JPH04205928A - Optical disk device - Google Patents

Abstract

PURPOSE:To allow stable recording and reproducing in a short period of time after switching of wavelengths by adding an offset to the output signal of a focus error detecting means, outputting this signal to a driving means and positioning an objective lens between the focusing position of the 1st wavelength and the focusing position of the 2nd wavelength. CONSTITUTION:This device has a laser beam source used at least at the two wavelengths, the objective lens 21, the focus error detecting means 50 which receives the luminous flux reflected by an optical disk MOD and generates a focus error signal and a driving means 57 which drives the objective lens 21 in the optical axis direction in accordance with the focus error signal. An offset adding means 55 which adds the offset to the output signal of the focus error detecting means 50 and outputs this signal to the driving means 57 so as to position the objective lens 21 between the focusing position at the 1st wavelength and the focusing position at the 2nd wavelength is provided. Then, the distance to move the objective lens 21 when the wavelengths are switched is shortened. The time until the objective lens 21 is moved to the prescribed position of the wavelength is shortened in this way.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an optical disc device that records and reproduces information on an optical disc, and particularly relates to an optical disc device that can perform stable focus servo even when the wavelength changes rapidly.

[Conventional technology]

A magneto-optical disk device capable of recording and reproducing is configured to image a light beam from a laser light source as a spot on a magneto-optical disk, which is a recording medium, to reproduce or record a signal. The output of the laser light source is low during playback and high during recording. In order to accurately record and reproduce signals, it is necessary to focus the laser beam onto the disk to a spot diameter that is close to the diffraction limit, and to accurately trace the recording track. For this reason, optical disc devices include detection means for detecting focus errors and tracking errors, and a drive device for driving the spot position of the laser beam in the optical axis direction of the optical system and in the radial direction of the disc based on the detection results. is provided. By the way, since the oscillation wavelength of a semiconductor laser used as a light source of a magneto-optical disk device shifts due to a change in output, the oscillation wavelength shifts instantaneously by several nm when switching between reproduction and recording. Therefore, if the optical system is not corrected for chromatic aberration, the focusing position will change rapidly. Changes in the focusing position due to such sudden shifts cannot be corrected by focus servo. Figures 9 to 14 show the relationship between the movement of the focal position due to a wavelength shift and the accompanying focus error signal. show. ■, ■, and ■ in each figure indicate respective points in time during playback, immediately after switching from playback to recording, and during recording. First, the positional relationship between the objective lens and the disk and the shape of a spot on the light receiving element for focus error detection will be explained based on FIGS. 9 and 10. Note that here, the astigmatism method is used to detect the focus error signal. In the astigmatism method, a cylindrical lens imparts astigmatism to a light beam reflected by a disk, and this light beam is received by a four-part light receiving element. The focus error signal is obtained by calculating the difference between the sums of the light receiving areas located diagonally across the light receiving element. The focus of the laser beam during playback (short wavelength, low power) and recording (long wavelength, high power) coincides with the disk surface as shown in Figure 9 ■ and ■, and the spot on the light receiving element is 10
As shown in FIGS. 2 and 3, the shape becomes circular, and the focus error signal FR becomes 0. On the other hand, immediately after the wavelength is switched, the objective lens is at the same position as during playback and the wavelength is switched for recording, so the focal position shifts by do from the recording surface of the disk, as shown in Figure 9 ■. As a result, the spot on the light receiving element becomes elliptical as shown in FIG. 10, and a focus error signal EO is generated. FIG. 11 is a characteristic diagram showing the relationship between the level of the focus error signal FE and the focal position of the laser beam. The X axis indicates the focal position of the laser beam, and the point at x=0 is the recording surface of the disk. Moreover, the y-axis shows the focus error signal FE, and the point of y=Q shows the error signal 0. The objective lens has a focus error signal FE
is controlled to be zero, that is, the focal position of the laser beam is controlled to coincide with the recording surface of the disk. When reproducing information, since the power of the laser beam is weak, the rate of change in the focus error signal with respect to movement of the focal position of the laser beam is small, and the relationship between the two is represented by a straight line A with a small slope. On the other hand, during recording, since the power of the laser beam is strong, the rate of change in the focus error signal with respect to the movement of the focal position of the laser beam is large, and the relationship between the two is represented by a straight line B with a large slope. The focus of the laser beam, which was at the position ■ during playback, instantaneously moves to the position shown in ■ due to the wavelength change caused by switching from playback to recording, and then returns to the position shown in ■ as the objective lens is moved by the focus servo. Return to position. The focus error signal generated by the focus movement dO during switching is EO. FIG. 12 shows the relationship between the position of the objective lens (horizontal axis) and the amplitude AP of the recording/reproduction signal (vertical axis). The amplitude AP of the recording/reproduction signal is maximum (indicated by level 1 in the figure) when the focal position of the laser beam coincides with the disc.When the focal position moves back and forth on the disc, it becomes curve A' and B'. drop along. Here, the curve A′ shows the time of reproduction, and the curve B° shows the time of recording. The inflection point of each curve indicates the position of the objective lens when the focal position of the laser beam matches the recording surface of the disk.
It can be seen that in order to match the focal position of the laser beam on the recording surface, the objective lens must be moved a distance do when switching from reproduction to recording. When the wavelength is switched to recording when the objective lens is at the focus position for reproduction, the amplitude of the recording and reproduction signal is curved B°
Then, the objective lens is driven by the focus servo to the in-focus position (2) during recording, and the recording/reproduction signal becomes maximum. FIG. 13 shows changes in the focus error signal FE as time t elapses, and FIG. 14 shows changes in the amplitude AP of the recording/reproduction signal as time t elapses. By switching from reproduction to recording, the focus error signal becomes EO, and the recording/reproduction signal is divided by the amount corresponding to EO. It takes time from switching until the focus error signal returns to 0 and the recording/playback signal reaches its maximum value.
It takes.

Problems to be Solved by the Invention) However, in the conventional magneto-optical disk device as described above, the focus error that occurs during wavelength switching is large, resulting in a large drop in the level of the recording and reproduction signal, making it difficult to perform accurate recording and reproduction. becomes impossible. Furthermore, since the amount of focus error that occurs during switching is large, it takes a long time for the servo mechanism to focus the objective lens on the disk, resulting in a problem that the uncontrollable period becomes long. [Objective of the Invention 1] The present invention has been made in view of the above-mentioned conventional problems, and provides an optical information reproducing system that is capable of accurately recording and reading information with little signal deterioration when switching the wavelength of a light source. The purpose is to provide equipment. [Means for Solving the Problems] In order to achieve the above object, an optical disc device according to the present invention includes a laser light source that is used with at least two wavelengths, and an objective lens that focuses the light beam from the laser light source on an optical disc. , a focus error detection means that receives the light beam reflected by the optical disk and generates a focus error signal, a drive means that drives the objective lens in the direction of its optical axis based on the focus error signal, and an output signal of the focus error detection means. It is characterized by comprising an offset adding means for adding an offset to and outputting it to the driving means so as to adjust the objective lens to be located at an intermediate position between the in-focus position at the first wavelength and the in-focus position at the second wavelength. shall be.

[Effect]

According to the above configuration, the laser beam is controlled to have a focus at a position separated from the disk in the same direction by the offset added to the focus error signal at each wavelength. Therefore, the distance that the objective lens must be moved when the wavelength is switched is shortened, and the time required for the focus servo mechanism to move the objective lens to a predetermined position corresponding to the wavelength is shortened. (Embodiment) An embodiment of the present invention will be described below with reference to the drawings. Figs. 1 to 8 show an embodiment in which the present invention is applied to an information recording/reproducing device for a magneto-optical disk. As shown in FIG. 1, this optical system includes a light source section 10°, an objective optical system 20, a prism block 30, and a signal detection optical system 4.
0. The light source unit 10 includes a semiconductor laser 11 that generates a diverging beam, a collimator lens 12 that converts the diverging beam into a parallel beam, two anamorphic prisms 13 and 14 that shape the cross section of the beam, and a mirror 15. Generates a parallel beam with a circular cross section. The objective optical system 20 includes an objective lens 21 that focuses the beam on the signal recording surface of the magneto-optical disk MOD, and a mirror 22. The objective lens 21 and the mirror 22 are provided in a head (not shown) that is slid in the radial direction X of the magneto-optical disk MOD. On the other hand, the light source section 10, prism block 30, and signal detection optical system 40,
It is fixed relative to the center of rotation of the disk. Further, the objective lens 21 is provided on an actuator provided within the head, and is driven at high frequency in the optical axis direction 2 thereof. The prism block 30 has two half mirror surfaces 31a.
, 31b, and a λ/2 plate 32
is connected to the first block via the polarization separation surface 33a.
and a total reflection surface 33b! It is composed of 112 blocks 33. A part of the light beam from the light source section 10 is reflected by the second half mirror surface 31b, and is focused by the focusing lens 34 onto the light receiving element 35 for automatic output adjustment of the semiconductor laser. On the other hand, the light beam reflected from the disk is reflected by the second half-mirror surface 31b, and the polarization direction is rotated by 45 degrees by the λ/2 plate, and the P-polarized light component is transferred to the polarization separation surface 33a.
The light is transmitted through the focusing lens 41a and focused onto the first light receiving element 4Za for detecting the first magnetic recording signal. Further, the S polarized light component is transmitted to the polarized light separating surface 33a and the total reflection surface 33.
b, and is focused via a focusing lens 41b onto a second light receiving element 42b for detecting magnetic recording signals. The polarization direction of the laser beam incident on the magneto-optical disk I'1OD corresponds to the magnetization direction of the disk at the position where the spot is imaged, and is rotated by the magnetic Kerr effect. By separating the S component into S components and detecting them with separate light receiving elements 42a and 42b, it is possible to read out the recording signal from the difference in intensity. Of the light reflected from the disk, the second half mirror surface 3
The component transmitted through 1b is reflected by the first half mirror surface 31a, is given an astigmatism by the cylindrical lens 44 via the focusing lens 43, and is focused onto the error detection light receiving element 45. The light receiving areas of the error detection light receiving element 45 are arranged as shown in FIG. These light-receiving areas include light-receiving areas A, B, C, and D that are divided into four areas along each axis in the direction x1 corresponding to the radial direction of the disk and the direction Y corresponding to the tangential direction. The focus error detection circuit 50 includes two adders 51 and 52 that take the sum of the outputs from the light receiving areas located diagonally on the light receiving element 45, a subtracter 53 that takes the difference between the outputs of these adders, and a subtracter 53 that takes the difference between the outputs of these adders. It includes an offset adjustment circuit 55 that adds an offset to the output of the calculator 53, and an automatic gain control (AGC) device W54 that adjusts the level of the output of the offset adjustment circuit. Focus error original signal F output from subtraction 1153
EO is expressed as FEO=(A+D)-(B+C) when the voltage corresponding to the light-receiving area of the light-receiving element is expressed with the same symbol. By adding the offset δ here, the focus error signal FICI output from the AGC circuit 54 is as follows: FEZ = FIEO + 6 = (A + D) - (B +
Indicated by (:)+6. The offset δ is set so that the focus error signal FEI becomes O when the objective lens is located between the focal positions of both wavelengths during recording and reproduction, that is, when the focal position is a predetermined distance from the recording surface of the disk. added to. The output focus error signal FEI is subjected to phase compensation by the phase compensation circuit 56, and then sent to the actuator 57.
supplied to Based on this signal, the actuator 57 drives the objective lens 21 in the optical axis direction so that FRI becomes 0. As mentioned above, in order to perform recording and reproduction accurately, it is a prerequisite that the spot diameter of the laser beam on the disk is kept within a certain value. Therefore, the deviation of the focal position from the disk during reproduction and recording due to offset must be within the depth of focus that can suppress the spot diameter within that range. Next, a focus adjustment operation in an embodiment having such a configuration will be explained with reference to FIGS. 3 to 8. ■, ■, and ■ in each figure indicate the time points during playback, immediately after switching from playback to recording, and during recording, respectively. First, the positional relationship between the objective lens and the disk and the spot shape on the light receiving element for focus error detection will be explained based on FIGS. 3 and 4. In a stable state during playback, the focus of the laser beam is at the third
As shown in the figure, the spot is located closer to the objective lens than the disk surface, and the spot on the light receiving element becomes an ellipse as shown in FIG. 4, and the focus error signal FEI becomes 0. On the other hand, immediately after the wavelength is switched, the objective lens is in the same position as during reproduction and the wavelength is switched to recording, so the focal position of the laser beam is changed to ■ as shown in Figure 3.
The spot on the photodetector becomes an ellipse with an opposite phase and a small eccentricity as shown in Figure 4 (■), and the focus error signal FEI is shifted by dO from the focal position of
become. In a stable recording state, the focus of the laser beam is at a predetermined distance from the disk surface as shown in Figure 5 (■), and the spot on the light receiving element is as shown in Figure 6 (■). It becomes an ellipse with the same phase as ■ and a large eccentricity,
The focus error signal EFI becomes 0. Next, based on FIG. 5, the focus error original signal FEO is
Explain the relationship between the level and the focal position of the laser beam. The X axis in the figure indicates the focal position of the laser beam, and x
The point at =0 is the recording surface of the disc. Further, the y-axis indicates the focus error original signal FEO, and the point y=Q indicates the state where the error original signal FEO=O, that is, the state where the focus error signal FEI=-6. The objective lens is controlled so that the focus error signal FEI becomes 0, that is, the focal position of the laser beam is 6 offsets away from the recording surface of the disk. When reproducing information, the power of the laser beam is weak, so when the focal position of the laser beam moves away from the recording surface of the disk, the rate of change in the focus error original signal is small, and the relationship between the two is represented by a straight line A with a small slope. be done. On the other hand, during recording, since the power of the laser beam is strong, the rate of change in the focus error original signal with respect to the movement of the focal position of the laser beam is large, and the relationship between the two is represented by a straight line B with a large slope. Therefore, the focus error original signal FEO
Even when the same offset δ is added to , the focused point (2) during reproduction is more stable at a position farther from the disk than the focused point (2) during recording. By the way, when a reading error occurs regarding playback,
Information can be obtained by re-reading, but
If incorrect information is recorded, it is impossible to determine whether the information is incorrect unless it is read out. Therefore, higher accuracy is required during recording than during playback. . The accuracy of playback and recording also depends on the spot diameter of the laser beam focused on the disc.
Particularly during recording, it is preferable that the focal point does not move away from the disk. By adding the offset δ, the stable positions during reproduction and recording are different, and the focus shift during recording is reduced, so that the above-mentioned requirements of the optical disc device are met. The focus of the laser beam, which was at the position ■ during playback, instantaneously moves to the position shown in ■ due to the wavelength change caused by switching from playback to recording, and then returns to the position shown in ■ as the objective lens is moved by the focus servo. Return to position. The focus error signal FEI generated by the focus movement dO during switching is El (<EO). Further, the focus movement from the switching time (2) to the recording time (2) is di (<do). FIG. 6 shows the relationship between the position of the objective lens (horizontal axis) and the amplitude AP of the recording/reproducing signal (vertical axis). The amplitude AP of the recording/reproducing signal is maximum when the focal position of the laser beam coincides with the disc (indicated by level 1 in the figure), and as the focal position moves forward and backward of the disc, it changes along curves A' and B'. and decreases. Here, the curve B° indicates the time of reproduction, and the curve B° indicates the time of recording. The inflection point of each curve indicates the focal position of the objective lens when the recording/reproduction signal is at its maximum, and the amplitude of the recording/reproduction signal during reproduction reaches its maximum from the offset objective lens position during reproduction. The distance to the position is d3, and the distance from the offset objective lens position during recording to the focal position where the amplitude of the recording/reproduction signal during recording is maximum is d3. When switching to recording mode (■→■), move the objective lens to d.
I can understand that I have to move it by 1. When the wavelength is switched to recording when the objective lens is at the focus position (■) for reproduction, the amplitude of the recording and reproduction signal decreases downward in the figure to the point (■) where it intersects curve B"0.
Next, the objective lens is moved to a curve B° so that the focus error signal FEI becomes 0 due to the function of the focus servo circuit.
It passes through the top and is driven to the recording position ■. FIG. 7 shows changes in the focus error signal FEI as time t elapses, and FIG. 8 shows changes in the amplitude AP of the recording/reproduction signal as time t elapses. During reproduction, since the focal position of the laser beam is shifted by δ, the recorded and reproduced signal is detected at a level lowered by this shift. Note that during recording, the focal position is closer to the disk than during playback, so the recording level of the signal is less degraded as shown by the dashed line in the figure. When switching from playback to recording, the focus error signal FEI becomes El, The recording/reproducing signal decreases by an amount corresponding to El. It takes time t1 from switching until the focus error signal returns to 0 and the recording/reproduction signal reaches its maximum value. According to the apparatus of this embodiment, there is little signal deterioration during wavelength switching, and stable recording operation can be started in a relatively short time t1 after switching. In addition, in the embodiment, a case has been described in which a fixed offset is applied for both reproduction and recording, and the focal point is removed in both reproduction and recording, but the present invention is not limited to the embodiment. For example, by adding an offset only during playback, it is also possible to perform shimmer recording at the recording focal point.

【effect】

As explained in detail above, according to the present invention, at least the amount of driving of the objective lens to correct the focal shift due to wavelength switching is sufficient, so that stable recording and playback can be performed in a short time after switching. reach.

[Brief explanation of the drawing]

1 to 8 are diagrams illustrating one embodiment of an optical disc device according to the present invention, in which FIG. 1 is an explanatory diagram showing the configuration of the entire optical system, FIG. 2 is a block diagram of a focus control system,
Figure 3 is an explanatory diagram showing the positional relationship between the objective lens and the disk, Figure 4 is an explanatory diagram showing the spot shape on the light receiving element at each time, and Figure 5 is a focus error signal during recording and reproduction. Figure 6 is a graph showing the relationship between the objective lens position and the amplitude of the recording/reproducing signal, and Figure 7 is a graph showing the change in focus error signal over time. FIG. 8 is a graph showing changes in the amplitude of the recording/reproduction signal over time. Figures 9 to 14 are diagrams explaining a conventional optical disc device, Figure 9 is an explanatory diagram showing the positional relationship between the objective lens and the disc, and Figure 10 is a spot shape on the light receiving element at each point in time. Figure 11 is a characteristic diagram showing the relationship between the level of the focus error signal and the focal position of the laser beam during recording and reproduction, and Figure 12 is the relationship between the position of the objective lens and the amplitude of the recording and reproduction signal. A graph showing the relationship between
FIG. 13 is a graph showing changes in the focus error signal over time, and FIG. 14 is a graph showing changes in the recording/reproduction signal over time. 11... Semiconductor laser (laser light source) 21... Objective lens FIOD... Magneto-optical disk 45... Light receiving element for error detection 50... Focus error detection circuit

Claims (2)

[Claims] (1) A laser light source used with at least two wavelengths, an objective lens that forms an image of the light beam from the laser light source on an optical disk, and a focus error that receives the light beam reflected by the optical disk and generates a focus error signal. a detection means; a drive means for driving the objective lens in its optical axis direction based on the focus error signal; and an output signal of the focus error detection means that adds an offset to the output signal and outputs it to the drive means, so that the objective lens An optical disc device comprising an offset adding means for adjusting the focus position to be located midway between the focus position at the first wavelength and the focus position at the second wavelength. (2) The focus error detection means includes an astigmatism generating element that generates astigmatism in the light beam reflected by the optical disk, and two that intersect at 45 degrees with respect to the principal meridian direction of the astigmatism generating element. a light receiving element having at least four light receiving areas separated by orthogonal axes of the book and receiving convergent light having an astigmatism difference transmitted through the astigmatism generating element; and a light receiving element located diagonally to each of the light receiving elements. 2. The optical disc device according to claim 1, further comprising an arithmetic circuit that generates a focus error signal by calculating a difference between output sums from the light receiving areas.