Abstract:
This invention provides an optical component which can keep the light transmittance high while ensuring good wiping resistance characteristics, and can also realize intended optical characteristics based on a fine structure, and an optical pickup apparatus including the optical component. An optical component according to this invention includes a first optical surface on which a fine structure is formed and a second optical surface on which no fine structure is formed. The number of layers of an antireflection film on the first optical surface is set to be smaller than the number of layers of an antireflection film on the second optical surface.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical component and optical pickup apparatus and, more particularly, to an optical component and optical pickup apparatus which improve optical performance.  
         [0003]     2. Description of the Prior Art  
         [0004]     Recently, as short-wavelength red semiconductor lasers have been put into practice, DVDs (Digital Versatile Disks) have been commercially available, which are high-density optical disks almost equal in size to CDs (Compact Disks) as conventional optical disks (also called optical information recording media) and having large capacities. However, an optical pickup apparatus designed to record/reproduce information on/from CDs or DVDs alone are not sufficient in terms of the value of products. For this reason, in order to increase the added value, a so-called compatible optical pickup apparatus which can record/reproduce information on/from both CDs and DVDs has also been developed.  
         [0005]     CDs and DVDs differ in specifications (light source wavelength, numerical aperture, transparent substrate thickness, and the like). In order to properly record and/or reproduce information on/from both types of optical disks by using a single objective lens, therefore, some implementation is needed. In order to meet this need, a diffraction structure is provided on an optical surface of an objective lens so as to obtain aberration characteristics suitable for CDs and DVDs.  
         [0006]     Meanwhile, there has been developed a next-generation high-density optical disk, one step advanced from CDs and DVDs. A condensing optical system for an optical information recording/reproducing apparatus (to be also referred to as an optical pickup apparatus) using such a next-generation optical disk as a medium is required to decrease the diameter of a spot condensed onto an information recording surface through an objective lens so as to record recording signals at a higher density or reproduce high-density recording signals. In order to realize this, it is necessary to shorten the wavelength of a laser serving as a light source or increase the numerical aperture (NA) of the objective lens. A blue-violet semiconductor laser having a wavelength of 450 nm or less is expected to be commercialized as a short-wavelength laser light source.  
         [0007]     Research and development on a high-density optical disk system have rapidly progressed, which can record/reproduce information by using such a blue-violet semiconductor laser light source having a wavelength of 450 nm or less. For example, an optical disk designed to record/reproduce information under specifications of an NA of 0.85 and a light source wavelength of 405 nm (such an optical disk will be referred to as a “high-density DVD” hereinafter in this specification) can record information of 20 to 30 GB per surface with a diameter of 12 cm, which is equal to that of a DVD (NA=0.6, light source wavelength=650 nm, and storage capacity=4.7 GB). An objective lens which has a diffraction structure to form an appropriate condensed light spot on the information recording surface of such a high-density DVD has also been developed (see patent reference 1: Japanese Unexamined Patent Publication No. 2002-236253).  
         [0008]     An optical component of an optical pickup apparatus has been contrived to increase its transmittance so as to efficiently use the laser beam emitted from a light source. For example, an antireflection film is formed on an optical surface of an objective lens or the like to suppress the amount of light reflected by the optical surface by using interference of light (see patent reference 2: Japanese Unexamined Patent Publication No. 2002-55207).  
         [0009]     When, however, an antireflection film is to be formed on an objective lens used for a compatible optical pickup apparatus like that described above, an antireflection effect must be realized for each of light beams of different wavelengths incident on the film. In general, the thickness of an antireflection film needs to be increased to ensure a wide wavelength range in which an antireflection effect can be realized. However, an increase in film thickness makes the shape (the shape of a corner portion, in particular) of the above diffraction structure dull. This may make it impossible to obtain desired diffraction characteristics.  
         [0010]     In addition, in order to obtain a diffraction effect in a short wavelength region of 450 nm or less, a smaller diffraction structure is required. Therefore, an antireflection film has a greater influence on the shape of the diffraction structure. This makes it difficult to obtain required diffraction characteristics for an objective lens used for an optical pickup apparatus for high-density DVDs, in particular.  
         [0011]     In addition, an optical component on which an antireflection film is formed is required to have good so-called wiping properties, i.e., suppressing peeling of the antireflection film by wiping out foreign substances adhering to an optical surface. However, forming a thick antireflection film on a diffraction structure considerably degrades the wiping properties.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention has been made in consideration of the above problems, and has as its object to provide an optical component and optical pickup apparatus which can keep the light transmittance high while ensuring high wiping resistance, and can also realize intended optical characteristics based on a fine structure.  
         [0013]     In order to achieve the above object, according to the first aspect of the present invention, there is provided an optical component comprising: a first optical surface having a fine structure; a second optical surface without having the fine structure; a first antireflection film provided on the first optical surface, in which the first antireflection film includes at least one layer; and a second antireflection film provided on the second optical surface, in which the second antireflection film includes a plurality of layers, wherein the number of layers of the first antireflection film is less than the number of layers of the second antireflection film.  
         [0014]     With this arrangement, for example, since the number of layers of the antireflection film on the first optical surface on which the fine structure such as a diffraction structure is formed is relatively small, the thickness of the antireflection film can be made relatively thin. This makes it easy to maintain the shape of the fine structure after film formation, and makes it possible to prevent a deterioration in the optical characteristics of the fine structure. In addition, the wiping characteristics can be improved. On the other hand, since no fine structure is formed on the second optical surface, even increasing the thickness of the antireflection film causes no deterioration in optical characteristics and no deterioration in wiping characteristics. Therefore, a sufficient antireflection function can be realized by increasing the number of layers of an antireflection film.  
         [0015]     According to the second aspect of the present invention, there is provided an optical component wherein the fine structure in the first aspect is a ring-like phase-difference-generating structure.  
         [0016]     According to the third aspect of the present invention, in the optical component described in the first or second aspect, the second antireflection film consists of seven layers.  
         [0017]     According to the fourth aspect of the present invention, in the optical component described in the first or second aspect, the second antireflection film consists of eight layers to ten layers.  
         [0018]     According to the fifth aspect of the present invention, in the optical component described in any one of the first to fourth aspects, the first antireflection film consists of one layer.  
         [0019]     According to the sixth aspect of the present invention, in the optical component described in any one of the first to fourth aspects, the first antireflection film consists of two layers.  
         [0020]     According to the seventh aspect of the present invention, in the optical component described in any one of the first to fourth aspects, the first antireflection film consists of three layers.  
         [0021]     According to the eighth aspect of the present invention, in the optical component described in any one of the first to fourth aspects, the first antireflection film consists of four layers to nine layers.  
         [0022]     According to the ninth aspect of the present invention, there is provided an optical component comprising: a first optical surface having a fine structure; a second optical surface without having the fine structure; and an antireflection film provided only on the second optical surface.  
         [0023]     With this arrangement, even forming an antireflection film does not change the fine structure on the first optical surface, thereby preventing a deterioration in its optical characteristics and improving the wiping characteristics.  
         [0024]     According to the 10th aspect of the present invention, in the optical component described in the ninth aspect, the fine structure described in the ninth aspect is a ring-like phase-difference-generating structure.  
         [0025]     According to the 11th aspect of the present invention, in the optical component described in the ninth or 10th aspect, the antireflection film consists of seven layers.  
         [0026]     According to the 12th aspect of the present invention, in the optical component described in the ninth or 10th aspect, the antireflection film consists of eight layers to ten layers.  
         [0027]     According to the 13th aspect of the present invention, in the optical component described in any one of the first to eighth aspects, the optical component is an objective lens for an optical pickup apparatus.  
         [0028]     With this arrangement, the performance of the optical pickup apparatus can be improved. However, the optical component of the present invention is not limited to an objective lens and may include a coupling lens, expander lens, parallel plate, and the like.  
         [0029]     According to the 14th aspect of the present invention, in the optical component described in the ninth to 12th aspects, the optical component is an objective lens for an optical pickup apparatus.  
         [0030]     According to the 15th aspect of the present invention, in the optical component described in the 13th aspect, the objective lens is used for a so-called compatible optical pickup apparatus which is capable of condensing each of light beams of different wavelengths emitted from a plurality of light sources mounted in the optical pickup apparatus, onto an information recording surface of optical information recording media corresponding to each of the light beams.  
         [0031]     According to the 16th aspect of the present invention, in the optical component described in the 14th aspect, the objective lens is used for a so-called compatible optical pickup apparatus which is capable of condensing each of light beams of different wavelengths emitted from a plurality of light sources mounted in the optical pickup apparatus, onto an information recording surface of optical information recording media corresponding to each of the light beams.  
         [0032]     According to the 17th aspect of the present invention, in the optical component described in the 13th or 15th aspect, the objective lens is used for an optical pickup apparatus for so-called high-density DVDs, which is capable of condensing a light beam of a wavelength λ (λ≦450 nm) onto an information recording surface of an optical information recording medium.  
         [0033]     According to the 18th aspect of the present invention, in the optical component described in the 14th or 16th aspect, the objective lens is used for an optical pickup apparatus for so-called high-density DVDs, which is capable of condensing a light beam of a wavelength λ (λ≦450 nm) onto an information recording surface of an optical information recording medium.  
         [0034]     According to the 19th aspect of the present invention, there is provided an optical component for use in an optical pickup apparatus, comprising: a plurality of optical surfaces; a first antireflection film provided on one optical surface of the optical surfaces, in which the first antireflection film includes at least one layer; and a second antireflection film provided on the other optical surface of the optical surface, in which the second antireflection film includes a plurality of layers, wherein the optical component is arranged on an optical path through which a first light beam of a wavelength λ 1  (390 nm≦λ 1 ≦450 nm) passes, and at least one of a second light beam of a wavelength λ 2  (635 nm≦λ 2 ≦670 nm) and a third light beam of a wavelength λ 3  (740 nm≦λ 3 ≦810 nm) passes in the optical pickup apparatus, and the following conditional formula is satisfied: m 1 &lt;m 2  where m 1  is the number of layers of the first antireflection film, and m 2  is the number of layers of the second antireflection film.  
         [0035]     With this arrangement, when a phase-difference-generating structure is to be formed on one optical surface of an optical component used for an optical pickup apparatus which compatibly records and/or reproduces information on/from a high-density DVD and either one of optical disks such as, for example, DVD and CD, or a high-density DVD and both of optical disks such as, for example, DVD and CD, the loss of light on the optical surface can be reduced and its wiping characteristics can be improved by setting a small number of layers for an antireflection film on the optical surface on which the phase-difference-generating structure is provided.  
         [0036]     As an optical pickup apparatus which compatibly records and/or reproduces information, there is known an optical pickup apparatus which can record and/or reproduce information on/from a high-density DVD having a protective layer thickness of 0.6 mm by using a blue-violet semiconductor laser, and can record and/or reproduce information on/from a DVD and/or a CD by using a red semiconductor laser, or an optical pickup apparatus which can record and/or reproduce information on/from a high-density DVD having a protective layer thickness of 0.1 mm by using a blue-violet semiconductor laser, and can record and/or reproduce information on/from a DVD and/or a CD by using a red semiconductor laser. The present invention can also be applied to a case wherein the optical component is comprised of two objective lenses in such an optical pickup apparatus. More specifically, the adverse effects produced as the number of layers of an antireflection film increases can be suppressed by setting the number of layers (m 1 ) of an antireflection film provided on one of the four optical surfaces of the two objective lenses which has a phase different adding function or an acute convex shape to be smaller than that (m 2 ) on each remaining optical surfaces having a less acute convex shape.  
         [0037]     According to the 20th aspect of the present invention, in the optical component described in the 19th aspect, a phase-difference-generating structure is formed on the one optical surface. Note, however, that the one optical surface is not limited to the optical surface having the phase-difference-generating structure, and may be an optical surface having an acuter convex shape (a smaller radius of curvature) than the remaining optical surface.  
         [0038]     According to the 21st aspect of the present invention, in the optical component described in the 19th or 20th aspect, wherein the following conditional formula is satisfied: Φ 1  &gt;Φ 2  where Φ 1  is an effective diameter of the one optical surface when the first light beam passes through the optical component, and Φ 2  is an effective diameter of the other optical surface when the first light beam passes through the optical component.  
         [0039]     With this arrangement, when the optical pickup apparatus which compatibly records and/or reproduces information is used for a high-density DVD and either one of optical disks such as, for example, DVD and CD, or a high-density DVD and both of optical disks such as, for example, DVD and CD, it becomes possible to record and/or reproduce information on/from a plurality of optical disks.  
         [0040]     According to the 22nd aspect of the present invention, in the optical component described in any one of the 19th to 21st aspects, the number of layers m 2  is seven.  
         [0041]     According to the 23rd aspect of the present invention, in the optical component described in any one of the 19th to 21st aspects, the number of layers m 2  is eight to ten.  
         [0042]     According to the 24th aspect of the present invention, there is provided an optical pickup apparatus comprising a light source and a condensing optical system including the optical component of the first to eighth aspects, wherein the optical component is capable of condensing a light beam from a light source onto an information recording surface of an optical information recording medium.  
         [0043]     According to the 25th aspect of the present invention, there is provided an optical pickup apparatus comprising a light source and a condensing optical system including the optical component of the ninth to 18th aspects, wherein the optical component is capable of condensing a light beam from a light source onto an information recording surface of an optical information recording medium.  
         [0044]     According to the 26th aspect of the present invention, there is provided an optical pickup apparatus comprising a plurality of light sources and a condensing optical system including the optical component of the 19th to 23rd aspects, wherein the optical component is capable of condensing each of light beams from the light sources onto information recording surfaces of optical information recording media corresponding to each of the light beams, respectively.  
         [0045]     The term “fine structure” used in this specification indicates a structure having such a function as generating an optical path difference. A stepped configuration for generating an optical path difference or a phase-difference-generating structure, etc., is an example of the optical path-difference-generating structure.  
         [0046]     In addition, the term “phase-difference-generating structure” used in this specification indicates a structure which can realize a phase-difference-generating function, and the phase-difference-generating function indicates a function of applying a special effect to an incident light beam by generating a predetermined phase difference to the light beam. For example, a “diffraction structure” is an example of this phase-difference-generating structure.  
         [0047]     Further, the term “diffraction structure” indicates a portion of a surface of an optical component on which a relief is provided to condense or diverge a light beam by using diffraction. As such a relief shape, there is known a shape obtained by forming concentric rings, centered on an optical axis, on a surface of an optical component with each ring having a serrated or staircase-like cross section when viewed from a plane including the optical axis. The above relief shape includes such a shape, which is referred to as a “diffraction ring”, in particular.  
         [0048]     In this specification, an objective lens indicates, in a narrow sense, a lens with a condensing effect which is placed nearest to the optical information recording medium side to oppose it in a state wherein the optical information recording medium is loaded in an optical pickup apparatus, and indicates, in a broad sense, a lens which can be moved in at least the optical axis direction by an actuator, in addition to the above lens.  
         [0049]     As is obvious from the respective aspects described above, according to the present invention, an optical component and optical pickup apparatus can be provided, which can maintain high light transmittance while ensuring high wiping resistance, and can also realize intended optical characteristics based on, for example, a fine structure.  
         [0050]     The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiment incorporating the principle of the present invention are shown by way of illustrative examples. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0051]      FIG. 1  is a schematic view showing the arrangement of an optical pickup apparatus according to the first embodiment;  
         [0052]      FIG. 2  is a schematic view showing the arrangement of an optical pickup apparatus according to the second embodiment;  
         [0053]      FIG. 3  is a sectional view of an objective lens; and  
         [0054]      FIG. 4  is a sectional view of another objective lens. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0055]     A few preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.  
         [heading-0056]     (First Embodiment)  
         [0057]     The first embodiment will be described.  FIG. 1  is a schematic view showing the arrangement of an optical pickup apparatus according to the first embodiment. Referring to  FIG. 1 , a light beam (wavelength: 390 to 450 nm) from a semiconductor laser as a light source passes through a beam splitter  2  and strikes an objective lens  4  forming a diffraction structure on a light-source-side surface (first optical surface). A light beam emerging from a medium-side surface (second optical surface) of the objective lens  4  on which no diffraction structure is formed is condensed onto the information recording surface of an optical information recording medium  5  which is a high-density DVD. Light reflected by the optical information recording medium  5  passes through the objective lens  4  and is reflected by the beam splitter  2  in a direction different from the semiconductor laser  1 . After astigmatism is generated by an astigmatism generating lens  6 , this light is received by a photodetector  7 . Note that although not shown, this apparatus includes a focusing mechanism which integrally moves the objective lens in the optical axis direction (ditto for the second embodiment to be described below).  
         [heading-0058]     (Second Embodiment)  
         [0059]     The second embodiment will be described below.  FIG. 2  is a schematic view showing the arrangement of an optical pickup apparatus according to the second embodiment. Referring to  FIG. 2 , a light beam (wavelength: 635 nm to 670 nm) from a first semiconductor laser  11 A as a first light source passes through beam splitters  12 A and  12 B and strikes an objective lens  14  forming a diffraction structure on a light-source-side surface (first optical surface). A light beam emerging from a medium-side surface (second optical surface) of the objective lens  14  on which no diffraction structure is formed is condensed on the information recording surface of a first optical information recording medium  15 A (DVD in this case). Light reflected by the first optical information recording medium  15 A passes through the beam splitter  12 B and is reflected by the beam splitter  12 A in a direction different from the first semiconductor laser  11 A. The reflected light is then received by a photodetector  17 A.  
         [0060]     In contrast to this, referring to  FIG. 2 , a light beam (wavelength: 390 nm to 450 nm) from a second semiconductor laser  11 B as a second light source passes through a beam splitter  18  and is reflected by the beam splitter  12 B. The reflected light beam is incident on the objective lens  14  forming a diffraction structure on the light-source-side surface (first optical surface). A light beam emerging from the medium-side surface (second optical surface) of the objective lens  14  on which no diffraction structure is formed is condensed onto the information recording surface of a second optical information recording medium  15 B (CD in this case, but the high-density DVD is preferable). Meanwhile, light reflected by the second optical information recording medium  15 B passes through the objective lens  14  and is reflected by the beam splitters  12 B and  18 . The reflected light beam is received by a photodetector  17 B. Note that so-called two lasers in one package as a light source can be obtained by forming the first semiconductor lasers  11 A and  11 B into one unit on the same substrate. Likewise, in addition to light beams of the above two types of wavelengths, a light source of another type of wavelength (wavelength: 740 to 810 nm), i.e., a total of three types of light beams, may pass through the objective lens  14 .  
         [0061]      FIG. 3  is a sectional view showing an example of an objective lens which can be used for an optical pickup apparatus using any one of the combinations of the three types of light beams in  FIGS. 1 and 2 . For the sake of easy understanding,  FIG. 3  shows an exaggerated view of the diffraction structure D. Referring to  FIG. 3 , the objective lens has the diffraction structure D which has a ring-like shape with a serrated cross section centered on the optical axis and is formed on only a first optical surface S 1 . No diffraction structure is therefore formed on a second optical surface S 2 . Obviously, the first optical surface may be set on the light source side, and the second optical surface may be set on the medium side.  
         [0062]     A ring pitch p of the diffraction structure D (in a direction perpendicular to the optical axis) is 10 to 100 μm, and a groove depth (a difference in level in the optical axis direction) h of the diffraction structure D is several μm.  
         [0063]      FIG. 4  is a sectional view showing an example of an objective lens which can be used for an optical pickup apparatus using any one of the combinations of two to three types of light beams in  FIG. 2 . In the second embodiment, the objective lens is comprised of two elements. More specifically, the objective lens is comprised of a plate-like element P on the light source side (the left side in  FIG. 4 ) and a lens L on the optical disk side (the right side in  FIG. 4 ). A phase-difference-generating structure M is formed on an optical surface S 1  of the plate-like element P which is located on the light source side by shifting its surface in a ring form in the optical axis direction. A phase-difference-generating structure D having a diffraction structure with a serrated cross section in the optical axis direction is formed on an optical surface S 2  of the ring pitch p which is located on the optical disk side. An optical surface S 3  of the lens L which is located on the light source side and an optical surface S 4  of the lens L which is located on the optical disk side have aspherical shapes and no phase diffraction structure. The following are the number of layers of antireflection films on the respective optical surfaces and their effective diameters:  
         [0064]     (The number of layers m 1  on S 1  (optical surface), the number of layers m 1  on S 2 , the number of layers m 2  on S 3 , the number of layers m 2  on S 4 )=(7, 5, 7, 7), (5, 7, 7, 7), (7, 7, 5, 7), (5, 5, 8, 10), (7, 7, 10, 9), or (8, 8, 10, 10)  
         [0065]     Effective diameter (Φ 1 ) of S 1  at the time of passage of light beam from second semiconductor laser  11 B: 3.7 mm  
         [0066]     Effective diameter (Φ 1 ) of S 2  at the time of passage of light beam from second semiconductor laser  11 B: 3.7 mm  
         [0067]     Effective diameter (Φ 2 ) of S 3  at the time of passage of light beam from second semiconductor laser  11 B: 3.6 mm  
         [0068]     Effective diameter (Φ 2 ) of S 4  at the time of passage of light beam from second semiconductor laser  11 B: 2.3 mm  
         [0069]     Note that the above values are merely examples, and the present invention is not limited to them.  
       EXPERIMENTAL EXAMPLE  
       [0070]     A film formation experiment on a plurality of antireflection films was conducted by stacking layers made of materials of different reflectances on the objective lens in  FIG. 3 .  
         [0071]     Table 1 shows the thicknesses of one-layer films to 10-layer films respectively designed for a case where the wavelength of transmitted light is 390 to 450 nm and a case where the wavelength of transmitted light is 635 to 670 nm. In addition to the cases of these two types of wavelengths, Table 1 shows the thicknesses of a one-layer film to a 10-layer film designed for a case where the wavelength of transmitted light is 740 to 810 nm.  
                                                                                                                                                                               TABLE 1                           (Table 1-1)                (1)   (2)   (3)   (4)   (5)           One-layer   Two-layer   Two-layer   Three-layer   Four-layer           Arrangement   Arrangement   Arrangement   Arrangement   Arrangement                        7                           6       5       4                   L material                           T 4  = 0.2˜0.3       3               L material   H material                       T 3  = 0.2˜0.3   T 3  = 0.2˜0.3       2       L material   L material   H material   H material               T 2  = 0.2˜0.33   T 2  = 0.2˜0.3   T 2  = 0.4˜0.6   T 2  = 0.2˜0.3       First Layer   L material   H or M   H or M   L material   L material           T 1  = 0.2˜0.3   material   material   T 1  = 0.2˜0.3   T 1  = 0.4˜0.6               T 1  = 0.02˜0.12   T 1  = 0.04˜0.6       Base Material   plastic or   plastic or   plastic or   plastic or   plastic or           glass   glass   glass   glass   glass                    (Table 1-2)                (6)   (7)   (8)   (9)   (10)           Four-layer   Five-layer   Five-layer   Six-layer   Seven-layer           Arrangement   Arrangement   Arrangement   Arrangement   Arrangement                        7                   L material                           T 7  = 0.27˜0.31       6               L material   H material                       T 6  = 0.21˜0.28   T 6  = 0.14˜0.18       5       L material   L material   H material   L material               T 5  = 0.2˜0.3   T 5  = 0.21˜0.28   T 5  = 0.48˜0.52   T 5  = 0.04˜0.07       4   L material   H material   H material   M material   H material           T 4  = 0.2˜0.3   T 4  = 0.4˜0.5   T 4  = 0.48˜0.52   T 4  = 0.31˜0.34   T 4  = 0.15˜0.30       3   H material   L material   M material   L material   L material           T 3  = 0.4˜0.6   T 3  = 0.07˜0.1   T 3  = 0.31˜0.34   T 3  = 0.10˜0.13   T 3  = 0.08˜0.10       2   L material   H material   L material   M material   H material           T 2  = 0.2˜0.3   T 2  = 0.03˜0.06   T 2  = 0.10˜0.13   T 2  = 0.09˜0.11   T 2  = 0.06˜0.08       First   L material   L material   M material   L material   L material       Layer   T 1  = 0.2˜0.3   T 1  = 0.3˜0.6   T 1  = 0.09˜0.11   T 1  = 0.01˜0.6   T 1  = 0.04˜0.07       Base   plastic or   plastic or   plastic or   plastic or   plastic or       Mate-   glass   glass   glass   glass   glass       rial                    (Table 1-3)                (11)   (12)   (13)   (14)           Seven-layer   Eight-layer   Nine-layer   10-layer           Arrangement   Arrangement   Arrangement   Arrangement                        10               L material                       T 10  = 0.25˜0.29       9           L material   H material                   T 9  = 0.20˜0.24   T 9  = 0.12˜0.16       8       L material   H material   L material               T 8  = 0.20˜0.24   T 8  = 0.41˜0.46   T 8  = 0.03˜0.06       7   L material   H material   L material   H material           T 7  =   T 7  = 0.41˜0.46   T 7  = 0.40˜0.45   T 7  = 0.25˜0.29           0.29˜0.33       6   H material   L material   H material   L material           T 6  =   T 6  = 0.40˜0.45   T 6  = 0.07˜0.11   T 6  = 0.49˜0.55           0.20˜0.24       5   L material   H material   L material   H material           T 5  =   T 5  = 0.07˜0.11   T 5  = 0.02˜0.06   T 5  = 0.06˜0.10           0.03˜0.07       4   H material   L material   H material   L material           T 4  =   T 4  = 0.02˜0.06   T 4  = 0.36˜0.42   T 4  = 0.10˜0.14           0.24˜0.29       3   L material   H material   L material   H material           T 3  =   T 3  = 0.36˜0.42   T 3  = 0.05˜0.08   T 3  = 0.18˜0.22           0.10˜0.15       2   H material   L material   H material   L material           T 2  =   T 2  = 0.05˜0.08   T 2  = 0.04˜0.07   T 2  = 0.04˜0.08           0.06˜0.09       First   L material   H material   L material   H material       Layer   T 1  =   T 1  = 0.04˜0.07   T 1  = 0.1˜0.35   T 1  = 0.08˜0.12           0.35˜0.39       Base   plastic or   plastic or   plastic or   plastic or       Mate-   glass   glass   glass   glass       rial                  
 
 The thickness of each layer is the thickness at the positions of lens central portions S 1 C and S 2 C (see  FIG. 3 ). 
 
         [0073]     Note that the film thickness of a given layer can be obtained by
 
 Ti=nidi/λ   0 
 
 where 
        Ti: film thickness of i th  layer (optical film thickness)     ni: refractive index of i th  layer     di: geometrical film thickness of i th  layer (nm)     λ 0 : design wavelength (nm)        
 
         [0079]     The following were used as materials for film formation 
    (1) low-refractive-index material (L material): aluminum fluoride, magnesium fluoride, or silicon oxide: refractive index of 1.30 to 1.50     (2) medium-refractive-index material (M material): aluminum oxide, yttrium oxide, or cerium oxide: refractive index of 1.55 to 1.70     (3) high-refractive-index material (H material): zirconium oxide, tantalum oxide, titanium oxide, or hafnium oxide: refractive index of 1.75 to 2.50    
 
         [0083]     Each optical surface of an objective lens was coated with one of the above materials alone or a mixed material containing it as a main component.  
         [0084]     A material (base material) making an objective lens as an optical component includes acrylic resin and polycarbonate resin. More specifically, a transparent plastic resin such as ZEONEX (trade name; available from ZEON CORPORATION) or a glass material is used. A plastic resin to be used is not limited to the above resins and includes all kinds of resins suitable as materials for the optical component.  
         [0085]     In addition, an underlayer may be provided between the base material and the first layer to improve the durability of the film. A lens surface is used facing an optical information recording medium, e.g., S 2  of the lens shape shown in  FIG. 3  or S 4  of the lens shape shown in  FIG. 4 , is required to have high abrasion resistance. For this reason, an underlayer formed from a silicon oxide film having a thickness of 0.1μ to 10μ is sometimes provided for such a lens surface.  
         [0086]     Coating methods include a vacuum deposition method, sputtering method, CVD method, atmospheric plasma method, application method, mist method, and the like. In this example, the vacuum deposition method was used.  
       EXPERIMENTAL EXAMPLE 1  
       [0087]     No antireflection coating was formed on S 1  of the objective lens made of Zeonex resin in the shape shown in  FIG. 3  through which two types of light beams of wavelengths of 405 nm and 650 nm pass, and a 7-layer antireflection coating having the thickness indicated by (10) in Table 1 was formed on S 2 . With regard to the arrangement of the respective layers on S 2 , the layer nearest to the material surface of the objective lens is regarded as the first layer, and the layer farthest from the material surface is regarded as the seventh layer. All the following layers are counted in the same manner.  
         [0088]     Table 2 shows the specifications of the seven layers on S 2 .  
                                                                 TABLE 2                                                   More                   Design       Preferable               Refractive   Wavelength   Thickness   Thickness           Material   Index   (nm)   (nm)   (nm)                                    First   silicon   1.45-1.47   480   18-19   18.5       Layer   oxide       Second   mixed   1.95-2.01   480   17-18   17.5       Layer   material           of           tantalum           oxide and           titanium       Third   silicon   1.45-2.01   480   29-31   30.0       Layer   oxide       Fourth   mixed   1.95-2.01   480   55-57   56.2       Layer   material           of           tantalum           oxide and           titanium       Fifth   silicon   1.45-1.47   480   19-20   19.3       Layer   oxide       Sixth   mixed   1.95-2.01   480   36-38   36.8       Layer   material           of           tantalum           oxide and           titanium       Seventh   silicon   1.45-1.47   480   96-98   97.2       Layer   oxide                 As a mixed material of tantalum oxide and titanium, OA600 (trade name; available from K.K. Optron) was used.             
 
       EXPERIMENTAL EXAMPLE 2  
       [0089]     A one-layer antireflection coating having the thickness indicated in (1) in Table 1 was formed on S 1  of an objective lens identical to the one in Experimental Example 1, and a 7-layer antireflection coating having the thickness indicated in (10) in Table 1 was formed on S 2 .  
         [0090]     Table 3 shows the specifications of the one layer on S 1 . Table 4 shows the specifications of the seven layers on S 2 .  
                               TABLE 3                                       More               Design       Preferable           Refractive   Wavelength   Thickness   Thickness       Material   Index   (nm)   (nm)   (nm)                   magnesium   1.35-1.38   540   95-110   97.6       fluoride                  
 
         [0091]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 More 
               
               
                   
                   
                   
                 Design 
                   
                 Preferable 
               
               
                   
                   
                 Refractive 
                 Wavelength 
                 Thickness 
                 Thickness 
               
               
                   
                 Material 
                 Index 
                 (nm) 
                 (nm) 
                 (nm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 First 
                 silicon 
                 1.45-1.47 
                 480 
                 17-19 
                 18.2 
               
               
                 Layer 
                 oxide 
               
               
                 Second 
                 mixed 
                 1.94-2.02 
                 480 
                 16.5-18   
                 17.2 
               
               
                 Layer 
                 material 
               
               
                   
                 of 
               
               
                   
                 zirconium 
               
               
                   
                 oxide and 
               
               
                   
                 Ti 
               
               
                 Third 
                 silicon 
                 1.45-1.47 
                 480 
                 29.1-31.3 
                 30.1 
               
               
                 Layer 
                 oxide 
               
               
                 Fourth 
                 mixed 
                 1.94-2.02 
                 480 
                 54.8-57.1 
                 55.3 
               
               
                 Layer 
                 material 
               
               
                   
                 of 
               
               
                   
                 zirconium 
               
               
                   
                 oxide and 
               
               
                   
                 Ti 
               
               
                 Fifth 
                 silicon 
                 1.45-1.47 
                 480 
                 19.1-20   
                 19.3 
               
               
                 Layer 
                 oxide 
               
               
                 Sixth 
                 mixed 
                 1.94-2.02 
                 480 
                 35.8-38.2 
                 36.8 
               
               
                 Layer 
                 material 
               
               
                   
                 of 
               
               
                   
                 zirconium 
               
               
                   
                 oxide and 
               
               
                   
                 Ti 
               
               
                 Seventh 
                 silicon 
                 1.45-1.47 
                 480 
                 95.8-98.1 
                 97.2 
               
               
                 Layer 
                 oxide 
               
               
                   
               
               
                   As a mixed material of zirconium oxide and Ti, OH-5 (trade name; available from K.K. Optron) was used.    
               
             
          
         
       
     
       EXPERIMENTAL EXAMPLE 3  
       [0092]     A two-layer antireflection coating having the thickness indicated in (2) in Table 1 was formed on S 1  of an objective lens identical to the one in Experimental Example 1, and a 7-layer antireflection coating having the thickness indicated in (10) in Table 1 was formed on S 2 .  
         [0093]     Table 5 shows the specifications of the two layers on S 1 . Note that the specifications of the seven layers on S 2  are the same as those shown in Table 4.  
                                                                 TABLE 5                                                   More                   Design       Preferable               Refractive   Wavelength   Thickness   Thickness           Material   Index   (nm)   (nm)   (nm)                                    First   tantalum   1.90-2.01   500   16-32   21.9       Layer   oxide       Second   silicon   1.45-1.47   500    85-115   112       Layer   oxide                  
 
       EXPERIMENTAL EXAMPLE 4  
       [0094]     A three-layer antireflection coating having the thickness indicated in (4) in Table 1 was formed on S 1  of an objective lens identical to the one in Experimental Example 1, and a five-layer antireflection coating having the thickness indicated in (7) in Table 1 was formed on S 2 .  
         [0095]     Table 6 shows the specifications of the three layers on S 1 . Table 7 shows the specifications of the five layers on S 2 .  
                                                                 TABLE 6                                                   More                   Design       Preferable               Refractive   Wavelength   Thickness   Thickness           Material   Index   (nm)   (nm)   (nm)                                    First   aluminum   1.55-1.70   500   63-94    74.7       Layer   oxide       Second   tantalum   1.90-2.01   500   100-150    124.6       Layer   oxide       Third   silicon   1.45-1.47   500   68-100   85.5       Layer   oxide                  
 
         [0096]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 More 
               
               
                   
                   
                   
                 Design 
                   
                 Preferable 
               
               
                   
                   
                 Refractive 
                 Wavelength 
                 Thickness 
                 Thickness 
               
               
                   
                 Material 
                 Index 
                 (nm) 
                 (nm) 
                 (nm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 First 
                 silicon 
                 1.45-1.47 
                 500 
                 150-170 
                 164.2 
               
               
                 Layer 
                 oxide 
               
               
                 Second 
                 OH-5 
                 1.94-2.02 
                 500 
                 11-14 
                 12.2 
               
               
                 Layer 
               
               
                 Third 
                 silicon 
                 1.45-1.47 
                 500 
                 25-29 
                 27.9 
               
               
                 Layer 
                 oxide 
               
               
                 Fourth 
                 OH-5 
                 1.94-2.02 
                 500 
                 110-120 
                 116.7 
               
               
                 Layer 
               
               
                 Fifth 
                 silicon 
                 1.45-1.47 
                 500 
                 80-90 
                 84.2 
               
               
                 Layer 
                 oxide 
               
               
                   
               
             
          
         
       
     
       EXPERIMENTAL EXAMPLE 5  
       [0097]     A four-layer antireflection coating having the thickness indicated in (5) in Table 1 was formed on S 1  of an objective lens identical to the one in Experimental Example 1, and a five-layer antireflection coating having the thickness indicated in (8) in Table 1 was formed on S 2 .  
         [0098]     Table 8 shows the specifications of the four layers on S 1 . Note that the specifications of the five layers on S 2  are the same as those shown in Table 4.  
                                                                 TABLE 8                                                   More                   Design       preferable               Refractive   Wavelength   Thickness   Thickness           Material   Index   (nm)   (nm)   (nm)                                    First   silicon   1.45-1.47   500   165-175   170.8       Layer   oxide       Second   OH-5   1.94-2.02   500   50-68   60.4       Layer       Third   OA-600   1.95-2.01   500   50-60   53.0       Layer       Fourth   silicon   1.45-1.47   500   80-90   85.5       Layer   oxide                  
 
       EXPERIMENTAL EXAMPLE 6  
       [0099]     A five-layer antireflection coating having the thickness indicated in (8) in Table 1 was formed on S 1  of an objective lens identical to the one in Experimental Example 1, and a seven-layer antireflection coating having the thickness indicated in (10) in Table 1 was formed on S 2 .  
         [0100]     Table 9 shows the specifications of the five layers on S 1 . The specifications of the seven layers on S 2  are the same as those shown in Table 4.  
                                                                 TABLE 9                                                   More                   Design       Preferable               Refractive   Wavelength   Thickness   Thickness           Material   Index   (nm)   (nm)   (nm)                                    First   aluminum   1.55-1.70   520   25-35   31.1       Layer   oxide       Second   silicon   1.45-1.47   520   40-45   42.7       Layer   oxide       Third   aluminum   1.55-1.70   520    95-100   99.5       Layer   oxide       Fourth   OH-5   1.95-2.01   520   120-130   126.0       Layer       Fifth   silicon   1.45-1.47   520   80-95   89.0       Layer   oxide                  
 
       EXPERIMENTAL EXAMPLE 7  
       [0101]     A six-layer antireflection coating having the thickness indicated in (9) in Table 1 was formed on S 1  of an objective lens identical to the one in Experimental Example 1, and a seven-layer antireflection coating having the thickness indicated in (10) in Table 1 was formed on S 2 .  
         [0102]     Table 10 shows the specifications of the six layers on S 1 . The specifications of the seven layers on S 2  are the same as those shown in Table 2.  
                                                                 TABLE 10                                                   More                   Design       Preferable               Refractive   Wavelength   Thickness   Thickness           Material   Index   (nm)   (nm)   (nm)                                    First   silicon   1.45-1.47   500   15-20   17.1       Layer   oxide       Second   aluminum   1.55-1.70   500   24-30   29.9       Layer   oxide       Third   silicon   1.45-1.47   500   35-40   37.6       Layer   oxide       Fourth   aluminum   1.55-1.70   500   85-91   89.7       Layer   oxide       Fifth   OH-5   1.95-2.01   500   118-123   120.9       Layer       Sixth   silicon   1.45-1.47   500   86-93   88.7       Layer   oxide                  
 
       EXPERIMENTAL EXAMPLE 8  
       [0103]     A seven-layer antireflection coating was formed on S 1  of the objective lens made of Zeonex resin in the shape shown in  FIG. 3  through which three types of light beams of wavelengths of 405 nm, 650 nm, and 780 nm pass, and a 10-layer antireflection coating having the thickness indicated by (14) in Table 1 was formed on S 2 .  
         [0104]     Table 11 shows the specifications of the seven layers on S 1 . Table 12 shows the specifications of the 10 layers on S 2 .  
                                                                 TABLE 11                                                   More                   Design       Preferable               Refractive   Wavelength   Thickness   Thickness           Material   Index   (nm)   (nm)   (nm)                                    First   silicon   1.45-1.47   480   115-129   121.3       Layer   oxide       Second   zirconium   1.8-2.2   480   14-20   16.1       Layer   oxide       Third   silicon   1.45-1.47   480   37-48   42.1       Layer   oxide       Fourth   zirconium   1.8-2.2   480   58-67   63.1       Layer   oxide       Fifth   silicon   1.45-1.47   480   12-17   15.0       Layer   oxide       Sixth   zirconium   1.8-2.2   480   47-55   51.3       Layer   oxide       Seventh   silicon   1.45-1.47   480    94-109   101.1       Layer   oxide                  
 
         [0105]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 12 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 More 
               
               
                   
                   
                   
                 Design 
                   
                 Preferable 
               
               
                   
                   
                 Refractive 
                 Wavelength 
                 Thickness 
                 Thickness 
               
               
                   
                 Material 
                 Index 
                 (nm) 
                 (nm) 
                 (nm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 First 
                 zirconium 
                 1.8-2.2 
                 530 
                 24-30 
                 26.8 
               
               
                 Layer 
                 oxide 
               
               
                 Second 
                 silicon 
                 1.45-1.47 
                 530 
                 19-24 
                 22.2 
               
               
                 Layer 
                 oxide 
               
               
                 Third 
                 zirconium 
                 1.8-2.2 
                 530 
                 50-58 
                 52.9 
               
               
                 Layer 
                 oxide 
               
               
                 Fourth 
                 silicon 
                 1.45-1.47 
                 530 
                 41-49 
                 45.1 
               
               
                 Layer 
                 oxide 
               
               
                 Fifth 
                 zirconium 
                 1.8-2.2 
                 530 
                 19-25 
                 22.4 
               
               
                 Layer 
                 oxide 
               
               
                 Sixth 
                 silicon 
                 1.45-1.47 
                 530 
                 174-199 
                 187.6 
               
               
                 Layer 
                 oxide 
               
               
                 Seventh 
                 zirconium 
                 1.8-2.2 
                 530 
                 67-78 
                 72.8 
               
               
                 Layer 
                 oxide 
               
               
                 Eighth 
                 silicon 
                 1.45-1.47 
                 530 
                 17.9-18   
                 16.4 
               
               
                 Layer 
                 oxide 
               
               
                 Ninth 
                 zirconium 
                 1.8-2.2 
                 530 
                 33-41 
                 37.9 
               
               
                 Layer 
                 oxide 
               
               
                 10th Layer 
                 silicon 
                 1.45-1.47 
                 530 
                  93-106 
                 99.5 
               
               
                   
                 oxide 
               
               
                   
               
             
          
         
       
     
         [0106]     An underlayer which is made of silicon oxide and has a thickness of 0.2μ to 2μ may be provided between the base material and the first layer of the 10-layer antireflection coating on S 2 .  
       EXPERIMENTAL EXAMPLE 9  
       [0107]     A seven-layer antireflection coating was formed on S 1  of the objective lens made of Zeonex resin in the shape shown in  FIG. 4  through which three types of light beams of wavelengths of 405 nm, 650 nm, and 780 nm pass, a seven-layer antireflection coating having the thickness indicated in (11) in Table 1 was formed on S 2 , a 10-layer antireflection coating having the thickness indicated by (14) in Table 1 was formed on S 3 , and a 10-layer antireflection coating having the thickness indicated by (14) in Table 1 was formed on S 4 .  
         [0108]     Table 13 shows the specifications of the seven layers on each of S 
         [0109]      1  and S 2 . Note that the specifications of the 10 layers on each of S 3  and S 4  are the same as those shown in Table 12.  
                                                                 TABLE 13                                                   More                   Design       Preferable               Refractive   Wavelength   Thickness   Thickness           Material   Index   (nm)   (nm)   (nm)                                    First   silicon   1.45-1.47   480   115-129   121.7       Layer   oxide       Second   hafnium   1.7-2.2   480   12-20   16.2       Layer   oxide       Third   silicon   1.45-1.47   480   37-48   42.3       Layer   oxide       Fourth   hafnium   1.7-2.2   480   58-68   62.9       Layer   oxide       Fifth   silicon   1.45-1.47   480   12-17   15.0       Layer   oxide       Sixth   hafnium   1.7-2.2   480   47-56   51.0       Layer   oxide       Seventh   silicon   1.45-1.47   480    95-109   103.0       Layer   oxide                    
       COMPARATIVE EXPERIMENTAL EXAMPLE 1  
       [0110]     A seven-layer antireflection coating with specifications similar to those in Table 2 (Experimental Example 1) was formed on S 1  of an objective lens identical to the one in Experimental Example 1, and a seven-layer antireflection coating with specifications similar to those in Table 4 (Experimental Example 2) was formed on S 2 .  
       COMPARATIVE EXPERIMENTAL EXAMPLE 2  
       [0111]     A 10-layer antireflection coating with specifications similar to those in Table 12 (Experimental Example 9) was formed on each of S 1  and S 2  of an objective lens identical to the one in Experimental Example 9, and a seven-layer antireflection coating with specifications similar to those in Table 11 (Experimental Example 8) was formed on each of S 3  and S 4 .  
         [0112]     Light condensing characteristics, the amounts of light transmitted, and wiping resistance (the peeling resistance of each film which was obtained when a swab impregnated with isopropyl alcohol was slid on S 1  of each lens while being pressed on it with a load of 10 g), which indicated the degrees of deterioration in diffraction structures, were evaluated with respect to Experimental Examples 1 to 9 and Comparative Experimental Examples 1 and 2 described above under the same conditions. Tables 14 and 15 show evaluation results and evaluation criteria.  
                                                     TABLE 14                                   Light   Amount               Condens-   Of           ing   Light   Wiping           Character-   Trans-   Resis-           istics   mitted   tance                                        Experimental Example 1   ◯   Δ   —           Experimental Example 2   ◯   Δ   ◯           Experimental Example 3   ◯   ◯   ◯           Experimental Example 4   ◯   ◯   ◯           Experimental Example 5   ◯   ◯   ◯           Experimental Example 6   Δ   ◯   Δ           Experimental Example 7   Δ   ◯   Δ           Experimental Example 8   Δ   ◯   Δ           Experimental Example 9   Δ   ◯   Δ           Comparative   X   ◯   X           Experimental Example 1           Comparative   X   ◯   X           Experimental Example 2                      
 
         [0113]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 15 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                 ◯ (level at 
                   
                   
               
               
                   
                 which no 
                 Δ (level at 
               
               
                   
                 practical 
                 which no 
                 X (level at 
               
               
                   
                 problem 
                 practical 
                 which some 
               
               
                   
                 occurs at 
                 problem 
                 problem 
               
               
                   
                 all) 
                 occurs) 
                 occurs) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Light 
                 information 
                 information 
                 crosstalk 
               
               
                 Condensing 
                 can be 
                 can be 
                 occurs and 
               
               
                 Characteris- 
                 properly 
                 properly 
                 information 
               
               
                 tics 
                 reproduced by 
                 reproduced by 
                 cannot be 
               
               
                   
                 optical 
                 optical 
                 stably repro- 
               
               
                   
                 pickup 
                 pickup 
                 duced 
               
               
                   
                 apparatus 
                 apparatus 
               
               
                   
                 without any 
                 without any 
               
               
                   
                 crosstalk 
                 crosstalk 
               
               
                 Amount of 
                 lens 
                 lens 
                 lens 
               
               
                 Light 
                 transmittance 
                 transmittance 
                 transmittance 
               
               
                 Transmitted 
                 is 90% or 
                 is 85% or 
                 is less than 
               
               
                   
                 more with 
                 more with 
                 85% with 
               
               
                   
                 respect to 
                 respect to 
                 respect to 
               
               
                   
                 laser beam to 
                 laser beam to 
                 laser beam to 
               
               
                   
                 be used 
                 be used (no 
                 be used (some 
               
               
                   
                 (excellent 
                 practical 
                 practical 
               
               
                   
                 transmittance) 
                 problem 
                 problem 
               
               
                   
                   
                 occurs) 
                 occurs) 
               
               
                 Wiping 
                 no peeling 
                 no peeling 
                 peeling 
               
               
                 Resistance 
                 occurs after 
                 occurs after 
                 occurs after 
               
               
                   
                 50 times of 
                 20 times of 
                 20 times of 
               
               
                   
                 wiping 
                 wiping 
                 wiping 
               
               
                   
               
             
          
         
       
     
         [0114]     As indicated by Table 14, with regard to an objective lens in the shape shown in  FIG. 3 , it was understood that the light condensing and wiping resistance characteristics in Comparative Experimental Example 1 could not meet the required level, whereas the light condensing and wiping resistance characteristics and the amounts of light transmitted in Experimental Examples 1 to 8 met the required levels. In addition, with regard to an objective lens in the shape shown in  FIG. 4 , it was found that the light condensing and wiping resistance characteristics in Comparative Experimental Example 2 could not meet the required level, whereas the light condensing and wiping resistance characteristics and the amount of light transmitted in Experimental Example 9 met the required levels.  
         [0115]     The present invention has been described with reference to several embodiments and the above plurality of experimental examples. Obviously, however, the present invention should not be interpreted as limited to the above embodiments and experimental examples, and can be modified and improved as needed.