Study of mode propagation with 632.8-nm laser in tapered fiber

The material dispersion of a tapered fiber is described by Sellmeier's equation. The dependence of refractive index on wavelength and doping concentration is discussed. A He-Ne laser with the output wavelength of 632.8 nm is used in the experiment. When the cutoff frequency of the fiber is less than the laser frequency, the guiding modes of a single-mode fiber (at 1550 nm) are investigated. The results show that the original single-mode fiber becomes a multi-mode waveguide. The propagation and mode coupling...