Self-starting high-order mode oscillation fiber laser.

In this paper, we proposed and demonstrated two kinds of all few-mode fiber lasers with self-starting high-order mode (HOM) oscillation. The fundamental mode can be completely suppressed by using a bandpass filter with a few-mode fiber pigtail. In the continuous-wave (CW) regime, the fiber laser directly oscillates in HOM with a signal-to-noise ratio as high as 70 dB, and the slope efficiency is up to 46%. The self-starting HOM mode-locked pulse can be easily achieved by employing a saturable absorber. The HOM oscillation pulsed fiber laser stably operates at 1063.72 nm with 3dB of 0.05 nm, which can deliver cylindrical vector beams with a high mode purity of over 98%. To our knowledge, this is the first demonstration for self-starting HOM direct oscillation in stable CW and pulsed operation states without additional adjustment. This compact and stable HOM fiber laser with a simple structure can have important applications in materials processing, optical trapping, and spatiotemporal nonlinear optics. Moreover, this work may offer a promising approach to realizing high-power fiber laser with arbitrary HOMs stable output.

All-fiber bidirectional mode-locked laser to generate cylindrical vector beam asynchronous noise-like pulses.

We proposed and demonstrated a bidirectional mode-locked fiber laser to generate cylindrical vector beam (CVB) asynchronous pulses based on a graded index multimode fiber. A homemade fused taper two-mode fiber optical coupler (TMF-OC) is employed as a mode converter. The central wavelength for clockwise (CW) pulses can be tuned from 1030.32 nm to 1041.04 nm due to the filtering effect based on multimode interference, that of counterclockwise (CCW) pulses is from 1030.81 nm to 1039.28 nm. When the central wavelengths are 1033.22 nm and 1032.71 nm for CW direction and CCW direction respectively, CVB asynchronous noise-like pulses with a repetition rate difference of ∼436.9 Hz can be obtained. The purity of CVB in CW direction and CCW direction is 95.7% and 93.4% respectively. This bidirectional mode-locked fiber laser with CVB output can be better applied to laser gyroscopes, asynchronous sampling, and dual-comb technique, and impel the interdisciplinary studies in the future.

Generation of cylindrical vector beams in a linear cavity mode-locked fiber laser based on nonlinear multimode interference.

In this paper, a linear cavity mode-locked pulsed fiber laser generating cylindrical vector beams (CVBs) is proposed and demonstrated based on a nonlinear multimode interference. A homemade long-period fiber grating with a broad bandwidth of 121 nm is used as a mode converter inside the cavity. The saturable absorber was formed by single-mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) structure. By controlling the pump power, the operation states are switchable among continuous-wave, Q-switched mode-locked (QML), and mode-locked regimes. The repetition rate of the QML CVB pulse envelope varies from 57.4 kHz to 102.7 kHz at the pump range of 118 to 285 mW. When increasing pump power to 380 mW, mode-locked CVB pulse repetition rate of 3.592 MHz, and pulse duration of 4.62 ns are achieved. In addition, the maximum single-pulse envelope energy can reach 510 nJ, and 142 mW average-power CVBs with a slope efficiency of as high as 20.2% can be obtained. Moreover, azimuthally and radially polarized beams can be obtained with mode purity over 95% in different operating regimes. The proposed fiber laser has a simple structure, and the operation is controllable in both temporal and spatial domains, which presents a flexible pulsed CVB source for application of laser processing, time or mode division multiplexing system, and spatiotemporal nonlinear optics.

High-efficiency mode-locked fiber laser with a switchable oscillating transverse mode in an all few-mode fiber linear cavity.

In this paper, an oscillating transverse mode switchable mode-locked fiber laser with a few-mode fiber linear cavity is proposed and demonstrated. An artificial filter is used to realize the mode gain modulation of the laser. The stable mode-locked pulsed operation with switchable wavelength is easily achieved and the oscillating transverse mode can be flexibly switched between the fundamental mode and high-order mode by adjusting the polarization controller. The mode-locked fiber laser directly oscillates in the high-order mode stably with a slope efficiency of as high as 12%, and the corresponding operating wavelength, repetition rate as well as pulse duration are 1054.07 nm, 22.662 MHz, 31.5 ps, respectively. Besides, a cylindrical vector beam with a high mode purity of 98.6% is obtained by removing the degeneracy of the LP11 mode. This compact and high-efficiency mode-locked fiber laser operating in switchable transverse mode has the potential application for laser processing, particle trapping, bioimaging, and mode division multiplexing system.

Tailoring the spectrum and spatial mode of Yb-doped random fiber laser.

In this paper, we make a comprehensive study on tailoring the spectrum and transverse mode of random fiber lasers (RFLs). By simply temperature tuning, the mode gain profile of RFL can be flexibly and precisely manipulated. The spectrum of laser output can be easily tailored in single-wavelength, dual-wavelength, and three-wavelength, respectively. Meanwhile, the operating transverse mode is also optional among LP01 mode, LP11 mode, and hybrid mode. The slope efficiency of 17.9% and 27.3% are obtained for LP11 mode and LP01 mode operation, respectively. Besides, the coherence control can be confirmed by making speckle contrast measurements. This high-efficiency RFL with the customizable spectrum and spatial mode would have unique applications in wavelength or mode division multiplexing systems, speckle-free imaging, secure communication, and information encryption.

High-power cylindrical vector beam fiber laser based on an all-polarization-maintaining structure.

We propose and demonstrate an all-polarization-maintaining (PM) high-power cylindrical vector beam (CVB) fiber laser based on the principle of mode superposition. The non-degenerated LPy 11a is generated from the oscillator with the maximum power of 11.9W, whose slope efficiency is 24.4%. Then the stable single TE01 vector beam is achieved by the superposition of LPy 11a and LPx 11b in an all-PM architecture, its output power is 3.1W and mode purity of 91.2%. Due to the all-PM architecture, our configuration is free of adjusting polarization controller (PC) and reliable during long-term operation. This laser could be used as a high-power CVBs source for a wide range of applications towards scientific research and industrial field.

Few-mode random fiber laser with a switchable oscillating spatial mode.

Random fiber lasers are of tremendous interest to diverse applications for optical fiber sensing, speckle-free imaging. To date, random fiber lasers with fundamental mode oscillation have been well developed. However, controllable oscillating spatial mode in random fiber lasers have not been reported yet. Here, we propose and demonstrate a few-mode random fiber laser with a switchable oscillating spatial mode based on mode injection locking. An external signal light is injected to realize the locking of transverse mode in this random fiber laser and the direct oscillations of the fundamental mode, hybrid mode, and high order mode can be realized, respectively. This random fiber laser operates in the high-order LP11 mode stably with a threshold of as low as 88 mW. High efficiency and high purity cylindrical vector beams can be obtained by removing the degeneracy of the LP11 mode. This work may pave a path towards random fiber lasers with controllable spatial modes for specific applications in mode division multiplexing, imaging, and laser material processing.

Structural and digestive characters of a heteropolysaccharide fraction from tea (Camellia sinensis L.) flower.

Tea (Camellia sinensis L.) flower polysaccharides (TFPS) have various health-promoting functions. In the present work, the structure of a purified TFPS fraction, namely TFPS-1-3p, and its in vitro digestive properties were investigated. The results demonstrated that TFPS-1-3p was a typical heteropolysaccharide consisting of rhamnose (Rha), arabinose (Ara), galactose (Gal) and galacturonic acid (GalA) with a molecular weight of 47.77 kDa. The backbone of TFPS-1-3p contained â†’ 4)-α-d-GalpA(-6-OMe)-(1 â†’ 4)-α-GalpA-(1 â†’ and â†’ 4)-α-d-GalpA(-6-OMe)-(1 â†’ 2,4)-α-l-Rhap-(1 â†’ linkages. The branch linkages in TFPS-1-3p contained â†’ 6)-ß-d-Galp-(1→, →3,6)-ß-d-Galp-(1→, →5)-α-l-Araf-(1 â†’ and â†’ 3,5)-α-l-Araf-(1 â†’. Subsequently, TFPS-1-3p could not be degraded under simulated human gastrointestinal conditions but could be of use to human fecal microbes, thereby lowering the pH and increasing the production of short-chain fatty acids (SCFAs) of the gut microenvironment and altering the composition of the gut microbiota. The relative abundance of Fusobacterium_mortiferum Megasphaera_elsdenii_DSM_20460, Bacteroides thetaiotaomicron, Bacteroides plebeius and Collinsella aerofaciens increased significantly, potentially contributing to the degradation of TFPS-1-3p.

Environmental implications of reduced electricity consumption in Wuhan during COVID-19 outbreak: A brief study.

Due to the COVID-19 outbreak, Wuhan was locked down from 23 January 2020 to 8 April 2020, a total of 76 days. It is well known that the electricity consumption is a direct reflection of human activity. During the lockdown of Wuhan, most of human activities were forbidden. The reduction in human activity would inevitably lead to a reduction in electricity consumption. At the same time, anthropogenic emissions of air pollutants would also be reduced with the reduction of human activity. In this study, the correlation between electricity consumption and air pollutants during lockdown was discussed in detail. The result showed that the drop in pollutants concentrations in January should be attributed to the washout effect of rainfall rather than the lockdown. The decrease of electricity consumption in the secondary industry might play a significant role on the decrease of PM 2.5 and NO2 concentrations in Wuhan in February 2020. The decrease in NO2 concentration in March should be attributed to the reduction of pollutants emissions from the tertiary industry, which means that more attention should be paid to the control of NO2 emission in the tertiary industry. Due to reduced emissions from local sources, the role of long-range transport sources might be more significant during the lockdown of Wuhan. By PSCF analysis, southeast of Wuhan could be the major potential emission sources of PM 2.5 , especially in the northern part of Jiangxi province. It was suggested that stricter regulation of pollutants emissions should be implemented in this area.