Spectroscopic study of bismuth-doped silica glass

摘要

In 2001, Fujimoto and Nakatsuka reported the observation of an intense luminescence around 1.2 μm in bismuth-doped silica glasses co-doped with aluminium [1]. Since this, there have been other reports on this broadband emission (FWHM > 200 nm) in bulk materials and in fibres fabricated by Modified Chemical Vapour Deposition (MCVD). In the latter case, laser emission has been obtained with conventional solid state [2] or fibre [3] laser pumping sources around 1060 nm with efficiency up to 24%. Despite these demonstrations, the luminescent centre responsible for this emission is still unidentified and none of the different hypothesis reported in the literature (Bi{sup}+, Bi{sup}(5+), Bi cluster, {[AlO{sub}(4/2)]{sup}-, Bi{sup}+} complexes) is totally admitted. Here, spectroscopic investigations have been performed on bismuth-doped preforms and fibres at temperature from 10 K to room temperature to identify the nature of the luminescent centre and to understand its fluorescence dynamics. It is well known that the luminescence of these bismuth-doped glasses is very sensitive to the excitation wavelength, more especially when pumped around 800 nm. This sensitivity is characterized by an extension of the luminescence to longer wavelengths combined to a shift of the luminescence peak [3]. Performing low temperature measurements on bismuth-doped preforms, we show that the near infrared luminescence spread over at least three emission bands centered at 1050, 1260 and 1430 nm when pumped in the region 700-850 nm whereas it consists in one main luminescence peak at 1140 nm when excited at 500 or 1000 nm (see Figure 1). The relative intensity of these bands depends on the excitation wavelength. The richness of the energy level organisation is confirmed by the observation of green up-conversion emission when the sample is excited around 800 nm (see Figure 2).