JPS6141112A - Optical device for optical pickup - Google Patents

Abstract

PURPOSE:To minimize the horizontal aberration, etc., and to attain the highly accurate reading by providing a beam splitter between a colimate lens and lazer source and constituting the collimate lens of a flat convex lens and a flat concave lens. CONSTITUTION:A beam splitter 12 is provided between a collimate lens 11 and a laser diode, and said lens 11 is constituted of a flat convex lens 11a with its surface pointing to the laser source side and a flat concave lens 11b which is positioned closer to the laser source than the lens 11a and has its surface pointing to the laser source side. A reflected beam from a recording medium is converged by the collimate lens 11, reflected by the beam splitter 12 as it is, and detected by a light receiving element. Thus the collimate lens 11 can correct the spherical aberration, and minimize the vertical aberration, etc., thereby executing the highly accurate reading.

Description

[Detailed Description of the Invention] [Technical Field] The present invention relates to an optical pickup installed in a compact disc player, etc., and particularly relates to an optical device for an optical pickup in which a beam splitter is interposed between a collimating lens and a laser diode. .

[Technical Background and Problems of the Prior Art] FIG. 4 schematically shows a conventional optical pickup.

In the conventional optical pickup 10, a diverging beam emitted from a laser diode 2 is made into a parallel beam by a collimator lens 3a, and reaches an objective lens 5 via a beam splitter 3b, a quarter-wave plate 3c, and a total reflection prism 4. The parallel beam is converged by the objective lens 5 and irradiated onto the spherical surface 1a of the optical disk 1 as a beam spot. The reflected beam from the recording surface 1a returns along the above path,
It is reflected in the 80'' direction by the beam splitter 3b. This reflected beam is further converged by the condensing lens 3d, passes through the cylindrical lens 6, and is detected by the light receiving element 7. The intensity modulation of this detected light causes the recording surface 1a to be Pit is detected and digital information can be read.

In this manner, in the conventional optical pickup, the collimating lens 3a is placed next to the laser diode 2, and the beam is converted into parallel light before being sent to the beam splitter 3b. The reason for this arrangement is as follows. Conventionally, the output of the laser diode 2 was small, so the light receiving element 7
In order to obtain a sufficient amount of received light, it is necessary to reflect most of the reflected beam from the optical disc l to the light receiving element 7. Therefore, a polarizing prism has been used as the beam splitter 3b. This polarizing prism can reflect most of the amount of incident light if it is parallel light, but in the case of convergent light, it is difficult to reflect the amount of light without attenuating it. Therefore, parallel light had to be sent to the beam splitter 3b.

In this way, in the conventional configuration shown in FIG. 4, the reflected beam from the optical disk 1 is reflected at 80 degrees as parallel light by the beam splitter 3b. Therefore, in order to converge the beam of the light receiving element 7, it is necessary to provide a condenser lens 3d. Therefore, the number of components of the optical member increases, which hinders miniaturization of the optical pickup.

However, recently, there have been many laser diodes 2 with high output, so as the beam splitter 3b,
Even with a non-polarizing prism, it is now possible to obtain an amount of reflected light above a predetermined level. Also.

Polarizing prisms that can reflect divergent or convergent light have also been developed. As a result, the collimating lens 3a
is placed closer to the prism 4 than the beam splitter 3b,
A configuration has been developed in which the reflected light from the optical disk l is converged by a collimating lens 3a, the converged light is reflected by a beam splitter 3b, and the converged light is converged directly onto the light receiving element 7. However, the conventional collimating lens 3a is When used as is and a beam splitter 3b is interposed between this and the laser diode 2, the optical characteristics change, as compared in FIGS. 5 and 6.
The spherical aberration also becomes larger in the usage state shown in FIG. 6 than in the usage state shown in FIG. 5. Laser diode 2 as shown in Figure 5
The collimating lens 3a based on the design without the beam splitter 3b interposed between the two has a transverse aberration curve shown in FIG. 7, but when used with the beam splitter 3b interposed as shown in FIG. As shown in FIG. 8, the lateral aberration becomes large. Note that FIGS. 7 and 8 are diagrams in which the height of the incident light is plotted on the horizontal axis and the transverse aberration is plotted on the vertical axis.

[Object of the present invention]

The present invention has been made by focusing on the above-mentioned conventional problems, and in a beam splitter interposed between a collimating lens and a laser diode, the spherical aberration due to the interposed beam splitter can be reduced by using the collimating lens. It is an object of the present invention to provide an optical device for an optical pickup that can perform highly accurate reading by minimizing lateral aberrations and the like.

[Configuration of the present invention]

In the optical device for an optical pickup according to the present invention, as shown in FIG.
1 and the laser diode 2, and the collimating lens 11 is located between a plano-convex lens lla whose plane face is directed toward the laser diode z side, and a plano-convex lens lla whose plane is oriented toward the laser diode z side. a plano-concave lens llb whose plane is directed, and a plano-convex lens ll which also forms the collimating lens 11.
The distance to the focal point of a is f1, and the radius of curvature of plano-concave lens flb is r
3. The distance between the plano-convex lens lla and the plano-concave lens llb is d
2. Letting the entire focal length be f, (a) 0 , 6< (L1/f) <0 , 85(
b) It satisfies the following conditions: -0,3<(d2/r3)<-0,05. Note that, as shown in FIG. 1, the beam splitter 12 is in close contact with the flat surface of the plano-concave lens llb.

The above condition (a) is the numerical aperture N of the collimating lens 11.
0 condition (b) is a condition for correcting spherical aberration. . When this exceeds the lower limit, it is over-corrected, and when it exceeds the upper limit, it is under-corrected.

As shown in FIG. 3, this optical device is installed inside an optical pickup. That is, the beam splitter 12 is placed next to the laser diode 2, and the collimating lens 11 is placed next to the beam splitter 12. A total reflection prism 4 is arranged at the tip of the collimating lens 11, and an objective lens 5 is placed above it.
is provided, and this objective lens 5 faces the optical disc l. Further, a light receiving element 7 is provided on the side of the beam splitter 12 via a cylindrical lens 6.

In this optical pickup, a beam emitted from a laser diode 2 passes through a beam splitter 12 and is then converted into a parallel beam by a collimating lens 11. The light is then sent from the total reflection prism 4 to the objective lens 5. In addition, the reflected beam of the optical disc 1 is collimated by the collimating lens 1.
1, the light is reflected by the beam splitter 12 in this converged state, passes through the cylindrical lens 6, and is focused on the light receiving element 7.

[Example of the present invention]

Examples of the optical device for an optical pickup described above will be shown below (refer to FIG. 1 for each reference numeral).

(Example 1) f = 1 ft /f-0, [i71 d2/r3
=-0,097rl=0.4013dt-0
,0785nl -1,80955r2: Plane dz
-0,0798n2-1r3--0.818d3
-0.0545 n3 =1.51143r4: Plane d4 =0.273 n4-1.78Ei7
9 (Example 2) f = 1 f t / f - 0,835dz /
T3=-0,085r1=0.387dl
=0.118 nl -1,801+55r2: Plane dz-0,0540n2-1r3=-〇,837
d3-0.0530 na-1,8085
5r4: Plane d4-0.285 n4=1.7
8879 (Example 3) f-L ft /f-0834dz/r3=-0,2
7rl = 0.839 dl-0,1118nl
=1.78879r2: Plane dz -0,198n
2 =1r3 =-(1,845d3-0.07113
n3 =1.51143r4: Plane d4=0
．． 330 rL+ =1.78Ei79 However, d
l, dz + d3 + d4 are the wall thickness and spatial distance of each lens and beam splitter, r+ +
r'2, r3, r4 are the radius of curvature of each lens, n
l + n2 + n3 1 n4 is the refractive index of each lens and beam splitter for λ=788.5 nm, f is the focal length of the entire system, and f is the focal length of the plano-convex lens 11a.

FIG. 2 shows v-! of (Example 3) among the above embodiments.
8! : In the table, the sheep are eaten and eaten 1.
.

In Figure 2, the height of the incident light is plotted on the horizontal axis.
, the lateral aberration (IL garden) is plotted on the vertical axis, and the lateral aberration curve in the case of f-15, 14 is shown. As shown in the diagram in FIG. 2, the lateral aberration is kept very small. Therefore, the accuracy of reading information is increased, and an excellent optical pickup can be constructed.

[Effects of the present invention]

As described above, according to the present invention, since the beam splitter is interposed between the collimating lens and the laser diode, the reflected beam from the recording medium is converged by the collimating lens, and is directly reflected by the beam splitter.
It is detected by the light receiving element 7. Therefore, the condensing lens 3d on the light-receiving element side as shown in FIG. 4 becomes unnecessary, and the number of components and size can be reduced.

Further, by combining a plano-convex lens and a plano-concave lens as a collimating lens, and by forming the lens under the conditions described in claim 2, aberrations can be optimally corrected.

Furthermore, as shown in the embodiment of FIG. 1, if the beam splitter is brought into close contact with the flat surface of the plano-concave lens of the collimating lens, the beam splitter can be easily positioned and assembled. Furthermore, it becomes possible to downsize the optical device.

[Brief explanation of the drawing]

Figures 1 to 3 show the present invention. Figure 1 is a configuration diagram of an optical member, Figure 2 is a transverse aberration diagram showing aberrations in (Example 3), and Figure 3 is an optical pickup. FIG. 4 is a perspective view showing the configuration of a conventional optical pickup.
6 are explanatory diagrams of the structure of an optical member showing the conventional problems, and FIGS. 7 and 8 are diagrams showing lateral aberration curves corresponding to FIGS. 5 and 6. 1... Optical disk, 2... Laser diode, z&
. . . Cover glass of laser diode, 7 . . . Light receiving element, 11 . . . Collimating lens, lla . 111 Figure 1

Claims (2)

[Claims] (1) In an optical pickup that is equipped with a collimating lens that converts the beam emitted from the laser diode into parallel light and a beam splitter that reflects the beam reflected from the recording medium in the direction of the light receiving member, the beam splitter is the collimating lens. The collimating lens is provided between the laser diode and the collimator lens, and includes a plano-convex lens with a plane facing toward the laser diode, and a plano-concave lens located closer to the laser diode with its plane facing toward the laser diode. An optical device for an optical pickup, comprising: (2) Assuming that the focal length of the plano-convex lens forming the collimating lens is f_1, the radius of curvature of the plano-concave lens is r_3, the distance between the plano-convex lens and the plano-concave lens is d_2, and the overall focal length is f, (a) 0. 6<(f_1/f)<0.85 (b)-0.3<(d_2/r_3)<-0.05. Optical device for optical pickup.