Spectroscopy and 400 nm-induced photodarkening measurements in Sm³⁺-doped aluminophosphosilcate fibre

We present initial measurements of the basic spectroscopic parameters and photodarkening characteristics for a Sm³⁺-doped aluminophosphosilicate (APS) glass fibre with a large phosphorous-to-aluminium ratio that is pumped at 400 nm using an InGaN diode laser. The spectroscopic parameters are compared with previous studies. We demonstrate that the presence of a P:Al ratio ∼9 leads to the formation of P-related colour centres only, causing absorption in the visible part of the spectrum that directly overlaps with the strongest Sm³⁺ transitions at 600 nm and 650 nm. We propose an explanation for the photoionization of the P-related defect precursors in high P:Al ratio APS glass based on excited state absorption from the upper state of Sm³⁺ that is followed by subsequent high-energy UV photons emission. We also establish that large P:Al ratios in APS fiber can effectively inhibit the reduction of Sm³⁺ to Sm²⁺. These findings lay the groundwork for further development of visible fibre lasers based on Sm³⁺-doped silicate glass.