Abstract:
An excitation signal is selected and transmitted, and a signal is received containing information produced by a remote object in response to the excitation signal. The information is evaluated to attempt to identify the object, the excitation signal is adjusted on the basis of the evaluation, the adjusted excitation signal is transmitted, and a resulting signal is received and evaluated. A variation involves transmitting an excitation signal, transmitting an optical probe signal, receiving an optical signal which is a reflection of the probe signal and which contains information produced by a remote object in response to the excitation signal, and evaluating the information for the purpose of identifying the object.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to technology for detecting hidden features within an opaque environment and, more particularly, to techniques for detecting such hidden features without physical contact with the structure or material of the opaque environment. 
     BACKGROUND OF THE INVENTION 
     There are a variety of situations in which it is desirable to be able to detect a hidden feature, characteristic or object disposed within an opaque environment, without any direct physical contact with the material or structure of the opaque environment. One specific example is the detection of landmines that are buried a short distance between the surface of the earth. Pre-existing systems have been developed for the purpose of attempting to detect features within an opaque environment, and have been generally adequate for their intended purposes. However, these pre-existing systems have not been satisfactory in all respects. 
     In this regard, pre-existing systems typically require a relatively short stand-off distance for interrogation, and have relatively high false alarm rates, including both false positives and false negatives. Further, pre-existing systems tend to have limited inspection rates, such as the rate-of-advance along the ground of a vehicle carrying a system that is being used to detect buried landmines. Moreover, some pre-existing systems are configured only to detect objects, without necessarily identifying them. Other systems that have some identification capabilities do not tend to provide accurate and efficient identification of objects. 
     SUMMARY OF THE INVENTION 
     From the foregoing, it may be appreciated that a need has arisen for a method and apparatus for efficient and accurate detection and identification of features disposed within an opaque environment. One form of the invention involves: selecting an excitation signal; transmitting the excitation signal; receiving a signal which contains information produced by a remote object in response to the excitation signal; evaluating the information for the purpose of identifying the remote object; adjusting the excitation signal based on the evaluation of the information; and repeating the transmitting, receiving and evaluating with the adjusted excitation signal. 
     A different form of the invention involves: transmitting an excitation signal; transmitting an optical probe signal; receiving an optical signal which is a reflection of the probe signal and which contains information produced by a remote object in response to the excitation signal; and evaluating the information for the purpose of identifying the remote object. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawing, which is a diagrammatic view of a detection system that embodies aspects of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The accompanying drawing is a diagrammatic view of an apparatus which is a detection system  10  that embodies aspects of the present invention. The purpose of the system  10  is to locate and identify features disposed within an opaque environment, such as an object  12  which is a landmine buried beneath the surface  14  of the earth  13 . However, detection of landmines is merely one exemplary application for the subject matter of the invention. The present invention can alternatively be utilized in a variety of other contexts. In this regard, the present invention can be utilized in medical applications (for example to detect and identify tumors or gallstones), or in dental applications (for example to detect cavities). Further, the invention can be used to detect objects embedded in walls, to map underground structures, to detect defects and voids in titanium or other metals, or to detect defects embedded in composites. There are a variety of other applications in which the invention can be applied. 
     Referring to the drawing, the system  10  is supported on a not-illustrated vehicle of a known type, so that the system  10  can be moved relative to the earth  13 , in a direction from left to right in the drawing. The not-illustrated vehicle can be a terrestrial vehicle, an airborne vehicle, a space-based vehicle, or some other type of vehicle. The system  10  is designed not only with the ability to determine that an object  12  is present, but also to determine what type of object has been detected. Thus, in the case of the object  12 , the system  10  is designed not only to detect the presence of the object  12 , but also to determine whether it is a landmine, or some other type of object such as a rock, tree root, discarded bottle, or discarded can. In fact, the system  10  is capable not only of determining what type of object is present, but also additional detail about the object. For example, in the case of a landmine, the system  10  can determine not only that an object is a landmine, but what type of landmine it is (anti-tank mine, anti-personnel mine, and so forth). 
     The system  10  includes an exciter laser, which in the disclosed embodiment is a diode laser  31 . The diode laser  31  includes one or more laser diodes of a known type, and is driven by a modulator/driver circuit  33 . The modulator/driver circuit  33  is, in essence, a controlled power supply of a known type which controls the current supplied to the diodes of the laser  31 , and thus the intensity of light produced by the diodes. Although the laser  31  in the drawing is a diode laser in which the intensity of the beam can be modulated, it would alternatively be possible to use a continuously operating laser and to temporally modulate the output to produce a modulated optical beam. 
     The exciter laser  31  periodically emits an excitation signal  36  in the form of one or more beams of laser light. Upon absorption in the ground  13  or in the air near the surface  14  of the ground  13 , the beam or signal  36  generates an acoustic excitation along the surface  14 , as well in the subsurface, through thermo-elastic and/or ablative effects. 
     Each pulse of energy emitted by the laser  31  is modulated, in a manner discussed later, so as to achieve a desired acoustic spectrum within the ground. More specifically, the laser modulation can be realized through direct injection-current modulation of the laser diodes in the laser  31 . A corresponding acoustic spectrum is then generated within the ground  13 , and substantially replicates the modulation format of the signal  36  from the excitation laser  31 , subject to acoustic transmission within the ground. The acoustic modes within the ground propagate and scatter due to inhomogeneities, such as the buried object  12 . A small portion of these scattered acoustic waves arrive back at the surface  14 , resulting in small but detectable surface displacements, on the order of 0.001 wavelength, with surface velocities on the order of 10 to 100 microns per second. 
     A second laser is used to detect these vibrations at the surface  14 , and in particular is the probe laser shown at  51  in the drawing. The probe laser  51  includes one or more lasers, such as laser diodes, which produce a signal  53  containing one or more beams of laser light. In the disclosed embodiment, the beam  53  is somewhat weaker than the beam  36 , because less energy is needed for detection than to effect excitation. The signal  53  is essentially a plane wave, as indicated diagrammatically by several parallel lines  54 . The signal  53  passes through a splitter plate  57  of a known type, and impinges on the surface  14  of the ground  13 . 
     A portion of the signal  53  which impinges on the surface  14  of the ground  13  will be reflected by the surface,  14 . Due to the fact that the surface  14  is experiencing vibrations resulting from the excitation signal  36 , the vibrations will be superimposed on the reflected energy of the signal  53 . Since the surface  14  can be relatively uneven, the reflected energy from the signal  53  will be scattered in a variety of directions. Nevertheless, a small portion of the reflected energy will travel in directions which are approximately opposite to the direction of travel of the signal  53 , as indicated diagrammatically at  61  and  62 . This reflected energy  61 - 62  defines an optical wavefront which is modulated by the ground vibrations, and which is indicated diagrammatically at  66 . This wavefront  66  is reflected by the splitter  57 , and enters a vibration sensor section or module  71  of a known type. 
     The vibration sensor module  71  essentially effects optical measurement of vibrations of a remote object, either in the plane of the object or in a direction normal to the plane of the object, or both. The vibration sensor module  71  includes an optical element  73 , which effectively cleans up the reflected wavefront  66  by optically converting it into a modulated plane wave, which is indicated diagrammatically at  74 . The plane wave  74  then impinges on a vibrometer  76  of a known type. The vibrometer  76  converts the plane wave  74  into electrical signals, which are supplied at  78  to a signal processor circuit  81 . The performance of the vibrometer  76  is improved by the fact that the optical element  73  has converted the wavefront  66  into a modulated plane wave. 
     Although the drawing shows one specific type of vibration sensor module  71 , it would alternatively be possible to use a variety of other types of optical devices. Examples include a module which combines the probe laser and heterodyne detector into a common package, as well as Doppler or micro-Doppler receivers. The devices can either process a single spatial mode (or speckle), or can process highly aberrated beams as well as depolarized beams to improve the system performance and signal-to-noise figure. In the latter case, beam clean-up techniques can be used prior to a single-speckle vibrometer, such as the beam clean-up techniques performed by the optical element  73  in the disclosed embodiment. Another technique is to employ an adaptive photodetector, which performs the combined functions of photodetection and beam cleanup (or wavefront matching) on a single detector device. In still another approach, multiple probe and/or excitation beams can emulate a phased array system for enhanced spatial resolution, sensitivity, or full-frame imaging through parallel processing. 
     The signal processor circuit  81  can be implemented with a known type of device, such as a digital signal processor (DSP) or a microcontroller, which includes a central processor, random access memory and read only memory. The read only memory in the signal processor  81  stores a computer program which is executed by the central processor, and which analyzes the information in the electrical signals received at  78  from the vibration sensor module  71 . This information will be representative of the detected vibrations at the surface  14 , which in turn will be representative of whatever is present below the surface  14 , including not only the object  12  but also the earth  13 . 
     The information stored in the memory of the signal processor  81  includes a look-up table  86 , which stores a variety of predetermined data patterns that correspond to a variety of different objects which might be encountered, such as respective patterns characteristic of a rock, a tree root, a can, a bottle, a landmine and so forth. In fact, these patterns can include a variety of different data patterns representing a variety of different objects of a given type, such as a variety of different landmines. In addition, for each given object, a variety of different data patterns may be present to represent that object under respective different environmental conditions. For example, for a given type of landmine, there may be a variety of data patterns representing acoustic properties of that landmine when respectively buried in dry earth, mud, gravel, sand, and so forth. 
     The signal processor  81  compares the vibration information in the electrical signals received at  78  to the patterns stored in the look-up table  86 , in order to determine whether a buried object appears to be present and, if so, what that object appears to be. Thus, for example, to the extent that the signal processor  81  receives electrical signals at  78  which contain information representing vibrations caused in part by the landmine  12 , the signal processor  81  compares this information to the predefined data in the look-up table  86 , in order to determine (1) that an object is present and (2) that the detected vibrations appear to be similar to those which a landmine would be expected to produce. 
     For purposes of this discussion, assume that the signal processor  81  determines that the object  12  is present, and then makes a preliminary determination that the object  12  appears to be a landmine. The signal processor  81  then produces electrical control signals at  91  which effect control of the excitation laser  31  through the modulator/driver circuit  33 . The signals  91  change the characteristics or format of the excitation signal  36 , in order to adjust the acoustic modes excited in the ground  13  by the signal  36  so as to achieve acoustic modes that couple most effectively to the acoustic modes of the type of object which the signal processor  81  believes is present, and not to other types of objects. The format of the energy can be matched to multi-spectral modes of the object  12  which is being analyzed. 
     As one aspect of this, the resonant frequencies of one type of object, such as a landmine, are significantly different from the resonant frequencies of other objects, such rocks, tree roots, cans and bottles. Thus, the adjusted excitation signal  36  produces acoustic modes within the ground  13  to which the particular object  12  is especially responsive, causing the object  12  to have a more pronounced effect on the overall vibrations produced within the earth  13  and thus at surface  14  in response to the excitation energy. 
     The probe laser  51  and vibration sensor module  71  will in turn detect the modified vibrations which are now occurring at the surface  14  as a result of the modified excitation signal, and will forward information  78  representative of these modified vibrations to the signal processor  81 . The signal processor  81  will then analyze the modified vibrations, in order to evaluate whether, in comparison to the original signals, they do or do not seem to embody any enhancement that is likely to be due to resonance relating to the detected object  12 . If there is such an enhancement, then the object is likely to be a landmine, thereby confirming the prior tentative assessment by the signal processor circuit  81 . On the other hand, if the modified vibrations do not reflect an enhancement of the type that would result from resonance of the object  12 , then the circuit  81  will conclude that the object is probably not a landmine, and will try various other modified excitation signals in order to determine what other type of object it is. If necessary, the signal processor  81  can effect a series of successive adjustments in the signals  91  which control the excitation laser  31 , until it makes a determination of what type of object is present, for a given set of environmental conditions. 
     Thus, through a somewhat iterative process, the signal processor  81  effects feedback control of the excitation signal  36 , in order to progressively narrow in on an accurate identification of what type of object is present at  12 . The signal processor  81  can output information at  96 , for example to a display or an external computer, to indicate not only that an object  12  has been detected, but also to indicate whether the detected object is a landmine, a rock, a tree root, a bottle, a can or some other type of object. 
     The signal processor  81  can be operated in a first mode in which the output information  96  contains some form of indication for every object  12  which is encountered. Alternatively, the signal processor  81  can be operated in a second mode in which it is looking for a particular type of object, such as a landmine, and in which it therefore limits the output information  96  to indications of detected objects that it has concluded are landmines. 
     The excitation source  31  and modulator  33  could be replaced with a plurality of devices that each produce a modulating beam directed to a respective different location on the surface  14 . The probe laser  51  and vibration sensor module  71  could be configured to transmit and receive multiple beams, with each beam interrogating surface vibrations at a respective different location on the surface  14 . Respective electrical signals representing each of the multiple output signals from the vibration sensor module would then be provided to a common processor, similar to the processor  81 . The processor would then produce appropriate output information, corresponding to that shown at  96  in the drawing, and would also produce a plurality of control signals corresponding to the control signal  91 , in order to control each of the multiple excitation beams. This extended system would provide increased sensitivity and spatial resolution, as well as increased inspection and imaging speed, in comparison to a system of the type shown in the drawing, which has a single excitation beam and a single probe beam. The multiple beam system would also be more robust when confronted with buried objects that are under and obscured by other objects, such as rocks, tree roots, wet regions, and so forth. 
     As mentioned above, the lasers  31  and  51  in the illustrated embodiment use laser diodes. However, it would alternatively be possible for each of these lasers to be implemented with some other type of laser device. For example, the excitation laser  31  could be some other type of laser which can be modulated, either through direct modulation of the laser (for example through modulation of pump diodes in a diode-pumped fiber laser), or through use of an external modulator in association with a continuous-wave or quasi-continuous-wave laser. 
     As discussed above, the signal processor  81  in the disclosed embodiment effects feedback control using a relatively straightforward process which involves use of a look-up table  86 . However, it would alternatively be possible to use any of a variety of more sophisticated techniques, separately or in combination, including fast Fourier transforms, sensor fusion algorithms, neural network classifiers, hyperspectral acoustic image processing, feature extraction techniques, and so forth. Use of neural network techniques would permit the signal processor  81  of the system  10  to be “trained” in realtime through on-the-spot evaluation of the detailed, complex transform frequency components of the detected information. 
     The present invention provides a number of advantages. One such advantage results from adjustment of the excitation signal with a closed loop system, thereby permitting the system to optimally excite acoustic modes that can most effectively couple to the acoustic modes of the type of object which is of interest. The system thus discriminates against objects which are not of interest, while minimizing background clutter and noise, thereby minimizing false positives from extraneous objects. The invention thus enhances the sensitivity and ability of the system to detect and classify the presence of a selected feature or object. 
     Moreover, through feedback control of the excitation signal, the energy of the excitation signal is optimally utilized, thereby providing enhanced performance, and a higher rate-of-advance for a vehicle on which the system is mounted. Adjustment of the excitation signal with feedback control also permits more effective classification of a buried object in a variety of ways, including not only identification of the object, but also identification of shape, density, two-dimensional or three-dimensional mapping, and/or discrimination between solid and hollow objects. The system can identify and discriminate objects more rapidly than pre-existing systems that use open-loop architectures. The system also reduces the occurrence of false alarms, including both false positive and false negatives. The system is capable of effecting non-contact interrogation with a relatively long stand-off distance, and can be utilized in a variety of different applications. 
     Although one embodiment has been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.