Single-frequency lasers have been undergoing prominent advances and finding increased applications since its creation in 1990s. High rare-earth doping level is essentially required for a rare-earth doped single-frequency laser. However, the doping level of rare-earth ions in commercial silica fibers is commonly less than 1 wt% owing to the dissolution rate limit of silica glass to rare-earth ions.

In a recent study, a research team from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has made new progress on the Yb doping level in high SiO₂ content glass fibers. Related work was published on Optics & Laser Technology.

In their experiment, a unique method for fabrication of silica-clad active fibers ultrahighly doped with rare-earth ions was provided, which was based on sol-gel preparation. With the help of silica tube inner wall coating and fusion-tapering technique, high dissolution rate to rare-earth ions in silica fiber could be realized.

The fiber fabrication process in the experiment were briefly divided into three steps: silica tube inner wall coating, sintering of the dried layer and two-stage fusion-tapering process.

Researchers successfully designed and developed ~1.03 μm aluminosilicate-core silica-clad optical fibers with Yb³⁺ doping level up to 5.7 wt% and signal-to-noise ratio up to 70 dB. The fibers were easy to be spliced with commercial silica-based passive fiber components such as FBGs.

This newly developed fibers overcame the problem of low dissolution rate to Yb³⁺ ions in high SiO₂ content glass fibers, indicating the potential to be made into short-cavity high-gain silica fiber, which may be applicable to silica-based single-frequency seed laser.

It is worth noting that the fiber core composition can extend to any heavily rare-earth doped system as long as it can be realized in the sol form with good dispersion. Therefore, the unique method will pave the way for fabrication of ~1um, ~1.5 um and even ~2 um single-frequency seed lasers in silica fibers.

The research was supported by National Natural Science Foundation of China.

Schematic diagram of the procedure for fabricating a highly rare-earth doped silica-clad fiber and fiber properties. (Image by SIOM)

Article website:
https://doi.org/10.1016/j.optlastec.2020.106425

Contact:
Mr. CAO Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: caoyong@siom.ac.cn