Abstract:
A stable optical reference oscillator (SORO) consisting of conventional low power CW source of coherent optical radiation having a relatively narrow bandwidth is mixed, i.e., beat against a sample of an incoherent laser transmit signal and the phase of the resultant signal is recorded. This is then compared to the phase of an ideal pulse of a perfect laser transmitter which was previously generated and recorded. The result is a phase correction term which is used in the subsequent signal processing of the received signals to realign the received laser pulses so that they are phase coherent.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority of Provisional application No. 60/638,515, filed Dec. 27, 2004, the entire contents of which are incorporated herein by reference. 
     
    
     GOVERNMENT INTEREST  
       [0002]     This invention was made in the performance of U.S. Air Force Contract No. F33615-02-C-1257 for the U.S. Government, and therefore the U.S. Government has an interest in this application. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     This invention relates generally to processing signals generated by and utilized by electronic system sensors such as radars, and more particularly to, but not limited to, laser radars also referred to as “ladars”.  
       DESCRIPTION OF RELATED ART  
       [0004]     Lasers, by virtue of their short wavelengths, present many benefits to sensor technology. These benefits include but are not limited to compactness, ruggedness, high power, and high pulse rates. Sensors utilizing radiation in the radio frequency (RF) bands have been developed using many techniques to extract maximum information from their surroundings. Most significant and far reaching of these techniques is coherent processing, which requires that each pulse be known completely down to its phase. With this requirement for coherency, it is difficult to achieve directly at RF and virtually impossible within the realm of lasers. The present invention is directed to overcoming this inherent limitation by an indirect method of providing coherency to the received signals.  
         [0005]     The present invention will be described hereinafter in terms of laser synthetic aperture radar (SAR). A typical example of such a system is shown and described in a publication entitled “Synthetic-Aperture-Radar Imaging with a Solid-State Laser”, Thomas J. Green, Jr. et al., which was published in  Applied Optics  on Oct. 20, 1995, in Vol. 34, No. 30, at pp. 6941-6943. As is well known, the SAR technique utilizes the relative transverse motion between the radar and the target by increasing the effective aperture dimension in the direction of motion through appropriate Doppler signal processing. Reference to the publication cited above may be resorted to for a further understanding of this type of technology.  
       SUMMARY  
       [0006]     The direct method of achieving coherency in a radar system employing pulsed lasers would require that the transmit lasers produce coherent pulses of energy. “Coherent” means that each pulse in the laser signal is identical to the other pulses down to its phase characteristic. For a high power pulse laser required for measurement systems such as air-to-ground laser radars, this is extremely difficult, if not impossible, to achieve due to the fact that in such lasers, the phase is completely random pulse-to-pulse, modulo 2π.  
         [0007]     It is an object of the present invention, therefore, to provide a method and apparatus for improving laser signal sensors and more particularly, to overcoming the inherent limitation of the lack of coherency in a relatively high power pulsed laser transmit signal source of a laser radar system. This is achieved by the inclusion of an indirect source of coherence comprised of a stable optical reference oscillator (SORO) consisting of a conventional low power CW source of coherent optical radiation having a relatively narrow bandwidth in frequency for use as a local oscillator for the laser transmit signal pulses. The SORO signal is mixed, i.e., beat against a sample of the incoherent laser transmit signal and the phase of the resultant signal is recorded. This is then compared to the phase of an ideal pulse of a perfect laser transmitter which was previously generated and recorded. The result is a phase correction term which is used in the subsequent signal processing of the received signals to realign the received laser pulses so that they are phase coherent.  
         [0008]     In one aspect of the subject invention, it is directed to a system that uses stretch processing, a technique well known in the art, in a synthetic aperture laser (SAR) radar. As such, it includes a deramp laser where the signal is also mixed with the SORO signal and the phase recorded. This phase is also compared against the phase of an ideal pulse with a second phase correction term being generated. Both phase corrections are then used in the post processing portion of the system to correct the received laser signal data for both linearity in slow time and coherency in fast time following detection and prior to SAR processing.  
         [0009]     Further scope of applicability for the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and the specific example, while indicating the preferred embodiment of the invention, it is provided by way of illustration only since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the following detailed description. 
     
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
       [0010]     The present invention will become more fully understood from the following detailed description and the accompanying drawings, which are provided by way of illustration only and thus are not meant to be limitative of the invention, and wherein:  
         [0011]      FIG. 1  is a simplified block diagram broadly illustrative of the invention implemented in a synthetic aperture laser radar sensor utilizing stretch signal processing and having a transmit signal correction channel and a deramp signal correction channel;  
         [0012]      FIG. 2  is a more detailed block diagram of the invention shown in  FIG. 1 ;  
         [0013]      FIG. 3  is a diagram illustrative of transmit and receive waveforms utilized in the subject invention for linearly modulated transmit frequency known in the art as chirp and stretch signal processing in the receive mode;  
         [0014]      FIG. 4  is a diagram broadly illustrative of the operational sequence of events occurring in the system shown in  FIG. 1 ;  
         [0015]      FIG. 5  is a block diagram illustrative of a portion of the processor shown in  FIG. 3  for correcting coherency of the received signal data in fast time;  
         [0016]      FIG. 6  is a block diagram of a portion of the processor shown in  FIG. 5  for providing deramp compensation or transmit compensation of the received laser pulse signals;  
         [0017]      FIG. 7  is a diagram illustrative of the linearity characteristic of the linear frequency modulated transmit laser pulse signal according to the subject invention;  
         [0018]      FIG. 8  is a block diagram of a portion of the processor and data storage unit shown in  FIG. 2  for improving the linearity of the laser transmit signal in slow time;  
         [0019]      FIG. 9  is a graphical representation of the signal spectrum of the transmit laser before and after a deskew operation performed in the portion of the processor shown in  FIG. 4 ; and  
         [0020]      FIG. 10  is illustrative of the impulse response (IPR) before and after coherence correction provided by the apparatus shown in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     Referring now to the drawings, wherein like reference numerals refer to like components throughout,  FIG. 1  is broadly illustrative of a block diagram of a linear frequency modulated (chirp) laser radar system in accordance with the subject invention that includes a stable optical reference oscillator (SORO)  10  which is used as a narrow bandwidth (BW) local oscillator and is comprised of a low power, CW source of coherent optical radiation which is used as a phase reference for a pair of optical transmit and deramp lasers  12  and  14 . A transmit signal consisting of a train of laser pulses, each having a respective random initial phase, is transmitted and received from a target, not shown, via a transmit/receive (T/R) optics subassembly  16 . A sample of the incoherent pulse signal from the transmit laser  12  is mixed, i.e. heterodyned, in an optical signal mixer  18  with the SORO  10  providing an output of a phase difference signal corresponding to the phase difference between the two signals. This phase difference signal is fed to a narrow bandwidth transmit correction channel  20  including signal detector  22 , a signal sampler  24 , and an analog-to-digital (A/D) converter  26  so as to provide a digitized phase difference signal. The sampled digital phase difference signal is fed to a digital signal processor and data storage unit  28  for subsequent use as will be shown hereinafter in connection with  FIG. 4  where it will be compared against the phase of an ideal transmitted laser pulse.  
         [0022]     The received laser signal transmitted from transmit/receive (T/R) optics  16  is detected, as shown in  FIG. 1 , in a conventional heterodyne optical detector  30 . Since the preferred embodiment of the invention is intended to use stretch signal processing of the return signals, a deramp laser signal is generated by the deramp laser  14  which is fed to the detector  30  where it is heterodyned with the return laser pulses. The output of detector  30  is fed to an A/D converter  32  in a main return channel  34  which feeds into the processor/data storage  28 .  
         [0023]     Additionally, a portion of the deramp laser signal from the deramp laser  14  is fed to a second optical signal mixer  36 , which beats a portion of the SORO signal with the deramp laser signal with a second phase difference signal being generated and fed into a narrow bandwidth deramp correction channel  38 , which also includes a detector  40 , a signal sampler  42 , and an A/D converter  44  similar to the transmit correction channel  20 . The second digitized phase difference signal which is also a digitized phase signal is also fed to the processor/data storage unit  28  for subsequent use as will be explained when  FIG. 4  is considered.  
         [0024]     Referring now to  FIG. 2  which is intended to be further illustrative of the preferred embodiment of the subject invention shown in  FIG. 1 , in addition to the SORO  10 , the transmit laser  12 , the receive deramp laser  14 , and the T/R optics  16 , a ramp signal generator  13  is connected to the transmit laser  12 . The frequency of transmit laser pulses is frequency modulated by a ramp (chirp) waveform  15  as shown in  FIG. 3 . With respect to the deramp laser  14 , it is driven by a separate ramp generator  17  which generates a waveform  19  as also shown in  FIG. 3 .  FIG. 2  also discloses that the detector  22  in the transmit correction channel  20  is comprised of a complex signal mixer having inphase (I) and quadrature (O) inputs. The same can be said of the detector  40  in the deramp correction channel  38 . It is significant to note also that the samplers  24  and  22  in  FIG. 2  provide an analog-to-digital conversion sampling rate which satisfies Nyquist for error frequencies. Accordingly, digital phase data simply are fed to the processor data storage  28  which is shown in  FIG. 2  coupled to a SAR processing unit.  
         [0025]      FIG. 2  also discloses that the transmit laser  12  is coupled to the transmit/receive optics  16  via a pulsed laser amplifier  21 . The heterodyne detector  30  shown in  FIG. 1  is shown in  FIG. 2  including a set of narrow band coherent receive detectors  48  which receive individual pulses included in the laser receive signal. The detected pulse signals are fed to a bandpass intermediate frequency (IF) filter  50  whose output is then digitally converted by means of a narrow band A/D converter shown by reference numeral  32 . As in most, if not all radar systems, all operations are controlled by a master clock and synchronizer  52 .  
         [0026]     Once samples of the digitized transmit correction phase signal, the deramp correction phase signal and the detected return signals are fed into the processor  28 , a sequence of events as shown in  FIG. 4  takes place.  FIG. 4  is intended to be merely illustrative of a functional sequence, while the apparatus used to implement these functions is shown in  FIG. 5 .  
         [0027]     In  FIG. 4 , reference numeral  54  is intended to denote a portion of the processor/data storage unit  28  shown in  FIG. 1 . Reference numerals  56 ,  58  and  60  denote three successive received return laser pulses. Correction for the deramp signal  19  is applied first via a receive chirp correction from deramp correction channel  38 , because all of the errors caused by the deramp signal are consecutively lined up in time immediately after reception. Transmit signal corrections are not yet ready for applications because the dramp process necessarily offsets the return signal slightly in time as shown by reference numerals  62 ,  64  and  66 . This effect is overcome by a deskew operation in which a quadratic phase (Q) is applied to the return pulses  62 ,  64  and  66 , forcing them to become phase coherent as shown by reference numerals  68 ,  70  and  72 . After the deskew operation, the transmit signal corrections are applied, resulting in IF signals  74 ,  76  and  78 . These signals are then fed to a range Fast Fourier Transform (FFT) unit in a conventional SAR processor  46  for generating a display with appropriate azimuth motion compensation, autofocus, scan beam removal and azimuth FFT operations being applied to the image signal data.  
         [0028]     Referring now to  FIG. 5 , the components required for implementing a phase coherency is shown in a phase correction processor portion  79  of the processor/data storage unit  28  shown in  FIG. 2 . It is comprised of a deramp compensation functional element  80 , a first digital memory section  82  in which an “ideal” transmit pulse waveform is generated and stored, a complex (I-Q) multiplier  84 , and a FFT section  82 . These sub-assemblies provide a deramp signal correction signal providing such signals as shown by reference numerals  62 ,  64  and  66  in  FIG. 4  while a transmit compensation functional element  88 , a second memory portion  92 , also used to store the waveform of an ideal transmit pulse, a complex multiplier  90 , and an inverse IFFT  92  produce a transmit signal correction following the deramp corrected signals being fed to a complex multiplier  94  along with an output from the quadratic (O) phase generator  96  providing such signals as shown by reference numerals  68 ,  70  and  72  in  FIG. 4 .  
         [0029]      FIG. 6  is intended to illustrate that the deramp and transmit compensation units  80  and  88  shown in  FIG. 5  include a deskewing functional element  98  and a complex conjugate functional element  100 . These elements feed the I-Q data from A/D converters  26  or  44  ( FIG. 2 ) to a complex multiplier  102  where an “ideal” complex reference function of the ideal pulse waveform is applied either from the memory portion  82  or  90 , whereupon their complex components are fed into the complex multiplier  84  or the complex multiplier  90  as shown in  FIG. 5 .  
         [0030]     While coherence correction of the received laser signal is carried out in “fast time”, a correction of the linearity of the linear frequency modulation of the transmit laser pulse is carried out in “slow” time as will now be explained. As shown in  FIG. 7 , reference numeral  104  depicts an actual frequency variation  104  of the ramped frequency of laser pulses generated by the transmit laser  12 , which is inherently unstable in frequency to some extent and having a pulsewidth T, and a bandwidth B. A desired or ideal linear variation of the ramped frequency is shown by reference numeral  106 . Linearity of the ramped frequency of the transmit pulse is achieved in slow time in a control loop as shown in  FIG. 8  by sampling one half of the digitized phase correction signal from A/D converter  26  over one-half the frequency bandwidth B/2. This portion of the signal is fed to a linearity correction unit  108  of the processor  28 , as shown in  FIG. 8  of the transmit correction channel  20  and which includes a digital tracking filter  110  connected to the A/D converter  26 , a polynomial generator consisting of a set of digital signal accumulators  112   1  . . .  112   n , and a digital/analog converter (DAC)  114 . The accumulators couple their combined output into the DAC  114  which couples a frequency corrective signal to the ramp generator  13  in the transmit correction channel  12  via signal lead  116 .  
         [0031]     Thus, two separate and distinct functions are implemented in the subject invention, namely: (1) coherence is imparted to received laser signals generated by an incoherent transmit source; and (2) a linearity correction function is supplied to the ramp generator  13  which controls the frequency of the transmitted pulses rom the T/R optics  16 .  
         [0032]      FIGS. 9 and 10  respectively illustrate the spectrum of the transmitted signal and the impulse response (IPR) of the received return signal. Reference numeral  118  of  FIG. 9  depicts the spectrum of the transmitted laser signal, while reference numeral  120  represents the quadratic phase of the deskew operation. The deskew operation is shown off-centered at the peak of the quadratic phase term; however, this is solved by providing a predistorted reference for a conventional 2D match filter used in the SAR processor to generate a synthetic aperture image generated with the SAR display shown in  FIG. 4 . Reference numeral  122  of  FIG. 10  depicts the IPR before phase and linearity corrections have been made, while a solid line of waveform  124  depicts the IPR after the corrections are made and having a very high single main lobe and a set of side lobes of significantly reduced side lobes as opposed to the multiple main lobes in the IPR  122 .  
         [0033]     The modulation techniques utilized herein are not meant to be limited to single pulse linear FM stretch and linear FM chirp techniques, but are also applicable for pulse doublets, pulse triplets, pulse n-lets, non-linear FM, bi-phase coding, quadri-phase coding, n-phase coding, Barker coding, Frank coding, complementary coding and Costas coding.  
         [0034]     Having thus shown and described what is at present considered to be the preferred embodiment of the invention, it is to be noted that alterations and changes coming within the spirit and scope of the invention as set forth in the appended claims are herein meant to be included.