Fabricating distributed feedback laser gratings with bismuth and gold focused ion beams

Fabricating first order gratings for laterally coupled distributed feedback (LC-DFB) lasers can be challenging due to aspect ratio dependent etching. Developments in focused ion beam (FIB) processing and source technology introduce the potential to mill grating structures directly into a ridge waveguide (RWG) laser structure without the requirement for electron beam lithography and inductively coupled plasma etching. In this work, we investigate the suitability of using bismuth and gold FIBs to mill Bragg gratings for DFB laser diodes directly into as well as adjacent to InP RWG laser structures. We explore the milling strategy on the profile of the fabricated grating structures. We observe that the single pixel line scanning routine is favorable when milling directly onto an RWG and using a concentric scanning method is better when milling the grating structures adjacent to the RWG. We conclude that milling off-ridge is more promising using this technique, as gratings with sidewall angles of 18° have been milled to a depth of over 350 nm. Based on modeling, a grating milled adjacent to the ridge with a dose of 60 000 μC/cm2 would provide a coupling strength of 73 cm−1. Such values would be suitable for DFB laser devices with cavity lengths as low as 200 μm.