Optical fiber LMR pressure sensor

Abstract

The invention discloses an optical fiber LMR pressure sensor which comprises a metal shell, the top of the shell is open, a metal diaphragm covers the metal shell, a force transmission metal rod is fixed to the center of the bottom end face of the metal diaphragm, a fluoride layer is deposited at the lower end of the force transmission metal rod, and a gallium-doped zinc oxide thin film layer is deposited on the fluoride layer; the single-mode optical fiber is located in the metal shell, and a cladding of the middle section of the single-mode optical fiber is removed to expose a fiber core of the middle section; the micro capillary tube with an opening in the side surface is fixedly sleeved on the cladding at the two ends of the single-mode optical fiber, the micro capillary tube is filled with matching liquid which has the same refractive index as the cladding and immerses the bare core, and the matching liquid can be used as a liquid cladding of the bare core; the gallium-doped zinc oxide thin film layer is located above the liquid cladding, and the space range between the gallium-doped zinc oxide thin film layer and the bare core is called as a transition layer. According to the sensor, the influence of stress transmission on the optical fiber can be avoided, the sensor has a wide sensing range, and high sensitivity can be guaranteed.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and more particularly to an optical fiber LMR pressure sensor. Background Art

[0002] Pressure measurement plays an important role in industrial equipment, life sciences, chemical manufacturing and other fields. With the continuous development of application scenarios, the increasing performance requirements such as high sensitivity, high precision, wide range and fast response speed are the main challenges facing pressure monitoring equipment today. Currently, many scholars have developed fiber optic pressure sensors based on optical components and structures such as fiber Bragg Grating (FBG) and Fabry-Pérot interferometer (FPI). Compared with traditional electrical component pressure sensors, they have advantages such as anti-electromagnetic interference, pressure and high temperature resistance, high sensitivity and fast response speed.

[0003] In addition to the aforementioned advantages, lossy mode resonance (LMR) sensors have been a research hotspot for nearly a decade due to their unique resonant properties. Compared to surface plasmon resonance (SPR) sensors, LMR sensors can be excited by both TM and TE polarizations, facilitating multi-parameter sensing and improving accuracy. For coating materials, SPR sensors often use metals such as Ag and Au as excitation materials, but their dispersion properties limit their operation over a wide wavelength range. LMR sensors, 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). When phase matching conditions are met, these materials can excite LMR sensors with both polarizations and are more affordable than metals. Therefore, with its superior sensing performance and economic advantages, the development of LMR pressure sensors holds enormous potential for application.

[0004] In sensor development, wide range and high sensitivity are key performance targets pursued by researchers. Existing research has shown that the resonant wavelength of LMR sensors can shift significantly in response to changes in the external refractive index. Currently, the most commonly used structures for LMR sensors are prisms and optical fibers. D-shaped fiber structures exhibit excellent performance. Because they are not cylindrical, TM and TE resonances can be observed directly on the coating surface, and they facilitate material deposition. Numerous studies have demonstrated that LMR sensors based on D-shaped fibers offer significant advantages in sensitivity and full-width at half maximum. However, due to the polishing process, D-shaped fibers have poor mechanical strength, making them difficult to operate in high-stress and high-strain environments. Similarly, for prism structures, surface pressure also causes deformation in multiple material layers, and the effects vary depending on the material and deposition method. These inherent limitations of optical fibers and enhanced sensitivity hinder the development of LMR sensors for pressure measurement. Existing research on electrical pressure sensors often employs the addition of compensating elements for pressure compensation. However, methods for preventing the effects of pressure on the optical fiber and for performing pressure compensation in LMR sensors have not been reported. These unresolved challenges directly limit the development of high-performance pressure sensors.

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

[0006] Existing patent ZL201910345917.8 discloses a microstructured fiber optic sensor based on loss mode resonance. This sensor uses a semicircular photonic optical fiber to reduce losses. Utilizing a double-layer membrane structure and external rubber, it sensitively converts external pressure changes into changes in rubber volume, further altering the medium's refractive index, which is then displayed through wave motion, thereby accurately measuring external air pressure. However, under the influence of external air pressure, both the rubber volume and the photonic crystal fiber are affected by pressure, resulting in cross-sensitivity caused by pressure. The invention does not provide a method to compensate for this cross-sensitivity, which affects accuracy.

[0007] The existing patent CN109459164A proposes a fiber optic pressure sensor and a method for manufacturing the same. The sensor includes a signal transmitting and receiving device, a transmission optical fiber, and a pressure detection device. In terms of the pressure detection device, the deformation of the elastic diaphragm causes the refractive index of the refractive index sensitive medium to change, which affects the interference light in the Fabry-Perot resonant cavity and is used to measure the pressure applied from the side of the transmission optical fiber. In the sensor proposed in this patent, the function of the diaphragm is to sense external pressure and transfer stress to the liquid cavity; the liquid in the closed liquid cavity is a pressure-sensitive material, which functions to change the refractive index in the Fabry-Perot resonant cavity. The above patent can measure pressure from the side direction, but does not take into account the effect of the stress transfer of the liquid on the liquid cavity under the closed liquid cavity, thereby changing the cavity length of the Fabry-Perot resonant cavity and affecting the accuracy.

[0008] Existing patent CN112254847A proposes a fiber optic FP pressure sensor based on hydraulic principles. This sensor consists of a glass housing with an open top. A bowl-shaped metal diaphragm and a single-mode optical fiber coated with a reflective film are connected at both ends of the glass housing. 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 length of the FP cavity through external pressure. In the sensor proposed in this patent, the metal diaphragm senses the pressure in the hydraulic cavity and deforms, thereby changing the length of the FP cavity. The liquid in the sealed hydraulic cavity transmits stress. This patent can measure very low pressures with a simple structure. However, at higher pressures, the bowl-shaped metal diaphragm, with a thickness of 50μm, will severely deform, affecting accuracy.

[0009] Therefore, how to provide a fiber optic LMR pressure sensor that can avoid the influence of stress transmission on the optical fiber, has a wide sensing range, and ensures high sensitivity is an urgent problem that needs to be solved by those skilled in the art. Summary of the Invention

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

[0011] In order to achieve the above object, the present invention adopts the following technical solutions:

[0012] An optical fiber LMR pressure sensor, comprising:

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

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

[0015] A microcapillary tube with a side opening, the microcapillary tube being fixed on the cladding at both ends of the single-mode optical fiber, the internal space of the microcapillary tube being a non-enclosed cavity with an upper opening, the non-enclosed cavity being filled with a matching liquid having the same refractive index as the cladding and immersing the bare core, the matching liquid serving as a liquid cladding for the bare core;

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

[0017] As can be seen from the above technical solution, compared with the prior art, the present invention provides a fiber-optic LMR pressure sensor. In this sensor, a metal diaphragm senses external pressure and deforms, causing a force-transmitting metal rod beneath the diaphragm to move the gallium-doped zinc oxide thin film downward into a matching liquid. This changes the height of the transition layer, which in turn alters the distribution and effective refractive index of the optical field (information such as the intensity, position, and direction of light transmitted within the fiber core). This change affects the mode loss from the fiber core to the gallium-doped zinc oxide thin film, enabling pressure sensing. By analyzing the mode loss of the liquid-clad fiber, external pressure changes can be calculated. This sensor uses a matching liquid instead of the fiber cladding, and forms a non-enclosed cavity by opening the upper end of a microcapillary tube. This allows the gallium-doped zinc oxide thin film to be immersed in the liquid while preventing stress transmission, thereby preventing the influence of pressure on the fiber, while also 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 liquid is prepared by mixing ethanol and glycerol in a certain ratio, and the refractive index of the matching liquid 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, a BK7 glass layer for enhancing the adhesion of the fluoride layer is provided at the lower end of the force-transmitting metal rod.

[0023] The present invention employs a fiber-optic LMR pressure sensor in which the cladding of the middle section of the optical fiber is removed. The fibers at both ends are nested using microcapillaries, and the bare core is filled with a refractive index matching liquid. In this case, the middle section of the optical fiber and the liquid cladding can be approximated as a D-shaped fiber. Above the optical fiber is a pressure transmission device consisting of a metal diaphragm, a metal rod, CYTOP fluoride, and gallium-doped zinc oxide. When pressure acts on the metal diaphragm from above, the diaphragm deforms and transmits the displacement of its center point directly through the metal rod to the analytical material at the bottom, stimulating LMR. Because the cladding is replaced with a matching liquid, external pressure only changes the distance of the transition layer and does not directly contact the fiber core. Therefore, the sensor does not affect the optical fiber and its liquid cladding under pressure, avoiding cross-sensitivity caused by pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0025] Figure 1 This is a schematic diagram of the main structure of a fiber optic LMR pressure sensor provided by the present invention.

[0026] Figure 2 This is a schematic diagram of the cross-sectional structure of a fiber optic LMR pressure sensor provided by the present invention.

[0027] Figure 3 This is a schematic diagram of a fiber optic LMR pressure sensor provided by the present invention hidden behind a metal shell.

[0028] Figure 4 A schematic diagram of the relationship between the metal diaphragm deflection and diaphragm thickness of a fiber optic LMR pressure sensor provided by the present invention within the range of 0-1 MPa.

[0029] Figure 5 This is a schematic diagram of the resonance wavelength and transition layer distance of a fiber optic LMR pressure sensor provided by the present invention. DETAILED DESCRIPTION

[0030] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0031] In the description of the present invention, it should be understood that the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore should not be understood as limiting the present invention.

[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 the technical features being referred to. Thus, a feature identified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0033] In the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," "connect," "fixed," etc. should be understood broadly. For example, they may refer to fixed connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0034] The present invention provides a fiber optic LMR pressure sensor that achieves high sensitivity sensing over a wide pressure range. Specifically, the sensor is suitable for measuring pressures within the range of 0 to 5 MPa, with an average sensitivity of 4.399 nm / MPa within 1 MPa.

[0035] See also Figure 1-Figure 5 As shown, an embodiment of the present invention discloses a fiber optic LMR pressure sensor, which is encapsulated by a metal shell 1 on the outside and has a fiber optic LMR pressure sensing chamber on the inside.

[0036] The metal shell 1 is made of a spring steel cylinder (spring steel metal cylinder) with an outer diameter of 30mm, an inner diameter of 20mm, a height of 60mm and an opening at the 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 higher Young's modulus and a lower Poisson's ratio. Its diaphragm has a lower deflection under the same pressure, which can increase the range of the sensor). A 10mm long and wide metal diaphragm with a height of 10mm is welded and fixed at the center of the bottom end surface of the metal diaphragm 2. m, and a force-transmitting metal rod 3 made of spring steel. A fluoride layer 4 (CYTOP) with a length and width of 1 mm is deposited on the lower end of the force-transmitting metal rod 3 using spin coating, serving as an analyte (a solid substitute material used to match the refractive index of the environment). A gallium-doped zinc oxide thin film layer 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 layer 5 is 60 nm to 80 nm. Of course, a layer of BK7 glass can also be attached to the end of the metal rod using SU-8 glue to enhance the adhesion of the fluoride layer 4 (CYTOP).

[0037] The single-mode optical fiber 6 is located in 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 with a diameter of 8.2 μm.

[0038] A microcapillary tube 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 fixed to the cladding 61 at both ends of the single-mode optical fiber 6 using epoxy glue. The interior space of the microcapillary tube 7 is a non-enclosed cavity with an upper opening. The non-enclosed cavity is filled with a matching liquid 8 having the same refractive index as the cladding 61 and immersing the bare core. The matching liquid 8 can serve as the liquid cladding of the bare core. In this case, the middle section of the optical fiber and the liquid cladding can be approximately D-shaped optical fiber.

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

[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 a transition layer (the present invention needs to consider the distance between the two materials, that is, the influence of the thickness of the transition layer on the resonance wavelength).

[0041] Please refer to the following for details: Figure 2When fluoride-gallium-doped zinc oxide (GZO) is immersed in a liquid cladding, the GZO is used to excite LMR. The fluoride-GZO material serves as the analyte. Gallium-doped zinc oxide (GZO) is a lossy dielectric with a positive real part of its dielectric constant, and its absolute value is greater than the imaginary part, thus stimulating LMR. As a low-refractive-index solid optical material, fluoride-GZO's refractive index (1.34) is lower than that of the fiber core, preventing the light field within the core from transitioning to the fluoride-GZO. Furthermore, its solid nature facilitates material deposition. By varying the pressure applied to the diaphragm, the distance between the core and the GZO material—that is, the height of the transition layer—can be directly altered. This in turn changes the distribution of the light field (information such as the intensity, position, and direction of light transmitted within the core) and the effective refractive index. This change affects the core-to-GZO mode loss, enabling pressure sensing. By analyzing the mode loss of the liquid-cladding fiber, the external pressure change can be calculated. Since the deflection of 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, tuning of different ranges and sensitivities can be achieved by adjusting the size of the diaphragm.

[0042] See also Figure 4 , a schematic diagram of the relationship between the diaphragm deflection and diaphragm thickness of a fiber optic LMR pressure sensor in the range of 0-1MPa when the diaphragm diameter is 20mm and the thickness is 2.2mm.

[0043] See also Figure 5 When the diaphragm diameter is 20 mm and the thickness is 2.2 mm, within the range of 0-1 MPa, the thickness of gallium-doped zinc oxide is 64 nm. A schematic diagram of the resonance wavelength and transition layer distance of a fiber optic LMR pressure sensor membrane is provided. In the figure, it can be clearly seen that the resonance wavelength drifts with the change of the thickness of the transition layer, verifying the regulating effect of the transition layer distance on the resonance wavelength and illustrating the feasibility of the present invention.

[0044] This patent proposes a fiber-optic LMR pressure sensor. This sensor is primarily based on a diaphragm and liquid-cladding fiber structure, using GZO as a lossy layer. (Galium-doped zinc oxide (GZO) is a lossy dielectric with a positive real part and a greater absolute value than the imaginary part, which can excite LMR. Loss refers to the transition of the light field from the fiber core to the GZO thin film, resulting in mode losses within the core.) By introducing the liquid cladding and diaphragm, the structure can be equivalent to a D-shaped fiber with adjustable polishing depth. When subjected to pressure, the diaphragm transfers displacement to the coating. Pressure changes can easily change the distance between the fiber and the coating, effectively increasing the sensor's range while maintaining the fiber's stability. (Existing LMR pressure sensor solutions all use all-solid materials, which inevitably transfer stress. Under high stress, the fiber is fragile and prone to breakage.) The present invention uses a solid-liquid structure, eliminating stress transfer and only changing the thickness of the transition layer, without affecting the fiber. Therefore, it can operate under higher pressure environments. Furthermore, the use of the diaphragm as an elastic element allows the center displacement to change by only about 1 micron, even under higher pressures, by adjusting the material and dimensions. This sensor structure protects the optical fiber while avoiding cross-sensitivity between the optical fiber and the coating material caused by pressure. Experimental results demonstrate a wider measurement range and higher sensitivity than other studies, addressing the limited range and low accuracy issues of existing LMR pressure sensing solutions. Constructed using compact metal and optical components, the structure is portable and suitable for high-sensitivity measurements over a wide range.

[0045] The present invention provides experimentally measured ranges and sensitivities under three different configurations, as shown in the table below:

[0046] Diaphragm diameter and thickness Average sensitivity Maximum sensitivity Range R=20mmd=0.2mm 4.663nm / kPa 9.992nm / kPa 0-1kPa R=20mmd=2.2mm 4.399nm / MPa 9.4269nm / MPa 0-1MPa R=26mmd=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 the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0048] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one 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 limited to the embodiments shown herein but is intended to conform to 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), wherein the top of the metal shell (1) is open and covered with a metal diaphragm (2), a force-transmitting metal rod (3) is fixed at the center of the bottom end surface of the metal diaphragm (2), a fluoride layer (4) as an analyte is deposited on the lower end of the force-transmitting metal rod (3), and 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), the single-mode optical fiber (6) being located in the metal shell (1), the cladding (61) of the middle section of the single-mode optical fiber (6) being removed to expose the fiber core (62) of the middle section, the fiber core (62) of the middle section being a bare core; A microcapillary tube (7) with a side opening, the microcapillary tube (7) is fixed on the cladding (61) at both ends of the single-mode optical fiber (6), the internal space of the microcapillary tube (7) is a non-enclosed cavity with an upper opening, the non-enclosed cavity is filled with a matching liquid (8) having the same refractive index as the cladding (61) and immersing the bare core, and 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 layer, and the space between the gallium-doped zinc oxide thin film layer (5) and the bare core is called a transition layer.

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

4. The optical fiber 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 to 80 nm.

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

6. The optical fiber 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).

Images