Optical pickup and opto-magnetic signal reproducing apparatus

Abstract: An optical pickup includes a polarization component separation element (43) which is formed as a unit or single piece of birefringence material. The separation element is not laminated, obviating the need to carefully match the optical axes of the laminated pieces. Multiple reflections within the birefringent crystal which comprises the separation element improve the separation and resolution of the polarized components which are extracted from a laser beam reflected back off the recording medium surface.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an optical pickup. More particularly, the present invention relates to an optical pick-up for an opto-magnetic signal reproducing apparatus. Description of the Related Art Fig. 1 shows an optical pickup for an opto-magnetic disc apparatus which is disclosed in US-A-5,479,387 issued on December 26, 1995 in the name of Ando et al. The optical pickup 1 is composed of a plurality of optical components 2 to 10. The optical pickup 1 condenses a laser beam emitted from a laser diode 2 and reflected from a opto-magnetic disc 11, via a grating element 3, a beam splitter 4, a collimator lens 5 and an objective lens 6, to form an image on a photodetector 9 via a Wollaston prism 7 and a multi-lens 8. However, this type of optical pickup suffers from a drawback that miniaturization of the overall apparatus is difficult. This arrangement also suffers from the drawback that improvement in reliability is difficult because a plurality of optical components must be individually assembled. This renders the arrangement unsuitable for use in compact opto-magnetic disc types of reproducing apparatus wherein a small-sized highly reliable optical pickup is vital. Figs. 2A and 2B show a structure disclosed in US-A-4,873,429 issued on October 10 1989 in the name of Kume et al. and reflecting the preamble of claim 1, which has been proposed for use as an optical pickup in a compact disc player arrangement. This optical pickup 21 incorporates a laser diode LD, a crystal element 23, a semiconductor substrate 24 on which photodetectors PD1 and PD2 and the like of elements are provided. As shown, all these elements are enclosed in a housing 22. The optical pickup 21 is arranged so that an opening in the housing 22 is sealed with a transparent member 25, and so that a non-polarizing semi-transparent film 26 is provided on an inclined surface of the trapezoidally sectioned crystal beam splitter element 23. A second adhesive film 27 is disposed between the beam splitter 23 and photodetector PD1 while a third réflexion film 28, which reflects the laser light reflected in the crystal, is provided on the upper surface of the crystal beam splitter element 23. However, this optical pickup 21 cannot be used for the reproduction of opto-magnetic signals because the optical pickup 21 is a non-polarization type optical system. Document JP-A-04 038 631 further discloses an optical head device wherein a part of a laser beam emitted from a laser diode is transmitted through a half-mirror film and separated into ordinary rays and abnormal rays by a uniaxial birefringence plate. SUMMARY OF THE INVENTION It is accordingly an object of this invention to provide an optical pickup and opto-magnetic signal reproducing apparatus which are both small in size and highly reliable as compared with convention counterparts. The above and other objects are achieved by the provision of an optical pickup apparatus comprising a semiconductor substrate on which a plurality of light receiving elements are formed; a light emitting element for emitting a light beam, said light emitting element being mounted on said semiconductor substrate; and a separation prism for separating said light beam and mounted on said light receiving elements, characterized in that said separation prism is a polarizing separation prism consisting of a biaxial crystal of birefringence material and includes means for reflecting said light beam at least once in said polarizing separation prism or in the vicinity of an interface of the prism. The returned light ray reflected from the opto-magnetic signal recording medium is assuredly separated in a predetermined direction which is determined by an optical axis of the birefringence material unit. BRIEF DESCRIPTION OF THE DRAWINGS The nature, principle and utility of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters and wherein : Fig. 1 is a schematic side view showing an optical pickup used for the reproduction of the opto-magnetic signal; Figs. 2A and 2B are schematic side views showing a known optical pickup used for the reproduction of a compact disc; Fig. 3 is a schematic sectional view depicting the effect of birefringence; Fig. 4 is a schematic diagram showing an inherent polarizing direction seen from a side on which a ray is incident; Fig. 5 is a schematic diagram showing a relation between a wave-normal direction and a direction in which ray energy progresses; Figs. 6A, 6B, 7A and 7B are schematic diagrams illustrating separation of a spot group following reflection in a polarization component separation element in accordance with a first separation technique according to the invention, wherein Figs. 6A and 6B illustrate the separation of the various light spots and Figs. 7A and 7B show the disposition of photodetectors in relation to the polarization component separation element, Fig. 7A being a side view and Fig. 7B a plan view; Figs. 8A, 8B, 9A and 9B are schematic diagrams illustrating separation of a spot group following reflection in a polarization component separation element in accordance with a second separation technique according to the invention; Fig. 10 is a schematic side sectional view showing the structure of an optical pickup according to the present invention; Fig. 11 is a schematic side view showing the attachment position of the optical pickup in an opto-magnetic signal reproducing apparatus; Fig. 12 is a block diagram showing the circuit arrangement of an opto-magnetic signal reproducing apparatus according to the present invention; and Fig. 13 is a schematic diagram explaining the polarization component separation element for reflecting the ray four times within a crystal. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of this invention will be described with reference to the accompanying drawings: (1) Optical pickup (1-1) Birefringence The optical pickup according to the present invention features the use of a material which exhibits birefringence as a "polarization component separation element" wherein incident light is split and separated on the basis of its polarization components and wherein the separated components are multiply-reflected within the element to promote separation. The present invention further features the use of a unit, or single piece, of this material which exhibits birefringence to form a beam splitting prism or polarization component separation element. This structure avoids a need to laminate multiple pieces of birefringent material one upon another in a manner which requires the axial directions of the respective refraction ellipsoids, which represent the characteristic of the different pieces of birefringence material, to be carefully taken into consideration. It should be noted that the polarized light can be separated from the incident light ray as it passes into the element without being reflected inside the element. However, in such cases, it is important that each refraction direction is different at the interface depending on a difference in an electric field density vector D (that is, the difference in a wave-normal direction k). This phenomenon is depicted in Fig. 3 which shows the separation between an ordinary ray (o-ray) and an extraordinary ray (e-ray without walk-off). More specifically, Fig. 3 shows a situation wherein converging light (in air) is incident on an uniaxial crystal YVO 4 (ne=2.1893 and no=1.9734 when λ = 780nm). It.will be noted that the o-ray represents a light ray which follows Snell's law while the e-ray represents a light ray which does not obey this law. The embodiments of the invention feature the incident ray being reflected once or more in the separating element, and are such that birefringence constitutes a vital feature. Birefringence refers to a phenomenon wherein a vector S (equal to an electric field vector E of light wave x magnetic field vector H of the light wave) of the ray (a direction in which an optical energy progresses) is different when a vibration direction of the electric field density vector D is different even if a wave-normal vector k in the crystal is the same. Birefringence is explained in more detail below. The refraction of a given light ray which progresses through birefringent material can be represented using the lengths of two axes of an ellipsoid which are formed when the refraction ellipsoid is cut with a plane surface passing through the origin so as to be perpendicular to a wavefront direction vector k of the ray. The axial direction at this time is a direction of an electric field density vector D which corresponds to the refractive direction (see Fig. 4). When polarized light is allowed to be incident at an angle of 45°, and is subsequently separated, the difference between the vectors D1 and D2 (see Fig. 4) can be used to derive the opto-magnetic signal. The difference in the refraction is the difference of the wave-normal vector k (see Fig. 5). However, since the refractive index changes with the difference of the wave-normal vector k, the change in the refractive index needs to be taken into consideration. The wave-normal vector k which is generated with the difference in the refraction can be determined in this manner with a set of the refractive index and an inherent polarizing vector D by using the fact that the wave-normal vector k follows Snell's law. For reference, the difference in the wave-normal vector k is sufficient only for separating the polarization components. However, when an attempt is made to grasp assuredly how the separated ray progresses, it is required to consider that the wave-normal vector k is shifted, or undergoes walk-off, from the ray vector S. The ray vector S is calculated using the following formula. Initially, an electric field vector E is obtained as ε -1 D by using the fact that the electric field vector E in the birefringent material satisfies the equation D = εE, and that D and ε are already known (since the refractive index ellipsoid is already known, ε is also already known from the equation n 2 = ε/ε 0 . Further, for the magnetic field vector H and the flux density vector B, seen in Fig. 5, the equation H/ /B/ /k x D is established. Since the direction of the magnetic field vector H is known, the direction of the ray vector S is determined from the previously determined electric field vector E using S = E x H. This equation is established at the time of reflection. The birefringent material separates generally in two directions at the interface in a manner similar to the aforementioned case because the birefringent material exhibits different refractive indices with respect to the ray wave surface vector k and the inherent vector D. With respect to the aforementioned point, the e-ray (with walk-off) in Fig. 3 shows a case where the ray is tracked. By way of reference, Fig. 3 is a view showing the situation wherein reflection is not induced within the crystal. (1-2) Polarization component separation element (Y-direction separation type) Next, the structure of a polarization component separation element according to this aspect of the invention will be explained. The polarization component separation element is set so that an optical axis of a uniaxial crystal is on a surface perpendicular to a normal line of the reflection surface as shown in Fig. 6A. The polarization component separation element is used in a manner wherein the incident ray is reflected at least once within the crystal. In this embodiment, for example, "oeo" designates the ingredient of ray of " o rdinary ray → e xtraordinary ray → o rdinary ray", separated by double reflection. Under these conditions, a P-polarization component and a S-polarization component are separated in the direction of the Y-axis as shown in Fig.6B. Figs.7A and 7B show how, under the above conditions, the light spots focus on light photodetectors PD1, PD2 and PD3 and wherein spots "ooo" and "eee", which are disclosed in more detail below condense in an obliquely separated manner on serially arranged photodetectors PD2 and PD3. In the case of this polarization component separation element, the optical properties of the crystal are suppressed so that neither a phase difference nor a reflection rate difference are generated in the P-polarization component and the S-polarization component before and after reflection. When the optical properties are suppressed, the relation between an inherent polarization component vector D, and the S-polarization component, and the relation between the inherent polarization component vector D and the P-polarization component are conserved before and after the reflection. Besides, the inherent refractive indices are consistent before and after the reflection. consequently, the separation of the ray on the reflection surface is not achieved and only the two spots "ooo" and "eee" are completely separated and respectively focussed on the photodetectors PD2 and PD3. For reference, even when the phase difference and the reflection rate difference are generated and the spot is separated before and after the reflection, the first separated light spot is somewhat expanded by the reflection process, but the second light spot is clearly separated into two spot groups. When the differential output obtained from these two spots is determined, the opto-magnetic signal can be obtained. It should be noted that the aforementioned polarization component separation element has a uniaxial crystal structure. In the case where the crystal has a biaxial crystal structure, a direction which corresponds to the refractive index having a larger difference of the refractive index from the intermediate refractive index out of the refractive directions of the crystal is set in a surface perpendicular to the normal line of the reflection surface. Also in this case, the incident ray is reflected at least once within the crystal. By way of reference, two approximate refractive indices out of three are set as "n o " and the remaining one is set to "n e ", so that the refractive indices can be easily coped with. Further, in this structure, if a direction which corresponds to the intermediate refractive index out of the three of the crystal is set in a direction parallel to a normal line of the reflection surface, the difference between "n o " and "n e " is made ostensibly larger (the separation angle of the ray can be made larger) so that a beam splitting function which is usable in practice can be achieved. In the case of biaxial crystals, an effect can be obtained which is similar to one obtained in the case of the uniaxial crystal. Consequently, the polarization component separation element can be selected from a wide selection of crystal material, which thus enables a large design freedom. (1-3) Polarization component separation element (X-direction separation type) It should be noted that the above description has been given with respect to a polarization component separation element in which a spot is subject to further separation in the Y direction. However, in accordance with another aspect of the invention described below, a second separation occurs in the X direction. As shown in Fig. 8A, this polarization component separation element is used so that an optical axis of an uniaxial crystal is set in a surface parallel to a normal line of an incident surface of a primary ray incident on the crystal and a normal line of the reflection surface and an incident ray is reflected at least once in the crystal. At this time, the P-polarization component and the S-polarization component are separated in the direction of the X-axis with the result depicted in Fig. 8B and Figs. 9A and 9B. In this manner, the separation direction of the polarization components can be changed only by changing the position of the optical axis even when the same uniaxial crystal is used, enabling latitude in the design of the optical system using a detection method of a servo error and PD separation method. Incidentally, if a biaxial crystal is used in the polarization component separation element, a direction which corresponds to the refractive index having a larger difference of refractive index from the intermediate refractive index out of the refractive index directions of the crystal, is set in a surface parallel to a normal line of the incident surface of a primary ray incident on the crystal and parallel to a normal line of the reflection surface. Even with such a procedure, a similar effect can be obtained as described above. Further, in this structure, a direction which corresponds to the intermediate refractive index out of the three refractive indices of the crystal is set so that the direction becomes parallel to both of an incident surface of a primary ray incident on the crystal and a normal line of the reflection surface thereof. Thereby, the difference between "n o " and "n e " is ostensibly enlarged so that the ray separation angle is enlarged with the result that a polarizing beam splitter which functions similarly to those in actual use can be achieved. Further, in the case of such biaxial crystals, an effect can be obtained which is similar to one obtained with a uniaxial crystal. Consequently, a wide selection of crystal material and freedom in design with respect to the polarization component separation element is possible. Since no attention needs to be paid to the precision with which lamination is carried out, a highly precise polarization component separation element can be readily manufactured. In addition, as the polarization component separation element consists of a single piece of birefringence material, the polarization component separation element does not exhibit any change with the passage of time. (2) Opto-magnetic signal reproducing apparatus Fig. 10 shows an optical pickup which uses the polarization component separation element described in the previous section. Furthermore, Fig. 11 shows an optical system of an opto-magnetic signal recording and reproducing apparatus using the optical pickup. Fig. 12 shows, in block diagram form, a circuit arrangement suitable for use with the polarization component separation element embodiments of the invention. As shown in Fig. 10 the optical pickup 41 comprises a laser diode LD and a polarization component separation element 43 on a semiconductor substrate 42 on which photodetectors PD1 through PD3 are integrated. The polarization component separation element 43 shown in this figure is a crystal element of the type described above and in which the ray is reflected twice within the crystal. Here, the polarization component separation element 43 has a trapezoidal-shaped section, and is arranged so that the laser light is reflected by the polarizing beam splitter film 44 which is formed on an inclined surface of the element 43 opposite the laser diode LD, so as to be irradiated onto a recording surface of the opto-magnetic disc. By way of reference, the polarizing beam splitter film 44 includes Ts=35% and Tp=70%. The returned light which is reflected from the recording surface of the opto-magnetic disc is introduced into the crystal 43 by way of the polarizing beam splitter film 44 so that a part of the returned light is directed to the photodetector PD1 on the bottom of the polarization component separation element 43 via a semi-transparent film 45. On the other hand, the laser light which is reflected off the bottom of the polarization component separation element 43 is directed to the reflection film 46 provided on the upper surface of the polarization component separation element 43 to be again reflected and condensed onto the photodetectors PD2 and PD3 at the bottom of the polarization component separation element 43. Electric signal outputs from these photodetectors PD1 through PD3 are current-to-voltage converted via an I/V amplifier 53 and then supplied to a matrix amplifier 54 (see Fig. 12). At this time, the matrix amplifier 54 outputs a difference in the output signals obtained from the photodetectors PD2 and PD3 as an opto-magnetic signal S3, and also outputs the sum of the output signals obtained from the photodetectors PD1 through PD3 or at least one of the output signals as pit signal S4. The matrix amplifier 54 calculates and outputs a focus error signal S2 from at least one of the output signals of the photodetectors PD2 and PD3 to calculate and output a tracking error signal S1 from at least one of the photodetectors PD1 through PD3. A demodulating and decoding circuit 55 reproduces information recorded in the opto-magnetic disc 54 on the basis of the input opto-magnetic signal S3 and the pit signal S4, and outputs this information to a processing circuit at the rear stage. In this embodiment the polarization component separation element 43 can take the form of LN (LiNbO 3 , uniaxial crystal) or a KTP (KTiOPO 4 , biaxial crystal). According to the structure mentioned above, an optical pickup can be miniaturized and made compact by using an optical pickup which utilizes a polarization component separation element 43 and a light emitting element which satisfy the conditions described above. Reduction in adjusting processes and improvement in reliability can be achieved by this type of optical pickup. Further, a small-size opto-magnetic signal reproducing apparatus compared with the conventional counterparts can be realized by utilizing such a small-size optical pickup. (3) Alternative embodiments In the above-disclosed embodiments, the light ray which is incident on the polarization component separation element is reflected only twice within a birefringent crystal. However, the number of reflections is not limited to two, the number of reflections can be three or more. When the number of reflections is increased, the spot separation can be improved along with the manufacturing tolerance. Furthermore, in this case, the polarization component separation element can be made thin, while adequate spot separation can still be achieved. By way of example, Fig. 13 shows a polarization component separation element 43' in which the number of reflections in the crystal is four, as shown at surface reflections a, b, c, d. The effect of the increased number of reflections on the separation resolution is self-evident from the lower portion of this figure. The aforementioned embodiments have been described in connection with an optical pickup for use in an opto-magnetic disc. However, the present invention is not limited to this use and the present invention can be also applied to an optical pickup used for reading information from a rewritable compact disc (MO-CD) or an optical recording medium such as an optical tape. Similarly, the present invention is not limited to use with opto-magnetic disc reproducing apparatus and can be applied to an optical apparatus such as an optical tape recorder. As described above, in accordance with the present invention, by forming a polarization component separation element using a single piece (unit) of birefringent material, precise lamination of multiple elements is not necessary. While the present invention has been described in connection with the preferred embodiments, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention which is limited only by the appended claims.

Claims: An optical pickup apparatus comprising: a semiconductor substrate (42) on which a plurality of light receiving elements (PD1-PD3) are formed; a light emitting element (LD) for emitting a light beam, said light emitting element being mounted on said semiconductor substrate (42); and a separation prism (43) for separating said light beam and mounted on said light receiving elements (PD1-PD3), characterized in that said separation prism (43) is a polarizing separation prism consisting of a biaxial crystal of birefringence material and includes means (46) for reflecting said light beam at least once in said polarizing separation prism or in the vicinity of an interface of the prism. The optical pickup apparatus according to claim 1, characterized in that a direction which corresponds to an intermediate refractive index out of the three refractive index directions of said biaxial crystal is set in a direction parallel to a normal line of the reflection surface of said reflection means (46). The optical pickup apparatus according to claim 1, characterized in that said polarizing separation prism (43) has a first surface for receiving said beam, a second surface for transmitting a part of said beam and for reflecting another part of said beam, and a third surface for reflecting said beam, said second surface being disposed over said light receiving elements (PD1-PD3). The optical pickup apparatus according to claim 3, characterized in that said first surface is opposite said light emitting element (LD). The optical pickup apparatus according to claim 3, characterized in that said first surface supports a polarizing beam splitter film (44). An opto-magnetic signal reproducing apparatus comprising an optical pickup apparatus according to any one of claims 1 to 5, and a signal processing circuit (53-55) for reproducing an opto-magnetic signal on basis of output from said light receiving elements (PD1-.PD3).

Inventors: HATTORI MASATO, MATSUMOTO SHUICHI, NISHI NORIAKI, SAITO KIMIHIRO, HATTORI, MASATO, MATSUMOTO, SHUICHI, NISHI, NORIAKI, SAITO, KIMIHIRO
Applicants: SONY CORP, SONY CORPORATION
Application year: 1996
Main classification: G11B7
IPC codes: G11B7, G11B11