Abstract:
In the case of a disk with a high NA which enables high-density recording, since the coma aberration increases as the numerical aperture, NA, increases, a disk protective layer needs to be thin very much, and a damage or scratch imparted to a surface of the disk affects largely the record and reproduction of data. An object of the invention is to reduce the generation of damage to a storage disk due to a collision between a lens holder or an objective lens and the storage disk. With a view to attaining the object, according to the invention, a longitudinal direction of a protector  11  provided on a holder is made substantially parallel to a straight line which connects the rotational center of a motor with the center of an objective member.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a storage disk unit which plays back or implements recording on a storage disk and more particularly to a disk unit having a protector which is formed at a location which is or is likely to be brought into contact with a storage disk for preventing a damage thereto.  
         [0003]     2. Description of the Related Art  
         [0004]     In recent years, in association with development in digital technologies, there has been a demand for high-density storage disks such as optical disks, hard disks and MO&#39;s. Due to this, once a scratch is imparted to such a storage disk, there will be caused a damage of far more digital data than digital data that would be damaged when a scratch is imparted to a conventional storage disk, and hence, there has been a demand for countermeasures thereagainst.  
         [0005]     In particular, in the area of optical disks, there have been developed various types of optical disks. In portable personal computers which can be carried and so-called note-type personal computers, it is assumed that an impact is applied to an optical disk unit accidentally while playing back or implementing recording on a CD or DVD, leading to a concern that a lens holder or an objective lens is then brought into collision with a surface of the optical disk to thereby damage the surface. In addition, in an optical disk unit having a function to gather light with the same lens for CD and DVD, there occurs a problem that an objective lens collides against an optical disk due to a difference in working distance therebetween, and this may result in a case where the objective lens and/or a recordable surface of the optical disk is damaged.  
         [0006]     Furthermore, in recent years, in order to realize such high-density optical disks, objective lenses of a high numerical aperture or NA are used for pickups, and short-wavelength laser light sources are used as light sources. When the numerical aperture of an objective lens is increased, a space (a working distance W) between the objective lens and the optical disk is inevitably narrowed as long as the diameter of the lens is not increased. As this occurs, an interval between the objective lens and the optical disk gets narrowed very much. Due to this, there occurs a collision between the objective lens and the optical disk due to the degree of precision of the optical disk itself and the deflection of the optical disk in operation, and there is caused a higher possibility that a damage is made onto the objective lens and/or the recordable surface of the optical disk.  
         [0007]     To deal with the problem that the objective lens and the recordable surface of the optical disk of such an optical disk unit are damaged, an objective lens collision preventive protector portion is provided on the periphery of the objective lens in such a manner as to protrude further towards the optical disk than the objective lens. By providing such an objective lens collision preventive protector portion, even in the event that there occurs an abnormal approach between the objective lens and a disk protective layer, since the objective lens collision preventive protector comes into contact with the disk protective layer earlier than the objective lens, a damage to the objective lens can be prevented. In addition, by using resin or the like as the objective lens collision preventive protector portion, even in the event that the objective lens collision preventive protector portion comes into collision with the disk preventive layer, a damage to the surface of the disk protective layer can be prevented.  
         [0008]     As the objective lens collision preventive protector portion, there are disclosed techniques in which a closest portion to an optical disk is formed of a material which is softer than the optical disk (refer to, for example, JP-A-2-54433), a silicone rubber is mounted on the periphery of an objective lens (refer to, for example, JP-A-11-312322), a cushion material of wool felt or the like is mounted on the periphery of an objective lens (refer to, for example, Japanese Patent No. 2593998), and an elastic element such as rubber is mounted on an objective lens itself (refer to, for example, JP-A-2000-242958).  
         [0009]     Even an objective lens collision preventive protector portion like the one disclosed in the aforesaid patent documents Nos. 1 to 4 can be considered to fulfill its function sufficiently for a disk having a relatively thick disk protective layer as of the conventional CD or DVD. In the case of the disk with the high NA which enables high-density recording, however, since the coma aberration increases as the numerical aperture increases, the disk protective layer needs to be thin very much. The fact that the disk protective layer is thin means that a damage or scratch imparted to the surface of the disk affects the record and reproduction of data more than before, and hence, there is a demand for countermeasures thereagainst.  
         [0010]     An object of the invention is to reduce the generation of a damage to a storage disk resulting from the collision between the lens holder or the objective lens and the storage disk.  
         [0011]     A longitudinal direction of a protector provided on a holder is made substantially parallel to a straight line which connects the rotational center of a motor with the center of an objective member.  
         [0012]     According to a disk unit of an embodiment of the invention, it is possible to reduce the occurrence of a damage to a storage disk due to a collision between a protector and the storage disk. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is an exemplary diagram of an embodiment of the invention.  
         [0014]      FIG. 2  is an exemplary diagram showing a comparison example to the embodiment of the invention.  
         [0015]      FIG. 3  are exemplary diagrams showing possible cross sections according to the embodiment of the invention.  
         [0016]      FIG. 4  is a graph showing cross sectional shapes of the embodiment of the invention and a comparison example.  
         [0017]      FIG. 5  are exemplary diagrams showing the embodiment of the invention and the comparison example.  
         [0018]      FIG. 6  is an exemplary diagram showing a direction in which fragments are thrown which are generated due to a collision between a storage disk and a protector.  
         [0019]      FIG. 7  is an exemplary diagram of the embodiment of the invention.  
         [0020]      FIG. 8  are exemplary diagrams showing possible cross sections according to the embodiment of the invention.  
         [0021]      FIG. 9  are exemplary diagrams of the embodiment of the invention.  
         [0022]      FIG. 10  are exemplary diagrams showing possible cross sections according to the embodiment of the invention.  
         [0023]      FIG. 11  is a diagram showing the whole of experimental equipment according to the embodiment of the invention.  
         [0024]      FIG. 12  is an exemplary diagram showing a section of the whole of the experimental equipment according to the embodiment of the invention.  
         [0025]      FIG. 13  is a diagram showing an optical disk unit according to the embodiment of the invention.  
         [0026]      FIG. 14  is a diagram showing part of the optical disk unit according to the embodiment of the invention.  
         [0027]      FIG. 15  is a diagram showing part of an optical pickup unit according to the embodiment of the invention.  
         [0028]      FIG. 16  is a graph showing the results of a test carried out according to the embodiment of the invention.  
         [0029]      FIG. 17  is a graph showing the results of a test carried out according to the embodiment of the invention.  
         [0030]      FIG. 18  is a diagram showing the contents of the test carried out according to the embodiment of the invention.  
         [0031]      FIG. 19  is a graph showing the results of a test carried out according to the embodiment of the invention.  
         [0032]      FIG. 20  is a diagram showing a test method according the embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     Hereinafter, an embodiment of the invention will be descried in detail using the accompanying drawings. Note that dimensions given on the drawings are such as to facilitate the understanding of the invention and are different from actual dimensions.  
         [0034]      FIG. 11  is a perspective view of an objective lens holder of experimental equipment for obtaining the depth of a damage mark given onto an optical disk through a collision test, and  FIG. 12  is a sectional exemplary diagram showing the whole of the experimental equipment for obtaining the depth of a damage mark given onto an optical disk through a collision test. An optical pickup actuator resulting from a modification made to an optical pickup actuator which is removed from an optical disk drive unit UJDA-710 manufactured by Panasonic Communications Co., Ltd. is used for an optical pickup actuator  122  used in experimental equipment  121  shown in  FIGS. 11 and 12 .  
         [0035]     As shown in  FIG. 11 , on an objective lens holder  123  of the optical pickup actuator  122  removed from the disk drive unit UJDA-710 manufactured by Panasonic Communications Co., Ltd., a test piece  124  on which a protective member is formed is mounted on an objective lens mounting portion  125  in place of an objective lens which should have been mounted originally.  
         [0036]     The test piece  124  is made up of a base portion  126  and a protective member forming portion  127 . The base portion  126  is sized so as to be just fitted in the objective lens mounting portion. For the protective member forming portion  127 , protective member forming portions of different shapes are used properly depending on purposes, and a resin material is dropped onto the protective member forming portion  127  to form a protective member through polymerization. The optical pickup actuator  122  on which the test piece  124  configured as described above is mounted is set at a position shown in  FIG. 12  relative to an optical disk  128  or is set so as to produce a distance of 0.3 mm between a distal end of the protective member of the test piece  124  and the optical disk  128  in such a state that the objective lens holder  123  is at rest. In addition, in order to prevent the fall of the test piece  124 , the test piece  124  may be fixed to the objective lens mounting portion  125  of the objective lens holder  123  using, for example, an adhesive.  
         [0037]     Using the experimental equipment  121  that has been described above, a collision test is to be carried out with the following points observed to obtain the depth of a damage mark inflicted onto the optical disk  128  and a wear amount of a protector surface layer material by causing the optical disk  128  to spin. Although not shown, there is a flexible substrate provided on the optical pickup actuator to which signals are imparted from an external unit to displace the objective lens holder  123  in a focusing direction and a tracking direction, and terminals are provided on the flexible substrate to which the various signals are imparted. By giving a sine-wave signal of a maximum voltage amplitude of 1.3V and a frequency of 2 kHz between two terminals among the terminals so provided which function to displace the objective lens holder  123  in the focusing direction, the objective lens holder  123  is caused to oscillate in a vertical direction, so as to cause the protective member forming portion  127  of the test piece  124  to collide against the same location on the optical disk  128  intermittently. In the case of the experimental equipment  121 , the rotational speed of the optical disk  128  is adjusted so that the linear velocity at a location thereof where the protective member of the test piece  124  is brought into contact becomes 10.56 m/sec. The number of times of collision between the protective member forming portion  127  and the same location on the optical disk  128  is 500 times. In the experimental equipment  121  shown in  FIGS. 11 and 12  which is set as has been described above, a contact time between the protective member and the optical disk  128  in a single collision is 0.2 second.  
         [0038]     In an experiment carried out in the way described above, the depth of a damage mark inflicted onto the optical disk  128  was measured and the result of the measurement was made to be a damage mark depth in the damage infliction test.  
         [0039]     In addition, a Japanese DVD-R which was not treated with a hard coating was used as the optical disk in this experiment. An optical interference three-dimensional measuring machine manufactured by Micromap was used to measure the depth of a damage mark.  
         [0040]     A protector and a lens holder according to an embodiment of the invention are shown in  FIG. 1 , and those according to a comparison example are shown in  FIG. 2 . In general, when a protector  11  collides against a storage disk  14 , a stress resulting from the contact is considered to be dispersed to a greater extent as the contact area increases. In this case, as shown in  FIG. 1 , a resulting damage mark can be reduced by making a longitudinal direction of the protector  11  parallel to a straight line which connects the rotational center of a spindle  25  which constitutes the rotational center of the storage disk  14  with the center of an objective lens  13  which is an objective member. In other words, the longitudinal direction of the protector  11  is disposed parallel to a rotational axis of the storage disk  14  and a plane which passes through the center of the objective lens  13 . Furthermore, in other words, the longitudinal direction of the protector  11  is disposed parallel to a radial direction of the storage disk  14 . Furthermore, in other words, the longitudinal direction of the protector  11  is disposed parallel to a direction which extends across data tracks of the storage disk  14 .  
         [0041]     When used herein, parallel means that the length of the longitudinal, data-track traversing direction of the protector  11  is longer than the length of a tangential direction of the protector  11  to the data tracks, and the longitudinal direction of the protector  11  may only has to be within +/−45° relative to a rotational normal of the storage disk  14 . By disposing the longitudinal direction of the protector  11  parallel to the direction which extends across the data tracks lying in a location on the storage disk  14  which the protector  11  is made to face, the length of the tangential direction of the protector  11  to the data tracks can be minimized, thereby making it possible to reduce the infliction of a damage to the storage disk. This effect increases as an angle relative to the rotational normal of the storage disk  14  decreases, whereby the infliction of a damage to the storage disk can be reduced further.  
         [0042]     By adopting the configuration described above, while the contact area between the protector and the storage disk is increased, an opportunity where the protector is brought into contact with the same location on the storage disk can be reduced, whereby it becomes possible to reduce the damage to the storage disk due to a collision that would be able to occur between the protector and the storage disk while data is being written to or read out of the storage disk.  
         [0043]     In addition, when the longitudinal direction of the protector  11  is disposed parallel to a tangent to the rotational direction of the storage disk  14  as shown in  FIG. 2 , a damage to the storage disk  14  becomes larger than with a protector  11  whose contact area with the storage disk  14  is reduced as with Comparison Example 1, which will be described later on. Data on the depth of a damage mark inflicted on the storage disk by the protector according to the embodiment of the invention is shown in (Table 1), the data including for comparison depths of damage marks inflicted by comparison examples. Comparison Example 1 is a protector in which a tangential direction of the protector to the rotational direction of the storage disk is substantially equal to a normal direction thereof to the rotational direction of the storage disk, while Embodiment 1 is a protector which is made to extend longer in the tangential direction of the protector to the rotational direction of the storage disk. When comparing Example 1 with Comparison Example 1, the depth of a damage mark in Example 1 is suppressed to on the order of one tenth the depth of a damage mark in Comparison Example 1. This is because the damage to the storage disk is considered to be suppressed by an increased contact area between the storage disk and the protector. Comparison Example 2 is such that the protector of Example 1 is disposed to extend in the tangential direction to the rotational direction of the storage disk, and although the contact area is increased in this case, a damage inflicted on the storage disk is increased. It is seen from this that when attempting to increase the contact area between the storage disk and the protector, it is much effective to form the protector in such a manner as that the normal direction thereof to the rotational direction of the storage disk is elongated.  
         [0044]     Since the storage disk spins, it is considered that the longer the normal direction of the protector to the rotational direction of the storage disk is elongated the longer the length of the protector becomes which comes into contact with the same location on the storage disk, and the damage inflicted on the storage disk is increased. Due to this, it is considered that an opportunity is reduced in which the protector comes into contact with the same location on the storage disk by making the longitudinal direction of the protector provided on the holder parallel to a straight line which connects the rotational center of the rotational driving member with the center of the objective member and that the damage to the storage disk can be reduced compared to a case where the longitudinal direction of the protector is made normal to the straight line.  
                                         TABLE 1                                   Samples   Damage Mark Depths (nm)                                        (Example 1)   23           (Comparison Example 1)   220           (Comparison Example 2)   417                      
 
         [0045]      FIG. 3  shows examples of cross sections of the protector according to the embodiment of the invention which are taken along a shorter side or widthwise. While as the length of the protector along the shorter side thereof becomes shorter, the opportunity of the protector coming into contact with the same location on the storage disk can be reduced, with the protector formed into a shape in which the cross section of the protector becomes rectangular, in the event that the length of the shorter side of the protector is made short, the strength of the protector is deteriorated, resulting in the failure of the protector. As shown in FIGS.  3 ( a ) to ( c ), by forming the protector in such a manner that the cross section thereof takes a shape which protrudes towards the storage disk or is formed into a shape which widens towards its bottom, the strength of the protector is increased while the contact point with the storage disk is reduced. In addition, as shown in  FIG. 3 ( d ), a shape is also effective in which a plurality of convex shapes are aligned. With the shapes which protrude towards the storage disk, air flowing in the direction of the protector is compressed by a flow of air generated by the rotation of the storage disk, whereby a repulsion force is generated between the storage disk and the protector, and there is also provided an advantage that an impact generated by a collision can be reduced. In this case, when the cross section taken along the shorter side of the protector includes an arc-like shape, air flows in smoothly, increasing the advantage of reducing the impact. Namely, buoyancy is generated by air generated by the rotation of the storage disk, whereby the impact caused when the protector comes into collision with the storage disk can be mitigated.  
         [0046]     Data on the depth of a damage mark inflicted on the storage disk by the protector according to the embodiment of the invention is shown in (Table 2), the data including for comparison the depth of a damage mark inflicted by a comparison example. Data resulting from actual measurements of cross sectional shapes of Example 2 and Comparison Example 3 are shown in  FIG. 4 . It is seen that with a shape which protrudes towards the storage disk like Example 2, the damage to the storage disk can be reduced largely.  
                                         TABLE 2                                   Samples   Damage Mark Depths (nm)                                        (Example 2)   21           (Comparison Example 3)   296                      
 
         [0047]      FIG. 5  shows an exemplary diagram of a lens holder having only one protector. With only one protector provided, as shown in  FIG. 5 ( b ), there is caused a risk that the objective lens or the lens holder comes into collision with the storage disk earlier than the protector. Due to this, when at least two protectors are provided on both the sides of the objective lens as shown in  FIG. 1 , the collision of the objective lens or the lens holder with the storage disk can be prevented. Although there is no specific limitation on the number of protectors because with two or more protectors provided, the intended object can be attained, the number of protectors to be provided is suitably in the range from 2 to 4 when considering the productivity. When a pair of protectors are positioned in such a manner as to hold the objective lens therebetween while being arranged in the tangential direction to the rotational direction of the storage disk, the objective lens or the lens holder can be prevented from coming into collision with the storage disk.  
         [0048]     Thus, by providing the protectors on both the sides of the objective lens, the objective member or the holder can be prevented from coming into collision with the storage disk earlier than the protectors.  
         [0049]      FIG. 6  shows an exemplary diagram of the lens holder. In the event that a protector resides at a position shown in  FIG. 6 , fragments of the protector generated by a collision of the protector with the storage disk are thrown in a direction indicated by an arrow, resulting in a high possibility that the fragments so thrown adhere on to the objective lens. Consequently, the fragment can effectively be prevented from being thrown on to the objective lens by attempting not to dispose the protector in an area on the lens holder which is defined as lying further upstream in the rotational direction and radially closer to a rotational center of the storage disk than the objective lens. To describe specifically, the objective lens can be prevented from getting dirty to thereby increase the reliability in wiring or reading data by attempting not to dispose the protector in an area  16  which is surrounded by a square in  FIG. 7 .  
         [0050]     Thus, by disposing the protector at a position which lies on the holder except for the area defined as lying further upstream in the rotational direction and radially closer to the rotational center of the storage disk than the objective member, the fragments that are generated by the collision between the protector and the storage disk can be prevented from adhering on to the objective member.  
         [0051]      FIG. 8  shows exemplary diagrams of the protective member forming portion of the lens holder  12 . As shown in  FIG. 8 , the protective member can be prevented from flowing to other locations by raising or lowering the portion where the protective member is to be formed, and the protective member forming area can be controlled easily. By utilizing this method, the shape of the protective member can be controlled with good accuracy even by a simple and highly productive production method like one in which a protective member is dropped to be cured using a dispenser or the like.  
         [0052]     While in general, a highly rigid and hard material is used for the lens holder  12 , by making up the protector  11  of the protective member which is formed on the base material which makes up the lens holder  12 , a soft material can be disposed on a surface which is brought into collision with the storage disk, and therefore, the damage to the storage disk can be reduced further. Furthermore, when a photo-setting urethane acrylate is used, the productivity can be increased and the production costs can be reduced.  
         [0053]      FIG. 9  shows exemplary diagrams of an upper surface of the protective member forming portion of the lens holder. When attempting to form a protective member on a protective member forming portion  17  having an elongated shape as shown in  FIG. 9 ( a ), there occurs a failure in which a protective member does not spread over the whole of the protective member forming portion but only part thereof is applied with the protective member. To solve the problem, when forming one or more grooves  18  on the protective member forming portion as shown in  FIG. 9 ( b ), the protective member is made to easily spread over the whole of the protective member forming portion  17 , and the control of the shape thereof can be ensured.  
         [0054]     Thus, by providing at least one groove on the protective member forming portion  17 , when applying a protective member such as a resin material on to the holder with a dispenser or the like, since the protective member spreads quickly over the whole of the protective member forming portion, the application of the protective member can be facilitated, and the variability of the shape of the protective member applied can be suppressed.  
         [0055]      FIG. 10  shows exemplary diagrams of cross sections of the protective member forming portion of the lens holder which are taken along the longitudinal direction thereof.  FIG. 10 ( a ) shows an example in which no groove is formed on the protective member forming portion, and in case no special molding is implemented only with the protective member applied thereto, a center point is increased in height, and a longitudinal contact area between the storage disk and the protective member is reduced. When forming a groove which passes through the center of the protective member forming portion as shown in  FIG. 10 ( b ), there occurs a contraction when the protective member is polymerized, whereby the center point is slightly lowered in height, thereby making it possible to increase the contact area. When making use of this phenomenon, the shape of the protective member can freely be controlled according to purposes. There is no specific limitation on the shape of the groove  18  formed on the protective member forming portion, and hence, a groove which is triangular circular in cross section may be adopted as shown in FIGS.  10 ( c ), ( d ).  
         [0056]     Thus, by forming a straight or curved groove which passes through substantially the center of the protective member forming portion when viewed from the storage disk side, the shape of an apex of the protective member, that is, a portion which comes into collision with the storage disk can be controlled.  
         [0057]     Next, an optical disk unit to which the protector  11  of the embodiment is applied will be described.  
         [0058]      FIG. 13  is a drawing which shows an optical disk unit according to the embodiment of the invention. In an optical disk unit  301  shown in  FIG. 13 , reference numeral  302  denotes a cover, and the cover  302  is made up of an upper cover  302   a  and a lower cover  302   b , and the cover  302  is made into a bag-like configuration having an opening  302   c  at one end portion thereof. In the cover  302 , a tray  303  is held in such a manner as to be freely inserted and ejected in an X direction shown in  FIG. 13 , and the tray  303  is made of a material such as a resin material which is light in weight. A bezel  304  is provided on the tray  303  at a front portion thereof, and this bezel  304  is made to cover the opening  302   c  when the tray  303  is accommodated within the cover  302 . An eject button  305  is exposed on the bezel  304 , and when this eject button  305  is pressed, the tray  303  is ejected to protrude slightly from the cover  302  in the X direction shown in  FIG. 13  by means of a mechanism, not shown, whereby the tray  303  is allowed to be pulled out or inserted into the cover  302  in the X direction.  
         [0059]     A pickup module  306  is mounted on the tray  303 . A spindle motor  25 , which is a rotational driving member for rotationally driving an optical disk  2 , is provided on the pickup module  306 , and furthermore, and a base plate  19  is movably provided on the pickup module  306  in such a manner as to move towards or away from the spindle motor  25 . Although a detailed description will be made later on, a lens holder  12  is provided on the base plate  19  in such a manner as to be move elastically relative to the base plate  19 . In the base plate  19 , a base plate cover  19   f  is mounted on a surface facing an information writeable or recordable surface of the optical disk  2  which is attached to the spindle motor  25 , so as to cover at least part of components such as a flexible substrate  29 , the lens holder  12  and the like which are mounted on the base plate  19 , whereby the components mounted on the base plate  19  can be prevented from coming into contact with the optical disk  2 , and on the contrary, these components can be protected against dust and electrical noise.  
         [0060]     Reference numerals  307 ,  308  denote rails which are held on the lower cover  302   b  and, moreover, are brought into engagement with both side portions of the tray  303 , respectively. The rails  307 ,  308  are made to slide in the X direction in which the tray  303  is inserted and ejected within a predetermined range relative to the lower cover  302   b  and the tray  303 .  
         [0061]     Next, the base plate  19  provided on the pickup module  306  will be described in greater detail.  
         [0062]      FIG. 14  is an overall perspective view of the base plate  19 . Note that the base plate  19  is shown in such a state that the components such as the base plate cover  19   f  and the flexible substrate are removed from the base plate  19  for the purpose of explanation. In addition, in the figure, a focusing direction denotes a direction in an optic axis direction of objective lenses  10 ,  13 . The focusing direction is also a direction which is parallel to a rotational axis of the spindle motor  25 . A tracking direction is a direction which is parallel to a surface of a suspension holder  27  to which a suspension  28  is fixed and which is normal to the focusing direction. A tangential direction is a direction which is normal to both the focusing direction and the tracking direction.  
         [0063]     The base plate  19  is movably mounted on shafts  21 ,  22 .  
         [0064]     In general, a short-wavelength optical unit  1  for emitting and receiving short-wavelength light, a long-wavelength optical unit  3  for emitting and receiving long-wavelength light and the lens holder  12  on which the objective lenses  10 ,  13  are mounted are provided on the base plate  19 .  
         [0065]     The lens holder  12  is elastically supported on the suspension holder  27  by the suspension  28 . The suspension holder  27  is fixed to a yoke member  23  by means of a method such as bonding, and the yoke member  23  is also joined to the base plate  19  by means of a method such as bonding.  
         [0066]     Next, the configuration of the lens holder  12  will be described in greater detail.  
         [0067]      FIG. 15  is a drawing which shows a movable portion of an optical pickup unit of the embodiment of the invention.  
         [0068]     As shown in  FIG. 15 , the objective lenses  10 ,  13 , which are objective members, focusing coils  130 ,  131 ,  132 ,  133 , tracking coils  134 ,  135 , mass balancers  146 ,  147 , an optical component, a color killer diffraction lens, a one fourth wavelength plate and the like are provided on the lens holder  12  which is a movable portion of the optical pickup unit of the embodiment for driving the objective lenses  10 ,  13 , the optical component, the color killer diffraction lens and the one fourth wavelength plate being not shown in the drawing.  
         [0069]     In addition, protectors  11  are provided in such a manner as to surround the objective lenses  10 ,  13 , and a longitudinal direction of each of the protectors  11  so provided is disposed parallel to the tracking direction. As shown shaded with oblique lines in  FIG. 15 , a straight groove  18  which passes through substantially the center of a protective member forming portion  17  and a groove  18  which is provided in such a manner as to surround the straight groove  18  are provided on the protector  11 .  
         [0070]     Note that while in this embodiment, the objective lenses  10 ,  13  are described as being used as the objective members which face the storage disk  14  and the lens holder  12  is described as being used as the holder having the objective members and the protectors  11 , the objective members and the holder are not limited thereto and are, hence, altered properly according to the type of storage disk  14  such as a hard disk or an MO which reads or writes information.  
       EXAMPLES  
       [0071]     Hereinafter, examples of the invention will be described.  
       Example 1  
       [0072]     A photo-setting urethane acrylate resin was applied using a dispenser to a test piece having a protective member forming portion  17  which is formed into 1 mm×2 mm in such a manner as to realize an apex height of the order of 300 μm, and the resin material so applied is exposed by on the order of 500 mJ by means of a UV emitting machine so as to be set. The resin material so set was attached to an objective lens holder with an adhesive in such a manner that a longitudinal direction thereof becomes normal to the rotational direction of the storage disk, and a surface of the resin material was sanded with a sanding disk in such a manner as to realize a height of the order of 150 μm. The test piece so completed was made as Example 1.  
       Example 2  
       [0073]     A photo-setting urethane acrylate resin was applied using a dispenser to a test piece having a protective member forming portion  17  which is formed into 0.5 mm×2 mm in such a manner as to realize an apex height of the order of 300 μm, and the resin material so applied is exposed by on the order of 500 mJ by means of the UV emitting machine so as to be set. The resin material so set was attached to an objective lens holder with an adhesive in such a manner that a longitudinal direction thereof becomes normal to the rotational direction of the storage disk, and a surface of the resin material was sanded with a sanding disk in such a manner as to realize a height of the order of 150 μm. The test piece so completed was made as Example 2.  
       Comparison Example 1  
       [0074]     A photo-setting urethane acrylate resin was applied using a dispenser to a test piece having a protective member forming portion  17  which is formed into 1 mm×1 mm in such a manner as to realize an apex height of the order of 300 μm, and the resin material so applied is exposed by on the order of 500 mJ by means of the UV emitting machine so as to be set. The resin material so set was attached to an objective lens holder with an adhesive in such a manner that a longitudinal direction thereof becomes normal to the rotational direction of the storage disk, and a surface of the resin material was sanded with a sanding disk in such a manner as to realize a height of the order of 150 μm. The test piece so completed was made as Comparison Example 1.  
       Comparison Example 2  
       [0075]     A photo-setting urethane acrylate resin was applied using a dispenser to a test piece having a protective member forming portion  17  which is formed into 1 mm×2 mm in such a manner as to realize an apex height of the order of 300 (m, and the resin material so applied is exposed by on the order of 500 mJ by means of the UV emitting machine so as to be set. The resin material so set was attached to an objective lens holder with an adhesive in such a manner that a longitudinal direction thereof becomes tangential to the rotational direction of the storage disk, and a surface of the resin material was sanded with a sanding disk in such a manner as to realize a height of the order of 150(m. The test piece so completed was made as Comparison Example 2.  
       Comparison Example 3  
       [0076]     A photo-setting urethane acrylate resin was applied using a dispenser to a test piece having a protective member forming portion  17  which is formed into 0.5 mm×2 mm in such a manner as to realize an apex height of the order of 300 (m, and the resin material so applied is exposed by on the order of 1 mJ by means of the UV emitting machine so that a surface of the resin is set. The resin material so set was attached to an objective lens holder with an adhesive in such a manner that a longitudinal direction thereof becomes normal to the rotational direction of the storage disk, and the resin material so attached is then exposed by on the order of 500 mJ in such a state that a crystal plate is pressed thereagainst so as to realize a height of the order of 150 (m, whereby the resin material was set completely. Then, the crystal plate was removed from the resin material, and the test piece so completed was made as Comparison Example 3.  
         [0077]     Next, a protective member to be formed on a base material of the lens holder  12  will be described in detail using FIGS.  16  to  20 .  
         [0078]     The experimental equipment shown in  FIG. 11  was used for a collision test that was carried out to obtain the depth of a damage mark on the optical disk and the amount of wear of a protector surface layer material.  
         [0079]     As shown in  FIG. 11 , on an objective lens holder  123  of an optical pickup actuator  122  removed from a disk drive unit UJDA-710 manufactured by Panasonic Communications Co., Ltd., a test piece  124  on which a protective member is formed is mounted on an objective lens mounting portion  125  in place of an objective lens which should have been mounted originally.  
         [0080]     The test piece  124  is made up of a base portion  126  and a protective member forming portion  127 . The base portion  126  is sized so as to be just fitted in the objective lens mounting portion. Here, the protective member forming portion  127  has a diameter of 1 mm and a height of 1 mm, and a resin material is dropped onto the protective member forming portion  127  to form a protective member through polymerization. The protective member so formed is then formed into a spherical shape having a maximum thickness of about 100 (m. The optical pickup actuator  122  on which the test piece  124  configured as described above is mounted is set at a position shown in  FIG. 12  relative to an optical disk  128  or is set so as to produce a distance of 0.3 mm between a distal end of the protective member of the test piece  124  and the optical disk  128  in such a state that the objective lens holder  123  is at rest. In addition, in order to prevent the fall of the test piece  124 , the test piece  124  may be fixed to the objective lens mounting portion  125  of the objective lens holder  123  using, for example, an adhesive.  
         [0081]     Using the experimental equipment  121  that has been described above, a collision test is to be carried out with the following points observed to obtain the depth of a damage mark inflicted onto the optical disk  128  and the amount of wear of a protector surface layer material by causing the optical disk  128  to spin. Although not shown, there is a flexible substrate provided on the optical pickup actuator to which signals are imparted from an external unit to displace the objective lens holder  123  in a focusing direction and a tracking direction, and terminals are provided on the flexible substrate to which the various signals are imparted. By giving a sine-wave signal of a maximum voltage amplitude of 1.3V and a frequency of 2 kHz between two terminals among the terminals so provided which function to displace the objective lens holder  123  in the focusing direction, the objective lens holder  123  is caused to oscillate in a vertical direction, so as to cause the protective member forming portion  127  of the test piece  124  to collide against the same location on the optical disk  128  intermittently. In the case of the experimental equipment  121 , the rotational speed of the optical disk  128  is adjusted so that the linear velocity at a location thereof where the protective member of the test piece  124  is brought into contact becomes 10.56 m/sec. The number of times of collision between the protective member forming portion  127  and the same location on the optical disk  128  is 500 times. In the experimental equipment  121  shown in  FIGS. 11 and 12  which is set as has been described above, a contact time between the protective member and the optical disk  128  in a single collision is 0.2 second.  
         [0082]     In an experiment carried out in the way described above, the depth of a damage mark inflicted onto the optical disk  128  and the amount of wear of the protective member were measured and the results of the measurements are shown in  FIGS. 16 and 17  as damage mark depths and wear amounts.  
         [0083]     In addition, a Japanese DVD-R which was not treated with a hard coating was used as the optical disk in this experiment. An optical interference three-dimensional measuring machine manufactured by Micromap was used to measure the depth of the damage marks. The wear amounts were obtained by measuring the thickness of the protective member before and after the experiment from a side thereof using a microscope.  
         [0084]     In addition, a DMS110 manufactured by Seiko Co., Ltd. was used to measure dynamic viscous elasticity, and an AUTOGRAPH DCS-50M manufactured by Shimazu Seisakusho Co., Ltd. was used to measure tensile stress and breaking strength, and those so measured will be discussed below.  
         [0085]      FIG. 16  shows a graph which plots a damage mark depth and a wear amount relative to a tensile stress at 100% stretch, and  FIG. 17  shows a graph which plots a damage mark depth and a wear amount relative to dynamic viscous elasticity. In addition, numerical data on the damage mark depths plotted in  FIGS. 16 and 17  are shown in (Table 3). According to the data, the damage inflicted on the optical disk decreased as the dynamic viscous elasticity or the tensile stress at 100% stretch of the protective member decreased, and when attempting to describe this fact in a general expression, it is said that the softer the protective member becomes, the smaller the damage mark inflicted on the optical disk becomes. In particular, when the tensile stress at 100% stretch was 0.49 kgf/mm2 or smaller, the depth of the damage mark inflicted on the optical disk became 0 nm. The dynamic viscous elasticity then was 3.7×10 6  Pa, and it can be said that with a protective member which is softer than this, no damage mark would have been inflicted on the optical disk under this experimental conditions.  
                                                 TABLE 3                                       Tensile Stress at               Dynamic Viscous   100% Stretch   Damage Mark           Elasticity(Pa)   (Kgf/mm2)   Depth (nm)                                    (Comparison   2.9 × 10 9         295       Example 8)       (Comparison   1.2 × 10 8         140       Example 7)       (Comparison   5.8 × 10 6         12       Example 6)       (Comparison   4.6 × 10 6         12       Example 5)       (Comparison   4.6 × 10 6     0.73   74       Example 4)       (Comparison   3.7 × 10 6     0.49   0       Example 3)       (Comparison       0.21   0       Example 4)       (Comparison       0.11   0       Example 5)                    
         [0086]     It is seen in  FIG. 17  that when the dynamic viscous elasticity decreases, the wear amount of the protective member increases. This can be said to be an inevitable result because it can easily be assumed that when the film gets soft, its toughness is lost, and the protective member tends to wear easily. Although a limit wear amount of the protective member is determined by the design of an optical pickup actuator, with a Blu-ray disk (BD) whose working distance is small, there occurs a risk that the objective lens comes into collision with the optical disk, and a wear amount of the order of 10 μm is considered to be a limit. According to the data shown in  FIG. 16 , the dynamic viscous elasticity of Comparison Example 4 is 4.6×10 6  Pa, and the dynamic viscous elasticity of Comparison Example 3 is 4.6×10 6  Pa, and it is clear that the wear amounts thereof are small as materials having substantially the same dynamic viscous elasticity or tensile stress at 100% stretch. While  FIG. 4  shows a relationship between breaking strength and wear amount, it is clear from what is shown therein that the wear amount decreases as the breaking strength increases. From this, the wear amount can be suppressed to 10 μm or smaller with the breaking strength being 0.43 kgf/mm 2  or larger, and an optical disk such as BD which has the small working distance can be used.  
                                             TABLE 4                                   Breaking Strength   Wear Amount           (kgf/mm)   (μm)                                        (Comparison Example 4)   0.43   7.75           (Example 3)   0.49   6.65           (Example 4)   0.71   3.88           (Example 5)   1.72   1.00                      
 
         [0087]      FIG. 18  is an exemplary diagram showing a method for measuring bonding strength. A liquid crystal polymer plate  51  and a polycarbonate plate  52  are affixed to each other so that an overlapping portion becomes 3 cm 2  using a polymer which is used as a protective member. An end portion of the polycarbonate plate so affixed was pulled by a push-pull gauge and a value obtained by dividing a stress resulting when the plate was separated by the overlapping area was made to be a bonding strength. Polymers used in this test are made by adding properly monomers shown in (Table 7) and (Table 8) and photopolymerization initiators shown in (Table 9) to urethane acrylates shown in (Table 5) by adjusting volumes thereof according to applications, mixing them in a water bath and getting the mixture polymerized.  
                               TABLE 5                                   Manufactures   Trade Names   Product Nos.                           Toa Gosei Chemicals   ARONICS   M-1100                   M-1200                   M-1210                   M-1310                   M-1600           Mitsui Takeda Chemicals   TAKERAC   UV-1050                   UV-1070                   UV-6012N                   UV-6012NB                   UV-6013                   UV-6023                   UV-6026                   UV-6030                   UV-6030                   UV-6033S                   UV-6034N                   UV-6040                   UV-9203                        
         [0088]    
       
         
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                   
               
               
                 Chemical Designation 
                 Structural Formula 
               
               
                   
               
             
             
               
                   
                   
               
               
                 Phenol EO modified acrylate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Nonyl Phenol EO modified acrylate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Bisphenol AEO modified diacrylate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Isobornyl acrylate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Isobornyl methacrylate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 N-vinyl-2-pyrrolidone 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Benzyl methacrylate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Neopentyl glycol acrylic benzoate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
         [0089]    
       
         
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                   
               
               
                 Chemical Designation 
                 Structure Formula 
               
               
                   
               
             
             
               
                   
                   
               
               
                 2-hydroxy acrylate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 2-hydroxy methacrylate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
         [0090]    
       
         
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                   
               
               
                 Chemical Designation 
                 Structural Formula 
               
               
                   
               
             
             
               
                   
                   
               
               
                 2,2-dimethosy- 1,2-diphenylethane-1-on 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 1-hydroxy-cyclohexyl- Phenyl-ketone 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 Benzophenone 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 2-methyl-1-[4-(methylthio)phenyl]- 2-monphelinopropane 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 2-benzyl-2-dimethylamino- 1-(4-moliphelinophenyl)-butanone-1 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 2-hydroxy-2-methyl- 1-phenyl-propane-1-on 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 1-[4-(2-hydroxyethoxy)phenyl]- 2-hydroxy-2-methyl-1-propane-1-on 
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
         [0091]    
       
         
               
               
               
             
           
               
                   
                 TABLE 9 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                 Manufactures 
                 Product Names 
               
               
                   
                   
               
             
             
               
                   
                 Shinetsu Chemicals 
                 KP-356 
               
               
                   
                   
                 KP-357 
               
               
                   
                   
                 KP-358 
               
               
                   
                   
                 KP-359 
               
               
                   
                 Big Chemy Japan 
                 BYK-300 
               
               
                   
                   
                 BYK-310 
               
               
                   
                   
                 BYK-315 
               
               
                   
                   
                 BYK-320 
               
               
                   
                   
                 BYK-322 
               
               
                   
                   
                 BYK-330 
               
               
                   
                   
                 BYK-340 
               
               
                   
                   
                 BYK-344 
               
               
                   
                   
                 BYK-350 
               
               
                   
                   
                 BYK-354 
               
               
                   
                   
                 BYK-UV3500 
               
               
                   
                   
                 BYK-UV3510 
               
               
                   
                   
                 BYK-UV3570 
               
               
                   
                   
               
             
          
         
       
     
         [0092]      FIG. 19  shows a graph which plots a relationship between bonding strength and the number of hydroxyl groups contained in a polymer. It is seen from this that with a hydroxyl group contained, the bonding strength increases and that the bonding strength increases as the amount of hydroxyl group increases. While the bonding strength varies depending on kinds and volumes of materials, since the same tendency as that resulting from this experiment is obtained in the event that an experiment is carried out by adjusting the amount of hydroxyl group using a material of the same system, no example as the protective member is raised here.  
         [0093]     By using protective members like those raised as Example 3, Example 4, Example 5, the objective lens collision preventive protector portion  112  not only protects the objective lens  110  and the optical disk  101  against damage but also makes itself a material which has superior wear resistance. By making the objective lens collision preventive protector portion  112  the material which is difficult to wear, a time is extended which is to be spent from its original state (a state in which an interval between the optical disk  101  and the objective lens  110  is larger than an interval between the optical disk  110  and the objective lens collision preventive protector portion  112 ) until the objective lens collision preventive protector portion  112  wears due to collision with the optical disk  101 , and the continuity of the function to protect the objective lens  110  is extended further. In addition, by making the objective lens collision preventive protector portion  112  difficult to wear, the amount of the constituent components of the objective lens collision preventive protector portion  112  which are separated from the protector portion due to its wear is reduced, and a risk of the constituent components so separated adhering to the optical disk  101  and/or the objective lens  110  is reduced, whereby the possibility of disruption of data reading or data writing to the optical disk  101  can be reduced.  
         [0094]     Thus, the infliction of damage on the storage disk due to a collision between the lens holder or the objective lens with the storage disk can be reduced, and furthermore, the infliction of damage on the optical disk and adhesion of the protective member to the optical disk due to its wear can be reduced, whereby data reading from the storage disk or data writing to the optical disk can be performed over a long period of time in a stable fashion with fewer errors.  
         [0095]     Being made of the soft material, the protective member illustrated in this embodiment absorbs an impact force generated when the lens holder collides against the storage disk, whereby the infliction of damage on the storage disk due to the collision between the protective member and the storage disk can be reduced. Furthermore, the use of the protective member having the large breaking strength can reduce the wear of the protective member, whereby the continuity of the function of the protective member can be extended, and the adhesion of the constituent fragments separated from the protective member to the optical disk can be reduced. In addition, since it has the soft and tough film quality, the protective member can be used for an impact absorbing member, a damage preventive member or the like. In particular, in storage disks such as optical disks, hard disks and MO&#39;s which are liable to be damaged easily, the infliction of damage on a storage disk due to a collision with a data recording/playing back unit can be reduced largely. The protective member of the invention is preferably applied, in particular, to the Blu-ray disk whose working distance is very small among those storage disks. In addition, since the protective member of the invention is good at bonding to a liquid crystal polymer, the protective member is advantageous in that the protective member is difficult to be separated or fall from the lens holder base material and can also be used as a liquid crystal polymer adhesive.  
         [0096]     As has been described heretofore, by making the tensile stress at 100% stretch 0.49 kgf/mm 2  or smaller, the infliction of damage on storage disks such as optical disks, hard disks and MO&#39;s due to collision therewith can be reduced. Here, the tensile stress at 100% stretch will be described using  FIG. 20 . The protective member is formed into a strip-like film sample  61  of the order of 5 mm×40 mm×60 μm, and the film so formed is made to be chucked with chucking jigs  62  of a tensile tester at both ends thereof so that a film exposing portion becomes approximately 20 mm. Then, the film is pulled at the ends thereof at steady speed by the tensile tester for measurement of stress applied then, and a stress applied at 100% stretch, that is, a stress applied when the film exposing portion has been extended from 20 mm to 40 mm is divided by the cross sectional area of the film. The resulting value is made to be the tensile stress at 100% stretch.  
         [0097]     In addition, by providing the protective members not only for the objective lens but also on the periphery thereof, the infliction of damage on the storage disk due to an accidental collision that would be able to occur while recording or playing back the storage disk can be reduced.  
         [0098]     Additionally, by making the breaking strength of the protective member 0.43 kgf/mm 2  or larger, even in the event that an accidental collision between the protective member and the storage disk happens, the infliction of damage on the storage disk can be reduced, and furthermore, the protective member is made difficult to be abraded, whereby the continuity of the function of the protective member can be extended and the adhesion of fragments of the protective member that would otherwise result from abrasion thereof to the storage disk can also be reduced. Here, the breaking strength will be described using  FIG. 20 . The protective member is formed into a strip-like film sample  61  of the order of 5 mm×40 mm×60 μm, and the film so formed is made to be chucked with chucking jigs  62  of a tensile tester at both ends thereof so that a film exposing portion becomes approximately 20 mm. Then, the film is pulled at the ends thereof at steady speed by the tensile tester for measurement of stress applied then, and a stress applied when the film fails is divided by the cross sectional area of the film. The resulting value is made to be the breaking strength.  
         [0099]     In addition, by using urethane acrylate for the protective member, a soft protective member whose ensile stress at 100% stretch 0.49 kgf/mm 2  or smaller can be prepared, whereby the infliction of damage on the storage disk due to an accidental collision that would be able to occur while recording or playing back the storage disk can be reduced. Here, the monomer is such as to be referred generally to as a reactive diluent and to make up part of the constituent elements of the protective member.  
         [0100]     Additionally, the protective member is a polymerized mixture of a small amount of urethane acrylate and a large amount of monomer and can be prepared as the soft protective member having the tensile stress at 100% stretch of 0.49 kgf/mm 2  or smaller and the breaking strength of 0.43 kgf/mm 2  or larger by mixing 5 to 10 percent by weight of urethane acrylate and a monomer together and polymerizing the mixture, whereby the infliction of damage on the storage disk due to an accidental collision that would be able to occur while recording or playing back the storage disk can be reduced.  
         [0101]     In addition, as the monomer, a monomer is used which contains 0.1 to 5 percent by weight of diacrylate monomer and 90 percent by weight of monomer which has a molecular weight of 150 to 500 and an annular construction, and the monomers are mixed and polymerized so as to prepare the protective member having the tensile stress at 100% stretch of 0.49 kgf/mm 2  or smaller and the breaking strength of 0.43 kgf/mm 2  or larger, whereby the infliction of damage on the storage disk can be reduced even in the event that an accidental collision occurs between the protective member and the storage disk, and furthermore, the protective member is made difficult to be abraded, whereby the continuity of the function of the protective member can be extended and the adhesion of fragments of the protective member that would otherwise result from abrasion thereof to the storage disk can also be reduced.  
         [0102]     In addition, as the protective member, by mixing 30 percent by weight of urethane acrylate and 70 percent by weight of monomer whose molecular weight of 150 to 500 and which has an annular construction and polymerizing the mixture, a protective member having the tensile stress at 100% stretch of 0.21 kgf/mm 2  or smaller and the breaking strength of 0.71 kgf/mm 2  or larger, whereby the performance of reducing the infliction of damage on the storage disk and extending the continuity of the function of the protective member and the performance of reducing the adhesion of the constituent fragments of the protective member to the storage disk can be increased.  
         [0103]     Additionally, as the protective member, by utilizing a polymer having at least hydroxyl group, the bonding force to the base material of the lens holder can be increased. In particular, while a liquid crystal polymer is known as having high dimension accuracy and mechanical strength and is hence often used as a material for the lens holder, the liquid crystal polymer is also known generally as having a small bonding force to a resin material. According to the embodiment of the invention, however, the bonding force to the liquid crystal polymer can be increased without largely reducing the performance as the protective member.  
         [0104]     In addition, as the protective member, a polymer is used which has at least one of hydroxy-based acrylate monomer and hydroxy-based methacrylate monomer. The amount of hydroxyl group can easily be controlled by having the hydroxyl group in the monomer, whereby the bonding force can be controlled according to applications. Due to this, a polymer used for the protective member can also be used as a liquid crystal polymer adhesive such as one used, for example, to bond the lens holder and the objective lens together.  
         [0105]     Additionally, as the protective member, by using a polymer in which urethane acrylate and various types of monomers are mixed together and polymerized or 20 percent by weight of urethane acrylate, 30 percent by weight of monomer whose molecular weight is 150 to 500 and which has an annular structure and 50 percent by weight of at least one of hydroxy-based acrylate monomer and hydroxy-based methacrylate monomer, a protective member having the tensile stress at 100% stretch of 0.11 kgf/mm 2  or smaller and the breaking strength of 1.72 kgf/mm 2  or larger, whereby the performance of reducing the infliction of damage on the storage disk and extending the continuity of the function of the protective member and the performance of reducing the adhesion of the constituent fragments of the protective member to the storage disk can be increased. Moreover, the protective member so prepared can be imparted a strong bonding force to the lens holder, whereby the separation or fall of the protective member from the lens holder can be reduced which would otherwise occur by virtue of an impact generated by a collision of the protective member with the storage disk.  
         [0106]     Additionally, by using a material containing a liquid crystal polymer as the base material of the lens holder, a lens holder results which has a strong bonding force to the protective member, a high dimension accuracy and a high mechanical strength.  
         [0107]     In addition, by dropping a mixture of at least urethane acrylate and a monomer to the periphery of the objective lens and setting the mixture so dropped so as to have the tensile stress at 100% stretch of 0.49 kgf/mm 2  or smaller, the protective member can be produced by a simple production method, thereby making it possible to increase largely the productivity.  
         [0108]     Additionally, by devising the shape of a protective member setting portion of the lens holder, the shape of the protective member can freely be altered, whereby the protective member can be modified in shape in any way according to applications.  
       Examples  
       [0109]     Hereinafter, examples of the invention will be described. When weighing constituent materials, an error of the order of +/−5% poses no problem in terms of performance, and therefore, an error something like that is to be permitted. In addition, there is no specific limitation on emitting machines and lamps which are used for ultraviolet ray emission.  
         [0110]     As to test sample preparing methods, test samples were prepared as follows and used for the relevant tests. A test piece sample for the collision test was prepared by dropping a resin, which has not yet been polymerized, on the test piece  124  using a dispenser for polymerization thereon, and a sample for the dynamic viscous elasticity and tensile tests was prepared by applying a material to a glass plate to which a releasant is applied or a fluoroplatic film having good releasability using a doctor blade technique for polymerization thereon and cutting the glass plate or film into a predetermined size.  
       Example 3  
       [0111]     While urethane acrylate is expressed by a general structural formula as CH2═CHCOO—R′—OCNH—(R—NHCOO-(polyol)-OOCNH) n —R—NHCOO—R′—-OCOCH═CH2, since a detailed structure is unknown, examples of commercially available urethane acrylates are shown in (Table 5).  
         [0112]     10 g of UN6012N among the urethane acrylates shown in (Table 5), 10 g of each of phenol EO modified acrylate and isobornyl acrylate among the monomers shown in (Table 6), 0.4 g of 2-benzyl-2-dimethylamino-1-(4-moliphelinophenyl)-butanone-1 among the photopolymerization initiators shown in (Table 8) and 0.3 g of BYK344 among the additives shown in (Table 9) were weighed respectively and mixed well together while being warmed in a water bath, and the mixture was irradiated with ultraviolet ray by 500 mJ/cm 2  so as to be polymerized. The resulting protective member was made to be Example 3.  
       Example 4  
       [0113]     30 g of UN6012NB among the urethane acrylates shown in (Table 5), 35 g of each of phenol EO modified acrylate and isobornyl acrylate among the monomers shown in (Table 6), 0.4 g of 2-benzyl-2-dimethylamino-1-(4-moliphelinophenyl)-butanone-1 among the photopolymerization initiators shown in (Table 8) and 0.3 g of BYK344 among the additives shown in (Table 9) were weighed respectively and mixed well together while being warmed in a water bath, and the mixture was irradiated with ultraviolet ray by 500 mJ/cm 2  so as to be polymerized. The resulting protective member was made to be Example 4.  
       Example 5  
       [0114]     20 g of UN6012NB among the urethane acrylates shown in (Table 5), 30 g of isobornyl acrylate among the monomers shown in (Table 6), 50 g of hydroxy-based acrylate monomer in (Table 7), 0.4 g of 2-benzyl-2-dimethylamino-1-(4-moliphelinophenyl)-butanone-1 among the photopolymerization initiators shown in (Table 8) and 0.3 g of BYK344 among the additives shown in (Table 9) were weighed respectively and mixed well together while being warmed in a water bath, and the mixture was irradiated with ultraviolet ray by 500 mJ/cm 2  so as to be polymerized. The resulting protective member was made to be Example 5.  
       Comparison Example 4  
       [0115]     TB3075 by 3-Bond was irradiated with ultraviolet ray by 500 mJ/cm 2  so as to be polymerized, and the resulting protective member was made to be Comparison Example 4.  
       Comparison Example 5  
       [0116]     KJR9022 by Shinetsu Chemicals was well mixed under a formula recommended by the manufacturer, and the mixture was polymerized 240 minutes at 150° C., and the resulting protective member was made to be Comparison Example 5.  
       Comparison Example 6  
       [0117]     10 g of UN6012N among the urethane acrylates shown in (Table 5), 90 g of phenol EO modified acrylate among the monomers shown in (Table 6), 0.4 g of 2-benzyl-2-dimethylamino-1-(4-moliphelinophenyl)-butanone-1 among the photopolymerization initiators shown in (Table 8) and 0.3 g of BYK344 among the additives shown in (Table 9) were weighed respectively and mixed well together while being warmed in a water bath, and the mixture was irradiated with ultraviolet ray by 500 mJ/cm 2  so as to be polymerized. The resulting protective member was made to be Comparison Example 6.  
       Comparison Example 7  
       [0118]     30 g of UN6012N among the urethane acrylates shown in (Table 5), 70 g of phenol EO modified acrylate among the monomers shown in (Table 6), 0.4 g of 2-benzyl-2-dimethylamino-1-(4-moliphelinophenyl)-butanone-1 among the photopolymerization initiators shown in (Table 8) and 0.3 g of BYK344 among the additives shown in (Table 9) were weighed respectively and mixed well together while being warmed in a water bath, and the mixture was irradiated with ultraviolet ray by 500 mJ/cm 2  so as to be polymerized. The resulting protective member was made to be Comparison Example 7.  
       Comparison Example 8  
       [0119]     KR5235 by Shinetsu Chemicals was polymerized 60 minutes at 200° C., and the resulting protective member was made to be Comparison Example 8.  
         [0120]     The disk unit according to the embodiment of the invention can be used for an impact absorbing material, a damage infliction preventive material and the like. In particular, in storage disks such as optical disks, hard disks and MO&#39;s which are liable to be damaged easily, the infliction of damage on the storage disk due to a collision with the data recording/playing back unit can be reduced largely. The protective member of the invention is preferably applied, in particular, to the Blu-ray disk whose working distance is very small among those storage disks.  
         [0121]     This application based upon and claims the benefit of priority of Japanese Patent Application No 2005-224885 filed on May 8, 2003, and Japanese Patent Application of the No 2005-350371 filed on May 12, 2005, the contents of which are incorporated herein by reference in its entirety.