A system and methods for dividing an optical beam in a hollow metallized waveguide are disclosed. The method includes directing an optical signal into a first section of a hollow metallized waveguide. The optical signal is adiabatically expanded in a second section of the hollow metallized waveguide coupled to the first section. The optical signal is split with an in-plane optical beam splitter located in a third section of the hollow metallized waveguide coupled to the second section.