Figure 4. Phase-noise spectra and beat-note spectral measurement.

(a) Phase-noise spectra measured for two, free-running 193 THz fibre lasers (black), fibre lasers independently locked to two, 3 mm disk resonators (blue), and fibre lasers independently locked to two, 1.2 m long spiral resonators (red). To test measurement reproducibility, resonators were characterized on multiple days. Measurement of the 1.2 m long resonators on a second day is shown as the dark-red trace. The data for the spiral resonator show an average suppression by 26 dB of the fibre laser noise when locked to the spiral resonators. In comparison, 10 dB of noise suppression is observed using the 3 mm device. The servo-control noise bumps appear at around 200 kHz for the locked phase-noise spectra. The inset is a plot of the noise suppression at 100 Hz and 1 kHz offset frequencies plotted versus resonator length for each of the resonators tested. (b) The electrical spectrum of the fibre lasers’ beat note for both free-running and locked configurations. Spectral narrowing and noise suppression are apparent in the locked spectrum.