Abstract: The present invention includes a method and system for producing radiation of a desired frequency with temperature invariance. Two or more radiation sources that produce an output are included. A temperature difference between the sources is sensed and a temperature difference signal is produced. A control unit controls a heat flux to one or more or between two or more of the sources based on the temperature difference signal. The control unit may provide the heat flux by self-heating by a supplied current or by heater/coolers. The outputs of the two or more radiation sources are mixed in or on a nonlinear medium. The mixing of the outputs produces beat frequencies and a desired beat frequency or frequencies may be selected by a resonant structure. The beat frequencies are invariant with fluctuations in ambient temperature.