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The main factors limiting the power scaling of high-peak-power high-brightness quasi-continuous wave (QCW) fiber laser oscillator and a possible solution are analyzed in this paper. The impacts of fiber length and grating parameters on the output power are studied by rate equations. Under the optimized parameters, a high-brightness QCW fiber laser oscillator with a peak power of 7.3 kW is presented experimentally by using a 24 meter long spindle-shaped Yb-doped fiber with a constant core cladding ratio, and that is the best performance of peak power and brightness in near-single-mode QCW fiber lasers at present to the best of our knowledge. The corresponding pulse energy is 0.57 J and the beam quality M2 factor is about 1.43 under the condition of repetition frequency of 1 kHz and pulse width of 100 μs. The details are analyzed, and also the limiting factors of further power scaling, strategies of suppressing nonlinear effects and the principle of optical device parameter optimization. [ABSTRACT FROM AUTHOR]

The integration of multi-access edge computing (MEC) and RAFT consensus makes it feasible to deploy blockchain on trustful base stations and gateways to provide efficient and tamper-proof edge data services for Internet of Things (IoT) applications. However, reducing the latency of storing data on blockchain remains a challenge, especially when an anomaly-triggered data flow in a certain area exceeds the block generation speed. This letter proposes an intelligent transaction migration scheme for RAFT-based private blockchain in IoT applications to migrate transactions in busy areas to idle regions intelligently. Simulation results show that the proposed scheme can apparently reduce the latency in high data flow circumstances. [ABSTRACT FROM AUTHOR]

We demonstrated highly-efficient pulsed mid-infrared generation based on intracavity difference frequency mixing inside a continuous-wave (CW) PPLN optical parametric oscillator (OPO). The near-infrared pulsed fiber source was located at 1120 nm with variable pulse width and repetition rate. The pump source of OPO was a CW linearly-polarized high-power 1060 nm fiber laser. The 1120 nm and 1060 nm pump beams were simultaneously incident into the OPO cavity. The 1060 nm pump beam built parametric oscillation firstly and generated high-power resonant signal beam inside the cavity. By controlling the PPLN temperature properly, the phase-matched difference frequency generation (DFG) occurred between the signal beam and 1120 nm pulsed pump beam in the PPLN crystal. Finally, the pulsed 1120 nm pump beam was successfully transformed to 3593 nm mid-infrared radiation at every power level and temporal characteristic. Both the slope efficiency and pump-to-idler conversion efficiency of the intracavity DFG process reached over 20%. In addition, the pulse shape of newly generated 3593 nm idler beam was basically the same as 1120 nm pump beam. The experimental results exhibited huge potential in frequency down conversion of low-power laser sources with special temporal characteristics. [ABSTRACT FROM AUTHOR]

High-power fiber lasers have been widely used in various industrial manufacturing and military defense applications. During the development of fiber lasers in the past decades, the thermal effect has always been one of the biggest obstacles. It is crucial to study the temperature characteristics and overcome the thermal restrictions for a better output performance. With the systematic design and optimization in this article, optical frequency domain reflectometry (OFDR) can achieve in situ distributed temperature measurement of the fiber core in high-power fiber lasers. This allows a better study of the temperature characteristics and thermal effects. The fiber-core distributed temperature of a kilowatt-level fiber oscillator is first demonstrated based on this method here. The splicing point between the high reflectivity fiber Bragg grating (HR-FBG) and gain fiber withstands the highest temperature, reaching 101.6 °C at a 1.47 kW output. In addition, the temperature of the gain fiber gradually decreases from 92 °C to 30 °C along the pumping direction. The internal temperatures of the combiner and HR-FBG are also measured to evaluate their performances in the high-power regime. The temperature distributions in the experiment agree well with the theoretical simulation. [ABSTRACT FROM AUTHOR]

In this paper, we construct a high power monolithic fiber laser oscillator based on a commercial ytterbium-doped fiber with core/inner cladding diameter of 25/400 μm. The output performances of the fiber oscillator are experimentally investigated in the conditions of either with or without an external feedback. In the presence of external feedback, a very strong stimulated Raman scattering (SRS) (∼9 dB below the signal light) emerges at the output power of ∼3.56 kW, where the onset of the transverse mode instability (TMI) limits further power scaling. By large angle cleaving the signal arm of the forward combiner to avoid external feedback, the maximum output power of 5.07 kW is achieved with no sign of TMI. The Raman stokes light is ∼35 dB below the signal light and the M2 factor is measured to be ∼1.6. To directly characterize its power stability and engineering capability, the fiber oscillator is tested for continuous 5-h operation at ∼5.07 kW with power fluctuation less than 0.42%. During the test process, the output power, spectrum, beam quality, and temporal signal perform excellent stability. The experimental results reveal that thresholds of SRS and TMI strongly depend on the external feedback in high power fiber oscillators. [ABSTRACT FROM AUTHOR]

We report, to the best of our knowledge, the first demonstration of a continuous-wave, dual-wavelength (DW) mid-infrared optical parametric oscillator (OPO) based on the stimulated Raman scattering (SRS) effect of periodically poled lithium niobate (PPLN). The OPO was pumped by two independent fiber lasers, one being fixed at 1018 nm and the other being tunable. The high-power 1018-nm source built parametric oscillation, induced SRS effect in PPLN and generated 1495-nm signal beam as well as 1506-nm Raman beam. The tunable fiber laser was set at 1080 nm and the phase-matched difference frequency generation (DFG) occurred between the 1506-nm Raman beam and 1080-nm pump beam. Finally, DW idler beam located at 3189 and 3819 nm was obtained whose power ranged from 8.044 to 8.115 W, in which the 3189-nm idler power made up the majority of about 8 W and the maximum 3819-nm idler power reached 0.154 W. The slope and conversion efficiency of DFG and OPO processes was also discussed separately. [ABSTRACT FROM AUTHOR]

We report on frequency down-conversion of dual-wavelength (DW) Raman fiber laser in a periodically poled lithium niobate-based optical parametric oscillator. The DW pump source was fixed at 1060 and 1111 nm that was obtained based on stimulated Raman scattering effect by combining a home-made linearly polarized 1060-nm fiber laser and 137-m-long polarization-maintaining passive fiber. The total pump power went through three stages, in the latter two of which the 1111-nm wave appeared. In the entire experiment, the 1060-nm pump beam achieved parametric oscillation and generated 1626-nm signal beam and 3056-nm idler beam. The 1111-nm pump beam generated 3503-nm idler beam based on difference frequency generation (DFG) between itself and 1626-nm signal beam in the second stage. With power enhancement, it built independent parametric oscillation in the third stage and generated 1621-nm signal beam as well as 3530-nm idler beam. The DW idler power ranged from 3.94 to 7.78 W in the latter two stages. The power and efficiency characteristics in frequency conversion processes of 1060- and 1111-nm pump beams were also analyzed separately. [ABSTRACT FROM AUTHOR]

We report on a widely tunable, dual-wavelength, continuous-wave, mid-infrared optical parametric oscillator (OPO), pumped by a dual-wavelength fiber source that was realized by combining two fiber lasers in series. The OPO was designed as a four-mirror singly resonant cavity, in which a periodically poled lithium niobate (PPLN) crystal was used as nonlinear crystal. Two idler emissions with different wavelengths were observed, one being basically fixed at 3140 nm and the other being tunable from 3227 to 3413 nm. Under the maximum pump power, the total idler power was over 6 W and the corresponding pump-to-idler slope efficiency reached 11.4%. The generated signal beam had just one central wavelength around 1600 nm. Through theoretical analysis and experimental verification, it was analyzed that two nonlinear processes, OPO and intracavity difference frequency generation (DFG), simultaneously occurred in the resonant cavity. The experimental phenomena revealed large widened DFG gain spectrum, which was in absolute contrast to simulation results based on classical DFG theory. It indicated great potential in practical application such ultra-wide nonlinear frequency conversion. [ABSTRACT FROM AUTHOR]

Estimating the carbon stock of forests, and further studying their spatiotemporal variations is an important prerequisite for assessing forest carbon sequestration capability. This paper simulated aboveground carbon (AGC) of bamboo forest in Zhejiang Province, China, during 2003–2014 based on the combination of an improved BIOME-BGC (BioGeochemical Cycles) model and bamboo forest map extracted from moderate resolution imaging spectroradiometer data, and analyzed the characteristic and impact factors of the AGC spatiotemporal variations based on the geostatistical method. The modeled AGC density significantly correlated with the plot data from continuous forest resource inventory, and led to an average correlation coefficient and normalized root mean square error (NRMSE) of 0.81% and 17.15%, respectively. The stands with high AGC density were distributed in the mountainous areas in the northwest, southwest, and northeast of the province, while those with low AGC density were located in the basin, high altitude areas, and coastal areas. During the study period, the AGC density and total AGC storage increased from 12.02 to 18.15 Mg C ha−1 and 9.22 to 16.41 Tg, respectively. The spatial heterogeneity of AGC was revealed by an exponential semivariance model, and characterized by significant autocorrelation. The dominant factors contributing to the spatial heterogeneity of AGC were structure relevant factors, especially meteorological and terrain factors. The autocorrelation range of AGC in 2003 was 24.90 km, but decreased dramatically after that, indicating that the influence of external factors (i.e., management measures) on AGC increased gradually. [ABSTRACT FROM AUTHOR]

Bamboo forest has great potential in climate change mitigation. However, the spatiotemporal pattern of carbon storage of global bamboo forest is still cannot be accurately estimated, because the lack of an accurate global bamboo forest distribution information. In this paper, the global bamboo forest distribution was mapped with the following steps. To begin with, training samples were obtained based on investigation data, statistic data, and literature data. Then, a decision tree was constructed for mapping the global bamboo forest distribution by integrating Landsat 8 OLI and MODIS data. Finally, the global bamboo forest area was estimated using a pixel unmixing algorithm. The constructed decision tree succeeds in extracting global bamboo forest based on remote sensing data, where the overall accuracy of classification was 78.81%. The estimated total global bamboo forest area was 30538.35 × 103 ha, with a low root-mean-square error of 611.1 × 103 ha. The estimated bamboo forest area of each province in China and each country were high consistent with the National Forest Inventory in China and Food and Agriculture Organization of the United Nations statistic results (average R2 > 0.9), respectively. Therefore, the global bamboo forest map yielded a satisfactory accuracy in both classification and area estimation, and could provide accurate and significant support for global bamboo forest resource management and carbon cycle research. [ABSTRACT FROM PUBLISHER]

In order to overcome the shortcomings of traditional wavefront achieving techniques, which cannot capture the wavefront in the case that an obstacle presents in front of the reference source, we propose and demonstrate a new wavefront measuring method by exploiting a plenoptic configuration. By processing the 4-D light-field information captured by plenoptic configuration, the obstacle's information can be distinguished and eliminated, and the preserved scene target's information can be utilized to extract the wavefront information. The numerical simulation using partially occluded and extended scene targets show that this technology is feasible, even in the situation of strong wavefront aberrations. The technology proposed in this paper can have wide applications in astronomical imaging and surveillance systems. [ABSTRACT FROM PUBLISHER]

We report a multiwavelength mid-infrared laser based on intracavity difference frequency generation in an MgO-doped periodically poled LiNbO3 optical parametric oscillator, which was pumped by a dual-wavelength fiber MOPA consisting of two parts: dual-wavelength seed and power amplifier. The maximum output power of the linearly polarized pump laser was 65.3 W, and the wavelengths were 1060 and 1080 nm. Three idler emissions with different wavelengths of 3127, 3303 and 3346 nm were obtained under the maximum pump power, and the total idler output reached 7.38 W, indicating a 15% pump-to-idler slope efficiency. The signal laser generated in the experiment had two wavelengths, which were 1594 and 1604 nm under the maximum pump power. Three nonlinear progresses occurred in the experiment, two of them being optical parametric oscillation and the one left being difference frequency generation. [ABSTRACT FROM PUBLISHER]

A novel approach for active coherent beam combining of a large number of fiber lasers based on cascaded phase control is demonstrated. The principle of this approach is briefly introduced, and its feasibility is verified by coherent beam combining of 16 fiber lasers. In the experiment, eight fiber lasers in each sub-array are coherently combined by employing stochastic parallel gradient descent algorithm with a residual phase error of $\lambda $ /28, and then, the combined beams from the two sub-arrays are phase locked by using a single-frequency dithering technique with a residual phase error of $\lambda $ /24. It is scalable to phase lock of hundreds of fiber lasers, because high control bandwidth can be obtained by using cascaded phase control. [ABSTRACT FROM PUBLISHER]