A 'measure' advance for ultrashort laser light

The Tata Institute of Fundamental Research, Mumbai has pioneered a method to measure ultrahigh power, ultrashort laser pulses in a comprehensive manner. The paper reporting this has been published in the journal Optica.

The laser, a marvel of the modern world, has produced the shortest time duration pulses ever known to humans. Not just that, it has also provided a means to pack large amounts of light energy in that extremely short duration, leading to astronomically large 'peak' powers, in magnitude thousands of times larger than the total electrical power consumption of the world.

Measuring the temporal shape of these pulses is, however, no mean task and while scientists have devised clever ways over the past few decades, there remain several significant challenges.

For one, the time distortions that a short pulse suffers whenever it goes through a medium. And the larger the power, the more severe these distortions.

Yet another major complication has to do with the pulse time profile being different at different points within the laser beam itself. Most often, scientists may not bother about these variations across the beam spatial extent and assume a single temporal profile.

However, the larger the beam and/or the more the length it traverses in a medium, the more critical these distortions become, dramatically changing the pulse. And at ultrahigh peak powers it is imperative to know what the time duration is at different points across the spatial extent of the beam.

The TIFR team used a specially designed instrument to measure the time profiles across spatial points in the ultrashort laser beam. They used an optical technique named 'spectral interferometry' at different spatial locations across the beam simultaneously, to achieve this. The team collaborated with Umea University, Sweden on this study.

With the scientific world marching towards peak laser powers never imagined before (tens of thousand trillion watts) in laser beams spread over diameters of several tens of centimeters, this method will not only be extremely useful but essential.

These ultrahigh power lasers emit pulses every once in a while—once over many seconds / minutes / hours. The earlier techniques of measurement needed to sample multiple pulses before estimating the pulse profile and were extremely cumbersome.

The TIFR advance solves this too. It works for a single pulse.

As laser peak powers shoot through the roof, the normal solid optical components cannot handle them as they breakdown by ionization. The technology is therefore moving towards using ionized matter or 'plasma' itself, to design these optical components. And these plasmas can be highly unstable and cause further distortions in the spatio-temporal profiles of the pulse incident on them. The TIFR method is perfectly suited to measure these distortions.

A one-shot solution for all ultrahigh peak power lasers? That is the promise.