August 13, 2009--Osram Opto Semiconductors (Sunnyvale, CA and Regensburg, Germany) has developed a direct green-emitting indium gallium nitride (InGaN) laser diode that has an optical output of 50 mW in pulsed mode at a wavelength of 515 nm (true green is considered to be the spectral range of 515 to 535 nm). In pulsed-mode operation at room temperature, the laboratory prototype has a threshold current density of around 9 kA/cm². (Osram had previously developed a blue-green laser diode.)
[Note: Sumitomo Electric Industries (Hyogo, Japan) has also recently developed a true-green InGaN electrically pumped laser diode: its version operates at 531 nm, uses a semi-polar (2021) plane, and operates at an unspecified optical power. For more information, see the Laser Focus World September 2009 Newsbreaks.]
Compared with semiconductor lasers based on existing technology that operate with frequency doubling, direct-emitting green laser diodes are more compact, offer greater temperature stability, are easier to control, and have higher modulation capability at several hundred megahertz.
Good for picoprojectors
Even though small second-harmonic-generation-based external-cavity green lasers are readily available, the advantages of direct-emitting laser diodes make them better candidates for red-green-blue (RGB) displays. Blue and red-emitting laser diodes already exist; a green-emitting counterpart would make RGB laser "picoprojectors" much cheaper and easier to build, and feasible for use in cell phones and digital cameras.
As for Osram's green laser diode, the German Ministry for Education and Research is sponsoring the MOLAS research project (until March 2011, FKZ 13N9373), which involves technologies for ultracompact and mobile laser-projection systems. As part of this project, Osram Opto Semiconductors is developing efficient laser light sources based on InGaN for mobile projection systems. (Osram already offers direct-emitting blue InGaN laser diodes for commercial applications.)
