Fig. 5: Self-oscillation analysis in the time domain.

Self-oscillation amplitudes in the time domain and the corresponding spectrograms with various direct currents from 6.36 mA (a), 6.56 mA (b), 6.86 mA (c) to 7.26 mA (d). The distortion of the oscillation signal becomes more and more significant with the increasing of the bias current, and meanwhile, the high-order harmonics gradually appear