Molded-glass optical element for focusing servos

A double-knife-edge method for focusing error detection has been integrated into one molded-glass optical element (MGOE). The first surface of the MGOE consists of two parts, a slant plane and a sphere; the second surface is an asphere; Groove cross noise caused by a geometrical aberration of the MGOE is simulated. When the second surface is at sphere, there is a focus offset of 3 µm for a photodetector shift of ±10 µm, whereas the aspherical surface causes no focus offset. There was less than 0.3µm groove cross noise in the MGOE in the experiment.