JPH0194536A - Optical head device - Google Patents

Abstract

PURPOSE:To improve the focus error detecting sensitivity of the title device by arranging a convergent optical system to convert a light flux divergent angle into a smaller divergent angle between a light source and an optical element to generate an astigmatism. CONSTITUTION:A light flux 9 emitted from a light source 7 of a semiconductor laser, etc., is converted into a light flux 8 with the smaller divergent angle. The divertent light flux 8 is reflected by a half mirror formed on a first surface 19 of parallel plates 10, passes through an objective lens 16, is made into a convergent light flux, and made incident on an optical disk 1. The light flux including information reflected by an information recording surface 2 transmits a half mirror surface 19 of the parallel plates 10, thereafter, reflected by mirror arranged at a rear surface 18, made into a light flux 8' possessing the astigmatism, and made incident on a photo detector 13. That is, by the function of the convergent lens 6, the divergent angle of the light flux 8 is set at 1/2 of the divertent angle of the light flux 9, and a desired lead-in range can be obtained even with the parallel plates 10 of the same width. Thus, the accuracy of the focus error detection can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an optical head device, and more particularly to an optical head suitable for reading signals from an information recording carrier such as an optical disk or for use as an information carrier.

[Prior art]

As a focus detection device as mentioned above, for example, Japanese Patent Application Laid-Open No.
The so-called astigmatism method as shown in No. 57013 is known. An example of this is shown in FIG. Here, a light beam 8 emitted from a light source 7 such as a semiconductor laser is reflected by a first surface 19 of a parallel plate 10 and focused onto an information track 2 of an optical disc 1 by an objective lens 16 . Then, the return light beam reflected by the optical disk 1 passes through the objective lens 16 again, enters the parallel plate 10 from the first surface 19, and enters the parallel plate 10 from the first surface 18.
The light is internally reflected and emitted from the first surface, and is detected by the photodetector 13. This returned light beam 8' causes astigmatism by passing through the parallel plate 10, and the shape of the light beam on the photodetector 13 changes from circular to elliptical depending on the focusing state on the optical disc 1. Focus detection is performed by detecting this change on 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 its mechanical structure, and as a result, the accuracy of focus error detection decreases.

For example, if the effective diameter of the light beam is about 4 mm, the parallel flat glass needs to have a thickness of about 2.5 mm to maintain surface accuracy.

In the known example shown in FIG. 4, the optical thickness of the parallel plate is doubled to about 5 mm because the back surface is a reflective surface. A parallel plate obliquely placed in a light beam produces an amount of astigmatism 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, if n=1.51.1=45°, i'-27
, 9°, and the astigmatism amount ΔP2 is calculated as follows.

= 1.3474647mm Now, as standard values for a normal optical head for compact discs, N and A on the disc side are 0.47, and N and A on the laser side (7) are 0.104, that is, from the laser to the disc. The imaging lateral magnification β on the surface is set to 1/4.5 times. The pull-in range of the focus error signal at this time is calculated as follows.

The value of this retraction range of 33 μm is approximately four times the desired retraction range of 7 to 9 μm, and this corresponds to the fact that the optical sensitivity for focus error detection is low.

Next, the second problem is that large comatic aberration and astigmatism occur simultaneously with the obliquely disposed thick parallel flat plates.

FIG. 5 schematically shows the shape of a light spot on a sensor of an optical head device that detects a focus error using an astigmatism method.

Focus position where a circular light spot should originally occur (Fig. 5 (
In b)), not only is the rotational symmetry disturbed, but also the focal line in Figs. 5(a) and 5
In Figure (c), the shape of the light beam is also disturbed.

Such disturbances not only have a negative effect on focus error detection, such as a decrease in detection sensitivity and a decrease in the linear region of detection characteristics, but also increase crosstalk from tracking errors, push-pull method, heterodyne method, etc. No. 1
This results in deterioration of detection characteristics when using the beam tracking method.

[Purpose of the invention]

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

The above object of the present invention is achieved by disposing a converging optical system that converts a beam divergence angle into a smaller divergence angle between a light source and an optical element that generates astigmatism.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram showing a first embodiment of the present invention. A light beam 39 emitted from a light source 41 such as a semiconductor laser is converted by a converging lens 40 into a light beam 42 with a smaller angular spread. Even if the converging lens 40 used here has spherical surfaces on both sides, by designing it as a so-called abranatic lens, it is possible to achieve substantially aberration-free divergence angle conversion.

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

The light beam containing information reflected by the information recording surface 2 of the optical disk passes through the half-mirror surface 19 of the parallel plate 10, and then is reflected by the reflecting mirror disposed on the back surface 18, and is reflected by the surface 1.
After passing through the light beam 9, the light beam becomes a light beam 8' having astigmatism and enters the photodetector 13.

In this embodiment, the divergence angle of the light beam 8 can be reduced to 1/2 of the divergence angle of the light beam 9 by the function of the converging lens 6. As a result, the design imaging magnification of the objective lens 16 is β=-1/4
．． 5, it becomes possible to set β' = -1/9, and even if the parallel plate 10 of the same thickness is used, the retraction range is 8 μm, and high sensitivity is achieved.

Note that when the optical head device is constructed without using the converging lens 6, the utilization rate of the luminous flux emitted from the light source 7 is 1/4.
As a result, the amount of light detected by the photodetector 13 also decreases, resulting in an optical head with poor focus error detection and signal readout S/N ratio as a whole.

A second embodiment of the present invention will be described using FIG. 2. The semiconductor laser light source includes a semiconductor laser 7, an aspherical glass molded lens 6 that also serves as a laser window, and a package part 6.
It is composed of 1.

A diverging light beam 9 with a large spread angle emitted from the semiconductor laser 7 is converted into a diverging light beam 8 with a smaller spread angle by the converging lens 6, and is converted into a diverging light beam 8 with a smaller spread angle, and is converted to a half mirror surface 19 disposed on the first surface of the prism 10'. The light is reflected by the beam and enters the objective lens 16. The light beam reflected by the information recording surface 2 on the optical disk 1 travels backward along the optical path, is refracted and enters the prism 10', and is reflected by the reflective beam disposed on the second surface 18 of the prism 10'. After being reflected by the surface, it is totally reflected by the surface 19 and exits the prism from the third surface 17 of the prism 10', becoming a light beam 8'.

When the optical path is developed, this prism 10 becomes equivalent to a parallel plate, and as a result, the 4-split sensor 13 performs focus error detection using the astigmatism method.

In this embodiment, unlike the first embodiment, the projected light flux 8
Since the received light beam 8' and the received light beam 8' are spatially separated, the lens 6 can be placed close to the reflecting surface 19, and the optical head device can be made more compact. In addition, it is difficult to design the prism 10' to have a short optical path length due to its structure, and when the beam diameter is set to 3.5 mm, the thickness is 1 mm.
It even reaches 0mm. According to this embodiment, the design magnification of the objective lens 16 is set from β = -1/4.5 to β' = -1/18 in this case as well, so that the retraction range 8
Focus error detection with a high sensitivity of μm is achieved.

Furthermore, reducing the beam angle has a great effect on correcting coma aberration.

When the amount of comatic aberration is expressed 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 angle by A times with lens 6 is 16 times, and the thickness is 10 m.
A parallel flat plate of m is effectively the same as a parallel flat plate of thickness 0.6 mm. The reduction magnification of the angle of divergence of the light flux can be designed to various values, but in practice it is preferably 0.7 times to 0.2 times.

In the present invention, reducing the slope angle of the light beam from the light source can be achieved by various means. Figure 3 is the second
1 is a modification of the second embodiment of the invention illustrated in the figure; A minute concave lens element 6' is disposed immediately in front of the semiconductor laser 7, and the laser window is constituted by the convex lens 6. This-
The function of the set of lenses creates a configuration similar to a so-called telephoto type lens, making it possible to configure a compact light projection optical system. After the diverging light beam 8 is reflected by the half mirror surface 19, it becomes a parallel light beam by the collimator lens 16', and becomes a convergent light beam by the objective lens 16'. The lens combination 16', 16' of this embodiment has the same function as the objective lens 16, but the infinite speed imaging objective lens 1
Since only the lens 6' is driven in the focus and tracking directions, there are advantages such as the mass of the movable part is small, the frequency response is good, and there is little change in imaging performance and focus error detection characteristics.

In this embodiment, the prism 10' has a scalene shape,
The light beam 8' is emitted at an angle with respect to the horizontal plane. Although this design value is set for the purpose of making the device thinner, the present invention is equally effective in this case as well.

In the above description, the effects of the present invention have been explained with respect to an optical head device using a parallel plate optical member, but the same effect can be obtained with a prism having an appropriate apex angle instead of a parallel plate.

Furthermore, by making the concave lens 6' and the convex lens 6 shown in FIG. 3 rotationally asymmetric, it is possible to improve the rotational symmetry of the spread of the light beam from the semiconductor laser and to correct the astigmatism of the semiconductor laser itself. is also possible.

In addition to the spherical lenses and aspheric lenses described in the specification, optical elements that can be used in the present invention include gradient index lenses, hologram lenses, and the like.

〔Effect of the invention〕

As explained above, the present invention provides a focus error detection device that uses astigmatism caused by a parallel plate, a non-parallel plate, or a prism, in which a converging optical system is provided to reduce the divergence angle of the light beam emitted from the light source. This made it possible to improve the accuracy of focus error detection. Furthermore, as a secondary effect, the amount of coma aberration could be reduced. As a result, the effects of improved focus error detection performance and improved tracking error detection performance were obtained.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a first embodiment of the invention, FIG. 2 is a schematic diagram showing a second embodiment of the invention, and FIG. 3 is a modification of the second embodiment of the invention. FIG. 4 is a schematic diagram of a conventional optical head device, and FIG. 5 is a diagram illustrating the influence of coma aberration on the conventional optical head device. 1... Optical disc, 2... Information recording surface, 6
... Luminous flux spread angle conversion lens, 7... Semiconductor laser, 8... Diverging luminous flux, 9.
...Divergent flux before conversion, 10...Parallel plate, 1σ...
・Prism, 13... 4-split sensor 16... Objective lens,
17... Luminous flux exit surface, 18... Reflective mirror surface, 1
9...Half mirror surface.

Claims (2)

[Claims] (1) a light source that emits a diverging light beam; a converging optical system that is disposed in the diverging light beam and converts the divergence angle of the light beam into a smaller angle; and a focusing means that focuses the light beam onto an information carrier;
an optical element that reflects the converted light flux on a surface and guides it to the condensing means, and that transmits the reflected light from the information carrier to generate an aberration; An optical head device consisting of a photodetector for detection. (2) The optical head device according to claim 1, wherein the converging optical system converts the divergence angle of the light beam from the light source by a factor of 0.7 to 0.2.