Growth and characterisation of single-crystal fibres for sensing applications
The laser heated pedestal growth technique has been successfully employed to grow pure and doped sapphire crystal fibres for characterisation as suitable sensor materials. Source materials used were polycrystalline and crystalline sapphire rods while fibres with typical diameters in the range 80 - 170 mm were grown. Pure sapphire fibres, both a- and c-axis, were found to grow easily with no complications such as melt instability. C-axis fibre growth was readily initiated while a-axis fibres required an appropriate a-axis oriented seed crystal. Dip-coating has been used to prepare suitably coated sapphire source rods for growth into doped fibres. Doped fibres grown included Cr3+:, Er3+:, Er3+:Yb3+: and Yb3+:Er3+:Al2O3. Er3+:Yb3+:Al2O3 fibres have been prepared with approximately equal concentration of both dopants while a 10:1 Yb3+ to Er3+ concentration ratio was used for preparing Yb3+:Er3+:Al2O3 fibres. Ruby fibres were also found to grow easily although brownish-green deposits have been observed on some of these fibres. Large transmission losses have been found in fibres with these deposits. Acid cleaning was not effective in removing these deposits, suggesting that they have diffused beneath the surface of the fibres. This was attributed to the condensation of chromium oxide on the fibre surface during growth. Growth of rare earth-doped fibres was initially problematic due to the constant breaking-off of the crystallising fibres from the melt. This was thought to be due to the flexibility of the small diameter source fibres used as well as the high concentration levels of doping. Replacing these small fibres with larger source rods thus permitted RE-doped fibres with relatively good optical quality to be grown. Fibres were grown with typical growth rates of 0.5 - 1 mm/min.