Abstract: A waveguide having an adjustable index of refraction (or an adjustable optical path length, or for providing an adjustable optical phase delay) based in part on thermal effects in the waveguide. In one example, the waveguide may include a core for guiding a light beam through the waveguide; at least one cladding; liquid crystal material disposed within the waveguide; and at least one temperature control element, such as resistive heater, for receiving at least one control signal to control a temperature of at least a portion of the liquid crystal material; wherein the index of refraction (or the optical path length, or the optical phase delay of the light beam) of the waveguide is altered by an amount that is controlled by the control signal.