Mette Marie Jørgensen

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.

Optimizing single mode robustness of the distributed modal filtering rod fiber amplifier

High-power fiber amplifiers for pulsed applications require large mode area (LMA) fibers having high pump absorption and near diffraction limited output. 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 diameter. However, achieving ultra low NA for single mode (SM) guidance is challenging, thus different design strategies must be applied. The distributed modal filtering (DMF) design enables SM guidance in ultra low NA fibers with very large cores, where large preform tolerances can be compensated during the fiber draw. 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, which can cover a large fraction of the Yb emission band including wavelengths of 1030 nm and 1064 nm. Design parameters tolerating refractive index fabrication uncertainties of ± 10⁻⁴ are targeted to yield stable SM bandwidths.

Thermal Effects on the Single-Mode Regime of Distributed Modal Filtering Rod Fiber

Power scaling of fiber laser systems requires the development of innovative active fibers, capable of providing high pump absorption, ultralarge effective area, high-order mode suppression, and resilience to thermal effects. Thermally induced refractive index change has been recently appointed as one major limitation to the achievable power, causing degradation of the modal properties and preventing to obtain stable diffraction-limited output beam. In this paper, the effects of thermally induced refractive index change on the guiding properties of a double-cladding distributed modal filtering rod-type photonic crystal fiber, which exploits resonant coupling with high-index elements to suppress high-order modes, are thoroughly investigated. A computationally efficient model has been developed to calculate the refractive index change due to the thermo-optical effect, and it has been integrated into a full-vector modal solver based on the finite-element method to obtain the guided modes, considering different heating conditions. Results have shown that the single-mode regime of the distributed modal filtering fiber is less sensitive to thermal effects with respect to index-guiding fibers with the same effective area. In fact, as the pump power is increased, their single-mode regime is preserved, being only blue-shifted in wavelength.

Single mode dye-doped polymer photonic crystal lasers

Dye-doped polymer photonic crystal (PhC) lasers fabricated by combined nanoimprint and photolithography are studied for their reproducibility and stability characteristics. We introduce a phase shift in the PhC lattice that substantially improves the yield of single wavelength emission. Single mode emission and reproducibility of laser characteristics are important if the lasers are to be mass produced in, e.g., optofluidic sensor chips. The fabrication yield is above 85% with highly reproducible wavelengths (within 0.5%), and the temperature dependence on the wavelength is found to be −0.045 or −0.066 nm K-1, depending on the material.

Hybrid Ytterbium-doped large-mode-area photonic crystal fiber amplifier for long wavelengths.

A large-mode-area Ytterbium-doped photonic crystal fiber amplifier with build-in gain shaping 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 gain shaping through distributed spectral filtering of amplified spontaneous emission. The fiber properties are ideal for amplification in the long wavelength regime of the Ytterbium gain spectrum above 1100 nm, and red shifting of the maximum gain to 1130 nm is demonstrated.

Influence of index contrast in two dimensional photonic crystal lasers

The influence of index contrast variations for obtaining single-mode operation and low threshold in dye doped polymer two dimensional photonic crystal (PhC) lasers is investigated. We consider lasers made from Pyrromethene 597 doped Ormocore imprinted with a rectangular lattice PhC having a cavity in the middle of the crystal structure. We demonstrate that the index contrast, neff,high/neff,low, is an essential parameter for achieving low threshold, and we identify a trade-off between low threshold and single-mode operation.

Thermal-recovery of modal instability in rod fiber amplifiers

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.

Thermal effect-resilient design of large mode area double-cladding Yb-doped photonic crystal fibers

The effects of thermally-induced refractive index change on the guiding properties of different large mode area fibers have been numerically analyzed. A simple but accurate model has been applied to obtain the refractive index change in the fiber cross-section, and a full-vector modal solver based on the finite-element method has been used to calculate the guided modes of the fibers operating at high power levels. The results demonstrate that resonant structures added to the fiber cross-section can be exploited to provide efficient suppression of high-order modes with a good resilience to thermal effects.

Cross-correlated imaging of single-mode photonic crystal rod fiber with distributed mode filtering.

Photonic crystal bandgap fibers employing distributed mode filtering design provide near diffraction-limited light outputs, a critical property of fiber-based high-power lasers. Microstructure of the fibers is tailored to achieve single-mode operation at specific wavelength by resonant mode coupling of higher-order modes. We analyze the modal regimes of the fibers having a mode field diameter of 60 µm by the cross-correlated (C(2)) imaging method in different wavelength ranges and evaluate the sensitivity of the modal content to various input-coupling conditions. As a result, we experimentally identify regimes of resonant coupling between higher-order core modes and cladding band. We demonstrate a passive fiber design in which the higher-order modal content inside the single-mode guiding regime is suppressed by at least 20 dB even for significantly misaligned input-coupling configurations.

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.