Abstract:
A spectrometer includes a micro-ring grating device having coaxially-aligned ring gratings for diffracting incident light onto a target focal point, a detection device for detecting light intensity, one or more actuators, and an adjustable aperture device defining a circular aperture. The aperture circumscribes a target focal point, and directs a light to the detection device. The aperture device is selectively adjustable using the actuators to select a portion of a frequency band for transmission to the detection device. A method of detecting intensity of a selected band of incident light includes directing incident light onto coaxially-aligned ring gratings of a micro-ring grating device, and diffracting the selected band onto a target focal point using the ring gratings. The method includes using an actuator to adjust an aperture device and pass a selected portion of the frequency band to a detection device for measuring the intensity of the selected portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application 61/089,261 filed on Aug. 15, 2008, which is hereby incorporated by reference in its entirety. 
     
    
     ORIGIN OF THE INVENTION 
       [0002]    The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without payment of any royalties thereon or therefor. 
     
    
     TECHNICAL FIELD 
       [0003]    The present invention relates generally to spectrometers, and in particular to spectrometers having circular or ring-shaped gratings and an adjustable aperture. 
       BACKGROUND OF THE INVENTION 
       [0004]    The field of spectroscopic analysis or spectroscopy pertains to the study of the dispersion of light into different colors based on the component wavelengths of the light. By analyzing the absorption and dispersion of incident light and other radiation by matter, scientists are able to study various properties of the matter such as temperature, mass, luminosity, composition, etc. Optical instruments known as spectrometers are used to measure and study such light dispersion, and play an essential role in the study and design of various scientific monitoring devices, for example multi-spectral imaging (MSI) systems, hyper-spectral imaging (HSI) systems, and the like. 
         [0005]    In a conventional spectrometer, incident light passes through a first linear opening or slit in a first mirror or optical lens. A beam of incident light passing through the slit illuminates a prism or a linear grating device. The grating device may have a series of vertically-aligned linear gratings which diffract the incident light into its component colors, with each color corresponding to a particular frequency band of the electromagnetic spectrum. 
         [0006]    Spectrometers may include multiple slits, with the first slit positioned in front of the linear grating device to initially select light in a relatively narrow frequency band. The linear grating device spreads this portion of the light beam at different wavelength-dependent angles. A second slit in another mirror or lens may be positioned in front of a photon detection device to allow selective passage of a narrower band of the incoming light beam from the linear grating device. In this manner, a specific wavelength or set of wavelengths may be selected for spectral analysis. The photon detection device detects the intensity of light of the specific wavelengths. In some spectrometer systems the linear grating device may be rotated to receive and diffract light of different wavelengths. The photon detection device then transits an electrical signal describing the intensity of the detected light to a recorder which records the signal at the predetermined wavelengths. 
         [0007]    In certain conventional spectrometer designs the second slit and the photon detection device described above may be replaced with a multi-channel position-sensitive device such as a Charged Coupled Detector (CCD) array. As noted above, conventional spectrometers typically use linear gratings in conjunction with linear aperture slits, and thus an additional dimension is ordinarily required to effectively spread the incident light beam. Position-sensitive detectors such as CCD arrays may eliminate the need for a linear aperture slit, but nevertheless may require the additional dimension in order to function properly. The added dimension may result in a relatively bulky spectrometer design, with the lower size limit of a typical spectrometer being on the order of a few centimeters. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, a spectrometer is provided herein that includes a series of coaxially-aligned annular or micro-ring gratings rather than the conventional linear gratings described above. The spectrometer also includes an adjustable aperture device having a circular opening or aperture rather than a linear slit of conventional spectrometer designs. As a result, the spectrometer of the present invention may be miniaturized to provide significant size advantages relative to the spectrometer devices of the prior art. 
         [0009]    Within the scope of the invention, the micro-ring gratings are concentrically aligned with respect to a common optical axis of the spectrometer. The micro-rings have a calibrated gap width between adjacent rings, and have a calibrated ring size or width. The micro-ring gratings are used to generate a particular grating or dispersion effect on incident light, thus directing a portion of the incident light onto a target optical focal point. An opaque surface such as a metallic disk or other suitably opaque member may be positioned at the center of the concentric micro-ring gratings in order to substantially block a zero-order direct beam, as that term is properly understood in the art. 
         [0010]    Outer micro-ring gratings diffract the incident light beam according to the beam&#39;s wavelength, and focus beams of selected wavelengths at different focal points along the optical axis of the spectrometer. The circular aperture passes a portion of a selected wavelength or frequency band of the incident light beam to a detection device, which in turn detects desired spectral characteristics of the selected portion, e.g., intensity. The detected intensities or other characteristics may be recorded for later spectral analysis. By changing the distance between the micro-ring gratings and the aperture device, such as by moving the aperture device closer to or farther away from the grating device using a linear actuator, various wavelengths may be scanned to obtain the desired spectral information. 
         [0011]    In particular, a spectrometer includes a micro-ring grating device having a plurality of coaxially-aligned ring gratings. The grating device diffracts a predetermined frequency band of incident light onto a target focal point. The spectrometer includes a detection device operable for detecting a predetermined spectral characteristic, e.g., intensity, at least one actuator, and an adjustable aperture device. The aperture device defines a circular aperture circumscribing the target focal point, with the aperture device being operable for directing a selected portion of the predetermined frequency band to the detection device. The aperture device is selectively adjustable using one or more of the actuators to thereby select the portion of the predetermined frequency band for transmission to the detection device. A linear actuator may be connected to the aperture device and configured for selectively adjusting the focal length between the grating device and the aperture device. Another actuator may be used for changing the diameter of the circular aperture. An opaque member may be positioned along an optical axis of the spectrometer and configured for blocking a zero-order direct beam. 
         [0012]    A method is also provided for detecting the intensity of a selected frequency band of incident light. The method includes directing the incident light onto a plurality of coaxially-aligned ring gratings of a micro-ring grating device, and then diffracting a predetermined frequency band of the incident light onto a target focal point using the ring ratings. The method also includes using at least one actuator to selectively adjust an adjustable aperture device to thereby pass a selected portion of the frequency band to a detection device, and then detecting the intensity of the selected portion using the detection device. 
         [0013]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    FlG.  1  is a schematic prior art illustration of a conventional spectrometer having linear gratings and a linear aperture slit; 
           [0015]      FIG. 2  is a schematic illustration of a spectrometer having concentric micro-ring gratings and an adjustable aperture device in accordance with an embodiment of the present invention; 
           [0016]      FIG. 3  is schematic illustration of wavelength selection using the spectrometer shown in  FIG. 2 ; and 
           [0017]      FIG. 4  is a schematic illustration of an actuator that is usable with the spectrometer of  FIG. 2  for providing a variable aperture size according to another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Referring to the drawings wherein like reference numbers represent like components throughout the several figures, and beginning with  FIG. 1 , a conventional spectrometer  10  may include a linear grating device  12  having a series of linear gratings  14  suitable for diffracting incident light (arrows  11 B) as noted above. As will be understood by those of ordinary skill in the art of optics, the term “grating” refers to an optical element configured for diffracting incident light. Gratings have a regular pattern which splits and diffracts incident light into several beams travelling in directions that depend on the spacing between gratings and the wavelength of the incident light. The spectrometer  10  may include vertically-aligned mirrors  16  defining the linear gratings  14 . Additionally, the spectrometer  10  may include a pair of lenses or mirrors  18  and  20  each defining respective first and second linear apertures or slits  19  and  21 . 
         [0019]    Source light (arrows  11 A) is directed toward the mirror  18  from a source  22 , e.g., an object, organism, matter, or other substance serving as the subject of the spectral analysis at hand. The source light passes from the source  22  through the slit  19  in mirror  18 . A portion of the source light (arrows  11 A) is directed toward the grating device  12  as the incident light (arrows  11 B). The linear gratings  14  diffract the incident light (arrows  11 B) passing through slit  19  at different angles according to the particular wavelength of the incident light. The diffracted light (arrows  11 C) passes in turn through the slit  21  in mirror  20 . A selected portion (arrows  11 D) of the diffracted light (arrows  11 C) is directed toward a detection device  24 , e.g., a photon detector or other suitable detection device capable of determining a desired spectral characteristic, e.g., the intensity of the selected light (arrows  11 D). 
         [0020]    As noted above, the slit  19  in mirror  18  enables passage of a selected band of the original source light (arrows  11 A). The detection device  24 , which is typically but not necessarily configured as a photon detector, may be positioned adjacent to the slit  21  to receive photons of a particular wavelength. After detection, the linear grating device  12  may be rotated as needed to receive different wavelengths of the incident light (arrows  11 B). Also after detection, the detection device  24  may transmit an electrical signal (arrow i) such as a proportional voltage level or other suitable signal to a recording device  26 , e.g., a data logger or other computer device configured for recording the required information. Thereafter, the recorder  26  may be used to record the desired spectral characteristic of the selected light (arrows  11 D). 
         [0021]    As noted above, in some conventional spectrometers the slit  19  and the detection device  24  may be replaced with a multi-channel position sensitive detector (not shown), e.g., a Charged Coupled Detector (CCD) array. Such spectrometers typically require additional space to direct the incident light (arrows  11 B) according to its wavelength(s), and therefore the lower limit of the size of the conventional spectrometer  10  of  FIG. 1  may be limited to a few centimeters due to the extra length required for a CCD and/or the rotation of the linear grating device  12 . 
         [0022]    Referring to  FIG. 2 , a spectrometer  110  constructed in accordance with the present invention includes a micro-ring grating device  112  having annular binary gratings or refractive micro-rings  30  interposed with a series of opaque micro-rings  36 . The grating device  112  may also include an opaque center area  32  and an adjustable aperture device  40 . A portion of the incident light (arrows  11 B of  FIG. 2 ) reaches and passes through the grating device  112  as the diffracted light (arrows  11 C). The diffracted light (arrows  11 C) passing through the grating device  12  is thus directed onto a target focal point C. 
         [0023]    The adjustable aperture device  40  defines a circular opening or aperture  42  circumscribing the focal point C, or mother focal point depending on the position of the aperture device  40 . That is, the focal point C may differ with other specified wavelengths of light to provide alternate focal points, e.g., the focal points D or E as shown in  FIG. 3  and described below. Opaque center area  32  is configured to substantially block transmission of the incident light (arrows  11 B) through the grating device  112  as noted above. The opaque center area  32  may be configured as, for example, one or more opaque circular discs or other suitable light blocking structure. The center area  32  may be constructed of a suitable metallic material, e.g., gold, aluminum, silver, and/or a combination thereof, or of any other suitable opaque material. 
         [0024]    The aperture device  40  may be selectively adjusted within the scope of the invention. As used herein, the term “adjusted” refers to one or more features or dimensions of the aperture device  40  being variable, e.g., the position of the aperture device  40  with respect to the grating device  112 , the diameter of the aperture  42 , etc. The aperture device  40  may be placed a focal length Z away from the grating device  112 . The aperture device  40  allows passage of selected light (arrow  11 D) of a selected wavelength through the aperture  42  while simultaneously blocking light of other non-selected wavelengths. 
         [0025]    The position of the aperture device  40  and the distance between the grating device  112  and the aperture  42  may be varied using one or more linear actuators (A)  44 , e.g., piezoelectric actuators, micro-motors, MEMS (Micro-Electro-Mechanical Systems) linear drivers, and/or other suitable linear translators, to name just a few. Also, by moving the aperture device  40  into the direction of light propagation as indicated by double-sided arrow B in  FIG. 2 , one may choose the specific wavelength of the diffracted light (arrows  11 C) which may pass through the aperture  42 . 
         [0026]    The light which passes through the aperture  42  is nearly monochromatic with a narrow wavelength distribution or Δλ. The detection device  28  (see  FIG. 3 ) positioned adjacent to the aperture  42  converts the intensity of the selected light (arrows  11 D) into a proportional voltage or other suitable electric signals suitable for receipt and processing by a recording device (R)  26  shown in the prior art  FIG. 1 . Therefore, full spectral information may be obtained by recording the intensity of the selected light (arrows  11 D) passing through the aperture  42 . As will be discussed below with reference to  FIG. 3 , another opaque structure  43  such as but not limited to an opaque disc may be positioned between the aperture  42  and the detection device  28  (see  FIG. 3 ) to block any stray light emitted by photons of a slightly different wavelength than those of the non-selected wavelengths. 
         [0027]    Referring to  FIG. 3  in conjunction with  FIG. 2 , wavelength selection may be achieved using the micro-ring grating device  112  and the aperture device  40  of the present invention as described above. The incident light (arrows  11 B) passing through the grating device  112  for a given focal length Z passes through aperture  42  and is detected by the detection device  28 . In order to detect other wavelengths, one may either move the grating device  112  closer to the aperture device  40  so that alternate focal points D or E are located within the aperture  42 , i.e., circumscribed by the aperture  42 , or one may move the aperture device  40  closer to the grating device  112  to accomplish the same result. 
         [0028]    It should be appreciated that a portion of the incident light (arrows  11 B), e.g., from a telescope or a microscope, passes through the circular binary or refractive micro-rings  30  of the grating device  112  as shown in  FIG. 2 . Another portion of the incident light is stopped or reflected away by the opaque micro-rings  36 . The center area  32  may be configured as an opaque disc according to one embodiment so that formation of a zero-order direct beam is prevented, as that term is properly understood in the art. The size of the center area  32  and each of the micro-rings  30  and  36  may be used to control the focal point of the diffracted light (arrows  11 C) of a selected wavelength. The focal point is located a distance Z from the center of the micro-ring grating device  112 , with the value of Z determined by the wavelength of the diffracted light (arrows  11 C). 
         [0029]    If one of the micro-rings of the grating device  112  is in a negative zone plate in a negative zone plate design, as those terms are understood in the art, the radius of the opaque micro-rings  36  may be determined using the following equation: 
         [0000]      r n =K√{square root over (n)} 
         [0000]    , where n is between 0˜1, 2˜3, 4˜5, etc., and where the constructive focal points are: 
         [0000]    
       
         
           
             
               Z 
               = 
               
                 
                   K 
                   2 
                 
                 λ 
               
             
             , 
             
               
                 
                   K 
                   2 
                 
                 
                   3 
                    
                   λ 
                 
               
               ; 
               
                 
                   K 
                   2 
                 
                 
                   5 
                    
                   λ 
                 
               
             
             , 
             
               etc 
               . 
             
           
         
       
     
         [0030]    It should be appreciated that focal point is inversely proportional to wavelength (λ), and that a negative zone plate is just one example of circular ring gratings. A fractal zone plate may also be used as the required circular ring grating. Different ring grating designs are also possible as long as the focal point distance Z changes significantly with the wavelength of the diffracted light (arrows  11 C). Because a zero-order direct-beam is blocked by the center area  32 , and because each wavelength (λ) has a different focal point, the detection device  28  receives photons of selected wavelengths, i.e., the diffracted light (arrows  11 C), which are determined by the value of Z between the grating device  112  and the aperture  42 . 
         [0031]    Still referring to  FIG. 3  in conjunction with  FIG. 2 , since the spectrometer  110  in its entirety is configured in the line of light propagation the spectrometer  110  does not require additional dimensions to properly spread the incident light beam (arrows  11 B) of  FIG. 2 . Furthermore, within the scope of the present invention rotation of the grating device  112  is not required. As a result, the spectrometer  110  may be miniaturized from between approximately a few micrometers and approximately a few millimeters. 
         [0032]    The various micro-rings of the micro-ring grating device  112  may be fabricated with different sizes. Electron-beam lithography, nano-imprint lithography, a focused ion beam, or other suitable methods may be used to fabricate rings from a metallic layer, e.g., Ag (silver), Au (gold), Al (aluminum), and/or combinations and compounds thereof, on glass and double side-polished sapphire disc. The rings may be fabricated with e-beam lithography, with an overall size of approximately 150 micrometers (μ) diameter and approximately 720μ diameter according to one embodiment. The aperture  42  may be fabricated with an E-beam lithography having a width of approximately 20μ in another embodiment. 
         [0033]    An improved aperture may be built by positioning the opaque structure  43 , for example an opaque disk as noted above, between the aperture  42  and the detection device  28  in order to block central stray-lights from the photons of slightly different wavelengths or the non-selected wavelengths. Also, the diameter of the aperture  42  may be varied and optimized using a MEMS structure, e.g., by using at least one Micro-Electro-Mechanical System or MEMS device  140  as shown in  FIG. 4  and discussed below. 
         [0034]    Referring to  FIG. 4 , the MEMS device  140  according to one embodiment may have a first fixed or movable component  52  which overlaps with a second movable component  54 . The components  52  and  54  are both structurally connected to at least one motion driving component  56 . For example, driving component  56  may be connected to the second component  54  such that a variable aperture  142  is enabled. An actuator (A)  44  may move the driving component  56  to affect the required diametric change in the aperture  142 . 
         [0035]    It should be appreciated that the size and shape of the aperture  142  is controlled by the component  56 . Proper selection of the size of the aperture  42  of  FIGS. 2 and 3  or the variable aperture  142  shown in  FIG. 4  ensures balance of the spectral resolution and collected photon intensity. While a circular shape for the apertures  42 ,  142  is set forth above, those of ordinary skill in the art will appreciate that apertures of any appropriate size or shape may also be used without departing from the intended scope of the invention. For simplicity a single spectrometer  110  is shown in  FIGS. 2 and 3 . However, one of ordinary skill in the art may construct an array of such spectrometers  110 , or of any of the components thereof, e.g., for multi-spectral and hyper-spectral imaging, without departing from the intended scope of the invention. 
         [0036]    Furthermore, because the dimension of the spectrometer  110  can be relatively small, and because the grating device  112 , apertures  42 ,  142 , and detection device  28  are located in the line of light propagation, one may use multiple spectrometers  110  for hyper-spectral imaging (HSI) to build a 3-D data cube of 2-D images at multiple wavelengths. 
         [0037]    For optical properties of a micro-ring grating of approximately 720μ overall size, for example, a PSI′ (Point-Spread-Function) at focal point Z may be calculated in order to estimate the focus size for the selected light  11 D (see  FIGS. 2 and 3 ). This simulation may be made with green light of wavelength 532 nm at the focal point Z=12.1 mm, which optimized for a 530 nm wavelength. Most of the PSI′ may be confined within a diameter of approximately 20μ. In order to find the spectral resolution, a PSI′ may be calculated at multiple focal points, with different wavelengths as shown in  FIGS. 2 and 3 . For example, Spectral Resolution (Δλ) of Full-Width-Half-Maximum may be obtained at different focal points for red light (633 nm), green light (532 nm) and deep blue light (405 nm). 
         [0038]    The following examples are provided to further illustrate the invention, and are not intended to be limiting thereof. 
       EXAMPLE 1 
       [0039]    The PSI′ (Point-Spread-Function) of different wavelengths may be obtained from about 590 nm to about 680 nm at a focal point Z=10.2 mm, optimized for the red color, i.e., λ=633 nm. The PSI′ may be maximized at approximately 633 nm and decreases as the wavelength varies from approximately 633 nm. Full-Width-Half-Maximum (FWHM) Δλ is approximately 30 nm for a red focal point in this example. 
       EXAMPLE 2 
       [0040]    The PSF of different wavelengths may be obtained from approximately 497 nm to about 572 nm at a focal point Z=12.1 mm optimized for the green color, i.e., λ=532 nm. The PSF may be maximized at approximately 532 nm and decreases as the wavelength varies from approximately 532 nm. FWHM Δλ is approximately 25 nm for a green focal point in this example. 
       EXAMPLE 3 
       [0041]    The PSF of different wavelengths may be obtained from approximately 375 nm to approximately 435 nm at a focal point Z=115.9 mm optimized for deep blue color, i.e., λ=405 nm. The PSF may be maximized at approximately 405 nm and decreases as the wavelength varies from approximately 405 nm. FWHM (Δλ) is approximately 20 nm for a deep blue focal point in this example. 
         [0042]    While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.