JPH0362342A - Optical head - Google Patents

Abstract

PURPOSE:To realize high-speed recording, to increase recording density to improve the stability of a servo in reproduction and to reduce the effect of birefringence by providing an aperture with its opening degree appropriately variable in the luminous flux to be injected into a magneto-optical memory element. CONSTITUTION:The laser beam emitted from a laser diode 1 is successively transmitted through a collimator lens 2, a half mirror 3, the aperture 4 and an objective lens 5 and condensed on the magnetic recording film 6b of a magneto-optical disk 6 to irradiate the film. The laser light reflected by the film 6b is passed through the objective lens 5, aperture 4 and half mirror 3, then passed through a half mirror 7 and an analyzer 8 and injected respectively into photodetectors 9 and 10. The aperture 4 is provided with a circular light transmitting part 4a, an annular changeover part 4b around the part 4a and a frame 4a, an annular changeover part 4b around the part 4a and a frame 4c, a light transmitting state is switched to a light shielding state by energizing the changeover part 4b, and the opening degree is variable in two steps.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention focuses a beam of light used for recording, reproducing, or erasing information and irradiates it onto a magnetic recording film. The present invention relates to an optical head that detects reflected or transmitted light beams.

[, Conventional technology]

In recent years, magneto-optical memory devices have been in the spotlight as rewritable optical memory devices. These magneto-optical memory elements include, for example, magneto-optical cards and magneto-optical disks, of which magneto-optical disks are constructed by having a magnetic recording film between disk-shaped substrates. be. To reproduce information on this magneto-optical disk, a linearly polarized laser beam is focused by an objective lens and is incident perpendicularly to the magnetic recording film of the magneto-optical disk, thereby changing the direction of magnetization of the magnetic recording film. Phenomenon in which the plane of polarization of reflected light rotates according to (Kerr effect)
The rotation change is detected by a photodetector as a change in the amount of light after passing through an analyzer. On the other hand, information is recorded by injecting a relatively high-power laser beam into the magnetic recording film to raise its temperature and applying a predetermined magnetic field to change the magnetization direction of the magnetic recording film.

By the way, with the demand for faster recording on magneto-optical disks, there is a need to increase the recording sensitivity (quickly raise the temperature of a predetermined part of the magnetic recording film), and in an optical head, light is focused on the magnetic recording medium. It is conceivable to increase the NA (numerical aperture) of the objective lens in order to improve the light intensity of the light spot. There is also a demand for higher density recording.
In this case, since the shortest bit length becomes shorter, it is necessary to reduce the spot diameter of the recording laser beam accordingly, and for this purpose as well, it is necessary to increase the NA of the objective lens.

[Problem to be solved by the invention]

However, when the NA of the objective lens is increased, the depth of focus becomes smaller, and as a result, focus servo and tracking servo systems become sensitive to disk inclination and eccentricity, making it necessary to improve servo characteristics. Furthermore, if the servo characteristics are to be increased, tolerances for the mechanical characteristics (shape characteristics, etc.) of the disk become stricter.

Furthermore, if a material with large vertical birefringence, such as polycarbonate, is used for the disk substrate, the higher the NA (that is, the larger the angle of oblique incidence), the more susceptible the vertical birefringence will be to the reproduction. Another problem is that the signal quality deteriorates.

[Means for Solving the Problems] In order to solve the above problems, an optical head according to the present invention condenses a light beam with an objective lens and irradiates the magnetic recording film of a magneto-optical memory element. In an optical head that detects a beam of light reflected or transmitted through a recording film,
It is characterized by providing an aperture whose aperture can be changed as appropriate in the light beam incident on the magneto-optical memory element.

[Operation] According to the above configuration, when recording and erasing information,
High-speed recording can be achieved by increasing the NA of the objective lens by increasing the opening degree of the aperture, increasing the intensity of the light spot formed on the magnetic recording film, and quickly raising the temperature of the magnetic recording film. Furthermore, by increasing the NA, the spot system becomes smaller and the recording density increases.

On the other hand, when reproducing information, the NA of the objective lens can be reduced by lowering the opening of the aperture, and the stability of the servo can be increased by increasing the depth of focus.
By making the value small, a portion of the light beam that enters the magneto-optical disk at a large angle of incidence is eliminated, and the influence of birefringence can be reduced to prevent deterioration of signal quality.

〔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 a parallel beam, and the reflected beam is directed in a 90° direction. Half mirror 3 for changing the opening, aperture 4 for changing the opening degree as appropriate
, and an objective lens 5 for condensing the laser beam, so that the light is condensed and irradiated onto the magnetic recording film 6b of the magneto-optical disk (magneto-optical memory element) 6.

The magneto-optical disk 6 includes a transparent substrate 6a and a magnetic recording film 6b, and a polycarbonate substrate, for example, is used as the transparent substrate 6a. The laser beam reflected by the magnetic recording film 6b passes through the objective lens 5,
After passing through the aperture 4 and the half mirror 3, a half mirror 7 that branches an optical path into one for servo signal detection and one for information signal detection (servo signal detection is not shown);
The light passing axis is set at 45 on the optical path for detecting the information signal mentioned above.
After passing through the analyzer 8 which is set and arranged at
0 respectively.

As shown in FIG. 4, the aperture 4 includes a circular light transmitting section 4a, a ring-shaped switching section 4b and a frame section 4c around the circular light transmitting section 4a. That is, the light transmitting part 4a is a part that can always transmit light, while the switching part 4b is a part that can be switched between a light transmitting state and a light blocking state by energization, and the aperture 4 has a variable opening degree in two stages. It is supposed to be done.

The switching section 4b includes a liquid crystal material and an analyzer. The analyzer has a light passing axis in a predetermined direction, and this axis is provided so as to be shifted from the polarization direction of the laser light, which is linearly polarized light (through a polarizer or the like).

The method of shifting is set so that the direction in which birefringence occurs and linearly polarized light changes when a predetermined voltage is applied to the liquid crystal material matches the light passing axis of the analyzer. Therefore, when a voltage is applied to the liquid crystal material, as shown in FIG.
increases, while when no voltage is applied, the switching section 4b is in a light blocking state, as shown in FIG.
The NA of the objective lens 5 becomes smaller.

Assuming that the aperture 4 changes its aperture diameter steplessly, the relationship between the aperture diameter, the beam diameter (spot diameter) on the recording medium, and its center intensity is shown in FIG. 3, respectively. Here, the maximum diameter of aperture 4 is ao,
The beam diameter at that time is ro, the center intensity at that time is Io, the changing aperture diameter is a, the beam diameter at each time is rl, and the center intensity at each time is ■. As is clear from this figure, r/r shows an upward trend as a/a o decreases, while I/I O shows an increasing trend as a/a o decreases.
As , decreases, it shows a downward trend.

According to the above configuration, when recording and erasing information,
The opening degree of the aperture 4 is increased, that is, the switching unit 4b is energized to make it a light transmitting state, the NA of the objective lens 5 is increased, and the intensity of the light spot formed on the magnetic recording film 6b is increased to increase the magnetic field. High-speed recording can be achieved by quickly raising the temperature of the recording film 6b. Furthermore, by increasing the NA, the spot system becomes smaller and the recording density increases.

On the other hand, when reproducing information, the opening degree of the aperture 4 is lowered, that is, the switching unit 4b is de-energized to put it in a light-blocking state, the NA of the objective lens 5 is decreased, the depth of focus is increased, and the servo can increase the stability of Furthermore, by reducing the NA, a portion of the light beam that enters the magneto-optical disk at a large angle of incidence is eliminated, thereby reducing the influence of birefringence and preventing deterioration of signal quality.

In this embodiment, the aperture 4 is replaced by a half mirror 3.
Although it is interposed between the collimator lens 2 and the objective lens 5, the present invention is not limited thereto. For example, it may be interposed between the collimator lens 2 and the half mirror 3. Also, regarding aperture 4, we have shown one that uses liquid crystal, but
For example, the aperture diameter may be controlled mechanically, and the specific configuration is not critical.

〔Effect of the invention〕

As described above, the optical head according to the present invention condenses a light beam using an objective lens, irradiates the magnetic recording film of a magneto-optical memory element, and detects the light flux reflected or transmitted by this magnetic recording film. In the optical head, an aperture is provided in the light beam incident on the magneto-optical memory element, the opening degree of which can be varied as appropriate.

As a result, when recording and erasing information, the temperature of the magnetic recording film can be quickly raised to achieve high-speed recording, and the spot system can be made small to increase the recording density. On the other hand, when reproducing information, it is possible to improve servo stability and reduce the influence of birefringence, thereby preventing deterioration of signal quality.

[Brief explanation of drawings]

1 to 4 show one embodiment of the present invention. FIG. 1 is a schematic configuration diagram of an optical head. Figure 2 (a) is an explanatory diagram showing a state in which the aperture switching section is in a light transmitting state and the NA of the objective lens is increased in order to perform recording and erasing appropriately, and Figure 2 (b) is an explanatory diagram showing a state in which the aperture switching section is in a light transmitting state and the NA of the objective lens is increased to perform recording and erasing appropriately. FIG. 4 is an explanatory diagram showing a state in which the optical system is in a light-blocking state and the NA of the objective lens is made small in order to perform reproduction appropriately. FIG. 3 is a graph showing the relationship between the opening diameter of the aperture, the spot diameter, and its center intensity. FIG. 4 is a plan view of the aperture. 1 is a laser diode, 2 is a collimator lens 3 is a half mirror, 14 is an aperture, 4a is a light transmitting part, 4b is a switching part, 5 is an objective lens, 6 is a magneto-optical disk (magneto-optical memory element), 7 is a half mirror 18 is an analyzer, 9.1
0 is a photodetector.

Claims (1)

[Scope of Claims] 1. In an optical head that collects a light beam with an objective lens and irradiates it onto a magnetic recording film of a magneto-optical memory element, and detects the light flux that is reflected or transmitted through this magnetic recording film, An optical head characterized in that an aperture is provided in a light flux incident on a magnetic memory element, the opening degree of which can be changed as appropriate.