Patent Publication Number: US-2009219525-A1

Title: System and method for portable raman spectroscopy

Description:
BACKGROUND 
     Raman spectroscopy is useful for analyzing matter. There is a need to make Raman spectrometers portable so that they are more useful in existing applications, and so that they can be used in new applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a scanning Raman spectrometer, according to some embodiments. 
         FIG. 1B  illustrates the scanning Raman spectrometer of  FIG. 1B  in a different mode. 
         FIG. 2  illustrates a Raman spectrometer, according to some embodiments. 
         FIG. 3  illustrates a display for a Raman spectrometer, according to some embodiments. 
         FIG. 4  illustrates a scanning spectrometer and coordinates for scanning, according to some embodiments. 
         FIG. 5  illustrates a method, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description of example embodiments is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
     It is desired to remotely detect the chemical composition of solids, liquids, and gasses. Specific applications that benefit from remote chemical detection include, but are not limited to, tailpipe and smoke-stack emission analysis, petrochemical gas leaks, liquid chemical spills, drug detection, puddles and hazardous material detection, CO 2  detection in vehicles crossing international borders, airport scanning such as automatic airport scanning, etc. Various entities such as by the Department of Defense of the United States, or the Department of Homeland Security of the United States could use the present subject matter. 
     In various embodiments, the present subject matter uses an optical method to detect the molecular composition of remote specimens. Examples of specimens include, but are not limited to, a solid object, a gas cloud and a liquid puddle. Various embodiments report a molecular composition via a spectrometer that is part of a detector. Additional embodiments report a higher-level composition (e.g., petroleum, plant matter, narcotics, etc.) via determining constituents to the higher-level composition. 
     In some examples, the components necessary to perform analysis are integrated into a small form factor that is portable. The portable detector can be used in a “point and shoot” mode in some embodiments to detect an object at a fixed specific location. In additional embodiment, the detector can be used in a scanning mode where the chemical composition of a large area can be determined and optionally mapped as chemical composition as a function of spatial coordinate. Various embodiments include a spectrometer coupled to a scanning mechanism to position the housing in alignment with a plurality of specimens. Non-portable scanners are additionally possible, such as scanners permanently installed at an airport or at another place. 
     Optical detection methods include, but are not limited to, active infrared (“IR”) absorption, passive IR absorption, laser induced fluorescence (“LIF”) detection, and Raman detection. Passive and active IR techniques detect the chemical composition of liquids and gasses over kilometer distances, in various embodiments. This ability is enabled by the high sensitivity of infrared absorption. While IR techniques offer a solution for long range gas and liquids detection, the associated costs can be expensive. 
     Active IR absorption provides a solution having reduced cost for chemical composition detection at a range over one or more meters. Active IR absorption schemes typically require a back-reflecting surface, such as a tree, the ground, or a wall. Embodiments of the present subject matter relying on Raman spectroscopy do not require a back-reflecting surface since the Raman mechanism uses non-directional scattered light. 
     In various embodiments, laser induced fluorescence (“LIF”) provides a means for remotely detecting the chemical composition of objects. However, the sensitivity of LIF can be limited because the light emitted from the chemical(s) of interest is typically at a single wavelength. This limited detection channel can be a source of false positives. In contrast, embodiment of the present subject matter relying on Raman spectroscopy monitor scattered light comprised of several wavelengths. 
       FIGS. 1A-B  illustrates a detector  100  that scans, according to some embodiments. According to several examples, the detector  100  includes one of several spectrometers disclosed herein. In some embodiments, the detector  100  includes a Raman spectrometer  102  as disclosed herein. In various embodiments, the Raman spectrometer  102  measures a Raman spectrum of at least one of the specimens  104 A-N. Some embodiments of the detector  100  include a distance measuring device  106 . In various embodiments, the distance measuring device  106  is mechanically coupled to the Raman spectrometer  102 , such as via a housing or a circuit board. In some embodiments, the distance measuring device  106  optically determines distance to at least one specimen of the specimens  104 A-N. Means for measuring distance include, but are not limited to, measuring with a graduated counter such as a ruler or a vehicle odometer, sound, such as via sonar, and optically, such as via a range-finding laser, including range finding lasers that measure time of flight, multiple frequency phase-shift and/or interferometery. The location of one of the specimens  104 A-N can be determined, in various embodiments, by using the distance to the specimen as discussed herein. 
     Various embodiments include a scanning mechanism  108 . In various embodiments, the scanning mechanism  108  is coupled to the Raman spectrometer  102  to align the Raman spectrometer  102  and the distance measuring device  106  with each of the specimens  104 A-N. The scanning mechanism  108  includes a gimbal, in various embodiments, but the present subject matter is not so limited. 
     Various embodiments include an interface  110 . In various embodiments, the interface  110  displays a location and a Raman spectrum for at least one specimen of the specimens  104 A-N. An interface  110  can output a signal carrying information, in some embodiments, via wires, optics, or wirelessly. In some embodiments, the interface includes a display. Displays contemplated include, but are not limited to, screens including touch screens, text bars, indicator lights, mechanical flags, and other displays. 
     In various embodiments, a photodetector is coupled to the Raman spectrometer  102  such that an image of the specimens is formed. In some of these embodiments, the interface indicates the location of one of the specimens  104 A-N, the Raman spectrum for that specimen, and a picture of that specimen via the photodetector. Some embodiments draw a virtual line in the display between the detector and the specimen of interest. 
     In various examples, light from the Raman spectrometer  102  is incident upon a one of the specimens  104 A-N. In various embodiments, scattered light  112 A-N is transmitted back to the detector  100  and detected, such as by optics of the detector  100  and by other detecting components. Raman spectroscopy inelastically scatters light using an illumination source, such as a laser. The scattered light is shifted in wavelength based on the unique vibrational properties of the molecules that make up the specimen. Recording the intensity and wavelength of the scattered light can provide an identification of the unknown substance. According to various embodiments, the optical path of the focusing lenses is varied in order to illuminate and collect light at different distances. In various embodiments, components of a detector are arranged to fit into a small form factor, such as the size of a backpack, so that it can be carried by a person. Embodiments which are large and are permanently fixed to a structure such as a building are additionally possible. 
     According to various embodiments, a housing houses several components of the detector  100 . In various embodiment, the housing houses a battery to power a spectrometer, a distance measuring transceiver, a scanning mechanism and an interface, with the spectrometer, the distance measuring device, and the battery each disposed in the housing. In some embodiments, the housing includes a seal such that the housing is submersible in water. 
       FIG. 2  illustrates a Raman spectrometer  102 , according to some embodiments. In various embodiments, the portable Raman detector includes a laser light source  202 . Examples of laser light sources include, but are not limited to, infrared lasers, ultraviolet lasers, and other lasers. In some embodiments, the laser light source  202  doubles as a range finding laser. Various embodiments include a range finding laser coupled to the Raman spectrometer to optically determine distance to the at least one specimen. Visible lasers for the range finger are used in some embodiments to encourage accurate aiming as well as eye safety. These are useful to encourage aiming and eye safety in embodiments in which the laser used to evoke Raman scatter is not visible. 
     Various embodiments of the spectrometer  102  include a photo-detector array  206 . In various embodiments, the photo-detector array  206  detects Raman scattering, but the present subject matter is not so limited. Examples of photo-detector arrays to detect Raman scattering include, but are not limited to, charge-coupled devices (“CCD”). 
     In various embodiments, the spectrometer  102  includes optics  204 A-N. Optics add functionality including, but not limited to, focusing laser light and collecting laser light such as Raman scattered light. Various embodiments include a slit  208  coupled to the laser light source  202  and aligned with the path of the laser. Various embodiments include one or more mirrors  210 A-N. It is indicated that several mirrors can be used as the present subject matter is not limited to the particular configuration illustrated. Other systems and geometries are possible without departing from the present subject matter. Some embodiments include a beam splitter  212  that can direct light in two directions. Various embodiments include a grating  214  aligned with the path of the laser. A dispersive grating is used in various embodiments. Various embodiments include a collimating lens aligned with the laser path of the laser. 
     Various embodiments include a computer  216 . In various examples, the computer  216  interprets a signal from the photo-detector array  206 . In some embodiments, the computer  216  controls one or more mechanisms of the spectrometer, such as the optics  204 A-N. In some examples, the computer controls the laser light source  202  to provide Raman excitation light in a first mode, and range finding and distance measuring in a second mode. 
     Various embodiments include an interface  218  to display a location and a Raman spectrum for at least one specimen. Some embodiments include a housing and a battery to power the Raman spectrometer, the range finding laser, and the interface, with the Raman spectrometer, the distance measuring device, and the battery each disposed in the housing. Some embodiments include a composition information circuit coupled to Raman spectrometer to associate the Raman spectrum with a specified composition. This circuit can be part of the computer  216 , the interface  218  or another subsystem of the spectrometer  102 . The interface  218  coupled with computer  216  can be used to not only display images and Raman spectra, but also provide an interface where a user can control the optics  204 A-N, the laser light source  202 , the slit  208  and the photo-detector array  206 . 
     Some embodiments include a video recorder  220  that records photographs or videos. Data captured by the video record  220  can include the visible spectrum, but the present subject matter is not so limited. In various embodiments, the images captured by the video recorder  220  are associated with one or more recorded spectra or locations. 
     In various embodiments, the detector  102  is computer controlled via the computer  216 . In various embodiments, the computer  216  is autonomous. In some examples, the computer  216  measures one or more specimens in order to improve a Raman measurement to yield an improved signal to noise ratio (S/N). In one example, the computer  216  monitors the output of the photo-detector array  206  at the wavelength of the excitation laser light source  202  while controlling the optics  204 A-N. In various embodiments, the output of the photo-detector array  206  is improved by controlling the optics  204 A-N. In other examples, the computer automatically improves the output of the photo-detector at the laser excitation wavelength by controlling the parameters or operation of any combination of the of the components that comprise the detector  102 , including the optics  204 A-N, the power of the laser light source  202 , the slit width  208  or the integration time of the photo-detector array. 
       FIG. 3  illustrates a display for a Raman spectrometer, according to some embodiments. In various embodiments, the display includes an example wave diagram  302  with counts on the y-axis and wave number on the x-axis (e.g., cm −1 ). The example wave diagram  302  is not based on real data and is provided for explanatory purposes. In various embodiments, the intensity of backscattered light is measured, and the focus adjusts optics or other computers to find a signal maxima. The maximized signal of interest includes the Rayleigh scattered light at the wavelength of the excitation source as described above, in various embodiments. Such auto-focusing can be part of a computer or an interface, according to several embodiments. Embodiments that use manual focus are additionally possible. 
     Some embodiments include a text display  304  that is the result of interpretation of the measured waveform and comparison of the waveform with a specified waveform to determine a higher-level composition. The text display  304  could also display more specific molecular information, such as hydrogen sulfide, benzene, etc. 
     Various embodiments include a picture display  314 . The illustrated picture display  314  shows a tree, a rock, and a brick structure. The illustration shows that a laser path  306  has been directed toward the tree. It could optionally be directed  308  toward a rock, or directed  310  toward a brick structure. The optional paths may or may not be shown in the display according to various embodiments. In some embodiments, a user can touch the picture illustration to select a specimen. The illustration shows that a small scan of each specimen, such as the small scan  312 , has been made to determine a wave number according to a statistical approach, such as by average. Although the small scan is showing, an instant sample with one laser path is additionally possible, as are larger scan paths. 
       FIG. 4  illustrates a scanning spectrometer and coordinates for scanning, according to some embodiments. In various embodiments, a detector  402  can be used in a point and shoot mode by pointing at a specimen and then shooting the specimen and recording a Raman spectrum. This can be aided by a visible laser, in some examples. Although the detector  402  is illustrated resting on a tri-pod  404 , the present subject matter is not so limited, and other mounts, such as robot and vehicle mounts are possible. Portable detectors are used in some embodiments for detector  402 . 
     In additional embodiments, a scan mode is used, scanning according to coordinates  406 . Examples of possible coordinates include, but are not limited to, Cartesian, cylindrical, spherical or semi-spherical coordinate scanning scheme. In various embodiments, a detector is adjusted, such as by controlling a motor such as a stepper motor, to different positions such that the head is aligned with a specified coordinate system. The illustration shows spherical coordinates. For example, a series of measurements could be made 0 degrees from an equator, then 5 degrees along a latitude of the equator, the 5 degrees along the latitude and 5 degrees from a longitude, etc. Recording a plurality of measurements of specimens according to a coordinate system allows for mapping of the specimens, so long as distance information relating to the specimens is known. For example, a computer can combine coordinate information with distance information to obtain a three dimensional location for a specimen that can then be mapped. 
     Along a coordinate path, multiple measurements  408 A-N can be made. The present subject matter includes embodiments in which samples are also collected randomly while moving a detector  402  in a scanning mode. In various embodiments, a scanning mode rotates the detector  402  and adjusts the focus and collection distance automatically. In various embodiments, distance to the specimen is sensed while scanning. In some embodiments, a map is constructed from the collected data and displays chemical composition as a function of spatial coordinate. Embodiments which provide for remote control, such as from a guard post, are additionally possible. Remote control can include activation/deactivation, aiming, and control of scanning modes, among other adjustments. 
     According to several embodiments, the point and shoot measurement mode uses a fixed distance and position to measure a specific target. In some examples, it is advantageous to use an ultraviolet (“UV”) wavelength laser for spectroscopy. In various examples, a UV laser increases the sensitivity of the measurement compared to visible and IR based technology. In some embodiment, sensitivity is increased because the Raman scattering cross section is larger at UV wavelengths compared to longer wavelengths. In some examples, using a UV laser increases the measurement sensitivity since the noise is reduced as a result of lower background radiation from the sun compared to other wavelengths. An additional benefit of using a UV laser is that a high power excitation beam can be used while remaining safe for the human eye. 
     In various embodiments, each measurement collects photons that are Raman scattered from the chemical target of interest. The data, or spectrum indicated includes photon counts as a function of wavelength shift from the excitation laser. The unique spectrum obtained can be compared to a data library to identify the chemical target of interest. 
     Each collected photon counts towards a measurable signal which competes with noise throughout the system. In various embodiments, a measurement is indicated as successful if the ratio of signal to noise (“S/N”) is above a specified threshold. In some examples, a successful measurement has S/N greater than or equal to 3, but the present subject matter is not so limited. In order to improve S/N, the detector  402  can automatically direct itself, such as toward a specimen having a higher concentration and volume of the target composition. The system can additionally vary power of the measurement laser, focus optics configuration, distance from the chemical target, measurement time (a.k.a. integration time), and slit width. In various embodiments, noise is determined by details of the measurement electronics, background light from the sun, and in certain scenarios, the strength of the signal. 
       FIG. 5  illustrates a method, according to some embodiments. The illustrated method starts at  502 . At  504 , the method includes scanning a plurality of specimens with a laser by moving the laser according to coordinates for laser movement and measuring a distance for each of the plurality of specimens. At  506 , the method includes associating location information with each of the specimens of the plurality of specimens based on its distance from the laser and its coordinates for laser movement. At  508 , the method includes recording a Raman spectrum for the plurality of specimens. At  510 , the method includes associating a Raman spectrum with each specimen of the plurality of specimens. At  512 , the method includes indicating a Raman spectrum and location information for at least one specimen. 
     Various optional methods are possible. Some method embodiments include associating the Raman spectrum with a composition and indicating the composition of the specimen. Some embodiments include recording the Raman spectrum by exciting the specimen with the laser. Various embodiments include indicating a map including a plurality of specimens and respective composition and location information for each specimen. Some embodiments include scanning an area around a specimen, recording a plurality of Raman spectrum during the scan, and indicating a composition based on a statistical analysis of the plurality of Raman spectrum. Some embodiments include positioning the laser in a specified area via a self-powered vehicle. Additional embodiments include automatically scanning the plurality of specimens according to predetermined coordinates for laser movement. Still further embodiments include manually controlling the scanning and indicating a Raman spectrum and corresponding location for each specimen concurrent to the manual control. Additional embodiments include displaying a picture of the specimen concurrent to the manual control. Also, some embodiments include displaying a visible laser incident unto the specimen. Some embodiments display composition information concurrent with location. Methods including combinations of the optional methods are possible. 
     The Abstract is provided to comply with 37 C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.