Rok Petkovšek

A laser probe measurement of cavitation bubble dynamics improved by shock wave detection and compared to shadow photography

High-intensity light from a laser pulse can produce optical breakdown in a liquid, followed by a shock wave and the growth of a cavitation bubble. When the bubble reaches its maximum radius the liquid pressure causes it to collapse, which in turn initiates the growth of another bubble. The oscillations can repeat themselves several times, and a shock wave is emitted after every collapse. In our study the breakdown was induced in distilled water by a Nd:YAG pulsed laser, which was designed for ocular photodisruption. The main focus of our experiments was measurement of the cavitation bubble and the shock waves using an optical probe based on deflections of a laser beam. The applied experimental setup made it possible to carry out one- or two-dimensional scanning of the cavitation bubble based on automatic control of the experiment. Since the beam-deflection probe (BDP) allowed simultaneous measurements of the cavitation bubble and the shock waves, we developed a method for reducing the measurement noise of...

Investigation of a cavitation bubble between a rigid boundary and a free surface

When a high-intensity laser pulse is focused into a liquid the energy is converted into mechanical energy via an optodynamic process. The conversion starts with plasma formation; this is followed by shock-wave propagation and the expansion of a cavitation bubble. A cavitation bubble developed near boundaries results in an asymmetrical collapse, with the generation of a liquid jet during the bubble’s rebound. In the case of a free surface this liquid jet is directed away from the surface and the oscillation times are prolonged. On the other hand, in the case of a rigid boundary, the liquid jet is directed toward the boundary and the oscillation times are shortened. We present measurements of a cavitation bubble oscillating between a free surface and a rigid boundary using deflections of a laser beam as the optical probe. Shadow photography was used simultaneously as a comparison during the experiments. With the beam-deflection probe we also measured the shortening of the oscillation times near a free surface as well as the prolongation of oscillation times near a rigid boundary. In order to explain this shortening of the cavitation-bubble oscillation times near a free surface, Rayleigh’s model was extended and compared with our experimental results.When a high-intensity laser pulse is focused into a liquid the energy is converted into mechanical energy via an optodynamic process. The conversion starts with plasma formation; this is followed by shock-wave propagation and the expansion of a cavitation bubble. A cavitation bubble developed near boundaries results in an asymmetrical collapse, with the generation of a liquid jet during the bubble’s rebound. In the case of a free surface this liquid jet is directed away from the surface and the oscillation times are prolonged. On the other hand, in the case of a rigid boundary, the liquid jet is directed toward the boundary and the oscillation times are shortened. We present measurements of a cavitation bubble oscillating between a free surface and a rigid boundary using deflections of a laser beam as the optical probe. Shadow photography was used simultaneously as a comparison during the experiments. With the beam-deflection probe we also measured the shortening of the oscillation times near a free surfa...

Optodynamic characterization of the shock waves after laser-induced breakdown in water

Plasma and a cavitation bubble develop at the site of laser-induced breakdown in water. Their formation and the propagation of the shock wave were monitored by a beam-deflection probe and an arm-compensated interferometer. The interferometer part of the setup was used to determine the relative position of the laser-induced breakdown. The time-of-flight data from the breakdown site to the probe beam yielded the velocity, and from the velocity the shock-wave pressure amplitudes were calculated. Two regions were found where the pressure decays with different exponents, pointing to a strong attenuation mechanism in the initial phase of the shock-wave propagation.

High-power pulsed diode-pumped Er:ZBLAN fiber laser

We report on the operation and performance of a gain-switched Er:ZBLAN fiber laser based on an active pulsed diode pump system. The produced laser pulses offer high peak powers while retaining the high average powers and efficiency of the cw regime. The measured pulse duration was about 300 ns and nearly independent of the pump repetition frequency. The maximum obtained 68 W of peak power is the highest reported, to our knowledge, for diode-pumped Er:ZBLAN fiber lasers, and the 2 W of average power at the repetition frequency of 100 kHz is 2 orders of magnitude higher than previously reported average power in a pulsed regime. The obtained slope efficiency was 34%.

A beam-deflection probe as a method for optodynamic measurements of cavitation bubble oscillations

High-intensity light from a laser pulse can produce laser-induced breakdown in a liquid followed by a shock wave and the growth of a cavitation bubble. When the bubble reaches its maximum radius, the pressure of the surrounding liquid causes it to collapse; this results in bubble oscillations. The cavitation bubbles oscillations and the corresponding shock waves were measured from the deflections of a laser beam. These deflections were detected using a fast quadrant photodiode, built into the optical probe. The precise relative-positioning system and the small diameter of the beams waist made it possible to detect and analyse the signals from the shock wave and the cavitation bubble. Here, we have demonstrated that a method based on a beam-deflection probe can be used to measure the fast phenomena that follow immediately after laser-induced breakdown as well as the whole dynamics of the bubble oscillations, which corresponds to a three-orders-of-magnitude larger time scale.

Gain-switched Yb-doped fiber laser for microprocessing

The gain-switched fiber laser presents the simplest construction among pulsed lasers in the nanosecond region and consequently is also very robust. These properties make it potentially appropriate for industrial applications, especially in some types of microprocessing. However, careful design of such lasers is important in order to reach the required pulse parameters (peak power and pulse duration). To design and optimize a gain-switched fiber laser for microprocessing, a numerical model using time and spatial dependencies was developed and reported in this paper. The effects of pump power and laser length on the pulse duration and peak power were investigated by modeling gain-switched operation. Further, the results of modeling were compared to data from an experimental setup based on a Yb3+-doped gain-switched fiber laser, revealing good agreement.

Q-switching of a fiber laser with a single crystal photo-elastic modulator

A study of using a single crystal photo-elastic modulator for active Q-switching of a fiber laser is presented. The modulator, which oscillates in a longitudinal eigenmode, was realized with LiTaO(3). This induces due to the photo-elastic effect a modulated artificial birefringence which modulates the polarization of passing light. When used together with a polarizer inside a laser cavity the laser photon life time is strongly modulated and the laser may start to emit laser pulses. We realized this with a fiber laser based on a 5m long double clad Nd-doped fiber. The pulse repetition frequency was 400 kHz and the pulse duration 300ns.

Optodynamic monitoring of the laser drilling of through-holes in glass ampoules

We present an accurate and reliable method for the detection of wall perforation during the excimer-laser micro-drilling of glass ampoules and vials. The method is based on the detection of shock waves generated in the air during the drilling process using a laser-beam deflection probe. An analysis of the detected optodynamic signals gives important information about the progress of the drilling process, and we take this as the basis for the presented online process-monitoring method. We show that a significant change in the signals amplitude is observed when the wall of a liquid-filled ampoule is perforated and the exit process point is in contact with the liquid, and that this signal change can serve as an indicator of wall perforation. We have verified this optodynamic method by examining the processed holes using optical and electron microscopy as well as with a non-destructive gas-leakage test method. The described method was employed for the production of test ampoules used for the adjustment of a high-voltage leak-detection device in a pharmaceutical production line. Holes with a diameter of less than 10 µm were produced in the walls of 0.5 mm thick glass ampoules using a XeCl excimer laser.

Burst shaping in a fiber-amplifier chain seeded by a gain-switched laser diode

A low-power source, such as a gain-switched laser diode, usually requires several amplification stages to reach sufficient power levels. When operating in burst mode, a correct input burst shape must be determined in order to compensate for gain saturation of all amplifier stages. In this paper we report on closed-form equations that enable saturation compensation in multiamplifier setups, which eliminates the need for an adaptive feedback loop. The theoretical model is then evaluated in an experimental setup.

Single stage Yb-doped fiber laser based on gain switching with short pulse duration

A simple solution for producing nanosecond laser pulses can be obtained using gain-switched fiber lasers. In this paper, we present an optimized single stage gain-switched ytterbium-doped fiber laser. Three fiber lengths were tested to show the impact of length on the laser output pulse. A pulse as short as 28 ns at 1.4 kW peak power and a maximum peak power of nearly 2 kW at 41 ns pulse duration was achieved. The laser possess a linear polarized output, very good beam quality of M2 < 1.1, and a spectral bandwidth of 0.11 nm.