JPH02294937A - Optical pickup device - Google Patents

Abstract

PURPOSE:To reduce an influence caused by the birefringence of a disk substrate to a focus error signal by providing an astigmatism generation photodetector having astigmatism generating function and photodetector function, and two quadripartite optical detector on a detecting system. CONSTITUTION:Reflected laser beam is made incident from the vertical magnetizing film 3b of a magneto-optical disk 3 respectively through an objective lens 14, beam splitter 12, 1/2 wavelength board 40 and detecting lens 41 on an astigmatism generation photodetector 42. For the magneto-optically deflected laser beam, the size of the quantity of light is changed by the photodetector 42. In the laser beam splitted by transmission and reflection, P polarized light transmitted through the photodetector 42 forms a spot on the light receiving surface of a first quadripartite optical detector 44 and S polarized light reflected on a polarizing film 42a of the photodetector 42 forms a spot on the light receiving surface of a second quadripartite optical detector 45. Since the focus error signal is generated based on the outputs of both detectors 44 and 45, the influence caused by the birefringence does not appear in the focus error signal.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical pickup device, and more particularly to an optical pickup device that sequentially outputs or writes information from a magneto-optical recording medium using a laser beam.

2. Description of the Related Art An optical pickup device schematically shown in FIG. 6 has been known. This device consists of a light source system 1 and a detection system 2, and records and reproduces information by irradiating a magneto-optical disk 3 with a laser beam.

The light source system 1 includes a semiconductor laser 11 which is a laser beam source,
Collimator lens 12, P polarization reflectance RP is 20%.
and a beam splitter 13 with an S polarization reflectance Rs of 100%.
, and an objective lens 14.

The detection system 2 includes a 1/2 wavelength plate 20 and a polarization beam splitter (hereinafter referred to as F, PBS) 2 that separates the light into both P and S polarizations.
1. Detection lens 22a, cylindrical lens 23 which is an astigmatism generating element, 4 divided by two orthogonal line segments.
4-split photodetector 24 with two light-receiving surfaces, detection lens 2
2b, and a two-split photodetector 25 having two light-receiving surfaces divided into two by one line segment. Here, in the detection system 2, the 1/2 wavelength plate 20 has its optical axis set at 22.5 points with respect to the incident surface of the beam splitter 13.
They are arranged at an angle of .degree., so that the separation of both P and S polarized light by the PBS 21 is equivalent for both the corresponding photodetectors 24 and 25. J: 7! for linearly polarized light entering the detection system 2 without providing the 1/2 wavelength plate 20 like this! Even when rotated at an angle of I5°, the 1/2 wavelength plate 20
The same effect can be obtained.

The operation of this device is as follows: First, a laser beam from a semiconductor laser 1 is collimated into a parallel laser beam by a collimator lens 12, passes through a beam splitter 3, and passes through an objective lens 14.
The light is focused toward the magneto-optical disk 3 at iJ. The focused laser beam focused on the magneto-optical disk 3 is reflected by the perpendicular magnetization film 3b via the substrate 3a, and this repulsed laser beam is converted into a parallel laser beam by the objective lens 14 and sent to one of the detection systems 2 by the beam splitter 13. The signal is directed to the PBS 21 via the /2 wavelength plate 20.

The PBS 21 separates the light from the beam splitter 13 into both P and S polarization. , P-polarized light transmitted through the PBS 21 is condensed by the detection lens 22a to become a focused laser beam, which forms a spot on the light receiving surface of the -c4 split photodetector 24 via the astigmatism generating element 23.

The S-polarized light reflected by the PBS 21 forms a spot on the light receiving surface of the two-split photodetector 25 via the detection lens 22b.

The shape of the spot on the 4-split photodetector 24 becomes a circle when the laser beam is focused on the magneto-optical disk 3;
When the lens is out of focus, it becomes an ellipse that extends in the diagonal direction of the four divisions due to astigmatism.

In addition, the dividing line of the two-split photodetector 25 is optically aligned with the track direction of the disk, and when the laser beam is on the track, the light intensity on the two light receiving surfaces is the same, and from When it is off, there will be a difference in the amount of light between the two light receiving surfaces.

Furthermore, the laser beam irradiated onto the magneto-optical disk 3 is linearly polarized due to the characteristics of the semiconductor laser 1.
This linearly polarized light is recorded in the magnetization direction of the perpendicularly magnetized film 3b (recorded information).
Rotates either left or right depending on the location. This rotation direction is determined by the four-split photodetector 24 and the two-split photodetector 25.
This appears as a difference in the amount of received light J'3i-J, 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 detection system 2, and each light receiving output from the four light receiving surfaces 24a to 24d of the 4-split photodetector 2/I is generated by two light receiving outputs at each of two pairs of diagonal positions. Surface (24a
and 24c, 24b, and 24d), and the difference between the added outputs is extracted as a focus error signal. Furthermore, the two light receiving surfaces 25a of the two-split photodetector 25,
The difference between the respective light reception outputs of 25b is extracted as a racking error signal of 1 and 1. Furthermore, the difference between the total received light output of the 4-split photodetector 24 and the total received light output of the 2-split photodetector 25 is extracted as a magneto-optical reproduction signal, which removes in-phase noise components and removes linearly polarized light. Information recorded on the magneto-optical disk 3 is differentially detected in all directions of rotation.

By the way, as for the transparent substrate 3a for magneto-optical disk,
A glass substrate with excellent optical uniformity is suitable, but a plastic substrate is desired from the standpoint of mass production and low cost.
Polycarbonate 1~ substrates, which are used in discs, etc., are becoming popular. However, these polycarbonates 1 to 1 have extremely high birefringence, and therefore the following phenomenon occurs.

Since the focused laser beam focused on the disk is narrowed down into a conical shape, the wavefront of the focused laser beam is perpendicular to the substrate 3a at the center, but as it approaches the periphery of the wavefront, it becomes less intrusive to the substrate 3a. The angle becomes larger. Therefore, the light at the periphery of the wavefront of the reflected laser beam receives retardation due to birefringence of the substrate 3a and becomes elliptically polarized light.

Therefore, the intensity distribution on the light receiving surfaces of the photodetectors 24 and 25 becomes non-uniform due to the elliptically polarized light components, and as shown in FIG. One diagonal pair (24
b, 24d) is different from the other diagonal pair (24a, 24C).
) is generated. This phenomenon causes an offset in the focus error signal from the photodetector, and an unbalanced cross-track component on the photodetector.

Therefore, when performing recording on a magneto-optical disk using polycarbonate 1 as a substrate, which has a large birefringence, it is better to use a magneto-optical disk with a substrate made of glass, PMMA, etc., which has almost no birefringence. There is a problem in that the 1-to-rack cross-crossing noise to the 1-to-1 rack signal increases.

In addition, the structure of the conventional optical pickup device for magneto-optical recording and reproducing equipment has a large number of parts as described above, and the configuration is complicated. There is also the problem.

1. It is an object of the present invention to solve the above-mentioned drawbacks and to reduce the influence of the birefringence of the disk substrate on the focus error signal while consolidating and simplifying each constituent functional element that makes up the device. Our company provides optical pickup devices for magneto-optical recording and reproducing devices.

The present invention is a focus error signal generating device that is applied to an optical pickup device that reads information by irradiating a focused laser beam onto an optical recording medium, and which outputs a focused laser beam that enters from the recording medium into two light beams. an astigmatism generation/analysis means having an astigmatism generation function that separates the laser beam into a bundle and imparts astigmatism thereto, and an analyzer function that detects the polarization of the focused laser beam; and an optical path of the two-ray bundle. a pair of 4-split photodetectors each having a light-receiving surface divided into 4 parts along two orthogonal line segments; the astigmatism generation analyzing means includes a pair of transparent double-sided parallel plates and a polarizing film supported between the double-sided parallel plates; The polarizing film is characterized in that the polarizing film is inclined at a predetermined angle with respect to the wavefront of the incident laser beam.

An example of the present invention will now be described with reference to the drawings.

FIG. 1 (aHb) is a schematic diagram of the optical pickup mounting according to this embodiment. Incidentally, in FIG. 1, members having the same symbols as those in FIG. 6 indicate the same parts, and therefore their explanations will be omitted. The detection system 4 of this device includes a 1/2 wavelength plate 40, a detection lens 41, an astigmatism generating analyzer/I2, a 4-split photodetector of ml, and a second
It is composed of four-divided photodetectors 44 and 45. These 4-split photodetectors detect a nocus error signal from a laser beam having astigmatism for automatically controlling the distance between the magneto-optical disk and the objective lens by the side of the focus servo machine.

The astigmatism generating analyzer 42 is an element in which a polarizing film 42a is sandwiched between a pair of transparent double-sided parallel plates having the same monthly rate and the same thickness. The astigmatism generating analyzer 42 is tilted at a predetermined angle with respect to the wavefront of the laser beam generated by the person, thereby generating an astigmatism that imparts astigmatism to the laser beam. It has an analyzer function that detects the presence or absence of polarization of the Lely beam whose vibration plane has been magneto-optically rotated, and almost 100% of the P-polarized light component is transmitted and almost 100% of the S-polarized light component is reflected. Since the astigmatism generating analyzer 42 has double-sided parallel plates formed symmetrically with respect to the polarizing film A2a, it is possible to obtain the same amount of astigmatism on both sides of the polarizing film 42a. like,
If the astigmatism generating analyzer 42 can separate the laser beam into three ray bundles and obtain astigmatism for these two ray bundles, the astigmatism generating analyzer 42 can be used instead of a symmetrical double-sided parallel plate with different thicknesses on both sides. It may be formed of double-sided parallel plates or real parallel plates made of materials with different ratios. For example, when a double-sided parallel plate with a high refractive index and a double-sided parallel plate with a low refractive index are combined, the thickness of the plate with the higher refractive index is made thinner than the plate with the lower refractive index, and the astigmatism is formed. Form an aberration generating analyzer. The first and second four-split photodetectors 4, 4, and 45 are arranged such that both light receiving surfaces are located in mirror image positions with respect to the polarizing film of the astigmatism generating analyzer 42.

Furthermore, instead of using the 1/2 wavelength plate 40, this detection system 4 is similar to the detection system 2 described with reference to FIG. The same effect can be obtained even if the rotation is avoided by 45 degrees.

The operation of the apparatus of this embodiment is as follows: First, the magneto-optical disk 3
The reflected laser beam from the perpendicularly magnetized film 3b is focused by the objective lens 1/'I and directed to the beam splitter 13. The laser beam from the beam splitter 13 passes through a 1/2 wavelength plate 40, is focused by a detection lens 41, and is input into an astigmatism generating analyzer 42. Here, the optically polarized laser beam passes through an analyzer 42 and is subjected to changes in the magnitude of the light beam. Minute N1 due to transmission and reflection
In the laser beam d3, the P-polarized light transmitted through the analyzer 42 forms a spot on the light-receiving surface of the first 4-split photodetector 44, and is reflected by the polarizing film 42a of the analyzer 42. The S-polarized light forms a spot on the light receiving surface of the second 4-split photodetector 45.

FIG. 2 shows the beam splitter 13, the astigmatism generating analyzer 42, and the first and second 4-split photodetectors 44.4!
) is a diagram schematically showing the positional relationship between the two. The dividing lines on the light receiving surfaces of the first and second four-part photodetectors 44.4.5 are optically aligned with the 1-to-rack direction of the magneto-optical disk 3 and the direction orthogonal thereto. Further, the elliptical spots 1 to 1 at the time of astigmatism in the astigmatism generating element 42 are formed so as to extend in the direction of the diagonal positions of the four light-receiving surfaces as shown in the figure.

FIG. 3 is a diagram showing spots formed on the light-receiving surfaces of the first and second 4-split photodetectors 44, 45;
) shows the first 4-split photodetector, and FIG. 1(b) shows the second 4-split photodetector.

When the optical disk 3 is in focus, S○ is not formed on the spot 1. When the focus is shifted to either the far point or the near point, circle-shaped spots 1 to S1 are formed, and when the focus is shifted to the other point, spots 1 to 8 are formed.
Elliptical slots 1 to S2 are formed in a direction perpendicular to 1. Further, when elliptically polarized light is generated as described above, a portion Sp where the amount of light is strong is generated around the laser beam, as indicated by diagonal lines in the slots 1 to So, for example.

When the focus is out of focus, in the first 4-split photodetector 44, the light-receiving surface 4 at the diagonal position in the spot 1-82 direction
4. b, 446 becomes stronger, and the light receiving surface 4.b, located at a diagonal position in the direction of spot S1. /I a, 4. /l.
The light of C becomes weaker. In addition, a second 4-split photodetector 4
5, the light intensity of the light-receiving surfaces 45a and 45G at diagonal positions in the spot S1 direction becomes strong, and the light intensity of the light-receiving surfaces 45b and 45d at diagonal positions in the spot 1-S2 direction becomes weak.

FIG. 4 is a circuit diagram according to an embodiment, and as shown in the figure, in the first 4-split photodetector 44, the light receiving surfaces 44b and 44d at diagonal positions are connected to each other. The addition output of each light receiving output of the light receiving surfaces 44a and 44G at the other diagonal positions is subtracted, and in the second 4-split photodetector 45, the addition output of each light receiving output of the light receiving surfaces 44a and 44G at the other diagonal positions is subtracted. A certain light-receiving surface/15
From the summed output of the respective light receiving outputs of 45d and 45d, the light receiving surface 14 at the other diagonal position is determined. The summed output of each light reception output of 5a and /I5c is subtracted.

Then, the subtraction output from the first 4-division photodetector 44 and the subtraction output from the second 4-division filters A1 to Detector/15 are added to generate a focus error signal.

The detection system 2 is adjusted so that the focus error signal becomes zero when the diameter of the beam irradiated onto the magneto-optical disk 3 becomes the minimum.

Since the subtracted output from the first 4-split photodetector 44 is added with the same polarity, each 4-split photodetector 44. cancel each other out, and the effect of the birefringence of the substrate does not appear on this added output, that is, the focus error signal.Also, since the above focus error signal is generated based on the two 4-split photodetectors, the 6th Compared to the conventional ones as shown in FIG.

Note that the adjacent light receiving surface 45 of the second 4-split photodetector 45
The sum of the light receiving outputs of the other adjacent light receiving surfaces 45a and /I5d is subtracted from the sum of the light receiving outputs of the light receiving surfaces b and 45C, and a racking error signal of 1 to 1 is generated. Roughly speaking, each of the light receiving surfaces 448 to 41 of the first 4-split photodetector 44
From the total output of the received light output of d, the second 4-split photodetector 45
The total output of the light receiving outputs of each of the light receiving devices 45a to 45d is subtracted, and the magneto-optical reproduction signal is differentially detected.

In the above embodiment, the first and second quadrant photodetectors 44,
For 4.5, first subtract the received light output at the same diagonal position,
Although the focus error signal is generated by adding the subtracted outputs, the same effect can be obtained by configuring the circuit as shown in FIG. 5, for example.

That is, regarding the light receiving output of each of the four-divided light receiving surfaces of the first and second four-divided photodetectors 44 and 45, spots 1 to 45
The outputs of the four light-receiving surfaces (44a, 44c, 45a and /45C) that have large outputs with respect to S+ (FIG. 3) are added, and the outputs of the four light-receiving surfaces (4. /l.b, 4.4d, 45b and 45
The outputs for d) may be added and a focus error signal may be generated based on the difference between the summed outputs j.

Although each of the above embodiments has been described with respect to information reproduction, it goes without saying that an appropriate focus error signal can be generated using the same configuration when information is recorded or erased. Further, even if the optical pickup device does not perform recording/reproduction on a magneto-optical disk, the same effect can be obtained as long as the detection system uses an astigmatism-generating analyzer.

Furthermore, if the detection system deviates from 45 degrees with respect to linearly polarized light, if the polarizing film characteristics are uneven, or if there is unevenness in the sensitivity or distribution of the light receiving element, the output of each light receiving surface may become unbalanced. , an offset component may occur in the focus error. Therefore, when calculating the output of each historical light receiving surface,
The offset 1 component may be removed by at least one variable resistance means. This variable resistance means is
For example, in the embodiment shown in Fig. 4, the adder is placed on the one-manpower side, and in the embodiment shown in Fig. 5, it is placed on the one-manpower side of the subtracter. It is only necessary to remove the offset components by appropriately disposing them between the output obtained and the output finally obtaining the focus error.

As explained above, according to the optical pickup device of the present invention, the detection system includes a pair of transparent double-sided parallel plates made of the same material and of the same thickness, and a polarizing film supported between them. Polarized light j with respect to the wavefront of the laser beam to be used
14! By using an astigmatism generation analysis means tilted at a predetermined angle, one analyzer is configured to have both an astigmatism generation function and an analyzer function, making the device compact and lightweight. It is possible to reduce the number of parts and the cost. Furthermore, two beams of light separated by the astigmatism generating/analyzing means, ie, P. Both S-polarized light components are received by a photodetector divided into four parts, and the non-uniformity of the intensity distribution in the first photodetector and the second photodetector is carefully offset. Since a focus error signal is generated and the effect of the magnitude of birefringence of the disk on the focus error signal is eliminated, it is possible to Regardless of the magnitude of refraction, it is possible to eliminate deviations in the position of the focus 1 to focus A and cross-crossing noise of the focus 1 to rack.In addition, the light receiving output that generates the focus A error signal is doubled compared to the conventional method. As a result, the fuselage becomes stable.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the optical system of an optical pickup device according to an embodiment of the present invention, FIG. 2 is a schematic diagram showing the positional relationship of a photodetector in the optical system of the embodiment, and FIG. The figure is a front view of the light-receiving surface of the photodetector showing the state of the laser beam spot 1 in the embodiment, FIG. 4 is a circuit diagram of the optical pickup device in the embodiment, and FIG. 5 is a circuit in another embodiment. 6 is a schematic diagram showing the optical system of a conventional optical pickup device, FIG. 7 is a circuit diagram of a conventional optical pickup device, and FIG. 8 is a schematic diagram of the optical system of a conventional optical pickup device. Figure C is a front view of the light-receiving body of the photodetector showing the state of FIG. Explanation of symbols of main parts 42... Astigmatism generating analyzer 44... First quarter υj photodetector 45...
・Second 4-split photodetector 1 person from Tokyo

Claims (7)

[Claims] (1) A focus error signal generating device in an optical pickup device that reads information by irradiating a focused laser beam onto an optical recording medium, the device separating the focused laser beam incident from the recording medium into two beam bundles and dividing the focused laser beam into two beams. astigmatism generating and analyzing means having an astigmatism generating function for imparting astigmatism and an analyzer function for detecting the polarization of the focused laser beam; A pair of 4 light-receiving surfaces divided into four by two line segments
It consists of a split photodetector, and a calculation means connected to each of the photodetectors and for calculating signal outputs emitted from these and outputting at least a focus error signal, and the astigmatism generating and analyzing means is a transparent 1. A device comprising a pair of double-sided parallel plates and a polarizing film sandwiched between the double-sided parallel plates, the polarizing film being tilted at a predetermined angle with respect to the wavefront of an incident laser beam. 2. The apparatus according to claim 1, wherein each of said double-sided parallel plates is made of the same material and has the same thickness. (3) The device according to claim 1 or 2, wherein the four-split photodetector is arranged such that both light-receiving surfaces are located in mirror image positions with respect to the polarizing film. (4) The output of each light-receiving surface of the 4-split photodetector is supplied to the calculation means, and the calculation means calculates the sum output of all the light-receiving surfaces whose light reception output is large for one astigmatism and the residual. 4. The apparatus according to claim 3, wherein a difference output from a sum output of all light receiving surfaces is determined, and the difference output is used as a focus error signal. (5) The arithmetic means includes a first addition means for calculating the sum output of the light-receiving surfaces where the light-receiving output is large for one astigmatism, corresponding to each of the four-split photodetectors; a second addition means for calculating the sum output of the light-receiving surface; and a subtraction means for calculating the difference between the sum output of the first addition means and the sum output of the second addition means; 5. The apparatus according to claim 4, further comprising third adding means for calculating the sum of the difference outputs of the corresponding subtracting means. (6) The calculation means includes a first addition means for calculating the sum output of all the light receiving surfaces in which the light receiving output is large with respect to the astigmatism of one of all the light receiving surfaces of the four-split photodetector; 5. The apparatus according to claim 4, further comprising a second addition means for determining the sum output of the remaining light-receiving surfaces, and a subtraction means for determining the difference between the sum outputs of the first and second addition means. (7) The device according to any one of claims 1 to 6, wherein the calculation means includes at least one variable resistance means.