Optical pickup

Abstract: An optical pickup irradiates a record surface of an optical storage medium (1) with light and receives the light reflected by the record surface. The reflected light includes data signal light corresponding to data recorded on the optical storage medium and other light. The optical pickup comprises a light source (5), a data photo detector (3) and another photo detector (4), arranged together on a base (2), a beam splitter (8), an introductive optical element (7), and a lens (9). The light source (5) emits the light onto the record surface. The data photo detector (3) receives the data signal light, and the other photo detector (4) receives other (non-signal) light. The beam splitter (8) transmits the light from the optical record medium (1) into the data signal light and the other light. The introductive optical element (7) introduces the data signal light to the data photo detector (3) and the other light to the other photo detector (4). The lens (9) condenses the light from the light source (5) transmitted through the beam splitter on the optical record medium (1) and introduces the reflected light from the optical record medium to the beam splitter (8).

Description: The present invention relates to an optical pickup reading data stored in an optical disk and so on. Recently, an electronic apparatus using a semiconductor laser has come into wide use, such as an optical disk apparatus. For an optical disk apparatus, a semiconductor laser is used as a laser light source to write and read data into/from storage media. An optical head and an optical pickup are general terms for a semiconductor laser and an optical apparatus which applies laser light emitted from the semiconductor laser to a spot on a data record surface, separates the light reflected by the data record surface into a focus error signal component, a track error signal component and a data signal component and detects them. Personal computers in which an optical disk apparatus is implemented have been developed to use a large storage medium such as an optical disk. Thus, since it is desired to make a pocket personal computer compact and to reduce the power consumption of such a personal computer, it is necessary to make an optical disk apparatus compact and lightweight when the optical disk apparatus is implemented in the personal computer. Particularly, when the occupied space and the weight of the optical pickup increase, a carriage supporting the optical pickup becomes large, therefore, a seek time increases because it takes a lot of time to move the carriage. Thus, it is necessary to reduce the number of parts, to make the occupied space small and to make the optical pickup lightweight. Moreover, in the conventional optical pickup, units for detecting a focus error signal component, a track error signal component and a data signal component are arranged separately. Thus, there are problems that the weight increases in accordance with increase of the number of parts and it is difficult to make the occupied space small. Accordingly, it is desirable to solve those problems, to reduce the space occupied by an optical pickup in an apparatus for writing data into an optical storage medium such as an optical disk and reading data stored in the optical storage medium, and to make the apparatus compact and to reduce the weight of the apparatus. JP-A-07 249 242 discloses an optical pickup according to the preamble of claim 1, and a method according to the preamble of claim 8. According to this prior art, a return beam from an optical disk is made incident on a polarized light separating film and thence to a uniaxial birefringence crystal to form a normal and an abnormal beam. The normal and abnormal beams are separated and directed onto respective parts of a light detector. According to a first aspect of the present invention, there is provided an optical pickup for irradiating a record surface of an optical storage medium with light and receiving the light reflected by the record surface, said reflected light composed of a data signal component corresponding to data recorded on the optical storage medium and another component, said optical pickup comprising: a light source for emitting the light to the record surface of the optical storage medium; a first photo detector for receiving the data signal component of the reflected light; a second photo detector for receiving the other component; and a base having a surface on which the light source, the first photo detector and the second photo detector are arranged; characterized by: a light separation optical element which transmits the light from the light source and separates the reflected light from the optical record medium into the data signal component and the other component; and an optical compensation element for condensing the data signal component and the other component received from the light separation optical element; wherein the light separation optical element makes the data signal component incident on the optical compensation element at a predetermined angle so that the optical compensation element condenses the data signal component on the first photo detector, and transmits the other component so that the optical compensation element condenses the other component on the second photo detector. According to a second aspect of the present invention, there is provided a method of irradiating a record surface of an optical storage medium with light and of receiving the light reflected by the record surface, said reflected light composed of a data signal component corresponding to data recorded on the optical storage medium and another component, said method comprising a step of emitting rays from a light source, arranged on a base, to the record surface; characterised by the further steps of: separating the reflected light from the optical record medium into the data signal component and the other component at a light separation optical element; condensing the data signal component and the other component, received from the light separation optical element, at an optical compensation element; and arranging the light separation optical element so as to: make the data signal component incident on the optical compensation element at a pre-determined incident angle, so as to condense the data signal component on the first photo detector; and transmit the other component, so as to condense the other component on the second photo detector. Thus, in embodiments of the invention, the light source emits the light to the record surface of the optical storage medium. The first photo detector (PD) receives the data signal component of the reflected light. The second PD receives the other component. The base is provided with a surface on which the light source, the first PD and the second PD are arranged. The light separation optical element transmits the light from the light source and separates the reflected light from the optical record medium into the data signal component and the other component. The optical compensation element introduces (guides) the data signal component to the first PD and the other component to the second PD. The optical compensation element also condenses the light from the light source, which is then transmitted through the light separation optical element and incident on the optical record medium. In embodiments of the present invention, the first PD, the second PD and the light source are arranged in the same surface, therefore, it is possible to reduce the space occupied and number of parts required for the optical pickup. It is also possible to reduce the seek time. In this optical pickup, it is possible to use a collimating lens or the like as the optical compensation element. The collimating lens, however, is designed such that light introduced vertically is gathered at a predetermined position. Thus, when light is introduced obliquely, aberration occurs and a condensed beam becomes large. To overcome this, the optical pickup may further comprise an optical member arranged between the first PD and the optical compensation element. The optical member may guide the reflected light transmitted through the optical compensation element to the first light receiving device. For example, the optical member may be adhered to a surface of the first PD device. The optical member is arranged in this way, whereby it is possible to receive the data signal by the first PD reliably. A diffractive optical element (e.g. hologram) may be interposed between the optical compensation element and the PDs. The diffractive optical element may comprise a tracking signal diffraction part, which diffracts a tracking signal contained in the other component and is used for a tracking control to position an optical spot at a predetermined track on the record surface, and a focus signal diffraction part, which diffracts a focus signal contained in the other component and is used for a focus control to keep the optical spot on the record surface. The second PD may comprise a tracking signal receiving part for receiving a tracking signal and a focus signal receiving part for receiving a focus signal. The optical pickup is structured like this, whereby the tracking signal and the focus signal can be received separately. The light separation optical element may comprise a polarizing beam splitter reflecting the data signal component and transmitting the other component. The light separation optical element may further comprise a polarizing separation element separating the data signal component reflected by the polarizing beam splitter into components perpendicular to each other. In both cases, it is preferable to arrange the polarizing beam splitter and the polarizing separation member in a manner that each component separated by the polarizing separation member is introduced to the optical compensation element at a predetermined angle of incidence. The polarizing separation element is arranged between the polarizing beam splitter and the first PD. As the polarizing separation element, an uniaxial doubly refracting crystal prism, a polarizing beam splitter or the like may be used. The polarizing separation element may be adhered to a surface of the first PD. Reference is made, by way of example, to the accompanying drawings, in which: FIG. 1 is a view illustrating an optical pickup according to the first embodiment of the present invention; FIG. 2 is a view illustrating an arrangement sample of photo detectors provided on a silicon wafer; FIG. 3 is a plan view illustrating a structure of a hologram; FIG. 4 is a view explaining separation of polarizing components by a polarizing separation element; FIG. 5 is a structural view illustrating an optical pickup for aberration correction; FIG. 6 is a view illustrating an optical pickup of the second embodiment according to the present invention; and FIG. 7 is a view illustrating an optical pickup of the third embodiment according to the present invention. FIG. 1 illustrates a whole structure of an optical pickup according to the first embodiment. In this embodiment, an explanation is given by using a MO (magneto-optical) disk in which data are stored by varying a magnetization direction in a storage magnetization domain formed on a surface of an optical disk, as an example of optical storage media. The optical pickup has a stem (housing) 2 which is a frame having an opening in its ceiling (top surface), and a semiconductor laser 5 and a silicon wafer 13 which are arranged on the bottom (base) of the stem 2. The semiconductor laser 5 is an example of a light source, and emits linearly polarized laser light including only p-polarized light in this embodiment. The laser light has a mean oscillating wave length of 685 nm and a divergence angle of 12°-15°. In the silicon wafer 13, a data signal photo detector 3 as a first photo detector and an error signal photo detector 4 as a second photo detector are provided. The data signal photo detector 3, as shown in FIG. 2, is provided with a data signal photo detector 3a receiving a p polarizing component of the reflected light from the MO disk 1 and a data signal photo detector 3b receiving a s polarizing component (magneto optical signal component) of the reflected light. The error signal photo detector 4 is provided with photo detectors for tracking error detection 4b including two photo detectors and for focus error detection 4a including two photo detectors arranged between the two photo detectors for the tracking error detection 4b. The opening of the stem 2 is covered by a clear glass plate 14. A hologram 6 is fixed to the upper surface of the glass plate 14 in a manner that the center thereof is arranged at the optical axis of the semiconductor laser 5. The hologram 6, namely, a diffractive optical element transmits the light from the semiconductor laser 5, and diffracts the reflected light from the MO disk 1 so as to condense the reflected light to (i.e. onto) the error signal photo detector 4. The hologram 6, as shown in FIG. 3, is divided into two areas 6a diffracting the tracking signal light and two areas 6b diffracting the focus signal light. Each of these four areas 6a, 6b has an individual diffraction grating constant and an individual diffraction direction. For example, one of the tracking signal areas 6a diffracts the reflected light so as to condense the reflected light to one of the tracking error photo detectors 4b, and the other diffracts and condenses the reflected light to the other tracking error photo detector 4b. Moreover, one of the focus signal areas 6b diffracts the reflected light so as to condense the reflected light to one of the focus error photo detector 4a in the error signal photo detector 4, and the other diffracts and condenses the reflected light to the other focus error photo detector 4a. The hologram 6 may be produced by the 2P method in which a hologram is formed on a surface of a BK7 glass substrate by photo polymer, or may be produced by etching of a glass substrate or by injection using a resin or glass in a metal pattern. The hologram 6, for example, transmits p polarized light (source light) emitted from the semiconductor laser 5 at zero-order diffraction efficiency of 70%, and diffracts a focus signal and a tracking signal in light reflected by the MO disk (hereinafter, called reflected light) at first-order diffraction efficiency of 30% so as to condense the reflected light to the error signal photo detector 4. The optical pickup is provided with a collimating lens 7, a polarizing beam splitter (PBS) 8 and an objective lens 9. The collimating lens 7 converts the source light emitted from the semiconductor laser 5 from divergent rays to parallel rays. When the data signal component in the reflected light from the MO disk 1 falls on the collimating lens 7 at a predetermined angle of incidence, this data signal component is condensed by the collimating lens 7 so as to strike the data signal photo detector 3. When the tracking signal component and the focus signal component in the reflected light fall on the collimating lens 7 at a right angle, these signal components are transmitted and condensed by the collimating lens 7 so as to strike the hologram 6. To make the collimating lens 7 lightweight, it is desirable that the collimating lens 7 is manufactured by injection molding of plastic materials. Then, the polarizing beam splitter (PBS) 8 is an optical element of a doubly refracting crystal, such as an optical element having a transmittance of p polarized light not less than 85% and a reflectance of s polarized light not less than 96%. The PBS 8 transmits the source light (including only p polarized light) passing through the collimating lens 7 not less than 85%. Then, the PBS 8 transmits the p polarized light (similar to the polarizing detection of the source light) in the reflected light from MO disk 1 not less than 85% and reflects the p polarized light not more than 15%. The PBS 8 also reflects the s polarized light (data signal light) in the reflected light with an efficiency of about 96%. Now, as to the light exposed to the record magnetized domain on the record surface of the MO disk 1, the plane of polarization is rotated by Kerr effect, therefore, the reflected light includes much p polarized light and not much s polarizing light. Since the PBS 8 reflects p polarized light about 15% and reflects s polarized light about 100%, it is possible to increase the proportion of s polarized light to the p polarized light. An non-phase-difference reflecting film is coated on the rear surface of the polarizing separation film, so that it is possible to reflect the reflected light falling on the PBS 8 without varying the phase thereof. A polarizing separation element 10 is arranged at the outgoing surface of the reflected light of the PBS 8. The polarizing separation element 10 separates the data signal light reflected by the PBS 8 into polarized light (p polarized light of 15% reflected by the PBS 8) similar to the source light and different polarized light (s polarized light reflected by the PBS 8) from the source light, whereby each kind of separated polarized light falls on the collimating lens 7 at a predetermined angle of incidence. As the polarizing separation element 10, it is possible to use an uniaxial doubly refracting crystal prism, a Wollaston prism or the like. In this embodiment, an uniaxial doubly refracting crystal prism is used. Then, as the uniaxial doubly refracting crystal prism, may be used mountain (rock) crystal, rutile, calcite, lithium niobate, KDP(KH 2 PO 4 ), ADP(KH 4 H 2 PO 4 ) or the like. This embodiment is explained by using lithium niobate which exhibits a relatively great difference in refractive index between ordinary light and extraordinary light. The uniaxial doubly refracting crystal prism, as shown in FIG. 4, is formed in a manner that the crystal axial direction (ordinary light direction) and the polarization axis of p polarized light are intersected at angle of 45°. In this case, it is possible to make the amounts of ordinary light and extraordinary light in the incident light (indicated by arrows A, B in FIG. 4) substantially equal rather than that the polarization axis of p polarized light and the crystal axial direction are set in the same direction. Additionally, the refractive index of the uniaxial doubly refracting crystal prism is set in a manner that, when the reflected light exits from the PBS 8, the reflected light falls on the collimating lens 7 at a predetermined angle of incidence. The predetermined angle of incidence is set in a manner that the light which comes out from the collimating lens 7 falls on the data signal photo detector 3 and/or an optical member 11 (see FIG. 5) without passing through the hologram 6. The optical member 11 is arranged on the data signal photo detector 3 since aberration is considered. The objective lens 9 condenses the source light which passes through the PBS 8 on the record surface of the MO disk 1, and corrects the reflected light from the record surface of the MO disk 1 from divergent rays to parallel rays so that it falls on the PBS 8. Anti-Reflecting (AR) coating is applied to surfaces of the hologram 6, the collimating lens 7 and the objective lens 9 to avoid a loss in quantity of light by surface reflection. Particularly, it is preferable to apply a coating independent of an angle of incidence to the collimating lens 7 and the hologram 6 when incident light from oblique directions is taken into consideration. Hereinafter, an explanation will be given of the operation and the effect of the optical pickup according to this embodiment. In the optical pickup, the semiconductor laser 5 emits light when data are written into or read from a MO disk. The source light emitted from the semiconductor laser 5 transmits through the glass plate 14 and the hologram 6 and falls on the collimating lens 7. The collimating lens 7 corrects the source light from divergent rays into a parallel beam which leaves the lens 7. Then, the source light oollimated by the collimating lens 7 falls on the PBS 8. The source light includes p polarized light, therefore, most of the source light falling on the PBS 8 is transmitted and falls on the objective lens 9. The objective lens 9 focuses the collimated source light on the record surface of the MO disk 1. Next, the source light exposed to the record surface of the MO disk 1 is reflected by the record surface. At that time, as to the light exposed to the record magnetized domain on the record surface, the plane of polarization is slightly rotated by Kerr effect in accordance with the magnetization direction, therefore, the reflected light includes lots of p polarized light and little s polarized light. Then, the reflected light falls on the objective lens 9 and is corrected from divergent rays to parallel rays. The reflected light introduced from the objective lens 9 falls on the incident surface of the PBS 8 obliquely. At that time, the PBS 8 transmits 85% of the p polarized light and reflects the remaining 15% p polarized light and the s polarized light. The p polarized light transmitted by the PBS 8 passes through the collimating lens 7 and falls on the hologram 6. The p polarized light falling on the tracking signal diffraction areas 6a is diffracted to be gathered at the tracking signal photo detector 4b on the silicon wafer 13. The p polarized light falling on the focus signal diffraction areas 6b is diffracted to be gathered at the focus signal photo detector 4a on the silicon wafer 13. On the contrary, the reflected light including the p polarized light and the s polarized light are refracted by the uniaxial doubly refracting crystal prism acting as the polarizing separation element 10, and fall on the collimating lens 7 at a predetermined angle of incidence. Then, the collimating lens 7 introduces the p polarized light and the s polarized light to the data signal photo detector 3. At that time, the s polarized light and the p polarized light fall on the data signal photo detector 3 without passing through the hologram 6, therefore, there is no loss in quantity of light by the diffraction of the hologram 6. According to this embodiment, the semiconductor laser 5, the error signal photo detector 4 and the data signal photo detector 3 are arranged on the same surface, therefore, it is possible to reduce the space taken up and the number of parts in the optical pickup. Thus, it is possible to reduce the weight of the whole optical pickup, to reduce the weight of a carriage loading the optical pickup, and to reduce the driving force to move the carriage. It is also possible to move the carriage speedily and to shorten a seek time. Additionally, aberration occurs in light falling on the collimating lens 7 when the light exits from the collimating lens 7, when the angle of incidence becomes large by restriction on an optical system. Therefore, it is necessary to shorten the distance between the collimating lens 7 and photo detectors. Thus, as shown in FIG. 5, an optical member 11 of a predetermined refractive index is arranged on the data signal photo detector 3 so as to correct aberrations. In this case, signal light exiting the collimating lens 7 is introduced to the optical member 11, and refracted by the optical member 11 so as to be gathered on the data signal photo detector 3. Hereinafter, explanations will be given of other embodiments of the optical pickup according to the present invention. These explanations will be confined to structures which are different from the first embodiment. FIG. 6 is a view illustrating the second embodiment of the optical pickup. As shown in FIG. 6, no polarizing separation element is attached to the PBS 8, and a polarizing separation element 12 is arranged on the data signal photo detector 3 instead of the optical member 11 in FIG. 5. As the polarizing separation element 12, it is possible to use a small-size polarizing beam splitter, a laminated polarizing separation element (LPS), an uniaxial doubly refracting crystal prism or the like. In this embodiment, an uniaxial doubly refracting crystal prism, e.g. rutile crystal formed in a prism,is used. Additionally, the uniaxial doubly refracting crystal prism is formed in a manner that an incident surface on which the reflected light from the collimating lens 7 falls is inclined at 8° from the horizontal direction and the optical axis is inclined at 45° from the p polarizing axis. Concerning an optical pickup structured like this, it is unnecessary to attach a polarizing separation element to the PBS 8, therefore, it is possible to reduce the number of parts and the weight in the optical pickup. FIG. 7 is a view illustrating the third embodiment of an optical pickup. As shown in FIG. 7, the semiconductor laser 5 is arranged on a wafer in a manner that the light from the semiconductor laser 5 is introduced in a horizontal direction, and a prism 13 which changes the direction of light from the semiconductor laser 5 into a vertical direction is arranged on the wafer. Additionally, the polarizing separation member 10 may be attached to the PBS 8 in FIG. 7. According to the above embodiment, since a light source, a first light receiving device and a second light receiving device are arranged on the same surface, it is possible to reduce the space taken up and the number of parts for the optical pickup, therefore, it is possible to reduce the weight of the optical pickup. This invention being thus described, it will be obvious that it may be varied in various ways within the scope of the following claims.

Claims: An optical pickup for irradiating a record surface of an optical storage medium (1) with light and receiving the light reflected by the record surface, said reflected light composed of a data signal component corresponding to data recorded on the optical storage medium (1) and another component, said optical pickup comprising: a light source (5) for emitting the light to the record surface of the optical storage medium; a first photo detector (3) for receiving the data signal component of the reflected light; a second photo detector (4) for receiving the other component; and a base (2) having a surface on which the light source (5), the first photo detector (3) and the second photo detector (4) are arranged; characterized by : a light separation optical element (8) which transmits the light from the light source (5) and separates the reflected light from the optical record medium (1) into the data signal component and the other component; and an optical compensation element (7) for condensing the data signal component and the other component received from the light separation optical element (8) ; wherein the light separation optical element (8) makes the data signal component incident on the optical compensation element (7) at a predetermined angle so that the optical compensation element (7) condenses the data signal component on the first photo detector (3), and transmits the other component so that the optical compensation element (7) condenses the other component on the second photo detector (4). An optical pickup according to claim 1, further comprising a diffractive optical element (6) for transmitting the light from the light source (5) and diffracting the other component from the optical compensation element (7) so as to separate it into at least two signal components. An optical pickup according to claim 1 or 2, wherein said optical compensation element (7) condenses the light from the light source (5) from divergent rays to parallel rays. An optical pickup according to claim 1, 2, or 3, further comprising an optical member (11) arranged between the first photo detector (3) and the optical compensation element (7), said optical member (11) introducing the data signal component from the optical compensation element to the first photo detector (3). An optical pickup according to claim 2, wherein said diffractive optical element (6) comprises a tracking signal diffraction part (6a) diffracting tracking signal light contained in the other component and used for a tracking control to position an optical spot at a predetermined track on the record surface, and a focus signal diffraction part (6b) diffracting focus signal light contained in the other component and used for a focus control to keep the optical spot on the record surface. An optical pickup according to claim 5, wherein said second photo detector (4) comprises a tracking signal receiving part (4b) for receiving the tracking signal light and a focus signal receiving part (4a) for receiving focus signal light. An optical pickup according to any preceding claim, further comprising a polarizing separation element (10) for separating the data signal component separated by the light separation optical element (8) into polarized components whose polarizations are perpendicular to each other. A method of irradiating a record surface of an optical storage medium (1) with light and of receiving the light reflected by the record surface, said reflected light composed of a data signal component corresponding to data recorded on the optical storage medium (1) and another component, said method comprising a step of emitting rays from a light source (5), arranged on a base (2), to the record surface; characterised by the further steps of: separating the reflected light from the optical record medium (1) into the data signal component and the other component at a light separation optical element (8) ; condensing the data signal component and the other component, received from the light separation optical element (8), at an optical compensation element (7); and arranging the light separation optical element (8) so as to: make the data signal component incident on the optical compensation element (7) at a pre-determined incident angle, so as to condense the data signal component on the first photo detector (3); and transmit the other component, so as to condense the other component on the second photo detector (4).

Inventors: HASEGAWA SHINYA, SHIBANO MOTOMICHI, HASEGAWA, SHINYA, SHIBANO, MOTOMICHI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1997
Main classification: G11B7
IPC codes: G11B11, G11B7