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BACKGROUND OF THE INVENTION
This invention relates to optical radars in which a high power CO.sub.2
laser is used as a transmitter and in which target enchoes are heterodyned
with the output of a local oscillator laser to yield an intermediate
frequency signal in the RF region, for example in the VHF or UHF band. The
desired target information is then extracted from the intermediate
frequency signal.
CO.sub.2 lasers are preferred as the transmitters of optical radars because
of the high electrical efficiency and high power characteristics thereof,
because the emitted radiation thereof is in the infrared region at
approximately 10 microns wavelength and is thus both convert and eye safe,
and also because the atmospheric low-loss transmission window which exists
between 8 and 14 microns makes possible long range optical transmission.
High powered CO.sub.2 transmitter lasers necessarily involve moderate to
large Fresnel number optical cavities which have inherently unacceptable
temporal and modal stability. The temporal instabilities arise when the
differential optical loss among competing high order transverse and
longitudinal modes is low, hence the laser oscillator indiscriminately
"mode hops". Moreover, without some form of intracavity optical
dispersion, a high gain CO.sub.2 laser transmitter can oscillate on any
number of vibrational-rotational transitions in the 9 to 11 micron
spectral region, and while gratings or prisms may be employed to provide
intracavity optical dispersion, these elements invariably and considerable
optical loss.
These inherently unstable large CO.sub.2 lasers can be stabilized or
controlled by injecting into the cavity thereof a small sample of the
desired frequency, wavelength and mode of operation, as long as the high
power laser cavity has the required optical design to support this
frequency or wavelength of oscillation. Under these conditions, the
injected signal will force the higher powered device to operate on the
injected transition and transverse mode. The source of the desired
injection signal is usually another smaller CO.sub.2 laser which, due to
its smaller cavity dimensions, has much better temporal, mode and
frequency stability, and which can in addition be provided with an
accurate frequency stabilization system, which may include, for example, a
Stark cell as an absolute frequency reference.
Heterodyne optical radars require highly stable transmitters and local
oscillators. If the desired radar signature is of the Doppler type, any
frequency drift between the transmitter and local oscillator will have the
same effect in the intermediate frequency (IF) signal thereof as radial
target movement. The prior art includes homodyne type optical radars in
which a frequency stable local oscillator laser has had a portion of its
output injected into the high power transmitter laser so that both lasers
operate at the same frequency. Such a homodyne radar cannot distinguish
the sense of radial movement of moving targets since it in effect has a
zero intermediate frequency. Further, homodyne radars have the additional
disadvantage that they do not produce any video signal for stationary
targets and they produce only extremely low frequency video signals for
targets with slow radial motion, and this limits the detection of low
speed radially moving targets.
Some of these disadvantages can be overcome by injecting the output of a
single local oscillator laser into the cavity of the larger CO.sub.2
transmitter and selecting an axial mode therein which has a frequency
different from the injected frequency. This results in a heterodyne radar
with a non-zero IF which can distinguish the sense of target radial
movement, but the selection or choice of the intermediate frequency is
constrained by the available axial modes of the transmitter, and further
it may require operation of the transmitter laser off of its line center
where the output beam power is not a maximum.
In contrast with these prior art optical radars the present invention
provides a more versatile heterodyne radar in which the transmitter is
injection-controlled so that it operates at a highly stable frequency
which is offset in frequency by a fixed and controllable amount from the
local oscillator laser. The amount of frequency offset determines the
intermediate frequency.
SUMMARY OF THE INVENTION
The invention comprises a heterodyne type of CO.sub.2 optical radar
including a frequency-stabilized first local oscillator laser with a
second local oscillator laser controlled to operate at a frequency offset
from that of said first local oscillator by the amount of the desired
intermediate frequency of the radar set. The output of the first local
oscillator is injected into the high power CO.sub.2 transmitter laser for
stabilization and frequency control purposes, and the output of the second
local oscillator laser is heterodyned with the received target echoes to
yield the intermediate frequency signal.
It is thus an object of the invention to provide a high power CO.sub.2
optical radar set of the heterodyne type which comprises a highly stable,
high powered transmitter and a highly stable local oscillator operating at
a fixed frequency offset from said transmitter, and whereby the amount of
said fixed frequency offset can be determined by the system designer in
accordance with operational requirements of said radar set.
A further object of the invention is to provide a heterodyne CO.sub.2 radar
with a high powered, frequency stabilized transmitter laser and with twin
low powered local oscillator lasers, the first of which has a portion of
its output injected into said transmitter laser for stabilization
purposes, and wherein the output of both of said local oscillator lasers
are applied to a detector which derives the difference or beat frequency
of said local oscillator lasers, said beat frequency being applied to a
frequency control system which maintains the difference frequency of said
twin lasers at the desired intermediate frequency of said radar, and
wherein the second of said twin lasers is applied to the mixer of said
radar where it is heterodyned with the received target echo return signals
to produce the radar's intermediate frequency signal.
Another object of the invention is to provide a heterodyne optical Doppler
radar which has an accurately controlled intermediate frequency and which
can accurately permit measurement of the radial velocities as well as
fine-grained Doppler signatures of moving targets, and the positions of
stationary targets, if the high power transmitter thereof emits a pulsed
output.
These and other objects and advantages of the invention will become
apparent from the following detailed description and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the invention which includes a ring laser
transmitter.
FIG. 2 shows how a conventional linear transmitter can be used in the
circuit of FIG. 1.
FIG. 3 shows one embodiment of the twin local oscillator lasers.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
It is preferred that the transmitter laser of the optical radar of this
invention be of the ring type since it is more convenient to inject a
control laser beam into such a laser and because the ring resonator is
directionally isolated from the injection source. The diagram of FIG. 1
shows a heterodyne type of optical radar with such a CO.sub.2 ring laser,
9, as the transmitter thereof. Such a laser resonator comprises four
separate mirrors, M9-M12, arranged at the corners of a rectangle (or
square), with the intracavity optical path arranged along the rectangle
sides between the mirrors. The four arrows labelled .lambda..sub.circ
indicates the circulating optical flux within the ring laser 9. The
partially reflective mirror M9 permits the injection laser beam
.lambda..sub.inj from the first local oscillator 15 to enter the laser 9.
Mirror M9 also permits a sample of the circulating flux,
.lambda..sub.samp, to exit the ring and actuate detector D1 which is part
of an automatic loop length control servo which maximizes the transmitter
optical flux by adjusting the transducer P3, which is attached to and
drives mirror M12. The detector D1 converts the sampled laser beam to an
electrical signal and applies the signal to ring control circuit 11, which
has its output connected to P3. The ring control circuit may comprise, for
example, a hill climbing servo which adjusts the position of mirror M12
through P3 until maximum laser power is achieved. This will occur when the
total length of the ring laser is an integral number of wavelengths of the
line center frequency.
The partially reflective mirror M10 is the front or output mirror of the
transmitter laser and the output beam, .lambda..sub.xmitt, passes
therethrough and thence through duplexer 17 to a scanner (not shown) which
deflects or scans the beam in a desired manner. It should be noted that in
the laser 9, the wavelengths indicated by the sumbols .lambda..sub.inj,
.lambda..sub.circ, .lambda..sub.samp, and .lambda..sub.xmitt, are all the
same wavelength. As stated above, the injected laser beam from the first
local oscillator causes the inherently unstable high powered transmitter
laser to oscillate on a single transverse and longitudinal mode and at a
stable frequency of the desired optical transition determined by the
frequency of the first local oscillator. With the injection system, no
dispersive optical element is needed in the high powered device.
The first local oscillator 15 comprises a laser cavity defined by rear
mirror M1 mounted on and driven by length-controlling transducer P1, and
partially reflective front mirror M2. The laser is provided with a prior
art type of frequency stabilization system, 3, which receives a sample of
the laser's output reflected from mirror M3. The system 3 controls the
position of mirror M1 via transducer P1 to achieve the desired wavelength,
.lambda..sub.1. The system 3 may for example comprise a frequency
reference in the form of a Stark cell which determines and controls the
oscillating frequency of the laser 15. Such a Stark cell stabilization
system is shown and described in detail in a co-pending application
entitled FREQUENCY STABILIZED LASER, Ser. No. 639,558, filed on Aug. 10,
1984.
A portion of the output of first local oscillator 15 is reflected from
partially reflective mirror M4 to mirror M8 and thence into the ring laser
9, as explained above. The second local oscillator 13 is similar to the
first one and includes a rear mirror M1 attached to and driven by
length-controlling transducer P2. The laser output beam at wavelength
.lambda..sub.2 passes through front mirror M2. A portion of this output is
reflected from mirror M6 and passes through mirror M5 to detector D2,
together with a sample of the output of the first local oscillator which
is reflected from M5 to D2. The wavelengths of the two local oscillators
are arranged to differ in frequency by the desired intermediate frequency
of the heterodyne radar. The laser cavity cross sections, the lasing
medium, CO.sub.2, and the CO.sub.2 pressures of the two local oscillators
may be identical but the cavity lengths will, for example, be controlled
so that they operate at different axial modes within the same line, to
yield a frequency difference or frequency offset in the RF region, for
example at 150 megaHz. A servo system comprising the detector D2, offset
frequency control system 5, and length-controlling transducer P2 attached
to rear mirror M2 of the second local oscillator 13, maintains the desired
offset frequency at a constant value. The two local oscillator laser beams
at wavelengths .lambda..sub.1 and .lambda..sub.2 applied to detector D2
produce therein an electrical difference or beat frequency which is
applied to the circuitry 5, which may, for example, include a frequency
discriminator of the Foster-Seeley type having a center frequency equal to
desired offset or intermediate frequency. The error signal produced by the
discriminator will adjust the transducer P2 in such a direction as to
maintain the frequency offset or difference between the two local
oscillators at a constant fixed value equal to the discriminator center
frequency.
The output of the second local oscillator 13 is mixed with the received
laser target echoes to generate the intermediate frequency. The target
echoes, labelled .lambda..sub.rec, pass through the scanner (not shown)
and are reflected by the duplexer 17, pass through partially reflective
mirror M7 and thence to detector or mixer D3. A portion of the output of
the second local oscillator 13 at wavelength .lambda..sub.2 passes through
mirror M6 and is reflected from mirror M7 to D3. The heterodyne receiver 7
is connected to the output of D3 and this receiver comprises one or more
intermediate frequency stages turned to the offset frequency between the
two local oscillators. This frequency is c/.lambda..sub.1
-c/.lambda..sub.2. The intermediate frequency stages within the receiver 7
would be followed by a second detector and/or Doppler frequency processing
circuitry, and some sort of display device for target information.
The target returns from stationary targets will be unchanged in frequency
from the transmitter signal. Targets which are moving radially inward
along the transmitted beam will cause increases in the target return
frequency and consequent shortening of the received wavelength,
.lambda..sub.rec. Outwardly moving targets would have the opposite effect
on received wavelength. These Doppler frequency shifts yield basic
information regarding moving targets. Frequency stable lasers are
essential to an accurate optical Doppler radar since any frequency drift
of either the transmitter of the local oscillator lasers will cause a
frequency error in the intermediate frequency signal which may be
interpreted as target movement by the Doppler processing circuitry of the
heterodyne receiver 7.
The twin local oscillators 13 and 15 may conveniently be of the waveguide
type formed in a common ceramic block. FIG. 3 is a cross sectional view of
such twin waveguide lasers. The ceramic block 31 has a pair of parallel
channels 35 and 37 formed in one surface thereof. Another ceramic block or
plate 33 is bonded to the top of block 31 to form the two laser cavities.
The cavities 35 and 37 would then be filled with the lasing medium and
appropriate excitation provided. The twin lasers may comprise cavities
approximately 15 cm long with square cross sections 2.25 mm on a side,
filled with CO.sub.2 -N.sub.2 -He gas mixture at a total pressure of 90
torr. Such lasers are capable of producing 7 watts of continuous wave
laser power. These types of twin lasers can, by virtue of their high
degree of electrical, mechanical and optical commonality, maintain a
stable difference frequency of 100 MHz to within 30 KHz for period of
seconds without using active stabilization systems. The twin lasers of
FIG. 3 may be provided with a common RF excitation source by means of two
planar electrodes 41 and 43 applied to the bottom of ceramic block 31 and
to the top of ceramic cover 33, as shown. The RF excitation generator 45
has its output connected across these electrodes to provide a transverse
field within the laser cavities.
The high power ring laser 9 may comprise four gain sections arranged in a
rectangle, square or X-configuration with a total peripheral gain length
of approximately 148 cm, with the cavity cross sections being square with
2.25 mm sides and filled with a CO.sub.2 -N.sub.2 -He gas mixture at 100
torr total pressure. Such a laser with suitable excitation is capable of
producing a continuous output beam of approximately 90 watts.
The embodiment of FIG. 2 utilizes a conventional non-ring or linear laser
19 as the transmitter. This transmitter comprises a single cavity with
mirrors 21 and 23 at each end thereof. Both of these mirrors are partially
reflective so that the laser output can pass through front mirror 21 to
duplexer 17 and the injection laser beam from the first local oscillator
can be injected through rear mirror 23, after reflection from M8. An
isolator 12 is placed in the path of the injection beam to prevent
retroreflection of the output of the transmitter laser back to the first
local oscillator.
While the invention has been described in connection with illustrative
embodiments, obvious variation therein will occur to those skilled in the
art without the exercise of invention, accordingly the invention should be
limited only by the scope of the appended claims.
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