Laser diodes are small, powerful light sources that are found, for example, in DVD drives. They are very interesting for projectors: Just like LEDs, they provide high-contrast images with high color saturation, but they deliver much more optical power. With their concentrated light beam they generate sharp images without projection lenses, at all distances and on curved or bent surfaces. They also can be modulated, which means their brightness can be directly controlled without using expensive micro-mirrors or micro-displays. Today’s mini-projectors with lasers already measure only a few centimeters. The green lasers now in use are still relatively large and expensive, however, because they need not only an infrared laser diode but also an optical crystal for converting the infrared light into the green spectral range (frequency doubling). This is where the direct green laser offers a solution.

As a result, there is currently a race to produce direct emitting green laser diodes with wavelengths greater than 515 nm. This year the 500 nm mark was surpassed for the first time. The problem is the InGaN material system. High indium concentrations are needed for green light with a wavelength above 515 nm, but these degrade the crystal quality and thus reduce the efficiency of light generation. Furthermore, piezoelectric fields occur in the crystal. The researchers at Osram Opto Semiconductors were able to produce laser-quality InGaN structures using a modified epitaxy method. The 515 nm laser has 50 milliwatts of optical power in pulsed mode at room temperature and exhibits good temperature stability. The results demonstrate that the InGaN material system can deliver the high outputs required for green lasers.

This will enable mobile projectors such as those found in cell phones to become even smaller and more powerful. Osram is part of Siemens Industry and is developing efficient laser light sources using InGaN for mobile projection systems as part of the MOLAS project funded by the German Ministry for Education and Research.

Reference Number: RN 2009.10.03.1e