Optical fiber pressure sensor and sensing method thereof

Abstract

An optical fiber (10) for measuring pressure includes a core (12) for guiding an optical signal along a length of the core (12) and a cladding (14) including a plurality of stress-applying members (16) disposed about the core (12). The plurality of stress-applying members (16) are disposed symmetrically about the core (12) and parallel to the core (12) to induce enhanced symmetric shear stress upon application of an 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 sensing method for sensing pressure along the length of an optical fiber. Background Art

[0002] A variety of sensors have been developed using optical fibers to measure temperature, pressure, force, strain, and other parameters. The advantages of optical fiber sensors include their small size, low cost, flexibility, and the ability to be embedded in other structures. However, most existing optical fiber pressure sensors require complex manufacturing and measurement methods and are not suitable for use in harsh environments such as oil and gas pipelines or underground wells.

[0003] A typical fiber optic pressure sensor consists of a fiber Bragg grating (FBG) formed in the core of an optical fiber by doping the fiber with a material such as germanium and then exposing the sides of the fiber to an interference pattern that produces a sinusoidal variation in the core's refractive index. The central wavelength of the spectral envelope reflected by the FBG varies linearly with temperature and strain. This variation can therefore be measured to derive the strain in the sensor's environment. However, such FBG pressure sensors are difficult to manufacture and operate, and large numbers of these discrete sensors are required along the length of the fiber for high-sensitivity pressure sensing in harsh environments such as underground wells.

[0004] Another type of fiber optic pressure sensor includes polarization-maintaining fiber, also known as birefringent fiber. This fiber includes a stress-applying component positioned on one or both sides of an elliptical or circular core, applying cladding stress to the core and inducing birefringence. However, using this type of sensor in most applications requiring high pressure sensitivity is difficult. Additional equipment must be installed with these sensor devices to increase their pressure sensitivity, which, however, makes them difficult to manufacture and creates stability and repeatability issues.

[0005] Therefore, an object of the present invention is to provide a fiber optic pressure sensor with high pressure sensitivity for use in harsh environments. Summary of the Invention

[0006] In one aspect of the present invention, there is provided an optical fiber for measuring pressure, comprising:

[0007] a fiber core for guiding an optical signal along a length of the fiber core; and

[0008] a cladding comprising a plurality of stress applying members disposed around the core;

[0009] The invention is characterized in that a plurality of stress applying members are arranged parallel to the fiber core and symmetrically around the fiber core to induce enhanced symmetrical shear stress when external pressure is applied while preventing birefringence.

[0010] In one embodiment, the cladding is made of silica, and the stress-applying member is made of at least one of borosilicate (B2O3+SiO2), Al2O3+La2O3+SiO2, or F+SiO2 rods, or air holes. The difference in mechanical properties between the silica-based cladding and the stress-applying member produces enhanced symmetrical shear stress when an external force is applied. Advantageously, this helps improve the strain and pressure sensitivity of the optical fiber.

[0011] In one embodiment, the stress applying members are arranged along the length of the optical fiber parallel to the core and symmetrically around the core. Advantageously, this facilitates pressure measurement along the entire length of the optical fiber.

[0012] In one embodiment, the optical fiber is a single mode optical fiber having stress applying members disposed within holes provided in the cladding. Advantageously, this helps provide an optical fiber having improved strain and pressure sensitivity but with standard dimensions and standard operating requirements.

[0013] Advantageously, the symmetrical arrangement of the stress applying components around the core helps prevent birefringence, different modal polarization sensitivities, and polarization mode dispersion of optical signals transmitted through the optical fiber.

[0014] In one embodiment, optical fiber may be used in a variety of applications including measuring external pressure in underground areas, oil wells, other harsh environments, and health monitoring of civil and mechanical structures.

[0015] In another aspect of the present invention, there is provided a method of measuring pressure along the length of an optical fiber, the method comprising:

[0016] Providing an optical fiber having a core, a cladding surrounding the core, and a plurality of stress applying members disposed in the cladding parallel to and symmetrically around the core;

[0017] receiving an optical signal at a first end via an optical fiber;

[0018] Transmitting light signals through an optical fiber through the core without causing birefringence;

[0019] receiving a scattered light signal at a first end of the optical fiber; and

[0020] The scattered light signal is analyzed using a scatterometry cell to determine the strain in the optical fiber.

[0021] In one embodiment, the plurality of stress-applying components include a pair of rods or a pair of air holes made of at least one of borosilicate (B2O3+SiO2), Al2O3+La2O3+SiO2, or F+SiO2 and are positioned within the cladding to generate enhanced symmetrical shear stress upon application of external pressure. Advantageously, this helps to improve the strain and pressure sensitivity of the optical fiber.

[0022] Advantageously, the symmetrical arrangement of the stress applying components about the core helps prevent birefringence, different modal polarization sensitivities and polarization mode dispersion of optical signals transmitted through the optical fiber.

[0023] Advantageously, the optical fiber provides improved strain sensitivity compared to standard single mode optical fibers. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] It will be convenient to further describe the invention with reference to the accompanying drawings which illustrate possible arrangements of the invention. Other arrangements of the invention are possible and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

[0025] Figure 1 A schematic cross-sectional view of an optical fiber having a symmetrical stress applying member according to an embodiment of the present invention is shown.

[0026] Figure 2 It shows that when the external pressure changes Figure 1 A graph showing the frequency shift of the scattered light signal in the optical fiber.

[0027] Figure 3 A flow chart is shown which illustrates the steps of Figure 1 The steps involved in the method of measuring pressure using an optical fiber length are shown. DETAILED DESCRIPTION

[0028] Reference Figure 1 , shows an optical fiber 10 having a plurality of symmetrical stress applying components 16 according to an embodiment of the present invention. The optical fiber 10 includes a core 12 for guiding an optical signal along the length of the core 12 and a cladding 14 including a plurality of stress applying components 16 disposed about the core 12. The plurality of stress applying components 16 are longitudinally disposed within the cladding 14 parallel to and symmetrically about the core 12 to induce enhanced symmetrical shear stress when external pressure or force is applied to the optical fiber 10.

[0029] In one embodiment, the optical fiber 10 is used to measure pressure or force along the length of the optical fiber 10. The core 12 of the optical fiber 10 is made of germanium-doped silica (Ge02+ Si02) with a refractive index of n1. The core 12 is surrounded by a cladding layer 14 made of silica (Si02) with a refractive index of n2 (n1> n2). In one embodiment, the stress-applying member 16 disposed within the cladding layer 14 is made of borosilicate (B203+ Si02) rod. In an alternative embodiment, the stress-applying member 16 is made of Al203+ La203+ Si02or F+ Si02rod. In another embodiment, an air hole is disposed as the stress-applying member 16. The mechanical property difference between the silica-based cladding layer 14 and the stress-applying member 16 results in enhanced symmetric shear stress when a uniform external pressure or external force is applied to the optical fiber 10. Advantageously, this helps to improve the strain and pressure sensitivity of the pressure sensor based on the optical fiber 10.

[0030] In one embodiment, the optical fiber 10 can be used to measure external pressure or external force in underground oil wells and other harsh environments. In an embodiment, the optical fiber 10 can be used for structural health monitoring of civil structures. In yet another embodiment, the optical fiber 10 can be used for structural health monitoring of mechanical structures such as railway tracks. When the fiber is exposed to static pressure, the force is converted to strain and the cable is elongated due to Poisson's effect. In the prior art, the stress-applying member has different mechanical properties such that the birefringence effect can be utilized to measure pressure. However, in accordance with the present invention, the symmetric disposition of the stress-applying member 16 around the core 12 helps to produce symmetric shear stress and prevent the occurrence of birefringence while passing the optical signal through the core 12. The pressure or force on the optical fiber 10 is measured by analyzing the effect of strain on the scattering of the optical signal transmitted through the core 12. Without birefringence, the different modal polarization sensitivity and polarization mode dispersion of the optical signal transmitted through the optical fiber 10 is prevented, thereby improving the measurement accuracy for a given strain as compared to the previously known methods.

[0031] In one embodiment, a method of manufacturing an optical fiber 10 for pressure measurement is disclosed. The method includes the steps of forming a preform comprising a core 12 made of germanium-doped silica (Ge02+ Si02) with a refractive index of n1. The core 12 is surrounded by a cladding layer 14 made of silica (Si02) with a refractive index of n2 (n1> n2). Two holes parallel and symmetric to the core 12 are drilled through the cladding layer 14 orthogonally to introduce a stress-applying member 16. The stress-applying member 16 can be of any desired shape such as cylindrical or polygonal. The thus formed preform is drawn or extruded to form a single mode optical fiber 10 having the core 12 at the center and the cladding layer 14 comprising the stress-applying member 16 around the core 12.

[0032] In another embodiment, the manufacturing of the optical fiber 10 includes the following steps: forming a preform of the optical fiber 10 by stacking silica rods around a germanium doped silica rod inside a large silica tube. This arrangement of the germanium doped silica rod forms the core 12 and the arrangement of the silica rods 14 forms the cladding layer 14. The orthogonal pairs of borosilicate (B2O3+ SiO2), Al2O3+ La2O3+ SiO2 or F+ SiO2 rods or air holes stacked symmetrically around the germanium doped silica rod form the stress applying parts 16. The stacked rods in the silica tube are then fused and drawn to form an intermediate preform. The intermediate preform thus formed is drawn or extruded to form a single mode optical fiber 10 having a core 12 located at the center and a cladding layer 14 including the stress applying parts 16 around the core 12.

[0033] Typically, the optical fiber 10 thus formed has a size of about 125 pm, wherein the core 12 has a size of about 8.2 pm and each stress applying part 16 has a size of 36 pm.

[0034] With respect to Figure 2 , a plot of the frequency shift in the scattered light signal through the optical fiber 10 upon application of varying external pressure is shown. The mechanical property difference of the silica based cladding layer 14 and the stress applying parts 16 placed parallel to and on either side of the core 12 and in a plane passing through the core 12 causes enhanced symmetric shear stress upon application of external pressure or force to the optical fiber 10. The enhanced symmetric shear stress on the optical fiber 10 provides amplification of the applied force to strain conversion upon application of external force, which helps 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 fiber (SMF). Further, the optical fiber 10 based pressure sensor with the stress applying parts 16 provides negligible hysteresis upon application of external force compared to a standard single mode fiber.

[0035] With reference to Figure 3, a flow chart illustrating the steps involved in a method for measuring pressure using an optical fiber 10 according to an embodiment of the present invention is shown. The method for measuring pressure or force along the length of the optical fiber 10 includes the following steps: As shown in block 100, an optical fiber 10 is provided having a core 12, a cladding 14 surrounding the core 12, and a plurality of stress-applying components 16 positioned parallel to and symmetrically around the core 12 in the cladding 14. As shown in block 102, an optical signal is received at a first end of the optical fiber 10. As shown in block 104, the optical fiber 10 transmits the received optical signal through the core 12 without inducing birefringence. The optical signal is transmitted through the optical fiber 10 without birefringence, different modal polarization sensitivities, and polarization mode dispersion. The optical signal transmitted through the optical fiber 10 is scattered, and its amplitude is affected by the 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 scatterometry unit is used to analyze the scattered optical signal to determine the strain in the optical fiber 10. In one embodiment, a scatterometry unit identifies the unique scatter spectrum or intensity of the scattered light signal and subsequently determines the strain in the optical fiber 10. An orthogonal pair of stress-applying components 16, such as borosilicate rods 16, placed in the cladding 14 induces enhanced symmetric shear stress upon application of external pressure, which in turn provides improved strain sensitivity compared to standard single-mode optical fibers.

[0036] Those skilled in the art will recognize that the optical fiber-based pressure sensor of the present invention may also include additional symmetrical stress-applying components surrounding the fiber core to further improve strain sensitivity.

[0037] Those skilled in the art will also recognize that the present invention may also include further modifications to the fiber or process that do not affect the overall function of the fiber or process.

Claims

1. An optical fiber (10) for measuring pressure, comprising: a fiber core (12) for guiding an optical signal along a length of the fiber core (12); as well as a cladding layer (14) comprising a plurality of stress applying members (16) arranged around the core (12), The invention is characterized in that the plurality of stress applying members (16) are arranged in orthogonal hole pairs parallel to and symmetrically arranged around the fiber core (12) to induce enhanced symmetrical shear stress when external pressure is applied while preventing birefringence.

2. The optical fiber (10) according to claim 1, wherein The stress applying member (16) is made of at least one of borosilicate (B2O3+SiO2), Al2O3+La2O3+SiO2 or F+SiO2 rods or empty air holes.

3. The optical fiber (10) according to claim 1, wherein The difference in mechanical properties between the cladding layer (14) and the stress applying member (16) causes enhanced symmetrical shear stress when external pressure is applied.

4. The optical fiber (10) according to any one of the preceding claims, wherein The optical fiber (10) is a single-mode optical fiber.

5. The optical fiber (10) according to any one of the preceding claims, wherein The optical fiber (10) is used in a variety of applications, including measuring external pressure in underground wells and health monitoring of civil and mechanical structures.

6. A method of measuring pressure along the length of an optical fiber (10), comprising: An optical fiber (10) is provided having a core (12), a cladding (14) surrounding the core (12), and a plurality of stress applying members (16), the plurality of stress applying members (16) being positioned within pairs of orthogonal holes in the cladding (14) parallel to and symmetrically disposed around the core (12); receiving an optical signal at a first end via the optical fiber (10); transmitting the optical signal through the optical fiber (10) through the optical core (12) without causing birefringence; receiving a scattered light signal at the first end of the optical fiber (10); and The scattered light signal is analyzed using a scatterometry unit to determine strain in the optical fiber (10).

7. The method according to claim 6, wherein: The plurality of stress applying members (16) include a pair of rods or air holes made of at least one of borosilicate (B2O3+SiO2), Al2O3+La2O3+SiO2 or F+SiO2 and are placed in the cladding layer (14) to generate enhanced symmetrical shear stress when external pressure is applied.

8. The method according to claim 6, wherein: The optical signal is transmitted through the optical fiber (10) without birefringence, different modal polarization sensitivity and polarization mode dispersion.

9. A method according to any one of the preceding claims, wherein The optical fiber (10) provides improved strain sensitivity compared to standard single-mode optical fibers.

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