JPH087311A - Optical disc apparatus - Google Patents

Abstract

PURPOSE:To control a focus position to be optimum at all times by detecting a focus offset value when an amplitude of an RF signal of a plurality of detecting signals detected by an optical head is maximum and servo controlling the focus position. CONSTITUTION:While a control part 6 feeds an offset level OFS gradually changing as shown in the lower Fig. to a focus servo control part 4, an amplitude of an RF signal RFS which is the sum of RF signals of a plurality of detecting signals sent from an optical head 2 is detected by a detecting part 5. The control part 6 supplies, upon receipt of the amplitude, an offset level OFS of the maximum amplitude to the focus servo control part 4, whereby a focus position is controlled. Since a position of a spot light is controlled while a focus offset value when the amplitude of the RF signal of the sum of RF signals of the plurality of detecting signals detected by the optical head 2 is maximum is fed to the focus servo control part 4, the focus position can be controlled to the optimum at all times even if the astigmatism is observed in the spot light due to the heat or change with time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk apparatus for automatically adjusting a focus offset value for giving an optimum spot light using a signal reflected from an optical disk.

[0002] An optical disc apparatus that automatically adjusts a focus offset value that gives an optimum spot light by using a signal reflected from an optical disc, and a focus in which a total amplitude of RF signals among received light signals received by an optical head is maximum. An offset value is given to the focus servo control unit to irradiate the optical disc with spot light and to receive multiple reflected light beams for the purpose of controlling the focus position to the optimum focus position even if astigmatism occurs. Head for receiving light by using a focusing device, and a focus servo control unit for controlling the focus position of spot light based on a focus error signal obtained from a plurality of light receiving signals received by the optical head and a given focus offset value And the focus offset value given to this focus servo control section Corresponding to the sequential change, a sum signal amplitude detection unit for detecting the sum amplitude of the RF signals among the light reception signals received by the plurality of light receivers of the optical head is provided. The focus servo control section is provided with a focus offset value that maximizes the amplitude detected by the control section to control the focus position.

An optical disk device is expected to be a large storage capacity device because the track interval can be extremely narrowed to several microns. On the other hand, it is indispensable to focus a spot light for recording / reproducing on an optical disk in an optimum state.

[0004]

2. Description of the Related Art Generally, as shown in FIG. 9A, an optical head 12 moves an optical head 12 in a radial direction of an optical disk 11 with respect to the optical disk 11 rotated about a rotation axis 11b by a rotation mechanism 11a such as a motor. By positioning to the desired track position with a motor (not shown),
Recording / reproducing on / from the optical disk 11 by the optical head 12 is performed.

The optical head 12 squeezes radiation emitted from a semiconductor laser 20, which is a light source, through a lens 21a, a beam splitter 22, a λλ wavelength plate 23, a mirror 24, and an objective lens 25. Optical disk 1
1 and the reflected light from the optical disk 11 is taken into the four-divided photodetector 28 via the objective lens 25, the mirror 24, the λλ reflector 23, the beam splitter 22, and the lens 21b to generate a light receiving signal. It is configured as follows.

In the optical disk device, a large number of tracks or bits are formed in the radial direction of the optical disk 11 at intervals of several microns. Since the focal position shifts, a spot light of 1 micron or less needs to follow these positional shifts.

Therefore, the objective lens 2 of the optical head 12 is
A focus actuator (focus coil) 2 for adjusting the focus position by moving 5 in the vertical direction of FIG.
7 and a track actuator (track coil) 26 for moving the objective lens 25 in the horizontal direction shown in the figure to adjust the irradiation position in the track direction. Corresponding to this, the focus servo control unit 14 that drives the focus actuator 27 in a mode of feeding back the focus actuator 27 by generating the focus error signal FES from the plurality of light receiving signals received by the light receiver 28, and the plurality of light receiving devices 28 receiving light. A track servo control unit 13 is provided which drives the track actuator 26 in a mode in which a track error signal TES is generated from the light reception signal and the track actuator 26 is fed back.

In this case, when the focus servo control unit 14 is used to control the focus position of the spot light in a feedback manner, the focus position changes depending on the focus offset value given to the focus servo control unit 14. By sequentially changing the values as shown in FIG. 9C, the amplitude of the track error signal TES at that time is obtained as shown in FIG. 9B, and the offset value that maximizes this is obtained by the focus servo control unit 14.
Was to be given to. Here, the larger the amplitude of the track error signal TES is, the more stable the servo control is, and in particular, the more stable the servo control can be when the track servo is pulled.

As another method, a focus offset at which DCSUM is maximized is also obtained.
The optimum focus position can be detected. TES / DCSUM
The maximum point is slightly different, but when the light beam has no astigmatism, the difference between them is constant, so that one can estimate the other.

[0010]

However, the optical head 1
Optical disk 11 due to heat during use, aging, etc.
As shown in FIG. 10 (a), when astigmatism is generated in the spot light irradiated with the light beam, the spot light corresponds to (A) shown in FIG. 10 (b) or an intermediate point between (A) and (B). If the focus offset value is given to the focus servo control unit 14, there is a problem that an optimum focus position is not focused. Here, DCSUM represents all signals including a DC component, and RF represents the amplitude of a read signal. That is, the light beam at the DCSUM maximum point (A) ′ in FIG. 10A has aberrations such that it becomes longer in the groove direction. Further, such a light beam which is long in the traveling direction has a large focus servo offset generated during recording, so that the servo is disturbed and the recording state deteriorates.

According to the present invention, a focus offset value at which the amplitude of the sum of the RF signals among the light reception signals received by the optical head is maximized is given to the focus servo control unit, so that even if astigmatism or the like occurs, it is optimal. It is intended to control the focus position to be appropriate.

[0012]

FIG. 1 is a block diagram showing the principle of the present invention. In FIG. 1A, an optical head 2 irradiates an optical disk 1 with spot light and receives reflected light.

The track servo control section 3 performs track following control based on the track error signal.
The focus servo control unit 4 controls the focus position based on the focus error signal and the focus offset value.

The sum signal amplitude detector 5 detects the amplitude of the sum of the RF signals of the plurality of light receiving signals received by the optical head 2. The control unit (MPU) 6 performs various controls.

[0015]

According to the present invention, the sum signal amplitude detecting section 5 operates from the optical head 2 while the control section 6 gives the focus servo control section 4 an offset level OFS that gradually changes as shown in FIG. An offset level at which the control unit 6 that has received the notification of these amplitudes detects the amplitudes (envelopes) that have been generated by detecting and detecting the RF signals that are the sum of the RF signals of the plurality of received light signals that have been detected. OFS
Is given to the focus servo control unit 4 to control the focus position. At this time, the amplitude is detected after differentiating the RF signal as needed.

Therefore, the position of the spot light is determined while the focus servo control section 4 is provided with a focus offset value (offset level) that maximizes the total amplitude of the RF signals of a plurality of received light signals received by the optical head 2. By controlling, even if astigmatism occurs in the spot light due to heat, aging, or the like, it becomes possible to always perform optimum focus position control.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the structure and operation of one embodiment of the present invention will be described in detail with reference to FIGS.

In FIG. 2, a control unit 6 is an MPU (microprocessor), and a focus servo control unit 4
Is controlled using the servo-on signal FSV, the focus zero-cross signal FZC, and the off-focus signal FOS, and the servo control operation of the track servo control unit 3 is performed using the servo-on signal TSV, the track zero-cross signal TZC, and the off-track signal TOS. And controls the movement of the optical head 2 by controlling a motor (not shown).

The memory 6a has a focus offset value Ve.
The memory 6b is a work area for detecting the focus offset value. The memory 6b is designed to store the amplitude values of the RF signals in a plurality of sectors for one offset value.

The FES creating circuit 40 is composed of the four-division light receiver 28.
The focus error signal FES is generated from the light receiving signals SV _a to SV _d of the respective light receivers 28a to 28d by performing the calculation of the following formula (1).

[0021] _{(SV a + SV c) -} (SV b + SV d) ·············· (1) AGC (Automatic Gain Control) circuit 42, all described below focus error signal The AGC is performed by dividing the total signal (total reflection level) DSC from the signal generation circuit by using the total reflection level as a reference value, and correcting the fluctuation of the irradiation beam intensity and the reflectance.

The zero cross detector 43a detects a zero cross point of the focus error signal FES and outputs a focus zero cross signal FZC to the MPU 6.
The off-focus detection circuit 43b determines that the focus error signal FES is equal to or greater than a certain value V _{0 in the} plus direction,
Also, the off-focus signal FOS is output to the MPU 6 by detecting that it has become equal to or less than the constant value −V _{0 in the} minus direction, that is, the off-focus state.

The phase compensating circuit 44 adds a value obtained by differentiating the input focus error signal FES and a proportional component of the focus error signal FES to advance a high-frequency phase.

The servo switch 45 closes the servo loop when the servo-on signal FSV from the MPU 6 is on, and opens the servo loop when the servo-on signal FSV is off. The offset giving circuit 46 adds the focus offset value V _F given from the MPU 6 to the input.

The power amplifier (PA) 47 amplifies the output of the focus offset giving circuit 46 and gives the focus drive current FVD to the focus actuator 27.

The TES generation circuit 30 includes a four-divided photodetector 28
The following equation (2) is calculated from the received light signals SV _a to SV _d from the above to generate the track error signal TES.

[0027] _{(SV a + SV b) -} (SV c + SV d) ·············· (2) the total signal generation circuit 31, a light reception signal SV _a through SV _d added, This is for creating an all-signal DSC having a total reflection level.

The AGC circuit 32 uses the track error signal T
ES is divided by the total signal (total reflection level) DSC from the total signal generation circuit 31, and the AGC using the total reflection level as a reference value
In order to correct the fluctuation of the irradiation beam intensity and the reflectance.

The zero-cross detector 34a detects the zero-cross point of the track error signal TES and outputs the track zero-cross signal TZC to the MPU 6. The off-track detection circuit 34b outputs the track error signal TES
Is less than or equal to a constant value V _{0 in the} plus direction and less than or equal to a constant value −V _{0 in the} minus direction, that is, the off-track state is detected.
The OS is output to the MPU 6.

The phase compensation circuit 37 adds a value obtained by differentiating the input track error signal TES and a proportional component of the track error signal TES to advance the phase in a high frequency band.

The servo switch 38 closes the servo loop when the servo-on signal TSV from the MPU 6 is on, and opens the servo loop when the servo-on signal TSV is off. Inversion amplifier 39a
Is to invert the output of the servo switch 32.

The power amplifier (PA) 39 amplifies the output of the inverting amplifier 39a and supplies the track drive current TVD to the track actuator 26. Next, the configuration and operation of the sum signal amplitude detector 5 in FIG. 3 will be described in detail with reference to FIG.

FIG. 3A shows the RF signal generating circuit 50. This is the sum signal RFI which is obtained by inputting the received light signals SV _a to SV _d to the resistor R _g via the capacitor C as shown in the figure and obtaining the sum of them,
RF2 is created. This RF signal is created as shown in FIG. 4A when data is recorded on the optical disc 1, for example. If you have not recorded any data,
As shown in FIG. 4A, the RF signal is detected only from the formatted ID part.

FIG. 3B shows the RF envelope creating circuit 51. This RF envelope creation circuit 51 is composed of a differentiation circuit and a detection circuit. The differentiation circuit is
Comprised of a capacitor C ₁ and a resistor R ₁ or R, FIG.
(A) Differentiating the RF signal created by the RF signal creation circuit 50. This differentiation facilitates extraction of a pulse signal of a predetermined repetition frequency in the RF signal, for example, a pulse signal of the ID section formatted on the optical disc 1 from the received light signal. The detection circuit is for creating an envelope of the RF signal differentiated by the differentiating circuit and detecting the amplitude of the RF signal. The RF envelope creation circuit 51 creates an envelope signal after the detection shown in FIG. 4B from the RF signal shown in FIG. Here, FIG.
FIG. 4A shows an envelope where data is recorded, and FIG. 4B shows an envelope where data is not recorded and only the ID part is formatted as shown in FIG.

FIG. 3C shows the sector mark detection circuit 5.
3, the shift register 54-1 and the sample holder 52 are shown. The sector mark detection circuit 53 uses the RF signal differentiated by the differentiation circuit of FIG.
The detection is performed as shown in (d). The shift register 54-1 corresponds to the delay 54 in FIG. 2, and generates a sample pulse delayed by a delay T corresponding to a predetermined number of clocks. Due to this delay T, as shown in FIG. 4D, the latter half of the VFO (pulse signal for performing frequency pull-in, which has the same pattern in any sector) detected after the sector mark of the ID portion Generates a sample pulse. As shown in FIG. 4 (c), the sample holder 52 converts the envelope from the detected envelope of FIG. 4 (b) to the VFO portion of the ID section at the on portion of the sample pulse generated by the shift register 54-1. The amplitude is sampled and held. As a result, it is possible to sample and hold the amplitude in the VFO portion having the same pattern in any sector and detect the stable amplitude. The sampled and held amplitude is converted into a digital value by the A / D 55 in FIG.
Will be notified.

[0036] As described above, the light-receiving signal SV _a through SV _d
Of the RF signals are generated and differentiated, and then an envelope is generated. Further, from this envelope, sampling is performed at a VFO portion in which the same pattern appears in any sector of the ID portion, and then the digital signal is obtained by the A / D 55. By notifying the MPU 6 of the amplitude value of the RF signal converted into a value, the amplitude can be stably detected in any sector, and low-speed and inexpensive A / D can be performed without using the high-speed A / D. It is possible to convert the amplitude sampled and held by using it into a digital value. From the amplitude of the RF signal detected in this way, FIG.
The optimum focus offset value is measured according to the order shown in the flowchart. This will be described below.

In FIG. 5, the track servo is turned on. As described later, this is the T when the MPU6 is on.
This means that the SV is notified to the servo switch 38 and the track servo system is closed to operate the track servo.

Is set to OFSET = 0. This means that the focus offset value is “0” as an initial value. Detects the falling of the home position signal. This means that the falling of the home position signal, which is detected by one pulse each time the optical disc 1 makes one rotation, is detected as shown in FIG. 6 ２１21 (this indicates that there is 21 in ○, the same applies hereinafter). It means to do. In response to this detection, the MPU 6 sets the focus offset value V _F = 0 in the register 461 of the offset applying circuit 46 as described later,
A focus offset value of “0” is given via the D / A 462.

Is A = N. This means that the value calculated by the following equation (3) is substituted for the variable A. N = [the number of sectors in one track]-[the number of sectors required for kickback]-[the number of sectors to catch the falling edge of the index] (3) For example, the number of sectors in one track 17, when the number of sectors required for kickback is 1 and the number of sectors for catching the fall of the home position signal is 2, N = 14.

Determines whether or not sector pulse = 1. If yes, perform. If NO, the process waits.
Determines whether sector pulse = 0. If yes, perform. If NO, the process waits. Thus, one sector pulse is detected.

Is the SEDRF value (RF
The digital value of the amplitude value of the signal is read and stored in the memory 6b. This is the amplitude value of the RF signal read from the A / D 55 (22 RF corresponding to 23 sample pulse in FIG. 6).
Signal amplitude value) as shown in the memory 6b of FIG. 1 and is stored in association with the number of sectors.

A = A-1. This means that the value of the variable A is "-1". Is the variable A =
It is determined whether it is 0 or not. If YES, ◯ 10 is performed. In the case of NO, the following is repeated.

By repeating the above, 1 of the optical disc 1
The sample pulses shown in FIG. 6 (23) are transmitted N times with respect to the rotation, and the amplitude value of the RF signal sampled when these sample pulses are turned on is stored in the memory 6b.

In the case of ○ 10, one track kickback is performed.
This means returning one track and irradiating the original track with spot light. This is unnecessary if the track is not spiral.

In the case of (11), OFSET = OFSET + 1. This means that the offset level OFS is increased by one as shown in FIG. ○ 12
Determines whether OFSET = maximum + 1. In the case of YES, step 13 is performed. If no, do the following:

In the case of ◯ 13, OFSET having the maximum SEDRF (amplitude value of RF signal) is obtained from the memory. This means that, for example, a total sum of N SEDRFs stored in the memory 6b in FIG.
This means that the largest offset value is determined as the largest OFSET.

At 14, the track servo is turned off.
As described above, with the same offset level OFS set for one rotation of the disk, the amplitude values of the N RF signals are obtained and sequentially stored in the memory 6b. By obtaining the offset level OFS having the largest sum of the above, it is possible to set the optimum focus offset value even if astigmatism or the like occurs in the optical head 2.

FIG. 6 is a view for explaining focus offset measurement. In FIG. 6, the ∘21 home position signal is a signal transmitted by one pulse each time the optical disk 1 makes one rotation.

[0049] ○ 22RF signals, as described with reference to FIGS. 3 and 4, the sum signal of the four RF signals of the light receiving signal SV _a through SV _d (read signal). The 23 sample pulse is the sample pulse of FIG.

The 24 offset level OFS is provided to the offset providing circuit 46 in FIG. Next, the configuration and operation of the focus servo control unit 4 will be described with reference to FIG.

[0051] In FIG. 7, under the state where MPU6 outside illustrated gave focus offset value V _F to be set in the register 461 focus offset amplitude of the sum of the RF signal is measured in advance the maximum of the light receiving signal , the servo loop closed by entering FSV oN the servo switch 45, to four without receiving signal SV _a detected by the four-division light receiver 2 not shown of the optical head will be described later from SV _d equation (4) Focus error signal F represented by
The servo control is performed so that the focus position follows the predetermined position on the optical disc 1 by creating the ES and performing the servo control in a feedback mode. Hereinafter, the configuration will be described in detail.

In the figure, the FES creation circuit 40 receives the light receiving signal S
V_aAnd SV_cAnd input resistance r ₁, R₃Gay through
G₁And the light receiving signal SV_bWhen
SV _dAnd input resistance r₂, R_FourVia the gain G₂
The output of the summing amplifier 401 and the summing amplifier 400
Force G₁・ (SV_a+ SV_c) To the output of summing amplifier 401
G₂・ (SV_b+ SV_d) And subtracting amplifier 402
And is represented by the following equation (4) from the summing amplifier 402.
The focus error signal FES is output.

FES = {G ₁ · (SV _a + SV _c ) −G ₂ · (SV _b + SV _d )} (4) The AGC circuit 42 receives the focus error signal FES from the first operational amplifier 420. And the first operational amplifier 4
A first FET (field effect transistor) for controlling the voltage division of the input side of the first operational amplifier 420 according to the output of 20.
421 and the total reflection level signal DCS
A second operational amplifier 422 that controls the input / output terminal 421, and a second FET 4 that controls the voltage dividing of the input side of the second operational amplifier 422.
23, the first FET 421 is controlled by the total reflection level signal DCS output from the operational amplifier 422, the gain of the operational amplifier 420 is controlled, and the focus error signal F
To get ES.

The second FET 423 is connected to the second FET 42
This is for compensating for the non-linear characteristic of No. 1 and providing a linear characteristic. The zero-cross detector 43a is an AGC circuit 4
2 and a comparator 430 for comparing the focus error signal FES output from the second and the zero cross potential,
The comparator 430 outputs the zero-cross signal FZC.

The off-focus detection circuit 43b has an AGC
The focus error signal FES output from the circuit 42 is compared with a fixed value V _0, and when FES> V ₀ , a first comparator 431 that sends out a “high” output, the focus error signal FES and a fixed value (−V ₀ ),
When FES <−V ₀ , the second high-level output is sent out.
And a comparator 432. The sum of both comparators 431 and 432 is the off-focus signal FOS.
Output as

The phase compensating circuit 44 includes a differential circuit including a resistor r _g and a capacitor C _{g provided} to the operational amplifier 440, and a resistor R
A circuit that is connected to a proportional circuit of _g and outputs a signal of a sum of a derivative of the focus error signal FES of the AGC circuit 42 and a proportional component, that is, a signal FCS obtained by adding a phase advance element to the focus error signal FES to the servo switch 45. Is.

The focus offset supply circuit 46, a summing amplifier 460, a register 461 which holds the focus offset level V _F from MPU 6, D for converting the contents held in the register 461 into an analog signal
/ A462, the output F of the servo switch 45
The offset level from the D / A 462 is added to CS and output to the power amplifier 47.

The operation of the track servo controller 3 will be described with reference to FIG. In FIG. 8, a TSV (not shown) in which the MPU 6 is turned on is input to the servo switch 38 to close the servo loop, and four light reception signals SV _a to SV detected by the four-division light receiver in the optical head 2 (not shown). _The track error signal T expressed by _d from _d described below by equation (5)
By creating an ES and performing servo control in such a manner that the ES is fed back, servo control is performed so as to follow a track on the optical disc 1. Hereinafter, the configuration will be described in detail.

In the figure, the TES generation circuit 30 receives the light receiving signal S
Summing amplifier 300 for summing V _a and SV _b via input resistors r ₁ and r ₂ , respectively, and received light signals SV _c and SV _d
, Via the input resistances r ₃ and r ₄ , respectively, and the output G ₁ · (SV _a + S
V _b ) to the output G ₂ · (SV _c + S) of the summing amplifier 301
V _c ), and outputs a track error signal TES represented by the following equation (5).

TES = {G₁・ (SV_a+ SV_b) -G₂・ (SV_c+ SV_d)｝ ... (5) All the signal generation circuits 31_aOr SV_dTo
Input resistance r_FiveOr r₈Adder amplifier 31 for adding via
The total reflection level DCS (= SV _a+ SV_b
+ SV_c+ SV_d) Is output.

AGC circuit 32, zero cross detection circuit 34
a, the off-track detection circuit 34b is the same as 42, 43 in FIG.
The description is omitted because it is the same as a and 43b.

[0062]

As described above, according to the present invention,
Since the focus offset value that maximizes the amplitude of the RF signal of the plurality of received light signals received by the optical head 2 is detected and provided to the focus servo control unit 4 to servo-control the focus position, the heat Even if astigmatism occurs in the spot light due to aging, the optimum focus position control can always be performed. Further, after the amplitude is sequentially held at a portion where the same pattern of the ID portion of the sector of the optical disc 1 is detected (for example, a VFO portion) and converted into a digital value and collected, the focus offset value of the maximum amplitude value is measured. Therefore, a stable amplitude can be measured, and it can be converted to a digital value using a low-speed and inexpensive A / D. Further, the sum of the RF signals uses the differential signal to detect the amplitude value of the RF signal, and the differential circuit (see FIG. 3B).
See) to obtain a high-pass filter effect,
Since there is no influence of the output fluctuation due to the DC component, it is possible to always stably obtain the optimum focus offset value. Further, the amplitude value is obtained in the VFO portion having the same pattern in all the sectors, and the amplitude value can be obtained under the same condition for each offset value, and the optimum focus offset value is always stabilized. It becomes possible to obtain it.

[Brief description of drawings]

FIG. 1 is a principle configuration diagram of the present invention.

FIG. 2 is a configuration diagram of an embodiment of the present invention.

FIG. 3 is a configuration diagram of a sum signal amplitude detection unit.

FIG. 4 is an operation waveform diagram of a sum signal amplitude detection unit.

FIG. 5 is a focus offset measurement flowchart.

FIG. 6 is an explanatory diagram of focus offset measurement.

FIG. 7 is a configuration diagram of a focus servo control unit.

FIG. 8 is a configuration diagram of a track servo control unit.

FIG. 9 is an explanatory diagram of a conventional technique.

FIG. 10 is an explanatory diagram of a conventional technique.

[Explanation of symbols]

 1: Optical disc 2: Optical head 3: Track servo control unit 4: Focus servo control unit 5: Sum signal amplitude detection unit 6: Control unit (MPU)

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shigeru Arai 1015 Uedanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (72) Inventor Toshitaka Iwamoto 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Fujitsu Limited ( 72) Inventor Hidenori Saito 1015 Uedanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited

Claims (1)

[Claims] An optical head (2) for irradiating an optical disk (1) with spot light and receiving reflected light using a plurality of light receivers, and a plurality of optical heads received by the optical head (2). A focus servo control unit (4) for controlling the focus position of the spot light based on the focus error signal obtained from the light receiving signal and the given focus offset value; and a focus offset value given to the focus servo control unit (4). An optical disc apparatus comprising: a focus offset adjusting unit (5, 6) for adjusting; and the focus offset adjusting unit (5, 6) sequentially changes a focus offset value during a focus offset value adjusting operation to adjust the focus. Means for providing to the servo control unit (4), and for each of the changed focus offset values When irradiating a VFO pattern in an ID portion on the optical disk (1), a DC component is removed from an RF signal of a received light signal received by a plurality of light receivers of the optical head (2) to remove the DC signal. A sum signal amplitude detecting unit (5) for detecting a sum amplitude of the RF signal from the differential signal, including a differentiating circuit for outputting a sum differential signal; and an amplitude detected by the sum signal amplitude detecting unit (5). Means for obtaining a focus offset value for which the value becomes large, and setting the focus offset value as a focus offset value during a focus servo control operation of the focus servo control unit (4). apparatus.