Lasers offer a BRIGHTER future

Materials World magazine
1 Aug 2009
High-power 635nm red laser bar. Courtesy of OSRAM Opto Semiconductors GmbH.

Healthcare, telecommunications and display technology may benefit from a ‘new generation’ of high-brightness semiconductor lasers that emit across the spectrum.

BRIGHTER, a 22-partner integrated EU project, is exploiting improvements in laser structures and novel techniques for shaping and combining beams to achieve higher power, higher brightness laser diodes that overcome the limitations of existing technology.

‘Broad-area diode lasers are widely used to achieve high powers, however, these are
susceptible to modal instabilities, filamentation and catastrophic optical damage,’ explains Dr Steve Bull from the University of Nottingham, UK, a partner in the programme. ‘These lasers suffer from poor brightness. The challenge is to create lasers that can be focussed [on a small spot] and efficiently coupled into fibres.’
Coupling is to combine multiple laser beams into an optical fibre to achieve higher powers.

Tapered lasers are being used in BRIGHTER as they have good beam quality and high output power. The Ferdinand Braun Institute in Berlin, Germany, part of the consortium, has achieved nearly diffraction-limited output powers of 12W from an infrared tapered laser. An eight Watt version of this laser has been frequency-doubled to give a record 1.5W of green light, which is useful for laser displays.

Super large optical cavities have been designed to improve the beam characteristics, reducing the far-field divergence, while maintaining high output powers. Quantum dot materials are being incorporated into some of the lasers produced as they provide a more stable wavelength as a function of temperature.

In terms of combining several laser beams, researchers from the Fraunhofer Institute for Laser Technology in Aachen, Germany, another consortium member, have developed a module which uses step mirrors and polarisation coupling to combine 16 single emitters into a 50µm fibre. This module is designed to achieve a power of 50W.

Information overload

A range of other fibre-coupled modules for telecommunication applications are being developed. In contrast to modules based on single emitters, these use high-power laser bars. Bull comments, ‘Near infrared lasers are being developed to pump erbium-doped fibre amplifiers and Raman amplifiers. Quantum dot lasers are also being used – the aim being to produce a
cooler-less telecoms module’.

A single mode fibre-coupled module below one Watt has been developed for free space optical communications. By using a different wavelength laser, it serves as a high-power pump for Raman amplification.

‘For optical wireless systems, modulation speed is critical and excellent results have been achieved with novel split-contact tapered lasers,’ says Bull. ‘BRIGHTER has achieved a world record, controlling the output of a 1.7W laser at 1Gbps using a modulation current of just 80mA.’

Medical marvel

As part of the project, red-emitting laser bars with improved performance have also been developed to replace argon-ion pumped dye lasers for photodynamic therapy (PDT) for cancer treatment.

‘A limitation of PDT is the penetration depth of light into tissue, which restricts this treatment to tumours on or just under the skin,’ explains Dr Tilmann Trebst of Biolitec AG in Germany, another project partner. ‘However, coupling the light through optical fibres
opens the path for interstitial PDT of large tumours and organs.’

The development of red laser bars in the wavelength range around 635nm, where photosensitisers are already available, is challenging. ‘Several laser parameters like carrier injection efficiency, laser mode confinement and the catastrophic optical damage threshold are closely linked and often require a trade-off,’ says Bull. ‘We have made progress by re-designing the laser cavity and improving thermal management by using copper-diamond heat spreaders between the chip and heatsink. Optical powers in excess of 4.5W have been demonstrated, making these devices suitable PDT excitation sources.’

Further information: BRIGHTER