JP2019039723A - Refractive index measurement device and method - Google Patents

Abstract

To enable a device with high resolving power and a compact size to measure a refractive index, when compared with conventional techniques.SOLUTION: A refractive index measurement device comprises: a fiber optical comb resonator that has an optical transmission path including a multi-mode interference optical fiber configured to have a crudless multi-mode optical fiber arranged in a prescribed medium, controls an optical resonator to a prescribed amount of dispersion to thereby have the optical resonator juxtaposed with comb tooth-shaped equal intervals in a spectrum, in which a plurality of phase-synchronized wavelengths is put in an optical resonance state, and generates an optical frequency comb; an optical detector that photoelectrically converts an optical signal to be generated by the fiber optical com resonator into an AC electric signal; and measurement control means that measures a refractive index of light in the medium on the basis of a repetition frequency of the AC electric signal to be output from the optical detector.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a refractive index measuring apparatus and method.

  The refractive index is an index indicating the speed of light in a substance, and contributes to light refraction and reflection. In addition, since the refractive index is derived from the dielectric constant inherent to the substance representing the interaction between light and the substance, accurate evaluation of the refractive index also leads to the evaluation of the state of the substance. For example, in the industrial field, refractive index measurement is used for quality evaluation of liquid substances (identification of liquid substances, measurement of liquid concentration / mixing ratio) and characteristic evaluation of optical material parts (optical glass, optical fiber, etc.). . It is also important in the bio / biochemical field, such as biomolecule detection and chemical reaction evaluation.

  Conventional refractive index measurement methods include a minimum declination method, a critical angle method, a V-block method, an interference method, a surface plasmon resonance method, and a multimode interference method (see, for example, Patent Documents 1 and 2).

JP 2017-090259 A JP 2012-251963 A

Hiroyuki Sada, “Current Status and Development of Optical Frequency Combing Technology”, Optics, General Report, Japan Society of Applied Physics, Vol. 41, No. 9, 2012 Takeo Minamikawa et al., “Development of Fiber Optic Comb Resonator Type Strain Sensor”, Proceedings of 57th Meeting on Lightwave Sensing Technology, LST57-21, June 2016 C. R. Biazoli et al. , "Multimode interference-tapped fiber refractive index sensors," Applied Optics, 51 (24), pp. 199 5941-5945 (2012) A. A. Jasim et al. , “Refractive index and strain sensing using inline Mach-Zehnder interferometer compiling perfluorinated 99-Plastic optical fibre, 19 C. Bobby Mathews et al. , "Design and Development of a Cholesterol Sensor Based on the Refractive Index Sensing of Tilted Fiber Bragg Grating," The Interventional Criteria. Xuelian Shi et al. , "Refractive index sensor based on tilted fiber Bragg grating and stimulated Brillouin scattering," Optics Express, 20 (9), 10853 (2012). Changyu Shen et al. , "Characteristics of Refractive Index Sensor Based on Adjusting Gap Fiber Bragg Grating," Proceeding of SPIE-OSA-IEEE AsiaPiSPI. 7990, 79900K (2011). Galina Nemova et al. , "Modeling of Plasmon-Polariton Refractive-Index Hollow Core Fiber Sensor Assisted by a Fiber Bragg Grating," JOURNAL OF LITE 24, NO. 10, 3789 (2006) Darran K.D. C. Wu et al. , "Ultrasensitive photonic fiber refractive index sensor," Optics Letters, Vol. 34, no. 3, 2009 Kwang Jo Lee et al. "Refractive index sensor based on a polymer fiber directional coupler for low index sensing," Optics Express, Vol. 22, no. 14, 17497 (2014).

However, these methods are based on the measurement of analog quantities such as an optical spectrum waveform and an optical intensity signal, and the resolution is limited to about 10 ⁻⁴ to 10 ⁻⁵ depending on the performance of the optical measuring device to be measured. In order to further improve the quality of industrial products such as liquid substances and optical material parts, it is necessary to improve the resolution of current refractive index sensors. Further, in addition to high resolution, the apparatus needs to be small so that it can be used in various situations.

For example, in Abbe's refractometer, in many cases, sampling is required, so that continuous measurement is difficult. For high accuracy, a precise optical system is necessary (upsizing) and the resolution is 10 ^−. It was about ⁴ .

  An object of the present invention is to solve the above problems and to provide a refractive index measuring apparatus and method capable of measuring a refractive index with a small-sized apparatus having a higher resolution than that of the prior art.

The refractive index measuring device according to the present invention is
It has an optical transmission line that includes a multimode interference optical fiber sensor in which a cladless multimode optical fiber is arranged in a predetermined medium, and the optical resonator is controlled to a predetermined dispersion amount, so that it has a comb-like shape in the spectrum. A fiber optical comb resonator in which a plurality of wavelengths that are arranged at equal intervals and phase-synchronized with each other in an optical resonance state and generate an optical frequency comb,
A photodetector for photoelectrically converting an optical signal generated by the optical resonator into an AC electrical signal;
Measurement control means for measuring the refractive index of light in the medium based on the repetition frequency of the AC electrical signal output from the photodetector is provided.

Moreover, the refractive index measurement method according to the present invention is:
A fiber optical comb resonator having an optical transmission line including a multimode interference optical fiber sensor in which a cladless multimode optical fiber is arranged in a predetermined medium controls the fiber optical comb resonator to a predetermined dispersion amount. A plurality of wavelengths that are arranged in equal intervals and phase-synchronized in a comb-like shape in the spectrum to generate an optical frequency comb in an optical resonance state;
A photodetector that photoelectrically converts an optical signal generated by the fiber optical comb resonator into an AC electrical signal;
The measurement control means includes a step of measuring and controlling the refractive index of the light in the medium based on the repetition frequency of the AC electric signal output from the photodetector.

  According to the refractive index measuring apparatus and method according to the present invention, it is possible to measure the refractive index with a small apparatus having a higher resolution than that of the prior art.

It is a block diagram showing an example of composition of a refractive index measuring device concerning an embodiment. It is a schematic side view which shows the structural example of the multimode interference fiber sensor 5 of FIG. FIG. 3 is a spectrum diagram showing the experimental results of the refractive index measuring apparatus of FIG. 1 and showing the presence or absence of a multimode interference fiber sensor 5 and the spectral waveform shift characteristics due to water (ethanol 0%). It is a spectrum figure which is an experimental result of the refractive index measuring apparatus of FIG. 1, and shows the spectrum waveform shift characteristic by the density | concentration of ethanol. It is an experimental result of the refractive index measuring apparatus of FIG. 1, Comprising: It is a graph which shows the wavelength shift amount with respect to the density | concentration of ethanol. It is a graph which shows the wavelength characteristic of the fiber refractive index in the refractive index measuring apparatus of FIG. FIG. 3 is a graph showing experimental results of the refractive index measuring apparatus of FIG. 1 and fluctuations (standard deviation) of repetition frequency with respect to each gate time when the number of samplings N = 100. It is an experimental result of the refractive index measuring apparatus of FIG. 1, Comprising: It is a graph which shows the measurement result of the refractive index by the change of a repetition frequency.

  Hereinafter, a refractive index measuring apparatus according to an embodiment of the present invention will be described.

Since the refractive index is derived from the dielectric constant inherent to the substance representing the interaction between light and the substance, accurate evaluation of the refractive index also leads to the evaluation of the state of the substance. For example, in the industrial field, refractive index measurement is used for quality evaluation of liquid substances (identification of liquid substances, measurement of liquid concentration / mixing ratio) and characteristic evaluation of optical material parts (optical glass, optical fiber, etc.). . However, as described above, the resolution of the conventional refractive index measurement method is limited to the order of 10 ⁻⁴ to 10 ⁻⁵ , and further higher resolution is required to improve the quality evaluation accuracy. .

  Here, if a “disturbance / RF conversion function” unique to a fiber optical comb (for example, see Non-Patent Document 1) resonator is used, various physical quantities can be converted into RF signals (repetition frequency frep). . Since the frequency is a physical quantity that can be measured with extremely high accuracy, this approach can be expected to improve the accuracy of various physical measurements. In the previous example, the resonator fiber itself was used as a strain sensor (see, for example, Non-Patent Document 2).

  In contrast, in this embodiment, a multi-mode interference (MMI) fiber sensor (see, for example, Patent Document 2; hereinafter referred to as an MMI fiber sensor), which is one of fiber refractive index sensors, is incorporated in a fiber optical comb resonator. (Refractive index sensing optical comb), characterized in that the refractive index measurement is made highly accurate by reading out the refractive index change repeatedly at the frequency frep.

FIG. 1 is a block diagram illustrating a configuration example of a refractive index measuring apparatus according to an embodiment. In FIG. 1, the refractive index measuring device according to the present embodiment is
(1) a semiconductor laser light source 1 for exciting a fiber-optic comb resonator;
(2) WDM coupler 2;
(3) an Er-doped optical fiber 3 that is an optical amplification means;
(4) polarization controller 4;
(5) MMI fiber sensor 5;
(6) an isolator 6;
(7) an optical fiber coupler 7;
(8) an isolator 8;
(9) a photodetector 9;
(10) a measurement control device 10;
These optical processing elements 1 to 9 are connected via single mode optical fibers 11 to 19. Specifically, it is as follows.

  As is well known, the semiconductor laser light source 1 generates laser light for exciting the Er-doped optical fiber 3 in the fiber optical comb resonator, and the laser light signal is input to the WDM coupler 2 via the single mode optical fiber 11. The Of the input optical signals, the WDM coupler 2 inputs an optical signal in a predetermined wavelength band to the Er-doped optical fiber 3. The luminescence light generated by the excitation of the Er-doped optical fiber 3 is the single mode optical fiber 12, the WDM coupler 2, the single mode optical fiber 16, the isolator 6, the single mode optical fiber 17, the optical fiber coupler 7, the single mode optical fiber 15, the MMI fiber sensor 5, and the single. The ring type fiber optical comb resonator is circulated in one direction (clockwise) through the mode optical fiber 14, the polarization controller 4 and the single mode optical fiber 13. Here, when the total dispersion amount of the resonator is set to an appropriate value and converted into a predetermined polarization state by the polarization controller 4, a plurality of wavelengths that are phase-synchronized and arranged in parallel at equal intervals in a comb-like shape in the spectrum. The optical resonance state (mode synchronization) is established, and the optical frequency comb circulates in the resonator.

  The optical signal generated by the optical comb resonator is input from the optical fiber coupler 7 to the photodetector 9 through the single mode optical fiber 18, the isolator 9 and the single mode optical fiber 19. The photodetector 9 photoelectrically converts the input optical signal into, for example, an RF band electrical signal (several tens of MHz to several tens of GHz) and outputs the result to the measurement control device 10. The measurement control device 10 has a spectrum analyzer function and a frequency count function capable of sampling an input RF signal and measuring spectrum characteristics and frequency characteristics. Here, the measurement control device 10 includes an operation unit 10a for the user to operate the measurement control device 10 (adjustment of sampling frequency, measurement frequency, etc.) and a display unit 10b for displaying measurement results such as spectrum characteristics. Have.

  FIG. 2 is a schematic side view showing a configuration example of the MMI fiber sensor 5 of FIG. In FIG. 2, a case 21 is filled with a medium 21 that is a liquid such as ethanol or water, and the cladless multimode optical fiber 20 connected between the single mode optical fibers 14 and 15 is immersed in the medium 21. Are arranged as follows. In FIG. 2, the single mode optical fiber 14 includes a core portion 14c and a surrounding cladding portion 14a, and the single mode optical fiber 15 includes a core portion 15c and a surrounding cladding portion 15a. Reference numeral 20 c denotes a core part of the multimode optical fiber 20. Here, as is well known, the Goose-Henschen shift amount in the MMI fiber sensor 5 depends on the refractive index of the medium 21. When the change in the seepage length indicated by 100 in FIG. 2 changes the optical resonator length nL, The repetition frequency frep of the optical signal generated by the optical frequency comb of FIG. 1 can be changed.

  That is, the optical resonator of the refractive index measuring apparatus of FIG. 1 is configured as a ring-type mode-locked Er fiber laser resonator, and an MMI fiber sensor 5 is inserted into the optical resonator. A mode-locking operation is obtained by nonlinear polarization rotation by. The mode-locked spectrum obtained from the optical resonator shows a spectrum shape centered on the interference wavelength of the MMI fiber sensor 5. Here, when the refractive index of the medium 21 of the MMI fiber sensor 5 changes, it is considered that a laser spectrum shift occurs.

  The inventors measured the frequency shift amount of the laser spectrum accompanying the refractive index change by using water and an ethanol aqueous solution as the medium 21 and changing the concentration ratio thereof.

  FIG. 3A is an experimental result of the refractive index measurement apparatus of FIG. 1 and is a spectrum diagram showing the presence / absence of the MMI fiber sensor 5 and the spectral waveform shift characteristics due to water (ethanol 0%). As is clear from FIG. 3A, it can be seen that the spectral peak is shifted to the short wavelength side by inserting the MMI fiber sensor 5.

  FIG. 3B is a spectrum diagram showing an experimental result of the refractive index measuring apparatus of FIG. 1 and showing a spectral waveform shift characteristic depending on the concentration of ethanol. As is clear from FIG. 3B, it can be seen that the spectral peak is shifted to the longer wavelength side by increasing the concentration of the aqueous ethanol solution in the MMI fiber sensor 5.

  FIG. 4 is a graph showing experimental results of the refractive index measuring apparatus of FIG. 1 and showing the wavelength shift amount with respect to the ethanol concentration. As can be seen from FIG. 4, by increasing the concentration of the aqueous ethanol solution in the MMI fiber sensor 5 in the range of 0 to 80%, the spectral peak shifts to the longer wavelength side, and the concentration of the aqueous ethanol solution is 80% or more. It can be seen that the spectral peak is shifted to the short wavelength side when the value is increased.

  FIG. 5 is a graph showing the wavelength characteristic (near 1.55 μm) of the fiber refractive index in the refractive index measuring apparatus of FIG. The refractive index of the single-mode optical fiber depends on the wavelength. When the refractive index of the medium 21 increases, the spectral peak shifts to the longer wavelength side, and when the refractive index of the optical fiber increases, the repetition frequency frep decreases. Therefore, the following equation is obtained.

  FIG. 6 is a graph showing experimental results of the refractive index measuring apparatus of FIG. 1 and showing fluctuation (standard deviation) of the repetition frequency with respect to each gate time when the number of samplings N = 100. As is clear from FIG. 6, when the gate time for measurement in the measurement control device 10 increases, the standard deviation of the repetition frequency frep decreases and becomes the minimum in 0.1 seconds (101 in FIG. 6). That is, the minimum standard deviation is 0.083 Hz with a gate time of 0.1 seconds. Note that the increase in the standard deviation in 0.1 seconds or more is considered to be due to a temperature disturbance with respect to the optical resonator. This frequency fluctuation limits the resolution of the refractive index measurement.

FIG. 7 is a graph showing experimental results of the refractive index measuring apparatus of FIG. As is clear from FIG. 7, the frequency change (difference) of each sample with respect to water was obtained. The frequency change rate is 2.78 [Hz / ethano%], and the refractive index change per unit frequency is 1.08 × 10 ⁻⁴ [RIU / Hz]. At this time, the resolution of the refractive index was 8.93 × 10 ⁻⁶ @standard deviation = 0.083 [Hz], and a resolution comparable to the world top level was obtained.

  As described above, it can be confirmed that the laser spectrum is shifted due to the change in the refractive index. From this, it can be seen that even if the MMI fiber sensor 5 is inserted into the optical resonator, it functions as a refractive index sensor. This refractive index dependent laser spectral shift is considered to lead to a repetition frequency change through wavelength dispersion of the resonator fiber.

  In the above embodiment, a ring-shaped optical resonator is configured. However, the present invention is not limited to this, and a linear-type, 8-shaped, 9-shaped optical resonator, or the like may be configured. . In the fiber optical comb resonator, instead of the polarization controller 4, a saturable absorber / carbon nanotube / graphene / electro-optic modulator may be used as a mode-locking element.

  Instead of the MMI fiber sensor 5, a tapered MMI fiber sensor (for example, see Non-Patent Document 3), a Mach-Zehnder interferometer-type fiber sensor (for example, Non-Patent Document 4), a fiber Bragg grating type fiber sensor (for example, Non-Patent Document 3) Patent Literatures 5 to 7), surface plasmon polaron type fiber sensor (for example, see Non-Patent Literature 8), and directional coupler type fiber sensor (for example, see Non-Patent Literatures 9 and 10) may be used.

In the present embodiment, a high-accuracy refractive index measuring apparatus using a fiber optical comb resonator and utilizing an original “resonator disturbance / RF frequency conversion function” has been disclosed. When a refractive index change is applied as a disturbance to the MII fiber sensor 5 which is a sensing unit of the fiber optical comb resonator, the optical resonator length expands and contracts, and the RF frequency signal called the optical comb interval changes. The most accurate national standard (cesium atomic clock, frequency uncertainty 10 ^-15 ) is ^{prepared and} can be measured as discrete quantities (digital quantities), and the frequency can be measured with extremely high accuracy and high dynamic range. become. Therefore, by reading out the refractive index change as the RF frequency, it is possible to significantly improve the refractive index measurement.

  The embodiment according to the present invention includes a “resonator disturbance / RF frequency conversion function” using an optical frequency comb light, which is an extreme light source in terms of frequency stability, and “high-precision frequency measurement” based on a frequency standard. Based on digital frequency measurement with good consistency. This method is completely different from the conventional refractive index measurement based on analog optical measurement, and has a possibility of breaking the limit in the current refractive index measurement. In addition, by utilizing the features of the MMI fiber sensor 5, practicality such as environmental resistance, possibility of measuring dangerous substances, non-electromagnetic induction, and easy handling can be provided.

  Refractive index measurement is widely used in the industrial field and bio / medical field, and if higher accuracy can be realized, more strict quality control and high-quality medical application will be possible. For example, stabilization of the production process can be expected by introducing it into various liquid production processes such as beverages, seasonings, fats and oils and bioethanol in the industry. Good results have been obtained in a principle confirmation experiment using a water / ethanol mixed solution using the above-described refractive index measuring apparatus.

In addition, the following physical quantities etc. can be measured by measuring a refractive index, for example.
(1) sugar content of the beverage,
(2) Concentration of seasoning liquid
(3) Concentration of tea,
(4) Seawater salinity,
(5) ethanol concentration,
(6) Concentration of water-soluble cutting oil,
(7) Concentration of cleaning solution,
(8) Oil concentration
(9) Detection of protein (antigen) in blood or body fluid.

  As described above in detail, according to the refractive index measuring apparatus and method according to the present invention, it is possible to measure the refractive index with a small apparatus having a higher resolution than that of the prior art.

1 Semiconductor laser light source,
2 WDM coupler,
3 Er-doped optical fiber,
4 Polarization controller,
5 Multi-mode interference optical fiber sensor (MMI fiber sensor),
6 Isolator,
7 Optical fiber coupler,
8 Isolator,
9 Photodetector,
10 Measurement control device,
10a operation unit,
10b display unit,
11-19 single mode optical fiber,
14a, 15a Clad portion of single mode optical fiber,
14c, 15c Single mode optical fiber core,
20 Cladless multimode optical fiber,
20c Cladless multimode optical fiber core,
21 medium,
30 housing.

Claims (2)

It has an optical transmission line that includes a multimode interference optical fiber sensor in which a cladless multimode optical fiber is arranged in a predetermined medium, and the optical resonator is controlled to a predetermined dispersion amount, so that it has a comb-like shape in the spectrum. A fiber optical comb resonator in which a plurality of wavelengths that are arranged at equal intervals and phase-synchronized with each other in an optical resonance state and generate an optical frequency comb,
A photodetector for photoelectrically converting an optical signal generated by the optical resonator into an AC electrical signal;
A refractive index measuring apparatus comprising: a measurement control unit that measures a refractive index of light in the medium based on a repetition frequency of an AC electrical signal output from the photodetector. An optical resonator having an optical transmission line including a multi-mode interference optical fiber sensor in which a cladless multi-mode optical fiber is arranged in a predetermined medium controls the fiber optical comb resonator to a predetermined dispersion amount. A plurality of wavelengths that are arranged in parallel at equal intervals and phase-synchronized with each other in a comb-like shape in an optical resonance state and generate an optical frequency comb;
A photodetector that photoelectrically converts an optical signal generated by the fiber optical comb resonator into an AC electrical signal;
And a step of measuring a refractive index of light in the medium based on a repetition frequency of an AC electrical signal output from the photodetector.

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