WAVEGUIDE SENSOR

Abstract: A species-specific metal clad segment on a waveguide, including a planar waveguide, allows controlled light leakage of light propagating through the waveguide by total internal reflection, to measure refractive index and identify chemical species. A waveguide sensor is designed for a particular chemical species by selecting a metal clad with an affinity for the species and by matching the refractive indexes of the waveguide body, clad, metal clad segment, and chemical species. Dual or multiple measurement methods use a pair of multiple metal clad segments of different specificity. The metal clad segment may include another material to provide a suitable refractive index while having the desired affinity for the chemical species or to provide a catalyst to react the species to form a reaction product which is more readily detected.

Description: WAVEGUIDE SENSOR BACKGROUND OF THE INVENTION The invention relates generally to chemical senso using optical waveguides and more particularly to refracti index type chemical sensors using optical waveguides. Optical waveguides, structures which guide light total internal reflection, have been used in chemical sensors Most waveguide chemical sensors are fiber optic chemic sensors (FOCS) based on optical fibers which guide lig through the core by total internal reflection at the core/cla interface using a clad of lower refractive index than the core U.S. Patent 4,929,049 describes a fiber opti refractive index sensor using a thin film metal clad whic allows modulation of transmitted signal by changes in t refractive index of the surrounding medium. U.S. Pate 5,026,139 expands the concept to include selection of thin fil metal clads which are specific to analytes of interest and t use combinations of different clad materials of differe specificities to determine particular analytes. U.S. Patent 4,913,519 to Klainer, et al., describe an optical sensor for detection of ice formation or othe chemical species in which a striped clad is formed on a fibe optic core or other waveguide structure. The spaced stripe of clad material have a refractive index less than the ligh guiding means and substantially equal to the refractive inde of the chemical species to be detected. When the species i not present, the gaps between the clad strips allow ligh leakage so that little or no light is transmitted through th waveguide. When the species is present, it fills the gaps t form a complete clad which totally internally reflects ligh within the waveguide so that light is transmitted therethrough P.C.T. Patent Application PCT/CH86/00072 t Tiefenthaler, et al., describes an optical waveguide sensor t selectively determine the presence of substances and th variations of refractive index in measured substances. Th sensor is formed of a planar waveguide film, or a narrow strip with a diffraction grating formed thereon. The thin fil waveguide is formed of an oxide or polymer. The grati provides input or output coupling or acts as a Bragg reflect within the waveguide. The sample interacts with the wavegui in the region around the grating. In order to selectivel establish the presence of specific substances, a layer o selective chemisorption material is formed on the thin fil waveguide. The remainder of the waveguide surface may b covered by a low-index protective layer. According t Tiefenthaler the presence of a chemisorbed layer and/or th modification of the refractive index of the measuremen substance modifies the refractive index within the waveguid film, which produces measurable changes in the propagation o light waves therein. The metal clad FOCS of U.S. Patent 5,026,139 ar advantageous for detecting a wide variety of chemical species however, a disadvantage of the FOCS configuration is the smal surface area available and the difficulty in attaching som clad materials to the fiber optic surface. It would be highl advantageous to apply the principles of the metal clad FOC sensors to a planar waveguide of the type shown b Tiefenthaler. The larger surface area of the planar waveguid makes it much easier to apply the desired coatings particularly where clad materials of specific crystallographi orientation are utilized. The improved coatings provide sensor which can operate at high temperature with a lon lifetime which is highly desirable for in-situ long-ter monitoring applications. The ability to form patterned plana coatings also facilitates the production of waveguide sensor which use combinations of clad materials of differen specificities to detect a chemical species of interest. SUMMARY OF THE INVENTION According to one aspect of the invention, there i provided a waveguide sensor for measuring refractive index an identifying chemical species of a surrounding medium, comprising a light guiding means having a refractive index Nl; a clad having a thin film metal clad segment of metal or metal oxide or other metal compound or alloy formed on a portion of the light guiding means, the metal clad segment having refractive index N3; and an effective thickness and lig transmissivity which in combination with the surrounding medi produces a localized refractive index and a controlled leaka of light which modulates the transmission of light through t light guiding means as a function of the refractive index the surrounding medium, the remainder of the clad having refractive index N2 less than Nl. According to another aspect of the invention, the is provided a method of detecting the refractive index a identifying chemical species of a medium, providing a wavegui sensor comprising light guiding means having a refractive ind Nl, and a clad having a thin film metal clad segment of met or metal oxide or other metal compound or alloy formed on portion of the light guiding means, the metal clad segme having a refractive index N3 and an effective thickness a light transmissivity which in combination with a surroundi medium produces a localized refractive index and a controll leakage of light which modulates the transmission of lig through the light guiding means as a function of the refracti index of the surrounding medium, the remainder of the cl having a refractive index N2 less than Nl; contacting the th film metal clad segment with the medium; inputting a lig signal into the light guiding means; detecting the changes intensity of a light signal transmitted from the light guidi means. The invention may provide a waveguide sensor whi is sensitive to the refractive index of a sample. T waveguide sensor may be specific to various chemical speci on the basis of refractive index, and may use thin film met clads. A waveguide sensor with multiple species-specific cl segments may be provided. The invention may provide a refractive index bas waveguide sensor which uses planar waveguides. The waveguide sensor is formed of a planar wavegui body having a conventional clad of lower refractive ind formed on a surface thereof to provide total intern reflection of light within the waveguide. A portion of t conventional clad is replaced by at least one species-specif clad segment which produces controlled leakage of light as function of the refractive index of the surrounding mediu The species-specific clad segments are thus only partly but n totally transmissive to light. The species-specific cl segment is designed to produce a localized refractive index the interface with the waveguide body when the clad segme contacts a surrounding medium; this localized index is great than that of the waveguide body so light can leak from t waveguide. The interaction of the sample with the wavegui through the specific clad segment thus modulates th transmission of light through the waveguide as a function o the refractive index of the medium. The changing refractiv index of the medium (liquid or vapor or water emulsion) whic contacts the clad changes the transmission characteristics o the waveguide which results in a signal (light intensity change at the detector. The clad segment is of a thickness an is sufficiently light porous or light transmissive such tha the leakage of light through the clad is modulated as function of the refractive index of the surrounding mediu which may be in liquid or vapor (gas) state. The species specific clad segments may be thin film metal clads selecte from Pt, Au, Rh, Pd, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Cu, Ag Zn, Cd, Fe, Ru, Co, Ir, Os, Ni, and oxides thereof, and othe metal compounds or alloys thereof. Different cladmaterialshavedifferent specificitie to various chemical species. The specificity of differen clads can be used to make simultaneous or multiple measurement which yield a better result by subtracting out the effects o other substances. Single waveguides can be clad with plurality of different species-specific clad segments Alternatively, multiple waveguide sensors each with a differen species-specific clad could be used in combination. The cla segments are chosen with various specificities so tha information about the particular analyte of interest can b calculated from the detected signals from each sensor or fro the plurality of clad segments on a single waveguide sensor. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Figure 1 is a perspective view of a planar wavegui having a clad formed thereon. Figure 2 is a perspective view of a planar wavegui having a clad formed thereon including a clad segment whi introduces controlled light leakage. Figures 3A and 3B are side views illustrating lig transmission in the waveguides of Figures 1 and respectively. Figure 3C is a top plan view of the clad segment the waveguide of Figure 2 showing clad porosity and absorpti of sample molecules on the clad. Figure 4 is a top plan view of a planar wavegui showing various configurations and orientations of cl segments. Figure 5 is a graph of light output intensity as function of light leakage. Figure 6A is a schematic view of a measurement syst using a planar waveguide with a species-specific clad segmen Figures 6B-F are various embodiments of measureme systems utilizing planar waveguides with two or more specie specific clad segments. Figure 7 is a side view of a planar wavegui measurement system including means for blocking light whi propagates straight down the waveguides. Figure 8 is a side view of a waveguide with a cl segment formed of two layers of different materials. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is a waveguide sensor with at least o clad segment wherein the index of refraction of the wavegui clad segment and waveguide body have been selected relative the index of the species of interest to produce a controll leakage of light from the waveguide through the clad segme when the species of interest contacts the clad segment. T transmission properties of the waveguide thus vary as t refractive index of the species of interest present in t surrounding medium (in liquid or vapor or water emulsion form) and thus provide a measure of the refractive index. The cl segments are also chosen for their specificity to particul species of interest so that the sensor response identifies t species. The invention applies generally to all wavegui sensors and particularly to planar waveguides. In accordance with the invention, the waveguide bo and clad segment materials are selected with indexes Nl, N3 optimize the effect of the refractive index of the chemica species of interest. Both selective affinity and refractiv index must be considered to get required specificity an sensitivity. In a mixture it is the average refractive inde which is measured, i.e. the refractive index is one numbe which is determined by all the species present and thei concentrations. Sorting, or enhancing the response of particular species or group of compounds, is thus prerequisite to a refractive index measurement. The proper matching of the refractive indexes Nl, N2 N3, N4 in configuring the waveguide sensor forms a further par of the invention and produces an optimized sensor tailored t a particular species. The species of interest has a refractiv index N4. The clad segment is formed of a material with inde N3 which has a .known affinity for the particular chemica species. When the species of interest contacts the cla segment on the waveguide, a refractive index N* is produce which is a function of both N3 and N4. The choice of the cla segment material, as well as various characteristics of th clad segment such as thickness and porosity, in combinatio with the chemical species of interest thus produce th refractive index N* seen by the waveguide sensor in th interaction region. For the waveguide to leak light, the inde N* must be greater than Nl so a suitable material for th waveguide can be selected. Once the waveguide material i selected, the remaining clad material with index N2 can b selected. The index N2 must be less than Nl and is preferabl at least about 10% less so that the critical angle i relatively large and a significant amount of the input ligh signal will be totally internally reflected down the waveguide. The index N2 and the position of the clad segment(s) shou also be selected so that a significant portion of the total internally reflected light beam is incident on the cl segment(s) . As shown in Figure 1 an optical waveguide 10 formed of a planar optical waveguide body 12 having refractive index Nl and a clad 14 having a refractive index less than Nl formed thereon. The waveguide body 12 forms t light guiding means through which the light is guided, simil to the core of an optical fiber. The optical wavegui operates by total internal reflection of light at the wavegui body 12/clad 14 interface since the index N2 is less than for all light within an angle determined by the ratio N2/N (Only details of the sensing surface containing clad 14 a shown, although the opposed surface would be mounted on suitable substrate or be coated with a suitable coating produce total internal reflection within the waveguide 12 The waveguide 12 also has a pair of end surfaces 16 f inputting and outputting a light signal. As shown in Figure 2, the clad 14 on waveguide bo 12 contains a clad segment 18 of different refractive index N With reference to Figure 2, the chemical species of intere has an index N4. The material of clad segment 18 is chosen that it is specific to a species of interest and has an ind N3 which in combination with the index N4 of the species interest when present in the surrounding medium and the ind Nl of the waveguide produces a controlled light leakage fr which the presence of the species of interest can determined. Although the entire clad 14 could in so circumstances be used for detecting the presence of t chemical species of interest, in most cases the clad 14 wi contain at least one clad segment 18 which is specifical designed to perform that function while the clad 14 serves guide the light wave within the waveguide. Only the segment( 18 interact with the species of interest and change the lig transmission characteristics of the waveguide. The operation of the waveguides of Figures 1 and to transmit light are illustrated in Figures 3A and 3 respectively. In Figure 3 the light is input through one e face 16 of waveguide 12 and is totally internally reflected the interface between waveguide 12 and clad 14 until it output through the other end face 16. (As previous described, total internal reflection also occurs on the oppos surface of waveguide 12 from clad 14 by suitable means.) E faces 16 are preferably at angles as shown to eliminate lig reflection on entrance and exit from the waveguide. In Figu 3B, clad 14 continues to produce total internal reflection the light within the waveguide 12 but clad segment 18 allo a controlled amount of the light to leak from the waveguid thereby reducing the light output signal from the waveguid Light leakage is controlled by the selection of the index of the waveguide and index N3 of the clad segment so that i combination with the index N4 of the species of interest, t effect is optimized. As illustrated in Figure 3C, the light transmissivit or leakage is controlled by the clad segment 18 which is show as having pores (cells) 19 into which atoms or molecules 17 o the species of interest are absorbed. The combination of th clad 18 and absorbed species 17 produces a localized refractiv index which is used to control light leakage. As illustrated in Figure 4, the clad segments 18a b,c,d,e can take a variety of shapes and orientations includin stripes in any direction on the surface or square or circula areas. The ease of producing patterned coatings on a plana waveguide surface, using conventional technology such a photoresist, masking, and etching from the semiconductor field provides great flexibility in making clad segments whic maximize contact with the sample. The light intensity of the signal transmitted throug the waveguide, thus will vary as a function of the presence o the chemical species of interest which causes a light los during the transmission. The result of changes in the outpu signal are illustrated in Figure 5. The dynamic range of th sensor is designed to provide easy measurement of the specie in the concentration range of interest, i.e. the sensor i designed to produce a large signal change for a selected rang of species concentration. By appropriate choice of the senso parameters, the light loss that occurs over a measurement ran of interest is a significant percentage of the total lig signal (no loss) so that the signal difference can be easi measured. Choice of light intensity and wavelength also c be used to obtain the desired dynamic range. A simple sensor system 29 utilizing the wavegui sensor is illustrated in Figure 6A. The waveguide 26 havi a clad segment 28 formed thereon is placed between a source 2 and detector 24. Suitable optics 23, 25 such as lense filters, polarizers, etc. may be utilized between the sour and detector and the waveguide, if necessary. The signal fro detector 24 is input into processing means 27 such as a digit computer to record, display, and otherwise process the signals Although clad segment 28 is shown as a stripe oriented alo the axis between the source and detector, other orientatio or shapes of the clad segment could be used. When t surrounding medium which contacts the clad segment 28 on t surface of waveguide 26 contains a species of interest fo which the waveguide 26/clad segment 28 have been designe light leakage occurs so that the detected light intensity o the signal propagated through the waveguide decreases. Ligh sources including lasers (e.g., diode lasers), lamps, or LE can be used. The detector may be a photodiode or laser diode Various embodiments of the dual/multiple measuremen approach are shown in Figures 6B-F. As shown in Figure 6B waveguide 26 is formed with a pair of spaced parallel cla segments 28a,b each of which has a different specificity to particular chemical species or group of chemical species. pair of sources 22a,b and a pair of detectors 24a,b ar positioned around the waveguide 26, with one of each pai aligned with one of the clad segments 28a,b. By using highl directional sources 22a,b, or by using additional optics, th light beam from source 22a to detector 24a will be affected b clad segment 28a but not significantly by clad segment 28b, an light from source 22b to detector 24b will be affected only b clad segment 28b; i.e., the light beams from the two source will propagate along separate channels or portions of t waveguide and each will only be affected by one clad segment The outputs of detectors 24a,b can then be processed to provid desired information. Alternative configurations include the use of single source with a pair of detectors or a single detecto with a pair of sources. The particular configuration wil depend partly on the spectral characteristics of availabl sources and detectors. Using such arrangements, the effect of each of the clad segments 28a,b can be measured and th signals processed, e.g. by processing means 27 (Figure 6A) suc as a digital computer, to provide desired information abou particular chemical species. Of course, the principles can b generalized to more than two clad segments. As shown in Figure 6C, a waveguide 26 with space parallel clad segments 28a,b is positioned between a singl source 22 and detector 24. A mechanical shutter or chopper 3 is positioned between the waveguide 26 and detector 24 in orde to alternately pass the signal produced by one clad segmen while blocking the effect produced by the other clad segment. In the configurations of 6B and C, the clad segments should b oriented parallel to the axis between the source and detecto to define somewhat separate channels wherein different effect on signal transmission can be detected, either simultaneousl (Figure 6B) or alternately (Figure 6C) . However, if suitable sources and detectors ar available, a more general configuration of waveguide 26 wit clad segments 28a,b can be positioned between a single sourc 22 and single detector 24 as shown in Figure 6D. For example, dichromatic LED lamps and color sensors are available. Th specificity and sensitivity of a clad segment to a chemical species can also be a function of the wavelength of the light. Thus, a light signal of two wavelengths, each of which is affected differently by the two clad segments can be input and the variation at the two frequencies can be detected. Alternatively, two sources of different wavelengths and/or two detectors can be used. When the available sources and/or detectors do not allow simultaneous measurement of the effects produced by two different clad segments, it may be possible to perform sequential measurements of these effects. For example, as shown in Figure 6E, the waveguide 26 with clad segments 28a, is positioned between a source 22 and detector 24. Sample fro reservoir 32 is flowed through a control valve 36 into a pai of flow channels 34a,b which contact clad segments 28a,b respectively. The sample can first be flowed through channe 34a to produce a response due to clad segment 28a and the flowed through channel 34b to produce a response due to cla segment 28b. In addition to measuring the light transmitte through the waveguide, it is also possible to measure the ligh lost through the clad segment. For example, in Figure 6F waveguide has a plurality of circular clad segments 28a,b,c, and is positioned between a source 22 and detector 24a. Fibe optic or other suitable detectors 38a,b,c,d are positioned t measure the light leakage through clad segments 28a,b,c,d respectively. The signals from light sensors 38a,b,c,d pas through a multiplexer 40 into a second detector 24b so that th light leakage signals can be measured. The ability to accurately measure the intensit changes caused by leakage through the clad segment i important. As shown in Figure 7, a waveguide structure 12/1 with clad segment 18 is positioned between a source 22 an detector 24. Light from the source 22 which enters waveguid body 12 through end face 16 and is incident on clad 14 at a angle less than the critical angle defined by the refractiv index ratio will be totally internally reflected a illustrated. However, some light will propagate substantiall down the center of the waveguide; this is particularly so whe source 22 is not highly directional. This unreflected ligh passes straight down the waveguide and provides a larg background signal at detector 24. To reduce or eliminate thi problem, detector masking means 42 can be positioned o detector 24 and/or detector masking means 44 can be positione on end face 16 to substantially block the unreflecte background light which passes straight down the center of th waveguide while permitting substantially all the totall internally reflected light to reach the detector. The refractive index and sensitivity of the cla segment are important parameters of the waveguide sensor However, in some circumstances the available materials may n provide the desired values. One approach, as illustrated Figure 8 is to form clad segment 18 of two different material e.g. a thin metal layer 46 overcoated by a polymer layer 48 or vice-versa, which together produce the desired properties For example, metal layer 46 can be chosen for its affinity the species and polymer 48 (which is permeable to the species can provide the desired refractive index N3 of clad segment 1 (N3 will be determined by index N5 of layer 46 and index N6 o layer 48) . The refractive index of select metals, at variou wavelengths, are listed in Table 1 and the refractive index o select polymers is shown in Table 6. Combinations of these ca be used as described. A further possibility to provide selectivity of th clad segment where a suitable material is not available, bu a good material is available which is specific to a reactio product of the species, is to include a catalyst in the cla segment. Clad segment 18 can include a catalyst embedded o dispersed therein, or a separate catalyst layer can be applie on the species-specific clad layer, e.g. in Figure 8, layer 4 is the specific clad material and layer 48 is the catalyst The catalyst will catalyze a reaction of the species o interest to form a reaction product to which the clad segmen has strong affinity. As an example, TCE could be detected b a waveguide sensor having a clad segment specific to HC1 an a catalyst to react with the TCE to form HC1. In accordance with the invention, the principles o U.S. Patent 5,026,139 relating to the use of thin film meta clads and dual measurement techniques using species-specifi metal clads of different specificity has been generalized fro fiber optic waveguides to all waveguides, particularly plana waveguides. The specificity or affinity of the clad has bee further defined in terms of various parameters, particularl surface structure as well as crystallographic orientation. methodology for tailoring a waveguide sensor to a particula chemical species by appropriate choice of the refractiv indexes is provided. Multiple clad segments are used t identify a particular species from a group of species whic interact with some clad materials. A clad segment of tw different materials, one to provide refractive index and th other to provide species selectivity, can be used, as well a a clad segment with a catalyst to react the species of interes to provide a reaction product for which the clad segment ha a strong affinity. The substrate is usually glass or quartz. Thi material has a refractive index of 1.47(6) at 400 n , 1.46(2 at 500 nm and 1.45(7) at 650 nm. Special substrates can b made of a variety of materials, e.g., NaCl, N=1.54(4); AgCl N=2.07(l); Ag 2 S0 4 , N=1.75(8); MgF 2 , N=1.378; Li 2 S0 4 , N=1.46(5) and A1 2 (S0 4 ) 3 , N=1.47. In fact, any material which can be mad into a plate which can propagate light is a possible substrate In the waveguide structures according to th invention, the clad segments are typically about a monolaye to about 50 A thick. The waveguide body must have sufficien rigidity so that the light propagation pattern is not affecte by flexing. A preferred type of clad that may be utilized for th clad segments on the refractive index based waveguide senso is the metal clad disclosed in U.S. Patent 5,026,139 which i herein incorporated by reference. These thin metal clads ca be chosen with an effective light porosity or tranmissivit when in contact with a species of interest to change the ligh transmission through the waveguide as a function of th refractive index of the chemical species of interest presen in the surrounding medium. The light porosity of the clad i a function of the clad material, its thickness and its physica characteristics such as physical porosity and crystallin structure. The metal clad is selected of a suitable thicknes to produce a gradient refractive index at the interface betwee the waveguide body and the clad segment as a result of chemica species of interest from the surrounding medium being absorbe on the metal clad. This localized refractive index at th boundary between the waveguide body and clad segment i directly proportional to the amount of the chemical specie adsorbed/absorbed on the clad segment and also its refractiv index. The amount of light leaking from the waveguide throug the clad depends on the amount and refractive index of th chemical species. The light leakage substantially modulate the light signal transmitted through the core, producing detectable measure of the presence of the chemical species a its refractive index. The metal coatings are key to the selection proces They are selected both because they have specific affinity various species and because they have appropriate refracti indexes. The index of refraction of several metals, f several wavelengths, are given in Table 1. The affinity depends on the metal selected, th thickness of the coating and the method of attachment. Th crystal structure and orientation is very important in th selectivity process. Furthermore, the more ordered th crystal, the better it behaves as a sensing material, e.g. Ni(lll) and Ir(llO) both absorb benzene, but the Ni(lll) i ordered while the Ir(llO) is disordered and the Ni(lll performs much better. The type of absorption, i.e. mechanical, chemical, electrostatic, solvation, etc. als determines the effectiveness of the metal coating. The metal claddings that may be utilized as the cla segments on the waveguide include: 1) group IVB elements: Titanium (Ti) , Zirconiu (Zr) , Hafnium (Hf) ; 2) group VB elements: Vanadium (V), Niobium (Nb) Tantalum (Ta) ; 3) group VIB elements: Chromium (Cr) , Molybdenu (Mo) , Tungsten (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) , Rutheniu (Ru) , Cobalt (Co) , Rhodium (Rh) , Nickel (Ni) Palladium (Pd) , Platinum (Pt) , Iridium (Ir) Osmium (Os) ; 7) Oxides of the above metals; 8) Metal compounds or alloys such as: a) group II-VI compounds: including but no limited to ZnSe, ZnTe, CdS, CdTe, MgTe ZnS; b) group III-V compounds: including but no limited to BN, GaAs, InAs, InSb, A1N, GaN InN; c) group I-VII compounds: including but no limited to CuCl, CuBr, Agl; d) sphalerite structure compounds: includin but not limited to Ga 2 Te 3 , ZnSnP 2 , ZnSnAs 2 e) Wurtzite structure compounds: includin but not limited to Sic, MnS, MnSe, MnTe Al 2 Se 3 ; f) group I, III, VI compounds: including bu not limited to CuGaTe 2 , CuGaSe 2 , CuLaS 2 ; g) group II, IV, V compounds: including bu not limited to ZnGeP 2 , CdGeP 2 , ZnGeAs 2 ZnSnAs 2 . The metal coatings can be deposited on the waveguid surface by sputtering, brush coating, vacuum deposition, plasm coating, or by any other technique that can form either homogeneously- or heterogeneously-coated surface. Variou lithographic techniques can be used to produce the appropriat clad segment patterns. Either inorganic or organometalli compounds can be used to deposit metal coatings on th waveguide. The metals can also be deposited from thei elemental states. The final coating deposited on the waveguid can be in the form of either elemental metal or any of it oxides or other metal compounds or alloys. The clad segments are made of materials which hav a strong affinity for the chemical species of interest. Th affinity is determined by a number of different factors suc as chemical or mechanical adsorption, pore size, electrostati attraction, or ability to dissolve the species. The affinit of the clad segment for the species is illustrated in Figur 3C in which atoms or molecules 17 of the species are adsorbe in pores 19 (indicated by cell structure) formed in cla segment 18. The metal clads show varying sensitivities o selectivities for different chemical species. The variatio of sensor response to various chemical species is illustrate in Table 2 where V 0 is the detector voltage output in th absence of the chemical species (i.e., no light leakage) an V is the detector output voltage in the presence of a give amount of the chemical species. Certain metals ar particularly sensitive to certain chemical species whil relatively insensitive to others. Each chemical species ca be detected by a number of different metal clads. TABLE 2 V 3.87 4.19 4.39 4.10 5.80 3.79 5.89 2.28 3.62 4.57 6.74 1.07 1.27 1.77 4.92 6.78 5.00 1.65 Table 3 lists a number of illustrative metal surface and various gases which have been found to be adsorbed on th metal surface. The adsorbability of the gases may depend o the crystal form and/or orientation of the metal layer. In th process of forming the metal clad segment on the waveguide particular crystal forms of the metal can be reproducibl obtained by controlling the process parameters, e.g temperature and pressure, so that a sensor tailored to specific gas can be obtained. Thus, a sensor with a metal cla segment of preselected crystal structure can be produced fo use with a desired analyte. Table 4 similarly lists a numbe of illustrative metal surfaces and various organic compound (hydrocarbons) which have been found to be adsorbed on th metal surface. Certain metals, and in some cases certai crystal forms of some metals, are thus particularly suitabl for the metal clad segment for detection of certai hydrocarbons. The lists of Tables 3 and 4 are only fo purposes of illustration and are not exhaustive of all possibl combinations. Further information about adsorption of gase by metal surfaces is contained in Chemistry In Two-Dimensiona Surfaces. Garbor A. Somorjai, Cornell Univ. Press, Ithaca, NY, 1981, particularly Tables 5.2-5.7, pp. 210-241, which ar herein incorporated by reference. TABLE 3 SURFACE ADSORBED GAS Al(lll) , (100) , (110) Au(lll) (100) H 2 S, CO, Xe Cu(lll) 0 2 , CO, Cl 2 , H 2 S, Xe (100) 0 2 , CO, N 2 (110) 0 2 , CO, H 2 0, H 2 S, Xe Cu/Ni(lll) CO (110) O -2,>, CO, H 2 S Fe(lll) 0 2 , NH 3 , H 2 (100) 0 2 , CO, H 2 S, H 2 , NH 3 , H 2 0 (110) 0 2 , CO, N 2 , H 2 , H 2 S Fe/Cr(100) , (110) Mo (111) 0 2 , H 2 S (100) 0 2 , CO, H 2 , N 2 , H 2 S (110) 0 2 , CO, C0 2 , H 2 , N 2 , H 2 S Ni(lll) 0 2 , CO, C0 2 , H 2 , NO, H 2 S, H 2 Se, Cl 2 (100) 0 2 , CO, C0 2 , H 2 , H 2 S, Te, Xe (110) 0 2 , CO, H 2 , NO, H 2 0, H 2 S, H 2 Se NiO(lOl) H 2 , H 2 S , Cl 2 Pt(lll) 0 2 , CO, H 2 , NO, H 2 0, S 2 , N (100) 0 2 , CO, H 2 , NO, N, H 2 S, S 2 (110) 0 2 , CO, C 3 0 2 , NO, H 2 S Rh(lll), (100) 0 2 , CO, C0 2 , H 2 , NO (110) 0 2 , CO Zn(0001) Znθ(lθlθ) TABLE 4 SURFACE ADSORBED GAS Fe ( 100 ) C 2 H 4 Ir (lll) 2 H 2 , C 2 H 4 , cyclohexane, benzene ( 100) C 2 H 2 , C C 22 HH 44 ,, bbeeinzene ( 110) C 2 H 4 , benzene Mo ( 100) CH, Ni ( lll) CH 4 , C 2 H 2 , c 2 H 4 , C 2 H 6 , cyclohexan (100) benzene ( 110) CH C 2 H 2 , C 2 H 4 , C 2 H 6 , benzene CH C 2 H 4 , C 2 H 6 , C 5 H 12 Pt ( lll) C 2 H 2 , C 2 H 4 , n-butane, n-petane, n-hexan n-heptane, n-octane, cyclohexan benzene, toluene, naphthalene, pyridin m-xylene, mesitylene, T-butylbenzen T-butylbenzene, aniline, nitrobenzen ( 100) cyanobenzene C 2 H 2 , C 2 H ' benzene, naphthalen pyridine, toluene, W-xylene, mesitylen T-butylbenzene, W-butylbenzene, anilin ( 110) nitrobenzene, cyanobenzene, C 2 N 2 HCN, C ~ 2 N " 2 Rh ( lll) , ( 100 ) , ( 331) C 2 H 2 , C 2 H 4 The specificity of different metal clads to vario analytes allows more specific analysis of an analyte mixed wi other substances. The invention further includes multip measurement methods and apparatus. Since more than one cl may be available for a particular analyte, but each clad wi not have the same absorption coefficients to other analyte a more complex measurement method and sensor structure can used to determine the presence of a selected analyte out of mixture of several different analytes. To detect analyte A a mixture of analytes A and B, it would be most desirable find a single metal surface that is specific to A. Howeve if one cannot be found, two sensors can be used, one which h a clad which responds to both A and B and one which has a cl which responds to B but not A. By subtracting the measureme obtained from the second sensor from that of the first senso a measure of analyte A is obtained. Alternatively, if a specific metal clad is n available for either A or B, then clads are chosen which ha different absorption coefficients for A and B, and t concentration of A and B are obtained by solving an algorit for two unknowns. The dual sensor approach can be extended multiple sensors for more than two analytes and can implemented in a variety of ways, using either a sing waveguide or multiple waveguides. The crystallographic orientation of the met coatings is particularly important since the affinity various chemical species is often strongly dependent on t crystallographic form of the metal. The planar waveguide a highly advantageous structure for depositing clad segmen having preferred crystallographic orientations, sin deposition on a flat surface is much easier to perform usi known deposition techniques. As shown in Table 5, pairs of clad segments, ea responsive to different combinations of different chemic species, can be used to detect a particular species subtracting one response from another when a particular cl which responds directly to a single species is not availabl TABLE 5 Mo (110) 0 2 , CO, C0 2 , H 2 , N 2 , H 2 S Mo (100) 0 2 , CO, H 2 , N 2 , H 2 S [Mo (110)] -[Mo (100)] CO, Ru (101) 0 2 , CO, NO Co (100) 0 2 , CO [RU (101)] - [Co (100)] NO In addition to the crystallographic orientation the surface, the surface structure itself provides specificit For example, Pt(lll) adsorbs n-butane, n-pentane, n-hexane, a n-octane. If the surface structures are considered, preferen can be attained. The preferred structures of the Pt(ll surface for these species are: For these linear molecules, therefore is the general surface structure. It is importan ' t t (oinote) th for cyclohexane, this changes and the surface structure becom /4 -Λ 1 5 J . Therefore, according to the invention, by forming clad segment of a particular material, with a particul crystallographic orientation and a specific surface structur specificity for individual species can be better achieved. The Pt(lll) example illustrates the use of differe surface structures of a material to select between close related compounds with very similar chemical structures. T ability to distinguish between compounds with very simil chemical structures, without doing a detailed spectr analysis, is very important to sensor technology. The sa approach is applicable to separate benzene, toluene, a xylene. Whereas the surface structures give good speci separation, improvements can be made on specificity by furth focusing on refractive index differences. This can accomplished in two (2) ways: (i) undercoating or overcoati the metal film; and (ii) adjusting the refractive index of t substrate (Nl) by selecting a suitable material for t substrate. The overcoating (or undercoating) of the metal fil will, among other materials, be a polymer. These materials nominally, have refractive indexes between 1.34 and 1.64 Examples of these are given in Table 6. TABLE 6 Compound Refractive Inde Pol (tetrafluoroethylene-co-hexafluoropropylene) 1.338 Poly(pentafluoropropyl acrylate) 1.385 Poly(butyl acrylate) 1.466 Poly(n-butyl methacrylate) 1.481 Cellulose Nitrate 1.500 Poly (N-benzyl methacrylamide) 1.596 It is possible to find metal surfaces which wil selectively adsorb target compounds. This can range from single species to a group of compounds. These surfaces ar anything but perfect when it comes to completely isolating particular target and secondary species sorting may help. A an example, in environmental analysis, separation and analysis of the organic chlorides is an important task. Typical compounds are chloroform [N = 1.448(6)], trichloroethylen (TCE) [N = 1.477(5) ] , and the chlorophenols [N 2 . chloro = 1.557(9) , N 3 - chioro = 1-556(5), N 4 _ chloro = 1.541(9)]. The metal is chosen to selectively adsorb the organic chlorides while an overcoating (or undercoating) is used to control light leakage. For the above compounds, if the overcoating has 1.448 < N < 1.477 (e.g. , cellulose acetate butyrate) only the chloroform controls the light. As shown in Table 7, proper selection of overcoating makes species concentrations available by difference. TABLE 7 Typical ffeasuced Species Compound Nc Overcoat No Species 1 Chloroform 1.448 Cellulose acetate 1. 460 1 butyrate 2 TCE 1.477 Poly(n-butyl 1.480 1+2 methacry1ate) 3 4-chlorophenol 1.541 Poly(N-allyl 1.547 1+2+3 methacrylate) 4 3-chlorophenol 1. 556 Epoxy Resin 1.557 1+2+3 5 2-chlorophenol 1. 557 Poly (benzyl 1. 568 1+2+3+4 methacrylate) For example, to obtain inputs on 3-chloropheno (species 4) subtract 3 from 4, i.e., [(1+2+3+4) - (1+2+3) = 4] Changes and modifications in the specificall described embodiments can be carried out without departing fro the scope of the invention which is intended to be limited onl by the scope of the appended claims.

Claims: CLAIMS 1. A waveguide sensor for measuring refracti index and identifying chemical species of a surrounding mediu comprising: a light guiding means having a refractive ind Nl; a clad having a thin film metal clad segment metal or metal oxide or other metal compound or alloy form on a portion of the light guiding means, the metal clad segme having a refractive index N3; and an effective thickness and light transmissivit which in combination with the surrounding medium produces localized refractive index and a controlled leakage of ligh which modulates the transmission of light through the ligh guiding means as a function of the refractive index of th surrounding medium, the remainder of the clad having refractive index N2 less than Nl. 2. The sensor of Claim 1 wherein the light guidin means is a planar waveguide body. 3. The sensor of Claim 1 further comprising: a light source operatively associated with th light guiding means for inputting a light signal into the ligh guiding means; detection means operatively associated with th light guiding means for detecting a light signal from the ligh guiding means. 4. The sensor of Claim 1 wherein the metal cla segment is selected from: a) group IB elements: Copper (Cu) , Silve (Ag) , Gold (Au) ; b) group IIB elements: Zinc (Zn) , Cadmiu (Cd) ; c) group IVB elements: Titanium (Ti) Zirconium (Zr) , Hafnium (Hf) ; d) group VB elements: Vanadium (V) , Niobi (Nb) , Tantalum (Ta) ; e) group VIB elements: Chromium (Cr) Molybdenum (Mo) , Tungsten (W) ; f) group VIII elements: Iron (Fe) , Rutheni (Ru) , Cobalt (Co) , Rhodium (Rh) , Nick (Ni) , Palladium (Pd) , Platinum (Pt) Iridium (Ir) , Osmium (Os) ; g) Oxides of the above metals; h) group II-VI compounds: ZnSe, ZnTe, Cd CdTe, MgTe, ZnS; i) group III-V compounds: BN, GaAs, InA InSb, A1N, GaN, InN; j) group I-VII compounds: CuCl, CuBr, Agl k) sphalerite structure compounds: Ga 2 Te ZnSnP 2 , ZnSnAs 2 ; 1) Wurtzite structure compounds: Sic, Mn MnSe, MnTe, Al 2 Se 3 ; m) group I, III, VI compounds: CuGaTe CuGaSe 2 , CuLaS 2 ; n) group II, IV, V compounds: ZnGeP 2 , CdGeP ZnGeAs 2 , ZnSnAs 2 . 5. The sensor of Claim 4 wherein the light guidi means is a planar waveguide body. 6. The sensor of Claim 4 further comprising: a light source operatively associated with t light guiding means for inputting a light signal into the lig guiding means; detection means operatively associated with t light guiding means for detecting a light signal from the lig guiding means. 7. The sensor of Claim 1 wherein the metal cl segment has a preselected crystallographic orientation whi is specific to a chemical species of interest. 8. The sensor of Claim 1 wherein the metal cl segment has a preselected surface structure which is specif to a chemical species of interest. 9. The sensor of Claim 8 wherein the metal cl segment is formed of Pt(lll) with a surface structure selecte from: 10. The sensor of Claim 1 further comprising plurality of metal clad segments, each having differen selectivity to chemical species of interest. 11. The sensor of Claim 10 further comprising: a light source operatively associated with th light guiding means for inputting a light signal into the ligh guiding means; detection means operatively associated with th light guiding means to detect signals produced by modulatio by each metal clad segment; processing means for calculating informatio about a particular compound of interest from the signal produced by modulation by the metal clad segments. 12. The sensor of Claim 10 comprising a pair o metal clad segments selected from: a) Be (0001) and Cu (100) ; b) Mo (100) and Be (0001) ; c) Co (100) and Cr (100) ; d) Mo (110) and Mo (100); e) Ru (101) and Co (100) ; and further comprising means for detecting a signa produced by each of the pair of metal clad segments and mean for subtracting the signals produced by each of the pair o metal clad segments. 13. The sensor of Claim 1 further comprising a overlayer or underlayer of a second material on the metal cla segment, the metal clad segment having a strong affinity f a chemical species of interest, the second material having suitable refractive index to produce controlled light leaka in the presence of the chemical species. 14. The sensor of Claim 13 wherein the seco material is a polymer with a refractive index in the range o about 1.34 to about 1.64. 15. The sensor of Claim 14 wherein the polymer i selected from poly(tetrafluoroethylene-co-hexafluoropropylene) poly(pentafluoropropyl acrylate), poly(butyl acrylate) poly(n-butyl methacrylate) , cellulose nitrate, and pol (N-benzyl ethacrylamide) . 16. The sensor of Claim 13 wherein the metal cla segment adsorbs organic chlorides, and the second material i selected from cellulose acetate butyrate, poly(n-but methacrylate), poly(N-allyl methacrylate), epoxy resin, a poly(benzyl methacrylate) . 17. The sensor of Claim 1 further comprising catalyst on the metal clad segment for reacting with t species of interest to form a reaction product, wherein t metal clad segment is specific to the reaction product. 18. The sensor of Claim 17 wherein the catalys reacts with TCE to form HC1 and the metal clad segment i specific to HCl. 19. The sensor of Claim 2 wherein the waveguide bod is formed of Si0 2 (glass/quartz) , NaCl, AgCl, Ag 2 S0 4 , MgF 2 Li 2 S0 4 , and A1 2 (S0 4 ) 3 . 20. A method of detecting the refractive index an identifying chemical species of a medium, comprising: providing a waveguide sensor comprising ligh guiding means having a refractive index Nl, and a clad havin a thin film metal clad segment of metal or metal oxide or othe metal compound or alloy formed on a portion of the ligh guiding means, the metal clad segment having a refractive inde N3 and an effective thickness and light transmissivity whic in combination with a surrounding medium produces a localize refractive index and a controlled leakage of light whic modulates the transmission of light through the light guidin means as a function of the refractive index of the surroundin medium, the remainder of the clad having a refractive index N less than Nl; contacting the thin film metal clad segment wit the medium; inputting a light signal into the light guidin means; detecting the changes in intensity of a ligh signal transmitted from the light guiding means. 21. The method of Claim 20 wherein the metal cla segment is selected from: a) group IVB elements: Titanium (Ti) Zirconium (Zr) , Hafnium (Hf) ; b) group VB elements: Vanadium (V) , Niobiu (Nb) , Tantalum (Ta) ; c) group VIB elements: Chromium (Cr) , Molybdenum (Mo) , Tungsten (W) ; d) group IB elements: Copper (Cu) , Silve (Ag) , Gold (Au) ; e) group IIB elements: Zinc (Zn) , Cadmiu (Cd) ; f) group VIII elements: Iron (Fe) , Rutheniu (Ru) , Cobalt (Co) , Rhodium (Rh) , Nicke (Ni) , Palladium (Pd) , Platinum (Pt) , Iridium (Ir) , Osmium (Os) . 22. The method of Claim 20 comprising forming the light guiding means of a planar waveguide body. 23. The method of Claim 20 wherein the metal clad segment is a species-specific metal clad segment. 24. The method of Claim 20 further comprisi forming the metal clad segment with a preselect crystallographic orientation which is specific to a chemic species of interest. 25. The method of Claim 20 further comprisi forming the metal clad segment with a preselected surfa structure which is specific to a chemical species of interes 26. The method of Claim 20 further comprisi forming a plurality of metal clad segments, each having different selectivity to chemical species of interest. 27. The method of Claim 26 wherein the step detecting comprises detecting changes in intensity produced each of the plurality of segments and comparing the changes intensity produced by each of the plurality of segments determine the presence of a species of interest. 28. The method of Claim 20 further comprisi forming an overlayer or underlayer of a second material on t metal clad segment, the metal clad segment having a stro affinity for a chemical species of interest, the seco material having a suitable refractive index to produ controlled light leakage in the presence of the chemic species. 29. The method of Claim 20 further comprising addi a catalyst to the metal clad segment for reacting the speci of interest to form a reaction product and forming the met clad segment specific to the reaction product. 30. A method of forming a refractive index wavegui chemical sensor comprising: providing a waveguide body of refractive ind Nl; forming a clad on the waveguide body, the cl having a thin film metal clad segment of metal or metal oxi or other metal compound or alloy on a portion of the wavegui body, the metal clad segment having a refractive index N3 a an effective thickness and light transmissivity which combination with a surrounding medium produces a localiz refractive index and a controlled leakage of light whi modulates the transmission of light through the waveguide bo as a function of the refractive index of the surroundi medium, the remainder of the clad having a refractive index less than Nl. 31. The method of Claim 30 further comprisi forming the metal clad of a metal selected from: a) group IB elements: Copper (Cu) , Silv (Ag) , Gold (Au) ; b) group IIB elements: Zinc (Zn) , Cadmi (Cd) ; c) group IVB elements: Titanium (Ti) Zirconium (Zr) , Hafnium (Hf) ; d) group VB elements: Vanadium (V) , Niobiu (Nb) , Tantalum (Ta) ; e) group VIB elements: Chromium (Cr) Molybdenum (Mo) , Tungsten (W) ; f) group VIII elements: Iron (Fe) , Rutheniu (Ru) , Cobalt (Co) , Rhodium (Rh) , Nicke (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, A1N, GaN, InN; j) group I-VII compounds: CuCl, CuBr, Agl; k) sphalerite structure compounds: Ga 2 Te 3 ZnSnP 2 , ZnSnAs 2 ; 1) Wurtzite structure compounds: Sic, MnS MnSe, MnTe, Al 2 Se 3 ; m) group I, III, VI compounds: CuGaTe 2 CuGaSe 2 , CuLaS 2 ; n) group II, IV, V compounds: ZnGeP 2 , CdGeP 2 ZnGeAs 2 , ZnSnAs 2 . 32. The method of Claim 30 wherein the step o forming the metal clad segment on the portion of the waveguide body is performed by sputtering, vacuum deposition, plasma coating, brush coating, or spin coating of said metal. 33. The method of Claim 30 further comprising matching the refractive indexes Nl, N2, N3 and the refractive index N4 of the species of interest to control light leakage through the clad segment when the species of interest contacts the clad segment. 34. The method of Claim 30 further comprising applying an over/underlayer of a second material on the metal clad segment, the metal clad segment having a strong affinity for a chemical species of interest, the second material having a suitable refractive index to produce controlled light leakage in the presence of the chemical species. 35. The method of Claim 30 further comprising applying a catalyst on the metal clad segment for reacting with a species of interest to form a reaction product, wherein the metal clad segment is specific to the reaction product. 36. Apparatus for detecting the presence of a particular chemical species, comprising: at least one waveguide body, each waveguide body having at least one thin film metal clad segment on a waveguide body, each metal clad segment having a different sensitivity to various chemical species; a light source operatively associated with each waveguide body to input a light signal into each waveguide body; detectionmeans operatively associatedwith each waveguide body to detect signals produced by modulation by each metal clad segment; processing means for calculating information about one or more species from the detected signals produced by modulation by each metal clad segment. 37. The apparatus of Claim 36 wherein each meta clad segment is formed on a separate waveguide body. 38. The apparatus of Claim 36 comprising a pluralit of metal clad segments all formed on a single waveguide bod with the metal clad segments arranged in a pattern on the waveguide body. 39. The apparatus of Claim 38 comprising a pair of spaced parallel metal clad segments on a waveguide body and a light source producing a pair of parallel light beams which propagate down the waveguide body, one beam in alignment with each of the metal clad segments, wherein each beam is affected by only one of the metal clad segments. 40. The apparatus of Claim 38 comprising a pair of spaced parallel metal clad segments and alternate viewing means positioned between the waveguide body and detection means for alternately passing a light beam affected by one of the clad segments while blocking a light beam affected by the other clad segment. 41. The apparatus of Claim 38 comprising a pair of metal clad segments, each of which affects light of a different wavelength, and a light source and detection means which produce and detect light at the two wavelengths. 42. The apparatus of Claim 38 further comprising means for alternately flowing a sample over one of the metal clad segments while preventing flow over the other metal clad segments. 43. The apparatus of Claim 36 further comprising light blocking means positioned on at least one of the waveguide body and detection means for substantially blocking background light passing straight down the center of the waveguide body from reaching the detection means. 44. The apparatus of Claim 36 wherein the detection means includes means for detecting light passing through the metal clad segments.

Application year: 1992
Main classification: G02B6
IPC codes: G01N21, G02B6