JP2733519B2 - Focus control method and device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention focuses a beam train composed of a plurality of beams on a storage medium with a single objective lens, and uses each of the beams in parallel. The present invention relates to a multi-beam optical head for recording and reproducing information, and more particularly, to a focus control method and a device for adjusting a distance of an objective lens from a storage medium surface.

[Conventional technology]

FIG. 2 is a block diagram showing an example of a focus control device to which a conventional focus control method of the multi-beam optical head 1 is applied. The focus control device 20 is composed of a beam separation optical system 3, a focus error detection device 21, and a differential amplifier 6. The focus of the multi-beam incident on the objective lens 14 of the optical head 1 from the surface 2 of the storage medium 8. Control the distance.

The optical head 1 includes a three-beam semiconductor laser 11, a collimator lens 12, a beam splitter 13, and an objective lens 14. The beam separation optical system 3 includes a condenser lens 15, a cylindrical lens 16,
The focus error detecting device 21 includes a four-divided photodetector 17 and adders 18 ₁ and 18 ₂ .

FIG. 2 shows only a part related to focus control, and omits a data detection optical system and the like.

The three beams emitted from the three-beam semiconductor laser 11 are collimated by a collimator lens 12, a beam splitter 13, and an objective lens 14.
And condensed on the surface 2 of the storage medium 8 to connect the spots. 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 detecting device 21. The focus error detection device 21 detects a focus error using only the central beam of the three beam trains, and when the focus of the objective lens 14 is shifted toward the front of the surface 2 of the storage medium 8 (to the objective lens 14 side). large becomes larger when the displacement of the first focus error signal becomes smaller when displaced rearward surface 2 to the first error signal output terminals 19 _1, also the rearward decreases when displaced forward The second focus error signal is output to a _second error signal output terminal 192. First error signal output terminal 19 ₁ and the second error signal output terminal 19 ₂ is connected to different input terminals of the differential amplifier 6, the output terminal of the differential amplifier 6 for driving the objective lens 14 It is connected to the coil of the focus actuator (the focus actuator and the coil are not shown). Now, objective lens
When 14 focus of shifts in front of the surface 2 of the storage medium 8, a first error signal output terminal 19 ₁ of the voltage increases, the second error signal output terminals 19 _{and second} voltage smaller, the differential amplifier 6
A positive current flows from the output terminal to the coil of the focus actuator, and the objective lens 14 is driven to approach the storage medium 8. In the opposite case, 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.

Conventional methods used in single-beam optical heads, such as an astigmatism method, a critical angle method, a knife edge method, and a Foucault method, are used for detecting a focus error. FIG. 2 shows the case of the astigmatism method as an example. The light paths of the three beams are separated by a beam separation optical system 3 including a condenser lens 15 and a cylindrical lens 16 and connect the spots. The quadrant photodetector 17 is arranged at the spot of the center beam as shown (strictly speaking, the plane of the quadrant photodetector is arranged perpendicular to the optical axis). Since the spot on the surface of the quadrant photodetector 17 is given an aberration by the cylindrical lens 16,
When the focal point of the objective lens 14 is on the surface 2 of the storage medium 8, the objective lens 14 has a circular shape. The two elements in the vertical direction on the drawing of the four-split photodetector 17 are connected to the adder 18 ₁ and the two elements in the horizontal direction are connected to the adder 18 _2, and the output of the adder 18 ₁ is output as the first error signal output. the terminal 19 _1, connects the output of the adder 18 ₂ to ₂ second error signal output terminal 19.

[Problems to be solved by the invention]

Since the above-mentioned conventional focus control method uses one beam at the center of the beam train, the focus of the beam coincides with the surface of the storage medium, but the curvature of field of the objective lens for the other beams is different. Therefore, the focal point is located before the surface of the storage medium, and the spot on the surface of the storage medium becomes large. As a result, the quality of the recording / reproducing signal decreases as the beam becomes closer to the end, and the number of beams that can be used for recording / reproduction is reduced. There was a disadvantage of being restricted.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned conventional disadvantages, improve the quality of a recording / reproducing signal of a multi-beam optical head, and increase the number of beams that can be used for recording / reproducing.

[Means for solving the problem]

According to the focus control method of the present invention, a beam train composed of a plurality of beams is condensed on a storage medium by a single objective lens, and an objective of a multi-beam optical head that records and reproduces information in parallel using each beam is provided. A focus control method for adjusting a distance of a lens from a storage medium, comprising separating an optical path of each of multiple beams emitted from the optical head, and selecting an arbitrarily selected first, second,... A first focus error signal that decreases as the position of the focal point of the beam from the objective lens shifts from the first side to the second side with respect to the reflecting surface of the storage medium as a reference plane, and .. Are generated for each of the first, second,..., N-th beams, as the position of the first focus shifts from the first side to the second side. Each of the error signal and the second focus error signal , The second,..., The N-th beam are added to generate first and second addition error signals, respectively, and the difference between the first and second addition error signals is generated. The distance of the objective lens from the storage medium is adjusted to be equal to

A focus control apparatus according to the present invention is directed to a multi-beam optical head that focuses a beam train composed of a plurality of beams on a storage medium with a single objective lens, and records and reproduces information in parallel using each beam. A focus control device for adjusting a distance of a lens from a storage medium, comprising: a beam separation optical system configured to separate optical paths of individual beams of a multi-beam emitted from the optical head; 2, a first focus error that decreases as the position of the focal point of the N-th beam by the objective lens shifts from the first side to the second side with the reflecting surface of the storage medium as a reference plane. And a second focus error signal that increases as the position of the focal point shifts from the first side to the second side.
Each of the first, second,..., Nth focus error detection devices generated for each Nth beam, and the first focus error signal and the second focus error signal generated by each focus error detection device To the first, second, ... Nth
A first and a second adder for adding first and second addition error signals by adding the first and second addition error signals, and a subtractor for generating a difference between the first and second addition error signals. An adjuster for adjusting the distance of the objective lens from the storage medium such that the difference is equal to a set value.

[Action]

In the focus control method of the present invention, the k-th (k = 1, 2,...)
When N) beam is incident on the objective lens, the surface of the recording medium (hereinafter, referred to as a reflecting surface) as a reference plane, the first, second focus error signal corresponding to the position of the focal point F _k of the k-beam each a _k, and B _k, when the focal point F _k is on the reflecting surface, the first, the second focus error signal _{^{▲ a 0 k ▼, ▲ B}} 0 k ▼ and (constant, respectively).

Now, when the focus _Fk is in the first example (for example, on the side of the objective lens with respect to the reference plane), Conversely, when the focus _Fk is on the second side, A _k −B _k <C _k (4) Therefore, the difference A _k −B _k between the first and second focus error signals is _represented by C _k
By comparing with, the position of the focal point _Fk with respect to the reflection surface can be known. Further, the first and second addition error signals S ₁ and S ₂ are as follows, respectively.

Therefore, if the focal points F _k (k = 1,
2,... N) are all on the first side, the difference ΔS between the first and second addition signals is become. (In the following It is written. If all the focal points F _k (k = 1, 2,..., N) are on the second side, then from equation (4), ΔS <ΣC _k (7). Therefore, when the position of the objective lens is adjusted so that ΔS = ΣC _k (8), N focal points F _k (k = 1 to N) are distributed on both sides of the reflection surface. Conversely, the reflection surface is located at an approximately average position of the N focal points F _k (k = 1 to N). A _k and B _k are expressed as functions of the position of the focal point F _k with respect to the reflecting surface by the optical constants of the optical system and the circuit constants of the electric circuit. Therefore,
The objective lens can be accurately positioned at the average position of the N focal points.

Also, for all beams k = 1 to N, ＡA ⁰ _k ▼
= ＢB ⁰ _k ▼ If the device is adjusted so that ΣC _k
= 0. Therefore, in this case, the position of the objective lens is adjusted so that ΔS = 0 (9), that is, ΣA _k = ΣB _k (10) from equation (8).

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

The optical paths of the arbitrarily selected N beams are separated by a beam separation optical system and input to the first to Nth focus error detection devices. Each of the focus error detection devices generates first and second focus error signals A _k and B _k (k = 1 to N). The first and second adders sum the first and second focus error signals for all of the first to Nth beams, respectively, to obtain the first and second adder error signals S ₁ = ΣA _k , S ₂ = Generate ΣB _k . The subtractor generates a difference ΔS = S ₁ −S ₂ between the first and second addition error signals S ₁ and S ₂ . The adjuster adjusts the position of the objective lens so that ΔS becomes equal to the set value ΣC _k .

In this way, the objective lens can be positioned such that the reflection surface is located at the average position of the focal point 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 a focus control method and apparatus according to the present invention.

In this embodiment, a three-beam optical head 1 is used in the same manner as the apparatus shown in FIG. 2, and the beam reflected by the storage medium 8 and again passed through the objective lens 14 is reflected at a right angle by the beam splitter 13 to be focused. The light enters the control device 10. In the present embodiment, of the three beams, one of the central beam (hereinafter, referred to as a central beam) and one of the beams at both ends (hereinafter, referred to as a central beam).
(Referred to as an end beam) is used for focus control of the objective lens 14.

Focus control device of this embodiment, the beam splitting optical system 3, the focus error detection device 4 _1, 4 _2, the adder 5 _1, 5 _2, differential amplifier (subtracter)
6, is constituted by a controller 7. The beam separation optical system 3 separates and converges the optical paths of the three beams constituting the incident beam. Focus error detection system 4 ₁ receives the central beam, the
1, a second focus error signal A ₁ , B ₁ (hereinafter, focus error signals A ₁ , B ₁
Is written). The focus error detection system 4 ₂ receives an end beam, first, second focus error signal A _2, B ₂ (hereinafter, referred to as a focus error signal A _2, B ₂₎ to produce a. In both of the focus error signals A ₁ and A ₂ , the focal points of the center beam and the end beam incident on the objective lens 14
The signal becomes smaller as it moves from the side (upper side) to the opposite side (lower side). Each of the focus error signals B ₁ and B ₂ is a signal that increases as the focus of the center beam and the end beam incident on the objective lens 14 moves from top to bottom in FIG. In the present embodiment, the first and second focus error signals ＡA ⁰ _k ，, generated by the focus error detection devices 4 ₁ and 4 ₂ when the focus of the center beam and the end beam is on the reflection surface 2. The optical system and the electric circuit are adjusted so that ＢB ⁰ _k （(k = 1, 2) becomes equal to each other. The adders 5 ₁ and 5 ₂ add the focus error signals A ₁ and A ₂ , and B ₁ and B ₂ , respectively, to generate addition error signals S ₁ and S ₂ . The differential amplifier (subtractor) 6 generates a difference ΔS between the addition error signals. The controller 7 moves the objective lens 14 so that ΔS becomes 0 (see equation (9)).

The beam separation optical system 3 of the present embodiment comprises a condenser lens 15 and a cylindrical lens 16 as in the apparatus shown in FIG.
Similarly to the focus error detection system 4 _1, 4 ₂ electrical circuit for detecting a focus error of the second view, respectively quadrant photodetector 40 _1,
Is composed of 40 ₂ and two adders 41 _1, 41 _2, 42 _1, 42 _2.
The adders 41 ₂ and 42 ₂ are cylindrical lenses of the quadrant photodetectors 40 ₁ and 40 ₂
Connected to photodetectors on a diagonal line parallel to the 16 buses to generate focus error signals B ₁ and B ₂ (second focus error signals);
The adders 41 ₁ and 42 ₁ are connected to the photodetectors on the other diagonal line (the diagonal line in the vertical direction in FIG. 1) of the quadrant photodetectors 40 ₁ and 40 ₂ , respectively. The focus error signals A ₁ and A ₂ (first focus error signals) are generated.

 Next, the operation of this embodiment will be described.

In a manner similar to that described in the apparatus of FIG. 2, of the beam reflected by the reflecting surface 2 of the storage medium, the central beam on the quadrant photodetector 40 _1, also either end beam 4 divided each a spot on the optical detector 40 _2. Now, the focal point of the center beam incident on the objective lens 14 is reflected on the reflection surface 2 of the storage medium.
When the overlying, the spot on the quadrant photodetector 40 ₁ becomes circular, while in front of the reflecting surface 2 of the storage medium for focus end beam image plane Curved aberration (objective lens 14 side) since the deviation becomes elliptic longitudinal (long perpendicular diagonal directions generatrix of the cylindrical lens 16) in the quadrant photodetector 40 on ₂ spots on the drawings. Accordingly, the adder 41 ₁ and 42 sum A ₁ of the _first voltage (first error detection signal) + A _2, i.e. adder 5 ₁ of the output voltage (first addition error signal S ₁₎ adder 41 ₂
And the adder 42 sums B ₁ of ₂ voltage (second error detection signal)
+ B _2, i.e. greater than the adder 5 _second output voltage (second addition error signal S _2), from the output terminal of the differential amplifier 6 coils focus actuator (controller) (not shown) to the + of A current flows and the objective lens 14 is driven so as to approach the storage medium 8. Opposite to the above-described, the focal end beam incident on the objective lens 14 is on the reflecting surface of the storage medium 8, 4 spots on photodetector 40 ₂ are rounded, whereas, enters the objective lens 14 since the focus of the central beam is shifted backward from the reflection surface 2 of the storage medium 8 for image plane Curved aberration, 4 spots on photodetector 40 ₁ becomes horizontally long ellipse in the drawing. Therefore, adder 5 ₁ voltage (first addition error signal S ₁₎ is smaller than the adder 5 _second voltage (second addition error signal S _2), the focus actuator (adjusting the output terminal of the differential amplifier 6 Current flows through the coil of the device, and the objective lens 14 is driven to move away from the storage medium 8.
As described above, the objective lens 14 is positioned and controlled so that the distance of the objective lens 14 from the surface of the storage medium 8 is substantially equal to the average value of the focal length of the center beam and the focal length of the end beam.

In the embodiment of FIG. 1, a three-beam semiconductor laser is used and the number of beams is three. However, the light source is not limited to the semiconductor laser, and the number of beams is not limited to three. . When the number of beams is even, the two beams at the center have the same condition, so any one beam is represented as the center beam. Although the case where the astigmatism method is used for focus error detection has been described, the focus position can be similarly controlled by using a critical angle method, a knife edge method, a Foucault method, or the like.

〔The invention's effect〕

As described above, according to the present invention, the position of the focal point of the first, second,..., N-th beam arbitrarily selected from the plurality of beams of the plurality of beam optical heads is determined by using the reflection surface of the storage medium as the reference surface. A first focus error signal that decreases as the position shifts from the side to the second side, and the position of the focus is changed to the first position.
.. Are generated for each of the first, second,..., N-th beams, and the first focus error signal and the second focus are increased. By adding each of the error signals for all of the first, second,..., N-th beams, and adjusting the position of the objective lens so that the difference between the respective addition results becomes equal to a set value, Deterioration of the recording / reproducing signal due to the enlargement of the beam spot can be suppressed, and the number of beams usable for recording / reproducing can be increased when the same objective lens is used.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of a focus control method and apparatus according to the present invention, and FIG. 2 is a block diagram showing an example of a focus control apparatus to which a conventional focus control method of a multi-beam optical head is applied. 1 ...... optical head, 2 ...... reflecting surface (the surface of the storage medium 8), 3 ...... beam splitting optical system, 4 _1, 4 ₂ ...... focus error detection system, 5 _1, 5 _2, 41 _1, 41 ₂ , 42 ₁ , 42 ₂ ... adder, 6 ... differential amplifier (subtractor), 7 ... adjuster, 8 ... storage medium, 10 ... focus control device, 11 ... 3 beam semiconductor laser, 12 …… Collimator lens, 13 …… Beam splitter, 14 …… Objective lens, 15 …… Condenser lens, 16 …… Cylindrical lens, 40 ₁ , 40 ₂ … 4 split photo detector, A ₁ , A ₂ … ... First focus error signal, B ₁ , B ₂ ... Second focus error signal, S ₁ , S ₂ ... Addition error signal, ΔS... Difference between addition error signals S ₁ , S ₂ .

Claims (2)

(57) [Claims] 1. An objective lens of a multi-beam optical head for condensing a beam train composed of a plurality of beams on a storage medium with one objective lens and recording and reproducing information in parallel using each beam. In a focus control method for adjusting a distance from a medium, the optical paths of individual beams of a multi-beam emitted from the optical head are separated, and the objectives of arbitrarily selected first, second,. A first focus error signal that decreases as the position of the focal point by the lens shifts from the first side to the second side with respect to the reflection surface of the storage medium as a reference plane; The second focus error signal, which increases as it shifts from the side to the second side, becomes the first, second,.
, And the first focus error signal and the second focus error signal are added for all of the first, second,. Generating an error signal; generating a difference between the first and second addition error signals; and adjusting a distance of the objective lens from the storage medium so that the difference becomes equal to a set value. Control method. 2. An objective lens of a multi-beam optical head for condensing a beam train composed of a plurality of beams on a storage medium with one objective lens and recording and reproducing information in parallel using each beam. A focus control device for adjusting a distance from a medium; a beam separation optical system for separating optical paths of individual beams of the multi-beams emitted from the optical head; arbitrarily selected first, second,... A first focus error signal that decreases as the position of the focal point of the beam from the objective lens shifts from the first side to the second side with respect to the reflecting surface of the storage medium as a reference plane, and Are generated for each of the first, second,..., N-th beams, and the second focus error signal increases as the position of the first position shifts from the first side to the second side. 2, ..., Nth focus error detection device, Each of the first focus error signal and the second focus error signal generated by the focus error detection device is added to the first, second,. A first and a second adder respectively for generating an addition error signal; a subtractor for generating a difference between the first and second addition error signals; and an objective lens so that the difference becomes equal to a set value. A focus control device comprising an adjuster for adjusting a distance from a storage medium.