Numerical simulation of ultrawideband supercontinuum generation covering a 2-20 µm waveband in cascaded all-soft-glass fiber.

We designed a cascaded all-soft-glass fiber structure and simulate midinfrared 2-20 µm ultrawideband supercontinuum (SC) generation numerically. The cascaded fiber structure consists of a 1.5 m I n F 3 fiber, a 0.2 m chalcogenide photonic crystal fiber, and a 0.2 m tellurium-based chalcogenide photonic crystal fiber. Using a 2 µm pulse pumping this cascaded structure, the generated SC covering the wavelengths longer than 20 µm has been demonstrated theoretically. The 30 dB bandwidth reaches 20.87 µm from 1.44 to 22.31 µm. The effect of different pulse widths on SC generation is considered. With the increase of peak power and the decrease of pulse width, the energy of SC in the 15-20 µm waveband increases gradually. The mechanism of SC broadening process has also been analyzed. The SC generation of more than 20 µm in this cascade structure is caused by the self-phase modulation, soliton effects, four-wave mixing, and redshifted dispersive wave. This method demonstrates the possibility of generating ultrawide bandwidth SCs up to a 20 µm waveband by a commercial 2 µm pump source and all-fiber structure.

High-efficiency mode-locked erbium-doped ZBLAN fiber laser around 2.8 µm by directly depositing Bi2S3 particles onto a cavity mirror.

Mid-infrared (MIR) pulsed lasers near a 3 µm waveband show great potential for the high absorption of water molecules and many important gas molecules. A passively Q-switched mode-locked (QSML) E r 3+-doped fluoride fiber laser with a low laser threshold and high slope efficiency around a 2.8 µm waveband is reported. The improvement is achieved by depositing bismuth sulfide (B i 2 S 3) particles onto the cavity mirror directly as a saturable absorber and using the cleaved end of the fluoride fiber as output directly. -QSML pulses begin to appear with the pump power of 280 mW. The repetition rate of the QSML pulses reaches a maximum of 33.59 kHz with the pump power of 540 mW. When the pump power is further increased, the output of the fiber laser switches from the QSML to the continuous-wave mode-locked operation with the repetition rate of 28.64 MHz and the slope efficiency of 12.2%. The results indicate that B i 2 S 3 is a promising modulator for the pulsed lasers near a 3 µm waveband, which paves the way for further development of various applications in MIR wavebands, including material processing, MIR frequency combs, and modern healthcare.

Simulation and analysis of supercontinuum generation in the waveband up to 25 µm.

The ultra-wideband supercontinuum generation (SCG) in a Te-based chalcogenide (ChG) photonic crystal fiber (PCF) is simulated in the mid-infrared (MIR) waveband. The PCF core and cladding materials are Ge20As20Se15Te45 and Ge20As20Se17Te43, respectively. The supercontinuum (SC) broadening affected by the core diameter and fiber absorption is considered. The selected PCFs at different pumping wavelengths can demonstrate the generation of ultra-wideband MIR supercontinuum according to the simulated results. We consider SC broadening with and without fiber absorption. A SC range from 3 to 25 µm is demonstrated by simulation in a PCF with a core diameter of 8 µm and a pump wavelength of 6 µm considering the fiber absorption. With the increase of the peak power and the pulse width and the decrease of the core diameter, the degree of coherence gradually degraded. To the best of our knowledge, this is the first demonstration of the possibility of SCG up to the waveband of 25 µm in fiber. Our results highlight the potential of a novel Te-based chalcogenide multi-material PCF for SCG. We also provide a way to generate the SCs to longer wavebands than 20 µm in fiber, especially up to the far-infrared waveband.

Geometrical constraint experimental determination of Raman lidar overlap profile.

A simple experimental method to determine the overlap profile of Raman lidar is presented in this paper. Based on Mie and Raman backscattering signals and a geometrically constrained condition, the overlap profile of a Raman lidar system can be determined. Our approach simultaneously retrieves the lidar ratio of aerosols, which is one of the most important sources of uncertainty in the overlap profile determination. The results indicate that the overlap factor is significantly influenced by the lidar ratio in experimental methods. A representative case study indicates that the correction of the overlap profile obtained by this method is practical and feasible.

The impact of synoptic patterns on summertime ozone pollution in the North China Plain.

Surface ozone pollution is a challenging environmental issue in most parts of China. In particular, the North China Plain (NCP) region suffers from the severest ozone pollution throughout the country. In addition to the emission of precursors, ozone concentration is closely related to meteorological conditions resulting from regional atmospheric circulation. In this study, we investigate the relationship between synoptic patterns and summertime ozone pollution in the NCP using the objective principal component analysis in T-mode (T-PCA) classification method. Four dominant synoptic patterns are identified during the summers of 2014-2018. The heaviest ozone pollution is found to be associated with a high pressure anomaly over the Northwest Pacific and a distinct low pressure center in Northeast China. The southwesterly wind surrounding the low pressure center brings dry, warm air from inland South China, resulting in a high temperature, low humidity environment in the NCP, which favors the chemical formation of surface ozone. Locally, this type is associated with a moderate planetary boundary layer height (PBLH) of ~860 m and a stronger warm anomaly within the boundary layer than the upper level. We also notice a non-linear relationship between surface ozone concentration and the PBLH, i.e., ozone concentration first increases with PBLH till ~0.9 km, and then remains stable. This initial increase may relate to enhanced mixing with upper levels where ozone concentration is typically higher than that near the surface. However, when PBLH further increases, this downward mixing effect is balanced with the stronger upward turbulent mixing so that surface ozone shows little change. The synoptic patterns identified here, however, is unlikely responsible for the observed increasing trend in ozone concentration over the NCP region. Our study sheds light on the meteorological contribution to surface ozone pollution in North China and provides a reference for the pollution control and prediction.