High power fiber lasers have been widely used in biomedical, laser processing and national defense security fields because of their high electro-optic efficiency, good beam quality and compact structure. Coherent polarization synthesis is a kind of high-power fiber laser output Useful ways. Coherent polarization synthesis technology can overcome the non-linear effects, thermal effects, mode instability and other factors to achieve a higher brightness of the laser output. However, the synthesis technique places strict requirements on factors such as light source characteristics (modes, polarizations, spectral lines), composite element characteristics (dynamic jitter, thermal aberration) and phase-locked residuals of a phase control system, making the development difficult. National Defense University Liu Zejin research group through phase-locked control system optimization, defocus aberration compensation, high-precision optical path control, through four narrow-line, linear polarization optical fiber amplifier coherent polarization synthesis achieved 5.02 kW near-diffraction limit synthesis laser Output, the synthesis efficiency of 93.8%. The results reported in the "China Laser" 2017 the fourth period The experimental system used in the synthesis of structural schematic diagram shown in Figure 1 (a) below. Narrow linewidth seed lasers (NBLs) are first split into four by a beamsplitter (BS) and then injected sequentially into a phase modulator (PM) and a fiber delay line (DL). Each laser through the DL injected into the three full Paul partial cascade fiber amplifier (A1, A2, A3) for power upgrade. The amplified beam is subjected to coherent polarization synthesis after the collimated output of the self-developed defocus compensation collimation system (CO). Figure 1 (b) shows the corresponding relationship between the output power of the four-way amplifier and the pump power. The amplified four-way laser power is 1.29 kW, 1.14 kW, 1.12 kW, 1.8 kW respectively. Further power increase is limited by the pump power. Figure 1 (c) for the synthesis of power and synthetic efficiency with the total incident power of the corresponding relationship. When the total injection power is 5.35 kW, the synthetic laser power is 5.02 kW and the system synthesis efficiency is 93.8%. Figure 1 (a) Schematic diagram of the experimental protocol; (b) Four-channel amplifier output power and pump power of the corresponding relationship; (c) synthetic power and synthesis efficiency with the total incident power diagram. The results show that when the combined laser power is 5.02 kW, the measured laser beam quality M2 <1.3, which represents the highest output power of the international coherent polarization synthesis system.