towards small wavelengths the control circuit compensates for this by heating the laser if the wavelength drifts towards a larger wavelength the control circuit cools the laser when a wavelength monitor of this kind is used for wavelength division multiplex transmission systems it is effective to select wavelength filters with a periodic characteristic it is known to user fabry perot filters here in the simplest case one uses for example fibre fabry perot interferometers with an air gap as dispersive element due to the use of fabry perot interferometers the period of the interferometer can be selected to correspond to the wavelength pattern for the wavelength division multiplex the error curve in the case of the use of such a periodic filter is illustrated in fig3 here the error signal of a periodic wavelength filter is plotted over the wavelength it can be seen that due to the periodic function the value u201c zero u201d always occurs at the intersection point between reference signal and filter function whereby the desired wavelengths can be set in the spacing of the free spectral region of the function in this way it is possible to set a laser at different wavelengths this reduces the number of different lasers to be used by an operator of a wavelength division multiplex transmission system in.