Frequency conversion through spontaneous degenerate four wave mixing (FWM) is investigated in large mode area hybrid photonic crystal fibers. Different FWM processes are observed, phasematching between fiber modes of orthogonal polarization, intermodal phasematching across bandgaps, and intramodal phasematching within the same transmission band as the one containing the pump laser. Furthermore first and second order Ra- man scattering is observed. The interplay between the different FWM processes and Raman scattering are investigated.
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 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.
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.
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.
A novel design of Yb-doped double-cladding hybrid photonic crystal fibers, with three rows of high-index inclusions
on each side of the core, has been numerically analyzed with a full-vector modal solver based on the
finite element method. Simulation results have demonstrated that a stronger filtering effect and a higher bending
tolerance can be obtained with an increased number of high-index resonators. Moreover, the unique bending
properties of hybrid fibers with anti-symmetric design have been investigated and explained through the comparison
with the ones of hybrid symmetric fibers, with high-index inclusions of the same dimension on both sides
of the core.
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