EPFL’s chip-scale erbium laser offers broad wavelength tunability

13 Jun 2024

Output exceeds 10 mW – outperforming many conventional systems, say its developers.

Fiber lasers use an optical fiber doped with rare-earth elements (erbium, ytterbium, neodymium, etc.) as their optical gain source. They emit high-quality beams, they are efficient and durable, and they are typically smaller than gas lasers. According to scientists at EPFL (L’École Polytechnique Fédérale de Lausanne), Switzerland, fiber lasers are now the “gold standard” for low phase noise, meaning that their beams remain stable over time.

But despite all that, there is a growing demand for miniaturizing fiber lasers on a chip-scale level. Erbium-based fiber lasers are of interest because they meet all the requirements for maintaining a laser’s high coherence and stability. But miniaturizing them has been met by challenges in maintaining their performance at small scales.

Now, scientists led by Dr. Yang Liu and Professor Tobias Kippenberg at EPFL have built the first ever chip-integrated erbium-doped waveguide laser that approaches the performance with fiber-based lasers, combining wide wavelength tunability with the practicality of chip-scale photonic integration. The study is published in Nature Photonics.

Chip-scale laser

The researchers developed their chip-scale erbium laser using a state-of-the-art fabrication process. They began 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 Dr. Liu.

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 performance and achieve precise wavelength control, the researchers engineered an innovative intra-cavity design featuring microring-based Vernier filters, a type of optical filter that can select specific frequencies of light.

Power, precision, stability, and low noise

The filters allow for dynamic tuning of the laser’s wavelength over a broad range, making it versatile and usable in various applications. This design supports stable, single-mode lasing with an impressively narrow intrinsic linewidth of just 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 side modes. This ensures a stable output across the light spectrum for high-precision applications.

The new laser features output power exceeding 10 mW and a side mode suppression ratio greater than 70 dB, outperforming many conventional systems. It also has a narrow linewidth, which means the light it emits is “very pure and steady”, says EPFL, which is important for coherent applications such as sensing, gyroscopes, LiDAR, and optical frequency metrology.

The microring-based Vernier filter gives the laser broad wavelength tunability across 40 nm within the C- and L-bands, surpassing legacy fiber lasers in both tuning and low spectral spurs metrics, while remaining compatible with current semiconductor manufacturing processes.