A. Dogariu and G. Popescu, Measuring the phase of spatially coherent polychromatic fields, Phys. Rev. Lett., vol. 89, 243902 (2002).

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Realistic optical fields are never completely monochromatic and the interpretation of the concept of phase is not straightforward. It has been pointed out by Wolf that, if a strict condition of spatial coherence is imposed, the statistically averaged behavior of a polychromatic field can be described by an associated monochromatic wave.We show that, for spatially coherent polychromatic optical fields, the measurement of the phase of the second-order correlations determines the phase of this associated, spatially coherent field.We verify this prediction using a novel interferometric technique for measuring the cross-spectral density of a steady-state optical field.