Patent ID: 8094539

Claim:
An optical information detection method comprising the steps of: dividing a light beam emitted from a light source into a first light beam and a second light beam; focusing the first light beam onto an optical information recording medium; guiding a signal light reflected from the optical information recording medium to a first photo detector, a second photo detector, a third photo detector, and a fourth photo detector, each photo detector having regions A, B, C, and D; guiding the second light beam as a reference light to the first, second, third, and fourth photo detectors without focusing the second light beam onto the optical information recording medium; causing the signal light and the reference light to optically interfere with each other on the first, second, third, and fourth photo detectors such that phase relationships between the signal light and the reference light are different from each other by 180 degrees on the first and second photo detectors, by 180 degrees on the third and fourth photo detectors, and by 90 degrees on the first and third photo detectors; computing a first differential between a signal from a region A, B, C, or D of the first photo detector and a signal from the corresponding region of the second photo detector, then computing the square of the first differential; and computing a second differential between a signal from the corresponding region of the third photo detector and a signal from the corresponding region of the fourth photo detector, then computing the square of the second differential; and computing signals S′(A), S′(B), S′(C), and S′(D), each representing the sum of the square of the first differential and the square of the second differential for a respective region A, B, C, or D; and computing a focus error signal by calculating the value of S′(A)+S′(C)−S′(B)−S′(D).