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.
In recent years, much advance in the field of high-power femtosecond laser technology has been made. The high pulse
power of femtosecond laser systems leads to various interesting phenomena, such as a very high power density and the
formation of a plasma in the propagation medium, which is usually air. The possible application of femtosecond lasers
for infrared countermeasure (CM) applications, other than direct illumination of the detector, was suggested by several
authors. The goal of these countermeasures is to divert missiles from their interception course. We quantitatively
examined several suggested CM applications of femtosecond lasers from the literature. Our analyses show that
application of femtosecond laser beams for dazzling of missile seeker heads is not in any way promising, neither via
white-light generation in the ultra-short laser pulses nor via glowing plasma. In both these cases the generated light
intensity is too low to successfully compete with the signal of a typical target. Taking into account literature reports, we
conclude that the most promising application of femtosecond laser beams in the field of countermeasure applications
seems to be related to inducing damage of the optical components of the seeker system.
We discuss the influence of indium segregation-induced disorder effects in InxGa1-xN alloys. Changes of the transport
mechanism between InN and InxGa1-xN with x=0.58 were demonstrated by means of temperature-dependent conductivity
measurements. Furthermore, an increase of (i) full width at half maximum of photoluminescence (PL) and (ii) the Stokes
shift between PL and absorption was seen for samples approaching an In content of 0.5, which can also be attributed to
growing disorder. Hydrostatic pressure dependent PL measurements of In-rich InGaN alloys are diacussed. Due to the
fact that PL in InGaN originates from regions with higher-than-average In content, the luminescence pressure coefficient
dEE/dp should not be associated with the average In content, but with the In content which is in accordance with the
energy of the photon emission. This correction leads to a reduction of the large bowing of dEE/dp (associated with the
band gap) which was reported earlier. Furthermore, it is shown that the electron concentration in InN has a significant
influence on the measured value of dEE/dp.
Metalorganic vapor phase epitaxy (MOVPE) and plasma assisted molecular beam epitaxy (MBE) were used as alternative techniques to fabricate similar group-III-nitride laser structures. Utilization of high-pressure-grown GaN substrates resulted in reduction of threading dislocation density down to 105 cm-2. Light amplification features of the measured structures were evaluated by means of the variable stripe length method. Maximum peak modal gain values of 180 cm-1 for the MOVPE-grown sample and 315 cm-1 for the MBE-grown one were reached at corresponding pump power of 464 kWcm-2. Temperature-dependent photoluminescence measurements yielded activation energies of 41 meV nad 22 meV for MOVPE- and MBE-grown samples, respectively. Saturation lengths of 350 &mgr;m and 250 &mgr;m determined for MOVPE and MBE structures indicate reduced rate of nonradiative recombination compared to heteroepitaxy on foreign substrates. Differences in nonradiative recombination processes between the investigated structures lead to deviations in threshold for stimulated emission in favor of the MBE-grown sample.
In nitride heterostructures and devices, effects related to (i) polarization induced electric fields (PIEFs) and (ii) spatial segregation of indium leading to exciton/carrier localization are of major significance. However, separate investigation of these effects is not straightforward since they give rise to identical observations, such as a Stokes shift of the luminescence with respect to absorption and a blue shift of luminescence with increasing pump intensity. In this work, we review the usefulness of measurements of the hydrostatic pressure dependence of InGaN luminescence for the verification of the presence of PIEFs in quantum structures and light emitting devices. Additionally, we show that the pressure coefficient is not or only slightly sensitive to the degree of localization in the InGaN alloy. Thus, the variation of the luminescence pressure coefficient in different quantum structures can be almost entirely assigned to changes in the magnitude of internal electric field. Using this knowledge, we demonstrate how the degree of PIEF screening in InGaN based LDs can be evaluated.
High pressure grown GaN bulk crystals, because of their low defect density, are atractive for the use as substrates for blue-violet laser diode fabrication. These laser diodes are characterized by a low density of dislocations (8×104-1×105 cm-2) and thus they possibly have the best crystalline quality ever reported for this type of nitride devices. Previously, we demonstrated that these lasers are able to emit a very high optical power under pulse operation. In the present paper we will demonstrate the details of their room temperature CW operation, giving good prognostics for the further development of these devices. Preliminary estimation of the internal losses indicated a very low internal absorption in the range of 5 cm-1. The characterization of the aged devices did not reveal any dark lines or facet degradation. A correlation between the device lifetime and p-type layers growth methods will be suggested here.
High-power laser diodes emitting in the violet - UV region are needed for many applications related to data storage, full color laser projectors, pollution screening etc. This type of device is difficult to fabricate by using the presently available technology of epitaxial growth which employs the lateral overgrowth scheme to reduce the dislocation density in the active layer of the device. This paper presents a new generation of wide stripe laser diodes, which structures were coherently grown on bulk, nearly defect free GaN substrates. Thanks to a low and homogeneously distributed dislocation density (3×105cm-3), these devices are able to emit a very large optical power in excess of 2.5 W with a slope efficiency per facet of around 0.3 W/A and threshold current densities of 5-10 kA/cm2. The use of wide 15 μm stripe lowers the optical power density on the mirrors, and helps avoiding their optical damage. We believe that these devices clearly show the potential of homoepitaxy for high-power lasers applications.
In this paper we discuss the applicability of high-pressure grown bulk GaN crystals as substrates for device oriented MOVPE homoepitaxy. First, we fabricated light emitting diodes as a step towards realization of our target device: a blue light emitting laser diode. Our homoepitaxialy grown LEDs are characterized by excellent electrical characteristics and very satisfactory optical properties. Building on the experience gained during this first stage of our research we have been able to fabricate pulse current operated laser diodes emitting light at a wavelength between 397 and 430 nm. We believe that this fast progress clearly demonstrates the usefulness of bulk GaN substrates for optoelectronic devices, especially for high power laser diodes.
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