Kristian Rymann Hansen

Theoretical analysis of mode instability in high-power fiber amplifiers

We present a simple theoretical model of transverse mode instability in high-power rare-earth doped fiber amplifiers. The model shows that efficient power transfer between the fundamental and higher-order modes of the fiber can be induced by a nonlinear interaction mediated through the thermo-optic effect, leading to transverse mode instability. The temporal and spectral characteristics of the instability dynamics are investigated, and it is shown that the instability can be seeded by both quantum noise and signal intensity noise, while pure phase noise of the signal does not induce instability. It is also shown that the presence of a small harmonic amplitude modulation of the signal can lead to generation of higher harmonics in the output intensity when operating near the instability threshold.

Distributed mode filtering rod fiber amplifier delivering 292W with improved mode stability.

We demonstrate a high power fiber (85 μm core) amplifier delivering up to 292 Watts of average output power using a mode-locked 30 ps source at 1032 nm. Utilizing a single mode distributed mode filter bandgap rod fiber, we demonstrate 44% power improvement before the threshold-like onset of mode instabilities by operating the rod fiber in a leaky waveguide regime. We investigate the guiding dynamics of the rod fiber and report a distinct bandgap blue-shifting as function of increased signal power level. Furthermore, we theoretically analyze the guiding dynamics of the DMF rod fiber and explain the bandgap blue-shifting with thermally induced refractive index change of the refractive index profile.

Thermally induced mode coupling in rare-earth doped fiber amplifiers.

We present a simple semianalytical model of thermally induced mode coupling in multimode rare-earth doped fiber amplifiers. The model predicts that power can be transferred from the fundamental mode to a higher-order mode when the operating power exceeds a certain threshold, and thus provides an explanation of recently reported mode instability in such fiber amplifiers under high average-power operation. We apply our model to a simple step-index fiber design, and investigate how the power threshold depends on various design parameters of the fiber.

Thermo-optical effects in high-power Ytterbium-doped fiber amplifiers

We investigate the effect of temperature gradients in high-power Yb-doped fiber amplifiers by a numerical beam propagation model, which takes thermal effects into account in a self-consistent way. The thermally induced change in the refractive index of the fiber leads to a thermal lensing effect, which decreases the effective mode area. Furthermore, it is demonstrated that the thermal lensing effect may lead to effective multi-mode behavior, even in single-mode designs, which could possibly lead to degradation of the output beam quality.

Impact of gain saturation on the mode instability threshold in high-power fiber amplifiers

We present a coupled-mode model of transverse mode instability in high-power fiber amplifiers, which takes the effect of gain saturation into account. The model provides simple semi-analytical formulas for the mode instability threshold, which are valid also for highly saturated amplifiers. The model is compared to recently published detailed numerical simulations of mode instability, and we find reasonably good agreement with our simplified coupled-mode model.

Echinostoma caproni in rats: worm population dynamics and host blood eosinophilia during primary infections with 6, 25 and 50 metacercariae and resistance to secondary and superimposed infections

Hooded-Lister rats were inoculated with 6, 25, 50 or 100 metacercariae of the intestinal trematodeEchinostoma caproni. Worm establishment and the pattern of egg excretion were followed during the course of primary infections with 6, 25 and 50 metacercariae. Peripheral blood eosinophilia was followed at all infecton levels. After 1 month, worm recovery and faecal egg output showed a gradual decline with increasing duration of infection. High worm burdens were expelled later than smaller worm burdens, and egg output persited longer in animals exhibiting a high initial egg out-put. The level of blood eosinophilia increased with increasing degree of infection and with the level of egg output. A marked concomitant resistance to superimposed infection was observed on the challenge of rats harbouring 21- and 49-day-old infections with 50 metacercariae. In addition, rats were partially resistant to secondary infection at challenge day 14 following anthelmintic removal of primary 7-day-old infections with 50 metacercariae and were completely resistant at challenge day 7 following elimination of a primary 14-day-old primary infection.

Estimating modal instability threshold for photonic crystal rod fiber amplifiers

We present a semi-analytic numerical model to estimate the transverse modal instability (TMI) threshold for photonic crystal rod amplifiers. The model includes thermally induced waveguide perturbations in the fiber cross section modeled with finite element simulations, and the relative intensity noise (RIN) of the seed laser, which seeds mode coupling between the fundamental and higher order mode. The TMI threshold is predicted to ~370 W - 440 W depending on RIN for the distributed modal filtering rod fiber.

Fiber amplifiers under thermal loads leading to transverse mode instability

Transverse mode instability (TMI) in rare-earth doped fiber amplifiers operating above an average power threshold is caused by intermodal stimulated thermal Rayleigh scattering due to quantum defect heating. We investigate thermally induced longitudinal waveguide perturbations causing power transfer from the fundamental mode (FM) to the higher order mode (HOM) by a nonlinear gain, which depends on the FM-HOM frequency shift and position along the fiber. We take temperature and mode profile evolution along the fiber into consideration to engineer fiber designs with increased TMI threshold and operation stability at higher average powers.

Modal instability of rod fiber amplifiers: a semi-analytic approach

The modal instability (MI) threshold is estimated for four rod fiber designs by combining a semi-analytic model with the finite element method. The thermal load due to the quantum defect is calculated and used to numerically determine the mode distributions on which the expression for the onset of MIs is highly dependent. The relative intensity noise of the seed laser in an amplifier setup is used to seed the mode coupling between the fundamental and higher order mode, and lead to MI threshold values of 174 W – 348 W of extracted output power for the four rod fibers having core diameters in the range 53 μm – 95 μm.

Resonant filtered fiber amplifiers

In this paper we present our recent result on utilizing resonant/bandgap fiber designs to achieve high performance ytterbium doped fiber amplifiers for achieving diffraction limited beam quality in large mode area fibers, robust bending performance and gain shaping for long wavelength operation of Yb-doped amplifiers.