JP2019168701A - Far-infrared spectroscopy device - Google Patents

Abstract

SOLUTION: There is provided a far-infrared spectroscopy device comprising: a variable wavelength far-infrared light source which generates first far-infrared light; an illumination optical system which irradiates a sample with the first far-infrared light; a nonlinear optical crystal for detection which uses pump light to convert second far-infrared light from the sample into near-infrared light; and a far-infrared light imaging optical system which forms an image of the sample on the nonlinear optical crystal for detection, in which the position irradiated with the first far-infrared light on the sample does not depend upon a wavelength of the first far-infrared light.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a far-infrared spectrometer.

  The far-infrared electromagnetic wave having a wavelength of about 25 μm to about 4 mm is also called a terahertz wave, and has both radio wave transparency and light straightness. The far-infrared electromagnetic wave is expected to be effective for identification of substances because the absorption spectrum in this area has peaks unique to many substances. Conventionally, however, there is no small and easy-to-use light source that emits light in this region, and it is difficult to handle a detector that is cooled with liquid helium or the like. Therefore, conventionally, electromagnetic waves in the far infrared region have been used only for limited research purposes.

  In the 1990s, light sources and detectors using femtosecond lasers that were small and did not require cooling were put into practical use. At present, general-purpose spectroscopic measuring devices based on time domain spectroscopy are also commercially available, and research and development for various fields such as security, biosensing, medical / pharmaceuticals, industry, agriculture, etc. are being promoted. In such industrial application, it is required to perform quantitative analysis of components.

JP 2003-302666 A

  In industrial applications, quantitative analysis of ingredients is a key. Although there are attempts at quantitative analysis using time-domain spectroscopy, it is difficult to measure 1 to 3 THz, which is effective for detecting hydrogen bonds or molecular networks, and measurement through shielding materials such as paper and packaging materials. There are problems such as being difficult to measure and difficult to measure highly scattered powder. On the other hand, a method using a frequency-variable coherent light source easily obtains a high output in the region of 1 to 3 THz, and is effective for analysis through a shield and analysis of powder.

  However, in the above method, since the direction in which the far-infrared light is emitted changes when the frequency is changed, there is a problem that the irradiation position on the sample is changed and the accuracy of quantitative analysis is lowered.

  Therefore, the present invention provides a far-infrared spectroscopic device capable of reducing a shift in the irradiation position of far-infrared light due to a frequency change.

  For example, in order to solve the above problems, the configuration described in the claims is adopted. The present application includes a plurality of means for solving the above-described problems. For example, a variable wavelength far-infrared light source that generates first far-infrared light, and a sample of the first far-infrared light. An illumination optical system for irradiating the light; a detection nonlinear optical crystal that converts second far-infrared light from the sample into near-infrared light using pump light; and an image of the sample for the detection nonlinear optical A far-infrared light imaging optical system that forms an image on a crystal, and the irradiation position of the first far-infrared light on the sample does not depend on the wavelength of the first far-infrared light. An infrared spectrometer is provided.

  ADVANTAGE OF THE INVENTION According to this invention, the shift | offset | difference of the irradiation position of the far infrared light by a frequency change can be reduced. Further features related to the present invention will become apparent from the description of the present specification and the accompanying drawings. Further, problems, configurations and effects other than those described above will be clarified by the description of the following examples.

It is the figure which looked at the structural example of the spectrometer which used the light of the far infrared region in 1st Example of this invention from the side. It is the figure which looked at the structural example of the spectrometer which used the light of the far infrared region in 1st Example of this invention from the upper surface. It is a top view which shows the irradiation area | region of the sample in 1st Example of this invention. It is a figure which shows the mode of propagation of the far-infrared light in 1st Example of this invention. It is a figure explaining the adjustment method of the incident angle at the time of making the far-infrared light which permeate | transmitted the sample in 1st Example of this invention enter into the nonlinear optical crystal for a detection. It is a figure explaining the adjustment method of the incident angle at the time of making the far-infrared light which permeate | transmitted the sample in 1st Example of this invention enter into the nonlinear optical crystal for a detection. It is a figure which shows the mode of generation | occurrence | production of the far-infrared light in 1st Example of this invention. It is a figure which shows each wave number vector of far-infrared light, pump light, and seed light. It is a figure which shows the structural example of the incident angle adjustment optical system in 1st Example of this invention. It is a figure which shows the structural example of the spectrometer which used the light of the far infrared region in 2nd Example of this invention. It is a figure explaining the irradiation position of the far-infrared light in the sample surface in 2nd Example of this invention. It is a figure explaining the irradiation position of the far-infrared light in the sample surface in 2nd Example of this invention, and its correction method. It is a figure explaining the irradiation position of the far-infrared light in the sample surface in 2nd Example of this invention, and its correction method. It is a figure which shows the structural example of the light source for pump lights of the wavelength variable far-infrared light source in 2nd Example of this invention. It is the figure which looked at the structural example of the spectrometer which used the light of the far infrared region in 3rd Example of this invention from the side. It is the figure which looked at the structural example of the spectrometer which used the light of the far infrared region in 3rd Example of this invention from the upper surface. It is a top view which shows the irradiation area | region of the sample in 3rd Example of this invention. It is the figure which looked at the structural example of the spectrometer which used the light of the far infrared region in 4th Example of this invention from the side. It is the figure which looked at the structural example of the spectrometer which used the light of the far infrared region in 4th Example of this invention from the upper surface. It is a top view which shows the irradiation area | region of the sample in 4th Example of this invention. It is the figure which looked at the structural example of the spectrometer which used the light of the far infrared region in 5th Example of this invention from the side. It is an enlarged view of the surface of the cylindrical lens of the illumination optical system in 5th Example of this invention. It is an enlarged view of the surface of the cylindrical lens of the illumination optical system in 5th Example of this invention. It is a figure explaining the groove structure of the surface of the cylindrical lens of the illumination optical system in 5th Example of this invention. It is a figure explaining the groove structure of the surface of the cylindrical lens of the illumination optical system in 5th Example of this invention. It is a figure explaining the groove | channel structure of the surface of the lens used for the optical system which adjusts the optical path length in a 5th Example of this invention, and a far-infrared light imaging optical system. It is a figure which shows the structural example of the conventional far-infrared spectroscopy apparatus using the light of a far-infrared area | region.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. The accompanying drawings show specific embodiments in accordance with the principle of the present invention, but these are for the understanding of the present invention, and are never used to interpret the present invention in a limited manner. is not.

  The following example is a far-infrared spectrometer for analyzing a sample using light in the far-infrared region in an inspection process such as analysis of the content of chemical substances in a sample or inspection of foreign components or foreign substances. About. Here, the light in the far-infrared region is, for example, light having a wavelength of about 25 μm to about 4 mm. There are numerical ranges of various wavelengths as the definition of “far infrared region”, but the light in the far infrared region described below is interpreted in the widest range of ranges defined in all fields. It should be. The term “terahertz wave” is assumed to be included in the far-infrared region described above.

[First embodiment]
1A and 1B show an example of the overall configuration of a far-infrared spectrometer according to the first embodiment. As an example, the far-infrared spectrometer is an apparatus that measures an absorption spectrum of the sample 200 using light transmitted through the sample 200.

  The far-infrared spectrometer includes a variable wavelength far-infrared light source 100 that generates far-infrared light, an illumination optical system 150 that irradiates the sample 200 with far-infrared light, a sample stage 202 on which the sample 200 is mounted, a sample Far-infrared light imaging optical system 170 for imaging far-infrared light from 200 on nonlinear optical crystal 132 for detection, and far-infrared light from sample 200 is converted to near-infrared light using pump light A non-linear optical crystal for detection 132, a photodetector (sensor) 290, a signal processing unit 400, and a control unit 500 are provided.

  The variable wavelength far infrared light source 100 includes a light source 110 for pump light 115, a variable wavelength light source 120 for seed light 125, an incident angle adjusting optical system 121, a nonlinear optical crystal (a nonlinear optical crystal for generating far infrared light). ) 130. The wavelength variable far-infrared light source 100 has a configuration in which two laser beams having different wavelengths (pump light 115 and seed light 125) are put into a nonlinear optical crystal 130 to generate far-infrared light by differential frequency generation or parametric generation. Use.

  For example, MgO: LiNbO 3 may be used as the nonlinear optical crystal 130, and a short pulse Q-switched YAG laser may be used as the light source 110 of the pump light 115. In this configuration, the light from the wavelength tunable light source 120 is used as the seed light 125, enters the nonlinear optical crystal 130 at a slight angle with respect to the pump light 115 through the incident angle adjusting optical system 121 and the mirror 126. Thereby, far-infrared light can be obtained by parametric generation.

  A Si prism 140 may be attached to the nonlinear optical crystal 130. According to this configuration, the generated far infrared light can be efficiently extracted. If the wavelength of the seed light 125 is changed between about 1066 nm to 1076 nm, and the incident angle to the nonlinear optical crystal 130 is adjusted by the incident angle adjusting optical system 121, the frequency of the generated far infrared light is 0.5 THz to It can be changed in the range of about 3THz.

  The far-infrared light thus obtained is irradiated onto the irradiation region 205 on the sample 200 using the illumination optical system 150. FIG. 1C is a plan view showing an irradiation area of the sample 200.

  The illumination optical system 150 is an anamorphic imaging optical system including at least three cylindrical lenses 152, 154, and 156. Here, “anamorphic” means that optical properties are different from each other in two orthogonal planes including the optical axis. Specifically, in the “imaging optical system”, anamorphic means that magnifications are different from each other in two orthogonal planes including the optical axis.

  The illumination optical system 150 will be described more specifically. In the plane of FIG. 1B, the cylindrical lenses 152 and 154 have optical power. The far-infrared light emission region is disposed on the front focal plane of the cylindrical lens 152. The light emitting region here is a linear light emitting region along the pump light 115 (for example, a linear region passing through B0 and A0 in FIG. 2). Further, the cylindrical lens 154 is disposed so that the rear focal plane of the cylindrical lens 152 and the front focal plane of the cylindrical lens 154 coincide. Further, the sample 200 is disposed on the rear focal plane of the cylindrical lens 154.

  An aperture stop may be installed on the rear focal plane of the cylindrical lens 152 (that is, the front focal plane of the cylindrical lens 154). According to this configuration, the optical system is strictly a bilateral telecentric optical system in the plane of FIG. 1B, but an aperture stop is not essential here. By using an aperture stop, the use range of far-infrared light can be limited.

  The illumination optical system 150 is an afocal optical system, and may function as a substantially bilateral telecentric optical system from the spread characteristics of the far-infrared beam emitted from the nonlinear optical crystal 130.

  In addition, the cylindrical lens 156 has optical power in the plane of FIG. 1A. The cylindrical lens 156 is disposed so that the far-infrared light emission region is imaged on the sample 200.

  The illumination optical system 150 is an imaging optical system that collimates the far-infrared light within the first section including the optical axis of the far-infrared light from the wavelength-variable far-infrared light source 100 and collects it again on the surface of the sample 200. And a condensing optical system that condenses the far-infrared light from the wavelength-variable far-infrared light source 100 on the surface of the sample 200 in a second cross section orthogonal to the first cross section. Specifically, the far-infrared light emitted from the variable wavelength far-infrared light source 100 is a linear light source (FIG. 2) along the beam of the pump light 115. The far-infrared light emitted from the wavelength-variable far-infrared light source 100 spreads in the plane of FIG. 1A, but becomes a parallel light beam in the plane of FIG. 1B. In the plane of FIG. 1A, the far-infrared light is converged on the irradiation region on the sample 200 by the cylindrical lens 156. On the other hand, in the plane of FIG. 1B, far-infrared light is once condensed by the cylindrical lens 152, converted into a parallel beam again by the cylindrical lens 154, and irradiated onto the sample 200. As described above, the illumination optical system 150 can image the linear light-emitting region formed by the variable wavelength far-infrared light source 100 on the sample 200 by reducing the longitudinal direction of the linear region.

  The illumination optical system 150 is afocal in the first cross section including the optical axis of the far infrared light from the wavelength variable far infrared light source 100. The illumination optical system 150 is an afocal system (that is, the cylindrical lens 154 is arranged so that the rear focal plane of the cylindrical lens 152 and the front focal plane of the cylindrical lens 154 coincide with each other). It is possible to irradiate the sample 200 with the far-infrared light substantially parallel in the plane of FIG. Further, the far infrared light transmitted through the sample 200 can be efficiently taken into the far infrared light imaging optical system 170.

  In this way, by using the illumination optical system 150 as an imaging optical system, it is possible to ensure the stability of illumination when the wavelength of the far infrared light emitted from the variable wavelength far infrared light source 100 is changed. It becomes. That is, by making the illumination optical system 150 an imaging optical system, the irradiation position of the far infrared light on the sample 200 does not depend on the wavelength of the far infrared light from the wavelength variable far infrared light source 100.

  In order to change the wavelength of the far-infrared light, the wavelength of the seed light 125 is changed and the incident angle to the nonlinear optical crystal 130 is adjusted. At that time, the emission direction of the generated far infrared light changes in the in-plane direction of FIG. 1B (for example, θ1 to θ2 in FIG. 1B). Even in this case, the illumination optical system 150 is an imaging optical system, and a far-infrared light spot is formed on the surface of the sample 200 by making a conjugate relationship (imaging relationship) between the far-infrared light emission region and the surface of the sample 200. Can be prevented from moving. Even if the wavelength of the far-infrared light is changed, the irradiation position of the sample 200 does not shift, so that the amount of illumination light does not change, and stable illumination can be ensured. On the other hand, when the illumination optical system 150 is not an imaging optical system, the illumination position of the illumination light may be completely different, and stable imaging becomes difficult. However, this is not the case when imaging is performed with a fixed wavelength, or when the change range of the wavelength is small and the change in the direction of far-infrared light is sufficiently small.

  In addition, by making the illumination optical system 150 an anamorphic optical system, the linear emission region of the wavelength variable far-infrared light source 100 is reduced in the longitudinal direction of the linear region, and the sample 200 is illuminated as a spot. can do. According to this configuration, two-dimensional data can be obtained by scanning the spot on the sample 200 as described below.

  The sample 200 to be imaged is mounted on the stage 202. The stage 202 includes a mechanism that can move in at least one direction. For example, the stage 202 is movable in the x direction in FIG. According to this configuration, by moving the sample 200 in the x direction, the irradiation region 205 can be scanned on the surface of the sample 200, and data of a line-shaped region of the sample 200 can be acquired. Further, the stage 202 may be an xy stage movable in the x direction and the y direction. By combining the scanning in the x direction and the scanning in the y direction, two-dimensional data (image) of a wider area of the sample 200 can be obtained.

  The far-infrared light transmitted through the sample 200 is converted into near-infrared light having a wavelength of about 1066 nm to 1076 nm by the detection nonlinear optical crystal 132. The converted near-infrared light is photoelectrically converted by a photodetector 290 having sensitivity to the near-infrared light, and detected as a detection signal. The near-infrared light detector (sensor) 290 may be a light receiving element (1D array detector) in which a plurality of light receiving elements are arranged in a one-dimensional array, or a plurality of light receiving elements are arranged in a two-dimensional array. A light receiving element (2D array detector) may be used. Near-infrared 1D array detectors and 2D array detectors are relatively easily available, have a fast response speed, and can be used at room temperature. These detectors are therefore suitable for industrial applications.

  As in the above example, when the far-infrared light is wavelength-converted to near-infrared light using the detection nonlinear optical crystal 132, a part of the pump light 115 is branched, and after adjustment, the detection nonlinear optical crystal 132 To enter. For example, the pump light 115 is branched into transmitted light and reflected light by a polarization beam splitter (hereinafter referred to as PBS) 127. The transmitted light that has passed through the PBS 127 is incident on the nonlinear optical crystal 130. The reflected light (wavelength conversion pump light) 235 reflected by the PBS 127 enters the detection nonlinear optical crystal 132.

  The pump light 235 for wavelength conversion is incident on the nonlinear optical crystal for detection 132 at the same timing as the timing at which the far-infrared light pulse transmitted through the sample 200 is incident. Therefore, although not shown on the optical path of the wavelength conversion pump light 235, a delay optical system (for example, an optical path length correction stage) for adjusting the timing of the optical pulse, and the polarization direction A half-wave plate or the like for adjusting the wavelength is provided as necessary. According to this configuration, a beam with a clean profile can be used for wavelength conversion when detecting far-infrared light. Thereby, the efficiency of wavelength conversion can be increased and the detection sensitivity can be increased.

  The far-infrared light transmitted through the sample 200 is guided to the detection nonlinear optical crystal 132 using the far-infrared light imaging optical system 170. For example, the far-infrared light imaging optical system 170 is an afocal imaging optical system including at least two lenses 177 and 179. The lens 177 is disposed on the stage 178a. The lens 179 is disposed on the stage 178b. The stages 178a and 178b are movable in at least one direction (here, the y direction). A mechanism for moving at least the lens on the sample 200 side in at least one direction among the two lenses 177 and 179 may be provided. An image of the surface of the sample 200 is formed in the detection nonlinear optical crystal 132 through the Si prism 142. As the detection nonlinear optical crystal 132, LiNbO3 or MgO: LiNbO3 may be used. Each light passing through the detection nonlinear optical crystal 132 and the nonlinear optical crystal 130 is received and processed by the termination processing unit 240 (see FIG. 1B).

  If there is no margin in the output of the light source 110 for the pump light 115, the pump light passing through the nonlinear optical crystal 130 may be guided to the detection nonlinear optical crystal 132 and reused (see, for example, FIG. 6). . Since the quality of the beam of the pump light used for wavelength conversion is lowered, the detection efficiency is lowered, but the pump light can be used efficiently for generating far infrared light and converting the wavelength to the near infrared.

  The signal processing unit 400 takes in the signal photoelectrically converted by the photodetector 290. The signal processing unit 400 generates a signal proportional to the light transmitted through the sample 200 and a distribution image thereof based on the position information of the stage 202 at the time of signal capture. The signal processing unit 400 calculates an absorption spectrum by comparing the acquired image data with spectral image data (reference data) when there is no sample accumulated in the storage area of the signal processing unit 400, and absorbs the absorption spectrum. It is also possible to obtain a two-dimensional spectrum distribution (absorption spectrum image).

  The control unit 500 controls the entire apparatus. For example, the control unit 500 controls the variable wavelength far infrared light source 100, the stages 202, 178a, and 178b, and the signal processing unit 400. The control unit 500 functions as a user interface. For example, the control unit 500 may include a display unit that displays signals and data (spectral information) acquired by the signal processing unit 400. When acquiring the data of the sample 200 with the wavelength fixed, the control unit 500 controls the wavelength variable far-infrared light source 100 to generate the specified far-infrared light, and moves the stage 202 and the photodetector. Controls the synchronization of data acquisition at 290. In addition, when acquiring the data of the sample 200 by changing the wavelength, the control unit 500 sets the wavelength of the variable wavelength far-infrared light source 100, and moves the stage 202 and acquires data with the photodetector 290. Control the synchronization.

  In the present embodiment, an example in which a short pulse Q-switched YAG laser is used as the pump light source 110 of the variable wavelength far-infrared light source 100 has been described, but the present invention is not limited to this. A mode-locked laser may be used as the light source 110 for pump light because the line width of the basic spectrum is narrow. Since the mode-locked laser repeats quickly, it is possible to measure at a higher speed.

  Here, how the far infrared light is generated will be described with reference to FIG. 4A. FIG. 4A shows an example in which far-infrared light is generated by parametric generation using MgO: LiNbO 3 as the nonlinear optical crystal 130.

Pump light 115 is incident on the nonlinear optical crystal 130, and seed light 125 is incident on the nonlinear optical crystal 130 at an angle θ. If the wavelength θ of the seed light and the angle θ with respect to the pump light are set so as to satisfy the following conditions, the far infrared light 145 can be generated with high efficiency. The frequency (ω _T ) of the far-infrared light 145 generated in the nonlinear optical crystal 130 is obtained from the following formula using the respective frequencies (ω _p and ω _s ) of the pump light 115 and the seed light 125 from the energy conservation law. Is required (where ω is an angular frequency).

On the other hand, the generation efficiency of the far-infrared light 145 increases when the momentum conservation law holds. That is, high efficiency is obtained when the following relational expression and the condition of FIG. 4B (phase matching condition) are satisfied between the emission direction of the far-infrared light 145 and the directions of the pump light 115 and the seed light 125. Here, it is assumed that the wave number vectors of the far-infrared light 145, the pump light 115, and the seed light 125 are k _T , k _p , and k _s .

  Therefore, by setting the wavelength and the incident direction (θ) of the seed light 125 so as to satisfy these conditions, it becomes possible to generate far-infrared light (terahertz light) with high efficiency.

  In this embodiment, the incident angle adjusting optical system 121 adjusts the incident angle of the seed light 125 to the nonlinear optical crystal 130. FIG. 5 shows a configuration example of the incident angle adjusting optical system 121.

  The incident angle adjusting optical system 121 includes a lens 122, an optical deflector 123, and an imaging optical element 124. Light from the wavelength tunable light source 120 is guided by a fiber 128. The light from the fiber 128 forms a beam waist near the front focal plane of the imaging optical element 124 through the lens 122 and the optical deflector 123. According to this configuration, the beam that has passed through the imaging optical element 124 becomes a beam having a long Rayleigh length (that is, close to a collimated state) and enters the nonlinear optical crystal 130.

  On the other hand, the imaging optical element 124 is configured to image the surface of the optical deflector 123 on the incident surface of the nonlinear optical crystal 130. Thus, it is possible to realize a condition that when the beam is shaken by the optical deflector 123, the beam position does not change on the incident surface of the nonlinear optical crystal 130, and only the incident angle changes.

  As the optical deflector 123, a reflective deflector such as a galvanometer mirror or a mirror using MEMS technology may be used, or a transmissive optical deflector may be used. In other words, any optical deflector 123 may be used as long as the angle can be controlled.

  In this example, a concave mirror is used as the imaging optical element 124. However, a lens may be used as the imaging optical element 124 as long as the incident surfaces of the optical deflector 123 and the nonlinear optical crystal 130 can be in an imaging relationship. If a reflective optical deflector such as a galvanometer mirror is used as the optical deflector 123 and a concave mirror is used as the imaging optical element 124, the optical path can be folded and folded, so the incident angle adjusting optical system 121 is compact. Can be formed.

  When the incident angle adjusting optical system 121 is mounted linearly, a transmissive optical deflector may be used as the optical deflector 123, and a lens may be used as the imaging optical element 124. Furthermore, depending on mounting restrictions, one of the optical deflector 123 and the imaging optical element 124 may be configured by a reflective optical element, and the other may be configured by a transmissive optical element.

  In this embodiment, as the imaging optical element 124, a single imaging optical element such as a single lens or a single concave mirror can be used. Therefore, the optical system can be formed compactly.

  Further, according to the incident angle adjusting optical system 121 of this example, when the wavelength of the wavelength tunable light source 120 is changed, the incident angle θ of the seed light 125 to the nonlinear optical crystal 130 is controlled and set by the optical deflector 123. This makes it possible to set the incident angle θ to the nonlinear optical crystal 130 with high accuracy. Therefore, it is possible to improve the stability of the far-infrared light output when the wavelength of the seed light 125 is changed and the stability of the absorption spectrum measurement. As a result, highly accurate quantitative measurement is possible.

  Next, how the far-infrared light propagates will be described with reference to FIG. The far-infrared light emitted from the wavelength-variable far-infrared light source 100 is a linear light source along the beam of the pump light 115. Since this linear light source is inclined with respect to the optical axis (z-axis in FIG. 2), care must be taken when guiding the beam to the nonlinear optical crystal for detection 132.

  In FIG. 2, far-infrared light generated in the nonlinear optical crystal 130 passes through the illumination optical system 150, the sample 200, and the far-infrared light imaging optical system 170 until it enters the detection nonlinear optical crystal 132. The light path is shown.

Here, two points (A ₀ and B ₀ ) on the linear light source of far-infrared light along the beam of pump light 115 will be described as an example. The A ₀ point and the B ₀ point are imaged in the vicinity of the sample 200 by the illumination optical system 150, but since the B ₀ point is farther from the illumination optical system 150 than the A ₀ point, the B point that is the image is It is formed in the vicinity of the illumination optical system 150 near the sample 200. These points A and B are guided to the detection nonlinear optical crystal 132 by the far-infrared light imaging optical system 170 again. Here, since point B is farther from the far-infrared light imaging optical system 170 than point A, the image of point B ′ is far-infrared light coupled from point A ′ in the nonlinear optical crystal 132 for detection. It is formed at a position close to the image optical system 170. Therefore, the nonlinear optical crystal 130 and the detection nonlinear optical crystal 132 are arranged so that the incident direction of the pump light 115 at the nonlinear optical crystal 130 and the incident direction of the pump light 235 at the detection nonlinear optical crystal 132 are substantially parallel. Is done. Since the incident direction of the pump light 115 at the nonlinear optical crystal 130 and the incident direction of the pump light 235 at the detection nonlinear optical crystal 132 are substantially parallel, both the points A ′ and B ′ are pump light for wavelength conversion. It becomes possible to superimpose on the 235 beam, and far infrared light can be efficiently converted into near infrared light.

  Next, a method for adjusting the incident angle when the far-infrared light transmitted through the sample 200 is incident on the detection nonlinear optical crystal 132 will be described with reference to FIG.

  In order to convert the far-infrared light that has passed through the sample 200 into the near-infrared light at the detection nonlinear optical crystal 132 with high efficiency, the far-infrared light for detection of the far-infrared light that has passed through the sample 200 is detected. It is necessary to optimize the incident angle. In this embodiment, the incident angle is adjusted by adjusting the positions of the lenses 177 and 179 constituting the far-infrared light imaging optical system 170.

  3A and 3B show the far-infrared light imaging optical system 170. FIG. Points A and B are two points on the sample 200, and these points A and B are imaged at points A ′ and B ′ in the nonlinear optical crystal for detection 132 by the far-infrared light imaging optical system 170. ing.

  3A shows a reference state, and FIG. 3B shows a state in which the incident angle of the far-infrared light transmitted through the sample 200 to the nonlinear optical crystal for detection 132 is changed with respect to the state of FIG. 3A. First, the stage 178a is driven to move the lens 177 by the distance S in the -y direction. If the lens 179 is not moved in this state, the image points A ′ and B ′ of the points A and B move in the −y direction within the detection nonlinear optical crystal 132. Therefore, in order to correct this deviation, the lens 179 is moved in the + y direction by the distance S ′ by driving the stage 178b. By performing such control, the incident angles θ of the far-infrared light incident on the points A ′ and B ′ can be changed (adjusted) while the positions of the points A ′ and B ′ are the same as those in FIG. 3A. It becomes possible. The distances S and S ′ are not necessarily the same value, and may not be the same value due to the focal length.

  The effects of this embodiment will be described below. Conventionally, changing the frequency of the light source for generating far-infrared light changes the direction in which the far-infrared light is emitted, so the irradiation position on the sample changes, and quantitative analysis of component distribution unevenness and changes in signal detection efficiency There has been a problem of lowering the accuracy. On the other hand, according to the present embodiment, illumination light from a light source is irradiated onto an analysis target (sample 200) by an anamorphic imaging optical system, and transmitted light (or reflected light) from the sample 200 is used. Is converted using a detection nonlinear optical crystal 132 and detected using a photodetector 290. According to this structure, the shift | offset | difference of the irradiation position of the far infrared light by a frequency change can be reduced. Therefore, component distribution unevenness and change in signal detection efficiency do not occur, and the accuracy of quantitative analysis can be improved.

  Further, when the optical system is complicated in order to reduce the deviation of the irradiation position of the sample, there is a problem that the attenuation by the optical system of far-infrared light is significant, and measurement through a shield becomes difficult. On the other hand, in this embodiment, the configuration for reducing the deviation of the irradiation position of the sample is a simple configuration, the attenuation of far-infrared light is small, and analysis through a shield is possible. According to the present embodiment, it is possible to realize quantitative analysis of various forms of samples including powder regardless of the presence or absence of a shield.

[Second Embodiment]
FIG. 6 shows the configuration of the spectroscopic device in the second embodiment. Constituent elements described in the above-described embodiments are denoted by the same reference numerals and description thereof is omitted. The differences from the first embodiment of FIG. 1 are mainly (i) the configuration of the illumination optical system 150 and the far-infrared light imaging optical system 170, (ii) the arrangement of the nonlinear optical crystal 132 for detection, and (iii) The pump light used for generating far-infrared light passing through the nonlinear optical crystal 130 is guided to the detection nonlinear optical crystal 132 for reuse.

  The light source 110 of the pump light 115 includes, as main components, a short-pulse Q-switched YAG laser 111, a polarization separation system including a polarization beam splitter (hereinafter referred to as PBS) 114 and a quarter-wave plate 116, a laser And an amplifier unit (here, solid-state amplifier 118) for amplifying the output. For example, the output beam of the YAG laser 111 is collimated by the lens 112 and amplified by the solid-state amplifier 118 through a polarization separation system composed of the PBS 114 and the quarter-wave plate 116.

  More specifically, the output beam of the YAG laser 111 is reflected by the mirror 181, collimated by the lens 112, reflected by the mirror 182, and enters the PBS 114. The beam that has passed through the PBS 114 is reflected by the mirror 183 through the quarter-wave plate 116 and the solid-state amplifier 118. The reflected beam enters the PBS 114 through the solid-state amplifier 118 and the quarter-wave plate 116. Thereafter, the beam is emitted from the PBS 114 through the mirror 184 as pump light 115. By using the solid-state amplifier 118, the output of the YAG laser 111 is amplified, and powerful far-infrared light having peak power of kW level can be extracted from the nonlinear optical crystal 130.

  In this embodiment, the illumination optical system 150 includes a cylindrical lens 151 and a condenser lens 155. Far-infrared light emitted from the wavelength-tunable far-infrared light source 100 having a linear light emitting region along the beam of the pump light 115 is converted into a parallel light beam by the cylindrical lens 151, and is spotted on the sample 200 by the condenser lens 155. It is focused on.

  The far-infrared light transmitted through the sample 200 is guided to the detection nonlinear optical crystal 132 by the far-infrared light imaging optical system 170. The far-infrared light imaging optical system 170 is an imaging optical system that images the surface of the sample 200 in the detection nonlinear optical crystal 132. The far-infrared light imaging optical system 170 includes a lens 176, a mirror 172, and a condenser lens 174. Specifically, the far-infrared light transmitted through the sample 200 is collimated by the lens 176, reflected by the mirror 172, and condensed by the condenser lens 174 on the detection nonlinear optical crystal 132.

  FIG. 7A shows far-infrared light spots 205a, 205b, and 205c on the surface of the sample 200. FIG. As described above, the far-infrared light emitted from the wavelength-variable far-infrared light source 100 changes its emission direction in the plane of FIG. 6 when wavelength scanning is performed (for example, θ1 in FIG. 6). ~ Θ2). Therefore, for example, when the wavelength of the seed light 125 is changed to change the frequency of the generated far infrared light from 1 THz to 3 THz, the far infrared light spot on the surface of the sample 200 is 1 THz on the low frequency side. It is at the position 205c, and at the intermediate frequency 2THz, it is at the position 205b, and at 3THz on the high frequency side, it is at the position 205a. When the frequency is changed in this way, the irradiation position on the sample 200 is different. For this reason, the position dependence of the concentration distribution affects the absorption spectrum, and an accurate spectrum cannot be obtained. Therefore, in this embodiment, the stage 202 of the sample 200 is controlled in accordance with the frequency of the far infrared light so that the irradiation position does not change even if the frequency of the far infrared light changes.

  The control unit 500 moves the stage 202 according to a change in the irradiation position of the far infrared light that can be generated on the surface of the sample 200 when the wavelength of the seed light 125 is changed. With this configuration, when the wavelength of the far infrared light from the variable wavelength far infrared light source 100 changes, the far infrared light can be irradiated to the same position of the sample 200. That is, the irradiation position of the far infrared light on the sample 200 does not depend on the wavelength of the far infrared light from the wavelength variable far infrared light source 100.

  For example, when the wavelength of the far infrared light from the tunable far infrared light source 100 is 1 THz on the low frequency side, since the far infrared light spot is at the position of 205c, the control unit 500 sets the stage 202 to -y Drive in the direction (see FIG. 7C). In addition, when the far-infrared light wavelength from the tunable far-infrared light source 100 is 3 THz on the high frequency side, since the far-infrared light spot is at the position 205a, the control unit 500 moves the stage 202 in the + y direction. (See FIG. 7B). When the wavelength of the seed light 125 is changed, the control unit 500 moves the stage 202 in accordance with the change in the irradiation position of the far infrared light that can be generated on the surface of the sample 200, and the same position of the sample (FIG. 7A to FIG. 7). It is configured to irradiate a spot of far-infrared light on the center) in FIG. 7C. According to this configuration, it is possible to avoid the influence of the position dependence of the concentration distribution on the absorption spectrum, and to perform accurate and stable spectrum measurement.

  In this embodiment, the pump light that has passed through the nonlinear optical crystal 130 is used as wavelength conversion pump light. Specifically, pump light that has passed through the nonlinear optical crystal 130 is reflected by the mirror 210 and guided to the detection nonlinear optical crystal 132 as pump light 235 for wavelength conversion. This configuration is suitable when the pump light 115 can be used efficiently and the power of the pump light 115 is not sufficient. However, because of reuse, the wavelength conversion pump light 235 may have a time waveform broken. Therefore, the conversion efficiency to near-infrared light and the beam quality of the converted near-infrared light may deteriorate. When there is a margin in the power of the pump light 115, it is desirable to split the pump light into two for generating far infrared light and for converting into near infrared light, as shown in FIG.

  FIG. 8 shows another configuration example of the light source 110 for the pump light 115 used in the variable wavelength far infrared light source 100. The configuration of the light source 110 shown in FIG. 8 is applicable to other embodiments. In FIG. 8, the same components as those in FIG. 6 are denoted by the same reference numerals.

  The tunable far-infrared light source 100 in this example includes lenses 119a and 119b that correct the thermal lens effect generated in the solid-state amplifier 118. That is, the difference from the example shown in FIG. 6 is that two lenses 119 a and 119 b for correcting the thermal lens effect are arranged between the solid-state amplifier 118 and the mirror 183. When the output of the YAG laser 111 is amplified using the solid-state amplifier 118, the amplified beam may be narrowed by the thermal lens effect, and the PBS 114 and the quarter-wave plate 116 may be damaged. Therefore, an optical system for correcting a thermal lens generated in the solid amplifier 118 is provided between the solid amplifier 118 and the folding mirror 183. Amplified light emitted from the solid-state amplifier 118 is incident on the correction lenses 119a and 119b, and light that has passed through the lenses 119a and 119b is incident on the PBS 114 and the quarter-wave plate 116.

  Specifically, the combination of the concave lens 119a and the convex lens 119b is disposed between the solid-state amplifier 118 and the mirror 183 so that the combined focal length is negative. Further, the synthesized main surface is configured to be located in the vicinity of the solid-state amplifier 118. According to this configuration, it becomes possible to return the beam which has become a convergent beam by the thermal lens to a parallel beam and hit the mirror 183, and the beam reflected by the mirror 183 follows the reverse optical path and enters the solid amplifier 118 again. In addition, a configuration in which the light is emitted back to a parallel light beam by the thermal lens is possible. Since the beam output from the solid-state amplifier 118 becomes a parallel light beam, the energy density on the polarization beam splitter 114 and the quarter-wave plate 116 can be suppressed, and damage can be avoided.

  Furthermore, the focal length of the correction system can be adjusted by configuring the optical system for correcting the thermal lens effect with the two lenses 119a and 119b. Even when the gain of the solid-state amplifier 118 is changed and the thermal lens effect is changed, it is possible to cope with the adjustment of the surface interval of the lenses 119a and 119b, and a system with a high degree of freedom can be configured. In this embodiment, the optical system for correcting the thermal lens is provided between the solid-state amplifier 118 and the folding mirror 183. This can be expected to prevent damage to the optical system. This is because the beam has passed through the solid-state amplifier 118 only once at this position, and the beam power has not increased so much.

  Conventionally, a coherent light source having a variable frequency requires a high-power laser, so that the optical system is damaged by the thermal lens effect, and there is a problem that stable measurement is difficult. On the other hand, in this embodiment, by providing the two lenses 119a and 119b as an optical system for correcting the thermal lens effect, damage to the optical system can be avoided and stable measurement can be performed. The configuration of the light source 110 for the pump light 115 shown in FIG. 8 can be applied to other embodiments.

[Third embodiment]
9A and 9B show an example of the overall configuration of a far-infrared spectrometer according to the third embodiment. FIG. 9C is a plan view showing an irradiation region of a sample in the third embodiment. Constituent elements described in the above-described embodiments are denoted by the same reference numerals and description thereof is omitted.

  The far-infrared spectrometer of the present embodiment measures an absorption spectrum using the reflected light of the sample 200. The components from the wavelength tunable far infrared light source 100 to the illumination optical system 150 and the components after the far infrared light imaging optical system 170 are tilted with respect to the surface of the sample 200 around the sample 200 surface. In the far-infrared spectrometer of the present embodiment, far-infrared light is incident on the sample 200 obliquely, and reflected light from the sample 200 is detected.

  According to this configuration, the reflected light from the sample 200 can be detected by arranging the illumination optical system 150 and the far-infrared light imaging optical system 170 to be inclined with respect to the surface of the sample 200. Thereby, measurement of the sample 200 with low transmittance or measurement of the spectral characteristics of the surface of the sample 200 becomes possible. A mechanism capable of changing the incident angle from the illumination optical system 150 and the angle to the far-infrared light imaging optical system 170 may be provided. As a result, it is possible to measure the incident angle dependence of the spectral characteristics.

[Fourth embodiment]
10A and 10B show examples of the overall configuration of a far-infrared spectrometer according to the fourth embodiment. FIG. 10C is a plan view showing a sample irradiation region in the fourth embodiment. Constituent elements described in the above-described embodiments are denoted by the same reference numerals and description thereof is omitted.

  In this embodiment, each of the illumination optical system 150 and the far-infrared light imaging optical system 170 further includes an optical system that bends the optical path of the far-infrared light and corrects the optical path length of the far-infrared light. More specifically, an optical system 158a and a mirror 159a for adjusting the optical path length are arranged between the illumination optical system 150 and the sample 200b. Further, a mirror 159b and an optical system 158b for adjusting the optical path length are disposed between the sample 200b and the far-infrared light imaging optical system 170. In this embodiment, the sample 200b is disposed on the yz plane of the stage 202b. According to this configuration, it is possible to measure an absorption spectrum using the reflected light of the sample 200b.

  If the layout is such that the far-infrared light path is simply bent by the mirrors 159a and 159b and the reflected light of the sample is detected, the optical path length of the far-infrared light is extended, so that the sample surface cannot be focused. Therefore, as a feature of the present embodiment, an optical system 158a for correcting the optical path length is disposed in front of the mirror 159a, and an optical system 158b for correcting the optical path length is disposed in the subsequent stage of the mirror 159b. Since the optical system is arranged symmetrically about the sample 200b, the optical systems 158a and 158b may be the same. The optical systems 158a and 158b need to have an action of extending the optical path length, and thus can be configured with, for example, one concave lens. According to this configuration, additional parts can be minimized, which is effective when mounting restrictions are severe.

  On the other hand, the optical systems 158a and 158b may be configured by a combination of a convex lens and a concave lens. According to this configuration, it is possible to adjust the focal length of the concave lens obtained by combining these lenses by adjusting the interval between these lenses. Therefore, an off-the-shelf lens can be used. Further, the focal position can be adjusted by adjusting the distance between the lenses.

  1A and 1B is only in that the mirrors 159a and 159b and optical systems 158a and 158b for adjusting the optical path length are added, and that the sample 200b is held on the yz plane. Therefore, a mechanism for taking in and out the mirrors 159a and 159b and the optical systems 158a and 158b and a mechanism for replacing the sample 200b and the stage 202b with the sample 200a and the stage 202a may be provided (see FIG. 10B). By removing the mirrors 159a and 159b and the optical systems 158a and 158b from the optical path of far-infrared light and replacing the stage 202b with the stage 202a, it becomes possible to easily switch the measurement by reflected light to the measurement by transmitted light. .

[Fifth embodiment]
FIG. 11A shows an example of the overall configuration of a far-infrared spectrometer according to the fifth embodiment. Constituent elements described in the above-described embodiments are denoted by the same reference numerals and description thereof is omitted.

  The difference from the above-described embodiment is that an optical element (for example, a lens) used for the far-infrared illumination optical system 150, an optical element (for example, a lens) used for optical systems 158a and 158b for adjusting the optical path length, and An optical element (for example, a lens) used in the far-infrared light imaging optical system 170 is provided with a structure for preventing reflection as shown in FIGS. 11A to 11C. At least one optical element used in the illumination optical system 150 and the far-infrared light imaging optical system 170 has a groove-shaped process that reduces reflection of far-infrared light.

  In FIG. 11A, far-infrared light generated from the nonlinear optical crystal 130 for generating far-infrared is mainly composed of a polarization component that vibrates in the vertical direction (x-axis direction in the drawing) in the drawing. In order to prevent reflection, the surface of the optical element described above includes a plurality of V-shaped grooves whose longitudinal direction is a direction (y-axis direction) orthogonal to the polarization direction. The plurality of grooves are formed substantially in parallel. 11B and 11C are enlarged views of the surface 152a of the cylindrical lens 152 of the illumination optical system 150 as an example. The plurality of grooves have a depth d and are formed with a period p.

  In the above configuration, it is known that if the groove period p is sufficiently small with respect to the wavelength of far-infrared light, it is equivalent to the case where the refractive index of the interface gradually changes for incident far-infrared light. Yes. Therefore, as an example, it is preferable that the surface of the optical element described above includes grooves having a period of 1/3 or less of the wavelength of far-infrared light (that is, the interval between the plurality of groove shapes is the wavelength-variable far-infrared light source 100. 1/3 of the wavelength of the first far-infrared light from or the second far-infrared light from the sample 200). More preferably, the surface of the optical element described above includes a groove having a period of 1/5 or less of the wavelength of far-infrared light. More preferably, the surface of the optical element described above includes grooves having a period of about 1/10 of the wavelength of far-infrared light. For example, since the wavelength of 1 to 3 THz far-infrared light has a wavelength of 100 to 300 μm, grooves having a period of about 10 μm, which is 1/10 of the wavelength, may be formed. For the purpose of reflection reduction, it is desirable that the groove period is small, but if it is less than about 10 μm, which is 1/10 of the wavelength, it is difficult to ensure the processing accuracy, or the processing cost is high, so it is practical. descend. Therefore, it is desirable to set it to about 1/10 of the wavelength. The depth d of the groove is preferably about the same as the period p to about 10 times the period p. This is because, for the purpose of reducing reflection, it is desirable that the refractive index change on the surface of the optical element be gentle on the wavelength scale. That is, if the groove is shallow, the refractive index change is not relaxed and the effect of reducing the surface reflection cannot be obtained. On the other hand, at a depth exceeding 10 times the period p, machining becomes difficult and the accuracy of the shape cannot be maintained. When the groove shape is disturbed, scattered light is generated and the reflection reduction effect is lowered.

  Since the reflection on the surface of the optical element becomes larger as the difference in the refractive index on the surface becomes larger, it is possible to reduce the surface reflection by creating a structure equivalent to a gradual change in the refractive index. In addition, when forming a V-shaped groove | channel, you may form the part which maintained the original surface shape between adjacent grooves. As shown in FIG. 11C, between the plurality of groove shapes, the optical axis of the first far infrared light from the wavelength variable far infrared light source 100 or the second far infrared light from the sample 200 is orthogonal. A flat surface (part w) in a direction parallel to the surface is formed. The width of the flat surface is larger than the wavelength of visible light. According to this configuration, although the effect of reducing the surface reflection is weakened, the optical system can be aligned using visible light, and the optical system can be easily adjusted. If the V-shaped groove is completely connected (the w portion in FIG. 11C does not remain), the alignment visible light beam is refracted by the groove structure, and therefore the same optical path as that of the far-infrared light is used. I will not follow. However, when the width of the w portion is made larger than the wavelength of visible light for alignment (for example, 1 μm or more), the alignment visible light beam passing through the w portion follows the same optical path as that of the far infrared light. Therefore, alignment using a visible light beam can be performed while reducing surface reflection of far-infrared light.

  FIG. 12A is an example in which the groove structure described above is formed on the surfaces of the cylindrical lenses 152 and 154 of the illumination optical system 150. FIG. FIG. 12B shows an example in which the groove structure described above is formed on the surface of the cylindrical lens 156 of the illumination optical system 150. FIG. 12C shows an example in which the groove structure described above is formed on the surfaces of the optical systems 158a and 158b for adjusting the optical path length and the lenses 177 and 179 used in the far-infrared light imaging optical system 170. In the example of FIG. 12A, grooves that are orthogonal to the generatrix of the cylindrical lenses 152 and 154 are formed. In the example of FIG. 12B, a groove parallel to the generatrix of the cylindrical lens 156 is formed. In the example of FIG. 12C, the spherical lens has a groove structure. These lenses for far-infrared light are often made of resin. Since resin lenses can be manufactured by molding using a mold, even those having a groove structure on the surface can be mass-produced relatively easily.

  The present invention is not limited to the above-described embodiments, and includes various modifications. The above embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. Also, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment. Moreover, the structure of another Example can also be added to the structure of a certain Example. Further, with respect to a part of the configuration of each embodiment, another configuration can be added, deleted, or replaced.

  The processes of the signal processing unit 400 and the control unit 500 described above can also be realized by a program code of software that realizes these functions. In this case, a storage medium in which the program code is recorded is provided to the system or apparatus, and the computer (or CPU or MPU) of the system or apparatus reads the program code stored in the storage medium. In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the program code itself and the storage medium storing it constitute the present invention. As a storage medium for supplying such a program code, for example, a flexible disk, CD-ROM, DVD-ROM, hard disk, optical disk, magneto-optical disk, CD-R, magnetic tape, nonvolatile memory card, ROM Etc. are used.

  The processes and techniques described herein are not inherently related to any particular device, and can be implemented by any suitable combination of components. In addition, various types of devices for general purpose can be used. In some cases, it may be beneficial to build a dedicated device to perform the processing described here. That is, a part of the signal processing unit 400 and the control unit 500 described above may be realized by hardware using electronic components such as an integrated circuit.

  Furthermore, in the above-described embodiments, control lines and information lines are those that are considered necessary for explanation, and not all control lines and information lines on the product are necessarily shown. All the components may be connected to each other.

100… wavelength tunable far-infrared light source
110… Light source
111… Q-switched YAG laser
112… Lens
114… Polarized beam splitter
115… Pump light
116 ... quarter-wave plate
118… Solid-state amplifier (amplifier unit)
119a ... Concave lens (thermal lens correction lens)
119b ... Convex lens (thermal lens correction lens)
120 ... wavelength tunable light source
121 ... Incident angle adjustment optical system
122… Lens
123… Optical deflector
124… Imaging optics
125… Seed light
126… Mirror
130… Nonlinear optical crystal
132… Nonlinear optical crystal for detection
140, 142 ... Si prism
150… Illumination optics
170… Far-infrared light imaging optical system
178a, 178b… stage
200, 200a, 200b ... Sample
202, 202a, 202b… stage

Claims (1)

A tunable far-infrared light source that generates first far-infrared light;
An illumination optical system for irradiating the sample with the first far-infrared light;
A non-linear crystal for detection that converts the second far-infrared light from the sample into near-infrared light using pump light for wavelength conversion;
A far-infrared light imaging optical system that forms an image of the sample on the nonlinear optical crystal for detection; and
A first light source and a second light source having different wavelengths, a non-linear optical crystal for generating far-infrared light that makes light emitted from the first and second light sources incident, and an incident angle adjusting optical system,
A configuration in which the far-infrared light generating nonlinear optical crystal is irradiated with light from the first and second light sources to generate far-infrared light by difference frequency generation or parametric generation;
The incident angle adjusting optical system is
A lens, an optical deflector, and a single imaging optical element are arranged in this order on the optical path between the second light source and the far-infrared light generating nonlinear optical crystal, and the single imaging optical element A far-infrared spectrometer, wherein a beam waist is formed near the front focal plane.

Images