JP2017009408A - Dielectric spectroscopic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure reproducibility and measurement accuracy of the measurement even when a refraction index of the fiber changes due to the environmental variation.SOLUTION: A phase modulation spectrum circuit 4 handles a fiber from a coupler 13A to a radiator 16 and the fiber from a coupler 13B to a detector 17 as an equivalent delay length, and provides a delay line 20 of the delay length equivalent to a space propagation length of a terahertz wave emitted from a radiator 16 to the detector 17 between the coupler 13B and the detector 17 and phase modulators 14A, 14B and between splitters 12A, 12B, couplers 13A, 13B and the splitter 12A and the couplers 13A, 13B. An optical path from the splitter 12A to each of the couplers 13A, 13B and the optical path from the splitter 12B to each of couplers 13A, 13B are made equal. As a result, a refractive index variation of the optical path length can be made equal, and a phase change accompanied with an environmental change can be suppressed, and the data can be obtained at a high accuracy.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an apparatus for noninvasively measuring the concentration of blood components such as humans and animals, and more particularly to a dielectric spectroscopic apparatus using photoelectric conversion.

  As aging progresses, the response to adult diseases has become a major issue. In particular, blood tests such as blood sugar levels are a heavy burden on patients because blood collection with a needle is necessary. 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. Generally, it is expressed as a linear combination of relaxation curves based on the Cole-Cole equation, and the complex dielectric constant of the medium or solvent is calculated. 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.

  As a conventional measurement method, there is a dielectric spectroscopic device using photoelectric conversion (photomixing) 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

  However, if the refractive index of the fiber fluctuates due to environmental fluctuations such as temperature, and the difference between the two optical path lengths fluctuates, there is a problem that the amplitude and phase of the signal become unstable and measurement reproducibility and measurement accuracy cannot be obtained. .

  The present invention has been made in view of the above, and an object thereof is to ensure measurement reproducibility and measurement accuracy even when the refractive index of a fiber changes due to environmental fluctuations.

  The dielectric spectroscopic device according to the present invention includes a first light source that outputs a first optical signal, a second light source that outputs a second optical signal having a frequency different from that of the first optical signal, and the first light signal. The first splitter for demultiplexing one optical signal, the second splitter for demultiplexing the second optical signal, and the phases of the first optical signal demultiplexed by the first splitter, respectively. The first and second phase modulators that are electrically modulated, the first optical signal whose phase is modulated, and the second optical signal that is demultiplexed by the second splitter are multiplexed. The first and second couplers for outputting the third and fourth optical signals and the third optical signal are photoelectrically converted to generate millimeter wave or terahertz electromagnetic waves to irradiate the object to be measured. A radiator and the electromagnetic wave transmitted or reflected by the object to be measured; and the electromagnetic wave and the fourth optical signal. A detector for homodyne mixing, and an optical path length from the first splitter to the first coupler is equal to an optical path length from the first splitter to the second coupler, and the second splitter The optical path length from the first coupler to the first coupler is equal to the optical path length from the second splitter to the second coupler.

  In the dielectric spectroscopic device, the first splitter is formed on a first planar optical circuit, the second splitter and the first and second couplers are formed on a second planar optical circuit, and the first 1. First and second waveguides as first and second phase modulators and first and second modulation electrodes are formed on a phase modulation substrate, and the first and second splitting destinations of the first splitter are the first and second phase modulators. A first waveguide is connected to a second waveguide, and the first and second waveguides are connected to the first and second couplers.

  In the dielectric spectroscopic device, the first and second couplers, the first and second waveguides, and the first and second waveguides centering on a line connecting the input of the first splitter and the input of the second splitter. The second modulation electrode is arranged symmetrically.

  According to the present invention, measurement reproducibility and measurement accuracy can be ensured even when the refractive index of the fiber changes due to environmental fluctuations.

It is a block diagram which shows the structure of the dielectric spectroscopy apparatus in this Embodiment. It is a top view which shows the structure of the phase modulation optical circuit used with the said dielectric spectroscopy apparatus. It is a figure which shows the measurement system of a dielectric spectroscopy sensor. It is a figure which shows the measurement system of another dielectric spectroscopy sensor.

  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, an oscillator 15, a radiator 16, a detector 17, a lock-in amplifier 18, and a monitor. 19 and a delay line 20.

  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 into two, and the splitter 12B demultiplexes the second optical signal into two.

  The phase modulators 14A and 14B are phase modulators that use an electro-optic crystal that can be electrically phase-modulated by a control signal. Be placed. A control signal having a single frequency w from the oscillator 15 is applied to the phase modulators 14A and 14B to perform serodyne phase modulation, and a frequency shift equivalent to the modulation frequency w is generated in the first optical signal. Note that the phase modulators 14A and 14B may be arranged at the subsequent stage of the splitter 12B, and a frequency shift equivalent to the modulation frequency w may be generated in the second optical signal.

  Each of the couplers 13A and 13B combines the first optical signal phase-modulated by the phase modulators 14A and 14B and the second optical signal demultiplexed by the splitter 12B. The optical signal combined by the coupler 13A is input to the radiator 16 through the fiber. The reference light combined by the coupler 13B is input to the detector 17 through the fiber in which the delay line 20 is disposed.

  The delay line 20 is disposed on a fiber connecting the coupler 13B and the detector 17. The reference light combined by the coupler 13B is input to the detector 17 through the delay line 20. The delay line 20 may be a normal optical fiber, but 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.

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

  The detector 17 is irradiated with the reference light combined by the coupler 13B, receives the electromagnetic wave radiated from the radiator 16 and transmitted through the sample 100, and performs homodyne mixing of the electromagnetic wave and the reference light to generate an electric wave of frequency w. Output a signal. The detector 17 can be realized by mounting a THz mixer composed of an SBD (Schottky barrier diode) with an antenna, a photomixer composed of an UTC-PD with an antenna, and an optical fiber in the same package. . The detector 17 may be a photoconductive antenna (PCA: Photo-Conductive Antenna). In the photoconductive antenna, the conductance between the antennas is modulated by the difference frequency between two continuous wave optical signals included in the reference light by irradiation of the reference light.

  The lock-in amplifier 18 detects the amplitude and phase by synchronously detecting the electrical signal output from the detector 17, and the monitor 19 processes the amplitude and phase detected by the lock-in amplifier 18.

  In the terahertz wave band, the complex dielectric constant is measured from the transmitted signal by irradiating the sample 100 as a measurement target with a terahertz wave by a free space method using a pseudo optical system using parabolic mirrors 31A and 31B. When the parabolic mirrors 31A and 31B are used, since the multiple reflection in the lens does not occur as compared with the case where the lens is used, the measurement value is stabilized.

  In the conventional dielectric spectroscopic apparatus, 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 at the detector coincide. In the present embodiment, the fiber from the coupler 13A to the radiator 16 and the fiber from the coupler 13B to the detector 17 have the same delay length, and further, the detection from the radiator 16 is performed between the coupler 13B and the detector 17. A delay line 20 having a length (delay length) equivalent to the spatial propagation length of the terahertz wave emitted to the device 17 is provided.

  Here, the effect of the delay line will be described.

The refractive index temperature coefficient dns / dT of the fiber is 6.6 × 10 ⁻⁶ / ° C., and the refractive index temperature coefficient dna / dT of air is −1.4 × 10 ⁻⁷ / ° C. The length of the fiber from coupler 13A to the radiator 16 _{L 1,} the length of the fiber from coupler 13B to the detector 17 and _{L 2.} The space propagation length of the terahertz wave and L _3. The conditions for homodyne detection are as follows.

n _s L ₁ + L ₃ = n _s L ₂
Here, _ns is the refractive index of the fiber core 1.4. The refractive index of air is 1.

  If the temperature change is ΔT, the optical path length difference ΔL can be expressed by the following equation.

ΔL = (L ₁ −L ₂ ) (dns / dT) ΔT + L ₃ (dna / dT) ΔT

  The delay difference Δτ is as follows.

Δτ = 2πΔL / λ
Here, λ is the wavelength of the terahertz wave. For example, when L ₁ −L ₂ = 20 cm and L ₃ = 28 cm and a change of ΔT = 1 ° C. occurs, the wavelength λ of the terahertz wave is λ = 0.3 mm at 1 THz, so the delay difference Δτ = 0.03 rad = 1.8 degrees of phase signal variation associated with a temperature.

In this embodiment, the length of the delay line 20 as L _4, together two optical path length difference, and the following respective lengths.

n _s L ₁ + L ₃ = n _s L ₂ + L ₄
That is, by setting L ₁ ≈L ₂ and L ₃ ≈L ₄ , ΔL≈0 can be obtained, and Δτ≈0 can be obtained, so that the fluctuation of the phase signal can be reduced with respect to the temperature fluctuation. Also, when there is a change in the refractive index of air due to humidity, the change can be canceled in the same manner. It should be noted that the delay line 20 may be disposed between the coupler 13A and the radiator 16 so as to satisfy the above expression, or the delay line 20 may not be provided.

  As described above, it was shown that the phase signal of the terahertz wave is affected by the temperature fluctuation due to the difference in the initial fiber length. Such an effect can be prominent also in the optical path from the splitters 12A and 12B to the couplers 13A and 13B.

The lengths of the fibers from the splitter 12A to the coupler 13A and from the splitter 12A to the coupler 13B are D ₁₁ and D ₁₂ . The lengths of the fibers from the splitter 12B to the coupler 13A and from the splitter 12B to the coupler 13B are D ₂₁ and D ₂₂ . Further, the lengths of the LN crystals in the phase modulators 14A and 14B are assumed to be equal.

  If the temperature change is ΔT, the optical path length difference ΔL can be expressed by the following equation.

ΔD = (D ₁₁ −D ₁₂ ) (dns / dT) ΔT + (D ₂₁ −D ₂₂ ) (dns / dT) ΔT

  The delay difference Δτ is as follows.

Δτ = 2πΔD / λ
Here, λ is the wavelength of the light wave. For example, when D ₁₁ −D ₁₂ = 2 mm and D ₂₁ −D ₂₂ = 2 mm and a change of ΔT = 1 ° C. occurs, the delay difference Δτ = 0.1 rad≈when the wavelength λ of the light wave is 1.55 μm. Variations in the phase signal associated with a temperature of 11 degrees occur. In the conventional configuration, the LN crystals are not arranged symmetrically. When the length of the LN crystal is D _LN , the refractive index temperature coefficient dnl / dT of the LN crystal is 5-50 × 10 ⁻⁶ / ° C., and is affected by D _LN (dnl / dT) ΔT. D _LN is typically a few centimeters and is estimated to be further affected. Therefore, in the present embodiment, the phase modulators are arranged so that D ₁₁ = D ₁₂ and D ₂₁ = D ₂₂ and the optical path lengths are made uniform.

  FIG. 2 is a plan view showing the configuration of the phase modulation optical circuit according to the present embodiment. The phase modulation optical circuit 4 shown in the figure is a circuit having a configuration from the splitters 12A and 12B to the couplers 13A and 13B in the dielectric spectroscopic device shown in FIG. 1, and the first and second inputs are used as axes. It is formed symmetrically.

  The phase modulation optical circuit 4 includes planar optical circuits 41A and 41B and a phase modulation unit.

  The planar optical circuit 41A is formed with a connection portion between the first input, the splitter 12A, and the phase modulation unit. The first optical signal input from the first input is branched into two by the splitter 12A, and input from the connection portion to the waveguides 44A and 44B formed in the phase modulation portion 42.

  In the phase modulation section 42, waveguides 44A and 44B and modulation electrodes 43A and 43B for the waveguides 44A and 44B are formed. A modulation voltage is supplied to the modulation electrodes 43A and 43B from the outside, and a voltage is applied to a portion through which the light wave passes to modulate the phase of the light wave that passes.

  In the planar optical circuit 41B, a second input, a splitter 12B, a connection portion with the phase modulation unit 42, couplers 13A and 13B, and first and second outputs are formed. The second optical signal input from the second input is branched into two by the splitter 12B and input to the couplers 13A and 13B. The phase-modulated first optical signal is input to the couplers 13A and 13B from the waveguides 44A and 44B of the phase modulation unit 42, respectively. In each of the couplers 13A and 13B, the second optical signal and the phase-modulated first optical signal are combined and output to the first output and the second output.

  The phase modulation unit 42 can use an LN crystal substrate using an electro-optic effect. A sawtooth waveform signal is applied to the phase modulation unit 42 by applying a sawtooth waveform signal having a single frequency w (for example, 1 GHz) from the oscillator 15, and a frequency shift equivalent to the modulation frequency w is generated in the optical signal. An optical signal having a frequency F2 (= F1 + 2w) is output. The control voltage of the phase modulator 42 can be expressed as Vm (t) = N2Vπwt by the product of an integer N and 2Vπ (Vπ is a control voltage whose phase changes by π).

  When there is one phase modulator, a frequency transition 2N times the frequency w of the control voltage occurs, and the frequency of the signal detected by the detector becomes 2N times the frequency of the control voltage (see Non-Patent Document 2).

  As the planar optical circuits 41A and 41B, quartz, polymer, silicon or compound semiconductor waveguides (planar light wave circuit PLC: Planar Lightwave Circuit) formed on a semiconductor substrate such as silicon can be used. For the phase modulation unit 42, an electro-optic polymer may be used, and a carrier injection type PIN phase modulator formed on a silicon substrate or a compound semiconductor substrate or a phase modulator based on a thermo-optic effect may be used. The planar lightwave circuit may be formed monolithically.

  By mounting the phase modulation optical circuit 4 on a temperature-adjustable board, the influence of the environment-dependent temperature can be further reduced.

  Next, a measurement system of a dielectric spectroscopic sensor using a focusing lens will be described.

  FIG. 3 is a diagram showing a measurement system of the dielectric spectroscopic sensor of the present embodiment. In the example of FIG. 3, a transmission type is arranged in the measurement system of the dielectric spectroscopic sensor, and the amplitude and phase of a transmission signal transmitted through a liquid such as an aqueous solution or oil are measured. The terahertz wave radiated from the radiator 16 passes through the lens 51 and enters the sample cell of the dielectric constant measurement cell 53 held by the fixing jig 54. The size of the sample cell is, for example, several millimeters × several millimeters or more as the beam size or more. The material of the window plate 52 for fixing the sample may be a high resistance Si, Z cut crystal, HDPE, TPX, Tsurupica, etc., and a material having a high transmittance is selected according to the measurement frequency. The terahertz wave that has passed through the sample cell passes through the lens 51 and is received by the detector 17. Note that the sample cell may have a flow cell configuration including an inlet and an outlet. Moreover, you may measure solid.

  FIG. 4 is a diagram showing a measurement system of another dielectric spectroscopic sensor of the present embodiment. In the example of FIG. 4, a sample cell is disposed on the ATR prism 55 made of silicon, and the amplitude and phase of the reflected signal reflected by the sample cell are measured. The terahertz wave radiated from the radiator 16 passes through the lens 51 and the ATR prism 55 and is reflected at the interface between the sample cell of the dielectric constant measurement cell 53 held by the fixing jig 54 and the ATR prism 55. The sample cell is sealed in the dielectric constant measurement cell 53 by the window plate 52. The terahertz wave reflected by the sample cell passes through the ATR prism 55 and the lens 51 and is received by the detector 17.

  As described above, according to the present embodiment, the fiber from the coupler 13A to the radiator 16 and the fiber from the coupler 13B to the detector 17 have the same delay length, and the gap between the coupler 13B and the detector 17 is set. And a delay line 20 having a delay length equivalent to the spatial propagation length of the terahertz wave emitted from the radiator 16 to the detector 17, and between the splitters 12A and 12B, the couplers 13A and 13B, and the splitter 12A and the couplers 13A and 13B. In the phase modulation optical circuit 4 provided with the phase modulators 14A and 14B, the optical paths from the splitter 12A to the couplers 13A and 13B and the optical paths from the splitter 12B to the couplers 13A and 13B are made equal in length. Long refractive index fluctuations can be made equal, phase changes caused by environmental fluctuations can be suppressed, and data can be acquired with high accuracy. That.

11A, 11B ... Continuous wave light source 12A, 12B ... Splitter 13A, 13B ... Coupler 14A, 14B ... Phase modulator 15 ... Oscillator 16 ... Radiator 17 ... Detector 18 ... Lock-in amplifier 19 ... Monitor 20 ... Delay line 31A, 31B Parabolic mirror 4 ... Phase modulation optical circuit 41A, 41B ... Planar optical circuit 42 ... Phase modulation unit 43A, 43B ... Modulation electrode 44A, 44B ... Waveguide 51 ... Lens 52 ... Window plate 53 ... Dielectric constant measurement cell 54 ... Fixing jig 55 ... ATR prism 100 ... Sample

Claims (3)

A first light source that outputs a first optical signal;
A second light source that outputs a second optical signal having a frequency different from that of the first optical signal;
A first splitter for demultiplexing the first optical signal;
A second splitter for demultiplexing the second optical signal;
First and second phase modulators that electrically modulate the respective phases of the first optical signal demultiplexed by the first splitter;
The first and second optical signals, which are combined with the first optical signal whose phase is modulated and the second optical signal demultiplexed by the second splitter, are output as third and fourth optical signals, respectively. Two couplers,
A radiator for photoelectrically converting the third optical signal to generate an electromagnetic wave of millimeter wave or terahertz wave and irradiating the object to be measured;
A detector that receives the electromagnetic wave transmitted or reflected from the object to be measured and performs homodyne mixing of the electromagnetic wave and the fourth optical signal;
The optical path length from the first splitter to the first coupler is equal to the optical path length from the first splitter to the second coupler, and the optical path length from the second splitter to the first coupler is 2. A dielectric spectroscopic apparatus characterized in that the optical path length from the second splitter to the second coupler is made equal. Forming the first splitter on a first planar optical circuit;
Forming the second splitter and the first and second couplers on a second planar optical circuit;
Forming first and second waveguides and first and second modulation electrodes on the phase modulation substrate as the first and second phase modulators;
The demultiplexing destination of the first splitter is connected to the first and second waveguides, and the first and second waveguides are connected to the first and second couplers. Item 2. The dielectric spectroscopic apparatus according to Item 1.   The first and second couplers, the first and second waveguides, and the first and second modulation electrodes are centered on a line connecting the input of the first splitter and the input of the second splitter. 3. The dielectric spectroscopic apparatus according to claim 2, wherein the dielectric spectroscopic apparatus is arranged symmetrically.

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