Intensity thresholds for the onset of optical damage in α-phase proton-exchanged waveguides on undoped ${\mathrm{LiNbO}}_3$ have been increased from the substrate value to a factor 500 greater. This has been achieved by increasing the exchange time, which in turn increases the $\left[{\mathrm{Fe}}^{2+}\right]∕\left[{\mathrm{Fe}}^{3+}\right]$ ratio in the guide. Intensity thresholds have been measured with a single-beam configuration, while $\left[{\mathrm{Fe}}^{2+}\right]∕\left[{\mathrm{Fe}}^{3+}\right]$ ratios have been determined from decay measurements during optical erasure of photorefractive gratings. In heavily Fe-doped guides, thresholds are too small to be measured, but the dependence of the ${\mathrm{Fe}}^{2+}$ absorption on the exchange time appears similar to undoped samples.