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Noninvasive optical measurements of the shockwave propagation velocity using multiple pulse illumination allow deducing the shockwave pressure amplitude through Hugoniot relations and an appropriate equation of state of the medium. This technique is particularly useful for spatially resolved measurements near the shockwave emission site. Due to diffraction, however, a shockwave front can significantly change its morphology, rendering precise velocity measurements non-trivial. As solution we propose a wave front evolution (WaFE) velocimetry technique, which applies Huygens principle. We take a shadowgraph of the wave front at subsequent times as initial condition for the acoustic Helmholtz equation and numerically propagate the fronts in time. From the instance of time, when two subsequently taken wave front shadows numerically interfere and form one sharp wave front, the local shock velocity is obtained and the local shock pressure amplitude measured. With artificial test images, it is shown that this technique has excellent sub-pixel accuracy, robustness to noise, and can work with low contrast images and even overlapping and interfering wave fronts. The software is made available freely and can be applied to general shock front velocity measurements. We apply WaFE to determine the anisotropic shockwave emission from an elongated laser-induced plasma in water from shadowgraphs of the shockwave front imaged four times onto the same camera frame using multiple pulse illumination at a repetition rate of 60 MHz. The direction dependence of attenuation of the shockwave pressure amplitude is measured at distances of 50–300 μm to the plasma. [ABSTRACT FROM AUTHOR]

Laser microstructuring has been studied extensively in the last decades due to its versatile, contactless processing and outstanding precision and structure quality on a wide range of materials. A limitation of the approach has been identified in the utilization of high average laser powers, with scanner movement fundamentally limited by laws of inertia. In this work, we apply a nanosecond UV laser working in an intrinsic pulse-on-demand mode, ensuring maximal utilization of the fastest commercially available galvanometric scanners at scanning speeds from 0 to 20 m/s. The effects of high-frequency pulse-on-demand operation were analyzed in terms of processing speeds, ablation efficiency, resulting surface quality, repeatability, and precision of the approach. Additionally, laser pulse duration was varied in single-digit nanosecond pulse durations and applied to high throughput microstructuring. We studied the effects of scanning speed on pulse-on-demand operation, single- and multipass laser percussion drilling performance, surface structuring of sensitive materials, and ablation efficiency for pulse durations in the range of 1–4 ns. We confirmed the pulse-on-demand operation suitability for microstructuring for a range of frequencies from below 1 kHz to 1.0 MHz with 5 ns timing precision and identified the scanners as the limiting factor even at full utilization. The ablation efficiency was improved with longer pulse durations, but structure quality degraded. [ABSTRACT FROM AUTHOR]

In a growing number of applications, fast and localized pressure measurement in aqueous media is desired. To perform such measurements, a custom-made single-mode fiber-optic probe hydrophone (FOPH) was designed and used to measure the pressure pulse generated by laser-induced breakdown (LIB) in water. The sensor enabled sub-nanosecond pressure rise time measurement. Both the rise time and the duration of the shockwave were found to be shorter in the direction perpendicular to the breakdown generating laser beam, compared to the shockwave observed in the parallel direction. Simultaneous high-frame-rate imaging was used to qualitatively validate the novel hydrophone data and to observe the shockwave evolution. The measurements were performed also on pressure pulses emitted during the generation of miniature ($150~\boldsymbol \mu \text{m}$ diameter) laser-induced bubbles at very small distances (down to $40~\boldsymbol \mu \text{m}$), further demonstrating the capabilities of the small-size sensor and the ability to measure locally. The results improve understanding of LIB shockwave characteristics dependence on laser pulse energy and duration. [ABSTRACT FROM AUTHOR]

In this manuscript we present a true pulse-on-demand laser design concept using two different approaches. First, we present a fiber master oscillator power amplifier (MOPA) based quasi-continuous wave (CW) laser, working at high modulation bandwidths, for generation of nanosecond pulses. Second, we present a hybrid chirped pulse amplification (CPA)-based laser, combining a chirped-pulse fiber amplifier and an additional solid-state amplifier, for generation of femtosecond pulses. The pulse-on-demand operation is achieved without an external optical modulator/shutter at high-average powers and flexible repetition rates up to 40 MHz, using two variants of the approach for near-constant gain in the amplifier chain. The idler and marker seed sources are combined in the amplifier stages and separated at the out using either wavelength-based separation or second harmonic generation (SHG)-generation-based separation. The nanosecond laser source is further applied to high throughput processing of thin film materials. The laser is combined with a resonant scanner, using the intrinsic pulse-on-demand operation to compensate the scanner's sinusoidal movement. We applied the setup to processing of indium tin oxide (ITO) and metallic films on flexible substrates. [ABSTRACT FROM AUTHOR]

To avoid a devastating effect of eye vision impairment on the information flow from the eye to our brain, enormous effort is being put during the last decades into the development of more sensitive diagnostics and more efficient therapies of retinal tissue. While morphology can be impressively imaged by optical coherence tomography, molecular-associated pathology information can be provided almost exclusively by auto-fluorescence-based methods. Among the latter, the recently developed fluorescence lifetime imaging ophthalmoscopy (FLIO) has the potential to provide both structural information and interacting pictures at the same time. The requirements for FLIO laser sources are almost orthogonal to the laser sources used in phototherapy that is expected to follow up the FLIO diagnostics. To make theranostics more effective and cheaper, the complete system would need to couple at least the modalities of low-power high-repetition-rate FLIO and precision high-pulse energy-adjustable repetition rate phototherapy. In addition, the intermediate-power high repetition rate for two-photon excitation would also be desired to increase the depth resolution. In our work, compact fiber-laser based on high-speed gain-switched laser diode has been shown to achieve adaptable/independently tunable repetition rate and energy per pulse allowing coupled fluorescence lifetime diagnostics via two-photon excitation and phototherapy via laser-induced photodisruption on a local molecular environment in a complex ex vivo retinal tissue. [ABSTRACT FROM AUTHOR]

This paper presents an investigation of pulse-on-demand operation in fibre and hybrid lasers. Two methods for efficient gain control that enable the generation of laser pulses at arbitrary times with controlled pulse parameters are presented. The method of direct modulation of the pump power in the high-power laser oscillator is shown to generate pulses with a duration in the nanosecond range, with repetition rates varying during operation from a single shot to over 1 MHz. An advanced method using a combination of marker and idler seeding a fibre amplifier chain is investigated. Such a system can easily achieve repetition rates of several tens of MHz. The lasers' performances were successfully tested in a real environment on an industrial platform for laser transfer printing. Similar concepts were used for a laser source with ultrashort laser pulses (femtosecond range) on demand by using a mode-locked seed as a source and a solid-state amplifier to achieve high pulse energy and peak power. [ABSTRACT FROM AUTHOR]

In this manuscript, an implementation of a tunable nonlinear phase compensation method is demonstrated on a typical femtosecond hybrid laser consisting of a fiber pre-amplifier and an additional solid-state amplifier. This enables one to achieve constant laser pulse parameters over a wide range of pulse repetition rates in such a laser. As the gain in the solid-state amplifier is inversely proportional to the input power, the shortfall in the solid-state gain at higher repetition rates must be compensated for with fiber pre-amplifier to ensure constant pulse energy. This increases the accumulated nonlinear phase and consequently alters the laser pulse parameters such as pulse duration and Strehl ratio. To overcome this issue, the nonlinear phase must be compensated for, and what is more it should be compensated for to a different extent at different pulse repetition rates. This is achieved with a tunable CFBG, used also as a pulse stretcher. Using this concept, we demonstrate that constant laser pulse parameters such as pulse energy, pulse duration and Strehl ratio can be achieved in a hybrid laser regardless of the pulse repetition rate. [ABSTRACT FROM AUTHOR]