An optical fiber LMR pressure sensor

Abstract

The utility model discloses a kind of optical fiber LMR pressure sensors, comprising: metal shell, its shell top is open, which is covered with metal diaphragm, the bottom end surface center of metal diaphragm is fixed with force transmission metal stick, the lower end of force transmission metal stick is deposited with fluoride layer, fluoride layer is deposited with gallium-doped zinc oxide film layer;Single-mode optical fiber, it is located in metal shell, the cladding of single-mode optical fiber removes middle section to expose the fiber core of middle section;Side surface opening microcapillary, it is cladded on the cladding of single-mode optical fiber both ends, microcapillary is filled with matching liquid with same refractive index and will be immersed with cladding, matching liquid can be as the liquid cladding of bare core;Gallium-doped zinc oxide film layer is located above liquid cladding, the space range between gallium-doped zinc oxide film layer and bare core is called transition layer. The sensor can avoid the influence of stress transmission on optical fiber, both have relatively wide sensing range, and also can ensure higher sensitivity.

Description

Technical Field

[0001] This utility model relates to the field of fiber optic sensing technology, and more specifically to a fiber optic LMR pressure sensor. Background Technology

[0002] Pressure measurement plays a vital role in industrial equipment, life sciences, and chemical manufacturing. With the continuous development of application scenarios, the increasing demands for high sensitivity, high accuracy, wide range, and fast response speed are the main challenges facing pressure monitoring equipment today. Currently, many researchers have developed fiber optic pressure sensors based on optical elements, using structures such as fiber optic gratings (FBG) and Fabry-Pérot interferometers (FPI). Compared to traditional electrical pressure sensors, these sensors offer advantages such as resistance to electromagnetic interference, pressure and high temperature resistance, high sensitivity, and fast response speed.

[0003] Lossy mode resonance (LMR) sensors, in addition to the advantages mentioned above, have become a research hotspot in the past decade due to their unique resonance characteristics. Compared with surface plasmon resonance (SPR) sensors, LMR can be excited by both TM and TE polarizations, which helps improve the sensing accuracy of multiple parameters. For coating materials, SPR commonly uses metals such as Ag and Au as excitation materials, but their dispersion characteristics limit their operation over a wide wavelength range. LMR, on the other hand, can use various oxides or polymers such as indium tin oxide (ITO), titanium dioxide (TiO2), tin oxide (SnO2), and gallium-doped ZnO (GZO) as excitation materials. When phase matching conditions are met, these materials can all excite LMR with two polarizations, and they are also more cost-effective than metal materials. Therefore, with its superior sensing performance and economic advantages, LMR pressure sensors have enormous application potential.

[0004] In sensor development, wide range and high sensitivity are important performance indicators pursued by researchers. Existing research has shown that the resonant wavelength of LMR sensors drifts significantly under changes in external refractive index. Currently, the most commonly used structure for LMR sensors is a prism and optical fiber. Among them, the D-shaped optical fiber structure exhibits good performance. Because it is not cylindrical, TM and TE resonances can be directly observed on the coating surface, and it is convenient to set up material deposition. Numerous studies have shown that LMR sensors based on D-shaped optical fibers have significant advantages in sensitivity and full width at half maximum (FWHM). However, due to the polishing process, the mechanical strength of D-shaped optical fibers is relatively poor, making it difficult to operate under high stress and large strain environments. Similarly, for prism structures, surface pressure causes deformation in multiple material layers, and the effects vary depending on the material and deposition method. These intrinsic limitations of optical fibers and materials used for sensitization hinder the development of LMR sensors in the field of pressure measurement. Existing research on electrical pressure sensors often uses the addition of compensation elements for pressure compensation. However, in LMR sensors, methods for avoiding the influence of pressure on the optical fiber and for pressure compensation have not been reported. This unresolved problem directly limits the technological development of high-performance pressure sensors.

[0005] There are few existing pressure sensing solutions based on LMR. After searching, the following existing technologies were found:

[0006] Existing patent ZL201910345917.8 discloses a microstructure fiber optic sensor based on loss mode resonance. This sensor reduces loss by utilizing a semi-circular photonic fiber and employs a double-layer film structure and an external rubber layer to sensitively convert changes in external pressure into changes in the rubber volume, which in turn alters the refractive index of the medium and is then reflected in the wave movement, thereby achieving accurate measurement of external air pressure. However, under the influence of external air pressure, both the rubber volume and the photonic crystal fiber are affected, resulting in pressure-induced cross-sensitivity. This invention does not provide a compensation method for this cross-sensitivity, affecting accuracy.

[0007] Existing patent CN109459164A discloses an optical fiber pressure sensor and its fabrication method. This sensor includes a signal transmitting and receiving device, a transmission optical fiber, and a pressure detection device. Regarding the pressure detection device, deformation of an elastic diaphragm causes a change in the refractive index of the refractive index-sensitive medium, affecting the interference light within the Fabry-Perot resonant cavity and used to measure pressure applied from the side of the transmission optical fiber. In the sensor proposed in this patent, the diaphragm senses external pressure and transmits stress to the liquid cavity; the liquid in the closed liquid cavity is a pressure-sensitive material, which changes the refractive index within the Fabry-Perot resonant cavity. While this patent can measure pressure from the side, it does not consider the effect of stress transmission from the liquid on the closed liquid cavity, which alters the cavity length of the Fabry-Perot resonant cavity and affects accuracy.

[0008] Existing patent CN112254847A proposes a fiber optic FP pressure sensor based on hydraulic principles. This sensor includes a glass housing with an open top, with a bowl-shaped metal diaphragm connected to both ends of the glass housing to a single-mode optical fiber coated with a reflective film. A sealed FP cavity is formed between the metal diaphragm and the top end face of the single-mode optical fiber. Pressure sensing is achieved by changing the cavity length of the FP cavity through external pressure. In this patented sensor, the metal diaphragm senses the pressure in the hydraulic cavity and deforms, thereby changing the cavity length of the FP cavity; the liquid in the sealed hydraulic cavity acts as stress transfer. While this patent can achieve the measurement of minute pressures with its simple structure, under higher pressures, the 50μm thick bowl-shaped metal diaphragm undergoes severe deformation, affecting accuracy.

[0009] Therefore, how to provide a fiber optic LMR pressure sensor that avoids the influence of stress transmission on optical fibers, has a wide sensing range, and ensures high sensitivity is a problem that urgently needs to be solved by those skilled in the art. Utility Model Content

[0010] In view of this, the present invention provides a fiber optic LMR pressure sensor that can avoid the influence of stress transmission on optical fibers, has a wide sensing range, and can also ensure high sensitivity.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] A fiber optic LMR pressure sensor, comprising:

[0013] A metal shell, the top of which is open and covered with a metal diaphragm, a force-transmitting metal rod is fixed at the center of the bottom end face of the metal diaphragm, a fluoride layer as an analyte is deposited at the lower end of the force-transmitting metal rod, and a gallium-doped zinc oxide thin film layer as an excitation layer is deposited on the fluoride layer.

[0014] A single-mode optical fiber, wherein the single-mode optical fiber is located inside the metal shell, and the cladding of the middle section of the single-mode optical fiber is removed to expose the core of the middle section, which is a bare core.

[0015] A microcapillary with a side opening is fitted onto the cladding at both ends of the single-mode optical fiber. The internal space of the microcapillary is an open-top cavity. The open-top cavity is filled with a matching liquid that has the same refractive index as the cladding and immerses the bare core. The matching liquid can serve as a liquid cladding for the bare core.

[0016] The gallium-doped zinc oxide thin film layer is located above the liquid cladding, and the space between the gallium-doped zinc oxide thin film layer and the bare core is called the transition layer.

[0017] As can be seen from the above technical solution, compared with the prior art, this utility model discloses a fiber optic LMR pressure sensor. The metal diaphragm senses external pressure and deforms, causing a force-transmitting metal rod below the diaphragm to move the gallium-doped zinc oxide thin film layer downwards into the matching liquid. This changes the height of the transition layer, thereby altering the distribution of the optical field (intensity, position, direction, etc. of light propagating inside the fiber core) and the effective refractive index. This change affects the mode loss from the fiber core to the gallium-doped zinc oxide thin film layer, thus achieving pressure sensing. By analyzing the mode loss of the liquid-clad fiber, the external pressure change can be calculated. This sensor uses a matching liquid instead of the fiber cladding and forms a non-closed cavity through an opening at the upper end of a microcapillary. This allows the gallium-doped zinc oxide thin film layer to be immersed in the liquid while avoiding stress transmission, thus preventing the influence of pressure on the fiber and simultaneously generating LMR.

[0018] Furthermore, the microcapillary has an inner diameter of 125 μm, an outer diameter of 160 μm, and a length of 3 mm.

[0019] Furthermore, the matching solution is prepared by mixing ethanol and glycerol in a certain proportion, and the refractive index of the matching solution is 1.4467.

[0020] Furthermore, the thickness of the fluoride layer is at least 3 μm, and the thickness of the gallium-doped zinc oxide thin film layer is 60 nm-80 nm.

[0021] Furthermore, the metal diaphragm is made of spring steel.

[0022] Furthermore, the lower end of the force-transmitting metal rod is provided with a BK7 glass layer to enhance the adhesion of the fluoride layer.

[0023] This invention discloses a fiber optic LMR pressure sensor. The cladding of the middle section of the optical fiber is removed, and the fibers at both ends are nested using microcapillaries. A refractive index matching liquid is filled into the bare core, making the middle section of the fiber and the liquid cladding approximately a D-shaped fiber. Above the fiber is a pressure transmission device composed of a metal diaphragm, a metal rod, CYTOP fluoride, and gallium-doped zinc oxide. When pressure is applied to the metal diaphragm from above, the diaphragm deforms, and the displacement of its center point is directly transmitted through the metal rod to the analytical material at the bottom, thus activating LMR. Because the cladding is replaced with a matching liquid, external pressure only changes the distance of the aforementioned transition layer and does not directly contact the fiber core. Therefore, this sensor does not affect the optical fiber and its liquid cladding under pressure, avoiding cross-sensitivity caused by pressure. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this utility model or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0025] Figure 1 This is a front view schematic diagram of a fiber optic LMR pressure sensor provided by this utility model.

[0026] Figure 2 A schematic diagram of the cross-sectional structure of an optical fiber LMR pressure sensor provided by this utility model.

[0027] Figure 3 This is a schematic diagram of a fiber optic LMR pressure sensor with its metal casing hidden, as provided by this utility model.

[0028] Figure 4 This is a schematic diagram showing the relationship between the deflection of a metal diaphragm and the thickness of a fiber optic LMR pressure sensor in the range of 0-1 MPa.

[0029] Figure 5 A schematic diagram showing the resonant wavelength and transition layer distance of an optical fiber LMR pressure sensor provided by this utility model. Detailed Implementation

[0030] The embodiments of this utility model are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this utility model, and should not be construed as limiting this utility model.

[0031] In the description of this utility model, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this utility model and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this utility model.

[0032] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this utility model, "a plurality of" means two or more, unless otherwise explicitly specified.

[0033] In this utility model, unless otherwise explicitly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this utility model according to the specific circumstances.

[0034] This invention provides a fiber optic LMR pressure sensor that achieves high sensitivity sensing over a wide pressure range. Specifically, the sensor is suitable for pressure measurement within the range of 0–5 MPa, and achieves an average sensitivity of 4.399 nm / MPa within the range of 1 MPa.

[0035] See Figures 1-5 As shown in the figure, this utility model embodiment discloses an optical fiber LMR pressure sensor, which is externally encapsulated by a metal shell 1 and internally contains an optical fiber LMR pressure sensing chamber.

[0036] The metal shell 1 is made of a spring steel cylinder (a metal cylinder made of spring steel) with an outer diameter of 30mm, an inner diameter of 20mm, a height of 60mm, and an open top. A metal diaphragm 2 made of spring steel with a thickness of 2.2mm and a diameter of 30mm is welded to the top opening of the metal shell 1. (Spring steel has a high Young's modulus and a low Poisson's ratio, and its diaphragm has low deflection under the same pressure, which can increase the range of the sensor.) A 1mm long and wide, 10mm high metal diaphragm is welded and fixed at the center of the bottom end face of the metal diaphragm 2. The rod 3 is a force-transmitting metal rod made of spring steel. At the lower end of the rod 3, a fluoride layer 4 (CYTOP), 1 mm in length and width, is deposited using spin coating as an analyte (a solid substitute material used to match the refractive index of the environment). A gallium-doped zinc oxide thin film 5 (GZO), serving as an excitation layer, is deposited on the fluoride layer 4 using radio frequency (RF) magnetron sputtering technology. The thickness of the fluoride layer 4 is at least 3 μm, and the thickness of the gallium-doped zinc oxide thin film 5 is 60 nm-80 nm. Alternatively, a layer of BK7 glass can be adhered to the end of the metal rod using SU-8 adhesive to enhance the adhesion of the fluoride layer 4 (CYTOP).

[0037] The single-mode fiber 6 is located inside the metal shell 1. The cladding 61 of the middle section of the single-mode fiber 6 is removed to expose the fiber core 62 of the middle section. The fiber core 62 of the middle section is a bare core with a diameter of 8.2 μm.

[0038] A microcapsule 7 with a side opening, an inner diameter of 125 μm, an outer diameter of 160 μm, and a length of 3 mm is located at the bare core position and is fixed to the cladding 61 at both ends of the single-mode fiber 6 with epoxy resin. The internal space of the microcapsule 7 is an open cavity with an upper opening. The open cavity is filled with a matching liquid 8 with the same refractive index as the cladding 61 and which immerses the bare core. The matching liquid 8 can serve as the liquid cladding for the bare core. At this time, the middle section of the fiber and the liquid cladding can be approximated as a D-shaped fiber.

[0039] It should be noted that using liquid materials can avoid stress transmission. Therefore, the cladding of the optical fiber is polished and filled with a liquid of the same refractive index. Specifically, it is a liquid with a refractive index of 1.4467, which is modulated by mixing ethanol and glycerol in a certain ratio, such as 23.5% and 76.5%. The ratio of ethanol to glycerol in this matching liquid can be precisely controlled by using an Abbe refractometer, and the refractive index of the single-mode optical fiber cladding is measured to be 1.4467 using a refractometer.

[0040] Among them, the gallium-doped zinc oxide thin film layer 5 is located above the liquid cladding, and the space between the gallium-doped zinc oxide thin film layer 5 and the bare core is called the transition layer (this utility model needs to consider the distance between the two materials, that is, the influence of the thickness of the transition layer on the resonant wavelength).

[0041] Please refer to the details. Figure 2When fluoride CYTOP-gallium-doped zinc oxide (GZO) is immersed in the liquid cladding, the GZO is used to excite LMR, and the fluoride CYTOP material is used as the analyte. GZO acts as a lossy dielectric, with a positive real part of its dielectric constant and an absolute value greater than the imaginary part, thus enabling LMR excitation. As a low-refractive-index solid optical material, CYTOP's refractive index (1.34) is lower than the fiber core's refractive index, preventing the optical field within the core from transitioning to the CYTOP. Furthermore, its solid nature facilitates material deposition. By changing the pressure applied to the diaphragm, the distance between the core and the GZO material, i.e., the height of the transition layer, can be directly altered. This changes the distribution of the optical field (intensity, position, and direction of light propagating within the core) and the effective refractive index. This change affects the mode loss from the core to the GZO, thus enabling pressure sensing. Therefore, by analyzing the mode loss of the liquid-clad fiber, the external pressure change can be calculated. Since the deflection at the center point of the diaphragm changes linearly with pressure, and the amount of change is closely related to the thickness and radius of the metal diaphragm, different ranges and sensitivities can be tuned by adjusting the size of the diaphragm.

[0042] Please see Figure 4 A schematic diagram showing the relationship between diaphragm deflection and diaphragm thickness in a fiber optic LMR pressure sensor with a diaphragm diameter of 20 mm and a thickness of 2.2 mm, within the range of 0-1 MPa.

[0043] Please see Figure 5 When the diaphragm diameter is 20 mm and the thickness is 2.2 mm, the thickness of gallium-doped zinc oxide is 64 nm in the range of 0-1 MPa. A schematic diagram of the resonant wavelength of a fiber optic LMR pressure sensor membrane and the distance to the transition layer is shown. In this figure, it can be clearly seen that the resonant wavelength drifts with the change of the thickness of the transition layer, which verifies the modulation effect of the transition layer distance on the resonant wavelength and illustrates the feasibility of this utility model.

[0044] This utility model patent proposes an optical fiber LMR pressure sensor. The sensor is primarily based on a diaphragm and liquid-clad optical fiber structure, using gallium-doped zinc oxide (GZO) as the loss layer (GZO is a lossy medium with a positive real part of its dielectric constant, and its absolute value is greater than the absolute value of its imaginary part, which can excite LMR; loss refers to the mode loss within the fiber core due to the transition of the optical field from the fiber core to the GZO thin film). By introducing a liquid cladding and a diaphragm, this structure can be equivalent to a D-shaped optical fiber with adjustable polishing depth. When the diaphragm is under pressure, it transfers displacement to the coating. Changes in pressure easily alter the distance between the optical fiber and the coating, effectively increasing the sensor's range while the optical fiber is not under pressure. (Existing LMR pressure sensor solutions all use all-solid materials to fabricate the sensor, which cannot avoid stress transmission; under high stress, the optical fiber is fragile and easily broken. This utility model uses a solid-liquid structure, avoiding stress transmission, only changing the thickness of the transition layer without affecting the optical fiber. Therefore, it can operate under higher pressure environments. Furthermore, by using an elastic element like a diaphragm, the center displacement can be adjusted by modifying the material and size, ensuring that even under greater pressure, the change is only about 1 micrometer). This sensor structure protects the optical fiber while avoiding cross-sensitivity between the fiber and the coating material due to pressure. Experimental results show a wider measurement range and higher sensitivity compared to other studies, solving the problems of small range and low accuracy in existing LMR pressure sensing schemes. The structure is built using small-sized metal and optical components, making the finished product portable and suitable for high-sensitivity measurements over a wide range.

[0045] This invention presents experimentally measured range and sensitivity under three different configurations, as shown in the table below:

[0046] R = 20mm d = 0.2mm 4.663 nm / kPa 9.992nm / kPa 0-1kPa R = 20mm d = 2.2mm 4.399nm / MPa 9.4269nm / MPa 0-1MPa R = 26mm d = 6mm 0.959nm / MPa 2.055nm / MPa 0-5MPa

[0047] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0048] The above description of the disclosed embodiments enables those skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A fiber optic LMR pressure sensor, characterized in that, include: A metal shell (1) has an open top covered with a metal diaphragm (2). A force-transmitting metal rod (3) is fixed at the center of the bottom end face of the metal diaphragm (2). A fluoride layer (4) as an analyte is deposited at the lower end of the force-transmitting metal rod (3). A gallium-doped zinc oxide thin film layer (5) as an excitation layer is deposited on the fluoride layer (4). A single-mode optical fiber (6) is located inside the metal shell (1). The cladding (61) of the middle section of the single-mode optical fiber (6) is removed to expose the core (62) of the middle section. The core (62) of the middle section is a bare core. A microcapillary (7) with a side opening is fitted onto the cladding (61) at both ends of the single-mode optical fiber (6). The internal space of the microcapillary (7) is an open-top non-closed cavity. The non-closed cavity is filled with a matching liquid (8) that has the same refractive index as the cladding (61) and immerses the bare core. The matching liquid (8) can serve as a liquid cladding for the bare core. The gallium-doped zinc oxide thin film layer (5) is located above the liquid cladding, and the space between the gallium-doped zinc oxide thin film layer (5) and the bare core is called the transition layer.

2. The fiber optic LMR pressure sensor according to claim 1, characterized in that, The microcapillary (7) has an inner diameter of 125 μm, an outer diameter of 160 μm, and a length of 3 mm.

3. The fiber optic LMR pressure sensor according to claim 1, characterized in that, The matching liquid (8) is prepared by mixing ethanol and glycerol in a certain proportion, and the refractive index of the matching liquid (8) is 1.4467.

4. The fiber optic LMR pressure sensor according to claim 1, characterized in that, The thickness of the fluoride layer (4) is at least 3 μm, and the thickness of the gallium-doped zinc oxide thin film layer (5) is 60 nm-80 nm.

5. The fiber optic LMR pressure sensor according to claim 1, characterized in that, The metal diaphragm (2) is made of spring steel.

6. The fiber optic LMR pressure sensor according to claim 1, characterized in that, The lower end of the force-transmitting metal rod (3) is provided with a BK7 glass layer for enhancing the adhesion of the fluoride layer (4).

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