US20200371023A1 - Far-Infrared Light Source and Far-Infrared Spectrometer - Google Patents
Far-Infrared Light Source and Far-Infrared Spectrometer Download PDFInfo
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- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
- G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/35—Non-linear optics
- G02F1/39—Non-linear optics for parametric generation or amplification of light, infrared or ultraviolet waves
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- G01N2201/064—Stray light conditioning
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- G02F2203/00—Function characteristic
- G02F2203/13—Function characteristic involving THZ radiation
Definitions
- the present invention relates to a far-infrared light source.
- Electromagnetic wave in a far-infrared region of wavelengths approximately ranging from 25 ⁇ m to 4 mm is also called teraherz wave which combines features of penetrating property of electric wave and rectilinearity of light.
- An absorption spectrum in the far-infrared region has peaks specific to many substances and hence, the electromagnetic wave in the far-infrared region is expected to be effectively used for identification of substances.
- no compact, convenient light source for emission in this wavelength region has been available.
- a detector was also hard to use because of the necessity of cooling with liquid helium. Accordingly, the electromagnetic wave in the far-infrared region has been used in only a limited research application.
- a far-infrared spectrometer includes: a wave-length variable far-infrared light source for emitting a first far-infrared light; an illumination optical system for illuminating a sample with the first far-infrared light; a nonlinear optical crystal for detection which converts a second far-infrared light from the sample to a near-infrared light by means of a pump light; and a far-infrared light imaging optical system for forming an image of the sample at the nonlinear optical crystal for detection, and where an irradiation position of the first far-infrared light on the sample is independent of the wavelength of the first far-infrared light (see the abstract herein).
- Patent Literature 1 W02017/013759A1
- Non-Patent Literature 1 Kosuke Murate et. Al., “Expansion of the tuning range of injection-seeded terahertz-wave parametric generator up to 5 THz”, Appl. Phys. Express 9,082401 (2016)
- an emission direction of the far-infrared light changes in conjunction with the change in the frequency of the far-infrared light. This results in the change of location irradiated with the far-infrared light on the sample and decrease in the accuracy of quantitative analysis. Particularly, in a case where the bandwidth of the far-infrared light is expanded to perform measurement in a 1 to 5 THz range, the emission direction of the far-infrared light significantly changes, making it difficult to properly configure the optical system.
- the invention has an object to provide a far-infrared light source adapted to reduce shift in the location irradiated with the far-infrared light even when the frequency of the far-infrared light is changed.
- the far-infrared light source of the invention is configured such that the variation in the emission angle of the far-infrared light in the nonlinear optical crystal in conjunction with the change in the frequency of far-infrared light is substantially compensated by the variation in the refractive angle of the far-infrared light at the interface between the nonlinear optical crystal and a prism in conjunction with the change in the frequency of the far-infrared light.
- the far-infrared light source according to the invention is adapted to reduce the shift in the irradiation position of the far-infrared light despite the change in the frequency of the far-infrared light.
- the far-infrared light source of the invention enables broadband spectroscopic measurement.
- FIG. 1 is a configuration diagram showing a far-infrared light source 100 according to a first embodiment of the invention
- FIG. 2 is a vector diagram showing a relation of far-infrared light 145 , seed light 125 and pump light 115 ;
- FIG. 3 is a graph showing a relation of an angle ⁇ T_p formed by a direction K T of the far-infrared light 145 with respect to a pump light direction (k p ) in a nonlinear optical crystal 131 ;
- FIG. 4 is a graph showing a relation between the frequency of far-infrared light 145 and refractive index
- FIG. 5 is a diagram showing refraction of the far-infrared light 145 at an interface between a nonlinear optical crystal 131 and a Si prism 140 ;
- FIG. 6 is a graph showing the results of ⁇ T_LN calculation on plural ⁇ T_Si ;
- FIG. 7 is a graph showing an example where ⁇ T_p and ⁇ T_LN have substantially the same change characteristic for frequency
- FIG. 8 is a vector diagram showing a relation between ⁇ T_p and ⁇ T_LN ;
- FIG. 9 is a graph showing differences between ⁇ T_LN and ⁇ T_p when ⁇ T_Si is 57° and 50°;
- FIG. 10 is a graph showing changes in ⁇ T_Si when ⁇ p is 4.5° and 0°;
- FIG. 11 is a configuration diagram showing a far-infrared light source 100 according to a second embodiment of the invention.
- FIG. 12 is a graph showing an example where both a direction ⁇ T_s of the far-infrared light 145 in the nonlinear optical crystal 131 with respect to a direction ⁇ S of the seed light 125 , and the direction ⁇ T_LN have substantially the same change characteristic for frequency;
- FIG. 13 is a vector diagram showing a relation between ⁇ T_s and ⁇ T_LN ;
- FIG. 14 is a configuration diagram showing a far-infrared spectrometer 1000 according to a third embodiment of the invention.
- FIG. 15 is a configuration diagram showing a far-infrared spectrometer 1000 according to a fourth embodiment of the invention.
- FIG. 1 is a configuration diagram of a far-infrared light source 100 according to a first embodiment of the invention.
- the far-infrared light source 100 includes: a light source 110 , a light source 120 , an optical system 121 , a nonlinear optical crystal 131 , a Si prism 140 , a rotary stage 138 and a linear stage 139 .
- the light source 110 emits a pump light 115 .
- the pump light 115 is incident on a nonlinear optical crystal 131 .
- the light source 110 can be employed, for example, a short pulse Q-switch YAG laser.
- the light source 120 emits a seed light 125 .
- the seed light 125 is incident on the nonlinear optical crystal 131 via the optical system 121 .
- the light source 120 can be employed, for example, a wavelength tunable semiconductor laser.
- the optical system 121 includes: a lens 122 , a light deflector 123 and an imaging optical element 124 .
- the seed light 125 is so guided by an optical fiber as to be illuminated onto the lens 122 . Passing through the lens 122 and the light deflector 123 , the seed light forms a beam waist in vicinity of a front focal plane of the imaging optical element 124 . In this configuration, the seed light 125 through the imaging optical element 124 is incident on the nonlinear optical crystal 131 as a beam having a long Rayleigh length (namely, beam in almost collimated state).
- the imaging optical element 124 is configured to image the seed light 125 deflected by the light deflector 123 on an entering surface of the nonlinear optical crystal 131 .
- an incident position with respect to the entering surface of the nonlinear optical crystal 131 does not change but only an incidence angle changes.
- the light deflector 123 may be employed a reflection light deflector such as Galvano-mirror and MEMS mirror or a transmission light deflector. Namely, the light deflector 123 can be employed in any type that can control the angle of the seed light 125 .
- the imaging optical element 124 can be employed, for example, a concave mirror. Any other optical element (such as lens) is also usable if such an element is capable of imaging the light deflected by the light deflector 123 on the entering surface of the nonlinear optical crystal 131 .
- the reflection light deflector such as Galvano-mirror is used as the light deflector 123 and the concave mirror is used as the imaging optical element 124 , the optical system 121 can be made compact because an optical path is folded in this configuration.
- a transmission light deflector may be used as the light deflector 123 and a lens may be used as the imaging optical element 124 .
- either one of the light deflector 123 and the imaging optical element 124 may be constituted by a reflection optical element and the other may be constituted by a transmission optical element.
- the nonlinear optical crystal 131 may be employed, for example, a rod of MgO:LiNbO 3 crystal (LN crystal) having a length of about 50 mm.
- the nonlinear optical crystal 131 is mounted on a stage constituted by the rotary stage 138 and the linear stage 139 .
- the rotary stage 138 angularly tilts the nonlinear optical crystal 131 so as to allow the pump light 115 to form a predetermined angle ⁇ p to a side plane of the nonlinear optical crystal 131 .
- the linear stage 139 adjusts a y-position of the nonlinear optical crystal 131 ( FIG.
- the rotary stage 138 and the linear stage 139 may be employed in a suitable mechanism that can implement such a function.
- the Si prism 140 is attached to a side wall of the nonlinear optical crystal 131 .
- the Si prism is capable of reducing the reflection of far-infrared light from the side plane of the nonlinear optical crystal 131 and efficiently extracting the far-infrared light from the nonlinear optical crystal 131 .
- the Si prism 140 and the nonlinear optical crystal 131 are so configured as to allow the far-infrared light to enter substantially at right angles to the exiting surface of the Si prism 140 . This configuration is adapted to reduce variation in the emission direction of the far-infrared light when the wavelength of the far-infrared light is changed.
- the pump light 115 is so powerful that a reflected light thereof can have sufficient power for damaging the optical system of the seed light 125 . It is effective to insert an optical isolator in the optical system 121 so as to attenuate return light. However, this approach is feasible only in cases where the cost of the isolator is acceptable or there are more than enough mounting spaces.
- the above-described configuration is capable of extracting a strong far-infrared light even from a high frequency range of 3 THz or more for the following reason.
- a far-infrared light generated due to parametric generation is absorbed by the nonlinear optical crystal 131 particularly in the high frequency range of 3 THz or more.
- the pump light close to the side plane of the nonlinear optic crystal 131 , the far-infrared light can be extracted while the generated far-infrared light is not attenuated so much by absorption.
- the far-infrared light can be generated in conjunction with difference frequency generation or parametric generation which is induced by making two laser beams of different wavelengths (pump light 115 and seed light 125 ) incident on the nonlinear optical crystal 131 at predetermined angles.
- the first embodiment provides an example where a LN crystal was used as the nonlinear optical crystal 131 , and the far-infrared light was generated by means of parametric generation.
- a wavelength of the seed light 125 , and an angle ⁇ between the seed light 125 and the pump light 115 were so set as to satisfy the following equation 1 and equation 2.
- the wavelength ( ⁇ T ) of the far-infrared light 145 generated in the nonlinear optical crystal 131 can be calculated on the basis of principle of energy conservation by applying the frequency ⁇ p of the pump light 115 and the frequency ⁇ S of the seed light 125 to the following equation 1, where ⁇ denotes angular frequency.
- FIG. 2 is a vector diagram showing a relation of the far-infrared light 145 , the seed light 125 and the pump light 115 .
- the generation efficiency of the far-infrared light 145 increases in a case where the momentum conservation low holds. Specifically, a high generation efficiency can be obtained when the following relational equation 2 (phase matching condition) is established among the emission direction of far-infrared light 145 /the direction of pump light 115 /the direction of seed light 125 .
- k T , k P , k S denotes a wavenumber vector of the far-infrared light 145 , the pump light 115 and the seed light 125 , respectively.
- the far-infrared light can be generated with high efficiency by setting the wavelength and the incident direction ( ⁇ ) of the seed light 125 so as to satisfy the above conditions.
- the following description is made on a configuration which is adapted to suppress the change in the emission direction of the far-infrared light 145 irrespective of the change in the frequency of the far-infrared light 145 , when the above-described conditions are satisfied.
- the far-infrared light source 100 of the first embodiment is adapted to change the frequency of the generated far-infrared light 145 in the range of about 0.5 THz to 5 THz by changing the wavelength of the seed light 125 is changed in the range of 1066 nm to 1084 nm and adjusting the incidence angle of the seed light 125 to the nonlinear optical crystal 131 is adjusted by the optical system 121 .
- the following description is made on assumption that the frequency of the far-infrared light 145 is changed in this range.
- FIG. 3 is a graph showing a relation of an angle ⁇ T_p formed between a direction K T of the far-infrared light 145 and a pump light direction (k p ) in a nonlinear optical crystal 131 .
- the angle ⁇ T_p depends upon the frequency of the far-infrared light 145 .
- This example indicates that the emission direction of the far-infrared light 145 varies by 6.7° in conjunction with the change in the frequency of the far-infrared light 145 in the range of 0.5 THz to 5 THz.
- refractive indexes n of the nonlinear optical crystal 131 for the pump light 115 and the seed light 125 at a wavelength ⁇ were calculated using the following equation 3 and according to literature (Zelmon et al. JOSA B Vol. 14, NO. 12, pp. 3319 to 3322).
- a refractive index n NL of the nonlinear optical crystal 131 for the far-infrared light at a wavelength ⁇ T was calculated using the following equation 4 and according to literature (D. R. Bosomworth et al. Appl. Phys. Lett. No. 9, pp. 330 (1996)).
- FIG. 4 is a graph showing a relation between the frequency of far-infrared light 145 and the refractive index n NL .
- the graph of FIG. 4 represents the calculation results based on the equation 4.
- FIG. 5 is a diagram showing refraction of the far-infrared light 145 at an interface between the nonlinear optical crystal 131 and the Si prism 140 .
- the far-infrared light 145 generated in the nonlinear optical crystal 131 is incident on the Si prism 140 .
- ⁇ T_LN denotes a traveling direction of the far-infrared light 145 in the nonlinear optical crystal 131
- ⁇ T_Si denotes a traveling direction of the far-infrared light 145 in the Si prism 140 .
- a refractive angle at the interface is calculated according to Snell's law represented by the following equation 5 using the above refractive index n NL and the refractive index n Si of the Si prism 140 for the far-infrared light wavelength ⁇ T .
- n Si ⁇ ⁇ sin ⁇ ( ⁇ 2 - ⁇ T ⁇ ⁇ _ ⁇ ⁇ Si ) n NL ⁇ ⁇ sin ⁇ ( ⁇ 2 - ⁇ T ⁇ ⁇ _ ⁇ ⁇ L ⁇ ⁇ N ) ( 5 )
- FIG. 6 is a graph showing the results of ⁇ T_LN calculation made on plural directions ⁇ T_Si .
- the frequency range of about 0.5 THz to 5 THz of the far-infrared light 145 the wavelength dependency of the Si refractive index is low so that a good approximation by n Si ⁇ 4.3 holds.
- ⁇ T_LN for given ⁇ T_Si can be calculated by using the equation 4 and the equation 5.
- the direction O T LN was calculated for each of 42°, 50°, 57° and 60° of ⁇ T_Si .
- ⁇ T_p and ⁇ T_LN respectively have a specific characteristic to frequency change of the far-infrared light 145 . If a condition that substantially equalizes these characteristics with each other can be found, then under such a condition, both the direction ⁇ T_p and the direction ⁇ T_LN change substantially the same way with the frequency change of the far-infrared light 145 . By previously setting a difference between these parameters as ⁇ p under such a condition, the angle of the far-infrared light 145 emitted from the Si prism 140 is substantially equalized at all times irrespective of the frequency change of the far-infrared light 145 . In the following description, a condition that establishes such a relation is found according to FIG. 3 and FIG. 6 .
- the variation width of ⁇ T_LN with respect to frequency differs according to the value of ⁇ T_Si . Therefore, it is considered that a ⁇ T_Si value that provides a variation width of ⁇ T_LN to frequency equal to the variation width of 6.7° as shown in FIG. 3 can be found. Specifically, when a difference of ⁇ T_p (6.7° as shown in FIG. 3 ) at opposite ends of the frequency range of the far-infrared light 145 is equal to a difference of ⁇ T_LN at opposite ends of the frequency range of the far-infrared light 145 (a difference between an upper end and a lower end of each curve as shown in FIG.
- ⁇ T_p and ⁇ T_LN can be said to have substantially the same change characteristic for the frequency change of the far-infrared light 145 .
- ⁇ T_Si 57°
- both characteristic curve exhibits the variation width of 6.7°. Therefore, it is apparent that this condition is satisfied.
- FIG. 7 shows an example where ⁇ T_p and ⁇ T_LN have substantially the same change characteristic for frequency.
- the dotted line is the characteristic curve shown in FIG. 3 .
- both ⁇ T_p and ⁇ T_LN have substantially the same change characteristic for the frequency of the far-infrared light 145 .
- ⁇ T_Si 57°
- ⁇ T_p and ⁇ T_LN have substantially the same change characteristic for the frequency of the far-infrared light 145 .
- FIG. 8 is a vector diagram showing a relation between ⁇ T_p and ⁇ T_LN .
- a difference between ⁇ Tp and ⁇ T_LN corresponds to 4.5° shown in FIG. 7 .
- ⁇ T_Si can be substantially maintained constant by conforming ⁇ P to this difference.
- FIG. 9 is a graph showing differences between ⁇ T_LN and ⁇ T_p when ⁇ T_Si is at 57° and at 50°. The graph indicates that the difference in the former case is substantially maintained constant at 4.5° but the difference in the latter case varies by 1.5° with the frequency change.
- FIG. 10 is a graph showing changes of ⁇ T_Si when ⁇ p is 4.5° and 0°.
- This angular change is further expanded when the light is emitted into the air from the Si prism 140 .
- this phenomenon has been the cause of extremely difficult optical system design of the far-infrared spectrometers.
- the invention is advantageous from the viewpoint of optical system design because the invention is adapted to reduce this angular change.
- the first embodiment is configured to previously determine such a ⁇ T_Si (emission direction of the far-infrared light 145 ) as to ensure that both ⁇ T_p (the angle of the far-infrared light 145 to the pump light 115 ) and ⁇ T_LN (the angle of the far-infrared light 145 to the interface between the nonlinear optical crystal 131 and the Si prism 140 ) substantially equally change when the wavelength of the far-infrared light 145 is changed.
- the rotary stage 138 has the difference between ⁇ T_p and ⁇ T_LN as ⁇ p (the incidence angle of the pump light 115 to the nonlinear optical crystal 131 ).
- the far-infrared light 145 can be emitted into the air substantially at constant angle.
- FIG. 11 is a configuration diagram showing a far-infrared light source 100 according to a second embodiment of the invention.
- the second embodiment differs from the first embodiment in that the incidence angle of the seed light 125 is fixed while the incidence angle of the pump light 115 is variable by means of the optical system 121 .
- the other components are the same as those of the first embodiment and hence, the following description is principally made on differences.
- a lens system 127 is employed in the optical system 121 in place of the lens 122 .
- the lens system 127 expands the pump light 125 before making the light incident on the light deflector 123 .
- the wavelength of the far-infrared light 145 is changed by adjusting the incidence angle of the pump light 115 in a manner to satisfy the equation 1 and the equation 2 according to the change in the wavelength of the seed light 125 .
- the second embodiment is also adapted to maintain the emission direction of the far-infrared light substantially constant by changing the angle of the nonlinear optical crystal 131 to the pump light 115 just as in the first embodiment.
- FIG. 12 is a graph showing an example where both the direction ⁇ T_s of the far-infrared light 145 in the nonlinear optical crystal 131 with respect to the direction ⁇ S of the seed light 125 , and the direction ⁇ T_LN have substantially the same change characteristic for frequency.
- FIG. 13 is a vector diagram showing a relation between ⁇ T_s and ⁇ T_LN .
- the incidence angle of the seed light 125 is fixed.
- the traveling direction of each light is expressed by way of the relation between ⁇ T_s and ⁇ T_LN .
- the fact also remains in the second embodiment that the difference still exists between ⁇ T_p and ⁇ T_LN .
- the notation in FIG. 13 is for convenience in description.
- the second embodiment it is important to incline the entering surface and the exiting surface of the nonlinear optical crystal 131 at an angle ⁇ (on the order of 80 to 89°) to the side plane thereof. This is effective to prevent the pump light 115 specularly reflected by the entering surface or exiting surface of the nonlinear optical crystal 131 from being incident on the optical system of the seed light 125 .
- FIG. 14 is a configuration diagram showing a far-infrared spectrometer 1000 according to a third embodiment of the invention.
- the far-infrared spectrometer 1000 is an apparatus for measuring an absorption spectrum of a sample 200 by using light through the sample 200 , for example.
- the far-infrared spectrometer 1000 includes: a far-infrared light source 100 ; an illumination optical system 151 ; a stage 202 ; a far-infrared light imaging optical system 240 ; a nonlinear optical crystal 132 ; a photoelectric detector 290 ; a signal processor 400 ; and a control unit 500 .
- the illumination optical system 151 illuminates the sample 200 with the far-infrared light emitted from the far-infrared light source 100 .
- the stage 202 carries the sample 200 thereon.
- the far-infrared light imaging optical system 240 images the far-infrared light transmitted through the sample 200 on the nonlinear optical crystal 132 .
- the nonlinear optical crystal 132 converts the far-infrared light transmitted through the sample 200 to a near-infrared light by using a pump light 235 .
- the far-infrared light source 100 includes the components described in the first embodiment.
- the light source 110 includes as principal components: (a) a short pulse Q-switch YAG laser 111 ; (b) a polarization separation system including a polarization beam splitter (hereinafter, referred to as PBS) 114 and a quarter-wave plate 116 ; and (c) an amplifier unit (solid-state amplifier 118 ) for amplifying a laser output.
- An output beam from the YAG laser 111 is collimated by a lens 112 , subjected to the polarization separation system and amplified by a solid-state amplifier 118 .
- the beam passed through the PBS 114 is amplified by means of the quarter-wave plate 116 and the solid-state amplifier 118 and is reflected by a mirror 183 .
- the reflected beam is amplified again by the solid-state amplifier 118 and is incident on the PBS 114 via the quarter-wave plate 116 .
- the beam is emitted as a pump light 115 via the PBs 114 and a mirror 184 .
- the output from the from the YAG laser 111 is amplified by the use of the solid-state amplifier 118 so that a powerful far-infrared light having a peak power on the order of kilowatts can be extracted from the nonlinear optical crystal 131 .
- the position of the linear stage 139 is set such that the pump light 115 is reflected by the side plane of the nonlinear optical crystal 131 at a location slightly toward the entering side from the exiting surface of the nonlinear optical crystal 131 .
- the illumination optical system 151 can be constituted by, for example, one cylindrical lens 180 and one condenser lens 190 .
- the far-infrared light generated in the nonlinear optical crystal 131 is emitted from a linear light emission region along the beam of the pump light 115 , forming parallel rays as seen in a drawing surface of FIG. 14 or a spread beam as seen in a plane vertical to the drawing surface.
- the cylindrical lens 180 having an optical power only in an in-plane direction vertical to the drawing surface is used for collimating the beam in the in-plane direction vertical to the drawing surface.
- a parallel pencil is formed.
- the parallel pencil is focused onto the sample 200 by the condenser lens 190 so that a spot on the sample 200 is irradiated with the light.
- the far-infrared light transmitted through the sample 200 is guided by the far-infrared light imaging optical system 240 to the nonlinear optical crystal 132 via a Si prism 142 .
- the far-infrared light imaging optical system 240 is constituted by two condenser lenses 220 , 230 and one mirror 225 , serving as an imaging optical system for forming an image of a surface of the sample 200 in the nonlinear optical crystal 132 .
- the far-infrared light transmitted through the sample 200 is collimated by the condenser lens 220 , reflected by the mirror 225 , and focused in the nonlinear optical crystal 132 by the condenser lens 230 via the Si prism 142 .
- the incidence angle formed by the far-infrared light through the sample 200 to the nonlinear optical crystal 132 in the Si prism 42 is optimized similarly to the angle ⁇ P of the nonlinear optical crystal 131 to the pump light 115 .
- the far-infrared light is made to travel through the nonlinear optical crystal 132 substantially at a constant angle irrespective of the change in the wavelength of the far-infrared light.
- the pump light 235 split from the beam of the pump light 115 is made incident on the nonlinear optical crystal 132 .
- the far-infrared light transmitted through the sample 200 and guided by the nonlinear optical crystal 132 can be converted again to the near-infrared light having a wavelength in vicinity of the range from 1066 nm to 1084 nm.
- the photoelectric detector 290 photoelectrically converts the generated near-infrared light so as to output the conversion result as a detection signal.
- a far-infrared absorption spectrum of the sample 200 can be obtained by recording the detection signals while scanning the wavelength of the generated far-infrared light.
- the photoelectric detector 290 may be a photosensitive device (1D array detector) including plural photosensitive elements arranged in 1D array or a photosensitive device (2D array detector) including plural photosensitive elements arranged in 2D array.
- the 1D array detector and 2D array detector for near-infrared light are relatively easy to acquire, features quick response and operates at normal temperatures. Therefore, these detectors are suitable for industrial applications.
- the pump light 235 is made incident on the nonlinear optical crystal 132 in synchronism with the incidence of pulses of the far-infrared light transmitted through the sample 200 .
- a delay optical system such as delay optical length correction stage
- a half wave plate for adjustment of polarization direction or the like can be disposed on the optical path of the pump light 235 , as needed.
- a beam with well-organized time profile can be used as the pump light 235 . This results in enhanced wavelength conversion efficiency and detection sensitivity.
- the pump light transmitted through the nonlinear optical crystal 131 may be guided to the nonlinear optical crystal 132 for reuse.
- the detection efficiency decreases because of the degraded beam quality of the pump light 235 .
- the configuration provides efficient use of the pump light 115 for double purposes of generating the far-infrared light and for wavelength conversion to that of near-infrared light.
- the signal processor 400 retrieves a detection signal outputted from the photoelectric detector 290 .
- the signal processor 400 generates a distribution image of signals proportional to light transmitted through the sample 200 on the basis of the position of the stage 202 at the time of signal retrieval.
- the signal processor 400 is also capable of calculating the absorption spectrum by comparing the acquired image data and reference data (spectral image data without a sample) stored in a storage region of the signal processor 400 , so as to acquire two-dimensional distribution of absorption spectrum (absorption spectral image).
- the control unit 500 controls the whole system. For instance, the control unit 500 controls the far-infrared light source 100 , the stage 202 and the signal processor 400 . As for the far-infrared light source 100 , the control unit controls the light source 110 , the optical system 121 , the rotary stage 138 , and the linear stage 139 .
- the control unit 500 also provides a user interface.
- the control unit 500 may also include a display part for displaying the signal and data (spectroscopic information) acquired by the signal processor 400 .
- control unit 500 controls the far-infrared light source 100 so as to generate a specified far-infrared light and synchronizes the transfer of the stage 202 with data acquisition by the photoelectric detector 290 .
- the far-infrared spectrometer 1000 optimizes the orientation ⁇ P of the pump light 115 in the nonlinear optical crystal 131 , so as to reduce the change in the emission direction of the far-infrared light to about 1.0° in the total angular range ( ⁇ 1 to ⁇ 2) in the air even though the frequency of the far-infrared light is changed in a broad range of 0.5 to 5 THz.
- This provides sufficient reduction of change in the irradiation position on the sample 200 , too.
- the change in the irradiation position on the sample 200 is on the order of ⁇ 0.5 mm. This value is negligible in practical use in a case where an irradiation spot size on the sample 200 exceeds 1 mm. In a case where more accurate measurement is required, this deviation can be corrected by transferring the stage 202 in the y-direction.
- the third embodiment illustrates the example where the short pulse Q-switch YAG laser 111 is used as the light source 110
- the invention is not limited to this. What is required is a narrow spectral line width and hence, a mode-locked laser, for example, may be used as the light source 110 .
- the mode-locked laser provides faster measurement because of fast repetition operation. It goes without saying that the far-infrared light source 100 described in the second embodiment is also usable in the third embodiment.
- FIG. 15 is a configuration diagram showing a far-infrared spectrometer 1000 according to a fourth embodiment of the invention.
- the fourth embodiment differs from the third embodiment in the configurations of the optical systems subsequent to the far-infrared light source 100 .
- the other components are the same as those of the third embodiment and hence, the following description is principally made on differences.
- An illumination optical system ( 150 to 190 ) for irradiating the sample 200 with the far-infrared light includes: for example, three cylindrical lenses 155 , 170 , 180 ; a slit 160 ; an angle variable mirror 165 ; and a condenser lens 190 .
- the two cylindrical lenses 155 and 170 have an optical power in a drawing surface of FIG. 15 .
- the cylindrical lens 155 is so set as to image the far-infrared light emitted from the nonlinear optical crystal 131 in vicinity of a reflection plane of the angle variable mirror 165 .
- the far-infrared light emitted from the nonlinear optical crystal 131 is changed in the emission direction on the order of one to several degrees in the drawing surface of FIG. 15 depending upon the frequency. However, the far-infrared light is not changed in the incident position on the reflection plane of the angle variable mirror 165 .
- the angle of the angle variable mirror 165 is adjusted such that the far-infrared light reflected by the angle variable mirror 165 invariably travels in the y-direction in FIG. 1 .
- the far-infrared light reflected by the angle variable mirror 165 is made to follow the same optical path irrespective of the frequency.
- the cylindrical lens 170 forms the parallel pencil in the drawing surface of FIG. 1 by collimating the far-infrared light reflected by the angle variable mirror 165 .
- the slit 160 mounted on a linear stage 161 is disposed in vicinity of a back focal plane of the cylindrical lens 155 . Scanning the frequency of the far-infrared light changes the direction of the far-infrared light 145 . This is followed by the change in a focusing position to the slit 160 .
- the linear stage 161 transfers the slit 160 in a manner to follow the change in the focusing position.
- far-infrared light components generated in the nonlinear optical crystal 131 but unnecessary for measurement (background light covering a broad band, referred to as “TPG light”) can be reduced by blocking. This results in improved SN ratio of spectral measurement across a broad frequency band.
- divergent beams emitted from the Si prism 140 are collimated by the cylindrical lens 180 .
- the condenser lens 190 applies the collimated light onto the spot on the sample 200 .
- a far-infrared light imaging optical system ( 210 to 230 ) includes a diffuser 210 in addition to the condenser lenses 220 , 230 .
- the sample 200 has high transmittance for the far-infrared light, scattering less light, the far-infrared light incident on the nonlinear optical crystal 132 may be superimposed with a coarse speckle pattern.
- the speckle pattern can be made very small by sufficiently diffusing the far-infrared light by means of multiple scattering in the diffuser 210 .
- contrast can be reduced, resulting in an enhanced stability of the detection signal.
- the variations of the detection signal may also be reduced by time-integrating the detection signals by rotating the diffuser 210 to move the speckle pattern.
- the far-infrared light is converted again to the near-infrared light having the wavelength in vicinity of the range from 1066 nm to 1084 nm by making the pump light 235 incident on the nonlinear optical crystal 132 .
- the near-infrared light is detected by the photoelectric detector 290 via a lens 270 and a ND (neutral density) filter 280.
- the far-infrared spectrometer 1000 according to the fourth embodiment of the invention is adapted to reduce the directional change of the far-infrared light 145 emitted from the far-infrared light source 100 . Furthermore, the far-infrared spectrometer 1000 according to the fourth embodiment is configured to guide the far-infrared light 145 to the angle variable mirror 165 , which cancels the directional variation of the far-infrared light 145 .
- This configuration can reduce the shift of the irradiation position of the far-infrared light due to the frequency change without using an optical system having a large aperture. Therefore, the broadband spectroscopic measurement can be accomplished by a compact optical system. Further, the invention can enhance the accuracy of quantitative analysis by obviating component distribution variations, change in signal detection efficiency and the like.
- the invention is not limited to the above-described embodiments but includes a variety of modifications.
- the foregoing embodiments, for example, are the detailed illustrations to clarify the invention.
- the invention is not necessarily limited to those including all the components described above.
- Some component of one embodiment can be replaced by some component of another embodiment.
- some component of one embodiment can be added to the arrangement of another embodiment. A part of the arrangement of each embodiment permits addition of some component of another embodiment, the omission thereof or replacement thereof.
- the above embodiments are applicable to analysis of contents of chemical components in samples, or to sample analysis by the use of light in a far-infrared region in test processes such as of exotic component or foreign matter.
- the light in the far-infrared region according to the above embodiments is light having wavelength ranging from 25 ⁇ m to 4 mm, for example. While a variety of numerical ranges of wavelength are known as the definition of the “far-infrared region”, the light in the far-infrared region in the above embodiments should be construed as the light having the widest range of the ranges defined in all areas.
- the term “terahertz wave” is included in the above-described far-infrared region.
- the difference between ⁇ T_LN and ⁇ T_p may be corrected by means of the rotary stage 138 .
- ⁇ p may be changed by rotating the stage according to the change in the wavelength of the far-infrared light. This is effective to reduce in some degree the change in the emission direction of the far-infrared light in the Si prism 140 .
- the signal processor 400 and the control unit 500 can be implemented by using hardware such as circuit devices mounting these functions, or otherwise implemented by program codes of software implementing these functions.
- a storage medium recording the program codes is provided such that an arithmetic device such as CPU (Central Processing Unit) can run the program by retrieving the program codes stored in the storage medium.
- the program codes retrieved from the storage medium per se implement the functions of the embodiment while the program codes per se and the storage medium recording the program codes constitute the invention.
- Examples of usable recording medium providing such program codes include flexible disk, CD-ROM, DVD-ROM, hard disk, optical disk, magnetic optical disk, CD-R, magnetic tape, non-volatile memory card, ROM and the like.
Abstract
Description
- The present invention relates to a far-infrared light source.
- Electromagnetic wave in a far-infrared region of wavelengths approximately ranging from 25μm to 4 mm is also called teraherz wave which combines features of penetrating property of electric wave and rectilinearity of light. An absorption spectrum in the far-infrared region has peaks specific to many substances and hence, the electromagnetic wave in the far-infrared region is expected to be effectively used for identification of substances. Heretofore, however, no compact, convenient light source for emission in this wavelength region has been available. Further, a detector was also hard to use because of the necessity of cooling with liquid helium. Accordingly, the electromagnetic wave in the far-infrared region has been used in only a limited research application.
- In the 1990's, a light source and detector employing a femtosecond laser, which is small and does not require cooling, have been put to practical use. At present, general-purpose spectrometry systems based on time domain spectroscopy have been commercially available. Research and development for applications in various fields such as security, biosensing, medical care/pharmaceutical technology, industries and agriculture are being conducted. Quantitative analysis of components is required for the implementation of such industrial applications.
- The following patent literature 1 describes about the far-infrared light source. This literature discloses the following technique where a far-infrared spectrometer includes: a wave-length variable far-infrared light source for emitting a first far-infrared light; an illumination optical system for illuminating a sample with the first far-infrared light; a nonlinear optical crystal for detection which converts a second far-infrared light from the sample to a near-infrared light by means of a pump light; and a far-infrared light imaging optical system for forming an image of the sample at the nonlinear optical crystal for detection, and where an irradiation position of the first far-infrared light on the sample is independent of the wavelength of the first far-infrared light (see the abstract herein).
- In the quantitative analysis using the time domain spectroscopy, it is difficult to obtain high output in a 1 to 3 THz bandwidth effective for the detection of hydrogen bond or molecular network. Therefore, the quantitative analysis has been faced with many problems such as difficulty in taking measurement through a shielding material such as paper or packing material and difficulty in taking measurement of powder sample intensively scattering light. On the other hand, a method using a frequency tunable coherent light source easily provides high output in the 1 to 3 THz bandwidth and is also effective for analysis through the shielding material or for the analysis of powder sample. More recently, a method expanding the bandwidth up to 5 THz has been also reported (Non-patent literature 1).
- Patent Literature 1: W02017/013759A1
- Non-Patent Literature 1: Kosuke Murate et. Al., “Expansion of the tuning range of injection-seeded terahertz-wave parametric generator up to 5 THz”, Appl. Phys. Express 9,082401 (2016)
- In the method of the patent literature 1, an emission direction of the far-infrared light changes in conjunction with the change in the frequency of the far-infrared light. This results in the change of location irradiated with the far-infrared light on the sample and decrease in the accuracy of quantitative analysis. Particularly, in a case where the bandwidth of the far-infrared light is expanded to perform measurement in a 1 to 5 THz range, the emission direction of the far-infrared light significantly changes, making it difficult to properly configure the optical system.
- In the light of the above-described problems, the invention has an object to provide a far-infrared light source adapted to reduce shift in the location irradiated with the far-infrared light even when the frequency of the far-infrared light is changed.
- The far-infrared light source of the invention is configured such that the variation in the emission angle of the far-infrared light in the nonlinear optical crystal in conjunction with the change in the frequency of far-infrared light is substantially compensated by the variation in the refractive angle of the far-infrared light at the interface between the nonlinear optical crystal and a prism in conjunction with the change in the frequency of the far-infrared light.
- The far-infrared light source according to the invention is adapted to reduce the shift in the irradiation position of the far-infrared light despite the change in the frequency of the far-infrared light. Thus, the far-infrared light source of the invention enables broadband spectroscopic measurement. The problems, components and effects other than the above will become apparent from the following description of the embodiments hereof.
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FIG. 1 is a configuration diagram showing a far-infrared light source 100 according to a first embodiment of the invention; -
FIG. 2 is a vector diagram showing a relation of far-infrared light 145,seed light 125 andpump light 115; -
FIG. 3 is a graph showing a relation of an angle θT_p formed by a direction KT of the far-infrared light 145 with respect to a pump light direction (kp) in a nonlinearoptical crystal 131; -
FIG. 4 is a graph showing a relation between the frequency of far-infrared light 145 and refractive index; -
FIG. 5 is a diagram showing refraction of the far-infrared light 145 at an interface between a nonlinearoptical crystal 131 and aSi prism 140; -
FIG. 6 is a graph showing the results of θT_LN calculation on plural θT_Si; -
FIG. 7 is a graph showing an example where θT_p and θT_LN have substantially the same change characteristic for frequency; -
FIG. 8 is a vector diagram showing a relation between θT_p and θT_LN; -
FIG. 9 is a graph showing differences between θT_LN and θT_p when θT_Si is 57° and 50°; -
FIG. 10 is a graph showing changes in θT_Si when θp is 4.5° and 0°; -
FIG. 11 is a configuration diagram showing a far-infrared light source 100 according to a second embodiment of the invention; -
FIG. 12 is a graph showing an example where both a direction θT_s of the far-infrared light 145 in the nonlinearoptical crystal 131 with respect to a direction θS of theseed light 125, and the direction θT_LN have substantially the same change characteristic for frequency; -
FIG. 13 is a vector diagram showing a relation between θT_s and θT_LN; -
FIG. 14 is a configuration diagram showing a far-infrared spectrometer 1000 according to a third embodiment of the invention; and -
FIG. 15 is a configuration diagram showing a far-infrared spectrometer 1000 according to a fourth embodiment of the invention. -
FIG. 1 is a configuration diagram of a far-infrared light source 100 according to a first embodiment of the invention. The far-infrared light source 100 includes: alight source 110, alight source 120, anoptical system 121, a nonlinearoptical crystal 131, aSi prism 140, arotary stage 138 and alinear stage 139. - The
light source 110 emits apump light 115. Thepump light 115 is incident on a nonlinearoptical crystal 131. Thelight source 110 can be employed, for example, a short pulse Q-switch YAG laser. Thelight source 120 emits aseed light 125. Theseed light 125 is incident on the nonlinearoptical crystal 131 via theoptical system 121. Thelight source 120 can be employed, for example, a wavelength tunable semiconductor laser. - The
optical system 121 includes: alens 122, alight deflector 123 and an imagingoptical element 124. Theseed light 125 is so guided by an optical fiber as to be illuminated onto thelens 122. Passing through thelens 122 and thelight deflector 123, the seed light forms a beam waist in vicinity of a front focal plane of the imagingoptical element 124. In this configuration, theseed light 125 through the imagingoptical element 124 is incident on the nonlinearoptical crystal 131 as a beam having a long Rayleigh length (namely, beam in almost collimated state). - The imaging
optical element 124 is configured to image theseed light 125 deflected by thelight deflector 123 on an entering surface of the nonlinearoptical crystal 131. When thelight deflector 123 deflects theseed light 125, therefore, an incident position with respect to the entering surface of the nonlinearoptical crystal 131 does not change but only an incidence angle changes. - The
light deflector 123 may be employed a reflection light deflector such as Galvano-mirror and MEMS mirror or a transmission light deflector. Namely, thelight deflector 123 can be employed in any type that can control the angle of theseed light 125. - The imaging
optical element 124 can be employed, for example, a concave mirror. Any other optical element (such as lens) is also usable if such an element is capable of imaging the light deflected by thelight deflector 123 on the entering surface of the nonlinearoptical crystal 131. In a case where the reflection light deflector such as Galvano-mirror is used as thelight deflector 123 and the concave mirror is used as the imagingoptical element 124, theoptical system 121 can be made compact because an optical path is folded in this configuration. - In a case where the
optical system 121 is linearly mounted, a transmission light deflector may be used as thelight deflector 123 and a lens may be used as the imagingoptical element 124. In some implementation constraint, either one of thelight deflector 123 and the imagingoptical element 124 may be constituted by a reflection optical element and the other may be constituted by a transmission optical element. - The nonlinear
optical crystal 131 may be employed, for example, a rod of MgO:LiNbO3 crystal (LN crystal) having a length of about 50 mm. The nonlinearoptical crystal 131 is mounted on a stage constituted by therotary stage 138 and thelinear stage 139. By adjusting the angle of the nonlinearoptical crystal 131, therotary stage 138 angularly tilts the nonlinearoptical crystal 131 so as to allow the pump light 115 to form a predetermined angle θp to a side plane of the nonlinearoptical crystal 131. Thelinear stage 139 adjusts a y-position of the nonlinear optical crystal 131 (FIG. 1 is accompanied by a coordinate system) so that thepump light 115 may be reflected by the side plane of the nonlinearoptical crystal 131 at alocation 136 slightly shifted toward the entering side from an exiting surface of the nonlinearoptical crystal 131. Therotary stage 138 and thelinear stage 139 may be employed in a suitable mechanism that can implement such a function. - The
Si prism 140 is attached to a side wall of the nonlinearoptical crystal 131. The Si prism is capable of reducing the reflection of far-infrared light from the side plane of the nonlinearoptical crystal 131 and efficiently extracting the far-infrared light from the nonlinearoptical crystal 131. TheSi prism 140 and the nonlinearoptical crystal 131 are so configured as to allow the far-infrared light to enter substantially at right angles to the exiting surface of theSi prism 140. This configuration is adapted to reduce variation in the emission direction of the far-infrared light when the wavelength of the far-infrared light is changed. - In a case where the nonlinear
optical crystal 131 is tilted at ϕ=1 to 10° with respect to thepump light 115, it is important to configure the nonlinear optical crystal such that the entering surface and the exiting surface of the nonlinearoptical crystal 131 is tilted at an angle α (approximately 80 to 89° with respect to the side plane thereof. In the case of α=90°, on the other hand, thepump light 115 is specularly reflected by the entering surface or exiting surface of the nonlinearoptical crystal 131 and incident on an optical system of theseed light 125. This may result in the destruction of thelight source 120. Even though an antireflection coating is applied to the entering surface of the nonlinearoptical crystal 131, thepump light 115 is so powerful that a reflected light thereof can have sufficient power for damaging the optical system of theseed light 125. It is effective to insert an optical isolator in theoptical system 121 so as to attenuate return light. However, this approach is feasible only in cases where the cost of the isolator is acceptable or there are more than enough mounting spaces. - The above-described configuration is capable of extracting a strong far-infrared light even from a high frequency range of 3 THz or more for the following reason. A far-infrared light generated due to parametric generation is absorbed by the nonlinear
optical crystal 131 particularly in the high frequency range of 3 THz or more. However, by putting the pump light close to the side plane of thenonlinear optic crystal 131, the far-infrared light can be extracted while the generated far-infrared light is not attenuated so much by absorption. - The far-infrared light can be generated in conjunction with difference frequency generation or parametric generation which is induced by making two laser beams of different wavelengths (pump light 115 and seed light 125) incident on the nonlinear
optical crystal 131 at predetermined angles. The first embodiment provides an example where a LN crystal was used as the nonlinearoptical crystal 131, and the far-infrared light was generated by means of parametric generation. A wavelength of theseed light 125, and an angle θ between theseed light 125 and thepump light 115 were so set as to satisfy the following equation 1 andequation 2. - The wavelength (ωT) of the far-
infrared light 145 generated in the nonlinearoptical crystal 131 can be calculated on the basis of principle of energy conservation by applying the frequency ωp of thepump light 115 and the frequency ωS of theseed light 125 to the following equation 1, where ω denotes angular frequency. -
{tilde over (ω)}T={tilde over (ω)}p−{tilde over (ω)}s (1) -
FIG. 2 is a vector diagram showing a relation of the far-infrared light 145, theseed light 125 and thepump light 115. The generation efficiency of the far-infrared light 145 increases in a case where the momentum conservation low holds. Specifically, a high generation efficiency can be obtained when the following relational equation 2 (phase matching condition) is established among the emission direction of far-infrared light 145/the direction of pump light 115/the direction ofseed light 125. In the equation, kT, kP, kS denotes a wavenumber vector of the far-infrared light 145, thepump light 115 and theseed light 125, respectively. -
k p =k s +k T (2) - The far-infrared light can be generated with high efficiency by setting the wavelength and the incident direction (θ) of the
seed light 125 so as to satisfy the above conditions. The following description is made on a configuration which is adapted to suppress the change in the emission direction of the far-infrared light 145 irrespective of the change in the frequency of the far-infrared light 145, when the above-described conditions are satisfied. - With the wavelength of the
pump light 115 set to 1064.4 nm, the far-infraredlight source 100 of the first embodiment is adapted to change the frequency of the generated far-infrared light 145 in the range of about 0.5 THz to 5 THz by changing the wavelength of theseed light 125 is changed in the range of 1066 nm to 1084 nm and adjusting the incidence angle of theseed light 125 to the nonlinearoptical crystal 131 is adjusted by theoptical system 121. The following description is made on assumption that the frequency of the far-infrared light 145 is changed in this range. -
FIG. 3 is a graph showing a relation of an angle θT_p formed between a direction KT of the far-infrared light 145 and a pump light direction (kp) in a nonlinearoptical crystal 131. As indicated by theequation 2, the angle θT_p depends upon the frequency of the far-infrared light 145. This example indicates that the emission direction of the far-infrared light 145 varies by 6.7° in conjunction with the change in the frequency of the far-infrared light 145 in the range of 0.5 THz to 5 THz. - In the frequency range shown in
FIG. 3 , refractive indexes n of the nonlinearoptical crystal 131 for thepump light 115 and theseed light 125 at a wavelength λ were calculated using thefollowing equation 3 and according to literature (Zelmon et al. JOSA B Vol. 14, NO. 12, pp. 3319 to 3322). A refractive index nNL of the nonlinearoptical crystal 131 for the far-infrared light at a wavelength λT was calculated using the following equation 4 and according to literature (D. R. Bosomworth et al. Appl. Phys. Lett. No. 9, pp. 330 (1996)). -
-
FIG. 4 is a graph showing a relation between the frequency of far-infrared light 145 and the refractive index nNL. The graph ofFIG. 4 represents the calculation results based on the equation 4. -
FIG. 5 is a diagram showing refraction of the far-infrared light 145 at an interface between the nonlinearoptical crystal 131 and theSi prism 140. The far-infrared light 145 generated in the nonlinearoptical crystal 131 is incident on theSi prism 140. In the figure, θT_LN denotes a traveling direction of the far-infrared light 145 in the nonlinearoptical crystal 131, while θT_Si denotes a traveling direction of the far-infrared light 145 in theSi prism 140. A refractive angle at the interface is calculated according to Snell's law represented by thefollowing equation 5 using the above refractive index nNL and the refractive index nSi of theSi prism 140 for the far-infrared light wavelength λT. -
-
FIG. 6 is a graph showing the results of θT_LN calculation made on plural directions θT_Si. In the frequency range of about 0.5 THz to 5 THz of the far-infrared light 145, the wavelength dependency of the Si refractive index is low so that a good approximation by nSi≈4.3 holds. In this frequency range, therefore, θT_LN for given θT_Si can be calculated by using the equation 4 and theequation 5. In this example, the direction OT LN was calculated for each of 42°, 50°, 57° and 60° of θT_Si. - As shown in
FIG. 3 andFIG. 6 , θT_p and θT_LN respectively have a specific characteristic to frequency change of the far-infrared light 145. If a condition that substantially equalizes these characteristics with each other can be found, then under such a condition, both the direction θT_p and the direction θT_LN change substantially the same way with the frequency change of the far-infrared light 145. By previously setting a difference between these parameters as θp under such a condition, the angle of the far-infrared light 145 emitted from theSi prism 140 is substantially equalized at all times irrespective of the frequency change of the far-infrared light 145. In the following description, a condition that establishes such a relation is found according toFIG. 3 andFIG. 6 . - According to
FIG. 6 , the variation width of θT_LN with respect to frequency differs according to the value of θT_Si. Therefore, it is considered that a θT_Si value that provides a variation width of θT_LN to frequency equal to the variation width of 6.7° as shown inFIG. 3 can be found. Specifically, when a difference of θT_p (6.7° as shown inFIG. 3 ) at opposite ends of the frequency range of the far-infrared light 145 is equal to a difference of θT_LN at opposite ends of the frequency range of the far-infrared light 145 (a difference between an upper end and a lower end of each curve as shown inFIG. 6 ), θT_p and θT_LN can be said to have substantially the same change characteristic for the frequency change of the far-infrared light 145. According to the example shown inFIG. 6 , when θT_Si=57°, both characteristic curve exhibits the variation width of 6.7°. Therefore, it is apparent that this condition is satisfied. -
FIG. 7 shows an example where θT_p and θT_LN have substantially the same change characteristic for frequency. The solid line extracts θT_LN when θT_Si=57° fromFIG. 6 . The dotted line is the characteristic curve shown inFIG. 3 . As shown inFIG. 7 , when θT_Si =57°, both θT_p and θT_LN have substantially the same change characteristic for the frequency of the far-infrared light 145. It is noted, however, that there is a difference of about 4.5° between these parameters. Then, this difference can be offset by setting the angle of the nonlinearoptical crystal 131 to θP=4.5°. That is, when the frequency of the far-infrared light 145 is changed, θT_Si can be substantially maintained constant. -
FIG. 8 is a vector diagram showing a relation between θT_p and θT_LN. A difference between θTp and θT_LN corresponds to 4.5° shown inFIG. 7 . When the frequency of the far-infrared light 145 is changed, θT_Si can be substantially maintained constant by conforming θP to this difference. -
FIG. 9 is a graph showing differences between θT_LN and θT_p when θT_Si is at 57° and at 50°. The graph indicates that the difference in the former case is substantially maintained constant at 4.5° but the difference in the latter case varies by 1.5° with the frequency change. -
FIG. 10 is a graph showing changes of θT_Si when θp is 4.5° and 0°. When the conditions explained with reference toFIG. 7 andFIG. 8 are satisfied, θT_Si can be substantially maintained constant against the frequency change of the far-infrared light 145 (A variation width is less the 0.3°.) by setting θP=4.5°. In a conventional case of θP=0°, on the other hand, the emission direction of the far-infrared light 145 changes by 5° or so because θT_Si≈50° in vicinity of 0.5 THz and θT_Si ≈45° in vicinity of 5.0 THz. This angular change is further expanded when the light is emitted into the air from theSi prism 140. Heretofore, this phenomenon has been the cause of extremely difficult optical system design of the far-infrared spectrometers. The invention is advantageous from the viewpoint of optical system design because the invention is adapted to reduce this angular change. - The first embodiment is configured to previously determine such a θT_Si (emission direction of the far-infrared light 145) as to ensure that both θT_p (the angle of the far-
infrared light 145 to the pump light 115) and θT_LN (the angle of the far-infrared light 145 to the interface between the nonlinearoptical crystal 131 and the Si prism 140) substantially equally change when the wavelength of the far-infrared light 145 is changed. Therotary stage 138 has the difference between θT_p and θT_LN as θp (the incidence angle of the pump light 115 to the nonlinear optical crystal 131). Thus, the far-infrared light 145 can be emitted into the air substantially at constant angle. -
FIG. 11 is a configuration diagram showing a far-infraredlight source 100 according to a second embodiment of the invention. The second embodiment differs from the first embodiment in that the incidence angle of theseed light 125 is fixed while the incidence angle of thepump light 115 is variable by means of theoptical system 121. The other components are the same as those of the first embodiment and hence, the following description is principally made on differences. - A
lens system 127 is employed in theoptical system 121 in place of thelens 122. Thelens system 127 expands thepump light 125 before making the light incident on thelight deflector 123. In the second embodiment, the wavelength of the far-infrared light 145 is changed by adjusting the incidence angle of the pump light 115 in a manner to satisfy the equation 1 and theequation 2 according to the change in the wavelength of theseed light 125. The second embodiment is also adapted to maintain the emission direction of the far-infrared light substantially constant by changing the angle of the nonlinearoptical crystal 131 to the pump light 115 just as in the first embodiment. -
FIG. 12 is a graph showing an example where both the direction θT_s of the far-infrared light 145 in the nonlinearoptical crystal 131 with respect to the direction θS of theseed light 125, and the direction θT_LN have substantially the same change characteristic for frequency. In the second embodiment, these directions have substantially the same change characteristic when θT_Si=42°, and exhibit constant angular difference of about 4.4° in the wavelength range of 0.5 to 5.0 THz. In spite of changing the frequency of the far-infrared light 145, therefore, the embodiment can maintain θT_Si=42° by setting the angle θS of the nonlinearoptical crystal 131 to theseed light 125 to 4.4°. -
FIG. 13 is a vector diagram showing a relation between θT_s and θT_LN. In the second embodiment, the incidence angle of theseed light 125 is fixed. For convenience of explanation, therefore, the traveling direction of each light is expressed by way of the relation between θT_s and θT_LN. However, the fact also remains in the second embodiment that the difference still exists between θT_p and θT_LN. Hence, it is noted that the notation inFIG. 13 is for convenience in description. - In the second embodiment, as well, it is important to incline the entering surface and the exiting surface of the nonlinear
optical crystal 131 at an angle α (on the order of 80 to 89°) to the side plane thereof. This is effective to prevent thepump light 115 specularly reflected by the entering surface or exiting surface of the nonlinearoptical crystal 131 from being incident on the optical system of theseed light 125. -
FIG. 14 is a configuration diagram showing a far-infrared spectrometer 1000 according to a third embodiment of the invention. The far-infrared spectrometer 1000 is an apparatus for measuring an absorption spectrum of asample 200 by using light through thesample 200, for example. - The far-
infrared spectrometer 1000 includes: a far-infraredlight source 100; an illuminationoptical system 151; astage 202; a far-infrared light imagingoptical system 240; a nonlinearoptical crystal 132; aphotoelectric detector 290; asignal processor 400; and acontrol unit 500. The illuminationoptical system 151 illuminates thesample 200 with the far-infrared light emitted from the far-infraredlight source 100. Thestage 202 carries thesample 200 thereon. The far-infrared light imagingoptical system 240 images the far-infrared light transmitted through thesample 200 on the nonlinearoptical crystal 132. The nonlinearoptical crystal 132 converts the far-infrared light transmitted through thesample 200 to a near-infrared light by using apump light 235. - The far-infrared
light source 100 includes the components described in the first embodiment. Thelight source 110 includes as principal components: (a) a short pulse Q-switch YAG laser 111; (b) a polarization separation system including a polarization beam splitter (hereinafter, referred to as PBS) 114 and a quarter-wave plate 116; and (c) an amplifier unit (solid-state amplifier 118) for amplifying a laser output. An output beam from the YAG laser 111 is collimated by a lens 112, subjected to the polarization separation system and amplified by a solid-state amplifier 118. The beam passed through thePBS 114 is amplified by means of the quarter-wave plate 116 and the solid-state amplifier 118 and is reflected by a mirror 183. The reflected beam is amplified again by the solid-state amplifier 118 and is incident on thePBS 114 via the quarter-wave plate 116. Subsequently, the beam is emitted as apump light 115 via thePBs 114 and amirror 184. The output from the from the YAG laser 111 is amplified by the use of the solid-state amplifier 118 so that a powerful far-infrared light having a peak power on the order of kilowatts can be extracted from the nonlinearoptical crystal 131. - The nonlinear
optical crystal 131 has its entering surface and exiting surface processed to form an angle α=84° to the side plane, for example. An angle ϕ of therotary stage 138 is so set as to provide orientation of the pump light 115 in the nonlinear optical crystal 131 (angle to the crystal side plane) θP=4.5. In the third embodiment, the angle is defined as ϕ=3.9°. Thus, the embodiment is adapted to allow the reflected pump light 115 from the surface of the nonlinearoptical crystal 131 to exit downward with respect to the drawing surface ofFIG. 14 , while satisfying θP=4.5°. The position of thelinear stage 139 is set such that thepump light 115 is reflected by the side plane of the nonlinearoptical crystal 131 at a location slightly toward the entering side from the exiting surface of the nonlinearoptical crystal 131. - The illumination
optical system 151 can be constituted by, for example, onecylindrical lens 180 and onecondenser lens 190. The far-infrared light generated in the nonlinearoptical crystal 131 is emitted from a linear light emission region along the beam of thepump light 115, forming parallel rays as seen in a drawing surface ofFIG. 14 or a spread beam as seen in a plane vertical to the drawing surface. Hence, thecylindrical lens 180 having an optical power only in an in-plane direction vertical to the drawing surface is used for collimating the beam in the in-plane direction vertical to the drawing surface. Thus, a parallel pencil is formed. The parallel pencil is focused onto thesample 200 by thecondenser lens 190 so that a spot on thesample 200 is irradiated with the light. - The far-infrared light transmitted through the
sample 200 is guided by the far-infrared light imagingoptical system 240 to the nonlinearoptical crystal 132 via aSi prism 142. The far-infrared light imagingoptical system 240 is constituted by twocondenser lenses mirror 225, serving as an imaging optical system for forming an image of a surface of thesample 200 in the nonlinearoptical crystal 132. Specifically, the far-infrared light transmitted through thesample 200 is collimated by thecondenser lens 220, reflected by themirror 225, and focused in the nonlinearoptical crystal 132 by thecondenser lens 230 via theSi prism 142. The incidence angle formed by the far-infrared light through thesample 200 to the nonlinearoptical crystal 132 in theSi prism 42 is optimized similarly to the angle θP of the nonlinearoptical crystal 131 to thepump light 115. Specifically, the far-infrared light is made to travel through the nonlinearoptical crystal 132 substantially at a constant angle irrespective of the change in the wavelength of the far-infrared light. - The
pump light 235 split from the beam of thepump light 115 is made incident on the nonlinearoptical crystal 132. Thus, the far-infrared light transmitted through thesample 200 and guided by the nonlinearoptical crystal 132 can be converted again to the near-infrared light having a wavelength in vicinity of the range from 1066 nm to 1084 nm. Thephotoelectric detector 290 photoelectrically converts the generated near-infrared light so as to output the conversion result as a detection signal. A far-infrared absorption spectrum of thesample 200 can be obtained by recording the detection signals while scanning the wavelength of the generated far-infrared light. Thephotoelectric detector 290 may be a photosensitive device (1D array detector) including plural photosensitive elements arranged in 1D array or a photosensitive device (2D array detector) including plural photosensitive elements arranged in 2D array. The 1D array detector and 2D array detector for near-infrared light are relatively easy to acquire, features quick response and operates at normal temperatures. Therefore, these detectors are suitable for industrial applications. - The
pump light 235 is made incident on the nonlinearoptical crystal 132 in synchronism with the incidence of pulses of the far-infrared light transmitted through thesample 200. To ensure the synchronized timings, a delay optical system (such as delay optical length correction stage), a half wave plate for adjustment of polarization direction or the like can be disposed on the optical path of thepump light 235, as needed. In this configuration, a beam with well-organized time profile can be used as thepump light 235. This results in enhanced wavelength conversion efficiency and detection sensitivity. - In case where the output power of the
light source 110 is not sufficient, the pump light transmitted through the nonlinearoptical crystal 131 may be guided to the nonlinearoptical crystal 132 for reuse. In this case, the detection efficiency decreases because of the degraded beam quality of thepump light 235. However, the configuration provides efficient use of thepump light 115 for double purposes of generating the far-infrared light and for wavelength conversion to that of near-infrared light. - The
signal processor 400 retrieves a detection signal outputted from thephotoelectric detector 290. Thesignal processor 400 generates a distribution image of signals proportional to light transmitted through thesample 200 on the basis of the position of thestage 202 at the time of signal retrieval. Thesignal processor 400 is also capable of calculating the absorption spectrum by comparing the acquired image data and reference data (spectral image data without a sample) stored in a storage region of thesignal processor 400, so as to acquire two-dimensional distribution of absorption spectrum (absorption spectral image). - The
control unit 500 controls the whole system. For instance, thecontrol unit 500 controls the far-infraredlight source 100, thestage 202 and thesignal processor 400. As for the far-infraredlight source 100, the control unit controls thelight source 110, theoptical system 121, therotary stage 138, and thelinear stage 139. Thecontrol unit 500 also provides a user interface. For example, thecontrol unit 500 may also include a display part for displaying the signal and data (spectroscopic information) acquired by thesignal processor 400. In a case where data on thesample 200 is acquired with fixed frequency, thecontrol unit 500 controls the far-infraredlight source 100 so as to generate a specified far-infrared light and synchronizes the transfer of thestage 202 with data acquisition by thephotoelectric detector 290. - The far-
infrared spectrometer 1000 according to the embodiment optimizes the orientation θP of the pump light 115 in the nonlinearoptical crystal 131, so as to reduce the change in the emission direction of the far-infrared light to about 1.0° in the total angular range (θ1 to θ2) in the air even though the frequency of the far-infrared light is changed in a broad range of 0.5 to 5 THz. This provides sufficient reduction of change in the irradiation position on thesample 200, too. Specifically, in a case where a lens having a focal length of 50 mm is used as thecondenser lens 190, the change in the irradiation position on thesample 200 is on the order of ±0.5 mm. This value is negligible in practical use in a case where an irradiation spot size on thesample 200 exceeds 1 mm. In a case where more accurate measurement is required, this deviation can be corrected by transferring thestage 202 in the y-direction. - While the third embodiment illustrates the example where the short pulse Q-switch YAG laser 111 is used as the
light source 110, the invention is not limited to this. What is required is a narrow spectral line width and hence, a mode-locked laser, for example, may be used as thelight source 110. The mode-locked laser provides faster measurement because of fast repetition operation. It goes without saying that the far-infraredlight source 100 described in the second embodiment is also usable in the third embodiment. -
FIG. 15 is a configuration diagram showing a far-infrared spectrometer 1000 according to a fourth embodiment of the invention. The fourth embodiment differs from the third embodiment in the configurations of the optical systems subsequent to the far-infraredlight source 100. The other components are the same as those of the third embodiment and hence, the following description is principally made on differences. - An illumination optical system (150 to 190) for irradiating the
sample 200 with the far-infrared light includes: for example, threecylindrical lenses slit 160; an anglevariable mirror 165; and acondenser lens 190. - In the drawing surface of
FIG. 15 , the twocylindrical lenses FIG. 15 . Thecylindrical lens 155 is so set as to image the far-infrared light emitted from the nonlinearoptical crystal 131 in vicinity of a reflection plane of the anglevariable mirror 165. The far-infrared light emitted from the nonlinearoptical crystal 131 is changed in the emission direction on the order of one to several degrees in the drawing surface ofFIG. 15 depending upon the frequency. However, the far-infrared light is not changed in the incident position on the reflection plane of the anglevariable mirror 165. The angle of the anglevariable mirror 165 is adjusted such that the far-infrared light reflected by the anglevariable mirror 165 invariably travels in the y-direction inFIG. 1 . Thus, the far-infrared light reflected by the anglevariable mirror 165 is made to follow the same optical path irrespective of the frequency. Thecylindrical lens 170 forms the parallel pencil in the drawing surface ofFIG. 1 by collimating the far-infrared light reflected by the anglevariable mirror 165. - The
slit 160 mounted on alinear stage 161 is disposed in vicinity of a back focal plane of thecylindrical lens 155. Scanning the frequency of the far-infrared light changes the direction of the far-infrared light 145. This is followed by the change in a focusing position to theslit 160. Thelinear stage 161 transfers theslit 160 in a manner to follow the change in the focusing position. Thus, far-infrared light components generated in the nonlinearoptical crystal 131 but unnecessary for measurement (background light covering a broad band, referred to as “TPG light”) can be reduced by blocking. This results in improved SN ratio of spectral measurement across a broad frequency band. - In a direction vertical to the drawing surface, divergent beams emitted from the
Si prism 140 are collimated by thecylindrical lens 180. Just as in an in-plane direction of the drawing, thecondenser lens 190 applies the collimated light onto the spot on thesample 200. - A far-infrared light imaging optical system (210 to 230) includes a
diffuser 210 in addition to thecondenser lenses sample 200 has high transmittance for the far-infrared light, scattering less light, the far-infrared light incident on the nonlinearoptical crystal 132 may be superimposed with a coarse speckle pattern. As the result, a minor positional shift of thesample 200 may result in a significant change in the detection signal. The speckle pattern can be made very small by sufficiently diffusing the far-infrared light by means of multiple scattering in thediffuser 210. Thus, contrast can be reduced, resulting in an enhanced stability of the detection signal. Alternatively, the variations of the detection signal may also be reduced by time-integrating the detection signals by rotating thediffuser 210 to move the speckle pattern. - The far-infrared light is converted again to the near-infrared light having the wavelength in vicinity of the range from 1066 nm to 1084 nm by making the
pump light 235 incident on the nonlinearoptical crystal 132. The near-infrared light is detected by thephotoelectric detector 290 via alens 270 and a ND (neutral density)filter 280. - Just as the first to third embodiments, the far-
infrared spectrometer 1000 according to the fourth embodiment of the invention is adapted to reduce the directional change of the far-infrared light 145 emitted from the far-infraredlight source 100. Furthermore, the far-infrared spectrometer 1000 according to the fourth embodiment is configured to guide the far-infrared light 145 to the anglevariable mirror 165, which cancels the directional variation of the far-infrared light 145. This configuration can reduce the shift of the irradiation position of the far-infrared light due to the frequency change without using an optical system having a large aperture. Therefore, the broadband spectroscopic measurement can be accomplished by a compact optical system. Further, the invention can enhance the accuracy of quantitative analysis by obviating component distribution variations, change in signal detection efficiency and the like. - The invention is not limited to the above-described embodiments but includes a variety of modifications. The foregoing embodiments, for example, are the detailed illustrations to clarify the invention. The invention is not necessarily limited to those including all the components described above. Some component of one embodiment can be replaced by some component of another embodiment. Further, some component of one embodiment can be added to the arrangement of another embodiment. A part of the arrangement of each embodiment permits addition of some component of another embodiment, the omission thereof or replacement thereof.
- The above embodiments are applicable to analysis of contents of chemical components in samples, or to sample analysis by the use of light in a far-infrared region in test processes such as of exotic component or foreign matter. The light in the far-infrared region according to the above embodiments is light having wavelength ranging from 25μm to 4 mm, for example. While a variety of numerical ranges of wavelength are known as the definition of the “far-infrared region”, the light in the far-infrared region in the above embodiments should be construed as the light having the widest range of the ranges defined in all areas. The term “terahertz wave” is included in the above-described far-infrared region.
- In the above embodiments, even under the conventionally used condition θP≈0°, the difference between θT_LN and θT_p may be corrected by means of the
rotary stage 138. Specifically, θp may be changed by rotating the stage according to the change in the wavelength of the far-infrared light. This is effective to reduce in some degree the change in the emission direction of the far-infrared light in theSi prism 140. - In the above embodiments, the
signal processor 400 and thecontrol unit 500 can be implemented by using hardware such as circuit devices mounting these functions, or otherwise implemented by program codes of software implementing these functions. In this case, a storage medium recording the program codes is provided such that an arithmetic device such as CPU (Central Processing Unit) can run the program by retrieving the program codes stored in the storage medium. In this case, the program codes retrieved from the storage medium per se implement the functions of the embodiment while the program codes per se and the storage medium recording the program codes constitute the invention. Examples of usable recording medium providing such program codes include flexible disk, CD-ROM, DVD-ROM, hard disk, optical disk, magnetic optical disk, CD-R, magnetic tape, non-volatile memory card, ROM and the like. - The processes and techniques described herein can be also implemented by suitable combinations of components. Further, a variety of general-purpose devices are usable. In some cases, it is useful to build a dedicated device for performing the processes described herein. That is, a part of the above-described
signal processor 400 andcontrol unit 500 may be implemented by hardware employing electronic components such as an integrated circuit. - The foregoing embodiments illustrate only some control lines and information lines that are considered necessary for explanatory purpose but not necessarily illustrate all the control lines and information lines of the product. All the configurations may be interconnected.
-
- 100: far-infrared light source
- 110: light source
- 111: YAG laser
- 112: lens
- 114: polarizing beam splitter
- 115: pump light
- 116: quarter-wave plate
- 118: solid-state amplifier (amplifier unit)
- 120: light source
- 121: optical system
- 122, 270: lens
- 123: light deflector
- 124: imaging optical element
- 125: seed light
- 130, 131, 132: nonlinear optical crystal
- 140, 142: Si prism
- 155, 170, 180: cylindrical lens
- 190, 220, 230: condenser lens
- 151: illumination optical system
- 160: slit
- 161, 139: linear stage
- 162, 138: rotary stage
- 165: angle variable mirror
- 240: far-infrared light imaging optical system
- 200: sample
- 202: stage
- 210: diffuser
- 280: ND filter
- 290: photoelectric detector
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JP2002072269A (en) * | 2000-08-30 | 2002-03-12 | Inst Of Physical & Chemical Res | Method and device for generating terahertz wave |
JP3747319B2 (en) | 2002-04-09 | 2006-02-22 | 独立行政法人理化学研究所 | Terahertz wave generator and its tuning method |
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