Interferometric measurement device and interferometric measurement method

Abstract

An interference measurement device (1A) is provided with a light source (10), an interference optical system (20A), a photomultiplier tube (30), an interference intensity measurement unit (40), an electric field amplitude calculation unit (50), and an analysis unit (60). The light source (10) outputs light in a frequency band (frequency band of light including terahertz waves) in which the photomultiplier (30) has sensitivity. The photomultiplier (30) has sensitivity to a frequency band of light including terahertz waves, and outputs an electrical signal having a value corresponding to the intensity of incident light. The electric field amplitude calculation unit (50) converts the value of the intensity (V) of the interference light measured by the interference intensity measurement unit (40) into the value of the electric field amplitude (E) on the basis of the relationship between the value of the electric field amplitude of the light incident on the photomultiplier tube (30) and the value of the electric signal output from the photomultiplier tube (30). The value of the electric field amplitude (E) of the interference light is obtained for each value of the time difference ([Delta] t) corresponding to the optical path length difference ([Delta] d). As a result, an interference measurement device and an interference measurement method are achieved, whereby Fourier spectroscopy based on interference measurement using terahertz waves can be accurately performed at a high speed.

Description

Technical Field

[0001] This disclosure relates to interferometric measuring apparatus and interferometric measuring methods. Background Technology

[0002] Terahertz waves are light in the intermediate frequency band between light and radio waves (around 1 THz). Because they exhibit unique absorption spectra for analytes such as pharmaceuticals that are not observed in other wavelength bands, they are expected to be used for the identification of analytes. Various analytical techniques utilizing terahertz waves are known.

[0003] Terahertz time-domain spectrophotometry (THz-TDS) can analyze an object by measuring the time waveform of terahertz waves transmitted, reflected, or totally reflected through it, and then performing a Fourier transform on the time waveform of the electric field amplitude of the terahertz waves obtained from this measurement (Non-Patent Document 1). Hereinafter, this will be referred to as "Prior Art 1". In Prior Art 1, a lock-in amplifier is used when measuring the time waveform of the terahertz waves.

[0004] By using a terahertz wave light source with a variable output wavelength, the terahertz waves that are transmitted, reflected, or totally reflected by the analyte can be dispersed and detected, enabling analysis of the analyte (Non-Patent Document 2). Hereinafter, this will be referred to as "Prior Art 2". In Prior Art 2, a thermal detector is used when detecting terahertz waves.

[0005] Furthermore, analysis of the analyte can also be performed by Fourier transform infrared spectroscopy (FTIR) based on interferometry using terahertz waves, using the same measurement principle as FTIR (Non-Patent Document 3). Hereinafter, this will be referred to as "Prior Art 3". In Prior Art 3, a thermal detector is used when detecting the interference of terahertz waves.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Publication No. 2022-538534

[0009] Non-patent literature

[0010] Non-patent literature 1: Jens Neu et al., "Tutorial: An introduction to terahertztime domain spectroscopy (THz-TDS)", J. Appl. Phys., 124, 231101 (2018), pp.231101-1-231101-14

[0011] Non-patent literature 2: K. Murate et al., "Perspective: Terahertz wave parametric generator and its applications", J. Appl. Phys., 124, 160901 (2018), pp.160901-1-160901-10

[0012] Non-patent literature 3: Masashi Yamaguchi et al., "Terahertz wave generation innitrogen gas using shaped optical pulses", J. Opt. Soc. Am. B, Vol.26, No.9 (2009), pp.A90-A94

[0013] Non-patent literature 4: Simon Lehnskov Lange et al., "Ultrafast THz-driven electron emission from metal metasurfaces", J. Appl. Phys., 128, 070901(2020), pp.070901-1-070901-21 Summary of the Invention

[0014] The technical problem that the invention aims to solve

[0015] In the aforementioned prior art 1, a long integration time of the lock-in amplifier is required when measuring the time waveform of terahertz waves. Furthermore, in the aforementioned prior art 2 and 3, the measurement time is long due to the use of thermal detectors with slow response. Including these prior art 1 to 3, existing analytical techniques using terahertz waves require long measurement times.

[0016] In their research on high-speed analysis techniques using terahertz waves, the inventors explored the use of a photomultiplier tube as described in Patent Document 1. The photomultiplier tube described in Patent Document 1 is sensitive to the frequency band of light containing terahertz waves and can output an electrical signal corresponding to the intensity of the incident light.

[0017] This photomultiplier tube includes an electron-emitting section that releases electrons upon light incidence, an electron-multiplying section that multiplies the released electrons, and a signal output section that collects the multiplied electrons and outputs them as a current signal or an optical image converted by a phosphor. These electron-emitting, electron-multiplying, and signal output sections are housed inside a casing maintained at a vacuum. A window is provided in this casing, through which light enters the electron-emitting section.

[0018] The electron emission section has a metamaterial structure (metasurface) formed on the main surface of the substrate that corresponds to the frequency band of the light of the target object, and can release photoelectrons when light is incident on the metasurface. The electron multiplier section includes multi-stage dynodes or microchannel plates. When the electron multiplier section includes a microchannel plate, imaging of the incident light intensity distribution is possible.

[0019] The inventors have discovered that by using the aforementioned photomultiplier tube instead of a thermal detector in the structure of prior art 3, it is possible to achieve high-speed Fourier spectroscopy based on interferometry using terahertz waves. On the other hand, they have discovered that by simply processing the output signal from the aforementioned photomultiplier tube in the same way as conventional FTIR, it is impossible to perform accurate analysis of the object being analyzed.

[0020] The present invention is based on the insights of the inventors described above, and its purpose is to provide an interferometric measurement device and method capable of performing Fourier spectroscopy based on interferometric measurement using terahertz waves at high speed and with high accuracy.

[0021] Technical means to solve the problem

[0022] The present invention is an interferometric measurement device. The interferometric measurement device comprises: (1) a photomultiplier tube that is sensitive to a frequency band containing terahertz waves and outputs an electrical signal corresponding to the intensity of the incident light; (2) a light source that outputs light in a frequency band to which the photomultiplier tube is sensitive; (3) an interferometric optical system that divides the light output from the light source into two branches as a first branch and a second branch, and combines the first branch and the second branch after passing through the object to be analyzed and incident on the photomultiplier tube, wherein the optical path length difference between the first branch and the second branch is variable; and (4) an interferometric intensity measuring unit that measures the intensity of the incident light based on the electrical signal output from the photomultiplier tube. The intensity of the interference light between the first branch light and the second branch light incident on the photomultiplier tube; (5) the electric field amplitude calculation unit, which converts the value of the intensity of the interference light measured by the interference intensity measurement unit into the value of the electric field amplitude based on the relationship between the value of the electric field amplitude of the light incident on the photomultiplier tube and the value of the electrical signal output from the photomultiplier tube, and calculates the value of the electric field amplitude of the interference light for each value of the time difference corresponding to the difference in optical path length; and (6) the analysis unit, which performs a Fourier transform based on the dependence of the value of the electric field amplitude of the interference light obtained by the electric field amplitude calculation unit on the value of the time difference, thereby performing the analysis of the object to be analyzed.

[0023] The embodiment of the present invention is an interferometric measurement method. The interferometric measurement method is a method of analyzing an object using a photomultiplier tube, a light source, and an interferometric optical system. (1) The photomultiplier tube is sensitive to the frequency band of light containing terahertz waves and outputs an electrical signal corresponding to the intensity of the incident light. (2) The light source outputs light in the frequency band to which the photomultiplier tube is sensitive. (3) The interferometric optical system divides the light output from the light source into two branches as the first branch light and the second branch light. The first branch light that has passed through the object being analyzed is combined with the second branch light and incident on the photomultiplier tube. The difference in optical path length between the first branch light and the second branch light is variable. The interferometric measurement method includes: (4) an interference intensity measurement step, based on the photomultiplier tube... The electrical signal output by the photomultiplier tube is used to measure the intensity of the interference light between the first branch light and the second branch light incident on the photomultiplier tube; (5) Electric field amplitude calculation step: based on the relationship between the value of the electric field amplitude of the light incident on the photomultiplier tube and the value of the electrical signal output from the photomultiplier tube, the value of the intensity of the interference light measured in the interference intensity measurement step is converted into the value of the electric field amplitude, and the value of the electric field amplitude of the interference light is obtained for each value of the time difference corresponding to the difference in optical path length; and (6) Analysis step: based on the dependence of the value of the electric field amplitude of the interference light obtained in the electric field amplitude calculation step on the value of the time difference, Fourier transform is performed, thereby analyzing the object of analysis.

[0024] The effects of the invention

[0025] According to embodiments of the present invention, Fourier spectroscopy based on interferometric measurements using terahertz waves can be performed at high speed and with high accuracy. Attached Figure Description

[0026] Figure 1 This is a diagram showing an example of the structure of an interferometric measuring device.

[0027] Figure 2 This is a diagram showing other structural examples of an interferometric measuring device.

[0028] Figure 3 This is a block diagram showing the structure of the photomultiplier tube 30.

[0029] Figure 4 It is a graph showing the relationship between the intensity of the interference light measured by the interference intensity measuring unit 40 based on the electrical signal output from the photomultiplier tube 30 and the time difference Δt.

[0030] Figure 5 It is a graph showing the relationship between the intensity of the interference light and the time difference Δt, as shown in Non-Patent Document 3.

[0031] Figure 6 (a) is a graph representing the input-output characteristics of the photomultiplier tube 30, and (b) is a graph representing the time dependence of the voltage signal V output from the photomultiplier tube 30.

[0032] Figure 7 (a) is a graph showing the dependence of the optical path length difference Δd between Vp and p, and (b) is a graph showing the dependence of the time difference Δt between Vp and p.

[0033] Figure 8 (a) is a graph showing the dependence of the electric field amplitude E of the interfering light on the time difference Δt, and (b) is a graph showing the amplitude spectrum and phase spectrum of the electric field amplitude E of the interfering light.

[0034] Figure 9 It is a graph showing the relationship between the output value V of the photomultiplier tube 30 obtained through fitting and the electric field amplitude E of the incident terahertz wave (FN formula).

[0035] Figure 10 It is a table that records the correspondence between the electric field amplitude E of the incident terahertz wave calculated using the FN formula and the output value V of the photomultiplier tube 30.

[0036] Figure 11 (a) is a graph showing the dependence of the electric field amplitude E of the interfering light on the time difference Δt, and (b) is a graph showing the phase spectrum of the electric field amplitude E of the interfering light.

[0037] Figure 12(a) is a graph representing the amplitude spectrum of the electric field amplitude E of the interference light, (b) is a graph representing the spectrum of the absorption coefficient α (ω), and (c) is a graph representing the spectrum of the refractive index n (ω).

[0038] Figure 13 It is a graph showing the relationship between the output value V of the photomultiplier tube 30 and the electric field amplitude E of the incident terahertz wave. Detailed Implementation

[0039] Hereinafter, embodiments of the interferometric measuring apparatus and method will be described in detail with reference to the accompanying drawings. Furthermore, in the description of the drawings, the same elements are labeled with the same symbols, and repeated descriptions are omitted. The invention is not limited to these examples, but is intended to include all modifications within the meaning and scope shown and equivalent to the claims.

[0040] The interferometric measuring apparatus of this embodiment can employ the interferometric measuring apparatus 1A described below ( Figure 1 ) or interferometric measuring device 1B ( Figure 2 The structure of ).

[0041] Figure 1 This is a diagram showing an example of the structure of an interferometric measuring device. The interferometric measuring device 1A shown in the figure includes a light source 10, an interferometric optical system 20A, a photomultiplier tube 30, an interferometric intensity measuring unit 40, an electric field amplitude calculation unit 50, and an analysis unit 60.

[0042] The light source 10 outputs light from the photomultiplier tube 30 within a frequency band that has a sensitivity range (including the frequency band of terahertz waves). The light output by the light source 10 can be pulsed light or continuous light.

[0043] Examples of terahertz wave light sources capable of outputting pulsed light include structures combining femtosecond laser sources (e.g., Ti sapphire laser sources) and nonlinear optical crystals (e.g., ZnTe), as well as injection-seeded THz parametric oscillators (is-TPGs). Examples of terahertz wave light sources capable of outputting continuous light include resonant tunneling diodes (RTDs), IMPATT diodes (Impact Avalanche and Transit Time Diodes), quantum cascade laser sources, and THz gas laser sources.

[0044] The interferometric optical system 20A includes a beam splitter 21, a mirror 23, and a mirror 24, and has the structure of a Michelson interferometer. The beam splitter 21 branches the light output from the light source 10 into two branches: a first branch and a second branch. The first branch is output to mirror 23, and the second branch is output to mirror 24. Additionally, the beam splitter 21 receives the first branch reflected by mirror 23 and the second branch reflected by mirror 24, and combines these input first and second branch beams for output to the photomultiplier tube 30. The beam splitter 21 can be constructed from, for example, silicon or ITO mirrors.

[0045] The object of analysis, S, is positioned on the optical path of the first branch of the beam between beam splitter 21 and mirror 23. The object of analysis, S, can also be positioned on the optical path of the second branch. Mirrors 23 and 24, or either one, can move in a direction perpendicular to the reflecting surface, thus the difference in optical path length between the first and second branches is variable.

[0046] The photomultiplier tube 30 is sensitive to the frequency band containing terahertz waves and outputs an electrical signal corresponding to the intensity of the incident light. Details of the photomultiplier tube 30 will be provided later. Figure 3 Please provide an explanation.

[0047] The interference intensity measurement unit 40 measures the intensity of the interference light between the first branch light and the second branch light incident on the photomultiplier tube 30 based on the electrical signal output from the photomultiplier tube 30 (interference intensity measurement step).

[0048] The electric field amplitude calculation unit 50, based on the relationship between the electric field amplitude of the light incident on the photomultiplier tube 30 and the value of the electrical signal output from the photomultiplier tube 30, converts the intensity V of the interference light measured by the interference intensity measurement unit 40 into the value of the electric field amplitude E, and calculates the value of the electric field amplitude E of the interference light for each value of the time difference Δt corresponding to the optical path length difference Δd (electric field amplitude calculation step). The optical path length difference Δd is equivalent to twice the difference in distance from the beam splitter to each of the two mirrors. There is a relationship between the optical path length difference Δd and the time difference Δt: Δt = Δd / c. c is the speed of light in vacuum.

[0049] The analysis unit 60 performs a Fourier transform based on the dependence of the value of the electric field amplitude E of the interference light obtained by the electric field amplitude calculation unit 50 on the value of the time difference Δt, thereby performing an analysis of the object S (analysis step).

[0050] The details of the processing performed by the interference intensity measurement unit 40, the electric field amplitude calculation unit 50, and the analysis unit 60 will also be described later.

[0051] The interferometric measurement method of this embodiment is a method for analyzing the object S to be analyzed using the light source 10, the interferometric optical system 20A and the photomultiplier tube 30 described above, and includes an interference intensity measurement step, an electric field amplitude calculation step and an analysis step.

[0052] Figure 2 This is a diagram showing other structural examples of an interferometric measuring apparatus. The interferometric measuring apparatus 1B shown in this diagram includes a light source 10, an interferometric optical system 20B, a photomultiplier tube 30, an interferometric intensity measuring unit 40, an electric field amplitude calculation unit 50, and an analysis unit 60. Compared with interferometric measuring apparatus 1A (… Figure 1 Compared to the structure of ), the interferometric measuring device 1B ( Figure 2 The difference is that it has an interference optical system 20B instead of an interference optical system 20A.

[0053] The interferometric optical system 20B includes beam splitters 21 and 22, and mirrors 23-26, and has the structure of a Mach-Zehnder interferometer. Beam splitter 21 branches the light output from light source 10 into a first branch and a second branch, outputting the first branch to mirror 23 and the second branch to mirror 24. The second branch output to mirror 24 is reflected by mirrors 24, 25, and 26, respectively. Beam splitter 22 receives the first branch reflected by mirror 23 and the second branch reflected by mirror 26, combining these input first and second branch beams and outputting them to photomultiplier tube 30.

[0054] The object of analysis, S, is positioned on the optical path of the first branch light that travels from beam splitter 21 to beam splitter 22 via mirror 23. The object of analysis, S, can also be positioned on the optical path of the second branch light. Mirrors 24 and 25 are movable, thereby allowing the difference in optical path length between the first and second branch lights to be variable.

[0055] Compared to the Mach-Zehnder interferometer, the Michelson interferometer is preferred because it has fewer optical components (beam splitters, mirrors) and a simpler mechanism for variable optical path length differences. Therefore, it is superior to the interferometric measuring device 1B ( Figure 2 Compared to the structure of ), the preferred interferometric measuring device 1A ( Figure 1 The structure of ).

[0056] Figure 3 This is a block diagram showing the structure of the photomultiplier tube 30. The photomultiplier tube 30 has an electron release section 31, an electron multiplier section 32, and a signal output section 33 arranged inside a housing 34 that is maintained at a vacuum. A window section 35 is provided in the housing 34.

[0057] When light ν passes through the window 35, the electron release unit 31 releases electrons e due to the incident light. The electron release unit 31 is a photoelectric conversion unit designed to be sensitive to the frequency band of light containing the terahertz wave of the detection object. For example, a metamaterial structure (metasurface) is formed on the main surface of the substrate, which can release electrons e when light is incident on the metasurface.

[0058] The electron multiplier 32 multiplies the electrons e released from the electron release unit 31. The electron multiplier 32 includes a multistage dynode or a microchannel plate. The electron multiplication rate in the electron multiplier 32 corresponds to the voltage applied to the multistage dynode or the microchannel plate.

[0059] The signal output unit 33 collects the electrons e multiplied by the electron multiplier 32 and outputs them as a current signal J. The interference intensity measuring unit 40 can input the current signal J output from the signal output unit 33, or it can input the voltage signal after the current signal J has been converted into a voltage signal by the IV conversion circuit.

[0060] Figure 4 This is a graph showing the relationship between the intensity of the interference light, measured by the interference intensity measuring unit 40 based on the electrical signal output from the photomultiplier tube 30, and the time difference Δt. As shown in the graph, there is a peak in the intensity of the interference light at the position where the time difference Δt = 0, and there are also peaks in the ranges of time difference Δt = -16 to -11 ps and time difference Δt = +11 to +16 ps, respectively. On the other hand, the intensity of the interference light is very small in the range of time difference Δt outside these peak ranges.

[0061] The peak in the interference light intensity located at the time difference Δt = 0 is produced by interference between the first and second branch beams without multiple reflections, with a full width at half maximum (FWHM) of approximately 0.4 ps. It can be confirmed that the interference light intensity can be determined using a photomultiplier tube 30.

[0062] The interference light intensity peaks located in the time difference ranges Δt = -16 to -11 ps and Δt = +11 to +16 ps, respectively, are believed to be generated by the etalon effect of the terahertz waves in the window 35 of the photomultiplier tube 30 or the substrate of the electron emission section 31. These interference light intensity peaks generated by the etalon effect are sufficiently far away from the interference light intensity peak located at the time difference Δt = 0, and will not become noise.

[0063] For example, if the window 35 of the photomultiplier tube 30 is made of synthetic quartz (refractive index 1.5) and has a thickness of 1 mm, a reflected pulse is generated at a time interval of 13.3 ps due to the etalon effect. If the substrate of the electron emission section 31 of the photomultiplier tube 30 is made of silicon (refractive index 3.3) and has a thickness of 0.525 mm, a reflected pulse is generated at a time interval of 12.0 ps due to the etalon effect.

[0064] The time intervals (13.3 ps, 12.0 ps) of these reflected pulses generated by the etalon effect are sufficiently greater than the full width at half maximum (FWHM) of the interference light intensity peak located at the time difference Δt = 0 (approximately 0.4 ps). Therefore, the influence of the etalon effect of the terahertz wave in the window 35 of the photomultiplier tube 30 or the substrate of the electron emission section 31 is absent.

[0065] Figure 5 This is a graph showing the relationship between the intensity of the interference light and the time difference Δt, as shown in Non-Patent Document 3. A thermal detector is used in Non-Patent Document 3. (Comparison) Figure 4 and Figure 5 The time difference Δt dependence of the intensity of the interference light is quite different in the photomultiplier tube 30 and the thermal detector.

[0066] The time difference Δt dependence of the intensity of the interference light obtained using the photomultiplier tube 30 has a low correlation with the waveform of the terahertz wave incident on the photomultiplier tube 30. Therefore, accurate analysis of the object being analyzed cannot be performed simply by processing the output signal from the photomultiplier tube 30 in the same way as existing FTIR methods.

[0067] This can be attributed to the fact that the relationship between the electric field amplitude of the terahertz wave incident on the photomultiplier tube 30 and the value of the electrical signal output from the photomultiplier tube 30 differs between the photomultiplier tube 30 and the thermal detector. This invention is based on this insight of the inventors.

[0068] Figures 6-8 This diagram illustrates the processing procedures of the interference intensity measurement unit 40, the electric field amplitude calculation unit 50, and the analysis unit 60. The interference intensity measurement unit 40 performs... Figure 6 (b) and Figure 7 The processing described in (a) is performed by the electric field amplitude calculation unit 50. Figure 7 (b) and Figure 8 The processing described in (a) is performed by the analysis unit 60. Figure 8 The treatment described in (b) is also mentioned. Figure 6 The horizontal axis scale, vertical axis scale, and (a) of the graph. Figure 7 The vertical axis of (b) is logarithmic, while the other horizontal and vertical axes are linear.

[0069] Figure 6 Figure (a) is a graph representing the input-output characteristics of the photomultiplier tube 30. The horizontal axis represents the electric field amplitude E of the light 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 the figure, the input-output characteristics of the photomultiplier tube 30 are not linear. The input-output characteristics of this photomultiplier tube 30 need to be determined beforehand.

[0070] Figure 6 (b) is a graph showing the time dependence of the voltage signal V output from the photomultiplier tube 30. The optical path length difference Δd is set to a certain value, and the time variation of the voltage signal V output from the photomultiplier tube 30 is calculated. The amplitude Vp-p of the voltage signal V is read from this graph.

[0071] Figure 7 (a) is a graph showing the dependence of the optical path length difference Δd on Vp-p. This graph is obtained by calculating Vp-p for each value of the optical path length difference Δd.

[0072] Figure 7 (b) is a graph showing the dependence of the time difference Δt between Vp and p. There is a relationship between the optical path length difference Δd and the time difference Δt: Δt = Δd / c. Therefore, the time difference Δt can be obtained from the optical path length difference Δd.

[0073] Figure 8 (a) is a graph showing the dependence of the electric field amplitude E of the interfering light on the time difference Δt. This is achieved by utilizing the input-output characteristics of the photomultiplier tube 30. Figure 6 (a) can be obtained from Figure 7 (b) is transformed into Figure 8 (a)

[0074] Figure 8 (b) is a graph showing the amplitude and phase spectra of the electric field amplitude E of the interfering light. This is obtained by performing a Fourier transform on the dependence of the electric field amplitude E of the interfering light on the time difference Δt.

[0075] Thus, in this embodiment, the input-output characteristics of the photomultiplier tube 30 (the relationship between the electric field amplitude E of the light incident on the photomultiplier tube 30 and the electrical signal (voltage signal V) output from the photomultiplier tube 30) are determined in advance. Using this relationship, the magnitude Vp-p of the amplitude of the voltage signal V output from the photomultiplier tube 30 is converted into the electric field amplitude E of the interferometric light. By using the photomultiplier tube 30 and performing this conversion, Fourier spectrometry based on interferometry using terahertz waves can be performed quickly and accurately.

[0076] As described above, the input-output characteristics of the photomultiplier tube 30 ( Figure 6(a) is not linear. The output value from the photomultiplier tube 30 can be expressed as a polynomial with the electric field amplitude E of the light incident on the photomultiplier tube 30 as the variable, or it can be expressed as the following equation (1) (non-patent document 4) which represents the efficiency of electron release from the metasurface. This equation represents the current J released from the metasurface. FN The relationship between the electric field amplitude E of the incident terahertz wave and the electric field amplitude E is known as the Fowler-Nordheim relations (hereinafter referred to as the "FN formula").

[0077]

[0078] In this FN formula, a FN and b FN These are referred to as FN constants, and are fixed values. β is the field enhancement factor, which is approximately 400 in Non-Patent Document 4. Φ is the work function of the metasurface material of the electron release section 31; if it is gold, it is 3.5 eV. F and ν F These are constants. When the electric field amplitude of the incident terahertz wave is small, t can be... F and ν F Each of them is set to 1. In this case, the FN formula is expressed by the following equation (2).

[0079]

[0080] The FN formula represents the current J released from the electron emission section 31 of the photomultiplier tube 30. FN The relationship between the electric field amplitude E of the incident terahertz wave and the output value of the photomultiplier tube 30 can also be expressed in the same way.

[0081] We need to find a in the FN formula. FN and b FN Each of these values ​​was determined. Therefore, by setting the electric field amplitude E of the incident terahertz wave to various values, the output value V of the photomultiplier tube 30 was measured, and by performing fitting processing using these measured values, a could be obtained. FN and b FN Their respective values. Figure 9 This is a graph showing the relationship (FN formula) between the output value V of the photomultiplier tube 30 obtained through fitting processing and the electric field amplitude E of the incident terahertz wave. In this graph, five measured values ​​are represented by circles.

[0082] To calculate the electric field amplitude E of the incident terahertz wave based on the output value V of the photomultiplier tube 30 using the FN formula, the following steps are performed. The output value V of the photomultiplier tube 30 is obtained by calculating the electric field amplitude E of the incident terahertz wave using the FN formula. Figure 10 This table records examples of the correspondence between the electric field amplitude E of the incident terahertz wave calculated using the FN formula and the output value V of the photomultiplier tube 30. The electric field amplitude calculation unit 50 calculates the electric field amplitude E of the incident terahertz wave that is closest to the fitted value based on the actual output value V of the photomultiplier tube 30. Alternatively, the electric field amplitude E of the incident terahertz wave can also be obtained through interpolation calculation.

[0083] The analysis unit 60 performs a Fourier transform based on the dependence of the electric field amplitude E of the interference light obtained by the electric field amplitude calculation unit 50 on the time difference Δt, thereby analyzing the object S to be analyzed. The specifics are as follows.

[0084] In the interferometric measuring device 1A, which includes an interferometric optical system 20A with a Michelson interferometer structure ( Figure 1 In the analysis, terahertz waves pass through the object S twice. Let the phase refractive index of object S be n(ω), the extinction coefficient of object S be k(ω), and the complex refractive index of object S be n'(ω) = n(ω) + ik(ω). ω is the angular frequency of the terahertz wave. If the frequency of the terahertz wave is f, then ω = 2πf. π is pi. i is the imaginary unit.

[0085] The electric field amplitude of the interference light obtained under the condition of having the object of analysis S is set as E. sample (ω), where E is the electric field amplitude of the interference light obtained without the object S being analyzed. ref The ratio T(ω) between the two can be expressed by the following equation (3). as The interfacial amplitude transmittance from air to the analyte S is expressed by equation (4) below. sa The interfacial amplitude transmittance from the analyte S to air is expressed by the following equation (5). d is the thickness of the analyte S. c is the speed of light in a vacuum.

[0086]

[0087]

[0088]

[0089] By decomposing equation (3) into real and imaginary parts, we obtain equations (6) to (8) below. Φ(ω) is the phase spectrum. α(ω) is the absorption coefficient. In the interferometric measuring device 1A ( Figure 1In the analysis unit 60, the analysis unit can analyze the object S based on these.

[0090]

[0091]

[0092]

[0093] In the interferometric measuring device 1B, which includes an interferometric optical system 20B with a Mach-Zehnder interferometer structure ( Figure 2 In the interferometric apparatus 1B, the terahertz wave passes through the analyte S once. Therefore, instead of equations (3), (6), and (7) above, equations (9) to (11) are obtained. Figure 2 In the analysis unit 60, the analysis unit can analyze the object S based on these.

[0094]

[0095]

[0096]

[0097] Figure 11 and Figure 12 It is a graph representing an example of the results of a measurement or analysis. Figure 11 The curve of (a) was obtained by the electric field amplitude calculation unit 50. Figure 11 (b) and Figure 12 The curves (a) to (c) are obtained by the analysis unit 60.

[0098] Figure 11 (a) is a graph showing the dependence of the electric field amplitude E of the interference light on the time difference Δt. Figure 11 (b) is a graph representing the phase spectrum of the electric field amplitude E of the interference light. Figure 12 Figure (a) is a graph representing the amplitude spectrum of the electric field amplitude E of the interference light. These graphs show the cases with and without the analyte S. Lactose is used as the analyte S.

[0099] Figure 12 (b) is a graph representing the spectrum of the absorption coefficient α(ω). Figure 12 (c) is a graph representing the spectrum of refractive index n(ω). These graphs show the analysis results of this embodiment and the THz-TDS analysis results of prior art 1.

[0100] Comparing the analysis results of this embodiment with the THz-TDS analysis results of prior art 1, it can be seen that the positions of the absorption peaks appearing in the spectrum of the absorption coefficient α(ω) are consistent between the two. Therefore, it can be said that this embodiment can be used to analyze the analyte S.

[0101] In the interference intensity measurement step, preferably, the interference intensity measurement unit 40 adjusts the measurement range when measuring the intensity of the interference light based on the electrical signal output from the photomultiplier tube 30, suppresses the saturation of the measured value of the interference light intensity, and suppresses the measured value of the interference light intensity from falling below the noise level.

[0102] That is, when the optical path length difference Δd changes, the amplitude Vp-p of the voltage signal V output from the photomultiplier tube 30 changes by about three orders of magnitude. At this time, if the measurement range for measuring the intensity of the interference light is fixed, the measured value of the interference light intensity may sometimes saturate, or the measured value may fall below the noise level. By adjusting the measurement range for measuring the intensity of the interference light, the interference intensity measuring unit 40 can suppress the saturation of the measured value of the interference light intensity and prevent the measured value of the interference light intensity from falling below the noise level.

[0103] In the interference intensity measurement step, preferably, the interference intensity measurement unit 40 measures the magnitude of a specific frequency component obtained by performing a Fourier transform on the time waveform of the electrical signal output from the photomultiplier tube 30 as the intensity of the interference light.

[0104] That is, the interference intensity measurement unit 40 preferably uses the FFT function of an oscilloscope. Using the FFT function allows for a logarithmic transformation of the electrical signal value, thus suppressing saturation of the measured value of the interference light intensity even without adjusting the measurement range, and preventing the measured value of the interference light intensity from falling below the noise level. The intensity of the interference light is determined by measuring the magnitude of a specific frequency component obtained through the Fourier transform.

[0105] Generally, the lower the frequency, the higher the signal-to-noise ratio (SN ratio). High-frequency components depend on the type of photomultiplier tube, so it is preferable to analyze low-frequency components as well. However, at a frequency of 0 Hz (i.e., the DC component), there is no meaningful difference based on the signal. Furthermore, it is preferable to avoid components that coincide with the repetition frequency of the pulsed light output from the light source 10. Therefore, it is preferable that the interference intensity measuring unit 40 measures the lowest possible frequency components as the intensity of the interference light, excluding the DC component and the repetition frequency of the pulsed light.

[0106] In the interference intensity measurement step, preferably, the interference intensity measurement unit 40 adjusts the applied voltage to the photomultiplier tube 30, and sets the dynamic range or sensitivity when measuring the intensity of the interference light based on the electrical signal.

[0107] That is, such as Figure 9 As shown, the output value V of the photomultiplier tube 30 increases non-linearly with the increase of the electric field amplitude E of the incident terahertz wave. There is a tendency for the increase in the output value V of the photomultiplier tube 30 to become sluggish as the electric field amplitude E of the incident terahertz wave increases. Furthermore, by changing the voltage applied to the electron multiplication section 32 of the photomultiplier tube 30, the sensitivity curve representing the relationship between the output value V of the photomultiplier tube 30 and the electric field amplitude E of the incident terahertz wave can be shifted to the left or right.

[0108] Figure 13 This is a graph showing the relationship between the output value V of the photomultiplier tube 30 and the electric field amplitude E of the incident terahertz wave. The graph shows three sensitivity curves A to C for different voltage values ​​applied to the electron multiplier section 32 of the photomultiplier tube 30. It is assumed that the specifications of the light source 10 are fixed, and that the range ΔE of the electric field amplitude E of the incident terahertz wave is also fixed.

[0109] If a voltage is increased when applying it to sensitivity curve A, the sensitivity curve shifts to the left, becoming sensitivity curve B. The variation range ΔV of the output value V of photomultiplier tube 30 in sensitivity curve A is also shown. A In contrast, in sensitivity curve B, the range of variation ΔV of the output value V of photomultiplier tube 30 is... B The smaller size allows for an increase in the dynamic range of the measurement, which, for example, is beneficial for spectroscopic measurements using a photomultiplier tube 30.

[0110] Conversely, if a smaller voltage is applied to sensitivity curve A, the sensitivity curve shifts to the right, becoming sensitivity curve C. The range of change ΔV of the output value V of photomultiplier tube 30 in sensitivity curve A is also shown. A In contrast, in sensitivity curve C, the range of variation ΔV of the output value V of photomultiplier tube 30 is... C The size increases, thus increasing the measurement sensitivity, which, for example, is beneficial for sensing using the photomultiplier tube 30.

[0111] In this way, by adjusting the voltage applied to the photomultiplier tube 30, the interference intensity measuring unit 40 can set the dynamic range or sensitivity for measuring the intensity of the interference light based on the electrical signal.

[0112] The photomultiplier tube 30 can measure the intensity of incident light and also perform imaging of the intensity distribution of incident light. When the electron multiplier section 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 object S can be performed.

[0113] The interferometric measuring device and interferometric method are not limited to the above-described implementation methods and structural examples, and can be modified in various ways.

[0114] The first embodiment of the above-described interferometric measuring apparatus comprises: (1) a photomultiplier tube that is sensitive to the frequency band of light containing terahertz waves and outputs an electrical signal corresponding to the intensity of the incident light; (2) a light source that outputs light in the frequency band to which the photomultiplier tube is sensitive; (3) an interferometric optical system that divides the light output from the light source into two branches as a first branch and a second branch, combines the first branch and the second branch after passing through the object to be analyzed, and incident the combined light onto the photomultiplier tube, wherein the optical path length difference between the first branch and the second branch is variable; and (4) an interferometric intensity measuring unit that measures the intensity of the light output from the photomultiplier tube based on the electrical signal. (5) An electric field amplitude calculation unit, which, based on the relationship between the value of the electric field amplitude of the light incident on the photomultiplier tube and the value of the electrical signal output from the photomultiplier tube, converts the value of the intensity of the interference light measured by the interference intensity measurement unit into the value of the electric field amplitude, and calculates the value of the electric field amplitude of the interference light for each value of the time difference corresponding to the difference in optical path length; and (6) an analysis unit, which performs a Fourier transform based on the dependence of the value of the electric field amplitude of the interference light obtained by the electric field amplitude calculation unit on the value of the time difference, thereby performing analysis on the object of analysis.

[0115] The second type of interferometric measurement device can also be configured such that, in the structure of the first type, the interference intensity measuring unit adjusts the measurement range when measuring the intensity of the interference light based on the electrical signal output from the photomultiplier tube, suppresses the saturation of the measured value of the interference light intensity, and suppresses the measured value of the interference light intensity to below the noise level.

[0116] The third type of interferometric measurement device can also be configured such that, in the structure of the first or second type, the interferometric intensity measuring unit measures the magnitude of a specific frequency component obtained by performing a Fourier transform on the time waveform of the electrical signal output from the photomultiplier tube as the intensity of the interferometric light.

[0117] The fourth type of interferometric measurement device can also be configured such that, in any of the first to third types, the interference intensity measuring unit adjusts the applied voltage to the photomultiplier tube and sets the dynamic range or sensitivity for measuring the intensity of the interference light based on the electrical signal.

[0118] The fifth type of interferometric measurement device can also be configured such that, in any of the first to fourth types of structures, the photomultiplier tube is capable of imaging the intensity distribution of incident light, and the analysis unit performs analytical imaging of the object to be analyzed.

[0119] The first method of the above-described interferometric measurement method is a method of analyzing an object using a photomultiplier tube, a light source, and an interferometric optical system. (1) The photomultiplier tube is sensitive to the frequency band of light containing terahertz waves and outputs an electrical signal corresponding to the intensity of the incident light. (2) The light source outputs light in the frequency band to which the photomultiplier tube is sensitive. (3) The interferometric optical system divides the light output from the light source into two branches as the first branch light and the second branch light. The first branch light that has passed through the object being analyzed is combined with the second branch light and incident on the photomultiplier tube. The difference in optical path length between the first branch light and the second branch light is variable. The interferometric measurement method includes: (4) an interferometric intensity measurement step. Step 1: Based on the electrical signal output from the photomultiplier tube, measure the intensity of the interference light between the first branch light and the second branch light incident on the photomultiplier tube; (5) Electric field amplitude calculation step: Based on the relationship between the value of the electric field amplitude of the light incident on the photomultiplier tube and the value of the electrical signal output from the photomultiplier tube, convert the value of the intensity of the interference light measured in the interference intensity measurement step into the value of the electric field amplitude, and calculate the value of the electric field amplitude of the interference light for each value of the time difference corresponding to the difference in optical path length; and (6) Analysis step: Based on the dependence of the value of the electric field amplitude of the interference light obtained in the electric field amplitude calculation step on the value of the time difference, perform Fourier transform, and thereby analyze the object of analysis.

[0120] The second type of interferometric measurement method can also be configured such that, in the structure of the first type, in the interferometric intensity measurement step, the measurement range when measuring the intensity of the interferometric light is adjusted based on the electrical signal output from the photomultiplier tube, the saturation of the measured value of the intensity of the interferometric light is suppressed, and the measured value of the intensity of the interferometric light is suppressed to fall below the noise level.

[0121] The third type of interferometric measurement method can also be configured such that, in the structure of the first or second type, in the interference intensity measurement step, the magnitude of a specific frequency component obtained by performing a Fourier transform on the time waveform of the electrical signal output from the photomultiplier tube is measured as the intensity of the interferometric light.

[0122] The fourth type of interferometric measurement method can also be configured such that, in any of the first to third types of structures, in the interferometric intensity measurement step, the applied voltage to the photomultiplier tube is adjusted, and the dynamic range or sensitivity when measuring the intensity of the interferometric light is set based on the electrical signal.

[0123] The fifth type of interferometric measurement method can also be configured such that, in any of the first to fourth types of structures, a photomultiplier tube capable of imaging the intensity distribution of incident light is used as a photomultiplier tube, and analytical imaging of the object to be analyzed is performed in the analysis step.

[0124] Industrial availability

[0125] This invention can be used as an interferometric apparatus and method for high-speed and accurate Fourier spectroscopy based on interferometric measurements using terahertz waves.

[0126] Explanation of symbols

[0127] 1A, 1B…interference measuring device, 10…light source, 20A, 20B…interference optical system, 21, 22…beam splitter, 23~26…mirror, 30…photomultiplier tube, 31…electron release unit, 32…electron multiplier unit, 33…signal output unit, 34…house, 35…window, 40…interference intensity measuring unit, 50…electric field amplitude calculation unit, 60…analysis unit, S…analyte.

Claims

1. An interferometric measuring device, wherein, have: A photomultiplier tube is sensitive to the frequency band of light containing terahertz waves and outputs an electrical signal corresponding to the intensity of the incident light. A light source that outputs light in a frequency band to which the photomultiplier tube has sensitivity; An interference optical system that branches the light output from the light source into two branches, namely the first branch light and the second branch light, combines the first branch light that has passed through the object being analyzed with the second branch light and incident it onto the photomultiplier tube. The difference in optical path length between the first branch light and the second branch light is variable. An interference intensity measuring unit measures the intensity of the interference light between the first branch light and the second branch light incident on the photomultiplier tube based on the electrical signal output from the photomultiplier tube. The electric field amplitude calculation unit calculates the electric field amplitude of the interference light, which is measured by the interference intensity measurement unit, based on the relationship between the value of the electric field amplitude of the light incident on the photomultiplier tube and the value of the electrical signal output from the photomultiplier tube. It then calculates the electric field amplitude of the interference light for each value of the time difference corresponding to the optical path length difference. and The analysis unit performs a Fourier transform based on the dependence of the electric field amplitude of the interference light obtained by the electric field amplitude calculation unit on the value of the time difference, thereby analyzing the object to be analyzed.

2. The interferometric measuring device according to claim 1, wherein, The interference intensity measuring unit adjusts the measurement range when measuring the intensity of the interference light based on the electrical signal output from the photomultiplier tube, suppresses the saturation of the measured value of the interference light intensity, and suppresses the measured value of the interference light intensity from falling below the noise level.

3. The interferometric measuring device according to claim 1 or 2, wherein, The interference intensity measuring unit measures the magnitude of a specific frequency component obtained by performing a Fourier transform on the time waveform of the electrical signal output from the photomultiplier tube, and uses this as the intensity of the interference light.

4. The interferometric measuring apparatus according to any one of claims 1 to 3, wherein, The interference intensity measuring unit adjusts the voltage applied to the photomultiplier tube and sets the dynamic range or sensitivity for measuring the intensity of the interference light based on the electrical signal.

5. The interferometric measuring apparatus according to any one of claims 1 to 4, wherein, The photomultiplier tube is capable of imaging the intensity distribution of incident light, and the analysis unit performs analytical imaging of the object being analyzed.

6. An interferometric measurement method, wherein, It is a method that uses photomultiplier tubes, light sources, and interferometric optical systems to analyze the object being analyzed. The photomultiplier tube is sensitive to the frequency band containing terahertz waves and outputs an electrical signal corresponding to the intensity of the incident light. The light source outputs light in a frequency band that the photomultiplier tube is sensitive to. The interferometric optical system branches the light output from the light source into two branches: a first branch and a second branch. The first branch, having passed through the object being analyzed, is combined with the second branch and then incident on the photomultiplier tube. The difference in optical path length between the first and second branches is variable. The interferometric measurement method has the following features: The interference intensity measurement step is based on the electrical signal output from the photomultiplier tube to measure the intensity of the interference light between the first branch light and the second branch light incident on the photomultiplier tube. The electric field amplitude calculation step is based on the relationship between the value of the electric field amplitude of the light incident on the photomultiplier tube and the value of the electrical signal output from the photomultiplier tube. The value of the intensity of the interference light measured in the interference intensity measurement step is converted into the value of the electric field amplitude. For each value of the time difference corresponding to the optical path length difference, the value of the electric field amplitude of the interference light is obtained. and The analysis step involves performing a Fourier transform based on the dependence of the electric field amplitude value of the interference light obtained in the electric field amplitude calculation step on the value of the time difference, thereby analyzing the object being analyzed.

7. The interferometric measurement method according to claim 6, wherein, In the interference intensity measurement step, the measurement range for measuring the intensity of the interference light is adjusted based on the electrical signal output from the photomultiplier tube, thereby suppressing the saturation of the measured value of the interference light intensity and preventing the measured value of the interference light intensity from falling below the noise level.

8. The interferometric measurement method according to claim 6 or 7, wherein, In the interference intensity measurement step, the magnitude of a specific frequency component obtained by performing a Fourier transform on the time waveform of the electrical signal output from the photomultiplier tube is measured as the intensity of the interference light.

9. The interferometric measurement method according to any one of claims 6 to 8, wherein, In the interference intensity measurement step, the applied voltage to the photomultiplier tube is adjusted, and the dynamic range or sensitivity for measuring the intensity of the interference light is set based on the electrical signal.

10. The interferometric measurement method according to any one of claims 6 to 9, wherein, The photomultiplier tube used is one capable of imaging the intensity distribution of incident light, and the analytical imaging of the object to be analyzed is performed in the analysis step.

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