JPH0432033A - Detection system of focus error signal - Google Patents

Abstract

PURPOSE:To facilitate the adjustment by arranging photo diodes divided the direction orthogonal to grooves of a diffraction grating in the vicinity of the focus position of a beam obtained by a condenser lens system and detecting a focus error signal by the output of these photo diodes. CONSTITUTION:Photo diodes 11 to 13 are arranged which are divided in the direction orthogonal to grooves of a diffraction grating 9 provided in the vicinity of the focus position of a first-order or higher-order beam obtained by the condenser lens system, and the focus error signal is detected by the output of photo diodes 11 to 13. When the beam passing the diffraction grating 9 is convergent or divergent light, center positions of plural beam spots formed on the focal plane except the beam spot of the 0-order beam are moved in the direction perpendicular to grooves of the diffraction grating 9. Since the extent of this movement corresponds to the change of the focus position, the extent of movement is detected by divided photo diode 11 to 13 to obtain the focus error signal. Thus, adjustment is simplified.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for detecting a focus error signal of an optical pickup that records or reproduces information on an optical recording medium using a laser beam.

[Conventional technology]

When recording or reproducing information on an optical recording medium (hereinafter referred to as an optical disk), optical pickup may be affected by factors such as the rotation of the optical disk, the tilt when loading the optical disk, the warpage of the optical disk itself, and the fluctuation of the turntable. A surface runout of about 0.5 to 1 angle always occurs between the optical disc and the optical disc.

In order to read the information on the optical disk, it is necessary to converge the light beam to near the diffraction limit using an objective lens and irradiate it onto the optical disk.
It is necessary to send an error correction signal to the actuator.

Focus error detection methods include an astigmatism method, a critical angle prism method, a knife method, and a wedge prism method, each of which has its own advantages and disadvantages.

[Problem to be solved by the invention]

Although various focus error detection methods have been considered, there is no method that is superior in terms of ease of adjustment, error detection sensitivity, focus dynamic range, and number of parts. For example, in the astigmatism method, the optical system can be made smaller and the detection sensitivity is high, but the adjustment of the quadrant photodiode is complicated and the focus dynamic range is small.

Furthermore, although adjustment is easier in the Knifezi method compared to the astigmatism method, a portion of the light beam is blocked, resulting in a loss of light quantity, and when the dynamic range becomes large, the polarity is reversed. The wedge prism method is an improvement over the Knifezi method in terms of light loss, but the adjustment of the prism and photodiode is complicated.

An object of the present invention is to provide a focus error detection method that has high focus error detection sensitivity, a wide focus dynamic range, and is easy to adjust without increasing the number of parts.

[Means for Solving the Problems] Therefore, in the present invention, a laser beam is focused on an optical recording medium on which information is recorded, and the information recorded on the recording medium is read by the reflected light. A condensing lens system incorporating a diffraction grating for condensing the reflected light, and a condensing lens system incorporating the diffraction grating near the focal position of the primary beam or higher-order beam obtained by the condensing lens system. A photodiode divided in a direction perpendicular to the groove is arranged, and a focus error signal is detected from the output of the photodiode.

[Effect]

When collimated light passes through a diffraction grating and enters a condenser lens system, a plurality of beam spots are formed on the focal plane of the condenser lens system. Here, if the beam to be passed through the diffraction grating is convergent light or diverging light, the center position of the multiple beam spots formed on the focal plane will be in the direction perpendicular to the grooves of the diffraction grating, except for the zero-order beam spot. Move to.

Since this amount of movement corresponds to a change in the focus position, a focus error signal can be obtained by detecting the amount of movement using a two-split photodiode.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, a light beam 23 emitted from a laser diode 1 is turned into parallel light by a collimator lens 2.
Polarized beam splinter 3. Passes through a 174-wavelength plate 4, and its direction can be changed by 90° by a reflecting mirror 5. A beam spot 7 is then formed on the information recording surface of the optical disc 8 by the objective lens 6. This beam spot 7 is modulated by the information bits on the optical disk 8, becomes reflected light, returns along the optical path, and is separated from the incident light by the polarizing beam splitter 3 along the way. The separated reflected light passes through a diffraction grating 9 that distributes the light beam intensity in three directions, mainly 0th order and ±1st order, and is converted into three light beams, -1st order beam 20. 0th order beam 21. +1
This becomes the next beam 22. The respective light beams are connected to photodiodes 11 and 12 installed on a flat surface.
．． 13. The photodiode 12 for detecting the zero-order beam is for detecting an HF signal.

FIG. 2 shows changes in the position and size of the beam spot on the photodiodes 11, 12, 13 due to changes in the divergence and convergence state of the returned light. In the figures, (A) shows the case where the returned light is divergent light, (B) shows the case where the returned light is collimated light, and (C) shows the case where the returned light is convergent light. And the focus error signal is the photodiode 11.12.13
It is obtained by connecting the detection surfaces of (a + d) - (b + c) and performing detection.

In another embodiment, the same effect can be obtained by reversing the return optical system from the previous embodiment and changing the order of the condenser lens 10 and the diffraction grating 9, as shown in FIG.

In still another embodiment, as shown in FIG.
Diffraction grating 9 incorporating and used in the present invention
The beam is oriented so that it separates the light beam in a direction perpendicular to the direction in which it is to be separated.

At the focal point of the condenser lens 10, there is a photodiode 1516.1 divided as shown in FIG. ,1
8.19 By setting (f+i) −(g0h
), and (k−/)
A tracking error signal by the three-beam method can be detected by (j), and an HF signal can be detected by (j).

〔Effect of the invention〕

As described above, according to the present invention, it is possible to detect a focus error signal with high detection sensitivity and a large focus dynamic range. Furthermore, compared to conventional systems, it is possible to provide an optical system for detecting focus error signals that does not require a complicated optical path system and is easy to adjust.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of the present invention, FIG. 2 is an explanatory diagram explaining the embodiment of FIG. 1, and FIGS. 3 and 4 are schematic diagrams showing other embodiments of the present invention. The configuration diagram, FIG. 5, is an explanatory diagram illustrating the embodiment of FIG. 4. l...Laser diode 2...Collimator lens 3...Polarizing beam subrifter 4...1/4 wavelength plate 5...Reflection mirror 6...Objective lens 7...Beam spot 8...・Optical disk 9...Diffraction grating 10...Condensing lens 11.12, 13, 15, 16, 17.18.19...
・Photodiode 20...-1st order beam 21...0th order beam 22...+1st order beam

Claims (1)

[Claims] A laser beam is focused on an optical recording medium on which information is recorded,
An optical pickup that reads information recorded on the recording medium using the reflected light includes a condenser lens system incorporating a diffraction grating that condenses the reflected light, and a primary beam obtained by the condenser lens system. Alternatively, a photodiode divided in a direction perpendicular to the grooves of the diffraction grating is arranged near the focal point of the higher-order beam, and a focus error signal is detected by the output of the photodiode. A focus error signal detection method featuring: