Abstract
In this work we investigate transverse mode instability (TMI) in the presence of pump intensity noise and a controlled perturbation of the input coupling for a rod-type fiber amplifier using spatially and temporally resolved imaging (ST). We show that inherent pump intensity noise from the power supply can define significant peaks in the resulting TMI spectrum. ST measurements show that the TMI in the transition region consists of different orientations of LP11. This finding indicates that the simple picture of TMI being seeded by the combination of a static initial fraction of LP11 and pump or signal intensity noise is not valid for our measurements. Furthermore we present seeding of TMI by perturbing the input coupling dynamically. ST measurements of the resulting TMI as a function of perturbation frequency provides quantitative information regarding the frequency response of the non-linear coupling coefficient. Finally, ST measurements of the resulting TMI as a function of signal power shows that the TMI experiences an exponential gain long before visible beam fluctuations appear.
Highlights
For many years, single mode fiber amplifiers have dominated the field of high power and high beam quality coherent light sources
In this work we investigate transverse mode instability (TMI) in the presence of pump intensity noise and a controlled perturbation of the input coupling for a rod-type fiber amplifier using spatially and temporally resolved imaging (ST)
In this work we have investigated the behaviour of TMI in the presence of pump intensity noise and perturbation of the input coupling with the spatially and temporally resolved imaging
Summary
Single mode fiber amplifiers have dominated the field of high power and high beam quality coherent light sources. Several intensity-dependent non-linear effects such as Raman scattering and self-phase modulation limiting in particular the peak power of ultrafast systems have been mitigated by increasing core size and reducing fiber length. Some early work utilized high-speed cameras to show transfer of power between different transverse modes [16,17] and since the simpler photodiode (PD) method proposed by Otto et al [17] has been standard when measuring TMI. This method consists of a PD measuring the signal power after a pinhole cropping part of the light and mainly measures the TMI threshold. Interesting fluctuations are observed and explained using simple mode simulations
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