Patent Publication Number: US-2023160765-A1

Title: Optical fiber pressure sensor and method of sensing thereof

Description:
FIELD OF INVENTION 
     The invention relates to an optical fiber based pressure sensor and a method of sensing pressure along the length of the optical fiber. 
     BACKGROUND 
     A wide variety of sensors have been developed using optical fibers for measuring temperature, pressure, force, strain and other parameters. The advantages of optical fiber sensors are their small size, low cost, flexibility and capability to be embedded into other structures. However, most of existing optical fiber pressure sensors require complex fabrication and measurement methods and are inconvenient to be employed in harsh environments such as oil and gas pipes or subterranean oil wells. 
     A typical optical fiber pressure sensor includes a fiber Bragg grating formed in the core of the optical fiber by doping an optical fiber with a material such as germanium and then exposing the side of the fiber to an interference pattern to produce sinusoidal variations in the refractive index of the core. The center wavelength of the spectral envelope reflected by the fiber Bragg grating changes linearly with temperature and strain. Thus, such changes can be measured to derive strain in the environment of the sensor. 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 the fiber in harsh environments such as subterranean wells. 
     Another type of optical fiber pressure sensor includes a polarization maintaining optical fiber, also known as a Birefringent fiber. Such optical fibres include stress applying parts provided on one side or on either side of the core having an elliptic section, or a circular core such that the cladding stress is applied to the core to induce birefringence. However, it is very difficult to use this sensor in most applications requiring high pressure sensitivity. Additional devices need to be installed together with these sensor devices to increase their pressure sensitivity, however this makes it more difficult to manufacture and creates stability and repeatability problems. 
     An aim of the invention therefore is to provide an optical fiber pressure sensor with high pressure sensitivity for use in harsh environments. 
     SUMMARY OF INVENTION 
     In an aspect of the invention, there is provided an optical fiber for measuring pressure comprising:
         a core for guiding optical signals along a length of the core; and   a cladding layer including a plurality of stress applying parts disposed around the core;   characterized in that the plurality of stress applying parts are disposed parallel to and symmetrically around the core to induce intensified symmetric shear stress upon application of external pressure while preventing birefringence.       

     In one embodiment, the cladding layer is fabricated from silica and the stress applying parts are fabricated from at least one of borosilicate (B 2 O 3 +SiO 2 ), Al 2 O 3 +La 2 O 3 +SiO 2  or F+SiO 2  rods or air holes. The difference in mechanical properties of the silica based cladding layer and the stress applying parts produces intensified symmetrical shear stress upon application of external force. Advantageously, this helps to increase the strain and pressure sensitivity of the optical fiber. 
     In one embodiment, the stress applying parts are disposed parallel to and symmetrically around the core along the length of the fiber. Advantageously this helps in pressure measurements along the length of the entire optical fiber. 
     In one embodiment, the optical fiber is a single-mode optical fiber having the stress applying parts arranged within the holes provided in the cladding layer. Advantageously this helps to provide an optical fiber with improved strain and pressure sensitivity but having standard dimensions and standard handling requirements. 
     Advantageously the symmetrical arrangement of the stress applying parts around the core helps to prevent birefringence, dissimilar modal polarization sensitivity and polarization mode dispersion of the optical signal transmitted through the optical fiber. 
     In one embodiment, the optical fiber can be used for a plurality of applications including measuring external pressure in subterranean regions, oil wells, other harsh environments and health monitoring of civil and mechanical structures. 
     In another aspect of the invention, there is provided a method of measuring pressure along a length of an optical fiber, the method comprising:
         providing an optical fiber having a core, a cladding layer surrounding the core and a plurality of stress applying parts placed in the cladding layer parallel to and symmetrically around the core;   receiving, via the optical fiber, an optical signal at a first end;   transmitting, via the optical fiber, the optical signal through the core without inducing birefringence;   receiving, at the first end of the optical fiber, a scattered optical signal; and   analysing the scattered optical signal using a scattering measurement unit to determine strain in the optical fiber.       

     In one embodiment, the plurality of stress applying parts includes a pair of rods fabricated from at least one of borosilicate (B 2 O 3 +SiO 2 ), Al 2 O 3 +La 2 O 3 +SiO 2  or F+SiO 2  or air holes and are placed in the cladding layer to produce intensified symmetric shear stress upon application of external pressure. Advantageously, this helps to increase the strain and pressure sensitivity of the optical fiber. 
     Advantageously the symmetrical arrangement of the stress applying parts around the core helps to prevent birefringence, dissimilar modal polarization sensitivity and polarization mode dispersion of the optical signal transmitted through the optical fiber. 
     Advantageously the optical fiber provides improved strain sensitivity compared to a standard single-mode optical fiber. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       It will be convenient to further describe the present invention with respect to the accompanying drawings that 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. 
         FIG.  1    illustrates a schematic sectional view of an optical fiber having symmetric stress applying parts according to an embodiment of the invention. 
         FIG.  2    is a plot illustrating the frequency shift in the scattered optical signal through the optical fiber in  FIG.  1    upon application of varying external pressures. 
         FIG.  3    illustrates a flow diagram showing the steps involved in a method of measuring pressure along a length of an optical fiber shown in  FIG.  1    according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With regard to  FIG.  1    there is illustrated an optical fiber  10  having a plurality of symmetric stress applying parts  16  according to an embodiment of the invention. The optical fiber  10  comprises a core  12  for guiding optical signals along a length of the core  12  and a cladding layer  14  including the plurality of stress applying parts  16  disposed around the core  12 . The plurality of stress applying parts  16  are disposed longitudinally parallel to and symmetrically around the core  12 , within the cladding layer  14  to induce intensified symmetric shear stress upon application of external pressure or force on the optical fiber  10 . 
     In one embodiment, the optical fiber  10  is used for the measurement of pressure or force along a length of the optical fiber  10 . The core  12  of the optical fiber  10  is fabricated from germanium doped silica (GeO 2 +SiO 2 ) having a refractive index n 1 . The core  12  is surrounded by the cladding layer  14  fabricated from silica (SiO 2 ) having a refractive index of n 2 , (n 1 &gt;n 2 ). In one embodiment, the stress applying parts  16  disposed within the cladding layer  14  are fabricated from borosilicate (B 2 O 3 +SiO 2 ) rods. In alternate embodiments, the stress applying parts  16  are fabricated from Al 2 O 3 +La 2 O 3 +SiO 2  or F+SiO 2  rods. In another embodiment, air holes are provided as the stress applying parts  16 . The difference in mechanical properties of the silica based cladding layer  14  and the stress applying parts  16  produces intensified symmetrical shear stress upon application of homogeneous external pressure or force on the optical fiber  10 . Advantageously, this helps to increase the strain and pressure sensitivity of the optical fiber  10  based pressure sensor. 
     In one embodiment, the optical fiber  10  can be utilized for the measurement of external pressure or force in subterranean oil wells and other harsh environments. In an embodiment, the optical fiber  10  can be utilized for the structural health monitoring of civil structures. In a yet another embodiment, the optical fiber  10  can be utilized for the structural health monitoring of mechanical structures such as railway tracks. When the fiber is exposed to hydrostatic pressure, the force is converted to strain and the cable elongates due to the Poisson effect. In the prior art, the stress applying parts have different mechanical properties such that the effects of birefringence can be used to measure pressure. However, according to the invention the symmetric arrangement of the stress applying parts  16  around the core  12  helps to create symmetrical shear stress and to prevent the occurrence of birefringence while passing the optical signals through the core  12 . The pressure or force on the optical fiber  10  is measured by analysing the effect of strain on the scattering of the optical signal transmitted through the core  12 . The absence of birefringence prevents dissimilar modal polarization sensitivity and polarization mode dispersion of the optical signals transmitted through the optical fiber  10  thereby improving the accuracy of measurements for a given strain compared to previously known methods. 
     In one embodiment, a method of fabricating the optical fiber  10  for pressure measurements is disclosed. The method includes the steps of forming a preform comprising the core  12  fabricated from germanium doped silica (GeO 2 +SiO 2 ) having a refractive index n 1 . The core  12  is surrounded by the cladding layer  14  fabricated from silica (SiO 2 ) having a refractive index of n 2 , (n 1 &gt;n 2 ). Two orthogonal pairs of holes, parallel and symmetrical to the core  12  are drilled through the cladding layer  14  to incorporate the stress applying parts  16 . The stress applying parts  16  can be of any desired shape such as cylindrical or polygonal shape. The preform thus formed is drawn or extruded to form a single-mode optical fiber  10  having the core  12  at the centre and the cladding layer  14  including the stress applying parts  16  surrounding the core  12 . 
     In another embodiment, the fabrication of the optical fiber  10  includes the steps of forming the preform of the optical fiber  10  by stacking silica rods around the germanium-doped silica rod within 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 . An orthogonal pair of borosilicate (B 2 O 3 +SiO 2 ), Al 2 O 3 +La 2 O 3 +SiO 2  or F+SiO 2  rods or air holes stacked symmetrically around the germanium-doped silica rod forms the stress applying parts  16 . The stacked rods in the silica tube is then fused and drawn to form the intermediate preform. The intermediate preform thus formed is drawn or extruded to form a single-mode optical fiber  10  having the core  12  at the center and the cladding layer  14  including the stress applying parts  16  surrounding the core  12 . 
     Typically, the optical fiber  10  thus formed has a dimension of approximately 125 μm with the core  12  having a dimension of approximately 8.2 μm and each of the stress applying parts  16  has a dimension of 36 μm. 
     With regard to  FIG.  2    there is illustrated a plot of the frequency shift in the scattered optical signal through the optical fiber  10  upon application of varying external pressures. The difference in mechanical properties 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 lying in planes passing through the core  12  induces intensified symmetrical shear stress upon application of the external pressure or force on the optical fiber  10 . The enhanced symmetrical shear stress on the optical fiber  10  provides magnification of applied force to strain conversion upon the 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 optical fiber (SMF). Further, the optical fiber  10  based pressure sensor with the stress applying parts  16  provides negligible hysteresis compared to the standard single-mode optical fiber upon application of external force. 
     With regard to  FIG.  3    there is illustrated a flow diagram showing the steps involved in a method of measuring pressure using the present optical fiber  10 , according to an embodiment of the invention. The method of measuring pressure or force along a length of the optical fiber  10  comprises the step of providing the optical fiber  10  having the core  12 , cladding layer  14  surrounding the core  12  and the plurality of stress applying parts  16  placed in the cladding layer  14  parallel to and symmetrically around the core  12 , as shown in block  100 . An optical signal is received at a first end of the optical fiber  10 , as in block  102 . The optical fiber  10  transmits the received optical signal through the core  12  without inducing birefringence, as shown in block  104 . The optical signal is transmitted through the optical fiber  10  in absence of birefringence, dissimilar modal polarization sensitivity and polarization mode dispersion. The optical signal transmitted through the optical fiber  10  is scattered with magnitude affected by the strain on the optical fiber  10 . The scattered optical signal is received at the first end of the optical fiber  10 , as in block  106 . A scattering measurement unit is utilized to analyse the scattered optical signal to determine the strain in the optical fiber  10 , as shown in block  108 . In one embodiment, the scattering measurement unit identifies the unique scattering spectrum or intensity of the scattered optical signal and subsequently determine the strain in the optical fiber  10 . The orthogonal pair of stress applying parts  16  such as the Borosilicate rods  16  placed in the cladding layer  14  induces intensified symmetric shear stress upon application of external pressure, which in turn provides improved strain sensitivity compared to a standard single-mode optical fiber. 
     It will be appreciated by persons skilled in the art that the present optical fiber based pressure sensor may also include additional symmetrical stress applying parts around the core to further improve the strain sensitivity. 
     It will also be appreciated by persons skilled in the art that the present invention may also include further additional modifications made to the fiber or method which does not affect the overall functioning of the fiber or method.