JPH02294936A - Method and device for controlling focal point - Google Patents

Abstract

PURPOSE:To improve quality for the recording and regenerative signal of a multi-beam optical head and to increase the number of beams used for recording and reproducing by generating a focus error signal by using all the beams of a beam train. CONSTITUTION:Out of the beams reflected on the reflecting surface 2 of a recording medium 8, the central beam forms a spot on a quadripartite optical detector 401 and any one end beam forms a spot on a quadripartite optical detector 402 respectively. When the focal point of the central beam incident on an objective lens 14 is on the reflecting surface 2, the spot on the detector 401 is made circular and the spot on the detector 402 is made elliptical. Accordingly, the output voltage (first addition error signal) of an adder 51 is larger than the output voltage (second addition error signal) of an adder 52. Thus, the objective lens 14 is driven to be closed to the recording medium 8 by the output of a differential amplifier 6. Even in a reverse case, operation is samely executed and the objective lens is driven to be distant from the recording medium 8.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention focuses a beam train consisting of a plurality of beams onto a storage medium using one objective lens, and uses each of the beams to perform parallel processing. The present invention relates to a multi-beam optical head for recording and reproducing information, and more particularly to a focus control method and apparatus for adjusting the distance of an objective lens from a storage medium surface.

[Prior Art] FIG. 2 is a block diagram showing an example of a focus control device to which a conventional focus control method for a multi-beam optical head 1 is applied. This focus control device 20 is composed of a beam separation optical system 3, a focus error detection device 21, and a differential amplifier 6, and controls the focus of the multi-beam incident on the objective lens l4 of the optical head from the surface 2 of the storage medium 8. Control distance.

The optical head 1 includes a 3-beam semiconductor laser 11 and a collimator 1.
- Consists of a lens 12, a beam splitter 13, and an objective lens 14, and the beam separation optical system 3 includes a condenser lens 15,
It is composed of a cylindrical lens 16, and a focus error detection device 2
1 is composed of a 4-split photodetector 17 and an adder 181182.

FIG. 2 shows only the parts related to focus control, and the data detection optical system and the like are omitted.

The three beams emitted from the three-beam semiconductor laser 11 are transmitted through collimator 1, lens 12, and beam splitter? 3. The light passes through the objective lens 14 and is focused onto the surface 2 of the storage medium 8, connecting the spots 1 and 1, respectively. Each beam reflected by the storage medium 8 passes through the objective lens 14 again, is reflected at a right angle by the beam splitter 13, and is guided to the focus error detection device 21. When the focus error detection device 21 detects a focus error using only the central beam of the three beam arrays, and the focus of the objective lens 14 shifts to the front of the surface 2 of the storage medium 8 (toward the objective lens 14 side). A first focus error signal that becomes larger when the plane 2 shifts backward and becomes smaller when it shifts toward the rear of the surface 2 is sent to the first error signal output terminal 19, and a first focus error signal that becomes smaller when it shifts toward the front and becomes larger when it shifts backwards is sent to the first error signal output terminal 19. The second focus error signal is output to the second error signal output terminal 192. The first error signal output terminal IL and the second error signal output terminal 19■ are connected to different manual terminals of a differential amplifier 6, and the output terminal of the differential amplifier 6 is used as a focal point for driving the objective lens 14. (Focus actuator and coil not shown). Now, if the focus of the objective lens l4 shifts to the front of the surface 2 of the storage medium 8, the first error signal? The voltage at the output terminal 191 becomes large, the voltage at the second error signal output terminal l9 becomes small, a positive current flows from the output terminal of the differential amplifier 6 to the coil of the focus actuator, and the objective lens l4 is memorized. It is driven to approach the medium 8.

In the case opposite to the above, the operation is reversed, and the focal point of the objective lens 14 is controlled so as to always be located on the surface 2 of the storage medium 8.

The astigmatism method, critical angle method, and knife edge method are used to detect focus errors. Conventional methods used in single-beam optical heads, such as the Foucault method, are used. In the case of FIG. 2, the astigmatism method is shown as an example. The three beams pass through a condensing lens 15. The optical paths are separated by a beam separation optical system 3 consisting of a cylindrical lens, and the respective spots are connected. A 4-split photodetector 17 is placed in the spot portion of the central beam as shown in the figure (strictly speaking, the surface of the 4-split photodetector is placed perpendicular to the optical axis). The spot on the surface of the 4-split photodetector 17 is aberrated by the cylindrical lens l6, so if the focus of the objective lens 14 is on the surface 2'' of the storage medium 8, it will be circular, front or rear? If so, it will be a vertically long circle or a horizontally long circle on the drawing. 4
The two vertical elements of the split photodetector 17 in the drawing are connected to the adder 18, and the two horizontal elements are connected to the adder 18■, and the output of the adder 181 is connected to the first error signal output terminal 19. , to,
The output of adder 182 is connected to second error signal output terminal 192.

[Problems to be Solved by the Invention] The conventional focus control method described above uses one beam at the center of the beam array, so although the beam is focused on the storage medium surface, other beams are As for the image distortion of the objective lens, the focal point is located in front of the storage medium surface, and the spot on the storage medium surface becomes larger.As a result, the quality of the recorded and reproduced signal decreases as the beam approaches the edges. However, the disadvantage was that the number of beams that could be used for recording and reproduction was limited.

An object of the present invention is to solve the above-mentioned conventional drawbacks, improve the quality of recording and reproduction signals of a multi-beam optical head, and increase the number of beams that can be used for recording and reproduction.

[Means for Solving the Problems] The focus control method of the present invention focuses a beam array consisting of a plurality of beams onto a storage medium using one objective lens, and records information in parallel using each beam. A focus control method for adjusting the distance of an objective lens of a multi-beam light head (1) for reproduction from a storage medium, the method comprising separating the optical paths of individual beams of the multi-beams emitted from the optical head and arbitrarily selecting one. of the first, second, ... Nth beams,
a first focus error signal that decreases as the focus position of the objective lens shifts from the first side to the second side with the reflective surface of the storage medium as a reference plane; A second focus error signal that increases as the focus shifts from the first side to the second side is used as the second focus error signal.
The first focus error signal and the second focus error signal are generated for each of the second, second, ... Nth beams, and each of the first focus error signal and the second focus error signal is generated for each of the first, second, ... Nth beams. generate first and second addition error signals, respectively, generate a difference between the first and second addition error signals, and store the storage medium of the objective lens so that the difference is equal to a set value. Adjust the distance from.

The focus control device of the present invention is a multi-beam optical head that focuses a beam array consisting of a plurality of beams onto a storage medium using one objective lens, and records and reproduces information in parallel using each beam. A focus control device for adjusting the distance of an objective lens from a storage medium, the device comprising: a beam separation optical system for separating the optical paths of individual beams of the multi-beam emitted from the optical head; an arbitrarily selected first beam separation optical system; of the second,...Nth beam,
A first focus error signal that decreases as the focus position of the objective lens shifts from the first side to the second side using the reflective surface of the storage medium as a reference plane; The second focus error signal, which becomes sharper as the focus shifts from the first side to the second side, is
2nd, . . . generated for each Nth beam, respectively.
The second..., Nth focus error detection devices and the first focus error signal and the second focus error signal generated by the respective focus error detection devices are transmitted to the first, second..., Nth focus error detection devices.・・・
・For the Nth focus error detection device, the first and second
The first . a second adder; a subtracter that generates a difference between the first and second addition error signals; and an adjuster that adjusts the distance of the objective lens from the storage medium so that the difference is equal to a set value. has.

[Operation] In the focus control method of the present invention, the k-th (k1, 2,...
・N) First and second focus error signals corresponding to the position of the focal point Fk of the k-th beam when the beam is incident on the objective lens, with the surface of the recording medium (hereinafter referred to as the reflective surface) as the reference plane. are respectively A, , Bk, and when the focal point Fk is on the reflective surface, the first and second focus error signals are A: . B: (each constant).

Now, when the focal point F is on the first side (for example, the objective lens side with respect to the reference plane), Δ1>Al
(])B*<B:
(2). '． Ay. Bk>A:
Bro = C K = constant (3) Conversely, if the focal point Fk is on the second side, then A. B
b<Ck (4) Therefore, the first and second
By comparing the difference AkBk between the focus error signals with GK, it is possible to know the position of the focal point F with respect to the reflecting surface. Further, the first and second addition error signals S. ,S
2 are as follows:

Therefore, if the focus Fk (k=1
．． 2. ...N) is on the first side, the first side. The difference of the second addition signal to 8 is expressed by equation (3). (
In the following, it is written as Σ=Σ. ) Also, the focal point Fk (k=1.2,...N
) is on the second side, then from equation (4), △S Σ
It becomes C K (7). Therefore, ΔS; ΣC. If you adjust the position of the objective lens so that it becomes (a), N focal points Fk (k
=1 to N) are distributed on both sides of the reflective surface. In other words, the reflecting surface is located at approximately the average position of the N focal points F, (k-1 to N). Ak and Bk are expressed as functions of the position of the focal point F with respect to the reflecting surface using optical constants of the optical system and circuit constants of the electric circuit. therefore,
The objective lens can be precisely positioned at the average position of N focal points.

Also, for all beams k=1 to N, AS=B−
If the device is adjusted so that ΣC5-0 is obtained. Therefore, in this case, from equation (8), ΔS=O' (9) or ΣA
k-ΣB. Adjust the position of the objective lens so that it becomes (10).

Next, the focus control device of the present invention is a device to which the above focus control method is applied.

The optical paths of N arbitrarily selected beams are separated by a beam separation optical system, and each of the N beams is inputted to the first to Nth focus error detection devices. Each focus error detection device outputs first and second focus error signals Ak. Bk (k=1~N
) is generated. The first and second adders sum up the first and second focus error signals for all of the first to N beams, respectively, to obtain first and second added error signals S+=ΣAk,S.

Generate ΣBk. The subtractor generates the difference between the first and second added error signals S, , S2, △S '' S + S 2.The adjuster adjusts the position of the objective lens so that S becomes equal to the set value ΣC, Adjust.

In this way, the objective lens can be positioned so that the reflective surface is at the average position of the focus of each beam.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the focus control method and apparatus of the present invention.

In this embodiment, a three-beam optical head 1 is used as in the apparatus shown in FIG. The light is input to the control device 10. In addition, in this embodiment, among the three beams, the central beam (
One of the beams at both ends (hereinafter referred to as the end beam) is selected and used to control the focus of the objective lens l4.

The focus control device of this embodiment includes a beam separation optical system 3, a focus error detection device 4, 4, and an adder 5. ．． It is composed of a differential amplifier (subtractor) 6 and an adjuster 7. The beam separation optical system 3 separates and converges the optical paths of three beams constituting the incident beam. The focus error detection device 41 receives the central beam and generates first and second focus error signals AI and Bl (hereinafter referred to as focus error signals A+.B1). Focus error detection device 4. The first and second focus error signals A2, B2 (hereinafter referred to as focus error signals A2.
82) is generated. Focus error signals A, . A2 is the focal point of the center beam and end beam incident on the objective lens 14, or the objective lens l with the reflecting surface 2 as a reference plane.
This is a signal that becomes smaller as it moves from the 4 side (upper side) to the opposite side (lower side).Focus error signal BB2
Both are signals that increase as the focus of the center beam and end beam incident on the objective lens l4 moves from top to bottom in FIG.

In addition, in this embodiment, when the focal points of the center beam and the end beams are on the reflecting surface 2'', the first and second focal points are generated by the focus error detection devices 41 and 42. Difference signal A: ,
The optical system and electric circuit are adjusted so that B:ac=1.2) are equal to each other. The adder 51.5■ adds the focus error signals A1 and A2, Bl and B2, respectively, and outputs the added error signals S+ . Generate S2. A differential amplifier (subtractor) 6 generates a difference ΔS between the addition error signals. The adjuster 7 moves the objective lens 14 so that ΔS becomes O (see equation (9)).

The beam separation optical system 3 of this embodiment is composed of a condenser lens 15 and a cylindrical lens 16, similar to the apparatus shown in FIG. Also, the focus error detection devices 41, 42 are each divided into four photodetectors 40, . 402 and two adders 41■. 41 ■, 42
■, 422. Adders 412, 422
is a 4-split photodetector 40. , 40. , cylindrical lens 1
6 are connected to diagonal photodetectors parallel to the generatrix of B, . B2 (second focus error signal);
, 40 ■, which is perpendicular to the diagonal? two diagonals (
In FIG. 1, each of the two photodetectors (lower diagonal line) is connected to a focus error signal A+. A2 (1st
(focus error signal).

Next, the operation of this embodiment will be explained.

Similarly to the apparatus shown in FIG. 2, among the beams reflected by the reflective surface 2 of the storage medium, the central beam is on the 4-split photodetector 401, and either end beam is on the 4-split photodetector 401, and one of the end beams is on the 4-split photodetector 401 Tie each subbot 1 on the detector 40■. Now, when the focus of the central beam incident on the objective lens l4 is on the reflective surface 2 of the storage medium, the spot on the 4-split photodetector 40 becomes circular, while the focus of the end beam is on the image plane W. Due to curvature aberration, the distance from the reflective surface 2 of the storage medium (objective lens 14
Since the second spot of the four-split photodetector 40 is shifted to the side), the second spot of the four-split photodetector 40 becomes a vertically elongated ellipse (long in the direction of the diagonal line perpendicular to the generatrix of the cylindrical lens 16) in the drawing. Therefore, the sum of the voltages (first error detection signal) of adders 41 and 42 (first error detection signal) Δl+A2, that is, the output voltage of adder 51 (first addition error signal S1) is added? device 41■ and adder 42.

The sum of the voltages (second error detection signal) BI+B2, that is, the output voltage of the adder 5 (second addition error signal S2), is greater than the voltage output from the output terminal of the differential amplifier 6 to the focus actuator (adjustment A current of 100 Ω flows through a coil (not shown) of the device, and the objective lenses 1 and 4 are driven to approach the storage medium 8. Contrary to the above, if the focal point of the off-edge beam incident on the objective lens l4 is on the reflective surface of the storage medium 8, the spot on the four-split photodetector 402 will be circular; Since the focus of the beam is shifted backward from the reflecting surface 2 of the storage medium 8 due to field curvature aberration, the spot on the 4-split photodetector 40 becomes a horizontally long ellipse in the drawing. Therefore, the voltage of the adder 51 (the first
The addition error signal S+) becomes smaller than the voltage of the adder 5 (second addition error signal S2), and a current force of ?1 is applied from the output terminal of the differential amplifier 6 to the coil of the focus actuator (adjuster). As a result, the objective lens 14 is driven away from the storage medium 8. As described above, the positioning of the objective lens 14 is controlled so that the distance of the objective lens 14 from the surface of the storage medium 8 is approximately equal to the average value of the focal length of the center beam and the focal length of the end beams.

In the embodiment shown in FIG. 1, a three-beam semiconductor laser is used, and the number of beams used is three, but the light source is not limited to a semiconductor laser, and the number of beams is not limited to three. . If the number of beams is even, the two beams at the center have the same conditions, so one of the beams is represented as the center beam. Although we have shown the case where the astigmatism method is used to detect focus errors, other methods include the critical angle method and the knife edge method. The focal position can be similarly controlled using the Foucault method or the like.

[Effect of the Invention 1] As explained above, the present invention provides the first, second, ... Nth beams arbitrarily selected from the plurality of beams of the multi-beam optical head
a first focus error signal that decreases as the position of the focal point of the beam shifts from the first side to the second side using the irregular surface of the storage medium as a reference plane; A second focus error signal that increases as the beam shifts from the side to the second side is generated for each of the first, second, . . . , for all of the first, second, ... Nth beams, and adjust the position of the objective lens so that the difference between the respective addition results is equal to the set value. As a result, it is possible to suppress the deterioration of the recording/reproducing signal due to the enlargement of the end beam spot, and
This has the effect of increasing the number of beams that can be used for recording and reproduction when the same objective lens is used.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the focus control method and device thereof according to the present invention, and FIG. 2 is a block diagram showing an example of a focus control device to which a conventional focus control method for a multi-beam optical head is applied. 1 optical head, 2 reflective surface (surface of storage medium 8), 3 beam separation optical system, 41. 42 Focus error detection device,] 9? ■． 4+,. 41. , 42+. 42. Adder, differential amplifier (subtractor), adjuster, storage medium, focus control device, 3-beam semiconductor laser, collimator lens, beam splitter, objective lens, condensing lens, cylindrical lens, 4024-split photodetector , A2 first focus error signal, B2 second focus error signal, S2 added error signal, difference between added error signals St and S2.

Claims (1)

[Claims] 1. Objective of a multi-beam optical head that focuses a beam array consisting of a plurality of beams onto a storage medium using one objective lens, and records and reproduces information in parallel using each beam. In a focus control method for adjusting the distance of a lens from a storage medium, the optical path of each of the multi-beams emitted from the optical head is separated, and arbitrarily selected first, second, ... Nth beams are separated. of the beam of
a first focus error signal that decreases as the focus position of the objective lens shifts from the first side to the second side with the reflective surface of the storage medium as a reference plane; The second focus error signal, which increases as the focus shifts from the first side to the second side, is
generated for each of the second, ... Nth beams, and add each of the first focus error signal and the second focus error signal for all of the first, second, ...Nth beams. generates first and second addition error signals, respectively, generates a difference between the first and second addition error signals, and outputs data from the storage medium of the objective lens such that the difference is equal to a set value. A focus control method characterized by adjusting distance. 2. A beam train consisting of a plurality of beams is focused onto a storage medium by one objective lens, and each beam is used to record and reproduce information in parallel. A focus control device that adjusts the distance includes: a beam separation optical system that separates the optical paths of individual beams of the multi-beams emitted from the optical head; and arbitrarily selected first, second, ... Nth beams. of the beam,
A first focus error signal that decreases as the position of the focus by the objective lens shifts from the first side to the second side using the reflective surface of the storage medium as a reference plane; The second focus error signal, which increases as the focus shifts from the first side to the second side, is
The respective first beams generated for each second,..., Nth beam are
, second, . . . , Nth focus error detection device;・
・For the Nth focus error detection device, the first and second
a subtracter that generates a difference between the first and second addition error signals; and an objective lens that generates an addition error signal such that the difference is equal to a set value. 1. A focus control device comprising an adjuster for adjusting a distance from a storage medium.