Dr. Aram Mooradian, Novalux Inc.
Although Novalux Inc. has only recently joined the Defense Advanced Research Projects Agency’s SHEDS program, it has been developing high-power, high-efficiency surface-emitting diode lasers for many years. These devices have significant advantages over conventional edge-emitting semiconductor lasers, including a high-quality output beam and the ease with which they can be manufactured. Our challenge under the program is to boost the electrical efficiency of these lasers to 80 percent.
Figure 1. A 4-in. surface-emitting diode laser array features more than 5000 finished devices.
Surface-emitting lasers are readily manufactured in an array format (Figure 1) and can be tested at the wafer level with either an external mirror or with a mirror fabricated directly onto or in the wafer (Figure 2). Wafer-level testing leads to lower-cost manufacturing and a higher yield of the finished product.
Figure 2. A wafer scan measures the center wavelength of the internal Fabry-Perot mirror. The total wavelength spread over most of the usable area of the wafer is 3 nm at a wavelength around 980 nm.
Novalux has carried out extensive design and testing of its devices and has demonstrated Telcordia reliability for its mounted, high-power, 980-nm NECSELs (Novalux Extended-Cavity Surface-Emitting Lasers) with a mean time to failure of more than 1 million hours.
These devices have produced more than 1 W CW in a low-order, multimode, 980-nm beam. In a round, TEM00 beam with an M2 of ~1.05, they have produced more than 0.5 W at 980 nm from a 150-μm gain aperture. Higher power is achieved from larger gain apertures, with CW power increasing roughly linearly with the diameter.
Multimode, CW NECSELs have operated at the company with about 30 percent power efficiency, and CW TEM00 lasers at 25 °C are capable of about 18 percent efficiency. One of the primary efficiency limitations in these devices is the series electrical resistance, mostly due to the low conductivity of the Bragg mirrors. Achieving the efficiency goal of the SHEDS program will require fabricating devices with significantly reduced series resistance and reduced threshold current, while maintaining high quantum efficiency.
One important advantage of vertical-cavity lasers is that high output powers can be realized without causing catastrophic damage to the laser itself, because the emitting areas are much larger than those of edge-emitting diode lasers. Another advantage of their design are the narrowband Bragg reflectors, which provide wavelength control with temperature (~0.07 nm/°C) that is significantly better than can be achieved with simple edge emitters.
Power efficiencies for vertical-cavity surface-emitting lasers of 57 and 50 percent have been reported, respectively, by groups at the University of Ulm in Germany and at Sandia National Laboratories in Albuquerque, N.M.1,2 The power efficiency goal of Phase 1 of the SHEDS program is 65 percent, a significant increase over the current state of the art. Nonetheless, we believe this can be accomplished by implementing known techniques to reduce series resistance. These techniques include suppression of the interface Schottky barriers in the Bragg mirrors by “spike” doping during the growth process, efficient carrier confinement, tailored doping profiles in the Bragg mirrors and gain optimization to reduce laser threshold.
Efficient cooling also is a key to achieving the desired power and efficiency in both individual emitters and arrays. But thermal crosstalk between adjacent emitters in an array does not become a significant issue for an array pitch (center-to-center distance divided by the device diameter) as small as 2.5. Measurements at Novalux have shown that for an array pitch as small as 1.5, the maximum power per emitter is maintained at the same level as when the emitter operates alone. Based on the results thus far, a CW power level of 80 W from 0.25 cm2 appears obtainable, which is one of the primary SHEDS goals.
Attaining 80 percent efficiency will require new schemes, some of which will come from the understanding gained in the early phases of the program. Reduction of the series resistance and threshold values by a factor of five to 10 while maintaining the optical power efficiency at its present value could allow the overall power conversion efficiency to approach the target of the SHEDS program. This will be a very significant challenge.
1. R. Jager et al (Feb. 13, 1997). 57% wallplug efficiency oxide-confined 850 nm wavelength GaAs VCSELs. ELECTRONICS LETTERS, pp. 330-331.
2. K.L. Lear et al (Feb. 2, 1995). Selectively oxidised vertical cavity surface emitting lasers with 50% power conversion efficiency. ELECTRONICS LETTERS, pp. 208-209.
Meet the author
Aram Mooradian is the founder and chief technology officer of Novalux Inc. in Sunnyvale, Calif.; e-mail: firstname.lastname@example.org.