The paper discusses the construction of the double-ended interferometer (DEI) at the National Institute for Standards (NIS) for contactless calibration of gauge blocks. The new measuring procedure avoids any contact to the working faces of the gauge block and maintains their quality from possible damage and scratches caused by repeated wringing during periodic calibration. The resulted uncertainty is reduced due to the absence of the auxiliary platen and the corresponding errors of wringing film and phase change. An optical technique based on polarised light is used to measure the surface roughness of the gauge block that influences the measured length. By using the principle angle of incidence, the polarised light technique can be used as an alternative to the stack method to measure the phase change correction with improved accuracy. The constructed interferometer uses multi-wavelength laser sources in illumination to produce a synthetic wavelength that can be suitable to measure the length of the gauge block of interest. The interferogram is analysed by dedicated software to extract the phase information. Optical setup , alignment, measurement, and uncertainty are presented. The comparable calibration results for some gauge blocks of the new technique and the conventional Köster comparator confirm the reliability of the constructed double-ended interferometer.