WO2012156714A1 - Tapered fibre optic, sensor and method of use - Google Patents

Tapered fibre optic, sensor and method of use Download PDF

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Publication number
WO2012156714A1
WO2012156714A1 PCT/GB2012/051066 GB2012051066W WO2012156714A1 WO 2012156714 A1 WO2012156714 A1 WO 2012156714A1 GB 2012051066 W GB2012051066 W GB 2012051066W WO 2012156714 A1 WO2012156714 A1 WO 2012156714A1
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WO
WIPO (PCT)
Prior art keywords
fibre optic
liquid
end part
light
cross
Prior art date
Application number
PCT/GB2012/051066
Other languages
French (fr)
Inventor
Matthew PARTRIDGE
Michael A. Collins
Renata JARZEBINSKA
Original Assignee
Leksing Limited
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Filing date
Publication date
Application filed by Leksing Limited filed Critical Leksing Limited
Publication of WO2012156714A1 publication Critical patent/WO2012156714A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/55—Specular reflectivity
    • G01N21/552—Attenuated total reflection
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
    • G01D5/3537—Optical fibre sensor using a particular arrangement of the optical fibre itself
    • G01D5/3538—Optical fibre sensor using a particular arrangement of the optical fibre itself using a particular type of fiber, e.g. fibre with several cores, PANDA fiber, fiber with an elliptic core or the like
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
    • G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
    • G01F23/28—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring the variations of parameters of electromagnetic or acoustic waves applied directly to the liquid or fluent solid material
    • G01F23/284—Electromagnetic waves
    • G01F23/292—Light, e.g. infrared or ultraviolet

Definitions

  • This invention relates to a tapered fibre optic, a sensor utilising the fibre optic, and a method of use.
  • the sensor is suitable in particular for measuring or testing the properties of the surface of a liquid.
  • Fibre optics are often utilised for the transmission of data such as telephone cabling, the optical signal travelling along the cable by virtue of repeated total internal reflections at the boundary between the fibre and its surrounding cladding. In data transmission applications it is intended that little or none of the transmitted energy passes into the cladding since that reduces the strength of the transmitted signal. The total internal reflection is dependent upon the relative refractive indices of the materials at either side of the fibre optic boundary. The cladding material, and in particular its refractive index, is therefore of great significance in the operation of the fibre optics. Fibre optics used for data transmission are usually of substantially consistent cross-section.
  • Fibre optics are also utilised as sensors, and in such applications the fibre optics may be tapered, i.e. the fibre optics have a varying cross-sectional diameter or are otherwise modified using gratings and similar devices incorporated into the internal structure of the fibre to allow energy to pass out of the fibre.
  • Sensor applications utilise the fact that some of the transmitted energy passes out of the fibre optic and into the surrounding material. Changes in the material surrounding the fibre optic, and/or the presence or absence of a material surrounding the fibre optic, can affect the transmitted energy, and can be detected to provide information about the surrounding material.
  • Fibre optics are acutely sensitive to mechanical interference. This makes fibre optics suitable for use also as strain gauges, for example, the application of tension to the fibre optic altering the transmission of light therealong.
  • the present invention seeks to provide an improved testing apparatus utilising a tapered fibre optic.
  • a tapered fibre optic having a first end part with a first cross-sectional dimension, a second end part with a second cross-sectional dimension, and a central part between the first end part and the second end part, the central part having a cross-sectional dimension which is smaller than the first and second cross-sectional dimensions, the central part of the fibre optic having a predetermined curvature.
  • Providing a curved tapered fibre optic, and in particular a fibre optic in which the central (narrowed) part is curved increases the utility of the fibre optic in sensing applications, and furthermore enables the sensitivity of the sensor to be increased.
  • the fibre optic is intended for use with at least a portion of its central part surrounded by a material under test, the fibre optic being sensitive to the portion which is surrounded, whereby changes in the length of the surrounded portion affect the transmission of light through the fibre optic and can thereby be detected.
  • the fibre optic sensor is particularly suitable for the sensing of the level of a liquid within a container.
  • the central part of the fibre optic can be arranged to lie below the end parts, and in particular below the level of the liquid.
  • the curvature of the fibre optic results in the fibre optic passing through the surface of the liquid at an angle, and preferably a shallow angle. Changes in the level (height) of the liquid meniscus relative to the fibre optic therefore result in a change in the length of the fibre optic which lies within the liquid, which change in length can be detected by a change in the transmission of light along the fibre optic.
  • the fibre optic sensor is also suitable for determining changes in the surface tension or surface pressure of a liquid.
  • the surface tension of the liquid will cause a meniscus to form around the fibre optic, the size of the meniscus, and therefore the total length of the fibre optic which is surrounded by the liquid, being dependent upon the liquid's surface tension. Changes in the liquid's surface tension will cause changes in the size of the meniscus and therefore changes in the transmission of light along the fibre optic.
  • the fibre optic sensor is also particularly suitable for determining properties of a monolayer of material, as commonly used in Langmuir testing procedures. Such testing procedures utilise a shallow container filled primarily with a carrier such as water, with a small amount of the material to be tested floating thereupon. It is arranged that the amount of material to be tested is so small that a single layer (or monolayer) of molecules of the material exists on the carrier surface.
  • the surface tension of the monolayer can be determined by the fibre optic sensor of the present invention.
  • the monolayer could be tested by a "Wilhelmy plate", specifically a plate suspended from a sensitive weighing machine with part of the plate beneath the surface of the liquid and another part of the plate above the surface of the liquid.
  • the surface tension of the monolayer causes a tensile force upon the plate, which can be determined by measuring the effective weight of the plate.
  • the invention according to one aspect therefore provides a new type of surface tension or surface pressure sensor based on the measurement of the meniscus forming properties of a liquid surface.
  • the invention according to this aspect takes advantage of the fact that, as the surface tension of a liquid changes, the meniscus that the liquid forms at the interface with a surface, also changes.
  • the sensor is based on the optical measurement of this change in the shape and size of the meniscus using a fixed fibre optic sensor. Tapered fibres have been previously used for sensing due to their high sensitivity to changes in the refractive index of the surrounding material.
  • Fig.1 shows a view of part of the fibre optic sensor of the present invention, in use in a preferred application
  • Fig.2 shows an enlarged view of the circled part of Fig .1 .
  • the fibre optic sensor 10 comprises a tapered fibre optic 12 having a first end part 14 and a second end part 16. Between the end parts 14, 16 is a central part 20. Notwithstanding the reference to “end parts”, it is not necessary that the end parts be at or adjacent to the terminal ends of the fibre optic. It is, however, necessary that the central part lie between the respective end parts.
  • the central part 20 is substantially equally spaced from the first end part 14 and the second end part 16, but that is not necessarily the case, and in other embodiments the central part is closer to the first end part, or closer to the second end part, as desired for the particular application.
  • the first end part 12 has a first cross-sectional dimension
  • the second end part 16 has a second cross-sectional dimension
  • the first end part 14 and the second end part 16 both having a larger cross-sectional dimension than the central part 20.
  • the first cross-sectional dimension and the second cross- sectional dimension are the same, but that is not necessary in all applications.
  • the fibre optic 12 is of circular cross-section so that the cross-sectional dimensions are diameters, and whilst a circular cross-section is typical of fibre optics that is not necessarily the case for the performance of the invention.
  • a tapered fibre optic which is suitable for use in the invention is available from Fibercore, of University Parkway, Southampton Science Park, Victoria, SO16 7QQ, England and is identified as fibre type SM750.
  • the invention is, however, not limited to the use of this particular fibre, and it can utilise both multi- mode and single-mode fibres.
  • suitable fibres may be made of plastics, doped glass, or any other material capable of supporting the total internal reflection required.
  • Fig.1 also includes a schematic representation of a light source 22 and a light detector 24, connected to the respective terminal ends of the fibre optic in known fashion.
  • the light source 22 emits light at a predetermined intensity, and in a predetermined wavelength range, either continuously or stroboscopically, as desired.
  • the light detector 24 can detect the light being transmitted through the fibre optic 12, and in particular can detect changes in the light being transmitted.
  • one end of the fibre optic carries a mirror and the other end carries the source and detector.
  • the light source 22 and the light detector 24 are connected to the opposed ends of the fibre optic 12.
  • the fibre optic 12 is flexible and the light source 22 and the light detector 24 are fixed in their relative positions. Since the light source 22 and light detector 24 are separated by a distance less than the length of the fibre optic 12 they cause the fibre optic 12, and in particular its central part 20, to adopt the desired curvature.
  • the fibre optic is substantially inflexible and is formed so that its central part has the desired curvature.
  • the sensor 10 can for example be used to determine the depth D of the liquid 26 by detecting changes in the transmitted light.
  • the depth D of the liquid changes, the length L of the fibre optic which is surrounded by the liquid 26 changes correspondingly.
  • the transmission of light from the source 22 to the detector 24 is dependent upon the total internal reflections which occur at the boundary between the fibre optic 12 and the air 30 (for that length of the fibre optic 12 which is not submerged in the liquid), and the total internal reflections which occur at the boundary between the fibre optic 12 and the liquid 26 (for the length L of the fibre optic 12 which is submerged in the liquid).
  • the fibre optic sensor 10 can be calibrated for different depths D of liquid 12, i.e. the sensor can have a look-up table or the like whereby a particular value of transmitted light energy can be translated into a particular depth.
  • the curvature of the fibre optic 12 does not significantly affect its ability to transmit light. In any event, however, it is intended that the fibre optic 12 be fixed in position during use (and be calibrated in that position), so that the curvature will not change in use. The curvature does, however, permit the light source 22 and the light detector 24 to be placed above the surface of the liquid 26.
  • Fig.2 shows an enlarged view of the circled part of Fig .1 , and in particular one of the two positions at which the fibre optic 12 engages the surface of the liquid 26.
  • the surface tension in the liquid 26 causes a meniscus 32 to form around the fibre optic 12, i.e. the liquid 26 "creeps" up the fibre optic 12.
  • the curvature of the meniscus, and the degree to which the liquid will creep up the fibre optic, is dependent upon the surface tension or surface pressure of the liquid 26, and changes in the liquid 26 can be detected by way of changes in the surface tension manifested by changes in the length L. It will be understood that a shallow angle a results in a relatively large change in the (liquid) length L for a small change in the surface tension, thereby increasing the sensitivity of the sensor to the liquid's surface tension.
  • the liquid 26 can if desired be water or other carrier in a Langmuir trough.
  • the surface of the liquid will in that case carry a monolayer of a material under test, for example stearic acid.
  • Useful information about the stearic acid, and changes in the stearic acid, can be determined by way of its surface tension and the resulting submerged length L of the fibre optic 12.
  • the application described above utilises the interface between a liquid and air, but other applications can utilise the interface between two liquids, i.e. the device can be used to measure the surface tension of a first liquid which is underneath a second liquid.
  • one of the light source and light detector could be immersed in the liquid, or else those embodiments utilising a mirror or grating serving a similar purpose at one end of the fibre optic could have the mirror or grating immersed. Such embodiments are not preferred, however, as they will be less sensitive to changes in the liquid surface (since the fibre optic will pass through the liquid surface only once).
  • tapering of the fibre optic promotes the escape of energy from the fibre optic and thereby increases the utility of the fibre optic in sensing applications. Tapering is not, however, the only means of promoting the escape of energy and it would be possible to use an alternative means in addition to, or instead of, providing a tapering fibre optic.
  • a grating structure can be incorporated into the fibre structure, it being known that a grating structure can also promote the escape of energy from the fibre.

Abstract

This invention relates to a tapered fibre optic (12), a sensor (10) utilising the fibre optic, and a method of use. The tapered fibre optic has a first end part (14) with a first cross- sectional dimension, a second end part (16) with a second cross - sectional dimension, and a central part (20) between the first end part and the second end part, the central part having a cross - sectional dimension which is smaller than the first and second cross - sectional dimensions. The central part of the fibre optic has a predetermined curvature and is adapted to be immersed in a liquid (26), the total internal reflection of the fibre optic depending upon the liquid. The sensor comprises the tapered fibre optic, a light source (22) and a light detector (24). The sensor can be used to determine changes in the liquid level, or changes in the surface properties of the liquid, or of a monolayer floating on the surface of the liquid.

Description

TAPERED FIBRE OPTIC, SENSOR AND METHOD OF USE
FIELD OF THE INVENTION
This invention relates to a tapered fibre optic, a sensor utilising the fibre optic, and a method of use. The sensor is suitable in particular for measuring or testing the properties of the surface of a liquid.
BACKGROUND TO THE INVENTION
Fibre optics are often utilised for the transmission of data such as telephone cabling, the optical signal travelling along the cable by virtue of repeated total internal reflections at the boundary between the fibre and its surrounding cladding. In data transmission applications it is intended that little or none of the transmitted energy passes into the cladding since that reduces the strength of the transmitted signal. The total internal reflection is dependent upon the relative refractive indices of the materials at either side of the fibre optic boundary. The cladding material, and in particular its refractive index, is therefore of great significance in the operation of the fibre optics. Fibre optics used for data transmission are usually of substantially consistent cross-section.
Fibre optics are also utilised as sensors, and in such applications the fibre optics may be tapered, i.e. the fibre optics have a varying cross-sectional diameter or are otherwise modified using gratings and similar devices incorporated into the internal structure of the fibre to allow energy to pass out of the fibre. Sensor applications utilise the fact that some of the transmitted energy passes out of the fibre optic and into the surrounding material. Changes in the material surrounding the fibre optic, and/or the presence or absence of a material surrounding the fibre optic, can affect the transmitted energy, and can be detected to provide information about the surrounding material.
Fibre optics are acutely sensitive to mechanical interference. This makes fibre optics suitable for use also as strain gauges, for example, the application of tension to the fibre optic altering the transmission of light therealong.
SUMMARY OF THE INVENTION
The present invention seeks to provide an improved testing apparatus utilising a tapered fibre optic.
According to the present invention there is provided a tapered fibre optic having a first end part with a first cross-sectional dimension, a second end part with a second cross-sectional dimension, and a central part between the first end part and the second end part, the central part having a cross-sectional dimension which is smaller than the first and second cross-sectional dimensions, the central part of the fibre optic having a predetermined curvature.
Providing a curved tapered fibre optic, and in particular a fibre optic in which the central (narrowed) part is curved, increases the utility of the fibre optic in sensing applications, and furthermore enables the sensitivity of the sensor to be increased.
The fibre optic is intended for use with at least a portion of its central part surrounded by a material under test, the fibre optic being sensitive to the portion which is surrounded, whereby changes in the length of the surrounded portion affect the transmission of light through the fibre optic and can thereby be detected.
The fibre optic sensor is particularly suitable for the sensing of the level of a liquid within a container. The central part of the fibre optic can be arranged to lie below the end parts, and in particular below the level of the liquid. The curvature of the fibre optic results in the fibre optic passing through the surface of the liquid at an angle, and preferably a shallow angle. Changes in the level (height) of the liquid meniscus relative to the fibre optic therefore result in a change in the length of the fibre optic which lies within the liquid, which change in length can be detected by a change in the transmission of light along the fibre optic.
The fibre optic sensor is also suitable for determining changes in the surface tension or surface pressure of a liquid. Thus, the surface tension of the liquid will cause a meniscus to form around the fibre optic, the size of the meniscus, and therefore the total length of the fibre optic which is surrounded by the liquid, being dependent upon the liquid's surface tension. Changes in the liquid's surface tension will cause changes in the size of the meniscus and therefore changes in the transmission of light along the fibre optic.
The fibre optic sensor is also particularly suitable for determining properties of a monolayer of material, as commonly used in Langmuir testing procedures. Such testing procedures utilise a shallow container filled primarily with a carrier such as water, with a small amount of the material to be tested floating thereupon. It is arranged that the amount of material to be tested is so small that a single layer (or monolayer) of molecules of the material exists on the carrier surface. The surface tension of the monolayer can be determined by the fibre optic sensor of the present invention. Previously, the monolayer could be tested by a "Wilhelmy plate", specifically a plate suspended from a sensitive weighing machine with part of the plate beneath the surface of the liquid and another part of the plate above the surface of the liquid. The surface tension of the monolayer causes a tensile force upon the plate, which can be determined by measuring the effective weight of the plate. The invention according to one aspect therefore provides a new type of surface tension or surface pressure sensor based on the measurement of the meniscus forming properties of a liquid surface. The invention according to this aspect takes advantage of the fact that, as the surface tension of a liquid changes, the meniscus that the liquid forms at the interface with a surface, also changes. The sensor is based on the optical measurement of this change in the shape and size of the meniscus using a fixed fibre optic sensor. Tapered fibres have been previously used for sensing due to their high sensitivity to changes in the refractive index of the surrounding material. It has been found that these sensitive fibres can be further enhanced to detect minute changes in a liquid meniscus by mounting them at a shallow angle to the liquid surface. Using this shallow angle curved tapered fibre optic it has been found to be possible to accurately measure the change in the meniscus and subsequently demonstrate good correlation to surface pressure measurements recorded using a Wilhelmy plate. This has been demonstrated in practice using an invented fibre optic sensor mounted in a Langmuir trough during Stearic acid and Calix-4- resorcarene isotherms.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention will now be described in more detail, by way of example, with reference to the accompanying schematic drawing, in which:
Fig.1 shows a view of part of the fibre optic sensor of the present invention, in use in a preferred application; and
Fig.2 shows an enlarged view of the circled part of Fig .1 .
DETAILED DESCRIPTION
The fibre optic sensor 10 comprises a tapered fibre optic 12 having a first end part 14 and a second end part 16. Between the end parts 14, 16 is a central part 20. Notwithstanding the reference to "end parts", it is not necessary that the end parts be at or adjacent to the terminal ends of the fibre optic. It is, however, necessary that the central part lie between the respective end parts.
In this embodiment the central part 20 is substantially equally spaced from the first end part 14 and the second end part 16, but that is not necessarily the case, and in other embodiments the central part is closer to the first end part, or closer to the second end part, as desired for the particular application.
The first end part 12 has a first cross-sectional dimension, the second end part 16 has a second cross-sectional dimension, the first end part 14 and the second end part 16 both having a larger cross-sectional dimension than the central part 20. In this embodiment the first cross-sectional dimension and the second cross- sectional dimension are the same, but that is not necessary in all applications. Also, in this embodiment the fibre optic 12 is of circular cross-section so that the cross-sectional dimensions are diameters, and whilst a circular cross-section is typical of fibre optics that is not necessarily the case for the performance of the invention.
A tapered fibre optic which is suitable for use in the invention is available from Fibercore, of University Parkway, Southampton Science Park, Southampton, SO16 7QQ, England and is identified as fibre type SM750. The invention is, however, not limited to the use of this particular fibre, and it can utilise both multi- mode and single-mode fibres. Also, suitable fibres may be made of plastics, doped glass, or any other material capable of supporting the total internal reflection required.
Fig.1 also includes a schematic representation of a light source 22 and a light detector 24, connected to the respective terminal ends of the fibre optic in known fashion. The light source 22 emits light at a predetermined intensity, and in a predetermined wavelength range, either continuously or stroboscopically, as desired. The light detector 24 can detect the light being transmitted through the fibre optic 12, and in particular can detect changes in the light being transmitted. In another embodiment according to the invention, one end of the fibre optic carries a mirror and the other end carries the source and detector.
It will be understood that references herein to "light" are not necessarily limited to visible light, although visible light can be used in many applications. The present invention is therefore not limited to particular wavelengths within the electromagnetic spectrum.
In known fashion, the light source 22 and the light detector 24 are connected to the opposed ends of the fibre optic 12. In this embodiment, the fibre optic 12 is flexible and the light source 22 and the light detector 24 are fixed in their relative positions. Since the light source 22 and light detector 24 are separated by a distance less than the length of the fibre optic 12 they cause the fibre optic 12, and in particular its central part 20, to adopt the desired curvature. In other embodiments the fibre optic is substantially inflexible and is formed so that its central part has the desired curvature.
In common with other fibre optics used as sensors, it is necessary that a proportion of the energy of the transmitted light be transferred into the surrounding medium. The sensor 10 can for example be used to determine the depth D of the liquid 26 by detecting changes in the transmitted light. Thus, as the depth D of the liquid changes, the length L of the fibre optic which is surrounded by the liquid 26 changes correspondingly. The transmission of light from the source 22 to the detector 24 is dependent upon the total internal reflections which occur at the boundary between the fibre optic 12 and the air 30 (for that length of the fibre optic 12 which is not submerged in the liquid), and the total internal reflections which occur at the boundary between the fibre optic 12 and the liquid 26 (for the length L of the fibre optic 12 which is submerged in the liquid). Since the refractive index of air 30 will differ from the refractive index of the liquid 26, changes in the length L will affect the transmission of light along the fibre optic, which changes can be detected by the detector 24. The fibre optic sensor 10 can be calibrated for different depths D of liquid 12, i.e. the sensor can have a look-up table or the like whereby a particular value of transmitted light energy can be translated into a particular depth. The curvature of the fibre optic 12 does not significantly affect its ability to transmit light. In any event, however, it is intended that the fibre optic 12 be fixed in position during use (and be calibrated in that position), so that the curvature will not change in use. The curvature does, however, permit the light source 22 and the light detector 24 to be placed above the surface of the liquid 26. It also permits the angle a at which the fibre optic 12 engages the surface of the liquid 26 to be relatively shallow. It will be understood that a shallow angle a results in a relatively large change in the (liquid) length L for a small change in the depth D, thereby increasing the sensitivity of the sensor to liquid depth. Fig.2 shows an enlarged view of the circled part of Fig .1 , and in particular one of the two positions at which the fibre optic 12 engages the surface of the liquid 26. The surface tension in the liquid 26 causes a meniscus 32 to form around the fibre optic 12, i.e. the liquid 26 "creeps" up the fibre optic 12. The curvature of the meniscus, and the degree to which the liquid will creep up the fibre optic, is dependent upon the surface tension or surface pressure of the liquid 26, and changes in the liquid 26 can be detected by way of changes in the surface tension manifested by changes in the length L. It will be understood that a shallow angle a results in a relatively large change in the (liquid) length L for a small change in the surface tension, thereby increasing the sensitivity of the sensor to the liquid's surface tension.
The liquid 26 can if desired be water or other carrier in a Langmuir trough. The surface of the liquid will in that case carry a monolayer of a material under test, for example stearic acid. Useful information about the stearic acid, and changes in the stearic acid, can be determined by way of its surface tension and the resulting submerged length L of the fibre optic 12. In applications in which the liquid depth D is likely to decrease, or the size of the meniscus 32 is likely to decrease, it may be necessary to treat the surface of the fibre optic 12 with a coating which can prevent (or at least reduce) the liquid sticking to the surface and thereby providing an artificially elevated result or a delay in the detection of a reducing depth D or in the detection of a reducing meniscus 32.
The application described above utilises the interface between a liquid and air, but other applications can utilise the interface between two liquids, i.e. the device can be used to measure the surface tension of a first liquid which is underneath a second liquid.
It will be understood that the invention could also be performed with the light source and light detector immersed in the liquid and with the central part above the liquid.
It will also be understood that one of the light source and light detector could be immersed in the liquid, or else those embodiments utilising a mirror or grating serving a similar purpose at one end of the fibre optic could have the mirror or grating immersed. Such embodiments are not preferred, however, as they will be less sensitive to changes in the liquid surface (since the fibre optic will pass through the liquid surface only once).
It will be appreciated that the tapering of the fibre optic promotes the escape of energy from the fibre optic and thereby increases the utility of the fibre optic in sensing applications. Tapering is not, however, the only means of promoting the escape of energy and it would be possible to use an alternative means in addition to, or instead of, providing a tapering fibre optic. Specifically, a grating structure can be incorporated into the fibre structure, it being known that a grating structure can also promote the escape of energy from the fibre.

Claims

1 . A tapered fibre optic having a first end part with a first cross-sectional dimension, a second end part with a second cross-sectional dimension, and a central part between the first end part and the second end part, the central part having a cross-sectional dimension which is smaller than the first and second cross-sectional dimensions, the central part of the fibre optic having a predetermined curvature.
2. The tapered fibre optic according to claim 1 having a circular cross-section.
3. The tapered fibre optic according to claim 1 or claim 2 in which the first cross-sectional dimension and the second cross-sectional dimension are substantially identical.
4. A sensor incorporating the tapered fibre optic according to any one of claims 1 -3, the sensor including a light source adapted to transmit light along the fibre optic and a light detector adapted the detect the light which has been transmitted along the fibre optic.
5. The sensor according to claim 4 in which the light source is located adjacent to the first end part and the light detector is located adjacent to the second end part.
6. The sensor according to claim 5 in which respective ends of the fibre optic are connected to the light source and the light detector, and the relative positions of the light source and the light detector determine the curvature of the central part.
7. The sensor according to any one of claims 4 to 6 in which the central part of the fibre optic is positioned at a level below the level of the first end part and the second end part.
8. The sensor according to any one of claims 4 to 7 in which the central part of the fibre optic is located within a container.
9. The sensor according to claim 8 in which the light source and the light detector are located outside the container.
10. A method of using a sensor according to any one of claims 4-9, including the steps of:
immersing the central part of the tapered fibre optic in a liquid,
locating the first end part and the second end part out of the liquid, and detecting changes in the transmission of light along the fibre optic.
1 1 . The method according to claim 10 in which the fibre optic passes through the surface of the liquid at an angle.
12. The method according to claim 1 1 in which the angle is acute.
13. The method of any one of claims 10-12 including the additional step of:
determining changes in the level of the liquid by way of the detected changes in the transmission of light.
14. The method of any one of claims 10-12 including the additional step of:
determining changes in the surface tension of the liquid by way of the detected changes in the transmission of light.
15. The method of any one of claims 10-12 including the additional steps of: floating a monolayer of material upon the liquid, and
determining changes in the properties of the monolayer of material by way of the detected changes in the transmission of light.
PCT/GB2012/051066 2011-05-13 2012-05-14 Tapered fibre optic, sensor and method of use WO2012156714A1 (en)

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WO2000017608A1 (en) * 1998-09-18 2000-03-30 European Community Represented By Commission Of The European Communities Fiberoptic coupler sensor and a measurement method
US6192176B1 (en) * 1998-02-23 2001-02-20 General Electric Company Compact optical system with turn and color mixing
US7060964B1 (en) * 2002-11-12 2006-06-13 Ifos, Inc. Reflection-mode fiber sensing devices
WO2007127512A2 (en) * 2006-01-31 2007-11-08 Drexel University Ultra sensitive tapered fiber optic biosensor for pathogens, proteins and dna
GB2454192A (en) * 2007-10-30 2009-05-06 Evanesco Ltd Fibre optic discrimination sensor

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5966477A (en) * 1996-07-25 1999-10-12 Johnson; Douglas M. Fiber optic sensor enclosure system
US6192176B1 (en) * 1998-02-23 2001-02-20 General Electric Company Compact optical system with turn and color mixing
WO2000017608A1 (en) * 1998-09-18 2000-03-30 European Community Represented By Commission Of The European Communities Fiberoptic coupler sensor and a measurement method
US7060964B1 (en) * 2002-11-12 2006-06-13 Ifos, Inc. Reflection-mode fiber sensing devices
WO2007127512A2 (en) * 2006-01-31 2007-11-08 Drexel University Ultra sensitive tapered fiber optic biosensor for pathogens, proteins and dna
GB2454192A (en) * 2007-10-30 2009-05-06 Evanesco Ltd Fibre optic discrimination sensor

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