[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Photo Darkening Suppression in Highly Yb-Doped Aluminophosphosilicate Fiber by Addition of Cerium

摘要： 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]. We here present three fibers with high Yb concentration of ~0.5mol% and with equimolar Al and P composition, all fabricated by modified chemical vapor deposition and solution doping technique. Fiber #1 contains no Ce while Fiber #2 and Fiber #3 contain Ce with different Ce/Yb oxide ratio of ~0.24 and ~0.625, respectively. The index profiles of the three fibers are shown in Fig. 1(a). As expected, the core NA increases with Ce concentration. Compositional concentration profile in Fiber #3 measured by energy dispersive X-ray analysis (EDX) is shown in Fig. 1(b). P2O5, Al2O3, Yb2O3 and CeO2 were found to be around 5 mol%, 4.6 mol%, 0.48 mol% and 0.3 mol% respectively, in the core. Hence the Ce/ Yb oxide ratio was observed to be ~0.625, which according to the reported works is supposed to yield quite good PD suppression while creating no additional thermal issues. All the fibers have similar Yb, Al and P contents with only difference in the Ce concentration. Fiber#2 contains ~0.12mol% of Ce2O3. The background losses in the fibers were found to be <25dB/Km after a cut-back method measurement. This is considered low for fibers with such high Yb concentration. Finally, the fibers were tested for PD loss using core pumped configuration with 634 nm as a probe wavelength and 976 nm as pump wavelength. Population inversion levels of ~45% was maintained in all the fibers. The PD losses for the fibers are shown in Fig. 1(c). Fiber#1, Fiber #2 and Fiber#3 has PD losses of 70 dB/m, 30 dB/m and 7 dB/m respectively. Thus, it can be clearly seen that addition of Ce to Yb doped aluminophosphosilicate fiber reduces PD significantly, with the ratio of Ce:Yb playing a major role as well. In conclusion, we report the first fabrication of Ce and Yb doped aluminophosphosilicate fiber with very high absorption, and extremely low PD.