has sensitivity in the terahertz wave band and outputs an electrical signal value corresponding to the incident light intensity of first interference light, which is interference light of the terahertz wave generated by combining the first branched light and the second branched light in the beam splitter; a detector that detects second interference light, which is interference light of the reference light generated by combining the first branched light and the second branched light in the beam splitter; an interference intensity measuring unit that measures the intensity of the first interference light based on the electrical signal output from the photomultiplier tube; an analysis unit that performs a Fourier transform based on the intensity of the first interference light measured by the interference intensity measurement unit and the detection result of the second interference light by the detector, thereby analyzing an object to be analyzed that is placed on an optical path through which the terahertz wave passes.

[0012] Conventionally, when incorporating a mechanism using reference light for measuring optical path length differences into an interferometer, light with a relatively short wavelength is generally used as the reference light in order to measure the optical path length difference as precisely as possible. In contrast, the interferometer described above in [1] intentionally uses light with a relatively long wavelength (3 μm or longer) as the reference light. This enables optical path length difference measurement with suitable resolution and dynamic range in Fourier spectroscopy analysis using terahertz waves. Furthermore, while beam splitters for transmitting terahertz waves generally do not transmit visible light to near-infrared light, by using reference light with a wavelength of 3 μm or longer (i.e., mid-infrared to far-infrared light), a common beam splitter can be used for both the terahertz wave and the reference light. This allows the optical path of the terahertz wave for generating the first interference light and the optical path of the reference light for generating the second interference light to be set to the same optical path (i.e., the first optical path and the second optical path). As a result, it is possible to accurately measure the optical path length difference in the first interference light based on the second interference light of the reference light generated along the same optical path as the terahertz wave. According to the above configuration, it is possible to perform measurement while continuously changing the optical path length difference between the first branched light and the second branched light and appropriately monitoring the change in the optical path length difference in real time based on the second interference light, so that interference measurement using terahertz waves can be performed quickly and suitably.

[0013] [2] The analysis unit converting the intensity of the first interference light measured by the interference intensity measuring unit into an electric field amplitude value based on a relationship between an electric field amplitude value of the light incident on the photomultiplier tube and an electric signal value output from the photomultiplier tube, and calculating the electric field amplitude value of the first interference light for each value of the time difference corresponding to the optical path length difference; The interference measurement device according to [1], wherein the object to be analyzed is analyzed by performing a Fourier transform based on the dependency of the calculated electric field amplitude value of the first interference light on the value of the time difference.

[0014] According to the configuration [2] above, the electric field amplitude value of the first interference light corresponding to each value of the time difference corresponding to the optical path length difference is calculated by conversion based on the relationship between the electric field amplitude value and the electrical signal value, thereby enabling fast and accurate Fourier spectroscopy by interference measurement using terahertz waves.

[0015] [3] The interference optical system is configured to be able to change the optical path length of the first optical path by driving the first mirror, and is configured to be able to change the optical path length of the second optical path by driving the second mirror. [1] The interference measuring device of [1] or [2].

[0016] According to the configuration [3] above, one mirror (for example, the first mirror) can be used as an initial setting mirror that adjusts the optical path length difference to near zero in the initial state, and the other mirror (for example, the second mirror) can be used as a mirror that changes the optical path length difference during interferometric measurement, thereby realizing a configuration that is suitable for interferometric measurement.

[0017] [4] The interference optical system further includes a third mirror; the second mirror is configured to be rotationally driven so that the optical path length of the second optical path is changed; The interferometric measurement device according to any one of [1] to [3], wherein the second optical path is composed of an optical path that runs from the beam splitter to the third mirror via the second mirror, and an optical path that is reflected by the third mirror and returns to the beam splitter via the second mirror.

[0018] According to the configuration [4] above, by rotating the second mirror configured as a rotating mirror, it becomes possible to perform measurement while changing the optical path length difference at high speed, compared to when the second mirror is configured as a mirror that can translate in a direction perpendicular to the reflecting surface, and as a result, it is possible to speed up analysis by the interferometer.

[0019] [5] The interferometer according to any one of [1] to [4], wherein the detector is any one of a quantum cascade detector, an MCT detector, a superlattice infrared detector, an InSb detector, an InAs detector, and an InAsSb detector.

[0020] According to the configuration [5] above, by using a detector that operates at high speed and with high sensitivity as a photodetector for the mid-infrared light region, the second interference light (interference light of the reference light) can be detected efficiently.

[0021] [6] The light source includes a first light source that outputs the terahertz wave and a second light source that outputs the reference light; The interferometer according to any one of [1] to [5], wherein the second light source is constituted by a quantum cascade laser element.

[0022] In order to detect the second interference light with high accuracy in the detector, it is preferable to use light with as short (narrow) a spectral linewidth (wavelength linewidth) as possible as the reference light. According to the configuration [6] above, by using a quantum cascade laser element having the property of outputting light with a relatively narrow spectral linewidth as the second light source, it is possible to improve the detection accuracy of the second interference light in the detector.

[0023] [7] The spectroscopic analysis device according to any one of [1] to [5], wherein the light source is configured by a single quantum cascade laser element that has a double upper level subband level structure and is capable of generating the terahertz wave and the reference light by multiple intersubband luminescence transitions.

[0024] According to the configuration of [7] above, it is possible to output terahertz waves for interferometry and reference light for measuring optical path length differences using a single light source (quantum cascade laser element). As a result, the number of light sources can be reduced compared to when the light source is configured with two light sources (i.e., a first light source that outputs terahertz waves and a second light source that outputs reference light). In addition, a component (e.g., a second beam splitter separate from the beam splitter) for adjusting the terahertz waves output from the first light source and the reference light output from the second light source so that they are incident on the beam splitter coaxially is also not required. Therefore, according to the configuration of [7] above, it is possible to simplify the configuration of the light source and the interference optical system.

[0025] [8] Further comprising a light source control unit that controls the driving of the light source; the light source includes a first light source that outputs the terahertz wave and a second light source that outputs the reference light, The interferometric measuring device according to any one of [1] to [6], wherein the light source control unit drives the first light source and the second light source in synchronization with each other in a pulsed manner.

[0026] According to the configuration [8] above, by synchronously driving the first light source and the second light source in a pulsed manner, it is possible to eliminate the need to cool the first light source and the second light source, and it is possible to extend the device life of the first light source and the second light source compared to when the first light source and the second light source are driven continuously.

[0027] [9] The interference optical system is configured to be able to change the optical path length of the second optical path by driving the second mirror; The interference measuring device according to any one of [1] to [8], wherein the width of the periodic change in the optical path length difference between the first branched light and the second branched light caused by driving the second mirror is 3 mm or more.

[0028] According to the configuration of [9] above, it is possible to obtain an appropriate resolution and measurement dynamic range with respect to changes in the optical path of the terahertz wave. [Effects of the Invention]

[0029] According to one aspect of the present disclosure, it is possible to provide an interferometry device that can perform interferometry using terahertz waves quickly and suitably. [Brief explanation of the drawings]

[0030] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of an interference measurement apparatus according to the first embodiment. [Figure 2] FIG. 2 is a diagram showing an example of the configuration of the photomultiplier tube 30. As shown in FIG. [Figure 3] 3(a) is a graph showing the input / output characteristics of the photomultiplier tube 30. FIG. 3(b) is a graph showing the time dependency of the voltage signal V output from the photomultiplier tube 30. [Figure 4] FIG. 4 is a diagram showing an example of the intensity of the interference light IL2 according to the optical path length difference detected by the detector 40. In FIG. [Figure 5] Fig. 5(a) is a graph showing the dependency of Vp-p on the optical path length difference Δd, and Fig. 5(b) is a graph showing the dependency of Vp-p on the time difference Δt. [Figure 6] Fig. 6(a) is a graph showing the time difference Δt dependency of the electric field amplitude value E of the interference light, and Fig. 6(b) is a diagram showing the amplitude spectrum and phase spectrum of the electric field amplitude value E of the interference light. [Figure 7] 7(a) is a graph showing the time difference Δt dependency of the electric field amplitude value E of the interference light, and FIG. 7(b) is a graph showing the phase spectrum of the electric field amplitude value E of the interference light. [Figure 8] Fig. 8(a) is a graph showing the amplitude spectrum of the electric field amplitude value E of the interference light. Fig. 8(b) is a graph showing the spectrum of the absorption coefficient α(ω). Fig. 8(c) is a graph showing the spectrum of the refractive index n(ω). [Figure 9] FIG. 9 is a diagram showing an example of the configuration of an interference measuring apparatus according to the second embodiment. [Figure 10] FIG. 10 is a diagram showing an example of pulse synchronization control in the interference measurement apparatus of the second embodiment. [Figure 11]FIG. 11 is a diagram showing an example of the configuration of an interference measuring apparatus according to the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0031] Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the drawings. In the following description, the same or equivalent elements will be designated by the same reference numerals, and redundant description will be omitted.

[0032] [First embodiment] An interference measurement apparatus 1A of the first embodiment will be described with reference to Figures 1 to 8. As shown in Figure 1, the interference measurement apparatus 1A includes a light source 10, an interference optical system 20, a photomultiplier tube 30, a detector 40, and a measurement device 50.

[0033] The light source 10 outputs a terahertz wave L1 for interference measurement and a reference light L2 for measuring the optical path length difference. In this embodiment, the light source 10 includes a light source 11 (first light source) that outputs (emits) the terahertz wave L1 and a light source 12 (second light source) that outputs (emits) the reference light L2.

[0034] The terahertz wave L1 output from the light source 11 may be pulsed light or continuous light. Examples of light sources capable of outputting pulsed terahertz waves include a combination of a femtosecond laser light source (e.g., a Ti:sapphire laser light source) and a nonlinear optical crystal (e.g., ZnTe), and an injection-seeded terahertz parametric oscillator (is-TPG). Examples of light sources capable of outputting continuous terahertz waves include resonant tunneling diodes (RTDs), impact avalanche and transit time diodes (IMPATT diodes), quantum cascade laser light sources, and THz gas laser light sources.

[0035] The reference light L2 output from the light source 12 is light having a wavelength of 3 μm or more. The reference light L2 is, for example, mid-infrared to far-infrared light having a wavelength of 3 μm or more and 100 μm or less. A quantum cascade laser element can be used as a light source that suitably outputs such reference light L2. In this embodiment, the light source 12 is configured with a quantum cascade laser element that outputs mid-infrared light having a wavelength of 10 μm. Note that the reference light L2 may be pulsed light or continuous light. However, when both the terahertz wave L1 and the reference light L2 are pulsed light, control is required to output the terahertz wave L1 and the reference light L2 in a synchronized manner. In this embodiment, a case where the terahertz wave L1 is pulsed light and the reference light L2 is continuous light will be described, and a case where both the terahertz wave L1 and the reference light L2 are pulsed light will be described in a second embodiment.

[0036] The interference optical system 20 has a beam splitter 21, a beam splitter 22, a mirror 23 (first mirror), a rotating mirror 24 (second mirror), a mirror 25 (third mirror), a beam splitter 26, and a lens 27. The beam splitters 21, 22, and 26 are all made of silicon. By using a beam splitter made of silicon, both the terahertz wave L1 and the reference light L2 can be suitably split into a component reflected by the beam splitter and a component transmitted through the beam splitter.

[0037] The beam splitter 21 is disposed between the light source 10 (light sources 11, 12) and the beam splitter 22. The beam splitter 21 is a component for multiplexing the terahertz wave L1 output from the light source 11 and the reference light L2 output from the light source 12, and for making the terahertz wave L1 and the reference light L2 coaxially incident on the beam splitter 22 at the subsequent stage. The beam splitter 21 is disposed so that the terahertz wave L1 is incident on the surface of the beam splitter 21 opposite to the incident surface of the reference light L2. In the example of FIG. 1, the component of the terahertz wave L1 that has passed through the beam splitter 21 and the component of the reference light L2 that has been reflected by the beam splitter 21 are multiplexed and incident on the beam splitter 22.

[0038] The beam splitter 22 splits the terahertz wave L1 and reference light L2 (in this embodiment, the terahertz wave L1 and reference light L2 combined by the beam splitter 21) output from the light source 10 (light sources 11, 12) and incident coaxially into split light La (first split light) and split light Lb (second split light). As a result, in the interference optical system 20, an optical path Pa (first optical path) through which the split light La passes and an optical path Pb (second optical path) through which the split light Lb passes are formed.

[0039] As an example, the branched light La is light containing components of the terahertz wave L1 and the reference light L2 reflected by the beam splitter 22. The optical path Pa of the branched light La is an optical path along which the branched light La from the beam splitter 22 is reflected by the mirror 23 and re-enters the beam splitter 22 (the exit surface of the beam splitter 22 from which the branched light La is emitted). In this embodiment, the optical path Pa is made up of an optical path (outgoing path) from the beam splitter 22 to the mirror 23 and an optical path (returning path) from the mirror 23 to the beam splitter 22.

[0040] As an example, the branched light Lb is light containing components of the terahertz wave L1 and the reference light L2 that have passed through the beam splitter 22. The optical path Pb of the branched light Lb is an optical path along which the branched light Lb from the beam splitter 22 is reflected by the rotating mirror 24 and re-enters the beam splitter 22 (the exit surface of the beam splitter 22 from which the branched light Lb is emitted). In this embodiment, the optical path Pb is composed of an optical path (outgoing path) from the beam splitter 22 to the mirror 25 via the rotating mirror 24 and the lens 27, and an optical path (returning path) that is reflected by the mirror 25 and returns to the beam splitter 22 via the lens 27 and the rotating mirror 24. The lens 27 is an optical member disposed between the rotating mirror 24 and the mirror 25 to allow the branched light Lb reflected by the mirror 25 to reach the mirror 25 and to allow the branched light Lb reflected by the mirror 25 to reach the rotating mirror 24 again, even when the rotation angle of the rotating mirror 24 becomes large. Lens 27 converts both the terahertz wave L1 and the reference light L2 into focused or parallel light. Lens 27 may be made of, for example, a plastic material such as Tsurupica (registered trademark) or ZEONEX (registered trademark), silicon, or the like. Alternatively, instead of lens 27, a reflective convex mirror or an off-axis parabolic mirror that is independent of the wavelength of the incident light may be used as an optical member disposed between rotating mirror 24 and mirror 25.

[0041] The mirror 23 is disposed downstream of the beam splitter 22 on the optical path Pa. In this embodiment, the mirror 23 is configured to be movable in parallel in a direction D1 along the optical path Pa (a direction perpendicular to the reflecting surface of the mirror 23). The position of the mirror 23 in the direction D1 is set, for example, so that the optical path difference Δd between the branched light La (optical path Pa) and the branched light Lb (optical path Pb) is close to zero in the initial state. The branched light La incident on the reflecting surface 23a of the mirror 23 is reflected by the reflecting surface 23a and returned to the beam splitter 22. In this embodiment, the analyte S is disposed between the beam splitter 22 and the mirror 23 on the optical path Pa. The analyte S is, for example, a sugar such as lactose or sucrose.

[0042] The object to be analyzed S may be placed on the optical path through which the terahertz wave L1 passes. For example, the object to be analyzed S may be placed on the optical path Pb (e.g., between the beam splitter 22 and the rotating mirror 24). However, in this embodiment, the optical path Pb changes due to the driving of the rotating mirror 24 during measurement, and therefore the conditions of the terahertz wave L1 (the component of the terahertz wave L1 contained in the branched light Lb) that pass through the object to be analyzed S (e.g., the position through which the terahertz wave L1 passes and the amplitude of the terahertz wave L1 at the position through which the terahertz wave L1 passes) are not constant. This may result in unexpected measurement errors, etc. From this perspective, it is preferable that the object to be analyzed S be placed on the optical path Pa, which does not change during measurement. Alternatively, the analyte S may be placed outside the optical paths Pa and Pb on the optical path through which the terahertz wave L1 passes (for example, at a position between the light source 11 and the beam splitter 21, a position between the beam splitters 21 and 22, a position between the beam splitters 22 and 26, or a position between the beam splitter 26 and the photomultiplier tube 30). When the analyte S is placed outside the optical paths Pa and Pb, it is possible to prevent a decrease in the interference signal (interference light IL) due to the influence of distortion of the wavefront of the analyte S. On the other hand, when the analyte S is placed outside the optical paths Pa and Pb, it is possible to obtain information about the absorption spectrum of the analyte S, but it is not possible to obtain information about the refractive index of the analyte S. In other words, by placing the analyte S on the optical path Pa or the optical path Pb, it is possible to obtain information about both the absorption spectrum and the refractive index of the analyte S.

[0043] The rotating mirror 24 is disposed downstream of the beam splitter 22 on the optical path Pb. The rotating mirror 24 is configured to be rotatable (swingable) within a predetermined angle range around a central axis of an axis X that extends in any direction perpendicular to the optical path Pb (in the example of FIG. 1, the direction perpendicular to the paper surface). The axis X of the rotating mirror 24 is located at a position offset from the optical path Pb. The rotating mirror 24 is configured to be rotatable at high speed around the axis X as a central axis so as to change the distance between the beam splitter 22 and the reflecting surface 24a of the rotating mirror 24 on the optical path Pb. For example, when the angle when the optical path lengths of optical path Pa and optical path Pb are the same (i.e., when the optical path length difference Δd between branched light La and branched light Lb is 0) is defined as angle θr, the rotating mirror 24 is configured to rotate within a predetermined angle θmax in a direction in which the optical path length of optical path Pb becomes longer (i.e., in a direction in which the distance from the beam splitter 22 to the reflecting surface 24a on optical path Pb becomes longer (clockwise direction in FIG. 1)). That is, the rotating mirror 24 is configured to be rotatable at a predetermined frequency so that the angle of the rotating mirror 24 periodically changes between the angle θr when the optical path length difference Δd becomes 0 and the angle "θr+θmax" when the optical path length difference Δd (in this embodiment, "the optical path length of optical path Pb - the optical path length of optical path Pa") becomes a maximum dmax.

[0044] The width (dmax in this embodiment) by which the optical path length difference Δd changes periodically due to the rotational driving of the rotating mirror 24 is set to, for example, 3 mm or more. As an example, the position of the rotating mirror 24 (i.e., the position of the axis X) is adjusted so that the optical path length difference Δd changes by about 1 μm when the angle of the rotating mirror 24 is changed by 0.0025°. In this case, by rotating the rotating mirror 24 with "θmax=25°", the optical path length difference Δd can be changed periodically between a state where the optical path length difference Δd is 0 (i.e., when the angle of the rotating mirror 24 is θr) and a state where the optical path length difference Δd is dmax (=10 μm) (i.e., when the angle of the rotating mirror 24 is "θr+θmax").

[0045] The mirror 25 is disposed after the rotating mirror 24 on the optical path Pb. The position of the mirror 25 is fixed. The branched light Lb incident on the reflecting surface 24a of the rotating mirror 24 is reflected by the reflecting surface 24a and travels via the lens 27 toward the reflecting surface 25a of the mirror 25. The branched light Lb is then reflected by the reflecting surface 25a of the mirror 25 and travels again via the lens 27 back to the reflecting surface 24a of the rotating mirror 24, where it is reflected again by the reflecting surface 24a and re-enters the beam splitter 22 (the exit surface of the beam splitter 22 from which the branched light Lb is emitted).

[0046] In the interference optical system 20, as described above, the optical path length difference Δd is variable by rotating the rotating mirror 24. Furthermore, the beam splitter 22 combines the branched light La that re-enters the beam splitter 22 via the optical path Pa and the branched light Lb that re-enters the beam splitter 22 via the optical path Pb. In this embodiment, the component of the branched light La that re-enters the beam splitter 22 and is transmitted through the beam splitter 22 and the component of the branched light Lb that re-enters the beam splitter 22 and is reflected by the beam splitter 22 are combined to produce interference light IL, which is emitted from the beam splitter 22 toward the beam splitter 26.

[0047] The interference light IL includes interference light IL1 (first interference light) of the terahertz wave L1 generated by multiplexing the branched light La and the branched light Lb, and interference light IL2 (second interference light) of the reference light L2 generated by multiplexing the branched light La and the branched light Lb. The interference light IL1 is light formed by interference between the terahertz wave L1 contained in the branched light La and the terahertz wave L1 contained in the branched light Lb. The interference light IL2 is light formed by interference between the reference light L2 contained in the branched light La and the reference light L2 contained in the branched light Lb.

[0048] The beam splitter 26 splits the interference light IL emitted from the beam splitter 22 into light reflected by the beam splitter 22 and light transmitted through the beam splitter 22 .

[0049] The photomultiplier tube 30 is disposed downstream of the beam splitter 26 at a position toward which the light transmitted through the beam splitter 26 is directed. The photomultiplier tube 30 is sensitive to the terahertz wave band (i.e., the wavelength range of the terahertz wave L1) and outputs an electrical signal value corresponding to the incident light intensity of the interference light IL1. In this embodiment, the photomultiplier tube 30 outputs an electrical signal value corresponding to the incident light intensity of the interference light IL1 contained in the component of the interference light IL that has transmitted through the beam splitter 26.

[0050] 2 is a block diagram showing the configuration of the photomultiplier tube 30. The photomultiplier tube 30 has an electron emitting section 31, an electron multiplier section 32, and a signal output section 33 arranged inside a housing 34, the inside of which is maintained at a vacuum. A window section 35 is provided in the housing 34.

[0051] When light v that has passed through the window 35 is incident on the electron emitter 31, the electron emitter 31 emits electrons e in response to the incident light. The electron emitter 31 is a photoelectric conversion unit designed to have sensitivity to a band of light that includes the terahertz wave L1 to be detected. The electron emitter 31 has a configuration in which, for example, a metamaterial structure (metasurface) is formed on the main surface of a substrate, and can emit electrons e in response to light incident on the metasurface.

[0052] The electron multiplier unit 32 multiplies the electrons e emitted from the electron emitter 31. The electron multiplier unit 32 includes multiple stages of dynodes or microchannel plates. The electron multiplication factor in the electron multiplier unit 32 corresponds to the voltage applied to the multiple stages of dynodes or microchannel plates. The signal output unit 33 collects the electrons e multiplied by the electron multiplier unit 32 and outputs them as a current signal J. The interference intensity measuring unit 51, which will be described later, may input the current signal J output from the signal output unit 33, or may input a voltage signal obtained after the current signal J has been converted by an IV conversion circuit. In this embodiment, the voltage signal is input to the interference intensity measuring unit 51 as the electrical signal value output by the photomultiplier tube 30.

[0053] FIG. 3(a) is a diagram showing an example of the input / output characteristics of the photomultiplier tube 30. The horizontal axis represents the electric field amplitude value E of the light (interference light IL1) incident on the photomultiplier tube 30. The vertical axis represents the electrical signal (voltage signal V) output from the photomultiplier tube 30. As shown in this diagram, the input / output characteristics of the photomultiplier tube 30 are not linear. Such input / output characteristics of the photomultiplier tube 30 are determined in advance. For example, by performing a fitting process using multiple (five in this example) measured values ​​shown in FIG. 3(a), a fitting function R indicating the relationship between the electric field amplitude value E and the voltage signal V can be obtained. Such fitting function R is stored in advance in the measurement device 50, for example.

[0054] 3(b) is a graph showing the time dependence of the voltage signal V output from the photomultiplier tube 30. Such voltage signal V corresponds to the optical path difference Δd between the branched light La (optical path Pa) and the branched light Lb (optical path Pb). In other words, the amplitude Vp-p of the voltage signal V output from the photomultiplier tube 30 corresponds to the optical path difference Δd. The analysis unit 52, which will be described later, can read Vp-p from the output result of the photomultiplier tube 30, such as that shown in FIG. 3(b).

[0055] The detector 40 is disposed downstream of the beam splitter 26, at a position toward which the light reflected by the beam splitter 26 is directed. The detector 40 is sensitive to the wavelength range of the reference light L2 and detects the interference light IL2. In this embodiment, the detector 40 outputs an electrical signal value corresponding to the incident light intensity of the interference light IL2 contained in the component of the interference light IL reflected by the beam splitter 26. The detector 40 is configured, for example, by any one of a quantum cascade detector, an MCT detector, a superlattice infrared detection element, an InSb detection element, an InAs detection element, and an InAsSb detection element. With the above configuration, the detector 40, which operates with high sensitivity and at high speed as a photodetector in the mid-infrared light region, can efficiently detect the interference light IL2 of the reference light L2.

[0056] 4 is a diagram showing an example of the intensity of the interference light IL2 depending on the optical path length difference Δd detected by the detector 40. The horizontal axis of the graph shown in FIG. 4 represents the optical path length difference Δd (in this embodiment, the delay amount of the branched light Lb relative to the branched light La), and the vertical axis represents the intensity of the detected interference light IL2. When the optical path length difference Δd is an integer multiple of the wavelength of the reference light L2 (including the case where the optical path length difference Δd is 0), the reference light L2 contained in the branched light La and the reference light L2 contained in the branched light Lb overlap, resulting in the highest intensity of the interference light IL2. On the other hand, when the optical path length difference Δd is an odd multiple of the half wavelength of the reference light L2, the reference light L2 contained in the branched light La and the reference light L2 contained in the branched light Lb cancel each other out, resulting in the lowest intensity of the interference light IL2. Therefore, when the optical path length difference Δd is continuously changed at a constant frequency by rotating the rotating mirror 24 as described above, when the wavelength λ of the reference light L2 is set to 0.63 μm, a signal value showing the upper waveform in FIG. 4 is obtained, and when the wavelength λ is set to 10 μm, a signal value showing the lower waveform in FIG. 4 is obtained. In other words, when the angle of the rotating mirror 24 is set to an angle corresponding to a certain optical path length difference, a signal value with an intensity corresponding to that optical path length difference can be obtained. Therefore, based on the intensity of the interference light IL2 detected by the detector 40, the optical path length difference Δd at the time when the interference light IL2 is detected can be accurately estimated.

[0057] The measuring device 50 performs Fourier spectroscopy analysis by interference measurement using the terahertz wave L1 based on the output value of the photomultiplier tube 30 and the output value of the detector 40. The measuring device 50 has an interference intensity measuring unit 51 and an analyzing unit 52. The measuring device 50 may be configured by, for example, a computer system including a processor, memory, storage, a communication device, etc. Each function of the measuring device 50 (the interference intensity measuring unit 51 and the analyzing unit 52) ​​is executed by these hardware elements operating according to a predetermined program.

[0058] The interference intensity measuring unit 51 measures the intensity of the interference light IL1 based on the electrical signal (the voltage signal V described above in this embodiment) output from the photomultiplier tube 30. In this embodiment, the interference intensity measuring unit 51 acquires the magnitude Vp-p of the amplitude of the voltage signal V as shown in FIG. 3(b) at each measurement time point (for example, a time point corresponding to each pulse of the terahertz wave L1, which is pulsed light) as a value indicating the intensity of the interference light IL1.

[0059] The analysis unit 52 analyzes the object to be analyzed S placed on the optical path Pa or the optical path Pb (in this embodiment, the optical path Pa) by performing a Fourier transform based on the intensity (Vp-p) of the interference light IL1 measured by the interference intensity measurement unit 51 and the detection result of the interference light IL2 by the detector 40. In this embodiment, as an example, the analysis unit 52 executes the following first process and second process.

[0060] (First process) As a first process, the analysis unit 52 converts the intensity (Vp-p) of the interference light IL1 measured by the interference intensity measurement unit 51 into an electric field amplitude value E based on the relationship (in this embodiment, the fitting function R shown in FIG. 3(a)) between the electric field amplitude value E of the light incident on the photomultiplier tube 30 and the electric signal value (voltage signal V) output from the photomultiplier tube 30, and calculates the electric field amplitude value E of the interference light IL1 for each value of the time difference Δt corresponding to the optical path length difference Δd. Note that the relationship between the optical path length difference Δd and the time difference Δt is "Δt = Δd / c," where c is the speed of light in a vacuum.

[0061] For example, the analysis unit 52 can estimate the optical path length difference Δd at each measurement time point based on the detection result of the interference light IL2 by the detector 40 (i.e., the signal value corresponding to the intensity of the interference light IL2). As a result, the analysis unit 52 can determine Vp-p corresponding to each value of the optical path length difference Δd by correlating Vp-p (see FIG. 3(b)) measured by the interference intensity measurement unit 51 at each measurement time point with the optical path length difference Δd corresponding to each measurement time point. As a result, a graph showing the dependency of Vp-p on the optical path length difference Δd is obtained, as shown in FIG. 5(a).

[0062] Note that the process for obtaining Vp-p (the graph in FIG. 5A) corresponding to each value of the optical path length difference Δd using the detection result of the interference light IL2 is not limited to the above process. As another example, the analysis unit 52 may be configured to perform a correction process using the detection result of the interference light IL2 by the detector 40. For example, the analysis unit 52 may obtain Vp-p (hereinafter referred to as "first information") corresponding to each value of the optical path length difference Δd without using the detection result of the interference light IL2 by the detector 40, and then correct the first information based on the detection result of the interference light IL2 by the detector 40. For example, when the rotating mirror 24 is continuously driven at a constant frequency, the angle of the rotating mirror 24 at each measurement time point (i.e., the optical path length difference Δd corresponding to the angle) can be estimated with a certain degree of accuracy based on the elapsed time from the start of driving the rotating mirror 24. In this way, the first information can be obtained by associating the Vp-p measured at each measurement time point with the optical path length difference Δd estimated for each measurement time point. The analysis unit 52 may compare the optical path length difference Δd at each measurement time point estimated based on the elapsed time as described above with the optical path length difference Δd at each measurement time point accurately estimated based on the detection result of the interference light IL2 by the detector 40, and correct the waveform of the first information according to the comparison result, thereby obtaining final information (information on Vp-p corresponding to each value of the optical path length difference Δd).

[0063] Next, the analysis unit 52 converts the optical path difference Δd into the time difference Δt using the above-mentioned relationship between the optical path difference Δd and the time difference Δt, "Δt=Δd / c," thereby determining Vp-p corresponding to each value of the time difference Δt corresponding to the optical path difference Δd. As a result, a graph showing the dependency of Vp-p on the time difference Δt is obtained, as shown in Figure 5(b).

[0064] Next, the analysis unit 52 converts the intensity (Vp-p) of the interference light IL1 into an electric field amplitude value E based on a fitting function R (see FIG. 3(a)) that indicates the input / output characteristics of the photomultiplier tube 30. As a result, a graph is obtained that indicates the time difference Δt dependency of the electric field amplitude value E of the interference light IL1, as shown in FIG. 6(a).

[0065] (Second process) The analysis unit 52 analyzes the analyte S by performing a Fourier transform based on the dependency of the electric field amplitude value E of the interference light IL1 calculated by the first process on the value of the time difference Δt. The above-mentioned Fourier transform results in a graph showing the amplitude spectrum (solid line) and phase spectrum (dashed line) of the electric field amplitude value E of the interference light IL1, as shown in (b) of Figure 6. An example of analysis (identification) of the analyte S by the analysis unit 52 will now be described.

[0066] In an interferometer 1A equipped with an interference optical system 20 having the configuration of a Michelson interferometer, the terahertz wave L1 passes through the object to be analyzed S twice. The phase refractive index of the object to be analyzed S is n(ω), the extinction coefficient of the object to be analyzed S is k(ω), and the complex refractive index of the object to be analyzed S is n'(ω) = n(ω) + ik(ω). ω is the angular frequency of the terahertz wave. If the frequency of the terahertz wave L1 is f, then ω = 2πf. π is the constant of the circumference of a circle. i is the imaginary unit.

[0067] The electric field amplitude value of the interference light IL1 obtained when the object to be analyzed S is placed is E sample (ω), and the electric field amplitude value of the interference light IL1 obtained when the object to be analyzed S is not placed is E ref (ω), and the ratio T(ω) is expressed by the following equation (1): as is the interface amplitude transmittance from air to the object to be analyzed S, and is expressed by the following equation (2): sa is the interfacial amplitude transmittance from the object to be analyzed S to air, and is expressed by the following formula (3): d is the thickness of the object to be analyzed S; and c is the speed of light in a vacuum.

[0068]

number

[0069]

number

[0070]

number

[0071] By decomposing the above equation (1) into a real part and an imaginary part, the following equations (4) to (6) are obtained. φ(ω) is the phase spectrum. α(ω) is the absorption coefficient. In the interferometric measurement apparatus 1A, the analysis unit 52 can analyze the analysis target S from these.

[0072]

number

[0073]

number

[0074]

number

[0075] 7 and 8 are graphs showing an example of the results of measurement and analysis by the interferometer 1A. The graph in Fig. 7(a) is obtained by the first processing of the analysis unit 52 described above (a graph corresponding to the graph in Fig. 6(a)). The graphs in Fig. 7(b) and Fig. 8(a) to (c) are obtained by the second processing of the analysis unit 52 described above (a graph corresponding to the graph in Fig. 6(b) and graphs derived from that graph).

[0076] FIG. 7(a) is a graph showing the time difference Δt dependency of the electric field amplitude value E of the interference light IL1. FIG. 7(b) is a graph showing the phase spectrum of the electric field amplitude value E of the interference light IL1. FIG. 8(a) is a graph showing the amplitude spectrum of the electric field amplitude value E of the interference light IL1. These figures show the cases where the analyte S is present and where it is not present. In this example, lactose was used as the analyte S.

[0077] Fig. 8(b) is a graph showing the spectrum of the absorption coefficient α(ω). Fig. 8(c) is a graph showing the spectrum of the refractive index n(ω). These figures show the analysis results of this embodiment and the analysis results of Prior Art 1 using THz-TDS.

[0078] As can be seen from a comparison between the analysis results of this embodiment and the analysis results of THz-TDS of Prior Art 1, the positions of the absorption peaks appearing in the spectra of the absorption coefficient α(ω) are consistent between the two. This proves that the analyte S can be analyzed by this embodiment.

[0079] [Effects of the first embodiment] Conventionally, when a mechanism using reference light for measuring optical path length differences is incorporated into an interferometer, it is common to use light with a relatively short wavelength (for example, a wavelength of 0.63 μm as shown in the upper part of FIG. 4) as the reference light in order to measure the optical path length differences as precisely as possible. In contrast, the interferometer 1A deliberately uses light with a relatively long wavelength (3 μm or more) (in this embodiment, light with a wavelength of 10 μm as shown in the lower part of FIG. 4) as the reference light L2. This enables optical path length differences to be measured with suitable resolution and dynamic range in Fourier spectroscopy using terahertz waves L1.

[0080] More specifically, as in this embodiment, when the sweep width (10 mm) of the optical path length difference Δd is relatively long and the rotating mirror 24 is rotated within an appropriate angle range (e.g., a rotation width of 25°), a slight rotation of the rotating mirror 24 by only 0.0025° results in a change in the optical path length difference Δd of approximately 1 μm. Therefore, when light with a wavelength of 0.63 μm as shown in the upper part of FIG. 4 is used as the reference light L2, the resolution is too fine (i.e., the wavelength is shorter than the change width (1 μm) of the optical path length difference Δd) to properly measure the change in the optical path length difference Δd. In contrast, in this embodiment, by using light with a relatively long wavelength (e.g., 10 μm) as shown in the lower part of FIG. 4 as the reference light L2, a change in the optical path length difference Δd of approximately 1 μm can be properly measured.

[0081] Furthermore, while beam splitters for transmitting terahertz waves L1 generally do not transmit visible light to near-infrared light, by using reference light L2 with a wavelength of 3 μm or more (i.e., mid-infrared to far-infrared light), a common beam splitter 22 can be used for both the terahertz waves L1 and the reference light L2. This allows the optical path of the terahertz waves L1 for generating interference light IL1 and the optical path of the reference light L2 for generating interference light IL2 to be set to the same optical path (i.e., optical path Pa and optical path Pb). As a result, it becomes possible to accurately measure the optical path length difference Δd in the interference light IL1 based on the interference light IL2 of the reference light L2 generated along the same optical path as the terahertz waves L1. With the above configuration, it is possible to perform measurements while continuously changing the optical path length difference Δd between the branched lights La and Lb and appropriately monitoring the change in the optical path length difference Δd based on the interference light IL2 of the reference light L2 in real time, thereby enabling high-speed and efficient interferometric measurement using terahertz waves L1.

[0082] Furthermore, the analysis unit 52 is configured to perform the first process and the second process described above to analyze the analyte S. According to the above configuration, the electric field amplitude value E of the interference light IL1 corresponding to each value of the time difference Δt corresponding to the optical path length difference Δd can be calculated by conversion based on the relationship between the electric field amplitude value E and the electric signal value (Vp-p) (for example, the fitting function R shown in FIG. 3(a)), thereby enabling fast and accurate Fourier spectroscopy by interference measurement using the terahertz wave L1.

[0083] Furthermore, the interference optical system 20 is configured to be able to change the optical path length of the optical path Pa by driving the mirror 23, and to be able to change the optical path length of the optical path Pb by driving the rotating mirror 24. With the above configuration, one mirror (for example, mirror 23) can be used as an initial setting mirror (a mirror whose position is fixed during interference measurement) for adjusting the optical path length difference Δd to be close to zero in the initial state, and the other mirror (for example, rotating mirror 24) can be used as a movable mirror for changing the optical path length difference Δd during interference measurement, thereby realizing a configuration suitable for interference measurement.

[0084] Furthermore, rotating mirror 24 is configured to be rotationally driven so as to change the optical path length of optical path Pb, which is made up of an optical path that goes from beam splitter 22 via rotating mirror 24 to mirror 25, and an optical path that is reflected by mirror 25 and returns to beam splitter 22 via rotating mirror 24. According to the above configuration, by rotating mirror 24, it becomes possible to perform measurement while changing the optical path length difference Δd at high speed, compared to when a mirror that can translate in a direction perpendicular to the reflecting surface, like mirror 23, is used instead of rotating mirror 24. As a result, the analysis by interferometry apparatus 1A can be performed at high speed.

[0085] Furthermore, the light source 12 that outputs the reference light L2 is configured by a quantum cascade laser element. In order to detect the interference light IL2 of the reference light L2 with high accuracy in the detector 40, it is preferable to use light with as short (narrow) a spectral linewidth (wavelength linewidth) as possible as the reference light L2. By using a quantum cascade laser element that has the property of outputting light with a narrow spectral linewidth compared to a lamp light source or the like as the light source 12, it is possible to improve the detection accuracy of the interference light IL2 in the detector 40. From the above viewpoint, it is more preferable that the light source 12 is a distributed feedback (DFB) quantum cascade laser element.

[0086] Furthermore, the width of the periodic change in the optical path length difference Δd caused by driving the rotating mirror 24 is 3 mm or more (10 mm in this embodiment). According to the above configuration, it is possible to obtain an appropriate resolution and measurement dynamic range for the change in the optical path of the terahertz wave L1.

[0087] [Second embodiment] 9 and 10, an interferometry apparatus 1B of the second embodiment will be described. The interferometry apparatus 1B differs from the interferometry apparatus 1A in that it has a measurement apparatus 50B instead of the measurement apparatus 50. The measurement apparatus 50B differs from the measurement apparatus 50 in that it further has a light source control unit 53. The interferometry apparatus 1B also differs from the interferometry apparatus 1A in that both the light source 11 and the light source 12 are pulse-driven (i.e., both the terahertz wave L1 and the reference light L2 are pulsed light). Below, the differences between the interferometry apparatus 1B and the interferometry apparatus 1A will be described, and redundant explanations of configurations similar to those of the interferometry apparatus 1A will be omitted.

[0088] The light source control unit 53 controls the driving of the light source 11 and the light source 12. Specifically, the light source control unit 53 synchronizes and pulse-drives the light source 11 and the light source 12. For example, as shown in FIG. 10 , the light source control unit 53 generates a pulse P having a pulse width W1 at a constant period (a period longer than the pulse width W3 of the reference light L2 described later) and outputs the pulse P to the light source 11 and the light source 12 as a trigger signal. This allows the light source 11 and the light source 12 to output the terahertz wave L1 as pulsed light having a pulse width W2 and the reference light L2 as pulsed light having a pulse width W3 at the same timing (the generation timing of the pulse P). For example, the pulse width W1 of the pulse P is set to about 50 fs, the pulse width W2 of the terahertz wave L1 is set to about 1 ps, and the pulse width W3 of the reference light L2 is set to a value (for example, about 100 ns) sufficiently larger than the pulse width W2 of the terahertz wave L1.

[0089] The interferometer 1B can perform measurements similar to those of the interferometer 1A, and by synchronously driving the light sources 11 and 12 in a pulsed manner, it is possible to eliminate the need to cool the light sources 11 and 12. Furthermore, the device life of the light sources 11 and 12 can be extended compared to when the light sources 11 and 12 are continuously driven.

[0090] [Third embodiment] An interferometry apparatus 1C of the third embodiment will be described with reference to Fig. 11. The interferometry apparatus 1C differs from the interferometry apparatus 1A in that it has a single light source 10C instead of the light source 10 consisting of light sources 11 and 12. The interferometry apparatus 1C also differs from the interferometry apparatus 1A in that it has an interferometry optical system 20C instead of the interferometry optical system 20. The interferometry optical system 20C differs from the interferometry optical system 20 in that the beam splitter 21 is omitted. Below, the differences between the interferometry apparatus 1C and the interferometry apparatus 1A will be described, and redundant explanations of configurations similar to those of the interferometry apparatus 1A will be omitted.

[0091] The light source 10C is configured by a single quantum cascade laser element that has a double upper level subband level structure and is capable of generating the terahertz wave L1 and the reference light L2 through multiple intersubband radiative transitions. The light source 10C can be configured by a quantum cascade laser element having a configuration similar to that disclosed in Patent Document 1, for example.

[0092] As an example, the light source 10C is configured to be able to generate first light of a first frequency ω1 and second light of a second frequency ω2 by a plurality of intersubband radiative transitions, and to generate a difference frequency ω between the first frequency ω1 and the second frequency ω2 by difference frequency generation between the first light and the second light. THzThe reference light L2 is configured to be capable of generating light (terahertz wave L1) of (=|ω1-ω2|). For example, the first light and the second light are mid-infrared light. One of the first light and the second light (e.g., the first light) may be light with a constant wavelength, and the other of the first light and the second light (e.g., the second light) may be light with a variable wavelength (or broadband light). In this case, the first light with a constant wavelength can be suitably used as the reference light L2. Furthermore, by using the second light as wavelength-variable light (or broadband light), the wavelength sweep width of the terahertz wave L1 can be increased.

[0093] The interferometer 1C can output the terahertz wave L1 for interferometry and the reference light L2 for measuring the optical path length difference using a single light source 10C (quantum cascade laser element). As a result, the number of light sources can be reduced compared to when the light source for outputting the terahertz wave L1 and the reference light L2 is configured with two light sources (i.e., light source 11 that outputs the terahertz wave L1 and light source 12 that outputs the reference light L2). Furthermore, a component for adjusting the terahertz wave L1 output from light source 11 and the reference light L2 output from light source 12 so that they are incident on the beam splitter 22 coaxially (e.g., the beam splitter 21 provided before the beam splitter 22 in the first embodiment) is not required. Therefore, the interferometer 1C can simplify the configuration of the light source and the interference optical system.

[0094] [Variations] Although several embodiments of the present disclosure have been described above, the present disclosure is not limited to the configurations shown in the above embodiments. The materials and shapes of each configuration are not limited to the specific materials and shapes described above, and various materials and shapes other than those described above can be used. Furthermore, some of the configurations included in the above embodiments may be omitted or modified as appropriate, or may be combined in any manner.

[0095] For example, the photomultiplier tube 30 may be capable of imaging the intensity distribution of incident light. When the electron multiplier unit 32 includes a microchannel plate (e.g., an image intensifier), imaging of the intensity distribution of incident light is possible. By using such a photomultiplier tube 30, analytical imaging of the analyte S becomes possible.

[0096] The analysis unit 52 converted the intensity (Vp-p) of the interference light IL1 measured by the interference intensity measurement unit 51 into an electric field amplitude value and performed a Fourier transform based on the dependency of the electric field amplitude value on the time difference Δt to analyze (identify) the analyte S. However, the analysis unit 52 may analyze the analyte S using a method other than the above. For example, the analysis unit 52 may analyze the analyte S using a method other than converting the intensity (Vp-p) of the interference light IL1 corresponding to each value of the time difference Δt corresponding to the optical path length difference Δd (see (b) of FIG. 5) into an electric field amplitude value. Even in such a case, the same effects as those of the above-described interferometric measurement apparatus 1A can be achieved. That is, it is possible to accurately measure the optical path length difference Δd in the interference light IL1 based on the interference light IL2 of the reference light L2 generated along the same optical path as the terahertz wave L1. As a result, it is possible to perform measurements while continuously changing the optical path length difference Δd and appropriately monitoring the change in the optical path length difference Δd based on the interference light IL2 in real time, thereby enabling high-speed and efficient interferometric measurement using the terahertz wave L1.

[0097] The components constituting the interference optical systems 20 and 20C may be appropriately changed or omitted as long as measurements can be performed using the above-described interferometry apparatuses 1A to 1C. Similarly, the arrangement of the components constituting the interference optical systems 20 and 20C may also be appropriately changed. Furthermore, optical components other than those described in the above embodiments may be added to the interference optical systems 20 and 20C as appropriate. For example, a lens may be disposed between the beam splitter 26 and the photomultiplier tube 30 to increase the efficiency of incidence of the interference light IL1 on the photomultiplier tube 30. Similarly, a lens may be disposed between the beam splitter 26 and the detector 40 to increase the efficiency of incidence of the interference light IL2 on the detector 40. [Explanation of symbols]

[0098] 1A, 1B, 1C...interferometry device, 10, 10C...light source, 11...light source (first light source), 12...light source (second light source), 20, 20C...interference optical system, 21, 22, 26...beam splitter, 23...mirror (first mirror), 24...rotating mirror (second mirror), 25...mirror (third mirror), 30...photomultiplier tube, 40...detector, 51...interference intensity measurement unit, 52...analysis unit, 53...light source control unit, IL1...interference light (first interference light), IL2...interference light (second interference light), L1...terahertz wave, L2...reference light, La...branched light (first branched light), Lb...branched light (second branched light), Pa...optical path (first optical path), Pb...optical path (second optical path), S...analyte.

Claims

1. a light source that outputs a terahertz wave for interference measurement and a reference light for measuring an optical path length difference having a wavelength of 3 μm or more; an interference optical system including: a beam splitter that splits the terahertz wave and the reference light output from the light source and incident coaxially into a first branched light and a second branched light; a first optical path that reflects the first branched light from the beam splitter on a first mirror and causes it to re-enter the beam splitter; and a second optical path that reflects the second branched light from the beam splitter on a second mirror and causes it to re-enter the beam splitter, wherein the first branched light and the second branched light that re-enter the beam splitter are combined, and an optical path length difference between the first branched light and the second branched light is variable; a photomultiplier tube that has sensitivity in the terahertz wave band and outputs an electrical signal value corresponding to the incident light intensity of first interference light, which is interference light of the terahertz wave generated by combining the first branched light and the second branched light in the beam splitter; a detector that detects second interference light, which is interference light of the reference light generated by combining the first branched light and the second branched light in the beam splitter; an interference intensity measuring unit that measures the intensity of the first interference light based on the electrical signal output from the photomultiplier tube; an analysis unit that performs a Fourier transform based on the intensity of the first interference light measured by the interference intensity measurement unit and the detection result of the second interference light by the detector, thereby analyzing an object to be analyzed that is placed on an optical path through which the terahertz wave passes.

2. The analysis unit converting the intensity of the first interference light measured by the interference intensity measuring unit into an electric field amplitude value based on a relationship between an electric field amplitude value of the light incident on the photomultiplier tube and an electric signal value output from the photomultiplier tube, and calculating the electric field amplitude value of the first interference light for each value of the time difference corresponding to the optical path length difference; 2. The interference measurement apparatus according to claim 1, wherein the object to be analyzed is analyzed by performing a Fourier transform based on the calculated dependency of the electric field amplitude value of the first interference light on the value of the time difference.

3. 2. The interferometer according to claim 1, wherein the interference optical system is configured to be able to change the optical path length of the first optical path by driving the first mirror, and is configured to be able to change the optical path length of the second optical path by driving the second mirror.

4. the interference optical system further includes a third mirror; the second mirror is configured to be rotationally driven so that the optical path length of the second optical path is changed; 2. The interferometry device according to claim 1, wherein the second optical path is composed of an optical path that runs from the beam splitter, via the second mirror, to the third mirror, and an optical path that is reflected by the third mirror, passes through the second mirror, and returns to the beam splitter.

5. 2. The interferometric measurement device according to claim 1, wherein the detector is any one of a quantum cascade detector, an MCT detector, a superlattice infrared detector, an InSb detector, an InAs detector, and an InAsSb detector.

6. the light source includes a first light source that outputs the terahertz wave and a second light source that outputs the reference light, The interferometer according to claim 1 , wherein the second light source is configured by a quantum cascade laser element.

7. 2. The interferometry device according to claim 1, wherein the light source is configured by a single quantum cascade laser element that has a double upper level subband level structure and is capable of generating the terahertz wave and the reference light by a plurality of intersubband radiative transitions.

8. a light source control unit that controls the driving of the light source; the light source includes a first light source that outputs the terahertz wave and a second light source that outputs the reference light, The interference measurement apparatus according to claim 1 , wherein the light source control unit pulse-drives the first light source and the second light source in synchronization with each other.

9. the interference optical system is configured to be able to change the optical path length of the second optical path by driving the second mirror, 2. The interferometer according to claim 1, wherein the width of the periodic change in the optical path length difference between the first branched light and the second branched light caused by driving the second mirror is 3 mm or more.

Images