Atanaska D. Deleva

Controlled time-delayed-pulses operation of a two-wavelength combined dye-Ti:Al2O3 laser

Abstract We have developed and investigated a 0.53 μm Nd: YAG pumped combined dye-Ti 3+ :Al 2 O 3 laser, which emits two independently tunable near-infrared wavelengths in a single beam with relative time decay controllable in the range 0–250 ns. The emission is produced in a simple dual-channel resonator, where an intracavity interference wedge is used both as an efficient channel coupler and as a spectral selector. The delay between the pulses arises from the different temporal behavior of the dye and the Ti 3+ :Al 2 O 3 laser emission and is controlled by varying the pump intensity or the cavity length.

Lidar Measurements of Atmospheric Dynamics over High Mountainous Terrain

Results of lidar observations on the dynamics of atmospheric layers over the ridge and northeast slope of Vitosha Mountain situated close to Sofia, Bulgaria, are reported and compared with results for layers over surrounding plain and urban zones. Measurements are carried out at 1064 nm wavelength with a lidar range resolution of 15 m, by using one of the aerosol channels of a combined Raman‐aerosol lidar based on a powerful Nd:YAG laser. Two cases of distinctly different weather conditions are studied. The mountainous area, situated from 6 km to 12 km away from the lidar station, is scanned reiteratively within 7°–9° slope‐angle range. Multiple series of lidar profiles are registered with average time of 2–5 s. Lidar signals are then range corrected and processed statistically. Normalized standard deviation is used as a characteristic of the atmospheric dynamics. The spatial‐temporal evolution of atmospheric density fluctuations is shown on colormap plots. Intensive air dynamics over the mountain is reve...

Remote monitoring of aerosol layers over Sofia during Sahara dust transport episode (April, 2012)

In this work we present results of lidar remote sensing of aerosol layers in the atmosphere above Sofia during an episode of Sahara dust transport, 02-07 April, 2012. The investigations were made using two lidar systems, one equipped with a CuBr-vapor laser, emitting at wavelength 510.6 nm, and a second one - with Nd:YAG laser, at wavelengths 1064 nm and 532 nm. The results of lidar measurements are presented in terms of vertical atmospheric backscatter coefficient profiles and color maps of the aerosol stratification evolution. The involved into discussions ceilometer data (CHM 15k ceilometer) and satellite data from CALIPSO lidar, enhance the synergy of observations. Conclusion about atmospheric aerosol’s origin was made upon analyses of the information of weather-forecast maps provided by the Forecast system of Barcelona Supercomputing Centre, which are accessible via Internet. Additional information was provided by calculations of the backward air mass trajectories, using online software of NOAA about HYSPLIT model. The comparison between the data from the two lidars and the ceilometer showed similar behavior of aerosol layers development in the atmosphere above Sofia. All information about aerosol layers origin, their altitude above ground, persistence during lidar observations, confirmed the conclusion of observation of a long-distance Sahara dust transport beyond Balkans and Sofia. An interesting completion of CALIPSO lidar and ground based lidars results of measurement is presented in case of thick opaque cloud layer in the atmosphere, which slices the path of lidar sensing in both directions.

Lower Troposphere Observation over Urban Area with Lidar at 1064 nm

An episode of relatively thick (till ∼3 km) aerosol formation over the urban area of Sofia city was observed by lidar at a wavelength of 1064 nm. The lidar is part of Sofia lidar station at the Institute of Electronics of Bulgarian Academy of Sciences. Analysis of the weather conditions during the measurement period explains the stable persistence of such formation of human-activity aerosol over the town for the days of observation 20, 21, 23, and 24 June, 2011. The estimated top of the Planetary Boundary Layer for the measurement dated 23 June showed unusually high altitude ∼2200 m above ground. The results are presented in terms of vertical atmospheric backscatter coefficient profiles and color maps of the aerosol stratification evolution.

Raman-elastic-backscatter lidar for observations of tropospheric aerosol

In this work, a combined Raman-elastic backscatter lidar is described. It is based on a Q-switched powerful frequencydoubled Nd:YAG laser (output pulse power: up to 1 J at 1064 nm; up to 100 mJ at 532 nm; pulse duration 15 ns FWHM; repetition rate 2 Hz). A Cassegrain telescope (35 cm diameter, 200 cm focal length) collects backscattered radiation from atmospheric particles and molecules. Lidars spectral receiving module consists of three cannels. The first two channels separate and detect elastic-backscattered lidar signals at laser wavelengths 1064 nm and 532 nm, respectively. In the third lidar channel, a radiation at a wavelength of 607 nm is selected and detected, resulting from Raman backscattering of laser second harmonic (532 nm) by atmospheric nitrogen molecules. Experimental profiles of the aerosol backscattering coefficient demonstrating measurement abilities of the system are shown and discussed. Particularly, attention is paid to profiles containing signals from high-altitude cirrus clouds, low-altitude stratus clouds, hazy atmospheric areas and Saharan dust over the city of Sofia.

Vertical Profiling of Atmospheric Backscatter with a Raman‐Aerosol Lidar

Aerosols have a strong impact on the planet’s thermal balance, air quality, and a variety of atmospheric processes and phenomena. In this work we present some results from a long term lidar observation of tropospheric aerosols over the city of Sofia, Bulgaria, within the framework of the European project “EARLINET‐ASSOS.” Vertical profiles of the aerosol backscattering coefficient and range corrected lidar signals are processed and analyzed. The temporal evolution and the spatial distribution of atmospheric aerosol fields are illustrated by 2D‐colormaps in height‐time coordinates. We present here several cases of aerosol loading: transport of Saharan dust (at altitudes from 3 km to 5 km), highly situated layers (from 9 km to 15 km), and anthropogenic smog (up to 2 km). All measurements were performed by using the two aerosol spectral channels of a combined Raman‐aerosol lidar developed in the Laser Radar Lab, Institute of Electronics, Bulgarian Academy of Sciences. It is based on a Q‐switched powerful fre...

Intercomparison of the three lidar systems in Sofia

In this work the results of an experimental assessment of the operation accuracy of three lidar systems in Sofia City are presented. The investigation was carried out performing an inter-comparison of profiles of the atmospheric aerosol backscattering coefficient simultaneously measured by the lidars. All three lidar systems are involved in regular measurements within the frame of the EARLINET-ASOS (European Aerosol Research Lidar Network: Advanced Sustainable Observation System) European project. Twenty-four lidar stations, distributed over 16 countries in Europe are associated in this network. The main objective of the project is the establishment of a large statistical database of the aerosol distribution on the European scale. In order for the data to be reliable, preliminary tests of the quality of work of the individual lidar systems should be performed. That was the reason to perform the present study. The experiments on simultaneous operation of the lidar systems were carried out according to the fixed EARLINET-ASOS schedule starting in April 2006. The mean difference in the aerosol backscatter for all profiles obtained simultaneously and the standard deviation of the differences were calculated and compared.

Self-seeded Ti: Sapphire laser with an active feedback mirror

Abstract A single-longitudinal mode (SLM) dual-cavity pulsed Ti:sapphire laser is presented. It is self-seeded by means of an active feedback mirror (AFM) consisting of a partially reflecting mirror and additional active medium. The use of AFM results in an increase of both the laser output energy and overall conversion efficiency by a factor of about 1.4 compared to results achieved with a passive feedback mirror, under equivalent conditions. A reliable, temporally stable SLM regime of operation is achieved in the 740–820 nm spectral range.

Injection seeding in a dual-cavity gain-switched Ti:Sapphire laser

Abstract An experimental investigation of a pulsed tunable injection-seeding system pumped by a Nd:YAG laser is presented. The slave laser (SL) of the system is a gain-switched Ti:Sapphire laser with a dual-channel competitive cavity which consists of a main ring channel and an auxiliary linear one with different lengths. The use of this configuration results in full and reliable suppression of both the perturbing reverse radiation from the SL to the master laser (ML) and the broadband spectral background of the SLs ring channel. This is achieved irrespective of the ratio between the energies of the SL and ML, fluctuations of laser parameters, and spectral detuning. We take advantage of the gain-switched regime of the Ti:Sapphire laser to avoid simultaneous competition between the SLs channels; this allows us to obtain spectrally pure, unidirectional output radiation with high seeding and overall efficiency. The principle of operation of the ring-linear-cavity Ti:Sapphire slave laser provides a natural optical isolation between the SL and ML; this makes such lasers suitable for use as regenerative amplifiers when seeded by single-frequency diode lasers as well as in chirped-pulse-amplification systems to amplify ultrashort laser pulses without the necessity of optical isolators.

Multiply-self-seeded pulsed Ti:Sapphire laser

Multiply-self-seeded single-longitudinal-mode (SLM) pulsed Ti:Sapphire (Ti:S) laser is reported. The compact laser cavity contains a novel multipass selective reflector (MPSR) based on a grazing-incidence diffraction grating. It operates equivalently to a succession of coupled SLM Littman-type cavities. The multiple self-seeding, provided by the MPSR, results in a considerable decrease of both the laser threshold energy and the pulse build-up time as well as an increase of the tuning range, as compared to those of the conventional Littman-configuration Ti:S lasers. Results of the experimental and theoretical investigation of the laser are presented and discussed. Output energy of about 8 mJ at a slope efficiency of 33% and overall efficiency of about 14%, in combination with a high spectral purity of the output radiation, are achieved. Experimental comparison of the MPSR laser with Littman-type SLM configurations with and without nonselective retroreflector is also done.