Far-infrared spectrometer and far-infrared spectrometer

Abstract

This invention provides a far-infrared spectrometer and a far-infrared spectroscopic method that can accurately measure the absorption spectrum derived from a sample. [Solution] In a far-infrared spectrometer 100 comprising a detector 225 that detects light obtained by irradiating a sample with far-infrared light, and a signal processing unit 230 that calculates the absorption spectrum of the sample from the signal from the detector, a first spectrum detected by the detector is obtained when the wavelength of the far-infrared light is changed and irradiated into the measurement optical path with the sample not on the measurement optical path, and a second spectrum detected by the detector is obtained when the wavelength of the far-infrared light is changed and irradiated into the measurement optical path with the sample on the measurement optical path. Then, the difference between the spectral signals is calculated at frequency intervals determined according to the absorption peak due to water vapor, and the frequency of the absorption peak due to water vapor is selected according to a comparison of the difference with a threshold. Based on the selected frequency, data removal is performed on the spectral signal.

Description

[Technical Field]

[0001] The present invention relates to a far-infrared spectrometer and a far-infrared spectrometer. [Background technology]

[0002] Far-infrared light in the 0.1 THz to 10 THz range is also known as terahertz waves. This frequency band is intermediate between radio waves and light, and terahertz waves are characterized by their combination of the straight-line propagation of light and high transmittance. Furthermore, because terahertz waves have an energy corresponding to the excitation width of phonon modes, absorption peaks originating from lattice vibrations and intermolecular vibrations can be obtained.

[0003] Because this absorption peak can be observed at a frequency specific to the substance, it can be used for non-destructive identification of materials. Applying these characteristics, industrial applications are expected in imaging technologies such as hazardous materials testing and pharmaceutical testing, as well as quantitative analysis of components.

[0004] One of the terahertz spectroscopy methods is terahertz time-domain spectroscopy (THz-TDS), which has been put into practical use since the 1990s and has become common ever since. In this THz-TDS method, a femtolaser is used as the light source to acquire the time waveform of a broadband terahertz pulse. By performing a fast Fourier transform on this time waveform, an absorption spectrum can be obtained.

[0005] In the field of pharmaceutical testing, research and development is progressing on destructive and non-destructive testing of pharmaceuticals using infrared light, ultraviolet light, Raman scattering light, and terahertz waves. In particular, research and development is underway to support in-line methods that involve introducing spectroscopic equipment during the manufacturing process, and expectations are rising for terahertz spectroscopy, which enables non-destructive testing.

[0006] However, since there are numerous absorption peaks due to water vapor in the terahertz band, conventional terahertz spectroscopy requires filling the optical path with a gas such as dry air to reduce this effect. This makes it difficult to implement in-line drug testing. Furthermore, such humidity changes may cause changes in the crystalline form and properties of the drug itself. For this reason, there is a need for a spectrometer that can measure samples under humid conditions without using dry air.

[0007] As a method for realizing terahertz spectroscopy measurements under humid conditions, a method is known that uses data from an existing database of water vapor absorption peak locations and applies weights to each frequency for smoothing (Patent Document 1). However, with this method using information from an existing database, it was difficult to correctly determine the frequency over a sufficient bandwidth due to differences in the characteristics of the method used to acquire the database data and the method used to actually measure the sample spectrum. Without correct determination, adjusting the threshold alone was insufficient to reduce the water vapor absorption peak over a sufficient bandwidth, posing a challenge to obtaining practical performance. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Republished Gazette No. 08 / 001785 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The present invention provides a far-infrared spectrometer and a far-infrared spectroscopy method that can effectively reduce absorption peaks originating from water vapor with minimal computational effort, even in any humid air atmosphere, and accurately measure the absorption spectrum originating from a sample. [Means for solving the problem]

[0010] To solve the above problems, the far-infrared analyzer according to the present invention comprises a holding mechanism configured to hold a sample in humid air, a detector that detects light obtained by irradiating the sample with far-infrared light, and a signal processing unit that calculates the absorption spectrum of the sample from the signal from the detector. The signal processing unit obtains a first spectrum detected by the detector when the far-infrared light is irradiated into the measurement optical path while changing the wavelength of the far-infrared light when the sample is not on the measurement optical path, and a second spectrum detected by the detector when the far-infrared light is irradiated into the measurement optical path while changing the wavelength of the far-infrared light when the sample is on the measurement optical path. The signal processing unit then calculates the difference between the first spectrum, the second spectrum, or the spectrum obtained based on the first and second spectra at frequency intervals determined according to the width of the absorption peak due to water vapor, selects the frequency of the absorption peak due to water vapor according to a comparison of the difference with a threshold, and performs data removal in the signal of the first spectrum, the second spectrum, or the spectrum obtained based on the first and second spectra based on the selected frequency. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a far-infrared spectrometer and a far-infrared spectroscopic method that can accurately measure the absorption spectrum originating from a sample by effectively reducing the influence of absorption peaks originating from water vapor with a small amount of computation, even in a humid air atmosphere. [Brief explanation of the drawing]

[0012] [Figure 1A] This is a schematic diagram illustrating the configuration of the far-infrared spectrometer 100 according to the first embodiment. [Figure 1B] This figure shows that when a high-intensity pump light of frequency fp is incident on a nonlinear optical crystal having a second-order nonlinear susceptibility χ(2), a signal light of frequency fs and an idler light of frequency fi are parametrically generated. [Figure 1C]This is a wave vector diagram illustrating the momentum conservation law for generating terahertz waves using pump light and seed light. [Figure 1D] This figure shows the relationship between the measurement frequency f and the spectral intensity values ​​acquired for each frequency. [Figure 2A] This diagram illustrates the detailed configuration of the spectrum processing unit 230. [Figure 2B] This is a conceptual diagram illustrating specific examples of spectral removal and interpolation processing in the spectral removal / interpolation unit 260. [Figure 2C] This is a block diagram showing the determination procedure in the water vapor absorption peak frequency determination unit 250 of the far-infrared spectrometer of the first embodiment. [Figure 3A] This graph illustrates a specific example of determining the water vapor absorption peak frequency by calculating the difference dk = r(k+δ)-rk of the reference spectrum and comparing it with a threshold value t. [Figure 3B] This graph shows an example of a reference spectrum obtained under humid air conditions and the spectrum after removing and interpolating the water vapor absorption peak. [Figure 4] This graph shows examples of absorption spectra where the absorption peaks of water vapor and the sample are mixed, and an example of an absorption spectrum of the sample obtained by reducing the influence of the water vapor absorption peak. [Figure 5] This is a block diagram of a comparative example. [Figure 6A] This graph shows an example of a reference spectrum acquired at Δ=5GHz. [Figure 6B] This graph shows an example of a spectrum obtained by removing the intensity values ​​attenuated by the water vapor absorption peak from the reference spectrum in Figure 6A. [Figure 6C] This is a block diagram showing an example of the configuration of a water vapor absorption peak frequency determination unit 250b according to a modification of the first embodiment, and the processing procedure. [Figure 7] This is a block diagram showing the processing of the water vapor absorption peak frequency determination unit 250c in the second embodiment. [Figure 8] This graph shows the result of removing spectral intensity values ​​attenuated by water vapor absorption, based on the second embodiment. [Figure 9] This figure illustrates a process performed in a far-infrared spectrometer according to the third embodiment to reduce the influence of water vapor absorption on the sample spectrum. [Figure 10] This figure illustrates a procedure for reducing the influence of water vapor absorption peaks from the absorption spectrum of a sample, as performed in a far-infrared spectrometer according to the fourth embodiment. [Figure 11] This figure illustrates the details of the water vapor absorption peak frequency determination unit 250d of the far-infrared spectrometer according to the fourth embodiment. [Modes for carrying out the invention]

[0013] This embodiment will be described below with reference to the attached drawings. In the attached drawings, functionally identical elements may be indicated by the same number. The attached drawings show embodiments and implementation examples in accordance with the principles of this disclosure, but these are for the purpose of understanding this disclosure and are not to be used in any way to restrictively interpret this disclosure. The descriptions in this specification are merely typical examples and do not limit the claims or applications of this disclosure in any way.

[0014] While this embodiment is described in sufficient detail for those skilled in the art to implement the disclosure, it is important to understand that other implementations and forms are possible, and that the configuration and structure can be modified and various elements replaced without departing from the scope and spirit of the technical idea of ​​this disclosure. Therefore, the following description should not be interpreted as limiting it to this embodiment.

[0015] (First Embodiment) Referring to Figure 1A, the configuration of the far-infrared spectrometer 100 according to the first embodiment will be described. This far-infrared spectrometer 100 includes, as an example, a light source unit 210, an optical system 220, and a spectrum processing unit 230 (signal processing unit). The spectrum processing unit 230 may consist of a general-purpose computer capable of performing various calculations and a computer program for spectrum processing.

[0016] The light source unit 210 and the optical system 220 employ a terahertz parametric method using an is-TPG (Injection-seeded Terahertz Parametric Generation) light source, which generates high-intensity light (in this case, terahertz light) from two light sources of different wavelengths and a nonlinear optical crystal. The is-TPG method offers superior peak power and wavelength resolution compared to the aforementioned TDS method. While the following description will use an is-TPG light source as an example, the present invention is also applicable to the TDS method.

[0017] Details of the light source unit 210 will be explained with reference to Figure 1A. The light source unit 210 consists, for example, of a light source 211 as a pump light source, a polarizing beam splitter 212, a light source 213 as a seed light source, optical elements 214, 215, a mirror 216, etc. The light source unit 210 (is-TPG light source unit) includes two light sources 211 and 213 that emit near-infrared light. The two light sources 211 and 213 emit near-infrared light of different wavelengths.

[0018] The light source 211 is a pump light source, and for example, a microchip laser can be used. The light emitted from the light source 211 (pump light) is split into two directions by the polarizing beam splitter 212, with one beam directed to the nonlinear optical crystal 221a via the mirror 216 and the other directed to the nonlinear optical crystal 221b.

[0019] Light source 213 is a seed light source, and for example, a tunable semiconductor laser can be used. Optical element 214 is an angle-controllable reflector, and for example, a galvanometer mirror can be used. Optical element 215 can for example, a concave mirror can be used.

[0020] The light emitted from the light source 213 (seed light) is incident on the nonlinear optical crystal 221a via optical elements 214 and 215. The incident seed light at this time is guided at an angle (horizontal direction of the paper) that satisfies the phase matching angle described later with respect to the pump light, by controlling the angle of optical element 214. At this time, since the incident surface on the nonlinear optical crystal 221a is in an imaging relationship with the surface of optical element 214, the irradiation position of the seed light on the incident surface of the nonlinear optical crystal 221a does not change even if the angle of optical element 214 is changed.

[0021] Next, the details of the optical system 220 will be described with reference to Figure 1A. As an example, the optical system 220 includes a sealed chamber 229, a near-infrared photodetector 225, a control unit 226, a dry air supply unit 227, and a dry air inflow control unit 228.

[0022] Furthermore, a terahertz light generation mechanism, consisting of a nonlinear optical crystal 221a and a Si prism 222a pressed together, is installed inside the sample chamber, which is composed of a sealed chamber 229. Terahertz light of an arbitrary wavelength is generated in the terahertz light generation mechanism from pump light incident on the nonlinear optical crystal 221a and wavelength-tunable seed light. A beam damper BD1 is provided near the nonlinear optical crystal 221a to block unwanted light passing through the nonlinear optical crystal 221a.

[0023] Furthermore, an optical mechanism consisting of a nonlinear optical crystal 221b and a Si prism 222b pressed together is also arranged inside the sample chamber. The nonlinear optical crystal 221b and Si prism 222b may have the same mechanism (structure) as the nonlinear optical crystal 221a and Si prism 222a, and function as a detection light (near-infrared light) generation unit. Light from the Si prism 222a is guided to the nonlinear optical crystal 221b via the Si prism 222b by the light guide optical system 223.

[0024] Furthermore, a sample stage ST and a moving stage RM that holds the sample stage ST are provided near the midpoint between the nonlinear optical crystals 221a and 221b inside the sample chamber. The sample stage ST, the moving stage RM, and the sample chamber constitute a holding mechanism for holding the sample in humid air (humid conditions). The sample stage ST and the moving stage RM are positioned so that the sample loaded onto the sample stage ST is inserted into and removed from the optical path of the terahertz light described above.

[0025] The principle of generating terahertz light using a nonlinear optical crystal will be explained in detail with reference to Figures 1B and 1C. Figure 1B shows the second-order nonlinear susceptibility χ. (2) This figure shows that when a high-intensity pump light of frequency fp is incident on a nonlinear optical crystal with (≠0), a signal light of frequency fs and an idler light of frequency fi are generated due to nonlinear polarization. In this case, if light of frequency fs is incident simultaneously with the pump light, the light of frequency fi can be amplified (this also holds if fs and fi are swapped), and this is called parametric amplification.

[0026] Pump light and seed light are incident non-coaxially on a nonlinear optical crystal, and terahertz waves are generated and amplified by the parametric amplification described above. The frequencies of the pump light and seed light, fpump and fseed, are determined to satisfy the following equation [Equation 1], where the frequency of the generated terahertz wave is fTHz. Furthermore, regarding the angle between the optical axis of the pump light and the optical axis of the seed light incident non-coaxially, when the wave vector of the pump light is denoted as →kpump and the wave vector of the seed light as →kseed, the wave vector kTHz of the generated terahertz wave is determined to satisfy the following equation [Equation 2].

[0027] [Mathematics 1] fpump-fseed=fTHz [Math 2] →kpump-→kseed=→kTHz

[0028] In Figure 1A, terahertz light can be parametrically generated by using the light from the light source 211 and the light from the seed light source 213 incident on the nonlinear optical crystal 221a in such a way that the aforementioned [Equation 1] and [Equation 2] are simultaneously satisfied.

[0029] The generation of detection light in the nonlinear optical crystal 221b shown in Figure 1A is based on the same principle as the terahertz light. Near-infrared light is parametrically generated in the nonlinear optical crystal 221b using terahertz light generated from the nonlinear optical crystal 221a and pump light from the polarizing beam splitter 212. The resulting near-infrared light becomes the detection light incident on the near-infrared photodetector 225.

[0030] Terahertz light transmitted through the sample placed on the sample stage ST, along with pump light from the polarizing beam splitter 212, is incident on the nonlinear optical crystal 221b, generating the aforementioned near-infrared light. This near-infrared light is used as detection light and guided to the near-infrared photodetector 225, where the intensity of the detection light is obtained as a detection signal value.

[0031] When acquiring the absorption spectrum of a sample, the frequency f of the measurement light is set from the measurement start frequency f1 [THz] to the measurement end frequency f n The detection signal value is acquired sequentially while changing the frequency in multiple ways at a predetermined frequency interval Δ up to [THz].

[0032] Figure 1D illustrates the relationship between these acquired signal values. After setting the measurement light frequency f to the measurement start frequency f1, first, a detection signal value (reference signal value) x1 is acquired when the sample is not on the terahertz optical path. Next, the sample movement stage RM is moved to place the sample on the terahertz optical path (sample stage ST) and a detection signal value (sample signal value) y1 is acquired. When measuring multiple samples simultaneously, the detection signal value of the next sample is then acquired. Furthermore, the terahertz optical path (measurement optical path) is shielded by a light-shielding section (not shown) and detection signal values ​​z1 as noise signal values ​​are acquired sequentially.

[0033] The process continues similarly; after setting the measurement light frequency f to f2, the detected signal values ​​are acquired in the following order: reference signal value x2, sample signal value y2, and noise signal value z2, in the same manner as above. This is then set to the measurement end frequency f n The process is repeated n times, obtaining a reference signal value xi, a sample signal value yi, and a noise signal value zi (i=1~n) at each frequency. n is determined by the measurement start / end frequencies f1 and fn and the measurement frequency interval Δ, as shown in [Equation 3], and represents the number of data points of the detected signal value obtained.

[0034] [Math 3] n=1+(fn-f1) / Δ

[0035] With respect to the series of detection signals obtained in this way (n-th order vector data), the series of signals xi acquired without the sample on the terahertz optical path is used as a reference spectrum (x1, x2, ..., x n ) is defined as the sample spectrum (y1, y2, ..., y n ) is defined as. Furthermore, the sequence of signal values ​​zi obtained by blocking the terahertz optical path is defined as the noise spectrum (n1, n2, ..., n n ) is defined as follows.

[0036] While some light originates solely from the pump light and others from the nonlinear optical crystal 221b, the influence of light generated independently of the terahertz light produced by the nonlinear optical crystal 221 is included in the noise spectrum.

[0037] The reference spectrum, sample spectrum, and noise spectrum are stored in the control unit 226 and then transmitted to the spectrum processing unit 230. The operation of the pump light source 211, seed light source 213, optical element (galvanometer mirror) 214, translation stage RM, detector 225, and dry air inflow control unit 228 can be controlled by the control unit 226.

[0038] In the spectrum processing unit 230, the absorption spectrum of the sample is calculated based on the reference spectrum, the sample spectrum, and the noise spectrum. The absorption spectrum can be calculated by comparing the reference spectrum with the sample spectrum to determine at what wavelengths the sample spectrum is attenuated and to what extent. In this embodiment, the far-infrared spectrometer is configured to detect terahertz light transmitted through the sample, but it is also possible to configure it to detect reflected light.

[0039] The configuration of the spectrum processing unit 230 is shown in Figure 2A. Specifically, the spectrum processing unit 230 comprises a noise processing unit 240, a water vapor absorption peak frequency determination unit 250, a spectrum removal / interpolation unit 260, and an absorption spectrum calculation unit 270. The water vapor absorption peak frequency determination unit 250 further comprises a water vapor absorption peak half-width input unit 160, a difference interval calculation unit 165, a difference calculation unit 170, a threshold processing unit 180, and a frequency list storage unit 190, as shown in Figure 2A. The spectrum removal / interpolation unit 260 comprises spectrum removal units 261a and 261b and interpolation units 265a and 265b. Each of these processes will be explained below.

[0040] The noise processing unit 240 has the function of subtracting the noise spectrum from the reference spectrum and the sample spectrum respectively (blocks 140a and 140b) to obtain a spectrum that can be thresholded.

[0041] Furthermore, the water vapor absorption peak frequency determination unit 250 determines two spectral intensity values ​​x of the reference spectrum. j and x k It has the function of calculating the difference value, performing thresholding using a threshold value t, and identifying the frequency at which the water vapor absorption peak is influencing the results.

[0042] Furthermore, the spectrum removal and interpolation unit 260 has the function of removing the spectrum of frequencies extracted by the water vapor absorption peak frequency determination unit 250a from the output of the noise processing unit 240 using the spectrum removal unit 261a and 261b, and then performing interpolation processing using the interpolation unit 265a and 265b.

[0043] The absorption spectrum calculation unit 270 has a function of calculating an absorption spectrum based on the reference spectrum and the sample spectrum that have been subjected to spectrum removal and interpolation processing by the spectrum removal and interpolation unit 260.

[0044] The dry air supply unit 227 is connected to the sealed chamber 229 and supplies dry air into the sealed chamber 229. The inflow of dry air is controlled by the dry air inflow control unit 228, and the inside of the sealed chamber can be kept in a wet to dry state.

[0045] Referring to FIG. 2A, the details of the configuration of the spectrum processing unit 230 will be described, and a procedure for outputting an absorption spectrum derived from a sample under wet conditions will be described.

[0046] The measurement frequency f is sequentially switched to the frequencies determined by the measurement start frequency f1 [THz], the measurement end frequency f n [THz], and the frequency interval Δ [THz] (f = (f1, f2,..., f n )), and the reference spectrum, the sample spectrum, and the noise spectrum are acquired. That is, the reference spectrum I0 = (x 1、 x 2、 …, x n ) obtained when the sample is not on the terahertz optical path is set as 110, and the noise spectrum N = (n1, n2,..., n n ) obtained by performing measurement with the terahertz optical path blocked is set as 120, and they are respectively input to the noise processing unit 240. Also, the sample is placed on the terahertz optical path, and the sample spectrum I1 = (y1, y2,..., y n ) obtained by transmitting or reflecting terahertz light through the sample is set as 130 and input to the noise processing unit 240.

[0047] As shown in [Equation 4] and [Equation 5], the noise processing unit 240 subtracts the noise spectrum N from each of the reference spectrum Io and the sample spectrum I1 in blocks 140a and 140b to obtain spectra R0 = (r1, r2,..., r n ), R1 = (r ’ 1, r ’ 2,..., r ’ n) obtain.

[0048] [Math 4] r k =x k -n k [Number 5] r' k =y k -n k

[0049] The spectrum R0 obtained by subtracting the noise spectrum N from the reference spectrum Io is input to the water vapor absorption peak frequency determination unit 250, where a determination process (selection process) for the frequency of the absorption peak due to water vapor is performed, and the determined frequency is stored.

[0050] The water vapor absorption peak full width at half maximum input section 160 is the full width at half maximum δ of the observed water vapor absorption peak. w This is the input section. The difference interval calculation unit 165 calculates the difference interval δ (frequency interval) when calculating the difference for the spectrum R0. The difference interval δ is, as an example, the full width at half maximum δ of the water vapor absorption peak. w It can be calculated based on the following. For example, the difference interval δ is δ × Δ = full width at half maximum δ w It can be calculated as an integer approximating twice the value. The threshold processing unit 180 determines the magnitude of the difference value and the threshold t, and stores a specific frequency in the frequency list storage unit 190 according to this result. Details of this process will be described later.

[0051] Next, the spectral removal and interpolation processing performed by the spectral removal and interpolation unit 260 will be explained. As described above, the spectral intensity values ​​acquired at the frequencies stored in the frequency list storage unit 190 are all discarded (removed) from spectra R0 and R1, leaving them in an empty state. Linear interpolation is applied to the spectral intensity values ​​of the frequencies remaining before and after the empty portion (data removal portion) to restore the discarded spectral intensity values. This makes it possible to restore spectra with reduced water vapor influence.

[0052] Refer to Figure 2B to explain a specific example of this spectral removal and interpolation process. In Figure 2B, the black circles represent the spectral intensity values ​​obtained at frequencies stored in the frequency list storage unit 190. Difference value d k As a result of determining that the value is greater than the threshold t, the portion is determined to be the spectral frequency of water vapor, and that frequency is stored in the frequency list storage unit 190. Therefore, the intensity value data of the black circle plot is discarded, and the removed data is linearly interpolated using the intensity values ​​of the remaining triangular plot to restore the white circle plot.

[0053] Next, the operation of the absorption spectrum calculation unit 270 will be explained. The absorption spectrum calculation unit 270 comprises an absorbance calculation unit 271 and a smoothing processing unit 275. The absorbance calculation unit 271 determines the absorbance A (absorption spectrum) of the sample to be measured according to the following [Equation 6].

[0054] [Number 6] A = -log 10 (R1 / R0)

[0055] The smoothing section 275 smooths the spectrum using, for example, a Savitzky-Golay filter or a simple moving average method. The spectrum converted to absorbance is referred to as the sample absorption spectrum. Through the above procedure, an absorption spectrum with reduced influence from water vapor absorption can be obtained from the spectrum output section 280 compared to the spectrum obtained under humid conditions.

[0056] Referring to Figure 2C, an example of the processing of the water vapor absorption peak frequency determination unit 250 will be explained in detail. In the difference calculation unit 170, the spectral intensity value r of the spectrum R0 acquired at the k-th frequency is calculated. (k) And the spectral intensity value of R0 obtained at the (k+δ)th frequency r (k+δ) The difference d k This is calculated as shown in [Equation 7].

[0057] [Number 7] d k =r(k+δ) -r (k)

[0058] Here, the difference interval δ is determined by the difference interval calculation unit 165 based on the full width at half maximum δw input to the water vapor absorption peak full width at half maximum input unit 160, as described above. The difference interval δ is δ × Δ = δ w It is best to use an approximate integer value that corresponds to about twice the frequency width. Specifically, the difference interval δ can be determined as shown in [Equation 8], which is expressed using the frequency interval Δ and the INT function (with decimal places truncated).

[0059] [Number 8] δ = INT(2δw / Δ + 0.5) (where δ >= 1)

[0060] A threshold value t (t>0) for determining the water vapor absorption peak frequency is input from the threshold input unit 175. The threshold processing unit 180 determines the relationship between this threshold value t and the difference dk, and determines (selects) the water vapor absorption peak frequency according to the result of this determination. Specifically, after the threshold value t is input, the threshold processing unit 180 performs the threshold determination shown in [Equation 9] and [Equation 10]. By determining the value of the threshold value t based on the minimum amount of attenuation that water vapor absorption has on the spectrum R0, the frequency at which water vapor absorption is observed can be extracted with high accuracy.

[0061] [Number 9] d k >t [Number 10] d k <-t

[0062] d k When [equation 9] is satisfied, the k-th frequency is determined to be the water vapor absorption peak frequency. On the other hand, d k When [equation 10] is satisfied, the (k+δ)th frequency is determined to be the water vapor absorption peak frequency. The frequency determined to be the water vapor absorption peak frequency is stored in the frequency list storage unit 190. This determination is repeated from k=1 to k=(n-δ).

[0063] The far-infrared spectrometer 100 determines the measurement frequency f and performs measurements while increasing the measurement frequency f from the measurement start frequency f1 to the measurement end frequency, thereby obtaining the reference spectrum I0, sample spectrum I1, and noise spectrum N. The reference spectrum I0, sample spectrum I1, and noise spectrum N can be continuously acquired while keeping them fixed at a certain measurement frequency f. Therefore, the water vapor absorption peak frequencies appearing in the reference spectrum I0 and sample spectrum I1 match with each measurement, and the same is true when measuring multiple samples. Thus, the processing of the water vapor absorption peak frequency determination unit 250a only needs to be performed once for each pair of measurements of the reference spectrum I0 and sample spectrum I1, and does not need to be performed individually.

[0064] An embodiment of the first embodiment is shown in Figures 3A and 3B. In Figures 3A and 3B, the measurement start frequency f1 is 0.9 THz, and the measurement end frequency f n The image shows the results obtained when the frequency was set to 2.5 THz and the frequency interval Δ was set to 10 GHz, and the reference spectrum, sample spectrum, and noise spectrum consisting of n=161 data points were acquired. In addition, the difference interval δ was set to 1 (equivalent to 10 GHz) and the threshold t was set to 50.

[0065] Figure 3A shows the difference D of the reference spectrum Io. k =r (k+δ) -r (k) This graph plots the data for each measurement frequency. In Figure 3, the threshold lines t=50 and -50 are shown as straight lines. Based on the threshold judgment, data acquired at 41 frequencies out of n=161 were subject to removal or interpolation.

[0066] As shown in Figure 3A, according to the identified water vapor absorption peak frequency, Figure 3B shows an example of data before and after the removal of the spectrum of the water vapor absorption peak (spectrum R0 in Figure 2A and spectrum R'0 after spectrum removal and interpolation). By removing the spectrum attenuated by the effect of water vapor absorption and linearly interpolating that portion, a reference spectrum R0' with reduced water vapor influence can be obtained.

[0067] Although not shown in the diagram, similarly for the sample spectrum R1, based on the data obtained as shown in Figure 3A, the spectrum of the water vapor absorption peak can be removed and linear interpolated to obtain a spectrum R1' with reduced water vapor absorption.

[0068] An example of the absorption spectrum obtained using spectra R0' and R1', which reduce the influence of the water vapor absorption peak, is shown in the left graph of Figure 4. The spectrum of the same sample obtained using spectra R0 and R1, which still show the influence of the water vapor absorption peak, is shown in the right graph of Figure 4. The procedure for outputting the spectrum in the right graph of Figure 4 is the same as shown in the comparative example in Figure 5. The sharp absorption caused by water vapor (indicated by inverted triangles) is reduced in the left graph of Figure 4, and the absorption originating from the sample (indicated by white inverted triangles) is clearly observed.

[0069] The operation of the control unit 226 will now be explained in more detail. The control unit 226 controls the wavelength of the tunable laser used in the seed light source unit 213 to generate terahertz waves of any desired frequency. To adjust the change in the optical axis of the seed light (horizontal direction of the paper) due to this wavelength change, the angle of the galvanometer mirror used in the optical element 214 (horizontal direction of the paper) is also controlled simultaneously. In addition, the direction of the optical axis of the terahertz light generated from the nonlinear optical crystal 221a changes slightly depending on its frequency (horizontal direction of the paper). The control unit 226 controls the sample stage RM according to the amount of change in the direction of the optical axis of the terahertz light (horizontal direction of the paper). This makes it possible to position the sample stage ST in an appropriate position for each measurement frequency. Furthermore, the control unit 226 can, if necessary, open and close the dry air inflow control unit 228 to adjust the humidity inside the chamber 229 to a level suitable for the sample.

[0070] This section describes the differences in properties between the absorption peaks of water vapor and those of the sample. The full width at half maximum (FMAX) of the absorption peak of a solid sample can be observed at a much wider frequency range compared to the absorption peak of water vapor (in the apparatus of this embodiment, the FMAX of the water vapor absorption peak is approximately 5 GHz, while the absorption of the solid sample is approximately 50 GHz). This broad absorption peak width is thought to originate from the collective vibrations of multiple molecules in the solid sample. Therefore, by removing the water vapor absorption peak, it is possible to interpolate the spectrum without significantly damaging the absorption peak originating from the sample.

[0071] Another specific application example of the first embodiment (Figure 2A) is shown below. Figure 6A shows an example of a reference spectrum consisting of n=401 detection signal values ​​obtained under measurement conditions of measurement start frequency f1=0.87THz, measurement end frequency fn=2.87THz, and Δ=5GHz in a humid environment. Using the method described above, the reference spectrum I0, sample spectrum I1, and noise spectrum N are obtained, and spectra R0 and R1 are obtained by subtracting the noise spectrum N from the reference spectrum I0 and sample spectrum I1. The observed full width at half maximum of the water vapor absorption peak is, here, δ w Since the frequency was 0.005 THz, δ was determined to be 2 from [Equation 8] above. The threshold t was set to 50.

[0072] Figure 6A shows an example of the reference spectrum R0 obtained under the above conditions. Figure 6B shows an example of the spectrum R0 after excluding data at frequencies stored in the frequency list storage unit 190 as a result of threshold determination. It can be seen that the frequencies affected by water vapor absorption can be accurately determined, and the influence of the water vapor absorption peak can be effectively removed.

[0073] Figure 6C shows a modified example of the first embodiment. The water vapor absorption peak frequency determination unit 250 in Figure 2A uses one difference d k The water vapor absorption peak frequency is detected by calculating the difference between the two values ​​and performing a threshold determination. In contrast, the modified method in Figure 6C calculates the difference between the two values ​​and detects the water vapor absorption peak frequency according to the result of the threshold determination. Specifically, the difference c shown in [Equation 11] below is used.k , difference d k Simultaneously, the data is acquired and the determination process is performed by the water vapor absorption peak frequency determination unit 250b shown in Figure 6C. k is forward difference (forward difference), c k This is a difference defined as a backward difference. In other words, in this modified example, the forward difference d is the difference between the signal intensity of the reference spectrum R0 at one frequency and the signal intensity of the reference spectrum R0 at a frequency with an interval Δ greater than that. k This is calculated. In addition, a backward difference c is calculated as the difference between the signal intensity of the reference spectrum R0 at one frequency and the signal intensity of the reference spectrum R0 at a frequency with an interval Δ smaller. k The difference between two values ​​c is calculated. k d k When either of the two conditions in [Equation 12] is met, the signal intensity value r k The frequency f obtained k The frequency list is stored in the frequency list storage unit 190.

[0074] [Number 11] c k =r (k) -r (k-δ) [Number 12] c k <- t or d k >t

[0075] c k and d (k-δ) Since the calculation formula is the same, when the same difference interval δ is used, the same frequency as in the example in Figure 2A is stored in the frequency storage unit 190. Thereafter, the removal and interpolation of data acquired at this frequency can be performed in the same manner as described above.

[0076] As described above, the far-infrared spectrometer of the first embodiment can effectively reduce absorption peaks originating from water vapor with a small amount of computation, even in any humid air atmosphere, and accurately measure the absorption spectrum originating from the sample.

[0077] (Second Embodiment) Next, a far-infrared spectrometer according to the second embodiment will be described with reference to Figure 7. The overall configuration of the apparatus is the same as that of the first embodiment, so redundant explanations will be omitted. The configuration of the water vapor absorption peak frequency determination unit 250c in this second embodiment of the far-infrared spectrometer differs from that of the first embodiment. According to this second embodiment, the influence of the water vapor absorption peak can be effectively reduced even in the frequency range where the detected light intensity of the reference spectrum is low. The intensity of light obtained by parametric generation shows a frequency dependence specific to the nonlinear optical crystal used. Therefore, it is conceivable that within a certain measurement frequency range, there may be a frequency band with low detected light intensity and a frequency band with high intensity.

[0078] Secondly, the water vapor absorption peak frequency determination unit 250c of the embodiment includes, in addition to the components of the water vapor absorption peak frequency determination unit 250 of the first embodiment, a difference calculation unit 171 and a threshold input unit 179. The difference calculation unit 171 uses the difference d used in the first embodiment. k And separately from the threshold t, as shown in [Equation 13], the relative difference d is obtained by relativeizing the difference itself with the reference spectral data. k The threshold input unit 179 receives a threshold t' which is different from the threshold t. The threshold processing unit 180 then calculates the difference d k In addition to detecting the water vapor absorption peak frequency by comparing it with a threshold t, the relative value difference d k The water vapor absorption peak frequency is detected by comparing the signal intensity with a threshold t. This allows for the detection of the water vapor absorption peak even when the signal intensity fluctuates within the measurement frequency range. For example, the threshold t' can be set to 0.25.

[0079] [Number 13] d k '=d k / r k

[0080] difference d k The water vapor absorption peak frequency determined solely from the threshold t matches the frequency of the data removed in Figure 6B. Figure 8 shows the relative difference d. kThe frequency data newly stored in the frequency list storage unit 190 by threshold determination performed separately using ' and threshold t' is shown. The frequency of 1.005 THz, where the intensity of the reference spectrum was low and the attenuation due to water vapor absorption was small, is d k This indicates that the determination was made using ' and t'. In the second embodiment, the difference d k and relative value difference d k An example of using ' in combination was shown, but relative value difference d k A configuration that uses only ' to detect the water vapor absorption peak frequency can also be adopted. Relative value difference d k Even using only ', it is possible to reduce the influence of water vapor absorption peaks in the low frequency range of the detected light intensity of the reference spectrum, and in this case, the effect can be obtained with a simpler process.

[0081] A modified version of the second embodiment is described below. In this modified version, instead of [Equation 11] described above, the ratio of spectral signal intensities at different sampling points (e.g., adjacent sampling points) (signal intensity ratio) is calculated as shown in [Equation 14], and this signal intensity ratio d k The water vapor absorption peak frequency is detected based on the following.

[0082] Specifically, in the difference calculation unit 171 in Figure 7, as shown in [Equation 14], the signal intensity ratio d of the reference spectrum is calculated. k The following is calculated. Furthermore, the threshold determination formulas are defined as shown in [Equation 15] and [Equation 16]. When [Equation 15] is satisfied, the (k+δ)th frequency is stored in the frequency list storage unit 190, and when [Equation 16] is satisfied, the kth frequency is stored. The subsequent procedure is the same as in the second embodiment. According to this modification, the influence of water vapor absorption peaks can be effectively reduced even in the frequency region where the detected light intensity of the reference spectrum is low. The relative value difference dk' and signal intensity ratio d shown in [Equation 17] k From the relationship, the signal intensity ratio d k A modified version that uses " for determining the water vapor absorption peak frequency is equivalent to using the relative value difference dk'.

[0083] [Number 14] dk ” = r k+δ / r k [Number 15] d k ” < 1 - t' [Number 16] d k ” > 1 + t' [Number 17] dk' = d k / r k = (r k+δ - r k ) / r k = r k+δ / r k - 1 = d k '' - 1

[0084] Note that, instead of the signal strength ratio d k ”, as shown in [Number 18] below, the difference d k of the natural logarithm of r lnk may also be used.

[0085] [Number 18] d lnk = ln(r k+δ ) - ln(r k ) = ln(r k+δ / r k )

[0086] [Number 18] takes the natural logarithm of the ratio of [Number 14]. Since the natural logarithm is a function whose slope can be approximated by 1 in the vicinity of 1, the threshold determination may be performed using [Number 15] and [Number 16]. That is, an effect almost equivalent to the case where threshold processing is performed on the relative value conversion difference can be obtained.

[0087] (The Third Embodiment) Next, the far-infrared spectroscopic apparatus according to the third embodiment will be described with reference to FIG. 9. Since the overall configuration of the apparatus is the same as that of the first embodiment, duplicate explanations will be omitted. The far-infrared spectroscopic apparatus of this third embodiment is different from that of the first embodiment in the configurations of the noise processing unit 240 and the spectrum data removal / interpolation unit 260b.

[0088] In the first embodiment, the water vapor absorption peak frequency determination unit 250 acquires the reference spectrum I0 and determines the water vapor absorption peak frequency each time a measurement is taken, but in the third embodiment, this procedure is omitted. In the spectral data removal and interpolation unit 260b, the data removal unit 261a and interpolation unit 265a, which target the reference spectral data R0, are omitted, and the data removal and interpolation operations are performed in the data removal unit 261b and interpolation unit 265b, targeting only the sample spectral data R1. In other words, in the third embodiment, spectral data due to the influence of water vapor is removed from the sample spectral data R1, which includes the water vapor absorption peak, and the interpolation operation is performed. The specific procedure will be described below with reference to Figure 9.

[0089] In this third embodiment, the reference spectrum Io and noise spectrum No. are acquired separately from the sample spectrum I1, which is the measurement result of the sample to be measured, under dry air conditions, and input to the noise processing unit 240. Then, when measuring the sample to be measured, the sample is placed on the sample stage ST under humid air, and the sample spectrum I1 and noise spectrum N1 are acquired in the same manner as in the previously described embodiment.

[0090] The reference spectrum I0 and noise spectrum N0 are preferably n-order row vector data of the same order, acquired at the same measurement start frequency, measurement end frequency, and frequency interval as the sample spectrum I1 and noise spectrum N1. However, it is also possible to use data that satisfies the following conditions.

[0091] The reference spectrum I0 and the noise spectrum N0 are measured at the measurement start frequency f1' and the measurement end frequency f n When acquired with a frequency interval Δ, the order n' of the vector data of the sample spectrum I0 and noise spectrum N0 is determined as shown in [Equation 19] below. The total measured frequencies of the reference spectrum I0 and noise spectrum N0 are given by the frequency sequence F'=(f1', f2'…, f n It is denoted as '). The order of the vector data of the sample spectrum I1 and the noise spectrum N1 is m, and the total measured frequencies are given as a frequency series F=(f1, f2, ..., fm In this case, the reference spectrum I0 and the noise spectrum N0 may be n'-order vector data obtained at a measurement frequency F' that satisfies [Equation 20].

[0092] [Number 19] n'=1+(f n '-f1') / Δ' [Number 20] n'≧m and F'∋F

[0093] The water vapor absorption peak frequency determination process is performed in the water vapor absorption peak frequency determination unit 250d on the spectrum R1 obtained by subtracting the noise spectrum N1 from the sample spectrum I1. The configuration of the water vapor absorption peak frequency determination unit 250d itself may be the same as in the embodiment described above.

[0094] In the third embodiment, it is not necessary to acquire the reference spectrum simultaneously with the sample, thus shortening the measurement time compared to the first and second embodiments. Furthermore, since the reference spectrum can be acquired under dry air conditions, the reference spectrum data can be used directly without going through the water vapor absorption peak frequency determination unit 250d.

[0095] (Fourth embodiment) Next, a far-infrared spectrometer according to the fourth embodiment will be described with reference to Figures 10 to 11. The overall configuration of the apparatus is the same as that of the first embodiment, so redundant explanations will be omitted. The configuration of the water vapor absorption peak frequency determination unit 250e and the spectral data removal / interpolation unit 260c in this fourth embodiment of the far-infrared spectrometer differs from that of the first embodiment.

[0096] In this fourth embodiment, first, the reference spectrum Io and the noise spectrum N oThe sample spectrum I1 and noise spectrum N1 are acquired and input to the noise processing unit 240. The noise processing unit 240 then acquires the reference spectrum R0 and sample spectrum R1, respectively, with the noise spectrum N0 and N1 removed. The absorbance calculation unit 271 calculates the absorption spectrum of the sample from [Equation 6]. The water vapor peak frequency determination unit 250e determines the water vapor absorption peak frequency from this absorption spectrum. Using the determined result, the spectrum data removal / interpolation unit 260C performs a reduction process for the water vapor absorption peak. Note that, similar to the third embodiment, the reference spectrum I o and noise spectrum N o The first spectrum (I1) may be obtained under dry air conditions, while the second spectrum (N1) may be obtained under humid air conditions.

[0097] The details of the determination operation in the water vapor absorption peak frequency determination unit 250e of the fourth embodiment will be explained with reference to Figure 11. This water vapor absorption peak frequency determination unit 250e is configured to determine the water vapor absorption peak frequency by obtaining the difference in intensity values ​​of the absorption spectrum of the sample, which is calculated based on the reference spectrum R0 and the sample spectrum R1, and performing a threshold determination similar to that of the above-described embodiment.

[0098] The difference interval δ is the total width at half maximum of the observed water vapor absorption peak δ. w Based on this, it can be determined as shown in [Equation 8], similar to the embodiment described above.

[0099] The intensity value of the absorption spectrum of the sample obtained at the kth frequency is a k When expressed as such, the difference calculation unit 171 performs the following, as shown in [Equation 21], a k+δ and a k The difference d k The threshold processing unit 181 calculates this difference d. k If [equation 9] is satisfied, then the frequency f k+δ If [number 10] is satisfied, then the frequency f k These are stored in the frequency list storage unit 190.

[0100] [Number 21] d k =a (k+δ) -a (k)

[0101] The intensity value ak of the input absorption spectrum has the sign inverted with that of the sample spectrum A1 (Figures 11, 141a). Accordingly, the combination of the frequency to be stored (added) to the frequency list storage unit 190 and the threshold determination formula is reversed compared to the threshold processing unit 180 in the first embodiment (d k >When t, f k+δ The frequency list is stored in the frequency list storage unit 190, d k (When t is true, fk is stored in the frequency list storage unit 190).

[0102] As shown in Figure 10, the absorbance values ​​obtained at frequencies stored in the frequency list storage unit 190 are removed from the absorption spectrum calculated by the absorbance calculation unit 271 by the data removal unit 261a. The removed absorbance values ​​are restored by the interpolation unit 265a by linearly interpolating the absorbance values ​​before and after. Furthermore, the spectrum is smoothed by the smoothing unit 275 as needed, and the absorption spectrum is output.

[0103] As described above, according to the apparatus of the fourth embodiment, an absorption spectrum A1' with reduced influence of water vapor can be obtained using only the absorption spectrum of the sample.

[0104] In the fourth embodiment, since it is not necessary to acquire the reference spectrum Ro simultaneously with the sample spectrum R1, the measurement time can be shortened compared to the first and second embodiments. Furthermore, since absorbance calculation data is used, it is possible to omit the input of the reference spectrum, sample spectrum, and noise spectrum, as well as the processing of the noise processing unit 240. In addition, since processing can be performed without using the reference spectrum, sample spectrum, or noise spectrum, it is possible to reduce the influence of the water vapor absorption peak even when this data is unavailable.

[0105] The present invention is not limited to the embodiments described above, and various modifications are included. For example, the embodiments described above are described in detail to make the present invention easier to understand, and are not necessarily limited to those having all the configurations described. Furthermore, it is possible to replace parts of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add configurations from other embodiments to the configuration of one embodiment. In addition, it is possible to add, delete, or replace parts of the configuration of each embodiment with other configurations. [Explanation of symbols]

[0106] 211...Near-infrared light source (pump light), 212...Polarizing beam splitter, 213...Near-infrared light source (seed light), 214, 215...Optical elements, 216...Mirror, 221a...Nonlinear optical crystal (terahertz light generation unit), 222a...Si prism (terahertz light generation unit), 221b...Nonlinear optical crystal (detection light generation unit), 222b...Si prism (detection light generation unit), 223...Light guide optical system, ST...Sample stage, RM...Moving stage, 225...Near-infrared photodetector, 226...Control unit, 227...Dry air supply unit, 228...Dry air inflow control unit, 229...Sealed chamber, 230...Spectrum processing unit, 240...Noise processing unit, 250...Water vapor absorption peak frequency determination unit, 260...Spectrum removal / interpolation unit, 270...Absorption spectrum calculation unit.

Claims

1. A holding mechanism configured to hold a sample in humid air, A detector that detects light obtained by irradiating the sample with far-infrared light, A signal processing unit that calculates the absorption spectrum of the sample from the signal from the detector, Equipped with, The signal processing unit, When the sample is not on the measurement optical path, the wavelength of the far-infrared light is changed and the far-infrared light is irradiated into the measurement optical path, and the first spectrum detected by the detector is obtained, and when the sample is on the measurement optical path, the wavelength of the far-infrared light is changed and the far-infrared light is irradiated into the measurement optical path, and the second spectrum detected by the detector is obtained. At frequency intervals determined according to the width of the absorption peak due to water vapor, the intensity ratio for each frequency interval is calculated for the first spectrum, the second spectrum, or the absorption spectrum obtained based on the first and second spectra, and the frequency of the absorption peak due to water vapor is selected according to a comparison of the intensity ratio with a threshold. Based on the selected frequencies, data removal is performed. By performing interpolation based on the remaining data before and after the portion where the data was removed, the data in the portion where the data was removed is restored. The signal processing unit performs the data removal as follows: Perform the operation on the first spectrum, or, instead of the data removal, acquire the first spectrum under dry conditions and perform the operation on the second spectrum, or Perform the following on the signal of the absorption spectrum obtained based on the first spectrum and the second spectrum: A far-infrared spectrometer characterized by the following features.

2. The signal processing unit acquires the noise spectrum obtained by performing a measurement with the measurement optical path shielded from light, The far-infrared spectrometer according to claim 1, comprising subtracting a noise spectrum from the first spectrum and the second spectrum, and performing data removal in the first spectrum or the second spectrum after the subtraction.

3. The signal processing unit sets the measurement optical path to a humidified condition and acquires the first spectrum and the second spectrum. The far-infrared spectrometer according to claim 1, configured to select the frequency of the absorption peak due to water vapor based on the intensity ratio of the first spectrum.

4. The far-infrared spectrometer according to claim 1, wherein the signal processing unit is configured to set the measurement optical path to dry conditions to acquire the first spectrum and to select the frequency of the absorption peak due to water vapor based on the intensity ratio of the second spectrum.

5. The far-infrared spectrometer according to claim 1, wherein the signal processing unit is configured to select the frequency of the absorption peak due to water vapor based on the intensity ratio of the absorption spectrum obtained based on the first spectrum and the second spectrum.

6. The far-infrared spectrometer according to claim 1, further comprising a dry air supply unit for supplying dry air to the inside of a sample chamber containing the sample, and a dry air inflow control unit for controlling the inflow of the dry air.

7. A step of obtaining a first spectrum detected by the detector when the wavelength of the far-infrared light is changed and the far-infrared light is irradiated into the measurement optical path while the sample is not in the measurement optical path, and a second spectrum detected by the detector when the wavelength of the far-infrared light is changed and the far-infrared light is irradiated into the measurement optical path while the sample is in the measurement optical path, A step of calculating the intensity ratio for each frequency interval for the first spectrum, the second spectrum, or the absorption spectrum obtained based on the first and second spectra, at frequency intervals determined according to the width of the absorption peak due to water vapor, and selecting the frequency of the absorption peak due to water vapor according to a comparison of the intensity ratio with a threshold, A step of performing data removal based on the selected frequencies, A step of restoring the data of the portion from which the data was removed by interpolating based on the remaining data before and after the portion from which the data was removed, Includes, The process of performing the aforementioned data removal is: Perform the operation on the first spectrum, or, instead of the data removal, acquire the first spectrum under dry conditions and perform the operation on the second spectrum, or Perform the following on the signal of the absorption spectrum obtained based on the first spectrum and the second spectrum: Far infrared spectroscopy method.

8. The far-infrared spectroscopy method according to claim 7, comprising: obtaining a noise spectrum obtained by performing a measurement with the measurement optical path shielded from light; subtracting the noise spectrum from the first spectrum and the second spectrum; and performing data removal in the first spectrum or the second spectrum after the subtraction.

9. A holding mechanism configured to hold a sample in humid air, A detector that detects light obtained by irradiating the sample with far-infrared light, The system comprises a signal processing unit that calculates the absorption spectrum of the sample from the signal from the detector, The signal processing unit, When the sample is not on the measurement optical path, the wavelength of the far-infrared light is changed and the far-infrared light is irradiated into the measurement optical path, and the first spectrum detected by the detector is obtained, and when the sample is on the measurement optical path, the wavelength of the far-infrared light is changed and the far-infrared light is irradiated into the measurement optical path, and the second spectrum detected by the detector is obtained. At frequency intervals determined according to the width of the absorption peak due to water vapor, the logarithm of the spectral intensity value is taken for either the first spectrum, the second spectrum, or the absorption spectrum obtained based on the first and second spectra, the difference in the logarithms of the spectral intensity values ​​for each frequency interval is calculated, and the frequency of the absorption peak due to water vapor is selected according to a comparison of the difference with a threshold. Based on the selected frequencies, data removal is performed. By performing interpolation based on the remaining data before and after the portion where the data was removed, the data in the portion where the data was removed is restored. The signal processing unit performs the data removal as follows: Perform the operation on the first spectrum, or, instead of the data removal, acquire the first spectrum under dry conditions and perform the operation on the second spectrum, or Perform the following on the signal of the absorption spectrum obtained based on the first spectrum and the second spectrum: A far-infrared spectrometer characterized by the following features.

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