JPS58220248A - Optical pickup - Google Patents

Abstract

PURPOSE:To simplify the structure of an optical system, by using the beams of different order numbers which are obtained through a holographic lens to detection of a focus error and a tracking error respectively. CONSTITUTION:The beam projected from a light source are focused on a disk 8 through an objective lens 7, and the reflected beam is radiated to the 1st and 2nd photoelectric detectors 21 and 22 via the lens 7, a 1/4 wavelength plate 6, a polarized beam splitter 4 and a holographic lens 19. For the parallel luminous flux made incident to the lens 19, the converging beams having astigmatism are radiated as the +1-order beams L1 as shown by 17a and 17b. At the same time, a 0-order beam L0 receiving no effect of hologram goes straight toward the detectors 21 and 22. Therefore no beam splitter is required to separate the luminous flux.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical pickup for extracting information from a medium (disc, tape, tram) on which information is recorded in an optically detectable form.

Conventionally, the most well-known pickup of this type is one for optical □ discs. This optical disc pickup has the function of reproducing signals individually and
It is necessary to have a function that can detect tracking errors and focus errors caused by disk eccentricity, surface wobbling, etc. Methods for detecting tracking errors include (1) two-beam method, (2) wobbling method, and (3)
The far field method is known. Also, known methods for detecting focus errors include (1) astigmatism method, (2) knife edge method, (3) critical angle method, and (4) skew beam method. The far-field method is most commonly used to detect tracking errors, and the astigmatism method is most commonly used to detect focus errors because of its high accuracy and simple pickup configuration.

FIG. 1 is an explanatory diagram of the configuration of a conventional optical pickup according to the above method. A beam of light emitted from a laser diode (hereinafter abbreviated as LD) 1 is made into parallel light by a collimator 2, and then shaped by a prism 3 into a beam having a circular cross-sectional shape. This is because the light emitting surface of the LD is oval, so that the parallel light beam after passing through the collimator 2 also has an oval cross-sectional shape. The luminous flux shaped into a circular beam is passed through a polarizing beam splitter (hereinafter referred to as P
BS) 4 and a beam splitter (hereinafter abbreviated as 88) 5, and enters the wavelength plate 6. Here, the PH 14 is placed so that linearly polarized laser light is transmitted therethrough. The light beam passing through the rabbit wave plate becomes circularly polarized light, and is imaged by the objective lens 7 onto the shidisk 8 as a minute spot of bait with a diameter of 1 to 2 μm. The reflected light from the disk passes through the objective lens 7 again, becomes parallel light, and passes through the wavelength plate 6, but this light flux becomes linearly polarized light whose polarization direction is 90°i4 with the light flux emitted from the LDl. There is. A part of this luminous flux is reflected by BH3 and is detected by photoelectric detector 9.
The light flux that enters the BH 5 and passes through the BH 5 is completely reflected by the PH 10 and enters the condenser lens 10 , and further passes through the cylindrical lens 11 and enters the photodetector 12 . Here, the detector 9 is used for tracking error detection, and the detector 12 is used for focus error detection and reproducing information recorded on the disc 8.

The principle will be explained below.

FIG. 2 and FIG. 6 show the principle of tracking error detection and the tracking error detection circuit, respectively. In order to detect tracking errors by the far-field method described herein, it is necessary that projections (or grooves) such as 81 are formed on the disk 8 in a concentric or spiral shape. Alternatively, it may be a bit string, such as a playback-only video, disc, digital audio disc, etc. Figure 2 shows concentric circles with protrusions,
Alternatively, a case is depicted in which a spiral track is provided in advance. In FIG. 2, 8b is a disk medium (recording medium) with high reflectance, and 8c is a transparent protective film. The light beam irradiated onto the disk B by the objective lens 7 is irradiated by the disk medium 8b, passes through the objective lens 7 again, and becomes a parallel beam. Since the height of the protrusion 8a is approximately equal to the optical path length λ, the intensity distribution of the parallel light beam when the protrusion 8a is on the optical axis of the objective lens 70 (FIG. 2(b)) is as follows near the optical axis. The intensity decreases due to light interference, and increases as the distance from the optical axis increases. If the protrusion 8a is shifted to the left or right from the optical axis of the objective lens, the projection 8a will be at its maximum on the right or left side of the optical axis, as shown in FIG. 2(a) or FIG. 2(b). Shows intensity distribution. Therefore, a two-split detector 9 consisting of two photodiodes 9a and 9b is placed in the part of the parallel light beam, and its output I9
If the output 101 from the differential amplifier 100 is differentially amplified as shown in FIG. 3 (&), the output 101 from the differential amplifier 100 is
), a signal is obtained that changes depending on the amount of deviation between the optical axis of the objective lens and the nose of the protrusion 8 on the disk. This becomes the tracking error signal. This signal is applied to the two-dimensional drive device 14 shown in FIG. 1, and is used for tracking to move the entire or part of the pickup including the objective lens in the radial direction of the disk (radial direction of the disk, that is, the direction of arrow 15 in FIG. 1). (arrow 16a).

Next, focus error detection and information reproduction will be explained. FIG. 4(m) U shows the shape of a spot formed by the condenser lens 10 and cylindrical lens 11 of FIG. The light beam reflected by the disk and returned as parallel light passes through the condenser lens 10 and the cylindrical lens 11, and then has a focal length such as 17a in the plane of the paper and 17b in the plane perpendicular to the plane of the paper. They have different light gathering properties.

Therefore, at the positions of i3a drops 13b and 13c, the images become 18a and 18b*18c, respectively.

Therefore, at the position 16b, 12
If the 4-split detector 12 consisting of 4 photodiodes ae 12b, 12ce, and 12d is placed perpendicular to the optical axis, and the distance between the objective lens and the disk is shorter than the focusing distance, as shown in Fig. 5. When the distance between the objective lens and the disc is longer than the focusing distance in a vertically elongated shape like (,), the horizontally elongated shape like in Fig. 5 (,), or when in focus, A circular light beam as shown in FIG. 5(b) comes to be incident on the detector 12.

If the outputs from 12a threat 12b * 12ce 12d are respectively 112 Todoroki @ 112b, ■ 12 @, 112d, \ (112a + 11zc) (112b + 11zd
) (1) If we perform the calculation, we get
A signal (
focus error signal) is obtained. FIG. 6 shows an electrical system for processing the output signal from the quadrant detector 12. As shown in FIG.

The summing amplifier 102 has 12ae 12b*12ce
An amplifier is used to calculate the sum of the output signals from 12a.
The output 103 becomes a reproduction signal of information recorded on the disc. Further, the amplifier 104 is for performing the calculation of the above-mentioned formula (1), and its output 105 is:
This results in a focus error signal that changes depending on the distance between the objective lens and the disk as shown in Figure (d).

This signal is applied to the two-dimensional drive 14 and is used for focusing, moving all or part of the pickup, including the objective lens, in a direction perpendicular to the disk (arrow 16b in FIG. 1).

The pickup shown in FIG. 1 constructed according to the above principle is different from the pickup constructed according to other principles (for example,
For tracking error detection, 2B A method, 7A method,
Although it can be configured more easily than the skew beam method for detecting errors, it has the following drawbacks.

(1) There is a lot of loss in the amount of light due to the beam splitter 5. Furthermore, if the loss of the amount of light is reduced, the amount of light incident on the tracking error detection detector 9 will be reduced, making it difficult to obtain a good tracking error signal.

(2) Optical elements requiring high machining accuracy, ie, beam splitter 5, condensing lens 109, and optical lens 11 are required.

The present invention aims to solve these drawbacks and provide an optical pickup that is simple in construction, compact, and lightweight. A graphic lens is provided, and light from the lens is detected by a first photoelectric detector and a second photoelectric detector to obtain a tracking error signal, a focus error signal, and an information reproduction signal. .

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

FIG. 7 is an explanatory plan view of an optical system of an optical pickup according to an embodiment of the present invention. A beam from a light source is focused by an objective lens 7 onto a track of a disk 8. The reflected beam, accompanied by stored information, passes through the objective lens, the wave plate 6 and the polarizing beam splitter 4, and is emitted toward the first photodetector and the second photodetector. . A holographic lens 19 is arranged between the polarizing beam splitter 4 and the photoelectric detector. This holographic lens 19 is one that generates convergent light having astigmatism with respect to a reference plane wave that is perpendicularly incident on the hologram surface. Therefore, if the parallel light beam reflected from the disk surface enters the holographic lens 19, two kinds of dotted lines 1 and 1 are shown in FIG.
As shown in 7a and 17b, the convergent light with aberration is +
Zero-order light L0, which is emitted as primary light and is not affected by the hologram, travels straight toward the photoelectric detector. This zero-order light L0 is shown as a concentrated line. In this way, the holographic lens 19 splits the incident light into a plurality of order lights having different focal lengths. Therefore, this +1st order light L1
If the focus error signal is detected using the Servo signal can be obtained.

As the shape of the photoelectric detector, the shape shown in FIG. 8 can be considered. In this case, the first photoelectric detector 21 that generates an output for detecting a focus error is arranged at the center, and on the outer circumferential side,
A second photoelectric detector 22 that generates an output for tracking difference detection is integrally arranged and formed. and the first
The photoelectric detector 21 includes four photodiodes 21a to 2.
1d, and the second photoelectric detector 22 is composed of two diodes 22&.22b. These diodes 21a to 21d*22ae22b correspond to 12a to 12d 9m@9b in FIG.

The above-mentioned first photoelectric detector 21 and second photoelectric detector 22
is arranged as shown in FIG. 9 at the minimum scattering circle position 20 (FIG. 7) of the +1st order light when the light/spot on the disk 8 is in focus. The four photodiodes 21a to 21d of the first photoelectric detector 21 are not so large as to be able to receive the amount of +1st order light having astigmatism. Using the outputs I211 to I21a, the above (1
) operation similar to the formula (I21 Luju I21・) −(I21b + I21
a) By performing ``° (2), positive and negative outputs corresponding to the front focus and rear focus can be obtained, and based on this output, the whole or part of the pickup including the objective lens 7 is moved in a direction perpendicular to the disk. Focusing is performed by moving the lens to the center of the lens.In this way, a focusing mechanism based on the astigmatism method is obtained.

Note that the reproduction signal of the recorded information is obtained by calculating the sum of the outputs of the respective photodiodes 21a to 21d, as in the case of the prior art.

Further, the two photodiodes 221 and 22b of the second photoelectric detector 22 receive zero-order light.

Tracking is performed by differentially amplifying the outputs I22a e I22b and moving the entire or part of the pickup including the objective lens 7 in the radial direction of the desk 8 based on the outputs. In this way, a tracking mechanism based on the far field method is obtained.

As for the configuration of the optical system, in the embodiment shown in FIG. 7, the polarizing beam splitter 4 and the holographic lens 19 are separated, but in order to further simplify the structure, the embodiment shown in FIG. A table structure can also be used. That is, by bonding the polarizing beam splitter 4 and the holographic lens 19 (FIG. 10(A)), by directly forming the hologram 19' on the end face of the polarizing beam splitter 4 (
Figure 10 (B)), 6 or photoelectric detector 21@22
A hologram 191 is formed on the cover glass (10th
A method such as that shown in Figure (0) can also be considered.

Next, a method for manufacturing the holographic lens used in the above examples will be explained.

First, a manufacturing method that does not use a laser will be described. First, a pattern as shown in FIG. 11 is created. This is a drawing of Fresnel's ring plate with different vertical and horizontal scales. Using this pattern, an amplitude-type Fresnel zone plate can be created using photolithography technology. Alternatively, create a zone plate with a phase structure on a glass substrate using an etching method.
By molding the same thing out of plastic, phase-type Nofresnel zone plates can be mass-produced at low cost.

Next, a method for optically manufacturing a holographic lens using a known laser will be described. FIG. 12 is an explanatory diagram of this. First, the object beam OB, which has been made into a parallel beam by a collimator (not shown), is incident on an optical system that causes astigmatism, which is composed of a spherical lens 23 and a cylindrical lens 24. , focus. At this time, the convergence angle of the light beam M in the meridian part (plane of the paper) is θm1, and the concentration angle of the light flux S in the spherical plane (plane perpendicular to the plane of the paper) is θm. Further, a photoreceptor 25 such as a photographic plate is placed perpendicular to the optical axis at a distance f in front of the position where the circle of least confusion is formed (approximately in the middle of the focal point formed by the meridian rays and the spherical rays).・Here, a half mirror 26 is placed between the cylindrical lens 24 and the photoconductor 25, and the reference beam RB is made into a parallel beam by a collimator (not shown), and the half mirror 26 is parallelized to the photoconductor 25 coaxially with the optical axis of the object beam. Make it incident. Interference fringes are formed on the photoreceptor 25 by the object light and the reference light. Photoreceptor 25 exposed in this way
When a collimated laser beam is incident on a holographic lens that has been developed to
It acts as an optical element having the same function as the holographic lens shown in 9. The astigmatism difference Δt1 focal length f1 and convergence angles θm and θB of the holographic lens are the same as the values shown in FIG. 12, respectively.

Although holographic lenses are easy to mass-produce, they have the disadvantage of having more loss than conventional lenses. However, in this example, since they are used as optical elements in the signal processing system, recording is possible. Even if it is used in a disc device, it will not affect the intensity of the minute spot irradiated onto the disc.
It will increase by the amount that is not used. On the other hand, in this type of optical pickup, the incident light on the photoelectric detector is sufficiently large, so that there is almost no problem with the reduction in light intensity due to loss in the holographic lens.

Note that the reference wave for the holographic lens used in the above embodiments does not necessarily have to be a plane wave; in that case, any light beam determined depending on the configuration of the optical system and usage conditions can be used as the reference wave to generate the holographic lens. All you have to do is design it.

Further, although the holographic lens in the above embodiment is of a transmission type, it goes without saying that it may be of a reflection type as shown in FIG.

Since the optical pickup according to the above embodiment records a beam having p1 astigmatism using a holographic lens, there is no need to use a cylindrical lens used in a conventional pickup. 1. Moreover, there is an advantage that the photoelectric detector can be placed at only one position, and the two-split detector and the four-split detector do not need to be placed at separate positions as in the conventional method. The shape of the photoelectric detector of the above pickup is not limited to the one shown in Figure 8, but can simultaneously detect changes in spot shape due to astigmatism of the +1st order light and far field light intensity distribution of the 0th order light. Any structure may be used.

In the above embodiment, the first photoelectric detector and the second photoelectric detector were formed integrally (FIG. 8).
The present invention is not limited to this embodiment, and both photoelectric detectors may have separate structures. Figure 14 shows
An example of a separate rain detector is shown. Here, the holographic lens 190 is configured to shift and emit the primary light. In this case, 0 for tracking
Since all of the secondary light is incident on the detector 22, the output of the tracking signal is increased compared to the above embodiment.

Further, the method of detecting focus error and tracking error with respect to the tracks of the disk is not limited to the method shown in the above-mentioned embodiments, but may be any other method as long as it achieves the purpose. Needless to say, it's good.

Furthermore, although the above-mentioned embodiment shows the case where the 00th order light and the +1st order light are used from the photographic lens, the present invention is not limited to this embodiment; It is sufficient to use lights of different orders at different distances.

As is clear from the above description, according to the optical pickup according to the present invention, different orders of light from a single holographic lens are used for focus error detection and tracking error detection, respectively. There is no need to insert a beam splitter to separate the light beams as is the case with conventional techniques, and this has the advantage that the structure of the optical system is extremely simple.

[Brief explanation of the drawing]

Figure #11 is an explanatory diagram of the configuration of a conventional optical pickup, Figure 2 is an explanatory diagram of the principle of the far field method for tracking, Figure 6 is an explanatory diagram of the method of generating a tracking error signal and its signal characteristic diagram, FIG. 4 is a diagram illustrating the principle of the astigmatism method for focusing, FIG. FIG. 7 is an explanatory plan view of the optical system of an optical pickup according to an embodiment of the present invention, FIG. 8 is an explanatory diagram of the structure of the first photoelectric detector and the second photoelectric detector, and FIG. 9 is an explanatory diagram thereof. FIG. 10 is an explanatory diagram of an embodiment in which the structure of the optical system is simplified. FIG. 11 is an explanatory diagram of the kth reduction exposure pattern for producing a holographic lens. 12
The figure is an explanatory diagram of a method for producing a holographic lens, Fig. 16 is an explanatory diagram of an example using a reflective holographic lens, and Fig. 14 is a diagram showing the first and second photodetectors separated. FIG. 1... Semiconductor laser, 2... Collimator lens,
6... Beam shaping prism, 4... Polarizing beam splitter, 5... Beam splitter, 6... Wave plate, 7... Objective lens, 8... Disk, ? ...
2-split detector, 10... convex lens, 11... cylindrical lens, 12...-4-split detector, 13...
Least confusion circle position, 14... Two-dimensional drive device, 15...
- Radial direction of disk, 16b in 16a... Driving direction of objective lens, 17a*17b... Light beam with aberration, 18... Spot shape, 19... Holographic lens, 20... circle of least confusion position, 21... first photoelectric detector, 22... second photoelectric detector, 23.
... Spherical lens, 24 ... Cylindrical lens, 2
5...Photoreceptor. Agent Patent Attorney Sanro Kimura・'11゜96°"8 °b) ((1;) □□ '6 figure, t-124 13th figure 14 figure 305-

Claims (4)

[Claims] (1) An objective lens that focuses light from a light source onto a track on a medium on which information is recorded in a photoelectrically detectable form; a holographic lens that splits the light into a plurality of orders of light; a photoelectric detector that receives a certain order of the light from the nuclear lens and generates an output for detecting a focus error with respect to the track of the objective lens; and a 20th photoelectric detector that receives order light of and generates an output for detecting a tracking error with respect to the track of the objective lens. (2) The first photoelectric detector is placed near the focusing position of the certain order light, and corresponds to the focusing shape of the certain order light that changes as the objective lens is displaced in its optical axis direction. Claim 1, characterized in that it generates an output.
Optical pickup described in section (3) The second photoelectric detector is placed at a position where the other order light is not focused, and generates an output corresponding to the intensity distribution of the another order light that changes with the tracking displacement of the objective lens. An optical pickup according to claim 1 or 2, characterized in that: (4) The second photodetector is integrally formed around the first photodetector, as set forth in claim 1, 2, or 3. optical pickup.