Fiber optic refractive index sensor using a metal clad
BACKGROUND 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 optical waveguide which transmits light by total internal reflection (TIR) at the core/clad interface. The critical angle A c  for TIR is determined by the ratio of the refractive index N 2  of the clad to the refractive index N1 of the core: A c  =sin -1  (N 2  /N 1 ). Thus the index of the clad must be less than the core for TIR to occur. 
     Optical fibers have been used in a wide variety of sensors, known as &#34;optrodes&#34; or &#34;fiber optic chemical sensors&#34; (FOCS), which are designed to measure the presence of various chemical species or the value of various parameters such as pressure or temperature. In most cases a signal from a reactant, e.g. a fluorescent signal from a fluorescent dye which interacts with the desired chemical species or is affected by the desired physical parameter, is transmitted through the fiber 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. application Ser. No. 046,986 filed May 6, 1987 (now U.S. Pat. No. 4,846,548) is directed to a more generalized fiber optic sensor methodology in which the principle of detection is based on how the operating 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 many different sensors which are sensitive to particular species or even groups of species. However, it would also be desirable to have available a more general sensor which can detect a wide variety of species. Since different species usually have different refractive 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. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a fiber optic sensor which measures refractive index. 
     It is also an objective of the invention to provide a fiber optic sensor 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 thin film metal clad. In one embodiment, the sensor has a fluorescent tip formed of a fluorescent dye immobilized on the tip of the fiber. An excitation signal is transmitted through the fiber 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 that an incident signal is transmitted back. In a third embodiment, the source and detector are positioned at opposite ends of the fiber so a transmitted signal is detected. The change in refractive index of the medium surrounding the fiber changes the transmission characteristics which results in a signal change at the detector. In one embodiment, platinum (Pt) paint is applied on the side of a fiber core and is then heated with a 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 (Au), rhodium (Rh). and palladium (Pd) are suitable for the thin film metal clad. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIGS. 1A,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. 
     FIG. 2 is sensor system for measuring refractive index of a sample. 
     FIG. 3 is a graph of signal intensity as a function of solvent refractive index for a series of different solvents. 
     FIG. 4 is a graph of the response of a gasoline sensor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is a fiber optic sensor with a thin metal clad on a fiber optic core which varies its transmission properties as a function of the refractive index of the surrounding medium, thus providing a measure of refractive index. 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 third embodiment, input light from a source at one end of the fiber is transmitted through the fiber and the transmitted signal is measured by a detector at the other end of the fiber. 
     As shown in FIG. 1A, sensor 10 has a core 12 with a fluorescent tip 14 and a thin metal clad 16. In a preferred embodiment core 12 is a silica fiber core, e.g. an Ensign Bickford MaxCore HCS fiber, with a core diameter of 400, 600 or 1000 microns and an attenuation of 10dB/km at 597μm. About one inch of the end of a fiber is stripped to the bare core, e.g. by burning off the cladding. A fluorescent chemical (dye), e.g. Rhodamine B, is attached to the distal end of the fiber core. One method of attaching the 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 shrink tubing which is then filled with methanol or other solvent to 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 fluorescence sack. Optical adhesive, e.g. Norland #61, is then coated on the outside of the entire fluorescence sack surface of the shrink tubing to completely seal the fluorescer. 
     In the embodiment of FIG. 1B a reflective tip 18 is formed onto the end of fiber optic core 12 with its thin film metal clad 16. In the linear (dual end) embodiment of FIG. 1C, a portion of fiber optic core is clad with thin metal film 16. 
     In order for the core to transmit light efficiently, the core 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 solvents, the light is transmitted with an efficiency which depends on the refractive index of the solvent. The solvent, in essence, becomes the clad. The lower the refractive index of the solvent, the more light is transmitted through the core. 
     In accordance with the invention, a thin metal clad on the fiber core enhances the change in light transmission of the core as a function of the refractive index of the surrounding solvent. 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 embodiment 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 hexachloroplatinate(N)hydrate and torched. Thin films of other materials such as gold (Au), rhodium (Rh) and palladium (Pd) may also be formed. 
     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 other metals including Au, Rh and Pd, can be applied to the side of the fiber core by painting, sputtering, or other suitable method such as plating from solution. The layer is heated, if necessary, to remove organic components. A very thin film, e.g. a few monolayers or less, is desired because the evanescent wave in the fiber core only penetrates a short depth into the clad. The thickness of the thin film is important; an effective thickness allows the transmission of the fiber core to be strongly affected by the refractive index of the surrounding solvent. The thickness should be less than the penetration depth of the evanescent wave propagating through the fiber core so the surrounding solvent can affect transmission. 
     A sensor system 20 is illustrated in FIG. 2 A sensor 10, as shown in FIG. 1A, 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 at 488nm producing 5mW power, is 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 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 solvent in sample region 24. Similar systems can be formed with the sensor of FIG. 1B where the reflected signal is measured instead of a fluorescent signal, or with the sensor of FIG. 1C where the detector is placed at the opposite end of the fiber from the source and the transmitted signal is measured. 
     FIG. 3 illustrates the response of a general 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 different 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. FIG. 4 shows the response of a refractive index sensor to repetitive samples of gasoline. 
     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 the scope of the appended claims