Abstract Phase shifters are necessary in order to design optical components for fibre optics networks such as mode selectors, frequency filters and some interterometric devices. We propose a new type of passive phase shifters, which we have called delay lines. These are based on modifying the waveguide widths instead of modifying the length of the waveguides, as was the usual method so far. In this paper we discuss in detail the delay difference between the two planar waveguides that compose our shifter and present simple closed equations to design it. We also carry out a comparative study between our delay lines and the traditional bent shifters.