Fig. 1. Principle of laser spectral purification based on distributed weak perturbation. (a) A novel laser configuration. (b) Evolution of laser phase fluctuation and noise coupling strength with different round trips. (c) Spectral distribution at different noise levels.

Fig. 2. Simulation results of laser linewidth evolution. (a) Output linewidth evolution with the feedback length under different feedback coefficients. (b) Linewidth curve at the different feedback ratios. Two-dimensional pseudocolor maps of the spectra vary (c) with the length and (d) with feedback ratio.

Fig. 3. An on-chip laser system based on weak external distributed perturbation. ISO: optical isolator; OC: optical coupler; PD: photoelectric detector; DAS: data acquisition system.

Fig. 4. Power spectra of the RF signal generated by beating two distributed feedback DFB lasers and spectral purity. (a) Comparison curves of the frequency spectrum from beat frequency signal. (b) Lorentz fitting curve of the linewidth with a distributed feedback. (c) Comparison curves of the frequency noise PSD. (d) Comparison curves of the RIN spectrum, where a red curve indicates the compressed result.

Fig. 5. Coherence of on-chip laser with different feedback ratios. (a) Evolution curves of the frequency noise PSD under different feedback ratios. (b) Evolution curves of output linewidth with the feedback ratio.

Fig. 6. Self-adaptive compression process of laser linewidth. (a) Transient spectrum and (b) corresponding Lorentzian linewidth when switching on the feedback. (c) Transient spectrum and (d) corresponding Lorentzian linewidth when tuning the frequency of the main laser cavity.