The past 30 years have seen spectacular progress in the development of techniques for measuring the complete temporal field, and even the complete spatiotemporal field, of ultrashort laser pulses. The challenge has been to measure a pulse without the use of a shorter event or an independent known reference pulse, neither of which is typically available. We begin with autocorrelation, the first such “self-referenced” pulse-measurement method ever proposed, which measures only a rough pulse length, and we describe its limitations. One such limitation is the presence of a somewhat unintuitive “coherent artifact,” which occurs for complicated pulses and also when averaging over a train of pulses whose shapes vary from pulse to pulse. We then describe the most important modern techniques capable of measuring the complete temporal intensity and phase of even complicated ultrashort pulses, as well as their ability (or inability) to measure such unstable pulse trains. A pulse reliably measured with such a device can then be used as a reference pulse in conjunction with another technique, such as spectral interferometry or holography, to measure pulses otherwise unmeasurable by a self-referenced technique. Examples include techniques for measuring low-intensity pulse(s) and for measuring the complete spatiotemporal intensity and phase of arbitrary pulse(s). This Tutorial is limited to well-established, proven methods, but other methods whose description proves instructive will be discussed.

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.

For essentially all applications, laser pulses must avoid variations in their intensity and phase within a pulse and from pulse to pulse. Currently available devices work very well for both long (>10ns) and short (<100ps) pulses. But intermediate (~ns) pulses remain difficult to measure and, not surprisingly, are the least stable. Here we describe a simple, elegant, complete, all-optical, single-shot device that measures ~ns pulses and that does not require a reference pulse or assumptions about the pulse shape. It simultaneously achieves a very high spectral resolution of <1pm and a very large delay range of ~10ns (several meters of light travel). It accomplishes both goals using high-efficiency, high-finesse etalons: one to generate high angular dispersion for a high-resolution spectrometer, and another to tilt the pulse front by ~89.9° without distorting it in time. Using this device, we completely measure microchip and fiber-amplifier pulses.

Frequency-resolved optical gating (FROG) is widely used to measure ultrashort laser pulses, also providing an excellent indication of pulse-shape instabilities by disagreement between measured and retrieved FROG traces. However, FROG requires—but currently lacks—an extremely reliable pulse-retrieval algorithm. So, this work provides one. It uses a simple procedure for directly retrieving the precise pulse spectrum from the measured trace. Additionally, it implements a multi-grid scheme, also quickly yielding a vastly improved guess for the spectral phase before implementing the entire measured trace. As a result, it achieves 100% convergence for the three most common variants of FROG for pulses with timebandwidth products as large as 100, even with traces contaminated with noise. Here we consider the polarizationgate (PG) and transient-grating (TG) variants of FROG, which measure amplified, UV, and broadly tunable pulses. Convergence occurs for all of the >20,000 simulated noisy PG/TG FROG traces considered and is also faster.