EARLINET, the European Aerosol Research Lidar NETwork, established in 2000, is the first coordinated lidar network
for tropospheric aerosol study on the continental scale. The network activity is based on scheduled measurements, a rigorous quality assurance program addressing both instruments and evaluation algorithms, and a standardised data
exchange format. At present, the network includes 27 lidar stations distributed over Europe.
EARLINET performed almost continuous measurements since 15 April 2010 in order to follow the evolution of the
volcanic plume generated from the eruption of the Eyjafjallajökull volcano, providing the 4-dimensional distribution of
the volcanic ash plume over Europe. During the 15-30 April period, volcanic particles were detected over Central Europe
over a wide range of altitudes, from 10 km down to the local planetary boundary layer (PBL). Until 19 April, the
volcanic plume transport toward South Europe was nearly completely blocked by the Alps. After 19 April volcanic
particles were transported to the south and the southeast of Europe. Descending aerosol layers were typically observed
all over Europe and intrusion of particles into the PBL was observed at almost each lidar site that was affected by the
volcanic plume. A second event was observed over Portugal and Spain (6 May) and then over Italy on 9 May 2010. The
volcanic plume was then observed again over Southern Germany on 11 May 2010.
Phase-unifying mirrors used as output couplers of laser resonators allow to improve the output laser beam divergence. In this paper, a phase-unifying unstable cavity has for the first time been applied to a high-power, commercially available XeF laser (Lamda Physik 210 i), and a laser output beam of 280 mJ, of 30 ns duration (FWHM), and with a divergence close to the diffraction limited value has been obtained at λ = 351 nm.
KEYWORDS: Mirrors, Reflectivity, Reflectors, Gas lasers, Laser resonators, Optical components, Near field optics, Ion beams, Thin films, Sputter deposition
Multiple spot mirrors are non-conventional optical components which present a number of equally spaced circular spots with a high reflectance that decreases to zero outside the spots. It is believed that the use of multiple spot mirrors as full reflectors in plane-parallel laser cavities should allow to improve the laser beam optical quality and get a good fillimg of the active medium. A double-spot mirror and a four-spot mirror for a XeCl laser cavity (λ = 308 nm) have been fabricated by depositing HfO2 and SiO2 thin films by reactive ion beam sputtering. The design employed allows to obtain a maximum reflectance (79%) on the spot area and a reflectance lower than 0.5% outside the spots by shaping the thickness of one of the inner layer. To this aim, a properly shaped mask was put inside the vacuum chamber during the deposition of the graded layer. The realization and the optical characterization of these devices are presented in this paper.
We report in this paper lidar measurements of two events of transport of dust from the Sahara region to the Mediterranean Sea, observed in Lecce, Italy (40°20'N,18°06'E). The observations have been made in the framework of the EARLINET project, in which aerosol measurements are performed in all Europe by 21 stations on a coordinate basis. Lidar measurements are compared with informations available from other sources. General results for the lidar ratio are also presented.
The return Raman Lidar signal contains a strong elastically scattered component (at X) that is useful for profiling clouds and aerosols and also weaker inelastically scattered components that provide chemical-specific information. For profiling water vapor, we use components produced by vibrational Raman effect that produces energy shifts characteristic of the molecules in the atmosphere (3652 cm' for water vapor, 233 1 cm1 for nitrogen). The aim of this paper is to process lidar backscattered signal that contains water vapor and aerosol information in order to improve their recovery. Since they are affected by different kinds of noise, an appropriate filtering, with an improved recovery, represents a way to get good estimates of the above components. Water vapor and aerosols are two significant atmospheric components that are generally detected for a better knowledge of weather and climate. In spite of optical filters included in the experimental apparatus used for this paper, there is a need of further filtering, by using signal digital filtering. To discriminate noises from the main signal that is backscattered from sky, we are investigating on the use of appropriate digital filtering to be utilized in order to retrieval a noiseless signal. This approach is different from the current one that uses a poissonian averaging of collected data. In our investigation, we prefer to employ filters that preserve either amplitude information or phase one. Different kinds of filtering procedures have been used in order to isolate the main signal from noise.
A XeF (351 nm) Raman lidar has been used to monitor tropospheric aerosols over the Sallentum peninsula of Italy and the vertical profile measurements of the aerosol extinction and backscatter coefficient and of the lidar ratio are presented in the paper. The measurements have been performed on a fixed schedule along one year and reveal that the aerosols are confined to lower altitudes and are characterized by larger lidar ratio values during autumn and winter months.
The joint effect of structural properties and electric field distribution on the laser damage threshold of HfO2 thin films is investigated in this work. Hafnium dioxide thin films of different optical thicknesses and with different structural properties have been realized employing two different deposition techniques: ion-assisted electron beam evaporation and dual-ion-beam sputtering technique. Laser damage thresholds of the sample have been measured at 308 nm by the photoacoustic beam deflection technique. It will be shown that samples presenting lower packing densities and lower peak values of the electric field intensity have higher damage threshold.
We report on water vapor and Ozone measurements made by a Raman LIDAR system recently built up in Lecce, Italy (40 degrees 20'6' N, 18 degrees 6'41' E). The system uses an excimer laser at 248 nm and detects the Raman backscattered radiation from O2, N2 and H2O and can operate in daytime. Atmospheric transmission function, which is mainly due to Ozone absorption, is found not to vary significantly at the operative LIDAR range (200 - 1000 m). We report two cases of daily evolution of water vapor.
Low Frequency Fluctuations (LFF) in diode lasers with optical feedback from an external reflector have been experimentally investigated by determining the control parameter region for their occurrence and by measuring statistical distributions of the return times between fluctuations. Two LFF regimes with different return time statistics have been identified. As asymmetry in the temporal dependence of the output intensity from the front and rear facets of the diode has been detected and explained in terms of coherent properties of the optical field.
We have performed an investigation of the optical feedback on laser diodes from an external mirror. We have examined at different values of the laser current the switching time distribution between different states and the power spectrum of the laser diode light intensity.
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