Pulses guided through solid

17 Jun 2002

High-power femtosecond pulses have for the first time been guided through fused silica.

Self-guiding

A team of French physicists has for the first time made a laser beam travel for long distances in a solid (Phys Rev Lett 87 213902).

Using a phenomenon known as self-guiding, Andre Mysyrowicz and co-workers from the Ecole Nationale Supérieure de Techniques Avancées and the French Atomic Energy Authority (CEA), observed high-intensity femtosecond pulses travelling 20 mm in silica. The development shows that self-guided pulses obey the same rules in solids as they do in gases and may lead to optical computers.

High-intensity femtosecond pulses can self-organise and travel through a solid. In order to see this effect, the peak power of the original beam needs to be above a threshold level.

This critical level depends on the refractive index of the two media involved, air and silica, as well as the wavelength. A change in refractive index causes the medium to act as a lens that focuses the laser ­ hence the name self-guiding. The threshold power for self-guiding to occur in silica was 2.7 MW.

Propagating pulses

The researchers used ultrashort pulses of 160 fs from a titanium:sapphire laser operating at 800 nm. The laser system operated at a repetition rate of 200 kHz and the output beam was positioned accurately near the front surface of the silica sample.

Mysyrowicz and colleagues also used a CCD camera to monitor the shape and duration of the pulses as they travelled through the silica. Using modelling, they found that self-guiding in solids was almost identical to that in air.

Mysyrowicz explained the mechanisms involved: "This unusual propagation regime is established through a dynamic competition between the optical Kerr effect, which tends to focus the beam, and multiphoton excitiation that tends to defocus the beam."

The team hopes to improve the conditions for self-guiding and study other important media in ultrashort laser physics. "This technique might become useful, although futuristic, in optical computing and for the shortening of pulses without the need for a compression stage," Mysyrowicz told Optics.Org.