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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates generally to laser radar systems, and more
particularly to continuous wave (CW) laser radar systems.
As is known, CW laser radar systems have a wide variety of applications,
one of such applications being to provide range tracking of a target as
well as imaging and classification of such target, that is, providing
details of the shape of the target to permit such target to be identified.
Such application requires the CW laser radar to provide precise target
range information. A conventional C laser radar utilized for such
application provides amplitude modulation (AM) of the transmitted CW laser
beam by passing the CW beam through a modulator, such as an electro-optic
crystal, biased with a time-varying signal at the AM frequency. Target
range is measured by determining the phase shift of the AM envelope of the
target reflected return signal relative to a reference AM envelope signal.
Since phase shift measurement accuracy of a few degrees may be
conventionally obtained, and since very high modulation frequencies (i.e.,
on the order of 1 MHz) may be achieved using conventional AM electro-optic
crystals, amplitude modulated CW laser radar systems are capable of
measuring target range quite accurately.
However, amplitude modulated CW laser radar systems with high AM
frequencies have poor target resolution. That is, it is difficult to
resolve a selected target from other, unselected targets or from
atmospheric clutter such as near-field clutter through the use of
amplitude modulation. Such poor resolution is due to the fact that
scatterers of energy at different ranges (for example, selected and
unselected targets, atmospheric clutter, etc.) will return signals to the
radar system which have AM envelopes at different relative phases with
respect to the reference AM envelope. Thus, the measured phase shift
between the return signals and the reference signal is an average phase
shift contributed to by several sources other than the selected target,
thereby substantially preventing the selected target from being
distinguished from other targets or from atmospheric clutter.
Additionally, since the AM frequency is typically selected to be
relatively high in order to obtain accurate target range measurements, the
AM waveform has a relatively short ambiguous range interval. The AM
frequency could be lowered to increase the ambiguous range interval, but
the AM waveform would have poorer accuracy and would still have poor
target resolution.
Another CW laser radar system utilizes frequency modulation (FM) rather
than AM to provide target range information. The CW signal is periodically
"chirped" in frequency at a predetermined repetition rate and by a
predetermined amount to thereby impose FM on the CW beam. The repetition
frequency of the FM modulation (i.e., the FM modulation frequency) is
typically much lower than the modulation frequency in conventional AM
systems, leading to long ambiguous range intervals. Further, the FM
waveform provides relatively high target resolution according to the FM
modulation frequency. Thus, the FM radar system is capable of
distinguishing a selected target from other targets and from near-field
atmospheric clutter. However, due to inherent FM bandwidth and chirp
limitations, inaccuracies are experienced in imposing FM on a CW laser
beam. Thus, the accuracy to which target range may be measured with an
FM-CW laser beam may not be sufficient to provide accurate information on
the details of the target, thereby decreasing the probability of measuring
target characteristics and/or degrading the capability of the laser radar
system to identify and classify the target.
SUMMARY OF THE INVENTION
In accordance with the present invention, apparatus is provided comprising:
means for generating a continuous wave (CW) beam of electromagnetic
energy; means for modulating the frequency of the continuous wave beam of
energy; and means for modulating the amplitude of the frequency modulated
continuous wave beam of energy to produce an amplitude and frequency
modulated continuous wave beam of electromagnetic energy. With such
arrangement, a continuous wave beam of energy is produced having frequency
and amplitude modulation imposed thereon, which, when transmitted in a
radar system, provides such radar system with accurate target range
measurements unambiguous over large ranges. Further, such transmitted
AM-FM CW signal provides high resolution between target-reflected return
signals of such AM-FM CW signal and returns from near-field atmospheric
clutter.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing features of the present invention, and the advantages
thereof, may be fully understood from the following detailed description
read in conjunction with the accompanying drawings wherein:
FIG. 1 is a schematic and block diagram of a preferred embodiment of a
laser radar system according to the present invention;
FIG. 2 is a schematic and block diagram of the receiver portion of the
laser radar system of FIG. 1;
FIG. 3A-3G illustrate frequency- vs. -time waveforms of the signals at
various points in the laser radar system of FIG. 1 and are helpful in
understanding the invention; and
FIG. 4 is a schematic and block diagram of a preferred embodiment of a
component of the receiver portion of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a preferred embodiment of the
amplitude-modulated-frequency-modulated (AM-FM) CW laser radar system 10
of the present invention is shown. AM-FM laser radar system 10 comprises
CW laser 20, here a CO.sub.2 laser producing a continuous wave beam of
electromagnetic energy having a nominal wavelength (approximately 10.6
.mu.m) and frequency. The output of CW laser 20 is coupled through
interferometer section 30, beam expander section 40, a conventional beam
scanner section 42 and afocal telescope 44 and transmitted to a target,
not shown. Portions of the transmitted beam reflected by such target are
received by afocal telescope 44, coupled through scanner 42 and beam
expander 40 (which in this direction functions to compress the diameter of
the beam), and directed to interferometer 30, where such received portions
are combined in a manner to be described with a local oscillator (L.O.)
beam, here derived from CW laser 20. The combined beams are applied to
detector 50, which produces a beat frequency signal representative of the
target range and Doppler velocity. Such beat frequency signal is fed to
receiver section 60, where here the range of the target is calculated and
applied to utilization device 70, which here comprises a CRT, for imaging
the target thereon with sufficient accuracy and resolution to enable an
operator to identify the shape of the target with sufficient certainty to
"classify" such target. The scanning of the CRT and of scanner 42 are
synchronized in a conventional manner by a control signal generated by
system controller 100. Alternately, utilization device 70 may comprise a
computer which is programmed to automatically classify a target according
to the signal coupled thereto from received section 60. To obtain the
precise target range information necessary for such target identification
and classification, radar system 10 further comprises a frequency
modulator (FM) unit 80 and an amplitude modulator (AM) unit 90 to
simultaneously produce frequency modulation (FM) and amplitude modulation
(AM) on the CW beam produced by laser 20. Receiver section 60 responds in
a manner to be described to the frequency modulation on the received
portions of the target-reflected AM-FM beam to produce a first signal
representative of the approximate range of the target, such first signal
having high resolution and relatively long ambiguous range intervals,
thereby distinguishing the target from other targets and from atmospheric
clutter. The receiver 60 also responds to the amplitude modulation on such
received AM-FM beam to produce a second signal representative of the
accurate range of the target, such second signal having poor resolution
and short ambiguous range intervals. That is, the second signal represents
variations in the approximate range of the target. The first and second
signals are applied to utilization device 70, which utilizes the second
accurate but poorly resolved and ambiguous range signal as a "vernier" on
the first inaccurate but highly resolved and unambiguous range signal to
provide a target range measurement which is accurate, has high resolution
and is unambiguous over long range intervals. System controller 100
synchronizes such received electronic signal with the scanning of scanner
42 in a conventional manner, thereby allowing the highly resolved and
unambiguous range signal to be displayed on a CRT-type screen to allow an
operator to identify the shape of such target precisely enough to
determine what the target is (i.e., to "classify" the target), with such
target also being resolved from other targets and from atmospheric
clutter.
Frequency modulator unit 80 here comprises FM driver 82 and piezoelectric
transformer (PZT) 84 controlling the position of one of the end mirrors
(not shown) in CW laser 20. FM driver 82 comprises a conventional waveform
generator responsive to a signal from system controller 100 here to apply
a continuous, periodic electrical control signal, here a triangular
waveform having an up-ramp and a down-ramp to PZT 84. PZT 84 responds to
such control signal by periodically moving the position of the end mirror
of laser 20 coupled thereto, thereby continuously changing the optical
length of the resonant cavity of laser 20 from a nominal length in
accordance with the shape of the periodic waveform (here triangular)
applied to PZT 84. Laser 20 responds to the movement of PZT 84 by changing
the frequency of the CW beam produced thereby from the nominal frequency
by an amount corresponding to the control signal applied to PZT 84. Thus,
here laser 20 resonates at continuously different frequencies and thus
periodically modulates the frequency of the CW beam produced by laser 20.
Here, the frequency of the CW beam is modulated in triangular modulation
pattern corresponding to the shape of the waveform produced by FM driver
82.
The frequency modulated CW beam (FM-CW) is polarized by laser 20 to a
predetermined polarization, here linear polarization such as
p-polarization, by conventional means, such as by a Brewster plate (not
shown) disposed within laser 20. The p-polarized FM-CW beam is directed to
interferometer section 30 which comprises beamsplitter 31, Brewster plate
32, quarter-waveplate 33, mirror 34, half-waveplate 35 and beam combiner
36, arranged as shown. Also included in interferometer section 30 is AM
unit 90, comprising AM driver 92 and AM modulator 94. Beamsplitter 31
divides the FM-CW beam incident thereon from CW laser 20 into a transmit
beam and a local oscillator (L.O.) beam, each beam having a nominal
amplitude. The transmit beam is applied to Brewster plate 32 through AM
modulator 94, which here comprises a conventional electro-optic crystal
such as a Cd-Te crystal. AM driver 92 responds to a signal from system
controller 100 to here apply a continuous, periodic electrical control
signal, here a sine wave, to AM modulator 94 to vary the indicies of
refraction of AM modulator 94 in accordance with such continuous, periodic
electrical control signal to produce corresponding changes in the
amplitude of the FM-CW beam coupled by AM modulator 94 to a Brewster plate
32, here by attenuating the nominal amplitude of the transmit beam in
accordance with such control signal. Thus, AM unit 90 continuously
modulates the amplitude of the FM-CW beam applied to Brewster plate 32.
The AM-FM CW beam applied to Brewster plate 32 remains p-polarized and
thus coupled directly therethrough and is incident on quarter-waveplate
33, which rotates such p-polarized beam to produce
right-circular-polarization on the beam coupled to beam expander section
40.
The L.O. beam is p-polarized and reflects from mirror 34 through
half-waveplate 35 and is incident on beam combiner 36. Half waveplate 35
rotates the polarization of the L.O. beam incident on combiner 36 from
p-polarization to s-polarization for purposes to be described. It is here
noted that the L.O. beam may be generated from a separate CW laser, not
shown, rather than from CW laser 20. In such case, the separate laser may
also be frequency modulated, as is known, such as by FM unit 80. The
nominal frequency of the CW L.O. beam may be adjusted to be offset from
that of CW laser 20, as is known.
The diameter of the laser beam is increased by beam expander 40, the
expanded beam being transmitted toward a target by scanner 42 and afocal
telescope 44. Scanner 42 responds in a conventional manner to a control
signal from controller 100 to scan the transmitted beam in a predetermined
pattern, as is known. Portions of the right circularly polarized,
transmitted beam which are reflected by the target are received by afocal
telescope 44 and directed to beam expander section 40 via scanner 42 as a
left circularly polarized beam. Beam expander section 40 compresses such
received portions into a narrow beam and directs the beam through
quarter-waveplate 33 onto Brewster plate 32. Quarter-waveplate 33 rotates
the polarization of the left-circularly polarized target-reflected beam to
s-polarization, which is reflected by Brewster plate 32 onto beam combiner
36. The s-polarized target return beam is mixed with the s-polarized L.O.
beam by combiner 36, and the resultant beam directed through lens 49 onto
optical detector 50. Optical detector 50 is a conventional photo-voltaic
or photoconductive device which produces an electrical output signal in
response to the beat frequency between the target-reflected-return beam
and the L.O. beam, as is known. In a homodyne system, such as is shown in
FIG. 1, wherein the same laser 20 produces the transmitted and L.O. beams,
such beat frequency signal will have a frequency component due to the
Doppler velocity of the target (f.sub.D) and a frequency component due to
target range (f.sub.R) It is noted that if a separate laser is used to
produce an L.O. beam having a frequency offset from the CW frequency of
laser 20 (i.e., in a heterodyne system), such beat frequency signal will
have an additional frequency component corresponding to such offset
frequency. The beat frequency signal produced by detector 50 is coupled to
receiver section 60 for processing in a manner described below.
Detector 50 also produces a signal representative of the frequency of the
L.O. beam and applies such signal to stabilizer 52 to maintain the
frequency stability of CW laser 20 by controlling the position of the
second end mirror (not shown) with piezoelectric (PZT) stack 54 to thereby
adjust the resonant frequency of laser 20 by varying the optical length of
the resonant cavity of laser 20, as is known. Here, stabilizer 52
superimposes an AC signal, here having a frequency of 1 KHz, on a DC
positioning signal nominally applied to PZT stack 54 by stabilizer 52.
Such AC signal dithers the position of the cavity mirror driven by PZT
stack 54 in order to scan laser 20 about the center of the selected laser
transition line (such as the P-20 line) and generate an error signal to
control the amplitude of the DC positioning signal applied to PZT stack
54. This is performed periodically during the operation of system 10 to
maintain CW laser 20 at the center of the selected transition. A
commercially available unit, such as a Lansing Research Company Model
80214, may be used as stabilizer 52. It is noted that a dedicated detector
(not shown), responsive solely to the output of CW laser 20, may be used
to stabilize laser 20, as is known, rather than using detector 50 for such
stabilization.
Referring additionally to FIG. 2, receiver section 60 is seen to comprise
FM subsection 160 and AM subsection 260. FM subsection 160 comprises
conventional input amplifier 161, which increases the power of the beat
frequency signal fed to receiver 60 by detector 50. The output of
amplifier 161 is coupled to the input port of mixer 162, the output port
of such mixer 162 being applied to FM processor unit 164. The mixer port
of mixer 162 is applied with a control signal f.sub.o from VCO 168 to
maintain the average frequency of the signal produced by mixer 162
constant, thereby minimizing the required bandwidth of the devices
comprising FM processor unit 164. A preferred embodiment of mixer 162 is
described in detail hereinafter in connection with FIG. 4.
FM processor unit 164 here comprises a conventional SAW (surface
accoustical wave) analyzer 170 which responds to the frequency of the
signal applied thereto from mixer 162 to produce an output signal
representative of such frequency, as is described in detail hereinafter.
Other devices which perform this function, such as a spectrum analyzer,
may be substituted for the SAW analyzer 170. Here, SAW analyzer 170 is
manufactured by Andersen Laboratories, 128 Blue Hills Avenue, Bloomfield,
Conn. 06002, and has a center frequency of f.sub.SAW and a predetermined
bandwidth and resolution. The output of SAW analyzer 170 is coupled to
post-processor 172 which here computes three signals from the output
signal of SAW analyzer 170 in a manner to be discussed: a signal V.sub.R
(coarse) representing the approximate range of the target; V.sub.D
representing the Doppler velocity of the target; and V.sub.L, a control
signal coupled through integrator 174 to VCO
(voltage-controlled-oscillator) 168 to adjust the output frequency of VCO
168 to hold the average frequency of the output signal from mixer 162
constant at a predetermined frequency, as will be discussed. The signal
V.sub.R (coarse) is an approximate measurement (i.e., is relatively
inaccurate) of target range. However, as will be discussed, such
approximate range measurement has relatively long ambiguous range
intervals and allows high resolution between a target and atmospheric
clutter and between such target and other targets. Signals V.sub.D and
V.sub.R (coarse) are coupled to utilization device 70, as shown, via lines
176, 178, respectively.
The output of mixer 162 is additionally coupled through cable 165 to AM
subsection 260, where such signal is coupled through delay line 262 for
purposes to be discussed and applied as inputs to a plurality of., here
three, mixers 264a, 264b, 264c. A single output of VCO 266 is coupled to
the mixer ports Of mixers 264a-264c for purposes to be discussed. The
input of VCO 266 is derived from the output of SAW analyzer 170 via
clutter threshold 171, as shown. The output ports of mixers 264a, 264b,
264c are coupled through a plurality of narrowband filters 268a, 268b,
268c, respectively, and are combined in conventional combiner/AM envelope
detector 270 to produce a signal on line 272 which corresponds to the
envelope of the amplitude modulation superimposed on the transmitted beam,
and hence on the beat frequency signal. The phase of the envelope signal
on line 272 is compared with the reference phase of the AM envelope signal
applied onto the FM-CW output of laser 20 by AM driver 92 in phase
detector 274, the difference in phase between the two signals representing
an accurate measurement of the range of the target, such range measurement
having poor resolution and short ambiguous range intervals, as will be
discussed. That is, it is difficult to resolve one target from another or
a target from atmospheric clutter with such accurate range measurement
signal. Such accurate, ambiguous and poorly resolved target range signal
is denoted here as V.sub.R (fine) and is coupled to utilization device 70,
as shown, on line 276. Alternately, the phase of the individual output
signals of filters 268a-268c may be detected, rather than detecting the
phase of the combined, envelope signal.
In operation, and referring also to FIG. 3, which illustrates frequency-
vs. -time waveforms 3A-3G representing the transmitted and received
signals at various points A-G in system 10, the AM-FM CW signal which is
transmitted toward the target at point A of FIG. 1 is shown as waveform
3A. As shown, such transmitted signal comprises a carrier signal
component, having a nominal frequency f.sub.c, and a pair of sideband
components: an upper sideband component having a nominal frequency of
f.sub.c +f.sub.m ; and a lower sideband component wherein the nominal
frequency is f.sub.c -f.sub.m. The symbol f.sub.c denotes the carrier
frequency of the CW laser output of laser 20, which for a laser having an
operating wavelength of 10.6 .mu.m, is approximately 3.times.10.sup.7 MHz.
The designation f.sub.m denotes the frequency of the amplitude modulation
envelope superimposed onto the output of CW laser 20 by AM unit 90, such
frequency f.sub.m typically being many orders of magnitude less than the
carrier frequency f.sub.c. In passing, it is noted that the range of a
target as measured by an AM system is given by the well-known formula:
##EQU1##
where c is the speed of light (3.times.10.sup.8 m/s) and .theta.
represents the phase difference between the amplitude modulation envelope
of the transmitted and received signals as determined here by phase
detector 274. A little thought reveals that the above-described range
measurement is ambiguous when such phase difference .theta. equals 2.pi..
Thus, the ambiguous target range is seen to be:
##EQU2##
and is a short range interval for any substantial f.sub.m (such as on the
order of 1 MHz).
Referring again to waveform 3A, the carrier and upper and lower sideband
components each undergo a predetermined frequency excursion B within a
predetermined time period T due to the frequency modulation imposed by FM
modulator 80 on the CW signal generated by laser 20. The symbol T
represents the duration of the up-ramp and the down-ramp produced by FM
driver 82 and here is selected to yield a relatively long ambiguous range
interval for target ranges measured from the FM superimposed on the CW
laser beam produced by laser 20. Typically, the FM period (i.e., 2T) thus
selected is such that the resulting FM modulation frequency (i.e., the
rate of repetition dF/dt of the frequency modulation) is several orders of
magnitude less than the AM frequency (f.sub.m) A little thought thus
reveals that the ambiguous range interval for the lower frequency FM
signal is much greater than the ambiguous range interval for the higher
frequency AM signal.
Thus, it is seen that AM modulator 90 and FM modulator 80 produce an AM-FM
waveform on the CW output of laser 20. That is, radar system 10 produces a
frequency and amplitude modulated beam of continuous wave electromagnetic
energy. Such AM-FM CW laser beam is transmitted toward a target (not
shown) via beam expander 40, scanner 42 and afocal telescope 44. Portions
of the transmitted AM-FM CW beam are reflected by clutter (not shown)
disposed near the system 10 (i.e., within the short ambiguous range
interval of the AM waveform). Other portions of the AM-FM CW signal are
reflected by a target or targets and return to radar system 10. Both
clutter-reflected and target-reflected return signals are received as
AM-FM CW energy by afocal telescope 44, scanner 42 and beam expander 40
and are directed, as discussed, by Brewster plate 32 onto beam combiner
36. The frequency of the target return signal over time is represented at
point B in FIG. 1 and waveform 3B of FIG. 3. As shown, the carrier
component of the return signal has a nominal frequency of f.sub.c
+f.sub.D, with the upper sideband component having a nominal frequency of
f.sub.c +f.sub.m +f.sub.D and the nominal frequency of the lower sideband
component being f.sub.c -f.sub.m +f.sub.D. The symbol f.sub.D here denotes
the Doppler frequency shift of the received signal, which manifestly is
the same for the carrier and sideband components. As is known, f.sub.D is
represented by the equation:
##EQU3##
where V is the velocity of the target relative to radar system 10 (f.sub.D
being positive for an approaching target, which is assumed here), and
.lambda. is the wavelength of the transmitted signal, here on the order of
10.6 .mu.m. The received signals at point B are also frequency modulated
by the amount B in a time period T, as shown, and are delayed in time by
an amount t.sub.R, representing the range delay of the target, from the
transmitted signals at point A. The range delay t.sub.R is represented by
the well-known formula:
##EQU4##
where c is the speed of light and R is the range of the target.
The L.O. beam applied to beam combiner 36 is represented by waveform 3C,
which is an FM-CW waveform of nominal frequency f.sub.c with a modulation
frequency B applied thereto in a period T. It is noted that the L.O. beam
is not applied with any amplitude modulation by AM modulator 90, as can
also be seen by observation of FIG. 1. The L.O. beam is, as discussed,
combined with the return beam by combiner 36 and the combined beams
directed to detector 50 via lens 49.
Detector 50 responds to the signals incident thereon as discussed to
produce a beat frequency signal on line 51, indicated as point D in FIG. 2
and shown as waveform 3D in FIG. 3. The beat frequency signal is a CW
signal having any FM components removed in the mixing process of detector
50. The frequency of such signal is the beat or difference frequency
between the return signal and the L.O. signal, except during the
transitions of the FM waveform between the up-ramp and down-ramp, such
transition periods being ignored, as is known. Since the return signal is
an AM signal, the beat frequency signal is an AM signal as well, having a
carrier component and upper and lower sideband components. During the
"up-ramp" of the FM signal, the beat frequency of the carrier output
component of the detector is f.sub.D -f.sub.R, the frequency of the upper
sideband component is f.sub.D -f.sub.R +f.sub.m, and that of the lower
sideband component is f.sub.D -f.sub.R -f.sub.m. The symbol f.sub.R
denotes the frequency shift in the output of detector 50 due to the range
of the target. As is well-known:
##EQU5##
where: R is the target range, B is the peak-to-peak FM deviation, c is the
speed of light, and T equals the duration of the FM ramp. Here it is
assumed that the absolute value of f.sub.R is less than the absolute value
of f.sub.D. As is known, the frequency f.sub.R is positive during the FM
up-ramp and negative during the down-ramp of the FM. Hence, during the
down-ramp, the carrier output component of detector 50 has a beat
frequency of f.sub.D +f.sub.R, while the beat frequency of the upper and
lower sideband components thereof are f.sub.D +f.sub.R +f.sub.m and
f.sub.D +f.sub.R -f.sub.m, respectively. The AM-beat-frequency output of
detector 50 | | |
