Patent Publication Number: US-8526292-B2

Title: Manufacturing method for optical recording medium, optical recording medium, optical information device, and information reproducing method

Description:
This application is a Divisional of U.S. application Ser. No. 13/001,192, filed Dec. 23, 2010 now U.S. Pat. No. 8,189,452 which is a national stage application of International Application No. PCT/JP2010/002710, filed Apr. 14, 2010. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to an optical recording medium for information recording or reproducing by irradiated light, a manufacturing method for the optical recording medium, an optical information device for recording or reproducing information with respect to the optical recording medium, and an information reproducing method for reproducing information from the optical recording medium; and more particularly to an interlayer structure of an optical recording medium having three or more information recording surfaces. 
     2. Background Art 
     There are known optical discs called as DVD or BD (Blu-ray disc), as examples of the commercially available high-density and large-capacity optical information recording media. In recent years, the optical discs have become widely used as recording media for recording images, music, and computer-readable data. There also has been proposed an optical disc having plural recording layers, as disclosed in Patent Literature 1 and Patent literature 2, to further increase the recording capacity. 
       FIG. 14  is a diagram showing an arrangement of a conventional optical recording medium and optical head device. An optical recording medium  401  includes a first information recording surface  401   a  closest to a surface  401   z  of the optical recording medium  401 , a second information recording surface  401   b  second closest to the surface  401   z  of the optical recording medium  401 , a third information recording surface  401   c  third closest to the surface  401   z  of the optical recording medium  401 , and a fourth information recording surface  401   d  farthest from the surface  401   z  of the optical recording medium  401 . 
     A divergent beam  70  emitted from a light source  1  is transmitted through a collimator lens  53 , and incident into a polarized beam splitter  52 . The beam  70  incident into the polarized beam splitter  52  is transmitted through the polarized beam splitter  52 , and converted into circularly polarized light while being transmitted through a quarter wavelength plate  54 . Thereafter, the beam  70  is converted into a convergent beam through an objective lens  56 , transmitted through a transparent substrate of the optical recording medium  401 , and collected on one of the first information recording surface  401   a , the second information recording surface  401   b , the third information recording surface  401   c , and the fourth information recording surface  401   d  formed in the interior of the optical recording medium  401 . 
     The objective lens  56  is so designed as to make a spherical aberration zero at an intermediate depth position between the first information recording surface  401   a  and the fourth information recording surface  401   d . A spherical aberration corrector  93  shifts the position of the collimator lens  53  in an optical axis direction. Thereby, spherical aberration resulting from collecting light on the first through the fourth information recording surfaces  401   a  through  401   d  is removed. 
     An aperture  55  restricts the opening of the objective lens  56 , and sets the numerical aperture NA of the objective lens  56  to 0.85. The beam  70  reflected on the fourth information recording surface  401   d  is transmitted through the objective lens  56  and the quarter wavelength plate  54 , converted into linearly polarized light along an optical path displaced by 90 degrees with respect to the outward path, and then reflected on the polarized beam splitter  52 . The beam  70  reflected on the polarized beam splitter  52  is converted into convergent light while being transmitted through a light collecting lens  59 , and incident into a photodetector  320  through a cylindrical lens  57 . Astigmatism is imparted to the beam  70  while the beam  70  is transmitted through the cylindrical lens  57 . 
     The photodetector  320  has unillustrated four light receiving sections. Each of the light receiving sections outputs a current signal depending on a received light amount. A focus error (hereinafter, called as FE) signal by an astigmatism method, a tracking error (hereinafter, called as It) signal by a push-pull method, and an information (hereinafter called as RF) signal recorded in the optical recording medium  401  are generated, based on the current signals. The FE signal and the TE signal are amplified to an intended level, subjected to phase compensation, and then supplied to actuators  91  and  92 , whereby focus control and tracking control are performed. 
     In this example, the following problem occurs, in the case where the thickness t 1  between the surface  401   z  of the optical recording medium  401  and the first information recording surface  401   a , the thickness t 2  between the first information recording surface  401   a  and the second information recording surface  401   b , the thickness t 3  between the second information recording surface  401   b  and the third information recording surface  401   c , and the thickness t 4  between the third information recording surface  401   c  and the fourth information recording surface  401   d  are equal to each other. 
     For instance, in the case where the beam  70  is collected on the fourth information recording surface  401   d  to record or reproduce information on or from the fourth information recording surface  401   d , a part of the beam  70  is reflected on the third information recording surface  401   c . The distance from the third information recording surface  401   c  to the fourth information recording surface  401   d , and the distance from the third information recording surface  401   c  to the second information recording surface  401   b  are equal to each other. Accordingly, the part of the beam  70  reflected on the third information recording surface  401   c  forms an image on a backside of the second information recording surface  401   b , and reflected light from the backside of the second information surface  401   b  is reflected on the third information recording surface  401   c . As a result, the light reflected on the third information recording surface  401   c , the backside of the second information recording surface  401   b , and the third information recording surface  401   c  may be mixed with reflected light from the fourth information recording surface  401   d  to be read. 
     Further, the distance from the second information recording surface  401   b  to the fourth information recording surface  401   d , and the distance from the second information recording surface  401   b  to the surface  401   z  of the optical recording medium  401  are equal to each other. Accordingly, a part of the beam  70  reflected on the second information recording surface  401   b  forms an image on the backside of the surface  401   z  of the optical recording medium  401 , and reflected light from the backside of the surface  401   z  is reflected on the second information recording surface  401   b . As a result, the light reflected on the second information recording surface  401   b , the backside of the surface  401   z , and the second information recording surface  401   b  may be mixed with reflected light from the fourth information recording surface  401   d  to be read. 
     As described above, there is a problem that reflected light from the fourth information recording surface  401   d  to be read is superimposed and mixed with reflected light which forms an image on the backside of the other surface, with the result that information recording/reproducing is obstructed. Light containing reflected light which forms an image on the backside of the other surface has a high coherence, and forms a brightness/darkness distribution on a light receiving element by coherence. Since the brightness/darkness distribution is varied depending on a change in phase difference with respect to reflected light from the other surface, resulting from a small thickness variation of an intermediate layer in an in-plane direction of an optical disc, the quality of a servo signal and a reproduction signal may be considerably deteriorated. Hereinafter, the above problem is called as a back focus problem in the specification. 
     In order to prevent the back focus problem, Patent literature 1 discloses a method, wherein the interlayer distance between the information recording surfaces is gradually increased in the order from the surface  401   z  of the optical recording medium  401  so that a part of the beam  70  may not form an image on the backside of the second information recording surface  401   b  and the backside of the surface  401   z  simultaneously when the beam  70  is collected on the fourth information recording surface  401   d  to be read. The thicknesses t 1  through t 4  each has a production variation of ±10 μm. It is necessary to set the thicknesses t 1  through t 4  to different values from each other, also in a case where the thicknesses t 1  through t 4  are varied. In view of this, a difference in the thicknesses t 1  through t 4  is set to e.g. 20 μm. In this example, the thicknesses t 1  through t 4  are respectively set to 40 μm, 60 μm, 80 μm, and 100 μm, and the total interlayer thickness t(=t 2 +t 3 +t 4 ) from the first information recording surface  401   a  to the fourth information recording layer  401   d  is set to 240 μm. 
     In the case where the thickness of a cover layer from the surface  401   z  to the first information recording surface  401   a , and the thickness from the fourth information recording surface  401   d  to the first information recording surface  401   a  are equal to each other, light reflected on the fourth information recording surface  401   d  is focused on the surface  401   z , and reflected on the surface  401   z . The light reflected on the surface  401   z  is reflected on the fourth information recording surface  401   d , and guided to the photodetector  320 . A light flux which forms an image on the backside of the surface  401   z  does not have information relating to pits or marks, unlike a light flux which forms an image on the backside of the other information recording surface. However, in the case where the number of information recording surfaces is large, the light amount of light returning from the information recording surfaces is reduced, and the reflectance of the surface  401   z  is relatively increased. As a result, coherence between a light flux reflected on the backside of the surface  401   z , and a light flux reflected on a targeted information recording surface to be recorded or reproduced is generated in the similar manner as in the case of a light flux reflected on the backside of the other information recording surfaces, which may considerably deteriorate the quality of a servo signal and a reproduction signal. 
     In view of the above problem, Patent literature 2 proposes a distance between information recording layers (information recording surfaces) of an optical disc. Patent literature 2 discloses the following structure. 
     An optical recording medium has four information recording surfaces, wherein the first through the fourth information recording surfaces are defined in the order from a side closest to a surface of the optical recording medium. The distance from the medium surface to the first information recording surface is set to 47 μm or less. The thicknesses of intermediate layers between the first through the fourth information recording surfaces are combination of a range from 11 to 15 μm, a range from 16 to 21 μm, and a range of 22 μm or more. The distance from the medium surface to the fourth information recording surface is set to 100 μm. The distance from the medium surface to the first information recording surface is set to 47 μm or less, and the distance from the medium surface to the fourth information recording surface is set to 100 μm. 
     An optical disc system is adapted to detect light incident from a medium surface and reflected on an information recording surface. Accordingly, a refractive index of a transparent material constituting a transparent member from the medium surface where light is transmitted to the information recording surface also affects the quality of a servo signal and a reproduction signal. However, there is no consideration and description about the refractive index in the disc structures disclosed in Patent literature 1 and Patent literature 2. Thus, both of the publications do not consider an influence of a refractive index of a transparent material on the quality of a servo signal and a reproduction signal. 
     CITATION LIST 
     Patent Literature 
     Patent literature 1: JP 2001-155380A 
     Patent literature 2: JP 2008-117513A 
     SUMMARY OF INVENTION 
     In view of the above, an object of the invention is to provide an optical recording medium manufacturing method, an optical recording medium, an optical information device, and an information reproducing method that enable to improve the quality of a servo signal and a reproduction signal. 
     A manufacturing method for an optical recording medium according to an aspect of the invention is a manufacturing method for an optical recording medium having (N−1) (where N is a positive integer of 4 or more) information recording surfaces, wherein, assuming that shape-wise thicknesses of a cover layer and first through (N−1)-th intermediate layers of the optical recording medium having refractive indexes nr 1 , nr 2 , . . . , and nrN are respectively tr 1 , tr 2 , . . . , and trN in the order from a surface of the optical recording medium where light is incident, the thicknesses tr 1 , tr 2 , . . . , and trN are converted into thicknesses t 1  , t 2 , . . . ,and tN of layers having a predetermined refractive index “no” which makes a divergent amount equal to a divergent amount of a light beam resulting from the thicknesses tr 1 , tr 2 , . . . , and trN; a difference DFF between the sum of a thickness “ti” through a thickness “tj”, and the sum of a thickness “tk” through a thickness “tm” is set to 1 μm or more (where i, j, k, and m are each any positive integer satisfying i≦j&lt;k≦m≦SN); and the thicknesses t 1  , t 2 , . . . , and tN are calculated by products of a function f(n) expressed by the following formula (1), and the thicknesses tr 1 , tr 2 , . . . , and trN:
 
 f ( n )=−1.088 n   3 +6.1027 n   2 −12.042 n +9.1007   (1)
 
     in the formula (1), n=nr 1  , nr 2 , . . . , and nrN. 
     In the above arrangement, assuming that shape-wise thicknesses of a cover layer and first through (N−1)-th intermediate layers of an optical recording medium having refractive indexes nr 1 , nr 2 , . . . , and nrN are respectively tr 1  , tr 2 , . . . , and trN in the order from a surface of the optical recording medium where light is incident, the thicknesses tr 1 , tr 2 , . . . , and trN are converted into thicknesses t 1 , t 2 , . . . , and tN of layers having a predetermined refractive index “no” which makes a divergent amount equal to a divergent amount of a light beam resulting from the thicknesses tr 1 , tr 2 , . . . , and trN. Further, a difference DFF between the sum of a thickness “ti” through a thickness “tj”, and the sum of a thickness “tk” through a thickness “tm” is set to 1 μm or more (where i, j, k, and m are each any positive integer satisfying i≦j&lt;k≦m≦N). Furthermore, the thicknesses t 1 , t 2 , . . . , and tN are calculated by products of the function f(n) expressed by the above-described formula (1), and the thicknesses tr 1 , tr 2 , . . . , and trN. 
     According to the invention, since the difference DFF between the sum of the thickness “ti”through the thickness “tj”, and the sum of the thickness “tk” through the thickness “tm” is set to 1 μm or more, it is possible to prevent light from forming an image on the backside of the surface of the optical recording medium, and suppress coherence between reflected light from the information recording surfaces to thereby improve the quality of a servo signal and a reproduction signal. Further, since the distance between the surface of the optical recording medium and the information recording surface closest to the surface of the optical recording medium can be set to a large value, deterioration of a reproduction signal in the case where there is a damage or a smear on the surface of the optical recording medium can be suppressed. 
     The objects, characteristics and advantages of the present invention will be more apparent after reading the following detailed description along with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a schematic arrangement of an optical recording medium embodying the invention, and an optical head device. 
         FIG. 2  is a diagram showing a layer structure of the optical recording medium in the embodiment of the invention. 
         FIG. 3  is a diagram showing reflected light from a fourth information recording surface, in the case where a beam is collected on the fourth information recording surface. 
         FIG. 4  is a diagram showing reflected light from a third information recording surface and a second information recording surface, in the case where a beam is collected on the fourth information recording surface. 
         FIG. 5  is a diagram showing reflected light from the second information recording surface and a surface of the optical recording medium, in the case where a beam is collected on the fourth information recording surface. 
         FIG. 6  is a diagram showing reflected light from the third information recording surface, a first information recording surface, and the second information recording surface, in the case where a beam is collected on the fourth information recording surface. 
         FIG. 7  is a diagram showing a relation between a difference in interlayer thickness, and an amplitude of an FS signal. 
         FIG. 8  is a diagram showing a relation between an interlayer thickness of an optical recording medium having information recording surfaces of reflectances substantially equal to each other, and a jitter. 
         FIG. 9  is a diagram showing a layer structure of an optical recording medium as a modification of the embodiment of the invention. 
         FIG. 10  is an explanatory diagram showing a refractive index dependence of a factor for converting a shape-wise thickness in terms of an actual refractive index into a thickness in terms of a standard refractive index. 
         FIG. 11  is an explanatory diagram showing a refractive index dependence of a factor for converting a thickness in terms of a standard refractive index into a shape-wise thickness in terms of an actual refractive index. 
         FIG. 12  is an explanatory diagram showing a refractive index dependence of a factor for converting a shape-wise thickness in terms of an actual refractive index into a thickness in terms of a standard refractive index, based on a spherical aberration. 
         FIG. 13  is a diagram showing a schematic arrangement of an optical information device embodying the invention. 
         FIG. 14  is a diagram showing an arrangement of a conventional optical recording medium and optical head device. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     In the following, an embodiment of the invention is described referring to the accompanying drawings. The following embodiment is merely an example embodying the invention, and does not limit the technical scope of the invention. 
     Firstly, an optical recording medium embodying the invention is described referring to  FIGS. 1 and 2 . 
       FIG. 1  is a diagram showing a schematic arrangement of an optical recording medium embodying the invention, and an optical head device.  FIG. 2  is a diagram showing a layer structure of the optical recording medium in the embodiment. An optical head device  201  irradiates blue laser light whose wavelength λ is 405 nm onto an optical recording medium  40  to reproduce a signal recorded in the optical recording medium  40 . Since the arrangement and the operation of the optical head device  201  shown in  FIG. 1  are substantially the same as the arrangement and the operation of the optical head device shown in  FIG. 14 , detailed description thereof is omitted herein. 
     The optical recording medium  40  as an example has four information recording surfaces. As shown in  FIG. 2 , the optical recording medium  40  has, in the order from a side closest to a surface  40   z  of the optical recording medium  40 , a first information recording surface  40   a , a second information recording surface  40   b , a third information recording surface  40   c , and a fourth information recording surface  40   d.    
     The optical recording medium  40  is further provided with a cover layer  42 , a first intermediate layer  43 , a second intermediate layer  44 , and a third intermediate layer  45 . The thickness t 1  of the cover layer  42  represents a thickness of a substrate from the surface  40   z  to the first information recording surface  40   a , the thickness t 2  of the first intermediate layer  43  represents a thickness of the substrate from the first information recording surface  40   a  to the second information recording surface  40   b , the thickness t 3  of the second intermediate layer  44  represents a thickness of the substrate from the second information recording surface  40   b  to the third information recording surface  40   c , and the thickness t 4  of the third intermediate layer  45  represents a thickness of the substrate from the third information recording surface  40   c  to the fourth information recording surface  40   d.    
     The distance d 1  (≈t 1 ) represents a distance from the surface  40   z  to the first information recording surface  40   a , the distance d 2  (≈t 1 +t 2 ) represents a distance from the surface  40   z  to the second information recording surface  40   b , the distance d 3  (≈t 1 +t 2 +t 3 ) represents a distance from the surface  40   z  to the third information recording surface  40   c , and the distance d 4  (≈t 1 +t 2 +t 3 +t 4 ) represents a distance from the surface  40   z  to the fourth information recording surface  40   d.    
     Now, problems to be solved in the case where an optical recording medium has four information recording surfaces are described. Coherence between reflected light from multiple surfaces is described referring to  FIGS. 3 through 7 , as a first problem to be solved. 
       FIG. 3  is a diagram showing reflected light from the fourth information recording surface  40   d , in the case where a beam is collected on the fourth information recording surface  40   d .  FIG. 4  is a diagram showing reflected light from the third information recording surface  40   c  and the second information recording surface  40   b , in the case where a beam is collected on the fourth information recording surface  40   d .  FIG. 5  is a diagram showing reflected light from the second information recording surface  40   b  and the surface  40   z , in the case where a beam is collected on the fourth information recording surface  40   d .  FIG. 6  is a diagram showing reflected light from the third information recording surface  40   c , the first information recording surface  40   a , and the second information recording surface  40   b , in the case where a beam is collected on the fourth information recording surface  40   d.    
     As shown in  FIG. 3 , a light flux collected on the fourth information recording surface  40   d  for information reproducing or recording is split into the following light beams by semi-translucency of an information recording layer (an information recording surface). 
     Specifically, a light flux collected on the fourth information recording surface  40   d  for information reproducing or recording is split into: a beam  70  shown in  FIG. 3 ; a beam  71  (back focus light with respect to an information recording surface) shown in  FIG. 4 , a beam  72  (back focus light with respect to a medium surface) shown in  FIG. 5 , and a beam  73  shown in  FIG. 6 . 
     As shown in  FIG. 3 , the beam  70  is a beam reflected on the fourth information recording surface  40   d  and emitted from the surface  40   z . As shown in  FIG. 4 , the beam  71  is a beam reflected on the third information recording surface  40   c , focused and reflected on the backside of the second information recording surface  40   b , reflected on the third information recording surface  40   c , and emitted from the surface  40   z . As shown in  FIG. 5 , the beam  72  is a beam reflected on the second information recording surface  40   b , focused and reflected on the backside of the surface  40   z , reflected on the second information recording surface  40   b , and emitted from the surface  40   z . As shown in  FIG. 6 , the beam  73  is a beam which is not focused on the surface  40   z  and the backsides of the information recording surfaces, but is reflected in the order of the third information recording surface  40   c , the backside of the first information recording surface  40   a , and the second information recording surface  40   b , and emitted from the surface  40   z.    
     First, let us consider a case that the refractive indexes of the cover layer  42 , the first intermediate layer  43 , the second intermediate layer  44 , and the third intermediate layer  45  are equal to each other. In this case, the refractive indexes of the respective corresponding layers are set to “no”. 
     For instance, in the case where the distance (thickness t 4 ) between the fourth information recording surface  40   d  and the third information recording surface  40   c , and the distance (thickness t 3 ) between the third information recording surface  40   c  and the second information recording surface  40   b  are equal to each other, the beam  70  and the beam  71  pass a common optical path when exiting from the surface  40   z . Accordingly, the beam  70  and the beam  71  are incident into a photodetector  320  with an identical light flux diameter. Similarly, in the case where the distance (thickness t 4 +thickness t 3 ) between the fourth information recording surface  40   d  and the second information recording surface  40   b , and the distance (thickness t 2 +thickness t 1 ) between the second information recording surface  40   b  and the surface  40   z  are equal to each other, the beam  70  and the beam  72  pass a common optical path when exiting from the surface  40   z . Accordingly, the beam  70  and the beam  72  are incident into the photodetector  320  with an identical light flux diameter. In the case where the distance (thickness t 2 ) between the second information recording surface  40   b  and the first information recording surface  40   a , and the distance (thickness t 4 ) between the fourth information recording surface  40   d  and the third information recording surface  40   c  are equal to each other, the beam  70  and the beam  73  pass a common optical path when exiting from the surface  40   z . Accordingly, the beam  70  and the beam  73  are incident into the photodetector  320  with an identical light flux diameter. 
     The light intensities of the beams  71  through  73  as reflected light from multiple surfaces are small, as compared with the light intensity of the beam  70 . However, coherent contrast does not depend on a light intensity but depends on a light intensity ratio of light amplitude, and the light amplitude is a square root of the light intensity. Accordingly, even a small difference between light intensities results in a large coherent contrast. In the case where the beams  70  through  73  are incident into the photodetector  320  with an identical light flux diameter, an influence by coherence between the beams is large. Further, a light receiving amount by the photodetector  320  is greatly varied, resulting from a small change in thickness between the information recording surfaces, which makes it difficult to stably detect a signal. 
       FIG. 7  is a diagram showing a relation between a difference in interlayer thickness, and an amplitude of an FS signal.  FIG. 7  shows an amplitude of an FS signal (the sum of light intensities) with respect to a difference in interlayer thickness, in the case where the light intensity ratio between the beam  70 ; and the beam  71 , or the beam  72 , or the beam  73  is set to 100:1, and the refractive indexes of the cover layer  42  and the first intermediate layer  43  are each set to about 1.60 (1.57). Referring to  FIG. 7 , the axis of abscissas indicates a difference in interlayer thickness, and the axis of ordinate indicates an amplitude of an FS signal. The FS signal amplitude is a value obtained by normalizing light solely composed of the beam  70  to be detected by the photodetector  320  by a DC light amount, assuming that there is no reflection from the other information recording surfaces. In this embodiment, an interlayer means a layer between a surface of the optical recording medium and an information recording surface, and a layer between information recording surfaces adjacent to each other. As shown in  FIG. 7 , it is obvious that the FS signal is sharply changed when the difference in interlayer thickness becomes about 1 μm or less. 
     Similarly to the beam  72  shown in  FIG. 5 , in the case where the difference between the thickness t 1  of the cover layer  42 , and the sum (t 2 +t 3 +t 4 ) of the thicknesses of the first through the third intermediate layers  43  through  45  is 1 μm or less, a problem such as variation of the FS signal also occurs. 
     As a second problem to be solved, an exceedingly small interlayer distance between adjacent information recording surfaces causes an influence of crosstalk from the adjacent information recording surface. In view of this, an interlayer distance of a predetermined value or more is necessary. Accordingly, various interlayer thicknesses are investigated, and an interlayer thickness which minimizes the influence is determined. 
       FIG. 8  is a diagram showing a relation between an interlayer thickness of an optical recording medium having information recording surfaces having reflectances substantially equal to each other, and a jitter. The refractive index of the intermediate layer is set to about 1.60. Referring to  FIG. 8 , the axis of abscissas indicates an interlayer thickness, and the axis of ordinate indicates a jitter value. As the interlayer thickness is reduced, the jitter is deteriorated. The interlayer thickness where the jitter starts increasing is about 10 μm, and in the case where the interlayer thickness becomes 10  82  m or less, the jitter is seriously deteriorated. Therefore, an optimum minimum value of the interlayer thickness is 10 μm. 
     Referring to  FIG. 2 , an arrangement of the optical recording medium  40  in the embodiment of the invention is described. In the embodiment, the structure of a four-layer disc (the optical recording medium  40 ) is defined in such a manner as to secure the following conditions (1) through (3) in order to eliminate an adverse effect of reflected light from the other information recording surfaces or a disc surface, considering a thickness variation among products. 
     Condition (1): The difference between the thickness t 1  of the cover layer  42 , and the sum (t 2 +t 3 +t 4 ) of the thicknesses t 2  through t 4  of the first through the third intermediate layers  43  through  45  is set to 1 μm or more. In other words, the thicknesses t 1 , t 2 , t 3 , and t 4  satisfy |t 1 −(t 2 +t 3 +t 4 )|≧1 μm. 
     Condition (2): The difference between any two values of the thicknesses t 1 , t 2 , t 3 , and t 4  is set to 1 μm or more in any case. 
     Condition (3): The difference between the sum (t 1 +t 2 ) of the thickness t 1  of the cover layer  42  and the thickness t 2  of the first intermediate layer  43 , and the sum (t 3 +t 4 ) of the thickness t 3  of the second intermediate layer  44  and the thickness t 4  of the third intermediate layer  45  is set to 1 μm or more. In other words, the thicknesses t 1 , t 2 , t 3 , and t 4  satisfy |(t 1 +t 2 )−(t 3 +t 4 )|≧1 μm. 
     There are other combinations of interlayer thicknesses. However, in the case where the thickness t 1  of the cover layer is set to a value approximate to the sum (t 2 +t 3 +t 4 ) of the thicknesses t 2  through t 4  of the first through the third intermediate layers  43  through  45 , there is no need of considering the other combinations. Therefore, description on the other combinations is omitted herein. 
       FIG. 9  is a diagram showing a layer structure of an optical recording medium as a modification of the embodiment of the invention. An optical recording medium  30  shown in  FIG. 9  has three information recording surfaces. As shown in  FIG. 9 , the optical recording medium  30  has, in the order from a side closest to a surface  30   z  of the optical recording medium  30 , a first information recording surface  30   a , a second information recording surface  30   b , and a third information recording surface  30   c . The optical recording medium  30  is further provided with a cover layer  32 , a first intermediate layer  33 , and a second intermediate layer  34 . 
     The thickness t 1  of the cover layer  32  represents a thickness of a substrate from the surface  30   z  to the first information recording surface  30   a , the thickness t 2  of the first intermediate layer  33  represents a thickness of the substrate from the first information recording surface  30   a  to the second information recording surface  30   b , and the thickness t 3  of the second intermediate layer  34  represents a thickness of the substrate from the second information recording surface  30   b  to the third information recording surface  30   c.    
     The distance d 1  (≈t 1 ) represents a distance from the surface  30   z  to the first information recording surface  30   a , the distance d 2  (≈t 1 −t 2 ) represents a distance from the surface  30   z  to the second information recording surface  30   b , and the distance d 3  (≈t 1 +t 2 +t 3 ) represents a distance from the surface  30   z  to the third information recording surface  30   c.    
     In the foregoing description, the structure of the four-layer disc is concretely described. In the case where a three-layer disc as shown in  FIG. 9  is produced, the structure of the three-layer disc (the optical recording medium  30 ) is defined in such a manner as to secure the following conditions (1) and (2). 
     Condition (1): The difference between the thickness t 1  of the cover layer  32 , and the sum (t 2 +t 3 ) of the thicknesses t 2  and t 3  of the first intermediate layer  33  and the second intermediate layer  34  is set to 1 μm or more. In other words, the optical recording medium  30  satisfies |t 1 −(t 2 +t 3 )|≧1 μtm. 
     Condition (2): The difference between any two values of the thicknesses t 1 , t 2 , and t 3  is set to 1 μm or more in any case. 
     Concerning a (N−1)-layer disc (where n is a positive integer equal to or more than 4), the above condition generally means that a difference between the sum of the thickness “ti” through the thickness “tj”, and the sum of the thickness “tk” through the thickness “tm” is necessarily set to 1 μm or more, assuming that t 1  is a thickness of the cover layer, and t 2  through tN are thicknesses of the first through the N-th intermediate layers, where i, j, k, and m are each any positive integer satisfying i≦j&lt;k≦m≦N. The cover layer thickness corresponds to a distance from the surface of the optical recording medium to the information recording surface closest to the medium surface. The above description is applied to the description that a distance from the surface of the optical recording medium to the information recording surface second closest to the medium surface is defined as d 2 , a distance from the surface of the optical recording medium to the information recording surface third closest to the medium surface is defined as d 3 , and a distance from the surface of the optical recording medium to the information recording surface fourth closest to the medium surface is defined as d 4  in the same manner as described above. 
     Further, all the intermediate layer thicknesses are each set to 10 μm or more to solve the second problem. 
     The foregoing description has been made based on the premise that the refractive indexes of the cover layer and the intermediate layers are equal to the standard value, and all the refractive indexes of the cover layer and the intermediate layers are equal to each other. In the following, described is a case that the refractive indexes of the cover layer and the intermediate layers are different from the standard value, or the refractive indexes of the cover layer and the intermediate layers are different from each other among the layers. 
     The back focus problem as the first problem occurs because the size and the shape are similar to each other between signal light, and reflected light from the other information recording surface on the photodetector  320 . In the case where the refractive index is set to about 1.60, it is possible to avoid the back focus problem, as far as a difference between the focus position of signal light, and the focus position of reflected light from the other information recording surface is smaller than 1 μm in the optical axis direction on the side of the optical recording medium. When the refractive index is set to about 1.60, crosstalk resulting from an adjacent information recording surface, as the second problem, occurs in the case where a defocus amount of signal light is smaller than 10 μm on an adjacent track. 
     In both of the cases, a defocus amount is an important factor to be considered. The defocus amount corresponds to the size of reflected light from the other information recording surface, or the size of a virtual image of reflected light from the other information recording surface at a position where signal light is focused. Let it be assumed that the radius of reflected light from the other information recording surface, or the radius of a virtual image of reflected light from the other information recording surface is RD. Since reflected light from the other information recording surface whose radius is RD is projected onto the photodetector  320 , coherence and the magnitude of crosstalk depend on the size of the reflected light. The size of the reflected light may be defined as a divergent amount of light resulting from an interlayer thickness. The inventors found that in order to avoid the back focus problem and the crosstalk problem in the case where the refractive index is set to a value other than 1.60, it is necessary to define a condition that makes a defocus amount i.e. the size of reflected light from the other information recording surface or the size of a virtual image of reflected light from the other information recording surface substantially equal to each other. The above technique may be defined as a technique of converting an interlayer thickness, referring to a divergent amount of light resulting from an interlayer thickness. 
     Since the size of the photodetector is fixed, as the radius of a light beam is increased, the density of light to be incident into the photodetector is decreased. As the density of light is decreased, crosstalk is decreased. Thus, the magnitude of crosstalk depends on the size of reflected light. 
     A condition that makes a defocus (the size of reflected light from the other information recording surface, or the size of a virtual image of reflected light from the other information recording surface) with respect to a layer having a refractive index “nr” different from a standard refractive index “no” and a shape-wise thickness “tr” equal to a defocus with respect to a layer having the standard refractive index “no” and a shape-wise thickness “to” is expressed by the following formulas (2) and (3).
 
 NA=nr ·sin(θ r )= no ·sin(θ o )  (2)
 
 RD=tr ·tan(θ r )= to ·tan(θ o )  (3)
 
     In the formulas, NA represents a numerical aperture of an objective lens  56  for converging light onto an optical recording medium. For instance, NA=0.85. The symbols θr and θo respectively represent convergence angles of light in materials having the refractive index “nr” and the refractive index “no”. The symbol RD represents a radius of reflected light from the other information recording surface, or a radius of a virtual image of reflected light from the other information recording surface. The symbols “sin” and “tan” respectively represent a sine function and a tangent function. The standard refractive index “no” is set to e.g. 1.60, and more preferably set to 1.57. 
     The convergence angle θr is expressed by the following formula (4), and the convergence angle θo is expressed by the following formula (5), based on the formula (2).
 
θ r =arcsin( NA/nr )  (4)
 
θ o =arcsin( NA/no )  (5)
 
     In the formulas, arcsin represents an inverse sine function. 
     The thickness “to” is expressed by the following formula (6), and the thickness “tr” is expressed by the following formula (7), based on the formula (3).
 
 to=tr ·tan(θ r )/tan(θ o )  (6)
 
 tr=to ·tan(θ o )/tan(θ r )  (7)
 
     The thickness “to” is calculated using the formula (6) to derive the thickness of a layer having the refractive index “no” with respect to the shape-wise thickness “tr” of a layer having the refractive index “nr”. 
     Conversely, the thickness “tr” is calculated using the formula (7) to derive the shape-wise thickness “tr” of a layer having the refractive index “nr” with respect to the thickness “to” of a layer having the refractive index “no”. 
     The factor portion in the formula (6) i.e. tan(θr)/tan(θo) is expressed as a function f(nr) of the refractive index “nr” in  FIG. 10 . The factor portion in the formula (7) i.e. tan(θo)/tan(θr) is an inverse number 1/f(nr) of the function f(nr). The factor portion tan(θo)/tan(θr) is expressed as the inverse number 1/f(nr) of the function f(nr) of the refractive index “nr” in  FIG. 11 . 
       FIG. 10  is an explanatory diagram showing a refractive index dependence of a factor for converting a shape-wise thickness in terms of an actual refractive index into a thickness in terms of a standard refractive index.  FIG. 11  is an explanatory diagram showing a refractive index dependence of a factor for converting a thickness in terms of a standard refractive index into a shape-wise thickness in terms of an actual refractive index. 
     Since both of the function f(nr) and the inverse number 1/f(nr) of the function f(nr) have a smooth curve, the function f(nr) and the inverse number 1/f(nr) of the function f(nr) can be expressed by polynomial expressions. The inventors found that it is possible to obtain an approximate polynomial expression with precision of about 0.1% by using a third expression. Specifically, the function f(nr) is expressed by a third expression as represented by the following formula (8), and the inverse number 1/f(nr) of the function f(nr) is expressed by a third expression as represented by the following formula (9).
 
 f ( n )=−1.088 n   3 +6.1027 n   2 −12.042 n +9.1007   (8)
 
1 /f ( n )=0.1045 n   3 −0.6096 n   2 +2.0192 n −1.0979  (9)
 
     To simplify the expressions, in the formula (8) and the formula (9), the refractive index “nr” is abbreviated as “n”. 
     As shown in  FIG. 10 , approximation of the function f(nr) expressed by six points normally corresponds to approximation of a fifth expression. However, as the order is increased, the function f(nr) may fluctuate or the calculation thereof may become complex. On the other hand, as the order is reduced, precision of the function f(nr) may be lowered. 
     The invention has been made based on a necessary and sufficient condition that thickness precision of a disc is about 0.1 μm. Accordingly, securing precision higher than the required value is meaningless. In view of this, the inventors derived the aforementioned formulas (8) and (9), based on a new finding that establishing a third expression is a necessary and sufficient condition to satisfy thickness precision of about 0.1 μm. 
     Specifically, assuming that shape-wise thicknesses of a cover layer and first through (N−1)-th intermediate layers of an optical recording medium having refractive indexes nr 1 , nr 2 , . . . , and nrN are respectively tr 1 , tr 2 , . . . , and trN in the order from the surface of the optical recording medium where light is incident, the thicknesses tr 1 , tr 2 , . . . , and trN are converted into thicknesses t 1 , t 2 , . . . , and tN of layers having a predetermined refractive index “no” which makes a divergent amount equal to a divergent amount of a light beam resulting from the thicknesses tr 1 , tr 2 , . . . , and trN. Further, a difference DFF between the sum of a thickness “ti” through a thickness “tj”, and the sum of a thickness “tk” through a thickness “tm” is set to 1 μm or more (where i, j, k, and m are each any positive integer satisfying i≦j&lt;k≦m≦N). Furthermore, the thicknesses t 1 , t 2 , . . . , and tN are calculated by products of the function f(n) expressed by the above-described formula (8), and the thicknesses tr 1 , tr 2 , . . . , and trN. In the formula (8), n=nr 1 , nr 2 , . . . , and nrN. 
     Further, assuming that shape-wise thicknesses of a cover layer and first through (N−1)-th intermediate layers of an optical recording medium having refractive indexes nr 1 , nr 2 , . . . , and nrN are respectively tr 1 , tr 2 , . . . , and trN in the order from the surface of the optical recording medium where light is incident, targeted values of the thicknesses tr 1 , tr 2 , . . . , and trN are calculated by converting thicknesses t 1 , t 2 , . . . , and tN of layers having a predetermined refractive index “no” into the thicknesses tr 1 , tr 2 , . . . , and trN which makes a divergent amount equal to a divergent amount of a light beam resulting from the thicknesses t 1 , t 2 , . . . , and tN. Further, a difference DFF between the sum of a thickness “ti” through a thickness “tj”, and the sum of a thickness “tk” through a thickness “tm” is set to 1 μm or more (where i, j, k, and m are each any integer satisfying i≦j&lt;k≦m≦n). Furthermore, the thicknesses t 1 , t 2 , . . . , and tN are calculated by products of the inverse number of the function f(n) expressed by the formula (9), and the thicknesses t 1 , t 2 , . . . , and tN. In the formula (9), n=nr 1  , nr 2 , . . . , and nrN. 
     As an example, there is described a relation between the thickness t 1  of the cover layer, and the sum of the thicknesses t 2  through t 4  of the first through the third intermediate layers of the four-layer disc (the optical recording medium  40 ). Let us consider a case that all the refractive indexes of the layers are set to a standard refractive index “no” i.e. set to 1.60, the thickness t 1  of the cover layer is set to 54 μm, the thickness t 2  of the first intermediate layer is set to 10 μm, the thickness t 3  of the second intermediate layer is set to 21 μm, and the thickness t 4  of the third intermediate layer is set to 19 μm. The sum of the thickness t 2  of the first intermediate layer through the thickness t 4  of the third intermediate layer becomes 50 μm. In this case, the difference between the thickness t 1  of the cover layer, and the sum of the thicknesses t 2  through t 4  of the first through the third intermediate layers is 4 μm, which is significantly larger than 1 μm. 
     If, however, the refractive index “nr” of the cover layer is set to 1.70, a different result is obtained, even if the shape-wise thickness tr 1  of the cover layer remains the same i.e. set to 54 μm. It is obvious, from the formulas (4) and (6) or from  FIG. 10 , that the thickness tr 1  of a layer having the refractive index “nr” is converted into the thickness t 1  of a layer having the standard refractive index “no” by multiplying the thickness tr 1  by 0.921. As a result, the thickness t 1  of the layer having the refractive index “no” is set to: t 1 =0.921×tr 1 =49.7 μm, which is smaller than 50 μm i.e. the sum of the thicknesses t 2  through t 4  of the first through the third intermediate layers. 
     Conversely, it is obvious from the formulas (5) and (7) or from  FIG. 11  that a difference between the thickness tr 1  of the cover layer, and the sum of the thicknesses t 2  through t 4  of the first through the third intermediate layers is set to 1 μm or more, and the thickness tr 1  of the cover layer is set to 51 μm or more by multiplying the thickness t 1  of a layer having the refractive index “no” by 1.086. In other words, the thickness tr 1  of the layer having the refractive index “nr” is set to: tr 1 =51×1.086≈55.4 μm. Accordingly, it is necessary to set the shape-wise thickness tr 1  of the cover layer to 55.4 μm or more, in the case where the refractive index “nr” is set to 1.70. The above example is merely an example, and the invention may embrace a value parameter other than the above. Further, in the case where the refractive index is a numerical value other than the ones shown in  FIG. 10  or  FIG. 11 , a factor may be calculated by substituting the refractive index in the formula (8) or the formula (9). 
     It is also necessary to satisfy a specific condition about the thickness of the cover layer and the thicknesses of the intermediate layers from another aspect. It is desirable to set the cover layer thickness and the intermediate layer thicknesses in a predetermined range including a standard value to perform a stable focus jumping operation. A focus jumping operation is an operation of changing a focus position from a certain information recording surface to another information recording surface. In performing a focus jumping operation, it is desirable to secure a focus error signal of good quality with respect to a targeted information recording surface by e.g. moving a collimator lens  53  prior to a focus jumping operation to stably obtain a focus error signal with respect to the targeted information recording surface. In view of this, it is desirable to set a difference in spherical aberration between information recording surfaces in a predetermined range. 
     If the refractive index is changed, the spherical aberration amount is changed, even if the thickness is unchanged. Accordingly, it is desirable to set a targeted value or an allowable range of an intermediate layer thickness in such a manner that the spherical aberration amount lies in a predetermined range. 
     Referring back to the description on the back focus problem, in the case where the refractive index of a predetermined layer (the cover layer or the intermediate layer) is nr(min)≦nr≦nr(max), the thickness “tr” of the layer having the refractive index “nr” can be obtained by implementing the formulas: θr(min)=arcsin(NA/nr(min)) and θr(max)=arcsin(NA/nr(max)), and using the formula: to=tr·tan(θr)/tan(θo) in the similar manner as described above. Thus, the thickness range of the intermediate layers may be determined. 
     The optical recording medium in the embodiment is not limited to one of a rewritable disc, a recordable disc, and a read only disc, but may be any of these discs. 
     As described above, signal fluctuation and signal quality deterioration resulting from the back focus problem occur, in the case where the sizes or the shapes are the same with each other between signal light, and reflected light from the other information recording surface on the photodetector. A state that the sizes or the shapes are the same with each other between signal light, and reflected light from the other information recording surface on the photodetector means a state that focus positions appear to be the same with each other between signal light, and reflected light from the other information recording surface, including a virtual image of reflected light from the other information recording surface. Optical paths of signal light and reflected light from the other information recording surface are partly different from each other in a transparent substrate of an optical disc. In the case where defocus amounts resulting from a difference in optical path are equal to each other, the focus position of signal light, and the focus position of reflected light from the other information recording surface appear to be the same with each other. In the case where divergences of convergent light i.e. the radii of convergent light are the same with each other between signal light, and reflected light from the other information recording surface, it is determined that defocus amounts resulting from a substrate thickness are equal to each other. 
     In view of the above, calculation based on the divergent radius R of a light spot resulting from a substrate thickness is made in order to determine whether the back focus problem can be avoided by setting the shape-wise thickness “tr” in terms of the refractive index “nr”. In this example, the shape-wise thickness indicates a material thickness, and may also be called as a physical thickness. 
     An interlayer coherence resulting from a reduced intermediate layer thickness can be avoided, if the spot configuration (the radius R) on an adjacent layer is sufficiently large. In view of this, calculation based on the divergent radius R of a light spot resulting from a substrate thickness is made in order to determine whether the interlayer coherence can be avoided by setting the shape-wise thickness “tr” in terms of the refractive index “nr”. 
     Assuming that the thickness of a cover layer or an intermediate layer is “t”, the numerical aperture of a light spot is NA (NA=0.85), and the convergence angle of light in a substrate is θ, since NA=n·sin(θ), θ=arcsin(NA/n). In this formula, “arcsin” represents an inverse sine function. The divergent radius R of a light spot can be calculated by R=t·tan(θ). 
     The standard refractive index is defined as “no”, the thickness of a layer having the standard refractive index “no” is defined as “to”, and the convergence angle of light in a substrate of the layer is defined as “θo”. The standard refractive index “no” is set to e.g. 1.60. The layer (targeted layer) constituting a thickness portion of a transparent substrate of an actual optical disc is indicated with the suffix “r”, the refractive index of the targeted layer is defined as “nr”, the shape-wise thickness of the targeted layer is defined as “tr”, and the convergence angle of light in a substrate is defined as “θr”. In this case, the convergence angles θo and θr are respectively expressed by: θo=arcsin(NA/no) and θr=arcsin(NA/nr). 
     The divergent radius R of a light spot is expressed by: R=tr·tan(θr)=to·tan(θo). Accordingly, the thickness “to” of a layer having the standard refractive index “no” is expressed by: to=tr·tan(θr)/tan(θo))=tr·f(nr). 
     The function f(nr) is a factor for deriving the thickness “to” of a layer having the standard refractive index “no” with respect to the shape-wise thickness “tr”, and is the function shown in the graph of  FIG. 10 . 
     For instance, let us consider a four-layer disc having four layers of information recording surfaces. The four-layer disc (the optical recording medium  40 ) has, in the order from the surface (a light incident surface)  40   z  of the disc, the first information recording surface  40   a , the second information recording surface  40   b , the third information recording surface  40   c , and the fourth information recording surface  40   d . The four-layer disc is further provided with the cover layer  42  between the light incident surface  40   z  and the first information recording surface  40   a , the first intermediate layer  43  between the first information recording surface  40   a  and the second information recording surface  40   b , the second intermediate layer  44  between the second information recording surface  40   b  and the third information recording surface  40   c , and the third intermediate layer  45  between the third information recording surface  40   c  and the fourth information recording surface  40   d.    
     Let it be assumed that the shape-wise thickness of the cover layer  42  is tr 1 , and the actual refractive index of the cover layer  42  is nr 1 ; the shape-wise thickness of the first intermediate layer  43  is tr 2 , and the actual refractive index of the first intermediate layer  43  is nr 2 ; the shape-wise thickness of the second intermediate layer  44  is tr 3 , and the actual refractive index of the second intermediate layer  44  is nr 3 ; and the shape-wise thickness of the third intermediate layer  45  is tr 4 , and the actual refractive index of the third intermediate layer  45  is nr 4 . 
     Converting the thicknesses tr 1 , tr 2 , tr 3 , and tr 4  of the cover layer  42  and the first through the third intermediate layers  43  through  45  respectively into the thicknesses t 1 , t 2 , t 3 , and t 4  of the cover layer  42  and the first through the third intermediate layers  43  through  45  each having the standard refractive index “no”, based on a defocus amount, yields a result: t 1 =tr 1 ×f(nr 1 ), t 2 =tr 2 ×f(nr 2 ), t 3 =tr 3 ×f(nr 3 ), and t 4 =tr 4 ×f(nr 4 ). 
     Normally, the thickness of the cover layer is larger than the thickness of the intermediate layer. In view of this, the four-layer disc should satisfy all the conditions: |t 1 −(t 2 +t 3 +t 4 )|≧1 μm, |t 2 −t 3 |1 μm, |t 3 −t 4 )|≧1 μm, and |t 2 −t 41 |≧1 μm to avoid the back focus problem. 
     Further, the four-layer disc should satisfy all the conditions: t 2 ≧10 μm, t 3 ≧10 μm, and t 4 ≧10 μm to avoid the interlayer coherence. In other words, the thicknesses t 1 , t 2 , t 3 , and t 4  of the cover layer  42 , the first intermediate layer  43 , the second intermediate layer  44 , and the third intermediate layer  45  are each set to 10 μm or more. 
     As described above, the optical recording medium  40  includes the first information recording surface  40   a  closest to the light incident surface  40   z  of the optical recording medium  40 , the second information recording surface  40   b  second closest to the surface  40   z , the third information recording surface  40   c  third closest to the surface  40   z , the fourth information recording surface  40   d  fourth closest to the surface  40   z , the cover layer  42  having a refractive index nr 1  different from the predetermined refractive index “no” and formed between the surface  40   z  and the first information recording surface  40   a , the first intermediate layer  43  having a refractive index nr 2  different from the refractive index “no” and formed between the first information recording surface  40   a  and the second information recording surface  40   b , the second intermediate layer  44  having a refractive index nr 3  different from the refractive index “no” and formed between the second information recording surface  40   b  and the third information recording surface  40   c , and the third intermediate layer  45  having a refractive index nr 4  different from the refractive index “no” and formed between the third information recording surface  40   c  and the fourth information recording surface  40   d.    
     Further, the shape-wise thicknesses tr 1 , tr 2 , tr 3 , and tr 4  of the cover layer  42 , the first intermediate layer  43 , the second intermediate layer  44 , and the third intermediate layer  45  are respectively converted into the thicknesses t 1 , t 2 , t 3 , and t 4  of the respective layers having the predetermined refractive index “no”. 
     Furthermore, a defocus amount generated in a layer having a refractive index nrα and a thickness trα (satisfying: 1≦α≦4 (where α is a positive integer)) is equal to a defocus amount generated in a layer having the refractive index “no” and a thickness tα (satisfying: 1≦α≦4 (where α is a positive integer)). 
     Furthermore, the thicknesses t 1 , t 2 , t 3 , and t 4  satisfy |t 1 −(t 2 +t 3 +t 4 )|≧1 μm, a difference between any two values of the thicknesses t 1 , t 2 , t 3 , and t 4  is set to 1 μm or more in any case, and |(t 1 +t 2 )−(t 3 +t 4 )|≧1 μm. 
     Thus, the thicknesses t 1 , t 2 , t 3 , and t 4  obtained by converting the shape-wise thickness tr 1 , tr 2 , tr 3 , and tr 4  of the cover layer  42 , the first intermediate layer  43 , the second intermediate layer  44 , and the third intermediate layer  45  satisfy |t 1 −(t 2 +t 3 +t 4 )|≧1 μm, a difference between any two values of the thicknesses t 1 , t 2 , t 3 , and t 4  is set to 1 μm or more in any case, and |(t 1 +t 2 )−(t 3 +t 4 )|≧1 μm. This enables to prevent light from forming an image on the backside of the surface of the optical recording medium, and suppress coherence between reflected light from the information recording surfaces to thereby improve the quality of a servo signal and a reproduction signal. 
     Further, since the distance between the surface of the optical recording medium, and the information recording surface closest to the surface of the optical recording medium can be set to a large value, deterioration of a reproduction signal in the case where there is a damage or a smear on the surface of the optical recording medium can be suppressed. 
     Further, in the case where the thickness of a layer having the refractive index nrα is set to trα (satisfying: 1≦α≦4 (where α is a positive integer)), the convergence angle of light in the layer having the refractive index nrα is set to θrα (satisfying: 1≦α≦4 (where α is a positive integer)), the thickness of a layer having the refractive index “no” is set to tα (satisfying: 1≦α≦4 (where α is a positive integer)), and the convergence angle of light in the layer having the refractive index “no” is set to θo, the thickness trα is converted into the thickness tα based on the following formula (10).
 
 tα=tr α·(tan(θrα)/tan(θo))  (10)
 
     Preferably, the range of the thickness tα of a layer having the refractive index “no” and whose spherical aberration amount lies in a predetermined allowable range may be converted into a range of the thickness trα of a layer having the refractive index nrα, and the thickness trαmay be included in the range of the thickness trα after conversion. 
     Generally, it is necessary to set the performance of a light spot in the range of the Marechal Criteria. If the performance of a light spot exceeds the range of the Marechal Criteria, a signal may be extremely deteriorated. In view of this, the ranges of the respective conditions are defined in such a manner that a spherical aberration amount generated in a layer having the refractive index “no” lies in a range of 70 mλ or less, which is the range of the Marechal Criteria. 
     In this embodiment, the refractive indexes nr 1 , nr 2 , nr 3 , and nr 4  are each different from the refractive index “no”. The invention is not specifically limited to the above. Alternatively, the refractive indexes nr 1 , nr 2 , nr 3 , and nr 4  may each be equal to the refractive index “no”. The modification is advantageous in that the manufacturing method for the optical recording medium can be standardized without depending on the value of the refractive index. 
     As another example, let us consider a case that a three-layer disc having three recording layers is produced. A three-layer disc (the optical recording medium  30 ) has, in the order from the surface (a light incident surface)  30   z  of the disc, the first information recording surface  30   a , the second information recording surface  30   b , and the third information recording surface  30   c . The three-layer disc is further provided with the cover layer  32  between the light incident surface  30   z  and the first information recording surface  30   a , the first intermediate layer  33  between the first information recording surface  30   a  and the second information recording surface  30   b , and the second intermediate layer  34  between the second information recording surface  30   b  and the third information recording surface  30   c.    
     Let it be assumed that the shape-wise thickness of the cover layer  32  is tr 1 , and the actual refractive index of the cover layer  32  is nr 1 ; the shape-wise thickness of the first intermediate layer  33  is tr 2 , and the actual refractive index of the first intermediate layer  33  is nr 2 ; and the shape-wise thickness of the second intermediate layer  34  is tr 3 , and the actual refractive index of the second intermediate layer  34  is nr 3 . 
     Converting the thicknesses tr 1 , tr 2 , and tr 3  of the cover layer  32 , the first intermediate layer  33 , and the second intermediate layer  34  respectively into the thicknesses t 1 , t 2 , and t 3  of the cover layer  32 , the first intermediate layer  33 , and the second intermediate layer  34  each having the standard refractive index “no”, based on a defocus amount, yields a result: t 1 =tr 1 ×f(nr 1 ), t 2 =tr 2 ×f(nr 2 ), and t 3 =tr 3 ×f(nr 3 ). 
     Normally, the thickness of the cover layer is larger than the thickness of the intermediate layer. In view of this, the three-layer disc should satisfy all the conditions: |t 1 −(t 2 +t 3 )|≧1 μm, and |t 2 −t 3 |1μm to avoid the back focus problem. 
     Further, the three-layer disc should satisfy all the conditions: t 2 ≧10 μm, and t 3 ≧10 μm to avoid the interlayer coherence. In other words, the thicknesses t 1 , t 2 , and t 3  of the cover layer  32 , the first intermediate layer  33 , and the second intermediate layer  34  are each set to 10 μm or more. 
     As described above, the optical recording medium  30  includes the first information recording surface  30   a  closest to the light incident surface  30   z  of the optical recording medium  30 , the second information recording surface  30   b  second closest to the surface  30   z , the third information recording surface  30   c  third closest to the surface  30   z , the cover layer  32  having a refractive index nr 1  different from the predetermined refractive index “no” and formed between the surface  30   z  and the first information recording surface  30   a , the first intermediate layer  33  having a refractive index nr 2  different from the refractive index “no” and formed between the first information recording surface  30   a  and the second information recording surface  30   b , and the second intermediate layer  34  having a refractive index nr 3  different from the refractive index “no” and formed between the second information recording surface  30   b  and the third information recording surface  30   c.    
     Further, the shape-wise thicknesses tr 1 , tr 2 , and tr 3  of the cover layer  32 , the first intermediate layer  33 , and the second intermediate layer  34  are respectively converted into the thicknesses t 1 , t 2 , and t 3  of the respective layers having the predetermined refractive index “no”. 
     Furthermore, a defocus amount generated in a layer having the refractive index nrα and the thickness trα (satisfying: 1≦α≦3 (where α is a positive integer)) is equal to a defocus amount generated in a layer having the refractive index “no” and the thickness tα (satisfying: 1≦α≦3 (where α is a positive integer)). 
     Furthermore, the thicknesses t 1 , t 2 , and t 3  satisfy |t 1 −(t 2 +t 3 )|≧1 μm, and a difference between any two values of the thicknesses t 1 , t 2 , and t 3  is set to 1 μm or more in any case. 
     Thus, the thicknesses t 1 , t 2 , and t 3  obtained by converting the shape-wise thickness tr 1 , tr 2 , and tr 3  of the cover layer  32 , the first intermediate layer  33 , and the second intermediate layer  34  satisfy |t 1 −(t 2 +t 3 )|≧1 μm, and a difference between any two values of the thicknesses t 1 , t 2 , and t 3  is set to 1 μm or more in any case. This enables to prevent light from forming an image on the backside of the surface of the optical recording medium, and suppress coherence between reflected light from the information recording surfaces to thereby improve the quality of a servo signal and a reproduction signal. 
     Further, since the distance between the surface of the optical recording medium and the information recording surface closest to the surface of the optical recording medium can be set to a large value, deterioration of a reproduction signal in the case where there is a damage or a smear on the surface of the optical recording medium can be suppressed. 
     Further, in the case where the thickness of a layer having the refractive index nrα is set to trα (satisfying: 1≦α≦3 (where α is a positive integer)), the convergence angle of light in the layer having the refractive index nrα is set to θrα (satisfying: 1≦α≦3 (where a is a positive integer)); the thickness of a layer having the refractive index “no” is set to tα (satisfying: 1≦α≦3 (where α is a positive integer)), and the convergence angle of light in the layer having the refractive index “no” is set to θo, the thickness trα is converted into the thickness tα based on the following formula (11).
 
 tα=tr α(tan(θ r α)/tan(θ o ))  (11)
 
     In the three-layer disc as well as the four-layer disc, preferably, the range of the thickness tα of a layer having the refractive index “no” and whose spherical aberration amount lies in a predetermined allowable range may be converted into a range of the thickness trα of a layer having the refractive index nrα , and the thickness trα may be included in the range of the thickness trα after conversion. 
     In the case where the layer between the medium surface and the information recording surface or each layer between the information recording surfaces is constituted of plural material layers having refractive indexes different from each other, at first, the thicknesses of the material layers are calculated in terms of the standard refractive index. Specifically, the actual thickness of each material layer having the refractive index “nr” is converted into the thickness of each material layer having the standard refractive index “no”, based on a defocus amount, by multiplying the shape-wise thickness by the function value “f”. Then, the thicknesses of the material layers after conversion are integrated. 
     For instance, in the case where a cover layer having the shape-wise thickness tr 1  is constituted of a first cover layer having the thickness tr 11  and the refractive index nr 11 , a second cover layer having the thickness tr 12  and the refractive index nr 12  . . . , and the N-th cover layer having the thickness tr 1 N and the refractive index nr 1 N, converting the shape-wise thickness of the cover layer into the thickness t 1  of the cover layer having the standard refractive index “no”, based on a defocus amount, yields a result: t 1 =Σtr 1 k×f(nrk). In this formula, Σ represents an integration from 1 through N with respect to “k”. 
     In the case where an objective lens having a large numerical aperture (NA) is used, spherical aberration sharply changes depending on the thickness of a transparent substrate through which light is transmitted. If the spherical aberration is large, the sensitivity of a focus error signal, serving as an index to be used in focus control, may be different from the design sensitivity, or focus error signal deterioration such as a decrease in signal amplitude may occur. 
     Accordingly, in the case where focus control is started from a state that focus control is not performed, or stability in focus jumping is obtained, it is desirable to correct spherical aberration with respect to a targeted layer for focus control in advance. In view of this, it is desirable to set the thickness from the medium surface to an information recording layer, and the thickness of an intermediate layer in a predetermined range including a standard value. 
     The focus jumping operation is an operation of changing a focus position from a certain information recording surface to another information recording surface. The standard value or a predetermined range for a focus jumping operation should be defined, referring to the spherical aberration for the above reason. Accordingly, in the case where the refractive index is set to a value other than the standard value, the shape-wise thickness is changed depending on the refractive index. 
     In view of the above, for instance, the layer thickness of a multilayer optical disc is designed as follows. First, the refractive index of a material constituting a transparent substrate is defined. Next, the shape-wise thickness from the medium surface to an information recording surface, and the shape-wise thicknesses of intermediate layers are determined in accordance with the obtained refractive index, referring to the spherical aberration. Since it is impossible to set a production error to zero, the shape-wise thickness is determined including an error range. The shape-wise thickness from the medium surface to an information recording surface, and the shape-wise thicknesses of intermediate layers may be determined, using a numerical value table or a chart. The spherical aberration is proportional to the layer thickness. Accordingly, the shape-wise thickness from the medium surface to an information recording surface, and the shape-wise thicknesses of intermediate layers may be determined by calculating a conversion factor g(nr) depending on a refractive index in accordance with a wavelength or a numerical aperture, and using the calculated conversion factor g(nr). 
     For instance, blue light of a wavelength 405 nm is converged on an information recording surface through a substrate having a refractive index of 1.60 and a thickness of 0.1 mm. An objective lens having a numerical aperture of 0.85 converges blue light of a wavelength 405 nm without aberration. The thickness ts(nr) (unit: mm) of a substrate which minimizes the aberration when the refractive index of the substrate is changed is calculated. As a result of the calculation, the conversion factor g(nr) is set to: g(nr)=ts(nr)/0.1. 
       FIG. 12  is an explanatory diagram showing a refractive index dependence of a factor for converting a shape-wise thickness in terms of an actual refractive index into a thickness in terms of a standard refractive index, based on a spherical aberration.  FIG. 12  shows a conversion factor g(nr) derived by the inventors. Since both of the conversion factor g(nr) and the inverse number (1/g(nr)) of the conversion factor g(nr) have a smooth curve, the conversion factor g(nr) and the inverse number (1/g(nr)) of the conversion factor g(nr) can be expressed by polynomial expressions. The inventors found that it is possible to obtain an approximate polynomial expression with precision of about 0.1% by using a third expression. Specifically, the function g(nr) is expressed by a third expression as represented by the following formula (12).
 
 g ( n )=−1.1111 n   3 +5.8143 n   2 −9.8808 n +6.476   (12)
 
     To simplify the expressions, in the formula (12), the refractive index “nr” is abbreviated as “n”. 
     A proper relation between a substrate thickness and a refractive index is also disclosed in JP 2004-288371A and JP 2004-259439A. However, the relation between a substrate thickness and a refractive index disclosed in JP 2004-288371A and JP 2004-259439A is different from the formula (12). Accordingly, the relation between a substrate thickness and a refractive index disclosed in JP 2004-288371A and JP 2004-259439A does not accurately express the relation between a substrate thickness and a refractive index, which makes a spherical aberration constant, as shown in  FIG. 12 . In this embodiment, a substrate thickness which makes a third-order spherical aberration constant is obtained depending on a refractive index by actually tracing a light ray, without performing an approximation. Thus, in this embodiment, the accurate relation between a substrate thickness and a refractive index is successfully defined. 
     The thicknesses of the cover layer and the first through the (N−1)-th intermediate layers are set in such a range that spherical aberration lies in a predetermined range. Targeted values of the thicknesses tr 1 , tr 2 , . . . , and trN are calculated by products of the thicknesses t 1 , t 2 , . . . , and tN, and the function g(n) expressed by the above-described formula (12) to set the thicknesses of the cover layer and the first through the (N−1)-th intermediate layers in such a range that spherical aberration lies in a predetermined range. In the formula (12), n=nr 1 , nr 2 , . . . , and nrN. 
     The shape-wise thickness of a cover layer can be obtained, based on the shape-wise thickness from the medium surface to an information recording surface, and the shape-wise thicknesses of intermediate layers, which have been calculated in the above-described manner. Then, these thicknesses are converted into thicknesses of the respective corresponding layers each having the standard refractive index “no”, referring to a defocus amount in the above-described manner. Alternatively, the shape-wise thicknesses of the cover layer and the intermediate layers of an actually fabricated optical disc may be obtained. Then, determination is made as to whether the back focus problem and the interlayer coherence as described above can be avoided, whether the design range is proper, and whether the quality of the fabricated optical disc has passed, using the thicknesses of the respective corresponding layers after conversion. 
     The thickness from the medium surface to an information recording surface can be calculated based on the sum of the cover layer thickness and the intermediate layer thicknesses. In the case of a three-layer disc, the shape-wise thickness from the medium surface to the first information recording surface is set to tr 1 , the shape-wise thickness from the medium surface to the second information recording surface is set to (tr 1 +tr 2 ), and the shape-wise thickness from the medium surface to the third information recording surface is set to (tr 1  +tr 2 +tr 3 ). 
     In the case of a four-layer disc, the shape-wise thickness from the medium surface to the first information recording surface is set to tr 1 , the shape-wise thickness from the medium surface to the second information recording surface is set to (tr 1 +tr 2 ), the shape-wise thickness from the medium surface to the third information recording surface is set to (tr 1 +tr 2 +tr 3 ), and the shape-wise thickness from the medium surface to the fourth information recording surface is set to (tr 1 +tr 2 +tr 3 +tr 4 ). 
     The optical recording medium in the embodiment enables to prevent light from forming an image on the backside of the surface of the optical recording medium, and suppress coherence between reflected light from the information recording surfaces to thereby improve the quality of a servo signal and a reproduction signal. Further, in the above arrangement, a guideline for producing the products can be clearly set by setting the guideline for designing the thickness of the optical recording medium depending on the refractive index in the above-described manner. 
     As described above, the shape-wise thicknesses tr 1 , tr 2 , tr 3 , and tr 4  of the cover layer  42 , the first intermediate layer  43 , the second intermediate layer  44 , and the third intermediate layer  45  of the optical recording medium  40  having four information recording surfaces are determined depending on the refractive indexes nr 1 , nr 2 , nr 3 , and nr 4 , referring to a spherical aberration. Then, the thicknesses tr 1 , tr 2 , tr 3 , and tr 4  are respectively converted into the thicknesses t 1 , t 2 , t 3 , and t 4  of the respective layers having the predetermined refractive index “no”, referring to a defocus amount. Then, the thicknesses tr 1 , tr 2 , tr 3 , and tr 4  are calculated by products of the thicknesses t 1 , t 2 , t 3 , and t 4 , and the function g(n) expressed by the above-described formula (12) to set the thicknesses t 1 , t 2 , t 3 , and t 4  in such a range that the spherical aberration lies in a predetermined range. Thereafter, the thicknesses t 1 , t 2 , t 3 , and t 4  are calculated by products of the function f(n) expressed by the above-described formula (8), and the calculated thicknesses tr 1 , tr 2 , tr 3 , and tr 4 . Further, the re-calculated thicknesses t 1 , t 2 , t 3 , and t 4  satisfy the following formula (13).
 
|( t 1 +t 2)−( t 3 +t 4)|1 μm  (13)
 
     Further, the shape-wise thicknesses tr 1 , tr 2 , and tr 3  of the cover layer  32 , the first intermediate layer  33 , and the second intermediate layer  34  of the optical recording medium  30  having three information recording surfaces are determined depending on the refractive indexes nr 1 , nr 2 , and nr 3 , referring to a spherical aberration. Then, the thicknesses tr 1 , tr 2 , and tr 3  are respectively converted into the thicknesses t 1 , t 2 , and t 3  of the respective layers having the predetermined refractive index “no”, referring to a defocus amount. Then, the thicknesses tr 1 , tr 2 , and tr 3  are calculated by products of the thicknesses t 1 , t 2 , and t 3 , and the function g(n) expressed by the above-described formula (12) to set the thicknesses t 1 , t 2 , and t 3  in such a range that the spherical aberration lies in a predetermined range. Thereafter, the thicknesses t 1 , t 2 , and t 3  are calculated by products of the function f(n) expressed by the above-described formula (8), and the calculated thicknesses tr 1 , tr 2 , and tr 3 . Further, the re-calculated thicknesses t 1 , t 2 , and t 3  satisfy the following formula (14).
 
| t 1−( t 2 +t 3)|1 μm  (14)
 
     Next, an example of an optical information device which performs a focus jumping operation is described. 
       FIG. 13  is a diagram showing a schematic arrangement of an optical information device embodying the invention. The optical information device  150  reproduces or records information with respect to plural information recording surfaces by moving a light spot of laser light to be irradiated onto the optical recording medium  40  from a predetermined information recording surface to another information recording surface of the optical recording medium  40 . 
     The optical information device  150  converges a light spot of laser light onto a predetermined information recording surface of the plural information recording surfaces to reproduce information from the predetermined information recording surface. In the case where information is reproduced from another information recording surface of the plural information recording surfaces different from the predetermined information recording surface, the optical information device  150  shifts the light spot of laser light from the predetermined information recording surface to the another information recording surface to reproduce the information from the another information recording surface. 
     The optical information device  150  includes a driving device  151 , a turntable  152 , an electric circuit  153 , a clamper  154 , a motor  155 , and an optical head device  201 . The optical head device  201  in  FIG. 13  has the same arrangement as the arrangement of the optical head device  201  shown in  FIG. 1 , and an optical recording medium  40  in  FIG. 13  has the same arrangement as the arrangement of the optical recording medium  40  shown in  FIG. 2 . 
     The optical recording medium  40  is placed on the turntable  152 , and is fixedly supported by the clamper  154 . The motor  155  rotates the turntable  152  to thereby rotate the optical recording medium  40 . The driving device  151  coarsely drives the optical head device  201  to a track on the optical recording medium  40  where intended information is recorded. 
     The optical head device  201  shifts the focus position of laser light to be irradiated onto the optical recording medium from a certain information recording surface to another information recording surface to reproduce or record information with respect to the plural information recording surfaces. 
     The optical head device  201  transmits a focus error signal and a tracking error signal to the electric circuit  153  in correspondence to a positional relation with respect to the optical recording medium  40 . The electric circuit  153  transmits a signal for finely moving the objective lens  56  to the optical head device  201  in accordance with the focus error signal and the tracking error signal. The optical head device  201  performs focus control and tracking control with respect to the optical recording medium  40 , based on a signal from the electric circuit  153 . The optical head device  201  reads out information from the optical recording medium  40 , writes (records) information into the optical recording medium  40 , or erases information from the optical recording medium  40 . 
     The electric circuit  153  controls and drives the motor  155  and the optical head device  201 , based on a signal to be obtained from the optical head device  201 . The electric circuit  153  mainly controls the focus jumping sequence. Specifically, the electric circuit  153  controls the optical head device  201  in such a manner as to correct spherical aberration generated in an information recording surface as a focus jumping destination, before shifting the focus position. A concrete spherical aberration correction method for the optical head device  201  has been described in the foregoing description. 
     The optical information device  150  in the embodiment is operable to correct spherical aberration generated in an information recording surface as a focus jumping destination by shifting the collimator lens  53  with respect to the optical recording medium  40  before a focus jumping operation is performed, and thereafter shift the focus position. This enables to improve the quality of a focus error signal with respect to a targeted information recording surface to thereby stably perform a focus jumping operation. 
     The aforementioned embodiment mainly includes the features having the following arrangements. 
     A manufacturing method for an optical recording medium according to an aspect of the invention is a manufacturing method for an optical recording medium having (N−1) (where N is a positive integer of 4 or more) information recording surfaces, wherein, assuming that shape-wise thicknesses of a cover layer and first through (N−1)-th intermediate layers of the optical recording medium having refractive indexes nr 1  , nr 2 , . . . , and nrN are respectively tr 1  , tr 2 , . . . , and trN in the order from a surface of the optical recording medium where light is incident, the thicknesses tr 1 , tr 2 , . . . , and trN are converted into thicknesses t 1 , t 2 , . . . , and tN of layers having a predetermined refractive index “no” which makes a divergent amount equal to a divergent amount of a light beam resulting from the thicknesses tr 1 , tr 2 , . . . , and trN; a difference DFF between the sum of thicknesses ti through tj, and the sum of thicknesses tk through tm is 1 μm or more (where i, j, k, and m are each any positive integer satisfying i≦j&lt;k≦m≦N); and the thicknesses t 1 , t 2 , . . . , and tN are calculated by products of a function f(n) expressed by the following formula (15), and the thicknesses tr 1 , tr 2 , . . . , and trN:
 
 f ( n )=−1.088 n   3 +6.1027 n   2 −12.042 n +9.1007   (15)
 
     in the formula (15), n=nr 1 , nr 2 , . . . , and nrN. 
     In the above arrangement, assuming that shape-wise thicknesses of a cover layer and first through (N−1)-th intermediate layers of the optical recording medium having refractive indexes nr 1 , nr 2 , . . . , and nrN are respectively tr 1 , tr 2 , . . . , and trN in the order from a surface of the optical recording medium where light is incident, the thicknesses tr 1 , tr 2 , . . . , and trN are converted into thicknesses t 1 , t 2 , . . . , and tN of layers having a predetermined refractive index “no” which makes a divergent amount equal to a divergent amount of a light beam resulting from the thicknesses tr 1 , tr 2 , . . . , and trN. Further, a difference DFF between the sum of a thickness “ti” through a thickness “tj”, and the sum of a thickness “tk” through a thickness “tm” is set to 1 μm or more (where i, j, k, and m are each any positive integer satisfying i≦j&lt;k≦m≦N). Furthermore, the thicknesses t 1 , t 2 , . . . , and tN are calculated by products of the function f(n) expressed by the above-described formula (15), and the thicknesses tr 1 , tr 2 , . . . , and trN. 
     As described above, since the difference DFF between the sum of the thickness “ti” through the thickness “tj”, and the sum of the thickness “tk” through the thickness “tm” is set to 1 μm or more, it is possible to prevent light from forming an image on the backside of the surface of the optical recording medium, and suppress coherence between reflected light from the information recording surfaces to thereby improve the quality of a servo signal and a reproduction signal. Further, since the distance between the surface of the optical recording medium and the information recording surface closest to the surface of the optical recording medium can be set to a large value, deterioration of a reproduction signal in the case where there is a damage or a smear on the surface of the optical recording medium can be suppressed. 
     A manufacturing method for an optical recording medium according to another aspect of the invention is a manufacturing method for an optical recording medium having (N−1) (where N is a positive integer of 4 or more) information recording surfaces, wherein, assuming that shape-wise thicknesses of a cover layer and first through (N−1)-th intermediate layers of the optical recording medium having refractive indexes nr 1 , nr 2 , . . . , and nrN are respectively tr 1 , tr 2 , . . . , and trN in the order from a surface of the optical recording medium where light is incident, targeted values of the thicknesses tr 1 , tr 2 , . . . , and trN are calculated by converting thicknesses t 1 , t 2 , . . . , and tN of layers having a predetermined refractive index “no” into the thicknesses tr 1 , tr 2 , . . . , and trN which makes a divergent amount equal to a divergent amount of a light beam resulting from the thicknesses t 1 , t 2 , . . . , and tN; a difference DFF between the sum of a thickness “ti” through a thickness “tj”, and the sum of a thickness “tk” through a thickness “tm” is set to 1 μm or more (where i, j, k, and m are each any positive integer satisfying i≦j&lt;k≦m≦N); and the thicknesses t 1 , t 2 , . . . , and tN are calculated by products of an inverse number 1/f(n) of a function f(n) expressed by the following formula (16), and the thicknesses t 1 , t 2 , . . . , and tN:
 
1 /f ( n )=0.1045 n   3 −0.6096 n   2 +2.0192 n −1.0979  (16)
 
     in the formula (16), n=nr 1 , nr 2 , . . . , and nrN. 
     In the above arrangement, assuming that shape-wise thicknesses of a cover layer and first through (N−1)-th intermediate layers of the optical recording medium having refractive indexes nr 1 , nr 2 , . . . , and nrN are respectively tr 1 , tr 2 , . . . , and trN in the order from a surface of the optical recording medium where light is incident, targeted values of the thicknesses tr 1 , tr 2 , . . . , and trN are calculated by converting thicknesses t 1 , t 2 , . . . , and tN of layers having a predetermined refractive index “no” into the thicknesses tr 1 , tr 2 , . . . , and trN which makes a divergent amount equal to a divergent amount of a light beam resulting from the thicknesses t 1 , t 2 , . . . , and tN. Further, a difference DFF between the sum of a thickness “ti” through a thickness “tj”, and the sum of a thickness “tk” through a thickness “tm” is set to 1 μm or more (where i, j, k, and m are each any positive integer satisfying i≦j&lt;k≦m≦N). Furthermore, the thicknesses t 1 , t 2 , . . . , and tN are calculated by products of an inverse number 1/f(n) of the function f(n) expressed by the above-described formula (16), and the thicknesses t 1 , t 2 , . . . , and tN. 
     As described above, since the difference DFF between the sum of the thickness “ti” through the thickness “tj”, and the sum of the thickness “tk” through the thickness “tm” is set to 1 μm or more, it is possible to prevent light from forming an image on the backside of the surface of the optical recording medium, and suppress coherence between reflected light from the information recording surfaces to thereby improve the quality of a servo signal and a reproduction signal. Further, since the distance between the surface of the optical recording medium and the information recording surface closest to the surface of the optical recording medium can be set to a large value, deterioration of a reproduction signal in the case where there is a damage or a smear on the surface of the optical recording medium can be suppressed. 
     In the manufacturing method for an optical recording medium, preferably, thicknesses of the cover layer and the first through the (N−1)-th intermediate layers may be set in such a range that a spherical aberration lies in a predetermined range. 
     In the above arrangement, since the thicknesses of the cover layer and the first through the (N−1)-th intermediate layers are set in such a range that a spherical aberration lies in a predetermined range, it is possible to suppress the spherical aberration in the cover layer and the first through the (N−1)-th intermediate layers having the thicknesses tr 1  , tr 2 , . . . , and trN. 
     In the manufacturing method for an optical recording medium, preferably, targeted values of the thicknesses tr 1  , tr 2 , . . . , and trN may be calculated by products of the thicknesses t 1  , t 2 , . . . , and tN, and a function g(n) expressed by the following formula (17) to set thicknesses of the cover layer and the first through the (N−1)-th intermediate layers in such a range that a spherical aberration lies in a predetermined range:
 
 g ( n )=−1.1111 n   3 +5.8143 n   2 −9.8808 n +6.476   (17)
 
     in the formula (17), n=nr 1  , nr 2 , . . . , and nrN. 
     In the above arrangement, targeted values of the thicknesses tr 1 , tr 2 , . . . , and trN are calculated by products of the thicknesses t 1  , t 2 , . . . , and tN, and the function g(n) expressed by the above-described formula (17) to set thicknesses of the cover layer and the first through the (N−1)-th intermediate layers in such a range that a spherical aberration lies in a predetermined range. 
     The above arrangement enables to easily calculate the targeted values of the thicknesses tr 1  , tr 2 , . . . , and trN capable of suppressing the spherical aberration. 
     In the manufacturing method for an optical recording medium, preferably, the refractive index “no” may be set to 1.60. 
     In the above arrangement, it is possible to convert the shape-wise thicknesses of the cover layer and the first through the (N−1)-th intermediate layers into the thicknesses t 1  , t 2 , . . . , and tN of the respective layers having a refractive index of 1.60. 
     In the manufacturing method for an optical recording medium, preferably, the thicknesses t 1 , t 2 , . . . , and tN may each be set to 10 μm or more. 
     In the above arrangement, it is possible to reduce an influence of crosstalk from an information recording surface adjacent to the targeted information recording surface by setting each of the thicknesses t 1  , t 2 , . . . , and tN to 10 μm or more to thereby reduce coherence between reflected light from the information recording surfaces. 
     An optical recording medium according to another aspect of the invention is an optical recording medium having (N−1) (where N is a positive integer of 4 or more) information recording surfaces. The optical recording medium includes: a cover layer formed between a surface of the optical recording medium where light is incident, and the first information recording surface closest to the medium surface; and first through (N−1)-th intermediate layers formed between the respective first through N-th information recording surfaces, wherein, assuming that shape-wise thicknesses of the cover layer and the first through the (N−1)-th intermediate layers of the optical recording medium having refractive indexes nr 1 , nr 2 , . . . , and nrN are respectively tr 1 , tr 2 , . . . , and trN in the order from the surface of the optical recording medium where light is incident, the thicknesses tr 1 , tr 2 , . . . , and trN are converted into thicknesses t 1 , t 2 , . . . , and tN of layers having a predetermined refractive index “no” which makes a divergent amount equal to a divergent amount of a light beam resulting from the thicknesses tr 1 , tr 2 , . . . , and trN; a difference DFF between the sum of a thickness “ti” through a thickness “tj”, and the sum of a thickness “tk” through a thickness “tm” is set to 1 μm or more (where i, j, k, and m are each any positive integer satisfying i≦j&lt;k≦m≦SN); and the thicknesses t 1 , t 2 , . . . , and tN are calculated by products of a function f(n) expressed by the following formula ( 18 ), and the thicknesses tr 1 , tr 2 , . . . , and trN:
 
 f ( n )=−1.088 n   3 +6.1027 n   2 −12.042 n +9.1007   (18)
 
     in the formula (18), n=nr 1 , nr 2 , . . . , and nrN. 
     According to the above arrangement, the optical recording medium includes a cover layer formed between a surface of the optical recording medium where light is incident, and the first information recording surface closest to the medium surface; and first through (N−1)-th intermediate layers formed between the respective first through N-th information recording surfaces. Assuming that shape-wise thicknesses of the cover layer and the first through the (N−1)-th intermediate layers of the optical recording medium having refractive indexes nr 1 , nr 2 , . . . , and nrN are respectively tr 1 , tr 2 , . . . , and trN in the order from the surface of the optical recording medium where light is incident, the thicknesses tr 1 , tr 2 , . . . , and trN are converted into thicknesses t 1 , t 2 , . . . , and tN of layers having a predetermined refractive index “no” which makes a divergent amount equal to a divergent amount of a light beam resulting from the thicknesses tr 1 , tr 2 , . . . , and trN. Further, a difference DFF between the sum of a thickness “ti” through a thickness 1″, and the sum of a thickness “tk” through a thickness “tm” is set to 1 μm or more (where i, j, k, and m are each any positive integer satisfying i≦j&lt;k≦m≦N) Furthermore, the thicknesses t 1 , t 2 , . . . , and tN are calculated by products of the function f(n) expressed by the above-described formula (18), and the thicknesses tr 1 , tr 2 , . . . , and trN. 
     As described above, since the difference DFF between the sum of the thickness “ti” through the thickness “tj”, and the sum of the thickness “tk” through the thickness “tm” is set to  1 μm or more, it is possible to prevent light from forming an image on the backside of the surface of the optical recording medium, and suppress coherence between reflected light from the information recording surfaces to thereby improve the quality of a servo signal and a reproduction signal. Further, since the distance between the surface of the optical recording medium and the information recording surface closest to the surface of the optical recording medium can be set to a large value, deterioration of a reproduction signal in the case where there is a damage or a smear on the surface of the optical recording medium can be suppressed. 
     An optical recording medium according to another aspect of the invention is an optical recording medium having a plurality of information recording surfaces. The optical recording medium includes a cover layer having a refractive index nr 1 , and formed between a surface of the optical recording medium where light is incident and the first information recording surface closest to the medium surface; a first intermediate layer having a refractive index nr 2 , and formed between the first information recording surface and the second information recording surface second closest to the medium surface; a second intermediate layer having a refractive index nr 3 , and formed between the second information recording surface and the third information recording surface third closest to the medium surface; and a third intermediate layer having a refractive index nr 4 , and formed between the third information recording surface and the fourth information recording surface fourth closest to the medium surface, wherein shape-wise thicknesses tr 1 , tr 2 , tr 3 , and tr 4  of the cover layer, the first intermediate layer, the second intermediate layer, and the third intermediate layer are respectively determined depending on the refractive indexes nr 1 , nr 2 , nr 3 , and nr 4 , referring to a spherical aberration, the thicknesses tr 1 , tr 2 , tr 3 , and tr 4  are respectively converted into thicknesses t 1 , t 2 , t 3 , and t 4  of the respective layers having a predetermined refractive index “no”, referring to a defocus amount, the thicknesses tr 1 , tr 2 , tr 3 , and tr 4  are calculated by products of the thicknesses t 1 , t 2 , t 3 , and t 4 , and a function g(n) expressed by the following formula (19) to set the thicknesses t 1 , t 2 , t 3 , and t 4  in such a range that the spherical aberration lies in a predetermined range, the thicknesses t 1 , t 2 , t 3 , and t 4  are calculated by products of a function f(n) expressed by the following formula (20), and the calculated thicknesses tr 1 , tr 2 , tr 3 , and tr 4 , and the re-calculated thicknesses t 1 , t 2 , t 3 , and t 4  satisfy the following formula (21):
 
 g ( n )=−1.1111 nr   3 +5.8143 nr   2 −9.8808 nr +6.476  (19)
 
 f ( n )=−1.088 nr   3 +6.1027 nr   2 −12.042 nr +9.1007  (20)
 
|( t 1 +t 2)−( t 3 +t 4)|1 μm  (21)
 
     in the formulas (19) and (20), n=nr 1 , nr 2 , nr 3 , and nr 4 . 
     In the above arrangement, the optical recording medium includes a cover layer having a refractive index nr 1 , and formed between a surface of the optical recording medium where light is incident and the first information recording surface closest to the medium surface; a first intermediate layer having a refractive index nr 2 , and formed between the first information recording surface and the second information recording surface second closest to the medium surface; a second intermediate layer having a refractive index nr 3 , and formed between the second information recording surface and the third information recording surface third closest to the medium surface; and a third intermediate layer having a refractive index nr 4 , and formed between the third information recording surface and the fourth information recording surface fourth closest to the medium surface. Shape-wise thicknesses tr 1 , tr 2 , tr 3 , and tr 4  of the cover layer, the first intermediate layer, the second intermediate layer, and the third intermediate layer are respectively determined depending on the refractive indexes nr 1 , nr 2 , nr 3 , and nr 4 , referring to a spherical aberration. Further, the thicknesses tr 1 , tr 2 , tr 3 , and tr 4  are respectively converted into thicknesses t 1 , t 2 , t 3 , and t 4  of the respective layers having a predetermined refractive index “no”, referring to a defocus amount. Furthermore, the thicknesses tr 1 , tr 2 , tr 3 , and tr 4  are calculated by products of the thicknesses t 1 , t 2 , t 3 , and t 4 , and the function g(n) expressed by the above-described formula (19) to set the thicknesses t 1 , t 2 , t 3 , and t 4  in such a range that the spherical aberration lies in a predetermined range. Thereafter, the thicknesses t 1 , t 2 , t 3 , and t 4  are calculated by products of the function f(n) expressed by the above-described formula (20), and the calculated thicknesses tr 1 , tr 2 , tr 3 , and tr 4 , and the re-calculated thicknesses t 1 , t 2 , t 3 , and t 4  satisfy the above-described formula (21). 
     As described above, since the thicknesses t 1 , t 2 , t 3 , and t 4  obtained by conversion from the shape-wise thicknesses tr 1 , tr 2 , tr 3 , and tr 4  of the cover layer, the first intermediate layer, the second intermediate layer, and the third intermediate layer satisfy the relation: |(t 1 +t 2 )−(t 3 +t 4 )|1 μm, it is possible to prevent light from forming an image on the backside of the surface of the optical recording medium, and suppress coherence between reflected light from the information recording surfaces to thereby improve the quality of a servo signal and a reproduction signal. Further, since the distance between the surface of the optical recording medium, and the information recording surface closest to the surface of the optical recording medium can be set to a large value, deterioration of a reproduction signal in the case where there is a damage or a smear on the surface of the optical recording medium can be suppressed. 
     An optical recording medium according to another aspect of the invention is an optical recording medium having a plurality of information recording surfaces. The optical recording medium includes a cover layer having a refractive index nr 1 , and formed between a surface of the optical recording medium where light is incident and the first information recording surface closest to the medium surface; a first intermediate layer having a refractive index nr 2 , and formed between the first information recording surface and the second information recording surface second closest to the medium surface; and a second intermediate layer having a refractive index nr 3 , and formed between the second information recording surface and the third information recording surface third closest to the medium surface, wherein shape-wise thicknesses tr 1 , tr 2 , and tr 3  of the cover layer, the first intermediate layer, and the second intermediate layer are respectively determined depending on the refractive indexes nr 1 , nr 2 , and nr 3 , referring to a spherical aberration, the thicknesses tr 1 , tr 2 , and tr 3  are respectively converted into thicknesses t 1 , t 2 , and t 3  of the respective layers having a predetermined refractive index “no”, referring to a defocus amount, the thicknesses tr 1 , tr 2 , and tr 3  are calculated by products of the thicknesses t 1 , t 2 , and t 3 , and a function g(n) expressed by the following formula (22) to set the thicknesses t 1 , t 2 , and t 3  in such a range that the spherical aberration lies in a predetermined range, the thicknesses t 1 , t 2 , and t 3  are calculated by products of a function f(n) expressed by the following formula (23), and the calculated thicknesses tr 1 , tr 2 , and tr 3 , and the re-calculated thicknesses t 1 , t 2 , and t 3  satisfy the following formula (24):
 
 g ( n )=−1.1111 nr   3 +5.8143 nr   2 −9.8808 nr +6.476  (22)
 
 f ( n )=−1.088 nr   3 +6.1027 nr   2 −12.042 nr +9.1007  (23)
 
| t 1−( t 2 +t 3)|≧1 μm  (24)
 
     in the formulas (22) and (23), n=nr 1 , nr 2 , and nr 3 . 
     According to the above arrangement, the optical recording medium includes a cover layer having a refractive index nr 1 , and formed between a surface of the optical recording medium where light is incident and the first information recording surface closest to the medium surface; a first intermediate layer having a refractive index nr 2 , and formed between the first information recording surface and the second information recording surface second closest to the medium surface; and a second intermediate layer having a refractive index nr 3 , and formed between the second information recording surface and the third information recording surface third closest to the medium surface. Shape-wise thicknesses tr 1 , tr 2 , and tr 3  of the cover layer, the first intermediate layer, and the second intermediate layer are respectively determined depending on the refractive indexes nr 1 , nr 2 , and nr 3 , referring to a spherical aberration. The thicknesses tr 1  , tr 2 , and tr 3  are respectively converted into thicknesses t 1 , t 2 , and t 3  of the respective layers having a predetermined refractive index “no”, referring to a defocus amount. The thicknesses tr 1 , tr 2 , and tr 3  are calculated by products of the thicknesses t 1 , t 2 , and t 3 , and the function g(n) expressed by the above-described formula (22) to set the thicknesses t 1 , t 2 , and t 3  in such a range that the spherical aberration lies in a predetermined range. Thereafter, the thicknesses t 1 , t 2 , and t 3  are calculated by products of the function f(n) expressed by the above-described formula (23), and the calculated thicknesses tr 1 , tr 2 , and tr 3 , and the re-calculated thicknesses t 1 , t 2 , and t 3  satisfy the above-described formula (24). 
     As described above, since the thicknesses t 1 , t 2 , t 3 , and t 4  obtained by conversion from the shape-wise thicknesses tr 1 , tr 2 , and tr 3  of the cover layer, the first intermediate layer, and the second intermediate layer satisfy the relation: |t 1 −(t 2 +t 3 )|1 μm, it is possible to prevent light from forming an image on the backside of the surface of the optical recording medium, and suppress coherence between reflected light from the information recording surfaces to thereby improve the quality of a servo signal and a reproduction signal. Further, since the distance between the surface of the optical recording medium, and the information recording surface closest to the surface of the optical recording medium can be set to a large value, deterioration of a reproduction signal in the case where there is a damage or a smear on the surface of the optical recording medium can be suppressed. 
     An optical information device according to another aspect of the invention is an optical information device for reproducing or recording information with respect to the optical recording medium having any one of the above arrangements, wherein the optical information device moves a light spot of laser light to be irradiated onto the optical recording medium from a predetermined information recording surface to another information recording surface of the plurality of the information recording surfaces to thereby reproduce or record information with respect to the plurality of the information recording surfaces. According to this arrangement, a light spot of laser light to be irradiated onto the optical recording medium is moved from a predetermined information recording surface to another information recording surface of the plurality of the information recording surfaces to thereby reproduce or record information with respect to the plurality of the information recording surfaces. 
     An information reproducing method according to yet another aspect of the invention is an information reproducing method for reproducing information from the optical recording medium having any one of the above arrangements. The information reproducing method includes a step of converging a light spot of laser light onto a predetermined information recording surface of the plurality of information recording surfaces; a step of reproducing information from the predetermined information recording surface; and a step of moving the light spot of laser light from the predetermined information recording surface to another information recording surface of the optical recording medium different from the predetermined information recording surface to thereby reproduce information from the another information recording surface, in the case where the information is reproduced from the another information recording surface. 
     According to the above arrangement, a light spot of laser light is converged onto a predetermined information recording surface of the plurality of information recording surfaces to reproduce information from the predetermined information recording surface. Further, in the case where information is reproduced from another information recording surface of the optical recording medium different from the predetermined information recording surface, the light spot of laser light is moved from the predetermined information recording surface to the another information recording surface to reproduce the information from the another information recording surface. 
     The embodiments or the examples described in the detailed description of the invention are provided to clarify the technical contents of the invention. The invention should not be construed to be limited to the embodiments or the examples. The invention may be modified in various ways as far as such modifications do not depart from the spirit and the scope of the invention hereinafter defined. 
     The inventive multilayer optical disc (the inventive optical recording medium), the inventive optical recording medium manufacturing method, the inventive optical information device, and the inventive information reproducing method enable to maximally suppress an influence of reflected light from an information recording surface other than a targeted information recording surface at the time of reproducing from the targeted information recording surface, even if the refractive indexes of the cover layer and the intermediate layer are different from the standard value, to thereby reduce an influence on a servo signal and a reproduction signal to be used in an optical head device. Thus, the invention is useful to an optical recording medium for information recording or reproducing by irradiated light, a manufacturing method for the optical recording medium, an optical information device for recording or reproducing information with respect to the optical recording medium, and an information reproducing method for reproducing information from the optical recording medium. 
     Thus, the invention provides an optical recording medium capable of securing a reproduction signal of good quality, having a large capacity, and having compatibility with an existing optical recording medium.