Combined Folding Mirror and Collimator Lens for an Optical Pickup Unit
FIELD OF THE INVENTION The present invention relates generally to optical disc drives and, more particularly, to an optical element for use in an optical pickup unit of an optical disc drive.
BACKGROUND OF THE INVENTION Optical information recording has been developed as a high-density recording method. The optical storage medium is in the form of a disc or a card. A conventional optical storage medium drive apparatus has a light source, such as a laser, for emitting a light beam, an optical system for guiding the light beam, a focus lens for focusing the light beam on the surface of the storage medium, and a light detector for converting the light beam reflected from the surface of the storage medium into an electric signal. Referring to Figures 1A and IB of the drawings, there is illustrated schematic plan and side view representations of the principal components of a DVD pickup unit according to the prior art. As shown, the illustrated DVD pickup unit comprises a laser diode 115, a grating 114, a photo-detector pre-amp integrated circuit IC 110, a beam splitter 111, a collimator lens 112, an objective lens 113, a folding mirror 116 and a polarizer 117. In operation, the laser diode 115 generates a laser beam, which then passes through the grating 114 to the beam splitter 111. At the beam splitter 111, part of the laser beam is reflected to the collimator 112 and collimated by the same. The collimated light from the collimator 112 is then reflected by the folding mirror 116 to the polarizer 117. The output light from the polarizer 117 is then focused by the objective lens 113 onto the information surface of the optical disc (not shown) being read. The reflected light from the optical disc (not shown) then propagates in the reverse direction through the polarizer 117, the folding mirror and the collimator 112 to the beam splitter 111. At the beam splitter, part of the reflected light transmits straight through the beam splitter to the photo-detector pre-amp IC 110. In response to the reflected light, the photo-detector pre-amp IC 110 generates a number of opto-electrical signals that can be processed in a conventional manner to retrieve the data stored on the optical disc. It is becoming increasingly desirable to produce an optical pickup unit OPU which is as compact as possible, and half-height or slim OPU's are becoming increasingly common. However, known OPU's are considered for some applications to be too thick. Furthermore,
conventional optical disc drives are quite complex in structure due to the fact that each single optical component can only provide a single optical function. For example, a folding mirror is used solely for reflecting the light incident on it to other directions, and a collimator is solely used to collimate the light passing through it. As a result, the total number of optical components constituting the optical pickup unit is large, thus increasing the structural complexity of the optical system of the pickup unit and leading to a constraint in respect of making the OPU slimmer, as well as increasing the cost of materials and the manufacturing process, and limiting the production yield due to the number of components, and the resultant process equipment, gluing operations and manpower required to assemble a conventional optical pickup unit.
SUMMARY OF THE INVENTION We have now devised an improved arrangement, and it is an object of the present invention to provide an optical element for use in an optical pickup unit of an optical drive system which is capable of reducing the height/size of the optical pickup unit relative to prior art arrangements. In accordance with the present invention, there is provided an optical element comprising a prism having first and second radiation transmissive plane surfaces and an inner radiation reflective surface, said first and second plane surfaces being provided with respective first and second aspherical lenses through which radiation respectively enters, is reflected from the inner radiation reflective surface and leaves said prism, said first and second aspherical lenses being adapted to collimate said radiation. As a consequence, the folding mirror and the collimator lens are combined in only one component. This reduces the number of components required to realize the optical pickup unit, thereby reducing its height/size relative to prior art arrangements, which is important for the continued efforts in manufacturing smaller and more compact platforms including optical drive systems. Moreover, it makes it possible to reduce the cost of materials, processing costs and cycling time required during production. In one exemplary embodiment of the invention, the aspherical lenses may be formed integrally with the first and second plane surfaces of the prism. Alternatively, the lenses may be attached to the plane surfaces. The prism is preferably in the form of a half cubic mirror, wherein the aspherical lenses are provided on first and second plane surfaces which are substantially at right angles to each other, with the reflective surface being provided on the hypotenuse side of the half cubic mirror.
In one exemplary embodiment, the optical element defined above may be formed in a substantially cubic beam splitter, and have a third plane surface comprising a servo lens. The third plane surface is preferably substantially cylindrical in shape. In yet another embodiment, the optical element may include a photo-detector and/or a radiation source. The present invention extends to an optical pickup unit including an optical element as defined above, and to an optical drive system including such an optical pickup unit. These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiments described herein.
BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which: Figures 1A and IB respectively are schematic plan and side views of an optical pickup unit according to the prior art; Figure 2 is a schematic diagram illustrating the principal components of an optical drive system according to a first exemplary embodiment of the present invention; and Figure 3 is a schematic diagram illustrating the principal components of an optical drive system according to a second exemplary embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION Referring now to Figure 2 of the drawings, an optical element 10 in accordance with a first exemplary embodiment of the present invention essentially comprises a combined folding mirror and collimator lens, consisting of a half cubic mirror having first and second radiation transmissive plane surfaces 14, 16 and a hypotenuse surface 17 which is coated with a reflective material such that it is capable of reflecting a laser beam incident thereon. The first and second plane surfaces are provided with respective first and second bi-convex aspherical lenses 18, 20 which together have the functionality of a collimator for collimating a divergent light beam from a light source. An optical drive system including such an optical element comprises a laser diode 22, a beam splitter 24 (which may or may not be polarizing, as required) and a photo-detector pre-amp IC 26, as before. Thus, the folding mirror and collimator lens of prior art arrangements are combined into a single optical element according to the above-described exemplary embodiment of the
present invention. As a result, the height of a conventional half-height or slim OPU can be further reduced. In addition, because one component is effectively eliminated relative to prior art optical drive systems, the material and process costs are reduced, as is the cycling time during production.
Referring to Figure 3 of the drawings, an optical element 30 according to another exemplary embodiment of the present invention essentially comprises a combined (polarizing or otherwise) beam splitter, optional servo lens, folding mirror and collimator lens, consisting of a cubic beam splitter having first and second radiation transmissive plane surfaces and an inner hypotenuse surface 37 which is coated with a reflective material such that it is capable of reflecting a laser beam incident thereon. The first and second plane surfaces are provided with respective first and second bi-convex aspherical lenses 18, 20 which together have the functionality of a collimator for collimating a divergent light beam from a light source. The reflective material of the inner hypotenuse surface 37 is such that it is capable of transmitting part of the light reflected from an optical disc (not shown) through the first aspherical lens 18 to a photo-detector pre-amp IC 26. In addition, a third plane surface of the cubic beam splitter is provided with a specially- shaped surface 40, i.e. a cylindrical type shape, which acts as a servo lens to generate a focus error signal, as will be apparent to a person skilled in the art. In an optical drive system including an integrated module such as that described above with reference to Figure 3 of the drawings, further comprises the photo-detector pre- amp IC 26 and a laser diode 22, as before. The integrated module according to this exemplary embodiment of the present invention makes the resultant OPU very compact, which is important in the development of a small-factor platform. Small- factor platforms are, in turn, useful in the exploration of further applications in the field of, for example, mobile telephones, etc. Still further, the new design reduces the number of components required, thereby decreasing material and process costs, and improving OPU reliability. Still further, in yet another exemplary embodiment of the present invention, the laser diode and/or the photo-detector pre-amp IC could also be integrated into the optical element, to form an even more integrated device.
The optical element as depicted in Figure 2 or in Figure 3 is integrated into an optical pickup unit of an optical disc drive, said optical pickup unit comprising in particular an objective lens. Basically, objective lens can be used to collimate a divergent beam, whereas a
collimator lens can be used to focus a parallel beam, as seen in a backward light-path from the disc in an optical pickup unit. There are some differences between collimator lens of the present invention and objective lens of the prior art, laying in that: (1) there is strict control in wavefront aberration particularly for forward light-path because of required size/shape of light beam spot on disc. Since the function of collimator lens, namely collimation, is different from objective lens (focusing), the geometrical configurations of lenses are different. Conventionally, for a collimator lens the radius of curvature (absolute value) at the vertex of front surface is (much) larger than that of rear surface while the reverse is true for objective lens; (2) the numerical aperture NA of an objective lens is almost standard and larger, e.g.
NA=0.43 for CD and NA=0.6 for DVD while NA of collimator lens is more variable and smaller, which causes design (radius or curvature, shape/pattern of surface) and manufacture (coating/molding) of collimator lens to be much easier, hence yield and quality are better. In an optical pickup unit the collimator lens is always closest to folding mirror, although the collimator lens could be in front of, or behind, the folding mirror, while in some cases, the collimator lens is located between objective lens and folding mirror. Therefore, combining folding mirror and collimator lens is more applicable than combining a folding mirror and objective lens in order to make optical pickup unit more compact. In addition, the prism-type objective lens is heavier and hence adversely affects sensitivity and dynamics of actuator.
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice- versa. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.