Upconversion fiber lasers are becoming their own family of visible lasers. They benefit from the intrinsically provided frequency conversion. The laser active medium is an optical fiber, with the large surface providing efficient cooling. Excellent transversal mode quality follows from the optical properties of the fiber. With narrow-bandwidth emissions in the red, green, and blue available, applications range from display technology to medical and biological issues, enabling fiber lasers to replace lower-power bulky ion or metal-vapor lasers. Therefore, research efforts aim to enhance the emission at different spectral lines and to improve the overall efficiency by applying diode-laser pumping.
At the Friedrich-Schiller University of Jena (Jena, Germany), Andreas Tünnermann and his coworkers Holger Zellmer and Peter Riedel continue the work they started a few years ago at the Laser Zentrum Hannover (Hannover, Germany). There, they demonstrated laser emission in the red (635 nm) and green (520 nm) out of praseodymium/ytterbium (Pr/Yb) doped double-clad ZBLAN (zirconium barium lanthanum aluminum sodium fluoride) fibers pumped by a Ti:sapphire laser at 840 nm (see Laser Focus World, May 1998, p.15). However, output in the blue was still fairly weak and unstable.
The team has recently reported 150-mW output at 491 nm (blue-green) for several hours without degradation in a single-clad fiber with Ti:sapphire laser pumping and expects further power scaling by again applying the double-clad fiber concept. The breakthrough in the blue came from a redesign of the laser cavity and a strong spectral selection in favor of the 491-nm line.