High aspect ratio DBR gratings for narrow linewidth red lasers

In interferometry, spectroscopy, and holography, HeNe lasers or semiconductor lasers with external wavelength stabilization have been widely used to achieve the required narrow linewidth at 633 nm. Semiconductor lasers utilizing wavelength selective grating fabricated on semiconductor chip enable the miniaturization of laser systems while also providing numerous advantages such as low energy consumption, reliability and tunability by temperature and injection current. Moreover, DBR gratings reduce the complexity of the laser system compared to external cavity solutions where integration of multiple components is required. DBR gratings on semiconductor surface are fabricated without epitaxial regrowth step that could degrade the performance and lifetime of the device. In addition, low-order surface gratings are providing higher reflectivity than high-order gratings which could lead to a decreased emission linewidth and the output power. To this end, at red emission region low-order gratings require a small pitch, which in combination with the required etching depth for surface gratings leads to high aspect ratio gratings. In this work, Modulight demonstrates the fabrication of high aspect ratio low-order DBR surface gratings to optimize the device performance. DBR gratings are fabricated by electron beam lithography (EBL) method to achieve the required tight pitch patterns. EBL gratings are etched to AlGaAs or AlInP cladding layer using inductively coupled plasma reactive ion etching (ICP-RIE) to produce the desired high aspect ratio structures with smooth vertical side profiles. These two cladding materials are compared based on the etching profiles and device performance.