CN110470633B - Multi-core fiber grating refractive index sensitive sensor with in-situ self-compensation characteristic - Google Patents
Multi-core fiber grating refractive index sensitive sensor with in-situ self-compensation characteristic Download PDFInfo
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- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
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- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
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- G02B6/02138—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating using beam interference based on illuminating a phase mask
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Abstract
The invention provides a multicore fiber grating refractive index sensitive sensor with in-situ self-compensation characteristics, which comprises a multicore fiber, wherein the multicore fiber comprises a cladding and a plurality of fiber cores in the cladding, the fiber cores are divided into a middle fiber core and a peripheral fiber core, and gratings with periodically-changed refractive indexes are engraved on each fiber core; a groove is formed in the cladding outside the grating, so that an evanescent field of the optical fiber is exposed; the groove is plated with a sensitive film. Because the core mold of the peripheral fiber core is exposed to the environment, the peripheral fiber core is sensitive to the environmental refractive index and is also influenced by multiple factors of temperature and strain; the middle fiber core is not exposed in the environment, so that the middle fiber core is not influenced by the ambient refractive index and is only influenced by temperature and strain; the peripheral fiber core is compensated through the middle fiber core, so that in-situ self-compensation of the sensor can be realized.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a multi-core optical fiber grating refractive index sensitive sensor with an in-situ self-compensation characteristic.
Background
Accurate measurement of the refractive index of the environment plays a crucial role in many aspects, such as detection of the concentration of various reagents in chemical analysis, as well as gas concentration detection based on changes in the refractive index and monitoring of the degree of corrosion of the steel reinforcement. Currently widely used refractometers are commonly used to measure the refractive index of some mixed solutions to determine the liquid solubility. There are other techniques for monitoring the change in the refractive index of the environment, including planar laser induced fluorescence, invasive thermal marking, interferometry, phase locked loop ultrasound, and the like. However, the above methods all suffer from one or more of the following disadvantages: the method has the advantages of low test resolution, high price, large detection instrument volume, difficult operation, large influence of subjective factors of detection personnel and the like.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the multi-core fiber grating refractive index sensitive sensor with the in-situ self-compensation characteristic is provided, and the same-optical-path self-compensation of environmental influence factors can be realized.
The technical scheme adopted by the invention for solving the technical problems is as follows: a multi-core fiber grating refractive index sensitive sensor with in-situ self-compensation characteristic is characterized in that: the sensor comprises a multi-core fiber, wherein the multi-core fiber comprises a cladding and a plurality of fiber cores in the cladding, the fiber cores are divided into a middle fiber core and a peripheral fiber core, and gratings with periodically changed refractive indexes are engraved on each fiber core; a groove is formed in the cladding outside the grating, so that an evanescent field of the optical fiber is exposed; the groove is plated with a sensitive film.
According to the scheme, the peripheral fiber cores are distributed in a radial or axial symmetry mode.
According to the scheme, interference light generated after ultraviolet light penetrates through the phase mask plate is used for exposing the multi-core optical fiber loaded with hydrogen to write in the grating.
According to the scheme, the groove is obtained by a chemical etching method, a physical grinding method or a laser burning method.
According to the scheme, the length of the groove in the length direction of the fiber core is 10-40 mm.
According to the scheme, the depth of the groove in the cross section direction of the multi-core fiber is-5-20 um away from the outer surface of the cladding.
According to the scheme, the thickness of the sensitive film is 5-200 nm.
According to the scheme, the sensitive film is a refractive index sensitization type film, a gas sensitive type film or a corrosion sensitive type film.
A multi-core fiber grating refractive index sensitive sensing system with in-situ self-compensation characteristic is characterized in that: the sensing system comprises a light source, a splitter, a beam splitter, a demodulating device and the sensor; one end of the beam splitter is welded with the multi-core optical fiber of the sensor, and a plurality of jumper wires are arranged at the other end of the beam splitter and respectively correspond to each fiber core in the multi-core optical fiber; light emitted by the light source is transmitted through optical fibers, sequentially passes through the splitter and the beam splitter, reaches the sensor, is reflected back, obtains optical signals with different reflection spectrum peak values through each jumper wire of the beam splitter, and is demodulated by the demodulating device; the self-compensation effect on the environmental interference factors is realized by demodulating the grating reflection spectrum peak positions of the middle fiber core and the peripheral fiber core.
According to the system, the demodulation device is a spectrometer.
The beneficial effects of the invention are as follows: because the core mold of the peripheral fiber core is exposed to the environment, the peripheral fiber core is sensitive to the environmental refractive index and is also influenced by multiple factors of temperature and strain; the middle fiber core is not exposed to the environment, so that the middle fiber core is not influenced by the ambient refractive index and is only influenced by temperature and strain; the peripheral fiber core is compensated through the middle fiber core, so that the in-situ self-compensation of the sensor can be realized.
Drawings
Fig. 1 is a schematic system structure according to an embodiment of the present invention.
Fig. 2 is a schematic structural diagram of a sensor according to an embodiment of the invention.
FIG. 3 is a spectrum diagram of an embodiment of the present invention under different refractive index environments.
FIG. 4 is a graph showing the relationship between the average wavelength shift of the center and side cores and the refractive index when the refractive index is measured according to an embodiment of the present invention.
In the figure: 1. the optical fiber grating sensor comprises a light source, 2 demodulation devices, 3 splitters, 4 splitters, 5 sensors, 5.1 sensitive films, 5.2 peripheral fiber cores, 5.3 middle fiber cores, 5.4 gratings and 5.5 cladding layers.
Detailed Description
The invention is further described below with reference to specific examples and the accompanying drawings.
The invention provides a multicore fiber grating refractive index sensitive sensor with in-situ self-compensation characteristics, as shown in figure 2, the sensor 5 comprises a multicore fiber, the multicore fiber comprises a cladding 5.5 and a plurality of fiber cores in the cladding 5.5, the fiber cores are divided into a middle fiber core 5.3 and a peripheral fiber core 5.2, and each fiber core is engraved with a grating 5.4 with periodically-changed refractive index; a groove is formed in the cladding 5.5 outside the grating 5.4, so that an evanescent field of the optical fiber is exposed; the groove is internally plated with a sensitive film 5.1.
The multi-core optical fiber is a special optical fiber with the core number more than 1, and comprises a seven-core optical fiber, a five-core optical fiber, a three-core optical fiber, a two-core optical fiber and the like. The middle part is provided with a middle fiber core 5.3, the periphery is provided with a peripheral fiber core 5.2, and the peripheral fiber cores 5.2 are distributed in a radial or axial symmetry mode.
The sensor manufacturing method of the embodiment comprises the following steps:
step 1: the multi-core fiber after hydrogen loading is exposed by interference light generated after ultraviolet light penetrates through the phase mask plate, a grating 5.4 with periodically changed refractive index is written, and each fiber core of the multi-core fiber after exposure is successfully written with the grating 5.4.
Step 2: the evanescent field of the multi-core optical fiber is exposed by a surface micromachining technology, and the method mainly comprises a chemical etching method (such as HF acid), a physical grinding method, a laser burning method and the like. The depth of the obtained groove along the cross section direction of the multi-core fiber is-5-20 um away from the outer surface of the cladding, and the sensitivity of the sensor is related to the depth; the length of the groove in the length direction of the fiber core is 10-40 mm.
And step 3: and (3) cleaning the multi-core optical fiber subjected to micro-processing in the step (2) by using deionized water, and putting the cleaned multi-core optical fiber into an evaporation machine to plate a layer of sensitive film 5.1 on a micro-processing area (namely a groove). In the embodiment, the thickness of the sensitive film 5.1 is 5-200 nm, and the thickness of the sensitive film 5.1 is related to the sensitivity of the sensor. The change of the external environment can cause the change of the refractive index of the sensitive film, and the sensitive film 5.1 comprises a refractive index sensitization type film, such as a graphene film; gas sensitive films including metal Pt and Pt alloy films; and corrosion sensitive films such as Fe-C films.
And finally, taking out the coated multi-core fiber grating, and fusing the multi-core fiber grating with the matched beam splitter through a manual fiber fusion splicer. After the multi-core optical fiber is welded with the matched beam splitter, the channel corresponding to each fiber core is numbered and connected with a jumper wire, so that the test is convenient.
The invention also provides a multi-core fiber grating refractive index sensitive sensing system with in-situ self-compensation characteristic, as shown in fig. 1, the sensing system comprises a light source 1, a splitter 3, a beam splitter 4, a demodulating device 2 and the sensor 5; one end of the beam splitter 4 is welded with the multi-core optical fiber of the sensor 5, and a plurality of jumper wires are arranged at the other end of the beam splitter 4 and respectively correspond to each fiber core in the multi-core optical fiber; light emitted by the light source 1 is transmitted through optical fibers, sequentially passes through the splitter 3 and the beam splitter 4, then reaches the sensor 5, is reflected back, and light signals with different reflection spectrum peak values are obtained through each jumper wire of the beam splitter 4 and are demodulated by the demodulating device 2; the self-compensation effect on the environmental interference factors is realized by demodulating the grating reflection spectrum peak positions of the middle fiber core and the peripheral fiber core. In this embodiment, the demodulation device is a spectrometer.
Wherein each core of the sensor 5 is 1 channel, thus producing multiple reflection spectra. The peripheral core 5.2 is sensitive to the ambient refractive index due to the fact that the core mode of the peripheral core 5.2 is exposed to the ambient, and of course is also subject to multiple effects of temperature and strain. The middle fiber core 5.3 is not exposed to the environment, so that the influence of the environmental refractive index is avoided, the influence of the temperature and the strain is only realized, and the sensor in-situ self-compensation can be realized by compensating the peripheral fiber core 5.2 through the middle fiber core 5.3.
The invention has the structural characteristics of a plurality of peripheral fiber cores 5.2 and a middle fiber core 5.3, so that the fiber has multiple advantages of in-situ self-compensation, stability, repeatability, miniaturization, low cost, high feasibility, easy preparation and the like.
The first embodiment is as follows:
the embodiment provides a multicore fiber grating refractive index sensitive hydrogen sensor with in-situ self-compensation characteristics, wherein a sensitive film of the embodiment is a metal Pt film, and the refractive index of the metal Pt film is influenced by environmental factors. The multi-core fiber adopted in the embodiment is a seven-core fiber, and the micro-processing mode is etching by using 20% HF acid.
The specific operation method of the present embodiment is as follows,
taking a section of hydrogen-loaded seven-core optical fiber with the length of 50cm, stripping a coating layer with the length of 2 cm at the middle section by using an optical pliers, fixing the seven-core optical fiber at the front end of a laser by using a clamp, exposing the seven-core optical fiber with the coating layer removed by using a phase mask method, and writing a Bragg grating. Wherein the diameter of the seven-core optical fiber is 150 um, the core diameter is 4.1 um, and the core interval is 41.5 um. The laser frequency was 300 Hz, the energy was 20 mJ and the exposure time was 5 s.
And etching the coating layer removing region of the seven-core optical fiber on which the Bragg grating is written by using 20% HF for 30 min, wherein the diameter of the etched region optical fiber is 100 mu m.
And cleaning and drying the corroded seven-core optical fiber by using deionized water, and putting the seven-core optical fiber into a magnetron sputtering vacuum coating machine to sputter a layer of Pt film with the power of 50W and the thickness of 10 nm to be used as a hydrogen sensitive film.
And taking out the seven-core optical fiber plated with the Pt film, and welding one end of the seven-core optical fiber with the seven-core optical fiber beam splitter through a welding machine. The other end of the beam splitter is divided into seven channels by jumper wires, and the seven channels correspond to seven fiber cores of the seven-core optical fiber respectively.
In the optical fiber hydrogen sensing system implemented by using the above sensor provided by this embodiment, the light source is a SLED light source of 1550 nm, the demodulation device is a spectrometer, the splitter is a 3dB coupler, and the splitter is a seven-channel splitter. Therefore, 7 reflection peaks can be generated, namely a reflection spectrum peak value 1 of the middle fiber core grating, and a reflection spectrum peak value 2, a reflection spectrum peak value 3, a reflection spectrum peak value 4, a reflection spectrum peak value 5, a reflection spectrum peak value 6 and a reflection spectrum peak value 7 of the peripheral fiber core grating. The peak value 1 is irrelevant to the refractive index of the hydrogen sensitive film and does not contain hydrogen concentration information, and is used as a reference peak to compensate the influence of peak position change caused by the change of a light source and an external environment; the peak value 2, the peak value 3, the peak value 4, the peak value 5, the peak value 6 and the peak value 7 are not only related to the change of a light source and an external environment, but also change with the same amplitude along with the refractive index of the hydrogen sensitive film, so that the hydrogen sensitive film can be used as a sensing peak to detect the influence caused by the change of the refractive index of the film due to external hydrogen. The self-compensation effect on environmental interference factors is realized by demodulating the reflection spectrum peak positions of the grating of the middle core and the grating of the peripheral core.
Fig. 3 is a wave spectrum diagram of the sensor of the present embodiment under different refractive index environments. As can be seen from the figure, as the refractive index increases, a phenomenon of spectral shift occurs, thereby showing the sensitivity of the sensor to the refractive index on the data.
FIG. 4 is the relationship between the average wavelength drift of the center core and the side cores and the refractive index when the sensor measures the refractive index, and it can be seen from the graph that the wavelength drift of the side cores increases in the form of a quadratic function with the increase of the refractive index, while the wavelength drift of the center core hardly changes with the change of the refractive index and approaches to zero. Therefore, the in-situ self-compensation of sensing can be realized based on that the side core is sensitive to the refractive index, the middle core is insensitive to the refractive index, and the side core and the middle core are both linearly sensitive to the temperature.
Example two:
the sensitive film in the first embodiment is changed into a graphene film to cover the area of the seven-core fiber subjected to HF acid corrosion, so that the sensitivity enhanced multi-core fiber grating refractive index sensor with the in-situ self-compensation characteristic can be formed. The principle is that graphene covers the area of the seven-core optical fiber subjected to HF acid corrosion, and the graphene can induce optical signals transmitted in the optical fiber, so that optical field distribution around the optical fiber is increased, and finally the sensitivity of the sensor is improved.
Example three:
in the first embodiment, the surface micromachining mode is changed from 20% HF corrosion to femtosecond laser machining of the annular groove, and then the multi-core fiber grating refractive index sensitive hydrogen sensor with in-situ self-compensation characteristic is formed. The femtosecond laser processing principle is also to expose evanescent field on the surface of the fiber core at the periphery of the optical fiber. The shape and area of the machined groove is related to the sensitivity of the sensor.
The above embodiments are only used for illustrating the design idea and features of the present invention, and the purpose of the present invention is to enable those skilled in the art to understand the content of the present invention and implement the present invention accordingly, and the protection scope of the present invention is not limited to the above embodiments. Therefore, all equivalent changes and modifications made in accordance with the principles and concepts disclosed herein are intended to be included within the scope of the present invention.
Claims (6)
1. A multi-core fiber grating refractive index sensitive sensor with in-situ self-compensation characteristic is characterized in that: the sensor comprises a multi-core optical fiber, wherein the multi-core optical fiber comprises a cladding and a plurality of fiber cores in the cladding, the fiber cores are divided into a middle fiber core and a peripheral fiber core, and gratings with periodically changed refractive indexes are engraved on each fiber core; the cladding outside the grating is provided with a groove so that only the evanescent field of the peripheral optical fiber is exposed; a sensitive film is plated in the groove;
the peripheral fiber cores are distributed in a radial or axial symmetry manner;
the grating is written in by exposing the multi-core fiber loaded with hydrogen through interference light generated after ultraviolet light penetrates through the phase mask plate; the length of the groove along the length direction of the fiber core is 10-40 mm, and the depth of the groove along the cross section direction of the multi-core fiber is-5-20 um away from the outer surface of the cladding.
2. The sensor of claim 1, wherein: the groove is obtained by a chemical etching method, a physical grinding method or a laser burning method.
3. The sensor of claim 1, wherein: the thickness of the sensitive film is 5-200 nm.
4. The sensor of claim 1, wherein: the sensitive film is a refractive index sensitization type film, a gas sensitive type film or a corrosion sensitive type film.
5. A multi-core fiber grating refractive index sensitive sensing system with in-situ self-compensation characteristic is characterized in that: the sensing system comprises a light source, a splitter, a beam splitter, a demodulating device and a sensor according to any one of claims 1 to 4; one end of the beam splitter is welded with the multi-core optical fiber of the sensor, and a plurality of jumper wires are arranged at the other end of the beam splitter and respectively correspond to each fiber core in the multi-core optical fiber; light emitted by the light source is transmitted through optical fibers, sequentially passes through the splitter and the beam splitter, reaches the sensor, is reflected back, obtains optical signals with different reflection spectrum peak values through each jumper wire of the beam splitter, and is demodulated by the demodulating device; the self-compensation effect on the environmental interference factors is realized by demodulating the grating reflection spectrum peak positions of the middle fiber core and the peripheral fiber core.
6. The sensing system of claim 5, wherein: the demodulation device is a spectrometer.
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