Big impact from tiny laser

A massive European effort to develop high-brightness semiconductor lasers could transform healthcare, telecoms and display applications and make Europe an undisputed leader in the field.

The semiconductor lasers developed by the Brighter project offer high power and very high efficiency in a small, relatively low-cost package and they have direct applications in cancer treatment and imaging, high-bandwidth fibre-optic communications, laser-based projectors, heads-up displays and even TV screens.

This 23-partner integrated project had a $27.7 million budget, with EU funding of $16.6 million. It followed on and further advanced two earlier projects Ultrabright and Bright.

“We did not start from zero. Many of the partners from earlier projects joined this effort to develop very high-quality semiconductor lasers for specific, real-world applications,” notes Michel Krakowski, coordinator of the Brighter project.

“There are many semiconductor lasers and many application fields but, certainly for lasers in the spectral range between 355 up to 1060 nm, Brighter has developed technology and become one of the leaders in the field,” he adds.

The project also tackled fundamental issues in science, wafer production and manufacturing, as well as education and training in laser technology among a vast range of activities.

The range and complexity of Brighter’s work is so large that it is impossible to go through each achievement individually but some illustrations convey the breadth, depth and scope of its work.

Take green lasers. No semiconductor material exists capable of emitting laser light in green, which occupies the 530 nm range of the spectrum. But there are materials capable of lasing at 1060 nm.

By doubling the frequency of the 1060 nm laser Brighter was able to halve its wavelength, due to the inverse relationship between the two. By this method Brighter used frequency doubling to produce a green laser at 531 nm, with output power up to more than 1.5 W, which is a world record for green frequency doubled diode laser according to the project.

While frequency doubling is a well-known technique in principle, there was nonetheless a host of practical and scientific problems to overcome and the Brighter project met and matched them all.

Similarly, external cavities on a lasing semiconductor material offer a wide range of tuning options.

Cavities can contain a grating, which helps to stabilise the beam of a laser, and they can also be used to double the frequency of light emitted, thereby occupying another region of the spectrum.

Finally, cavities can be used to help couple together a series of lasers, combining their power to create a much brighter laser using a so-called Talbot cavity. Coupling lasers in this manner has important applications in telecoms.

Brighter developed external cavities and the required expertise to manufacture them for a wide range of lasers, greatly enhancing the flexibility of the lasers in the process.

To a lasing expert these principles may seem ho-hum. They have been around for a long time. But the range of applications these lasers can be used for - and the degree to which Brighter pushed the current state of the art - is a truly impressive testament to their research effort.

In practical terms, the project has improved the quality, efficiency, brightness and power of semiconductor lasers across a range of spectra and, in the process, has racked up a number of world firsts.

“We have a red laser bar at 635 nm with an output of 4.5 W. With the red tapered laser at 650 nm, Brighter has achieved an output up to 1 W with a good beam quality. The M2 [a measure of beam quality] is 1.3 for this laser, close to a perfect beam,” explains Krakowski.

Wall plug efficiency, or the quantity of current required to achieve a certain output of power, is a particular strength of the project. For an infrared laser at 980 nm the research achieved an efficiency of 70%.

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