JPS61153858A - Thermal recording and optical reproducing device - Google Patents

Abstract

PURPOSE:To prevent a decrease in the amplitude of a playback signal by using a long circular beam which is expanded only in the width direction of a pit during recording and also using a beam which is as wide as a formed pit or narrower during reproduction. CONSTITUTION:When a beam spot BW on the surface of a medium 1 is put in a defocusing state during recording by placing an objective 9 closer to the medium surface, the beam becomes long and circular because of astigmatism while having the diameter extended in the width direction of a pit at right angles to the moving direction of the medium shown by an arrow, thereby forming the pit P. The objective 9 is put farther in reproduction than in the recording to focus the beam, and then the diameter of the beam spot on the medium surface which is extended in the width direction of the pit P returns to its original size and the spot becomes round. Therefore, even if the beam Br forms a short spot P, neither of areas at both sides of the pit is irradiated. Consequently, reflected light from the medium contains no reflected light component from both side areas of the pit and the playback signal never decreases in amplitude.

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention irradiates a laser beam onto a recording medium, and utilizes the heat generated by the laser beam and a reversal magnetic field applied as necessary to record microscopic particles with different optical properties. The present invention relates to an improvement in a recording and reproducing apparatus that forms pits, records information based on the presence or absence of pits or the pit length, and optically reproduces the recorded information.

・(Background of the invention) ・With the development of the information society, there is a need for a recording method that can record a large amount of information at high density. A method has been developed in which micro pits are formed along the course, such as pits with a minimum length of 2 microns, and information is recorded based on the presence or absence of these pits or the length of the pits.

As a means for forming □ bits, a method has been developed in which a medium is irradiated with a laser beam focused to a minute diameter, and the resulting heat is used to form a cipit.

For example, (a) a method in which the medium is melted or sublimated by heat to create a recess, and this recess is used as a bit;
Create an area where the reflectance is different from the original state, and
(c) Using heat to reduce the coercive force of the nine perpendicular magnetizations aligned upward or downward in the medium, at the same time applying a reversal magnetic field to reverse the direction of magnetization, and then irradiating the laser beam. Thermal information recording methods have already been proposed, and some have been put into practical use, such as one in which the magnetic field is stopped, an area is created where the direction of magnetization is opposite to the original state, and this area is used as a pit (magneto-optical recording). There is.

The information recorded in this way is created by irradiating the medium with a laser beam, taking advantage of the fact that the optical properties of the formed pits are different from the surrounding areas, and then using the beam reflected by the medium or the medium. It is reproduced by detecting optical changes in the transmitted beam. In this case, the optical properties are different even if the pit is just a "dent" and the "dent" causes diffraction or interference, and if there is no "<", the diffraction or interference does not occur. Include in that.

By the way, since it is inconvenient to use separate devices for recording and reproduction in such a thermal recording optical reproduction method, the same device is often used for both purposes.

In that case, the secondary optical system including the light source is also used (however, the beam intensity is strong during recording and weak during playback).
During reproduction, part or all of the optical system used during recording is used. Ninth, the beam diameter of the irradiated surface of the recording medium has been the same during recording and reproduction, and the cross-sectional shape of the beam at the irradiated surface of the medium is both a perfect circle. As information is recorded, as higher frequency signals (pits with shorter bit lengths) are recorded, the reproduced signal amplitude (solid line) gradually increases to the theoretical value determined from the MTF performance of the objective lens, as shown in Figure 2. (
It turns out that the value decreases further than the dashed line).

Note that in the optical reproduction method, it has already been found that the reproduction signal amplitude is determined by the wavelength of the reproduction light and the MTF performance of the objective lens.

(Object of the Invention) Therefore, an object of the present invention is to make the reproduced signal amplitude close to the theoretical value without decreasing it even when recording a high frequency signal in the same device that performs recording and reproduction. .

(Summary of the invention) High-frequency signals have a short bit length, but the relationship between the shape (B) on the medium irradiation surface of the beam and the shape of the formed pin (P) is determined by the bit length. When I investigated the relationship by changing , I found the relationship as shown in Figure 3 (the arrow in Figure 3 indicates the direction of movement of the medium).

That is, as the bit length becomes shorter, the width of the pit tends to become narrower.

Considering the reason for this tendency, the light intensity distribution of the beam cross section on the medium irradiation surface of the laser beam is concentric and becomes lower toward the periphery.

On the other hand, when a beam is irradiated onto a medium moving at a constant velocity, the temperature at a certain point on the medium increases as shown in FIG. 4, and then decreases when the beam is no longer irradiated. The horizontal axis in Fig. 4 is time (1), the vertical axis is the temperature (1) of the medium, and t =
Start irradiation at o and increase irradiation at 1=11. The profile of such a temperature curve can be modeled by the following equation, assuming that the thermal time constant tτ depends on the properties of the medium.

(When OtI≧t≧0, T=T, (1-・戎) (b) When t>t, -1°T=T. (1-0/r)x, -(t-t,) /τ(To
is the highest temperature reached at thermal equilibrium), in this case the medium temperature (
Minimum temperature required to form pits in the medium (T
V) If t- is not exceeded, pits will not be formed.

Therefore, if the beam irradiation time is shortened to form a short pit, the temperature of the medium irradiated at the periphery of the beam will not rise so much and no pits will be formed. As a result, it is thought that the width of the pit becomes smaller than the diameter on the medium irradiation surface of the beam.

In such a situation, if the same beam is used for reproduction with reduced intensity, the diameter of the beam is wider than the width of the formed pit. This includes reflected light or transmitted light from. This results in a decrease in the contrast of the reproduced light in the optical reproduction method that utilizes changes in the optical properties of the reproduced light.

As a result, in a method in which reproduced light from a pit is converted into an electrical signal by a photodetector or the like, the amplitude of the reproduced electrical signal decreases. For example, if a pit is an area that reflects reproduction light and an area other than the bit is an area that absorbs reproduction light, generally speaking, the reproduction light reflected by the medium is weak between the pits, and the reproduction light is weak between the pits. Then become stronger. When such reproduction light is converted into an electrical signal, an electrical signal having a wave shape of troughs and peaks is obtained corresponding to the reproduction light of weak 9-strong.

However, when the electric signal based on the reproduced light from the pit is weakened, the height of the peak becomes lower, and the amplitude of the wave-shaped electric signal eventually becomes smaller.

Therefore, it is conceivable to record by increasing the beam diameter during recording, but this would create a contradiction in that the length of the pit would become longer, making it impossible to record high-frequency signals.

As a result of extensive research, the present inventors came up with the idea of widening the beam only in the width direction during recording, that is, making the cross-sectional shape of the beam elliptical, and narrowing it during playback to make it equal to or smaller than the width of the pit. This led to the realization of the present invention. In this way, since the beam diameter is small in the length direction of the pit, high-frequency signals can be recorded, and since the beam diameter is large in the width direction of the pit, the bit width will not become narrow even with high-frequency signals with a short irradiation time. As a result, the reproduced signal amplitude does not decrease.

Therefore, the present invention irradiates a recording medium moving at high speed with a laser beam, forms pits on the medium using heat generated by the irradiation and a reversal magnetic field applied as necessary, and determines the presence or absence of the pits and the length of the pits. The recorded information is recorded by irradiating a recording medium that moves at high speed with a laser beam using part or all of the optical system at the time of recording, and is reflected by the medium. - Detecting optical changes in the transmitted laser beam In a thermal recording optical reproducing device that reproduces data from K, during recording, the cross-sectional shape of the laser beam on the medium irradiation surface is perpendicular to the direction of movement of the medium. An apparatus characterized in that an elliptical laser beam having a major axis is used, and during reproduction, a laser beam having a cross-sectional shape equal to or smaller than the width of a formed pit is used.

I will provide a.

The beam cross-sectional shape at the medium irradiation surface is widened (
A specific method for narrowing the optical path during playback is to insert a cylindrical lens into the optical path, create astigmatism, and change the distance between the medium and the objective lens of the pickup during recording and playback. The most preferable method is to use a defocused state during recording and a focused state II+ (circle of least confusion state) during playback, since the only new component required is a cylindrical lens.Instead of inserting a cylindrical lens, it is possible to Astigmatism can be obtained even if the beam splitter (prism type) and objective lens used in the section are finished with cylindrical surfaces, and the same effect can be achieved. A1 is inserted into the optical path, and A1
The diffraction phenomenon of light is controlled by the 0 modulator, and during reproduction,
By generating a single perfect circular beam and, during recording, generating several light beams 1r in close proximity, a pseudo-elliptical beam can be generated.

Hereinafter, the magneto-optical case will be explained in detail as an example.

(Example) First, the overall configuration of the magneto-optical recording and reproducing apparatus of this example is explained in the fifth example.
As shown in the figure. In the figure, (1) is a disk-shaped magneto-optical recording medium, (2) is a spindle motor for rotating the medium, G) is a semiconductor laser, (4) is a laser drive circuit, (5) is a collimator lens, (6) is a polarizing plate, (7
) is the beam splitter, (8) is the cylindrical lens, % (9) is the objective lens, and Ql is the objective/z rotational system (,
only the objective lens moves up and down), aυ is the external magnetic field generating means,
0 is a polarizing plate, a3 is a condensing lens, and ■ is a photoelectric conversion element.

Here, a cylindrical lens (8) is installed in the optical path in order to make the cross-sectional shape of the beam oval during recording. Several embodiments are possible depending on whether the cylindrical lens (8) is convex or concave, and whether the arrangement direction is perpendicular or parallel to the track direction. and the direction of the degree meridian (
(direction that has a convergent effect) is arranged parallel to the track direction.

To explain the recording process, a drive current is input to the laser (3) from the drive circuit (4) based on information input from the outside, and the laser beam is thereby
It emits light as shown in the figure and becomes a friend. The emitted beam is corrected into parallel light by a collimator lens (5), and then made into linearly polarized light by a polarizing plate (6).

(However, if the degree of polarization of the light emitted from the semiconductor laser (3) that is the light source is sufficiently high, the polarizing plate (6) may be omitted.)
This polarized beam is reflected by a beam splitter (7), passed through a cylindrical lens (8), and then passed through an objective lens (9).
The light is focused and irradiated onto the medium <1). In this case, when the objective lens is brought close to the medium surface and brought into a defocused state, the beam focus (By) on the medium surface will change in the direction perpendicular to the moving direction (track direction) of the medium due to astigmatism.
It has an elliptical shape with a diameter extending in the width direction of the bit. Note that the beam does not need to be polarized during recording.

Simultaneously with the beam irradiation, when a switching magnetic field is applied by the external magnetic field generating means az, a bit with switching magnetization is formed. The relationship between the shape of the formed bit and the cross-sectional shape of the beam on the medium irradiation surface is shown in FIG. 1(a).

In the figure, (fly) represents the cross-sectional shape of the beam during recording, P) represents the bit shape, and the arrow indicates the direction of movement of the medium. In addition, (Br) is a conventional perfect circular beam subbox) 1
- and shows a perfectly circular beam spot during playback.

As shown in Figure 1 (1), an elliptical beam expanded in the width direction of the bit is used during recording, so even if the pit length is short, the width of the bit formed is be greater than or equal to the beam cross-sectional diameter.

L'-f-(3) is caused to emit light at low power, and the resulting laser beam is converted into parallel light using a t-collimator lens, which is then converted into linearly polarized light using a polarizing plate (6). This straight! 1IfI
The A light beam is reflected by the beam splitter (7),
The light passes through the lens (8) and is focused by the objective lens (9) to the medium (
1) Irradiate with K.

In this case, the objective lens (9) is moved further away than during recording, so that the objective lens (9) is in focus. When it is set to l (circle of least confusion II), the beam diameter (Br) on the medium surface returns to its original diameter extending in the width direction of the beam (P) and becomes a narrow perfect circle.

Therefore, the beam (Br) is as shown in FIG. 1(b).
Even short bits do not illuminate the area on either side of the bit. The reflected light from the next medium also does not include reflected light components from both side areas of the bit, so that the reproduced signal amplitude does not decrease.

The reflected light from the medium α) passes through the objective lens (9)% lens (8) again, reaches the beam splitter (7), transmits there, passes through the polarizing plate azt-, and enters the photoelectric conversion element (IJ).
K light is received, and it is converted into an electrical signal at ζ. Media 0) here uses a magneto-optical recording medium, so there is no magnetic field between the orbit and other areas. The polarization plane of the reflected light differs depending on the effect, and as a result, depending on the polarizing plate Q, the amount of light differs between the bit (p) and the other areas by a strength of 9 or less. However, according to the present invention, since the amount of light from the bits does not decrease or increase illegally, the amplitude of the electrical signal does not become illegally narrowed, and as a result, good signal reproduction is performed even in the high frequency region.

In this embodiment, since the astigmatism is utilized, the beam shape during reproduction is a perfect circle, and if the width of the beam is equal to or smaller than the width of the formed bit, the shape will be There are no particular restrictions. In addition, in this example, the cylindrical lens was fixed and only the objective lens was moved up and down, but the same effect is not limited to this, and the same effect can be obtained with a configuration in which the 7 lindrical lens and the objective lens are moved up and down together. .

(Effects of the Invention) As described above, according to the present invention, an oval beam expanded only in the width direction of the bit is used during recording, and a beam that is narrower than or equal to the width of the formed pit is used during reproduction. , even if the bit length becomes short (even if the signal becomes a high frequency signal), the reproduced signal amplitude does not decrease and approaches the theoretical value.

[Brief explanation of drawings]

FIG. 1(a) is an explanatory diagram showing the relationship between the shape of the beam (Bw) on the medium irradiation surface and the bits formed when recording according to the embodiment of the present invention. FIG. 1(b) is an explanatory diagram showing the relationship between the shape of the beam (Br) on the medium irradiation surface and the formed bit (G) when reproducing according to the embodiment of the present invention. Figure 2 shows the results when recording and playing back using a conventional device.
It is a graph showing changes in reproduced signal amplitude with respect to the frequency of an information signal, where the solid line shows the measured value and the broken line shows the theoretical value. FIG. 3 is an explanatory diagram showing the relationship between the shape of the recording/reproducing beam (B) on the medium irradiation surface and the formed bit pattern when recording is performed using a conventional apparatus. I! FIG. 4 is a graph showing the relationship between the medium temperature (1) and the elapsed time (1) when the medium is irradiated with a beam and the surface of the medium is imaginary. FIG. 5 is a conceptual diagram showing the overall configuration of a magneto-optical recording/reproducing apparatus according to the first embodiment of the present invention. FIG. 6 is a graph showing the relationship between laser beam intensity and elapsed time during recording. [Explanation of symbols of main parts] Bw... Shape B of the beam on the medium irradiation surface during recording
r... Shape B on the medium irradiation surface of the beam during reproduction
...Beam P...Bit l...Recording medium 3...Laser 5...Collimator lens 6.12...Polarizing plate 7...Beam splitter 8...7 Lindrical lens 9...Objective lens 10...Objective lens drive system 13...Condensing lens 14...Photoelectric conversion element

Claims (1)

[Claims] A recording medium moving at high speed is irradiated with a laser beam, and pits are formed on the medium by the heat generated by the irradiation and a reversal magnetic field applied as necessary, and information is recorded based on the presence or absence of the pits or the length of the pits. The recorded information is obtained by irradiating a recording medium that moves at high speed with a laser beam using part or all of the optical system during recording, and by calculating the optical information of the laser beam that is reflected by or transmitted through the medium. In a thermal recording optical reproducing device that performs reproduction by detecting a change, during recording, an elliptical laser beam whose cross-sectional shape at the medium irradiation surface has a major axis in a direction perpendicular to the direction of movement of the medium is used. . An apparatus characterized in that during reproduction, a laser beam having a cross-sectional shape equal to or smaller than the width of the formed pit is used.