Giorgos Chourdakis

Analysis of the receiver response for a noncoaxial lidar system with fiber-optic output

The return signal of a noncoaxial lidar system with fiber-optic output is examined. The dependence of the overlap regions and the overlap factor of the system on the fiber diameter is calculated for several inclination angles between the laser beam and the optical receiver axes. The effect of central obstruction is included and both cases of Gaussian and quasi-Gaussian laser beam profiles are treated. The irradiance spatial distribution on the focal plane of the system is calculated and experimentally determined. Finally, an alignment procedure of the lidar system is described based on the comparison between the range-corrected lidar signal and the range-corrected exponentially attenuated Rayleigh backscattered coefficient.

Aerosol Optical Depth Measured at Different Coastal Boundary Layers and Its Links with Synoptic-Scale Features

This paper presents the results of measurements of aerosol optical properties which were made between 2006 and 2008 within the framework of various international projects in different locations such as Spitsbergen, northern Norway and Crete. The investigations were made under different baric topography conditions and in various seasons of the year which facilitated the investigations of spatial and temporal dependencies between upper troposphere mass state and spectral variations of aerosol properties. The results of aerosol optical depth (AOD) measurements showed significant episodes during which jet stream events (300 hPa surface) over the Arctic were present. The mean spectral characteristics of AOD from “before” and “after” the event differ by 0.14 versus the “during” phase of the episode. The macrometeorological relative topography charts shown also the relationships between the 500 hPa, close sea-level pressure SLP (1,000 hPa) charts surfaces and the attenuation caused by aerosol scattering and absorption in vertical profiles during the afternoon hours.

Pulsed HF and Er:YAG laser radiation transmission through sapphire and fluoride glass fibers

A pulsed Er:YAG laser and a preionized pulsed HF laser were used for the evaluation of the transmission properties of sapphire fibers and fluoride glass fibers. Both lasers provided an output of 100 mJ/pulse, with a repetition rate of 1Hz at 3 micrometers wavelength. The pulse duration of the two lasers was 200 ns for the HF and 80 microsecond(s) for the Er:YAG. The sapphire fibers were 0.25 m long with diameters varying from 585 to 850 micrometers . The fluoride fibers were flexible, 1.6m long, with inner diameters varying from 350 to 600 micrometers . The evaluation of the fiber properties include both straight and bend loss characteristics, the influence of the input power to the output power and the permitted misalignment of the fiber in respect to the optical axis.

One-year observations of the vertical structure of Saharan dust over Athens, Greece monitored by NTUA's lidar system in the frame of EARLINET

Suspended aerosol particles play a significant role in the Global Change issue. Every year huge quantities of Saharan dust are transported to the European continent and North Atlantic Sea. Since May 1, 2000, an elastic backscatter lidar station is in operation in Athens, Greece, in the frame of the European LIDAR Network (EARLINET), aiming at the monitoring of extreme aerosol events over the Eastern Mediterranean Sea. During the first year of operation 20 cases of Saharan dust transport to the East Mediterranean Sea (EMS), were successfully observed. The lidar data are correlated with meteorological and satellite observations, including the aerosol index (AI data from the Total Ozone Mapping Spectrometer (TOMS) and the SeaWiFS satellite images. The analysis of the data collected so far, made possible a first statistical approach of the vertical and horizontal extent and of the seasonal variation of free tropospheric Saharan dust layers over the EMS area. The main results of this work are: (1) multiple dust layers of variable thickness (0.5 - 4 km) are systematically observed in the altitude region 2 - 6 km, (2) Saharan dust outbreaks take place all year round, with more pronounced frequencies during the summer period and can persist for a few days (1- 5), (3) 24 - 48 hours are usually needed for the Saharan air masses to reach the EMS area, and finally (4) the TOMS AI values can reach 1.5 - 3.5 over Greece. These lidar data gave the aerosol vertical structure over the Eastern Mediterranean Sea during Saharan dust outbreaks and can be a direct input for global radiative transfer models.

Pulsed 3-μm laser radiation transmission through hollow plastic and hollow glass waveguides

The transmission properties of four different waveguides were investigated: (1) dielectric coated silver hollow glass waveguides, with diameters varying from 0.7 mm to 1 mm, (2) polyamide waveguides, with diameters varying from 0.7 mm to 1 mm, (3) a silica glass waveguide, with a 0.7 mm diameter and (4) a plastic waveguide, with a 0.7 mm diameter. Two different lasers were used for the evaluation of these waveguides: (1) a pulsed Er:YAG laser, emitting at 2.96 micrometers and (2) a corona preionized pulsed HF laser, emitting at 2.78 micrometers . Both straight and bent loss characteristics of the waveguides were studied.

Compact mobile lidar system based on the LabVIEW code: applications in urban air pollution monitoring in Athens, Greece

The LIDAR technique is an efficient tool for continuous monitoring of air pollution over urban areas, with high temporal and range resolution. The urban areas of Athens, Greece, exhibit high air pollution levels, especially those regarding suspended particulates, mainly linked with car traffic and industrial emissions. In this paper, we present the first mobile Greek LIDAR system, based on the LabVIEW code, now located at the Athens Technical University Campus, nearby the urban area of the city. The LIDAR dataset acquired, under various air pollution and meteorological conditions, gives specific indications of the diurnal variation of the backscattering coefficient and relative backscatter of the suspended particulates in the first 2500 - 3000 m ASL over the city of Athens. The LIDAR dataset acquired is analyzed in conjunction with meteorological data (temperature, humidity) and air pollution data (O3 CO, NOx), acquired at the same site, and conclusions are drawn.

Air-pollution monitoring and atmospheric parameter profiling over Athens, Greece, using a compact mobile laser remote sensing system

In this paper we present the basic features of the laser remote sensing (lidar) system located at the National Technical University of Athens (NTUA), Greece. This compact and mobile lidar is in full operation since late 1999. This paper includes basic features of the lidar technique (technical description, temporal and spatial resolution, range etc.), a brief presentation of the instrumentation used and an outline of the algorithms applied to retrieve useful information from the backscattered lidar signals. We also present some examples of aerosol profiling measurements over Athens, which include monitoring of clouds and suspended aerosols (i.e. during Saharan dust and photochemical air pollution events), as well as the evolution of the Planetary Boundary Layer (PBL) over the Greater Athens Area (GAA).

Performance studies of a TEA–TEA HF oscillator–amplifier laser system with semiconductor and corona preionization

Abstract A TEA–TEA HF (SF 6 :C 3 H 8 :He) oscillator–amplifier laser system with one common switching element and one common power supply unit was designed. Two different preionization schemes were employed, one using a corona discharge on a glass plate and another one using semiconductor slabs. For both arrangements the small signal gain coefficient g 0 and the gain value G were determined and the dependence of g 0 and G on the charging voltage and the active gas concentrations was investigated. Measurements of the performance characteristics of the HF laser oscillator (i.e. beam divergence, pulse energy, pulse duration, etc.) are presented and the improvement of the output laser beam profile is discussed in detail via 2D and 3D figures obtained with the help of a beam analyzer. The laser system presented is suitable as a research tool for a number of applications and among them some medical ones (microsurgery, dentistry etc.).

Output beam quality investigation of medical infrared and midinfrared waveguides and fibers

The effect of the mid-infrared laser radiation propagation through waveguides and fibers on the quality of the laser beam is investigated. Fluorocarbon fibers and metallic hollow waveguides with dielectric coating were investigated, using an Er:YAG laser (2.94 micrometers ), an HF laser (2.78 micrometers ) and a CO2 laser (10.6 micrometers ). A comparison was made between the different waveguides and fibers, regarding the quality of the output beam.

Development and in-vitro/in-vivo trials of an endoscopic laser diagnosis/surgery prototype

A laser laboratory prototype for laparoscopic, endoscopic or angioplastic surgery was developed, incorporating an Er:YAG laser as the therapeutic tool in endoscopic/laparoscopic surgery and angioplasty and a N2 laser induced fluorescence system in order to monitor the surgical process. Additionally, a laser radiation transmission system with waveguides and fibers was developed, to transmit the laser radiation, through an endoscope or laparoscope. The system was used in in vivo animal trials, giving promising results for the application of the device in semi-operative applications, in combination with systems of improved endoscopic imaging.