JP3506723B2 - Optical information reader - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information reading device for reading information by irradiating an optical recording medium with laser light. [0002] As a conventional optical information reading device,
For example, the one shown in FIG. 5 has been proposed. In this device, light from a semiconductor laser 1 is made incident on a beam splitter 3 as parallel light by a collimator lens 2,
After the light transmitted through the beam splitter 3 is totally reflected by the tracking mirror 4, a parallel plate beam splitter 5 is formed.
The light transmitted through the beam splitter 5 is applied to the optical disk 7 via the objective lens 6, and the light reflected by the beam splitter 5 is incident on the two-divided photodetector 8. The return light reflected by the optical disk 7 is made incident on the beam splitter 3 via the objective lens 6, the beam splitter 5, and the tracking mirror 4, and the return light reflected by the beam splitter 3 is detected by the two-part photodetector 9. It is designed to receive light. In this apparatus, as shown also in a plan view of FIG. 5, the outputs of the light receiving areas 8a and 8b of the two-segment photodetector 8 are supplied to a difference detector 10, whereby the tracking mirror 4 is provided.
Is obtained, and the difference signal S is used as a servo signal. That is, the optical disk 7
Is eccentric in the track direction, the tracking mirror 4
Turns in the direction of arrow A so that the light spot follows the same track, and accordingly, the light spot on the two-divided photodetector 8 moves in the direction of arrow B. Here, the light spot on the two-divided photodetector 8 is generally set such that when the tracking mirror 4 is at the neutral position, its center is located on the dividing line of the light receiving regions 8a and 8b. Therefore, when the tracking mirror 4 rotates and the light spot on the two-segment photodetector 8 moves, the difference signal S obtained from the difference detector 10 becomes non-zero and is proportional to the rotation angle of the tracking mirror 4. A signal will be obtained. Further, in this apparatus, as shown in a plan view of FIG. 5, the outputs of the light receiving areas 9a and 9b of the two-segment photodetector 9 are supplied to a difference detector 11, whereby a track is obtained by a push-pull method. An error signal (TE) is obtained. In addition, regarding detection of the focus error signal,
The description is omitted because it is not related to the present invention. FIG. 6 shows another conventional optical information reading apparatus. In this device, the beam splitter 5 is omitted in the configuration shown in FIG. 5, and the transmitted light of the tracking mirror 4 on the outward path is received by the two-divided photodetector 8. In this case, when the tracking mirror 4 rotates in the direction of the arrow A, the transmitted light of the tracking mirror 4 is changed to an arrow on the two-segment photodetector 8 as parallel light as shown in the plan view in FIG. Since it moves in the B direction, the difference detector 10 outputs a difference signal S proportional to the rotation angle of the tracking mirror 4.
Is obtained. [0006] As still another optical information reading apparatus, the one shown in FIG. 7 has been proposed. This device receives the reflected light on the outward path of the parallel plate beam splitter 5 with a photodetector 12 comprising one light receiving area,
The output light amount of the semiconductor laser 1 is controlled based on the output. However, in the apparatus shown in FIG. 5, a parallel plate beam splitter 5 is provided.
, The rotation angle of the tracking mirror 4 is detected via the optical axis. Therefore, when the beam splitter 5 causes an angle change as indicated by an arrow C due to an ambient temperature change or an aging change,
Even though the tracking mirror 4 is at the neutral position, there is a problem that the center of the light spot on the two-segment photodetector 8 is shifted from the division line and an offset is generated in the difference signal S. The optical disk 7 of the beam splitter 5
The surface 5a on the side is generally provided with an anti-reflection film. If the effect of the anti-reflection film is insufficient due to manufacturing variations, the reflected light on the surface 5a is reduced to 2 as indicated by the broken line. The light enters the split photodetector 8, and this light overlaps with light that is normally reflected by the back surface 5 b of the beam splitter 5 to cause interference. In addition, this interference state changes due to a change in the thickness of the wavelength order of the beam splitter 5 due to a change in the ambient temperature. Thus, when the interference state changes, 2
Since the light quantity distribution of the light spot on the split photodetector 8 changes, the difference signal S does not become 0 even when the tracking mirror 4 is at the neutral position, and an offset occurs. Further, in the device shown in FIG. 6, offset of the difference signal does not occur due to a change in ambient temperature or aging, but since the angle is detected by the light transmitted through the tracking mirror 4, the detection sensitivity is low. There is a problem. Further, in the apparatus shown in FIG. 7, similarly to FIG. 5, the normal reflected light on the front surface 5b of the beam splitter 5 and the undesired reflected light on the back surface 5a are separated. Since interference occurs, the light amount ratio between the light incident on the photodetector 12 and the light incident on the objective lens 6 changes. Therefore, although the light amount of the semiconductor laser 1 does not actually change, There is a problem that the light amount incident on the photodetector 12 changes and the light amount control of the semiconductor laser 1 is performed. Japanese Patent Application Laid-Open No. Hei 3-254448 discloses that a wedge prism having a half mirror, a total reflection film and a polarizing film is used to separate a forward path and a return path from each other.
There has been proposed a device in which a polarization component of light on an outward path is separated and received by a photodetector. In this publication,
Although it is not described that the wedge prism has a tracking mirror function, if the tracking mirror function is provided, the multiple reflection light in the wedge prism does not exit in the direction of the photodetector, so that interference occurs. However, as in FIG. 5, the position of the light spot incident on the photodetector is displaced by the displacement of the wedge prism due to a change in the ambient temperature, aging, or the like, so that an offset occurs in the servo signal. The present invention has been made in view of the above-mentioned conventional problems, and stably converts a desired signal without being affected by an ambient temperature change or an aging change, or by interference with stray light. It is an object of the present invention to provide an optical information reading device appropriately configured to obtain the information. According to the first aspect of the present invention, the above object is achieved by causing light from a semiconductor laser to enter a rotatable beam splitter having a tracking function. In an optical information reading apparatus in which one light beam separated by a beam splitter is irradiated on a recording medium via an objective lens and the other light beam is received by a photodetector, the beam splitter is a non-parallel first light beam.
A light beam reflected by the first surface is applied to the recording medium through the objective lens, transmitted through the first surface, and transmitted to the second surface. The light beam reflected by the first surface and transmitted again through the first surface is received by the photodetector, and the rotation angle of the beam splitter is detected. FIG. 1 shows a first embodiment of the present invention. In this embodiment, light from the semiconductor laser 1 is incident on the beam splitter 3 as parallel light by the collimator lens 2, and light transmitted through the beam splitter 3 is incident on a tracking mirror 21 formed of a non-parallel flat plate. The tracking mirror 21 is provided with, for example, a dielectric multilayer film having a high refractive index and a low refractive index on its surface (first surface) 21a so that approximately 90% of the incident light amount is reflected. The light reflected by the front surface 21a of the tracking mirror 21 is condensed on the optical disk 7 via the objective lens 6, passes through the front surface 21a, is reflected by the back surface (second surface) 21b, and further passes through the front surface 21a. As shown in the plan view of FIG. 1, light is received by a two-segment photodetector 8 having two light-receiving regions 8a and 8b divided by a dividing line perpendicular to the sheet of FIG. 10 to obtain a difference signal S. The reflected light (return light) from the optical disk 7
Is a two-segment photodetector having two light receiving areas 9a and 9b which are reflected by the surface 21a of the tracking mirror 21 via the objective lens 6 and further reflected by the beam splitter 3 and divided by a dividing line perpendicular to the paper surface. At 9, the outputs are supplied to a difference detector 11 to obtain a track error signal (TE) by a push-pull method. Here, the angle α of the tracking mirror 21 is
Is transmitted through the front surface 21a on the outward path and
The light that is reflected at the surface mirror 21a and further reflected at the surface 21a and is multiply reflected within the tracking mirror 21 is divided into two light detectors 8
In the return path, return light is transmitted through the front surface 21a of the tracking mirror 21, reflected by the back surface 21b, and light transmitted through the front surface 21a is reflected by the front surface 21a as shown by a broken line. The optical path is set to an angle that does not enter the two-divided photodetector 9 by taking an optical path different from that of the divided light. In such a configuration, the difference signal S obtained from the difference detector 10 is set to 0 when a spot on the optical disk 7 is on a recording track. Also,
The groove direction of the optical disk 7 is perpendicular to the paper surface. Therefore, when the optical disc 7 is decentered and the tracking mirror 21 rotates, the light spot on the two-segment photodetector 8 moves, and the difference signal S obtained from the difference detector 10 becomes non-zero. A signal proportional to the rotation angle of the tracking mirror 21 is obtained from the detector 10, as in the case of FIG. According to this embodiment, since the tracking mirror 4 and the beam splitter 5 in FIG. 5 are also used as one tracking mirror 21 of a non-parallel plate, the difference signal S has an offset due to temperature change and aging. Will not occur. Further, on the outward path, the light that is multiple-reflected in the tracking mirror 21 does not enter the two-segment photodetector 8, so that interference does not occur on the two-segment photodetector 8. Therefore, even if the thickness of the tracking mirror 21 changes due to a change in the ambient temperature, the offset as shown in FIG. 5 does not occur. Further, since the angle of the tracking mirror 21 is detected based on the reflected light on the back surface 21b of the tracking mirror 21 in the forward path, FIG.
The detection sensitivity can be made higher than in the above. In FIG. 1, the two-divided photodetector 8 is used.
The sum of the outputs of the two light receiving areas 8a and 8b can be detected, and this sum signal can be used for controlling the amount of light emitted from the semiconductor laser 1. FIG. 2 shows a second embodiment of the present invention. In this embodiment, in the configuration shown in FIG. 1, a condenser lens 22 is added between the tracking mirror 21 and the two-segment photodetector 8 to condense light incident on the two-segment photodetector 8. The other configuration and operation are the same as those in FIG. If the light incident on the two-divided photodetector 8 is condensed by the condenser lens 22 in this way, the detection sensitivity of the rotation angle of the tracking mirror 21 can be increased, and the two-divided photodetector 8 can be reduced in size. In addition, the size and cost of the apparatus can be reduced. FIG. 3 shows a first reference example of the optical information reading device developed with the present invention. In this reference example, light from the semiconductor laser 1 is incident on a collimator lens 2 as parallel light, and is incident on a beam splitter 25 formed of a non-parallel plate. The light reflected on the surface (first surface) 25a of the beam splitter 25 is The amount of light emitted from the semiconductor laser 1 is controlled by receiving light with the photodetector 26 having one light receiving region, and the front surface 25a and the back surface (second
The light sequentially transmitted through the surface 25b is focused on the optical disk 7 via the objective lens 6. The reflected light (return light) from the optical disk 7 passes through the objective lens 6, is reflected by the back surface 25 b of the beam splitter 25, and is received by the two-divided photodetector 9. Here, the angle α of the beam splitter 25
In the forward path, the light that is regularly reflected on the surface 25a and the light that passes through the surface 25a and is multiple-reflected in the beam splitter 25 and emitted from the surface 25a interfere with each other, and a spot on the photodetector 26 The angle is set to several degrees to several minutes so that an interference fringe is formed. In such a configuration, on the outward path, the amount of light going to the objective lens 6 via the beam splitter 25 and
The sum with the amount of light going to the photodetector 26 side is always constant, and the brightness of the interference fringes on the photodetector 26 is split to the objective lens 1 side. This spectral ratio is determined by the dielectric or metal thin film applied to the front surface 25a or the back surface 25b of the beam splitter 25. Therefore, when the ambient temperature changes and the thickness of the beam splitter 25 changes, the bright and dark portions of the interference fringes are switched at a constant cycle. It becomes smaller in inverse proportion to the number of stripes. Therefore, the output light amount of the semiconductor laser 1 can be accurately controlled by the output of the photodetector 26, and a desired output light amount can be stably maintained. In the case of this reference example, light going to the objective lens 6 also causes interference fringes. However, since this light is focused on the optical disk 7 by the objective lens 6, there is no problem. FIG. 4 shows a second reference example of the optical information reading apparatus developed together with the present invention. This reference example shows a separation optical system having a fixed part 31 and a movable part 32. The fixed portion 31 includes a semiconductor laser 1, a collimator lens 2, a beam splitter 3,
Non-parallel plate beam splitter 25, photodetector 2
A 6 and 2 split photodetector 9 is provided, a movable section 32 is provided with a reflection prism 33 and an objective lens 6, and the light from the semiconductor laser 1 is converted into parallel light by the collimator lens 2 and then transmitted through the beam splitter 3. The beam is then incident on the beam splitter 25, and the surface 25
The light reflected by a is received by the photodetector 26 to control the amount of light emitted from the semiconductor laser 1, and the light transmitted through the beam splitter 25 is collected on the optical disk 7 via the reflection prism 33 and the objective lens 6. Make it light. Here, the angle α of the beam splitter 25
In the same way as in the first reference example, in the outward path, light that is regularly reflected by the surface 25a and light that passes through the surface 25a and is multiply reflected in the beam splitter 25 and emitted from the surface 25a interfere with each other. The angle is set to several degrees to several minutes so that an interference fringe is formed on the spot on the photodetector 26. The return light from the optical disk 7 is made incident on the beam splitter 25 through the objective lens 6 and the reflection prism 33, and the return light transmitted through the beam splitter 25 is reflected by the beam splitter 3 to detect two-divided light. The light is received by the container 9. In addition, the fixing part 31 has a substantially sealed structure, also using the beam splitter 25 as a light entrance / exit window. According to the second embodiment, since the movable section 32 can be made light, high-speed access is possible. Further, since the fixing portion 31 has a substantially hermetic structure, it is possible to effectively prevent dust from entering the fixing portion 31. Therefore,
Although the output power of the semiconductor laser 1 has not changed, it is possible to effectively prevent the output power from being undesirably changed due to a decrease in detection output due to the attachment of dust to the photodetector 26. Can always be accurately controlled. As described above, according to the first aspect of the present invention, the beam splitter that separates the light from the semiconductor laser and leads one to the recording medium and the other to the photodetector, A flat plate having a non-parallel first surface and a second surface,
The light beam reflected by the first surface is guided to the recording medium,
The light flux transmitted through the surface of the first surface, reflected by the second surface, and transmitted again through the first surface is received by the photodetector to detect the rotation angle of the beam splitter. The rotation angle of the beam splitter can be stably detected without being affected by the change or interference with stray light.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a first embodiment of the present invention. FIG. 2 is a diagram showing a second embodiment. FIG. 3 is a diagram showing a first reference example of an optical information reading device developed with the present invention. FIG. 4 is a view showing a second reference example. FIG. 5 is a diagram for explaining a conventional technique. FIG. 6 is a diagram for explaining a conventional technique. FIG. 7 is a diagram for explaining a conventional technique. [Description of Signs] 1 Semiconductor laser 2 Collimator lens 3 Beam splitter 21 Tracking mirror 21a Surface (first surface) 21b Back surface (second surface) 6 Objective lens 7 Optical disk 8, 9 Two-part photodetector 10, 11 Difference Detector 22 Condensing lens

Claims (1)

(57) [Claim 1] Light from a semiconductor laser is made incident on a rotatable beam splitter having a tracking function,
In an optical information reading device, one of the light beams separated by the beam splitter is irradiated on a recording medium through an objective lens, and the other light beam is received by a photodetector.
The beam splitter is formed of a flat plate having a first surface and a second surface that are non-parallel, and irradiates a light beam reflected by the first surface to the recording medium via the objective lens, and the first surface , And a light beam reflected by the second surface and transmitted through the first surface again is received by the photodetector to detect a rotation angle of the beam splitter. Optical information reader.