Spectroscopic measuring device

Abstract

To provide a spectroscopic measurement device that can improve accessibility of a measurement unit with respect to a measured object.SOLUTION: A spectroscopic measurement device 1 comprises: a light source unit 11 that outputs pump light La, and probe light Lb; a terahertz wave generation unit 21 that generates a terahertz wave T by input of the pump light La; a terahertz wave detection unit 23 to which the terahertz T and probe light Lb are input, and which modulates the probe light Lb based on the refractive index by a change in refractive index owing to an electro-optical effect accompanied by the input of the terahertz wave T; an optical detection unit 13 that detects the probe light Lb modulated by the terahertz wave detection unit 23; a main body unit 2 configured to include the light source unit 11 and the optical detection unit 13; a measurement unit 3 configured to include the terahertz wave generation unit 21 and the terahertz wave detection unit 23, in which the main body unit 2 and measurement unit 3 are optically connected by a polarization holding fiber F.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a spectroscopic measurement device.

Background Art

[0002] Terahertz waves are electromagnetic waves having a frequency of about 0.01 THz to 100 THz, which corresponds to the intermediate region between light waves and radio waves, and have intermediate properties between light waves and radio waves. As an application of terahertz waves, a technique for acquiring information on a measurement object by measuring the time waveform of the electric field amplitude of terahertz waves transmitted or reflected by the measurement object has been studied.

[0003] Examples of spectroscopic measurement devices using terahertz waves include the devices described in Patent Documents 1 and 2. This spectroscopic measurement device includes a light source unit that outputs pump light and probe light, a terahertz wave generation unit that generates terahertz waves by inputting the pump light, a terahertz wave detection unit that inputs the terahertz waves acting on the measurement object and the probe light and modulates the probe light with the terahertz waves, and a photodetector that detects the probe light modulated by the terahertz wave detection unit.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the field of time-domain spectroscopy, including spectroscopic measurements using terahertz waves, ensuring the optical path length in the measurement optical system is extremely important, and once the layout of the optical system is determined, it is not easy to change it. For example, in the spectroscopic measurement apparatus described in Patent Document 1, ensuring the optical path length of each of the optical systems—the one that inputs the pump light to the terahertz wave generation unit, the one that inputs the probe light to the terahertz wave detection unit, and the one that applies the terahertz waves generated in the terahertz wave generation unit to the object being measured and inputs them to the terahertz wave detection unit—is particularly important. For this reason, once the positions of mirrors and other components constituting these optical systems are determined, the positions of measurement units such as measurement prisms also remain basically unchanged.

[0006] However, if the position of the measuring unit remains constant, the shape and orientation of the object being measured that can be accessed (e.g., by contact with the measuring surface) may be limited. Furthermore, the types of equipment and manufacturing lines that can incorporate the spectroscopic measurement device may also be limited. Therefore, in order to expand the applications of spectroscopic measurement devices in industrial fields, it is necessary to improve the accessibility of the measuring unit to the object being measured.

[0007] This disclosure was made to solve the above-mentioned problems and aims to provide a spectroscopic measuring device that can improve the accessibility of the measuring unit to the object being measured. [Means for solving the problem]

[0008] A spectroscopic measuring device relating to one aspect of this disclosure comprises a light source unit that outputs pump light and probe light; a terahertz wave generation unit that generates terahertz waves upon input of pump light; a terahertz wave detection unit that receives terahertz waves and probe light respectively, and modulates the probe light based on the refractive index as the refractive index changes due to the electro-optic effect associated with the input of terahertz waves; and a photodetection unit that detects the probe light modulated by the terahertz wave detection unit. The device comprises a main body unit comprising the light source unit and the photodetection unit, and a measurement unit comprising the terahertz wave generation unit and the terahertz wave detection unit, with the main body unit and the measurement unit being optically connected by a polarization-maintaining fiber.

[0009] This spectroscopic measurement device samples the change in refractive index of the terahertz wave detection unit using probe light, due to the electro-optic effect associated with the input of terahertz waves. This allows the electric field waveform of the terahertz wave acting on the object being measured to be acquired, and information about the object can be obtained based on the electric field waveform. Furthermore, in this spectroscopic measurement device, the measurement unit is optically connected to the main unit by a polarization-maintaining fiber. This allows the measurement unit to be freely positioned relative to the main unit, improving the accessibility of the measurement unit to the object being measured. Restrictions on the shape and orientation of the object being measured that can access the measurement unit are eliminated, and the number of devices and manufacturing lines that can incorporate the spectroscopic measurement device increases, thereby expanding the applications of spectroscopic measurement devices in industrial fields. By using a polarization-maintaining fiber, the polarization state of the probe light input to the terahertz wave detection unit is maintained, thus maintaining the detection efficiency of the probe light in the optical detection unit.

[0010] The measuring unit may have a measuring surface on the optical path of the terahertz wave between the terahertz wave generating unit and the terahertz wave detection unit. By providing a measuring surface on a measuring unit that can be freely positioned relative to the main unit, the accessibility of the measuring unit to the object being measured can be further improved.

[0011] The measurement unit may further include a waveplate positioned on the optical path from which the probe light modulated by the terahertz wave detection unit is directed toward the photodetector. In polarization-maintaining fibers, the refractive index differs between the first polarization component and the second polarization component orthogonal thereto, which can cause a time lag between the first and second polarization components of the propagated light. Therefore, by positioning the waveplate appropriately in the measurement unit, the detection efficiency of the probe light in the photodetector can be ensured.

[0012] The waveplate is a λ / 8 waveplate, and in the measurement unit, terahertz waves may be incident from one side of the terahertz wave detection unit, and probe light may be incident from the other side of the terahertz wave detection unit and reflected back to the photodetector. With this configuration, for example, when the initial polarization state of the probe light is linearly polarized, by causing the probe light to be incident on the λ / 8 waveplate twice, before and after input to the terahertz wave detection unit, it becomes possible to appropriately separate the polarization components of the probe light into a first polarization component and a second polarization component. Therefore, the detection efficiency of the probe light in the photodetector can be suitably ensured.

[0013] The waveplate is a λ / 4 waveplate, and in the measurement unit, terahertz waves and probe light may be incident from one side of the terahertz wave detection unit, and the probe light may also be transmitted to the other side of the terahertz wave detection unit toward the photodetector. With this configuration, for example, when the initial polarization state of the probe light is linearly polarized, by causing the probe light that has passed through the terahertz wave detection unit to be incident once on the λ / 4 waveplate, it becomes possible to appropriately separate the polarization components of the probe light into a first polarization component and a second polarization component. Therefore, the detection efficiency of the probe light in the photodetector can be suitably ensured.

[0014] At least one of the optical components constituting the main body may be optically connected by a polarization-maintaining fiber. This increases the degree of freedom in arranging the optical components within the main body. This also contributes to miniaturization of the main body.

[0015] The main unit may include an adjustment unit for adjusting the difference in optical path length between the pump light and the probe light. This allows for flexible adjustment of the timing of input of the terahertz wave and probe light to the terahertz wave detection unit. By sweeping the input timing of the probe light to the terahertz wave detection unit using the adjustment unit, the electric field waveform of the terahertz wave acting on the object being measured can be suitably acquired.

[0016] The adjustment unit is configured to include a plurality of mirrors and a stage for moving the positions of the mirrors, and the pump light or the probe light may propagate through free space without relying on an optical fiber. In this case, the adjustment unit can be constructed with a simple configuration.

[0017] The adjustment unit may be constituted by a polarization-maintaining fiber. In this case, the degree of freedom in the arrangement of the optical components within the main body portion can be further increased. Also, further miniaturization of the main body portion can be achieved.

[0018] The light source unit may be constituted by a fiber laser. In this case, miniaturization of the light source unit becomes possible, and further miniaturization of the main body portion can be achieved.

[0019] The laser light output from the light source unit may be ultrashort pulsed light on the order of femtoseconds. By using ultrashort pulsed light, the band of the terahertz wave generated in the terahertz wave generation unit is expanded, and it becomes possible to widen the measurement band of the spectroscopic measurement device.

Advantages of the Invention

[0020] According to the present disclosure, the accessibility of the measurement unit to the measurement object can be improved.

Brief Description of the Drawings

[0021] [Figure 1] It is a perspective view showing an embodiment of a spectroscopic measurement device according to one aspect of the present disclosure. [Figure 2] It is a block diagram showing the components of the spectroscopic measurement device shown in FIG. 1. [Figure 3] It is a schematic diagram showing the principle of electro-optic sampling by the spectroscopic measurement device shown in FIG. 1. [Figure 4] (a) and (b) are schematic diagrams showing the relationship between the electric field waveform of the terahertz wave acting on the measurement object and the polarization state of the modulated probe light. [Figure 5] (a) and (b) are schematic diagrams showing the polarization state of the probe light according to a comparative example. [Figure 6](a) and (b) are schematic diagrams showing the polarization states of the probe light according to the examples. [Figure 7] It is a block diagram showing the components of the main part of the spectroscopic measurement apparatus according to the modified example.

Mode for Carrying Out the Invention

[0022] Hereinafter, with reference to the drawings, a preferred embodiment of the spectroscopic measurement apparatus according to one aspect of the present disclosure will be described in detail.

[0023] FIG. 1 is a perspective view showing an embodiment of a spectroscopic measurement apparatus according to one aspect of the present disclosure. The spectroscopic measurement apparatus 1 shown in the figure is configured as an apparatus for measuring optical parameters of a measurement object (not shown) by electro-optic sampling. The measurement object is a substance such as a liquid, powder, or solid. Examples of the optical parameters to be measured include a refractive index, an absorption coefficient, a complex dielectric constant, and the like.

[0024] As shown in FIG. 1, the spectroscopic measurement apparatus 1 includes, for example, a box-shaped main body 2 having a rectangular parallelepiped shape, and a measurement unit 3 including a handy-type housing 4 having a measurement surface M. The main body 2 and the measurement unit 3 are physically connected by a flexible cable 5 that houses a polarization-maintaining fiber F described later. Thereby, in the spectroscopic measurement apparatus 1, the measurement unit 3 can be freely moved with respect to the main body 2, and the measurement surface M can be arranged at an arbitrary position within the range of the length of the cable 5. In FIG. 1, the measurement surface M is provided on the side surface of the housing 4 of the measurement unit 3, but the position of the measurement surface M on the housing 4 is arbitrary.

[0025] FIG. 2 is a block diagram showing the components of the spectroscopic measurement apparatus shown in FIG. 1. As shown in FIG. 2, the main body 2 of the spectroscopic measurement apparatus 1 includes a light source unit 11 that outputs a laser beam L that becomes pump light La and probe light Lb, a light guiding optical system 12A that guides the pump light La, a light guiding optical system 12B that guides the probe light Lb, and a light detection unit 13 that detects the probe light Lb.

[0026] In this embodiment, each component of the main body 2 is composed of fiber-based optical components, and these components are all optically connected to each other by polarization-maintaining fibers F. Fiber-based optical components are components that can be optically connected to each other by fibers. Fiber-based optical components may also include components in which at least one of the input and output ends of the optical fiber is an optical fiber. The polarization-maintaining fiber F has a structure in which circular stress-applying portions are arranged on both sides of the core, for example. In the polarization-maintaining fiber F, a function to suppress polarization fluctuations due to fluctuations in birefringence is exhibited by applying non-axisymmetric stress to the core to induce large birefringence.

[0027] The measurement unit 3 of the spectroscopic measurement device 1 includes a terahertz wave generation unit 21 that generates terahertz waves T in response to the input of pump light La, a measurement prism 22 that constitutes the measurement surface M (see Figure 1) described above, a terahertz wave detection unit 23 to which the terahertz waves T and probe light Lb are input, and a λ / 8 wave plate 24.

[0028] The light source unit 11 is the part that outputs laser light L, which becomes pump light La and probe light Lb. In this embodiment, the light source unit 11 is composed of an ultrashort pulse fiber laser, and the laser light L output from the light source unit 11 is ultrashort pulse light on the order of femtoseconds. The pulse width of the laser light L is, for example, 200 fs or less. In this embodiment, the laser light L has, for example, a pulse width of 60 fs, a repetition frequency of 100 MHz, and an average output of 50 mW. The wavelength of the laser light L is, for example, 1560 nm, which is in the eye-safe band. The laser light L output from the light source unit 11 is split into pump light La and probe light Lb by the coupler 14.

[0029] The optical guide optical system 12A of the pump light La branched by the coupler 14 includes an optical path length adjustment unit (adjustment unit) 15A, a dispersion correction patch cord 16A, and a modulation unit 17. The optical path length adjustment unit 15A is the part that sweeps the incidence timing of the probe light Lb to the terahertz wave detection unit 23 with respect to the terahertz wave T. In this embodiment, the optical path length adjustment unit 15A is composed of a delay stage which includes a plurality of mirrors and a stage that moves the positions of the mirrors. Therefore, in the optical path length adjustment unit 15A, the pump light La propagates in free space without relying on an optical fiber (polarization-maintaining fiber F). The pump light La is input to the dispersion correction patch cord 16A with a predetermined delay relative to the probe light Lb given by the delay stage.

[0030] The dispersion correction patch code 16A is the part that corrects the wavelength dispersion of light (in this case, pump light La) passing through the polarization-maintaining fiber F. The dispersion correction patch code 16A is composed of dispersion compensation fibers of a length that cancels out (inflates inverse dispersion) the wavelength dispersion that occurs depending on the length of the polarization-maintaining fiber F that constitutes the optical guide optical system 12A. As a result, the pump light La is input to the modulation unit 17 in a state that is adjusted so that the pulse width when incident on the terahertz wave generation unit 21 is minimized.

[0031] The modulation unit 17 is the part that periodically switches the on / off state of the pump light La directed toward the terahertz wave generation unit 21. The modulation unit 17 is composed of, for example, an optical chopper. The optical chopper is a disc-shaped member on which light-shielding ribs are provided radially from the center at a predetermined phase angle. By periodically switching the on / off state of the pump light La by the rotation of the optical chopper, the timing of the generation of terahertz waves T in the terahertz wave generation unit 21 is controlled. A signal indicating the repetition frequency of the on / off state of the pump light La by the optical chopper is output to the lock-in amplifier 32, which is part of the photodetection unit 13 and will be described later. Note that the modulation unit 17 is not limited to modulation by an optical chopper, but may also perform other modulation using acousto-optic elements or electro-optic elements.

[0032] The optical guide optical system 12B of the probe light Lb branched by the coupler 14 is equipped with an optical path length adjustment unit (adjustment unit) 15B, a dispersion correction patch cord 16B, and a circulator 18. The optical path length adjustment unit 15B is the part that adjusts the time difference between the input of the terahertz wave T and the probe light Lb to the terahertz wave detection unit 23. In this embodiment, the optical path length adjustment unit 15B is composed of a delay stage similar to the optical path length adjustment unit 15A located in the optical guide optical system 12A. Therefore, in the optical path length adjustment unit 15B, the probe light Lb propagates in free space without relying on an optical fiber (polarization-maintaining fiber F). The probe light Lb is input to the dispersion correction patch cord 16B with a predetermined delay relative to the pump light La given by the delay stage.

[0033] In this embodiment, the dispersion correction patch cord 16B is composed of a dispersion compensation fiber similar to the dispersion correction patch cord 16A located in the optical guide optical system 12A. As a result, the probe light Lb is input to the circulator 18 in a state adjusted so that the pulse width when incident on the terahertz wave detection unit 23 is minimized. The circulator 18 is an optical component having three ports for light and for separating two beams of light traveling in opposite directions. The probe light Lb input to port 1 of the circulator 18 is output from port 2 toward the measurement unit 3. The probe light Lb that returns from the measurement unit 3 to port 2 is output from port 3 and input to the photodetector 13 via the polarization separation element 19.

[0034] The pump light La that has passed through the modulation unit 17 and the probe light Lb output from port 2 of the circulator 18 reach the measurement unit 3 via the polarization-maintaining fiber F housed in cable 5 (see Figure 1), and are emitted out of the fiber from GRIN lenses 20A and 20B. In other words, the optical connection of the pump light La and probe light Lb between the main unit 2 and the measurement unit 3 is realized by two refractive index-distributed lenses (GRIN lenses) 20A and 20B located in the measurement unit 3. The pump light La input from the modulation unit 17 to the measurement unit 3 via one of the GRIN lenses 20A is input to the terahertz wave generation unit 21.

[0035] The terahertz wave generation unit 21 is the part that generates terahertz waves T by the incidence of pump light La. For example, when the wavelength of the laser light L used as the pump light La is 1560 nm, the terahertz wave generation unit 21 can be composed of an organic nonlinear optical crystal such as DAST (4-dimethylamino-N-methyl-4-stilbazolium tosylate). The pulse width of the terahertz waves T generated from this crystal is generally around a few ps, and it contains frequency components in a band of approximately 0.1 THz to 5 THz. The terahertz waves T generated in the terahertz wave generation unit 21 are input to the measurement prism 22.

[0036] The method of this embodiment is called total internal reflection attenuation (ATR) spectroscopy, and measurement is performed with the object to be measured in contact with the measurement surface M. The measurement prism 22 has, for example, an input surface, an output surface, and a total internal reflection surface. The total internal reflection surface of the measurement prism 22 is the surface corresponding to the measurement surface M described above. The measurement surface M is located on the optical path of the terahertz wave T between the terahertz wave generation unit 21 and the terahertz wave detection unit 23. The terahertz wave T enters the measurement prism 22 from the input surface, undergoes total internal reflection at the measurement surface M, and is then emitted out of the measurement prism 22 from the output surface. The terahertz wave T emitted out of the measurement prism 22 is input to the terahertz wave detection unit 23. The evanescent component generated during the total internal reflection of the terahertz wave T at the measurement surface M acts on the object to be measured placed on the measurement surface M. This allows information about the object to be obtained using the terahertz wave T.

[0037] The terahertz wave detection unit 23 is the part that detects terahertz waves T acting on the object being measured. The terahertz wave detection unit 23 is made of an electro-optic crystal, for example, a (111) plane cut out of GaAs. In this embodiment, terahertz waves T are incident on one side of the terahertz wave detection unit 23, and probe light Lb is incident on the other side of the terahertz wave detection unit 23. In the terahertz wave detection unit 23, the refractive index changes due to the electro-optic effect associated with the input of terahertz waves T, and the probe light Lb is modulated based on this refractive index.

[0038] The probe light Lb is input to the measurement unit 3 via the other GRIN lens 20B located in the measurement unit 3. The probe light Lb input to the measurement unit 3 passes through the λ / 8 wave plate 24 and is incident on the other side of the terahertz wave detection unit 23. The probe light Lb modulated by the terahertz wave detection unit 23 is reflected by the terahertz wave detection unit 23 and heads towards the photodetector 13. The probe light Lb passes through the λ / 8 wave plate 24 again and returns to the main unit 2 via the GRIN lens 20B. The probe light Lb that has returned to the main unit 2 passes through the circulator 18 and then through the polarization separation element 19 before being input to the photodetector 13.

[0039] The photodetector 13 is the part that detects the probe light Lb modulated by the terahertz wave detection unit 23. In this embodiment, the photodetector 13 is composed of a balance detector 31 and a lock-in amplifier 32. A polarization separation element 19 is placed in front of the photodetector 13. The polarization separation element 19 is composed of, for example, an optical fiber fusion type polarization beam splitter. An optical fiber fusion type polarization beam splitter is an optical element that separates the first polarization component and the second polarization component of light by passing it through an optical fiber. The first polarization component is, for example, the component in the same direction as the polarization of the laser light L output from the light source unit 11, and the second polarization component is the component in the direction perpendicular to the first polarization component. Hereinafter, for convenience of explanation, the first polarization component will be referred to as the longitudinal polarization component, and the second polarization component will be referred to as the transverse polarization component.

[0040] The balance detector 31 detects the difference between the longitudinal and transverse polarization components of the probe light Lb separated by the polarization separation element 19. The balance detector 31 outputs a difference signal based on the difference between the longitudinal and transverse polarization components to the lock-in amplifier 32. By performing difference detection, the intensity fluctuation component of the probe light Lb is removed. When performing difference detection, it is preferable that the sensitivity is adjusted so that the intensity of the difference signal from the balance detector 31 becomes zero when the terahertz wave T is not incident on the terahertz wave detection unit 23. In addition, an optical attenuator may be placed between the polarization separation element 19 and the balance detector 31. By placing an optical attenuator, the intensity of the longitudinal polarization component and the intensity of the transverse polarization component input to the balance detector 31 can be easily adjusted.

[0041] The lock-in amplifier 32 synchronously detects the difference signal output from the balance detector 31 based on the repetition frequency of the on / off switching of the pump light La by the optical chopper. The detection signal output from the lock-in amplifier 32 takes a value that depends on the electric field strength of the terahertz wave T input to the terahertz wave detection unit 23.

[0042] Generally, the pulse width of a terahertz wave T is on the order of picoseconds. In contrast, the pulse width of the probe light Lb is on the order of femtoseconds, which is several orders of magnitude smaller than that of the terahertz wave T. Therefore, the optical path length adjustment unit 15A sweeps the incidence timing of the probe light Lb to the terahertz wave detection unit 23, thereby obtaining the electric field waveform (time waveform of the electric field amplitude) of the terahertz wave T. Based on this electric field waveform of the terahertz wave T, the optical parameters of the object being measured can be obtained.

[0043] Figure 3 is a schematic diagram illustrating the principle of electro-optic sampling using the spectroscopic measurement device shown in Figure 1. As shown in the figure, pulsed terahertz waves T are used in the electro-optic sampling in the spectroscopic measurement device 1. Upon input of terahertz waves T, a change in refractive index occurs in the terahertz wave detection unit 23 due to the electro-optic effect. The electro-optic effect is a phenomenon in which the optical properties of a medium change due to an electric field incident on the medium. The probe light Lb modulated by the terahertz wave detection unit 23 based on the refractive index changed by the electro-optic effect becomes probe light Lb that contains electric field waveform information of the terahertz wave T. Therefore, by sampling the change in refractive index in the terahertz wave detection unit 23 with probe light Lb, the electric field waveform of the terahertz wave T acting on the object being measured can be obtained.

[0044] Figures 4(a) and 4(b) are schematic diagrams showing the relationship between the electric field waveform of the terahertz wave acting on the object being measured and the polarization state of the modulated probe light. As shown in these figures, the change in refractive index at the terahertz wave detection unit 23 can be read as a change in the polarization state of the probe light Lb that has passed through the terahertz wave detection unit 23. In this case, for example, it is preferable to set the initial polarization state of the probe light Lb directed toward the terahertz wave detection unit 23 to linear polarization, and the polarization state of the probe light Lb input to the polarization separation element 19 to circular polarization when the electric field intensity of the terahertz wave T is zero (cases A and D in Figure 4(a)).

[0045] For example, when the electric field strength of the terahertz wave T is zero (cases A and D in Figure 4(a)), no change in refractive index occurs in the terahertz wave detection unit 23, and the polarization state of the probe light Lb that has passed through the terahertz wave detection unit 23 remains the same as the initial polarization state. In this case, if the polarization state of the probe light Lb input to the polarization separation element 19 is circularly polarized, the output from the balance detector 31 will be zero. On the other hand, when the electric field strength of the terahertz wave T is greater than zero (cases B and C in Figure 4(a)), a change in refractive index occurs in the terahertz wave detection unit 23 according to the electric field strength of the terahertz wave T. In this case, if the polarization state of the probe light Lb that has passed through the terahertz wave detection unit 23 changes to elliptically polarized light with a phase corresponding to the change in refractive index, the output from the balance detector 31 will be a non-zero value corresponding to the electric field strength of the terahertz wave T.

[0046] Next, we will explain the method for reconstructing the electric field waveform of terahertz waves based on the polarization state of the probe light. As mentioned above, in the spectroscopic measurement device 1, the main body 2 and the measurement unit 3 are optically connected by polarization-maintaining fibers F. In addition, the optical components constituting the main body 2 are also optically connected to each other by polarization-maintaining fibers F. In the polarization-maintaining fiber F, a non-axisymmetric stress is applied to the core, and the refractive index for the longitudinal polarization component of the propagating light is different from that for the transverse polarization component. For this reason, in light propagating through the polarization-maintaining fiber F, a timing difference may occur between the output of the longitudinal polarization component and the transverse polarization component.

[0047] Here, as a comparative example, consider the case shown in Figure 5(a), where a terahertz wave T is incident from one side of the terahertz wave detection unit 23 and a probe light Lb is incident from the other side of the terahertz wave detection unit 23, and for example, a λ / 4 wave plate 101 is placed between the circulator 18 and the polarization separation element 19. In this case, as shown in Figure 5(b), if the probe light Lb directed toward the terahertz wave detection unit 23 (A in Figure 5(a)) is linearly polarized and the probe light Lb modulated by the terahertz wave detection unit 23 (B in Figure 5(a)) is elliptically polarized, then the polarization state of the probe light Lb directed from the circulator 18 toward the polarization separation element 19 (C in Figure 5(a)) will be two linearly polarized states with a timing difference.

[0048] If the probe light Lb passes through the λ / 4 wave plate 101 just before being input to the polarization separation element 19, the polarization state of the probe light Lb that has passed through the λ / 4 wave plate 101 (D in Figure 5(a)) will be two circularly polarized signals with a timing difference. In this case, the longitudinal and transverse polarization components of the probe light Lb separated by the polarization separation element 19 become equal, and the difference value output from the balance detector 31 becomes zero. Therefore, even though a change in refractive index occurs in the terahertz wave detection unit 23, the polarization state of the probe light Lb detected by the photodetector 13 will be the same as in the case of linear polarization, and it is conceivable that a detection result of 0 for the electric field intensity of the terahertz wave T will be obtained.

[0049] In contrast, in the spectroscopic measurement device 1, as shown in Figure 6(a), a λ / 8 wave plate 24 is positioned between the terahertz wave detection unit 23 and the GRIN lens 20B (see Figure 2) in the measurement unit 3, on the optical path from the terahertz wave detection unit 23 toward the photodetector 13. In this configuration, as shown in Figure 6(b), the polarization state of the probe light Lb (A in Figure 6(a)) heading from the circulator 18 toward the λ / 8 wave plate 24 is linearly polarized, while the polarization state of the probe light Lb (B in Figure 6(a)) modulated by the terahertz wave detection unit 23 becomes elliptical polarization with a phase corresponding to the change in refractive index.

[0050] The polarization state of the probe light Lb (C in Figure 6(a)) that is reflected by the terahertz wave detection unit 23 and passes through the λ / 8 wave plate 24 again becomes elliptic polarization with different ellipticities, while the polarization state of the probe light Lb (D in Figure 6(a)) that travels from the circulator 18 towards the polarization separation element 19 becomes two linearly polarized signals with different timings. When the difference between the longitudinal and transverse polarization components of the probe light Lb separated by the polarization separation element 19 is detected by the balance detector 31, the value is non-zero, so the electric field waveform of the terahertz wave T based on the polarization state of the probe light Lb can be easily reconstructed.

[0051] The timing difference between the two linearly polarized signals in the probe light Lb is generally on the order of picoseconds. Therefore, in balance detectors below the MHz band, which are commonly used for this type of spectroscopic measurement, the timing difference between the two linearly polarized signals in the probe light Lb is not separated, and they are treated as if they were input to the balance detector simultaneously, and difference detection is performed. Consequently, even if the linearly polarized component of the probe light Lb after passing through the λ / 8 waveplate 24 is separated in time by the action of the polarization-maintaining fiber F, it does not affect the measurement of the terahertz wave T.

[0052] As explained above, in the spectroscopic measurement device 1, the change in refractive index of the terahertz wave detection unit 23 due to the electro-optic effect associated with the input of a terahertz wave T is sampled by the probe light Lb. This makes it possible to obtain the electric field waveform of the terahertz wave T acting on the object being measured, and to obtain information about the object being measured based on the electric field waveform. In addition, in the spectroscopic measurement device 1, the measurement unit 3 is optically connected to the main unit 2 by a polarization-maintaining fiber F. This makes it possible to freely position the measurement unit 3 (measurement surface M) relative to the main unit 2, thereby improving the accessibility of the measurement unit 3 to the object being measured.

[0053] The limitations on the shape and orientation of the object being measured that can access the measurement unit 3 can be eliminated, and the number of devices and manufacturing lines that can incorporate the spectroscopic measurement device 1 will increase, thereby expanding the applications of the spectroscopic measurement device 1 in industrial fields. In the spectroscopic measurement device 1, the polarization state of the probe light Lb input to the terahertz wave detection unit 23 is maintained by using a polarization-maintaining fiber F, so that the detection efficiency of the probe light Lb in the photodetector 13 can also be maintained.

[0054] Furthermore, in the spectroscopic measurement device 1, since the terahertz wave detection unit 23 is made of an electro-optic crystal, electromagnetic compatibility (EMC) measures are unnecessary. For example, if the terahertz wave detection unit 23 is made of a photoconductive antenna, the terahertz wave detection unit 23 utilizes electrical signals, so the measurement unit 3 needs to be designed with a housing 4 that includes electromagnetic shielding in addition to noise countermeasures. In contrast, in electro-optic sampling where the terahertz wave detection unit 23 is made of an electro-optic crystal, the terahertz wave detection unit 23 does not utilize electrical signals, so the housing 4 of the measurement unit 3 can be made of plastic or the like without electromagnetic shielding.

[0055] The spectroscopic measurement device 1 includes a waveplate positioned on the optical path toward the photodetector 13 for probe light Lb modulated by the terahertz wave detection unit 23. In this embodiment, the waveplate is a λ / 8 waveplate 24, and in the measurement unit 3, terahertz waves T are incident from one side of the terahertz wave detection unit 23, and probe light Lb is incident from the other side of the terahertz wave detection unit 23 and reflected toward the photodetector 13. With this configuration, for example, if the initial polarization state of the probe light Lb is linearly polarized, by causing the probe light Lb to be incident on the λ / 8 waveplate 24 twice, before and after input to the terahertz wave detection unit 23, it becomes possible to appropriately separate the polarization component of the probe light Lb into a longitudinal polarization component and a transverse polarization component. Therefore, the detection efficiency of the probe light Lb in the photodetector 13 can be suitably ensured.

[0056] In the spectroscopic measurement device 1, at least one of the optical components constituting the main body 2 is optically connected by a polarization-maintaining fiber F. In this embodiment, all optical components constituting the main body 2, except for the optical path length adjustment units 15A and 15B, are fiber-based optical components, and all of these components are optically connected by polarization-maintaining fibers F. This increases the degree of freedom in arranging the optical components within the main body 2. This also contributes to miniaturization of the main body 2.

[0057] The spectroscopic measurement device 1 includes optical path length adjustment units 15A and 15B, which adjust the difference in optical path length between the pump light La and the probe light Lb, and the main body 2 is configured accordingly. This allows for flexible adjustment of the timing of input of the terahertz wave T and the probe light Lb to the terahertz wave detection unit 23. For example, by sweeping the input timing of the probe light Lb to the terahertz wave detection unit 23 using the optical path length adjustment unit 15A, the electric field waveform of the terahertz wave T acting on the object being measured can be suitably acquired.

[0058] In the spectroscopic measurement device 1, each of the optical path length adjustment units 15A and 15B is configured to include multiple mirrors and a stage for moving the mirrors, allowing the pump light La or probe light Lb to propagate in free space without relying on optical fibers. This allows the optical path length adjustment units 15A and 15B to be constructed with a simple configuration. Furthermore, in the spectroscopic measurement device 1, the light source unit 11 is configured with a fiber laser. This enables miniaturization of the light source unit 11, and further miniaturization of the main unit 2.

[0059] This disclosure is not limited to the embodiments described above. For example, in the above embodiments, the waveplate is composed of a λ / 8 waveplate 24, and in the measurement unit 3, a terahertz wave T is incident from one side of the terahertz wave detection unit 23, and probe light Lb is incident from the other side of the terahertz wave detection unit 23 and reflected to the photodetector 13 side. However, the configuration of the measurement unit 3 can take other forms.

[0060] For example, as shown in Figure 7, the waveplate may be composed of a λ / 4 waveplate 41, and in the measurement unit 3, the terahertz wave T and probe light Lb may be incident from one side of the terahertz wave detection unit 23, and the probe light Lb may also be transmitted to the other side of the terahertz wave detection unit 23 toward the photodetector 13. With this configuration, for example, if the initial polarization state of the probe light Lb is linearly polarized, by causing the probe light Lb that has passed through the terahertz wave detection unit 23 to be incident once on the λ / 4 waveplate 41, the polarization state of the probe light Lb toward the polarization separation element 19 can be made into two linearly polarized states with a timing difference. Therefore, it becomes possible to appropriately separate the polarization component of the probe light Lb into a longitudinal polarization component and a transverse polarization component, and the detection efficiency of the probe light Lb in the photodetector 13 can be suitably ensured.

[0061] In the example shown in Figure 7, a fiber coupling element 42 is positioned downstream of the λ / 4 wave plate 41 to guide the probe light Lb modulated by the terahertz wave detection unit 23 from the measurement unit 3 to the main unit 2. The fiber coupling element 42 is composed of, for example, a lens, and directs the probe light Lb that has passed through the λ / 4 wave plate 41 into the polarization-maintaining fiber F that optically connects the main unit 2 and the measurement unit 3.

[0062] Furthermore, in the above embodiment, each of the optical path length adjustment units 15A and 15B is configured to include a plurality of mirrors and a stage for moving the positions of the mirrors, and the pump light La or probe light Lb propagates in free space without relying on optical fibers. However, at least one of the optical path length adjustment units 15A and 15B may be configured with polarization-maintaining fiber F. In this case, the degree of freedom in arranging optical components within the main body 2 can be further increased. In addition, the main body 2 can be further miniaturized.

[0063] One example of a configuration in which the optical path length adjustment sections 15A and 15B are made using polarization-maintaining fibers F is to wind a polarization-maintaining fiber F of a predetermined length around a piezoelectric element. With this configuration, the fiber length of the polarization-maintaining fiber F wound around the piezoelectric element can be adjusted by applying a voltage to the piezoelectric element and changing its volume. Therefore, the optical path length of the pump light La or probe light Lb can be controlled with high precision.

[0064] Furthermore, in the above embodiment, a dispersion correction patch cord 16A is placed in the optical guide optical system 12A of the pump light La, and a dispersion correction patch cord 16B is placed in the optical guide optical system 12B of the probe light Lb. However, the dispersion correction patch cords 16A and 16B do not necessarily have to be used. In this case, the spectroscopic measurement device 1 can be configured using a laser light source that has been pre-dispersed so that the pulse widths of the pump light La and probe light Lb, which have propagated a certain distance, are as short as possible when they are incident on the terahertz wave generation unit 21 and the terahertz wave detection unit 23. Alternatively, a polarization-maintaining patch cord may be used instead of the dispersion correction patch cords 16A and 16B, or a combination of a polarization-maintaining patch cord and a dispersion correction patch cord may be used.

[0065] Furthermore, in the above embodiment, in the measurement unit 3, waveplates (λ / 8 waveplate 24, λ / 4 waveplate 41) are arranged on the optical path of the probe light Lb modulated by the terahertz wave detection unit 23 toward the photodetector unit 13. However, other elements capable of adjusting the polarization state of the probe light Lb may be placed instead of waveplates. Examples of such elements include a Faraday rotator. In the case of a configuration in which the probe light b does not travel back and forth across the waveplate, as shown in the example in Figure 7, a polarization rotator (e.g., a quartz polarization rotator) can also be used.

[0066] Furthermore, while the above embodiment illustrates a method of measurement based on total reflection attenuation spectroscopy (ATR) with the object to be measured in contact with the measurement surface M, the measurement method in the spectroscopic measurement device 1 is not limited to this. For example, a terahertz wave T emitted from the measurement surface M into free space may be irradiated onto the object to be measured, and the reflected light may be incident on the measurement surface M again and detected by the terahertz wave detection unit 23. Alternatively, a terahertz wave T emitted from the measurement surface M into free space may be irradiated onto the object to be measured, and the transmitted light may be reflected by a mirror or the like and incident on the measurement surface M again and detected by the terahertz wave detection unit 23.

[0067] Alternatively, the measuring unit 3 may be configured without a measuring surface M on its housing 4, with the object to be measured being placed inside the housing 4. In this case, for example, the measuring prism 22 may be omitted from the measuring unit 3, and instead, the object to be measured may be placed between the terahertz wave generating unit 21 and the terahertz wave detection unit 23. The terahertz wave T generated by the terahertz wave generating unit 21 may be irradiated onto the object to be measured, and the transmitted light may be detected by the terahertz wave detection unit 23. [Explanation of symbols]

[0068] 1...Spectroscopic measurement device, 2...Main unit, 3...Measurement unit, 11...Light source unit, 13...Photodetector unit, 15A, 15B...Optical path length adjustment unit (adjustment unit), 21...Terahertz wave generation unit, 23...Terahertz wave detection unit, 24...λ / 8 wave plate (waveplate), 41...λ / 4 wave plate (waveplate), F...Polarization-maintaining fiber, L...Laser light, La...Pump light, Lb...Probe light, M...Measurement surface, T...Terahertz wave.

Claims

1. A light source unit that outputs pump light and probe light, A terahertz wave generating unit that generates terahertz waves upon input of light from the pump, A terahertz wave detection unit receives the terahertz wave and the probe light respectively, and modulates the probe light based on the refractive index, as the refractive index changes due to the electro-optic effect associated with the input of the terahertz wave. The system comprises a photodetector that detects the probe light modulated by the terahertz wave detection unit, A main body comprising the light source unit and the light detection unit, The measurement unit comprises the terahertz wave generation unit and the terahertz wave detection unit, The main body and the measuring unit are optically connected by a polarization-maintaining fiber. The measurement unit further includes a waveplate positioned on the optical path toward the photodetector, through which the probe light modulated by the terahertz wave detection unit is directed. In the measurement unit, the probe light that has passed through the waveplate is incident on the polarization-maintaining fiber that optically connects the main body and the measurement unit. The aforementioned waveplate is a waveplate that gives a phase difference of λ / 8 between the two vertically polarized components of incident light. In the measurement unit, the terahertz wave is incident from one side of the terahertz wave detection unit, and the probe light that has passed through the waveplate is incident from the other side of the terahertz wave detection unit, and the probe light that is reflected by the terahertz wave detection unit and heads toward the photodetector from the other side of the terahertz wave detection unit passes through the waveplate again, in a spectroscopic measurement device.

2. A light source unit that outputs pump light and probe light, A terahertz wave generating unit that generates terahertz waves upon input of light from the pump, A terahertz wave detection unit receives the terahertz wave and the probe light respectively, and modulates the probe light based on the refractive index, as the refractive index changes due to the electro-optic effect associated with the input of the terahertz wave. The system comprises a photodetector that detects the probe light modulated by the terahertz wave detection unit, A main body comprising the light source unit and the light detection unit, The measurement unit comprises the terahertz wave generation unit and the terahertz wave detection unit, The main body and the measuring unit are optically connected by a polarization-maintaining fiber. The measurement unit further includes a waveplate positioned on the optical path toward the photodetector, through which the probe light modulated by the terahertz wave detection unit is directed. In the measurement unit, the probe light that has passed through the waveplate is incident on the polarization-maintaining fiber that optically connects the main body and the measurement unit. The aforementioned waveplate is a waveplate that gives a phase difference of λ / 4 between the two vertically polarized components of incident light. The measurement unit is a spectroscopic measurement device in which the terahertz wave and the probe light are incident from one side of the terahertz wave detection unit, and the probe light is transmitted to the other side of the terahertz wave detection unit and directed toward the light detection unit side.

3. The spectroscopic measurement apparatus according to claim 1 or 2, wherein the measurement unit has a measurement surface on the optical path of the terahertz wave between the terahertz wave generation unit and the terahertz wave detection unit.

4. The spectroscopic measuring apparatus according to any one of claims 1 to 3, wherein at least one of the optical components constituting the main body is optically connected by a polarization-maintaining fiber.

5. The spectroscopic measuring apparatus according to any one of claims 1 to 4, wherein the main body includes an adjustment unit for adjusting the difference in optical path length between the pump light and the probe light.

6. The spectroscopic measuring apparatus according to claim 5, wherein the adjustment unit comprises a plurality of mirrors and a stage for moving the positions of the mirrors, and the pump light or the probe light propagates in free space without relying on optical fibers.

7. The spectroscopic measurement apparatus according to claim 5, wherein the adjustment unit is composed of polarization-maintaining fibers.

8. The spectroscopic measurement apparatus according to any one of claims 1 to 7, wherein the light source is configured with a fiber laser.

9. The spectroscopic measurement apparatus according to any one of claims 1 to 8, wherein the laser light output from the light source is ultrashort pulse light on the order of femtoseconds.

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