Abstract

Background: Bismuth doped optical fibers (BDFs) have potential application in the area of broadband VIS and NIR luminescence in the range from 350-700 nm and 1100 to 1600 nm without presence of any rare-earth elements. Such fibers are studied intensively and used successfully in fiber amplification and lasing particularly in NIR region where no emission from any rare-earths are obtained. They have found that proper modeling for spectral behavior of Bi doped glasses and fibers, is still not available due to unpredictable oxidation states of Bi. Therefore there is scope for detail study for designing and fabricating BDFs with well organized spectral characteristics due to its long interaction length.

Methods: Bi doped nano-phase separated yttria-alumino-silica (YAS) glass optical fibers are fabricated using modified chemical vapour deposition (MCVD) associated with Sloution doping (SD) technique. The micro-structures of the core of perform and fiber samples are analyzed by TEM along with electron diffraction (ED) using thin film sampling technique. The concentration distributions of the dopants in core are analysed by EPMA. Core refractive index (RI) profile is generated using preform analyzer model: PKL 2600 of Photon Kinetics. Using the measured RI of the core numerical aperture (NA) of preforms is calculated. The absorption spectra of BDFs are measured by cutback method at room temperature to avoid external interference. Photoluminescence spectra (PL) of BDFs in VIS region are obtained by FLS920 spectrometer (Edinburg Instrument) at room temperature using ~30 cm long BDFs at 532 nm excitation of 184 mW.

Results: TEM images and ED patterns suggest that the core glasses are composed of amorphous nano-phase separated particles of size around 2-3 nm which are the Bi rich zones in the core. EPMA data show a uniform distribution of dopants within core. Absorption bands for bismuth active centres (BACs) appear mainly at ~515 nm, ~690 nm along with weak bands at ~800 nm, ~960nm and ~1200 nm and observe an overlapping zone within 400-1150 nm regime. Using Gaussian multi-peak fit technique we deconvolute them. The deconvoluted absorption bands are ~480, ~525, ~565, ~675, ~810, ~970 and ~1200 nm. The presence of multiple absorbance suggest the presence of multiple oxidation state of bismuth basically Bi⁰, Bi⁺, Bi²⁺ in the doped core glass. A schematic energy diagram is proposed based on the deconvoluted absorption bands. Using this diagram, the emission band, within 520 to 840 nm under 532 nm excitation, is explained.

Conclusion: Fabrication process of nano phase separated Bi-YAS optical fibers through MCVD assisted with SD technique is discussed. Phase separation of core glass is analyzed by TEM. Almost uniform dopant distribution in the core obtained through EPMA analysis. Different absorption bands of Bi active centres at VIS to NIR have been identified using suitable Gaussian multi-peaks fitting technique. The analyses confirm the presence of multiple oxidation states of Bi into the fabricated BDFs. Intense broad PL from 520 nm to 840 nm are observed under 532 nm pumping. This unique feature would be helpful for fabricating wavelength tunable broadband fiber laser source from VIS to NIR region.

Keywords: Absorption spectra, Bi doped fibers, nanophase separated core glass, visible fluorescence.

Graphical Abstract