JP2017194361A - Dielectric spectroscopic device - Google Patents

Abstract

[PROBLEMS] To reduce the generation of stray light and interference ripples on a spectrum to improve measurement reproducibility and measurement accuracy. A measurement sample 100 is arranged on a parallel surface of an attenuated total reflection prism 30 having a trapezoidal cross-sectional shape, a radio wave absorber 31 is provided on a surface facing the arrangement surface, and electromagnetic waves are incident from an inclined surface. Electromagnetic waves are emitted from the inclined surface. As a result, it is possible to reduce the generation of stray light and the interference ripples on the spectrum due to the effects of diffraction and multiple reflections that occur in the attenuated total reflection prism 30, and the effect of improving measurement reproducibility and measurement accuracy is achieved. [Selection] Figure 1

Description

  The present invention relates to a technique for measuring a component concentration of a measurement object using dielectric spectroscopy.

  As aging progresses, the response to adult diseases has become a major issue. Tests such as blood glucose levels are a heavy burden on patients because they need to collect blood. Therefore, a non-invasive component concentration measuring apparatus that does not collect blood has attracted attention.

  An apparatus using dielectric spectroscopy has been proposed as a noninvasive component concentration measuring apparatus. Dielectric spectroscopy irradiates the skin with electromagnetic waves, absorbs the electromagnetic waves according to the interaction of blood components to be measured, for example, glucose molecules and water, and observes the amplitude and phase with respect to the frequency of the electromagnetic waves. The dielectric relaxation spectrum is calculated from the amplitude and phase with respect to the frequency of the observed electromagnetic wave. In general, the complex dielectric constant is calculated by expressing as a linear combination of relaxation curves based on the Cole-Cole equation. In the measurement of biological components, for example, the complex dielectric constant has a correlation with the amount of blood components such as glucose and cholesterol contained in blood, and is measured as an electrical signal (amplitude, phase) corresponding to the change. A calibration model is constructed by measuring the correlation between the complex dielectric constant change and the component concentration in advance, and the component concentration is calibrated from the measured change in the dielectric relaxation spectrum.

  In material analysis using electromagnetic waves such as microwaves, the complex dielectric constant of a material is evaluated using various methods such as resonance method and coaxial reflection method. At a high frequency such as the THz band, it is common to measure the complex dielectric constant of a measurement object by a free space method using a pseudo optical system using a lens or a parabolic mirror. The free space method is also used in the millimeter wave band as described in Non-Patent Document 1.

  As a conventional measuring apparatus, there is a dielectric spectroscopic apparatus using photoelectric conversion (photo mixing) in a frequency band from microwave to millimeter wave or more (see Patent Document 1). The dielectric spectroscopic device of Patent Document 1 photoelectrically converts an optical signal in which two continuous light waves having different frequencies are combined to generate an electromagnetic wave, for example, a terahertz wave, irradiates the object to be measured with the generated terahertz wave, The configuration is such that the terahertz wave transmitted through the measurement object is received and the reference light synthesized by modulating the phase of one of the two continuous light waves is input to perform homodyne mixing. For example, a detector that performs homodyne mixing uses a photoconductive antenna, and the conductance between the antennas is modulated by the difference frequency between two continuous light waves included in the reference light by irradiation with the reference light. In a conventional dielectric spectroscopic device, when homodyne detection of electromagnetic waves, it is necessary that the two optical path length differences coincide with each other when mixing with a detector. Therefore, the propagation length of the terahertz wave propagating in space, the length of the fiber through which light propagates, and the like are adjusted.

JP 2013-32933 A

Andrew P. Gregory, and Robert N. Clarke, “A Review of RF and Microwave Techniques for Dielectric Measurements on Polar Liquids”, IEEE Transactions on Dielectrics and Electrical Insulation, August 2006, Vol. 13, No. 4, pp.727- 743 Jae-Young Kim, Ho-Jin Song, Katsuhiro Ajito, Makoto Yaita, and Naoya Kukutsu, “Continuous-Wave THz Homodyne Spectroscopy and Imaging System With Electro-Optical Phase Modulation for High Dynamic Range”, IEEE Transactions on terahertz science and technology, March 2013, Vol. 3, No. 2, pp.158-164 Hirotaka Naito, Yuichi Ogawa, Keiichiro Shiraga, Naoshi Kondo, Tsunao Hirai, Ikuo Osaka, and Asuka Kubota, “Inspection of Milk Components by Terahertz Attenuated Total Reflectance (THz-ATR) Spectrometer Equipped Temperature Controller”, IEEE / SICE International Symposium on System Integration, December 2011, pp. 192-196 M. Exter and D. Grischkowsky, “Optical and electronic properties of doped silicon from 0.1 to 2 TZh”, Applied Physics Letter, April 1990, Vol. 56, No. 17, pp.1694-1696

  When evaluating a liquid sample such as an aqueous solution or oil, the measurement is generally performed by sealing the liquid sample in a sample cell or supplying the liquid sample by a flow. In the dielectric measurement cell, window materials that transmit the terahertz wave are provided on the incident surface and the exit surface of the terahertz wave. In continuous wave electromagnetic wave spectroscopy measurement, it is known that multiple reflection of terahertz waves depending on the thickness of the window material occurs. It is not easy to eliminate the influence of multiple reflections from the thickness of two window materials, the sample thickness, and the dielectric constant. For example, a method of reducing interference ripples on a spectrum by taking a moving average is taken.

  Since water exhibits very strong absorption in the infrared region, a liquid sample is placed on an attenuated total reflection prism (ATR prism) made of high-resistance silicon, and the liquid sample is evaluated by transmitting electromagnetic waves into the prism. It is known (Non-Patent Document 3).

  However, in wideband electromagnetic waves, stray light is generated and interference ripples occur in the spectrum due to the effects of diffraction and multiple reflection that occur in the attenuated total reflection prism due to differences in beam diameter and focusing position due to wavelength aberration. Since these noises cause fluctuations in the amplitude and phase of the signal due to the influence of the refractive index fluctuation of the prism due to environmental fluctuations such as temperature, there is a problem that measurement reproducibility and measurement accuracy cannot be obtained.

  The present invention has been made in view of the above, and an object of the present invention is to reduce the generation of stray light and interference ripples on the spectrum, and improve measurement reproducibility and measurement accuracy.

  The dielectric spectroscopic device according to the present invention includes a radiator that photoelectrically converts an optical signal composed of two continuous light waves having different frequencies to generate an electromagnetic wave, and a trapezoidal cross-sectional shape that is measured on one of parallel planes. An object is arranged, the electromagnetic wave is incident from one inclined surface, the attenuated total reflection prism that emits the electromagnetic wave from the other inclined surface, and the other parallel surface of the attenuated total reflection prism, Detection using a radio wave absorber having a dielectric constant equivalent to that of an attenuated total reflection prism and reference light synthesized by modulating the phase of at least one of the two continuous light waves, and receiving the electromagnetic wave and performing homodyne detection And a container.

  According to the present invention, generation of stray light and interference ripples on a spectrum can be reduced, and measurement reproducibility and measurement accuracy can be improved.

It is a block diagram which shows the structure of the dielectric spectroscopy apparatus in this Embodiment. It is a figure which shows the example which used the lens for the measurement system. It is a figure which shows the attenuation | damping total reflection prism in this Embodiment. It is the graph which calculated the absorption coefficient and refractive index of the electromagnetic wave absorber in this Embodiment. It is an example of numerical analysis when a metal is disposed at the bottom of the attenuated total reflection prism. It is an example of numerical analysis when a radio wave absorber is disposed at the bottom of the attenuated total reflection prism. It is a figure which shows the modification of the attenuation | damping total reflection prism in this Embodiment. It is a figure which shows another modification of the attenuation | damping total reflection prism in this Embodiment. It is a figure which shows another modification of the attenuation | damping total reflection prism in this Embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a block diagram showing a configuration of a dielectric spectroscopic apparatus according to the present embodiment.

  1 includes a continuous wave light source 11A, 11B, splitters 12A, 12B, couplers 13A, 13B, phase modulators 14A, 14B, a first photomixer 16, a second photomixer 17, a lock-in amplifier 18, A monitor 19 and a delay line 20 are provided. The measurement sample 100 is disposed on the attenuated total reflection prism 30 between the parabolic mirrors 41A and 41B on the electromagnetic wave path. A radio wave absorber 31 having a dielectric constant equivalent to that of the prism material is provided at the bottom of the attenuated total reflection prism 30.

  The continuous wave light sources 11A and 11B output continuous wave optical signals having different frequencies. In the following description, the continuous wave optical signal output from the continuous wave light source 11A is referred to as a first optical signal, and the continuous wave optical signal output from the continuous wave light source 11B is referred to as a second optical signal.

  The splitter 12A demultiplexes the first optical signal. The splitter 12B demultiplexes the second optical signal.

  The coupler 13A combines the first optical signal demultiplexed by the splitter 12A and the second optical signal demultiplexed by the splitter 12B. The optical signal combined by the coupler 13A is input to the first photomixer 16 through the fiber.

  The first photomixer 16 photoelectrically converts the optical signal combined by the coupler 13A, and generates an electromagnetic wave (millimeter wave or terahertz wave) having a frequency that matches the frequency difference between the first optical signal and the second optical signal. As the first photomixer 16, for example, a single traveling carrier photodiode (UTC-PD) can be used. The first photomixer functions as a radiator that generates electromagnetic waves.

  The phase modulators 14A and 14B are phase modulators using an electro-optic crystal that can be electrically phase-modulated by a control signal. The phase modulator 14A is disposed between the splitter 12A and the coupler 13B. The phase modulator 14B is disposed between the splitter 12B and the coupler 13B. A sawtooth phase modulation is performed by applying a sawtooth control signal having a single frequency fm from the oscillator 15 to the phase modulators 14A and 14B, and a frequency shift equivalent to the modulation frequency fm is generated in the optical signal. The phase modulator 14A is disposed at a position for modulating one of the phases of the first optical signal demultiplexed by the splitter 12A. The phase modulator 14A may be disposed between the splitter 12A and the coupler 13A. The phase modulator 14B is arranged at a position where one of the phases of the second optical signal demultiplexed by the splitter 12B is modulated. The phase modulator 14B may be disposed between the splitter 12B and the coupler 13A. The phase modulators 14A and 14B invert the control signals and perform phase modulation by push-pull control. In addition, the structure provided with either one of phase modulator 14A, 14B may be sufficient.

  The coupler 13B combines the first optical signal and the second optical signal that have been phase-modulated by the phase modulators 14A and 14B, respectively.

  The delay line 20 is disposed on a fiber connecting the coupler 13B and the second photomixer 17, and the optical signal combined by the coupler 13B is input to the second photomixer 17 through the delay line 20. The delay line 20 can be realized with low loss by connecting a lens collimator to the fiber end, making two lenses face each other, and propagating parallel light. In the dielectric spectroscopic device, when homodyne detection of electromagnetic waves, the length of the fiber through which light propagates is adjusted in advance so that the two optical path length differences at the time of mixing by the detector coincide. In the present embodiment, the fibers from the coupler 13A to the first photomixer 16 and the fibers from the coupler 13B to the second photomixer 17 have the same delay length, and further, between the coupler 13B and the second photomixer 17. The delay line 20 has a length (delay length) equivalent to the spatial propagation length of the electromagnetic wave emitted from the first photomixer 16 to the second photomixer 17. Alternatively, the delay line 20 may be disposed between the coupler 13A and the first photomixer 16, or the delay line 20 may not be provided.

  The second photomixer 17 is irradiated with the optical signal combined by the coupler 13B, and receives the electromagnetic wave radiated from the first photomixer 16, transmitted through the attenuated total reflection prism 30, and reflected by the measurement sample 100. Then, homodyne mixing is performed to output an electric signal having a frequency fm. The second photomixer 17 can be realized by mounting the UTC-PD with an antenna and the optical fiber in the same package. The second photomixer 17 may be a photoconductive antenna (PCA: Photo-Conductive Antenna). The second photomixer functions as a detector that detects homodyne electromagnetic waves.

  The lock-in amplifier 18 detects the amplitude and phase by synchronously detecting the electrical signal output from the second photomixer 17 (see Non-Patent Document 2). The monitor 19 processes the amplitude and phase detected by the lock-in amplifier 18.

  In the terahertz wave band, an electromagnetic wave is incident on the attenuated total reflection prism 30 to irradiate the measurement sample 100 to be measured by a free space method using a pseudo optical system using a lens or a parabolic mirror, and the attenuated total reflection prism 30 is irradiated. The complex permittivity is measured by detecting the electromagnetic wave transmitted through the. When the parabolic mirrors 41A and 41B are used, since the multiple reflection in the lens does not occur compared to the case where the lens is used, the measurement value is stabilized. As shown in FIG. 2, lenses 42A and 42B may be used instead of the parabolic mirrors 41A and 41B, and the attenuated total reflection prism 30 with the measurement sample 100 placed on the center of the lenses 42A and 42B may be disposed. The attenuated total reflection prism 30 is disposed on the support base 35 to adjust the irradiation position of the electromagnetic wave.

  FIG. 3 is a diagram showing the attenuated total reflection prism in the present embodiment.

  The attenuated total reflection prism 30 is a dove type having a trapezoidal cross section. The electromagnetic wave is irradiated with a plane wave on a predetermined position of one inclined surface of the attenuated total reflection prism 30 and reflected by the measurement sample 100 arranged on the upper surface (a wide surface of parallel surfaces), and then the other inclined surface. Emits from the surface. The width and height of the attenuated total reflection prism 30 are adjusted according to the beam size of the irradiated electromagnetic wave. The angle θ of the inclined surface of the attenuated total reflection prism 30 is designed so that the incident electromagnetic wave is irradiated to the measurement sample 100 at 45 degrees.

  When electromagnetic waves are incident and emitted, approximately 30% of the electromagnetic waves are reflected at the interface between silicon and air, which is the material of the attenuated total reflection prism 30. A stray light component is generated by a change in beam diameter or a spherical wave while multiple reflections occur on the entrance surface and the exit surface. Further, even when electromagnetic waves are incident on the upper surface side from a predetermined position on one inclined surface of the attenuated total reflection prism 30, it is emitted after being reflected by the upper and lower surfaces of the attenuated total reflection prism 30, as indicated by arrows in the figure. Is done. Furthermore, an unnecessary wave enters due to the influence of diffraction caused by electromagnetic waves entering the edge of the attenuated total reflection prism 30. These stray light components and unwanted waves are combined with the optical system on the emission side through multiple reflections, resulting in measurement signal errors.

In the present embodiment, a radio wave absorber 31 is provided at the bottom of the attenuated total reflection prism 30, and the radio wave absorber 31 absorbs electromagnetic waves such as stray light without reflecting it. The radio wave absorber 31 uses low resistance silicon of 1 kΩ or less, and reduces the dielectric constant difference from the attenuated total reflection prism 30. The absorption coefficient n and the refractive index α of low-resistance silicon subjected to p-type or n-type high-concentration impurity doping (10 ¹⁵ to 10 ¹⁶ cm ⁻³ ) can be expressed by the Drude model (Non-patent Document 4). For example, FIG. 4 shows an example in which the absorption coefficient n and the refractive index α are calculated with the impurity concentration N _c = 2 × 10 ¹⁶ cm ⁻³ . As shown in the figure, a relatively flat absorption coefficient n and refractive index α can be realized at 0.1 to 2 THz. The radio wave absorber 31 desirably has the same dielectric constant as the material of the attenuated total reflection prism 30. As the radio wave absorber 31, rubber or ceramics in which a magnetic material is dispersed may be used. Alternatively, electromagnetic waves are attenuated by multiple internal scattering caused by conductive materials such as carbon dispersed in porous materials such as foamed polystyrene and sponge that transmit radio waves well, and dielectric fillers having a size comparable to the wavelength. May be. A metal pattern using a metamaterial that absorbs electromagnetic waves in a wide band may be formed. In addition to the bottom portion of the attenuated total reflection prism 30, the radio wave absorber 31 may be provided on the side surface (the side surface that is not the electromagnetic wave incident surface / outgoing surface).

  Next, a numerical analysis example of the effect of removing unnecessary reflected waves of the radio wave absorber according to the present embodiment will be described.

  FIG. 5A is an example in which a metal is disposed at the bottom of the attenuated total reflection prism and the time waveform of the signal intensity of the output pulse waveform when the incident wave to the attenuated total reflection prism is applied as a pulsed plane wave is numerically analyzed. FIG. 5B is an example of numerical analysis when a radio wave absorber (low resistance silicon) is arranged at the bottom of the attenuated total reflection prism. In both cases, at 0.175 nsec, the incident pulse-like plane wave reaches the exit surface of the attenuated total reflection prism through a desired propagation path. When a metal is arranged, it can be seen that an unnecessary reflected wave reaches the emission surface at 0.26 nsec. It can also be seen that small unnecessary reflected waves after 0.2 nsec, which are considered to be other stray light components, can be relatively reduced when the radio wave absorber is arranged.

  Next, a modified example of the attenuated total reflection prism will be described.

  FIG. 6 is a diagram showing a modification of the attenuated total reflection prism in the present embodiment. In the attenuated total reflection prism 30 shown in the figure, an antireflection structure 32 is provided on an incident surface and an output surface of an electromagnetic wave. Since the reflection is reduced when the electromagnetic wave enters and exits the attenuated total reflection prism 30, the occurrence of interference ripple and stray light due to multiple reflection can be reduced. As the antireflection structure 32, it is possible to use a quarter-λ dielectric film or a structure in which irregularities of 1/10 or less of the wavelength of the electromagnetic wave are formed on the surface, like the optical antireflection structure.

  FIG. 7 is a diagram showing another modified example of the attenuated total reflection prism in the present embodiment. In the attenuated total reflection prism 30 shown in the figure, an antireflection structure 32 is further provided at the bottom of the attenuated total reflection prism 30 in FIG. The dielectric constant and thickness of the antireflection structure 32 provided at the bottom are appropriately designed according to the material of the attenuated total reflection prism 30 and the dielectric constant of the radio wave absorber 31. Since the antireflection structure 32 is provided at the bottom of the attenuated total reflection prism 30, the radio wave absorber 31 does not need to have the same dielectric constant as that of the attenuated total reflection prism 30.

  FIG. 8 is a diagram showing still another modification of the attenuated total reflection prism in the present embodiment. In the attenuated total reflection prism 30 shown in the figure, a temperature controller 33 is provided under the radio wave absorber 31 of the attenuated total reflection prism 30 in FIG. The temperature controller 33 is a metal plate provided with, for example, a Peltier element and a heater. By using silicon having high thermal conductivity for the radio wave absorber 31, it is possible to efficiently control the temperature of the measurement sample 100 and the attenuated total reflection prism 30, and to suppress the refractive index fluctuation of the attenuated total reflection prism 30. is there.

  As described above, according to the present embodiment, the measurement sample 100 is arranged on the parallel surface of the attenuated total reflection prism 30 having a trapezoidal cross section, and the radio wave absorber 31 is provided on the surface facing the arrangement surface. By reducing the incidence of stray light and spectral interference ripple due to the effects of diffraction and multiple reflection occurring in the attenuated total reflection prism 30 by entering the electromagnetic wave from the inclined surface and emitting the electromagnetic wave from another inclined surface As a result, measurement reproducibility and measurement accuracy can be improved.

DESCRIPTION OF SYMBOLS 11A, 11B ... Continuous wave light source 12A, 12B ... Splitter 13A, 13B ... Coupler 14A, 14B ... Phase modulator 15 ... Oscillator 16, 17 ... Photomixer 18 ... Lock-in amplifier 19 ... Monitor 20 ... Delay line 30 ... Attenuation total reflection Prism 31 ... Radio wave absorber 32 ... Antireflection structure 33 ... Temperature controller 35 ... Support base 41A, 41B ... Parabolic mirror 42A, 42B ... Lens 100 ... Measurement sample

Claims (3)

A radiator that generates an electromagnetic wave by photoelectrically converting an optical signal in which two continuous light waves having different frequencies are combined;
A cross-sectional shape is trapezoidal, an object to be measured is arranged on one of parallel surfaces, the electromagnetic wave is incident from one inclined surface, and the attenuated total reflection prism emits the electromagnetic wave from the other inclined surface;
A radio wave absorber having a dielectric constant equivalent to that of the attenuated total reflection prism, disposed on the other parallel surface of the attenuated total reflection prism;
A detector that receives a reference beam synthesized by modulating the phase of at least one of the two continuous light waves, receives the electromagnetic wave, and performs homodyne detection;
A dielectric spectroscopic apparatus comprising:   2. The dielectric spectroscopic apparatus according to claim 1, further comprising an antireflection structure in at least one of the inclined surface and the attenuated total reflection prism and the radio wave absorber.   The dielectric spectroscopic apparatus according to claim 1, further comprising a temperature control unit that controls a temperature of the attenuated total reflection prism.

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