III-V Direct-bandgap semiconductor optical filter

An optical filter suitable for use in a duplex optical communication system in which wave energy of a first optical wavelength longer than a given optical wavelength is employed for communication in one direction and wave energy of a second optical wavelength shorter than said given wavelength is employed for communication in a direction opposite to said one direction, said optical filter comprising a plate of III-V direct-bandgap semiconductor having a thickness between opposed faces thereof in a range of 50-400 micrometers, said direct bandgap exhibiting an energy bandgap corresponding to said given optical wavelength, whereby said filter passes said first optical wavelength with substantially negligible attenuation and rejects said second optical wavelength with substantially complete absorption.