Optical apparatus

Abstract: An optomagnetic recording polarization optical apparatus comprises a laser diode (86), first and second photodetectors (87,88), a prism (91), a first polarization beam splitter (92), a λ/2 plate (94), a second polarization beam splitter (95), fixed on a semiconductor substrate (84,85).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a pickup structure for optically reading recorded data from a recording medium. Description of the Prior Art In an optical disk player or the like, as shown in Fig. 1A, a light-emitting-receiving composite element called a laser coupler 11 is used in an optical pickup. In the laser coupler 11, photodiodes 13 and 14 are formed on an Si substrate 12, and a prism 15 is fixed on the photodiodes 13 and 14 by an adhesive. A submount 16 serving as another Si substrate is fixed on the Si substrate 12 by soldering or the like. A photodiode 17 is formed on the submount 16, and a laser diode 21 is fixed on the submount 16 by soldering or the like. In order to reproduce data recorded on a optical disk 22 using the laser coupler 11, part of light 23 emitted from the laser diode 21 is reflected on a surface 15a of the prism 15, transmitted through an objective lens 24, and incident on the optical disk 22. The part of the light 23 which is reflected by the optical disk 22, transmitted through the objective lens 24, and incident on the surface 15a of the prism 15 is refracted and enters into the prism 15. The first half of the light 23 which enters into the prism 15 is incident on the photodiode 13, the second half reflected by the photodiode 13 is totally reflected by a surface 15b of the prism 15, and is incident on the photodiode 14. The photodiode 13, as shown in Fig. 1B, is divided into three parts A, B and C, and the photodiode 14, as shown in Fig. 1C, is divided into three parts D, E and F. An output obtained from these photodiodes 13 and 14 and expressed by: (A + C + E) - (B + D + F) is used as a focus error signal. Therefore, as described above, 50% of the light 23 which enters into the prism 15 through the surface 15a must be incident on the photodiode 13, and the remaining 50% must be reflected by the photodiode 13. For this reason, in the conventional pickup used in an unpolarization optical system recording scheme such as an optical disk, as shown in Fig. 3, an optical semitransparent film 25 (half mirror) having a reflectance of 50% is deposited on a portion of the prism 15 opposite to the photodiode 13. Note that an SiO 2 film 26 serving as a protection film is formed on the surface of the Si substrate 12, and an adhesive layer 27 is interposed between the SiO 2 film 26 and the optical semitransparent film 25 on the photodiode 13. Furthermore, the photodiode 17 receives the light 23 emitted from the rear surface of the laser diode 21 to perform automatic power control. An unpolarization optical system recording scheme can be used for only reading data. As a recording medium capable of repetitively recording and erasing data, an optomagnetic disk serving as a polarization optical system recording scheme is known. A basic optical system of the above optomagnetic disk is shown in Fig. 2. Multi-mode laser light emitted from a semiconductor laser 46 passes through a collimating lens 45 and a beam shaping prism 44 and is incident on a special beam splitter 43. In the special beam splitter 43, 1 ○ the linearly polarized component of the laser light is highly purified, and 2 ○ weak signal light is separated from the laser light. A light-receiving optical system splits light reflected by the special beam splitter 43 into light components to a data signal detecting system and a focus tracking control signal detecting system, and the light component incident on the data signal detecting system is guided to a differential optical system. In addition, the light component incident on the focus tracking control signal detecting system is guided to a focus error signal detecting optical system and to a tracking error signal detecting optical system. As a conventional pickup for recording and reading data on/from the above optomagnetic disk, a pickup obtained by assembling parts such as a polarization beam splitter and fixing them with an adhesive or the like is known. The conventional pickup obtained by assembling these parts does not easily have a small size and a small thickness. A pickup used in a polarization optical system recording scheme must have a structure in which a laser coupler obtained by integrating a laser diode, a microprism and the like is formed to equally split light which enters into the prism into light components to two or more photodiodes. A further prior art device is known from EP 0 439 876 A. OBJECTS AND SUMMARY OF THE INVENTION An object of the present invention is to provide a laser coupler obtained by integrating a laser diode, a microprism and the like to form a pickup using a polarization optical system recording scheme to solve difficulty in realizing a compact, small pickup caused by assembling and adhering conventional parts. The invention is defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiments of the invention, however, the drawings are not intended to imply limitation of the invention to a specific embodiment, but are for explanation and understanding only. Fig. 1A is a side view showing a conventional structure, and Figs. 1B and 1C are plan views showing a photodiode of the conventional structure in Fig. 1A; Fig. 2 is a view showing the structure of a conventional pickup of an optomagnetic disk which is not integrated with a laser coupler; Fig. 3 is a side view showing another prior art; Fig. 4 is a side view showing an embodiment of the present invention; Fig. 5 is a side view showing a positional relationship between an optomagnetic disk and the optical device of this embodiment; Fig. 6 is a side view for explaining light emitted from a laser diode in this embodiment; Fig. 7 is a side view showing a part of this embodiment of the present invention; Fig. 8 is a side view showing a part of a modification of this embodiment; Fig. 9 is a side view showing a part of another modification of this embodiment; and Fig. 10 is a side view showing a part of a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below. As shown in Fig. 5, in a pickup for reading an optomagnetic recording signal from an optomagnetic disk 82, an optomagnetic recording laser coupler 81 is constituted by integrating optical elements other than an objective lens 83. As shown in Fig. 4, the optomagnetic recording laser coupler 81 includes an Si substrate 84. A submount 85 serving as another Si substrate is fixed on the Si substrate 84 by a silver paste or the like, and a laser diode 86 is fixed on the submount 85 by soldering or the like. Two photodiodes 87 and 88 are formed on the Si substrate 84 at positions different from the position where the submount 85 is fixed. The photodiode 87 is constituted like the diode 14 shown in Fig. 1B by three parts A to C divided in a direction perpendicular to the alignment direction of the photodiodes 87 and 88, and the photodiode 88 is constituted by three parts D to F divided in the direction of the parts A to C. A prism 91 having a parallelogram side section and a height of about 1 mm is adhered to the photodiodes 87 and 88. A polarization beam splitter 92 which reflects 100% of an S-polarized light component and transmits 100% of a P-polarized light component is formed on an inclined surface of the prism 91 opposite to the photodiode 87, and a total reflection mirror 93 is formed on the inclined surface opposite to the inclined surface on which the polarization beam splitter 92 is formed. A λ/2 plate 94 and a polarization beam splitter 95 which reflects an S-polarized light component within a range of 1 to 100%, in a normal state, 20 to 80%, and transmits 100% of a P-polarized light component are integrally fixed on the upper surface of the prism 91 above the photodiode 87. Note that the reflectance for an S-polarized light component in the polarization beam splitter 95 is determined by a ratio of the intensity of the S-polarized light component returned from the optomagnetic disk 82 to the intensity of the P-polarized light component serving as a signal component, i.e., the degree of enhancement of the signal. In addition, a stray light protecting film 96 serving as a light-absorbing film or a reflecting film is formed on the upper surface of the prism 91 at a position wherein the λ/2 plate 94 is not formed. All the above optical elements fixed on the Si substrate 84 are sealed in a package 97. A transparent window (not shown) is formed at a portion of the package 97 above the polarization beam splitter 95. In this embodiment, as shown in Fig. 6, light 98 emitted from the laser diode 86 is an S-polarized light component having an electric field component polarized perpendicularly to the drawing surface in Fig. 6. This light 98 is incident on the polarization beam splitter 95, and only part of the light 98 is reflected upward. The light 98 reflected by the polarization beam splitter 95, as shown in Fig. 5, is focused on the optomagnetic disk 82 by the objective lens 83, and the polarization plane of the light 98 is rotated by about ±0.3° in accordance with a magnetic signal recorded on the optomagnetic disk 82. As a result, although 100% of the light 98 incident on the optomagnetic disk 82 is an S-polarized light component, 0.5% of the light 98 reflected by the optomagnetic disk 82, i.e., 0.5% of the light 98 returned from the optomagnetic disk 82, is a P-polarized light component as a signal component. The P-polarized light component has an electric field component polarized parallel to the drawing surface in Fig. 5. The light 98 returning from the optomagnetic disk 82 is incident on the polarization beam splitter 95 again, 100% of the P-polarized light component serving as the signal component of the light 98 is transmitted through the polarization beam splitter 95, and only part of the S-polarized light component of the light 98 is transmitted through the polarization beam splitter 95. The light 98 transmitted through the polarization beam splitter 95 is incident on the λ/2 plate 94, and the polarization direction of the light 98 is rotated through 45° by the λ/2 plate 94. As shown in Fig. 7, of the light 98 which has the polarization direction rotated through 45° by the λ/2 plate 94, is incident on the prism 91, and reaches the polarization beam splitter 92, the P-polarized light component is transmitted through the polarization beam splitter 92, and the S-polarized light component is reflected by the polarization beam splitter 92. The light 98 transmitted through the polarization beam splitter 92 is incident on the photodiode 87. The light 98 reflected by the polarization beam splitter 92 is focused in the prism 91 once, is reflected by the total reflection mirror 93, and is incident on the photodiode 88. That is, components of the light 98 which are not modulated by the optomagnetic disk 82 are split to the photodiodes 87 and 88 at a ratio of 1 : 1. For this reason, as in the case of Fig. 1B, a focus error signal is obtained by differential detection expressed by: FE = (A + C + E) - (B + D + F) In addition, an optomagnetic signal can be obtained by differential detection expressed by: RF = (A + B + C) - (D + E + F) Figs. 8 and 9 show two modifications of the third embodiment. In each of these modifications, the polarization beam splitter 92 which reflects 100% of an S-polarized light component and transmits 100% of a P-polarized light component is arranged, and the light 98 returned from the optomagnetic disk 82 are split into light components before and after the focal point of the light 98. Therefore, a focus error signal can be differentially detected, and the same function as that of the third embodiment can be obtained. Fig. 10 shows a second embodiment. According to the second embodiment, a photodiode 101 for directly monitoring an emission output from a laser diode 86 to control the amount of a current injected in the laser diode 86 is formed on an Si substrate 84, and a half mirror 102 for guiding light 98 to the photodiode 101 is arranged. The second embodiment has almost the same arrangement as that of the embodiment shown in Fig. 4 except for the abovedescribed arrangement. Note that the photodiode 101 may be adhered to a polarization beam splitter 95 or a prism 91. In this case, the half mirror 102 is not required. As has been described above, according to the present invention, in a pickup used in a polarization optical system recording scheme, a laser coupler in which the amounts of light incident on two or more photodiodes are properly controlled to preferably perform differential detection can be obtained.

Claims: An optical apparatus for reading data from an optomagnetic recording medium, said apparatus comprising: a substrate (84,85); first and second photodetectors (87,88) formed on said substrate (84); a prism (91) fixed on said substrate (84), said prism having a reflecting face (93) arranged to reflect light propagating inside the prism toward said second photodetector (88); a first polarization beam splitter (92) fixed either on a further face of said prism or on said substrate (84) and located such that a component of light passing through the prism and impinging on said beam splitter is reflected toward said prism reflecting face (93) and directed thereby toward said second photodetector (88), whereas a further component of said light is transmitted through said beam splitter and impinges onto said first photodetector (87); a λ/2 plate (94) fixed on a further face of said prism (91), such that light passing through said plate and entering said prism may impinge onto said first polarization beam splitter (92); a second polarization beam splitter (95) fixed on the side of said λ/2 plate which is opposite to said prism (91); a laser diode (86), fixed on said substrate (85), for emitting light having a linearly polarized component toward said second polarization beam splitter (95), the arrangement being such that a component of light from said laser diode is reflected by said second polarization beam splitter (95) away from said substrate and may impinge on said optomagnetic recording medium, whereas a component of light reflected back from said medium is transmitted through said second polarization beam splitter (95) and said λ/2 plate (94), and impinges onto said first polarization beam splitter (92). An optical apparatus according to claim 1, further comprising: a third photodetector (101) formed on said substrate (84), and a half mirror (102) fixed on said substrate (84) for reflecting part of light emitted from said laser diode (86) toward said third photodetector. An optical apparatus according to claims 1 or 2, wherein said substrate (84,85) is a semiconductor substrate.

Inventors: KOJIMA CHIAKI, TANIGUCHI TADASHI, KOJIMA, CHIAKI, TANIGUCHI, TADASHI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: G11B7
IPC codes: G11B7, G11B11