JPS6161240A - Optical system signal reproducing device - Google Patents

Abstract

PURPOSE:To make focus error detection by an astigmatism method with simple constitution that does not use a cylindrical lens by placing the first transparent parallel flat plate in the light path between a collimator lens and a light detector and slanting it to the optical axis. CONSTITUTION:The parallel flat plate 8 is rotated around an axis that corresponds optically to recording tracks of an optical recording medium, and consequently, it is slanted by an angle theta to the optical axis, and further, it is placed in the state rotated by 45 deg. around the optic axis. A focus error signal is optained by the difference of sum of detection output of detecting sections 10a, 10c of the first and third quadrants of a light detector 10 and the sum of detection output of detecting sections 10b, 10d of the second and fourth quadrants by astigmatism generated by the slanted parallel flat plate 8. A tracking error signal is obtained by a difference in detection output of light detectors 11, 12. An objective lens 6 is shifted in the direction (y) perpendicular to the direction (x) of recording tracks of the recording medium 7 by the tracking error signal.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an optical signal reproducing device using a semiconductor laser light source as a laser source, and detects a focus error using an astigmatism method using a simple optical system. The aim is to provide a device that can do this.

[Background of the invention]

Conventionally, in focus error detection using the astigmatism method, for example
As shown in Japanese Patent No. 39123), there is known an optical system that uses a condenser lens with a uniform condensing direction and a cylindrical lens with a unidirectional condensing direction. Furthermore, as shown in Japanese Patent Application Laid-Open No. 57-205833, a photodetector divided into four quadrant photodetectors is used to detect astigmatism of a cylindrical lens. There is.

This method does not require an auxiliary beam, is simple in configuration, and does not have any effects such as changes in the brightness of the light emitted from the light source or changes in the reflectance of the disc, so it is suitable for optical video disc players and other devices. Widely used in audio disc players. However, there was a problem in that an expensive and special optical component called a cylindrical lens was required to generate astigmatism.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical signal reproducing device that can perform focus error detection using an astigmatism method with a simple configuration without using a cylindrical lens.

[Summary of the invention]

In order to achieve the above object, a diverging beam from a semiconductor laser light source is made into a parallel beam by a collimating lens, the said parallel beam is focused by an objective lens, and is irradiated onto the optical recording medium. In an optical reproducing device that obtains a reproduced signal by irradiating a reflected beam onto a photodetector, a transparent first parallel plate is provided in an optical path between the collimating lens and the photodetector and is tilted with respect to the optical axis. The optical e-coat 91+φF snoring pattern is arranged so that the beam is focused and astigmatism is produced, and the front P
A focus error signal is obtained by detecting the focused shape of the reflected beam using a photodetector.

It is known that astigmatism occurs in the transmitted and refracted light when a transparent parallel plate is placed with its optical axis tilted in focused light.The thickness of the parallel plate is t, and the refraction height is n. , the astigmatism difference Δ that occurs when the inclination angle θ is as follows. (Japanese Unexamined Patent Publication No. 58-143445) On the other hand, the focal length of the objective lens is f. bj, the focal length of the collimating lens is ' COt r the detection range of the focus error signal (when the fluctuation of the optical recording medium when the focus error signal changes from the minimum value to the maximum value is Δ°, it is generated by a parallel plate) The astigmatism difference Δ that should be
This is because it fluctuates twice as much.

In the present invention, the relational expression obtained from the 111.121 equation...
... (3) Set each variable to satisfy the following.

For example, Δ1sll! 9μm fCol=18m fobj-4,5 road mt511 θ-45', then t? to7g+1, and a generally available parallel glass plate with a thickness of about 1 inch can be used.

[Embodiments of the invention]

A first embodiment of the present invention will be described below with reference to FIG.

A diverging beam 2 emitted from a semiconductor laser light source (laser diode) 1 is converted into a 0th-order diffracted beam 2 by a diffraction grating 3.
It is separated into aI +1st order diffraction beam 2b and -1st order diffraction beam 2c, and each of the separated beams 2al 2bl 2
The beams c are incident on the collimating lens 5 through the prism 4 having a beam splitter surface 4a, and are formed into parallel beams 5a, and then incident on the objective lens 6J/C to form a convergent beam 6a, which is formed in a line on the recording medium 7. Line up and focus. Note that each beam 2al focused on the recording medium 7
2bl 21? , a +1st order diffraction beam 2b and a -1st order diffraction beam 2c are arranged in the direction y perpendicular to the recording track direction X, and are positioned before and after the 0th order diffraction beam 2c. Each reflected beam 2a reflected by the recording medium 7.

The beams 2', b, and 2c travel backwards, are reflected by the beam splitter surface 4a of the prism 4, and then enter the photodetector 10°11.12 through the transparent parallel plate 8 and further through the concave lens 9. The parallel plate 8 is rotated around an axis that optically corresponds to the recording track of an optical recording medium (not shown),
As a result, it is tilted by an angle θ with respect to the optical axis, and further rotated by 45° around the original axis.

Photodetectors 11 and 12 are photodetectors 1 for obtaining reproduction signals.
1, and receive the +1st and -1st order diffracted beams 2b and 2C, respectively, and still receive the 0th order diffracted beam 2c.
is incident on the photodetector 1o. Note that the present invention mainly relates to focus error detection using an astigmatism method, and uses a diffraction grating 5. +1st order diffraction beam 2b, -1st order diffraction beam 2c, concave lens 10. f, detectors 11 and 12 are not directly related to the essence of the present invention.

FIG. 2 shows the detection surface of the photodetector 11.10.12.

Note that the photodetector 10 consists of original detection parts IDa, 10b, 10c, and 10d divided into four quadrants. The reproduced signal is transmitted to each detection section 10a, 10b of the photodetector 10,
It is obtained by the sum of the detection outputs of 10c and 10d.

A focus error signal is transmitted from a parallel plate 8 arranged in an inclined manner.
Due to the astigmatism generated by
and the sum of the detection outputs of the detection units 10b and 10d in the fourth quadrant.
2 a' is approximately circular in a focused state, that is, in an optimal focused state, and approximately elliptical in a defocused state. The reason why it is not perfectly circular or elliptical is because coma aberration occurs. Using this focus error signal, the objective lens 6 is moved in the axial direction to create an optimal focus state.

The tracking error signal is obtained by the difference between the detection outputs of the photodetectors 11 and 12. This tracking error signal causes the objective lens 6 to move in the direction y perpendicular to the recording track direction X of the recording medium 7.

According to the first embodiment, an inexpensive and simple optical component such as the parallel plate 8 can be used in place of the cylindrical lens, which is an expensive and special optical component that has been conventionally used to generate astigmatism.

Next, a second embodiment of the present invention will be described with reference to FIG.

A diverging beam 2 from a semiconductor laser light source 1 passes through a diffraction grating 3
is separated into each diffracted beam 2a+2b, 2c (not shown), and each of the separated beams 2al 2bl
The light 2C enters a transparent parallel plate 20 which is arranged at an angle of 45° with respect to the light source and rotated by 45° around the original axis. The surface of the parallel plate 20 on the collimating lens 5 side is a beam splitter surface 20a, and the diverging beam 2 from the laser light source 1 is reflected by the surface, so it enters the collimating lens 5 without generating aberration, and is split into a parallel beam. After that, the beams are incident on the objective lens 6 to form a focused beam, and are aligned and focused on the recording medium 7. Each of the reflected beams 2a, 2b, 2c reflected by the recording medium 7 travels backwards, passes through the parallel plate 20 and is refracted to generate astigmatism, and then passes through the concave lens 9.
The light is incident on the photodetectors 10, 11 and 12 through. Note that other items such as the focus error signal are the same as in the first embodiment, so their explanation will be omitted.

According to the second embodiment, the cylindrical lens, which is an expensive special optical component conventionally used to generate astigmatism, can be omitted. In addition, the beam splitter surface 4a
Instead of the expensive prism 40, a cheaper parallel plate 20 with a beam splitter surface 20a can be used.

Next, a third embodiment of the present invention will be described with reference to FIG.

FIG. 4 shows a partially modified configuration of FIG. 1. That is, another transparent parallel plate 21 tilted in the opposite direction to the parallel plate 8 is placed in the optical path between the parallel plate 8 and the photodetector 10 to correct comatic aberration. . The figure shows a configuration in which the absolute values of the refractive index, W width, and inclination angle of the parallel plate 8 and the parallel plate 21 are set to the same values to completely correct coma aberration, but this is not limited to this example, and other configurations may also be used. It goes without saying that coma aberration can be corrected with

Next, a fourth embodiment of the present invention will be described with reference to FIG.

FIG. 5 shows a partially modified configuration of FIG. 1. That is, another transparent parallel plate 22 tilted in the opposite direction to the parallel plate 8 is arranged in the optical path between the parallel plate 8 and the photodetector 10, and the surface opposite to the parallel plate 8 is reflected. Side 2
2a, and the photodetector 10 is internally reflected by the reflective surface 22a.
This corrects coma aberration by guiding the It goes without saying that in this case as well, coma aberration can be corrected with various configurations, as in the third embodiment.

Next, a fifth embodiment of the present invention will be described with reference to FIG.

FIG. 6 shows a partially modified configuration of FIG. 1. That is, a concave lens 9 tilted in the opposite direction to the parallel plate 8 is arranged in the optical path between the parallel plate 8 and the photodetector 10 to correct comatic aberration. Also in this case, comatic aberration can be corrected with various configurations.

Next, a sixth embodiment of the present invention will be described with reference to FIG.

FIG. 7 shows a partially modified configuration of FIG. 1. That is, a concave lens 23 tilted in the opposite direction to the parallel plate 8 is arranged in the optical path between the parallel plate 8 and the photodetector 10, and the surface on the same side as the parallel plate 8 is concave, and the plane on the opposite side is reflective. surface 23a, and the photodetector 1 is internally reflected by the reflective surface 23a.
Comatic aberration is corrected by guiding the aberration to 0.

Also in this case, comatic aberration can be corrected with various configurations.

Next, a seventh embodiment of the present invention will be described with reference to FIG. FIG. 8 shows a partially modified configuration of FIG. 2. That is, in the optical path between the parallel plate 20 and the photodetector 10, another transparent parallel plate 21 tilted in the opposite direction to the parallel plate 8 is arranged to correct comatic aberration. Also in this case, comatic aberration can be corrected with various configurations.

Next, an eighth embodiment of the present invention will be described with reference to FIG.

FIG. 9 shows a partially modified configuration of FIG. 2. That is, another transparent parallel plate 22 tilted in the opposite direction to the parallel plate 20 is arranged in the optical path between the parallel plate 20 and the photodetector 10, and the surface opposite to the parallel plate 20 is reflected. The coma aberration is corrected by internally reflecting the light on the reflecting surface 22a and guiding it to the photodetector 10. Also in this case, comatic aberration can be corrected with various configurations.

Next, a ninth embodiment of the present invention will be explained with reference to FIG. FIG. 10 shows a partially modified configuration of FIG. 2. That is, in the optical path between the parallel plate 20 and the photodetector 10, there is a concave lens 9 inclined in the opposite direction to the parallel plate 8.
is arranged to correct coma aberration. In this case as well, Kotsu aberration can be corrected with various configurations.

Next, a tenth embodiment of the present invention will be described with reference to FIG. FIG. 11 shows a partially modified configuration of the second M. That is, a concave lens 23 inclined in the opposite direction to the parallel plate 20 is arranged in the optical path between the parallel plate 8 and the photodetector 10, and the surface on the same side as the parallel plate 20 is concave, and the plane on the opposite side is reflective. The coma aberration is corrected by internally reflecting the light on the reflecting surface 23a and guiding it to the photodetector 10. Also in this case, comatic aberration can be corrected with various configurations.

Note that in the embodiments described above, a collimating lens was used, but the present invention can also be applied to a configuration that does not use a collimating lens. In this case, a parallel flat plate may be placed between the objective lens and the photodetector.

Further, it is also applicable to a configuration using a converging lens (convex lens) in addition to the collimating lens. In this case,
A parallel plate may be placed during the focusing between the focusing lens and the photodetector.

〔Effect of the invention〕

As described above, according to the present invention, a cheap and simple optical component called a parallel plate can be used instead of a cylindrical lens, which is an expensive and special optical component conventionally used to generate astigmatism. . Also, instead of a prism with a beam splitter surface, a parallel plate with a cheaper beam splitter surface can be used. Further, coma aberration can be corrected by arranging another parallel plate tilted in the opposite direction to the parallel plate or a concave lens in the optical path between the parallel plate and the photodetector.

[Brief explanation of the drawing]

1 is a side view of the first embodiment of the present invention, FIG. 2 is a front view of the detection surface of the photodetector, FIG. 3 is a side view of the second embodiment, and FIGS. Figures 6, 7, 8, 9, 10, and 11 are respectively 3.4, 5, 6,
7.8,9°10 is a side view of the main parts of the embodiment. 1... Semiconductor laser light source, 2... Beam, 4...
Prism, 4a...Beam splitter surface, 5...Collimating lens, 6...Objective lens, 7...Recording medium, 8...Parallel plate, 9...Concave lens, 1o...
Photodetector, 21...Parallel plate, 22...Parallel plate,
22a... Reflective surface, 23... Concave lens, 23a...
・Reflective surface.

Claims (1)

[Claims] 1. A diverging beam emitted from a semiconductor laser light source is incident, and a collimating lens converts the beam into a parallel beam. In the optical signal reproducing apparatus, the optical signal reproducing apparatus is provided with an objective lens for irradiating an optical recording medium with an optical recording medium, and a photodetector into which a reflected beam reflected by the optical recording medium is incident and which obtains a reproduced signal from the incident beam. In the optical path between the collimating lens and the photodetector, a first parallel flat plate made of a transparent flat plate is arranged at an angle with respect to the optical axis, and the reflected beam reflected by the optical recording medium has astigmatism. What is claimed is: 1. An optical reproducing device characterized in that the focus error signal is obtained by detecting a focused shape of the reflected beam using the photodetector. 2. A surface of the first parallel plate on the collimating lens side is a beam splitter surface, and the diverging beam from the semiconductor laser light source is reflected by the beam splitter surface and guided to the collimating lens. An optical signal reproducing device according to claim 1. 3. In the optical path between the first parallel plate and the photodetector,
a transparent second parallel plate tilted in the opposite direction to the first parallel flat plate;
The optical signal reproducing device according to claim 1 or 2, wherein coma aberration is corrected by arranging parallel flat plates. 4. In the optical path between the first parallel plate and the photodetector,
3. The optical signal reproducing device according to claim 1, wherein a concave lens tilted in a direction opposite to that of the first parallel plate is arranged to correct coma aberration. 5. A surface of the second parallel flat plate opposite to the first parallel flat plate is a reflective surface, and the reflected beam is reflected by the reflective surface and guided to the photodetector. An optical signal reproducing device according to claim 3. 6. A patent claim characterized in that a surface of the concave lens opposite to the first parallel plate is a reflective surface, and the reflected beam is reflected by the reflective surface and guided to the photodetector. The optical reproducing device according to item 4. 7. The first parallel plate, the second parallel plate, and the concave lens have a direction x that optically corresponds to the recording track of the optical recording medium, and a direction perpendicular to the recording track. When the optically corresponding direction is y, x
A straight line (y
The optical signal reproducing device according to claim 1, 3, or 4, wherein the optical signal reproducing device is arranged to be rotated by a predetermined angle about x or y=−x.