Abstract

Photodarkening (PD) suppressed low numerical aperture (NA), high absorption, low loss, large mode area (LMA) fiber is an important requirement for high power fiber lasers, in both pulsed and continuous wave operation regimes. We have reported the fabrication of high absorption (~1200 dB/m core absorption) Yb doped aluminophosphosilicate fiber with raised Ge cladding to realize low NA step index fiber [1]. The presence of Aluminium (Al) and phosphorus (P) in the core helps in lowering the PD loss by a significant margin, but some PD loss is still observed. In this work we intend to further lower the PD loss by adding Cerium (Ce) to this equimolar P:Al core. Previous reports suggests that addition of Ce to Yb:Al core significantly reduce the PD loss when the ratio of Ce oxide to Yb oxide ratio is ~0.5-0.7 [2–4]. However, like Yb, Ce has a high molar refractivity, and all dopants (Yb, Ce and Al) push up the core NA. In our work, equimolar P:Al does not contribute to index of the core, and only the Ce and Yb raise the core NA. Furthermore, the reported raised Ge cladding technique can compensate the core NA to a desired level [1].

© 2019 IEEE