Chaotic cavity boosts stability of high-power laser

Chaotic cavity boosts stability of high-power laser
28 Aug 2018
Chaotic cavity
Chaotic cavity: Quantum chaos is produced within the D-shaped cavity. (Courtesy: Stefan Bittner et al)

(more at link: https://physicsworld.com/a/chaotic-cavity-boosts-stability-of-high-power-laser/ )

Unwanted fluctuations in the output of high-power lasers can be reduced by using a laser cavity that allows light to bounce around chaotically. That is the counterintuitive conclusion of scientists in the US, UK and Singapore, whose research could also boost our understanding of weather patterns and turbulent fluid dynamics.

High-power lasers have an immense range of applications, from materials processing to surgery, but keeping light emission stable is difficult. Complex non-linear interactions of a laser’s active medium with the light field can lead to chaotic fluctuations that degrade its output and reduce its usefulness. Researchers have tried to suppress these fluctuations, but this can restrict the laser’s power.

An ideal laser would transmit all its power at a single frequency, with all its wavefronts perfectly parallel. In reality, however, the desired longitudinal modes in a traditionally-shaped laser cavity inevitably excite transverse modes as well.
Making waves

“It’s like a ship propagating through water,” explains Ortwin Hess, a theoretician at Imperial College London specializing in quantum nanophotonics: “You’ve got waves pushed in front of it and waves pushed aside created in the wake.”

Narrow laser cavities supporting only one transverse mode generally remain stable. High-power lasers, however, require large laser cavities and, within these, multiple transverse modes can pile up, leading rapidly to chaotic fluctuations in the light output.

Attempts to tame these fluctuations have focused on suppressing these multiple transverse modes to make the cavity field resemble that of a small laser. Such strategies can be moderately successful, says Hess, but the cavities remain inherently unstable.

“The characteristic thing about a semiconductor laser is that, within the semiconductor active material, light and matter interact quite intimately,” explains Hess. “It’s a bit like the light changing the viscosity of a very viscous fluid while propagating through it.” Injecting a stable control pulse, for example, may successfully suppress the multiple transverse modes at one pumping current, but increasing the current further may cause them to reappear.