An oblique-incidence Foucault 
prism method for multi-layer 
optical disc focusing, the focusing device involved is: the 
laser light emitted by the 
laser and collimated is focused by the incident lens and enters the 
optical disc surface in a way that the 
optical axis is inclined relative to the 
optical disc surface,  After being reflected on the information recording surface of the optical disc, it is focused and imaged by a reflective lens, and then through the Foucault 
prism set at the imaging point, the defocus 
signal is formed by receiving the 
light spot through a two-quadrant photoelectric 
receiver; the method is as follows: 1) Make the optical disc away from the 
laser  The position of the focal point moves toward the laser focal point to realize coarse focusing; 2) The laser beam forms a 
light spot on the surface of the two-quadrant photoelectric 
receiver through the above-mentioned focusing device; 3) Let the defocus 
signal T be the upper quadrant 
photocurrent of the two-quadrant photoelectric 
receiver  and the ratio of the difference between the 
photocurrent of the lower quadrant and the total 
photocurrent; 4) change the position of the two lenses with T to control the two-dimensional torque device of the position of the two lenses to reduce the amount of defocus; 5) repeat the above 2) to 4)  process until the accurate focused illumination of the laser focus point on the information recording surface of the optical disc is realized.