kW-level monolithic single-mode narrow-linewidth all-solid photonic bandgap fiber amplifier.

Further power scaling of narrow-linewidth fiber lasers is critical for beam combining. Using all-solid photonic bandgap fibers with large effective mode areas and strong higher-order-mode suppression is an interesting approach. Previously, we demonstrated ∼400W single-frequency single-mode power at 1064 nm from a 50/400 photonic bandgap fiber amplifier, limited only by transverse mode instability (TMI). In this work, we demonstrate a TMI-limited single-mode power of 1.37 kW from a monolithic fiber amplifier with a 25/400 photonic bandgap fiber, the highest output power from a photonic bandgap fiber demonstrated to date, to the best of our knowledge. The spectral linewidth is broadened to ∼8GHz to suppress stimulated Brillouin scattering.

Degradation of difenoconazole in water and soil: Kinetics, degradation pathways, transformation products identification and ecotoxicity assessment.

Difenoconazole is a widely used triazole fungicide that has been frequently detected in the environment, but comprehensive study about its environmental fate and toxicity of potential transformation products (TPs) is still lacking. Here, laboratory experiments were conducted to investigate the degradation kinetics, pathways, and toxicity of transformation products of difenoconazole. 12, 4 and 4 TPs generated by photolysis, hydrolysis and soil degradation were identified via UHPLC-QTOF/MS and the UNIFI software. Four intermediates TP295, TP295A, TP354A and TP387A reported for the first time were confirmed by purchase or synthesis of their standards, and they were further quantified using UHPLC-MS/MS in all tested samples. The main transformation reactions observed for difenoconazole were oxidation, dechlorination and hydroxylation in the environment. ECOSAR prediction and laboratory tests showed that the acute toxicities of four novel TPs on Brachydanio rerio, Daphnia magna and Selenastrum capricornutum are substantially lower than that of difenoconazole, while all the TPs except for TP277C were predicted chronically very toxic to fish, which may pose a potential threat to aquatic ecosystems. The results are important for elucidating the environmental fate of difenoconazole and assessing the environmental risks, and further provide guidance for scientific and reasonable use.

The effect of freezing and thawing on water flow and MCPA leaching in partially frozen soil.

Limited knowledge and experimental data exist on pesticide leaching through partially frozen soil. The objective of this study was to better understand the complex processes of freezing and thawing and the effects these processes have on water flow and pesticide transport through soil. To achieve this we conducted a soil column irrigation experiment to quantify the transport of a non-reactive tracer and the herbicide MCPA in partially frozen soil. In total 40 intact topsoil and subsoil columns from two agricultural fields with contrasting soil types (silt and loam) in South-East Norway were used in this experiment. MCPA and bromide were applied on top of all columns. Half the columns were then frozen at -3 °C while the other half of the columns were stored at +4 °C. Columns were then subjected to repeated irrigation events at a rate of 5 mm artificial rainwater for 5 h at each event. Each irrigation was followed by 14-day periods of freezing or refrigeration. Percolate was collected and analysed for MCPA and bromide. The results show that nearly 100% more MCPA leached from frozen than unfrozen topsoil columns of Hov silt and Kroer loam soils. Leaching patterns of bromide and MCPA were very similar in frozen columns with high concentrations and clear peaks early in the irrigation process, and with lower concentrations leaching at later stages. Hardly any MCPA leached from unfrozen topsoil columns (0.4-0.5% of applied amount) and concentrations were very low. Bromide showed a different flow pattern indicating a more uniform advective-dispersive transport process in the unfrozen columns with higher concentrations leaching but without clear concentration peaks. This study documents that pesticides can be preferentially transported through soil macropores at relatively high concentrations in partially frozen soil. These findings indicate, that monitoring programs should include sampling during snow melt or early spring in areas were soil frost is common as this period could imply exposure peaks in groundwater or surface water.

Comparison of phase modulation schemes for coherently combined fiber amplifiers.

Optical linewidth broadening through both white noise (WNS) and pseudo-random binary sequence (PBRS) phase modulation are effective techniques for suppressing stimulated Brillouin scattering (SBS) in high- power fiber amplifiers. However, detailed studies comparing both coherent beam combining and SBS suppression of these phase modulation schemes have not been reported. In this study, a passive fiber cutback experiment is performed comparing the SBS threshold enhancement factor of a PRBS and WNS broadened seed as a function of linewidth and fiber length. Particularly, assuming an optimal PRBS pattern is chosen, pseudo-random modulation provides superior SBS suppression than WNS for a given fiber length and signal linewidth. Furthermore, two WNS and PRBS modulated 150 W fiber lasers are coherently combined to measure and compare the combining efficiency, beam quality, and coherence as a function of optical path length difference. Notably, the discrete spectral density of PRBS modulation provides a re-coherence effect where the lasers periodically come back into phase. Overall, this may reduce path length matching complexity in coherently combined fiber laser systems.

400-W near diffraction-limited single-frequency all-solid photonic bandgap fiber amplifier.

An ytterbium-doped large-mode area photonic bandgap fiber is used to demonstrate 400 W of single-frequency output at 1064 nm with excellent beam quality and minimal stimulated Brillouin scattering. The fiber possesses all-solid microstructures embedded in the cladding and a core composed of phosphosilicate with a diameter of ∼50 µm. As the signal power is pushed beyond 450 W, there is degradation in the beam quality due to the modal instability. We briefly discuss techniques to alleviate this problem in future designs. To the best of our knowledge, the 400-W single-frequency near diffraction-limited output far exceeds the current state-of-the-art from such type of fiber amplifier.