Fig. 1. Experimental setup of kW-level laser power amplification based on MOPA structure ^[30]

Fig. 2. Experimental results^[30]. (a) kW-level laser power based on MOPA structure; (b) high resolution spectrum of phase modulated seed sources; (c) (d) spectra of forward and backward propagating signal at 1 kW

Fig. 3. Experimental setup and results of high-power narrow-linewidth laser power amplification with white noise phase modulation combined with SPGD algorithm polarization control ^[34]. (a) Schematic diagram of experimental setup; (b) single frequency laser output after phase modulation and laser output spectrum after power amplification ; (c) far field intensity distributions of the linear polarized laser beam

Fig. 4. Experimental setup and results of single frequency high-power narrow-linewidth fiber amplifier system based on cascaded WNS modulation ^[19]. (a) Experimental setup; (b) laser amplification power and PER versus pump power; (c) measured beam quality at 2540 W

Fig. 5. PRBS modulation signal optimization^[41]. (a) Spectrum of PRBS modulated signal and phase modulated signal after low-pass filtering; (b) measured spectrum of PRBS7 with 8.5 GHz modulation frequency after 2.2 GHz low-pass filtering (insert: wide spectrum of 4 GHz); (c) measured spectrum of PRBS9 with 8.5 GHz modulation frequency after 2.2 GHz low-pass filtering (insert: wide spectrum of 4 GHz)

Fig. 6. Experimental structure of the four-stage Yb-doped fiber amplifier

Fig. 7. Principle of the high-order phase modulation to broaden the seed source bandwidth using various electrical driving signals^[44]. (a) A single-frequency signal; (b) white noise; (c) a broadband signal with a rectangular-like envelope; (d) a broadband signal with a steep triangle-like envelope

Fig. 8. Amplitude envelope simulation diagram of P-turning sequence under different P values^[44]

Fig. 9. Experimental setup of the proposed high-power narrow-linewidth fiber laser based on seed source spectrum broadening^[44]

Fig. 10. Experimental results. (a) Broadened optical spectra of the seed source different P values; (b) relationship between laser output signal and reverse return signal under the modulation of cascaded white noise and P-turning sequence ^[44]

Fig. 11. Experimental results. (a) FPGA and DAC signal generation module; (b) forward output power and reverse power of the laser using cascaded WNS and real-time P-turning sequence with P=7/8 as driving signals, respectively (inset: self-pulse phenomenon observed by OSC)^[46]

Fig. 12. Experimental setup and results with chirped diode laser (ChDL) to suppress SBS effect^[47]. (a) Experimental setup, distributed feedback Bragg (DFB) laser, electro-optic modulator (EOM), photodiodes (PD), and cladding mode stripper (CMS); (b) backward power versus output power for various chirps, and for a seed with a bandwidth of 40 GHz, produced by random phase modulation

Fig. 13. Simulation result. (a) Phase (above) as a function of time, and frequency (below) as a function of time for the sawtooth (solid line) and triangle (dashed line) frequency chirp; (b) simulation diagram of laser power and backward Stokes wave power under different modulation formats^[43]

Fig. 14. Envelope simulation of multi-tone driving signals with different bandwidths and shapes

Fig. 15. Time-domain and frequency-domain simulation diagrams of the binarized multi-tone signals. (a) Time-domain; (b) frequency-domain

Fig. 16. Optical spectra of broadened seed sources. (a) Different bandwidths; (b) different spectrum types

Fig. 17. Optimization results of Pareto algorithm. (a) Relationship between threshold power of SBS effect and RMS linewidth of laser; (b) relationship between product of SBS threshold and length and RMS linewidth ^[15]