Abstract:
A spectrometer employs multiple filters having complex filter spectra that can be generated robustly from received light over short optical path lengths. The complex filter spectra provide data that can be converted to a spectrum of the received light using compressed sensing techniques. The result is a more compact, easily manufactured spectrometer.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     CROSS REFERENCE TO RELATED APPLICATION 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates to systems for measuring light spectra, for example, for absorption, transmission, or reflection spectroscopy, and in particular to an optical spectrometer providing compact and robust construction. 
     Optical spectrometers provide a measurement of light intensity over multiple frequencies. An optical spectrometer may measure the spectrum of an unknown light source or be used with a known light source to measure absorption of a material when light from the known light source passes through or is reflected from the measured material before being detected. This latter absorption spectrum is derived by subtracting the spectrum of the light received at the detector from the spectrum of the known light source. 
     Distinguishing the intensity of different frequencies of light, as needed for determining a spectrum, is normally accomplished by using an analyzing filter system and a broadband light detector, the latter which does not distinguish among frequencies and is ideally equally sensitive to all frequencies of interest. The analyzing filter system is changed as a function of time so that the broadband light detector receives different frequencies of light as a function of time. In this way the time varying signal from the broadband detector may be decoded into a spectrum providing the intensity of different frequencies of light. 
     For determining the spectrum of an unknown light source, the analyzing filter system may be applied directly to the light source. For absorption spectroscopy, the analyzing filter system may be placed after a sample to be analyzed, to receive reflected or absorbed light from the sample as illuminated by a known light source. This light is then passed to the detector. Alternatively, the analyzing filter system may be placed in front of the sample to modify the light from a known light source before it is reflected or absorbed by the sample and then received by the broadband detector. 
     Conventional optical spectrometers may use a frequency dispersive element as an analyzing filter system, such as a diffraction grating. The angle of incidence of the light on the diffraction grating may be changed to generate a series of narrowband monochromatic beams each approximating a single spectral line. Independent measurements of the different monochromatic beams by the detector allow a full spectrum to be assembled. Generally, a slit may be used to successively isolate each narrowband monochromatic beam for sequential measurement by a single detector, or the monochromatic beams may be measured in parallel by a multi-detector array. 
     Fourier transform spectrometers may use an interferometer as an analyzing filter assembly to produce a light beam having a multi-frequency spectrum approximating a periodic sinusoid with regular zero values for frequencies within the range of interest. This sinusoidal spectrum is generated by reflecting a broadband light beam back on itself so that the light is subject to constructive and destructive interference at different frequencies. The period of the interference may be changed, for example, by moving a mirror of an interferometer, so that the sinusoid of the spectrum is modulated. Generally, higher optical frequencies will have a higher rate of modulation so that measurements made with the broadband detector may be processed by the Fourier transform to reveal a spectrum. 
     Both of these types of spectrometers require relatively large optical paths for good resolution and may further require complex precision machinery to move optical elements during the measurement process. As a result, low-cost and compact spectrometers, potentially useful in a variety of applications, are difficult to produce. 
     SUMMARY OF THE INVENTION 
     The present invention provides an analyzing filter system for a spectrometer that provides a set of uncorrelated and varying filter spectra over an extremely short optical path. Although the filter spectra are complex and appear largely random, they can be assembled into absorption spectrum mathematically, for example, by compressed sensing techniques. The result is a spectrometer that can be both compact and robust. 
     Specifically then, the present invention provides spectrometer that includes a frequency filter receiving a light beam and modifying the light beam according to a set of different filter spectra each defining a frequency-dependent attenuation of the received light to provide a corresponding set of filtered light beams each associated with a different filter spectra. Each different filter spectra is a broadband spectrum with substantially non-periodic variations in value as a function of frequency. A broadband light detector receives the set of filtered light beams to provide a corresponding set of independent measures of each filtered light beam. Finally, an electronic computer receives the independent measures of the set of filter light beams to generate a spectrum derived from the set of independent measures, the spectrum indicating intensity as a function of frequency for different light frequencies over a range of frequencies. 
     It is thus a feature of at least one embodiment of the invention to provide an alternative to standard diffractive or Fourier transform spectrometry using a frequency filter providing a complex non-periodic output. 
     The different filter spectra are substantially uncorrelated with each other. 
     It is thus a feature of at least one embodiment of the invention to provide a frequency filter that provides an efficient analysis of a received light signal with a reduced number of different filter spectra. 
     The different filter spectra may be statistically random. 
     It is thus a feature of at least one embodiment of the invention to permit the use of novel filter structure designs, for example, those providing complex interference patterns. 
     The frequency filter may be a set of optical structures arranged so that a given ray of light through the frequency filter interacts sequentially and repeatedly with the optical structures in an optical resonance. 
     It is thus a feature of at least one embodiment of the invention to provide a spectrometer having an extremely short external optical path made possible by the high internal optical path achievable with optical resonance. 
     The frequency filter may be a photonic crystal of a matrix with periodic light-disrupting elements. 
     It is thus a feature of at least one embodiment of the invention to take advantage of the spectral filtering properties of photonic crystals and the like having regular structures. 
     Alternatively, the frequency filter may provide multiple layers having different refractive indices. 
     It is thus a feature of at least one embodiment of the invention to permit the use of a wide variety of structures including non-crystalline irregular structures for the generation of the necessary filter spectra. 
     The electronic computer may include a memory store holding the different filter spectra to generate the spectrum, the different filter spectra being represented in memory by at least one of stored spectral values or an algorithm approximating the spectral values. 
     It is thus a feature of at least one embodiment of the invention to provide measures of the filter spectra that may be used for constructing the spectrum from highly disorganized filter spectra. 
     The spectrometer may further include a temperature sensor sensing temperature of the frequency filter, and the memory store may hold different filter spectra associated with different temperatures and the electronic computer selects among the different filter spectra associated with different temperatures according to a temperature measured by the temperature sensor. 
     It is thus a feature of at least one embodiment of the invention to accommodate temperature sensitivity in the frequency filter by a compensation system. 
     The electronic computer may generate the spectrum by compressed sensing which reconstructs the spectrum from the set of filter spectra and the set of independent measures by finding a solution to an undetermined system of equations under the assumption of sparseness. 
     It is thus a feature of at least one embodiment of the invention to allow the construction of the spectrum from multiple measurements subject to complex filter spectra of the type produced by the frequency filter of the present invention. 
     The broadband light detector may be a two-dimensional array of light sensors in rows and columns across a plane and the frequency filter may be positioned over the light sensors so that different regions of the frequency filter simultaneously provide light to different light sensors of the array and wherein each of the different regions of the frequency filter provides a different filter spectra. 
     It is thus a feature of at least one embodiment of the invention to provide a robust spectrometer with few or no moving parts. By assigning different filter spectra to different light sensors, the multiple independent measures may be made simply by electronically addressing the different light sensors without moving a filter mechanism. 
     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 
         FIG. 1  is a block diagram of a spectrometer constructed according to one embodiment of the present invention providing an integrated frequency filter and light sensor array each with multiple elements; 
         FIG. 2  is a set of successively exploded perspective views of the frequency filter and light sensor of  FIG. 2  showing a detector element in the form of a photonic crystal; 
         FIG. 3  is a cross-section along line  3 - 3  of  FIG. 2  of the photonic crystal element; 
         FIG. 4  is a figure similar to that of  FIG. 3  showing an alternative multilayer interference element; 
         FIG. 5  is a signal processing flowchart showing the generation of a spectrum using the frequency filter of claim  1 ; 
         FIG. 6  is a representation of a stored filter spectrum showing correction for temperature; 
         FIG. 7  is an alternative embodiment of the spectrometer using a single broadband detector and movable filter; 
         FIG. 8  is an alternative optical path for the spectrometer in which a material to be analyzed is placed in an optical path between the filter system and the light detector; and 
         FIG. 9  is a signal processing flowchart similar to that of  FIG. 5  showing characterization of each of the elements of the frequency filter array. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a spectrometer  10  of the present invention may be configured to make absorption spectrographic measurements of a sample material  12 . In this configuration, one or more light sources  14  having a known spectral output, for example, light emitting diodes or incandescent bulbs, may shine a light on the sample material  12  to be reflected into a collimator  16 . It will be understood that in a minor variation on this arrangement, the light sources  14  may be positioned to transmit light through the sample material  12 . 
     Light from the collimator is directed as a received light beam  18  generally along axis  20  normal to the surface of a generally planar frequency-filter plate  22 . The frequency-filter plate  22  may have a set of filter elements  24 , for example, in square tiles arranged in rows and columns over the two-dimensional area of the planar frequency-filter plate  22 . Each of these filter elements  24  will have a different filter spectrum, being a description of selected light transmission of the filter element  24  as a function of light frequency. The filter elements  24  break the received light beam  18  into a series of filtered light beams  26 , the latter each filtered according to the different filter spectra of the filter elements  24 . 
     The filtered light beams  26  may be received by broadband light detector elements  28  of a light detector array  30  being, for example, a charge-coupled device camera or the like. Each of the light detector elements  28  may comprise one or more pixels of that camera device. Generally each of the light detector elements  28  is responsive to light over a range of frequencies to be measured by the spectrometer  10  and is relatively frequency insensitive, being unable to distinguish among different frequencies by frequency measurement. The frequency response of the detector elements  28  need not be uniform for all frequencies, but is desirably well-characterized. 
     The combination of the frequency-filter plate  22  and the light detector array  30  allows parallel acquisition of multiple independent measures of the filtered light beams  26  each subject to different filter spectra  48  by electronically scanning through the light detector elements  28  as is understood in the art. It will be appreciated that the same approach may be used with a one-dimensional frequency-filter plate  22  and corresponding light detector array  30  each having multiple columns of a single row. 
     Each filter element  24  may provide for a series of optical structures  32  that affect the propagation of electromagnetic waves from received light beam  18  through the filter elements  24  to create complex interference patterns and optical resonances  36  between structures  32 . While the inventor does not wish to be bound by a particular theory, these optical resonances  36  and the standing waves are believed to contribute to the formation of distinct filter spectra by the filter detector elements in the short optical path length through the thickness of the filter element  24  along the axis  20 . 
     Referring also to  FIG. 3 , each of the optical structures  32  may, for example, be a small diameter blind bore hole (circular, rectangular, or other cross-section) in a transparent matrix  34  of the filter element  24 . Generally the structures  32  may be placed with a spatial periodicity  33  along two axes separated by an axis angle  35 . The spatial periodicity  33  is selected according to the spectral range of the received light beam  18  to be between 1/10 and 10 times the central wavelength  40  of the received light beam  18 . The central wavelength  40  is the wavelength of the median frequency of the spectrum to be produced. 
     In one embodiment, the spatial periodicity  33  may be 2.5 micrometers and the axis angle maybe 60 degrees. The thickness  42  of the filter element  24  will generally be no more than 100 times the central wavelength  40 . A typical filter element  24  will have a width and height 100 times periodicity  33  to allow sufficient light flux through the filter element  24 . 
     The multiple filtered light beams  26  may exit each of the filter elements  24  of the frequency-filter plate  22  having a distinct complex filtered spectrum  48  (for example, shown in  FIG. 6 ) as will be discussed below. The filtered spectrum  48  is substantially stable in a very short free-space optical path distance after exiting the filter element  24 , for example, less than 10 centimeters and typically much less than one centimeter or at a scale allowing the frequency-filter plate  22  to be directly attached to the light detector array  30 . “Free-space” optical path means the optical path length outside of the filter elements  24 . 
     Referring now to  FIG. 4  in an alternative embodiment, the detector elements  24 ′ may provide for multiple layers  44  generally perpendicular to the axis  20  of the received light beam  18 . Each layer  44  may have a different index of refraction so as to create partial reflections at the interfaces between layers  44 . In one example embodiment, a top layer may be a 400-nanometer thick layer of silicon followed by a 200-nanometer thick layer of silicon dioxide followed by a 500-nanometer thick layer of silicon followed by a 200-nanometer thick layer of silicon dioxide. Generally each of the layers will have a thickness selected according to the central wavelength  40  to create the desired interference patterns and generally lying between 10 nanometers and 100 micrometers. Each layer creates one response function and together create a complex filtered spectrum  48 . 
     As noted, the interface between each layer creates reflections which generate optical resonances  36  and standing waves. The embodiment of the filter elements  24  uses a non-periodic structure but, again, provides a stable filtered spectrum  48  in a filtered light beam  26  proximate to the exit point at a bottom surface of the filter element  24 ′. 
     Referring now to  FIGS. 1, and 6 , the filter spectra  48  of each of the filter elements  24  may be used to construct a spectrum of the received light beam  18  through the use of an electronic computer  50  (shown in  FIG. 1 ) having one or more processors  52  communicating with a memory  54  holding a stored program  56  as will be described. The memory  54  also includes stored representation  58  of the filter spectra  48  and in some embodiments a measured spectrum  70  of the light sources  14 . 
     The computer  50  may communicate with a temperature sensor  60  for reading a temperature of the frequency-filter plate  22  and may receive independent measures  62  of each filtered light beam  26  from the light detector array  30 . As will be discussed below, the computer  50  may then generate a spectrum  64 , for example, displayed on a graphics terminal  68  or used in numeric form by other processes. Depending on the application, spectrum  64  may describe either of the (1) intensity of the received light beam  18  as a function of the light frequency, generating a light spectrum or (2) in this example, a difference between a spectrum of the light sources  14  and the received light beam  18  generating an absorption spectrum. 
     Referring now to  FIG. 5 , in the example of the generation of an absorption spectrum, the light sources  14  may provide light beam  72  with an emission spectrum  70  that may be transmitted to the sample material  12  to be partially absorbed or reflected by the test sample material  12 . This modification of the light beam  72  by the sample material  12  is according to an absorption spectrum  73  of the sample material  12  being an intrinsic property of the sample material  12 . Generally the spectrum  64  therefore is a measure of the absorption spectrums  73 . As is understood in the art, this absorption spectrum  73  may be used to identify or otherwise characterized the sample material  12 . 
     Reflected or transmitted light  74  from the sample material  12  will have a spectrum  75  being a combination of the emission spectrum  70  and the spectrum  73 . This light  74  may be received by the detector elements  24  of the frequency-filter plate  22 . As noted above, each of the detector elements  24  have a different filter spectrum  48  and create a set of filtered light beams  26  each having a characteristic spectrum  76  being generally a combination of spectra  70 ,  73 , and  48  for the particular filter element  24 . These filtered light beams  26  are received by the light detector array  30  which generates multiple independent measures  80  of the light intensity of the filtered light beams  26  (each independent measure indicated by a different index variable i) associated with different filter elements  24 . The independent measures  80  will generally be the integral of the spectrum  76  over the area of the detector element  28  as slightly modified by the sensitivity spectrum  82  of the detector elements  28  of the light detector array  30 . 
     These above-described steps may be repeated for each test of the sample material  12  or different sample material  12 . 
     For the generation of an absorption spectrum  64 , the filter characteristics of the optical path from the light source  14  through the filter elements  24  and the light detector array  30  must be characterized without the sample material  12 . This latter measurement may generate a set of different independent measures  80 ′ corrected for the particular light source  14  and light detector array  30 . In particular, the light source  14  may be used to directly illuminate the frequency-filter plate  22  to generate a working filter spectrum  84  for each of the filter elements  24  which may then be detected by each of the detector elements  28  to generate the independent measures  80 ′ for each of the filter elements  24 . 
     The independent measures  80 ′ may be subtracted from the independent measures  80  to determine difference independent measures  80 ″ for each filter element  24  according to the index variable i and representing the modification of the light beam  72  by the sample material  12  as may be expressed in an absorption spectrum  64 . In the case of the generation of a light spectrum, the subtraction process and the collection of independent measures  80 ′ is not required. 
     The difference independent measures  80 ″ may then be analyzed to determine the spectrum  64 . As part of this process, each of the filter spectra  48  augmented by the effects of the spectra  70  and  82  must be determined. Referring to  FIG. 9 , this step may be accomplished, for example, using conventional spectrographic techniques such as a scanning slit spectroscopy machine or a Fourier transform spectroscopy machine. With a scanning slit system (as depicted in  FIG. 9 ), broadband light source  14  having emission spectrum  70  may be received by a scanning slit monochromator  90  of a type known in the art to produce a time-varying monochromatic light beam  92  indicated by dynamic spectrum  94 . This time-variable monochromatic light beam  92  may be received by each filter element  24  and the resulting transmission through the filter element  24  detected by detector element  28  to determine a corrected filter spectra  48 ′ collectively describing the filter spectra  48  as influenced by the effective emission spectrum  70  of the light source  14  and sensitivity spectrum  82  of the light detector array  30 . More simply, the corrected filter spectra  48 ′ is the product of filter spectra  48 , emission spectrum  70  and sensitivity spectrum  82 . Alternatively, or in addition, each of the filter spectra  48 , emission spectrum  70 , and sensitivity spectrum  82  may be determined independently, for example at the factory, using a conventional spectroscope, and the necessary combined effect of these associated components. 
     Referring back to  FIG. 5 , each of the corrected filter spectra  48 ′ may be used with the independent measures  80  to deduce the spectrum  64  by a variety of techniques that attempt to solve for an absorption spectra  64  that could generate independent measures  80 ″ given the known corrected filter spectra  48 ′. Generally this problem is represented by a system of linear equations:
 
 V   1   ·S=a   1  
 
 V   2   ·S=a   2  
 
 V   2   ·S=a   2  
 
     (etc.) 
     where V i  are the corrected filters spectra  48 ′, a j  are the independent measures  80 ″ and S is the absorption spectrum  64 . This system of linear equations will generally be an undetermined linear system, that is, having more unknowns than equations and therefore an infinite number of solutions. In order to choose a solution, a compressed sensing program  98  may be used with an additional assumption of smoothness or sparseness of the absorption spectrum  64 . The smoothness or sparseness assumption basically allows the user to arbitrarily set a resolution of the ultimate absorption spectrum  64 . 
     One type of compressed sensing is minimum basis pursuit, generally known in the art and described, for example, in Candes, E. J.; Romberg, J.; Tao, T., Robust Uncertainty Principles: Exact Signal Reconstruction From Highly Incomplete Frequency Information, IEEE Transactions on Information Theory (Volume: 52, Issue: 2), pages 489-509 (February 2006). 
     Referring now to  FIG. 6 , the particular corrected filter spectra  48 ′ will be a function of temperature of the spectrometer  10  and primarily the temperature of the frequency-filter plate  22 . Generally, at higher temperatures, expansion of the material of the transparent matrix  34  (shown in  FIG. 3 ) will cause a dilation  100  of the corrected filter spectrum  48 ′. Accordingly, a number of different corrected filter spectra  48 ′ may be stored for use by the compressed sensing program  98 , each stored corrected filter spectrum  48 ′ being associated with different temperatures measured by temperature sensor  60  contemporaneously with the acquisition of the received light beam  18  during the process of  FIG. 9 . The multiple measurements at different temperatures may be stored as data points in a lookup table, or compressed algorithmically to provide for a method of generating corrected filter spectra  48 ′ procedurally, for example, providing a dilation based on temperature) according to techniques well known in the art. 
     Referring now to  FIG. 7 , in an alternative embodiment of the spectrometer  10 , a single detector element  28  may be used and a frequency-filter plate  22 , for example, having a linear array of detector elements  24 , may be mechanically indexed with respect to a single detector element  28  for successive characterizations of the received light beam  18  along axis  20 . This approach reduces the need for multiple light detector elements  28  and/or allows for a single more expensive and possibly more linear light detector element  28 . 
     Referring now to  FIG. 8 , it will be appreciated that the spectrometer  10  described above may be used to characterize the spectrum of a light source  14  (a light spectrum) that is unknown and directed directly into collimator  16  (as mentioned above). Alternative, the spectrometer  10  may be used to generate an absorption spectrum with sample material  12  located along an optical path between the frequency-filter plate  22  and the light detector array  30  with substantially the same analysis as described above. 
     Referring now again to  FIG. 6 , generally the filter spectra  48  and thus the corrected filter spectra  48 ′ are extremely complex compared to the monochromatic filter spectra produced by an optical grating or the sinusoidal filter spectra produced by an interferometer used with Fourier transform spectroscopy. The filter spectra  48  and  48 ′ are generally broadband covering a full range of frequencies  102  that will be measured in the spectra  64  and non-periodic, that is a spectrum of the spectra  48  would indicate multiple frequencies not a single frequency or filter spectra  48  and  48 ′ would not exhibit any strong autocorrelation peaks. Generally the spectra  48  and  48 ′ appear to be random and are statistically random. A statistically random sequence is one that contains no recognizable patterns or regularities but may not necessarily be truly random. 
     The depictions of the spectra  48  and  48 ′ in the figures are highly simplified and should not be relied upon for understanding the actual spectra. As used herein, the term photonic crystal means a structure having periodic dielectric or other structures that disrupt the propagation of electromagnetic waves by absorption and scattering or the like. 
     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. 
     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. 
     References to an electronic computer can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
     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.