Mirrors with a graded reflectance profile have been used for many years in unstable cavities for improving the optical quality of the laser output beams. All the variable reflectivity mirrors are realized with multilayer-coatings containing one or more profiled layers inside the stack. They generally exhibit high reflectance in the central area and very low reflectance in the external area. In particular, phase-unifying (PU) mirrors are graded mirrors properly designed in order to obtain a low wave-front distortion in the transmitted laser beam. In this paper, the laser damage resistance properties of a PU mirror designed for XeF excimer lasers (351 nm) have been studied. The laser-induced damage threshold has been measured by a XeF laser on the high and low reflectivity areas. A correlation between the damage threshold values and the standing wave electric field profile, which settles inside the two coating structures during laser irradiation, has been found.
This paper is a work-in progress report on the development of sol-gel coatings for high power laser systems in the near-UV, infrared region. Silica, titania and titania-silica acid catalysed sols were prepared by using tetraethoxysilane and titanium isoproxide as precursors. Single and multi-layer coatings were generated by dipping on fused silica substrates. The single films were heated at 500°C and 900°C after deposition in order to investigate the role of the sintering temperature either on the optical properties and on the film laser-induced damage threshold at 1064 nm (Nd:YAG c.w. laser) and 351 nm (XeF excimer laser). The ageing effects due to the exposure to humidity was investigated by testing the damp heat resistance of the coatings in agreement with the ISO environmental test for optical coatings. The silica coatings have been assessed before and after the damp heat test with regard to their laser-induced-damage resistance, reflectance and transmittance properties. The optical parameters (refractive index and extinction coefficient) have been determined by UV-VIS-NIR spectrometry. A global fit procedure based on the simultaneous characterisation of several samples was used for the evaluation of the optical properties of the materials both as single films and inside multi-layer stacks.
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.
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.
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 laser induced damage in optical coatings plays a limiting role in the laser reliability. Either the laser cavity mirrors and the optical components employed outside the cavity are often damaged by the laser radiation, delivered by high power lasers. Their resistance becomes more critical at shorter wavelengths, therefore the research is focused on the ultraviolet region where excimer lasers are finding many applications. Laser damage studies were performed at 248 nm (KrF laser) and 308 nm (XeCl laser) on MgF2, HfO2 and SiO2 films and multilayer coatings. Results about the dependence of the damage threshold on both the deposition technique and standing-wave electric field profile inside the coating, are reported.
The aim of this work is to investigate the influence of the standing-wave electric field profile on the laser damage resistance of HfO2 thin films. To this end, HfO2 thin films of different optical thickness and deposited by the electron beam evaporation technique at the same deposition conditions have been analyzed. Laser damage thresholds of the samples have been measured at 308 nm (XeCl laser) by the photoacoustic beam deflection technique and microscopic inspections. The dependence of the laser damage threshold on the standing-wave electric field pattern has been analyzed.
The ion beam assistance during the film growth is one of the most useful method to obtain dense film along with improved optical and structural properties. Afnia material is widely used in optical coating operating in the UV region of the spectrum and its optical properties depend on the production method and the physical parameters of the species involved in the deposition process. In this work afnia thin films were evaporated by an e-gun and assisted during the growth process. The deposition parameters, ion beam energy, density of ions impinging on the growing film and the number of arrival atoms from the crucible, have been related to the optical and structural properties of the film itself. The absorption coefficient and the refractive index were measured by spectrophotometric technique while the microstructure has been studied by means of x-ray diffraction. A strictly correlation between the grain size, the optical properties and the laser damage threshold measurements at 248 nm was found for the samples deposited at different deposition parameters.
The photoacoustic beam deflection technique at (lambda) equals 248 nm was employed for the measurement of the laser damage threshold of single films of magnesium fluoride deposited on superpolished calcium fluoride and fused silica substrates. Different samples were investigated. All films have similar thickness but are deposited by various techniques in different laboratories. The samples are fully characterized, both from the optical and the structural point of view, in the framework of a European Project on 'UV-coatings'. The results of these measurements are reported, along with the data on laser damage threshold, to find out which are the most significant parameters for laser applications.
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.
SiO2 and HfO2 thin films have been deposited on polished fused silica substrates by the ion assisted electron beam evaporation technique in different deposition conditions. The role of the assisting ion beam parameters either on the otpical and structural film properties and the film damage threshold at 308 nm has been investigated. Laser damage thresholds have been determined by the photoacoustic mirage technique.