JPH0629784Y2 - Optical device for optical pickup - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical pickup used in a video disc player, an optical disc memory device, or the like, in which a recording surface of a disc is adjacent to an optical pickup without using an apodized filter. The present invention relates to an optical device that can prevent crosstalk between tracks.

[Background of the invention]

FIG. 3 is an element layout diagram showing an optical device of an optical pickup installed in a conventional video disc player or the like.

The laser beam output from the semiconductor laser 1 passes through the polarization beam splitter 2, is collimated by the collimator lens 3, is condensed by the objective lens 5, and a minute spot is formed on the recording surface Da of the optical disc D. It A diffraction grating 7 is arranged in the optical path of the laser beam, and three spots due to the laser beam are formed on the recording surface of the optical disc D. The reflected beam from the optical disc D passes through the quarter-wave plate 4, is reflected by the polarization beam splitter 2 in the orthogonal direction, passes through the light receiving lens 9, and is detected by the light receiving element 6 such as a photodiode. RF recorded by pits on the recording surface Da of the optical disc D by the light receiving element 6
The signal is read.

Reference numeral 8 in FIG. 4 denotes an apodized filter. The apodized filter 8 is integrated with the diffraction grating 7, for example. The apodized filter 8 has a beam transmission surface 8a at the center and semi-transmission surfaces 8b at both sides sandwiching the transmission surface 8a.

FIG. 5 (A) shows the light quantity distribution of the laser beam incident on the objective lens 5 when the apodized filter 8 is not provided, and FIG. 6 (A) is provided with the apodized filter 8 7 shows the light quantity distribution of the laser beam incident on the objective lens 5 in such a case. Further, FIG. 5 (B) shows the light quantity distribution of the beam spot formed on the recording surface Da of the optical disc D when the apodized filter 8 is not provided, and FIG. 6 (B) shows the apodized filter. The light amount distribution of the beam spot formed on the recording surface Da of the optical disc D when the filter 8 is provided is shown. If the apodized filter 8 is not provided, a spot K1 due to first-order diffraction is formed around the main spot K0 of the beam due to the diffraction phenomenon of the beam that has passed through the diffraction grating 7, as shown in FIG. 5 (B). It is formed. Therefore, when the main spot K0 scans the track T2, the spot K1 due to the first-order diffraction may be applied to the pits p of the tracks T1 and T3 that are adjacent to each other on the left and right. Since information is recorded by an analog signal in a video disc player or the like, if the spot K1 due to the first-order diffraction is applied to the pits p of the adjacent tracks T1 and T3 and crosstalk occurs, the S / N ratio of the reproduction of the RF signal is deteriorated. And a phenomenon such as noise on the screen occurs.

On the other hand, when the apodized filter 8 is used, as shown in FIG.
%), The amount of light on both sides of the effective diameter 10 of the laser beam is limited. Therefore, as shown in FIG. 6 (A),
The amount of light in the left and right portions of the laser beam incident on the objective lens 5 is reduced. As a result, as shown in FIG. 6 (B),
The light quantity of the spot formed on the recording surface Da of the optical disc D due to the first-order diffraction decreases.

[Problems to be solved by the invention]

However, when the apodized filter 8 is provided as a means for lowering the level of the spot due to the first-order diffraction in the conventional optical device shown in FIG. 3, the number of parts of the optical member increases as much as the apodized filter 8 is provided,
High cost.

If the apodized filter 8 is used, the filter 8
Since the numerical aperture of the objective lens 5 is substantially changed by the semi-transmissive surface 8b, the spot diameter indicated by φ in FIG. 6 (B) is widened and the intensity level of the main spot is lowered. The decrease in the intensity level of the main spot lowers the detection level by the light receiving element 6, and the increase in the spot diameter φ reduces the modulation degree. Further, when the apodized filter 8 is used, the aberration due to the semi-transmissive surface 8b also increases.

The present invention solves the above-mentioned conventional problems. Crosstalk between adjacent tracks occurs on the recording surface of an optical disk without using an apodized filter and adding a new optical element. An optical device of an optical pickup capable of preventing the above problem.

[Specific means for solving problems]

The present invention is directed to a semiconductor laser, an objective lens that focuses a laser beam emitted from the semiconductor laser on a recording surface of an optical disc, an optical system arranged on an optical path from the semiconductor laser to the objective lens, and an optical disc recording. In an optical device of an optical pickup including a light receiving element that detects reflected light reflected from a surface, an antireflection film coated on at least one surface of an optical element and an objective lens forming the optical system is The strip-shaped portion, which is thinner than the effective diameter at the center, is thin and both edge portions sandwiching the strip-shaped portion are thick.

In the present invention, one of the optical elements arranged on the optical path from the semiconductor laser to the recording surface of the optical disc, for example, the antireflection film coated on the surface of the collimator lens or the objective lens, is more effective than the effective diameter of the lens. It is thin in the thin strip-shaped range and thick in both sides sandwiching it. The beam transmittance at the portion where the antireflection film is thickly coated on both sides is lower than that at the portion where the strip-shaped thin coating is performed. Therefore, the light amount of the laser beam focused on the recording surface of the optical disk is reduced at both side portions thereof, and the intensity level of the first-order diffraction spot generated by both side portions of the beam can be reduced.

[Example of device]

An embodiment of the present invention will be described below with reference to the drawings of FIGS. 1 to 2 (B).

FIG. 1 is an element layout diagram of an optical device of an optical pickup installed in, for example, a video disc player, 2 (A)
The figure is a cross-sectional view of the collimator lens, and FIG. 2 (B) is a plan view of the collimator lens.

In FIG. 1, reference numeral 1 is a semiconductor laser, 2 is a polarization beam splitter, 3 is a collimating lens, 4 is a quarter-wave plate, 5 is an objective lens, 6 is a light receiving element such as a photodiode, 7 is a diffraction grating, and 9 is It is a light receiving lens.

As shown in FIG. 2 (B), for example, the collimator lens 3
An antireflection film 20 is coated on the front and back surfaces of the. This antireflection film 20 is made of magnesium fluoride (MgF
2) and the like are formed by coating by a vacuum deposition method or a sputtering method. The antireflection film 20 is thinly coated on the band-shaped portion A having a width W at the center of the lens, and is thickly coated on both side portions B sandwiching the strip-shaped portion A. The difference in the thickness of the coating layer is formed, for example, by first thinly coating the front surface of the lens 3 with the antireflection film 20 and then masking the band-shaped portion A, and further depositing the antireflection film 20 in multiple layers on the entire lens. . This makes it possible to change the coating thickness of the strip portion A and the other portion B. Further, in FIG. 2 (A), the strip-shaped portion A on one surface of the collimator lens 3 is shown.
The coating layer of the antireflection film 20 is thinly formed, but the antireflection film 20 of the central strip portion A is thinly coated on both front and back surfaces of the lens 3, and both side portions B sandwiching this are thickly coated on both front and back surfaces. It is also possible to do so.

The difference in the thickness of the coating layer due to the antireflection film 20 is
It may be formed not only on the collimator lens 3 but also on one side or both sides of the objective lens 5. Alternatively, both the collimator lens 3 and the objective lens 5 may be provided with different thicknesses of the antireflection film 20. In addition, the difference in the thickness of the antireflection film 20
It is also possible to provide it on one side or both sides of other optical elements such as the beam splitter 2, the quarter-wave plate 4, and the diffraction grating 7.

As described above, the antireflection film 20 is formed on the central strip portion A of the collimator lens 3, the objective lens 5 or other optical elements.
If the coating layer is thinned and the coating layers are thickened on both side portions B, the light transmittance in the range of B is lower than that of the strip portion A. Therefore, almost the same optical characteristics as when an apodized filter is provided can be obtained, the level of the first-order diffraction spot K1 shown in FIG. 5 (B) can be lowered, and crosstalk between adjacent tracks can be prevented. Like

[Effect of device]

As described above, according to the present invention, crosstalk between adjacent tracks due to the first-order diffraction spot can be eliminated without using an apodized filter. In addition, the antireflection film can be formed by the same means as applied to a normal optical lens,
Moreover, changing the thickness of the coating layer
It can be easily implemented by masking the strip-shaped portion. Therefore, the optical device can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is a layout view of elements showing an optical device of an optical pickup according to the present invention, FIG. 2 (A) is a cross-sectional view of a collimator lens, and FIG. 2 (B) is a band-shaped antireflection coating provided on the collimator lens. FIG. 3 is a plan view showing a film, FIG. 3 is a layout view of elements showing an optical device of a conventional optical pickup, FIG. 4 is a plan view of an apodized filter, and FIG. 5 (A) is provided with an apodized filter. Diagram showing the light intensity distribution of the laser beam incident on the objective lens when not provided, Fig. 5 (B) shows the light intensity of the beam spot formed on the recording surface of the optical disc when the apodized filter is not provided. A diagram showing the distribution,
Fig. 6 (A) is a diagram showing the light amount distribution of the laser beam incident on the objective lens when an apodized filter is provided, and Fig. 6 (B) is a case where an apodized filter is provided. FIG. 6 is a diagram showing a light quantity distribution of a beam spot formed on a recording surface of an optical disc. D ... Optical disc, 1 ... Semiconductor laser, 2 ... Polarization beam splitter, 3 ... Collimating lens, 5 ... Objective lens, 6 ... Light receiving element, 20 ... Antireflection film, A ...
Band, B ... Both sides.

Claims (1)

[Scope of utility model registration request] 1. A semiconductor laser, an objective lens for focusing a laser beam emitted from this semiconductor laser on a recording surface of an optical disc, an optical system arranged on an optical path from the semiconductor laser to the objective lens, and an optical disc of the optical disc. In an optical device of an optical pickup comprising a light receiving element for detecting reflected light reflected from a recording surface, an antireflection film coated on at least one surface of an optical element and an objective lens forming the optical system. Is an optical device of an optical pickup, characterized in that a strip-shaped portion thinner than the effective diameter at the center thereof is thin and both edge portions sandwiching the strip-shaped portion are thick.