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Description  |
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BACKGROUND OF THE INVENTION
This invention relates generally to an imaging lidar (light detection and
ranging) system. More particularly, this invention relates to an imaging
UV/visible fluorosensing and Raman lidar system having the ability to
simultaneously measure temporally, spatially and spectrally resolved laser
backscatter from on the land, on or beneath the surface of bodies of water
and in the atmosphere.
There are numerous civilian and military applications which require or
could benefit from the remote and non-destructive sensing and probing of
the spectrally-dependent optical properties of a scene. Such applications
include (1) detection and classification of oil spills and oil seepage on
land and the ocean; (2) remotely measuring the atmospheric release of a
pollutant or a target chemical such as a chemical associated with illegal
drug production and chemical warfare agents (which might be monitored in a
treaty verification control agreement or on a battlefield); (3) the
measurement of sub-oceanic surface biogenic fluorescence spectra or
differential reflectance spectral images in a complex scene to enhance
object or substance detection; (4) measurement of wavelength dependent
fluorescence decay laws and performance of spontaneous and stimulated
Raman spectroscopy to measure such parameters as water temperature, sea
salinity, water turbidity (due to gaseous or solid dispersements),
subsurface chemical pollution as well as performance of vibrational
spectral identification of hydrocarbons and target chemicals.
Prior art methods are known for the remote probing of the spectrally
dependent optical properties of a scene. Such prior art uses either
"passive interrogation" where the sensor system casts no light of its own
upon the land, sea or atmosphere; or "active interrogation" wherein an
intense narrow spectral bandwidth light source, such as a laser, probes
the optical properties of the different media. Examples of such "active"
systems are described in "Laser Remote Sensing" by Raymond M. Measures,
published by John Wiley & Sons, Inc. (1984). In general, while these prior
art "active" systems have the ability to perform measurements of
temporally resolved spectral reflectance, fluorescence and Raman
scattering, such prior art "active" systems are deficient in their
inability to produce quantitative areal, volumetric, radiometric and
spectrometric imagery of a scene with geodetic measurements. Examples of
passive remote sensing systems are described in F. E. Hoge, R. N. Swift
and J. K. Yungel, "Active-Passive Ocean Color Measurements: 2
Applications", Appl. Opt. 25 (1986) 48-57.
Sensor systems for remote detection and imaging of objects in a
backscattering medium are known and described in, for example, U.S. Pat.
Nos. 4,862,267, 4,964,721, 4,967,270, 5,013,917 and U.S. Ser. No. 565,631
filed Aug. 10, 1990, all of which are assigned to the assignee hereof and
fully incorporated herein by reference. In general, these imaging lidar
systems utilize one or more laser transmitters which generate short pulses
of light and project these pulses down toward an object or target
enveloped by a backscattering medium. One or more gated camera receivers
detect the pulses of light reflected from the target after an appropriate
time delay. These detected reflected pulses of light are then converted to
a video image of the target.
While well suited for their intended purposes, the above-mentioned imaging
lidar systems generally do not have the ability to perform measurements of
temporally resolved spectral reflectance, fluorescence and Raman
scattering. As discussed, there is a perceived need for such measurements;
and therefore known imaging lidar systems are not adequate in this regard.
SUMMARY OF THE INVENTION
The above-discussed and other drawbacks and deficiencies of the prior art
are overcome or alleviated by the imaging UV/visible fluorosensing and
Raman lidar system of the present invention. In accordance with the
present invention, an optical sensor is provided for simultaneously
measuring temporally, spatially and spectrally resolved laser backscatter
from on the land, on or beneath the surface of bodies of water and in the
atmosphere. The present invention utilizes "active" interrogation or
"passive" interrogation for remotely and non-destructively probing the
spectrally-dependent optical properties of a scene.
In the "active" mode, the optical sensor of this invention comprises a
transmitter (preferably a tunable solid state laser) which emits pulses of
coherent light through a variable or adjustable field-of-view telescope
whereupon the light pulses are then propagated towards a scene (e.g.,
land, sea or atmosphere). Thereafter, laser backscatter is collected by a
second variable field-of-view telescope and directed to an imaging system
and spectrally dispersive optical subsystem. The light collected for the
imaging system is focussed through either a laser bandpass or laser line
filter onto the face of a time-gated image intensifier followed by a
charge-coupled device (CCD) camera. The light collected for spectral
dispersion subsystem is anamorphically demagnified to a slit-shape and
subsequently fed into an input slit on a spectrometer (e.g., a flat-field
monolithic spectrograph or cascaded prism dispersers followed by a streak
camera). The output of the spectrometer is focussed onto the photocathode
of an intensifier and CCD camera as in the imaging system. The digitized
output of the spectrally dispersive and imaging systems is sent to a
computer, where data is concurrently logged on the laser wavelength,
geodetic position, time-gate and range information, laser power,
intensifer gains and other information relevant to remote sensor
operation. The data may then be stored in analog or digital video format.
The optical sensor described above includes at least three different
operating modes, namely imaging, fluorosensing and Raman detection. The
present invention provides an improved technique for performing imaging,
fluorosensing and Raman detection relative to prior art techniques in that
in addition to its ability to perform measurements of temporally resolved
spectral reflectance, fluorescence and Raman scattering, it can provide
quantitative imagery of a scene with geodetic measurements.
In accordance with a feature of this invention, the spectrometer is
preferably either a cascaded prism disperser(s) followed by a streak
camera, prismatic predispersers or a monolithic spectrometer. In the
latter case, the monolithic spectrometer is comprised of a single piece of
glass, plastic and other optical materials that are permanently bonded
together and fabricated from focussing lenses, diffraction gratings,
prismatic pre-dispersers, laser line filters and entrance/exit slits. The
one-piece integral design of this spectrometer is an important feature of
the present invention in that this invention is well suited for
applications in dusty, high vibration environments over large temperature
ranges.
The above-described and other features and advantages of the present
invention will be appreciated and understood by those of ordinary skill in
the art from the following detailed description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings, wherein like elements are numbered alike in
the several FIGURES:
FIG. 1 is a flow chart depicting the several operational modes and
sub-modes for the optical sensor of the present invention;
FIG. 2 is a block schematic diagram depicting the structural components for
the optical sensor of the present invention;
FIG. 3 is a schematic diagram of a spectrograph with a line filter as its
input;
FIG. 4 is a schematic diagram of a prism spectrograph with an intensified
detector array; and
FIG. 5 is a schematic diagram of a temporally and spectrally resolved
emission detection system with cascaded prism disperser and streak camera.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention comprises an optical sensor which remotely and
non-destructively probes the spectrally, temporally and distance-dependent
optical properties of a scene. FIG. 1 is a flow chart depicting the
functions or operational modes of this invention (referred to as a Time
Gated, Spectrally Dispersive Imaging Active Remote Sensor).
There are essentially three different major modes of operation for the
present invention: imaging, fluorosensing and Raman detection. There are
also ancillary operational modes for each of the major modes (as described
in FIG. 1). While it is recognized that these operational modes may, under
some circumstances, have significant overlap in the operation of the
present invention, these modes are useful for broad descriptive
categorization of the end use of this invention. Prior to describing each
of these operational modes in detail, the components (or hardware) of the
remote sensing system of this invention will be described with reference
to FIG. 2.
The present invention is schematically shown in the block diagram of FIG.
2. The optical train begins with the output 12 of a tunable solid state
laser 10. A tunable solid state laser 10 whose gain medium has a small
thermal lens constant is preferred in order to have the added capability
of variable repetition rate and probe wavelengths. More conventional laser
technology lasers such as Nd.sup.3+ :YAG pumped by flashlamps (where
constant laser repetition rate is critical to maintain stable operation)
are less desirable for this invention than diode or flash lamped pumped
tunable solid state lasers. At present, variable repetition rate, tunable,
Ti.sup.3+ :Al.sub.2 O.sub.3 (titanium sapphire), Cr.sup.3+ BeAlO.sub.x
("Alexandrite") or Cr.sup.3+ :LiSrAlF.sub.6 lasers appear to be the most
appropriate contemporary laser technologies. It may be most desirable to
utilize a tunable optical parametric oscillator. Detailed examples of
suitable tunable solid state lasers are described in U.S. application Ser.
No. 632,377, filed Dec. 21, 1990, assigned to the assignee hereof and
incorporated herein by reference.
The laser light 12 is directed into a beam splitter 14, where a small
portion 16 of the light is sent to a broad band laser wavemeter and power
monitor 18 (such as a thermopile or rapid response pyroelectric joule
meter) to continuously monitor laser 10. The high temporal bandwidth joule
meter 18 is important in that it accurately measures the outgoing laser
pulse energy thereby facilitating absolute radiometric assessment of the
backscatter magnitude. The remainder of the laser light 20 is sent to a
first adjustable field-of-view telescope 22 and propagated at 24 towards a
scene 26, which could be land, sea or atmosphere. Then, laser backscatter
28 is collected by a second telescope 30 with a variable field-of-view,
which under most circumstances matches that of the telescope 22. In an
alternative embodiment, a single variable field-of-view telescope may be
used in place of the discrete telescopes 22, 30. A suitable field-of-view
telescope may have a field-of-view of 10 milliradians and a 24 inch
collecting aperture. The collecting telescope output 32 is then directed
by a mirror or beam splitter 34 to an imaging system (at 36) and a
spectrally dispersive optical subsystem (at 38).
Collected light 36 that goes to the imaging system is focussed through a
laser bandpass filter 40 onto the face of a gated camera 42 comprised of a
timed-gated image intensifier followed by a charge-coupled device camera.
Camera 42 is described in more detail in FIG. 4 of aforementioned U.S.
Pat. No. 4,862,257.
Light 38 that is collected for spectral dispersion is anamorphically
demagnified to a slit-shape using either prisms, astigmatic lenses or
preferably a fiber optic bundle; and subsequently fed into an input slit
on a spectrometer 46. The output of the spectrometer 46 is then focussed
or fiber optically coupled onto the photocathode of an intensifier and CCD
camera 42. Alternatively, light 38 is sent to a temporally and spectrally
resolved optical subsystem identified generally at 47. FIGS. 3 and 4
depict various embodiments of suitable hardware assemblies for such
spectral dispersion. FIG. 3 depicts a spectrograph with a line filter as
its input while FIG. 4 depicts a prism spectrograph with an intensified
detector array. In FIG. 3, light 38 is delivered to a prism pre-disperser
48 followed by a laser line rejection filter 50 and an entrance slit 52.
The light is then reflected off a collimating mirror 54 to a diffraction
grating 56. In turn, the light is sent to a focussing mirror 58 and
finally to a detector array 60 to record the dispersed spectrum.
In FIG. 4, the light 38 is delivered to a laser line rejection filter 62
followed by a prism disperser 64. The output from prism disperser 64 is
then delivered to an intensifier 66/detector array assembly 68.
Spectrometer 46 may also comprise a single piece of glass, plastic and
other optical materials that are permanently bonded together and
fabricated from focussing lenses, diffraction gratings, prismatic
pre-dispersers, laser line filters and entrance/exit slits as described in
U.S. patent application No. 417,623 filed Oct. 4, 1989 entitled
"Monolithic Optical Programmable Spectrograph", (now U.S. Pat. No.
5,026,160) the entire contents of which is incorporated herein by
reference. The one piece integral design of the spectrometer disclosed in
U.S. patent application Ser. No. 417,623 (now U.S. Pat. No. 5,026,160 is
well suited for use in this invention in that the present invention is
particularly useful in dusty, high vibration environments over large
temperature ranges. Prior art standard design spectrometers, such as
double and triple Raman spectrographs would be difficult to qualify for
airborne applications or to military specifications.
As mentioned, light 38 may also be directed to a temporally and spectrally
resolved optical subsystem 47. Subsystem 47 utilizes a spectrograph shown
in detail in FIG. 5. In FIG. 5, light 38 is fed to a laser line rejection
filter 70 and then through a pair of cascaded dispersive prisms 72, 74 to
a streak camera tube 76 and a two dimensional detector array 78.
Referring again to FIG. 2, the digitized outputs 80, 82 from the spectrally
dispersive system and from the imaging system are sent to a computer 84,
where data is concurren | | |
