JP2711474B2 - Focus adjustment device for optical pickup device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a focus adjustment device for an optical pickup device,
In particular, the light reflected on the recording medium is radiated through an optical demultiplexer onto two systems of photodetectors arranged in front and behind the focal point, and the focus is determined by the output deviation of each photodetector. The present invention relates to an improvement of a focus adjustment device that performs position control of an adjustment mechanism.

[Prior art]

2. Description of the Related Art Various types of focus adjustment devices for an optical pickup device have been known. In particular, light reflected on a recording medium is converged to two focal points via an optical demultiplexer, and the two systems are adjusted. FIGS. 1 and 2 show the basic principle of a focus adjustment device which is a premise of the present invention, in which the position control of a focus adjustment mechanism is performed based on output deviations of two systems of photodetectors disposed in front and behind a focal point. This will be described with reference to FIG. In this specification, the tracking direction is treated as the X axis, the disk rotation direction as the Y axis, and the focusing direction as the Z axis. Light emitted from the semiconductor laser 1 is converted into a parallel light beam by a collimating lens 2 and is incident on an optical block 3 including beam shaping prisms 3a and 3b and a beam splitter (a boundary surface G between the glasses 3b and 3c). Since the optical cross section of the laser beam shaped into a parallel light beam by the collimating lens 2 generally has an elliptical intensity distribution due to the properties of the laser beam, the perfect circular correction is performed by the beam shaping prism 3a. That is, assuming that the major axis of the ellipse is perpendicular to the plane of the paper, the laser beam is made incident on the surface E of the beam shaping prism 3a made of, for example, material BK7 (BSC7) at an incident angle θ, and the refraction angle When refracted by φ, the beam system becomes CO in the short axis direction.
It expands by Sφ / COSθ and becomes almost circular. Further, the wavelength λ of the laser light generally has a property that slightly changes depending on the temperature and the light emission intensity, and the refraction angle φ also changes depending on the temperature and the light emission intensity. Therefore, a prism 3b made of glass such as SF11 (FD11), which has a wavelength dependence and a temperature dependence with respect to the refractive index, is provided, and the laser beam is made incident on the boundary surface F between the glass 3a and 3b at an incident angle α. The wavelength dependence and the temperature dependence of the laser beam are compensated by refraction at the refraction angle β. The boundary surface G between the glass 3b and the glass 3c forms a phaseless beam splitter, and the P-polarized light in the laser beam is 70 to 80%.
Is transmitted through the interface G, and most of the S-polarized light is
Reflected by A 45 degree inclined surface H is formed on the bottom surface of the glass 3c, and is coated. The P-polarized light transmitted through the boundary surface G is incident on the inclined surface H at an incident angle of 45 degrees, is totally reflected without phase, and its light beam is emitted from the optical block 3 in the Z-axis direction. The light emitted from the optical block 3 is condensed by an objective lens 4 on a disc (not shown), and the light reflected by the disc is again converted into a parallel light beam by the objective lens 4 and is incident on the glass 3c. Incident on the boundary surface G. Now, in the case of the magneto-optical disk system, when the beam spot is reflected on the disk recording surface, its polarization plane is rotated by a Kerr angle due to perpendicular magnetization forming information pits. Therefore, when the reflected light beam enters the boundary surface G, 20-30% of the P-polarized light and the S-polarized light are reflected, and this reflected light beam enters the half-wave plate 5 and is transmitted to the beam splitter 7 described later. ,
The plane of polarization is tilted 45 degrees. The light emitted from the half-wave plate 5 passes through the condenser lens 6 and is incident on the beam splitter 7, where the light is split into a P component and an S component by the beam splitter 7, and is focused on the focal points F and F '. Detectors 8 and 9 are respectively disposed in front of the focal point F and behind the focal point F ', and light receiving surfaces A and B having the same diameter are formed near the optical axis of each of the detectors 8 and 9. When the spot diameter of the light beam S incident on each of the detectors 8 and 9 is φ _SA and φ _SB , each spot diameter φ _SA and φ _SB exceeds the diameter of the light receiving surfaces A and B with a margin during focusing. Preferably, the light receiving amounts of the respective light receiving surfaces A and B are respectively set to a
And b, the separation distances of the detectors 8 and 9 from the focal points F and F 'are set so that the received light amounts a and b are substantially equal. Now, in such a mechanism, when the disk surface is closer to the objective lens 4 than when focused, the reflected light converges farther than the focal planes F and F '. Therefore,
Defining with each phi _SA and phi _SB the spot diameter of the light beam incident on each detector 8.9, to each of the spot diameter is established the relationship φ _SA> φ _SB. This means that the light incident on the light receiving surface A is more diffused than the light incident on the light receiving surface B.
<B is satisfied. On the other hand, when the disk surface is farther from the objective lens 4 than when focused, the reflected light converges before the focal planes F and F '. Therefore, each detector 8, 9
The relation of φ _SA > φ _SB is established for the spot diameter of the light beam incident on the light receiving surface B, and the light incident on the light receiving surface B is diffused more than the light incident on the light receiving surface A. The relationship of a> b is established for the light quantity. Therefore, as shown in FIG. 6, the outputs of the light receiving surfaces A and B are applied to the differential amplifier 12, and the deviation ab of the detected light amount of each system is obtained.
If the objective lens 4 is driven so that converges to 0, the focus can be adjusted. Reference numeral 13 denotes a driver including a compensation circuit, and reference numeral 14a denotes a focus coil.

[Problems to be solved by the invention]

As described above, in a focus adjustment device in which the reflected light from the recording medium is distributed to two systems of detectors by a beam splitter and the servo system is operated by a deviation of the detected light amount, the light amount detected by the two systems of detectors Is a prerequisite for normal operation. Therefore, in general, the position of the half-wave plate 5 is adjusted so that the amounts of light received by the two detectors 8 and 9 are balanced. However, the position adjustment of the half-wave plate 5 requires extremely precise adjustment. Problem. Also, even if the position of the half-wave plate 5 is precisely adjusted, the detection light quantity imbalance in the two detectors caused by the influence of crosstalk between tracks and birefringence on the disk plane, etc. Cannot be handled by the conventional method, and there was no guarantee that the focusing accuracy could always be ensured.

[Means for solving the problems]

The present invention has been made in view of such a problem, and an object of the present invention is to provide a focus adjustment device capable of performing focus adjustment without being affected by an imbalance in detection light amounts between two detectors. In summary, the focus adjusting device of the optical pickup device according to the present invention comprises: a light source which is focused on a recording medium by an objective lens whose focus can be adjusted; The light is converged to two focal points via a duplexer, and two optical detectors are disposed in front of and behind the two focal points, respectively, and the objective lens is determined by an output deviation of the two optical detectors. And a focus adjusting device of an optical pickup device that servo-controls the light beam; each of the two photodetectors is arranged so that a light beam spot radiated by the condenser lens around its optical axis is A central light receiving surface having a smaller area and a peripheral light receiving surface surrounding the central light receiving surface and having an area including the central light receiving surface larger than the light beam spot; Heart of vessel Value obtained by multiplying the difference between the total received light amount of the light receiving surface and the peripheral light receiving surface and the total received light amount of the center light receiving surface of the other type of photodetector and the peripheral light receiving surface by the component ratio of the central light receiving amount to the total light receiving amount at the focal point Is provided as a correction value.
A value obtained by subtracting the calculated correction value from the center light receiving amount deviation of the photodetectors of the respective systems is used as a focus error signal, and the objective lens is servo-controlled so that the focus error signal converges to zero. It has been done. Preferably, the focus adjustment device of the optical pickup device of the present invention is based on the above-mentioned condition; the two systems of photodetectors are arranged equidistantly in front of and behind the focal points of each system, and The photodetector has a central light receiving surface and a peripheral light receiving surface having the same shape.

[Action]

In the present invention as well, the reflected light is split into two by an optical demultiplexer and projected onto photodetectors disposed before and after the focal point. The total amount of received light, which is the sum of the amount of light received at the center light-receiving surface and the amount of light received at the peripheral light-receiving surface of the photodetector of each system, is not affected by focus fluctuation. Changes according to the degree of Therefore, in the present invention, the objective lens is basically driven by the deviation of the center light reception amount of the photodetectors of each system. However, the center light reception amount has an imbalance in the light reception amount between the photodetectors of each system. In this case, an imbalance component of the amount of received light is included. As described above, the total received light amount obtained by adding the received light amount of the central light receiving surface and the received light amount of the peripheral light receiving surface of the photodetector of each system is not affected by the focus fluctuation, but the effect of the imbalance of the received light amount is Therefore, if the difference between the total light receiving amounts of the photodetectors in each system is calculated, the calculated difference indicates the imbalance in the total light receiving amount of the photodetectors in each system. Also, at the focal point, the spot diameter of the light beam projected on each system of photodetectors shows a constant value, so that the component ratio of the central light receiving amount to the total light receiving amount also shows a constant numerical value. If a value obtained by multiplying the imbalance of the total amount of received light of the photodetector in each system calculated above by the constant component ratio is set as a correction value, this correction value is applied to the central light receiving surface during the imbalance of the amount of received light. The corresponding components will be shown. Then, the value obtained by subtracting the correction value calculated as described above from the deviation of the center light receiving amount of the photodetectors of each system is used as the focus error signal. Will be eliminated, and if the objective lens is servo-controlled so that this focus error signal converges to 0, the objective lens will always track the focal point regardless of the imbalance in the amount of light of the two photodetectors. become.

【Example】

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is also used in the description of the present embodiment because the structure of the device is basically the same as that of the above-described conventional example in the present invention, and the overall configuration of the optical pickup device according to the present embodiment is further described. As shown in the perspective view of FIG.
Details of the actuator for driving the actuator are shown in the plan view of FIG. 3A and the front view of FIG. 3B. In FIG. 2, the same reference numerals as those in FIG. 1 denote the same elements as those described in FIG. 1, and a duplicate description will be omitted. Elements not described so far will be described in detail. Reference numeral 15 denotes an actuator main body to which the objective lens 4 is fixed, and connection legs 16 and 17 having elasticity are connected to one leg 15a of the actuator main body 15 by hinge portions 16a and 17a, respectively. The other ends of the connection plates 16 and 17 are connected to a mounting member 18 fixed to an optical seek mechanism (not shown) by hinge portions 16b and 17b so that the connection plates 16 and 17 maintain a parallel state. . Although the connection plates 16 and 17 themselves have elasticity, these outer edges are formed with bending portions 16c and 17c for reinforcement, respectively, so that a force for moving the actuator body 15 in the Z-axis direction is provided. , The connection plates 16 and 17 are not substantially bent, and the four hinge portions 16a and 17a
The actuator body 15 is moved in the Z-axis direction without tilting the optical axis of the objective lens 4 due to bending of 16b and 17b. A focus coil 14a is fixed to the other leg 15b of the actuator body 15, and a U-shaped yoke 19 fixed to the optical seek mechanism is inserted into a winding shaft of the focus coil 14a. Has a magnet 20 fixed thereto. Therefore, when the focus coil 14a is energized, a force is generated that moves the actuator body 15 in the Z-axis direction. Reference numeral 14b denotes a tracking coil. When a current is applied to the tracking coil 14b, a force is generated in a direction in which the actuator main body 15 turns around the hinge portions 16b and 17b (that is, a force for moving the actuator main body 15 in the X-axis direction). However, since the present invention relates to focus control,
Tracking coil 14b is not directly relevant to the present invention. Next, as a feature of the present invention, as shown in FIG. 1, the size around the central light receiving surfaces A and B of the respective detectors 8 and 9 is sufficient to cover the light beam spot S in any state. Peripheral light receiving surfaces A 'and B' are provided. Next, FIG. 4 shows an example of a driving circuit which is a feature of the present invention. Reference numeral 21 denotes a subtraction circuit for calculating a deviation between the output a of the central light receiving surface A of the detector 8 and the output b of the central light receiving surface B of the detector 9, and the output (ab) of the subtraction circuit 21 is used for each system. The output deviation of the central light receiving surfaces A and B of the detectors 8 and 9 is shown. The output (ab) of the subtraction circuit 21 is applied to the in-phase input of the subtraction circuit 22. 23 is an addition circuit for adding the output a of the central light receiving surface A of the detector 8 and the output a 'of the peripheral light receiving surface A'.
The output (a + a ′) of 23 indicates the total amount of light received by the detector 8. The output (a + a ') of the adding circuit 23 is applied to the positive phase input of the subtracting circuit 24. Similarly, reference numeral 25 denotes an addition circuit for adding the output b of the central light receiving surface B of the detector 9 and the output b 'of the peripheral light receiving surface B'.
The output (b + b ') of the addition circuit 25 indicates the total amount of light received by the detector 9. The output (b + b ') of the adding circuit 25 is
Applied to the negative phase input of Since the subtraction circuit 24 subtracts the negative-phase input from the normal-phase input, the output (a + a ')-(b + b') is obtained when the total received light amount of the detector 8 and the total received light amount of the detector 9 are unbalanced. 8 indicates an imbalance between the total received light amount of the detector 8 and the total received light amount of the detector 9. The output of the subtraction circuit 24 is multiplied by a predetermined constant K by a voltage divider 26 and applied to the negative phase input of the subtraction circuit 22. Here, the constant K of the voltage divider 26 indicates the component ratio (a / a + a ') of the center received light amount a to the total received light amount (a + a') of the detector 8 at the time of focusing. In this embodiment, since the detector 8 and the detector 9 have the same shape, the constant K is equal to the total amount of received light (b +
Needless to say, the component ratio (b / b + b ') of the center light reception amount b to b') is also shown. Then, as described above, the output of the subtraction circuit 24 is
The negative phase input of the subtraction circuit 22 is applied to the central light receiving surface A during the imbalance of the total received light amount of the detectors 8 and 9 during focusing.
And components corresponding to B. The non-inverting input of the subtraction circuit 22 includes the output (a−) of the subtraction circuit 21 indicating the output deviation of the central light receiving surfaces A and B of the detectors 8 and 9.
Since b) is added, the focus error signal FES, which is the output of the subtraction circuit 22, is based on the output deviation of the central light receiving surfaces A and B of the two detectors 8 and 9 and the total light reception of the detectors 8 and 9 during focusing. A value obtained by removing a component corresponding to the center light receiving surface during an imbalance in the amount is shown. The focus error signal FES output from the subtraction circuit 22 is applied to the driver 13 to control the focus coil 14a. Incidentally, as is apparent from the above description, the focus error signal FES is expressed by the following (Equation-1). FES = (ab) −K {(a + a ′) − (b + b ′)}
(Equation-1) Next, the operation of the present embodiment will be described with reference to the above matters and FIGS. 5 (A) to 5 (C). FIG. 5A shows a case where the disk surface 30 is in focus with respect to the objective lens 4, and FIG. The fifth
FIG. 4C shows a case where the disk surface 30 is closer to the objective lens 4 than at the time of focusing. First, a case where the amounts of incident light on the two detectors 8 and 9 are balanced will be described. When the amounts of incident light on the two detectors 8 and 9 are balanced, the total amount of received light (a
+ A ') and (b + b') have the same value.
(A + a ')-(b + b') in 1) becomes 0.
That is, the output of the subtraction circuit 21 in FIG.
(Equation-1) can be rewritten as (Equation-2) because the output is 22. FES = ab (Equation-2) First, when the disk surface 30 is focused on the objective lens 4, the light split by the beam splitter 7 converges at the focal points F and F '. The spot diameter φ _{SA of} the light beam S incident on the detector 8 and the light beam S incident on the detector 8
If the spot diameters φ _SB are equal and show the same degree of diffusion, and the amounts of incident light on the two systems of detectors 8 and 9 are balanced, the amount of received light per unit area of the detectors 8 and 9 is also equal. The values of a and b in the above (Equation-2) are also equal. Therefore, the focus error signal FE in (Equation 2)
Since S becomes 0, the servo system acts so as to maintain the current state. Next, when the disk surface 30 is farther from the objective lens 4 than when focused, the light split by the beam splitter 7 converges before the focal points F and F '. As shown in B), the relationship of φ _SA <φ _SB is established between the spot diameter φ _{SA of the} light beam projected on the detector 8 and the spot diameter φ _SB of the light beam projected on the detector 9. Since the incident light beam is more diffused than the incident light beam on the light receiving surface A, the relationship of a> b is established between a and b in the above (Equation-2). Therefore, (Equation-2) can be transformed into (Equation-3), and the driver 13 supplies a positive current to the focus coil 14a. FES = ab> 0 (Equation-3) where; (a> b) When a positive current is supplied to the focus coil 14a, the actuator body 15 is moved upward by an electromagnetic force acting between the focus coil 14a and the yoke 19. And the objective lens 4 approaches the disk surface 30. When the disk surface 30 is closer to the objective lens 4 than at the time of focusing, the light split by the beam splitter 7 converges deeper than the focal points F and F '. As shown in), the relationship φ _SA > φ _SB is established between the spot diameter φ _SA of the light beam projected on the detector 8 and the spot diameter φ _SB of the light beam projected on the detector 8, and the incident light on the light receiving surface A Since the light beam is more diffused than the light beam incident on the light receiving surface B, a between a and b in the above (Equation-1) is a
<B is satisfied. Therefore, (Equation-2) can be transformed into (Equation-4), and the driver 13 supplies a negative current to the focus coil 14a. FES = ab <0 (Equation-4); (a <b) The actuator body 15 is moved downward by an electromagnetic force acting between the focus coil 14a and the yoke 19 when a negative current is supplied to the focus coil 14a. To move the objective lens 4 away from the disk surface 30. Next, a case where the amounts of incident light on the two detectors 8 and 9 are imbalanced will be described. When the total amount of received light (a + a ′) and (b + b ′) of the two detectors 8 and 9 has a relationship of R ₁ : R ₂ (where R ₁ + R ₂ = 2 and R ₁ ≠ R ₂ ), the focal point is Similarly, the relationship of R ₁ : R ₂ is established between the light receiving amounts a and b of the central light receiving surfaces of the respective detectors 8 and 9. The output (a + a ')-(b + b') of the subtraction circuit 24, which is the content of the curly brackets in (Equation-1), indicates an imbalance between the total light receiving amount of the detector 8 and the total light receiving amount of the detector 9. This is as described above. The output of the subtraction circuit 24 is multiplied by a predetermined constant K by a voltage divider 26 and given to the negative-phase input to the subtraction circuit 22. The constant K multiplied by the voltage divider 26 is the output of the detector 8 at the focal point. The component ratio (a / a + a ') of the center received light amount a to the total received light amount (a + a') and the component ratio (b / b + b ') of the center received light amount b to the total received light amount (b + b') of the detector 9 at the focal point are shown. The component corresponding to the imbalance of the center received light amount at the focal point among the light amount imbalances occurring in the total received light amounts of the detectors 8 and 9 is given to the negative phase input of the subtraction circuit 22. Become. On the other hand, the detectors 8.9
The output of the subtraction circuit 21 indicating the deviation (ab) of the center light receiving amount of the two detectors is added as described above.
When an imbalance occurs in the total amount of received light of No. 9, the output (ab) of the subtraction circuit 21 naturally also includes a light amount imbalance component. However, as described above, in this embodiment, the focus error signal FES is created by subtracting the output of the voltage divider 26 from the output of the subtraction circuit 21 by the subtraction circuit 22, and the output of the voltage divider 26 is output from each of the detectors 8 and 9. Since the component corresponding to the imbalance of the center received light amount at the focal point out of the light amount imbalance occurring in the total received light amount is shown, the two systems of detectors 8.9 from the focus error signal FES near the focal point are shown.
Will be removed. Of course, the constant K set by the voltage divider 26 is the component ratio of the center received light amount to the total received light amount at the focal point (a / a +
a '), the amount of correction of the light quantity imbalance component becomes excessive or deficient as the distance from the focal point increases, but the light quantity imbalance component converges to 0 near the focal point as described above. The objective lens 5 follows the in-focus position irrespective of the light amount imbalance in the detectors of the system. Next, it will be explained that the focus error signal FES converges to 0 at the focal point algebraically. When the relationship of R ₁ : R ₂ is established as described above between the total received light amounts (a + a ′) and (b + b ′) of the two detectors 8 and 9, the total received light amount (b + b ′) of the detector 9 becomes ( Expression-
5), and at the focal point, the center received light amount b of the detector 9 is expressed by (Equation-6). By substituting this into (Equation-1), (Equation-1) can be developed into the following (Equation-7), and the focus error signal is finally obtained regardless of the light quantity imbalance between the two detectors 8 and 9. Converges to 0, so the entire servo system also has detectors 8.9
The objective lens 4 operates so as to follow the in-focus state irrespective of the light amount imbalance between the two. As is apparent from the above description, the present invention basically drives the objective lens by servo so that the center light receiving amount deviation of the two detectors converges to zero, and generates the total light receiving amount of each detector. By subtracting the component corresponding to the center light reception amount at the focal point during the light amount imbalance from the above center light amount deviation, the influence of the light amount imbalance at the focal point can be removed. Therefore, in the above description, two detectors 8 and 9 are used to simplify the description and the circuit configuration.
Are described as having the same shape. However, even when the same detector cannot be used due to the arrangement space or the like, the gains of various input signals are appropriately weighted on the circuit, and the focus of each detector is adjusted. It is also possible to use a detector having a different shape by appropriately adjusting the separation distance from the position and the like. Further, since the present invention relates to focus adjustment, an example in which a peripheral light receiving surface is simply provided on the outer periphery of the center light receiving surface for simplification of description and circuit configuration has been described, but these detectors are used for focusing control. In addition, it is generally used for tracking control or the like. In such a case, the center light receiving surface or the peripheral light receiving surface is generally divided into two right and left sides, and tracking control is performed with a left / right light amount ratio. When the center light receiving surface and the peripheral light receiving surface are divided into two in this way, the present invention can be applied as it is by adding an adding circuit for adding the left and right light receiving amounts to the stage preceding the circuit shown in FIG. can do.

【effect】

As described above, according to the present invention, the imbalance of the incident light amount is removed from the focus error signal at the in-focus point even if the incident light amount is imbalanced with respect to the two detectors arranged before and after the focal position. As a result, the objective lens can stably track the in-focus position regardless of the imbalance in the amount of incident light. Therefore, according to the present invention, the requirement for the adjustment accuracy of the half-wave plate for adjusting the amount of light distributed to the two detectors is reduced, and the adjustment is facilitated. In addition, since the crosstalk can be considered as an imbalance of the incident light amount resulting from the groove, a focus adjustment error due to the crosstalk can be prevented.

[Brief description of the drawings]

FIG. 1 is a view for explaining an optical path of an optical pickup device according to an embodiment of the present invention, FIG. 2 is a perspective view of the optical pickup device, and FIG. 3 (A) is an actuator of the optical pickup device shown in FIG. FIG. 3B is a front view of an actuator portion of the optical pickup device shown in FIG. 1, FIG. 4 is a circuit diagram of a focusing servo system of the optical pickup device according to the embodiment of the present invention, and FIG. FIG. 5 (A) shows the detector 8 when the disc is at the in-focus position of the objective lens.
FIG. 5B is an explanatory view of a projection pattern for the detectors 8 and 9 when the disc is at a farther distance than the focus position of the objective lens, and FIG.
FIG. 6C is an explanatory diagram of a projection pattern on the detectors 8 and 9 when the disk is at a shorter distance than the focus position of the objective lens. FIG. 6 is a circuit diagram of a focusing servo system of the conventional optical pickup device. DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser, 2 ... Collimating lens 3 ... Optical block, G ... Beam splitter 4 ... Objective lens, 5 ... 1/2 wavelength plate 6 ... Condensing lens, 7 ... Beam splitter 8. 9: Detector, AB: Central light receiving surface A ', B': Peripheral light receiving surface 14a: Focus coil 15: Actuator body 19: Yoke, 20: Magnet 21, 22, 24 ... Subtraction Circuits 23 and 25: Addition circuit, 26: Voltage divider 30: Disk

Continuation of front page (72) Inventor Masaaki Takagi 16-20, Shimura 2 Itabashi-ku, Tokyo Copal Inc. (56) References JP-A-61-156536 (JP, A) JP-A-60-93044 (JP, A) JP-A-59-56234 (JP, A)

Claims (2)

(57) [Claims] 1. An optical system according to claim 1, wherein said light is focused on a recording medium by a focus-adjustable objective lens and reflected by said recording medium to two focal points via said objective lens, condenser lens and optical demultiplexer. An optical pickup device which converges, and two optical detectors are respectively disposed in front of and behind the two focal points, and the objective lens is servo-controlled by an output deviation of the two optical detectors. In the focus adjusting device, each of the two photodetectors has a center light receiving surface having an area smaller than a light beam spot irradiated by the condensing lens around the optical axis thereof, and a photodetector around the center light receiving surface. And a peripheral light receiving surface whose area including the central light receiving surface is larger than the light beam spot, and the total light receiving amount of the central light receiving surface and the peripheral light receiving surface of the photodetector of one system and the other system. Photodetector core Means for calculating, as a correction value, a value obtained by multiplying a difference between a total light receiving amount of the light receiving surface and a peripheral light receiving surface by a component ratio of the central light receiving amount to the total light receiving amount at the focal point; A value obtained by subtracting the calculated correction value from the received light amount deviation is used as a focus error signal, and the objective lens is servo-controlled so that the focus error signal converges to zero. Focus adjustment device. 2. A focus adjusting device for an optical pickup device according to claim 1, wherein said two systems of photodetectors are arranged equidistantly in front of and behind a focal point of each system. A focus adjusting device for an optical pickup device, wherein each of the photodetectors has a central light receiving surface and a peripheral light receiving surface having the same shape.