Hollow core waveguide for high quality CO₂ laser beam delivery: exploitation of bend-induced mode coupling

The mode coupling behaviour inside a bent hollow waveguide is discussed, and experimental results for modal power coupling using single fundamental Gaussian mode beam inputs are presented. It is shown, theoretically and experimentally, that the mode coupling caused by the bending of the waveguide is periodic, and that the power transferred to higher order modes from the fundamental mode (for a given bend radius) may be coupled back to the fundamental, if the radiation propagates along a certain length of the bend. Thus, it is possible to configure a bent waveguide in such a manner as to produce maximum power in the fundamental mode, allowing the preservation of the excellent beam quality launched at the input to the hollow fibre, without any significant reduction in the overall fractional power transmission in comparison with the straight fibre. © 1995.