Studies On High Beam Quality Pump Coupler With Large Mode Area Fiber

One of the key components of fiber laser or amplifier is the all-fiber signal and pump combiner.The output mode area can directly determine the output laser beam quality of the amplifier.At present,there are few studies on high-beam quality pump couplers with output cores above 40?m.In this paper,the theory and experiment of high beam quality pump coupler with large mode area are studies based on the Taper-Fused Fiber Bundle(TFB)technique.A multi-stage taper scheme is proposed to realize the mode area adaptation of signal light in the large mode area pump coupler,and the corresponding experimental research is carried out.The main content of the paper includes the following aspects:1.The design scheme of the cascaded taper coupler is proposed.By using the signal transition fiber of the larger mode field,the coupler signal light is transmitted from the small mode field signal fiber(10?m)to the larger mode field signal transition fiber(30?m)to the mode field adaptation of the large mode field output fiber(50?m).The fabrication parameters of the coupler are calculated preliminarily,and the influence of the different fused fiber bundle taper ratio on the output light field is analyzed.2.The numerical simulation model of the pump coupler with large mode area output fiber is established.The different modes of the output light field of the fused fiber bundle and the output fiber,the transmittance of the signal light,and the pump coupling efficiency of the pump fiber under different taper ratios are simulated,In order to verify the possibility of the actual experimental scheme,the signal fiber--signal transition fiber--output fiber cascaded mode area adapter are fabricated experimentally.The results show that the design of the coupler is feasible.3.A large mode area(6+1)×1 high beam quality pump coupler with an output fiber core of 50?m is fabricated based on the casing method.The development of large mode field pump coupler has been completed through overcoming the key technologies of fused fiber bundle cutting and high signal light transmittance etc..The signal light transmittance of the coupler is greater than 98%,the average pump coupling efficiency of 6-ports are greater than 98%,the single-port pump power carrying capacity is greater than 1.5 kW,and the two-ports total power are 3 kW,the coupler signal light beam quality evaluation factor M~2=1.59.Based on the developed coupler,a high beam quality amplifier system is built.The total output power of the amplifier is 2.34 kW,the slope efficiency is 73.7%,and the beam quality evaluation factor M2 is 2.83 at the maximum power.