Yellow lasers emitted at 565–590 nm have a wide range of research prospects for their existing and potential applications in ophthalmology, astronomy (e.g., laser guide star), medical treatment for acne melasma, Bose–Einstein condensation, and scientific research [1–5]. At present, visible light lasers operating in the red, green, and blue spectral regions have been well developed [6–16], but yellow lasers are relatively difficult and still rely heavily on dye lasers [17] or nonlinear frequency conversion (e.g.,  frequency doubling [18–20], sum frequency [21], and four-wave mixing [22]) of near-infrared lasers, and suffer from high maintenance cost, a complex system, and degenerated performance. Therefore, researchers always desire an alternative solution for a yellow laser source, which has the advantages of high performance, compactness, low cost, and being maintenance free. The yellow fiber lasers can meet these demands, so there is a strong motivation to develop compact fiber lasers in the yellow spectral region.

In recent years, the frequency downconversion using trivalent rare-earth ion-doped crystals or fibers (e.g.,  fluoride fiber) is a fascinating way to directly obtain visible emission [8]. Unfortunately, emission spectra of most crystals or fibers doped with rare-earth ions (e.g., Pr3+, Er3+, Ho3+, Tm3+, or Nd3+) cannot cover the yellow range, which results in no significant progress in the high-power yellow fiber lasers. Recently, dysprosium (Dy3+)-doped fluoride fibers have been developed and exhibit the strong fluorescence emission in yellow spectral band (from F49/2 to H613/2 transition) [23–25], providing the potential of highly efficient yellow laser direct generation. However, up to now, little research progress in Dy3+-doped fiber yellow lasers (usually <10mW yellow power [26,27]) has been made. The main challenges for a long time have been: (1) the manufacture of Dy3+ fluoride fibers with low loss and high gain is relatively immature; (2) the commercially available high-power GaN blue laser sources with high beam quality are scarce; (3) the construction of yellow fiber laser cavity with high efficiency and simple structure is relatively difficult. Recently, thanks to the continuous breakthrough of both rare-earth-doped fluoride fiber manufacturing technology and high-power blue GaN laser diode (LD), high-power yellow-light generation in Dy3+-doped fiber laser can be expected. Most recently, our research group reported a yellow wavelength-tunable (568–582 nm) Dy3+:ZBLAN fiber laser [28], but the output power was still limited to only 142 mW due to the unoptimized cavity designs. Therefore, it is necessary to carry out in-depth research to obtain high-power yellow-light fiber laser for practical applications.

In this paper, we proposed and demonstrated a watt-level high-power yellow fiber laser for the first time. First, in order to obtain high-efficiency emission and high-power output, we experimentally optimized the output coupling and gain-fiber length of the yellow laser. Subsequently, we further demonstrated a yellow fiber laser with an output coupler reflectivity of 4% and a gain fiber length of ∼1.8m according to the optimized results. The laser directly generated 1.12 W yellow laser output with a central wavelength of ∼575nm and a slope efficiency of 33.6%, which has the advantages of simple structure, high performance, and low cost.

In conclusion, we demonstrated a high-efficiency, watt-level yellow fiber laser with a central wavelength of ∼575nm by directly pumping Dy3+-doped ZBLAN fiber with a 450-nm LD. We carried out experimental optimization of the effects of the output coupling and gain-fiber length on the yellow laser performance, revealing that ∼96% output coupling and ∼1.8m gain fiber are optimal for high-power output and high-quality spectrum of the yellow fiber laser. According to the optimization results, the watt-level yellow-laser emission has been directly generated without additional frequency conversion elements (e.g., nonlinear crystal). The maximum output power is up to 1.12 W (1–2 orders higher than those reported previously), and the slope efficiency is as high as 33.6%. To the best of our knowledge, this is the largest output power from yellow fiber lasers so far. This work provides a new paradigm for compact high-power fiber lasers in the yellow spectral region.

Acknowledgment. Prof. Zhengqian Luo acknowledges the Program for Young Top Notch Talents of Fujian Province and the Program for Nanqiang Young Top Notch Talents of Xiamen University.