Abstract:
A method and system of combining gated intensifiers and advances in solid-state, short-pulse laser technology, compact systems capable of producing high resolution (i.e., approximately less than 20 centimeters) optical images through a scattering medium such as dense clouds, fog, smoke, etc. may be achieved from air or ground based platforms. Laser target designation through a scattering medium is also enabled by utilizing a short pulse illumination laser and a relatively minor change to the detectors on laser guided munitions.

Description:
This application claims priority to Provisional Patent Application Ser. No. 60/174,364, titled “High-Resolution Imaging And Target Designation Thru Clouds Or Smoke” filed Jan. 4, 2000, incorporated herein by reference. 
    
    
     The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the imaging of objects and more particularly to an approach for imaging of objects through a medium. 
     BACKGROUND OF THE INVENTION 
     Imaging through clouds and/or fog by radar is a well established technique. However, conventional radar based reconnaissance systems provide insufficient resolution for many applications. Synthetic aperture radar systems have been recently developed which provide high resolution images at moderate range (approximately 6 km). These systems suffer from various limitations associated with the extensive computations required to produce an image and are not adequate for target designation. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention include a method comprising: generating pulsed electromagnetic radiation having a predetermined wavelength through a scattering medium towards a target; receiving reflections of the pulsed electromagnetic radiation through the scattering medium from the target; and forming a temporally gated image of the target using a ballistic component of the reflected pulsed electromagnetic radiation. 
     Further aspects of the present invention include an apparatus comprising: a means for generating pulsed electromagnetic radiation having predetermined wavelength through a scattering medium towards a target; a means for receiving reflections of the pulsed electromagnetic radiation through said scattering medium from the target; and a means for forming a temporally gated image of the target using a ballistic component of the reflected pulsed electromagnetic radiation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated into and form a part of the disclosure, 
     FIG. 1 is a schematic representation of an embodiment of the system; 
     FIG. 2 is a schematic representation of an optical pod used in the system of FIG. 1; 
     FIG. 3 is a schematic representation of a gated intensifier used in the optical pod of FIG. 2; and 
     FIG. 4 is a graph showing the arrival of photons at the gated intensifier. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The method and system disclosed are concerned with imaging and target designation through obscurants (e.g., clouds, rain, fog, smoke, etc.) in the optical regime. The method and system disclosed herein are applicable to imaging through many different types of obscurants. For exemplary purposes, water droplets (e.g., clouds, fog) will be used in order to provide a quantitative analysis of a typical deployment scenario. FIG. 1 discloses a typical example of operation. An aerial platform such as an aircraft  100  (e.g., unmanned aerial vehicle (UAV)) having an optical pod  102  (shown in detail in FIG.  2  and discussed further below) provides illumination to and receives return signals from a target area  104  having a length d 1  (e.g., approximately 100 meters (m)). The aircraft will typically operate in the range of approximately 5000 to 15,000 feet above the target area to conduct an operation. However, the aircraft may operate as low as 1000 feet or greater than 15,000 feet to conduct an operation. The illumination signal will include a short path signal I 1  and long path signal I 2 . The return signal will be made up of a short path signal R 1  and a long path signal R 2 . The illumination and return signals will travel through medium  106  such as clouds, rain, etc. during the transmission and reception of information from the target area  104 . The illumination signal and return signal are comprised of photons. To describe attenuation of photons incident on the target  104  by the medium  106  such as clouds the extinction coefficient, α ext , is used. 
     The extinction coefficient, α ext , is the sum of the scattering coefficient, α scat  and the absorption coefficient, α abs . The absorption coefficient for water over the wavelength of interest is sufficiently small that it may be neglected and the focus may be placed on the scattering coefficient. The scattering coefficient, α scat , is composed of two principal contributions: Rayleigh scattering from individual molecules and Mie scattering from water droplets, dust, smoke, etc. 
     Rayleigh scattering from atmospheric gases is described by,            α   Ray1          (   λ   )       =         8            π   3          (       n   2     -   1     )       2         3      N                   λ   4                           6   +     3      δ         6   -     7      δ                                  
     where N is the number of molecules per unit volume, n is the refractive index of the medium, λ is the radiation wavelength, and δ is the depolarization factor. Over the wavelength range of interest, the depolarization factor, δ, may be approximately 0.035. From this expression and a model of the density distribution of the atmosphere, it is possible to calculate the loss of optical radiation due to Rayleigh scattering over the propagation distance of interest. 
     At atmospheric pressure and T=15° Celsius (C.), the following results were obtained: 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Wavelength (μm) 
                 α Ray1  (km −1 ) 
                 τ Ray1   
                 Transmission Ray1   
               
               
                   
                   
               
             
             
               
                   
                 0.36 
                 6.680 × 10 −2   
                 0.5653 
                 0.5682 
               
               
                   
                 0.40 
                 4.303 × 10 −2   
                 0.3641 
                 0.6948 
               
               
                   
                 0.45 
                 2.644 × 10 −2   
                 0.2238 
                 0.7995 
               
               
                   
                 0.50 
                 1.716 × 10 −2   
                 0.1452 
                 0.8648 
               
               
                   
                 1.06 
                 8.458 × 10 −2   
                 0.0072 
                 0.9928 
               
               
                   
                 1.3  
                 3.739 × 10 −2   
                 0.0033 
                 0.9967 
               
               
                   
                   
               
             
          
         
       
     
     The second column is the Rayleigh scattering coefficient at sea level (pressure (P)=1 atmosphere (atm)) and the third column is the optical thickness of the atmosphere for the wavelength of interest. Specifically, it is the scattering coefficient integrated over the column density of the atmosphere from sea level to the outer atmosphere,          τ   Ray1     =       ∫   0   ∞              α   Ray1          (     λ   ,   z     )               z                                
     The attenuation of an incident beam by Rayleigh scattering is given simply by, 
     
       
         Transmission=Exp[−τ Rayl ( Z )]=Exp[−∫α Rayl (λ, z ) dz]   
       
     
     This is shown in the fourth column above. For the long wavelength region of approximately 850 to approximately 1060 nanometers (nm) the effect of Rayleigh scattering may be negligible and is neglected hereafter. 
     Mie scattering from atmospheric gases is described as follows. Although the absorption of light by clouds is weak, it is normally impossible to “see” through a thick cloud. This is due to the phenomenon of Mie scattering by water droplets within the clouds. Photons incident on a medium  106  such as clouds as shown in FIG. 1, are scattered by water droplets of radius, a, and refractive index, n=1.33. The extinction cross section is enhanced by a scattering enhancement factor, Q ext , over the geometrical cross section due to diffraction from the droplet. The magnitude of diffraction is determined by the ratio of the particle size to the wavelength of the incident light through the parameter, x=ka=2 Πa/λ. Mie scattering produces light scattered primarily in the forward direction. As light passes through a cloud of such particles, it will undergo many very small deflections. The light may be regarded as comprising discrete photons which travel along straight paths except for an occasional small deflection. A distribution of photons that initially head in a single direction as a collimated beam, will be spread into a range of angles by passage through the cloud. The probability of observing an undeflected photon will diminish exponentially with cloud thickness, not because the light is absorbed but because it is removed from the original direction to appear in a scattered direction. The scattering enhancement, Q ext , is as follows:          Q   ext     =     2   -       4   p        sin                 p     +       4     p   2            (     1   -     cos                 p       )                                
     where p=2Δ(ka)=2 ka(n−1)=4π(n−1)a/λ. 
     Clouds and fog consist of a distribution of small water droplets, typically having a mean radius of several microns, larger than the wavelength of visible or near-infrared laser light. The extinction coefficient, α ext , is dominated by the drops in the range of approximately 5 to 20 μm. In this range, the scattering parameter, p, varies from approximately 20 to 100 for λ=1 μm light. In this regime, the approximation that Q ext =2 is sufficient. 
     The scattering coefficient, α scat , is determined by integrating the cross section over the distribution of scatterers, 
     
       
         
           
             
               α 
               scat 
             
             = 
             
               
                 N 
                  
                 
                   
                     ∫ 
                     0 
                     ∞ 
                   
                    
                   
                     
                       f 
                        
                       
                         ( 
                         a 
                         ) 
                       
                     
                      
                     
                       σ 
                        
                       
                         ( 
                         a 
                         ) 
                       
                     
                      
                     
                        
                       a 
                     
                   
                 
               
               = 
               
                 N 
                  
                 
                   
                     ∫ 
                     0 
                     ∞ 
                   
                    
                   
                     
                       f 
                        
                       
                         ( 
                         a 
                         ) 
                       
                     
                      
                     
                       Q 
                        
                       
                         ( 
                         a 
                         ) 
                       
                     
                      
                     π 
                      
                     
                         
                     
                      
                     
                       a 
                       2 
                     
                      
                     
                        
                       a 
                     
                   
                 
               
             
           
         
                 
         
             
         
      
     
     where f(a) is the normalized distribution function for the scatterers, i.e., f(a)da is the probability of finding a droplet with radius in the range a to a+da. A common distribution function used for clouds and fog is the following:          f        (   a   )       =         [     u   r     ]       u   +   1              a   u       u   !            exp        [       -   ua     /   r     ]                                
     where u is an integral parameter characterizing the full width at half maximum of the distribution, and a=&lt;r&gt; is the most probable radius. An average radius approximation of the scattering coefficient is given by, 
     
       
         α scat   =N&lt;Qσ   geom   &gt;=N&lt;Q&gt;π&lt;a&gt;   2   
       
     
     Clouds are described in terms of their water content, M (gm/m3). Since the mass of a given droplet is simply, 4πp water α 3 . The number of droplets is,        N   =       3      M       4      π                   a   3          p   water                                
     Combining equations and the resulting scattering coefficient is,          α   scat     =       3      MQ       4        〈   a   〉          p   water                                
     For a given water content, the scattering coefficient decreases with increasing droplet size. The average water content of various types of clouds may fall in the range of approximately 0.1 to 0.3 gm/m 3 . A typical cumulus cloud with a water content of 0.1 gm/m′ and an average radius of 6 gm has a droplet density of 1×10 8  droplets/m 3  Assuming M=0.1 gm/m3 and &lt;a&gt;=6 μm, the following is obtained, α scat  is equal to approximately 0.025 rri′. 
     The scattering coefficient may then be used to determine the image of the target  104 . Photon migration through a scattering medium may be categorized into three major signal components: first, ballistic (coherent) photons which arrive first at a receptor after striking a target area by travelling over the shortest most direct path; second, the snake (quasi-coherent) photons which arrive later than the ballistic photons and which deviate, only to a very slight extent, off a straight-line propagation path; and third, the diffusive (incoherent) photons which experience comparatively more scattering than do ballistic or snake photons and, therefore, deviate considerably more from the straight-line propagation path followed by the ballistic and snake photons. Ballistic photons are used in the system and method described herein to obtain an accurate image of the target  104  since they carry the original image information. 
     The ballistic photons may be exponentially attenuated by scattering: 
     
       
           I ( z )= I   0 exp[−α scat   z]   
       
     
     For a medium droplet cloud of typical water content, 0.1 gm/m 3  and a length of 500 meters, the transmission is approximately exp[−0.016*500]=3.35×10 −4 , i.e., approximately only 1 out of every 3000 photons passes in a straight line through the cloud. Attenuation of the ballistic signal as a function of distance through the scattering medium. For different cloud conditions (e.g., water content, particle size), the extinction coefficient and therefore the transmission of the ballistic signal may be different. It is also evident that the ballistic photons travel the shortest distance to the image plane and it is only these ballistic photons that carry any original image information. Furthermore, since the optical path from a point on the object through the imaging system will be the same for all ray paths comprising the image, all ballistic photons will arrive at the image plane at approximately the same time provided they were emitted from the object simultaneously. 
     As discussed above, FIG. 1 discloses a aircraft flying within or above the clouds at an altitude of approximately 5,000 to 15,000 ft. An optical pod  102  mounted on the bottom of the plane may emits bursts, for example, of 1 nanosecond (nsec) radiation at a repetition rate of 2 kilohertz (kHz). The details of the optical pod  102  containing the receiver are shown in FIG.  2 . The optical pod  102  comprises a laser  110 , a passive transmitter section  102   a  and a receiving telescope  102   b.  The transmitting section  102   a  of the optical pod  102  is designed to eliminate the need for the aircraft to maintain a specific altitude. Laser  110  may be a compact, diode-pumped short-pulse laser, preferably operating in the wavelength bands of interest approximately 350 to 380 nm and approximately 850 to 1100 nm. These two wavelength ranges are ideal for active imaging for the reasons discussed below. First, these wavelength ranges are easily transmitted by the atmosphere and are only weakly attenuated by typical obscurants (e.g., water vapor, smoke). Second, the short-wavelength (approximately less than 1000 nm) is well suited to conventional optical imaging systems with the resolution limited only by the numerical aperture of the imaging system and atmospheric distortion. Third, the range is well suited to diode-pumped solid-state lasers and sensors. For examplary purposes, the specifications of a laser system may be the following: 
     Repetition Rate: Variable from 2 to 10 kHz 
     Average Power: 300 W or 200 W 
     Pulse Duration: 2.5 nsec or 1.5 nsec 
     The laser  110  produces a laser pulse which passes through a 5 to 10% beam splitter  112  producing a first “trigger” pulse  114  and a second primary “illuminator” pulse  116 . The trigger pulse  114  which is characterized by low divergence and having approximately 5 to 10% of the pulse energy is directed through the medium  106  towards the ground. The remainder of the laser pulse passes through an optical delay section  118  of approximately 4 to 10 nsec and preferably 6 nsec. The illuminator pulse  116  passes through a variable telescope  120  which allows the pilot of the aircraft  100  to control the divergence of the illuminator beam and hence the illuminated area on the ground  104 . 
     Receiving telescope  102   b  is made up of an acquisition mirror  120  mounted on a gimble  132 . Acquisition mirror  120  captures on-axis light  136  which is a reflection from the illuminator pulse  116  striking the target area  104  and directs it towards a primary mirror  136 . The acquisition mirror  120  directs off-axis light  138  towards an aperture stop  134  (e.g., baffles). The primary mirror  136  directs the light towards a focus  138 . Focus  138  concentrates the light on a gated intensifier  124 . FIG. 3 is an enlarged version of the gated intensifier  124 . The gated intensifier includes a photocathode  140 , a microchannel plate  142 , a phosphor screen  144 , a fiber optic taper  146  and a charge-coupled device (CCD)  148 . Photons  150  create electrons on the photocathode  140 . These electrons are accelerated by a field across the microchannel plate  142  wherein they produce a cascade of secondary electrons by collisions with the walls inside an individual channel. This cascade of secondary electrons produces an amplification factor of approximately 103 per incident electron for a typical microchannel plate  142  biased at approximately 1000 Volts (V). Gating is accomplished by pulsing the voltage applied across the plate  142 . As discussed in detail below, the trigger pulse reflection from the target area  104  will perform the gating function. When the field is applied, photoelectrons emitted from the photocathode  140  experience gain within the plate  142 . When the field is off, there is no amplification and the primary electron never makes it through the channel of the microchannel plate  142 . The amplified electrons strike the phosphor screen  144  at the rear of the intensifier  124 . The fluorescence from this phosphor screen  144  is then imaged through fiber optic taper  146  onto the multipixel charge coupled device (CCD) array  148 . CCD  148  is a multi-frame imager that may operate with gate times as short as 50 picoseconds (psec). Gated intensifiers  124  are commercially available with gate times as short as 400 psec and a dark current (noise) of approximately 1 count/pixel during the gate. Photodiode  122  is also coupled to the gated intensifier  124  and will provide the trigger pulse information. 
     In operation, both the trigger and illuminator pulses  114 ,  116  travel the approximately 10 μsec from the aircraft  100  to the target area  104  and one pulse is simply delayed from the other by approximately 6 nsec. The trigger pulse produces two return signals. The first return trigger pulse signal  128  is the Mie backscatter from the medium  106  on the outgoing path and is detected by photodiode  122  mounted on the bottom of the pod  102  near the gated intensifier  124 . The time-dependent amplitude of this first return trigger pulse signal provides a measure of the density of the medium  106  and the distance from the aircraft  100  to the bottom of the medium. FIG. 4 shows this first return trigger pulse signal as reference numeral  180  on a graph of volts versus time during the reconnaissance time period. The second return trigger pulse signal is the return from the ground and is detected by photodiode  122 . The leading edge of the second return trigger pulse (i.e., the ballistic component) as illustrated by reference numeral  182  in FIG.  4  and activates the high voltage on the microchannel plate  142  of the gated intensifier  124 . The high voltage pulse is pre-programmed to have a gate width of approximately 8 nsec and follow the photodiode trigger pulse by 6 nsec. With this technique, the altitude of the aircraft  100  is removed from the problem. All triggering of the receiver electronics is optical. The gated intensifier  124  is now turned on and receives the ballistic component (reference numeral  184  in FIG. 4) of the reflection of the illuminator pulse from the target area  104 . FIG. 4 also shows receipt of the mie forward scatter from the medium  106  (reference numeral  186 ). The time, t 1 , from the receipt of the first return trigger pulse signal to the arrival of the ballistic component may be approximately 20 μsec. The time of flight distance between the shortest ray path I 1  and longest ray path I 2  may be only approximately 0.5 nsec which is much less than the gate width. For reconnaissance at a slant, this difference increases but can be compensated by both optical and mathematical means. By using a gated detector, it is possible to detect only the ballistic photons while discriminating against all others. The gated detector is turned on only for a very short duration to allow the ballistic photons to be detected The gated intensifier  124  is connected to a processor (not shown) which analyzes the image information received from the ballistic photons to prepare an image of the target area  104 . 
     The energy required to produce an illuminator pulse  116  that will provide the ballistic photons also needs to be calculated. The photons of the illuminator pulse  116  traverse the cloud layer and strike the ground, whereupon they undergo both absorption and scatter from ground terrain. The photons reflected from the ground must again pass through the medium  106  to reach the optical pod  102  on the aircraft  100 . In order to calculate the laser power required of the laser  110  to produce a high contrast image of the target area  104 , the first step is to estimate the signal strength. The signal from a single resolution element on the ground can be written as the product of several factors: 
     Signal (S)=[laser pulse energy incident on ground element] 
     x [albedo of ground element] 
     x [attenuation by medium (e.g., cloud)] 
     x [area of receiver/area scattered into by ground] 
     x [Quantum efficiency of photocathode] 
     The following definitions apply: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Ep 
                 Initial laser pulse energy into ground element 
               
               
                 Eg 
                 Laser pulse energy reflected by ground element = Ep*(albedo of 
               
               
                   
                 ground element) 
               
               
                 Ag 
                 area on ground covered by laser beam without cloud 
               
               
                 Ar 
                 area of receiver 
               
               
                 h 
                 height of source from ground 
               
               
                 η 
                 Quantum efficiency of photocathode 
               
               
                 L 
                 thickness of cloud 
               
               
                   
               
             
          
         
       
     
     It may be assumed that the object of interest is a pure diffuse scatterer, i.e., it scatters radiation uniformly into 2π. Thus, 
     
       
           S (photons/element)=ηE p (photons/element)*(albedo of ground element) 
       
     
      *exp[−α ext   L ]*(π r   2   mirror /2 πh   2 ) 
     Inverting leads to the following: 
     
       
           Ep (photons/element)= S (photons/element)/(η(albedo of ground element) 
       
     
     
       
         *exp[−α ext   L ]*( d   mirror   /h ) 2 /8) 
       
     
     Using a typical albedo of 0.35, a 30 centimeter (cm) primary mirror  136  and an altitude of 10,000 feet leads to the following:                  E   p          (photons/element)       =                8   *   2          (photoelec/element)     /                                (     η   *   0.35   *     exp        [       -   016     *   200     ]       *       (         1   r     /   10     ,   000     )     2       )                 =                1.1   ×     10   11            (photoelec/element)     /   η                                    
     The quantum efficiency, η, of a photochathode  140  used in the receiver section  102   b  of the optical pod  102  is the number of photoelectrons produced per incident photon. The quantum efficiency varies strongly with wavelength due to the work function and electron affinity of the cathode material. Various cathode materials that may be suitable include fused silica input window, S-20, S-25, GaAs or InGaAs coated specialty windows from Coming (Coming #7056), a cluster compound, and Si/Cs/O. The quantum efficiency of various cathode material operating in the visible and near infrared are preferred since operation in the visible region of the spectrum may be problematic for two principal reasons: 1) the increased sensitivity of the human eye in the visible regime reduces the maximum permissible exposure (MPE) level to values sufficiently low that it would be difficult to certify the system as eye safe at the power levels required for ballistic imaging for airborne reconnaissance and 2) the Mie scatter from the cloud would be readily observable by an enemy in military operations, thereby eliminating the ability for covert reconnaissance. For these reasons, as previously discussed, a practical illumination system should operate at wavelengths in the range of approximately 350 to 380 nm or approximately 850 to 1100 nm. 
     Using a quantum efficiency of η=0.5%, the laser pulse energy per element becomes, 
     
       
           Ep= 2.2×10 13  (photons/element) 
       
     
     For the wavelength 1064 nm, this translates to an incident pulse energy of 
     
       
           Ep= 4.2 microJoules/element 
       
     
     This is the number of photons (laser energy) incident on a single ground element. A typical ground resolution would be approximately 8 inches (20 cm) corresponding to an area per ground element of 315 cm 2 . Hence, the incident photon fluence is, 
     
       
         φ p   =Ep/A= 4.2 μJ/315 cm 2 =13 nanoJoules (nJ)/cm 2   
       
     
     This result is significant since it is more than 105 times below the maximum permissible exposure (MPE) for eye safety at 1060 nm (0.01 J/cm 2  for a 1 sec exposure) as established by the American National Standards Institute (ANSI). 
     Since the illuminating laser pulse has also traversed the cloud, the initial laser energy per element is,                E   initial     =       E   p          exp              [       α   ext        L     ]                   =     2.2   ×     10   13          (     photons   /   element     )     *     exp        (     .16   *   200     )                     =     5.4   ×     10   14          (     photons   /   element     )                                    
     Note that there are no area or solid angle factors in this expression since the divergence of the outgoing beam can be arbitrarily adjusted to achieve the desired field of view. 
     A typical reconnaissance imaging system may have 10 6  elements in the focal plane array. With each element corresponding to a ground resolution of 8 inches, the field of view would be approximately 4×10 4  m 2  or 200×200 m. Hence, the total laser energy required to achieve a ballistic image of a 0.2 km×0.2 km field of view is,                E   total     =       E   initial     *     (     Number                 of                 elements     )                   =     5.4   ×     10   20                   photons                 =     101                 Joules                                  
     This energy can be delivered to the field of view in a single pulse or by multiple pulses. For a typical airspeed of 200 mph (90 m/sec), the aircraft  100  would reside over the field of view for approximately 2 sec. As a result, the average power required would be approximately 50 Watts. 
     In an alternative embodiment a receiver is no longer mounted adjacent to the transmitter as shown in FIG. 2 but rather may be mounted on laser guided munitions. Note that there is no difference required in any of the control systems associated with the laser guided munition from that associated with conventional laser designation. The only difference is that the detector must be gated to receive only the ballistic signal and thereby discriminate against any light scattered from the smoke or cloud. This may be accomplished by the same two pulse scheme described earlier. The gated detector simply replaces the conventional detector within the munition. A typical targeting scenario would include the following: 1) Reconnaissance officer identifies target through clouds utilizing ballistic photon reconnaissance, 2) Field of view of outgoing laser transmitting telescope is adjusted to apply laser radiation on the target only, 3) Information regarding pulse format from illuminating laser is transmitted to shooting asset thereby insuring that the munition will only pick up ballistic photon signals which originate from a specific designator, 4) munition is fired and acquires target, and 5) flight to target is controlled by conventional laser guidance software. 
     The foregoing discussion is illustrative only and is not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein.