OPTICAL HEAD

Abstract: An optical head for recording, reproducing or erasing information in a magnetooptical memory device by irradiating a laser beam to a storage medium (6). A means (10) is inserted in an optical system of a signal detecting section for separating an outer portion of a light flux at least partially.

Description: BACKGROUND OF THE INVENTION The present invention relates to optical heads for recording, reproducing or erasing information on optical memory devices by irradiating a storage medium with a laser beam. The invention further relates to magneto-optical heads for recording information on memory devices or reproducing or erasing the recorded information by irradiating a storage medium which is composed of a magnetic thin film with a laser beam. It has recently been observed that the use of an optical disk or magnetooptical disk as a large-capacity memory device is attracting a great deal of attention. The substrate of such optical or magnetooptical disks are generally composed of glass or plastic material. Normally acrylic or polycarbonate plastic is employed due to the remarkable merits of these materials in both productivity and handling facility. However, such plastic substrates have some disadvantages in that, as compared with a glass substrate, double refraction is prone to occur (particularly in polycarbonate) and in that the optical characteristics are inferior. The occurrence of double refraction is especially liable to be induced in plastic substrates produced by injection molding. For reproducing signals in any optical disk of reproduction-only or, DRAW types or of erasable types that are based on phase transition, a technique of eliminating return light to a laser diode (LD) through an optical isolator is adopted by using a combination of a polarized beam splitter (PBS) and a quarter wavelength plate. The light emitted from the laser diode is passed through the polarized beam splitter so that P-wave light alone is linearly polarized and, in a structure where the optical axis of the quarter wavelength plate is set to form an angle of 45 degrees with the oscillation surface, the linearly polarized light is converted into circularly polarized light before being incident upon the substrate. The reflected light is passed through the quarter wavelength plate again to become S-wave linearly polarized light, which is then introduced to a photo detector by the beam splitter without being returned to the laser diode. However if double refraction takes place in the substrate, the light reflected from the substrate is elliptically polarized and therefore fails to become linearly polarized light even after passage through the quarter wavelength plate. Return light to the laser diode is therefore generated and the quantity of light introduced to the photo detector is reduced. Meanwhile, in magnetooptical disks, signal reproduction is performed by the use of linearly polarized light. In these devices where double refraction in the substrate of disk exists, the linearly polarized light is harmfully affected by elliptical polarization to eventually cause considerable deterioration of the reproduced signal quality. SUMMARY OF THE INVENTION It is an aim of the present invention to alleviate the foregoing problems. According to the present invention, there is provided an optical head for recording, reproducing or erasing information in a thin film data storage medium carried on a light-transmissive substrate, using a laser beam which irradiates the medium, the head having first and second light sensing means arranged to receive light of a beam reflected from the medium, and masking light filter means arranged to separate an outer portion of said reflected beam from an inner portion, wherein the first light sensing means receives said inner portion and derives an information signal therefrom, and said second light sensing means receives said outer portion and derives a control signal therefrom. By insertion of such filter in the optical system of the signal detecting section to separate an outer portion of a light flux at least partially, elliptically polarized light generated in the outer portion of the light flux is cut so circularly polarized light in the central portion is taken out to an optical signal detection system, whereby reproduced signals of satisfactory quality can be obtained. BRIEF DESCTIPTION OF THE DRAWINGS: The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: Fig. 1 is a schematic block diagram of a known reproducing optical system. Fig. 2 is a plan view showing the shape of a light cutting filter; Fig. 3 graphically shows the characteristics representative of reduction caused in a quantity of return light by the light cutting; Figs. 4 (a) and 4 (b) are a side view and a plan view respectively showing a partial vicinity of an optical disk; Fig. 5 is a schematic block diagram of a reproducing optical system in the present invention for a magnetooptical memory device; Fig. 6 (a) illustrates how reflected light is polarized when linearly polarized light is incident; Fig. 6 (b) is a side view showing a partial vicinity of a magnetooptical disk; Fig. 7 graphically shows curves representing the degree of modulation; and Fig. 8 shows the structure of a reflecting mirror. DETAILED DESCRIPTION OF THE INVENTION: In advance of describing preferred embodiments of the present invention, an explanation will be first given on double refraction in a disk substrate. Double refraction occurring in the substrate of an optical or magnetooptical disk can be classified into two types, one in the direction of the substrate plane and the other in the direction perpendicular thereto. The former 'in-plane' double refraction occurs when there is a difference between the refractive indices in two mutually different directions within the substrate plane. It affects polarized light equally at any position of the light flux. The latter, perpendicular double refraction, occurs when there is a difference between the refractive index in the direction perpendicular to the substrate and the refractive index in the direction of the plane. Its effect on the polarized light varies depending on the position of the light flux. Since the central light flux portion is incident substantially perpendicularly upon the substrate, the light flux is affected merely by the 'in-plane' double refraction while being almost unaffected by the perpendicular double refraction. In contract therewith, the outer light flux portion, incident obliquely upon the substrate, is affected by the perpendicular double refraction as well as by the in-plane double refraction. When the direction of the incident polarized light is parallel or perpendicular to the plane of incidence, the linearly polarized light remains unchanged. In a polycarbonate substrate produced by injection molding, it is usual that 'in-plane' double refraction is of the order of 10⁻⁶ while perpendicular double refraction is of the order of 10⁻⁴ which is greater by two figures than the value of the 'in-plane' double refraction. Therefore, elimination of the undesired polarization caused by the double refraction in the substrate is attained principally by improving the perpendicular double refraction. It is supposed here that two orthogonal directions in the substrate plane are defined as an x-axis and a y-axis respectively, and a direction perpendicular to the substrate is defined as a z-axis. Then the refractive index n s in the direction perpendicular to the plane of incidence is expressed as n s = 1/√ sin²α/n x ² + cos²α/n y ² 1 where n x , n y and n z are refractive indices in the x-axis, y-axis and z-axis directions respectively; α is the angle formed by the x-axis with the plane of incidence; and ϑ is the angle of refraction. Meanwhile, the refractive index n p in the direction of the plane of incidence is expressed as n p = 1/√ cos²ϑ(cos²α/n x ² + sin²α/n y ²) + sin²ϑ/n z ² 2 Assume now that light linearly, polarized in the x-axis direction is incident upon a disk with the values of n s and n p calculated from Eqs. 1 ○ and 2 ○ respectively. Figs. 4 (a) and (b) show the state of polarization of reflected light at individual positions of the light flux in an optical disk. And Figs. 6 (a) and (b) show such state of polarization in a magnetooptical disk. First a description will be given of an optical disk which serves as an optical memory device. Fig. 4 (a) is a side view of a partial vicinity of an optical disk, where there are shown a condenser lens a , a plastic substrate b , a recording medium c , and a light beam d . Fig. 4 (b) is a plan view of Fig. 4 (a) and shows the state of polarization of reflected light at individual positions of a light flux. In Fig. 4 (b), the circularly polarized light remains unchanged in the central light flux portion, but the ellipticity becomes gradually smaller with displacement toward the outer portion and thereby causes an increase in the quantity of return light to the laser diode, whereby the quantity of light introduced to the photo detector is reduced. In Figure 1, there are shown a laser diode (LD) 1, a collimator lens 2, a polarized beam splitter 3, a quarter wavelength plate 4 with its optical axis set at an angle of 45 degress, a condenser lens 5, an optical disk 6 consisting of a polycarbonate substrate 6a and a recording medium 6b, a half mirror 7, and a photo detector 8. In the above structure, reflected light from the optical disk 6 is divided by the half mirror 7 into a component for an optical servo system and a component for an optical signal detection system, and the light flux of the latter component is fed to the photo detector 8. The output of the detector 8 is then obtained as a reproduced signal. A light cutting filter 10 composed of a nontransparent material such as metal, paper or plastic is inserted between the half mirror 7 and the photo detector 8 so as to cut an outer portion of the light flux at least partially. The light cutting filter 10 serves to cut the elliptically polarized portion of the light flux. Fig. 2 illustrates the shape of such light cutting filter 10. It is formed with a circular through-hole 11 having a diameter smaller than the outer diameter A of the light flux represented by a dotted line at the center, so as to cut an outer portion of the light flux where the ellipticity is small. Generally the quantity of the signal light is reduced when the reflected light from the optical disk 6 is elliptically polarized, and the quantity of the return light to the laser diode 1 is increased. As a result, noise included in the signal is increased to eventually deteriorates the signal-to-noise ratio. Fig. 3 graphically shows changes in both the signal light S from the polycarbonate substrate 6a and the return light R to the laser diode 1, wherein the area ratio of the through-hole 11 to the light beam is plotted along the abscissa, and the quantity of the signal light is plotted along the ordinate on the basis of a value 1 which is obtained in an exemplary case of using a glass disk. The conditions in Fig. 3 are such that the lens has a numerical aperture (NA) of 0.5, and the poly carbonate substrate 6a has a refractive index of n o = 1.585 in the x-axis direction, a refractive index of (n o + 5 x 10⁻⁶) in the y-axis direction and a refraction index of (n o - 6 x 10⁻⁴) in the z-axis direction. From the above graphic representation, it is seen that the return light to the laser diode 1 is remarkably reduced in quantity by cutting the light flux portion where the ellipticity is small. In this case the signal light S is also reduced in quantity, but its absolute value is merely of the order of several µw which is substantially free from causing deterioration of the signal-to-noise ratio. A device of this type is described in co-pending application EP-A-0 241 007 which is available as prior art only by virtue of Article 54(3). In the apparatus described above, the light flux is partially cut by the filter 10 after being divided by the half mirror 7 into a component for the servo system and a component for the signal detection system. However, a similar effect may be obtained by dividing the light flux, instead of such partial cutting, into a portion with a small ellipticity for the servo system and using the remaining portion for the signal detection system, and still it is possible to diminish reduction of the signal light in quantity. As described hereinabove, by using the optical head of the present invention, reproduced signals of high quality can be obtained even by use of a plastic substrate where double refraction is prone to occur. The magnetooptical head of the invention is equipped with a means for separating a light flux, which is reflected from a recording medium, into its central portion and remaining portion, wherein the central light flux portion is used as a magnetooptical signal while the remaining light flux portion is used as an optical servo signal. When a signal is actually read out from a magnetooptical disk, a laser light beam is condensed through a substrate by a condenser lens having a numerical aperture (NA) of 0.5 to 0.6. In this stage, if double refraction occurs in the substrate, the linearly polarized light incident thereon is elliptically polarized during its passage through the substrate. Fig. 6 (b) is a side view showing a partial vicinity of a magnetooptical disk 105, and Fig. 6 (a) is a plan view thereof showing how a reflected light flux is polarized at individual positions. In Fig. 6 (a), linearly polarized light remains unchanged on the axis, while the ellipticity becomes gradually greater toward the outer portion from the axis in the direction to form an angle of 45 degrees, hence deteriorating the reproduced signal quality. Therefore, high-quality signal reproduction can be attained by using merely the central portion of the light flux as a magnetooptical signal. Fig. 5 is a schematic block diagram of a reproducing optical system in a magnetooptical head of the present invention. In this block diagram, there are shown a laser diode 201, a collimator lens 202, a polarized beam splitter 203, a condenser lens 204, a magnetooptical disk 205 having a magnetic thin film 205a as a storage medium, a reflecting mirror 206, an analyzer 207, photo detectors 208 and 209, a servo lens 210, and a quartered light receiver 211. According to the present invention, a central portion of a light flux reflected from the magnetooptical disk 205 is passed through the reflecting mirror 206 to be used as a magnetooptical read signal, while the remaining outer portion of the light flux is reflected by the mirror 206 to be used as an optical servo signal. Fig. 7 graphically shows the result of calculation of the modulation degree. It is seen therefrom that, if 30 percent or so of the reflected light is used as a magnetooptical signal, then the modulation degree becomes about 80 percent of the reference value in the case without any double refraction and is therefore sufficient for practical operation. In this graphic representation, r₀ is the diameter of a light flux incident upon the reflecting mirror 206, and r₁ is the diameter of a light flux passed through the opening 206a in the reflecting mirror 206. (Refer to Fig. 8) As described hereinabove, according to the magnetooptical head of the present invention, high signal quality can be attained even by the use of a plastic substrate. Although described in relation to a magnetooptical disk, the invention may also be applied to an optical disk system.

Claims: An optical head for recording, reproducing or erasing information in a thin film data storage medium (205a) carried on a light-transmissive substrate (205), using a laser beam which irradiates the medium, the head having first and second light sensing means arranged to receive light of a beam reflected from the medium, and masking light filter means (206) arranged to separate an outer portion of said reflected beam from an inner portion, wherein the first light sensing means (208,209) receives said inner portion and derives an information signal therefrom, and said second light sensing means (210,211) receives said outer portion and derives a control signal therefrom. An optical head according to Claim 1 where said masking light filter means (206) comprises a partially reflecting mirror. An optical head according to any one of claims 1 and 2 wherein the optical head is a magneto-optical head and wherein the thin film data storage medium is magnetic thin film (205a).

Inventors: DEGUCHI, TOSHIHISA, KATAYAMA, HIROYUKI, MIEDA, MICHINOBU, MIYAKE, TOMOYUKI, MURAKAMI, YOSHITERU, OHTA, KENJI, TAKAHASHI, AKIRA, VAN, KAZUO
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1987
Main classification: G11B7
IPC codes: G11B7, G11B11