A wavelength modulated interferometer is presented, based on a telecom laser source. A solution for the target distance dependency of wavelength modulated interferometers is presented. With the introduction of a delay line, the modulation depth variation is reduced. A method and a sensor head for the reduction of the thermal dependency of this delay line are presented. The resulting interferometer has sub-nanometer non linearity errors and a thermal dependency of below 3 nmK¡1 with a working range of 1meter.