Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/28—Investigating the spectrum
  • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
• • 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
• • 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
  • 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
  • G01N2021/775—Indicator and selective membrane
• • 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

Definitions

• This invention relates to optical sensors based on long period gratings.
• the invention provides methods, sensors, and systems for optical sensors based on long period gratings interrogated by optical waveguide loop ring-down spectroscopy, and /or coated with a solid phase microextraction film.
• LPGs Long period gratings
• FBGs fiber Bragg gratings
• LPGs Fiber Bragg gratings
• FBGs fiber Bragg gratings
• the LPG has a much longer period (typically 10 ⁇ m to 1 mm) compared to the Bragg grating ( ⁇ 1 ⁇ m).
• LPGs couple light from the mode propagating along the fiber core to modes associated with co-propagating cladding modes of the fiber. Due to the high losses typically experienced by cladding modes, the LPG behaves as a notch filter.
• LPGs act as notch filters with low back reflection.
• the band rejection of an LPG can have a width of typically 30 nm and the loss at the peak can approach -30 dB (James et al. 2003).
• the wavelengths of the core mode that couples into the cladding modes are characterized by the phase-matching condition ⁇ f (equation 1 )
• n eff(Core is the effective core refractive index which is a function of wavelength, core refractive index ni and cladding refractive index n 2
• n g is the effective cladding refractive index of the i th mode, which is a function of wavelength, cladding refractive index n 2 and surrounding refractive index n 3
• ⁇ is the period of the LPG.
• LPGs can hence be used as sensitive sensors for changes in refractive indices (Bhatia 1999; Chong et al. 2004; Lee et al. 2003; Shu et al. 1999).
• aromatic compounds in a hydrocarbon matrix have been detected by changes in an LPG spectrum (Allsop et al. 2001), producing a wavelength change of -0.4 nm for a concentration of xylene of 0.5% (vol) in a paraffin solution. The detection limit was reported as 0.04% (-400 ppm).
• LPGs as sensors have been to coat the LPG with a reactive coating that undergoes a chemical and/or physical change when exposed to the analyte.
• CMC carboxymethylcellulose
• PEI polyethylenimine
• a method for detecting one or more compounds in a test medium comprising: providing an optical sensor comprising a long period grating and a solid phase microextraction film; exposing the optical sensor to the test medium such that said one or more compounds of interest are selectively partitioned into the solid phase microextraction film; and comparing at least one optical property of the sensor exposed to the test medium with at least one corresponding optical property of the sensor in absence of the test medium; wherein a difference in said at least one optical property is indicative of detection of said one or more compounds.
• the long period grating is disposed on an optical fiber.
• the optical fiber is a single-mode optical fiber
• the solid phase microextraction film comprises PDMS.
• partitioning of the one or more compounds of interest into the solid phase microextraction film is reversible.
• the method further comprises providing an array of two or more optical sensors each comprising a long period grating and a solid phase microextraction film.
• an optical sensor for detecting one or more compounds in a test medium, comprising: an optical waveguide comprising a long period grating; and a solid phase microextraction film disposed on said long period grating; wherein said one or more compounds are selectively partitioned into the solid phase microextraction film; and wherein said partitioning of said one or more compounds alters at least one optical property of the long period grating.
• the optical waveguide is an optical fiber.
• the optical fiber is a single-mode optical fiber
• the solid phase microextraction film comprises PDMS.
• partitioning of the one or more compounds of interest into the solid phase microextraction film is reversible.
• the senor further comprises an array of two or more optical sensors each comprising a long period grating and a solid phase microextraction film.
• a system for detecting one or more compounds in a test medium comprising: one or more optical sensors as described above; a light source; a detector for detecting light having passed through said one or more sensors; and means for evaluating one or more properties of said detected light.
• a method for detecting one or more compounds in a test medium comprising: providing an optical sensor comprising a long period grating; measuring and comparing at least one optical property of the sensor exposed to the test medium with at least one corresponding optical property of the sensor in absence of the test medium, a result of said comparison being indicative of detection of the one or more compounds; wherein measuring at least one optical property comprises using fiber loop ring-down spectroscopy.
• the method further comprised disposing a solid phase microextraction film on the long period grating.
• a method for detecting one or more compounds in a test medium comprising: providing an optical sensor comprising a long period grating and a solid phase microextraction film; exposing the optical sensor to the test medium such that said one or more compounds are selectively partitioned into the solid phase microextraction film; and measuring and comparing at least one optical property of the sensor exposed to the test medium with at least one corresponding optical property of the sensor in absence of the test medium, a result of said comparison being indicative of detection of the one or more compounds; wherein measuring at least one optical property comprises using fiber loop ring-down spectroscopy.
• said fiber loop ring-down spectroscopy comprises: providing an optical waveguide loop attached to said optical sensor; launching in the optical waveguide loop an intensity-modulated light at a reference phase; detecting a phase of said light along the optical waveguide loop; and comparing the detected phase of said light along the loop with the reference phase; wherein comparing the detected phase and the reference phase provides information about said at least one optical property of the optical sensor.
• said fiber loop ring-down spectroscopy comprises: providing an optical waveguide loop attached to said optical sensor; illuminating the optical waveguide loop with a plurality of light pulses; detecting roundtrips of said light pulses at one or more locations along the loop; and determining ring-down time of said light pulses; wherein said ring-down time is indicative of at least one optical property of the optical sensor.
• a system for detecting one or more compounds in a test medium comprising: an optical sensor comprising a long period grating, the optical sensor having optical properties which are altered when exposed to the one or more compounds; an optical waveguide loop attached to said optical sensor; a light source for launching in the optical waveguide loop an intensity-modulated light at a reference phase; a detector for detecting a phase of said light along the optical waveguide loop; and means for comparing the detected phase of said light along the loop with the reference phase; wherein comparing the detected phase and the reference phase provides information about said optical properties of the optical sensor.
• a system for detecting one or more compounds in a test medium comprising: an optical sensor comprising a long period grating, the optical sensor having optical properties which are altered when exposed to the one or more compounds; an optical waveguide loop attached to said optical sensor; a light source for illuminating the optical waveguide loop with a plurality of light pulses; a detector for detecting roundtrips of said light pulses at one or more locations along the loop; and means for determining ring-down time of said light pulses; wherein said ring-down time is indicative of at least one optical property of the optical sensor.
• the above systems may further comprise a solid phase microextraction film disposed on said long period grating; wherein said one or more compounds are selectively partitioned into the solid phase microextraction film; and wherein said partitioning of the one or more compounds alters at least one optical property of the long period grating.
• partitioning of the one or more compounds of interest into the solid phase microextraction film is reversible.
• the at least one optical property is refractive index
• the light may be of at least one wavelength selected from infra-red (IR), visible, and ultra-violet
• the optical waveguide loop may comprise a single-mode optical fiber.
• the methods and systems of the invention may employ a plurality of optical sensors as described above, the plurality of sensors being multiplexed and/or in an array, for detecting a plurality of compounds of interest.
• individual sensors or groups of sensors may each have a solid phase microextraction film corresponding to a distinct compound of interest.
• Figure 1 is a schematic diagram of an experimental setup incorporating a LPG into a fiber optic ring-down spectroscopy loop.
• Figure 2 is plot of the transmission spectrum in air of the LPG which was spliced into the fiber loop of the setup shown in Figure 1.
• Figure 3 is a plot of the instrumental phase angle offset correction.
• the fiber loop connecting the laser and detector (see Figure 1) was replaced by a patch consisting of a short length of SMF-28 optical fiber.
• the modulation frequency dependent phase shift between the photodiode detector signal and the laser modulation constitutes an instrumental correction factor which must be subtracted from phase shift measurements made using the fiber loop.
• Figure 4 is a plot of modulation frequency dependence of the phase angle difference between the fiber loop of Figure 1 and the patch used to obtain the data in Figure 3, at three different laser frequencies. The slope of these plots gives the negative ring-down time for the fiber loop.
• Figure 5A is a plot showing the attenuation spectra of a LPG with periodicity of
• FIG. 5B is a plot showing change in attenuation maximum wavelength with refractive index for two LPGs with different periodicities, as indicated.
• the attenuation maximum at around 1570 nm of the LPG used in Figure 5A shifts by about 16 nm as the refractive index of the surrounding solution is changed to match the refractive index of the cladding.
• the periodicity affects the sensitivity of the monitored mode to refractive index, seen as a greater wavelength response for the 252 ⁇ m LPG.
• the maximum wavelength response is when the refractive index approaches the refractive index of the cladding independent of the periodicity.
• Figure 6 is a plot showing dependence of the fiber loop ring-down time on the composition of a solution surrounding the LPG, obtained using the setup of Figure 1.
• the volume fraction of a solution containing dimethylsulfoxide (DMSO) and water surrounding the LPG was varied and the cavity ring-down time was determined.
• the solution temperature was 22°C.
• the laser wavelength was set at 1520 nm and the modulation frequency was 130 kHz.
• Figure 7A is a plot showing temporal evolution of the position of the attenuation maximum of a LPG.
• the entire LPG was coated with a thin film of PDMS and submersed in a saturated solution of toluene in water. As the toluene partitioned into the
• FIG. 7A shows temporal evolution of the attenuation maximum wavelength for lower concentrations of toluene for the LPG of Figure 7A, with equilibration in about 5 min. The shift in wavelength after 5 min exposure is linearly related to the toluene concentration (data not shown). The signal did not stabilize at 1000 ppm toluene concentration.
• Figure 7C is a plot of spectra of an LPG coated with a film of PDMS, measured upon immersion in air, water and after submersion in a saturated solution of xylenes
• Figure 8B is a plot of the change in ring-down time of the PDMS-coated LPG of Figure 8A, as a function of the concentration of xylene in water. The laser wavelength was fixed at 1590 nm.
• FIG. 9 shows film:solution partition constant (K fe ) values for various compounds partitioning from water into three siloxane films: pure polydimethylsiloxane (PDMS), PDMS doped with amine groups by adding 10% (v/v) 3-aminopropyltriethoxy silane to PDMS precursor before film formation (APTES), and PDMS doped with phenyl groups by adding 10% (v/v) diphenyldiethoxy silane to PDMS precursor before film formation (PDPS).
• K fe film:solution partition constant
• Figure 10A is a plot showing refractive index of PDMS films as a function of the mol% of titanium doping of the PDMS.
• the doped PDMS was prepared by adding tetraethoxy titanium to PDMS during polymerization. Films were deposited on glass slides and refractive index was measured using a refractometer.
• Figure 10B is a plot showing refractive index of 6 mol% titanium-doped PDMS films as a function of mol% diphenyl siloxane doping of the PDMS.
• the doped PDMS was prepared by adding diethoxydiphenyl silane and tetraethoxy titanium during polymerization.
• Figure 11 shows the change in refractive index of a 12.2% titanium PDMS film, or a 7.7% APTES PDMS film, for various analytes. Films were deposited on glass slides and refractive index was measured using a refractometer. The slides were exposed to water saturated with the analyte and the change in refractive index was normalized to a 100 ppm solution concentration. The numbers in parentheses indicate the refractive index of the analytes in pure liquid form.
• solid phase microextraction relates to the extraction or partitioning of a compound of interest (e.g., an analyte or target species) from a mixture of compounds, into a solid phase, based on affinity of the compound of interest for the solid phase.
• a compound of interest e.g., an analyte or target species
• partitioning of the compound of interest into the solid phase material is reversible.
• affinity does not refer to interaction characteristic of protein-ligand complexes, as SPME does not involve such reactions.
• the solid phase material does not react with the compound of interest, and accordingly does not undergo a chemical change when exposed to the compound of interest. Rather, SPME is based on partitioning of the compound of interest from a mixture or medium into the solid phase.
• Such partitioning may involve the compound dissolving into the solid phase, wherein compound particles become substantially surrounded by the solid phase. Without being bound by theory, it is believed that at least in some situations such partitioning is based on the free energy gain of the system when the molecule of interest leaves the mixture or medium and moves into the solid phase. For example, in an aqueous system, unfavourable interactions of a compound of interest with water may drive the transfer (i.e., entropy).
• the mixture of compounds is typically a fluid, and may be a gas or a liquid. Where the fluid is a liquid, the mixture may be aqueous.
• the compound of interest may itself be a solid, liquid, or gas, e.g., dispersed as particles in the mixture of compounds.
• the solid phase material is applied, e.g., as a film, to the outside surface of an optical waveguide, such as an optical fiber.
• Compounds of interest partitioned into the solid phase material detectably alter at least one optical property of the fiber, thus rendering the fiber an optical sensor for the compound of interest.
• the affinity of the compound of interest for the solid phase material is characterized as the filnxsolution partition constant (K fs ), defined as the ratio of the concentration of a compound in the film to the concentration of the compound in solution at equilibrium.
• SPME film material is the polymer polydimethylsiloxane (PDMS), which extracts compounds from a mixture of compounds in close correlation to hydrophobicity. This is best described by the close correlation between K fs for compounds in PDMS films and K ow , the octanohwater partition constant (Mayer et al. 2000). Large values of K fs indicate preconcentration of analyte in the polymer matrix, with typical logK fs values for organic compounds such as polycyclic aromatic hydrocarbons (PAHs) in the range of 2-5 (Mayer et al. 2000; Brown et al. 2001).
• PAHs polycyclic aromatic hydrocarbons
• the selectivity patterns of polymer films such as PDMS can be altered by doping the polymer. This may be accomplished by incorporating specific chemical functional groups into the polymer at various levels. For example, a PDMS film doped with phenyl groups was shown to have affinity for the aromatic compound toluene (Matejec et al. 2003). SPME based on polymer coatings on optical fibers has been used for detection of extracted compounds through absorption of the evanescent radiation in the SPME coating (Krska et al. 1993; Mizaikoff 1999).
• PDMS is a good matrix for these measurements due to its optical properties: it is clear and has a refractive index (typically 1.41) which acts as a cladding to maintain light propagation in the waveguide.
• refractive index typically 1.41
• the main drawback of these approaches has been the need for direct absorption of radiation by the extracted compounds. This places significant limitations on the light sources and detectors which can be used.
• An alternative is to measure a more generic parameter from the polymer film, such as film refractive index. This is different than the evanescent detection scheme mentioned above, as detection of refractive index changes requires detecting changes in the light propagating in the waveguide.
• a method for detecting one or more compounds of interest in a test medium using an optical sensor comprising a LPG and a SPME film.
• the method comprises exposing the optical sensor to the test medium such that said one or more compounds of interest are selectively partitioned into the solid phase microextraction film, and comparing at least one optical property of the sensor exposed to the test medium with at least one corresponding optical property of the sensor in absence of the test medium, wherein the comparison is indicative of detection of the one or more compounds of interest.
• the invention also provides an optical sensor comprising a LPG and a SPME film, and a system for carrying out the method of detecting one or more compounds of interest.
• the SPME film is applied to the entire LPG, or a portion of the LPG.
• the combination of LPG and SPME coating provides for determining optical properties of the coating, which affect the optical spectrum of the LPG.
• a change in an optical property of the coating is related to the optical property of the analyte (e.g., refractive index of analyte), but this is not necessarily the case.
• the term "detecting” is intended to mean determining the presence and/or concentration and/or identity and/or optical property(ies) of one or more compounds of interest (i.e., one or more analytes).
• the term "test medium” is intended to refer to any medium in which one or more compounds of interest may be found, and which may substantially surround the optical sensor so as to facilitate detection of the analyte(s).
• a test medium may be solid, semi-solid, or fluid such as liquid or gas.
• This aspect of the invention provides for determination of the presence, concentration, optical properties, and/or identity of an analyte partitioned into the SPME film, using the resulting shift in optical loss spectrum of the LPG. This may be determined either by recording a spectrum of wavelengths or by recording the loss at a fixed wavelength.
• SPME provides not only selectivity for the analyte, but also enhances the local concentration of the analyte in the film by many orders of magnitude (e.g., 100 to 1000-fold).
• the chemical selectivity of the measurement arises from the chemical specificity (i.e., formulation) of the SPME film.
• the formulation of the film may be prepared so as to maximize selectivity for an analyte of interest.
• the SPME film is a polymer, for example, PDMS. Such embodiment is particularly well suited to aqueous mixtures of compounds.
• the selectivity of a SPME material may be enhanced by providing for specific chemical or physical interactions of the analyte with the polymer matrix.
• the SPME material may be doped with, e.g., functional groups to enhance specificity to an analyte (see Example 4).
• the film may be a polymer or composite material selected from those listed in Table 1 , with corresponding selectivity to various analytes as shown.
• the SPME film is a material or combination of materials selected from, for example, polymers, zeolites, porous glass, antibodies, ion exchange resins, sol- gels, and Iigands.
• the selectivity of a measurement may be enhanced by multiplexing two or more LPG-SPME sensors into a sensor array, and using for example, multivariate analysis to extract chemical composition of mixtures.
• SPME increases the analyte concentration near the fiber cladding by two or three orders of magnitude over the analyte concentration in the mixture. This leads to large changes in the refractive index of the polymer matrix which are directly dependent on the concentration change in solution.
• Polymers with affinities to particular classes of chemicals are used, which provides crude chemical selectivity. The effect may be enhanced by multiplexing an array of two or more of such sensors, each with slightly different polymer selectivities, and extracting the exact composition using multivariate analysis.
• Fiber-loop ring-down spectroscopy is capable of measuring very small changes in optical losses in optical waveguides.
• the technique is fully compatible with single mode optical fibers typically used for LPGs.
• FLRDS allows for measurement of absolute optical loss independently of power fluctuations of the light source.
• pulse FLRDS described in detail in our U.S. Patent No. 6,842,548, issued January 11, 2005, and in Brown et al. (2002), a nanosecond laser pulse is injected into an optical waveguide loop and the optical losses are determined from the time it takes for the intensity of the round trip signal to decay to 1/e of its initial value, i.e., the ring-down time.
• ⁇ is the modulation frequency and ⁇ 0 is a frequency dependent offset phase angle that depends on the inherent time delays in the electronic and optical components.
• the phase angle measurements can be done very fast and we have demonstrated a time resolution of 200 ms on a system that was not optimized.
• a method for detecting one or more compounds in a test medium comprising providing an optical sensor comprising a long period grating, measuring and comparing at least one optical property of the sensor exposed to the test medium with at least one corresponding optical property of the sensor in absence of the test medium, said comparison being indicative of detection of the one or more compounds, wherein measuring at least one optical property comprises using fiber loop ring-down spectroscopy.
• the invention also provides a system for carrying out such method.
• This aspect of the invention provides for determination of optical loss introduced into an optical fiber loop by the LPG, using pulsed or phase shift FLRDS.
• optical loss may be caused by variables such as mechanical (e.g., stress, strain, vibration) and/or environmental (e.g., chemical, thermal) factors acting on the LPG
• the invention provides a method and apparatus to accurately and rapidly detect, characterize, and/or quantify those factors. Measurements may be carried out by scanning the light source and determining the resulting spectrum in relation to such variable(s), or by keeping the light source wavelength constant and determining optical loss resulting from such variable(s) after calibration. It should be noted that both refractive index and evanescent wave absorption change the loss properties of the LPG.
• the refractive index change induced by the analyte may shift the attenuation band of the LPG into the wavelength region interrogated by FLRDS (or any other means of interrogating the LPG), thereby increasing the loss at this wavelength.
• FLRDS any other means of interrogating the LPG
• the laser is tuned to the wavelength where the attenuation band of the LPG is maximum in absence of the analyte, then the shift of the attenuation spectrum of the LPG in the presence of the analyte will cause the loss to decrease at this wavelength.
• a benefit of this aspect of the invention is that the optical loss of the LPG can be obtained without having to rely on the linearity and stability of the light source or detector, making the system considerably more robust and field-suitable when compared to a typical intensity-based measurement.
• FLRDS has the greatest sensitivity at low optical losses, compared to most other optical loss measurements.
• FLRDS can be implemented using either a scanning laser source or a fixed wavelength source. In the former case, one can obtain the full attenuation spectrum with particular sensitivity to regions of low optical loss, whereas the latter case may be more suitable in a field-deployable application.
• the changes in optical loss are monitored by FLRDS in accordance with the variation of an environmental parameter, for example, the refractive index of the medium surrounding the LPG.
• a method for detecting one or more compounds in a test medium comprising providing an optical sensor comprising a long period grating and a solid phase microextraction film, exposing the optical sensor to the test medium such that said one or more compounds of interest are selectively partitioned into the solid phase microextraction film, and measuring and comparing at least one optical property of the sensor exposed to the test medium with at least one corresponding optical property of the sensor in absence of the test medium, the comparison being indicative of detection of the one or more compounds, wherein measuring at least one optical property comprises using fiber loop ring-down spectroscopy.
• the invention also provides sensors and systems for carrying out such method.
• FLRDS is used to determine optical loss introduced into the optical loop by the LPG, the optical loss resulting from partitioning of an analyte into the SPME film of the LPG.
• FLRDS may be used as described above; that is, in phase shift or pulsed operation, scanning the light source or keeping it at a constant wavelength, and the like.
• the LPG and SPME are used as described above, wherein the chemical selectivity of the measurement arises from the chemical specificity of the SPME formulation, which may be enhanced as described above.
• the invention is further described by way of the following non-limiting examples.
• Example 1 Sensor using a long period grating and fiber loop ring-down spectroscopy.
• the experimental setup is shown in Figure 1. At its center is a LPG 10, with a length of 22 mm and a periodicity of 274 ⁇ m, which was custom-made by Avensys Inc. (Pointe-Claire, Quebec, Canada) using Dow Corning SMF-28 single mode optical fiber.
• the attenuation spectrum of the LPG in air at around 1.5 ⁇ m is shown in Figure 2.
• the LPG was spliced into a loop 12 made of the same SMF-28 fiber using a fusion splicer to make a total length of the loop of about 11 m.
• An ANDO AQ4320D tunable diode laser 14 was modulated using a function generator 24, and the amplitude modulated light with a power of 5 mW and a bandwidth of 200 MHz was coupled into the loop using a 99.5/0.5 2x2 directional coupler 16 (Lightel Technologies).
• the intensity of light circulating within the loop was monitored using a 99.5/0.52x1 tap 18 (Lightel Technologies) and a fast InGaAs photodiode detector 20.
• the phase angle shift, ⁇ was measured using a 200 MHz lock-in amplifier 22 (Stanford Research Systems 844), where the reference was obtained from the synchronized output of the modulated laser diode. The error in the measurements was approximately 0.02 degree according to the manufacturer.
• the ring-down time of the fiber loop was determined from the slope of a plot of tan ⁇ against the modulation frequency, ⁇ (Tong et al. 2004).
• the inset in Figure 1 shows shematically analyte particles partitioning into an SPME film disposed on the LPG, as described in Example 3.
• the ring-down time at 1520 nm was 683 ns, while that measured at 1527 nm (circles) was 520 ns.
• Examination of Figure 2 indicates that the higher loss of the LPG at 1527 nm compared with that at 1520 nm is responsible for the shorter ring- down time at this laser wavelength.
• the effect of a change of refractive index in the attenuation spectrum of the LPG was determined by immersing the LPG in solutions of dimethylsulfoxide (DMSO) in water where the mole fraction was modified from 30% to 100% in DMSO. From the mole fraction the refractive index was calculated using the Lorentz-Lorenz equation.
• Figure 5A shows the effect of the surrounding refractive index on the attenuation spectrum.
• the attenuation maxima corresponding to higher order cladding modes shift dramatically as the refractive index of the surrounding solution approaches and then matches the refractive index of the cladding.
• the optical losses change dramatically as the refractive index changes.
• Figure 5A shows that the optical losses at 1550 nm increase by more than 1 dB as the refractive index of the surrounding solution increases.
• a change in refractive index of 0.000015 can be detected assuming a phase resolution of 0.02°.
• one explanation for the shape of the curve in Figure 6 is that, as the volume fraction of DMSO increases, the ring-down time decreases sharply as the refractive index of the test medium approaches that of the optical fiber cladding. As the volume fraction of DMSO increases further, the ring-down time increases as the refractive index of the test medium exceeds that of the optical fiber cladding.
• Example 2 Sensor using a long period grating and solid phase microextraction The combination of solid-phase microextraction with long period gratings but without the use of optical ring-down detection was tested in a preliminary experiment.
• An LPG was coated with PDMS (Dow Corning 100% silicone rubber) by evaporating the solvent from a 100 mg/ml solution of the PDMS precursor material in methylene chloride.
• Attenuation spectra of the LPG were recorded in straight transmission while the film was submersed in a saturated solution of toluene in water (concentration approx. 500 ppm).
• the change in position of the attenuation maximum was recorded as a function of time ( Figure 7A) and was fitted using a simple exponential growth function.
• a partitioning time constant of t fs 19 min was obtained for partitioning of toluene into the polymer and of only 1.55 min for removal of toluene during rinsing with water.
• Example 3 Sensor using a long period grating, solid phase microextraction, and fiber loop ring-down spectroscopy
• a sensor based on the combination of an LPG with SPME and interrogated by FLRDS is described.
• this example relates to a sensor for detecting hydrophobic contaminants in water.
• the inset in Figure 1 shows shematically analyte particles partitioning into an SPME film disposed on the LPG.
• An LPG with a comparably weak attenuation spectrum was coated with PDMS (Dow Corning 100% silicone rubber) by evaporating the solvent from a 100 mg/ml solution of PDMS precursor material in methylene chloride.
• PDMS Dow Corning 100% silicone rubber
• the thickness of the resulting film was between about 10 and 500 ⁇ m and its refractive index was close to 1.41.
• the attenuation spectrum of the LPG was recorded using fiber-loop ring-down spectroscopy in the continuous wave (i.e., phase shift) implementation.
• the phase angle of the synchronized output from a periodically driven laser was referenced to the detector output, and from the difference of the phase angles the ring-down time was calculated ( Figure 8A).
• the ring-down times were recorded over part of the attenuation spectrum of the LPG they provided for a very sensitive determination of optical loss.
• the attenuation spectrum was then modified by immersing the polymer-coated LPG in a test medium.
• Example 4 Film:solution partition constant (K fs ) of modified PDMS films
• Figure 9 shows the logK fs values measured for a variety of analytes in pure
• the modified PDMS films were doped with amine groups (by adding 10% (v/v) 3-aminopropyltriethoxy silane to PDMS precursor before film formation) (APTES), and with phenyl groups (by adding 10% (v/v) diphenyldiethoxy silane to PDMS precursor before film formation) (PDPS).
• APTES aminopropyltriethoxy silane
• PDPS diphenyldiethoxy silane
• PDPS diphenyldiethoxy silane
• Example 5 Refractive index changes in films
• Table 2 shows the refractive index (at 1.55 ⁇ m wavelength) of several PDMS and modified PDMS polymer films. Modified films were made from the polymerization of dichlorodimethyl silane with one or more of the following: trichloromethyl silane, diethoxydiphenyl silane, 3-aminopropyltriethoxy silane or tetraethoxy titanium (IV). All percentages in Table 2 are in mole percent.
• Figure 10 shows the ability to modify the film refractive index to a specific value by changing the levels of titanium (Figure 10A) or diphenyl siloxane ( Figure 10B) substituent in the film.
• FIG. 11 shows changes in refractive index for various analytes partitioned into either a 12.2% titanium PDMS film, or a 7.7% APTES PDMS film. Films were exposed to water saturated with the analyte. The corresponding change in refractive index was normalized to change for a 100 ppm solution of the analyte.
• Khaliq S., James S.W., Tatam R.P. (2001) Fiber-optic liquid-level sensor using a long- period grating, Optics Letters 26:1224-1226.
• Khaliq S., James S.W., Tatam R.P. (2002) Enhanced sensitivity fibre optic long period grating temperature sensor, Measurement Science & Technology 13:792-795.

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