FOCUSING STATE DETECTING AND CONTROLLING DEVICE FOR OPTICAL DISC REPRODUCING DEVICE

PURPOSE:To simplify an optical system, by detecting a focusing state from the timing of output change in a two-split photodetector located in an optical path of return light from a disc and controlling of focus digitally. CONSTITUTION:In a just-focusing state where a recording plane 24 is at a suitable position with respect to an objective lens 23, output signal IA and IB reflected from a disc and outputted from two-split photodetectors 26a and 26b change as shown in a figure while a beam spot S impinges on edges 25a and 25b of a pit. Thus, an output comparing a difference signal (IA-IB) with a 0 level is sampled by an output comparing a sum signal (IA+IB) with the 0 level and the output drives a focus controlling coil 27. Since the change of timing decreasing the output signal IA and IB from two photodetectors located in an optical path is used, a special optical element such as a cylindrical lens is not needed, allowing to perform the simplicity of the system.