JPH0438627A - Optical head device - Google Patents

Abstract

PURPOSE:To enable a changeover between the push-pull method and the 3-beam method under the state of fixing a branching filter means such as a diffraction grating, etc., by arranging a liquid crystal panel having stripe-like electrodes and a polarizing plate between a light source and an objective lens and impressing a voltage selectively on electrodes. CONSTITUTION:Since the polarizing plate 10 is set in its direction to transmit a light beam transmitted through the liquid crystal panel when no voltage is impressed on the stripe-like transparent electrodes 12, light beams 9 incident upon other liquid crystal panel 2 are all transmitted through the polarizing plate 10. On the other hand, when a voltage V more than a threshold is impressed, the voltage is impressed on every other piece of the stripe-like transparent electrodes, so that a transmission part and a nontransmission part of the light are alternately formed in the width of the stripe-like transparent electrode 12. By this method, the push-pull system at the time of recording a tracking servo sensor signal or the 3-beam system at the time of reproducing can thus be selected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical head device, and more particularly to a tracking sensor in an optical disk device that optically records and reproduces information.

[Prior art] Optical disk devices concentrically and
Alternatively, a tracking servo sensor method is required to record or reproduce signals in a spiral manner. Various methods have been proposed for this tracking servo sensor method, including the push-pull method, which uses diffracted light from signal pits or guide grooves, and the three-beam method, which uses sub-beams. There is a law.

Each method will be briefly explained.

The push-pull method is a method in which a diffracted light distribution by a guide groove formed on an information recording medium is projected onto a two-split photodetector, and a tracking servo sensor signal is obtained from the differential output of the two-split photodetector.

The three-beam method is a method in which a light beam is divided by a splitting means such as a diffraction grating, and a tracking sabot sensor signal is obtained by differential output of sub-beams located on both sides of a main beam for recording and reproduction.

Next, problems with each method will be explained.

Since the push-pull method uses diffraction caused by the guide groove, it is affected by the depth of the guide groove. FIG. 4 shows the amplitude of the tracking servo sensor signal (push-pull signal) with respect to the groove depth. As can be seen from the figure, the groove depth is maximum when it is λ/8 and minimum when it is λ/4. On the other hand, in the three-beam method, since there is a sub-beam, the sub-beam may disturb the recording signal or cause an offset during recording. The offset will be explained with reference to FIG.

In the figure, (17) is a recording track, which is assumed to be a guide groove. (18) is a land portion.

(19) and (20) are spots focused on the information recording medium, (19) is the main beam, (20a), (20
b) is the secondary beam. (21) is the portion recorded by the main beam (19). In the figure, (a), (
b) Shows the state in which the symbiotic beam is tracked on the guide groove. At this time, the sub beams (20a), (20
Looking at the differential output in b), in the case of figure (b), it is (20a)
, (20b), the same signal output is obtained, so the differential output is zero and no offset occurs. However, figure (a
), the leading beam (20a) passes through the non-recording area, while the trailing beam (20b) passes through the recording area, so the signal output from both beams is different. An offset will occur.

As mentioned above, both methods have problems, and about λ/4
Optical head devices that record and play back compact discs (hereinafter referred to as CDs) that have pit rows formed at a depth of Depending on the type, it is necessary to insert or remove a splitting means such as a diffraction grating.

This will be explained with a diagram.

FIG. 6 is a diagram showing an optical system of a conventional optical head device. In the figure, (1) is a semiconductor laser which is a light source, (
2) is a diffraction grating, (3) is a collimator lens, and (4) is a beam splitter for separating light beams. (5) is an objective lens, and (6) is an information recording medium. (7) is a convex lens for condensing the light beam reflected by the information recording medium (6) and deflected by the beam splitter (4) onto the photodetector (8).

Next, the operation will be explained.

By moving the diffraction grating (2) as shown by the arrow in the figure, the light beam (
9) Can be put in and taken out. Therefore, when recording on a W10 disc, it is removed from the light beam and the tracking servo sensor signal is obtained by the push-pull method. On the other hand, W10
When reproducing a disk or CD disk, it is inserted into the light beam, and tracking servo sensor signals are obtained by the 3-beam method. That is, by moving the diffraction grating (2) in and out, an optimal tracking servo sensor system can be adopted.

[Problems to be Solved by the Invention] Since the conventional optical head device is configured as described above, the diffraction grating must be moved, and the angle of the diffraction grating is shifted, causing the sub-beam on the information recording medium to There was a problem in that the alignment direction and the drag direction changed, resulting in a decrease in the tracking servo sensor signal.

This invention was made in order to solve the above-mentioned problems, and provides an optical head device that can switch between push-pull method and three-beam method with a fixed splitting means such as a diffraction grating. With the goal.

[Means for Solving the Problems] The optical head device according to the present invention uses a liquid crystal panel as a wavelength splitting means, and makes the electrodes of the liquid crystal panel have a narrow stripe shape so as to function as a diffraction grating. It is something that

[Function] The liquid crystal panel of the demultiplexing means in this invention allows the demultiplexing means to be inserted into or taken out of the light beam without moving the position of the liquid crystal panel by turning on/off the voltage applied to the electrodes. You can get the same effect.

[Example 1] An example of the present invention will be described below with reference to the drawings. First, the liquid crystal panel will be briefly explained in the third drawing. The liquid crystal (14) is sandwiched between two glass plates (16), and furthermore, polarizing plates (10) and (15) are arranged on both sides. At V-0 when no voltage is applied (Fig. 3(a)), the linearly polarized light (9) transmitted through the polarizing plate (15) on the incident side is reflected by the liquid crystal (14).
When transmitted, the polarization direction is 90° due to the optical rotation of the liquid crystal.
The light rotates and passes through the output polarizing plate (10) arranged so that the polarization axis is perpendicular to the input polarizing plate (15).

On the other hand, when a voltage V higher than the threshold voltage vth is applied (Fig. 3(b)), the optical rotation of the liquid crystal (14) decreases, and the amount of light transmitted through the output side polarizing plate (10) decreases with respect to the voltage. It decreases as it increases. By utilizing such a transmittance control effect and configuring electrodes in a one-dimensional array, a one-dimensional comb-shaped transmission control element can be formed. In addition, the above liquid crystal (
14) is a TN (Twisted N) with an optical rotation angle of 90°.
An example of using a liquid crystal in a normally white mode has been described. As for the type of liquid crystal phase, the magnitude of the angle of optical rotation, etc., modifications are known in addition to those described above.

Next, the apparatus of the embodiment will be explained with reference to FIGS. 1 and 2.

FIG. 1 is a block diagram of an optical system of an optical head device according to an embodiment of the present invention. Components (2) of the demultiplexing means, (
Since everything other than 10) is the same as the conventional example, the explanation will be omitted.

In the figure, (2) is a liquid crystal panel, and (10) is a polarizing plate. The structure of the liquid crystal panel will be explained with reference to FIG. In the figure, a liquid crystal (14) is sandwiched between two glass substrates (16), and a striped transparent electrode (12) is arranged between the glass substrates (16), and the width thereof is several tens of μm. The structure is such that a voltage can be applied every other line.

Next, the operation will be explained.

When no voltage is applied to the striped transparent electrode (12), the third
As explained in the figure, the direction of the polarizing plate (10) is set so that the light flux that has passed through the liquid crystal panel is transmitted, so all the light flux (9) that has entered the liquid crystal panel (2) is transmitted through the polarizing plate (10).
10).

That is, it is not subjected to the demultiplexing action by the demultiplexing means (2).

On the other hand, when a voltage V higher than the threshold value is applied, the voltage is applied to every other striped transparent electrode (12), so light is transmitted through the width of the striped transparent electrode (12). Parts and non-transparent parts alternate. In other words, a diffraction grating exists, and the light beam (9) incident on the liquid crystal panel (2) is split. Therefore, in a system equipped with the optical head device of the present invention, the tracking servo sensor is electrically switched so that no voltage is applied to the transparent electrode (12) during recording and a voltage is applied during playback. It is possible to select between the push-pull method when recording signals and the 3-beam method when reproducing signals. In addition, in Figure 3, there is also a polarizing plate (Step 5) on the incident side.
However, since the light beam (9) emitted from the semiconductor laser (1) is generally linearly polarized light, there is no need for an incident-side polarizing plate (15) to convert the light beam into linearly polarized light.

In the above embodiment, the width of the - transparent electrode (12) is set to several tens of μm.
Although the diffraction grating was formed by turning on and off every other wire on the order of m, the pitch and duty of the diffraction grating were controlled by making the width of the transparent electrode (12) narrower and creating a structure that could be controlled one by one. I can do something.

[Effects of the Invention] As described above, according to the present invention, the liquid crystal is used as a splitting means, the transparent electrode is formed into a stripe shape, and the effect of a diffraction grating can be applied to the liquid crystal panel simply by turning on/off the voltage applied to the transparent electrode. Since the configuration is such that it can be held or not held, a tracking servo sensor system suitable for operation in an optical head device can be used without mechanical movement.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an optical system of an optical head device according to an embodiment of the present invention, FIG. 2 is a perspective view of a liquid crystal panel, FIG. 3 is a perspective view for explaining the operation of the liquid crystal panel, and FIG. The figure shows the relationship between the guide groove depth and the push-pull signal in the push-pull method.
The figure is a configuration diagram of an optical system of a conventional optical head device. In the figure, (1) is a semiconductor laser, (2) is a liquid crystal panel, (4) is a beam splitter, (5) is an objective lens,
(8) is a photodetector, (9) is a light beam, and (10) is a polarizing plate. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] An objective lens that condenses a light beam from a light source and emits it onto an information recording medium and receives a reflected light beam from the information recording medium, and separates the output light beam from the light source and the reflected light beam. An optical head device including a beam splitter and a photodetector for detecting a tracking servo sensor signal that receives the light beam separated by the beam splitter, comprising: a liquid crystal panel having a striped electrode between the light source and the objective lens; An optical head device comprising a polarizing plate and means for selectively applying a voltage to the electrodes.