BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects and features of the present invention will become more apparent from the following description of embodiments taken in conjunction with the accompanying drawings.
FIG. 1 shows a configuration of an optical pickup device according to an embodiment of the present invention;
FIG. 2 schematically shows a transmittance property of a filter unit according to an embodiment of the present invention;
FIGS. 3A and 3B show configuration examples of the filter unit according to the embodiment;
FIGS. 4A, 4B, and 4C show configuration examples of the filter unit according to the embodiment;
FIGS. 5A and 5B show configuration examples of the filter unit according to the embodiment;
FIGS. 6A and 6B show configuration examples of the filter unit according to the embodiment;
FIGS. 7A and 7B show a pattern of the filter unit according to the embodiment and an irradiation state of stray light;
FIG. 8 shows light intensity distribution of the signal light and stray light according to the embodiment;
FIGS. 9A and 9B show a pattern of the filter unit according to the embodiment and an irradiation state of the stray light;
FIGS. 10A and 10B show a pattern of the filter unit according to the embodiment and an irradiation state of the stray light;
FIG. 11 shows a modification of the optical pickup device according to the embodiment;
FIGS. 12A and 12B show a pattern of the filter unit according to the embodiment and an irradiation state of the stray light;
FIGS. 13A and 13B show a pattern of the filter unit according to the embodiment and an irradiation state of the stray light;
FIGS. 14A and 14B illustrate a method of forming the filter unit according to the embodiment;
FIG. 15 shows a modification of the optical pickup device according to the embodiment;
FIGS. 16A and 16B show a pattern of the filter unit according to the embodiment and a reflectance property;
FIG. 17 shows a configuration of an optical pickup device according to the related art;
FIGS. 18A and 18B show an irradiation state of a laser beam on a disk and light intensity distribution;
FIGS. 19A and 19B show states of a main beam and sub-beams on a photodetector;
FIGS. 20A and 20B illustrate an optical path of the stray light according to the related art;
FIG. 21 shows an irradiation state of the stray light according to the related art;
FIG. 22 shows light intensity distribution of the signal light and the stray light according to the related art;
FIGS. 23A and 23B illustrate a technique of suppressing the stray light according to the related art; and
FIGS. 24A and 24B illustrate a technique of suppressing the stray light according to the related art.
However, the drawings are used for illustration by way of example, and the present invention is not limited thereto.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a configuration of an optical pickup device of an embodiment according to the present invention. The same component as in the configuration of FIG. 17 is designated by the same numeral, and the description thereof will be omitted.
In the present embodiment, a filter 20 is arranged in an optical path between a collimator lens 14 and an objective lens 15. In the filter 20, a filter unit 20a (see FIG. 7A) is partially formed, and the filter unit 20a gives a different transmittance depending on the incident angle. The influence of the stray light on the sub-beam is suppressed by the filter 20. Therefore, the accuracy of various signals such as the tracking error signal is ensured at a practical level.
FIG. 2 schematically shows a transmittance property in the filter unit 20a. The filter unit 20a has transmittance distribution which has a width of ±α while centered at an incident angle A. The incident angle A becomes 0° when the light is incident in parallel with an optical axis of the filter unit 20a. As another embodiment, obviously the incident angle A may be 0°.
Then, a configuration of the filter unit 20a will be described.
FIGS. 3A and 3B show a case in which the filter unit 20a is configured by forming a multi-layer film 202 on a substrate 201. As shown in FIG. 3B, the multi-layer film 202 is formed by laminating periodic multi-layer films including a plurality of paired layers A and B with a center layer C vertically sandwiched between the periodic multi-layer films. Each of the paired layers A and B is made of two kinds of materials.
For example, the substrate 201 is made of SiO2, the center layer is made of TiO2, and materials 1 and 2 constituting the paired layers 1 and 2 are formed by TiO2 and SiO2.
In the present configuration example, as the number of paired layers is increased, the width α of the transmittance distribution shown in FIG. 2 is decreased. The incident angle A can be changed by appropriately adjusting parameters. For example, a thickness P of the center layer is set to 0.441 μm, a thickness R of the material 1 of the paired layer is 0.144 μm, a thickness Q of the material 2 is set to 0.07 μm, and the number of pairs is set to eight in both the upper and lower portions. In this case, a transmittance property of A=0° and α=0.39° is obtained for a wavelength of 405 nm.
FIGS. 4A, 4B, and 4C show a case in which the filter unit 20a is configured by forming a waveguide layer 212 and a fine grating 213 on a substrate 211. As shown in FIGS. 4A and 4B, the fine grating 213 is formed in such a manner that projections having a constant width D and a height H are arranged at a constant pitch L on the waveguide layer 212 having a thickness G. Similarly to the case of FIGS. 3A and 3B, in the present configuration example, the incident angle A and the width α can be changed by appropriately adjusting the parameters.
For example, the substrate 211 is made of glass, the waveguide layer 212 is made of CORNING #7059 glass (manufactured by CORNING INCORPORATED), the projection of the grating layer 213 is made of TiO2, and a recess of the grating layer 213 is in air. Assuming the parameters to be G=0.160 μm, L=0.200 μm, D=0.120 μm, and H=0.231 μm, the transmittance property of A=0+ and α=0.23° is obtained for the wavelength of 405 nm. In this case, the transmittance distribution is in a one-dimensional direction (thickness direction of the projection). In order to develop the transmittance distribution in a two-dimensional direction, the two elements of FIG. 4A may be prepared and superposed while grating directions differ from each other by 90°, or another waveguide layer 212 and another grating layer 213 whose grating direction is changed by 90° may be formed on the back side of the substrate 211. As shown in FIG. 4C, the grating layer 213 of FIGS. 4A and 4B may be divided at a constant pitch in the longitudinal direction to form a grating layer having a two-dimensional periodic structure.
FIGS. 5A and 5B show a case in which the filter unit 20a is configured by a fine blind structure. As shown in FIG. 5B, in the fine blind structure, absorption layers 221 having a thickness T and a width U are arranged at pitch S, and a transmission layer 222 is arranged between the absorption layers 221. Similar to the cases of FIGS. 3A and 3B and FIGS. 4A, 4B, and 4C, in the present configuration example, the incident angle A and the width α can be changed by appropriately adjusting the parameters.
For example, the transmission layer 222 is made of SiO2 and the absorption layer 221 is made of aluminum. Assuming that S=19.8 μm, T=0.2 μm, and U=940 μm, the transmittance property of A=0° and α=0.31° is obtained for the wavelength of 405 nm. Such a structure can be obtained as follows. For example, transparent sheets on which aluminum is evaporated are laminated and cut in a desired length (U). Although the absorption layer 221 is made of aluminum as a design example, the present invention is not limited thereto. Any material which absorbs the wavelength of the light source used in the optical pickup device may be used as the absorption layer 221. For example, an adhesive agent may be used.
In the configuration example of FIGS. 5A and 5B, the transmittance distribution is given in a one-dimensional direction (pitch direction of the absorption layer 221). In order to give the transmittance distribution in a two-dimensional direction, the two elements of FIGS. 5A and 5B are prepared and superposed while absorption walls differ from each other by 90°, or a two-dimensional fine blind structure in which through-holes 232 having a constant diameter are arranged at a constant pitch in an absorption material 231 is used as shown in the top view of FIG. 6A. As shown in FIG. 6B, a relatively large two-dimensional blind structure is prepared, the structure is heated and stretched, and a part which is reduced in the similar shape is cut to obtain the deep and fine two-dimensional blind structure. A first half of this producing method is well known as a method of producing an optical fiber.
Thus, the configuration examples of the filter unit 20a have been described. However, the configuration of the filter unit 20a is not limited to the above configuration examples, and the filter unit produced by other methods and structures may obviously be used.
CONFIGURATION EXAMPLE 1
FIG. 7A shows a configuration of the filter 20.
In this configuration example, the filter units 20a are formed in two rectangular regions which are separated from each other in a disk tangential direction with an incident beam optical axis (center of incident beam) at the center. The two rectangular regions have two-fold rotational symmetry relative to the optical axis of the incident laser beam. For example, the transmittance property of the filter unit 20a is set to A=0° and α=0.20°. The numeral 20b designates a transparent portion.
FIG. 7B shows a state in which the laser beam is incident on a light receiving surface of the photodetector 17 when the filter unit of FIG. 7A is used. In FIG. 7B, a boundary of a stray light extinction portion does not indicate a boundary of existence of the light, but indicates a position on which the light whose transmittance is reduced to 0.5 in FIG. 2 is incident. The position of stray light extinction portion is controlled by adjusting the positions where the filter units 20a of FIG. 7A are arranged.
In the stray light beams incident on the filter units 20a, the larger incident angle to the filter unit 20a the stray light beam has, the more the stray light beam is reduced. Accordingly, although the stray light is also incident on the stray light extinction portion of FIG. 7B, because the stray light is reduced to a negligible level at the sub-beam incident position as shown in FIG. 8, the stray light has little influence on the generation of the error signal.
In the signal light (light reflected from the recording layer which is of the recording and reproducing target) of the main beam, because the whole light is incident on the filter unit 20a at the incident angle of 0°, the extinction portion is not generated in the spot of the signal light as shown in FIG. 7B. Therefore, a good RF signal can be obtained.
Because the signal light of the sub-beam incident on the filter unit 20a in the state in which the signal light is slightly shifted from the parallel light, an extinction portion “M” where the signal light is reduced by the filter unit 20a is generated in the spot of the sub-beam. However, as shown in FIG. 7A, the two filter units 20a are arranged in the disk tangential direction, and the extinction portions “M” generated by the filter units 20a are rotated by 90° by the astigmatism action of the cylindrical lens 16. Therefore, as shown in FIG. 7B, the extinction portions “M” are arranged in the direction in which the extinction portions “M” have a little influence on the push-pull signal (the direction in which a difference between signals is not taken). Accordingly, the extinction portions “M” have an extremely little influence on the tracking error signal, and the good tracking error signal can be obtained even when the extinction portions “M” are generated.
Because the two filter units 20a are arranged in the two-fold rotational symmetry relative to the optical axis of the incident laser beam, the two extinction portions “M” generated in the sub-beam spot region are in the two-fold rotational symmetry relative to the laser beam optical axis on the sensor pattern. The two extinction portions “M” have an equal influence on the two signals which are subtracted when the push-pull signal is generated based on the sub-beam spot, so that the influence of the extinction portion on the push-pull signal can be prevented.
Thus, in the present configuration example, the adverse influence of the stray light on the signal light can be prevented without impairing various signals at practical level.
CONFIGURATION EXAMPLE 2
FIG. 9A shows another configuration of the filter 20.
In this configuration example, the filter unit 20a is formed in the region except a circular portion 20b near the incident beam center. For example, the transmittance property of the filter unit 20a is set to A=0° and α=0.20°.
FIG. 9B shows a state in which the laser beam is incident on the light receiving surface of the photodetector 17 when the filter unit of FIG. 9A is used. As shown in FIG. 9B, in the present configuration example, the light is reduced around the whole circumference of the main beam. Accordingly, as in the configuration example 1, the influence of the stray light on the sub-beam signal light can be prevented.
In the present configuration example, because the light is reduced in the peripheral region of the sub-beam signal light by the filter unit 20a, a spot diameter of the sub-beam signal light is slightly smaller than that of the configuration example 1. According to the study performed by the inventors, the spot diameter of the sub-beam signal light corresponds substantially to the diameter of the circular portion 20b.
However, the spot diameter of the sub-beam signal light can be adjusted to a practical level by optimizing the diameter of the circular portion 20b of the filter 20 according to an optical system of the individual optical pickup device.
In the present configuration example, too, because no extinction portion is generated in the spot of the main beam signal light, a good RF signal can be obtained. Thus, in the present configuration, the adverse influence of the stray light on the signal light can also be prevented without impairing various signals at practical level.
In the above two configuration examples, the inclination angle A is set to 0° in the transmittance property of the filter unit 20a. This is because the filter 20 is arranged at the right angle to the parallel light optical axis. However, when the filter 20 is arranged at the right angle to the parallel light optical axis, there may be a problem that the laser beam reflected from the surface of the filter 20 in the optical path toward the objective lens 15 is incident on the photodetector 17. In order to avoid the problem, preferably the filter 20 is arranged in a manner slightly oblique to the parallel light optical axis. In this case, it is necessary that the inclination angle A in the transmittance property of the filter unit 20a be equalized to the inclination angle of the filter 20.
CONFIGURATION EXAMPLE 3
FIG. 10A shows still another configuration of the filter 20.
In this configuration example, the filter unit 20a of the configuration example 1 shown in FIG. 7A is extended in the disk radial direction. In the case of the configuration example 1, when the filter 20 is fixed in the optical path, the stray light extinction portion shown in FIG. 7B is vertically moved according to the movement in the radial direction of the objective lens 15 due to the tracking operation, and a stray light portion having strong light intensity is possibly incident on a spot range of the sub-beam signal light. On the contrary, in the present configuration example, because the filter unit 20a is extended in the disk radial direction, the stray light portion having strong light intensity is never incident on the spot range of the sub-beam signal light, even when the stray light extinction portion is vertically moved according to the displacement of the objective lens 15. Therefore, a good error signal can be generated.
In the present configuration example, the problem caused by the displacement of the objective lens 15 is solved by extending the filter unit 20a in the disk radial direction. Alternatively, as shown in FIG. 11, the filter 20 is fixed to a holder 30 which holds the objective lens 15, and the filter 20 is synchronized with the objective lens 15 to solve the problem.
CONFIGURATION EXAMPLE 4
FIG. 12A shows still another configuration of the filter 20.
In this configuration example, the filter unit 20a of the configuration example 1 shown in FIG. 7A is formed into a circular shape. In this case, the stray light extinction portion becomes a circular shape as shown in FIG. 12B. The present configuration example is easily applied to the case in which the filter unit 20a is produced by the method shown in FIG. 6B. The two filter units 20a are arranged in the two-fold rotational symmetry relative to the optical axis of the incident laser beam. The effect of this configuration example is equal to that of the configuration example 1.
In the present configuration example, as in the configuration example 3, the filter unit 20a is extended in the disk radial direction, or the filter 20 is synchronized with the objective lens 15, which allows the problem caused by the displacement of the objective lens 15 to be prevented.
CONFIGURATION EXAMPLE 5
FIG. 13A shows still another configuration of the filter 20.
In this configuration example, the filter unit 20a is formed into a stripe shape in the tangential direction. When the filter 20 of the present configuration example is used, the laser beam incidence onto the light receiving surface of the photodetector 17 becomes the state shown in FIG. 13B. In this case, because the stray light incident on the filter 20 is in the state close to the parallel light in the vicinity of the filter central portion, the light receiving surface of the photodetector 17 is irradiated with the light while the light is not reduced by the filter unit 20a. Therefore, as shown in FIG. 13B, the vicinity of the quadrant sensor 171 is irradiated with the stray light. Similarly to the case shown in FIGS. 7A and 7B, the extinction portion “M” is generated in the spot of the sub-beam signal light, and the extinction portion “M” of the present configuration example is one in which the two extinction portions “M” of FIG. 7B are connected to each other.
The effect of the present configuration example is equal to that of the configuration example 1. In the present configuration example, because the filter unit 20a is formed into a stripe shape, the filter 20 is easily formed. For example, in the present configuration example, a plurality of filter units 20a are produced on a large-area substrate, and the substrate is cut into a desired size to obtain the filter 20.
In the present configuration example, similarly to the case shown in FIG. 11, the problem caused by the displacement of the objective lens 15 can be prevented by synchronizing the filter 20 with the objective lens 15.
(Filter Unit Forming Method)
A method of forming a pattern of the filter unit 20a in the above configuration examples will be described.
FIG. 14A shows a forming method using a widely known photolithography technique, and the method of FIG. 14A is applied to form the filter unit 20a having the structure shown in FIGS. 3A, 3B, 4A, 4B, and 4C. That is, according to this method, the filter structure is formed all over the substrate surface, a mask material is formed on the filter structure in a desired pattern, and the filter structure exposed to the outside of the mask is removed by, e.g., ion beam etching. Then, the mask material is removed to obtain the filter unit 21a having the desired pattern.
FIG. 14B shows a method of fixing an already-patterned filter member onto the substrate with e.g., an adhesive agent, and the method of FIG. 14B is applied to form the filter unit 20a having the structure shown in FIGS. 5A, 5B, 6A, and 6B.
In the above embodiment, the filter unit is arranged on the substrate. Alternatively, the filter unit may be formed on the surface of another optical component (for example, quarter-wave plate) located in the optical path of the optical pickup device.
(Modification of Optical System)
In the above description, the transmission type filter 20 is arranged in the optical path. Alternatively, a reflection type filter can be arranged in the optical path to exert the same stray light removing function. FIG. 15 shows a configuration where the reflection type filter unit is used. The same component as in the configuration of FIG. 17 is designated by the same numeral, and the description thereof will be omitted.
In the configuration example of FIG. 15, a rise mirror 21 is arranged between the collimator lens 14 and the objective lens 15, and the filter unit 21a is formed in a predetermined pattern on the mirror surface.
FIG. 16A shows a pattern of the filter unit 21a on the reflecting surface 21b. This means that the effect equivalent to that of the configuration example 1 shown in FIGS. 7A and 7B is obtained. FIG. 16B schematically shows dependence of the reflectance on the incident angle in the filter unit 21a. In the case where an inclination angle B (see FIG. 15) of the mirror surface is set to 45°, the incident angle A is set to 45°. The filter unit 21a of the present configuration example can be formed with a so-called wire grid structure (grating structure made of a conductive material) as well as a multi-layer film structure and a structure in which the waveguide and the fine grating structure are combined.
As described above, according to the present embodiment, the influence of the stray light on the sub-beam signal light can effectively be prevented. Therefore, a good error signal can be generated.
In the present embodiment, as shown in FIG. 2, the transmittance distribution of the filter units 20a and 21a is not steeply formed like a rectangular shape, but is changed relatively gently. Even when the incident angle of the sub-beam (signal light) is slightly shifted from the desired incident angle, the sub-beam (signal light) is not rapidly reduced by the filter units 20a and 21a, so that the reduction of the sub-beam (signal light) on the sensor pattern can effectively be prevented.
In the present embodiment, there is a trade-off relationship between the prevention of the stray light from being incident on the sensor pattern for receiving the sub-beam and the securement of light quantity of the sub-beam (signal light). Accordingly, as shown in FIG. 2, the relatively gentle transmittance distribution of the filter units 20a and 21a can perform the adjustment such that the reduction of the sub-beam (signal light) is decreased as much as possible while incidence of the stray light on the sensor pattern for receiving the sub-beam is permitted to a practical level. Moreover, the relatively gentle transmittance distribution can relatively widen an attachment attitude allowable error of the filter 20 in producing the optical pickup device. Therefore, the position adjustment can be facilitated in attaching the filter 20.
The present invention is not limited to the above described embodiments. It should be understood that various modifications of the present invention can appropriately be made without departing from the scope of the technical idea shown in claims.
Patent Application
20080019254
