Fig. 3. Spectral characterization of self-injection locked soliton microcombs.

a Photographs of the DFB laser diode and the Si₃N₄ photonic chip containing 9 microresonators of 30.6 GHz FSR. Experimental setup: the RF mixer is utilized to study RF signal above 26 GHz in combination with a local oscillator (LO). TC: temperature controller. CC: current controller. AFG: arbitrary function generator. OSA: optical spectral analyzer. ESA: electrical spectral analyzer. FPD: fast photodetector. b The optical spectrum of the laser self-injection locked single-soliton microcomb. Inset: optical spectrum of the free-running DFB. c Comparison of laser phase noise in the free-running and SIL regimes. The phase noise is measured by heterodyning of the system output and CW Laser. The Lorentzian linewidth of the free-running diode is 119 kHz, for the SIL DFB—1.1 kHz. Also, the phase noise in multi-frequency regime of the DFB laser diode is presented (see Supplementary Note 1 for details). d Phase noise of the soliton repetition rate signal in different SIL regimes and of LO. The phase noises of 35.4 GHz repetition rate signal are measured directly by high-RF electronics, without RF mixer. Inset: soliton repetition rate signal.