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WO2022069687A1 - Dispositif de détection à fibre optique microstructurée - Google Patents

Dispositif de détection à fibre optique microstructurée Download PDF

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Publication number
WO2022069687A1
WO2022069687A1 PCT/EP2021/077043 EP2021077043W WO2022069687A1 WO 2022069687 A1 WO2022069687 A1 WO 2022069687A1 EP 2021077043 W EP2021077043 W EP 2021077043W WO 2022069687 A1 WO2022069687 A1 WO 2022069687A1
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Prior art keywords
optical fiber
cladding
refractive index
mode
fiber
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PCT/EP2021/077043
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English (en)
Inventor
Thomas Geernaert
Hugo Thienpont
Olga RUSYAKINA
Tigran Baghdasaryan
Francis Berghmans
Christophe Caucheteur
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Universite de Mons
Vrije Universiteit Brussel VUB
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Universite de Mons
Vrije Universiteit Brussel VUB
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    • 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
    • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
    • G01N21/4133—Refractometers, e.g. differential
    • 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
    • G01N21/75—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated
    • G01N21/77—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator
    • G01N21/7703—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator using reagent-clad optical fibres or optical waveguides
    • G01N21/774—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator using reagent-clad optical fibres or optical waveguides the reagent being on a grating or periodic structure
    • 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
    • G01D5/35306—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 using an interferometer arrangement
    • G01D5/35309—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 using an interferometer arrangement using multiple waves interferometer
    • G01D5/35316—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 using an interferometer arrangement using multiple waves interferometer using a Bragg gratings
    • 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
    • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
    • G01N21/43—Refractivity; Phase-affecting properties, e.g. optical path length by measuring critical angle
    • G01N21/431—Dip refractometers, e.g. using optical fibres
    • G01N2021/432—Dip refractometers, e.g. using optical fibres comprising optical fibres
    • 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
    • G01N21/75—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated
    • G01N21/77—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator
    • G01N21/7703—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator using reagent-clad optical fibres or optical waveguides
    • G01N2021/7706—Reagent provision
    • G01N2021/7709—Distributed reagent, e.g. over length of guide
    • 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
    • G01N21/75—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated
    • G01N21/77—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator
    • G01N2021/7769—Measurement method of reaction-produced change in sensor
    • G01N2021/7776—Index
    • 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
    • G01N21/553—Attenuated total reflection and using surface plasmons
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
    • G02B6/021—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the core or cladding or coating, e.g. materials, radial refractive index profiles, cladding shape
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02319—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by core or core-cladding interface features
    • G02B6/02333—Core having higher refractive index than cladding, e.g. solid core, effective index guiding
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02319—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by core or core-cladding interface features
    • G02B6/02338—Structured core, e.g. core contains more than one material, non-constant refractive index distribution in core, asymmetric or non-circular elements in core unit, multiple cores, insertions between core and clad
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02385—Comprising liquid, e.g. fluid filled holes

Definitions

  • the present invention relates to the field of optical fiber sensors and more particularly relates to refractive index sensing using gratings in microstructured optical fibers.
  • lab-on-fiber bio-sensing probes are based on fiber Bragg grating (FBG) technology, of which tilted FBGs in standard step-index fibers are one particularly promising candidate, as demonstrated for example in Luß, M. et al. "Rapid detection of circulating breast cancer cells using a multiresonant optical fiber aptasensor with plasmonic amplification", ACS sensors 5.2 (2020): 454-463.
  • FBG fiber Bragg grating
  • Tilted FBGs as described in Albert, J., et al. "Tilted fiber Bragg grating sensors", Laser & Photonics Reviews, 7: 83-108 (2013), contain refractive index modulation planes that are slanted at some tilt angle relative to the fiber axis, resulting in the excitation of a set of cladding modes.
  • the excited cladding modes are sensitive to surrounding refractive index changes.
  • the fiber is often covered with a thin layer of metal to allow for the excitation of a surface plasmon wave on the metal/external medium boundary if light of an excited cladding mode has a particular wavelength and propagation angle with respect to the boundary, i.e. when the longitudinal part of the photon momentum matches the momentum of the surface plasmon wave.
  • tilted FBG probes exploiting the surface plasmon resonance effect well-suited for refractive index sensing and bio-sensing applications.
  • SPR surface plasmon resonance
  • a disadvantage of tilted FBG probes is that larger tilt angles are required for the sensing of small effective index fluids surrounding the fiber, e.g. water or gases, via cladding mode resonance. At large tilt angles, the optical bandwidth occupied by the resonant cladding modes of the tilted FBG transmission spectrum is very large, e.g. extending over hundred nanometres or more, which hampers efficient spectral multiplexing of several tilted FBGs along a same probe.
  • the present invention relates to an optical fiber for refractometry of a fluid surrounding the fiber.
  • the fluid may be a liquid.
  • the fluid may be an aqueous substance, e.g. an aqueous solution.
  • the optical fiber comprises a solid core portion, a cladding portion surrounding the core portion, and a tilt-free Bragg grating formed in the core portion.
  • the tilt-free Bragg grating is configured to couple light of a core-guided mode to at least one higher-order cladding mode.
  • a cross-section of the cladding portion is microstructured through an arrangement of holes such that an effective refractive index associated with one of said higher-order cladding modes varies as a function of the surrounding fluid refractive index and is inferior to 1,35.
  • the at least one higher-order cladding mode may have a direction opposite to the propagation direction of the core-guided mode.
  • the tilt-free Bragg grating enables a simplified spectral sensing response. Only a few spectral lines are part of the spectral response curve and are located in a narrow spectral band. Microstructuring the cladding portion of the optical fiber through an arrangement of holes enables the coupling between the core-guided mode and higher-order cladding modes, at least one of which has a refractive index that is inferior to 1,35. Therefore, the optical fiber-based refractive index sensing device is well-suited for biochemical sensing applications in aqueous solutions that have a refractive index that is close to that of water. A smaller index mismatch between the higher-order cladding mode and the surrounding fluid results in an increased sensitivity of the device. A sensitivity slope of the optical fiber with respect to refractive index changes of the surrounding fluid may be at least 10 nm/RIU, preferably at least 20 nm/RIU, for instance at least 50 nm/RIU.
  • the arrangement of holes may be a regular lattice of air holes and the optical fiber may be provided as a photonic crystal fiber.
  • a refractive index profile of the core portion may be non-uniform in the optical fiber cross-section that contains the grating.
  • a plasmonic detection area may be provided on an outer surface of the cladding portion.
  • a metal layer coating of the plasmonic detection area is arranged such that light that is coupled into one of said higher-order cladding modes induces a surface plasmon wave at an outwardly facing boundary of the metal layer coating.
  • the outwardly facing boundary of the metal layer faces away from the cladding portion.
  • the plasmonic detection area may further include a layer of carbon nanomaterial which is provided on the outwardly facing boundary of the metal layer.
  • Carbon nanomaterials have the additional advantage that they can enhance the interaction between the surface plasmon and the surrounding liquid, further improving the sensitivity and/or resolution of the device.
  • the outer surface of the optical fiber that is contacting the surrounding fluid may comprise a selective coating for selectively immobilizing target analytes in the surrounding fluid. This is of benefit in biochemical sensing applications in which a refractive index change occurs only upon binding of a specific target analyte.
  • the optical fiber comprises a plurality of tilt-free Bragg gratings that are formed at different locations along the core portion.
  • Each Bragg grating is configured to couple light at a different wavelength of the core-guided mode to at least one corresponding higher-order cladding mode.
  • Individual Bragg gratings may be configured to excite more than one higher-order cladding mode.
  • the cross-section of the cladding portion of each tilt-free Bragg grating is microstructured through an arrangement of holes such that the effective refractive index of one of the higher-order cladding modes associated with each Bragg grating varies as a function of the surrounding fluid refractive index and is inferior to 1,35.
  • Such embodiments enable spectrally multiplexed refractive index sensing.
  • the spectral response curves associated with each one of the Bragg gratings preferably does not overlap with adjacent response curves.
  • a plurality of individual plasmonic detection areas may be provided on an outer surface of the cladding portion to increase the sensitivity of each Bragg grating location.
  • Each plasmonic detection area is associated with only one corresponding Bragg grating of the plurality of Bragg gratings and comprises a metal layer coating.
  • Each one of these metal layer coating is arranged such that light that is coupled into one of said higher-order cladding modes induces a surface plasmon wave at an outwardly facing boundary of the metal layer coating relative to the cladding portion.
  • a number of excitable cladding modes can be limited by adjusting the fiber microstructure. Consequently, fewer spectral lines, attributable to excited cladding modes, are populating a narrower region of a transmission spectrum recorded with an optical fiber according to embodiments of the invention as compared to tilted fiber Bragg gratings. This simplifies the spectral analysis and enables reliable spectral multiplexing of multiple refractive index sensing sites arranged on the same optical fiber.
  • a spectral position of excitable cladding modes can be controlled by adjusting the fiber microstructure.
  • This allows the design of optimized optical fibers according to embodiments of the invention, in which the spectral region for refractive index sensing is determined by the spectral position of the excitable cladding modes and closely matched to the refractive index of the surrounding fluid.
  • This increases the sensitivity of the refractive index sensing optical fiber, because excited cladding modes tend to be the more sensitive to the surrounding fluid the weaker the refractive index contrast between the cladding mode and the fluid is.
  • refractive index sensing in aqueous solutions can be performed with high sensitivity.
  • the spectral region for refractive index sensing occupied by the excitable higher-order cladding modes can be designed to be in the vicinity of the refractive index of water, for instance within less than 5 %, less than 3 %, or less than 1 % of the refractive index of water at normal temperature and pressure (20°C, 1 atm).
  • Such a spectral region for refractive index sensing is particularly well-suited for bio- or biochemical sensing applications that are frequently carried out in aqueous solutions. Nonetheless, embodiments of the invention may also be used to sense refractive index changes in oil-based fluids.
  • refractive index sensing can be performed with bent-free and unmodified optical fibers, e.g. not tapered or polished, which reduces the loss of light travelling along the fiber and also reduces the risk of wear and damage of the optical fiber.
  • the latter can gain importance if the refractive index sensing is performed in body tissues for instance, where brittle fibers or wear losses could damage the tissues that are sensed.
  • mechanical stability of the optical fiber is also important for the repeated use thereof.
  • Embodiments of the present invention therefore have the advantage of increasing the lifetime of the refractive index sensing optical fiber.
  • unmodified optical fibers may be not drilled or not etched.
  • an unmodified optical fiber may have a cross-section geometry of its core and cladding substantially constant along the axis of propagation and the fiber core may be not exposed.
  • the optical fiber is adapted for contacting the fluid on its outer surface rather than receiving the fluid in the holes of its cladding portion. This facilitates the fabrication of the optical fiber probe and allows for easier reusability thereof the probe, for example after having cleaned the fiber outer surface.
  • the present invention relates to a lab-on-fiber system which comprises an optical fiber according to the first aspect, a light source for supplying light to the core-guided mode of the optical fiber, and a photodetecting means for detecting light transmitted by the optical fiber.
  • the present invention also relates to a method of refractive index sensing in fluids and comprises the steps of: contacting at least one fluid with an optical fiber according to the first aspect, coupling input light of a plurality of wavelengths into the optical fiber, collecting transmitted light at an output of the optical fiber to obtain a transmission spectrum of the optical fiber, determining a spectral shift of an excited higher-order cladding mode in the transmission spectrum of the optical fiber, and deriving from the spectral shift a refractive index of the aqueous solution.
  • FIG. 1 shows an optical fiber for refractometry of a surrounding fluid in cross-sectional view, according to an embodiment of the invention.
  • FIG. 2 is a cross-section of an optical fiber for refractometry with multiple FBGs, according to an embodiment of the invention.
  • FIG. 3 shows transmission spectra before and after gold layer deposition on an optical fiber probe comprising a straight (tilt-free) FBG inscribed into a hexagonal lattice PCF (crosssection in the inset), according to embodiments of the invention.
  • FIG. 4 is a schematic of an experimental setup for refractive index sensing of a fluid surrounding an optical fiber according to embodiments of the invention.
  • FIG. 5 displays the transmission spectra of nine different fluids in a sensing window of an optical fiber in accordance with embodiments of the invention.
  • FIG. 6 is a magnified view on the most sensitive resonance dip in the transmission spectra of FIG. 5, before and after the application of a smoothing filter.
  • FIG. 7 is a graph illustrating the resonant wavelength location and transmission amplitude of an observed resonance dip in the optical fiber transmission spectrum as a function of the surrounding fluid refractive index.
  • FIG. 8 displays the transmission spectra obtained for an optical fiber according to an embodiment of the invention, in which two FBGs were patterned into the different locations of the fiber core portion.
  • FIG. 9 shows how resonance wavelengths for selected modes show different dependencies on temperature variations of water, as can be used for temperature calibration in embodiments according to the present invention.
  • FIG. 10 shows a schematic representation of an experiment of bio-detecgtion of HER2 protein using an embodiment according to the present invention.
  • FIG. 11(a) shows transmission spectra after immersing the single FBG-equipped PCF sensor in HER2 protein with concentration of IO -6 g/mL and
  • FIG. 11(b) shows a zoomed view on the most sensitive resonance dip whose wavelength position is tracked as a function of immersion time. Fitting of the Gaussian function to each dip is shown with a blue color. The arrow connects the first and the last measurements indicating a wavelength red-shift.
  • FIG. 12 shows evolution of the wavelength position vs time when the sensor is exposed to anti-HER2 aptamers and mercaptohexanol for surface blocking
  • FIG. 13 shows evolution of the wavelength position vs time when the sensor is immersed in four concentrations of HER2 protein. In between consecutive concentrations, PBS is used to clean the fiber surface.
  • FIG. 14(a) shows an envelope fitted to the transmission spectra recorded when the single FBG equipped PCF sensor is immersed in IO -6 g/mL of HER2 protein.
  • FIG. 14(b) shows a zoomed view on the envelope dips fitted with Gaussian functions (red color) for tracking their wavelength position as a function of immersion time. The arrow connects the first and the last measurements indicating a wavelength red-shift.
  • FIG. 15 shows the evolution of the wavelength position of an envelope (red) and of a single resonance (black) vs time of sensor's immersion in four concentrations of HER2 protein.
  • FIG. 16 shows the transmission spectrum of the PCF sensor with two spectrally separated FBGs.
  • FIG. 17 shows transmission spectra of two FBGs used for refractive index sensing in aqueous solutions, with resulting sensitivities for (a) bare and (b) gold coated PCF sample.
  • FIG. 18 shows envelope fitting to the transmission spectra recorded when the (a) 1st and (b) 2nd gratings of the multiplexed PCF sensor are immersed in IO -6 g/mL of HER2 protein.
  • Arrows on the left figures indicate the most sensitive resonances.
  • Zoomed views on the envelope dips fitted with Gaussian functions (red color) are shown on the right. The arrows connect the first and the last measurements indicating a wavelength red-shift.
  • FIG. 19 shows the evolution of the wavelength position of an envelope (red) and of a single resonance (black) vs time for the (a) 1st and (b) 2nd FBG immersed in three concentrations of HER2 protein.
  • the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the invention.
  • FIG. 1 a longitudinal cross-section of an optical fiber 10 according to a first embodiment of the invention is depicted.
  • the optical fiber 10 has an elongated cylindrical body and comprises an inner solid core portion 11 and, surrounding the core portion, a microstructured cladding portion 12.
  • the optical fiber 10 preferably comprises a silica-based glass material. Nonetheless, embodiments of the invention can also be conceived with optical fibers other than silica glass fibers, for instance with polymer fibers.
  • the cladding portion 12 is microstructured by the presence of a plurality of holes 13 that extend longitudinally through the cladding portion 12 of the fiber 10. Furthermore, a tilt-free, e.g. straight, Bragg grating 14 is provided in the core portion 11 of the fiber. In a straight Bragg grating the boundary surfaces of the varied refractive index regions in the core portion are substantially perpendicular to the longitudinal fiber axis, e.g. a residual tilt angle is less than one arc degree.
  • the Bragg grating 14 may be formed by a photo-inscription process, using a laser beam and a phase mask.
  • An exemplary Bragg grating may extend about 1 cm in a longitudinal direction of the fiber 10 and may have a fixed grating period in the range from 400 nm to 600 nm, e.g. between 520 nm and 580 nm.
  • the Bragg grating 14 reflects core-guided light at the corresponding Bragg wavelength.
  • Core portion 11 is preferably configured for single-mode operation, i.e. a single coreguided mode is supported in the core portion 11 of the optical fiber 10 which, in unmodified optical fibers as used in embodiments of the invention, is insensitive to the refractive index of a fluid surrounding the fiber. Additionally, the Bragg grating 14 is capable of coupling core-guided light into one or more higher-order cladding modes of the fiber.
  • cladding modes when excited, are observable as dips (attenuation modes) in the transmission spectrum of the fiber.
  • the particular wavelength at which coupling to higher-order cladding modes occurs depends on the optical properties of the higher-order cladding mode, e.g. the confinement, spatial extent and effective refractive index of the higher-order cladding mode.
  • the excited cladding modes extend both into the core portion 11, to obtain good coupling efficiencies, and far enough into the cladding portion to be influenced by the refractive index (Rl) of a surrounding fluid when contacted with the fiber 10 (e.g. evanescent tail sensing).
  • the cladding modes do not disappear in the transmission spectrum when the Rl of the surrounding fluid is increased, except for the cladding modes that are coupled to the surface plasmon wave and appear as attenuated modes in the transmission spectrum. It is desirable for many non-plasmonic bio- or biochemical sensing applications that the effective Rl of excited higher-order cladding modes closely matches the Rl values typically encountered for aqueous solutions in those applications, e.g. about 1,32 at 1550 nm wavelength. In contrast thereto, plasmonic-enhanced configurations of the optical fiber typically study excited higher-order cladding modes, as well as an SPR-attenuated mode, which have a slightly larger effective refractive index than the surrounding fluid.
  • Interaction of the coupled light in the higher-order cladding mode(s) and the effective Rl values that are obtainable for these cladding mode(s) is controlled by the arrangement of holes 13 in the cladding portion 12 of the fiber 10.
  • a hexagonal lattice of air holes 13 centered on the solid core portion 11 can be provided in the cladding portion 12.
  • lattices of holes are not restricted to hexagonal lattices; embodiments of the invention may use other lattice types, e.g. square or circular lattices, birefringent fiber varieties, etc.
  • a unit cell of a lattice of holes does not have to be limited to contain only a single hole, but may contain more than a single hole, e.g.
  • the different holes of the unit cell may have different sizes, e.g. different diameters and/or different hole-to-hole distances.
  • Lattices of holes are considered to be regular if it can be generated by a replication of a unit cell, wherein locations of replica of the unit cell can be expressed as a linear combination of two lattice vectors over the field of integers.
  • various parameters of the cladding portion 12 can be controlled to ensure that the effective Rl value of at least one excitable higher-order cladding mode is less than 1,36, e.g. less than 1,355, e.g. less than 1,35, and yet still sensitive to Rl changes of the surrounding fluid.
  • Such parameters include the outer diameter of the cladding portion (e.g. 50 pm to 150 pm) or the number and distribution of holes in the cladding portion. The latter includes, for instance, the selection of different 2D lattices of holes in a photonic crystal fiber, different lattice constants or hole-to-hole distances, different filling factors (e.g. air hole filling factor), different hole sizes (e.g.
  • a transverse cross-section of the optical fiber may have a circular shape.
  • embodiments of the invention are not limited to circularly shaped transverse fiber cross-sections and may include other shapes, e.g. oval or ell iptical ly shaped transverse fiber cross-sections.
  • PCF photonic crystal fiber
  • the hole arrangement may be optimized based on a simulation and experimentally verified thereafter. An agreement between simulation and experiment can be evaluated by comparing the predicted and experimentally verified locations and strengths of resonance lines representative of excited higher order cladding modes in the transmission spectrum.
  • a commercial finite-difference eigenmode solver may be used, which is set up to calculate the effective Rl and overlap integral with the fundamental core-guided mode of each cladding mode supported by the photonic crystal fiber of given cladding diameter D, air hole diameter d and lattice pitch A. Modal losses and polarization states of each cladding mode can be obtained from the simulation as well. It has been shown, e.g. in Eggleton, B. J. et al., "Cladding-mode-resonances in air-silica microstructure optical fibers", J. Light. Technol.
  • a design optimization strategy may then be based on the study of the fundamental space filling mode (FSM).
  • FSM is the mode with the largest effective index that is allowed to propagate in the microstructured cladding portion of the PCF.
  • the modes that propagate in the fiber core portion have effective index values in between the FSM and the fundamental coreguided mode, whereas modes with effective index values lower than that of the FSM are guided by the microstructured cladding. It is thus reasonable to select parameter combinations of cladding diameter D, air hole diameter d and lattice pitch A for which the FSM has a predicted effective index that is slightly above the refractive index of the fluid to be sensed, e.g.
  • n F sM 1,34 and assuming water (Rl of 1,32 at wavelengths X ⁇ 1550 nm) as the fluid. This ensures that higher- order cladding modes have effective indices below the predicted FSM effective index and a spread of the cladding mode effective indices (overlap integral strength as a function of effective index) is likely to include the Rl of water.
  • the FSM of the hexagonal PCF geometry can be calculated efficiently in an elementary piece of the cladding portion, i.e. in the unit cell, wherein modal polarization state maintaining boundary conditions are applied. This efficient simulation of the FSM by focussing on the unit cell allows large parameter sweeps to be carried out more quickly.
  • these sweeps can be repeated or refined if the assumption that the spread of the cladding mode effective indices includes the Rl of water is proven wrong during the subsequent, more detailed eigenmode simulations of the whole PCF.
  • a lower or higher initial effective index of the FSM can be selected and the more detailed eigenmode simulations are repeated for the new parameter combinations for which this updated initial effective index of the FSM has been predicted. This process can be reiterated until the spread of the cladding mode effective indices effectively includes the Rl of water.
  • the parameter combinations that are swept and for which the FSM effective index is predicted may be given as the FSM effective index which is plotted as a function of X/A, for various air-filling ratios d/A and the wavelength fixed to 1550 nm.
  • the choice of possible parameter combinations may be restricted by design limitations, for example lattice pitch values d that are feasible in PCF fabrication, e.g. 2 pm ⁇ d ⁇ 10 pm.
  • those parameter combinations are selected for which the eigenmode simulations predicts cladding mode resonances overlapping with the Rl of the fluid, e.g. water, and which yield the strongest overlap integral values.
  • random perturbations may be added to the selected parameter combinations in order to study fabrication tolerances, e.g. by running eigenmode simulations for the perturbed parameter sets.
  • Selected parameter combinations that are robust in view of the applied perturbations can be approved for fabrication.
  • Experimental verification can have an important impact onto the next design cycle.
  • the inventors compared the numerically calculated spectra to the experimentally obtained transmission spectra in PCFs with different air- filling ratios and found that the ratio of the resonance strength of the confined mode to the resonance strength of the strongest cladding mode is adequately correlated between the simulations and experiments. This correlation is taken into account when performing novel PCF designs and can be used as a new selection criterion for parameter combinations.
  • spectral shifts of the guided higher-order cladding mode or modes may be tracked to derive corresponding Rl changes for the surrounding fluid.
  • Such changes may be caused by pressure or temperature changes in the fluid, pressure or strain applied to the optical fiber, or by binding events in bio-sensing applications in which the optical fiber comprises a functional coating which is adapted to selectively immobilize analytes or bio-markers that are present in the surrounding fluid. The latter also allows the study of binding curves and/or concentration measurements. Amplitude changes can be also tracked (together with the wavelength shifts) to provide additional information during the surface bio-functionalization process.
  • a fiber coating comprising functional groups or polymer chains whose conformation is changing as a function of the pH value of the surrounding fluid may be used in pH sensing applications.
  • amplitude changes of excitable leaky cladding modes may be tracked and analyzed to provide additional information during the surface biofunctionalization process as well.
  • a set of cladding modes may be excitable and their resonant wavelength positions and amplitude levels tracked in the transmission spectrum of the optical fiber.
  • a sensitivity for the Rl sensing of surrounding fluids is increased by providing a plasmonic detection area 18 on the outer surface of the cladding portion 12.
  • the plasmonic detection area 18 comprises a thin metal layer coating 15, e.g. tens of nanometers thin, which enables the excitation of surface plasmon waves with strongly enhanced electric fields near the metal layer/fluid interface.
  • Surface plasmonic waves are excited at specific wavelengths by coupling light from the excited higher-order cladding modes, i.e. if the longitudinal momentum of the photons in the cladding mode matches the momentum of the surface plasmon mode at the metal layer/fluid interface.
  • This surface plasmon resonances shows up as an attenuation dip in the transmission spectrum of the fiber 10.
  • Metal layer coatings may comprise gold, silver, copper, or other metallic materials suitable for SPR excitation.
  • the plasmonic detection region 18 may comprise a layer of carbon nanomaterial 16, such as a sheet of graphene or single-wall carbon nanotubes, which is provided on the metal layer coating 15.
  • Single-wall carbon nanotubes preferably have a single-chirality.
  • the layer of carbon nanomaterial 16 has the benefit of causing a larger interaction of the evanescent field with the surrounding fluids (and with the analytes therein, if present), whereby a sensitivity of the optical fiber for Rl sensing is further increased.
  • sheets of graphene or single-wall carbon nanotubes may be directly attached to an outer surface of the cladding portion 12.
  • the optical fiber 10 may have a coating 17 on its outermost surface, which allows for the selective immobilization of target analytes in the fluid, e.g. specific biomolecules, e.g. proteins. This is useful for biosensing applications, e.g. applications which study binding events in- situ or in-vivo.
  • the functional coating 17 may be provided on the cladding portion 12 as outermost surface of the fiber 10 if no plasmonic detection region is provided, or on the coated metal layer equipped with the carbon nanomaterial 16 if a plasmonic detection region is provided.
  • the functional coating 17 may comprise antibodies which are linked to the outermost surface of the fiber 10, e.g. using linker molecules. In addition thereto, non-specific binding to the outermost surface of the fiber 10 may be prevented or reduced by adding surfactants to the functional coating 17, e.g. PEG-molecules.
  • FIG. 2 shows a second embodiment of the invention in which the optical fiber probe 20 comprises three cascaded Bragg gratings 24a-c that are formed in the core portion 11.
  • Each of the plurality of Bragg gratings 24a-c is configured to have a different Bragg wavelength at which incident core-guided light, e.g. confined to the fundamental mode of the fiber 20, is reflected back. This may be achieved by properly selecting a different grating period for each Bragg grating 24a-c and/or by locally altering the effective refractive index of the core portion, e.g. by changing a doping concentration or a laser fluence used for photo-inscription of the gratings.
  • the coating layers on the outer surface of the fiber cladding e.g. the metal layer coating 15 and the graphene layer 16
  • the coating layers on the outer surface of the fiber cladding may be patterned into three corresponding patches, e.g. three multiplexed plasmonic detection regions 18a-c.
  • surface plasmon waves can be excited independently with respect to each patch of metal layer coating 15 and the corresponding SPR dips tracked in the transmission spectrum of the optical fiber 20.
  • Each patch of coating layers may be equipped with a different functionalization, e.g.
  • the graphene layer 16 of each patch may be functionalized with a different functional group or biomarker, e.g. antibody. This allows for detecting multiple analytes in a fluid sample contacting the optical fiber probe 20.
  • one or more patches of coating layers, e.g. plasmonic detection areas may be equipped with a same functional group or biomarker, but the individual patches are at a distance and well separated from each other such that different fluid samples can be contacted with the optical fiber probe 20 at the locations of the individual patches. This has the advantage that one plasmonic detection area can be contacted with a reference fluid, e.g.
  • aqueous solution without analytes serve as a self-reference for the one or more further plasmonic detection regions provided along the optical fiber probe 20, which are each contacted with a same or different test fluid sample, e.g. the same aqueous solution comprising analytes.
  • a surface functionalization may be provided relative to the outer surface of the cladding portion of the optical fiber without an intermediate metal layer coating, or the bare outer surface of the cladding portion may be used for Rl sensing.
  • FIG. 8 A transmission spectrum for an optical fiber with multiplexed Rl sensing regions is shown in FIG. 8. Therein, two straight FBGs with grating periods of 541 nm and 567,5 nm have been formed in the core portion of a single photonic crystal fiber with hexagonal air hole lattice. Next to the two strong Bragg core modes, two confined cladding modes are observed that are insensitive to Rl changes of the surrounding fluid, as well as two spectrally multiplexed sets of higher-order cladding modes for Rl sensing.
  • the inset 31 shows the cross-section of a hexagonal lattice PCF that can be used, in embodiments of the invention, for high sensitivity, in-line refractive index (Rl) measurements of aqueous media surrounding the fiber.
  • Aqueous media e.g. water solutions or buffer solutions
  • An exemplary PCF comprises a hexagonal lattice of air holes, e.g. 6 rings, with a missing hole at the origin which is centered on the PCF core portion.
  • a lattice constant (pitch) may be 2,5 pm and an air-filling factor may be approximately 0,4.
  • the cladding diameter of the PCF may range between 86 pm and 125 pm.
  • the lattice of air holes may be formed in a silica glass matrix, whereas the solid core portion at the center of the PCF may be doped, e.g. comprising germanium with a doping concentration of about 8,5 mol%.
  • a doped core portion of the PCF may have a diameter that is approximately equal to the diameter of the air holes, e.g. about 1 pm.
  • Straight FBGs may be inscribed in the optical fiber, e.g. PCF, using a phase mask and a 193 nm ArF excimer pulsed laser.
  • the pulsed laser may be configured to output an average laser power of 250 mW and an approximate fluence at the photosensitive core of 2,8 kW/cm 2 .
  • a period of the phase mask may be 1082 nm, resulting in a first order FBG in the telecom C-band.
  • Hydrogen loading of the optical fiber e.g. at 60 degree Celsius for 60 hours at 205 bar
  • the optical fiber may be annealed at 70°C for 20 hours and conventional (nonspecialty) single-mode pigtailed fiber portions may be spliced to both end facets of the PCF to facilitate spectral interrogation.
  • the recorded transmission spectra 30 of a straight FBG with 541 nm grating period inscribed in the PCF are also reported in FIG. 3 for the case of an uncoated PCF in air (solid line) and a gold-coated PCF in air (dashed line).
  • a first set of strong cladding mode resonances is observed in a sensing window 32, e.g. limited to a wavelength region ranging from 1490 nm to 1495 nm, with associated effective refractive indices ranging between 1,325 and 1,334.
  • the resonant cladding modes preferably extend both in the core portion as well as near the interface between the cladding portion and the surrounding fluid, e.g.
  • n 1,3152 for distilled water at 25°C extrapolated to an optical wavelength of 1550 nm, according to the Sellmeier fit for distilled water obtained by Daimon, M., and Masumura, A.," Measurement of the refractive index of distilled water from the near-infrared region to the ultraviolet region", Applied Optics 46, 3811-3820 (2007).
  • the Bragg resonance 31 that originates from the reflected fundamental core-guided mode is centered at 1546,4 nm which corresponds to an effective index of 1,4292. Only the core mode appears in the reflection spectrum, confirming the single-mode behaviour of the PCF.
  • a saturated resonance is observed at 1511 nm and has a corresponding effective index of 1,3638.
  • This saturated resonance is associated with a higher-order leaky mode which exhibits an insignificant sensitivity to the refractive index of the surrounding fluid.
  • the resonance amplitude of this mode is very strong because most of the modal power resides in the core portion as well as in between the two to three inner rings of air holes in the microstructured cladding portion, proximate to the core portion. Consequently, there is a high overlap with both the fundamental mode and the inscribed grating structure.
  • a second set 33 of resonant cladding modes is located in the spectral range between the Bragg resonance of the fundamental mode at 1546,4 nm and the saturated resonance at 1511 nm.
  • the effective indices associated with the second set 33 range between 1,3790 and 1,4130 and are therefore more distant to the refractive indices typically encountered for aqueous solutions.
  • the number of resonant cladding modes in the transmission spectra of the PCFs is small. It is the microstructured cladding portion of the PCF that controls and limits the number of resonant cladding modes that can be excited through the FBG.
  • cladding modes can be efficiently excited also in microstructured optical fibers, e.g. in PCFs, the core portions of which comprise a straight, i.e. non-tilted, Bragg grating.
  • a thin metal layer may be provided on the outer surface of the cladding portion.
  • an at least 35 nm thick layer of gold may be deposited around the fiber circumference by sputtering (in argon-filled chamber at IO -4 mbar) in two steps.
  • the metal layer coating e.g. gold coating, results in a reduction of the strength of the cladding mode resonances and in a wavelength red-shift of about 500 pm.
  • FIG. 4 shows an experimental setup which can be used for refractive index sensing of a fluid which contacts an optical fiber according to embodiments of the invention.
  • the optical fiber refractometer (PCF sensor) is secured by two fiber clamps such that the inscribed FBG is positioned straight in a small reservoir of test fluid, e.g. anhydrous lithium chloride salt (LiCI) dissolved in distilled water at different concentrations.
  • test fluid e.g. anhydrous lithium chloride salt (LiCI) dissolved in distilled water at different concentrations.
  • SiCI anhydrous lithium chloride salt
  • a tuneable laser source is coupled to the optical fiber (PCF sensor) and programmed to deliver laser light to the optical fiber (PCF sensor) while performing a wavelength sweep between 1480 and 1550 nm, e.g. a stepped wavelength sweep at 60 nm/s tuning speed and 0,6 pm step size.
  • the transmitted power at each scanning step is recorded by a photodetector (PD) and may be amplified subsequently, e.g. by a built-in transimpedance amplifier of the photodetector.
  • a broadband laser light source may be coupled to the optical fiber (PCF sensor) and the transmission spectrum at the output of the optical fiber (PCF sensor) may be detected by an optical spectrometer.
  • An oscilloscope may be provided to synchronize the wavelength sweep of the laser source (TLS) with an output voltage of the photodetector (PD), allowing a more reliable transmission spectrum to be obtained.
  • a control unit (Computer) is connected with the laser source (TLS) and the photodetector (PD), e.g. via the oscilloscope (Osc.), and is configured for sending control signals to the laser source (TLS) to start and stop the wavelength sweep and for receiving signals from the photodetector (PD) for the acquisition of a transmission spectrum.
  • the control unit may comprise data storage means to store acquired transmission spectra.
  • a signal generator of the control unit may replace the oscillator in some embodiments of the invention.
  • the optical fiber PCF sensor
  • a light source e.g. tuneable narrowband laser source or broadband light source, e.g. LED having more than 5 nm emission linewidth
  • a photodetector may be provided in a lab-on-fiber system.
  • the light source and/or the photodetector may be directly attached to the optical fiber, e.g. directly spliced, glued, or pigtail-connected to the microstructured fiber or to a single-mode fiber segment which itself has been spliced to the microstructured fiber.
  • a polarization controller may be inserted in an optical pathway between the laser source (TLS) and the optical fiber (PCF sensor) to ensure that the polarization state of input light to the optical fiber (PCF sensor) is linear.
  • the SPR dip in the transmission spectra is excited, e.g. the resulting sensitivity is increased, if the linearly polarized input light to the optical fiber (PCF sensor) corresponds to p-polarized light at the cladding portion/metal layer coating boundary.
  • intermediate polarisation states may be admitted and SPR may occur for both s- polarisation and p-polarisation, but with different sensitivities with respect to phase and amplitude changes. This may depend on the particular mode that is selected for tracking.
  • a polarization-maintaining optical fiber (PCF sensor) and/or polarization-maintaining spliced single-mode fibers (SMF) may be used to accurately control a polarization state of the input light.
  • PCF sensor polarization-maintaining optical fiber
  • SMF polarization-maintaining spliced single-mode fibers
  • the control unit may be configured to track resonant cladding modes and/or SPR dips (e.g. tracked wavelength shifts and amplitude changes) over repeated transmission spectrum acquisitions, e.g. over subsequent wavelength sweeps of the laser source, with effective refractive indices in the region of interest to compare their sensitivities.
  • the tracked wavelength position and power level of the fundamental Bragg resonance (fundamental core mode), or of a spatially confined mode that is insensitive to Rl changes of the surrounding fluid may be used to compensate for any fluctuation in the ambient conditions of the optical fiber (PCF sensor), e.g. temperature fluctuations in the surrounding fluid or fluctuations of the laser source power.
  • PCF sensor optical fiber
  • FIG. 5 shows the transmission spectra of a gold-coated PCF according to an embodiment of the invention, when immersed into each one of nine different solutions. Only a limited spectral range of the recorded transmission spectra is plotted in FIG. 5, corresponding to the sensing window 32 comprising the first set of resonant cladding modes of FIG. 3.
  • the gold-coated PCF was rinsed with distilled water between consecutive Rl measurements of the prepared solutions. Moreover, a transmission spectrum of the air-exposed PCF was recorded between consecutive Rl measurements of the prepared solutions to ensure that the PCF-based refractometer had returned to its initial state.
  • Each of the nine solutions had been prepared with a different Rl value, which had been determined independently by measurement with an Abbemat MW refractometer (IO -6 RIU resolution, measured at 436,6 nm) and by extrapolating the Rl value at 1550 nm from the measurement via a Sellmeier fit for distilled water, as described previously.
  • For Rl changes on the order of 10' 4 between the prepared solutions wavelength shifts of the SPR dip of only a few picometers are expected in the transmission spectra.
  • an SPR-attenuated mode is present in the transmission spectra of the gold-coated PCF in FIG. 5.
  • the centre wavelength of the SPR-attenuated mode is located at 1493,3 nm (corresponding to the phase-matched surface plasmon wave) and indicated by a vertical dashed line.
  • a transmission amplitude change is negative for increasing Rl values of the prepared solutions at the shorter wavelength side of the SPR-attenuated mode, e.g. at 1493 nm, but positive at the longer wavelength side, e.g. at 1493,8 nm.
  • the largest wavelength shift for a resonant cladding mode in the transmission spectra is observed at 1491,5 nm (e.g. about two nanometres below the SPR-attenuated mode which is also observed for SPR sensors in step-index fibers assisted with tilted FBGs), corresponding to an effective Rl of 1.3277, which is slightly larger than the Rl of the surrounding solutions under test.
  • FIG. 6 A more detailed view of this most sensitive wavelength region is depicted in FIG. 6, with the acquired noisy spectra shown in sub-plot (a) and the smoothened spectra after application of a Savitzky-Golay smoothing filter shown in sub-plot (b).
  • the black arrow indicates the red-shifting of the resonance wavelength and increasing resonance losses at higher Rl values of the surrounding solution.
  • the minima of the dips in the transmission spectra may be approximately identified by applying a peak search algorithm to the inverted and smoothened transmission curves.
  • a Gaussian kernel can be fit to each approximated peak location, allowing both the wavelength position and the transmission amplitude to be obtained as a function of the Rl value of the surrounding solution, as shown in FIG. 7.
  • Linear functions can be fit to the extracted wavelength position and transmission amplitude of the tracked cladding mode resonance dip, which describe their dependence on the Rl value of the surrounding solution.
  • the FBGs used may be FBGs having a short-period grating, e.g. a few hundreds of nm, and not having long-period gratings having hundreds of micrometer long periods.
  • the core-guided mode typically is insensitive to Rl changes of the surrounding fluid, which also includes temperature induced Rl changes of the fluid to be sensed.
  • the fiber as a whole is temperature sensitive and the effective Rl of the core-guided mode is therefore also drifting as a function of temperature. Therefore, according to some embodiments, from the tracked Rl drifts of the core-guided mode, the temperature fluctuations of the fiber and the fluid system can be extracted and used to compensate for temperature drifts, e.g. in the spectrum of the higher-order cladding modes.
  • Fiber-based Rl sensors are known to exhibit cross-sensitivity to temperature changes, which therefore affect the Rl measurements.
  • a priori knowledge of the modal sensitivity to temperature changes allows correcting the final Rl measurements for spectral shifts due to temperature variations.
  • a gold-coated FBG- equipped PCF was immersed in a temperature-controlled bath filled with distilled water.
  • the immersion allowed for a stable and uniform temperature and avoids air circulation that may affect the measurement accuracy.
  • this situation mimicked the actual conditions for one of the target Rl sensing experiments in the present example as the fiber probe in the present example was intended for being immersed in aqueous solutions.
  • Temperature control and monitoring was enabled by placing the bath on a hot plate (Torrey Pines Scientific HS40) and measuring the temperature with an external needle-type digital thermometer (IKA ETS-D5) with a resolution of 0.1°C placed next to the grating.
  • IKA ETS-D5 external needle-type digital thermometer
  • Modes that are less confined in the core region are affected by changes in both temperature and external index.
  • the confined cladding mode has a slightly lower thermal sensitivity of 9.0 pm/°C but close to that of the core mode. This mode is partially affected by the surrounding index due to leakage of the optical power in the mode through the microstructure to the interface between fiber and surrounding medium.
  • Modes 1 and 2 that are exploited for SPR sensing in aqueous solutions show even lower sensitivity to temperature.
  • these modes are strongly influenced by the thermo-optic coefficients of water and gold. Whereas the effect of temperature changes on the permittivity of gold is small, distilled water features a large negative thermo-optic coefficient of approximately -1.4xl0 -4 RIU/°C, which lowers the slope of the temperature sensitivity line for modes 1 and 2.
  • PCFs with a single grating and multiplexed gratings are able to detect HER2 protein with a limit of detection of 8.62 nM (or 10' 6 g/mL).
  • another demodulation technique is demonstrated that focuses on tracking the wavelength position of the envelope. This technique results in sensitivities several times higher than the single resonance tracking method.
  • results are shown of refractive index sensing with multiplexed gratings in aqueous media, where non-zero sensitivities for both gratings are indicated.
  • the second grating excited in the transmission spectrum at larger wavelengths is affected by the modal dispersion. Dispersion lowers the effective index values of the excited resonances which, in turn, leads to a difference in sensitivities between the first and the second gratings.
  • a proper design should narrow down the wavelength span occupied by a single grating, which will allow to bring (at least) two gratings closer to each other in the spectrum and to reduce the dispersion effect leading to similar sensitivities from the sensing areas.
  • a non-tilted short-period fiber Bragg grating was inscribed into the core of a hexagonal lattice photonic crystal fiber (PCF).
  • the transmission spectrum of the written FBG demonstrated excitation of strong cladding mode resonances with effective indices in the region of water/ phosphate buffered saline (PBS), which are therefore suitable for bio-sensing.
  • PBS water/ phosphate buffered saline
  • the signal was enhanced with a surface plasmon resonance after sputtering a gold layer on the fiber cladding.
  • the proposed sensor was relatively simple to fabricate and to reuse owing to the external location of the sensing region.
  • HER2+ Human Epidermal Growth Factor Receptor-2
  • thiolated aptamers As a receptor to attach the protein, we used thiolated aptamers.
  • the bio-functionalization process of the fiber surface includes, as shown FIG. 10:
  • PBS phosphate buffered saline
  • the surface was washed with PBS for 5 min.
  • the senor was immersed in a specific concentration of HER2 protein for 10 min. In case more than one protein concentrations were investigated, the lowest concentration was inserted first, then the remained concentrations were inserted with a gradual increase of the concentration. In between consecutive concentrations, the sensor surface was washed with PBS.
  • the bio-detection setup used in the present exemplary embodiment included a light source (tunable laser), a polarization rotator (half wave plate), a light receiver (photodetector), and an oscilloscope to synchronize the signals of the light source and the light receiver.
  • the PCF sample that comprised the sensor area was spliced from both sides with standard optical fibers. The sensor area was immersed in the bath with the corresponding solution (PBS/aptamers /mercaptohexanol /proteins). Transmission spectra of the PCF sensor were recorded and analyzed for the wavelength sensitivity.
  • a single FBG was used for HER2 bio detection, whereby a PCF sample with a single FBG and gold thickness of 23.5 nm sputtered two times on opposite sides of the fiber was used.
  • the sensor's response to four concentrations of HER2 protein starting from lowest IO -9 g/mL and increasing up to IO -6 g/mL with a factor of 101 was studied. These concentrations correspond to a range from 8.62 pM to 8.62 nM taking for the calculations HER2 protein's molecular mass of 116 kDa.
  • FIG. 11(a) shows a part of the transmission spectrum with the most sensitive resonance (indicated with an arrow) recorded after immersing the sensor in HER2 protein with concentration IO -6 g/mL.
  • the zoomed view of the resonance dip is shown on the right Fig. 11(b).
  • Blue parts correspond to Gaussian functions fitted to find central wavelength positions of the resonances.
  • the arrow connects the first and the last points of the sensor-protein interaction. It shows a clear red shift in the wavelength position originated from attachment of the protein onto the fiber surface and as a result, increased refractive index of the surrounding medium.
  • FIG. 12 shows evolution of the resonance position during the bio-functionalization process (aptamers immobilization + surface blocking) with PBS immersions in between.
  • First interaction between the sensor and the surrounding material (either anti-HER2 aptamers or mercaptohexanol) leads to an abrupt increase in the wavelength position.
  • the wavelength position continues growing at a lower rate until it reaches saturation point, when the most part of the fiber surface has been interacted with the material. Further immersion does not lead to significant changes in the spectral position of the resonances.
  • Fig. 13 shows wavelength evolution of the resonance when the fiber is immersed in four concentrations of HER2 protein.
  • the largest wavelength shift of ⁇ 30 pm is recorded for the concentration of IO -6 g/mL.
  • Some wavelength shifts are observed for lower concentrations as well; however, the amplitude of these shifts is close to the shifts originated from PBS which is used as a 'neutral' medium in this experiment. Therefore, we conclude that the limit of detection of HER2 protein for this particular PCF sensor with a single resonance tracking technique is IO -6 g/mL (8.62 nM).
  • Sensitivity of the device according to the exemplary embodiment can also be estimated by tracking the wavelength shift of an envelope that connects several transmission resonance dips from the sensing region.
  • the envelope is collectively 'dragged' by multiple resonances that are coupled to the plasmonic wave to a greater or a lesser extent depending on the proximity of their modal effective index to that of the plasmonic resonances.
  • FIG. 14(a) Envelope fitting to the spectra recorded after immersing the sensor in IO -6 g/mL of HER2 protein, is shown in FIG. 14(a) with blue curves.
  • the zoomed view of the resonance dip is shown on FIG. 14(b).
  • Red curves correspond to fitted gaussian functions.
  • the arrow connects the first and the last points of the sensor-protein interaction and shows a red shift in the wavelength position.
  • FIG.15 compares two peak-demodulation techniques: tracking the envelope (red) and single resonance (black, same as in FIG. 13). It is clear that envelope tracking results in larger wavelength shifts, for both protein and PBS. Again, protein concentrations lower than 10' 6 g/mL result in shifts with similar amplitudes as those for PBS. The shift of 310 pm is observed for concentration 10' 6 g/mL making it the limit of detection for this PCF sensor.
  • FIG. 16 shows transmission spectrum of the PCF sample in air, right after inscription of both gratings. For visualization, regions corresponding to each grating are highlighted with different colors. Both fundamental core modes and regions of cladding modes used for bio-detection are also indicated in the figure. Spectral separation of two FBGs is possible due to the fact that the number of strong resonances at wavelengths lower than the sensing region, is minimized. Therefore, the wavelength span that the 1st FBG occupies, does not interfere with the span of the 2nd FBG.
  • Fig. 17 shows sensing regions of separated gratings (FBG1 and FBG2) for the non-coated PCF sample (a) and the same PCF sample coated two times with 27.5 nm (b).
  • FBG1 and FBG2 separated gratings
  • the modal effective index of the resonance responsible for sensing in the 2nd FBG is lower than that in the 1st FBG.
  • This PCF sample showed different sensitivities for both FBGs in both bare and gold- coated configurations.
  • another non-coated multiplexed PCF sample showed sensitivities of 38 and 39 nm/RIU for FBG1 and FBG2, respectively. This means that even though the sensitivity of the multiplexed sensor is affected by modal dispersion, similar sensitivity value for two FBGs is possible for particular polarization states.
  • FIG. 18 shows envelope fitting for the 1st (a) and for the 2nd (b) grating. Arrows on the left figures indicate the most sensitive resonances. For both gratings, the envelope width was optimized to obtain the highest wavelength sensitivity. Note that for both single and multiplexed samples, the envelope dip is positioned close to the most sensitive resonance which means that this resonance, together with other resonances involved in envelope 'dragging', contributes to the envelope shift.
  • FIG. 19 shows comparison of the envelope (red) and single resonance tracking techniques (black) for two gratings.
  • the envelope tracking results in larger wavelength shifts for both protein and PBS.
  • the largest wavelength shift is observed for concentration IO -6 g/mL (19 pm with the single resonance, and 350 pm with the envelope tracking).
  • the ratio of the wavelength shifts calculated with the envelope tracking (64 pm) and with the single resonance tracking (33 pm) is close to 2 which is lower than for the 1st FBG. Nevertheless, the largest shift is clearly observed for IO -6 g/mL which makes this concentration to be the limit of detection of this PCF sample for both gratings.
  • the above example shows the response of a single-FBG and multiplexed-FBG PCF-based sensor to several concentrations of HER2 protein.
  • the maximal sensitivity was achieved to be more than 300 pm for concentration IO -6 g/mL of HER2 protein. This concentration corresponds to 8.621 nM and it is the limit of detection of our PCF sensor when HER2+ protein is detected. Calculated wavelength and amplitude shifts for both sensors are listed in Table 1.
  • the second FBG of the multiplexed sensor although affected by the modal dispersion due its location at higher wavelengths, shows a non-zero sensitivity to both aqueous solutions in refractive index sensing and to HER2+ protein in bio-sensing experiments.
  • the dispersion problem can be tackled by the proper PCF design which reduces the wavelength span occupied by both gratings and thus brings them closer in the spectral position.

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Abstract

L'invention concerne une fibre optique pour la réfractométrie d'un fluide encerclant la fibre et un procédé de détection d'indice de réfraction. La fibre optique (20) comprend une partie d'âme solide (11) et une partie de gaine (12) encerclant la partie d'âme (10). Un réseau de Bragg sans inclinaison (14, 24a) est formé dans la partie d'âme et configuré pour coupler une lumière d'un mode guidé d'âme avec au moins un mode de gaine d'ordre supérieur. Une section transversale de la partie de gaine est microstructurée par le biais d'un agencement de trous (13) de telle sorte qu'un indice de réfraction efficace associé à l'un desdits modes de gaine d'ordre supérieur varie en fonction de l'indice de réfraction de fluide environnant et soit inférieur à 1,35.
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CN113310461A (zh) * 2021-04-23 2021-08-27 中铁第一勘察设计院集团有限公司 温度不敏感的光纤二维倾角传感器
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CN118168587A (zh) * 2024-04-02 2024-06-11 华中科技大学 一种圆环型小周期长周期光纤光栅传感器及其制备方法和应用
CN118091829A (zh) * 2024-04-23 2024-05-28 西北大学 一种具有二维弯曲识别能力的单模光纤包层波导光栅及制备方法
CN120593806A (zh) * 2025-06-13 2025-09-05 大连理工大学 实现应变与温度解耦的光纤传感器的设计方法，对应的测量装置及测量方法

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