JPS6259369B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus error signal extraction device for obtaining a focus error signal for accurately detecting a focus error, that is, a focus error of a light spot on an object having a portion that diffracts light.

Generally, in the part of an object that has a diffraction effect that has a part that diffracts light, concave parts called pits are lined up in a line, but the moving speed, that is, the repetition frequency, is higher than the frequency range of the focus control target. It's quite expensive. Therefore, conventionally, a focus error signal has been obtained by ignoring the recesses on the target object and assuming that the reflective surface of the target object is substantially flat. However, the concave portion is a portion where light is diffracted, and in the conventional method, a harmful signal component is mixed in as a focus error signal, whether theoretically or not.

A general method for detecting a focus error will be explained using FIG. 1. 1 is a disk as a target object, 2 is a lens whose lower half (shaded area in the figure) is shielded from light, and 3 is a two-split photodetector. The light beam I reflected from the disk 1 is focused by a lens 2 onto a two-split photodetector 3. Therefore, 2
If the dividing line of the two-split photodetector 3 is placed at a position that divides the light intensity of the light spot on the split photodetector 3 into two, the two-split light will be split when the focus is exactly as shown in Figure 1a. The difference between both outputs of the detector 3 becomes zero. Next, when the disk 1 is displaced and the light spot is not focused on the disk 1 as shown in FIG. The difference between both outputs of the detector 3 does not become zero, and a focus error is detected.
If the lens 2 has an aberration such as spherical aberration, it will not be possible to focus all the light beams on the two-split photodetector 3 even when they are in focus, and some of them will be lost as shown in Figure 1. It becomes like b. Also, the position of the two-split photodetector 3 is in the optical axis direction (left and right in the figure).
If the two-split photodetector 3 is out of focus, change the position of the two-split photodetector 3 to the optical axis as shown in Fig. It must be adjusted in the direction perpendicular to the direction (up and down in the figure).

In this way, even if the light spot is exactly focused on the disk 1, due to the aberration of the lens 2 and the positional shift of the two-split photodetector 3, the focus control will actually be in the state shown in Figure 1c. The reality is that it is working.

In the case shown in FIG. 1c, what kind of inconvenience occurs when the light spot hits the pit portion of the disk 1 will be explained with reference to FIG. 2. Second
In the figure, 4 is a pit, 5 is a light spot focused on the pit 4, 6 is the distribution of the amount of light reflected from the disk 1 on the lens 2, 6a is a bright area, and 6b is a dark area. Assuming that the position of the two-split photodetector 3 is adjusted so that the difference in output between the two parts is exactly zero in the part of the disk 1 where there is no pit 4, the split piece 3 of the two-split photodetector 3 is on the pit 4.
One of the divided pieces 3 a and 3 b is brighter than the other divided piece 3 b. In other words, an error signal indicating that the object is out of focus is generated even though the object is exactly in focus. Since the output of this erroneous signal changes when the disk 1 changes, it is difficult to foresee and take countermeasures.

The electric signal generated when the optical spot 5 alternately passes through the pit 4 portion and the non-pit portion on the disk 1 will be explained with reference to FIG. FIG. 3a shows a state in which the pits 4 are lined up in a line. Third
FIG. b shows the focus error signal, which is zero in the non-pitted area, but does not become zero in the four-pitted area due to the reasons explained with reference to FIG. 2 and other unexplained reasons. Moreover, this value changes depending on the situation.
FIG. 3c shows a concavo-convex signal created by the sum of both outputs of the two-split photodetector 3. This unevenness signal is synchronized with the erroneous focus error signal shown in b, and therefore, the erroneous focus error signal shown in b can be removed using this unevenness signal.

In view of the above-mentioned points, the present invention provides a focus error signal extracting device that extracts only the focus error signal of the non-pitted portion using the unevenness signal. I will explain.

Fig. 4 is a configuration diagram of the optical system, in which 7 is a semiconductor laser diode, 8 is a collimating lens, 9 is a beam splitter, 10 is an objective lens, 11 is a disk, 12 is a photodetector lens, and 13 is a 2-split beam. It is a detector. The light emitted from the semiconductor laser diode 7 is collimated by a collimator lens 8, and is focused onto a disk 11 through a beam splitter 9 and an objective lens 10. The reflected light from the disk 11 bounces off the objective lens 10 and the beam splitter 9 again, and is made incident on the two-split photodetector 13 by the photodetector lens 12. The photodetector lens 12 is half light-shielded like the lens 2 shown in FIG.

FIG. 5 is a circuit block diagram showing the overall configuration, in which 14 is a concavo-convex signal reproducing section, 15 is a differential focus error extraction section, 16 is a non-pit focus error signal extraction section, 17 is an amplification section, and 18 is a focus actuation section. It's Yueta. The light reflected from disk 11 is 2
enters the split photodetector 13, and enters the split photodetector 13.
Both outputs are input to the concavo-convex signal reproducing section 14 and the differential focus error extracting section 15. The concavo-convex signal reproducing section 14 reproduces the concave-convex signal by the sum of both outputs from the two-split photodetector 13. On the other hand, the differential focus error extraction section 15 generates a focus error signal based on the difference between the two outputs from the two-split photodetector 13. This focus error signal is input to the pit-free focus error signal extraction section 16. The pit-free focus error signal extracting section 16 is comprised of a gate circuit and the like, and extracts the focus error signal of only the pit-free section using the unevenness signal as a synchronizing signal. This pit-free focus signal is amplified by an amplifier 17 and drives a focus actuator 18.

As explained above, according to the focus error extraction device according to the present invention, only the focus error signal of the non-pitted area is extracted using the unevenness signal, so that it is possible to obtain an accurate focus error signal with a simple configuration. , and its industrial utility value is extremely large.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the principle of the focus error detection method.
Fig. 2 is an explanatory diagram of a state in which a light spot is incident on a pit portion, Fig. 3 is a waveform diagram of a general focus error signal and unevenness signal, and Figs. 4 and 5 show an embodiment of the present invention. , Figure 4 is a configuration diagram of the optical system, Figure 5
The figure is an overall circuit block diagram. 7...Semiconductor laser diode, 8...Collimating lens, 9...Beam splitter, 10...
Objective lens, 11... Disk, 12... Lens for photodetector, 13... Two-split photodetector, 14...
Concave/convex signal reproducing section, 15... Differential focus error extraction section, 16... No-pit focus error signal extraction section, 17... Amplifying section, 18... Focus actuator.

Claims (1)

[Claims] 1. A light source, a light focusing means for focusing light emitted from the light source onto a target object, a light detection means for detecting reflected light from the target object, and a light detection means for detecting light reflected from the light detection means on the target object. A concavo-convex signal reproducing means for optically reproducing the concave-convex recording signal, and a focus error signal indicating that the focused light spot has shifted from the focus on the target object from the output signal of the light detecting means. and a non-pit portion focus error for extracting a focus error signal of only the non-pit portion on the information track of the target object from the output of the differential focus error extraction device in synchronization with the unevenness signal. What is claimed is: 1. A focus error signal extraction device comprising: signal extraction means.