Abstract:
The present invention relates to thermal detectors and the application of such to devices and methods of detecting the infrared images using thermal detectors. For example, by using optical measuring systems in combination with at least one light source to measure changes position of a movable anchored surface coupled to an absorption surface such that the movable anchored surface changes position due to absorption of infrared radiation by the absorption surface. In another example, by combining a detector pixel (infrared radiation sensitive) with an optical measuring device such as an interferometer.

Description:
[0001]     This Application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/709,791, filed on Aug. 22, 2005, which is entirely incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to the field of imaging technology. Particularly, it involves the field of infrared (IR) imaging device using a light source, diffraction grating and a detector array having at least two detector pixels. More particularly, the invention may be used by an uncooled opto-mechanical IR detector with optical readout.  
       BACKGROUND  
       [0003]     It is well known that Infrared (IR) imaging technology has found many important application areas such as military, automobile heads-up-displays (HUD), medical diagnostics, surveillance for security, victim search for rescue teams, non-destructive testing for industrial applications, and geo-thermal event detection for environmental control. The devices used to generate the thermal map of the targets are passive devices that collect the optical signal radiated from the targets according to Planck&#39;s radiation law assuming that the targets behave as black bodies. According to the well known Planck&#39;s radiation law, the energy emitted per unit volume per unit wavelength from a blackbody (EA) is proportional to the temperature of the blackbody.  
         [0004]     Therefore, it is possible to generate the thermal map of the target by detecting the radiated energy since the only parameters that determine the amount of energy are the temperature and the radiation wavelength. The interested spectrum of the radiation is classified as infrared (IR) band and IR band is further divided into bands as a function of wavelength as follows: 
        Near IR (NIR): 0.78-1 micron     Short wavelength IR (SWIR): 1-3 micron     Mid-wavelength IR (MWIR): 3-5 micron     Long-wavelength IR (LWIR): 8-14 micron     Very long-wavelength IR (VLWIR): 14-100 micron        
 
         [0010]     Each of these specific bands has their own properties, and specific detector technologies have been developed for these bands. For infrared imaging applications, WIR and LWIR bands, where the transmittance of the atmosphere is high, are especially important. Transmittance through air is reduced by several factors as a result of scattering and absorption processes. The bands of operation should be selected specifically to each application. For instance, MWIR band is favorable if the target is relatively hot and the weather is clear, on the other hand LWIR band offers high sensitivity in hazy weather conditions.  
         [0011]     Infrared imaging devices can basically be classified into two main groups according to the detection mechanism. Photon detectors deal directly with the interaction of the incoming photons with the electrons in the detector material. On the other hand, thermal detectors are the devices whose properties can be modulated by its temperature. Therefore, the detection mechanism of thermal detectors is indirect in the sense that the incoming radiation is first converted into heat energy and the generated heat energy is used to change an appropriate material property of the detector.  
         [0012]     Information relevant to this invention can also be found in United States Patent and Parent Publication U.S. Pat. Nos. 6,576,572, 6,643,025, 6,753,969, 20040130728, 20060138347, and 20060181712 issued to Degertekin, et al.; each of the foregoing in United States Patent and Patent Publication Nos. is hereby incorporated herein by reference.  
         [0013]     Each one of the above descriptions or references, however, suffers from disadvantages including; for example, one or more of the following, the invention is not directed at thermal detectors, or if directed at thermal detectors, the thermal detector has poor sensitivity, poor response time and noise. Other disadvantages of known thermal detectors include that they are not easily scalable and that they do not integrate a diffraction grating into each detector pixel to be combined with an optical readout to decrease the noise and improve the sensitivity.  
       SUMMARY  
       [0014]     It is an object of the present invention to provide a thermal detector and related method comprising an absorption surface coupled to a movable anchored surface wherein the movable anchored surface changes position due to absorption by the absorption surface; at least one light source; and an optical measuring system to measure the movable anchored surface changes using the at least one light source. In a preferred embodiment, the movable anchored surface is a movable leg or a flexible membrane, and more preferably is made of at least two materials having sufficient differences in the coefficient of expansion to cause measurable mechanical movement (displacement) in response to thermal changes (absorption). Preferably, the optical measuring system also includes at least one detector and an interferometer comprising a reflector surface and an interference means forming a gap between the reflector surface and the interference means wherein the movable anchored surface changes position due to absorption by the absorption surface and the movable anchored surface is coupled to the interferometer to make the gap variable wherein the at least one detector detects variations in the gap and produces a detector signal related thereto. In another preferred embodiment the at least one light source is selected from the group consisting of light emitting diodes, laser diodes, vertical cavity surface emitting lasers, lasers and combinations thereof The absorption surface is a thermal absorption surface. Also preferably, the interference means is selected from a diffraction grating or a Fabry-Perot surface. It is also preferable that the movable anchored surface is coupled to the group consisting of either an interference means or a reflector surface, and more preferably that the movable anchored surface is at least two materials having different coefficients of thermal expansion to cause measurable mechanical movement in response to thermal changes; for example, a bimaterial comprising silicon nitride and aluminum (or gold) or silicon dioxide and aluminum (or gold) and combinations thereof Also, in a preferred embodiment, the absorption surface comprises silicon nitride, infrared absorbing thin metal films and combinations thereof, and more preferably the reflector surface comprises polysilicon, light reflecting thin metal films and combinations thereof In yet another preferred embodiment, the invention includes at least one more thermal detector to form an infrared imaging system when coupled with an infrared lens (collimating or focusing means). In a further preferred embodiment of the invention, the optical imaging system further comprises a read-out imaging system, and more preferably the read-out imaging system is selected from the group consisting of one dimensional detectors, two dimensional detectors, and CCD cameras. Also preferably, the read-out imaging system further comprises a processing device to control the light source and process the detector signal.  
         [0015]     The novel features that are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its structure and its operation together with the additional object and advantages thereof will best be understood from the following description of the preferred embodiment of the present invention when read in conjunction with the accompanying drawings. Unless specifically noted, it is intended that the words and phrases in the specification and claims be given the ordinary and accustomed meaning to those of ordinary skill in the applicable art or arts. If any other meaning is intended, the specification will specifically state that a special meaning is being applied to a word or phrase. Likewise, the use of the words “function” or “means” in the Description of Preferred Embodiments is not intended to indicate a desire to invoke the special provision of 35 U.S.C. § 112, paragraph 6 to define the invention. To the contrary, if the provisions of 35 U.S.C. § 112, paragraph 6, are sought to be invoked to define the invention(s), the claims will specifically state the phrases “means for” or “step for” and a function, without also reciting in such phrases any structure, material, or act in support of the function. Even when the claims recite a “means for” or “step for” performing a function, if they also recite any structure, material or acts in support of that means of step, then the intention is not to invoke the provisions of 35 U.S.C. §112, paragraph 6. Moreover, even if the provisions of 35 U.S.C. §112, paragraph 6, are invoked to define the inventions, it is intended that the inventions not be limited only to the specific structure, material or acts that are described in the preferred embodiments, but in addition, include any and all structures, materials or acts that perform the claimed function, along with any and all known or later-developed equivalent structures, materials or acts for performing the claimed function. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a perspective view of one preferred embodiment of one of a thermal detector.  
         [0017]      FIG. 2  is a side view of one preferred embodiment of the invention.  
         [0018]      FIG. 3  is a diagrammatic view of a preferred embodiment of a light source for the invention.  
         [0019]      FIG. 4  is a diagrammatic view of a preferred embodiment of the invention.  
         [0020]      FIG. 5  is a diagrammatic view of another preferred embodiment of the invention.  
         [0021]      FIG. 6  is a side diagrammatic view of another preferred embodiment of one of a thermal detector. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0022]     The infrared imaging device of this invention is a thermal detector in which the incoming radiation is first converted into heat energy and then the heat energy is converted into mechanical energy that deflects the detector pixels of the detector array. To achieve mechanical deflection, the working principle of thermostats is employed. Additionally, one embodiment of the current invention measures the deflection using an interferometer having a diffraction grating and a light source. In a preferred embodiment the pixels in the detector array have structures that are connected to a movable anchored surface (preferably suitable bimaterial layers) as shown in  FIG. 1 .  
         [0023]     Referring to  FIG. 1 , there is depicted a preferred embodiment of the one detector pixel  100  having an absorption surface  110 , a reflector surface  120  and a movable anchored surface  130  (specifically a movable leg in this preferred embodiment). The absorption surface  110  can be chosen from light absorptive material such as silicon nitride (SiN) or thin metal films such as aluminum (Al) or gold (Au) and the reflector surface  120  can be chosen from any reflective material such as polysilicon or thin metals films such as aluminum (Al) or gold (Au) or polysilicon. In a preferred embodiment, the thickness of the absorption surface  110  of SiN would be 300 nanometers and for thin metals would range from 10 to 30 nanometers. In a preferred embodiment, the thickness of the reflector surface  120  for thin metals would be at least 70 nanometers. Additionally, the detector pixel  100  may have anchors  140  and thermal isolation legs  150 . In the preferred embodiment shown in  FIG. 1 , the absorption surface  110  is a thermal absorption surface such that it absorbs heat energy (infrared radiation). The movable leg  130  of the preferred embodiment is composed of any two compatible materials that have different thermal expansion rates; for example, SiN and Al or silicon dioxide (SiO 2 ) and Al.  
         [0024]      FIG. 2  shows a preferred embodiment of a thermal detector  160  having a detector pixel  100  coupled to an optical measuring system comprising a detector  400 ; a light source  300  and an interferometer having an interference means  200  (such as a diffraction grating) and a reflector surface (not shown in  FIG. 2 , but  120  in  FIG. 1 ) that form a gap  70  there between; for example, the optical measuring system may comprise in one preferred embodiment a diffraction grating, the reflector surface, a light source and a detector. In this preferred embodiment, the diffraction grating  200  is preferably coupled to a transparent substrate  220  while the reflector surface  120  contained in the pixel detector  100  is coupled to the a movable anchored surface  130  contained in the pixel detector  100  (see  FIGS. 1 and 2 ). More preferably, the movable anchored surface  130  is a movable leg. Preferably, the transparent substrate  220  can be chosen from quartz or other transparent materials. In this preferred embodiment, the detector pixel  100  is separated from the diffraction grating  200  creating a gap  70 . Preferably, the distance of the gap  70  is chosen to be one-quarter of the center of the wavelength range to be measured. The reflector surface  120  is illuminated by a light source  300 . Preferably, the light source  300  can be chosen from light emitting diodes, lasers, laser diodes, lasers and vertical cavity surface emitting diodes. As shown in the preferred embodiment in  FIG. 2 , detectors  400  can be set next to an entrance for the light source  300 , or in an alternative embodiment set next to the light source  300  itself In the preferred embodiment shown in  FIG. 2 , the light reflected by the reflector surface  120  of the detector pixel  100  (see  FIGS. 1 and 2 ) is diffracted to form reflected light orders; for example the zero order reflection  350  (I 0 ) is directly reflected and higher order reflections (1 st )  351 , (3 rd )  353 , etc. are reflected at greater angles and with lessening intensity. Additionally, a voltage (V)  235  may be applied for tuning and calibration purposes. In an alternative embodiment (not shown), the reflector surface  120  would be fixed and the movable anchored surface  130  would be coupled to the interference means  200 ; for example, a silicon substrate (infrared transmissive but not visible light transmissive) can be used as the reflector surface  120  while the interference means  200  could be coupled to the movable anchored surface  130  such that infrared light would pass through a first side of the silicon substrate to heat the absorption surface but visible light from the light source directed towards a second side (opposite the first side) of the silicon substrate and through the interference means  200  would be reflected.  
         [0025]     In a preferred embodiment of the invention shown in  FIG. 3 , the light source  300  may be split into many beams so that it may illuminate multiple thermal detectors having detector pixels  100  such as one thousand detector pixels or more using a micro-optical element  500  such as a fan out diffraction grating and a lens  510 . In an alternative embodiment shown in  FIG. 4 , the light source  300 , may be scanned across an array  180  of at least two thermal detectors  160 ; for example, using a microelectromechanical scanner  600 , a mirror  610 , and a first lens  620  such as a convex lens. In this preferred embodiment, the reflector surface  120  of the detector pixels  100  (see  FIG. 1  also) reflects light through an interference means  200  which in this preferred embodiment is a focal plan array comprising one or more diffraction grating to create reflected light orders, and in this preferred embodiment, the first order reflection  351  is focused by the first lens  620  on a optical component  650  (a second lens) to focus it on the detector  400 . The optical measuring system, in a preferred embodiment, includes a read-out imaging system to image the focal plane image of the at least two detector pixels. In a preferred embodiment, the light source  300  is controlled by a processing device  700  such as a special or general-purpose computer or a microprocessor. The light source  300  may be controlled by a microprocessor  700  such that the light may be modulated by frequency, wavelength and/or intensity; and more preferably, the modulation of the light source  300  may then be multiplexed with the information received from the detector  400  such that an image of the thermal emission of the black body may be constructed ( 800 ). In a more preferred embodiment of the invention, some of the thermal detectors  160  may be designated for reference so that their reflected order signals may be used to cancel noise from the thermal detectors  160  that are active.  
         [0026]     In yet another preferred embodiment shown in  FIG. 5 , a light source  300  is scanned across a detector array  180  through a lens  620  using a light source directing device  605  (for example a lens) and a two dimensional scanner  615  such as a rotating stage. Preferably, the light source  300  can be modulated; for example, using a laser combined with an acousto-optic modulator (AOM) or a laser diode and more preferably the modulated light source  300  may be controlled using either frequency, wavelength, amplitude (intensity) or combinations thereof by a processing device  700  such as a general or specific purpose computer or microprocessor. In this preferred embodiment, the detector array  180  comprising an interference means (in a focal plane array) and the detector pixels are placed in a controlled environment  662 ; for example, a temperature stabilized vacuum is used for thermal detectors. In this preferred embodiment, infrared radiation  50  is directed through an infrared lens  60  and through a silicon substrate  65  (transmissive to infrared light but not visible light) which is used as the reflector surface  120  while the interference means  200  is coupled to the movable anchored surface  130  (preferably, a movable leg) such that infrared light would pass through a first side of the silicon substrate  65  to heat the absorption surface  110  (see  FIG. 1 ) but visible light from the light source  300  is directed towards a second side (opposite the first side) of the silicon substrate and reflected from the silicon substrate  65  through the interference means  200 . The reflected order light (zero order reflected order light  350  and first order reflected order light  351 ) is then reflected through at least one optical component  650 . In this preferred embodiment, the first order reflected order light  351  is directed through a polarizing beam splitter  655 , a quarter wave plate  667 , a spatial filter  674 , and a second lens  676  to a two dimensional detector  900 ; for example, a CCD camera. In this preferred embodiment, the quarter wave plate rotates the light polarization by  90  degrees (by the light passing through it twice) such that it increases the light reflected to the detectors and minimizes any light that would be reflected to the light source  300 . The zero order reflected order light  350  is directed by the polarizing beam splitter  655  to a mirror  675  and through a third lens  688  to focus on a zero order detector  685 ; for example a photo diode or array of photo diodes. The processing device  700  can use the signal received from the two dimensional detector  900  to modulate the light source  300  so that the dynamic range can be expanded. For example, the light source illumination (intensity or amplitude) can be modulated using feedback from the CCD camera corresponding to each detector pixel  100  to increase the optical detection dynamic range. Likewise, the frequency and wavelength could be modulated in real time with feedback from the two dimensional detector  900 . In this preferred embodiment, the zero order reflected order light may be detected for noise cancellation and signal equalization.  
         [0027]      FIG. 6  shows an alternate preferred embodiment of a thermal detector  160  comprising an optical measuring system using a Fabry-Perot (FP) interferometer. As shown in the alternative preferred embodiment in  FIG. 6 , the thermal detector  160  comprises an absorption surface  110  coupled to a reflector surface  120  and a movable anchored surface  130  (preferably a flexible membrane) coupled to an anchor  140  coupled to a Fabry-Perot surface  195  (also known as a distributed Bragg reflector (DBR) surface for Fabry-Perot interferometer (FP) in this preferred embodiment) thereby forming a gap  70 ; and more preferably the Fabry-Perot surface  195  is coupled to a transparent substrate  220 . The incoming infrared radiation  50  is absorbed by an absorption surface  110  having a reflector surface  120  such that the absorbed heat causes movable anchored surface  130  (preferably in this embodiment a bimaterial cantilever or a flexible membrane device) to deflect and change the gap  70  which changes the FP interferometer. If the gap is biased to a sensitive region (such as the mid-point of the infrared wavelength range of interest) the measurement of the reflected light will give information about the deflection in the bimaterial cantilever proportional to infrared radiation  50 . More preferably, the gap is chosen to be one-quarter of the center of the wavelength range to be measured to enhance the absorption. In this read-out configuration, no diffraction grating is needed and the absorption surface  110  and read-out cavity of the Fabry-Perot interferometer can be biased to the optimum points independent of each other.  
         [0028]     In the preferred embodiments of this invention the interferometer may be of any known type; for example reflecting surface, diffraction grating, Fabry-Perot, etc. Further, the interference means of the preferred embodiments of the invention may include one or more elements such as a single diffraction grating, two diffraction gratings, multiple diffraction gratings; a single Fabry-Perot surface or multiple Fabry-Perot surface; more preferably when two or more gratings are employed the gratings are oriented at different angles or have different grating periods to provide enhanced deflection measurement range due to varying amounts of deflection at different parts of the detector pixel. In a preferred embodiment of the invention the light source may comprise one or more narrow spectrum sources, and more preferrably coherent light sources; for example, the light source can use multiple wavelenghts to enhance the dynamic range of detection such as the preferred use of two coherent (laser) radiation have wavelengths of 620 nm and 670 nm.  
         [0029]     The preferred embodiment of the invention is described above in the Drawings and Description of Preferred Embodiments. While these descriptions directly describe the above embodiments, it is understood that those skilled in the art may conceive modifications and/or variations to the specific embodiments shown and described herein. Any such modifications or variations that fall within the purview of this description are intended to be included therein as well. Unless specifically noted, it is the intention of the inventor that the words and phrases in the specification and claims be given the ordinary and accustomed meanings to those of ordinary skill in the applicable art(s). The foregoing description of a preferred embodiment and best mode of the invention known to the applicant at the time of filing the application has been presented and is intended for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in the light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application and to enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.