Abstract: A transmitter for multiplexing three optical channels at different wavelengths has (a) first, second and third laser diode means delivering linearly polarized optical beams at respective wavelengths .lambda.2, .lambda.3 and .lambda.4 which are mutually different; (b) an assembly located in the path of the light of wavelength .lambda.2 emitted by said first diode means and including a plate which is substantially quarter-wave for the wavelength (2 .lambda.2-.lambda.4), a filter having a band pass centered on wavelength .lambda.2, a plate quarter-wave which is substantially for wavelength .lambda.4 and a polarizer, the first diode means being oriented so that its beam is transmitted by the polarizer. A first light beam combiner directs light from said third diode means onto the polarizer along a direction of 90.degree. from the direction of the beam of wavelength .lambda.2 and with the same polarization direction, so that the beam of wavelength .lambda.4 is reflected towards said filter.