Abstract:
A compact LADAR transmitting and receiving apparatus includes a pulse laser generating pulses of light; a transmitter collimating and directing the pulses of light toward a target; a receiver collecting reflected pulses of light, the reflected pulses of light having been reflected from the target, the receiver comprising a tapered fiber bundle; a sensor operatively connected to the tapered fiber bundle, where the sensor comprises a photosensitive region and outputs a photocurrent; an amplifier amplifying the photocurrent; and a power divider splitting the amplified photocurrent between a high gain channel and a low gain channel; a RF interface accepting the high gain channel, the low gain channel, and an undelayed sample of a pulse of light generated from the pulse laser as input; a processing unit accepting output from the RF interface; and a display unit displaying output from the processing unit.

Description:
GOVERNMENT INTEREST 
       [0001]    The embodiments herein may be manufactured, used, and/or licensed by or for the United States Government without the payment of royalties thereon. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The embodiments herein generally relate to a LADAR transmitting and receiving apparatus, and, more particularly, to a compact LADAR transmitting and receiving apparatus provided at a reduced cost and with reduced power requirements. 
         [0004]    2. Description of the Related Art 
         [0005]    Laser Detection And Ranging (LADAR) is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. LADAR may be used in a variety of contexts for elastic backscatter light detection and ranging (LIDAR) systems. Although the acronym LADAR is usually associated with the detection of hard targets and the acronym LIDAR is usually associated with the detection of aerosol targets, there has been no real standard on their use and both acronyms may be used interchangeably to describe the same laser ranging system. 
         [0006]    While a LADAR system may perform similar functionality to a radar system, LADAR uses a much shorter wavelength of the electromagnetic spectrum compared to radar. For example, LADAR systems typically operate in the ultraviolet, visible, or near infrared spectrums. This gives a compact LADAR the ability to image a target at a high spatial resolution and allows LADAR systems to be made more physically compact. 
         [0007]    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. 
         [0008]    Lasers provide one solution to this problem regarding non-metallic detection. The beam power 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. 
         [0009]    The transmitter and receiver functions (transceiver) of current LADAR systems typically rely on a mono-static optical system (i.e., the transmitted and received beams are co-axial) that is a complex assembly of beam splitters, polarizers, and steering mirrors. This arrangement is generally expensive, very difficult to align, prone to losing alignment, subject to narcissus, and requiring much more space than desired for a compact LADAR system. 
         [0010]    In addition, current compact LADAR systems have generally been flawed by one or more factors including, low pixelization, insufficient range or range resolution, image artifacts, no daylight operation, large size, high power consumption, and high cost. Current LADAR systems frequently use a wide bandwidth photo detector/amplifier system with a small detector, and a low shunt capacitance, leading to an undesirable signal-to-noise ratio. 
       SUMMARY 
       [0011]    In view of the foregoing, an embodiment herein provides a compact Laser Detection And Ranging (LADAR) system comprising a pulse laser generating pulses of light; a transmitter collimating and directing the pulses of light toward a target; a receiver collecting reflected pulses of light, the reflected pulses of light having been reflected from the target, the receiver comprising a tapered fiber bundle; a sensor operatively connected to the tapered fiber bundle, wherein the sensor comprises a photosensitive region and outputs a photocurrent; an amplifier amplifying the photocurrent; and a power divider splitting the amplified photocurrent between a high gain channel and a low gain channel; a radio frequency (RF) interface accepting the high gain channel, the low gain channel, and an undelayed sample of a pulse of light generated from the pulse laser as input; a processing unit accepting output from the RF interface; and a display unit displaying output from the processing unit. 
         [0012]    In such a LADAR system, the pulse laser may comprise an Erbium fiber laser. In addition, the amplifier may comprise a microwave amplifier. Furthermore, the sensor may comprise an InGaAs PIN photo detector. Moreover, such a LADAR system may further comprise an analog-to-digital converter (ADC) coupled to the RF interface and the processing unit and sampling input from the RF interface to produce sampling data. Additionally, the processing unit may comprise a field programmable gate array (FPGA) processing sampling data from the ADC. Moreover, the processing unit may comprise a first-in-first-out (FIFO) memory storing sampling data from the ADC. 
         [0013]    In such a LADAR system, the transmitter may comprise a microelectromechanical system (MEMS) mirror to direct pulses of light. In addition, the processing unit may control the MEMS mirror. Moreover, such a LADAR system may further comprise a telescope coupled to the MEMS mirror, where the pulses of light generated from the pulse laser are reflected from the MEMS mirror and are amplified in scan angle by the telescope. In addition, such a LADAR system may further comprise a high voltage amplifier producing analog voltages that controls a pointing direction of the MEMS mirror. Additionally, such a LADAR system may further comprise a digital-to-analog converter (DAC) coupled to the high voltage amplifier. Furthermore, the processing unit may be coupled to the DAC, and the processing unit may generate a digital signal to control the pointing direction of the MEMS mirror. Moreover, the processing unit may control the pulse laser in generating the pulses of light. 
         [0014]    Another embodiment herein provides an apparatus for receiving pulses of light from a LADAR transmitter comprising a tapered fiber bundle; a plurality of sensors coupled to the tapered fiber bundle, wherein each sensor comprises a photosensitive region and outputs a photocurrent; a plurality of microwave amplifiers, wherein each amplifier is coupled to a sensor and amplifies the photocurrent; a plurality of feedback circuits, wherein each feedback circuit is coupled to an input and output of a microwave amplifier and raises the bandwidth of the circuit comprised of the sensor and the microwave amplifier; and a microwave combiner combining the photocurrent from each of the plurality of microwave amplifiers to a combined output. 
         [0015]    In such an apparatus, the feedback circuit may comprise a single capacitor, a single resistor, and a single inductor. Moreover, the feedback circuit may comprise an LC circuit. In addition, the microwave amplifier may comprise a wideband monolithic microwave integrated circuit (MMIC) amplifier. Additionally, each of the plurality of sensors may comprise an InGaAs PIN photo detector. 
         [0016]    In addition, an embodiment herein provides a method of processing pulses of light from a LADAR transmitter, the method comprising receiving the pulses of light via a tapered fiber bundle; producing a plurality of photocurrents, wherein each photocurrent is produced from a sensor coupled to the tapered fiber bundle and each sensor comprises a photosensitive region to output the photocurrent; amplifying the plurality of photocurrents using a plurality of microwave amplifiers, wherein each amplifier is coupled to a sensor and amplifies a photocurrent; providing a plurality of feedback circuits, wherein each feedback circuit is coupled to an input and output of a microwave amplifier and raises the bandwidth of the circuit comprised of the sensor and the microwave amplifier; combining the plurality of amplified photocurrents using a microwave combiner, wherein the number of inputs the microwave combiner accepts is as least as great as the number of microwave amplifiers; and processing and displaying the combined output. 
         [0017]    These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The embodiments herein will be better understood from the following detailed description with reference to the drawings, in which: 
           [0019]      FIG. 1  illustrates a schematic diagram of LADAR transceiver according to the embodiments described herein; 
           [0020]      FIG. 2  illustrates a schematic diagram of an L-C circuit used with an LADAR receiver according to the embodiments described herein; 
           [0021]      FIG. 3  illustrates a schematic diagram of a LADAR receiver according to the embodiments described herein; 
           [0022]      FIG. 4  is a flow diagram illustrating a preferred method according to an embodiment herein; and 
           [0023]      FIG. 5  illustrates a schematic diagram of a computer architecture used in accordance with the embodiments herein. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein. 
         [0025]    The embodiments herein provide a LADAR transmitting and receiving apparatus (or a LADAR transceiver), which facilitates the development of a compact, low-cost, and low-power LADAR imager for various applications, including small unmanned ground vehicles, and supports autonomous navigation, obstacle/collision avoidance, and target detection and identification functions in various applications, including small ground robots. Referring now to the drawings, and more particularly to  FIGS. 1 through 5 , where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments. 
         [0026]      FIG. 1  illustrates a schematic diagram of LADAR transceiver  1  according to an embodiment herein. In  FIG. 1 , a trigger signal from field-programmable gate array (FPGA)  10  commands laser  15  (e.g., an Erbium fiber laser) to emit a short pulses of light (e.g., 2-3 ns pulses, at 200 kHz). These pulses of light are collimated and then directed to the surface of mirror  20  (e.g., may be embodied as a small microelectromechanical system (MEMS) mirror). In addition, analog voltages from high voltage amplifier (HV Amp)  25  control the pointing direction of mirror  20 . As the pulses of light are reflected from mirror  20 , they are subsequently fed into telescope  30  to amplify the reflected angle. 
         [0027]    Light backscattered upon hitting a target is collected on the large face of fiber bundle  35 , which may be tapered. Tapering fiber bundle  35  effectively increases the diameter of photo detector  40  and thereby increases the signal-to-noise ratio. Photocurrent from photo detector  40  is fed into amplifier  45 , which may be embodied as a monolithic 50 ohm microwave amplifier. The output of amplifier  45  is fed into power divider  50  splits the signal into low gain channel  50   a  and high gain channel  50   b . In radio frequency (RF) interface board  55 , both low gain channel  50   a  and high gain channel  50   b  are adjusted in amplitude and limited in amplitude to produce output channels  55   a  and  55   b . In addition, inputs to RF interface board  55  are summed with a photocurrent from an undelayed sample of the original transmitted light signal, shown as T-zero  55   c  in  FIG. 1 . T-zero  55   c  pulse of the transmitted signal is optionally used as a reference to determine target range. Output channels  55   a  and  55   b  are subsequently feed into analog-to-digital converter (ADC)  60 , shown in  FIG. 1  as a two channel 8-bit ADC, via input channels  60   a  and  60   b . ADC  60  optionally samples input channels  60   a  and  60   b  at a 1.5 giga-samples-per-second (GSPS) rate. This sampling data is fed to FPGA  10 , which stores the sampling data as a function of time from ADC  60  in memory  65 . Memory  65  is optionally a first-in first-out register (FIFO), and starts storing ADC  60  sampling data upon transmission of the laser  15  pulse. In addition to storing sampling data from ADC  60 , FPGA  10  determines the range to the pixel, and formats the data for acquisition by computer  70  for display. FPGA  10  also controls the pointing direction of mirror  20  (e.g., via digital-to-analog converter (DAC)  12 ) and directs the laser  15  to emit a pulse. 
         [0028]    To increase the receiver capture area for a given size of photo detector  40 , a tapered fiber bundle  35  may be used to magnify the apparent size of a photo detector (e.g., a 1 mm diameter photo detector can theoretically increase its effective diameter to 3.1 mm at the front of a tapered fiber bundle, when the tapered fiber bundle  35  has a magnification ratio equal to 3.1:1). The theoretical maximal effect of magnifying tapered fiber bundle  35  is often not reached if photo detector  40  is a commercially packaged photo detector since the packaging of the commercial photo detectors typically cannot couple the output of tapered fiber bundle  35  directly against the detector surface of photo detector  40  to capture all of the light. In addition, the capacitance of photo detector  40  may limit the output bandwidth of amplifier  45  (e.g., a photo detector with a 1 mm diameter detecting surface may limit bandwidth to about 85 MHz when fed directly into a 50 ohm microwave amplifier). This issue is addressed via L-C circuit  75  (shown in  FIG. 2 ) between photo detector  40  and amplifier  45  input to extend the bandwidth with a tolerable level of pulse distortion and stretching. 
         [0029]    As shown in  FIG. 2 , photo detector  40  may include a PIN InGaAs photo detector. A PIN InGaAs photo detector is preferably used for photo detector instead of an avalanche photo detector. In addition,  FIG. 2  shows L-C circuit  75  with 47 nH inductors 76 and 25 pF capacitors  77 . Those skilled in the art, however, would recognize that L-C circuit  75  is not limited to what is illustrated in  FIG. 2  or described above and could easily modify L-C circuit to other configurations without undue experimentation. 
         [0030]    As shown in  FIG. 1 , photocurrent from photo detector  40  may be amplified by using one or more amplifiers  45 . Each amplifier  45  in  FIG. 1  is shown as a surface mounted monolithic microwave amplifier, which are typically low-cost. Furthermore, each amplifier  45  may have a bandwidth ranging from 1 to 7 GHz and yield power gains to 20 db with noise figures around 2.4 db. Although amplifier  45  is shown in  FIG. 1  as a surface mounted monolithic microwave amplifier, embodiments described herein are not to such a configuration, and those skilled in the art could readily devise a different configuration without undue experimentation. Thus,  FIG. 1  uses a low power and low noise figure model (as provided by amplifiers  45 ) for the first stage of photocurrent amplification. Output from a first amplifier  45  is then split by power divider  50  to create a low gain channel ( 50   a ) and a high gain channel ( 50   b ), which is fed into RF interface board  55 . 
         [0031]    Recent advances in Erbium fiber lasers have allowed Erbium fiber lasers to be manufactured as physically smaller units, and at a lower cost, than what was previously available. The electrical and optical parameters of these new Erbium fiber lasers are identical to the previous fiber laser technology except that the peak power has been reduced, e.g., the peak power of a low-cost Erbium fiber laser may be one-fourth of the peak power of a current fiber laser.  FIG. 3  shows receiver  78  that compensates for a loss in signal power when using a laser with reduced peak power, according to the embodiments described herein. Similar to  FIGS. 1 and 2 ,  FIG. 3  includes fiber bundle  35  to enhance the optical capture area.  FIG. 3 , however, uses a plurality of chip detectors  80  instead of a single photo detector  40 . For example, since each PIN chip detector  80  may be only 1 mm square, four PIN chip detectors  80  may be coupled to the output of fiber bundle  35 .  FIG. 3  illustrates a single PIN chip detector  80 . The output of each PIN chip detector  80  is fed into a separate microwave amplifier  85 . The output of each microwave amplifier  85  is fed into an n-way microwave combiner  90 . Microwave combiner  90  yields target signal  90   a  that has an improved signal-to-noise ratio compared to the individual output of any of microwave amplifier  85 . 
         [0032]    Although  FIG. 3  uses a four-way microwave combiner  90 , to match the number of PIN chip detectors  80  and microwave amplifiers  85  combinations (only one of which is shown in  FIG. 3 ), the embodiments described herein are not limited to a four-way microwave combiner and those skilled in the art could readily increase or decrease the number of inputs to microwave combiner  90  to match increases or decreases in the number of PIN chip detectors  80  and microwave amplifiers  85  combinations used. 
         [0033]    The circuit shown in  FIG. 3  also includes feedback circuit  92  positioned as a feedback loop for the output of microwave amplifier  85  to raise the overall bandwidth. In feedback circuit  92 , capacitor  93  comprises a large capacitor used to decouple DC signal  94  at amplifier  85  output from the input. Inductor  95  (e.g., a 68 nH inductor) cuts-off the feedback path at high frequencies, where the phase shifts in amplifier  85  may cause conditions for oscillations. In addition, resistor  96  (e.g., a 360Ω resistor) is effectively the actual feedback element over the bandwidth of interest. To further achieve stable (e.g., non-oscillating) performance, amplifier  85  may include a wideband monolithic microwave integrated circuit (MMIC) amplifier (e.g., to provide gain up to 7 GHz). 
         [0034]      FIG. 4 , with reference to  FIGS. 1 through 3 , illustrates a flow diagram according to an embodiment herein. In step  100 , the method of  FIG. 4  describes receiving pulses of light via a tapered fiber bundle (e.g., tapered fiber bundle  35 ). Step  101  describes producing a plurality of photocurrents (e.g., as produced from photo detector  40 ). Step  102  describes amplifying the plurality of photocurrents using a plurality of microwave amplifiers (e.g., microwave amplifiers  45 ). Next, at step  103 , the method of  FIG. 4  provides a plurality of feedback circuits (e.g., feedback circuit  92 ). Step  104  describes combining the plurality of amplified photocurrents using a microwave combiner (e.g., microwave combiner  90 ). In step  105 , the method of  FIG. 4  describes processing and displaying the combined output (e.g., via computer  70 ). 
         [0035]    The techniques provided by the embodiments herein may be implemented on an integrated circuit chip (not shown). The chip design is created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer transmits the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
         [0036]    The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
         [0037]    The embodiments herein can comprise hardware and software elements. The embodiments that are implemented in software include but are not limited to, firmware, resident software, microcode, etc. In addition, the hardware elements described herein may be simulated in software. For example, computer models of analog hardware elements described herein (e.g., lasers, microwave amplifiers, resistors, capacitors, and inductors) may be used in conjunction with emulators for discrete hardware elements described herein (e.g., FPGA emulators) to simulate operational parameters for the software elements of the embodiments described herein. Furthermore, the embodiments herein can take the form of a computer program product accessible from a computer-usable or computer-readable medium providing program code for use by or in connection with a computer or any instruction execution system. For the purposes of this description, a computer-usable or computer readable medium can be any apparatus that can comprise, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. 
         [0038]    The medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device) or a propagation medium. Examples of a computer-readable medium include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Current examples of optical disks include compact disk-read only memory (CD-ROM), compact disk-read/write (CD-R/W) and DVD. 
         [0039]    A data processing system suitable for storing and/or executing program code will include at least one processor coupled directly or indirectly to memory elements through a system bus. The memory elements can include local memory employed during actual execution of the program code, bulk storage, and cache memories which provide temporary storage of at least some program code in order to reduce the number of times code must be retrieved from bulk storage during execution. 
         [0040]    Input/output (I/O) devices (including but not limited to keyboards, displays, pointing devices, etc.) can be coupled to the system either directly or through intervening I/O controllers. Network adapters may also be coupled to the system to enable the data processing system to become coupled to other data processing systems or remote printers or storage devices through intervening private or public networks. Modems, cable modem and Ethernet cards are just a few of the currently available types of network adapters. 
         [0041]    A representative hardware environment for practicing the embodiments herein is depicted in  FIG. 5 . This schematic drawing illustrates a hardware configuration of an information handling/computer system in accordance with the embodiments herein. The system comprises at least one processor or central processing unit (CPU)  110 . The CPUs  110  are interconnected via system bus  112  to various devices such as a random access memory (RAM)  114 , read-only memory (ROM)  116 , and an input/output (I/O) adapter  118 . The I/O adapter  118  can connect to peripheral devices, such as disk units  111  and tape drives  113 , or other program storage devices that are readable by the system. The system can read the inventive instructions on the program storage devices and follow these instructions to execute the methodology of the embodiments herein. The system further includes a user interface adapter  119  that connects a keyboard  115 , mouse  117 , speaker  124 , microphone  122 , and/or other user interface devices such as a touch screen device (not shown) to the bus  112  to gather user input. Additionally, a communication adapter  120  connects the bus  112  to a data processing network  125 , and a display adapter  121  connects the bus  112  to a display device  123  which may be embodied as an output device such as a monitor, printer, or transmitter, for example. 
         [0042]    The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.