Optics: measuring and testing – Velocity or velocity/height measuring – With light detector
Reexamination Certificate
2001-01-12
2002-10-22
Tarcza, Thomas H. (Department: 3662)
Optics: measuring and testing
Velocity or velocity/height measuring
With light detector
C356S005090, C356S005150
Reexamination Certificate
active
06469778
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a coherent laser radar system and a target measurement method for measuring physical information such as a target distance, velocity, density distribution and velocity distribution of the target, and particularly to a coherent laser radar system and a target measurement method utilizing a pulsed laser oscillating a single-wavelength (single-frequency) pulsed laser beam as a light source.
2. Description of Related Art
As devices for measuring physical information such as the target distance, velocity, density distribution and velocity distribution of the target, there are a pulse Doppler radar system utilizing microwaves or millimeter waves and a coherent laser radar system utilizing light waves (laser beam). Because of the difference between their frequencies, the former can perform wide-range, long-distance measurements, whereas the latter can perform measurements at high spatial resolution and high velocity resolution.
In soft target measurements, such as measurements of wind velocity and wind velocity distribution, the pulse Doppler radar system handles raindrops and particles of mist or cloud in atmosphere as scatterers, and computes the wind velocity from the Doppler shift of the echo. Accordingly, it is difficult for the pulse Doppler radar system to measure the clear-air turbulence because not enough echo is captured in clear weather in which there are no raindrops, particles of mist or cloud in the atmosphere.
In contrast with this, the coherent laser radar system can measure the wind velocity and wind velocity distribution even in clear weather because it utilizes the laser beam and hence can achieve enough scattering intensity in aerosol in the atmosphere. Thus, the coherent laser radar system installed in an airport or aircraft is expected to serve as a device for detecting obstacles such as turbulence. There are two types of coherent laser radar systems: one employs as its light source a pulsed laser that oscillates a single frequency pulsed laser beam; and the other uses as its light source a continuous wave (CW) laser that oscillates a single frequency continuous laser beam.
FIG. 16
is a block diagram showing a configuration of a conventional coherent-laser radar system disclosed in U.S. Pat. No. 5,237,331, for example. The conventional coherent laser radar system utilizes an injection-seeding pulsed laser as its light source.
In
FIG. 16
, the reference numeral
101
designates a CW laser light source for oscillating a single-frequency CW laser beam;
102
designates an optical divider for dividing part of the CW laser beam as a local beam;
103
designates a frequency shifter for shifting the frequency of the CW laser beam;
104
designates an injection-seeding pulsed laser for generating a pulsed laser beam utilizing the CW laser beam as a seed beam;
105
designates an optical divider for dividing the pulsed laser beam;
106
designates a beam splitter for reflecting the light beam supplied from the optical divider
105
using the difference in polarization direction, and for transmitting the light beam supplied from a quarter-wave plate
107
;
107
designates the quarter-wave plate for converting a linearly polarized beam with a certain polarization direction with respect to the crystallographic axis to a circularly polarized beam, and for converting a circularly polarized beam into a linearly polarized beam;
108
designates a transceiver optics for supplying a scanning optics
109
with a beam from the quarter-wave plate
107
, and for supplying the quarter-wave plate
107
with a beam from the scanning optics
109
along the same optical path; and
109
designates the scanning optics for transmitting a transmitted beam to a target, and for receiving a scattered beam from the target as a received beam.
The reference numeral
110
designates an optical divider for dividing the local beam;
111
designates an optical coupler for coupling the local beam divided by the optical divider
110
with the pulsed laser beam divided by the optical divider
105
;
112
designates an optical coupler for coupling the local beam divided by the optical divider
110
with the received beam passing through the beam splitter
106
;
113
designates a photodetector for detecting a light beam output from the optical coupler
111
;
114
designates a photodetector for detecting a light beam output from the optical coupler
112
;
115
designates an A/D converter for converting the electric signals which are detected and generated by the photodetectors
113
and
114
into digital signals; and
116
designates a signal processor for computing the physical information such as the target distance, velocity, density distribution and velocity distribution in response to the two digital detection signals output from the A/D converter
115
.
The reference numeral
117
designates a controller for controlling an adjuster
118
in response to the signal supplied from the signal processor
116
; and
118
designates the adjuster such as a piezoelectric device for adjusting the cavity length of the injection-seeding pulsed laser
104
.
FIG. 17
is a block diagram showing a configuration of the signal processor
116
of the conventional coherent laser radar system. In this figure, the reference numeral
121
designates a memory unit for temporarily storing the digital signals fed from the A/D converter
115
;
122
designates a time gate for selecting from the digital signals stored in the memory unit
121
the digital signals corresponding to the received beam from a particular range;
123
designates a window processor for executing window processing such as Hanning window processing or Hamming window processing;
124
designates an FFT section for carrying out fast Fourier transform (FFT); and
125
designates a Doppler frequency detector for detecting the Doppler frequency in response to the signal passing through the Fourier transform.
Next, the operation of the conventional radar system will be described.
FIG. 18
is a timing chart illustrating the operation of the conventional coherent laser radar system.
The CW laser light source
101
oscillates the CW laser beam at a single frequency f
0
(that is, at a single wavelength), and supplies it to the optical divider
102
. The optical divider
102
divides the CW laser beam into two portions. A first one of the two CW laser beams is used as the local beam, and the second one is supplied to the frequency shifter
103
. The local beam is further divided into two portions by the optical divider
110
, and they are supplied to the optical couplers
111
and
112
. On the other hand, the frequency shifter
103
increases the frequency of the CW laser beam by f
IF
, and supplies the injection-seeding pulsed laser
104
with the CW laser beam with the frequency f
0
+f
IF
as the seed beam.
The injection-seeding pulsed laser
104
oscillates the single frequency (that is, single wavelength) pulsed laser beam in the axial mode at a frequency closest to the seed beam. The pulsed laser beam output from the injection-seeding pulsed laser
104
is divided by the optical divider
105
into two parts, and a first part is incident on the beam splitter
106
, whereas a second part is incident on the optical coupler
111
.
The pulsed laser beam output from the injection-seeding pulsed laser
104
, linearly polarized beam with a particular polarization direction, is reflected off the beam splitter
106
and is incident on the quarter-wave plate
107
. The quarter-wave plate
107
converts it to the circularly polarized beam which is transmitted the to a target as the transmitted beam via the transceiver optics
108
and the scanning optics
109
.
The pulsed laser beam thus transmitted to the target is scattered by the target, and part of the scattered beams is incident on the scanning optics
109
.
The scattered beam from the target, that is, the received beam, reversely proceeds along the same optical path as the transmitted beam through the scanning optics
1
Asaka Kimio
Hirano Yoshihito
Ooga Yasuisa
Sugimoto Etsuo
Suimoto Kyoko
Andrea Brian
Mitsubishi Denki & Kabushiki Kaisha
Suimoto Kyoko
Tarcza Thomas H.
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