JP2008268162A - Frequency sweeping type terahertz spectrum measurement device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem wherein a rotation angle control by a frequency sweep mechanism takes too much time, when a high-speed frequency sweep is needed, and there is a problem in accuracy, when conducting wide band, high-accuracy and high-speed terahertz spectral measurements, according to the frequency variation due to the temperature change and component wear of the frequency sweep mechanism and the rotation angle control type, or the like. <P>SOLUTION: This terahertz spectrum measurement device is using a reference light in a means which carries out intensity compensation of terahertz lights T<SB>0</SB>-T<SB>6</SB>, T<SB>S</SB>and a means which takes a standard frequency from the absorption spectrum of a gas cell and then calibrates the measured spectrum frequency in real-time basis. According to this procedure, the terahertz spectrum measurement device can sweep the rotation angle continuously and at a high speed in the predetermined frequency range, by using a smooth analog drive mechanism, or the like. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a frequency calibration method and means for a spectrum measuring apparatus that sweeps and takes a coherent terahertz wave.

In general, frequency calibration in spectrum measurement is performed using an emission line spectrum of a light source, an absorption spectrum of a gas cell, a reference position sensor of a frequency sweep mechanism, or the like.
However, since the terahertz wave is an electromagnetic wave in an unexplored region, a frequency calibration method in terahertz spectrum measurement has not been developed. In particular, conventional frequency calibration methods and means in wideband, high accuracy, and high speed measurement are not necessarily appropriate.
New Experimental Chemistry Course Basic Technology 3 Light [1] p194-p198 Japanese Patent Application No. 2004-16889 JP 2000-136965 A JP2004-108808 J. et al. Phys. D: Appl. Phys. 36 (2003) p. 2958-2961 The Japan Academy, Ser. B, Vol. 82, No9 (2006)

At present, in the frequency calibration of the terahertz spectrum, it is difficult to obtain a desired terahertz emission line spectrum because an existing terahertz light source is not generalized.
Further, when frequency sweeping is performed in correspondence with the reference position of the frequency sweeping mechanism, there is a frequency variation due to temperature change of the frequency sweeping mechanism, member wear, position sensor sensitivity, PID rotation angle control method, etc. There is a problem of accuracy in terahertz spectrum measurement with high accuracy and high speed.
Furthermore, when a high-speed frequency sweep is required, such as tracking a chemical reaction or measuring a terahertz spectrum of a moving object, it takes too much time to control the rotation angle of a diffraction grating or the like by the frequency sweep mechanism.
The purpose of the present invention is to eliminate the above-mentioned drawbacks by using a gas cell having a terahertz light intensity correction means and a desired absorption spectrum, a frequency sweeping mechanism, a frequency sweeping method, and a frequency calibration method. It is an object to provide a frequency sweep type terahertz spectrum measuring apparatus.

In the invention according to claim 1, in the case of frequency calibration of the terahertz spectrum of the measurement sample, the reference light is branched from the terahertz light that is irradiated by sweeping the frequency of the sample,
The frequency calibration is carried out using the means for correcting the intensity of the terahertz light and the means for taking the reference frequency from the absorption spectrum of one or more gas cells in which the type, pressure, temperature, etc. of the gas such as water vapor are set. Features.

According to the present invention, even if the intensity of the terahertz light irradiated to the sample fluctuates with the frequency sweep, the intensity is corrected, so that the influence on the spectrum measurement is avoided.
In addition, since the reference frequency measurement of the gas cell is performed simultaneously with the terahertz spectrum measurement, the frequency calibration of the terahertz spectrum is accurate.
Instead of the gas cell, the half width of the spectrum is large, but a liquid cell or a solid cell can be used as appropriate.

  The invention according to claim 2 is characterized in that one frequency to be calibrated is adjacent to a control point section of a calibration curve, that is, between the two reference frequencies, or adjacent to a plurality of the reference frequencies.

According to the present invention, for example, the one-frequency calibration is performed using a calibration curve set by a cubic spline function or the like having a plurality of the reference frequencies taken by the gas cell as control points. Interpolation becomes more accurate because gas cells having a large terahertz absorption spectrum are appropriately prepared and are adjacent to each other in the vicinity of the frequency to be calibrated or adjacent to each other.
In addition, even if there are mechanical and optical material quality errors and changes over time in frequency sweep, or changes in position control characteristics and temperature characteristics, the reference frequency is synchronized with the terahertz spectrum measurement of the sample. Since it is set above, high-precision frequency calibration is possible.

  According to a third aspect of the present invention, in the frequency sweep, the rotation angle of the predetermined frequency section is continuously and rapidly swept using a smooth analog drive mechanism without performing the rotation angle control of a diffraction grating or the like for setting the frequency. It is characterized by.

According to the present invention, for example, positioning of an accurate rotation angle by digital control becomes unnecessary, and a measurement sample or a sample is synchronized with a frequency sweep by a smooth frequency sweep mechanism including equipment such as a voice coil motor, a piezoelectric actuator, and a spring. Since the spectrum of the gas cell is measured, the time for taking the terahertz spectrum is greatly shortened, and a terahertz spectrum measuring apparatus with high accuracy, small size, and low price can be realized.
For example, even when an analog conduction mechanism such as a ball screw or a linear motor is driven with a mechanical / electrical discrete value by a stepping motor or a digital power source, if the quantization level is increased, that is, the sweep frequency If the frequency driving means is sufficiently finer than the sampling interval, the present invention can be similarly applied.

  According to the present invention, it is possible to perform high-quality terahertz spectrum measurement even if there is an error in mechanical and optical material quality in frequency sweeping, a change over time, or a change in position control characteristics and temperature characteristics depending on the measurement environment. There is an advantage.

  In terahertz spectrum measurement, means for correcting the intensity of terahertz light and means for taking a reference frequency from the absorption spectrum of the gas cell are applied, which will be described in the following examples.

  FIG. 1 is a schematic diagram of a frequency sweep type terahertz spectrum measuring apparatus according to an embodiment of the present invention relating to claim 1, and the reference numerals are the same as those in FIG.

In this example, when the infrared light of a YAG laser light source Y1, _{Y 2} wavelength 1064nm is irradiated chromium forsterite crystals, pointed to wavelength selection by the diffraction grating (not shown) from the infrared laser light source _Cr 1, Cr ₂ Infrared laser beams I ₁ and I ₂ are emitted.
For example, the wavelength from Cr ₁ 1217.734nm, _{I 1} frequency 246.1884THz, Cr ₂ wavelength 1222.701Nm, is emitted to _{I 2} frequency 245.1884THz, taking angular alignment with the optical mixer B in GaP crystals When incident, terahertz light T ₁ having a difference frequency of ₁ THz comes out in a predetermined direction.
Thus, the difference frequency between the infrared laser light I ₁ and the infrared laser light I ₂ determines the frequency of the terahertz light T _1. For example, the wavelength of I ₁ is fixed and the wavelength of I ₂ is changed to the red wavelength. When the rotation angle of the diffraction grating installed in the outer laser light source Cr ₂ is changed by the rotation mechanism R and incident on the GaP crystal in the same manner as described above, terahertz light having a difference frequency between I ₁ and I ₂ is predetermined from the GaP crystal. Come out in the direction.
Incidentally, the phase phase matching of I ₁ and I ₂ is performed by adjusting the timing of the Q switches (not shown) of the YAG laser light sources Y 1 and Y ₂ .

Terahertz light _{T 1} is branched as reference light _{T 2} by a beam splitter _{H 1} to the measuring terahertz light _{T 3,} further, the reference light _{T 2} are using a beam splitter _{H 2, H 3, H 4} , each of the terahertz light Terahertz sensors S _{2 to} S ₄ and S are branched into T ₄ , T ₅ , T ₆ , and intensity-correcting terahertz light T ₀ and pass through the water vapor gas cell C ₁ , the carbon monoxide gas cell C ₂ , and the Freon gas cell C _3. Enter ₀ .

Accordingly, the gas absorption characteristics of the gas cells C ₁ , C ₂ , and C ₃ are sensed by the terahertz sensors S _{2 to} S ₄ , and exhibit the spectrum as shown in FIG. 2 (FIG. 2). Can be taken.
At this time, the gas cells C ₁ , C ₂ , and C ₃ are supplied with predetermined gas types and gas pressures from the gas containers G _{1 to} G ₃ , and are strictly temperature-controlled in the thermostats w _{1 to} w ₃ . .

In the sensor S ₀ , the intensity of the terahertz light T ₁ is corrected because the YAG laser light sources Y 1 and Y ₂ , the chrome forsterite laser light sources Cr ₁ and Cr ₂ , the terahertz light source GaP, optical system components, and the like fluctuate in output due to changes with time and frequency characteristics. Used for.

When the rotation angle of the diffraction grating is changed by the rotation mechanism R and the frequency of the terahertz light T ₃ is swept, the terahertz light Ts transmitted through the measurement sample X is detected by the terahertz sensor S ₁ for each frequency, and the terahertz spectrum is measured. The

  As described above, in the terahertz spectrum measurement according to the present invention, the intensity correction and the frequency calibration measurement based on the absorption spectrum of the gas cell can be performed simultaneously using the reference light, so that the mechanical and optical material quality of the measurement system Even if there is a change in position control characteristics, temperature characteristics, etc. depending on the measurement error, changes over time, or the measurement environment, it can be accurately performed.

FIG. 2 is an explanatory diagram for frequency calibration of terahertz spectrum measurement in an embodiment relating to claim 2 of the present invention.
The horizontal axis is the frequency F of the absorption spectrum of the gas cell, the vertical axis is the light absorption rate δ, the gas cell is indicated by δ ₁ to δ ₃ , the sample is indicated by δx, and the spectrum diagrams of the gas cells C _{1 to} C ₃ are t ₁ to t _3. , spectrum of the measurement sample X is shown by t _X.
Here, F _{1 to} F ₁₀ are gas cells, F _X is a terahertz absorption spectrum that appears in a measurement sample, and F _X is accurately frequency calibrated using F _{1 to} F ₁₀ .
The frequency calibration procedure will be described with reference to FIG.

FIG. 3 shows an embodiment relating to claim 2 of the present invention and describes the procedure of frequency calibration. The horizontal axis represents the sweep frequency f of the terahertz frequency sweep mechanism, and the vertical axis represents the frequency F of the absorption spectrum of the gas cell.
For example, when the sweep frequency f of the frequency sweep mechanism is the frequency sweep of the terahertz light T ₁ and the absorption spectrum F of the gas cell is obtained in the relationship shown in FIG. 3A, the frequency Fx of the terahertz absorption spectrum in the measurement sample. is that can be accurately measured using a f _X calibration curves L calculated from the relationship of f and F in FIG. 3 (1).
The calibration curve can also be used by setting a predetermined section with a plurality of spline functions using the relationship between f and F.

FIG. 4 is an embodiment relating to claim 3 of the present invention and explains a high-speed sweep of the frequency in the measurement of the reflection spectrum of the sample.
As described above, the frequency of the terahertz light applied to the measurement sample using the frequency sweep mechanism is simultaneously calibrated and measured by the absorption spectrum of the gas cell, which is the reference frequency. If the rotating mechanism is a smooth operating mechanism, it is possible to perform high-accuracy spectrum measurement with a sweep having hysteresis characteristics even at a high speed sweep.
For example, when an infrared laser beam Cr ₂ having a frequency width is incident on the diffraction grating Gr, infrared light I ₂ having a specific frequency that satisfies the diffraction condition is emitted in the direction of the arrow, but the change of the diffraction condition, that is, the frequency sweep is rotated. The mechanism R is rotated around a spring bearing (not shown).
Rotation angle and the sweep rate is set continuously by the high-speed rotation of the servo motor through members Si on the opposite side of the spring bearing Sp ₁ is a lever fulcrum, the spring member Sp, a transmission mechanism such as a ball screw Sc Is done.
Conventionally, since the rotation angle of the rotating mechanism R to accurately set had increased frequency accuracy of the infrared light I _2, it takes time to angle control, also hard to develop constraints and measurement environment of the member to be used Met.

In the figure, the reference numerals are the same as those in FIG. 1, and another infrared light I ₁ that simultaneously irradiates the terahertz light source GaP to generate terahertz light having a difference frequency is omitted.
The terahertz light T ₁ emitted from the GaP is irradiated to the measurement sample X through the half mirror H ₁ , and the reflected light Ts is sensed by the sensor S ₁ to measure the spectrum diagram of FIG.
On the other hand, the reference beam T ₂ are the same as that of FIG. 1, the frequency calibration by gas cell C ₁ is carried out, for example, the same processing as FIG. 3 with reference to FIG. 4 (2).

  By using the absorption spectrum of an imminent gas cell such as water vapor as a reference frequency and using a simple analog frequency sweep mechanism, a high-speed and high-accuracy terahertz spectrum can be obtained. It can be used in the field.

It is the figure explaining the structure outline | summary of an apparatus. It is a figure explaining the absorption spectrum of the gas cell. It is a figure explaining calibration of a spectrum. It is a figure explaining the high-speed sweep mechanism of a frequency.

Explanation of symbols

Y ₁ , Y ₂ YAG laser light source Cr ₁ , Cr ₂ infrared laser light source I ₁ , I ₂ infrared laser light B mixer GaP terahertz light source T _{0 to} T ₆ , Ts terahertz light R rotating mechanism H _{1 to} H ₄ beam Splitter C ₁ Water vapor gas cell C ₂ Carbon monoxide gas cell C ₃ Freon gas cell S _{0 to} S ₄ Terahertz sensor G _{1 to} G ₃ Gas container w _{1 to} w ₃ Thermostatic chamber X Measurement samples δ, δ _{1 to} δ ₃ , δ _X light absorption rate _t 1 _~t 3, _{t X} spectrogram _{F, F 1 ~F 10, F} X absorption spectrum of the frequency f, _{f X} sweep frequency Gr diffraction grating L calibration curve Sp ₁ spring bearing Sp spring member Si member Sc ball screw

Claims (3)

In frequency calibration of the terahertz spectrum of the measurement sample,
From the absorption spectrum of one or more gas cells in which the reference light is branched from the terahertz light that is irradiated with the frequency sweep to the sample, the intensity of the terahertz light is corrected, and the type, pressure, temperature, etc. of the gas such as water vapor are set. A frequency sweep type terahertz spectrum measuring apparatus characterized in that the frequency is calibrated using means for taking a reference frequency.   2. The frequency sweep type terahertz spectrum measuring apparatus according to claim 1, wherein one frequency to be calibrated is adjacent to a control point section of a calibration curve, that is, between the two reference frequencies, or adjacent to a plurality of the reference frequencies. .   In the frequency sweep, the rotation angle of the predetermined frequency section is continuously and rapidly swept using a smooth analog drive mechanism without performing the rotation angle control of a diffraction grating or the like for setting the frequency. The frequency sweep type terahertz spectrum measuring apparatus according to claim 1 or 2.