Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers

Abstract

We report on the observation and experimental characterization of a threshold-like onset of mode instabilities, i.e. an apparently random relative power content change of different transverse modes, occurring in originally single-mode high-power fiber amplifiers. Although the physical origin of this effect is not yet fully understood, we discuss possible explanations. Accordingly, several solutions are proposed in this paper to raise the threshold of this effect.

Bending performance of large mode area multi-trench fibers

“…MTFs with a core diameter of 30µm ensuring HOMs losses larger than 10dB/m and 1dB/m and the FM losses smaller than 0.1dB/m and 0.01dB/m respectively, over wide range of bend radius have been achieved. The A eff of the FM remains larger than 380μm 2 . Furthermore, the mode area scaling to 790μm 2 for a 46µm core diameter at 20cm bend radius has been achieved but at the expense of reduced losses ratio between HOMs and FM.…”

“…Bending losses of the MTF can be analyzed using standard conformal mapping [17] with additional stress perturbations [18] by following the standard equation: ( ) ( ) 2 2 eq 2r n r,φ n r * 1 cosφ , ρR…”

“…Moreover, it has recently been found that presence of Higher Order Modes (HOMs) in fiber lasers and amplifiers can cause detrimental modal instability at high power [2]. The impact of the bend also becomes stronger with increasing A eff [3].…”

5 kW Near-Diffraction-Limited and 8 kW High-Brightness Monolithic Continuous Wave Fiber Lasers Directly Pumped by Laser Diodes

“…4 shown, the beam quality factor was measured to be ∼1.732 and ∼1.779 in the x and y directions, respectively, for the laser beam with >5000 W laser power using a beam profile analyzer (M2-200S, Ophir Photonics) based on 4-sigma method; near diffraction-limited beam quality was achieved. Mode instability is a phenomenon of beam quality degradation for high power fiber laser systems [16], [17]. The >5000 W laser beam was monitored and no mode instability was observed.…”

Estimating modal instability threshold for photonic crystal rod fiber amplifiers

“…New complex fiber designs favor large differential gain between the fundamental mode (FM) and higher order mode (HOM); for instance by using resonant structures in the cladding to effectively remove HOMs from the core [2], or delocalizing HOMs at low signal powers [3]. Currently, the onset of transverse modal instability (TMI) degrading the output beam quality sets the upper limit for power scaling [4][5][6]. TMI sets in as the average signal power reaches a certain threshold, typically on the order of a few hundred W. A phase shift between a thermally induced index grating at high power levels and the signal causes power transfer from the FM to a HOM leading to TMI.…”