In an optical head for use in an optical data recording/reproducing
apparatus of the present invention, light emitted from a semiconductor
laser is split into a main beam that is zeroth order light and sub-beams
that are plus and minus first-order diffracted light. A focus error signal
is detected from each of the main beam and sub-beams. A diffractive
optical lens causes the main beam and sub-beams to differ in light
intensity distribution from each other when incident to an objective lens.
As a result, when a disk or optical recording medium has a thickness
error, the focus error signal derived from the main beam and focus error
signals derived from the sub-beams differ in zero-crossing point from each
other. The thickness error is detected on the basis of the difference
between the zero-crossing points.
