Figure 4: Ultrafast mid-infrared fibre laser based on Cd₃As₂ saturable absorber.

(a) Schematic of the 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 a pump power of 57.6 mW. (c) CW mode-locked (CWML) pulses at a pump power of 286.9 mW. There is no discernable envelope modulation, indicating stable operation. (d) Output optical spectrum. It is observed that the centre wavelength of CW operation (by moving the focussed beam onto a spot of the gold mirror clear of the Cd₃As₂ sample) was red-shifted to 2864.3 nm, as the decreased intracavity loss led to a lower initial Stark manifold of the ⁵I₆ energy level. Meanwhile, the full-width at half-maximum (FWHM) was decreased to 1.7 nm as a result of fewer required spectral Fourier components. (e) Radio-frequency (RF) spectrum at a scanning span of 0.8 MHz with a resolution bandwidth of 10 kHz. The repetition rate and signal-to-noise ratio were 14.28 MHz and 54 dB, respectively. The inset shows the RF spectrum with a broader scanning range from 0 to 160 MHz. The smooth roll-off of the clean harmonic frequency components indicated that no Q-switched modulation and multiple pulsing were presented in this operation regime. (f) Autocorrelation trace measured with an intensity autocorrelator. The blue points are the experimental results and the red line is the fitting result using a Sech² function. The FWHM of the autocorrelation trace is 9.8 ps, corresponding to a pulse duration of 6.3 ps (a deconvolution factor of 1.54 is used to account for the Sech² pulse shape). For b–d,f, the intensities are normalized by the maximum value of measured physical quantities.