CA2078784A1 - Fiber optic refractive index sensor using metal cladding - Google Patents

Abstract

A refractive index FOCS has a fiber optic core (12) with a partly light transmissive thin metal film clad (16) of an effective thickness and light transmissivity so that transmission through the core is strongly affected by the refractive index of a surrounding liquid or vapor medium (24). The metal clad (16) and surrounding medium (24) produce a localized refractive index at the core interface which modulates light transmission through the core as a function of the medium refractive index. The clad (16) is made of platinum, or also of gold, rhodium, palladium, nickel, iron, cobalt, ruthenium, iridium, osmium, zinc, copper, silver, chromium, molybdenum, tungsten, vanadium, niobium, tantalum, titanium, zirconium, or hafnium. The clad (16) is also made of oxides of these metals, or metal compounds or alloys. With a fluorescent tip (14), the changes in the fluorescent signal are a measure of the medium refractive index. With a reflective tip (18), the changes in the reflected signal are measured. In a linear configuration, source and detector are placed at opposite ends of the fiber and changes in the transmitted signal are measured as a function of refractive index. Multiple measurements with multiple clads of different specificity can be made. The multiple clads can be on a single fiber or on separate fibers.

Description

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WO ~l/1't~56 PCI~US91/0213~

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FIBER OPTIC REFRACTIVE INDEX SENSOR
USING METAL CLADDING
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;Backqround of the Invention . , The invention relates generally to fiber optic sensors, and more particularly to fiber optic sensors for measuring refractive index.
A fiber optic is an optica' waveguide which transmits light by total internal reflection ~TIR) at the core/clad i~interface. The cxitical angle Ac for TIR is cletermined by the ratio oE the refractive index Nz of the clad to the r~xactive index N1 of the core: Ac = sinl (N2/N1)~ Thus the index of the clad must be less than the core for TIR to occur.
~iOptical fibers have been used in a wide variety of sensors, known as "optrodes" or "fiber optic chemical sensors"
(FOCS), which are designed to measure the presence of various chemical species or the value o:E various parameters such as pressure or temperature. In Tnost cases a 5ignal from a reactant, e.g. a fluorescent signal from a fluorescent dye ; which interacts with the desired chemical species or is ~a~fected by the desired physical parameter, is transmitted ; through the~iber to a detector. These sensors are generally limited by being specific to a single chemical species or physical parameter; thus each sensor is based on~ its own~
unique chemistry.~ U.~S. Patent 4,846,548 issued July 11, 1989 25 ~ is directed~ to a ~more generalized fiber optic sensor methodology in which the principle of detection is based on~
how the operatlng characteristics of the fiber~itself~are ;modified as the~result of the presence of the desired species.
The;use of this sensor principle allows the~;fabrication of 30 ;~ many different senso~s which are s~nsiti~e to particular G~e~ F ev ~ grcups f ,pecie-. Howev~, it woul~ b-W091/14956 P~T/US91t ~

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desirable to have available a more general sensor which can detect and differentiate a wide variety of species. Since different species usually ha~e different r~fractive indexes, a single sensor which can measure refractive index would be ~ ~ able to detect the presence of different species. Thus, such - a sensor would not be species specific but would be a more universal detector.
According to one aspect of the invention, there is provided a fiber optic sensor for measuring refractive inde~
of a surrounding medium, comprising a fiber optic core; a thin film metal clad of metal or metal o~ide or other metal compound or alloy formed on and surrounding the fiber optic core, the metal clad having an effective thickness and light transmissivity which in combination with the surrounding medium produces a localized refractive index and a controlled leakage of light which modulate~ the transmission o~ light through the fiber optic core as a function of the refractive ` index of the surrounding medium.
:,,'! According to another aspect of the inve.ntion, there is `1 20 provided a method of detecting the refractive index of a medium, comprising providing a fiber optic sensor comprising a fiber optic core and a thin film metal clad of metal or metal oxide or other metal compound or alloy formed on and :~ .
surrounding the fiber optic core, the metal clad having an 2S effective thickness and light transmissivity which in combination with a surrounding medium produces a localized ; refractive index and a controlled leakage of light which modulates the transmission of light through the fiber optic core as a function of the refracti~e index o~ the surrounding medium; contacting the thin film metal clad with the medium;
inputti~g a light signal into the core; detecting changes in intensity of a light slgnal transmitted from the core.

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Summary of the Invention The invention may provide a fiber optic sensor which measures refractive index, and which can identify a wide variety of different species on the basis of different refractive indexes.
- The invention is a fiber optic sensor for measuring refractive index which has a fiber optic core with a light ~: porous thin film metal clad which produces a controlled leakage of light as a function of the refractive index of the surrounding medium. The ~hin film may be thus only partly, . but not totally, transmissive to light. The thin film metal clad may be designed to produce a localized refractive index at the core interface when the clad contacts a surrounding medium which modulates the transmission of light through the core as a function of the refractive index of the medium. In ~ one embodiment, the sensor has a fluorescent tip formed of a `` fluorescent dye immobilized on the tip of the fiber. An excitatio~ signal is transmitted through the ~iber to the tip and the fluorescent emission is detected through the fiber.
In a second embodiment a (silvered) reflective tip is formed at the end of the fiber so t:hat an incident signal is transmitted bac~. In a third embodiment, the source and I detector are positioned at opposite ends of the fiber so a transmitted signal is detected. The change in refractive ` 25 index of the medium (liquid or vapor or water emulsion) ` surrounding the fiber changes the transmission characteristics '!~ which results in a signal change at the detector. The thin metal film is of a thickness and is sufficiently light porous or light transmissive such that the leakage of light through the thin film is modùlated as a function of the refractive . index of the surrounding medium, which may be in liquid or '~ vapor (gas) state. In one embodiment, platinum (Pt) paint is ~ applied on the side of a fiber core and is then heated with a :, ' ~

WO91/14956 PC~/U~91/0 5 ' ,~ A ~ ~J ~

torch to remove the organic base, leaving a thin porous platinum film. Another method of forming the clad is to paint with hydrogen hexachloroplatinate(n)hydrate and torch. In addition to platinum, other metals including gold lAu), rhodium (Rh), and palladium (Pd) are suitable for the thin film metal clad. Additional metals for thie thin film clad include Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Cu, Ag, Zn, Fe, Ru, Co, Ir, Os and Ni. Alternatively, in place of elemental metal, the clad may be formed of metal oxides or metal compounds or alloys. The metal coatings are deposited by various coating techniques~
; Different metal clads have specificity to one or more analytes. The specificity of di.fferent clads can be used to make simultaneous measurements w.hich yield a better result by subtracting out the effects of other substances. A single fiber optic core can be clad with a metal clad having plurality of adjacent segments, each segment being made of a ` different metal. Alternately, multiple fiber optic sensors, each with a di~ferent metal clad can be used in combination.
y 20 The clads are chosen with various specificities so that '. information about the particular analyte of interest can be calculated from the detected signals from each sensor.

Brief Description o~f the Drawings -~
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` ~ In the accompanying drawings: ;
Figures lA,B,C show a fiber optic sensor with thin film metal clad and fluorescent tip, reflective tip, and a linear design, respectively, for measuring refractive index.
Figure 2 is a sensor system for measuring refractive index of a sample.
Figure 3 is a graph of signal intensity as a function of solvent re~ractive index for a series of different solvents.
, . , Figure 4 is a graph of the response of a gasoline sensor . ~.

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Figures SA,B,C illustrate the operation of the refractive index FOCS.
Figure 6A shows a fiber optic core with metal clad which S is made of adjacent segments of different metals.
Figure 6B is a schematic view of a multiple sensor system.
.. . .
Detailed Descri~tion of the Preferred Embodiments ~` The invention is a fiber optic sensor with a light porous ; l0 or partly light transmissive thin metal clad of suitable thickness on a fiber optic core which varies its transmission . properties as a ~unction of the refractive index of the surrounding medium (liquid or vapor or water emulsion), thus ' providing a measure of refracti.ve index. The metal clad is formed on and surrounds a length of the core. In one `! embodiment, excitation light is transmitted through the fiber to a fluorescent tip and the returning fluorescent signal is detected. In a second embodiment, input light is reflected by a (silvered~ reflective tip, and the reflected signal is detected. In a t~ird embodiment, input light from a source at '! one end of the fiber is transmitted through the fi~er and the ~ transmitted signal is measured by a detector at the other end -` of the fiber.
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' - As ~hown in Figure lA, sensor l0 has a core 12 with a fluorescent tip 14 and a thin metal clad 16 formed on and surrounding a length of core 12. In a preferred embodiment core 12 i5 a silica fiber~ core, e.g. an Ensign-Bickford -~ ~ MaxCore HCS fi~er, with a core diameter of 400, 600 or l000 `microns, a refractive index of about 1.4, and an attenuation 30 ~ of l0dB/km at 597nm. About one inch of the end of a fiber is stripped to the bare core, e.g. by burning off the cladding or i removing the cladding chemically. A fluorescent chemical ``:i . :

WO~l/14956 PCT~S91/0 ~

3 s (dye), e.g. Rhodamine B, is attached to the distal end of the fiber core. The fluorescen~ material m~y be immobilized or impregnated at the tip in any suitable manner. One me~hod of attaching the fluorophore (fluorescer) is to close a length of shrink tubing onto the fiber core, leaving the distal end open. A grain of Rhodamine B is placed in the open end of the shrinX tubing which is then filled with methanol or other solvent ~o dissolve the dye. A small drop of cyanoacrylate ester (super glue) is then placed on the open end and the tip is heated, e.g. with a hot air gun, to seal the open end to form a fluorescent sack. Optical adhesive, e.g. Norland ~61, is then coated on the outside of the entire fluorescent sack surface of the shrink tubing to completely seal the fluorophore.
In the embodiment of Fig. lB a reflective tip 18 is ~ormed onto the end of fiber opt:ic core 12 with its thin film metal clad 16. In the linear (dual end) embodiment of Fig.
lC, a portion of fiber optic core 12 is clad with thin metal ~ilm 16.
The principle of operation of an optical fiber depends on ~ the refractive index of the material at the core interface.
'~`! In order for the core to transmit light efficiently, the core `i must be clad with a material of lower refractive index than the core. With the clad removed, light is transmitted very inefficiently. As the core is placed into various media, the light is transmitted with an efficiency which depends on the refractive index of the medium. The medium, in essence, becomes the clad. The lower the refractive index of the medium, the more light is tra~smitted through the core. If the medium has a higher refractive index than the core, thèn no re~lection will occur and all the light will be 105t. . .
In accordance with the invention, a thin metal clad of an effective light porosity or transmissivity on the fiber core enhances the change in light transmission or the core as a :

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function of the refractive index of the surrounding medium.
The light porosity of the clad is a function of the clad material, its thickness and its physical characteristics such as physical porosity and crystalline structure. The metal clad is o~ suitable thickness to produce a gradient refractive index at the core metal clad interface as a result of material from the surrounding medium that is absorbed by the metal clad. The preferred thickness is in the range c~ greater than about 50A up to about 10 microns. A thickness of about one ~uarter the wavelength of the excitation light may be used. :~
This localized refractive index a~ the boundary is directly proportional to the amount of material adsorbed/absorbed and also its refractive index, so that the amount of light leaking through the fiber depends on the refractive index o~ the medium. This substantially modulates a light signal transmitted through the core, producing a detectable measure , of refractive index. Thus, the thin metal clad on the core amplifies the effect and provides greater sensitivity, producing a simple, rugged and practical refractive index sensor. In the first embocliment platinum (Pt) paint (Engelhard ~A4338 platinum ink) was placed on the side of the bare core. The organic components base of the paint was ~, removed by burning off with a torch to leave a thin shiny Pt coat on the fiber core. Alternatively the core is painted with hydrogen hexachIoroplatinate~N)hydrate and torched. Thin films of other materials such as gold (Au), rhodium (Rh) and palIadium (Pd~ may also be formed. Nickel (Ni) is also of ` particular interest.
.~ The metal claddings that respond in a similar way to liquids or vapors with-different refractive indexes include:
1) group IVB elements: Titanium(Ti), Zirconium ; -(Zr), Hafnium (~f) 2) group VB elements: Vanadium (V), Nio~ium (Nb), Tantalum (Ta) ~, ': - ' ' '~

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3) group ~IB elements: Chromium (Cr), Molybdenum (Mo), T'ungsten (W) ; 4~ group IB elements: Copper (Cu), Silver (Ag), Gold (Au) 5) group IIB elements: Zinc (Zn), Cadmium (Cd) 6) group VIII elements: Iron (Fe), Ruthenium (Ru), Cobalt (Co), Rhodium (Rh), Nickel (Ni), Pall~di~m (Pd), Platinum (Pt), Iridium lIr), Osmium (Os) . 10 7) Oxides of the above metals . 8) Metal compounds or alIoys such as:
- a) group II-VI compounds: including but not limited to ZnSe, ZnTe, CdS, CdTe, MgTe, ZnS
b) group III-V compounds: including but not .. 15 limited to BN, GaAs, Il~s, InSb, AlN, GaN, InN
~ c) group I-VII compounds: including but not :~ limited to CuC1, CuBr, AgI
: d) sphalerite struct:ure compounds: including . but not limited to Ça~Te3, ZnSnPz, ZnSnAs2 e) Wurtzite structure compounds: including ~ ut not limited to SiC, MnS, MnSe, MnTe, A12Se3 :;!, f) group I, III, VI compounds~ including but .
not limited to CuGaTe2, CuGaSe2, CuLaS2 . g) group II, IV, V compounds: including but : ~ 2:s: not limited to ZnGeP2, CdGeP2, ZnGeAs2, ZnSnAs2 .-~.
These metal coatings can be deposited on the fiber optic . surface by sputtering, brush coating, vacuum deposition, :' : plasma coating or by any other technig~ie that can form either a homogeneously or heterogeneously coated surface. Either ~:
~ inorganic or organometallic oompounds could be employed to d posit metal coating on the fiber. Metals can also be deposited from their ~el-mental states. Final coating : deposited on the fiber can be in the form of either elemental :--~ metal or any of its oxides, or a metal compound or alloy. -:-. ., , : ..

4~56 PCT/US91/0213g i~x~i?
2~7~ S ~2f~ :-The metal ¢lad, according to the invention, must be of a suitable and effective thickness (neither too thin nor too thick) so that the core does not see a constant index ~e.g.
either the metal itself or the actual index of the medium).
Instead the clad must be of such a thickness that the core sees an index which is related to the index of the medium in the immiediate vicinity of the core, and which significantly modulates light transmission through the core for a wide range of values of refractive index of the medium, including values qreater than the index of the core.
When the Pt coated core was placed into various solvents, the sensitivity was markedly increased over the core alone.
Thus a thin film of Pt or the other matals listed above can be applied to the side of the fiber core by painting, sputtering, or other suitable method such as plating from solution. The ~'J, layer is heated, if necessary, to remove organic components.
~ A very thin film, e.g., down to a few monolayers or less, in `~ some cases is desired if it is sufficiently partly ~1i transmissive to produce the desired controlled light leakage.
.~ 20 The thickness of the thin film in combination with other ~ properties of the clad that determine its transmissivity to '~! light is important; an effective thickness allows the transmission of the fiber core to be strongly affected by the ~ refractive index of the surrounding medium. The clad must ; 2~ have an e~fective light porosity or transmissivity that produces a controlled leakage of light as a function of the ` refractive index~ of any surrounding medium. The clad must thus be only partly, or not ~otally, transmissive to light The clad must~produce a re~ractive index sensitive modulation of the light transmission through the core. The thickness should be greater than about 50 A and less than about 10 microns to produce significant variations in the detector signal 50 the surrounding medium can affect tr;ansmission. A
typical thickness may be about~a quarter wavelength of the "
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input light, which for visible light 4000-7500 A is a thickness on the order of 1000-2000 A. Preferably, the clad thickness is in the range of 50 A to 10 microns.
A sensor system 20 is illustrated in Figure 2. A sensor lO, as shown in Figure lA, is formed on one end of an optical fiber 22 and placed in a sampling region 24. Light from a light source 26, e.g., an argon laser, or a tungsten halogen lamp or a light emitting diode, i5 focused by lens 28 through dichroic mirror 30 and into the opposite end 32 of fiber optic 22. This excitation light 34 is transmitted by the fiber to sensor 10 to- excite the fluorsphore (fluorescer). The returning fluorescent signal 36 is reflected at dichroic mirror 30 and passes through lens 38 and filter 40 into detector 42. The signal received by detector 42 is a measure of the refractive index of the medium in sample region 24.
Similar systems can be formed with the sensor of Figure lB
where the reflected signal is measured instead of a fluorescent signal, or with the sensor of Figure lC where the detector is placed at the opposi.te end of the fiber from the ` 20 source and the transmitted signal is measured. While visible light may be used, a wider range of wavelengths, from , ultraviolet to infrared may also be used to practice the 'j invention.
Figure 3 illustrates the response of a general ~ 5 refractive-index FOCS according to the invention where a `~ number of different solvents are sequentially flowed over the -` ~ sensor. The intensity variations at the detector clearly differentiate between dlfferent solvents (different refractive indexes). The sensor quickly recovers from one sample solvent to another and the measurements are reproducible, i.e., a similar detector intensity is produced by repeating a solvent.
The i~dex of a glass core is ~bout 1.4 and a modulated signaI
v is produced even for a medium with a larger refractive index.
-~ This occurs ~ecause the~metal clad produces a localized index .~........................................................................ . .

WO~ 956 P~T/US91/02135 2 ~ 3 i~

at the core-clad interface which is less -than the index of the medium. Figure 4 shows the response of a refractive index sensor to repetitive samples of gasoline. The intermediate peaks at the beginning and end are gasoline vapor while the rest is liquid.
The operation of the refractive index FOC~ is illustrated in Figures 5A,B and C. A fiber optic core 50 has a thin film ~ metal clad 52 replacing a portion of the regular clad 51.
- Light from a source 54 is input into the core. Tip 56 at end of the fiber may include a fluorophore, or may be reflective tor an altexnate linear or dual end fiber optic arrangement may be used). The intensity of the light signal ; ~rom the fiber optic core 50 is measured by detector 58 and -` depends on the refractive index of the medium which depends on the concentration of the particular chemical species present.
As shown in Figure 5A, in the absence of analyte, e.g., ` hydrocarbons, the return signal is high (in light intensity) showing higher voltage V0. The index produced at the core interface is produced by the metal clad and air, s~ it i5 very ~`~ 20 low and results in the maximum light transmission. As the concentration of analyte on the FOCS surface increases, the returnin~ light signal and hence voltage drops and is directly ;~i proportional to ~he concentration of the analyte and to the refractive index of the analyte. Figure SB shows the case for 100% sample concentration (highest refractive index of the medium) where light transmission is most reduced (maximum light leakage since the metal clad-medium produce a high localized index) and~the detector signal is a minimum Vl.
Fig~re 5C shows the case for an intermediate sample ~ 30 concentration ~tlower refractive index medium) where an .~ '`t intermediate detector signal Vz is produced (since the localized index and light leaka~e are intermediate).
In the examples given below in Table I, the initial signal (voltage) is given as V0 and the final signal (volts) - .

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is given as V. In qualitative tests, V0 was obtained in empty - 4 L glass jars and V was obtained in the presence of analytevapor which was present as liquid-vapor equilibrium tsaturated or near saturated vapor~.

In the quantitative measurements described below, V0 = voltage in the absence of analyte ~ V = voltage in the presence of a given amount of - analyte~
,:
~` As soon as the FOCS is withdrawn ~rom the analyte atmosphere, the original signal level is attained showing the instant re.versibility of the sensor. The ~ollowing empirical equation clescribes the performance of the FOCS.

metal coating + analyte = change in refractive index =
` modulation of light traveling through the fiber optic Table I shows the detector values for a number of different metal clads and for a number of analytes measured with each metal clad. Accordingly a particular metal clad can be designed to detect a particular analyte, although each analyte can be detected by a number of different metal clads.
j ~0 Cer~ain metals are particularly sensitive to certain analytes. ;
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. Table I
Ru Vo V
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Gasoline 7.27 3.87 Benzene 7.00 - 4.19 ~ .
Yylene7.3c 4.39 - . .
.. ~. Toluene 7.07 4.10 '`; : !
V
. Gasoline 6.85 5.80 ~;.'i Benzene 5.32 3.79 :' .,,l Zr ~;~
~i Gasoline 8.20 5.89 ::
~ Benzene &,00 2.28 `~i Xylene 7.~8 3.62 : :::
~ Toluene 7.51 4.57 ~:
`~ Ni Gasoline 9.46 . 6.74 Ben~en~ 7.4& 1.07 : . .:
: Xvlene 8.56 1.27 Toluene ~8.16 1.77 Pd Gasoline 7.51 4.92 Benzene 8.00 6.~78 . ~ .
Xylene : 7.44 5.00 Benzene : : ~ &.96 1.65 W091/14956 PCT/US91 ~ $

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Table II lists a number of illustrative metal surfaces and various gases which have been found to be adsorbed on the metal surface. The adsorbability of the gases may depend on the crystal form and/or orientation of the metal layer.
; 5 However, in the process of formin~ the metal clad on the fiber optic core, particular crystal forms of the metal can be reproducibly obtained by controlling the process parameters, e.g. temperature and pressure, so that a sensor tailored to a specific gas can be obtained. Thus, a sensor with a metal clad of preselected crystal structure can be produced for use with a desired analyte. Table III similarly lists a number of illustrative metal surfaces and various organic compounds ~hydrocarbons) which have been found to be absorbed on the metal surface. Certain metals, and in some cases certain crystal forms of some metals, are thus particularly suitable for the metal clad for detection of certain hydrocarbons. The ~` lists in Tables II and III are only for purposes o~
;;l illustration and are not exhallstive of all possible combinations. Further information about adsorption of gases by metal surfaces is contained in Chemistry In Two-Dimensional Surfaces, Gabor ~. Somorjai, Cornell Univ. Press, Ithaca, NY, l ~ 1981 particularly Tables 5.2 - 5.7, pp. 210-241 which are : ~ herein incorporated by reference.
The specificity of different metal clads to various ~`'JI 25 analytes allows more specific analysis of an analyte mixed ,?`~ with other~substances. The invention ~urther includes ~, multiple measurement methods and apparatus. Since~more than `l~ ~ one clad may be available for a particular analyte, but each clad will not~have the same absorption coefficients to other analytes, a more complex measurement method and sensor ~;
structure can be used to determine the presence o~ a selected ; analyte oùt of a mixture o~ several different analytes. To ` ~ detect analyte A in a mixture of analytes A and B, it would be most deslrable to find a single metal surface that is specific ;

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to A. However, if one cannot be found, two sensors can be -~ used, one which has a clad which responds to both A and B and one of which has a clad which responds to B but not A. By subtracting the measurement obtained from the second sensor from that of the first sensor, a measure of analyte A is obtained. Alternatively, if a specific metal clad is not available for either A or B, then clads are chosen which have ;
different absorption coefficients for A and B, and the concentration of A and B are obtained by solving an algorithm lo for two unknowns. The dual sensor approach can be extended to multiple sensors for more than two analytes and can be ` implemented in a variety of ways, using either a single fiber optic or multiple fiber optics.
One embodiment of the dual/m~ltiple measurement approach ~i 15 is to form the metal clad 16 o~ Figs. lA-C of an alloy or two (or more) metals which are specific to the analytes of interest. In a second embodiment, as shown in Fig. 6A, metal ~ clad 16 on core 12 is made up of a plurality of adjacent ;~ segments 16a, 16b, etc. each Qf which is a different metal ~ 20 with preferential absorption coefficients to one or more of `;, the analytes of interest. In a third embodiment, illustrated in Fig. 6C, a detection system 60 has a first fiber optic 22a with a first metal clad sensor lOa and a second fiber optic 22b with a second metal clad sensor lOb, each of which is connected to a respective source/detector 62a, 62b. There is no limit to the nu~ber of fibers which can be used to perform ; the requisite analysis of a sample. Source/detector units 62a, 62b could be combined into a single unit or share some cumponents, e.g., a common source or other pairts of the 30 ~` optical train. The function of ~ource/detector units 62a, 62b is to provlde a source signal to each sensor lOa, lOb and to produce a detector signal from each sensor lOa, lOb. Each ~ ~ s nsor lOa, lOb is of any of the types shown in Figs. lA-C and 1 ~ ; has a different metal~clad from the other. Each metal clad is :i . : :, W~9l/l4956 PCT/US91/0 ~
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preselected for its affinity to particular analytes. The combination is chosen to improve the specificity to the analyte o~ interest. The detector signals from source/detector units 62a, 62b are input into a processing unit 64 which performs the necessary data analysis to yield information about the ~nalyte(s) of interest. Inasmuch as ~ metal absorbers are not perfect, e.g., those showing 11 0 1l - absorption m~y absorb slightly and those showing complete absorption may not coilect everything, the processor contains ~0 an algorithm which properly weights the sensor response according to the absorption coefficient of the metal for the target species. Although the illustrative embodiment of Fig.
6B has been shown with two sensors, the method and apparatus ; can, of course, be extended to more than two sensors each with a different metal clad, including different crystal forms of the same metals.

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SURFACE ADSORBED G~S
Al(lll), (100~, (llO) 2 :
Au(lll) 2 (lOO) H?S, CO, ~e Cu(lll) 2~ CO, C12, H~S, Xe (lOOj 2~ CO, Nf?
(l10`f 2~ CO, H~fG, H2S, Xe Cu/Ni(lll) CO
. (110) 2~ CO, H2S
.~ Fe(lll) 2. NH3. H2 (lOO) 2~ CO, H2S, H2, NH3, H20 ` (llO) 2~ CO, N2, H2. H2S
.~ Fe/Cr(lOO), (llO) 2 .
.~ Mo(lll) 2~ H2S
(100) 2l CO, H2, l~2~ H2S
(110) 2~ CO, C02, H2, N2~ H2S
f '; ' .- Ni(lll) 2~ CO, CO~, H2, NO, H2S, H25e, Cl?
f~ ~ (100) 2- Co, C02, H2, H2S, Te, Xe : (llO) 2. CO, H2, NO, H20, H2S. H2Se -~
NiO(lOl) H2, H2S, C12 ` ~ Pt(lll) 2~ CO, H2, NO, H20, S2, N
(lOG) 2~ CO, H2, NO, N, H2S, S2 :::
~ (llO) 2~ CO, C302, NO, H2S
:`f~ Rh(lll), (100) 2~ CO, C02, H'f, NO ~
0) ~ 2~ CO : :
zn(00011 : 2 ZnO(lOlO) 2 - ~;
: 1 : . . :

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TABLE I I I .
SURFACE ADSC!RBED G~S
- Fe(100) C2H4 Ir(lll) C2H2, C2H4, cyclohe~ane, benzene (lOO) C~H2, C2H4, benzene (110) C~H4, ben~ene ;
Mo(100) CH4 ~'i(711) CH4, C~H2, C2H~, C2H6, cyclohe~ane, ben-en~
(lOO) CH4, C2H2, C2H~, C2~6 benzene (110) C~l~;, C2H4, C2H6~ C5H12 . .
P~(lll) C2H2, C2H4, n-bu~ane, n-pen~ane, n-he~an~, n-hep~ane, n-occane, cyclohexane, ben~e~e, , toluene, naph~halene, pyridine, ~-~vlene, mesitylene, T-bu~ylben7ene, N-bu~ylben7ene, aniline, nicrobenzene, cyanobenzene (100) C2H2, C2H4, ben~ene naph.halene, p~ridine, ~oluene, M-~cvlene, mesi~ylene, T-bu~lbenzen , `' ~-butylben7~ne, aniline, ni.robenzen~, `i cyanobenzen~3, C2N2 ;l (110) HCN, C2N2 `,Rh(lll), (100), (331) C2H2, C2~4 As an example of the multi-sensor approach,: from Table II, .:
Mo(111): is reactive to 2 and H2S while A1(111) is sensitive to 2 but not to H2S. Therefore,-the measurement obtained with : the A1 ~lad sensor can be subtracted from the measurement with :~
S ~: : the Mo clad sensor to provide a more accurate measure of H2S.
s~ another example, Cu :and Ni both respond to numerous gases, ~.
:; including~ CO, so: Cu and :Ni sensors could be used in combination. However Cu/Ni(~lll) appears to be specific to CO, demonstrating the~ ability to use an alloy for: a specific measurement~, bu~ ~Cu/Ni(110) is not as specific: to CO so is not .
as good, showing the preference of certain c7~ystal forms or . ~.
orientations~.~As-a third example, to measure C2H6, use Ni(llI) . ~:
: which responds to~H4, CzH2, C2H4 and C2H6, subtract measurements from Mo(100~ for CH4, Fe(100) for C2H4, Ir~100) for C2Hz and:CzH~

~ WO91/14956 PCTt~S91/0213S ;

2 ~ 3 -~ 1 and benzene, Ir(llO) for C2Hs and benzene.
Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by - 5 the scope of the appended claims. -.`"
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Claims (25)

CLAIMS:

1. A fiber optic sensor for measuring refractive index of a surrounding medium, comprising:
a fiber optic core;
a thin film metal clad of metal or metal oxide or other metal compound or alloy formed on and surrounding the fiber optic core, the metal clad having an effective thickness and light transmissivity which in combination with the surrounding medium produces a localized refractive index and a controlled leakage of light which modulates the transmission of light through the fiber optic core as a function of the refractive index of the surrounding medium.

2. The sensor of Claim 1 wherein the metal clad has a thickness of greater than about 50 .ANG. and less than about 10 microns.

3. The sensor of Claim 1 further comprising:
a light source operatively associated with the fiber optic core for inputting a light signal into the core;
detection means operatively associated with the fiber optic core for detecting a light signal from the fiber optic core.

4. The sensor of Claim 1 wherein the metal clad is selected from:
a) group IB elements: Copper (Cu), Silver (Ag), Gold (Au);
b) group IIB elements: Zinc (Zn), Cadmium (Cd);
c) group IVB elements: Titanium (Ti), Zirconium (Zr), Hafnium (Hf);
d) group VB elements: Vanadium (V), Niobium (Nb), Tantalum (Ta);
e) group VIB elements: Chromium (Cr), Molybdenum (Mo), Tungsten (W);
f) group VIII elements: Iron (Fe), Ruthenium (Ru), Cobalt (Co), Rhodium (Rh), Nickel (Ni), Palladium (Pd), Platinum (Pt), Iridium (Ir), Osmium (Os);
g) oxides of the above metals:
h) group II-VI compounds: ZnSe, ZnTe, CdS, CdTe, MgTe, ZnS;
i) group III-V compounds: BN, GaAs, InAs, InSb, AlN, GaN, InN;
j) group I-VII compounds: CuCl, CuBr, AgI;
k) sphalerite structure compounds: Ga2Te3, ZnSnP2, ZnSnAs2;
l) Wurtzite structure compounds: MnS, NnSe, SiC, MnTe, Al2Se3;
m) group I, III, VI compounds: CuGaTe2, CuGaSe2, CuLaS2;
n) group II, IV, V compounds: ZnGeP2, CdGeP2, ZnGeAs2, ZnSnAs2.

5. The sensor of Claim 4 wherein the metal clad has a thickness of greater than about 50 .ANG. and less than about 10 microns.

6. The sensor of Claim 4 further comprising:
a light source operatively associated with the fiber optic core for inputting a light signal into the core;
detection means operatively associated with the fiber optic core for detecting a light signal from the fiber optic core.

7. The sensor of Claim 1 further comprising a fluorescent material immobilized at a tip of the fiber optic core with the thin film metal clad adjacent to the fluorescent tip.

8. The sensor of Claim 1 further comprising a reflective tip formed at a tip of the fiber optic core with the thin film metal clad adjacent to the reflective tip.

9. The sensor of Claim 3 wherein the light source is operatively associated with one end of the fiber optic core and the detection means is operatively associated with the other end of the fiber optic core.

10. The sensor of Claim 4 further comprising a fluorescent material immobilized at a tip of the fiber optic core with the thin film metal clad adjacent to the fluorescent tip.

11. A method of detecting the refractive index of a medium, comprising:
providing a fiber optic sensor comprising a fiber optic core and a thin film metal clad of metal or metal oxide or other metal compound or alloy formed on and surrounding the fiber optic core, the metal clad having an effective thickness and light transmissivity which in combination with a surrounding medium produces a localized refractive index and a controlled leakage of light which modulates the transmission of light through the fiber optic core as a function of the refractive index of the surrounding medium, contacting the thin film metal clad with the medium;
inputting a light signal into the core;
detecting changes in intensity of a light signal transmitted from the core.

12. The method of Claim 11 wherein the metal clad is selected from;
a) group IB elements: Copper (Cu), SiIver (Ag), Gold (Au);
b) group IIB elements: Zinc (Zn), Cadmium (Cd);
c) group IVB elements: Titanium (Ti), Zirconium (Zr), Hafnium (Hf);
d) group VB elements: Vanadium (V), Niobium (Nb), Tantalum (Ta);
e) group VIB elements: Chromium (Cr), Molybdenum (Mo), Tungsten (W);
f) group VIII elements: Iron (Fe), Ruthenium (Ru), Cobalt (Co), Rhodium (Rh), Nickel (Ni), Palladium (Pd), Platinum (Pt), Iridium (Ir), Osmium (Os);
g) oxides of the above metals;
h) group II-VI compounds: ZnSe, ZnTe, CdS, CdTe, MgTe, ZnS;
i) group III-V compounds: BN, GaAs, InAs, InSb, AlN, GaN, InN;
j) group I-VII compounds: CuCl, CuBr, AgI;
k) sphalerite structure compounds: Ga2Te3, ZnSnP2;
ZnSnAs2;
l) Wurtzite structure compounds: MnS, MnSe, SiC, MnTe, Al2Se3;
m) group I, III, VI compounds: CuGaTe2, CuGaSe2, CuLaS2;
n) group II, IV, V compounds: ZnGeP2, CdGePz, ZnGeRs2, ZnSnAs2.

13. The method of Claim 11 wherein the metal clad is formed of a thickness of greater than about 50 .ANG. and less than about 10 microns.

14. The method of Claim 11 wherein the step of providing a sensor is performed by providing a sensor further comprising a fluorescent tip on the fiber optic core, the step of inputting a light signal is performed by inputting a light signal of a wavelength selected to excite fluorescence of the tip and the step of detecting is performed by detecting changes in intensity of a fluorescent signal from the core.

15. The method of Claim 11 wherein the step of providing a sensor is performed by providing a sensor further comprising a reflective tip on the fiber optic core and the step of detecting is performed by detecting the intensity of a light signal reflected from the reflective tip back through the fiber optic core.

16. The method of Claim 11 further comprising inputting the light signal at one end of the fiber optic core and detecting a transmitted signal at the other end of the fiber optic core.

17. A method of forming a refractive index fiber optic chemical sensor comprising:
providing a fiber optic core;
forming a thin film metal clad of metal or metal oxide or other metal compound or alloy on and surrounding the fiber optic core, the metal clad having an effective thickness and light transmissivity which in combination with a surrounding medium produces a localized refractive index and a controlled leakage of light which modulates the transmission of light through the fiber optic core as a function of the refractive index of the surrounding medium.

18. The method of Claim 17 further comprising forming the metal clad of a metal selected from:
a) group IB elements: Copper (Cu), Silver (Ag), Gold (Au);
b) group IIB elements: Zinc (Zn), Cadmium (Cd);
c) group IVB elements: Titanium (Ti), Zirconium (Zr), Hafnium (Hf);
d) group VB elements: Vanadium (V), Niobium (Nb), Tantalum (Ta);
e) group VIB elements: Chromium (Cr), Molybdenum (Mo), Tungsten (W);
f) group VIII elements: Iron (Fe), Ruthenium (Ru), Cobalt (Co), Rhodium (Rh), Nickel (Ni), Palladium (Pd), Platinum (Pt), Iridium (Ir), Osmium (Os);
g) oxides of the above metals:
h) group II-VI compounds: ZnSe, ZnTe, CdS, CdTe, MgTe, ZnS;
i) group III-V compounds: BN, GaAs, InAs, InSb, AlN, GaN, InN;
j) group I-VII compounds: CuCl, CuBr, AgI;
k) sphalerite structure compounds: Ga2Te3, ZnSnP2, ZnSnAs2;
l) Wurtzite structure compounds: MnS, MnSe, SiC, MnTe, Al2Se3;
m) group I, III, VI compounds: CuGaTe2, CuGaSe2, CuLaS2;
n) group II, IV, V compounds: ZnGePz, CdGeP2, ZnGeAs2, ZnSnAs2.

19. The method of Claim 17 further comprising forming the metal clad of a thickness of greater than about 50 .ANG. and less than about 10 microns.

20. The method of Claim 17 wherein the step of forming a metal clad on the core is performed by sputtering, vacuum deposition, plasma coating, brush coating, or spin coating of said metal.

21. The sensor of Claim 1 wherein the thin film metal clad comprises a plurality of adjacent segments of different materials of selected sensitivity to components of the surrounding medium.

22. The sensor of Claim 21 further comprising:
a light source operatively associated with the fiber optic core for inputting a light signal into the core;
detection means operatively associated with the fiber optic core to detect signals produced by each segment of the core;
processing means for calculating information about a particular component of interest from the signals produced by the metal clad segments.

23. Apparatus for detecting the presence of a particular chemical species, comprising:
a plurality of fiber optic sensors, each sensor having a thin film metal clad on a fiber optic core, each metal clad being of a different material of selected sensitivity to various chemical species;
a light source operatively associated with the plurality of sensors to input a light signal into each sensor;
detection means operatively associated with the sensors to detect signals produced by each metal clad;
processing means for calculating information about one or more species from the detected signals produced by each metal clad.

24. The apparatus of Claim 23 wherein each sensor is formed on a separate fiber optic.

25. The apparatus of Claim 23 wherein the plurality of sensors are all formed on a single fiber optic with the metal clads forming adjacent segments on the fiber optic core.