The laser used to attain ultra-high, so-called relativistic intensities are almost always lasers amplified to very high pulse energies on the order Joules. This energy level constrains them to function at rather low repetition rates of typically a few Hz, so as to leave cool-down time between successive laser shots. Moreover, these amplifiers limit the attainable pulse durations to no less than a few tens of femtoseconds. One might call this the brute-force approach: while it allows generating very high optical intensities, this architechture remains limited in terms of temporal resolution, spatial beam quality, shot-to shot stability and simply productivity (on the order of only 1 shot per second).

The laser developed by the PCO group, which is the heart of the “Salle Noire” laboratory at LOA, takes a different approach. It only produces a few millijoules per pulse, which allows it to function at a much high repetition rate of 1 kHz, but also to seed a so-called post-compression stage which gives access to much shorter pulse durations of a few femtoseconds only.

After a double-CPA amplifier chain including an XPW filter stage for temporal contrast cleaning, the post-compression is based on a long stretched hollow-core fiber filled with helium. The nonlinear interaction of the laser pulses with that gas produces 3.5-fs pulses of 3.5 mJ energy, with a remarkable spatial beam quality thanks to the beam guiding in the fiber. Therefore, the modest pulse energy of this laser is compensated by a much shorter pulse duration, which combines to a peak power of 1 terawatt (10¹² W). The Salle Noire of LOA houses the only laser worldwide offering this kind of power in pulses less than 2 cycles long. Its excellent spatial quality then allows focusing its beam to very small dimensions to concentrate that power in space. In the focal spot of about 2 μm diameter, the intensity reaches a value of 1 TW/(π.1 μm)² ~10¹⁹ W/cm² !