We show that the coherent modulation of a beam from an incoherent source, such as an incandescent lamp, induces correlations between the normal modes describing the beam. Further, we show that it is possible to modulate the thermal beam without altering the coherence propreties, i.e., the statistical properties of the beam. We demonstrate the general properties of modulated incoherent beams by solving exactly the equations of motion for a model of a beam from a thermal source modulated by an electro-optic light-modulation crystal. When the incoherent beam is gaussian, we show that the modulation process does not alter the coherence properties of the incoherent beam.
CHARLES R. WILLIS, "Coherent Modulation of a Thermal Beam," J. Opt. Soc. Am. 60, 921-924 (1970)