Electrically-pumped external-cavity surface-emitting laser diodes

Vertical-cavity surface-emitting laser diodes (SELDs) continue to be the topic of much research, and recent results show that they can produce CW powers of above 10 mW without heatsinking. For many applications, a major problem with SELDs is that they only operate in a single transverse mode at low powers. The reason for multiple transverse modes is a combination of the thermal lensing, spatial hole burning, and non-uniform current injection. We demonstrate the use of an external cavity to maintain single transverse mode operation at all currents. More specifically we have reduced the reflectivity of the top mirror from above 99% to 85% and placed the SELD in an external cavity. This results in transverse modes controlled by the external cavity, not by the SELD. The modes of the external cavity can then be controlled by the use of an aperture in the external cavity or by adjusting the cavity length. In order to provide more uniform current spreading, we designed a novel structure (AlGaAs/GaAs) in which the SELD was grown on a p-doped substrate with the p-doped mirror on the bottom and the n-doped mirror on top. We have observed that the current can spread uniformly over a device as large as 100 μm in diameter