JPH0421936A - Optical head device - Google Patents

Abstract

PURPOSE:To prevent the adverse influence of birefringence even when the birefringence exists by providing an optical means to guide a beam from a light source as a linearly polarized beam so that the light source and a polarizing face can be coincident with the advanced and delayed phase axes of a transparent substrate. CONSTITUTION:A second lambda/4 plate 15 and a Faraday rotor 16 are rotated and controlled in the direction of an optical axis so that the advanced and delayed phase axes of the birefringence of a substrate 12 can be coincident with the beam polarizing face on a tape T. The beam made incident in this way is reflected on the optical tape T and returned back to an optical system 6 and at such a time, the reflected linearly polarized beam is rotated at 45 deg. on the polarizing face by the rotor 16. When the beam returns to the plate 15, the polarizing face is rotated at 90 deg. in comparison with the incident time. Therefore, this beam is transformed to a circularly polarized beam so that the rotational direction can be made opposite to the incident time, and when the beam is transmitted through a lambda/4 plate 10, it is transformed to the linearly polarized beam so that the polarizing face can be orthogonal to that of the incident time. Then, the beam is almost fully reflected by S waves in a polarized beam splitter 8 and made incident to a photoelectric converting element. Thus, the adverse influence caused by the birefringence of the substrate can be prevented.

Description

[Detailed description of the invention] (b) Industrial application fields The present invention relates to an optical head device.

(B) Conventional Technology As an example of an optical head device, the one disclosed in Japanese Patent Application Laid-open No. 149036/1985 (G11B71085) is known. This type of optical head device is shown in FIG. In the figure,
(1) is a rotating cylinder, which has a cylindrical shape with a bottom and is rotationally driven by a motor (2). The rotating cylinder (1
) has a prism (3) placed on the center of rotation,
Further, an objective lens (5) is attached to the side surface of the rotating cylinder (1) facing the prism (3) through a through hole (4).

A fixed optical system (6) is arranged above the rotating cylinder (1), and this fixed optical system (6) is connected to the rotating cylinder (1).
A circularly polarized beam is emitted such that its tip axis coincides with the rotation axis of 1). The fixed optical system (6) includes a semiconductor laser (7) that emits a linearly polarized beam, a polarizing beam splitter (8), a collimator lens λ (9), a four-plate (10), and a photoelectric conversion element (11). ). The polarizing beam splitter (8) is a semiconductor laser (
The position is adjusted so that the beam from 7) becomes a P wave. Therefore, the beam from the semiconductor laser (7) is substantially completely transmitted through this polarizing beam splitter (8). and,
The beam transmitted through the polarizing beam splitter (8) is converted into a parallel beam by a collimator lens (9), and then converted from linearly polarized light to circularly polarized light by one plate (1o).

A circularly polarized beam is incident on the prism (3) from the fixed optical system (6). After being reflected by the prism (3), the beam is guided to the objective lens (5). on the other hand,
An optical tape (T) is obliquely wound approximately half the circumference around the circumferential surface of the rotating cylinder (1) by a guide means (not shown). The beam guided to the objective lens (5) is then focused onto the optical tape by the objective lens (5).

FIG. 3 is a sectional view showing the structure of the optical tape (T).

The optical tape (T) includes a transparent substrate (12) and a recording layer (1).
3) and a reflective layer (14). The light beam is incident from the substrate (12) side, passes through the substrate (12), and is focused on the recording layer (13). Here, the substrate (12) is in sliding contact with the circumferential surface of the rotating cylinder (1) and is absolutely necessary to protect the recording layer (13). The substrate (12) is formed of a PET (polyethylene terephthalate) film or the like, and has a thickness of about 10 to 20 μm. The recording layer (13) and the reflective layer (14) are formed on this substrate (1).
2) Formed on top by coating or vapor deposition. The beam focused on the recording layer (12) as described above is transmitted to the reflective layer (14).
), the light travels the same optical path as above and enters the polarizing beam splitter (8). Here, if the polarization state of this beam does not change between the time of incidence on the optical tape (T) and the time of reflection, it remains as circular hole polarization, then it passes through the fourth plate (10) and enters the polarization beam splitter (8). On the other hand, since it is converted into a linearly polarized beam that becomes an S wave, it is almost totally reflected laterally by the polarizing beam splitter (8) and the photoelectric conversion element (
11). Therefore, the reflected beam from the optical tape (T) is not incident on the semiconductor laser (7).

However, in the case of the above conventional example, the polarization state of the beam reflected from the optical tape (T) has actually changed to elliptical polarization, so that the reflected beam when incident on the polarizing beam splitter (8) is It does not become a linearly polarized beam as described above, but becomes an elliptical polarized beam. Therefore, a part of the reflected beam passes through the polarizing beam splitter (8) and enters the semiconductor laser (7). If the returned light is incident on the semiconductor laser (7) in this way, the oscillation of the semiconductor laser (7) will be disturbed, which will cause problems during recording and reproduction.

The reason why the polarization state of the reflected beam from the optical tape (T) is not circularly polarized but elliptical as described above is because the optical tape (T)
This is because birefringence exists in the substrate (12) of ).

Such birefringence occurs based on the orientation direction of molecules in the material. In other words, when molecules are oriented in one direction, a phase difference occurs between a beam component parallel to the orientation direction and a beam component perpendicular to the beam transmitted through the material. In other words, such a material has a fast axis or a slow axis in the molecular orientation direction and in a direction perpendicular to this direction, respectively. In the case of the substrate (12) of the above-mentioned conventional example, this substrate (12) is a tape-like film, and since stretching tension is always applied in the longitudinal direction of the tape during molding, the molecules of this substrate (]2) , is oriented in the stretching direction, that is, in the longitudinal direction of the tape. Therefore, such a substrate (12) has a fast axis or a slow axis in the longitudinal direction of the tape.

When a circularly polarized beam is incident on such a substrate (12) as described in Section 2, the beam also undergoes a phase shift based on the fast axis and the slow axis, and as a result, the beam becomes Substrate (1
2), the light changes from circular polarization to elliptical polarization while passing through it.

(c) Problems to be Solved by the Invention The present invention solves the problems that occur when the optical tape substrate has birefringence, and the polarization state of the beam changes due to the birefringence. I will try to solve it.

(d) Means for Solving the Problems In view of the above problems, the present invention provides an optical head that records and/or reproduces information by irradiating a beam from the transparent substrate side onto a medium having a transparent substrate and a recording layer. The apparatus is characterized by comprising a light source and optical means for guiding the beam from the light source as a linearly polarized beam so that the plane of polarization coincides with the fast axis or slow axis of the transparent substrate.

(e) Effect When a linearly polarized beam is made incident on a material such that its plane of polarization has a predetermined angle (excluding 90°) with respect to the fast axis or slow axis of the material, the beam progresses. Since it has a phase axis component and a slow axis component, a propagation phase difference occurs in each element flJ, and therefore, this beam becomes an elliptical polarized beam by passing through this material.

On the other hand, a linearly polarized beam enters a material such that its plane of polarization coincides with the fast or slow axis of the material (4=
When it is set to 1, this beam has only a fast axis component or a slow axis component, so the above-mentioned phase difference does not occur, and therefore, this beam is transmitted as linearly polarized light.

Therefore, as in the present invention, when a beam from a light source is incident on a medium such that its polarization plane coincides with the fast axis or slow axis of the substrate, the beam undergoes a change in polarization state due to the substrate. I don't accept it.

(f) Example Hereinafter, an example of the present invention will be described using FIG. 1. In the figure, the same parts as those in FIG. 2 used in the conventional example are given the same reference numerals, and the explanation thereof will be omitted.

That is, in this embodiment, the second four plates (]5) are placed in the optical path between the prism (3) and the fourth plate (10) in the rotating cylinder (1).
The prism (3) and objective lens (
5), a Faraday rotator (16) is placed between them.

According to this embodiment, the circularly polarized beam from the fixed optical system (6) is converted into a linearly polarized beam by the second plate (15), and then the plane of polarization is changed by the Faraday rotator (16). It is rotated by 45° and then focused onto the optical tape (T) by the objective lens (5). Here, the state of the polarization plane of the beam incident on the Faraday rotator (16) can be adjusted by rotating the second four plates (15) about the optical axis with respect to the rotating cylinder (1). Further, the state of the polarization plane of the beam on the optical tape can be adjusted by rotating the Faraday rotator (16) around the optical axis. Therefore, by adjusting the second four plates (15) and the Faraday rotator (16), the optical chip (
The polarization plane of the beam incident on the optical tape (T) can be made to coincide with the longitudinal direction or the width direction of the optical tape (T).

As mentioned above, the fast and slow axes of birefringence existing in the substrate (12) of the optical tape (T) are parallel to the longitudinal direction or the width direction of the optical tape (T), so the second 4 boards (1
5) and the Faraday rotator (16) in the optical axis direction to match the polarization plane of the beam on the optical tape (T) with the longitudinal or width direction of the optical tape (T). The plane of polarization can be made to coincide with the fast axis or slow axis of the substrate (12).

The beam incident on the optical tape (T) in this manner is reflected by the optical tape (T) and returns to the fixed optical system (6) along the same optical path as above. At this time, the linearly polarized beam reflected by the optical tape (T) is rotated by Faraday rotation (16
), the plane of polarization is rotated by 45° again, so when it returns to the second four plates (15), the plane of polarization is rotated by 90' compared to the time of incidence. Therefore, this beam is converted by the second λ 4 plate (15) into a circularly polarized beam whose rotation direction is opposite to that at the time of incidence. Therefore, when this beam passes through the plate (15) in the fixed optical system, it is converted into a linearly polarized beam whose polarization plane is perpendicular to that at the time of incidence. Such a beam is passed through a polarizing beam splitter (8
), it is an S wave with respect to the polarized beam brick (8
), and is then incident on the photoelectric conversion element (11).

Although one embodiment of the present invention has been described above, the present invention is not limited to this embodiment, and various other polarizations are possible. For example, a Faraday rotator (16) may be placed in the optical path between the prism (3) and the objective lens (5). ) and Faraday rotator (16
) can be arranged in other ways as long as it is in the optical path between the 4th plate (10) and the optical tape (T). Moreover, although the above embodiment is an example in which the present invention is applied to a helical scan type optical head device, it is also possible to use it in other optical head devices. Furthermore, in the above embodiment, the beam polarization plane on the optical tape' (T) is adjusted by the second λ plate (5) and the Faraday rotator (16), but other optical means may be used. It's okay.

(g) Effects of the Invention As described above, according to the present invention, even if birefringence exists in the substrate of an optical recording medium, it is possible to provide an optical head device in which the beam condition is not adversely affected by this birefringence. This makes it suitable for use in an optical head device that records and/or reproduces information on a medium whose substrate has birefringence.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a conventional example, and FIG. 3 is a diagram showing the structure of an optical tape. (7)...Semiconductor laser, (10) (15) and (1
6)...Four plates λ Second single plate and Faraday rotator (optical means) (12)...Substrate.

Claims (1)

[Claims] (1) An optical head device that records and/or reproduces information by irradiating a beam from the transparent substrate side onto a medium having a transparent substrate and a recording layer, wherein a light source and a polarization plane are located on the transparent substrate. An optical head device comprising: optical means for guiding a beam from the light source as a linearly polarized beam so as to coincide with a fast axis or a slow axis.