JPH03192541A - Optical head device - Google Patents

Abstract

PURPOSE:To reduce the size of the optical head device by splitting respective reflected light beams from plural spots, and installing a photodetector at such a position that the reflected light beams from the light spots are not put one over another in front and behind a convergence position. CONSTITUTION:Reflected light from a recording medium 5 is split by a beam splitter 3 and converged again by a focusing lens 6. A splitting prism 8 splits the beam from the lens 6 and the two split beams form spots on the array type photodetector 7 in front of and behind the convergence point of the lens 6. Consequently, spots 41 and 43 before the convergence and spots 42 and 44 after the convergence are formed on the detector 7. In this constitution, the spots before and after the convergence are formed on the divided type photodetector 7 to lead a focus-error signal out and the size is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical head device that records and reproduces information using light.

[Conventional technology]

Optical recording such as optical disks is being put into practical use as a large-capacity file device. In order to further improve the performance of optical discs, there is a need to improve the transfer rate in recording and reproducing information.

One possible method for improving this transfer rate is to use a multi-beam optical head as shown in FIG.

In other words, by simultaneously forming a plurality of spots (two in the illustrated case) on the recording medium, it is possible to record and reproduce multiple tracks in parallel, and to perform a check operation by reproducing immediately after recording.
Effective transfer rate can be improved. In FIG. 7, light emitted from an array type semiconductor laser 1 is transmitted through a collimating lens 2° beam splitter 3. The light is focused onto the recording medium 5 via the condenser lens 4, forming two spots.

In the figure, two beams are shown by a solid line and a dotted line. The reflected light from the recording medium 5 is separated by the beam splitter 3 and condensed again by the converging lens 6 to the knife lens 31.
The light is focused on an array type photodetector 7 through the .

Since this method uses the Knifezi method, as shown in FIG. 8, defocus is detected by a change in the spot shape 22 on the two-split photodetector element 23 constituting the photodetector 7. FIG. 8(a) shows when the recording medium 5 approaches the condensing lens 4, FIG. 8(b) shows when it is in focus, and FIG. The shapes of the spots are shown in each case. In a normal focused state, the spot 22 is located on the two-split photodetector 2.
It is located on the dividing line of 3.

[Problem to be solved by the invention]

In order to downsize the entire optical head device, it is necessary to reduce the focal length of the converging lens.

Reducing the focal length of the converging lens simultaneously reduces the spot on the photodetector. However, in a split-type photodetector, the dividing line of the photodetecting element requires a specific width of a low-sensitivity portion. Therefore, a spot smaller than this width cannot receive light, and the size of the spot that can receive light is limited.

As a method for detecting focus errors in the far field with a large spot, an astigmatism method can be considered. However, since it uses a cylindrical lens, it is not suitable for miniaturization.

An object of the present invention is to provide an optical head device that does not cause the above-mentioned problems.

[Means to solve the problem]

The present invention includes: a first optical system that simultaneously forms a plurality of light spots on a recording medium; a second optical system that converges and divides each reflected light from the plurality of spots; It is characterized by having photodetectors installed in front and behind the convergence position and at positions where the reflected lights from the respective light spots do not overlap.

〔Example〕

FIG. 1 shows the configuration of an embodiment of the present invention. This optical head device includes an array type semiconductor laser 1. having two light emitting points. Collimating lens 2. Beam splitter 3. Condensing lens 4. Converging lens 6, splitting prism 8. It is composed of an array type photodetector 7.

The splitting prism 8 splits the beam from the converging lens 6 and is arranged so that the two split beams form spots on the array photodetector 7 in front and behind the converging point of the converging lens 6. ing.

The light emitted from the array type semiconductor laser 1 is passed through a collimating lens 2. Beam splitter 3. The light is focused on the recording medium 5 via the fi light lens 4, forming two spots.

The reflected light from the recording medium 5 is separated by a beam splitter 3, condensed again by a converging lens 6, and split by a splitting prism 8 so that the spots before and after convergence are received by an array type photodetector 7. Split beam array type photodetector 7
and four spots 41 on the array type photodetector 7.
, 42.43.44. Spots 41 and 42 are spots formed by two divided beams of one light emitting point, spot 41 is a pre-convergence spot, and spot 42 is a post-convergence spot. Spots 43 and 44 are spots formed by two divided beams from the other light emitting point, spot 43 is a pre-convergence spot, and spot 44 is a post-convergence spot.

The shape of the spot on the array type photodetector 7 is shown in FIG. A pre-convergence spot 41 is formed on the 3-split detection element 11, 12, 13, and a post-convergence spot 42 is formed on the 3-split detection element 15, 16, 17. On the other hand, the detection element 14
A pre-convergence spot 43 is formed on the detection element 18, and a post-convergence spot 44 is formed on the detection element 18.

When the converging lens 6 has a short focus, the splitting prism 8 is rotated 90 degrees around the optical axis to change the shape of the array detector 7 to the third one.
It is also effective to do as shown in the figure.

The principle of detecting focus errors in this detector will be explained with reference to FIG. The detection element is of a three-part type as described above. When the recording medium 5 approaches the condenser lens 4 (FIG. 4(a) L focused state)
In the case of far distance (FIG. 4(C)), the shapes of the spots 41 and 42 differ on the two three-split detection elements as shown in the figure.

Therefore, a focus error signal can be obtained from the difference between the detection elements inside and outside of each spot. Specifically, a focus error signal is obtained from the difference between the sum of the amounts of light received by the detection elements 11.13.16 and the sum of the amounts of light received by the detection elements 12.15.16.

If the detectors are arranged so that spots from different light emitting points do not overlap near the focal point position, each signal can be detected independently. Therefore, during the focus pull-in operation, it is sufficient to turn on only one light emitting point.

As a method for detecting focus errors, a two-segment photodetector element 24.2 having a circular dividing line as shown in FIG.
5.26.27 may also be used. Further, as shown in FIG. 6, it is also possible to further divide the sensor into two in the orthogonal direction so as to simultaneously obtain track error signals using the push-pull method.

Although one embodiment of the present invention has been described above, the same structure can be applied even if the number of light emitting points is three or more. Furthermore, the light source may be of a type that combines two laser beams instead of an array type.

〔Effect of the invention〕

According to the present invention, a plurality of light spots are simultaneously formed on a recording medium, reflected light from the plurality of spots is converged and divided, and the spot before convergence is divided by one of the divided reflected lights. To obtain an array type optical head device that can be easily miniaturized by forming a converged spot on a split type photodetector by the other reflected light and extracting a focus error signal from the photodetector. I can do it.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIGS. 2 and 3 are diagrams each showing spots on a photodetector, and FIG. 4 is a diagram for explaining the principle of focus error signal detection. , Fig. 5 and Fig. 6 are diagrams showing other examples of photodetectors, Fig. 7 is a diagram showing a conventional optical head device, and Fig. 8 is a diagram showing a focus error signal in the optical head device of Fig. 7. FIG. 3 is a diagram for explaining the detection principle. Array type semiconductor laser collimating lens Beam splitter Condensing lens Recording medium 6 ・ ・ ・ 7 ・ ・ ・ 8 ・ ・ ・ 11 to 18° 22 ・ ・ 23 ・ ・ 31 ・ ・ 41 to 44 ・ ・ ・・Array type photodetector・・Split prism 24 to 27・・Photodetection element・・Spot shape・・2 split type photodetector・・Knifetsu・・Spot

Claims (1)

[Claims] (1) A first optical system that simultaneously forms a plurality of light spots on a recording medium, a second optical system that converges and divides each reflected light from the plurality of spots, and converges the divided reflected lights. What is claimed is: 1. An optical head device comprising: a photodetector installed in front and behind the position and at a position where the reflected lights from the respective light spots do not overlap.