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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a coherent detection laser radar system
for detecting objects
2. Description of the Related Art
There are two basic types of laser radar systems for detecting objects;
those based on direct detection of optical radiation and those based on
coherent detection of optical radiation. In a laser radar system based on
direct detection of optical radiation, a beam of radiation is transmitted
to an object, scattered off of the object, and the scattered or reflected
portion is detected. In a laser radar system based on coherent detection,
radiation scattered off of the object (return radiation or return beam),
as well as radiation remaining within the laser radar system (a local
oscillator beam) are detected.
FIGS. 1a, 1b and 1c show conventional coherent detection laser radar
systems. FIG. 1a shows a heterodyne coherent detection laser radar
detecting system, and FIGS. 1b and 1c show homodyne coherent detection
laser radar systems.
In a heterodyne coherent detection laser radar system, the return radiation
is mixed with radiation from a second laser. As shown in FIG. 1a, a laser
beam generated by a first laser 102, having a frequency f.sub.1, is
transmitted through a beam splitter 120 to a scanning device 121, and
scanned across the object to be detected. As the beam is scattered off the
object, part of the scattered radiation (the return radiation or return
beam) is reflected back into the system (designated in the figures by a
dashed line with arrows), reflected by beam splitter 120 to beam combiner
122 and directed into an optical detector 130. At the same time, a second
laser 112 (a local oscillator) generates a second laser beam (the local
oscillator beam) having a frequency f.sub.2, which is transmitted to beam
combiner 122 and mixed with the return radiation at the optical detector
130.
Optical detector 130 converts the optical energy of the mixed beams into an
electrical signal which can be processed and displayed by components (not
shown) according to conventional techniques.
In conventional homodyne laser radar detection systems, only one laser is
utilized. As shown in FIG. 1b, laser 102 outputs a laser beam having a
frequency f.sub.1 which is split by a beam splitter 170 into two beams 172
and 174. Beam 172 is transmitted through beam splitter 120, scanned across
the object to be detected, reflected back into the system, and input into
optical detector 130 in a manner similar to that described above with
respect to the heterodyne system of FIG. 1a. Beam 174 is reflected off
mirror 176 into an optical frequency shifter 180 where the frequency
f.sub.1 of the beam 174 is shifted by .DELTA.f to a new frequency f.sub.2.
The output beam of the optical frequency shifter is combined with the
return beam 178 by beam combiner 122 and input to the optical detector
130.
As is known, laser radar detecting systems using coherent optical detection
require the simultaneous detection of two optical beams having different
frequencies. The coherent lasers used in conventional laser radar systems
have finite linewidths which translate to finite bandwidth signals at the
output of detector 130. Further, system backscatter that is mixed with the
local oscillator beam makes it difficult to detect signals corresponding
to a slowly moving object to be imaged. For example, FIG. 6a shows the
output of detector 130 of FIG. 1c in the frequency domain. This signal has
a finite -3dB bandwidth determined by the coherence linewidth of the laser
and the sample time of the signal. A CO.sub.2 gas laser can have a
linewidth on the order of 75 kHz. The peak frequency of this signal
corresponds to the frequency of the local oscillator beam f.sub.lo minus
the frequency of the return radiation f.sub.r. If the object to be imaged
is at rest, f.sub.lo -f.sub.r =0 and the peak frequency is at DC. A
similar spike results from the system of FIG. 1a, but at a frequency equal
to f.sub.2 -f.sub.1. This spike also occurs in the system of FIG. 1b, but
at a frequency corresponding to the shift imparted by optical frequency
shifter 180.
As shown in FIG. 6b, a signal corresponding to an object to be detected
must fall outside the frequency range of the signal components
corresponding to the local oscillator beam and internal backscatter to
avoid being swamped out by these large signal components. Because the
signal from the object to be detected is derived from the transmitted
beam, the transmitted beam must be sufficiently frequency shifted to
discriminate the signal corresponding to the object to be detected.
Conventionally, this frequency shift is obtained using modulating devices,
shown as dashed boxes 190 and 192 in FIGS. 1a1c. The modulating devices
192 may comprise, for example, acousto-optic or electro-optic modulators
alone or in combination with polarizers and/or birefringent retardation
plates, etc., to modulate the frequency of or pulse the transmitted
radiation and/or the local oscillator beam. Control circuitry 190 provides
the necessary signals to drive the modulators 192 in accordance with
conventional techniques In FIGS. 1a-1c, the control circuitry 190 directly
modulates the output of the laser 102 and/or laser 112. Alternatively,
modulating device 192 can be positioned after laser 102 to modulate the
laser beam output by the laser in accordance with signals from control
circuitry 190. In FIG. 1b, modulating device 190 can also be used to
directly modulate the output of frequency shifting device 180.
Although the modulating means 190 and 192 allow the conventional systems of
FIGS. 1a-1c to detect objects at rest, significant disadvantages result
from the use of modulators 190 and 192. They are expensive and require
complicated control circuitry to synchronize the movement of the scanning
means 121 with the modulation of the laser beam to be transmitted. This
significantly decreases the signal processing speed of conventional
systems.
Alternatively, the systems of FIGS. 1a-1c can be used without the
modulation devices 190 and 192, but only to detect moving objects. This
implementation is based on the principles of the Doppler effect which
imparts the necessary frequency shift to the transmitted radiation if the
object is moving with sufficient speed. This implementation is
disadvantageous in that the system cannot be used to detect objects at
rest or objects moving at a speed which does not impart a sufficient
Doppler frequency shift to the transmitted radiation to allow proper
detection of the object to be detected
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a laser
radar detecting system and method for detecting objects, particularly
slowly moving objects or objects at rest, without the requirement of
conventional modulation devices and techniques.
Additional objects and advantages of the invention will be set forth in the
description which follows or may be learned by practice of the invention.
The objects and advantages of the invention may be realized and obtained
by means of the instrumentalities and combinations particularly pointed
out in the appended claims.
To accomplish these and other objects, and in accordance with the purposes
of the invention as embodied and broadly described herein, a laser radar
system for detecting objects is provided comprising a laser for generating
a beam of radiation having a first frequency; a device for generating a
local oscillator beam of radiation; a scanning device for scanning the
beam of radiation across the object; a device for controlling the scanning
rate of the scanning means; a device for receiving return radiation
scattered by the object; a device for combining the return radiation with
the local oscillator beam of radiation; a device for generating an
electrical signal corresponding to the sum of thO return radiation and the
local oscillator beam of radiation; and a filter device for filtering a
portion of the electrical signal uniquely corresponding to the object; the
scanning rate being selected to shift the first frequency by an amount
sufficient to allow the filter device to filter the portion of the
electrical signal corresponding to the object.
To further accomplish these and other objects, and in accordance with the
purposes of the invention as embodied and broadly described herein, a
method of detecting an object in a laser radar system comprises the steps
of generating a laser beam of radiation having a first frequency;
generating a local oscillator beam of radiation; scanning the laser beam
of radiation across an object at a scanning rate sufficient to shift the
first frequency; receiving return radiation scattered by the object;
combining the return radiation with the local oscillator beam of
radiation; generating an electrical signal corresponding to the sum of the
return radiation and the local oscillator beam of radiation; and filtering
a portion of the electrical signal corresponding to the object; the
scanning rate being selected to shift the first frequency by an amount
sufficient to allow the portion of the electrical signal corresponding to
the object to be filtered.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part
of the specification, illustrate a presently preferred embodiment and
method of the invention and, together with the general description given
above and the detailed description of the preferred embodiment and method
given below, serve to explain the principles of the invention. In the
drawings:
FIGS. 1a, 1b and 1c represent conventional coherent detention laser radar
systems;
FIG. 2 is a block diagram of a preferred embodiment of a laser radar system
in accordance with the present invention;
FIG. 3 is a diagram of a preferred embodiment of scanning device as shown
in FIG. 2;
FIG. 4 is a diagram of a preferred embodiment of a control device as shown
in FIG. 2;
FIG. 5 represents a diagram demonstrating Doppler induced frequency
modulation due to scanning motion, in accordance with the present
invention; and
FIGS. 6a and 6b represent amplitude versus frequency plots of optical
detector output signals.
DESCRIPTION OF THE PREFERRED EMBODIMENT AND METHOD
Reference will now be made in detail to the presently preferred embodiment
and method of the invention as illustrated in the accompanying drawings.
FIG. 5 illustrates the concept behind imparting an adequate Doppler induced
frequency shift to the transmitter beam in accordance with the present
invention. The velocity, hence, the peak frequency deviation of scanning
mirror 304 is largest at points A and B on the scanning mirror (for
convenience only the y scanning mirror 304 of scanning device 208 is shown
in FIG. 5). Accordingly, increasing the scanning field of view increases
the maximum Doppler frequency shift. Consider the case of driving the
mirow 304 with a sine wave of angular frequency d.alpha./dt. The component
of velocity normal to the mirror surface at a distance r from the axis of
rotation is v where,
##EQU1##
The component of velocity v.sub.R contributing to the Doppler shift in
frequency is thus,
v.sub.R =cos(.alpha.(t)).multidot.v
Combining these two expressions, v.sub.R can be written as,
##EQU2##
where .alpha.(t) is the time varying angle between the mirror plane and
the transmitted optical beam given by,
##EQU3##
where .alpha..sub.o is the angle that the mirror makes with the
transmitted radiation when no scanning occurs. For example, .alpha..sub.o
may be 45 degrees.
Since v.sub.R <<c (speed of light), the resulting Doppler shift due to
v.sub.R is
##EQU4##
is .lambda. the wavelength of the transmitted radiation. Substituting the
expression for v.sub.R into the expression for .DELTA.f.sub.d gives,
##EQU5##
The maximum Doppler shift of the transmitted radiation due to the scanning
motion of the mirror 304 can then be written as,
##EQU6##
where R is the maximum distance of the transmitted and received photons
from the scanning axis 314. If both the transmitted and return beams are
Doppler shifted by (1/2) max.DELTA.f.sub.d, the highest frequency
component at the output of optical detector 222 due to the y-scanning
motion is,
##EQU7##
Hence, the expression for max.DELTA.f.sub.d, the maximum detectable
frequency depends on the scanning rate
##EQU8##
the wavelength of the transmitted radiation .lambda., the maximum distance
R of the transmitted and the received photons from the scanning axis 314
of the scanning mirror 304 and the angle .alpha. that the mirror makes
with the transmitted radiation when no scanning occurs. Thus, in addition
to increasing the scanning rate
##EQU9##
the scanning mirrors, the diameter of the transmitter beam can be expanded
to obtain scanning induced Doppler frequency shifts significant enough to
detect the signal due to radiation scattered by the object.
Consider, for example, a CO.sub.2 laser source expanded to a beam diameter
of 3 cm (R=1.5 cm), and a scanning frequency
##EQU10##
300.pi. rad/sec or 150 Hz. Substituting these values into the expression
for max.DELTA.f.sub.d, yields
max.DELTA.f.sub.d =5.3MHz, and
0.ltoreq..DELTA.fd.ltoreq.5.3MHz
A bandpass filter (BPF) 224 is selected to filter out the signals in the
desired frequency range based on predetermined system parameters to
separate the return beam signal from the local oscillator signal and/or
internal backscatter. Proper selection of the passband frequencies of BPF
224 (as shown in FIG. 6b), allows only the Doppler shifted signal
corresponding to the return radiation scattered by the object to be
detected, hence, slowly moving objects and objects at rest can be detected
by the present invention, without the requirement of adding conventional
modulating devices.
As shown in FIG. 2, a laser radar detecting system in accordance with the
preferred embodiment of the present invention comprises a laser 200, a
beam splitter 202, first and second beam expanders 206 and 220, a scanning
device 208, a beam combiner 214, an optical detector 222, a bandpass
filter (BPF) 224, an envelope detector 226, a control device 230, a
display device 228, a quarter-wave plate 204, a half-wave plate 218, and
reflecting mirrors 212 and 216, interconnected as shown.
The laser 200 may comprise, for example, a CO.sub.2 laser or any other gas
laser, a solid state laser or a semi-conductor laser (narrow linewidth).
For example, a 5 watt CO.sub.2 gas laser can be used for long distance
object detection.
Beam splitter 202 may comprise a standard amplitude typ beam splitter,
where most of the radiation is transmitted in the direction of beam
expander 206 and the remainder is reflected to the mirror 216 (for
example, 90 percent of the beam is transmitted to the beam expander 206
and the remaining 10 percent is reflected to the mirror 216).
Beam expanders 206 and 220 may comprise conventional Newtonian-type or
Galilean-type beam expanders.
The quarter-wave plate 204 and the half-wave plate 218 may comprise
conventional bifringent transmission wave plates. The fast and slow axes
of the quarter-wave plate and half-wave plate are rotated to a 45.degree.
angle and a 90.degree. angle, respectively, with respect to the plane of
polarization of the radiation from laser 200.
The scanning device 208 may comprise, for example, scanning mirrors as
shown in FIG. 3. In operation, the laser beam 201 output by laser 200 is
reflected and scattered by the x-scanning mirror 302 and the y-scanning
mirror 304, which are rotated about their respective axes 312 and 314 by
servo motors 322 and 324, respectively. Control device 230 provides
signals along lines 342 and 344 which drive the servo motors 322 and 324,
respectively, at a sufficient rate to impart an adequate Doppler frequency
shift to the transmitted radiation. Signals indicating the positions of
mirrors 302 and 304 are output from the servo motors 322 and 324 via lines
352 and 354, respectively, to the control device 230 to synchronize
scanning of the object to be detected with scanning of the display device
electron beam (not shown).
The control device 230 may comprise, as shown in FIG. 4, for example,
signal generators 402 and 404 to generate servo motor control signals for
controlling the positions of the scanning mirrors 302 and 304,
respectively. Any conventional signal generator can be used, provided the
frequency of the output signal of signal generators 402 and 404 is
sufficient to drive the scanning mirrors at a rate sufficient to impart an
adequate Doppler frequency shift to the transmitted radiation. X and y
scanner drivers/servo controllers 422 and 424 amplify the signals received
on lines 412 and 414 to drive the servo motors 322 and 324, respectively.
At the same time, x and y scanner drivers 422 and 424 receive return
signals 352 and 354 from the servo motors 322 and 324 and generate
voltages proportional to the positions of the scanning mirrors 302 and
304, respectively. These voltages are output on lines 432 and 424 to drive
the x and y axis of display device 228 to synchronize the scanning of the
object with the scanning of the electron beam on the display device 228.
An example of the x and y servo controller/scanner device is the General
Scanning Inc. DX series drivers.
Optical detector 222 produces a signal proportional to the square of the
sum of the electric field due to the local oscillator beam E.sub.lo and
the return beam E.sub.r. Any conventional wideband optical detector
providing this mixing method and signal output can be utilized as the
detector 222.
Beam combiner 214, band pass filter 224 (active or passive), envelope
detector 226, and reflecting mirrors 212 and 216 may comprise conventional
devices for producing the corresponding described functions.
Referring to FIGS. 2-4, the operation of the preferred embodiment of the
present invention will now be described. Laser 200 outputs a beam of
radiation 201 with a predetermined polarization. The beam of radiation 201
is split by a beam splitter 202 so that, for example, 90percent of the
optical power (transmitter beam) is transmitted toward beam expander 206,
and, for example, 10 percent (local oscillator beam) is reflected to
mirror 216. The local oscillator beam is reflected by mirror 216 toward
half-wave plate 218, which rotates the beam by 90.degree. and transmits
the beam to the beam expander 220. Beam expander 220 increases the
diameter of the local oscillator beam by, for example, a factor of three,
in order to, for example, to simplify alignment.
The transmitter beam is transmitted through beam splitter 202 toward the
quarter-wave plate 204 which causes the linearly polarized transmitter
beam to become circularly polarized. The circularly polarized transmitter
beam is then expanded by the beam expander 206 by, for example, a factor
of six and transmitted to scanning device 208. In response to drive
signals from control device 230, scanning device 108 rapidly scans the
transmitter beam across the object to be detected. The scanning mirrors
302 and 304 must be driven at a rate sufficient to cause the desired
Doppler frequency shift in the transmitter beam. For example, as shown in
the calculations above, if the transmitted beam has a radius of 1.5cm, the
scanning frequency should be selected to be 150Hz.
The scanned transmitter beam is scattered off the object to be detected 210
in all directions, including directly back into the detecting system. This
direct reflection (the return beam ) is received by the scanning device
208, directed back toward the beam expander 206, where it is compressed
by, for example, a factor of six before being transmitted to quarter-wave
plate 204, where the return beam is linearly polarized. Mirror 212
reflects the return beam toward the beam combiner 214 where it is combined
with the local oscillator beam and transmitted to optical detector 222.
Mirror 212 and beam combiner 214 are provided such that the return beam
and the local oscillator beam enter the optical detector 222 with parallel
face fronts and polarizations.
As discussed, the optical detector 222 produces a signal proportional to
the sum of the electric field due to the local oscillator beam E.sub.lo
and the return beam E.sub.r. The detector output signal, as shown in FIG.
6a, comprises frequency components corresponding to the local oscillator
beam and backscatter, and different frequency components corresponding to
the object to be detected as a result of the Doppler frequency shift
imparted by the scanning mirrors 302 and 304. Accordingly, the passband of
bandpass filter 224 is selected to filter the signal uniquely
corresponding to the object to be detected from the rest of the optical
detector output signal.
The filtered signal is input to an envelope detector 226 which outputs a
voltage to display device 228 proportional to the peak voltage levels of
the signal output from the bandpass filter 224. Display device 228 may
comprise a standard oscilloscope with control over the x- and y-scanning
of the electron beam, control over the intensity (or z-axis) of the
electron beam, and some type of persistence or memory control. Control
device 230 outputs voltages on lines 432 and 434 to the x- and y-axis of
the display device 228 to scan the electron beam of the display device 228
in synch with the scanning of the scanning mirrors 302 and 304. At the
same time, the output signal from envelope detector 226 is applied to the
z-axis of the display device to provide an image or the object to be
detected as the electron beam scans the display device 228.
Thus, the present invention allows slowly moving objects and objects at
rest to be detected without the requirement of adding conventional
modulating devices. Further, the present invention can detect objects in
motion, if the passband of BPF 224 is variable, similar, for example, to a
tracking filter, or if the passband region of BPF 224 is large enough that
the signal corresponding to the object falls within this passband.
Additional advantages and modifications will readily occur to those skilled
in the art. Therefore, the invention in its broader aspects is not limited
to the specific details, representative devices, and illustrative examples
shown and described. Accordingly, departures may be made from such details
without departing from the spirit or scope of the general inventive
concept as defined by the appended claims and their equivalents.
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