Compact hyperspectral mid-infrared spectrometer

Abstract

An infrared spectrometer for operating in the mid-infrared spectral range is presented, wherein the spectrometer includes a spectral filter based on a Bragg reflector, the spectral filter being operable to provide an interrogation signal, the spectral content of the interrogation signal being dispersed along a first direction at a filter aperture. The filter aperture is imaged through a sample by a thermal imaging camera to create a focused image based on the interrogation signal and the absorption characteristics of the sample. Thus, embodiments according to the present disclosure may be smaller, less complex, and less expensive than infrared spectrometers known in the prior art.

Description

[0001] Related Applications

[0002] This application claims the benefit of U.S. Provisional Application No. 62 / 801,762 (Attorney Docket: CIT-8189-P), filed on February 6, 2019, which is incorporated herein by reference. If there is any contradiction or inconsistency in the language between this application and one or more cases incorporated by reference that may affect the interpretation of the claims in this case, the claims in this case should be interpreted consistent with the language in this case. Field of the Invention

[0003] The present invention relates generally to spectroscopy and more particularly to a hyperspectral spectrometer for use in mid-infrared spectroscopy. Background of the Invention

[0005] Infrared spectroscopy enables analysis of the chemical composition of a sample by interrogating the material with light (or alternatively, radiation) within a spectral range and determining the absorption spectrum of the sample material. Infrared spectroscopy is widely used in many applications such as medical diagnostics, oil exploration, environmental monitoring and drug testing.

[0006] Each chemical and compound has a unique absorption spectrum, which manifests itself as a specific set of wavelengths at which light is measurably absorbed. Thus, the material-identifying "absorption peaks" for a sample provide a chemical "fingerprint" that enables the chemical composition of the sample to be determined. The mid-infrared (MIR) spectral range (i.e., the wavelength range from approximately 5 microns to 25 microns) is particularly attractive for spectroscopy because it typically includes a large number of absorption peaks for most chemical species; therefore, the MIR spectral range is often referred to as the "fingerprint region" for most molecules.

[0007] Prior art spectrometers are generally based on Fourier transform infrared (FTIR) spectroscopy. Unfortunately, FTIR spectroscopy requires bulky, highly complex interferometers with long optical paths and optical components (such as mirrors and beam splitters), the alignment of which must be precisely maintained. As a result, such systems are bulky, complex and expensive to manufacture and maintain during their service life.

[0008] Although attempts have been made to miniaturize FTIR spectroscopy systems for operation in the MIR spectral range, such systems still require the fabrication and alignment of many precisely controlled structures. In addition, they require the sample to be positioned very close to the sensor surface. Therefore, prior art miniaturized FTIR spectroscopy systems are not well suited for use in many applications, such as point-of-care health monitoring, contamination investigations, and pharmaceutical testing.

[0009] Furthermore, operation in the MIR spectral range results in additional challenges due to the fact that the set of materials suitable for use in optical components operable at MIR wavelengths is limited and such materials are often prohibitively expensive.

[0010] Therefore, to date, conventional infrared spectrometers for operating in the MIR spectral range have remained rather bulky and complex, and generally suffer from a narrow bandwidth or wavelength resolution that is too coarse to effectively identify many chemicals and compounds.

[0011] The need for a small, low-cost spectrometer capable of performing high quality chemical and / or molecular analysis of a sample has heretofore remained unmet in the prior art.

[0012] Overview

[0013] Advances in the art have been made in accordance with aspects of the present disclosure directed to an infrared spectrometer comprising a thermal imaging camera and a spectral filter having a filter aperture at which a portion of a source signal is provided as an interrogation signal comprising multiple wavelength signals. The camera images the filter aperture through a sample and focuses the image of the filter aperture at a focal plane array comprising multiple detector elements. The absorption characteristics of the sample are determined based on the output signal from the camera and used to estimate the chemical composition of the sample. Embodiments of the present invention are particularly suitable for use in hyperspectral imaging spectrometers, hyperspectral detection spectrometers, and high-resolution spectrometers.

[0014] Like the prior art, the spectrometer according to the present disclosure employs a spectral filter including a Fabry-Perot (FP) cavity having a pair of Bragg reflector-based mirrors, wherein the spacing between the mirrors varies linearly along a first direction to produce a cavity length that varies linearly along the direction. A source signal is filtered by the spectral filter to achieve an interrogation signal, the spectral content of which includes a wavelength signal dispersed along the first direction at a filter aperture of the spectral filter. The interrogation signal propagates through a sample, and then the absorption characteristics of the sample are determined.

[0015] In contrast to the prior art spectrometers, a compact thermal imaging camera including a focusing lens and a focal plane array is used according to an embodiment of the present disclosure to image a filter aperture through a sample and focus the image of the filter aperture onto its focal plane array. The resulting image thus includes multiple wavelength signals as affected by absorption in the sample, and each wavelength signal is detected at a different detector element of the focal plane array. By using a compact thermal imager to image the filter aperture through a sample, a spectrometer according to the present disclosure can be more compact and lower cost than a prior art spectrometer. In addition, by positioning the filter aperture so that its image is focused at the detector element, the divergence of the light in the interrogation signal is reduced, thereby reducing the spectral range around the center wavelength of each wavelength signal. In addition, the readout rate of the infrared imaging camera is fast enough to make spectral acquisition times of one second or less possible.

[0016] One illustrative embodiment is a mid-infrared spectrometer for analyzing a sample, wherein the spectrometer includes: a black body radiation source for providing a source signal including a plurality of wavelength signals; a spectral filter for filtering the source signal to provide an interrogation signal such that the wavelength signals are dispersed along a first direction at a filter aperture; and a thermal imaging camera including an imaging lens and a focal plane array, the focal plane array including a plurality of detector elements. The camera images the filter aperture through the sample to form an image focused on the detector elements such that each detector element receives a different wavelength signal. The detector elements are then read out as output signals that are used by a processor to estimate the chemical composition of the sample.

[0017] The spectral filter is a Fabry-Perot cavity based on a Bragg reflector, whose reflector comprises alternating layers of silicon and air. The reflectors are separated by linearly increasing distances along a first axis aligned with the row direction of the focal plane array. In some embodiments, the first axis is slightly misaligned with the row direction of the focal plane array to enable finer spectral resolution. In some embodiments, the reflectors comprise alternating layers of different high and / or low refractive index materials, such as other semiconductors (e.g., germanium, gallium arsenide, etc.), and the like.

[0018] Furthermore, the spectral filter is configured such that the spectral range of the interrogation signal is limited such that only one wavelength signal is passed at each position along the first axis. In some embodiments, the spectral filter limits the spectral range of the interrogation signal to a spectral range narrower than an operating range of the detector element.

[0019] In some embodiments, the operating spectral range is different from the MIR spectral range, such as a long wavelength infrared spectral range, a near infrared spectral range, a visible spectral range, or a different spectral range.

[0020] In some embodiments, the spectral filter is configured to reduce the dispersion angle of the wavelength signal in the source signal when providing the interrogation signal; therefore, at least some of the wavelength signals received at the detector elements include a narrower wavelength range around their central wavelength. In addition, the divergence of the wavelength signal is also reduced due to the fact that the spectral filter is positioned so that its image is focused on the detector array.

[0021] According to an embodiment of the present disclosure is a spectrometer 100 for analyzing a sample 110, the spectrometer comprising: a source 102 providing a source signal 112 having a first spectral range SR1, the first spectral range comprising a plurality of wavelength signals λ1-λn, wherein each of the plurality of wavelength signals is characterized by a different wavelength; a spectral filter 104 receiving the source signal at a first surface 116-1 and providing at least a portion of the source signal as an interrogation signal 114 at a filter aperture 118 included in a second surface 116-2, wherein the interrogation signal has a second spectral range SR2, the second spectral range comprising the plurality of wavelength signals, and wherein the plurality of wavelength signals are dispersed along a first direction at the filter aperture; and a camera 106 comprising a lens 124 and a plurality of detector elements 130, wherein the camera operates to provide an output signal 132 based on light incident on the plurality of detector elements; wherein the camera and the spectral filter are arranged such that the camera images the filter aperture through the sample and forms an image 118' of the filter aperture focused on the plurality of detector elements such that each detector element receives a different image portion of the image.

[0022] Another embodiment according to the present disclosure is a spectrometer 100 for analyzing a sample 110, the spectrometer comprising: a source 102 providing a source signal 112 having a first spectral range SR1 including a plurality of wavelength signals λ1-λn, wherein each of the plurality of wavelength signals is characterized by a different wavelength; a spectral filter 104 receiving the source signal at a first surface 116-1 and providing at least a portion of the source signal as an interrogation signal 114 at a filter aperture 118 included in a second surface 116-2, the interrogation signal having a second spectral range SR2 including a plurality of wavelength signals, wherein the plurality of wavelength signals are dispersed along a first direction at the filter aperture; and a camera 106 comprising a plurality of detector elements 130 and a lens 124 having a focal length f1 defining an object distance 128, wherein the camera operates to provide an output signal 132 based on light incident on the plurality of detector elements; wherein the camera and the spectral filter are arranged such that the filter aperture is located at the object distance, and wherein the sample is between the lens and the spectral filter.

[0023] According to another embodiment of the present disclosure, a method for analyzing a sample 110 is provided, the method comprising: providing an interrogation signal 114 at a filter aperture 118 of a spectral filter 104, wherein the interrogation signal comprises a plurality of wavelength signals λ1-λn dispersed along a first direction at the filter aperture; imaging the spectral filter through the sample to form an image 118' focused at a focal plane array 122 comprising a plurality of detector elements 130, wherein the image is based on the interrogation signal and an absorption spectrum of the sample, and wherein each detector element receives a different image portion of the image and provides a detector output based on an intensity of the image portion received by the detector element; and providing an output signal 132 comprising a plurality of detector outputs. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic diagram of an illustrative embodiment of a MIR spectrometer in accordance with the present disclosure is depicted.

[0026] Figure 2 Operations of a method for performing mid-infrared spectroscopy are depicted in accordance with an illustrative embodiment.

[0027] Figure 3 A schematic diagram depicting a cross-sectional view of a spectral filter according to the present disclosure is shown.

[0028] Figure 4A A simulated transmission spectrum of the spectral filter 104 is depicted.

[0029] Figure 4B Measured transmission spectra as a function of position across the width of the spectral filter in accordance with the present disclosure are depicted.

[0030] Figure 5 A schematic diagram of the spectrometer 100 is depicted, showing the optical configuration of the three representative wavelength signals as they propagate from the source 102 to the FPA 122 .

[0031] Figure 6 A schematic diagram depicting a cross-section of a portion of a spectral filter according to the present disclosure.

[0032] Figure 7 A graph of wavelength shift as a function of incident angle is depicted. Graph 700 shows that for a cavity length equal to 4.5 microns, the incident angle (θi Si )’s resonant frequency position shifts.

[0033] Fig. 8A The transmittance of a spectral filter according to the present disclosure is depicted over a range of cavity lengths and incident angles.

[0034] Figure 8BThe total transmittance of the spectral filter 104 at three different positions along the x-direction is depicted.

[0035] Figures 9A-9H A comparison of transmission spectra as measured by a conventional FTIR system and a spectrometer according to the present disclosure for different sample materials is depicted.

[0036] Detailed Description

[0037] Figure 1 A schematic diagram of an illustrative embodiment of a MIR spectrometer according to the present disclosure is depicted. The spectrometer 100 includes a source 102 , a spectral filter 104 , a camera 106 , and a processor 108 , wherein the source 102 , the spectral filter 104 , and the camera 106 are aligned along an optical axis A1 passing through a sample 110 .

[0038] Figure 2 Operations of a method for performing MIR spectroscopy according to an illustrative embodiment are depicted. Figure 1 And reference Figure 3-4B Method 200 is described. Method 200 begins with operation 201 , where spectral filter 104 receives source signal 112 from source 102 .

[0039] Source 102 is a conventional broadband blackbody light source that provides source signal 112. In the depicted example, source signal 112 has a continuous spectral range with a spectral width SR1 extending from λmin to λmax with a center wavelength λc1. In the depicted example, SR1 extends from λmin of approximately 2 microns to λmax of approximately 20 microns.

[0040] At operation 202 , the spectral filter 104 provides an interrogation signal 114 based on the source signal 112 .

[0041] The spectral filter 104 is a Bragg reflector-based Fabry-Perot (FP) cavity filter having outer surfaces 116-1 and 116-2, wherein a portion of the surface 116-2 defines a filter aperture 118. The spectral filter 104 is configured to receive the source signal 112 at the surface 116-1 and pass a portion of the source signal as an interrogation signal 114 to the filter aperture 118, such that the interrogation signal includes wavelength signals λ1 to λn dispersed along the x-direction at the filter aperture. For the purposes of this specification (including the appended claims), the term "wavelength signal" is defined as an optical signal whose spectral content is characterized by a central wavelength and a narrow spectral range surrounding it.

[0042] As is apparent to those skilled in the art, the size of a spectrometer is dictated by the required propagation length of light within the spectrometer. Typical prior art spectrometers require long propagation lengths, which limits their miniaturization. Furthermore, long propagation lengths require precisely aligned optical elements, such as mirrors and beam splitters, which increases their complexity and manufacturing cost. Furthermore, these optical components must remain carefully aligned throughout the useful life of the system, which also increases operating costs.

[0043] However, compared to conventional spectrometers, by configuring the spectral filter 104 as a spectral filter based on a FP cavity, embodiments according to the present disclosure have greater potential for miniaturization and reduced complexity. These advantages arise in part from the fact that a FP cavity is an optical resonant cavity in which light at the resonant frequency of the cavity is reflected back and forth multiple times, thereby multiplying the path length of the photons in the light by the number of round trips they make within the cavity.

[0044] Figure 3 A schematic diagram of a cross-sectional view of a spectral filter according to the present disclosure is depicted. Spectral filter 104 includes substantially identical mirrors 302-1 and 302-2 separated by a small, non-uniform spacing that defines an optical cavity 304 (hereinafter referred to as "cavity 304").

[0045] Each of the reflectors 302-1 and 302-2 (collectively referred to as reflectors 302) is a multilayer Bragg reflector designed to achieve high reflectivity (>99.9999%) over the extent of the spectral range SR2 of the interrogation signal 114. Each of the reflectors 302 includes at least one Bragg layer pair 312 disposed on a surface of a substrate 306, wherein the Bragg layer pair 312 includes a high refractive index (HR) layer 308 and a low refractive index (LR) layer 310.

[0046] The HR layer 308 comprises a material having a relatively high refractive index and has a thickness as measured within its material that is substantially equal to an odd multiple of one quarter of the target wavelength λt in the spectral range SR2 (i.e., kλt / 4, where k=1, 3, 5, ...). In a similar manner, the LR layer 310 comprises a relatively low refractive index material and has a thickness within its material that is substantially equal to an odd multiple of one quarter of the target wavelength λt. It should be noted that the target wavelength is a matter of design choice and is selected to enable operation of the spectral filter 104; thus, in practice λt can be any wavelength within the spectral range SR2. Preferably, however, the target wavelength λt has a value equal to or nearly equal to the center wavelength of the spectral range. In the depicted example, λt is selected to be approximately 9 microns.

[0047] In the depicted example, substrate 306 is a conventional single crystal silicon substrate, and each reflector includes two Bragg layer pairs, each Bragg layer pair having a HR layer 308 and a LR layer 310. HR layer 308 is an amorphous silicon layer deposited using plasma enhanced chemical vapor deposition (PECVD) such that the amorphous silicon has a refractive index of approximately 2.8 and a thickness of approximately 803 nm. LR layer 310 is an air layer having a thickness of approximately 2.25 microns. It should be noted that the materials and design parameters provided for reflector 302 are merely exemplary, and numerous alternative designs of reflectors for spectral filter 104 are within the scope of the present disclosure. However, embodiments including Bragg reflectors made of alternating layers of silicon and air are afforded significant advantages over the prior art because the fabrication of silicon layers is a well-known and well-understood process and can be accomplished at low cost. Furthermore, the mechanical and optical properties of silicon are well suited for their use in reflector 302.

[0048] The spacing between the reflectors 302 at any point (i.e., the cavity length of the cavity 304) specifies what wavelengths pass through the spectral filter 104 at that point and what wavelengths are reflected by the spectral filter. When the spacing between the reflectors (i.e., the cavity length L) is equal to an integer m times of half a wavelength (i.e., when L=mλ / 2), light having a wavelength λ resonates back and forth between the reflectors inside the optical resonant cavity. Therefore, light having a wavelength λ is transmitted through the cavity with low loss, while light characterized by other wavelengths is reflected by the FP cavity. Therefore, the cavity length specifies the wavelength of light that selectively passes through the FP cavity.

[0049] Mirrors 302-1 and 302-2 are arranged at a non-zero angle Φ to each other along the x-direction so that the cavity length of cavity 304 varies linearly along the x-direction. Thus, at each position x, mirrors 302-1 and 302-2 separate a cavity length L(x) that increases from Lmin at position xm to Lmax at position xn. In the depicted example, Lmin is approximately 3.28 microns and Lmax is approximately 7.05 microns. In some embodiments, the cavity length decreases linearly along the x-direction. In some embodiments, the spacing and / or angle between mirrors 302 is controllable.

[0050] The linearly increasing cavity length of the spectral filter 104 enables it to receive the source signal 112 as a multi-spectral input optical signal at each position x and selectively pass a narrow transmission peak (i.e., a wavelength signal) within the spectral range SR2. In other words, at each position along the x direction, the spectral filter 104 selectively passes a signal λi of a different wavelength, where m≤i≤n, so that its spectral content is dispersed along the x direction from λ1 at position xm to λn at position xn. In the depicted example, λ1 is approximately 7.5 microns and λn is approximately 12 microns.

[0051] Figure 4A Depicted is a simulated transmission spectrum of the spectral filter 104. The graph 400 shows that for a cavity length equal to half the target wavelength λt (ie, a cavity length of 4.5 microns in the depicted example), a single narrow wavelength signal corresponding to λt passes through the spectral filter.

[0052] Figure 4B The measured transmission spectrum as a function of position across the width of the spectral filter according to the present disclosure is depicted. The graph 402 shows that at position x, a single wavelength signal passes through the spectral filter 104. However, it should be noted that the transmission spectrum shown in the graph 402 was acquired using an FTIR spectrometer with a millimeter scale aperture. Therefore, the peak in the measured spectrum is artificially broadened due to the variation in cavity thickness within the millimeter aperture region in each measurement.

[0053] In some embodiments, mirrors 302 are arranged at a second angle to each other along the y direction so that the cavity length of cavity 304 also has a linear gradient along the y direction. In such embodiments, the spectral filter is configured to disperse the spectral content of interrogation signal 114 along each of the x and y directions.

[0054] Returning now to method 200 , at operation 203 , camera 106 images filter aperture 118 through sample 110 and sample holder 120 to form image 118 ′ on focal plane array (FPA) 122 .

[0055] Camera 106 is a thermal imaging camera that includes FPA 122, lens 124, and camera aperture 126. In some embodiments, camera 106 is a different camera that operates to image filter aperture 118.

[0056] FPA 122 includes a two-dimensional array of substantially identical detector elements 130-1 through 130-n (collectively referred to as detector elements 130), each of which is adapted to detect light within spectral range SR2. In the depicted example, each of detector elements 130 is a bolometer configured to enable detection of light within a wavelength range from about 7.5 microns to about 13.5 microns.

[0057] One aspect of the present disclosure is that operation in the wavelength range from about 7.5 microns to about 13.5 microns corresponds to an atmospheric transparency window and a transmission passband in the infrared spectral range. Thus, only light corresponding to a particular wavelength passing through the spectral filter 104 reaches its corresponding detector element 130. Thus, embodiments according to the present disclosure do not require additional optical filters, such as those required in prior art infrared spectrometers, which enables such embodiments to have lower complexity and / or cost.

[0058] In some embodiments, FPA 122 includes various suitable photodetectors. Alternative photodetectors suitable for use in FPA 122 include, but are not limited to, semiconductor detectors, cadmium mercury telluride detectors, and the like.

[0059] The lens 124 is an imaging lens adapted to operate over the spectral range SR2. The lens 124 is configured to form an image 118' of a filter aperture of the spectral filter 104 on the FPA 122. The lens 124 has a focal length f1 and an object distance 128, which defines a separation between the spectral filter 104 and the lens 122 that enables the lens to form a focused image of the spectral filter on the FPA 122. As will be apparent to one skilled in the art after reading this specification, the lens 124 may include any lens design or configuration (e.g., multi-element, compound, telescopic, diffractive, refractive, etc.) that enables it to form a focused image of the spectral filter 106 on the FPA 122.

[0060] In the depicted example, the focal length f1 of the lens 124 is approximately 6.35 cm. By employing a lens with a short focal length, the spectrometer 100 can be made compact without the need to include additional optical components, such as beam splitters, mirrors, and lasers for beam collimation. Thus, a spectrometer according to the present disclosure can be significantly smaller and less complex than prior art spectrometers. For example, in the depicted example, the spectrometer 100 has a total length of only approximately 10 cm along the y direction without the need for the optical path from the source 102 to the FPA 122 to be folded.

[0061] The camera aperture 126 is a clear aperture that is optionally included in the camera 106 to function as a camera “stop” that reduces the divergence angle of light received at some or all of the detector elements 130 .

[0062] The spectrometer 100 is arranged so that the filter aperture 118 is located at an object distance 128, which is defined by the focal length f1 of the lens 124 as it images the filter aperture through the sample 110 and the sample holder SH. Thus, the camera 106 focuses the filter aperture on the FPA 122 to form an image 118'. In some embodiments, the spectrometer 100 is arranged so that different planes within the spectral filter 104 are located at the focal length f1.

[0063] It should be noted that although the detector element 130 can detect light having any wavelength in the range from about 7.5 microns to about 13.5 microns, there are one or more higher order resonance peaks at wavelengths greater than 12 microns, which may cause interference in the spectrometer 100. Therefore, the spectral range SR2 of the spectral filter 104 is capped at the maximum wavelength signal λn of 12 microns, thereby limiting the measurement range of the spectrometer 100 to 12 microns or below.

[0064] Image 118' is a convolution of the dispersed spectral content of interrogation signal 114 and the absorption characteristics of sample 110. Thus, image 118' includes absorption peaks whose positions, amplitudes, and inflection points provide a "spectral fingerprint" indicative of the chemical composition of sample 110.

[0065] Image 118' includes image portions 132-1 through 132-n corresponding to wavelength signals λ1 through λn, respectively.

[0066] At operation 204 , the detector elements 130 are read out by the camera 106 and provided to the processor 108 as output signals 132 including detector outputs 134 - 1 through 134 - n .

[0067] Because the spectral content of the interrogation signal 114 is dispersed along the x-direction at the filter aperture 118, the detector elements 130-1 to 130-n detect image portions 132-1 to 132-n, respectively, and provide detector outputs 134-1 to 134-n. The amplitude of each of the detector outputs 134-1 to 134-n is based on the intensity of the wavelength signal (i.e., the image portion) incident on its corresponding detector element. As discussed below, the wavelength range around the central wavelength of each wavelength signal is a function of the optical design of the spectrometer 100, the size of the detector elements 130, the spacing between adjacent detector elements, and the angle of incidence of light included in the wavelength signal.

[0068] Another aspect of the present disclosure is that because the readout rate of the microbolometer array can be as fast as 9 Hz, embodiments according to the present disclosure can obtain a molecular absorption spectrum of a sample in less than one second without the need for scanning optics as is typically necessary in the prior art.

[0069] At operation 205 , the processor 108 generates an estimate of the chemical composition of the sample 110 based on the output signal 132 .

[0070] Processor 108 is a conventional processor including circuitry operable to receive output signal 132, execute instructions, store and retrieve data, and estimate material composition of sample 110, among other functions.

[0071] It should be noted that, in general, method 200 is preceded by a calibration routine in which the optical transfer function of spectrometer 100 is determined by performing at least some of operations 201 to 205 with an empty sample holder SH. Such calibration provides processor 108 with a baseline against which output signal 132 can be compared to form an estimate of the chemical composition of sample 110.

[0072] As one skilled in the art will appreciate after reading this specification, the transmittance of the spectral filter 104 may be affected by the angle at which light is received from the source 102; thus, the wavelength of light impinging on each detector element 130 may be based on the cavity length and the angle of incidence of the light received from the spectral filter.

[0073] However, one aspect of the present disclosure is that the range of wavelengths incident on at least some of the detector elements 130 (i.e., the range of wavelengths around the center wavelength of at least some of the wavelength signals) can be reduced by:

[0074] i. focusing the image of the filter aperture 118 onto the detector element of the FPA 122; or

[0075] ii. providing a spectral filter 104 such that it is configured to reduce the divergence angle of the wavelength signal; or

[0076] iii. Combination of i and ii.

[0077] Because the filter aperture 118 is focused on the detector elements 130 of the FPA 122, each detector element receives a different wavelength signal corresponding to a different portion of the cavity 304, where each portion of the cavity has a different cavity length. In addition, the detector elements 130 are configured so that they are very small; therefore, the variation in cavity length across the cavity portion from which each detector element receives light is substantially insignificant and is ignored for the purposes of the discussion provided herein.

[0078] It should be noted that the angular spread (i.e. divergence angle) of the propagation direction of the photons arriving at the detector element 130 is a function of the size of the source 102, the size of the lens 124 and the distance between the lens and the spectral filter 104. When the light in the source signal 112 is incident on the spectral filter 104, the angle (relative to the optical axis A1) during its propagation reduces the factor based on the refractive index difference at the surface 116-1. By forming the spectral filter so that it includes a Bragg reflector with a high refractive index material, the light passing the spectral filter is highly refracted. Therefore, when the wavelength signal included in the light is changed from the source signal 112 to the inquiry signal 114, they become more collimated, thereby reducing their divergence angle.

[0079] Figure 5 A schematic diagram of spectrometer 100 is depicted, showing the optical configuration of three representative wavelength signals as they propagate from source 102 to FPA 122 . Figure 5 The paths and shapes of wavelength signals λ1 and λn received at the outermost detector elements of FPA 122 (i.e., detector elements 130-1 and 130-n) and the center wavelength signal (i.e., wavelength signal λc) received at the center detector element of FPA 122 (i.e., detector element 130-c) are shown.

[0080] In the source signal 112, the waveguide signals λ1 to λn all have the same divergence angle. In other words, θd1-1, θd1-c, θd1-n all have the same value. In the depicted example, the value is about 5.4°, which produces incident angles of 7.5° and 12.9° for the outermost rays LRmin and LRmax in each of the wavelength signals λ1 and λn, respectively.

[0081] Figure 6 A schematic diagram of a cross section of a portion of a spectral filter according to the present disclosure is depicted. Cross section 600 is the region of mirror 302-1 where light ray LRmax in wavelength signal λ1 of source signal 112 is incident.

[0082] As in Figure 6 As indicated in FIG. 1 , by virtue of the large difference in refractive index between air and the material 306 of the substrate (ie, silicon), according to Snell's law, the angle θi of a ray propagating in air is 空气 The factor based on the refractive index difference is reduced at the surface 116-1 (relative to the optical axis A1) to (θi Si ).

[0083] In the depicted example, air has a refractive index of 1.0, silicon of substrate 306 has a refractive index of approximately 3.4, and each of HR layers 308 has a refractive index of 2.8. As discussed above, when LRmin and LRmax are received at surface 116-1, their incident angles (θi 空气 ) (i.e., 7.5° and 12.9°, respectively). However, when received at FPA 122 after passing through spectral filter 104, these incident angles are reduced to 2.67° and 4.57°, respectively. For an exemplary cavity length of 4.5 microns, at an angle of 4.57° (θi ) compared to an incident angle of 0°, Si ) produces a wavelength shift of approximately 0.174 microns for light incident on detector element 130.

[0084] Figure 7 Graph 700 shows that for a cavity length equal to 4.5 microns, the incident angle (θi Si ) (light propagates in the substrate 306 at this incident angle) is shifted.

[0085] It should be noted however that the spectral range around the central wavelength of the wavelength signal received at each detector element is a function of the cavity length through which the wavelength signal passes and the angle of incidence of its outermost rays on the detector element.

[0086] Fig. 8A The transmittance of a spectral filter according to the present disclosure is depicted over a range of cavity lengths and angles of incidence. Graph 800 includes traces 802 and 804 representing the transmittance of extreme external light within the dispersion angle of the wavelength signal.

[0087] It can be seen from graph 800 that the divergence of the transmittance of spectral filter 104 is greatest at each end of cavity 304, while its divergence is very small at the center of the cavity because light passing through that point is substantially aligned with the center of lens 126. The larger divergence at the very ends of spectral cavity 104 occurs because, for the same divergence angle, light passes through the spectral filter eccentrically, resulting in a larger angle of incidence.

[0088] Figure 8B The total transmittance of the spectral filter 104 at three different positions along the x-direction is depicted. The spectrum shown in graph 806 is obtained from the integration of all divergence angles along the x-direction at positions at either end xm and xn of the cavity 304 and at the center xc of the cavity. In the depicted example, the cavity lengths L(xm), L(xc), and L(xn) are equal to 3.7, 4.7, and 5.7 microns, respectively.

[0089] Figures 9A-9H Depicted is a comparison of transmission spectra of different sample materials as measured by a conventional FTIR system and a spectrometer according to the present disclosure.

[0090] Graphs 900A-D illustrate the transmission spectra of glucose, acetaminophen, polystyrene, and low density polyethylene, respectively, measured using a conventional FTIR system.

[0091] Graphs 900E-H illustrate the transmission spectra of glucose, acetaminophen, polystyrene, and low density polyethylene, respectively, measured using a spectrometer similar to spectrometer 100 .

[0092] As shown in graphs 900A-H, the performance of spectrometer 100 is comparable to that of more expensive, more complex and larger FTIR systems. In addition, each measurement result made using spectrometer 100 is obtained in less than one second, which is significantly faster than can be achieved using traditional FTIR systems.

[0093] It should be understood that the present disclosure teaches only one example of illustrative embodiments and that many variations of the present invention may be readily devised by those skilled in the art after reading the present disclosure and that the scope of the present invention will be determined by the appended claims.

[0094] Aspects of the present disclosure may be implemented in one or more of the following embodiments.

[0095] 1) A spectrometer 100 for analyzing a sample 110, the spectrometer comprising:

[0096] A source 102 providing a source signal 112 having a first spectral range SR1, the first spectral range comprising a plurality of wavelength signals λ1-λn, wherein each wavelength signal of the plurality of wavelength signals is characterized by a different wavelength;

[0097] a spectral filter 104 receiving the source signal at a first surface 116-1 and providing at least a portion of the source signal as an interrogation signal 114 at a filter aperture 118 included in a second surface 116-2, wherein the interrogation signal has a second spectral range SR2, the second spectral range includes the plurality of wavelength signals, and wherein the plurality of wavelength signals are dispersed along a first direction at the filter aperture; and

[0098] a camera 106 comprising a lens 124 and a plurality of detector elements 130, wherein the camera is operative to provide an output signal 136 based on light incident on the plurality of detector elements;

[0099] Therein, the camera and the spectral filter are arranged such that the camera images the filter aperture through the sample and forms an image 118' of the filter aperture focused on the plurality of detector elements such that each detector element receives a different image portion 132 of the image.

[0100] 2) A spectrometer according to 1), wherein the spectral filter comprises a first reflector 302-1 and a second reflector 302-2 that jointly define an optical cavity 304, the optical cavity having a cavity length L(x) that varies linearly along the first direction, and wherein each of the first reflector and the second reflector is a multilayer Bragg reflector comprising at least one Bragg layer pair, the Bragg layer pair comprising a first layer 308 of a first material having a first refractive index and a second layer 310 of a second material having a second refractive index lower than the first refractive index.

[0101] 3) The spectrometer according to 2), wherein the first material comprises a material selected from the group consisting of silicon and germanium, and the second material is air.

[0102] 4) The spectrometer according to 1), wherein the lens has a first focal length defining an object distance 128, and wherein the spectral filter and the camera are arranged such that the filter aperture is located at the object distance.

[0103] 5) The spectrometer according to 1), further comprising a processor 108 for estimating the chemical composition of the sample based on the output signal.

[0104] 6) The spectrometer of 1), wherein each detector element of the plurality of detector elements comprises a bolometer operative to detect light in the second spectral range.

[0105] 7) Spectrometer according to 6), wherein each bolometer is characterized by an operating spectral range larger than the second spectral range.

[0106] 8) A spectrometer according to 1), wherein a first wavelength signal among the multiple wavelength signals is characterized by a first dispersion angle θd1 at the first surface, and wherein the first wavelength signal is characterized by a second dispersion angle θd2 at the filter aperture, the second dispersion angle being smaller than the first dispersion angle.

[0107] 9) A spectrometer 100 for analyzing a sample 110, the spectrometer comprising:

[0108] A source 102 providing a source signal 112 having a first spectral range SR1, the first spectral range comprising a plurality of wavelength signals λ1-λn, wherein each wavelength signal of the plurality of wavelength signals is characterized by a different wavelength;

[0109] a spectral filter 104 receiving the source signal at a first surface 116-1 and providing at least a portion of the source signal as an interrogation signal 114 at a filter aperture 118 included in a second surface 116-2, the interrogation signal having a second spectral range SR2 including the plurality of wavelength signals, wherein the plurality of wavelength signals are dispersed along a first direction at the filter aperture; and

[0110] a camera 106 comprising a plurality of detector elements 130 and a lens 124 having a focal length f1 defining an object distance 128, wherein the camera is operative to provide an output signal 136 based on light incident on the plurality of detector elements;

[0111] wherein the camera and the spectral filter are arranged such that the filter aperture is located at the object distance, and wherein the sample is between the lens and the spectral filter.

[0112] 10) The spectrometer according to 9), wherein the spectral filter comprises:

[0113] a first reflector 302-1 comprising a first Bragg reflector comprising at least one Bragg layer pair 312 having a first layer 308 comprising a first material and a second layer 310 comprising air, wherein the first material has a refractive index greater than a refractive index of air; and

[0114] a second reflector 302-2 comprising a second Bragg reflector including at least one Bragg layer pair including the first layer and the second layer, wherein the second reflector comprises the filter aperture;

[0115] wherein the first reflector and the second reflector are arranged such that they define a first angle Φ along the first direction, and wherein the first angle is a non-zero angle, and

[0116] Wherein, the first reflector and the second reflector are separated by a cavity length L(x) that varies linearly along the first direction.

[0117] 11) A spectrometer according to 9), wherein a first wavelength signal λ1 among the multiple wavelength signals is characterized by a first dispersion angle θd1 at the first surface, and wherein the first wavelength signal is characterized by a second dispersion angle θd2 at a first detector element 130-1 among the multiple detector elements, and the second dispersion angle is smaller than the first dispersion angle.

[0118] 12) The spectrometer according to 9), further comprising a processor 108 for estimating the chemical composition of the sample based on the output signal.

[0119] 13) A spectrometer according to 9), wherein the camera and the spectral filter are arranged so that the camera images the filter aperture through the sample and focuses the image 118' of the filter aperture on the multiple detector elements so that each detector element receives a different image portion 132 of the image.

[0120] 14) A method for analyzing a sample 110, the method comprising:

[0121] providing an interrogation signal 114 at a filter aperture 118 of the spectral filter 104, wherein the interrogation signal has a second spectral range comprising a plurality of wavelength signals λ1-λn dispersed along a first direction at the filter aperture;

[0122] imaging the filter aperture through the sample to form an image 118' focused at a focal plane array 122 comprising a plurality of detector elements 130, wherein the image is based on the interrogation signal and an absorption spectrum of the sample, and wherein each detector element receives a different image portion 132 of the image and provides a detector output 134 based on an intensity of the image portion received by that detector element; and

[0123] An output signal 136 is provided comprising the outputs of the plurality of detectors.

[0124] 15) The method according to 14) also includes estimating the chemical composition of the sample based on the output signal.

[0125] 16) The method according to 14) also includes providing the spectral filter so that the spectral filter includes a first reflector 302-1 and a second reflector 302-2 that jointly define an optical cavity 304, the optical cavity having a cavity length L(x) that varies linearly along the first direction, and wherein each of the first reflector and the second reflector is a multilayer Bragg reflector including at least one Bragg layer pair, the Bragg layer pair including a first layer 308 of a first material having a first refractive index and a second layer 310 of a second material having a second refractive index lower than the first refractive index.

[0126] 17) The method according to 16), wherein the spectral filter is provided so that the first material includes a material selected from the group consisting of silicon and germanium, and the second material is air.

[0127] 18) The method according to 16) further comprises:

[0128] providing a camera 106 for imaging the filter aperture through the sample, wherein the camera comprises a lens 124 and the plurality of detector elements, and wherein the lens has a focal length f1 defining an object distance 128; and

[0129] The spectral filter is positioned such that the filter aperture is at the object distance.

[0130] 19) The method according to 18), wherein each detector element of the plurality of detector elements comprises a bolometer characterized by an operating spectral range greater than and including the second spectral range.

[0131] 20) The method according to 16) further comprises:

[0132] providing a source 102 transmitting a source signal 112 to the spectral filter, wherein the source signal comprises the plurality of wavelength signals, and wherein a first wavelength signal of the plurality of wavelength signals is characterized by a first dispersion angle θd1 when the first wavelength signal is received at the spectral filter;

[0133] The interrogation signal is provided at the filter aperture so that the first wavelength signal has a second dispersion angle θd2 that is smaller than the first dispersion angle.

Claims

1. A spectrometer (100) for analyzing a sample (110), the spectrometer comprising: A source (102) providing a source signal (112) having a first spectral range (SR1), the first spectral range comprising a plurality of wavelength signals (λ1-λn), wherein each wavelength signal of the plurality of wavelength signals is characterized by a different wavelength; a spectral filter (104) receiving the source signal at a first surface (116-1) and providing at least a portion of the source signal as an interrogation signal (114) at a filter aperture (118) included in a second surface (116-2), wherein the interrogation signal has a second spectral range (SR2), the second spectral range includes the plurality of wavelength signals, and wherein the plurality of wavelength signals are dispersed along a first direction at the filter aperture; and a camera (106) comprising a lens (124) and a plurality of detector elements (130), wherein the camera is operative to provide an output signal (136) based on light incident on the plurality of detector elements; wherein the camera and the spectral filter are arranged such that the camera images the filter aperture through the sample and forms an image (118') of the filter aperture focused on the plurality of detector elements such that each detector element receives a different image portion (132) of the image, each image portion corresponding to a different wavelength signal among the plurality of image portions; and Wherein the lens and the spectral filter are configured to reduce the divergence angle of the wavelength signal of each image portion at its respective detector element.

2. The spectrometer according to claim 1, wherein: The spectral filter comprises a first reflector (302-1) and a second reflector (302-2) that together define an optical cavity (304), the optical cavity having a cavity length (L(x)) that varies linearly along the first direction, and wherein each of the first reflector and the second reflector is a multilayer Bragg reflector comprising at least one Bragg layer pair, the Bragg layer pair comprising a first layer (308) of a first material having a first refractive index and a second layer (310) of a second material having a second refractive index lower than the first refractive index.

3. The spectrometer according to claim 2, wherein: The first material includes a material selected from the group consisting of silicon and germanium, and the second material is air.

4. The spectrometer according to claim 1, wherein: The lens has a first focal length defining an object distance (128), and wherein the spectral filter and the camera are arranged such that the filter aperture is located at the object distance.

5. The spectrometer of claim 1, further comprising a processor (108) for estimating a chemical composition of the sample based on the output signal.

6. The spectrometer according to claim 1, wherein: Each detector element of the plurality of detector elements comprises a bolometer operative to detect light within the second spectral range.

7. The spectrometer according to claim 6, wherein: Each bolometer is characterized by an operating spectral range greater than said second spectral range.

8. The spectrometer according to claim 1, wherein: A first wavelength signal of the plurality of wavelength signals is characterized by a first dispersion angle (θd1) at the first surface, and wherein the first wavelength signal is characterized by a second dispersion angle (θd2) at the filter aperture, the second dispersion angle being less than the first dispersion angle.

9. A spectrometer (100) for analyzing a sample (110), the spectrometer comprising: A source (102) providing a source signal (112) having a first spectral range (SR1), the first spectral range comprising a plurality of wavelength signals (λ1-λn), wherein each wavelength signal of the plurality of wavelength signals is characterized by a different wavelength; a spectral filter (104) receiving the source signal at a first surface (116-1) and providing at least a portion of the source signal as an interrogation signal (114) at a filter aperture (118) included in a second surface (116-2), the interrogation signal having a second spectral range (SR2) including the plurality of wavelength signals, wherein the plurality of wavelength signals are dispersed along a first direction at the filter aperture; and a camera (106) comprising a plurality of detector elements (130) and a lens (124) having a focal length (f1) defining an object distance (128), wherein the camera is operative to provide an output signal (136) based on light incident on the plurality of detector elements; wherein the camera and the spectral filter are arranged such that the filter aperture is located at the object distance, and wherein the sample is between the lens and the spectral filter; wherein the camera and the spectral filter are arranged such that the camera images the filter aperture through the sample and focuses an image (118') of the filter aperture onto the plurality of detector elements such that each detector element receives a different image portion (132) of the image, each image portion corresponding to a different wavelength signal among the plurality of image portions; and Wherein the lens and the spectral filter are configured to reduce the divergence angle of the wavelength signal of each image portion at its respective detector element.

10. The spectrometer according to claim 9, wherein: The spectral filter comprises: a first reflector (302-1) comprising a first Bragg reflector comprising at least one Bragg layer pair (312) having a first layer (308) comprising a first material and a second layer (310) comprising air, wherein the first material has a refractive index greater than a refractive index of air; and a second reflector (302-2) comprising a second Bragg reflector including at least one Bragg layer pair including the first layer and the second layer, wherein the second reflector comprises the filter aperture; wherein the first reflector and the second reflector are arranged such that they define a first angle (Φ) along the first direction, and wherein the first angle is a non-zero angle, and Wherein the first reflector and the second reflector are separated by a cavity length (L(x)) that varies linearly along the first direction.

11. The spectrometer according to claim 9, wherein: A first wavelength signal (λ1) among the plurality of wavelength signals is characterized by a first dispersion angle (θd1) at the first surface, and wherein the first wavelength signal is characterized by a second dispersion angle (θd2) at a first detector element (130-1) among the plurality of detector elements, the second dispersion angle being smaller than the first dispersion angle.

12. The spectrometer of claim 9, further comprising a processor (108) for estimating a chemical composition of a sample based on the output signal.

13. A method for analyzing a sample (110), the method comprising: providing an interrogation signal (114) at a filter aperture (118) of a spectral filter (104), wherein the interrogation signal has a second spectral range, the second spectral range comprising a plurality of wavelength signals (λ1-λn) dispersed along a first direction at the filter aperture; providing a camera (106) for imaging the filter aperture through the sample, wherein the camera comprises a lens (124) and a plurality of detector elements, and wherein the lens has a focal length (f1) defining an object distance (128); imaging the filter aperture through the sample to form an image (118') focused at a focal plane array (122) comprising the plurality of detector elements (130), wherein the image is based on the interrogation signal and an absorption spectrum of the sample, and wherein each detector element receives a different image portion (132) of the image and provides a detector output (134) based on the intensity of the image portion received by the detector element, each image portion corresponding to a different wavelength signal among the plurality of image portions; and further wherein the lens and the spectral filter are configured to reduce a divergence angle of the wavelength signal of each image portion at its respective detector element; and An output signal (136) is provided that includes the outputs of the plurality of detectors.

14. The method of claim 13, further comprising estimating a chemical composition of the sample based on the output signal.

15. The method of claim 13, further comprising providing the spectral filter such that the spectral filter comprises a first reflector (302-1) and a second reflector (302-2) that together define an optical cavity (304), the optical cavity having a cavity length (L(x)) that varies linearly along the first direction, and wherein, Each of the first reflector and the second reflector is a multilayer Bragg reflector including at least one Bragg layer pair including a first layer (308) of a first material having a first refractive index and a second layer (310) of a second material having a second refractive index lower than the first refractive index.

16. The method according to claim 15, wherein: The spectral filter is provided such that the first material comprises a material selected from the group consisting of silicon and germanium, and the second material is air.

17. The method according to claim 15, further comprising: The spectral filter is positioned such that the filter aperture is at the object distance.

18. The method according to claim 17, wherein: Each detector element of the plurality of detector elements includes a bolometer characterized by an operating spectral range greater than and including the second spectral range.

19. The method according to claim 15, further comprising: providing a source (102) for transmitting a source signal (112) to the spectral filter, wherein the source signal comprises the plurality of wavelength signals, and wherein a first wavelength signal of the plurality of wavelength signals is characterized by a first dispersion angle (θd1) of the first wavelength signal when received at the spectral filter; Wherein, the interrogation signal is provided at the filter aperture so that the first wavelength signal has a second dispersion angle (θd2) smaller than the first dispersion angle.

Images