CN216144696U - Optical fiber Mach-Zehnder refractive index sensor of liquid drop type air cavity - Google Patents
Optical fiber Mach-Zehnder refractive index sensor of liquid drop type air cavity Download PDFInfo
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- CN216144696U CN216144696U CN202121637488.0U CN202121637488U CN216144696U CN 216144696 U CN216144696 U CN 216144696U CN 202121637488 U CN202121637488 U CN 202121637488U CN 216144696 U CN216144696 U CN 216144696U
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Abstract
The utility model discloses an optical fiber Mach-Zehnder refractive index sensor of a liquid drop type air cavity, and belongs to the technical field of optical fiber refractive index sensors. The optical fiber comprises a first single-mode fiber (1), a photonic crystal fiber (2) and a second single-mode fiber (5) from front to back in sequence, a collapse region (4) and an optical fiber tapering region (3) are formed at the rear end of the photonic crystal fiber (2) in sequence, a liquid drop type air cavity (31) and a silica cladding (33) are formed in the optical fiber tapering region (3), one beam of light is transmitted in the liquid drop type air cavity (31), the other beam of light is transmitted in the silica cladding (33), the two beams of light are coupled when reaching the second single-mode fiber (5), and Mach-Zehnder interference occurs. The sensor can measure the refractive index by measuring the deviation of the wave trough wavelength of the interference signal of the sensor, can directly demodulate and realize real-time detection.
Description
Technical Field
The utility model relates to an optical fiber Mach-Zehnder refractive index sensor of a liquid drop type air cavity, and belongs to the technical field of optical fiber refractive index sensors.
Background
The on-line Mach-Zehnder interferometer has the advantages of small volume, high sensitivity, high resolution, low cost, excellent anti-electromagnetic interference characteristic and the like, and can realize light beam interference by using a single optical fiber without using an optical fiber coupler, so that the on-line Mach-Zehnder interferometer is widely applied to sensing measurement of refractive index, temperature, humidity, strain and liquid level.
For refractive index measurements, a large overlap region between the propagating light field and the surrounding medium is required. The overlap region is increased by reducing the size of the in-line mach-zehnder interferometer waveguide, thereby achieving higher sensitivity. In the existing in-line mach-zehnder interferometer, interference occurs between the core mode and the excited cladding mode, or between the fundamental mode and the higher-order modes of the multimode fiber. The beams participating in the interference propagate mainly in silica, with a very small difference in effective refractive index between them. Therefore, an interference arm several centimeters long is often required to obtain an interference spectrum with sufficient phase difference, which increases the difficulty of packaging in practical applications and makes it difficult to obtain a mach-zehnder interferometer of a micrometer scale.
Disclosure of Invention
The utility model aims to overcome the defects of the prior art, provides a compact micron-sized optical fiber Mach-Zehnder refractive index sensor based on a liquid drop type air cavity,
the technical scheme of the utility model is as follows:
the utility model relates to an optical fiber Mach-Zehnder refractive index sensor with a liquid drop type air cavity, which sequentially comprises a first single-mode optical fiber, a photonic crystal optical fiber and a second single-mode optical fiber from front to back, wherein air holes are regularly arranged in a cladding of the photonic crystal optical fiber, a collapse region and an optical fiber tapering region are sequentially formed at the rear end of the photonic crystal optical fiber through a fusion splicing technology, a liquid drop type air cavity and a silica cladding are formed in the optical fiber tapering region, and the liquid drop type air cavity is coated by the silica cladding;
the light is transmitted in the first single-mode fiber, when reaching the collapse region of the photonic crystal fiber, the light is divided into two beams by one beam, wherein one beam of light is transmitted in the droplet-type air cavity, and the other beam of light is transmitted in the silica cladding to form evanescent waves; the two beams of light are coupled when reaching the second single-mode fiber and are transmitted through the fiber core of the second single-mode fiber; the two beams are coupled to generate Mach-Zehnder interference, and the interference intensity can be expressed as follows:
wherein the content of the first and second substances,I 1 andI 2 the intensities of the transmitted light of the core mode and the cladding mode of the second single mode optical fiber,φthe phase difference between the cladding mode and the core mode of the second single-mode optical fiber is expressed as:
in the formula (I), the compound is shown in the specification,Lexpressed as the length of the droplet-type air chamber,λexpressed as the wavelength of the incident light,Δn eff expressed as the difference between the effective refractive index of the cladding mode and the core mode of the second single mode fiber, the effective refractive index of the cladding mode changes with the environmental refractive index; as can be seen from formulas 1 and 2, the intensity of the transmitted light varies with the variation of the phase difference; the corresponding valley wavelengths for the m-order and m-2 order interference fringes can be expressed as:
effective refractive index difference between cladding mode and core mode of second single mode optical fiberΔn eff Can be expressed as:
the variation of the m-order interference fringe troughs can be expressed as:
in the formulaΔnThe change of the effective refractive index of the cladding mode along with the change of the environmental refractive index.
Further, the ambient refractive index of the sensor ranges from 1.4 to 1.42, and the sensitivity is up to 3133.3 nm/RIU.
Further, the first single-mode fiber is connected with an amplified spontaneous emission light source ASE, and the second single-mode fiber is connected with a spectrum input end of the fiber sensing analyzer OSA.
Furthermore, the diameter of the lumbar vertebra of the optical fiber tapering region is 40-60 microns, the length of the droplet-type air cavity is 200-350 microns, the diameter of the central air cavity of the tapering region is 37-50 microns, and the wall thickness is 1-8.5 microns.
Further, the medium of the droplet-type air cavity is air, and the refractive index n of the airO=1, the silica clad medium is silica, the silica refractive index nSiO2=1.45。
Advantageous effects
The optical fiber Mach-Zehnder refractive index sensor with the liquid drop type air cavity has the advantages of common optical fiber sensors, is not easy to be interfered by electromagnetism, and is full in optical fiber, simple in structure, miniaturized and convenient to manufacture. In addition to this, the present sensor has some unique advantages: (1) the optical fiber Mach-Zehnder refractive index sensor is provided with the ultra-short liquid drop type air cavity 31, and the ultra-compact structure enables the optical fiber Mach-Zehnder refractive index sensor to be suitable for detecting the refractive index of trace liquid and obtaining interference signals with high contrast. (2) The interference wall thickness is controlled, and the sensitivity can be very high and reaches more than 2803 nm/RIU. (3) The refractive index is measured by measuring the deviation of the wave trough wavelength of the interference signal of the sensor, and the measured value is a monotonic function in the free frequency spectrum range of the interference fringe, so that the direct demodulation can be realized, and the real-time detection can be realized.
Drawings
FIG. 1 is a schematic diagram of a drop-type air cavity optical fiber Mach-Zehnder refractive index sensor according to the present invention;
FIG. 2 is a schematic illustration of a refractive index measurement experiment of the sensor of FIG. 1;
FIG. 3 is a graph of the transmission of a sensor in air for a droplet-type air cavity having a cavity length of 208 microns;
FIG. 4 is a graph of experimental results of sensor wavelength versus ambient refractive index for a droplet-type air cavity having a cavity length of 208 microns and a wall thickness of 8.1 microns;
FIG. 5 is a graph of simulation results of sensor wavelength versus ambient refractive index for a droplet-type air cavity having a cavity length of 330 microns and a wall thickness of 1.15 microns;
wherein: first single mode optical fiber 1
Photonic crystal fiber 2
Optical fiber tapering region 3
Droplet-type air chamber 31
Collapse zone 4
A second single mode fibre 5.
Detailed Description
For better understanding of the technical solutions of the present invention, the following detailed descriptions will be provided for the working procedures and advantages of the present invention with reference to the accompanying drawings and specific embodiments.
The utility model relates to a liquid drop type optical fiber Mach-Zehnder refractive index sensor with an air cavity, which is a compact micron-sized sensor based on the Mach-Zehnder interference and evanescent wave principle.
Specifically, referring to fig. 1, the optical fiber mach-zehnder refractive index sensor of the droplet-type air cavity is composed of two common single-mode fibers (SMF) and one photonic crystal fiber 2 (PCF), which are a first single-mode fiber 1, the photonic crystal fiber 2, and a second single-mode fiber 5 in sequence from front to back. The first single-mode fiber 1 is connected with an Amplified Spontaneous Emission (ASE) which provides a light source; the second single mode fibre 5 is connected to the spectral input of a fibre-Optic Sensing Analyser (OSA) for analysing the spectrum, fig. 2 shows the refractive index testing apparatus of the sensor.
The photonic crystal fiber comprises a photonic crystal fiber 2 and is characterized in that air holes 21 regularly arranged in a cladding of the photonic crystal fiber 2 are formed in the cladding of the photonic crystal fiber 2, a collapse region 4 and a fiber tapering region 3 are sequentially formed at the rear end of the photonic crystal fiber 2 through a fusion welding technology, a droplet-shaped air cavity 31 and a silica cladding 33 are formed in the fiber tapering region 3, and the droplet-shaped air cavity 31 is coated by the silica cladding 33. The diameter of the lumbar vertebra of the optical fiber tapering region 3 is 40-60 microns, the length of the droplet-shaped air cavity 31 is 200-350 microns, the diameter of the central air cavity of the tapering region is 37-50 microns, and the wall thickness is 1-8.5 microns. The medium of the droplet-type air cavity 31 is air, and the refractive index n of the airO=1, the silica cladding 33 medium is silica, the silica refractive index nSiO2=1.45。
Light is firstly transmitted in the first single-mode fiber 1, and when the light reaches the collapse region 4 of the photonic crystal fiber 2, the light is divided into two beams by one beam, wherein one beam is transmitted in the droplet-type air cavity 31, and the other beam is transmitted in the silica cladding 33, so that an evanescent wave is formed.
The thickness of the silica cladding 33 is defined as the wall thickness, and evanescent waves are generated by a mode field partially propagating in the silica cladding 33 and act with the environment, and the effective refractive index of a cladding mode of the evanescent waves changes along with the refractive index of the environment. When the two beams of light reach the second single-mode optical fiber 5 through the collapse region 4, the two beams of light are coupled and transmitted through the fiber core of the second single-mode optical fiber 5. The two beams are coupled to generate Mach-Zehnder interference, and the interference intensity can be expressed as follows:
wherein the content of the first and second substances,I 1 andI 2 respectively of second single-mode optical fibres 5The intensity of the transmitted light of the core mode and the cladding mode,φthe phase difference between the cladding mode and the core mode of the second single-mode optical fiber 5 is expressed as:
in the formula (I), the compound is shown in the specification,Lshown as the length of the droplet-type air chamber 31,λexpressed as the wavelength of the incident light,Δn eff expressed as the difference between the effective refractive index of the cladding mode and the core mode of the second single mode fiber 5, the effective refractive index of the cladding mode varies with the ambient refractive index; as can be seen from equations (1) and (2), the intensity of the transmitted light changes with the change in the phase difference. The corresponding valley wavelengths for the m-order and m-2 order interference fringes can be expressed as:
effective refractive index difference between cladding mode and core mode of second single mode optical fiber 5Δn eff Can be expressed as:
wherein the content of the first and second substances,λ m the wave trough wavelength corresponding to the m-level interference fringe;λ m-2 the wave trough wavelength corresponding to the m-2 level interference fringe.
Due to the effect of evanescent waves, the effective refractive index of the cladding mode is influenced by the ambient refractive index, so that the wave trough wavelength also changes, and the ambient refractive index can be demodulated by tracking the change of the wave trough wavelength. The variation of the m-order interference fringe troughs can be expressed as:
in the formulaΔnThe change of the effective refractive index of the cladding mode along with the change of the environmental refractive index.
The utility model relates to a method for manufacturing an optical fiber Mach-Zehnder refractive index sensor based on a liquid drop type air cavity, which comprises the following steps of:
step 1: taking a single-mode optical fiber jumper wire, cutting the single-mode optical fiber jumper wire from the middle into two single-head single-mode optical fibers which are a first single-mode optical fiber 1 and a second single-mode optical fiber 5 respectively, and removing a tail coating layer;
step 2: taking a solid-core photonic crystal fiber 2, and removing a coating layer;
and step 3: vertically cutting the tail end of the first single-mode fiber 1 and one end of the photonic crystal fiber 2 by using a fiber cutter to protect the cut end faces; the other end of the single-ended single-mode fiber 1 is connected with an amplified spontaneous emission light source;
and 4, step 4: welding the two end faces of the first single-mode fiber 1 and the photonic crystal fiber 2 cut in the step 3 in a manual mode by using an optical fiber welding machine;
and 5: vertically cutting the tail end of the second single-mode fiber 5 and the other end of the photonic crystal fiber 2 by using a fiber cutter to protect a cutting surface; the other end of the second single-mode fiber 5 is connected with the light source input end of the fiber sensing analyzer and is used for observing optical signals;
step 6: welding the two end faces in the step 5 by using a manual mode of an optical fiber welding machine, wherein the photonic crystal fiber 2 is slightly far away from the electrode during welding; the edges at the welding points are firstly welded, air discharged from the center due to collapse of the cladding air holes 21 of the photonic crystal fiber 2 is captured to form an air cavity, and additional discharge is carried out until a spherical air cavity with the diameter of 50-60 microns is formed;
and 7: the left end of the optical fiber is fixed, the spherical air cavity part is unidirectionally tapered by an optical fiber tapering machine to form a liquid drop type air cavity 31 enclosed in the optical fiber tapering area 3, and the liquid drop type air cavity Mach-Zehnder refractive index sensor with a sandwich structure is formed.
FIG. 2 shows a device for measuring the refractive index of a sensor, which is used to prevent the sensor from bending, and after the sensor is manufactured, the sensor is straightened and fixed on a glass slide, and then the glass slide is placed on an optical platform. The method comprises the following steps of respectively using different solutions and glycerol solutions with different concentrations as refractive index samples, dropping a detected liquid on a sensor by using a dropper, recording the transmission spectrum of the sensor, repeatedly washing a sensing head by using water and alcohol after each group of experiments are finished, and carrying out next group of measurement after drying to select a wave trough as a monitoring object.
Fig. 3 is an experimental transmission spectrum of a sensor with a cavity length of 208 μm and a wall thickness of 8.1 μm in air in the droplet-type air cavity 31, and fig. 4 is an experimental result graph of the relationship between the wavelength of the sensor in glycerol solutions with different concentrations and the ambient refractive index, and the transmission spectrum valley is red-shifted. The sensitivity is 148.4nm/RIU when the ambient refractive index is in a sensitivity chart between 1.34 and 1.37; the sensitivity was 226.67nm/RIU when the ambient refractive index ranged from 1.39-1.41.
As the wall thickness decreases, the overlap area with the environment increases and the sensitivity increases. Particularly, when the wall thickness is reduced to the wavelength level, the refractive index sensitivity can be greatly improved when a fundamental mode evanescent field transmitted in the cladding acts on the environmental liquid to be measured, fig. 5 is a simulation result diagram of the relationship between the wavelength of the sensor with the liquid drop type air cavity length of 330 microns and the wall thickness of 1.15 microns and the environmental refractive index, and it can be seen from the diagram that the sensor has higher sensitivity in the environmental refractive index range of 1.32-1.42, particularly between 1.4-1.42, and the sensitivity is as high as 3133.3 nm/RIU.
The above description is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the embodiments of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (5)
1. The optical fiber Mach-Zehnder refractive index sensor with the liquid drop type air cavity is characterized by sequentially comprising a first single-mode optical fiber (1), a photonic crystal optical fiber (2) and a second single-mode optical fiber (5) from front to back, wherein air holes (21) are regularly arranged in a cladding of the photonic crystal optical fiber (2), a collapse region (4) and an optical fiber tapering region (3) are sequentially formed at the rear end of the photonic crystal optical fiber (2) through a fusion splicing technology, a liquid drop type air cavity (31) and a silica cladding (33) are formed in the optical fiber tapering region (3), and the liquid drop type air cavity (31) is coated by the silica cladding (33).
2. The optical fiber mach-zehnder refractive index sensor of claim 1, characterized in that the ambient refractive index of the sensor ranges between 1.4-1.42 with a sensitivity up to 3133.3 nm/RIU.
3. The optical fiber mach-zehnder refractive index sensor according to claim 1, characterized in that the first single mode fiber (1) is connected to an amplified spontaneous emission light source ASE and the second single mode fiber (5) is connected to a spectral input of an optical fiber sensing analyzer OSA.
4. The optical fiber Mach-Zehnder refractive index sensor of claim 1, characterized in that the optical fiber tapering region (3) has a lumbar diameter of 40-60 microns, the droplet type air cavity (31) has a length of 200-350 microns, the central air cavity of the tapering region has a diameter of 37-50 microns, and the wall thickness is 1-8.5 microns.
5. The optical fiber mach-zehnder refractive index sensor according to claim 1, characterized in that the medium of the droplet-type air cavity (31) is air, and the air refractive index nO=1, the silica clad (33) medium is silica, the silica refractive index nSiO2=1.45。
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