Write and read objective lens for high density storage
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a write and read objective lens for encoding and recovery of information upon a storage member and, more particularly, to a write and read objective lens for the encoding of high-density information upon a storage member, and further, a write and read objective for the recovery of high-density information which is optically encoded on a storage member. 
     2. Description of the Prior Art 
     For a write and read objective lens for encoding and recovery of information upon a storage member, it is necessary that the write and read objective for encoding and recovery of information on a storage member be utilizable with a plurality of wavelengths of light. This is required due to the use of the write and read objective lens as both the encoding objective lens and further as a reproducing lens for the recovery of the encoded information, the specific requirements for encoding and recovery differ greatly. A write objective lens must be of a high numerical aperture to provide a minimum spot size at the object, this provides for the highest signal to noise ratio of the recorded information and further provides a higher storage density of information upon the storage member. 
     To provide the maximum available power through the objective lens for use in encoding information, the objective lens must be as transparent as possible at the wavelength of light provided for the encoding of information (Λ=4420 nm). This can be accomplished by providing a multi-layer, anti-reflection coating on the lens surfaces. For an objective lens to be used in reproducing systems of encoded information, it is required to guarantee a resolving power of at least 1μ due to the fact that the objective lens must recover the very small signals recorded at a high density. Moreover, the information recovered from the disc, which rotates at high speed, provides information for directing the objective lens to follow the recorded track, information for automatic focusing in addition to the stored video information. To insure that the objective lens recovers those kinds of information correctly, the flatness of field focussed by the objective lens should be high. To prevent the objective lens from contacting the storage member which would result in the destruction of the storage member and the objective lens, the working distance of the objective lens should be long. 
     Further, to accomplish automatic focussing, the objective lens should be compact and light in weight. As the light source provided for generation of light provided to the objective lens for recovery of information is generally of the monochromatic type (Λ=6328 nm), it is effective for increasing the signal to noise ratio of the recovered signal as amplified after recovery by a photo detector that the objective lens be as highly transparent to light of this wavelength as possible. Therefore, to increase the transparency, it is necessary to provide a multi-layer, anti-reflection coating on the lens surfaces and to minimize the number of lenses constituting the objective lens. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a primary object of the present invention to provide a write and read objective lens for encoding and recovery of information upon a storage member which is utilizable at a plurality of wavelengths of light and further provide a large working distance and high flatness of field and resolving power. 
     As shown in FIG. 1, the write and read objective lens for encoding and recovery of information is a lens system comprising a first, second, third and fourth lens, said first lens being a positive meniscus lens with its concave surface positioned toward the object side, said second lens being a positive achromatic doublet, said third lens being a negative meniscus lens with its convex surface positioned toward the object side and said fourth lens being a positive lens. The write and read objective lens for encoding and recovery of information upon a storage member satisfies the following conditions when reference symbol f represents the focal length of the lens system as a whole. Reference symbol f 3  represents the focal length of the third lens, reference symbol f 4  represents the focal length of the fourth lens, reference symbol r 6  represents the radius of curvature of the surface on the image side of the second lens, reference symbol r 7  represents the radius of curvature of surface on the object side of the third lens, reference symbol d 1  represents the thickness of the first lens and reference symbol d 7  represents the thickness of the third lens. 
     
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1.         1.24 ≧ r.sub.6 /f                                       
                            ≧ 1.17                                 
2.         0.46 ≧ d.sub.1 /f                                       
                            ≧ 0.37                                 
3.         0.27 ≧ |r.sub.6 |/|r.sub.7   
           |       ≧ 0.21                                 
4.         1.20 ≧ |f.sub.3 |/|f.sub.4   
           |       ≧ 1.00                                 
5.         0.45 ≧ d.sub.5 /f                                       
                            ≧ 0.40                                 
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     When in the lens system of the above-mentioned configuration, r 6  /f becomes larger than the upper limit of the condition (1), i.e., r 6  f&gt;1.24 distortion will increase and spherical aberration in the zonal and marginal portions will be overcorrected. When, on the contrary, r 6  /f becomes smaller than the lower limit, i.e., r 6  f&lt;1.17, astigmatism will be undercorrected. When d 1  /f in the condition (2) becomes d 1  /f&gt;0.46, the working distance becomes short and it becomes impossible to attain the object of the present invention. Moreover, spherical aberration will be undercorrected and the astigmatic difference becomes large. When, on the contrary, it becomes d 1  /f&lt;0.37, coma and spherical aberration in the marginal portion will become unfavorable though the working distance becomes large. It is this condition which corrects additional spherical aberrations introduced by subsequent elements interposed in the long conjugate of the objective lens. If it becomes |r 6  |/|r 7  |&gt;0.27, in the condition (3), spherical aberration and coma will be overcorrected. If it become | r 6  |/|r 7  |&lt;0.21, coma will be somewhat undercorrected. If it becomes |f 3  |/|f 4  |&gt;1.20  in the condition (4), coma and spherical aberration in the marginal portion will become large. If it becomes |f 3  |/|f 4  |&lt;1.00, symmetry of coma will become unfavorable. Moreover, astigmatism will be undercorrected and will increase in a negative direction toward the marginal portion. If, in the condition (5), it becomes d 5  /f&gt;0.45, spherical aberration will be somewhat overcorrected and, at the same time, the astigmatic difference becomes large. If, on the contrary, it becomes d 5  /f&lt;0.40, coma will be somewhat overcorrected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     EMBODIMENT 1 
     FIG. 1 shows a sectional view of the objective lens according to the present invention; and 
     FIGS. 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, 2j, 2k, and 2l, respectively, show graphs illustrating aberration curves of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the write and read objective lens for use with a wavelength of light suitable for the encoding of information upon a storage member is as follows: 
     
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r.sub.1 =  -6.5338                                                        
           d.sub.1 = 3.9509 n.sub.1 = 1.7401 ν.sub.1 = 44.77           
r.sub.2 =  -4.6723                                                        
           d.sub.2 = 3.2154                                               
r.sub.3 =  21.6825                                                        
           d.sub.3 = 2.0000 n.sub.2 = 1.7980 ν.sub.2 = 25.43           
r.sub.4 =  12.0589                                                        
           d.sub.4 = 0.0000                                               
r.sub.5 =  +12.0589                                                       
           d.sub.5 = 3.8669 n.sub.3 = 1.4992 ν.sub.3 = 56.41           
r.sub.6 =  -10.9260                                                       
           d.sub.6 = 5.2846                                               
r.sub.7 =  +44.0165                                                       
           d.sub.7 = 1.5000 n.sub.4 = 1.7490 ν.sub.4 = 27.58           
r.sub.8 =  +11.2958                                                       
           d.sub.8 = 1.8043                                               
r.sub.9 =  +14.7279                                                       
           d.sub.9 = 3.4780 n.sub.5 = 51.48 ν1.548 5 = 64.17           
r.sub.10 = -26.4865                                                       
           f = 8.9494 f.sub.3 = 19.8969 f.sub.4 = -17.8722                
           1 = 1.5                                                        
RMS WAVEFRONT ERRORS .0029                                                
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     EMBODIMENT 2 
     The preferred embodiment of the write and read objective lens for use with a wavelength of light suitable for the recovery of high density information upon a storage member is as follows: 
     
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r.sub.1 =  -6.5338                                                        
           d.sub.1 = 3.9509 n.sub.1 = 1.7637 ν.sub.1 = 44.77           
r.sub.2 =  -4.6723                                                        
           d.sub.2 = 3.2154                                               
r.sub.3 =  +21.6825                                                       
           d.sub.3 = 2.0000 n.sub.2 = 1.8442 ν.sub.2 = 25.43           
r.sub.4 =  +12.0589                                                       
           d.sub.4 = 0.0000                                               
r.sub.5 =  +12.0589                                                       
           d.sub.5 = 3.8669 n.sub.3 = 1.5117 ν.sub.3 = 56.41           
r.sub.6 =  -10.9260                                                       
           d.sub.6 = 5.2846                                               
r.sub.7 =  +44.0165                                                       
           d.sub.7 = 1.5000 n.sub.4 = 1.7887 ν.sub.4 = 27.58           
r.sub.8 =  +11.2958                                                       
           d.sub.8 = 1.8043                                               
r.sub.9 =  +14.7279                                                       
           d.sub.9 = 3.4780 n.sub.5 = 1.5260 ν.sub.5 = 64.17           
r.sub.10 = -26.4865                                                       
           f = 9.1383 f.sub.3 = 18.8846 f.sub.4 = -17.4855                
           1 = 1.5                                                        
RMS WAVEFRONT ERRORS = .0063                                              
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     In the above-mentioned embodiments, reference symbols r 1  through r 10 , respectively, represent radii of curvature of respective lens surfaces. Reference symbols d 1  through d 9 , respectively, represent thicknesses of respective lenses and air spaces between respective lenses. Reference symbols n 1  through n 5 , respectively, represent refractive indices of respective lenses. Reference symbols ν 1  through ν 5 , respectively, represent Abbe&#39;s numbers of respective lenses. Reference symbol f represents the focal length of the lens system as a whole. Reference symbol f 3  represents the focal length of the third lens. Reference symbol f 4  represents the focal length of the fourth lens and reference symbol l represents the working distance of the lens system. As explained in the above, the lens system according to the present invention fully satisfies the requirements for a reproducing objective lens for videodiscs. This is also evident from the favorably corrected aberrations shown in respective graphs of aberration curves.