Apparatus for optical inspection of a sample includes a radiation source, adapted to irradiate a spot on the sample with coherent radiation, and collection optics, adapted to collect the radiation scattered from the spot so as to form a beam of scattered radiation. A diffractive optical element (DOE) is positioned to intercept the beam of scattered radiation and is adapted to deflect a first portion of the beam by a predetermined offset relative to a second portion of the beam, and then to optically combine the first portion with the second portion to generate a product beam. A detector is positioned to receive the product beam and to generate a signal responsive thereto, which is processed by a signal processor so as to determine an autocorrelation value of the product beam.