The laser is one of the most fantastic and versatile tools ever invented by men, and its story began long ago with the observation of many natural optical phenomena. However, the true nature of light started to emerge only in the 19^th century, was completely understood at the beginning of the 20^th century and, finally, in 1960 the various basic and technical knowledges coalesced for the realization of the first laser device. Since then the laser field exploded almost exponentially, and thousands of different materials, in the state of solids, liquids, vapors, gases, plasmas, and elementary particles have lased up to now from less than 1 Å to more than 1 mm. Lasers have been applied in various fields with such outstanding results that many practical aspects of human life cannot be fulfilled anymore without them. Moreover, the laser light has started an unprecedented revolution in optics because of its unique properties unrivaled by any other natural and artificial light source. In particular, it contributed to the development of new chemical and physical processes which, besides pervasive applications, have been used also to fabricate new photonic materials, with the notable consequence of advancing and enriching the laser field itself. In general, although a few sectors show signs of maturity, the laser field as a whole looks well and alive, and there will be a new golden age also as a consequence of impressive technological advances.

A general comparative introduction on laser diodes and diode-pumped solid-state laser systems is presented. The main advantages and limits of these devices are reported. Low-to-medium power systems are mainly discussed in this report. With reference to the work carried out in our laboratory, we review on laser action of crystals at wavelengths different from their main laser transitions. In particular, red light generation through intra-cavity second harmonic generation processes is discussed, and the related problems of efficiency, beam quality and thermal effects are pointed out. Finally, as an example of system with great applicative perspectives both in scientific and technological fields, all-solid-state ultrashort pulse laser systems via passive mode-locking techniques are reviewed. The example of their application to the field of nonlinear microscopy is presented in some detail.

The laser dye coumarin-153 has been incorporated in nanocomposite organic/inorganic matrices composed of surfactant/silica or poly(propyleneoxide)/silica. Cetyltrimethylammonium bromide or Triton X-100 are used as surfactant, while polyethers of different chain lengths have been employed. The polyether is covalently bound to silica through urea bridges (Ureasil). The position of the fluorescence emission band and the resulting laser lines depend on the polarity of the nanoscale environment due to solvatochromic effects. Polarity is related with material composition so that the nature of the matrix affects the lasing behavior of the incorporated dye.

Mixed fluoride crystals doped with rare-earth ions are interesting both as active media for lasers and as intermediate spectroscopic models between simple crystals and glasses. This behavior is related to the variety of optical centers that are present in these mixed systems, which also influences the stimulated-emission process. Cubic crystals of yttrofluorite (CaF₂-YF₃-NdF₃) of space group Fm3m belong to this kind of multicenter system. Previous works on neodymium-doped yttrofluorite crystals containing high concentrations of yttrium fluoride and neodymium have shown that Nd³⁺ can generate stimulated emission at two wavelengths: 1054 nm and 1063 nm. In this work, together with the laser dynamics we present for the first time the study of upconversion processes that produce ultraviolet and visible emissions under lasing and nonlasing conditions in yttrofluorite mixed crystals with 15 wt% of YF₃ and 1, 4, 8, and 16 wt% of NdF₃.

In the present work a modification of the well-known z-scan technique is presented. It is based on the direct measurement of the beam dimensions in the far field rather than the transmittance of the irradiance through an aperture. More specifically, the quantity measured in the case of a circular Gaussian incident beam is the beam radius in the far field, while in the more general case of an elliptic Gaussian beam the measured quantities are the lengths of the principal semiaxes of the beam. It is worth emphasizing that the latter case is more interesting in practice since even a very small misalignment of the resonator of an actual laser system can easily induce astigmatism to the beam. The measurements were performed through a CCD camera in connection with a laser beam profiler. The advantages of the proposed modification compared to the classic z-scan technique are the elimination of the sensitivity in beam pointing instability as well as the drastic reduction of the sensitivity in energy fluctuations. Furthermore, the application of the standard z-scan technique in the case of an elliptic Gaussian incident beam is extremely difficult. The technique has been tested using the standard nonlinear optical material CS₂ as a sample.

Free Electron Lasers (FELs) came on the scenario of coherent sources during the late seventies. Since that days the interest in this "new" kind of device has increased very rapidly in the world in spite of the complexity of this systems respect to the conventional Lasers. The main reason for this interest is due to the fact that many are the unique features not present on other sources. First of all the tunability; FELs can be designed, in principle, to operate around any frequency, this means that they can cover the all electromagnetic spectrum. Moreover a wide tunability can be performed around the central frequency acting on the different elements that constitute the device.

We report a review of the developments of the table-top soft X-ray lasers, which have been carried on in the last years utilizing different experimental techniques. A comparison between the performances of the different devices is illustrated.

The linear electro-optic coefficients r₃₃ and r₁₃ of a highly nonlinear organic chromophore were measured via an experimental method based on waveguide sample geometry. The technique used is simple and accurate and the interpretation is straightforward. Viscous solution of chromophore and PMMA were prepared in chlorophorm (CHCl₃) and films were made by drop casting of these solutions onto metallic substrates. A DC poling electric field V_p was applied, in order to polarize the doped molecules and a linearly polarized beam from a He-Ne laser was left to pass through the film. A polarizer placed in front of a photodiode was used to determine the polarization state of the beam, allowing us to measure the magnitude of the induced birefringence Δn. The electro-optic coefficients difference (Δn=r₃₃-r₁₃) was calculated via the induced birefringence. The dependence of the coefficients r₃₃ and r₁₃ on the concentration of chromophore in PMMA was also investigated. By applying an AC electric field onto the film the dynamic electro-optic effects can be studied.

Plasmas produced by capillary discharges have recently received great attention. This could be due to the possible application (soft x-ray laser microscopy and lithography), as well as to the interest in studying the physical phenomena (fusion research, intense sources of continuum and line emission). The soft x-ray lasing has been realized in a gas filled table-top system, utilizing a capillary discharge to excite 46.9 nm transition of Ne-like argon. To obtain lasing at wavelengths below 46 nm in the Ne-like isoelectronic sequence, excitation of higher Z-ions should be used. Highly uniform, high-density metal and dielectric vapor plasmas can be used for the Z-scaling of the Ar IX laser. The technique of the production of ablative plasmas with high-voltage μs discharges utilizing experimental set-up with open-geometry capillaries is discussed. Applications of the developed metal-vapor generator for the Z-scaling of the Ne-like soft x-ray laser, recently realized by our group, is considered.

Solid-state laser sources are required for numerous applications in industry and science. High output power while preserving a diffraction limited beam quality results in high brightness operation. However, conventional laser systems suffer from thermally induced phase distortions in the active medium, which considerably reduce the beam quality. Proper cooling, diode pumping, as well as active media with high quantum efficiency reduce the thermal load. But the still remaining phase distortions result in a reduction of beam quality. Phase cojugate mirrors are suitable to compensate for phase distortions in master oscillator power amplifier systems (MOPA). Stimulated Brillouin scattering (SBS) in commercial multimode silica fibers leads to reliable and stable phase conjugation. A further advantage is, that the fiber core diameter can be select in respect of the systems pulse energy to obtain the necessary intensity for high reflectivity SBS. Three systems with phase conjugators have been investigated. A pulse pumped, passively q-switched Nd:YAP System which delivers an average output power of 315 W with M² = 2.6. The pulse energy is about 160 mJ with a pulse width of 140 ns at 2 kHz repetition rate. Another pulse pumped MOPA system based on Nd:YAG with depolarization compensation delivers an average output power up to 124 W with M² = 2.2. Due to active q-switching the pulse repetition rate and peak power of this system are variable in a wide range. Furthermore a continuously pumped amplifier arrangement with nearly diffraction limited output of 120 W average power has been achieved at 10 kHz repetition rate.

A novel Mach-Zehnder interferometer terminated at two different frequencies which realizes for a single photon quantum state (qubit) the nonlinear frequency conversion has been realized. The information-preserving character of the nonlinear process allows to transfer the coherence of the input to the output state. The results of this experiment can have relevant applications in quantum information technology.

Potassium 4S_1/2 - 6S_1/2 two-photon excitation initiates the emission of several internally generated photons. For the first time two emission lines one close and one below the potassium 4P_3/2 level are reported. Radiation emitted below the 4P_3/2 - 4S_1/2 level is due to a Parametric Four Wave Mixing (PFWM) process initiated by the internally generated radiation from 5P_3/2 - 4S_1/2 level. The line emitted close to the 4P_3/2 level is produced by a two-step Four Wave Mixing (FWM) process. Direct one photon driving of 4S_3/2 - 4P_3/2 transition produces similar to HEP radiation.

We report internal laser cooling in Yb³⁺ doped KPb₂Cl₅ crystal. From the quantum efficiency values measured in the heating and cooling regions by using the photothermal deflection technique, we have obtained a room temperature cooling efficiency of 0.2% in a sample doped with about 5 x 10¹⁹ ions/cm³. Excitation spectra obtained under high irradiation fluences show an excess of fluorescence with regard to those obtained at low fluences, which agrees with the prediction of a model based on the photon-ion-phonon interaction.

Experimental results are presented related to the wave mixing in K. The influence of the medium number density, the incident radiation intensity and the detuning from resonance on the spectral, spatial and temporal characteristics of the radiation emitted near the 5P-4S transition is investigated for the 4S_1/2 - 6S_1/2 two-photon excitation. The different processes involved and the quantum interference is examined.

Line-splitting phenomena, occurring upon strong saturation conditions at high laser power, were investigated on DFWM lineshape in iron atomic vapors. Measurements have been peformed monitoring the a⁵D₂-y⁵D⁰₂ atomic transitions. A mechanism to explain the physical nature of the splitting effect is proposed and discussed.

In this paper we review the general problem of Optical Beam Quality for beams that are not simple axial symmetric gaussian beams. We discuss the theoretical studies and experimental results obtained on the beams emitted by compact, medium power, diffusion cooled CO₂ lasers and laser arrays. Our case-studies include a linear array of 9 CO₂ lasers and an annular laser aperture with array-like emission produced by a Talbot cavity. We show that aspects of beam propagation that were founded on Gaussian beam theory need to be re-evaluated when considering these more complex beams. For example different polarization states in array elements lead to different field distributions in the far field and thus to a different beam quality. Furthermore we comment on the limited information given for such structured beams by traditional Beam Quality Factors, such as M², established for nearly-Gaussian beams.

We present an optical system which simultaneously makes homogeneous the spatial energy distribution and varies the spot size of any light beam, including beams having asymmetric intensity spikes, without replacement of lenses and without changing the total length of the optical system. This homogenizer technology (patent pending) includes a software to design the optimum optical system to achieve the wished output beam performance, as well as to know how the beam shape changes when changing the position of each optical element. In this paper, we illustrate the principle of the new homogenizer, the design software performance and the experimental results achieved applying our technology to a large volume excimer laser.

New and original monocrystalline samples having a continuous longitudinal concentration gradient are used to study dynamical processes of resonant transitions in rare earth doped laser crystals like Y₂O₃ sesquioxide and YAG garnet. This fast and simple method allows to measure the radiative lifetime, the influence of radiation trapping, impurity quenching and RE pairs or clusters on the excited-state lifetime as a functon of the dopant concentration. Examples in Yb³⁺, Er³⁺ and Ho³⁺ rare earth ions are presented.

Diode pumping of a stress-free, Nd(5%at.):KGd(WO4)2, grown by top nucleated floating crystal (TNFC) method, is presented. The diode-pumped laser was operated both in the free-running and passively Q-switched operating modes. Under optimized conditions of resonator and optics, the disk (1.3 mm in thickness), produced at room temperature an efficient free-running, TEM₀₀ output with maximum power of 0.4 W, with 75% slope efficiency and 51% total laser efficiency. The results presented here indicate the potential Nd:KGd(WO4)2 crystals grown by TNFC growth method, as candidates for concentrated, stress free diode pumped microlasers in a large variety of wavelengths, including the eye-safe range.

In this work the optical properties of Yb:YAl₃(BO₃)₄ have been studied for a concentration of 2 at%. The low temperature polarized absorption and emission spectra, together with site selective spectroscopy, show the presence of different Yb³⁺ sites in the YAB crystal host.

We consider a gas of two-level atoms of two species (for example, natural Rb vapor consisting of two isotopes) in the field of laser radiation tuned close to resonance with the certain transition of a specific iostope. This radiation may turn to be rather close to a near lying transition of other isotope (within the linewidth of laser radiation), so it is necessary to take into account the possibility of excitation of other isotope in order to evaluate the enrichment degree if it is aimed at the isotope separation by means of selective excitation with subsequent chemcial reaction (in our example the rubidium hydride formation) of the excited isotope which is believed to take place at a rate much higher than that in the ground state of the atom.

Lithium fluoride (LiF) films irradiated by low energy electrons were employed as active spacers in all-solid, dielectric optical microcavities emitting in the visible spectral range. We present the results of optical characterization of the spontaneous emission from F₂ color centers embedded in a LiF layer confined inside a planar microcavity. These structures seem promising for the realization of novel kinds of solid-state miniaturized emitting devices.

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).

New developments are reported on the over one century old topic of collisional energy transfer in a nitrogen afterglow with active oxygen species without a metal catalyst and even at much higher pressures than those in previous experiments. This report is centered mainly on the changes in the normal distribution of intensities of various electronic-vibrational transitions of the first positive band system (1pbs) of molecular nitrogen, (formula available in paper) in the visible and the IR part of the spectrum, some of which are reported for the first time. A study of the variation of the concentrations of active oxygen species is also reported, in order to establish the mechanism of the energy transfer that is responsible for the enhanced emissions of the 1 pbs of N₂. Also the present experimental results show that, contrary to previous research over the last more than 40 years, the observed enhancement of emissions of the N₂ 1pbs results from homogeneous collisional intersystem excitation transfer into (formula available in paper) induced by excited molecular oxygen. Probing of this enhancement of emissions for lasing is suggested. Possible implications of this phenomenon are commented, some of which even suggest reinterpretation of energy transfer experiments between some activated nitrogen and oxygen species.

Laser beams can be used in many industrial applications including machining whereby it constitutes an alternative to traditional material removal techniques and can be used to process a variety of materials including metals, ceramics, glass, plastics, and composites. Laser machining is characterized by a number of advantages such as absence of tool wear, tool breakage, chatter, machine deflection and mechanically induced material damage, phenomena which are usually associated with traditional machining processes. However, as with all manufacturing processes, optimum operating parameters have to be determined. These include the laser power, the spot size of the focused laser beam, etc. In order to arrive at a set of optimum setting variables, either experiments can be conducted and optimum conditions can be found based on the results, or a process model can be created and evaluated for the selection of setting variables, which will optimize the desired performance measures. Analytical and numerical modeling have contributed to the understanding of laser processing, but there are still many questions to be answered. This paper attempts to give an overview of laser machining modeling and experimentation techniques, elucidating recent developments and research trends.

Specimens of 2024 Al alloy (Al-4,4%Cu-1,5%Mg-0,6%Mn) were annealed at 400°C for two (2) hrs. After annealing, the alloy specimens were surface treated with a high power KrF Excimer Laser. The wavelength of laser radiation was 248 nm and the time duration of each pulse was 30 ns. The surface treatment of 2024 Al alloy was conducted by changing the value of power of the incident laser radiation. The radiation frequency was 7Hz, the overlapping was 50% and the pulses per step (pps) were 50. The changes in the power of incident laser radiation led the surface layers of the alloy to undertake various modifications. These processes were carefully studied by observing the surface modifications of the alloy with an Optical Microscope, a Scanning Electron Microscope, a X-Rays Diffractometer and a Surface Profilometer.

Molecular two-photon absorption (TPA) has gained great interest over recent years owning to each application in various fields, including spectroscopy, microscopy, 3-D optical data storage, optical power limitation and microfabrication. The requirement for organic molecules with large TPA cross-sections is therefore essential. In the present work the two-photon absorption properties of a series of pyrylium-based chromophores were investigated. The molecules of this series were synthesized by systematically changing the chemical structure of a specific substituent of an initial, strong two-photon absorbing, molecule. Very large TPA cross-sections (1800 GM) and high quantum yields were achieved in the visible red and NIR region of the spectrum. The relation between the chemical structure of the molecules and their TPA performance was discussed. Furthermore, the photobleaching efficiency of these molecules doped in polymer matrices was investigated. Parallel successive patterns were recorded in a polymer matrix via photobleaching. The accuracy of recorded patterns reveals the great potential of these chromophores as memory materials.

The concept of equilibrium has a fundamental importance in thermodynamics and is related to the reversibility of any chemical and physical process involving a temperature cycle or a phase change. It is of major interest also for the models of high energy beam processing; a recent ablation model has shown that the non-equilibrium character of laser irradiation with pulse lengths τ of the order of some ns is essential to understand the mechanisms of thermal sputtering, in particular when overheating of a molten layer of metal surface is allowed to occur. It is known in fact that if the heating or surface recession rates overcome the nucleation rate of vapor bubbles, then the surface temperature T_s of the molten layer can increase above the thermodynamic boiling point T_b. This situation entails by consequence the rising of a temperature gradient within the boiling layer, whose thickness λ is controlled by the condition T ≥ T_b.

It is shown that a two-photon dissociation of HgBr₂ molecular vapor can be used as a non-linear fast opening passive mode locking technique for excimer laser. Pulses below our resolution limit of 400 ps and with a modulation better than 85% are reported.

The most known feature of polytetrafluoroethylene (PTFE) is its adhesion behavior: it is hydrophobic and oleophobic at the same time. This can cause serious problems and obstacles during the surface treatment and fixing of PTFE objects. During our experiments Teflon films were irradiated by an ArF excimer laser beam in presence of liquid photoreagents containing amine groups (aminoethanol, 1,2-diaminoethane, triethylene-tetramine). In consequence of the treatment the adhesion of the modified surfaces significantly increased, the samples could be glued and moistened. The adhesion strength of the glued surfaces was measured in the function of the applied laser fluence. The adhesion strength increased drastically between 0 - 1 mJ/cm² and showed saturation above 1 mJ/cm² at approximately 5 - 9 MPa values depending on the applied photoreagents. On the basis of our experiments it was found that the treatment with triethylene-tetramine was the most effective. The surface chemical modifications of the treated Teflon samples can be due to the incorporation of amine groups into the surface layer.

Despite a lot of researches on Pulsed Laser Deposition (PLD) for film production over the years, it has largely been an unsuccessful technology because of the poor quality of laser deposited films, generally due to particle contamination. A way of improving the quality of thin films was recently developed at LPC by using MHz-repetition-rate IR laser pulses of ps-range pulse duration. As a result, Ultrafast PLD outperforms other methods in terms of surface quality, deposition rate and energy efficiency by up to three orders of magnitude. In addition, it is desirable to use short laser wavelength to more easily atomize the materials and also favor the material ablation through photochemical processes during the laser-matter interaction. An optical two-crystal tripling system has then been developed to produce high energy UV photons from a mode locked Nd:YVO₄ laser (1.064 μm, 5.7 MHz, 12 ps, up to 1.5 W). A large number of UV photons can be obtained with this optical scheme providing optimum laser characteristics for efficient PLD. This is further applied to the synthesis of various high quality thin films, including polymer films and nonlinear optical films for photonic applications.

The micromachining process of transparent materials by laser induced backside wet etching (LIBWE) was investigated. Fused silica targets were irradiated by an ArF excimer laser at 2.14 J/cm² fluence and naphthalene solved in methyl-methacrylate with different concentrations were used as absorbing liquid. The absorption coefficient of thse solutions was measured by a plano-concave microcuvette and it found to be between 39426 and 62350 1/cm depending on the concentration of napthalene. It was demonstrated that the etch rate depends on the absorption coefficient linearly, while the roughness does not. The dependence of the etch rate can be explained as follows. The absorbed energy in the interface of the solution and the fused silica increases when increasing the absorption coefficient resulting in higher temperature liquid layer at the surface of the fused silica causing higher etch rate.

In comparison to standard fibers, fluorescent plastic optical fibers have quite different absorption and emission spectra, especially in the UV-A region up to the NIR-region depending on the co-dopant. These properties can be used for new applications in the field of sensors. In addition to a brief description of the mechanism of light-interaction within the dye-doped core material, an overview is given of the most promising applications.

Polymer Optical Fibers (POF) are the most promising solution for the "last 100 m" ion data communication. The combine the inherent benefits of all optical fibers such as high bandwidth, total electromagnetic immunity with additionally amazing simplicity in handling. This paper will show the evolution of POF for datacom applications, starting from standard step index types via multi-step-index- or multi-core-types to recently published micro-structured POF.

A special device, which monitors the traffic in active optical fibers, is presented. The system can be adjusted to indicate the transmission of light above a certain threshold depending on the requirement of the fiber link. The system is based on low cost commercially available opto-electronic and passive fiber optic components for the 1310 and 1550 nm wavelength windows.

The term of 3D laser processing has been used so far to describe a group of different three-dimensional laser processing concepts. At each of these concepts the 3D aspect refers to a different manipulation of one or more laser beams, as to process and/or produce three-dimensional geometries by performing material removal, welding or heat treating. The most important concepts are focused mainly in laser machining and laser welding processes by incorporating one or two laser beams simultaneously. This paper overviews a number of these concepts that have been developed in research or industrial level, along with their advantages, drawbacks and fields of application.

Having shown that intra-cavity Er³⁺ absorption can stabilized one frequency out of the many frequencies emitted by a broad band F.P. laser, we shall see that the wavelength can be determined in an absolute way for InGaAsP F.P. lasers with somewhat different gain curves and mode spacing still with side modes rejection ratios as large as 30 - 40 dB. The gain curve of a multimode Fabry Perot (F.P.) laser is shaped with intra-cavity Er³⁺-doped single crystal absorption so achieving single mode selection. Absolute wavelengths are predetermined by combinations of crystals of various thicknesses. Finally tentative selection by Er³⁺-doped amorphous materials such as glass or sol-gel layers are presented indicating new roads for low cost laser at absolute frequency.

Atmospheric absorption and scattering phenomena of optical waves might be used for verify the type, the concentration, the distance and the thickness of the absorbed/scattered gases/pollutants. The active detection techniques of atmospheric gases/pollutants with a laser, briefly introduced in our project, are still the most modern and reliable methods. With those techniques may be located and measured targets in a molecular or bigger size to achieve an accurate remote sensing of the atmospheric gases/pollutants. For the photo-receivers of these detectors we propose a new special optical input converter that corresponds the intensity of the incident optical radiation to the frequency of an electronic pulsed signal, and then this signal manipulation might become with a common receiver of radio-electric signals.

Surface chlorophyll-a concentrations and other biogeochemical parameters of seawater were measured by using a mobile lidar apparatus in the Italy - New Zealand and New Zealand - Italy transects along the Mediterranean and Red Sea, Indian and Pacific Oceans. Experiments were carried out during the oceanographic campaign by means of the ENEA lidar fluorosensor aboard the research vessel Italica (November - December 2001; March - April 2002). The lidar measurements were compared to in situ data collected by other groups and to remote imagery acquired by the Sea-viewing Wide Field-of-view Sensor.

A high resolution Amplitude Modulation Laser Radar (AM-LR) sensor has recently been developed, aimed at accurately reconstructing 3D digital models of real targets -- either single objects or complex scenes. The sensor sounding beam can be swept linearly across the object or circularly around it, by placing the object on a controlled rotation platform. Both intensity and phase shift of the back-scattered light are then collected and processed, providing respectively a shade-free photographic-like picture and accurate range data in the form of a range or depth image, with resolution depending mainly on the laser modulation frequency. Starting from the sample points, with an uncertainty that can be made as small as 100 μm, the complete object surface can be reconstructed by using specifically developed software tools. The system has been successfully applied to scan different types of real surfaces (stone, wood, bones) and is expected to have significant applications in industrial machining, artwork cataloguing and medical diagnostics. Examples of 3D reconstructions are presented and the relevance of this technology for reverse engineering applied to artwork restoration and conservation is briefly discussed.

Light emitting diodes (LED) are usually the second choice optoelectronic devices for optical communications with moderate bandwidth efficiency and bit rates, compared to semiconductor lasers. Laser applications dominate the long distance communication networks. However considering the cost efficiency, LED seems to be a very good solution, especially in short distance communication networks. From the LED's response in applied electrical signals, obviously there is a limit in maximum transmitted bit rate, posing an upper boundary in the available amount of information transmitted by LED. The idea of multiple access technique using Digital Pulse Width Modulation (DPWM) is needed in order to cure the weakness of LED's bit rate efficiency. Combining multiple access with source coding allows the transmission of 4 Mb/s using normal 2 Mb/s PCM line over short distance communication links.

The communication of humans with their surrouding is achieved through their senses and the related organs. Visual communication using the eyes is made possible because the various sources of light, natural i.e. the sun or the lightning, or artificial such as Lasers, emit electromagnetic radiation which is either reflected or scattered by surfaces. This radiation received by eyes is processed in the brain where the images of the environment are developed. The luminous processing can be either macro- or microscopic. The macroscopic processing is the result of light coming from the sun or from wide range lamps, while the microscopic results from light coming from wide range lamps, mercury lamps, lasers or electron beam. The microscopic processing is the subject we are dealing with in this presentation.

Laser welding is used when it is essential to limit the size of the heat-affected zone (HAZ), to reduce the roughness of the welded surface and to eliminate mechanical effects. Solid-state lasers operating in a continuous or pulsed mode can function as welding sources. Present-day lasers can provide vey high levels of power per unit area. The application of solid-state Nd-doped lasers -- such as neodymium-doped yttrium-aluminum-garnet (Nd:YAG) lasers -- in industry verifies the fact that they are reliable, safe to operate, and simple to control. They can emit power in a pulse as high as 10⁷ W or more and can process materials at an extremely high rate. Lasers with high peak power pulses are capable of better material procesing than their average power rating might indicate. High peak power overcomes the thermal diffusivity and reflectivity of precious metals, copper, and aluminum. They can also weld large volumes with a single pulse. In general, increasing lasing efficiency and power is a prerequisite for increased quality and capacity in laser material processing. Nd:YAG laser welding covers a large variety of techniques capable of producing welds in various metals, ranging from a few micrometers to tens of millimeters in thickness. This paper attempts to give an overview of recent developemnts and research trends.

The ENEA lidar ground-based station of Brasimone has been recently upgraded in order to perform resonance atmospheric Na measurements. A high resolutin dye laser was inserted in the existing transmitter optical line, and a Na vapor cell (MOF) magneto optical filter) was integrated in the receiver in order to scan the dye laser over the Na fine structure doublet. A complete set of laboratory measurements have been performed, before sending the optical beam through the open atmosphere, in order to characterize the MOF (Na vapor, temperature and magnetic field intensity) and to determine the experimental settings that will be used in the final system configuration.

The influence of the experimental parameters on the determination of the optical nonlinearities of materials by the z-scan technique is investigated. The experimental parameters considered in this study are: the sample's length, the self-focusing, the cell's lensing and/or wedging, the laser repetition rate, the temporal and spatial profile of the laser beam and the multiple internal reflections. As shown, these parameters can seriously affect the macroscopic nonlinear optical response of a medium.

The lowest thickness of contiguous polytetrafluoroethylene (PTFE) thin films prepared by pulsed laser deposition is influenced by the size of the particulates emitted from the target. Since the particulates reach the substrate at random, by the time a contiguous layer is formed its average thickness can be several times as much as the mean size of the particulates. During our experiments the size distribution of the particulates emitted during ArF excimer laser deposition from pressed PTFE pellets was studied as the function of the applied ablating fluence in the range of 1.1 - 6.2 J/cm². The size distribution of the particulates could be described with a first order exponential decay function. The value of the decay constant varied between 3.94 and 6.15 depending on the laser fluence. With the knowledge of the size distribution of the depositing particulates a theoretical model was used for simulating the growth of the thin film. The minimum number of the pulses required to obtain a contiguous layer and its thickness could be estimated. The thinnest layer could be obtained at the lowest investigated fluence.

In this work, we have measured the second order hyperpolarizability of some organic materials with (EFISH) method and also calculated the second order hyperpolarizability of 13 organic compound with Mopac6 software and investigated the different factors that affect the amount of second order hyperpolarizability and ways to increase it.

A series of pyrylium dyes is studied concerning their Non-Linear Absorption (NLA) in order to assess their applicability as optical limiters. The research aims at combining the selective material preparation through the systematic change of the molecules' substituents and the linear and nonlinear optical characterization. The open z-scan technique at various excitation intensities was used for the nonlinear absorption characterization. With this method, the third and fifth order contributions to the susceptibility can be separated and quantified. Femtosecond pulses at 760, 790 and 840 nm were used to induce the NLA. All wavelengths lay in the two-photon absorption spectral region of the dyes. In most of the cases studied an efficient Excited State Absorption (ESA), fifth order nonlinearity, following Two-Photon Absorption (TPA), third order nonlinearity, was observed above a threshold of the excitation intensity (I_thr). This was indicated through an increase of the value of the NLA coefficient above I_thr. If TPA was the only nonlinear mechanism, the NLA coefficient would be intensity independent. The third and fifth order NLA coefficients as well as the ESA cross section values were extracted. High values of the ESA cross sections of the order of 10^-15 cm² were calculated indicating that the pyrylium dyes are potential candidates in optical limiting applications.

New analytical expressions have been found for the mode volume, the output beam waist and radius of curvature of the Generalized Self-Filtering Unstable Resonator (GSFUR), in terms of the cavity parameters. This permits an easy comparison with the confocal SFUR, in order to choose the most appropriate resonator for a given active medium. Contrary to the general feeling, the SFUR choice results the best in term of mode volume for low (<14) absolute magnification values.

A laser based photoacoustic system has been designed and built at ENEA in order to monitor bio-gases produced in different processes involving living cells. Experiments aimed to reveal traces of ethylene emitted from tomato plantlets and fruits under temperature stress are here presented and discussed.

The efficient coloration of LiF material, in the form of bulk and films, by EUV and soft X-rays emitted by a laser-plasma source is demonstrated. The short penetration depth of soft-X-rays is exploited to obtain high spatial resolution luminescent patterns while the high dynamic range of proportionality between X-ray dose and coloration is exploited for using LiF as image detector in micro-radiography and soft X-ray microscopy applications.

Contemporary methods for minimally invasive interventions are gaining wide acceptance in various everyday operations, offering extremely localized treatment, reduced suffering and practically no risk for the patient as well as great benefits to diagnostic examinations and surgeries that require continuous monitoring. Many established endoscope systems offer the aforementioned advantages without the risks and restrictions of the computer-aided tomography techniques but with limitations in the resolution and chromatic representation provided. A microscanning specific-endoscope device has been developed aiming to provide superior resolution and chromatic representation in comparison with the above endoscopes. The key technology employed in the design of this endoscope relies on the use of tiny microelectro-mechanical silicon mirrors for the scanning of three laser beams over the target tissue area. The so-developed microscanning endoscope system provides color imaging with high resolution at near video rates targeting at macroendoscopy applications. The optical design and implementation of this endoscope system will be presented in this communication together with a brief description of the overall endoscope device developed. In addition results are given from the study of the metamery effect that is utilized in the realized endoscope, together with a presentation of the procedure followed for the objective evaluation of its optical performance and first results from system operation.

The effect of the 2.94 μm Er:YAG laser radiation propagation through sapphire fibers with diameters varying from 250 μm to 550 μm, on the quality of the laser beam is investigated. A comparison was made between the fibers performance in free-running and Q-switched Er:YAG laser radiation.

Experimental results are presented concerning the electronic nonlinear optical response of C₆₀ and higher fullerenes, i.e. C₇₀, C₇₆, C₈₂, C₈₄ and C₉₂ under 100 fsec, 800 nm laser excitation. The Optical Kerr Effect technique is employed for the determination of the magnitude of the third order nonlinear susceptibility.

A new blue emitting conjugated polymer is studied for Amplified Spontaneous Emission (ASE) and laser action in liquid solutions and solid matrices. Both processes were observed simultaneously when the liquid polymeric sample is placed into a resonator consisting of a mirror and a grating. This observation permits the distinguishing between the main differences of ASE and laser action such as tunability and coherence. The stimulated emission efficiency can be strongly affected by the presence of polymer chain aggregates into the sample. In order to study this effect mixtures of the polymer with good and poor liquid solvents were prepared and the ASE action was examined. The polymer molecules tend to aggregate when they approach the poor solvent molecules. Thus, by varying the poor solvent quantity into the liquid samples, the aggregates concentration can be systematically controlled. It is shown that the ASE behavior exhibits a rapid suppress with the aggregates concentration. Performing spectroscopic and stimulated emission cross section measurements in isolated and aggregated polymer solutions, it is proved that in the latter media ASE is caused by the intrachain excitons while the spontaneous emission is caused by the aggregates. The suppress of the ASE is due to the increase of aggregates at the expense of isolated chains.