Optimizing high-power Yb-doped fiber amplifier systems in the presence of transverse mode instabilities

Abstract

The average output power of Yb-doped fiber amplifier systems is currently limited by the onset of transverse mode instabilities. Besides, it has been recently shown that the transverse mode instability threshold can be significantly reduced by the presence of photodarkening in the fiber. Therefore, reducing the photodarkening level of the core material composition is the most straightforward way to increase the output average power of fiber amplifier systems but, unfortunately, this is not always easy or possible. In this paper we present guidelines to optimize the output average power of fiber amplifiers affected by transverse mode instabilities and photodarkening. The guidelines derived from the simulations do not involve changes in the composition of the active material (except for its doping concentration), but can still lead to a significant increase of the transverse mode instability threshold. The dependence of this parameter on the active ion concentration and the core conformation, among others, will be studied and discussed.

Research of the Mode Instability Threshold in High Power Double Cladding Yb‐doped Fiber Amplifiers

“…However, all of them will produce the increasing of the heat load, and this will result in decreasing of the MI threshold. According to the previous report, the amplifier with counter‐propagating pump configurations has a higher MI threshold comparing to the co‐propagating pump configurations. Moreover, it has a higher SRS threshold.…”

“…It includes to reduce pump absorption level by modifying the fiber design. Until now, the influence of various fiber laser parameters on MI, such as pump wavelength, pump directions, signal wavelength, core/cladding radio have been studied theoretically and experimentally. However, from our knowledge, no strategy or experiment about reducing the pump absorption level by broadening the pump linewidth has ever been reported until now.…”

Power scaling limits in high power fiber amplifiers due to transverse mode instability, thermal lensing, and fiber mechanical reliability

“…It is observed that TMI sets in well before the conventional TL limit is reached, following closely a (1/d0) 2 dependence in very good agreement with our theory. It should be noted that use of the alternative TMI power threshold [7], which in this case takes the simplified form PTMI2 = 34(ηlaser/ηheat)L, would have resulted in a TMI threshold of ~105W, independent of core diameter. …”

“…In these studies, different TMI power threshold formulae were considered giving different power limits. In [5] the TMI threshold is considered to be reached at a fixed average heat load of Q0=34W/m, independent of core diameter [7], and predicts a diode-pumped power limit of 70kW, defined though by SRS and TL. In [6] the TMI threshold scales inversely with core area and the diode-pumped power limit is ~28kW.…”

Photodarkening as a heat source in ytterbium doped fiber amplifiers

“…In general, minimizing PD by core material composition is advised as the solution for increasing the output power of high-power fiber lasers limited by TMI. When not possible, active ion concentration and the core conformation should be optimized to reduce TMI [8].…”