Systems and methods for controlling light phase difference in interferometric waveguides at near field transducers by selectively heating the light source are provided. One such system for controlling light phase at the NFT of an interferometric waveguide includes a laser, a heater configured to heat the laser, a splitter configured to receive light from the laser and to split the light into a first waveguide arm and a second waveguide arm, the first waveguide arm and the second waveguide arm converging at a junction about opposite the splitter, and the NFT proximate the junction and configured to receive the light, where the first waveguide arm is longer than the second waveguide arm by a preselected distance, and where the heater is configured to generate and maintain a preselected phase difference in the light arriving at the NFT via the first waveguide arm and the second waveguide arm.