Alexander S. Silenok

Bone-ablation mechanism using CO2 lasers of different pulse duration and wavelength

Bone ablation using different pulse parameters and four emission lines of 9.3, 9.6, 10.3, and 10.6 μm of the CO2 laser exhibits effects which are caused by the thermal properties and the absorption spectrum of bone material. The ablation mechanism was investigated with light- and electron-microscopy at short laser-pulse durations of 0.9 and 1.8 μs and a long pulse of 250 μs. It is shown that different processes are responsible for the ablation mechanism either using the short or the long pulse durations. In the case of short pulse durations it is shown that, although the mineral components are the main absorber for CO2 radiation, water is the driving force for the ablation process. The destruction of material is based on explosive evaporation of water with an ablation energy of 1.3 kJ/cm3. Histological examination revealed a minimal zone of 10–15 μm of thermally altered material at the bottom of the laser drilled hole. Within the investigated spectral range we found that the ablation threshold at 9.3 and 9.6 μm is lower than at 10.3 and 10.6 μm. In comparison the ablation with a long pulse duration is determined by two processes. On the one side, the heat lost by heat conduction leads to carbonization of a surface layer, and the absorption of the CO2 radiation in this carbonized layer is the driving force of the ablation process. On the other side, it is shown that up to 60% of the pulse energy is absorbed in the ablation plume. Therefore, a long pulse duration results in an eight-times higher specific ablation energy of 10 kJ/cm3.

Starting mechanisms and dynamics of bubble formation induced by a Ho:Yttrium aluminum garnet laser in water

The starting mechanisms and dynamics of laser-induced bubble formation at a submerged fiber tip in distilled water were experimentally investigated using pressure measurements and fast flash videography. A fiber guided Ho:YAG laser operating in the free running (τ=200 μs) and Q-switched (τ=45 ns) mode at a wavelength of λ=2.12 μm was used as a light source. It is shown that the beam profile at the distal fiber tip (multimode fiber d=300 μm) exhibits hot spots that result in an inhomogeneous temperature distribution in the heated water volume. Depending on the laser irradiance, three different bubble formation processes are distinguished: bubble formation by heating, by rarefraction (cavitation), and by a combination of these two processes. For laser irradiances of less than 0.5 MW/ cm2 bubble formation takes place at temperatures near the critical point of water (T=280 °C). A rapid decrease in the threshold temperature for bubble formation was found for laser irradiances between 0.5 and 1 MW/cm 2. At lase...

Dynamics in laser cutting of soft media

A high speed camera was used to record time-resolved the development and the shape of the holes drilled by Er:YAG laser pulses of high intensity. The experimental results suggest that above a critical radiant exposure the dynamics of the drilling mechanism changes and driven by a high vapor pressure gradient, hydrodynamic instabilities within the hole occur. They are evidenced by dramatic local variations of the channel diameter and by a decrease of the normalized recoil momentum. These instabilities influence strongly the extent, and velocity of the ejection. It is shown that, when cutting soft tissue, the extent of the zone of thermal damage is determined by the amount of hot liquefied material that remains in the hole and acts as a heat source.

Channel propagation in water and gelatin by a free‐running erbium laser

Precise cutting of biological tissue is possible with the Er:YAG laser because of the strong absorption of radiation exhibited by water containing media at a wavelength of 2.94 μm. To achieve control of the depth of drilled channels a thorough knowledge of the channel propagation mechanism is required. The channel propagation process of pulsed erbium laser radiation in liquid water, and in gelatin with a high water content, as substitutes for biological tissue is investigated experimentally and modeled theoretically. We explain the propagation process with a hydrodynamic model, which describes the channel propagation process in terms of energy, mass, and momentum balance equations, which influence the evaporation pressure at the phase boundary. As the key feature, the theory takes into account the deformability of cold material below the zone of absorption. We show that by modeling this hydrodynamic effect with the Bernoulli equation we can explain the channel propagation velocity in water and gelatin as ...

Fused silica fibers for the delivery of high-power UV radiation

A set of all silica step index fibers was produced using PCVD and PMCVD methods. The influence of the fiber production technology and chemical composition of silica core on the transmission characteristics of fibers in the UV range, as well as on the processes of color centers formation, was studied. The experiments were carried out for the excimer laser wavelengths 308 nm (XeCl) and 248 nm (KrF).

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.

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.

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.

Efficiency of stone fragmentation by long pulses of a Q-switched Nd:YAG laser

The effect of long pulsed Nd:YAG laser (pulse duration 300 ns) with the fundamental and second harmonic wavelength on the fragmentation of different urological and gall stones has been investigated. With 200 and 400 micrometers fibers in a contact application, all types of stones could be fragmented with energies less than 120 mJ (400 micrometers fiber) or 45 mJ (200 micrometers fiber). By use of a double pulse-simultaneous application of second harmonic and fundamental radiation the efficiency of fragmentation could be increased and the energy threshold decreased.

Mechanism of UV laser-induced absorption in fused silica fibers

Abstract An alteration of absorption spectra of fused silica fibers under delivery of high-power (ultraviolet) UV laser radiation (4th harmonic of the Nd:YAG laser, wavelength 266 nm) was studied in comparison with their photoluminescent properties. Tested fibers were produced by various technologies based on PMCVD methods. The nature of defects responsible for UV absorption in fibers and the mechanisms of their photogeneration are discussed.