JPS5914136A - Focus control optical system of optical disk - Google Patents

Abstract

PURPOSE:To produce astigmatism without setting an astigmatism generating optical system into an image forming optical system, by providing an optical element having rotary symmetry between a polarized beam splitter and a photodetecting element with an inclination set to the optical axis. CONSTITUTION:The dispersed luminous flux delivered from a semiconductor laser 1 are converted into parallel luminous fluxes through a collimator lens and then condensed on an optical disk D through a polarized beam splitter 3, a lambda/4 plate 4 and a focus lens 5. This reflected light goes backward and is reflected by a polarizing surface 3' of the beam splitter 3. This reflected light is focused with astigmatism on a 4-split photodetecting element 8 through an astigmatism generating lens 7 to obtain a focus signal. Thus it is possible to produce astigmatism without using an astigmatism optical system like a cylindrical lens, etc.

Description

[Detailed Description of the Invention] Technical Field The present invention relates to the configuration of an optical system used for focus control of optical discs such as digital digital discs and video discs, particularly focus control using the astigmatism method. It is related to.

Prior Art Conventionally, several methods have been proposed as force control methods for optical dinuks in this building, such as the non-Fish aberration method, the critical angle method, and the Naifetsu method. Although it has its advantages and disadvantages, the astigmatism method is currently most widely used.

In the above-mentioned astigmatism method 2, the reflected light from the optical dinuque is polarized and separated by a polarizing beam nuplitter installed in the middle of the imaging optical system that focuses the laser beam onto the optical dinucu, and then the reflected light from the optical dinucu is separated by a cylindrical force lens. The image formation gap and astigmatism are caused to occur on the light receiving element. In this way, the cylindrical lens used to generate astigmatism is not suitable for M production like a spherical lens due to the force of the lens, and even if it is difficult to measure the shape after processing, the Newtonian A simple measurement method has not yet been established.

Japanese Unexamined Patent Publication No. 54-39101 discloses a system using a point-symmetric optical element such as a spherical lens in place of a cylindrical lens. However, in this method, an astigmatism generating system is incorporated into the optical system for forming an image on the optical disk, and astigmatism is generated in the image forming spot on the optical disc. This causes problems with the image formation performance on Dinuku.

In other words, the imaging performance of this type of optical dinuque is required to have wavefront aberration of λ/4 or less, but generating astigmatism as described above has a negative impact on the imaging performance. This has an adverse effect on the appearance, and the necessary imaging performance cannot be effectively ensured.

Purpose of the Invention The present invention was made in view of the conventional problems, and it is possible to achieve an optical system without using a cylindrical lens and without incorporating an optical system for generating astigmatism into an imaging optical system. The basic objective is to provide a focus control optical system 2 for an optical disc that can generate necessary astigmatism on a light receiving element.

SUMMARY OF THE INVENTION To achieve the above object, this invention uses laser light from a laser light source. A polarized beam beam exists in the imaging optical system that forms an image on an optical disk.

A rotationally symmetrical optical element is placed between the liter and the photodetector, tilted with respect to the optical axis, and the reflected light from the original disk, whose polarization has been separated by the polarizing beam sinter, is generated on the photodetector with astigmatism. The image is formed in this state.

Various rotationally symmetrical optical elements can be used to generate astigmatism, but the collimating lens used in the imaging optical system must be one lens at a time. Since they can be shared, the laziness effect can be increased.

Furthermore, in the case where the rotationally symmetrical optical element for generating astigmatism and the collimating lens are shared, it is preferable to use an aspherical plastic lens.

Plastic lenses are easy to manufacture, but as is well known, there is a big problem with focal length fluctuations due to temperature changes.

However, when both are used together, the collimating lens and the rotationally symmetrical optical element for generating astigmatism can be used.
So, is it possible that the focal length of the plastic lens changes due to temperature changes? It can be put to good use.

Embodiments Hereinafter, embodiments of the invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows an embodiment of the invention.

Semiconductor laser as a laser light source1. The divergent light beam (wavelength around 800 nm) emitted from the lens enters the collimating lens 2, where it is converted into a parallel light beam. The polarizing beam nullifier 3 placed next to the collimating lens 2 is
One that transmits most of the polarized light component (P polarized light wave) within the plane of the paper and reflects the polarized light component (S polarized light wave) perpendicular to the plane of the paper 'T:
Therefore, only the P-polarized light wave passes through the polarization beam splitter 3. The transmitted P-polarized light wave is converted into circularly polarized light by the quarter-wave plate 4, and then enters the focus lens 5.
The focus lens 5 forms an image as a spot on the optical disc 71). The spot focused on the optical disc D is reflected by the A1 reflective film of the optical disc D and returns to the focus lens 5 again. In this case, the circularly polarized light becomes circularly polarized light of opposite rotation when viewed from the direction of travel.

1. When the focus lens 5 is controlled to the correct focusing position with respect to the optical dinuque D by the tracking and focus control servo mechanism 6 that drives and controls the focus lens 5, the above reflected light flux is By passing through the focus lens 5, it becomes a parallel beam of light again,
The light is incident on the 1/4 wavelength plate 4. The light flux incident on this 1/4 wavelength plate 4 is reversed so that the direction of the circularly polarized light reaches n14, so the light flux transmitted through the 1/4 wavelength plate 4 is perpendicular to the plane of the paper ft, 5i) F/+ light wave It has become. Therefore, most of the light is reflected toward the light receiving element 8 by the polarizing beam splitter 3 or its polarizing plane 3, and astigmatism occurs when the polarizing beam splitter 3 and the light receiving element 8 are arranged at an angle. The light is focused onto the light receiving element 8 by the optical lens 7 .

Firstly, the light rays in the plane of the paper that have passed through the astigmatism generating lens 7 are shown as solid lines, and the rays that are perpendicular to the plane of the paper are shown as dotted lines. In the light-receiving element 8, an image is formed as a nuvotte based on astigmatism.

This Nupot is shown in Figure 2 (al, (b), (C)
As shown in each figure, the optical diode D of the focus lens 5 is
(a) indicates the so-called rear focus position, (bl indicates the in-focus position, and (C1 indicates the so-called front focus position). By detecting all such changes in the focus position, ,] In the unlikely event that the Earl of Kanu f'a [that happens].

That is, as shown in the third figure, four divided cells A, B, "'C, and D forming the light receiving element 8 are divided cells A and C arranged in the vertical direction and divided cells B arranged in the horizontal direction. .

D, and the difference between the output sums of each set ([A+C)-(B+
I))) When detecting k, a positive difference signal is obtained at the rear focus, zero when in focus, and a negative difference signal at the front focus. Therefore, by inputting this differential signal to the servo mechanism 6 as a focus signal, the focusing lens 5 can be controlled.

FIG. 4 shows one embodiment of the present invention. In this embodiment, a polarizing beam splitter 13 is arranged in a diverging beam between a semiconductor laser 11 and a collimating lens 12. In addition, a flat lens 17 is used as an astigmatism generating lens. Note that 14 is a quarter wavelength plate, 15 is a focus lens, and 16 is a servo mechanism thereof, and an xsH light receiving element.

In this case, since the convergence 1i of the collimating lens 12 can be used, the flat lens 17 can be used as a rotationally symmetrical optical element for generating astigmatism.

Next, referring to Example 2 shown in FIG. 1, the structure of a lens used for generating astigmatism will be described.

This astigmatic Ill difference generating lens 7 can be used in common with the aspherical planar lens and collimating lens 2 having the following configuration.

However, the inside indicates an aspheric surface, and the aspheric coefficient is ε−
1, A1=O, A2=0.27045X10-3A
−0,12207X10−5 P, the aspheric coefficient is the tangential plane blade
If X is the distance at a distance of 1 at a tangential plane of 1° at the incident height Y, then it is defined by the following general formula for the aspherical surface.

The spherical aberration and sine conditions of this aspherical plastic lens are shown in FIG. Also, what about the occurrence of astigmatism when this aspherical plastic lens is tilted with respect to the optical axis? It is shown in FIG.

As is clear from Fig. 5), this aspherical plastic lens satisfies the sine condition, so even when it is tilted with respect to the optical axis, the image formation state on the light receiving element 8 is As shown in Section 2 (2), a good state of one balanne can be obtained corresponding to the focusing state of the focus lens 5. Therefore, a focus signal with excellent responsiveness can be obtained.

Referring to FIG. 6, when the aspherical lens is arranged at an angle of θ with respect to the perpendicular to the parallel light beam reflected from the beam splitter 3, the parallel light beam becomes convergent light.

Here, if the distance between the imaging point P of the ray in the plane of the paper and the paraxial imaging point is AT, and the distance between the imaging point Q of the ray perpendicular to the plane of the paper and the paraxial imaging point is As, then , the distance A・r, A
s changes continuously as a function of the tilt angle O as shown in the graph. Therefore, an arbitrary astigmatism shape can be obtained depending on the inclination angle θ, and the focus control optical system can be easily adjusted by adjusting the inclination angle θ.

The above-mentioned inclination angle θ may be set as follows.

The focus signal obtained in the above-mentioned non-breath aberration method generally exhibits an output characteristic as shown in FIG.
Peaks A and B are shown before and after α.

These peaks A and B are obtained when the astigmatism image becomes linear on the light receiving element 8. When the focus signal is between peaks A and B, the focus lens 5 can be servo-driven to the focus point 70, that is, it can be retracted. As is clear from the above, the positions +α of peaks A and B are correlated with the tilt angle θ, and by setting this peak position α to a preferable value, the tilt angle θ
It will be possible to determine.

In this regard, in the past, the curvature of the cylindrical lens? Once determined, the pull-in range α is fixed, so if α was to be changed, it was necessary to redesign and re-prototype. In contrast, in the present invention, it is possible to easily change the pull-in range α by simply changing the above-mentioned inclination angle θ.

Here, assuming that the focus M f of the focus lens 5 is the focus f of the astigmatism generating lens 7, the retraction range β of the astigmatism generation lens 7 corresponding to the retraction range 1α of the focus lens 5 is, be. Therefore, the astigmatism AT
, A The inclination angle θ may be selected so that the difference is 5.

More specifically, the above-mentioned aspherical plastic lens (f = 14.468) is used as the astigmatism generating lens 7,
When using a lens with a focal length of 4.5 aperture as the focus lens 5, in order to set α to about +30μ, AT-As=0.62rrrta, and from FIG. 6, the tilt angle θ that satisfies this value is approximately 13
It turns out that it is sufficient to use °.

As the adjustment mechanism for the inclination angle θ, for example, the adjustment mechanism shown in FIG. 8 can be used.

That is, the astigmatism-generating lens 7 is fitted into the lens holder 30 and fixed by the set screw 31! ll fixed. The lens holder 30 is provided with shafts 32, 32' that protrude on both sides in the diametrical direction, and these shafts 32, 32' are rotatably supported on both walls of the adjustment holder 33.
ing. A coil spring 34 is hooked between one shaft 32 and an adjustment holder 33, and the lens holder 30 is biased around the hour hand at the 81st stroke. The lower end of this lens holder 30 is brought into contact with the tip of an adjustment bolt 35 screwed into a screw hole provided in the vertical wall of the adjustment holder 33, and by adjusting this adjustment bolt 35, the The angle of inclination θ of the point aberration generating lens 7 may be adjusted.

Furthermore, referring to temperature changes in plastic lenses, as is well known, the lens back of plastic lenses changes to an extent that cannot be ignored due to the influence of temperature. For this reason, if one of the collimating lens and the astigmatism lens is plastic and the other is a Gala 7 lens, or if the focal lengths of the two are different even though they are the same plastic, the image formation state of the focus lens and the light receiving section may change depending on the temperature. If the setting position of the focus signal is not correct, the correct focus signal cannot be obtained.

However, if these are used in common as in the non-embodiments, not only the cost but also the focus control performance will be significantly improved.

FIG. 9 shows another embodiment of the present invention, in which a spherical mirror 27 is used as a rotationally symmetrical optical element for generating astigmatism.

That is, the light from the semiconductor laser 21 is transmitted through the collimating lens 2.
2, the side blade) is incident on the polarizing beam splitter 23, reflected by the polarizing beam splitter 23, and 174
The light passes through the wavelength plate 24 and the optical lens 25 and forms an image on the light beam D. Hikari Dinuku D71. The reflected light passes through the focus lens 25, the 1/4 wave f-plate 24, and the polarizing beam splitter 23 in order, and then passes through the spherical mirror 27 for astigmatism generation.
, and an image is formed on the light receiving element 28 . In this case, by adjusting the inclination angle of the astigmatism generating spherical mirror 27, the necessary astigmatism can be generated on all the light receiving elements 28.

As is clear from the description of the various embodiments above, the present invention makes it possible to eliminate the need for the conventionally used cylindrical lens for generating astigmatism, and it is possible to manufacture the cylindrical lens in its place. By using a rotationally symmetrical optical element that is easy to use, it is possible to significantly reduce manufacturing costs and achieve good mass productivity.

According to this invention, there is no need to incorporate an astigmatism generating optical system into the imaging optical system for imaging the laser beam onto the optical disk, which adversely affects the imaging performance. Never.

Furthermore, according to the present invention, the state of astigmatism generation can be adjusted simply by adjusting the tilt of the rotationally symmetrical optical element for astigmatism generation with respect to the optical axis. It can be simplified.

Furthermore, according to the present invention, the rotationally symmetrical optical element for generating astigmatism and the collimating lens can all be used in common, improving mass production. Furthermore, if both are made of an aspherical planar lens, -M manufacturability can be further improved, and mass productivity can be further improved.

[Brief explanation of drawings]

Fig. 1 is a configuration explanation (8) of a focus control optical system for an optical disc showing the first embodiment of the present invention, and Fig. 2 is an explanatory diagram showing the state of astigmatism occurring on an optical element in the astigmatism method. , FIG. 3 is a circuit diagram of a focus signal detection circuit used in the astigmatism method, FIG. 4 is an explanatory diagram of the configuration of a focus control optical system for an optical disc showing a second embodiment of the present invention, and FIG.
The figure shows the aberration diagram of the aspherical planutic lens used in this study, Figure 6 shows the change in the astigmatism state when the #i angle θ of the aspherical plastic lens is changed, and Figure 7 is the output characteristic diagram of the Focanu signal, Fig. 8 [11,C
I'll respectively show a front view and a longitudinal cross-sectional view showing an example of a mechanism for adjusting the inclination angle of an optical element for generating astigmatism. The system is □□□. 1.11.21... Semiconductor laser 2.12, 22°° collimating lens 3.13.23
...Polarizing beam splitter 4.14.24...1/
4 wavelength plate 5.15.25...Focanu lens 6.16.26
... Servo mechanism 7.17.27 ... Rotationally symmetrical optical element for generating astigmatism 8.18.28 ... Light receiving element Patent Applicant Minolta Camera Co., Ltd. Agent Patent attorney Maeda Hao et al. 2 people Figure 3 Figure 6 Masaotake 10.011

Claims (1)

[Scope of Claims] (1) A laser light source, an imaging optical system including at least a collimating lens and a focus lens, for forming an image on the optical dinuque in the laser light, and an imaging optical system inserted in the middle of the imaging optical system. a polarizing beam splitter; a light receiving section that receives the light beam after the reflected light from the optical disk is separated by the polarizing beam splitter; A rotationally symmetrical optical element for generating astigmatism and a focus control optical system with an optical diode. (2. A focus control optical system for an optical disc according to claim 1, wherein the rotationally symmetrical optical element is the same lens as the collimating lens. (3) A focus control optical system for an optical disc according to claim 2, characterized in that the collimating lens and the rotationally symmetrical optical element are the same aspherical plastic lens. D7ri focus flilJ optical system.