Semiconductor laser diodes are widely employed in many different applications because of their compactness and cost-effectiveness. However, when a very narrow linewidth is required, such as in high data-rate coherent communications or high precision metrology, making the laser on a small chip becomes much more challenging. The reason is that we have only two ways to reduce the linewidth of a laser: increasing the photon lifetime of the cavity and increasing the laser power. Yet, in both cases, loss is the main obstacle. A strategy is to extend the cavity length by coupling the gain chip to a passive feedback circuit. It may sound simple, but the design and the optical material of the feedback circuit, and the chip-to-chip optical coupling do really make the difference.

In this work, Y. Fan and coworkers successfully used a Si₃N₄ feedback circuit extending the laser cavity of an InP gain chip to a roundtrip optical length of half a meter. The passive architecture is made of a 33-mm-long low-loss (< 0.1 dB/cm) spiral waveguide followed by several microring-resonator filters. The microring filters inserted in the laser cavity enable single-mode oscillation in the 1550 nm wavelength range with ultranarrow linewidth and high side-mode suppression. Moreover, since Si₃N₄ is a wide-bandgap material, nonlinear absorption does not take place, despite the high intra-cavity optical power enhanced by the high-finesse spectral filtering. Blending all these ingredients, the result is a laser diode with a world-record narrow line of 40 Hz, 70-nm wavelength tunability, 23 mW output power and more than 60 dB side mode suppression.

These numbers are amazing, but even more exciting is the fact that we are not yet at the limit. Si₃N₄ waveguides with propagation loss as low as 0.1 dB/m, that is two orders of magnitude below the ones used in this work, are available today. Therefore, on-chip lasers with sub-Hertz linewidth are maybe not too far from appearing.

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