This article provides an overview of the principles of ultrashort pulse lasers and current research trends, targeting users of femtosecond lasers for visualization purposes. Femtosecond lasers, now commercially available, are increasingly compact and widely adopted in visualization techniques. Despite their ease of use, they often do not reach their full potential as light sources. To address this, a fundamental understanding of femtosecond laser pulse generation is essential. This article quantitatively presents these fundamentals using easily understandable charts and graphs. In the latter part, recent trends in generating ultra-short pulses are introduced. Previously, oscillators primarily generated the shortest pulses in laser systems. However, contemporary approaches incorporate techniques such as nonlinear pulse compression from longer pulses, the concept of time lenses, and electrical modulation of continuous waves to achieve shorter pulses. These concepts are elucidated in a manner conducive to a thorough grasp of the basics.

Our group has been studying the fundamental phenomena of micro-shock waves generated by femtosecond laser-induced shock waves to utilize them as effective microscale stimulation for regenerating tissue cells. By now, we have confirmed that micro-shock waves are generated in liquid by optical focusing of a femtosecond laser. And we also visualized them by using a high-speed camera to investigate the wavefront structure and generated compression waves around the focus (plasma emission). Thus, we have been observing micro shock waves and bubble behavior. In this paper, we introduce some of the research that our group has conducted to date, and then introduce visualization methods for observing the propagation behavior of shock wavefront that accompanies the initial generation of femtosecond laser-induced underwater shock waves. In addition, we also introduce the information that can be obtained from the images.

High-speed photography has widely contributed to the development of natural science as an indispensable technology for understanding high-speed phenomena. Optical imaging using ultrashort pulse lasers has played an important role in the measurement of ultrafast phenomena that cannot be captured by electrical high-speed cameras. Nowadays, it is possible to satisfy all temporal resolution requirements from femtoseconds to seconds. However, it is still difficult to measure phenomena lasting over a wide range of timescales from femtoseconds to seconds in a single shot, due to the temporal resolution of electrical high-speed cameras and the limited number of frames of optical imaging methods. This article describes an imaging method to overcome these limitations: a single-shot multi-timescale ultrafast photography based on all-optical ultrashort pulse manipulation. As a demonstration, we introduce single-shot measurements of ultrashort laser processing of glass on pico-, nano-, and milli-second timescales (~10–100 ps, ~1–10 ns, and ~1–100 ms).

This paper describes Femtosecond Laser Electronic Excitation Tagging (FLEET) for air flow velocity and density measurement. FLEET uses the emitted light which is obtained by focusing a femtosecond laser on nitrogen molecules as a tag for fluid measurement. Because FLEET uses nitrogen molecules in the air, air flow information can be obtained using only a laser system and an imaging system. Therefore, it is suitable for measuring flow fields that include the center of vortices, the wake of objects, and large accelerations and decelerations caused by expansion waves and shock waves. These flow fields are generally not suitable for measurements with particle image velocimetry. In this paper, we first provide an overview of the emission of nitrogen molecules in FLEET. Next, the measurements of underexpansion jet and uniform flow are described as examples of velocity measurement. The uniform flow measurement was performed in large supersonic wind tunnel. Finally, we discuss density measurement using FLEET emission lifetime. As an example of density measurement, we introduce the measurement of an underexpanded jet.