At the crossroads of growing demand for fiber lasers and chip-scale lasers, researchers at École Polytechnique Fédérale de Lausanne (EPFL) have developed a chip-integrated erbium-doped waveguide laser that approaches the performance of fiber-based lasers. It combines wide wavelength tunability with the practicality of chip-scale photonic integration. Fiber lasers use an optical fiber doped with rare-earth elements as their optical gain. These lasers are favored for their stable high-quality beams, high output power, as well as their efficiency, low-maintenance requirements, durability, and their smaller size compared to gas lasers. Responding to the demand for chip-scale fiber lasers, the researchers turned to erbium as the gain source. Erbium-based fiber lasers meet the requirements for maintaining high coherence and stability. Miniaturization, however, has been difficult to achieve due to challenges in maintaining their characteristic high performance. The researchers started by constructing a meter-long, on-chip optical cavity based on ultralow-loss silicon nitride photonic integrated circuit. “We were able to design the laser cavity to be meter-scale in length despite the compact chip size, thanks to the integration of these microring resonators that effectively extend the optical path without physically enlarging the device,” said Yang Liu, a researcher in EPFL’s Laboratory of Photonics and Quantum Measurements. Researchers at EPFL developed a hybrid integrated erbium-doped photonic integrated circuit-based laser (pictured), overcoming issues in frequency tunability to meet growing demand for chip-scale fiber lasers. Courtesy of Yang Liu/EPFL. The team then implanted the circuit with high-concentration erbium ions to selectively create the active gain medium necessary for lasing. Finally, they integrated the circuit with a III-V semiconductor pump laser to excite the erbium ions to enable them to emit light and produce the laser beam. To refine the laser’s performance and achieve precise wavelength control, the researchers engineered an innovative intracavity design featuring microring-based Vernier filters, a type of optical filter that can select specific frequencies of light. The filter allows dynamic tuning of the laser's wavelength across 40 nm within the C- and L-bands, which surpasses legacy fiber lasers in both tuning and low spectral spurs (unwanted frequencies) metrics, while remaining compatible with current semiconductor manufacturing processes. The design supports stable, single-mode lasing with a narrow intrinsic linewidth of 50 Hz. It also allows for significant side mode suppression — the laser’s ability to emit light at a single, consistent frequency while minimizing the intensity of other frequencies (‘side modes’). In practice, this allows “clean” and stable output across the light spectrum for high-precision applications. The chip-scale erbium-based fiber laser features output power exceeding 10 mW and a side mode suppression ratio greater than 70 dB, outperforming many conventional systems. Its narrow linewidth enables it to emit pure and steady light desirable for coherent applications like sensing, gyroscopes, lidar, and optical frequency metrology. The ability to produce chip-scale erbium fiber lasers could reduce overall costs and increase accessibility for portable integrated systems across telecommunications, medical diagnostics, and consumer electronics. Additionally, it could scale down optical technologies in applications like lidar, microwave photonics, optical frequency synthesis, and free-space communications. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-024-01454-7).