Origin of thermal modal instabilities in large mode area fiber amplifiers
Spotlight summary: Over the last decade, fiber laser systems have gained the reputation of being a power-scalable concept with power-independent, nearly-diffraction-limited beam quality. This reputation is grounded on the unparalleled development rate that this technology has undergone in recent years. Thus, in a comparatively short time, fiber lasers evolved from mere Watt-level laboratory prototypes to full-fledged multi-kW commercial systems with a deep market penetration. This rapid progress has been enabled by the excellent thermal management resulting from the high surface-to-active-volume ratio offered by the fiber geometry. However, with the extremely high powers being extracted from active fibers nowadays, it is natural that sooner or later some limitations would arise. It was widely assumed that these limitations would appear in the form of non-linear effects due to the very high intensities and long interaction lengths in the fiber core. However, the impressive recent advances in fiber design have successfully mitigated these effects to the point that a new unexpected limitation has become the most dominant one: mode instabilities.
The term “mode instabilities” refers to the threshold-like degradation of the output beam of a fiber laser system once a certain average output power has been reached. This effect is so severe that it threatens to put the brakes on the once relentless power scaling of fiber laser systems. Moreover, this effect is particularly damaging for the fiber community because it undermines one of the pillars on which the reputation of fiber systems is cemented: their power-independent, nearly-diffraction-limited beam quality. This explains the high interest that mode instabilities have awoken in the community and the great expectation with which every new advance within the topic is met.
In this context, in their paper “Origin of thermal modal instabilities in large mode area fiber amplifiers” Benjamin Ward et al. succeed in simulating the main experimental characteristics of this effect, which was reported for the first time by our group. This important contribution confirms the validity of the first explanations of the origin of mode instabilities, proposed shortly after the description of the effect, which are based on the existence of a thermally-induced long period index grating being generated in the active fiber through modal beating. Such an induced grating, which is not static but moves along the active fiber, always has the right periodicity to transfer energy between the transverse modes that originally gave rise to it.
When researching the literature, it soon becomes evident that this effect is not exclusive of optical fibers. Similar phenomena have been observed for years in experiments as diverse as gas-filled hollow-core compression stages, intra-cavity high-harmonic generation experiments, and gas and liquid media (where they are typically referred to as stimulated thermal Rayleigh scattering). Actually, from a broad point of view, the effect of mode instabilities can be classified as a relatively new manifestation of the two-wave mixing phenomena that was extensively studied decades ago. In most of the fields mentioned above this kind of phenomenon remains an unsolved problem. However, optical fibers offer unique possibilities to control mode instabilities, thanks to their waveguiding nature. This has already been proven with the introduction of advanced fiber designs, with which an increase of the mode instability threshold when compared to other fiber designs with similar mode-field diameters has already been demonstrated.
At this stage, understanding the physics of mode instabilities is of capital importance in order to come up with solutions that mitigate the effect and thus raise its threshold of ocurrence. This is precisely where the importance of Benjamin Ward’s paper lays. This paper presents the most detailed numerical simulations of the effect to date, and much can be learned from them. For example, it is very interesting to see that the modal content is only really unstable in the last section of the fiber. Moreover, this unstable section becomes longer with higher output powers. Furthermore, from the simulations the authors have identified longitudinal heat flow as one of the factors leading to the triggering of the effect. Thus, among an arsenal of mitigation strategies, they propose trying to minimize the maximum longitudinal thermal gradient, e.g., by creating an opposite gradient on the heat sink. We are sure that the authors are currently working on these mitigation strategies and we are already looking forward to the results. In any case, the work of Ward et al. is good news for the fiber community since it suggests that the effect can, in principle, be effectively mitigated by either changes in the fiber design or by external means.
The kind of advanced fiber models developed by Ward et al. and by several other researchers around the world will become invaluable in the near future for proposing and developing mitigation strategies for mode instabilities in high-average-power fiber-laser systems. In fact, with the rate of progress in the understanding of this effect that we have seen over the last year, we are convinced that some successful experimental demonstrations of the increase of the mode instability threshold will be reported in the coming months.
--Cesar Jauregui, Jens Limpert, and Andreas Tuennermann
Technical Division: Optoelectronics
ToC Category: Lasers and Laser Optics
|OCIS Codes:||(140.3510) Lasers and laser optics : Lasers, fiber|
|(140.4480) Lasers and laser optics : Optical amplifiers|
|(140.6810) Lasers and laser optics : Thermal effects|
|(190.4370) Nonlinear optics : Nonlinear optics, fibers|
|(060.5295) Fiber optics and optical communications : Photonic crystal fibers|
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