Near-infrared luminescence of bismuth in fluorine-doped-core silica fibres

Abstract

Photoluminescence spectra and decay kinetics of bismuth inclusions in silica optical fibres containing fluorine additive in the core glass are studied in the vicinity of a wavelength of 1420 nm at temperatures of 80-900 K under a continuous wave (CW) and a pulsed diode laser pump at a wavelength of 808 nm. At high fluorine concentration and low temperatures, luminescence decay kinetics becomes essentially bi-exponential, typical lifetimes being 720 and 1200 µs. Hydrogen and deuterium loading at pressures of up to 125 bar leads to a decrease of the steady-state luminescence intensity and lifetime. We attribute this to the appearance of an energy transfer bridge from bismuth clusters to vibrational degrees of freedom of diatomic molecules. It is found that in the presence of H(2) or D(2) molecules experiencing random walking in silica, luminescence decay kinetics stop following a single exponential function even in fluorine-free silica-core fibre, deviation from the single exponent being greater at higher temperatures. The induced quenching rate increases with the increase of temperature as well and is greater for H(2) molecules. All conditions being equal, the equilibrium concentration of hydrogen molecules is greater in heavily fluorinated silica. At temperatures below ~250 K, the presence of dissolved molecules has no effect, which speaks for the primary importance of having rotational degrees of freedom of migrating interstitial diatomic molecules in an excited state for effective quenching of bismuth electronic excitations. It is found that the influence of dissolved deuterium is weaker than that of hydrogen. We attribute this feature to a greater angular momentum of the D(2) molecule and correspondingly smaller energy of the molecule's rotational quantum. The results of the experiments show that bismuth clusters mainly located in voids of the silica network, rather than bismuth point defects, are responsible for near-infrared luminescence.

Effects of Post Treatments on Bismuth-Doped and Bismuth/ Erbium Co-doped Optical Fibres

“…Especially, the influence of dissolved D 2 is weaker than that of H 2 , due to a greater angular momentum of the D 2 and correspondingly smaller energy of the molecule's rotational quantum. In addition, such experimental results provide additional evidence for a cluster rather than a point defect model for bismuth defects in silica being responsible for NIR luminescence [32].…”

“…In addition, the reduction of emission of BAC-Si has also been observed in SBi-H, SBi-HF and GSBi after annealing at high temperature, as listed in Table 1 [31,32]. The features, behaviours and causes of these phenomena vary case by case.…”

“…However, their interpretations are not convincible owing to the limited data. Later on, thermal effects on emission of BAC-Si at 830 nm and 1420 nm under 808 nm pumping have been studied in bismuth-doped silicate fibres (SBi) [31,32], bismuth-doped germanosilicate fibre (GSBi) [31] as well as bismuth-doped silicate tube (SBi*) [33] (Table 1). It is observed that the emission of BAC-Si at ~1420 nm shows an increasing trend when treating at a specific high temperature, along with the reduction of emission at 830 nm.…”

“…Another possibility is the reconfiguration of clusters in a greater porosity in SBi-HF [32], resulting in an irreversible reduction of their luminescence. In addition, the dissociation of BAC-Si due to the oxidation by GeO 2 is also observed in GSBi [31], although it is a reversible process.…”

“…In bismuth and fluorine-doped-core silicate fibres, H 2 and D 2 loading at pressures of up to 125 bar leads to a decrease of the steady-state luminescence intensity and lifetime [32]. It is attributed to the appearance of an energy transfer bridge from bismuth clusters to vibrational degrees of freedom of diatomic molecules.…”