A multi-mode interference (MMI) device, comprising a hollow core
multi-mode waveguide optically coupled to at least one hollow core input
waveguide, is described in which the internal surfaces of the hollow core
waveguides carry a reflective coating. The coating may be a low
refractive index material at the wavelength of operation, such as a
metal, or a multiple layer dielectric stack. Resonators and optical
amplifiers using such (MMI) devices are also described.