Patent Application
20050135202
This nonprovisional application is based on Japanese Patent Application No. 2003-425636 filed with the Japan Patent Office on Dec. 22, 2003 the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to optical miniaturized modules employed to optically record or reproduce information on or from optical disks or similar information storage media, and optical pickup devices equipped therewith, and particularly to magneto-optic optical miniaturized modules and magneto-optic optical pickup devices.
2. Description of the Background Art
The optical pickup device includes an optical miniaturized module and an objective lens. The module includes a source of light, polarization separation means and the like is formed integrally. Objective lens 113 is arranged between the module and a magneto-optic storage medium 130 and formed to focus laser light on the medium 130 recording surface.
The module includes a source of light 111 providing lased light, a grating 116 receiving the laser light from the source of light 111 to separate the light into three beams, and a base 140 arranged between the source of light 111 and objective lens 113. Base 140 has an upper surface provided with first polarization separation means 112 receiving light reflected from medium 130 to separate the reflected light in the direction of the radius of medium 130.
Base 140 has a lower surface provided with second polarization separation means 114a, 114b receiving the separated reflected light for further separation. Below the second polarization separation means 114a, 114b is arranged a photodetector 115 receiving the further separated reflected light. Photodetector 115 includes a group of light receiving portions 115a and a group of light receiving portions 115b.
The first polarization separation means 112 is implemented by a polarization hologram implemented by a double refraction diffraction grating.
The first polarization separation means 112 or a polarization hologram is formed to provide a phase difference φ of approximately 70° for an ordinary ray and that of approximately 130° or approximately 230° for an extra ordinary ray. Furthermore, it is also formed to provide a 0th-order diffraction efficiency of 67% and +1st-order diffraction efficiencies in total of 27% for ordinary ray, and a 0th-order diffraction efficiency of 18% and ±1st-order diffraction efficiencies in total of 76% for extra ordinary ray. Medium 130 has information reproduced in accordance with Kerr effect. Medium 130 provides reflected light, which has a polarization plane rotated (Kerr rotated) in accordance with information recorded on medium 130. The first polarization separation means 112 formed to provide the above-described diffraction efficiencies provides a multiplied Kerr rotation angle of light reflected by medium 130. In other words, the first polarization separation means 112 has an enhancement function providing a multiplied Kerr rotation angle of light reflected by medium 130.
The second polarization separation means 114a, 114b is polarization separation means for detecting a behavior of a magneto-optical signal. The second polarization separation means 114a separates a +1st-order diffracted beam 112a formed by the first polarization separation means 112. The second polarization separation means 114b separates a −1st-order diffracted beam 112b formed by the first polarization separation means 112.
The second polarization separation means 114a, 114b, as well as the first polarization separation means 112, is implemented by a polarization hologram formed of a double refraction diffraction grating having such a structure as shown in
Group 115a includes light receiving portions 118, 119a, 119b and 120 to receive laser light divided by grating 116 into three beams in a direction perpendicular to that of the radius of medium 130. The group 115a light receiving portions receive a beam transmitted through the second polarization separation means and a −1st-order diffracted beam therefrom.
Similarly, group 115b includes light receiving portions 121, 122a, 122b and 123 to receive light divided by grating 116 into three beams. The group 115b light receiving portions receive a beam transmitted through the second polarization separation means and a +1st-order diffracted beam therefrom. As shown in
Group 115a at a row indicated by an arrow 114a0 receives a laser beam of light transmitted through the second polarization separation means 114a and at a row indicated by an arrow 114aB receives the −1st-order diffracted beam from the second polarization separation means 114a. Similarly group 115b at a row indicated by an arrow 114b0 receives a laser beam of light transmitted through the second polarization separation means 114b and at a row indicated by an arrow 114bA receives the +1st-order diffracted beam from the second polarization separation means 114b. Throughout the description of the background art, the left- and right-hand sides as seen in a direction X (the direction of the radius of the medium) in
Group 115a has a center light receiving portion, as seen in a direction Y (the medium's tangential direction), divided into light receiving portions 119a and 119b formed to be capable of receiving the second polarization separation means' transmitted and −1st-order diffracted beams, respectively, separately. Similarly, group 115b has light receiving portions 112a and 112b formed to be aligned in direction X.
Furthermore, light receiving portions 119a, 119b, 122a, 112b are divided by two lines parallel to direction X to detect a focus error signal by differential 3-division method. These portions are each divided into three portions to provide a center narrow portion and larger side portions sandwiching the center portion.
A reproduced magnet-optic signal MO1 is represented by the following equation:
MO1=(S119a−S119b)+(S122b−S122a) (1)
wherein a detected signal obtained at each light receiving portion is indicated by a reference character indicating the light receiving portion preceded by the letter “S”. In expression (1) the first term (S119a−S119b) is a differential signal by detected signals of the transmitted and −1st-order diffracted beams provided through the second polarization separation means 114a and the second term (S122b−S122a) is a differential signal by detected signals of the transmitted and +I st-order diffracted beams provided through the second polarization separation means 114b.
In a suitable embodiment described in Japanese Patent Laying-Open No. 08-297875 magnet-optic signal MO1 is formed of a signal obtained by detecting a differential between the transmitted and −1st-order diffracted beams of the second polarization separation means 114a and a signal obtained by detecting a differential between the transmitted and +1st-order diffracted beams of the second polarization separation means 114b. In the present invention a system employing expression (1) to form a magnet-optic signal will be referred to as a “first MO signal generation system.”
In
With reference to
In the first MO signal generation system, for the group of light receiving portions 115a the second polarization separation means' transmitted and −1st-order diffracted beams alone are used, and for the group of light receiving portions 115b the second polarization separation means' transmitted and +1st-order diffracted beams alone are used. In other words, of the 1st-order diffracted beams of the second polarization separation means 114a and 114b, one diffracted beam is used for one group and the other diffracted beam for the other group.
For the photodetector, a photodetector can also be suggested that employs both +1st-order and −1st-order diffracted beams of the second polarization separation means 114a and 114b. A photodetector can also be proposed that has the group of light receiving portions 115a shown in
MO2={S119a−(S119b+S119c)}+{S122a−(S122b+S122c)} (2)
In the following description a system employing expression (2) to obtain magneto-optic signal MO2 will be referred to as a “second MO signal generation system.”
Japanese Patent Laying-Open No. 08-297875 does not describe the second polarization separation means' diffraction efficiency. However the second polarization separation means' disclosed configuration in theory allows a 0th-order diffraction efficiency of 100% for ordinary ray and a +1st-order diffraction efficiencies of 40.5% for extra ordinary ray.
In detecting a differential signal of a magneto-optic signal, to reduce a common mode noise attributed to variation in intensity of laser light output from a source of light and variation in reflectance of a magneto-optic storage medium it is preferable that an information signal be detected by using ordinary and extra ordinary rays' components having substantially equal quantities of light.
If the information signal is obtained from a difference between a signal of a transmitted beam (a 0th-order diffracted beam) and a signal of a +1st-order diffracted beam or a difference between a signal of a transmitted beam and a signal of a −1st-order diffracted beam, as described in the first MO signal generation system, the ordinary and extra ordinary rays' components have their quantities of light at an unbalanced ratio of 100:45, and common mode noise is insufficiently reduced.
The angle formed by the direction of the optical axis of the second polarization separation means and that of polarization of incident light can be shifted from 45° to provide a ratio in quantity of light of 1:1. This, however, results in reduced carrier level and hence reduced carrier to noise ratio (C/N). Accordingly the imbalance of the ratio in quantity of light between the ordinary and extra ordinary rays' components is preferably reduced by adopting the second MO signal generation system obtaining an information signal from a difference between the sum of ±1st-order diffracted beams and a 0th-order diffracted beam.
In the second MO signal generation system ordinary and extra ordinary rays' components have their quantities of light at a ratio of 100:81, and as compared with the first MO signal generation system, the second MO signal generation system resolves the imbalance of the ratio in quantity of light and significantly reduces common mode noise. The second MO signal generation system, however, requires light receiving portions 119c and 122c, as shown in
Internal to an optical pickup device, laser light emitted from a source of light is diff-used by a package or similar members and a portion thereof thus reaches the photodetector, resulting in a noise signal. In other words, stray light causes noise in a detected signal. Accordingly, it is preferable that a light receiving portion have a small surface area. However, light receiving portions 119c and 122c need to be formed to have a large surface area, and this causes an increased noise component and hence a decreased C/N.
As described above, conventional optical miniaturized modules and optical pickup devices can hardly achieve improved C/N. In particular, the conventional optical miniaturized modules and optical pickup devices can hardly adjust aberration of reflected light at the photodetector nor control a spot in position, profile and the like.
The present invention contemplates an optical miniaturized module and optical pickup device capable of controlling the position, profile, length or width of a spot of reflected light at a photodetector. The present invention also contemplates an optical miniaturized module and optical pickup device that can provide improved C/N.
The present optical miniaturized module includes: a photodetector having a plurality of light receiving portions to receive light reflected by a magneto-optic storage medium; a first polarization separation element separating the reflected light and lasing light output from a source of light and traveling toward the medium; and a second polarization separation element receiving the reflected light from the first polarization separation element to diffract at least a portion of the reflected light and guide diffracted light to the photodetector, the second polarization separation element including a diffraction element having a plurality of diffraction areas, the diffraction element being formed to diffract the reflected light through the diffraction areas in different directions, the diffraction element being formed such that two of the diffraction areas provide diffracted beams directed to one of the light receiving portions. The module can control a position, profile, length or width of a spot on the photodetector.
In the present invention preferably the diffraction element includes first and second diffraction areas and is formed to allow both of +1st-order diffracted beams of the first diffraction area, both of 1st-order diffracted beams of the second diffraction area, and beams transmitted through the first and second diffraction areas to align on the photodetector generally in a straight line. The photodetector can receive a plurality of above-described spots spaced by a reduced distance so that the light receiving portion can have a reduced surface area and C/N can also be improved.
In the present invention preferably the diffraction element includes first and second diffraction areas and is formed such that of a set of a +1st-order diffracted beam of the first diffraction area and a −1st-order diffracted beam of the second diffraction area and a set of a −1st-order diffracted beam of the first diffraction area and a +1st-order diffracted beam of the second diffraction area, at least one set has its two diffracted beams at least partially overlapping at the light receiving portion. The photodetector can receive the above described spot reduced in area. The light receiving portion can have a reduced surface area and C/N can also be improved.
In the present invention preferably the diffraction element is divided by a line into two areas one having a grating smaller in pitch than the other's grating, the gratings being formed substantially along the line and curved to be concave or convex as seen in a direction from the one to the other areas. The photodetector can receive reflected lights such that they are adjacent, overlap and the like. The light receiving portion can have a reduced surface area and C/N can also be improved.
In the present invention preferably the diffraction element includes first and second lines orthogonal to each other and the second line defines two diffraction areas, one being further divided by the first line into first and second diffraction areas, and the other being further divided by the first line into third and fourth diffraction areas. The photodetector can receive spots of light such that they are adjacent, overlap and the like. The light receiving portion can have a reduced surface area and C/N can also be improved.
In the present invention preferably the diffraction element is formed such that of a set of a +1st-order diffracted beam of the first diffraction area and a +1st-order diffracted beam of the second diffraction area and a set of a −1st-order diffracted beam of the first diffraction area and a −1st-order diffracted beam of the second diffraction area, at least one set has its two diffracted beams at least partially overlapping at the light receiving portion. Alternatively, the diffraction element is formed such that of a set of a +1st-order diffracted beam of the third diffraction area and a +1st-order diffracted beam of the fourth diffraction area and a set of a −1st-order diffracted beam of the third diffraction area and a −1st-order diffracted beam of the fourth diffraction area, at least one set has its two diffracted beams at least partially overlapping at the light receiving portion. The light receiving portion can have a reduced surface area and C/N can also be improved.
In the present invention preferably the diffraction element includes a polarization hologram. By changing the polarization hologram's grating in configuration or the like, each diffraction area's diffracted beam can readily be adjusted in direction and/or the like.
In the present invention preferably the polarization hologram has a grating at least partially curved. The photodetector can receive a spot further reduced in area so that further improved C/N can be achieved.
In the present invention preferably the photodetector is formed to receive at one of the light receiving portions one of beams transmitted through the second polarization separation element and ±1st-order diffracted beams corresponding to one of the beams transmitted through the second polarization separation element. The photodetector can have a reduced number of light receiving portions and hence a simplified structure. Furthermore the photodetector can also be miniaturized.
In the present invention preferably the module includes a magneto-optic signal detector obtaining a differential signal between a detected signal of a diffracted beam of the second polarization separation element and a detected signal of a beam transmitted through the second polarization separation element. This allows the present invention to be applied to the above-described optical miniaturized module including the above magnet-optic signal detector.
The present optical pickup device includes the above optical miniaturized module. The device can control a position, profile, length or width of a spot on the photodetector. The device can also provide improved C/N.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
With reference to
The optical miniaturized module is formed integral with a package 25 having a substantially rectangular profile as seen in a plane, and formed in the form of a box so that it can internally accommodate each member. In package 25 at a lower portion is arranged a source of light 11 to lase light upward. Over the source of light 11 is arranged a grating 16 to divide the lasing light into three beams. Grating 16 is formed in a flat plate.
Package 25 has an upper portion provided with a base 40 having an internal portion hollowed and an upper surface provided with first polarization separation means implemented by a polarization hologram 12 formed to transmit the lasing light received from the source of light 11 while diffract reflected light received from the medium. In the present embodiment a diffracted beam on the left hand of
On a lower surface of base 40 on optical paths of the ±1st-order diffracted beams of polarization hologram 12 second polarization separation means implemented by diffraction elements are implemented by polarization holograms 14a and 14b arranged to sandwich the laser light's optical axis J and formed to diffract at least a portion of reflected light.
In package 25 at a lower portion is arranged a photodetector 15 receiving ±1st-order diffracted and transmitted beams of the second polarization separation means or polarization holograms 14a, 14b. The second polarization separation means is provided to diffract at least a portion of reflected light received from the first polarization separation means and guide the diffracted light to photodetector 15. Photodetector 15 has a group of light receiving portions 15a and a group of light receiving portion 15b arranged to receive +1st-order and −1st-order diffracted beams 12a and 12b, respectively. Polarization holograms 14a, 14b are each arranged on an optical path of reflected light immediately in front of photodetector 15.
Base 40 is internally hollowed and formed to have a height (or a length in a direction Z) of 1.45 mm. An optical substrate 3 has a lower surface spaced from the light receiving portion by a distance (in direction Z) of approximately 0.56 mm. The source of light 11 is adapted to output laser light of 785 nm.
The optical miniaturized module underlies an objective lens 13 arranged to focus light at the medium 130 recording surface. The optical pickup device includes objective lens 13 and the optical miniaturized module.
The first polarization separation means or polarization hologram 12 is similar to a conventional polarization hologram. For example, as described in Japanese Patent Laying-Open No. 08-297875, it has a surface provided with parallel bars having a substantially fixed pitch.
In the present embodiment polarization hologram 12 is, as shown in
The first polarization separation means is only required to separate reflected light received from the medium from the lasing light traveling toward the medium, and other than the above may for example include a polarization beam splitter. Furthermore in the present embodiment the first polarization separation means is implemented by a polarization hologram having an enhancement function.
The optical miniaturized module in the present embodiment is distinguished from a conventional optical miniaturized module by a configuration of polarization hologram 14a, 14b and that of photodetector 15.
Furthermore, polarization hologram 14a is arranged to substantially bisect +1st-order diffracted beam 12a of polarization hologram 12 of
In polarization hologram 14a the first and second areas 14a1 and 14a2 have gratings different in geometry and the like. More specifically, the first diffraction area 14a1 grating 60a has a smaller pitch than the second diffraction area 14a2 grating 60b. In other words, grating 60a is formed of bars spaced by a smaller distance than grating 60b is.
The first and second diffraction areas 14a1 and 14a2 gratings have a tendency to have a pitch generally maintained or increased as seen toward the positive direction in direction X. (Although areas P9 through to P7 provide a slightly decreasing pitch, it is substantially maintained.)
Furthermore, while at line 31 serving as the border of the first and second areas 14a1 and 14a2 gratings 60a and 60b are interrupted, in each area grating 60a, 60b is not interrupted. For example, in each area, grating 60a, 60b have their pitches smoothly changing in direction X. Furthermore in each area grating 60a, 60b has smoothly changing curvature.
Furthermore in areas 14a1, 14a2 their respective gratings 60a, 60b are each formed of substantially parallel bars. Strictly, while the pitch of the grating on the line perpendicularly bisecting line 31 is slightly different from that of the grating at an end of polarization hologram 14a, in each area 14a1, 14a2 the bars are substantially parallel.
Furthermore in areas 14a1, 14a2 their respective gratings 60a, 60b are formed in symmetry with respect to the line (not shown) perpendicularly bisecting line 31.
Gratings 60a, 60b are each formed to have substantially the same direction as line 31. Furthermore, gratings 60a, 60b are slightly curved to protrude as seen at the second area 14a2 toward the first area 14a1, or in the negative direction as seen in direction X.
The second polarization separation means or polarization hologram 14b is formed such that its grating and that of polarization hologram 14a are in symmetry for example in geometry with respect to optical axis J of lased light emitted from the source of light 11 (see
Polarization hologram 12 is formed to diffract a main beam of light at an angle of approximately −18° (in the negative direction as seen in direction X). The polarization hologram 12+1st-order diffracted beam is incident on polarization hologram 14a at approximately −18°. Polarization hologram 14a is formed so that the first and second areas 14a1 and 14a2 diffract the main beam's +1st-order diffracted beam at setting angles of approximately −28° and approximately −9°, respectively. The setting angles are angles relative to a downward direction (the negative direction as seen in direction Z) in
The
Group 15a includes light receiving portions 18, 19a, 19b, 19c and 20. Light receiving portions 18 and 20 as seen in a plane are each a rectangle having a length in direction X. Light receiving portions 19a and 19b as seen in a plane are each substantially square and also divided in three to detect a focus error signal by differential 3 division method. Each subarea has a length parallel to direction X. The three subareas form a center area and adjacent side areas. The center area is smaller in width than the adjacent side areas. The two adjacent side areas are substantially equal in area. Light receiving portion 19c is formed to be substantially square as seen in a plane.
The source of light provides lased light which is in turn divided by grating 16 in three, and the medium provides a reflection thereof which is in turn separated by the second polarization separation means or polarization holograms 14a, 14b (see
In
Group 15B is formed to be capable of receiving a beam transmitted through and +1st-order diffracted beams provided by the second polarization separation means or polarization hologram 14b. More specifically, the transmitted, and +1st-order and −1st-order diffracted beams impinges on rows indicated by arrows 14b0, 14bA and 14bB, respectively.
Thus group 15b is also formed to be capable of receiving nine of subbeams of laser light. Furthermore, group 15b is formed in symmetry with group 15a with respect to optical axis J of the lased light. Light receiving portion 23 is in symmetry with light receiving portion 20, and light receiving portions 22a, 22b and 22c are in symmetry with light receiving portions 19a, 19b, and 19c, respectively. Light receiving portion 21 is in symmetry with light receiving portion 18.
Furthermore in the present embodiment the optical miniaturized module includes a magneto-optic signal detection means detecting a differential signal of detected signals obtained from the light receiving portions that correspond to the second polarization separation means' diffracted and transmitted beams.
In the present description a diffracted beam of polarization separation means implemented by a polarization hologram that should preferentially be determined in design (i.e., a diffracted beam of a side presetting a position at which a diffracted beam arrives) is set as a +1st-order diffracted beam, and a diffracted beam opposite the +1st-order diffracted beam is set as a −1st-order diffracted beam. As such, the symbols “+” and “−” in the “+1st-order diffracted beam” and “−1st-order diffracted beam” do not indicate absolute directions but rather relative directions. The −1st-order diffracted beam's angle of diffraction, a position at which the beam is collected, and the like are determined as depending on a polarization hologram designed with the +1st-order diffracted beam considered.
In
Polarization hologram 12 forms +1st-order and −1st-order diffracted beams 12a and 12b which are in turn incident on polarization holograms 14a and 14b, respectively. These functions are similar to those of a conventional optical pickup device.
The first diffraction area 14a1 provides a diffracted beam including +1st-order and −1st-order diffracted beams 20a and 20b and the second diffraction area 14a2 provides a diffracted beam including +1st-order and −1st-order diffracted beams 21a and 21b. In
As shown in
The second polarization separation means or polarization hologram 14a is formed so that each diffraction area can diffract reflected light in a different direction. Polarization hologram 14a has two diffraction areas different in angle of diffraction, and each diffraction area generates +1st-order diffracted beams. In polarization hologram 14a the first diffraction area 14a1 grating 60a has a smaller pitch than the second diffraction area 14a2 grating 60b. Such configuration allows +1st-order and −1st-order diffracted beams 20a and 21b to overlap on a surface (or a receiving portion) of photodetector 15, as shown in
The grating of each diffraction area of polarization hologram 14a is not limited in geometry, pitch and curvature to the present embodiment. It depends on the laser light's wavelength and angle of incidence, the polarization hologram's material and distance from the photodetector, and the like. Accordingly, it is adjusted as appropriate for use. For example in the present embodiment the gratings are generally parallel and curved to protrude as seen in one direction. However, they are not limited thereto, and one area may have a grating that is generally unparallel and having an inflection point to have a further curved geometry.
The other second polarization separation means or polarization hologram 14b is functionally similar to polarization hologram 14a, allowing the first diffraction area's +1st-order diffracted beam and the second diffraction area's −1st-order diffracted beam to overlap on the photodetector. Furthermore, the first and second diffraction areas' −1st-order and +1st-order diffracted beams, respectively, also overlap on the photodetector.
The
When the spots of the rows indicated by arrows 114aA and 14aA, respectively, located, at their respective groups of light receiving portions farther from the center of the optical miniaturized module are compared in profile the latter is smaller in length as seen in direction X. For example, the
Furthermore, as shown in
Table 1 indicates lengths and surface areas of light receiving portions of a conventional photodetector and those of light receiving portions of the photodetector of the present invention. A light receiving portion has a length (as seen in direction X) including a uniform margin of 20 μm from its respective spot's end, as the precision of the light receiving portion's attachment and a production error in producing the light receiving portion are considered. As each spot's width (in direction Y) is generally equivalent, each light receiving portion's width W is set to be 80 μm.
In accordance with the present invention light receiving portion 19c has a length L19c shorter than a length L119c of light receiving portion 119c of conventional art. In contrast, when the light receiving portion 19b length L19b is compared with the light receiving portion 19b length L119b, length L119b based on conventional art is shorter than length L19b of the present invention. When each light receiving portion's surface area is calculated, however, the present light receiving portions 19b and 19c total surface area (of 13,040 μm2) is 13% smaller than the conventional light receiving portion 119b and 119c total surface area (of 14,960 μm2). Length L22b and L22c of light receiving portions 22b and 22c of group 15b are also similar to group 15a, and the light receiving portions can provide a reduced total surface area.
Thus the present optical miniaturized module can selectively reduce a large spot in size and allows a photodetector as a whole to have light receiving portions having a reduced surface area. As a result, stray light causing noise can be received on a reduced surface area so that the present optical miniaturized module can provide better C/N than a conventional optical miniaturized module. Furthermore, the photodetector can be reduced in size and the optical miniaturized module can be miniaturized.
As described above, the second polarization separation means includes a diffraction element having a plurality of diffraction areas each diffracting reflected light in a different direction and more than one diffraction area's diffracted light is directed to one light receiving portion so that diffracted light's aberration can be adjusted for example to reduce a spot in length to control the spot in profile. Furthermore, their respective diffracted beams can be overlapped or the like to control the position of the diffracted beams on the photodetector and the length of the spot thereof
The present light receiving portions 18, 20 are formed to be larger in length as seen in direction X than conventional light receiving portions 118, 120. Light receiving portions 118, 120 are formed not to receive the +1st-order diffracted beam of the row indicated by arrow 114aA, whereas light receiving portions 18, 20 are formed to also receive ±1st-order diffracted beams of the row indicated by arrow 14aA. Such configuration can increase signals of light receiving portions 20 and 18 and hence a tracking error signal calculated therefrom.
Furthermore in the present embodiment the diffraction element is formed so that a plurality of diffracted beams overlap at light receiving portion. Such configuration can provide a spot having a smaller area and hence light receiving portion having a smaller surface area. Alternatively, the diffraction element may be formed so that a portion of a plurality of reflected lights does not overlap or none of them overlap and they are sufficiently adjacent to each other. Such configuration also allows light receiving portions to have a reduced surface area.
Furthermore, as shown in
Furthermore in the present embodiment the photodetector is formed to receive at a single light receiving portion the second polarization separation means' one transmitted beam and ±1st-order diffracted beams corresponding to the transmitted beam. For example, in
The present invention does not necessarily allow each and every light receiving portion to have a reduced spot area. A spot can be smaller, whereas another can be larger. Accordingly the second polarization separation means is formed to provide a reduced total surface area of light receiving portions receiving diffracted reflected light.
Furthermore, if diffracted beams provided by the second polarization separation means do not overlap on the photodetector, a light receiving portion can be divided to correspond to their respective spots and they can be added together in subsequently processing an optical signal. Preferably, however, the light receiving portion is formed so that diffracted beams to be added together collected at a plurality locations can be received at a single light receiving portion. Such configuration allows the photodetector to have a reduced number of light receiving portions and hence a simplified configuration, and can also contribute to increased productivity of optical miniaturized modules.
Furthermore the present optical pickup device includes the above described optical miniaturized module and an optical lens. Such configuration allows the optical pickup device to be capable of controlling a position, profile and the like of a spot of reflected light at a light receiving portion and also providing improved C/N.
Reference will be made to
The source of light 11 and photodetectors 7a, 7b are arranged internal to package 39. The source of light 1.1 is arranged in package 39 on a bottom surface substantially at a center portion. Photodetectors 7a, 7b are arranged adjacent to the source of light 11. The source of light 11 is formed to be capable of providing lased light and on the laser beam's optical axis J a grating 5 is arranged to separate the laser light into three beams. Support plate 38 has a bottom surface provided with an optical substrate 3. Grating 5 is provided at a surface of optical substrate 3. On optical axis J at a surface of optical substrate 1 the first polarization separation means is implemented by a polarization hologram 2.
Magneto-optic storage medium 130 reflects light which is in turn diffracted by the first polarization separation means or polarization hologram 2 and thus separated into a +1st-order diffracted beam 2a and a −1st-order diffracted beam 2b. On an optical path of +1st-order diffracted beam 2a of polarization hologram 2 is arranged a phase plate 9a and the second polarization separation means implemented by a polarization hologram 4a. On an optical path of −1st-order diffracted beam 2b of polarization hologram 2 is arranged a phase plate 9b and the second polarization separation means implemented by a polarization hologram 4b. Support plate 38 has a hollowed portion at the center. Phase plates 9a and 9b are fixed by a fixture means (not shown) in the hollowed portion. Photodetectors 7a, 7b are arranged to be capable of receiving diffracted and transmitted beams from polarization holograms 4a, 4b.
In the present embodiment optical plates 1 and 3 each have a thickness of 0.35 mm and an index of refraction of 1.52. Support plate 38 has a thickness of 1.45 mm. Optical plate 3 has a bottom surface spaced from a light receiving portion by a distance (as seen in direction Z) of approximately 0.75 mm. The source of light 11 is adapted to provide lased light of 785 nm.
External to the optical miniaturized module on optical axis J are arranged a collimator lens 17 and objective lens 13 focusing the laser light at the medium 130 recording surface.
Furthermore, polarization hologram 2 is formed to provide a 0th-order diffraction efficiency of 77% and +1st-order diffraction efficiencies of 11% for P polarization, and a 0th-order diffraction efficiency of 0% and ±1st-order diffraction efficiencies of 44% for S polarization.
Polarization hologram 4a is arranged so that the polarization hologram 2 +1st-order diffracted beam 2a arrives on the circle substantially at the center portion. In
Polarization hologram 4b is divided by the second line 35 into two diffraction areas, and one of the areas is divided by the first line 34 into the first and second diffraction areas 4b1 and 4b2 and the other of the areas is divided by the first line 34 into the third and fourth diffraction areas 4b3 and 4b4.
The first and second lines 34 and 35 provide an intersection, which overlaps the center of the circle of polarization hologram 4b. In other words, polarization hologram 4b is divided at a main surface thereof by the first and second lines 34 and 35 into substantially four equal portions.
Polarization hologram 4b is arranged such that a main beam of light of −1st-order diffracted beam 2b of polarization hologram 2 arrives substantially at the center. In
In the present embodiment gratings 60g and 60h have their respective pitches in symmetry with respect to line 34 and so do gratings 60i and 60j. Furthermore, the gratings 60i, 60j pitch is smaller than the gratings 60g, 60h pitch. In other words, when the second line 35 is seen as a border, one diffraction area's grating has a pitch smaller than the other diffraction area's grating.
Gratings 60g, 60h, 60i, 60j are formed to be substantially parallel to direction Y. It should be noted, however, that the diffraction areas defined by line 35 are each curved to protrude in the negative direction as seen in direction X.
The first and second diffraction areas 4b1 and 4b2 gratings 60g and 60h have a tendency to have a pitch increased as seen toward the positive direction in direction X. The third and fourth diffraction areas 4b3 and 4b4 gratings 60i and 60j have a tendency to have a pitch generally maintained or increased as seen toward the positive direction in direction X. (Although areas P19 through to P17 provide a slightly decreasing pitch, it is substantially maintained.)
Furthermore at the second line 35 the gratings are interrupted. In each area, however, the corresponding grating is not interrupted. For example, while gratings 60g and 60j are interrupted, gratings 60g, 60h, 60i, 60j in their respective areas are each uninterrupted. Furthermore, at line 34 gratings 60g and 60h are interrupted and at line 34 gratings 60i and 60j are interrupted.
Furthermore within areas 4b1, 4b2, 4b3, 4b4 their respective gratings 60g, 60h, 60i, 60j each have their bars formed substantially parallel to each other in substantially the same direction as the second line 35, similarly as has been described in the first embodiment in connection with a polarization hologram serving as the second polarization separation means.
Polarization hologram 2 is formed to diffract a main beam of light at an angle of approximately −19° (in the negative direction as seen in direction X). The polarization hologram 2 −1st-order diffracted beam is incident on polarization hologram 4b at approximately −19°. Polarization hologram 4b is formed so that the first and second areas diffract the main beam's +1st-order diffracted beam at a setting angle of approximately −34° and the third and fourth areas diffract the main beam's +1st-order diffracted beam at a setting angle of approximately −3.5°. The setting angles are angles relative to a downward direction in
It should also be noted that
In the present embodiment the optical pickup device includes the optical miniaturized module and in addition thereto a collimator lens 17 and objective lens 13, as shown in
In
The laser light separated into the three beams passes through optical substrate 1 having polarization hologram 2, collimator lens 17 and objective lens 13 and illuminates medium 130. Medium 130 provides reflects light which in turn passes through objective and collimator lenses 13 and 17 to be incident on polarization hologram 2 serving as the first polarization separation means.
If medium 130 is an optical storage medium having information reproduced in accordance with optical Kerr effect, the reflected light has a polarization plane performed by Kerr-rotation effect in accordance with the information of medium 130. Accordingly, the reflected light slightly has an S polarized component. Polarization hologram 2 is formed to provide a 0th-order diffraction efficiency of 77% and ±1st-order diffraction efficiencies of 11% for P polarization, and a 0th-order diffraction efficiency of 0% and ±1st-order diffraction efficiencies of 44% for S polarization. Polarization hologram 2 has a function apparently multiplying the reflection's Kerr rotation angle.
Polarization hologram 2 provides −1st-order diffracted beam 2b which in turn passes through phase plate 9b and is incident on polarization hologram 4b. Polarization hologram 4 provides transmitted and diffracted beams which are in turn detected by photodetector 7b. Polarization hologram 4b is the second polarization separation means employed to detect a magneto-optic signal and from this polarization separation means' diffracted beam, focusing and tracking error signals are not detected. Polarization hologram 2 provides +1st-order diffracted beam 2a which in turn passes through phase plate 9a and is incident on polarization hologram 4a formed at optical substrate 3. Polarization hologram 4a separates the reflected-light into a transmitted beam and ±1st-order diffracted beams which are in turn detected by photodetector 7a. Polarization hologram 4a is the second polarization separation means employed to detect a magneto-optical signal, a servo signal and the like.
In polarization hologram 2 P and S polarizations have a phase difference (of several tens degrees) caused therein. By arranging phase plates 9a, 9b on optical paths of ±1st-order diffracted beams 2a, 2b of polarization hologram 2, a phase difference introduced in ±1st-order diffracted beams 2a, 2b can be corrected.
Magneto-optic storage media are reproduced in systems, which include a typical, magneto-optic storage medium reproduction system employed for example in mini disk players, Domain Wall Displacement Detection (DWDD) system and the like. If medium 130 is a DWDD magneto-optic storage medium, a phase difference is introduced in P and S polarizations of reflected light. In the present embodiment the optical miniaturized module has photodetectors 7b and 7a detecting −1st-order and +1st-order diffracted beams 2b and 2a, respectively, and each can independently detect a magneto-optic signal. By forming phase plate 9b to correct both the phase difference attributed to polarization hologram 2 and that attributed to the DWDD system, photodetector 7b can detect a magneto-optic signal of the DWDD system while photodetector 7a can detect an ordinary magneto-optic record signal. Thus in the present embodiment the optical miniaturized module allows two magneto-optic signals to be detected by a single unit.
When optical pickup devices perform focus servo, the knife-edge method is often employed as it contributes to reduced crosstalk (or reduced mixing of a push-pull signal). In particular, an optical pickup device that employs a polarization hologram having area 4a1 in the form of a half-moon as shown in
In
With reference to
When a signal output from a light receiving portion formed to receive transmitted light and diffracted light is represented by the light receiving portion's reference character preceded by the letter “S” a magneto-optic signal MO1 can be represented by:
MO1=S71−(S70+S72+S73+S74a+S75a+S74b+75b) (3).
A differential signal between detected signals of light diffracted by the second polarization separation means and that transmitted therethrough can thus be obtained. In the present embodiment a magneto-optic signal detection means is implemented by a signal calculation circuit (not shown) and by this circuit the above-described magneto-optic signal is calculated.
Furthermore, a focus error signal FES is represented by:
FES=S72−S73 (4).
Furthermore a tracking error signal TES1 by detection of a push-pull signal is represented by:
TES1=S74a−S75a (5).
Furthermore, a tracking error signal TES2 by the DPP method is represented by:
TES2=TES1−k{(S76+S77)−(S78+S79)) (6).
Photodetector 7b receiving a beam transmitted through and +1st-order diffracted beams provided by polarization hologram 4b will be described.
As shown in
Spots 41A-41D indicate spots of +1st-order diffracted beams and spots 42A-42D indicate those of −1st-order diffracted beams. In
Polarization hologram 4b is formed so that spots 41A, 41B of +1st-order diffracted beams by diffraction areas 4b1, 4b2 and spots 41C, 41D of +1st-order diffracted beams by diffraction areas 4b3, 4b4 are diffracted in direction X toward opposite sides with a transmitted beam (not shown) sandwiched therebetween. More specifically, the second polarization separation means or polarization hologram 4b with line 35 serving as a border provides two diffraction areas providing diffracted beams diffracted in opposite directions. Similarly, spots 42A, 42B of −1st-order diffracted beams through diffraction areas 4b1, 4b2 and spots 42C, 42D of −1st-order diffracted beams through diffraction areas 4b3, 4b4 are diffracted in opposite direction, as seen in direction X, with a transmitted beam (not shown) sandwiched therebetween.
Polarization hologram 4b is formed so that spots 41A and 4B partially overlap on the photodetector and so do spots 41C and 41D.
Furthermore, polarization hologram 4b is also formed so that spots 42A and 42B are adjacent to each other on the photodetector and so are spots 42C and 42D.
Furthermore, polarization hologram 4b is also formed so that spots 41C, 41D, 42A and 42B overlap or are adjacent to each other on photodetector 7b and so do or are spots 41A, 41B, 41C and 42D.
Polarization hologram 4b is thus formed so that diffracted beams overlap or are adjacent to each other on opposite sides of a transmitted beam.
Thus, of a set of +1st-order diffracted beams of the first and second diffraction areas, respectively, and a set of −1st-order diffracted beams of the first and second diffraction areas, respectively, at least one set has its two diffracted beams at least partially overlapping at a light receiving portion so that the light receiving portion can receive a small spot.
Alternatively, of a set of +1st-order diffracted beams of the third and fourth diffraction areas, respectively, and a set of −1st-order diffracted beams of the third and fourth diffraction areas, respectively, at least one set has its two diffracted beams at least partially overlapping at a light receiving portion so that the light receiving portion can receive a small spot.
Alternatively, diffracted beams proceeding in their respective directions can be brought to be adjacent to each other so that a light receiving portion can receive a small spot.
Furthermore, polarization hologram 4b is formed so that spots 41A and 41B of +1st-order diffracted beams through diffraction areas 4b1 and 4b2, respectively, arrive at the photodetector after they traverse each other so that they overlap on the photodetector and so do spots 41C and 41D of +1st-order diffracted beams through diffraction areas 4b3 and 4b4, respectively. Such configuration can prevent diffracted beams' spots remote in direction Y so that they can arrive at the photodetector's light receiving portion such that they overlap or are adjacent to each other in direction Y. For example, in direction Y, +1st-order diffracted beams through diffraction areas 4b1, 4b2 can overlap and −1st-order diffracted beams through diffraction areas 4b3, 4b4 can be adjacent to each other and thus arrive.
In the present embodiment polarization hologram 4b is in the form of a circle as seen in a plane. The polarization hologram is however not limited thereto. It is only required to be formed such that the first and second lines provide an intersection substantially overlapping the center of an incident laser beam of light (e.g., a main beam's position).
As shown in
A light receiving portion 86 is formed to receive transmitted beams of a main beam and light receiving portions 85 and 87 are formed to receive +1st-order diffracted beams. Light receiving portion 85 receives a spot of diffracted light corresponding to a collection of the
As shown in
The
For each light receiving portion, lengths L85, L87 L185, L87 are indicated as measured in a direction parallel to direction X and widths W85, W87, W185, W187 are indicated as measured in direction Y.
In the present embodiment the second polarization separation means or a polarization hologram can control not only a spot's length but also width to adjust the spot's width. Accordingly, light receiving portions are also compared in width. In Table 2, a spot's length and width are used to calculate a light receiving portion's surface area to compare a conventional light receiving portion and the present light receiving portion. The present light receiving portions 85 and 87 total surface area (of 28,863 μm2) is smaller than the conventional light receiving portions 185 and 187 total surface area (of 33,035 μm2). In particular, a reduction of approximately 13% in surface area can be achieved.
The polarization hologram's transmitted light is received at light receiving portions 86 and 186, which are not indicated in Table 2 as their respective spot sizes and their own sizes do not have a substantial difference.
Thus the second polarization separation means of the present invention allows a light receiving portion to have a reduced surface area and a photodetector to be miniaturized. Furthermore, the light receiving portion's reduced surface area can contribute to an improved C/N.
When a signal output from each light receiving portion shown in
MO2=S86−(S85+S87) (7).
Thus a differential signal of diffracted and transmitted beams provided through the second polarization separation means are calculated to calculate a magneto-optic signal. The operation is performed by magneto-optic signal detection means (not shown).
Note that light receiving portions 85 and 87 receive all +1st-order diffracted beams, and an unbalance with a quantity of light of transmitted light received at light receiving portion 86 can be reduced and common mode noise can sufficiently be reduced.
In the present embodiment the second polarization separation means or polarization hologram 4b is formed in symmetry with respect to the first line. However, it is not limited thereto and may have a grating formed for each of four diffraction areas defined by the first and second lines.
The present embodiment's optical miniaturized module is applicable for example to an optical pickup device employing an MO disk and can achieve an effect similar to that described above.
The other function and effect are similar to those of the optical miniaturized module and optical pickup device of the first embodiment.
The present invention can provide an optical miniaturized module and optical pickup device capable of controlling a position, profile, length or width of a spot of reflected light at a light receiving portion. The present invention can also provide an optical miniaturized module and optical pickup device capable of providing improved C/N of a magneto-optic signal.
Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-425636(P) | Dec 2003 | JP | national |
