GB2257248A - Optical disk tracking system including tangential component correction - Google Patents

Description

22 3 724i - 1 ERROR DETECTOR FOR OPTICAL DISK APPARATUS The present

invention relates to an error detector for an optical disk apparatus which produces a tracking error signal, a focus error signal, etc. depending on outputs from a light receiving portion divided into plural zones, and more particularly to an error detector by which, when there occurs a position shift between a spot of light returned from a disk and the light receiving portion, such a position shift can be corrected.

Fig. 16 shows the optical system of a known opto-magnetic disk apparatus.

The sys tem shown i n Fi g. 16 i s o f the so-cal 1 ed separated optical type comprising a fixed optical system 1 and a movable optical system 2. The movable optical system 2 i s moun ted on an op t i ca 1 head and dr i ven by a 1 i near mo to r indicated by reference character M along the recording surface of an opto-magnetic disk in the radial direction thereof. The movable optical system 2 comprises an object lens 21 facing the recording surface of the disk, and a total reflection prism 22 positioned just below the object lens 21 on its optical axis. - A laser beam emitted from a semiconductor laser 11 provided in the fixed optical system 1 passes through a collimator lens 12 for conversion into a parallel light beam. This parallel light beam is reflected by a beam splitter 13 and then by a mirror 14 of a galvanometric mirror unit for delivery to the total reflection prism 22 of the movable optical system 2. The beam reflected by the total reflection prism 22 is focused by the object lens 21 onto the recording surface of the disk to form a minute spot. The light returned from the recording surface of the disk passes through the beam splitter 13 via the object lens 21, the total reflection prism 22 and the mirror 14. The light having passed through the beam splitter 13 is condensed by a condensing lens 15 and then separated by a polarizing separator 16 into three rays BO, B1,- B2.

The polarizing separator 16 comprises, for example, a Wollaston prism and separates the light returned from the disk into the rays B1, B2 for detecting the polarized components due to a Kerr rotatory angle and the ray BO for detecting both focus and tracking error signals.

The separated light rays are received by a pin photodiode 17. The pin photodiode 17 has a light receiving portion 17a divided into four zones and a pair of light - 3 error are detected receiving portions 17b, 17c located on both sides of the portion 17a. The light ray BO separated by the polarizing separator 16 is received by the four-divided light receiving portion 17a from which a tracking error and a focus To obtain a focus error signal by the astigmatism method, though not shown, a member for producing astigmatism, such as a cylindrical lens, is disposed between the beam splitter 13 and the polarizing separator 16.

On the other hand, the light rays B1 and B2 separated into different polarized components (P-wave component and S-wave component) are received by the light receiving portions 17b and 17c, respectively, for detecting a modulated playback signal (MO signal).

Fig. 17 shows a circuit arrangement for detecting the signal and the tracking error signal.

The difference between quantities of light received by 7c is determined the two light receiving portions l7b and 1 by a calculator 25 to obtain the MO signal depending the azimuthal direction of a Kerr rotatory angle The tracking error signal is detected by the so-called push-pull method using calculators 26, 27, 28 which cooperatively perform calculations of (Ial + Idl) - (IbI + Icl) for received light outputs lal, Ibl, Icl, Idl from four divided zones a, b, c, d of the light receiving portion 17a. Fig. 18 shows the recording surface of an opto-magnetic disk. In an MO signal recording area (E), there are formed a land 30 which serves as the signal recording surface, and a pair of grooves 31 on both sides of the land 30. In Fig. 18, (a) indicates a condition that the spot of the laser beam is positioned on the land 30 with no tracking error, and (b) or (c) indicates a condition that the spot is partially overlapped with the groove 31, causing a tracking error. As shown in Fig. 17, the reflected light from the spot formed on the recording surface of the disk is focused on the four-divided light receiving portion 17a to form a received light spot S. When the spot is partially overlapped with the groove 31 as indicated by (b) or (c) in Fig. 18, there occurs a shade Sa in any part of edges of the received light spot S on the light receiving portion 17a. On which side the quantity of the received light is reduced by the shade can be detected from an output of the calculator 28 and, therefore, the tracking error signal can be obtained.

In response to the detection of the tracking error signal, the mirror 14 is driven by a galvanometric mirror driver 18 about an imaginary axis indicated by 0 in Fig. 16. The detection light is thereby deflected in the radial (RAD) direction (or in the direction of T in Fig. 16) with respect to the recording surface of the disk for correcting the tracking error.

The focus error signal is detected by the astigmatism method. This method is effected by utilizing a function of the aforesaid cylindrical lens (not shown). More specifically, when the spot is focused just on the land 30, a substantially circular image (the received light spot S) is formed on the four-divided light receiving portion 17a. However, when the focus position is deeper or shallower with respect to the land 30, the received light spot on the light receiving portion 17a becomes substantially ellipsoidal as indicated by S1 or S2. Therefore, the focus error signal can be detected by using calculators different.from those shown in Fig. 17 to perform calculations of (Ial + Icl) - (Ibl + Idl) for the received light outputs. In response to the detection of the focus error signal, the object lens 21 is moved along its optical axis in the movable optical system 2 for correction of the focus error.

In the optical disk apparatus that the tracking error signal and the focus error signal are detected by the fourdivided light receiving portion 17a as explained above, a position shift between the light receiving portion 17a and the received light spot S gives a large influence on detection accuracy of the error signals.

Forexample, if a fixed shift 6aas shown inFig. 19 is present between the center of the light receiving porti- - 6 on 17a and the center of the received light spot S due to a position shift of any of components making up the Optical system or an inclination of the mirror 14 of the movable mirror unit in an undesired direction, the difference between the quantities of light received by the left and right zones of the light receiving portion, i.e., between (A + C) and (B + D), cannot be accurately detected even with the shade Sa appearing on the left side of the received light spot S as shown, thus causing an offset in the detected signal. As a prior method of dealing with such a fixed shift of the received light spot S, for example, a bias voltage applied to the tracking error signal is regulated in the adjusting stage so as to cancel out the difference, caused by the position shift of the received light spot S, between the quantities of light received by the left and right zones of the light receiving portion.

However, the above adjustment using a bias voltage has limitations in a correctable amount of the shift 6a and, therefore, the optical components require to be positioned with a fairly high degree of accuracy. In addition, thetime-dependent shift due to hardening of adhesives in the bonded regions of the optical components, after once adjusted, cannot be dealt with.

Furthermore, a position shift 6b directing as shown in Fig. 20, for example, may occur in some cases. The 1 1 shift in this direction corresponds to a shift in the tangential (TAN) direction on the recording surface of the disk. If that position shift &a is present, the respective calculated results of the quantities of light received by the four divided zones of the light receiving portion 17a become incorrect, causing crosstalk from the tracking error signal to the focus error signal. As a result, the focus error cannot be accurately corrected even by moving the object lens 21.

Such crosstalk will be explained below using numerical expressions.

As mentioned above, a tracking error signal Te is given by Gal. + Idl) (Ibl + Icl) and a focus error signal Fe is given by Ual + Icl) - (Ibl + Idl). Supposing now that neither tracking error nor focus error occur under a condition that the received light spot S is shifted by 8b in the TAN direction as shown in Fig. 20, the shade Sa does not exist and the received light spot S is of true circular shape. In this case, the tracking error signal Te and the focus error signal Fe are given below like the above:

Te = (Ial + M1) - (Ibl + Icl) Fe = (lal + Icl) - (Ibl + Idl) Here, because of Ial = Ibl and Icl = Idl, the error signals Te and Fe are both zero.

Next, let it be supposed that no focus error occurs 8 under a condition that the received light spot S is shifted by &b in the TAN direction as with the above, but the shade Sa is formed on the left side due to the tracking error, as shown, to thereby reduce the received light' output from the lef t-hal f zones by a relative to that from the right-half zones. Assuming for simplicity of calculations that the received light output from the right-half zones is increased simply by a, the tracking error signal Te is expressed below:

Te = Gal + Idl) (1 + a) - (Ibl + Icl) The following resultS because Ial = Ibl and Icl Idl:

Te = Gal + Idl).a This represents an amount of the tracking error. On the other hand, the focus error signal Fe is calculated below:

Fe = (Ial(l + a) + Icl) - {Ibl + Idl(l + a) = (Ial - Idl)-a Thus, in spite of no actual focus error. there produces the output as if any focus error occurs, meaning the occurrence of crosstalk.

The present invention..eKsto solve the above-mentioned problems in the prior art, and its object is to provide an error detector for an optical disk apparatus which can

0 - 9 correct a position shift between a received light spot and a light receiving portion at any desired time.

An error detector for an optical disk apparatus acc o r d i n g t o a first aspect of the present inventioni s a r r a n ge d such that detection light emitted from a light source and reflected by a disk is received by a light receiving portion divided into a plurality of zones, the error detector being featured in comprising a mirror surface detecting unit for detecting, based on received light outputs from the respective zones of said light receiving portion, that said detection light has reached a mirror surface area of said disk, and then outputting a mirror surface detection signal; detection means for detecting, based on said mirror surface detection signal and said received light outputs, a fixed shift amount between light returned from a recording surface area of said disk and said light receiving portion in at least one of the tracking direction and the tangential direction; and correction means for applying a correction signal to correct said shift amount based on a detection output from said detection means.

Incm error detector for an optical disk apparatus according to a second aspect of the invention said correction means comprises cancel signal generating means for generating a cancel signal based on the shift amount detected by said detection means, and crosstalk - canceling means for canceling out a crosstalk component of a focus error signal based on said cancel signal.

In an error detector for an optical disk apparatus according to a third aspect of the present invention said correction means comprises a gain calculating unit for calculating gains of said received light outputs based on said mirror surface detecting signal and said received light outputs, an output amplifying unit for producing amplified outputs of said received light outputs based on said gains and said received light outputs, and an error signal generating unit for generating error signals of said detection light based on said amplified outputs.

With the present invention of claim 1, by way of example, when a received light spot is moving over the mirror surface area of the disk, the direction and amount of a position shift of the received light spot are detected by the light receiving portion divided into four zones. Based on the detected output, a signal for correcting the shift in the tracking direction or the tangential direction is produced and then combined with a tracking error signal for driving a light deflector, for example. The fixed position shift between the received light spot and the light receiving portion is thereby corrected.

The present invention of claim 2 is made with an attention in mind that when the detection light returned W from the disk forms the received light spot of which center is shifted from the center of the light receiving portion, there establishes a certain mathematical relationship between such a shift amount and an amount of crosstalk acting on the focus error signal. Specifically, the direction and amount of the shift of the returned light spot are detected from the received light outputs resulted when the spot of the detection light moves into the mirror surface area of the disk, and the detected shift amount is subjected to arithmetic operations using predetermined equations to calculate the component to be canceled out. This cancel component is subtracted from the focus error signal to cancel out a crosstalk component contained in the focus error signal, and a focus correction driver is operated in accordance with the focus error signal free from any crosstalk.

With the present invention of claim 3, when light spot moves into the mirror surface area of received light outputs of respective zones of the light receiving portion are detected. Arithmetic operations are carried out based on the received light outputs to calculate gains for the received light outputs from the respective zones. These gains are calculated to have such values that if a position shift is present between the received light spot and the light receiving portion, such a position a received the disk K1631:JK - 12 shift will not affect on the tracking error signal and the focus error signal. The received light outputs from the respective zones are amplified with the calculated gains. The focus error signal and the tracking error signal are then produced based on the amplified values.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a top plan view showing one embodiment of a movable mirror unit suitable for an optical disk apparatus using an error detector according to the present invention.

Fig. 2 is an exploded perspective view of the movable mirror unit.

Fig. 3 is a top plan view for explaining operation of the movable mirror unit.

Fig. 4 is a circuit block diagram showing an error detecting section in a light receiving unit.

Fig. 5 is a circuit block diagram of a correction circuit for correcting a tangential error.

Fig. 6 is a circuit block diagram showing a correction unit of a second embodiment according to the present invention of claim 1.

Fig. 7 is a block diagram showing means for detecting outputs from the light receiving unit.- Fig. 8 is a circuit block diagram showing an error detector for an optical disk apparatus according to the 1 R 13 present invention of claim 2.

Fig. 9 is a circuit block diagram showing one example of practical arrangement of an error signal generating unit in the error detector of Fig. 8.

Fig. 10M is a circuit block diagram showing practical arrangement of an cancel signal generating unit, and Fig. 1O(B) is a circuit block diagram showing one example of a crosstalk canceling unit.

Fig. 11 is an explanatory view showing a position shift between a received light spot and a light receiving portion.

Fig. 12 is a circuit block diagram showing an error detector for an optical disk apparatus according to the present invention of claim 3.

Fig. 13 is a circuit block diagram showing one exampl of practical arrangement of an output amplifying unit and an error signal generating unit in the error detector of Fig. 12.

Fig. 14 is a circuit block diagram showing a part of practical arrangement of a gain calculating unit and a sample/hold unit in the error detector of Fig. 12.

Fig. 15 is a graph showing the relationship between a shift amount of the received light spot on a disk in the tracking direction and ratios of respective outputs from a four-divided light receiving portion to a total output; e 14 - Fig. 15(A) represents a state that there occurs no Position shift between the received light spot and the light receiving portion, and Fig. 15(B) represents a state that there occurs a position shift in the tangential direction between the received light spot and the light receiving portion.

Fig. 16 is a perspective view showing layout of components of an optical system of an opto-magnetic disk apparatus.

Fig. 17 is a circuit block diagram showing an arrangement of a light receiving unit of the opto-magnetic disk apparatus.

Fig. 18 is a plan view showing a structure of the recording surface of an opto-magnetic disk.

Fig. 19 is an explanatory view of a four-divided light receiving portion, for explaining problems in the prior art.

Fig. 20 is an explanatory view of respective light receiving zones of the four-divided light receiving portion, for explaining problems in the prior art.

Preferred embodiments of the present invention will be hereinafter described with reference to the drawings.

To begin with, a first embodiment according to the present invention of claim I will be explained by referring - 15 to Figs. 1 to 5.

Fig. 1 is a top unit suitable for an detector according to exploded perspective Fig. 3 is a top plan movable mirror unit.

plan view showing a movable mirror optical disk apparatus using an error the present invention, Fig. 2 is an view of the movable mirror unit, and view for explaining operation of the Denoted by reference numeral 14 is a mirror and 14a is a mirror surface thereof. 41 is a movable holder for holding the mirror 14. A tracking drive coil 42 for driving the mirror in the tracking direction is fixed to a pair the rear side of the movable holder tangential drive coils 43 for the tangential direction is bonded of the of projections 41a on 41. Further, a pair of driving the mirror in and fixed to the opposite upper and lower surfaces coil 42.

Denoted by 44 is a yoke serving as a stationary member. A pair of magnets 45 is fixed to the upper and lower surfaces of a central projection 44a of the yoke 44. Both the coils 42 and 43 fixed to the movable holder 41 are interposed between the magnets 45 and upper and lower opposite projections 44b of the yoke facing the magnets 45. The magnets 45 are positioned to face not only upper and lower sides 42a of the coil 42, but also the coils 43.

A support member 46 is bonded to the front surfaces of 16 the upper and lower opposite projections 44b of the yoke 44, and a metallic leaf spring 47 is bonded at its center to the center of the front surface of the support member 46. The opposite ends of the leaf spring 47 are bonded to support surfaces 41b on the rear side of the movable holder 41. While the movable holder 41 is supported to the yoke 44 through the leaf spring 47, the movable holder 41 can be actuated depending on deformations of a free portion 47a of the leaf spring 47 which is bonded to neither the support member 46 nor the support surfaces 41b. In the movable mirror unit of this embodiment, the movable holder 41 can be actuated in two directions. Specifically, a torsional deformation of the free portion 47a of the leaf spring allows the movable holder 41 to displace in the direction of a about an imaginary axis J1 shown in Fig. 3, whereas a flexural deformation (bending deformation) of the free portion 47a of the leaf spring allows the movable holder 41 to displace in the direction of 13 about an imaginary axis J2. Additionally, as shown in Fig. 1, an elastic member 48 such as formed of rubber or elastomer is bonded to the leaf spring 47. The elastic member 48 serves to suppress vibration of the leaf spring.

Operation of the movable mirror unit of the above structure will now be explained.

First, when the tracking drive coil 42 is energized, 17 - there produce electromagnetic forces due to both electric currents flowing through the upper and lower sides 42a of the coil 42 in the direction of X and magnetic flux direct ing from the magnets 45 to the opposite projections 44b across the coil, whereby the movable holder 41 is driven in the direction of a about the imaginary axis JI Also, there produce electromagnetic forces due to both electric currents flowing through respective lateral sides 43a of the tangential drive coils 43, provided on the upper and lower surfaces of the coil 42, in the direction of Y and the aforesaid magnetic flux, whereby the movable holder 41 is driven in the direction of 13 about the imaginary axis J2. With the above operation, the light reflected by the mirror surface 14a of the mirror 14 is deflected in the two directions to shift the reflection angle.

It is to be noted that while the movable mirror unit shown in Figs. I to 3 employs the leaf spring as a resilient member for supporting the movable holder 41, a linear spring may be used in place of the leaf spring. The leaf spring or the linear spring may be formed of metallic spring material or resin. Further, while the above-stated movable mirror unit is designed to drive the movable holder in the two orthogonal directions, the movable holder may be driven in more than two directions. Additionally, the magnets may be mounted to the movable holder 41 and the coils 42, 43 may be provided on the stationary member side.

The optical disk apparatus using the movable mirror unit will be next described.

Where the movable mirror unit shown in Figs. 1 to 3 is used in the beforementioned optical disk apparatus shown in Fig. 16, the mirror 14 is arranged in such an orientation as shown in Fig. 16. Then, the mirror 14 of the movable mirror unit is driven to displace in the direction of a about the imaginary axis J1 and to shif t in the direction of jO about the imaginary axis J2.

Fig. 4 is a circuit block diagram showing an error signal detecting section in the light receiving unit, and Fig. 5 is a circuit block diagram showing a correction circuit to shift the mirror 14 of the movable mirror unit in the direction of 13 depending on an error output in the tangential direction.

As with the light receiving unit shown in Fig. 17, the tracking error output is obtained using calculators 26, 27, 28 which cooperatively perform calculations of (B + D) (A + C) for respective quantities of light received by four light receiving zones A. B, C, D. Depending on the tracking error output thus obtained, the electric current supplied to the tracking drive coil 42 of the movable mirror unit is controlled, causing the mirror 14 to deflect in the direction of a about the imaginary axis J1 so that the R detection iight is corrected in the tracking direction (in the direction of T in Fig. 16). Simultaneously, depending on the above tracking error output, a linear motor M shown in Fig. 16 is also driven to move a movable optical system 2 in the direction of T for the additional tracking correction.

Though not shown in Fig. 4, the focus error output is obtained using other calculators which cooperatively per- form calculations of (B + C) - (A + D) for respective quantities of the received light. Depending on the focus error output thus obtained, an object lens 21 is driven by a focus actuator (not shown) in the direction of its optical axis for the focus correction.

Additionally, in the error signal detecting section of Fig. 4, the received light outputs of the four-divided light receiving portion 17a are calculated by calculators 51, 52, 53 to obtain an output of (A + B) - (C + D). From this output, the amount and the direction of a position shift 8b (see Fig. 20) of a received light spot S with respect to a light receiving portion 17a in the tangential direction can be detected.

As shown in Fig. 5, the tangential error output (i.e., the output from the calculator 53 in Fig. 4) based on the outputs from the four-divided light receiving portion 17a is applied to a sample/hold circuit 55 for being sampled and held therein. The tangential error output thus held in the circuit 55 is applied to a drive circuit 57 via a lowpass filter 56, and a corresponding electric current is supplied to the tangential drive coils 43 of the aforesaid movable mirror unit.

The sample/hold circuit 55 is set to a sampling/holding mode in response to a mirror surface detection signal. This mirror surface detection signal is obtained, by way of example, as follows. The recording surface of an optomagnetic disk shown in Fig. 18 is divided into a plurality of sectors in the rotating direction of the disk. An MO signal recording area (E) is formed in each of the divided sectors, while a mirror surface area (F) free from any grooves 31 is also present in the sector at a predetermined location.

No shades due to the grooves 31 appear in the received light spot S formed by the light which is returned from the s,pot scanning over the mirror surface area to reach the light receiving area 17a. Therefore, when the spot moves into the mirror surface area, the quantity of the returned light is maximized. With a calculator 54 shown in Fig. 4 used to calculate the sum of the quantity of light received by a light receiving portion 17b and the quantity of light received by a light receiving portion 17c, an output of the calculator 54 exhibits a maximum value corresponding to the 21 position of the mirror surface area. Accordingly, by detecting the output of the calculator 54 in comparison with a predetermined threshold level, the movement of the spot into the mirror surface area is detected.

Operation of the apparatus of this embodiment will be next explained.

In the opto-magnetic disk apparatus shown in Fig. 16, a laser beam emitted from a semiconductor laser 11 provided in a fixed optical system 1 passes through a collimator lens 12 for conversion into a parallel light beam. This parallel light beam is reflected by a beam splitter 13 and then by the mirror 14 of the aforesaid movable mirror unit for delivery to a total reflection prism 22 of the movable optical system 2. The beam reflected by the total reflection prism 22 is focused by the object lens 21 onto the recording surface of the disk to form a minute spot. The light returned from the recording surface of the disk passes through the beam splitter 13 via the object lens 21, the total reflection prism 22 and the mirror 14. The light having passed through the beam splitter 13 is condensed by a condensing lens 15 and then separated by a polarizing separator 16 into three rays BO, Bl, B2 which are detected by the light receiving portions 17a, 17b, 17c of a pin photodiode 17, respectively.

In the circuit shown in Fig. 4, while the spot is moving in the MO signal recording area (E), the output of the calculator 28 becomes the tracking error signal and, in accordance with a correction drive signal based on that calculator output, an electric current is supplied to-the tracking drive coil 42 of the movable mirror unit so that the mirror 14 is deflected in the direction of a about the imaginary axis Jl or the linear motor M is driven for the tracking correction. Further, the object lens 21 is driven depending on the focus error output for the focus correction.

When the spot scanning over the recording surface of the disk moves into the mirror surface area (F) shown in Fig. 18, the received light output from the calculator 54 shown in Fig. 4 is maximized, whereupon the mirrorsurface detection signal is produced. In the correction circuit of Fig. 5, the sample/hold circuit 55 is switched over to the sampling/holding mode in response to the mirror surface detection signal from the calculator 54. The received light output from the four-divided light receiving portion 17a, i.e., the output of the calculator 53, is thereby sampled. Let it now be assumed that there is a position shif t 6b in the tangential direction between the center of the four-divided light receiving portion 17a and the center Os of the received light spot S due to an error in the mount position of some component, a change in orientation of the mirror, or other factors. Because of no shades of the grooves 31 appearing in the received light spot S of the light returned from the mirror surface area, the amount and direction of the shif t Sb can be detected depending on the received light output from the calculator 54, i. e., (A + B) - (C + D). The sample/hold circuit 55 samples and holds the output of the calculator 54. Then, the held value is applied to the drive circuit 57 via the low-pass filter 56, and a corresponding electric current is supplied to the coils 43 of the movable mirror unit.

As a result, the mirror 14 of the movable mirror unit is displaced in the direction of G about the imaginar,y axis J2, so that the mirror 14 is driven in a direction to el iminate the posi tion shif t Sb and the center of the received I ight spot S is corrected to coincide wi th the center of the four-divided light receiving portion 17a.

Note that the sampling and holding by the sample/hold circuit 55 may be performed whenever the mirror surface area (F) which exists plural in number along the circumference of the disk is detected, or may be performed once or twice for each rotation of the disk.

Additionally, by using the above-mentioned apparatus, it is possible to not only correct the position shift between the I ight receiving portion 17a and the received light spot S caused by an offset in the mount position of some optical component, but also servo-control the Position shift of the spot in the tangential direction at all times.

It should be understood that the present invention is not limited to the above embodiment. For example, th-e means for detecting the mirror surface area may not rely on the quantities of light received by the light receiving portions 17b and 17c. Alternatively, if the received light outputs from the four divided zones A, B, C, D of the light receiving portion 17a are all added to give a total output, this output is maximized in the mirror surface area and, therefore, the mirror surface detection signal can be produced from the total output. It is also practicable to compute a period of time from the time at which the spot passes the area to be addressed to the time at which the spot passes the mirror surface area, and set the sample/hold circuit 55 into the sampling/holding mode at the time the spot passes the mirror surface area.

Next, a second embodiment according to the present invention of claim 1 will be described with reference to Figs. 6 and 7. An error detector of the second embodiment is employed in the optical disk apparatus in which the galvanometric mirror 14 is finely moved in the direction of ct to deflect the laser beam for the tracking correction.

Fig. 6 is a circuit block diagram of a correction unit according to the second embodiment.

- A tracking correction drive signal 58 is obtained from the four-divided light receiving portion 17a. As shown in Fig. 7, the output 58 of the four-divided light receiving portion 17a is obtained using calculators 26, 27, 28 which cooperatively perform calculations of (B + D) - (A + C) for respective quantities of light received by four light receiving zones A, B, C, D. The tracking correction drive signal 58 is inputted to a galvanometric mirror driver 18 via a phase compensation circuit 60. An output of the phase compensat i on c i rcui t 60 i s app I i ed to a phase compensation circuit 64 and also to a linear motor M via an equivalent filter 65 which is constituted to be electrically equivalent to the galvanometric mirror driver 18.

The galvanometric mirror driver 18 finely drives the galvanometric mirror 14, shown in Fig. 16, in the direction of a so that the spot on the recording surface of the disk is deflected in the direction of T for fine correction of the tracking. Simultaneously, a head mounting thereon the movable optical system 2 shown in Fig. 16 is driven by the linear motor M for coarse correction of the tracking.

Additionally, in the correction unit of Fig. 16, the output 58 of the four-divided light receiving portion 17a (the output of the calculator 28 in Fig. 7: the tracking correction drive signal) is also inputted to a sample/hold circuit 62 for sampling and holding thereof. The output - 26 thus held is applied via a low-pass filter 63 to reduce the input signal to the galvanometric mirror driver 18.

The sample/hold circuit 62 is set to a sampling/holding mode in response to a mirror surface detection si,gnal 54A. This mirror surface detection signal 54A is obtained, by way of example, as follows. The recording surface of an opto-magnetic disk shown in Fig. 18 is divided into a plurality of sectors in the rotating direction of the disk. An MO signal recording area (E) is formed in each of the divided sectors, while a mirror surface area (F) free from any grooves 31 is present in the boundary region between adjacent twos of the sectors.

No shades due to the grooves 31 appear in the received light spot S formed by the light which is returned from the spot scanning over the mirror surface area to reach the light receiving portion 17/a. Therefore, when the spot moves into the mirror surface area, the quantity of the returned light is maximized. With a calculator 54 shown in Fig. 7 used to calculate the sum of the quantity of light received by a light receiving portion l7b and the quantity of light received by a light receiving portion 17c, an output of the calculator 54 exhibits a maximum value corresponding to the position of the mirror surface area. Accordingly, by detecting the output of the calculator 54 in comparison with a predetermined threshold level, the - 27 movement of the spot into the mirror surface area is detected.

Correcting operation with this embodiment will be next explained.

In the opto-magnetic disk apparatus shown in Fig. 16, a laser beam emitted from a semiconductor laser 11 provided in an optical system 1 on the stationary side passes through a collimator lens 12 for conversion into a parallel light beam. This parallel light beam is reflected by a beam splitter 13 and then by the galvanometric mirror 14 for delivery to a total reflection prism 22 of an optical system 2 on the movable side. The beam reflected by the total reflection prism 22 is focused by an object lens 21 onto the recording surface of the disk to form a minute spot. The light returned from the recording surface of the disk passes through the beam splitter 13 via the object lens 21, the total reflection prism 22 and the galvanometric mirror 14. The light having passed through the beam splitter 13 is condensed by a condensing lens 15 and then separated by a polarizing separator 16 into three rays BO, B1, B2 which are detected by the light receiving portions 17a, 17b, 17c of a pin photodiode 17, respectively.

In the circuit shown in Fig. 6, while the spot is moving in the MO signal recording area (E), the galvanometric mirror 14 and the linear motor M are driven depending on the output 58 of the four-divided light receiving portion 17a (i.e., the tracking correction drive signal) for the tracking correction.

When the spot scanning over the recording surface of the disk moves into the mirror surface area (F) shown in Fig. 18, the received light output from the calculator 54 shown in Fig. 7 is maximized, whereupon the mirror surface detection signal 54A is produced. In the correction unit of Fig. 6, the sample/hold circuit 62 is switched over to the sampling/holding mode in response to the mirror surface detection signal 54A. The received light output from the four-divided light receiving portion 17a, i.e., the output 58 of the calculator 28, is thereby sampled. Let it now be assumed that there is a position shift &a between the center of the four-divided light receiving portion 17a and the center Os of the received light spot S due to in the mount position of some component or other Because of no shades of the grooves 31 appearing received light spot S of the light returned from surface area, the amount and direction of the sh be detected from the received light output of the., (B + D) - (A + C). The sample/hold an error reasons.

in the the mirror if t a can calcula tor 28, i.e circuit 62 samples and holds the output of the calculator 28. Then, the held value is applied via the low-pass filter 63 so as to reduce an input signal to the galvanometric mirror driver 18.

As a result, the driving force is applied to the galvanometric mirror 14 in a direction to eliminate the position shift &a so that the center of the received- light spot S is corrected to coincide with the center of the four-divided light receiving portion 17a. The output applied via the low-pass filter 63 so as to reduce the input signal to the galvanometric mirror driver 18 is a DC component, and the galvanometric mirror 14 is driven in accordance with the signal resulted by subtracting that DC component from the correction drive signal for the tracking correction. Therefore, when the spot moves into the MO signal recording area (E) again, the normal tracking correction is performed under a condition that the center of the received light spot S is held coincident with the center of the four-divided light receiving portion 17a.

Note that the sampling and holding by the sample/hold circuit 62 may be performed whenever the mirror surface area (F) which exists plural in number along the circumference of the disk is detected, or may be performed once or twice for each rotation of the disk.

The focus correction is carried out based on the focus error signal detected by the four-divided light receiving portion 17a, but will not be explained herein.

It is a matter of course that while the means based on - a mechanical method utilizing the movable mirror has been explained as a light deflector used for the tracking correction in this embodiment, it may be otherwise based on a method utilizing diffraction of light by ultrasonic waves, or a method utilizing changes in a refraction angle due to the electro-optic effect. Further, the means for detecting the mirror surface area may not rely on the quantities of light received by the light receiving portions 17b and 17c. For example, if the received light outputs from the four divided zones A, B, C, D of the light receiving portion 17a are all added to give a total output, this output is maximized in the mirror surface area and, therefore,,the mirror surface detection signal 54A can be produced from the total output. It is also practicable to compute a period of time from the time at which the spot passes the recording area to the time at which the spot passes the mirror surface area, and set the sample/hold circuit 62 into the sampling/holding mode at the time the spot passes the mirror surface area.

Additionally, the above embodiment has been explained as obtaining the tracking error signal by the push-pu 11 method utilizing the fact that the spot is partly over lapped with the grooves 31 on the recording surface of the disk. However, the tracking error signal may be detected by any other methods such as a sample servo method using 1 31 - pits, for example.

Next, an embodiment according to the present invention of claim 2 will be described with reference to Figs. 8 to Fig. 8 is a circuit block diagram of an error detector for an optical disk apparatus according to the present invention of claim 2.

An optical system of the optical disk apparatus is of the same structure as that shown in Fig. 16.

Using the received light outputs from the four-divided light receiving portion 17a of the pin photodiode 17 in Fig. 16, an error signal generating unit 71 calculates a tracking error signal (TE), a shift amount (TAN) of the received light spot in the tangential direction, and a focus error signal (FE).

Fig. 9 shows a practical circuit arrangement of the error signal generating unit 71. Received light outputs from the respective zones a, b, c, d of the four-divided light receiving portion 17a are amplified as voltage values by associated current-to-voltage converters and amplifiers (all not shown), and the resultant voltage values are subjected to arithmetic operations by calculators 76 to 84. Outputs of these calculators are as follows:

Output of the calculator 81 (tracking error signal) TE = (Ia + ld) - (Ib + lc) 32 - Output of the calculator 78 (shift amount in the tangential direction) TAN = (Ic + Id) - (Ia + Ib) Output of the calculator 84 (focus error signal) FE = (la + Ic) - (Ib + Id) A mirror surface detecting unit 72 shown in Fig. 8 detects whether or not the spot of the defection light has reached a mirror surface area of a disk. The recording surface of an opto-magnetic disk shown in Fig. 18 is divided into a plurality of sectors in the rotating direction of the disk. An MO signal recording area (E) is formed in each of the divided sectors, while a mirror surface area (F) free from any grooves 31 is present in the boundary region between adjacent twos of the sectors. When the spot is positioned in the mirror surface area, no shades due to the grooves 31 appear in the received light s.pot S formed on the light receiving portion 17a. Therefore, when the spot moves into the mirror surface area, the quantity of the returned light is maximized. In the mirror surface detecting unit 72, the sum of the quantity of light received by a light receiving portion 17b and the quantity of light received by a light receiving portion 17c is calculated by a calculator, for example, the portions 17b and 17c being adapted to obtain an MO signal. An output of this calculator exhibits a maximum value corresponding to - 33 the position of the mirror surface area. Accordingly, by detecting the output of the calculator in comparison with a predetermined threshold level, the movement of the spot into the mirror surface area is detected.

When the mirror surface detecting unit 72 detects that the spot of the detection light has reached the mirror surface area of the disk, a cancel signal generating unit 73 generates a cancel signal for canceling out crosstalk based on the outputs TE, TAN produced from the calculators 81, 78 at that time.

Fig. 10(A) shows a practical arrangement of the cancel signal generating unit 73. It is to be noted that an output of the cancel signal generating unit 73 is obtained when the detection signal is issued from the mirror surface detecting unit 72, i.e., when the spot of the detection light has reached the mirror surface area. Since the output TE of the calculator 81 is obtained by calculating the difference between the quantities of light returned and received by lef t-hal f zones and right-hal f zones of the four-divided light receiving portion 17a shown in Fig. 11, the TE resulted when the spot of the detection light has reached the mirror surface area takes a value depending on a shift amount y of the center Os of the received light spot S in the tracking direction. Likewise, the TAN resulted when the spot of the detection light has reached the - 34 mirror surface area takes a value depending on a shift amount x of the center Os of the received light spot S in the tangential direction. Accordingly, the outputs TE and TAN resulted when the spot of the detection light has. reached the mirror surface area are indicated by (y) and (x), respectively.

In the cancel signal generating unit 73, the product (x-y) of (x) and (Y) is determined by a multiplier 91 and then multiplied by a certain constant A4 in a constant setting unit 92 using a combination of resistors or the like to obtain a value of (A4-x-y). Also, the value of (x) is multiplied by a certain constant A6 in a constant setting unit 93 to obtain a value of (A6-x). The values of (A4-x-y) and (A6-x) are sent to hold circuits 95a, 95b upon turning-on of analog switches 94a, 94b when the detection output is issued from the mirror surface detecting unit 72, respectively, to be held therein. Thus, the values of (A4-x-y) and (A6-x) are updated and held in the hold circuits 95a, 95b whenever the mirror surface area is detected. Alternatively, the values of (A4-x- y) and (A6-x) are updated and held in the hold circuits 95a, 95b at the any other timing such as once for a predetermined rotation of the disk.

When the spot of the detection light on the recording surface of the disk is out of the mirror surface area, i.e., during the normal playback operation, the tracking error signal TE obtained at that time is multiplied in a multiplier 95 by the value of (A6-x) held in the hold circuit 95a to calculate a value of (A6-x-TE). Then, a cancel signal CFE = (A4-x-y) + (A6-x-TE) is obtained by an adder 96.

In a crosstalk (component) canceling unit 74 shown in Fig. 8, the above cancel signal CFE is subtracted from the focus error signal FE at that time to thereby obtain a focus error signal Fe which has been corrected, i.e., in which the crosstalk component has been canceled out.

Fig. 10(B) shows one example of circuit arrangement of the crosstalk canceling unit 74. In a subtracter 97, the cancel signal CFE is subtracted from the focus error signal FE to obtain the corrected focus error signal Fe = FE CFE. Thereafter, in a control circuit 75 shown in Fig. 8, focus error correction control is performed in accordance with the corrected focus error signal Fe and tracking error correction control is performed in accordance with the tracking error signal TE obtained from error signal generating unit 71.

The procedure of deriving the above correction signal CFE = (A4-x-Y) + (A6-x-TE) will now be explained.

When the shift amounts x and y are present between the center of the recel ved light spot S and the four-divided - 36 1, when S is at the center of portion 17a, i.

light receiving portion 17a as shown in Fig. 11, the amount of crosstalk to the focus error signal, i.e., the cancel signal CFE, can be expressed by the following equation of higher degree with the shift amounts x, y and the tracking error signal TE being as variables:

CFE = F(x, Y, TE) Here, since the shift amounts x, y are such an extent that their terms of higher degree are negligible, the above equation can be simplified as follows:

CFE = A1 + A2.x + A3.y + A4.x.y + (A5 + A6.x + A7.y + A8.x.y)-TE Coefficients of this equation will be explained below.

In Fig. 1 the center of the received light spot the four-divided light receiving e., when x = 0 and y = 0 hold, CFE = A1 + A5-TE Under this condition, there occurs no crosstalk even if the tracking error signal TE is produced. Namely, because of CFE 0.

Al = A5 0 is resulted.

When the received light spot S is shifted in the RAD direction, i.e., when x = 0 and y 0 hold, CFE = A1 + A3.y + A7.y.TE Since no crosstalk occurs under this condition like the above case, A1 = A3 = A7 = 0 is resulted.

37 - Furthermore, in the following equation:

CFE = A2-x + A4-x-Y + (A6-x + A8-x-y)-TE the respective coefficients are calculated below by simulat i o n:

A2 = 8.7 x 10-3 A4 = - 21 A6 = 0.51 A8 = - 3.9 x 10-10 Here, since A2 and A8 are very small values as compared with A4 and A6, the former coefficients can be omitted.

Thus, the component to be canceled out (corresponding to the crosstalk component) CFE is given by:

CFE = A4-x-y + A6-x-TE In this embodiment, by using the above value as the correction amount which is subtracted from the focus error signal, the crosstalk to the focus error signal caused when the center Os of the received light spot S is shifted from the center of the light receiving portion 17a as shown in Fig. 11, is canceled out. In the practical apparatus, therefore, a focus actuator is actuated in accordance with the focus error signal Fe resulted from canceling out the crosstalk component, so that the object lens 21 is driven in the d.irection of its optical axis to perform the focus error correcting operation.

Note that while the cancel component CFE is determined 38 - in the above embodiment using the multiplier 91, the constant setting units 92, 93 and so forth in Fig. 10, it may be determined through any other arithmetic operations.

It should be understood that the present invention is not limited to the above embodiment. For example, the means for detecting the mirror surface area may not rely on the quantities of light received by the 1-ight receiving portions 17b and 17c. Alternatively, if the received light outputs from the four divided zones a, b, c, d of the light receiving portion 17a are all added to give a total output, this output is maximized in the mirror surface area and, therefore, the mirror surface detection signal can be produced from the total output. It is also practicable to compute a period of time from the time at which the spot passes the area to be addressed to the time at which the spot passes the mirror surface area, and produce the mirror surface detection signal by calculating the time at which the spot passes the mirror surface area.

Next, an embodiment according to the present invention of claim 3 will be described with reference to Figs. 12 and 15.

Fig. 12 is a circuit block diagram of an error detector for an optical disk apparatus according to the present invention of claim 3.

An optical system of the optical disk apparatus is of - 39 the same structure as that shown in Fig. 16.

The pin photodiode 17 in Fig. 16 is denoted by a block "PD" in Fig. 12. Received light outputs of a four-divided light receiving portion 17a of the pin photodiode 17 are sent to an output amplifying unit 101 and also a gain calculating unit 103 via a sample/hold unit 106.

A mirror surface detecting unit 102 detects whether or not a spot of the detection light has reached a mirror surface area of a disk. The recording surface of an optomagnetic disk shown in Fig. 18 is divided into a plurality of sectors in the rotating direction of the disk. An MO signal recording area (E) is formed in each of the divided sectors, while a mirror surface area (F) free from any grooves 31 is present in the boundary region between adjacent twos of the sectors. When the spot is positioned in the mirror surface area, no shades due to the grooves 31 appear in the received light spot S formed on the light receiving portion 17a. Therefore, when the spot moves into the mirror surface area, the quantity of the returned light is maximized. In the mirror surface detecting unit 102, the sum of the quantity of light received by a light receiving portion 17b (1 portion) and the quantity of light received by a light receiving portion 17c U portion) calculated by a calculator, for example. An output o calculator exhibits a maximum value corresponding to i S f th the is - 40 position of the mirror surface area. Accordingly, by detecting the output of the calculator in comparison with a predetermined threshold level, the movement of the spot into the mirror surface area can be detected.

The gain calculating unit 103 calculates gains based on the detection outputs from the four-divided light receiving portion 17a resulted while the mirror surface area is being detected. Then, the output amplifying unit 101 is set to provide gains equal to the calculated results of the gain calculating unit 103, and the received light outputs from four divided zones of the light receiving portion 17a with the respective gains. Based on these outputs amplified by the output amplifying unit 101, a tracking error signal and a focus error signal are produced in an error signal generating unit 104. A control circuit 105 is operated in accordance with these error signals to correct the tracking error and the focus error. In the apparatus shown in Fig. 16, the coarse tracking correction operation is performed by moving a movable optical system 2 via a linear motor M, and the fine tracking correction Derformed by f operation is inely moving a galvanometric mirror 14.

Additionally, the focus error correcting operation is performed by finely moving an object lens 21 in the direc tion of its optical axis.

Practical structures and operations of the above circuit components will now be explained.

Fig. 13 shows the output amplifying unit 101 and the error signal generating unit 104. The received light outputs from four divided zones a, b, c, d of the light receiving portion 17a of the pin photodiode 17 are amplified by amplifiers 101a, 101b,-101c, 101d with gains Ga, Gb, Gc, Gd, respectively. Based on outputs Ia, Ib, Ic, Id thus amplified, the error signal generating unit 104 produces a tracking error signal Te and a focus error signal Fe. In the error signal generating unit 104, there are provided calculators 104a to 104f which perform arithmetic operations to obtain those error signals Te and Fe as follows:

Te = (Ia + ld) - (Ib + lc) Fe = (Ia + Ic) - (Ib + Id) The above outputs Ia, Ib, Ic, Id are resulted by amplifying the received light outputs of the respective zones a, b, c, d in a playback operative state, etc. with the gains Ga, Gb, Gc, Gd. In actual equipment, the received light outputs of the respective zones a, b, c, d are subjected to current-to-voltage conversion beforehand and the amplifiers 101a, 101b, 101c, 101d perform voltage amplification.

Fig. 14 shows a practical arrangement of the gain calculating unit 103 for setting the gains Ga, Gb, Gc, Gd and the sample/hold unit 106. The gain calculating unit 103 has an adder 103a and four calculators 103b, 103c, 103d, 103e. The gain calculating unit 103 performs arithmetic operations based on detection outputs IaO, IbO ' " ICO, IdO from the respective zones a, b, c, d of the four-divided light receiving portion 17a resulted while the detection light is detecting a mirror surface area (F) of the disk (see Fig. 18). Specifically, movement of the spot of the detection light into the mirror surface area (F) on the recording surface of the disk in Fig. 18 is detected by the mirror surface detecting unit 102, and the outputs IaO, IbO, IcO, IdO from the respective zones a, b, c, d at that time are held in sample/hold circuits 106a to 106d to be used for subsequent arithmetic operations. In Fig. 14, the adder 103a adds all the outputs IaO, IbO, IcO, IdO resulted while the mirror surface area is being detected. The calculators 103b, 103c, 103d, 103e respectively divide the total output from the adder 103a by IaO, IbO, IcO, IdO and further divide the respective quotients by 4, followed by outputting the divided results. Thus, the outputs of the gain calculating unit are as follows:

Ga = MaO + IbO + IcO + IdO)/C Iao Gb = MaO + lbO + lcO + IdO)/4) lbO Gc = MaO + IbO + IcO + ldO)/4) IcO Gb = (GaO + IbO + IcO + IdO)/4) IdO 1 - 43 zone a, b, c, resulted when and the light each other in d ( i. e., both the centers of receiving Fig. 20, from each zone. This spot is positioned out received light output to that in the case of each of the zones a, b, Then, the respective gains of the amplifiers 101a, 101b, 101c, 101d shown in Fig. 13 are set based on the calculated results in the gain calculating unit 103. This gain setting may be performed whenever the spot of the detection light detects the mirror surface area (F) of the disk, or once for each rotation of the disk, or once for each disk. In the actual playback operation, the received light outputs detected by the respective zones a, In the above equations, the term put in () represents a four-divided average value of the total received light output resulted while the mirror surface area (F) of the disk is being detected. Therefore, the output Ga indicates a ratio of the four-divided average value to the received light output IaO from the zone a. This is equally applied to Gb, Gc, Gd as well. In other words, Ga, Gb, Gc, Gd eachindicate a ratio of the received light output from each the four-divided average value) the received light spot S portion 17a are coincident with to the actual received light output means that when the received light of the center, the ratio of the in the case of producing such a shift producing no shift is obtained for c, d.

b, c, d of the four-divided light receiving portion 17a are amplified with the respective set gains, following which the tracking error signal and the focus error signal are produced from the amplified outputs for the error correction.

These error signals will be explained below using calculating equations. By replacing the term {), which indicates the four-divided average values, with X in the above expressions of the gains, light received outputs la, Ib, Ic, Id from the respective zones after the amplification in Fig. 13 are given as follows on an assumption that light received outputs from the zones a, b, c, d before the amplification are Ial, Ibl, Icl, Idl, respectively.

Ia = Ga.Ial. = MIal/IaC Ib = Gb.1bl = MIbl/IbO) Ic = Gc.Icl = MIcl/IcC Id = Gd.Idl = MIdl/IdO) Accordingly, the tracking error signal Te and the focus error signal Fe calculated by the error signal generating unit 104 are expressed below:

Te = (Ia + Id) - (Ib + Ic) = MIaMaO + Idl/IdO) - (Ibl/IbO + Icl/IcOM Fe = Ga + lc) - (Ib + Id) = XMaMaO + Icl/lcO) - (Ibl/IbO + Idl/IdOM Here, when neither tracking error nor focus error are - 45 caused, Ial = IaO, Ibl. = IbO, Icl. = IcO and Idl = IdO hold and, therefore, Te and Fe are both zero.

Let it now be assumed that a tracking error is caused to make the received light outputs of the right zones a, d as much as (1 + a) times the received light outputs of the left zones b, c. In this case, Te is given by:

Te = (Ia + Id) - (Ib + Ic) = MIal(l + a)/IaO + Idl(l. + a)/IdO) - (Ibl/IbO + Icl/lcOM = XE(GaMaO + IdUIdO) - (Ibl/IbO + Icl/IcM + MaMa01).a) + (IdUIdO).a)l Here, because of Ial = IaO, Ibl = IbO, lel = IcO and Idl IdO, Te is given by; Te = 2.X.a which represents an amount of the tracking error.

Likewise, the focus error signal Fe is calculated as follows:

Fe = (1a + Ic) - (Ib + Id) = X-a(l - 1) = 0 It is thus found that there occurs no crosstalk.

In the above-mentioned embodiment, the gains Ga to Gd of the amplifiers 101a to 101d in the output amplifying unit 101 shown in Fig. 13 are determined by the gain calculating unit 103 shown in Fig. 14 which calculates the ratios of the four-divided average value of the quantity of - 46 light received under a condition of detecting the mirror surface area to the quantity of light received by each of the four divided zones a. b, c, d under a condition of detecting the mirror surface area. However, the gains may be calculated by any other methods.

One alternative calculation method is explained below. Fig. 15 shows results of the simulation used to derive the following arithmetic operations.

In Fig. 15, the axis of abscissa represents a spot position on an optical disk in the tracking direction and the axis of ordinate represents detection rates, i.e., ratios of the outputs Ial, Ibl, Icl, Idl from the respective zones of the four-divided light receiving portion 17a to the total output (lal + Ibl + Icl + Idl) therefrom. Thus, in Fig. 15, the ratios of Ial, Ibl, Icl, Idl to the total output are indicated by IA, IB, IC, ID. The value of 0 (zero) on the axis of abscissas in Fig. 15 represents the spot position (a) in Fig. 18, i.e., the spot position in a state of no tracking error.

Fig. 15(A) indicates changes in the ratios IA, IB, ID resulted when the spot is deflected to the positions (b), (a) and (c) in Fig. 18 under a condition that the center of the received light spot S is coincident with the center of the light receiving portion 17a in Fig. 20. On the other hand, Fig. 15(B) indicates changes in the ratios IA, IB, IC, ID resulted when the spot is deflected to the positions (b), (a) and (c) in Fig. 18 under a condition that the amount of aposition shift 6 of the spot is 5 gm in the upward direction in Fig. 20 (i.e., in the tangential direction).

From comparison between Figs. 15(A) and 15(B), it will be found that there is correlation in changes of the outputs resulted from deflecting the spot in the tracking direction between the condition where the center of the received light spot S and the center of the light receiving portion 17a are coincident with each other in the fourdivided light receiving portion 17a shown in Fig. 20.and the condition where the position shift occurs in the tangential direction, and that the characteristic curves shown in Fig. 15(A) can be obtained by setting certain coefficients for IA, IB, IC, ID shown in Fig. 15(B).

In view of the above correlation, gains of the respective amplifiers in the output amplifying unit are set as follows:

Ga = (0.25/IaO) (0.5/(IaO + IdO)) Gb = (0.25/IbO) (0.5/(IbO + IcO)) Gc = (0.25/IcO) (0.5/(IbO + IcO)) Gd = (0.25/IdO) (0.5/(IaO + IdM Then, in Fig. 13, the outputs lal, Ibl, Icl, Idl from the four divided zones a, b, c, d of the light receiving portion 17a are amplified by the amplifiers 101a, 101b, 101c, 101d with the above set gains Ga, Gb, Ge, Gd, respectively. From the resultant outputs of; Ia = Ga Ib = Gb Ge Id = Gd lc = Ial Ibl Icl Idl the tracking error signal Te and the focus error signal Fe are calculated in a like manner to the above. As a result, in the case of no tracking error, both Te and Fe becomes zero. Meanwhile, when the quantities of light received by the zones a, d are as much as (1 + a) times the quantities of light received by the zones b, c, there hold:

Te = 0.25.a.W(IaO + H0) Fe = 0 It is thus found that no crosstalk to the focus error signal is caused when there occurs a tracking error.

It should be understood that the present invention is not limited to the above embodiment. For example, the means for detecting the mirror surface area may not rely-on the quantities of light received by the light receiving portions 17b and 17c. Alternatively, if the received light outputs from the four divided zones a, b, c, d of the light receiving portion 17a are all added to give a total output, this output is maximized in the mirror surface area and, R 49 therefore, the mirror surface detection signal can be produced from the total output. It is also practicable to compute a period of time from the time at which the spot passes the area to be addressed to the time at which the spot passes the mirror surface area, and produce the mirror surface detection signal by calculating the time at which the spot passes the mirror surface area.

According to the present invention of claim 1, even when there is present a position shift between a light receiving portion and a received light spot, such a position shift can be corrected at any desired time. Therefore, offsets not only in the tracking direction but also in the tangential direction can be corrected, and optical components require not to be assembled with a higher degree of accuracy than usual. Additionally, a position shift between the spot and the light receiving portion caused after completion of products can also be corrected at any desired time.

According to the present invention of claims 2 and 3, even when there is present a position shift between a light receiving portion and a received light spot, such a position shift can be electrically corrected at any desired time and the crosstalk of a tracking error signal to a focus error signal is prevented. Therefore, the correction of a focus error effected by moving an object lens can be - 50 accurately achieved.

1 K1631:JK

Claims (4)

1. An error detector for an optical disk apparatus in which detection light emitted from a light source and reflected by a disk is received by a light receiving portion divided into a plurality of zones, said error detector comprising: a mirror surface detecting unit responsive to received light outputs from the respective zones of said light receiving portion, to detect that said detection light has reached a mirror surface area of said disk, and then outputting a mirror surface detection signal; detection means responsive to said mirror surface detection signal and said received light outputs, to detect a fixed shift amount between light returned from a recording surface area of said disk and said light receiving portion in at least one of the tracking direction and the tangential direction, and correction means for applying a correction signal to correct said shift amount based on a detection output from said detection means.

2. An error detector for an optical disk apparatus according to claim 1, wherein said correction means comprises cancel signal generating means for generating a cancel signal based on the shift amount detected by said detection means, and crosstalk cancelling means for cancelling out a crosstalk component of a focus error signal based on said cancel signal.

3. An error detector for an optical disk apparatus according to claim 1, wherein said correction means comprises a gain calculating unit for calculating gains of said received light outputs based on said mirror surface detecting signal and said received light outputs; an output amplifying unit for producing amplified outputs of said received light outputs based on said gains and said received light outputs; and an error signal 1 K1631:= generating unit for generating error signals of said detection light based on said amplified outputs.

4. An error detector for an optical disk apparatus substantially as hereinbefore described with reference to, and as illustrated by, any of Figs. 1 to 15 of the accompanying drawings.

1