The invention relates to a free space optical communications device (10) multiplexed in wavelengths of between 400 nm and 1600 nm, said device including demultiplexing means (11) that are designed so as to separately dissociate a number n1 of wavelengths from one another, the demultiplexing means (11) including one or more detectors (110) having a number n2 of optical filters (111) and active elements (112) which correspond to the number n1 of wavelengths, each active element (112) being arranged to selectively detect one wavelength from among said wavelengths (Id . . . Àn) via an optical filter (111) separate from the active element (112) which is included in a housing (113), the optical filter (111) being in contact with a protection means (114), inserted between the optical filter (111) and the active element (112), said protection means (114) closing said housing (113).