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 the Shanghai Institute of Optics and Fine Mechanics of the 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.
The 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 the 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)
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