WO2016139754A1 - テラヘルツ波発生装置及びそれを用いた分光装置 - Google Patents
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- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
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- 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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- G02F1/01—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 for the control of the intensity, phase, polarisation or colour
- G02F1/03—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 for the control of the intensity, phase, polarisation or colour based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect
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- 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
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
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- H01S3/0057—Temporal shaping, e.g. pulse compression, frequency chirping
Definitions
- the present invention relates to a terahertz wave generating device and a spectroscopic device using the terahertz wave generating device.
- the far infrared region wave is an electromagnetic wave having a frequency range of about 0.1 THz to 10 THz.
- Far-infrared region waves are superior in permeability to many substances such as paper, wood, or plastic compared to infrared, which is an electromagnetic wave in a higher frequency band, while millimeter waves, which are electromagnetic waves in a lower frequency band, Compared to this, there are features that are superior in straightness and resolution.
- the absorption spectrum inherent to many substances is included in the frequency band of far-infrared waves. Taking advantage of these characteristics, there is a method of irradiating a far-infrared region wave onto a target substance and observing the transmitted wave or reflected wave.
- the above observation method it is possible to observe the internal structure of the target object, the presence / absence of defects / foreign substances, the difference in material and content, etc. in a non-destructive manner while the target object is placed in a permeable container. it can. Therefore, it is assumed that the above observation method can be applied to material inspection, structure inspection, chemical inspection, and the like.
- a narrow-band spectral light source can be realized simultaneously with the realization of a wide band, so that the utility as a general-purpose spectral light source has expanded.
- a pulsed Q-switched YAG laser that excites a nonlinear crystal has a driving frequency limited to about 100 Hz in order to ensure life and reliability.
- This limitation of the driving frequency limits the performance of the terahertz wave (for example, far infrared region light) generator.
- the terahertz wave generator is applied to a general-purpose spectroscope, the above drive frequency limitation becomes a factor limiting the spectral detection speed.
- the terahertz wave generator it is necessary to increase the driving frequency (about 100 Hz) of the Q-switched YAG laser (hereinafter referred to as a pump light laser).
- a pump light laser There is a limit to the driving frequency of the pump light laser. Therefore, it is difficult to improve the drive frequency of the pump light laser itself.
- the present invention provides a technique capable of improving the generation efficiency of terahertz waves without changing the drive frequency of the pump light laser.
- the present application includes a plurality of means for solving the above-described problems.
- a fixed wavelength pump light laser that generates pump light having a single wavelength, a seed light, and a wavelength of the seed light.
- a tunable laser a delay element that delays a pulse of the pump light, the seed light, a first pump light that is not delayed by the delay element, and a delay that is delayed by the delay element.
- a terahertz wave generation device including a first nonlinear crystal that generates a terahertz wave when the second pump light thus incident is incident thereon.
- a fixed wavelength pump light laser that generates single wavelength pump light, a wavelength tunable laser that generates seed light, and makes the wavelength of the seed light variable
- the delay element that delays the pulse of the pump light, the seed light, the first pump light that is not delayed by the delay element, and the second pump light that is delayed by the delay element are incident.
- a spectroscopic device includes a first nonlinear crystal that generates a terahertz wave by a detector and a detector that detects the terahertz wave irradiated on a sample.
- the generation efficiency of terahertz waves can be improved without changing the drive frequency of the pump light laser.
- a terahertz wave is generally an electromagnetic wave having a frequency of 0.1 THz to 100 THz.
- a terahertz wave is defined as including an electromagnetic wave of several tens of GHz to several hundreds of THz.
- the terahertz wave can be used for various measurements such as imaging and spectroscopic measurement, non-destructive inspection, and the like.
- the terahertz wave generator is applied to a far-infrared spectrometer will be described as an example, but the present invention can also be applied to apparatuses other than the far-infrared spectrometer.
- terahertz wave is used as a term including the above-described far-infrared light region.
- the far-infrared spectrometer is used in inspection processes such as analysis of chemical substance component distribution in a sample, inspection of different components or foreign substances.
- This spectroscopic device is a device that enables component analysis of a sample from frequency characteristics such as transmitted light that has been transmitted through the sample using light in the far infrared region or reflected light that has been reflected.
- FIG. 1 is a schematic configuration diagram of a reference example of a far-infrared spectrometer.
- the far-infrared spectrometer includes two types of laser light sources (pump light laser 110 and variable wavelength far-infrared light source 120), a nonlinear crystal (first nonlinear crystal) 130 for the light source, and illumination as main components.
- far-infrared light the generation part of the far-infrared region wave (hereinafter referred to as far-infrared light) until the far-infrared light is received by the signal detection system (light detector 200) will be described.
- a configuration of a wavelength tunable light source for generating far-infrared light a configuration in which far-infrared light is generated by differential frequency generation or parametric generation generated when two types of laser beams having different wavelengths are irradiated onto a nonlinear crystal.
- the pump light laser 110 is a fixed wavelength pump light laser that generates single wavelength pump light, and is, for example, a short-pulse Q-switched YAG laser.
- a tunable far-infrared light source (hereinafter referred to as a seed light source) 120 is a light source capable of generating seed light having a desired wavelength.
- the variable wavelength far infrared light source 120 can change the wavelength of the seed light. More specifically, the wavelength of the seed light may change continuously.
- the “desired wavelength” means a wavelength within the range of the laser specification that can change the wavelength.
- the pump light from the pump light laser 110 is changed in azimuth angle of linearly polarized light by a half-wave plate (hereinafter referred to as ⁇ / 2 plate) 330, and then transmitted light and reflected light by a polarizing beam splitter (hereinafter referred to as PBS) 350. Fork.
- the transmitted light passes through the ⁇ / 2 plate 331 and enters the nonlinear crystal 130.
- the seed light from the seed light source 120 enters the nonlinear crystal 130 through the two mirrors 121 and 122 at a slight angle with the pump light (transmitted light transmitted through the PBS 350).
- the generated far-infrared light can be efficiently extracted.
- the wavelength of the seed light source 120 is changed between about 1066 nm to 1076 nm, and the incident angle of the seed light to the nonlinear crystal 130 is accordingly in the range of about 2 ° to 4 °. Enter with.
- the generation frequency of the generated far infrared light can be changed in the range of about 0.5 THz to 3 THz.
- the far-infrared light thus obtained is shaped into a desired beam shape through the illumination optical system 140.
- the shaped beam is applied to the sample 150 to be measured.
- the beam diameter irradiated on the sample 150 is narrowed down to about 1 mm via the illumination optical system 140.
- the sample 150 is irradiated with a beam of about ⁇ 10 mm without narrowing the beam diameter.
- the far-infrared spectrometer may include an XY stage (not shown) for mounting the sample 150 to be measured.
- Spectral spectrum information (map information) distributed in the two-dimensional direction can also be obtained by moving the sample 150 in the two-dimensional axis direction by the XY stage.
- the illumination optical system 140 may include a deflection unit for changing the beam irradiation direction.
- a deflection unit for changing the beam irradiation direction of the measurement beam by the deflection unit and scanning the sample 150 with the beam in one or two-dimensional directions.
- spectral spectrum information maps in the two-dimensional direction on the sample 150 is obtained. Is possible as well.
- FIG. 2 is an example of spectral characteristics of far-infrared frequency and absorbance. As shown in FIG. 2, the characteristics are easy to understand when the horizontal axis represents the frequency of far-infrared light and the vertical axis represents absorbance (or transmittance).
- a resonance phenomenon due to crystallinity can be detected as a change in absorption spectrum.
- a resonance phenomenon in pharmaceuticals and the like, a difference in crystal structure, which is called a crystal polymorph that is frequently generated when crystallizing from a liquid, a difference between crystallinity and non-crystallinity, and a crystal polymorph
- a resonance phenomenon related to the structural form of a molecule such as an optical isomer that takes the form of a mirror-image (so-called symmetrically-opposite structure).
- the far-infrared light transmitted through or reflected by the sample 150 is generally easily detected by a photoelectric conversion type detector (photodetector 200) in order to obtain intensity information. Convert to
- the far-infrared light 280 that has passed through the sample 150 is reflected by the mirror 160.
- the reflected far-infrared light is shaped by the detection optical system 170 and enters the nonlinear crystal 180.
- a part of the pump light (reflected light reflected by the PBS 350) is incident on the nonlinear crystal 180.
- Near-infrared light around 1066 nm to 1076 nm is generated by wavelength conversion using both far-infrared light and part of pump light.
- LiNbO3 or MgO: LiNbO3 may be used as the nonlinear crystal 180.
- the beam shape of the near-infrared light is shaped by the detection optical system 190, and the near-infrared light is photoelectrically converted by the photodetector 200 having sensitivity to the near-infrared light and detected as a detection signal.
- the control unit 210 controls the pump light laser 110 and the seed light source 120. Further, the control unit 210 performs spectral control of the detection signal detected by the photodetector 200 and outputs spectral information 220.
- the near-infrared light detector 200 may be a single light receiving element, a light receiving element in which a plurality of light receiving elements are arranged in a one-dimensional array (1D array detector), or a two-dimensional array.
- a light receiving element (2D array detector) or the like may be used.
- Near-infrared light receiving elements are suitable for industrial applications because of their high operating speed, low cost, easy handling, and many types.
- the control unit 210 may include a display unit (such as a display) that displays a frequency spectrum (spectral information 220) in which information corresponding to the intensity of the signal from the photodetector 200 is represented.
- a display unit such as a display
- spectral information 220 information corresponding to the intensity of the signal from the photodetector 200 is represented.
- the quantum energy of the far infrared light (h ⁇ value, h: Planck constant, ⁇ : far infrared frequency) Therefore, a part of the pump light (reflected light reflected by the PBS 350) is branched by the PBS ⁇ 350 and is incident on the nonlinear crystal 180 after adjustment. Specifically, a part of the pump light (reflected light reflected by the PBS 350) passes through the pump light irradiation optical system 270 and is reflected by the mirror 344. A part of the reflected pump light is adjusted in azimuth by the ⁇ / 2 plate 332 and enters the nonlinear crystal 180.
- the pulse of the laser (reflected light reflected by PBS 350) branched from the pump light is incident at the same timing as the pulse of the far-infrared light (terahertz wave) 280 is incident on the nonlinear crystal 180. It is necessary to make it.
- the far-infrared spectrometer includes an optical path length correction stage 320.
- the optical path length correction stage 320 includes a moving mechanism 323 including a plurality of mirrors 321 and 322. By moving the position of the multiple mirrors 321 and 322 by the moving mechanism 323, the optical path length of the laser branched from the pump light (reflected light reflected by the PBS 350) is adjusted, and the incident timing to the nonlinear crystal 180 is adjusted. adjust.
- the laser branched from the pump light (reflected light reflected by the PBS 350) enters the optical path length correction stage 320 via the two mirrors 341 and 342, and the optical path length is adjusted.
- the laser that has passed through the optical path length correction stage 320 enters the pump light irradiation optical system 270 via the mirror 343.
- the frequency of the seed light source 120 is controlled according to the driving frequency to obtain far infrared light having a desired single wavelength.
- the far-infrared light is set to various stepwise frequencies, whereby the spectral information 220 is acquired.
- the measurement time when acquiring the spectral information 220 is determined by the driving frequency of the Q-switched YAG laser (pump light laser 110).
- FIG. 3 is an example of a timing chart of far-infrared region wave generation.
- FIG. 3 shows a configuration in which far-infrared light having a corresponding frequency is generated from the frequency of the seed light source 120 synchronized with 100 Hz.
- the pulse interval is 10 msec.
- the control unit 210 controls the emission of the seed light from the seed light source 120 and the frequency thereof in synchronization with the driving of the pump light laser 110.
- the set frequency of the seed light source 120 is set in increments of 10 GHz.
- the control unit 210 controls the emission of the seed light at frequencies f1, f2,..., Fm in synchronization with the driving of the pump light laser 110.
- the frequencies f1, f2,..., Fm of the seed light change linearly with respect to the time axis.
- the pulse timing of the far-infrared light (terahertz wave) 280 is the same timing as the laser pulse of the pump light laser 110, and the frequency of the generated far-infrared light 280 ranges from 1 THz to 0.01. THz increments.
- the transmittance of far-infrared light 280 may be poor.
- the data has a poor SN ratio.
- repeated measurement may be performed and an average value may be obtained from accumulated data.
- the measurement of the two-dimensional distribution (map) of the sample using the XY stage is assumed.
- FIG. 4 is a schematic configuration diagram of the far-infrared spectrometer according to the first embodiment.
- the same components as those in FIG. 4 are identical to FIG. 4, the same components as those in FIG. 4, the same components as those in FIG. 4, the same components as those in FIG. 4, the same components as those in FIG. 4, the same components as those in FIG. 4, the same components as those in FIG. 4, the same components as those in FIG. 4, the same components as those in FIG.
- the inventor has found a configuration capable of high-frequency driving of terahertz waves using the same pump light laser 110 as in the reference example of FIG. As a result, the speed of spectroscopic measurement can be increased in the far-infrared spectrometer.
- the pulse width by pulse emission (CCW emission) is about 500 psec.
- the repetition driving frequency of light emission is about 100 Hz, and the pulse interval is as long as 10 msec. Therefore, a new Q-switched YAG laser pulse is inserted in the 10 msec interval.
- the terahertz wave generator in the far-infrared spectrometer splits the fundamental wave of 100 Hz (pump light from the pump light laser 110) and branches in order to increase the generation of far-infrared light. One of them is delayed to generate a delayed wave.
- the terahertz wave generator superimposes this delayed wave on the original fundamental wave to generate a synthesized wave. By combining the fundamental wave and the delayed wave, the frequency is doubled, and the generation speed (efficiency) of the far infrared region wave can be doubled.
- the pulse width of the original pump light laser 110 is smaller than 1 nsec (pulse width ⁇ 1 nsec).
- the delay time is 1 nsec or more (delay time ⁇ 1 nsec)
- the frequency of the synthesized wave can be doubled.
- a delay element can be used to generate a delayed wave. For example, since the speed of light is 3 ⁇ 10 8 m / sec, the light travels 0.3 m in 1 nsec.
- n 3.41
- one of the branched fundamental waves may be passed through a delay element (material) having a length> 88 mm.
- the delay time may be adjusted by appropriately changing the length thereof.
- the branched excitation laser is passed through the delay element to generate a delayed pulse wave, without changing the drive frequency of the pump light laser 110 itself. This makes it possible to increase the frequency of terahertz waves. Therefore, it is possible to speed up spectroscopic analysis using variable frequency.
- the fundamental wave may be branched into three or more, and two or more delay elements may be parallelized.
- the generation speed of the terahertz wave can be increased to three times or more.
- the laser power for excitation exceeds the threshold inherent to the crystal.
- Increasing the number of branches and increasing the number of parallels reduces the energy of each laser pulse. Therefore, it can be assumed that the speed limit in this method depends on the laser power.
- the irradiation light path of the pump light laser is divided into the irradiation light path and the branching light path.
- the pump light traveling in the branching optical path is delayed by a predetermined time and then returned to the original irradiation optical path, and superimposed on the non-delayed pump light.
- the superimposed pump light is applied to the nonlinear crystal.
- the terahertz wave generator in the far-infrared spectroscopic device includes an optical element that branches the irradiation light path of the pump light laser 110, a delay element 240 arranged in the branching light path, and pump light of the original irradiation light path. And an optical element for superimposing the pump light delayed by the delay element.
- ⁇ / 2 plate 330 is disposed at the subsequent stage of the pump light laser 110.
- the azimuth angle of the linearly polarized light is changed by the ⁇ / 2 plate 330.
- a polarizing beam splitter (hereinafter referred to as PBS) 351 is disposed at the subsequent stage of the ⁇ / 2 plate 330.
- the pump light is branched by PBS 351 so that the amount of reflected wave (light branched to delay element 240) 291 and the amount of transmitted wave 292 are adjusted.
- the mirror 241 is disposed on the branch optical path, and the reflected wave 291 is reflected by the mirror 241 and guided to the delay element 240.
- the delay element 240 gives a predetermined delay time to the pulse of the reflected wave 291 and generates a delayed wave 293 in which the transmission time is delayed.
- the delayed wave 293 is reflected by the mirror 242 and enters the PBS 352.
- PBS352 is placed on the original irradiation light path.
- the transmitted wave (first pump light) 292 and the delayed wave (second pump light) 293 are superimposed via the PBS 352.
- the superposed light wave (hereinafter, synthesized wave 300) is incident on a polarizing plate 340 disposed at the subsequent stage of PBS 352.
- the polarizing plate 340 extracts polarized light having the same azimuth angle from polarized components orthogonal to each other. Thereafter, the synthesized wave 300 passes through the ⁇ / 2 plate 333 and has a desired azimuth angle.
- the spectral ratio of the synthesized wave 300 is adjusted by the PBS 350.
- the PBS 350 branches the reflected wave (light traveling toward the nonlinear crystal 180 through the optical path length correction stage 320) 295 and the transmitted wave (light traveling toward the nonlinear crystal 130) 296.
- the azimuth angle (S-polarized light / perpendicular to the paper surface) is adjusted by the ⁇ / 2 plate 334 in order to enter the nonlinear crystal 130.
- the optical path length of the reflected wave 295 is adjusted by the optical path length correction stage 320.
- the timing at which the pulse of the reflected wave 295 enters the nonlinear crystal 180 coincides with the timing at which the generated far-infrared light (terahertz wave) 280 pulse enters the nonlinear crystal 180.
- the mechanism for adjusting the optical path length is arranged in the front stage of the pump light irradiation optical system 270, but is arranged in the rear stage of the pump light irradiation optical system 270. May be.
- the seed light from the seed light source 120 enters the nonlinear crystal 130 through the two mirrors 121 and 122 at a slight angle with the transmitted light 296 transmitted through the PBS 350.
- the control unit 210 controls the emission of the seed light from the seed light source 120 and its frequency (or wavelength) in synchronization with the pulse timing of the synthesized wave 300.
- the set frequency of the seed light source 120 is set in increments of 10 GHz.
- the delay time of the delayed wave 293 can be calculated in advance. Therefore, the pulse timing of the synthesized wave 300 can be calculated in advance.
- the pulse timing of the synthesized wave 300 is set in advance.
- the control unit 210 controls the emission of the seed light at the frequencies f1, f2,..., Fm in synchronization with the pulse timing of the synthesized wave 300.
- the processing after the synthesized wave 300 is branched by the PBS 350 is the same as that in FIG.
- a part of the synthetic wave 300 (reflected light reflected by the PBS 350) and the far-infrared light 280 enter the nonlinear crystal 180 at the same timing.
- the wavelength of far infrared light is converted to near infrared light.
- the near-infrared light is detected as a detection signal by the photodetector 200 having sensitivity to the near-infrared light.
- the display unit connected to the control unit 210 displays a frequency spectrum in which information corresponding to the intensity of the signal from the photodetector 200 is expressed with respect to the frequency of the far infrared light (terahertz wave) 280.
- FIG. 5 is an example of a far-infrared wave generation timing chart according to the first embodiment, and an example of generating a delayed wave delayed by 3 msec will be described.
- the driving frequency of the pump light laser 110 is 100 Hz
- the pulse interval is 10 msec, as in FIG. 3.
- the delay time by the delay element 240 is set to 3 msec.
- the pulse of the synthetic wave 300 includes the pulse timing of the pump light (first pump light) from the pump light laser 110 and the pulse timing of the delayed wave 293 (second pump light). And the generation of these pulses is repeated.
- the interval of the pulses of the synthesized wave 300 between the pulse of the pump light (first pump light) from the pump light laser 110 and the pulse of the delayed wave 293 (second pump light) is non-equal.
- the interval between the pulse 501 of the first pump light and the pulse 502 of the second pump light is 3 msec, whereas the pulse 502 of the second pump light and the pulse 503 of the first pump light are The interval between is 7 msec.
- the delay time delayed by the delay element 240 is 3 msec, and the period of the pump light from the pump light laser 110 is 10 msec. Therefore, the delay time delayed by the delay element 240 is shorter than half the period of the pump light from the pump light laser 110 (5 msec).
- the delayed wave 293 is combined with the fundamental wave while being slightly shifted from the transmitted wave (fundamental wave) 292 of the pump light from the pump light laser 110.
- the delay time may be about 1 nsec. Therefore, the delay time delayed by the delay element 240 only needs to be larger than the pulse width of the pump light from the pump light laser 110, and is, for example, twice or more the pulse width of the pump light from the pump light laser 110. .
- the period of the pump light from the pump light laser 110 may be at least twice the pulse width of the pump light from the pump light laser 110.
- the pulse width of the pump light from the pump light laser 110 is about 500 psec
- the period of the pump light from the pump light laser 110 is 10 msec. Therefore, the period of the pump light from the pump light laser 110 is 10 7 times or more the pulse width of the pump light from the pump light laser 110. Therefore, the period of the pump light from the pump light laser 110 may be 10 n (for example, 1 ⁇ n ⁇ 7) times or more the pulse width of the pump light from the pump light laser 110.
- this embodiment uses the feature that the pulse width of the terahertz wave is short. For example, in the case of a light wave having a large pulse width, a longer delay time is required, resulting in a problem that the delay element becomes enormous.
- the pulse width of the terahertz wave is short, the delayed wave need only be shifted slightly from the fundamental wave. That is, it is not necessary to shift the delay wave by exactly half with respect to the period of the fundamental wave.
- the present embodiment is characterized in that the frequency of the terahertz wave generator can be increased by generating a composite wave of a fundamental wave and a delayed wave slightly shifted from the fundamental wave.
- the delay time for generating the delayed wave may be very small.
- the pulse width of the pump light from the pump light laser 110 is about 500 psec, the delay time may be about 1 nsec, and the delay time becomes very small.
- the period of the pump light from the pump light laser 110 is very large compared to the pulse width of the pump light, the degree of freedom for inserting a delayed wave within one period is high.
- the pulse width of the pump light is small and the delay time is small, it is possible to put a plurality of delayed waves in the pump light, and further high frequency driving by multistage branching is possible. Even at this time, it is possible to insert a plurality of delayed waves between half the period of the fundamental wave.
- the controller 210 controls the frequency (or wavelength) of the seed light from the seed light source 120 according to the delay time delayed by the delay element 240. Specifically, as shown in FIG. 5, control unit 210 controls the emission of seed light at frequencies f 1, f 2,..., Fm in synchronization with the pulse timing of synthesized wave 300. Note that f1 ⁇ f2 ⁇ f3... ⁇ Fm.
- the frequency of the generated far infrared light 280 is from 1 THz to 0.01 THz. That is, the first wavelength seed light (seed light corresponding to the frequency f1), the first terahertz wave generated by the first pump light 501, and the second wavelength seed light (seed light corresponding to the frequency f2). And the second terahertz wave generated by the delayed wave (second pump light) 502 have different wavelengths.
- the frequencies f1, f2,... Of the seed light are set so as to change nonlinearly with respect to the time axis according to the pulse timing of the synthesized wave 300.
- the seed light frequencies f1, f2,... May be changed in a curved manner with respect to the time axis.
- the driving frequency 100 Hz of the pump light laser 110 incident on the nonlinear crystal 130 can be doubled to 200 Hz.
- the time varying from 1 to 3 THz can be halved, and the detection time can be shortened.
- the delay time can be multiplexed by providing the delay elements 240 in multiple stages. For example, when two delay elements are arranged and the fundamental wave is branched into three, a driving frequency three times as high can be realized. That is, the driving frequency 100 Hz of the pump light laser 110 can be tripled to 300 Hz. Note that it can be assumed that it is necessary to secure a conversion threshold in the nonlinear crystal 130 because attenuation of the laser intensity due to the division occurs.
- the delay element 240 will be described below. If the pulse width by pulse light emission (CCW light emission) is 500 psec, the combined wave frequency can be doubled if the delay time is about 1 nsec.
- the speed of light is 3 ⁇ 10 8 m / sec, and the light travels a distance of 0.3 m / 1 nsec.
- the delay element 240 is not limited to the element having a fixed value as described above, and has an electro-optic effect in which the refractive index changes depending on the applied voltage.
- An element such as an element having a Pockels effect or an element having a Kerr effect can also be used.
- the delay element 240 is connected to a voltage control unit (not shown) for applying a voltage.
- the voltage control unit controls the delay time by the delay element 240 by controlling the voltage applied to the delay element 240.
- the control unit 210 is electrically connected to the delay element 240 or the voltage control unit.
- the controller 210 is configured to monitor the applied voltage to the delay element 240.
- the control unit 210 calculates the delay time by the delay element 240 according to the monitoring result of the applied voltage, and controls the seed light frequencies f1, f2,..., Fm according to the pulse timing of the synthesized wave 300. Note that the control unit 210 may have the function of the voltage control unit.
- the efficiency of terahertz wave generation depends on the driving frequency of the pump light laser.
- the efficiency of generating terahertz waves can be improved without changing the drive frequency of the pump light laser. Therefore, when the terahertz wave generator of this example is applied to a far-infrared spectrometer, the generation efficiency of far-infrared region waves can be increased and the spectroscopic measurement time can be shortened.
- a Q-switched YAG laser is used as the pump light laser 110, its frequency is 100Hz, and the pulse width is 500psec.
- this method is applied even if the laser frequency is higher. it can.
- the above method makes it possible to increase the driving frequency by the beam branching, delay, and synthesis described above when the time difference between the pulse interval time determined by the laser frequency and the pulse width is large.
- a short pulse Q-switched YAG laser is used as the pump light laser 110
- a mode-locked laser may be used as long as the basic spectral line width is narrow. . Since there are some types of lasers that repeat quickly, high-speed measurement may be possible.
- the control unit 210 is configured to vary the frequency of the seed light source 120 in accordance with the pulse of the synthesized wave 300.
- the fundamental wave is branched into two, one of the branched waves is delayed by a predetermined time, and then combined with the fundamental wave to obtain twice the driving frequency.
- the fundamental wave is branched into three, and two delay elements having different delay times are arranged for two of them. Then, the two delayed waves generated from the two delay elements are combined with the fundamental wave. As a result, it is possible to obtain a drive frequency three times as high. In this way, it is possible to branch the fundamental wave in multiple stages, set different delay times for each branch, and generate their combined waves.
- the constraint condition for increasing the drive frequency is that the power down due to the branch of the fundamental wave from the pump light laser 110 (Q-switched YAG laser) is less than the wavelength conversion threshold in the nonlinear crystal 130.
- the threshold value When the laser power incident on the nonlinear crystal 130 is less than the threshold value, it becomes impossible to generate a terahertz wave from the nonlinear crystal 130, so that it cannot be used as a light source for generating a terahertz wave. Therefore, it can be assumed that the number of branches that can secure pulse energy exceeding the threshold value is the maximum branch in the present system, and the maximum driving frequency.
- FIG. 6 is a schematic configuration diagram of a far-infrared spectrometer according to the second embodiment.
- This example is a far-infrared spectrometer capable of directly detecting far-infrared light.
- the first embodiment it is necessary to branch the pump light and to control the incident timing of the synthesized wave and the terahertz wave to the nonlinear crystal 180.
- far infrared light is not converted. They do not need to detect directly. That is, in the example of FIG. 6, the configuration required in FIG. 1, for example, the branching of pump light (branching at PBS 350), the optical path length correction stage 320, the nonlinear crystal 180, and the like are not necessary.
- the far-infrared region detector 250 is disposed at the subsequent stage of the detection optical system 170.
- the far-infrared light 280 that has passed through the sample 150 is reflected by the mirror 160, and the reflected far-infrared light is shaped by the detection optical system 170 and enters the far-infrared region detector 250.
- the far-infrared region detector 250 is a bolometer-type detection element that performs thermal conversion
- the detection element has a time constant on the order of msec. Therefore, the drive frequency of the pump light laser 110 can be increased only within the time constraint of the time constant of the bolometer.
- the far-infrared light is not frequency-converted by the nonlinear crystal 180, it is not necessary to branch the pump light, and it is not necessary to control the incident timing of the synthesized wave and the terahertz wave to the nonlinear crystal 180.
- Has advantages. Therefore, far-infrared light can be detected with a simple configuration compared to the first embodiment.
- the far-infrared spectrometer includes a lock-in control unit (lock-in amplifier) 260.
- the lock-in control unit 260 controls the reading of the detection signal from the far infrared region detector 250.
- the lock-in control unit 260 monitors the signal branched from the synthesized wave 300 (synchronization signal), and performs time control for reading the detection signal from the far infrared region detector 250.
- a beam sampler 370 is disposed downstream of the polarizing plate 340 for branching the synthesized wave 300.
- the beam sampler 370 branches a part of the synthesized wave 300.
- a part of the branched composite wave 300 is detected by the light receiving element 360.
- the lock-in control unit 260 locks in the detection signal from the far-infrared region detector 250 using the detection signal detected by the light receiving element 360 as a synchronization signal. With this configuration, it is possible to reduce the influence of noise.
- lock-in control is applied to the configuration for directly detecting far-infrared light.
- the configuration of the lock-in control is the same as that of the first embodiment and the third embodiment described later. Is also applicable.
- lock-in control is applied, detection with higher sensitivity is possible.
- FIG. 7 is a schematic configuration diagram of a far-infrared spectrometer related to the third embodiment.
- the pump light before entering the nonlinear crystal 130 is branched, but in this embodiment, the pump light after passing through the nonlinear crystal 130 is used.
- a mirror 381 is disposed at the subsequent stage of the nonlinear crystal 130.
- the synthesized wave 300 (the synthesized wave of the transmitted wave 292 and the delayed wave 293) that has passed through the nonlinear crystal 130 enters the optical path length correction stage 320 via the mirror 382.
- Other configurations are the same as those in FIG.
- the total amount of pump light from the pump light laser 110 can be used for generation of terahertz waves by the nonlinear crystal 130 as compared with the method of branching before the nonlinear crystal 130 of FIG. . Therefore, it is possible to construct an efficient terahertz wave generating apparatus with little loss of laser light for generating terahertz waves.
- this invention is not limited to the Example mentioned above, Various modifications are included.
- the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described.
- a part of the configuration of one embodiment may be replaced with the configuration of another embodiment, and the configuration of another embodiment may be added to the configuration of one embodiment.
- the functions of the control unit 210 of the embodiment may be realized by software program codes.
- a storage medium in which the program code is recorded is provided to the system or apparatus, and the computer (or CPU or MPU) of the system or apparatus reads the program code stored in the storage medium.
- the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the program code itself and the storage medium storing it constitute the present invention.
- a storage medium for supplying such program code for example, a flexible disk, CD-ROM, DVD-ROM, hard disk, optical disk, magneto-optical disk, CD-R, magnetic tape, nonvolatile memory card, ROM Etc. are used.
- control lines and information lines are those that are considered necessary for the explanation, and not all control lines and information lines on the product are necessarily shown. All the components may be connected to each other.
- Pump light laser 120 Variable wavelength far infrared light source (seed light source) 121: Mirror 122: Mirror 130: Nonlinear crystal 140: Illumination optics 150: Sample 160: Mirror 170: Detection optical system 180: Nonlinear crystal 190: Detection optical system 200: Photodetector 210: Control unit 220: Spectral information 240: delay element 241 and 242: Mirror 250: Far-infrared detector 260: Lock-in control unit 270: Pump light irradiation optical system 320: Optical path length correction stage 321 and 322: Mirror 323: Movement mechanism 330, 331, 332, 333, 334: Half-wave plate ( ⁇ / 2 plate) 340: Polarizing plate 341, 342, 343, 344: Mirror 350, 351, 352: Polarizing beam splitter (PBS) 360: Light receiving element 370: Beam sampler 381, 382, 383: Mirror
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Abstract
Description
図1は、遠赤外分光装置の参考例の概略構成図である。遠赤外分光装置は、主要な構成要素として、2種類のレーザ光源(ポンプ光レーザ110と波長可変遠赤外光源120)と、光源用の非線形結晶(第1の非線形結晶)130と、照明光学系140と、サンプル(試料)150と、検出用の非線形結晶(第2の非線形結晶)180と、光検出器200と、レーザの発光時間制御と検出信号の分光制御を行う制御部210とを備える。
図4は、第1実施例に関わる遠赤外分光装置の概略構成図である。図4において、図1と同じ構成要素については同じ符号を付して、説明を省略する。
図6は、第2実施例に関わる遠赤外分光装置の概略構成図である。本例は、遠赤外光を直接検出できる遠赤外分光装置である。
図7は、第3実施例に関わる遠赤外分光装置の概略構成図である。第1実施例では、非線形結晶130に入射する前のポンプ光を分岐させているが、本実施例では、非線形結晶130の通過後のポンプ光を利用する。
120 :波長可変遠赤外光源(シード光光源)
121 :ミラー
122 :ミラー
130 :非線形結晶
140 :照明光学系
150 :試料
160 :ミラー
170 :検出光学系
180 :非線形結晶
190 :検出光学系
200 :光検出器
210 :制御部
220 :分光情報
240 :遅延素子
241、242 :ミラー
250 :遠赤外領域検出器
260 :ロックイン制御部
270 :ポンプ光照射光学系
320 :光路長補正ステージ
321、322 :ミラー
323 :移動機構
330、331、332、333、334 :2分の1波長板(λ/2板)
340 :偏光板
341、342、343、344 :ミラー
350、351、352 :偏光ビームスプリッタ(PBS)
360 :受光素子
370 :ビームサンプラー
381、382、383 :ミラー
Claims (20)
- 単一波長のポンプ光を発生する波長固定のポンプ光レーザと、
シード光を発生し、当該シード光の波長を可変とすることが可能な波長可変レーザと、
前記ポンプ光のパルスを遅延させる遅延素子と、
前記シード光と、前記遅延素子によって遅延されていない第1のポンプ光と、前記遅延素子によって遅延された第2のポンプ光とが入射されることによってテラヘルツ波が発生する第1の非線形結晶と、
を備えることを特徴とするテラヘルツ波発生装置。 - 請求項1に記載のテラヘルツ波発生装置において、
前記第2のポンプ光が前記遅延素子によって遅延された時間に応じて、前記波長可変レーザから出力される前記シード光の波長を制御する制御部をさらに備えることを特徴とするテラヘルツ波発生装置。 - 請求項1に記載のテラヘルツ波発生装置において、
前記波長可変レーザから出力される前記シード光の周波数は、時間に対して非線形に変化することを特徴とするテラヘルツ波発生装置。 - 請求項1に記載のテラヘルツ波発生装置において、
前記第1のポンプ光と前記第2のポンプ光の合成波のパルスの間隔は非等間隔であることを特徴とするテラヘルツ波発生装置。 - 請求項1に記載のテラヘルツ波発生装置において、
前記第1のポンプ光の周期が当該第1のポンプ光のパルス幅の2倍以上であることを特徴とするテラヘルツ波発生装置。 - 請求項5に記載のテラヘルツ波発生装置において、
前記第2のポンプ光が前記遅延素子によって遅延される時間は、前記第1のポンプ光の周期の半分より短いことを特徴とするテラヘルツ波発生装置。 - 請求項1に記載のテラヘルツ波発生装置において、
前記遅延素子が、固有の屈折率を有する遅延物質であることを特徴とするテラヘルツ波発生装置。 - 請求項1に記載のテラヘルツ波発生装置において、
前記遅延素子が、加圧電圧により屈折率が変化する素子であり、
前記加圧電圧をモニタすることにより、前記波長可変レーザから出力される前記シード光の波長を制御する制御部をさらに備えることを特徴とするテラヘルツ波発生装置。 - 請求項1に記載のテラヘルツ波発生装置において、
試料に照射された前記テラヘルツ波を検出する検出器と、
前記第1のポンプ光及び前記第2のポンプ光の合成波のパルス発生のタイミングに基づいて、前記検出器からの信号の読み出しタイミングを制御するロックインアンプと、
をさらに備えることを特徴とするテラヘルツ波発生装置。 - 請求項1に記載のテラヘルツ波発生装置において、
試料に照射された前記テラヘルツ波と、前記第1のポンプ光又は前記第2のポンプ光とが入射されることによって近赤外光を発生する第2の非線形結晶をさらに備え、
前記第2の非線形結晶には、前記第1の非線形結晶を通過した後の前記第1のポンプ光又は前記第2のポンプ光が入射されることを特徴とするテラヘルツ波発生装置。 - 請求項1に記載のテラヘルツ波発生装置において、
第1の波長のシード光と前記第1のポンプ光によって生じる第1のテラヘルツ波と、第2の波長のシード光と前記第2のポンプ光によって生じる第2のテラヘルツ波は波長が異なることを特徴とするテラヘルツ波発生装置。 - 請求項11に記載のテラヘルツ波発生装置において、
前記第1のテラヘルツ波及び前記第2のテラヘルツ波を試料に照射することで生じる透過光または反射光を検出する検出器と、
前記テラヘルツ波の周波数に対して前記検出器からの信号の強度に相当する情報が表される周波数スペクトルを表示する表示部と、を有することを特徴とするテラヘルツ波発生装置。 - 単一波長のポンプ光を発生する波長固定のポンプ光レーザと、
シード光を発生し、当該シード光の波長を可変とすることが可能な波長可変レーザと、
前記ポンプ光のパルスを遅延させる遅延素子と、
前記シード光と、前記遅延素子によって遅延されていない第1のポンプ光と、前記遅延素子によって遅延された第2のポンプ光とが入射されることによってテラヘルツ波が発生する第1の非線形結晶と、
試料に照射された前記テラヘルツ波を検出する検出器と、
を備えることを特徴とする分光装置。 - 請求項13の分光装置において、
前記第2のポンプ光が前記遅延素子によって遅延された時間に応じて、前記波長可変レーザから出力される前記シード光の波長を制御する制御部をさらに備えることを特徴とする分光装置。 - 請求項13に記載の分光装置において、
前記波長可変レーザから出力される前記シード光の周波数は、時間に対して非線形に変化することを特徴とする分光装置。 - 請求項13に記載の分光装置において、
前記第1のポンプ光と前記第2のポンプ光の合成波のパルスの間隔は非等間隔であることを特徴とする分光装置。 - 請求項13に記載の分光装置において、
前記第1のポンプ光の周期が当該第1のポンプ光のパルス幅の2倍以上であることを特徴とする分光装置。 - 請求項17に記載の分光装置において、
前記第2のポンプ光が前記遅延素子によって遅延される時間は、前記第1のポンプ光の周期の半分より短いことを特徴とする分光装置。 - 請求項13の分光装置において、
前記第1のポンプ光及び前記第2のポンプ光の合成波のパルス発生のタイミングに基づいて、前記検出器からの信号の読み出しタイミングを制御するロックインアンプをさらに備えることを特徴とする分光装置。 - 請求項13の分光装置において、
前記試料に照射された前記テラヘルツ波と、前記第1のポンプ光又は前記第2のポンプ光とが入射されることによって近赤外光を発生する第2の非線形結晶をさらに備え、
前記第2の非線形結晶には、前記第1の非線形結晶を通過した後の前記第1のポンプ光又は前記第2のポンプ光が入射され、
前記検出器が、前記近赤外光を検出することを特徴とする分光装置。
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