JPH07118082B2 - Optical head device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical head device, and more particularly to an optical head suitable for reading a signal from an information recording carrier such as an optical disk or for use as an information carrier.

[Prior Art] As the focus detecting device as described above, for example, Japanese Patent Laid-Open No.
A so-called astigmatism method as shown in No. 013 is known. This example is shown in FIG. Here, the light beam 8 emitted from the light source 7 such as a semiconductor laser is reflected by the first surface 19 of the parallel plate 10 and is focused on the information track 2 of the optical disk 1 by the objective lens 16. Then, the return light flux reflected by the optical disk 1 again passes through the objective lens 16 and enters the parallel plate 10 from the first surface 19, is internally reflected by the second surface 18, and is reflected by the first surface 19.
The light is emitted from the surface 19 and is detected by the photodetector 13. This return light beam 8'has astigmatism caused by passing through the parallel flat plate 10, and the shape of the light beam on the photodetector 13 changes from a circular shape to an elliptic shape according to the focused state of the optical disc 1. . Focus detection is performed by detecting this change in the four-divided light-receiving surface of the photodetector 13.

However, the conventional device as described above has two problems.

The first problem is that it is impossible to reduce the thickness of the parallel plate 10 to a desired thickness due to the mechanical structure, and as a result, the accuracy of focus error detection is reduced. For example, if the effective diameter of the light flux is about 4 mm, the parallel plate 1 glass needs to have a thickness of about 2.5 mm in order to maintain the surface accuracy.
In the known example shown in FIG. 4, the optical thickness of the parallel plate is about 5 mm, which is twice as large as the back surface is the reflecting surface. A parallel plate obliquely installed in the light beam produces an astigmatism amount given by the following equation.

Here, d is the thickness of the flat plate, n'is the refractive index, i is the incident angle from the air to the flat plate, and i'is the refraction angle in the flat plate.

Now, assuming that n = 1.51 and i = 45 °, i ′ = 27.9 °, and the astigmatism amount ΔP ₂ is calculated as follows.

Now, as the standard value of the optical head for ordinary compact disks, the NA on the disk side is 0.47, and the NA on the laser side is
Is 0.104, that is, the lateral imaging magnification β from the laser to the disk surface is −1 / 4.5. The pull-in range of the focus error signal at this time is calculated as follows.

The value of the pull-in range of 33 μm is about four times as large as the desirable pull-in range of 7 to 9 μm, which directly corresponds to the low optical sensitivity of the focus error detection.

Next, as a second problem, a large coma aberration is generated at the same time as the astigmatism due to the thick parallel plates obliquely installed.

FIG. 5 schematically shows the shape of an optical spot on a sensor of an optical head device for detecting a focus error using the astigmatism method. At the in-focus position (Fig. 5 (b)) where an originally circular optical spot should occur, not only the rotational symmetry is disturbed, but also a sufficiently thin focal line should be originally produced. Also in FIG. 6C, the luminous flux shape is disturbed.

Such disturbance not only adversely affects the focus error detection such as a decrease in detection sensitivity and a decrease in the linear region of the detection characteristic, but also an increase in crosstalk due to a tracking error, a push-pull method or a heterodyne method. 1 of etc.
When the beam tracking method is used, the detection characteristics are deteriorated.

[Object of the Invention]

The object of the present invention is to solve the above-mentioned drawbacks of the conventional device, to sufficiently improve the focus error detection sensitivity with a simple configuration, and at the same time, to prevent crosstalk from tracking error and to improve tracking error detection performance. The object is to provide an achievable optical head device.

The above-mentioned object of the present invention comprises a light source that emits a divergent light beam, a condensing unit that condenses the divergent light beam on an information carrier, and a parallel plate or a prism that is arranged between the light source and the light condensing unit. An optical element and a photodetector for detecting a light beam emitted from the optical element are provided, and a divergent light beam emitted from the light source is reflected from the first surface of the optical element, and the information carrier is provided by the light converging means. The reflected light from the information carrier, which is condensed above, is incident on the optical element from the first surface through the light condensing means, and is the back surface of the first surface.
After being internally reflected by the surface of, and transmitted or reflected by the first surface, it becomes a light beam having astigmatism and exits from the optical element, and the light beam having the astigmatism is detected by the photodetector. Thus, in the optical head device for detecting the focus position error, the converging optical system for converting the divergence angle of the divergent light beam emitted from the light source into a smaller divergence angle and guiding the divergence light flux to the optical element between the light source and the optical element. This is accomplished by arranging the system.

〔Example〕

 Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing a first embodiment of the present invention. A divergent light beam 9 emitted from a light source 7 such as a semiconductor laser is converted by a converging lens 6 into a divergent light beam 8 having a smaller angular spread. Even if the converging lens 6 used here is spherical on both sides, by designing it as a so-called aplanatic lens, it becomes possible to achieve divergence angle conversion with substantially no aberration.

The divergent light beam 8 is reflected by the half mirror formed on the first surface 19 of the parallel plate 10, passes through the objective lens 16, and
It becomes a convergent light flux and enters the optical disc 1.

The light flux containing information reflected by the information recording surface 2 of the optical disk is the first surface 19 which is the half mirror surface of the parallel plate 10.
After passing through the first surface 19, it is reflected by a reflecting mirror provided on the second surface 18 which is the back surface of the first surface 19, and after passing through the first surface 19, a light beam 8'having astigmatism is formed. Incident on the photodetector 13.

In this embodiment, the divergence angle of the light beam 8 can be halved by the action of the converging lens 6. As a result, it is possible to change the design imaging magnification of the objective lens 16 from β = 1 / 4.5 to β ′ = − 1/9, and even if the parallel plate 10 having the same thickness is used, the pulling range is That is, it becomes 8 μm, and high sensitivity is achieved.

When the optical head device is configured without using the converging lens 6, the utilization rate of the luminous flux emitted from the light source 7 becomes 1/4, and as a result, the amount of light detected by the photodetector 13 also decreases, As a whole, the optical head is inferior in focus error detection and signal readout S / N ratio.

A second embodiment of the present invention will be described with reference to FIG. The semiconductor laser light source includes a semiconductor laser 7, an aspherical glass mold lens 6 also serving as a laser window, and a package portion 61.
It is composed of

The divergent light beam 9 with a large divergence angle emitted from the semiconductor laser 7 becomes a divergent light beam 8 with a smaller divergence angle by the converging lens 6, and the half mirror surface 19 disposed on the first surface of the prism 10 '. Is reflected by the objective lens 16
Incident on. The light beam reflected by the information recording surface 2 on the optical disk 1 travels backward in the optical path, is refracted and enters the prism 10 ', and is a reflecting surface provided on the second surface 18 of the prism 10'. After being reflected by, the surface 19 is totally reflected and prism 10 '
The light is emitted from the third surface 17 of the prism to the outside of the prism 10 'and becomes a light beam 8'having astigmatism.

This prism 10 'becomes equivalent to a parallel plate when the optical path is turned, and as a result, the astigmatism method focus detection is performed by the four-division sensor 13.

In the present embodiment, unlike the first embodiment, the translucent light beam 8
Since the light receiving light beam 8'and the received light beam 8'are spatially separated, the lens 6 can be arranged close to the reflecting surface 19 and the optical head device can be made compact. Further, it is difficult to design the prism 10 'to have a short optical path length due to its structure, and the thickness reaches 10 mm when the beam diameter is 3.5 mm. According to the present embodiment, also in this case, by setting the design magnification of the objective lens 16 from β = −1 / 4.5 to β ′ = − 1/18, a highly sensitive focus error detection with a pull-in range of 8 μm is achieved. To be done.

Further, reducing the divergence angle of the light flux also has a great effect on the correction of coma aberration. When the amount of coma aberration is displayed as lateral aberration on the sensor, this value is proportional to twice the spread angle.

Therefore, the improvement rate of the amount of coma aberration by increasing the beam divergence angle by 1/4 by the lens 6 is 16 times, and the parallel plate with a thickness of 10 mm is effectively the same as the parallel plate 10 with a thickness of 0.6 mm. Become. The reduction ratio of the luminous flux divergence angle can be designed to various values, but in practical use, it is desirable to be 0.7 times to 0.2 times.

In the present invention, the reduction of the spread angle of the light flux from the light source can be realized by various means. Figure 3 is second
It is a modification of the second embodiment of the present invention shown in the drawings. A minute concave lens element 6 ′ is arranged immediately before the semiconductor laser 7, and the laser window is composed of the convex lens 6. Due to the function of this pair of lenses, a configuration similar to that of a so-called telephoto type lens is achieved, and a compact projection optical system can be configured. The divergent light beam 8 is reflected by the half mirror surface 19 and then becomes a parallel light beam by the collimator lens 16 ″ and a convergent light beam by the objective lens 16 ′. The lens combination 16 ′, 16 ″ of this embodiment is the objective lens 16 Although it has the same function as, but only the infinity imaging objective lens 16 'is driven in the focusing and tracking directions, the moving part mass becomes small, the frequency response becomes good, and the imaging performance and There are advantages such as little change in the focus error detection characteristic.

In this embodiment, the prism 10 'has an unequal side shape, and the light beam 8'is emitted with an inclination with respect to the horizontal plane.
This is a design value like this for the purpose of reducing the thickness, but the present invention is equally effective in this case as well.

In the above description, the effect of the present invention has been described with respect to the optical head device using the optical member of the parallel plate, but the same effect can be obtained by using a prism having an appropriate apex angle other than the parallel plate.

Further, by making the concave lens 6'and the convex lens 6 of FIG. 3 rotationally asymmetrical, the rotational symmetry of the spread of the light beam from the semiconductor laser is improved and the astigmatism of the semiconductor laser itself is corrected. Will also be possible.

In addition to the spherical lens and aspherical lens described in the specification, the optical element that can be used in the present invention includes a gradient index lens, a hologram lens, and the like.

〔The invention's effect〕

As described above, the present invention is a parallel plate, or in a focus error detection device using astigmatism by a prism, by providing a converging optical system for reducing the divergence angle of the light beam emitted from the light source, It was possible to improve the accuracy of focus error detection. Further, as a secondary effect, the amount of coma aberration could be reduced. As a result, the effect of improving the focus error detection performance and the effect of improving the tracking error detection performance were obtained.

[Brief description of drawings]

1 is a schematic view showing a first embodiment of the present invention, FIG. 2 is a schematic view showing a second embodiment of the present invention, and FIG. 3 is a second view of the present invention.
FIG. 4 is a schematic diagram showing a modification of the embodiment, FIG. 4 is a schematic diagram of a conventional optical head device, and FIG. 5 is a diagram for explaining the influence of coma aberration in the conventional optical head device. 1 ... Optical disc, 2 ... Information recording surface, 6 ... Luminous flux divergence angle conversion lens, 7 ... Semiconductor laser, 8 ... Divergence luminous flux, 9 ... Divergence luminous flux before conversion, 10 ... Parallel plate, 10 ’…
… Prism, 13 …… 4-division sensor, 16 …… Objective lens, 17 …… Light flux exit surface, 18 …… Reflecting mirror surface, 19 …… Half mirror surface.

Claims (2)

[Claims] 1. An optical element comprising a light source that emits a divergent light beam, a condensing unit that condenses the divergent light beam on an information carrier, and a parallel plate or a prism disposed between the light source and the light condensing unit. And a photodetector for detecting a light beam emitted from the optical element, the divergent light beam emitted from the light source is reflected by the first surface of the optical element, and is focused on the information carrier by the light converging unit. The condensed light is reflected, and the reflected light from the information carrier passes through the condensing means to enter the optical element from the first surface, and is internally reflected by the second surface which is the back surface of the first surface. After passing through or reflecting on the first surface, a light beam having astigmatism is emitted from the optical element, and a light beam having the astigmatism is detected by the photodetector to obtain a focus position error. In an optical head device for detecting, the light source An optical head device is provided between the optical element and the optical element, and a converging optical system for converting the divergence angle of the divergent light beam emitted from the light source into a smaller divergence angle and guiding the divergence light flux to the optical element. 2. The optical head device according to claim 1, wherein the converging optical system converts the divergence angle of the divergent light beam from the light source to 0.7 to 0.2 times.