Focus tracking registration for optical reproducing systems

In an optical reproducing system having an information signal stored in a track on a carrier to be derived by reading the storage track with a focused reading beam of energy, which system includes means for effecting periodic lateral relative displacement of said beam with respect to said track at a predetermined wobble frequency to develop a radial tracking error signal and means responsive to said radial error signal to maintain radial tracking registration of said track and said beam, the improvement which comprises a focus tracking arrangement having:

• a pair of photoreceptors disposed on opposite sides of a reference plane that includes (a) the optical axis of said reading beam during conditions of radial tracking registration and (b) the tangent of the storage track;
• means for differentially combining the output signals of said photoreceptors to develop, during operating intervals in which said beam and said carrier are out of focus registration, a focus error signal of said wobble frequency having a phase related to the sense of focus misregistration and magnitude related to the extent of misregistration;
• a synchronous phase detector;
• means for supplying to said detector said focus error signal and a reference signal phase synchronized to the periodic displacement of said beam to develop a focus tracking signal;
• and
• means responsive to said focus tracking signal for maintaining said beam and carrier in focus registration.