There is great difficulty in implementing lateral gratings in GaSb-based lasers. Commercially, single-frequency GaSb lasers have been fabricated using metal gratings deposited laterally to the ridge-waveguide (RWG) stripe. The disadvantage of this is that the laser performance is compromised by additional optical loss due to radiation absorption by the metal. Fabricating lasers in this way limits the potential for high-power performance. A better method is to etch gratings into the semiconductor, but generally, patterning these grating structures is difficult because of nonuniformity of the grating pattern and etching difficulty due to sub-micrometer dimensions.
Laterally coupled distributed feedback (LC-DFB) Type-I GaSb-based diode lasers were demonstrated that exhibit low internal loss comparable to laser devices without grating structures, approximately 5 cm–1. The successful performance of these lasers is attributed to the optimized fabrication of uniformly etched lateral gratings along the laser ridge waveguide.
Using a unique E-beam resist spinning technique, a method was developed that results in uniform application of the resist such that the pattern is directly adjacent to the ridge sidewall. This is done by bonding the wafer to a 4-in. (≈10-cm) silicon platform such that the length of the ridges is parallel to the vector of resist spreading, in contrast to standard methods of spinning where the ridge orientation is not fixed to this vector, resulting in a non-uniform thickness of the resist on either side of the ridge.
The lasers have been fabricated with etched lateral gratings and have been shown to be superior in performance to devices with metal gratings. As a result, the lasers have been considered for high-power applications such as lidar and spectrometers that require lasers with greater than 10 mW.
Lidar currently uses solid-state lasers. The new technology is 100 times smaller and has fewer components with possibly the same performance. By bonding the wafers on a stable platform such that the length of the RWG structures is parallel to the radial flow of the resist spread, a uniform resist film thickness along both RWG sidewalls is achieved.