Fiber Sensing Device

Abstract

To achieve a fiber sensing device for detecting a physical quantity of a measurement object with high sensitivity and high accuracy without being affected by ambient temperature.SOLUTION: A fiber sensing device includes: an optical frequency comb resonator 100 having a fiber sensor 10 in a resonator and generating an optical frequency comb whose repetition frequency changes with respect to change in a detected physical quantity of the measurement object; a local oscillator for generating a high frequency reference signal of a prescribed frequency; and a mixing element 27 for receiving the optical frequency comb to output a high frequency signal of a high-order harmonic component of the repetition frequency and a beat signal of the high frequency reference signal of the local oscillator.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a fiber sensing device using an optical frequency comb resonator. [Background technology]

[0002] Fiber sensing using the optical / electrical frequency conversion function of a fiber optical frequency comb resonator (fiber sensing optical frequency comb) is a fiber sensing technique with an electrical frequency readout that enables high-precision measurements. For example, in biosensing and refractive index sensing, a fiber sensor is placed inside a fiber optical frequency comb resonator, and the optical spectral shift dependent on the physical quantity being measured is converted into a frequency shift of the optical RF signal (repetition rate) via the refractive index dispersion of the resonator fiber. In strain sensing, the optical frequency comb resonator fiber itself is used as a sensor to measure the frequency shift of the optical RF signal due to strain. In both cases, the optical RF signal can be observed as an electrical frequency signal using a photodetector, enabling high-precision measurements using RF measurement equipment based on an electrical frequency standard.

[0003] Among the physical quantities measured, refractive index measurement is particularly useful. The refractive index is an index that indicates the speed of light in a material and is derived from the material's inherent dielectric constant, which describes the interaction between light and the material. Therefore, accurate refractive index measurement techniques can be used to evaluate optical components, industrial products, beverages, food, and more. Furthermore, by chemically modifying the surface of a refractive index sensor with a molecular recognition layer, it can be used to detect the content of toxic chemicals or identify viruses and proteins. Highly sensitive sensors for refractive index detection have attracted attention, including SPR sensors (Patent Document 1), which utilize the surface plasmon resonance phenomenon, and multimode interference (MMI) fiber sensors (Patent Document 2), which utilize the multimode interference and Gooss-Henschen shift effect within optical fibers.

[0004] One example of an MMI fiber sensor is based on a cladless multimode optical fiber with single-mode optical fibers at both ends (Patent Document 3). Light propagates through this cladless multimode optical fiber, undergoing total internal reflection. When a target substance, such as a solution or suspension, comes into contact with the side of the cladless multimode optical fiber, a Gooss-Henschen shift effect occurs, resulting in a phase change at the total reflection surface depending on the refractive index. As a result, light passing through the MMI fiber sensor exhibits a sample-dependent optical spectral shift. Measuring this optical spectral shift allows the refractive index and concentration of the target substance to be determined.

[0005] MMI fiber sensors are compatible with optical fiber optics, and it has already been demonstrated that incorporating them into a fiber optic frequency comb resonator enables highly sensitive detection of changes in the refractive index (or concentration) of an object as a change in the optical RF signal frequency (Patent Document 3). Furthermore, in the field of dual optical frequency comb spectroscopy, a technique has been reported in which the frequency fluctuations of an optical RF signal are cancelled out by mechanically sharing two fiber optic frequency comb resonators (Non-Patent Document 1). There are also methods for receiving terahertz-band RF signals using electro-optic crystal materials (Patent Document 4) or photoconductive antennas (Non-Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-257631 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-251963 [Patent Document 3] Japanese Patent Application Publication No. 2019-039723 [Patent Document 4] Japanese Patent Application Laid-Open No. 2002-031658 [Non-patent literature]

[0007] [Non-Patent Document 1] Y. Nakajima, Y. Kusumi, and K. Minoshima, “Mechanical sharing dual-comb fiber laser based on an all-polarization-maintaining cavity configuration”, Optics Letters, Vol. 46, Issue 21, pp. 5401-5404 (2021). [Non-patent document 2] S. Yokoyama, R. Nakamura1, M. Nose, T. Araki, and T. Yasui, “Terahertz spectrum analyzer based on a terahertz frequency comb”, Optics Express, Vol. 16, Issue 17, pp. 13052-13061 (2008). Summary of the Invention [Problem to be solved by the invention]

[0008] However, while refractive index measurements incorporating an MMI fiber sensor into an optical frequency comb resonator can detect refractive index changes of about 0.001, the signal frequency change is only a few tens of hertz, which is extremely small compared to the RF signal frequency, so the amplification of frequency changes during detection has been an issue.In addition, optical frequency comb resonators are generally susceptible to the effects of ambient temperature, and temperature drift during measurements can cause errors, which has also been an issue. [Means for solving the problem]

[0009] A fiber sensing device according to one aspect of the present invention comprises an optical frequency comb resonator having a fiber sensor within a resonator and generating an optical frequency comb whose repetition frequency changes in response to a detected change in a physical quantity to be measured; a local oscillator generating a high-frequency reference signal of a predetermined frequency; and a mixing element that receives the optical frequency comb and outputs a high-frequency signal of a higher-order harmonic component of the repetition frequency and a beat signal of the high-frequency reference signal of the local oscillator.

[0010] The mixing element may be a photoconductive antenna element that receives the optical frequency comb, internally generates high-order harmonic components of a repetition frequency as a high-frequency photocarrier signal, and photoconductively mixes the high-order harmonic photocarrier signal with a high-frequency reference signal of the local oscillator to generate a beat between the high-order harmonic photocarrier signal and the high-frequency reference signal.

[0011] The mixing element may be an electro-optic crystal that receives the optical frequency comb and a high-frequency reference signal from the local oscillator, generates a sideband equal to the frequency of the high-frequency reference signal in a series of optical frequency modes that make up the optical frequency comb, and generates an optical beat with an optical frequency mode of an adjacent frequency.

[0012] The photoconductive antenna element may include a photoconductive film, a metal parallel transmission line, and a metal antenna facing each other on the surface of the photoconductive film, the optical frequency comb being focused in a gap (gap) between the metal antennas to generate the high-order harmonic photocarrier signal in the photoconductive film in the gap, and the high-frequency reference signal being supplied to the metal antenna from the side opposite the light-receiving surface of the optical frequency comb.

[0013] The electro-optic crystal may be configured such that the optical frequency comb and the high-frequency reference signal of the local oscillator are incident on the same axis from the same direction or from opposite directions with a predetermined polarization direction, and the crystal axis is arranged so as to maximize the sideband generation efficiency.

[0014] The frequency of the high-frequency reference signal may be preset to be adjacent to the high-order harmonic component when a sensor portion of the optical frequency comb resonator is not in contact with the object to be measured.

[0015] The optical frequency comb resonator may further include a calculation unit that determines the order of the higher harmonic from the amount of change in beat frequency when the repetition frequency of the optical frequency comb is changed by a known amount while the sensor portion of the optical frequency comb resonator is not in contact with the object to be measured.

[0016] Another aspect of the present invention provides a fiber sensing device comprising: a first optical frequency comb resonator having a first fiber sensor within a resonator and generating a first optical frequency comb; a second optical frequency comb resonator having a second fiber sensor within the resonator and generating a second optical frequency comb; a generating element that receives the first optical frequency comb and outputs high-order harmonic components of the repetition frequency into free space as a high-frequency radio wave signal; and a mixing element that outputs a high-frequency signal of the high-order harmonic components of the repetition frequency of the second optical frequency comb and a beat signal of the high-frequency radio wave signal output from the generating element, wherein only one of the first fiber sensor and the second fiber sensor is an active sensor whose repetition frequency changes in response to a change in a physical quantity detected when the sensor comes into contact with a measurement object, and the other fiber sensor is a dummy sensor.

[0017] The mixing element may be a first photoconductive antenna that receives the second optical frequency comb, internally generates high-order harmonic components of a repetition frequency as a high-frequency photocarrier signal, and photoconductively mixes the high-order harmonic radio wave signal output from the generating element to generate a beat frequency of the high-order harmonic photocarrier signal and the high-order harmonic radio wave signal.

[0018] The generating element may be a second photoconductive antenna, a nonlinear optical crystal, or a uni-traveling-carrier photodiode that receives the first optical frequency comb and generates high-order harmonic components of the repetition frequency as a high-frequency radio wave signal.

[0019] The mixing element may be an electro-optic crystal that receives the second optical frequency comb and the high-order harmonic radio wave signal output from the generating element, generates a sideband equal to the frequency of the high-order harmonic radio wave signal in an optical frequency mode that constitutes the second optical frequency comb, and generates an optical beat with an optical frequency mode of an adjacent frequency.

[0020] The first photoconductive antenna element may include a photoconductive film, a metal parallel transmission line and a metal antenna facing each other on a surface of the photoconductive film, the first optical frequency comb being focused in a gap between the metal antennas to generate the high-order harmonic photocarrier signal in the photoconductive film in the gap, and the high-order harmonic radio wave signal being supplied to the metal antenna from the side opposite to the light-receiving surface of the second optical frequency comb.

[0021] The second photoconductive antenna element may include a photoconductive film, a metal parallel transmission line and a metal antenna facing each other on a surface of the photoconductive film, the first optical frequency comb being focused in a gap between the metal antennas to which a bias voltage is applied, and the high-order harmonic radio wave signal being output from the metal antenna on the opposite side to the light-receiving surface of the first optical frequency comb.

[0022] The optical frequency comb may further include a calculation unit that measures the repetition frequency of the first optical frequency comb, the repetition frequency of the second optical frequency comb, and a beat frequency, and determines the order of the high-order harmonic.

[0023] The dummy sensor may be located adjacent to the active sensor. [Effects of the Invention]

[0024] According to one aspect of the present invention, the mixing element has a repetition frequency f rep Higher order (mth order) harmonic component mf rep and the frequency f of the high frequency reference signal THz The difference frequency component f b (f b =mf rep -f THz or f b =f THz -mf rep ) appears, and when the sensor is in contact with the object, the change in the repetition rate of the optical frequency comb is amplified m times. As a result, even a small frequency change relative to the RF frequency can be detected with sufficient measurement sensitivity.

[0025] Furthermore, according to another aspect of the present invention, by generating a high-frequency radio wave signal equivalent to the high-frequency reference signal from a dummy sensing optical frequency comb resonator having a configuration equivalent to that of the active sensing optical frequency comb resonator, it is possible to almost completely compensate for the temperature characteristics of the optical frequency comb resonator, thereby realizing a fiber sensing device that is excellent not only in detection sensitivity but also in detection accuracy and stability. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a block diagram of a fiber sensing device according to a first embodiment of the present invention. [Figure 2] FIG. 1 is a conceptual diagram showing the function of a multi-mode interference (MMI) fiber sensor according to a first embodiment of the present invention. [Figure 3] 10 is a graph showing the optical spectrum measurement results of a ring-type mode-locked Er fiber optical frequency comb incorporating an MMI fiber sensor. [Figure 4] 10 is a graph showing the optical RF signal spectrum measurement results of a ring-type mode-locked Er fiber optical frequency comb incorporating an MMI fiber sensor. [Figure 5] 10 is a graph showing the relationship between the ethanol concentration of an ethanol solution and the frep shift measured using a ring-type mode-locked Er fiber optical frequency comb incorporating an MMI fiber sensor. [Figure 6] 5A to 5C are diagrams illustrating the operation of an example of the photoconductive antenna according to the first embodiment of the present invention. [Figure 7] FIG. 3 is an explanatory diagram showing a frequency amplification (multiplication) effect in the first embodiment of the present invention. [Figure 8] 3A to 3C are diagrams illustrating the operation of an example of an electro-optic crystal according to the first embodiment of the present invention. [Figure 9] FIG. 10 is a block diagram of a fiber sensing device according to a second embodiment of the present invention. [Figure 10]FIG. 10 is a block diagram of a fiber sensing device having a more precise temperature compensation function in a second embodiment of the present invention. [Figure 11] 4 is a graph showing experimental results in the first embodiment of the present invention. [Figure 12] 10 is a graph showing experimental results in the second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] Hereinafter, an embodiment (first embodiment) according to one aspect of the present invention will be described in detail with reference to the drawings. In the block diagrams of Figures 1, 8, 9, and 10, double lines indicate propagation paths for optical signals, and single lines indicate propagation paths for electrical signals. (First embodiment)

[0028] In Fig. 1, a pumping laser diode 1, a wavelength division multiplexing (WDM) coupler 2, an erbium-doped optical fiber (EDF) 3, an isolator 5, and an optical fiber coupler 6 are loop-connected by a single-mode optical fiber 10 to form a sensing optical frequency comb resonator 100. This resonator has a repetition rate of f rep In a typical optical frequency comb, the ring resonator generates an optical frequency comb with a repetition rate of f rep is expressed by the following equation (1). f rep = c / nL (1) Here, c is the speed of light, n is the group refractive index of the fiber, and L is the resonator length of the loop. The optical path length of the entire resonator is then expressed as nL.

[0029] In this embodiment, a sensor 4 is inserted into the loop of the sensing optical frequency comb resonator 100. The sensor 4 may be a multimode interference (MMI) fiber sensor. Figure 2 shows a conceptual diagram of an MMI fiber sensor. This sensor is composed of a cladless multimode optical fiber with single-mode optical fibers (10a, 10b) at both ends. Light supplied from the input single-mode optical fiber 10a is diffracted and propagates through the cladless multimode optical fiber while undergoing total reflection at the side, toward the output single-mode optical fiber 10b. Of the light propagating in the cladless multimode optical fiber, only light with wavelengths that satisfy the multimode interference conditions is output from the output single-mode optical fiber 10b. Furthermore, when the object to be measured comes into contact with the side of the cladless multimode optical fiber, a change occurs in the Goos-Henschen shift effect, causing a phase delay during total reflection and a change in the effective optical path length. As a result, the multimode interference wavelength exhibits sample refractive index dependence, resulting in a shift in the optical frequency comb spectrum. This optical spectral shift is converted into a change in the optical cavity length via the refractive index dispersion of the cavity fiber, resulting in a change in the repetition rate (f rep The amount of the Goos-Henschen shift effect depends on the refractive index of the object being measured, so if this frequency change can be detected, the refractive index of the object can be calculated.

[0030] In the present invention, the sensor 4 inserted into the loop of the sensing optical frequency comb resonator is not limited to an MMI fiber sensor. Instead of an MMI fiber sensor, an SPR sensor, a fiber Bragg grating (FBG) sensor, or an offset-core fiber sensor may be used. Furthermore, the side of a cladless multimode optical fiber may be surface-modified to selectively adsorb specific biological substances (chemical substances, viruses, proteins, etc.). In this case, the presence or amount of the substance can be determined from changes in the repetition frequency. Furthermore, since the single-mode optical fiber 10 has the property that its optical resonator length changes due to external forces such as bending or stretching, this property can be utilized to use the single-mode optical fiber 10 itself as a strain sensor or vibration sensor. Furthermore, although not shown, a calculation unit for identifying the order m of the higher-order harmonic may be provided. This calculation unit calculates the repetition frequency f of the optical frequency comb. rep is measured when the sensor part of the sensing optical frequency comb resonator is not in contact with the object to be measured, and then the repetition frequency is changed by a known amount, and the change in beat frequency is divided by the change in repetition frequency to obtain the order m.

[0031] Figure 3 shows a graph measuring the change in the optical spectrum wavelength of the sensing optical frequency comb resonator when the refractive index of the measurement object is changed. Using an ethanol aqueous solution as the measurement object, the ethanol concentration was increased in 2.5% increments from 0% to 15% by volume, and the optical spectrum was measured each time. The measurements were carried out with the sensor (MMI fiber sensor) 4 completely immersed in the ethanol aqueous solution being measured. Although Figure 3 shows a tentative correlation between the refractive index of the measurement object and the optical spectrum shift, the optical spectrum shift is small relative to the optical spectrum width, which leads to errors when back-calculating the refractive index from the optical spectrum shift.

[0032] Therefore, we focused on the shift in the optical RF signal spectrum of the optical frequency comb. Figure 4 shows the measurement results of the optical RF signal spectrum (optical frequency comb spectrum) of a ring-type mode-locked Er fiber optical frequency comb incorporating a multimode interference fiber sensor. The repetition frequency f obtained from the output of the sensing optical frequency comb resonator 100 in response to changes in ethanol concentration was rep The optical frequency comb spectrum was measured. Compared to the case where the wavelength of the optical frequency comb spectrum is plotted on the horizontal axis (Fig. 3), the spectrum (broadening) appears very sharp, and the optical spectrum shift relative to the spectral width is more pronounced. Furthermore, the ethanol concentration (refractive index) and the repetition frequency f rep Figure 5 shows a graph plotting the relationship between the change in the repetition frequency f and the change in the refractive index. It was confirmed that if the frequency change of the sensing optical frequency comb resonator can be measured, it can be converted into a change in the refractive index with an error of about 0.0005. rep Although temperature drift may be superimposed on the measured values, in Figure 5, the ambient temperature was controlled by indoor air conditioning.

[0033] In this way, by using a sensing optical frequency comb resonator, the change in sample concentration (refractive index) is repeated at a frequency of f rep However, a small change in the refractive index can be read out by f rep The shift amount also becomes minute, making accurate measurement difficult. rep By amplifying (multiplying) the shift amount by, for example, 1000 to 10,000 times depending on the repetition frequency, f rep The amount of shift can be detected. The configuration will be explained below.

[0034] In FIG. 1, the optical frequency comb generated by the sensing optical frequency comb resonator 100 is supplied to the photoconductive antenna element 27 via the isolator 7. In this embodiment, the photoconductive antenna element 27 functions as a mixing element. First, the repetition frequency (f rep ) harmonic components (mf rep) generates a high-frequency photocarrier signal having a frequency equal to the high-order harmonic component inside. Meanwhile, a high-frequency reference signal source 101 supplies a high-frequency photocarrier signal having a frequency (f THz ) is supplied as the received signal. As a result, a beat signal (frequency f) is generated between the high-order harmonic component of the photocarrier signal and the high-frequency reference signal. b ) is obtained. The operation at this time will be described below with reference to FIG. 6 along with a specific configuration example of the photoconductive antenna element 27.

[0035] 6, photoconductive antenna 27 is composed of a photoconductive film and a metal parallel transmission line and a metal antenna provided on the surface of the film so as to face each other. Photoconductive antennas having such an outer shape are generally used as means for detecting terahertz waves (Non-Patent Document 2). In this embodiment, the antenna is used in a completely different way.

[0036] First, an optical frequency comb is irradiated into the gap between these metal antennas. If the photon energy of the optical frequency comb exceeds the band gap of the photoconductive film, photocarriers are generated in the photoconductive film. These photocarriers act as optical switches between the antennas, generating harmonic components (mf rep ) is used to perform high-frequency modulation. Meanwhile, a high-frequency reference signal, which is an electromagnetic wave, is irradiated from the opposite side of the metal antenna to the laser light incident side. This high-frequency reference signal is received by the metal antenna. Here, the received high-frequency reference signal acts as a local oscillator signal within the photoconductive antenna. As a result, within the photoconductive antenna, the high-order harmonic photocarrier signal (optical RF signal) resulting from the optical frequency comb and the high-frequency reference signal are mixed (photoconductive mixing), and the beat signal between the two becomes a high-frequency current and flows into the current amplifier 28.

[0037] This is shown in Figure 7. In Figure 7(a), the horizontal axis represents frequency components and the vertical axis represents intensity. THzThe high frequency reference signal provided by the high frequency reference signal source 101 is shown as a single spectral line. On the other hand, the photocarrier signal generated by the optical frequency comb has a frequency interval f rep Here, the optical frequency comb is supplied from the sensing optical frequency comb resonator 100 incorporating the sensor 4, and the frequency interval of the photocarrier signal is f rep is the repetition rate of the optical frequency comb resonator, f rep is equivalent to

[0038] When the high-frequency reference signal and the photocarrier signal are mixed, a beat signal having a spectrum as shown in Figure 7(b) is generated. That is, the high-order harmonic components in the photocarrier signal spectrum are at the frequency f of the high-frequency reference signal. THz The frequency of the nearby component is mf rep Then, the difference frequency f b (=|mf rep -f THz |) appears in the baseband. Here, m is called the order of the higher harmonic components of the photocarrier signal. For frequency components other than the baseband, mf rep Therefore, the low-pass filter 29 filters out the difference frequency f b If we separate and extract the difference frequency f b can be detected as

[0039] This repetition frequency f rep changes according to the amount of light detected by the sensor 4 of the optical frequency comb resonator, and at this time the optical frequency comb changes in a so-called accordion-like manner. This property is also inherited by the photocarrier signal spectrum. For example, the repetition frequency f rep is Δf rep When the frequency changes by only mf rep ) is mΔf rep However, it is difficult to directly detect this change due to the ultra-high frequency. On the other hand, the frequency of the high-frequency reference signal is constant (=f THz), so the beat signal mΔf rep Only Δf changes. rep The shift amount can be multiplied by m and detected as an electrical frequency signal.

[0040] Frequency of the high-frequency reference signal, f THz can be set arbitrarily, but the sensitivity will be higher if the order m is set to a larger value, i.e., closer to the frequency of the photocarrier signal of a higher harmonic. rep However, if the order m is too large, the high frequency characteristics of the photocarrier signal may appear, which may result in a decrease in SN ratio. The optimum value of the order m depends on the frequency characteristics of the photoconductive antenna element and the repetition frequency f rep Depends on.

[0041] Also, if the order m is too high, when the refractive index changes significantly, the difference frequency f b In case that the frequency interval of the photocarrier signal (i.e., the dynamic range) is exceeded, the order m may be made variable. In this case, the detected difference frequency f b If a numerical process is implemented to always divide the shift amount by the order m, it can always be converted into the change amount when m=1 and understood.

[0042] 6 is used as the mixing element in this embodiment, but the present invention is not limited to this configuration. Instead of a photoconductive antenna, an electro-optic crystal material (Patent Document 4) such as zinc telluride (ZnTe) crystal that generates sidebands of laser light according to the frequency of an applied high-frequency radio wave signal may be used as the mixing element.

[0043] FIG. 8 shows a modified example of this embodiment in which an electro-optic crystal element is used as the mixing element. In FIG. 8, the sensing optical frequency comb resonator 100, the high-frequency reference signal source 101, and the isolator 7 function in the same manner as those shown in FIG. 1. In FIG. 8, the high-frequency reference signal passes through the beam splitter 271, and the optical frequency comb is reflected by the beam splitter 271, and both enter the electro-optic crystal element 272 from the same direction, coaxially, and with a predetermined polarization direction. The optical frequency comb propagating inside the electro-optic crystal element 272 is subjected to the electro-optic Pockels effect caused by the high-frequency reference signal, and generates sidebands in each optical frequency mode. That is, the optical frequency mode series (frequency a rep ) with the frequency f of the high frequency reference signal THz sidebands (frequency af) with a frequency spacing equal to rep +f THz ) appears. The optical frequency mode with the frequency closest to this sideband [frequency = (a + m)f rep ] and the optical beat frequency [=(af rep +f THz )-(a+m)f rep =f THz -mf rep ] is the difference frequency f b Nothing but that.

[0044] The directions of the optical frequency comb and the high-frequency reference signal incident on the electro-optic crystal member 272 may be opposite to each other as long as they are coaxial. The crystal axis of the electro-optic crystal member 272 is preferably arranged so as to maximize the efficiency of generating the sidebands. The difference frequency f b For detection, the same method as in FIG. 1 can be used.

[0045] (Second embodiment) A second embodiment of the present invention will now be described. Figure 9 shows a block diagram of the second embodiment of the present invention. In this figure, the sensing optical frequency comb resonator 100, and its constituent components, the laser diode 1, wavelength-division multiplexing coupler 2, erbium-doped optical fiber 3, isolator 5, optical fiber coupler 6, and single-mode optical fiber 10, are the same as those shown in Figure 1. The sensor 4, photoconductive antenna 27, current amplifier 28, and low-pass filter 29 of the optical frequency comb resonator are also the same as those shown in Figure 1. In this embodiment, the high-frequency reference signal source 101 is realized using a dummy sensing optical frequency comb resonator with approximately the same configuration as the sensing optical frequency comb resonator 100.

[0046] Here, the dummy sensing optical frequency comb resonator is an optical frequency comb resonator that has almost the same configuration as the sensing optical frequency comb resonator 100, but does not have the sensor 4. To distinguish it from the dummy sensing optical frequency comb resonator, the sensing optical frequency comb resonator 100 will hereinafter be referred to as an active sensing optical frequency comb resonator. Also, hereinafter, the high-frequency reference signal source 101 may be referred to as the dummy sensing optical frequency comb resonator 101.

[0047] In this embodiment, high-frequency reference signal source 101 has a dummy sensing optical frequency comb resonator composed of laser diode 11, wavelength division multiplexing coupler 12, erbium-doped optical fiber 13, isolator 15, optical fiber coupler 16, and single-mode optical fiber 110. These components have the same functions as those used in active sensing optical frequency comb resonator 100. The optical frequency comb generated by this dummy sensing optical frequency comb resonator is supplied to generating element 22 via isolator 17.

[0048] The generating element 22 converts the optical frequency comb into an electrical signal and outputs it as a high-frequency reference signal. The generating element 22 receives the (first) optical frequency comb generated by the high-frequency reference signal source 101 and converts it into a repetition frequency (f rep1) high-order (m-th) harmonic components (mf rep1 ) into free space as a high-frequency radio wave signal. For example, it may be a nonlinear optical crystal or a uni-traveling-carrier photodiode (UTC-PD). Also, although it is used as a mixing element in the previous embodiment, a photoconductive antenna (second photoconductive antenna) may also be used.

[0049] In this embodiment, the high-frequency reference signal is generated using an optical frequency comb resonator equivalent to the active sensing optical frequency comb resonator 100. Therefore, by selecting an appropriate order m, the frequency f of the high-frequency reference signal can be THz MF rep1 Moreover, the repetition frequency (f rep2 ) changes, the high frequency reference signal also changes in the same manner, making it possible to perform some temperature compensation.

[0050] If more precise temperature compensation is required, a dummy sensor 40 can be inserted into the fiber optic frequency comb resonator of the high-frequency reference signal source 101, as shown in FIG. 10. In this embodiment, the dummy sensor 40 is preferably a sensor with characteristics equivalent to those of the sensor 4. However, it is desirable that the area around the dummy sensor 40 has the same refractive index and temperature environment as the measurement sample and is filled with a reference solution (e.g., distilled water or pure water) whose refractive index is kept constant. By placing this dummy sensor 40 close to the sensor 4, it is possible to cancel out all temperature characteristics occurring in the measurement system, including the temperature characteristics of the sensor.

[0051] In this embodiment, laser diode 1 and laser diode 11 are provided as excitation light sources for the optical frequency comb resonators in active sensing optical frequency comb resonator 100 and high-frequency reference signal source 101 (dummy sensing optical frequency comb resonator), respectively. However, light emitted from a single laser diode may be split into two by a beam splitter and supplied to each optical frequency comb resonator.

[0052] In addition, in this embodiment, (second) photoconductive antenna 27 is used as the mixing element, but the present invention is not limited to this configuration. Instead of a photoconductive antenna as the mixing element, an electro-optic crystal member that generates sidebands in an optical frequency comb according to the frequency of a high-frequency radio wave signal, as shown in FIG. 8, for example, may be used.

[0053] Furthermore, in this embodiment, the output of the dummy sensing optical frequency comb resonator 101 is converted into a high-frequency reference signal, and the output of the active sensing optical frequency comb resonator 100 is supplied as an optical signal to the mixing element 27. However, the respective resonators may be interchanged. In other words, the output of the active sensing optical frequency comb resonator 101 may be converted into a high-frequency reference signal, and the output of the dummy sensing optical frequency comb resonator 101 may be supplied as an optical signal to the mixing element 27. [Example]

[0054] (First Example) A first embodiment of the present invention will be described below. In this embodiment, the results of actually verifying the frequency amplification effect when the sensor 4 is not disposed in the sensing optical frequency comb resonator 100 shown in FIG. 1 and m=1426 are shown. In this embodiment, first, the repetition frequency f rep The sensing optical frequency comb resonator 100 is set so that the frequency f of the high-frequency reference signal is 56, 124, 408 Hz. THz , the repetition frequency f rep The frequency is set to approximately 1426 times (frequency 80,033,760,000 Hz) of the photoconductive antenna 27. A bowtie type photoconductive antenna is used as the photoconductive antenna 27.

[0055] In this state, the repetition frequency f rep When the frequency of high-order harmonics increases by 25Hz (m×f rep ) and a high-frequency reference signal (frequency f THz ) beat signal (frequency f bThe results are shown in Figure 11. rep Beat frequency before change f b was 354,192Hz, while f rep After the change, the difference frequency (f b ) was -35,651.8Hz. Here, the minus sign means f THz is m×f rep It means higher.

[0056] Furthermore, the difference frequency f b Dividing the absolute value of by 25 Hz gives 1426.072, which is the frequency of the high frequency reference signal, f THz The above results are approximately equal to the value of the order m used when setting the frequency f THz This means that by setting m times the repetition frequency, the frequency transition accompanying the change in the refractive index of the object to be measured is amplified m times and detected.

[0057] (Second Example) A second embodiment of the present invention will be described below. In this embodiment, the results of actually verifying the frequency amplification effect when the sensor 4 is not disposed in the active sensing optical frequency comb resonator 100 shown in FIG. 9 are shown. In this embodiment, first, the repetition frequency f rep1 250,000,000 Hz, the repetition rate f of the dummy sensing optical frequency comb resonator 101 rep2 is set to 250,000,050 Hz. Furthermore, a dipole type photoconductive antenna is used for both the generating element 22 and the photoconductive antenna 27.

[0058] By inputting the active sensing optical frequency comb into the dipole-type photoconductive antenna of the generating element 22, f rep1 Higher harmonic components (frequency mf rep1) is generated in space. The high-frequency reference signal (frequency 0.1 to 2 THz) propagating through space is incident on a dipole-type photoconductive antenna, which is the photoconductive antenna 27, together with a dummy sensing optical frequency comb, to perform photoconductive mixing. As a result, a beat signal (frequency f b The frequency spectrum of the signal is shown in Figure 12(a). The broadband spectrum is clearly visible in the filled-in area.

[0059] Inside Fig. 12(a) is the difference in repetition frequency interval between the active sensing optical frequency comb and the dummy sensing optical frequency comb (Δf rep =f rep2 -f rep1 ) harmonic components (mΔf rep ) is included. To confirm this, the graph with the frequency scale enlarged is shown in Figure 12(b). Δf rep (=50Hz) intervals. For example, the 111,400Hz signal has a Δf rep By using this, the f rep1 The shift can be measured by multiplying it by 2,228.

[0060] When the temperature of the measurement environment changes, the effect is proportional to the repetition rate (f rep1 ,f rep2 ) appears similarly in f rep1 and f rep2 The difference (Δf rep ) can be cancelled out, resulting in Δf rep As a result, highly sensitive measurements can be achieved without being affected by temperature changes in the measurement environment. [Industrial Applicability]

[0061] The present invention can measure the refractive index of an object with high sensitivity and precision, and is not easily affected by changes in the surrounding environment, such as temperature. Therefore, it can be applied to a wide range of fields, including chemistry, biochemistry, medicine, and food, such as detecting proteins in blood and body fluids, measuring the concentration of water-soluble cutting oils, measuring the concentration of seasonings, and hydrogen sensors. [Explanation of symbols]

[0062] 1, 11 Laser diode 2, 12 wavelength division multiplexing coupler 3, 13 Erbium-doped optical fiber 4 sensors 40 Dummy Sensor 5, 15 Isolator 6, 16 Fiber Optic Coupler 7, 17 Isolator 10, 110 single mode optical fiber 22 Generator 27 Photoconductive antenna 271 Beam Splitter 272 Electro-optical crystal components 28 Current Amplifier 29 Low-pass filter 100 (Active) Sensing Optical Frequency Comb Resonator 101 High-frequency reference signal source (dummy sensing optical frequency comb resonator)

Claims

1. a first optical frequency comb resonator having a first fiber sensor within the resonator and generating a first optical frequency comb; a second optical frequency comb resonator having a second fiber sensor within the resonator and generating a second optical frequency comb; a generating element that receives the first optical frequency comb and outputs high-order harmonic components of a repetition frequency into free space as a high-frequency radio wave signal; a mixing element that outputs a high-frequency signal of a high-order harmonic component of the repetition frequency of the second optical frequency comb and a beat signal of the high-frequency radio wave signal output from the generating element; A fiber sensing device in which only one of the first fiber sensor and the second fiber sensor is an active sensor whose repetition frequency changes in response to a change in a physical quantity detected when the sensor comes into contact with an object to be measured, and the other fiber sensor is a dummy sensor.

2. 2. The fiber sensing device according to claim 1, wherein the mixing element is a first photoconductive antenna that receives the second optical frequency comb, internally generates high-order harmonic components of a repetition frequency as a high-frequency photocarrier signal, and photoconductively mixes the high-order harmonic radio wave signal output from the generating element with the high-order harmonic photocarrier signal to generate a beat frequency of the high-order harmonic radio wave signal.

3. The fiber sensing device described in claim 1, characterized in that the generating element is a second photoconductive antenna, a nonlinear optical crystal, or a uni-traveling carrier photodiode that receives the first optical frequency comb and generates high-order harmonic components of the repetition frequency as a high-frequency radio wave signal.

4. The fiber sensing device described in claim 1, characterized in that the mixing element is an electro-optic crystal that receives the second optical frequency comb and the high-order harmonic radio wave signal output from the generating element, generates a sideband in the optical frequency mode that constitutes the second optical frequency comb equal to the frequency of the high-order harmonic radio wave signal, and generates an optical beat with an optical frequency mode of an adjacent frequency.

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