Miniaturizing Mode-Locked Laser to Optical Chip Size

Setting out to improve a technology that usually requires bulky, bench-top equipment, Quishi Guo and colleagues have shrunk a mode-locked laser (MLL) to the size of an optical chip with an integrated nanophotonic platform. The results show promise for developing ultrafast nanophotonic systems for a wide range of applications. Mode-locked lasers (MLLs) can produce coherent ultrashort pulses of light at extremely fast speeds – on the order of picoseconds and femtoseconds. These devices have enabled numerous technologies in photonics, including extreme nonlinear optics, two-photon microscopy, and optical computing. However, most MLLs are expensive, power-demanding, and require bulky discrete optical components and equipment. As a result, the use of ultrafast photonic systems has generally been limited to table-top laboratory experiments. What's more, so-called "integrated" MLLs meant to drive nanophotonic platforms suffer from critical limitations like low peak power and a lack of controllability. Through hybrid integration of a semiconductor optical amplifier chip with a novel thin-film lithium niobate nanophotonic circuit, Guo et al. created an integrated MLL the size of an optical chip. According to the authors, the MLL generates ultrashort ~4.8 picosecond optical pulses at around 1065 nanometers with a peak power of ~0.5 Watts – the highest output pulse energy and peak power of any integrated MLLs in nanophotonic platforms. In addition, Guo et al. showed that the repetition rate of the integrated MLL can be tuned over ~200 megahertz range, and that the laser's coherence properties can be precisely controlled, providing a route toward a fully stabilized on-chip nanophotonic frequency comb source.