Technical Insight
Osram cranks up the output power of green and blue lasers
Lasers from the lab hit up to 250 mW in the green and 4W in the blue
Speaking to delegates at the International Conference on Nitride Semiconductors in Washington, Adrian Avramescu from Osram Opto Semiconductors revealed significant advances in the performance of the company's green and blue laser diodes.
According to Avramescu, Osram's green lasers, which emit around 520 nm, can now deliver a kink-free output up to 250 mW and have a wall-plug efficiency of 8.7 percent at 150 mW. Meanwhile, the company's blue lasers can produce an output up to 4 W, and at 1.6 W they have a conversion efficiency of 30 percent.
Avramescu pointed out that the applications for green lasers include small projectors, speciality lighting, assisting with surveying and head-up displays. Small projectors, which could soon feature in mobile phones, combine red, green and blue lasers with a two-axis mirror to form a technology known as flying-spot laser projection. “You build your image up pixel by pixel,” explained Avramescu.
Requirements for lasers used in small projectors include an output of about 80 mW, or 20 lumens. Blue lasers that the company launched in 2009 meet this, while the direct green variants that hit the market in 2012 fall short of that mark, producing 50 mW, equating to 12-13 lumens. In addition, the emission profile from these chips is not an ideal Guassian profile that is preferred for projection applications, due to interactions with the substrate.
All these weaknesses have now been remedied in the lab, due to various efforts at understanding the behaviour of the green laser diode. First-generation devices produced a sub-linear output above 50 mW, and investigating this decline in efficiency leads
the engineers at Osram to discover that the laser's differential gain varies with temperature. Self-heating and a high threshold current are partly to blame, along with imperfection injection.
Research efforts were also directed at material issues. Dark spots were found with dimensions of 1-20 μm, alloy fluctuations were uncovered at 50 nm to 500 nm length scales, and transmission electron microscopy uncovered defects in the quantum wells, such as dislocations.
Improving the quality of the material and making proprietary modifications to laser design enabled the fabrication of higher output green lasers with improved beam quality. Kink-free output is now possible up to 250 mW, while at 80 mW – the output power required for small projectors – efficiency is 7.5 percent and 7 percent at 25 °C and 60 °C, respectively.
Avramescu also explained that Osram's more powerful blue laser is even suitable for projectors in homes and offices, which require 2000 lumen sources. In 2012, the company launched a 1.4 W blue laser in a TO56 package, and now it has raised the bar to 2.5 W at 25 °C. Packaging is very important at these power densities, and by optimising this, it is even possible to crank the output up to 4 W. However, this is driving the chip in an “over-stressed” regime, warns Avramescu.
