Extended-Cavity Single-Frequency Semiconductor Lasers using Ring Filters in Low-Loss SiN Technology

Extended-cavity semiconductor lasers with high spectral purity and wide spectral coverage are important for a large range of applications including fiber-optic communications [1], optical sensing [2], or applications in space, for instance in atomic clocks [3]. Monolithic diode lasers for such tasks, e.g., distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers, approach their limits since they typically show either a small tuning range [4] or considerable spectral linewidths at the MHz level [5]. These limitations can be largely removed in hybrid lasers, where the gain from a semiconductor optical amplifier chip is receiving spectrally filtered feedback from a second chip fabricated from dielectric material. The dielectric chip carries an integrated-optical waveguide circuit with which highly selective filtering and a long photon lifetime can be realized beyond what is typically possible in semiconductor platforms.