Bending diameter dependence of mode instabilities in multimode fiber amplifier

Abstract

We systematically investigated the dependence of mode instability (MI) thresholds on bending diameters in ytterbium-doped fiber amplifiers. The MI thresholds of a fiber amplifier, which was based on a multimode ytterbium-doped fiber, were measured at various bending diameters. It is found that the correlation between MI thresholds and fiber bending diameters depends on the operation state of the active fiber. When the multimode active fiber is in the few-mode operation, the MI threshold is successfully raised from 717 W to 1084 W by increasing the fiber bending diameter from 11 cm to 60 cm. When the multimode active fiber operates in quasi-single mode, the MI threshold is raised from 717 W to 953 W by reducing the bending diameter from 11 cm to 8 cm. According to our experiments and previous reports, it is suggested that the influence of changing fiber bending diameter on MI might be closely related to the mode coupling effect.

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

“…Besides, the similar results have been reported in the Ref. [43] without specific physical explanations.…”

“…It is similar to the situation when the bending diameter ranges from 11 cm to 60 cm in the Ref. [43] (the V-value of gain fiber is 5.7735). On the other hand, when the bending diameter decreases, the bend-induced mode coupling becomes serious which means the higher-order mode can be excited and the higher power ratio of the HOMs account for [16] (see Fig.…”

“…Secondly, we found that the IM-FWM threshold can also be suppressed by the increment of bending diameter, which is not revealed in the Ref. [43]. The main reason is that with the decrease of bending diameter, as mentioned above, the bend-induced mode coupling between the LP 11 mode and LP 01 mode become serious.…”

“…its V-value was 4.729. On the one hand, when the bending diameter is larger than 30 cm, the corresponding bend-dependent modal losses of LP 11 (<10 dB/m) cannot effectively wear out the amplification of this mode [28], [43]. It is similar to the situation when the bending diameter ranges from 11 cm to 60 cm in the Ref.…”

“…Nowadays, majority of studies were carried out to show that both the TMI and IM-FWM can be suppressed by the reduction of bending diameter which can enhance the loss of high-order modes [14], [15]. However, considering the bend-induced coupling will be relieved with the increment of bending diameter [42], [43], it is interesting how these two thresholds will be varied by increasing Fig. 7.…”