JPH01298540A - Optical head - Google Patents

Abstract

PURPOSE:To convert a stable optical spot by correcting a change in the astigmatic difference of a laser light source by the chromatic abberation of a collimeter lens. CONSTITUTION:Beams projected from a semiconductor laser 1 are condensed by a collimeter lens 21, made parallel beams and then converged onto the recording face of a disk 9 through a shaping prism 4, a deflecting beam splitter 6, a 1/4 wavelength plate 7, and an objective lens 22 to form an optical spot 10. The beams reflected by the disk 9 is reflected by the beam splitter 6 through the lens 22 and the plate 7 and forms an image on a photodetector 13 through a detecting lens 11 and a cylindrical lens 12, so that focus and tracking error signals and a reproduced signal can be detected by a method similar to an ordinary method. In this case, a change in the astigmatic difference of the laser light source is corrected by the chromatic abberation of the collimeter lens 21. Consequently, a change in the astigmatic difference to be generated when the output of the semiconductor laser 1 is changed can be suppressed and the stable spot diameter can be maintained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical head for an optical disk device that optically records or reproduces information on an information recording medium. This relates to the optical head used.

2. Description of the Related Art As an optical head for an optical disk device for recording and reproducing information on an information recording medium using a semiconductor laser, an optical head mounted on a boat is shown in FIG.

In FIG. 6, the diverging light emitted from the semiconductor laser (2) is converted into a parallel light beam by the collimator lens 2 and becomes a light beam 3. The light beam 3 is passed through a shaping prism 4,
The beam width in one direction is expanded so that the light distribution shape is approximately circular. The approximately circular light beam 5 is P-polarized and enters the polarizing beam splitter 6, passes through the 1/4 wavelength plate 7, is focused by the objective lens 8, and forms a light spot 10 on the disk 9 coated with the information recording medium. form. The light beam 5 reflected from the disk 9 passes through an objective lens 8 and a quarter-wave plate 7.
The light becomes S-polarized light, and is reflected by the polarization beam splitter 6. The light beam reflected here is further passed through the detection lens 11. The light is focused by a cylindrical lens 12 and received by a photodetector 13. The photodetector 13 is configured to detect a reproduction signal, a focus error signal by a so-called astigmatism method, and a tracking error signal by a push-pull method.

Problems to be Solved by the Invention In the optical head configured as described above, when switching from recording to reproduction or from reproduction to recording, the output of the semiconductor laser 1 changes sharply, and the wavelength and astigmatism difference accordingly change. Elephant changes drastically. The output, wavelength, and astigmatism of the semiconductor laser I are shown in Figure 3.

The relationship is as shown in FIG. As shown in Figure 3, although the wavelength changes discontinuously due to mode hopping, there is a constant relationship with the output of the semiconductor laser.

Further, as shown in FIG. 4, the astigmatism difference due to the deviation of the light emitting point in the vertical and horizontal directions on the PN junction surface of the semiconductor laser is large at low output and small at high output, around f.

On the other hand, the output of the semiconductor laser required for recording and reproduction varies depending on the material of the information recording medium and the rotation speed of the disk, but in recent years, generally about 35 to 5 Qmw is used for recording and about 2 to 4 mw for reproduction. - As an example, if the output P1 during reproduction emitted from the semiconductor laser I is 47nW and the output P2 during recording is 59mW, the change in wavelength is 5
The change in the astigmatism difference is about 4 μm.

In the above optical head, the position of the optical spot 10 changes due to the chromatic aberration of the collimator lens 2 and the objective lens 8, causing defocus. With conventional lenses that do not correct six chromatic aberrations, a change in wavelength of 5 nm is generally about 1 to 2 μm. Defocus occurs. This change is 2. Because of the sharpness of the control system, the control system remains unchanged and the defocus state continues for a certain period of time, causing problems such as increases in bits, errors, and rate. However, collimator lens 2 and objective lens 8
By correcting chromatic aberration in the wavelength range used, defocus caused by this wavelength change can be suppressed.

On the other hand, even if the chromatic aberration of the collimator lens and objective lens is corrected, the change in the astigmatism of the semiconductor laser 1 remains;
A problem has arisen in that the diameter of the optical spot 10 that is imaged on the disk 9 becomes larger, deteriorating the recording frequency characteristics or the crosstalk characteristics.

Means for Solving the Problems The present invention is as described above! i! ! In order to solve this problem, the optical head consists of a laser light source, a collimator lens that focuses the diverging light from the laser light source, a shaping prism that shapes the shape of the light beam of the focused beam into a substantially circular shape, and this shaping. An objective lens that focuses the emitted light beam onto an information recording medium and a detection optical system that detects reflected light from the information recording medium are provided, and due to the chromatic aberration of the collimator lens,
It is configured to correct changes in astigmatism of the laser light source.

Effect: With the above configuration, the change in astigmatism caused by the semiconductor laser is canceled out by the astigmatism caused by the chromatic aberration of the collimator lens, and a stable light spot can be focused on the disk.

EXAMPLE An example of the present invention will be described below with reference to FIGS. 1 to 5. Incidentally, in FIG. 1, those having the same configuration as the conventional optical head (FIG. 6) are designated by the same numbers, and detailed description thereof will be omitted.

FIG. 1 shows a configuration diagram of an optical head according to an embodiment of the present invention.
As shown in FIG. 1, the light emitted from the semiconductor laser I is focused by the collimator lens 21 and becomes a parallel beam of light, and then
Shaping prism 4. A polarizing beam splitter 6, a quarter wavelength plate 7 and an objective lens 22 focus the light onto the recording surface of the disk 9 to form a light spot 10. The light reflected from the disk 9 passes through the objective lens 22 and the quarter-wave plate 7 again, is reflected by the polarizing beam splitter 6, and is sent to the detection lens 11. An image is formed on the photodetector 13 by the cylindrical lens I2, and focused and focused in the same manner as in the conventional example.
A tracking error signal and a reproduction signal are detected.

In the above configuration, the collimator lens 21 has chromatic aberration as shown in FIG. 2 in the wavelength range used. In other words, the change in working distance ΔWD is approximately in the relationship ΔWD (μm)=0.25×Δλ (nm) with respect to the change in wavelength Δλ.

On the other hand, the semiconductor laser l changes from the reproduction output PI to the recording output P2.
As shown in Figure 4, the astigmatism difference when changing to
This occurs because the light emitting point in the direction perpendicular to the PN junction surface does not move, but only the light emitting point in the parallel direction moves approximately 4 μm closer to the light emitting point in the perpendicular direction. Therefore, as shown in FIG. 5, the focusing position of the light spot 10 focused on the disk 9 in the direction corresponding to the perpendicular direction does not change, but the focusing position in the direction corresponding to the parallel direction is dp as shown below. only changes.

ap=ΔAs
, is the focal length of the objective lens 22, f2 is the focal length of the collimator lens 21, and m is the magnification of the shaping prism 4. In this example, f, -4m, f, 2m8m, m-2,
Although it was set to 0, a value of this order is often used.

Therefore, if the astigmatism difference is 4 μm as described above, the amount of change in the focusing position will be 0.25 μm, as shown in Figure 5(a).
(The focusing position in each direction will shift.

Furthermore, as the wavelength of the semiconductor laser 1 changes as shown in FIG. 3 as the output changes, the collimator lens 2
Due to the chromatic aberration of 1, the focal position of the light spot 10 changes. When the output of the semiconductor laser l changes from Pl to P2, the wavelength increases by about 5 nm, so as mentioned above, the working distance of the collimator lens 21 of this embodiment is ΔWD-0,25X5=1.25 (μm) This change causes the focusing position of the optical spot IO to move in the direction corresponding to the vertical direction of the PN junction surface.
1ds is: ds=ΔWDX (f, /f2)2 -1.25X (4/8)2 =0.31 (μm)
The focusing position movement amount dp in the direction corresponding to the horizontal direction is dP-
ΔWDX (r 1/12)2x (1/m2)-1,
25X (4/8)2XI/22=0.06 (μm).

Therefore, due to the chromatic aberration of the collimating lens 21, an astigmatism difference of 0.025 μm occurs in the optical substrate 10 on the disk 9. This direction is as shown in FIG. 5(b), and is opposite to the shift in the focusing position of the light spot 10 due to the astigmatism difference of the semiconductor laser 1 (FIG. 5(a)), so they are different from each other. They cancel each other out and the astigmatism difference disappears.

Therefore, the conventional problems can be solved.

Note that by correcting the astigmatism difference in this way, some chromatic aberration of the collimator lens 21 remains, and the light spot 10
Chromatic aberration also causes a shift in the focal position of the lens, but this problem can be solved by correcting excessive chromatic aberration corresponding to the residual chromatic aberration of the collimator lens 21 in the objective lens 22.

Effects of the Invention The optical head of the present invention can suppress the change in astigmatism that occurs when the output of the semiconductor laser is changed, so the optical band that forms an image on the disk can be stabilized without astigmatism. The spot diameter can be maintained, and deterioration of recording frequency characteristics or crosstalk characteristics due to this can be prevented.

[Brief explanation of the drawing]

FIG. 1 is a principle diagram of an optical head in an embodiment of the present invention.
FIG. 2 is an explanatory diagram of the chromatic aberration of the collimator lens in the example, and FIG. 3 is a characteristic diagram showing the wavelength change of the semiconductor laser.
Figure 4 is a characteristic diagram showing changes in astigmatism of a semiconductor laser;
FIG. 5 is an explanatory diagram of the astigmatism difference in the embodiment, and FIG. 6 is a diagram of the principle of a conventional optical head. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 4... Shaping prism, 9... Disc, 10... Light spot, 21... Collimator lens, 22・・・
...Objective lens. Name of agent: Patent attorney Toshio Nakao (1 person) Figure 3 Figure 4 Hand number output (my)

Claims (2)

[Claims] (1) A laser light source, a collimator lens that focuses the diverging light from the laser light source, a shaping prism that shapes the light distribution shape of the focused light beam into a substantially circular shape, and a shaping prism that converts the shaped light beam into an information recording medium. an objective lens that focuses on the
It is composed of a detection optical system that detects the reflected light from the information recording medium, and due to the chromatic aberration of the collimator lens,
An optical head characterized in that the change in astigmatism of the laser light source is corrected. (2) The optical head according to claim (1), wherein chromatic aberration of the collimator lens is corrected by an objective lens.