Theoretical study of pump power distribution on modal instabilities in high power fiber amplifiers

Abstract

Influence of pump power distribution on modal instabilities (MI) is studied numerically, which reveals that the MI threshold is dependent on the pump power distribution in fiber amplifiers and can be increased by optimizing the pump power distribution without any adjustment of other amplifier parameters. It shows that amplifiers with backward or bi-direction pump schemes have a higher threshold than those employing forward pump schemes. For backward pumped amplifiers employing fiber with core/clad diameter being 20/400 µm, the MI threshold yields a 42% increase compared to the forward pumped ones. For bi-direction pumped amplifiers, there exists an optimal power ratio between forward and backward pump power, which results in the highest threshold power. When amplifiers with core/clad diameter being 20/400 µm employ a bi-direction pump scheme at the optimal backward pump power fraction, the threshold can be increased by a factor of approximately 60% with respect to the forward pump configuration. The threshold increment factor reduces as the gain saturation effect weakens. It also shows that the MI threshold can be increased by employing multi-point side pump schemes.

Towards the Enhancement of the TMI Threshold in Monolithic High-Power Fiber System by Controlling the Pump Distribution and the Seed Power

“…The strength of the TMI at 3030 W is far stronger than that at 2980 W. So it is thought that the true TMI threshold is below 3030 W and the TMI threshold begins to decline with the ratio of the backward pump power increased further, which agrees with the reported result in Ref. [54]. As a result, the 2980 W is regarded as the final TMI threshold under the bi-directional pump regime, with a leap of 1640 W compared with the co-pump regime.…”

“…For the co-pump regime and the counter-pump regime, the calculated TMI thresholds are close to experimental ones within reasonable errors. Due to stronger gain saturation effect in counter-pump regime [54], calculated TMI threshold is higher than that under the co-pump regime, which agrees with the experimental result. For the bi-directional pump regime, the calculated TMI threshold displays a rising trend with the ratio of backward pump power increasing, which also accords well with the experimental results and the conclusion in Ref.…”

“…By contrast, the completely all-optical strategies, such as optimizing the pump distribution and the injected seed power without any change of the system during operation, are more convenient to enhance the TMI threshold based on the simplified system. Yet, the effect of optimizing the pump distribution on improving the TMI threshold was only systematically exploited in theory scope [26], [54]. For the monolithic fiber laser system, the investigation in experiment scope was less conducted.…”

Experimental Investigations on TMI and IM-FWM in Distributed Side-Pumped Fiber Amplifier

“…3 (i.e., the threshold is 1625 W), it can be known that the counter-pumping case is the most beneficial, while the co-pumping one is the least beneficial to suppress the TMI in the fiber amplifier. These results are consistent with [40], [41], compared with co-pumping case, the counter-pumping case has lower upper state populations, which can increase the TMI threshold. By comparing Figs.…”

Experimental study of the influence of mode excitation on mode instability in high power fiber amplifier

“…The amplified seed laser is coupled into the main amplifier through a mode field adapter. To increase the TMI threshold, the counter pumping configuration has been employed in the main amplifier stage 10 . The main amplifier is made up of a counter (6 + 1) × 1 signal/pump combiner, a 18 m long Nufern 25/400 large mode area Yb-doped fiber with pump absorption factor of 0.6 dB/m at 915 nm, two cladding power strippers (CPSs) and an output quartz block holder (QBH).…”