A brighter future

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

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 €16.2m budget, with EU funding of €9.7m. 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 355nm up to 1060nm, Brighter has developed state-of-the-art technology and become one of the leaders in the field,” he adds.

The Brighter project also tackled fundamental issues in science, wafer production and manufacturing, as well as education and training in laser technology. 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 breath, depth and scope of its work.

Take green lasers. No semiconductor material exists capable of emitting laser light in green, which occupies the 530nm range of the spectrum. But there are materials capable of lasing at 1060nm. By doubling the frequency of the 1060nm 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 531nm, with output power up to more than 1.5W, 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. Cavities can also 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 enhances the flexibility of the lasers in the process.

To a lasing expert, these well-known 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, it has racked up several world firsts.

“We have a red laser bar at 635nm with an output of 4.5W, which is state of the art. With the red tapered laser at 650nm, Brighter has achieved an output power up to 1W 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 amount of current required to achieve a certain output of power, is a particular strength of the Brighter project. For their infrared laser at 980nm, Brighter achieved an efficiency of 70%.

The list of successes goes on: first to develop a green laser of this quality; first to develop infrared lasers of high brightness; first to achieve high modulation. Here, Brighter achieved a world record, controlling the output of a 1.7W laser using only an 80mA modulation current. “That is 20W per amp - a huge number - as well as a world record,” reveals Krakowski.

The upshot of all this work and all these results is that Europe now has an enormously strong standing worldwide in the development and fabrication of lasers in these spectra. This opens up the prospect of valuable, high-impact applications in healthcare, telecoms and entertainment. Even more importantly, Europe also now has the expertise to realise these applications in the short to medium term.

Telecoms, healthcare and display technology will be the major beneficiaries of the new generation of semiconductor lasers. Better cancer treatment, wider bandwidth and smaller, better displays could be on their way. The Brighter project, which is co-funded by the European Union, has set a series of world firsts in lab-based records for semiconductor lasers in the red, infrared and green spectra. These results will not languish on the testing bench however. The Brighter project began its work with three, hugely important applications already in its sights.

One of the most compelling is a new type of cancer treatment and imaging. Photodynamic therapy is a very promising treatment for cancer patients that can maximise the benefits, while minimising the harm, from chemotherapy.

Photodynamic therapy works by introducing an inactive chemotherapy drug into the patient. This drug then seeks out and attaches itself to cancer cells. The drug is then activated by laser. It is a very promising and potentially highly effective treatment that only releases drugs at cancer cell sites,

In Brighter, the scientists developed a range of lasers for different elements of the treatment. Powerful and highly reliable red lasers – one at 635nm another at 650nm – were developed to activate different drugs. Meanwhile, a high-powered blue laser provides fluorescent spectroscopy, to show that the drug has reached the target site. Finally, a high-powered ultraviolet laser provides auto-fluorescent imaging of the cancer site.

Both clinical and experimental animal studies are currently underway for different elements of the treatment, and any promising results are likely to be commercialised. “We have end users, medical doctors, working within the project to ensure that the technologies developed are best suited to their needs,” explains Michel Krakowski, coordinator of the Brighter project.

Telecommunications too, is a compelling application and Brighter has developed a range of elements to respond to pressing needs in the sector.

“We have developed a range of lasers and their associated technology to considerably boost bandwidth across optical fibres for data-intensive telecoms,” explains Krakowski. The upshot is more bandwidth in the same pipe, an application that will go a long way to meeting the challenge of rising demand for bandwidth.

This was a non-trivial problem. In addition to developing a semiconductor of the required quality and power, the Brighter project had to develop methods for coupling lasers together, to create an even more powerful light source, and then coupling the light to optical fibres.

In displays, Brighter hopes to usher in new products or better, smaller and more efficient versions of existing products. Current applications in sight are extremely small, powerful and efficient light projectors for film, presentations and other applications, such as heads-up-displays and mobile projectors.

The technology could even be adapted for use with mobile phones, allowing them to project extremely high quality video for television applications, for example. In the coming months, the high-performance red and green laser modules developed in the Brighter project will be tested for feasibility for display applications.

For all these applications, Brighter has developed a vast range of fabrication and design techniques for semiconductor lasers, including doping, deposition, and external cavities on the semiconductor material to “tune” the laser, stabilise it, or couple it with other lasers, depending on the required results.

Brighter’s work did not stop at theory or application, either. The team engaged in a surprisingly large-scale effort to disseminate new knowledge gained on laser production, characterisation, design and fabrication. These educational materials cover a vast range of the latest thinking on lasers and present them in a series of tutorials and presentations on the project and its work. The aim here is to make effective materials available for students and others, and to expand the pool of expertise in this field in Europe and around the world. Brighter’s online materials and CD-ROMs are freely available to anyone.

There are 46 PhD students involved – 21 directly funded through Brighter with a further 25 contributing to and/or benefiting from the project, notes Krakowski. “We made a special effort with dissemination because we wanted to spread the impact of our work and raise the profile of the field.”

The EU-funded project is now expected to end in January 2010, by which time it plans to have fully demonstrated Europe’s expertise in semiconductor laser technology, and its ability to create compelling new devices over the next two to five years.

The Brighter integrated project has received funding from the ICT strand of the EU’s Sixth Framework Programme for research.