This paper studies the method of using a high-power fiber combiner to form a Gaussian beam into a flat-top beam, and divides the factors that affect the shaping effect of the flat-top beam. The theoretical analysis model of the fiber power combiner is established based on the waveguide theory, and the excitation characteristics of the fiber mode in the output fiber are analyzed. Based on the beam propagation method, the propagation and superposition characteristics of the highorder modes excited in the fiber combiner are simulated, and the flat-top beam synthesis method is studied. According to the simulation model, a high-power fiber combiner was made, and a high-power laser beam combining experiment was carried out. Finally, a flat-top beam with a beam quality of 10 and an output power of 20kW is obtained. During the experiment, we also found that the uniformity of the flat-top beam increases as the output power of the fiber laser increases.
A high power short-cavity random fiber laser employing the gain mechanism of the Yb-doped fiber and the half-open cavity structure and the temporal optical rogue waves (RWs) behavior are observed and investigated in the paper. The record output power without the stimulated Raman scattering (SRS) is promoted to 26.6 W in the YDRFL with the GDF length of 120 m. The stochastic pulses and temporal optical RWs are observed and demonstrated in the short cavity YDRFL for the first time. It is found that the proportion of RWs depends on the GDF length which can also affect the stability of output lasing. The research results reveal that achieving the relative stable output power requires the greater pump power for the shorter GDF length, although decreasing the GDF length will promote the maximum output power of the YDRFL without the SRS.
Using self-designed large core Yb-doped fiber (100 μm / 400 μm)1 and high-power fiber combiner, a laser output with average power of 3 kW, repetition rate of 60 kHz, pulse width of 150 ns, single pulse energy of 50 mJ is achieved by combining three laser modules in an all fiber-fusion structure, where each output power is more than 1000 W; and the beam combining efficiency is 98.3%. By controlling the pulse delay of the three modules, the three pulses are managed to be overlapped completely, and the overlapping rate is more than 97%. This is the highest value among all known fiber pulsed laser output using self-developed fiber and pulse beam combiner.