Fig. 2
From: Electrically pumped photonic integrated soliton microcomb
Electrically pumped soliton microcomb via laser injection-locked soliton formation. a Transmission spectrum of a Si3N4 microresonator of 1.02 THz free spectral range (FSR), featuring two sets of resonances: the fundamental transverse electric (TE) mode family (marked by red circles) and one high-order TE mode family. b The laser spectrum of the multi-frequency laser diode chip used in this experiment, corresponding to state i in g. c Measured and fitted heterodyne beat signal between the free running laser diode and a narrow linewidth reference laser (Toptica CTL1550, short-time linewidth ~10 kHz), showing 60 MHz full-width at half-maximum (FWHM) of Voigt profile. d (state ii in g): Spectra of single longitudinal mode that is injection locked to a selected resonance of the microresonator. f (state iii in g): Spectrum of the Kerr frequency comb that stems from the laser injection locking. Inset: One resonance of the fundamental TE mode showing mode splitting due to backscattering, with the estimated 118 MHz coupling strength \(\left( {\frac{\gamma }{{2\pi }}} \right)\) between the forward and backward propagating modes. e Heterodyne beat signal between the injection-locked laser and a narrow linewidth reference laser. The measured beat signal is fitted with Voigt profile with FWHM ~186 kHz (cf. Methods). RBW: Resolution bandwidth. g Typical transmitted power trace measured at the chip output facet, by current modulation imposed on the laser diode, in which different states are marked: (i) noisy, multi-frequency lasing without injection locking; (ii) laser injection locking to a microresonator resonance, and simultaneous formation of low-noise single-longitudinal-mode lasing (the orange region); (iii) formation of Kerr frequency comb (the green region)
