JPH0489645A - Signal detecting method for magneto-optical recording - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Industrial applications The present invention relates to a signal detection method in magneto-optical recording.

Conventionally, in magneto-optical recording, a recording medium is irradiated with linearly polarized light,
Recorded information was obtained by detecting reflected or transmitted light from a recording medium as information light and detecting rotation of the plane of polarization due to the direction of magnetization of the recording medium. Furthermore, in order to accurately irradiate a spot light onto recording bits of a recording medium, a focus error signal and a tracking error signal are detected from the information light, and these are used as servo signals to servo drive an actuator.

FIG. 4 shows the basic configuration of a signal detection method in a conventional magneto-optical recording medium.

The light from the laser diode 11 passes through the collimator lens 13, the shaping prism 15, and the beam splitter 17, and is directed to the disk 2 by the objective lens 19 of the actuator.
Converged on one side. The reflected light from the surface of the disk 21 passes through the beam splitter 17, 172 wavelength plate 23, and the condenser lens 51. Converged by the cylindrical lens 53,
enters a polarizing beam splitter 56 that acts as an analyzer,
The light is separated and incident on a photodiode 57, 59 as a spot.

The difference between the output signals from the two photodiodes 57 and 59 is output as a differential signal, and the information recorded as the direction of magnetization is read out. On the other hand, the focus error signal is detected by the photodiode 57 using the astigmatism method, and the objective lens is servo driven in the focus direction Fo.

On the other hand, the tracking error signal is transmitted through the photodiode 5.
9 by a push-pull method (far field method), and using this as a servo signal, the objective lens 19 is driven in the tracking direction Tr.

However, in this method, since an astigmatic method is used to detect the focus error signal, it is necessary to use a cylindrical lens, making it difficult to miniaturize the optical head (actuator).

In addition, since a polarizing beam splitter is used, S-polarized light and P-polarized light
The polarized light is separated vertically by two photodiodes 57.
．． 59 is arranged as shown in FIG. 4, making it impossible to downsize.

Detecting the magneto-optical signal by separating the light into P-polarized light and S-polarized light using a polarizing prism and taking this differential output is as follows:
It is described in JP-A-1-189051 and JP-A-1-189052. However, in these methods, the light used for the focus error detection system and tracking error detection system is separated before the birefringent plate, so
The optical system and detection system become complicated.

SUMMARY OF THE INVENTION The present invention provides a method for detecting signals in magneto-optical recording, which has a simplified structure and can reduce the weight of the entire head.

The signal detection method in magneto-optical recording of the present invention focuses information light from a magneto-optical recording medium through a birefringent material made by cutting a uniaxial crystal obliquely to the optical axis, and detects the ordinary light component. The present invention is characterized in that it separates a spot light and an abnormal light component into a spot light, and detects a differential force rough focus error signal of the spot light diameter.

Kui - November □ When light enters a birefringent material made by cutting a uniaxial crystal obliquely to the optical axis, it is separated into an ordinary light component and an extraordinary light component. Since these two have different refractive indexes, when light is converged through a birefringent material, the imaging position of the ordinary light component and the extraordinary light component will be different due to refraction, for example, the diameter of the irradiation spot on the light receiving element provided on the same plane are different. Therefore, by detecting this positional displacement, a focus error signal, that is, the amount of deviation of the actuator objective lens from the proper focus position can be detected.

Embodiment FIG. 1 is a block diagram showing an embodiment of the signal detection method of the present invention.

The light from the laser diode 11 passes through the collimator lens 1
3, it becomes a parallel light beam, passes through a shaping prism 15 and a beam splitter 17, is converged by an objective lens 19 mounted on an actuator, and is irradiated with a spot beam onto the disk 21. The plane of polarization of the reflected light from the disk 21 is rotated in the direction of magnetization. This reflected light enters the objective lens 19 again and becomes a parallel light beam, passes through the beam splitter 17. 1/2 wavelength plate 23, and is converged by the condenser lens 19. The light is incident on the plate 33.

Here, the optical axis of the crystal of the birefringent plate 33 is tilted with respect to the optical axis, and the birefringent plate 33 allows the incident light to have an ordinary light component (S
polarized light) and an extraordinary light component (P-polarized light). Therefore, the S-polarized light is made incident on the first light-receiving element (photodiode) 37, while the P-polarized light is made incident on the second light-receiving element 39, and differential signals are obtained by taking differential outputs of these.
That is, magneto-optical signals can be detected. 41 indicates a differential amplifier. Birefringent materials have higher extinction ratios than conventional polarizing beam splitters, which improves the C/N ratio.

Furthermore, due to the difference in refractive index between ordinary light and extraordinary light, the birefringent plate 3
Both imaging distances from 3 are different. Figures 2A and 2
Figure B is an explanatory diagram showing this state. Figure 2B (Second
As can be seen from the enlarged view of part B in Figure A), the P-polarized light is focused closer to the birefringent plate than the S-polarized light.

For example, when the objective lens 19 is at the neutral point position, the focal position Fp of P-polarized light and the focal position Fp of S-polarized light are
If the first and second light-receiving elements 37.39 are placed on the same plane at an intermediate position with s and almost perpendicular to both lights, the two light-receiving elements 37.39 will have a circle with the same diameter. Spot light is incident. A focus error occurred and disc 2
When the distance between 1 and the objective lens 19 changes from the appropriate value,
Since the circular spot diameter changes on the first and second light receiving elements 37 and 39, a focus error signal can be detected as a difference in the circular spot diameter.

FIGS. 3A and 3B are explanatory diagrams showing an example of a light receiving element used for such detection. In the first and second light receiving elements 37.39, the light receiving surface is divided into four bands a, b, c, d or a', b', a', d', and the widths of these bands are , a=b+c=d(b=C), a'=
b' + c' = d'(b':=c'). Now, considering an ideal case, when the focus position is at the proper position, the S-polarized spot light 43s and the P-polarized spot light 43P are transmitted to the first and second light receiving elements, as shown in FIG. 3A. 37.39 are incident on the center of the light receiving surface with the same spora 1 to diameter.

On the other hand, when a focus error occurs, the spot light diameters of the spot lights 43s and 43p on the light receiving surface change as shown in FIG. 3B, for example. Therefore, a focus error signal is detected by the differential concentric circle method, and this is used as a servo signal to servo drive the objective lens. Specifically, the focus error signal is determined by the following calculation from the amount of light on the light receiving surfaces of the first and second light receiving elements 37 and 39.

(a+d+b'ri+c') -(a'+d' +b+
c) Furthermore, when a tracking error occurs, the light intensity distribution becomes biased, so a tracking signal can be obtained by the following calculation using one of the light-receiving elements using the conventional push-pull method.

(a + b) - (c + d) or (a' + b')
-(c' + d') Furthermore, when using the light receiving element 37.39 with the above configuration, the tracking error signal can be taken as the difference between the light receiving signals from the two light receiving elements 37.39, as in the following signal calculation. This reduces noise level and improves servo accuracy.

((a0b) (c0d)) ((c'+d')-
(a'+b')) Examples of the birefringent material include rutile crystal, calcite, and quartz.

Effects of the Invention According to the present invention, information light from a magneto-optical recording medium is focused through a birefringent material in a uniaxial direction, divided into S-polarized light and P-polarized light, and is Detecting the focus error signal eliminates the need for a cylindrical lens, simplifies the configuration, and facilitates weight reduction of the entire optical head.

In addition, it is no longer necessary to separate S-polarized light and P-polarized light in the vertical direction, as in the conventional method using a polarizing beam splitter, and it becomes possible to arrange the detection elements for S-polarized light and P-polarized light on the same plane. The configuration can be simplified.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention, FIG. 2A is an explanatory diagram showing the main part thereof, and FIG. 2B is an enlarged view of the vicinity of section B in FIG. 2A. FIGS. 3A and 3B are explanatory diagrams showing the light receiving element and its irradiation state with sporato light. FIG. 4 is an explanatory diagram showing a conventional example. 11... Laser diode 13... Collimator lens 15... Shaping prism 17... Beam splitter 21... Disc 23... 1
72 Wave plate 31...Condensing lens 33...
Birefringent plate 37...first light receiving element 39, second light receiving element 41...differential amplifier 43s...S
Polarized light 43p...P polarized light

Claims (1)

[Claims] 1. Information light from a magneto-optical recording medium is focused through a birefringent material made by cutting a uniaxial crystal obliquely to the optical axis, and separated into a spot light of the ordinary light component and a spot light of the extraordinary light component. A signal detection method in magneto-optical recording, characterized in that a focus error signal is detected from the difference in the diameter of the spot light.