JP3327929B2 - Optical pickup device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup device, and more particularly to an optical system of an optical pickup device for reading information from an optical recording medium such as a magneto-optical disk using a laser beam.

[0002]

2. Description of the Related Art Conventionally, an optical pickup device as shown in FIG. 6 has been known. Such an apparatus includes an optical system 2 for irradiating light from a light source provided to a magneto-optical disk 1 as an optical recording medium to extract reflected light, a focus error signal and a tracking error signal based on a change in the reflected light. Signal generating means 3 for generating a signal and a reproduction signal.

The optical system 2 includes a semiconductor laser 11, which is a laser beam source, a collimator lens 12, a P-polarized reflectance Rp.
Is 20% and the S-polarized light reflectance Rs is 100%. The signal generating means 3 includes a half-wave plate 20, a polarization beam splitter (hereinafter, PBS) for splitting into both P and S polarized lights.
21, a detection lens 22a, a cylindrical lens 23 as an astigmatism generating element, a quadrant photodetector 24 having four light receiving surfaces divided into four by two orthogonal line segments, a detection lens 22b, and a single line. It is composed of a two-segment photodetector 25 having two light-receiving surfaces divided into two parts. Here, in the signal generating means 3, the half-wave plate 20 is disposed with its optical axis inclined at 22.5 ° with respect to the incident surface of the beam splitter 13,
And the separation of the S-polarized light and the S-polarized light is made equal to the corresponding photodetectors 24 and 25. In addition, even if the optical system 2 is rotated at an angle of 45 ° with respect to the incident linearly polarized light without providing the half-wave plate 20, the same effect as the half-wave plate 20 can be obtained.

The operation of such a device is as follows.
The laser beam from 1 is converted into a parallel laser beam by the collimator lens 12, passes through the beam splitter 13, and is condensed by the objective lens 14 toward the magneto-optical disk 1.
The focused laser beam condensed on the magneto-optical disk 1 is reflected by the perpendicular magnetization film 1b via the substrate 1a, and this reflected laser beam is converted into a parallel laser beam by the objective lens 14 and is converted by the beam splitter 13 into a signal generated by the signal generating means 3. 1/2
It is directed to the PBS 21 via the wave plate 20.

[0005] The PBS 21 separates the light from the beam splitter 13 into P and S polarized lights. P transmitted through PBS21
The polarized light is condensed by the detection lens 22a to become a focused laser beam, and is transmitted through the astigmatism generating element 23 to the quadrant photodetector 2.
A spot is formed on the light receiving surface of No. 4. The S-polarized light reflected by the PBS 21 forms a spot on the light receiving surface of the two-segment photodetector 25 via the detection lens 22b.

When the laser beam is focused on the magneto-optical disk 1, the spot shape in the quadrant photodetector 24 becomes a circle, and when the laser beam is out of focus, the diagonal position of the quadrant is shifted due to astigmatism. It becomes an elliptical shape that extends. Also, the dividing line of the two-segment photodetector 25 is optically coincident with the track direction of the disk, and when the laser beam is on the track, the amounts of light on the two light receiving surfaces are the same, and the light is off the track. When there is, there is a difference between the light amounts of the two light receiving surfaces.

The laser beam applied to the magneto-optical disk 1 is linearly polarized by the characteristics of the semiconductor laser 11, and this linearly polarized light is either left or right depending on the magnetization direction (recording information) of the perpendicular magnetization film 1b. Rotated in the direction of. This rotation direction appears as a difference in the amount of received light between the four-divided photodetector 24 and the two-divided photodetector 25, and a magneto-optical reproduction signal is obtained from the difference in the amount of received light.

FIG. 7 is a circuit diagram of the signal generating means 3 and shows four light receiving surfaces 24a to 24a of the four-divided photodetector 24.
Each light receiving output by 4d is added for each of two light receiving surfaces (24a and 24c, 24b and 24d) at two diagonal positions, and a difference between the added outputs is taken out as a focus error signal. The difference between the respective light receiving outputs of the two light receiving surfaces 25a and 25b of the two-segment photodetector 25 is extracted as a tracking error signal. Further, the difference between the total received light output of the four-split photodetector 24 and the total received light output of the two-split photodetector 25 is taken out as a magneto-optical reproduction signal, whereby in-phase noise components are removed and linearly polarized light is removed. The direction of rotation, that is, the information recorded on the magneto-optical disk 1 is differentially detected.

By the way, a transparent substrate 1 for a magneto-optical disk
As a, a glass substrate excellent in optical uniformity is suitable, but plasticization is desired from the viewpoint of mass productivity and cost, and a polycarbonate substrate used for a compact disk or the like is promising. However,
Since this polycarbonate has a large birefringence, the following phenomenon occurs.

Since the focused laser beam focused on the disk is narrowed down to a conical shape, the center of the wavefront of the focused laser beam is perpendicular to the substrate 1a. Incident angle increases. For this reason, the light in the peripheral portion of the wave front of the reflected laser beam receives the retardation due to the birefringence of the substrate 1a and becomes elliptically polarized light.

Therefore, the photodetector 2 is formed by the elliptically polarized light component.
The intensity distribution on the light receiving surfaces 4 and 25 becomes non-uniform, and as shown in FIG. 8, one of the four light receiving surfaces of the four-divided photodetector 24 receiving the spot So at the focused position has a diagonal pair ( 24b, 24d) is received by the other diagonal pair (24
a, 24c).
Due to this phenomenon, an offset occurs in the focus error signal from the photodetector, and the track crossing component on the photodetector becomes unbalanced.

For this reason, when recording / reproducing a magneto-optical disk using polycarbonate having a large birefringence as a substrate, the best performance is obtained compared with the case where a magneto-optical disk using glass or PMMA having little birefringence as a substrate is used. There is a problem that the focus position shifts and noise crossing the track to the focus error signal increases. Light receiving surface 2
In the so-called push-pull method in which the difference between the respective light receiving outputs 5a and 25b is used as a tracking error signal, if the orthogonal relationship between the disk recording surface and the laser beam shifts, the projected position of the reflected light projected on each light receiving surface becomes one. , Each light receiving output becomes unbalanced, and an offset occurs in the tracking error signal. Also, when the objective lens is shifted from the neutral position in the tracking direction, reflected light is shifted by the shifted amount, so that each light receiving output is also unbalanced, causing an offset in the tracking error signal.

Further, the push-pull method is based on the principle of detecting a change in the distribution of the diffracted light component due to the track in the reflected light. The change in the distribution of the diffracted light component also affects the focus error signal, The tracking error signal sometimes leaked into the focus error signal.

[0014]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points.
Therefore, the purpose is to
Reduced the effect of birefringence on focus error signal
An object of the present invention is to provide an optical pickup device.

[0015]

According to a first aspect of the present invention, an optical pickup device irradiates light from a light source to an optical recording medium to extract reflected light, and a focus and tracking error signal based on a change in the reflected light. An optical pickup device including focus and tracking error detection means for generating a reproduction signal, and reproduction signal detection means, wherein the optical pickup device corresponds to a central portion in a beam spot of the reflected light.
Three reflecting (or transmitting) surfaces at different positions , and
When the three reflective (or transmissive) surfaces are arranged in approximately one line
Specifically, of the three reflecting (or transmitting) surfaces, light passing through one surface closest to the optical axis of the reflected light is supplied to the focus error detecting means, and light passing through the remaining surface is detected as the tracking error detection light. Reflection (or transmission) feeding means
Means.

[0016]

In the optical pickup device having such a structure,
Has three reflecting (or transmitting) surfaces, and the three
Light passing through one of the surfaces closest to the optical axis of the reflected light
Is supplied to the focus error detecting means, and the remaining surface is
The transmitted light is supplied to the tracking error detecting means.

[0017]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a block diagram of an optical pickup device configured according to the first embodiment of the present invention. In addition,
6 are given the same reference numerals. In the figure, an optical pickup device includes an optical system 32 for irradiating light from a light source provided on the optical disk 1 as an optical recording medium to extract reflected light, and a change in reflected light from the magneto-optical disk 1. Signal generating means 3 for generating a focus error signal, a tracking error signal and a reproduction signal based on
3

The optical system 32 includes a semiconductor laser 35 as a light source for emitting a laser beam, and a collimator lens 36.
, A prism 37, a beam splitter 38 having a P-polarized reflectance Rp of 20% and an S-polarized reflectance Rs of 100%, and an objective lens 39. On the other hand, the signal generating means 33 includes a half-wave plate 41, a detection lens 42, a transparent parallel flat plate 43 as astigmatism imparting means, and a polarizing beam splitter (hereinafter, referred to as P and S polarized light). P
BS) 44 and a so-called six-segment photodetector 45
And a pair of photodetectors 46 and 47. The parallel flat plate 43 has a non-polarized reflection film on its beam incident surface. The reflection film has a reflectance (transmittance) of about 50%, for example, as shown in FIG. One transmission surface 43a, 43b, 43c is formed, and the other part except these is a total reflection surface. As shown in the figure, the transmission surfaces 43a to 43c are each formed in a circular shape, and are arranged in the central portion within the beam spot (inside the dotted line) of the parallel flat plate 43. The parallel plate 43 imparts astigmatism to the laser beam by inclining the laser beam incident on the parallel plate 43 at a predetermined angle with respect to the optical axis of the laser beam. A converging means for converging light is provided.

The six-split photodetector 45 includes a tracking error signal for automatically controlling the displacement of the objective lens 39 in the tracking direction by a tracking servo mechanism, and the magneto-optical disk 1 and the objective lens 39 by a focus servo mechanism. And a focus error signal for automatically controlling the in-focus state, and the other photodetectors 46 and 47 obtain a magneto-optical reproduction signal.

Next, the operation of the optical pickup device will be described. First, a laser beam from a semiconductor laser 35 is converted into a parallel laser beam by a collimator lens 36, the optical path of which is bent by a prism 37, passes through a beam splitter 38, and is transmitted to a magneto-optical disk 1 by an objective lens 39.
Light is collected toward. The focused laser beam condensed on the magneto-optical disk 1 is reflected by the perpendicular magnetization film 1b via the substrate 1a, and this reflected laser beam is converted into a parallel laser beam by the objective lens 39, and the beam is split by the beam splitter 38 into the signal generating means 33. Through the half-wave plate 41. 1 /
The parallel laser beam having passed through the two-wavelength plate 41 is supplied to a detection lens 42.
After the convergence is imparted by the above, it reaches the parallel flat plate 43. The central portion of the optical axis of the focused laser beam that has reached the parallel plate 43 is transmitted in the direction of the optical path by the transmission surfaces 43a, 43b, and 43c of the parallel plate, and is given astigmatism, so that the six-segment photodetector 45 is provided. Reach

On the other hand, the focused laser beam reflected by the parallel plate 43, that is, the outer portion of the beam and a portion reflected by the transmission surface pass through the PBS 44 without being transmitted, thereby forming a pair of photodetectors. 46 and 4
The light is separated into P and S polarized light toward. Then, a magneto-optical reproduction signal is obtained according to the difference between the amounts of light received by the photodetectors 46 and 47.

FIG. 3 shows a circuit for generating a tracking and focus error signal in the optical pickup device shown in FIG. As shown in the figure, the six-segment photodetector 45 has a four-segment photodetector 48 having four light receiving surfaces divided into four by two orthogonal line segments, and a pair of light beams interposed therebetween and arranged in the track direction. It comprises detectors 49a and 49b. In such a quadrant photodetector 48, the sum output of the respective light receiving outputs of the light receiving surfaces 48a and 48c at one diagonal position is subtracted from the sum output of the respective light receiving outputs of the light receiving surfaces 48b and 48d at the other diagonal position. Then, a focus error signal is generated. Also, a pair of photodetectors 49a and 49b
, The photodetector 49a based on the push-pull method
The tracking error signal is generated by subtracting the light receiving output of the photodetector 49b from the light receiving output of.

As described above, this six-segment photodetector 45
, By the transmission surface provided to the parallel plate 43,
Of the light reflected from the magneto-optical disk 1, only the light at the center of the optical axis is guided. The light in this portion is reflected almost perpendicularly to the substrate 1a of the magneto-optical disk 1, and is hardly affected by the birefringence of the substrate 1a. Therefore, the intensity distribution of the light incident on the light receiving surface of the four-split photodetector 48 becomes uniform over the entire light receiving surface without including an elliptically polarized light component based on birefringence.

The reflected light transmitted through the transmitting surfaces 43a and 43c is projected on the photodetectors 49a and 49b, respectively, and the movement of the beam due to the disk skew and the lens shift in each of the photodetectors 49a and 49b. Even if there is, the difference output is not affected, so that the offset of the tracking error signal hardly occurs. In addition, the quadrant photodetector 4
8, the reflected light of the central portion of the track where the diffracted light component is small is projected, so that the phenomenon of the tracking error signal leaking into the focus error signal is reduced.

In the first embodiment, the transmission surface for transmitting a part of the reflected light from the magneto-optical disk 1 in the optical path direction is provided on the main surface of the parallel plate 43 for generating astigmatism. However, the configuration according to the second embodiment as shown in FIG. 4 can provide the same effect as that of the first embodiment. The optical pickup device shown in FIG. 4 has the same structure as the first device shown in FIGS.
The configuration is the same as that of the optical pickup device according to the embodiment, and the operation thereof is also the same, so that the details are not described.

That is, in FIG. 4, the parallel flat plate 43 '
Has three reflecting surfaces made of, for example, a reflecting film having a reflectance of 50% on the surface opposite to the incident surface. This reflecting surface is also disposed near the center portion in the incident beam spot as in FIG. 2, and the optical axis center portion of the focused laser beam incident from the detection lens 42 is formed by the parallel flat plate 43 '. Provided with astigmatism and the reflection surface 43a
, 43b 'and 43c' are reflected to the side of the optical path, and 6
The light reaches the split photodetector 45.

On the other hand, the laser beam that has passed through the parallel flat plate 43 ', that is, the outer portion of the beam and a portion that has passed through the reflecting surface, pass through the PBS 44 without being reflected, and are thus reflected by a pair of photodetectors 46 and 47. Is polarized and separated. The generation of the error signal and the reproduction signal by each photodetector is as described in the first embodiment.

In this embodiment, contrary to the optical system (including the signal generating means 33) in the first embodiment, the transmission component of the parallel plate is guided to the photodetector for detecting the reproduction signal. Since the central portion of the optical path of the reflected light is guided to the photodetector for detecting an error signal, the same effect as in the first embodiment can be obtained. FIG. 5 is a block diagram of an optical pickup device according to a third embodiment of the present invention.

In this embodiment, unlike the configuration shown in FIG. 1, the half-wave plate 41 is provided in the reflected light path reflecting the parallel plate 43. According to such a configuration, the influence of birefringence can be reduced even if the transmission surface formed on the parallel plate 43 is not a non-polarizing reflection film. For example, the reflectance of the P-polarized component is 50%, and the reflectance of the S-polarized component is 50%. Rate 100
%. This reflectivity is due to the fact that the Kerr effect in the magneto-optical reproducing method rotates the P-polarized component into the S-polarized component. This way,
It is possible to guide a magneto-optical signal component to the photodetectors 46 and 47 for detecting a read signal without using a non-polarizing reflection film.

[0030]

According to the present invention, as described in detail above,
In an optical pickup device, three reflection (or transmission)
Iii) having a surface, and the optical axis of the reflected light among the three surfaces
Detecting the focus error by detecting the light passing through the closest one surface
And the light passing through the remaining surface is
And the convergence is improved.
Reflection affected by birefringence of the disk substrate due to having
The light outside the light does not enter the photodetector. Follow
The intensity distribution of light incident on the light receiving surface of the photodetector is uniform.
Of glass substrates and polycarbonate substrates
Regardless of the size of the birefringence of the substrate material like a disk,
Since the shift of the best focus position can be eliminated
is there.

[0031]

[Brief description of the drawings]

FIG. 1 is a block diagram of an optical pickup device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an incident surface of a parallel flat plate.

FIG. 3 is a circuit diagram of signal detection means in the optical pickup device of FIG.

FIG. 4 is a block diagram of an optical pickup device according to a second embodiment of the present invention.

FIG. 5 is a block diagram of an optical pickup device according to a third embodiment of the present invention.

FIG. 6 is a block diagram showing an example of a conventional optical pickup device.

FIG. 7 is a circuit diagram of signal detection means in the optical pickup device of FIG.

FIG. 8 is a front view of a light receiving surface of a photodetector showing a state of a spot of a laser beam due to birefringence, and is a view for explaining non-uniformity of intensity distribution of the spot.

[Explanation of Signs of Main Parts]

32 optical system 33 signal generating means 35 semiconductor laser 36 collimator lens 37 prism 38 beam splitter 39 objective lens 41 half-wave plate 42 detection lens 43 43 ': transparent parallel flat plate 44: polarizing beam splitter 45: six-segment photodetector 46, 47 for focus and tracking error detection Reproduced signal detection photodetector 43a, 43b, 43c ... semi-transmissive film 48 …… Focus error detection photodetectors 49a, 49b …… Tracking error detection photodetectors

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-101641 (JP, A) JP-A-3-1387841 (JP, A) JP-A-3-78128 (JP, A) JP-A-2- 12620 (JP, A) JP-A-62-154338 (JP, A) JP-A-1-223653 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G11B 7 / 09-7 / 135

Claims (5)

(57) [Claims] 1. An optical system for irradiating an optical recording medium with light from a light source to extract reflected light, and a focus and tracking error detecting means for generating a focus and tracking error signal and a reproduction signal based on a change in the reflected light. and an optical pickup apparatus including a reproduction signal detecting means, said has three reflective (or transmissive) plane at a position corresponding to a central portion of the reflected light, and the three reflective (or transmissive
Ii) The surfaces are arranged substantially in a line and the three reflections
(Or transmission) surface which is closest to the optical axis of the reflected light
An optical pickup device comprising: a reflection (or transmission) unit that supplies light passing through one surface to the focus error detection unit and supplies light passing through the remaining surface to the tracking error detection unit. 2. An optical system for irradiating an optical recording medium with light from a light source to extract reflected light, and a focus and tracking error detecting means for generating a focus and tracking error signal and a reproduction signal based on a change in the reflected light. And an optical pickup device including a reproduction signal detecting means, wherein only a part of a central portion of the reflected light is reflected (or transmitted).
And a reflection (or transmission) means for supplying to the focus error detection means and supplying a part other than the central portion of the reflected light to the reproduction signal detection means. 3. The reflection (or transmission) means reflects (or transmits) only a part of a central portion of the reflected light,
3. The optical pickup device according to claim 2, wherein the optical pickup device supplies the tracking error to a tracking error detecting unit. 4. The reflection (or transmission) means comprises:
Of the reflected light for supplying to the focus error detecting means.
Reflection (or transmission) of part of the central part
It has a radiation (or transmission) part and its reflection (or transmission) part
Is a given reflection (or transmission) rate, and its reflection (or
Is the light transmitted (or reflected) in the
The signal is supplied to raw signal detection means.
The other is the optical pickup device of the mounting come to 3. 5. The reflection (or transmission) means comprises:
Astigmatism in the emitted light, and the reflection giving the astigmatism
Supplying light to the focus error detecting means.
The optical pick-up according to any one of claims 1 to 4,
Backup device.