JP2716793B2 - Optical pickup device using dual grating - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup device using a dual grating for detecting a signal of reflected light from an optical information recording medium.

2. Description of the Related Art A conventional example in which a grating is used for detecting a focus error signal and a track error signal will be described with reference to FIG. After the light emitted from the semiconductor laser 1 is collimated by the collimating lens 2, the beam splitter 3
The light passes through a mirror 4 and is condensed by an objective lens 5 to irradiate the surface of a magneto-optical disk 6 as an optical information recording medium,
Thereby, information recording and the like are performed. 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 is rotated by 45 ° in the direction of deflection of the light by the half-wave plate 8, and is condensed by the condenser lens 9. After that, the focused light is HOE10 (Holographic
Optical Element) and the 4
Diffracted light K and transmitted light T are separated by different kinds of holograms. At this time, the diffracted light K is divided into four lights and guided to the light-receiving element 11 divided into six parts. On the other hand, the transmitted light T transmitted as it is without being affected by the HOE 10 passes through the deflecting prism 12 and is divided into two parts. The light receiving element 13 is guided.

In this case, the detection of each signal in the signal detection optical system 7 is as follows: the magneto-optical signal is (S ₇ −S ₈ ), the track error signal is (S ₅ −S ₆ ), and the focus error signal is (S ₁ + S ₄ ). − (S ₂
+ S ₃ ). 6 (a), (b) and (c) show that the shape of the spot irradiated on the light receiving element 11 changes depending on the focus state of the magneto-optical disk 6. FIG. FIG. 6 (b) shows a state where the magneto-optical disk 6 is in focus. At this time, the difference in the amount of received light is 0 and no focus error signal is detected, but as shown in FIGS. 6 (a) and (c). Out of focus, front pin,
When the rear focus state is reached, the difference in the amount of received light does not become 0, and thus a focus error signal is detected.

As described above, by adopting an optical element for signal detection using a grating having both a signal detection function of a focus error signal and a track error signal and a polarization separation function for detection of a magneto-optical signal, a conventional optical signal is obtained. The configuration of the pickup device can be reduced in size and weight.

Problems to be Solved by the Invention In the conventional device as described above, the wavelength of light emitted from the semiconductor laser 1 changes according to a change in temperature or the like. The diffraction angle of the diffracted light of the light incident on the grating (hologram) of the HOE 10 changes according to the change in the wavelength. Therefore, in this case, in the structure in which the six light receiving elements 11 are arranged on the side of the diffracted light, the diffracted light K may not be collected at a predetermined position with respect to the wavelength fluctuation. . As a result, if the position shift of the diffracted light K occurs as described above, the focus error signal, the track error signal, and the magneto-optical signal may not be able to be detected accurately, which may cause a problem in the reliability of signal detection. become.

Further, in the case of this apparatus, since the HOE 10 itself does not have a light condensing ability, a condensing lens 9 having a light condensing function is provided on the optical path of the signal detection optical system 7, and the HOE 10 is arranged in the light 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 accuracy and labor are required for the assembling, resulting in poor production efficiency.

In addition, in addition to the device using HOE10 described above, in the case of an optical pickup device using a dual grating having gratings on both front and back surfaces, a light receiving element for signal detection can be installed in the same place, so that a small , It can reduce the weight and is strong against the wavelength, but since the light passes through the grating twice,
The light use efficiency and the diffracted wave efficiency are worse than those that pass the grating once, which causes a problem in the stability of signal detection.

Means for Solving the Problems Therefore, in order to solve such a problem, the reflected light from the optical information recording medium is first passed through a condenser lens on an optical path guided to a signal detection optical system. The pitch of the surface grating formed on the side where the reflected light is incident on the surface grating is smaller than the pitch of the back surface grating formed opposite thereto, and the angle of incidence of the reflected light on the surface grating is reduced by the Bragg of the surface grating. A dual grating set larger than the angle was arranged, and a light receiving element for detecting transmitted light and diffracted light obtained by dividing the dual grating by the dual grating was provided.

Therefore, by making the reflected light from the optical information recording medium incident on the surface grating different from the pitch of the back surface grating at an incident angle slightly deviated from the Bragg angle of the front surface grating, the incident angle on the back surface grating is also reduced. In this case, the light is incident at a state slightly deviated from the Bragg angle of the light, thereby the combined diffraction efficiency of the light that has passed through the dual grating, the pitch of the grating on both the front and back surfaces is equal, and the incident angle is made incident at the Bragg angle Can be higher than sometimes.

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

A 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 on a light path through a condensing lens 16 and a dual grating 17.
Is provided on the optical path of the light that has passed through the dual grating 17, and a light receiving element 18 which is divided into upper and lower parts for detecting a signal is provided.

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

In such a configuration, first, the overall flow of the signal detection optical system 15 will be described. The reflected light 14 from the magneto-optical disk is condensed by a condenser lens 16 and then transmitted through a front grating 19 and a back grating 20 of a dual grating 17 and a light obtained by diffracting both of them. The light is received by the element 18, whereby a focus error signal, a track error signal, and a magneto-optical signal can be detected.

Next, the dual grating 17 according to the present invention will be described. First, the reason why the pitch of the front surface grating 19 is smaller than the pitch of the back surface grating 20 will be described. The transmitted light T
In order to receive the light and the diffracted light K, it is necessary to separate the two condensed spots so that they do not overlap. Therefore, by making the pitches of the gratings 19 and 20 on the front and back surfaces different, the optical axis of the transmitted light T and the optical axis of the diffracted light K are not parallel, and after being transmitted through the dual grating 17, they are separated from each other. As a result, the required distance of the light receiving surface of the light receiving element 18 divided into two can be secured. When the pitches of the gratings 19 and 20 on the front and back surfaces are equal, the focal point of the diffracted light K is located closer to the front side than the focal point of the transmitted light T, and the diameter of the converged spot on the surface of the light receiving element 18 Will be different. 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 so as 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 on the surface grating 19 is set to be larger than the black angle of the surface grating 19 will be described. FIG. 3 shows the relationship between the incident angle θ of the surface grating 19 and the diffraction efficiency. According to this figure, the diffraction efficiency does not decrease so much when the incident angle slightly deviates from the Bragg angle. However, when the incident angle deviates to some extent from the Bragg angle, the diffraction efficiency decreases at an accelerated rate. Now, assuming that the incident angle of the reflected light 14 is equal to the Bragg angle of the surface grating 19, the diffraction efficiency of the surface grating 19 becomes the largest value. Since the emission angles are equal, the light also enters the back surface grating 20 at the same angle of incidence as that of the front surface grating 19. However, since the Bragg angle of the backside grating 20 is different from its incident angle, the diffraction efficiency of the backside grating 20 is reduced. In this case, in particular, as the pitch difference between the gratings 19 and 20 on the front and back surfaces increases, the diffraction efficiency of the back surface grating 20 decreases at an accelerated rate from the relationship shown in FIG. Accordingly, the diffraction efficiency becomes higher in the front grating 19, but becomes considerably lower in the back grating 20.As a result, the overall diffraction efficiency after passing through the dual grating 17 is low. Become.

However, as in the present invention, by setting the angle of incidence of the reflected light 14 on the surface grating 19 to a value slightly larger than the Bragg angle of the surface grating 19, the angle of incidence on the back surface grating 20 is also reduced. Although they are slightly shifted from the Bragg angles, the shift amounts of the incident angles from the Bragg angles are all small. Therefore, from the relationship shown in FIG. The efficiency can be increased as compared with the case where the incident angle is equal to the Bragg angle of the surface grating 19 described above. As described above, if the diffraction efficiency of the dual grating 17 is high as a whole, the light use efficiency can be increased, thereby increasing the sensitivity of the signal detection of the focus error signal, the track error signal, and the magneto-optical signal, and stabilizing it. The detected signal can be detected.

Effect of the Invention The present invention is designed to make the reflected light from the optical information recording medium incident on the surface grating different from the pitch of the back surface grating at an incident angle slightly shifted from the Bragg angle of the front surface 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 that has passed through the dual grating can be reduced by equalizing the pitch of the grating on both the front and back surfaces. Can be increased as compared with the case where the incident light is made to coincide with the Bragg angle of the surface grating. Therefore, if the diffraction efficiency of the light passing through the dual grating can be increased as a whole, the light use efficiency can be improved, thereby increasing the signal detection sensitivity of a focus error signal, a track error signal, and the like. A stable signal can be detected.

[Brief description of the drawings]

FIG. 1 is an optical path diagram of a signal detection optical system showing an embodiment of the present invention, FIG. 2 is an optical path diagram showing a relationship between incident angles of light incident on the dual grating, and FIG. 3 is an incident angle of diffraction efficiency. FIG. 4 is a configuration diagram showing a conventional example,
FIG. 5 is an optical path diagram of the signal detection optical system, and FIG. 6 (a).
(B) and (c) are explanatory diagrams showing the principle of detecting a focus error signal. 14 ... reflected light, 15 ... signal detection optical system, 16 ... condensing lens, 17 ... dual grating, 18 ... light receiving element,
19 front surface grating, 20 back surface grating, T transmitted light, K diffracted light

Claims (1)

(57) [Claims] An information recording medium is recorded and reproduced by irradiating light emitted from a laser light source to an optical information recording medium, and reflected light from the optical information recording medium is guided to a signal detection optical system to provide a focus error signal and a light. In an optical pickup device that performs signal detection such as a track error signal, the reflected light is formed on an optical path guided to the signal detection optical system, via a condenser lens, on a side where the reflected light is first incident. A dual grating in which the pitch of the front grating is smaller than the pitch of the back grating formed opposite thereto, and the incident angle of the reflected light to the front grating is set to be larger than the Bragg angle of the front grating. In addition, a light receiving element for detecting the transmitted light and the diffracted light obtained by dividing the light into two by the dual grating is provided. Optical pickup device using a dual grating to.