A low-numerical-aperture (NA) concept enables large-mode-area fiber with better single-mode operation ability, which is beneficial for transverse mode instability and nonlinear effects suppression. In this contribution, we reported a high-power fiber amplifier based on a piece of self-developed large-mode-area low-NA fiber with a core NA of 0.049 and a core/inner cladding diameter of 25/400 µm. The influence of the pump wavelength and fiber length on the power scaling potential of the fiber amplifier is systematically investigated. As a result, an output of 4.80 kW and a beam quality factor of ∼1.33 were finally obtained, which is the highest output power ever reported in a fiber amplifier exploiting the low-NA fiber. The results reveal that low-NA fibers have superiority in power scaling and beam quality maintenance at high power levels.

锥形光纤能够有效兼顾非线性效应抑制和模式控制，具备实现高功率、高光束质量光纤激光的潜力。近期国防科技大学研制了锥形掺镱光纤，采用1018 nm光纤激光后向级联泵浦实现了20.2 kW 激光输出，光束质量β因子平均值优于2，拉曼抑制比为33 dB。研究结果展示了锥形光纤在实现万瓦级高光束质量激光方面的优势。

Systemic blood circulation is one of life activity’s most important physiological functions. Continuous noninvasive hemodynamic monitoring is essential for the management of cardiovascular status. However, it is difficult to achieve systemic hemodynamic monitoring with the daily use of current devices due to the lack of multichannel and time-synchronized operation capability over the whole body. Here, we utilize a soft microfiber Bragg grating group to monitor spatiotemporal hemodynamics by taking advantage of the high sensitivity, electromagnetic immunity, and great temporal synchronization between multiple remote sensor nodes. A continuous systemic hemodynamic measurement technique is developed using all-mechanical physiological signals, such as ballistocardiogram signals and pulse waves, to illustrate the actual mechanical process of blood circulation. Multiple hemodynamic parameters, such as systemic pulse transit time, heart rate, blood pressure, and peripheral resistance, are monitored using skin-like microfiber Bragg grating patches conformally attached at different body locations. Relying on the soft microfiber Bragg grating group, the spatiotemporal hemodynamic monitoring technique opens up new possibilities in clinical medical diagnosis and daily health management.

Overview: Surface-enhanced Raman scattering (SERS) affords a rapid, highly sensitive, and nondestructive approach for label-free and fingerprint diagnosis of a wide range of chemicals. This technique has been applied in explosives detection, pre-cancer diagnosis, food safety, and forensic analysis, where a small number of hazardous substances can seriously affect health of human beings. Thus, it is of great significance to prepare high-performance SERS sensors. In general, the signal intensity of SERS is determined by the following three factors: 1) The enhancement effect of surface nanostructure on local electric fields; 2) The number of molecules to be detected in hot spots; 3) Performance of the Raman spectrometer. Therefore, in order to achieve high-performance SERS detection of trace molecules, current research focuses on how to increase the density of hot spots and the number of analyte molecules in the detection area. An ultrafast laser has an ultra-short pulse width and ultra-high peak power, so it can interact with the majority of materials with high processing accuracy and excellent controllability. Meanwhile, it can rapidly construct a variety of large-area micro/nano-structures on material surfaces based on facile digital programming strategies. In addition, combined with multi-beam parallel fast scanning technology, low-cost and high-efficiency machining can be realized without a special requirement for the machining environment. Based on the above advantages, the ultrafast laser has become one of the important means for the fabrication of micro/nano-structures. This is important for the commercial preparation of high-performance SERS sensors. In this paper, we focus on two aspects to introduce the ultrafast laser preparation of high-performance SERS sensors, including how to increase the density of hot spots and the number of analyte molecules in the detection region. Ultrafast lasers can prepare micro/nano-structures with local field enhancement effects by both "bottom-up" and "top-down" processing strategies. The first is based on the "bottom-up" principle, where the reduction, deposition or polymerization of atoms, molecules or other nanoparticles is controlled by ultrafast lasers to achieve additive manufacturing of micro/nano-structures. The other is based on the "top-down" principle, where materials are removed by the ultrafast laser ablation to rapidly achieve hierarchical micro/nanostructures. These structures provide abundant active hot spots for SERS detection. In particular, the superhydrophobic surfaces prepared by the ultrafast laser are one of the most effective methods to achieve the enrichment of analyte molecules. Raman scattering can be excited more effectively by enriched molecules, which is conducive to obtaining higher detection limits and realizing ultra-trace detection. Finally, a prospect for the development of laser-prepared SERS substrates is provided.

高功率光纤激光器具有高效率、小体积、低成本、抗回光能力强等突出优点，在工业加工等应用领域中具有明显的竞争优势。近期，国防科技大学基于光纤耦合半导体激光器（LD）直接泵浦的主振荡功率放大器（MOPA）实现了单纤20.27 kW的功率输出。放大器采用纯后向泵浦方案，中心波长1080 nm，光光效率达到84.8%，拉曼散射抑制比大于50 dB。通过优化光纤和器件的设计，可进一步提升激光器的功率和光束质量。

In this work, a confined-doped fiber with the core/inner-cladding diameter of 40/250 μm and a relative doping ratio of 0.75 is fabricated through a modified chemical vapor deposition method combined with the chelate gas deposition technique, and subsequently applied in a tandem-pumped fiber amplifier for high-power operation and transverse mode instability (TMI) mitigation. Notably, the impacts of the seed laser power and mode purity are preliminarily investigated through comparative experiments. It is found that the TMI threshold could be significantly affected by the seed laser mode purity. The possible mechanism behind this phenomenon is proposed and revealed through comprehensive comparative experiments and theoretical analysis. Finally, a maximum output power of 7.49 kW is obtained with the beam quality factor of approximately 1.83, which is the highest output power ever reported in a forward tandem-pumped confined-doped fiber amplifier. This work could provide a good reference and practical solution to improve the TMI threshold and realize high-power high-brightness fiber lasers.

当前光纤激光功率提升受限于模式不稳定效应及非线性效应，为了克服上述功率提升限制因素，自主设计并研制了大模场部分掺杂光纤。最终，采用自研部分掺杂光纤及后向级联泵浦方案，成功实现了10.1 kW的光纤激光输出，对应的光束质量因子(M²)为2.16。