(a) Schematic diagram of fibre laser setup, where LD, PBS, and DM are diode laser, polarized beam splitter, and dichroic mirror, respectively. (b) Q-switched mode-locked pulses at pump power 57.6 mW. (c) CW mode-locked (CWML) pulses at pump power 286.9 mW. No discernable envelope modulation indicates stable operation. (d) Output optical spectrum. Because reduction in intracavity loss results in a lower initial Stark manifold of the ⁵I₆ level, center wavelength of continuous wave operation is red-shifted to 2864.3 nm. At the same time, since required spectral Fourier component is small, the full width at a half maximum reduces to 1.7 nm. (e) Radio frequency (RF) spectra have scan range 0.8 MHz and resolution bandwidth 10 kHz, with repetition rate and signal-to-noise ratios being 14.28 MHz and 54 dB, respectively. Inset shows RF spectra with broader scans ranging from 0 to 160 MHz. (f) Autocorrelation curve measured with intensity autocorrelator. Blue points are experimental results, and red line refers to the fitting results from sech function. This figure is adapted from Ref. [40], Nature Publishing Group.