JPS6341137B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an autofocus servo circuit used in a device, such as an optical disk, which records or reproduces information on an information track by optical means.

FIG. 1 is an explanatory diagram showing an example of an optical system of an optical disc, and FIG. 2 is a circuit diagram for conventional focus error detection. In FIG. 1, 1 is a semiconductor laser for reading information, 2 is a collimating lens that converts the light beam emitted from the semiconductor laser 1 into a parallel light beam;
3 is a focusing lens, 4 is a disk that stores information,
5 is a beam splitter that reflects the reflected light from the disk 4, 6 is a spherical lens that focuses the reflected light from the beam splitter 5, and 7 is a cylindrical lens having a lens function in the x direction.

In FIG. 2, reference numeral 10 denotes a photodetector composed of four divided photodetection elements 10A to 10D;
A light spot emitted from the cylindrical lens 7 in the figure is received. 11 is a photodetecting element 10A
and a summing amplifier that adds the output signals of 10C and 12
13 is a differential amplifier that obtains the difference between the output signals of both summing amplifiers 11 and 12; and 14 is an output terminal for a focus error signal ε.

The light emitted from the semiconductor laser 1 is turned into a parallel beam by the collimating lens 2, and is then transformed into a parallel beam by the converging lens 3.
The information track surface of the disk 4 is irradiated with a focused spot having a diameter of, for example, about 1 μm. The reflected light from the disk 4 passes through a condenser lens 3, is reflected by a beam splitter 5, passes through a spherical lens 6, a cylindrical lens 7, and
Here, a convergence point 15 in the x direction and a convergence point 16 in the y direction perpendicular to the plane of the paper form different light beams, which are irradiated onto the photodetector 10 in FIG. 2. The cross-sectional shapes (spot shapes) of the light beams at positions 8a, 8b, and 8c between the convergence points 15 and 16 are as shown in 9a, 9b, and 9c shown in FIG. 1, respectively. Here, the photodetector 10 is installed at a position 8b where the shape of the light spot is circular, and when the distance between the lens 3 and the disk 4 is a predetermined value and the focus is on, the light A circular light spot is irradiated onto the light receiving surface of the detector 10. If the distance between the lens 3 and the disk 4 changes from a predetermined value and the focus shifts, this means that the photodetector 10 is installed from 8b to 8b.
Corresponding to the change in the direction a or 8c, the shape of the light spot irradiated onto the light receiving surface of the photodetector 10 changes as shown in 9a or 9c. The photodetector 10 includes each photodetector element 10A to 10
As shown in Figure 2, the boundary line of D is in the x and y directions.
They are installed at an angle of 45 degrees, and each of the photodetecting elements 10A to 10D provides an electrical signal corresponding to the shape (area) of the light spot irradiated thereon.

The summing amplifier 11 includes photodetecting elements 10A and 10C.
The summing amplifier 12 adds the outputs A and C of the
The outputs B and D of the photodetecting elements 10B and 10D are added. Differential amplifier 13 subtracts the outputs of both adder amplifiers 11 and 12, and obtains focus error signal ε=(A+C)−(B+D) at output terminal 14. According to such a circuit, when the disk 4 approaches the lens 3, the spot shape on the photodetector 10 becomes an ellipse long in the y-axis direction as shown in 9a, and a positive voltage is obtained from the output terminal 14. Conversely, when the disk 4 moves away from the lens 3, the spot shape on the photodetector 10 becomes an ellipse long in the x-axis direction as shown in 9c.
A negative voltage is obtained from the output terminal 14. The focus error signal ε obtained from the output terminal 14 in this way indicates the degree of focus matching, and the lens 3 is adjusted so that the value of the focus error signal ε becomes a constant value (for example, zero). The distance between the lens 3 and the disk 4 is maintained at a predetermined value.

FIG. 3 is an explanatory diagram showing the relationship between the diffracted light from the track on the disk 4 and the spot shape on the photodetector 10. A and B are the diffracted light from the track,
C and D show the spot shapes on the photodetector 10, respectively. B shows the diffraction pattern when the spot hits the center of the track 17, which is symmetrical as shown in 18. In addition, in this case, the diffraction pattern on the photodetector 10 is symmetrical as shown in 20 of C. It becomes a spot shape. B shows the diffraction pattern when the spot hits the edge of track 17, and as shown in 19, it is asymmetrical. In this case, the photodetector 10 has an asymmetrical spot shape as shown in d-21. Furthermore, if the radiation pattern (transverse mode) of the semiconductor laser 1 is biased (including the higher-order mode pattern), the bright portion 28 will also be biased as shown in D.

In this way, when the reproduction spot scans the track 17, the spot shape on the photodetector 10 is no longer circular, but as long as it is horizontally symmetrical or vertically symmetrical, the focus from the differential amplifier 13 is Although it does not cause a disturbance to the error signal ε,
If the spot shape is asymmetrical as shown in d, it will be a disturbance.

Therefore, in conventional focus sensor type devices, in order to avoid disturbance caused by the reproducing spot crossing the track, the shape of the spot irradiated onto the photodetector 10 is made vertically or horizontally symmetrical. , it was necessary to precisely adjust and arrange the photodetector 10 and other optical elements. Furthermore, it was necessary to select a semiconductor laser 1 with less bias and to devise an optical system.

Here, the present invention aims to realize an autofocus servo circuit that can simplify the adjustment of the photodetector 10 and other optical systems, and can loosen the criteria for selecting a semiconductor laser.

The circuit according to the present invention is provided with a tracking error detection means that receives a signal from a photodetector as an input signal and outputs a tracking error signal indicating a tracking error of a light beam, and converts the focus error signal into a tracking error signal. The feature is that it is modified accordingly.

FIG. 4 is a block diagram showing an example of a circuit according to the present invention. In this figure, TE is tracking error detection means that is a feature of the present invention. This means includes a summing amplifier 22 that adds signals from the photodetecting elements 10A and 10D, a photodetecting element 1
Adding amplifier 2 that adds signals from 0B and 10C
3. It is composed of a differential amplifier 24 that obtains the difference between the outputs of these two adding amplifiers 22 and 23, and an amplifier 25 with a gain μ (μ is positive or negative polarity). 26
2 is an summing amplifier that adds the outputs of the differential amplifier 13 and the amplifier 25, and 27 is an output terminal that takes out the tracking error signal output from the differential amplifier 24. Other parts are similar to the circuit of FIG. 2.

In this circuit, the output terminal 27 has (A
A tracking error signal εt expressed as +D)-(B+C) is obtained. In addition, the output terminal 14
As shown in equation (1), a focus error signal ε ₀ corrected by the tracking error signal εt is obtained.

ε ₀ =ε+μ·εt=(A+C)−(B+D)+μ{(A+D)−(B+C)} (1) Next, the operation of this circuit will be explained with reference to FIGS. 5 to 7.

Figure 5 A, B, and C are all diffraction patterns of the reflected light from the track 17, where A is the left end of the track 17, B is the center of the track 17, and C is the right end of the track 17, respectively. This is the case. D, E, and F show the shapes of the light spots on the photodetector 10 in cases A, B, and C, respectively. Here, photodetecting elements 10A, 10B
A case where there are bright parts 35, 36, and 37 on the side is illustrated.

When the spot hits the left end of the track 17 as shown in (a), as is clear from (d), the output signal A from the photodetector 10A is the largest, followed by the output signal D from the photodetector 10D. The output signals B and C of the photodetecting elements 10B and 10C have the same magnitude. When the spot hits the center of the track 17 as shown in (b), as is clear from (e), the output signals A and B of the photodetecting elements 10A and 10B are equal, and the output signals A and B of the photodetecting elements 10C and 10B are equal.
The output signals C and D of D are also equal, and their magnitudes are A,
Smaller than B. When the spot hits the right end of the track 17 as shown in C, as is clear from F, the output signal B from the photodetecting element 10B is the largest, followed by the output signal 10C from the photodetecting element 10C. The output signal C of is large, and the photodetector element 1
The output signals A and D of 0A and 10D have the same magnitude.

From this, the focus error signal ε=(A+C)-(B+D) obtained from the differential amplifier 13 is
It changes as shown in FIG. 6 depending on the spot position. Further, the tracking error signal εt=(A+D)-(B+C) obtained from the differential amplifier 24 changes as shown in FIG. 7 depending on the spot position.

As is clear from FIGS. 6 and 7, the focus error signal ε and the tracking error signal εt show almost the same characteristics (tendencies) with respect to the spot tracking deviation, although there is a difference in sensitivity. There is. Therefore, in the addition amplifier 26 in FIG. 4, a signal μ·εt, which is the tracking error signal εt multiplied by μ, is added (or subtracted) to the focus error signal ε.
By doing so, it is possible to modify the focus error signal ε and obtain a focus error signal ε ₀ at the output terminal 14 in which the influence of track deviation has been removed or reduced. This signal ε ₀ is applied to a lens moving means (not shown), and the position of the lens 3 is adjusted so that ε ₀ becomes zero.

In the above embodiment, the focus error signal ε and the tracking error signal εt are obtained by using the output signal from the same photodetector 10, but the tracking error signal εt is obtained As the circuit means, for example, a circuit means that slightly vibrates the reproduction spot in a direction across the track at a constant frequency, and synchronously detects the change in the amplitude of the reproduction signal with the spot vibration signal to obtain a tracking error signal. It's okay. In other words, the output signal of the tracking error detection means is essentially a track crossing signal, and any type of tracking error detection means will generate the same signal for track crossing, and the track crossing interference component of the astigmatism method will generate the same signal for track crossing. It is clear that any type of tracking error detection means may be used. The circuit means for obtaining the focus error signal ε is also not limited to that of the embodiment. Further, although the case where track information is reproduced is taken as an example here, the present invention can also be applied to the case where information is recorded on a track.

As explained above, according to the present invention, the focus error signal is corrected by the tracking error signal, and the disturbance of the focus servo at the time of crossing the track is corrected by adding a simple electric circuit. It is possible to reduce and eliminate the noise and improve stability and reliability. Further, the adjustment of the optical system can be simplified, and the criteria for selecting a laser light source can be relaxed. Therefore, the yield can be improved.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing an example of the optical system of an optical disc, Fig. 2 is a circuit diagram for conventional focus error detection, and Fig. 3 shows the diffracted light from the track and the shape of the spot on the photodetector. FIG. 4 is a block diagram showing an example of a circuit according to the present invention. FIG. 5 is an explanatory diagram showing the relationship between the diffracted light from the track and the spot shape on the photodetector. This figure is a diagram showing the relationship between the spot position and the focus error signal, and FIG. 7 is a diagram showing the relationship between the spot position and the tracking error signal. 10...Photodetector, 11, 12, 22, 23,
26...Additional amplifier, 13, 24...Differential amplifier, 25...Amplifier, TE...Tracking error detection means.

Claims (1)

[Scope of Claims] 1. An autofocus servo device for a device that records or reproduces information by scanning a light beam over an information track, which includes a focus servo device for detecting a focus shift of the light beam onto the information track. The cass error detection means includes a cylindrical lens, a light amount detection means having four regions, and two sets of additions that take the sum of signals obtained from diagonally located regions among the regions of the light amount detection means. Focusing error detection means comprising an amplification means and a differential amplification means for taking the difference between the signals obtained from the two sets of summing amplification means; Tracking error detection means for detecting a deviation of the light beam from an information track. and; amplifying means having a positive or negative gain for amplifying or decreasing the signal value obtained from the differential amplifying means of the tracking error detecting means at a predetermined rate; and a signal obtained from the amplification means; and a summing amplification means for calculating the sum of the signal obtained from the focus error detection means and the signal obtained from the amplification means; An autofocus servo device characterized in that the signal is removed by addition or subtraction, and the corrected signal is output as a focus error signal.