Patent Publication Number: US-11644418-B2

Title: Far-infrared light source and far-infrared spectrometer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/765,238, filed May 19, 2020, which is a 371 of International Application No. PCT/JP2017/044695, filed Dec. 13, 2017, the disclosures of which are expressly incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a far-infrared light source. 
     BACKGROUND ART 
     Electromagnetic wave in a far-infrared region of wavelengths approximately ranging from 25 μm to 4 mm is also called terahertz 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&#39;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). 
     CITATION LIST 
     Patent Literature 
     
         
         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) 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     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. 
     Solution to Problem 
     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. 
     Advantageous Effects of Invention 
     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. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         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 θ 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 θ T_LN  calculation on plural θ T_Si ; 
         FIG.  7    is a graph showing an example where θ T_p  and θ T_LN  have substantially the same change characteristic for frequency; 
         FIG.  8    is a vector diagram showing a relation between θ T_p  and θ T_LN ; 
         FIG.  9    is a graph showing differences between θ T_LN  and θ T_p  when θ T_Si  is 57° and 50°; 
         FIG.  10    is a graph showing changes in θ T_Si  when θ 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 θ T_s  of the far-infrared light  145  in the nonlinear optical crystal  131  with respect to a direction θ S  of the seed light  125 , and the direction θ T_LN  have substantially the same change characteristic for frequency; 
         FIG.  13    is a vector diagram showing a relation between θ T_s  and θ T_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. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment: Equipment Configuration 
       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 . When the light deflector  123  deflects the seed light  125 , therefore, 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 . In a case where 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. 
     In a case where the optical system  121  is linearly mounted, a transmission light deflector may be used as the light deflector  123  and a lens may be used as the imaging optical element  124 . In some implementation constraint, 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 . By adjusting the angle of the nonlinear optical crystal  131 , the rotary stage  138  angularly tilts the nonlinear optical crystal  131  so as to allow the pump light  115  to form a predetermined angle θ 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.  1    is accompanied by a coordinate system) so that the pump light  115  may be reflected by the side plane of the nonlinear optical crystal  131  at a location  136  slightly shifted toward the entering side from an exiting surface of the nonlinear optical crystal  131 . 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. 
     In a case where the nonlinear optical crystal  131  is tilted at ϕ=1 to 10° with respect to the pump light  115 , it is important to configure the nonlinear optical crystal such that the entering surface and the exiting surface of the nonlinear optical 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, the pump light  115  is specularly reflected by the entering surface or exiting surface of the nonlinear optical crystal  131  and incident on an optical system of the seed light  125 . This may result in the destruction of the light source  120 . Even though an antireflection coating is applied to the entering surface of the nonlinear optical crystal  131 , 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. However, by putting 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. 
     First Embodiment: Principle of Far-Infrared Light Generation 
     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 θ 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 (ω 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 ω p  of the pump light  115  and the frequency ω S  of the seed light  125  to the following equation 1, where ω denotes angular frequency.
 
 ω   T = ω   p − ω   s   [Equation 1]
 
       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 . In the equation, 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.
 
[Equation 2]
 
 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. 
     First Embodiment: Principle of Reduction of Directional Change of Far-Infrared Light 
     With the wavelength of the pump light  115  set to 1064.4 nm, 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 θ 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 . As indicated by the equation 2, the angle θ 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. 
     In the frequency range shown in  FIG.  3   , refractive indexes n of the nonlinear optical crystal  131  for the pump light  115  and the seed light  125  at a wavelength λ 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 λ 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)). 
     
       
         
           
             
               
                 
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       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 . In the figure, θ T_LN  denotes a traveling direction of the far-infrared light  145  in the nonlinear optical crystal  131 , while θ 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&#39;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 λ T . 
                   [     Equation   ⁢         5     ]                                       n   Si     ⁢     sin   ⁡   (       π   2     -     θ     T   ⁢   _   ⁢   Si         )       =       n   NL     ⁢     sin   ⁡   (       π   2     -     θ     T   ⁢   _   ⁢   LN         )               (   5   )                 FIG.  6    is a graph showing the results of θ T_LN  calculation made on plural directions θ T_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 n Si ≈4.3 holds. In this frequency range, therefore, θ T_LN  for given θ T_Si  can be calculated by using the equation 4 and the equation 5. In this example, the direction θ T_LN  was calculated for each of 42°, 50°, 570 and 60° of θ T_Si .
 
     As shown in  FIG.  3    and  FIG.  6   , θ T_p  and θ 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 θ T_p  and the direction θ 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 θ 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   . 
     According to  FIG.  6   , the variation width of θ T_LN  with respect to frequency differs according to the value of θ T_Si . Therefore, it is considered that a θ T_Si  value that provides a variation width of θ 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 θ 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 θ 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.  6   ), θ T_p  and θ T_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 in  FIG.  6   , when θ 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 θ T_p  and θ T_LN  have substantially the same change characteristic for frequency. The solid line extracts θ T_LN  when θ T_Si =570 from  FIG.  6   . The dotted line is the characteristic curve shown in  FIG.  3   . As shown in  FIG.  7   , when θ T_Si =57°, both θ T_p  and θ T_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 nonlinear optical crystal  131  to θ P =4.5°. That is, when the frequency of the far-infrared light  145  is changed, θ T_Si  can be substantially maintained constant. 
       FIG.  8    is a vector diagram showing a relation between θ T_p  and θ T_LN . A difference between θ T_p  and θ T_LN  corresponds to 4.5° shown in  FIG.  7   . When the frequency of the far-infrared light  145  is changed, θ T_Si  can be substantially maintained constant by conforming Or to this difference. 
       FIG.  9    is a graph showing differences between θ T_LN  and θ T_p  when θ T_Si  is at  570  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 θ T_Si  when θ p  is 4.5° and 0°. When the conditions explained with reference to  FIG.  7    and  FIG.  8    are satisfied, θ T_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 θ P =4.5°. In a conventional case of θ P =0°, on the other hand, the emission direction of the far-infrared light  145  changes by 5° or so because θ T_Si ≈50° in vicinity of 0.5 THz and θ T_Si ≈45° in vicinity of 5.0 THz. This angular change is further expanded when the light is emitted into the air from the Si 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. 
     First Embodiment: Summary 
     The first embodiment is configured to previously determine such a θ T_Si  (emission direction of the far-infrared light  145 ) as to ensure that both θ T_p  (the angle of the far-infrared light  145  to the pump light  115 ) and θ 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 θ T_p  and θ T_LN  as Op (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. 
     Second Embodiment 
       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 . 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 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 θ T_s  of the far-infrared light  145  in the nonlinear optical crystal  131  with respect to the direction θ S  of the seed light  125 , and the direction θ T_LN  have substantially the same change characteristic for frequency. In the second embodiment, these directions have substantially the same change characteristic when θ T_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 θ T_Si =42° by setting the angle θ S  of the nonlinear optical crystal  131  to the seed light  125  to 4.4°. 
       FIG.  13    is a vector diagram showing a relation between θ T_s  and θ T_LN . In the second embodiment, the incidence angle of the seed light  125  is fixed. For convenience of explanation, therefore, the traveling direction of each light is expressed by way of the relation between θ T_s  and θ T_LN . However, the fact also remains in the second embodiment that the difference still exists between θ T_p  and θ T_LN . Hence, it is noted that the notation in  FIG.  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 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 . 
     Third Embodiment 
       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 . Subsequently, 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 nonlinear optical crystal  131  has its entering surface and exiting surface processed to form an angle α=840 to the side plane, for example. An angle ϕ of the rotary 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) θ P =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 nonlinear optical crystal  131  to exit downward with respect to the drawing surface of  FIG.  14   , while satisfying θ P =4.5°. 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. Hence, 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. Thus, 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 . Specifically, 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 θ P  of the nonlinear optical crystal  131  to the pump light  115 . Specifically, 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 . Thus, 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 . 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 the pump light  235 , as needed. In this configuration, 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. 
     In case where the output power of the light source  110  is not sufficient, the pump light transmitted through the nonlinear optical crystal  131  may be guided to the nonlinear optical crystal  132  for reuse. In this case, the detection efficiency decreases because of the degraded beam quality of the pump light  235 . However, 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. For example, the control unit  500  may also include a display part for displaying the signal and data (spectroscopic information) acquired by the signal processor  400 . In a case where data on the sample  200  is acquired with fixed frequency, the 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  according to the embodiment optimizes the orientation θ 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 (θ 1  to θ 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. Specifically, in a case where a lens having a focal length of 50 mm is used as the condenser lens  190 , 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. 
     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 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. 
     Fourth 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 . 
     In the drawing surface of  FIG.  15   , 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   . Thus, 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. Thus, 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. 
     In a direction vertical to the drawing surface, divergent beams emitted from the Si prism  140  are collimated by the cylindrical lens  180 . Just as in an in-plane direction of the drawing, 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 . In a case where 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. As the result, a minor positional shift of the sample  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 the diffuser  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 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 . 
     Fourth Embodiment: Summary 
     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-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. 
     &lt;Modifications of Invention&gt; 
     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 25 μm 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 θ P ≈0°, the difference between θ T_LN  and θ T_p  may be corrected by means of the rotary stage  138 . Specifically, θ 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 . 
     In the above embodiments, 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. 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  and control 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. 
     LIST OF REFERENCE SIGNS 
     
         
           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