The invention pertains to the fields of spectroscopy and spectral imaging.
Spectroscopy is the science of determining information about the spectral content of an electromagnetic radiation source. Thus, in its broadest sense, the science of spectroscopy encompasses basic photography cameras since a photograph contains spectral information about the observed scene, namely, the colors of light emanating from the observed scene. Hereinafter, we will sometimes use the term “light” as shorthand to refer to electromagnetic radiation of any wavelength. However, this is not intended to limit the discussion to electromagnetic radiation that is in the visible spectrum.
A spectroscope observes light from a source and determines spectral information about that light. The light source may be virtually anything, including, an object that produces its own light (such as a star, a laser, or the molecules involved in a phosphorescent chemical reaction), light that is reflected off of an object, and light that passes through an object. Spectral information about an original source of light can provide information about the chemical composition of the source of the light. Likewise, if one knows the spectral composition of the original light source, light reflected from or light transmitted through an object can provide information about the chemical composition of the object. For instance, the portion of the light spectrum that can and cannot pass through an object could disclose the chemical composition of the object. The same is true for light reflected from an object.
Spectroscopes with extremely high spectral resolution are useful in many applications including scientific and military applications. For instance, spy planes may carry cameras capable of capturing images containing very broad spectral information and very high spectral resolution in order to detect the existence of certain materials, to see through things that are opaque to the visible eye, and/or to provide highly detailed spectroscopic images.
One form of spectroscopy, known as standing wave spectroscopy, takes advantage of the constructive interference that occurs when a beam of light of a particular wavelength is reflected back on itself so that two beams of the same light interfere with each other.
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Thus, by measuring the intensity of the light detected at the detector and scanning the distance, d, between the reflector 106 and the detector 108, one can determine the spectral content of a light beam.
Thus, a detector 208 placed behind one of the reflectors 204 or 205 would detect light of an intensity that would vary as a function of the ratio of l to the wavelength content of the light in the cavity 203. Thus, by varying l, a Fabry-Perot cell can be used to determine the wavelength content of a light beam. Light at other wavelengths essentially will interfere partially destructively or constructively. Again, by varying the distance between the two reflectors, the cell can be used to determine the wavelength content of light in the cavity. A detector could be placed behind each reflector to increase the sensitivity of measurement. However, in theory, both detectors should detect essentially complementary signals, thus revealing identical information.
In theory, all light in a perfect Fabry-Perot cell will be transmitted through one of reflectors 204 and 205 (i.e., the amount of light entering the cell is equal to the amount of light exiting the cell per unit time), with the percentage of the light that is transmitted through each reflector 204, 205 depending on the distance between the two reflectors. For example, if l is ½ the wavelength of monochromatic light in the cell, then 100% of the light in the cell will be transmitted through reflector 204. If l is ¼ the wavelength of monochromatic light in the cell, then 100% of the light in the cell will be transmitted through reflector 205. At other distances, some percentage of the light may be transmitted through reflector 204 and the rest is transmitted through reflector 205.
However, no Fabry-Perot cell is perfect. In actuality, some light always is reflected and some always is transmitted. The Q of a Fabry-Perot cell is a measure of the quality of the cell. More specifically, the Q of a cell is the number of times that a light beam will bounce back and forth in the cell before the amount of light entering the cell is equal to the amount of light exiting the cell per unit time. The higher the Q in a Fabry-Perot cell, the narrower the FWHM. This, in turn, means that the cell is more sensitive to wavelength and produces a more robust output measurement.
One common problem with the manufacture of Fabry-Perot cells is the placement of the circuitry needed to move one of the reflectors (in order to vary l over time) and the circuitry of the detector. Generally, one the reflectors must have circuitry directly behind it in order to make the reflector translatable so as to vary the gap of the cavity. The detector therefore must be placed behind the other reflector because the light passing through the movable reflector cannot make it through the movement circuitry to be detected by a detector positioned behind that reflector. With the detector circuitry on one side of the cavity and the movement circuitry behind the other side of the cavity, it is difficult to provide an open pathway for light to initially enter the cavity.
The invention pertains to a new type of standing wave filter in which the detector is located within the cavity, rather than outside the cavity and methods of manufacturing such a filter.
A light beam 311 is directed into the cavity 307 through the first reflector 301 and the detector 305. The light beam 311 bounces back and forth in the cavity 307 between the first and second reflectors 301, 303. The detector 305 is semi-transparent so that light can pass through the detector in both directions to enable light to reflect back and forth between the two reflectors while simultaneously being at least partially detected by the detector 305. Since the detector 305 is mounted on the face of the first reflector 301, the light beam 311 reflected from the second reflector 303 will also impinge on the detector 305. For each round-trip pass through the cavity, the light beam 311 passes through the detector 305 twice. The detector 305 may be positioned anywhere within the cavity. However, as will be described in more detail below, one fabrication process lends itself to locating the detector directly on one of the reflectors, as shown.
As with a Fabry-Perot cell, the inventive cell can be tuned to detect any wavelength content of the light within the cavity by varying the optical cavity depth (e.g., by varying the gap distance between the reflectors or by varying the index of refraction within the gap) between the two reflectors. The spectrum of the light is measured by recording the strength of the detected signal as a function of the cavity depth.
Because the detector 305 is inside the cavity 307, it must be very thin (on the order of less than a wavelength of the light). As is well known, generally, the thinner the detector, the less light impingent on it is absorbed, i.e., detected (at least at thicknesses less than a wavelength of the impingent light). Generally, in a conventional Fabry-Perot cell, in which the detector is outside of the cavity, the detector can be made much thicker than the wavelength of the light being detected so that the detector will absorb substantially all of the impingent light. Contrarily, a detector such as detector 305 placed inside the cavity 307 generally should be significantly thinner than a wavelength of the light in the cavity. Hence, it is likely to be unable to absorb all of the light of each beam segment that impinges on it. However, the absorption efficiency of the detector is not a concern because it is inside the cavity, and therefore, receives light from all of the beam segments impingent on the reflector 301 on which it is mounted. Hence, all light in the cavity eventually will be absorbed by the detector 305, in any event.
More particularly, if we call the sensitivity of the detector 305 to the light 311, α, then the magnitude of the signal generated by the detector is a function of α and the amount of light hitting the detector. Thus, the spectroscope of
As will be described in more detail below, another advantage of the invention is that spectroscopes in accordance with the above-described principles can be readily manufactured using inexpensive and practical semiconductor manufacturing techniques. Moreover, a focal plane array of such spectroscopes can be manufactured using inexpensive and practical semiconductor manufacturing techniques. Even further, a focal plane array of such spectroscopes can be manufactured in which each spectroscope is independently wavelength tunable (e.g., the gaps between the reflectors of the cells can be varied individually for each cell). Accordingly, different cells in the array can be used independently and simultaneously to detect different wavelengths of light from different spots, and/or it is possible to form arrays comprised of multiple super-pixels, wherein each super-pixel comprises two or more cells focused on the same spot (or very close spots), but which are tuned to detect different wavelengths. This technique may be used to provide much faster image spectral data.
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A prototype structure substantially as described herein was fabricated in which the entire assembly reflector/detector 416 was approximately 220 nanometers thick. More particularly, the reflector 412 was approximately 15-20 nanometers thick and the detector was approximately 120 nanometers thick. Accordingly, the cavity 424 in the prototype could be as small as 120 nanometers in depth. The thicknesses disclosed are actual minimum values measured. Thicknesses can be smaller, but were limited in the prototype structure by the resolution of the particular fabrication equipment used and by the selection of readily available, and inexpensive, materials for use.
The above-described fabrication technique lends itself well to the fabrication of a focal plane array for spectroscopic imaging comprising millions of spectroscopic cells in which each cell is independently and simultaneously wavelength tunable. Accordingly, this technology may be used to build, at low cost and high production yield, high spatial resolution imaging devices (e.g., cameras) that have relatively high spectral resolution and individually tunable cells. In one exemplary embodiment of a focal plane array, the second reflector and the mechanics for moving the second reflector may be a MEMS Mirror Array. In one embodiment, we used a Fraunhofer Phase Former Kit available from Fraunhofer IPMS of Dresden, Germany. It is a piston-type MicroMirror Array (MMA) consisting of a segmented array of 240×200 mirror elements with a 40 micron pixel size. Each pixel can be electrostatically addressed and deflected independently by means of underlying integrated CMOS address circuitry at an 8 bit height resolution. MMA programming is performed in an interlaced line-by-line fashion.
Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.
This application is a non-provisional of U.S. provisional patent application No. 61/353,019 filed Jun. 9, 2010, U.S. provisional patent application No. 61/381,595 filed Sep. 10, 2010, U.S. provisional patent application No. 61/390,782 filed Oct. 7, 2010, and U.S. provisional patent application No. 61/493,066 filed Jun. 3, 2011, all of which are incorporated herein fully by reference.
Number | Date | Country | |
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61353019 | Jun 2010 | US | |
61385595 | Sep 2010 | US | |
61390782 | Oct 2010 | US | |
61493066 | Jun 2011 | US |