Vitali I. Konov

CVD diamond transmissive diffractive optics for CO2 lasers

Growing applications of CO2-lasers in many cases demand transmission optical elements (widows, lenses, etc.) which could reliably operate at high beam powers. In the present work we propose to produce CVD diamond transmissive diffractive optics. To realize this idea surface microstructuring by laser ablation was applied. Computer controlled selected area ablation of preliminary polished (plane) diamond plates with thickness of 0.3 mm was performed by scanning focused beam of KrF laser. As a test sample for radiation with (lambda) equals 10.6 micrometers a cylindrical lens with the aperture of 4 X 4 mm2 and focal length of 25 mm was produced. Numerical simulation of the lens was made for the fundamental mode TEM00. For tests of the diamond lens 20 W CO2 laser was used.

Laser-induced structure transformations of diamonds

Results are reported on the study of phase transformations in diamonds induced by nano- and picosecond pulses of KrF excimer laser (λ=248 nm) and second harmonic of a YAP:Nd laser (λ=539 nm). Main attention in the research was paid to i) laser-induced graphitization of high-quality CVD diamond plates and ii) laser-induced structure transitions in ion-implanted diamond single crystals. For CVD diamond, the thickness of the laser-graphitized surface layers was measured and the accumulation period for graphitization to occur was found to be longer for lower laser fluences. In the experiments with ion-implanted diamonds, multipulse laser irradiation at fluences lower than the graphitization thresholds resulted in progressive annealing, i.e., in an increase of the optical transmission and surface contraction. Under certain low-intensity irradiation conditions, it was also found that, competing with the annealing process, laser etching of the ion-implanted diamond occurred at extremely low rates of 10-4-10-3 nm/pulse. A correlation between the defect concentration distribution and graphitization thresholds in partially annealed ion-implanted diamonds is discussed.

Mechanism of channel propagation in water by pulsed erbium laser radiation

The channel propagation in liquid water caused by Er3+:YAG laser radiation at a wavelength of 2.94 micrometers is investigated experimentally and compared to a theoretical model. The propagation velocity of the phase front is measured with the beam delivered to the open water surface as well as with a fiber submersed in the water. For laser intensities up to 4 MW/cm2 the results agree with model calculation. We suggest that the hydrodynamic behavior of water is a key feature for the propagation mechanism.

Diamond diffractive optical elements for infrared laser beam control

Recently a new technique has been proposed for laser-assisted generation of phase microrelief to manufacture diamond diffractive lenses for the far IR range. In the present paper the realization of diamond diffractive optical elements (DOEs) is considered, able to focus an incoming CO2 laser beam into certain pregiven focal domains. Exemplarily, two completely different DOEs for different tasks of laser beam focusing have been designed by different methods, manufactured and finally investigated by means of optical experiment and computer simulation. Measured intensity distributions in the DOEs focal planes as well as measured diffraction efficiencies have been compared with related results of computer simulation, and have been found to be in good mutual concordance. Obtained first results indicate that technique of laser-assisted ablation can be effectively used for manufacturing of high quality diamond DOEs for laser beam focusing.

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.

Starting mechanisms of bubble formation induced by Ho:Tm:YAG laser in water

The starting mechanisms of laser induced bubble formation at the submerged fiber tip was investigated by pressure measurements and fast flash light videography. The radiation of a free running or Q-switched Ho:Tm:YAG-laser operating at a wavelength of (lambda) equals 2.1 micrometers was delivered via a fiber into a water filled cuvette. The spatial intensity distribution at the distal end of the fiber was investigated with a thermal image method. It was shown that the beam profile exhibits hot spots which result in an inhomogeneous temperature distribution of the water volume underneath the fiber tip. It was found that at high laser intensities (3 MW/cm2) micro bubbles can appear already at the very beginning of the laser pulse at average water temperatures below the boiling temperature. Corresponding to each sub-ablative laser spike a bipolar thermo-elastic pressure signal was recorded. We came to the opinion that the lowering of pressure by the negative component of the bipolar pressure transients leads to initiation of bubbles by cavitation at high laser intensities. When the laser intensity was reduced from 1 to 0.5 MW/cm2 a fast increase of the bubble formation temperature was found. At laser intensities less than 0.5 MW/cm2 bubble formation takes place at temperatures near the critical temperature of water.

Laser shaping of diamond for IR diffractive optical elements

Recently a new technique for laser-induced generation of phase relief to manufacture diamond diffractive lenses for the mid IR range has been proposed. In the present paper the realization of more complicated diamond diffractive optical elements (DOEs) is considered, able to transform a CO2 laser beam into arbitrary pre-given focal domains. Two DOEs for completely different tasks of laser beam focusing have been manufactured and finally investigated by means of various optical techniques. Measured intensity distributions in the DOEs focal planes as well as diffraction efficiencies have been compared with related results of computer simulation, and have been found to be in a good mutual concordance. The obtained results indicate that laser ablation technique can be effectively used to manufacture high quality diamond DOEs for laser beam focusing. Special attention is paid to the diamond surface graphitization in the process of laser ablation. Main parameters of excimer laser ablation are investigated and density of laser-induced graphite-like layer is defined. It was demonstrated experimentally that graphitized layer formed at different regimes of irradiation remains almost constant in thickness, but has different crystal structure.

Fiber optic magnetic field sensors based on Faraday effect in new materials

Magnetic materials with high Faraday effect were studied for fiber-optic magnetic fields and electric current sensors design. Different compositions of semimagnetic semiconductors were grown to measure high frequency magnetic fields (up to several GHz). For moderate frequency sensors < 50 MHz the most perspective material was found to be Bi-substituted yttrium-iron garnets. Several ways to achieve high sensitivity and wide frequency band are discussed. Sensors on the basis of the investigated materials for various spheres of applications were constructed.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Formation of nonstable color centers in fused silica fibers induced by high-power XeCl laser radiation

The transmission properties of fused silica fibers irradiated by high power high repetition rate XeC1 laser have been investigated. Laser-induced additional losses of radiation were measured as a function of intensity and of repetition rate.

Ultrafast electronic processes in CVD diamonds and GaAs: picosecond photoconductivity and high-voltage switching

An electrode-free transient photoconductivity technique was applied to investigate excitation, drift and recombination of non-equilibrium free charge carriers in high quality synthetic polycrystalline diamond films, natural diamond crystals and low-conductive GaAs with a time resolution better than 200 ps. Picosecond laser pulses of UV, visible and Ir spectral range were applied for single- photon excitation of free charge carriers with initial concentrations of (1012-1019) cm-3. Dependences of amplitude and duration of photocurrent on laser intensity/carrier density were measured. Lifetimes, drift mobilities and carrier photoexcitation cross sections as a function of electron concentration were estimated. Computer calculations of conduction and displacement currents, induced space charge and electric field spatial distribution have been performed for the real experimental conditions. Based on the obtained results, high voltage diamond-based switches triggered by ultra-short laser pulses have been designed. Special attention was paid to metal- dielectric interface investigation and ohmic contacts formation. The developed diamond-base module permitted to switch electric fields as high as 100 kV/cm within a time interval less than 200 ps. The amplitude of photocurrent reached 100 A and the electrical resistance reduce by a factor of 1010.