Fig. 1. PRBS phase modulation spectrum after lowpass filtering

Fig. 2. Spectrum after PRBS phase modulation when ratio of filter cutoff frequency to modulation frequency is 0.53 and modulation depth is 0.55$\mathrm{\pi }$

Fig. 3. Sinusoidal phase modulation spectra under different modulation frequencies and modulation amplitudes. (a) Modulation frequency is 9 GHz and modulation amplitude is 6π; (b) modulation frequency is 9 GHz and modulation amplitude is 3π; (c) modulation frequency is 4 GHz and modulation amplitude is 6π

Fig. 4. Sinusoidal phase modulation spectra when modulation frequency is 9.7 GHz and modulation amplitude is 0.458π

Fig. 5. Spectrum after cascaded PRBS and sinusoidal phase modulation

Fig. 6. Structural diagram of narrow linewidth fiber laser system

Fig. 7. Spectra and backward transmission power curves under different linewidths. (a) Spectra at root mean square (RMS) linewidths of 20 GHz and 46 GHz; (b) backward transmission power curves at RMS linewidths of 20 GHz and 46 GHz and SBS threshold versus output power

Fig. 8. Forward output spectra under different powers

Fig. 9. Laser output characteristic curve