JPH029021A - Optical head device - Google Patents

Abstract

PURPOSE:To prevent the wavelength fluctuation of a light source and the influence to be caused by the deformation of a light spot and to decrease the focusing shift by diffracting and separating reflected light generated from a recording medium from an optical axis of an image forming lens by a grating lens, placing a parting of a four-division photodetector in parallel to the diffracted direction, and selecting a distance to the grating lens. CONSTITUTION:A grating lens 25 diffracts and separates a laser reflected light beam of a disk 4 along the track direction 6 from an optical axis of an image forming lens 3. Subsequently, a four-division photodetector 9 is placed between focal lines 10, 11 to which an astigmatism light 5 is orthogonal. Next, by selecting a distance extending from a semiconductor laser 1 to the grating lens 25, a focusing shift generated by the wavelength fluctuation of the semiconductor is minimized. According to such constitution, against the wavelength fluctuation of a light source, a small light spot deformation and the focusing shift are decreased remarkably, and its shift is extremely small and can be reduced to <=0.24mum when the wavelength fluctuation is 10nm.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical head device, which is used for recording and reproducing so-called optical discs, digital audio discs, video discs, and the like.

[Conventional technology]

Conventionally, there are various configurations of optical head devices.
One of them uses a grating lens. An optical head device with this configuration is capable of extracting information light without using a half prism or a deflection beam splitter prism. It can be composed of an imaging lens that converges on the optical disk and a grating lens that guides reflected light from the optical disk surface to a four-split photodetector.

The reason why it is not necessary to use the prism as described above is as follows. In other words, the light emitted from the grating lens includes, in addition to the first-order diffracted light, the 0 light that directly passed through the grating lens.
There is a second diffracted light. Therefore, if this grating lens is placed between the semiconductor laser light source and the imaging lens as described above, and the 0th order diffracted light is used for the light that goes from the semiconductor laser to the disk surface, the thickness of the substrate of the grating lens will simply be used. It will be the same as if a transparent liquid equal to is inserted.

On the other hand, for the reflected light from the disk surface, if first-order diffracted light is used, the information light can be extracted off the optical axis without using a half prism or a deflection beam splitter prism. In other words, the grating lens acts as a beam splitter.

In this way, an optical head device can be configured by a light source, an imaging lens, a grating lens, and a 4-split photodetector, and as described above, a signal is generated from the information light as the first-order diffracted light extracted off the optical axis. Make it possible to obtain other brake signals as well. That is, in addition to signals, a focus error signal and a tracking error signal are also extracted from the information light, and these detection error signals are used to perform focus control and the like. There are also various types of focus error detection means. An example is one based on the astigmatism method. The focus error detection means of this astigmatism method uses a grating lens that converts a convergent spherical wave into a convergent wavefront having astigmatism, and is composed of a grating lens and a 4-split photodetector.

The grating lens in this case is composed of an interference pattern of diverging light from a semiconductor laser and a diverging wavefront having astigmatism.

In an optical head device that employs the above-described focus error detection means, a focus error signal and further a reproduction signal are obtained as follows.

As shown in FIG.
15 forms a detection surface. Code 16, 17
indicates a dividing line. The reflected light from the optical disk surface is
The image is formed by the imaging lens and separated off the optical axis by the grating lens, resulting in astigmatism and forming a front focal line and a rear focal line orthogonal to each other at the front and rear of the 4-split photodetector. A circular beam is formed on the four-split photodetector between the front focal line and the rear focal line, as shown in FIG. 5(b). When the disk surface approaches the imaging lens, the two orthogonal focal lines shift backward, and the beam on the photodetector becomes an ellipse as shown in Figure 5(a); When the two orthogonal focal lines move away from each other, the two orthogonal focal lines shift forward, forming an ellipse as shown in FIG. 5(c). Therefore, each element 12 of the 4-split photodetector
If the output voltage of ~15 is V (12) ~V (15), then V (12) +V (14) - V (13)
A focus error signal is obtained by detecting the signal of -V (15).

In this way, the focus error signal can be detected, and the reproduced signal from the disk is V (12) +V (13) +V (1
4) Can be detected by taking +V (15).

[Problem to be solved by the invention]

By the way, in optical head devices using this type of grating lens, focus offset caused by wavelength fluctuations of the light source used is a problem, and this becomes a bottleneck in putting such optical head devices into practical use.

That is, the practical focus offset tolerance of this type of optical head device is, for example, 5 μm.
That's about it. Additionally, in general, it is necessary to operate within a temperature range of ±40°C around 20°C. On the other hand, when a semiconductor laser is used as a light source, the amount of variation in oscillation cracking with respect to the temperature of the semiconductor laser is usually about 4.5 nm/20°C at a center temperature of 20°C. In other words, based on the above two conditions, the focus offset needs to be within 0.5 μm for a wavelength variation of about 10 nm. However,
In the conventional optical head device, the permissible focus offset value reaches 0.5 μm for a variation of 5 nm, making it difficult to put it into practical use.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical head device that can eliminate the above-mentioned drawbacks, can significantly reduce focus offset, and is thus practical.

[Means to solve the problem]

The optical head device of the present invention includes a light source, an imaging lens that focuses an image of the light source onto a recording medium, and is provided between the light source and the imaging lens, and converts light reflected from the recording medium into light from the imaging lens. a grating lens that diffracts and separates the reflected light from the axis along the track direction on the recording medium and converts the reflected light into astigmatic light; and a grating lens that is disposed between two orthogonal focal lines of the astigmatic light. The dividing line of the 4-split photodetector is arranged parallel to the diffraction direction, and the distance from the light source to the grating lens is minimized to minimize focus offset caused by wavelength fluctuations of the light source. The feature is that the distance can be set to

[Effect]

When the reflected light from the recording medium is diffracted and separated from the optical axis of the imaging lens by a grating lens, and then applied to a 4-split photodetector,
By arranging the dividing line of the 4-split photodetector parallel to the diffraction direction, it is possible to make focus offset less likely to occur, and also to minimize focus offset caused by wavelength fluctuations of the light source. By setting the distance to , it is possible to eliminate the influence of deformation of the light spot on the photodetection surface, making it possible to significantly reduce focus offset.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows an embodiment of the present invention. This embodiment shows a configuration in which a semiconductor laser is used as a light source, and the optical head device includes a semiconductor laser 1 and an imaging lens 3.
, a grating lens 25 , and a four-split photodetector 9 .

A radiation beam 2 is emitted from the semiconductor laser 1 to a disk 4 which is a recording medium, and an imaging lens 3 that focuses the image of the semiconductor laser 1 onto the disk 4 is arranged between the semiconductor laser 1 and the disk 4. There is.

The grating lens 25 is provided between the semiconductor laser 1 and the imaging lens 30. This grating lens 25 is connected to the disk 4
This is for diffracting and separating the reflected light from the imaging lens 3 along the track direction 6 on the disk 4 from the optical axis of the imaging lens 3, and converting the reflected light into astigmatic light 5 as shown in the figure.
This is used to detect reproduction signals, focus error signals, etc.

As shown in the figure, the four-split photodetector 9 detects two orthogonal focal lines of the astigmatic light 5, that is, a front focal line 10° and a rear focal line 11.
placed between.

The configuration of the 4-split photodetector 9 itself is as shown in FIG. 5, and as shown in FIG. 4
It consists of two photodetecting elements 12 to 15, and the photodetecting surface is numbered 1.
6. Divided into four parts as shown by the dividing line marked 17,
Output V according to the irradiation ratio of each beam spot
(12) to V (15) are taken out.

The method for detecting the reproduced signal and the focus error signal may basically be the method described above.

That is, regarding the reproduced signal, the sum of the outputs from each element 12 to 15 V (12) +V (13) +V (
14) Detect +V(15) as a reproduction signal.

Also, regarding focus error detection, focus error signal V (12) +V (14) -V (13) -V
This can be done by taking out (15).

Further, as a method for detecting a tracking error signal, which is another type of control signal, a known method can be applied.

That is, there are various methods for tracking error detection means, and one of them, for example, is a push-pull method.

This push-pull method uses a split-type photodetector to detect the imbalance of reflected light from the disk surface, and when the track is parallel to the split line 17, V (12)
+V (13) -V (14) -V (15), and if the track is parallel to the dividing line 16, ■(1
2) 10V (15) -V (13) -V (14
) the tracking signal can be obtained. In addition, as another method, focus error signal V (12) +V (14)
-v(13)-V(15) A method of generating a tracking error signal using an electronic circuit using a heterodyne method or a time difference method from the signal of -V(15) can also be applied.

The 4-split photodetector 9 is installed so that the dividing line of the 4-split photodetector 9 is arranged parallel to the diffraction direction in the case of diffraction separation by the grating lens 25, and regarding the grating lens 25, The distance from the semiconductor laser 1 to the grating lens 25 is set so as to minimize the focus offset caused by wavelength fluctuations of the semiconductor laser 1.

In this way, this embodiment uses the semiconductor laser 1 as a light source.
, an imaging lens 3 that focuses the image of the light source onto a disk 4 which is a recording medium, and an imaging lens 3 provided between the light source and the imaging lens 3,
A grating lens 25 that diffracts and separates the reflected light from the recording medium from the optical axis of the imaging lens 3 along the track direction 6 on the recording medium and converts the reflected light into astigmatic light 5;
In an optical head device composed of a four-split photodetector 9 arranged between two orthogonal focal lines 10 and 11,
The distance from the light source to the grating lens 25 is set so that the dividing line of the split photodetector 9 is arranged parallel to the diffraction direction, and in the grating lens 25, the focus offset caused by the wavelength fluctuation of the light source is minimized. is set optimally.

The optical head device with the above configuration can significantly reduce the focus offset, and even if a focus offset of about 0.5 μm is required in practice, it can fully meet this requirement, and is suitable for semiconductor lasers. When the wavelength variation of 1 is 10 nm, the focus offset is set to 0.2.
The thickness can be approximately 4 μm or less.

This focuses on the following points.

Focus offsets caused by wavelength variations in the light source used are caused by movement and slight deformation of the beam spot. The former cause can be eliminated by matching the direction of the dividing line of the photodetector 9 with the moving direction of the diffracted light.

In other words, in the case of an optical head device using a grating lens, the diffraction angle of the first-order diffracted light from the optical axis changes slightly as the wavelength of the light source changes, so the beam spot on the photodetector moves slightly. and deformation, resulting in focus offset.

For this reason, the following measures are adopted in the optical head device shown in FIG.

That is, FIG. 2(a) shows the beam spot on the photodetector 9 at the on-focus 4J[, but in this case, the focus error signal is The sum of the upper diagonal components V (12) + V (15) and v (
13) The difference from +v (14), that is, V (12)
10V (15) -V (13) -V (14) Therefore, as shown in Fig. 2(b), the moving direction of the beam spot caused by wavelength fluctuation is aligned with the dividing line of the photodetector. In this case, the focus error signal hardly changes.

In addition, since the moving direction of the beam spot coincides with the diffraction separation direction from the optical axis of the diffracted light, by matching this diffraction separation direction and the direction of the dividing line, focus offset is prevented ( Further, by making the direction of this dividing line coincide with the track direction, it is possible to prevent track offset from occurring.

In this way, it is possible to reduce the former cause, that is, the focus offset caused by the movement of the beam spot.

In addition to the above, the optical head device shown in FIG.
The latter cause, that is, focus offset caused by deformation, is also reduced, and focus offset is significantly reduced.

That is, simply preventing the focus offset caused by the movement of the beam spot as described above is not sufficient to reduce the focus offset to a practical value. Therefore, the inventor of the present invention found that the following focus offset depends on the distance between the light source and the grating lens 25 when the grating lens 25 as described above is used. In addition to the structure of the optical head device, a grating lens 25 is also provided.
We have decided to adopt a configuration that reduces focus offset to a practical value by optimally designing.

That is, since the grating lens 25 in FIG. 1 is formed from the interference pattern of the spherical wave from the semiconductor laser 1 which is the light source and the astigmatic light, the wave of the astigmatic light 5 converted by the grating lens 25 is The deformation of the beam shape due to the fluctuation depends on the distance dLIll from the grating lens 25 to the semiconductor laser 1, which is the light source, and this dLD determines the focus.
The offset also changes.

- As an example, FIG. 3 shows the dependence of the focus offset on the distance dLD between the light source and the grating lens. Note that this figure is for a case where the wavelength variation is 5 nm. As shown in Figure 3, dLo is 14.8m
The focus offset becomes close to zero near m. That is, it can be seen that the focus offset is minimized.

As described above, the optical head device according to the present invention takes advantage of the fact that the latter cause mentioned above, that is, the focus offset due to beam shape deformation, can be reduced by optimizing dLD. This is an experimental example of focus offset for wavelength variation over 10 nm of an optimized dLD.

That is, FIG. 4 shows the distance d between the grating lens 25 and the light source.
Lattice lens designed by optimizing LD25. (dLD=
16.5mm) and a grating lens designed without optimization (
d LD = 16.5 mm) is a diagram showing focus offset due to wavelength variation. According to Figure 4,
It was confirmed that by optimizing dllll, the focus offset could be reduced to 0.24 μm or less for a wavelength variation of 10 nm, making it practical.

In this way, the present optical head device can significantly reduce focus offset in a device using this type of grating lens by further changing only one design parameter of the grating lens.

〔Effect of the invention〕

As explained above, according to the present invention, an optical head device using a grating lens that can significantly reduce focus offset, has extremely small focus offset, and has the advantages described above. can be easily put into practical use.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the optical head device of the present invention. FIG. 2 is a diagram for explaining the state of the beam before and after wavelength variation of the light source on the four-split photodetector of the optical head device. Figure 3 is a diagram showing the dependence on the design distance from the light source to the grating lens for explaining the present invention, and Figure 4 is a diagram showing the dependence of focus offset due to wavelength fluctuation of the optical head device for explaining the present invention. FIG. 5 is a diagram illustrating the state of the light beam on the four-split photodetector. ■...Semiconductor laser 2...Radiation beam 3...Imaging lens 4...Disk (disk@) 5...Astigmatism light 6... ...Track direction 9...Four division photodetector 10...Front focal line 11...Back focal line 12-15...Photodetection elements 16, 17...Division Line 25... Lattice lens

Claims (1)

[Claims] (1) A light source, an imaging lens that focuses an image of the light source onto a recording medium, and is provided between the light source and the imaging lens, and directs the reflected light from the recording medium from the optical axis of the imaging lens to the recording medium. a grating lens that diffracts and separates the reflected light along the upper track direction and converts the reflected light into astigmatic light; and a 4-split photodetector disposed between two orthogonal focal lines of the astigmatic light. The dividing line of the 4-split photodetector is arranged parallel to the diffraction direction, and the distance from the light source to the grating lens is set to a distance that minimizes the focus offset caused by wavelength fluctuations of the light source. An optical head device characterized by comprising: