Fig. 4. DKS generation in LTOI microresonators.
a, Optical spectrum of a single soliton microcomb featuring a sech2-spectral profile with a 3 dB bandwidth of 4.9 THz, corresponding to an FWHM pulse duration of 63 fs at a pulse repetition rate, frep, of 81 GHz. Inset, the light generated during the rapid laser scan measured by filtering out the pump light. The soliton step is marked with light blue shading. Ipd, photodiode current. b, Optical spectrum of a single soliton with a repetition rate of 30.1 GHz; ΔTFWHM = 71 fs. Inset, the relative phase position inside the microresonator. c, Optical setup for soliton generation in x-cut LTOI microresonators. The orientation and TE polarization are also indicated in the schematic diagram of the fabricated LTOI chip. Rapid laser scans were generated using a single-sideband modulator (SSB) and voltage-controlled oscillator (VCO), continuous wave (CW) laser and an erbium-doped fibre amplifier (EDFA). FBG, fibre Bragg grating; FPC, fibre polarization controller. The soliton microcombs were analysed using an optical spectrum analyser (OSA) and the nonlinearly generated light and microwave beat notes were recorded with a PD and analysed with an OSC and electrical spectrum analyser (ESA), respectively. d, Variation of the Raman intensity with different LTOI crystal rotation angles. Inset, the angle between the orientation (z axis) and the polarization of the excitation laser. e, Optical spectrum of a three-soliton state with a repetition rate of 30.1 GHz. Inset, three solitons inside the microresonator. ΔTFWHM = 52 fs. f, Single-side band-phase noise-power spectral density (Sff) of a 30.1 GHz microwave beat note generated from the multisoliton state in e. Inset, spectrum of a microwave beat note with a resolution bandwidth of 30 Hz.