What is a laser? Should we simplify the definition of a laser to the extreme, we could say that a laser is an optical cavity with a gain sufficiently high to induce a positive feedback. However, when high performances are required, the concept itself of an optical cavity needs to be revisited. Many applications ask for lasers with wide tuneability (several tens of nanometers), very narrow linewidth (< 10 kHz), high side-mode suppression ratio (SMSR > 50 dB) and low lasing threshold. Meeting all these specifications in a small footprint and cost-effective integrated laser is definitely challenging.

In their work, A. Malik and coworkers efficiently combine the lasing properties of a quantum-dot gallium-arsenide gain medium with the possibility of engineering a sophisticated optical cavity on a silicon photonic platform. The proposed cavity architecture exploits a Vernier scheme with two thermally-controlled silicon microring resonators, with the addition of a tuneable Mach-Zehnder interferometer to further increase the SMSR. Results show single mode lasing in the O-band with 52 nm wavelength tunability, a SMSR of 58 dB and a record linewidth as low as 5.3 kHz. Even better metrics are expected if more complex cavity structures are employed, for instance including more resonators and other photonic devices.

Reading this paper, one realizes that the ultimate performance of integrated laser sources is only limited by our capability of designing, fabricating and finely controlling more and more complex optical cavities…which may actually look much different from the traditional concept of an optical cavity that we currently have.

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