Methods and techniques are presented to inhibit crystallization in optical waveguide structures, during high temperature annealing or deposition, thus preventing the formation of crystalline grains that scatter and/or absorb light. Dopant atoms or molecules are used to disrupt crystallization. The dopant atoms or molecules are selected to be transparent to the optical signal's wavelength range(s). Optical signals propagating in a waveguide that is fabricated with such techniques will experience reduced propagation loss or insertion loss. The passive dopants can also be used in active devices such as lasers or optical amplifiers that incorporate optically active dopants, as long as the passive dopants are chosen so that they do not interact with the active dopants.