Optical fiber pressure sensor and method for sensing the same

Abstract

An optical fiber (10) for measuring pressure, comprising a core (12) for guiding an optical signal along its length, and a cladding layer (14) including a plurality of stress-applying portions (16) disposed about the core (12). The plurality of stress-applying portions (16) are disposed parallel to and symmetrically about the core (12) to induce enhanced symmetric shear stress in response to the application of external pressure while preventing birefringence. The optical fiber (10) provides improved strain sensitivity compared to standard single-mode optical fibers.

Description

[Technical Field]

[0001] The present invention relates to an optical fiber-based pressure sensor and a method for sensing pressure along the length of an optical fiber. [Background Art]

[0002] A variety of sensors have been developed that use optical fibers to measure temperature, pressure, force, strain and other parameters. The advantages of optical fiber sensors are their small size, low cost, flexibility and ability to be incorporated into other structures. However, most existing optical fiber pressure sensors require complex fabrication and measurement methods, and are unsuitable for use in harsh environments such as oil and gas pipelines or underground oil wells.

[0003] A typical optical fiber pressure sensor includes a fiber Bragg grating formed in the core of an optical fiber by doping the optical fiber with a material such as germanium, then exposing the side surface of the fiber to an interference pattern to generate sinusoidal variations in the refractive index of the core. The center wavelength of the spectral envelope reflected by the fiber Bragg grating varies linearly with temperature and strain. Therefore, such variations can be measured to derive strain in the sensor's environment. However, such fiber Bragg grating pressure sensors are difficult to fabricate and operate, and a large number of these discrete sensors are required for high-sensitivity pressure sensing along the length of an optical fiber in harsh environments such as underground wells.

[0004] Another type of optical fiber pressure sensor includes polarization-maintaining optical fibers, also known as birefringent fibers. Such optical fibers include a stress-applying section on one or both sides of a core with an elliptical or circular cross-section, where cladding stress is applied to the core to induce birefringence. However, it is very difficult to use these sensors for most applications requiring high pressure sensitivity. Additional equipment needs to be installed with these sensor devices to increase their pressure sensitivity, however, this makes them more difficult to manufacture and introduces stability and repeatability problems. [Overview of the project] [Problems that the invention aims to solve]

[0005] The object of the present invention is therefore to provide a highly pressure-sensitive optical fiber pressure sensor for use in harsh environments. [Means for solving the problem]

[0006] In one embodiment of the present invention, A core, which is used to guide optical signals along its length, An optical fiber for measuring pressure, comprising a cladding layer including a plurality of stress-applying sections provided around a core, An optical fiber is provided, characterized in that a plurality of stress-applying sections are provided parallel to and symmetrically around the core in order to induce a strengthened symmetric shear stress in response to the application of external pressure while preventing birefringence.

[0007] In one embodiment, the cladding layer is fabricated from silica, and the stress-applying section is fabricated from at least one of borosilicate (B2O3+SiO2), Al2O3+La2O3+SiO2, or F+SiO2 rods or voids. The difference in mechanical properties between the silica-based cladding layer and the stress-applying section generates enhanced symmetrical shear stress in response to the application of external force. Advantageously, this helps to increase the strain and pressure sensitivity of the optical fiber.

[0008] In one embodiment, the stress application section is provided parallel to the core and symmetrically around the core along the length of the fiber. Advantageously, this is useful for measuring pressure along the entire length of the optical fiber.

[0009] In one embodiment, the optical fiber is a single-mode optical fiber having a stress-applying portion located within a hole provided in the cladding layer. Advantageously, this helps to provide an optical fiber with improved strain and pressure sensitivity, while still having standard dimensions and standard operating requirements.

[0010] Advantageously, the symmetrical arrangement of the stress-applying portion around the core helps prevent birefringence, heteromode polarization sensitivity, and polarization mode dispersion of optical signals transmitted through the optical fiber.

[0011] In one embodiment, optical fibers can be used for multiple applications, including measuring external pressure in underground areas, oil wells, and other harsh environments, as well as health monitoring of civil and mechanical structures.

[0012] In another embodiment of the present invention, a method for measuring pressure along the length of an optical fiber, The steps include providing an optical fiber having a core, a cladding layer surrounding the core, and a plurality of stress-applying portions arranged in the cladding layer parallel to the core and symmetrically around the core, The steps include receiving an optical signal at the first end via an optical fiber, The steps include transmitting an optical signal through a core via an optical fiber without inducing birefringence, The steps include receiving a scattered light signal at the first end of the optical fiber, A method is provided which includes the step of determining the strain in an optical fiber by analyzing the scattered light signal using a scattering measurement unit.

[0013] In one embodiment, the stress-applying sections include a pair of rods or voids made from at least one of borosilicate (B2O3+SiO2), Al2O3+La2O3+SiO2, or F+SiO2, and are arranged in the cladding layer to generate enhanced symmetrical shear stress in response to the application of external pressure. Advantageously, this helps to increase the strain and pressure sensitivity of the optical fiber.

[0014] Advantageously, the symmetrical arrangement of the stress-applying portion around the core helps prevent birefringence, heteromode polarization sensitivity, and polarization mode dispersion of optical signals transmitted through the optical fiber.

[0015] Advantageously, optical fibers offer improved distortion sensitivity compared to standard single-mode optical fibers.

[0016] It would be convenient to further describe the present invention with reference to the accompanying drawings illustrating possible arrangements of the present invention. Other arrangements of the present invention are possible, and consequently, the details of the accompanying drawings are not to be understood as superseding the outline of the prior description of the present invention. [Brief explanation of the drawing]

[0017] [Figure 1] A schematic cross-sectional view of an optical fiber having a symmetric stress application section according to one embodiment of the present invention is shown as an example. [Figure 2] This plot illustrates the frequency shift of the scattered light signal passing through the optical fiber in Figure 1 in response to the application of fluctuating external pressure. [Figure 3] A flowchart illustrating the steps involved in a method for measuring pressure along the length of an optical fiber shown in Figure 1, according to one embodiment of the present invention, is provided. [Modes for carrying out the invention]

[0018] With respect to Figure 1, an example of an optical fiber 10 having a plurality of symmetric stress-applying portions 16 according to one embodiment of the present invention is shown. The optical fiber 10 comprises a core 12 for inducing optical signals along the length of the core 12 and a cladding layer 14 including a plurality of stress-applying portions 16 provided around the core 12. The plurality of stress-applying portions 16 are provided within the cladding layer 14 parallel to the longitudinal direction of the core 12 and symmetrically around the core 12 in order to induce enhanced symmetric shear stress in response to external pressure or force applied to the optical fiber 10.

[0019] In one embodiment, an optical fiber 10 is used for measuring pressure or force along the length of the optical fiber 10. The core 12 of the optical fiber 10 is fabricated from germanium-doped silica (GeO2 + SiO2) having a refractive index n1. The core 12 is surrounded by a cladding layer 14 fabricated from silica (SiO2) having a refractive index n2 (n1 > n2). In one embodiment, a stress-applying section 16 provided within the cladding layer 14 is fabricated from a borosilicate (B2O3 + SiO2) rod. In an alternative embodiment, the stress-applying section 16 is fabricated from an Al2O3 + La2O3 + SiO2 or F + SiO2 rod. In another embodiment, a void is provided as the stress-applying section 16. The difference in mechanical properties between the silica-based cladding layer 14 and the stress-applying section 16 generates enhanced symmetrical shear stress in response to the application of uniform external pressure or force to the optical fiber 10. Advantageously, this helps to increase the strain and pressure sensitivity of the optical fiber 10-based pressure sensor.

[0020] In one embodiment, the optical fiber 10 can be utilized for measuring external pressure or force in underground oil wells and other harsh environments. In one embodiment, the optical fiber 10 can be utilized for structural health monitoring of civil engineering structures. In yet another embodiment, the optical fiber 10 can be utilized for structural health monitoring of mechanical structures such as railway tracks. When the fiber is exposed to hydrostatic pressure, the force is converted into strain, and the cable elongates due to the Poisson effect. In the prior art, stress applying portions have different mechanical properties such that the effect of birefringence can be used to measure pressure. However, according to the present invention, the symmetrical arrangement of the stress applying portions 16 around the core 12 serves to generate symmetrical shear stress and prevent the generation of birefringence while allowing an optical signal to pass through the core 12. Pressure or force on the optical fiber 10 is measured by analyzing the effect of strain on scattering of the optical signal transmitted through the core 12. The absence of birefringence prevents cross-mode polarization sensitivity and polarization mode dispersion of the optical signal transmitted through the optical fiber 10, thereby improving the measurement accuracy for a given strain compared to known methods.

[0021] In one embodiment, a method of manufacturing an optical fiber 10 for pressure measurement is disclosed. The method comprises the step of forming a preform comprising a core 12 made of germanium-doped silica (GeO2+SiO2) having a refractive index n1. The core 12 is surrounded by a cladding layer 14 made of silica (SiO2) having a refractive index n2, where n1>n2. To incorporate the stress applying portions 16, two orthogonal pairs of holes parallel and symmetrical relative to the core 12 are drilled through the cladding layer 14. The stress applying portions 16 can be of any desired shape, such as cylindrical or polygonal. The preform thus formed is drawn or extruded to form a single-mode optical fiber 10 having the core 12 in the center and the cladding layer 14 surrounding the core 12 and including the stress applying portions 16.

[0022] In another embodiment, fabricating optical fiber 10 comprises forming a preform for optical fiber 10 by stacking silica rods around a germanium-doped silica rod within a large silica tube. The arrangement of the germanium-doped silica rod forms the core 12, and the arrangement of the silica rods 14 forms the cladding layer 14. A pair of orthogonal borosilicate (B2O3+SiO2), Al2O3+La2O3+SiO2 or F+SiO2 rods or voids symmetrically stacked around the germanium-doped silica rod form the stress applying parts 16. The stacked rods within the silica tube are then melted and drawn to form an intermediate preform. The intermediate preform thus formed is drawn or extruded to form single-mode optical fiber 10 having the core 12 at the center and the cladding layer 14 surrounding the core 12 and including the stress applying parts 16.

[0023] Typically, the optical fiber 10 thus formed has a dimension of about 125 µm, the core 12 has a dimension of about 8.2 µm, and each of the stress applying parts 16 has a dimension of 36 µm.

[0024] Referring to FIG. 2, a plot of frequency shift of a scattered light signal through optical fiber 10 in response to application of varying external pressure is illustrated. The difference in mechanical properties between the silica-based cladding layer 14 and the stress applying parts 16, which are arranged parallel to and on either side of the core 12 and lie in a plane passing through the core 12, induces enhanced symmetrical shear stress in response to application of external pressure or force to the optical fiber 10. The enhanced symmetrical shear stress imparted to the optical fiber 10 provides an amplification of applied force-strain conversion in response to the application of an external force, thereby helping to improve the strain and pressure sensitivity of the optical fiber 10-based pressure sensor by at least 21% compared to a standard single-mode optical fiber (SMF). Furthermore, the optical fiber 10-based pressure sensor with stress applying parts 16 provides negligible hysteresis compared to standard single-mode optical fibers in response to application of an external force.

[0025] With respect to Figure 3, an illustrative flowchart shows steps involved in a method for measuring pressure using the optical fiber 10 according to one embodiment of the present invention. The method for measuring pressure or force along the length of the optical fiber 10 includes the step of providing an optical fiber 10 having a core 12, a cladding layer 14 surrounding the core 12, and a plurality of stress-applying portions 16 arranged in the cladding layer 14 parallel to the core 12 and symmetrically around the core 12, as shown in block 100. An optical signal is received at the first end of the optical fiber 10, as shown in block 102. As shown in block 104, the optical fiber 10 transmits the optical signal received through the core 12 without inducing birefringence. The optical signal is transmitted through the optical fiber 10 in the absence of birefringence, heteromode polarization sensitivity, and polarization mode dispersion. The optical signal transmitted through the optical fiber 10 is scattered to an extent that is affected by strain on the optical fiber 10. As shown in block 106, the scattered optical signal is received at the first end of the optical fiber 10. As shown in block 108, a scattering measurement unit is used to analyze the scattered light signal and determine the strain in the optical fiber 10. In one embodiment, the scattering measurement unit identifies the intrinsic scattering spectrum or intensity of the scattered light signal and subsequently determines the strain in the optical fiber 10. A stress-applying section 16, such as a pair of orthogonal borosilicate rods 16 arranged in the cladding layer 14, induces enhanced symmetric shear stress in response to the application of external pressure, which then provides improved strain sensitivity compared to a standard single-mode optical fiber.

[0026] Those skilled in the art will recognize that this optical fiber-based pressure sensor may also include an additional symmetrical stress-inducing section around the core to further improve strain sensitivity.

[0027] Those skilled in the art will also recognize that the present invention may include further additional modifications made to the fiber or method without affecting the overall function of the fiber or method. [Explanation of Symbols]

[0028] 10 Optical Fibers 12 cores 14. Clad layer, silica rod 16 Stress application section

Claims

1. The core (12) is made of germanium-doped silica (GeO₂ + SiO₂) and is used to guide optical signals along its length. A cladding layer (14) is made of silica (SiO₂) and surrounds the core (12), and includes a plurality of stress-applying portions (16) provided around the core (12). An optical fiber (10) for measuring pressure, comprising: The cladding layer (14) includes two pairs of orthogonal holes parallel and symmetrical to the core (12), and the plurality of stress-applying portions (16) are arranged within the holes in the cladding layer (14). The stress-applying portion (16) includes an Al₂O₃ + La₂O₃ + SiO₂ rod or void and is provided around the core (12) parallel to the longitudinal direction and symmetrically around the core (12), The difference in mechanical properties between the cladding layer (14) and the stress-applying portion (16) induces a symmetrical shear stress in the optical fiber (10) that is strengthened in response to the application of uniform external pressure or force. The optical fiber (10) is a single-mode optical fiber configured to measure pressure or force along the length of the optical fiber (10) by analyzing the effect of distortion on the scattering of optical signals transmitted through the core (12). The symmetrical arrangement of the stress application section (16) prevents birefringence-induced mode polarization sensitivity during pressure sensing. Optical fiber.

2. The optical fiber (10) according to claim 1, wherein the cladding layer (14) includes two orthogonal pairs of holes formed within the cladding layer (14) to accommodate the stress-applying portion (16).

3. A method for measuring pressure along the length of an optical fiber (10), The steps of providing the optical fiber (10) described in claim 1, The steps include receiving an optical signal at the first end via the optical fiber (10), The steps include transmitting the optical signal through the core (12) via the optical fiber (10), The steps include receiving a scattered light signal at the first end of the optical fiber (10), The process includes the step of analyzing the scattered light signal using a scattering measurement unit to determine the strain in the optical fiber (10), A method in which the pressure or force in the optical fiber (10) is determined based on the strain.

4. The stress-applying portion (16) is provided in the cladding layer (14) with Al ２ O ３ +La ２ O ３ +SiO ２ The method according to claim 3, including a rod.

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