Optical head for recording and reading information and objective lens for use therein

An objective lens used in an optical head having a convex lens and a chromatic aberration correction optical element wherein there is provided a positioning means that positions the center of gravity of the objective lens used in an optical head substantially on a straight line connecting supporting points for the objective lens used in an optical head.

BACKGROUND OF THE INVENTION

The present invention relates to an optical head for recording and reading information used for a compact disc (CD) device, a digital versatile disc (DVD) device and a magneto-optical disc device (MO) and to an objective lens used in the optical head, and in particular, to those capable of being applied to the aforementioned devices wherein a light source is a blue laser.

The optical head has an objective lens which forms an image of a laser beam on an information recording surface of an optical disk such as CD, DVD and MO as a spot and has a driving means which drives the objective lens for tracking and focusing.

In JP-A No. 5-210860, it is proposed that a position of the center of gravity of an optical head is made to agree positionally with a position of a driving point for the optical head for preventing resonance of the optical head caused in the course of tracking or focusing.

Further, in the objective lens, chromatic aberration is caused by wavelength changes resulted from output changes or temperature changes of a semiconductor laser representing a light source or is caused by changes of refractive index resulted from lens temperatures, however, the chromatic aberration cannot be corrected by the aforesaid one or two convex lenses. Therefore, correction of chromatic aberration conducted by the use of a diffracting lens is proposed in, for example, JP-A No. 6-82725, JP-A No. 6-242373, JP-A No. 7-294707, JP-A No. 11-95145, JP-A No. 11-337818, JP-A No. 2000-81566 and JP-A No. 2002-298422.

Since the objective lens having optical elements for correcting chromatic aberration is composed of a plurality of optical elements, the center of gravity thereof is in a position which is different from that of the center of gravity of the objective lens composed of one optical element. Therefore, if an objective lens having plural optical elements is mounted on a supporting frame of the optical head in the same way as in the conventional objective lens having one optical element, it results in that the objective lens is supported at the position deviated from the center of gravity of the objective lens.

Though the center of gravity of a movable portion in an optical head including an objective lens is made to agree positionally with a driving point, in JP-A No. 5-210860, there still is a problem that movements of the movable portion become complicated to cause a tilt and prevention of vibration becomes difficult, even when the center of gravity of the entire movable portion is made to agree with its driving point, if the center of gravity of the objective lens is deviated from its supporting point, which has been a problem.

In FIG. 12 of JP-A No. 5-210860, there is described an optical head wherein an objective lens is arranged at the position that is away from the driving point of a movable portion including an objective lens, and in FIG. 14, there is described an optical head wherein an objective lens is arranged on a line connecting driving points substantially. However, there is a possibility that the problems mentioned above are still caused on both optical heads described in FIG. 12 and FIG. 14 in JP-A No. 5-210860.

SUMMARY OF THE INVENTION

An object of the invention is to solve the problems of this kind.

The object stated above is attained by either one of the following Structures 1–5.

Structure 1: An objective lens used in an optical head having a convex lens and a chromatic aberration correcting optical element wherein there is provided a positioning means that positions the center of gravity of the objective lens used in an optical head substantially on a straight line connecting supporting points for the objective lens used in an optical head.

Structure 2: The objective lens used in an optical head described in Structure 1 wherein a lens frame is provided and the center of gravity mentioned above is a combined center of gravity of the entire objective lens including the convex lens, the chromatic aberration correcting optical element and the lens frame.

Structure 3: The objective lens used in an optical head described in Structure 1 or Structure 2 wherein the convex lens is of a one-element structure.

Structure 4: The objective lens used in an optical head described in Structure 1 or Structure 2 wherein the convex lens is of a two-element structure.

Structure 5: An optical head for recording and reading information equipped with an objective lens used for an optical head having a convex lens and a chromatic aberration correcting optical element wherein the center of gravity of the objective lens used for an optical head is positioned substantially on a straight line connecting supporting points for the objective lens used for an optical head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1shows an outline of an optical head for recording and reading information, and in the optical head, a light flux emitted from semiconductor laser1representing a light source is transmitted through beam splitter2representing a light composing and separating means to be transformed into a parallel light flux by collimator lens3and is transmitted through quarter-wave plate5to be stopped down by diaphragm7to a prescribed numerical aperture to be converged for image forming as a spot on an image recording surface of optical disc6representing an information recording medium such as CD, DVD and MO through objective lens4.

The reflected light flux modulated by information pits on the information recording surface of the optical disc6turns into a parallel light flux again through objective lens4, and it further passes through diaphragm7, quarter-wave plate5and collimator lens3to become a converged light, and is reflected on beam splitter2to pass through cylindrical lens8, and is subjected to correction of astigmatism and magnification change to be converged on a photosensitive surface of photodetector9. Numeral50in the figure represents a magnetic driving mechanism as a driving means for focusing and tracking, and it is constructed to drive objective lens4with magnetic driving mechanism50.

In reading of information, an output of photodetector9is used as reading information, and in recording of information, information is recorded on an information recording surface of optical disc6by light that is emitted from semiconductor laser1in which information has been modulated.

FIG. 2is a perspective view of an example of the optical head for recording and reading information relating to the embodiment of the invention. The objective lens driving mechanism in the example is of a wire suspension method wherein holder10representing a movable side member which holds objective lens4is supported on the side of apparatus frame13representing a fixed member, while being held under the condition to be movable in tracking direction T and focusing direction X (optical axis direction) by four wires11.

There is further provided magnetic driving mechanism50of a moving coil type as a driving means for moving the holder10in the tracking direction T and in the focusing direction X. This magnetic driving mechanism50is composed of driving coil unit20housed in the holder10and driving magnet30that is mounted on the side of apparatus frame13to face the driving coil unit20.

The holder10has therein trapezoidal lens holder10ahaving on its center objective lens4and frame portion10bthat extends downward in a square barrel shape whose one side is represented by the base of the lens holder10a. The holder10forms, in its inside, a rectangular holding space with the frame portion10band lens holder10a. On the lens holder10a, there is mounted objective lens4so that its optical axis may be in the direction of X.

A base end portion of each of four wires11which support the holder10is supported by supporting plate portion13athat is formed by lifting a part of apparatus frame13vertically, through flat-board-shaped gel pot12having a framework filled with gel agents. It is further soldered on wiring base board14that is mounted on the back of the supporting plate portion13a. The gel pot12is one for absorbing unwanted vibration caused on the wire11when the holder10is driven, while, the wiring base board14is one for supplying electricity to driving coil unit20by utilizing wire11.

On bottom plate portion13bof the apparatus frame13, there is fixed driving magnet30.

On the frame portion10b, there is attached driving coil unit20. The driving coil unit20is composed of tracking coil21, focusing coil22and yoke23. The yoke23is composed of central yoke main body231and yoke arms232on both sides. As shown inFIG. 3representing a diagram of the driving coil unit20, the tracking coil21is wound around the yoke main body231to have a winding portion that is in parallel with optical axis direction X, and the focusing coil22is wound around the yoke main body231to be perpendicular to the optical axis direction X. The yoke arm232of the driving coil unit20is fixed on frame portion10bof the holder10.

The tracking coil21and the focusing coil22are opposite to the driving magnet30, and the tracking coil21, the focusing coil22and the driving magnet30are arranged in the holding space that is formed inside the holder10.

Tracking signals generate driving power on the tracking coil21, which moves objective lens4in the direction of arrow T for tracking, and focusing signals generate driving power on the focusing coil22, which moves objective lens4in the optical axis direction X for focusing.

For example, as is studied in JP-A No. 5-210860, when the driving coil unit20is driven, resonance generated by moments which are determined by the positional relationship between the center of gravity of the driving coil unit20and its driving point result in a problem in tracking and focusing. Therefore, establishment of the center of gravity of the driving coil unit is important in a design of the optical head, and when objective lens4is composed of a plurality of optical elements as stated above, if the center of gravity of the objective lens unit is deviated from its holding position, there are caused problems that a tilt of the objective lens4comes into existence, or establishment of the center of gravity of the driving coil unit20at an appropriate position is difficult.

In the present embodiment, the problems stated above are solved by making the position of the center of gravity of the objective lens unit composed of plural optical elements to agree substantially with the supporting position for the objective lens unit.

Each ofFIGS. 4(a) and4(b) is a sectional view of an example of the objective lens for the optical head relating to the present embodiment of the invention. The objective lens4is composed of lens frame40that is called a bobbin, diffracting lens41and convex lens42.

The diffracting lens41is an optical element for correcting chromatic aberration, and it corrects chromatic aberration caused by changes in the refractive index of the lens resulting from changes in the oscillation wavelength of semiconductor laser1and from changes in temperatures. The diffracting lens41has a diffracting surface, and the diffracting surface has microscopic diffraction stripes which are formed to be ring-shaped zones having centers on the optical axis, as shown inFIG. 4(b). The diffracting lens41is one for correcting chromatic aberration by generating aberration so that chromatic aberration caused by convex lens42may be canceled, and it may be either a flat-board-shaped optical member having no refractive power or the one wherein a diffracting surface is provided on one surface of the lens having refractive power, and for example, it is possible to use those disclosed in JP-A No. 6-82725, JP-A No. 6-242373, JP-A No. 7-294707, JP-A No. 11-95145, JP-A No. 11-337818, JP-A No. 2000-81566 and JP-A No. 2002-298422 which are made of plastic mold lens. The numeral42is a convex lens that converges parallel light on an information recording surface of optical disc6, and transforms reflected radiation light on the information recording surface of optical disc6into parallel light, and a glass mold lens is preferably used for the illustrated single lens structure, and a plastic mold lens may also be used.

A flange portion of the diffracting lens41is fixed on lens frame40through a means of adhesion, and a flange portion of the convex lens42is stuck on the flange portion of the diffracting lens41so that the convex lens may be fixed on the diffracting lens41. The lens frame40is fitted to a circular hole provided on lens holder10ato be fixed through a means of adhesion.

In the course of connection between the lens frame40and the lens holder10a, when the center of gravity of the combination of the lens frame40, diffracting lens41and convex lens42is positioned at P1, positional relationship is established so that a line connecting supporting points P2and P3by lens holder10amay pass through combined center of gravity P1. Each of supporting points P2and P3is a point where line L that passes through the center in the thickness direction of lens holder10aintersects the line where lens holder10aand lens frame40are in contact each other. By establishing the combined center of gravity P1of the total of objective lens4including lens frame40, diffracting lens41and convex lens42to be positioned substantially on the straight line connecting supporting points P2and P3as stated above, the center of gravity of the movable portion is not moved in the optical axis direction X even when the objective lens4is mounted on the lens holder10a, which makes it easy to design a means to prevent vibration in tracking and focusing, and makes it possible to prevent vibration satisfactorily.

The positional relationship between lens holder10aand lens frame40is established by a supporting surface of flange40arepresenting a positioning means provided on the lens frame40. Therefore, the position of the supporting surface of flange40ais established in accordance with the position of the center of gravity of the objective lens40.

When the diffracting lens41is incorporated, the position of combined center of gravity P1is changed, in many cases, to a position that is deviated greatly toward the diffracting lens41from the position of the center of gravity of convex lens42itself.

Incidentally, it is also possible to omit lens frame40by providing, on a flange of diffracting lens41or convex lens42, a supporting surface that is engaged with lens holder10a.

FIG. 5shows an example wherein a convex lens is composed of two convex lenses42and43. For this convex lens of a two-element structure, a plastic mold lens is preferably used.

Even in the lens structure inFIG. 5, a supporting surface of flange40aof lens frame40is established so that center of gravity P1of objective lens4composed of lens frame40, diffracting lens41and convex lenses42and43may be positioned substantially on the line connecting supporting points P2and P3of lens holder10a.

FIG. 6shows an example wherein a lens supporting portion of lens holder10ais not flat-board-shaped but is curved. Even in the structure wherein lens holder10ais curved as shown inFIG. 6, each of supporting points P2and P3is a point where line S that passes through the center in the thickness direction of lens holder10aintersects the line where lens holder10aand lens frame40are in contact each other, in the same way as that explained above.

Incidentally, though there has been explained relationship between the center of gravity of objective lens4and the supporting point in the optical axis direction X, in establishment of the relationship between the center of gravity and the supporting point in the direction perpendicular to the optical axis direction X, the aforementioned center of gravity and the supporting point agree with each other automatically, because of the structure wherein lens frame40is fitted to a circular supporting hole provided on lens holder10a.

FIG. 7shows an example wherein convex lens42is supported by lens frame40and diffracting lens41is supported by the convex lens42. Namely, flange42ais provided on the convex lens42, and diffracting lens41is supported by the flange42a, and the flange42ais supported by the lens frame40. In this example, the position of the diffracting lens41is established on the basis of the position of the convex lens42.

In the invention described in either one of Structures 1–5, a tilt and a vibration of an objective lens which tend to be caused in the course of tracking or focusing can be prevented satisfactorily. Therefore, by using an optical element for correcting chromatic aberration, it is possible to realize an optical head for recording and reading information capable of reading and recording information stably even in the objective lens composed of plural optical elements or to realize an objective lens for use therein.