hi esakkimuthu
Optical wireless communications (OWC) is a form of optical communication in which unguided visible, infrared (IR), or ultraviolet (UV) light is used to carry a signal while Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications [ source- wikipedia].
i hope you wil observe the spectrum of optical signal. & owc seems to be parent and fso as child
but todays both are used interchangebly