JPH02206024A - Focus servo device - Google Patents

Abstract

PURPOSE:To obtain the just in-focus state at the time of reproducing as well as recording by using correcting voltages set independently of each other at the time of the reproducing operation and the recording operation, and correcting the offset of a focus error signal. CONSTITUTION:The reflected laser beam from an optical disk is detected by a photodetectors 1 and 2 of an optical head, and a focus error signal FE1 is outputted from a differential amplifier 5 and is processed in an AGC circuit 9 which has the gain controlled by a detection output sum signal passing a gain switching circuit 7 having the gain switched at recording and reproducing, a summing amplifier 6, and a gain switching circuit 8 having the gain switched at recording and reproducing, and an error signal FE3 corrected in accordance with the reflectivity change of the disk is obtained. This signal FE3 becomes an error signal EF4 of large phase margin by the increase of the servo gain in the low frequency in a phase compensating circuit 10, and the offset voltage which is outputted from an offset switching circuit 16 and is set in accordance with the recording or reproducing operation is added by an offset adder 15 to apply the focus servo, and thus, the just in-focus state is obtained in the reproducing operation as well as the recording operation.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical disc device for recording and reproducing information on an optical disc. The present invention relates to a focus servo device that maintains a just-focus state.

The problem to be solved by the conventional technology and invention is shown in Figure 4.
It is a principle block diagram of the focus adjustment mechanism built into the optical head of an optical disk device, Comprising: 13 is an optical disk, 14 is an objective lens.

In order to record and reproduce information, it is necessary to always focus the laser beam narrowed down by the objective lens 14 onto the information recording layer of the optical disc 13. This focus adjustment is performed by moving the objective lens 14 in the optical axis direction. This is done by displacing the The objective lens 14 and its lens barrel 21 are attached to the base of the optical head so as to be vertically movable, and the lens barrel 21 is provided with a drive coil 12 that constitutes an electromagnetic actuator together with a magnet and a spring (not shown).
By passing a current through the drive coil 31\-712, the objective lens 14 can be displaced in the optical axis direction. The focus servo device is a device that maintains a constant distance between the objective lens 14 and the optical disk 13 by adjusting the current of the drive coil 12, and always correctly focuses the laser beam on the information recording layer of the optical disk 13.

The general configuration of a conventional focus servo device is shown in FIG. Reference numerals 1 and 2 refer to photodetectors 1 and 2 provided in the optical head, which detect the reflected light of the laser beam applied to the optical disk 13 by the optical system of the optical head (generally consisting of an objective lens 14, a half mirror, etc.). ), but under ideal conditions, the amount of light received is the same in a just-focus state, but in a defocused state, one becomes more υ than the other depending on the direction of the shift (focus error).

The outputs of the photodetectors 1 and 2 are amplified by preamplifiers 3 and 4 and input to a differential amplifier 5 and an addition amplifier 6, and the difference signal (focus error signal) FE between the respective signals is
I and a sum signal (light reception level signal) FSI are generated.

The power of the laser beam is switched between the recording operation and the reproducing operation, and the intensity of light received by the photodetectors 1 and 2 naturally changes. In order to correct this effect, the focus error signal FE2 and the received light level signal FS2 are controlled by AGC (automatic gain control ) input to circuit 9.

Incidentally, the gain switching circuits 7 and 8 are not necessary for a read-only optical disc device.

The AGC circuit 9 changes the amplification gain of the focus error signal FE2 according to the received light level signal FS2, and outputs a focus error signal FE3 that compensates for changes in the reflectance of the optical disc 13. This focus error signal FEB is
The signal is passed through a phase compensation circuit 10 which increases the phase margin at the focus servo gain intersection to stabilize the servo operation and increase the servo gain at low frequencies, and becomes the focus error signal FE4. 11 is a drive circuit for the drive coil 12 of the focus adjustment actuator; 5.
\-7 An excitation current having a magnitude and polarity corresponding to the voltage and polarity of the focus error signal FE4 is caused to flow through the drive coil 12.

When the laser beam is accurately focused on the information recording layer of the optical disc 13 (just focus state), the focus error signal FE (F'E1, FE2.F
E3. The level of FE4) becomes zero, but if the focus shifts before and after the information recording layer (defocus state)
, the polarity and level of the focus error signal FE change depending on the direction and amount of the shift (focus error). By automatically controlling the magnitude and polarity of the excitation current of the drive coil 12 so that the level of the focus error signal FE becomes zero, the objective lens 14 and the optical disc 13 follow the surface runout of the optical disc 13. The distance between the images is adjusted to a constant value to maintain just focus.

FIG. 5 is a characteristic diagram of the focus error signal FE, and the horizontal axis is the appropriate value (
The vertical axis is the level of the focus error signal FE.

In Fig. 5, ■ is the ideal focus error signal FE.
The characteristic is that the level is zero at the just focus point υ, and the level increases and decreases symmetrically with respect to errors in each direction around this point.

However, in an actual device, the mounting of the photodetectors 1 and 2 depends on the positional accuracy, the mounting of the optical system for detecting focus errors related to the photodetectors 1 and 2 depends on the positional accuracy, the relative positional relationship between these, etc. As a result, the focus error signal characteristic becomes a characteristic as shown in (2) with a certain degree of offset from the ideal characteristic (2). Correct focus servo is impossible if characteristic (2) with such an offset remains, so it is necessary to correct it.

It is not impossible to achieve ideal characteristics (2) without offset by repeatedly fine-tuning the positions of the photodetectors 1, 2 and their optical systems and measuring the characteristics for each individual device.

However, it is impossible to make such adjustments in terms of time and cost in a mass production system, so conventionally, a correction voltage corresponding to the offset amount of the focus error signal is applied, and this correction is performed. By adding voltage to the focus error signal, the offset of the focus error signal is canceled and the ideal characteristic ■
I am trying to correct it.

However, even with such a correction means, there is a problem in that focus accuracy deteriorates either during a recording operation or during a reproducing operation. The reason for this will be explained next.

In the case of semiconductor lasers commonly used in optical disk devices, the far field pattern changes sharply depending on the power. That is, as shown in FIG. 6, the far field patterns during the recording operation and during the reproducing operation are dissimilar. Due to such a difference in the far field pattern, the offset amount of the focus error signal varies between the recording operation and the reproduction operation. However,
Conventionally, the configuration was such that offset correction was performed using the same correction voltage during recording and playback operations, so it was difficult to set the optimal correction voltage for both playback and recording operations, and it was difficult to set the optimal correction voltage for both playback and recording operations. If the correction voltage is set in accordance with the amount of offset during one operation, there is a problem that the offset correction becomes insufficient during the other operation, resulting in deviation from the just focus point.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a focus servo device that can maintain a laser beam in a just-focused state during both reproduction and recording operations.

Note that by adding an offset voltage to the focus error signal at the front and rear stages of the AGC circuit and setting each offset voltage to have a specific relationship, the spot diameter of the laser beam during recording operation can be sufficiently narrowed down. In addition, a focus servo circuit is known that improves the S/N ratio of a reproduced signal by widening the spot diameter during a reproduction operation (Japanese Patent Laid-Open No. 158038/1983). However, this cannot compensate for the offset fluctuation of the focus error signal due to the difference in the far field pattern of the nine-vane laser during recording and reproducing operations as described above.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides means for generating different correction voltages that are independently set depending on whether the optical disk device is in a reproducing operation or a recording operation. , a means for correcting a focus error signal using a start correction voltage generated by this means, and a configuration for controlling the displacement of an objective lens of an optical head in accordance with the corrected focus error signal. .

According to the above-described configuration, the focus error signal of the present invention is corrected by independently set optimum correction voltages in each case of the reproducing operation and the recording operation, so that the far field pattern of the semiconductor laser is different in the reproducing operation The laser beam can be maintained in a state in which it is just focused on the information recording layer of the optical disc both during the recording operation and during the recording operation.

1O Page Embodiment FIG. 1 is a block diagram of a focus servo device according to an embodiment of the present invention. In this figure, the same reference numerals as those in FIG. 3 are equivalent parts, so their explanation will be omitted.

The difference between this focus servo device and conventional devices is that an offset addition circuit 15 and an offset switching circuit 16
has been added.

When the R/W signal given from an external control system (not shown) indicates that the reproducing operation is being performed, the offset switching circuit 16 changes the offset amount of the focus error signal at the just focus point during the reproducing operation. The offset addition circuit applies a correction voltage to cancel the offset amount of the focus error signal during the recording operation when the R/W signal indicates that the recording operation is in progress. 15, respectively.

The offset addition circuit 15 is the offset switching circuit 1
This circuit adds the correction voltage supplied from 6 to the focus error signal FE4 and outputs a focus error signal FE5 whose offset is canceled.

A specific example of the offset addition circuit 15 and the offset switching circuit 16 is shown in FIG.

The offset switching circuit 16 shown here uses a variable resistor VRI to set a correction voltage for offset cancellation during a playback operation, sets a correction voltage during a recording operation using a variable resistor VR2, and sets a correction voltage for canceling an offset during a playback operation using a variable resistor VR2. W
Turn on the signal, close the analog switch SWI, output the correction voltage set by the variable resistor VRI to the outside, turn off the R/W signal during recording operation, and close the analog switch with the output of the inverter ICI that is turned on at this time. The configuration is such that Sw2 is closed and the correction voltage set by variable resistor VR2 is outputted to the outside.

Further, the offset addition circuit 15 is a general addition amplifier circuit including an operational amplifier OP1 and resistors R1, R2, R3, and R4.

As is clear from the explanation so far, the excitation current of the drive coil 12 is controlled by the focus error signal whose offset is corrected by separate correction voltages during the reproduction operation and the recording operation. By setting the optimum correction voltage according to the offset amount in each case of operation, a just focus state can be maintained during any operation.

Note that the position where the focus error signal is corrected by the correction voltage may be changed, and the function of the offset addition circuit 15 may be included in the phase compensation circuit 10 or the drive circuit 11.

Further, although there are various methods for optically detecting focus errors, the obtained focus error signals are the same, so the present invention can be applied regardless of the focus error detection method.

Effects of the Invention As is clear from the above description, the present invention has the advantage of correcting the offset of the focus error signal using correction voltages set separately during the playback operation and during the recording operation. This has the effect that the just focus state 13-\- can be maintained even in the event of a deviation during the recording operation.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a focus servo device according to an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a specific example of the offset switching circuit and offset addition circuit in FIG.
Figure 3 is a block diagram showing the general configuration of a conventional focus servo device, Figure 4 is a logical configuration diagram of the focus adjustment mechanism, Figure 5 is a characteristic diagram of the focus error signal, and Figure 6 is the recording operation. FIG. 3 is a diagram showing a far field pattern of a semiconductor laser during playback operation and reproduction operation. 1.2...Photodetector, 3,4...Preamplifier, 5...
Differential amplifier, 6... Addition amplifier, 7.8... Gain switching circuit, 9... AGC circuit, 10... Phase compensation circuit, 11... Drive circuit, 12... Focus adjustment 13... Optical microscope, 14... Objective lens, 15... Offset addition circuit, 16... Offset switching circuit. Name of agent: Patent attorney Shigetaka Awano and one other person

Claims (1)

[Claims] means for detecting a focus error of a laser beam for recording or reproduction from reflected light from an optical disk; and means for displacing an objective lens for focusing the laser beam onto an information recording layer of the optical disk relative to the optical disk. a means for generating a first correction voltage during a reproducing operation and a second correction voltage set independently of the first correction voltage during a recording operation; means for correcting the focus error signal output from the focus error detection means; and means for displacing the objective lens according to the corrected focus error signal by the means, thereby generating the corrected focus error signal. 1. A focus servo device for an optical disc device, comprising means for minimizing a focus error shown by.