Abstract:
An imaging method and apparatus using an unmodulated pulsed laser with a chirp modulated receiver is provided for producing 3D plus intensity imagery of targets in heavily cluttered locations The apparatus includes a laser for emitting a laser beam and synchronizing a receiver to receive a reflected laser signal and transform the reflected laser signal into a displayable image that includes intensity information.

Description:
GOVERNMENT INTEREST 
   The invention described herein may be manufactured, used, and licensed by or for the United States Government. 

   BACKGROUND 
   Laser detection and ranging (ladar) may be used in a military context for elastic backscatter light detection and ranging (lidar) systems. The acronym LADAR is usually associated with the detection of hard targets, while the acronym LIDAR is usually associated with the detection of aerosol targets. However, there has been no real standard on their use and both acronyms may be used interchangeably to describe the same laser ranging system. A ladar system is similar to a radar system with the exception that a much shorter wavelength of the electromagnetic spectrum is used, typically in the ultraviolet, visible, or near infrared spectrums. With a radar system, it is possible to image a feature or object of about the same size of the wavelength or larger. Due to diffraction limits, the laser radiation is easier to collimate than microwave radiation given realistic aperture constraints. This gives a compact ladar the ability to image a target with at a high spatial resolution. 
   In order for a ladar system target to reflect a transmitted electromagnetic wave, an object needs to produce a dielectric discontinuity from its surroundings. At radar frequencies, a metallic object produces a dielectric discontinuity and a significant specular reflection. However, non-metallic objects, such as rain and rocks produce weaker reflections, and some materials may produce no detectable reflection at all, meaning some objects or features are effectively invisible at radar frequencies. In a military context, metallic objects may be disguised by use of a non-metallic covering material. 
   Lasers provide one solution to this problem regarding non-metallic detection. The beam densities and coherency of lasers are excellent. Moreover, the wavelengths are much smaller than can be achieved with radio systems, and range from about 10 μm to around 250 nm. At such wavelengths, the waves are reflected very well from small objects such as molecules and atoms. This type of reflection is called diffuse “backscattering.” Both diffuse and specular reflection may be used for different ladar applications. 
   The Army Research Laboratory has demonstrated the capability of its previously patented chirped amplitude modulation (AM) ladar technique to produce high-resolution, range-resolved intensity imagery (3D+intensity) of targets in heavy clutter, under dense foliage canopy, and obscured by camouflage nets. This technique is covered in U.S. Pat. No. 5,877,851 for “Scannerless Ladar Architecture Employing Focal Plane Detector Arrays and FM-CW Ranging Theory” and in U.S. Pat. No. 5,608,514 for “High-Range Resolution Ladar,” which are hereby incorporated by reference. These previously patented techniques use a modulated continuous wave (CW) laser as an illumination source and a photon detection receiver. For some applications, a high-peak power illumination source is required to satisfy the design requirements, which is not easily achieved with a CW source. For those applications, it is advantageous to use a short-pulse laser as an illumination source due to their high-peak powers and commercial availability. However, since the duration of the laser pulse is extremely short, it is not feasible to chirp modulate the laser intensity as is required with the current ladar architecture Therefore, a modification to the existing ranging technique is required. 
   SUMMARY 
   Embodiments of the present disclosure provide apparatuses and methods for using Chirped Amplitude Modulation Ladar. Briefly described, one embodiment of the apparatus, among others, comprises: a laser source configured to emit an unmodulated pulsed laser beam, a reflection of which produces an optical signal; a receiver configured to receive the optical signal; a modulator configured to modulate a gain of the receiver to produce a modulated optical signal; an integrator configured to determine a state of the modulated optical signal; and a detection circuit adapted to measure the amplitude of the state of the modulated optical signal. 
   One embodiment of such a method, among others, can be broadly summarized by the following steps: triggering a laser to emit an unmodulated laser beam; modulating the gain of a receiver; receiving, at the modulating receiver, a reflected laser beam signal associated with the laser beam emission; and determining at least one characteristic associated with the reflected laser beam signal. 
   Other systems, methods, features, and/or advantages of the present disclosure will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and/or advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
       FIG. 1  is a top view of an environment in which an exemplary embodiment may be utilized. 
       FIG. 2  is a block diagram of an exemplary embodiment of a chirped amplitude modulation ladar using a short-pulse laser transmitter which may be utilized in the environment provided in  FIG. 1 . 
       FIG. 3  is a comparison graph of a sampled continuous chirp waveform and a step-chirp waveform for a small number of sample times as used in an exemplary embodiment according to  FIG. 2 . 
       FIG. 4  is a flow diagram of a method of using a short-pulse laser transmitter for chirped amplitude modulation ladar as used in an exemplary embodiment according to  FIG. 2 . 
       FIG. 5  is a graph of detected target signals for two individual targets at positions of 0.25 and 0.5 respectively. 
     FIG,  6  is a graph of a discrete Fourier transform of the reflected signal of the two targets of  FIG. 5 . 
       FIG. 7  is a graph of the superposition of the oscillations of the two frequencies corresponding to the range of each individual target of  FIG. 5 . 
       FIG. 8  is a graph of the magnitude of the discrete Fourier transform for the 2-target signal of  FIG. 7 . 
   

   DETAILED DESCRIPTION 
   Embodiments of an apparatus for chirped amplitude modulation ladar disclosed herein use a short-pulse laser as an illumination source. The illumination source is used to interrogate the state of the local oscillator waveform applied to any modulatable optical receiver compatible with the laser source and modulation waveform. The illumination source is not amplitude modulated with the modulation waveform, which can simplify its implementation and allow the use of commercial-off-the-shelf (COTS) lasers. The laser pulse width is preferably less than one-half the pulse width of the maximum modulation frequency, and may be as large as several tens of nanoseconds. This pulse width is compatible with commercially available high-peak power pulsed lasers. 
   Several sensors are being developed in the area of long-range detection in the classification of targets, such as the Laser Illuminated Viewing and Ranging (LIVAR®) imaging sensor and the Cost Effective Targeting System (CETS) that operate in the 1.55 micrometer wavelength operation. Both sensors function by triggering the laser to transmit a short pulse of light to illuminate the area of interest. Simultaneously, an image intensifier is turned on, or gated, for a short period of time after a predetermined amount of delay from the laser pulse trigger. The gating forms an image on the intensifier tube due to the scattered light from the laser and in a range swath proportional to the gate time, roughly 36 meters for a 120 ns gate, at a distance from the sensor set by the delay time. These sensors are useful for detecting and classifying targets in the open since they provide high resolution imagery with reasonable signal-to-noise ratio using the intensifier tube as a receiver and the laser as a controllable illumination source. The sensor has difficulty, however, in cluttered terrain since it cannot separate targets from clutter objects when they are within the same range swath. 
   One approach to solving this deficiency is to continue to reduce the gate-width to a few nanoseconds, thus decreasing the probability that a clutter object and the target are in the same range of the swath. Using this approach would require the sensor to acquire multiple gated images in order to build an effective overall image over a reasonable target size, making the gate timing delay difficult to implement. Also, generating such a narrow tube gate is very difficult to implement in a practical system. 
   Another approach is to modify the system so that the actual time of flight of the laser pulse is measured directly on the focal plane. This is the typical detection technique used with single or few detector ranging sensors, such as a range finder or scanned imaging ladar. However, is very difficult to implement in a large array of detectors. There is significant research to extend the state-of-the-art for such time of flight focal plane array sensors. However, there are still significant problems such as pixel cross-talk, clock noise, yield, and manufacturability which need to be addressed. 
   Another previous approach is to directly apply the ARL patented chirp AM ladar technique to the current sensor. To implement this approach, the modulation waveform, such as a continuous chirp signal, would be directly applied to the gain of the image intensifier tube and the intensity of a CW laser source. The two signals would then be multiplied, or mixed, in the detection process to produce an output signal whose frequency is proportional to the range of the target. Application of this technique has a number of advantages such as the ability to form multiple range cells inside of the current range swath and to image multiple targets in a given pixel. Unfortunately, to fully implement this technique requires a modulated high-peak-power laser illumination source, which may be difficult to design. 
     FIG. 1  depicts an environment in which an exemplary embodiment of chirped amplitude modulation ladar system  100  may be utilized. The environment includes target  110  surrounded by various non-target subjects  120 A-F, which may mask target  110  from conventional radar detection systems. Ladar system  100  emits laser pulse  105 . After laser pulse  105  reaches target  110 , reflection  115  is received at ladar system  100  where reflection  115  is processed. Target  110  can be discerned from non-target objects  120 A-F through processing performed in ladar system  100 , 
     FIG. 2  is a simplified block diagram of one exemplary embodiment of chirped amplitude modulation ladar (ladar system)  100  using a short-pulse laser transmitter. In this exemplary embodiment, direct digital synthesizer (DDS)  205  is operated in a stepped-frequency modulation mode for the chirp modulation source in order to modulate the gain of the receiver. However, any other ranging waveforms may be implemented as discussed in the references above, such as, but not limited to, continuous and stepped versions of saw-toothed frequency chirp, up-down frequency chirp, or a pseudo-random code. DDS  205  is also used to initiate gate and modulation circuitry  250 , which gates optical receiver  210  on and off to form the range swath. Any device capable of the gating function could also be used in conjunction with DDS  205 . 
   Modulatable optical receiver  210  shown in  FIG. 2  may be an electron-bombarded image intensifier tube with a voltage controllable optical gain for converting the signal received through receiver optics  235  into an electrical signal. However, any modulatable optical receiver compatible with laser source  215  and modulation waveform produced by gate and modulation circuitry  250 , such as, but not limited to, self-mixing detectors, electro-optic modulator followed by detector focal plane array, an avalanche photo-detector array, or a photon-counting detector array. In this embodiment, the optical receiver is communicatively coupled to an integrating read-out structure  220  to integrate the signal over the gate width. However, any detection circuit capable of measuring the signal amplitude can also accomplish the signal detection. Read-out structure  220  may be comprised of storage  260  and discrete Fourier transform module  270  to produce 4-D range-image file  280  for display. 
   Master controller  218  controls trigger circuit  230  which triggers short-pulse laser  215  in an exemplary embodiment. The laser is emitted through transmitter beam shaping optics  225  towards potential objects for which some measurement (e.g., distance) is desired. Pulse dectector  245  detects the laser pulse emission, and together with the master controller, signal a time-delay to be generated by time-delay generator  240 . The time delay generated by time-delay generator  240  is used by DDS  205  to adjust the output of gate and modulation circuitry  250 . 
     FIG. 3  is an illustrative comparison of a sampled continuous chirp waveform  300  and a step-chirp waveform  310  for a small number of sample times. By sampling the continuous chirp waveform  300  at discrete times  305 , the continuous chirp  300  can be replaced by a step-chirped waveform  310  whose stepped frequencies are equal to the instantaneous frequency of the continuous chirp at sample times  305 . When this is performed, the signal processing  320  of the sample of intermediate frequency signal derived from either chirped waveform is identical. Embodiments of a chirped amplitude modulation ladar may use either chirp waveform; however, for illustrative purposes, it is simpler to explain its operation mathematically using the stepped frequency chirped waveform. 
     FIG. 4  provides a flow diagram  400  for an exemplary embodiment as illustrated in  FIG. 2 . To begin the acquisition of a ladar, first, calculations are made for the time delay, gate time, chirp frequency deviation, number of frequency steps, and frequency step size. The time delay is calculated using the range to the target, r, and the speed of light, c, by 
             t   d     =         2   ⁢   R     c     .           
The gate time is determined similarly using the required range swath, Rs, by
 
             t   g     =         2   ⁢     R   s       c     .           
The chirped frequency deviation, ΔF, is determined by the range resolution, ΔR, using
 
             Δ   ⁢           ⁢   F     =       c     2   ⁢   Δ   ⁢           ⁢   R       .           
The number of frequency steps, N steps , required for an unaliased range measurement is 2Rs/ΔR and the frequency step size, F steps  is ΔF/N steps .
 
   Once these parameters are determined, in block  410 , the DDS may be programmed with a sinusoidal frequency F start  and the laser pulse is initiated in block  420 . The laser pulse exits the laser cavity and, in block  425 , a sample of the pulse is measured by the pulse detector and converted to an electrical signal. In block  430 , this electrical signal, which is a measure of the exact time of the laser emission, is delayed by t d  and used to trigger the DDS and gating circuit. In block  440 , the DDS gates or turns-on the electronic-bombarded image intensifier tube for t d  and concurrently modulates the gain of the image intensifier with the sinusoidal signal at frequency F start . 
   Substantially simultaneously, the laser pulse travels out to the target at range R, is scattered by the target, and a portion of the scattered signal is collected by the receiver optics and imaged onto the photocathode of the image intensifier tube in block  450 . The short-pulse of a laser light is detected by the photocathode and converted to an electrical signal that is amplified in the image intensifier tube by the instantaneous value of its modulated gain Each detecting element in the focal plane array then integrates the electrical signal over the gate time t g , amplifies the signal, and converts the signal to a digital number that is then stored in the memory device. After the digital data is stored, the master controller sets the DDS output frequency to F start +F step  in block  465  and repeats the process. This is continued until, in block  460 , the number of steps, N steps , necessary to complete the chirp waveform is determined to have been reached. In block  470 , the stored data is processed using discrete Fourier transforms to generate the 3-D plus intensity data set (4-D image), and displayed in an appropriate format in block  480 . Since the same modulation waveform is applied to every pixel of the receiver, the ranges and intensities of each scatterer within the gate limited range swath viewed by each pixel are measured, so that a 3-D plus intensity image of the scene can be formed using this technique. 
   As an illustrative case of the embodiment of  FIG. 2 , a target may be imaged at a range of 5 km over a range swath of 18 m with a range resolution of 0.564 m. Using these parameters, t d  is calculated at 33.3 μs, t g  is 120.0 ns, ΔF is 266 MHz, N step  is 64, and F step  is 4.16 MHz. For this example, a starting modulation frequency of 30 MHz is chosen. From the flow diagram of  FIG. 4 , to begin an image collection, the master controller sets the DDS output frequency to 30 MHz and initiates a laser pulse. This laser pulse is detected, converted to an electrical signal, delayed by the time delay generator by 33.3 μs, and used to trigger the DDS. The DDS gates the tube for 120 ns and simultaneously modulates the gain of the electron bombarded image intensifier tube at a 30 MHz frequency. 
   The target signal is detected by the photocathode, converted to an electrical signal, integrated in each signal in the read-out structure, amplified, converted to a digital number, and stored in memory. After the digital data is stored, the master controller sets the DDS output frequency to 34.16 MHz and repeats the process. This is continued until all 64-frequency steps necessary to complete the chirp waveform are finished. The stored data for each pixel over all frequency steps is then processed using discrete Fourier transforms to generate the 3-D plus intensity data set, and displayed on a stereo-graphics enabled computer screen. 
   Mathematically, the signals necessary to explain the ladar operation can be expressed using the following simplified equations:
     Transmitter function: δT; simplified short pulse initiated at time 0   Receiver gain function: ½(1+cos(ω step t))*Gate(t); where Gate=1 or 0   Target function at receiver δ(t−τ), where τ is the round trip delay time for the transmitter to the target and back to the receiver.   

   The target signal detected by the image intensifier tube is found by multiplying the target function and the receiver gain function, which, because of the property of the delta function, reduces to ½(1+cos(ω step τ))*Gate(τ). This signal is then integrated by the read-out structure over the gate-width to convert the optical signal to an electrical signal. 
   When the gating function G(τ) is 1, the signal measured by the receiver is proportional to the instantaneous gain modulation cos(ω step τ) at the round trip delay time.  FIG. 5  is a plot  500  of detected target signals  510 ,  520  for two individual targets at positions of 0.25 and 0.5 respectively. Target signals  510 ,  520  correspond to the gate width measured from the start of the gate for each frequency step using the parameters given in the example above. As with the original chirp AM ladar, each target signal oscillates at a frequency proportional to the target&#39;s range and, because the target signal is multiplied by the overall gating function; this range is limited to be inside of the range swath. As provided in  FIG. 6 , graph  600  provides a discrete Fourier transform of the target&#39;s signal, which maps the target frequency to target range inside of the gate, with the targets, 0.25 gate  610  and 0.5 gate  620 , appearing in the 8 th  and 16 th  range cell. 
   If, instead of a single target illuminated in a pixel, there are two targets simultaneously illuminated in the same pixel, represented by a target function δ(t−τ 1 )+δ(t−τ 2 ), the signal detected by the receiver at each frequency step is ¼(1+cos(ω step τ 1 ))*Gate(τ 1 )+½(1+cos(ω step τ 2 ))*Gate(τ 2 ) When both targets are within the gate time so that both Gate(τ 1 ) and Gate(τ 2 ) are 1, the signal collected during the chirp will oscillate at the superposition of the two frequencies corresponding to each individual target&#39;s range, as plotted in  FIG. 7 . Although waveform  700  appears complicated when plotted as a function of time. After transforming the signal to range space by taking the DFT, discrete Fourier transform, both targets are easily separated and detected in the plot of the magnitude of the discrete Fourier transform for the 2-target signal depicted by waveform  800  shown in  FIG. 8 . 
   Processing embodiments of the present disclosure can be implemented in hardware, software, firmware, or a combination thereof. In the preferred embodiment(s), processing is implemented in software or firmware that is stored in a memory and that is executed by a suitable instruction execution system. If implemented in hardware, as in an alternative embodiment, the processing can be implemented with any or a combination of the following technologies, which are all well known in the art: a discrete logic circuit(s) having logic gates for implementing logic functions upon data signals, an application specific integrated circuit (ASIC) having appropriate combinational logic gates, a programmable gate array(s) (PGA), a field programmable gate array (FPGA), etc. 
   The flow chart of  FIG. 4  shows the architecture, functionality, and operation of a possible implementation of the processing software. In this regard, each block represents a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that in some alternative implementations, the functions noted in the blocks may occur out of the order noted in  FIG. 4 . For example, two blocks shown in succession in  FIG. 4  may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. 
   The processing program, which comprises an ordered listing of executable instructions for implementing logical functions, can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “computer-readable medium” can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a nonexhaustive list) of the computer-readable medium would include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM) (electronic), a read-only memory (ROM) (electronic), an erasable programmable read-only memory (EPROM or Flash memory) (electronic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical). Note that the computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory. In addition, the scope of the present disclosure includes embodying the functionality of the preferred embodiments of the present disclosure in logic embodied in hardware or software-configured mediums. 
   It should be emphasized that the above-described embodiments of the present disclosure, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the disclosure Many variations and modifications may be made to the above-described embodiment(s) of the disclosure without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present disclosure and protected by the following claims.