Degenerate spontaneous four wave mixing is studied for the rst time in a large mode area hybrid photonic crystal ber, where light con nement is achieved by combined index- and bandgap guiding. Four wave mixing products are generated on the edges of the bandgaps, which is veri ed by numerical and experimental results. Since the core mode is in resonance with cladding modes near the bandedges an unconventional measurement technique is used, in this work named nonlinear spatial mode imaging.
We investigate the temporal dynamics of Modal instabilities (MI) in ROD fiber amplifiers using a 100 μm core rod fiber in a single-pass amplifier configuration, and we achieve ~200W of extracted output power before the onset of MI. Above the MI threshold, we investigate the temporal dynamics of beam fluctuations in both the transient and chaotic regime. We identify a set of discrete frequencies in the transient regime and a white distribution of frequencies in the chaotic regime. We test three identical rods using a multiple ramp-up procedure, where each rod is tested in three test series and thermally annealed between each test series. We find that the MI threshold degrades as it is reached multiple times, but is recovered by thermal annealing. We also find that the test history of the rods affects the temporal dynamics.
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.
We analyze the modal properties of an 85μm core distributed mode filtering rod fiber using cross-correlated (C2) imaging. We evaluate suppression of higher-order modes (HOMs) under severely misaligned mode excitation and identify a single-mode regime where HOMs are suppressed by more than 20dB.
A large-mode-area Ytterbium-doped photonic crystal fiber amplifier with efficient suppression of amplified spontaneous
emission is presented. The fiber cladding consists of a hexagonal lattice of air holes, where three rows are replaced with
circular high-index inclusions. Seven missing air holes define the large-mode-area core. Light confinement is achieved
by combined index and bandgap guiding, which allows for single-mode operation and distributed spectral filtering of
amplified spontaneous. The fiber properties give control of the gain shape and are ideal for amplification in the long
wavelength regime of the Ytterbium gain spectrum above 1100 nm.
High-power fiber amplifiers for pulsed applications require large mode area (LMA) fibers having high pump absorption
and near diffraction limited output. This improves the limiting factor of nonlinear effects, while maintaining good beam
quality. Photonic crystal fibers allow realization of short LMA fiber amplifiers having high pump absorption through a
pump cladding that is decoupled from the outer fiber. However, achieving ultra low NA for single-mode (SM) guidance
is challenging, and thus different design strategies must be applied to filter out higher order modes (HOMs). The novel
distributed modal filtering (DMF) design presented here enables SM guidance, and previous results have shown a SM
mode field diameter of 60 μm operating in a 20 nm SM bandwidth. The DMF rod fiber has high index ring-shaped
inclusions acting as resonators enabling SM guidance through modal filtering of HOMs. Large preform tolerances are
compensated during the fiber draw resulting in ultra low NA fibers with very large cores. In this paper, design
optimization of the SM bandwidth of the DMF rod fiber is presented. Analysis of band gap properties results in a
fourfold increase of the SM bandwidth compared to previous results, achieved by utilizing the first band of cladding
modes. This covers of a large fraction of the Yb emission band, where wavelengths of 1030 nm and 1064 nm can be
included.
We report on an ytterbium doped single mode distributed mode filtering rod fiber in an amplifier configuration
delivering high average output power, up to 292 watts, using a mode-locked 30ps source at 1032nm with good power
conversion efficiency. We study the modal stability of the output beam at high average output power levels and
demonstrate a 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 explain the improved performance
by thermally induced refractive index profile change.
High-power fiber lasers and amplifiers have gained tremendous momentum in the last 5 years. Many of the traditional manufacturers of gas and solid-state lasers are now pursuing the fiber-based systems, which are displacing the conventional technology in many areas. High-power fiber laser systems require reliable fibers with large cores, stable mode quality, and good power handling capabilities-requirements that are all met by the airclad fiber technology. In the present paper we go through many of the building blocks needed to build high-power systems and we show an example of a complete airclad laser system. We present the latest advancements within airclad fiber technology including a new 100 μm single-mode polarization-maintaining rod-type fiber capable of amplifying to megawatt power levels. Furthermore, we describe the novel airclad-based pump combiners and their use in a completely monolithic 350 W cw fiber laser system with an M2 of less than 1.1.
We perform modal characterization on an ytterbium-doped large mode area photonic-crystal-fiber (PCF) amplifier using spatial and spectral (S2) resolved imaging and compare results to conventional cutoff methods. We apply numerical simulations and step-index fiber experiments to calibrate our mathematical and experimental routines of our S2 imaging system. We systematically analyze higher-order-mode (HOM) content of a polarizing 40-μm core double-clad PCF amplifier with various launching and coiling configurations. We demonstrate a HOM suppression of more than −24 dB with variance of 2.3 dB.
Enabling Single-Mode (SM) operation in Large-Mode-Area (LMA) fiber amplifiers and lasers is critical, since a SM
output ensures high beam quality and excellent pointing stability. In this paper, we demonstrate and test a new design
approach for achieving ultra-low NA SM rod fibers by using a spatially Distributed Mode Filter (DMF). This approach
achieves SM performance in a short and straight rod fiber and allows preform tolerances to be compensated during draw.
A low-NA SM rod fiber amplifier having a mode field diameter of ~60μm at 1064nm and a pump absorption of 27dB/m
at 976nm is demonstrated.
We demonstrate modal characterization using spatial and spectral resolved (S2) imaging, on an Ytterbium-doped
large-mode-area photonic crystal fiber (PCF) amplifier and compare results with conventional cut-off methods.
We apply numerical simulations and step-index fiber experiments to calibrate our mathematical and experimental
routines of our S2 imaging system. We systematically analyze higher order mode (HOM) content of a polarizing
40μm core double-clad PCF amplifier with various launching and coiling configurations. We demonstrate a HOM
suppression of more than -24dB with variance of 2.3dB.
Power scaling of Yb-doped large-mode-area fibers drives the scaling of the mode area in order to suppress nonlinearities.
Two Yb-doped large-mode-area fibers were manufactured using the Direct Nanoparticle Deposition process: one with a
step refractive index profile and active ion confinement, and another with a tailored refractive index and active ion
confinement. The index tailoring and doping profiles were designed based on literature to enhance the beam quality of
the fibers. Both fibers exhibited a mode field diameter comparable to a 40μm step index fiber with 0.07 NA. The fibers
were characterized for their geometries, index profiles, and material composition profiles. Additional testing for beam
quality and nonlinearities in pulsed operation was conducted using a power amplifier setup. The beam quality
enhancement capability of the tested fibers was inconclusive due to incomparable launching conditions of the signal to
the fibers.
Photodarkening is a detrimental phenomenon known to affect ytterbium doped fibers. Methods to study the spectral and
temporal properties of the photodarkening induced loss were developed. The spectral shape of the photodarkening loss
measured from multiple aluminosilicate samples indicate that visible wavelength(s) could be used in benchmarking
fibers for their PD induced loss. Two principal methods, core and cladding pumping, were introduced to induce a known
and repeatable inversion to fiber samples. The photodarkening rate could be parameterized using a single variable,
inversion. More generally, the photodarkening rate was found to follow a simple power law and to be proportional to
[Yb]7±1 (the excited state Yb ion density). Two methods, stretched exponential and bi-exponential, were used to fit the
rate measurements. Both fitting methods were found suitable, with the bi-exponential method having more potential in
increasing the understanding of the mechanism(s) behind photodarkening. Coiling induced spatial changes in the
inversion and subsequent photodarkening performance were demonstrated for a large-mode-area fiber laser.
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