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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