Abstract:
In an objective lens shared by various kinds of optical discs using different wavelengths, damage due to collision between the disc and lens is avoided. A protective member is provided in the entire periphery or in a part of the periphery of a protective protrusion of the objective lens. The protective member may replace the protective protrusion.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an objective lens for focussing a plurality of laser beams having different wavelengths onto an optical disc.  
           [0003]    2. Description of the Related Art  
           [0004]    Nowadays, there are many kinds of optical recording media, including CDs and DVDs. It is to be noted that there is a tendency to use a single objective lens for various kinds of discs, that is, to focus beams on, for example, a CD and a DVD using the same objective lens.  
           [0005]    In the case of a CD, a beam having a wavelength of 780 nm is focussed, and in the case of a DVD, a beam having a wavelength of 650 nm is focussed, so that the so-called working distance between the disc and the objective lens differs according to the disc used. The working distance is 1.3 mm for the CD, and 1.7 mm for the DVD.  
           [0006]    In the focus servo mechanism for the objective lens, a pick-up  101  moves vertically as the disc  100  moves vertically during rotation due to small warps in the disc  100 , as shown in FIG. 1. Apart from the pick-up  101 , the servo mechanism includes a feedback circuit comprising a focus error signal detection circuit  102  and a phase compensation drive amplifier  103 . The servo mechanism maintains the proper working distance between the objective lens and the disc  100 .  
           [0007]    As shown in FIG. 2A, the objective lens  101 L moves vertically by the excitation of coils  101 C. As shown in FIG. 2B, during operation the focus servo maintains the proper working distance by moving the objective lens  101 L towards the disc  100  and then moving the objective lens  101 L away from the disc  100 .  
           [0008]    When focus servo moves the objective lens over wide ranges for different working distances, it frequently occurs that the objective lens collides with the back surface of the disc when operating at the smaller working distance. When the objective lens collides with the back surface of the disc, either the objective lens itself or the back surface of the disc is damaged.  
         SUMMARY OF THE INVENTION  
         [0009]    Accordingly, it is an object of the present invention to provide an objective lens which prevents damage of the lens and the back surface of the disc as a result of collisions.  
           [0010]    To achieve the above object, according to a first aspect of the present invention, there is provided an objective lens for focussing a plurality of laser beams having different wavelengths onto an optical disc. The objective lens comprises a lens main body and a protective protrusion integrally formed around the lens main body. The protective protrusion protrudes beyond the apex of the convex surface of the lens main body. At least the portion of the protective protrusion protruding beyond the apex of the convex surface of the lens main body is provided with a protective member.  
           [0011]    According to a second aspect of the invention, the protective member is provided at least partially in the periphery of the lens main body.  
           [0012]    According to a third aspect of the invention, the protective member is arranged to replace the entire protective protrusion of the lens main body.  
           [0013]    According to a fourth aspect of the invention, the protective member is arranged to replace the entire protective protrusion and is provided at least partially in the periphery of the lens main body.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a block diagram of a focus servomechanism;  
         [0015]    [0015]FIGS. 2A and 2B are schematic views illustrating the focus servomechanism;  
         [0016]    [0016]FIGS. 3A, 3B and  3 C are sectional views of objective lenses;  
         [0017]    [0017]FIG. 4 is a schematic view of a disc playback apparatus; and  
         [0018]    [0018]FIG. 5 is a schematic view of another disc playback apparatus. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    Embodiments of the present invention will now be described with reference to FIGS. 3A through 5. In FIGS. 3A, 3B and  3 C, the upper side of the lens is opposed to the back side of the disc. In FIG. 3A, a protective member  1   b  separate from a lens main body  1   a  is attached to a peripheral flat surface portion of the lens main body  1   a  of an objective lens  1  opposed to the back side of the disc.  
         [0020]    This protective member  1   b  is formed as a ring surrounding the lens main body  1   a  and is high enough so as to protrude beyond a plane  1   d  which is tangent to the apex of the convex surface  1   c  of the lens main body  1   a.    
         [0021]    The protective member  1   b  is preferably formed of an elastic material having a fixed level of hardness, such as rubber, plastic or other suitable materials. Examples of suitable rubber materials include: natural rubber, butyl, polyurethane Viton™ and other synthetic rubbers. Examples of suitable plastics include: nylon, Teflon, Delrin, Torlon and polycarbonate. Examples of other suitable materials include: cotton, paper and other soft materials.  
         [0022]    In FIG. 3B, the top portion of a protective protrusion  1   p  of the conventional objective lens  1  is cut away, and the protective member  1   b  is attached to the surface of the portion of the protective protrusion  1   p . The protective member  1   b  protrudes beyond a plane  1   d  which is tangent to the apex of the convex surface  1   c  of the lens main body  1   a . That is, the protective member  1   b  is only provided on the protective protrusion  1   p  surrounding the lens main body  1   a . In one embodiment, the protective protrusion  1   p  is integrally formed with the objective lens  1 . The protective member  1   b  is stacked on top of the protective protrusion  1   p . Due to the small working distance between the objective lens  1  and the disk, it is important to minimize the height of the protective member  1   b  above the apex of the convex surface  1   c  of the lens main body  1   a . If the protective member  1   b  protrudes to a larger degree than necessary it is liable to collide with the back surface of the disc, which is undesirable. That is, it is undesirable to simply stack the protective member  1   b  on the protective protrusion  1   p  well beyond the lens main body  1   a  since the protective member  1   b  would forcibly rub against the back surface of the disc. Further, the reproduction of signals would be hindered and the focus mechanism could be damaged since focus servo is constantly operating during the rotation of the disc.  
         [0023]    [0023]FIG. 3C illustrates an embodiment of the present invention wherein the protective member  1   b  is formed around the perimeter of the objective lens  1 . The protective member  1   b  may be molded or attached by other means including: bonding, adhesives and melting around the objective lens  1 . Again the protective member  1   b  protrudes beyond a plane  1   d which is tangent to the apex of the convex surface  1   c  of the lens main body  1   a.    
         [0024]    While in the examples shown in FIGS. 3A  3 B and  3 C the protective member  1   b  is formed continuously around the lens main body  1   a , it is possible in other embodiments to provide a plurality of protective members  1   b  at equal intervals or unequal intervals in the periphery of the lens main body  1   a . In still other embodiments, the protective protrusion  1   p  is a plurality of equal intervals or unequal intervals in the periphery of the lens main body  1   a . The height of the stacked protective member  1   b  and protective protrusion  1   p  is the same as the previously described embodiments. Furthermore, even if collision occurs, it is not the protective protrusion  1   p  but the protective member  1   b  that collides with the back surface of the disc. This protective member  1   b  may be formed by applying a protective material to the protective protrusion  1   p . Because the protective member  1   b  is made of a soft material, damage to the lens  1  and disc are substantially reduced as compared with objective lenses not having a protective member  1   b.    
         [0025]    [0025]FIGS. 4 and 5 show playback apparatuses for a disc  100  which includes: a laser beam source  10 , an aperture, a beam splitter  12 , a collimator lens  13 , a focusing lens  14 , a photodetector  15  and an objective lens  1 . The laser beam source  10  can be: a laser diode, a semiconductor laser, or the any other suitable laser source. The beam splitter  12  separates incident light from the laser beam source  10  and reflected light from the disc  100 . The collimator lens  13  produces a parallel beam for the objective lens  1 . The focussing lens  14  focuses light onto the photodetector  15 .  
         [0026]    The apparatus shown in FIG. 5 is similar to that shown in FIG. 4 except that in FIG. 5, the incident beam from the laser beam source  10  is transmitted through the beam splitter  12  and the output beam from the disk  100  is reflected by the beam splitter  12 . In the apparatus shown in FIG. 4 the incident beam from the laser beam source  10  is reflected by the beam splitter and the output beam from the disk  100  is transmitted through the beam splitter  12 . Although not shown in FIGS. 4 and 5, the objective lens  1  has a protective member around the periphery of the lens main body as illustrated in FIGS. 3A, 3B and  3 C.  
         [0027]    While only three examples are shown in FIGS. 3A, 3B and  3 C, various modifications are possible as long as the protective member  1   b  protrudes beyond a plane  1   d  tangent to the apex of the convex surface  1   c  of the lens main body  1   a . Further, the objective lens of this embodiment is applicable to any system in which a single objective lens is used to read from and/or write to a plurality of kinds of discs, requiring changes in the focal length and working distance.  
         [0028]    As described above, in accordance with the present invention, an objective lens has a protective member on its surface which protects it from colliding with the associated disc, whereby damage to the lens and disc is avoided.  
         [0029]    In the foregoing, an objective lens has been described. Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.