Coherent laser radar device

Details Coherent laser radar device Coherent laser radar device

2002-11-21

2004-07-20

Gregory, Bernarr E. (Department: 3662)

Optics: measuring and testing

Range or remote distance finding

With photodetection

C356S005010, C356S028000, C342S054000

Reexamination Certificate

active

06765654

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a laser radar device, and more particularly to a coherent laser radar device using a pulsed laser that oscillates at a single wavelength as a light source for the purpose of measuring physical information such as a distance, a velocity, a density distribution or a velocity distribution of a target.
BACKGROUND ART
A coherent laser radar device using a laser beam can measure a wind velocity or a wind velocity distribution even in fine weather because a sufficient scattering intensity is obtained even through aerosol existing in the atmospheres. For that reason, the coherent laser radar device is preferably located at an airport or mounted on an aircraft and expected as a device for detecting a hindrance including an air turbulence.
The coherent laser radar devices are of two types one of which employs a pulsed laser that oscillates at a single frequency as a light source and the other of which employs a CW (continuous wave) laser.
FIG. 9
is a structural diagram showing a laser radar device in which a coherent laser radar device using an injection seeding pulsed laser device as a light source as disclosed in U.S. Pat. No. 5,237,331 B by Sammy W. Henderson et al. is combined with a wavelength synchronizing circuit for stabilizing the wavelength of the laser radar light source as disclosed in JP 10-54760 A by Shoji and Hirano.
The laser radar device shown in
FIG. 9
includes a CW laser light source
1
that oscillates at a single frequency, a first optical divider
2
that branches a laser beam
19
from the CW laser light source
1
, a frequency shifter
3
, an injection seeding pulsed laser
4
, a beam splitter
5
, a ¼ wavelength plate
6
, a telescope
7
, a scanning optical system
8
, a first optical coupler
9
, a photo detecting portion
10
, a second optical divider
11
, a third optical divider
12
, a second optical coupler
13
, a signal processing device
16
, an adjusting mechanism
17
for a cavity length of the injection seeding pulsed laser
4
, and a control circuit
18
for the adjusting mechanism. Reference numeral
20
denotes a seed light from the frequency shifter
3
,
21
is a pulsed laser beam from the injection seeding pulsed laser
4
,
22
is an optical axis of a transmit/reception light,
23
is a transmission light,
24
is a reception light,
25
is a local light and
26
is a coupled light of the reception light
24
and the local light
25
due to the first optical coupler
9
.
Subsequently, the operation of the laser radar device shown in
FIG. 9
will be described. The laser beam
19
from the laser light source
1
that oscillates at a single frequency f
0
is branched by the first optical divider
2
into two beams one of which forms the local light
25
, and the other of which becomes a laser beam that increases in frequency by a frequency f
IF
by the frequency shifter
3
and is then supplied to the injection seeding pulsed laser
4
as the seed light
20
.
The injection seeding pulsed laser
4
conducts the pulse oscillation at the single frequency (single wavelength) in an axis mode having a frequency closest to the seed light
20
. The laser pulse
21
from the injection seeding pulsed laser
4
which is linearly polarized is reflected by the beam splitter
5
through the second optical divider
11
. Thereafter, the reflected light is transformed into a circularly polarized light by the ¼ wavelength plate
6
and then irradiated toward a target through the telescope
7
and the scanning optical system
8
as the transmission light
23
.
A scattered light from the target is received through a backward path of the transmission light. The reception light
24
becomes a linearly polarized light shifted from a polarization plane of the laser pulse
21
by 90 degrees due to the ¼ wavelength plate
6
and is then transmitted through the beam splitter
5
so as to be guided to the first optical coupler
9
. In the first optical coupler
9
, the reception light
24
and the local light
25
are coupled together, and the coupled light
26
is supplied to the photo detecting portion
10
.
In this example, the photo detecting portion
10
is structured as shown in FIG.
10
.
As shown in
FIG. 10
, the photo detecting portion
10
includes a first photo detector
27
and a second photo detector
28
. Each of the first and second photo detectors
27
and
28
is made up of a photodiode that functions as a square-law detector which conducts light coherent detection and a microwave amplifier that electrically amplifies a signal from the photodiode. The microwave amplifier is shown by the combination of a pre-amplifier and a post-amplifier in the figure. A detection output from the first photo detector
27
is outputted to the signal processing device
16
as a reception signal, and a detection output from the second photo detector
28
is outputted to the signal processing device
16
as a monitor signal.
Returning to
FIG. 9
, the coupled light
26
from the first optical coupler
9
is coherent-detected by the first photo detector
27
of the photo detecting portion
10
. A signal from the first photo detector
27
is inputted to the signal processing device
16
as the reception signal. The signal processing device
16
calculates a distance to the target in accordance with an arrival period of time of the reception signal (a period of time since the transmission of the transmission light to the target till the reception of the reception light from the target), analyzes the frequency of the reception signal to obtain a Doppler signal, and extracts the velocity of the target from the Doppler signal.
As described above, the injection seeding pulsed laser
4
is required to monitor a difference in frequency between the pulsed laser beam
21
and the local light
25
in order to obtain an accurate Doppler signal since the injection seeding pulsed laser
4
conducts the pulse oscillation at the single frequency in an axis mode having a frequency closest to the seed light
20
. For that reason, after a part of the laser pulse
21
and a part of the local light
25
are extracted as monitor lights from the second and third optical dividers
11
and
12
, respectively, and then coupled together by the second optical coupler
13
, the coherent detection is conducted by the second photo detector
28
within the photo detecting portion
10
a
. A signal from the second detector
28
becomes the monitor signal.
In the signal processing device
16
, a frequency difference (the frequency of the monitor signal) f
M
between the laser pulse
21
and the local light
25
and the oscillation timing of the laser pulse are obtained from the monitor signal. Assuming that the frequency of the local light
25
is f
0
, the respective frequencies f
s
, f
T
, f
R
, f
M
and f
sig
of the seed light, the laser pulse, the reception light, the monitor signal and the reception signal are represented by the following expressions.
f
s
=f
0
+f
IF
f
T
=f
s
+&Dgr;f
f
R
=f
T
+f
d
f
M
=f
IF
+&Dgr;f
f
sig
=f
M
+f
d
where &Dgr;f is a frequency difference between the laser pulse
21
and the seed light
20
, and f
d
is a Doppler frequency of the target. A difference between the frequency f
sig
of the reception signal and the frequency f
M
of the monitor signal is taken, thereby being capable of obtaining the Doppler frequency f
d
of the target.
In order that the injection seeding pulsed laser
4
stably obtains the injection seeding operation, the injection seeding pulsed laser
4
adjusts the cavity length of the pulsed laser by using a piezoelectric element as the adjusting mechanism
17
for the cavity length. The piezoelectric element that functions as the adjusting mechanism
17
of the cavity length is controlled by the control circuit
18
. In the signal processing device
16
, an error signal based on a value of the frequency difference f
M
between the laser pulse
21
and the local light
25
is transmitted to the control circuit
18
from the mo

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