JPH02267747A - Optical pickup device using dual grating - Google Patents

Abstract

PURPOSE:To improve the utilization efficiency of a light by making a reflected light from an optical information recording medium incident at an angle to be a little deviated from the Bragg angle of a surface grating and making even an incident angle on a back surface grating incident in a little deviating it from the Bragg angle. CONSTITUTION:The incident angle on a surface grating 19 of a reflected light 14 is set at a value to be a little larger than the Bragg angle of the surface grating 19. Thus, even the incident angle on a back surface grating 20 is made into one to be a little deviated from the Bragg angle of the grating 20. Here, by setting the incident angle on the surface grating 19 of the reflected light 14 so as to be a little larger than the Bragg angle of the surface grating 19, even the incident angle on the back surface grating 20 is made into one to be a little deviated from the Bragg angle of the back surface grating 20, and deviation quantities are both small. Consequently, the diffraction efficiency of the whole after passing a dual grating 17 can be made high. As a result, the sensitivities of the signal detections of a focus error signal, a track error signal, etc., are improved, and the stable signal detections are executed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical pickup device that uses dual gratings to detect signals of reflected light from an optical information recording medium.

BACKGROUND OF THE INVENTION A conventional example of using a grating to detect focus error signals and track error signals will be described with reference to FIG. After the light emitted from the semiconductor laser 1 is collimated by a collimating lens 2, it passes through a beam splitter 3 and a mirror 4, and is focused by an objective lens 5 and irradiated onto the surface of a magneto-optical disk 6 as an optical information recording medium. This allows information to be recorded. Further, the reflected light from the magneto-optical disk 6 is reflected by the beam splitter 3 and guided to the signal detection optical system 7. The reflected light guided to the signal detection optical system 7 has its polarization direction rotated by 45 degrees by the 1/2 wavelength plate 8, and becomes focused light by the condensing lens 9. After that, the focused light is HOE 10 (
Holographic 0pticalE leme
nt), and is separated into diffracted light and transmitted light T by four different types of holograms formed on its surface. At this time, the diffracted light is separated into four lights and guided to the six-divided light receiving element 11, while the transmitted light T, which has passed through without being affected by the HOE 10, passes through the polarizing prism 12 and is divided into two. The light is guided to the divided light receiving elements 13.

In this case, the detection of each signal in the signal detection optical system 7 is such that the magneto-optical signal is (S, -3,), the track error signal is (S, -S,), and the focus error signal is (S, -3,).
+S, )-(S, 10S, ) can be detected respectively. FIGS. 6(a), 6(b), and 6(c) show that the shape of the spot irradiated onto the light receiving element 11 changes depending on the focus state of the magneto-optical disk 6. FIG. 6(b) shows the state in which the magneto-optical disk 6 is in focus,
At this time, the difference in the amount of received light becomes O, and no focus error signal is detected. However, when the focus shifts and the front focus and back focus state occurs as shown in Figure 6 (a) and (C), the difference in the amount of received light becomes O. does not become 0, and as a result, a focus error signal is detected.

In this way, by adopting an optical element for signal detection using a grating that has a signal detection function for focus error signals and track error signals, and a polarization separation function for detecting magneto-optical signals, it is possible to The configuration of the pickup device can be made smaller and lighter.

Problems to be Solved by the Invention In the conventional device as described above, the semiconductor laser 1
The wavelength of the light emitted from the sensor changes depending on changes in temperature and the like. In accordance with this change in wavelength, the diffraction angle of the diffracted light of the light incident on the grating (hologram) of the HOEIO changes. Therefore, in this case, in a structure in which six light receiving elements 11 are arranged on the diffracted light side, there is a possibility that the diffracted light K will not be focused at a predetermined position due to the wavelength fluctuation. . As a result, if such a positional shift occurs in the diffracted light, there is a risk that the focus error signal, track error signal, and magneto-optical signal cannot be detected accurately, causing problems in the reliability of signal detection. become.

In addition, in the case of this device, since the HOE 10 itself does not have a light-converging horn, a condensing lens 9 having a condensing function is provided on the optical path of the signal detection optical system 7, and the HOE 10 is placed in the condensing optical path.
It is necessary to arrange a total of eight light receiving elements 11 and 13 at predetermined positions on the diffracted light side and the transmitted light side.

For this reason, considerable precision and labor are required for assembly, resulting in poor production efficiency.

In addition to the device using the HOE 10 described above, in the case of an optical pickup device using a dual grating that has gratings on both the front and back sides, the light receiving element for signal detection can be installed at the same location.
Although it can be made smaller and lighter and is more resistant to wavelength fluctuations, the light passes through the grating twice, so the light utilization efficiency and diffracted wave efficiency are lower than when it passes through the grating once. This causes a problem with the stability of signal detection.

In order to solve these problems, the reflected light from the optical information recording medium is first passed through a condensing lens on the optical path that is guided to the signal detection optical system. The pitch of the front grating formed on the side where the reflected light is incident is smaller than the pitch of the back grating formed opposite thereto, and the angle of incidence of the reflected light on the front grating is determined by the Bragg of the front grating. A dual grating set larger than the angle was provided, and a light receiving element was provided to detect transmitted light and diffracted light obtained by dividing the light into two parts by the dual grating.

Effect: Therefore, by making the reflected light from the optical information recording medium incident on the front grating at an incident angle slightly different from the Bragg angle of the front grating, which is different from the pitch of the back grating, the angle of incidence on the back grating can also be changed to the Bragg angle of the front grating. As a result, the combined diffraction efficiency of the light passing through the dual grating becomes
The pitch of the gratings on both the front and back surfaces is equal, and the incident angle can be made higher than when the incident angle is Bragg angle.

Embodiment An embodiment of the present invention will be described with reference to FIGS. 1 to 3. Here, a description of the overall configuration of the optical pickup device will be omitted, and only the signal detection optical system according to the present invention will be described.

A dual grating 17 is disposed on an optical path on which reflected light 14 from a magneto-optical disk (not shown) as an optical information recording medium is guided to a signal detection optical system 15 via a condensing lens 16. dual grating 1
On the optical path of the light that has passed through 7, there is provided a light receiving element 18 which is divided into upper and lower halves and which detects a signal.

The dual grating 17 has a surface grating 19 formed on the optical path on the side where the reflected light 14 first enters.
A backside grating 20 is formed at a position opposite to. In this case, the pitch of the front grating 19 is smaller than the pitch of the back grating 20, and the angle of incidence of the reflected light 14 on the front grating 19 is the Bragg angle of the front grating 19. ).

In such a configuration, first, the overall flow of the signal detection optical system 15 will be explained. The reflected light 14 from the magneto-optical disk is collected by a condensing lens 16, and is then split into two parts: the light that has passed through both the front grating 19 and the back grating 20 of the dual grating 17, and the light that has been diffracted together. The light is received by the element 18,
This allows detection of focus error signals, track error signals, and magneto-optical signals.

Next, the dual grating 17 according to the present invention will be explained. First, the reason why the pitch of the front surface grating 19 is made smaller than the pitch of the back surface grating 7-ing 2o will be described. Light-receiving element divided into upper and lower halves】8
In order to receive the transmitted light T and the diffracted light, it is necessary to separate the two focused spots to such an extent that they do not overlap.

Therefore, by making the pitches of the gratings 19 and 20 on both the front and back sides different, the optical axis of the transmitted light T and the optical axis of the diffracted light are no longer parallel, and they are further separated after passing through the dual grating 17. As a result, the required distance between the light receiving surfaces of the light receiving element 18 divided into two can be secured. In addition, gratings 19 and 2 on both the front and back sides
If the pitch of 0 is made equal, the focal point of the diffracted light will be located in front of the focal point of the transmitted light T, and the diameter of the focal spot on the surface of the light receiving element 18 will differ. Therefore, in order to make the shape of the condensed spot on the surface of the light receiving element 18 the same, it is necessary to adjust the pitch to be different. For this reason, the pitch of the front grating 19 of the dual grating 17 is made smaller than the pitch of the back grating 20.

Next, the reason why the angle of incidence of the reflected light 14 from the magneto-optical disk onto the surface grating 19 is set larger than the black angle of the surface grating 19 will be explained. FIG. 3 shows the relationship between the incident angle θ of the surface grating 19 and the diffraction efficiency. From this figure, the diffraction efficiency does not decrease much if the incident angle deviates from the Bragg angle by a small amount, but if the incident angle deviates to a certain extent from the Bragg angle, the diffraction efficiency decreases at an accelerating rate.

Now, if the incident angle of the reflected light 14 is set to the surface grating 1
9, the diffraction efficiency of the front grating 19 will be the largest value. However, in the case of incidence at such a Bragg angle, the incidence angle and the exit angle are equal, so the front grating 2o also has a diffraction efficiency. 1
It will be incident at the same angle of incidence as the angle of incidence of 9. However, since the Bragg angle of the back grating 20 is different from its angle of incidence, the diffraction efficiency of the back grating 20 is reduced. In this case, in particular, as the pitch difference between the front and back gratings 19, 20 becomes larger, the diffraction efficiency of the back grating 2o decreases at an accelerating rate, based on the relationship shown in FIG. Therefore, the diffraction efficiency becomes high in the surface grating 19, but
With back grating 20, it is quite low,
As a result, the overall combined diffraction efficiency after passing through the dual gating 17 becomes low.

However, according to the present invention, the angle of incidence of the reflected light 14 on the surface grating 19 is determined by
By setting the value to a value slightly larger than the Bragg angle of , the angle of incidence on the back grating 20 will also deviate slightly from the Bragg angle of the back grating 2o, but the amount of deviation of these incident angles from the Bragg angle are both small, so from the relationship shown in FIG. can. In this way, if the overall diffraction efficiency of the dual grating 17 is high, the light utilization efficiency can also be high.
Thereby, the sensitivity of signal detection of focus error signals, track error signals, and magneto-optical signals can be increased, and stable signal detection can be performed.

Effects of the Invention In the present invention, the reflected light from the optical information recording medium is made to enter the front grating, which is different from the pitch of the back grating, at an incident angle slightly deviated from the Bragg angle of the front grating. The incident angle is also slightly shifted from the Bragg angle of the back grating, so that the combined diffraction efficiency of the light passing through the dual grating can be adjusted to the same angle of incidence as the pitch of the front and back gratings is made equal. can be made higher than that in the case where the angle is made coincident with the Bragg angle of the surface grating. Therefore, if the diffraction efficiency of the multiplication table can be increased as a whole by passing through the dual grating in this way, the light utilization efficiency can also be improved, thereby increasing the detection sensitivity of signals such as focus error signals and track error signals. This allows for stable signal detection.

[Brief explanation of drawings]

Fig. 1 is an optical path diagram of a signal detection optical system showing one embodiment of the present invention, Fig. 2 is an optical path diagram showing the relationship between the incident angles of light incident on the dual grating, and Fig. 3 is an incident angle of diffraction efficiency. A waveform diagram showing dependence, Figure 4 is a configuration diagram showing a conventional example,
Figure 5 is an optical path diagram of the signal detection optical system, Figure 6 (a)
(b) and (c) are explanatory diagrams showing the principle of detection of a focus error signal. 14... Reflected light, 15... Signal detection optical system, 16.
...Condensing lens, 17...Dual grating,
18... Light receiving element, 19... Surface grating,
20... Back side grating, T... Transmitted light, K...
...Diffraction light applicant Rico Zufuzu Co., Ltd.

Claims (1)

[Claims] Light emitted from a laser light source is irradiated onto an optical information recording medium to record and reproduce information, and the reflected light from the optical information recording medium is guided to a signal detection optical system to detect focus error signals, track error signals, etc. In an optical pickup device that performs signal detection, the pitch of a surface grating formed on the optical path where the reflected light is guided to the signal detection optical system, on the side where the reflected light first enters through a condenser lens. is smaller than the pitch of the back grating formed opposite to the dual grating, and the incident angle of the reflected light to the front grating is set larger than the Bragg angle of the front grating. An optical pickup device using a dual grating, characterized in that a light receiving element is provided for detecting transmitted light and diffracted light obtained by dividing the light into two by the grating.