CN115683271A - Seawater depth sensor based on hollow optical fiber - Google Patents
Seawater depth sensor based on hollow optical fiber Download PDFInfo
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Abstract
The invention provides a seawater depth sensor based on a hollow optical fiber, and belongs to the field of optical fiber sensing. The optical fiber comprises a section of hollow optical fiber, wherein a part of the hollow optical fiber is polished to form a D-shaped platform, and the cross section of the hollow optical fiber is provided with an optical fiber core and a large air hole. The air hole is positioned in the center of the cross section of the optical fiber, and the fiber core of the optical fiber is positioned on the upper side of the central large air hole. A section of cladding region of the hollow optical fiber is polished to form a D-shaped platform so as to improve the sensing sensitivity, and a layer of pressure-sensitive material and a layer of gold film are plated on the inner side of the central air hole. The seawater depth sensor based on the hollow optical fiber can realize high-sensitivity sensing of seawater pressure in a large range, has the advantages of in-fiber integration, small volume, low loss and the like, can realize real-time detection of the seawater pressure, and is suitable for seawater depth measurement in a complex environment.
Description
Technical Field
The invention relates to a seawater depth sensor based on a hollow optical fiber, and belongs to the field of optical fiber sensing.
Background
With the gradual development and utilization of ocean resources in China, the comprehensive ocean information in the territorial waters of China is mastered, so that the ocean deep diving exploration, the ocean protection and the like have wide application values, and at present, the deep sea exploration operation is realized in most of the countries in the world.
The optical characteristics of light guided inside the optical fiber are very sensitive to the change of the external environment, including the amplitude, wavelength, phase and polarization state of the guided light, which can be obviously changed along with the change of the physical quantities such as the external pressure. The optical fiber sensor may be classified into a temperature sensor, a humidity sensor, a pressure sensor, and the like according to the application field. The optical fiber sensor has the characteristics of high sensitivity, easiness in integration, strong anti-electromagnetic interference capability, suitability for measuring physical quantities in complex environments and the like, and is widely applied to the field of ocean sensing.
The optical fiber pressure sensor is a novel and widely used pressure sensor, and can convert the change of the external pressure into the change of the light guide property in the optical fiber. The conventional pressure sensor usually uses an electrical signal as a carrier, and converts an external strain amount into a change of the electrical signal. This type of pressure sensor is typically bulky, complex to demodulate, and has poor resistance to electromagnetic interference, and is not suitable for measurements in complex sea water. Meanwhile, with the development of the optical fiber technology, the optical fiber pressure sensor gradually replaces the traditional pressure sensor taking an electric signal as a carrier by the advantages of simple structure, convenience in demodulation, good electromagnetic interference resistance and the like.
The optical fiber pressure sensor is mainly divided into four types according to different structures, wherein the types are as follows: a fiber grating pressure sensor, an interference type pressure sensor, a polarization type pressure sensor, and an SPR type pressure sensor. In recent years, surface Plasmon resonance (spr) technology has received attention from a number of researchers. The SPR effect occurs at the medium-metal interface, and when an evanescent wave formed when a light beam transmitted in the fiber core is totally reflected resonates with a surface plasmon wave in the metal layer, the energy of the reflected light sharply decreases, and an obvious loss peak appears in the output spectral line. When the external environment changes, the resonance peak of the loss spectrum can shift, and the wavelength corresponding to the loss peak is called as the resonance wavelength of SPR. The SPR-type sensors are very sensitive to changes in refractive index in the external environment and are therefore commonly used for the detection of biochemical analytes, drugs and chemical reagents, and are widely used in the medical and biochemical fields.
The evanescent wave is a small segment of light wave which is transmitted in the cladding and has the same order of magnitude as the incident wavelength and is called as an evanescent wave. If a layer of metal film is plated on the Surface of the cladding, at the moment, evanescent waves enter the metal film and influence the movement of free electrons in the metal film, the electrons in the cladding move together to form Surface plasma waves, and when the wavelength of incident light is changed and reaches a specific value, the incident light energy can be rapidly attenuated and converted into the energy of the Surface Plasma Waves (SPW).
The sensor has the characteristics of high sensitivity, strong anti-interference capability, simple and flexible structure, wider measurement range, low manufacturing cost and the like, and is widely used. In the research, the metal or metal oxide film of gold plating, silver and the like is usually used for enhancing the SPR effect, a layer of pressure-sensitive material is firstly deposited on the inner side of the central air hole, the elastic-optical coefficient of the layer of pressure-sensitive material is very high, the change of the external pressure can be converted into higher refractive index change to obtain higher pressure sensitivity, and compared with the condition without the pressure-sensitive material, the pressure sensing sensitivity is improved by more than 3.4 times to the maximum.
The patent No. CN208568143U discloses a seawater pressure sensor based on a diamond film, which is characterized in that a pressure-sensitive film made of a monocrystalline or polycrystalline boron-doped diamond material is adopted, a stress cavity is formed in a substrate, and a voltage signal generated by a composite electrode is matched and transmitted to a signal acquisition circuit at the rear end to realize pressure sensing. However, in the electrical sensing method, the sensing element has a large size, which is not favorable for integration, and the level signal is interfered by external electromagnetic interference, which results in unstable sensing result.
In a patent No. CN216870370U, a D-type photonic crystal fiber sensing device based on double-layer air hole arrangement deposits a silver film on a D-type interface of a photonic crystal fiber to excite an SPR effect, coats a graphene layer on the surface of the silver film, and detects the refractive index change of an external liquid through the change of a resonant peak of the photonic crystal fiber. But the defects are that the optical fiber is complex to manufacture, the oxidation and corrosion progress of the sensitive layer is aggravated by the direct contact of the sensitive layer and the liquid to be measured, and the optical fiber is easily influenced by other external parameters, so that a great measurement error is caused.
Disclosure of Invention
The invention aims to solve the problem of measuring seawater pressure under complex environmental conditions, and provides a seawater depth sensor based on a hollow optical fiber. The invention has the advantages of high sensitivity, strong anti-interference capability, small volume, simple structure, capability of being integrated in one optical fiber and the like.
The purpose of the invention is realized as follows: the sea water pressure sensing device comprises an optical fiber annular cladding, an air hole positioned in the middle of the optical fiber annular cladding, an optical fiber core arranged in the optical fiber annular cladding, a pressure-sensitive material medium layer and a metal film, wherein the pressure-sensitive material medium layer and the metal film are arranged on the surface of the air hole, the cross section of the optical fiber annular cladding is D-shaped, and the sea water pressure sensing device can sense the sea water pressure by detecting the deviation of resonance wavelength caused by the sea water pressure change by utilizing the SPR effect.
The invention also includes such structural features:
1. the metal film is gold, silver, copper or metal oxide.
2. The distance between the fiber core and the central air hole is 1 μm.
3. The pressure sensitive material is used as a sensing medium of the pressure sensor, and the used functional material is polycarbonate PC or other pressure sensitive materials.
4. The cross section of the optical fiber annular cladding layer is D-shaped: the thickness of the optical fiber annular cladding is reduced by polishing and grinding the optical fiber annular cladding, and a platform is formed, and the optical fiber core is positioned below the D-shaped platform and is not more than 1 mu m away from the central large air hole so as to excite the SPR effect.
Compared with the prior art, the invention has the beneficial effects that: the invention provides a seawater depth sensor based on a hollow optical fiber, which can be integrated into one optical fiber and exerts the advantage of in-fiber integration. The invention adopts the hollow optical fiber, and the hollow structure of the hollow optical fiber provides an internal sensing mechanism, thereby improving the sensitivity of liquid pressure sensing. The mode of coating the film in the central air hole is selected, so that the metal layer is protected from being corroded by chemical substances in liquid to be detected, and the oxidation degree of the metal layer when the metal layer is directly placed in air is reduced. The seawater pressure sensing sensitivity of the invention is closely related to the selection of the pressure sensitive material, and the pressure sensitivity of the invention can be greatly improved by selecting the optical material with larger elasto-optical effect and smaller Young modulus. In the current research stage, the Polycarbonate (PC) is selected as the pressure sensitive medium layer, so that the pressure sensitivity of the pressure sensitive sensor is greatly improved (no pressure sensitive layer: 70pm/MPa and 240pm/MPa of pressure sensitive layer), and the result shows that the pressure sensing sensitivity is improved by 3.4 times to the maximum extent compared with the pressure sensitive material. In subsequent researches, the pressure sensitivity can be further improved only by changing the material of the pressure sensitive medium layer. The invention has the characteristics of simple and flexible structure, good repeatability, large measurement range, no need of modulation and demodulation, strong anti-interference capability and the like, and can be used for measuring the depth of the seawater under the complex environmental condition.
Drawings
Fig. 1a is a schematic two-dimensional cross-sectional view of example 1 of the present invention.
Fig. 1b is a schematic two-dimensional cross-sectional view of example 2 of the present invention.
FIG. 2 is a stress distribution diagram of the present invention at an external liquid pressure of 10 MPa.
FIG. 3 is a graph of loss as the diameter of the center air hole of the present invention is varied.
FIG. 4 is a graph of loss curves for varying distances from the core of an optical fiber to the central air hole in accordance with the present invention.
FIG. 5 is a graph showing the loss at the time of change in the thickness of the metal film in the present invention.
FIG. 6 is a graph showing the loss when the thickness of the pressure sensitive medium layer is changed according to the present invention.
Fig. 7 is a graph of loss curves for different pressures according to the present invention.
FIG. 8 is a graph of the fitting results of the sensitivity of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
Referring to fig. 1a-8, the hollow optical fiber is composed of a D-type platform 1, an optical fiber annular cladding 2, an optical fiber core 3, an air hole 4, a pressure-sensitive material medium layer 5 and a metal film 6. The thickness of the annular cladding 2 of the optical fibre is reduced by polishing and grinding it and forms a platform. The diameter D =20 μm of the fiber core D =6.5 μm, a layer of pressure sensitive material is firstly deposited on the inner side of the air hole to enhance the pressure sensitivity, and then a layer of metal film is plated on the surface of the pressure sensitive material. The optical fiber sensor comprises a D-shaped platform, an optical fiber annular cladding, an optical fiber core and a central air hole. The fiber core is located near the central air hole, and the distance between the fiber core and the central air hole is 1 μm. A layer of pressure-sensitive medium material with higher elasto-optic coefficient is firstly deposited on the inner side of the air hole, the pressure-sensitive material is used as a sensing medium of the pressure sensor, and the used functional material is Polycarbonate (PC) or other pressure-sensitive materials. Plating a metal film on the surface of Polycarbonate (PC). The metal film is gold, silver, copper or metal oxide. By using the SPR effect, the seawater pressure is sensed by detecting the shift of the resonance wavelength caused by the change of the seawater pressure. The thickness of the pressure sensitive material medium layer 5 and the metal film 6 can be changed to realize the best sensing effect.
When in measurement, the sensing probe is placed in liquid to be measured, and different liquid depths correspond to different pressure magnitudes. Under the action of pressure, the effective refractive index of the pressure-sensitive material medium layer changes, so that the phase matching condition of the wave vector of the evanescent wave along the x-axis direction and parallel to the surface of the cladding and the excited SPW wave vector changes, which shows that the position of the resonance wavelength in the output loss spectrum shifts, and the magnitude of the pressure applied to the sensor at the moment is obtained by measuring the movement amount of the resonance wavelength, thereby realizing sensing.
A seawater depth sensor based on hollow optical fibers comprises a D-shaped platform 1, an optical fiber annular cladding 2, an optical fiber core 3, an air hole 4, a pressure-sensitive material medium layer 5 and a metal film 6. The thickness of the annular cladding 2 of the fiber is reduced by polishing and forming a flat land with a large air hole in the center of the fiber to enhance pressure sensitivity. The fiber core 3 is positioned below the D-shaped platform and is not more than 1 μm away from the central large air hole to excite the SPR effect.
The inner side of the central air hole is firstly coated with a layer of pressure sensitive material, the sensitive material with a large elastic-optical coefficient is selected, small pressure change can be converted into more obvious material refractive index change, polycarbonate (PC) is selected as the pressure sensitive material to be coated on the inner wall of the central air hole, and the first elastic-optical effect coefficient of the Polycarbonate (PC) is as follows: 2.45e-11 (m) 2 and/N), the second elastic-optical effect coefficient is as follows: 9.38e-11 (m) 2 and/N), compared with the common quartz material, the elastic-optical effect coefficient of the material is increased by more than 10 times, and the material is a good pressure sensitive material. Then, a metal film is plated on the surface of the polycarbonate to excite the SPR effect, and a gold film is selected here. The physical properties of gold are stable to oxidation relative to other metals, and the sensor loss peak using gold thin films is sharper and the full width at half maximum is smaller relative to metal oxides.
The mode of coating the film in the central air hole is selected, so that the pressure sensitive layer and the metal layer do not need to be in direct contact with the liquid to be detected, the design not only protects the metal layer from being corroded by chemical substances in the liquid to be detected, but also reduces the oxidation degree of the metal layer when the metal layer is directly placed in the air. When the COMSOL software is used for simulation, the pressure sensitive layer is coated on the inner side of the central air hole, and the pressure sensitive layer is subjected to larger external liquid pressure compared with the pressure sensitive layer coated on the outer side of the optical fiber sensor. Therefore, the seawater depth sensor based on the hollow optical fiber has higher stability and anti-interference capability and is suitable for seawater depth measurement in a complex environment.
In the scheme, the polishing depth of the D-shaped platform is 8 mu m.
In the above scheme, the diameter D of the large air hole in the center of the optical fiber sensor is 20 μm.
In the above scheme, the fiber core diameter d =6.5 μm.
In the above scheme, the thicknesses of the pressure-sensitive material dielectric layer and the metal film are respectively d PC =20nm and d AU ＝25nm。
The seawater depth sensor based on the hollow optical fiber provided by the invention utilizes the hollow optical fiber, and the pressure-sensitive material medium layer is added in the central air hole, so that the change of the external pressure is converted into the change of the effective refractive index of the pressure-sensitive material medium layer, and the phase matching condition of the transmission light is changed. The pressure applied to the sensor at the moment is obtained by measuring the movement amount of the resonance wavelength, so that sensing is realized.
The present invention may further comprise:
the relative positions and spacings of the fiber core 3 and the central air hole 4 can be varied.
The functional material used by the pressure sensitive layer is Polycarbonate (PC) or other pressure sensitive materials with larger elastic-optical coefficient.
The metal film is gold, silver, copper or other metal oxides.
The invention provides a seawater depth sensor based on a hollow optical fiber, which utilizes an elasto-optical effect to convert the change of external pressure into the change of a material refractive index, utilizes an SPR effect to convert the change of the material refractive index into the shift of a resonance peak of a loss spectrum, and the wavelength corresponding to the loss peak is called as resonance wavelength. The seawater pressure can be sensed by detecting the deviation of the resonance wavelength caused by the change of the seawater pressure. The inner wall of the central air hole is coated with a layer of Polycarbonate (PC) and a layer of gold film, the Polycarbonate (PC) has a higher elasto-optical effect coefficient, external pressure change can be converted into change of the refractive index of the medium layer, and finally the pressure change is converted into more obvious change of resonance wavelength which is beneficial to observation by utilizing the characteristic that the SPR effect is very sensitive to the refractive index change. In subsequent research, the pressure sensitivity can be further improved by only replacing the material of the pressure sensitive medium layer. The invention has the characteristics of flexible structure, simple manufacturing process, strong anti-interference capability and the like, and can be applied to the measurement of the depth of the seawater under the complex environmental conditions.
An optical fiber used as a carrier in a seawater depth sensor based on a hollow optical fiber is composed of a high germanium-doped optical fiber core, a low germanium-doped optical fiber annular cladding and a central large air hole, wherein a pressure-sensitive material medium layer and a metal film are plated on the inner wall of the central large air hole, the diameter of the optical fiber core is 6.5 mu m, the diameter of the optical fiber annular cladding is 54 mu m, the diameter of the central large air hole is 20 mu m, and the thicknesses of the pressure-sensitive material medium layer and the metal film are 25nm and 20nm respectively.
In the hollow single-core optical fiber, the refractive index of the material can be calculated by using a Sellmeier dispersion formula according to the material of the fiber core and the annular cladding of the optical fiber. The calculation formula is as follows:
wherein n is the refractive index of the material, X is the molar coefficient of doping of the material, and X adopted by the invention core ＝0.057，X clad =0.02, refractive indices of the fiber core and the fiber ring cladding are n core =1.4507 and n clad ＝1.4451。
Plating a nano gold film on the pressure-sensitive material medium layer with a thickness t AU =30nm, nano-meter can be calculated using Lorentz-Drude modelRelative dielectric constant of gold film. The Lorentz-Drude model expression is as follows:
wherein epsilon AU Denotes the dielectric constant,. Epsilon.of gold ∞ Dielectric constant at high frequency, ω D Is the plasma frequency, ω is the angular frequency of the incident light, γ D Is the damping frequency. In addition, Δ ε is a weight factor, Ω L Is oscillator strength gamma L Is the spectral width.
Example 1:
the cross-sectional schematic view of a hollow fiber-based seawater depth sensor designed by the invention is shown in figure 1 and comprises a D-shaped platform 1, a fiber annular cladding 2, a fiber core 3, an air hole 4, a pressure-sensitive material medium layer 5 and a metal film 6
In the present example, we simulated the fiber pressure distribution diagram on the sensor when the external liquid pressure is 10MPa by numerical calculation using the total vector finite element analysis software COMSOL, as shown in fig. 2. As shown, the pressure value of the fiber near the central air hole is greater than that of the cladding, and the central maximum pressure of the sensor is: 13.6MPa, the average pressure born by the pressure sensitive medium layer is as follows: 11.12MPa. Applying a pressure sensitive layer inside the central air hole increases its force effect.
Further analyzing the influence of each structural parameter on the inductive performance, and performing numerical simulation analysis on the inductive performance by using COMSOL software, wherein most of energy of a TM polarization mode in an optical fiber core is coupled to a Surface Plasmon Polariton (SPP) mode at a metal film-medium interface due to an SPR effect, the energy in the optical fiber core is rapidly reduced, and the loss of the mode in the optical fiber core can be calculated by using the following formula:
wherein λ represents the wavelength, im (n) eff ) The imaginary part of the effective index of refraction of the representation.
The sensor loss curve is shown in FIG. 3, with the change of the diameter D of the central air hole, and the distance D from the fiber core to the air hole is fixed h =1 μm and the outside pressure was 11MPa, and when the central air hole diameter D was changed from 19 μm to 21 μm, the loss peak was increased from 34.96dB/cm to 41.50dB/cm, indicating that much light leaked from the core and the loss peak was increased. And the loss resonance wavelength of the TM mode changes from 1701nm to 1860nm as the central air hole diameter D increases. In the graph, when D =20 μm, the loss peak was 37.29dB/cm, and the loss resonance wavelength was 1772nm.
When the diameter D =20 μm of the central air hole is fixed, the distance D between the fiber core and the central air hole is changed h The loss profile of the sensor is shown in fig. 4. When the distance d between the fiber core and the central air hole h =0.5 μm, the TM mode loss resonance wavelength λ =1780nm, and the loss peak is 19.96B/cm; when the distance d between the fiber core and the central air hole h =0.75 μm, the TM mode loss resonance wavelength λ =1777nm, and the loss peak is 32.09dB/cm; when the distance d between the fiber core and the central air hole h =1 μm, the TM mode loss resonance wavelength λ =1772nm, and the loss peak is 37.29dB/cm; when the distance d between the fiber core and the central air hole h =1.25 μm, TM mode loss resonance wavelength λ =1767nm, loss peak 35.16dB/cm; when the distance d between the fiber core and the central air hole h =1.5 μm, the TM mode loss resonance wavelength λ =1762nm, and the loss peak is 36.78dB/cm. From the simulation results, it is found that as the distance between the fiber core and the central air hole is gradually increased, the loss resonance wavelength of the TM mode is gradually decreased and shifted to a short wavelength. And when the distance d between the fiber core and the central air hole h When =1 μm, the maximum resonance loss peak appears to be 37.29dB/cm.
Thickness d of pressure sensitive medium layer PC =30nm, distance d between fiber core and central air hole h =1 μm, and the change in thickness of the gold thin film when the diameter of the central air hole D =20nmThe influence of the sensory properties is shown in fig. 5. When the thickness of the metal film is changed from 20nm to 25nm, 30nm, 35nm and 40nm, the loss resonance wavelength λ of the TM mode gradually increases, changing from 1612nm to 1708nm, 1772nm, 1811nm and 1834nm. Loss peak at t AU The maximum value was 72.54dB/cm at 25 nm. After reaching the maximum value, the loss peak value is continuously reduced as the thickness of the gold film is larger. This is because the increase in the thickness of the gold thin film causes a decrease in the energy coupled from the TM mode to the SPP mode, the loss peak also becomes broader with the increase in the thickness of the gold thin film, and the resonance wavelength is shifted red. In summary, as the thickness of the gold film becomes larger, the loss peak of the TM mode becomes larger and then smaller, and d is larger AU The maximum value is obtained 25 μm.
When the thickness t of the gold film is fixed AU =30 μm, and the influence of the thickness change of the pressure sensitive medium layer on the sensing characteristics of the present invention is shown in fig. 6 when other structural parameters are unchanged. When the dielectric layer thickness is changed from 25nm to 40nm, the loss spectrum moves to the right, and the loss resonance wavelength gradually increases from 1704nm to 1836nm. And the peak of the loss peak generally exhibits a gradually decreasing trend. When the thickness t of the pressure sensitive medium layer PC And when the loss peak value is not less than 20nm, the loss peak value is maximum and reaches 43.83dB/cm.
In summary, in order to achieve the best sensing effect, the structural parameters selected by the present invention are as follows: diameter of central air hole D =20 μm, distance D from fixed core to air hole h =1 μm, thickness d of the pressure-sensitive medium layer PC =20nm and thickness d of gold thin film AU ＝25nm。
The loss peak is shown as a function of wavelength as the ambient liquid pressure is varied from 0MPa to 50 MPa. The loss curve is red shifted with increasing external liquid pressure. When the measured values of the external pressure are respectively as follows: the loss peak values at 0MPa, 10MPa, 20MPa, 30MPa, 40MPa and 50MPa are respectively: 49.54dB/cm, 50.01dB/cm, 50.18dB/cm, 50.15dB/cm, 49.80dB/cm and 49.34dB/cm, and the wavelengths corresponding to the loss peak values are respectively as follows: 1842.7nm, 1845.1nm, 1847.5nm, 1849.9nm, 1852.3nm and 1854.7nm. The resonance wavelength is linearly fitted with the change of the external liquid pressure, the maximum pressure sensing sensitivity can reach 0.24nm/MPa, and the measurement precision value of the invention reaches 0.21MPa by using a spectrometer with the resolution of 0.05 nm.
Example 2:
the cross-sectional schematic view of the hollow optical fiber-based seawater depth sensor is shown in fig. 2 and comprises a D-shaped platform 1, an optical fiber annular cladding 2, an optical fiber core 3, an air hole 4, a metal film 5 and a pressure-sensitive material medium layer 6.
A layer of metal film is plated on the inner wall of the central air hole for exciting the SPR effect, and then the metal film is arranged on the surface of the metal film. A layer of pressure sensitive material is deposited to enhance pressure sensitivity. Such a design may further avoid the degree of oxidation of the metal film.
In conclusion, the invention discloses a hollow optical fiber-based seawater depth sensor, and belongs to the field of optical fiber sensing. The invention relates to a hollow optical fiber which is formed by polishing and grinding a part of hollow optical fiber to form a D-shaped platform. Wherein the cross section of the hollow optical fiber is provided with an optical fiber core and a large air hole. The air hole is positioned in the center of the cross section of the optical fiber, and the fiber core of the optical fiber is positioned on the upper side of the central large air hole. A section of cladding region of the hollow optical fiber is polished to form a D-shaped platform so as to improve the sensing sensitivity, and a layer of pressure-sensitive material and a layer of gold film are plated on the inner side of the central air hole. When the seawater pressure applied to the sensor changes, the refractive index of the pressure-sensitive material can obviously change due to the elasto-optical effect, and a resonance peak in a loss spectrum can shift. The one-to-one correspondence relationship between the seawater pressure and the resonance wavelength can be obtained by detecting the deviation of the resonance wavelength caused by the change of the seawater pressure, the seawater pressure is sensed by observing the position of the resonance wavelength in the output loss spectrum, and the seawater depth data can be obtained by combining other seawater parameters. The seawater depth sensor based on the hollow optical fiber can realize high-sensitivity sensing of seawater pressure in a large range, has the advantages of in-fiber integration, small volume, low loss and the like, can realize real-time detection of the seawater pressure, and is suitable for seawater depth measurement in a complex environment.
Claims (5)
1. The utility model provides a sea water depth sensor based on cavity optic fibre which characterized in that: the sea water pressure sensing device comprises an optical fiber annular cladding, an air hole positioned in the middle of the optical fiber annular cladding, an optical fiber core arranged in the optical fiber annular cladding, a pressure-sensitive material medium layer and a metal film, wherein the pressure-sensitive material medium layer and the metal film are arranged on the surface of the air hole, the cross section of the optical fiber annular cladding is D-shaped, and the sea water pressure sensing device can sense the sea water pressure by detecting the deviation of resonance wavelength caused by the sea water pressure change by utilizing the SPR effect.
2. The hollow optical fiber based seawater depth sensor of claim 1, wherein: the metal film is gold, silver, copper or metal oxide.
3. The hollow optical fiber-based seawater depth sensor of claim 1, wherein: the distance between the fiber core and the central air hole is 1 μm.
4. The hollow optical fiber based seawater depth sensor of claim 1, wherein: the pressure sensitive material is used as a sensing medium of the pressure sensor, and the used functional material is polycarbonate PC or other pressure sensitive materials.
5. The hollow optical fiber based seawater depth sensor of claim 1, wherein: the cross section of the optical fiber annular cladding layer is D-shaped: the thickness of the optical fiber annular cladding is reduced by polishing and grinding the optical fiber annular cladding, and a platform is formed, and the optical fiber core is positioned below the D-shaped platform and is not more than 1 mu m away from the central large air hole so as to excite the SPR effect.
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