JP2000331370A - Optical head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the use of a low-output laser and to reduce electric power consumption by improving the light utilization efficiency of an optical head of a three beam system. SOLUTION: Diffraction gratings 2 of a diffraction element 1 for branching the laser beam emitted from a laser beam source to three beams are formed as a region smaller than a luminous flux diameter in a direction corresponding to information tracks and larger than the luminous flux diameter in a direction orthogonal therewith, by which the degradation in the light utilization efficiency is suppressed. The expansion in the spot diameter of the sub-beams in the direction orthogonal with the information tracks on the optical disk relating to tracking performance is suppressed and the impairment of the tracking performance is averted by forming the diffraction grating grooves to a shape larger than the exit luminous flux diameter in the direction orthogonal with the information tracks.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head of an optical recording / reproducing apparatus for recording / reproducing information, which is used for a sound / image file, a document file, and an external memory device for a computer. .

[0002]

2. Description of the Related Art Conventionally, many so-called three-beam optical heads have been put to practical use. FIG. 4 is a diagram illustrating the configuration of this conventional optical head. In the figure, 21 is a laser light source, 22 is a diffraction element, 23 is a beam splitter, 24 is a collimating lens, 25 is an objective lens, 26 is an optical disk as an information recording medium, 27 is a plano-concave lens, and 28 is a photodetector.

In this optical head, a laser light source 21
The laser beam emitted from the
The beam is split into three beams of the next light and ± first order light, and each beam is reflected by the beam splitter 23, then changed into a parallel beam by the collimator lens 24, and condensed on the optical disk 26 by the objective lens 25. Irradiated to form three spots. The three laser beams reflected by the optical disk 26 pass through the objective lens 25, the collimator lens 24, and the beam splitter 23, and are separated by the plano-concave lens 27 into three light beams.
The light is condensed on 8 and converted into an electric signal by the photodetector 28.

FIG. 5 shows one surface 22 of the diffraction element 22.
FIG. 2 is a schematic diagram showing a relationship between a and a light beam incident on the element. Numeral 31 denotes a diffraction grating groove, and numeral 32 denotes a light beam diameter on the diffraction element 22 of a light beam transmitted through the objective lens 25 and focused on the optical disk 26. In this diffraction element 22,
A diffraction grating groove 31 is formed in a region through which a light beam emitted from the laser light source 21 is transmitted. The 0th-order light by the diffraction grating groove 31 forms a main beam, and the ± 1st-order lights form two sub-beams. .

FIG. 6 is a schematic diagram showing the appearance of spots on an optical disk when the optical head provided with the diffraction element shown in FIG. 5 is used. 41 is an optical disk, 42 is an information track, 43 is a main beam, and 44 is a sub beam. The two sub-beams are arranged shifted by 1/4 track to the left and right toward the information track with respect to the main beam. When the main beam deviates from the center of the track, one sub-beam approaches the center of the track, and the other sub-beam approaches the track. Since they are away from the center, there is a light amount difference between these reflected lights. Therefore, by detecting the difference between the sub-beam signals, which are the reflected lights, the deviation of the main beam from the track center and the direction of the deviation can be detected, and the detected signal is used as a tracking error signal.

Further, as an application of the above-mentioned conventional example, a technique for increasing the spot diameter of the sub-beam by making the range of the diffraction grating groove 31 on the diffraction element 22 smaller than the light beam diameter 32 is disclosed in Japanese Patent Publication No. 7-92927. It has been disclosed.

[0007]

However, in this conventional method, since the outgoing light beam is split into three beams by the diffraction element, the outgoing light amount of the laser light source and the converging spot light amount of the main beam on the optical disk are required. There is a problem that the light use efficiency, which is the ratio of the light utilization efficiency, decreases. for that reason,
In an optical head that requires a high-output spot light amount when recording or erasing information, it is necessary to use a high-output laser, which has been a problem along with an increase in power consumption due to a decrease in light use efficiency.

In order to solve this problem, the diffraction efficiency of the diffraction element is reduced to increase the transmitted light, and the light utilization efficiency is improved. Is required. For example, in a diffraction element having a diffraction efficiency of ± 1 order light of 10%, the accuracy of the diffraction efficiency may be 1% in order to make the variation of the sub-beam light amount 10%. In order to realize a degree of light quantity variation, the precision of the diffracted light rate needs to be 0.3%, which makes it difficult to manufacture a diffraction element.

[0009]

According to the present invention, in order to solve such a problem, in a diffraction element for splitting a laser beam emitted from a laser light source, a diffraction grating groove is formed in a direction corresponding to an information track. By making the region smaller than the light beam diameter of the light beam condensed on the optical disk through the objective lens and larger than the light beam diameter of the light beam in a direction orthogonal to the diameter, the light transmitted through the diffractive element is increased. The light use efficiency is improved as compared with the three-beam method.

In this means, provisional Japanese Patent Publication No. 7-929
When a diffractive element as disclosed in the publication No. 27 is used, since the spot diameter of the sub beam becomes large, the change of the sub beam signal when the main beam deviates from the track center becomes small, and the modulation degree of the tracking error signal becomes small. However, in the diffraction element of the present invention, by making the diffraction grating groove larger than the light beam in the direction orthogonal to the information track, the sub-beam spot diameter in this direction becomes larger. And the same tracking performance as in the case of using the conventional three-beam diffraction element can be obtained.

Further, the diffractive element of the present invention has a small influence of the variation of the diffracted light rate on the amount of the sub-beam, so that it is not necessary to make the diffraction efficiency high precision and can be easily manufactured.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The main difference between the optical head according to the present embodiment and the conventional example is the diffraction element 22, so the configuration and operation of this diffraction element will be described.

FIG. 1 is a schematic diagram showing the relationship between a diffraction element used in an optical head according to an embodiment of the present invention and a light beam incident on this element. In this figure, 1 is a diffraction element, 2 is a diffraction grating forming portion, 3 is a diffraction grating non-forming portion,
Reference numerals 4a and 4b denote boundaries between the diffraction grating forming section 2 and the non-diffraction grating forming section 3, 5 denotes a light beam diameter on the diffraction element, and 6 denotes a center line of the light beam 5 in a direction corresponding to the information track.

The diffraction element according to the present embodiment has a diffraction grating forming portion 2 and a diffraction grating non-forming portion 3 on a transmissive substrate 1 at boundaries 4a and 4b. Boundaries 4a, 4
The width in the information track direction of the diffraction grating forming portion 2 sandwiched between b is determined to be a predetermined width smaller than the diameter of the light beam 5.

When the light beam 5 is incident on the diffraction element, the component of the main beam transmitted through the diffraction element is a mixed light of the zero-order light in the diffraction grating forming section 2 and the transmitted light in the diffraction grating non-forming section 3. Becomes Since the transmittance of the non-diffraction grating forming portion 3 is approximately 100%, when the diffraction element of the present embodiment is used, the light passes through the diffraction element more than in the conventional case where the diffraction grating grooves are provided in the entire area of the light beam 5. The component of the main beam increases, and the light use efficiency improves.

For example, the transmittance of the 0th-order light of the diffraction grating forming section 2 is set to 70%, and the diffraction efficiency of the ± 1st-order light is set to 10%, respectively. Assuming that the area is determined to be １ ／, the component of the main beam passing through this diffraction element is 85% and the component of the sub beam is 5% with respect to the incident light amount, respectively. Under the same conditions, in the case of the conventional diffraction element, the main beam component is 70% and the sub beam component is 10%, so that the light use efficiency is improved by about 20% in this example.

In the optical head according to the present embodiment, an important factor for determining the light use efficiency is the width d of the diffraction grating forming section 2.
However, if the width d is expressed as d <D / 10 − (Equation 1) with respect to the light beam diameter D, for example, the zero-order light transmittance of the diffraction grating forming unit 2 becomes 7
If the diffraction efficiencies of the 0% and ± 1st-order lights are 10% each, the amount of light diffracted by the diffraction element to become a sub-beam component is about 2% or less of the incident light amount, and the tracking error due to the shortage of the sub-beam light amount. A problem is a decrease in the amplitude of the signal. If the width d of the diffraction grating forming section 2 is d> 3D / 4- (Equation 2) under the same conditions, the main beam component transmitted through the diffraction element is about 74% or less of the incident light amount. The effect of the invention aimed at improving the use efficiency cannot be obtained.

FIG. 2 is a schematic diagram showing the appearance of spots on an optical disk when the diffraction element shown in FIG. 1 is used. Reference numeral 11 denotes an optical disk, 12 denotes an information track, 13 denotes a main beam, and 14 denotes a sub beam. Generally, when a light beam is condensed by a condensing means such as an objective lens, the spot diameter is proportional to λ / NA, where NA is the numerical aperture of the condensed light beam and λ is the wavelength of the laser beam. Therefore, when the light beam diffracted by the diffraction grating forming unit in FIG. 2 is collected, the spot shape of the sub-beam 14 on the optical disk increases in the information track direction, but in the direction orthogonal thereto, the conventional diffraction element As in the case of the above, the diameter is almost equal to the diameter of the main beam.

If the spot diameter of the sub-beam increases in the direction perpendicular to the track, the change in the amount of reflected light from each sub-beam due to the track shift becomes small, and the amplitude of the tracking error signal is reduced, thereby deteriorating the tracking performance. In the diffractive element of the form (1), by suppressing the spot diameter from increasing in the direction orthogonal to the track of the sub beam, the same tracking performance as that obtained by using the conventional diffractive element can be obtained.

The diffraction grating forming section 2 does not need to be located at the center of the light beam 5, and the effect of improving the light use efficiency can be obtained similarly as shown in FIG. In the figure, the same components as those in FIG. 1 are denoted by the same reference numerals. Also in this case, if the NA of the direction orthogonal to the track of the diffracted light by the diffraction grating forming unit 2 is set to be equal to the NA of the light beam 5 as in the conventional case, the case where the conventional diffraction element is used is obtained. Equivalent tracking performance is obtained.

[0021]

As described above, according to the present embodiment, in the diffraction element that splits the laser beam emitted from the laser light source, the diffraction grating groove passes through the objective lens in the direction corresponding to the information track. A diffraction element having a diffraction efficiency that is smaller than the light beam diameter of the light beam condensed on the optical disc, and is larger than the light beam diameter of the light beam in a direction perpendicular to the light beam diameter. , The light use efficiency of the laser light source can be improved as compared with the case where the conventional three-beam method is used. As a result, a low-output laser can be used, and an optical head of an optical recording / reproducing apparatus with low power consumption can be provided. In addition, effects such as an increase in the degree of freedom in optical design due to an improvement in light use efficiency, and an improvement in C / N can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a diffraction element according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing spots on an optical disc according to the embodiment of the present invention.

FIG. 3 is a schematic view showing a diffraction element according to another embodiment of the present invention.

FIG. 4 is a diagram illustrating the configuration of a conventional optical head.

FIG. 5 is a schematic view showing a conventional diffraction element.

FIG. 6 is a schematic diagram showing spots on an optical disk in a conventional optical head.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 diffraction element 2 diffraction grating forming part 3 diffraction grating non-forming part 4 boundary between diffraction grating forming part and non-forming part 5 luminous flux diameter 6 center line in information track direction 11 optical disk 12 information track 13 main beam 14 sub-beam 21 laser light source 22 diffraction Element 23 Beam splitter 24 Collimating lens 25 Objective lens 26 Optical disk 27 Plano-concave lens 28 Photodetector 31 Diffraction grating groove 32 Beam diameter 41 Optical disk 42 Information track 43 Main beam 44 Sub beam

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Takuo Hayashi 1006 Kadoma Kadoma, Kazuma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. Terms (reference) 5D118 AA08 BF02 BF03 CA13 CC03 CD03 CF16 CG04 CG24 5D119 AA37 AA43 DA01 DA05 EA02 EB01 EC21 FA02 JA22

Claims (4)

[Claims] A light source; a diffractive element for branching the light from the light source into a first light beam that transmits without diffracting the light and a second light beam that diffracts and deflects the light; A light condensing means for condensing the first light beam and the second light beam on an information recording medium, wherein the diffraction element has a diffraction grating forming portion and a diffraction grating non-forming portion within a region through which light from the light source is transmitted. The diffraction grating forming section has an area smaller than the light beam diameter of the first light beam in a direction corresponding to the information track, and larger than the light beam diameter of the first light beam in a direction perpendicular to the information track. An optical head, characterized in that: 2. The method according to claim 1, wherein the second light flux is converged on the information recording medium at intervals of an integral multiple of about 1/2 of a track pitch.
2. The optical head according to claim 1, wherein the optical head is reflected by the information recording medium and used for detecting a tracking error signal. 3. An optical head according to claim 2, wherein said diffraction grating forming portion is provided substantially at the center of a region through which a light beam passes on said diffraction element. 4. The diffraction grating forming portion is a band-like region that is long in a direction perpendicular to the information track, and the width of the diffraction grating forming portion in the direction corresponding to the information track is d, and the beam diameter on the diffraction element is The optical head according to claim 2, wherein D / 10 <d <3D / 4.