JPS6339151A - Optical head - Google Patents

Abstract

PURPOSE:To prevent relative position deviation of light spots for erasure and recording/reproduction at tracking control and to contrive to relieve the load on an actuator through thin profile and light weight by constituting an objective lens of a reflectance distributed type single lens applied with color aberration correction. CONSTITUTION:The objective lens 23 is constituted of the reflectance distribution type single lens applied with color aberration correction. Thus, the thin profile and light weight are allowed for the objective lens, the load onto the actuator is relieved remarkably and the actuator is simplified and made thin in the profile. Since the erasure light beam 24 is made incident in the objective lens 23 as collimated light, the displacement of the objective lens 23 is made coincident with the displacement of the recording/reproducing light spot 17 and the erasure light spot 18 at tracking control, and the relative position deviation of the two light spots 17, 18 is canceled.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical information recording/reproducing device that can erase signals by annealing recorded bits of an optical disk.

2. Description of the Related Art In recent years, optical disk devices have been actively developed and commercialized as large-capacity information storage devices, such as still image disk file devices and correspondence file devices. An optical disk device is a device that irradiates a high-speed rotating disk with a laser beam to record information pits on its memory thin film, and then lowers the power of the same laser to read changes in the reflectance of the recorded bits. Now, when using a thin film made of tellurium oxide doped with germanium and soot, for example, as a memory thin film, when recording information bits, a light spot (about 0.8 μm in diameter) with high power density narrowed down to the diffraction limit is irradiated. do. In this way, the thin film is rapidly heated and cooled, transitioning to a state with low reflectance, and recording is completed. In addition, when erasing a recorded bit, the recorded bit is irradiated with a light spot shaped into an ellipse (ellipse diameter of about 10 μm) with low power density, and the thin film in this area is annealed (slowly cooled) to restore the original state. Erasing is completed by transitioning to the pre-recording state with high reflectance.

As described above, an optical head that emits a light spot that can be annealed in erasing recorded bits has been proposed as an optical head for an optical information recording/reproducing device having an erasing/rewriting function.

A conventional example of such an optical head will be described below with reference to the drawings. FIG. 7 is a diagram showing a schematic configuration of a conventional optical head, in which 1 is a recording/reproducing semiconductor laser, and wavelengths λ1. Laser light with a power of approximately smW is irradiated during recording and approximately 1 mW during reproduction. 2 is a semiconductor laser for erasing, which emits wavelengths λ2. Laser light with a power of about 10 mW is irradiated. 3.4 is a collimator lens, and the semiconductor laser 1
, 2, and 6.6 is a pair of cylindrical lenses that shape the light beam of the semiconductor laser 1 into a substantially circular shape. A is also a cylindrical lens, and in order to make the aperture light shape elliptical, a semiconductor laser 2 is used.
imparts astigmatism to the light beam. A polarizing beam splitter 8 reflects the S-polarized laser beam and transmits the P-polarized light beam.

9 and 10 are A wave plates, respectively. Reference numeral 11 denotes an optical filter, which transmits the light beam of wavelength λ1 and reflects the original beam of wavelength λ2. Reference numerals 12 and 13 are objective lenses, 14 is a disk having a memory thin film for recording and reproducing, and 16 is a cylindrical lens for detecting focal and negative control signals by an astigmatism method. Reference numeral 16 denotes a signal detector, which includes a plurality of light receiving elements for detecting a reproduction signal, a focus control signal, and a tracking control signal. Based on the focus control signal and tracking control signal obtained from the signal detector 1e, the objective lens 12 is subjected to focus control and tracking control by an actuator 22. 17 is the recording and reproduction state party spot irradiated with focus on disc 14, 18
indicates an elliptical focused erasing light spot, and 19 indicates a recording bit string.

Next, the operation of the conventional optical head configured as described above will be explained. First, in recording or reproducing, a recording/reproducing semiconductor laser 1 emits a light beam of a predetermined power. This S-polarized light beam is reflected by the polarization beam splitter 8, passes through the A-wavelength plate 9, and then is irradiated onto the disk 14 by the objective lens 12 as a recording/reproducing light spot 17. During recording, this light spot 17 increases its power and rapidly heats up the memory thin film of the disk 14.
Rapidly cool and change the state to change the reflectance.

This creates recording bits. During reproduction, the power of the light beam is lowered to irradiate the original spot 17 without causing a change in the state of the memory thin film on the disk 14, and the reflected light from there is focused by the objective lens 12, and the polarizing beam splitter 8 Return to.

At this time, the returned light beam is changed from S-polarized light to P-polarized light by reflection on the disk 14 and the A-wavelength plate 9. Therefore, it passes through the polarizing beam splitter 8, and the A wavelength plate 1
0. After passing through the optical filter 1, the aperture lens 13 narrows down the light to the signal detector 16. The cylindrical lens 16 imparts astigmatism to this aperture light to enable focus detection. It goes without saying that reflected light can also be detected in the light beam during recording as described above.

Next, in erasing, the erasing semiconductor laser 2 emits a light beam with a predetermined power. This S-polarized original beam is reflected by the polarizing beam splitter 8, passes through the auxiliary wave plate 10, and since it has a wavelength of λ, is reflected by the optical filter 1, returns to the A-wave plate 10, and passes there. The light becomes P-polarized light and then passes through the polarization beam splitter 8. The light then passes through the optical wave plate 9 and is focused onto the disk 14 by the objective lens 12 as an erasing light spot 18 . This light spot 18 is given astigmatism by the cylindrical lens 7 and has an elliptical shape. That is, since the power density is low and the recording bit is irradiated for a long time, the recording bit is heated by the thermal energy and then slowly cooled to receive the annealing effect.

Therefore, the state of the memory thin film in that area changes, and erasing is performed by returning to the reflectance before recording. Note that the reflected light of the erasing light spot 13 from the disk 14 is condensed by the objective lens 12, passes through the % wavelength plate again, and enters the polarizing beam splitter 8. At this time, since the light beam has changed from S-polarized light to P-polarized light, it is reflected by the polarizing beam splitter 8 and escapes in the direction of the recording/reproducing semiconductor laser 1. The optical axes of the erasing light beam and the recording/reproducing light beam are slightly shifted to prevent the escaped erasing light beam from being concentratedly incident on the recording/reproducing semiconductor laser 1.

Problems to be Solved by the Invention Since the optical head described above uses semiconductor lasers of two wavelengths, chromatic aberration of the optical system, particularly the objective lens, becomes a problem. In other words, when light of two wavelengths that have become ideal parallel rays using a collimator lens are condensed using an objective lens,
The focal points of the two lights generally do not coincide, and are located at slightly distant points on the optical axis, as shown in FIG. Recording and playback of signals,
In order to perform erasing efficiently, it is necessary to align the focal point on the recording surface of the optical disc. Conventionally, for this purpose, a method was used in which the erasing collimator lens 4 was slightly displaced on the optical axis, as shown in Figure 3, to align the erasing light spot 18 with the recording/reproducing light spot position. . By taking such measures, the optical performance on the axis is degraded, and at the same time, the following problems occur.

The erasing light beam 2o that exits the erasing collimator lens 4 is not an ideal parallel beam, and is slightly spread as shown by the broken line in Fig. 4 (2L) or as shown by the broken line in Fig. 5 (2L). The light flux is slightly convergent as shown in the figure. On the other hand, the recording/reproducing light beam 21 becomes an ideal parallel beam and enters the objective lens. In order to make the objective lens follow the track on the disk 14, its displacement is controlled in a direction perpendicular to the track. At this time, the displacement amount IR of the recording/reproducing light spot 1 matches the displacement amount xF of the objective lens, whereas the displacement amount xE of the erasing light spot 18 does not match ry. That is, during tracking control, if the displacement of the erasing light spot formed by the light beam incident on the objective lens as spread light is larger than the displacement of the objective lens (xE>xF), as shown in Figure 4, = 几), and in the case of convergent light, the opposite relationship exists as shown in Figure 5 (xE<xF
=xR).

In either case, there is a problem in that the erasing optical spot 18 is displaced relative to the recording/reproducing optical spot 17 that follows the track, causing deterioration in signal erasing performance. Ta.

This problem can be solved by correcting the chromatic aberration of the objective lens. However, when considering correction of chromatic aberration of an objective lens, a spherical lens system requires a combination of at least three lenses, and an aspheric lens system requires a combination of at least two lenses. Furthermore, in order to obtain a good chromatic aberration correction state, a configuration including at least one cemented surface must be adopted. This makes it difficult to manufacture the objective lens, increases the overall length of the lens, and increases its weight. Therefore, a burden is placed on the actuator that holds the objective lens and controls focusing and tracking, causing a problem of deterioration of control characteristics and signal recording/reproducing characteristics.

Means for Solving the Problems The present invention provides an optical disc recording medium capable of recording, reproducing, and erasing signals by irradiating an original spot with a recording/reproducing light beam of wavelength λ and an erasing light beam of wavelength λ2. The light beam is condensed by an objective lens to form an image, and the length λ1. Regarding the wavelength λ2, the optical head is characterized in that the objective lens is constituted by a gradient index single lens corrected for chromatic aberration.

By configuring the objective lens with a gradient index single lens in this way, the objective lens becomes thin and lightweight, and the burden on the actuator is significantly reduced. In addition, by correcting the chromatic aberration of the objective lens, it is possible to make the two wavelengths of light enter the objective lens as parallel light and to align the positions of the two light spots. At this time, relative positional deviation between the two spots due to displacement of the objective lens during tracking control does not occur.

Embodiment One embodiment of the present invention is shown in FIG. Components with the same numbers as those in FIG. 7 showing the conventional example have the same names and functions. The feature of the present invention is that the objective lens 23
It consists of a gradient index single lens with chromatic aberration correction.

The possibility of correcting chromatic aberration using graded index optical elements was discussed, for example, by S., D., and Fλntone in his academic paper (R.
Ochester University) shows the existence of a refractive index distribution shape with no wavelength dependence. As a specific example, Mr. Nishizawa has published Applied Optics, Vol. 19.
Jl, 7, in the manufacture of radially graded index lenses, the chromatic aberration can be reduced by appropriately selecting the type of ions to be implanted into the glass using the ion exchange method. In addition, for the axial graded refractive index V7 lens, Mr. D., T., and Moors
s, Vow, 21, A6
Chromatic aberration can be corrected by obtaining an appropriate dispersion distribution. All of the above references state that chromatic aberration correction is possible using a gradient index single lens. This shows that it has extremely high industrial value in that it has characteristics that cannot be achieved with an aspherical lens or grating lens, which may constitute an objective lens with a single lens.

With the objective lens having the above configuration, it is possible to make the erasing light beam 24 enter the objective lens as parallel light.

As a result, during tracking control, the objective lens 23
The displacement amount JCF, the recording/reproducing light spot displacement □, and the erasing light spot displacement ixE match, and the relative positional deviation between the two light spots is canceled. That is,
In the optical head according to the present invention shown in this embodiment, a relative positional deviation between the two light spots does not occur in principle, and an actuator for independently controlling the position of the erasing light spot is not required. In addition, the position adjustment of the erasing collimator lens 4, which was conventionally performed so that the image forming position of the erasing light spot matches the image forming position of the recording/reproducing light spot, can be performed independently as a collimation adjustment of the erasing light beam. You will be able to do this. At the same time, the performance of the on-axis erasing optical system can be maintained at its best.

Effects of the Invention As described above, according to the present invention, the objective lens becomes thin and lightweight, and the burden on the actuator is significantly reduced. This has the advantage of simplifying and thinning the structure of the actuator. That is, this greatly contributes to making the entire optical head thinner. At the same time, the effect is obtained that, in principle, relative positional deviation between the erasing light spot and the recording/reproducing spot during tracking control does not occur. This makes it possible to obtain a compact and easily adjustable optical head with good and stable erasing performance.

[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 a principle diagram showing an example of focal position shift due to chromatic aberration, and Fig. 3 is a principle diagram showing an example of focal position deviation due to chromatic aberration. Figures 4 and 6 are principle diagrams showing an example of means for matching focal positions, and Figures 6 and 6 are principle diagrams showing relative positional deviations between two light spots caused by displacement of the objective lens during tracking control. FIG. 7 is a principle diagram showing that a gradient index objective lens corrected for chromatic aberration prevents relative positional deviation between two light spots from occurring. FIG. 7 is a principle diagram of an optical head in a conventional example. 1... Semiconductor laser for distributed regeneration, 2...
・Semiconductor laser for erasing, 3... Collimator lens for recording and reproduction, 4... Collimator lens for erasing, 8...,... Polarizing beam splitter, 9.10.
...Nail plate, 16...Detector, 23...
...Objective lens, 22... Actuator. Name of agent: Patent attorney Toshio Nakao and 1 other person
1 Figure 2 Figure 3 Figure 4 (Slope)
(b) ((L) (b)
Figure 6

Claims (1)

[Claims] By irradiating a light spot, the wavelength λ_
The recording/reproducing light beam of 1 and the erasing light beam of wavelength λ_2 are focused and imaged by an objective lens, and the objective lens is a gradient index single lens in which chromatic aberration is corrected for the wavelengths λ_1 and λ_2. An optical head characterized by comprising: