JPH02187950A - Optical head - Google Patents

Abstract

PURPOSE:To prevent a linearly polarized light from being converted partially to a circularly polarized light, to avoid the lowering of the modulation degree of a signal, to increase an S/N and to contrive the improvement of the quality of a regenerative signal by interposing partially a 1/2 lambda plate on a luminous flux in a light beam. CONSTITUTION:A light beam from a laser diode 1 transmits successively through a collimator lens 2, a half mirror 3, a 1/2 lambda plate group 4 constituted by providing plural pieces of 1/2 lambda plates 4a, and a condensing lens 5 and radiates the recording film 6b of a magneto-optical disk 6. A reflected light from the disk 6 passes through a reverse path, passes through a half mirror 7, and an analyzer 8 disposed by setting a polarization axis to 45 deg. on a signal detection optical path, and is inputted to photodetectors 9, 10, respectively. Consequently, even in a part in which the azimuth of a polarized light in a luminous flux is not parallel (vertical) to the incident surface, the polarized light directions are varied in parallel to the incident surface, respectively by the 1/2 lambda plate group 4, and it is prevented that a linearly polarized light for reproducing the information is converted partially to an elliptically polarized light.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention focuses and irradiates a light beam used for recording, reproducing, or erasing information on a magneto-optical memory element.
The present invention also relates to an optical head that detects reflected light reflected by a magneto-optical memory element.

[Conventional technology]

In recent years, magneto-optical memory devices have been in the spotlight as optical memory devices in which information can be rewritten. Magneto-optical memory elements include, for example, magneto-optical cards and magneto-optical disks, of which a magneto-optical disk is constructed by having a magnetic recording film between disk-shaped substrates. Further, information reproduction on this magneto-optical disk is performed by using a substrate 15 constituting the magneto-optical disk 15, as shown in FIG. 5(a).
A linearly polarized laser beam 17 is focused by an objective lens 18 and incident perpendicularly to the magnetic recording film 15b under a,
Utilizing the phenomenon (Kerr effect) in which the plane of polarization of reflected light rotates depending on the direction of magnetization of the recording film 16, the rotation change is detected as a change in light intensity by a photodetector after passing through an analyzer (not shown). This is done by detecting.

Incidentally, glass and plastics are used as materials for the substrate 15a of the magneto-optical disk, and among them, plastics such as acrylic and polycarbonate are being considered from the viewpoint of productivity and ease of handling.

(Problem B to be solved by the invention) However, one of the drawbacks of these plastics is that they are more likely to cause birefringence than glass (especially polycarbonate) and have inferior optical properties. In particular, plastic substrates obtained by injection molding are prone to birefringence.
The linearly polarized light used in information reproduction becomes elliptically polarized light, greatly reducing the quality of the reproduced signal. That is,
When actually reading signals from a magneto-optical disk, the laser beam is normally focused onto the recording film 15b through the substrate 15a using a lens with an NA (numerical aperture) of 0.5 to 0.6. If the substrate has birefringence, linearly polarized light changes to elliptically polarized light while passing through the substrate 15a. and,
As described above, when the light becomes elliptically polarized, the degree of modulation of the signal decreases, and when the degree of modulation decreases, noise in the signal increases, resulting in a decrease in S/N.

There are two types of birefringence of the substrate: in-plane direction and vertical direction. Among these, birefringence in the in-plane direction means that there is a difference in refractive index in different in-plane directions of the substrates. Further, vertical birefringence means that there is a difference between the refractive index in the direction perpendicular to the substrate and the refractive index in the in-plane direction. Birefringence in the in-plane direction affects polarization in the same way at any position in the plane perpendicular to the optical axis of the light beam, but birefringence in the vertical direction has different effects on polarization depending on its position in the light beam. There is. In other words, at the center of the beam, the laser beam is incident perpendicularly to the substrate, so it is affected only by birefringence in the in-plane direction, and is almost unaffected by birefringence in the vertical direction. Since light is obliquely incident on the substrate, it is affected not only by birefringence in the in-plane direction but also by birefringence in the vertical direction. Note that if the direction of the incident polarized light is parallel or perpendicular to the plane of incidence, the linearly polarized light can reach the recording film as linearly polarized light even if it is incident obliquely.

The polycarbonate substrate obtained by injection molding has birefringence in the in-plane direction of the order of 10-6, while birefringence in the vertical direction is of the order of 10-4. It is usually two fingers larger than the direction. Therefore, birefringence in the vertical direction is the main factor in polarization changes due to birefringence of the substrate.

Here, the direction perpendicular to the main surface of the substrate is taken as the Z axis, and the Xl axis and the Y1 axis are taken in directions perpendicular to this, respectively, and the refractive index of each is taken as n and % ny % nz. Also, the angle between the laser beam incident surface and the X axis is α, and the refraction angle at the substrate is θ.
Then, the refractive index n5 in the direction perpendicular to the plane of incidence is expressed by the following first equation.

. . . Second equation When linearly polarized light polarized in the direction of the Xl axis is incident using the above equation, the result is as shown in FIG. 2(b). That is, as described above, the substrate 15a is
The linearly polarized light incident on the X, Y, and Y axes remains the same, but the ellipticity increases as it goes outward in a direction tilted by 45° from the X, Y, and Y1 axes. This causes the quality of the reproduced signal to deteriorate.

[Means to solve the problem]

In order to solve the above problems, an optical head according to the present invention condenses and irradiates a light beam toward a magneto-optical memory element, and also directs the light beam reflected by the magneto-optical memory element to a photodetector. In the optical end, an l/2λ plate is partially interposed on the light beam so as to change the polarization direction of a part of the light beam in a direction in which the influence of birefringence is small. It is characterized by the presence of

[For production]

According to the above configuration, it is possible to prevent linearly polarized light for signal reproduction from being partially circularly polarized due to birefringence of the substrate forming the magneto-optical memory element. This avoids a decrease in the degree of modulation of the signal, suppresses noise in the signal, and improves the S/N
Therefore, the quality of the reproduced signal can be improved. Therefore, sufficient reliability can be ensured even when using a magneto-optical memory element using a plastic substrate or the like obtained by injection molding, which is likely to cause birefringence. In addition, by interposing the 1/2 λ plate mentioned above instead of simply arranging a light shielding plate, the amount of light that passes through it does not change, so the amount of light beam during recording does not decrease. .

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 to 4.

In the optical head according to the present invention, as shown in FIG. 1, a laser beam irradiated from a laser diode 1 as a light source is passed through a collimator lens 2 to convert the laser beam into parallel light, and the optical path of the reflected light is The light passes sequentially through a half mirror 3 for changing the direction, a 1/2 λ plate group 4 comprising a plurality of 1/2 λ plates 4a, and a condensing lens 5 for condensing the laser beam. The light is focused and irradiated onto the magneto-optical recording film 6b of the magnetic disk (magneto-optical memory element) 6. The magneto-optical disk 6 has a transparent substrate 6a.
and a magneto-optical recording film 6b, and a polycarbonate substrate, for example, is used as the transparent substrate 6a. The laser beam reflected by the magneto-optical disk 6 passes through the aforementioned condenser lens 5.1/2λ plate group 4 and half mirror 3, and then branches into an optical path for servo signal detection and information signal detection. After passing through the half mirror 7 and the analyzer 8, which is disposed with the polarization axis set at 45° on the optical path for detecting the information signal, a photodetector constituting a magneto-optical signal detection section using a differential detection method is passed. 9 and 10, respectively.

As shown in FIG. 2, the 1/2λ plate group 4 changes the polarization direction of a part of the laser beam C into a direction in which the influence of birefringence due to the transparent substrate 6a is small. Four square 1/2λ plates 4 partially arranged on C
It is composed of a... Specifically, each 1/2λ
The plate 4a is the outer part of the light beam C (the part where it enters the transparent substrate 6a obliquely), and the part other than the part where the direction of the incident polarized light is parallel or perpendicular to the plane of incidence, i.e. , is disposed at a portion located at approximately 45 degrees with respect to the X-axis and Y-axis shown in the figure. And each 1/2λ
The polarization direction of the light transmitted through the plate 4a (direction A in the figure)
are adapted to be changed by 45 degrees in a direction relative to the incident plane (direction B in the figure), that is, by 45 degrees.As a means for this purpose, the 1/2λ plate 4a is configured to change the direction of polarization of the light beam C. It is arranged at a set angle of +22.5° or -22.5°. Here, the 1/2 λ plate 4a has a function of rotating the polarization direction of the light by twice the set angle.

Note that the shape of the 1/2 space plate 4a is not limited to the above-mentioned rectangular shape, but as shown in FIG. 3, a circular shape may also be used.
The specific shape, number, etc. are not limited.

According to the above configuration, even in a portion where the direction of polarization in the light beam C is not parallel (or perpendicular) to the plane of incidence, the direction of polarization is made parallel to the plane of incidence in the 1/2 λ plate group 4. Therefore, even in such a part, if the direction of polarized light is parallel (or perpendicular) to the plane of incidence, the linearly polarized light will be affected by birefringence even if it is incident obliquely. It is possible to take advantage of the characteristic that the linearly polarized light can reach the magneto-optical recording film with almost no interference, and it is possible to prevent the linearly polarized light used in information reproduction from becoming partially elliptically polarized. However, 1/2
By interposing the λ plate 4a, circular polarization is prevented as described above, and at the same time, the signal amount is reduced. In other words, there is no difference in signal amount (light amount) between the light before and after passing through the 1/2λ plate 4a, but the analyzer 8
By passing through the 1/2 λ plates 4a, the amount of the signal is reduced by the amount that has passed through the 1/2 λ plate 4a. However, l/2λ plate 4a
Since there is no difference in signal amplitude between the case where ... is interposed and the case where it is not interposed, there is no problem in signal detection by the analyzer 8, and as a result, the degree of modulation increases. As the degree of modulation increases, the S/N ratio increases and signal quality improves.

Thereby, sufficient reliability can be ensured even when using a magneto-optical disk using a plastic substrate etc. obtained by injection molding which is likely to cause birefringence. Furthermore, since the 1/2λ plate 4a is used instead of a light shielding plate, the amount of light transmitted therethrough does not change, so the amount of light beam does not decrease during recording.

Here, when the relationship between the intervening area of the 1/2 λ plate 4a, the signal amount S and the modulation degree m was investigated, it was as shown in FIG. In this figure, the horizontal axis is the ratio a/r between the distance a shown in Figure 2 and the radius r of the luminous flux C, and the vertical axis is 1/2λ
The signal amount and modulation degree when the plate 4a is not used are 1, and the signal amount and modulation degree when the 1/2λ plate 4a is used are shown. However, the NA of the lens used in this test
(Numerical aperture) is 0.6, a polycarbonate substrate is used as the substrate 6a, the refractive index n0 in the X-axis direction of the polycarbonate substrate is 1.585, and the birefringence in the in-plane direction is 5X10.
”, the birefringence in the vertical direction is set to 5×10−, and the transmittance of the half mirror is set to 0.03.

As is clear from this figure, in the installation configuration shown in FIG. 2, the degree of modulation is highest when a/r is 0.3.

〔Effect of the invention〕

As described above, the optical head according to the present invention condenses and irradiates a light beam toward a magneto-optical memory element, and guides the light beam reflected by the magneto-optical memory element to a photodetector. In the optical head, a 1/2 λ plate is partially interposed on the light beam so as to change the polarization direction of a part of the light beam in a direction in which the influence of birefringence is small. .

This can prevent the linearly polarized light used in information reproduction from becoming partially elliptically polarized. Therefore, a decrease in the degree of modulation of the signal is avoided, noise in the signal is suppressed, the S/N increases, and the quality of the reproduced signal can be improved. As a result, sufficient reliability can be ensured even when using a magneto-optical memory element using a plastic substrate or the like obtained by injection molding, which is likely to cause birefringence. Furthermore, since a 1/2λ plate is used instead of a light shielding plate, the amount of light passing through the plate does not change, so that the amount of light beam does not decrease during recording.

[Brief explanation of the drawing]

1 to 4 show an embodiment of the present invention, in which FIG. 1 is a schematic configuration diagram of an optical head, and FIG.
An explanatory diagram showing the arrangement of /2λ plate group, Figure 3 is 1/2λ
An explanatory diagram showing another example of the arrangement configuration of the plate group, FIG. 4 is a/
A graph showing changes in signal amount and modulation degree with respect to changes in r, FIG. 5, shows a conventional example, and FIG.
2 is an explanatory diagram showing how the light focused by the objective lens is incident on the magneto-optical disk, and FIG. ■ is a laser diode, 2 is a collimator lens, 3 and 7
The half mirror 14 is a group of 1/2 λ plates, 4a is a 1/2 λ plate, 5 is a condenser lens, 6 is a magneto-optical disk (magneto-optical memory element), 6a is a transparent substrate, 6b is a magneto-optical recording film, 8 is a magneto-optical recording film. Analyzers 9 and 10 are photodetectors.

Claims (1)

[Claims] 1. An optical head configured to condense and irradiate a light beam toward a magneto-optical memory element and guide the light beam reflected by the magneto-optical memory element to a photodetector, An optical head characterized in that a 1/2λ plate is partially interposed on the light beam so as to change the polarization direction of a portion of the light beam in a direction in which the influence of birefringence is small.