JPH0738255B2 - Optical head - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head including an optical element that produces astigmatism in a beam that returns from an optical recording medium and enters a photodetector. is there.

[Outline of Invention]

According to the present invention, in the above optical head, the first and second photodetectors, which are divided into three in the direction perpendicular to the optical axis of the beam and the focal line of the sagittal ray, are separated from each other by the focal line. By arranging at equal distance positions in the opposite direction, by using the difference between the detection outputs of the photodetector in the center of one photodetector and the photodetectors on both sides of the other photodetector as the focus error signal, Although it is small, it is possible to perform accurate focus adjustment.

[Conventional technology]

Since the optical head operates in a non-contact state with the optical recording medium, it is necessary to detect a focus error and perform focus adjustment.

There are various methods for detecting the focus error, and one of them is disclosed in Japanese Patent Laid-Open No. 60-171643.
The three-division photodetectors are integrally mounted on the outer surface of a single beam splitter so as to be positioned before and after the focal point of the detection beam, and the total difference in the detection output of the six photodetection units is obtained. Such a method is disclosed.

With an optical head using such a method, adjustment is easy, and normally, as shown in FIG.
The signal C, which is the difference between and, forms an S-shaped characteristic having a good characteristic as a focus error signal.

By the way, an optical head as shown in FIG. 8 has been considered which is not only easy to adjust but also can be further downsized. In this optical head, first and second photodetectors 12 and 13 are formed on a semiconductor substrate 11, and a semiconductor laser chip 14 and a prism 15 are arranged on the semiconductor substrate 11.

The surface 15a of the prism 15 facing the semiconductor laser chip 14 is inclined with respect to the optical axis of the beam 16 emitted from the semiconductor laser chip 14 and is a semi-transmissive reflecting surface.

The surface 15b of the prism 15 facing the semiconductor substrate 11 has a semi-transmissive reflective surface in the vicinity of the photodetector 12 and a fully transmissive surface in the vicinity of the photodetector 13. There is. A surface 15c of the prism 15 opposite to the surface 15b is a total reflection surface.

Therefore, a part of the beam 16 is reflected by the surface 15a and irradiates an optical recording medium (not shown), and a part of the beam 17 reflected by the optical recording medium passes through the surface 15a and enters the prism 15. Incident on.

A part of the beam 17 incident on the prism 15 is transmitted through the surface 15b, enters the photodetector 12, and is reflected by the surface 15b.
7 is reflected by the surface 15c and further transmitted through the surface 15b, and the photodetector 13
Incident on.

The photodetectors 12 and 13 are arranged in three photodetector sections 12a to 12c and 13a to 13c (Fig. 9) in a direction parallel to the surface of the semiconductor substrate 11 and a direction perpendicular to the optical axis of the beam 17. It is divided.

Then, similar to the optical head in JP-A-60-171643, the detection output from the central photodetector 12a of the photodetector 12 and the detection output from the photodetectors 13b and 13c on both sides of the photodetector 13. Of the photodetectors 12b and 12c on both sides of the photodetector 12 and the sum of the detection outputs from the photodetector 13a at the center of the photodetector 13 (not shown). ) Is also provided.

Therefore, in this optical head, the single prism 15 serves as a beam splitter for separating the irradiation beam 16 and the detection beam 17 and an optical element for making the beam 17 incident on the two photodetectors 12 and 13. It is very small as a whole.

[Problems to be solved by the invention]

However, the surface 15a of the prism 15 is inclined not only with respect to the optical axis of the beam 16 but also with respect to the optical axis of the converging beam 17,
Moreover, the refractive index changes with the surface 15a as the interface. Therefore, astigmatism occurs in the beam 17 that has passed through the surface 15a.

Therefore, in the optical head shown in FIG. 8, similar to the optical head disclosed in Japanese Patent Laid-Open No. 60-171643, light detection is performed at positions equidistant before and after the best image point which is the focal point of the beam 17. When the detectors 12 and 13 are arranged, the photodetectors 12 and 13 and the surface 15c
Above, as shown in FIG.
23 is formed.

That is, the spots 21 and 22 have mutually different widths in the direction of the three divisions of the photodetectors 12 and 13. To this end, the six photodetection units 12a to 12c and 13a to 13c are calculated by the above-mentioned arithmetic unit.
When the difference in the detection output of is obtained, the signal of FIG. 10 is obtained corresponding to the signal of FIG.

As is clear from FIG. 10, the focus error signal is output even when the optical recording medium is at the in-focus position, and normal focus adjustment cannot be performed. Even in such a case, the signal C can be used as the focus error signal by applying a bias to the signal C. However, a circuit for that purpose is required, and the configuration becomes complicated.

In addition to the astigmatism, the beam 17 transmitted through the surface 15a has
Coma aberration occurs in a direction perpendicular to the direction of the photodetectors 12 and 13 divided into three parts. For this reason, each of the spots 21 to 23 shown in FIG. 9 has an asymmetrical shape.

[Means for solving problems]

The optical head according to the present invention is inclined with respect to the optical axis of the beam 16 emitted from the light source 14, reflects the beam 16 and guides the beam 16 to the optical recording medium, and the beam 17 returned from the optical recording medium. In an optical head comprising a beam splitter 15 having a transparent first surface 15a,
A predetermined portion of the beam 17 transmitted through the first surface 15a is arranged between the focal line of the sagittal ray 17a of the beam 17 that has passed through the first surface 15a and the first surface 15a. The beam splitter 15 has a second surface 15b which transmits the light and reflects the rest, and the beam splitter 15 is arranged on the side opposite to the second surface 15b and is reflected by the second surface 15b. The beam splitter 15 has a third surface 15c that reflects all 17 and is divided into three in a direction perpendicular to the optical axis of the beam 17 and the focal line.
The first photodetector 12 having 12a to 12c has the second surface 15b.
Second light having a photodetection section 13a to 13c which is arranged in the optical path of the beam 17 that has passed through Detector
13 is in the optical path of the beam 17 reflected by the third surface 15c, and from the focal line in a direction opposite to the first photodetector 12, the focal line and the first photodetector 12 And a detection output from the photodetector on both sides of one of the first and second photodetectors 12 and 13 and the other light. The sum of the detection outputs from the photodetector in the center of the detector, the detection output from the photodetector in the center of the one photodetector, and the detection from the photodetectors on both sides of the other photodetector. An arithmetic unit for outputting a difference from the sum of outputs is provided, and the difference is used as a focus error signal.

[Action]

In the optical head according to the present invention, the first and second photodetectors 12, 13 which are divided into three in the direction perpendicular to the optical axis of the beam 17 and the focal line of the sagittal ray 17a are separated from each other by the focal line. They are arranged at equal distances in the opposite direction, and the difference between the detection outputs of the central photodetector of one photodetector and the photodetectors on both sides of the other photodetector is used as the focus error signal.

Therefore, even if astigmatism occurs on the first surface 15a of the beam splitter 15, a focus error signal that causes zero crossing at the focus position can be obtained.

〔Example〕

Hereinafter, first and second embodiments of the present invention will be described with reference to FIGS. 1 to 6.

1 to 3 show the first embodiment. This first
In the embodiment, the first and second photodetectors 12 and 13 are arranged at equidistant positions before and after the focal line of the sagittal ray 17a, not at equidistant positions before and after the best image point of the beam 17 for detection. It has substantially the same structure as the optical head shown in FIG.

And in this first embodiment, as is clear from FIG.
The focal line of the sagittal ray 17a is located on the surface 15c, and as is clear from FIG. 1, the focal line of the meridional ray 17b is located between the surface 15c and the photodetector 13. Therefore, the best image point is the focal line of sagittal ray 17a and the meridional ray 1
It is located in the middle of the focal line of 7b.

In the first embodiment, the photodetector 24 for output monitoring of the semiconductor laser chip 14 is formed on the semiconductor substrate 11, but even if such a photodetector 24 is formed, the semiconductor substrate 11 is The area is only 2.7 mm x 1.8 mm.

In the first embodiment as described above, when the optical recording medium is at the in-focus position, the spots 21, 22 and 23 of the beam 17 are formed on the photodetectors 12, 13 and the surface 15c as shown in FIG. It is formed.
Since the spots 21 and 22 are equidistant from each other from the focal line of the sagittal ray 17a, that is, the spot 23, the photodetector 1
The photodetectors 12 and 13 have the same width in the directions of the three divisions 2 and 13, and are accompanied by the focus error.
Expands and contracts in the direction of 3.

For this purpose, as shown in FIG. 5, the photodetectors 12a and 13b,
The signal A is obtained as the sum output of 13c, and the photodetectors 12b and 12c
And 13a, a signal B is obtained, and a signal C is obtained as a difference between the signals A and B.

Therefore, in the first embodiment, the signal C is S-shaped with good characteristics as the focus error signal without any correction for astigmatism.

Further, in this first embodiment, since the beam 17 is reflected by the surface 15c at the focal line of the sagittal ray 17a, that is, the position of the spot 23, the beam 17 is reflected at the position of the best focused image point. Compared to, it is less affected by scratches on the surface 15c.

Incidentally, not only in the example shown in FIG. 8 but also in this first example, the beam 17 transmitted through the surface 15a produces coma as well as astigmatism. However, as is clear from FIGS. 4 and 9, this coma aberration appears in the direction perpendicular to the direction of the photodetectors 12, 13 divided into three parts. Therefore, this coma does not affect any of the signals A to C in FIGS. 5 and 10.

FIG. 6 shows a second embodiment. In the second embodiment, the sectional shape of the prism 15 is different from the sectional shape of the prism 15 of the first embodiment, and the focal line of the sagittal ray 17a is the surface 1.
It has substantially the same configuration as that of the above-described first embodiment except that it is not located above 5c.

However, also in this second embodiment, the optical distances from the focal line of the sagittal ray 17a to the photodetectors 12 and 13 are equal to each other, and this second embodiment also has the same effects as the first embodiment. There is.

In the second embodiment, by selecting the size of the spot 23 of the beam 17 on the surface 15c, the influence of scratches on the surface 15c on the focus error signal can be further reduced as compared with the first embodiment. .

By the way, instead of equalizing the distances from the focal line of the sagittal ray 17a to the photodetectors 12 and 13 as described above,
It is also conceivable to make the distances from the focal line of the meridional ray 17b to the photodetectors 12, 13 equal to each other.

However, in that case, the direction of expansion and contraction of spots 21-23 is the first.
And the case of the second embodiment is the vertical direction. Therefore, the photodetectors 12 and 13 also need to be divided into three in the direction perpendicular to the case of the first and second embodiments, that is, the left and right directions in FIG.

However, even in this case, the direction in which coma aberration appears is the first
And the same direction as in the second embodiment. Therefore, the effects of coma appear on the signals A to C in FIG.
It is not possible to obtain a focus error signal in the shape of a letter.

Moreover, the height of the light emitting point of the semiconductor laser chip 14 is generally low in accuracy. When the photodetectors 12 and 13 are divided into three parts in the left-right direction in FIG. 4, as is apparent from FIG. 1, the semiconductor laser chip 14 is affected by the error in the height of the light emitting point. easy.

〔The invention's effect〕

In the optical head according to the present invention, the first beam splitter
Even if astigmatism is generated in the above aspect, a focus error signal that zero-crosses at the in-focus position can be obtained, so that accurate focus adjustment can be performed despite the small size.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 to 3 are a side view, a plan view and a perspective view, respectively, of a first embodiment of the present invention, and FIG. 4 is a plan view of a beam spot according to the first embodiment. FIG. 5 is a graph showing the focus error signal according to the first embodiment, and FIG. 6 is a side view of the second embodiment of the present invention. FIG. 7 is a graph showing a focus error signal according to a conventional example of the present invention, FIG. 8 is a side view of another example already considered, and FIG. 9 is a beam spot according to the example shown in FIG. A plan view and FIG. 10 are graphs showing focus error signals according to the example shown in FIG. In the reference numerals used in the drawings, 12, 13 ... Photodetectors 12a, 12b, 12c, 13a, 13b, 13c ... Photodetector 14 ... Semiconductor laser chip 15 ... Prism 15a, 15b ... Surface 16 , 17 …… beam 17a …… sagittal ray.

Claims (1)

[Claims] 1. A first surface which is inclined with respect to an optical axis of a beam emitted from a light source, reflects the beam and guides the beam to an optical recording medium, and transmits the beam returned from the optical recording medium. An optical head including a beam splitter having: a beam splitter that is disposed between a focal line of a sagittal ray of the beam that has passed through the first surface and the first surface and that has transmitted through the first surface. The beam splitter has a second surface that transmits a predetermined portion of the beam and reflects the remaining portion of the beam. The beam splitter is arranged on the opposite side of the second surface and is reflected by the second surface. The beam splitter has a third surface that reflects the entire beam, and the beam splitter has a third surface in a direction perpendicular to the optical axis of the beam and the focal line.
A first photodetector having a divided photodetector is arranged in the optical path of the beam transmitted through the second surface, and is perpendicular to the optical axis of the beam and the focal line. Direction 3
A second photodetector having a split photodetector is in the optical path of the beam reflected at the third surface and from the focal line in a direction opposite to the first photodetector. The focal line and the first photodetector are spaced apart from each other by an optical distance, and the photodetector is provided on both sides of one of the first and second photodetectors. Of the detection output and the sum of the detection output from the photodetector in the center of the other photodetector and the detection output from the photodetector in the center of the one photodetector and both sides of the other photodetector An optical head is provided, which is provided with a computing unit that outputs a difference from the sum of the detection outputs from the light detection unit, and uses the difference as a focus error signal.