By Yvonne Carts–Powell
Mirrors with higher power–handling capacity could enable pump lasers with higher output powers, which in turn are desirable for more powerful erbium–doped fiber amplifiers (EDFAs). Erbium–doped fiber amplifiers that can amplify more channels at once are necessary for high–channel–count WDM systems, but require a watt or more of pump power. The output power of 980–nm semiconductor pump lasers, however, has been limited to around 250 mW by several factors, one of which is damage to the end mirrors.
Researchers at ADC Telecommunications (Minneapolis, MN) and before that at Spectracom (a company bought by ADC) developed a way to grow high–quality mirrors on the cleaved facets of the indium gallium arsenide/gallium arsenide lasers that increases the power density that they can handle without damage. The epitaxial–mirror–on–facet (EMOF) process grows mirrors from a wide–bandgap semiconductor that is lattice–matched to the laser material to provide an interface with minimal defects. Lasers made with the EMOF process have been driven with 50–ns–long current pulses of more than 15 A, producing peak output powers of more than 3.5 W (see figure). In these tests reported at OFC, Devin Crawford and others measured power densities of over 4 × 108 W/cm2 at the laser facets without damage.1
After the laser gain region is grown by molecular beam epitaxy (MBE), the wafer is cleaved into bars. Metallization, dielectric deposition, and most processing have been carried out prior to mirror growth. Then ADC uses further MBE processing to grow compound–semiconductor mirror structures directly onto the cleaved–end surface of laser bars. Molecular beam epitaxy enables tight control of the mirror thickness, which translates to excellent control of reflectivity.
The growth temperature for the mirrors must be kept low to avoid damaging the structures fabricated on the laser before the mirrors are grown. Fortunately, the nature of the mirror material and the (110) orientation of the facet both lend themselves to a low MBE growth temperature.
In addition to having a crystal lattice that matches the laser and low growth temperature, the mirror material must have a wider bandgap than the laser emission, must be capable of being grown to an optically significant thickness, and must be electrically insulating.
Crawford's OFC presentation focused on 980–nm fiber–coupled lasers that produced 400 mW of fiber–coupled power. Several groups of researchers have presented fiber–coupled modules with powers around 500 mW, but these are not yet commercially available in bulk. ADC thinks that its process will prove commercially viable.
"Every manufacturer does something to the facet," explains product manager Denise Synstelien. Because the EMOF deposition is a batch process, and because the MBE provides good process control, the company believes EMOF may prove less expensive than other facet–modification processes. The company currently sells a Bellcore–qualified EMOF–based laser that produces 400 mW from the laser chip and 250 mW from the fiber–coupled module.
For more information contact Denise Synstelien at email@example.com.
1. D. Crawford et al., OFC 2002.