CN116818715A - Refractive index measurement system and demodulation method applicable to various measurement environments - Google Patents

Refractive index measurement system and demodulation method applicable to various measurement environments Download PDF

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CN116818715A
CN116818715A CN202310235615.1A CN202310235615A CN116818715A CN 116818715 A CN116818715 A CN 116818715A CN 202310235615 A CN202310235615 A CN 202310235615A CN 116818715 A CN116818715 A CN 116818715A
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refractive index
interferometer
optical fiber
fiber
coupler
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苑立波
王洪业
王剑
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Guilin University of Electronic Technology
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Guilin University of Electronic Technology
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
    • G01N21/4133—Refractometers, e.g. differential
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
    • G01N21/45—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
    • G01N21/45—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods
    • G01N2021/458—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods using interferential sensor, e.g. sensor fibre, possibly on optical waveguide

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  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
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Abstract

The invention provides a refractive index measurement system and a demodulation method which can be used for various measurement environments. The device consists of a light source, a single-mode fiber, an optical fiber circulator, an optical fiber 1 multiplied by 2 coupler, a refractive index sensing interferometer, a fiber integrated coupler, a double-core optical fiber, an end surface reflecting film, piezoelectric ceramics, a photoelectric detector, a data acquisition card, a computer and a piezoelectric ceramics control system. The dual-core optical fiber forms an interferometer through a fiber integrated coupler, and is connected with the refractive index sensing interferometer in parallel to form a vernier effect. The dual-core optical fiber interferometer is wound on the piezoelectric ceramic, and is used for controlling the refractive index measurement range and demodulating signals. According to the range of the measured refractive index, the voltage applied to the piezoelectric ceramic is adjusted to enable the measured refractive index value to be in a detectable range after the two interferometers are connected in parallel. The invention realizes the regulation and control of the refractive index measurement range by the piezoelectric ceramic, expands the detection interval and increases the detection sensitivity, and can be applied to the field of optical fiber sensing.

Description

Refractive index measurement system and demodulation method applicable to various measurement environments
Technical Field
The invention relates to a refractive index measurement system and a demodulation method which can be used in various measurement environments, and belongs to the technical field of optical fiber sensing.
Background
The optical interferometry is a technique based on the principle of light wave interferometry, and has been widely used in the measurement field. Optical interferometry has higher sensitivity and accuracy than conventional optical measurement methods. Any physical quantity such as bending, temperature, strain, refractive index, displacement, vibration, pressure, temperature, etc., can be measured by the optical fiber interferometer as long as it can be represented by a change in the optical path difference of the optical fiber.
With the increasing demands of modern technologies on sensors, the rapid development of integrated optical waveguide sensors has been driven to meet the demands for rapid and specific detection of physical properties of mechanical, biological, chemical, etc. substances. Unlike the conventional optical fiber sensor, the integrated optical fiber sensor can be easily miniaturized, and thus it is possible to perform large-scale integration in a small area to realize a miniaturized experimental platform. Over the past several decades, many different configurations of fiber optic sensors have been obtained for various physical parameters. In general, these conventional sensors can meet the basic requirements for physical parameter detection.
In recent years, certain special application fields put higher demands on the sensitivity of the optical fiber temperature and refractive index sensor, and researchers start to further apply the vernier effect as a sensitization means in optical detection. The vernier effect is originally a phenomenon that the measuring sensitivity is improved by utilizing the tiny scale difference between the main scale and the vernier in the vernier caliper, and the vernier effect is applied to optical fiber sensing, so that the sensitivity and the dynamic range of the optical fiber interferometer can be improved simultaneously. The vernier effect is applicable to interferometers having periodic spectra, and in order to create the vernier effect, two interferometers having close free spectral ranges are typically cascaded. In the spectrum after cascade connection, the contrast ratio at the same phase position is higher, the contrast ratio at the phase position phase difference pi is very low, so that a large envelope is formed, the sensitivity and the dynamic range are obviously amplified, and the amplification factor depends on the dynamic range proximity degree.
Zhao Yuxin et al (full-fiber liquid refractive index sensor sensitized based on vernier effect. Photonic report, 2019,48 (11): 224-230.) propose a refractive index sensor based on parallel connection of double FP interferometers, wherein a sensing cavity is made of single-mode fibers by dislocation fusion, is an open cavity structure, a reference cavity is obtained by welding single-mode fibers with hollow fibers, is a closed cavity, has similar free spectral ranges of the two interferometers, and is superimposed to generate vernier effect, thereby realizing the sensitization effect.
Patent CN 110057389A proposes an optical fiber sensor based on the side hole optical fiber dual mach-zehnder interference vernier effect, which utilizes a single mode optical fiber, a coreless optical fiber and a side hole optical fiber to form a dual mach-zehnder structure. Light transmitted along the single-mode fiber enters the fiber core of the side hole fiber, the hole communicated with the external environment and the closed hole through the coreless fiber. The three light paths are combined at the second coreless fiber and transmitted to the spectrometer through the single mode fiber. The fiber core of the side hole optical fiber is interfered with the light of the two holes at the same time to form a double Mach-Zehnder interferometer, the interferometer formed by the fiber core and the closed hole is used as a reference part, and the interferometer formed by the hole communicated with the external environment is used as a sensing part. The refractive index sensor has ultrahigh sensitivity.
The sensors for realizing the optical fiber vernier effect prepared at the present stage are all passive optical fiber devices, and the spectrum is not adjustable, so that the refractive index measurement range is limited. And the demodulation means are large-scale equipment such as a spectrometer, and the like, so that the spectrum is firstly extracted and then the data analysis processing is carried out, the cost is high, the volume is huge, and the demodulation process is complex.
Disclosure of Invention
The invention aims to provide a refractive index measuring system and a demodulation method which can be used in various measuring environments, the system can adjust the refractive index measuring range according to the medium characteristics required to be measured, the refractive index measurement can be realized by adopting the system no matter gas or liquid, meanwhile, large measuring equipment such as a spectrometer and the like is omitted, and signal demodulation can be realized by using the system. The system has simple integral structure and high integration level, the dual interferometers form vernier effect, the detection sensitivity is improved, the detection interval is adjusted and signals are demodulated by controlling the voltage applied to the piezoelectric ceramics, and the response speed is high.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
the system consists of a light source, a single-mode fiber, an optical fiber circulator, an optical fiber 1 multiplied by 2 coupler, a refractive index sensing interferometer, a fiber integrated coupler, a double-core optical fiber, an end surface reflecting film, piezoelectric ceramics, a photoelectric detector, a data acquisition card, a computer and a piezoelectric ceramic control system. The optical fiber circulator is a 3-port optical fiber circulator.
The double-core optical fiber is provided with two fiber cores, the two fiber cores are not coupled during transmission, and the end face of the double-core optical fiber is plated with a reflecting film.
The fiber integrated coupler is manufactured by adopting a tapering or thermal diffusion mode after a single-mode fiber and a double-core fiber are welded, and plays roles of beam splitting and beam combining. Light transmitted along the single-mode fiber can be dispersed into two fiber cores of the double-core fiber according to a specific light splitting ratio after passing through the coupler, the light transmitted along the two fiber cores is reflected by the end surface reflecting film, reversely transmitted along the two fiber cores, and then passes through the fiber integration coupler again, and the two light beams are overlapped to form the fiber integration Michelson interferometer. The prepared Michelson interferometer is wound on a piezoelectric ceramic. According to actual needs, a plurality of piezoelectric ceramics with different piezoelectric strain constants can be adopted to realize flexible control of the dual-core optical fiber Michelson interferometer.
The refractive index sensing interferometer is one of a Mach-Zhender interferometer, a Michelson interferometer, a Fabry-Perot interferometer and a Sagnac interferometer, and the interference spectrum of the refractive index sensing interferometer changes along with the change of the environment refractive index.
The light source is a narrow linewidth adjustable laser. Light emitted by the light source is transmitted to the optical fiber circulator through the single-mode optical fiber and then is divided into two paths through the 1 multiplied by 2 optical fiber coupler, one path is connected with a Michelson interferometer formed by the fiber integrated coupler, the double-core optical fiber and the end surface reflecting film, and the spectrum of the interferometer can be controlled through piezoelectric ceramics. The other path of light is connected with a refractive index sensing interferometer for detection, the two interferometers are connected in parallel by an optical fiber 1 multiplied by 2 coupler, spectra are overlapped, a vernier effect is formed, and the sensitivity of sensing is amplified.
After the vernier effect is formed, the range of refractive index that can be detected by the whole interferometer is limited because the formation of the vernier effect requires that the Free Spectral Ranges (FSRs) of the two interferometers are relatively close, and once the FSRs of the two interferometers differ too much, the vernier effect is not formed. Therefore, a sensor suitable for gas measurement is difficult to be applied to measurement of refractive index of liquid. The invention adopts piezoelectric ceramics to realize the control of the spectrum of the Michelson interferometer, thereby expanding the detectable environment of the system. By adjusting the spectrum of the Michelson interferometer, the spectrums of the two interferometers can be overlapped to form a vernier effect no matter what measuring environment the refractive index sensing interferometer is applied to.
Meanwhile, the Michelson interferometer part can also be used for demodulating signals, when the Michelson interferometer part is used for refractive index measurement, the sensing interferometer is influenced by the environment refractive index, so that the spectrum after superposition of the two interferometers is changed, the photoelectric detector detects output signals, the data acquisition card and the computer analyze the acquired signals, then the piezoceramics is applied with voltage through the piezoceramics control system to regulate and control the voltage, so that the interference spectrum is changed again, finally, the detected signals are identical to the initial signals, the applied voltage value at the moment is read, and the demodulation of the signals is realized by utilizing the voltage-refractive index curve obtained by calibration in advance.
The principle used by the invention is as follows:
taking a dual-core optical fiber Michelson interferometer and a Fabry-Perot interferometer as examples, according to the interference principle, the phase difference of light transmitted along two fiber cores of the dual-core optical fiber is
Wherein: n is n 1 ，n 2 Respectively represent the refractive index of the fiber core, L 1 ，L 2 Respectively representing the lengths of the two cores. Assuming that the spectral ratio of the fiber-integrated coupler is one-to-one, the resulting interference spectrum light intensity can be expressed as follows:
in which I 1 And I 2 The light intensity of incident light of two fiber cores of the interferometer is shown as I, and the light intensity of emergent light is shown as I. Two important parameters of the interferometer, one being the free spectral range and the other the sensitivity. By a means ofThe free spectrum range refers to the wavelength difference between two adjacent peaks in the interference spectrum, and for a dual-core optical fiber interferometer, the FSR is:
for the Fabry-Perot interferometer, assuming that a micro air cavity exists in the optical fiber, the refractive index of the fiber core of the optical fiber is about 1.45, and the refractive index of air is 1, the reflectivity of the two reflecting surfaces can be obtained to be only 3.37 percent based on the Fresnel reflection principle. Thus, the reflected light of more than two times can be ignored, and the reflected light can be simplified into a dual-beam interferometer model for analysis. In the micro air cavity, the output intensity of two beams of light reflected back by the end faces of two optical fibers after interference is as follows:
wherein I is 3 、I 4 The light intensity reflected by the reflecting surface is respectively n is the effective refractive index of air in the F-P cavity, L is the length of the F-P cavity, and lambda is the working wavelength of input light. The FSR of the interference spectrum can be expressed as:
when the two interferometers are connected in parallel, the phase difference of the two interferometers is respectivelyAnd->The input optical signals are superimposed by a quantitative interferometer, and the finally output spectrum is as follows:
if the optical path differences of the two interferometers are similar, thenCan be seen as a high frequency periodic function, < +.>Can be seen as a low frequency periodic function. It can be seen that the final spectrum contains a high frequency fine spectrum, consisting of +.>And (5) determining. In addition, a low-frequency envelope is included, mainly consisting of +.>And (5) determining.
For the spectrum of the interferometer after cascade connection, the FSR refers to the wavelength difference of two adjacent peaks on the envelope, the optical path difference of the two interferometers directly corresponds to the FSR of the two interferometers, so that the relation between the FSR of the envelope of the interferometer after cascade connection and the FSR of the two independent interferometers can be obtained by substitution
The dynamic range is amplified, and compared with one interferometer, the amplification factor is as follows:
when sensing, one interferometer is kept as a reference interferometer, no external influence is exerted, and the other interferometer is used as a sensing interferometer to sense the external environment change. After an environmental change, the envelope shifts, and from the analysis described above, it can be seen that the sensitivity of the parallel interferometer is amplified.
The beneficial effects of the invention are as follows:
1. the invention uses two interferometers to form vernier effect in parallel, which increases the sensitivity of the sensing system;
2. the piezoelectric ceramic is used as a control part of one interferometer, so that the measuring range is enlarged, and the problem of measuring different environments by adopting the same system is solved;
3. the piezoelectric ceramic regulation and control means are adopted to realize signal demodulation, large-scale equipment such as a spectrometer is not needed, the integration level of the system is improved, and data operation in the demodulation process is simplified.
Drawings
FIG. 1 is a schematic diagram of a high sensitivity refractive index measurement system that may be used in a variety of measurement environments;
FIG. 2 (a) is a schematic diagram of an asymmetric twin-core optical fiber structure, and FIG. 2 (b) is a schematic diagram of a symmetric twin-core optical fiber structure;
FIG. 3 (a) is an interference spectrum of the refractive index sensing interferometer in air, and FIG. 3 (b) is a spectrum of the piezoceramic-conditioned dual-core fiber Michelson interferometer; FIG. 3 (c) is a spectrum of the two interferometers superimposed;
FIG. 4 (a) is an interference spectrum of the refractive index sensing interferometer placed in water, and FIG. 4 (b) is a spectrum of the piezoceramic-tuned dual-core fiber Michelson interferometer; fig. 4 (c) is a spectrum of the two interferometers superimposed.
In the figure: 1. a light source; 2. a single mode optical fiber; 3. an optical fiber circulator; 4. an optical fiber 1 x 2 coupler; 5. refractive index sensing interferometers; 6. a fiber-integrated coupler; 7. a dual-core optical fiber; 8. an end surface reflection film; 9. piezoelectric ceramics; 10. a photodetector; 11. a data acquisition card; 12. a computer; 13. a piezoelectric ceramic control system.
Detailed Description
For the purpose of promoting an understanding of the principles and advantages of the invention, reference will now be made to the drawings in which there will be illustrated, by way of illustration, and not as an actual or complete description, the embodiments of the invention. All other embodiments, based on the described embodiments, which a person of ordinary skill in the art would obtain without making any inventive effort, are within the scope of the invention.
The invention discloses a refractive index measurement system and a demodulation method which can be used in various measurement environments, taking Michelson interferometer and Fabry-Perot interferometer as an example, the structure of the refractive index measurement system is shown in figure 1, and the refractive index measurement system consists of a light source, a single-mode fiber, an optical fiber circulator, an optical fiber 1 multiplied by 2 coupler, a refractive index sensing interferometer, a fiber integrated coupler, a double-core optical fiber, an end surface reflecting film, piezoelectric ceramics, a photoelectric detector, a data acquisition card, a computer and a piezoelectric ceramics control system. The dual-core optical fiber comprises two fiber cores, and typical dual-core optical fiber core arrangement is shown in fig. 2, wherein fig. 2 (a) is an asymmetric dual-core optical fiber, and fig. 2 (b) is a symmetric dual-core optical fiber. The dual-core optical fiber used in this embodiment is an asymmetric dual-core optical fiber, as shown in fig. 2 (a), where the refractive index of the two cores is different, and there is a refractive index difference. The refractive index sensing interferometer was a Fabry-Perot interferometer made by etching micro-holes in a single mode fiber using a femtosecond laser, the empty length being 0.15mm.
In the embodiment, when the dual-core optical fiber Michelson interferometer is manufactured, the coupler is manufactured by adopting a thermal diffusion technology, and the single-mode optical fiber and the dual-core optical fiber are welded in pairs, because the asymmetric dual-core optical fiber is adopted, the middle core can be completely connected with the single-mode optical fiber in pairs, and therefore, the dual-core optical fiber can be heated during thermal diffusion. And (3) placing a section of double-core optical fiber on the central shaft of a heating area of the furnace for heat diffusion treatment, and heating for a certain time to gradually change the concentration distribution of the medium-core dopant in the heat diffusion area of the double-core optical fiber into quasi-Gaussian distribution so as to realize the coupling of light beams. The length of the heating zone of the furnace is usually above the order of centimeters, ensuring that the refractive index in the gradient temperature field changes slowly to a quasi-gaussian distribution. During thermal diffusion, the optical beam analyzer is used for real-time monitoring, and heating is stopped when the split ratio of the two fiber cores is 1:1. And then plating a reflecting film on the tail end of the double-core optical fiber to prepare the double-core optical fiber Michelson interferometer.
Winding a double-core optical fiber interferometer serving as a demodulation interferometer on piezoelectric ceramics; the Fabry-Perot interferometer is a refractive index sensing interferometer for measuring the external refractive index. When the sensor is put into air, the spectrum of the refractive index sensing interferometer is as shown in FIG. 3 (a), and the FSR is 8.0nm. The voltage applied to the electroceramic changes the spectrum of the interferometer thereon so that the FSR of the two interferometers are similar, the final interference spectrum is as shown in fig. 3 (b), the FSR of which is 7.3nm. The two interferometers are connected through a fiber 1 x 2 coupler, the two interferometers are overlapped to form a vernier effect, the spectrum of the vernier effect is shown in fig. 3 (c), and the FSR of the spectrum envelope is about 80nm.
When the sensor is placed in water, the optical path difference of the refractive index sensing interferometer changes, resulting in a spectrum thereof as shown in fig. 4 (a), where the FSR becomes 6.0nm. At this time, the voltage applied by the piezoelectric ceramic was adjusted so that the FSR of the two interferometers was again similar, and the interference spectrum was 5.6nm as shown in FIG. 4 (b). After the two interferometers are superimposed again to form the vernier effect, the spectrum is shown in FIG. 4 (c), where the FSR is about 80nm. Therefore, no matter what refractive index interval the refractive index sensing interferometer is used for, FSRs of the two interferometers can be similar by adjusting the voltage applied by the piezoelectric ceramic, so that vernier effect is formed, and high-sensitivity detection is realized.
When the system is used, the sensor is firstly calibrated, all parts are connected according to the diagram shown in fig. 1, piezoelectric ceramics are regulated, so that the two interferometers form vernier effect, and the calibration is carried out through known refractive index values. During calibration, the output wavelength of the light source is adjusted first, so that the incident wavelength is the wavelength at which the spectrum output intensity is maximum after superposition, as indicated by the circle in fig. 4 (c), and the current light intensity is detected and recorded by the photoelectric detector as an initial value. Then, applying refractive index matching liquid with a determined value to the sensing interferometer, at the moment, the light intensity detected by the photoelectric detector can change, the data acquisition card uploads detected data to the data processing unit, the data processing unit applies voltage to the piezoelectric ceramic through the piezoelectric ceramic controller, the spectrum of the demodulation interferometer is changed, the signal detected by the photoelectric detector changes, the detected signal is the same as an initial value through adjustment of the applied voltage, the current voltage value and the refractive index value are recorded, and a curve related to the applied voltage and the sensing change quantity is finally obtained through changing the sensing change quantity, so that the calibration of the system is realized.
When the system is used for refractive index measurement, the sensing optical fiber is placed in an environment to be detected, when the refractive index of the measured environment changes, the detection signal changes, the detected signal is finally the same as the initial value by continuously adjusting the applied voltage, the voltage value at the moment is read, and the refractive index change quantity of the environment is determined through a calibrated curve.

Claims (5)

1. The refractive index measurement system capable of being used in various measurement environments consists of a light source, a single-mode fiber, an optical fiber circulator, an optical fiber 1 multiplied by 2 coupler, a refractive index sensing interferometer, a fiber integrated coupler, a double-core optical fiber, an end surface reflecting film, piezoelectric ceramics, a photoelectric detector, a data acquisition card, a computer and a piezoelectric ceramics control system; the fiber integrated coupler, the double-core optical fiber and the end surface reflecting film form a Michelson interferometer, and the output spectrum of the Michelson interferometer can be controlled by piezoelectric ceramics; light emitted by a light source is transmitted to an optical fiber circulator through a single mode fiber and then is divided into two paths through a 1 multiplied by 2 optical fiber coupler, one path is connected with a Michelson interferometer formed by a fiber integrated coupler, a double-core optical fiber and an end surface reflecting film, the other path is connected with a refractive index sensing interferometer for detection, the two interferometers are connected in parallel through the optical fiber 1 multiplied by 2 coupler, and spectra are overlapped to form a vernier effect; the spectrum of the Michelson interferometer is controlled by the voltage applied to the piezoelectric ceramic, so that the spectrums of the two interferometers can be overlapped to form a vernier effect no matter what measuring environment the refractive index sensing interferometer is applied to, and the measurement of the refractive index is realized.
2. The refractive index measurement system for a plurality of measurement environments according to claim 1, wherein: the fiber integrated coupler is made in a tapering or thermal diffusion mode and plays a role in beam splitting and beam combining.
3. The refractive index measurement system for a plurality of measurement environments according to claim 1, wherein: the piezoelectric ceramic is formed by cascading a plurality of piezoelectric ceramics with different piezoelectric strain constants, so that the spectrum of the fiber-integrated Michelson interferometer is flexibly controlled.
4. The refractive index measurement system for a plurality of measurement environments according to claim 1, wherein: the refractive index sensing interferometer is one of a Mach-Zhender interferometer, a Michelson interferometer, a Fabry-Perot interferometer and a Sagnac interferometer, and the interference spectrum of the refractive index sensing interferometer changes along with the change of the environment refractive index.
5. The demodulation method of the refractive index measurement system which can be used for various measurement environments is characterized in that: when the system is used for refractive index measurement, the sensing interferometer is influenced by the refractive index of the environment, so that the spectrum after superposition of the two interferometers is changed, the photoelectric detector detects an output signal, the data acquisition card and the computer analyze the acquired signal and feed back the acquired signal to the piezoelectric ceramic control system, voltage applied to the piezoelectric ceramic is changed, the interference spectrum after superposition is changed again, the finally detected signal is identical to the initial signal, the voltage value applied at the moment is read, and signal demodulation is realized by utilizing a voltage-refractive index curve obtained through pre-calibration.
CN202310235615.1A 2023-03-13 2023-03-13 Refractive index measurement system and demodulation method applicable to various measurement environments Pending CN116818715A (en)

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