Characterization and Properties of Large-core Fluorine-doped Fibers Based on Depressed Inner Cladding Design

The surface morphology and microstructure of fiber preform taper zones and fibers with different fluorine content and waveguide structure were characterized by stereomicroscopy, scanning electron microscopy and Raman spectroscopy. The loss and laser transmission efficiency of large-core fibers with fluorine-doped cladding were analyzed by fiber analysis system and self-made output laser probe. The results show that the fluorine volatilization phenomenon becomes more obvious with the increase of fluorine content. As a result, the surface defects of traditional large-core fibers with fluorine-doped cladding increase, such as cracks and pits. Simultaneously, the fiber loss increases slightly and the laser transmission efficiency decreases. Moreover, the fluorine volatilization and crystallization during the fabrication process of large-core fibers with fluorine-doped cladding are effectively suppressed through subsidence fluorine-doped inner cladding design. Thus, the fiber loss at 1 200 nm is 3.99 dB/km, and the laser transmission efficiency at 2 mu m of flat and spherical fibers is 88.9% and 88.4%, respectively. It can be obviously seen that its property is better than that of traditional fibers.