Evaluation of Picosecond Laser Electronic Excitation Tagging (PLEET) at Ultraviolet Wavelengths for High Speed Flows

Over the past few years, picosecond laser electronic excitation tagging (or PLEET) has emerged as a credible alternative to its femtosecond counterpart (FLEET) for high-speed measurements. The attractiveness of picosecond operation lies with its capacity to produce higher beam energies and at high repetition rates. Given its potentially superior conversion efficiency, the use of upconverted, ultraviolet (UV) light is studied here as a conceivable path forward for improved performance in the picosecond regime. Through various metrics such as the peak signal-to-noise ratio (SNR), three different excitation wavelengths namely, 266 nm, 355 nm and 532 nm, harmonics of the fundamental 1064 nm light, are carefully examined. The results show that UV excitation indeed produces superior performance relative to the longer wavelengths of 532 nm (and 1064 nm), even after considering the effect of energy conversion efficiency. For a given amount of fundamental (1064 nm) laser energy, 355 nm excitation produces the strongest PLEET signals, while 266 nm excitation generates the longest tagged lines. Most of these findings appear to be qualitatively substantiable using Gaussian-beam-approximation-type arguments, due to a predominantly linear dependence of the PLEET signal on the excitation laser intensity, the latter a feature that is consistent with previous work. These findings pave the way for the use of UV PLEET in high-speed flows, particularly the use of 355 nm light, which transmits well through most optical window material.