Serge V. Garnov

Self-focusing and optical damage in Cr:LiSAF and Cr:LiCAF

The Cr-doped Colquiriites, Cr:LiSAF, and Cr:LiCAF, are new solid-state laser materials with high gain cross-sections, very large gain bandwidths, and relatively long fluorescence lifetimes (approximately 100 microsecond(s) ) making them amenable to flashlamp pumping. These laser materials are capable of providing tunable high power laser radiation in the near IR and are very suitable for ultrashort pulse generation and amplification. For these high power laser applications, however, a knowledge of the optical damage thresholds and mechanisms in these materials is essential. We present results of measurements of bulk damage thresholds, of basic material properties, and of the nonlinear refractive indices of these materials. We have also observed optical damage due to self-focusing in a femtosecond chirped pulse Cr:LiSAF regenerative amplifier. We discuss these observations and give an estimate of the critical power of self-focusing for these materials.

Pulse-width dependence of laser damage in optical materials: critical analysis of available data and recent results for nanopicosecond region

A critical analysis of available theoretical and experimental data for a pulse-width dependence of laser induced damage thresholds (LIDT) in various optical materials is given along with new results of our recent measurements obtained in nano-picosecond range at 1064 nm wavelength for alkali halide crystals (NaCL, KCl, KBr). Three YAG:Nd laser oscillator- amplifier systems operating in Q-switch and mode-locking modes provided highly stable light pulses at 2 ns, 15 ns, and 50 ps with Gaussian spatial profile of beams were used in the experiments. Special attention was paid to the comparability of the LIDT test conditions for various pulse widths, implying the carefully controlled similarity of beam intensity spatial distributions and temporal profiles. The experimental data are analyzed on the basis of theoretical predictions for different damage mechanisms. It is concluded that pulse-width scaling of LIDT is still a problem, and an adequate approach to its solution is formulated.

Ultrafast laser-induced plasma diagnostics with time-spatial-resolved shadow and interferometric techniques

The experimental studies of laser-induced plasma accompanying the laser ablation of material have been conducted with the developed shadow-interferometric technique. High intensity single picosecond pulses of YAP:Nd laser were applied to ablate tested samples and time-delayed probing pulses of second harmonic illuminating the interaction area were used to make snap-shots of the expanded plasma plume. Both shadow and interferometric images of hot plasma were captured simultaneously with a CCD camera providing approximately equals 1.5 micron spatial and approximately equals 10 ps temporal resolution of the investigated processes. By varying the intensity of ablating pulses and the time-delay of probing pulses it was possible to study a highly inhomogeneous small-scaled plasma density and refractive index distribution within the plume. The longer time-delays allowed study of laser-initiated shock wave expansion in the surrounding atmosphere. A special attention was paid to the plasma formation arising at a through-hole laser drilling process was observed. In particular, it was shown that the cluster explosion can efficiently block the laser radiation resulting in decreasing the ablation rate. A computer modeling of optical visualization of small-scale plasma objects has been conducted. The analysis si of the experimental and numerical results has revealed a number of characteristic features of plasma images that should be taken into account at the qualitative and quantitative evaluations of the plasma parameters.

Ultrashort-pulse generation in solid state dye lasers

Generation of ultrashort pulses (picosecond range duration) in pyrromethene-580 (PM-580) and xanthene 11B dyes doped modified PMMA polymer lasers is reported. Active polymer elements were pumped by second harmonic radiation of the 50 picosecond Nd:YAG laser. A single-pulse and train-pulse laser generation was observed at different conditions (variable optical resonator length and pump energy) with a pulsewidth ranging from 140 ps to 30 ps. The train-pulse generation is attributed to mode-locking of a laser resonator. The lifetime (tau) e, of an excited quantum state of a laser transition of the PM- 580 dye molecules has been estimated ((tau) e equals 200 divided by 250 ps) from the pulse-train duration.

Laser damage of HR, AR-coatings, monolayers, and bare surfaces at 1064 nm

Laser induced damage thresholds and morphologies have been investigated in a variety of uncoated and coated surfaces including monolayers and multi-layers of different chemical compositions. Both antireflective (AR) and highly reflective (HR) were tested. Testing was done at 1064 nm with 25 picosecond and 8 nanosecond YAG:Nd laser single pulses. Spot diameter in the experiments varied from 0.09 to 0.22 mm. Laser damage measurement procedure consisted of 1-on-1 (single laser pulse in the selected site) and N-on-1 experiments including repeated irradiation by pulses of the same fluence and subsequently raised from pulse to pulse fluence until damage occurred. The highest picosecond damage thresholds of commercially available coatings averaged 12 - 14 J/cm2, 50% less than thresholds obtained in bare fused silica. Some coatings and bare surfaces revealed a palpable preconditioning effect (an increase in threshold of 1.2 to 1.8 times). Picosecond and nanosecond date were compared to draw conclusions about pulsewidth dependence. An attempt was made to classify damage morphologies according to the type of coating, class of irradiating, and damage level.

Comparison of laser-induced damage of optical crystals from the USA and USSR

In this paper we report the results of comparative measurements of laser-induced damage (LID) in optical material from the USA and the USSR. Bulk LID thresholds were measured in very pure alkali halide crystals at 1064 nm and 532 nm and surface LID thresholds in selected nonlinear optical crystals were measured at 10.6 microns. The results show that the LID thresholds of the USSR and the USA samples are approximately the same except for KC1 where the USA samples have a factor of 2 higher LID threshold electric field then the Soviet sample.

Laser-induced damage of Cr:LiSAF crystal at 0.85 μm

An experimental investigation of laser-induced damage of the Cr:LiSAF crystal under pulses from a Cr:LiSAF laser has been made. For comparison, the same measurements were also made with borosilicate glasses BK7 and K8. The experiments were performed with spot sizes from 0.7 to 50 micrometers . The morphology of the damage area, the spot size dependence, the statistics of the damage thresholds, and the truncation of the transmitted pulses were measured. It was found that for spot sizes of more 10 micrometers , the damage of LiSAF resulted from the presence of local bulk inhomogeneities. For smaller spot sizes the damage resulted from self-focusing. Finally, for spot sizes of less 1 micrometers , the damage resulted form the intrinsic laser-induced damage and its threshold was significantly higher than the threshold for BK7 glass. As a result the spot size dependence for Cr:SAF was found to be stronger than for borosilicate glass.

Energy coupling to materials ablated by intensive pulsed 1-μm radiation: plasma shielding and surface-modification effects

Optical transmission and reflectivity of ceramics and CVD diamond plates prior, in the process and after ablation by intensive nano- and picosecond pulses of Nd:YAG and Nd:YAP lasers are investigated. Laser induced surface layer modification effects and plasma influence are considered. Correlation between crater depth and plasma optical properties was found. The mechanisms of crater walls influence on plasma are discussed.

Photoinduced processes in silicate glasses exposed to IR femtosecond pulses

The optical properties variation in silicate glasses after exposure to high-power femtosecond laser radiation at 0.85 micrometers have been studied. The laser spectral line broadening leading to the supercontinuum generation in visible and UV spectral regions was observed in all studied glasses. Color center generation and intrinsic luminescence were found in boro- and alkali-silicate glasses. It is believed that these processes result from linear and/or two-photon absorption of the short-wavelength part of this supercontinuum which causes glass matrix ionization. No color center absorption in the visible region was observed in fused silica at irradiances up to the laser damage threshold. It was concluded that there is no significant ionization of fused silica under exposure to IR femtosecond laser pulses with irradiance below laser induced damage threshold.

Nonlinear coloration and damage of wide-gap glasses by femtosecond laser at 0.85 μm

The processes arising in glasses as an example of solid dielectrics under high-power femtosecond laser radiation have been investigated. In the investigation samples of fused silica were used as well as a wide selection of commercial borosilicate glasses K8 (Russia) and BK7 (USA) which have the band gap near 6 eV. The glass samples were irradiated with the output of a laser system allowing the production of laser pulses at 0.85 micrometer wavelength and duration approximately 10-13 s. It is found that color centers formation and intrinsic luminescence of borosilicate glasses are observed under high-power femtosecond radiation at 0.85 micrometer. These processes result from supercontinuum generation in the bulk glass and are followed by two-photon absorption of the short-wavelength part of this supercontinuum. These same processes lead to the damage of glasses under conditions of intensive ionization of material matrix, and show that the damage process is not connected with avalanche ionization.