Ultrafast pulse characterization method is updated and optimized for picosecond applications

When pulsed lasers entered the now common realm of femtoseconds, determining phase effects and other characteristics requiring extremely high time resolution became vital to their use. A technique known as frequency-resolved optical gating, or simply FROG, emerged in the field and has since become the standard approach to perform measurements of these ultrafast pulses.

FROG has been successful for obtaining femtosecond data, but over time periods that are only a few picoseconds long. It can’t capture the entire spectra of laser pulses in high-energy density physics experiments, for example, which can last for many picoseconds. New work by Aboushelbaya et al., however, provides a way to perform a single-shot FROG measurement on pulses as long as 25 picoseconds.

In a typical FROG device, pulses are split such that half is propagated along a path with a variable delay stage before the two components are recombined. The nonlinear recombination creates a signal, or autocorrelation, that can provide both time and frequency information at each delay step.

For measurements in a single-shot, the authors recombined the pulse halves on a barium borate crystal that generates an autocorrelation not by way of a delay stage, but by the path differences created by the relative incidence angles. Different points on the crystal’s axis have different delays between the pulse halves. This effectively maps the time delay axis onto the transverse spatial axis (position) of the crystal.

The new, relatively low-cost system also includes improvements to the analysis algorithms that retrieve phase information, and is suitable for implementation on any short-pulse laser facility.

Source: “Single-shot frequency-resolved optical gating for retrieving the pulse shape of high energy picosecond pulses,” by R. Aboushelbaya, A. F. Savin, L. Ceurvorst, J. Sadler, P. A. Norreys, A. S. Davies, D. H. Froula, A. Boyle, M. Galimberti, P. Oliveira, B. Parry, Y. Katzir, and K. Glize, Review of Scientific Instruments (2018). The article can be accessed at https://doi.org/10.1063/1.5044526 .