Abstract:
A detector for electromagnetic radiation such as millimeter wave and infrared employs a ring-shaped ferroelectric element having a temperature affected by an absorber for the radiation. The dielectric constant of the ferroelectric material is a strong function of the temperature near its Curie temperature. The resonant frequency of the ferroelectric element is detected by applying a swept-frequency signal to the circuit and detecting the frequency which enhances the energy of the pulse. A two-dimensional camera for the radiation employs a two-dimensional array of these ferroelectric resonant circuits and a system for rapidly interrogating their resonant frequencies on a sequential basis.

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, and licensed by or for the United States Government. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to elements and systems incorporating these elements for measuring electromagnetic radiation, particularly at frequencies in the millimeter wave range, specifically elements that employ ferroelectric components whose temperatures, and thus their dielectric constants, are affected by absorbers of the radiation in contact with them, the elements forming part of resonant circuits, and to techniques for measuring the resonant frequencies of these circuits using swept-frequency techniques. 
     BACKGROUND OF THE INVENTION 
     It has previously been proposed to provide detectors for electromagnetic energy, and particularly millimeter wave and infrared radiation that exploit the temperature-sensitive properties of ferroelectric devices. For example, U.S. Pat. No. 4,902,895 discloses a system wherein a ferroelectric material is placed between two electrodes or plates to form a capacitor and the charge accumulated in the capacitor during exposure to radiation is read out to measure the radiation. In U.S. Pat. No. 5,530,247 similar elements are incorporated into a two-dimensional array so that the charge build-up in the ferroelectric layers during exposure to radiation may be converted to a readable image and displayed by video screen. U.S. Pat. No. 6,534,767 includes a ferroelectric element connected in a resonant circuit. The resonant frequency of the circuit, which varies with the radiation incident on the ferroelectric element, is tracked by a phase-locked loop. 
     These systems are inherently expensive and relatively slow in operation. Independently, technologies for measuring frequency have advanced so significantly in recent years that systems based on frequency measurements have become highly precise, very fast in operation, and relatively low in cost. 
     SUMMARY OF THE INVENTION 
     The present invention takes advantage of modern frequency measurement techniques. It provides a system for measuring radiation, particularly in the millimeter and infrared wavelengths, that is extremely precise, simple in implementation and relatively low in cost. The system employs ferroelectric elements maintained in a constant temperature chamber at a temperature near their Curie temperature. Preferably their composition is such that this is near ambient temperature in order to reduce the power consumption required to maintain the temperature controlled chamber. A window in the chamber allows incoming radiation to fall on radiation absorbers associated with each of the ferroelectric elements. Radiation absorbers may be chosen that are frequency selective and that result in detectors that are sensitive to selected frequencies of radiation. The temperatures of these absorbers are increased in proportion to the radiation they absorb, thereby changing the temperatures of the ferroelectric elements and thus their dielectric constants. The ferroelectric elements are arranged in resonant circuits such that the resonant frequency of a given circuit shifts sharply as its associated ferroelectric element is heated and its temperature is changed relative to its Curie temperature. 
     In order to read out the change in resonant frequency of a given circuit due to heating of the associated ferroelectric element, which arises from the radiation incident on the element through the window of the temperature-controlled chamber, a constant-amplitude swept-frequency signal or “chirp” is applied to the circuit. The time-domain response of the resonant circuit to the chirp is no longer a constant-amplitude signal: at the instant at which the instantaneous frequency of the chirp sweeps through the resonance frequency, the amplitude develops a spike. If this output is applied to a circuit that records the position of the spike in time relative to the start of the original chirp, the resulting signal is a measurement of the resonant frequency of the circuit. Because this signal is a function of the temperature of the absorber, it records the level of incoming radiation at the absorber. 
     In one embodiment of the invention a plurality of the ferroelectric elements and associated absorbers are arranged in rows as “pixels” in a two-dimensional array. Each pixel consists of a ferroelectric element and its individual resonant circuit. A train of chirps is generated, each of which interrogates a specific row of the array. Each of the pixels in a given row is designed with different dimensions to differ slightly in resonant frequency with no incident radiation, so that each interrogation chirp in the sequence develops a series of spikes, one per pixel in the row. These spikes record the respective resonant frequencies of the pixels or circuits in that row, and thus the levels of incident radiation at the corresponding pixels. The output signal of a single “scan” of the array is a serial train of pulses, like the output from a CCD array. A “template” signal consisting of the output of the full array for uniform illumination at some reference radiation level (also called a “replica” in the radar literature) can be stored in a computer and used for comparison with the real array output; such a comparison will reveal shifts in the spike positions, which record the thermal contrast of the scene relative to the uniform illumination. These shifts may be used to generate a two-dimensional visual display of the incoming radiation pattern. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, advantages and applications of the present invention will be made apparent by the detailed description of a preferred embodiment of the present invention. The description makes reference the accompanying drawings in which: 
         FIG. 1  is a perspective view of the radiation detector of the preferred embodiment of the invention in partially exploded view; 
         FIG. 2  is a perspective view, partially broken away, of the radiation detector of  FIG. 1  located in a constant temperature chamber in accordance with the present invention; 
         FIG. 3  is a schematic diagram of a system incorporating a two-dimensional array of radiation sensors of the type illustrated in  FIG. 1 , designed to sequentially interrogate the radiation sensors with chirp pulses to detect their resonant frequencies and generate a two-dimensional display based on the incident radiation on the display; 
         FIG. 4  is a plot of waveforms occurring in the circuit of  FIG. 3 ; and 
         FIG. 5   a - FIG. 5   d  are plots of typical input and output signals from the circuit elements. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, a preferred embodiment of the radiation sensor of the present invention is constructed on top of a planar ground plane  8 , which may be made of a metal such as copper. A dielectric layer  10  is placed on the ground plane, and a metal microstrip transmission line  18  is placed on the dielectric layer  10 . A cylindrical mesa  12  of dielectric material is built up on the dielectric layer  10 . A ferroelectric ring  14  having a similar outer diameter as the mesa  12  is formed on the top of the mesa. These two dielectric structures together act as a single dielectric resonator, whose resonant frequency will change with temperature due to the change in dielectric constant of primarily the temperature sensitive ferroelectric ring  14 . The ring  14  may be replaced by a disc or any other shape capable of forming a resonant structure or cavity in other embodiments of the invention. A preferred material for the ferroelectric ring  14  is barium strontium titanate having the chemical formula Ba 1−x Sr x TiO 3 . The composition of the ferroelectric, i.e., the concentration of strontium x, is preferably adjusted so that the Curie temperature of the material is close to that of the ambient temperature to reduce the power requirements for the constant temperature chamber. Other appropriate ferroelectric materials may be used in place of Ba 1−x Sr x TiO 3  especially when the ambient temperature is above about 120° C. for BaTiO 3  or below about 90 K for SrTiO 3  or in order to improve other design properties. The ferroelectric ring may have a thickness in the range 0.1-100 microns. The diameter and thickness of the ring and mesa determine the resonant frequency of the resonator pixel in the circuit, whose values are chosen so that the value is identified with a particular resonator position in the detector array. The top of the ferroelectric ring is covered with a radiation absorber  16 , of appropriate material for efficient transfer of the desired electromagnetic radiation to thermal heat. The resonant frequency of the structure may change but in a predictable manner due to the properties of the absorber material, dielectric constant for non-electrically conducting materials and boundary conditions if conducting materials like copper, gold or silver, are used for transforming radiation into thermal heat. 
     The composite resonator consisting of the dielectric mesa  12  and the ferroelectric ring  14  can couple to the microwave transmission line  18  simply by its proximity to the line if the latter passes close by. Alternatively, the line  18  can either stop at the edge of the mesa  12 , then start again at a diametrically opposed point of the mesa  12  and continue on, or pass beneath the structure as a single uninterrupted strip of metal or be coupled using other geometries known in the literature. 
     As illustrated in  FIG. 2  the radiation sensor of  FIG. 1  is located within a controlled temperature chamber  20  which has a window  22  situated directly above the radiation absorber  16 . This window is transparent to the radiation to be measured, so that incident radiation increases the temperature of the absorber  16 . 
     In this configuration the ferroelectric ring  14  acts as a bandpass filter that will strongly transmit any signal whose wavelength satisfies the relationship λ=2πnr where n is an integer and r is the radius of the ring. This corresponds to a frequency 
             f   =       c   λ     =     c     2   ⁢   π   ⁢           ⁢   nr   ⁢     ɛ                 
where ∈ is the dielectric constant of the ferroelectric.
 
     As the energy absorber layer is exposed to radiation in the preferred wavelength range, which may be either the millimeter range or in the infrared range depending upon the absorbing material  16 , it converts the absorbed energy to heat. The resulting rise in temperature will change the effective ∈, that predominately depends on the temperature sensitive ferroelectric material, and hence will shift the frequency of the resonator by an amount that depends on the intensity of the radiation. This frequency shift is thus a direct measure of the intensity of radiation falling on the absorber  16 . 
     In order to determine the resonant frequency of a sensor subjected to radiation, and thus measure the intensity of the radiation, the resonant circuit formed by the ferroelectric element  14  is interrogated by a swept-frequency modulated signal or “chirp” that propagates along the microstrip  18 . This signal is a constant-amplitude sinusoidal pulse with a frequency that varies within the pulse duration in either an increasing or decreasing manner. When such a chirp signal is applied to the circuit consisting of the ferroelectric element  14 , the resonator will enhance the amplitude of the pulse at its resonant frequency as the pulse traverses the resonator. This will produce a spike in the output of the resonant circuit, which occurs at the time the swept frequency passes through the circuit&#39;s resonance. By identifying the frequency of the interrogating signal at the time the spike occurs in the output from the ferroelectric circuit, the resonant frequency of the circuit can be determined. The shift in this resonant frequency relative to the template signal determines the intensity of the illuminating radiation. 
     Since the ferroelectric material is fabricated so as to have a Curie point quite close to the ambient temperature background and kept constant by the chamber  20 , the increase in thermal energy caused by incoming radiation on the absorber  16  will cause a large change in the dielectric constant of the ferroelectric, leading to a relatively large change in the resonant frequency of the resonant circuit for small changes in the intensity of the absorbed radiation. 
     The swept-frequency technique may also be employed to interrogate a matrix of radiation sensors.  FIG. 3  illustrates a circuit in which twenty-five radiation sensors of the type illustrated in  FIG. 2  are arranged in a 5×5 array. Each row of the array,  50   a ,  50   b ,  50   c ,  50   d , and  50   e , henceforth referred to as a “stick”, includes five sensor elements  52  connected in series. Each element  52  in the series is constructed so as to have a different resonant frequency. A system clock  54  produces clocking square waves  56 , shown in  FIG. 4 , at a selected frequency, e.g., 5 KHz. The clock  54  is fed to a sawtooth generator  62  consisting of a simple integrating IC, which in turn provides output to a voltage controlled oscillator (VCO)  64 . 
     The same clock drives a generator of “blanking pulses”  58 , which allows every fifth clock pulse to pass and blocks the next four clock pulses. This blanking signal  60  ( FIG. 4 ) is sent to the first of a series of five AND gates  68   a ,  68   b ,  68   c ,  68   d ,  68   e , and simultaneously is fed to delay lines  70   a ,  70   b ,  70   c  and  70   d . The outputs of these lines are versions of the blanking signal  60 , delayed by one, two, three, and four clock pulses. These signals successively enable the AND gates  68   a - 68   e  to pass the signal from the voltage controlled oscillator  64 , which thus feeds chirps from the oscillator sequentially to their associated sticks  50   a - 50   e . Thus a chirp passes through the sensor line  50   a , then a chirp passes through the sensor line  50   b , next a chirp passes through the line  50   c , then  50   d , and finally  50   e . The outputs of the sensor lines, which will contain amplitude spikes as the chirps pass through the resonant frequencies of the sensors  52  forming part of each line, are sent to a correlator  74  along with a signal stored in a computer  72 , called a “replica”, which preferably represents a uniform scene at the average temperature of the scene under surveillance. Alternatively it may represent the outputs with no incident radiation. The output of this correlator is a series of amplitude pulses that can be fed to a vidicon  76 , where the scene imaging takes place, perhaps with color associated with each frequency for ease of observation. 
     The waveforms produced by the circuit of  FIG. 3  are illustrated in  FIG. 4 . The first line illustrates the clock outputs from the clock  56 . The blanking circuit produces the signal  60  shown on the second line. The clock signal also feeds the sawtooth generator  62 , which produces the sawtooth signal  78 . This signal drives the VCO, which in turn produces the sequence of chirp signals  80  shown in the next line. These chirps are applied to the adders  68   a - 68   e  and gated into the five sensor lines  50   a - 50   e.    
       FIGS. 5   a - d  shows typical output signals to and from the circuit elements. The constant amplitude chirp signal  90  of  FIG. 5   a  is shown in  FIG. 5   b  as distorted by the frequency spectrum  91  of the chain of resonators in the stick (each peak in the trace  91  derives from an individual resonator) and exits the stick looking like the trace  92  of  FIG. 5   c . After correlation, the signal  93  of  FIG. 5   d  shows strong amplitude peaks that can be smoothed by a low-pass filter (not shown) and sent to the vidicon  76 .