The Investigation Of All-fiber High-power Side-pumping Combiner

A signal/pump combiner,which is one of the most important fiber-based passive components for a fiber laser,is essential to couple the signal and pump light into the gain fiber of the amplifiers effectively.The output power level of a fiber laser can be determined by the power handling capability directly.Using side-pumping technique to fabricate a signal/pump combiner has many attractive characteristics,including uninterrupted signal fiber core and unlimited pump points,which can maintain high pump/signal transmission efficiency simultaneously,thus facilitating their potential applications in high power fiber laser and amplifiers.Therefore,it is of great importance to develop high power side-pumping combiner for high power fiber lasers with good beam quality.In this dissertation,through investigation of the high-power all-fiber side-pumping combiner has been demonstrated,which includes:The influence of the taper length of transition section,the mode distribution of the input pump light,the shape of the coating edge of the signal fiber as well as the number of pump ports,on the pump propagation properties of a side-pumping combiner,has been analyzed.The numerical results are very instructive for the design of the side-pumping combiner and the improvement of the fabrication technique.Based on the calculation results,it is pointed out that,in order to further improve the power handling capability of the component,one may face with the challenges including the unwanted beam quality degradation,the evident influence of input pump NA or brightness on the characteristics of the component,as well as difficulties of further increasing pump ports and maintaining high pump coupling efficiency simultaneously.The numerical model of signal propagation in a side-pumping combiner has been established.Based on this model,the numerical and experimental investigation has been demonstrated.The results show the beam quality degradation of the signal light is caused by the micro deformation and the thermal dopant diffusion of the signal fiber core during the heating process,while the signal insertion loss is mainly dominated by the thermal dopant diffusion effect.The simulation results indicate that a longer coupling region,a lower heating temperature and a shorter heating duration can suppress the beam quality degradation and reduce the signal insertion loss,which is validated in the experiments.This work provides instructions on the development of a side-pumping combiner with high beam quality and signal transmission efficiency based on tapered-fused technique.The effective methods and techniques of enhancing the pump power handling capability are put forward and discussed.By using these modified fabrication technique,a(2+1)×1 side-pumping combiner for high-power fiber laser based on tandem pumping has been developed numerically and experimentally for the first time.The total pump efficiency of the combiner is 98.1%with 2882 W input pump power at 1018 nm and an ytterbium-doped fiber amplifier based on tandem pumping with 1080 nm output of 2533 W is achieved based on the home-made combiner.The results show that,the brightness of the pump source is also a critical parameter that influences the performance of the side-pumping combiner.The combiner appears to have a better power handling capability when using 1018 nm fiber laser as the pump light,compared with laser diodes pumping.The numerical and experimental investigation about the loss mechanism of a two-stage-cascaded side-pumping combiner based on tapered-fused technique has been presented.The theoretical analysis indicates that the pump transmission efficiency is lower than expected due to the extra power loss when the pump light from the first stage propagates to the next stage.Besides,the heating load on the coating of the second stage is much more serious than that in the first stage,which is the dominant factor that limit the further enhancement of the pump transmission efficiency and the power handling capability of a cascaded structure.Both the simulations and experiments show that by increasing the taper waist diameter of the pump fiber in the first stage can suppress the extra pump loss in a cascaded scheme,which is the effective approach to achieve cascaded side-pumping structure with high pump transmission and power handling capability.The numerical and experimental investigation of extended L-band erbium-doped fiber amplifier(EDFA)is carried out by using C-band laser as the forward pump source Experimentally,a two-stage L-band EDFA(the 1st stage:standard 1480 nm laser diode(LD)pumping,the 2nd stage:C-band pumping)can finally achieve wideband 20 dB gain covering 1570-1618 nm and the maximum NF of 5.7 dB.The power transfer efficiency(PCE)is more than 50%higher compared to 1480 nm pumping for both stage.A bi-directionally dual wavelength pumped extended L-band EDFA is demonstrated,by using a standard 1480 nm pump laser as the forward pump source and a high-power C-band light source as the backward pump source.Experimentally,the L-band EDFA can finally achieve wideband 20 dB gain covering 1570-1619 nm and an average NF of 4.5 dB.The PCE is 40%higher compared to 1480 nm pumping for both direction.It is the first time that C-band light has been used as the pump source to extend the bandwidth of L-band EDFAs beyond 1610 nm.Considering the current availability of fiber lasers with high output power,the results show that C-band pumping should be considered for high performance extended L-band amplifiers,no longer suffering from the limitation of the output power from a single-mode 1480 nm LD.