Abstract:
A remote sensor element for spectrographic measurements employs a monolithic assembly of one or two fiber optics to two optical elements separated by a supporting structure to allow the flow of gases or particulates therebetween. In a preferred embodiment, the sensor element components are fused ceramic to resist high temperatures and failure from large temperature changes.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    - - - 
       CROSS REFERENCE TO RELATED APPLICATION 
       [0002]    - - - 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to instruments for the study of gases or particle laden fluids and in particular to an improved sensor system for providing spectrographic information about high-temperature or corrosive environments. 
         [0004]    Remote spectrographic measurements may be made using a fiber optic guide attached to a “U-bench” being an optical component supporting an opposed light emitter and light collector or an opposed light emitter and mirror across a gap. Gases to be analyzed may pass freely within the gap to absorb frequencies of the light creating a spectrographic signature. 
         [0005]    Precise alignment of the optical elements of the U-bench is normally obtained by close tolerance machining augmented by a separate alignment step where minor adjustments to the optical elements are made and then the optical elements fixed in a curing polymer such as epoxy. This two-step process allows a precision exceeding that obtained with normal mechanical tolerances alone. 
         [0006]    Knowledge about gaseous species can be important in the study and control of chemical reactions in high-temperature environments including internal combustion engines, coal gasifiers in power plants, or gases in high-temperature process furnaces. The epoxies used in constructing a typical sensor U-bench are normally not compatible with such high temperatures or corrosive environments. For this reason, construction of U-bench type sensors for these applications can be time consuming and expensive. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a high temperature U-bench constructed of a monolithic optical ceramic material. Two opposed lenses or an opposed lens and mirror together with the supporting structure of the U-bench are constructed of compatible materials and fused together to provide a system robust against high temperatures and wide temperature swings that might otherwise affect precision optical alignment or cause mechanical failure. 
         [0008]    In one embodiment, the invention provides a monolithic sensor assembly including an elongate member extending along an extension axis between a first and second end and having an opening between the first and second ends for passage of fluid material therethrough, and a light guiding element fusibly attached to the elongate member at the first end, the light guiding element having an optical axis generally parallel to the extension axis. A light receiving element is fusibly attached to the elongate member at the second end, the light receiving element having an optical axis aligned with the optical axis of the light guiding element and being spaced from the light guiding element by a region of the elongate member having the opening. At least one optical fiber is then fusibly attached to one of the light guiding element and light receiving element to receive light passing between the light guiding element and light receiving element through the fluid material. 
         [0009]    It is thus a feature of at least one embodiment of the invention to provide a monolithic optical sensor assembly in which the components are fused together to better resist high temperature environments. 
         [0010]    The elongate member and the light guiding element and light receiving element may be composed of at least one ceramic material selected from the group consisting of crystalline and amorphous ceramics. 
         [0011]    It is thus a feature of at least one embodiment of the invention to construct the monolithic sensor of compatible materials providing the desirable resistance to high temperature and corrosive environments. 
         [0012]    At least one ceramic material is selected from the group consisting of sapphire, silica glass, and zirconia. 
         [0013]    It is thus a feature of at least one embodiment of the invention to provide a sensor that may be fabricated out of readily available high-temperature materials. 
         [0014]    The elongate member and the light guiding element and light receiving element may be composed of an identical ceramic material. 
         [0015]    It is thus a feature of at least one embodiment of the invention to provide a monolithic structure that maintains optical alignment and integrity with extreme swings in temperature. 
         [0016]    The ceramic materials may be transparent. 
         [0017]    It is thus a feature of at least one embodiment of the invention to provide a monolithic sensor in which light transmitting optical elements may be fused to compatible support structure. 
         [0018]    The elongate member may be a substantially cylindrical tube and the light guiding element and light receiving element may provide cylindrical peripheries coaxially fitting into ends of the tube to be fused thereto. 
         [0019]    It is thus a feature of at least one embodiment of the invention to provide a simple mechanical self-alignment method suitable for use with standard ceramic shapes. 
         [0020]    The light guiding element may be a lens having an optical axis defining the optical axis of the light guiding element and the light receiving element may be a mirror substrate having a mirror surface defining an optical axis of the light receiving element. 
         [0021]    It is thus a feature of at least one embodiment of the invention to provide a sensor that may be tethered at only one end by optical fibers simplifying its placement in use. 
         [0022]    The mirror may be a dielectric mirror. 
         [0023]    It is thus a feature of at least one embodiment of the invention to provide a mirror surface that may withstand high temperatures by being constructed of two or more high-temperature dielectrics of different indices of refraction without the risk of possibly corroding metallic mirror surfaces. 
         [0024]    The mirror surface may have a concave surface facing the lens for optical focusing of light. 
         [0025]    It is thus a feature of at least one embodiment of the invention to provide improved light return for higher signal-to-noise ratio. 
         [0026]    The lens may have a diameter of greater than 0.5 mm and the optical fiber may have a diameter substantially less than 0.25 mm. 
         [0027]    It is thus a feature of at least one embodiment of the invention to provide a broad area collimated beam from a narrow single mode fiber for increasing the interaction between light and the fluid material to be measured. 
         [0028]    The lens may have a convex surface facing the mirror for optical collimation of light. 
         [0029]    It is thus a feature of at least one embodiment of the invention to provide improved beam area and capture of returned light. 
         [0030]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a simplified schematic representation of a spectrographic system employing the sensor of the present invention; 
           [0032]      FIG. 2  is an exploded perspective view of the elements of one embodiment of the sensor of the present invention prior to assembly including a lens, elongate support, mirror, and optical fiber; 
           [0033]      FIG. 3  is a cross-section along line  3 - 3  of  FIG. 2  showing the sensor after assembly of the elements by fusing; 
           [0034]      FIG. 4  is a detailed fragmentary view of  FIG. 3  showing the attachment of the optical fiber to the lens; 
           [0035]      FIG. 5  is a fragmentary perspective view of one end of the elongate support with an alternative lens design having a flange for simplified manufacturing; 
           [0036]      FIG. 6  is a cross-sectional view along line  6 - 6  of  FIG. 5  showing the fusing of the flange using an electric arc; 
           [0037]      FIG. 7  is a figure similar to that of  FIG. 3  showing an alternative embodiment providing for two fiber elements on opposed lenses; 
           [0038]      FIG. 8  is a figure similar to that of  FIG. 3  showing the attachment of two optical fibers to a single lens opposed to a mirror; 
           [0039]      FIG. 9  is a figure similar to that of  FIG. 8  showing the replacement of the convex lens with a planar lens or window element; 
           [0040]      FIG. 10  is a fragmentary perspective view similar to that of  FIG. 2  showing assembly of elements of a fiber and support to provide a window assembly; and 
           [0041]      FIG. 11  is a fragmentary cross-section similar to  FIG. 9  of the embodiment of  FIG. 10  showing the fusing of the elements. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0042]    Referring now to  FIG. 1 , a spectrographic system  10  may include a sensor head  12  that may fit within a high temperature gaseous environment  14  and communicate via a fiber optic light guide  16  having one or more optical fibers with a remote spectrometer  18 . 
         [0043]    The spectrometer  18 , in one embodiment, may include a polychromatic light source  20  providing a broad spectrum light output, for example, using an incandescent bulb in the form of a quartz tungsten-halogen lamp, or a wideband light emitting diode (LED), or broadband laser, each providing substantial energy, for example, in the range of 1000 nanometers to 3000 nanometers and preferably in the range of 1330 nanometers to 1360 nanometers and having a known spectral profile. The polychromatic light source  20  may direct a light beam to a beam splitter  22  or other similar device that in turn directs the light into the fiber optic light guide  16 . 
         [0044]    Generally, light passing through into fiber optic light guide  16  from the polychromatic light source  20  travels to the sensor head  12  to interact with the gas  60  of the gaseous environment  14  and to return along fiber optic light guide  16  where it is again received by the beam splitter  22  this time to pass to a spectrometer  24 . 
         [0045]    The spectrometer  24  may be, for example, a slit spectrometer providing a slit assembly  26  (possibly formed by a portion of fiber-optic  28  communicating with the beam splitter  22 ) and any necessary collimating optics followed by an optical grating  30 , the latter projecting a spectrum on a digital camera  32  or the like. The digital camera  32  may, for example, include a solid-state image detector  32  such as an InGaAs line scan camera commercially available from Xenics Leuven, Belgium. The spectrum may be analyzed by a computer  34  according to techniques known in the art to display absorption bands  36  or the like on a display terminal  38 . For example, the computer processing may compare a spectral profile of the received light to a known spectral profile of the transmitted light from the polychromatic light source  20 . Other measurements of this type well known in the art may be conducted by the spectrometer  24  and other forms of spectrometer  24  may also be employed including, for example, those described in U.S. Pat. No. 7,826,061 assigned to the same assignee as the present invention and hereby incorporated by reference. 
         [0046]    Referring now to  FIGS. 2 and 3 , the fiber optic light guide  16  attached to the sensor head  12  may be, in one embodiment, a single, single mode fiber  40 , attached at a center of a rear planar surface  42  of a cylindrical body of a lens  44  to be generally perpendicular thereto and aligned with the optical axis  41 , perpendicular to a plane of the rear planar surface  42 , at the point of attachment. The lens  44  provides a light receiving element for the sensor head  12 . 
         [0047]    Referring momentarily to  FIG. 4 , the single mode fiber  40  may provide for an outer cladding  48  surrounding an inner core  50  of different indices of refraction as is understood in the art. Typically, for the frequencies of interest described above, the core  50  will have a diameter of 20 micrometers or less and typically 10 micrometers or less while the cladding  48  will have a diameter significantly greater than 20 micrometers but less than 0.25 millimeters and typically of substantially 125 micrometers. In contrast, the cylindrical body of the lens  44  may have a diameter of greater than 0.5 millimeters and typically on the order of 1.2 millimeters. 
         [0048]    The attachment between the optical fiber  40  and the rear planar surface  42  of the lens  44  may be performed by fusing the two together by partial melting of each to an integrated monolithic structure with the material of the core  50  communicating directly to material of the lens  44 . Preferably, each of the materials of the core  50  and lens  44  will have similar indices of refraction. It will be appreciated that some mismatch between the materials of the core  50  and lens  44  may be accommodated by providing an angled interface (with respect to the direction of light propagation) that accommodates the difference in indices of refraction according to techniques well known in the art. This same technique may be used with other interfaces between optical fibers and optical elements of the sensor head  12  described below. 
         [0049]    Referring again to  FIGS. 2 and 3 , the cylindrical body of the lens  44  may be inserted into a support structure  52  preferably comprising a cylindrical tube extending along the axis  41  and having a bore diameter substantially equal to the outer diameter of the cylindrical body of the lens  44 . In this way the lens of  44  may slide into the support structure  52  at one end, while maintaining alignment between an optical axis of the lens  44  and axis  41 . Once inserted into the tubular support structure  52 , the lens  44  may be fused to the support structure  52  at zones  54  along an exposed interface between the tubular support structure  52  and cylindrical body of the lens  44 . This fusing may be performed by melting the materials of the support structure  52  and lens  44  together at the interface into a monolithic structure. 
         [0050]    A face of the cylindrical body of the lens  44  received within the support structure  52  and opposite rear planar surface  42  may provide a lens surface  46  being outwardly convex and having an optical axis aligned with axis  41  to collimate light received from the optical fiber  40  into a wider beam directed along the axis  41  into the support structure  52 . 
         [0051]    The support structure  52  may provide for a transverse passageway formed by openings  56  and  58  positioned across from each other along an axis perpendicular to the optical axis  41 . Openings  56  and  58  provide a channel allowing the passage of gas  60 , including species to be analyzed, across the optical axis  41  to receive light collimated and directed outward into the gas  60  by the lens surface  46 . 
         [0052]    An opposite end of the support structure  52  may receive a cylindrical mirror substrate  62  also having an outer diameter approximately equal to the bore diameter of the tubular support structure  52  so that the mirror substrate  62  may slide into the opposite end of the tubular support structure  52  maintaining its alignment with the axis  41  and then fused to the support structure  52  at zones  64  at an exposed interface between the support structure  52  and the cylindrical mirror substrate  62 . 
         [0053]    A front face of the cylindrical mirror substrate  62  facing the lens surface  46 , may provide for an outwardly planar mirror surface  66 , for example, using layered high temperature dielectric materials to create a dielectric mirror or a metallic layer to create a conventional mirror. The planar shape of the mirror surface  66  has an optical axis (surface normal) aligned with axis  41  to return light received from the lens surface  46  back along the optical axis  41  to the lens surface  46  for receipt thereby. A concave shape of the mirror surface  66  may be desirable when the lens  44  is replaced by a window as will be described below. The mirror formed by the cylindrical mirror substrate  62  and the mirror surface  66  provides a light receiving element. 
         [0054]    It will be appreciated that the components of the sensor head  12  may thus be mechanically assembled and retained by fusing without the need for heat-susceptible epoxy materials. In a preferred assembly technique, the lens  44  and cylindrical mirror substrate  62  are first assembled to the tubular support structure  52  with alignment with optical axis  41  promoted by the interfacing surfaces of these elements. These elements are then fused together. The optical fiber  40  may then be abutted without fusing against the rear planar surface  42  of the cylindrical body of the lens  44  and light transmitted through the optical fiber  40  to the lens  44 . The light received from the optical fiber  40  from the lens  44  is measured and the position of the optical fiber  40  adjusted to maximize that return of the light into the optical fiber  40  prior to fusing of the fiber  40  to the rear planar faces  42 . Upon proper positioning of the fiber  40 , the fusing process may be performed, for example, by an arc heat source as will be described below. 
         [0055]    In a preferred embodiment, each of the components of the sensor head  12  described including the fiber  40 , the lens  44 , the tubular support structure  52  and the mirror substrate  62  are constructed of a high temperature ceramic material, defined broadly herein as including both crystalline and amorphous ceramics. In one embodiment, each of the ceramics may be identical to provide both for compatibility for the fusing operation and to provide for similar coefficients of expansion to preserve optical alignment during radical temperature changes and eliminate possibly destructive stress. In any case, the coefficients of expansion for these different components may be matched for this purpose even when different materials are used. The ceramic materials may further all be transparent. Suitable ceramic materials include sapphire, silica glass, and zirconia, although other ceramics may also be used. Silica glass, also known as fused quartz, is a substantially pure silicon dioxide in contrast to soda-lime glass having ingredients such as sodium carbonate and calcium oxide. 
         [0056]    When silica glass is used, the sensor head  12  may withstand temperatures up to 1400 degrees Kelvin and provides a functional lifetime that may extend to decades without the need for replacement. 
         [0057]    Referring now to  FIG. 5 , fusing of the lens  44  and support structure  52  may be simplified by the addition of a radially extending flange  70  from a rear end of the cylindrical body of the lens  44  to provide a front radial surface  72  which may abut a rear radial surface  74  of the tubular support structure  52 . In this case an electrical arc  75  or similar heat source may be applied around the periphery of the tube of the support structure  52  to fuse the lens  44  to the support structure  52  in zones  54  removed from the fiber  40 . A similar configuration using the flange  70  may be used for the mirror substrate  62  or for dual lenses  44  and  76  to be described below. 
         [0058]    Referring now to  FIG. 7 , in an alternative embodiment, the mirror substrate  62  may be replaced with a second lens  76  being substantially identical to lens  44  but rotated to face lens  44  and having its optical axis also aligned with optical axis  41 . Lens  76 , like mirror substrate  62 , is fused to the support structure  52  in zones  64  and presents rear planar surface  78  corresponding to rear planar surface  42  to which a second optical fiber  80  may be attached. In this case optical fiber  40  may connect with the polychromatic light source  20  to provide light through lens  44  along the optical axis  41  to be received by lens  76  which may communicate that light to fiber  80  which may be received by the spectrometer  18 . This approach eliminates light loss inherent in beam splitter  22  shown in  FIG. 1 . The lens  76  thereby provides an alternative light receiving element. 
         [0059]    The lens  76  may be manufactured as described with respect to  FIG. 5  and may be constructed of the same material as lens  44  and include a lens surface  82  collimating light received from cylindrical body of the lens  76  for receipt through fiber  80 . Fiber  80  may be attached to the lens  76  in a similar manner described above with respect to fiber  40 , for example, by mechanically positioning and fusing fiber  40  close to the optical axis  41  and adjusting fiber  80  interactively by measuring the light received by fiber  80  from fiber  40  to obtain maximum light throughput. Fiber  80  may then be fused to rear planar surface  78 . 
         [0060]    Referring now to  FIG. 8 , the embodiment of  FIG. 2  may be modified to eliminate the need for a beam splitter  22  by attaching two optical fibers  40  and  40 ′ to the rear planar surface  42  close to the optical axis  41 . Fiber  40  may communicate to the polychromatic light source  20  while fiber  40 ′ may lead to the spectrometer  18 . Alignment of the fibers  40  and  40 ′ may be done by holding them in a predetermined spaced apart relationship and moving the centerline of the fibers  40  and  40 ′ together while measuring the light received from fiber  40 ′ with illumination of the fiber  40  to maximize that former value as described above. 
         [0061]    Referring now to  FIG. 9 , the lens  44  may be replaced with a window  90  (being essentially a lens  44  with a planar lens surface  46 ) having a planar face  92  opposite the rear planar surface  42  the latter which provides the connection to the fiber  40  as before. The planar face  92  simplifies the fabrication of the device and providing an achromatic optical element (operating at multiple light frequencies). 
         [0062]    Referring now to  FIGS. 10 and 11 , in yet a further embodiment, the window  90  of  FIG. 9  may be fabricated through the use of a tubular centering element  94  having an outer diameter  96  substantially equal to the inner diameter  98  of the tubular support structure  52  as described above with respect to  FIG. 2 . The tubular centering element  94  may have an inner diameter  100  equal to the outer diameter  102  of the light fiber  40 . The light fiber  40  may be inserted into the tubular centering element  94  and the two fused together to be monolithic while preserving the interface between different indices of refraction allowing light to be conducted through the window  90  formed by the fusing of tubular centering element  94  and fiber  40  to the front surface  92 . The fused tubular centering element  94  and fiber  40  are then inserted into the support structure  52  and a second fusing operation is conducted at regions  54  as described above. Or these two fusing processes may be conducted simultaneously. The natural surface tension induced contraction of the tubular centering element  94  when fused to the fiber  40  preserves the necessary alignment for this unfocused version. Alternatively an alignment step may be conducted by manipulating the window formed by the fused tubular centering element  94  and fiber  40  within the support structure  52  prior to fusing therein. 
         [0063]    It will be appreciated that the present invention need not be used with a polychromatic light source but may instead be used with a swept monochromatic light source. The term “optical element” as used herein should be understood to include generally lenses, mirrors and windows that transmit, reflect or refract a light beam and that may, but need not, cause convergence or divergence of the light beam. 
         [0064]    Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
         [0065]    When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0066]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.