Systems and methods for an isothermal thin film heater are provided. The isothermal thin film heater mitigates temperature variations in an optical circuit, such as an arrayed-waveguide grating. The thin film heater comprises a conductive plate and at least one heating element trace coupled to the conductive plate for heating the optical circuit. The heating element trace(s) is arranged around a periphery portion of the conductive plate such that concentric arcs are formed by the heating element trace(s). A sensor for sensing the temperature of the thin film heater and/or the optical circuit can also be provided.