Optical disc drive having a mechanism to reduce the noise level of a focus error signal
There is provided an optical disc drive, which includes a light source emitting a light beam, a beam dividing element that divides the light beam into main and sub-beams, an objective lens, an astigmatism producing unit that gives astigmatism to the main and sub-beams, a signal generation unit that has sensors for the beams and generates a focus error signal based on output signals of the sensors; and an optical element that has a function of adjusting intensity distribution of the main beam such that the main beam incident on the optical disc has a predetermined intensity distribution where a maximum intensity level is positioned at a predetermined height within a range of 70% of a pupil diameter, the predetermined height is not equal to an optical axis, and an intensity of the main beam decreases gradually from the predetermined height to a peripheral portion of the pupil.
1. An optical disc drive, comprising:
a light source that emits a light beam;
a beam dividing element that divides the light beam emitted by the light source into a main beam and a plurality of sub-beams such that each of the plurality of sub-beams has a light amount smaller than that of the main beam;
an objective lens that converges each of the main beam and the plurality of sub-beams onto a recording surface of an optical disc;
an astigmatism producer that gives astigmatism to each of the main beam and the plurality of sub-beams which passed through the objective lens after reflecting from the recording surface of the optical disc;
a signal generator that has sensors respectively provided for receiving the main beam and at least two of the plurality of sub-beams which passed through the astigmatism producer, each of the sensors comprising four sensor segments, the signal generator processing output signals of the sensors in accordance with a differential astigmatic method to generate a focus error signal;
an optical element that adjusts, without blocking transmission of a central portion of the main beam, an intensity distribution of the main beam by causing a maximum light amount of the main beam to be positioned away from a pupil center of the main beam, a radial distance between the position of the maximum light amount of the main beam and the pupil center to be within 70% of a pupil diameter of the main beam, and an intensity of the main beam to gradually decrease from the position of the maximum light amount to a periphery of the main beam,
wherein the main beam and each of the sub-beams are incident on the optical element, and
the optical element comprises a diffraction structure that provides a predetermined intensity distribution to an incident beam, and is provided on a surface of the objective lens.
2. The optical disc drive according to claim 1 , wherein the radial distance between the position of the maximum light amount of the main beam and the pupil center is within 50% of the pupil diameter of the main beam.
3. The optical disc drive according to claim 1 , wherein the radial distance between the position of the maximum light amount of the main beam and the pupil center is greater than 20% of the pupil diameter of the main beam.
4. The optical disc drive according to claim 1 , wherein the main beam has an intensity distribution equal to a Gaussian distribution after being reflected from the recording surface of the optical disc.
5. The optical disc drive according to claim 1 , wherein:
each of the sensors has four sensor segments, and
the signal generator generates the focus error signal in accordance with an astigmatic method.
6. The optical disc drive according to claim 1 , wherein the signal generator generates an RF signal based on an output signal of a sensor for the main beam.
7. The optical disc drive according to claim 1 , wherein the signal generator generates the focus error signal based on output signals of at least two of the sensors provided for the plurality of sub-beams.
8. The optical disc drive according to claim 7 , wherein the signal generator generates the focus error signal further based on an output signal of a sensor for the main beam.
9. The optical disc drive according to claim 1 , wherein the signal generator generates the focus error signal based on output signals of at least two of the sensors for the main beam and the plurality of sub-beams.
10. The optical disc drive according to claim 1 , wherein the optical element adjusts an intensity distribution of each of the sub-beams so that the main beam and the sub-beams all have the same intensity distribution.
11. The optical disc drive according to claim 1 , wherein the signal generator processes the output signals of the sensors in accordance with the differential astigmatic method to generate the focus error signal by receiving a first diagonal sum signal and a second diagonal sum signal from each of the sensors, combining the first diagonal sum signals to generate a combined first diagonal sum signal, combining the second diagonal sum signals to generate a combined second diagonal sum signal, and generating the focus error signal by calculating a difference between the combined first diagonal sum signal and the combined second diagonal sum signal.
12. The optical disc drive according to claim 11 , wherein the first diagonal sum signal of each sensor is a sum of an output level of a first sensor segment and an output level of a second sensor segment arranged diagonal to the first sensor segment, and the second diagonal sum signal of each sensor is a sum of an output level of a third sensor segment and an output level of a fourth sensor segment arranged diagonal to the third sensor segment.
13. The optical disc drive according to claim 1 , wherein the adjusted intensity distribution of the main beam gradually decreases from the maximum level to each of the periphery and pupil center.
14. The optical disc drive according to claim 1 , the adjustment of the intensity distribution of main beam being structured to decrease noise generated due to focus error signal resulting from a portion of the main beam being incident onto a sensor of a sub-beam.
15. The optical disc drive according to claim 1 , wherein maximum light amount of main beam is positioned away from pupil center by adjusting a diffraction efficiency of a diffraction structure on a surface of the objective lens.
16. An optical disc drive, comprising:
a light source that emits a light beam;
a beam dividing element that divides the light beam emitted by the light source into a main beam and a plurality of sub-beams such that each of the plurality of sub-beams has a light amount smaller than that of the main beam;
an objective lens that converges each of the main beam and the plurality of sub-beams onto a recording surface of an optical disc;
an astigmatism producer that gives astigmatism to each of the main beam and the plurality of sub-beams which passed through the objective lens after reflecting from the recording surface of the optical disc;
a signal generator that has sensors respectively provided for receiving the main beam and at least two of the plurality of sub-beams which passed through the astigmatism producer, each of the sensors comprising four sensor segments, the signal generator processing output signals of the sensors in accordance with a differential astigmatic method to generate a focus error signal;
an optical element that adjusts an intensity distribution of the main beam by causing, without blocking transmission of a central portion of the main beam a maximum light amount of the main beam to be positioned away from a pupil center of the main beam, a radial distance between the position of the maximum light amount of the main beam and the pupil center to be within 70% of a pupil diameter of the main beam, and an intensity of the main beam to gradually decrease from the position of the maximum light amount to a periphery of the main beam,
wherein the main beam and each of the sub-beams are incident on the optical element, and
the optical element comprises a coating that provides a predetermined intensity distribution to an incident beam, and is provided on a surface of the objective lens.
17. The optical disc drive according to claim 16 , wherein the signal generator processes the output signals of the sensors in accordance with the differential astigmatic method to generate the focus error signal by receiving a first diagonal sum signal and a second diagonal sum signal from each of the sensors, combining the first diagonal sum signals to generate a combined first diagonal sum signal, combining the second diagonal sum signals to generate a combined second diagonal sum signal, and generating the focus error signal by calculating a difference between the combined first diagonal sum signal and the combined second diagonal sum signal.
18. The optical disc drive according to claim 17 , wherein the first diagonal sum signal of each sensor is a sum of an output level of a first sensor segment and an output level of a second sensor segment arranged diagonal to the first sensor segment, and the second diagonal sum signal of each sensor is a sum of an output level of a third sensor segment and an output level of a fourth sensor segment arranged diagonal to the third sensor segment.
19. The optical disc drive according to claim 16 , wherein the adjusted intensity distribution of the main beam gradually decreases from the maximum level to each of the periphery and pupil center.
20. The optical disc drive according to claim 16 , the adjustment of the intensity distribution of main beam being structured to decrease noise generated due to focus error signal resulting from a portion of the main beam being incident onto a sensor of a sub-beam.