MXPA97002263A

MXPA97002263A - Attenuated total reflectance sensing

Info

Publication number: MXPA97002263A
Application number: MXPA/A/1997/002263A
Authority: MX
Inventors: A Stevenson William
Original assignee: Fostermiller Inc
Priority date: 1994-09-29
Filing date: 1997-03-26
Publication date: 1998-04-01

Abstract

A radiation transmission optical fiber for spectroscopic monitoring includes a transmission portion (16, 18) and a sensor portion;the transmission portion (16, 18) has a continuous core portion and a continuous cladding over the core portion;the sensor portion has the cladding removed from one side of the fiber and the core portion exposed from the same side leaving the continuous cladding intact over the opposite side of the core portion of the sensor.

Description

DETECTION OF ATTENUATED TOTAL REFLECTANCE Background of the Invention This invention relates to spectroscopic technology and more particularly to technology for analyzing material using total reflectance technology attenuated by optical fiber. Spectroscopy is frequently used in a qualitative and quantitative analysis of materials. Infrared radiation detection techniques are often advantageous over spectroscopic techniques that use radiation of shorter wavelengths, such as visible or ultraviolet light, as organic and biological materials have intense and relatively narrow, unique identification absorption peaks, characteristic , in the infrared region. Fourier transform infrared spectroscopic monitoring (FTIR) is useful in spectroscopy, as discussed, for example, in U.S. Patent Nos. Re. 33,789 to Stevenson; 5,070,243 of Borstein et al; 5,239,176 to Stevenson; and 4,852,967 from Cook. The material that is being analyzed or monitored can be gaseous, liquid or solid. This invention relates to the use of an optical fiber as a multiple internal reflection (MIR) sensor and more particularly to the technology of using optical fibers as MIR sensors to carry out measurements of both emission spectroscopy and absorptive spectroscopy. of a material of high absorption or high dispersion, a technique sometimes referred to as attenuated total reflectance (ATR) or evanescent wave spectroscopy. SUMMARY OF THE INVENTION In accordance with one aspect of the invention, a radiation transmission fiber is provided for spectroscopic monitoring that includes a transmission portion and a sensor portion. The transmission portion has a continuous core and a continuous coating on 100% of the transmission portion and between 40 to 60% of the sensor portion. The rest of the sensor portion has an exposed core surface (which is flattened in particular embodiments, but can be in other forms, such as cylindrical, as appropriate), both the coating and the core are mechanically removed by grinding and polishing with suitable optical abrasive compounds or chemically removed by etching with a suitable etchant such as potassium hydroxide, zirconium oxychloride or hydrogen fluoride. The fiber can be as short as one centimeter and in a particular embodiment, the sensor portion is about one centimeter long. The core of the sensor fiber preferably is of a chalcogen glass such as arsenic selenium tellurium, arsenic trisulfide, germanium selenium tellurium, arsenic germanium selenium; a heavy metal fluoride glass such as zirconium, barium, lanthanum, aluminum, sodium fluoride; fused silica or silicate glasses, or single crystal materials such as silver halides, thallium bromoiodide, and cesium or sapphire halide. Preferably, the core has an initial diameter before removal by cutting at least fifteen micras but less than one millimeter and a refractive index greater than 1.5. Preferably, the fiber includes structure for changing the mode structure of the light beam propagating within the fiber such as one or more acute bends and / or by means of conical transition portions such as spindles. In a particular embodiment, the transmission portion has a chalcogen glass core with a diameter of about 750 microns and a chalcogen glass coating layer of about 125 microns thick; the core and the coating of the sensor region are removed by cutting at about the center of the core or at a total depth of about 500 microns in a length of about one centimeter. The optical fiber in the transmission portion has a numerical aperture of 0.5, the glass core has a glass transition temperature of 136 ° C, a coefficient of thermal expansion of 23.6 x 10"6 / ° C and a refractive index at a wavelength of 10.6 microns of 2.81, while "that the glass coating has a glass transition temperature of about 105 ° C and a refractive index of about 2.18 at a wavelength of 10.6 microns.

When such a sensor is encapsulated or planted in a typical optical epoxy, the coating glass and / or core in the sensor region can be accurately cut to the desired depth using conventional optical grinding and polishing techniques. The optical epoxy provides a firm, tough support for the fiber and is ground and polished at the same rate as glass. This provides a continuous firm support and a mounting for the fiber that can be used to assemble and protect the fragile sensor in a variety of different ATR probes. The evanescent wave propagating on the polished surface of the core glass is not absorbed by the epoxy since the coating glass on the underside of the sensor region is the only part that is in close optical contact with the epoxy. By combining this asymmetrically exposed core sensor with optical mode alteration techniques such as simple bending in the form of a U or using biconical spindles, qualitative and quantitative spectral measurements can be achieved that match those obtained by the best core / cladding sensors tapering A major difference is the ease of reproducible manufacturing. The fibers can be bent precisely using simple abutments and can be permanently attached to a variety of suitable optical cements. Removal by cutting or removal of the sensor can be controlled very accurately using a variety of well-known grinding and polishing equipment and techniques.

A permanent protective support for the sensor can be provided by planting it in a hard, tough and durable optical cement that does not interfere with the operation of the sensor. This is particularly true when the sensor is to be used to monitor spectroscopically solids, abrasive powders, flowing viscous liquids, and high velocity gas streams. According to another aspect of the invention, there is provided a spectroscopic system that includes a radiation source for generating a broadband radiation beam, a detector, a spectrum analyzer apparatus, and an elongated radiation transmission fiber for arrangement in an absorption means comprising a transmission portion and a sensor portion, and a coupling structure for optically coupling the transmission fiber with the source for transmitting a radiation beam through the fiber to the sensor portion and for coupling the absorbed beam back to the detector and the spectrum analysis apparatus to analyze the absorption medium in which said sensor portion is disposed. The fiber length can vary from less than one centimeter to ten meters or more. The transmission and sensor portions are previously described. BRIEF DESCRIPTION OF THE DRAWINGS Other aspects and advantages of the invention will be observed upon advancing the following description of particular embodiments, in conjunction with the drawings, in which: Figure 1 is a diagrammatic view of a wave fiber optic sensor evanescent according to the invention, and Figure IA is a sectional view along line 1A-1A of Figure 1; Figure 2 is a diagrammatic view of another evanescent wave sensor according to the invention; Figure 3 is a diagrammatic view of an evanescent wave optical fiber sensor according to the invention, configured in a U; Figure 4 is a diagrammatic view of yet another evanescent wave fiber optic sensor configured in a U-shaped fold, with complementary bends in the transmission portion just before and after the sensor region, and all the fiber encapsulated in a optical cement; Figure 5 is a schematic diagram of a spectroscopic system employing the sensor of Figure 4; Figure 5A is an enlarged diagrammatic view; and Figure 5B is a sectional view along line 5B-5B of Figure 5A; Figures 6 and 7 are schematic diagrams of variations of the system of Figure 5; and Figure 8 is a graph of absorbance spectra of 100% isopropanol, obtained with a spectroscopic system of the type shown in Figure 5 and with an optical fiber sensor according to the invention sketched in Figure 4 and a sensor optimally designed tapered core / cladding according to U.S. Patent No. 5,239,176 to Stevenson. Description of the Particular Forms of Realization With reference to the diagrammatic views of FIGS. 1 and 1A, the optical fiber 10 includes the core portion 12 of chalcogen glass of arsenic, selenium, tellurium (AsSeTe) and a coating layer 14 of a chalcogen glass of arsenic sulfide (AsS) with a lower refractive index. The fiber 10 has transmission portions 16, 18, transmission core portions 12T, each having an outer diameter of approximately 750 microns, and transmission coating portions 14T, each having an outer diameter of approximately one millimeter. The sensor portion 20 has a length of about 4 cm, with the core portion 22 being semi-circular in shape and having a relatively flattened surface 24 that is approximately 750 microns wide, and a semi-circular coating 26 which is approximately 125 microns thick. As indicated, a beam of light 28 propagates at reflection angles within the numerical aperture of the fiber. More reflections per unit length occur in the sensor region 20 due to the reduced cross section of the core portion 22 in the sensor region 20. The fiber 10 is processed by encapsulating the entire fiber in a suitable optical cement and then grinding and polishing the sensor region 20 with suitable optical abrasives until the core portion 22 and the attached coating portion 26 are removed or removed by approximately 50% and a smooth, flattened surface of the sensor 24 is exposed. Figure 2 sketches a fiber 10 'containing spindles between the transmission portions 16' and 18 'and the exposed core surface 24' of the sensor portion 20 '. The spindles are in accordance with the teachings of U.S. Patent No. 5,239,176 to Stevenson and create more higher order modes in the sensor region 20 'and then restore these modes to the normal propagation modes in the transmission region , as shown by the beam trace 28 ', in this way creating a more sensitive sensor. Figure 3 outlines another sensor fiber 10"according to the invention in the form of a square U with a turn portion 19 in which the sensor region 20" is disposed. The sensor region 20"is approximately two centimeters long and includes the flattened core surface 24". The asymmetric rearrangement of the structure in mode to higher order modes is achieved in the folds by 90 ° in the transmission portions 16", 18". Figure 4 outlines another sensor fiber suitable for mounting in small diameter "needle probes" of approximately 5 mm, designed to make evanescent wave spectral measurements in confined spaces such as test tubes or small diameter cylinders. A tight U-bend 31, of about 2 mm radius, is combined with relatively low angle bends 29 to produce higher order modes in the sensor region 40 in a compact sensor. With reference to Figure 4, the sensor 30 includes an optical fiber of a similar type to the fibers of the sensors shown in Figures 1-3 and includes the core portion 32 of about 750 microns of external diameter and coating layer 34. about 125 microns thick. The fiber 31 is attached to a suitable epoxy optical cement 36, such that the terminal portions 38 at the end of the transmission portions 31T are flush with the end surface 37 of the epoxy 36. The fiber 30 is bent (in FIG. ) twice, each at an angle of about 15 ° and again to form a tight bend at U 31 radius of about 2 mm, the housing 36 having an external diameter of about 5 mm. The epoxy housing support 36 is polished with suitable optical abrasives until the core portion 32 and the attached coating portion 34 are removed approximately 50% and a smooth, flattened sensor surface 40 is formed at an angle of about 15. ° with the axis of the cylindrical portion of the housing 36. The U-shaped bend 31 of about 2 mm radius on the exposed surface 40, together with the relatively low angle bends 29, produces higher order modes in the sensor region 40 in a compact sensor that is about 5 mm in diameter and has a sensor surface 41 that is about 1 cm in length. A coating of hard, optically transparent material, such as magnesium fluoride, may be applied to the polished sensor surface 40 for use in contact measurement applications such as with abrasives, solids or human tissue. Additional aspects of the sensor shown in Figure 4, in combination with an FTIR analysis system, are shown in Figure 5. The sensor 30 is mounted on a stainless steel probe support tube 42 on which coupling cables are attached 44 by suitable optical cement 45 and has exposed coupling ends within the tube 42. Formed in the tube 42 is a wedge guide recess 46 and an annular recess receiving the O-ring 48. Coupled to the core fiber optic cable and Inlet transmission liner 44 is an FTIR 50 spectrometer of the Michelson interferometer type which includes an infrared source 54, beam splitter 56 and focusing mirrors 58, 60. Coupled to the core optical fiber output transmission cable and Liner 44 is an MCT (mercury cadmium telluride) detector 62, the connecting amplifier 64 and the output processor 66 that includes the display 68. A sensor 30 of the type shown in FIG. Figure 4 is inserted into the housing 42 with axial and radial alignment by a wedge 47 that links the wedge guide 46, and the end surfaces 38 of the sensor fiber 31 are biased against the end surfaces of the transmission cables 43, 44 by the fluoroelastomer 0-ring 48. A removable protective liner 70 may be disposed on the protruding portion of the sensor 30, the bevelled end surface 72 being flush with the sensor surface 40 for applications to monitor solid material such as particles abrasive or human tissue, and protruding slightly where the material to be monitored is a liquid. A sensor of the type according to the invention, sketched in Figure 4, was connected to the analyzer apparatus sketched in Figure 5. Measurements to determine sensor sensitivity, dynamic range, performance and signal-to-noise ratios were carried out as follows. The system was set for resolution of four wave numbers (4 cm "1), a scan time of one minute (52 scans), and a spectral range of 4,000 to 1,000 cm" 1. Anhydrous, pure isopropanol was used as the test analyte. With the sensor 30 connected, a spectrum of a single beam of the system was obtained in air. The sensor 30 was then immersed in isopropanol and a second spectrum of a single beam of the system was obtained with the sensor 30 immersed in isopropanol. The second spectrum was related to the first to produce an absorbance spectrum of isopropanol. A tapered core / cladding sensor of the type shown in U.S. Patent No. 5,239,176, with appropriate transmission cables, was then replaced by the sensor of Figure 4 and cables according to the invention, and similar spectra were obtained under the same experimental conditions. Figure 8 shows the comparative results. The core removed sensor (figure 4) showed a peak absorbance height at 1,126 cm "1 (a higher analytical peak for isopropanol) of 0.55 absorbance units, while the tapered sensor shows an absorbance value of 0.6 absorbance units for the same peak. The rms noise was measured for both spectra in the region between 1,810 and 1,850 cm "1. The total noise for the removed core sensor of Figure 4 was 0.00015 absorbance units, resulting in a signal-to-noise ratio of 3,667. The total noise for the tapered sensor of the type shown in U.S. Patent No. 5,239,176 was 0.00024 absorbance units between 1.810 and 1.850 cm "1, resulting in a signal-to-noise ratio of 2,500 to 1. In the global performance, the sensors are approximately equivalent. In another embodiment, shown in Figure 7, the fiber 10 includes a simple transmission portion 16 with the retro-reflector 92 at the remote end of the sensor portion 20 so that the transmitted beam, modified by absorbance in the sensor 20, be reflected back through the portion 16 to the beam splitter 94; and in another embodiment, shown in Figure 6, the fiber 10 has several sensors 20 created according to the invention along its length. Although particular embodiments of the invention have been described and shown, other embodiments will be apparent to those skilled in the art, and therefore the invention is not intended to be limited to the embodiments described, or to their details, and that aspects that depart from them can be foreseen, but within the spirit and scope of the invention.

Claims

CLAIMS 1. Radiation transmission fiber sensor apparatus for spectroscopic monitoring, comprising an optical fiber with a transmission portion and a sensor portion, said transmission and sensor portions having a continuous portion of core and a continuous coating on said core portion through all said transmit and sensor portions, characterized in that said coating in said sensor portion is of asymmetric configuration such that an uncoated core surface portion in said sensor portion is provided for exposure to the material that it will be monitored spectroscopically. 2. Spectroscopy apparatus comprising a radiation source for generating a radiation beam, an apparatus for analyzing spectrum, a sensor including an elongated radiation transmission fiber for arrangement in a material of interest comprising a transmission portion and a sensor portion, said transmitter and sensor portions having a continuous core portion and a continuous coating on said core portion through said transmit and sensor portions, and a coupling structure for coupling said transmission fiber to said sensor portion. source for transmitting a beam of infrared radiation through said fiber to said sensor portion and for coupling said fiber to said apparatus for analyzing in order to analyze the absorption medium in which said sensor portion is disposed, characterized in that said coating in said sensor portion is of asymmetric configuration such that a portion of the surface The uncoated core in said sensor portion is provided for exposure to the material to be monitored spectroscopically. 3. The apparatus of claim 2, wherein said source is of the Michelson interferometer type and generates a beam of infrared radiation. 4. The apparatus of claim 2 or 3, wherein said apparatus for analyzing is of the Fourier transform type. The apparatus of any of the preceding claims, further characterized by the provision of an encapsulation support structure surrounding said optical fiber with said sensor surface exposed on a surface of said support structure. The apparatus of any of the preceding claims, further characterized in that said coating in said transmission portion has a thickness sufficient to contain the evanescent field at wavelengths of analytical interest. The apparatus of any of the preceding claims, further characterized in that said core is selected from the group comprising chalcogen glass such as arsenic sulfide, selenide of arsenic germanium, or tellurium of germanium selenium, heavy metal fluoride glass, glass of oxide such as silica glass, and single crystal or polycrystalline materials such as thallium, bromoiodide, cesium halide or silver halide. The apparatus of any of the preceding claims, further characterized in that said sensor core portion has a diameter in the range of 15 to 1,000 microns and a refractive index greater than 1.5. The apparatus of any of the preceding claims, further characterized in that said transmission portion has a chalcogen glass core with a diameter of about 750 microns and a chalcogen glass coating layer of a thickness of about 100 microns; and the core surface exposed in said sensor region has a length of at least about one centimeter and a width of less than one centimeter. The apparatus of any of the preceding claims, further characterized in that said exposed surface is flattened and has a width of at least about one third of the diameter of said core portion. 11. The apparatus of any of the preceding claims, further characterized in that said fiber is formed U-shaped with a turn portion, and said exposed core surface extends towards said turn portion. The apparatus of any of the preceding claims, further characterized in that said fiber further includes bends of relatively low angle in separate portions of said turn portion. The system of claim 12, further characterized in that the angle of said low angle bends is about 15 °.

1 . The apparatus of any of the preceding claims, further characterized by the provision of a sensor housing structure and a structure for releasably retaining said sensor in said alloy structure.