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• Directly pump controlled compact DPSS laser for micro pulse generation. • Gain control in intracavity doubled laser for pulse on demand operation. • First investigation of high modulation intracavity doubled Nd:YVO 4 laser. • Identified optimal parameters for intracavity doubled directly pump modulated laser. • Shown photon lifetime influence on laser efficiency and maximum modulation frequency. Directly pump controlled pulse on demand operation of an intracavity frequency doubled Nd:YVO 4 laser is presented, capable of generating arbitrary delay between pulses in the output sequence with the upper limit in MHz range. Micro pulses on demand were achieved by the gain switching technique and intracavity doubling through the optimization between the conversion efficiency and achievable repetition rate. Further, the article presents first study of a high modulation intracavity doubled Nd:YVO 4 laser utilizing the crystals high emission cross section for fast gain switching. In the free space experimental setup, the highest repetition rate over 2 MHz and frequency doubling efficiency of 30% were achieved with the further scaling possible using optically bonded tightly packed architecture. The demonstrated pulse on demand operation of a compact micro pulsed laser at 532 nm enables full synchronization with the application. [ABSTRACT FROM AUTHOR]

Abstract: Laser induced breakdown in water represents an abrupt high intensity photoacoustic phenomenon. The pressure front that appears at its site represents an ultrasonic shock wave spreading into surrounding water. At the same time the cavitation bubble develops. The effect of both is particularly important in ocular microsurgery, where Q-switched lasers are used to vaporize the tissue in procedures such as posterior capsulotomy. Formation of the cavitation bubble and the propagation of the sound wave were analyzed by an optodynamic experimental set-up. It is based on two-dimensional scanning technique using a beam deflection probe. From the beam deflection probe signals the time of flights for both the sound wave and the cavitation bubble were determined. By using Hugoniot curves and the two-dimensional time of flight data field the pressure amplitude in the front was estimated. Simultaneously the expansion of the cavitation bubble was also analyzed and its energy was determined. [Copyright &y& Elsevier]

Abstract: Conversion of light to thermal and mechanical energy in the vicinity of the water–air interface was studied experimentally. The focus of an IR pulsed laser was moved in steps in vertical direction from the position inside the water upwards to the position slightly above the surface, in the air. The generated optodynamic waves were detected by probe beam deflection. Two probe beams were used simultaneously, both laying parallel to the surface, one inside the water and the other one in the air. The analysis and comparison of the waveforms captured in the two media provide information about the position and size of the optodynamic source relative to the interface, indicating the relative efficiency of thermal energy conversion that takes place in the vicinity of the interface both in the air and inside the water. Three positions of the optodynamic source can be determined from the waveforms: in the air, at the interface and inside the water. [Copyright &y& Elsevier]

• Bubble dynamics investigation near and through a hole in the flat surface. • Laser induced bubbles causes micro pumping action. • Bubble rebound found to be additional pumping driver beside the jetting mechanism. • Methodology of bubble tracking suitable for lower temporal resolution visualizations. • Bubble movement follows the power law of rigid boundary till it touches a hole edge. The laser- induced cavitation bubble, which collapses near a rigid boundary with a hole on a microscale is investigated for liquid pumping applications. The generated bubble and the hole have a comparable size on a scale of 100 μm. The dynamics of the process are visually tracked near the hole and through the hole. This is made possible by the translucent 3D-printed boundary. The main measurable quantities are the bubble oscillation times and the bubble movement. For the latter, we show that it follows the power law of the Kelvin impulse for the rigid boundary without a hole for small standoff distances up to the moment when the bubble touches the edge of the hole. Further, it was found that the bubble standoff distance has a negligible influence on the first oscillation time, while it increases by almost a factor of two for the second oscillation. During the second oscillation, the bubble enters the hole and displaces all the liquid in the direction of bubble propagation. This indicates a second pumping driver beside the jet produced during the collapse of the primary bubble. [ABSTRACT FROM AUTHOR]

• Ultrashort pulses generated with a compact DFB based fiber laser source. • Demonstration that weak spectral broadening is sufficient for spectral filtering. • Stimulated Raman scattering mitigation by a double-pass amplifier. • The optimization of the ultrashort pulse generation based on the tunable filter. In this work, ultrashort pulse generation based on optical fiber coupled distributed feedback (DFB) laser diode and pulse regeneration technique is presented. Two-stage Mamyshev filtering, double-pass amplification in the first stage, and a tunable filter are used to achieve output pulses of 3.6 ps duration from the gain switched DFB diode with no additional pulse compression at the source output. Optimization of spectral broadening and isolation of initial pulse core components are achieved by a tunable spectral filter within a fiber. The resulting pulses are characterized by highly sensitive frequency resolved optical gating (FROG). The concept is compatible with today's requirements for pulse on demand operation as it enable precise synchronization with external systems. Further, its architecture offers mitigation of stimulated Raman scattering (SRS), which usually represents one of the limiting factors for potential applications of such systems. The spectrally broadened output up to 2 nm can be used with well-established chirped pulse amplification systems. [ABSTRACT FROM AUTHOR]

• Jet parameters near a liquid-liquid interface dependent on anisotropy parameter. • Cavitation bubble penetrates the interface when produced in less dense liquid. • Liquid jet more intense when difference in density between both liquids is higher. Presented study analyses different jet parameters that appear during and after bubble collapse, namely bubble center displacement, length of a jet-pierced bubble and size of the rebounded bubble and their relation to anisotropy parameter in a vicinity of a liquid–liquid interface. In experiments cavitation bubble was produced with a focused pulsed laser and its dynamics was recorded by a high-speed camera. Two different types of liquid-liquid interfaces consisting of distilled water and two types of silicone oil were used for the study in order to be able to compare the influence of anisotropy parameter on the magnitude of considered jet parameters. The elongation of the cavitation bubble during jet protrusion was analysed and compared for both interface types. The study exhibits systemic approach to jet parameter consideration and presents an important step in understanding the behaviour of cavitation bubble near liquid-liquid interfaces. [ABSTRACT FROM AUTHOR]

• A method of tunable nonlinear phase pre-compensation in CPA laser system is presented. • Compact pre-compensation is achieved by tunable CFBG used also as a pulse stretcher. • Nonlinear phase between 0 and 14 rad is efficiently pre-compensated. • Detailed FROG analysis is performed to characterize output laser pulses. • The method is applied to a femtosecond fiber laser with pulse energies up to 70 µJ. We present here a detailed study of a compact and tunable nonlinear phase pre-compensation method based on a tunable chirped fiber Bragg grating (TCFBG) applied to an ultrafast fiber laser system. Frequency resolved optical gating (FROG) measurements reveal a significant variation in the output pulse shape due to the variation in accumulated nonlinear phase for varying pulse energy and repetition rate (even if the pulse energy is kept constant) of the laser system. The TCFBG, used as a stretcher and a pre-compensating element in this chirped fiber amplification (CPA) system, eliminates the need for additional pulse shaping components which greatly simplifies the system for tunable pre-compensation. We demonstrate an efficient pre-compensation of the accumulated nonlinear phase between 0 and 14 rad resulting in pulses with a constant pulse duration and Strehl ratio over a broad operating regime of the laser with repetition rates ranging from 100 kHz up to 30 MHz and pulse energies spanning from 1 µJ up to 70 µJ. [ABSTRACT FROM AUTHOR]

Aspects of laser-induced bubble dynamics in sample geometry closely resembling intraocular environment were studied. Laser pulses with a few millijoules of energy were focused into the sample, producing optical breakdown followed by formation of single bubbles. The sample consisted of a piece of polymethyl methacrylate (PMMA) material and a thin polyethylene (PE) elastic membrane which was carefully positioned parallel to the flat PMMA surface, creating a small and adjustable gap between them. The arrangement was placed in distilled water. Such geometry is frequently encountered in ophthalmology, specifically in posterior capsulotomy where there is a gap present between the intraocular lens and the posterior capsule. The formation and evolution of the laser-induced bubble was recorded with a high speed camera for different positions of the laser focus, i.e. inside the gap as well as outside the gap, in the water. The bubble dynamics was compared to the dynamics of the bubbles created near a rigid boundary alone and near an elastic membrane alone. The comparison showed that the evolution of the bubble created inside the gap can be described as a combination of the features which are characteristic for the rigid boundary and the elastic membrane cases, depending on relative dimensions of the gap and the bubble and on their relative positions. Possible influence on related medical procedures is discussed. [ABSTRACT FROM AUTHOR]

• Visualization of the growth of cavitation bubbles near the nucleation energy threshold. • Report on low and high conversion of excitation energy to the cavitation. • Visualization supported shockwave pressure measurement and modeling. • Laser induced cavitation bubbles of few μm diameter generated by 60 ps 515 nm laser. The nucleation and growth of cavitation bubbles few micrometers in size in water generated by a 60 ps 515 nm fiber laser is observed and visualized near nucleation threshold. The study is performed by monitoring the plasma size, the cavitation bubble size and the emitted shock waves. The latter two aspects are supported by the Gilmore model using a Noble-Abel-stiffened-gas (NASG) equations of state. For the first time, two types of cavitation events are identified and visualized that exhibit a difference of more than two orders of magnitude in the excitation energy converted to mechanical effects with minimal change in excitation laser pulse energy. The result is localized cavitation and reduced mechanical stress on water-based media with potentially positive implications for laser treatments of biological tissue. [ABSTRACT FROM AUTHOR]

• Piezoelectric detection simplifying the observation of cavitation thresholds in control valves. • Elimination of the sound noise of an industrial environment. • Removal of the subjectivity of the operator. • Detailed observation of the cavitation phenomenon in industrial monitoring. For industrial monitoring of fluid flows, the transition from a single-phase flow to a cavitation regime is most commonly determined by a microphone that measures the sound emitted in the vicinity of critical constrictions such as in the opening of the control valves. To measure the cavitation threshold, we developed an analogue and robust piezoelectric detection method. The evaluation of this method on the reference test orifice plate and on a chosen control valve shows that it eliminates the sound noise of an industrial environment, largely removes the subjectivity of the operator and enables a more detailed observation of the cavitation phenomenon. [ABSTRACT FROM AUTHOR]

• Laser patterning of thin films on flexible substrates. • Observation and analysis of patterning-induced surface effects. • Insight into rapid nanosecond ablation properties in thin film materials. • Demonstration of ultra-fast scanning utilization on in-demand electronic materials. Modern electronics facilitate the need for fast, efficient, and reliable methods for direct laser-based surface engineering of conductive thin film materials on flexible substrates. Recent advances in pulsed laser source development only incrementally increased the processing speeds, as those are limited by the available scanning systems. Our goal was to combine a high pulse repetition frequency high-power pulse-on-demand fiber laser source with an ultra-fast resonant scanner to achieve high throughput surface engineering. The enabling factor to compensate a resonant scanner's sinusoidal movement were the laser's intrinsic pulse-on-demand capabilities beyond simple pulse picking solutions. The high temporal resolution at full laser power was exploited for spatially controlled surface texturing, allowing a minimally 3 μm positioning accuracy throughout the scanner's range at up to 60 m/s scan speed with a 10 μm laser spot size. We applied the setup to processing of ITO and metallic films on flexible substrates for touchscreens, position sensors, or EM shielding. Surface modification and patterning of the conductive layer was successfully demonstrated while keeping the underlying surface intact. We employed a simple laser ablation model in comparison to the experimental data to improve the understanding of the ablation process. The resulting surface topography was observed and analysed. [ABSTRACT FROM AUTHOR]