Temporal dynamics of mode instabilities in high-power fiber lasers and amplifiers

Abstract

The temporal behavior of mode instabilities in active large mode area fibers is experimentally investigated in detail. Thus, apart from the onset threshold of mode instabilities, the output beam is characterized using both high-speed camera measurements with 20,000 frames per second and photodiode traces. Based on these measurements, an empiric definition of the power threshold of mode instabilities is introduced. Additionally, it is shown that the temporal dynamics show a transition zone between the stable and the unstable regimes where well-defined periodic temporal fluctuations on ms-timescale can be observed. Finally, it is experimentally shown that the larger the mode-field area, the slower the mode-instability fluctuation is. The observations support the thermal origin of mode instabilities.

13 kW monolithic linearly polarized single-mode master oscillator power amplifier and strategies for mitigating mode instabilities

“…The sudden degradation from 94% to 90% above 1.2kW was caused by MI while the slowly degradation of PER as lasing power increases may be caused by the temperature increase of the fiber. An InGaAs photo detector (150 MHz, 700-1800 nm, Thorlabs) with a pinhole of 1.5mm diameter was put in the center of the collimated beam to monitor the onset of MI [10,11,19]. Time traces at different output power are shown in Fig.…”

Theoretical analysis of mode instability in high-power fiber amplifiers

“…The output signal in this case shows the presence of additional symmetrically distributed sidebands at the second harmonic of the modulation frequency. Recent experiments have utilized a beam aperture measurement to investigate the temporal dynamics of TMI, in which the intensity in a small part of the output beam near field image is measured by a fast photo-diode [4,8,14]. The idea is that the temporal mode fluctuations result in an intensity variation, which is then recorded and analyzed.…”

“…These coupling coefficients can be evaluated numerically using standard quadrature methods for any given set of modes. Although our model can include an arbitrary number of transverse modes, the detailed experimental analysis of TMI has shown that only the FM and the first higher-order mode of LMA fibers are involved, except for fibers with very large mode areas [4]. In this paper we therefore include only the fundamental LP 01 mode and one of the two degenerate LP 11 modes of a simple step-index fiber (SIF).…”

“…However, some large-core fibers may support additional guided modes, such as LP 02 -like modes, in particular for operating powers for which the thermo-optic effect is strong enough to significantly alter the guiding properties of the fiber [6]. While the presence of an additional LP 31 mode has been reported in TMI of a large-pitch fiber with a mode field diameter of 75 μm [4], LP 02 -like modes have as yet not been observed to take part. To investigate this, we have calculated the nonlinear coupling coefficients for both LP 01 − LP 11 coupling and LP 01 − LP 02 coupling for a SIF with a V parameter of 5 and all other parameters the same as Fiber A.…”

“…The phenomenon manifests itself as a temporal fluctuation, typically on a ms timescale, of the output beam profile as the output power reaches a certain threshold. Detailed experimental investigations of TMI have shown that the power and relative phase of the light in the fundamental and higher-order modes of the amplifier fluctuates on a timescale which depends strongly on the core diameter of the fiber, and that only the first higher-order mode (HOM) is involved, except for fibers with very large core diameters [3,4]. While the onset of TMI does not in itself prevent the amplification of the signal beyond the instability threshold, the resulting dramatic decrease in beam quality renders the amplifier useless for applications that require a stable output beam.…”