Abstract:
A reproduction apparatus is provided for reproducing information stored on an optical medium. The apparatus includes a light source for illuminating the optical medium, the optical medium having at least one surface containing information stored thereon; a focusing arrangement operable to focus the light source on the at least one surface containing information thereon; and a detection arrangement operable to detect the information stored on the optical medium. The optical medium includes a first substrate having a first information surface; a semitransparent reflection film formed on the first information surface of the first substrate; a second substrate having a second information surface; a reflection film formed on the second information surface of the second substrate; and an adhesive layer for adhering the first substrate and the second substrate so that the first information surface and the second information surface face each other, wherein the thickness of the first substrate is at least 0.56 mm, the thickness of the adhesive layer is at least 30 μm, the total thickness of the first substrate and the adhesive layer is in the range of 0.59 mm to 0.68 mm, and the adhesive layer includes an ultraviolet-curable material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of application U.S. Ser. No. 10/267,601, filed on Oct. 9, 2002 now U.S. Pat. No. 6,737,144, which is a continuation of Ser. No. 09/865,308, filed May 25, 2001, now U.S. Pat. No. 6,489,002, which is a continuation of Ser. No. 09/698,569, filed Oct. 26, 2000, now U.S. Pat. No. 6,280,812, which is a continuation of Ser. No. 09/183,310, filed Oct. 30, 1998, now U.S. Pat. No. 6,143,426, which is a continuation of Ser. No. 08/895,787, filed Jul. 17, 1997, now U.S. Pat. No. 5,878,018, which is a continuation of Ser. No. 08/577,253, filed Dec. 22, 1995, now U.S. Pat. No. 5,726,969. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical recording medium where a light beam is focused on the recording medium and information is reproduced by detecting light reflected from the recording medium. Particularly, the present invention relates to an optical recording medium having dual information surfaces. 
   2. Description of the Related Art 
   In recent years, optical recording media have become more and more important as a means for storing sound information data, image information data, and various information apparatus data because they can store and reproduce a large amount of data. There are still requirements for further increasing the capacity of the optical recording media and reducing the size of optical recording/reproducing apparatuses. In order to satisfy these requirements, the storage capacity of the optical recording media needs to be further increased. 
   Compact disks (CDs) having one information surface, for example, are known as a conventional read-only optical recording medium. The CD includes a spiral information track composed of convex and concave portions (pits) formed on a surface of a disk-shaped resin substrate with a thickness of 1.2 mm. A reflection film made of aluminum and the like and a protection film are formed on the resultant information surface of the substrate by sputtering and the like. An identification label is then printed on the protection film. 
   The storage capacity of such a CD is small because the CD has only one information surface. In order to increase the storage capacity, a recording medium where two disks are adhered together, such as a 5″ magneto-optical (MO) disk, has been commercialized. The 5″ MO disk is classified into two types; a disk having one information surface (one-sided disk) and a disk having two information surfaces (double-sided disk). The one-sided disk includes a spiral guide groove composed of convex and concave portions formed on a surface of a disk-shaped resin substrate with a thickness of 1.2 mm. A dielectric film, a magneto-optical recording material film, another dielectric film, and a reflection film made of aluminum and the like are formed in this order on the resultant information surface of the substrate by sputtering and the like. Another resin substrate with a thickness of 1.2 mm is then adhered to the reflection film. The double-sided disk includes a spiral guide groove composed of convex and concave portions formed on a surface of a disk-shaped resin substrate with a thickness of 1.2 mm. A dielectric film, a magneto-optical recording material film, another dielectric film, and a reflection film made of aluminum and the like are formed in this order on the resultant information surface of the substrate by sputtering and the like. The thus-fabricated disk is adhered with another disk fabricated in the same manner. Conventional 5″ MO disk recording/reproducing apparatuses are designed to receive both the one-sided disk and the double-sided disk to accomplish the recording and reproduction. The user can select the one-sided disk when information to be recorded is small or the double-sided disk when it is large. The 5″ MO disk apparatuses are generally provided with only one optical head. Accordingly, when the double-sided disk is used, the disk needs to be taken out and turned over to continue the recording or reproduction. 
   In general, the information density of a recording medium is determined by the pitch of an information track and the information density in the tracking direction, i.e., the information linear density. In order to increase the information density of the recording medium, the track pitch should be small, while the linear density should be large. In recent years, there have been studies to increase the density of the optical recording medium by reducing the thickness of the substrate to 0.6 mm, for example, to reduce the aberration of a light beam passing through the substrate due to a tilt of the disk. 
   However, the above conventional techniques have the following problems. In the case of the conventional double-sided optical recording medium, if both the top and bottom surfaces of the recording medium are illuminated with light beams so as to record information or reproduce recorded information, little space is left on the surfaces of the recording medium for printing an identification label. This is inconvenient for handling the recording medium. Also, when the conventional double-sided optical recording medium is used for an optical reproduction apparatus having only one optical head, the optical recording medium needs to be taken out from the apparatus and turned over to continue the reproduction. In order to continue the reproduction automatically, two optical heads disposed above and below the recording medium are required. An apparatus having two optical heads is large in size and its cost is high. 
   Another problem is that when a new optical recording medium thinner than the conventional optical recording media is commercialized to increase the density of the recording medium, such a new optical recording medium is not compatible with the conventional recording/reproduction apparatus. 
   SUMMARY OF THE INVENTION 
   The optical recording medium of this invention includes: a first substrate having a first information surface; a semitransparent reflection film formed on the first information surface of the first substrate; a second substrate having a second information surface; a reflection film formed on the second information surface of the second substrate; and an adhesive layer for adhering the first substrate and the second substrate so that the first information surface and the second information surface face each other, wherein the thickness of the first substrate is 0.56 mm or more, the thickness of the adhesive layer is 30 μm or more, and the total thickness of the first substrate and the adhesive layer is 0.68 mm or less. 
   In one embodiment, the thickness of the first substrate is in the range of 0.56 mm to 0.6 mm, and the thickness of the adhesive layer is in the range of 40 μm to 60 μm. 
   In another embodiment, a recording material film is formed on the reflection film for the second substrate for recording and reproducing information. 
   In still another embodiment, the recording material film is made of a phase-change type recording material. 
   In still another embodiment, a label is formed on a surface of the second substrate. 
   In still another embodiment, a spiral track is formed on each of the first and second substrates, and the direction of the formation of the spiral track on the first substrate is the same as the direction of the formation of the spiral track on the second substrate when the spiral tracks are viewed from the side of a surface of the first substrate opposite to the first information surface. 
   In still another embodiment, a spiral track is formed on each of the first and second substrates, and the direction of the formation of the spiral track on the first substrate is reverse to the direction of the formation of the spiral track on the second substrate when the spiral tracks are viewed from the side of a surface of the first substrate opposite to the first information surface. 
   Alternatively, the optical recording medium of this invention includes: a first substrate having a first information surface; a semitransparent reflection film formed on the first information surface of the first substrate; a second substrate having a second information surface; a reflection film formed on the second information surface of the second substrate; and an adhesive layer for adhering the first substrate and the second substrate so that the first information surface of the first substrate faces a surface of the second substrate opposite to the second information surface, wherein the thickness of the first substrate is substantially the same as the thickness of the second substrate. 
   Alternatively, the optical recording medium of this invention includes: a first substrate having a first information surface; a semitransparent reflection film formed on the first information surface of the first substrate; a second substrate having a second information surface; a reflection film formed on the second information surface of the second substrate; an adhesive layer for adhering the first substrate and the second substrate so that the first information surface of the first substrate faces a surface of the second substrate opposite to the second information surface; and a label formed on the reflection film for the second substrate, wherein the thickness of the first substrate is substantially the same as the thickness of the second substrate. 
   Thus, the invention described herein makes possible the advantages of (1) providing an optical recording medium having dual information surfaces where a label can be easily printed on a surface of the recording medium, information can be automatically reproduced by use of one optical head, and the compatibility with an optical recording medium having one information surface can be secured, and (2) providing an optical recording medium which includes a substrate with a thickness different from the conventional standard but is compatible with conventional apparatuses so that information stored in the optical recording medium can be reproduced. 
   These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of an optical recording medium of Example 1 according to the present invention. 
       FIGS. 2A and 2B  are views showing optical paths of reflected light beams when information recorded on a first information surface and a second information surface, respectively, is reproduced in Example 1. 
       FIG. 3  is a sectional view of an optical recording medium of Example 2 according to the present invention. 
       FIGS. 4A and 4B  are views showing optical paths of reflected light beams when information recorded on a first information surface and a second information surface, respectively, is reproduced in Example 2. 
       FIG. 5  is a sectional view of an optical recording medium of Example 3 according to the present invention. 
       FIG. 6  is an enlarged sectional view showing a second optical disk of the optical recording medium of Example 3. 
       FIGS. 7A  to  7 C are graphs showing the measurement results of jitters obtained from trial-manufactured optical recording media. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will be described by way of examples with reference to the accompanying drawings. 
   EXAMPLE 1 
     FIG. 1  is a schematic sectional view showing an optical recording medium  101  of Example 1 according to the present invention. The optical recording medium  101  is a one-side read type recording medium composed of a first optical disk  102  and a second optical disk  103  adhered to each other. Such an optical recording medium can provide excellent performance as a digital video disk (DVD). 
   The first optical disk  102  includes a disk-shaped first substrate  104  having a first information surface  105  where a spiral information track composed of convex and concave portions (pits) is formed. A semi transparent first reflection film  106  is formed on the first information surface  105  of the first substrate  104  by sputtering and the like. The semitransparent first reflection film  106  is made of gold (Au), aluminum (Al), and the like, for example. The first reflection film  106  is supposed to have a property of reflecting part of laser light for reproduction while transmitting the remaining, as will be described later in detail. In order to realize this property, not only the selection of an appropriate material for the reflection film, but also the adjustment of the thickness thereof to an appropriate range are required. The thickness of the first reflection film  106  is preferably in the range of 5 to 20 nm. In Example 1, the thickness of the reflection film  106  is 10 nm. 
   The second optical disk  103  includes a disk-shaped second substrate  107  having a second information surface  108  where a spiral information track composed of convex and concave portions (pits) is formed. A second reflection film  109  is formed on the second information surface  108  of the second substrate  107  by sputtering and the like. The second reflection film  109  is made of aluminum (Al) and the like. 
   Information is recorded on the first and second information surfaces  105  and  108  with high density, i.e., a track pitch of about 0.74 μm and a minimum pit length of about 0.4 μm. The thickness of the second reflection film  109  is smaller than the lengths of the pits formed on the second information surface  108  so that the pits recorded on the second information surface  108  can be transferred well to the second reflection film  109 . Specifically, the thickness of the second reflection film  109  is preferably in the range of 30 to 150 nm. In Example 1, the thickness of the second reflection film  109  is 50 nm. 
   As shown in  FIG. 1 , an adhesive layer  110  is formed between the first optical disk  102  and the second optical disk  103  for adhering the two optical disks. The adhesive layer  110  is made of an acrylic ultraviolet (UV)-curable material, for example. Such a UV-curable material is applied to at least one of the optical disks  102  and  103 . Then, the two optical disks  102  and  103  are put in contact with each other via the UV-curable material, and illuminated with a UV ray to cure the UV-curable material and thus to adhere the two optical disks. Other thermosetting adhesives may also be used instead of the UV-curable material. 
   A label  111  is attached to the surface of the second optical disk  103 . A hole  112  (inner diameter: 15 mm) is formed in the center of the optical recording medium  101  for mounting the optical recording medium  101  on a driving motor. 
   Now, referring to  FIGS. 2A and 2B , the reproduction of information recorded on the first and second information surfaces  105  and  108  will be described. 
     FIG. 2A  shows the case where information recorded on the first information surface  105  is read, while  FIG. 2B  shows the case where information recorded on the second information surface  108  is read. A parallel light beam  201  is converged by a focusing lens  202  and illuminates the optical recording medium  101  from the side opposite to the side of the label  111 , i.e., from the side of the first substrate  104 . 
   The focusing lens  202  is designed to be used for optical recording media having a substrate with a thickness of 0.6 mm. Accordingly, a conventional disk having one information surface which has a substrate with a thickness of 0.6 mm is also applicable to the focusing lens  202 . 
   Referring to  FIG. 2A , when information recorded on the first information surface  105  is reproduced, the light beam  201  is controlled to be focused on the first information surface  105  by a known focusing control technique. A reflected light beam  203  reflected from the first reflection film  106  is detected by an optical detector  205  via a splitter  204 . Thus, the information is read. Referring to  FIG. 2B , when information recorded on the second information surface  108  is reproduced, the light beam  201  is controlled to be focused on the second reflection film  109  by the focusing control technique. A reflected light beam  206  reflected from the second reflection film  109  is detected by the optical detector  205 , and thus the information is read. For the reproduction of information recorded in the optical recording medium  101 , the wavelength of the light beam  201  needs to be 650 nm, and the numeral aperture (NA) of the focusing lens  202  needs to be about 0.6. 
   When information recorded on the first information surface  105  is reproduced, as shown in  FIG. 2A , the reflected light beams  203  and  206  pass through the focusing lens  202  and are received by the optical detector  205 . However, the size of the spot of the light beam  201  formed on the second reflection film  109  is in the order of several tens of micrometers, which is considerably larger than the track pitch (0.8 μm) and the minimum pit length (0.5 μm). Thus, a plurality of pits are illuminated with the light beam  201 . Accordingly, the reflected light beam  206  hardly includes an individual pit information component, and has a substantially constant light amount as if it is reflected from a surface where no pit is formed. Further, only part of the reflected light beam  206  passes through the focusing lens  202 , and the part of the reflected light beam  206  which has passed through the focusing lens  202  is not made parallel. Accordingly, the light amount of the part of the reflected light beam  206  which has reached the optical detector  205  is small. Thus, the pit information detected by the optical detector  205  is mostly composed of components modulated by the pits recorded on the first information surface  105 . 
   When information recorded on the second information surface  108  is reproduced, as shown in  FIG. 2B , the reflected light beams  203  and  206  pass through the focusing lens  202  and are received by the optical detector  205 . As in the above case, however, the size of the spot of the light beam  201  formed on the first reflection film  106  is in the order of several tens of micrometers, which is considerably large and illuminates a plurality of pits. Accordingly, the reflected light beam  203  hardly includes individual pit information components. Further, since part of the reflected light beam  203  which has passed through the focusing lens  202  is not made parallel, the light amount of the part of the reflected light beam  203  which has reached the optical detector  205  is small. Thus, the pit information detected by the optical detector  205  is mostly composed of components modulated by the pits recorded on the second information surface  108 . 
   Next, the relationship between the reflectances of the first and second reflection films  106  and  109  will be described. In the case where information recorded on the first information surface  105  is read, as the reflectance of the first reflection film  106  is higher, the light amount of the reflected light beam  203  is larger and the quality of the resultant reproduced signal is better. However, in the case where information recorded on the second information surface  108  is read, as the reflectance of the first reflection film  106  is higher, the light amount of the light beam  201  passing through the first reflection film  106  is smaller. Since the reflected light beam  206  reflected from the second reflection film  109  passes through the first reflection film  106  again, the light amount of the reflected light beam  206  is further reduced at the reading of the information recorded on the second information surface  108 . In other words, in the case where information recorded on the second information surface  108  is read, the light beam  201  passes through the first substrate  104 , the first reflection film  106 , and the adhesive layer  110  to reach the second reflection film  109 . The reflected light beam  206  reflected from the second reflection film  109  then passes through the adhesive layer  110 , the first reflection film  106 , and the first substrate  104  again. The light thus passes through the first reflection film  106  twice. Accordingly, if the reflectance of the first reflection film  106  is high, the light amount of the reflected light beam  206  becomes small at the reading of the information recorded on the second information surface  108 . In order to overcome this problem, in the optical recording medium  101  of Example 1 according to the present invention, the reflectances of the first and second reflection films  106  and  109  are set so that a light amount P 2  of the reflected light beam  206  at the reading of information recorded on the second information surface  108  is substantially the same as a light amount P 1  of the reflected light beam  203  at the reading of information recorded on the first information surface  105 . In this case, the relationship is expressed by k 2 =k 1 /(1−k 1 ) 2  where k 1  is the reflectance of the first reflection film  106  and k 2  is the reflectance of the second reflection film  109 . 
   The above expression is obtained in the following manner. The light amount P 1  of the reflected light beam  203  at the reading of information recorded on the first information surface  105  is expressed by P 1 =P 0 ×k 1 . The light amount P 2  of the reflected light beam  206  at the reading of information recorded on the second information surface  108  is expressed by P 2 =P 0 ×k 2 (1−k 1 ) 2 . Since P 1 =P 2 , the above expression is obtained. The reflectances k 1  and k 2  represent the percentage of the reflected light amount with respect to the incident light amount. Specifically, in the optical recording medium  101  of Example 1, the reflectance of the first reflection film  106  is in the range of 20 to 35%, while the reflectance of the second reflection film  109  is 60% or more. The reflectance of the second reflection film  109  is preferably as high as possible. However, in order to realize a reflectance closer to 100% using an inexpensive material such as aluminum, the film thickness needs to be about 0.6 to 0.8 μm. Since a high-density optical disk has a pit length of about 0.5 μm, a reflection film as thick as 0.6 to 0.8 μm lowers the level of the transfer of the information surface onto the reflection film. In order to prevent the lowering of the level of the transfer, the thickness of the second reflection film  109  is made equal to or less than the pit length of the second information surface  108 , i.e., 0.5 μm, and thus the reflectance is made 60% or more. 
   Next, the aberration of the light beam  201  focused by the focusing lens  202  will be described. In the optical recording medium  101  of Example 1, the optical path length of the light beam  201  when information recorded on the first information surface  105  is read and that when information recorded on the second information surface  108  is read are different by the total of the thickness of the first reflection film  106  and a thickness t 0  of the adhesive layer  110 . Since the thickness of the first reflection film  106  is 0.5 μm or less in the optical recording medium  101  of Example 1, it can be neglected. When the optical path length, i.e., the thickness through which the light beam passes varies, the light beam  201  focused by the focusing lens  202  generates an aberration. The aberration increases in proportion to about the fourth power of the NA of the focusing lens  202 . 
   The relationship between the focusing lens  202  and a thickness t 1  of the first substrate  104  will be described. Herein, the thickness t 1  of the first substrate  104  is considered to include the thickness of the first reflection film  106  because the thickness of the first reflection film  106  is negligible in comparison with the thickness t 1  of the first substrate  104  and the thickness t 0  of the adhesive layer  110 . 
   In general, the focusing lens  202  is designed in consideration of the thickness of a substrate of an optical disk. When the thickness of a substrate of an optical disk having one information surface is 0.6 mm, the focusing lens  202  is designed based on the thickness of the substrate of 0.6 mm. When the optical recording medium  101  having the first substrate  104  with the thickness t 1  of 0.6 mm is reproduced by use of this focusing lens  202 , no problem arises when information recorded on the first information surface  105  is read. However, when information recorded on the second information surface  108  is read, the thickness of the adhesive layer  110  is added to the thickness of the first substrate  104 . That is, if the thickness to t 0  of the adhesive layer  110  is 40 μm, 40 μm is added to the thickness t 1  of the first substrate  104 , 0.6 mm. This is substantially equal to the case where a substrate with a thickness of 0.64 mm is used. Accordingly, the aberration increases when information recorded on the second information surface  108  is read. This lowers the quality of the reproduced signal. In order to overcome this problem, when the focusing lens  202  designed for an optical recording medium having a 0.6 mm thick substrate is used, the thickness t 1  of the first substrate  104  is made 0.58 mm as a standard. Then, the thickness of a substrate of an optical recording medium having dual information surfaces is made slightly thinner than that of a substrate of an optical recording medium having one information surface. As a result, the thickness of the substrate is 0.58 mm when information recorded on the first information surface  105  is read, while it is 0.62 mm when information recorded on the second information surface  108  is read. In the latter case, the thickness of the substrate is equal to the distance between the incident surface of the first substrate  104  and the second information surface  108 . The differences between these thicknesses and the design value 0.6 mm for the focusing lens  202  are both 20 μm. Thus, substantially the same quality of reproduced signals can be obtained from the first information surface  105  and the second information surface  108 . Naturally, variations are generated in the thickness t 1  of the first substrate  104  and the thickness t 0  of the adhesive layer  110  in the fabrication process. With the above setting, however, the allowances of these variations are widened. 
   The relationship between the thickness t 1  of the first substrate  104  and the thickness t 0  of the adhesive layer  110  will be described in more detail with reference to  FIGS. 7A  to  7 C.  FIGS. 7A  to  7 C are graphs showing the measurement results obtained from various trial-manufactured optical recording media. The X-axis of these graphs is the distance between the incident surface of the first substrate  104  and the information surface, and the Y-axis is the jitter of the reproduced signal. The jitter is the value obtained by dividing the standard deviation value of the time-axis variation in the reproduced signal by the period of the channel clock. In  FIG. 7A , four types of the first optical disks  102  having the first substrate  104  with the thickness t 1  of 0.56 mm, 0.57 mm, 0.62 mm, and 0.63 mm were trial-manufactured. Each of these first optical disks  102  was adhered with the second optical disk  103  via the adhesive layer  110  with a thickness of 30 μm. Using the resultant optical recording media, reproduction of information was conducted, and the jitters were measured. The reference numeral  71  shows the jitters obtained when information recorded on the first information surface  105  is reproduced, while the reference numeral  72  shows the jitters obtained when information recorded on the second information surface  108  is reproduced. In  FIG. 7B , four types of the first optical disks  102  having the first substrate  104  with the thickness t 1  of 0.56 mm, 0.57 mm, 0.58 mm, and 0.61 mm were trial-manufactured. Each of these first optical disks  102  was adhered with the second optical disk  103  via the adhesive layer  110  with a thickness of 40 μm. Using the resultant optical recording media, reproduction of information was conducted, and the jitters were measured. The reference numeral  73  shows the jitters obtained when information recorded on the first information surface  105  is reproduced, while the reference numeral  74  shows the jitters obtained when information recorded on the second information surface  108  is reproduced. In  FIG. 7C , three types of the first optical disks  102  having the first substrate  104  with the thickness t 1  of 0.61 mm, 0.62 mm, and 0.63 mm were trial-manufactured. Each of these first optical disks  102  was adhered with the second optical disk  103  via the adhesive layer  110  with a thickness of 50 μm. Using the resultant optical recording media, reproduction of information was conducted, and the jitters were measured. The reference numeral  75  shows the jitters obtained when information recorded on the first information surface  105  is reproduced, while the reference numeral  76  shows the jitters obtained when information recorded on the second information surface  108  is reproduced. 
   In general, when information recorded on a disk. is reproduced, defocusing and off-tracking arise due to deflection and decentering of the disk, vibration and shock applied to the apparatus from outside, and the like. These deteriorate the jitter of the reproduced signal. The jitter of the reproduced signal is also deteriorated when the disk and the optical axis of the light beam are inclined against each other. This warp of the disk varies depending on a change of the environmental conditions such as humidity. A variation among optical heads in the fabrication process and a variation of each optical head with time should also be considered. Accordingly, in order to reproduce information recorded on a disk with high reliability, the jitter of a reproduced signal is about 10% at maximum in consideration of the deterioration of the jitter due to various factors described above. 
   In comparison between  FIGS. 7A and 7B , the following are observed. When the thickness t 0  of the adhesive layer  110  is 30 μm, the jitter of the reproduced signal obtained from the first information surface little changes. The value is kept at and around 9.5%, irrespective of the change in the thickness t 1  of the first substrate  104  from 0.56 mm to 0.63 mm. On the contrary, when the thickness to of the adhesive layer  110  is 40 μm, the jitter of the reproduced signal obtained from the first information surface is higher as the thickness t 1  of the first substrate  104  is thinner. This indicates that, when the thickness t 0  of the adhesive layer  110  is as thin as 30 μm, the influence of a leak signal from the second information surface is greater than the influence of the aberration due to the change in the thickness t 1  of the first substrate  104 . It is therefore expected that, if the thickness t 0  of the adhesive layer  110  is smaller than 30 μm, the leak signal from the second information surface will be greater and the quality of the reproduced signal will be eminently reduced. Accordingly, the thickness t 0  of the adhesive layer  110  should be 30 μm or more. 
   From  FIG. 7B , it is observed that the jitter of the reproduced signal obtained from the first information surface  105  starts increasing sharply when the thickness t 1  of the first substrate  104  is in the range of 0.58 to 0.56 mm. This is because, when the thickness t 0  of the adhesive layer  110  is 40 μm, the influence of the aberration due to the change in the thickness t 1  of the first substrate  104  becomes greater than the influence of a leak signal from the second information surface. The jitter of the reproduced signal changes substantially parabolically with the change in the thickness t 1  of the first substrate  104 . It is therefore expected that, if the thickness t 1  of the first substrate  104  is smaller than 0.56 mm, the jitter of the reproduced signal will sharply increase. Accordingly, the thickness t 1  of the first substrate  104  should be 0.56 mm or more. 
   The total of the thickness t 1  of the first substrate  104  and the thickness t 0  of the adhesive layer  110 , i.e., (t 0 +t 1 ) is the thickness of a substrate existing when information recorded on the second information surface  108  is reproduced. From  FIG. 7C , it is observed that the jitter of the reproduced signal starts increasing sharply when the thickness (t 0 +t 1 ) is in the range of 0.66 to 0.68 mm. The jitter changes substantially parabolically with the change in the thickness t 1  of the first substrate  104 . It is therefore expected that, if the thickness (t 0 +t 1 ) of the substrate is 0.69 mm, the jitter of the reproduced signal will exceed 10%. Accordingly, in order to obtain a jitter of the reproduced signal of 10% or less, the total of the thickness t 1  of the first substrate  104  and the thickness t 0  of the adhesive layer  110 , i.e., (t 0 +t 1 ) should be 0.68 mm or less. 
   The above values are very strict. In order to secure the reliability of the device, severe examination is required for each component of the device. Since the allowances are too narrow to allow mass production, the cost of each device becomes high. The allowances should be widened in order to manufacture the devices easily. This point will be described in more detail as follows. 
   When information recorded on the first information surface  105  is reproduced, as the thickness t 0  of the adhesive layer  110  is larger, the influence of a leak signal from the second information surface  108  is smaller. As is observed from the comparison between  FIGS. 7A and 7B , the thickness t 0  of the adhesive layer  110  is desirably 40 μm or more when information recorded on the first information surface  105  is reproduced. Further, when the thickness t 1  of the first substrate  104  is 0.56 mm or more, the jitter of the reproduced signal can be as small as 8%. 
   As is observed from  FIG. 7A , when the distance between the surface of the first substrate  104  and the second information surface  108  is 0.66 mm, the jitter of the reproduced signal is 7.5%. From  FIG. 7B , when the above distance is 0.65 mm, the jitter is 6.6%. From  FIG. 7C , when the above distance is 0.66 mm, 0.67 mm, and 0.68 mm, the jitter is 7%, 7.8%, and 8.8%, respectively. Thus, when the above distance exceeds 0.66 mm, the jitter of the reproduced signal sharply increases. Accordingly, the thickness of the substrate existing when information recorded on the second information surface  108  is reproduced, i.e., the total of the thickness t 1  of the first substrate  104  and the thickness t 0  of the adhesive layer  110  is desirably 66 mm or less. 
   When the focusing lens  202  is designed for an optical recording medium having a 0.6 mm thick substrate, the thickness of the substrate preferably varies with 0.6 mm as the center of the variation. Accordingly, when the thickness of the first substrate  104  is 0.56 mm or more, it can be defined as 0.58 mm±0.02 mm. Therefore, in order to obtain the total of the thickness of the first substrate  104  and the thickness of the adhesive layer  110  of 0.66 mm, the thickness of the adhesive layer  110  should be 60 μm or less. 
   From the above description, it is understood that, by setting the thickness t 0  of the adhesive layer  110  in the range of 40 to 60 μm and the thickness t 1  of the first substrate  104  in the range of 0.56 mm to 0.6 mm, the jitters of the reproduced signals obtained from the first and second information surfaces  105  and  108  are both low, and thus reproduced signals with excellent quality can be obtained. 
   Now, the direction of the spiral tracks of the first and second optical disks  102  and  103  will be described. For example, when the spiral track of the first optical disk  102  is formed from the inner side to the outer side and the spiral track of the second optical disk  103  is also formed from the inner side to the outer side, interactive reproduction can be realized by using one optical head for the reproduction from the dual information surfaces. For example, a program of a game having a plurality of branches may be recorded on the dual information surfaces separately. In the game, upon receipt of branching instruction, the program can instantaneously move from the first information surface  105  to the second information surface  108  or from the second information surface  108  to the first information surface  105  by focus jumping. 
   Alternatively, when the spiral track of the first optical disk  102  is formed from the inner side to the outer side, and the spiral track of the second optical disk  103  is formed from the outer side to the inner side, continuous reproduction can be easily realized by using one optical head for the reproduction from the dual information surfaces. That is, information is first reproduced from the first information surface  105  by moving the optical head from the inner side to the outer side of the disk when the optical head reaches the outermost side, the focusing is instantaneously jumped from the first information surface  105  to the second information surface  108 . Then, information recorded on the second information surface  108  is reproduced from the outer side to the inner side. This procedure allows for long-time continuous reproduction of a movie and the like. Such an optical recording medium including the first and second optical disks of which spiral tracks are formed in directions reverse to each other is obtained in the following manner: At the cutting of the original disks, signals are recorded in the first optical disk  102  by moving the optical head from the inner side to the outer side of the disk. When signals are to be recorded in the second disk  103 , the disk is rotated reversely, and the optical head is moved from the outer side to the inner side of the disk. 
   Thus, in the optical recording medium in Example 1, the first and second optical disks are adhered via an adhesive having a predetermined thickness. Information recorded on both the first and second information surfaces is reproduced by illuminating the surfaces with a light beam from one side of the optical recording medium. Thus, a label can be attached to the other side. Further, since information recorded on both the first and second information surfaces is reproduced only by changing the position of the focusing point by use of one optical head, interactive reproduction or long-time continuous reproduction of a movie is possible. This also reduces the cost of an recording/reproducing apparatus. Moreover, since the thicknesses of the first and second optical disks are the same, these optical disks little change in shape with the change in humidity. This facilitates the adhesion of these optical disks, and thus lowers the cost of the disks. 
   EXAMPLE 2 
   In Example 2, an optical recording medium which can be used for different types of optical recording/reproducing apparatuses designed for optical recording media having substrates with different thicknesses will be described. 
     FIG. 3  shows a schematic sectional view of an optical recording medium from which information can be read by both an apparatus designed for a recording medium with a 1.2 mm thick substrate and an apparatus designed for a recording medium with a 0.6 mm thick substrate. 
   An optical recording medium  301  of Example 2 is composed of a first optical disk  302  and a second optical disk  303  adhered to each other. The same information is stored in the first and second optical disks  302  and  303 . The first optical disk  302  includes a disk-shaped first substrate  304  with a thickness of 0.6 mm having a first information surface  305  where a spiral information track composed of convex and concave portions (pits) is formed. A semitransparent first reflection film  306  is formed on the first information surface  305  of the first substrate  304  by sputtering and the like. The second optical disk  303  stores completely the same information as that in the first optical disk  302  in the same way. The second optical disk  303  includes a disk-shaped second substrate  307  with a thickness of 0.6 mm having a second information surface  308  where a spiral information track composed of convex and concave portions (pits) is formed. A second reflection film  309  of aluminum and the like is formed on the second information surface  308  of the second substrate  307  by sputtering and the like. The reference numeral  310  denotes an adhesive layer made of a UV-curable material for adhering the first and second optical disks  302  and  303 . The reference numeral  311  denotes a label for identifying the optical recording medium. The reference numeral  312  denotes a hole for mounting the optical recording medium  301  on an optical recording/reproducing apparatus. 
   The reproduction of information recorded on the first and second information surfaces  305  and  308  will be described with reference to  FIGS. 4A and 4B .  FIG. 4A  shows the case where information recorded on the first information surface  305  is read by use of an apparatus designed for an optical recording medium having a 0.6 mm thick substrate.  FIG. 4A  is basically the same as FIG.  2 A. That is, a parallel light beam  201  is converged by a focusing lens  202  designed for a 0.6 mm thick substrate and illuminates the optical recording medium  301  from the side of the first substrate  304 . The light beam  201  is partly reflected from the first reflection film  306  and a reflected light beam  203  is detected by an optical detector  205  via a splitter  204 . Thus, the information is read. 
     FIG. 4B  shows the case where information recorded on the second information surface  308  is read by use of an apparatus designed for an optical recording medium having a 1.2 mm thick substrate. Referring to  FIG. 4B , when information recorded on the second information surface  308  is reproduced, a parallel light beam  401  is converged by a focusing lens  402  designed for a 1.2 mm thick substrate and illuminates the optical recording medium  301  from the side of the first substrate  304 . The light beam  401  passes through the first substrate  304 , the first reflection film  306 , the adhesive layer  310 , and the second substrate  307  and reaches the second information surface  308 . The light beam  401  is partly reflected from the second reflection film  309 , and a reflected light beam  406  passes through the second substrate  307 , the adhesive layer  310 , the first reflection film  306 , the first substrate  304 , and the focusing lens  402 . The reflected light beam  406  is then detected by an optical detector  405  via a splitter  404 . Thus, the information is read. 
   As shown in  FIG. 4A , when information recorded on the first information surface  305  is reproduced, the reflected light beams  203  and  206  pass through the focusing lens  202  and are received by the optical detector  205 . However, the size of the spot of the light beam  201  formed on the second reflection film  309  is as large as 1 mm or more, and thus a plurality of pits are illuminated with the light beam  201 . Also, the reflected light beam  206  which has passed through the focusing lens  202  is not made parallel. Accordingly, the light amount of the reflected light beam  206  which has reached the optical detector  205  is extremely small. Thus, pit information components obtained from the second information surface  308  are hardly detected by the optical detector  205 . 
   As shown in  FIG. 4B , as in the above case, when information recorded on the second information surface  308  is reproduced, the reflected light beams  403  and  406  pass through the focusing lens  402  and are received by the optical detector  405 . However, the size of the spot of the light beam  401  formed on the first reflection film  306  is as large as 1 mm or more, and thus a plurality of pits are illuminated with the light beam  401 . Also, the reflected light beam  406  which has passed through the focusing lens  402  is not made parallel. Accordingly, pit information components obtained from the first information surface  305  are hardly detected by the optical detector  405 . 
   The relationship between the reflectances of the first and second reflection films  306  and  309  in the optical recording medium  301  of Example 2 is basically the same as that of the optical recording medium  101  of Example 1. In the optical recording medium  301 , however, information recorded on the second information surface  308  is not required to be transferred to the second reflection film  309 . This enables the second reflection film  309  to be thickened, and thus a reflectance of 90% or more can be obtained. 
   As described above, when the optical recording medium  301  shown in  FIG. 3  is used as an apparatus designed for a 1.2 mm thick substrate, the light beam  401  passes through the first and second substrates  304  and  307 , the first reflection film  306 , and the adhesive layer  310 . When the thicknesses of the first and second substrates  304  and  307  are 0.6 mm, the total thickness exceeds 1.2 mm by the thickness of the adhesive layer  310 , though the thickness of the first reflection film  306  is negligible. This causes aberration. In order to solve this problem, the thickness of the adhesive layer  310  is preferably several tens of micrometers or less. Alternatively, the thickness of the second substrate  307  may be thinned by the thickness of the adhesive layer  310 . 
   The first and second information surfaces  305  and  308  may have different formats from each other. For example, information may be recorded on the second information surface  308  with the format of conventional CDs so that the information can be reproduced by widely-available CD players. In general, the density of CDs is low and the capacity thereof is only a quarter or so of that of the optical disk according to the present invention. Accordingly, for example, while the entire movie may be recorded on the first information surface  305 , an edited version of the movie shortened by cutting part thereof may be recorded on the second information surface  308 . In this case, since a light beam with a wavelength of  780  nm is used for reproduction from CDs, the first reflection film  306  should have the optical property of reflecting a 650 nm light beam and transmitting a 780 nm light beam. This increases-the reflected light amount and thus the S/N ratio of the resultant reproduced signal. 
   As described above, in Example 2, since the same information is recorded in the first and second optical disks, the same information can be read by both an apparatus designed for an optical recording medium-having a 1.2 mm thick substrate and an apparatus designed for an optical recording medium having a 0.6 mm thick substrate. Also, since both the first and second information surfaces are illuminated with a light beam from one side of the optical recording medium, a label can be attached to the other side. 
   EXAMPLE 3 
   In Example 3, an optical recording medium having a first information surface for reproduction only and a second information surface for recording and reproduction will be described. The reproduction or recording of information is conducted by illuminating the optical recording medium with a light beam from only one side. 
     FIG. 5  is an exaggerated sectional view of an optical recording medium  501  of Example 3. The optical recording medium  501  is composed of a first optical disk  502  for reproduction only and a second optical disk  503  for recording and reproduction adhered to each other. The first optical disk  502  includes a disk-shaped substrate  504  with a thickness of 0.6 mm having a first information surface  505  where a spiral information track composed of convex and concave portions (pits) is formed. A semitransparent reflection film  506  is formed on the first information surface  505  of the substrate  504  by sputtering and the like. The second optical disk  503  includes a substrate with a thickness of 0.58 mm having a second information surface where a spiral information track composed of minute convex and concave portions (grooves) is formed. The reference numeral  510  denotes an adhesive layer for adhering the first and second optical disks  502  and  503 . The reference numeral  511  denotes a label for identifying the optical recording medium. The reference numeral  512  denotes a hole for mounting the optical recording medium  501  on an optical recording/reproducing apparatus. 
   In the optical recording medium  501  shown in  FIG. 5 , as in the optical recording medium  101  shown in  FIG. 1 , the first information surface  505  for reproduction only and the second information surface for recording and reproduction are adhered via the adhesive layer  510  so that they are apart from each other by about 40 μm. The optical recording medium  501  is illuminated with a light beam from the side of the first optical disk  502 . 
   Referring to  FIGS. 5 and 6 , the second optical disk  503  will be described.  FIG. 6  is an enlarged exaggerated sectional view obtained by cutting the second optical disk  503  in a radial direction. A groove track  602  having convex and concave portions is formed on one surface of a substrate  601  of the second optical disk  503 . Then, a reflection film  603  made of aluminum and the like, a dielectric film  604  made of SiO 2  and the like, a recording material film  605 , and another dielectric film  606  are formed consecutively in this order by sputtering and the like. The reflection film  603  is disposed to enhance the sensitivity and protect the recording material film  605  from thermal shock by facilitating heat radiation. The recording material film  605  is formed, for example, by sputtering a phase-change type recording material containing tellurium (Te), antimony (Sb), and germanium (Ge) as main components. The dielectric films  604  and  606  are formed to protect the recording material film  605  from humidity or thermal shock. These dielectric films can be omitted. 
   The phase-change type recording material becomes crystalline when gradually cooled after heating and becomes amorphous when abruptly cooled after melting. This property is used in the phase-change type disk, where the crystalline state and the amorphous state of the phase-change type recording material is reversibly changed to each other, so that information can be over-written repeatedly as is done on magnetic disks such as floppy disks and hard disks. Information is recorded on the phase-change type disk as follows. The disk is rotated at a predetermined speed. While the tracking is controlled so as to locate a light beam along the groove track, the intensity of the light beam is changed between the strong amorphous level and the weak crystalline level depending on the signal to be recorded. For example, in the case where the recording is conducted so that the recording mark is in the amorphous state, a light beam with a light amount large enough to melt the film is radiated so as to form a mark in the amorphous state on the film. On the contrary, during the period when no mark is to be formed, a light beam with a light amount small enough to prevent the film from melting is radiated, so as to crystallize the position of the film. At this time, therefore, the position of the film is crystallized regardless of the previous state of the position, amorphous or crystalline. Thus, even the position of the film where information has been recorded is overwritten. The reproduction of the information recorded-on the phase-change type disk is conducted based on the principle that the reflectances of the amorphous state and the crystalline state are different from each other. For example, the disk is illuminated with a constant weak light beam, a reflected light beam from the disk is detected by an optical detector, and a detected variation in the reflected light amount is used to reproduce information. 
   As described above, the optical recording medium  501  of Example 3 is constructed to receive a light beam from the side of the first optical disk  502 . The reason is as follows. Information is recorded in the second optical disk  503  for recording and reproduction by use of heat obtained by absorbing the light beam. Thus, in order to conduct the recording using a light beam with a small light amount, about 60% of the light beam needs to be absorbed by the second optical disk. Accordingly, when the reflectance is about 20%, the transmittance is as small as 20%. If the optical recording medium is constructed to receive a light beam from the side of the second optical disk  503 , in reverse to the case of Example 3, the reflected light amount required at the reading of information recorded on the first information surface  505  will become extremely small. For example, the reflected light amount will be only 4% of the incident light amount after passing through the second optical disk  503  twice even if the reflectance of the reflection film  506  is 100%. The above trouble is avoided in the case of the optical recording medium  501  according to the present invention, which receives a light beam from the side of the first optical disk  502 . The absorbance of the second optical disk  503  can be as large as 60%, while the reflectance thereof can be 40%. Thus, when the reflectance of the reflection film  506  is 20%, for example, a reflected light amount of about 20% of the incident light amount is obtained in the case where information recorded on the first information surface  505  is read, and a reflected light amount of about 26% of the incident light amount is obtained in the case where information recorded in the second optical disk  503  is read. When information recorded in the first optical disk  502  for reproduction only is reproduced, the incident light beam is greatly modulated by pits formed on the first information surface  505 . Accordingly, a reproduced signal with sufficiently high quality can be obtained even when the reflectance of the first reflection film  506  is as low as 20%. 
   As described above, since the optical recording medium  501  of Example 3 is constructed to receive a light beam from the side of the first optical disk  502  for reproduction only, reproduction from both the first and second optical disks  502  and  503  can be conducted with high reliability. 
   Thus, in Example 3, the first and second optical disks  502  and  503  are adhered with an adhesion with a predetermined thickness, and the information surfaces of the two optical disks are illuminated with a light beam from one side of the optical recording medium. Accordingly, a label-can be attached to the other side of the optical recording medium. Also, since information recorded on the dual information surfaces can be reproduced only by changing the position of the focusing point of a light beam by use of one optical head, interactive reproduction is possible, and the cost of the optical recording/reproducing apparatus is reduced. Further, the optical disk for reproduction only and the optical disk for recording and reproduction are combined to form an optical recording medium. Accordingly, for example, information recorded in the optical disk for reproduction only may be processed and the processed information may be recorded in the optical disk for recording and reproduction. This makes it easy to handle the information since related information is stored in the same optical recording medium. Since the thicknesses of the first and second optical disks are the same, these optical disks little change in shape with the change in humidity. This facilitates the adhesion of the optical disks, and thus lowers the cost of the optical recording/reproducing medium. 
   Incidentally, a recording material film similar to the recording material film  605  used in Example 3 may be formed on the second information surface  308  of the optical recording medium of Example 2. Such a recording material film should be formed between the second information surface  308  and the second reflection film  309  of the second optical disk  303 . 
   Thus, according to the present invention, the first optical disk including the semitransparent reflection film formed on the first information surface where information is recorded and the second optical disk including the reflection film formed on the second information surface where information is recorded are adhered with a transparent adhesive so that the information surfaces are closer to each other. Accordingly, the information recorded on the dual information surfaces can be read by illuminating the surfaces with a light beam radiated from one side of the optical recording medium. Thus, nearly double the amount of information can be consecutively reproduced. Since a label can be attached to the other side of the optical recording medium, the identification of the optical recording medium is easy. 
   The thicknesses of the first substrate and the adhesive layer are set at predetermined values. Accordingly, the jitters of reproduced signals obtained from the first and second information surfaces are low, and thus reproduced signals with high quality can be obtained. 
   Alternatively, according to the present invention, the thicknesses of the first and second substrates are made substantially the same, and these substrates are adhered to each other so that the first information surface of the first substrate faces the surface of the second substrate opposite to the second information surface. Such an optical recording medium can be used for both an apparatus designed for an optical recording medium with a 1.2 mm thick substrate and an apparatus designed for an optical recording medium with a 0.6 mm thick substrate. 
   Alternatively, according to the present invention, the optical disk for reproduction only and the optical disk for recording and reproduction are adhered to each other. Information recorded in the optical disk for reproduction only may be processed, for example, and the processed information may be recorded in the optical disk for recording and reproduction. This makes it easy to handle the information since related information is stored in one optical recording medium. 
   Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.