Abstract: The light source has as pumping light source a semiconductor laser with a wavelength of 980 nm. This pumps by way of a coupler a monomode optical fibre which is doped with erbium ions. The erbium ions emit a wideband light spectrum. The spectrum has maximum values 1532 nm and 1556 nm. A minimum value lies between them at approximately 1538 nm. The light spontaneously emitted by the erbium ions is amplified in the same monomode optical fibre (erbium dopted fibre amplifier EDFA). A Fabry-Pérot filter is disposed 1 m before an end of the EDFA and adjusteed in such a way that it selectively transmits light with a wavelength of 1538 nm, whereby this part of the spectrum is preferably amplified during further passage through the EDFA. At the opposite end of the EDFA light leaves thereby with an approximately uniform spectrum.