Patent Publication Number: US-2021190693-A1

Title: Adjustable extended focus raman system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the priority benefit from U.S. Patent Application Ser. No. 62/950,837, filed Dec. 19, 2019, which is hereby incorporated by reference herein in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to an apparatus for conducting Raman spectroscopy which utilizes an aspheric diffuse ring producing optic, or ADRPO, to generate an illumination spot with a variable radial intensity and a simple movable optic to alter the diameter of the illumination spot. 
     BACKGROUND 
     Optical scanning methods such as Raman spectroscopy can provide useful information about unknown materials. By identifying unknown materials, certain threats posed by hazardous materials can be reduced or eliminated. While the problem of fluorescence can be avoided completely by removing the material from its container, in field applications this is not normally an option. Sampling can also cause contamination of the sample or cast doubts on the integrity of the sampling method by untrained personnel. 
     Ideally a Raman spectroscopy system can test a sample while it is still in its packaging container. A fundamental problem with sampling through a container wall is that the packaging materials cause excessive fluorescence signal potentially obscuring the Raman signal from the material under test (e.g. diffuse scattering). 
     Such diffusely scattering packaging materials are frequently encountered in many common applications. Examples of such include but are not limited to plastic bottles, paper labels, baggies, vials, blister packaging, etc. The diffusely scattering packaging materials encountered in such applications are often highly heterogeneous and can be made of various layers each having a different chemical make-up. 
     Therefore, a primary goal for analytical spectroscopy is to provide a system capable of determining the chemical composition of packaged materials in a non-destructive manner. 
     BRIEF SUMMARY 
     Systems, methods, and products to address these and other needs are described herein with respect to illustrative, non-limiting, implementations. Various alternatives, modifications and equivalents are possible. 
     In general, in a first aspect, the disclosure features an apparatus that produces an illumination spot on the sample surface that has a non-uniform radial intensity profile that can further be adjusted in diameter. The spot could be of uniform radial intensity but of much larger diameter than the sampling optics field of view. Alternatively, the spot could be of non-uniform radial intensity, with weaker portions near the collection area. These weaker portions produce lower background or interfering Raman signal, thus allowing a potentially higher signal to noise in the sample signal. It might be desirable to focus the light source beam to a tight focus exactly at the collection area, in order to measure the package&#39;s Raman spectrum directly in a more traditional Raman arrangement. This packaging spectrum could then be used to remove background signal from any other spectrum containing both package and content signals. The present disclosure shows how to move a single lens in the system to change the excitation between distributed and tight focus conditions. 
     The size of the spot can be altered from compact (e.g. about 50 micron) to large (e.g. about 5 mm) while remaining centered on the same location. The sampling optic could also be altered in size in conjunction with the size change of the illumination spot. The sampling optic could alternately be moved or resized in conjunction with the change in illumination spot. 
     For use in a handheld analysis device where compact size is advantageous. Additional mirrors and focusing elements may be employed in order to fit the optic design into the available constraints. Use of such off-the-shelf components for manipulating the beam path have been contemplated by the inventors. 
     For use in emergency situations where speed of analysis is a concern. Minimizing the time required, particularly the time required collecting data, it of importance to field personnel who may be operating in hazardous environments. 
     For use in handheld analysis device where simplicity and ruggedness of the moving elements is appreciated. The movement of just a single component in the optical design is advantageous as it reduces the complexity of the device and reduces the opportunities for mechanical failure. The simple linear motion also allows for more rugged solutions amenable to the handheld application space. 
     This invention is intended for use by field personnel in outdoor environments as opposed to laboratory personnel in controlled environments. This use model necessitates a high level of automation, ruggedness, and highly values ease-of-use. The automated system control of spot size and/or detection size allows for use of the device with a minimum level of expertise in spectroscopy. The desire to minimize the complexity and increase the robustness can also be appreciated in the field-use application. 
     We have disclosed a design producing a distributed illumination spot with lower intensity near the center, and a means of collapsing the spot into a tight focus. The lower intensity of the distributed spot near the center allows for collecting Raman scattering spectra of materials inside a package, while the tighter focus spot allows sampling the packaging material spectrum so that it may be later subtracted from spectra containing both sample and packaging components. 
     Embodiments of the apparatus and method can also include any of the other features or steps disclosed herein, as appropriate. The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description, drawings, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number refer to the figure number in which that element is first introduced. 
         FIG. 1  illustrates an item  100  including the optics  102 , user  104 , computer  108  and Raman spectrometer  106  in communication with one another. 
         FIG. 2  is an illustration of one embodiment of optical configuration  200  of optics  102  configured to achieve a variable focus distributed illumination spot. 
         FIG. 3  illustrates spot intensity distribution  300  in accordance with one embodiment. 
         FIG. 4  illustrates cumulative intensity plot  400  that shows the relative intensity by radius distance from a centroid in accordance with one embodiment. 
         FIG. 5  illustrates cumulative intensity plot  500  in accordance with one embodiment. 
         FIG. 6  illustrates optical arrangement  600  in accordance with one embodiment. 
         FIG. 7  illustrates false color spot image  700  in accordance with one embodiment. 
         FIG. 8  illustrates adjustable illumination spot diameter  800  in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     ““traditional” Raman measurement” refers to a Raman system where the illumination spot diameter remains fixed size and has a uniform radial distribution 
     “Aspheric diffuse ring producing optic” refers to One embodiment for producing the distributed spot includes an aspheric diffuse ring producing optic, or ADRPO. For example, embodiments of aspheric optics may include what is referred to as an axicon or conical optic which produces a ring of intensity but has higher order aspheric terms to produce the spread-out pattern. In the described example the aspheric optic may have coefficients of A1=−0.01, A2=−0.06, and A4=0.002, with all other terms being zero. 
     “Collimating lens” refers to optical elements that transform the incoming light direction to parallel paths 
     “Filter” refers to optical elements that remove some wavelengths of incoming light 
     “Focusing optics” refers to optical elements that transform the incoming light direction to a point in space 
     “Light source” refers to A light source used for excitation in a spectroscopy application. May include a laser that is adapted for Raman spectroscopy such as 785 nm or 1064 nm. The light source could also include a broad band source such as an LED. 
     “Moving mount” refers to a mechanical assembly allowing an optical element to move in a linear fashion 
     “Non-uniform radial spot distribution” refers to an illumination area that has an intentionally varying uniformity from center to edge 
     “Sample surface plane” refers to the surface of the sample under test where the illumination area is directed 
     “Steering mirrors” refers to optical elements used to change the direction of light path 
     “Uniform radial spot distribution” refers to an illumination area that has a nominally consistent uniformity from center to edge 
     “Voice coil” refers to Voice coil actuators (VCAs) are direct drive, limited motion devices that utilize a permanent magnet field and coil winding (conductor) to produce a force that is proportional to the current applied to the coil. 
     The subject invention is now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It may be evident, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the present invention. 
     As used herein, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or.” That is, unless specified otherwise, or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A, X employs B, or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances. Moreover, articles “a” and “an” as used in the subject specification and annexed drawings should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form. 
     Raman spectroscopy systems can be used in a variety of environments to identify unknown materials, to evaluate the threat posed by unknown materials, to provide positive identification of packaged raw materials, and to provide general security screening functions of a variety of substances. Raman spectroscopy systems can be implemented in a wide range of sizes, from portable, handheld instruments to larger systems in permanent laboratories. 
     As depicted in  FIG. 1 , user  104  interacts with Raman spectrometer  106  and computer  108  in order to take Raman spectroscopy measurements using the system. In one embodiment, computer  108  and Raman spectrometer  106  are packaged together as a portable system that can be operated on battery power for use outside a laboratory setting. The system may be a fully integrated device or may be configured as separate units for computer  108  and Raman spectrometer  106 . In some embodiments, computer  108  may be located separate from Raman spectrometer  106  providing the opportunity for increased computing power at a central location. One skilled in the art can envision various interconnections, both physical and wireless, between the components of the system. 
     One embodiment for producing the distributed spot includes an aspheric diffuse ring producing optic, or ADRPO. For example, embodiments of aspheric optics may include what is referred to as an axicon or conical optic which produces a ring of intensity but has higher order aspheric terms to produce the spread-out pattern. In the described example the aspheric optic may have coefficients of A1=−0.01, A2=−0.06, and A4=0.002, with all other terms being zero. 
     Turning to  FIG. 2 . The optical arrangement can be manipulated in order to change the diameter of the spot. For example, moving collimating lens  204  to the left (toward the optical fiber input) by about 3.0 mm, as shown in the following diagram results in a reduction in spot diameter. 
       FIG. 2  description of elements: optics  102  may include optical configuration  200  with various elements coupled to light source  202  that includes any type of light source known in the art that produces a beam of light useable for Raman spectroscopy. Light source  202  may be configured for direct input to optical configuration  200 , or light source  202  may be fiber optically coupled to optical configuration  200 . Collimating lens  204  may be designed to optimally capture substantially all of the light from the beam output from light source  202  and produce a substantially collimated beam. Collimating lens  204  can be movably mounted such that it can change position along the axis of the optical path. For example, the range of motion in the described embodiment is about 0.1 mm to about 10 mm to allow for a change in spot size on the sample surface to range from about 10 microns to about 10 mm. Collimating lens  204  directs the substantially collimated beam into aspheric diffuse ring producing optic  208  configured to produce a light pattern that is radially diffuse. The intensity of the output from aspheric diffuse ring producing optic  208  is more intense at the outer edge of the resulting pattern than in the center. While this pattern could be projected directly onto sample surface plane  216 , in practical application it is advantageous to use one or more steering mirrors  210 , one or more filters  206 , and focusing elements such as concave focusing lens  212  and focusing optics  214 , to direct the radially diffuse light pattern onto sample surface plane  216 . 
       FIG. 3  shows an image of spot intensity distribution  300 . The non-uniform distribution shows the higher relative intensity at the outer edges than in the center (e.g. centroid). A sample output from optical configuration  200  that shows the non-uniform distribution of the spot intensity. Of particular note is that the spot still has some level of intensity in the center of the distribution that gradually increases in intensity towards the edge which has the highest level of intensity. The diameter of the spot can be controlled in a range of about 10 microns to as large as about 10 mm in diameter. 
       FIG. 4  is an illustrative example of plot  400  of cumulative intensity versus radius distance from a centroid out to a 750-micron radius for a 1.5 mm diameter spot. As can be seen from the plot, the central region intensity is fairly low producing an intensity that is just above background levels. The intensity rapidly increases towards the outer extents of the profile, that is referred to herein as a “diffuse ring type pattern”. The specific level of illumination at the center, as well as the characteristics of the diffuse ring curve could be altered depending on one or more characteristics of optical configuration  200  (e.g. relative distance between collimating lens  204  and aspheric diffuse ring producing optic  208 ) that may include the optical properties of the aspheric diffuse ring producing optic  208  (ADRPO). 
       FIG. 5  compares the cumulative intensity in plot  500  of a diffuse spot with a uniform spot. In a traditional Raman spectroscopy arrangement, the illumination spot is of substantially uniform intensity across its diameter. As you can see in  FIG. 5 , the intensity of the traditional uniform radial spot distribution  504  is substantially linear from its origin to its edge. Comparatively, aspheric diffuse ring producing optic  208  produces non-uniform radial spot distribution  502  that shows a substantially non-linear cumulative intensity from origin to edge. 
       FIG. 6  is an illustrative example of optical arrangement  600  wherein movement of the collimating lens  204  using collimating lens moving mount  602  effects the spot diameter. In some embodiments collimating lens moving mount  602  comprises a linear positioning assembly capable of micron resolution. In some cases, collimating lens moving mount  602  comprises a voice coil, however other types of moving mount known in the art may be employed. For example, collimating lens moving mount  602  may include a piezo, motor, or other type of linear positioning assembly known in the art. 
     As the distance between collimating lens  204  and aspheric diffuse ring producing optic  208  changes (e.g.—collimating lens  204  is moved relative to the position of aspheric diffuse ring producing optic  208 ), the spot diameter will change. For example, as the distance increases the spot diameter becomes smaller and as the distance decrease the spot diameter becomes larger. 
       FIG. 7  is an illustrative example (using a false color spot image  700 ) of the spot focused to about a  900  micron diameter using optical configuration  200 , with the distance between collimating lens  204  and aspheric diffuse ring producing optic  208  configured to generate a spot with an intensity distribution that has enough uniformity for “traditional” Raman measurement. Thus, optical configuration  200  enables “traditional” Raman measurement, as well as measurements where the spot is made larger producing a more pronounced diffuse ring type pattern as the diameter increases. 
       FIG. 8  is an illustrative example of embodiments of the adjustable illumination spot diameter  800  comprising different diameters. A tightly focused spot  802  illustrates a substantially uniform intensity although it will be appreciated that some intensity distribution exists; medium spot  804  illustrates a discernable diffuse ring type pattern (e.g. higher degree of intensity distribution/non-uniformity than spot  802 ); medium-large spot  806  illustrates a more pronounced diffuse ring type pattern than medium spot  804  (e.g. higher degree of intensity distribution/non-uniformity than spot  804 ); and large diameter spot  808  illustrates a clearly defined diffuse ring type pattern (e.g. highest degree of intensity distribution/non-uniformity). Importantly, each of spots  802 ,  804 ,  806 , and  808  maintain a degree of non-uniform radial intensity distribution at all sizes. In other words, the degree of non-uniformity of the intensity distribution is directly related to the diameter of the spot. 
     While the present invention has been illustrated by a description of an exemplary embodiment and while this embodiment has been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.