Issues important for new ultrashort-pulse-measurement techniques include the generation of theoretical example traces for common pulses, validity ranges, ambiguities, coherent artifacts, device calibration sensitivity, iterative retrieval convergence, and feedback regarding measurement accuracy. Unfortunately, in the past, such issues have gone unconsidered, yielding long histories of unsatisfactory measurements. We review these issues here in the hope that future proposers of new techniques will consider them without delay, and, as an example, we address them for a relatively new technique: self-referenced spectral interferometry.

When a narrowband laser pulse evolves into a broadband pulse, for example, via transmission through a photonic crystal fiber, the resulting continuum usually exhibits massive shot-to-shot pulse-shape fluctuations. The continuum’s extreme complexity prevents its single-shot measurement, with the best estimates so far resulting from an average over many pulses. Here we solve this problem using cross-correlation frequency-resolved optical gating, achieving the necessary large spectral range using a polarization-gating geometry and the necessary large temporal range by significantly tilting the reference pulse. Furthermore, we have also discovered that we can simultaneously cancel the previously unavoidable longitudinal geometrical smearing by using a carefully chosen combination of pulse tilt and beam-crossing angle, thus simultaneously achieving the required temporal resolution. We hence make a complete measurement of an individual complex continuum pulse generated in photonic crystal fiber. By enabling measurement of single optical rogue waves, this technique could provide insight and perhaps even lead to the prediction of when mathematically similar, destructive oceanic rogue waves may occur.

Multiple pulsing occurs in most ultrashort-pulse laser systems when pumped at excessively high powers, and small fluctuations in pump power in certain regimes can cause unusual variations in the temporal separations of sub-pulses. Unfortunately, the ability of modern intensity-and-phase pulse measurement techniques to measure such unstable multi-pulsing has not been studied. Here we report calculations and simulations finding that allowing variations in just the relative phase of a satellite pulse causes the second pulse to completely disappear from a spectral interferometry for direct electric field reconstruction (SPIDER) measurement. We find that, although neither frequency-resolved optical gating (FROG) nor autocorrelation can determine the precise properties of satellite pulses due to the presence of instability, they always succeed in, at least, seeing the satellite pulses. Also, additional post-processing of the measured FROG trace can determine the correct approximate relative height of the satellite pulse and definitively indicate the presence of unstable multiple-pulsing.

We study multi-shot intensity-and-phase measurements of unstable trains of ultrashort pulses using two-dimensional spectral shearing interferometry (2DSI). We find that, like other interferometric ultrashort-laser pulse-measurement methods, it measures only the coherent artifact and so yields effectively only a lower bound to the pulse length. We also attempt to identify warning signs of pulse-shape instability in 2DSI and find that it responds to instability with reduced fringe visibility, although this effect is very small when using the small spectral shears appropriate for large temporal ranges. We conclude that 2DSI should be used with caution and large-shear measurements or alternative techniques should be used to verify the stability of the pulse train.

Even though a general theory of first-order spatiotemporal couplings exists in the literature, it is often difficult to visualize how these distortions affect laser pulses. In particular, it is difficult to show the spatiotemporal phase of pulses in a meaningful way. Here, we propose a general solution to plotting the electric fields of pulses in three-dimensional space that intuitively shows the effects of spatiotemporal phases. The temporal phase information is color-coded using spectrograms and color response functions, and the beam is propagated to show the spatial phase evolution. Using this plotting technique, we generate two- and three-dimensional images and movies that show the effects of spatiotemporal couplings.