JPS6074127A - Optical head - Google Patents

Abstract

PURPOSE:To obtain a sufficient out-of-focus detecting signal even in a part which is out of focus greatly, by constituting a titled head so that the inside diameter of an opening part of a projecting lens is larger than a specified value determined by various conditions of an optical system. CONSTITUTION:Reflected light from an optical disk 2 is made incident to a photodetector 58 for detecting a focus after passing through a projecting lens 54 by an optical system which is below an objective lens 40 at the time of a focused point. In this optical system, when a distance between the objective lens 40 and the projecting lens 54, a focal distance of the objective lens 40, and a radius of an opening part of the objective lens 40 are denoted as L, f0 and A, respectively, the inside diameter of the projecting lens 54 is made larger than 2A[1+1/ (f0+f0<2>/2delta)]. Provided that delta shows a moving distance of the objective lens 40 in case of reading a non-focused time from a focused time, and its value is regulated to -0.005mm.. Also, an allowable out-of-focus quantity which can obtain a large out-of-focus detecting signal is about 0.5-2.0mum, and an out-of-focus quantity of a part of the projecting lens 54 is 5mum (-0.005mm.). In this way, even if a large out-of-focus occurs, light is not vignetted in a part of the projecting lens 54, therefore, a large out-of-focus detecting signal is obtained.

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention is used in, for example, DADs, video discs, optical discs, COMs, etc., and is used to read information from an information storage medium using light and, in some cases, to record additional information. , relates to an optical head that can erase information.

[Technical background of the invention and its problems]

Defocus detection methods applied to the above types of optical heads include the astigmatism method, which detects the spot shape of the Rade light on a photodetector, and the detection method, which detects it by moving the spot of the Rade light on the photodetector. There are methods such as the Naifetsuji method.

However, in order to miniaturize the optical system, the inner diameter of the projection lens that projects light onto the photodetector that detects defocus (the diameter of the area at the projection lens that passes through this projection lens and goes to the photodetector) has been reduced. If it is made small, when the focus is out of focus, the light reflected by the information storage medium and passed through the objective lens will be eclipsed by the inner diameter of the projection lens, and a sufficient amount of light will not reach the photodetector. Therefore, there is a problem that the defocus detection signal becomes small.

[Purpose of the invention]

- The present invention has been made based on the above circumstances, and its purpose is to provide an optical head that is capable of obtaining a sufficient defocus detection signal even when the focus is significantly out of focus. Summary of the Invention In order to achieve the above object, the present invention provides an optical head capable of reading at least information from an information storage medium using a single focused beam of light, which captures the light reflected by the information storage medium and sends it to the rear. The objective lens comprises an objective lens, and a projection lens that projects the light sent from the objective lens onto a photodetector that detects at least a focal point. When the focal length of the lens is f6s and the radius of the aperture of the objective lens is A,
It is assumed that a=-0,005 of the above projection lens. ).

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 schematically explains an information recording/reproducing apparatus to which the optical head of the present invention is applied, and 2 in the figure is an optical disk (
information storage medium). This optical disc 2 is formed by laminating a pair of disc-shaped transparent plates 4, 6 with inner and outer spacers 8, 10 interposed therebetween, and an information recording layer is formed on the inner surfaces of the transparent plates 4, 6. Light reflective layer 12.14
is formed by vapor deposition. This light reflecting layer 12.1
A tracking guide 16 is formed in a helical manner on each of the tracking f-id 16, on which information is recorded in the form of bits. A hole is bored in the center of the optical disk 2, and when the optical disk 2 is placed on the turntable 18, the center spindle 20 of the turntable 18 is inserted into the hole of the optical disk 2, and the turntable 18 and the optical disk 2 are inserted into the hole. The rotation centers of are matched. A chuck device 22 is further attached to the center spindle 20 of the turntable 18, and the optical disc 2 is fixed on the turn tee Sol I8 by this chuck device 22. The turn chisel 18 is rotatably supported by a support (not shown) and rotated by a drive motor 24 at a constant speed.

An optical head 26 is provided so as to be movable in the radial direction of the optical disk 20 by a linear actuator 28 or a rotary arm, and a Rade device 30 for generating a Rade beam is provided within the optical head 26. When writing information to the optical disc 2, a Rade beam whose light intensity is modulated according to the information to be written is generated from the Rade device 30, and when reading information from the optical disc 2, a radar beam with a constant light intensity is generated. A radar beam is generated from the radar device 30. The Radhe beam generated by the Radhe device 30 is diverged by a concave beam 32, converted into a parallel beam by a convex lens 34, and directed to a polarizing beam sgelitter 36.

The parallel laser beam reflected by the polarizing beam splitter 36 passes through the quarter-wave plate 38 and enters the objective lens 40, and is focused by the objective lens 40 toward the light reflective layer 14 of the optical disc 2. Objective lens 4
0 is? When the objective lens 40 is supported movably in the optical axis direction by a chair coil 42 and positioned at a predetermined position, the beam waist of the focused Radhe beam emitted from the objective lens 40 is reflected by the light reflecting layer 14. is projected onto the surface, and a minimum beam spot is formed on the surface of the light reflective layer 14. In this state, the objective lens 40
It can be kept in focus and information can be written and read. When writing information, bits are formed in the tracking id 16 on the light reflective layer 14 by a radar beam whose light intensity is modulated, and when reading information, a radar beam having a constant light intensity is used to form a bit in the tracking id 16. The light intensity is modulated by the bits formed at 16 and reflected.

The diverging Radhe beam reflected from the light reflection layer 14 of the optical disk 2 is converted into a parallel beam by the objective lens 4θ when focused, passes through the quarter-wave plate 38 again, and is returned to the polarizing beam splitter 36. It will be done. As the Rede beam goes back and forth through the quarter-wave plate 38, the plane of polarization of the laser beam is rotated by 90 degrees compared to when it is reflected by the polarizing beam splitter 36, and the plane of polarization is rotated by this 90 degrees. The Rede beam is polarized by polarizing beam splitter 3.
6 and passes through this polarized beam splinter 36.

The laser beam that passes through the polarizing beam splitter is
The light is divided into two systems by a half mirror 44, and one of the systems is irradiated onto a first photodetector 48 by a convex lens 46J/C. The first signal detected by the first photodetector 48 includes information recorded on the optical disc 2, and is sent to the signal processing device 50 and converted into digital data.

From the other laser beam separated by the half mirror 44, only the component that passes through a region separated from the optical axis 53 by the light shielding plate 52 is extracted, and after passing through the projection lens 54, it is reflected by the mirror 56. The light is incident on the second photodetector 58. Here, the light shielding plate 52 may be formed of any one of a prism, an annular slit, a knife edge, and the like. The signal detected by the second photodetector 58 is processed by a focus signal generator 60, and the focus signal generated from the focus signal generator 6θ is given to a voice coil drive circuit 62.

The voice coil drive circuit 62 drives the voice coil 42 in response to the focus signal to maintain the objective lens 40 in focus. Note that in order to accurately trace the tracking guide 16 formed on the light reflective layer 14 of the optical disc 2, it is also possible to process the signal from the second photodetector 48 and operate the linear actuator 28. Also, if the objective lens 4θ is moved laterally,
Alternatively, although not shown, a Rubano mirror may be activated.

The optical system for detecting the time of focus shown in Fig. 1 is simplified as shown in Fig. 2, and the locus of the laser beam related to focus detection is shown in Fig. 3 (A) (→→ When the objective lens 4o is in the focused state, the beam waist is projected onto the light reflecting layer 14, and the minimum beam
A spot, i.e., a beam spot 4, is formed on the light reflective layer 14. Since the laser beam incident on the objective lens 40 from the laser device 30 is usually a parallel beam of light, the laser beam is formed on the focal point of the objective lens 4o. However, the laser device 3 is attached to the objective lens 40.
If the radar incident from 0 is slightly diverging or converging, the beam width) ld, objective lens 40
is formed near the focal point. Figures 1, 2 and 3 (
In the optical system shown in (a) and (b)←→, the light-receiving surface of the photodetector 58 is arranged on the convergence surface of the beam waist spot 64 in the focused state. Therefore,
When focused, an image of the beam waist spot 64 is formed at the center of the light receiving surface of the photodetector 58. That is, as shown in FIG. 3(a), a beam waist spot 64 is formed on the light reflection layer 14, and the laser beam reflected by the light reflection layer 14 is converted into a parallel beam by the objective lens 40. and is directed toward the light shielding plate 52. Only light components passing through a region spaced apart from the optical axis 53 are extracted by the light shielding plate 52, focused by the projection lens 54, and transmitted to the photodetector 5.
The beam is converged to a minimum on 8, and a beam waist image is formed thereon. Next, the objective lens 40
4, the beam waist is as shown in Fig. 3c.
The laser beam is reflected by the light reflecting layer 14 as shown in FIG. That is, a beam waist occurs between the objective lens 40 and the light reflective layer 14. In such an out-of-focus state, the beam waist usually occurs within the focal length of the objective lens 40, so assuming that the beam waist functions as a light spot, it is clear that the light reflecting layer 1
The laser beam reflected by the objective lens 4 and emitted from the objective lens e40 becomes a divergent laser beam due to the objective lens e4oK.
converted into a beam. Since the Radhe beam component passing through the light shielding plate 52 is also divergent, the laser beam component 52 is also divergent.
Even if the beam components are focused by the projection lens 54, they are not focused to the minimum on the light-receiving surface of the photodetector 58, and the beam components are also focused toward a point far away from the photodetector 58. Therefore, the laser beam component is projected upward in the figure from the center of the light-receiving surface of the photodetector 58, and a pattern larger than the beam spot image is formed on the light-receiving surface. Furthermore, when the objective lens 40 is separated from the light reflective layer I4 as shown in FIG. 3(c), the laser beam is reflected by the reflective layer 14 after forming a beam waist. In such an out-of-focus state, the beam waist is usually out of the focal length of the objective lens 40 and between the objective lens 40 and the reflective layer 1.
4, the two reflected laser beams directed from the objective lens 40 toward the light shielding plate 52 have convergence. Therefore, the Rade beam component that has passed through the light shielding plate 52 is further converged by the projection lens 54 to form a convergence point and then projected onto the light receiving surface of the photodetector 58.

As a result, a pattern larger than the image of the beam waist spot is formed on the light receiving surface of the photodetector 58 from the center upward and downward in the figure.

Next, find a minimum allowable value for the inner diameter of the projection lens 54. The above-mentioned change in the trajectory of the Rede, that is, the change in the trajectory of the light ray, can be explained in terms of Kinai optics as follows, and the value by which the laser beam component is deflected on the projection lens 54 can be determined.

The Kinai optical imaging system of the objective lens 40 can be expressed as shown in FIG. Here, fo is the focal length of the objective lens 40, and δ is the objective lens 40 from in-focus to out-of-focus, that is, the light reflection layer 1 of the optical disc 2.
The ray trajectory shown as a solid line in FIG.
0 and is focused by passing through a point spaced apart from the optical axis 53 by a distance ho. In the focused state shown in FIG. 3(a), δ is 0 as is clear, and in the out-of-focus state shown in FIG. Since the beam waist is formed by being reflected by the light reflecting layer 14, the beam waist will be twice as close to the objective lens 4oK. (If they are close, δ<0.) In addition, when the optical disc 2 is out of focus as shown in FIG.
Since the Lede beam is reflected from the light reflection layer 14 after forming the waist, it is essentially the same as forming the beam waist behind the light reflection layer 14, and the beam
The waist will be separated from the objective lens by 2δ. At the time of focusing, if the beam waist is formed at the focal position of the objective lens 40, when the optical disk 2 moves by δ, the beam waist and the main axis of the objective lens 4θ are formed as shown in FIG. The distance between the faces is (f.

+2δ). If the beam waist is regarded as a light spot, the angles βG and β1 in FIG. 4 are expressed by the following equations (1) and (2).

Mata, from the lens imaging formula Therefore, β1=βO+ ha/f.

FIG. 5 shows a light ray trajectory in the optical system of the projection lens 54, in which the projection lens 54 is a pair of combined lenses 54-1.
．． 54-2.

Here, the lenses 54-1 and 54-2 each have a focal length f
l+fi, the light shielding plate 52 is arranged at a position spaced apart by a from the main surface of the objective lens 40, the main surface of the lens 54-1 is arranged at a position spaced apart by L from the main surface of the objective lens 40, and further It is assumed that the main surface of the lens 54-2 is arranged at a distance H from the main surface of the lens 54-1, and the light receiving surface of the photodetector 58 is arranged at a distance t from the main surface of the lens 54-1. The light ray locus shown by a solid line in the figure shows the light transmission surface of the light tracing plate 52 that is focused by the objective lens 40 and is spaced apart from the optical axis 53 by y.

The distance y is expressed by the following equation (3).

y:llo −aβl where F(J)=(f o + f o /2δ)
-1, equation (3) is expressed by the following equation.

y=ho (1-aF(δ)) Curve/Song (4) Therefore, the position h1 from the optical axis 53 where the light ray passes on the main surface of the lens 54-1 is expressed by equation (6). .

hl -3' (L-a)β] When the focus is out of focus by δ, the spot diameter at the projection lens 54 located at a distance L from the objective lens 4o is (6) 2, y "" A( A is objective lens f4
0 aperture or exit pupil radius), a = Q, then

Here, a sufficiently large focal point without obscuring the light at the projection lens 54 is an allowable focal point I'! at which a detection signal can be obtained. Consider the range of amount. In the Pu5h-Pull method, which detects track misalignment using the diffraction pattern of the reflected light from the tracking guide of the focused light, the track misalignment detection signal no longer appears if the focus is significantly out of focus.

Figures 6 and 7 show that when the radar spot crosses the grooves (pre-groups) that extend in parallel in a spiral pattern at right angles, each groove is It shows the track deviation detection signal that appears. Each drawing represents the state of the track deviation detection signal when the focus is shifted in each direction. In the figure, δ is the amount of deviation of the light reflection layer 14 from the focal point of the light emitted from the objective lens 40, and the side farther from the objective lens 40 is positive. Also, the upper curve is the tracking signal 50
The lower curve of A1 shows the total signal 50B, and both zero points are 2 dlv from the top, and the lower side is positive.

From this figure, it can be seen that if the focus is out of focus by 3.0 μm or more, or 5.0 μm or more at most, the signal of the Pu 5h-Pu 11 method cannot detect the track deviation (note that the waveform of signal 50 is smoother). , and the larger the amplitude, the better.)

Further, when recording is performed by causing a state change such as making a hole in the light reflection layer 14 of the information storage i body 2, defocusing occurs and the spot on the light reflection layer 14 becomes large, making it difficult to perform recording. The spot size at on the light reflecting layer 14 at the time of focusing (for example, the numerical aperture (NA) value is NA
(NA here is treated as a symbol in a mathematical formula) Suppose that a parallel laser beam with an even intensity distribution is incident on an ideal lens with no aberrations, and after passing through this lens, the beam is condensed. The spot size %'at) is assumed to be 0.82λ aL=□ (7) NA when the wavelength of the Rede light is λ. Also, assuming that the intensity distribution at this time is similar to the Gaussian distribution, the radius of the annular zone where the intensity at the beam waist is 1/e of the center intensity is ω0, and the radius at a position shifted by 2 from there is ω0. Since the radius ω(2)=ωol can be expressed, this formula can be used for the radius ω on the light reflecting layer 14 when the focal point is buried by Z. Therefore, ωce)=ω. '(i+('',)2) πωo2 At this time, the central strength of the strip is 1+( """)2Z2ku1/lm1n0.17πλ r z+ <O-17"', Me1/I m I n-
1 (NA) 2 River...(9) Now, λ= 0.831tm, NA= 0.6,
If Iml n = 0.7, = 0.79 μm λ = 0.83 μm * NA- = 0.5,
If lm1n=0.7, then 0.44 X O,65 I Z I <, -□-= 1.14 μm0.25 The allowable defocus amount is about 0.5 to 2.0 μm.

Therefore, the amount of defocus within the range where the light is not obstructed at the projection lens 54 is 5 μm (-0,005 m+I+)
shall be.

From the above, even if the inner diameter (diameter) of the projection lens 54 is blurred, the light is not obstructed at the projection lens e54, and therefore a sufficiently large defocus detection signal can be obtained.

Although the above embodiment describes an optical head that uses the knife edge method as the defocus detection method, the present invention is not limited to this, and may be applied to an optical head that uses the astigmatism method, for example. Of course.

That is, this astigmatism method is applied to the light reflecting layer (information forming layer) of the optical disk (information storage medium) 71 as shown in FIG.
Rede light 7 reflected from 22 and passed through objective lens 73
A detection system optical lens 77 is a combination of a projection lens (spherical lens) 75 and a cylindrical lens 76 on the reflected optical path of 4.
and a photodetector 7 divided diagonally into four as a detection section.
8 are arranged in sequence to detect the spot shape of the Raded light 74 on the photodetector 28.The basic principle is as shown in FIGS. 9(a) and 9(b), the position P of the point light source 79 Even if they are the same, if the focal lengths of the lenses 80 & 80b are different, the image formation positions q and q will differ, and the width with which the light hits the plate 81 placed at position R will differ. On the other hand, the projection lens (spherical lens, e) 75
When the cylindrical lens 76 is combined with the cylindrical lens 76, the focal length of the composite lens differs between the plane containing the generatrix and the plane perpendicular to it. The shape will be oval.

Furthermore, when the position P of the point light source 79 is shifted, the spot shape of the light reflected on the plate 8 placed at the position R changes from an ellipse to a circle. This is then detected by a detector 78 divided into four parts, thereby detecting out-of-focus. therefore,
Focusing is performed by moving the objective lens 73 so that the spot of light reflected on the plate 81 placed at position R is in focus when it becomes circular.

Here, 82 shown in FIG. 8 is a lens driving coil that moves the objective lens 73 toward and away from the surface of the optical disk (information storage medium) 72 to adjust the focus.

By setting the inner diameter of the projection lens 75 in an optical system that performs such an astigmatism method in the same manner as in the above embodiment, similar effects can be obtained.

〔Effect of the invention〕

As explained above, according to the present invention, in an optical head capable of reading at least information from an information storage medium using focused light, an objective lens that captures the light reflected by the information storage medium and sends it backward; a projection lens that projects the light sent from the objective lens onto a photodetector for detecting at least defocus; f's When the radius of the aperture of the above objective lens is A,
The inner diameter of the above projection lens is as large as 2A (1+-i.1°27°) (however, δ = -0,005tm), so even if the focus is significantly out of focus, there is sufficient defocus. It has excellent effects such as being able to obtain a detection signal.

[Brief explanation of the drawing]

Figures 1 to 7 show one embodiment of the present invention.
The figure is a block diagram showing the information recording and reproducing device, Figure 2 is a diagram showing the optical system of the defocus detection system, and Figure 3 is a diagram showing the optical system of the defocus detection system.
is an explanatory diagram showing the trajectory of the laser beam when it is in focus and when it is out of focus, Figure 4 is a diagram for analyzing the trajectory of the ray passing through the objective lens, and Figure 5 is the diagram showing the trajectory of the ray passing through the projection lens. Diagrams for analysis, Figures 6 a) to 7)
Figures (a) to (g) are diagrams showing a track deviation detection signal with respect to the amount of defocus, Figure 8 is a diagram showing a defocus detection optical system showing another embodiment of the present invention, and Figure 9 is a diagram explaining its principle. 2... Information storage medium (optical disk), 40... Objective lens, 54... Projection lens, 58... Photodetector, 7
1... Information storage medium (optical disk), 73... Objective lens, 75... Projection lens, 78... Photodetector.

Claims (1)

[Claims] In an apparatus capable of reading at least information from an information storage medium using focused light, an objective lens that captures the light reflected by the information storage medium and sends it backward, and at least a defocusing of the light sent from the objective lens are provided. a projection lens for projecting an image onto a photodetector to be detected, the distance between the objective lens and the projection lens is determined, the focal length of the objective lens is determined by the radius of the aperture of the objective lens is determined by
When, the distance of the above projection lens is δ−0,005m
shall be. ).