Although visible femtosecond lasers based on nonlinear frequency conversion of Ti:sapphire femtosecond oscillators or near-infrared ultrafast lasers have been well developed, limitations in terms of footprint, cost, and efficiency have called for alternative laser solutions. The fiber femtosecond mode-locked oscillator as an ideal solution has achieved great success in the 0.9 to 3.5 μm infrared wavelengths, but remains an outstanding challenge in the visible spectrum (390 to 780 nm). Here, we tackle this challenge by introducing a visible-wavelength mode-locked femtosecond fiber oscillator along with an amplifier. This fiber femtosecond oscillator emits red light at 635 nm, employs a figure-nine cavity configuration, applies a double-clad Pr^{3 +} -doped fluoride fiber as the visible gain medium, incorporates a visible-wavelength phase-biased nonlinear amplifying loop mirror (PB-NALM) for mode locking, and utilizes a pair of customized high-efficiency and high-groove-density diffraction gratings for dispersion management. Visible self-starting mode locking established by the PB-NALM directly yields red laser pulses with a minimum pulse duration of 196 fs and a repetition rate of 53.957 MHz from the oscillator. Precise control of the grating pair spacing can switch the pulse state from a dissipative soliton or a stretched-pulse soliton to a conventional soliton. In addition, a chirped-pulse amplification system built alongside the oscillator immensely boosts the laser performance, resulting in an average output power over 1 W, a pulse energy of 19.55 nJ, and a dechirped pulse duration of 230 fs. Our result represents a concrete step toward high-power femtosecond fiber lasers covering the visible spectral region and could have important applications in industrial processing, biomedicine, and scientific research.

翠绿宝石晶体是一种性能优良的宽带可调谐激光增益介质，具有荧光寿命长、饱和能量密度高、吸收带宽较宽以及热机械性能优良等特点.除闪光灯外，翠绿宝石晶体还可以使用蓝光激光二极管、红光激光二极管、绿光激光器、黄光激光器等多种可见光光源进行泵浦.随着激光二极管技术的成熟及其商业化应用，利用红光激光二极管泵浦的翠绿宝石激光器逐渐成为固体激光领域的研究热点.本文首先介绍了翠绿宝石晶体的各种特性，并着重对基于闪光灯以及各类激光二极管泵浦的翠绿宝石激光器在国内外的发展进行综述，包括翠绿宝石连续激光、翠绿宝石调Q激光、翠绿宝石锁模激光、以及基于翠绿宝石的紫外激光.最后，阐述了翠绿宝石激光光源在医疗、雷达、显微等领域中的应用并对其未来发展提出了展望.

A power-scaled laser operation of Pr:YLiF₄ (YLF) crystal at 720.9 nm pumped by a 443.6 nm laser diode (LD) module was demonstrated. The 20 W module was used to pump the Pr:YLF crystal, and a maximum output power of 3.03 W with slope efficiency of 30.04% was obtained. In addition, a 5 W blue LD was also used to pump the Pr:YLF laser, and a maximum output power of 0.72 W was obtained at room temperature. The output power was limited by the wavelength mismatch between the single-emitter LD and the absorption peak of the crystal.

In this paper, recent advances in underwater wireless optical communication (UWOC) are reviewed for both LED- and LD-based systems, mainly from a perspective of advanced modulation formats. Volterra series-based nonlinear equalizers, which can effectively counteract the nonlinear impairments induced by the UWOC system components, are discussed and experimentally demonstrated. Both the effectiveness and robustness of the proposed Volterra nonlinear equalizer in UWOC systems under different water turbidities are validated. To further approach the Shannon capacity limit of the UWOC system, the probabilistic constellation shaping technique is introduced, which can overcome the inherent gap between a conventional regular quadrature amplitude modulation (QAM) format and the Shannon capacity of the channel. The experimental results have shown a significant system capacity improvement compared to the cases using a regular QAM.

The growing number of underwater activities is giving momentum to the development of new technologies, such as buoys, remotely operated vehicles, and autonomous underwater vehicles. The data collected by these vehicles need to be transmitted to a high-speed central unit. Clearly, wired solutions are not suitable, since they strongly impact the mobility. In this scenario, a promising solution is offered by underwater optical wireless communication (UOWC) technology, which can achieve both high-speed and wireless operation. Here, we provide a comprehensive survey on the challenges, the experimental realizations, and the state of the art in UOWC researches.