High-performance visible light holmium-doped all-fiber laser

Directly generating visible light from compact all-fiber lasers while maintaining high output characteristics has always been a research topic in laser technology. Here, Ji et al. proposed a method to develop dual-wavelength lasers using the excitation mechanism in holmium-doped ZBLAN fluoride glass fibers, and experimentally achieved high output performance of all-fiber lasers, especially operating in the deep red band under 640 nm pumping. Notably, a maximum continuous wave output power of 271 mW was achieved at 750 nm with a slope efficiency of 45.1%, which is the highest direct output power recorded in all-fiber lasers with a core diameter of less than 10 μm in the deep red band. In addition, the researchers developed a 1.2 μm all-fiber laser pumped by a 640 nm laser. The researchers extensively studied the correlation between these two laser generation processes and their performance at 750 nm and 1.2 μm wavelengths. By increasing the pump rate, the researchers observed effective population recycling through the high excited state absorption process, which effectively restored the population to the upper laser level of the deep red transition. In addition, the researchers determined the optimal conditions for this laser, identified the process of filling the excited state energy levels, and established the corresponding spectral parameters. This research shows great promise in improving the performance of lasers using other rare earth ions through excited state absorption processes, paving the way for the advancement of all-fiber ultrafast lasers.

All-fiber lasers are widely used due to their compact structure, excellent heat dissipation performance, and no need for optical cavity cleaning. They have a variety of applications such as precision machining measurement, biophotonics, and defense applications. High-power fiber lasers in the infrared optical region, especially 1 μm, 1.53 μm, and 2 μm, have been well studied using doped silicate glass fibers. These lasers have achieved optical powers exceeding kilowatts. In addition, visible light lasers have broken through the watt-level laser output. However, the output power of single-clad all-fiber lasers in the visible light band is still limited to 100 mW. This is mainly attributed to two main factors. First, fluoride fibers, which are the main body of visible laser generation, have a low damage threshold. Second, achieving high-performance visible light all-fiber laser mirrors has proven to be challenging.

In recent years, researchers have made significant progress in the development of ultrafast visible light lasers using various traditional methods to improve visible light mode locking, such as incorporating figure-of-eight cavities and free-space nonlinear polarization rotation in Dy, Ho, and Pr/Yb-doped fiber lasers. However, the output power of all-fiber mode-locked lasers is still limited to a few milliwatts, limiting their applications. Therefore, it is very important to continue exploring high-performance all-fiber visible lasers, because achieving continuous-wave output of visible light in an all-fiber structure is the basis for utilizing high-energy pulses.

Holmium-doped ZBLAN fluoride glass fibers have attracted widespread attention due to their wide spectral resources in the visible to near-infrared region. These fibers provide three main pumping options for the visible light generation process. Blue laser diode pumping produces efficient green laser output, although the beam quality is limited. On the other hand, due to the long energy level lifetime of 5I7, the maximum output power of the all-fiber deep-red laser is only 16 mW. Compared with green pumping, red pumping covers a wider range of energy levels, which is conducive to studying the interconnection and inversion between different energy levels. In addition, the implementation of high-performance red solid-state lasers and advanced plasma sputtering coating technology, which is known for its high damage threshold, has led to the emergence of deep-red lasers operating at the watt level. These studies provide additional evidence to support the enhancement of laser output characteristics through excited-state absorption processes that rely on deep-red and near-infrared excitation.