JP2000036121A - Optical head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect focusing error detection signals employing a single photodetector by forming optical spots at the front and the back of a focus on the photodetector and arranging the light receiving surfaces of the photodetector at the positions corresponding to the spots. SOLUTION: The polarization direction of light beams, which are made incident on a polarizing beam splitter 104, is rotated by approximately 90 deg. with respect to the forward light path direction by passing through a 1/4 wavelength plate 105 twice, and the beams are reflected by a polarization beam splitter 104, converged by a detection lens 109, passed through an optical path difference generating plate 110 and made incident on a photodetector 111. The photodetector 111 detects the focusing error detection signal, a track error detection signal, and data signals. The plate 110 is divided into two regions at an approximate center. The thicknesses of the respective regions are made different so that the optical distances of the passing light beams are made different by the different thicknesses and optical path difference is generated. The light receiving surface of the photodetector 111 is arranged to be located at the approximate center of the two focal distances of the luminous fluxes that are divided into two by the plate 110.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a CD, C
The present invention relates to a focus error detection system for an optical head of an optical disk device such as a D-ROM, MO, and DVD.

[0002]

2. Description of the Related Art An optical disk apparatus records and reproduces information by positioning a light spot narrowed by an objective lens of an optical head on a recording film of an optical disk. The optical head has a focus error detection system for detecting a positional deviation between the light spot and the recording film in order to position the light spot on the recording film with an accuracy of 1 μm or less. The focus error detection system detects the amount of positional deviation between the light spot and the recording film using optical components such as lenses and a photodetector, and moves the objective lens in accordance with the detected amount of reflected light from the optical disk. Thus, the light spot is controlled so as to be accurately positioned on the recording film.

A conventional focus error generation system is described in Yoshito Tsunoda, “Basics and Application of Optical Disk Storage”, IEICE, pp. Various systems have been proposed and put into practical use as described in 76-83. Each detection method has advantages and disadvantages, and a method that is considered to be the best from the usage conditions and costs is selected and used. The DVD, which is expected to rapidly expand in the market in the future, adopts a land-groove method in which information is recorded on both the land portion and the groove portion of the guide groove of the disk. In the land-groove method, diffracted light by the groove is used. Is expected to be greater than the conventional land type and groove type. The light diffracted by the groove is superimposed on the light incident on the focus error detection system and becomes noise for focus error detection. This noise is that even if the focus error signal becomes 0, the light spot is shifted from the recording film.
Alternatively, when the light spot crosses the groove, the focus error detection signal is oscillated to cause a problem such as oscillating the objective lens.

Therefore, the prior art will be described with an example of a differential detection method before and after the beam size, which has a relatively small influence of diffracted light. In the differential detection method before and after the beam size, as described below, two light detectors detect light including the same diffracted light, and the difference between the outputs of the respective detectors is taken as a focus error signal. In principle, it is hardly affected by diffracted light.

FIG. 3 is an explanatory diagram of an example of an optical head using the differential detection method before and after the beam size. FIG. 3-1 is an overall configuration diagram, and FIG. 3-2 shows a state of a light spot on a light receiving surface of a photodetector. The light emitted from the semiconductor laser 301 is converted into substantially parallel light by a collimating lens 302, the intensity distribution is changed from an elliptical shape to a circular shape by a beam shaping prism 303, passes through a polarization beam splitter 304, a quarter-wave plate 305, and rises. The light is reflected by the raising mirror 306, changes its path in a direction substantially perpendicular to the optical disk, and is condensed by the objective lens 307 to form a minute light spot 308. The way the light travels so far is defined as the going light path.

The light condensed by the objective lens 307 is reflected by a recording film of an optical disk (not shown),
7. The light enters the polarization beam splitter 304 via the rising mirror 306 and the quarter-wave plate 305. Here, the light incident on the polarizing beam splitter 304 is a ４ wavelength plate 30.
5, the polarization direction is rotated by about 90 ° with respect to the direction of the outgoing optical path. Therefore, the light is reflected by the polarization beam splitter 304, converged by the detection lens 309, and enters the half beam splitter 310.

[0007] In the half beam splitter 310, the light is split into reflection and transmission by approximately 1/2 in the light amount ratio. The light reflected by the half beam splitter 310 is transmitted to the detection lens 30
The light is incident on a photodetector 311 which is arranged at a predetermined distance W from the focal position of the lens 9 toward the detection lens 309. The light transmitted through the half beam splitter 310 is detected by the detection lens 3
The light is incident on a photodetector 312 arranged at a fixed distance W in a direction away from the detection lens 309 with respect to the focal position 09.

The photodetectors 311 and 312 are the same photodetector and the light receiving surface is divided into four areas a, b, c and d as shown in FIG. Signals such as a focus error detection signal are obtained by calculating outputs of respective regions of the photodetector. Regarding the focus error detection signal ΔAF of the subject of the present invention,

[0009]

ΔAF = (a1 + b1) − (a2 + b2)

For reference, the track error detection signal ΔTR
Is

[0011]

ΔTR = (d1−c1) + (c2−d2)

In the differential detection method before and after the beam size, the influence of diffracted light due to a guide groove formed on the recording film of the optical disk and irregularities called prepits, which become disturbances of the focus error detection signal, is caused by both the photodetectors 311 and 312. And the difference is reduced by taking the respective differences. For this reason, as described above, it can be said that a DVD or the like is a very excellent focus error detection system.

[0013]

However, in the differential detection method before and after the beam size, the use of two sets of photodetectors causes problems such as an increase in cost, an increase in adjustment locations, and a large installation space.

[0014]

As means for solving such a problem, in the present invention, the shape of the light receiving surface of the photodetector is devised and a simple optical element is provided between the detection lens and the photodetector. By providing an insertion, a small focus error detection system that can be detected by one detector is provided.

In the present invention, the optical path difference from the detection lens to the photodetector required in the differential detection method before and after the beam size described in the prior art is defined as a substantially parallel optical path having at least two regions having different optical distances in thickness. By using a transparent plate, light spots before and after the focal point are created on one photodetector. By providing a light receiving surface of a photodetector at a position corresponding to each light spot, a focus error detection system capable of detecting a focus error detection signal with one photodetector is provided.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is an explanatory diagram for explaining an embodiment of the present invention. FIG. 2 is a detailed view of the focus error detection system of FIG. The light emitted from the semiconductor laser 101 is converted into substantially parallel light by a collimating lens 102, and the intensity distribution is changed from an elliptical shape to a circular shape by a beam shaping prism 103. A polarization beam splitter 104 and a 波長 wavelength plate 105 are provided.
, Is reflected by the rising mirror 106, changes its path in a direction substantially perpendicular to the optical disk, and is condensed by the objective lens 107 to form a minute light spot 108. The way the light travels so far is defined as the going light path. The light condensed by the objective lens 107 is reflected by a recording film of an optical disc (not shown),
The light enters the polarization beam splitter 104 via the objective lens 107, the rising mirror 106, and the quarter-wave plate 105. Here, since the light incident on the polarization beam splitter 104 has passed through the quarter-wave plate 105 twice, the polarization direction is rotated by about 90 ° with respect to the direction of the going optical path. Therefore, the light is reflected by the polarization beam splitter 104, converged by the detection lens 109, passes through the optical path difference generating plate 110, and enters the photodetector 111.

In the photodetector 111, a focus error detection signal,
A track error detection signal and a data signal are detected. The optical path difference generating plate 110 is formed of a material having a high light transmittance such as glass or polycarbonate, and is divided into two regions substantially at the center, and the thicknesses of the respective regions are different.
The focus detection system will be described in detail with reference to FIG. It is arranged so that the divided part of the region is located substantially at the center of the light transmitted through the detection lens 109. The optical distance of the transmitted light differs depending on the different thicknesses t1 and t2, and an optical path difference occurs. That is, if the refractive index of the optical path difference generating plate 110 is n,

[0018]

An optical path difference of δ = (n−1) (t1−t2) is generated.

Further, the incident surface of the optical path difference generating plate 110, or the incident surface and the reflecting surface are slightly inclined, so that the two lights having the generated optical path difference do not overlap on the photodetector. I have. This angle may be such that the light patterns on the photodetector do not overlap when the light spot 108 is located at a distance of several tens of μm from the recording film.

The photodetector 111 is arranged such that the light receiving surface is located substantially at the center of the two focal positions of the light beam divided by the optical path difference generating plate 110.

FIG. 2-2 is a diagram showing the shape of the light receiving portion of the photodetector 111 and the light pattern on the light receiving surface. (1), (2), and (3) of FIG. 2-2 show light patterns when the position of the light spot 108 with respect to the recording film is different, respectively. (1) is when the light spot 108 is shifted to the optical head side with respect to the recording film, (2) is when it is on the recording film, and (3) is when it is shifted to the opposite side of the optical head. The state of a light pattern is shown. The light receiving surface is largely divided into two, each of which is divided into three, and a
1, b1, c1 and a2, b2, c2.

Of the two light beams whose optical path difference has been generated by the optical path difference generating plate, the longer one of the light paths enters the areas a1, b1 and c1, and the shorter one enters the areas a2, b2 and c2. When the light spot 108 is on the recording film, that is, in the case of (2), the light pattern on the light receiving surface of the photodetector is two semicircular patterns having substantially the same size. A portion surrounded by a 円 arc in the semicircular light spot indicates a diffraction pattern by the track groove. Focus error detection signal ΔAF
Is obtained by the following operation.

[0023]

ΔAF = (a1 + b1) − (a2 + b2) For reference, the track error detection signal ΔTR and the data detection signal S are:

[0024]

ΔTR = (a1 + a2) − (b1 + b2)

[0025]

S = a1 + b1 + c1 + a2 + b2 + c2

FIG. 4 is an explanatory view of a second embodiment of the present invention. According to the present invention, the focus error detection system can be miniaturized, so that the configuration shown in FIG. 4 is also possible. Semiconductor laser 40
The light emitted from 1 is transmitted through the polarizing beam splitter 402, becomes almost parallel light by the collimating lens 403, the intensity distribution is changed from elliptical to circular by the beam shaping prism 404, and is transmitted through the quarter wavelength plate 405. The light beam is reflected by the rising mirror 406, changes its path in a direction substantially perpendicular to the optical disk, and is condensed by the objective lens 407.
8 is formed. The way the light travels so far is defined as the going light path.

The light condensed by the objective lens 407 is reflected by a recording film of an optical disk (not shown),
7, rising mirror 406, beam shaping prism 404
, And is condensed by the collimator lens 403. The light condensed by the collimator lens 403 enters the polarization beam splitter 402. Here, the polarization beam splitter 40
2 is transmitted through the quarter-wave plate 405 twice, so that the polarization direction is almost 90 ° with respect to the direction of the going optical path.
It is spinning. Therefore, the light is polarized
2 is reflected by the optical path difference generating plate 402 and is transmitted through the optical detector 41.
Incident at 0.

In the photodetector 410, a focus error detection signal,
A track error detection signal and a data signal are detected. Said,
As in the embodiment, the optical path difference generating plate 409 is formed of a material having a high light transmittance such as glass or polycarbonate and is divided into two regions substantially at the center, and the thickness of each region is different. The optical path difference generating plate 409 is arranged so that the divided portion of the region is located substantially at the center of the light transmitted through the collimating lens 403.

Further, the incident surface of the optical path difference generating plate 409, or the incident surface and the reflecting surface are slightly inclined, so that the two lights having the optical path difference generated do not overlap on the photodetector. I have. This angle may be such that the light patterns on the photodetector do not overlap when the light spot 408 is located several tens of μm away from the recording film. Photodetector 4
Numeral 10 is arranged such that the light receiving surface is located substantially at the center of the two focal positions of the light beam divided into two by the optical path difference generating plate 409. The focus error detection signal, the track error detection signal, and the data detection signal are obtained by the same calculation as in the above embodiment.

As described above, the focus error detection system of the differential detection method before and after the beam size using the optical path difference generating plate has the disadvantage that the effect of the diffracted light due to the groove is small, but it is a disadvantage of the same method. A small and inexpensive focus error detection system can be provided without using two photodetectors.

Also, in this method, it is possible to separate the two light beams reaching the photodetector at a sufficiently large position of the light beam system with respect to the pattern on the photodetector. In contrast, a focus error detection error hardly occurs.

FIGS. 5 and 6 show another example of the optical path difference generating plate.

FIG. 5A is a diagram of the optical path difference generating plate by the trigonometric method. The circle on the front view shows the light incident on the optical path difference generating plate. Two arcs in the circle indicate a diffraction pattern by the groove of the optical disk. In this embodiment, the surface on the incident surface side is divided into three regions, and the center is perpendicular to the optical axis of the incident light. The surfaces on both sides are at an angle deviated from the perpendicular to the optical axis of the incident light. FIG. 5C is an explanatory diagram of a focus error detection system using the present optical path difference generating plate. The light narrowed down by the detection lens 502 is transmitted to an optical path difference generating plate 501.
When the light is transmitted, the light is separated into three light beams, and enters the light receiving surface 503 of the photodetector. FIG. 5-2 shows the light receiving surface 503 of the photodetector.
FIG. 5 is a diagram viewed from the incident side. The light receiving surface has seven areas a1,
It is divided into b1, c1, a2, b2, c2 and d1. The focus error detection signal ΔAF is obtained by the following calculation.

[0034]

ΔAF = (a1 + b1) − (a2 + b2) For reference, the track error detection signal ΔTR and the data detection signal S are

[0035]

ΔTR = (a1 + a2) − (b1 + b2)

[0036]

S = a1 + b1 + c1 + a2 + b2 + c2 + d1

Since the optical path difference generating plate of this embodiment does not use the strongest part of the light intensity distribution for focus error detection and TR error detection, the tolerance for fluctuations in optical axis deviation and the like is further increased.

FIG. 6A is a diagram of the optical path difference generating plate by the trigonometric method. The circle on the front view shows the light incident on the optical path difference generating plate. Two arcs in the circle indicate a diffraction pattern by the groove of the optical disk. In this embodiment, the surface on the incident surface side is divided into four parts as shown in the figure, and each region is given an angle deviated from the perpendicular to the optical axis of the incident light.

FIG. 6C is an explanatory diagram of a focus error detection system using the present optical path difference generating plate. When the light narrowed down by the detection lens 602 passes through the optical path difference generating plate 601, the light is separated into four light beams and enters the light receiving surface 603 of the photodetector. FIG. 6B is a diagram of the light receiving surface 603 of the photodetector viewed from the incident side. The light receiving surface has six areas a1, b1, c1, a2, b
2, c2. The focus error detection signal ΔAF is obtained by the following calculation.

[0040]

ΔAF = (a1 + b1) − (a2 + b2) For reference, the track error detection signal ΔTR and the data detection signal S are:

[0041]

ΔTR = (a1 + a2) − (b1 + b2)

[0042]

FIG. 12 is obtained as S = a1 + b1 + c1 + a2 + b2 + c2.

In the present embodiment, since the light for detecting the TR error is divided by the optical path difference generating plate, the offset of the TR error signal is less likely to occur particularly than in the case where the light is divided in the area of the light receiving surface of the photodetector. Become.

[0044]

According to the present invention, it is possible to provide an optical head having a focus error detection system which is inexpensive and small, and is less affected by diffracted light due to grooves on an optical disk.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an optical head according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing details of a focus error detection system of the present invention.

FIG. 3 is an explanatory view showing an example of a conventional optical head.

FIG. 4 is an explanatory view showing an optical head according to another embodiment of the present invention.

FIG. 5 is an explanatory diagram showing an optical head according to another embodiment of the present invention.

FIG. 6 is an explanatory diagram showing an optical head according to another embodiment of the present invention.

[Explanation of symbols]

101: semiconductor laser, 102: collimating lens, 1
03: beam shaping prism, 104: polarization beam splitter, 105: quarter-wave plate, 106: mirror, 107: objective lens, 108: light spot, 109
.., Detection lens, 110, optical path difference generating plate, 111, photodetector.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5D118 AA02 AA24 BA01 BB02 BF02 BF03 CA11 CA13 CC03 CC12 CD02 CD03 CD08 CF04 CG02 DA35 DA40 DA42 DB04 DB08 DB13 5D119 AA02 AA04 AA28 BA01 BB01 BB02 BB03 DA01 EA02 EA03 JA06

Claims (4)

[Claims] A light source; a collimating lens for making light emitted from the light source substantially parallel; an objective lens for condensing parallel light emitted from the collimating lens onto an information recording medium to form a minute light spot; Objective lens moving means for positioning the minute light spot at a predetermined position on the information recording medium, and the amount of deviation of the minute light spot from the predetermined position on the information recording medium is determined by using reflected light from the information recording medium. An optical head having a detecting means for detecting, a means for detecting a positional deviation of the minute optical spot in the vertical direction with respect to the information recording medium, and a positional deviation in the vertical direction with respect to the information recording medium, that is, a focus error detecting means, A detection lens that narrows down the reflected light from the information recording medium, a photodetector that receives the reflected light narrowed down by the detection lens, and a photodetector that is disposed between the detection lens and the photodetector. An optical element having at least first and second regions having different optical distances, wherein the photodetector has a position where a light beam transmitted through the first region of the optical element converges and transmitted through the second region. An optical head, which is located substantially at the center of a position where a light beam converges. 2. An optical element having first and second regions having different optical distances, so that light beams transmitted through the respective regions do not overlap on a photodetector. 2. The optical head according to claim 1, wherein both have an angle other than perpendicular to the optical axis of the incident light. 3. The optical head according to claim 1, wherein the photodetector has at least four divided areas. 4. The optical head according to claim 1, wherein the collimating lens also functions as a detecting lens.