High-Power UV Lasers: Tunable Ultraviolet Burst-Mode Laser System Produces High-Energy Pulses

摘要

While planar laser-induced-fluorescence (PLIF) imaging of aerodynamic and combustion flows has developed enormously since its inception in the early 1980s, the ability to directly capture the dynamics of unsteady and/or turbulent phenomena at high speed has been constrained by the difficulty of generating high-repetition-rate, broadly wavelength-tunable ultraviolet radiation with sufficient individual pulse energy. However, sources with all these characteristics have now been made possible by advances in the development of Nd:YAG-pumped optical-parametric-oscillator (OPO) systems operated in what has come to be known as 'pulse-burst' mode. In combination with sum-frequency mixing, such systems have produced demonstrated output of around 0.5 or 1.0 mJ per pulse at 226 or 313 nm, respectively - sufficient to enable instantaneous nitric oxide (NO) and hydroxyl (OH) imaging at repetition rates as high as 250 kHz. While reported burst-mode Nd:YAG systems differ in detail, they share certain basic characteristics. Specifically, reported systems typically use a low-power (on the order of 100 mW) master oscillator (typically Nd:YAG) that is 'sliced' into a burst train using either a pair of electro-optic Pockels cells or an acousto-optic deflector. In one exception, a repetitively Q-switched diode-pumped Nd:YVO4 laser is used as the master oscillator. The burst train is then amplified in a series of flashlamp-pumped Nd:YAG amplifiers. Typical reported burst sequences consist of between 8 and 40 pulses, with interpulse spacing as low as 1 microseconds, and individual pulse duration of between 6 and 25 ns.