JPH02236830A - Optical head - Google Patents

Abstract

PURPOSE:To control a servo without the influence of a disturbance by condensing the external beams of a luminous flux to the center area of a photo converter, condensing the internal beams to the external area, and generating a servo signal based on a light receiving output in the outer peripheral area. CONSTITUTION:The reflected beams from an optical disk 3 are condensed by an objective lens 14, and reflected to a PBS 41 by a beam splitter 13. The P-polarized luminous flux transmitted through the PBS 41 is made incident on a compound lens 42, and the beams made incident on the external peripheral part of the compound lens 42 are converged at the center of a five-divided photodetector 43. In addition, the luminous flux made incident on the inside is condensed to external peripheral areas 43a to 43d of the five-divided photodetector 43. Further based on the respective light receiving outputs of the areas 43a to 43d, servo signals such as focus and tracking error signals are generated. Thus the servo can be controlled without the influence of the disturbance.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention receives laser reflected light from a recording medium with a photodetector, and generates a servo signal such as a focus error signal or a tracking error signal based on the received light output. Regarding the optical head to be generated.

[Technical background of the invention and its problems]

Conventionally, there is an optical head of this type as shown in FIG. 7, which shows an optical head for magneto-optical recording and reproducing. In the figure, 1 is a light source system, 2 is a detection system, and 3 is a magneto-optical disk as a magneto-optical recording medium. In the light source system 1, 11 is a semiconductor laser as a laser light source, 12 is a collimating lens, and 13 is, for example, a P-polarized light reflectance Rp of 20%.
, a beam splitter with an S-polarization reflectance Rs of 100%,
14 is an objective lens.

In the detection system 2, 21 is a PBS (polarization beam splitter) that separates the reflected light from the beam splitter 13 into both P and S polarization, 22 is a detection lens, and 23 is a cylindrical lens, an inclined parallel plate, or a deformed prism (Tokuko Showa). 63-13135), etc., 24 is 4
Divided photodetector 25 is a two-divided photodetector. Although not shown in the figure, separation of both P and S polarized light by the PBS 21 is performed by both photodetectors 24.
The detection system 2 is tilted by 456 with respect to the linearly polarized light so that it is equivalent to 25. Also, in this way, the detection system 2
The same effect can be obtained by placing a 1/2 wavelength plate between the detection system 2 and the beam splitter l3 with its optical axis tilted by 22.5 degrees with respect to the incident plane of the beam splitter 13 instead of tilting the beam splitter l3. can get. The laser beam from the semiconductor laser 11 is collimated into a parallel beam by the collimating lens 12, and then sent to the beam splitter 1.
3 and is focused toward the magneto-optical disk 3 by the objective lens l4. The laser beam focused on the magneto-optical disk 3 is reflected by the perpendicular magnetic film 3b via the substrate 3a, and this reflected light is converted into a parallel beam by the objective lens 14 and sent to the PBS 21 of the detection system 2 via the beam splitter 13. directed towards.

The PBS 21 converts the light from the beam splitter 13 into P and S
The P-polarized light, which has been separated into both polarized lights and transmitted through the PBS 2 1, is focused by the detection lens 22 and forms a spot on the light-receiving surface of the 4-split photodetector 24 via the astigmatism generating element 23. The polarized light forms a spot on the light receiving surface of the two-split photodetector 25.

The shape of the spot in the 4-division photodetector 24 is circular when the magneto-optical disk 3 is in focus, and when it is out of focus, it becomes an ellipse extending in the direction of the diagonal position of the 4-division photodetector 24 due to astigmatism. Further, the dividing line of the two-part photodetector 25 is optically directed in the track direction of the disk. When the laser beam is on the track, the light intensity in the two areas is the same, and when it is off the track, there is a difference in the light intensity in the two areas. Further, the laser beam irradiated onto the magneto-optical disk 3 is made into linearly polarized light due to the characteristics of the semiconductor laser 11, and this linearly polarized light is oriented in either the left or right direction depending on the magnetization direction (recorded information) of the perpendicular magnetic film 3b. It is rotated. This rotation direction is determined by the 4-split photodetector 24 and the 2-split photodetector 25.
This appears as a difference in the amount of light received at , and a magneto-optical reproduction signal can be obtained from the difference in the amount of light received. FIG. 8 is a circuit diagram of the detection system 2, in which each light reception output by the four regions 24a to 24d of the four-division photodetector 24 is transmitted to two regions (
24a and 24c, and 24b and 24d), and the difference between the added outputs is taken out as a focus error signal. Further, the difference between the light reception outputs of the two regions 25a and 25b of the two-part photodetector 250 is extracted as a tracking error signal. Further, the difference between the total light reception output of the 4-split photodetector 24 and the total light reception output of the 2-split photodetector 25 is extracted as a magneto-optical reproduction signal, which removes in-phase noise components and Information recorded on the magnetic disk 3 is differentially detected. By the way, glass substrates with excellent optical uniformity are suitable as transparent substrates for magneto-optical disks, but plastic is desired from the standpoint of mass production and cost, and polycarbonate, which is used in compact disks, etc. The substrate is the most popular. However, due to the high birefringence of this polycarbonate, the following phenomenon occurs: The laser beam focused on the disk is narrowed down into a conical shape, so the rays become perpendicular to the substrate at the center of the light beam. , the angle of incidence on the substrate increases toward the periphery. Therefore, the light in the peripheral portion is subjected to retardation due to the birefringence of the substrate, and the peripheral portion of the light beam guided to the detection system 2 is made into elliptically polarized light as shown in FIG. In FIG. 9, the arrows indicate the direction of linearly polarized light when it is not rotated by magnetization, and the respective intensities of P-polarized light and S-polarized light separated by the PBS 21 are determined by the direction of straight line P and the direction of straight line S of elliptically polarized light and linearly polarized light. Corresponds to each component of the direction. Therefore, the elliptically polarized light component causes the photodetector 24 . 2
As shown in FIG. 10, the intensity distribution on the light-receiving surface of the photodetector 24 becomes non-uniform, and as shown in FIG. The amount of light received is greater than that of (24a, 24c),
An offset occurs in the focus error signal, and the cross-track component on the photodetector becomes unbalanced. For this reason, when adjusting the optical positional relationship of the detection system 2, for example, when using a magneto-optical disk with a substrate made of glass or PMMA, which has little or almost no birefringence, When recording and reproducing on a magneto-optical disk with large birefringence, there are problems in that the best focus position shifts and cross-track noise in the focus error signal increases.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to generate servo signals such as focus error signals and tracking error signals that are not affected by disturbances occurring in a light beam in an optical head, thereby enabling stable servo control.

[Summary of the invention]

In an optical head, the present invention uses an optical element such as a compound lens to condense the light inside the light beam onto the outer peripheral area of the light receiving surface of a photodetector, and to condense the light outside the light beam onto the central area of the light receiving surface. A servo signal is generated based on the received light output in the outer peripheral area of the light receiving surface.

〔Example〕

FIG. 1 is a diagram showing an optical system of an optical head according to an embodiment of the present invention, and shows an optical head for magneto-optical recording and reproducing.

Note that the same reference numerals as in FIG. 7 indicate the same elements, and the explanation thereof will be omitted. In the figure, 4 is a detection system, 41 is a PBS, 42 is a compound lens, 43 is a 5-segment photodetector, and 44 is a photodetector whose light receiving surface has a single area. Note that the detection system 4 is tilted by 456 degrees with respect to the light source system l, similar to the detection system 2 described with reference to FIG. Further, in this embodiment as well, a 1/2 wavelength plate can be used as described above. The reflected light from the magneto-optical disk 3 is focused by an objective lens l4 and reflected by a beam splitter l3 toward a PBS 4l. Of the light flux incident on PBS4 1, PBS4
The S-polarized light reflected by the PBS 41 is guided to the light-receiving surface of the photodetector 44, and the P-polarized light transmitted through the PBS 41 is focused on the light-receiving surface of the 5-split photodetector 43 via the compound lens 42. FIG. 3 is a cross-sectional view of the compound lens 42,
As shown in the figure, a cylindrical lens 42a is formed on one side of the compound lens 42. Further, on the opposite surface, a long focal length lens portion 42b with a large radius of curvature is formed at the center facing the cylindrical lens 42a, and further,
A short focus lens portion 42c with a small radius of curvature is formed on its outer periphery. On the other hand, as shown in FIG. 4, the light-receiving surface of the 5-part photodetector 43 consists of four roughly rectangular areas 43a to 43d on the outer periphery and a small circular area 4 in the center.
It is divided into 3e and 3e. FIG. 2 is a diagram explaining the action of the compound lens 42, in which the P-polarized light beam transmitted through the PBS 41 is
When the light is incident on the outer peripheral portion of the compound lens 42, that is, the portion other than the cylindrical lens 42a, the light is focused on the center of the 5-split photodetector 43 by the short focus lens portion 42c. Further, the light flux incident on the cylindrical lens 42a is given astigmatism and is focused on the outer peripheral areas 43a to 43d of the 5-divided photodetector 43. That is, as shown in FIG.
0, and most of the light inside the luminous flux is reflected in the outer peripheral areas 43a to 43d. , S3, and the light is received.

In addition, 'complex lens 42 and 5-segment photodetector 43
The positional relationship is set such that an astigmatic spot S+ -S:+ (FIG. 4) due to the cylindrical lens 42a is formed in a diagonal region of the light receiving surface. Further, the 5-divided photodetector 43 has an outer peripheral area 4
The dividing line separating the disks 3a to 43d is arranged so as to optically coincide with the track direction of the disk and the direction perpendicular thereto.

FIG. 6 is a diagram showing another embodiment of the compound lens 42', in which parts that perform the same functions as those in FIG. 3 are given the same reference numerals.

In the case shown in FIG. 6(a), a cylindrical lens 42a is formed in the center of one surface, a convex surface with a constant curvature is formed on the opposite surface, and the cylindrical lens 42a is formed in the center of one surface.
The outer periphery of the a' side surface is made convex to form a long focus lens portion 42b and a short focus lens portion 42c. In addition, in the case shown in FIG. 3(B), the long focal length lens portion 42b and the short focal length lens portion 42c are constructed of Fresnel lenses, and the cylindrical lens 42a is also formed into a thin shape by dividing the curved surface into small pieces in the same manner as the Fresnel lens. It is. It should be noted that no special explanation is required for the fact that the compound lens shown in the same figure also performs the same function as the compound lens 42 shown in FIG. 3.

FIG. 5 is a circuit diagram in an embodiment, in which a region 43 at one diagonal position in a 5-divided photodetector 43 is shown.
The summation output of the light reception outputs of the other diagonal regions 43b and 43d is subtracted from the summation output of the light reception outputs of the regions a and 43c, and this subtraction output is output as a focus error signal. Note that 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.

Furthermore, the summed output of the light-receiving outputs of the other adjacent areas 43a and 43d is subtracted from the summed output of the light-receiving outputs of the adjacent areas 43b and 43C, and the result is output as a tracking error signal.

Furthermore, the outer peripheral regions 43a to 43d and the central region 4
The light receiving output of the single photodetector 44 is subtracted from the light receiving output of the photodetector 3e, that is, the total light receiving output of the 5-divided photodetector 43, and the magneto-optical reproduction signal is differentially detected.

As mentioned above, each servo signal of the focus error signal and the tracking error signal is generated based on the received light output of the light inside the luminous flux, so there is no influence from external disturbances such as birefringence of the disk substrate, and the servo signal is stable. Servo control can be performed.In addition, in each of the above-mentioned optical systems, even when the optical head is defocused, the spot formed in the central area 43e of the 5-split photodetector 43 is in the outer peripheral area 43.
It is set so that it does not span a to 43d.

In addition, since the outer light focused by the objective lens 14 is focused at the center of the 5-split photodetector 43 without being given astigmatism, it is assumed that the light in FIG.
) beam diameter S. Even if the outer diameter of the center area 43e becomes larger than the outer diameter of the center area 43e, the amount of light is evenly distributed in the outer peripheral areas 43a to 43d, so that the focus error signal is not adversely affected. The lens 42 is disposed between the PBS 41 and the 5-split photodetector 43, and this compound lens 42 is connected to the beam splitter 13 and the PBS 4.
Instead of the photodetector 44 which is disposed between the photodetector 1 and the photodetector 44 and has a single light-receiving surface, a detector similar to the 5-split photodetector 43 may be used. in this case,
Since the same servo signal as described above is obtained by two 5-split photodetectors, by matching the polarities of the output signals from each detector, a servo signal with twice the output can be obtained, and servo control becomes more stable. In this case, if the birefringence of the disk substrate is large enough to affect the reproduction signal, it is possible to obtain a good optical/magnetic reproduction signal by not using the received light output in the central area of the 5-part photodetector as the reproduction signal. Obtainable.

In the above example, disturbances due to birefringence of the disk substrate in a magneto-optical disk were explained, but stray light of the optical element, uneven intensity distribution of the light beam, incident angle dependence of the optical element, vignetting of the light beam, and when accessing the optical head. Similar effects can be obtained with respect to various disturbances such as positional deviation of the spot caused by disk inclination or disk warpage.

Further, the present invention is not limited to an optical head for magneto-optical recording and reproducing.
It goes without saying that the present invention can also be applied to various optical heads that generate servo signals based on reflected light for other optical discs such as video discs and CDs.

[Effects of the Invention] As explained above, according to the optical head of the present invention, an optical element such as a compound lens condenses the light inside the light beam onto the outer peripheral area of the light-receiving surface of the photodetector, and also focuses the light inside the light beam on the outer peripheral area of the light receiving surface of the photodetector. The light is focused on the central area of the light receiving surface, and the servo signal is generated based on the light receiving output of the outer peripheral area of the photodetector, so it is possible to generate focus error signals and Servo signals such as tracking error signals can be obtained and stable servo control can be performed.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the optical system of the optical head according to the embodiment of the present invention, Fig. 2 is a diagram illustrating the action of the compound lens in the embodiment, Fig. 3 is a sectional view of the compound lens in the embodiment, and Fig. 4 The figure shows the light-receiving surface of the 5-split photodetector in the example, Figure 5 is the circuit diagram in the example, Figure 6 is a diagram showing another example of the compound lens in the example, and Figure 7 is the conventional optical Figure 8 is a diagram showing the optical system of the head, Figure 8 is a circuit diagram of a conventional optical head, Figure 9 is a diagram showing the polarization state of the light beam incident on the PBS, and Figure 10 is a diagram showing the unevenness of the spot intensity distribution due to birefringence. It is a figure explaining uniformity.

Claims (2)

[Claims] (1) In an optical head that collects the reflected light of the laser light irradiated onto the recording medium using an objective lens and generates a servo signal based on the received light output of this focused light, the light receiving surface is the central area. a photodetector that is divided into a region on its outer periphery; and a photodetector that focuses the light on the outside of the light beam collected by the objective lens onto the central region of the photodetector, and the light inside the light beam on the above-mentioned area. An optical head comprising: an optical element that focuses light on an area around the outer periphery of a photodetector, the optical head generating a servo signal based on a received light output from the area around the outer periphery of the photodetector. (2) The optical element has a cylindrical lens in its center, and the cylindrical lens generates astigmatism in the outer peripheral area, and generates a focus error signal based on the received light output in the outer peripheral area. The optical head according to claim 1, characterized in that: