JPS63209029A - Optical pick-up device - Google Patents

Abstract

PURPOSE:To fulfil requests such as light weight, small size and low cost and to obtain an optical pick-up device having high precision and reliability, which does not receive the effect of the variation of wavelength, by utilizing a grating lens optical system. CONSTITUTION:By combining two grating lenses such as first grating lens 11 which crosses an incident laser beam in an axis symmetry and a second grating lens 12 which converges the crossed beam on an optical disk 10, a favorable beam spot having no astigmation and a stable focusing function having no stagger can be realized without receiving the effect of the variation of wavelength of the incident laser beam. And a third grating lens 33 which diffracts a part of a reflected beam (signal light) from the optical disk, which goes back following a course opposite to going, and transmits a part of the beam is installed and the one (for example the diffracted light) is introduced to a photodetector 37 for focusing and the other (for example the transmitted light) is introduced to the photodetector 39 for tracking. Thus, the pick-up device for the optical disk which has high detecting precision and high reliability can be obtained.

Description

[Detailed Description of the Invention] [Summary] Two grating lenses are combined: a first grating lens that crosses an incident laser beam axially symmetrically, and a second grating lens that converges this crossed beam onto an optical disk. A newly developed grating lens system that achieves a good beam spot without aberration and stable focusing performance without deviation without being affected by wavelength fluctuations of the incident laser beam.
A third grating lens is attached to the grating lens system, which diffracts a part of the reflected beam (signal light) from the optical disk traveling backward through the grating lens system, and transmits a part of the reflected beam (signal light). By guiding one (for example, diffracted light) to a focusing photodetector and the other (for example, transmitted light) to a tracking photodetector, an optical disk pickup device with high detection accuracy and reliability can be realized.

[Industrial application field]

The present invention relates to an optical pink amplifier, and particularly to an optical disk pickup device using a grating lens system.

[Conventional technology]

Pink-up devices that write or read information on optical disks are being made lighter, smaller, and less expensive due to demands for smaller, higher-density, and shorter access times for the entire optical disk device.

In order to satisfy such demands, development of optical pickups using hologram elements has been progressing in recent years. this is,
A laser beam from a semiconductor laser light source is focused on an optical disk through a hologram, and the reflected beam (signal light) is polarized by a quarter-wave plate and taken out to a photodetector. Light weight, small size, and low cost can be realized, and in this respect, results that are quite satisfactory compared to the previous ones have been obtained.

[Problem that the invention seeks to solve]

However, as is well known, the diffraction angle of a hologram lens changes as the wavelength changes and aberrations occur, so as the oscillation wavelength of the semiconductor laser changes (the wavelength of the semiconductor laser beam source virtually always changes slightly), There are inherent problems such as changes in the focal point position and deterioration of the focal beam diameter, and as a result, optical pickups using holograms have not yet been fully put into practical use.

By the way, the applicant of this application previously filed Japanese Patent Application No. 61-22087.
In the specification of No. 0, a grating lens optical system that can always obtain a good beam spot free of aberrations without being influenced by the above-mentioned wavelength fluctuations of incident light and has accurate and stable focusing performance. proposed a system.

The present invention utilizes this grating lens optical system,
It is an object of the present invention to provide an optical pickup device that satisfies the requirements of light weight, small size, and low intensity, and is not affected by wavelength fluctuations, has high precision, and has high reliability.

Means for Solving Problem C] In order to achieve the above object, the optical pickup device according to the present invention includes a first grating lens that allows laser light from a laser light source to intersect axially symmetrically, and the first grating lens. A grating lens system includes a second grating lens that focuses the diffracted light that has passed through the optical signal onto a single point on the optical signal recording medium, and a second grating lens that allows only linearly polarized light in a specific direction to pass through and linearly polarized light in a direction perpendicular to the second grating lens. 1 which converts the linearly polarized light transmitted through or reflected by the polarized beam splitter into circularly polarized light and makes it incident on the grating lens system;
A part of the signal light whose polarization angle has been changed by 90 degrees by the quarter-wave plate and the quarter-wave plate from the grating lens system, which is reflected by the optical signal recording medium and follows a course opposite to the outgoing path, is transmitted and unified. a third grating lens integral with the polarized beam subrifter for diffraction;
The structure is characterized in that it includes a focusing photodetector that detects one of the transmitted light or diffracted light by the grating lens, and a tracking photodetector that detects the other of the transmitted light or diffracted light.

[Effect]

A polarizing beam splitter passes only a laser beam (linearly polarized light) in a specific direction, for example, only an S wave, and reflects linearly polarized light in a direction perpendicular thereto, for example, a P wave. Therefore, for example, only the P wave is reflected by the polarization beam subrifter and enters the grating lens system. At that time, the linearly polarized light is converted into circularly polarized light by a quarter wavelength plate (λ/4 plate).

In the grating lens system, the beam is diffracted by the first grating lens, and the diffracted lights are made to intersect symmetrically with the optical axis. Next, these diffracted lights are diffracted by the second grating lens, and the optical disc (
converges on one point on the optical signal recording medium). The reflected beam (signal light) reflected by the optical disk exits from the grating lens system following a course completely opposite to the outgoing path. At this time, the circularly polarized light is again converted into linearly polarized light by the λ/4 plate. Therefore, circularly polarized light occurs only between the λ/4 plate and the optical disk (including the grating lens system).

When the linearly polarized light (signal light) emitted from the grating lens system passes through the above-mentioned λ/4 plate, it is linearly polarized light whose polarization direction is 90° different from the linearly polarized light on the outgoing path (i.e., if the outgoing path is a P wave, the incoming path is a P wave). It passes through the polarizing beam splitter to become an S wave). The third grating lens integrally formed with the polarizing beam splitter has a function of a kind of half mirror, and part of the light transmitted through the polarizing beam splitter is transmitted, and part of it is diffracted. The transmitted light (or diffracted light) is guided to a focusing photodetector to perform focusing control, and the diffracted light (or transmitted light) is guided to a tracking detector to perform tracking control.

As will be described later, the grating lens system is completely unaffected by wavelength fluctuations of the laser beam and can always focus on one point on the optical disk. That is, aberrations that would otherwise occur due to wavelength variations are completely eliminated by using the grating lens system.

〔Example〕

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

First, the configuration of the grating lens system, which plays an important role in the present invention, will be briefly explained with reference to FIGS. 11 and 12. The detailed structure of this grating lens system is disclosed in the above-mentioned Japanese Patent Application No. 61-220870.

In FIG. 11, the grating lens system is the first. It has a configuration in which second inline grating lenses 11 and 12 are arranged on the same optical axis (dotted chain line), and a spherical wave diverging from a point P (coherent light source) on the optical axis is
The second grating lens 11 diffracts the light toward the optical axis, and once it crosses the optical axis, the second grating lens 1
2 to focus the light on a predetermined point Q on the optical axis.

The first grating lens 11 has a predetermined spatial frequency distribution that is rotationally symmetrical with respect to the optical axis, so that diffracted lights from any two points that are symmetrical with respect to the optical axis intersect on the optical axis. . Further, the second grating lens 12 has a predetermined spatial frequency distribution that is rotationally symmetrical with respect to the optical axis, so that the crossed diffracted lights are focused on one point (point Q) on the optical axis. be.

In the above configuration, mutually different wavelengths λ0. Consider two paths of light λ (λ0<λ). First, light with a wavelength λ is diffracted by a first in-line grating lens at a larger angle than light with a wavelength λ0, and after both of these diffracted lights intersect with the optical axis, they are passed through a second in-line grating lens. reach above the lens. The arrival points of these lights are on the same radius around the optical axis, and the distance from the optical axis for light with wavelength λ is farther than light with wavelength λ0. next,
These lights are diffracted by the second in-line grating lens, but at this time, the light with wavelength λ is diffracted at a larger angle than the light with wavelength λ0, so the distance between the two lights gradually narrows. , eventually intersect at one point.

Therefore, by providing the two inline grating lenses with a predetermined spatial frequency distribution, the above 2.
The point where the two lights intersect can be placed at a specified point on the optical axis.

The above can be said about the light incident on any point of the first in-line grating lens, and since the above spatial frequency distribution is rotationally symmetric with respect to the optical axis, the wavelength of the incident diverging spherical wave light is Even if the light changes, it will be focused on the predetermined point on the optical axis, and therefore no aberrations or focal position shifts will occur.

Next, a specific method for determining the spatial frequency distribution of the grating lenses 11, 12 will be described below in (i) to (iv) using FIG. Note that the distance between the point P and the grating lens 11 is j! l.

The distance between the two grating lenses 11.12 is d1
The distance between the grating lens 12 and point Q is assumed to be 22.

(i) First, consider a ray of wavelength λO that is emitted from point P and reaches point R1 at the outermost periphery of grating lens 11. This ray is diffracted at point R1 and grating lens 1
2, and is further diffracted here to reach point Q (solid line a in Figure 2). Then, the optical path described above (P→R1→rl−' Q), the spatial frequency F1 . f
l is determined.

(ii) Next, consider the case where the wavelength changes from λ0 to λ (>λO). The wavelength λ that progressed from point P to point R1
At point R1, the light ray is diffracted at a larger angle than when the wavelength is λO, and reaches point r2 on grating lens 12 (dashed line b).

Here, the spatial frequency 12 at point r2 is determined from the condition that the light is focused on point Q even when the wavelength is λ.

(iii) Returning to the case where the wavelength is λO, it is possible to conversely find from which point on the grating lens ll the light beam that is diffracted at point r2 and reaches point Q comes from (solid line C). Assuming that the point on the grating lens 11 is R2, the spatial frequency F2 at the point R2 is determined from the condition that the diffracted light at the point R2 reaches the point P.

(iv) Consider the case where the wavelength becomes λ again, and consider the above ゛(
Similarly to ii), the point r3 (not shown) on the grating lens 12 and its spatial frequency f3 are determined. Then, the wavelength is returned to λ0, and the point R3 (not shown) on the grating lens 11 and its spatial frequency F3 are determined in the same manner as in (i) above. In this way, point Rn (n-1
, 2, 3, ...) reaches the center of the grating lens 11, by repeating the steps (ii) and (iii) above,
The radial spatial frequency distribution at is determined. Note that the diameter of the second grating lens 12 is determined at the position of point rn.

By determining the spatial frequency distribution of the grating lenses 11° and 12 as described above, even if the light emitted from point P has a wavelength λ different from the reference wavelength λ0,
This can be focused on point Q without aberration.

The present invention realizes an optical pickup device using the grating lens optical system as described above, and the present invention will be described in detail with reference to FIG. 1 and subsequent figures, taking an optical disk as an example of an optical signal recording medium.

In FIG. 1, the above-mentioned grating lens system is indicated as a whole by 21, and the first grating lens 11 and the second grating lens 12 are arranged at the optical axis position. The laser converts the diverging light from the semiconductor laser 23 into a flat polarizing beam splitter (PBS: Po1ariz
The beam is polarized by a predetermined angle by the ed Beam 5plitLer ) 25 and enters the grating lens system 21 . Here, as is well known, the polarizing beam splitter 25 transmits only linearly polarized light in a specific direction, and reflects linearly polarized light in a direction perpendicular thereto.

That is, for example, only S waves (or P waves) are allowed to pass, and P waves (
or S waves) are reflected.

According to the present invention, the first grating lens 11 has one
/4 wavelength Fi (λ/4 plate) 29 is attached. still,
The quarter-wave plate 29 does not necessarily need to be physically attached to the first grating lens 11. Linearly polarized light (for example, P wave) reflected by the polarizing beam splitter 25
is converted into circularly polarized light by the quarter-wave plate 29, diffracted axially symmetrically by the first grating lens 11 as described above, and transmitted to the optical disc 10 by the second grating lens 12.
converges on one predetermined point.

As is well known, the optical disc device calculates the intensity of reflected light from bit 5 (generally depth = λ/8) on the optical disc 10.
The presence or absence of bits, and therefore the optical signal, is detected by using a beam probe narrowed to near the diffraction limit.

Although the wavelength of the beam from the semiconductor laser changes virtually constantly, by using a grating lens system, aberrations caused by wavelength changes can be absorbed as described above, and an extremely highly accurate optical signal can be obtained.

The beam (signal light) reflected by the optical disk 1O follows the completely opposite optical path to the grating lens system 21.
The light travels backwards inside the grating lens system 21 and exits from the grating lens system
/4 wavelength plate 29 changes the polarization angle by 90'' and becomes linearly polarized light. In other words, the beam has a polarization angle of λ/4 with respect to the incident light on λ/4 plate 29
The polarization angle differs by 90° from the light emitted from the plate 29 (signal light), and therefore the signal light passes through the polarization beam splitter 25.

According to the present invention, the third grating lens 33, which functions as a kind of half mirror, is integrally formed on the flat polarizing beam subrifter 25.

Thus, part of the signal light is diffracted by the third grating lens 33, and part of it is transmitted through the third grating lens 33. One of the transmitted light and the diffracted light, for example, the diffracted light B1, is sent to the focusing photodetector 37, and the other, for example, the transmitted light B2, is sent to the tracking photodetector 3.
9 respectively.

A grating lens can be easily converted into a half mirror like the one above by appropriately designing its stripes (grating).
The following characteristics are obtained.

The focusing photodetector 37 and the 1-roughing photodetector 39 are two-split photodetectors that are composed of two PIN photodiodes, which are known per se. The presence or absence of bits on the optical disc, ie, channel bit signals, is identified by the strength or weakness of the outputs of the two-split photodetectors 37 and 39.

In the illustrated embodiment, as shown in FIG. 2, the focusing photodetector 37 is placed slightly far from the imaging point (focus point), and when the optical disc 10 is accurately in focus, it detects the two-split light beam. The amount of light received by the two photodiodes of the device 37 is made equal. This state is shown in Figure 2(a).
is shown.

Therefore, when the optical disc IO is "far" from the in-focus position, the focusing photodetector 37 as shown in FIG. 2(b)
The output of the lower photodiode 37b becomes larger, and conversely, when the optical disk 1° is "closer", the output of the upper photodiode 37a becomes larger as shown in FIG. 2(C). .

The focus error detection method shown in FIG. 2 can be considered a modification of the so-called "Knifezi method."

Preferably, the dividing planes of the two photodiodes 37a and 37b of the focusing photodetector 37 are oriented in a direction perpendicular to the interference fringes of the third grating lens 33, as shown in FIG. There is. In other words, if there is a change in the wavelength of the light source, it is effectively absorbed within the specially configured grating lens system 21 as described above, and adverse effects such as defocusing are avoided, but the third grating lens 3
The change in the diffraction direction due to 3 cannot be avoided. Therefore, if the dividing surface of the focusing photodetector 37 is positioned in the direction in which this diffraction direction changes, that is, in the direction perpendicular to the interference fringes, oscillation will occur as shown in FIGS. 3(a), (b), and (c). Even if the diffraction angle of the third grating lens 33 changes in accordance with the change in wavelength, the image remains focused on the center of the dividing plane. That is, since the imaging position only changes on the dividing plane, the two photodiodes 37a, 37
It has no effect on the relative output characteristics of b.

FIG. 4A shows the arrangement of the tracking photodetector 39. In optical discs, tracking is performed using bits of the optical disc. The dividing planes of the two-split photodiodes 39a and 39b constituting the tracking photodetector 39 are located parallel to the row (track) of the bits 5. In FIG. 4A, the beam focused on the optical disk 10 is bit 5.
When the beam spot is located at the center of the photodiodes 39a and 39b, the beam spot is located at the center of the dividing plane of the photodiodes 39a and 39b, as shown in FIG. 4A (a). FIG. 4A (b) shows a case where the beam shifts to the left side of bit 5. In this case, the amount of light received by one photodiode 39a becomes partially dark, and conversely, when the beam shifts to the right side of the pit. Figure 4A (c)
As shown in FIG. 3, the amount of light received by the other photodiode 39b becomes partially dark. In addition, since the laser beams are crossed axially symmetrically within the grating lens system 21, the dark portion due to the positional deviation of the aberration beam is reversed and the photodiode 3
It appears near the dividing planes 9a and 39b. The state of the reversal is shown in FIG. 4B.

The tracking error detection method shown in Figure 4A is
This uses a bush-pull method that detects the difference in light intensity between the diodes 39a and 39b.

FIG. 5 shows an example of an actual structure of a pick amplifier for an optical disk 10 constructed based on the basic concept shown in FIG. In the figure, parts corresponding to the parts in FIG. 1 are indicated by corresponding numbers, and detailed explanation thereof will be omitted.

In FIG. 5, the first and second grating lenses 11
The S12 and 1/4 wavelength plates 29 are integrated in advance by a frame 53 and fixed to the housing 51. In addition, 40
is a spacer interposed between the first grating lens 11 and the quarter wavelength plate 29, and is optically useless.

The polarizing beam splinter 25 and the third grating lens 33 are integrally formed on both sides of the glass substrate 61 and similarly fixed at a predetermined position within the housing 51. Both the focusing photodetector 37 and the tracking photodetector 39 are mounted directly within the wall thickness of the housing 51. The semiconductor laser 23 is also fixed within the housing 51.

It is also possible to form a solid (for example, glass) structure between the first and second grating lenses, integrally form two grating lenses on both sides of a glass substrate in advance, and fix them in the housing.

In the optical pickup shown in FIG. 5, the convergent beam can be adjusted by adjusting the position of the semiconductor laser 23, and the detection section can be adjusted simply by adjusting the positions of the focusing photodetector 37 and the tracking photodetector 39. can.

According to the configuration shown in FIG. 5, the tracking photodetector 39 simply detects the difference in light intensity between the two photodiodes 39a139b, so the tracking photodetector 39 is placed close to the third grating lens 33. Therefore, the vertical width W of the housing 51 in FIG. 5 can be shortened.

When a focusing error or a tracking error is detected by the focusing photodetector 37 or the tracking photodetector 39, the optical system can be adjusted by, for example, moving the entire housing relative to the optical disk 10 using an appropriate actuator (not shown). This can be done by moving it in a predetermined direction.

FIG. 6 shows another embodiment of the present invention, in which the optical arrangement of the focusing photodetector 37 and the tracking photodetector 39 is reversed from that shown in FIG. The configuration is the same as that in FIG. 1 except that That is, in FIG. 6, the transmitted light B2 of the third grating lens 33 is guided to the focusing photodetector 37, and the diffracted light 81 is guided to the tracking photodetector 39.

The tracking photodetector 39 uses the bush-pull method to detect the difference in light intensity between the two photodiodes 39a and 39b, just as in the case of FIG. 4A, and its dividing plane is parallel to the track of the optical disk 10. It becomes. Therefore, as shown in FIG.
If there is a shift to the left or right with respect to the track), a difference in light amount will occur between the photodiodes 39a and 39b, and the shift can be detected.

Also, for the same reason as explained in relation to Figure 3,
The dividing planes of the photodiodes 39a and 39b are positioned in a direction perpendicular to the interference fringes of the third grating lens 33 as shown in FIG. Since the change in the diffraction angle of light occurs on the dividing plane of the photodiodes 39a and 39b, it does not have any adverse effect on tracking error detection. In FIG. 7.8, (a) shows the focused state, and (b) and (c) show the case where the beam is shifted left and right from the bit, respectively.

In the embodiment shown in FIG. 6, the focusing photodetector 37 is constituted by a known four-part photodetector as shown in FIG. The four-division photodetector 37 includes PIN photodiodes 37a, 37b, and 3 whose light-receiving areas are divided into four.
7c and 37d, and the presence or absence of bits on the optical disc (i.e., optical signal) is determined by the total output (It +
12 + 13 + 14). As shown in FIG. 9(a), the four-split photodetector 37 has photodiodes 37a, 37b, 37c, 3 in a focused state.
It is arranged so that the beam spot hits the center of 7d.

When the optical disc 10 approaches the second grating lens 12 from the in-focus position, the spot of the signal light on the 4-split photodetector 37 becomes a horizontally long ellipse as shown in FIG. As shown in Figure (b), it becomes a vertically long ellipse.

Therefore, the four areas 37a of the four-division light detection '2i37,
From the optical output of 37b, 37c, 37d (Il+12)
If you measure -(I3 +I4), it will be "0" when the optical disc is in focus, 0 when the optical disc approaches, and 〉O when it moves away. Therefore, this error signal (11+I2) −(13
Focusing can be performed by controlling the housing 51 up and down with an actuator (not shown) so that +I4) becomes O. This is a method called the astigmatism method, and in this case, the polarizing beam splitter 25 and the third grating lens 33 are integrally formed on a glass substrate 61.
(FIG. 10) becomes an astigmatism element.

FIG. 10 shows an actual pickup structure corresponding to the embodiment shown in FIG.
It is completely the same as that shown in FIG. 5 except that the positions of the tracking photodetector 7 and the tracking photodetector 39 are reversed. It will be easily understood that in the pickup shown in FIG. 10, the lateral width W' of the housing 51 can be shortened for the same reason as in the case of FIG.

In the embodiments described above, the tracking photodetector and the focusing photodetector are not limited to the above-mentioned 2-split or 4-split photodiodes, but it goes without saying that other photodetectors can be used.

In addition, in the above embodiment, the polarizing beam splitter 2
In No. 5, the arrangement was such that the outbound light is reflected and the return light is transmitted, but conversely, it is also possible to use an arrangement in which transmitted light is used for the outbound journey and reflected light is used for the return journey.

〔Effect of the invention〕

As described above, according to the present invention, by using a grating lens optical system that is not affected by changes in the oscillation wavelength and does not produce aberrations, it is lightweight, compact, and inexpensive, with high operational reliability for tracking control and focusing control. A high-performance optical pickup device can be realized.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the basic configuration of an optical pickup device according to the present invention, FIG. 2 is a diagram showing three types of beam detection states of the focusing photodetector shown in FIG. 1, and FIG.
A diagram for explaining that the focusing photodetector shown in the figure is not affected by changes in the oscillation wavelength, and Figure 4A shows three types of beam detection states of the tracking photodetector shown in Figure 1. Figure 4B is a diagram explaining that the "dark part" of the beam entering the tracking photodetector is reversed, and Figure 5 is an illustrative diagram showing the actual structure of the pickup using the configuration shown in Figure 1. 6 is a diagram similar to FIG. 1 showing a second embodiment of the present invention, FIG. 7 is a diagram showing three types of beam detection states of the tracking photodetector shown in FIG. 6, and FIG. The figure is a diagram explaining that the tracking photodetector shown in Fig. 6 is not affected by changes in the oscillation wavelength, and Fig. 9 is a diagram illustrating the 4-split light that constitutes the focusing photodetector shown in Fig. 6. Figure 10 shows the actual structure of the optical big amplifier corresponding to the embodiment shown in Figure 6. Figure 11 shows the grating lens optics used in the present invention. A diagram showing the basic configuration of the system, and FIG. 12 is a diagram explaining a method for determining the spatial frequency of the grating lens shown in FIG. 11. IO...Optical disk, 11...First grating lens, 12...Second
Grating lens, 21... Grating lens optical system, 23... Semiconductor laser, 25... Polarizing beam splitter, 29... Bow/4 wavelength plate, 33... Third grating lens, 37... Focusing photodetector for tracking, 39...photodetector for tracking. Fig. 3(b) (C) Fig. 4A shows the beam detection state of the tracking photodetector Fig. 4B shows an example of the actual structure of the optical pick amplifier Figure 5: A diagram showing the second embodiment of the present invention Figure 6 (b) Figure 7: Figure 8 (C) Figure 1119: Figure 10: Another example of the actual structure of the optical pickup

Claims (1)

[Claims] 1. A first grating lens (11) that axially symmetrically intersects laser light from a laser light source (23);
A grating lens system (21) in which a second grating lens (12) that focuses the diffracted light transmitted through the grating lens on a single point on the optical signal recording medium (10) is arranged on the optical axis; A polarizing beam splitter (25) that allows only linearly polarized light in a specific direction of the laser beam to pass through and reflects linearly polarized light in a direction perpendicular thereto, and converts the linearly polarized light transmitted through or reflected by the polarized beam splitter into circularly polarized light. a 1/4 wavelength plate (29) which makes the optical signal enter the grating lens system, and a 1/4 wavelength plate (29) from the grating lens system that is reflected by the optical signal recording medium and follows an optical path in the opposite direction. a third grating lens (33) integrated with the polarizing beam splitter that partially transmits the signal light whose polarization angle has been changed and partially diffracts the signal light; and one of the transmitted light and the diffracted light by the third grating lens. An optical pickup device comprising: a focusing photodetector (37) that detects the light; and a tracking photodetector (39) that detects the other of the transmitted light or the diffracted light.