Estimate the peak electric field strength, in 1 , of the guided electric field of a dielectric waveguide with a 5 m by 5 m cross-section surrounded by an infinite cladding on all sides. The vacuum wavelength is 1.55 um, the core and cladding have a refractive index of 1.49 and 1.45 respectively and the total launched power is 100 mW. The estimate should be based on reasonable assumptions about the profile and field of the bound mode(s) that are substantially excited: an exact calculation is not required.