JPH05142499A - Plastic beam splitter - Google Patents

Abstract

PURPOSE:To enable the mass production of the plastic beam splitter at a low cost while maintaining high plane accuracy by taking the advantage of the free shape characteristic possessed by plastic and further to enable the free control of the astigmatic difference in an astigmatism system without changing an inter-face, spacing (on the optical axis) and to form the spot shape at the time of focusing to a circular shape. CONSTITUTION:A plastic base material 9 consisting of PMMA and having an H-section shape in section is formed by an injection molding method. A lambda/4 film consisting of silicon (Si) is formed as a 1st layer 11 on the plastic base material 9 on the 1st surface 10 of a plane which is a reflection surface. Further, a lambda/4 film consisting of silicon dioxide (SiO2) is formed as a 2nd layer 12 and a non-polarizing film having a two-layered film structure is formed. The 2nd surface 13 is formed as a recessed cylindrical surface.

Description

Detailed Description of the Invention

[0001]

This invention relates to a semiconductor laser (L
The present invention relates to a plastic beam splitter in an optical pickup optical system using D) as a light source.

[0002]

2. Description of the Related Art In a bulk type optical system of an optical pickup, a basic configuration diagram of a finite system optical pickup which is currently the mainstream is shown in FIG. 7 (a tracking error signal uses a three-spot type using a diffraction grating, and a focus error signal is According to the point aberration method). In this optical system, the beam splitter 1
Guides part of the semiconductor laser light of the light source 3 that has passed through the diffraction grating 2 to the objective lens 4, while imparting astigmatism while branching the return light from the optical disk 5 to the photodetector light-receiving surface 6. There is a purpose. FIG. 8 shows a spot diagram of the photodetector light-receiving surface 6 in the optical system shown in FIG. 7, and the astigmatic difference is 0.48 mm.

Conventionally, the beam splitter 1 is a BK7.
The non-polarizing film was formed on the first surface 7 of the parallel plane plate using the optical glass as a substrate. The beam splitter 1 was manufactured by rough-grinding, medium-grinding, fine-grinding, polishing, and cleaning both surfaces of a large glass base material having a diameter of about 75 mm, and forming a non-polarizing film on one surface of the glass base material by a vacuum evaporation method. After that, it was cut with a diamond wheel so as to have a desired size of 5 × 9 × 2.4t. However,
Since it is a glass substrate, this cutting also involves many steps such as sticking, cutting, peeling and cleaning with wax, and automation of the manufacturing process is also difficult. That is, the glass beam splitter 1 has a problem that it is difficult to mass-produce the beam splitter 1 at low cost.

Therefore, in recent years, as described in Japanese Patent Application Laid-Open No. 2-281202, there has been known one in which a non-polarizing film composed of three or more thin film layers is formed by vapor deposition on a plastic substrate without using optical glass. There is.

[0005]

However, since the numerical aperture of the light beam on the photodetector side is about NA = 0.08, in order to achieve the required astigmatic difference of 0.5 mm in the astigmatism method, The plate thickness of the plastic beam splitter 1 must be 2.4 mm. The 5 × 9 mm size and the 2.4 mm thickness make it difficult to shorten the cycle while maintaining the required surface accuracy by the injection molding method or the like, and as a result, it has been difficult to manufacture low-cost parts. Further, the conventional plastic beam splitter has a film structure of three layers or more, and the reduction of the film formation time at the time of forming the non-polarization film has not been considered. Further, there is a problem that the astigmatic difference and the spot size cannot be freely controlled in the astigmatism method of focus signal detection.

The present invention has been made in view of the above-mentioned conventional problems, and by utilizing the free formability of plastics, it is possible to mass-produce at low cost while maintaining high precision surface accuracy, and further, the astigmatism method. It is an object of the present invention to provide a plastic beam splitter capable of freely controlling the astigmatic difference in (3) without changing the surface spacing (on the optical axis) and making the spot shape in focus circular.

[0007]

In order to achieve the above object, the present invention provides a non-polarizing film having a beam splitter of a laser optical system made of plastic and having a specific demultiplexing ratio on its first plane. While forming a second
A plastic beam splitter was constructed with the surface as a curved surface.

Examples of the plastic substrate used in the present invention include thermoplastic resins such as polymethylmethacrylate, polystyrene, polycarbonate, polyvinyl chloride, copolymers of methylmethacrylate and other vinyl monomers, and the like. Examples of the resin include polydiethylene glycol bisallyl carbonate. Among these, polymethylmethacrylate is particularly preferable from the viewpoints of transparency and birefringence.

Here, the non-polarizing film is preferably formed of a λ / 4 film of silicon and silicon oxide. Further, the second surface may be a cylindrical surface or a toric surface.

[0010]

According to the plastic beam splitter of the present invention having the above-mentioned structure, the base material can be formed by the injection molding method or the like, the mass productivity is high and the cost can be reduced even in a complicated shape. By giving curvature to the second surface, it becomes possible to freely control the spot shape at the time of focusing in the astigmatism method.

However, if only the second surface is a cylindrical surface, the spot shape at the time of focusing becomes a substantially triangular shape due to the influence of coma aberration. Therefore, if the second surface is a toric surface to correct this, it is almost the same. A circular spot can be obtained.

A wedge plate having a prism apex angle of 0.5 degrees can be used as a means for realizing an astigmatic difference equivalent to the spot diagram shown in FIG. 8 with a thickness of 1.0 mm, but the angle tolerance is ±. It has to be held for about 3 minutes, so it cannot be said that the structure is suitable for mass production.

[0013]

[Embodiment 1] FIG. 1 shows a plastic beam splitter of the present embodiment, in which a plastic base material 9 made of PMMA and having a H-shaped cross section with high rigidity is formed by an injection molding method or the like to obtain a free shape. ing. Then, the plastic base material 9 is formed on the first surface 10 which is a reflection surface.
A λ / 4 film of silicon (Si) is formed as the first layer 11 on the upper surface, and silicon dioxide (Si) is further formed as the second layer 12.
A λ / 4 film of O ₂ ) is formed, and a non-polarizing film having a two-layer film structure is formed. The target p-polarized light reflectance and s-polarized light reflectance at a wavelength of 780 nm are Rp = 35.9% and Rs = 36.5% under a 45 ° incidence condition, respectively. On the other hand, the second surface 13 is formed as a concave cylindrical surface having a curvature radius of about 600 mm, and the surface distance between the first surface 10 and the second surface 13 is 1.0 mm.

In order to manufacture the plastic beam splitter having the above-mentioned structure, first, the plastic base material 9 is molded by the injection molding method, and the plastic base material 9 is cleaned by the ultrasonic cleaning machine to remove dirt such as oil and fat. After that, preliminary drying was performed in order to remove water contained in the plastic substrate 9. And
The plastic base material 9 was loaded into the bell jar so that the first surface 10, which is the surface to be coated, faces the evaporation source. Next, the degree of vacuum in the bell jar is 2 × 10 ^-5 Tor
The substrate was heated to 60 to 70 ° C., and Si and SiO _{2 as} vapor deposition samples were set in the crucible for electron gun. After evacuation, start vacuum degree 2 × 10 ^-5 T
Starting vapor deposition from orr, coating center wavelength λ = 880n
m, and λ / 4 of Si as the first layer 11
A film was formed. Then, similarly, the second layer 12 is formed of SiO ₂
A λ / 4 film of i was formed.

Table 1 shows the constitution of the film and the refractive index.

[0016]

[Table 1]

FIG. 2 shows spectral characteristic data of a non-polarizing film formed of a two-layer film formed on the first surface 10 of the plastic beam splitter 14 having the above structure. Note that FIG. 2 is a design value in consideration of wavelength dispersion of the complex refractive index (n-ik) of silicon, and actually agrees well with the actually measured value of the coating film in this film configuration.

FIG. 3 is a basic configuration diagram of a finite system optical pickup using the plastic beam splitter 14 of this embodiment, and FIG. 4 shows a spot diagram on the light receiving surface 6 of the photodetector. In the plastic beam splitter 14 of this embodiment, the plane distance between the first surface 10 and the second surface 13 is only 1.0 mm, but the plate thickness of the plane parallel plate is 2.
The same astigmatic difference as the conventional product of 4 mm can be obtained. That is, it can be seen that an astigmatic difference of 0.48 mm and a spot size at the time of focusing, which are equivalent to those in FIG. 8, can be obtained.

[0019]

[Embodiment 2] FIG. 5 shows an embodiment of a plastic beam splitter 15 which can obtain a substantially circular spot at the time of focusing. A non-polarizing film is formed on the first surface 16 as in Embodiment 1 above. The second surface 17 is a toric surface. The toric surface has a radius of curvature R _Y in the Y-axis sectional direction of 2000 mm and a radius of curvature R _{X in} the X-axis sectional direction of 69.98 mm.

FIG. 6 shows a spot diagram obtained when the plastic beam splitter 15 of the present embodiment is used, and it can be seen that a substantially circular spot is obtained with an astigmatic difference of 1.2 mm.

[0021]

As described above, according to the plastic beam splitter of the present invention, it is possible to mass-produce inexpensively by the production means such as the injection molding method, the automation of the manufacturing process is facilitated, and the non-polarizing film is formed by the two-layer film. By forming it, the coating time can be shortened, and the astigmatic difference can be set freely, which increases the degree of freedom when designing the optical pickup. Especially, by making the second surface a toric surface, The spot shape at the time of focusing can be made circular.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing a plastic beam splitter according to a first embodiment of the present invention.

FIG. 2 is a design spectral characteristic diagram of p-polarized light and s-polarized light transmittance and reflectance in Example 1 of the present invention.

FIG. 3 is a basic configuration diagram of a finite optical pickup using the plastic beam splitter shown in FIG.

FIG. 4 is a spot diagram of a light receiving surface of a photodetector in the optical system shown in FIG.

FIG. 5 is a configuration diagram showing a plastic beam splitter according to a second embodiment of the present invention.

6 is a spot diagram of a light receiving surface of a photodetector in the optical system shown in FIG.

FIG. 7 is a basic configuration diagram of a finite system optical pickup using a conventional beam splitter.

8 is a spot diagram of a light receiving surface of a photodetector in the optical system shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Beam splitter 2 Diffraction grating 3 Light source (LD) 4 Finite system objective lens 5 Optical disk 6 Photodetector light receiving surface 9 Plastic substrate 7, 10, 16 1st surface 11 1st layer 12 2nd layer 8, 13, 17th 2 side 14, 15 plastic beam splitter

Claims (4)

[Claims] 1. A beam splitter of a laser optical system is formed of plastic, and a non-polarizing film having a specific demultiplexing ratio is formed on the plane of its first surface, while the second surface is curved. And a plastic beam splitter. 2. The plastic beam splitter according to claim 1, wherein the non-polarizing film is a λ / 4 film of silicon and silicon oxide. 3. The plastic beam splitter according to claim 1, wherein the second surface is a cylindrical surface. 4. The plastic beam splitter according to claim 1, wherein the second surface is a toric surface.