April 19, 2005, Lohja, Finland--Liekki, which produces nanoparticle optical fibers, has announced the availability of highly doped ytterbium fibers with greatly reduced photodarkening.
In a side-by-side testing comparison with conventional fibers, the Liekki direct-nanoparticle-deposition (DND) fibers showed less than half of the photodarkening degradation of conventional fibers with similar doping density made with the modified-chemical-vapor-deposition (MCVD) process commonly used in the industry. A white paper has been written by Liekki, "Photodarkening: Understanding and Mitigation," which describes the fibers in detail.
Photodarkening
Photodarkening is an impairment in doped fibers that manifests itself as increased attenuation over time. In extreme cases, under certain pump conditions, photodarkening results in considerable incremental attenuation over a timeframe of minutes. This severely limits the efficiency and lifetime of fiber lasers and fiber amplifiers. Photodarkening is known to exist in fibers doped with ytterbium, thulium, cerium, praseodymium, and europium. Liekki and the industry have been focused on photodarkening in ytterbium-doped fibers for industrial and military lasers. The underlying mechanism of photodarkening is not completely understood, and there are several factors that contribute to it. It is believed that the formulation of the core composition, inhomogeneities and impurities, the codopants used in conventional fiber processes (such as MCVD), and unwanted rare-earth dopants are key factors in photodarkening.
Measurements
Liekki is now incorporating a "best practice" for photodarkening evaluation into its quality assurance to guarantee a fully reliable product. A novel test and measurement technique includes the use of high pump intensity to enhance and accelerate the photodarkening process. This provides a worst-case estimate for long-term reliability. Secondly, application measurements are done on all fibers on a regular basis to monitor the performance. Extensive long-term reliability testing is also underway at Liekki.
Results and fiber process dependence
Liekki has measured a significant improvement in photodarkening in its new-generation ytterbium fibers with 1200-dB/m core absorption at 976 nm. This improvement, which translates into a significant benefit in product lifetime, was the result of a straightforward process improvement that built on the flexibility of Liekki's DND process. Additionally, Liekki compared its new DND fibers to fibers manufactured with standard fiber-processing techniques (MCVD). The comparison showed that the Liekki fiber-attenuation change with time was less than half that of the conventional fibers, again providing a significant reliability advantage.
"We always understood the optical advantages of our DND fiber process for high doping, large mode area, and flat refractive-index profile. Now we are beginning to appreciate the reliability advantages as it relates to photodarkening," said Simo Tammela, Liekki's chief technical officer. "We expect to use this learning in the fabrication of even more highly doped fibers with very low photodarkening in the near future."