A GaAs/AlGaAs distributed feedback semiconductor (DFB) laser with a laterally-coupled grating is demonstrated at a wavelength of 780.24 nm with an output power up to 60 mW. A mode expander and aluminum-free active layers have been used in the material epilayer to reduce the linewidth to 612 kHz while maintaining high output power. The fabricated laser demonstrates over 40 dB side-mode suppression ratio with tuning range > 0.3 nm, which is suitable for atom cooling experiments with the D2 87Rb atomic transition and provides substantial potential for the laser to be integrated into miniaturized cold atom systems.
The growing interest in quantum technology applications such as laser cooling and quantum sensing has generated a large demand for narrow linewidth and high-power laser sources in the visible and near-infrared wavelength emission range. Semiconductor lasers are ideal candidates for developing these sources as they combine low cost and low-power consumption with small size and unparalleled potential for integration.
This work presents experimental results on two complementary design strategies that can be effectively used to reduce the laser linewidth in a Distributed FeedBack (DFB) semiconductor laser: i) the optimisation of the epilayer structure and ii) the apodisation of the grating geometry. The design of the epilayer stack was optimised to vertically shift the optical mode towards the n-doped region so as to reduce the interaction with the more lossy p-doped region and therefore decrease the internal losses of the waveguide. Such design also reduces the modal overlap with the quantum well (QW) gain region. As predicted by the Schawlow-Townes relationship, both of these factors translate into a reduction of the laser linewidth. The DFB lasers were fabricated with a sidewall grating geometry that simplifies the fabrication process and allows to engineer the feedback profile. In this work, the gratings were apodised to alleviate the spatial hole burning, which substantially worsen the laser linewidth at high power levels. The optimisation of the epilayer and grating designs allowed to fabricate robust devices with measured linewidths as small as a few hundred kHz and power output of several tens of mW.