Blue GaN-based DFB laser diode with sub-MHz linewidth
Emily Trageser, Haojun Zhang, Sonya Palmer, Theodore J. Morin, Joel Guo, Jiaao Zhang, Evan Geske, Heming Wang, Andreas Boes, Shuji Nakamura, John E. Bowers, Steven P. DenBaars
Abstract
Distributed feedback laser diodes (DFBs) serve as simple, compact, narrow-band light sources supporting a wide range of photonic applications. Typical linewidths are on the order of sub-MHz for free-running III-V DFBs at infrared wavelengths, but linewidths of short-wavelength GaN-based DFBs are considerably worse or unreported. Here, we present a free-running InGaN DFB operating at 443 nm with an intrinsic linewidth of 685 kHz at a continuous wave output power of 40 mW. This performance is achieved using a first-order embedded hydrogen silsesquioxane (HSQ) surface grating. The frequency noise is measured using a cross-correlated self-heterodyne frequency discriminator, and two estimations of integrated linewidth are evaluated using 1/π integration and β-separation line integration methods.