Optical information detection method, optical pickup, and optical information recording and reproducing apparatus
An interference-type optical pickup, an optical information detection method, and an optical information recording and reproducing apparatus, which allow easy adjustment of a path difference between two lights, have a high signal amplification effect and are suitable for size reduction of an optical system, are provided. A signal light reflected from an optical disk and a reference light branched from the same light source and guided into detectors without being projected onto the optical disk are caused to interfere with each other on detectors. Detector outputs in four interference states in which phase relationships between the reference light and the signal light are different from each other by degrees are simultaneously obtained and calculated. Accordingly, a readout signal RF that is always stable and amplified with high quality is obtained even when an optical path changes due to disk undulations. The four detector outputs are simultaneously obtained by use of four quadrant photo detectors, and are calculated, to thereby also obtain a servo signal.
1. 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).
2. The optical information detection method according to claim 1 , further comprising:
obtaining a tracking error signal by calculating the value of S′(A)+S′(B)−S′(C)−S′(D).
3. An optical pickup, comprising:
a laser;
a first optical element configured to branch a light beam from the laser into a first light beam and a second light beam;
an objective lens configured to focus the first light beam onto a recording film surface of an optical information recording medium, and configured to receive a reflected light;
a reference light beam reflecting means provided in an optical path of the second light beam;
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;
a second optical element configured to branch a light beam obtained by combining the first light beam reflected by the optical information recording medium and the second light beam reflected by the reference light beam reflecting means, and configured to allow the branched light beams to enter the first photo detector and the second photo detector;
a third optical element configured to combine the first light beam reflected by the optical information recording medium and the second light beam reflected by the reference light beam reflecting means in a state in which a phase relationship is different from that of the light beam combined by the second optical element by 90 degrees, configured to branch the combined light beam, and configured to allow the branched light beams to enter the third photo detector and the fourth photo detector;
wherein phase relationships between the first light beam and the second light beam 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;
an operation circuit configured to perform an operation on outputs of the first, second, third and fourth photo detectors, wherein the operation circuit is configured to:
compute 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 compute the square of the first differential, and
compute 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 compute the square of the second differential, and
compute 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
compute a focus error signal by calculating the value of S′(A)+S′(C)−S′(B)−S′(D).
4. The optical pickup according to claim 3 , wherein the operation circuit is further configured to obtain a tracking error signal by calculating the value of S′(A)+S′(B)−S′(C)−S′(D).
5. The optical pickup according to claim 3 , wherein the first, second, third and fourth photo detectors are formed on a same substrate.
6. An optical information recording and reproducing apparatus comprising:
a motor configured to hold and to rotate an optical information recording medium on a rotary shaft;
a servo control circuit; and
an optical pickup configured to project a light onto the optical information recording medium and configured to detect a reflected signal, the optical pickup including:
a laser;
a first optical element configured to branch a light beam from the laser into a first light beam and a second light beam;
an objective lens configured to focus the first light beam onto a recording film surface of the optical information recording medium, and configured to receive a reflected light;
a reference light beam reflecting means provided in an optical path of the second light beam;
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
a second optical element configured to branch a light beam obtained by combining the first light beam reflected by the optical information recording medium and the second light beam reflected by the reference light beam reflecting means, and configured to allow the branched light beams to enter the first photo detector and the second photo detector;
a third optical element configured to combine the first light beam reflected by the optical information recording medium and the second light beam reflected by the reference light beam reflecting means in a state in which a phase relationship is different from that of the light beam combined by the second optical element by 90 degrees, configured to branch the combined light beam, and configured to allow the branched light beams to enter the third photo detector and the fourth photo detector; and
wherein phase relationships between the first light beam and the second light beam 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; and
an operation circuit configured to perform an operation on outputs of the first, second, third and fourth photo detectors, wherein the operation circuit is configured to:
compute 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 compute the square of the first differential, and
compute 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 compute the square of the second differential, and
compute 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
compute a focus error signal by calculating the value of S′(A)+S′(C)−S′(B)−S′(D);
wherein based on the focus error signal, the servo control circuit compensates for a focus error by controlling a position of the objective lens in an optical axis direction, and adjusts a position of the reference light beam reflecting means in the optical axis direction such that a path difference between a signal light and a reference light is within a coherence length of the semiconductor laser.
7. The optical information detection method according to claim 1 , further comprising:
obtaining a readout signal (RF) by calculating the value of S′(A)+S′(B)−S′(C)−S′(D), or by performing a square root operation to a result of calculating the value of S′(A)+S′(B)−S′(C)−S′(D).
8. The optical information detection method according to claim 1 , wherein the first, second, third and fourth photo detectors are formed on a same substrate.
9. The optical pickup according to claim 3 , wherein the operation circuit is configured to obtain a readout signal (RF) by calculating the value of S′(A)+S′(B)−S′(C)−S′(D), or by performing a square root operation to a result of calculating the value of S′(A)+S′(B)−S′(C)−S′(D).
10. The optical information recording and reproducing apparatus according to claim 6 , wherein the operation circuit is configured to obtain a tracking error signal by calculating the value of S′(A)+S′(B)−S′(C)−S′(D).
11. The optical information recording and reproducing apparatus according to claim 6 , wherein the first, second, third and fourth photo detectors are formed on a same substrate.
12. The optical information recording and reproducing apparatus according to claim 6 , wherein the operation circuit is configured to obtain a readout signal (RF) by calculating the value of S′(A)+S′(B)−S′(C)−S′(D), or by performing a square root operation to a result of calculating the value of S′(A)+S′(B)−S′(C)−S′(D).