JP6985695B2 - Refractive index measuring device and method - Google Patents

Description

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

The refractive index is an index showing the speed of light in a substance and contributes to the refraction and reflection of light. In addition, since the refractive index is derived from the permittivity peculiar to a substance, which represents the interaction between light and a substance, accurate evaluation of the refractive index leads to 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 fields of biochemistry and biochemistry, such as detection of biomolecules and evaluation of chemical reactions.

Conventional methods for measuring the refractive index include a minimum deviation method, a critical angle method, a V-block method, an interferometry, a surface plasmon resonance method, a multimode interferometry, and the like (see, for example, Patent Documents 1 and 2).

Japanese Unexamined Patent Publication No. 2017-090259 Japanese Unexamined Patent Publication No. 2012-251963

Hiroyuki Sasada, "Current Status and Development of Optical Frequency Comb Technology", Optics, Comprehensive Report, Japan Society of Applied Physics, Vol. 41, No. 9, 2012 Takeo Minamikawa et al., "Development of Fiber Optical Com Resonator Strain Sensor", Proceedings of 57th Meeting on Lightwave Sensing Technology, LST57-21, June 2016 C. R. Biazoli et al. , "Multimode interference tapered fiber refractive index sensors," Applied Optics, 51 (24), pp. 5941-5945 (2012) A. A. Jasim et al. , "Refractive index and refraction sensing inline Mach-Zehnder interferometer comprising perfluorinated graded-index plastic optics * or C. Bobby Mathews et al. , "Design and Development of a Cholesterol Sensor Based on the Refractive Index Sensing of Tilted Fiber Bragg Grating," The Engine Xueliang Shi et al. , "Refractive index sensor based on tiled fiber Bragg grating and stimulating and Brillouin scattering," Optics Express, 20 (9), 10853 (2012). Changeu Shen et al. , "Characteristics of Refractive Index Sensor Based on Adjusting Gap Fiber Bragg Grating," Proceeding of SPIE-OSA-IESOS. 7990, 79900K (2011). Galina Nemova et al. , "Modeling of Plasmon-Polariton Refractive-Index Hollow Core Fiber Sensors Assisted by a Fiber Bragg Grating," JOURNAL OF LIGHTWV. 24, NO. 10, 3789 (2006) Darran K. C. Wu et al. , "Ultrasenitive photonic crystal fiber fiber refractive index sensor," Optics Letters, Vol. 34, No. 3, 2009 Kwang Jo Lee et al. , "Refractive index sensor based on a polymer fiber director direct 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 optical spectrum waveforms and optical intensity signals, and the resolution remains at 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 the current refractive index sensor. In addition to increasing the resolution, the device also needs to be small so that it can be used in various situations.

For example, in the Abbe refractometer, it is difficult to continuously measure for sampling is required in most cases, a required precision optics for high accuracy (large), 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 device and a method capable of measuring a refractive index with a device having a high resolution and a small size as compared with the prior art.

The refractive index measuring device according to the present invention is
It has an optical transmission path including a multimode interference optical fiber sensor in which a cladless multimode optical fiber is arranged in a predetermined medium, and by controlling the optical cavity to a predetermined dispersion amount, a comb tooth shape in the spectrum. A fiber optical comb cavity that generates an optical frequency comb when multiple wavelengths that are juxtaposed at equal intervals and phase-synchronized are in an optical resonance state.
An optical detector that photoelectrically converts an optical signal generated by the optical resonator into an AC electric signal,
It is characterized by being provided with a measurement control means for measuring the refractive index of light in the medium based on the repetition frequency of an AC electric signal output from the photodetector.

Further, the method for measuring the refractive index according to the present invention is:
A fiber optical comb resonator having an optical transmission path 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. In the spectrum, multiple wavelengths that are juxtaposed at equal intervals in the shape of a comb and are phase-synchronized enter an optical resonance state and generate an optical frequency comb.
A step in which the photodetector 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 light in the medium based on the repetition frequency of the AC electric signal output from the photodetector.

According to the refractive index measuring device and method according to the present invention, the refractive index can be measured with a device having higher resolution and smaller size than the prior art.

It is a block diagram which shows the structural example of the refractive index measuring apparatus which concerns on embodiment. It is a schematic side view which shows the structural example of the multimode interference fiber sensor 5 of FIG. It is an experimental result of the refractive index measuring apparatus of FIG. 1, and is a spectral diagram which shows the presence / absence of a multimode interference fiber sensor 5 and the spectral waveform shift characteristic by water (ethanol 0%). It is an experimental result of the refractive index measuring apparatus of FIG. 1, and is a spectral diagram which shows the spectral waveform shift characteristic by the concentration of ethanol. It is an experimental result of the refractive index measuring apparatus of FIG. 1, and is a graph which shows the wavelength shift amount with respect to the 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. It is an experimental result of the refractive index measuring apparatus of FIG. 1, and is a graph which shows the fluctuation (standard deviation) of the repetition frequency with respect to each gate time when the sampling number N = 100. It is the experimental result of the refractive index measuring apparatus of FIG. 1, and is the graph which shows the measurement result of the refractive index by the change of a repetition frequency.

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

Since the refractive index is derived from the permittivity peculiar to a substance, which represents the interaction between light and a substance, accurate evaluation of the refractive index also leads to 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 improvement in resolution is required in order to improve the accuracy of quality evaluation. ..

Here, by using the "disturbance / RF conversion function" unique to the fiber optical comb (see, for example, Non-Patent Document 1) resonator, it is possible to convert various physical quantities into RF signals (repetition frequency prep). .. Since 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 preceding example, the resonator fiber itself was used as a strain sensor (see, for example, Non-Patent Document 2).

On the other hand, in the present embodiment, a multimode interference (MMI) fiber sensor (see, for example, Patent Document 2; hereinafter referred to as MMI fiber sensor), which is one of the fiber refractive index sensors, is incorporated in the fiber optical comb resonator. (Refractive index sensing optical comb), it is characterized in that the refractive index measurement is highly accurate by repeatedly reading out the refractive index change at the frequency prep.

FIG. 1 is a block diagram showing a configuration example of the refractive index measuring device according to the 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 optical comb resonator, and
(2) WDM coupler 2 and
(3) Er-added optical fiber 3 which is an optical amplification means and
(4) Polarization controller 4 and
(5) MMI fiber sensor 5 and
(6) Isolator 6 and
(7) Optical fiber coupler 7 and
(8) Isolator 8 and
(9) Photodetector 9 and
(10) Equipped with a measurement control device 10
These optical processing elements 1 to 9 are connected via a single mode optical fiber 11 to 19. Specifically, it is as follows.

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

The optical signal generated by the optical comb resonator is input from the optical fiber coupler 7 to the photodetector 9 via 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 electric signal (from several tens of MHz to several tens of GHz) and outputs it to the measurement control device 10. The measurement control device 10 has a spectrum analyzer function and a frequency counting function capable of sampling an input RF signal and measuring spectral 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 spectral characteristics. Have.

FIG. 2 is a schematic side view showing a configuration example of the MMI fiber sensor 5 of FIG. In FIG. 2, the housing 30 is filled with a medium 21 which 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. Arranged like this. In FIG. 2, the single-mode optical fiber 14 is configured to include a core portion 14c and a clad portion 14a around the core portion 14c, and the single-mode optical fiber 15 is configured to include a core portion 15c and a clad portion 15a around the core portion 15c. Further, 20c is a core portion of the multimode optical fiber 20. Here, as is known, the goose-henshen shift amount in the MMI fiber sensor 5 depends on the refractive index of the medium 21, and when the change in the seepage length shown by 100 in FIG. 2 changes the optical resonator length nL, The repetition frequency refraction 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 device of FIG. 1 constitutes a ring-type mode-synchronized Er fiber laser resonator, and the MMI fiber sensor 5 is inserted into the optical resonator. Mode-synchronized operation is obtained by non-linear polarization rotation due to. The mode-synchronized spectrum obtained from the optical resonator shows a spectral shape centered on the interference wavelength of the MMI fiber sensor 5. Here, it is considered that a laser spectrum shift occurs when the refractive index of the medium 21 of the MMI fiber sensor 5 changes.

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

FIG. 3A is an experimental result of the refractive index measuring device of FIG. 1, and is a spectral diagram showing the presence / absence of the MMI fiber sensor 5 and the spectral waveform shift characteristic 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 an experimental result of the refractive index measuring device of FIG. 1, and is a spectral diagram 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 long wavelength side by increasing the concentration of the ethanol aqueous solution in the MMI fiber sensor 5.

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

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

FIG. 6 is an experimental result of the refractive index measuring device of FIG. 1, and is a graph showing fluctuation (standard deviation) of the repetition frequency with respect to each gate time when the sampling number N = 100. As is clear from FIG. 6, as the gate time when measuring in the measurement control device 10 increases, the standard deviation of the repetition frequency prep 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. The increase in the standard deviation in 0.1 seconds or more is considered to be due to the temperature disturbance to the optical resonator. This frequency fluctuation limits the resolution of the refractive index measurement.

FIG. 7 is an experimental result of the refractive index measuring device of FIG. 1, and is a graph showing the measurement result of the refractive index due to a change in the repetition frequency. 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 the resolution was about the same as the world's top level.

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 in the optical resonator, it still functions as a refractive index sensor. It is considered that this refractive index-dependent laser spectrum shift leads to a repeating frequency change through the wavelength dispersion of the resonator fiber.

In the above embodiment, the ring-shaped optical resonator is configured, but the present invention is not limited to this, and a linear type, 8-shaped type, 9-shaped optical resonator, or the like may be configured. .. Further, in the fiber optical comb resonator, it is also possible to use a hypersaturated absorber / carbon nanotube / graphene / electro-optical modulator as a mode-synchronizing element instead of the polarization controller 4.

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

In the present embodiment, a high-precision refractive index measuring device using a unique "resonator disturbance / RF frequency conversion function" using a fiber optical comb resonator is disclosed. When a change in the refractive index 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 frequency has the highest accuracy national standard (cesium atomic clock, frequency uncertainty ^10-15 ) among various physical quantities and can be measured as a discrete quantity (digital quantity), so it is possible to measure with extremely high accuracy and high dynamic range. become. Therefore, by reading out the change in the refractive index as the RF frequency, it is possible to significantly improve the performance of the refractive index measurement.

The embodiments according to the present invention are "resonator disturbance / RF frequency conversion function" using optical frequency comb light which is an extreme light source from the viewpoint 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 the possibility of breaking the limit in the current refractive index measurement. Further, by utilizing the features of the MMI fiber sensor 5, practicality such as environmental resistance, measurable substance of dangerous substance, non-electromagnetic induction, and easy handling can be imparted.

Refractive index measurement is widely used in the industrial field and bio / medical field, and if higher accuracy can be realized, stricter quality control and higher quality medical application will be possible. For example, it can be expected to stabilize the manufacturing process by introducing it into various liquid manufacturing processes such as beverages, seasonings, fats and oils and bioethanol in the industrial world. Good results have been obtained in a principle confirmation experiment using a water / ethanol mixed solution using the above-mentioned refractive index measuring device.

By measuring the refractive index, for example, the following physical quantities can be measured.
(1) Sugar content of beverages,
(2) Concentration of seasoning liquid,
(3) Tea concentration,
(4) Salinity of seawater,
(5) Ethanol concentration,
(6) Concentration of water-soluble cutting oil,
(7) Concentration of cleaning solution,
(8) Concentration of fats and oils,
(9) Detection of protein (antigen) in blood or body fluid.

As described in detail above, according to the refractive index measuring device and method according to the present invention, the refractive index can be measured with a device having higher resolution and smaller size than the prior art.

1 Semiconductor laser light source,
2 WDM coupler,
3 Er added optical fiber,
4 Polarization controller,
5 Multimode interference optical fiber sensor (MMI fiber sensor),
6 isolator,
7 Fiber optic coupler,
8 isolator,
9 Photodetector,
10 Measurement control device,
10a operation unit,
10b display unit,
11-19 single mode optical fiber,
14a, 15a Single mode optical fiber clad part,
14c, 15c single mode optical fiber core,
20 cladless multimode optical fiber,
20c Cladless multimode optical fiber core,
21 medium,
30 housing.

Claims (2)

A cladless multimode optical fiber is placed in a predetermined medium, has an optical transmission path including a multimode interference optical fiber sensor showing a refraction-dependent optical spectrum shift, and controls an optical resonator to a predetermined dispersion amount. As a result, in the refractive index measuring device equipped with an optical resonator, which is a fiber optical comb resonator that generates an optical frequency comb, a plurality of wavelengths that are juxtaposed at equal intervals in the spectrum and are phase-synchronized are in an optical resonance state. There,
The optical fiber is an optical resonator fiber that converts a refractive index-dependent optical spectrum shift into an optical comb repeat frequency shift via the refractive index dispersion of the optical cavity fiber.
The refractive index measuring device is
An optical detector that photoelectrically converts an optical comb repeat frequency signal generated by the optical resonator into an AC electrical signal, and
A refractive index measuring device comprising a measurement control means for measuring the refractive index of light in the medium based on the repetition frequency of an AC electric signal output from the photodetector. A multimode interference optical fiber sensor in which a cladless multimode optical fiber is placed in a predetermined medium converts a change in refractive index into an optical spectrum shift, and
An optical cavity having an optical transmission path including a multimode interference optical fiber sensor controls the optical cavity to a predetermined dispersion amount, so that the optical resonators are juxtaposed at equal intervals in a comb-like shape in a spectrum and have multiple wavelengths synchronized with each other. Steps to generate an optical frequency comb in an optical resonance state,
The step in which the optical fiber with controlled refractive index dispersion converts the refractive index-dependent optical spectrum shift into an optical comb repeat frequency shift,
A step photodetector, which photoelectrically converts an optical comb repetition frequency signal that will be generated by the optical resonator to the AC electrical signal,
A method for measuring a refractive index, wherein the measurement control means includes a step of measuring the refractive index of light in the medium based on the repetition frequency of an AC electric signal output from the photodetector.

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