JP2010156544A - Terahertz light measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a terahertz light measuring device that has improved portability, can save alignment working hours of an optical system, and reduces the amount of attenuation of terahertz light by water vapor in a casing. <P>SOLUTION: The terahertz light measuring device includes: a casing 1; a terahertz light generation means 2 stored in the casing 1; an incidence means 3 stored in the casing 1 to allow terahertz light Lt generated from the generation means 2 to impinge on a target to be measured; a light reception means 4 stored in the casing 1 to receive terahertz light Ltr emitted from the target to be measured; a terahertz light detection means 5 stored in the casing 1 to detect the emitted terahertz light Ltr received by the light reception means 4; and a removing means 6 for removing H<SB>2</SB>O in the air in the casing 1. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an apparatus for measuring a spectral spectrum of terahertz light irradiated on a measurement target or a film thickness of the measurement target.

  Terahertz light is irradiated onto the object to be measured and the spectrum is measured, and physical properties such as a complex refractive index and a material structure of the object to be measured can be obtained from the spectrum (see, for example, Patent Document 1). Further, by using pulsed terahertz light, each film thickness of the multilayer film can be measured using a time-of-flight method (see, for example, Patent Document 2).

Terahertz light is an electromagnetic wave having a wavelength of 30 to 3000 μm (frequency is 0.1 to 10 THz). Therefore, when terahertz light propagates in the air, it is strongly absorbed by water vapor (H ₂ O) in the air, and the intensity is attenuated.

In the conventional terahertz light measuring apparatus, the constituent elements are arranged in an open space, so that the terahertz light propagates in the free space. As a result, it is strongly absorbed by water vapor in the air and the strength is attenuated. Therefore, the SN ratio of the signal detected by the terahertz photodetector is reduced. In addition, the absorption spectrum of H ₂ O is superimposed as noise on the measured spectrum, making it difficult to determine the physical properties of the measurement target.

Therefore, in order to solve the above-described conventional problems, a terahertz light measurement device has been developed in which components are housed in a housing and the inside of the housing is purged with nitrogen or the inside of the housing is evacuated (for example, a patent Reference 3).
JP 2002-277393 A Japanese Patent No. 4046158 Japanese Patent No. 3950818

  In the case of the conventional terahertz light measuring apparatus, it is necessary to always provide a purge nitrogen cylinder or a vacuum pump for making a vacuum in the terahertz light measuring apparatus. Therefore, the portability of the terahertz light measurement device is significantly impaired.

  Further, in the type in which the inside of the housing is evacuated, it is necessary to improve the sealing property and maintain the vacuum, so that the housing becomes complicated and large. Furthermore, when aligning the optical system as a component, it is necessary to break the vacuum of the housing, and alignment takes time.

  The present invention has been made in view of the above problems, and it is an object of the present invention to provide a terahertz light measurement apparatus that is excellent in portability, saves the alignment man-hours of the optical system, and is less attenuated by water vapor in the housing of terahertz light. To do.

The terahertz light measuring device of the present invention made to solve the above-described problems is a housing, terahertz light generating means accommodated in the housing, and terahertz light generated from the generating means as a measurement target. Incident means accommodated in the casing to be incident, light receiving means accommodated in the casing for receiving outgoing terahertz light emitted from the measurement target, and the outgoing terahertz light received by the light receiving means Terahertz light detecting means accommodated in the casing to be detected, and removing means accommodated in the casing and removing H ₂ O in the air in the casing.

The terahertz light measurement device may include a concentration detection unit that detects an H ₂ O concentration in the air in the housing.

  The concentration detection means includes a surface reflecting mirror housed in the housing, an incident control means for controlling the incident means so that the incident means causes the terahertz light to be incident on the surface reflecting mirror, and The light receiving means may include a light receiving control means for controlling the light receiving means so as to receive the terahertz light reflected from the surface reflecting mirror.

Since the removal means for removing H ₂ O in the air in the housing is only housed in the housing, the portability of the terahertz light measurement device is not impaired. Since it is not necessary to evacuate the inside of the housing, the housing does not increase in size. Since there is no need to break the vacuum, the alignment process of the optical system can be saved.

  By providing the concentration detection means, the removal means can be controlled so that the water vapor concentration in the housing is not more than a predetermined value.

A surface reflecting mirror, an incident control means for controlling the incident means so that the terahertz light is incident on the surface reflecting mirror, a light receiving control means for controlling the light receiving means to receive the terahertz light reflected from the surface reflecting mirror, With this, it is possible to obtain a spectrum of terahertz light propagating through an optical path passing through the surface reflecting mirror. Since an absorption line of H ₂ O proportional to the water vapor concentration in the housing appears in this spectral spectrum, the water vapor concentration in the housing can be detected without a separate concentration detecting means.

  The best mode for carrying out the present invention will be described with reference to the drawings.

  FIG. 1 is a top view (z-axis direction view) of the terahertz light measurement apparatus of the present invention, and FIG. 2 is a side view of the main part (y-axis direction view) of FIG.

  As shown in FIG. 1, the terahertz light measuring apparatus is configured by housing an optical element in two housings 1 and 1A. The housing 1 houses optical elements for generating and detecting terahertz light, and the housing 1A houses optical elements for generating and delaying ultrashort light pulse lasers. The housing 1 has a structure that forms a sealed space that is blocked from outside air and the housing space of the housing 1A.

As shown in FIGS. 1 and 2, the terahertz light measuring apparatus is configured to make a case 1 forming a sealed space, terahertz light generating means 2 accommodated in the case 1, and terahertz light Lt incident on a measurement target 20. The incident means 3 housed in the housing 1 to be received, the light receiving means 4 housed in the housing 1 that receives the outgoing terahertz light Ltr emitted from the measurement target 20, and the housing 1 that detects the outgoing terahertz light Ltr. Terahertz light detecting means 5 housed in the housing 1, removing means 6 for removing H ₂ O in the air contained in the housing 1, the surface reflecting mirror 7a constituting the water vapor concentration detecting means, terahertz light An incident control means 7b for controlling the incident means 3 so that Lt is incident on the surface reflecting mirror 7, and a light receiving control means 7c for controlling the light receiving means 4 so as to receive the outgoing terahertz light Ltr emitted from the surface reflecting mirror 7a; ,have

  In addition to the above, the terahertz light measuring apparatus pumps an ultrashort light pulse light source 8 housed in a housing 1A connected to the housing 1 and an ultrashort light pulse laser Lo generated from the ultrashort light pulse light source 8 by pumping light Lpu. And a light splitting means 9 for splitting the probe light Lpr, an optical delay means 10 for controlling the time delay of the probe light Lpr, and the like.

  The ultrashort optical pulse light source 8 is an Er-doped fiber laser, and generates an ultrashort optical pulse laser L0 having a pulse width of 17 fs, a repetition frequency of 50 MHz, a center wavelength of 1560 nm, and an output of 110 mW.

  The larger the repetition frequency, the higher the signal-to-noise ratio of the electric field amplitude time-resolved waveform of the outgoing terahertz light Ltr. However, if the repetition frequency is too large, the pulse interval is narrowed and the scan range in the time domain is narrowed. Accordingly, it is desirable to use a laser with an appropriate repetition frequency.

  The ultrashort optical pulse light source 8 is not limited to the above, and may be, for example, a Yb-doped fiber laser or a titanium sapphire laser.

  The light splitting means 9 is a beam splitter. The ultrashort optical pulse laser L0 is branched by the beam splitter 9 into the pump light Lpu and the probe light Lpr at a ratio of 10: 1 and travels on different optical paths. That is, the pump light Lpu has a pulse width of 17 fs and an output of 100 mW, and the probe light Lpr has a pulse width of 17 fs and an output of 10 mW.

  When the ultrashort optical pulse light source 8 is an Er-doped fiber laser including an SHG crystal, an ultrashort optical pulse comprising a fundamental wave pulse having a center wavelength of 1560 nm and an output of 100 mW and a second harmonic pulse having a center wavelength of 780 nm and an output of 10 mW. Since the laser L0 is generated, in this case, a two-color mirror may be used as the light dividing means 9.

  The pump light Lpu passes through the modulator 11 and is modulated. The modulation frequency by the modulator 11 is preferably higher if it is about 1/10 or less of the repetition frequency of the pump light Lpu. In this embodiment, a chopper is used for the modulator 11, and the pump light Lpu is modulated at 1 kHz.

  If an acousto-optic modulator (AOM), an electro-optic modulator (EOM), or the like is used as the modulator 11, high-speed modulation becomes possible.

  The modulated pump light Lpu is condensed on the terahertz pulse light generation means 4 by the lens 15 fitted in the wall of the housing 1. The terahertz pulse light generation means 4 is a DAST crystal. DAST is an abbreviation for 4-dimethylamino-N-methyl-4 stilbazolium tosylate.

When pump light Lpu is irradiated to DAST4, terahertz light Lt is generated by the χ ⁽²⁾ effect of the crystal. The terahertz light Lt generated from the DAST 4 is collimated by the off-axis parabolic mirror 16, and then condensed and irradiated to the measurement target 20 disposed outside the housing 1 through the window 17 by the incident means 3. The

  As shown in FIG. 2, the incident means 3 includes an off-axis parabolic mirror 3 a rotatably attached to a rotation axis 3 b in the z-axis direction, and the terahertz light collimated by the off-axis parabolic mirror 16. Lt is focused on the object 20 to be measured.

  The off-axis parabolic mirror 3a is rotated by 180 ° around the rotation axis 3b by a command signal S4 from the personal computer 22 described later by the incident control means 7b, and becomes an off-axis parabolic mirror 3a indicated by a dotted line. Therefore, the off-axis parabolic mirror 3a indicated by the dotted line collects and irradiates the terahertz light Lt onto the surface reflecting mirror 7a described later.

  The outgoing terahertz light Ltr specularly reflected by the object 20 to be measured is received by the light receiving means 4 and collimated, and then condensed and irradiated to the terahertz light detecting means 5 by the off-axis parabolic mirror 16A.

  As shown in FIG. 2, the light receiving means 4 has an off-axis parabolic mirror 4 a rotatably attached to a rotation axis 4 b in the z-axis direction, and outputs the outgoing terahertz light Ltr reflected by the measurement target 20. Receive light collimate.

  The rotating shaft 3b and the rotating shaft 4b are coaxial, and the off-axis parabolic mirror 3a and the off-axis parabolic mirror 4a are in a vertical position relationship in the z-axis direction.

  The off-axis parabolic mirror 4a is rotated by 180 ° around the rotation axis 4b by a command signal S4 from the personal computer 22 described later by the light receiving control means 7c, and becomes an off-axis parabolic mirror 4a indicated by a dotted line. Therefore, the off-axis parabolic mirror 4a indicated by the dotted line receives and collimates outgoing terahertz light Ltr reflected by the surface reflecting mirror 7 described later.

  A surface reflecting mirror 7 is arranged at the condensing irradiation position of the off-axis parabolic mirror 3a indicated by a dotted line. That is, the surface reflecting mirror 7 and the measurement target 20 are in a mirror image positional relationship with the rotation axes 3b and 4b as mirror surfaces.

  The terahertz light detection means 5 includes a silicon lens 51 and a photoconductive switch 52. The photoconductive switch 52 is formed by forming a dipole antenna on a low-temperature grown GaAs substrate. The gap portion of the dipole antenna is excited by the probe light Lpr, and the outgoing terahertz light Ltr is incident thereon to obtain the electric field amplitude time waveform. be able to.

  Reference numeral 19 denotes a preamplifier that amplifies the electric signal from the photoconductive switch 52, and the amplified signal S <b> 1 is input to the lock-in amplifier 21.

  The lock-in amplifier 21 extracts and amplifies a component synchronized with the modulation signal S2 of the chopper 11 from the signal S1 detected by the photoconductive switch 52.

  The probe light Lpr divided by the beam splitter 9 is converted into light having a central wavelength of 780 nm by the wavelength conversion element 18. The wavelength conversion element 18 is, for example, an SHG crystal such as lithium niobic acid.

  The probe light Lpr wavelength-converted by the wavelength conversion element 21 is condensed on the photoconductive switch 52 of the detection means 5 by the condensing lens 15A fitted in the wall of the housing 1 through the optical delay means 10.

  The optical delay means 10 includes an intersecting mirror 10a and a moving mechanism 10b for moving the intersecting mirror 10a in the direction of arrow A. The terahertz light Lt generated by pumping with the pump light Lpu is temporally related to the probe light Lpr. Delay or advance is generated. The moving mechanism 10 b is controlled by the personal computer 22. When the optical delay means 10 is a rapid scan type, the personal computer 22 may be synchronized with the cycle of the delayed sweep. By doing so, data can be acquired at a higher speed.

  The personal computer 22 records the position information of the optical delay means 10 and the signal from the lock-in amplifier 21, and also has a function of controlling the moving mechanism 10b, the incident control means 7b, and the light reception control means 7c of the optical delay means 10. ing.

  The removing means 6 inserted from the opening of the wall surface of the housing 1 includes a container 6a and a water vapor removing member 6b accommodated in the container 6a. When the container 6 a is inserted into the housing 1, the opening of the wall surface is closed with the buttocks of the container, so that the confidentiality of the housing 1 is maintained. A part of the wall surface located inside the housing 1 of the container 6a is formed of a wire mesh. The removing member 6b is silica gel, but may be aluminum oxide, calcium oxide, calcium chloride, silica alumina gel, or the like.

  As described above, the removing means 6 of the present embodiment inserts the container 6a containing the silica gel 6b into the housing 1, but is not limited thereto. For example, the opening of the wall surface of the housing 1 into which the container 6a of the removing means 6 is inserted is connected to the inlet of a commercially available dehumidifier (for example, a desiccant dehumidifier manufactured by Seibu Giken), and the outlet is connected to another opening. Also good. By doing so, water vapor in the air in the housing 1 entering from the inlet of the dehumidifier is removed by the dehumidifier, and the dehumidified air exits from the outlet of the dehumidifier and is circulated in the housing 1.

In the terahertz light measuring apparatus of the present invention, most of the optical path through which the terahertz light propagates is in the housing 1 and the housing 1 has the removing means 6 for removing H ₂ O. There is no attenuation.

  The housing 1 of the present embodiment has a structure that forms a sealed space that is shielded from the outside air, and it is desirable that the sealing property be higher. The removal performance of the removing means 6 may be lower as the sealing property is higher. In addition, when the housing | casing 1 is a structure with low airtightness, what is necessary is just to use the removal means 6 with high removal performance.

  Next, the operation of the terahertz light measurement apparatus (preliminary operation when there is an operation interval) will be described.

  First, the control signal S4 is issued from the personal computer 22 to the incident control means 7b and the light reception control means 7c, and the off-axis parabolic mirror 3a and the off-axis parabolic mirror 4a in the state indicated by the solid line are rotated with the rotation shaft 3b and the rotation shaft 4b. And is switched between an off-axis parabolic mirror 3a and an off-axis parabolic mirror 4a in the state indicated by the dotted line (see FIG. 2).

  The ultrashort light pulse laser Lo generated from the ultrashort light pulse light source 8 is split by the beam splitter 9 into pump light Lpu and probe light Lpr.

The pump light Lpu is intensity-modulated by the chopper 11 and then condensed and irradiated by the lens 15 in the axial direction of the DAST crystal 2. Then, the terahertz light Lt is generated by the χ ⁽²⁾ effect of the crystal 2.

  The terahertz light Lt is collimated by the off-axis parabolic mirror 16, and then condensed and irradiated on the surface reflecting mirror 7a by the off-axis parabolic mirror 3a of the incident means 3.

  The outgoing terahertz light Ltr reflected by the surface reflecting mirror 7a is received and collimated by the off-axis parabolic mirror 4a of the light receiving means 4, and collected by the off-axis parabolic mirror 16A via the silicon lens 51 to the photoconductive switch 52. Lighted.

  On the other hand, the probe light Lpr divided by the beam splitter 9 is wavelength-converted by the wavelength conversion element 18, passes through the optical delay means 10, and is condensed on the photoconductive switch 52 by the lens 15 </ b> A. By scanning the optical delay means 10, the electric field amplitude time-resolved waveform of the outgoing terahertz light Ltr is measured by the photoconductive switch 52. That is, the signal of the photoconductive switch 52 is amplified by the preamplifier 19, and further amplified by the lock-in amplifier 21, the data is accumulated by the personal computer 22, and the electric field amplitude time-resolved waveform of the output terahertz light Ltr as shown in FIG. Get.

  Next, this electric field amplitude time-resolved waveform is Fourier-transformed by the personal computer 22 to obtain a spectrum as shown in FIG.

The terahertz light Lt generated by the generating means 2 is absorbed by water vapor in the optical path while propagating through the generating means 2 → incident control means 3 → surface reflecting mirror 7a → light receiving control means 4 → detecting means 5. Therefore, FIG. 4 shows a spectrum when the water vapor concentration is high, but a large number of absorption lines B1 to B8 (abrupt drop in amplitude) are observed between 1 and ₂ THz due to the rotation and translation of the H ₂ O molecule. . Since this drop amount is proportional to the water vapor concentration, the water vapor concentration is measured from the drop amount of the absorption line.

  For example, paying attention to the B1 line, for example, an alarm is generated when the B1 line drops below an amplitude of 0.01, and when the alarm is issued, the operator replaces the silica gel 6b with a new baked silica gel.

  After the silica gel 6b is replaced and a predetermined time elapses, the above preliminary operation is performed again. If the preliminary operation is performed again and the alarm cannot be issued, the following operation is performed.

  First, the control signal S4 is issued from the personal computer 22 to the incident control means 7b and the light reception control means 7c, and the off-axis parabolic mirror 3a and the off-axis parabolic mirror 4a in the state indicated by the dotted lines are rotated with the rotation shaft 3b and the rotation shaft 4b. And is switched between an off-axis parabolic mirror 3a and an off-axis parabolic mirror 4a in the state indicated by the solid line.

  Next, the electric field amplitude time-resolved waveform of the outgoing terahertz light Ltr reflected by the measurement target 20 is obtained by operating in the same manner as the preliminary operation.

  Next, this electric field amplitude time-resolved waveform is Fourier transformed by the personal computer 22 to obtain a spectral spectrum.

In this spectrum, H ₂ O absorption lines are hardly superimposed, and for example, the complex refractive index of the measurement target 20 can be measured with high accuracy.

  In the terahertz light measuring apparatus according to the present embodiment, the surface reflecting mirror 7a conjugated with the measurement target 20 and the incident control for controlling the incident means 3 so that the incident means 3 irradiates the surface reflecting mirror 7a with the terahertz light. Means 7b and a light receiving control means 7c for controlling the light receiving means 4 so that the light receiving means 4 receives the terahertz light reflected from the surface reflecting mirror 7a, so that the water vapor concentration in the housing 1 is detected. However, the water vapor concentration detector 7 indicated by a dotted line may be separately arranged in the housing 1. As the water vapor concentration detector 7, a humidity sensor using metal oxide ceramics or the like can be used.

  In the present embodiment, the measurement target 20 is a reflective object. For example, a surface reflection mirror is placed at the position of the measurement target 20, and a transmission object (gas, liquid, etc.) is provided between the window 17 and the surface reflection mirror. May be inserted to measure the transmission spectrum.

  Next, experimental results when the humidity in the housing 1 is changed are shown below. In the experiment, the humidity in the housing 1 was set to three levels of 0% (vacuum), 7.8%, and 49%. The results are shown in FIGS. FIG. 5 is an electric field amplitude time-resolved waveform, and FIG. 6 is a spectrum obtained by Fourier transforming the waveform of FIG.

  An absorption line of 0.55 THz that is an evaluation standard is hardly observed in a vacuum, but is observed as a small concave portion at a humidity of 7.8%, and is observed as a large dip at a humidity of 49%. On the spectrum, the spectrum intensity at 0.55 THz is indicated by a number next to the broken line in FIG. The intensity is strongest in a vacuum (967.4), decreasing to 82.3% (796.3) at a humidity of 7.8% and 30% (241.5) at a humidity of 49%. If the absorption of water vapor increases so far, the fluctuation range of the time waveform shown in FIG. 5 also increases. Therefore, it is concluded that the hygroscopic action by silica gel is effective in reducing noise.

  In this experiment, 500 milliliters of silica gel is used per 12 liters of the volume of the casing 1, so that about 42 milliliters of silica gel is required per liter.

1 is a top view of a terahertz light measuring apparatus according to the present invention. It is a principal part side view of FIG. It is an electric field amplitude time-resolved waveform diagram. It is the spectrum which carried out the Fourier transform of the waveform of FIG. It is an electric field amplitude time-resolved waveform figure when the humidity in a housing | casing is changed. 6 is a spectrum obtained by Fourier transforming the waveform of FIG.

Explanation of symbols

1... Case 2... Terahertz light generating means 3 .. Incident means 4... Light receiving means 5. Terahertz light detection means 6 ... Removal means 7 ... Concentration detection means 7a ... Surface reflector 7b ... Incident control means 7c ... Light reception Control means Lt ... Terahertz light Ltr ... Emission terahertz light

Claims (3)

A housing,
Terahertz light generating means housed in the housing;
Incident means accommodated in the casing for allowing the terahertz light generated from the generating means to enter the measurement target;
A light receiving means accommodated in the housing for receiving the output terahertz light emitted from the measurement object;
Terahertz light detecting means housed in the casing for detecting the emitted terahertz light received by the light receiving means;
A terahertz light measuring apparatus comprising: removing means for removing H ₂ O in the air in the housing. The terahertz light measuring apparatus according to claim 1, further comprising a concentration detection unit that detects a concentration of H ₂ O in the air in the housing.   The concentration detection means includes a surface reflecting mirror housed in the housing, an incident control means for controlling the incident means so that the incident means causes the terahertz light to be incident on the surface reflecting mirror, and the light receiving means. The terahertz light measuring apparatus according to claim 2, further comprising: a light receiving control unit that controls the light receiving unit so as to receive the terahertz light reflected from the surface reflecting mirror.

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