Launch conditions and mode coupling in hollow-glass waveguides

The attenuation and input end heating of hollow-glass waveguides are greatly affected by the way in which the input laser energy is coupled into the guides. A theoretical discussion of coupling conditions is related to waveguide loss and local heating, which can occur when the coupling is not optimized. Our results indicate that the optimum coupling for maximum transmission is not necessarily equiva- lent to optimum coupling to the lowest order HE11 mode. Additionally, it is shown that the input end heating of these waveguides can be reduced substantially with minimal effect on transmission if the waveguide bore size is large.