JP2001052366A - Optical system of optical head - Google Patents

Abstract

(57) [Problem] To provide an optical system of an optical head capable of reducing a loss due to total reflection within an effective diameter of an objective lens while using an antireflection film having a small number of layers. An optical system of an optical head includes a semiconductor laser.
The light source unit 10 includes a light source unit 1 and a collimating lens 12, a beam shaping / separating prism 20, an objective lens 30, and a signal detection system 40. The lens surfaces 30a and 30b of the objective lens 30 have an anti-reflection film 31,
32 are formed. Assuming that the center emission wavelength λ0 of the semiconductor laser 11 is 680 nm, the design reference wavelength λ1 of the antireflection film 31 is set to 740 nm, and the design reference wavelength λ2 of the antireflection film 32 is set to 730 nm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system for an optical head used in a reproducing / recording apparatus for an optical disk such as a compact disk (CD) and a digital versatile disk (DVD), and more particularly to an optical system for an anti-reflection film of an objective lens. About settings.

[0002]

2. Description of the Related Art The optical system of an optical head includes a semiconductor laser that emits laser light, an objective lens that converges this laser light on the recording surface of an optical disk, and reflected light that has been reflected by the optical disk and transmitted through the objective lens to be incident light. It is composed of elements such as a beam splitter that separates from the optical path and a light receiving element that receives the separated reflected light and detects a signal.

The surface of the objective lens is coated with an antireflection film in order to efficiently transmit incident light from a semiconductor laser or reflected light from an optical disk. Since the wavelength used can be considered to be substantially a single wavelength in the optical head, an inexpensive single layer having a simple structure and an antireflection film having a small number of layers such as about two layers are used. For example, a single-layer antireflection film forms a thin film having a different refractive index on the surface of a lens, and the optical path difference between the light reflected on the surface of the thin film and the light reflected at the boundary between the thin film and the lens is の of the wavelength. Thus, the light is designed to have such a thickness that these lights cancel out by interference. The antireflection film designed in this manner has a quadratic spectral reflection characteristic in which the reflectance at the used wavelength exhibits a minimum value at the time of normal incidence, and the reflectance increases as the distance from the wavelength increases. In this specification, the wavelength at which the reflectance at the time of vertical incidence becomes the lowest is defined as the design reference wavelength of the antireflection film.

A conventional antireflection film applied to an objective lens is designed using the emission wavelength of a semiconductor laser as a design reference wavelength.

[0005]

However, since the lens surface of the objective lens is a curved surface, even when the laser light from the semiconductor laser is incident on the objective lens as parallel light, it is perpendicularly incident (the incident angle is almost 0 degree). The condition () is almost satisfied only in a very limited range around the optical axis, and in most other regions, the incident angle is larger than 0 degree. When the angle of incidence increases, the optical path difference between the reflected light at the thin film surface and the reflected light at the boundary between the thin film and the lens becomes smaller, so that the wavelength of the light canceled out by the interference becomes shorter than the design reference wavelength, and the original reflectance The reflectance for the laser light from the semiconductor laser whose emission is to be reduced is increased. For this reason, the conventional objective lens provided with the antireflection film has a problem that the loss due to the total reflection within the effective diameter cannot be reduced.

When the antireflection film has a multilayer structure, the reflectance in a relatively wide wavelength range including the emission wavelength of laser light can be suppressed, but the cost required for forming the film is small when the number of layers is small. This significantly increases the manufacturing cost of the objective lens.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems of the prior art, and uses a total reflection within an effective diameter of an objective lens while using an antireflection film having a small number of layers having wavelength dependency. An object of the present invention is to provide an optical system of an optical head that can reduce loss.

[0008]

In order to achieve the above object, an optical system of an optical head according to the present invention includes a light source for emitting a laser beam of a predetermined wavelength and a laser beam from the light source on a recording surface of an optical disk. In the configuration having an objective lens that converges to the objective lens, an anti-reflection film is applied to the surface of the objective lens, and the design reference wavelength of the anti-reflection film that has the lowest reflectance at normal incidence is set to be longer than the wavelength of the laser beam. Features.

If the design reference wavelength is set to be longer than the wavelength of the laser light as described above, the wavelength at which the reflectance becomes minimum is longer than the actual laser light wavelength for the vertically incident laser light. And the wavelength of the laser beam are matched, the reflectivity is higher than in the conventional example. on the other hand,
For laser light incident at an angle of incidence greater than 0 degrees, the wavelength at which the reflectance is lowest is shorter than the wavelength at which the reflectance is lowest at normal incidence. By shifting the reference wavelength to a side longer than the wavelength of the actual laser light, the reflectance at the wavelength of the actual laser light can be reduced as compared with the conventional example. The area that satisfies the condition of normal incidence, where the reflectivity is higher than the conventional one, occupies only a small percentage of the effective diameter of the objective lens near the optical axis. By reducing the reflectance, the total reflectance can be suppressed to be lower than in the past.

In a general optical head for DVD,
The design reference wavelength of the antireflection film is desirably about 50 to 60 nm longer than the wavelength of the laser light. In addition, the radius of curvature of the lens surface on the light source side is often set to be smaller than the radius of curvature of the lens surface on the optical disk so that the objective lens is advantageous for aberration correction. The smaller the radius of curvature is, the larger the incident angle in the peripheral portion is, so the average incident angle is larger on the lens surface on the light source side. Therefore, in such a case, the design reference wavelength of the antireflection film applied to the lens surface on the light source side is λ1, the design reference wavelength of the antireflection film applied to the lens surface on the optical disk is λ2, Wavelength λ0
It is desirable to satisfy the condition of λ0 <λ2 <λ1. More specifically, the design reference wavelength λ2 and the design reference wavelength λ1 can be made longer by about 50 nm and about 60 nm, respectively, than the wavelength λ0 of the laser beam.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an optical system of an optical head according to the present invention will be described below. FIG. 1 is an explanatory diagram of an objective lens included in the optical system of the optical head according to the embodiment, and FIG. 2 is an explanatory diagram illustrating a schematic configuration of the entire optical system. First, the overall configuration will be described with reference to FIG. The optical system according to the embodiment includes a light source unit 10, a beam shaping / separating prism 20, an objective lens 3
0, and a signal detection system 40. The light source unit 10 includes a semiconductor laser 11 and a collimating lens 12
And The power of the collimating lens 12 is set so that the laser light from the semiconductor laser 11 is converted into parallel light.

The beam shaping / separating prism 20 is formed by laminating two prisms 21 and 22. The beam shaping / separating prism 20 shapes the cross-sectional shape of the laser beam emitted from the light source unit 10 and reflects the reflected light from the optical disk D on the laminating surface. And is guided to the signal detection system 40.

The objective lens 30 is made of plastic such as PMMA. The first surface 30a on the light source unit 10 side and the second surface 30b on the disk side are both formed as aspheric surfaces.
This is a single-group lens. First surface 3 of objective lens 30
0a has a smaller radius of curvature than the surface 30b on the optical disk D side. The objective lens 30 focuses the laser light from the light source unit 10 on the recording surface of the optical disc D.

The signal detection system 40 includes a condenser lens 41 for condensing the reflected light from the beam shaping / separating prism 20;
It includes a cylindrical lens 42 and a sensor 43.
The laser light emitted from the semiconductor laser 11 is reflected by the disk and then enters the sensor 43 via the cylindrical lens 42. The signal from the sensor 43 is appropriately calculated and used for reproducing information recorded on the optical disk, or for generating a signal indicating a focusing error and a tracking error of the objective lens.

The lens surfaces 30a, 30b of the objective lens 30
1, anti-reflection films 31 and 32 each having a small number of layers, such as a single layer or about two layers, are formed as shown in FIG. In FIG. 1, the film thicknesses of the antireflection films 31, 32 are exaggerated from the actual scale for explanation. The setting of these antireflection films will be described.

The laser light from the light source 10 enters the objective lens 30 as parallel light. Here, light rays on the optical axis are denoted by L1, and light rays incident on points within the effective diameter that gradually increase in distance from the optical axis are denoted by L2, L3, and L4, respectively. The light ray L4 is a light ray incident on the outermost side within the effective diameter. The incident angles of the light beams L1, L2, L3, and L4 with respect to the objective lens 30 are α1, α2, α3, and α4, respectively. α1 is 0 degree, and the incident angle increases as the distance from the optical axis to the incident point increases, so that α1 <α2 <
The relationship α3 <α4 holds.

When the angle of incidence increases, the optical path difference between the light reflected on the surface of the antireflection film and the light reflected at the boundary between the antireflection film and the lens decreases, so that the wavelength at which the reflectance becomes minimum gradually increases. Be shorter. In the example of FIG. 1, the wavelength at which the reflectance of the light beams L2, L3, and L4 is the lowest gradually becomes smaller than the wavelength at which the reflectance of the light beam L1 is the lowest. For this reason, when the design reference wavelength of the antireflection films 31 and 32 is made to coincide with the center emission wavelength of the semiconductor laser 11 as in the related art, the reflectance of the light beams L2, L3 and L4 at the emission wavelength increases.

FIG. 3 shows the light beams L1, L2, L3 and L4 when the center emission wavelength λ0 of the semiconductor laser 11 is 680 nm and the design reference wavelength of the antireflection film 31 is 680 nm which is equal to λ0 as in the prior art. 4 is a graph showing the spectral reflectance of the sample. According to FIG. 3, the vertically incident light beam L1
For the light ray L2 whose incident angle is close to 0 °, the reflectance at the central emission wavelength of 680 nm can be reduced, but for the light rays L3 and L4 having larger incident angles, the reflectance at the central emission wavelength of 680 nm increases. You can see that. For this reason, the objective lens provided with the conventional antireflection film cannot reduce the loss due to the total reflection within its effective diameter.

In the embodiment, the design reference wavelengths λ 1 and λ 2 of the antireflection films 31 and 32 are changed to the emission wavelength λ of the semiconductor laser 11.
It is set larger than 0. Further, the radius of curvature of the lens surface 30a on the light source unit 10 side is equal to the lens surface 3 on the optical disk D side.
0b, the average incident angle of the lens surface 30a on the light source side is larger than that of the lens surface 30a.
It is set to satisfy the condition of λ1. Specifically, the center emission wavelength λ 0 of the semiconductor laser 11 is set to 680 n
m, the design reference wavelength λ1 of the antireflection film 31 is 740
nm, the design reference wavelength λ2 of the antireflection film 32 is 730 nm
It is.

FIG. 4 shows the light beams L1, L2, L when the center emission wavelength λ0 of the semiconductor laser 11 is 680 nm and the design reference wavelength λ1 of the antireflection film 31 is 740 nm.
It is a graph which shows the spectral reflectance about 3, L4. FIG.
According to the above, the reflectivity of the vertically incident light beam L1 and the light beam L2 whose incident angle is close to 0 degree is higher than that of the example of FIG. 3, but the reflectivity of the light beams L3 and L4 having a larger incident angle is higher. Since the lowest wavelength is shorter than the wavelength at which the reflectance is lowest at normal incidence, the center emission wavelength 680
It can be seen that the reflectance in nm is smaller than the example in FIG. The region satisfying the condition of normal incidence where the reflectivity is higher than that of the example of FIG. 3 occupies only a small proportion near the optical axis in the effective diameter of the objective lens 30, and most of the incident light at an angle larger than that. By reducing the reflectivity in the region of, the total reflectivity can be suppressed to be smaller than that in the example of FIG.

The single-layer antireflection film is designed so that the optical thickness nd obtained by the product of the refractive index n and the physical thickness d is １ ／ of the wavelength used. Therefore, in order to increase the design reference wavelength, the physical thickness d may be set to be large.

[0022]

As described above, according to the present invention, by setting the antireflection film of the objective lens to be longer than the light emission wavelength of the light source, the reflectance of the light beam incident at an angle larger than 0 degree is suppressed. And the total reflectance within the effective diameter of the objective lens can be kept low.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an objective lens included in an optical system of an optical head according to an embodiment.

FIG. 2 is an explanatory diagram illustrating a schematic configuration of the entire optical system of the optical head according to the embodiment.

FIG. 3 is a graph showing spectral reflectances for light beams having different incident angles when a conventional antireflection film is applied.

FIG. 4 is a graph showing spectral reflectances for light rays having different incident angles when the antireflection film of the embodiment is applied.

[Explanation of symbols]

 Reference Signs List 10 light source unit 11 semiconductor laser 12 collimating lens 20 beam shaping / separating prism 30 objective lens 31, 32 antireflection film 40 signal detection system

Claims (4)

[Claims] A light source that emits a laser beam of a predetermined wavelength;
An objective lens for converging the laser light from the light source on the recording surface of the optical disk, wherein an anti-reflection film is formed on the surface of the objective lens, and the design of the anti-reflection film has the lowest reflectance at normal incidence. An optical system for an optical head, wherein a reference wavelength is set longer than a wavelength of the laser light. 2. The optical system according to claim 1, wherein a design reference wavelength of the anti-reflection film is about 50 to 60 nm longer than a wavelength of the laser light. 3. The objective lens according to claim 1, wherein a radius of curvature of the lens surface on the light source side is smaller than a radius of curvature of the lens surface on the optical disk side, the design reference wavelength of the antireflection film formed on the lens surface on the light source side is λ1, and The design reference wavelength of the antireflection film applied to the lens surface on the side is λ2, and the wavelength of the laser light is λ0.
2. The optical system according to claim 1, wherein the following condition is satisfied: λ0 <λ2 <λ1. 4. The optical system according to claim 3, wherein the design reference wavelength λ2 is about 50 nm longer than the laser light wavelength λ0, and the design reference wavelength λ1 is about 60 nm longer.