Ultraviolet and High-Power Pulse Measurement

Abstract

The Issues in UV and High-power Pulse MeasurementSo far, we've discussed the general ideas behind FROG and an array of tricks for simplifying FROG measurements. And because nearly every ultrafast lab's workhorse laser is an infrared low-power Ti : Sapphire oscillator, our discussion revolved around SHG FROG, which is ideal for measuring such pulses. Consequently, in this chapter, we consider quite different problems: the measurement of ultraviolet and high-power pulses. These are interesting-and important-problems that are often neglected in discussions of measurement techniques.Indeed, many researchers believe it's not possible to measure UV pulses because SHG crystals don't exist in this wavelength range. While such crystals don't, in fact, currently exist, third-order FROG methods work beautifully in the UV, where third-order nonlinearities yield signal pulses at the same wavelength as the input pulse. Also, third-order nonlinearities are stronger in the UV, so even relatively weak pulses are measurable.On the other hand, many researchers believe that high-power pulse measurement is trivial; after all, there's plenty of intensity-which means that any nonlinearity will yield plenty of signal-so what's the problem? The problem is that high-power pulses have their own unique problems that seriously complicate their measurement. For a variety of reasons, not the least of which is the high complexity of amplifier systems that produce them, they tend to have poor spatial quality. Worse, they suffer from spatio-temporal distortions, such as spatial chirp, in which the intensity and phase vary from point to point across the beam. Such distortions violate the assumption made by essentially all pulse measurement techniques that the intensity and phase vs. time are the same throughout the beam. We'll discuss measuring the intensity and phase vs. time and space later, but for now, we'll see that FROG's ability to check the accuracy of a measurement will be crucial for such measurements. And to further complicate the problem of measuring high-power pulses, their temporal distortions can also be quite severe. And at the same time, our standards for them can be higher. For example, they can have satellite pulses several orders of magnitude less intense. Weaker satellites would be of no significance in low-power pulses, but, in high-power pulses, they can be intense enough to do some damage, so we really need to know about them. Finally, high-power pulses also tend to have significant fluctuations from shot to shot, necessitating single-shot methods. Interestingly, third-order FROG methods R. Trebino, Frequency-Resolved Optical Gating: The Measurement of Ultrashort Laser Pulses