Patent ID: 6134199
Filing Date: 2000-10-17
Classification: G11B

Abstract:
An apparatus for control of a focused beam of radiant energy, comprising:an optical element for directing said focused beam;an actuator operative on said optical element for displacing a focal point of said focused beam in a direction of a predetermined position;a detector receiving light from said beam and having first and second outputs responsive to a displacement of a focal point of said beam from said predetermined position;a circuit coupled to said outputs of said detector for producing an error signal representing a displacement of said focal point of said beam from said predetermined position;a servo coupled to said actuator and to said error signal, wherein said actuator is responsive to said servo to displace said focal point of said beam onto said predetermined position;wherein the improvement comprises:a local feedback loop coupled to said outputs of said detector, said loop comprising:a first periodic function generator responsive to said error signal; anda second periodic function generator responsive to said error signal, saidsecond periodic function generator having an output that differs from an output of said first periodic function generator by a phase angle;a first multiplier for multiplying said first output of said detector by said output of said first periodic function generator; anda second multiplier for multiplying said second output of said detector by said output of said second periodic function generator;wherein said outputs of said first and second multipliers are provided as inputs of said circuit.