JPH01241027A - Optical pickup - Google Patents

Abstract

PURPOSE:To stably perform spot control by using a motor-driven optical deflector using surface acoustic waves (SAW) by using a photodetector elongated in the direction perpendicular to the direction of tracks of an optical disk. CONSTITUTION:The title optical pickup adopts a tracking AF control system and uses a photodetector 12 elongated in the direction perpendicular to the track of an optical disk 11. In other words, this optical pickup is provided with an optical deflector 4 which uses surface acoustic waves for tracking and is inserted between a laser light source 1 and beam splitter 9 leading the information from the optical disk 11 to the photodetector 12 and adopts a tracking system which obtains the tracking information from the optical disk 11 based on the absolute intensity. Moreover, by using plural photodetectors arranged in parallel with each other in the direction perpendicular to the tracks of the disk 11, automatic focus control signals are obtained. Therefore, even if a deflecting element is inserted between the laser light source and beam splitter, stable control which does not rely on the movement of the light spot on the photodetector caused by the movement of the tracks can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical pickup used in an optical disk device.

[Conventional technology]

Optical disks are expected to be a device that can record audio, images, and computer information with high density. Accurately reading information from an optical disc requires automatic focusing technology (AF), which stably forms a minute spot on the disc, and tracking technology (T R), which allows the minute spot to follow and move on the pit row. ) is essential.

Conventionally, both AF and TR have used voice coils, leaf springs, etc. to control the optical spot.

Among these, it is expected that mechanical drive parts such as voice coils and leaf springs will become a bottleneck for high-speed optical access, and it is necessary to consider alternative means.

In addition, related to this type of device.

For example, Japanese Patent Application Laid-Open No. 49-60702 or U.S. Patent No. 3
No. 876842 is mentioned.

[To be solved by the invention: a Wl ] In spot control, a high-speed tracking means is necessary to shorten the access time, and candidates for this include an ultrasonic optical deflector or a surface acoustic wave (SAW). ) to L i N b
A high-speed optical deflector such as a SAW optical deflector generated in an Os thin film waveguide is promising.

The pickup configuration when such an optical deflector is used for an optical disk has not been sufficiently studied.

An object of the present invention is to provide an optical pickup having a configuration in which spot control can be stably performed using a surface acoustic wave (SAW) electrically driven optical deflector.

Here, as an example, consider an optical deflector using a Ti-diffused LiNb○3 thin film waveguide.

FIG. 2 is a diagram showing its configuration. When the surface acoustic wave 5AW 22 generated from the AQ electrode 21 is transmitted through the Ti-LiNb○8 crystal 20, it causes the crystal to have a different density of refractive index.

On the other hand, when the laser light transmitted through the thin film waveguide passes through 5AW22, it undergoes optical diffraction, and when it reaches the ultrasonic wavelength, nΔ(sinθ+sinα)=λ n: refractive index, λ: first-order diffracted wave in the direction of the wavelength of light. progresses. If 8 changes, α also changes, so the light can be deflected.

When considering how to take out the laser beam from such an optical deflector, there are two methods: taking it out from the end face as shown in Fig. 3, and using a grating coupler 41 etc. as shown in Fig. 4. It is difficult to bring back the light once it has come out. Since the latter is a grating, its light utilization efficiency is low, so even if it were to be used in an optical disk device, it would be placed between the laser light source and the beam splitter that guides the photodetector, as shown in Figure 1, and the light would only be emitted once. It is a good idea to allow it to pass through.

However, all optical pickups that have been put into practical use up to now have an optical deflector 51 disposed between the beam splitter 50 and the optical disc disk. That is, the laser beam passes through the optical deflector twice, once going from the laser to the optical disc and once returning. Conventional pickups that use a reflector as an optical deflector do not have any problems even after passing the light twice.

However, as mentioned above, the AO deflector and SAW deflector need to be passed through only once.

Next, we will discuss the problems caused by this difference.

Figure 5 shows the optical path to the photodetector when the light is deflected to two places by an optical system in which the deflector passes only once, and Figure 6 shows
A similar optical path is shown as the light travels back and forth.

In Figure 5, when the light is deflected and moves, the light spot moves on the photodetector, whereas in Figure 6, the optical path is corrected when the reflected light from the optical disk passes through the optical deflector on the return path. The light spot on the photodetector does not move because it travels backward along the original optical path.

[Means to try to solve the problem]

It is necessary to select a detection method that does not disturb the AF and TR control when the light moves on the photodetector as shown in FIG.

As for tracking, if you use a three-beam method using a single diffraction grating, or a method that requires only a difference in beam intensity and does not use unbalanced beam shape, such as a tracking method using pre-pits, etc., it is possible to On the other hand, if a sufficiently large photodetector is used, stable tracking control can be performed without disturbing the TR control.

On the other hand, regarding AF control, even if the laser spot moves on the photodetector due to optical deflection for tracking,
An AF spot control signal must always be obtained independently of the movement. The present invention proposes to use a long photodetector in the direction perpendicular to the track direction to obtain a sufficient control signal even if the laser spot moves.

[Effect]

The tracking method using an absolute intensity distribution that does not depend on the two-dimensional intensity distribution of light in the present invention includes, for example, a three-spot tracking method using a diffraction grating.

On the other hand, as a tracking method that uses the asymmetry of the distribution due to a single diffraction distribution, there is a bush-pull method. This is because the tracking signal is received by the photodetector divided in the direction along the track row, so when the light spot moves with tracking, the light goes from 0 to 0 from the center of the photodetector 71 as shown in FIG. , and it becomes impossible to determine whether the light distribution is asymmetric. On the other hand, for 3-spot 1-tracking, as shown in FIG.
and 81c is used as a tracking control signal. At this time, the distribution of ±1st-order light itself is not a problem. Even if the light spot moves from 81a=c to 82a-c as shown in FIG. 8, the photodetector 83a
” If c is large enough to cover the movement of the light spot, a sufficient tracking control signal can be obtained, and stable tracking control can be performed.

Regarding AF sub-control, most currently use the asymmetry of the light distribution on the light detector as an AF control signal based on the difference in the detection output of the light on the split photodetector. It is.

Even if tracking is slow and the spot moves on the photodetector, in order to obtain an AF multiplied signal, it is necessary to use a photodetector whose split photodetector is divided in the direction perpendicular to the track.

FIG. 10 shows an example of this, using photodetectors such as 91a to 91c, and measuring the intensity difference (91a
+91c) -91b is used as an AF control signal. In this case, when this detector is inserted into the optical path of the light reflected from the optical disk, the light distribution on the photodetection base will be as follows before and after the focal point of the laser beam.

It changes like 100 shown in FIG. At this time, the sum of the central light detection power 91b and the upper and lower outputs (91a + 91c)
The differential output between the two may be used as the AF control signal. If a long photodetector is prepared to sufficiently suppress the movement of the light spot, the AF sub-control will not be affected at all even if the light spot moves due to tracking. In this way, by using an appropriate tracking AF control method and a photodetector extending longitudinally in the direction perpendicular to the track, even if a deflection element is inserted between the laser light source and the beam splitter, the light caused by the movement of the track can be prevented. An optical pickup that can be controlled stably without depending on the movement of the optical spot on the detector can be constructed.

C Example] FIG. 1 is a basic configuration diagram of the present invention and shows an embodiment.

The laser beam emitted from the laser 1 is passed through the collimating lens 2. The light is propagated through a rutile prism 3 through a planar thin film waveguide 4 made of Ti-LiNbOa or the like. The thin film waveguide has an electrode that generates SAW, and vC○ (Vol.
control 0scillator)
When a high frequency (MHz or higher) signal is applied by 6, a SAW is generated, a diffraction grating is formed in the crystal, and the laser beam is deflected by diffraction. The deflection angle α is given by n A (sin θ+5ina)=λ.

A: Wavelength of the SAW, λ: Laser light θ is the angle of light incident on the SAW, which is a Bragg angle given by 2Δosinθ=λ (A o Lo = speed of S A W) when the center operating frequency Lo is set. If 8 changes, α will also change and light will be deflected. The laser beam that has undergone optical diffraction exits from the end face, passes through the cylinder and lens 7, and enters the parallel beam. Then the diffraction grating 8 for the tracking (TR) signal
, enters the beam splitter 9 , then passes through a focusing lens and reaches the optical disk 11 .

The laser beam from the optical disk on which information has been recorded is again passed through the objective lens 10. It passes through a beam splitter 9 and is directed onto a photodetector. The pattern of 12 lights on the photodetector is as shown in Figure 7, and 70 lights are generated by the diffraction grating.
The second-order light is a spot for tracking, and the center is the O-order diffracted light for AF and FM signals. Reference numeral 80 denotes a photodetector, which is a horizontal tanzaku type 5-split photodetector, and optical output signals include a tracking error signal obtained at 80a-80c, and an AF error signal obtained at 80cm (80b+80d).

The light spot follows the track and as the light spot moves,
It moves horizontally, but because it moves on a tanzaku-shaped photodetector,
The error signal is not affected by the movement of the light spot. Here, the tracking method that does not depend on the light pattern is ±1
This refers to any method in which a tracking signal is obtained based on the difference in the intensity of the next-order diffracted light, and the spatial distribution of that light is not affected by the tracking signal.

Signals are obtained by a plurality of photodetectors formed in a tanzag shape parallel to the direction perpendicular to the AF double signal track array.

〔Effect of the invention〕

According to the present invention, stable spot control of an optical disc is possible by defining the AF and TR methods when an optical deflector is disposed between a laser light source and a beam splitter on an optical disc.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of the present invention, FIG. 2 is a diagram for explaining the deflection principle of the SAW optical deflection element, and FIGS.
The figures are for explaining how to take out light from a thin film waveguide, Figure 5 is a diagram showing a one-way optical deflection optical system, and Figure 6 is a diagram for explaining how to extract light from a thin film waveguide.
The figure shows the optical system for reciprocating optical deflection, Figures 7 and 8 show the photodetector and the light pattern on the photodetector, and Figure 9 shows the photodetector for track control. FIG. 10 is a diagram showing a front photodetector for AF, and FIG. 11 is a diagram showing the control principle of AF. 1...Laser diffusion, 2...Collimating lens, Mikawa rutile prism, 4.20...Ti-LiNbO5 waveguide, 5, 21.SAW electrode, 6.VCO circuit, 7...
・Cylinder lens, 8... Diffraction grating, 50, 9...
Beam splitter, 10...Objective lens, 11...
optical disc, 12... photodetector, 22... SAW,
23... Incident light, 24... Polarized light, 41... Grating lens, 51... Optical deflector, 52... Optical disk, 71... Photodetector, 72.73... Light 81 a = c , 82
a ” c...each indicates a light spot, 83 a ”
c...Structure of tracking photodetector, 91a=c
...Structure of the front photodetector for AF, 100...AF inspection when the disk moves before and after the focal point 3 Figure %q diagram and ZC χ7θ diagram /l diagram

Claims (1)

[Claims] 1. In an optical pickup that reads minute information recorded on an optical disk, optical card, or optical tape, a surface acoustic wave for tracking is used between a laser light source and a beam splitter that guides information from the disk to a photodetector. Insert the optical deflector that was used, and use a tracking method that obtains tracking information from the optical disk only from the absolute intensity without depending on changes in the two-dimensional pattern of light, and autofocus control signals in the direction perpendicular to the track. An optical pickup characterized in that it is obtained by using a plurality of photodetectors arranged in parallel to each other.