Aluminium adds value to fibre lasers

Materials World magazine
1 Oct 2008

Applying aluminium instead of phosphorous dopant ions to optical fibre glass lasers may help manufacturers better tackle photodarkening, which reduces the lifetime of the device. This could aid materials processing where lasers are used for cutting, drilling and welding.

‘The photodarkening effect occurs because colour centres build up, absorbing the laser light, heating up the fibre and leading to degradation,’ says Magnus Engholm, lead researcher in the team at Mid Sweden University, Härnösand, which is working on the problem. Phosphorous dopant ions in the glass have been used to inhibit the effect, however, this approach has drawbacks, says Engholm. Researchers believe that the use of aluminium will open up new potential.

Engholm explains that phosphorous co-doped silica glass is softer, so the fibre is more likely to crack. Furthermore, the material increases the refractive index of the laser. ‘In high-power fibre lasers, one wants just a high enough index such that the light is guided but, if it is too high, the beam quality is reduced,’ he says.

The refractive index increases in relation to the concentration of co-dopants. When using phosphorous, relatively high concentrations are needed (>12at%) to prevent phase separation, where yterrbium ions in the fibre cluster together and result in optical losses. The ytterbium concentration must also increase by an equal amount to compensate for the drop in laser quality due to phosphorous. ‘The efficiency of the fibre laser is two to three times less,’ says Engholm.

‘Phosphorous co-doping gets a modified chemical vapour deposition dip in the centre of the fibre refractive index profile [and] also has higher background losses, which means that more pump and laser light will be absorbed in the glass and turned to heat.’

Aluminium does not have the same limitations. Lower concentrations can be used, without phase separation, enabling a lower refractive index in the core relative to the cladding. Engholm explains that this allows an even higher ytterbium use (without increasing the refractive index), which is necessary to produce the shorter fibres that in turn improve laser performance.

The research could have far-reaching effects in several applications, such as remote welding in metallurgy, as well as in glass engraving, and the marking of plastics and other surfaces. ‘With photodarkening out of the way, people will be able to make deeper markings into materials and also use fibre lasers in a wider range of materials,’ Engholm says.

Solid-state lasers and gas lasers could become less prevalent, he argues. ‘Fibre lasers are a particularly attractive option because the spot size is smaller, so it is possible to achieve a higher intensity over a smaller area and make fine cuts.’

He believes the team’s fibre glass composition can be manufactured on an industrial scale by optimising the existing process but without buying new machinery. They hope to commercialise the research within a year.