JPS61243951A - Optical head - Google Patents

Abstract

PURPOSE:To decrease a shift of an optical axis caused by a temperature variation, and to detect stably out-of-focus by using a ceramic material as an insulator placed between a holder for supporting a photodetector of an optical head, and the photodetector. CONSTITUTION:An information storage medium 21 is placed on a turntable 21, a chuck device 32 is installed to a center spindle 31 of this table 30, and the table 30 is rotated at a prescribed speed by a driving motor 33. In the radial direction of this medium 21, an optical head 34 having a light source 36, an objective lens, and the second photodetector 49 is placed so as to be rotatable. This detector 49 is supported by a holder 54 through an insulator 53, and the photodetector 49 is constituted of a photodetecting area 55, and a frame 56 for supporting it. Also, a ceramic material is used as the insulator 53 placed between this detector 49 and the holder 54, and the detector 49 and the insulator 53 are fixed by an adhesive agent 57 whose coefficient of thermal expansion is the same level as a metal. In such a way, the shift of an optical axis caused by a temperature variation is decreased, and out-of-focus is detected stably.

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention is capable of reading at least information from an information storage medium using focused light.
Playback-only information storage media such as D (compact disc) and video disc, image files, still image files, and DOM (computer output memory)
It is used for information storage media that can record and reproduce information by causing a state change such as making a hole in the recording layer using focused light, and also for erasable information storage media. On the other hand, the present invention relates to an optical head applied at least when performing reproduction or recording.

[Technical background of the invention and its problems]

In the above types of devices, when reading information from an information storage medium or writing new information to an information storage medium, the focal point of the focused light always coincides with the position of the recording layer or light reflection layer of the information storage medium. Must match.

Therefore, the device has an automatic defocus detection function and its correction function.

However, with this function, when the optical axis shifts, the spot for defocus detection moves on the photodetector that detects defocus, resulting in false detection as if the object is out of focus. Therefore, if the optical axis shifts due to changes in the external environment (temperature change, humidity change, mechanical imaging, impact, etc.), the focus will be blurred. In particular, it is easily affected by temperature changes.

That is, as shown in FIGS. 10 and 11, the light reflected by the information storage medium and guided by the projection lens 1 is projected onto the light detection area 3 of the photodetector 2. As shown in FIGS. Here, the frame 4 of the photodetector 2 itself serves as an electrode, and if the holder 5 that supports the photodetector 2 also has a large coefficient of thermal expansion, the movement of the holder 5 may cause the photodetector 2 to shift position. Metals are usually used because of this. For this reason,
For the purpose of insulating the photodetector 2 and the holder 5, an insulator 6 made of organic material such as baking material is placed around the photodetector 5, the holder 5 is attached on top of the insulator 6, and the holder 5 is attached to the projection lens. 1 or to the optical head body.

However, the characteristics of the insulator 6, such as the coefficient of thermal expansion and hygroscopicity, have been completely ignored.
Misalignment of the photodetector 2 occurs due to thermal expansion, moisture absorption, etc. Moreover, between the insulator 6 and the photodetector 2 and the insulator 6
and the holder 5 are fixed with adhesives 8, 8, and since this adhesive itself is an organic substance, it causes positional displacement even though the layer is thin. Therefore, it causes a malfunction in defocus detection.

[Purpose of the invention]

The present invention has been made based on the above circumstances, and its purpose is to prevent the optical axis from shifting due to temperature changes,
An object of the present invention is to provide an optical head capable of stably detecting defocus and the like.

[Summary of the invention]

In order to achieve the above object, the present invention is characterized in that the insulator that provides insulation between the photodetector and its holder is made of a ceramic material.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 1 to 5.

FIG. 3 shows an information recording/reproducing apparatus using an optical head according to the present invention, and numeral 21 in this figure is an optical disk (information storage medium). This optical disc 21 is formed by bonding a pair of disc-shaped transparent plates 22.23 with an inner and outer spacer 24.25 interposed therebetween.
There is a light reflecting layer 2 as an information recording layer on each inner surface of the
6.27 is formed by vapor deposition. A tracking guide 28 (see FIG. 4) is formed helically on each of the light reflecting layers 26, 27, and information in the form of bits 29 is recorded on this tracking guide 28. A hole is made in the center of the optical disc 21, and when the optical disc 21 is placed on the turntable 30, the center spindle 31 of the turntable 30 is inserted into the hole of the optical disc 21, and the turntable 30 and the optical disc The rotation centers of 21 are matched. turntable 30
The center spindle 31 further includes a chuck device 3.
2 is mounted on the turntable 30, and the optical disc 21 is fixed on the turntable 30 by this chuck device 32. The turntable 30 is rotatably supported by a support base (not shown) and rotated by a drive motor 33 at a constant speed.

Reference numeral 34 denotes an optical head that is movable in the radial direction of the optical disk 21 by a linear actuator 35 or a rotating arm.
6 is provided.

When writing information to the optical disc 21,
A laser beam L whose light intensity is modulated according to the information to be written is generated from the semiconductor laser 36, and when reading information from the optical disk 21, a laser beam L having a constant light intensity is generated from the semiconductor laser 36. Ru. A diverging laser beam L generated by a semiconductor laser 36 is converted into a parallel beam by a collimator lens 37 and directed to a polarizing beam splitter 38 . The parallel laser beam L reflected by the polarizing beam splitter 38 passes through a 1/4 wavelength plate 39 and enters the objective lens 4.
0 and is focused by the objective lens 40 toward the light reflective layer 26 of the optical disc 21 . Objective lens 4
0 is supported movably in the optical axis direction by a voice coil 41, and when the objective lens 40 is positioned at a predetermined position, the beam waist of the focused laser beam L emitted from the objective lens 40 is reflected by the light reflecting layer. 26 and a minimum beam spot is formed on the surface of the light reflective layer 26. In this state, the objective lens 40 is kept in focus, and information can be written and read. When reading information, a bit 29 is formed on the tracking guide 28 on the light reflection layer 26 by the laser beam L whose light intensity is modulated, and when reading the information, a laser beam L having a constant light intensity is used to form a bit 29. The beam L is intensity-modulated by a bit 29 formed on the tracking guide 28 and reflected.

The diverging laser beam L reflected from the light reflection layer 26 of the optical disk 21 is converted into a parallel beam by the objective lens 40 when it is focused, passes through the quarter-wave plate 30 again, and is returned to the polarizing beam splitter 38. . By the laser beam L going back and forth through the quarter-wave plate 39, the plane of polarization of the laser beam L is rotated by 90 degrees compared to when it is reflected by the polarizing beam splitter 38, and the laser beam L has a plane of polarization rotated by this 90 degrees. L is not reflected by the polarizing beam splitter 38 and passes through the polarizing beam splitter 38. The laser beam L that has passed through the polarizing beam splitter 38 is divided into two systems by a half mirror 42 , one of which is irradiated onto a first photodetector 44 by a first projection lens 43 . The first signal detected by the first photodetector 44 includes information recorded on the optical disc 21, and is sent to the signal processing device 45 and converted into digital data. The other laser beam divided by the half mirror 42 is extracted asymmetrically with respect to the optical axis 47 by a light shielding plate (light handling member) 46.
Photodetector 49 in the rear tube 2 that has passed through the second projection lens 48
is incident on the The signal detected by the second photodetector 49 is processed by a focus signal generator 50, and this focus signal is given to a voice coil drive circuit 51. The voice coil drive circuit 51 drives the voice coil 41 according to the focus signal to maintain the objective lens 40 in a focused state.

Next, the optical system for detecting the focused point shown in FIG. 3 can be simplified as shown in FIG. )(b
) (c). When the objective lens 40 is in the focused state, the beam waist is projected onto the light reflective layer 26, and a minimum beam spot, ie, a beam waist spot 52, is formed on the light reflective layer 26. Normally, the laser beam L incident on the objective lens 40 from the semiconductor laser 36 is a parallel beam of light, so the beam waist is formed on the focal point of the objective lens 40. However, if the laser beam L incident on the objective lens 40 from the semiconductor laser 36 is slightly divergent or convergent, the beam waist is formed near the focal point of the objective lens 40. Here, the light-receiving surface of the photodetector 40 is arranged at the imaging plane of the beam waist spot 52 in a focused state (it may be arranged near the imaging plane). Therefore, when the beam is focused, an image of the beam waist spot 52 is formed at the center of the light receiving surface of the photodetector 49.

That is, as shown in FIG. 5(a), a beam waist spot 52 is formed on the light reflection layer 26, and the laser beam L reflected by the light reflection layer 26 is directed to the objective lens 40.
The light is converted into a parallel light beam and directed toward the light shielding plate 46. The beam is then extracted asymmetrically with respect to the optical axis 47 by a light shielding plate 46, converged by a projection lens 48, and focused to a minimum on a photodetector 49, and a beam waist image is formed thereon. Next, when the objective lens 40 approaches the light reflection layer 26, a beam waist is generated as the laser beam L is reflected by the light reflection layer 26, as shown in FIG. 5(b). In other words, the beam waist reflects light from the objective lens 40! ! 26. In such an unfocused state, the beam waist is usually the same as the objective lens 40.
Assuming that the beam waist functions as a light point, it is clear that the light reflecting layer 2
The laser beam L reflected by the laser beam 6 and emitted from the objective lens 40 is converted into a diverging laser beam L by the objective lens 40. Since the laser beam L component that has passed through the light shielding plate 46 is also diverging, even if this laser beam L component is focused by the projection lens 48, it is not focused to the minimum on the light receiving surface of the photodetector 49 and is not detected. Vessel 4
It will be focused towards a point farther away than 9. Therefore, the laser beam L component is projected upward in the figure from the center of the light-receiving surface of the photodetector 49, and a pattern larger than the beam spot image is formed on the light-receiving surface. Furthermore, as shown in FIG. 5(C), the objective lens 4
0 is spaced apart from the light reflective layer 26, the laser beam L forms a beam waist and then passes through the light reflective layer 26.
reflected. At the time of such an out-of-focus point, the focal point is out of the focal length of the objective lens 40 and the objective lens 40 and the light reflecting layer 2
6, the reflected laser beam L directed from the objective lens 40 toward the light shielding plate 46 has a focusing property. Therefore, the laser beam L component that has passed through the light shielding plate 46 is further converged by the projection lens 48 to form a convergence point, and then projected onto the light receiving surface of the photodetector 49.

As a result, a pattern larger than the image of the beam waist spot 52 is formed on the light receiving surface of the photodetector 49 from the center downward in the figure.

Next, the photodetector 49 will be further explained.

As shown in FIGS. 1 and 2, the photodetector 49 is supported by a holder 54 with an insulator 53 in between. That is, the photodetector 49 includes a photodetection area 55 and a frame 56 that supports the photodetection area 55, and the frame 56 itself serves as an electrode. Furthermore, if the holder 54 has a large coefficient of thermal expansion, the movement of the holder 54 will cause the photodetector 49 to shift in position, so metal is usually used. Therefore, an insulator 53 is placed between the photodetector 49 and the holder 54 for the purpose of insulating them. A ceramic material is used for this insulator 53. Further, the photodetector 49 and the insulator 53 are fixed with an adhesive 57 whose coefficient of thermal expansion is on the same level as that of metal, or an adhesive 57 whose movement is sufficiently small compared to the allowable amount of movement. ing. Furthermore, the insulator 53
The fixation between the holder 54 and the holder 54 is achieved by fixing screws 58.58. Further, the holder 54 holding the photodetector 49 in this manner is attached to the holder 59 of the projection lens 48 with screws 60, 60.

According to the above configuration, the insulator 53 that provides insulation between the frame 56 of the photodetector 49 and the holder 54 is made of a ceramic material. Displacement of the photodetector 49 due to deformation of the insulator 53 due to environmental changes due to temperature, humidity, etc.
This makes it possible to prevent malfunctions in defocus detection.

Moreover, the photodetector 49 and the insulator 53 are fixed with an adhesive 57 whose coefficient of thermal expansion is on the same level as metal, or with an adhesive 57 that moves only sufficiently small relative to the allowable amount of movement.
7, and the insulator 53 and the holder 54 are fixed using the fixing screws 58, 58, so that misalignment can be roughly prevented.

Although the present invention is applied to the second photodetector 49 that performs defocus detection in the above embodiment, it is not limited thereto, and may also be applied to the first photodetector 44 that performs track deviation detection. good.

Furthermore, as optical systems for detecting the in-focus state, there are systems shown in FIGS. 6 to 9 in addition to those of the above embodiments, and the present invention can be applied to such optical systems as well. In addition, the light reflection li! of the optical disc 21! This method can be applied to any optical system in which a photodetector 49 is placed at or near the imaging position for i26, and the center of the spot moves on the photodetector 49 when the focus is out of focus, thereby detecting defocus. be able to. In the optical system shown in FIG. 6, the laser beam L is incident obliquely to the optical axis 47 of the objective lens 40 and is irradiated onto the light reflecting layer 26. In this case as well, the convergent laser beam L is irradiated from the objective lens 40 to the projection lens 48 as shown by the broken line, and when the light reflection layer 26 approaches, the projection lens 48 is irradiated from the objective lens 40 to the projection lens 48 as shown by the dashed line. A diverging laser beam L will be irradiated. Therefore, the laser beam L directed from the projection lens 48 to the photodetector 49 is deflected according to the degree of defocus, and the size of the spot pattern changes on the light receiving surface of the photodetector 49, and its projection position is deflected. It will be done. In the optical system shown in FIG. 7, a pipe rhythm 71 is provided between the projection lens 48 and the photodetector 49. Therefore, the laser beam L draws a trajectory shown by a solid line when it is in focus, and is deflected by the pipe prism 71 when it is out of focus. In the optical system shown in FIG. 8, a mirror 72 is provided at the imaging point of the beam waist determined by the objective lens 40 and the projection lens 48, and a lens 73 forms the image on the mirror 72 onto the photodetector 49. is mirror 72
and the photodetector 49. At the focused point, the laser beam L is focused onto the mirror 72 as shown by the solid line, whereas at the out-of-focus point, the convergent or diverging laser beam L shown by the broken line or the dashed line is directed toward the projection lens 48. As a result, the laser beam L is deflected by the mirror 72. Furthermore, in the optical system shown in FIG. 9, the objective lens 40 is irradiated with a laser beam L that passes through a region spaced apart from the optical axis 47 and is parallel to the optical axis 47. Also in this case, the objective lens 40 and the light reflecting layer 2
The laser beam L directed from the projection lens 48 to the photodetector 49 will be deflected depending on the distance between the projection lens 48 and the photodetector 49.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to read at least information from an information storage medium using focused light. It includes an objective lens, a photodetector that detects light emitted from the information storage medium, a holder that supports the photodetector, and an insulator that provides insulation between the holder and the photodetector. In the optical head, since the insulator is made of a ceramic material, the optical axis is difficult to shift due to temperature changes, and excellent effects such as stably detecting defocus can be achieved.

[Brief explanation of the drawing]

Figures 1 to 5 show one embodiment of the present invention.
The figure is a vertical side view showing the photodetector part, Fig. 2 is a front view also showing the photodetector part, Fig. 3 is a block diagram schematically showing the information recording/reproducing device, and Fig. 4 is for defocus detection. A perspective view showing the optical system; FIGS. 5(a), 5(b), and 9(c) are explanatory diagrams showing the locus of the laser beam when the optical system is in focus and out of focus; FIGS. 6 to 9 are Figures 10 and 11 showing other defocus detection optical systems show conventional examples. Figure 10 is a longitudinal cross-sectional side view showing the photodetector portion, and Figure 11 is the same photodetector portion FIG. 21... Information storage medium (optical disk), 36... Light source (semiconductor laser), 40... Objective lens, 49...
...Second photodetector, 53...Insulator, 54...Holder, 55...Photodetection area, 56...Frame. Applicant's Representative Patent Attorney Takehiko Suzue Figure 1 Figure 2rIA Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11

Claims (3)

[Claims] (1) A light source capable of reading at least information from an information storage medium using focused light; an objective lens for condensing the light emitted from the light source onto the information storage medium; and the information storage medium. In the optical head, the optical head includes a photodetector that detects light emitted from the photodetector, a holder that supports the photodetector, and an insulator that provides insulation between the holder and the photodetector, the insulator being An optical head characterized by being made of ceramic material. (2) The optical head according to claim 1, wherein the photodetector is used at least for detecting defocus. (3) The optical head according to claim 1, wherein the photodetector includes a photodetection area for detecting light and a frame for supporting the photodetection area.