Laser vibrometry based on coherent detection allows noncontact measurements of small-amplitude vibration characteristics of objects. This technique, commonly using the Doppler effect, offers high potential for short-range civil applications and for medium- or long-range applications in defense and security. Most commercially available laser Doppler vibrometers are for short ranges (up to a few tens of meters) and use a single beam from a low-power HeNe laser source (λ=633 nm). Medium- or long-range applications need higher laser output power, and thus, appropriate vibrometers typically operate at 1.5, 2, or 10.6 μm to meet the laser safety regulations. Spatially resolved vibrational information can be obtained from an object by using scanning laser vibrometers. To reduce measuring time and to measure transient object movements and vibrational mode structures of objects, several approaches to multibeam laser Doppler vibrometry have been developed, and some of them are already commercially available for short ranges. We focus on applications in the field of defense and security, such as target classification and identification, including camouflaged or partly concealed targets, and the detection of buried land mines. Examples of civil medium-range applications are also given.
This paper presents experimental work on the use of a CO2 laser for triggering of PIR sensors. Pyro-electric InfraRed sensors are often used as motion detectors for detection of moving persons or objects that are warmer than their environment. Apart from uses in the civilian domain, also applications in improvised weapons have been encountered. In such applications the PIR sensor triggers a weapon, when moving persons or vehicles are detected. A CO2 laser can be used to project a moving heat spot in front of the PIR, generating the same triggering effect as a real moving object. The goal of the research was to provide a basis for assessing the feasibility of the use of a CO2 laser as a countermeasure against PIR sensors.
After a general introduction of the PIR sensing principle a theoretical and experimental analysis of the required power levels will be presented. Based on this quantitative analysis, a set up for indoor experiments to trigger the PIR devices remotely with a CO2 laser was prepared. Finally some selected results of the experiments will be presented. Implications for the use as a countermeasure will be discussed.
Laser vibrometry based on coherent detection allows non-contact measurements of small-amplitude vibration
characteristics of objects. This technique, commonly using the Doppler effect, offers much potential for short-range civil
applications and for long-range applications in defence and security. Most commercially available laser vibrometers are
for short ranges (up to a few tens of metres) and use a single beam from a low power HeNe laser source (λ = 632 nm).
Long-range applications need higher laser output power, and thus appropriate vibrometers typically operate at 1.5 μm, 2
μm or 10.6 μm to meet the laser safety regulations.
Spatially resolved vibrational information can be obtained from an object by using scanning laser vibrometers. To reduce
measuring time and to measure transient object movements and mode structures of objects, several approaches to multibeam
laser Doppler vibrometry have been developed, and some of them are already commercially available for short
ranges.
In this paper we focus on applications in the field of defence and security such as target classification and identification,
including camouflaged or partly concealed targets, and the detection of buried land mines. Some examples of civil
medium-range applications are given also.
A theoretical and experimental study is performed to eliminate the effect of retroreflection of a camera by using masks. Theoretically, different types of masks are studied in terms of their capability to reduce the strength of the retroreflected signal while maintaining good camera image quality. Experimentally, the strength of retroreflected signals and image quality have been measured for different alignments of different masks. The results of these experiments in combination with the presented theory can be used to design masks that can reduce or even completely eliminate the strength of retroreflected signals, while simultaneously maintaining good image quality.
KEYWORDS: LIDAR, Target detection, Receivers, Radar, Laser systems engineering, Surveillance, Optical parametric oscillators, Sensors, Visibility, Signal to noise ratio
Coastal surveillance and naval operations in the littoral both have to deal with the threat of small sea-surface targets.
These targets have a low radar cross-section and a low velocity that makes them hard to detect by radar. Typical threats
include jet skis, FIAC's, and speedboats. Previous lidar measurements at the coast of the Netherlands have shown a very
good signal to clutter ratio with respect to buoys located up to 10 km from the shore where the lidar system was situated.
The lidar clutter is much smaller than the radar clutter due to the smoothness of the sea surface for optical wavelengths,
thus almost all laser light is scattered away from the receiver. These results show that due to the low clutter a search lidar
is feasible that can detect small sea-surface targets. Based on these promising results a search-lidar demonstrator project
has started end of year 2008. The system set-up of the search lidar demonstrator is presented and experimental results
near the coast of Holland are presented. By using a high rep-rate laser the search time is limited in order to be useful in
the operational context of coastal surveillance and naval surface surveillance. The realization of a search lidar based on a
commercially available high power and high rep-rate laser is presented. This demonstrator is used to validate the system
modeling, determine the critical issues, and demonstrate the feasibility.
Naval operations in the littoral have to deal with the threat of small sea-surface targets. These targets have a low radar
cross-section and low velocity, which makes them hard to detect by radar in the presence of sea clutter. Search lidars
can provide an alternative detection capability for small sea-surface targets at ranges up to 10 km (using commercially
available parts). The lidar clutter is much smaller than the radar clutter due to the smoothness of the sea surface for
optical wavelengths, thus almost all laser light is scattered away from the receiver. By using a high rep-rate laser the
search time is limited in order to be useful in the operational context of coastal surveillance and naval surface
surveillance. In the same scenario, a laser range profiler with a high bandwidth, fast laser receiver can be used to for
identification of the various contacts. To this end, the experimental results have to be compared to a database with premeasured
or pre-calculated range profiles of possible targets. The good match between experimental and simulated laser
range profiles implies that such a database can be constructed from 3D-target models, thus simplifying the database
creation.
Naval operations in the littoral have to deal with threats at short range in cluttered environments with both neutral and
hostile targets. There is a need for fast identification, which is possible with a laser range profiler. Additionally, in a
coastal-surveillance scenario a laser range profiler can be used for identification of small sea-surface targets
approaching the coast. A field trial in June 2007 at the coast of Norway was conducted to validate the concept of ship
identification with a laser range profile. A laser range profiler with a high bandwidth, fast laser receiver was used to
perform tests on the capability of a laser range profiler for identification. The ships in the field trial were of frigate size.
Good laser range profiles could be obtained up to a range of 10 km. The experimental results were compared with the
geometry of the ships and a simulated range profile based on a 3D target model. The good match between experimental
and simulated laser range profiles means that a database of laser range signatures can be constructed from 3D-models,
thus simplifying the database creation. It is shown that sea-surface targets can be distinguished by their laser range
profiles. A neural net approach could distinguish five ships with no false identification.
Coastal surveillance and naval operations in the littoral both have to deal with the threat of small sea-surface targets.
These targets have a low radar cross-section and a low velocity that makes them hard to detect by radar. Typical threats
include jet skis, FIAC's, and speedboats. Lidar measurements at the coast of the Netherlands have shown a very good
signal-to-clutter ratio with respect to buoys located up to 10 km from the shore where the lidar system was situated. The
lidar clutter is much smaller than the radar clutter due to the smoothness of the sea surface for optical wavelengths. Thus,
almost all laser light is scattered away from the receiver. These results show that due to the low clutter a search lidar is
feasible that can detect small sea-surface targets. A search-lidar demonstrator is presented and experimental results near
the coast of Holland are presented. By using a high rep-rate laser the search time is limited in order to be useful in the
operational context of coastal surveillance and naval surface surveillance. The realization of a search lidar based on a
commercially available high power and high rep-rate laser is presented. This demonstrator is used to validate the system
modeling, determine the critical issues, and demonstrate the feasibility.
Naval operations in the littoral have to deal with the threat of small sea-surface targets. These targets have a low radar
cross-section and low velocity, which makes them hard to detect by radar in the presence of sea clutter. Typical threats
include periscopes, jet skies, FIAC's, and speedboats. Search lidars on board naval vessels can provide detection
capability for small sea-surface targets. Lidar measurements at the coast have shown a very good signal-to-clutter ratio
with respect to buoys located up to 10 km from the shore were the lidar system was situated. The lidar clutter is much
smaller than the radar clutter due to the smoothness of the sea surface for optical wavelengths, thus almost all laser light
is scattered away from the receiver. These results show that due to the low clutter a search lidar is feasible that can detect
small sea-surface targets. The concept of a search lidar is presented and its performance is derived from system models.
By using a high rep-rate laser and a variable beam divergence the search time can be limited. The design of a search lidar
based on a commercially available high power and high rep-rate laser is shown.
Naval operations in the littoral have to deal with threats at short range in cluttered environments with both neutral and
hostile targets. On board naval vessels there is a need for fast identification, which is possible with a laser range profiler.
Additionally, in a coast-surveillance scenario a laser range profiler can be used for identification of small sea-surface
targets approaching the coast. An eye-safe 1.5 μm laser range profiler has been used to validate these claims.
Experimental results show that range profiles of sea-surface targets can be obtained at ranges of several km's. Sea-surface
clutter is shown to be negligible. Simulation shows that sea-surface targets can be distinguished from their range
profiles. The influence on the identification performance of range resolution and a-priori knowledge of the aspect angle
is presented. Classification has been tested on simulated range profiles of a number of small boats. With a range
resolution of 0.3 meter (comparable to our experimental set-up), these small boats could be identified.
Laser countermeasures against infrared focal plane array cameras aim to saturate the full camera image. In this paper we
will discuss the results of three different dazzling experiments performed with MWIR lasers and show that the obtained
results are independent of the read-out mechanism of the camera and can be explained by an expression derived from the
point spread function of the optics. This expression also allows us to estimate the required laser power to saturate a
complete focal plane array in a camera system. Simulated Images with simulated dazzling effects based on this
expression will be shown.
Small maritime targets, e.g., periscope tubes, jet skies, swimmers and small boats, are potential threats for naval ships under many conditions, but are difficult to detect with current radar systems due to their limited radar cross section and the presence of sea clutter. On the other hand, applications of lidar systems have shown that the reflections from small targets are significantly stronger than reflections from the sea surface. As a result, dedicated lidar systems are potential tools for the detection of small maritime targets. A geometric approach is used to compare the diffuse reflection properties of cylinders and spheres with flat surfaces, which is used to estimate the maximum detectable range of such objects for a given lidar system. Experimental results using lasers operating at 1.06 μm and 1.57 μm confirm this theory and are discussed. Small buoys near Scheveningen harbor could be detected under adverse weather over more than 9 km. Extrapolation of these results indicates that small targets can be detected out to ranges of approximately 20 km.
Countermeasures against heat seeking missiles require access to efficient laser sources, which should emit wavelengths at band I, II and IV. Efficient diode pumped solid-state lasers, combined with efficient non-linear wavelength shifters, allow the development of practical tuneable mid-IR countermeasure sources. The paper describes the requirements and the development of a tabletop laser source for study of DIRCM techniques. Jamming laser systems must be able of creating pulse sequences in the frequency range between 100 Hz and 10,000 Hz, including the capability to mix and sweep the jam frequency. A Nd:YVO4 pump laser with maximum pump power of 3 Watt and pulse length of 10 ns, and a maximum modulation frequency of 100 kHz was selected. A linear single resonant OPO cavity with 30 mm long, 1mm thick PPLN crystals was build. With the tabletop laser system we were able to generate wavelengths from 1.5 to 4 micron. In band I, at 2 micron we can generate between 400-550 mW, and in band II, from 3-4 micron we can generate 130-160 mW laser jam power. The beam quality (M2) is approximately 2.5. The power efficiency for the idler was 8.8%, while the slope power efficiency was 15%. Jam patterns are generated by use of an acousto-optic modulator.
Acoustic landmine detection (ALD) is a technique for the detection of buried landmines including non-metal mines. Since it gives complementary results with GPR or metal detection, sensor fusion of these techniques with acoustic detection would give promising results. Two methods are used for the acoustic excitation of the soil: laser excitation and loudspeaker excitation. A promising concept is using lasers for excitation and monitoring for complete stand-off detection. Results from a field test and laboratory experiments show the feasibility of laser excitation for ALD. In these experiments buried landmine surrogates were measured with ALD using a Nd-YAG laser at 1.06 μm for the acoustic generation and a Laser Doppler Vibrometer (LDV) system at 1.54 μm for the detection of soil vibrations. An analysis is given of the experimental results showing the potential and the inherent limitations of the technique. We discuss the relative merits of LDV detection versus microphone detection of the laser-induced acoustic vibration. It was found that the LDV has limitations with respect to microphone detection due to the influence of surface effects that are prominent in LDV but absent in microphone detection.
Acoustic landmine detection (ALD) is a technique for the detection of buried landmines including non-metal mines. An important issue in ALD is the acoustic excitation of the soil. Laser excitation is promising for complete standoff detection using lasers for excitation and monitoring. Acoustic excitation is a more common technique that gives good results but requires an acoustic source close to the measured area. In a field test in 2002 both techniques were compared side by side. A number of buried landmines were measured using both types of excitation. Various types of landmines were used, both anti-tank and anti-personnel, which were buried at various depths in different soil types with varying humidity. Two Laser Doppler Vibrometer (LDV) systems of two different wavelengths for the different approaches were used, one based on a He-Ne laser at 0.633 μm with acoustic excitation and one on an erbium fiber laser at 1.54 μm in the case of laser excitation. The acoustic excitation gives a good contrast between the buried mine and the surrounding soil at certain frequencies. Laser excitation gives a pulse response that is more difficult to interpret but is potentially a faster technique. In both cases buried mines could be detected.
We analyzed a time series of high resolution 8 - 13.4 micrometers scanner images of a sandbox with buried (3 - 10 cm depth) and unburied, metal and plastic AP and AT mines, surrogates and other targets. With a high resolution DUDA scanner operating in 8 - 13.4 micrometers all the surface laid targets were visible during the whole diurnal cycle. The buried targets were only visible during sunrise and sunset. The emissivity of the targets and sand could not be derived from the measured apparent temperatures and contact temperatures.
The conversion efficiency of a Raman cell at high pump energies has been increased with an astigmatic focus. Experimental and numerical results show that the increased conversion is due to the reduction of cascade second order Stokes. For our experimental setup, it is shown that other effects, namely Brillouin scattering, anti-Stokes generation, and ground-state depletion, are negligible. Another important advantage of an astigmatic focus is the reduced risk of optical breakdown due to the lower intensity at the focus with only a small gain reduction. Good agreement was found between experimental and numerical results. In particular, the pump energy where the conversion for an ordinary focus is identical to an astigmatic focus is reproduced by the calculations.
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