Design and fabrication of 780nm Fabry-Perot and distributed-feedback laser towards narrow linewidth

Cold atoms have important applications in high-precision metrology, sensing, navigation, etc. Laser cooling is crucial to acquire cold atoms^[1-2]. In particular, 780 nm narrow linewidth laser is a key enabling factor for cold Rb atoms. The linewidth of the 780nm laser should be below 1 MHz to exactly match the energy levels of Rb atoms. This far exceeds the linewidth limit of Fabry-Perot (FP) lasers. Distributed Feedback (DFB) lasers exhibit much narrower linewidths compared to FP lasers^[3]. However, the period of the grating for a 780nm DFB laser is much smaller than the period of the gratings of DFB lasers working at longer wavelength, e.g. at telecommunication wavelength. This presents a significant challenge in manufacturing 780nm DFB lasers^[4]. In this work we designed a 780nm DFB laser having a sidewall grating with a period of 360nm, a duty cycle of 50%, an etching depth of 550nm, and a ridge waveguide width of 1.2μm. The laser's epitaxial layer's fundamental properties were verified by a 780nm FP laser, with a 110mW single-sided output power. The DFB's grating was fabricated by electron-beam lithography (EBL) and Inductively Coupled Plasma (ICP) etching. Ultimately, our goal is to achieve a DFB laser with a linewidth below 1 MHz.