referring now to fig1 a and 1b , a capillary - action - type optical fiber pressure sensor is shown . the optical fiber 12 may consist of a core 13 and a cladding 14 . although the most commonly used core material is poly ( methyl methacrylate ) ( pmma ), the core may also be composed of other suitable materials such as an azypolymer or an amorphous perfluorinated polymer . the cladding 14 may consist of a fluorinated polymer or other suitable materials such as polyethylene ( pe ), polyvinyl chloride ( pvc ), propylene , crosslinked polyethylene , polyamide , silica , or fluoride - or phosphate - based glass fibers . the optical fiber 12 may include a distal end 15 and a proximal end 16 , and may either be a solid - core optical fiber or a hollow optical fiber . if a solid core optical fiber is used , one or more longitudinal evacuated ( hollow ) columns 18 may be machined or etched into the distal end 15 of the optical fiber 12 . the one or more columns 18 are referred to as “ evacuated ” or “ hollow ,” regardless of whether they may contain a liquid or other substance . the one or more evacuated columns 18 may either extend longitudinally along the core 13 for the entire length of the optical fiber 12 ( from the distal end 15 to the proximal end 16 ; as shown in fig1 a and 1b ), or may be a “ blind ” column that extends longitudinally along the core 13 of the optical fiber 12 for a distance from the distal end 15 but not reaching the proximal end 16 ( as shown in fig2 ). the distance over which the one or more blind columns 18 extend may be dependent on the needs of the user , the environment in which the optical fiber pressure sensor 10 will be used , the material of the optical fiber , etc . additionally , the one or more evacuated columns 18 may either have a single diameter or varying diameters along their length . if varying diameters are used , the one or more evacuated columns 18 may comprise discrete diameter values or diameter values that gradually increase or decrease along their length , so that the one or more evacuated columns 18 may have unique geometries . furthermore , the diameter of the one or more evacuated columns may have shapes other than a circle . for example , a blind column 18 may have an elliptical shape with a polygonal cross section . the one or more evacuated columns 18 may contain the gas 20 of the gaseous environment , which may freely ingress and egress when the optical fiber 12 is not submerged in a liquid . alternatively , the one or more evacuated columns 18 may contain one or more solids , liquids , or gels . under the theory of capillary action , the forces of adhesion and cohesion with a liquid enable the liquid to flow against gravity and upward into a capillary tube , such as the one or more evacuated columns 18 of the optical fiber pressure sensor 10 . not only may capillary action be used to measure liquid pressure , but it may also be used to keep the one or more solids , liquids , or gels contained within the one or more evacuated columns 18 . the distal end 15 of the optical fiber pressure sensor 10 may be placed into a liquid 22 whose pressure is to be measured using capillary action . the height the liquid reaches within the one or more evacuated columns 18 ( as shown in fig1 b ), or penetration , is a function of the pressure exerted by and the surface tension of the liquid 22 . the diameter of the one or more evacuated columns 18 may be sized to allow for capillary action ; that is , the diameter of the one or more evacuated columns 18 may be of a size that enables cohesive and adhesive forces of the liquid allow the liquid to rise and trap the gas 20 within the one or more evacuated columns 18 . for example , the diameter of the one or more evacuated columns 18 may be between 5 nm and 10 mm . in addition to the diameter of the one or more evacuated columns 18 , however , the material of the fiber 12 and its interaction with the liquid 22 may be considered . the material of the fiber 12 must interact with the liquid 22 so that the wetting angle ( the angle at which the liquid meets the solid surface ) of the liquid 22 is greater than 90 degrees to optimize liquid 22 penetration and ensure that the liquid 22 remains in the one or more evacuated columns 18 . if the material of the optical fiber 12 and the liquid 22 do not interact to produce a wetting angle of at least 90 degrees , a coating material 24 may be applied to the surface of the one or more evacuated columns 18 . the coating material may be applied in a thin film , such as a film with a thickness of approximately 75 nm or less . referring now to fig2 , the liquid pressure may be measured using a device 28 such as an interferometer ( such as a fabry - perot interferometer ), spectrometer , or optical coherence tomograph , which are well known in the art . as shown in fig2 , the evacuated column 18 may be a blind column that does not extend the full length of the optical fiber 12 . the pressure at which the liquid 22 enters the one or more evacuated columns 18 , may serve as the reference point for the device 28 . this device may be an interferometer or spectrometer , which are well known in the art . the device 28 may be coupled to the proximal end 16 of the optical fiber 12 . the device 28 may send waves of light ( generally depicted with arrows ) longitudinally through the core 13 of the optical fiber 12 to the distal end 15 of the optical fiber 12 , and may detect the waves of light ( generally depicted with arrows ) that are transmitted back to the device 28 . ( general directions of light are depicted with arrows in fig2 .) the volume of liquid 22 within the one or more evacuated columns 18 will affect the waves of light that are transmitted to and detected by the device 28 . the device 28 will then correlate the amount of transmitted light to the penetration of the liquid 22 within the one or more evacuated columns 18 , and then communicate the comparison as a pressure reading . now referring to fig3 a and 3b , a different measurement method may be conducted using capillary action . as an alternative to the pressure measuring system discussed above , environmental pressure may be measured using interferometry , spectrometry , or optical coherence tomography ( oct ) using the interface between a gaseous or liquid environment and one or more solids , liquids , or gels 29 contained within the one or more evacuated columns 18 of the optical fiber pressure sensor 10 . the one or more solids , liquids , or gels 29 may be prevented from falling out of the one or more evacuated columns 18 by the forces of adhesion and cohesion that occur in capillary action , and essentially create a membrane 30 . the location of this membrane 30 may be affected by the environmental pressure , the location being detected and correlated to a pressure measurement by interferometery , spectrometery , or oct . for example , as pressure increases , the membrane 30 may be located higher within the one or more evacuated columns 18 ( as shown in fig3 b ) than under lower pressure conditions ( as shown in fig3 a ). referring now to fig4 a and 4b , an optical - grating - type optical fiber pressure sensor is shown . as shown in fig4 a and 4b , the optical fiber pressure sensor may be a solid piece of material . the optical fiber 12 includes a distal end 15 and a proximal end 16 , and the distal end 15 includes a distal face 15 a . the distal face 15 a may be a cross section of the optical fiber 12 , and may be coated ( for example , if at least a portion of the distal face 15 a includes a thin layer of reflective coating material ) or uncoated . an optical grating 34 comprising a plurality of evenly spaced grooves 36 may be machined or etched onto the outer surface of the optical fiber 12 , which may diffract incident light . the plurality of grooves 36 may be approximately 5 μm or less in depth . additionally or alternatively , the optical grating 34 may comprise a frosting or increased opacity of certain areas within the fiber 12 at the distal end 15 , areas of ink deposition , or a thin layer of gold or other metal . the optical grating 34 may be located on the distal face 15 a of the fiber 12 . the distance 38 between the grooves 36 of the optical grating 34 may vary with pressure , and the magnitude of the variation may be a function of the material of the optical fiber 12 ( i . e . the compressibility of the material from which the optical fiber 12 is composed ). as pressure of the environment increases , the optical fiber 12 may become compressed and in turn reduce the distance 38 between the grooves 36 of the optical grating 34 ( as shown in fig4 b ). the insets of fig4 a and 4b show an exaggerated view of the reduction in distance 38 between the grooves 36 as affected by pressure . the reduction in distance 38 is a dynamic respond in direct correlation to pressure changes . as the distance 38 changes , the spectrum of light 30 diffracted by the optical grating 34 may also change . however , whatever the distance 38 between the grooves 36 , the distance 38 will be such that light is diffracted at wavelengths within the visible spectrum of light . the change of distance 38 between the grooves 36 affect the angle of diffraction of light , which can be measured with a spectrometer . even a small change in the distance 38 can result in a significant change in the diffraction angle . for example , if the wavelength of light is approximately 600 nm , a distance between the grooves 36 of approximately 1200 nm will result in a maximum angle of diffraction of 30 °. if the distance between the grooves 36 is increased by 5 nm , the angle of diffraction becomes approximately 29 . 86 °, which represents a change of more than 4 %. in this way , environmental pressure may be measured using interferometry , spectrometry , or optical coherence tomography ( oct ). referring now to fig5 the liquid pressure may be measured using a device 28 such as a spectrometer , which is well known in the art . the device 28 may be coupled to the proximal end 16 of the optical fiber 12 . the device 28 may send waves of light ( generally depicted with arrows ) longitudinally through the core 32 of the optical fiber 12 to the distal end 15 of the optical fiber 12 , and may detect the spectrum of light ( generally depicted with arrows ) that is transmitted back to the device 28 . the device 28 may correlate the changes in the transmitted spectrum of light with changes in the distance 38 between the grooves 36 of the optical grating 34 . the device 28 may then communicate the comparison as a pressure reading . it will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above . in addition , unless mention was made above to the contrary , it should be noted that all of the accompanying drawings are not to scale . a variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention , which is limited only by the following claims .
the present invention relates to optical fiber pressure sensors . in one embodiment , liquid pressure of an environment may be measured based on capillary action occurring within an optical fiber having one or more hollow columns within . in another embodiment , the change in environmental pressure may be measured based on light diffracted from an optical fiber having optical grating . in either embodiment , the pressure sensor is manufactured from a single optical fiber .