JP2002025092A - Optical disk device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that a method using an objective lens with a high numerical aperture is proposed in order to improve, the recording density of an optical disk causes increase in coma aberration due to a disk inclination in proportion to the height of a numerical aperture, and degradation in the recording and reproducing performance of the optical disk. SOLUTION: The coma aberration signal is obtained, by condensing the inner side and outer side of luminous flux on separate photodetectors by using a diffraction grating, before condensing light on a photodetector, and by calculating track deviation signals independently, respectively to obtain these differences. Thereby, the coma aberration signal can be detected more stably.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk device, and more particularly, to a technology for detecting a disk inclination.

[0002]

2. Description of the Related Art In recent years, the density of optical disks has been steadily increasing, and 4.7 GB DVD-ROMs have been put on the market instead of 0.65 GB CD-ROMs which are read-only optical disks for consumer use. As a recordable large-capacity optical disk, a 2.6 GB DVD-RAM has already been put to practical use, and a large-capacity version of 4.7 GB capacity is expected to be released in the first half of 2000. Such recordable DVDs are increasingly required not only as storage media for computers but also for video image recording that does not require rewinding or fast-forwarding.
By the end of the year, video recorders using optical disks have already been put on the market. A video recorder using a DVD-RAM is also expected to be supported from the 4.7 GB capacity version, and market expectations are high in terms of compatibility with CDs and DVD-ROMs.

The recording density of an optical disk is almost limited by the size λ / NA (λ: light wavelength, NA: numerical aperture of an objective lens) of a light spot for recording and reproducing. Therefore, in order to increase the capacity, it is necessary to shorten the wavelength or increase the numerical aperture. Regarding the wavelength, development of a blue-violet semiconductor laser having a wavelength of 410 nm has been progressing in recent years. As a method of increasing the NA, there is, for example, JP-A-11-195229.
This exceeds NA 0.8 by forming a two-group objective lens.

Since the amount of coma caused by the tilt of the disk increases in proportion to the cube of NA, frequent tilt detection is required to realize a high NA.

[0005] Regarding the inclination detection, see, for example,
There is. In this method, a part of a light beam is applied to a disk surface, and a tilt detection signal is obtained by a differential output of reflected ± 1st order light.

[0006]

In the first prior art, since the main purpose is to detect spherical aberration, there is no mention of tilt detection.

In the second prior art, since the detection light is not focused, the detection light is applied to about 1,000 tracks, and there is a possibility that offset may occur in the ± 1st order light due to uneven pitch, uneven reflectivity, and the like. is there.

SUMMARY OF THE INVENTION An object of the present invention is to accurately and stably detect a coma aberration due to a disc tilt deviation, and to detect a tracking error signal with a small offset to stably record / reproduce an optical disc. It is to provide a device.

[0009]

An optical disk apparatus according to the present invention for solving the above-mentioned problems basically includes a semiconductor laser, an optical system for condensing the light on an optical disk, and a focus of the condensed light. A variable focus mechanism that changes the position, a mechanism that receives grooved diffraction light from the optical disk, converts it into a push-pull signal and applies tracking servo, an arithmetic circuit that outputs a coma aberration detection signal from the signal, and an optical disk Splitter that splits the reflected light from the optical path from the semiconductor laser to the optical disk, a lens that condenses the reflected light, and a light-receiving element that receives the light condensed by the lens and converts it into an electric signal And an arithmetic circuit for obtaining a reproduction signal and a defocus signal from the electric signal of the light receiving element.

At this time, the reflected light split by the light splitting element is further separated into a first light flux near the optical axis and a second light flux in the peripheral portion and split so as to be focused on the light receiving element. Are added. This light splitting element substantially becomes a hologram. Then, the first and second track shift signals are independently detected for each of the first light beam and the second light beam, and a signal substantially proportional to coma is obtained from the difference signal between them. Also, by applying tracking servo with the second light beam, it is possible to reduce track offset when the disk is tilted.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a basic embodiment of an optical disk apparatus according to the present invention.

The light from the semiconductor laser 101 is converted into parallel light by a collimating lens 102, and
The light is transmitted through the substrate 03 and is focused on the recording film surface of the optical disk 108 through the substrate by the two groups of two objective lenses 104 and 105. The beam splitter corresponds to a first optical splitting device described in the claims. Two groups of two objective lenses 10
4 and 105 are mounted on a two-dimensional actuator 106 and driven in the optical axis direction and the radial direction of the optical disk. The light reflected from the optical disk 108 is the beam splitter 103
Is reflected to the light separation hologram 109, and the light near the optical axis (not shown) and the light in the peripheral portion are separated in different directions, and both are incident on the photodetector 112 through the cylindrical lens 111 by the condenser lens 110. ing. The photodetector 112 has a plurality of light receiving areas. The light is divided by the plurality of light receiving areas, detected, and converted into a photocurrent.
These are defocused signal detection circuit 113, tracking error signal detection circuit 114, coma aberration signal detection circuit 115,
The reproduction signal detection circuit 116 outputs each signal as a voltage signal. The out-of-focus signal is fed back as a drive signal in the focus direction of the two-dimensional actuator 106, and is controlled so that the best image point is always formed on the optical disc. The tracking error signal is fed back as a drive signal of the two-dimensional actuator 106 in the radial direction of the disk, and a coma signal detection circuit 115 outputs a disk tilt signal. With that signal,
Tilt control device 1 to keep disk and optical axis vertical
07 is tilted. The reproduction signal detection circuit 116 reproduces a signal recorded on an optical disk, including current-voltage conversion, waveform equalization processing, and binarization processing. Figure 1
, The collimating lens 102 is
9 and can be arranged between the beam splitter 103 and the first lens 104. In the present embodiment, a cylindrical lens 110 is arranged to show a case where an astigmatism method is used as a defocus detection method.

FIG. 2 is a schematic diagram of a pattern of the light separation hologram 109 in the embodiment of FIG. Incident light beam 20
A boundary 20 that is approximately equally divided by the amount of light for one diameter
2, the direction of the diffraction grating is made different between the inner region 203 and the outer region 204. As a result, the inside and outside of the light beam are separated and condensed on the detector 112.

FIG. 3 shows the photodetector 1 in the embodiment of FIG.
12 is a schematic diagram showing 12 light receiving surface patterns and a circuit operation method for obtaining a tracking error signal, a coma aberration signal, a defocus signal, and a reproduction signal from output signals thereof. The light separated into the inside and the outside of the light beam by the light separation hologram 109 is received by the four divided light receiving areas 301 and 302. Of these, 301 receives the inner beam diffracted light 304 as the outer beam diffracted light 303 and 302. The outputs of these light receiving regions are converted into voltages by a buffer amplifier 305, and the differential amplifiers 307, 308, 309,
Addition and subtraction operations of 310 and 311 are performed.

The tracking error signal uses a push-pull method. The inner light beam and the outer light beam are calculated so as to obtain the difference between the detected light amounts of the two areas divided by the radial diameter of the disk, and the inner light beam track shift signal and the outer light beam track shift signal are obtained. The light flux track shift signal is used as a tracking error signal. After that, the coma aberration signal is obtained by subtracting the inner light flux track shift signal from the outer light flux track shift signal. At this time, if the light amount ratio of the inner and outer divisions of the light beam is not uniform, the resistance 306 may be adjusted as a variable resistance.

In the ordinary push-pull system, the difference between the amounts of received light in two regions divided by the diameter in the tangential direction is calculated. However, in the embodiment of FIG. 1, the astigmatic defocus detection system is used. In the circle of least confusion due to astigmatism, the direction of the light beam rotates 90 °, and the diffraction pattern by the guide groove of the disk appears in the tangential direction. For this reason, the division is performed on the radial diameter. The defocus signal is obtained by subtracting the diagonal regions of the inner and outer luminous fluxes divided into four.

FIG. 4 shows a diffracted light pattern in the detection area. (A) shows the diffraction light pattern of the detection light beam.
401 is a 0th-order diffracted light, 402 is a 1st-order diffracted light, and 403 is a -1st-order diffracted light. The portion corresponding to the 0th-order diffracted light of the inner light beam is 404, and the portion corresponding to the 1st-order diffracted light is 405-1, -1.
The portion corresponding to the next-order diffracted light is 406. The detection of the inner light beam track shift signal is performed in an area where 404 and 405 overlap and an area where 404 and 406 overlap, and (b)
407. The detection region of the outer light beam track shift signal is a portion other than 407, and is indicated by 408 in FIG.

FIGS. 5, 6, 7 and 8 show simulation results. In the simulation, the light intensity distribution on the detector was obtained by Fourier integration based on the scalar diffraction theory. The calculation conditions were a wavelength of 400 nm, a rim intensity of 0.77,
Objective lens NA 0.85, track pitch 0.298μ
m, groove depth 0.14λ, reflectivity 18%, substrate thickness 0.1m
m land groove disk.

FIG. 5A shows a track shift signal amplitude when the inner light beam diameter is changed. The horizontal axis is the inner light beam diameter normalized by the light beam diameter, and the vertical axis is the track shift signal amplitude normalized by the mirror level.

FIG. 3B shows a position where the outer light beam track deviation signal becomes 0 when the inclination in the disk radial direction is given as coma aberration. The aberration amount is the Seidel aberration coefficient of 0.1
λ, 0.2λ, 0.3λ, 0.4λ, and 0.5λ.
0 on the vertical axis is the maximum position of the center intensity of the spot at each aberration. (C) shows a position where the inner light flux track shift signal of the same amount of coma becomes zero. From (b), it can be seen that the offset due to coma of the outer light beam track shift signal is extremely small. As for the inner light beam diameter, the offset of the outer light beam track shift signal at each aberration amount becomes substantially zero.
Around 7 is considered appropriate.

FIG. 6 shows the result of obtaining the track shift signal. The inner light beam diameter was 0.68. The horizontal axis represents the off-track amount, which is the amount of deviation between the maximum position of the center intensity of the spot and the track center. 0 is the track center on the groove, ± 0.
Reference numeral 298 denotes a track center on the land. (A) is Seidel's coma 0.5λ, (b) is no aberration, (c) is-
0.5λ. The signal amount is normalized such that the amplitude without aberration is 1 with respect to the mirror level. The outer light flux track shift signal, the inner light flux track shift signal,
The total light flux track shift signal is obtained. It can be seen that the inner light flux signal and the total light flux signal are shifted due to aberration, but the outer light flux track shift signal has almost no offset.

FIG. 7 shows a similar calculation in which an infinite length mark having a reflectivity of 10% and a width of 0.2 μm is placed on one of the lands in order to examine the influence of crosstalk due to a recording mark. The offset of the outer light flux track shift signal is very small. Adjacent marks have almost no effect.

FIG. 8 shows a result of calculating a coma aberration signal in the present invention using the result. The horizontal axis represents the amount of aberration, and the vertical axis represents the amount of the inner light beam track shift signal at the position of the outer light beam track shift signal 0. (A) shows a case without a mark, and (b) shows a case with a one-sided mark. In both cases, it can be seen that a signal proportional to coma is obtained in the range of ± 0.5λ. Since 0.5λ corresponds to a slope of about 1 °, it is sufficient as a detection range.

[0025]

According to the present invention, it is possible to accurately, easily and inexpensively detect a coma aberration in an optical disk device, and feed it back to a coma aberration compensating mechanism so that the quality of a condensed spot can be maintained at a high level and a stable high spot can be obtained. Recording and reproduction of an optical disk having a high density can be performed.

[Brief description of the drawings]

FIG. 1 is a basic embodiment of an optical disk device according to the present invention.

FIG. 2 is a light separation hologram 10 in the embodiment of FIG.
7 is a schematic view of a pattern of FIG.

FIG. 3 is a light receiving surface pattern of a photodetector 110 and a circuit calculation method in the embodiment of FIG.

FIG. 4 is a schematic diagram of a diffraction pattern.

FIG. 5 is a simulation result of a relationship between a light beam diameter and a signal amount.

FIG. 6 shows a simulation result of a track shift signal without a recording mark.

FIG. 7 shows a simulation result of a track shift signal with an adjacent recording mark.

FIG. 8 shows a coma aberration signal simulation according to the present invention.

[Explanation of symbols]

101 semiconductor laser, 102 collimating lens, 103 beam splitter, 104 first lens, 105 second lens, 106 two-dimensional actuator, 107 tilt control device, 108 optical disk, 109: light separation hologram, 110: cylindrical lens, 111: condenser lens, 112: photodetector, 113: defocus signal detection circuit, 114:
Tracking error signal detection circuit, 115 ° coma aberration signal detection circuit, 116 ° reproduction signal detection circuit, 201 °
Incident light flux, 202 ° boundary, 203 ° inner area, 20
4 ‥‥ outer area, 301, 302 ‥‥ 4 divided light receiving area,
303 ° outer light beam, 304 ° inner light beam, 305 °
Buffer amplifier, 306 ° resistor, 307, 308, 3
09, 310, 311 differential amplifier.

Claims (2)

[Claims] 1. A semiconductor laser, an optical system for condensing the light on an optical disk, a mechanism for detecting a tilt between a recording information surface of the optical disk and an optical axis of the optical system, and a reflected light from the optical disk A light splitting element that splits the light from the optical system, a lens that collects the branched reflected light, a light receiving element that receives the light collected by the lens and converts the light into an electric signal, In the optical disc device, which is at least composed of an arithmetic circuit for obtaining a reproduction signal and a tracking error signal from the electric signal, the branched reflected light is further separated into a first light flux near the optical axis and a second light flux in the peripheral portion. A second light branching element for branching so as to be focused on the light receiving element is added, and the first light flux and the second light flux are respectively detected for the first and second track shift signals, respectively, To their difference signal An optical disc device having a function of performing more tilt control. 2. The optical disk device according to claim 1, wherein tracking servo is performed using the second light beam.