Soraa Builds The Brightest Blue Lasers
University of California, Santa Barbara, spin-off Soraa claims to have broken the output power and efficiency records for blue laser diodes.
The west coast start-up, which is based in Goleta, California, has produced a 447 nm edge-emitting laser with a 900 micron cavity that delivers an output above 800 mW, and a shorter version with a 600 micron cavity that has a peak wall plug efficiency (WPE) of 23.2 percent. In addition, the company has made a 521 nm laser that delivers more than 60 mW and produces a WPE in excess of 1.9 percent. All the green and blue lasers have been built on undisclosed, non c-plane orientations of GaN using conventional semiconductor processing technology for this wide bandgap material. The devices were 1.5- 2.5 microns wide and featured a surface ridge laser architecture designed for single lateral mode operation. Soraa’s most powerful blue laser had a threshold current and voltage of 45 mA and 3.8V. Slope efficiency was 1.55 W/A at a case temperature of 20 degrees C. The shorter variant had a threshold current and voltage of 30 mA and 3.8V, a slope efficiency of 1.68 W/, and a WPE of 23.2 percent at a laser output of 180 mW. At an output of 100 mW, the efficiency of these laser chips are about 40 percent higher than those reported by Osram Opto Semiconductors at this year’s Photonics West meeting. Soraa says that this highlights the attractiveness of non c-plane lasers for applications requiring high efficiency, such as next generation displays. Reliability testing of five blue lasers revealed a mean lifetime of 10,000 hours. These tests used automatic power control to start driving lasers at 60 mW and the time it took for the output power to fall by 20 percent was recorded. The green lasers produced by the start up had a 600 micron cavity, a threshold voltage and current of 7.3 V and 130 mA, and a slope efficiency of 0.3 W/A. The 60 mW, CW output of this device compares favourably with the 50 mW output of Osram’s 524 nm laser, which the German outfit reported this summer. Soraa says that its success stems from only 15 months of effort, which indicates the great promise of green lasers built on non c-plane GaN. Turning to this orientation increases quantum well electron-hole overlap, reduces effective hole mass and boosts design freedom. Soraa’s engineers, which include the University of California, Santa Barbara, academics James Speck, Steven DenBaars and Shuji Nakamura, and former director of Philips Lumileds’ Advanced Laboratories, Mike Krames, have detailed the results of these blue and green lasers in the latest edition of Applied Physics Express.