Transverse Mode Control in Vertical-Cavity Surface - Laser Diodes Using AN External Cavity.

Current vertical-cavity surface-emitting laser diode (VCSEL) designs exhibit poor transverse mode properties. Spatial hole burning induces higher-order modes to reach threshold, limiting the single fundamental mode power to a few mW. This dissertation presents measurements of spatial holes in the spectrally-resolved spontaneous emission near field of weakly index guided VCSELs that correlate to regions of intense stimulated emission. Spatial hole burning is shown to produce a self focusing effect which causes the fundamental mode width to decrease with increasing output power, exacerbating the spatial hole burning problem and accelerating the transition to multimode operation. A simple two-dimensional model is developed which predicts a relationship between fundamental mode power and width that is in agreement with the data. In order to maintain a single fundamental transverse mode of constant size, despite any spatial hole burning effects, a VCSEL is coupled to an external cavity. These are the first electrically-pumped external-cavity surface -emitting laser diodes (X-SELs) reported. They yield single fundamental mode powers of 2.4 mW cw and over 100 mW pulsed. The mode diameters are as large as 36 mum. Two cavity geometries are presented, a "macrocavity" which utilizes a curved mirror placed 10 cm in front of the VCSEL in a concentric geometry and a "microcavity" which uses a flat mirror placed less than 700 μm in front of the diode. The macrocavity provides better mode selectivity and is attractive for mode locking. The microcavity is attractive because a single longitudinal mode lases, many lasers on the same chip can utilize the same mirror, and the structure is compact and lends itself to integration. The microcavity also reduces the amount of wavelength chirp exhibited during pulsed operation because the external cavity largely determines the lasing wavelength. The single fundamental mode X-SEL powers are limited by heating and a poor lateral overlap between the optical mode and the current injection. Preliminary results are presented from an advanced heat-sunk current-channeling device which should overcome these limitations. A three -fold reduction in the thermal resistance is measured.