Abstract:
A laser radar system is disclosed. The laser radar system includes a transmitting means that transmits a signal. The transmitted signal is formed by an optical signal that is amplitude modulated with a source signal thereby forming a transmitted signal having light packets at a repetition rate of the source signal. A receiving means that receives the transmitted signal after it is reflected from an object. A Doppler processor determines the Doppler frequency shift of the source signal from signals derived from the reflected signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally radar and range measurement systems and in particular to optical radar systems. 
     2. Description of Related Art 
     Conventional laser radar systems have several advantages over microwave radar systems. The primary advantage of this kind of radar is its very narrow beam width. This narrow beam width enables long-range performance with moderate transmit power. Conventional laser radar systems use techniques that are analogous to early rudimentary microwave systems. Conventional laser radar systems use short pulses to achieve fine range resolution, which limits range resolution as well as average power. Another drawback of conventional laser radar systems is the unsatisfactory way in which it deals with Doppler shifts. 
     Coherent optical detection is accomplished by mixing the incoming beam with a duplicate via a beam splitter. The Doppler shift appears in the beat frequency of the output of the beam splitter. However, coherent laser processing has two problems. 
     First, the Doppler frequencies for laser signals are many orders of magnitude higher than the Doppler frequencies for microwave signals. In applications of optical velocimetry to date initiated by the communications industry, all the emphasis has been on sensitivity issues rather than bandwidth issues (see, e.g., Kawai et al., “Ultrahigh-Sensitivity Self-Mixing Laser Doppler Velocimetry with Laser-Diode-Pumped Microchip LiNdP4O12 Lasers”, IEEE Photonics Tech. Let., Col. 11, No. 6, June 1999, which is incorporated herein by reference in its entirety). There has been more interest in the industry for the detection of small numbers of photons than there has been to accurately measure the speed of a detected object. Consequently, there has been little research done of interest to radar researchers and the radar art. 
     A second problem with laser radar is the phase noise of laser devices. Phase noise cancellation has been the subject of some recent research, again in the communications industry. However, it is still a formidable problem. Examples of research in this area include articles by Taylor, R., and Forrest, S., “Optically Coherent Direct Modulated FM Analog Link with Phase Noise Cancelling Circuit”, J. Lightwave Tech., Vol. 17, No. 4, April 1999; Gross, R., Olshansky, R., and Schmidt, M., “Coherent FM-SCM System Using DFB Lasers and a Phase Noise Cancellation Circuit”, IEEE Photonics Letters, Vol. 2, No. 1, January 1990; and Kwong, W., Prucnal, P., and Teich, M., “Coherent Subcarrier Fiber-Optic Communication Systems with Phase-noise Cancellation”, IEEE Transactions on Comm., Vol. 42, No. 6, June 1994. Each of the above-identified references is incorporated herein by reference in its entirety. 
     Fortunately, with the advent of very high-speed modulators, some aspects of modern microwave radar are now applicable to laser radar. A comparison between modern microwave radar, and modern laser radar is provided in the following table: 
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Equivalent Microwave 
                   
               
               
                   
                 (X-Band) Linear FM 
                 IR (800 nm) Laser 
               
               
                   
                 Radar 
                 Radar 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 1.5 m Range Resolution 
                 100 
                 MHz Bandwidth 
                 10 
                 nsec pulse 
               
               
                 0.075 m Range Resolu- 
                 2000 
                 MHz Bandwidth 
                 0.5 
                 nsec pulse 
               
               
                 tion 
               
             
          
           
               
                 Duty cycle 
                 0.20 
                 10 −5   
               
             
          
           
               
                 Peak Power 
                 3 
                 kW 
                 50 
                 MW 
               
               
                 2-way Doppler of 100 m/ 
                 6.7 
                 kHz 
                 250 
                 MHz 
               
               
                 s object 
               
               
                   
               
             
          
         
       
     
     Table 1 
     Table 1 is a comparison between microwave radar and infrared (IR) laser radar. This table shows clearly the difference in duty cycle and Doppler shifts as measured by the two systems. Several of the design implications as a result of these characteristics on laser radar systems are discussed in the following section. Those skilled in the art will appreciate that the peak power of radar is the average power divided by the duty cycle. Thus, for microwave radar, the peak power is greater than the average power by a factor of 5. For the laser the difference is a factor of 10 5 , thereby placing a very high power requirement on the laser. The contrast is even starker in relation to Doppler processing. Because of the poor short-term stability of a laser coupled with the 5+ order of magnitude, the difference between laser Doppler and microwave Doppler effectively precludes lasers from tasks requiring range-rate information. Since the vast majority of radar applications either require Doppler processing or would greatly benefit from it, the implications of this drawback significantly undermines the use of lasers in radar systems. 
     SUMMARY 
     Accordingly, it is an object of the present invention to provide an improvement in laser radar systems. 
     It is yet another object of the invention to provide a laser radar, that has the benefits of microwave radar, along with the benefits of laser radar. 
     The foregoing and other objects are achieved by a method of implementing a laser radar, the method comprising: amplitude modulating a laser beam with a source signal; transmitting a signal comprising light packets at a repetition rate of the source signal, wherein the transmitted signal is generated from amplitude modulating the laser beam; receiving a reflected signal that is a reflection of the transmitted signal; and determining a Doppler frequency shift from signal properties of the reflected signal. 
     Further, the foregoing and other objects are achieved by a laser radar system comprising: a transmitter that transmits a signal, wherein the transmitted signal is formed by an optical signal that is amplitude modulated with a source signal, the transmitted signal comprising light packets at a repetition rate of the source signal; a receiver that receives a reflected signal that is a reflection of the transmitted signal; and a Doppler processor that determines the Doppler frequency shift of the source signal from signals derived from the reflected signal. 
     Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific embodiments, while disclosing the preferred embodiments of the invention, are provided by way of illustration only inasmuch as various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood when the following detailed description is considered in conjunction with the accompanying drawings, which are provided by way of illustration only, and thus are not meant to be limitative of the present invention, and wherein: 
     FIG. 1 is illustrative of waveforms of a laser radar system of the present invention; 
     FIG. 2 is a block diagram of a laser radar system according to an embodiment of the present invention; 
     FIG. 3 is illustrative of output waveforms of a laser radar system of the present invention; and 
     FIG. 4 is illustrative of a pulse train of a laser radar system of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One way microwave radar achieves range resolution is via direct Linear Frequency Modulation (LFM) of the transmitted carrier. This type of modulation is efficiently and easily achieved at microwave frequencies. However, it is difficult for high power laser sources to achieve this type of modulation. It is possible, with some loss in efficiency, to achieve the same result by amplitude modulating the laser beams with a microwave signal that carries the desired LFM modulation. Therefore, the LFM microwave signal is used to amplitude modulate the laser, rather than the carrier being modulated with the LFM. Therefore, the laser transmits light packets whose repetition rate is that of the microwave source. Thus, the undesirable phase and Doppler characteristics of the optical signal disappear entirely and are replaced with those of the RF modulation, which are more easily controlled. This method effectively shifts the level of the modulation from the carrier to the pulse frequency. However, the narrow beam width of the laser is still in effect, as shown in FIG.  1 . 
     FIG. 2 shows a block diagram of an embodiment of the present invention. A transmitter  220  and a receiver  250  are shown. Those skilled in the art will appreciate that there can be many variations of the transmitter  220  and receiver  250 , without departing form the teachings of the present invention. For example, the laser can be amplitude modulated by an X-band microwave carrier, or any other carrier deemed suitable. In block  222 , the X-band microwave carrier is frequency modulated by a LFM waveform. Generating X-band LMF waveforms is well known in the art. An example of generating a LMF waveform is described in U.S. Statutory Invention Registration No. H1,767, which is incorporated herein by reference in its entirety. The LMF waveform is used to drive a light modulator in block  224 . The modulator can be a Mach-Zehnder light modulator, or any other suitable modulator, which will be appreciated by those skilled in the art. The Mach-Zehnder modulator is well known in the art and is described in pages 763-769 of Dorf. R., “The Electrical Engineering Handbook”, Chapter 30, 1993, which is incorporated herein by reference in its entirety. As shown in FIG. 2, the light modulator  224  comprises a Mach-Zehnder light modulator and amplitude modulates the output from a continuous wave (CW) laser source from block  226  with a linear frequency modulated (LFM) X-band sine wave  100  shown in FIG. 1 from the X-band oscillator  222 . Then, the modulated laser tone (FIG. 3) outputted from the light modulator  224  is amplified in block  228  which is shown in FIG. 2 to be a pulsed laser power amplifier. The output of the pulsed laser power amplifier  228  is transmitted in the form of light packets, shown in FIG. 1 by reference numeral  102 , and reflected by an object (not shown). 
     The return signal is amplitude detected, in block  252 , via a low noise optical detector. Block  254  then filters for X-band carrier and amplifies the output of block  252 . Then, the signal is mixed with the original X-band signal, and a quadrature detector separates the mixed signal as is well known in the art, in block  256 . The resulting signals from the quadrature detector are processed in the Doppler processor, in block  258  to determine the Doppler frequency shift. However, as those skilled in the art will appreciate, the Doppler frequency shift is that of the X-band signal, not the optical signal. Therefore, the radar system of the present invention has the Doppler processing properties and stability requirements of an X-band radar system. The Doppler frequency shift can be used in a conventional manner to determine information about an object, such as velocity and the like. Doppler processing is well known in the art and will not be described further herein. Examples of Doppler processing are described in pages 958-970 of Dorf. R., “The Electrical Engineering Handbook”, Chapter 39, 1993, which is incorporated herein by reference in its entirety and U.S. Pat. No. 6,208,288, which is also incorporated herein by reference in its entirety. 
     Phase coherence properties of the optical source, often a major concern for optical systems, are irrelevant because there is no coherent detection of the optical signal. As described above, it is actually the coherence properties of the X-band carrier that are used to implement a radar system according an embodiment of the present invention. 
     In view of the foregoing description and with particular reference to FIGS. 1 and 2, a method of implementing a laser radar according to the present invention will be apparent to those skilled in the art. The method comprises amplitude modulating a laser beam with a source signal. For example, the source signal is at a microwave frequency and is linear frequency modulated. A signal comprising light packets having a repetition rate of the source signal is transmitted, wherein the transmitted signal is generated from amplitude modulating the laser beam. A reflected signal is received that is reflection of the transmitted signal. Then, the Doppler frequency shift is determined from signal properties of the reflected signal. 
     As noted above in regard to the laser radar system, many alternative components can be used to practice the method. For example, the amplitude modulation can be performed by a Mach-Zehnder light modulator. The mixing can be performed in a quadrature detector that supplies the signals to a Doppler processor for the Doppler frequency shift determination. Those skilled in the art will appreciate that the various functions referred to herein refers to any combination of hardware, software and or combinations of hardware and software to implement the associated function. For example the Doppler processor can be a discrete signal processor, application specific integrated circuit, and the like with associated logic/software to perform the necessary functions. Alternatively, the Doppler processor can be a functional block of software that performs the desired function as part of a larger general purpose processing platform that may include other functions of the receiver, such as filtering, mixing, and the like. 
     Further, additional steps can be added to the method described above as will be apparent from the foregoing description. For example, the method can further comprise amplifying the signal to be transmitted prior to transmitting the signal. Upon reception the reflected signal can be amplitude detected. The reflected signal can be filtered for the source signal. Then, the filtered signal can be mixed with the source signal to generate signals for the Doppler frequency shift determination. Many alternative steps can be added or substituted without departing from the teachings of the present invention. 
     To evaluate the efficiency of a laser radar system according to the present invention, a slightly simplified version of a laser radar modulated by a single X-Band tone is analyzed. This results in a pulse train of light with a fifty-percent duty cycle, as shown in FIG.  3 . The total power of the pulse train can be determined as shown in FIG.  4  and as follows. After amplitude modulation, the useful information comes from the first side lobes, given as:          c   1     =       c     -   1       =       A   2          (     2   π     )                                
     The useful side-band power is therefore given as:          P   sb     =           (     c   1     )     2     +       (     c     -   1       )     2       =           A   2     4          [         (     2   π     )     2     +       (     2   π     )     2       ]       =           A   2     4     ·     8     π   2         =       2     π   2       ·     A   2                                    
     Thus, the efficiency is given as:        η   =         P   sb       P   tot       =           A   2     ·     2     π   2             A   2     ·     1   2         =       4     π   2       =     40      %                                  
     The present invention can use standard components such as the WaveStar™ ADM 40G Lucent Technologies, Inc., which is a fast single-laser optical transmission system. The WaveStar™ ADM 40G system is capable of delivering 40 gigabits per second (Gb/s) of capacity with a single laser over a single wavelength on a single fiber-optic cable, making it four times faster than typical commercially available single-laser systems. Conventional direct detectors such as the Electron Tubes, Inc. P30RS232 photomultiplier with six decades of dynamic range can also be used in the present invention. This device can detect from 10 to 10 8  photons per second and can interface with a computer via a RS-232 cable. The system according to the present invention can achieve efficiencies of 40% due to amplitude modulation, as shown in the above equations. 
     The pulse-compression has been described as LFM. However, those skilled in the art will appreciate that the invention is not limited to LFM. Additionally, the transmitting and receiving circuits can be configured in any known manner and comprised of other components then outlined above. For example, alternative laser sources modulators and optical detectors can be selected based on the specific laser radar system requirements, as will be appreciated by those skilled in the art. 
     Accordingly, the foregoing detailed description merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.